Managing the Oocyte Meiotic Arrest-Lessons from Frogs and Jellyfish

Abstract

During oocyte development, meiosis arrests in prophase of the first division for a remarkably prolonged period firstly during oocyte growth, and then when awaiting the appropriate hormonal signals for egg release. This prophase arrest is finally unlocked when locally produced maturation initiation hormones (MIHs) trigger entry into M-phase. Here, we assess the current knowledge of the successive cellular and molecular mechanisms responsible for keeping meiotic progression on hold. We focus on two model organisms, the amphibian Xenopus laevis, and the hydrozoan jellyfish Clytia hemisphaerica. Conserved mechanisms govern the initial meiotic programme of the oocyte prior to oocyte growth and also, much later, the onset of mitotic divisions, via activation of two key kinase systems: Cdk1-Cyclin B/Gwl (MPF) for M-phase activation and Mos-MAPkinase to orchestrate polar body formation and cytostatic (CSF) arrest. In contrast, maintenance of the prophase state of the fully-grown oocyte is assured by highly specific mechanisms, reflecting enormous variation between species in MIHs, MIH receptors and their immediate downstream signalling response. Convergence of multiple signalling pathway components to promote MPF activation in some oocytes, including Xenopus, is likely a heritage of the complex evolutionary history of spawning regulation, but also helps ensure a robust and reliable mechanism for gamete production.

Keywords: Clytia; Xenopus; maturation promoting factor (MPF); meiosis; meiotic maturation; oocyte; oogenesis.

Figure 1

Diverse signalling pathways initiated by maturation initiation hormones (MIHs) lead to a universal biochemical core, activating maturation promoting factor (MPF). Due to the diversity of molecular pathways controlling the resumption of meiosis, this scheme is not exhaustive but is based on a few examples among those studied. MIHs are released from somatic cells near the oocyte (follicle cells, ectoderm cells or others) in response to hormonal (luteinizing hormone (LH) in vertebrates or gonad-stimulating substance (GSS) in starfish) and environmental inputs (dark/light transition, temperature, sea water, etc.). MIH molecules have different identities between species, and the types of receptors mediating the MIH action are also diverse; see text for details. In amphibian and fish oocytes, steroids recruit canonical nuclear receptors unusually associated with membranes in oocytes, but also progestin-specific membrane receptors most probably acting through Gα_i and Gβγ. MIH receptor activation leads either to the downregulation of adenylate cyclase, a decrease in cAMP level and PKA activity in vertebrates, or the opposite regulation in various species of hydrozoans or nemerteans. In vertebrates, PKA substrates dephosphorylated following the drop in PKA activity include Arpp19 in Xenopus, Cdc25 in Xenopus and mouse, and Wee1B in mouse. PKA substrates remain to be identified in species where PKA activity increases. In starfish, the dissociation of Gα_i from Gβγ activates phosphoinositide 3-kinase (PI3K) which leads to serum- and glucocorticoid-regulated kinase (SGK) activation, independently of cAMP and PKA. In other species, cytoplasmic calcium release is critical in the pathway. In many vertebrate oocytes, the drop in PKA activity indirectly activates the synthesis of new proteins required for MPF activation, such as Cyclin B1 and, in Xenopus, the kinase Mos. In many other species, MPF is activated without the need for new synthesized proteins. The final step of MPF activation (orange box) is common to all animals. Cyclin B-Cdk1 is activated by Cdk1 dephosphorylation at T14 and Y15 due to the reverse of the balance of activities between its regulators, the phosphatase Cdc25 and the kinases Wee1/Myt1. This activation is accelerated by an auto-amplification loop. In parallel, the Cdk1 opposing enzyme, the PP2A phosphatase, is inhibited by Arpp19 phosphorylated by the kinase Greatwall (Gwl).

