Description

Mature oocytes arrest in meiosis by the end of oogenesis and need to be activated in order to proceed to embryonic development. This egg activation process encompasses a series of events that transition the oocyte to a developing embryo, including meiosis resumption, maternal protein translation/modification, maternal mRNA processing, and cytoskeleton and eggshell changes (reviewed in Avilés-Pagán and Orr-Weaver, 2018; Horner and Wolfner, 2008a; Kashir et al., 2014; Krauchunas and Wolfner, 2013). The triggers of egg activation vary across species, from mechanical pressure in arthropods to the fertilizing sperm in nematodes, echinoderms and vertebrates (reviewed in Horner and Wolfner, 2008a). Despite differences in trigger, a rise of intracellular calcium levels preceding downstream events is found in most species studied to date (reviewed in Swann and Lai, 2016). In Drosophila egg activation, the intracellular calcium rise is triggered by mechanical cues, which can be pressure exerted by the oviduct or from oocyte swelling in vivo (Heifetz et al., 2001) or in vitro due to osmotic pressure from a hypotonic buffer (Horner and Wolfner, 2008b; Page and Orr-Weaver, 1997). This calcium rise takes the form of a transient calcium wave starting from the pole(s) and traversing the oocyte (Kaneuchi et al., 2015; York-Andersen et al., 2015). This calcium wave is initiated by influx of environmental calcium through Trpm channels in response to the mechanical trigger (Hu and Wolfner, 2019). Further propagation of the calcium wave requires release of internal calcium from stores through the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) calcium channel (Kaneuchi et al., 2015). It remains unclear how the initiation of calcium waves triggers the activation of IP₃R during this process.

Phospholipase Cs (PLCs) are membrane-associated enzymes that mediate the cleavage of phospholipids, specifically the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP₂) to produce diacylglycerol (DAG) and IP₃. This reaction is involved in multiple signal transduction pathways (reviewed in Kadamur and Ross, 2013). In mammalian egg activation, a sperm-delivered PLC (PLCζ) is responsible for activating IP₃R to start the initial calcium rise (Saunders et al., 2002).

Because the propagation of calcium waves in Drosophila egg activation also requires IP₃R, we hypothesized that this is also mediated by PLC and the IP₃ pathway. The Drosophila genome encodes three PLCs: No receptor potential A (norpA), Small wing (sl) and Phospholipase C at 21C (plc21C). According to the RNAseq database, all three are expressed in Drosophila ovaries (Leader et al., 2017). It is possible that one or more of the three PLCs is involved in transducing the initial calcium influx signal to the IP₃ pathway to allow the calcium wave to propagate. Some PLCs can directly bind to Ca²⁺ and get activated in response to calcium signals (reviewed in Katan, 1998). All three Drosophila PLCs contain EF hand domains, which can potentially bind Ca²⁺ and directly transduce the ionic signal to downstream pathways (Lewit-Bentley and Réty, 2000).

To determine the role of phospholipase C (PLC) in calcium wave propagation during Drosophila egg activation, we screened each of the PLCs. We started by examining the role of Plc21C. We examined the calcium wave phenotype in oocytes from germline-specific CRISPR/Cas9 plc21C knockout females. These females were offspring from nos-Cas9; matα-GAL4-VP16; UAS-GCaMP3 crossed to gRNA-plc21C (see Methods). We observed a significant decrease in calcium wave incidence in oocytes from plc21C germline knockout females compared to controls (Fig.1A). To confirm these results, we isolated a null allele of plc21C (plc21C¹) from the offspring of the germline-knockout females (see Methods). This mutation is homozygous viable. We crossed it into the nos-GCaMP6m transgenic background (Hu and Wolfner, 2019) to allow us to visualize calcium dynamics in the germline of plc21C¹ females. We examined oocytes dissected from plc21C¹ homozygous females during in vitro egg activation and again found a significant decrease in calcium wave incidence compared to heterozygous controls (Fig.1A and C). In the few homozygous plc21C¹ oocytes that did show calcium waves, we did not observe a significant difference in calcium wave propagation speed compared to controls (Fig.1B). Taken together, our data show that calcium waves during Drosophila egg activation require Plc21C.

Since there were still calcium waves in a minority of plc21C¹ mutant oocytes, we suspected that Plc21C might function redundantly with other molecule(s). We thus examined the role of the two other PLCs encoded by the Drosophila genome, NorpA and Sl. norpA has an available, viable, null allele norpA³⁶ (Riesgo-Escovar et al., 1995). We crossed it into it the nos-GCaMP3-attP2 transgenic background (Kaneuchi et al., 2015) to visualize calcium dynamics in the germline. We isolated mature oocytes from norpA³⁶; nos-GCaMP3-attP2 females and imaged them during in vitro egg activation. We observed that calcium wave incidence and propagation speed did not differ between the oocytes of homozygous norpA³⁶ mutants and heterozygous controls (Fig.1A-B). Next, we examined calcium waves in sl germline knockout oocytes during in vitro activation. Mutant oocytes did not differ from control oocytes in calcium wave incidence or propagation speed (Fig.1A-B). We also isolated a null allele of sl (sl¹²) from the offspring of sl germline-knockout females (see Methods) and attempted to visualize calcium waves in oocytes from sl¹² females. However, sl¹² appeared to have combinatorial lethality with the nos-GCaMP6m transgene, as we were unable to isolate homozygous sl¹²; nos-GCaMP6m flies. The fluorescence signal strength of heterozygous nos-GCaMP6m was too low for us to visualize calcium waves. Although oocytes from germline specific sl knockout females displayed normal calcium wave incidence and propagation speed, it is possible that this knockout did not efficiently cause biallelic null mutations in most oocytes to reveal the function of Sl. Thus, we were unable to determine a requirement of sl for calcium wave propagation.

The presence of calcium waves in a minority of plc21C null oocytes suggests that Plc21C functions redundantly with other PLCs such as Sl or with other calcium signal relaying mechanisms to facilitate calcium wave propagation. These redundant mechanisms require further investigation. It also remains to be elucidated how Plc21C is activated by the initial calcium influx, whether through direct binding of Ca²⁺ to Plc21C or through other signal relaying molecules. Finally, we note that lack of Plc21C activity did not lead to the presence of initiated but only partially-propagated calcium waves, as was seen for IP3R knockdowns (Kaneuchi et al., 2015). The complete absence of calcium waves seen in most plc21C null oocytes suggests that Plc21C activity is needed at the earliest stages of (or to initiate) wave propagation in response to the Trpm-mediated calcium influx.

In wing imaginal discs, Plc21C is required for the intercellular calcium waves that regulate wing development via the IP₃ pathway (Brodskiy et al., 2019). Thus, Plc21C and the IP₃ pathway mediate both intracellular calcium waves during egg activation and intercellular calcium waves during tissue development.

This study identified the connection between calcium wave initiation and propagation during Drosophila egg activation. Based on this and our previous studies (Hu and Wolfner, 2019; Kaneuchi et al., 2015), we propose the following model for the mechanism of the calcium wave during Drosophila egg activation: mechanical pressure activates Trpm channels located on the plasma membrane of mature oocytes, allowing influx of external calcium. These Ca²⁺ ions then directly or indirectly activate Plc21C (and possibly other signal-relaying molecules), which catalyzes the reaction producing IP₃. IP₃ then binds to and activates its receptor to release calcium from internal stores, facilitating propagation of the calcium wave (Fig.1D). Our demonstration of the use of PLC to relay egg activation triggering signals to intracellular calcium rises reveals an important conservation in egg activation mechanisms between Drosophila and mammals.