Introduction
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. At skeletal maturity, bone mass is maintained through the balanced activity of osteoblasts and osteoclasts during the remodeling cycle. During skeletal aging, there is a shift in the balance that favors osteoclast over osteoblast activity, which results in net bone loss [3]. The amount and rate at which bone is gained during development and lost during aging are determined in large part by genetics [4–6] but also by physical activity and by alterations in the availability and response of bone cells to circulating hormones [7–9] and locally derived growth factors [10,11]. Whereas genetic-based studies have provided novel insights into the pathways that regulate bone development [12–14], relatively little is known about the etiology of age-related bone loss. 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 [8,18–20], and to alterations in bone cell apoptosis [21]. Bone loss in the elderly has also 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].
Aging has long been associated with an increase in marrow fat, where the generation of adipocytes is favored over osteoblasts [23]. Osteoblasts and adipocytes are derived from a common mesenchymal precursor cell present in bone marrow, and the factors that control this age-induced switch toward adipogenic differentiation is not well understood [24]. While several transcriptional regulatory proteins have been associated with cell fate determination of bone marrow mesenchymal cells, including peroxisome proliferator–activated receptor γ (PPARγ) and KLF5 [25–30], the role of RNA binding proteins in this process remains unknown. RNA binding proteins of the KH type are known regulators of cellular differentiation. For example, expression defects in the KH domain proteins NOVA and FMRP are known to cause paraneoplastic neurologic disorders [31] and the fragile X syndrome, respectively, in humans [32]. The phenotype of the quaking viable mice suggests a role for the QUAKING RNA binding protein in oligodendrocytes and myelination [33]. Indeed, the ectopic expression of the QKI-6/7 isoforms in vivo led to the formation of glial cells rather than neurons from neural progenitor, demonstrating its role in cell fate determination [34]. The loss of GLD-1 protein in Caenorhabditis elegans prevents the appearance of stem cells [35], and the absence of the KH domain protein HOW in Drosophila prevents muscle differentiation and results in flies with held-out-wings [36,37]. The Src substrate associated in mitosis of 68 kDa (Sam68) is also a member of the family of KH domain RNA binding proteins [38]; however, its physiologic role has remained undefined.
Sam68 was identified as an SH3 and SH2 domain interacting protein for Src family kinases and is also a known substrate of Src kinases [39–42] and of the breast tumor kinase [43]. Sam68 has been shown to facilitate the export of unspliced HIV RNA [44] and to regulate pre-mRNA processing [45]. In the present paper, we report the generation of Sam68−/− mice and analysis of their skeletal phenotype. Our data indicate that the absence of Sam68 confers resistance to age-related bone loss in mice such that old Sam68 have a higher bone mass than their wild-type littermates. We provide evidence that Sam68 regulates the differentiation of bone marrow stromal cells by showing that cells isolated from Sam68−/− animals had enhanced osteogenic activity and decreased adipogenic activity than those harvested from wild-type littermates. Furthermore, Sam68−/− mouse embryo fibroblasts (MEFs) were impaired in their capacity to differentiate into adipocytes, consistent with Sam68 being a regulator of bone marrow mesenchymal cell differentiation. These results also characterize a new animal model to study bone metabolism, regeneration, and repair during aging.