The transcriptional regulatory network in the developing spinal cord

Soo-Kyung Lee

Department of Molecular and Cellular Biology, Huffington Center on Aging, Baylor College of Medicine, Houston, TX 77030

LIM homeodomain (LIM-HD) and basic helix-loop-helix (bHLH) proteins are relatively well characterized transcription factors regulating cell type specification in the embryonic CNS. However, both the molecular mechanism by which these factors regulate gene expression and the identity of their target genes remain poorly understood. Our results demonstrate that LIM-HD factors Lhx3 and Isl1 specify two distinct neurons of the embryonic spinal cord in a combinatorial manner. Lhx3 and the LIM cofactor NLI (for nuclear LIM-interactor) form a V2-interneuron-specifying tetramer. In contrast, Lhx3 and NLI form a motoneuron-specifying hexamer when coexpressed with Isl1, which provides the molecular basis of combinatorial function of Lhx3 to specify two distinct neuronal cell types. During motoneuron specification, bHLH proteins Ngn2/NeuroM controlling neurogenesis are functionally synchronized with the hexamer to specify motoneurons in the embryonic spinal cord and in P19 stem cells. The mechanism underlying this cooperativity is based on transcriptional synergy. Our recent screening effort led to the finding of genomic fragments with the NLI:Isl1:Lhx3 hexamer binding sites, many of which are linked to candidate motoneuron genes. Together, these studeis define a gene regulatory network by which motoneurons are specified during embryonic development and provide critical insights into the basic principles for the proper formation of the nervous system.