Two-pore channel 1 and Ca2+ release-activated Ca2+ channels contribute to the acrosomal pH-dependent intracellular Ca2+ increase in mouse sperm

The acrosome is a lysosome-related vesicular organelle located in the sperm head. The acrosomal reaction (AR) is an exocytic process mediated by Ca²⁺ and essential for mammalian fertilization. Recent findings support the importance of acrosomal alkalinization for the AR. Mibefradil (Mib) and NNC 55-0396 (NNC) are two amphipathic weak bases that block the sperm-specific Ca²⁺ channel (CatSper) and induce acrosomal pH (pH_a ) increase by accumulating in the acrosomal lumen of mammalian sperm. This accumulation and pH_a elevation increase the intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) and trigger the AR by unknown mechanisms of Ca²⁺ transport. Here, we investigated the pathways associated with the pH_a increase-induced Ca²⁺ signals using mouse sperm as a model. To address these questions, we used single-cell Ca²⁺ imaging, the lysosomotropic agent Gly-Phe-β-naphthylamide (GPN) and pharmacological tools. Our findings show that Mib and NNC increase pH_a and release acrosomal Ca²⁺ without compromising acrosomal membrane integrity. Our GPN results indicate that the osmotic component does not significantly contribute to acrosomal Ca²⁺ release caused by pH_a rise. Inhibition of two-pore channel 1 (TPC1) channels reduced the [Ca²⁺ ]_i increase stimulated by acrosomal alkalinization. In addition, blockage of Ca²⁺ release-activated Ca²⁺ (CRAC) channels diminished Ca²⁺ uptake triggered by pH_a alkalinization. Finally, our findings contribute to understanding how pH_a controls acrosomal Ca²⁺ efflux and extracellular Ca²⁺ entry during AR in mouse sperm. KEY POINTS: The acrosomal vesicle is a lysosome-related organelle located in the sperm head. The acrosome reaction (AR) is a highly regulated exocytic process mediated by Ca²⁺ , which is essential for fertilization. However, the molecular identity of Ca²⁺ transporters involved in the AR and their mechanisms to regulate Ca²⁺ fluxes are not fully understood. In mammalian sperm, acrosomal alkalinization induces intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) increase and triggers the AR by unknown molecular mechanisms of Ca²⁺ transport. In this study, we explored the molecular mechanisms underlying Ca²⁺ signals caused by acrosomal alkalinization using mouse sperm as a model. TPC1 and CRAC channels contribute to [Ca²⁺ ]_i elevation during acrosomal alkalinization. Our findings expand our understanding of how the acrosomal pH participates in the physiological induction of the AR.