Transcription-dependent and transcription-independent functions of the classical progesterone receptor in Xenopus ovaries

Signal pathway of LH-induced expression of nuclear progestin receptor in vertebrate ovulation

Nuclear progestin receptor (PGR), which is induced in the follicles destined to undergo ovulation, is believed to be obligatory for rupture of the follicles during ovulation in vertebrates. Studies in some mammals and teleost medaka have revealed the outline of the central signaling pathway that leads to the PGR expression in the preovulatory follicles at ovulation. In this review, we summarize the current knowledge on what signaling mediators are involved in the LH-induced follicular expression of PGR at ovulation in these animals. LH-inducibility of follicular PGR expression is conserved. In both group of animals, activation of the LH receptor on the granulosa cell surface with LH commonly results in the increase of intracellular cAMP levels, while the downstream signaling cascades activated by high level of cAMP are totally different between mice and medaka. PGR is currently presumed to be induced via PKA/CREB-mediated transactivation and ERK1/2-dependent signaling in mice, but the receptor is induced via EPAC/RAP and AKT/CREB pathways in the teleost medaka. The differences and similarities in the signaling pathways for PGR expression between them is discussed from comparative and evolutionary aspects. We also discussed questions concerning PGR expression and its regulation needed to be investigated in future.

Chronic Exposure to Two Gestagens Differentially Alters Morphology and Gene Expression in Silurana tropicalis

Gestagens are active ingredients in human and veterinary drugs with progestogenic activity. Two gestagens-progesterone (P4), and the synthetic P4 analogue, melengestrol acetate (MGA)-are approved for use in beef cattle agriculture in North America. Both P4 and MGA have been measured in surface water receiving runoff from animal agricultural operations. This project aimed to assess the morphometric and molecular consequences of chronic exposures to P4, MGA, and their mixture during Western clawed frog metamorphosis. Chronic exposure (from embryo to metamorphosis) to MGA (1.7 µg/L) or P4 + MGA (0.22 µg/L P4 + 1.5 µg/L MGA) caused a considerable dysregulation of metamorphic timing, as evidenced by an inhibition of growth, narrower head, and lack of forelimb emergence in all animals. Molecular analysis revealed that chronic exposure to the mixture induced an additive upregulation of neurosteroid-related (GABA_A receptor subunit α6 (gabra6) and steroid 5-alpha reductase 1 (srd5α1) gene expression in brain tissue. Chronic P4 exposure (0.26 µg/L P4) induced a significant upregulation of the expression hypothalamic-pituitary-gonadal (HPG)-related genes (ipgr, erα) in the gonadal mesonephros complex (GMC). Our data suggest that exposure to P4, MGA, and their mixture induces multiple endocrine responses and adverse effects in larval Western clawed frogs. This study helps to better our understanding of the consequences of chronic gestagen exposure and suggests that the implications and risk of high gestagen use in beef cattle feeding operations may extend to the aquatic environment.

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

Managing the Oocyte Meiotic Arrest-Lessons from Frogs and Jellyfish

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.

In Vitro Reconstruction of Xenopus Oocyte Ovulation

Progesterone is widely used to induce maturation of isolated fully grown oocytes of the African clawed frog, Xenopus laevis. However, the hormone fails to release oocytes from the layer of surrounding follicle cells. Here, we report that maturation and follicle rupture can be recapitulated in vitro by treating isolated follicular oocytes with progesterone and low doses of the matrix metalloproteinase (MMP), collagenase, which are ineffective in the absence of the steroid. Using this in vitro ovulation model, we demonstrate that germinal vesicle breakdown (GVBD) and oocyte liberation from ovarian follicles occur synchronously during ovulation. Inhibition of the MAPK pathway in these experimental settings suppresses both GVBD and follicular rupture, whereas inhibition of MMP activity delays follicular rupture without affecting GVBD. These results highlight importance of MAPK and MMP activities in the ovulation process and provide the first evidence for their involvement in the release of oocytes from ovarian follicles in frogs. The in vitro ovulation model developed in our study can be employed for further dissection of ovulation.

