The Shape of Things to Come

Science's STKE  20 Apr 2004:
Vol. 2004, Issue 229, pp. tw140-TW140
DOI: 10.1126/stke.2292004TW140

Adult human mesenchymal stem cells (hMSCs) grown in vitro can differentiate into adipocytes, chondrocytes, or osteoblasts, depending on their exposure to growth factors and on the density of cells in the culture. Although the pathways whereby growth factors affect differentiation have been extensively studied, little is known about the mechanisms whereby density--which affects cell shape, proliferation, contact with other cells, and exposure to paracrine secretions--influences cell fate. To specifically investigate the effects of cell shape, McBeath et al. plated solitary hMSCs on "islands" of adherent substrate surrounded by regions to which they could not adhere. Cells grown on small islands (forced to remain rounded) developed markers for adipocytes, whereas cells grown on large islands (which flattened out) developed markers for osteoblasts. Cells grown under "osteogenic conditions" (low density and appropriate medium) were flat and showed increased activity of the small guanosine triphosphatase RhoA. Inhibiting the Rho effector ROCK (Rho kinase), which interfered with cytoskeletal tension without causing cell rounding, blocked osteogenesis in flattened cells. Undifferentiated hMSCs expressing constitutively active RhoA became round and differentiated into osteoblasts, whereas cells expressing dominant-negative RhoA became flattened and differentiated into adipocytes, in both cases overcoming the effects of soluble factors in the media. Constitutively active RhoA did not, however, promote osteogenesis when expressed in round cells grown on small islands, whereas constitutively active ROCK did. Thus, cell shape, RhoA, and ROCK all appear to play key roles in hMSC fate determination, with RhoA acting downstream of soluble differentiation factors and ROCK acting downstream of cell shape-mediated signaling as well.

R. McBeath, D. M. Pirone, C. M. Nelson, K. Bhadriraju, C. S. Chen, Cell shape, cytoskeletal tension, and RhoA regulate stem cell lineage commitment. Devel. Cell 6, 483-495 (2004). [Online Journal]