Genetic dissection of cadherin and catenin functions in spinal motor neuron development

A key step in the assembly of circuits in the vertebrate central nervous system (CNS) is the segregation of neurons into anatomically delineated nuclei or stratified laminae. In the spinal cord, functionally related groups of motor neurons cluster together into nuclei, termed motor pool, which are exemplars of CNS nuclear organization. Motor neurons within a pool possess common molecular identities, project to the same peripheral muscle targets, share dendritic architecture and receive proprioceptive inputs from sensory afferents innervating the same muscle. Transcriptional programs defining motor neuron identity direct the expression of downstream surface receptors controlling motor pool differentiation. I focus specifically on type I and type II classical cadherins and their involvement in CNS development.;To begin to address the role classical cadherins in motor neurons, I survey their expression during the time of columnar and pool organization. I find that both type I and type II cadherins are expressed in motor neurons either in a ubiquitous or pool-restricted pattern. These results suggest that classical cadherins serve general adhesive roles during motor neuron development, as well as provide pool-specific recognition specificity.;In order to test the functional requirement of cadherins in motor pool development, I examine motor neuron organization in single and multiple knockout mice for cadherin-8, -11, -20 and T-cadherin. Since eliminating these cadherins does not affect motor neuron columnar or pool organization, I instead focused on inactivating all classical cadherins by targeting the intracellular transducers of cadherin adhesion and signaling, beta-catenin and gamma-catenin. I generate single catenin mutants and show that beta-catenin and gamma-catenin are functionally redundant in motor neurons. To overcome this redundancy I ablate both catenins and find that under these genetic conditions several major aspects of motor neuron development are disrupted.;In beta/gamma-catenin double mutants, I find defects in motor neuron lateral migration, lateral motor column (LMC) divisional segregation, pool sorting and clustering, and compartmentalization between LMC and interneuron populations. These findings point to a central role for beta-catenin and gamma-catenin signaling in the nuclear organization of spinal motor neurons during development.;To examine the mechanism of catenin action in motor neurons, I show that cadherin protein localization and function is affected in beta/gamma-catenin double mutants and find no evidence that supports a role of Wnt signaling in motor neuron development. The precise link between catenin signaling and cadherin function in motor neurons remains to be further elucidated.;In addition to cell body positioning defects in beta/gamma-catenin double mutants, I also observe errors in muscle target connectivity and defects in dendritic growth and patterning. These preliminary findings indicate that cadherin-catenin signaling may also be involved in later stages of motor circuit development.;Lastly, I show that removing N-cadherin, a type I member, from motor neurons does not recapitulate the LMC divisional and motor pool mixing defects found in beta/gamma-catenin double mutants. This finding together with the expression patterns of type I and type II cadherins in motor neurons suggest that type I and type II members may contribute differentially to the observed catenin phenotypes.