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The evolutionary success of higher plants is largely attributed to their tremendous developmental
plasticity, which allows them to cope with adverse conditions. However, because these adaptations
require investments of resources, they must be tightly regulated to avoid unfavourable trade-offs.
Most of the resources required are macronutrients based on carbon and nitrogen. Limitations in the
availability of these nutrients have major effects on gene expression, metabolism, and overall plant
morphology. These changes are largely mediated by the highly conserved master kinase SNF1-RELATED
PROTEIN KINASE1 (SnRK1), which represses growth and induces catabolic processes. Downstream of
SnRK1, a hub of heterodimerising group C and S1 BASIC LEUCINE ZIPPER (bZIP) transcription factors has
been identified. These bZIPs act as regulators of nutrient homeostasis and are highly expressed in
strong sink tissues, such as flowers or the meristems that initiate lateral growth of both shoots and
roots. However, their potential involvement in controlling developmental responses through their
impact on resource allocation and usage has been largely neglected so far. Therefore, the objective of
this work was to elucidate the impact of particularly S1 bZIPs on gene expression, metabolism, and
plant development.
Due to the high homology and suspected partial redundancy of S1 bZIPs, higher order loss-of-function
mutants were generated using CRISPR-Cas9. The triple mutant bzip2/11/44 showed a variety of robust
morphological changes but maintained an overall growth comparable to wildtype plants. In detail
however, seedlings exhibited a strong reduction in primary root length. In addition, floral transition
was delayed, and siliques and seeds were smaller, indicating a reduced supply of resources to the shoot
and root apices. However, lateral root density and axillary shoot branching were increased, suggesting
an increased ratio of lateral to apical growth in the mutant. The full group S1 knockout
bzip1/2/11/44/53 showed similar phenotypes, albeit far more pronounced and accompanied by
growth retardation. Metabolomic approaches revealed that these architectural changes were
accompanied by reduced sugar levels in distal sink tissues such as flowers and roots. Sugar levels were
also diminished in leaf apoplasts, indicating that long distance transport of sugars by apoplastic phloem
loading was impaired in the mutants. In contrast, an increased sugar supply to the proximal axillary
buds and elevated starch levels in the leaves were measured. In addition, free amino acid levels were
increased in bzip2/11/44 and bzip1/2/11/44/53, especially for the important transport forms
asparagine and glutamine. The increased C and N availability in the proximal tissues could be the cause
of the increased axillary branching in the mutants.
To identify bZIP target genes that might cause the observed shifts in metabolic status, RNAseq
experiments were performed. Strikingly, clade III SUGARS WILL EVENTUALLY BE EXPORTED (SWEET)
8
genes were abundant among the differentially expressed genes. As SWEETs are crucial for sugar export
to the apoplast and long-distance transport through the phloem, their reduced expression is likely to
be the cause of the observed changes in sugar allocation. Similarly, the reduced expression of
GLUTAMINE AMIDOTRANSFERASE 1_2.1 (GAT1_2.1), which exhibits glutaminase activity, could be an
explanation for the abundance of glutamine in the mutants. Additional experiments (ATAC-seq, DAPseq, PTA, q-RT-PCR) supported the direct induction of SWEETs and GAT1_2.1 by S1 bZIPs. To confirm
the involvement of these target genes in the observed S1 bZIP mutant phenotypes, loss-of-function
mutants were obtained, which showed moderately increased axillary branching. At the same time, the
induced overexpression of bZIP11 in axillary meristems had the opposite effect.
Collectively, a model is proposed for the function of S1 bZIPs in regulating sink tissue development. For
efficient long-distance sugar transport, bZIPs may be required to induce the expression of clade III
SWEETs. Thus, reduced SWEET expression in the S1 bZIP mutants would lead to a decrease in apoplastic
sugar loading and a reduced supply to distal sinks such as shoot or root apices. The reduction in longdistance transport could lead to sugar accumulation in the leaves, which would then increasingly be
transported via symplastic routes towards proximal sinks such as axillary branches and lateral roots or
sequestered as starch. The reduced GAT1_2.1 levels lead to an abundance of glutamine, a major
nitrogen transport form. The combined effect on C and N allocation results in increased nutrient
availability in proximal tissues, promoting the formation of lateral plant organs. Alongside emerging
evidence highlighting the power of bZIPs to steer nutrient allocation in other species, a novel but
evolutionary conserved role for S1 bZIPs as regulators of developmental plasticity is proposed, while
the generation of valuable data sets and novel genetic resources will help to gain a deeper
understanding of the molecular mechanisms involved
Bone morphogenetic proteins (BMPs) are involved in various aspects of cell-cell communication in complex life forms. They act as morphogens, help differentiate different cell types from different progenitor cells in development, and are involved in many instances of intercellular communication, from forming a body axis to healing bone fractures, from sugar metabolism to angiogenesis. If the same protein or protein family carries out many functions, there is a demand to regulate and fine-tune their biological activities, and BMPs are highly regulated to generate cell- and context-dependent outcomes.
Not all such instances can be explained yet. Growth/differentiation factor (GDF)5 (or BMP14) synergizes with BMP2 on chondrogenic ATDC5 cells, but antagonizes BMP2 on myoblastic C2C12 cells. Known regulators of BMP2/GDF5 signal transduction failed to explain this context-dependent difference, so a microarray was performed to identify new, cell-specific regulatory components. One identified candidate, the fibroblast growth factor receptor (FGFR)2, was analyzed as a potential new co-receptor to BMP ligands such as GDF5: It was shown that FGFR2 directly binds BMP2, GDF5, and other BMP ligands in vitro, and FGFR2 was able to positively influence BMP2/GDF5-mediated signaling outcome in cell-based assays. This effect was independent of FGFR2s kinase activity, and independent of the downstream mediators SMAD1/5/8, p42/p44, Akt, and p38. The elevated colocalization of BMP receptor type IA and FGFR2 in the presence of BMP2 or GDF5 suggests a signaling complex containing both receptors, akin to other known co-receptors of BMP ligands such as repulsive guidance molecules.
This unexpected direct interaction between FGF receptor and BMP ligands potentially opens a new category of BMP signal transduction regulation, as FGFR2 is the second receptor tyrosine kinase to be identified as BMP co-receptor, and more may follow. The integration of cell surface interactions between members of the FGF and BMP family especially may widen the knowledge of such cellular communication mechanisms which involve both growth factor families, including morphogen gradients and osteogenesis, and may in consequence help to improve treatment options in osteochodnral diseases.
Puberty is an important period of life with physiological changes to enable animals to reproduce. Xiphophorus fish exhibit polymorphism in body size, puberty timing, and reproductive tactics. These phenotypical polymorphisms are controlled by the Puberty (P) locus. In X. nigrensis and X. multilineatus, the P locus encodes the melanocortin 4 receptor (Mc4r) with high genetic polymorphisms.
Mc4r is a member of the melanocortin receptors, belonging to class A G-protein coupled receptors. The Mc4r signaling system consists of Mc4r, the agonist Pomc (precursor of various MSH and of ACTH), the antagonist Agrp and accessory protein Mrap2. In humans, MC4R has a role in energy homeostasis. MC4R and MRAP2 mutations are linked to human obesity but not to puberty.
Mc4rs in X. nigrensis and X. multilineatus are present in three allele classes, A, B1 and B2, of which the X-linked A alleles express functional receptors and the male-specific Y-linked B alleles encode defective receptors. Male body sizes are correlated with B allele type and B allele copy numbers. Late-maturing large males carry B alleles in high copy number while early-maturing small males carry B alleles in low copy number or only A alleles. Cell culture co-expression experiments indicated that B alleles may act as dominant negative receptor mutants on A alleles.
In this study, the main aim was to biochemically characterize the mechanism of puberty regulation by Mc4r in X. nigrensis and X. multilineatus, whether it is by Mc4r dimerization and/or Mrap2 interaction with Mc4r or other mechanisms. Furthermore, Mc4r in X. hellerii (another swordtail species) and medaka (a model organism phylogenetically close to Xiphophorus) were investigated to understand if the investigated mechanisms are conserved in other species.
In medaka, the Mc4r signaling system genes (mc4r, mrap2, pomc, agrp1) are expressed before hatching, with agrp1 being highly upregulated during hatching and first feeding. These genes are mainly expressed in adult brain, and the transcripts of mrap2 co-localize with mc4r indicating a function in modulating Mc4r signaling. Functional comparison between wild-type and mc4r knockout medaka showed that Mc4r knockout does not affect puberty timing but significantly delays hatching due to the retarded embryonic development of knockout medaka. Hence, the Mc4r system in medaka is involved in regulation of growth rather than puberty.
