Involvement of epidermal growth factor receptors and mitogen-activated protein kinase in progestin-induction of sperm hypermotility in Atlantic croaker through membrane progestin receptor-alpha

Study on epigenotoxicity, sex hormone synthesis, and DNA damage of benzo[a]pyrene in the testis of male Ruditapes philippinarum

Research on the mechanisms of reproductive toxicity caused by persistent organic pollutants (POPs) in marine animals has received significant attention. One group of typical POPs, called polycyclic aromatic hydrocarbons (PAHs), has been found to cause various reproductive toxicities in aquatic organisms, including epigenotoxicity, reproductive endocrine disruption, DNA damage effects and other reproductive toxicity, thereby affecting gonadal development. Interestingly, male aquatic animals are more susceptible to the disturbance and toxicity of environmental pollutants. However, current studies primarily focus on vertebrates, leaving a large gap in our understanding of the reproductive toxicity and mechanisms of PAHs interference in marine invertebrates. In this study, male Ruditapes philippinarum was used as an experimental subject to investigate reproduction-related indexes in clams under the stress of benzo[a]pyrene (B[a]P) at different concentrations (0, 0.8, 4 and 20 μg/L) during the proliferative, growth, maturity, and spawning period. We analyzed the molecular mechanisms of reproductive toxicity caused by PAHs in marine bivalves, specifically epigenotoxicity, reproductive endocrine disruption, and gonadal damage-apoptotic effect. The results suggest that DNA methylation plays a crucial role in mediating B[a]P-induced reproductive toxicity in male R. philippinarum. B[a]P may affect sex hormone levels, impede spermatogenesis and testis development in clams, by inhibiting the steroid hormone synthesis pathway and downregulating genes critical for cell proliferation, testis development, and spermatid expulsion. Moreover, the spermatids of male R. philippinarum were severely impaired under the B[a]P stress, leading to reduced reproductive performance in the clams. These findings contribute to a better understanding of the reproductive toxicity response of male marine invertebrates to POPs stress.

Membrane Progesterone Receptors (mPRs, PAQRs): Review of Structural and Signaling Characteristics

The role of membrane progesterone receptors (mPRs), which belong to the progestin and adipoQ receptor (PAQR) family, in mediating rapid, nongenomic (non-classical) progestogen actions has been extensively studied since their identification 20 years ago. Although the mPRs have been implicated in progestogen regulation of numerous reproductive and non-reproductive functions in vertebrates, several critical aspects of their structure and signaling functions have been unresolved until recently and remain the subject of considerable debate. This paper briefly reviews recent developments in our understanding of the structure and functional characteristics of mPRs. The proposed membrane topology of mPRα, the structure of its ligand-binding site, and the binding affinities of steroids were predicted from homology modeling based on the structures of other PAQRs, adiponectin receptors, and confirmed by mutational analysis and ligand-binding assays. Extensive data demonstrating that mPR-dependent progestogen regulation of intracellular signaling through mPRs is mediated by activation of G proteins are reviewed. Close association of mPRα with progesterone membrane receptor component 1 (PGRMC1), its role as an adaptor protein to mediate cell-surface expression of mPRα and mPRα-dependent progestogen signaling has been demonstrated in several vertebrate models. In addition, evidence is presented that mPRs can regulate the activity of other hormone receptors.

The Interface of Nuclear and Membrane Steroid Signaling

Steroid hormones bind receptors in the cell nucleus and in the cell membrane. The most widely studied class of steroid hormone receptors are the nuclear receptors, named for their function as ligand-dependent transcription factors in the cell nucleus. Nuclear receptors, such as estrogen receptor alpha, can also be anchored to the plasma membrane, where they respond to steroids by activating signaling pathways independent of their function as transcription factors. Steroids can also bind integral membrane proteins, such as the G protein-coupled estrogen receptor. Membrane estrogen and progestin receptors have been cloned and characterized in vitro and influence the development and function of many organ systems. Membrane androgen receptors were cloned and characterized in vitro, but their function as androgen receptors in vivo is unresolved. We review the identity and function of membrane proteins that bind estrogens, progestins, and androgens. We discuss evidence that membrane glucocorticoid and mineralocorticoid receptors exist, and whether glucocorticoid and mineralocorticoid nuclear receptors act at the cell membrane. In many cases, integral membrane steroid receptors act independently of nuclear steroid receptors, even though they may share a ligand.

PAQR7: An intermediary mediating nongenomic progesterone action in female reproductive tissue

Progestin and adipoQ receptor 7 (PAQR7) as an indispensable member of membrane progestin receptors in the Progestin and adipoQ receptor (PAQR) family that mediates nongenomic progesterone actions, initiated rapidly at the cell surface. Previous research demonstrated the distribution of PAQR7, which was mainly expressed in reproductive tissues, including ovary and testis. In the male reproductive system, PAQR7 is involved in progestin-induced sperm hypermotility. However, reports studying PAQR7 in female reproductive tissue mainly concentrate on oocyte maturation in fish, its expression in the ovary and gestational tissue, and regulation of uterine functions in mammals. Despite recent advances, many aspects of progestin signaling through PAQR7 are still unclear, especially in female reproductive tissue. Therefore, we reveal the structure and characteristics of PAQR7 and conclude the putative progestin-induced action mediated by PAQR7 in female reproductive tissue, such as the development of ovarian follicles, apoptosis of granulosa cells, oocyte maturation, and development of certain diseases, among others, to review the function of PAQR7 in the female reproductive system in detail.

