Hind limb unloading of mice modulates gene expression at the protein and mRNA level in mesenchymal bone cells

Background: We investigated the extent, modalities and reversibility of changes at cellular level in the expression of genes and proteins occurring upon Hind limb unloading (HU) in the tibiae of young C57BL/6J male mice. We focused on the effects of HU in chondrogenic, osteogenic, and marrow mesenchymal cells. Methods: We analyzed for expression of genes and proteins at two time points after HU (7 and 14 days), and at 14 days after recovery from HU. Levels of mRNAs were tested by in situ hybridization. Protein levels were tested by immunohistochemistry. We studied genes involved in osteogenesis (alkaline phosphatase (AP), osteocalcin (OC), bonesialoprotein (BSP), membrane type1 matrix metalloproteinase (MT1-MMP)), in extracellular matrix (ECM) formation (procollagenases (BMP1), procollagenase enhancer proteins (PCOLCE)) and remodeling (metalloproteinase-9 (MMP9), RECK), and in bone homeostasis (Stro-1, CXCL12, CXCR4, CD146). Results: We report the following patterns and timing of changes in gene expression induced by HU: 1) transient or stable down modulations of differentiation-associated genes (AP, OC), genes of matrix formation, maturation and remodelling, (BMP1, PCOLCEs MMP9) in osteogenic, chondrogenic and bone marrow cells; 2) up modulation of MT1-MMP in these same cells, and uncoupling of its expression from that of AP; 3) transient down modulation of the osteoblast specific expression of BSP; 4) for genes involved in bone homeostasis, up modulation in bone marrow cells at distal epiphysis for CXCR4, down modulation of CXCL12, and transient increases in osteoblasts and marrow cells for Stro1.14 days after limb reloading expression returned to control levels for most genes and proteins in most cell types, except AP in all cells, and CXCL12, only in bone marrow. Conclusions: HU induces the coordinated modulation of gene expression in different mesenchymal cell types and microenvironments of tibia. HU also induces specific patterns of expression for homeostasis related genes and modulation of mRNAs and proteins for ECM deposition, maturation and remodeling which may be key factors for bone maintenance.