H-Ras Signaling Mediates Microglia Proliferation Contributing to Neuropathology in INCL Mice

In virtually all neurodegenerative diseases, including infantile neuronal ceroid lipofuscinosis (INCL), neuronal death follows microglial proliferation. Proliferating microglia release toxic cytokines, which are suggested to cause neuroinflammation and progressive death of viable neurons contributing to neurodegeneration. Despite this knowledge, the molecular mechanism(s) underlying microglial proliferation in neurodegenerative diseases remains poorly understood. INCL is a neurodegenerative lysosomal storage disease, caused by inactivating mutations in the CLN1 gene encoding palmitoyl‐protein thioesterase‐1 (PPT1). PPT1 depalmitoylates S‐acylated proteins (constitutes of ceroid), essential for their degradation by lysosomal hydrolases. Thus, PPT1‐deficiency causes lysosomal ceroid accumulation leading to INCL. S‐palmitoylation (also called S‐acylation) is a reversible post‐translational lipid‐modification, which regulates the function of many proteins, especially in the brain. In humans, a family of 23 palmitoyl‐acyltransferases (PATs), containing a DHHC (Asp‐His‐His‐Cys) motif (DHHC‐PATs), catalyze S‐palmitoylation. In contract, thioesterases, localized to the cytosol or to the lysosome, depalmitoylate S‐acylated proteins. We previously reported that cytosolic thioesterase, APT1, requires S‐palmitoylation for membrane‐anchorage and catalyzes its own depalmitoylation to promote dynamic palmitoylation (palmitoylation‐depalmitoylation), essential for its cytosol‐membrane shuttling. Notably, proto‐oncogene H‐Ras product also requires dynamic S‐palmitoylation for cytosol‐membrane shuttling for signaling. Moreover, S‐palmitoylated H‐Ras is a substrate of APT1. Despite the demonstration that APT1 self‐catalyzes depalmitoylation, the DHHC‐PAT(s) that S‐palmitoylates APT1, remains unknown. Here we report that DHHC5 and DHHC23 catalyze S‐palmitoylation of APT1 promoting its membrane‐anchorage. Moreover, in Cln1 −/− mice, which mimic INCL, the levels of S‐palmitoylated H‐Ras in the brain are significantly higher compared with those in WT controls. Unexpectedly, we found that the levels of DHHC5 and DHHC23 in Cln1 −/− mice are significantly reduced. These results may suggest Ppt1‐Apt1 cross‐talk is mediated via regulation of these two DHHCs. We propose that crosstalk between Apt1 and Ppt1 elevates membrane‐anchored, S‐palmitoylated‐H‐Ras, which transduces signals to stimulate astroglia proliferation contributing to rapid progression of neurodegeneration in Cln1 −/− mice. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .