The secreted protein thrombospondin 2 is an autocrine inhibitor of marrow stromal cell proliferation.
Marrow stromal cells (MSCs) are obtained in increased number from mice in which the thrombospondin 2 (TSP2) gene is disrupted, and these cells show increased DNA synthesis in vitro. To examine more closely the role of TSP2 in the physiology and osteogenic differentiation of MSCs, an in-depth characterization of TSP2-null MSCs was conducted. Determination of TSP2 protein content by Western analysis and RNA levels by reverse-transcription polymerase chain reaction (RT-PCR) indicated that MSCs are the primary source of TSP2 in the marrow and secrete abundant TSP2 into culture medium. Morphologically, the TSP2-null and wild-type (WT) cell populations were similar and by flow cytometry contained equivalent numbers of CD44+, Mac1+, intercellular adhesion molecule-1 (ICAM-1+), and ScaI+ cells. TSP2-null cells showed delayed mineralization associated with an increased rate of proliferation. Consistent with this finding, there was a decrease in expression of collagen and osteocalcin RNA by TSP2-null MSCs on day 7 and increased osteopontin expression on day 7 and day 14. In add-back experiments, recombinant TSP2 produced a dose-dependent decrease in proliferation. This reduction was associated with an accumulation of TSP2-treated cells in the G1 phase of the cell cycle and did not result from an increase in apoptosis. When TSP2 treatment was terminated, the cell population reentered the S phase. We conclude that the increased endosteal bone formation observed in TSP2-null mice results primarily from the failure of TSP2 to regulate locally MSC cell cycle progression.
Hankenson
Kurt D
KD
Department of Biochemistry, University of Washington, Seattle 98195-7350, USA.
Bornstein
Paul
P
D000818:Animals; D001854:Bone Marrow Cells / Q000166:cytology* / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D015870:Gene Expression; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D010012:Osteogenesis / Q000187:drug effects / Q000502:physiology; D010641:Phenotype; D012313:RNA / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000494:pharmacology; D017154:Stromal Cells / Q000166:cytology / Q000187:drug effects; D019699:Thrombospondins / Q000172:deficiency / Q000235:genetics / Q000494:pharmacology / Q000502:physiology*; D000099262:Thrombospondin 2
D012313:RNA; D011994:Recombinant Proteins; D019699:Thrombospondins; D000099262:Thrombospondin 2