Lack of TRIC-B dysregulates cytoskeleton assembly, trapping β-catenin at osteoblast adhesion sites.

The trimeric intracellular cation channel B (TRIC-B), encoded by TMEM38B, is a potassium (K(+)) channel present in the endoplasmic reticulum membrane, where it counterbalances calcium (Ca(2+)) exit. Lack of TRIC-B activity causes a recessive form of the skeletal disease osteogenesis imperfecta (OI), namely OI type XIV, characterized by impaired intracellular Ca(2+) flux and defects in osteoblast (OB) differentiation and activity. Taking advantage of the OB-specific Tmem38b knockout mouse (Runx2Cre;Tmem38b(fl/fl); cKO), we investigated how the ion imbalance affects the osteogenetic process. We found an abnormal cytoskeleton in the cKO OBs, with actin accumulation at OB adhesion sites. The reduced amount of active Ca(2+)-dependent actin-binding proteins myristoylated alanine-rich C-kinase substrate (MARCKS) and fascin, which modulate cytoskeletal actin dynamics, explains the altered cytoskeletal assembly. The actin clusters at adhesion sites trap β-catenin, a key structural protein at cell-cell junction sites, that abnormally accumulates despite the significant reduction in both N- and E-cadherins. Besides its structural fuction at cell borders, β-catenin also has a pivotal role as a transcription factor for proper osteoblastogenesis. Immunofluorescence of cKO nuclei revealed impaired nuclear β-catenin translocation, further validated in human fetal OB knocked out for TMEM38B, which was not rescued by specifically stimulating the canonical Wnt pathway. Thus, we demonstrated in vitro that alterations of intracellular Ca(2+) homeostasis, as a consequence of lack of TRIC-B, cause cytoskeleton disorganization in cKO OBs, resulting in abnormal β-catenin accumulation at cell adhesion sites and reduced nuclear β-catenin translocation, contributing to impaired osteoblastogenesis.