The proper functioning of the adult mammalian brain relies on the orchestrated regulation of neural activity by a diverse population of GABA (gamma-aminobutyric acid)-releasing neurons. Until recently, our appreciation of GABA-mediated inhibition focused predominantly on the GABA(A) (GABA type A) receptors located at synaptic contacts, which are activated in a transient or 'phasic' manner by GABA that is released from synaptic vesicles. However, there is growing evidence that low concentrations of ambient GABA can persistently activate certain subtypes of GABA(A) receptor, which are often remote from synapses, to generate a 'tonic' conductance. In this review, we consider the distinct roles of synaptic and extrasynaptic GABA receptor subtypes in the control of neuronal excitability.

Variations on an inhibitory theme : phasic and tonic activation of GABA(A) receptors

Palmitate modifies both peripheral and integral membrane proteins and its addition can be permanent or transient, which makes it unique among the lipid modifications of proteins. The presence of palmitate on a protein affects how the protein interacts with lipids and proteins in a membrane compartment, and the reversibility of palmitoylation allows different modes of trafficking between membrane compartments. Here, we review recent studies that have provided insights into the mechanisms that mediate the functional consequences of this versatile modification.

Palmitoylation: policing protein stability and traffic

γ-Aminobutyric acid type A (GABAA) receptors mediate the majority of fast synaptic inhibition in the mammalian brain, controlling activity both at the network and cellular level. The diverse functions of GABA in the central nervous system are matched not just by the heterogeneity of GABAA receptors, but also the complex trafficking mechanisms and protein-protein interactions that generate and maintain appropriate receptor cell surface localization. In this review, we discuss recent progress in our understanding of the dynamic regulation of GABAA receptor composition, trafficking to and from the neuronal surface, and lateral movement of receptors between synaptic and extrasynaptic locations. Finally, we highlight a number of neurological disorders, including epilepsy and schizophrenia, in which alterations in GABAA receptor trafficking occur.

GABAA receptor trafficking and its role in the dynamic modulation of neuronal inhibition

Protein palmitoylation, a classical and common lipid modification, regulates diverse aspects of neuronal protein trafficking and function. The reversible nature of palmitoylation provides a potential general mechanism for protein shuttling between intracellular compartments. The recent discovery of palmitoylating enzymes--a large DHHC (Asp-His-His-Cys) protein family--and the development of new proteomic and imaging methods have accelerated palmitoylation analysis. It is becoming clear that individual DHHC enzymes generate and maintain the specialized compartmentalization of substrates in polarized neurons. Here, we discuss the regulatory mechanisms for dynamic protein palmitoylation and the emerging roles of protein palmitoylation in various aspects of pathophysiology, including neuronal development and synaptic plasticity.

Protein palmitoylation in neuronal development and synaptic plasticity

Global Analysis of Protein Palmitoylation in Yeast

The neurotransmitter GABA activates heteropentameric GABA(A) receptors, which are composed mostly of alpha, beta, and gamma2 subunits. Regulated membrane trafficking and subcellular targeting of GABA(A) receptors is important for determining the efficacy of GABAergic inhibitory function. Of special interest is the gamma2 subunit, which is mostly dispensable for assembly and membrane insertion of functional receptors but essential for accumulation of GABA(A) receptors at synapses. In a search for novel receptor trafficking proteins, we have used the SOS-recruitment system and isolated a Golgi-specific DHHC zinc finger protein (GODZ) as a novel gamma2 subunit-interacting protein. GODZ is a member of the superfamily of DHHC cysteine-rich domain (DHHC-CRD) polytopic membrane proteins shown recently in yeast to represent palmitoyltransferases. GODZ mRNA is found in many tissues; however, in brain the protein is detected in neurons only and highly concentrated and asymmetrically distributed in the Golgi complex. GODZ interacts with a cysteine-rich 14-amino acid domain conserved specifically in the large cytoplasmic loop of gamma1-3 subunits but not in other GABA(A) receptor subunits. Coexpression of GODZ and GABA(A) receptors in heterologous cells results in palmitoylation of the gamma2 subunit in a cytoplasmic loop domain-dependent manner. Neuronal GABA(A) receptors are similarly palmitoylated. Thus, GODZ-mediated palmitoylation represents a novel posttranslational modification that is selective for gamma subunit-containing GABA(A) receptor subtypes, a mechanism that is likely to be important for regulated trafficking of these receptors in the secretory pathway.

https://www.jneurosci.org/content/jneuro/24/26/5881.full.pdf

The γ2 Subunit of GABAA Receptors Is a Substrate for Palmitoylation by GODZ

Calcium-modulating cyclophilin ligand regulates membrane trafficking of postsynaptic GABAA receptors

γ-Aminobutyric acid receptors (GABAA Rs) are the principal receptors that mediate neural inhibition in the brain. Changes in the function of GABAergic transmission are implicated in activity-dependent adaptation of neural excitability. Of particular interest are mechanisms that control the size of the postsynaptic GABAA-receptor pool, a major determinant of synaptic strength. Mechanisms of trafficking of postsynaptic GABAA Rs contribute to regulation of inhibitory synaptic transmission in response to changes in neural activity and extracellular stimuli. This review summarizes current information available on the receptor structures relevant for trafficking of GABAA Rs, the molecular composition of the submembrane cytoskeleton of inhibitory synapses, and the receptor interacting proteins that regulate the localization and trafficking of postsynaptic GABAA Rs during exocytosis, lateral diffusion and endocytic recycling, and degradative pathways.

Xu Yuan

Papers

The γ2 Subunit of GABAA Receptors Is a Substrate for Palmitoylation by GODZ

Calcium-modulating cyclophilin ligand regulates membrane trafficking of postsynaptic GABAA receptors

Trafficking of Postsynaptic GABAA Receptors by Receptor-Associated Proteins