Heterologous Gap Junctions between Granulosa Cells and Oocytes in Ayu (Plecoglossus altivelis): Formation and Role during Luteinizing Hormone-dependent Acquisition of Oocyte Maturational Competence

Abstract

Abstract. Gap junctions (GJs) are intercellular channels. Molecules with a molecular weight of 1 kDa or less can pass back and forth between adjacent cells through GJs. Communication between oocytes and the somatic cells that surround them via GJs is known to play key roles to initiate oocyte maturation in many vertebrates. However, little is known of the detailed functions of ovarian GJs during oocyte maturation in fish. The oocyte maturation of fish is induced by a maturation-inducing steroid (MIS). The sensitivity of oocytes to the MIS is known as oocyte maturational competence (OMC) and is induced by luteinizing hormone (LH). However, LH receptors are found on the surface of granulosa cells rather than oocytes. We therefore proposed that the LH signals received by granulosa cells were passed to oocytes via GJs. This review describes current knowledge of the role of GJs between granulosa cells and oocytes during the LH-induced acquisition of OMC in fish. ammalian oocytes are surrounded by cumulus cells, and their maturation is induced by luteinizing hormone (LH) stimulation. By contrast, fish oocytes are surrounded by a layer of granulosa cells, a basement membrane, and a further layer of theca cells (Fig. 1). LH stimulates these two layers to produce a maturation-inducing steroid (MIS). Oocyte maturation is induced by direct stimulation from the MIS. In most fish, the MIS is thought to be either 17,20β, [1].Sensitivity of oocyte to MIS is known to be crucial for oocyte maturation in many teleost species including our model fish ayu (Plecoglossus altivelis). This sensitivity, which is known as oocyte maturational competence (OMC), is induced by LH [1,2] (Fig. 2). LH therefore has two important functions in oocyte maturation: inducing the acquisition of OMC by stimulating the oocytes, and inducing MIS synthesis by the follicle cells. LH-induced acquisition of OMC and the subsequent MIS-induced oocyte maturation are separate steps both physiologically and mechanistically [1,2]. We found that incubating ayu post-vitellogenic follicle-enclosed oocytes in a culture medium containing MIS did not induce oocyte maturation [3]. However, pre-treating the follicle-enclosed oocytes with a culture medium containing human chorionic gonadotropin (hCG), which is known to have an LH-like function, caused them to respond to MIS and induced oocyte maturation [3]. We therefore proposed that the ayu follicle-enclosed oocytes acquired OMC via LH stimulation. However, the LH receptors are localized on the surface of the follicle cells, and are not present on the oocytes themselves [4]. Therefore, the mechanism by which LH induces the acquisition of OMC in cells without receptors has previously remained unclear.Our research has focused on the channels known as gap junctions (GJs) that are formed between granulosa cells and oocytes. Ions and second-messenger molecules can pass freely through GJs by means of diffusion [5] (Fig. 1). In the Atlantic croaker (Micropogonias undulatus) [6,7] and the red sea bream (Pagrus major) [8], an in...

New insights into molecular pathways associated with flatfish ovarian development and atresia revealed by transcriptional analysis

“…During this process, the germinal vesicle migrates towards the oocyte periphery, the nuclear envelope breaks down, the first meiotic division occurs, and the chromosomes proceed to second meiotic metaphase where they arrest; at this point, the oocyte will ovulate and becomes an egg [ 51 ]. Oocyte maturation is also accompanied by important changes in the cytoarchitecture and function of the ovarian follicle, since steroidogenic pathways in granulosa cells are switched from estrogen to progestagen production, intercellular communication oocyte-granulosa cells is resumed, and the zona radiata becomes more compacted [ 39 , 52 , 53 ]. At this stage, ovarian transcripts associated with the regulation of intracellular signalling pathways (such as mapk13 in follicular cells, and ywhae ), and cystoskeleton ( mybph , gsna , myo10 , LOC100005008 ), as well as in the assembly of the nucleosome ( hist1h2be , myst3 ), were significantly regulated in Senegalese sole, which is consistent with the important nuclear and cytoplasmic changes occurring in the ovarian follicle during oocyte maturation.…”

Maternal investment in fish oocytes and eggs: The molecular cargo and its contributions to fertility and early development

“…Both processes involve gonadotropins, and members of the Igf family. The increase in follicular gap junctions, composed of various forms of connexin proteins (York et al, 1993;Patiño and Kagawa, 1999;Bolamba et al, 2003), is a critical process in the acquisition of OMC (Yamamoto et al, 2008;Yamamoto and Yoshizaki, 2008;Yamamoto et al, 2011a). Both gonadotropins and Igf1 increase numbers of gap junctions (Yoshizaki et al, 1994;Chang et al, 1999Chang et al, , 2000Choi and Takashima, 2000;Choi and Takemura, 2001), and Lh, Fsh and Igf1 have connexin-specific and stage-specific effects on expression of connexins (Yamamoto et al, 2011a).…”

“…Membrane progestin receptors (mPRs: Zhu et al, 2003 andreview: Thomas, 2012) mediate the stimulatory effects of MIS on resumption of meiosis (Thomas et al, 2004;Tokumoto et al, 2006Tokumoto et al, , 2007Tokumoto et al, , 2012Hanna et al, 2006;Zhu, 2009, 2011), and their expression is upregulated during OM (Zhu et al, 2003;Tokumoto et al, 2006, Hanna andZhu, 2009;Tubbs et al, 2010). OMC can be induced by gonadotropins (Patiño and Sullivan, 2002;Patiño et al, 2003;Weber et al, 2007;Yamamoto and Yoshizaki, 2008), and gonadotropin-induced upregulation of the mPR early in OM is strongly associated with increased OMC (review: Thomas, 2012).…”

The spatiotemporal expression of multiple coho salmon ovarian connexin genes and their hormonal regulation in vitro during oogenesis

“…The temporal expression pattern and follicular localization of coho salmon cx43.2 showed a similar pattern to that of ayu cx34.9 [49]. In ayu, cx34.9 appears to contribute to formation of GJs between the oocyte and the granulosa cells, and may have an important role in transmitting the LH-signal received in the follicle cells to the oocyte via a second messenger such as cAMP during acquisition of OMC [14,15,49]. Although we have no empirical data on the function of coho salmon cx43.2 , it may have a similar role to ayu cx34.9 given the similarity of their spatiotemporal expression patterns.…”

“…Increases in ovarian GJs were also induced by insulin-like growth factor 1 (IGF1) treatment in red seabream [12]. Further, Yamamoto et al [14,15] found that culturing ovarian fragments with common GJ inhibitors prevented LH-induced acquisition of OMC in ayu, Plecoglossus altivelis , suggesting that ovarian GJ communication is essential for the LH-induced acquisition of OMC in this species. Thus, some ovarian GJs appear to be hormonally regulated and to have important roles during final maturation of the follicle in fishes.…”