Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions

Abstract

GABA A receptors form heteromeric GABA-gated chloride channels assembled from a large family of subunit genes. In cerebellum, distinct GABA A receptor subtypes, differing in subunit composition, are segregated between cell types and synaptic circuits. The cerebellum therefore represents a useful system to investigate the significance of GABA A receptor heterogeneity. For instance, studies of mice carrying targeted deletion of major GABA A receptor subunit genes revealed the role of a subunit variants for receptor assembly, synaptic targeting, and functional properties. In addition, these studies unraveled mandatory association between certain subunits and demonstrated distinct pharmacology of receptors mediating phasic and tonic inhibition. Although some of these mutants have a profound loss of GABA A receptors, they exhibit only minor impairment of motor function, suggesting activation of compensatory mechanisms to preserve inhibitory networks in the cerebellum. These adaptations include an altered balance between phasic and tonic inhibition, activation of voltageindependent K + conductances, and upregulation of GABA A receptors in interneurons that are not affected directly by the mutation. Deletion of the a1 subunit gene leads to complete loss of GABA A receptors in Purkinje cells. A striking alteration occurs in these mice, whereby presynaptic GABAergic terminals are preserved in the molecular layer but make heterologous synapses with spines, characterized by a glutamatergic-like postsynaptic density. During development of a1 0/0 mice, GABAergic synapses are initially formed but are replaced upon spine maturation. These findings suggest that functional GABA A receptors are required for long-term maintenance of GABAergic synapses in Purkinje cells.

Down-Regulation of GABAA Receptor via Promiscuity with the Vasoactive Peptide Urotensin II Receptor. Potential Involvement in Astrocyte Plasticity

“…We showed intense UT labeling in Purkinje cell bodies and ramifications as well as in glial Bergmann-GFAP positive long processes where it co-localizes with γ 1 and, to a lesser extent, with γ 2 subunits of the GABA A R, and in astrocytes co-cultured with granule neurons. Together, this corroborates previous studies establishing expression of α 5 , γ 1 and γ 3 subunits in the PCL [68], [69], and of γ 1 mRNA in Bergmann glia [70], [71]. We confirmed the coexpression of GABA A R subunits with UT in native human astrocytes and in the U87 glioma cell line.…”

Molecular and functional heterogeneity of GABAergic synapses

“…However, ablation of GABA A Rs by targeted deletion of an α subunit variant often results in the disruption of postsynaptic gephyrin clusters and formation of large intra-cellular gephyrin aggregates [56,[72][73][74][75][76], confirming the dependence of gephyrin on GABA A Rs for postsynaptic localization. Importantly, in thalamic neurons expressing both α1-GABA A R postsynaptically and α4-GABA A R extrasynaptically, gephyrin clustering was not rescued following α1 subunit deletion, indicating that α4-GABA A R cannot interact with gephyrin [75,56].…”

Regulation of GABAergic synapse development by postsynaptic membrane proteins

“…Early studies in GABA A R subunit knockout (KO) mice have indicated an important role for GABA A Rs in GABAergic synapse development. Genetic deletion of the α1 subunit abolished GABAergic synaptic transmission in mature cerebellar Purkinje neurons (Fritschy and Panzanelli, 2006; Patrizi et al, 2008). Interestingly, detailed analysis reveals an intricate mechanism underlying the loss of GABAergic synaptic transmission.…”