Selective attachment of osteoprogenitors to laminin

During endochondral ossification and bone remodeling, osteoprogenitors (OP) attach to the matrix and differentiate into osteoblasts. To identify matri x proteins binding specifically these precursors, fetal rat calvaria (RC) c ells were plated for 5-20 min in serum-free medium, on wells coated with va rious proteins and saturated with bovine serum albumin (BSA) to block nonsp ecific binding sites. Adherent cells were either counted or grown to assess bone colony (nodule) formation. As each nodule originates from the clonal division of one OF, the ratio (nodules/100 cells attached) measures the pro portion of OP among adherent cells. Of numerous purified matrix proteins te sted, laminin-1 and tenascin inhibited cell attachment, whereas fibronectin , bone sialoprotein, and type I collagen increased cell attachment and othe rs had no effect. Only laminin-1 and, to a lesser extent, tenascin, enriche d the cell population in OP. Laminin-1 acted time- and dose-dependently. In experiments in which cell attachment to laminin-coated but unsaturated wel ls was ensured by plating for 24 h in 10% fetal calf serum, laminin-1 had n o effect on cell attachment nor on OP differentiation. In contrast, repeate d plating of RC cells on laminin-1-coated/saturated wells depleted the popu lation in OF, confirming that OP selection was a cell-attachment effect. Wh en RC cell populations isolated by successive collagenase extractions were compared, the highest rate of OP enrichment on laminin-1 was obtained with the earliest populations, which were the most responsive to dexamethasone, a marker of early OP stages. In conclusion, laminin- recruits in vitro, thr ough a cell-1 attachment effect, OP present in early RC cell populations, o f which laminins are abundant extracellular matrix components. (C) 1999 by Elsevier Science Inc, All rights reserved.