Dynamic expression of murine HOXA5 protein in the central nervous system.

The Hox genes encode transcription factors that are indispensable for proper spatio-temporal patterning of the vertebrate body axes. As for other Hox genes, region-specific expression of Hoxa5 appears to be important for correct function during development. In mouse, Hoxa5 transcripts are differentially expressed in specific mesoderm-derived structures and in the most anterior domain of expression in the central nervous system (CNS), in contrast to indistinct patterns seen in the posterior CNS. However, the functional significance of any pattern of protein-coding RNAs must be verified by correlating the presence of the protein(s) and RNAs. Here, we describe the dynamic pattern of HOXA5 protein during mouse embryogenesis. The HOXA5 protein is detected as early as embryonic day (E) 9.0, and is found, as development proceeds, in several mesoderm-derived structures such as prevertebrae (pv), proximal forelimb bud, scapula, lung, trachea, and gut. In addition, the protein shows a strikingly restricted and dynamic expression pattern in the developing CNS, and is detected in both motor neurons and interneurons between E10.5 and E13.5. Moreover, this CNS region-specific HOXA5 protein pattern is more restricted than the pattern observed for the Hoxa5 transcripts. In many mesoderm-derived tissues affected by the Hoxa5 mutation, the expression pattern of HOXA5 protein corresponds to that of the putative functional Hoxa5 transcript. However, in the CNS, this correlation is exclusively demonstrated in the most anterior domain of expression. Overall, the HOXA5 protein pattern is consistent with its proposed role in positional specification in mesodermal structures, as well as in the embryonic neuraxis.