Diagnosis and Treatment of Male Infertility-Related Fertilization Failure

Abstract

Infertility affects approximately 15% of reproductive-aged couples worldwide, of which up to 30% of the cases are caused by male factors alone. The origin of male infertility is mostly attributed to sperm abnormalities, of which many are caused by genetic defects. The development of intracytoplasmic sperm injection (ICSI) has helped to circumvent most male infertility conditions. However, there is still a challenging group of infertile males whose sperm, although having normal sperm parameters, are unable to activate the oocyte, even after ICSI treatment. While ICSI generally allows fertilization rates of 70 to 80%, total fertilization failure (FF) still occurs in 1 to 3% of ICSI cycles. Phospholipase C zeta (PLCζ) has been demonstrated to be a critical sperm oocyte activating factor (SOAF) and the absence, reduced, or altered forms of PLCζ have been shown to cause male infertility-related FF. The purpose of this review is to (i) summarize the current knowledge on PLCζ as the critical sperm factor for successful fertilization, as well as to discuss the existence of alternative sperm-induced oocyte activation mechanisms, (ii) describe the diagnostic tests available to determine the cause of FF, and (iii) summarize the beneficial effect of assisted oocyte activation (AOA) to overcome FF.

Keywords: AOA; HOCA; ICSI; MOAT; MOCA; PLCZ1 mutations; fertilization failure; male infertility; oocyte activation deficiencies; phospholipase C zeta (PLCζ).

Figure 1

Oocyte activation pathway induced by the sperm factor PLCζ. The release of sperm PLCζ into the oocyte induces IP₃ production, which subsequently binds to its receptor (IP₃R) and provokes Ca²⁺ release from the endoplasmic reticulum. The Ca²⁺ oscillations will activate different oocyte kinases in a time-dependent order, allowing cortical granule exocytosis though PKC activation, extrusion of the second polar body and meiotic resumption via CaMKII activation and formation of pronuclei by MAPK inactivation. PLCζ: phospholipase C zeta; DAG: diacylglycerol; PIP2: phosphatidylinositol 4,5-biphosphate; IP₃: inositol 1,4,5-triphosphate; PKC: protein kinase C; MARCKS: myristoylated alanine-rich C-kinase substrate; CaMKII: Ca²⁺/calmodulin-dependent protein kinase II; WEE2: Wee1-like protein kinase 2; CDK1: cyclin-dependent kinase 1; CNB1: cyclin B; MPF: maturation-promoting factor; Emi2: early mitotic inhibitor 2; APC/C: anaphase-promoting complex/cyclosome; MAPK: Mos/mitogen-activated protein kinase; V: vesicle; N: nucleus; ER: endoplasmic reticulum; CG: cortical granule; PB: polar body; PN: pronuclei; MII: metaphase II.

Figure 2

Schematic representation of the mouse oocyte activation test (MOAT). (+): Positive control group (−): Negative control group; PIEZO-ICSI: Piezo-assisted intracytoplasmic sperm injection; OAD: oocyte activation deficiency.

Figure 3

Schematic representation of the diagnostic tests designed to study the calcium (Ca²⁺) oscillation pattern obtained after injection of human sperm into mouse and human oocytes. (A) Mouse oocyte calcium analysis (MOCA). (B) Human oocyte calcium analysis (HOCA). (+): Positive control group; PIEZO-ICSI: Piezo-assisted intracytoplasmic sperm injection; OAD: oocyte activation deficiency; A: amplitude; F: frequency.

Figure 4

Overview of the localization of the identified mutations in PLCζ. The amino acid sequence position of the protein domains and the identified mutations are indicated by the numbers. In red, mutations linked to male infertility by functional analysis. In green, mutations not associated to male infertility by functional analysis. In black, mutations not studied by functional analysis yet.

Figure 5

Representative Ca²⁺ signaling responses registered after in vitro fertilization in mouse oocytes in combination with different assisted oocyte activation methodologies. (a) Physiological Ca²⁺ oscillation pattern obtained after an injection of wild-type sperm into mouse oocytes (Control 2). (b) Ca²⁺ oscillation pattern observed after the parthenogenetic activation of mouse oocytes using SrCl₂ in combination with Cytochalasin D (Control 3). (c–f) In order, Ca²⁺ oscillation patterns obtained after injection of PLCζ-null sperm in combination with rhPLCζ (AOA1), SrCl₂ (AOA2), Ionomycin (AOA3), and the Zn²⁺ chelator TPEN (AOA4). Reproduced from Minerva et al., 2020 with permission [174].