Involvement of cAMP-dependent unique signaling cascades in the decrease of serine/threonine–phosphorylated proteins in boar sperm head

We previously suggested that protein phosphatase–dependent decrease of postacrosomal phosphorylated proteins may be necessary for the occurrence of acrosome reaction in livestock spermatozoa (Adachi et al., J Reprod Dev 54, 171–176, 2008; Mizuno et al., Mol Reprod Dev 82, 232–250, 2015). The aim of this study was to examine the involvement of the intracellular cAMP signaling cascades in the regulation of the decrease of postacrosomal phosphorylated proteins in boar spermatozoa. Boar ejaculated spermatozoa were incubated with cAMP analogs and then used for the immunodetection of serine/threonine–phosphorylated proteins and assessment of acrosome morphology. The protein phosphatase–dependent decrease of postacrosomal phosphorylated proteins was greatly promoted by the incubation with a cAMP analog Sp-5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole-3′,5'-monophosphorothioate (cBiMPS). This decrease was induced before the initiation of acrosome reaction and did not require the millimolar concentration of extracellular Ca2+ which was necessary for the initiation of acrosome reaction. Moreover, suppression of protein kinase A activity with an inhibitor (H89) had almost no influence on both decrease of phosphorylated proteins and occurrence of acrosome reaction in the spermatozoa incubated with cBiMPS. In addition, the prolonged incubation with a potentially exchange protein directly activated by cAMP-selective cAMP analog (8pM) could only partially mimic effects of cBiMPS on these events. These results indicate that the cAMP-dependent signaling cascades which are less dependent on protein kinase A may regulate the decrease of postacrosomal phosphorylated proteins in boar spermatozoa before the extracellular Ca2+-triggered initiation of acrosome reaction.