| Bone marrow derived mesenchymal stem cells (MSC) are multipotent, self-renewing, mesodermal-origin stem cells sequestered in the endosteal compartment. MSC are maintained in a relative state of quiescence in vivo but in response to a variety of physiological and pathological stimuli, proliferate and differentiate into osteoblasts, chondrocytes, adipocytes, or hematopoiesis-supporting stromal cells. Little is understood regarding the cellular or molecular events underlying MSC fate decisions. We evaluated gene expression of early passage hMSC cultured under serum free conditions.; In the absence of osteoinductive stimuli, these cells express low to moderate levels of the bone specific or bone associated transcription factors Runx-2 and Dlx-5, as well as the bone associated extracellular matrix proteins, osteopontin and osteocalcin. Expression of BMP-6, but not BMP-2, 4, and 7 was detected in hMSC. Expression of several Wnt family members was detected in hMSC, including the canonical Wnts, Wnt-4b, 5b, 7a, 8b, and 10b, and the non-canonical Wnt11. We also examined the osteogenic potential of hMSC in response to growth factors and found that human MSC (hMSC) cultured in defined, serum-free conditions respond to BMPs with osteogenic commitment, differentiation, and hydroxyapatite deposition. Addition of BMP-6 to hMSC induces the expression or up-regulation of a repertoire of osteoblast-related genes including type I collagen, osteocalcin, bone sialoprotein, and their regulatory transcription factors Cbfa1/Runx-2, and Osterix. This translates into increased production of osteogenic extracellular matrix (ECM) with subsequent hydroxyapatite deposition.; Osterix, a BMP induced osteoblast transcription factor expressed at the earliest stages of differentiation, appears to regulate expression of bone sialoprotein and osteomodulin, bone extracellular matrix proteins essential for the earliest stages of bone formation. At the latest stages of BMP induced hMSC osteoblast differentiation, we observed activation of Notch signaling, indicating that Notch may be required for terminal differentiation of hMSC and osteoblast function.; Wnt11, an endogenous factor expressed at moderate levels in murine pre-osteoblasts, enhances BMP osteoinduction of these cells. Murine calvarial pre-osteoblasts overexpressing Wnt11 have elevated levels of cytosolic and nuclear beta-cateninin, hallmarks of canonical Wnt signaling. These cells demonstrate increased osteoblast differentiation and function at a molecular and phenotypic level. These studies identify Wnt11 as a novel osteogenic factor. |
