Defects in dendrite and spine maturation and synaptogenesis associated with an anxious-depressive-like phenotype of GABAA receptor-deficient mice

Abstract

Mice that were rendered heterozygous for the γ2 subunit of GABAA receptors (γ2+/− mice) have been characterized extensively as a model for major depressive disorder. The phenotype of these mice includes behavior indicative of heightened anxiety, despair, and anhedonia, as well as defects in hippocampus-dependent pattern separation, HPA axis hyperactivity and increased responsiveness to antidepressant drugs. The γ2+/− model thereby provides strong support for the GABAergic deficit hypothesis of major depressive disorder. Here we show that γ2+/− mice additionally exhibit specific defects in late stage survival of adult-born hippocampal granule cells, including reduced complexity of dendritic arbors and impaired maturation of synaptic spines. Moreover, cortical γ2+/− neurons cultured in vitro show marked deficits in GABAergic innervation selectively when grown under competitive conditions that may mimic the environment of adult-born hippocampal granule cells. Finally, brain extracts of γ2+/− mice show a numerical but insignificant trend (p = 0.06) for transiently reduced expression of brain derived neurotrophic factor (BDNF) at three weeks of age, which might contribute to the previously reported developmental origin of the behavioral phenotype of γ2+/− mice. The data indicate increasing congruence of the GABAergic, glutamatergic, stress-based and neurotrophic deficit hypotheses of major depressive disorder.

GABAergic Control of Depression-Related Brain States

“…These same neurons are also recognized as a cellular substrate for the behavioral action of antidepressant drugs (Samuels & Hen, 2011). Consistent with a role of impaired pattern separation and hippocampal neurogenesis in the pathology of MDD, γ2 +/− mice exhibit marked deficits in the maturation and survival of adult-born hippocampal granule cells (Earnheart et al, 2007; Ren et al, 2014). Anxiety- and depression-related behavior and maturational deficits of adult-born hippocampal neurons similar to those of γ2 +/− mice have also been reported for mice lacking the α2 subunit (Duveau et al, 2011; Vollenweider, Smith, Keist, & Rudolph, 2011).…”

Dissociation of Golgi-associated DHHC-type Zinc Finger Protein (GODZ)- and Sertoli Cell Gene with a Zinc Finger Domain-β (SERZ-β)-mediated Palmitoylation by Loss of Function Analyses in Knock-out Mice

“…The ␥2 subunit is known to be required for postsynaptic clustering of GABA A Rs and for GABAergic inhibitory synapse formation (28 -30). This latter phenotype is specifically evident under competitive conditions, such as under artificial conditions where ␥2 ϩ/Ϫ neurons are grown in competition with WT neurons (22) or following sparse in utero electroporation of embryonic cortex with a ␥2 shRNA vector in vivo (30), and it may be physiologically relevant in the context of adult-born hippocampal neurons that must compete for innervation by axons of mature neurons (22). However, similar defects were not observed upon global deletion of the ␥2 subunit in homogeneous ␥2 KO cultures (28).…”

“…However, this phenotype was only observed when mutant neurons were grown and analyzed under conditions where they were forced to compete with wildtype (WT) neurons for GABAergic innervation, i.e. conditions we previously showed to be highly sensitive to reduced expression of the ␥2 subunit of GABA A Rs (22). By contrast, the cell surface expression and synaptic localization of GABA A Rs and the density of GABAergic synapses were unaffected in pure DKO cultures.…”

“…However, we recently showed that ␥2-GABA A Rs are critically important for GABAergic synapse formation selectively under conditions where GABA A R-deficient neurons are forced to compete with WT neurons for presynaptic innervation by WT neurons (22), i.e. conditions that mimic cultures that were sparsely transfected with shRNA constructs.…”

“…Neuron Cultures-Cortical cultures were prepared from E14 mouse embryos essentially as described previously (22,45) with the following modifications. For immunofluorescent stainings, the neurons were seeded on poly-L-lysine-coated coverslips at a density of 3.6 ϫ 10 4 cells/cm 2 .…”