Gbx2 regulates thalamocortical axon guidance by modifying the LIM and Robo codes.

Combinatorial expression of transcription factors forms transcriptional codes to confer neuronal identities and connectivity. However, how these intrinsic factors orchestrate the spatiotemporal expression of guidance molecules to dictate the responsiveness of axons to guidance cues is less understood. Thalamocortical axons (TCAs) represent the major input to the neocortex and modulate cognitive functions, consciousness and alertness. TCAs travel a long distance and make multiple target choices en route to the cortex. The homeodomain transcription factor Gbx2 is essential for TCA development, as loss of Gbx2 abolishes TCAs in mice. Using a novel TCA-specific reporter, we have discovered that thalamic axons are mostly misrouted to the ventral midbrain and dorsal midline of the diencephalon in Gbx2-deficient mice. Furthermore, conditionally deleting Gbx2 at different embryonic stages has revealed a sustained role of Gbx2 in regulating TCA navigation and targeting. Using explant culture and mosaic analyses, we demonstrate that Gbx2 controls the intrinsic responsiveness of TCAs to guidance cues. The guidance defects of Gbx2-deficient TCAs are associated with abnormal expression of guidance receptors Robo1 and Robo2. Finally, we demonstrate that Gbx2 controls Robo expression by regulating LIM-domain transcription factors through three different mechanisms: Gbx2 and Lhx2 compete for binding to the Lmo3 promoter and exert opposing effects on its transcription; repressing Lmo3 by Gbx2 is essential for Lhx2 activity to induce Robo2; and Gbx2 represses Lhx9 transcription, which in turn induces Robo1. Our findings illustrate the transcriptional control of differential expression of Robo1 and Robo2, which may play an important role in establishing the topography of TCAs.

Differentiation and functional incorporation of embryonic stem cell-derived GABAergic interneurons in the dentate gyrus of mice with temporal lobe epilepsy.

Cell therapies for neurological disorders require an extensive knowledge of disease-associated neuropathology and procedures for generating neurons for transplantation. In many patients with severe acquired temporal lobe epilepsy (TLE), the dentate gyrus exhibits sclerosis and GABAergic interneuron degeneration. Mounting evidence suggests that therapeutic benefits can be obtained by transplanting fetal GABAergic progenitors into the dentate gyrus in rodents with TLE, but the scarcity of human fetal cells limits applicability in patient populations. In contrast, virtually limitless quantities of neural progenitors can be obtained from embryonic stem (ES) cells. ES cell-based therapies for neurological repair in TLE require evidence that the transplanted neurons integrate functionally and replace cell types that degenerate. To address these issues, we transplanted mouse ES cell-derived neural progenitors (ESNPs) with ventral forebrain identities into the hilus of the dentate gyrus of mice with TLE and evaluated graft differentiation, mossy fiber sprouting, cellular morphology, and electrophysiological properties of the transplanted neurons. In addition, we compared electrophysiological properties of the transplanted neurons with endogenous hilar interneurons in mice without TLE. The majority of transplanted ESNPs differentiated into GABAergic interneuron subtypes expressing calcium-binding proteins parvalbumin, calbindin, or calretinin. Global suppression of mossy fiber sprouting was not observed; however, ESNP-derived neurons formed dense axonal arborizations in the inner molecular layer and throughout the hilus. Whole-cell hippocampal slice electrophysiological recordings and morphological analyses of the transplanted neurons identified five basic types; most with strong after-hyperpolarizations and smooth or sparsely spiny dendritic morphologies resembling endogenous hippocampal interneurons. Moreover, intracellular recordings of spontaneous EPSCs indicated that the new cells functionally integrate into epileptic hippocampal circuitry.

Recent advancements in stem cell and gene therapies for neurological disorders and intractable epilepsy.

The potential applications of stem cell therapies for treating neurological disorders are enormous. Many laboratories are focusing on stem cell treatments for CNS diseases, including spinal cord injury, Amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, multiple sclerosis, stroke, traumatic brain injury, and epilepsy. Among the many stem cell types under testing for neurological treatments, the most common are fetal and adult brain stem cells, embryonic stem cells, induced pluripotent stem cells, and mesenchymal stem cells. An expanding toolbox of molecular probes is now available to allow analyses of neural stem cell fates prior to and after transplantation. Concomitantly, protocols are being developed to direct the fates of stem cell-derived neural progenitors, and also to screen stem cells for tumorigenicity and aneuploidy. The rapid progress in the field suggests that novel stem cell and gene therapies for neurological disorders are in the pipeline.

Embryonic stem cell-derived neural precursor grafts for treatment of temporal lobe epilepsy.

Complex partial seizures arising from mesial temporal lobe structures are a defining feature of mesial temporal lobe epilepsy (TLE). For many TLE patients, there is an initial traumatic head injury that is the precipitating cause of epilepsy. Severe TLE can be associated with neuropathological changes, including hippocampal sclerosis, neurodegeneration in the dentate gyrus, and extensive reorganization of hippocampal circuits. Learning disabilities and psychiatric conditions may also occur in patients with severe TLE for whom conventional anti-epileptic drugs are ineffective. Novel treatments are needed to limit or repair neuronal damage, particularly to hippocampus and related limbic regions in severe TLE and to suppress temporal lobe seizures. A promising therapeutic strategy may be to restore inhibition of dentate gyrus granule neurons by means of cell grafts of embryonic stem cell-derived GABAergic neuron precursors. "Proof-of-concept" studies show that human and mouse embryonic stem cell-derived neural precursors can survive, migrate, and integrate into the brains of rodents in different experimental models of TLE. In addition, studies have shown that hippocampal grafts of cell lines engineered to release GABA or other anticonvulsant molecules can suppress seizures. Furthermore, transplants of fetal GABAergic progenitors from the mouse or human brain have also been shown to suppress the development of seizures. Here, we review these relevant studies and highlight areas of future research directed toward producing embryonic stem cell-derived GABAergic interneurons for cell-based therapies for treating TLE.