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Ca²⁺-stores in sperm: their identities and functions

¹ School of Biosciences, University of Birmingham, Birmingham B15 2TT, UK² Reproductive Biology and Genetics Research Group, The Medical School, University of Birmingham, Birmingham B15 2TT, UK³ Centre for Human Reproductive Science, Birmingham Women's Hospital, Birmingham B15 2TG, UK⁴ School of Health Sciences, Piaget Institute, Algarve 6300-025, Silves, Portugal⁵ Division of Maternal and Child Health Sciences, Medical School, Ninewells Hospital, University of Dundee, Dundee DD1 9SY, UK

Correspondence should be addressed to S Publicover at School of Biosciences, University of Birmingham, B15 2TT; Email: s.j.publicover{at}bham.ac.uk

Intracellular Ca²⁺ stores play a central role in the regulation of cellular [Ca²⁺]_i and the generation of complex [Ca²⁺] signals such as oscillations and waves. Ca²⁺ signalling is of particular significance in sperm cells, where it is a central regulator in many key activities (including capacitation, hyperactivation, chemotaxis and acrosome reaction) yet mature sperm lack endoplasmic reticulum and several other organelles that serve as Ca²⁺ stores in somatic cells. Here, we review i) the evidence for the expression in sperm of the molecular components (pumps and channels) which are functionally significant in the activity of Ca²⁺ stores of somatic cells and ii) the evidence for the existence of functional Ca²⁺ stores in sperm. This evidence supports the existence of at least two storage organelles in mammalian sperm, one in the acrosomal region and another in the region of the sperm neck and midpiece. We then go on to discuss the probable identity of these organelles and their discrete functions: regulation by the acrosome of its own secretion and regulation by membranous organelles at the sperm neck (and possibly by the mitochondria) of flagellar activity and hyperactivation. Finally, we consider the ability of the sperm discretely to control mobilisation of these stores and the functional interaction of stored Ca²⁺ at the sperm neck/midpiece with CatSper channels in the principal piece in regulation of the activities of mammalian sperm.