The Role of Pin1 in the fusion of osteoclasts and myoblasts

Abstract

Cell-cell fusion is critical for the conception, development, and physiology of multicellular organisms. Although cellular fusogenic proteins and the actin cytoskeleton are implicated in cell-cell fusion, it remains unclear whether and how they coordinate to promote plasma membrane fusion. Pin1 null mice showed low bone mass and increased TRAP staining in histological sections of long bones, compared to Pin1 wild-type mice. In vitro osteoclast forming assays with bone marrow-derived monocyte/macrophage revealed that Pin1-deficient osteoclasts were larger than wild-type osteoclasts and had higher nuclei numbers, indicating greater extent of fusion. Pin1 deficiency also highly enhanced foreign body giant cell formation both in vitro and in vivo. Among the known fusion proteins, DC-STAMP was significantly increased in Pin1−/− osteoclasts. Immunohistochemistry showed that DC-STAMP expression was also significantly increased in tibial metaphysis of Pin1 KO mice. I found that Pin1 bound and isomerized DC-STAMP and affected its expression levels and localization at the plasma membrane. Taken together, my data indicate that Pin1 controls of bone mass through the regulation of the osteoclast fusion protein DC-STAMP. The identification of Pin1 as a factor involved in cell fusion contributes to the understanding of osteoclast-associated diseases, including osteoporosis, and opens new avenues for therapeutic targets. When the effect of Pin1 was tested in myoblast cells it was found that Pin1 inhibits myoblast fusion and inhibition of Pin1 enhanced myoblast fusion. If Pin1 is involved in both osteoclast and myoblast fusion it can be a key molecule to modulate in various musculoskeletal diseases as well as sarcopenia related to other generalized ailments like cancer and ageing. The conformational regulation catalyzed by the PPIase, Pin1 is crucial for regulation of SMAD proteins. The inhibition of Pin1 activity enhanced muscle cell fusion while affected minimally the expression of key transcription factors of myogenesis, MyoD, MYF5, and Myogenin. Pin1 over-expressing myoblasts, however, failed to fuse. These findings reveal a specific role for Pin1 in myoblast fusion. Pin1 also modulates the TGF-ß signaling during muscle hypertrophy and in low dosage of inhibition it specifically reduced pSmad 3 only. Pin1 appear therefore to act as a mediator of the myogenic cell-cell fusion and hypertrophy mechanism underlying formation of functional muscle fibers. With all the experimental results taken together it can be inferred that Pin1 can modulate myoblast and osteoclast fusion. This makes Pin1 a very important therapeutic target for several common human diseases like myopathy and osteoporosis.

Key Words: Cell Fusion, Myoblast, Osteoclast, Pin1.