Regulation of Mesenchymal Progenitor Proliferation and Defferentiation by Thrombospondin-2 and Notch Signaling.

Abstract

Mesenchymal Stem Cells (MSC) give rise to osteoblasts, adipocytes and chondrocytes. A decrease in MSC number and a shift in their differentiation to the adipogenic fate is observed with aging and have been proposed as possible mechanisms for the pathogenesis of age associated osteoporosis. Mice deficient in thrombospondin-2 (TSP2) have an increase in MSC number and as a result, TSP2-null mice have an increase in bone mass. In-vitro, TSP2 promotes osteogenic differentiation and inhibits MSC proliferation. There is a reciprocal relationship between osteogenic and adipogenic differentiation such that factors which promote osteogenic differentiation inhibit adipogenic differentiation, therefore we hypothesized that TSP2 inhibits adipogenesis. MSC isolated from bone marrow as well as adipose tissue of TSP2-null mice display an increase in lipid accumulation relative to those isolated from wild type mice. In addition TSP2-null mice are heavier due to increased adiposity.

Recent studies have demonstrated that TSP2 modulates activation of notch signaling in various cell types. Activation of Notch signaling by plating MSC onto Jagged-1 increases primary MSC proliferation, and decreases osteogenic differentiation. Jagged-1 also increases lipid accumulation in part by increasing their proliferation. Expressing a dominant negative form of mastermind-like-1 (dnMAML) to selectively inactivate canonical notch signaling in vivo in osteoblasts results in vertebral malformations similar to Alagille Syndrome patients. dnMAML expression in early mesenchymal tissue results in decreased trabecular bone volume fraction at 8 weeks of age but has no apparent effect on trabecular bone at 6 or 14.5 months of age. Post-natal induction of global dnMAML expression results in decreased trabecular bone at 6 months of age without significantly altering CFU-F numbers suggesting the decrease in bone mass could be secondary to increased bone resorption. We evaluated the effect of TSP2 on Jagged-1 mediated notch activation and found that notch target gene expression is increased in TSP2-null MSC. Over-expressing TSP2 in C3H10T1/2 cells decreases Hey-1 expression and notch reporter activity. We propose a model in which TSP2 decreases notch activation resulting in a decrease in MSC proliferation and an increase in osteogenic differentiation which in turn leads to a decrease in the osteoprogenitor pool.