Introduction

Vertebrate eggs are arrested at the metaphase stage of the second meiosis (MII) when ovulated because they have an active Cdk1/cyclin B complex and inactive APC/C^Cdc20 (Heim et al., 2018). Release from MII initiates egg activation, the first hallmark of embryonic development (Ducibella et al., 2002; Schultz and Kopf, 1995). The universal signal of egg activation is an increase in the intracellular concentration of calcium (Ca²⁺) (Ridgway et al., 1977; Stricker, 1999). Ca²⁺ release causes the inactivation of the APC/C inhibitor Emi2, which enhances cyclin B degradation and induces meiotic exit (Lorca et al., 1993; Shoji et al., 2006; Suzuki et al., 2010a). In mammals, the stereotypical fertilization Ca²⁺ signal, oscillations, consists of transient but periodical Ca²⁺ increases that promote progression into interphase (Deguchi et al., 2000; Miyazaki et al., 1986). The sperm-borne Phospholipase C zeta1 (PLCζ) persistently stimulates the production of inositol 1,4,5-trisphosphate (IP₃) (Matsu-ura et al., 2019; Saunders et al., 2002; Wu et al., 2001) that binds its cognate receptor in the endoplasmic reticulum (ER), IP₃R1 and causes Ca²⁺ release from the egg’s main Ca²⁺ reservoir (Wakai et al., 2019). The intake of extracellular Ca²⁺ via plasma membrane channels and transporters ensures the persistence of the oscillations (Miao et al., 2012; Stein et al., 2020; Wakai et al., 2019, 2013).

Before fertilization, maturing oocytes undergo cellular and biochemical modifications (see for review (Ajduk et al., 2008)). The nucleus of immature oocytes, known as the germinal vesicle (GV), undergoes the breakdown of its envelope marking the onset of maturation and setting in motion a series of cellular events that culminate with the release of the first polar body, the correct ploidy for fertilization, and re-arrest at MII (Eppig, 1996). Other organelles are also reorganized, such as cortical granules migrate to the cortex for exocytosis and polyspermy block, mitochondria undergo repositioning, and the cytoplasm’s redox state becomes progressively reduced to promote the exchange of the sperm’s protamine load (Liu, 2011; Perreault et al., 1988; Wakai et al., 2014). Wide-ranging adaptations also occur in the Ca²⁺ release machinery to produce timely and protracted Ca²⁺ oscillations following sperm entry (Fujiwara et al., 1993; Lawrence et al., 1998), including the increase in the content of the Ca²⁺ stores, ER reorganization with cortical cluster formation, and increased IP₃R1 sensitivity (Lee et al., 2006; Wakai et al., 2012). The total intracellular levels of zinc (Zn²⁺) also remarkably increase during maturation, amounting to a 50% rise, which is necessary for oocytes to proceed to the telophase I of meiosis and beyond (Kim et al., 2010). Remarkably, after fertilization, Zn²⁺ levels need to decrease, as Emi2 is a Zn²⁺-associated molecule, and high Zn² levels prevent MII exit (Bernhardt et al., 2012; Shoji et al., 2014; Suzuki et al., 2010b). Following the initiation of Ca²⁺ oscillations, approximately 10 to 20% of the Zn²⁺ accrued during maturation is ejected during the Zn²⁺ sparks, a conserved event in vertebrates and invertebrate species (Converse and Thomas, 2020; Kim et al., 2011; Mendoza et al., 2022; Que et al., 2019; Seeler et al., 2021; Tokuhiro and Dean, 2018; Wozniak et al., 2020; Zhang et al., 2016). The use of Zn²⁺ chelators such as N,N,N,N-tetrakis (2-pyridinylmethyl)-1,2-ethylenediamine (TPEN) to create Zn²⁺-deficient conditions buttressed the importance of Zn²⁺ during meiotic transitions (Kim et al., 2010; Suzuki et al., 2010b). However, whether the analogous dynamics of Ca²⁺ and Zn²⁺ during maturation imply crosstalk and Zn²⁺ levels modulate Ca²⁺ release during fertilization is unknown.

IP₃Rs are the most abundant intracellular Ca²⁺ release channel in non-muscle cells (Berridge, 2016). They form a channel by assembling into tetramers with each subunit of ∼270kDa MW (Taylor and Tovey, 2010). Mammalian eggs express the type I IP₃R, the most widespread isoform (Fissore et al., 1999; Parrington et al., 1998). IP₃R1 is essential for egg activation because its inhibition precludes Ca²⁺ oscillations (Miyazaki and Ito, 2006; Miyazaki et al., 1992; Xu et al., 2003). Myriad and occasionally cell-specific factors influence Ca²⁺ release through the IP₃R1 (Taylor and Tovey, 2010). For example, following fertilization, IP₃R1 undergoes ligand-induced degradation caused by the sperm-initiated long-lasting production of IP₃ that effectively reduces the IP₃R1 mass (Brind et al., 2000; Jellerette et al., 2000). Another regulatory mechanism is Ca²⁺, a universal cofactor, which biphasically regulates IP₃Rs’ channel opening (Iino, 1990; Jean and Klee, 1986), congruent with several Ca²⁺ and calmodulin binding sites on the channel’s sequence (Sienaert et al., 1997; Sipma et al., 1999). Notably, Zn²⁺ may also participate in IP₃R1 regulation. Recent studies using electron cryomicroscopy (cryoEM), a technique that allows peering into the structure of IP₃R1 with a near-atomic resolution, have revealed that a helical linker (LNK) domain near the C-terminus mediates the coupling between the N-and C-terminal ends necessary for channel opening (Fan et al., 2015). The LNK domain contains a putative Zinc-finger motif proposed to be vital for IP₃R1 function (Fan et al., 2015; Paknejad and Hite, 2018). Therefore, the exponential increase in Zn²⁺ levels in maturing oocytes, besides its essential role in meiosis progression, may optimize the IP₃R1 function, revealing hitherto unknown cooperation between these cations during fertilization.

