GABAergic transmission regulates adult neurogenesis by exerting negative feedback on cell proliferation and enabling dendrite formation and outgrowth. Further, GABAergic synapses target differentiating dentate gyrus granule cells prior to formation of glutamatergic connections. GABAA receptors (GABAARs) mediating tonic (extrasynaptic) and phasic (synaptic) transmission are molecularly and functionally distinct, but their specific role in regulating adult neurogenesis is unknown. Using global and single-cell targeted gene deletion of subunits contributing to the assembly of GABAARs mediating tonic (α4, δ) or phasic (α2) GABAergic transmission, we demonstrate here in the dentate gyrus of adult mice that GABAARs containing α4, but not δ, subunits mediate GABAergic effects on cell proliferation, initial migration and early dendritic development. In contrast, α2-GABAARs cell-autonomously signal to control positioning of newborn neurons and regulate late maturation of their dendritic tree. In particular, we observed pruning of distal dendrites in immature granule cells lacking the α2 subunit. This alteration could be prevented by pharmacological inhibition of thrombospondin signaling with chronic gabapentin treatment, shown previously to reduce glutamatergic synaptogenesis. These observations point to homeostatic regulation of inhibitory and excitatory inputs onto newborn granule cells under the control of α2-GABAARs. Taken together, the availability of distinct GABAAR subtypes provides a molecular mechanism endowing spatiotemporal specificity to GABAergic control of neuronal maturation in adult brain.

To identify new potential therapeutic targets for neurodegenerative diseases, we initiated activity-based protein profiling studies with withanolide A (WitA), a known neuritogenic constituent of Withania somnifera root with unknown mechanism of action. Molecular probes were designed and synthesized, and led to the discovery of the glucocorticoid receptor (GR) as potential target. Molecular modeling calculations using the VirtualToxLab predicted a weak binding affinity of WitA for GR. Neurite outgrowth experiments in human neuroblastoma SH-SY5Y cells further supported a glucocorticoid-dependent mechanism, finding that WitA was able to reverse the outgrowth inhibition mediated by dexamethasone (Dex). However, further GR binding and transactivation assays found no direct interference of WitA. Further molecular modeling analysis suggested that WitA, although forming several contacts with residues in the GR binding pocket, is lacking key stabilizing interactions as observed for Dex. Taken together, the data suggest that WitA-dependent induction of neurite outgrowth is not through a direct effect on GR, but might be mediated through a closely related pathway. Further experiments should evaluate a possible role of GR modulators and/or related signaling pathways such as ERK, Akt, NF-B, TR, or Hsp90 as potential targets in the WitA-mediated neuromodulatory effects.

Murine stem cell-derived neurons have been used to study a wide variety of neuropsychiatric diseases with a hereditary component, ranging from autism to Alzheimer’s. While a significant amount of data on their molecular biology has been generated, there is little data on the physiology of these cultures. Different mouse strains show clear differences in behavioral and other neurobiologically relevant readouts. We have studied the physiology of early differentiation and network formation in neuronal cultures derived from three different mouse embryonic stem cell lines. We have found largely overlapping patterns with some significant differences in the timing of the functional milestones. Neurons from R1 showed the fastest development of intrinsic excitability, while E14Tg2a and J1 were slower. This was also reflected in an earlier appearance of synaptic activity in R1 cultures, while E14Tg2a and J1 were delayed by up to 2 days. In conclusion, stem cells from all backgrounds could be successfully differentiated into functioning neural networks with similar developmental patterns. Differences in the timing of specific milestones, suggest that control cell lines and time-points should be carefully chosen when investigating genetic alterations that lead to subtle deficits in neuronal function.

Neurodevelopmental diseases such as the Rett syndrome (RTT) have received renewed attention, since the mechanisms involved may underlie a broad range of neuropsychiatric disorders such as schizophrenia and autism. In vertebrates early stages in the functional development of neurons and neuronal networks are difficult to study. Embryonic stem cell-derived neurons provide an easily accessible tool to investigate neuronal differentiation and early network formation. We used in vitro cultures of neurons derived from murine embryonic stem cells missing the methyl-CpG-binding protein 2 (MECP2) gene (MeCP2-/y) and from wild type cells of the corresponding background. Cultures were assessed using whole-cell patch-clamp electrophysiology and immunofluorescence. We studied the functional maturation of developing neurons and the activity of the synaptic connections they formed. Neurons exhibited minor differences in the developmental patterns for their intrinsic parameters, such as resting membrane potential and excitability; with the MeCP2-/y cells showing a slightly accelerated development, with shorter action potential half-widths at early stages. There was no difference in the early phase of synapse development, but as the cultures matured, significant deficits became apparent, particularly for inhibitory synaptic activity. MeCP2-/y embryonic stem cell-derived neuronal cultures show clear developmental deficits that match phenotypes observed in slice preparations and thus provide a compelling tool to further investigate the mechanisms behind RTT pathophysiology.

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