Degeneration of neurons or muscle are observed in several human pathologies, including Alzheimer's, Parkinson's and the hereditary spastic paraplegias (HSP) for neuronal degeneration, and disuse atrophy, cancer cachexia, and sepsis for muscle degeneration. Despite many recent advances, the molecular mechanism(s) underlying these degenerative processes remain incompletely understood. To generate such mechanistic insights, the PIs have recently established a Drosophila model for the HSPs. The specific focus is in atlastin (atl, Spastic Paraplegia Gene 3A), which encodes an ER fusion protein. Based on the previous observation that atl knockdown in neurons causes progressive, age-dependent locomotor deficits, the we asked if this knockdown also caused progressive cellular degeneration. Investigations into the adult thoracic musculature revealed that atl loss from either neuron or muscle caused progressive degeneration associated with a number of other pathologies including accumulation of aggregates containing ubiquitin, increased reactive oxygen species (ROS), and activation of the JNK/Foxo stress response pathway. Administering the drug rapamycin, which inhibits the Tor kinase, or decreasing Tor gene dosage reversed many of these pathologies at least partially, indicating that atl loss might activate muscle Tor. Muscle Tor and Foxo activation have also been observed in denervation-induced muscle atrophy. In this application, experiments are proposed to elucidate the mechanisms by which atl loss causes progressive muscle pathologies.
Aim #1 will test the hypothesis that muscle Tor is activated by atl loss, determine if Tor activity is sufficient as well as necessary for atl loss phenotypes, and test the prediction that Tor activity promotes muscle degeneration by inhibiting autophagy.
Aim #2 will examine the causal relationship between activated Tor and increased ROS, and between ROS and the JNK/Foxo stress pathway. In particular, we will test two non-mutually exclusive hypotheses explaining Foxo activation; first, that activated Tor increases ROS, which in turn is responsible for JNK activation, and finally Foxo activation, and second, that activated Tor activates its target S6K, which in turn down-regulates insulin signaling, thus decreasing activity of the Foxo inhibitor Akt.
Aim #3 will test the hypothesis that neuronal atl loss activates muscle Tor by attenuating glutamatergic neuromuscular transmission. In particular, it will be determined if deletion of one glutamate receptor, previously shown to be sufficient to activate muscle Tor, will cause similar muscle pathologies as is observed by neuronal atl knockdown. In addition, it will be determined if neuronal atl loss confers neuronal phenotypes similar to those conferred by glutamate receptor deletion. Successful completion of these experiments will provide novel and critical mechanistic insights linking defective synaptic input conferred by atl loss to muscle degeneration. The PIs anticipate that these experiments will also provide mechanistic insights applicable to neuronal degeneration as well, which will give these experiments a broad medical relevance.
Degeneration of neurons or muscle is observed in several human pathologies, including Alzheimer's, Parkinson's, the hereditary spastic paraplegias (HSPs), and disuse atrophy, cancer cachexia, and sepsis. We have found that loss within neurons of the Drosophila orthologue of the HSP gene atlastin causes muscle degeneration. We will use this new model to identify the molecular mechanisms by which this degeneration takes place.