Increase of intracellular Ca2+ and relocation of E-cadherin during experimental decompaction of mouse embryos

To determine the role of intracellular Ca²⁺ in compaction, the first morphogenetic event in embryogenesis, we analyzed preimplantation mouse embryos under several decompacting conditions, including depletion of extracellular Ca²⁺, blocking of Ca²⁺ channels, and inhibition of microfilaments, calmodulin, and intracellular Ca²⁺ release. Those treatments induced decompaction of mouse morulae and simultaneously induced changes in cytosolic free Ca²⁺ concentration and deregionalization of E-cadherin and fodrin. When morulae were allowed to recompact, the location of both proteins recovered. In contrast, actin did not change its cortical location with compaction nor with decompaction-recompaction. Calmodulin localized in areas opposite to cell–cell contacts in eight-cell stage embryos before and after compaction. Inhibition of calmodulin with trifluoperazine induced its delocalization while morulae decompacted. A nonspecific rise of intracellular free Ca²⁺ provoked by ionomycin did not affect the compacted shape. Moreover, the same decompacting treatments when applied to uncompacted embryos did not produce any change in intracellular Ca²⁺. Our results demonstrate that in preimplantation mouse embryos experimentally induced stage-specific changes of cell shape are accompanied by changes of intracellular free Ca²⁺ and redistribution of the cytoskeleton-related proteins E-cadherin, fodrin, and calmodulin. We conclude that intracellular Ca²⁺ specifically is involved in compaction and probably regulates the function and localization of cytoskeleton elements.