Gap junction intercellular communication GJIC facilitates th

Gap junction intercellular communication (GJIC) facilitates the exchange of ions, metabolites, Ca2+, inositol phosphates, and/or cyclic nucleotides of up to 1.8 kD in size between HG-10-102-01 through contact-dependent mechanisms [3,4]. Oocyte growth and development (as depicted in Fig. 1) depends, at least partly, upon a supply of nutrients, amino acids, glucose metabolites, and nucleotides transmitted from follicle cells via gap junctions (GJ). Interconnection of ovarian cells via GJ is observed between the innermost layer of cumulus cells and the oocyte, between adjacent cumulus cells, between granulosa cells and also between cumulus and granulosa cells [5]. Gap junctions are made up of six connexin (CX) proteins to form a connexon, a hollow ring in the plasma membrane that enables communication between the cells when coupled with another connexon in an adjacent cell, with the capacity to change function based on protein isoform or post-translational modification, though this mechanism remains vague [6,7]. Localization of CX proteins is used for GJ identification in a variety of tissues [8] and the CX family of proteins is very diverse, with 20 proteins in mice and 21 in humans, each the product of a distinct gene [9]. A total of 8 CX proteins are known to be expressed in the ovary, with expression varying in a species-specific manner. The ovary of the pig expresses GJB2, GJB4, GJB1, GJA1, and GJA10 [10,11]. In sheep, ovarian GJB2, GJB1, GJA4, and GJA1 have been detected [[12], [13], [14]]. Similar to the sheep, the ovary of the cow expresses GJB2, GJB1, GJA4, and GJA1 [15,16]. In mouse and rat, GJB1, GJA4, GJA1, GJC1, and GJA10 have been detected in the ovary [17]. Pannexin genes have also been implicated in GJ channeling, but less is known about their functions [18]. Pannexin1 (Panx1) has been identified in human ovary and placenta, but no further work has been performed to describe Panx1 function in the female reproductive system [19]. Gap junction proteins are subject to hormonal regulation in various tissue types. Both E2 and P4 regulate GJIC in the reproductive system, heart, brain, and liver via complementary or opposing actions dependent on physiological context and tissue phenotype [20]. An increase in Gja1 mRNA and GJ formation is stimulated by E2 while, in contrast, an inhibitory effect is mediated by P4 in the female reproductive system [21,22]. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) also have similar effects on GJIC in reproductive tissues, where FSH stimulates upregulation of Gja1 mRNA and protein, and Gja1 expression is elevated as follicle size increases in response to FSH [[23], [24], [25], [26]]. In contrast, LH stimulates a reduction in Gja1 mRNA and protein [[27], [28], [29], [30]]. Alterations of CX expression are also detected throughout the stages of the estrous cycle. In sheep, GJB2 mRNA expression in the corpus luteum (CL) is at its highest at d10 of the estrous cycle and decreases in PGF-induced luteal regression [13]. In cows, GJB2 mRNA is highest during the second half of the estrous cycle and after luteal regression [31]. GJB1 mRNA and protein expression remains relatively stable throughout the estrous cycle [13,14]. In contrast, GJA4 mRNA expression is increased after hCG treatment in sheep, and expression of GJA4 protein and mRNA is greatest on d5 of the estrous cycle with a gradual decrease thereafter [12]. GJA1 mRNA expression in sheep decreases in the granulosa and theca cells after hCG treatment and in the luteal tissue, but is observed to increase in the CL on d5 of the estrous cycle [13,14]. The expression of GJA1 mRNA decreases after injection of GnRH and after luteal regression in the ovary of the cow, with higher GJA1 levels observed in the CL during the early luteal phase [31]. In addition to changes during the estrous cycle, high levels of GJB2 and GJA1 mRNA are detected throughout pregnancy in cows [31]. Ovarian GJ investigations have primarily focused on defining the function and role of ovarian GJA4 and GJA1. The generation of a Gja4-null mouse demonstrated arrest of folliculogenesis at the early antral follicular stage and oocytes that do not reach meiotic competence [32,33]. Interestingly, a genetic variant of Gja4 is associated with primary ovarian insufficiency in women [34] while another Gja4 gene variant is associated with polycystic ovarian syndrome in women [35]. Deficiency of Gja1 is postnatally lethal in mice, so Gja1−/− prenatal ovaries have been cultured ex vivo or via transplant, and these ovaries have retarded oocyte growth and arrested folliculogenesis [36,37]. Further supporting the roles of ovarian GJA4 and GJA1 are chimeric ovary studies which paired wild-type (WT) oocytes with Gja1-deficient somatic cells; Gja1-deficient oocytes with WT granulosa cells; WT oocyte with Gja4-deficient granulosa cells; or Gja4-deficient oocytes with WT granulosa cells [38,39]. In ovaries containing WT granulosa cells with Gja1-/- oocytes or WT oocytes and Gja4−/− granulosa cells, meiosis occurred and fertilization could be achieved. In contrast, ovaries with WT granulosa cells and Gja4-/- oocytes could not proceed with meiosis and did not achieve fertilization. In ovaries with Gja1-/- granulosa cells and WT oocytes, follicles remained in the early preantral stages and contained smaller oocytes [38]. Thus, both oocyte and granulosa cell GJIC is important for follicular development.