HOX11 Function in Musculoskeletal Development and Repair.

Abstract

Previous genetic analyses of Hox loss-of-function phenotypes have demonstrated that these genes are essential regulators of skeletal patterning. However, as studies of Hox function in limb development have been limited to the skeleton, our understanding of the cell types in which Hox genes function and the tissues they pattern is incomplete. Utilizing a Hoxa11eGFP allele, we show that Hox11 is expressed in the connective tissue of the outer perichondrium, tendons and muscle connective tissue of the zeugopod region throughout all stages of development. Hox11 is not expressed in differentiated cartilage or bone, or in vascular or muscle cells in these regions. Hox11 genes provide patterning information to the muscle and tendon of the forelimb zeugopod in addition to patterning the skeletal elements. In Hox11 double mutants, numerous muscles and tendons of the forelimb zeugopod are absent and others fail to separate into properly patterned muscle bundles. Analyses of Hox11 compound mutants, in which three of for Hox11 alleles are mutant, demonstrate that muscle and tendon patterning is not secondary to skeletal malformations. Despite the normal skeletal phenotype of these embryos, significant disruption in patterning is observed. Recent evidence suggests that Hox expression is maintained throughout life and conserves aspects of the regionally restricted pattern observed during embryogenesis. Analysis of Hoxa11 expression at adult stages demonstrates that Hox11 remains expressed in skeletal tissues and is expressed in cells of the fracture callus during repair. Fracture healing is impaired in Hox11 compound mutants. Ulnar fractures in these animals show reduced cartilage formation, demonstrating that Hox11 genes are important for the early phases of bone repair. Thus, Hox genes are not simply regulators of skeletal morphology, but are key factors that regulate regional patterning and integration of the musculoskeletal system. Our results shift the paradigm of Hox function in establishing the body plan from key regulators that direct skeletal morphology to factors that control the patterning and integration of all three tissue types of the musculoskeletal system: muscle, tendon and bone. Our data also demonstrates a continued role for Hox genes throughout the life of the organism in skeletal repair and remodeling.