In vitro steroid-induced meiosis in Rhinella arenarum oocytes: role of pre-MPF activation

In this work we showed the relationship between seasonal periods and the response of R. arenarum follicles and oocytes to different steroids. Using in vitro germinal vesicle breakdown (GVBD) assays, we demonstrated that P4 is the main steroid capable of inducing maturation in R. arenarum oocytes and follicles. In the second part of this work we showed that androgens can activate pre-maturation promoting factors (pre-MPFs) such as P4, by cytoplasm microinjection experiments. The results indicated that the steroids assayed induced oocyte and follicle maturation in a dose- and time-dependent manner. In oocytes, P4 was the most efficient steroid as a maturation inducer (EC50 of the reproductive period, 6 nM, EC50 of the non-reproductive period ≅ 30 nM). Androgens (DHEA, dehydroepiandrosterone; T, testosterone; and AD, androstenedione) were less efficient maturation inducers than P4 (EC50 reproductive period ≅ 50, 120 and 600 nM respectively). Similar results were obtained with intact follicles in both seasonal periods. Although the response of follicles to the different androgens was variable, in no case was it above the above the response induced by P4. Independently of the season, oocytes and follicles incubated in P4, P5 and T underwent GVBD after 6–10 h while oocytes and follicles incubated in DHEA and AD matured more slowly. Furthermore, we demonstrated that microinjection of mature cytoplasm from androgen-treated oocytes is sufficient to promote GVBD in immature recipient oocytes (DHEA, 57 ± 12%; AD, 60 ± 8%; T, 56 ± 13%). Thus, androgens such as DHEA, T and AD are as competent as P4 to activate pre-MPF.

The challenge presented by progestins in ecotoxicological research: a critical review

Around 20 progestins (also called gestagens, progestogens, or progestagens) are used today in assisting a range of medical conditions from endometrial cancer to uterine bleeding and as an important component of oral contraception. These progestins can bind to a wide range of receptors including progestin, estrogen, androgen, glucocorticoid, and mineralocorticoid receptor, as well as sex hormone and corticosteroid binding globulins. It appears that only five of these (four synthetic and one natural) progestins have so far been studied in sewage effluent and surface waters. Analysis has reported values as either nondetects or low nanograms per liter in rivers. Seven of the progestins have been examined for their effects on aquatic vertebrates (fish and frogs). The greatest concern is associated with levonorgestrel, norethisterone, and gestodene and their ability to reduce egg production in fish at levels of 0.8-1.0 ng/L. The lack of environmental measurements, and some of the contradictions in existing values, however, hampers our ability to make a risk assessment. Only a few nanograms per liter of ethynodiol diacetate and desogestrel in water would be needed for fish to receive a human therapeutic dose for these progestins according to modeled bioconcentration factors. But for the other synthetic progestins levels would need to reach tens or hundreds of nanograms per liter to achieve a therapeutic dose. Nevertheless, the wide range of compounds, diverse receptor targets, and the effect on fish reproduction at sub-nanogram-per-liter levels should prompt further research. The ability to impair female reproduction at very low concentrations makes the progestins arguably the most important pharmaceutical group of concern after ethinylestradiol.

Role of arachidonic acid cascade in Rhinella arenarum oocyte maturation

There are no studies that document the production of prostaglandins (PGs) or their role in Rhinella arenarum oocyte maturation. In this study, we analysed the effect of arachidonic acid (AA) and prostaglandins (PGs) on maturation, activation and pronuclear formation in R. arenarum oocytes. Our results demonstrated that AA was capable of inducing maturation in time-dependent and dose-dependent manner. Arachidonic acid-induced maturation was inhibited by indomethacin. PGs from AA hydrolysis, such as prostaglandin F2α (PGF2α) and, to a lesser extent, PGE2, induced meiosis resumption. Oocyte maturation in response to PGF2α was similar to that produced by progesterone (P4). Oocyte response to PGE1 was scarce. Rhinella arenarum oocyte PGF2α-induced maturation showed seasonal variation. From February to June, oocytes presented low sensitivity to PGF2α. In following periods, this response increased until a maximum was reached during October to January, a close temporal correlation with oocyte response to P4 being observed. The effect of PGF2α on maturation was verified by analysing the capacity of oocytes to activate and form pronuclei after being injected with homologous sperm. The cytological analysis of activated oocytes demonstrated the absence of cortical granules in oocytes, suggesting that PGF2α induces germinal vesicle breakdown (GVBD) and meiosis resumption up to metaphase II. In turn, oocytes matured by the action of PGF2α were able to form pronuclei after fertilization in a similar way to oocyte maturated by P4. In microinjection of mature cytoplasm experiments, the transformation of pre-maturation promoting factor (pre-MPF) to MPF was observed when oocytes were treated with PGF2α. In summary, our results illustrated the participation of the AA cascade and its metabolites in maturation, activation and pronuclei formation in R. arenarum.

