Hox transcription factors influence motoneuron identity through the integrated actions of both homeodomain and non-homeodomain regions

• Published in: Developmental dynamics 2012-04, Vol.241 (4), p.718-731
• Main Authors: Misra, Mala, Sours, Emily, Lance-Jones, Cynthia
• Format: Article
• Language: English
• Subjects: Animals; Avian Proteins - physiology; Cell Differentiation - genetics; chick; Chick Embryo - embryology; Chick Embryo - physiology; Gene Expression Regulation, Developmental; homeodomain; Homeodomain Proteins - physiology; Hox; LIM genes; motoneuron development; Motor Neurons - cytology; Motor Neurons - physiology; spinal cord; Spinal Cord - embryology; Transcription Factors - physiology
• ISSN: 1058-8388; 1097-0177
• DOI: 10.1002/dvdy.23763

Background: Hox transcription factors play a critical role in the specification of motoneuron subtypes within the spinal cord. Our previous work showed that two orthologous members of this family, Hoxd10 and Hoxd11, exert opposing effects on motoneuron development in the lumbosacral (LS) spinal cord of the embryonic chick: Hoxd10 promotes the development of lateral motoneuron subtypes that project to dorsal limb muscles, while Hoxd11 represses the development of lateral subtypes in favor of medial subtypes that innervate ventral limb muscles and axial muscles. The striking degree of homology between the DNA‐binding homeodomains of Hoxd10 and Hoxd11 suggested that non‐homeodomain regions mediate their divergent effects. In the present study, we investigate the relative contributions of homeodomain and non‐homeodomain regions of Hoxd10 and Hoxd11 to motoneuron specification. Results: Using in ovo electroporation to express chimeric and mutant constructs in LS motoneurons, we find that both the homeodomain and non‐homeodomain regions of Hoxd10 are necessary to specify lateral motoneurons. In contrast, non‐homeodomain regions of Hoxd11 are sufficient to repress lateral motoneuron fates in favor of medial fates. Conclusions: Together, our data demonstrate that even closely related Hox orthologues rely on distinct combinations of homeodomain‐dependent and ‐independent mechanisms to specify motoneuron identity. Developmental Dynamics 241:718–731, 2012. © 2012 Wiley Periodicals, Inc. Key findings: Hoxd10 and Hoxd11 have similar DNA‐binding homeodomains but exert opposing effects on motoneuron identity in the developing lumbosacral (LS) spinal cord: Hoxd10 promotes the development of anterior lateral subtypes while Hoxd11 promotes posterior medial subtypes. Mutant and chimeric Hoxd10 and Hoxd11 constructs were designed and misexpressed in the chick LS spinal cord to assess the functional roles of homeodomain versus non‐homeodomain regions. Hoxd10 requires both its homeodomain and non‐homeodomain regions to specify lateral motoneuron subtypes. The specification of medial motoneuron subtypes by Hoxd11 is governed entirely by its non‐homeodomain regions. Though their DNA‐binding domains are similar in structure and sequence, Hoxd10 and Hoxd11 influence the expression of downstream targets through distinct molecular mechanisms.