Role of Bone Mesenchymal Stromal Cells and Abnormal Bone Matrix in Promoting Increased Osteoclast Formation and Bone Resorption in Brtl Osteogenesis Imperfecta Mice

Osteogenesis Imperfecta (OI) is an inherited disorder of collagen that causes reduced bone mass, increased numbers of bone resorbing osteoclasts (OCs), and brittle bones prone to fracture. We used heterozygous Brtl OI mice, which contain a glycine substitution (G349C) in half of their type I collagen α1(I) chains and exhibit the hallmark characteristics of human Type IV OI, to help determine the mechanism(s) responsible for this increase in OCs. Past studies showed that the expression ratio of RANKL to OPG, key OC differentiation molecules produced by osteoblasts and their bone marrow mesenchymal stromal cell (MSC) precursors, was not altered and therefore not the reason for increased OCs in Brtl mice. Here, we isolated and directly co-cultured wild-type (WT) or Brtl bone marrow MSC populations with WT or Brtl bone marrow pre-OC populations in the presence of VD3 to promote OC development to learn which population(s) were responsible for increased OC formation in OI. We found that co-cultures containing Brtl MSCs significantly stimulated more OC formation both on plastic and on a bone substrate compared to WT MSCs, regardless of the pre-OC origin. This strongly indicates that MSC-derived regulatory signals dictate the extent of OC formation and bone resorption. Consistent with this, RT-PCR microarray analysis revealed substantial differences between Brtl and WT in the expression of various key signaling molecules by MSCs. Moreover, while we found that the abnormal bone matrix produced by Brtl MSCs does not affect OC development in terms of their size and number, it may independently contribute to the elevation of bone resorption seen in Brtl OI mice. Deciphering the molecular mechanisms responsible for increased bone loss in OI brittle bone disease may reveal new targets for the future development of more effective therapies to combat OI bone fragility