Naturally occurring steroids in Xenopus oocyte during meiotic maturation. Unexpected presence and role of steroid sulfates

In the ovary, oocytes are surrounded by follicle cells and arrested in prophase of meiosis I. Although steroidogenic activity of follicle cells is involved in oogenesis regulation, clear qualitative and quantitative data about the steroid content of follicles are missing. We measured steroid levels of Xenopus oocytes and follicles by gas chromatography-mass spectrometry. We show that dehydroepiandrosterone sulfate is the main steroid present in oocytes. Lower levels of free steroids are also detected, e.g., androgens, whereas progesterone is almost undetectable. We propose that sulfatation is a protective mechanism against local variations of active steroids that could be deleterious for follicle-enclosed oocytes. Steroid levels were measured after LH stimulation, responsible for the release by follicle cells of a steroid signal triggering oocyte meiosis resumption. Oocyte levels of androgens rise slowly during meiosis re-entry whereas progesterone increases abruptly to micromolar concentration, therefore representing the main physiological mediator of meiosis resumption in Xenopus oocyte.

The synthetic progestogen, Levonorgestrel, but not natural progesterone, affects male mate calling behavior of Xenopus laevis

Worldwide, more than 100 million women use hormonal contraceptives, which act through progestogenic modes of action. These man-made hormones can enter the aquatic environment as they are excreted via feces and urine. Xeno-progestins are able to interfere with the endocrine system of female aquatic vertebrates impairing oogenesis and reproduction. However, data on progestogenic effects on reproductive behavior of male aquatic vertebrates are lacking. To evaluate whether progestins affect the mating behavior of male Xenopus laevis, we exposed male frogs to three environmentally relevant concentrations (10(-7) M, 10(-8) M and 10(-10) M) of the synthetic progestin Levonorgestrel (LNG) and the corresponding natural steroid progesterone (PRG), respectively. LNG at all exposure concentrations increased the proportions of advertisement calling, indicating a sexually aroused state of the males. Furthermore LNG at 10(-7) M decreased the relative proportions of rasping, a call type indicating a sexually unaroused state of the male. PRG, on the other hand, did not affect any of those parameters. Temporal and spectral features of the advertisement call itself were not affected by any of the two exposure treatments. Since LNG exhibits slight androgenic activity, the results suggest that LNG effects on male mate calling behavior of X. laevis are due to its moderate androgenic but not to its progestogenic activities. However, although males' sexual arousal seems to be enhanced by LNG, the adverse effects of LNG on female reproduction presumably outweigh these enhancing effects and LNG exposure nonetheless might result in reduced reproductive success of these animals.

Nongenomic steroid- and ceramide-induced maturation in amphibian oocytes involves functional caveolae-like microdomains associated with a cytoskeletal environment

Stimulation of full-grown amphibian oocytes with progesterone initiates a nontranscriptional signaling pathway that converges in the activation of Cdc2/cyclin B and reentry into meiosis. We observed that cholesterol depletion mediated by methyl-beta-cyclodextrin (MbetaCD) inhibited meiotic maturation, suggesting involvement of membrane rafts. In the present study, we further characterized caveolae-like membranes from Rhinella arenarum oocytes biochemically and functionally. The identification by mass spectrometry of a nonmuscle myosin heavy-chain associated with caveolar membranes showed evidence of direct involvement of the underlying cytoskeletal environment in the structure of oocyte rafts. Biophysical analysis using the fluorescent probe Laurdan revealed that MbetaCD-mediated cholesterol depletion affected membrane lipid order. In line with this finding, cholesterol removal also affected the localization of the raft marker lipid GM1. Results demonstrated that ceramide is an effective inducer of maturation that alters the distribution of the raft markers caveolin-1, SRC, and GM1, while progesterone seems not to affect membrane microdomain integrity. Cholesterol depletion had a greater effect on ceramide-induced maturation, thus suggesting that ceramide is an inducer more vulnerable to changes in the plasma membrane. MbetaCD treatment delayed tyrosine phosphorylation and MAPK activation in progesterone-induced maturation. Functional studies regarding tyrosine phosphorylation raise the possibility that the hormone receptor is located in the nonraft membrane in the absence of ligand and that it translocates to the caveola when it binds to progesterone. The presence of raft markers and the finding of signaling molecules from MAPK cascade functionally associated to oocyte light membranes suggest that this caveolae-rich fraction efficiently recreates, in part, maturation signaling.

