TGF-β1 Directs Chondrogenic Differentiation of Monocytic Precursor Cells

Abstract

Bone marrow consists of two types of stem cell populations, the hematopoietic stem cell (HSC) and the mesenchymal stromal cell (MSC). While MSCs have been demonstrated to have the capacity to differentiate into osteoblasts, chondrocytes, and adipocytes, recent studies by our laboratory have demonstrated an HSC origin for osteoblasts, cancer-associated fibroblasts, and immature adipocytes, revealing the ability of HSCs to give rise to cell types outside of the hematopoietic lineage. Previous in vivo studies in our laboratory using a clonal cell transplantation model, demonstrate the ability of HSCs to give rise to hypertrophic chondrocytes during nonstabilized fracture repair. This work has led to the hypothesis that the HSC is an alternate source for chondrocytes. To begin addressing this hypothesis, in vitro studies were performed to determine appropriate culture conditions for HSC-derived chondroprogenitors. The importance of identifying alternate sources for chondrocytes is high, as there are currently no widely successful standard therapies for cartilage regeneration. Alcian Blue staining was used as an indicator of chondrogenic potential in determining glycosaminoglycan production. In addition to this, nodule formation was assessed in cultures in serum-containing chondrogenic media. More specific studies involved monocytic precursor cells cultured in serum-free conditions and directed towards the chondrogenic lineage via transforming growth factor beta-1 (TGF-β1). After induction, positive expression of the cartilage specific markers, Aggrecan and Collagen II, along with negative expression of F4/80, suggested chondrogenic differentiation. Future work will be aimed at delineating the mechanisms and factors governing HSC differentiation and maturation to chondrogenic lineages, with the potential to enhance stem cell therapies for cartilage repair.