GABA_B receptors

Abstract

Neural stem cells (NSCs) continually produce differentiated progeny within restricted areas of the adult mammalian brain. Not only do newly-generated neurons in the adult brain contribute to plasticity and remodeling of neuronal networks but also the neuronal network activity feeds back onto the NSCs and their progeny in order to modulate neurogenic output. A growing body of evidence indicates that neurotransmitters are fundamental niche signals that coordinate function of the neuronal network and the pace of adult neurogenesis. Among the neurotransmitters, gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the brain, modulates progression of multiple stages of the adult neurogenic lineage via activation of GABA_A receptor ion channels. Recent data implicate metabotropic GABA_B receptors, in addition to GABA_A receptors, in the regulation of NSC proliferation and differentiation, adding a new level of complexity to the regulation of adult neurogenesis by GABA.

Keywords: :

• neural stem cells
• adult neurogenesis
• neurotransmitters
• GABA
• GABA_B receptors
• cell proliferation
• hippocampus
• subventricular zone

Introduction

The mammalian central nervous system has a limited potential for self-renewal and adult NSCs can drive neurogenesis but only within restricted brain regions during postnatal life. The subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) and the subventricular zone (SVZ) of the wall of the lateral ventricles contain neurogenic NSCs that produce differentiated progeny through adulthood.¹ Typically, NSCs residing in both adult neurogenic regions divide asymmetrically to generate fast proliferating transient-amplifying intermediate progenitors (IPs) that after a limited number of divisions differentiate into migratory neuroblasts and subsequently neurons.^2,3 The developmental stages that cells go through during lineage progression as well as some of the molecular mechanisms regulating these processes are common, to some degree, between adult and embryonic neurogenesis. However, the new neurons generated during adult life not only integrate into a preexisting functional local circuitry but they also impact on its function.¹ Although it remains controversial, a growing body of evidence supports a significant contribution of newborn neurons in the adult brain to some hippocampus and olfactory bulb-dependent learning and memory tasks in rodents.¹ It has also been proposed that adult neurogenesis is required, to some extent, for mood control and antidepressant efficacy.^4,5 Given recent findings providing evidence for continued neurogenesis in adult humans at rates that suggest it may play a significant role in human brain function,^6,7 it is critical to further our understanding of factors that regulate adult neurogenesis in vivo. In contrast to embryonic NSCs, which continuously divide to contribute to neural tube development and growth, most adult NSCs divide infrequently and their ability to switch from quiescence to proliferation and back is tightly controlled by several mechanisms. Among them are intrinsic factors, niche signals as well as activity of the neuronal network.^8,9 Therefore, adult neurogenesis feeds into the network by adding new cells, but conversely, network activity influences the fate of NSCs and their progeny. Recently it has begun to emerge that the crosstalk between network function and adult neurogenesis can be mediated by neurotransmitter signaling.⁸ GABA is one of the most studied neurotransmitters in this context.¹⁰

The Impact of GABA on Adult Neurogenesis in Brief

GABA is the main inhibitory neurotransmitter in the central nervous system and acts via distinct receptor types, GABA_A and GABA_C ion channels and G-protein coupled GABA_B receptors. Besides its role in the communication between differentiated neurons, GABA has been shown to be a potent modulator of adult neurogenesis. Ionotropic GABA_A receptors mediate most of the effects of GABA on adult neurogenesis described to date.^8,10 Multiple steps of the adult neurogenic process including proliferation, migration, and differentiation are highly sensitive to GABA_A receptor signaling.¹⁰

In the SGZ, interneurons are the main local source of GABA¹¹ and evidence suggests that distinct cell types of the hippocampal neurogenic lineage respond to GABA released from specific interneuron subclasses.^12-14 In addition to interneurons, new glutamatergic granule cells can transiently release GABA during their differentiation.^15,16 Genetic loss of function experiments demonstrated that GABA_A receptor activity inhibits cell proliferation in the adult hippocampus.^12,17 In the same line, GABA_A receptor positive modulators such as benzodiazepines can prevent the pro-proliferative effects of antidepressants.^18,19 It remains controversial whether proliferation of IPs vs. NSCs is differentially regulated by GABA in the hippocampus.^12,20 However, it has been proposed that GABA released by parvalbumin-expressing interneurons acts through GABA_A receptors on the most primitive quiescent NSCs, blocking their proliferation.¹² In addition to its anti-proliferative effects, activation of GABA_A receptors promotes differentiation along the neuronal lineage, survival of the NSC progeny, neurite outgrowth, as well as synaptic integration of new granule neurons in the DG.^13,20-23

