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

Nianhui Zhang; Zechun Peng; Xiaoping Tong; A Kerstin Lindemeyer; Yliana Cetina; Christine S Huang; Richard W Olsen; Thomas S Otis; Carolyn R Houser

doi:10.1016/j.expneurol.2017.08.008

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

Exp Neurol. 2017 Nov:297:168-178. doi: 10.1016/j.expneurol.2017.08.008. Epub 2017 Aug 16.

Authors

Nianhui Zhang¹, Zechun Peng¹, Xiaoping Tong¹, A Kerstin Lindemeyer², Yliana Cetina¹, Christine S Huang¹, Richard W Olsen³, Thomas S Otis⁴, Carolyn R Houser⁵

Affiliations

¹ Department of Neurobiology, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, CA 90095, USA.
² Department of Medical and Molecular Pharmacology, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, CA 90095, USA.
³ Department of Medical and Molecular Pharmacology, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, CA 90095, USA; Brain Research Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.
⁴ Department of Neurobiology, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, CA 90095, USA; Brain Research Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.
⁵ Department of Neurobiology, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, CA 90095, USA; Brain Research Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA. Electronic address: houser@mednet.ucla.edu.

Abstract

While numerous changes in the GABA system have been identified in models of Fragile X Syndrome (FXS), alterations in subunits of the GABA_A receptors (GABA_ARs) 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 δ subunit of the GABA_AR, 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 Fmr1 KO mice, as well as reduced effects of two δ subunit-preferring pharmacological agents, THIP and DS2, supporting the suggestion that δ subunit-containing GABA_ARs are compromised in the Fmr1 KO mice. Immunohistochemistry demonstrated a small but statistically significant decrease in δ subunit labeling in the molecular layer of the dentate gyrus in Fmr1 KO mice compared to wildtype (WT) littermates. The discrepancy between the large deficits in GABA-mediated tonic inhibition in granule cells in the Fmr1 KO mice and only modest reductions in immunolabeling of the δ subunit led to studies of surface expression of the δ subunit. Cross-linking experiments followed by Western blot analysis demonstrated a small, non-significant decrease in total δ subunit protein in the hippocampus of Fmr1 KO mice, but a four-fold decrease in surface expression of the δ subunit in these mice. No significant changes were observed in total or surface expression of the α4 subunit protein, a major partner of the δ subunit in the forebrain. Postembedding immunogold labeling for the δ subunit demonstrated a large, three-fold, decrease in the number of symmetric synapses with immunolabeling at perisynaptic locations in Fmr1 KO mice. While α4 immunogold particles were also reduced at perisynaptic locations in the Fmr1 KO mice, the labeling was increased at synaptic sites. Together these findings suggest that, in the dentate gyrus, altered surface expression of the δ subunit, rather than a decrease in δ subunit expression alone, could be limiting δ subunit-mediated tonic inhibition in this model of FXS. Finding ways to increase surface expression of the δ subunit of the GABA_AR could be a novel approach to treatment of hyperexcitability-related alterations in FXS.

Keywords: Dentate gyrus; Fmr1 gene; Fragile X mental retardation protein; Fragile X syndrome; GABA receptor; Tonic inhibition.

Publication types

Research Support, N.I.H., Extramural

MeSH terms

Animals
Dentate Gyrus / metabolism*
Dentate Gyrus / pathology
Dentate Gyrus / ultrastructure
Fragile X Syndrome / genetics
Fragile X Syndrome / metabolism*
Fragile X Syndrome / pathology
Gene Expression
Male
Mice
Mice, Inbred C57BL
Mice, Knockout
Neural Inhibition / physiology*
Organ Culture Techniques
Protein Subunits / biosynthesis*
Protein Subunits / genetics
Receptors, GABA-A / biosynthesis*
Receptors, GABA-A / genetics

Substances

Protein Subunits
Receptors, GABA-A

Abstract

Publication types

MeSH terms

Substances

Grants and funding