Genetic or pharmacological interventions that decrease synaptic transmission often induce a signal produced in the postsynaptic cell that acts in a retrograde manner to increase transmitter release from the presynaptic cell. At some synapses, generation of this signal requires an increased concentration of the phospholipid phosphatidic acid (PA) in the postsynaptic cell, which then activates the ?Target of rapamycin? (Tor) kinase. Tor activation, in turn, increases translation of mRNAs encoding the retrograde signal. Although increased protein synthesis is the most prominent Tor output, Tor also inhibits autophagy by phosphorylating and inhibiting several Atg proteins; this autophagy impairment might explain the association of activated Tor with a number of neurodegenerative and muscle degenerative disorders. The Hereditary Spastic Paraplegias (HSPs) represent one family of neurodegenerative disorders caused by mutations in any of over 70 different genes. We recently developed a Drosophila model for the HSP caused by mutations in atlastin (atl, SPG3A), which encodes an ER fusion GTPase. Using this model, we found that neuronal atl loss both decreased evoked transmitter release at the larval neuromuscular junction and caused progressive muscle degeneration. Because this degeneration, as well as associated locomotor and muscle pathologies, was partially suppressed by either decreasing Tor gene dosage or by administering the Tor inhibitor rapamycin, we hypothesize that neuronal atl loss activates muscle Tor. In this proposal, we will determine the role of [PA] in mediating the muscle degeneration and related pathologies caused by atl loss.
In aim #1 we use mass spectrometry and an in vivo fluorescent PA reporter to test the hypothesis that neuronal atl loss increases muscle [PA].
In aim #2, we will determine the functional role of altered muscle [PA] in muscle degeneration. In particular, we will adjust muscle [PA] levels by overexpressing or introducing mutations in genes encoding PA-metabolizing enzymes including PLD, DAG Kinase and PA phosphatase. We hypothesize that increasing muscle [PA] will be sufficient to cause muscle degeneration whereas decreasing muscle [PA] will prevent muscle degeneration caused by neuronal atl loss. If we find, as expected that, increasing muscle [PA] is sufficient to cause muscle degeneration, we will then determine if this degeneration is dependent on Tor.
In aim #3 we will investigate the Tor dependent retrograde signaling pathway that occurs at the Drosophila larval neuromuscular junction. This pathway is triggered by deletion of gluRIIA, one of the two alternative subunits of post-synaptic muscle glutamate receptors. We will determine if muscle PA is both necessary and sufficient to generate this retrograde signal. If successful, these studies will establish a link between impaired synaptic transmission and postsynaptic cell degeneration and elucidate the role of postsynaptic PA in this process. We anticipate that these studies will provide critical mechanistic insights into the HSPs as well as in the process of degeneration in diseases such as Alzheimer?s with great clinical importance.
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 both impaired synaptic transmission and muscle degeneration in Drosophila lacking the orthologue of the HSP gene atlastin. We will use this new model to test the hypothesis that elevated muscle phosphatidic acid levels links the impaired synaptic transmission with muscle degeneration.