In Xiphophorus, expression co-localization of mc4r and mrap2 in X. nigrensis and X. hellerii fish adult brains was characterized by in situ hybridization. In both species, large males exhibit strikingly high expression of mc4r while mrap2 shows similar expression level in the large and small male and female. Differently, X. hellerii has only A-type alleles indicating that the puberty regulation mechanisms evolved independently in Xiphophorus genus. Functional analysis of Mrap2 and Mc4r A/B1/B2 alleles of X. multilineatus showed that increased Mrap2 amounts induce higher cAMP response but EC50 values do not change much upon Mrap2 co-expression with Mc4r (expressing only A allele or A and B1 alleles). A and B1 alleles were expressed higher in large male brains, while B2 alleles were only barely expressed. Mc4r A-B1 cells have lower cAMP production than Mc4r A cells. Together, this indicates a role of Mc4r alleles, but not Mrap2, in puberty onset regulation signaling. Interaction studies by FRET approach evidenced that Mc4r A and B alleles can form heterodimers and homodimers in vitro, but only for a certain fraction of the expressed receptors. Single-molecule colocalization study using super-resolution microscope dSTORM confirmed that only few Mc4r A and B1 receptors co-localized on the membrane. Altogether, the species-specific puberty onset regulation in X. nigrensis and X. multilineatus is linked to the presence of Mc4r B alleles and to some extent to its interaction with A allele gene products. This is reasoned to result in certain levels of cAMP signaling which reaches the dynamic or static threshold to permit late puberty in large males.
In summary, puberty onset regulation by dominant negative effect of Mc4r mutant alleles is a special mechanism that is found so far only in X. nigrensis and X. multilineatus. Other Xiphophorus species obviously evolved the same function of the pathway by diverse mechanisms. Mc4r in other fish (medaka) has a role in regulation of growth, reminiscent of its role in energy homeostasis in humans. The results of this study will contribute to better understand the biochemical and physiological functions of the Mc4r system in vertebrates including human.
Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal monogenic motoneuron disease in children with unknown etiology caused by mutations in the immunoglobulin μ-binding protein 2 (IGHMBP2) gene coding for DNA/RNA ATPase/helicase. Despite detailed knowledge of the underlying genetic changes, the cellular mechanisms leading to this disease are not well understood. In the Nmd2J ("neuromuscular disorder") mouse, the mouse model for the juvenile form of SMARD1 patients, in which similar pathological features as diaphragmatic paralysis and skeletal muscle atrophy are observed. Ex vivo studies in Nmd2J mice showed that loss of the motor axon precedes atrophy of the gastrocnemius muscle and does not correlate with neurotransmission defects in the motor endplate. The already described independent myogenic anomalies in the diaphragm and heart of the Nmd2J mouse raised the question whether spinal motoneuron degeneration develops cell autonomously. Ighmbp2 is predominantly localized in the cytoplasm and seems to bind to ribosomes and polysomes, suggesting a role in mRNA metabolism.
In this Ph.D. thesis, morphological and functional analyses of isolated Ighmbp2-deficient (Ighmbp2-def.) motoneurons were performed to answer the question whether the SMARD1 phenotype results from dysregulation of protein biosynthesis. Ighmbp2-deficient motoneurons show only negligible morphological alterations with respect to a slight increase in axonal branches. This observation is consistent with only minor changes of transcriptome based on RNA sequencing data from Ighmbp2-deficient motoneurons. Only the mRNA of fibroblast growth factor receptor 1 (Fgfr1) showed significant up-regulation in Ighmbp2-deficient motoneurons. Furthermore, no global aberrations at the translational level could be detected using pulsed SILAC (Stable Isotope Labeling by Amino acids in cell culture), AHA (L-azidohomoalanine) labeling and SUnSET (SUrface SEnsing of Translation) methods. However, a reduced β-actin protein level was observed at the growth cones of Ighmbp2-deficient motoneurons, which was accompanied with a reduced level of Imp1 protein, a known β-actin mRNA interactor. Live-cell imaging studies using fluorescence recovery after photobleaching (FRAP) showed translational down-regulation of eGFPmyr-β-actin 3'UTR mRNA in the growth cones and the cell bodies, although the amount of β-actin mRNA and the total protein amount in Ighmbp2-deficient motoneurons showed no aberrations. This compartment-specific reduction of β-actin protein occurred independently of a non-existent direct IGHMBPF2 binding to β-actin mRNA. Fgfr1, which was upregulated on the RNA level, did not show an increased protein amount in Ighmbp2-deficient motoneurons, whereas a reduced amount could be detected. Interestingly, a correlation could be found between the reduced amount of the Imp1 protein and the increased Fgfr1 mRNA, since the IMP1 protein binds the FGFR1 mRNA and thus could influence the transport and translation of FGFR1 mRNA. In summary, all data suggest that Ighmbp2 deficiency leads to a local but modest disturbance of protein biosynthesis, which might contribute to the motoneuron defects of SMARD1.
Wilms tumor protein 1 (WT1) is a suitable target to develop an immunotherapeutic approach against high risk acute myeloid leukemia (AML), particularly their relapse after allogeneic hematopoietic stem cell transplantation (HSCT). As an intracellular protein traversing between nucleus and cytoplasm, recombinant expression of WT1 is difficult. Therefore, an induction of WT1-specific T-cell responses is mostly based on peptide vaccination as well as dendritic cell (DC) electroporation with mRNA encoding full-length protein to mount WT1-derived peptide variations presented to T cells. Alternatively, the WT1 peptide presentation could be broadened by forcing receptor-mediated endocytosis of DCs.
In this study, antibody fusion proteins consisting of an antibody specific to the human DEC205 endocytic receptor and various fragments of WT1 (anti-hDEC205-WT1) were generated for a potential DC-targeted recombinant WT1 vaccine. Anti-hDEC205-WT1 antibody fusion proteins containing full-length or major parts of WT1 were not efficiently expressed and secreted due to their poor solubility and secretory capacity. However, small fragment-containing variants: anti-hDEC205-WT110-35, anti-hDEC205-WT191-138, anti-hDEC205-WT1223-273, and anti-hDEC205-WT1324-371 were obtained in good yields.
Since three of these fusion proteins contain the most of the known immunogenic epitopes in their sequences, the anti-hDEC205-WT191-138, anti-hDEC205-WT1223-273, and anti-hDEC205-WT1324-371 were tested for their T-cell stimulatory capacities. Mature monocyte-derived DCs loaded with anti-hDEC205-WT191-138 could induce ex vivo T-cell responses in 12 of 16 blood samples collected from either healthy or HSC transplanted individuals compared to included controls (P < 0.01). Furthermore, these T cells could kill WT1-overexpressing THP-1 leukemia cells in vitro after expansion.
In conclusion, alongside proving the difficulty in expression and purification of intracellular WT1 as a vaccine protein, our results from this work introduce an alternative therapeutic vaccine approach to improve an anti-leukemia immune response in the context of allogeneic HSCT and potentially beyond.
Spermiogenesis describes the differentiation of haploid germ cells into motile, fertilization-competent spermatozoa. During this fundamental transition the species-specific sperm head is formed, which necessitates profound nuclear restructuring coincident with the assembly of sperm-specific structures and chromatin compaction. In the case of the mouse, it is characterized by reshaping of the early round spermatid nucleus into an elongated sickle-shaped sperm head. This tremendous shape change requires the transduction of cytoskeletal forces onto the nuclear envelope (NE) or even further into the nuclear interior. LINC (linkers of nucleoskeleton and cytoskeleton) complexes might be involved in this process, due to their general function in bridging the NE and thereby physically connecting the nucleus to the peripheral cytoskeleton.
LINC complexes consist of inner nuclear membrane integral SUN-domain proteins and outer nuclear membrane KASH-domain counterparts. SUN- and KASH-domain proteins are directly connected to each other within the perinuclear space, and are thus capable of transferring forces across the NE. To date, these protein complexes are known for their essential functions in nuclear migration, anchoring and positioning of the nucleus, and even for chromosome movements and the maintenance of cell polarity and nuclear shape.
In this study LINC complexes were investigated with regard to their potential role in sperm head formation, in order to gain further insight into the processes occurring during spermiogenesis. To this end, the behavior and function of the testis-specific SUN4 protein was studied. The SUN-domain protein SUN4, which had received limited characterization prior to this work, was found to be exclusively expressed in haploid stages during germ cell development. In these cell stages, it specifically localized to the posterior NE at regions decorated by the manchette, a spermatid-specific structure which was previously shown to be involved in nuclear shaping. Mice deficient for SUN4 exhibited severely disorganized manchette residues and gravely misshapen sperm heads. These defects resulted in a globozoospermia-like phenotype and male mice infertility. Therefore, SUN4 was not only found to be mandatory for the correct assembly and anchorage of the manchette, but also for the correct localization of SUN3 and Nesprin1, as well as of other NE components. Interaction studies revealed that SUN4 had the potential to interact with SUN3, Nesprin1, and itself, and as such is likely to build functional LINC complexes that anchor the manchette and transfer cytoskeletal forces onto the nucleus.
