Programmed Switch in The Mitochondrial Degradation Pathways During Human Retinal Ganglion Cell Differentiation from Stem Cells is Critical for RGC Survival

Retinal ganglion cell (RGC) degeneration is the root cause for vision loss in glaucoma as well as in other forms of optic neuropathies. Genetic analysis indicated abnormal mitochondrial quality control (MQC) as a major risk factor for optic neuropathies. However, nothing is known on how MQC regulates human retinal ganglion cell (hRGC) health and survival. Human pluripotent stem cells (hPSCs) provide opportunity to differentiate hRGCs and understand the abnormal MQC associated hRGC degeneration in great detail. Degradation of damaged mitochondria is a very critical step of MQC, here we have used stem cell derived hRGCs to understand the damaged mitochondrial degradation pathways for hRGC survival. Using pharmacological methods, we have investigated the role of the proteasomal and endo-lysosomal pathways in degrading damaged mitochondria in hRGCs and their precursor stem cells. We find that upon mitochondrial damage with the proton uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP), hRGCs more efficiently degraded mitochondria than their precursor stem cells. We further identified that for degrading damaged mitochondria, stem cells predominantly use the ubiquitine-proteasome system (UPS) while hRGCs use the endo-lysosomal pathway. UPS inhibition causes apoptosis in stem cells, while hRGC viability is dependent on the endo-lysosomal pathway but not on the UPS pathway. This suggests manipulation of the endo-lysosomal pathway could be therapeutically relevant for RGC protection in treating glaucoma. Endo-lysosome dependent cell survival is also conserved for other human neurons as differentiated human cerebral cortical neurons also degenerated upon endo-lysosomal inhibition but not for the proteasome inhibition. SIGNIFICANCE STATEMENT Using human stem cells we have shown a switch in the mitochondrial degradation pathway during hRGC differentiation where endo-lysosomal pathway becomes the predominant pathway for cellular homeostasis and hRGC survival which is also true for human cortical neurons. These findings suggest manipulation of the endo-lysosomal pathway could be therapeutically relevant for RGC protection in treating glaucoma as well as for other neurodegenerative diseases.