Induction of sperm hypermotility through membrane progestin receptor alpha (mPRα): A teleost model of rapid, multifaceted, nongenomic progestin signaling

Rapid progestin effects on sperm physiology have been described in a variety of vertebrate species. Here, we briefly review the signaling pathways mediating rapid progestin induction of sperm hypermotility and increased fertility in two teleost species, Atlantic croaker and southern flounder. Acute in vitro treatment of teleost sperm with the progestin hormone, 20β-S, causes activation of progestin membrane receptor alpha (mPRα, or Paqr7) coupled to a stimulatory olfactory G protein (G_olf), resulting in increased cAMP and calcium concentrations and hypermotility upon activation in a hyperosmotic medium. Pharmacological tools were used to investigate the involvement of mPRα and several intracellular signaling pathways in the hypermotility response. Evidence was obtained using the specific mPRα agonist, Org OD 02-0, that this progestin action is mediated through mPRα and not through the nuclear PR. The results indicate that progestins induce hypermotility through activation of a membrane adenylyl cyclase (Acy)/cAMP pathway, an epidermal growth factor receptor (Egfr)/Mapkinase pathway, and a Pi3kinase/Akt/phosphodiesterase (Pde) pathway which result in increased sperm calcium concentrations within 10 s. The finding that inhibition of any one of these pathways is sufficient to prevent hypermotility along with the calcium increase suggests that activation of all of them and the associated calcium increase are required for the progestin hypermotility response. On the basis of these findings a model of progestin induction of sperm hypermotility in teleosts is proposed. As teleosts lack CatSper, the model described here is a non-CatSper mediated one and may therefore be applicable to a wide variety of nonmammalian vertebrates.

Sperm motility in fishes: (III) diversity of regulatory signals from membrane to the axoneme

Fish spermatozoa acquire potential for motility in the sperm duct where they are immotile. Osmolality of the seminal plasma is a key factor to maintain spermatozoa in the quiescent state in either freshwater or marine fishes. However, potassium (K⁺) ions prevent spermatozoa motility in salmonid and sturgeon fishes, while CO₂ inhibits spermatozoa motility in flatfishes. Once, spermatozoa are released at spawning, their motility is initiated in hypo-osmotic and hyper-osmotic environments in freshwater and marine fishes, respectively. Some substances produced by the testes (a progestin), or released from oocytes (peptides) induce spermatozoa hypermotility in some marine fishes including the Atlantic croaker and Pacific herrings, respectively. Duration of spermatozoa motility is short, lasting for a few seconds to few minutes in most fishes due to rapid depletion of energy required for the beating of the motility apparatus called axoneme. In the osmotic-activated spermatozoa, K⁺ and water effluxes occur in freshwater and marine fishes, respectively, which trigger spermatozoa motility signaling. In general, initiation of axonemal beating is associated with an increase in intracellular calcium (Ca²⁺) ions in spermatozoa of both freshwater and marine fishes and a post- or pre-increase in intracellular pH, while cyclic adenosine monophosphate (cAMP) remains unchanged. However, axonemal beating is cAMP-dependent in demembranated spermatozoa of salmonid and sturgeon fishes. Calcium from extracellular environment or intracellular stores supply required Ca²⁺ concentration for axonemal beating. Several axonemal proteins have been so far identified in fishes that are activated by Ca²⁺ and cAMP, directly or mediated by protein kinase C and protein kinase A, respectively. The present study reviews differences and similarities in complex regulatory signals controlling spermatozoa motility initiation in fishes, and notes physiological mechanisms that await elucidation.

Roles of progesterone receptor membrane component 1 and membrane progestin receptor alpha in regulation of zebrafish oocyte maturation