Taken together, the severe impact of SUN4 deficiency on the nucleocytoplasmic junction during sperm development provided direct evidence for a crucial role of SUN4 and other LINC complex components in mammalian sperm head formation and fertility.
Die Venusfliegenfalle, Dionaea muscipula, weckte aufgrund ihrer karnivoren Lebensweise schon sehr früh das Interesse vieler Wissenschaftler. Für karnivore Pflanzen, die auf Nährstoff-armen Böden wachsen, spielen Insekten als Beute und somit als Nährstofflieferant eine entscheidende Rolle. So können die Pflanzen durch die Verdauung der Beute mit wichtigen Makro- und Mikronährstoffen, wie Stickstoff, Phosphat, Kalium oder Natrium versorgt werden. Aus diesem Grund sollte im Rahmen meiner Arbeit ein besonderes Augenmerk auf die molekularen Mechanismen der Kationenaufnahme während der Nährstoffresorption gerichtet werden. Insbesondere die aus dem Insekt stammenden Nährstoffe Kalium und Natrium waren dabei von großem Interesse.
Im Allgemeinen sind Kaliumionen für Pflanzen eine essentielle anorganische Substanz und von großer physiologischer Bedeutung für die Entwicklung, den Metabolismus, die Osmoregulation, das Membranpotential und viele zelluläre Prozesse. Analysen der Kaliumaufnahme an Wurzeln von Modellpflanzen wie Arabidopsis thaliana und Reis zeigten, dass die Aufnahme von K+ ein Zusammenspiel von hoch-affinen K+-Transportern der HAK5-Familie und nieder-affinen Kaliumkanälen (AKT1/AtKC1) erfordert, die in ein komplexes
(De-)Phosphorylierungsnetzwerk eingebunden sind. In der vorliegenden Arbeit war es mir möglich das Netzwerk zur Kaliumaufnahme in den Drüsen der Venusfliegenfalle zu entschlüsseln. Es konnten Orthologe zum Kaliumtransporter HAK5 aus Arabidopsis (DmHAK5) und zum Kaliumkanal AKT1 (DmKT1) identifiziert und im heterologen Expressionssystem der Xenopus laevis Oozyten elektrophysiologisch charakterisiert werden. Dabei zeigte sich, das DmKT1 durch einen Ca2+-Sensor/Kinase-Komplex aus der CBL/CIPK-Familie phosphoryliert und somit aktiviert wird. Phylogenetische Analysen von DmKT1 bestätigten die Eingruppierung dieses Kaliumkanals in die Gruppe der pflanzlichen Shaker-Kaliumkanäle des AKT1-Typs. Die Transporteigenschaften zeigten zudem, dass DmKT1 bei hyperpolarisierenden Membranpotentialen aktiviert wird und einen K+-selektiven Einwärtsstrom vermittelt. In Oozyten konnte eine Kaliumaufnahme bis zu einer externen Konzentration von ≥1 mM beobachtet werden. DmKT1 repräsentiert also einen Kaliumkanal mit einer hohen Transportkapazität, der die nieder-affine Kaliumaufnahme in die Drüsenzellen der Venusfliegenfalle vermitteln kann.
Unterhalb einer externen Kaliumkonzentration von 1 mM würde der anliegende elektrochemische Kaliumgradient einen Kaliumausstrom und somit einen Verlust von Kalium favorisieren. Hoch-affine K+/H+-Symporter können durch die Ausnutzung des Protonengradienten eine Kaliumaufnahme im mikromolaren Bereich gewährleisten. In Wurzelhaaren von Arabidopsis vermittelt der Transporter AtHAK5 die Kaliumaufnahme unter Kaliummangelbedingungen. DmHAK5, ein Ortholog zu AtHAK5, ist in Dionaea Drüsen exprimiert und konnte zum ersten Mal im heterologen Expressionssystem der Xenopus Oozyten im Detail charakterisiert werden. Interessanterweise zeigte sich, dass DmHAK5 wie der K+-Kanal DmKT1 durch denselben CBL/CIPK-Komplex posttranslational reguliert und aktiviert wird. Die Transporteigenschaften von DmHAK5 wiesen auf einen Transporter mit einer breiten Substratspezifität hin, sodass sich DmHAK5 neben Kalium auch für Ammonium permeabel zeigte. Affinitätsuntersuchungen von DmHAK5 zu seinem Substrat Kalium klassifizierten das Protein als einen hoch-affinen Kaliumtransporter, der im Symport mit Protonen die Kaliumaufnahme im mikromolaren Konzentrationsbereich vermitteln kann.
Das Kaliumtransportmodul besteht also aus dem K+-selektiven Kanal DmKT1 und dem
K+/H+-Symporter DmHAK5, die die hoch- und nieder-affine Kaliumaufnahme in den Drüsenzellen während der Beuteverdauung in Dionaea muscipula Fallen ermöglichen. Beide Transportmodule werden Kalzium-abhängig durch die Kinase CIPK23 und den Ca2+-Sensor CBL9 auf posttranslationaler Ebene reguliert.
Zusammenfassend gelang es in dieser Arbeit Einblicke in die Kationenaufnahme während der Nährstoffresorptionsphase der Venusfliegenfalle, Dionaea muscipula, zu gewinnen. Dabei wurde klar, dass Dionaea muscipula im Laufe ihrer Evolution zu einer karnivoren Pflanze, nicht neue Transportmodule zur Nährstoffresorption aus der Beute entwickelte, sondern bekannte aus Wurzeln stammende Transportmodule umfunktionierte. Auf molekularer Ebene konnten die biophysikalischen Charakteristika der K+- und Na+-Transportproteine, sowie ihre Regulation entschlüsselt werden. Diese Erkenntnisse wurden schließlich in den Kontext des Beutefangs der Venusfliegenfalle gebracht und diskutiert.
LIM and SH3 protein 1 (LASP1) is a nucleocytoplasmic scaffolding protein. LASP1 interacts with various cytoskeletal proteins via its domain structure and is known to participate in physiological processes of cells. In the present study, a detailed investigation of the expression pattern of LASP1 protein in normal skin, melanocytic nevi and melanoma was carried out and the melanocyte–specific function of LASP1 was analyzed. LASP1 protein was identified in stratum basale of skin epidermis and a very high level was detected in nevi, the benign tumor of melanocyte. In the highly proliferative basal cells, an additional distinct nuclear localization of the protein was noted. In different tumor entities, an elevated LASP1 expression and nuclear localization, correlated positively with malignancy and tumor grade. However, LASP1 level was determined to be very low in melanoma and even reduced in metastases. Melanoma is distinguished as the first tumor tested to date – that displayed an absence of elevated LASP1 expression. In addition no significant relation was observed between LASP1 protein expression and clinicopathological parameters in melanoma.
The epidermal melanin unit of skin comprises of melanocytes and keratinocytes. Melanocytes are specialized cells that synthesize the photo protective coloring pigment, melanin inside unique organelles called melanosomes. The presence of LASP1 in melanocytes is reported for the first time through this study and the existence was confirmed by immunoblotting analysis in cultured normal human epidermal melanocyte (NHEM) and in melanoma cell lines, along with the immunohistostaining imaging in normal skin and in melanocytic nevi. LASP1 depletion in MaMel2 cells revealed a moderate increase in the intracellular melanin level independently of de novo melanogenesis, pointing to a partial hindrance in melanin release. Immunofluorescence images of NHEM and MaMel2 cells visualized co-localization of LASP1 with dynamin and tyrosinase concomitant with melanosomes at the dendrite tips of the cells. Melanosome isolation experiments by sucrose density gradient centrifugation clearly demonstrated the presence of LASP1 and the melanosome specific markers tyrosinase and TRP1 in late stage melanosomes.
The study identified LASP1 and dynamin as novel binding partners in melanocytes and provides first evidence for the existence of LASP1 and dynamin (a protein well–known for its involvement in vesicle formation and budding) in melanosomes. Co-localization of LASP1 and dynamin along the dendrites and at the tips of the melanocytes indicates a potential participation of the two proteins in the membrane vesicle fission at the plasma membrane.
In summary, a possible involvement of LASP1 in the actin–dynamin mediated membrane fission and exocytosis of melanin laden melanosome vesicles into the extracellular matrix is suggested.
The honeybee Apis mellifera is a social insect well known for its complex behavior and the ability to learn tasks associated with central place foraging, such as visual navigation or to learn and remember odor-reward associations. Although its brain is smaller than 1mm² with only 8.2 x 105 neurons compared to ~ 20 x 109 in humans, bees still show amazing social, cognitive and learning skills. They express an age – related division of labor with nurse bees staying inside the hive and performing tasks like caring for the brood or cleaning, and foragers who collect food and water outside the hive. This challenges foragers with new responsibilities like sophisticated navigation skills to find and remember food sources, drastic changes in the sensory environment and to communicate new information to other bees. Associated with this plasticity of the behavior, the brain and especially the mushroom bodies (MBs) - sensory integration and association centers involved in learning and memory formation – undergo massive structural and functional neuronal alterations. Related to this background my thesis on one hand focuses on neuronal plasticity and underlying molecular mechanisms in the MBs that accompany the nurse – forager transition.
