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Michael Gross
Assistant Professor, Biochemistry and Biophysics
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RESEARCH:
Transcriptional Networks in Spinal Cord Development
Our research group is interested in uncovering the molecular networks that control spinal cord development at the systems level. We focus on the development of the dorsal horn of the mouse spinal cord. The dorsal horn consists of a variety of association and relay interneurons that receive, process, and transmit all incoming touch and pain information to a variety of locations in the brain. By studying the ontogeny of this structure we hope to provide a basic scientific underpinning for the development of novel spinal cord regeneration and pain therapies.
The spinal cord develops from a proliferative epithelium that folds into a neural tube during early embryonic development. Molecular signals emanating from structures at the dorsal and ventral extremes of the neural tube induce dorsal-ventral (DV) domains of transcription factor expression in the proliferating ventricular zone of the neural tube. Thus, discrete combinations of developmental transcription factors specify distinct populations of proliferating neuronal precursors. Neurons are born when they exit the cell cycle, leave the ventricular zone, and enter the outer mantle layer of the developing neural tube. New combinations of developmental transcription factors begin to be expressed in each newly born population of neurons. These transcription factors appear prior to elaboration of projection patterns and neurotransmitter phenotypes and their elimination in knock-out mice frequently results in altered expression of genetically downstream transcription factors and the loss of adult neuronal cell types. We are interested in determining how developmental transcription factors control the transitions from specified precursor to newborn neuron to mature neuron.
Expression and knockout analyses allow us to place many of the known developmental transcription factors into genetic hierarchies. We are interested in converting such genetic hierarchies of transcription factors to networks of direct molecular interactions. To do this effectively, we need to identify all of the discrete populations that exist in the dorsal horn, establish comprehensive lists of transcription factors expressed in each population, and identify the population-specific enhancer elements that are utilized by developmental transcription factors to effect the developmental transitions described above. We will reach these long range goals by using mouse molecular genetics to indelibly label dorsal horn populations with fluorescent tags, microarray analyses to determine the RNA expression profiles in these populations, and high-throughput cis-element screens with specific combinations of factors to isolate population-specific enhancer elements.
We have recently demonstrated that the mouse homeobox gene Lbx1 is essential for the developmental bifurcation of two broad classes of dorsal horn interneurons. Lbx1 is expressed only in newborn neurons emerging from a discrete dorsal domain and contributes to the further specification of several types of association interneurons. Thus, Lbx1 is genetically between the two transitions we will examine. The onset of Lbx1 expression is likely controlled by a population specific enhancers that are acted upon by a discrete combinations of transcription factors in the proliferative zone. The action of the Lbx1 transcription factor itself appears to control the expression of several downstream transcription factors (Pax2, Lmx1b, Isl1, and Foxd3). Thus, it provides a suitable starting point to initiate investigations into the developmental transitions described above. We have replaced the Lbx1 coding region with green fluorescent protein and can now isolate the Lbx1 population from the neural tubes of mouse embryos by fluorescence activated cell sorting. Microarray analyses will be used to identify short lists of candidate Lbx1 cofactors and target genes.