Here, we examined whether crosstalk between Ca²⁺ and Zn²⁺ is required to initiate and sustain Ca²⁺ oscillations and maintain Ca²⁺ store content in MII eggs. We found that Zn²⁺-deficient conditions inhibited Ca²⁺ release and oscillations without reducing Ca²⁺ stores, IP₃ production, IP₃R1 expression, or altering the viability of eggs or zygotes. We show instead that Zn²⁺ deficiency impaired IP₃R1 function and lessened the receptor’s ability to gate Ca²⁺ release out of the ER. Remarkably, resupplying Zn²⁺ re-established the oscillations interrupted by low Zn²⁺, although persistent increases in intracellular Zn²⁺ were harmful, disrupting the Ca²⁺ responses and preventing egg activation. Together, the results show that besides contributing to oocyte maturation, Zn²⁺ has a central function in Ca²⁺ homeostasis such that optimal Zn²⁺ concentrations ensure IP₃R1 function and the Ca²⁺ oscillations required for initiating embryo development.

Results

TPEN dose-dependently lowers intracellular Zn²⁺ and inhibits sperm-initiated Ca²⁺ oscillations

TPEN is a cell-permeable, non-specific chelator with a high affinity for transition metals widely used to study their function in cell physiology (Arslan et al., 1985; Lo et al., 2020). Mouse oocytes and eggs have exceedingly high intracellular concentrations of Zn²⁺ (Kim et al., 2011, 2010), and the TPEN-induced defects in the progression of meiosis have been ascribed to its chelation (Bernhardt et al., 2011; Kim et al., 2010). In support of this view, the Zn²⁺ levels of cells showed acute reduction after TPEN addition, as reported by indicators such as FluoZin-3 (Arslan et al., 1985; Gee et al., 2002; Suzuki et al., 2010b). Studies in mouse eggs also showed that the addition of low µM (40-100) concentrations of TPEN disrupted Ca²⁺ oscillations initiated by fertilization or SrCl₂ (Lawrence et al., 1998; Suzuki et al., 2010b), but the mechanism(s) and target(s) of the inhibition remained unknown. To gain insight into this phenomenon, we first performed dose-titration studies to determine the effectiveness of TPEN in lowering Zn²⁺ in eggs. The addition of 2.5 µM TPEN protractedly reduced Zn²⁺ levels, whereas 5 and 10 µM TPEN acutely and persistently reduced FluoZin-3 fluorescence (Fig. 1A). These concentrations of TPEN are higher than the reported free Zn²⁺ concentrations in cells, but within range of those of found in typical culture conditions (Lo et al., 2020; Qin et al., 2011). We next determined the concentrations of TPEN required to abrogate fertilization-initiated oscillations. Following intracytoplasmic sperm injection (ICSI), we monitored Ca²⁺ responses while increasing TPEN concentrations. As shown in Fig. 1B, 5 and 10 µM TPEN effectively blocked ICSI-induced Ca²⁺ oscillations in over half of the treated cells, and the remaining eggs, after a prolonged interval, resumed lower-frequency rises (Fig. 1B-center panels). Finally, 50 µM or greater concentrations of TPEN permanently blocked these oscillations (Fig. 1B-right panel). It is noteworthy that at the time of addition, TPEN concentrations of 5 µM or above induce a sharp drop in basal Fura-2 F340/ F380 ratios, consistent with Fura-2’s high affinity for Zn²⁺ (Snitsarev et al., 1996). We next used membrane-permeable and -impermeable chelators to assess whether TPEN inhibited Ca²⁺ oscillations by chelating Zn²⁺ from intracellular or extracellular compartments. The addition of the high-affinity but cell-impermeable Zn²⁺ chelators DTPA and EDTA neither terminated nor temporarily interrupted ICSI-induced Ca²⁺ oscillations (Fig. 1C), although protractedly slowed them down, possibly because of chelation and lowering of external Ca²⁺ (Fig. 1C). These results suggest that chelation of external Zn²⁺ does not affect the continuation of oscillations. We cannot determine that EDTA successfully chelated all external Zn²⁺, but the evidence that the addition of EDTA to the monitoring media containing cell impermeable FluoZin-3 caused a marked reduction in fluorescence, suggests that a noticeable fraction of the available Zn²⁺ was sequestered (Supplementary Fig. 1A). Similarly, injection of mPlcζ mRNA in eggs incubated in Ca²⁺ and Mg²⁺-free media supplemented with EDTA, to maximize the chances of chelation of external Zn²⁺, initiated low-frequency but persistent oscillations, and addition of Ca²⁺ and Mg²⁺ restored the physiological periodicity (Supplementary Fig. 1B). Lastly, another Zn²⁺-permeable chelator, TPA, blocked the ICSI-initiated Ca²⁺ oscillations but required higher concentrations than TPEN (Fig. 1D). Collectively, the data suggest that basal levels of labile internal Zn²⁺ are essential to sustain the fertilization-initiated Ca²⁺ oscillations in eggs.

Figure 1.

We next evaluated whether Zn²⁺ depletion prevented the completion of meiosis and pronuclear (PN) formation. To this end, ICSI-fertilized eggs were cultured in the presence of 10 μM TPEN for 8h, during which the events of egg activation were examined (Fig. 1E and Table 1). All fertilized eggs promptly extruded second polar bodies regardless of treatment (Fig. 1E). TPEN, however, impaired PN formation, and by 4-or 7-h post-ICSI, most treated eggs failed to show PNs, unlike controls (Fig. 1E and Table 1). Together, these results demonstrate that depletion of Zn²⁺ terminates Ca²⁺ oscillations and delays or prevents events of egg activation, including PN formation.

Table 1.

