Signaling pathways involved in human sperm hyperactivated motility stimulated by Zn²⁺

Abstract

To fertilize the egg, sperm cells must reside in the female reproductive tract for several hours during which they undergo chemical and motility changes collectively called capacitation. During capacitation, the sperm develop a unique type of motility known as hyperactivated motility (HAM). The semen contains Zn in millimolar concentrations, whereas in the female reproductive tract, the concentration is around 1 µM. In this study, we characterize the role of Zn in human sperm capacitation focusing on its effect on HAM. Western blot analysis revealed the presence of GPR39-type Zn-receptor localized mainly in the sperm tail. Zn at micromolar concentration stimulates HAM, which is mediated by a cascade involving GPR39-adenylyl cyclase (AC)-cyclic AMP (cAMP)-protein kinase A-tyrosine kinase Src (Src)-epidermal growth factor receptor and phospholipase C. Both the transmembrane AC and the soluble-AC are involved in the stimulation of HAM by Zn . The development of HAM is precisely regulated by cAMP, in which relatively low concentration (5-10 µM) stimulated HAM, whereas at 30 µM no stimulation occurred. A similar response was seen when different concentrations of Zn were added to the cells; low Zn stimulated HAM, whereas at relatively high Zn , no effect was seen. We further demonstrate that the Ca -channel CatSper involved in Zn -stimulated HAM. These data support a role for extracellular Zn acting via GPR39 to regulate signaling pathways in sperm capacitation, leading to HAM induction.

The Role of Zinc in Male Fertility

“…This movement is characterized by an intensive and asymmetric beating of the middle and principal pieces of the flagella, which engender a strong driving force to infiltrate the extracellular matrix of oocytes [ 73 ]. It has been demonstrated that low concentrations of Zn 2+ (5–10 µM) stimulate hyper-activated motility in human sperm under capacitation conditions, whereas, at 30-µM Zn 2+ , no stimulation was observed [ 74 ]. We also know that relatively high concentrations of Zn 2+ (in the mM range) in the semen are inhibitory to sperm function, whereas, in the female reproductive tract, the concentration of Zn 2+ is much lower (1.0–1.5 µM) [ 75 ].…”

“…The cytoplasmic alkalization leads to the activation of the sperm-specific Ca 2+ channel CatSper [ 81 ], which is localized in the flagellum and mediates the development of capacitation-dependent hyper-activated motility [ 82 ] (see Figure 1 ). It has been shown that the CatSper inhibitor negatively impacts the stimulatory effect of Zn 2+ on human sperm HAM, indicating that CatSper mediates Zn 2+ -stimulated hyper-activated motility [ 74 ]. Hyper-activated motility develops during sperm capacitation [ 83 ], a process that depends on protein kinase A (PKA) activity [ 84 ].…”

“…CatSper-null sperm show normal PKA activity [ 82 , 85 ], indicating that PKA activity is not dependent on CatSper. We found that CatSper inhibition by NNC-55-0396(1S,2S-2-(2-[N-((3-benzimidazole-2-yl)propyl)-N-methyamino]ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtylcyclopropanecarboxylate-dihydrochloride hydrate) resulted in PKA inhibition, indicating that PKA activity depends on CatSper activity [ 74 ]. This possible contradiction could be answered as follows: Ca 2+ influx via CatSper causes membrane depolarization, leading to the activation of voltage-gated H + channel H v 1, which is the predominant alkalization system in human sperm [ 86 ].…”

“…Thus, the inhibition of Ca 2+ influx in CatSper-null sperm would not affect intracellular alkalization, allowing the activation of sAC/PKA. We suggested that Zn 2+ activates the PKA-Src-EGFR cascade, which is CatSper-dependent [ 74 ] (see Figure 1 ). Since CatSper activation is required for hyperactivation and, ultimately, for male fertility [ 82 ], it is not unexpected that zinc would be important for preparing the spermatozoa for hyperactivation and successful fertilization.…”

“…Extracellular Zn 2+ affects intracellular signaling pathways through its interaction with the Zn 2+ -sensing receptor (ZnR), also named GPR39 [ 65 ]. This receptor was found in the sperm tail and the acrosome [ 74 , 88 , 89 ], suggesting the possible involvement of Zn 2+ in sperm motility and acrosomal exocytosis. Our studies showed that Zn 2+ stimulates bovine sperm acrosomal exocytosis [ 88 ], as well as human sperm hyper-activated motility [ 74 ], both mediated by GPR39 (see Figure 1 ).…”