In the first part I investigated an endogenous and an internal factor that may contribute to the nurse - forager phenotype plasticity and the correlating changes in neuronal network in the MBs: sensory exposure (light) and juvenile hormone (JH). Young bees were precociously exposed to light and subsequently synaptic complexes (microglomeruli, MG) in the MBs or respectively hemolymph juvenile hormone (JH) levels were quantified. The results show that light input indeed triggered a significant decrease in MG density, and mass spectrometry JH detection revealed an increase in JH titer. Interestingly light stimulation in young bees (presumably nurse bees) triggered changes in MG density and JH levels comparable to natural foragers. This indicates that both sensory stimuli as well as the endocrine system may play a part in preparing bees for the behavioral transition to foraging.
Considering a connection between the JH levels and synaptic remodeling I used gene knockdown to disturb JH pathways and artificially increase the JH level. Even though the knockdown was successful, the results show that MG densities remained unchanged, showing no direct effect of JH on synaptic restructuring.
To find a potential mediator of structural synaptic plasticity I focused on the calcium-calmodulin-dependent protein kinase II (CaMKII) in the second part of my thesis. CaMKII is a protein known to be involved in neuronal and behavioral plasticity and also plays an important part in structural plasticity reorganizing synapses. Therefore it is an interesting candidate for molecular mechanisms underlying MG reorganization in the MBs in the honeybee. Corresponding to the high abundance of CaMKII in the learning center in vertebrates (hippocampus), CaMKII was shown to be enriched in the MBs of the honeybee. Here I first investigated the function of CaMKII in learning and memory formation as from vertebrate work CaMKII is known to be associated with the strengthening of synaptic connections inducing long term potentiation and memory formation. The experimental approach included manipulating CaMKII function using 2 different inhibitors and a specific siRNA to create a CaMKII knockdown phenotype. Afterwards bees were subjected to classical olfactory conditioning which is known to induce stable long-term memory. All bees showed normal learning curves and an intact memory acquisition, short-term and mid-term memory (1 hour retention). However, in all cases long-term memory formation was significantly disrupted (24 and 72 hour retention). These results suggests the necessity of functional CaMKII in the MBs for the induction of both early and late phases of long-term memory in honeybees. The neuronal and molecular bases underlying long-term memory and the resulting plasticity in behavior is key to understanding higher brain function and phenotype plasticity. In this context CaMKII may be an important mediator inducing structural synaptic and neuronal changes in the MB synaptic network.
The auditory system is an exquisitely complex sensory organ dependent upon the synchronization of numerous processes for proper function. The molecular characterization of hereditary hearing loss is complicated by extreme genetic heterogeneity, wherein hundreds of genes dispersed genome-wide play a central and irreplaceable role in normal hearing function. The present study explores this area on a genome-wide and single gene basis for the detection of genetic mutations playing critical roles in human hearing.
This work initiated with a high resolution SNP array study involving 109 individuals. A 6.9 Mb heterozygous deletion on chromosome 4q35.1q35.2 was identified in a syndromic patient that was in agreement with a chromosome 4q deletion syndrome diagnosis. A 99.9 kb heterozygous deletion of exons 58-64 in USH2A was identified in one patient. Two homozygous deletions and five heterozygous deletions in STRC (DFNB16) were also detected. The homozygous deletions alone were enough to resolve the hearing impairment in the two patients. A Sanger sequencing assay was developed to exclude a pseudogene with a high percentage sequence identity to STRC from the analysis, which further solved three of the six heterozygous deletion patients with the hemizygous, in silico predicted pathogenic mutations c.2726A>T (p.H909L), c.4918C>T (p.L1640F), and c.4402C>T (p.R1468X). A single patient who was copy neutral for STRC and without pathogenic copy number variations had compound heterozygous mutations [c. 2303_2313+1del12 (p.G768Vfs*77) and c.5125A>G (p.T1709A)] in STRC. It has been shown that STRC has been previously underestimated as a hearing loss gene. One additional patient is described who does not have pathogenic copy number variation but is the only affected member of his family having hearing loss with a paternally segregating translocation t(10;15)(q26.13;q21.1).
Twenty-four patients without chromosomal aberrations and the above described patient with an USH2A heterozygous deletion were subjected to a targeted hearing loss gene next generation sequencing panel consisting of either 80 or 129 hearing-relevant genes. The patient having the USH2A heterozygous deletion also disclosed a second mutation in this gene [c.2276G>T (p.C759F)]. This compound heterozygous mutation is the most likely cause of hearing loss in this patient. Nine mutations in genes conferring autosomal dominant hearing loss [ACTG1 (DFNA20/26); CCDC50 (DFNA44); EYA4 (DFNA10); GRHL2 (DFNA28); MYH14 (DFNA4A); MYO6 (DFNA22); TCF21 and twice in MYO1A (DFNA48)] and four genes causing autosomal recessive hearing loss were detected [GJB2 (DFNB1A); MYO7A (DFNB2); MYO15A (DFNB3), and USH2A]. Nine normal hearing controls were also included. Statistical significance was achieved comparing controls and patients that revealed an excess of mutations in the hearing loss patients compared to the control group. The family with the GRHL2 c.1258-1G>A mutation is only the second family published worldwide with a mutation described in this gene to date, supporting the initial claim of this gene causing DFNA28 hearing loss. Audiogram analysis of five affected family members uncovered the progressive nature of DFNA28 hearing impairment. Regression analysis predicted the annual threshold deterioration in each of the five family members with multiple audiograms available over a number of years.
Besides HIV and tuberculosis, malaria still is one of the most devastating infectious diseases especially in developing countries, with Plasmodium falciparum being responsible for the frequently lethal form of malaria tropica. It is a major cause of mortality as well as morbidity, whereby pregnant women and children under the age of five years are most severely affected. Rapidly emerging drug resistances and the lack of an effective and safe vaccine hamper the combat against malaria by chemical and pharmacological regimens, and moreover the poor socio-economic and healthcare conditions in malaria-endemic countries are compromising the extermination of this deadly tropical disease to a large extent. Malaria research is still questing for druggable targets in the parasitic protozoan which pledge to be refractory against evolving resistance-mediating mutations and yet constitute affordable and compliant antimalarial chemotherapeutics.
The parasite kinome consists of members that represent most eukaryotic protein kinase groups, but also contains several groups that can not be assigned to conservative ePK groups. Moreover, given the remarkable divergence of plasmodial kinases in respect to the human host kinome and the fact that several plasmodial kinases have been identified that are essential for the intraerythrocytic developmental cycle, these parasite enzymes represent auspicious targets for antimalarial regimens. Despite elaborate investigations on several other ePK groups, merely scant research has been conducted regarding the four identified members of the cyclin-dependent kinase-like kinase (CLK) family, PfCLK-1-4. In other eukaryotes, CLKs are involved in mRNA processing and splicing by means of phosphorylation of serine/arginine-rich (SR) proteins, which are crucial components of the splicing machinery in the alternative splicing pathway. All four PfCLKs are abundantly expressed in asexual parasites and gametocytes, and stage-specific expression profiles of PfCLK-1 and PfCLK-2 exhibited nucleus-associated localization and an association with phosphorylation activity. In the course of this study, PfCLK-3 and PfCLK-4 were functionally characterized by indirect immunofluorescence, Western blot analysis and kinase activity assays. These data confirm that the two kinases are primarily expressed in the nucleus of trophozoites and both kinases possess in vitro phosphorylation activity on physiological substrates. Likewise PfCLK-1 and PfCLK-2, reverse genetic studies exhibited the indispensability of both PfCLKs on the asexual life cycle of P. falciparum, rendering them as potential candidates for antiplasmodial strategies. Moreover, this study was conducted to identify putative SR proteins as substrates of all four PfCLKs. Previous alignments revealed a significant homology of the parasite CLKs to yeast SR protein kinase Sky1p. Kinase activity assays showed in vitro phosphorylation of the yeast Sky1p substrate and SR protein Npl3p by precipitated PfCLKs. In addition, four homologous plasmodial SR proteins were identified that are phosphorylated by PfCLKs in vitro: PfASF-1, PFSRSF12, PfSFRS4 and PfSR-1. All four parasite SR splicing factors are predominantly expressed in the nuclei of trophozoites. For PfCLK-1, a co-localization with the SR proteins was verified.