TPEN is a universal inhibitor of Ca²⁺ oscillations in eggs

Mammalian eggs initiate Ca²⁺ oscillations in response to numerous stimuli and conditions (Miyazaki and Ito, 2006; Wakai and Fissore, 2013). Fertilization and its release of PLCζ stimulate the phosphoinositide pathway, producing IP₃ and Ca²⁺ oscillations (Miyazaki, 1988; Saunders et al., 2002). Neurotransmitters such as acetylcholine (Ach) and other G-protein coupled receptor agonists engage a similar mechanism (Dupont et al., 1996; Kang et al., 2003), although in these cases, IP₃ production occurs at the plasma membrane and is short-lived (Kang et al., 2003; Swann and Parrington, 1999). Agonists such as SrCl₂ and thimerosal generate oscillations by sensitizing IP₃R1 without producing IP₃. The mechanism(s) of SrCl₂ is unclear, although its actions are reportedly directly on the IP₃R1 (Hajnóczky and Thomas, 1997; Hamada et al., 2003; Nomikos et al., 2015, 2011; Sanders et al., 2018). Thimerosal oxidizes dozens of thiol groups in the receptor, which enhances the receptor’s sensitivity and ability to release Ca²⁺ (Bootman et al., 1992; Evellin et al., 2002; Joseph et al., 2018). We took advantage of the varied points at which the mentioned agonists engage the phosphoinositide pathway to examine TPEN’s effectiveness in inhibiting their effects. mPlcζ mRNA injection, like fertilization, induces persistent Ca²⁺ oscillations, although mPlcζ’s tends to be more robust. Consistent with this, the addition of 10 and 25 µM TPEN transiently interrupted or belatedly terminated oscillations, whereas 50 µM acutely stopped all responses (Fig. 2A). By contrast, SrCl₂-initiated rises were the most sensitive to Zn²⁺-deficient conditions, with 2.5 µM TPEN nearly terminating all oscillations that 5 µM did (Fig. 2B). TPEN was equally effective in ending the Ach-induced Ca²⁺ responses (Fig. 2C), but curbing thimerosal responses required higher concentrations (Fig. 2D). Lastly, we ruled out that downregulation of IP₃R1 was responsible for the slow-down or termination of the oscillations by TPEN. To accomplish this, we examined the IP₃R1 mass in eggs (Jellerette et al., 2004) with and without TPEN supplementation and injection of mPlcζ mRNA. By 4h post-injection, PLCζ induced the expected down-regulation of IP₃R1 reactivity, whereas was insignificant in TPEN-treated and Plcζ mRNA-injected eggs, as it was in uninjected control eggs (Fig. 2F). These findings together show that Zn²⁺ deficiency inhibits the IP₃R1-mediated Ca²⁺ oscillations independently of IP₃ production or loss of receptor, suggesting a role of Zn²⁺ on IP₃R1 function (Fig. 2E).

Figure 2.

Zn²⁺ depletion reduces IP₃R1-mediated Ca²⁺ release

To directly assess the inhibitory effects of TPEN on IP₃R1 function, we used caged IP₃ (cIP₃) that, after short UV pulses, releases IP₃ into the ooplasm (Wakai et al., 2012; Walker et al., 1987). To exclude the possible contribution of external Ca²⁺ to the responses, we performed the experiments in Ca²⁺-free media. In response to sequential cIP₃ release 5 min apart, control eggs displayed corresponding Ca²⁺ rises that occasionally transitioned into short-lived oscillations (Fig. 3A). The addition of TPEN after the third cIP₃ release prevented the subsequent Ca²⁺ response and prematurely terminated the in-progress Ca²⁺ rises (Fig. 3B and inset). Pre-incubation of eggs with TPEN precluded cIP₃-induced Ca²⁺ release, even after 5 sec UV exposure (Fig. 3C). The addition of excess ZnSO₄ (100 µM) overcame TPEN’s inhibitory effects only if added before (Fig. 3E) and not after the addition of TPEN (Fig. 3D). Similar concentrations of MgCl₂ or CaCl₂ failed to reverse TPEN effects (Fig. 3F, G). Together, the results show that Zn²⁺ is required for IP₃R1-mediated Ca²⁺ release downstream of IP₃ production, appearing to interfere with receptor gating, as suggested by TPEN’s rapid termination of in-progress Ca²⁺ rises and ongoing oscillations.

Figure 3.

ERp44 is an ER luminal protein of the thioredoxin family that interacts with the IP₃R1, reportedly inhibiting its ability to mediate Ca²⁺ release (Higo et al., 2005). The localization of ERp44 in the ER-Golgi intermediate compartment of somatic cells correlates with Zn²⁺’s availability and changes dramatically after TPEN treatment (Higo et al., 2005; Watanabe et al., 2019). To rule out the possibility that TPEN suppresses the function of IP₃R1 by modifying the subcellular distribution of ERp44, we overexpressed ERp44 by injecting HA tagged-Erp44 mRNA into MII eggs and monitored the effect on Ca²⁺ release. TPEN did not alter the localization of ERp44 (Supplementary Fig. 2A), and overexpression of ERp44 modified neither the Ca²⁺ oscillations induced by agonists (Supplementary Fig. 2B) nor the effectiveness of TPEN to block them (data not shown). Thus, TPEN and Zn²⁺ deficiency most likely inhibits Ca²⁺ release by directly interfering with IP₃R1 function rather than modifying this particular regulator.

