610 Medizin und Gesundheit
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In this Doctoral Thesis we investigated the consequences of perturbed mitochondrial calcium handling in the context of a rare human disease, Barth syndrome, in which the altered phospholipid composition of the inner mitochondrial membrane affects the structural organization of several protein complexes, including the mitochondrial calcium uniporter. We discovered that loss of the mitochondrial calcium uniporter in cardiac, but not skeletal muscle mitochondria hinders the calcium-induced adaptation of mitochondrial oxidative metabolism during workload transitions. This mechano-energetic uncoupling impairs the physiological increase in contractile force during physical exercise and might predispose Barth syndrome patients to the development of arrhythmias.
Neutral sphingomyelinase-2 (NSM2) is a member of a superfamily of enzymes responsible for conversion of sphingomyelin into phosphocholine and ceramide at the cytosolic leaflet of the plasma membrane. Upon specific ablation of NSM2, T cells proved to be hyper-responsive to CD3/CD28 co-stimulation, indicating that the enzyme acts to dampen early overshooting activation of these cells. It remained unclear whether hyper-reactivity of NSM2-deficient T cells is supported by a deregulated metabolic activity in these cells. Here, we demonstrate that ablation of NSM2 activity affects metabolism of the quiescent CD4\(^+\) T cells which accumulate ATP in mitochondria and increase basal glycolytic activity. This supports enhanced production of total ATP and metabolic switch early after TCR/CD28 stimulation. Most interestingly, increased metabolic activity in resting NSM2-deficient T cells does not support sustained response upon stimulation. While elevated under steady-state conditions in NSM2-deficient CD4\(^+\) T cells, the mTORC1 pathway regulating mitochondria size, oxidative phosphorylation, and ATP production is impaired after 24 h of stimulation. Taken together, the absence of NSM2 promotes a hyperactive metabolic state in unstimulated CD4\(^+\) T cells yet fails to support sustained T cell responses upon antigenic stimulation.