Biogenesis of membrane proteins is controlled by the protein homeostasis (proteostasis) network. Our lab has been focusing on protein quality control of ?-aminobutyric acid type A receptors (GABAARs), the major inhibitory neurotransmitter-gated ion channels in mammalian central nervous system (CNS). Proteostasis deficiency in GABAARs causes loss of their surface expression on the plasma membrane and thus negatively affects GABAARs mediated inhibitory control in CNS, leading to epilepsy and other neurological diseases. Two well-characterized examples are the A322D and D219N mutation in the ?1 subunit that both cause extensive misfolding and expedited degradation of the mutant subunit in the endoplasmic reticulum (ER), resulting in familial epilepsy. In this thesis, I show that modulating the ER proteostasis network regulates the proteostasis of pathogenic A322D and D219N GABAARs and restores their functional surface expression. Firstly, I demonstrate that modest activations of the unfolded protein response (UPR)-ATF6 pathway or UPR-IRE1 pathway genetically enhances the plasma membrane trafficking of the ?1(A322D) protein in HEK293T cells. Secondly, I show that application of BIX, a specific potent ER resident HSP70 family protein BiP activator, significantly attenuates the degradation of ?1(A322D) subunits, enhances their forward trafficking and increases the functional surface expression of the mutant A322D and D219N GABAARs in human HEK293T cells and neuronal SH-SY5Y cells. In this thesis, I also show that modulating the ER proteostasis network regulates the proteostasis of wild type (WT) GABAARs and affects their surface expression. I show that ER-Golgi intermediate compartment protein-53 (ERGIC-53, aka LMAN1), which cycles between the ER and Golgi, is a trafficking factor for Cys-loop superfamily of neuroreceptors including WT GABAARs in the central nervous system. This is the first report of LMAN1 function in membrane protein trafficking as previously LMAN1 is a known cargo receptor for a number of soluble proteins.
