Project Summary There is no cure for osteogenesis imperfecta (OI or brittle bone disease), a group of heritable genetic disorders affecting 1:15,000 births. The most evident clinical hallmarks of OI are bone fragility and skeletal deformities. Mutations in trimeric intracellular cation channel B (TRIC-B), an endoplasmic reticulum potassium (K+) channel encoded by transmembrane protein 38 B (TMEM38B) gene and modulating Calcium (Ca2+) flux, cause the rare recessive form of the disease, namely OI type XIV. How a potassium channel is responsible for a heritable bone disease is puzzling the field. Calcium is indispensable for several cell activities including signal transduction pathways that regulate osteoblast differentiation and function. We here plan to identify the intra and extra-cellular mechanisms responsible for the skeletal defects and fragility caused by loss-of-function mutations in TRIC-B, with the goal to unveil a druggable target for OI therapy. We recently proved that the Calcium flux impairment due to osteoblast TRIC-B loss-of-function compromises the TGF-β/SMAD signaling. Based on preliminary data, we hypothesize that TMEM38B mutation generates an impairment of β-catenin/WNT pathway and of cell adhesion, which results in an increased bone fragility by compromising bone matrix structure and composition, thus reducing strength and toughness. Specific aims are to: 1) dissect TRIC-B role in osteoblast intracellular signaling affecting activity and adhesion; 2) determine the mechanism by which osteoblast specific absence of TRIC-B develops bone fragility. To address TRIC-B bone function, we generated an osteoblast specific TRIC-B knock-out mouse that reproduces patients’ skeletal outcome. The consequence of loss of TRIC-B on osteoblast WNT pathways and cell adhesion will be investigated by advanced molecular, biochemical and high-resolution imaging techniques (AIM 1). Bone strength and fracture toughness combined with second harmonic generation microscopy and spectroscopic analysis of the bone tissue collagen fiber structure and composition, respectively, will inform on the mechanism through which depletion of Tmem38b generates bone fragility (AIM 2). This work will shed light on TRIC-B role in osteoblasts and extracellular matrix to identify a druggable target for OI.