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Abstract
Sperm-specific phospholipase C zeta (PLCζ) is widely considered to be the physiological stimulus responsible for generating calcium (Ca2+) oscillations that induce egg activation and early embryonic development during mammalian fertilization. In the mammalian testis, PLCζ expression is detected at spermiogenesis following elongated spermatid differentiation. Sperm-delivered PLCζ induces Ca2+ release via the inositol 1,4,5-trisphosphate (InsP3) signaling pathway. PLCζ is the smallest known mammalian PLC isoform identified to date, with the simplest domain organization. However, the distinctive biochemical properties of PLCζ compared with other PLC isoforms contribute to its unique potency in stimulating cytosolic Ca2+ oscillations within mammalian eggs. Moreover, studies describing PLCζ “knockout” mouse phenotypes confirm the supreme importance of PLCζ at egg activation and monospermic fertilization in mice. Importantly, a number of clinical reports have highlighted the crucial importance of PLCζ in human fertilization by associating PLCζ deficiencies with certain forms of male factor infertility. Herein, we give an update on recent advances that have refined our understanding of how sperm PLCζ triggers Ca2 + oscillations and egg activation in mammals, while also discussing the nature of a potential “alternative” sperm factor. We summarise PLCζ localization in mammalian sperm, and the direct links observed between defective PLCζ protein in sperm and documented cases of male infertility. Finally, we postulate how this sperm protein can be used as a potential diagnostic marker, and also as a powerful therapeutic agent for treatment of certain types of male infertility due to egg activation failure or even in more general cases of male subfertility.
Highlights
Sperm-specific phospholipase C zeta (PLCζ) is widely considered to be the physiological stimulus responsible for generating calcium (Ca2+) oscillations that induce egg activation and early embryonic development during mammalian fertilization
Studies describing PLCζ “knockout” mouse phenotypes confirm the supreme importance of PLCζ at egg activation and monospermic fertilization in mice
We summarise PLCζ localization in mammalian sperm, and the direct links observed between defective PLCζ protein in sperm and documented cases of male infertility
Summary
Received: 09 September 2019 Accepted: 14 January 2020 Published: 29 January 2020. Citation: Saleh A, Kashir J, Thanassoulas A, Safieh-Garabedian B, Lai FA and Nomikos M (2020) Essential Role of Sperm-Specific PLC-Zeta in Egg Activation and Male Factor Infertility: An Update. Upon sperm-egg fusion, PLCζ is proposed to be delivered by the fertilizing sperm into the ooplasm, triggering the Ca2+ oscillations via the InsP3 signaling pathway, through the hydrolysis of its membrane-bound phospholipid substrate, PIP2 (Saunders et al, 2002; Nomikos, 2015) The importance of this sperm specific protein in mammalian fertilization has been further highlighted by numerous clinical studies directly linking defects or deficiencies in human PLCζ with documented cases of male factor infertility (Yoon et al, 2008; Heytens et al, 2009; Nomikos et al, 2011a, 2017b; Kashir et al, 2012b,c; Escoffier et al, 2016; Torra-Massana et al, 2019). Are such reduced frequency and amplitude oscillations due to fusion of a single sperm, or the cumulative effect of multiple sperm-egg fusion events? It is necessary that such experiments are performed to ascertain fully the conflicting data generated from knockout studies far
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