GABAA receptor-associated phosphoinositide 3-kinase is required for insulin-induced recruitment of postsynaptic GABAA receptors

doi:10.1016/j.neuropharm.2006.06.023

Neuropharmacology

Volume 52, Issue 1, January 2007, Pages 146-155

https://doi.org/10.1016/j.neuropharm.2006.06.023 Get rights and content

Abstract

Type A γ-aminobutyric acid (GABA_A) receptors mediate most of the fast inhibitory synaptic transmission within the vertebrate brain. The regulation of this inhibition is vital in modulating neural activity. One regulator of GABA_A receptor function is insulin, which can serve to enhance GABA_A receptor-mediated miniature inhibitory postsynaptic currents, via an increase in the number of receptors at the plasma membrane. We set out to investigate the molecular mechanisms involved in the insulin-induced potentiation of GABA_A receptor-mediated responses, by examining the role of phosphoinositide 3-kinase (PI3-K), a key mediator of the insulin response within the brain. We found that PI3-K associates with the GABA_A receptor, and this interaction is increased following insulin treatment. Additionally, the β2 subunit of the GABA_A receptor appears to mediate the insulin-stimulated association with the N-terminal SH2 domain of the p85 subunit of PI3-K. Our results imply a mechanism whereby insulin can regulate changes in synaptic transmission through its downstream actions on the GABA_A receptor.

Introduction

Most of the fast inhibitory synaptic transmission within the central nervous system is mediated by type A γ-aminobutyric acid (GABA_A) receptors (Mehta and Ticku, 1999). These receptors are vital in modulating neural activity and maintaining electrical balance. Structurally, GABA_A receptors are thought to be heteropentameric complexes (Tretter et al., 1997) made up of various subunits from different subfamilies (Macdonald and Olsen, 1994). So far, at least six α, three β, three γ, one δ, one ε, one π, and one θ subunits have been cloned from the mammalian nervous system (Barnard et al., 1998). In the mammalian brain, the most prevalent GABA_A receptor subunit combination is believed to be α1β2γ2 (McKernan and Whiting, 1996).

An emerging theme in the area of synaptic transmission is that of the dynamic nature of postsynaptic ionotropic receptors within the brain, whereby rapid changes in the number of these receptors at postsynaptic domains can modulate the efficacy of receptor-mediated synaptic transmission (Wan et al., 1997b, Nusser et al., 1998, Man et al., 2000, Lu et al., 2001, Malinow, 2003, Collingridge et al., 2004). As such, understanding of the molecular mechanisms underlying the rapid trafficking of these ligand-gated receptors at postsynaptic membranes has been a subject of intensive research.

We have previously shown that insulin produces a rapid recruitment of postsynaptic GABA_A receptors and hence potentiates the receptor-mediated miniature inhibitory postsynaptic currents (mIPSCs) (Wan et al., 1997b). Recent work from our lab further demonstrated that this recruitment is mediated by a facilitated plasma membrane insertion of GABA_A receptors that is dependent on Akt phosphorylation of GABA_A receptor β subunits (Wang et al., 2003). However, the detailed mechanisms by which insulin activates Akt and leads to facilitated receptor insertion remain to be determined.

It is well established that the activation of phosphoinositide 3-kinase (PI3-K) is a critical step in stimulating Akt kinase activity in many intracellular signaling pathways. PI3-Ks constitute a family of lipid kinases that is involved in mitogenic signaling, cell survival, cytoskeletal rearrangements, and vesicular trafficking (Cantley, 2002). In neuronal cells, there is a recruitment to, and activation of, Akt at postsynaptic domains of GABAergic synapses following insulin stimulation (Wang et al., 2003), but the underlying mechanisms remain to be specified. Since several subunits of GABA_A receptors contain tyrosine residues that can be phosphorylated both in vitro and in vivo (Moss et al., 1995, Valenzuela et al., 1995, Wan et al., 1997a), we hypothesize that insulin stimulation leads to tyrosine phosphorylation of one or more of the GABA_A receptor subunits and creates SH2-binding domains for PI3-K, promoting a complex formation between the PI3-K and GABA_A receptors, and that this association plays an essential role in insulin-mediated GABA_A receptor insertion into postsynaptic domains and hence potentiation of the receptor-mediated synaptic transmission. The present study tests this hypothesis.

Section snippets

Methods

All animals used in the study were housed, cared for, and used experimentally in accordance with the Guide for the Care and Use of Experimental Animals, Volumes I and II, published by the Canadian Council on Animal Care.

Results

Previous work from our laboratory has shown that insulin treatment can increase plasma membrane insertion of postsynaptic GABA_A receptors and hence potentiate the receptor-mediated IPSCs in cultured hippocampal neurons (Wan et al., 1997b, Wang et al., 2003). In order to determine whether or not PI3-K plays a role in the insulin-induced cell-surface amplification of GABA_A receptors, we examined the effects of wortmannin, a specific PI3-K inhibitor at nanomolar concentrations (Arcaro and Wymann,

Discussion

In the present work, we have established a role for PI3-K in the modulation of GABA_A receptors by insulin. We observed an increase in the amplitude and frequency of GABA_A receptor-mediated currents following insulin stimulation of hippocampal neuronal cultures that was dependent on PI3-K activation (Fig. 2). Activation of PI3-K has been shown to modulate the function of other neural ion channels. For example, insulin-like growth factor-1 (IGF-1)-mediated increases in calcium channel currents

Acknowledgments

This work was supported by grants from the Canadian Institutes of Health Research (CIHR), Howard Hughes Medical Institute (HHMI), and the Heart and Stroke Foundation of British Columbia and Yukon (HSFBC&Y) to Y.T.W. Y.T.W. is a CIHR Investigator, an HHMI International Scholar and a Michael Smith Foundation for Medical Research Senior Scholar, and is also the holder of HSFBC&Y Chair in Stroke Research at the University of British Columbia and the Vancouver Hospital and Health Sciences Centre. We

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GABAA receptor-associated phosphoinositide 3-kinase is required for insulin-induced recruitment of postsynaptic GABAA receptors

Abstract

Introduction

Section snippets

Methods

Results

Discussion

Acknowledgments

Virus Res.

Neuron

Curr. Opin. Cell Biol.

J. Biol. Chem.

Cell. Signaling

Neuron

Neuron

Neuron

Trends Neurosci

Brain Res. Rev.

J. Biol. Chem.

Cell

Trends Biochem. Sci.

Brain Res Mol. Brain Res.

J. Biol. Chem.

FEBS Lett.

Neuron

FEBS Lett.

Wortmannin is a potent phosphatidylinositol 3-kinase inhibitor: the role of phosphatidylinositol 3,4,5-trisphosphate in neutrophil responses

Biochem. J.

GABA_A receptor-associated phosphoinositide 3-kinase is required for insulin-induced recruitment of postsynaptic GABA_A receptors