Age-related Decline in Bone Mineral Transport and Bone Matrix Proteins in Osteoblasts from Stromal Stem Cells.

We studied osteoblast bone mineral transport and matrix proteins as a function of age. In isolated bone marrow cells from long bones of young (3-4 month) and old (18-19 month) mice, age correlated with reduced mRNA of mineral transport proteins: alkaline phosphatase (ALP), ankylosis (ANK), the Cl/H exchanger ClC3, and matrix proteins collagen 1 (Col1) and osteocalcin (BGLAP). Some proteins including the neutral phosphate transporter2 (NPT2) were not reduced. These are predominately osteoblast proteins, but in mixed cell populations. Remarkably, in osteoblasts differentiated from preparations of stromal stem cells (SSC) made from bone marrow cells in young and old mice, differentiated on perforated polyethylene terephthalate membranes, mRNA confirmed decreased expression with age of most transport-related and bone matrix proteins. Additional mRNAs in osteoblasts included ENPP1, unchanged, and ENPP2, reduced with age. Decrease with age in ALP activity and protein by Western Blot were also significant. Transport protein findings correlated with micro-computed tomography of lumbar vertebra showing that trabecular bone of old mice is osteopenic relative to young mice, consistent with other studies. Pathway analysis of osteoblasts differentiated showed that cells from old animals had reduced Erk1/2 phosphorylation and decreased Smad 2 mRNA, consistent with TGF-β pathway, and reduced β-catenin mRNA consistent with WNT pathway regulation. Our results show that decline in bone density with age reflects, selective changes resulting effectively in a phenotype modification. Reduction of matrix and mineral transport protein expression with age is regulated by multiple signaling pathways.