The Role of Transglutaminase 2 in GABAA Receptor Regulation in Autism

Background:  Autism is a neurodevelopmental disorder affecting 1 in 88 children in America and is approximately 5 times more common in boys than girls. Deficits in GABAergic neurotransmission have been implicated in the pathophysiology of autism. The strength of synaptic inhibition can be determined by the number of γ-amino butyric acid type A receptors (GABA_ARs) found at synaptic sites. The role of GABA_ARs in autism is substantiated by studies showing decreased GABA_AR subunit α₁ (GABA_AR α₁) protein levels in the prefrontal cortex (PFC) of subjects with autism. However, the molecular mechanisms that regulate GABA_AR α₁ trafficking in autism remain unclear. Our goal is to determine the role of transglutaminase 2 (TG2) in the regulation of GABA_AR α₁ signaling. TG2 is a calcium-dependent enzyme that plays an important role in posttranslational modification of proteins. Recent studies have indicated that calnexin, an endoplasmic reticulum chaperone involved in GABA_AR α1 regulation is a TG2 binding partner.

Objectives:  Our long term goal is to understand the regulatory mechanism of GABAAR signaling in autism as a prerequisite to the development of therapeutic protocols that can be used to attenuate the disease process. The objective of this proposal is to determine the role of transglutaminase 2 (TG2) in the regulation of GABA_AR α₁ signaling. The specific hypothesis behind the study is that increased TG2-dependent degradation of GABA_AR α₁ inhibits GABAergic neurotransmission with autism-like phenotype in mice.

Methods:  Human postmortem prefrontal cortex samples from ASD/Autism and control subjects were obtained from NICHD. Animal experiments were performed in C57BL/6 mice overexpressing Transglutaminase 2 in neurons and wild-type. In vitro studies were performed in isolated cortical neurons as well as HEK293 cell line. Protein levels were measured by western blotting, mRNA levels were measured by RT-PCR, TG2 activity was measured by transglutaminase activity assay kit from Sigma. Behavioral measures included open field, tail suspension, forced swim test, social interaction test, and marble bury test.

Results:  We found that mice with neuronal TG2 overexpression (TG2^+/+) show autism-like phenotype with reduced GABA_AR α₁ protein levels in the frontal cortex. Increases in mRNA, protein, and activity of TG2 were found in PFC of subjects with autism. Moreover, we observed a significant reduction in GABA_AR α₁ protein levels, but not mRNA in PFC of subjects with autism suggesting GABA_AR α₁ degradation at the posttranscriptional level. Interestingly, we found significant increases in calnexin in PFC of subjects with autism. Additional studies indicated a role of calnexin in GABA_AR regulation in our model.

Conclusions:  Our data suggests that calnexin functions downstream of TG2 and is involved in GABA_AR α₁ regulation in autism. We further demonstrate that TG2-dependent degradation of GABA_AR induces autism-like phenotype in mice and that calnexin mediates TG2-induced GABA_AR endocytosis.  Given the important role of GABA_AR in neuroplasticity, these findings on the regulatory mechanisms of GABA_AR may provide avenues to develop newer therapeutics for autism and related neurodevelopmental disorders.