The differentiation of osteoblasts and bone marrow adipocytes are closely associated yet mutually exclusive processes that are essential for maintaining bone homeostasis. Various diseases have been shown to develop once the delicate balance between adipogenesis and osteoblastogenesis is disrupted. Investigating the underlying molecular mechanisms of the osteoblasto-adipogenic switch under osteoporotic conditions will facilitate our understanding of the pathogenesis of osteoporosis and may eventually lead to the development of clinical therapeutic approaches for this life-threatening disease. While changes in cell morphology and cytoskeletal integrity can alter pre-committed mesenchymal stem cell (MSC) differentiation of certain lineages, previous studies have shown that cellular morphological changes can affect the early commitment of pluripotent MSCs via modulation of Ras homolog gene family, member A (RhoA) activity. The RhoA pathway regulates actin polymerization to promote the incorporation of globular-actin (G-actin) into filamentous-actin (F-actin). Actin polymerization releases G-actin bound myocardin-related transcription factors (MRTFs), which translocate to the nucleus and co-activate serum response factor (SRF) target gene expression. Exactly how the RhoA-actin-MRTF-SRF circuit is involved in the regulation of early commitment of MSCs remains poorly understood. Here we show that global MRTFA knockout mice (MRTFA KO) exhibited lower body weight, shorter femur and tibia lengths, and decreased trabecular bone volume. Furthermore, bone marrow MSCs isolated from MRTFA KO mice showed increased adipogenesis and brown fat gene expression as well as compromised osteoblastogenic differentiation as compared to WT controls. Treatment of WT bone marrow MSCs with the SRF inhibitor, CCG1423, mimicked these effects in that the compound inhibited osteoblastogenesis and promoted adipogenesis. Over-expression of MRTFA or SRF inhibited adipogenesis and enhanced osteoblastogenesis in C3H/10T1/2 cell lines, whereas over-expression of dominant-negative MRTFA or SRF variants had the opposite effects. In conclusion, our study identified MRTFA as a crucial regulator of skeletal homeostasis via regulating the balance between adipogenic and osteoblastogenic differentiation of the MSCs. Furthering our understanding of how the RhoA-actin-MRTFA-SRF circuit is involved in regulating the fate commitment of MSCs may ultimately lead to novel therapeutic strategies for treating osteoporosis and obesity.

Thermogenic brown adipose tissue generates heat via mitochondrial uncoupling protein-1 (UCP-1), increases whole-body energy expenditure and may protects against obesity and metabolic disorders. White adipocytes store excess energy in the form of triglycerides. UCP-1 positive adipocytes develop within white adipose tissue (beige or brite adipocytes) in response to cold exposure or β3 adrenergic agonists. It was known that beige adipocytes arise from a distinct lineage compared with brown adipocytes, but the developmental origin of the beige adipocytes is still unclear. Signaling pathways that control beige adipocyte determination and formation are essentially unknown. Here, we identified a novel signaling pathway that regulates the lineage specification of beige adipocytes. Bone morphogenetic protein 7 (BMP7), a known brown adipogenesis inducer, suppresses Rho-GTPase kinase (ROCK) and depolymerizes F-actin (filamentous actin) into G-actin (globular actin) in mesenchymal stem cells. G-actin regulates myocardin-related transcription factor A (MRTFA) that co-transactivates serum response factor (SRF) and promotes smooth muscle cell differentiation in various organs. Subcutaneous white adipose tissue from MRTFA-/- mice had enhanced accumulation of UCP-1+ adipocytes and elevated levels of brown-selective proteins. Compared with wild type (WT) controls, MRTFA-/- mice exhibited improved metabolic profiles and were protected from diet-induced obesity and insulin resistance, suggesting that the beige adipocytes are physiologically functional. Compared to WT mice, stromal vascular cells from MRTFA-/- mice expressed higher levels of distinct beige progenitor markers and reduced levels of smooth muscle markers. Our studies demonstrate a novel ROCK-actin-MRTFA/SRF pathway that contributes to the development of beige adipocytes.