To examine the mechanism of intercellular adhesion in the establishment of limb skeletal elements we have investigated the process of limb bud cell aggregation in vitro. Limb bud cells are aggregation-competent immediately after their trypsin:collagenase dissociation in the absence of calcium. This aggregation is largely Ca2(+)-independent (CI) and is completely and reversibly inhibited by cycloheximide. In contrast, when limb bud cells are first allowed to recover from Ca2(+)-free trypsin:collagenase dissociation, aggregation of the surviving population is exclusively Ca2(+)-dependent (CD) and completely and reversibly inhibited by cycloheximide. The presence of exogenous calcium during initial cell dissociation retains a functional CD aggregation mechanism. However, incubation of such cells with EGTA releases the CD component and converts the cells to a predominantly CI aggregation. Rabbits were immunized with limb bud cells exhibiting the recovered CD aggregation mechanism and the resulting immune sera were screened for their effect on cell aggregation. Relative to pre-immune sera, intact immune IgG agglutinated dissociated limb bud cells whilst immune Fab fragments inhibited their aggregation. The aggregation-inhibiting antiserum recognizes five major limb bud cell surface components with apparent molecular weights of 72K, 50K, 23K, 14.5K and 8.5K (K = 10(3) Mr), respectively. Limb bud cell surface plasma membranes were isolated by sucrose gradient density centrifugation and detergent-solubilized proteins coupled to Sepharose 4B with cyanogen bromide. Equivalent cell surface plasma membrane proteins were 125I-iodinated and applied to the affinity column. Limb bud cell surface protein affinity chromatography in the presence of exogenous calcium yields a single protein with an apparent molecular weight of approximately 8.5 K. This protein molecule elutes at 0.6 M NaCl, indicating a high affinity, is recognized by the aggregation-inhibiting antiserum, and is itself capable of inhibiting CD limb bud cell aggregation. Fab fragments prepared from rabbit antisera specifically directed against the affinity-purified material also inhibit CD limb bud cell aggregation and this inhibition is neutralized by the 8.5 K protein. Our data thus demonstrate that CD limb bud cell aggregation is not mediated by fibronectin and/or collagen type I and indicate that this process is governed by a novel 8.5 K cell adhesion molecule.

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