Starting a new life: Sperm PLC-zeta mobilizes the Ca2+signal that induces egg activation and embryo development
Cell Signalling Laboratory, WHRI, Institute of Molecular and Experimental Medicine, Cardiff University School of Medicine, Cardiff, UK.BioEssays (Impact Factor: 4.73). 11/2011; 34(2):126-34. DOI: 10.1002/bies.201100127
We have discovered that a single sperm protein, phospholipase C-zeta (PLCζ), can stimulate intracellular Ca(2+) signalling in the unfertilized oocyte ('egg') culminating in the initiation of embryonic development. Upon fertilization by a spermatozoon, the earliest observed signalling event in the dormant egg is a large, transient increase in free Ca(2+) concentration. The fertilized egg responds to the intracellular Ca(2+) rise by completing meiosis. In mammalian eggs, the Ca(2+) signal is delivered as a train of long-lasting cytoplasmic Ca(2+) oscillations that begin soon after gamete fusion and persist beyond the completion of meiosis. Sperm PLCζ effects Ca(2+) release from egg intracellular stores by hydrolyzing the membrane lipid PIP(2) and consequent stimulation of the inositol 1,4,5-trisphosphate (InsP(3) ) receptor Ca(2+) -signalling pathway, leading to egg activation and early embryogenesis. Recent advances have refined our understanding of how PLCζ induces Ca(2+) oscillations in the egg and also suggest its potential dysfunction as a cause of male infertility.
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- "PLCz exhibits highest homology with phospholipase C delta 1 (PLCd1), and a significant amount of research elucidating the regulation of PLCz function in the oocyte has been based upon our structural knowledge of PLCd1 (Saunders et al., 2002; Nomikos et al., 2013). Each characteristic PLC protein domain plays an important role in the release of Ca 2þ but in contrast to other PLC isoforms, PLCz does not possess a pleckstrin homology (PH) domain localised at the N-terminus (Saunders et al., 2002; Nomikos et al., 2012). Indeed, the PH domain of PLCd1 has been proven to mediate the hydrolysis of its substrate, phosphatidylinositol 4,5-biphosphate (PIP 2 ), in the plasma membrane to initiate cell signalling (Lomasney et al., 1996), and it was not clear until recently how PLCz interacted with PIP 2 in order to exert function. "
ABSTRACT: The development of a mammalian embryo is initiated by a sequence of molecular events collectively referred to as 'oocyte activation' and regulated by the release of intracellular calcium in the ooplasm. Over the last decade, phospholipase C zeta (PLCζ), a sperm protein introduced into the oocyte upon gamete fusion, has gained almost universal acceptance as the protein factor responsible for initiating oocyte activation. A large body of consistent and reproducible evidence, from both biochemical and clinical settings, confers support for the role of PLCζ in this fundamental biological context, which has significant ramifications for the management of human male infertility. Oocyte activation deficiency (OAD) and total fertilisation failure (TFF) are known causes of infertility and have both been linked to abnormalities in the structure, expression, and localisation pattern of PLCζ in human sperm. Assisted oocyte activators (AOAs) represent the only therapeutic option available for OAD at present, although these agents have been the source of much debate recently, particularly with regard to their potential epigenetic effects upon the embryo. Consequently, there is much interest in the deployment of sensitive PLCζ assays as prognostic/diagnostic tests and human recombinant PLCζ protein as an alternative form of therapy. Although PLCζ deficiency has been directly linked to a cohort of infertile cases, we have yet to identify the specific causal mechanisms involved. While two genetic mutations have been identified which link defective PLCζ protein to an infertile phenotype, both were observed in the same patient, and have yet to be described in other patients. Consequently, some researchers are investigating the possibility that genetic variations in the form of single nucleotide polymorphisms (SNPs) could provide some explanation, especially since >6000 SNPs have been identified in the PLCζ gene. As yet, however, there is no consistent data to suggest that any of these SNPs influence the functional ability of PLCζ. Other laboratories appear to be focussing upon the PLCζ promoter, which is bi-directional and shared with the actin filament capping muscle Z-line alpha 3 gene (CAPZA3), or seeking to identify interacting proteins within the ooplasm. The aim of this review is to provide a synopsis of recent progress in the application of PLCζ in diagnostic and therapeutic medicine, to discuss our current understanding of how the functional ability of PLCζ might be controlled, and thus how PLCζ deficiency might arise, and finally, to consider the potential implications of alternative sperm protein candidates, such as post-acrosomal WW-domain binding protein (PAWP), which has caused much debate and confusion in the field over the last few years.
