Abstract
The Src substrate associated in mitosis of 68 kDa (Sam68) is a KH-type RNA binding protein that has been shown to regulate several aspects of RNA metabolism; however, its physiologic role has remained elusive. Herein we report the generation of Sam68-null mice by homologous recombination. Aged Sam68−/− mice preserved their bone mass, in sharp contrast with 12-month-old wild-type littermates in which bone mass was decreased up to approximately 75%. In fact, the bone volume of the 12-month-old Sam68−/− mice was virtually indistinguishable from that of 4-month-old wild-type or Sam68−/− mice. Sam68−/− bone marrow stromal cells had a differentiation advantage for the osteogenic pathway. Moreover, the knockdown of Sam68 using short hairpin RNA in the embryonic mesenchymal multipotential progenitor C3H10T1/2 cells resulted in more pronounced expression of the mature osteoblast marker osteocalcin when differentiation was induced with bone morphogenetic protein-2. Cultures of mouse embryo fibroblasts generated from Sam68+/+ and Sam68−/− littermates were induced to differentiate into adipocytes with culture medium containing pioglitazone and the Sam68−/− mouse embryo fibroblasts shown to have impaired adipocyte differentiation. Furthermore, in vivo it was shown that sections of bone from 12-month-old Sam68−/− mice had few marrow adipocytes compared with their age-matched wild-type littermate controls, which exhibited fatty bone marrow. Our findings identify endogenous Sam68 as a positive regulator of adipocyte differentiation and a negative regulator of osteoblast differentiation, which is consistent with Sam68 being a modulator of bone marrow mesenchymal cell differentiation, and hence bone metabolism, in aged mice.
Highlights
During skeletal development, the anabolic activity of osteoblasts [1] is favored over the catabolic activity of osteoclasts [2], which results in a net gain in bone mass
Sections of paraffin-embedded wild-type E14.5 and E16.5 embryonic mice were immunostained with the well-characterized AD1 anti-substrate associated in mitosis of 68 kDa (Sam68) antibody, raised in rabbits against an immunizing peptide that corresponds to amino acids 330 to 348 of the mouse Sam68 protein [46]
The present study provides evidence that a physiologic role of Sam68 is to modulate bone marrow mesenchymal stem
Summary
The anabolic activity of osteoblasts [1] is favored over the catabolic activity of osteoclasts [2], which results in a net gain in bone mass. There is a shift in the balance that favors osteoclast over osteoblast activity, which results in net bone loss [3]. Increased bone resorption in elderly men and women is associated with a reduction in bone mass and an increase in circulating levels of bone biomarkers [15] These changes have been attributed primarily to nutritional deficits resulting in alterations in the parathyroid hormone–vitamin D axis [16], to gonadal hormone deficiency [17], to leptin levels and the sympathetic nervous system [8,18,19,20], and to alterations in bone cell apoptosis [21]. Bone loss in the elderly has been attributed to alterations in the response of bone marrow stromal cells to their microenvironment that favors differentiation down the adipocyte lineage rather than the osteoblast lineage [22]
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