Finally, a library of human and microbial CLK inhibitors and the antiseptic chlorhexidine (CHX) was screened to determine their inhibitory effect on different parasite life cycle stages and on the PfCLKs specifically. Five inhibitors out of 63 compounds from the investigated library were selected that show a moderate inhibition on asexual life cycle stages with IC50 values ranging between approximately 4 and 8 µM. Noteworthy, these inhibitors belong to the substance classes of aminopyrimidines or oxo-β-carbolines. Actually, the antibiotic compound CHX demonstrated an IC50 in the low nanomolar range. Stage-of-inhibition assays revealed that CHX severely affects the formation of schizonts. All of the selected CLKs inhibitors also affect gametocytogenesis as well as gametogenesis, as scrutinized in gametocyte toxicity assays and exflagellation assays, respectively. Kinase activity assays confirm a specific inhibition of CLK-mediated phosphorylation of all four kinases, when the CLK inhibitors are applied on immunoprecipitated PfCLKs. These findings on PfCLK-inhibiting compounds are initial attempts to determine putative antimalarial compounds targeting the PfCLKs. Moreover, these results provide an effective means to generate chemical kinase KOs in order to phenotypically study the role of the PfCLKs especially in splicing events and mRNA metabolism. This approach of functionally characterizing the CLKs in P. falciparum is of particular interest since the malarial spliceosome is still poorly understood and will gain further insight into the parasite splicing machinery.
Mitochondria are organelles of endosymbiotic origin, which play many important roles in eukaryotic cells. Mitochondria are surrounded by two membranes and, considering that most of the mitochondrial proteins are produced in the cytosol, possess import machineries, which transport mitochondria-targeted proteins to their designated location. A special class of outer mitochondrial membrane (OMM) proteins, the β-barrel proteins, require the sorting and assembly machinery (SAM) for their OMM integration. Both mitochondrial β-barrel proteins and the central component of the SAM complex, Sam50, have homologs in gram-negative bacteria. In yeast mitochondria, bacterial β-barrel proteins can be imported and assembled into the OMM. Our group demonstrated that this, however, is not the case for human mitochondria, which import only neisserial β barrel proteins, but not those of Escherichia coli and Salmonella enterica. As a part of this study, I could demonstrate that β-barrel proteins such as Omp85 and PorB of different Neisseria species are targeted to human mitochondria. Interestingly, only proteins belonging to the neisserial Omp85 family were integrated into the OMM, whereas PorB was imported into mitochondria but not assembled. By exchanging parts of homologous neisserial Omp85 and E. coli BamA and, similarly, of neisserial PorB and E. coli OmpC, it could be demonstrated in this work that the mitochondrial import signal of bacterial β barrel proteins cannot be limited to one short linear sequence, but rather secondary structure and protein charge seem to play an important role, as well as specific residues in the last β-strand of Omp85. Omp85 possesses five conserved POTRA domains in its amino-terminal part. This work additionally demonstrated that in human mitochondria, at least two POTRA domains of Omp85 are necessary for membrane integration and functionality of Omp85. In the second part of this work, the influence of Sam50 on the mitochondrial cristae structure was investigated. This work contributed to a study performed by our group in which it was confirmed that Sam50 is present in a high molecular weight complex together with mitofilin, CHCHD3, CHCHD6, DnaJC11, metaxin 1 and metaxin 2. This connection between the inner and outer mitochondrial membrane was shown to be crucial for the maintenance of the mitochondrial cristae structure. In addition, a role of Sam50 in respiratory complex assembly, suggested by a SILAC experiment conducted in our group, could be confirmed by in vitro import studies. An influence of Sam50 not only on respiratory complexes but also on the recently described respiratory complex assembly factor TTC19 was demonstrated. It was shown that TTC19 not only plays a role in complex III assembly as published, but also influences the assembly of respiratory complex IV. Thus, in this part of the work a connection between the OMM protein Sam50 and maintenance of cristae structure, respiratory complex assembly and an assembly factor could be established.
p21-aktivierte Kinasen regulieren zahlreiche zelluläre Prozesse, die während der Entwicklung, aber auch beispielsweise bei der Krebsentstehung, von zentraler Bedeutung sind. Mbt, das einzige Typ II PAK-Protein von Drosophila melanogaster, spielt eine Rolle bei der Gehirnentwicklung. Eine Nullmutation von mbt, mbtP1, bildet kleinere Gehirne mit stark verkleinerten Pilzkörpern aus. In dieser Arbeit wurde die Funktion von Mbt in Neuroblasten untersucht. Mbt wurde als Teil des apikalen Proteinkomplexes in Neuroblasten des Zentralhirns nachgewiesen. Die apikale Lokalisation von Mbt ist Zellzyklus-abhängig und wird über Bindung an Cdc42 reguliert. Sie ist essentiell für die Funktion von Mbt in Neuroblasten. Trotz apikaler Mbt-Lokalisation in Neuroblasten zeigte die mbt Nullmutante keine Defekte des basalen Mechanismus der asymmetrischen Zellteilung. Mud zeigte geringfügige Lokalisationsveränderungen, die auf einen möglichen Einfluss von Mbt hinweisen. Obwohl PAKs zentrale Regulatoren des Zytoskeletts sind, zeigte die mbtP1 Mutante keine offensichtlichen Veränderungen des Aktin- und Tubulin-Zytoskeletts. Armadillo, ein Aktin-assoziiertes Mbt-Substrat, zeigte ebenfalls keine Lokalisationsveränderung in Neuroblasten. Mbt steuert jedoch die apikale Anreicherung von Cno, einem weiteren Aktin-assoziierten Protein, in Neuroblasten. Darüber hinaus beeinflusst Mbt die Zellgröße von Neuroblasten, sowie deren Proliferationspotenzial und Überleben. mbtP1 Neuroblasten sind kleiner als wildtypische Neuroblasten, haben ein geringeres Proliferationsvermögen und eine geringere Überlebenswahrscheinlichkeit. Der Zelltod von Neuroblasten ist jedoch ein sekundärer Effekt. Daher kann eine Blockierung von Apoptose den adulten Pilzkörperphänotyp nicht retten. Signalwege, die Zellgröße und Proliferation regulieren, wurden auf eine Beteiligung von Mbt hin analysiert. mbtP1 induzierte leichte Effekte im Insulin-Signalweg und die Delokalisation eines nukleolären Proteins. Eine genetische Interaktion von mbtP1 mit Mutationen in Genen des klassischen MAPK-Signalweges identifzierte mbt als Positivregulator dieses Signalweges im Auge. Ein ähnlicher, schwächerer Effekt wurde auch bzgl. der Proliferation und Größe von Neuroblasten beobachtet. Eine 2D-Gelanalyse von Larvengehirnen identifizierte Bic und Hsp83 als mögliche von Mbt regulierte Proteine. Diese Arbeit charakterisiert eine bisher unbekannte Funktion der p21-aktivierten Kinase Mbt in neuronalen Stammzellen und liefert damit Ansatzpunkte für eine detaillierte Aufklärung der Funktionsmechanismen von Typ II PAKs bei der Regulation von Zellproliferation und Überleben
SYCE3, ein neues Synaptonemalkomplexprotein: Expression, funktionelle Analyse und Bindungspartner
(2011)
Der Synaptonemalkomplex ist eine evolutionär hoch konservierte Struktur. Er wird spezifisch während der Prophase I der Meiose ausgebildet und ist essentiell für die Segregation der homologen Chromosomen während der Meiose und auch für die Entstehung genetischer Vielfalt. Der Synaptonemalkomplex ist eine proteinöse Struktur, deren Aufbau dem einer Leiter ähnelt. Dabei werden die Leiterholme als Lateralelemente bezeichnet. Sie bestehen unter anderem aus den Proteinen SYCP2 und SYCP3 und assoziieren mit dem Chromatin der homologen Chromosomen. Die Stufen der Leiter bestehen hingegen aus Transversalfilamenten, deren Hauptkomponente parallele Homodimere des meiosespezifische Proteins SYCP1 sind. Dabei wird ein SYCP1 Dimer mit seinem C-Terminus in den Lateralelementen verankert und kann über seine N-terminale Domäne eine schwache Interaktion mit der N-terminalen Domäne eines gegenüberliegenden SYCP1 Dimers eingehen. Um diese Bindung zu stabilisieren werden Proteine des Zentralelements des Synaptonemalkomplexes benötigt: Während SYCE1 durch seine Interaktion mit SYCP1 die N-terminale Assoziation zweier gegenüberliegender SYCP1 Dimere stabilisiert, verknüpfen die zwei anderen zentralelementspezifischen Proteine SYCE2 und Tex12 lateral benachbarte SYCP1 Filamente und breiten so das SYCP1 Netzwerk entlang der chromosomalen Achsen