Role of Hox Genes in Integration of the Musculoskeletal System in Development

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DESCRIPTION (provided by applicant): A significant amount of data exists regarding the formation and differentiation of muscle, bone and tendon tissue individually, but little is understood regarding molecular mechanisms that allow these tissues to make appropriate connections with one another. This knowledge is critical for establishing useful therapies to repair these tissues after disruption from injury or disease. Hox genes perform fundamental roles in patterning the skeleton, but new preliminary data shows that Hox gene expression in the developing limb is not restricted to skeletal tissue, and these genes also appear to play less anticipated roles in patterning tendon and muscle tissue as well as in the integration of all three tissue types. Recent data additionally that shows that these genes are highly up-regulated following injury, suggesting a role in the healing and remodeling that occurs in response to injury. The long-term goal of this research is to understand how Hox genes regulate the region- specific formation and integration of the musculoskeletal system and how this information can be used to inform regenerative therapies following injury or disease. The objective of this proposal is to achieve an understanding of the cellular mechanisms of Hox function in the development and integration of muscle, tendon and bone in the developing forelimb and how these mechanisms are redeployed in response to injury. The central hypothesis is that Hox11 genes function to direct the patterning and the integration of the muscle, tendon and cartilage in the developing limb and that these genes are up-regulated in response to injury to allow proper repair and remodeling in vivo. How Hox11 genes contribute to the formation and patterning of muscle groups, tendons and cartilage elements within the developing limb will be assessed using existing null alleles and fluorescent reporter lines. The spatiotemporal control of Hox-mediated patterning and the establishment of connectivity in the musculoskeletal system will be examined by conditionally ablating Hox function using spatial and temporally restricted Cre deletor lines. Finally, the roles for Hox genes in tissue repair and skeletal remodeling will be determined by conditionally ablating Hox function in adulthood (after proper musculoskeletal patterning has been achieved) and examining the effects of loss of Hox function on fracture repair and subsequent skeletal remodeling. The contribution of the proposed research is significant because critical new knowledge regarding the regional regulation musculoskeletal integration will be gained, the tissue(s) from which this information is transmitted will be defined, and how these factors are redeployed in adulthood to repair injuries will be determined. The research proposed in this application is innovative in identifying factors and cell types involved in the integration of individual components of the musculoskeletal system as well as in the novel tools generated to allow an understanding of the function of Hox genes in properly patterned adult tissue during injury repair processes. Together, the information gained from these studies will provide a paradigm-shifting understanding of Hox function in musculoskeletal biology. PUBLIC HEALTH RELEVANCE: The connectivity between muscles, tendons and bones is critical for normal musculoskeletal function, yet a large number of patients suffer injuries, disease or genetic disorders that fail to heal or recapitulate proper interactions between these tissues. An increased understanding of the cellular mechanisms by which Hox genes, critical genetic determinants of musculoskeletal development, are deployed during development and injury repair processes will inform their use in musculoskeletal regenerative therapies.