Bone Ridge Patterning during Musculoskeletal Assembly Is Mediated through SCX Regulation of Bmp4 at the Tendon-Skeleton Junction

Abstract

SUMMARY During the assembly of the musculoskeletal system, bone ridges provide a stable anchoring point and stress dissipation for the attachment of muscles via tendons to the skeleton. In this study, we investigate the development of the deltoid tuberosity as a model for bone ridge formation. We show that the deltoid tuberosity develops through endochondral ossification in a two-phase process: Initiation is regulated by a signal from the tendons, whereas the subsequent growth phase is muscle-dependent. We then show that the transcription factor scleraxis (SCX) regulates Bmp4 in tendon cells at their insertion site. The inhibition of deltoid tuberosity formation and several other bone ridges in embryos in which Bmp4 expression was blocked specifically in Scx-expressing cells implicates BMP4 as a key mediator of tendon effects on bone ridge formation. This study establishes a mechanistic basis for tendon-skeleton regulatory interactions during musculoskeletal assembly and bone secondary patterning.

Higher BMP receptor expression and BMP-2-induced osteogenic differentiation in tendon-derived stem cells compared with bone-marrow-derived mesenchymal stem cells

“…Constitutively activated Smad1 overexpression was reported to induce myoblast osteogenic differentiation, whereas nuclear translocation of Smad4 inhibited myoblast myogenic differentiation [34]. It is not known whether Smad proteins and MSX2 also function to inhibit osteogenic differentiation and promote TDSC tenogenic differentiation, as reported in previous studies [30,32,34]; further study is required. Other factors, such as mechanical loading [35] and local extracellular matrix environment [9], may also function to suppress TDSC osteogenesis inside the tendon midsubstance.…”

“…However, the myogenic differentiation potential of TDSCs has not been formally investigated and requires further study. Despite this, the potential of TDSCs to establish and maintain the TBJ and musculotendinous junction is plausible based on a previous study [30]. It has been reported that bone ridge formation was initiated by tendons, whereas the subsequent growth phase was muscle dependent [30].…”

“…Despite this, the potential of TDSCs to establish and maintain the TBJ and musculotendinous junction is plausible based on a previous study [30]. It has been reported that bone ridge formation was initiated by tendons, whereas the subsequent growth phase was muscle dependent [30]. During initiation of bone ridge formation, scleraxis bound to the Bmp4 promoter and regulated its transcription in tendon cells at the insertion site.…”

“…BMP-12 and BMP-13, which are tenogenic, activated the same sets of receptors as BMP-2 [16,23]. Besides BMP-2, BMPs such as BMP-4 and BMP-7 are osteochondrogenic and have been reported to play important roles in bone and TBJ regeneration [30,31]. Whether TDSCs is also responsive to BMP-4 and BMP-7 and showed differential response compared with BMSCs requires further research.…”

“…During initiation of bone ridge formation, scleraxis bound to the Bmp4 promoter and regulated its transcription in tendon cells at the insertion site. BMP4 expression then bound to its receptor ALK3 in tuberosityforming chondrocytes, leading to BMP signaling activation and initiation of bone ridge formation [30]. A regulatory mechanism to prevent erroneous TDSCs differentiation to junctional cell types (bone, chondrocytes, muscles) in tendon midsubstances other than tenocytes therefore may be in place in order to maintain tendon homeostasis.…”