In the SVZ, migrating neuroblasts release GABA in a non-synaptic fashion.²⁴ GABAergic striatal interneurons that project into the SVZ have been identified as an additional local source of neurotransmitter.²⁵ GABA_A receptor activation limits proliferation of SVZ NSCs^24,26 and IPs²⁷ as well as migration of neuroblasts along the rostral migratory stream.²⁸ Once SVZ-derived neuroblasts reach the olfactory bulb, GABA promotes their maturation into neurons by accelerating dendritic growth.^29,30

The intracellular molecular mechanisms that regulate neurogenesis downstream of GABA_A receptor activation have not been fully elucidated. In immature cells GABA is excitatory due to high intracellular chloride concentration.¹⁰ In this condition, opening of the GABA_A receptor ion channels leads to an efflux of chloride anions and, consequently, membrane depolarization. GABA-mediated excitation can initiate calcium-sensitive signaling cascades that control a variety of cellular processes.¹⁰ For example, depolarizing GABA induces AKT/mTOR signaling and phosphorylation of the cAMP response element-binding protein (CREB) to promote dendritic growth in newborn hippocampal neurons.^20-22,31 Another interesting epigenetic mechanism that operates in SVZ NSCs is phosphorylation of the histone variant H2AX. Activation of the GABA_A receptors promotes H2AX phosphorylation in stem cells and inhibits their cell-cycle progression.²⁶

Contribution of GABA_B Receptor Signaling to the Regulation of Adult Neurogenesis

While GABA_A receptors have been shown to play multifaceted roles in the regulation of adult neurogenesis, the impact of GABA_B and GABA_C receptors in this context remained largely unexplored. GABA_B receptors are heterodimeric G-protein-coupled receptors composed of GABA_B1 and GABA_B2 subunits. Both subunits are required for proper receptor function³² and accordingly, lack of the GABA_B1 subunit abolishes GABA_B responses in neurons.³³ The GABA_B1 subunit exists in two isoforms (GABA_B1a and GABA_B1b) that are generated from the GABA_B1 gene by differential promoter usage. The GABA_B1a subunit isoform confers a preferential axonal distribution to the receptor and, accordingly, receptors containing the GABA_B1a vs. GABA_B1b isoform mediate distinct synaptic functions.^34,35 GABA_B receptors can modulate activity of voltage-gated calcium channels and inward-rectifying potassium channels, thereby regulating neuronal excitability.³² Besides their role in mature neurons, there is a growing body of evidence implicating GABA_B receptors in numerous aspects of neural development, ranging from progenitor cell proliferation and neuroblast migration to neuronal differentiation and synaptogenesis.^36-39 Very recently, pharmacological, electrophysiological and mouse genetics studies have begun to investigate a role of GABA_B receptors in the biology of adult NSCs and their progeny, suggesting that GABA_B signaling is an inhibitor of adult neurogenesis.

Within the SGZ of the adult mouse hippocampus, GABA_B receptors are expressed by multiple cell types of the neurogenic lineage including neural stem and progenitor cells, neuroblasts and newborn neurons.^16,40 Expression of GABA_B receptors in the adult SVZ has not been addressed in great detail. However, mRNA of several G-protein-coupled receptors, including the GABA_B1 receptor subunit, has been detected in SVZ NSCs.⁴¹ IPs and neuroblasts isolated from the early postnatal and young adult SVZ also express GABA_B1 and GABA_B2 mRNA.^42,43 SVZ-derived newborn interneurons in the adult olfactory bulb express GABA_B receptors, although presynaptic GABA_B regulation of GABA release seems to be absent in these cells.⁴⁴ It is not known whether selective control of alternative GABA_B1 promoters dynamically regulates expression of GABA_B1a and GABA_B1b gene products in distinct cell types of the adult neurogenic lineage. Functional GABA_B receptors form high-molecular-mass complexes with members of a subfamily of the KCTD (potassium channel tetramerization domain-containing) proteins.⁴⁵ KCTD auxiliary subunits associate with the carboxy terminus of GABA_B2 and influence agonist potency and G-protein signaling of GABA_B receptors. Interestingly, KCTD members show distinct expression profiles in the DG.⁴⁵ Whether newly-generated neurons in the adult brain express individual or combinations of KCTDs remains unexplored.

