No abstract

Dlx homeodomain transcription factors are essential during embryonic development for the production of forebrain GABAergic interneurons. Here we show that Dlx1 is also required for regulating the functional longevity of cortical and hippocampal interneurons in the adult brain. We demonstrate preferential Dlx1 expression in a subset of cortical and hippocampal interneurons which, in postnatal Dlx1 mutants, show a time-dependent reduction in number. This reduction preferentially affects calretinin(+) (bipolar cells) and somatostatin(+) subtypes (for example, bitufted cells), whereas parvalbumin(+) subpopulations (basket cells and chandelier cells) seem to be unaffected. Cell transplantation analysis demonstrates that interneuron loss reflects cell-autonomous functions of Dlx1. The decrease in the number of interneurons was associated with a reduction of GABA-mediated inhibitory postsynaptic current in neocortex and hippocampus in vitro and cortical dysrhythmia in vivo. Dlx1 mutant mice show generalized electrographic seizures and histological evidence of seizure-induced reorganization, linking the Dlx1 mutation to delayed-onset epilepsy associated with interneuron loss.

Dravet syndrome (DS) is a catastrophic pediatric epilepsy with severe intellectual disability, impaired social development and persistent drug-resistant seizures. One of its primary monogenic causes are mutations in Nav1.1 (SCN1A), a voltage-gated sodium channel. Here we characterise zebrafish Nav1.1 (scn1Lab) mutants originally identified in a chemical mutagenesis screen. Mutants exhibit spontaneous abnormal electrographic activity, hyperactivity and convulsive behaviors. Although scn1Lab expression is reduced, microarray analysis is remarkable for the small fraction of differentially expressed genes (~3%) and lack of compensatory expression changes in other scn subunits. Ketogenic diet, diazepam, valproate, potassium bromide and stiripentol attenuate mutant seizure activity; seven other antiepileptic drugs have no effect. A phenotype-based screen of 320 compounds identifies a US Food and Drug Administration-approved compound (clemizole) that inhibits convulsive behaviors and electrographic seizures. This approach represents a new direction in modeling pediatric epilepsy and could be used to identify novel therapeutics for any monogenic epilepsy disorder.

Cortical inhibitory circuits are formed by GABAergic interneurons, a cell population that originates far from the cerebral cortex in the embryonic ventral forebrain. Given their distant developmental origins, it is intriguing how the number of cortical interneurons is ultimately determined. One possibility, suggested by the neurotrophic hypothesis1-5, is that cortical interneurons are overproduced, and then following their migration into cortex, excess interneurons are eliminated through a competition for extrinsically derived trophic signals. Here we have characterized the developmental cell death of mouse cortical interneurons in vivo, in vitro, and following transplantation. We found that 40% of developing cortical interneurons were eliminated through Bax- (Bcl-2 associated X-) dependent apoptosis during postnatal life. When cultured in vitro or transplanted into the cortex, interneuron precursors died at a cellular age similar to that at which endogenous interneurons died during normal development. Remarkably, over transplant sizes that varied 200-fold, a constant fraction of the transplanted population underwent cell death. The death of transplanted neurons was not affected by the cell-autonomous disruption of TrkB (tropomyosin kinase receptor B), the main neurotrophin receptor expressed by central nervous system (CNS) neurons6-8. Transplantation expanded the cortical interneuron population by up to 35%, but the frequency of inhibitory synaptic events did not scale with the number of transplanted interneurons. Together, our findings indicate that interneuron cell death is intrinsically determined, either cell-autonomously, or through a population-autonomous competition for survival signals derived from other interneurons.

Impaired GABA–mediated neurotransmission has been implicated in many neurologic diseases including epilepsy, intellectual disability, and psychiatric disorders. Here we report that inhibitory neuron transplantation into the hippocampus of adult mice with confirmed epilepsy at the time of grafting dramatically reduced the occurrence of electrographic seizures and restored behavioral deficits in spatial learning, hyperactivity, and the aggressive response to handling. In the recipient brain, GABA progenitors migrated up to 1500 μm from the injection site, expressed genes and proteins characteristic for interneurons, differentiated into functional inhibitory neurons, and received excitatory synaptic input. In contrast to hippocampus, cell grafts into basolateral amygdala rescued the hyperactivity deficit but did not alter seizure activity or other abnormal behaviors. Our results highlight a critical role for interneurons in epilepsy and suggest that interneuron cell transplantation is a powerful approach to halt seizures and rescue accompanying deficits in severely epileptic mice.

No abstract

Epilepsy, a disease characterized by abnormal brain activity, is a disabling and potentially life-threatening condition for nearly 1% of the world population. Unfortunately, modulation of brain excitability using available antiepileptic drugs can have serious side effects, especially in the developing brain, and some patients can only be improved by surgical removal of brain regions containing the seizure focus. Here, we show that bilateral transplantation of precursor cells from the embryonic medial ganglionic eminence (MGE) into early postnatal neocortex generates mature GABAergic interneurons in the host brain. In mice receiving MGE cell grafts, GABA-mediated synaptic and extrasynaptic inhibition onto host brain pyramidal neurons is significantly increased. Bilateral MGE cell grafts in epileptic mice lacking a Shaker-like potassium channel (a gene mutated in one form of human epilepsy) resulted in significant reductions in the duration and frequency of spontaneous electrographic seizures. Our findings suggest that MGE-derived interneurons could be used to ameliorate abnormal excitability and possibly act as an effective strategy in the treatment of epilepsy.epilepsy ͉ inhibition ͉ neocortex ͉ therapy ͉ graft

Dravet syndrome is a catastrophic childhood epilepsy with early-onset seizures, delayed language and motor development, sleep disturbances, anxiety-like behaviour, severe cognitive deficit and an increased risk of fatality. It is primarily caused by de novo mutations of the SCN1A gene encoding a neuronal voltage-activated sodium channel. Zebrafish with a mutation in the SCN1A homologue recapitulate spontaneous seizure activity and mimic the convulsive behavioural movements observed in Dravet syndrome. Here, we show that phenotypic screening of drug libraries in zebrafish scn1 mutants rapidly and successfully identifies new therapeutics. We demonstrate that clemizole binds to serotonin receptors and its antiepileptic activity can be mimicked by drugs acting on serotonin signalling pathways e.g. trazodone and lorcaserin. Coincident with these zebrafish findings, we treated five medically intractable Dravet syndrome patients with a clinically-approved serotonin receptor agonist (lorcaserin, Belviq®) and observed some promising results in terms of reductions in seizure frequency and/or severity. Our findings demonstrate a rapid path from preclinical discovery in zebrafish, through target identification, to potential clinical treatments for Dravet syndrome.