Decreased surface expression of the 8 subunit of the GABAA receptor contributes to reduced tonic inhibition in dentate granule cells in a mouse model of fragile X syndrome

作者：Zhang, Nianhui; Peng, Zechun; Tong, Xiaoping; Lindemeyer, A. Kerstin; Cetina, Yliana; Huang, Christine S.; Olsen, Richard W.; Otis, Thomas S.; Houser, Carolyn R.^*

来源：Experimental Neurology, 2017, 297: 168-178.

DOI：10.1016/j.expneurol.2017.08.008

• GABA receptor
• Tonic inhibition
• Dentate gyrus
• Fmr1 gene
• Fragile X mental retardation protein
• Fragile X syndrome

While numerous changes in the GABA system have been identified in models of Fragile X Syndrome (FXS), alterations in subunits of the GAB(A)A receptors (GABA(A)Rs) that mediate tonic inhibition are particularly intriguing. Considering the key role of tonic inhibition in controlling neuronal excitability, reduced tonic inhibition could contribute to FXS-associated disorders such as hyperactivity, hypersensitivity, and increased seizure susceptibility. The current study has focused on the expression and function of the delta subunit of the GABA(A)R, a major subunit involved in tonic inhibition, in granule cells of the dentate gyrus in the Fmr1 knockout (KO) mouse model of FXS. Electrophysiological studies of dentate granule cells revealed a marked, nearly four-fold, decrease in tonic inhibition in the Fmrl KO mice, as well as reduced effects of two delta subunit-preferring pharmacological agents, THIP and DS2, supporting the suggestion that delta subunit-containing GABAARs are compromised in the Fmrl KO mice. Immunohistochemistry demonstrated a small but statistically significant decrease in delta subunit labeling in the molecular layer of the dentate gyrus in Fmrl KO mice compared to wildtype (WT) littermates. The discrepancy between the large deficits in GABA-mediated tonic inhibition in granule cells in the Fmrl KO mice and only modest reductions in immunolabeling of the delta subunit led to studies of surface expression of the 5 subunit. Cross-linking experiments followed by Western blot analysis demonstrated a small, non-significant decrease in total delta subunit protein in the hippocampus of Ftnr1 KO mice, but a four-fold decrease in surface expression of the delta subunit in these mice. No significant changes were observed in total or surface expression of the alpha 4 subunit protein, a major partner of the delta subunit in the forebrain. Postembedding immunogold labeling for the delta subunit demonstrated a large, three-fold, decrease in the number of symmetric synapses with immunolabeling at perisynaptic locations in Fmr1 KO mice. While alpha 4 immunogold particles were also reduced at perisynaptic locations in the Fmrl KO mice, the labeling was increased at synaptic sites. Together these findings suggest that, in the dentate gyrus, altered surface expression of the delta subunit, rather than a decrease in 8 subunit expression alone, could be limiting delta subunit-mediated tonic inhibition in this model of FXS. Finding ways to increase surface expression of the delta subunit of the GABA(A)R could be a novel approach to treatment of hyper excitability-related alterations in FXS.

• 出版日期2017-11
• 单位上海交通大学