Effects of RU486 on cyclooxygenase-2 gene expression, prostaglandin F2alpha synthesis and ovulation in Xenopus laevis

RU486 is a synthetic analog of progesterone and functions as a progesterone receptor antagonist. It binds to the progesterone receptor to prevent progesterone from occupying its receptor in many cellular systems. Early studies from our laboratory have shown that in Xenopus laevis ovarian follicles progesterone stimulates the expression of cyclooxygenase-2 (COX-2) gene which leads to a rapid increase in the production of prostaglandin F2alpha (PGF2alpha) and subsequent ovulation. In this study, we examined the effect of RU486 on the synthesis of COX-2 mRNA, production of PGF2alpha and ovulation in X. laevis. Ovarian tissue fragments were primed with human chorionic gonadotropin (hCG) and then incubated with progesterone (P4) alone or in the presence of varying concentrations of RU486 over a period of 12h. After the incubation ovulated oocytes were counted, COX-2 expression and synthesis of PGF2alpha were measured. Results demonstrated that RU486 attenuated the expression of COX-2 gene, reduced the synthesis of PGF2alpha, and inhibited ovulation in a dose-dependent manner. This finding suggests that progesterone receptor is an important regulator in the progesterone-cyclooxygenase-prostaglandin-mediated ovulation in amphibians.

Characterization and expression of the nuclear progestin receptor in zebrafish gonads and brain

The zebrafish nuclear progestin receptor (nPR; official symbol PGR) was identified and characterized to better understand its role in regulating reproduction in this well-established teleost model. A full-length cDNA was identified that encoded a 617-amino acid residue protein with high homology to PGRs in other vertebrates, and contained five domains characteristic of nuclear steroid receptors. In contrast to the multiplicity of steroid receptors often found in euteleosts and attributed to probable genome duplication, only a single locus encoding the full-length zebrafish pgr was identified. Cytosolic proteins from pgr-transfected cells showed a high affinity (K(d) = 2 nM), saturable, single-binding site specific for a native progestin in euteleosts, 4-pregnen-17,20 beta-diol-3-one (17,20 beta-DHP). Both 17,20 beta-DHP and progesterone were potent inducers of transcriptional activity in cells transiently transfected with pgr in a dual luciferase reporter assay, whereas androgens and estrogens had little potency. The pgr transcript and protein were abundant in the ovaries, testis, and brain and were scarce or undetectable in the intestine, muscle, and gills. Further analyses indicate that Pgr was expressed robustly in the preoptic region of the hypothalamus in the brain; proliferating spermatogonia and early spermatocytes in the testis; and in follicular cells and early-stage oocytes (stages I and II), with very low levels within maturationally competent late-stage oocytes (IV) in the ovary. The localization of Pgr suggests that it mediates progestin regulation of reproductive signaling in the brain, early germ cell proliferation in testis, and ovarian follicular functions, but not final oocyte or sperm maturation.

Candidates for membrane progestin receptors--past approaches and future challenges

Progestins have a broad range of functions in reproductive biology. Many rapid nongenomic actions of progestins have been identified, including induction of oocyte maturation, modulation of reproductive signaling in the brain, rapid activation of breast cancer cell signaling, induction of the acrosomal reaction and hypermotility in mammalian sperm. Currently, there are three receptor candidates for mediating rapid progestin actions: (1) membrane progestin receptors (mPRs); (2) progestin receptor membrane components (PGRMCs); and (3) nuclear progestin receptors (nPRs). The recently-described mPR family of proteins has seven integral transmembrane domains and mediates signaling via G-protein coupled pathways. The PGRMCs have a single transmembrane with putative Src homology domains for potential activation of second messengers. The classical nPRs, in addition to having well defined transcriptional activity, can also mediate rapid activation of intracellular signaling pathways. However, details of the mechanisms by which these three classes of progestin receptors mediate rapid intracellular signaling and their subcellular localization remain unclear. In addition, mPRs, nPRs and PGRMCs exhibit overlapping expression and functions in multiple tissues, implying potential interactions during oocyte maturation, parturition, and breast cancer signaling in individual cells. However, the overwhelming majority of studies to date have focused on the functions of one of these groups of receptors in isolation. This review will summarize recent findings on the three major progestin receptor candidates, emphasizing the different approaches used, some experimental pitfalls, and current controversies. We will also review evidence for the involvement of mPRs and nPRs in one of the most well-characterized nongenomic steroid actions in basal vertebrates, oocyte maturation, and conclude by suggesting some future areas of research. Clarification of the controversies surrounding the identities and localization of membrane progestin receptors may help direct future research that could advance our understanding of rapid actions of steroids.

