Transglutaminases factor XIII-A and TG2 regulate resorption, adipogenesis and plasma fibronectin homeostasis in bone and bone marrow

Abstract

Appropriate bone mass is maintained by bone-forming osteoblast and bone-resorbing osteoclasts. Mesenchymal stem cell (MSC) lineage cells control osteoclastogenesis via expression of RANKL and OPG (receptor activator of nuclear factor κB ligand and osteoprotegerin), which promote and inhibit bone resorption, respectively. Protein crosslinking enzymes transglutaminase 2 (TG2) and Factor XIII-A (FXIII-A) have been linked to activity of myeloid and MSC lineage cells; however, in vivo evidence has been lacking to support their function. In this study, we show in mice that TG2 and FXIII-A control monocyte-macrophage cell differentiation into osteoclasts as well as RANKL production in MSCs and in adipocytes. Long bones of mice lacking TG2 and FXIII-A transglutaminases, show compromised biomechanical properties and trabecular bone loss in axial and appendicular skeleton. This was caused by increased osteoclastogenesis, a cellular phenotype that persists in vitro. The increased potential of TG2 and FXIII-A deficient monocytes to form osteoclasts was reversed by chemical inhibition of TG activity, which revealed the presence of TG1 in osteoclasts and assigned different roles for the TGs as regulators of osteoclastogenesis. TG2- and FXIII-A-deficient mice had normal osteoblast activity, but increased bone marrow adipogenesis, MSCs lacking TG2 and FXIII-A showed high adipogenic potential and significantly increased RANKL expression as well as upregulated TG1 expression. Chemical inhibition of TG activity in the null cells further increased adipogenic potential and RANKL production. Altered differentiation of TG2 and FXIII-A null MSCs was associated with plasma fibronectin (FN) assembly defect in cultures and FN retention in serum and marrow in vivo instead of assembly into bone. Our findings provide new functions for TG2, FXIII-A and TG1 in bone cells and identify them as novel regulators of bone mass, plasma FN homeostasis, RANKL production and myeloid and MSC cell differentiation.

Transglutaminase activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics

“…The increased osteoclastogenesis was also seen in double FXIII‐A/TG2 deficient monocytes in vitro demonstrating that the two enzymes negatively regulate osteoclastogenesis. However, similarly, as in the work of Raghu et al () a chemical inhibitor of TGs blocked osteoclastogenesis in our study, which was attributed to expression of TG1 in wild type and FXIII‐A/TG2 deficient monocytes and osteoclasts (Mousa et al, ). Although we did not see a bone phenotype in either, individual TG2 or FXIII‐A knockouts (Mousa et al, ), Kim et al () showed recently that TG2 deficient mice exhibited increased osteoclast number and lower trabecular bone mass in vivo and that siRNA knockdown of TG2 gave rise to increased osteoclastogenesis in vitro (Kim et al, ).…”

“…Previous studies, including ours, have linked TG enzymes and their activity to monocyte, macrophage, and osteoclast function in vitro and in vivo (Adany et al, ; Akimov & Belkin, ; Bagoly et al, ; Chrobok et al, ; Kim et al, ; Mousa et al, ; Raghu et al, ) and our recent work demonstrated the relevance of TG2, FXIII‐A in regulation of osteoclastogenesis in mice as well as showed for the first time the presence of TG1 in osteoclasts (Mousa et al, ). In the present study, we have continued the exploration on the role of TGs in osteoclastogenesis and show that TG inhibitors are highly potent blockers of osteoclast differentiation, fusion and migration.…”

“…The work of Raghu et al showed that FXIII‐A deficiency in mice led to reduced osteoclastogenesis in vivo and in vitro and that transglutaminase inhibitor, cystamine inhibited osteoclastogenesis in vivo (Raghu et al, ). Furthermore, our recent work showed that deletion of both TG2 and FXIII‐A gene expression in a double knockout mouse model resulted in severe osteopenia caused by increased bone resorption (Mousa et al, ). The increased osteoclastogenesis was also seen in double FXIII‐A/TG2 deficient monocytes in vitro demonstrating that the two enzymes negatively regulate osteoclastogenesis.…”

Transglutaminase diseases: from biochemistry to the bedside

“…The phenotype of all 3 independent global TG2 knockout lines is normal at baseline, suggesting a negligible developmental role for TG2. Indeed, double-FXIII-A/TG2 knockout mice exhibit only a transient delay in bone mineral density and growth relative to wild-type mice (68), and double-TG1/ TG2 knockout mice show epidermal features similar to TG1 knockout mice (69). Most studies are thus focusing on the pathologic role of TG2 function.…”

Fibrinogen and factor XIII: newly recognized roles in venous thrombus formation and composition

“…Finally, it is clear that many biologic functions of FXIII have not been thoroughly elucidated. In addition to coagulation, FXIII may have roles in arthritis(50), adipogenesis(51), and cardiac repair(52, 53). It will be important to understand the impact of FXIII inhibition on these situations before the benefit of a FXIIIa antagonist can be realized in the clinic.…”