The Neuronal Role of Cc2d1a in Escrt-Dependent Endosomal Trafficking and Proteostasis

Protein homeostasis, or proteostasis, is essential for proper neuronal function and is governed by a complex cellular network regulating protein synthesis, trafficking, and deg-radation. Damaged, misfolded, or unwanted proteins are removed by multiple distinct degradation mechanisms, each dedicated to tackling a diverse range of protein targets. For degradation of water-insoluble membrane proteins, such as neurotransmitter recep-tors, cells employ the Endosomal Sorting Complexes Required for Transport (ESCRT)-dependent endolysosomal pathway, which traffics endocytosed membrane proteins from early endosomes through multivesicular bodies (MVBs) and finally to lysosomes for deg-radation. Notably, disruptions in neuronal proteostasis arising from dysfunction of protein degradation mechanisms contribute to at least a subset of neurodevelopmental disorders (NDDs), including autism spectrum disorder (ASD) and intellectual disability (ID). Here, we investigate the neuronal role of CC2D1A, a protein scaffold involved in traffick-ing proteins through the endolysosomal pathway for degradation. Recessive null variants in CC2D1A lead to ID and ASD in humans; however, the exact pathogenetic mecha-nisms underlying CC2D1A loss-of-function (LoF) are not fully understood. CC2D1A con-sists of four DM14 protein-binding domains and a membrane-targeting C2 domain. It regulates endosome biogenesis in Drosophila and non-neuronal cells by binding to CHMP4B, a component of the ESCRT-III complex. However, the endolysosomal role of CC2D1A in neurons has not previously been investigated. We have previously demonstrated that the protein-protein interaction between CC2D1A and CHMP4B is conserved in the mouse brain by co-immunoprecipitation and mass spectrometry. Here, we show that a subset of primary mouse cortical neurons deficient for CC2D1A accumulates ubiquitinated proteins at the surface of dramatically enlarged early endosomes in the soma, a phenotype reminiscent of previously published ESCRT dysfunction phenotypes. Late endosomes were mildly affected, while recycling endo-some morphology appeared unperturbed. Increasing the abundance of CHMP4B mono-mers in CC2D1A-deficient neurons prevented formation of the enlarged early endo-somes, corroborating the hypothesis that CC2D1A acts as a positive regulator of ESCRT-III function. Furthermore, CC2D1A is enriched at postsynaptic densities, and fu-ture studies should delineate its role in regulation of ESCRT-dependent endolysosomal pathway specifically at synapses.