Recently, pharmacological data provided the first evidence that modulation of GABA_B receptors may impact adult neurogenesis in the hippocampal DG.⁴⁶ Chronic but not acute treatment with a GABA_B receptor antagonist increased cell proliferation in the SGZ without affecting cell survival. The neurogenic effects of GABA_B receptor inhibition specifically affected the ventral hippocampus, a region involved in the regulation of stress and emotion.⁴⁶ Interestingly, GABA_B receptor inhibition induced behavioral responses similar to the effects of antidepressant drug treatment.⁴⁶ This work proposed a link between antidepressant and neurogenic effects of GABA_B receptor blockade. Another recent study demonstrated that impairing GABA_B signaling, either pharmacologically or in mice deficient for the GABA_B1 receptor gene, increases cell proliferation in the DG, while activating GABA_B receptors has the opposite effect.⁴⁰ In line with the results of Felice et al., cell survival was unaltered in GABA_B1 knockout mice.^40,46 GABA_B auto- or hetero-receptors can modulate neurotransmitter release³² and this may be altered in the GABA_B1-deficient hippocampus, leaving open the possibility that the neurogenic effects of GABA_B loss of function are non-cell autonomous. However, conditional deletion of the GABA_B1 gene in adult neural stem and progenitor cells suggested that these effects are, at least in part, cell autonomous.⁴⁰ Intriguingly, pharmacological blockade of GABA_B receptors induced Hes5-expressing adult NSCs to leave quiescence, indicating that GABA_B receptors may act very early within the neurogenic lineage.⁴⁰ It remains unclear whether heterogeneous subpopulations of adult neural stem and progenitor cells differentially respond to GABA_B receptor signaling.^9,47 These findings suggest that activation of GABA_B and GABA_A receptors may synergize to mediate the antimitotic effects of GABA.⁴⁸

GABA_A receptors regulate migration of neuroblasts in the postnatal SVZ and positioning of newborn neurons in the adult DG.^17,28 It is unclear whether GABA_B receptor function contributes to neuronal migration in the adult brain, but studies revealing that altering GABA_B signaling affects migration in the embryonic brain suggest that this might be the case.³⁹ GABA_B receptors promote both tangential and radial migration in the developing cerebral cortex by elevating the intracellular calcium concentration and inhibiting the cAMP/PKA/LKB1 pathway.^39,49 It will be interesting to test whether GABA_B receptors affect cell motility within the adult neurogenic niches, potentially by interfering with the adenylate-cyclase-cAMP-CREB axis.^22,50-52

During development, GABA_B receptors regulate neurite outgrowth in a variety of brain regions.³⁹ Immature granule neurons in the postnatal DG express functional GABA_B receptors that modulate presynaptic excitability and could potentially influence differentiation of these cells.¹⁶ Interestingly, even though dendritic length and morphology of newborn neurons were not analyzed, a decrease in Doublecortin expression and an increase in NeuN expression were detected in the DG of adult GABA_B1-deficient mice, suggesting that accelerated neuronal maturation, in addition to increased cell proliferation, contributes to enhanced hippocampal neurogenesis in the absence of functional GABA_B receptors.⁴⁰ However, and in apparent contrast to these findings, treatment with a GABA_B receptor antagonist had no significant effects on dendritic initiation in postnatally generated olfactory bulb interneurons.²⁹ This may reflect differences in GABA_B-mediated regulation of neuritogenesis in distinct brain areas.³⁹

Mechanistically, it is not known how GABA_B receptors regulate adult neurogenesis and the downstream molecular pathways involved. Presynaptic GABA_B receptors seem to be coupled to GIRK channels in immature granule neurons of the hippocampus¹⁶ and activation of GIRKs can induce changes in membrane excitability and in turn affect several cellular processes. GABA_B receptors are also able to modulate opening of calcium channels at the cell membrane as well as intracellular calcium mobilization by regulating phospholipase C, thereby influencing calcium-sensitive signaling cascades that could affect adult neurogenesis.^{20,25,39,53,54} Finally, GABA_B receptors regulate neural development through inhibition of adenylate-cyclase activity and cAMP signaling.³⁹ The adenylate-cyclase-cAMP pathway is a key signal transduction pathway that promotes adult neurogenesis, can be potentiated by activation of GABA_A and Beta3-adrenergic receptors in the SGZ neurogenic niche^22,52,55 and can mediate crosstalk between GABA_B and GABA_A receptors in neurons.⁵⁶ Thus, the adenylate-cyclase-cAMP pathway could potentially be an intersection point between the actions of GABA_B and other neurotransmitter receptors in the context of adult neurogenesis. Elucidating the molecular basis of neurotransmitter signaling in adult NSCs and their progeny is fundamental toward our understanding of how the brain coordinates crosstalk between activity of the neuronal network and generation of new cells. Much work is needed to deepen our comprehension of GABA_B receptor-mediated regulation of adult neurogenesis.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

I thank Dr P Knuckles for comments on the manuscript. This work was supported by the University of Basel.