Hox11 genes are required for regional patterning and integration of muscle, tendon and bone

Abstract

Development of the musculoskeletal system requires precise integration of muscles, tendons and bones. The molecular mechanisms involved in the differentiation of each of these tissues have been the focus of significant research; however, much less is known about how these tissues are integrated into a functional unit appropriate for each body position and role. Previous reports have demonstrated crucial roles for Hox genes in patterning the axial and limb skeleton. Loss of Hox11 paralogous gene function results in dramatic malformation of limb zeugopod skeletal elements, the radius/ulna and tibia/fibula, as well as transformation of the sacral region to a lumbar phenotype. Utilizing a Hoxa11eGFP knock-in allele, we show that Hox11 genes are expressed in the connective tissue fibroblasts of the outer perichondrium, tendons and muscle connective tissue of the zeugopod region throughout all stages of development. Hox11 genes are not expressed in differentiated cartilage or bone, or in vascular or muscle cells in these regions. Loss of Hox11 genes disrupts regional muscle and tendon patterning of the limb in addition to affecting skeletal patterning. The tendon and muscle defects in Hox11 mutants are independent of skeletal patterning events as disruption of tendon and muscle patterning is observed in Hox11 compound mutants that do not have a skeletal phenotype. Thus, Hox genes are not simply regulators of skeletal morphology as previously thought, but are key factors that regulate regional patterning and integration of the musculoskeletal system.

A unique stylopod patterning mechanism by Shox2 controlled osteogenesis

“…This is supported by the observations that at E14.5, Shox2 expression becomes predominantly restricted to the outer layer of the perichondrium that is characterized by relatively low levels of Runx2 expression ( Fig. 3E-H) and was thought to have a less definitive role in osteogenesis (Swinehart et al, 2013). It was proposed that skeletal patterning in the limb by Hox genes is largely exerted by their expression in the perichondrium (Swinehart et al, 2013;Villavicencio-Lorini et al, 2010).…”

“…3E-H) and was thought to have a less definitive role in osteogenesis (Swinehart et al, 2013). It was proposed that skeletal patterning in the limb by Hox genes is largely exerted by their expression in the perichondrium (Swinehart et al, 2013;Villavicencio-Lorini et al, 2010). Given that Shox2 deficiency causes a specific loss of perichondrial osteogenic cell type signature and leads to a depletion of osteogenic cells, we hypothesized that Shox2 functions to control stylopodial skeleton patterning through maintaining the fate of perichondrial osteogenic precursors that initially provide instructive cues for chondrocyte differentiation, and later differentiate to osteoblasts for bone formation.…”

Genetic Interactions Between Shox2 and Hox Genes During the Regional Growth and Development of the Mouse Limb

“…Future experiments are required to address any causal role of perichondrial Shox2 expression in driving the growth of the humerus or whether this expression is involved in later steps of skeletogenesis, such as the generation of bone. As Hox genes are also hypothesized to exert a large part of their effect on the developing skeleton from their expression in the perichondrium (Villavicencio-Lorini et al 2010;Swinehart et al 2013), their cell autonomous role in this structure should also be investigated. Of note, although Hox genes are required for normal Shox2 expression, such regulation cannot completely account for the role of Hox genes in growth, as Shox2 overexpression failed to fully rescue the Hox mutant phenotype.…”

“…Mouse Shox2 expression, in contrast, occupies both the developing stylopodal and zeugopodal domains even though its mutation primarily disrupts the developing stylopod (Cobb et al 2006). Within their broad segmental domains, Shox2 and at least some Hox genes are expressed in the proliferating chondrocytes and perichondrium of developing skeletal elements (Villavicencio-Lorini et al 2010;Swinehart et al 2013), being thus associated with their continual growth during development.Gene expression levels can be critical during development, with variation in expression leading to dose-dependent responses. The effects of dosage variation can be assessed by modulating the expression of a single gene or varying the expression of multiple genes, where genetic interactions, or epistasis, become important.…”

“…Mouse Shox2 expression, in contrast, occupies both the developing stylopodal and zeugopodal domains even though its mutation primarily disrupts the developing stylopod (Cobb et al 2006). Within their broad segmental domains, Shox2 and at least some Hox genes are expressed in the proliferating chondrocytes and perichondrium of developing skeletal elements (Villavicencio-Lorini et al 2010;Swinehart et al 2013), being thus associated with their continual growth during development.…”