Synaptic defects caused by mitochondrial complex I dysfunction

PROJECT SUMMARY Background and Objectives: Neurons and their synapses require high amounts of energy to sustain normal levels of activity. Mitochondria are the main energy source, producing ATP via oxidative phosphorylation. In turn, oxidative phosphorylation proceeds through the action of large protein complexes, like Mitochondrial Complex I (MCI). But much work shows that mitochondrial components in neurons and at synapses also do far more than generate ATP. Mitochondria buffer calcium, drive Reactive Oxygen Species (ROS) signaling, and influence cell survival. Using the Drosophila melanogaster neuromuscular junction (NMJ) as a model synapse, we found loss of MCI components impact distinct synaptic tissues in profoundly different ways. The objective of this proposal is to use this model to understand exactly how mitochondria impact discrete parts of the synapse. Specific Aims and Research Design: We found that MCI impairment causes profound cytological abnormali- ties at both the pre- and postsynaptic NMJ. But important differences emerged upon examination of individual tissues. NMJ activity is dampened when MCI is impaired in the postsynaptic muscle. NMJ activity can also be dampened when MCI is impaired in neurons – but curiously, this only happens when it is combined with other insults. This project has two specific aims. Aim 1 is to understand how loss of MCI in the muscle causes a diminishment of NMJ function. This phenotype is noteworthy for multiple reasons. First, the NMJ displays phe- notypes reminiscent of neurodegeneration when MCI is lost in muscle. Second, the NMJ is known to employ numerous muscle-to-nerve signaling paradigms to sustain normal activity. We can test if any of those retrograde signals are occluded. Aim 2 is to define how the presynaptic neuron eludes dysfunction when MCI is lost. Com- bining genetics, pharmacology, electrophysiology, and imaging we will test if known homeostatic signaling com- ponents and modalities are co-opted to maintain normal activity. The expected outcome of our work is new understanding of how mitochondrial function and dysfunction impinge upon discrete synaptic compartments. Health Relatedness: Impairment of mitochondria is associated with neurodegenerative conditions like Parkin- son’s Disease, neuromuscular conditions like Leigh Syndrome, as well as forms of epilepsy and ataxia. For any genetic disease or disorder caused by mitochondrial dysfunction, we require better cell-specific models to eluci- date what is happening on the levels of synapses and circuits. Based on our data, the tractable Drosophila NMJ is a good way to define how synapses react to normal mitochondrial function or dysfunction. In turn, this kind of foundational information from the NMJ model could edify downstream investigations into neurological conditions caused by mitochondrial dysfunction, like forms of Parkinson’s disease, epilepsy, and ataxia.