Zn²⁺ depletion diminishes the ER Ca²⁺ leak and increases Ca²⁺ store content

Our above cIP₃ results that TPEN inhibited IP₃R1-mediated Ca²⁺ release and interrupted in-progress Ca²⁺ rises despite the presence of high levels of environmental IP₃ suggest its actions are probably independent of IP₃ binding, agreeing with an earlier report showing that TPEN did not modify IP₃’s affinity for the IP₃R (Richardson and Taylor, 1993). Additionally, the presence of a Zn²⁺-binding motif near the C-term cytoplasmic domain of the IP₃R1’s channel, which is known to influence agonist-induced IP₃R1 gating (Fan et al., 2015), led us to posit and examine that Zn²⁺ deficiency may be disturbing Ca²⁺ release to the cytosol and out of the ER. To probe this possibility, we queried if pre-treatment with TPEN inhibited Ca²⁺ release through IP₃R1. We first used Thapsigargin (Tg), a Sarcoplasmic/ER Ca²⁺ ATPase pump inhibitor (Thastrup et al., 1990) that unmasks a constitutive Ca²⁺ leak out of the ER (Lemos et al., 2021); in eggs, we have demonstrated it is mediated at least in part by IP₃R1 (Wakai et al., 2019). Treatment with TPEN for 15 min slowed the Tg-induced Ca²⁺ leak into the cytosol, resulting in delayed and lowered amplitude Ca²⁺ responses (Fig. 4A; P<0.05). To test whether the reduced response to Tg means that TPEN left a temporarily increased Ca²⁺ content in the ER after it, we added the Ca²⁺ ionophore ionomycin (Io), which empties all stores independently of IP₃Rs. Io-induced Ca²⁺ responses were 3.3-fold greater in TPEN-treated cells, supporting the view that TPEN interferes with the ER Ca²⁺ leak (Fig. 4A; P < 0.05). We further evaluated this concept using in vitro aged eggs that often display reduced Ca²⁺ store content than freshly collected counterparts (Abbott et al., 1998). After culturing eggs in the presence or absence of TPEN for 2h, we added Io during Ca²⁺ monitoring, which in TPEN-treated eggs induced bigger Ca²⁺ rises than in control eggs (Fig. 4B; P <0.05). We confirmed that this effect was independent of IP₃R1 degradation because TPEN did not change IP₃R1 reactivity in unfertilized eggs (Fig. 4C; P <0.05).

Figure 4.

Next, we used the genetically encoded FRET sensor D1ER (Palmer et al., 2004) to assess the TPEN’s effect on the ER’s relative Ca²⁺ levels changes following the additions of Tg or Ach. TPEN was added 10 min before 10 µM Tg or 50 µM Ach, and we simultaneously monitored changes in cytosolic and intra-ER Ca²⁺ (Fig. 4D, E). For the first three min, the Tg-induced decrease in Ca²⁺-ER was similar between groups. However, while the drop in Ca²⁺ content continued in control eggs, in TPEN-treated eggs, it came to an abrupt halt, generating profound differences between the two groups (Fig. 4D; P <0.05). TPEN had even more pronounced effects following the addition of Ach, leading to a reduced-and prematurely terminated-Ca²⁺ release from the ER in treated eggs (Fig. 4E; P <0.05).

Lastly, we sought to use a cellular model where low labile Zn²⁺ occurred without pharmacology. To this end, we examined a genetic model where the two non-selective plasma membrane channels that could influx Zn²⁺ in maturing oocytes have been deleted (Bernhardt et al., 2017; Carvacho et al., 2016, 2013), namely, the transient receptor potential melastatin-7 (TRPM7) and TRP vanilloid 3 (TRPV3), both members of the TRP superfamily of channels (Wu et al., 2010). We found that eggs from double knockout females (dKOs) had lower levels of labile Zn²⁺ (Fig. 4F), and the addition of Tg revealed an expanded Ca²⁺ store content in these eggs vs. control WT eggs (Fig. 4G). Remarkably, in dKO eggs, the Ca²⁺ rise induced by Tg showed a shoulder or inflection point before the peak delaying the time to peak (Fig. 4G, inset; P <0.001). These results in dKO eggs show a changed dynamic of the Tg-induced Ca²⁺ release, suggesting that lower levels of labile Zn²⁺ modify ER Ca²⁺ release independently of chelators.

Ca²⁺ oscillations in eggs occur within a window of Zn²⁺ concentrations

We next examined if resupplying Zn²⁺ could restart the Ca²⁺ oscillations terminated by Zn²⁺ depletion. Zn pyrithione (ZnPT) rapidly increases cellular Zn²⁺ upon extracellular addition (Barnett et al., 1977; Robinson, 1964). Dose titration studies and imaging fluorimetry revealed that 0.01 µM ZnPT caused subtle and protracted increases in Zn²⁺ levels, whereas 0.1 µM ZnPT caused rapid increases in eggs’ Zn²⁺ baseline (Fig. 5A). We induced detectable Ca²⁺ oscillations by injection of mPlcζ mRNA followed by 50 µM TPEN (Fig. 5B), which terminated them. After 30 min, we added 0.1 µM ZnPT, and within 15 min the oscillations restarted in most TPEN-treated eggs (Fig. 5C). We repeated this approach using Thimerosal (Fig. 5D, E). Adding 0.1 µM ZnPT did not restore the Ca²⁺ oscillations retrained by TPEN, but 0.5 µM ZnPT did so (Fig. 5E). These results demonstrate that Zn²⁺ plays a pivotal, enabling role in the generation of Ca²⁺ oscillations in mouse eggs.

Figure 5.

Excessive intracellular Zn²⁺ inhibits Ca²⁺ oscillations

Zn²⁺ is necessary for diverse cellular functions, consistent with numerous amino acids and proteins capable of binding Zn²⁺ within specific and physiological ranges (Pace and Weerapana, 2014). Excessive Zn²⁺, however, can cause detrimental effects on cells and organisms (Broun et al., 1990; Hara et al., 2022; Sikora and Ouagazzal, 2021). Consistent with the deleterious effects of Zn²⁺, a previous study showed that high concentrations of ZnPT, ∼50 µM, prevented SrCl₂-induced egg activation and initiation of development (Bernhardt et al., 2012; Kim et al., 2011). We examined how ZnPT and excessive Zn²⁺ levels influence Ca²⁺ oscillations. Our conditions revealed that pre-incubation or continuous exposure to 0.1 µM or 1.0 µM ZnPT delayed or prevented egg activation induced by mPlcζ mRNA injection (Supplementary Fig. 3). We used these ZnPT concentrations to add it into ongoing oscillations induced by ICSI and monitored the succeeding Ca²⁺ responses. The addition of 0.05 to 10 µM ZnPT caused an immediate elevation of the basal levels of Fura-2 and termination of the Ca²⁺ oscillations (Fig. 6A-D). mPlcζ mRNA-initiated Ca²⁺ responses were also interrupted by adding 0.1 µM ZnPT, whereas untreated eggs continued oscillating (Fig. 6E, F). ZnPT also inhibited IP₃R1-mediated Ca²⁺ release triggered by cIP₃, suggesting that excessive Zn²⁺ directly inhibits IP₃R1 function (Fig. 6G).

