Control of Axon Degeneration by Palmitoylation - PROJECT SUMMARY Degeneration of neuronal axons is a hallmark of diverse neuropathological conditions, ranging from acute nerve injuries to chronic neuropathies and neurodegenerative diseases. If we better understood the mechanisms of axon degeneration, we could devise new therapies to potentially slow or prevent axon degeneration and ameliorate a range of neuropathologies. Recent findings suggest that one key event in axon degeneration is the loss of ‘axon survival factors’, labile proteins whose continuous supply from neuronal cell bodies to distal axons normally ensures axon integrity. The damage to axons that occurs after injury, or in chronic neuropathies, disrupts the supply of survival factors and triggers distal axon degeneration. The best-known axon survival factor, nicotinamide mononucleotide adenylyl transferase-2 (NMNAT2) requires the protein-lipid modification palmitoylation for its fast axonal transport, but how palmitoylation contributes to NMNAT2 degradation is not fully clear. I identified previously unrecognized phosphorylation sites on NMNAT2, whose phosphorylation by c-Jun N-terminal kinase (JNK) triggers selective degradation of palmitoyl-NMNAT2 in cultured cells. While addressing this question, I also realized that several other regulators of axon degeneration are palmitoylated, in addition to NMNAT2. Given that palmitoylation is the only reversible protein-lipid modification, I asked if acute depalmitoylation of NMNAT2 and/or other axonal proteins is important for axon degeneration. My results revealed that pharmacological inhibition of protein depalmitoylases indeed protects axons from degeneration and additional preliminary genetic data ascribe this protection to block of a recently described depalmitoylase, ABHD16A. However, axon protection by depalmitoylase inhibitors is independent of NMNAT2 and other known axon survival factors, suggesting that additional protein(s) are depalmitoylated after axonal damage to drive subsequent degeneration. The hypothesis that underlies this proposal is that palmitoylation plays broad, previously unrecognized roles in the control of axon degeneration. Experiments in Aim 1 will use a recently generated phospho-mutant knockin mouse to define the importance of phosphorylation by JNK for NMNAT2 degradation and subsequent axon degeneration, both in cultured neurons and in vivo. Aim 2A will determine whether ABHD16A is indeed the key depalmitoylase whose activity is required for axon degeneration and will provide the first cell biological characterization of ABHD16A localization in axons. In Aim 2B I will break new ground by comprehensively identifying axonal proteins that are rapidly depalmitoylated after axonal injury, in order to identify those protein(s) whose acute depalmitoylation drives axon degeneration. Together, the results of these studies will provide new insights into the cell biology of axonal (de)palmitoylation, will define key events that drive axon degeneration, and could reveal new therapeutic strategies to ameliorate a host of neuropathological conditions.
