Hox factors in the control of neuronal connectivity

Hox proteins appear to determine the selectivity of connections between motor neurons in the spinal cord and muscle targets in the limb, yet the mechanisms that determine this specificity are not known. One objective of our research is to elucidate the pathways downstream of Hox proteins that determine the guidance of motor axons to their targets.

Genetic dissection of Hox gene function in motor neuron connectivity.

While the contributions of Hox genes to the patterning of the limb and axial skeleton in mice have been well documented, few studies have examined Hox gene function in motor neuron identity and connectivity. We are addressing the genetic requirements for Hox genes in motor neuron differentiation and connectivity through analysis of Hox mutant mice. By breeding Hox mutants to reporter mice (Hb9-GFP) in which all motor axons are labeled with GFP we will explore the requirements for Hox genes in motor axon projection patterns.

What are the gene targets of Hox proteins that define the selection of muscle targets?

A potential output of Hox function is to control, either directly or indirectly, the expression of surface molecules that define the specificity of target selection. We plan to use two approaches to identify Hox targets. The first approach is based on the presumption that Hox factors control the expression of related members of large gene families. This strategy will involve screens against families of axon guidance molecules, to identify genes that are spatially coexpressed with Hox proteins. A second approach will involve microarray-based screens, using fluorescence-activated cell sorting (FACS) of motor neurons. Gene targeting in mouse, RNAi and misexpression in chick will be employed to assess the role of candidate gene targets in motor neuron connectivity.