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- "Ca 2+ oscillations during mammalian oocyte activation are generated in an IP 3 -mediated manner, requiring that phosphatidylinositol 4,5- bisphosphate (PIP 2 ) is hydrolysed to liberate IP 3 and diacylglycerol. This is consistent with the idea that the sperm factor is a phospholipase C (PLC) (Nomikos et al., 2012, 2013a). Accordingly, accumulating data over the past decade has supported the identity of this sperm factor to be a novel, testis-specific PLC isozyme, termed PLCzeta (PLCz) (Cox et al., 2002; Saunders et al., 2002). "
ABSTRACT: Mammalian oocyte activation is mediated by cytosolic calcium (Ca2+) oscillations initiated upon delivery of a putative ‘sperm factor’ by the fertilizing sperm. Previous studies suggest the identity of this sperm factor as the testis-specific phospholipase C-zeta (PLCζ). Recently, a post-acrosomal sheath WW domain-binding protein (PAWP) has been proposed as an alternative sperm factor candidate, following a report that human PAWP protein and cRNA elicited Ca2+ oscillations in mouse and human oocytes. Those Ca2+ oscillations were inhibited by a PAWP-derived peptide corresponding to a functional PPGY binding motif. Herein, using a series of human PAWP expression constructs, we demonstrate that both human PAWP protein and cRNA are, in our experiments, unable to elicit Ca2+ release following microinjection into mouse oocytes. Parallel experiments performed with human PLCζ elicited the characteristic Ca2+ oscillations present at mammalian fertilization, which produced oocyte activation and embryo development. Furthermore, sperm-induced Ca2+ oscillations were not inhibited by the PAWP-derived PPGY peptide following in vitro fertilization or intracytoplasmic sperm injection. Thus, the functional disparity with PLCζ leads us to conclude that human PAWP is neither sufficient nor necessary for the Ca2+ oscillations that initiate mammalian oocyte activation at fertilization.
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- "In all species studied, such Ca 2+ oscillations are both necessary and sufficient for the completion of all the events of oocyte activation and early embryonic development (Nomikos et al., 2012). Ca 2+ oscillations in mammalian oocytes occur as a result of inositol trisphosphate (IP 3 )mediated Ca 2+ release from internal stores such as the endoplasmic reticulum (ER) (Miyazaki et al., 1992), with the amplitude, duration and frequency of Ca 2+ oscillations being largely species specific (Swann et al., 2006; Nomikos et al., 2012). However, the sperm factor causing the Ca 2+ oscillations is not species specific because the injection of human sperm into mouse oocytes can cause Ca 2+ oscillations as well as oocyte activation (Vanden Meerschaut et al., 2013). "
ABSTRACT: A sperm-specific phospholipase C-zeta (PLCζ) is believed to play an essential role in oocyte activation during mammalian fertilization. Sperm PLCζ has been shown to trigger a prolonged series of repetitive Ca2+ transients or oscillations in oocytes that precede activation. This remarkable intracellular Ca2+ signalling phenomenon is a distinctive characteristic observed during in vitro fertilization by sperm. Previous studies have notably observed an apparent differential ability of PLCζ from disparate mammalian species to trigger Ca2+ oscillations in mouse oocytes. However, the molecular basis and confirmation of the apparent PLCζ species difference in activity remains to be provided. In the present study, we provide direct evidence for the superior effectiveness of human PLCζ relative to mouse PLCζ in generating Ca2+ oscillations in mouse oocytes. In addition, we have designed and constructed a series of human/mouse PLCζ chimeras to enable study of the potential role of discrete PLCζ domains in conferring the enhanced Ca2+ signalling potency of human PLCζ. Functional analysis of these human/mouse PLCζ domain chimeras suggests a novel role of the EF-hand domain in the species-specific differences in PLCζ activity. Our empirical observations are compatible with a basic mathematical model for the Ca2+ dependence of generating cytoplasmic Ca2+ oscillations in mammalian oocytes by sperm PLCζ.
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