PLC and IP3-evoked Ca2+ release initiate the fast block to polyspermy in Xenopus laevis eggs

The fast block to polyspermy breaks with convention

JGP study (Komondor et al. 2023. J. Gen. Physiol. https://doi.org/10.1085/jgp.202213258) reveals that conventional PLC activation pathways are not required for the fertilization-induced depolarization of Xenopus eggs that prevents the entry of additional sperm.

TMEM16A activation for the fast block to polyspermy in the African clawed frog does not require conventional activation of egg PLCs

Fertilization of an egg by more than one sperm, a condition known as polyspermy, leads to gross chromosomal abnormalities and is embryonic lethal for most animals. Consequently, eggs have evolved multiple processes to stop supernumerary sperm from entering the nascent zygote. For external fertilizers, such as frogs and sea urchins, fertilization signals a depolarization of the egg membrane, which serves as the fast block to polyspermy. Sperm can bind to, but will not enter, depolarized eggs. In eggs from the African clawed frog, Xenopus laevis, the fast block depolarization is mediated by the Ca2+-activated Cl- channel TMEM16A. To do so, fertilization activates phospholipase C, which generates IP3 to signal a Ca2+ release from the ER. Currently, the signaling pathway by which fertilization activates PLC during the fast block remains unknown. Here, we sought to uncover this pathway by targeting the canonical activation of the PLC isoforms present in the X. laevis egg: PLCγ and PLCβ. We observed no changes to the fast block in X. laevis eggs inseminated in inhibitors of tyrosine phosphorylation, used to stop activation of PLCγ, or inhibitors of Gαq/11 pathways, used to stop activation of PLCβ. These data suggest that the PLC that signals the fast block depolarization in X. laevis is activated by a novel mechanism.

Xenopus laevis lack the critical sperm factor PLCζ

Fertilization of eggs from the African clawed frog Xenopus laevis is characterized by an increase in cytosolic calcium, a phenomenon that is also observed in other vertebrates such as mammals and birds. During fertilization in mammals and birds, the transfer of the soluble PLCζ from sperm into the egg is thought to trigger the release of calcium from the endoplasmic reticulum (ER). Injecting sperm extracts into eggs reproduces this effect, reinforcing the hypothesis that a sperm factor is responsible for calcium release and egg activation. Remarkably, this occurs even when sperm extracts from X. laevis are injected into mouse eggs, suggesting that mammals and X. laevis share a sperm factor. However, X. laevis lacks an annotated PLCZ1 gene, which encodes the PLCζ enzyme. In this study, we attempted to determine whether sperm from X. laevis express an unannotated PLCZ1 ortholog. We identified PLCZ1 orthologs in 11 amphibian species, including 5 that had not been previously characterized, but did not find any in either X. laevis or the closely related Xenopus tropicalis. Additionally, we performed RNA sequencing on testes obtained from adult X. laevis males and did not identify potential PLCZ1 orthologs in our dataset or in previously collected ones. These findings suggest that PLCZ1 may have been lost in the Xenopus lineage and raise the question of how fertilization triggers calcium release and egg activation in these species.

Phosphate position is key in mediating transmembrane ion channel TMEM16A-phosphatidylinositol 4,5-bisphosphate interaction

TransMEMbrane 16A (TMEM16A) is a Ca2+-activated Cl- channel that plays critical roles in regulating diverse physiologic processes, including vascular tone, sensory signal transduction, and mucosal secretion. In addition to Ca2+, TMEM16A activation requires the membrane lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). However, the structural determinants mediating this interaction are not clear. Here, we interrogated the parts of the PI(4,5)P2 head group that mediate its interaction with TMEM16A by using patch- and two-electrode voltage-clamp recordings on oocytes from the African clawed frog Xenopus laevis, which endogenously express TMEM16A channels. During continuous application of Ca2+ to excised inside-out patches, we found that TMEM16A-conducted currents decayed shortly after patch excision. Following this rundown, we show that the application of a synthetic PI(4,5)P2 analog produced current recovery. Furthermore, inducible dephosphorylation of PI(4,5)P2 reduces TMEM16A-conducted currents. Application of PIP2 analogs with different phosphate orientations yielded distinct amounts of current recovery, and only lipids that include a phosphate at the 4' position effectively recovered TMEM16A currents. Taken together, these findings improve our understanding of how PI(4,5)P2 binds to and potentiates TMEM16A channels.

