Bernhard Lüscher

Postsynaptic clustering of major GABA A receptor subtypes requires the γ2 subunit and gephyrin

Most fast inhibitory neurotransmission in the brain is mediated by GABA A receptors, which are mainly postsynaptic and consist of diverse α and ß subunits together with the γ2 subunit. Although the γ2 subunit is not necessary for receptor assembly and translocation to the cell surface, we show here that it is required for clustering of major postsynaptic GABAA receptor subtypes. Loss of GABAA receptor clusters in mice deficient in the γ2 subunit, and in cultured cortical neurons from these mice, is paralleled by loss of the synaptic clustering molecule gephyrin and synaptic GABAergic function. Conversely, inhibiting gephyrin expression causes loss of GABAA receptor clusters. The γ2 subunit and gephyrin are thus interdependent components of the same synaptic complex that is critical for postsynaptic clustering of abundant subtypes of GABAA receptors in vivo.

Decreased GABAA-receptor clustering results in enhanced anxietyand a bias for threat cues

Patients with panic disorders show a deficit of GABAA receptors in the hippocampus, parahippocampus and orbitofrontal cortex. Synaptic clustering of GABAA receptors in mice heterozygous for the γ2 subunit was reduced, mainly in hippocampus and cerebral cortex. The γ2+/– mice showed enhanced behavioral inhibition toward natural aversive stimuli and heightened responsiveness in trace fear conditioning and ambiguous cue discrimination learning. Implicit and spatial memory as well as long-term potentiation in hippocampus were unchanged. Thus γ2+/– mice represent a model of anxiety characterized by harm avoidance behavior and an explicit memory bias for threat cues, resulting in heightened sensitivity to negative associations. This model implicates GABAA-receptor dysfunction in patients as a causal predisposition to anxiety disorders.

The GABAergic deficit hypothesis of major depressive disorder

Increasing evidence points to an association between major depressive disorders (MDDs) and diverse types of GABAergic deficits. In this review, we summarize clinical and preclinical evidence supporting a central and causal role of GABAergic deficits in the etiology of depressive disorders. Studies of depressed patients indicate that MDDs are accompanied by reduced brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and by alterations in the subunit composition of the principal receptors (GABAA receptors) mediating GABAergic inhibition. In addition, there is abundant evidence that suggests that GABA has a prominent role in the brain control of stress, the most important vulnerability factor in mood disorders. Furthermore, preclinical evidence suggests that currently used antidepressant drugs (ADs) designed to alter monoaminergic transmission and nonpharmacological therapies may ultimately act to counteract GABAergic deficits. In particular, GABAergic transmission has an important role in the control of hippocampal neurogenesis and neural maturation, which are now established as cellular substrates of most if not all antidepressant therapies. Finally, comparatively modest deficits in GABAergic transmission in GABAA receptor-deficient mice are sufficient to cause behavioral, cognitive, neuroanatomical and neuroendocrine phenotypes, as well as AD response characteristics expected of an animal model of MDD. The GABAergic hypothesis of MDD suggests that alterations in GABAergic transmission represent fundamentally important aspects of the etiological sequelae of MDDs that are reversed by monoaminergic AD action.

Neuronal circuitry mechanism regulating adult quiescent neural stem-cell fate decision

Adult neurogenesis arises from neural stem cells within specialized niches. Neuronal activity and experience, presumably acting on this local niche, regulate multiple stages of adult neurogenesis, from neural progenitor proliferation to new neuron maturation, synaptic integration and survival. It is unknown whether local neuronal circuitry has a direct impact on adult neural stem cells. Here we show that, in the adult mouse hippocampus, nestin-expressing radial glia-like quiescent neural stem cells (RGLs) respond tonically to the neurotransmitter γ-aminobutyric acid (GABA) by means of γ2-subunit-containing GABAA receptors. Clonal analysis of individual RGLs revealed a rapid exit from quiescence and enhanced symmetrical self-renewal after conditional deletion of γ2. RGLs are in close proximity to terminals expressing 67-kDa glutamic acid decarboxylase (GAD67) of parvalbumin-expressing (PV+) interneurons and respond tonically to GABA released from these neurons. Functionally, optogenetic control of the activity of dentate PV+ interneurons, but not that of somatostatin-expressing or vasoactive intestinal polypeptide (VIP)-expressing interneurons, can dictate the RGL choice between quiescence and activation. Furthermore, PV+ interneuron activation restores RGL quiescence after social isolation, an experience that induces RGL activation and symmetrical division. Our study identifies a niche cell–signal–receptor trio and a local circuitry mechanism that control the activation and self-renewal mode of quiescent adult neural stem cells in response to neuronal activity and experience.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses.

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

The γ2 subunit of GABAA receptors is required for maintenance of receptors at mature synapses

Abstract The γ2 subunit of GABA A receptor chloride channels is required for normal channel function and for postsynaptic clustering of these receptors during synaptogenesis. In addition, GABA A receptor function is thought to contribute to normal postnatal maturation of neurons. Loss of postsynaptic GABA A receptors in γ2-deficient neurons might therefore reflect a deficit in maturation of neurons due to the reduced channel function. Here, we have used the Cre-loxP strategy to examine the clustering function of the γ2 subunit at mature synapses. Deletion of the γ2 subunit in the third postnatal week resulted in loss of benzodiazepine-binding sites and parallel loss of punctate immunoreactivity for postsynaptic GABA A receptors and gephyrin. Thus, the γ2 subunit contributes to postsynaptic localization of GABA A receptors and gephyrin by a mechanism that is operant in mature neurons and not limited to immature neurons, most likely through interaction with proteins involved in trafficking of synaptic GABA A receptors.