aus. Dieser Prozess wird als Synapse bezeichnet und stellt eines der Schlüsselereignisse der Meiose dar. Fehler während dieses Prozesses führen meist zu Aneuploidie der entstehenden Gameten oder zum Abbruch der Meiose und somit zu Infertilität des betroffenen Organismus. In dieser Arbeit wurde mit SYCE3 ein neues Protein des murinen Synaptonemalkomplexes charakterisiert. Es konnte gezeigt werden, dass SYCE3 meiosespezifisch in Männchen und Weibchen exprimiert wird und Bestandteil des Zentralelements des Synaptonemalkomplexes ist. Hierbei zeigt es dasselbe Verteilungsmuster wie SYCP1 und SYCE1 und kann mit beiden Proteinen interagieren. Eine zusätzliche Interaktion konnte zwischen SYCE3 und SYCE2 nachgewiesen werden. Durch Untersuchungen an entsprechenden Knockout Mausmodellen konnte in dieser Arbeit außerdem gezeigt werden, dass SYCE3 in Abwesenheit von SYCP1 nicht an die chromosomalen Achsen rekrutiert werden kann. Die Ausbildung der Lateralelemente und auch die Anwesenheit der anderen zentralelementspezifischen Proteine SYCE1 und SYCE2 sind hingegen für die Anlagerung von SYCE3 an die chromosomalen Achsen nicht essentiell. Somit steht SYCE3 hinsichtlich seiner Bedeutung für die Paarung und die Synapse der homologen Chromosomen hierarchisch offenbar über den bisher beschriebenen Zentralelementproteinen SYCE1, SYCE2 und Tex12. Die funktionelle Bedeutung von SYCE3 für die Synapse der homologen Chromosomen und für den korrekten Ablauf der homologen Rekombination wurde im Rahmen dieser Arbeit durch die Herstellung und die Charakterisierung einer Syce3-/- Maus detailliert untersucht: Dabei führte der Knockout von SYCE3 zur Infertilität in beiden Geschlechtern, die gleichzeitig mit einer signifikanten Reduktion der Größe der entsprechenden Hoden und Ovarien im Vergleich zum Wildtyp einherging. Weitere Untersuchungen ergaben zudem, dass es in Syce3 defizienten Tieren zu einem Abbruch der Meiose kommt. Dabei hatte das Fehlen von SYCE3 keinen Einfluss auf die Ausbildung der Axialelemente. Die Initiation der Synapse hingegen war sowohl in Oocyten als auch in Spermatocyten in Abwesenheit von SYCE3 stark gestört. Darüber hinaus konnte in der vorliegenden Arbeit nachgewiesen werden, dass das Fehlen von SYCE3 Einfluss auf die homologe Rekombination nimmt: Zwar können sich frühe (DNA Doppelstrangbrüche) und intermediäre (Transitionsknoten) Rekombinationsereignisse in der Abwesenheit von SYCE3 ausbilden, die Prozessierung zu späten Rekombinationsstrukturen (Rekombinationsknoten) und die damit einhergehende Ausbildung von Crossing-over Strukturen fand jedoch nicht statt. Zusammengefasst wurde in dieser Arbeit gezeigt, dass das neue Synaptonemalkomplexprotein SYCE3 essentiell für die Fertilität von Mäusen ist. Durch den Knockout von Syce3 kann die Synapse zwischen den Homoligen nicht initiiert werden und es findet kein Crossing-over statt. Im Assembly Prozess des Synaptonemalkomplexes agiert SYCE3 oberhalb der anderen zentralelementspezifischen Proteine und unterhalb von SYCP1.
The present study was aimed at revealing the early signalling events during the interaction of the diazotrophic soil bacterium Azospirillum brasilense with its host plant Arabidopsis thaliana. Furthermore, taking advantage of the micro array technique, a comprehensive overview of Arabidopsis genes has been undertaken which are affected upon association with A. brasilense The characterization of the early responses of Arabidopsis plants upon inoculation with Azospirillum brasilense strain Sp7 clearly indicated parallels with the initial events in plant pathogen interaction. For instance, not only bacterial preprations (lysates) form Azospirillum elicited an apoplastic alkalinization of the culture medium, but also the live bacteria, which were even more effective. Besides, in a luminol based assay, the bacterial lysates triggered production of the reactive oxygen species (ROS) in the Arabidopsis leaf discs. Interestingly, the elongation factor receptor mutants (efr) were completely insensitive to Azospirillum, suggesting elongation factor Tu (EF-TU) recognition as elicitor by Arabidopsis. This hypothesis was further validated with a bioinformatic approach. The N terminus initial 26 amino acids from Azospirillum EF-TU gene (elf26) showed more similarity to the elf26 sequences of bacteria like Agrobacterium tumefaciens which elicit responses in the plants through EF-TU rather than Pseudomonas syringae where the potent elicitor is flagellin 22. Universal transcriptome profiling of Arabidopsis thaliana seedlings upon inoculation with Azospirillum brasilense over a time course of six, twenty four and ninty six hours revealed very little genetic responses in the early time points. However, a bulk of genes was differentially regulated in 96 hours post inoculation (96hpi). The nature of these genes indicated that the bacterial treatment, among others, greatly affect the processes like cell wall modification, hormone metabolism, stress and secondary metabolism. Additionally expression levels of a numer of transcription factors (TFs) related to basic helix loop helix (BHLH) and MYB domain containing TF families were altered with Azospirillum inoculation. Particularly the BHLH TFs were among the most highly regulated genes. The array results from Azospirillum treated plants were further compared with the already available data emnating from treatment with flagellin 22 (flg22), oligogalacturonides (OGs) and Agrobacterium tumefaciens. Noteworthy, very different set of genes were affected upon inoculation with Azospirillum in relation to other treatments. Secondly a cluster of proteins involved in the biosynthesis of aliphatic glucosinolates (GSL) were uniquely induced upon Sp7 exposure. Genes operating in flavonoid biosynthesis also showed a distinct regulation trend in the comparative analysis. Taken together, the study in question provides insights into the early signalling events in the context of Azospirillum-Arabidopsis association and the bacterial signals recognized by the plants. The array data, at the same time, elucidates the genetic factors of Arabidopsis triggered upon association with Azospirillum brasilense.
Bone Morphogenetic Proteins (BMPs) bilden zusammen mit den Activinen, Growth and Differentiation Factors (GDFs) und Transforming Growth Factor β (TGF-β) die Transforming Growth Factor β-Superfamilie von sekretierten Signalproteinen. Sie spielen eine wichtige Rolle in der Entwicklung, Erhaltung und Regeneration von Geweben und Organen. Die Signalvermittlung dieser Proteine erfolgt durch die Bindung von zwei verschiedenen Typen von Serin-/Threonin-Kinaserezeptoren, die als Typ-I- und Typ-II-Rezeptoren bezeichnet werden. Im ersten Schritt erfolgt die Bindung an den hochaffinen Rezeptor (im Fall von BMP-2 der Typ-I-Rezeptor), im nächsten Schritt wird der niederaffine Rezeptor in den Komplex rekrutiert. Bis heute sind lediglich sieben Typ-I- und fünf Typ-II-Rezeptoren bekannt, was auf eine Promiskuität in der Liganden-Rezeptor-Interaktion schließen lässt. Die Architektur beider Rezeptorsubtypen ist dabei relativ ähnlich. Beide bestehen aus einer ligandenbindenden extrazellulären Domäne, einer Transmembrandomäne sowie einer intrazellulären Kinasedomäne. Eine nacheinander ablaufende Transphosphorylierung der intrazellulären Domänen führt zu einer Phosphorylierung von SMAD-Proteinen, die dann als nachgeschaltete Vermittler fungieren und die Transkription regulierter Gene auslösen. Im Hauptteil dieser Arbeit wurden die initialen Schritte der Rezeptorkomplexformierung sowie die Mobilität der Rezeptoren mit Hilfe von fluoreszenzmikroskopischen Methoden untersucht. Dabei konnte festgestellt werden, dass für die Bildung eines Signalkomplexes eine bestimmte Schwellenkonzentration des Liganden nötig ist und dass der Mechanismus nach einem Alles-oder-Nichts-Prinzip wie ein Schalter funktioniert. Außerdem konnten Unterschiede in der Nutzung der gleichen Rezeptoren durch verschiedene Liganden festgestellt werden. Die anderen Teile der Arbeit befassen sich mit der Funktionalität der verschiedenen Rezeptordomänen in der Signalübermittlung, der Analyse von hoch- und niederaffinen Ligandenbindestellen auf ganzen Zellen sowie dem Einfluss des SMAD- und des MAPK-Signalwegs auf die Induktion der Alkalischen Phosphatase. Dabei konnte gezeigt werden, dass die Art der SMAD-Phosphorylierung allein vom Typ der Kinasedomäne abhängig ist, dass auf einer Zelle verschiedene Rezeptorpopulationen existieren, welche von unterschiedlichen Ligandenkonzentrationen angesprochen werden, und dass die Induktion der Alkalischen Phosphatase stark vom zeitlichen Verlauf der SMAD- und MAPK-Aktivierung abhängig ist.
