Fig. 1: Alb activates hHv1 in human sperm to initiate capacitation and allow the acrosome reaction.

a Left, Proton current traces in non-capacitated human sperm in the absence (top) and presence (bottom) of 80 μM Alb (20 mV steps from −60 and +60 mV). Right, hHv1 currents at the end of a test pulse to +60 mV with 80 μM Fab (white bar), 80 μM Alb (gray bar), or 80 μM Alb + 1 μM C6 (red bar). Values are normalized to mean current amplitude without Alb. C6 peptide blocked 86 ± 3% of the Alb-activated current, n = 5 cells. b Dose-response relationships for Alb potentiation of sperm proton current at +60 mV. The half-maximal effective concentration (EC₅₀) of Alb activation estimated from the fit to Hill relationship as 158 ± 16 µM with a coefficient of 1.09 ± 0.01, n = 5 cells. c Conductance-voltage relationships (G-V) for sperm proton currents in the absence (black squares) or presence of 800 µM Alb (gray circles). Curves were fit to a Boltzmann equation, n = 4 cells. d Left, Non-capacitated sperm were loaded with BCECF and changes of fluorescence intensity measured. 75 μM Alb (gray trace) increased pH_i and the increase was inhibited by 20 μM C6 (red trace). A 75 μM Fab alone had no effect (black trace). Right, BCECF signals were converted to ΔpH as described in Methods. Alb-induced cytoplasmic alkalization was concentration dependent (gray circles). The Alb-triggered pH_i increase was inhibited by C6 (red circles, * P = 0.03; one way ANOVA, Dunnett Test); Fab did not increase the pH_i (black circle, ** P = 0.005; one way ANOVA, Dunnett Test), n = 3–13 independent experiments (Supplementary Fig. 1a). e Non-capacitated sperm were loaded with Fluo-3 and changes in fluorescence measured. Alb potentiates the increase of [Ca²⁺]_i induced by progesterone (15 μM) in a concentration dependent manner (gray bars). The [Ca²⁺]_i increase potentiated by 75 μM Alb was 2.3-fold (** P = 0.002; one way ANOVA, Dunnett Test) and was fully suppressed by 20 μM C6 peptide (red bar), n = 4–7 independent experiments (Supplementary Fig. 1b, c). f Acrosomal exocytosis induced by 15 μM progesterone. Upon progesterone stimulation control capacitated sperm underwent the acrosome reaction (Cap, white bar, *** P = 0.0001; one way ANOVA, Dunnett Test), whereas non-capacitated sperm did not (Non-Cap, white bar). Incubation of C6 peptide (20 μM) with non-capacitated sperm had no effect (C6 + Non-Cap, white bar). In a concentration dependent manner, incubation with Alb increased the progesterone-induced acrosome reaction (gray bars, * P = 0.04; one way ANOVA, Dunnett Test) in non-capacitated sperm; addition of C6 (20 μM, red bar) fully inhibited exocytosis stimulated by 75 μM Alb, n = 3 independent experiments. Values are mean ± SEM. Source data are provided in the Source Data file.