Cell-cell Interactions in Early Embryogenesis: a Molecular Approach to the Role of Calcium

Compaction, a process of cell-cell adhesion between mouse blastomeres or between embryonal carcinoma (EC) cells requires calcium ions. A decompaction effect similar to that observed in the absence of Ca2+ is triggered by Fab fragments of rabbit anti-EC IgG. This effect occurs through the recognition of a specific cell-surface glycoprotein named uvomorulin. An 84,000 dalton fragment of uvomorulin (UMt) has been previously extracted by trypsin from EC cell membranes and purified. WE present evidence that effects of Ca2+ on compaction are transmitted through conformational changes in uvomorulin. First, Ca2+ protects UMt from further proteolysis by trypsin. Mn2+ and Sr2+ have similar effects, whereas this protection is reversed by La3+. Second, UMt can bind the monoclonal antibody De1 only in the presence of Ca2+ (half-binding at 10(-5) M Ca2+). This antigenic exposure also takes place in the presence of Mn2+ or Sr2+ and is reversed by La3+. Third, metal ions (Ca2+, Mn2+, Sr2+) that promote trypsin resistance and recognition by DE1 are found to trigger the compaction of morulae and EC cells. Metal ions (La3+) that reduce trypsin resistance and affinity for DE1 result in decompaction.