The role of HoxD10 in the development of motoneurons in the posterior spinal cord

Hox genes encode anterior-posterior identity during central nervous system development. Few studies have examined Hox gene function at lumbosacral (LS) levels of the spinal cord, where there is extensive information on normal development. Hoxd10 is expressed at high levels in the embryonic LS spinal cord, but not the thoracic (T) spinal cord. To test the hypothesis that restricted expression of Hoxd10 contributes to the attainment of an LS identity, and specifically an LS motoneuron identity, Hoxd10 was ectopically expressed in T segments in chick embryos via in ovo electroporation. Electroporations were carried out at early neural tube stages (stages 13-15) and at the onset of motoneuron differentiation (stages 17-18). Regional motoneuron identity was assessed after the normal period of motor column formation (stages 28-29). Subsets of motoneurons in transfected T segments developed a molecular profile normally shown by anterior LS LMCl motoneurons, including Lim 1 and RALDH2 expression. In addition, motoneurons in posterior T segments showed novel axon projections to two muscles in the anterodorsal limb, the sartorius and anterior iliotibialis muscles. These changes are accompanied by a significant reduction in the number of T motoneurons at stage 29. Analyses of Hoxd10 electroporated embryos at the onset of motor column formation (stage 18) suggest that early and high levels of Hoxd10 expression led to the death of some early differentiating motoneuron. Despite these adverse effects, our data indicate that Hoxd10 expression is sufficient to induce LS motoneuron identity and axon trajectories characteristic of motoneurons in the LS anterior spinal cord. Equivalent changes in motoneuron identity were not found with the ectopic expression of Hoxd9, a gene normally expressed in T as well as LS segments. In an additional series of experiments, Hoxd10 was overexpressed in LS segments via in ovo electroporation at early neural tube stages. Analyses at stage 29 indicated proportionate increases in LMCl (Lim 1+, RALDH2+) motoneurons, and proportionate decreases in LMCm and MMC motoneurons (Isl 1+) motoneurons. These findings suggest that Hoxd10 specifically promotes the development and/or survival of LMCl motoneurons.