Figure 6.

A noticeable feature of ZnPT is the increased basal ratios of Fura-2 AM. These changes could reflect enhanced IP₃R1 function and increased basal Ca²⁺ concentrations caused by Zn²⁺ stimulation of IP₃R1. This seems unlikely, however, because extended elevated cytosolic Ca²⁺ would probably induce cellular responses, such as the release of the second polar body, egg fragmentation, or cell death, neither of which happened. It might reflect, instead, Fura-2’s ability to report changes in Zn²⁺ levels, which seemed the case because the addition of TPEN lowered fluorescence without restarting the Ca²⁺ oscillations (Fig. 6F). To ensure the impact of ZnPT abolishing Ca²⁺ oscillations was not an imaging artifact obscuring ongoing rises, we simultaneously monitored cytoplasmic and ER Ca²⁺ levels with Rhod-2 and D1ER, respectively. This approach allowed synchronously observing Ca²⁺ changes in both compartments that should unfold in opposite directions. In control, uninjected eggs, the fluorescent values for both reporters remained unchanged during the monitoring period, whereas in mPlcζ mRNA-injected eggs, the reporters’ signals displayed simultaneous but opposite changes, as expected (Fig. 6H, I). The addition of ZnPT in uninjected eggs rapidly increased Rhod-2 signals but not D1ER’s, which was also the case in oscillating eggs, as the addition of ZnPT did not immediately alter the dynamics of the ER’s Ca²⁺ release, suggesting D1ER faithfully reports in Ca²⁺ changes but cannot detect changes in Zn²⁺ levels, at least to this extent; ZnPT progressively caused fewer and lower amplitude changes in D1ER fluorescence, consistent with the diminishing and eventual termination of the Ca²⁺ oscillations. Noteworthy, in these eggs, the basal D1ER fluorescent ratio remained unchanged after ZnPT, demonstrating its unresponsiveness to Zn²⁺ changes of this magnitude. The ZnPT-induced increases in Rhod-2 fluorescence without concomitant changes in D1ER values suggest that the changes in the dyes’ fluorescence do not represent an increase in basal Ca²⁺ and, more likely, signal an increase in intracellular Zn²⁺. We confirmed that both reporters were still in working order, as the addition of Io triggered Ca²⁺ changes detected by both reporters (Fig. 6H, I).

Discussion

The present study demonstrates that appropriate levels of labile Zn²⁺ are essential for initiating and maintaining IP₃R1-mediated Ca²⁺ oscillations in mouse eggs regardless of the initiating stimuli. Both deficient and excessive Zn²⁺ compromise IP₃R1 sensitivity, diminishing and mostly terminating Ca²⁺ oscillations. The results demonstrate that IP₃R1 and Zn²⁺ act in concert to modulate Ca²⁺ signals, revealing previously unexplored crosstalk between these ions at fertilization (Fig. 7).

Figure 7.

Zn²⁺ is an essential micronutrient for living organisms (Kaur et al., 2014) and is required for various cellular functions, such as proliferation, transcription, and metabolism (Lo et al., 2020; Maret and Li, 2009; Yamasaki et al., 2007). Studies using Zn²⁺ chelators have uncovered what appears to be a cell-specific, narrow window of Zn²⁺ concentrations needed for cellular proliferation and survival (Carraway and Dobner, 2012; Lo et al., 2020). Further, TPEN appeared especially harmful, and in a few cell lines, even low doses provoked oxidative stress, DNA fragmentation, and apoptosis (Mendivil-Perez et al., 2012). We show here that none of the Zn²⁺ chelators, permeable or impermeable, affected cell viability within our experimental observations, confirming findings from previous studies that employed high concentrations of TPEN to interrupt the Ca²⁺ oscillations (Lawrence et al., 1998) or inducing egg activation of mouse eggs (Suzuki et al., 2010b). Our data demonstrating that ∼2.5 µM is the threshold concentration of TPEN in eggs that first causes noticeable changes in basal Zn²⁺, as revealed by FluoZin, is consistent with the ∼2 to 5-µM Zn²⁺ concentrations in most culture media without serum supplementation (Lo et al., 2020), and with the ∼100 pM basal Zn²⁺ in cells (Qin et al., 2011). Lastly, the effects on Ca²⁺ release observed here with TPEN and other chelators were due to the chelation of Zn²⁺, as pretreatment with ZnSO₄ but not with equal or greater concentrations of MgCl₂ or CaCl₂ rescued the inhibition of the responses, which is consistent with results by others (Kim et al., 2010; Lawrence et al., 1998).

