Gregory B. Potter

Autistic-like behaviour in Scn1a +/− mice and rescue by enhanced GABA-mediated neurotransmission

Haploinsufficiency of the SCN1A gene encoding voltage-gated sodium channel NaV1.1 causes Dravet’s syndrome, a childhood neuropsychiatric disorder including recurrent intractable seizures, cognitive deficit and autism-spectrum behaviours. The neural mechanisms responsible for cognitive deficit and autism-spectrum behaviours in Dravet’s syndrome are poorly understood. Here we report that mice with Scn1a haploinsufficiency exhibit hyperactivity, stereotyped behaviours, social interaction deficits and impaired context-dependent spatial memory. Olfactory sensitivity is retained, but novel food odours and social odours are aversive to Scn1a+/− mice. GABAergic neurotransmission is specifically impaired by this mutation, and selective deletion of NaV1.1 channels in forebrain interneurons is sufficient to cause these behavioural and cognitive impairments. Remarkably, treatment with low-dose clonazepam, a positive allosteric modulator of GABAA receptors, completely rescued the abnormal social behaviours and deficits in fear memory in the mouse model of Dravet’s syndrome, demonstrating that they are caused by impaired GABAergic neurotransmission and not by neuronal damage from recurrent seizures. These results demonstrate a critical role for NaV1.1 channels in neuropsychiatric functions and provide a potential therapeutic strategy for cognitive deficit and autism-spectrum behaviours in Dravet’s syndrome.

Dlx1 and Dlx2 control neuronal versus oligodendroglial cell fate acquisition in the developing forebrain.

Progenitors within the ventral telencephalon can generate GABAergic neurons and oligodendrocytes, but regulation of the neuron-glial switch is poorly understood. We investigated the combinatorial expression and function of Dlx1&2, Olig2, and Mash1 transcription factors in the ventral telencephalon. We show that Dlx homeobox transcription factors, required for GABAergic interneuron production, repress oligodendrocyte precursor cell (OPC) formation by acting on a common progenitor to determine neuronal versus oligodendroglial cell fate acquisition. We demonstrate that Dlx1&2 negatively regulate Olig2-dependant OPC formation and that Mash1 promotes OPC formation by restricting the number of Dlx+ progenitors. Progenitors transplanted from Dlx1&2 mutant ventral telencephalon into newborn wild-type mice do not produce neurons but differentiate into myelinating oligodendrocytes that survive into adulthood. Our results identify another role for Dlx genes as modulators of neuron versus oligodendrocyte development in the ventral embryonic forebrain.

Specific deletion of NaV1.1 sodium channels in inhibitory interneurons causes seizures and premature death in a mouse model of Dravet syndrome

Heterozygous loss-of-function mutations in the brain sodium channel NaV1.1 cause Dravet syndrome (DS), a pharmacoresistant infantile-onset epilepsy syndrome with comorbidities of cognitive impairment and premature death. Previous studies using a mouse model of DS revealed reduced sodium currents and impaired excitability in GABAergic interneurons in the hippocampus, leading to the hypothesis that impaired excitability of GABAergic inhibitory neurons is the cause of epilepsy and premature death in DS. However, other classes of GABAergic interneurons are less impaired, so the direct cause of hyperexcitability, epilepsy, and premature death has remained unresolved. We generated a floxed Scn1a mouse line and used the Cre-Lox method driven by an enhancer from the Dlx1,2 locus for conditional deletion of Scn1a in forebrain GABAergic neurons. Immunocytochemical studies demonstrated selective loss of NaV1.1 channels in GABAergic interneurons in cerebral cortex and hippocampus. Mice with this deletion died prematurely following generalized tonic-clonic seizures, and they were equally susceptible to thermal induction of seizures as mice with global deletion of Scn1a. Evidently, loss of NaV1.1 channels in forebrain GABAergic neurons is both necessary and sufficient to cause epilepsy and premature death in DS.

Dlx1&2 and Mash1 Transcription Factors Control Striatal Patterning and Differentiation Through Parallel and Overlapping Pathways

Here we define the expression of ≈100 transcription factors in progenitors and neurons of the developing basal ganglia. We have begun to elucidate the transcriptional hierarchy of these genes with respect to the Dlx homeodomain genes, which are essential for differentiation of most GABAergic projection neurons of the basal ganglia. This analysis identified Dlx‐dependent and Dlx‐independent pathways. The Dlx‐independent pathway depends in part on the function of the Mash1 b‐HLH transcription factor. These analyses define core transcriptional components that differentially specify the identity and differentiation of the striatum, nucleus accumbens, and septum. J. Comp. Neurol. 512:556–572, 2009.

Lhx6 and Lhx8 Coordinately Induce Neuronal Expression of Shh that Controls the Generation of Interneuron Progenitors

Lhx6 and Lhx8 transcription factor coexpression in early-born MGE neurons is required to induce neuronal Shh expression. We provide evidence that these transcription factors regulate expression of a Shh enhancer in MGE neurons. Lhx6 and Lhx8 are also required to prevent Nkx2-1 expression in a subset of pallial interneurons. Shh function in early-born MGE neurons was determined by genetically eliminating Shh expression in the MGE mantle zone (MZ). This mutant had reduced SHH signaling in the overlying progenitor zone, which led to reduced Lhx6, Lhx8, and Nkx2-1 expression in the rostrodorsal MGE and a preferential reduction of late-born somatostatin(+) and parvalbumin(+) cortical interneurons. Thus, Lhx6 and Lhx8 regulate MGE development through autonomous and nonautonomous mechanisms, the latter by promoting Shh expression in MGE neurons, which in turn feeds forward to promote the developmental program of the rostrodorsal MGE.

