Cell penetrating peptides, targeting the regulation of store-operated channels, slow decay of the progesterone-induced [Ca2⁺]_i signal in human sperm

Previous work has provided evidence for involvement of store operated channels (SOCs) in [Ca2+]i signalling of human sperm, including a contribution to the transient [Ca2+]i elevation that occurs upon activation of CatSper, a sperm-specific cation channel localised to the flagellum, by progesterone . To further investigate the potential involvement of SOCs in the generation of [Ca2+]i signals in human sperm we have used cell-penetrating peptides containing the important basic sequence KIKKK, part of the STIM-Orai activating region/CRAC activation domain (SOAR/CAD) of the regulatory protein stromal interaction molecule 1 (STIM1). SOAR/CAD plays a key role in controlling the opening of SOCs, which occurs upon mobilisation of stored Ca2+. Resting [Ca2+]i temporarily decreased upon application of KIKKK peptide (3-4 min) but scrambled KIKKK peptide had a similar effect, indicating that this action was not sequence specific. However, in cells pre-treated with KIKKK the transient [Ca2+]i elevation induced by stimulation with progesterone decayed significantly more slowly than in parallel controls and in cells pre-treated with scrambled KIKKK peptide. Examination of single cell responses showed that this effect was due, at least in part, to an increase in the proportion of cells in which the initial transient was maintained for an extended period, lasting up to 10 minutes in a sub-population of cells. We hypothesise that SOCs contribute to the progesterone-induced [Ca2+]i transient and that interference with the regulatory mechanisms of SOC delays their closure, causing a prolongation of the [Ca2+]i transient.