Figure 2

Overview of oocyte growth and maturation in Clytia and in Xenopus. (A) In Clytia adult jellyfish, oocytes develop throughout adult life in the four gonads (green), situated on each of the radial gastrovascular canals connected to the central feeding organ (blue). (B) Stage I, II and III growing oocytes are sandwiched between an outer ectoderm and the endodermal layer of the central gastric cavity, which directly provides nutrients for growth. (C) A dark–light transition each dawn causes MIH to be released from specialised cells of the gonad ectoderm. MIH acts on an oocyte GPCR leading to MPF activation, manifest as Germinal Vesicle Breakdown (GVBD), then completion of meiotic divisions MI and M2, with the emission of polar bodies PB1 and PB2, before spawning as a G1 unfertilised egg after about two hours. (D) In the adult Xenopus ovary (pink), the meiotic cycle of oocytes is also arrested at diplotene of the first meiotic prophase. (E) Growing oocytes of Stages I-VI are tightly surrounded by one layer of follicle cells and a theca (blood vessels, collagen, fibroblasts) (magenta). (F) Ovulation is triggered by Luteinising hormone (LH) from the pituitary, which causes follicle cells to release steroid hormones, including progesterone (Pg). These act on oocyte membrane receptors to initiate a series of events culminating with variable timing in GVBD and completion of the first meiotic division before arrest as an unfertilised egg at M2 after a total of around 5–7 h.

Figure 3

Oocyte growth in Xenopus laevis. (A) A portion of the ovary from a recently ovulated female. Oocytes at successive stages of growth are indicated by Roman numerals. Note the absence of fully-grown Stage VI oocytes (characterised by a completely unpigmented equatorial band), which were ovulated. Scale bar 0.5 mm. (B) Micrograph of a paraffin section of a Stage V oocyte, hematoxylin and eosin staining. The animal pole of the oocyte is at the top left of the micrograph. Chromosomes and small nucleoli condense at the center of the nucleus. Scale bar 14 μm. (C) Micrograph of a paraffin section of a Stage VI oocyte showing the condensed lampbrush chomosomes. Scale bar 1.8 μm. The images were reproduced with permission (license number 4818271464898) from J.N. Dumont, Journal of Morphology; published by John Wiley and Sons, 1972 [61].

Figure 4

Gonad development and oocyte growth in Clytia hemisphaerica. (A) Schematic of gonad development in 1-day jellyfish (newly released from a polyp specialised for budding called the gonozooid), one-week and two-week old jellyfish; (B) Confocal image through the gonad of one-week old jellyfish, three different z planes; cyan—Hoechst (DNA), red—phalloidin (actin). In the bottom section, strong phalloidin staining of a surrounding fold of bell muscle is visible (m); (C) Gonad of two-week-old jellyfish, single confocal z plane. Grey—Hoechst (DNA). (D) Maximum projection of fully-grown Stage III oocytes at different stages of chromatin compaction. The fifteen pairs of homologous chromosomes, linked by chiasmata, are dispersed throughout the nucleus, mostly adjacent to the nuclear envelope. Grey—Hoechst (DNA). Asterix indicates possible stem cells. Arrows indicate different meiotic stages in early oocytes. Abbreviations: ect—ectoderm, GC—gastric cavity, rc—radial canal, gr—growing oocyte, l/z—leptotene/zygotene stage oocyte, p—pachytene stage oocyte, d—diplotene stage oocyte, m—muscle. Scale bars all 20 µm.

Figure 5

Models for release of the Xenopus oocyte prophase block: probable cooperation between several receptors and downstream pathways. The constitutively active Gα_s-coupled GPR185 maintains high cAMP and PKA activity, ensuring the prophase arrest (red pathway—right). The prophase release (green pathway—left) is triggered by steroids, mainly progesterone, with the potential contribution of IGF-1 and its receptor (IGF-1R). Progesterone could interact with its canonical nuclear receptor (iPR) but also a plasma membrane-bound receptor (mPR). Upon binding to progesterone, these receptors could inhibit the GPR185 pathway, and/or independently inhibit adenylate cyclase by recruiting Gα_i or inhibiting Gα_S. They could also launch positive downstream signalling independently of cAMP and PKA. These cascades converge to the synthesis of new proteins required for activating MPF.