The Section on Mammalian Molecular Genetics carries out a functional analysis of genes that control key steps of post-implantation development in the mammalian embryo. Transcription factors are major players in the cascade of molecular events that implement the body plan. Our group uses a loss-of-function approach to study in detail the function of the LIM/homeodomain (Lhx) class of transcription factors. Over the years, our studies have shown that LIM-homeodomain proteins are involved in early patterning events, in the development of the nervous system, and in orchestrating organ assembly. More recently, we identified two gene families, termed Ldb and Ssdp, respectively, that encode obligatory cofactors of Lhx gene action. Current work is focused on functions of the LIM-homeodomain proteins Lhx8 and Lhx2 and the LIM-domain-binding protein Ldb1 in embryonic development. Previously, we generated a strain of knockout mice with a null-deletion of the LIM-homeobox gene Lhx8. Our initial analysis of the mutant mice revealed an important role for Lhx8 in development of the palate. Since this gene is also abundantly expressed in the developing ventral forebrain, we recently examined the brain of mice with a targeted null mutation of the Lhx8 gene. Development of cholinergic neurons in the ventral telencephalon is severely impaired in these knockout mutants. As a result, these mice lack the nucleus basalis, a major source of cholinergic neuron input to the cerebral cortex. In addition, the number of cholinergic neurons in several other regions of the subcortical forebrain including the caudate putamen, septum, and magnocellular preoptic nucleus is reduced. Our marker analysis revealed that progenitor cells form in the absence of Lhx8 gene function. However, the gene is essential for terminal differentiation of the majority of cholinergic neurons in the ventral telencephalon. The loss of telencephalic cholinergic projection neurons is a hallmark of neurodegenerative disorders such as Alzheimer's disease and is likely to play a role in the cognitive impairments of these patients. For this reason, the Lhx8 null mutant mouse constitutes a valuable model for functional studies of cholinergic projection neurons in the context of memory and cognition. Another ongoing study concerns the function of the LIM-homeobox gene Lhx2 in brain development. Our initial Lhx2 knockout study had shown that this gene is essential for eye development and the formation of the telencephalon. More recently we observed that development of the ventral diencephalon and pituitary is also impaired in the null mutant embryo, indicating an important additional role of Lhx2 in forebrain development. The Section also continued its functional evaluation of protein complexes formed by LIM-homeodomain proteins and their associated cofactors. The Ssdp co-factors that we discovered are thought to form complexes with many transcription factors active during early stages of embryonic development. Whereas the vertebrate Ssdp gene family has several closely related members, the Drosophila ssdp gene is unique. Moreover, the existence of several mutant alleles of this gene makes the Drosophila system attractive for a genome wide search for downstream genes whose transcription is regulated by Ssdp and associated proteins. Since loss-of-function mutations in the Drosophila ssdp gene cause lethality at early pupal stages we chose to focus our search on the late third instar larval stage which precedes the pupal stage. The analysis was carried out in collaboration with the laboratory of Dr. Brian Oliver at NIDDK, using Drosophila genomic amplicon micro arrays, and the resulting wealth of data is currently being evaluated in an effort to identify direct targets of ssdp-mediated transcriptional regulation. In an unrelated project, our Section has identified in azoospermic patients a single nucleotide deletion in the SYCP3 gene that causes a truncation of a protein required for chromosomal pairing during meiosis. This study represents an important advance in our clinical understanding of the molecular basis of early meiotic arrest as a cause of non-obstructive azoospermia. Our experiments suggest that the truncated protein dominantly interferes with the function of the wild type allele and thus causes male infertility. Another separate project led to the identification of FoxP4, a forkhead transcription factor involved in embryonic development, cell cycle regulation and oncogenesis. Our group has also been involved in a diverse array of collaborative projects, aimed at revealing gene functions via knockout. These studies have shown that (1) the D5 dopamine receptor is involved in blood pressure regulation; (2) the vasopressin receptor 1a is a negative regulator of B cell receptor signaling; (3) nitric oxide acts as a negative regulator of cell proliferation in the adult mammalian brain; and (4) the glycose-6-phosphate transporter, deficient in glycogen storage disease type Ib, is an important immuno-modulatory protein. Finally, in a couple of monographs, I have summarized (a) the current stand of knowledge with regard to genes that participate in the formation of the pituitary gland and (b) international considerations with respect to stem cell research.
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