Specification of distinct motor neuron identities by the singular activities of individual Hox genes

Hox genes have been implicated in specifying positional values along the an teroposterior axis of the caudal central nervous system, but their nested a nd overlapping expression has complicated the understanding of how they con fer specific neural identity. We have employed a direct gain-of-function ap proach using retroviral vectors to misexpress Hoxa2 and Hoxb1 outside of th e normal Hox expression domains, thereby avoiding complications resulting f rom possible interactions with endogenous Hox genes. Misexpression of eithe r Hoxa2 or Hoxb1 in the anteriormost hindbrain (rhombomere1, r1) leads to t he generation of motor neurons in this territory, even though it is normall y devoid of this cell type. These ectopic neurons have the specific identit y of branchiomotor neurons and, in the case of Hoxb1-induced cells, their a xons leave the hindbrain either by fasciculating with the resident cranial motor axons at isthmic (trochlear) or r2 (trigeminal) levels of the axis or via novel ectopic exit points in r1. Next, we have attempted to identify t he precise branchiomotor subtypes that are generated after misexpression an d our results suggest that the ectopic motor neurons generated following Ho xa2 misexpression are trigeminal-like, while those generated following Hoxb 1 misexpression are facial-like, Our data demonstrate, therefore, that at l east to a certain extent and for certain cell types, the singular activitie s of individual Hox genes (compared to a combinatorial mode of action, for example) are sufficient to impose on neuronal precursor cells the competenc e to generate distinctly specified cell types. Moreover, as these particula r motor neuron subtypes are normally generated in the most anterior domains of Hoxa2 and Hoxb1 expression, respectively, our data support the idea tha t the main site of individual Hox gene action is in the anteriormost subdom ain of their expression, consistent with the phenomenon of posterior domina nce.