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

The neurotransmitter GABA activates heteropentameric GABAA receptors, which are composed mostly of α, β, and γ2 subunits. Regulated membrane trafficking and subcellular targeting of GABAA receptors is important for determining the efficacy of GABAergic inhibitory function. Of special interest is the γ2 subunit, which is mostly dispensable for assembly and membrane insertion of functional receptors but essential for accumulation of GABAA 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 γ2 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 γ1-3 subunits but not in other GABAA receptor subunits. Coexpression of GODZ and GABAA receptors in heterologous cells results in palmitoylation of the γ2 subunit in a cytoplasmic loop domain-dependent manner. Neuronal GABAA receptors are similarly palmitoylated. Thus, GODZ-mediated palmitoylation represents a novel posttranslational modification that is selective forγ subunit-containing GABAA receptor subtypes, a mechanism that is likely to be important for regulated trafficking of these receptors in the secretory pathway.

The GDP-GTP Exchange Factor Collybistin: An Essential Determinant of Neuronal Gephyrin Clustering

Glycine receptors (GlyRs) and specific subtypes of GABAA receptors are clustered at synapses by the multidomain protein gephyrin, which in turn is translocated to the cell membrane by the GDP-GTP exchange factor collybistin. We report the characterization of several new variants of collybistin, which are created by alternative splicing of exons encoding an N-terminal src homology 3 (SH3) domain and three alternate C termini (CB1, CB2, and CB3). The presence of the SH3 domain negatively regulates the ability of collybistin to translocate gephyrin to submembrane microaggregates in transfected mammalian cells. Because the majority of native collybistin isoforms appear to harbor the SH3 domain, this suggests that collybistin activity may be regulated by protein-protein interactions at the SH3 domain. We localized the binding sites for collybistin and the GlyR β subunit to the C-terminal MoeA homology domain of gephyrin and show that multimerization of this domain is required for collybistin-gephyrin and GlyR-gephyrin interactions. We also demonstrate that gephyrin clustering in recombinant systems and cultured neurons requires both collybistin-gephyrin interactions and an intact collybistin pleckstrin homology domain. The vital importance of collybistin for inhibitory synaptogenesis is underlined by the discovery of a mutation (G55A) in exon 2 of the human collybistin gene (ARHGEF9) in a patient with clinical symptoms of both hyperekplexia and epilepsy. The clinical manifestation of this collybistin missense mutation may result, at least in part, from mislocalization of gephyrin and a major GABAA receptor subtype.

Myc oncoproteins are phosphorylated by casein kinase II.

Casein kinase II (CK‐II) is a ubiquitous protein kinase, localized to both nucleus and cytoplasm, with strong specificity for serine residues positioned within clusters of acidic amino acids. We have found that a number of nuclear oncoproteins share a CK‐II phosphorylation sequence motif, including Myc, Myb, Fos, E1a and SV40 T antigen. In this paper we show that cellular myc‐encoded proteins, derived from avian and human cells, can serve as substrates for phosphorylation by purified CK‐II in vitro and that this phosphorylation is reversible. One‐ and two‐dimensional mapping experiments demonstrate that the major phosphopeptides from in vivo phosphorylated Myc correspond to the phosphopeptides produced from Myc phosphorylated in vitro by CK‐II. In addition, synthetic peptides with sequences corresponding to putative CK‐II phosphorylation sites in Myc are subject to multiple, highly efficient phosphorylations by CK‐II, and can act as competitive inhibitors of CK‐II phosphorylation of Myc in vitro. We have used such peptides to map the phosphorylated regions in Myc and have located major CK‐II phosphorylations within the central highly acidic domain and within a region proximal to the C terminus. Our results, along with previous studies on myc deletion mutants, show that Myc is phosphorylated by CK‐II, or a kinase with similar specificity, in regions of functional importance. Since CK‐II can be rapidly activated after mitogen treatment we postulate that CK‐II mediated phosphorylation of Myc plays a role in signal transduction to the nucleus.

THE BASIC REGION/HELIX-LOOP-HELIX/LEUCINE ZIPPER DOMAIN OF MYC PROTO-ONCOPROTEINS : FUNCTION AND REGULATION

A large body of evidence has been accumulated that demonstrates dominant effects of Myc proto-oncoproteins on different aspects of cellular growth. Myc is one of the few proteins that is sufficient to drive resting cells into the cell cycle and promote DNA synthesis. In line with this finding is that the constitutive expression of Myc in cells blocks their differentiation. These growth stimulating properties are most likely responsible for Mycs ability to initiate and promote tumor formation. Interestingly Myc can also sensitize cells to apoptosis, suggesting that this protein is part of a life-and-death switch. Molecularly Myc functions as a transcriptional regulator that needs to heterodimerize with Max to exert the biological activities described above and to regulate gene transcription. Myc and Max are just two members of a growing family of proteins referred to as the Myc/Max/Mad network. A hallmark of these proteins is that they possess a C-terminal basic region/helix – loop – helix/leucine zipper domain (bHLHZip). The bHLHZip domain specifies dimerization within the network and determines sequence specific DNA binding. Importantly this domain together with the N-terminal transactivation domain is essential for Myc biology. Here we have summarized the structural, functional, and regulatory aspects of the bHLHZip domain of Myc proteins.