Vesicular GABA Transporter Is Necessary for Transplant-Induced Critical Period Plasticity in Mouse Visual Cortex.

The maturation of GABAergic inhibitory circuits is necessary for the onset of the critical period for ocular dominance plasticity (ODP) in the postnatal visual cortex (Hensch, 2005; Espinosa and Stryker, 2012). When it is deficient, the critical period does not start. When inhibitory maturation or signaling is precocious, it induces a precocious critical period. Heterochronic transplantation of GABAergic interneuron precursors derived from the medial ganglionic eminence (MGE) can induce a second period of functional plasticity in the visual cortex (Southwell et al., 2010). Although the timing of MGE transplantation-induced plasticity is dictated by the maturation of the transplanted cells, its mechanisms remain largely unknown. Here, we sought to test the effect of blocking vesicular GABA loading and subsequent release by transplanted interneurons on the ability to migrate, integrate, and induce plasticity in the host circuitry. We show that MGE cells taken from male and female donors that lack vesicular GABA transporter (Vgat) expression disperse and differentiate into somatostatin- and parvalbumin-expressing interneurons upon heterochronic transplantation in the postnatal mouse cortex. Although transplanted Vgat mutant interneurons come to express mature interneuron markers and display electrophysiological properties similar to those of control cells, their morphology is significantly more complex. Significantly, Vgat mutant MGE transplants fail to induce ODP, demonstrating the pivotal role of vesicular GABAergic transmission for MGE transplantation-induced plasticity in the postnatal mouse visual cortex.SIGNIFICANCE STATEMENT Embryonic inhibitory neurons thrive when transplanted into postnatal brains, migrating and differentiating in the host as they would have done if left in the donor. Once integrated into the host, these new neurons can have profound effects. For example, in the visual cortex, such neurons induce a second critical period of activity-dependent plasticity when they reach the appropriate stage of development. The cellular mechanism by which these transplanted GABAergic interneurons induce plasticity is unknown. Here, we show that transplanted interneurons that are unable to fill synaptic vesicles with GABA migrate and integrate into the host circuit, but they do not induce a second period of plasticity. These data suggest a role for the vesicular GABA transporter in transplantation-mediated plasticity.

Plasticity of local GABAergic interneurons drives olfactory habituation.

Despite its ubiquity and significance, behavioral habituation is poorly understood in terms of the underlying neural circuit mechanisms. Here, we present evidence that habituation arises from potentiation of inhibitory transmission within a circuit motif commonly repeated in the nervous system. In Drosophila, prior odorant exposure results in a selective reduction of response to this odorant. Both short-term (STH) and long-term (LTH) forms of olfactory habituation require function of the rutabaga-encoded adenylate cyclase in multiglomerular local interneurons (LNs) that mediate GABAergic inhibition in the antennal lobe; LTH additionally requires function of the cAMP response element-binding protein (CREB2) transcription factor in LNs. The odorant selectivity of STH and LTH is mirrored by requirement for NMDA receptors and GABA(A) receptors in odorant-selective, glomerulus-specific projection neurons(PNs). The need for the vesicular glutamate transporter in LNs indicates that a subset of these GABAergic neurons also releases glutamate. LTH is associated with a reduction of odorant-evoked calcium fluxes in PNs as well as growth of the respective odorant-responsive glomeruli. These cellular changes use similar mechanisms to those required for behavioral habituation. Taken together with the observation that enhancement of GABAergic transmission is sufficient to attenuate olfactory behavior, these data indicate that habituation arises from glomerulus-selective potentiation of inhibitory synapses in the antennal lobe. We suggest that similar circuit mechanisms may operate in other species and sensory systems.

Central synaptic mechanisms underlie short-term olfactory habituation in Drosophila larvae.

Naive Drosophila larvae show vigorous chemotaxis toward many odorants including ethyl acetate (EA). Chemotaxis toward EA is substantially reduced after a 5-min pre-exposure to the odorant and recovers with a half-time of ∼20 min. An analogous behavioral decrement can be induced without odorant-receptor activation through channelrhodopsin-based, direct photoexcitation of odorant sensory neurons (OSNs). The neural mechanism of short-term habituation (STH) requires the (1) rutabaga adenylate cyclase; (2) transmitter release from predominantly GABAergic local interneurons (LNs); (3) GABA-A receptor function in projection neurons (PNs) that receive excitatory inputs from OSNs; and (4) NMDA-receptor function in PNs. These features of STH cannot be explained by simple sensory adaptation and, instead, point to plasticity of olfactory synapses in the antennal lobe as the underlying mechanism. Our observations suggest a model in which NMDAR-dependent depression of the OSN-PN synapse and/or NMDAR-dependent facilitation of inhibitory transmission from LNs to PNs contributes substantially to short-term habituation.

Activity Regulates Cell Death within Cortical Interneurons through a Calcineurin-Dependent Mechanism.

We demonstrate that cortical interneurons derived from ventral eminences, including the caudal ganglionic eminence, undergo programmed cell death. Moreover, with the exception of VIP interneurons, this occurs in a manner that is activity-dependent. In addition, we demonstrate that, within interneurons, Calcineurin, a calcium-dependent protein phosphatase, plays a critical role in sequentially linking activity to maturation (E15-P5) and survival (P5-P20). Specifically, embryonic inactivation of Calcineurin results in a failure of interneurons to morphologically mature and prevents them from undergoing apoptosis. By contrast, early postnatal inactivation of Calcineurin increases apoptosis. We conclude that Calcineurin serves a dual role of promoting first the differentiation of interneurons and, subsequently, their survival.