Phosphorylation State of Olig2 Regulates Proliferation of Neural Progenitors

The bHLH transcription factors that regulate early development of the central nervous system can generally be classified as either antineural or proneural. Initial expression of antineural factors prevents cell cycle exit and thereby expands the pool of neural progenitors. Subsequent (and typically transient) expression of proneural factors promotes cell cycle exit, subtype specification, and differentiation. Against this backdrop, the bHLH transcription factor Olig2 in the oligodendrocyte lineage is unorthodox, showing antineural functions in multipotent CNS progenitor cells but also sustained expression and proneural functions in the formation of oligodendrocytes. We show here that the proliferative function of Olig2 is controlled by developmentally regulated phosphorylation of a conserved triple serine motif within the amino-terminal domain. In the phosphorylated state, Olig2 maintains antineural (i.e., promitotic) functions that are reflected in human glioma cells and in a genetically defined murine model of primary glioma.

Hilar GABAergic Interneuron Activity Controls Spatial Learning and Memory Retrieval

Background Although extensive research has demonstrated the importance of excitatory granule neurons in the dentate gyrus of the hippocampus in normal learning and memory and in the pathogenesis of amnesia in Alzheimers disease (AD), the role of hilar GABAergic inhibitory interneurons, which control the granule neuron activity, remains unclear. Methodology and Principal Findings We explored the function of hilar GABAergic interneurons in spatial learning and memory by inhibiting their activity through Cre-dependent viral expression of enhanced halorhodopsin (eNpHR3.0)—a light-driven chloride pump. Hilar GABAergic interneuron-specific expression of eNpHR3.0 was achieved by bilaterally injecting adeno-associated virus containing a double-floxed inverted open-reading frame encoding eNpHR3.0 into the hilus of the dentate gyrus of mice expressing Cre recombinase under the control of an enhancer specific for GABAergic interneurons. In vitro and in vivo illumination with a yellow laser elicited inhibition of hilar GABAergic interneurons and consequent activation of dentate granule neurons, without affecting pyramidal neurons in the CA3 and CA1 regions of the hippocampus. We found that optogenetic inhibition of hilar GABAergic interneuron activity impaired spatial learning and memory retrieval, without affecting memory retention, as determined in the Morris water maze test. Importantly, optogenetic inhibition of hilar GABAergic interneuron activity did not alter short-term working memory, motor coordination, or exploratory activity. Conclusions and Significance Our findings establish a critical role for hilar GABAergic interneuron activity in controlling spatial learning and memory retrieval and provide evidence for the potential contribution of GABAergic interneuron impairment to the pathogenesis of amnesia in AD.

Missense mutation in mouse GALC mimics human gene defect and offers new insights into Krabbe disease

Krabbe disease is a devastating pediatric leukodystrophy caused by mutations in the galactocerebrosidase (GALC) gene. A significant subset of the infantile form of the disease is due to missense mutations that result in aberrant protein production. The currently used mouse model, twitcher, has a nonsense mutation not found in Krabbe patients, although it is similar to the human 30 kb deletion in abrogating GALC expression. Here, we identify a spontaneous mutation in GALC, GALCtwi-5J, that precisely matches the E130K missense mutation in patients with infantile Krabbe disease. GALCtwi-5J homozygotes show loss of enzymatic activity despite normal levels of precursor protein, and manifest a more severe phenotype than twitcher, with half the life span. Although neuropathological hallmarks such as gliosis, globoid cells and psychosine accumulation are present throughout the nervous system, the CNS does not manifest significant demyelination. In contrast, the PNS is severely hypomyelinated and lacks large diameter axons, suggesting primary dysmyelination, rather than a demyelinating process. Our data indicate that early demise is due to mechanisms other than myelin loss and support an important role for neuroinflammation in Krabbe disease progression. Furthermore, our results argue against a causative relationship between psychosine accumulation, white matter loss and gliosis.

A Time and a Place for Nkx2-1 in Interneuron Specification and Migration

The homeobox transcription factor, Nkx2-1, plays multiple roles during forebrain development. Using restricted genetic ablation of Nkx2-1, in this issue of Neuron, Butt et al. show that Nkx2-1 in telencephalic progenitors regulates interneuron subtype specification, while Nóbrega-Pereira et al. demonstrate that postmitotic Nkx2-1 regulates migration and sorting of interneurons to the striatum or cortex by controlling the expression of the guidance receptor, Neuropilin-2.

Chronic reduction in inhibition reduces receptive field size in mouse auditory cortex

Inhibitory interneurons regulate the responses of cortical circuits. In auditory cortical areas, inhibition from these neurons narrows spectral tuning and shapes response dynamics. Acute disruptions of inhibition expand spectral receptive fields. However, the effects of long-term perturbations of inhibitory circuitry on auditory cortical responses are unknown. We ablated ∼30% of dendrite-targeting cortical inhibitory interneurons after the critical period by studying mice with a conditional deletion of Dlx1. Following the loss of interneurons, baseline firing rates rose and tone-evoked responses became less sparse in auditory cortex. However, contrary to acute blockades of inhibition, the sizes of spectral receptive fields were reduced, demonstrating both higher thresholds and narrower bandwidths. Furthermore, long-latency responses at the edge of the receptive field were absent. On the basis of changes in response dynamics, the mechanism for the reduction in receptive field size appears to be a compensatory loss of cortico-cortically (CC) driven responses. Our findings suggest chronic conditions that feature changes in inhibitory circuitry are not likely to be well modeled by acute network manipulations, and compensation may be a critical component of chronic neuronal conditions.