Faculty Bibliography

Mutant alleles of MYH9 encoding a class II non-muscle myosin heavy chain-A (NMMHC-IIA) have been linked to hereditary megathrombocytopenia with or without additional clinical features that include sensorineural deafness, cataracts, and nephritis. To assess its biological role in the affected targets, particularly the inner ear, we have generated and characterized mice with Myh9 deficiency. These mice were generated using the XA136 ES cell line (BayGenomics, http://baygenomics.ucsf.edu/) carrying gene trap insertion in Myh9, within the intron flanking exons 4 and 5. Mice heterozygous for the Myh9 null allele, Myh9 +/- were expanded on C57BL/6J background. Intercross of the Myh9 +/- mice did not yield Myh9 -/- pups, indicating embryonic lethality, subsequently determined to occur at or before E7.5, thus precluding a post-natal analysis of the effects of complete Myh9 deficiency. The heterozygous mice were normal for their hearing, parameters of platelet integrity and renal function despite their Myh9 haplo-insufficiency. In addition, the age-dependent auditory threshold of the Myh9 +/- mice and their wild type littermates, spanning from 3 to 12 months of age, were similar indicating that Myh9 haplo-insufficiency does not contribute towards accelerated age-related hearing loss (AHL). The embryonic lethality associated with the complete Myh9 deficiency establishes a critical role for this non-muscle myosin in fetal development. The results of these studies do not support the Myh9 haploinsufficiency as a pathogenic factor in the etiology of auditory dysfunction

Cortical GABAergic interneurons in mice are largely derived from the subpallium. Work from our laboratory and others over the past five years has demonstrated that a developmental logic in space and time underlies the emergence of specific cortical interneuronal subtypes. Following on from the seminal work of the Rubenstein laboratory, we set out to fate map the output of the subpallial ganglionic eminences. Our initial approach utilized ultrasound backscatter microscopy to perform homotopic and heterotopic transplants of genetically marked progenitors from the lateral, medial and caudal ganglionic eminences (LGE, MGE and CGE, respectively) to unmarked host brains. The LGE, at least in the context of our transplant studies, did not appear to generate cortical interneurons. By contrast, we found that that approximately eighty percent of cortical interneurons arise from the MGE, while the remaining twenty percent were generated by the CGE. Hence, the majority of interneuron subtypes, including all fast spiking parvalbumin-positive basket cells and somatostatin-positive Martinotti cells appear to arise from the MGE. A more restricted set of cortical interneurons seems to be generated in the CGE, the majority of which are bipolar calretinin/VIP-positive interneurons. Complementing these results, we have recently demonstrated using inducible genetic fate mapping that the MGE produces specific cortical interneuron subtypes at discrete timepoints during development. These studies demonstrate that cortical interneurons arise from a precise developmental programme that acts in both space and time. Beyond this however, it seems likely that postmitotic events influence the specific function of subclasses of cortical interneurons. A primary challenge in the future will be to investigate what aspects of interneuron diversity are determined by intrinsic genetic programmes within each lineage versus those properties imposed by the local environment in the cortex

Seizure induction in laboratory animals is followed by many changes in structure and function, and one of these is an increase in neurogenesis-the birth of new neurons. This phenomenon may be relevant to temporal lobe epilepsy (TLE), because one of the regions of the brain where seizure-induced neurogenesis is most robust is the dentate gyrus-an area of the brain that has been implicated in the pathophysiology of TLE. Although initial studies predicted that neurogenesis in the dentate gyrus would be important to normal functions, such as learning and memory, the new neurons that are born after seizures may not necessarily promote normal function. There appears to be a complex functional and structural relationship between the new dentate gyrus neurons and preexisting cells, both in the animal models of TLE and in tissue resected from patients with intractable TLE. These studies provide new insights into the mechanisms of TLE, and suggest novel strategies for intervention that could be used to prevent or treat TLE