To identify how Zn²⁺ deficiency inhibits Ca²⁺ release in eggs, we induced Ca²⁺ oscillations using various stimuli and tested the effectiveness of membrane-permeable and impermeable chelators to abrogate them. Chelation of extracellular Zn²⁺ failed to terminate the Ca²⁺ responses, whereas membrane-permeable chelators did, pointing to intracellular labile Zn²⁺ levels as essential for Ca²⁺ release. All agonists used here were susceptible to inhibition by TPEN, whether their activities depended on IP₃ production or allosterically induced receptor function, although the effective TPEN concentrations varied across stimuli. Some agents, such as mPlcζ mRNA or thimerosal, required higher concentrations than SrCl₂, Ach, or cIP₃. The reason underlying the different agonists’ sensitivities to TPEN will require additional research, but the persistence of IP₃ production or change in IP₃R1 structure needed to induce channel gating might explain it. However, the universal abrogation of Ca²⁺ oscillations by TPEN supports the view drawn from cryo-EM-derived IP₃R1 models that signaling molecules can allosterically induce channel gating from different starting positions in the receptor by mechanically coupling the binding effect to the ion-conducting pore in the C-terminal end of IP₃R (Fan et al., 2015). The cytosolic C-terminal domain of each IP₃R1 subunit is alongside the IP₃-binding domain of another subunit and, therefore, well positioned to sense IP₃ binding and induce channel gating (Fan et al., 2015). Within each subunit, the LNK domain, which contains a Zn²⁺-finger motif (Fan et al., 2015), connects the opposite domains of the molecule. Although there are no reports regarding the regulation of IP₃R1 sensitivity by Zn²⁺, such evidence exists for RyRs (Woodier et al., 2015), which also display a conserved Zn²⁺-finger motif (des Georges et al., 2016). Lastly, mutations of the two Cys or two His residues of this motif, without exception, resulted in inhibition or inactivation of the IP₃R1 channel (Bhanumathy et al., 2012; Uchida et al., 2003). These results are consistent with the view that the C-terminal end of IP₃Rs plays a dominant role in channel gating (Bhanumathy et al., 2012; Uchida et al., 2003). We propose that TPEN inhibits Ca²⁺ oscillations in mouse eggs because chelating Zn²⁺ interferes with the function of the LNK domain and its Zn²⁺-finger motif proposed role on the mechanical coupling induced by agonist binding to the receptor that propagates to the pore-forming region and required to gate the channel’s ion-pore (Fan et al., 2022, 2015).

In support of this possibility, TPEN-induced Zn²⁺ deficient conditions altered the Ca²⁺-releasing kinetics in resting eggs or after fertilization. Tg increases intracellular Ca²⁺ by inhibiting the SERCA pump (Thastrup et al., 1990) and preventing the reuptake into the ER of the ebbing Ca²⁺ during the basal leak. Our previous studies showed that the downregulation of IP₃R1 diminishes the leak, suggesting it occurs through IP₃R1 (Wakai and Fissore, 2019). Consistent with this view, TPEN pre-treatment delayed the Ca²⁺ response induced by Tg, implying that Zn²⁺ deficiency hinders Ca²⁺ release through IP₃R1. An expected consequence would be increased Ca²⁺ content in the ER after Tg. Io that mobilizes Ca²⁺ independently of IP₃Rs (Toeplitz et al., 1979) induced enhanced responses in TPEN-treated eggs vs. controls, confirming the accumulation of Ca²⁺-ER in Zn²⁺ deficient conditions. We demonstrated that this accumulation is due to hindered emptying of the Ca²⁺ ER evoked by agonists in Zn²⁺-deficient environments, resulting in reduced cytosolic Ca²⁺ increases, as IP₃R1 is the pivotal intermediary channel between these compartments. Noteworthy, the initial phase of the Tg-induced Ca²⁺ release out of the ER did not appear modified by TPEN, as if it was mediated by a Zn²⁺-insensitive Ca²⁺ channel(s)/transporter, contrasting with the abrogation of Ach-induced ER emptying from the outset. Remarkably, independently of Zn²⁺ chelators, emptying of Ca²⁺ ER was modified in a genetic model of Zn²⁺-deficient oocytes lacking two TRP channels, confirming the impact of Zn²⁺ on Ca²⁺ release. It is worth noting that TPEN did not reduce but maintained or increased the mass of IP₃R1, which might result in the inhibition of Zn²⁺-dependent ubiquitin ligase Ubc7 by the Zn-deficient conditions (Webster et al., 2003). We cannot rule out that these conditions may undermine other conformational changes required to trigger IP₃R1 degradation, thereby favoring the accumulation of IP₃R1.

Despite accruing Zn²⁺ during oocyte maturation, fertilization witnesses a necessary Zn²⁺ release into the external milieu, known as “Zn²⁺ sparks” (Kim et al., 2011; Que et al., 2019, 2015). This release of Zn²⁺ is a conserved event in fertilization across species and is associated with several biological functions, including those related to fending off polyspermy (Kim et al., 2011; Que et al., 2019; Wozniak et al., 2020). The concomitant decrease in Zn²⁺ facilitates the resumption of the cell cycle and exit from the MII stage (Kim et al., 2011). Congruent with this observation, artificial manipulation that maintains high Zn²⁺ levels prevents egg activation (Kim et al., 2011), whereas lowering Zn²⁺ with chelators leads to egg activation without Ca²⁺ mobilization (Suzuki et al., 2010b). As posed by others, these results suggest that meiosis completion and the early stages of fertilization unfold within a narrow window of permissible Zn²⁺ (Kim et al., 2011, 2010). Here, we extend this concept and show that IP₃R1 function and the Ca²⁺ oscillations in mouse eggs require this optimal level of labile Zn²⁺ because the Ca²⁺ responses interrupted by TPEN-induced Zn²⁺-insufficiency are rescued by restoring Zn²⁺ levels with ZnPT. Furthermore, unopposed increases in Zn²⁺ by exposure to ZnPT abrogated fertilization-initiated Ca²⁺ oscillations and prevented the expected egg activation events. It is unclear how excess Zn²⁺ disturbs the function of IP₃R1. Nevertheless, IP₃R1s have multiple cysteines whose oxidation enhances the receptor sensitivity to IP₃ (Joseph et al., 2018), and it is possible that excessive Zn²⁺ aberrantly modifies them, disturbing IP₃R1 structure and function or, alternatively, preventing their oxidation and sensitization of the receptor. Lastly, we cannot rule out that high Zn²⁺ levels directly inhibit the receptor’s channel. These results reveal a close association between the Zn²⁺ levels controlling meiotic transitions and the Ca²⁺ release necessary for egg activation, placing the IP₃R1 at the center of the crosstalk of these two divalent cations.