Sperm MMP-2 is indispensable for fast electrical block to polyspermy at fertilization in Xenopus tropicalis

Sperm matrix metalloproteinase-2 (MMP-2) is necessary for frog fertilization. Monospermy is ensured by a fast, electrical block to polyspermy mediated by a positive fertilization potential. To determine the role of the MMP-2 hemopexin domain (HPX) in a fast block to polyspermy during fertilization of the frog, Xenopus tropicalis, we prepared mutant frogs deficient in mmp2 gene using the transcription activator-like effector nuclease method. mmp2 ΔHPX (-/-) sperm without MMP-2 protein were able to fertilize wild-type (WT; +/+) eggs. However, polyspermy occurred in some eggs. The mutant sperm generated a normal fertilization potential amounting to 10 mV, and were able to fertilize eggs at 10 mV, at which WT sperm never fertilized. Sensitivity during voltage-dependent fertilization decreased in mutant sperm. This study demonstrates for the first time that the genetic alteration of the MMP-2 molecule in sperm causes polyspermy during fertilization of a monospermic species. Our findings provide reliable evidence that sperm MMP-2 is indispensable for the fast, electrical block to polyspermy during Xenopus fertilization.

Zinc protection of fertilized eggs is an ancient feature of sexual reproduction in animals

One of the earliest and most prevalent barriers to successful reproduction is polyspermy, or fertilization of an egg by multiple sperm. To prevent these supernumerary fertilizations, eggs have evolved multiple mechanisms. It has recently been proposed that zinc released by mammalian eggs at fertilization may block additional sperm from entering. Here, we demonstrate that eggs from amphibia and teleost fish also release zinc. Using Xenopus laevis as a model, we document that zinc reversibly blocks fertilization. Finally, we demonstrate that extracellular zinc similarly disrupts early embryonic development in eggs from diverse phyla, including Cnidaria, Echinodermata, and Chordata. Our study reveals that a fundamental strategy protecting human eggs from fertilization by multiple sperm may have evolved more than 650 million years ago.

Ion channels and signaling pathways used in the fast polyspermy block

Fertilization of an egg by multiple sperms, polyspermy, is lethal to most sexually reproducing species. To combat the entry of additional sperm into already fertilized eggs, organisms have developed various polyspermy blocks. One such barrier, the fast polyspermy block, uses a fertilization-activated depolarization of the egg membrane to electrically inhibit supernumerary sperm from entering the egg. The fast block is commonly used by eggs of oviparous animals with external fertilization. In this review, we discuss the history of the fast block discovery, as well as general features shared by all organisms that use this polyspermy block. Given the diversity of habitats of external fertilizers, the fine details of the fast block-signaling pathways differ drastically between species, including the identity of the depolarizing ions. We highlight the known molecular mediators of these signaling pathways in amphibians and echinoderms, with a fine focus on ion channels that signal these fertilization-evoked depolarizations. We also discuss the investigation for a fast polyspermy block in mammals and teleost fish, and we outline potential fast block triggers. Since the first electrical recordings made on eggs in the 1950s, the fields of developmental biology and electrophysiology have substantially matured, and yet we are only now beginning to discern the intricate molecular mechanisms regulating the fast block to polyspermy.

Paul F. Cranefield Award to Anne E. Carlson

Trudeau recognizes the latest recipient of the Cranefield Award.

Phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ are both required to open the Cl- channel TMEM16A

Transmembrane member 16A (TMEM16A) is a widely expressed Ca²⁺-activated Cl^- channel with various physiological functions ranging from mucosal secretion to regulating smooth muscle contraction. Understanding how TMEM16A controls these physiological processes and how its dysregulation may cause disease requires a detailed understanding of how cellular processes and second messengers alter TMEM16A channel gating. Here we assessed the regulation of TMEM16A gating by recording Ca²⁺-evoked Cl^- currents conducted by endogenous TMEM16A channels expressed in Xenopus laevis oocytes, using the inside-out configuration of the patch clamp technique. During continuous application of Ca²⁺, we found that TMEM16A-conducted currents decay shortly after patch excision. Such current rundown is common among channels regulated by phosphatidylinositol 4,5-bisphosphate (PIP₂). Thus, we sought to investigate a possible role of PIP₂ in TMEM16A gating. Consistently, synthetic PIP₂ rescued the current after rundown, and the application of PIP₂ modulating agents altered the speed kinetics of TMEM16A current rundown. First, two PIP₂ sequestering agents, neomycin and anti-PIP₂, applied to the intracellular surface of excised patches sped up TMEM16A current rundown to nearly twice as fast. Conversely, rephosphorylation of phosphatidylinositol (PI) derivatives into PIP₂ using Mg-ATP or inhibiting dephosphorylation of PIP₂ using β-glycerophosphate slowed rundown by nearly 3-fold. Our results reveal that TMEM16A regulation is more complicated than it initially appeared; not only is Ca²⁺ necessary to signal TMEM16a opening, but PIP₂ is also required. These findings improve our understanding of how the dysregulation of these pathways may lead to disease and suggest that targeting these pathways could have utility for potential therapies.

The fast block to polyspermy: New insight into a century-old problem

Jaffe underscores new research that identifies key roles for IP3 and TMEM16a in the fast block to polyspermy.

The TMEM16A channel mediates the fast polyspermy block in Xenopus laevis

In their preceding paper, Wozniak et al. show that fertilization increases intracellular Ca²⁺ in Xenopus laevis eggs by activating an IP₃ signaling cascade. Here, they reveal that Ca²⁺ subsequently opens the Cl⁻ channel TMEM16A to allow Cl⁻ efflux, cell depolarization, and fast block to polyspermy.

In externally fertilizing animals, such as sea urchins and frogs, prolonged depolarization of the egg immediately after fertilization inhibits the entry of additional sperm—a phenomenon known as the fast block to polyspermy. In the African clawed frog Xenopus laevis, this depolarization is driven by Ca²⁺-activated Cl⁻ efflux. Although the prominent Ca²⁺-activated Cl⁻ currents generated in immature X. laevis oocytes are mediated by X. laevis transmembrane protein 16a (xTMEM16A) channels, little is known about the channels that contribute to the fast block in mature eggs. Moreover, the gamete undergoes a gross transformation as it develops from an immature oocyte into a fertilization-competent egg. Here, we report the results of our approach to identify the Ca²⁺-activated Cl⁻ channel that triggers the fast block. By querying published proteomic and RNA-sequencing data, we identify two Ca²⁺-activated Cl⁻ channels expressed in fertilization-competent X. laevis eggs: xTMEM16A and X. laevis bestrophin 2A (xBEST2A). By exogenously expressing xTMEM16A and xBEST2A in axolotl cells lacking endogenous Ca²⁺-activated currents, we characterize the effect of inhibitors on currents mediated by these channels. None of the inhibitors tested block xBEST2A currents specifically. However, 2-(4-chloro-2-methylphenoxy)-N-[(2-methoxyphenyl)methylideneamino]-acetamide (Ani9) and N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA) each reduce xTMEM16A currents by more than 70% while only nominally inhibiting those generated by xBEST2A. Using whole-cell recordings during fertilization, we find that Ani9 and MONNA effectively diminish fertilization-evoked depolarizations. Additionally, these inhibitors lead to increased polyspermy in X. laevis embryos. These results indicate that fertilization activates TMEM16A channels in X. laevis eggs and induces the earliest known event triggered by fertilization: the fast block to polyspermy.