Elucidating the Quality Control Pathway of KCC2, a Critical Synchronizer of Neuronal Development

Abstract

Neurodevelopment is a complex process orchestrated by a multitude of factors, including neuronal ion transporters. The neuron‐specific K+/Cl‐ co‐transporter 2 (KCC2) regulates γ‐aminobutyric acid (GABA) inhibitory neurotransmission. Mutations in KCC2 have been linked to multiple neurodevelopmental disorders, including epilepsy, schizophrenia, and autism. KCC2 is a complex multimeric membrane protein exhibiting reduced cell surface expression and/or activity when disease‐associated mutations are present. This observation is indicative of an underlying instability in the KCC2 protein, potentially making it susceptible to cellular quality control pathways that encourage proper folding and/or target misfolded proteins for degradation. Here, we sought to elucidate the quality control mechanisms that regulate KCC2 in three different model systems: yeast, HEK293 cells, and primary rat neurons. Assessment of protein stability in yeast revealed that KCC2 is targeted for endoplasmic‐reticulum‐associated degradation (ERAD). In contrast, experiments in HEK293 cells supported a more subtle role for ERAD in maintaining steady‐state levels of KCC2, but this system allowed for an analysis of KCC2 glycosylation and transport through the secretory system. In turn, KCC2 was remarkably stable in primary rat neurons. By capitalizing on the advantages of the HEK293 cell system, we have also obtained preliminary data that select disease associated KCC2 mutations are unstable relative to the wildtype protein. Together, this study represents a critical new analysis of the quality control mechanisms regulating KCC2 and suggests that KCC2‐associated diseases might be linked to the ERAD pathway.