Human Zona Pellucida Glycoproteins: Binding Characteristics With Human Spermatozoa and Induction of Acrosome Reaction

Abstract

Human zona pellucida (ZP) matrix is composed of four glycoproteins designated as ZP glycoprotein -1 (ZP1), -2 (ZP2), -3 (ZP3), and -4 (ZP4). Mutations in the genes encoding human ZP glycoproteins are one of the causative factors leading to abnormal ZP matrix and infertility in women. Relevance of the human ZP glycoproteins in 'sperm-oocyte' binding has been delineated by using either transgenic animal models expressing human zona proteins or purified native/recombinant human zona proteins. Studies based on the purified native/recombinant human zona proteins revealed that ZP1, ZP3, and ZP4 primarily bind to the capacitated acrosome-intact human spermatozoa whereas ZP2 binds to acrosome-reacted spermatozoa. On the contrary, human spermatozoa binds to the eggs obtained from transgenic mouse lines expressing human ZP2 but not to those expressing human ZP1, ZP3, and ZP4 suggesting that ZP2 has an important role in human 'sperm-oocyte' binding. Further studies using transgenic mouse lines showed that the N-terminus of human ZP2 mediate the taxon-specific human sperm-oocyte binding. Both glycans and protein-protein interactions have a role in human gamete interaction. Further studies have revealed that the purified native/recombinant human ZP1, ZP3, and ZP4 are competent to induce acrosome reaction. Human sperm binds to the mouse transgenic eggs expressing human ZP1-4 instead of mouse ZP1-3 proteins, penetrated the ZP matrix and accumulated in the perivitelline space, which were acrosome-reacted suggesting that human ZP2 in transgenic mouse model also induce acrosome reaction. In humans N-linked glycosylation of zona proteins have been shown to play an important role in induction of the acrosome reaction. Hence in humans, based on studies using transgenic mouse model as well as purified native/recombinant zona proteins, it is likely that more than one zona protein is involved in the 'sperm-oocyte' binding and induction of the acrosome reaction.

Keywords: ZP glycoproteins-mediated acrosome reaction; fertilization; human zona pellucida glycoproteins; mutations in genes encoding human zona glycoproteins; zona proteins binding to sperm.

FIGURE 1

Schematic diagram to depict the ability of human ZP glycoproteins to induce acrosome reaction. In humans, more than one zona protein is competent to induce AR. Incubation of capacitated human sperm with baculovirus-expressed recombinant ZP1, ‘ZP domain’ of ZP1 (273–551 aa residues), ZP3, C-terminal fragment of ZP3 (214–305 aa residues), and ZP4 led to a dose dependent increase in AR. Further, several studies have also shown that recombinant human ZP3 expressed in mammalian cells also induce AR. In addition, induction of AR has also been observed in presence of native ZP3 and ZP4 purified from human eggs. However, baculovirus-expressed recombinant ZP2 failed to induce AR.

FIGURE 2

Schematic diagram to illustrate binding of human sperm to transgenic mouse oocyte expressing human ZP2. No binding of human capacitated sperm is observed when incubated with mouse eggs expressing mouse ZP1, ZP2, and ZP3. However, zonae from transgenic mouse lines expressing either human ZP2, mouse ZP1, and mouse ZP3 or human ZP1-4 in place of mouse ZP1-3 or those expressing human ZP2 N-terminal fragment corresponding to 22–164 aa residues instead of mouse ZP2, showed binding with human sperm suggesting its important role in human sperm-egg binding. Agarose beads coated with human ZP2 peptide (39–154 aa residues) inhibited the binding of human sperm to transgenic mouse oocytes expressing human ZP2 (Baibakov et al., 2012; Avella et al., 2014, 2016).