Abrupt Zn²⁺ changes have emerged as critical signals for meiotic and mitotic transitions in oocytes, eggs, embryos, and somatic cells (Kim et al., 2011, 2010; Lo et al., 2020). Fertilization relies on prototypical Ca²⁺ rises and oscillations, and Zn²⁺ sparks are an egg activation event downstream of this Ca²⁺ release, establishing a functional association between these two divalent cations that continues to grow (Kim et al., 2011). Here, we show that, in addition, these cations actively crosstalk during fertilization and that the fertilization-induced Ca²⁺ oscillations rely on optimized IP₃R1 function underpinned by ideal Zn²⁺ levels set during oocyte maturation. Future studies should explore if artificial alteration of Zn²⁺ levels can extend the fertile lifespan of eggs, improve developmental competence, or develop methods of non-hormonal contraception.

Materials and Methods

Key resources table

N,N,N′,N′-tetrakis (2-pyridinylmethyl)-1,2-ethylenediamine (TPEN) and Zinc pyrithione (ZnPT) were dissolved in dimethyl sulfoxide (DMSO) at 10 mM and stored at -20°C until use. SrCl₂, CaCl₂, ZnSO₄, and MgCl₂ were freshly dissolved with double-sterile water at 1M and diluted with the monitoring media just before use. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) were reconstituted with double-sterile water at 0.5M and 10 mM, respectively, and the pH was adjusted to 8.0. Tris(2-pyridylmethyl) amine (TPA) was diluted in DMSO at 100 mM and stored at -20°C until use. Acetylcholine chloride and Thimerosal were dissolved in double-sterile water at 550 mM and 100 mM, respectively. Acetylcholine was stored at -20°C until use, whereas Thimerosal was made fresh in each experiment.

Mice

The University of Massachusetts Institutional Animal Care and Use Committee (IACUC) approved all animal experiments and protocols. Trpm7-floxed (Trpm7^fl/fl) Gdf9-Cre and Trpv3^−/− mice were bred at our facility. Trpm7^fl/fl mice were crossed with Trpv3^−/− to generate Trpm7^fl/fl; Trpv3^−/− mouse line. Female Trpm7^fl/fl; Trpv3^−/− mice were crossed with Trpm7^fl/fl; Trpv3^−/−; Gdf9-cre male to generate females null for Trpv3 and with oocyte-specific deletion for Trpm7. Ear clips from offspring were collected prior to weaning, and confirmation of genotype was performed after most experiments.

Egg Collection

All gamete handling procedures are as previously reported by us (Wakai and Fissore, 2019). MII eggs were collected from the ampulla of 6- to 8-week-old female mice. Females were superovulated via intraperitoneal injections of 5 IU pregnant mare serum gonadotropin (PMSG, Sigma, St. Louis, MO) and 5 IU human chorionic gonadotropin (hCG, sigma) at 48hr. interval. Cumulus-oocyte-complexes (COCs) were obtained 13.5 hr. post-hCG injection by tearing the ampulla using forceps and needles in TL-HEPES medium. COCs were treated with 0.26% (w/v) of hyaluronidase at room temperature (RT) for 5 min to remove cumulus cells.

Intracytoplasmic sperm injection (ICSI)

ICSI was performed as previously reported by us (Kurokawa and Fissore, 2003) using described setup and micromanipulators (Narishige, Japan). Sperm from C57BL/6 or CD1 male mice (7-12 weeks old) were collected from the cauda epididymis in TL-HEPES medium, washed several times, heads separated from tails by sonication (XL2020; Heat Systems Inc., USA) for 5 s at 4°C. The sperm lysate was washed in TL-HEPES and diluted with 12% polyvinylpyrrolidone (PVP, MW = 360 kDa) to a final PVP concentration of 6%. A piezo micropipette-driving unit was used to deliver the sperm into the ooplasm (Primetech, Ibaraki, Japan); a few piezo-pulses were applied to puncture the eggs’ plasma membrane following penetration of the zona pellucida. After ICSI, eggs were either used for Ca²⁺ monitoring or cultured in KSOM to evaluate activation and development at 36.5°C in a humidified atmosphere containing 5% CO₂.

Preparation and microinjection of mRNA

pcDNA6-mPlcζ-mEGFP, pcDNA6-CALR-D1ER-KDEL, and pcDNA6-humanERp44-HA were linearized with the restriction enzyme PmeI and in vitro transcribed using the T7 mMESSAGE mMACHINE Kit following procedures previously used in our laboratory (Ardestani et al., 2020). A poly(A) tail was added to the in vitro synthesized RNA (mRNA) using Tailing Kit followed by quantification and dilution to 0.5 μg/μL in nuclease-free water and stored at -80°C until use. Before microinjection, mPlcζ, D1ER, and ERp44 mRNA were diluted to 0.01, 1.0, and 0.5 μg/μL, respectively, in nuclease-free water, heated at 95°C for 3 min followed by centrifugation at 13400×g for 10 min at 4°C. Cytoplasm injection of mRNA was performed under microscopy equipped with micromanipulators (Narishige, Japan). The zona pellucida and the plasma membrane of MII eggs were penetrated by applying small pulses generated by the piezo micromanipulator (Primetech, Ibaraki, Japan). The preparation of the injection pipette was as for ICSI (Kurokawa and Fissore, 2003), but the diameter of the tip was ∼1 μm.

