All ETDs from UAB

Advisory Committee Chair

James H Meador-Woodruff

Advisory Committee Members

John Chatham

Sarah Clinton

Lori L McMahon

Vlad Parpura

Scott Wilson

Document Type

Dissertation

Date of Award

2015

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Mechanisms underlying the complex etiology of schizophrenia have long been a subject of scientific inquiry. Early investigations focused on identifying functional deficits in dopaminergic and glutamatergic neurotransmission, but evidence for disrupted GABAergic signaling has also emerged. Recent studies from our lab have identified disrupted N-glycosylation of glutamate receptor and transporter subunits and abnormal subcellular distribution of glutamate receptor subunits, suggesting a potential functional consequence of perturbed N-glycosylation in schizophrenia. N-glycosylation is the posttranslational enzymatic attachment of an oligosaccharide precursor to a protein. This modification plays a significant role in protein processing in the ER and Golgi, and is known to be an important regulator of forward trafficking. To determine if disruptions of N-glycosylation might represent a global mechanism underlying schizophrenia pathophysiology and not a neurotransmitter system-specific deficit, we used protein deglycosylation shift assays to determine which GABA(A) receptor (GABAAR) subunits are N-glycosylated in human cortex and to identify potential disruptions of GABAAR N-glycosylation in postmortem superior temporal gyrus (STG) in schizophrenia. We found that the alpha1, alpha4, and beta1-3 GABAAR subunits are N-glycosylated in human cortex, and further identified abnormal N-glycosylation of the alpha1, beta1, and beta2 subunits in schizophrenia. N-glycosylation differences of the alpha1 and beta1 GABAAR subunits were limited to disruptions of immature N-glycosylation, suggesting deficient processing in the ER, while the beta2 subunit demonstrated altered total N-glycosylation, indicative of disruptions further along the forward trafficking pathway. To ascertain potential functional consequences associated with deficits of N-glycosylation, we used a novel biochemical fractionation method to isolate ER- and synapse-enriched fractions from postmortem STG and measured the expression of abnormally N-glycosylated GABAAR subunits, finding significant isoform-specific increases of beta2 subunit expression in the ER, as well as altered ratios of beta1 and beta2 subunits and subunit isoforms in both the ER and synapse-enriched fractions. Together, these studies provide evidence that disrupted N-glycosylation of GABAAR subunits may impair the protein folding and subunit assembly of GABAARs in the ER leading to alterations in receptor stoichiometry at the synapse and contributing to the GABAergic signaling abnormalities in schizophrenia.

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