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CALMODULIN REGULATES CAPACITATION AND TRIGGERS THE ACROSOME REACTION IN MOUSE SPERMATOZOA.
Bendahmane, Malika1, Lynch, Christopher 1, Tulsiani, Daulat1, 1
ABSTRACT- Capacitated acrosome-intact mouse spermatozoa interact with specific sugar residues on neoglycoproteins (ngps) or solubilized zona pellucida (ZP), the egg's extracellular glycocalyx, prior to the initiation of calcium (Ca2+)-dependent signal transduction cascade that results in the fenestration and fusion of the sperm plasma membrane overlying the outer acrosomal membrane at multiple sites and exocytosis of acrosomal contents (i.e., induction of the acrosome reaction [AR]). The AR releases powerful enzymes (proteases, glycohydrolases, etc.) at the site of sperm-zona binding and is thought to be a prerequisite event that allows acrosome-reacted spermatozoa to penetrate the ZP and fertilize the egg. Since calmodulin (CaM), a putative Ca2+ sensor protein, plays a significant role in many cell signaling pathways and membrane fusion events by modulating the activity of key regulatory enzymes, ion pumps, and proteins that are associated with Ca2+ regulated exocytosis, we have used a pharmacological approach to examine the role of CaM in sperm capacitation and agonist-induced AR. Inclusion of CaM antagonists (calmodulin binding domain, calmidazolium, compound 48/80, ophiobolin A, W5, W7 and W13) either in in vitro capacitation medium or after sperm capacitation blocked the npg-/ZP-induced AR. Purified CaM largely reversed the AR blocking effects of antagonists during capacitation. Our results demonstrate that CaM plays an important role in priming (i.e., capacitation) of mouse spermatozoa as well as in the agonist-induced AR. These data allow us to propose that CaM regulates these events by modulating sperm membrane component(s). Supported in part by NIH grants HD34041 and HD25869.
KEY WORDS: Calmodulin, Capacitation, Acrosome Reaction, Mouse Sperm
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