Memory is dynamic: shortly after acquisition it is susceptible to amnesic treatments, gets gradually consolidated, and becomes resistant to retrograde amnesia (McGaugh, 2000). Associative olfactory memory of the fruit fly Drosophila melanogaster also shows these features. After a single associative training where an odor is paired with electric shock (Quinn et al., 1974; Tully and Quinn, 1985), flies form an aversive odor memory that lasts for several hours, consisting of qualitatively different components. These components can be dissociated by mutations, their underlying neuronal circuitry and susceptibility to amnesic treatments (Dubnau and Tully, 1998; Isabel et al., 2004; Keene and Waddell, 2007; Masek and Heisenberg, 2008; Xia and Tully, 2007). A component that is susceptible to an amnesic treatment, i.e. anesthesia-sensitive memory (ASM), dominates early memory, but decays rapidly (Margulies et al., 2005; Quinn and Dudai, 1976). A consolidated anesthesia-resistant memory component (ARM) is built gradually within the following hours and lasts significantly longer (Margulies et al., 2005; Quinn and Dudai, 1976). I showed here that the establishment of ARM requires less intensity of shock reinforcement than ASM. ARM and ASM rely on different molecular and/or neuronal processes: ARM is selectively impaired in the radish mutant, whereas for example the amnesiac and rutabaga genes are specifically required for ASM (Dudai et al., 1988; Folkers et al., 1993; Isabel et al., 2004; Quinn and Dudai, 1976; Schwaerzel et al., 2007; Tully et al., 1994). The latter comprise the cAMP signaling pathway in the fly, with the PKA being its supposed major target (Levin et al., 1992). Here I showed that a synapsin null-mutant encoding the evolutionary conserved phosphoprotein Synapsin is selectively impaired in the labile ASM. Further experiments suggested Synapsin as a potential downstream effector of the cAMP/PKA cascade. Similar to my results, Synapsin plays a role for different learning tasks in vertebrates (Gitler et al., 2004; Silva et al., 1996). Also in Aplysia, PKA-dependent phosphorylation of Synapsin has been proposed to be involved in regulation of neurotransmitter release and short-term plasticity (Angers et al., 2002; Fiumara et al., 2004). Synapsin is associated with a reserve pool of vesicles at the presynapse and is required to maintain vesicle release specifically under sustained high frequency nerve stimulation (Akbergenova and Bykhovskaia, 2007; Li et al., 1995; Pieribone et al., 1995; Sun et al., 2006). In contrast, the requirement of Bruchpilot, which is homologous to the mammalian active zone proteins ELKS/CAST (Wagh et al., 2006), is most pronounced in immediate vesicle release (Kittel et al., 2006). Under repeated stimulation of a bruchpilot mutant motor neuron, immediate vesicle release is severely impaired whereas the following steady-state release is still possible (Kittel et al., 2006). In line with that, knockdown of the Bruchpilot protein causes impairment in clustering of Ca2+ channels to the active zones and a lack of electron-dense projections at presynaptic terminals (T-bars). Thus, less synaptic vesicles of the readily-releasable pool are accumulated to the release sites and their release probability is severely impaired (Kittel et al., 2006; Wagh et al., 2006). First, I showed that Bruchpilot is required for aversive olfactory memory and localized the requirement of Bruchpilot to the Kenyon cells of the mushroom body, the second-order olfactory interneurons in Drosophila. Furthermore, I demonstrated that Bruchpilot selectively functions for the consolidated anesthesia-resistant memory. Since Synapsin is specifically required for the labile anesthesia sensitive memory, different synaptic proteins can dissociate consolidated and labile components of olfactory memory and two different modes of neurotransmission (high- vs. low frequency dependent) might differentiate ASM and ARM.
In this thesis we have used Drosophila melanogaster as a model organism to investigate proteins and their putative interacting partners that are directly or indirectly involved in the release of neurotransmitters at the synapse. We have used molecular techniques to investigate conserved synaptic proteins, synapsin and synapse associated protein of 47 kD (SAP47), and a putative interaction partner of SAP47, tubulin binding chaperone E-like (TBCEL). SAP47 and synapsins are highly conserved synaptic vesicle associated proteins in Drosophila melanogaster. To further investigate the role and function of Sap47 and Syn genes, we had earlier generated the null mutants by P-element mutagenesis (Funk et al., 2004; Godenschwege et al., 2004). Western blots and ELISA of brain homogenates from Sap47156 null mutants showed the presence of up-regulated phospho-synapsin in comparison to wild-type (CS) and the presence of up-regulated phospho-synapsin was partially abolished when a pan-neuronal rescue of SAP47 was performed by the Gal4- UAS technique. Thus, the results suggest a qualitative and quantitative modulation of synapsin by SAP47. At the transcript level, we did not observe any difference in content of Syn transcript in Sap47156 and wild-type CS flies. The question of a direct molecular interaction between SAP47 and synapsin was investigated by co-immunoprecipitation (Co-IP) experiments and we did not find any stable interactions under the several IP conditions we tested. The possibility of Sap47 as a modifier of Syn at the genetic level was investigated by generating and testing homozygous double null mutants of Sap47 and Syn. The Syn97, Sap47156 double mutants are viable but have a reduced life span and decreased locomotion when compared to CS. In 2D-PAGE analysis of synapsins we identified trains of spots corresponding to synapsins, suggesting that synapsin has several isoforms and each one of them is posttranslationally modified. In an analysis by Blue native-SDS-PAGE (BN-SDS-2D- PAGE) and Western blot we observed synapsin and SAP47 signals to be present at 700-900 kDa and 200-250 kDa, respectively, suggesting that they are part of large but different complexes. We also report the possibility of Drosophila synapsin forming homo- and heteromultimers, which has also been reported for synapsins of vertebrates. In parallel to the above experiments, phosphorylation of synapsins in Drosophila was studied by IP techniques followed by 1D-SDS gel electrophoresis and mass spectrometry (in collaboration with S. Heo and G. Lubec). We identified and verified 5 unique phosphorylation sites in Drosophila synapsin from our MS analysis. Apart from phosphorylation modifications we identified several other PTMs which have not been verified. The significance of these phosphorylations and other identified PTMs needs to be investigated further and their implications for synapsin function and Drosophila behavior has to be elucidated by further experiments. In a collaborative project with S. Kneitz and N. Nuwal, we investigated the effects of Sap156 and Syn97 mutations by performing a whole Drosophila transcriptome microarray analysis of the individual null mutants and the double mutants (V2 and V3). We obtained several candidates which were significantly altered in the mutants. These genes need to be investigated further to elucidate their interactions with Sap47 and Syn. In another project, we investigated the role and function of Drosophila tubulin- binding chaperone E-like (Tbcel, CG12214). The TBCEL protein has high homology to vertebrate TBCE-like (or E-like) which has high sequence similarity to tubulin-binding chaperone E (TBCE) (hence the name TBCE-Like). We generated an anti-TBCEL polyclonal antiserum (in collaboration with G. Krohne). According to flybase, the Tbcel gene has only one exon and codes for two different transcripts by alternative transcription start sites. The longer transcript RB is present only in males whereas the shorter transcript RA is present only in females. In order to study the gene function we performed P- element jump-out mutagenesis to generate deletion mutants. We used the NP4786 (NP) stock which has a P(GawB) insertion in the 5’ UTR of the Tbcel gene. NP4786 flies are homozygous lethal due to a second-site lethality as the flies are viable over a deficiency (Df) chromosome (a deletion of genomic region spanning the Tbcel gene and other upstream and downstream genes). We performed the P-element mutagenesis twice. In the first trial we obtained only revertants and the second experiment is still in progress. In the second attempt, jump-out was performed over the deficiency chromosome to prevent homologous chromosome mediated double stranded DNA repair. During the second mutagenesis an insertion stock G18151 became available. These flies had a P-element insertion in the open reading frame (ORF) of the Tbcel gene but was homozygous viable. Western blots of fresh tissue homogenates of NP/Df and G18151 flies probed with anti-TBCEL antiserum showed no TBCEL signal, suggesting that these flies are Tbcel null mutants. We used these flies for further immunohistochemical analyses and found that TBCEL is specifically expressed in the cytoplasm of cyst cells of the testes and is associated with the tubulin of spermatid tails in wild-type CS, whereas in NP/Df and G18151 flies the TBCEL staining in the cyst cells was absent and there was a disruption of actin investment cones. We also found enrichment of TBCEL staining around the actin investment cone. These results are also supported by the observation that the enhancer trap expression of the NP4786 line is localised to the cyst cells, similar to TBCEL expression. Also, male fertility of NP/Df and G18151 flies was tested and they were found to be sterile with few escapers. Thus, these results suggest that TBCEL is involved in Drosophila spermatogenesis with a possible role in the spermatid elongation and individualisation process.