Ca²⁺ and Zn²⁺ imaging

Before Ca²⁺ imaging, eggs were incubated in TL-HEPES containing 1.25 μM Fura2-AM, 1.25 μM FluoZin3-AM, or 2.2 μM Rhod2-AM and 0.02% Pluronic acid for 20 min at room temperature and then washed. The fluorescent probe-loaded eggs were allowed to attach to the bottom of the glass dish (Mat-Tek Corp., Ashland, MA). Eggs were monitored simultaneously using an inverted microscope (Nikon, Melville, NY) outfitted for fluorescence measurements. Fura-2 AM, FluoZin3-AM, and Rhod2-AM fluorescence were excited with 340 nm and 380 nm, 480 nm, and 550 nm wavelengths, respectively, every 20 sec, for different intervals according to the experimental design and as previously performed in the laboratory. The illumination was provided by a 75-W Xenon arc lamp and controlled by a filter wheel (Ludl Electronic Products Ltd., Hawthorne, NY). The emitted light above 510 nm was collected by a cooled Photometrics SenSys CCD camera (Roper Scientific, Tucson, AZ). Nikon Element software coordinated the filter wheel and data acquisition. The acquired data were saved and analyzed using Microsoft Excel and GraphPad using Prism software (Ardestani et al., 2020). For Figures 1A, 4A-C, 5A, and 6H-I, values obtained from FluoZin3-AM, Fura2-AM, or Rhod2-AM recordings were divided by the average of the first five recordings for each treatment that was used as the F₀.

To estimate relative changes in Ca²⁺-ER, emission ratio imaging of the D1ER (YFP/CFP) was performed using a CFP excitation filter, dichroic beamsplitter, CFP and YFP emission filters (Chroma technology, Rockingham, VT; ET436/20X, 89007bs, ET480/40m, and ET535/30m). To measure Ca²⁺-ER and cytosolic Ca²⁺ simultaneously, eggs that had been injected with D1ER were loaded with Rhod-2AM, and CFP, YFP, and Rhod-2 intensities were collected every 20 sec.

Caged IP₃

Caged-IP₃/PM (cIP₃) was reconstituted in DMSO and stored at -20°C until use. Before injection, cIP₃ stock was diluted to 0.25 mM with water and microinjected as above. After incubation in KSOM media at 37°C for 1-hr., the injected eggs were loaded with the fluorophore, 1.25 μM Fluo4-AM, and 0.02% Pluronic acid and handled as above for Fura-2 AM. The release of cIP₃ was accomplished by photolysis using 0.5 to 5-sec pulses at 360 nm wavelengths. Ca²⁺ imaging was as above, but Fluo4 was excited at 488 nm wavelength and emitted light above 510 nm collected as above.

Western blot analysis

Cell lysates from 20-50 mouse eggs were prepared by adding 2X-Laemmli sample buffer. Proteins were separated on 5% SDS-PAGE gels and transferred to PVDF membranes (Millipore, Bedford, MA). After blocking with 5% fat-free milk + TBS, membranes were probed with the rabbit polyclonal antibody specific to IP₃R1 (1:1000; a generous gift from Dr. Jan Parys, Katholieke Universiteit, Leuven, Belgium; Parys et al., 1995). Goat anti-rabbit antibody conjugated to horseradish peroxidase (HRP) was used as a secondary antibody (1:5000; Goat anti-Rabbit IgG (H+L) Cross-Adsorbed Secondary Antibody, HRP; Invitrogen, Waltham, Ma). For detection of chemiluminescence, membranes were developed using ECL Prime (Sigma) and exposed for 1–3 min to maximum sensitivity film (VWR, Radnor, PA). Broad-range pre-stained SDS–PAGE molecular weight markers (Bio-Rad, Hercules, CA) were run in parallel to estimate the molecular weight of the immunoreactive bands. The same membranes were stripped at 50°C for 30 min (62.5 mM Tris, 2% SDS, and 100 mM 2-beta mercaptoethanol) and re-probed with anti-α-tubulin monoclonal antibody (1:1000).

Immunostaining and confocal microscopy

Immunostaining was performed according to our previous study (Akizawa et al., 2021). After incubation with or without TPEN, MII eggs were fixed with 4% (w/v) paraformaldehyde in house-made phosphate-buffered saline (PBS) for 20 min at room temperature and then permeabilized for 60 min with 0.2% (v/v) Triton X-100 in PBS. Next, the eggs were blocked for 45 min with a blocking buffer containing 0.2% (w/v) skim milk, 2% (v/v) fetal bovine serum, 1% (w/v) bovine serum albumin, 0.1% (v/v) TritonX-100, 0.75% (w/v) glycine in PBS. Eggs were incubated overnight at 4°C with mouse anti-HA antibody (1:200) diluted in blocking buffer. Eggs were washed in blocking buffer 3X for 10 min, followed by incubation at room temperature for 30 min with a secondary antibody, Alexa Fluor 488 goat anti-mouse IgG (H + L) (1:400) diluted in blocking buffer. Fluorescence signals were visualized using a laser-scanning confocal microscope (Nikon A1 Resonant Confocal with six-color TIRF) fitted with a 63×, 1.4 NA oil-immersion objective lens.

Statistical analysis

Comparisons for statistical significance of experimental values between treatments and experiments were performed in three or more experiments performed on different batches of eggs in most studies. Given the number of eggs needed, WB studies were repeated twice. Prism-GraphPad software was used to perform the statistical comparisons that include unpaired Student’s t-tests, Fisher’s exact test, and One-way ANOVA followed by Tukey’s multiple comparisons, as applicable, and the production of graphs to display the data. All data are presented as mean±s.d. Differences were considered significant at P < 0.05.

Acknowledgements

We thank Ms. Changli He for technical support and Dr. James Chambers for support with confocal microscopy support. We thank all members of the Fissore lab for useful discussions and suggestions. We thank Jan B. Parys, K.U. Leuven, Belgium, for initial discussions and advice.

Competing interests

The authors declare no competing or financial interests.

Additional information

Funding Sources

Author contributions

Hiroki Akizawa, Data curation, Formal analysis, Validation, Investigation, Visualization, Writing—original draft, Writing—review and editing; Emily Lopes, Data curation, Formal analysis, Validation; Rafael A Fissore, Conceptualization, Formal analysis, Supervision, Funding acquisition, Methodology, Writing—original draft, Project administration, Writing—review and editing

Supplementary Figure Legends

Supplementary Figure 1.

Supplementary Figure 2.

Supplementary Figure 3.