Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing

Abstract

Uncovering the mechanisms of the sensitivity of bone healing to mechanical factors is critical for understanding the basic biology and mechanobiology of the skeleton, as well as for enhancing clinical treatment of bone injuries. This study refined an experimental method of measuring the strain microenvironment at the site of a bone injury during bone healing. This method used a rat model in which a well-controlled bending motion was applied to an osteotomy to induce the formation of pseudarthrosis that is composed of a range of skeletal tissues, including woven bone, cartilage, fibrocartilage, fibrous tissue, and clot tissue. The goal of this study was to identify both the features of the strain microenvironment associated with formation of these different tissues and the expression of proteins frequently implicated in sensing and transducing mechanical cues. By pairing the strain measurements with histological analyses that identified the regions in which each tissue type formed, we found that formation of the different tissue types occurs in distinct strain microenvironments and that the type of tissue formed is correlated most strongly to the local magnitudes of extensional and shear strains. Weaker correlations were found for dilatation. Immunohistochemical analyses of focal adhesion kinase and rho family proteins RhoA and CDC42 revealed differences within the cartilaginous tissues in the calluses from the pseudarthrosis model as compared to fracture calluses undergoing normal endochondral bone repair. These findings suggest the involvement of these proteins in the way by which mechanical stimuli modulate the process of cartilage formation during bone healing.

DC-STAMP: A Key Regulator in Osteoclast Differentiation

“…Collectively, our data suggest that DC‐STAMP, together with MMP‐2 and MMP‐9 (Galliera et al, ; Rocha et al, ), can be a useful tool for monitoring, in parallel with other bone turnover markers, to evaluate bone remodeling and tissue healing. Given that DC‐STAMP+ cells are homing to bone fracture sites, a step forward approach is to molecular engineer a chimeric DC‐STAMP protein by which DC‐STAMP is able to bring small peptides/proteins such as bone morphogenetic protein 2 (BMP‐2) or MMP‐9 (Kim et al, ; Matsumoto et al, ; Miller et al, ; Saito et al, ; Wang et al, ) with healing‐promoting effects to bone fracture sites.…”

Cartilage to bone transformation during fracture healing is coordinated by the invading vasculature and induction of the core pluripotency genes

“…Differentiation of the progenitor cells into chondrocytes or osteoblasts depends on the local mechanical microenvironment (Isaksson et al, 2006;Miclau et al, 2007;Miller et al, 2015). Progenitor cells undergo direct bone formation, or intramembranous ossification, along the periosteal and endosteal surfaces.…”

Mechanical Regulation of Microvascular Angiogenesis

“…In our previous in vivo segmental defect study, early ambulatory loading disrupted vascularization within regenerating bone, whereas delayed ambulatory loading enhanced vascular network formation (25); however, the specific parameters of loading that regulate angiogenesis are poorly defined. Informed by bone regeneration literature, we identified 5% strain as proregenerative, 10% as transitional, and 30% strain as inhibitory to healing (29). These loads (5%, 10%, and 30% strain) were applied to microvascular fragment-seeded hydrogels with a triangle wave at a frequency of 1 Hz, corresponding to typical gait frequency (28).…”

“…Ambulatory loading typically occurs with a frequency around 1 Hz (28), and strain magnitude is an important regulator of bone formation. Peak bone tissue regeneration occurs at low strains, whereas a fibrous response occurs at high strains; 10% strain is thought to be an approximate transition point between regenerative and non-healing responses (29). Further, while compressive forces are the most widely studied in bone, ambulatory loading includes additional modes of loading such as shear, which has been shown to reduce revascularization of bone (26).…”