Zur Gattung Bordetella zählen derzeit neun verschiedene Arten Gram-negativer Bakterien. Der strikt humanpathogene Keim B. pertussis ist der Erreger des Keuchhustens und stellt das wohl wichtigste Mitglied der Gattung dar. B. holmesii gehört zu den sogenannten neuen Bordetella-Arten und wurde 1995 erstmals von Weyant et al. beschrieben. In den letzten Jahren gewann B. holmesii als humanpathogener Keim, der Keuchhusten-ähnliche Erkrankungen verursacht, zunehmend an Bedeutung. Mit Ausnahme des BvgAS-Systems (Gerlach et al., 2004) konnten in B. holmesii bislang keine für die „klassischen“ Bordetella-Arten bekannten Virulenzfakoren nachgewiesen werden. Im Rahmen dieser Arbeit wurde ein zum Filamentösen Hämagglutinin homologer Faktor in B. holmesii identifiziert. Der fhaB-Locus aus B. holmesii unterscheidet sich deutlich von dem der anderen Bordetella-Arten, da die fhaB-Sequenz in B. holmesii (fhaBBH) stromaufwärts von einem putativen Membranprotein und stromabwärts von einem IS1001-ähnlichen IS-Element flankiert wird. Sowohl auf DNA- als auch auf Proteinebene weist das fhaBBH-Gen die größte Ähnlichkeit zu dem fhaB-Homolog aus B. avium auf, was die Vermutung bestärkt, dass B. holmesii phylogenetisch im Umfeld von B. avium anzusiedeln ist. Durch in silico-Analysen der FhaBBH-Aminosäuresequenz konnte eine N-terminale Signalpeptiddomäne sowie eine TPS-Domäne identifiziert werden. Dies lässt vermuten, dass FHABH an die bakterielle Oberfläche transportiert und möglicherweise sekretiert wird. Anhand von in silico-Analysen wurde zudem ein KGD-Motiv, nicht jedoch eine Heparinsulfat- und eine Kohlenhydratbindedomäne im FhaBBH identifiziert. Das KGD-Motiv könnte das RGD-Motiv im FHA der „klassischen“ Bordetella-Arten bei der Erkennung von Rezeptoren ersetzen. In Zellkultur-Experimenten konnte die Adäsionsfähigkeit von B. holmesii G7702 an A549-Zellen nachgewiesen werden. Eine fhaB-Deletionsmutante des entsprechenden Stammes zeigte dagegen eine signifikant niedrigere Adhäsionsrate an die menschlichen Lungenepithelzellen. Somit konnte gezeigt werden, dass FHA in B. holmesii eine Rolle als Adhäsionsfaktor spielt. Die im Vergleich zum Wildtyp stark verringerte Adhäsionsrate der bvgA-Mutante von B. holmesii G7702 lässt zudem auf die Beteiligung weiterer Adhäsionsfaktoren an der Wirtszelladhäsion in B. holmesii und deren Regulation durch das BvgAS-Systems schließen. Die Regulationsstudien haben gezeigt, dass die fhaB-Transkription in B. holmesii über zwei konstitutiv aktive Promotoren und einen bvg-abhängigen Promotor erfolgt. In vitro ist phosphoryliertes BvgABH in der Lage, spezifisch an die fhaB-upstream-Region aus B. holmesii zu binden. Innerhalb der fhaBBH-upstream-Region wurden drei putative BvgA-Bindestellen BS2-4 identifiziert, die Ähnlichkeiten zu inverted repeat-Anordnungen der BvgA-Konsensussequenz 5’- T/A T T C C/T T A -3’ aufweisen. Basierend auf den Ergebnissen der in vitro-Binde- und in vivo-Expressionsstudien zur Charakterisierung der einzelnen putativen BvgABH-Bindestellen wird ein Modell für die BvgABH-Bindung innerhalb des fhaB-Promotors in B. holmesii vorgeschlagen. Demnach wird zunächst die primäre BvgABH-Bindestelle BS2 mit der höchsten Affinität von BvgABH-P gebunden, wobei für BvgABH-P eine Dimerisierung angenommen wird. Anschließend bindet ein zweites BvgABH-P-Dimer an die als sekundäre Bindestelle bezeichnete BS3-Region, die sich stromabwärts von BS2 befindet. Eine möglicherweise darauf folgende unspezifische Anlagerung von zwei weiteren BvgABH-P-Dimeren an den 37 bp großen DNA-Abschnitt zwischen BS2 und BS3 ist, ebenso wie die Besetzung der niedrigaffinen Bindestelle BS4 durch ein weiteres BvgABH-P-Dimer, hauptsächlich auf kooperative Protein-Protein-Wechselwirkungen zurückzuführen. Das an BS4 gebundene BvgABH-P-Dimer befindet sich am weitesten stromabwärts und könnte zusammen mit der RNA-Polymerase die Initiation der Transkription gewährleisten. Ergebnisse aus den in vitro-Bindestudien sowie die unterschiedliche Zusammensetzung und Anordnung der putativen BvgA-Bindestellen der fhaB-Promotoren aus B. holmesii und B. pertussis deuten auf eine unterschiedliche Regulation dieser beiden Promotoren hin. So scheint der fhaB-Promotor aus B. holmesii, im Vergleich zum fhaB-Promotor aus B. pertussis, erst bei höheren Konzentrationen an phosphoryliertem BvgA gebunden und somit aktiviert zu werden. In B. holmesii wird vermutlich die konstitutive FHA-Synthese nach BvgAS-Stimulation durch die bvg-abhängige fhaBBH-Expression unterstützt, um eine erfolgreiche Infektion des Wirtes zu gewährleisten.
Viren durchliefen eine gemeinsame Evolution mit ihren Wirtsorganismen, die zu einer spezifischen Anpassung der Viren an ihren jeweiligen Wirt führte. Als Folge dessen verfügen viele Viren über ein eng begrenztes Wirtsspektrum. Gelegentlich machen Viren Veränderungen durch, die es ihnen erlauben, einen neuen Wirt zu infizieren und in ihm zu replizieren, wie dies in jüngster Vergangenheit beim humanen Immundefizienz-Virus oder beim Grippevirus geschehen ist. Spezies-übergreifende Infektionen sind für die meisten neuen und wiederauftauchenden Viruserkrankungen verantwortlich. Allerdings ist bisher wenig über die Mechanismen bekannt, die Viren auf einen bestimmten Wirt beschränken, und welche Faktoren Viren zur Überwindung der Spezies-Barriere und zur Vermehrung in einer neuen Wirtsspezies benötigen. Cytomegaloviren sind Prototypen der beta-Herpesvirus Unterfamilie und verfügen über eine ausgeprägte Spezies-Spezifität. Sie vermehren sich nur in Zellen der eigenen oder einer eng verwandten Wirtsspezies. Der molekulare Mechanismus, der dieser Spezies-Spezifität zugrunde liegt, ist noch weitgehend unbekannt und stellt deshalb das Thema dieser Arbeit dar. Initiale Beobachtungen zeigten, dass sich das Maus-Cytomegalovirus (MCMV) ausschließlich in menschlichen 293 und 911 Zellen, aber keiner anderen getesteten menschlichen Zelle vermehren ließ. Diese beiden Zelllinien sind mit Adenovirus E1-Genen transformiert, die den Transkriptions-Transaktivator E1A sowie zwei Apoptose-Inhibitoren (E1B-55k und E1B-19k) kodieren. Daher lag die Hypothese nahe, dass diese Funktionen benötigt werden, um eine MCMV-Replikation in menschlichen Zellen zu ermöglichen. Außerdem konnte gezeigt werden, dass normale menschliche Zellen nach Infektion rapide absterben, und zwar durch eine Caspase-9-vermittelte Apoptose. Die Induktion der Apoptose durch MCMV lässt sich durch Caspase-Inhibitoren unterdrücken, wodurch die virale Replikation wiederhergestellt wird. Dies deutet auf eine Schlüsselfunktion der Caspasen für diesen Prozess hin. Durch Überexpression eines mitochondrialen Apoptose-Inhibitors, d.h. eines Bcl-2-ähnlichen Proteins, in menschlichen Zellen ließ sich die Virus-induzierte Apoptose verhindern. Diese Zellen erlaubten ebenfalls eine effiziente MCMV-Replikation. Die Bedeutung Bcl-2-ähnlicher Proteine für die Spezies-übergreifende Cytomegalovirus-Infektion wurde sowohl durch die Integration korrespondierender Gene, alsauch durch die Integration anderer Inhibitioren der Apoptose oder von Kontroll-Genen in das MCMV Genom bestätigt. Nur rekombinante Viren, die ein Bcl-2-ähnliches Protein kodieren, konnten in menschlichen Zellen vermehrt werden. Ein einziges Gen des humanen Cytomegalovirus, das einen mitochondrialen Apoptose-Inhibitor kodiert, reichte aus, um eine MCMV-Replikation in menschlichen Zellen zu ermöglichen. Zusätzlich konnte gezeigt werden, dass dieselben Prinzipien für eine Replikation des Ratten-Cytomegalovirus in menschlichen Zellen gelten. Zusammenfassend kann festgestellt werden, dass die Induktion der Apoptose eine Spezies-übergreifende Infektion bei den Nagetier-Cytomegaloviren einschränkt.