Cytosolic Ca²⁺ as a multifunctional modulator is required for spermiogenesis in Ascaris suum

The dynamic polar polymers actin filaments and microtubules are usually employed to provide the structural basis for establishing cell polarity in most eukaryotic cells. Radially round and immotile spermatids from nematodes contain almost no actin or tubulin, but still have the ability to break symmetry to extend a pseudopod and initiate the acquisition of motility powered by the dynamics of cytoskeleton composed of major sperm protein (MSP) during spermiogenesis (sperm activation). However, the signal transduction mechanism of nematode sperm activation and motility acquisition remains poorly understood. Here we show that Ca²⁺ oscillations induced by the Ca²⁺ release from intracellular Ca²⁺ store through inositol (1,4,5)-trisphosphate receptor are required for Ascaris suum sperm activation. The chelation of cytosolic Ca²⁺ suppresses the generation of a functional pseudopod, and this suppression can be relieved by introducing exogenous Ca²⁺ into sperm cells. Ca²⁺ promotes MSP-based sperm motility by increasing mitochondrial membrane potential and thus the energy supply required for MSP cytoskeleton assembly. On the other hand, Ca²⁺ promotes MSP disassembly by activating Ca²⁺/calmodulin-dependent serine/threonine protein phosphatase calcineurin. In addition, Ca²⁺/camodulin activity is required for the fusion of sperm-specific membranous organelle with the plasma membrane, a regulated exocytosis required for sperm motility. Thus, Ca²⁺ plays multifunctional roles during sperm activation in Ascaris suum.