Although previous studies suggest membrane progesterone receptor alpha (mPRα/Paqr7) mediates 17, 20β-dihydroxy-4-pregnen-3-one (DHP) induction of oocyte maturation (OM) in zebrafish, critical information needed to establish mPRα as the receptor mediating OM is lacking. The relative potencies of progestins and specific mPRα agonists in inducing OM matched their relative binding affinities for zebrafish mPRα, supporting its role in OM. Microinjection of pertussis toxin blocked DHP induction of OM and the progestin-induced decrease in cyclic AMP levels, suggesting mPRα activates an inhibitory G protein (Gi). Microinjection of morpholino antisense oligonucleotides to zebrafish pgrmc1 blocked induction of OM by DHP which was accompanied by decreased levels of Pgrmc1 and mPRα on the oocyte plasma membranes. Similarly, treatment of denuded oocytes with a PGRMC1 inhibitor, AG205, blocked the gonadotropin-induced increase in plasma membrane mPRα levels and attenuated DHP induction of OM. Co-incubation with two inhibitors of epidermal growth factor Erbb2, ErbB2 inhibitor II and AG 879, prevented induction of OM by DHP, indicating the likely involvement of Erbb2 in mPRα-mediated signaling. Treatment with AG205 reversed the inhibitory effects of the Erbb2 inhibitors on OM and also inhibited insulin-like growth factor-1 induction of OM. Close associations between Pgrmc1 and mPRα, and between Pgrmc1 and Erbb2 were detected in zebrafish oocytes with in situ proximity ligation assays. The results suggest progestin induction of OM in zebrafish is mediated through an mPRα/Gi/Erbb2 signaling pathway that requires Pgrmc1 for expression of mPRα on oocyte membranes and that Pgrmc1 also is required for induction of OM through Erbb2.

Progestin is important for testicular development of male turbot (Scophthalmus maximus) during the annual reproductive cycle through functionally distinct progestin receptors

In teleost, sex steroid hormones are critical for reproduction. Progestin is known to promote spermiation. To further understand the functions of progestin via its receptors during the annual reproductive cycle in male turbot (Scophthalmus maximus), we observed testicular development, quantified several sex steroid hormones, detected the expression of progestin receptors, and measured various sperm parameters. Results showed that the turbot testicular structure was of the lobular type. During breeding season, a number of spermatocytes (stage III) developed into spermatids (stage IV), then differentiated into sperm during spermiogenesis (stage V), and finally regressed to spermatocytes (stage VI). Concomitant with testicular development, serum progesterone (P4) and 17α,20β-dihydroxy-4-pregnen-3-one (DHP) exhibited higher levels from stage IV to V than other stages. Furthermore, males with higher motility sperm showed higher levels of P4 and DHP compared with fish with lower motility sperm. These results indicated that P4 and DHP might induce spermatogenesis due to seasonal changes. Concurrently, in testes, the nuclear progesterone receptor (pgr) was expressed throughout the reproductive cycle and its level peaked during spermiogenesis while expression of membrane progestin receptor alpha (mPRα) did not change significantly. However, in sperm, mPRα expression was higher than in testes and had a significant positive correlation with curvilinear velocities (VCL), sperm motility, and motility duration. In conclusion, progestin appears to exert a direct pgr-mediated effect on spermiogenesis and improve sperm motility characteristics depending on the abundance of mPRα protein in sperm during spermiation.

Membrane progesterone receptors in reproduction and cancer

Progesterone is a sexual steroid hormone that has a critical role in reproductive processes in males and females of several species, including humans. Furthermore, progesterone has been associated with pathological diseases such as breast, gynecological and brain cancer, regulating cell proliferation, apoptosis, and metastasis. In the past, progesterone actions were thought to be only mediated by its intracellular receptor (PR). However, recent evidence has demonstrated that membrane progesterone receptors (mPRs) mediate most of the non-classical progesterone actions. The role of the different mPRs subtypes in progesterone effects in reproduction and cancer is an emerging and exciting research area. Here we review studies to date regarding mPRs role in reproduction and cancer and discuss their functions and clinical relevance, suggesting mPRs as putative pharmacological targets and disease markers in cancer and diseases associated with reproduction.

Progestin increases the expression of gonadotropins in pituitaries of male zebrafish

Our previous study showed that the in vivo positive effects of 17α,20β-dihydroxy-4-pregnen-3-one (DHP), the major progestin in zebrafish, on early spermatogenesis was much stronger than the ex vivo ones, which may suggest an effect of DHP on the expression of gonadotropins. In our present study, we first observed that fshb and lhb mRNA levels in the pituitary of male adult zebrafish were greatly inhibited by 3 weeks exposure to 10nM estradiol (E2). However, an additional 24h 100nM DHP exposure not only reversed the E2-induced inhibition, but also significantly increased the expression of fshb and lhb mRNA. These stimulatory effects were also observed in male adult fish without E2 pretreatment, and a time course experiment showed that it took 24h for fshb and 12h for lhb to respond significantly. Because these stimulatory activities were partially antagonized by a nuclear progesterone receptor (Pgr) antagonist mifepristone, we generated a Pgr-knockout (pgr(-/-)) model using the TALEN technique. With and without DHP in vivo treatment, fshb and lhb mRNA levels of pgr(-/-) were significantly lower than those of pgr(+/+) Furthermore, ex vivo treatment of pituitary fragments of pgr(-/-) with DHP stimulated lhb, but not fshb mRNA expression. Results from double-colored fluorescent in situ hybridization showed that pgr mRNA was expressed only in fshb-expressing cells. Taken together, our results indicated that DHP participated in the regulation of neuroendocrine control of reproduction in male zebrafish, and exerted a Pgr-mediated direct stimulatory effect on fshb mRNA at pituitary level.