Sensory inputs control the integration of neurogliaform interneurons into cortical circuits.

Neuronal microcircuits in the superficial layers of the mammalian cortex provide the substrate for associative cortical computation. Inhibitory interneurons constitute an essential component of the circuitry and are fundamental to the integration of local and long-range information. Here we report that, during early development, superficially positioned Reelin-expressing neurogliaform interneurons in the mouse somatosensory cortex receive afferent innervation from both cortical and thalamic excitatory sources. Attenuation of ascending sensory, but not intracortical, excitation leads to axo-dendritic morphological defects in these interneurons. Moreover, abrogation of the NMDA receptors through which the thalamic inputs signal results in a similar phenotype, as well as in the selective loss of thalamic and a concomitant increase in intracortical connectivity. These results suggest that thalamic inputs are critical in determining the balance between local and long-range connectivity and are fundamental to the proper integration of Reelin-expressing interneurons into nascent cortical circuits.

Identification and analysis of a glutamatergic local interneuron lineage in the adult Drosophila olfactory system.

BACKGROUND: The antennal lobe of Drosophila is perhaps one of the best understood neural circuits, because of its well-described anatomical and functional organization and ease of genetic manipulation. Olfactory lobe interneurons - key elements of information processing in this network - are thought to be generated by three identified central brain neuroblasts, all of which generate projection neurons. One of these neuroblasts, located lateral to the antennal lobe, also gives rise to a population of local interneurons, which can either be inhibitory (GABAergic) or excitatory (cholinergic). Recent studies of local interneuron number and diversity suggest that additional populations of this class of neurons exist in the antennal lobe. This implies that other, as yet unidentified, neuroblast lineages may contribute a substantial number of local interneurons to the olfactory circuitry of the antennal lobe. RESULTS: We identified and characterized a novel glutamatergic local interneuron lineage in the Drosophila antennal lobe. We used MARCM (mosaic analysis with a repressible cell marker) and dual-MARCM clonal analysis techniques to identify this novel lineage unambiguously, and to characterize interneurons contained in the lineage in terms of structure, neurotransmitter identity, and development. We demonstrated the glutamatergic nature of these interneurons by immunohistochemistry and use of an enhancer-trap strain, which reports the expression of the Drosophila vesicular glutamate transporter (DVGLUT). We also analyzed the neuroanatomical features of these local interneurons at single-cell resolution, and documented the marked diversity in their antennal lobe glomerular innervation patterns. Finally, we tracked the development of these dLim-1 and Cut positive interneurons during larval and pupal stages. CONCLUSIONS: We have identified a novel neuroblast lineage that generates neurons in the antennal lobe of Drosophila. This lineage is remarkably homogeneous in three respects. All of the progeny are local interneurons, which are uniform in their glutamatergic neurotransmitter identity, and form oligoglomerular or multiglomerular innervations within the antennal lobe. The identification of this novel lineage and the elucidation of the innervation patterns of its local interneurons (at single cell resolution) provides a comprehensive cellular framework for emerging studies on the formation and function of potentially excitatory local interactions in the circuitry of the Drosophila antennal lobe.

Neuronal activity and Wnt signaling act through Gsk3-beta to regulate axonal integrity in mature Drosophila olfactory sensory neurons.

The roles played by signaling pathways and neural activity during the development of circuits have been studied in several different contexts. However, the mechanisms involved in maintaining neuronal integrity once circuits are established are less well understood, despite their potential relevance to neurodegeneration. We demonstrate that maintenance of adult Drosophila olfactory sensory neurons requires cell-autonomous neuronal activity. When activity is silenced, development occurs normally, but neurons degenerate in adulthood. These detrimental effects can be compensated by downregulating Glycogen synthase kinase-3beta (Gsk-3beta). Conversely, ectopic expression of activated Gsk-3beta or downregulation of Wnt effectors also affect neuron stability, demonstrating a role for Wnt signaling in neuroprotection. This is supported by our observation that activated adult neurons are capable of increased Wingless release, and its targeted expression can protect neurons against degeneration. The role of Wnt signaling in this process is non-transcriptional, and may act on cellular mechanisms that regulate axonal or synaptic stability. Together, we provide evidence that Gsk-3beta is a key sensor involved in neural circuit integrity, maintaining axon stability through neural activity and the Wnt pathway.

Beta-thalassemia mutations in western India.

OBJECTIVE: To study occurrence of common mutations in the population of Gujarat and the most prevalent mutation in certain high-risk communities. METHODS: The mutation screening was carried out using ARMS-PCR in children with beta thalassemia. RESULTS: Population screening has identified certain communities like Sindhis, Lohana, Rajputs, and SC/ST/OBC to be at higher risk as compared to others. The most common mutation was IVS 1-5 (G-->C) followed by 619 bp deletions of the total cases coming to Gujarat. CONCLUSION: Molecular evaluation for Thalassemia should be considered for families whose ethnicity indicates origin from high-risk community.