Phosphatidylserine Decarboxylase Regulates Retinal Ganglion Cell Neurite Outgrowth with Altered Somal Membrane Fluidics and Mitochondrial Morphology

Mitochondrial lipid metabolism is an emerging regulator of neuronal regeneration, yet its role remains poorly defined. We investigated the function of phosphatidylserine decarboxylase (PSD), a mitochondrial enzyme that converts phosphatidylserine to phosphatidylethanolamine (PE), in retinal ganglion cell (RGC) regeneration. Using human glaucomatous degenerating optic nerves, we found PE was aberrantly accumulated with an elevated PSD expression and activity. In contrast, transcriptomes of regenerating RGCs present downregulated PSD, implicating PSD as a potential negative regulator of axonal growth. Using AAV2-mediated gene modulation, we evaluated how PSD knockdown (PSDKD) and PSD overexpression (PSDOE) alter RGC neurite outgrowth in vitro while evaluating effects on mitochondrial morphology, membrane fluidity by C-Laurdan staining, and lipidomes by LC-MS analysis. PSDOE did not support RGC neurite outgrowth, fragmented mitochondria, and increased polyunsaturated triacylglycerols. PSDKD significantly enhanced RGC neurite outgrowth and increased somal membrane fluidity accompanied by decreased cholesterol and saturated triacylglycerols. Notably, Doxorubicin, which attenuates PSD activity, increased neurite growth in PSDOE RGCs, supporting PSD's activity as a negative role for growth. Using the optic nerve crush degenerative model in C57BL/6 mice, we confirm PSDKD RGCs have higher growth competency in vivo. These findings indicate PSDKD positions RGCs in a more growth-permissive state.