Expression of cyclooxygenase genes and production of prostaglandins during ovulation in the ovarian follicles of Xenopus laevis

Involvement of cyclooxygenase (COX) and prostaglandin (PG) synthesis during ovulation in Xenopus laevis ovarian follicles was investigated. X. laevis COX cDNAs were isolated from ovarian tissues by using reverse transcription-polymerase chain reaction (RT-PCR) followed by rapid amplification of cDNA ends (RACE). In X. laevis ovary, expression of COX-2 but not COX-1 mRNA was up-regulated during gonadotropin-induced oocyte maturation and ovulation in vitro. Elevation of PGF2* synthesis was directly correlated with the up-regulation of COX-2 mRNA. Synthesis of PGE(2) also increased during periovulatory period, however, the concentrations were much lower than those of PGF2*Progesterone (P4) also induced up-regulation of COX-2 mRNA as well as ovulation. Actinomycin D (ActD) blocked P4-induced ovulation. The inhibition of ovulation by Act D was rescued by co-treatment with exogenous PGF2* in a dose dependent manner. A non-selective COX inhibitor (indomethacin) and selective COX-2 inhibitor (NS398) strongly inhibited the hCG- and P4-dependent production of PGF2*. Inhibitory effects of selective COX-1 inhibitor (SC560) on PGF2* production were lower than that of other inhibitors. Indomethacin and NS398 blocked P4-induced ovulation. NS398 did not block hCG-induced ovulation although it strongly suppressed PGF2* production. These results suggest that (1) in Xenopus ovarian follicles, PGF2* is synthesized during periovulatory period, similar to that in mammals, (2) PGF2* synthesis is regulated by de novo transcription of COX-2 but not COX-1, (3) PGF2* is necessary for P4-induced ovulation but may not be essential for hCG-induced ovulation, and other factor(s) may be involved in the hCG-induced ovulation.

A role for GPRx, a novel GPR3/6/12-related G-protein coupled receptor, in the maintenance of meiotic arrest in Xenopus laevis oocytes

Progesterone-induced Xenopus laevis oocyte maturation is mediated via a plasma membrane-bound receptor and does not require gene transcription. Evidence from several species suggests that the relevant progesterone receptor is a G-protein coupled receptor (GPCR) and that a second receptor-GPR3 and/or GPR12 in mammals-tonically opposes the progesterone receptor. We have cloned a novel X. laevis GPCR, GPRx, which may play a similar role to GPR3/GPR12 in amphibians and fishes. GPRx is related to but distinct from GPR3, GPR6, and GPR12; GPRx orthologs are present in Xenopus tropicalis and Danio rerio, but apparently not in birds or mammals. X. laevis GPRx is mainly expressed in brain, ovary, and testis. The GPRx mRNA increases during oogenesis, persists during oocyte maturation and early embryogenesis, and then falls after the midblastula transition. Microinjection of GPRx mRNA increases the concentration of cAMP in oocytes and causes the oocytes to fail to respond to progesterone, and this block is reversed by co-injecting GPRx with morpholino oligonucleotides. Morpholino injections did not cause spontaneous maturation of oocytes, but did accelerate progesterone-induced maturation. Thus, GPRx contributes to the maintenance of G2-arrest in immature X. laevis oocytes.

The role of Xenopus membrane progesterone receptor beta in mediating the effect of progesterone on oocyte maturation

Rapid, nongenomic membranal effects of progesterone were demonstrated in amphibian oocytes more than 30 y ago. Recently, a distinct family of membrane progestin receptors (mPRs) has been cloned in fish and other vertebrate species. In this study we explore the role of mPR in promoting oocyte maturation in Xenopus laevis. RT-PCR analysis indicates that Xenopus oocytes contain transcripts for the mPRbeta ortholog, similar to what has been reported in zebrafish oocytes, and Western blotting shows that the protein is expressed on the oocyte plasma membrane. Microinjection of mPRbeta-specific antibodies into oocytes resulted in a dramatic inhibition of progesterone-dependent oocyte maturation, whereas microinjection of mRNA encoding Myc-Xenopus mPR (XmPR)beta resulted in an accelerated rate of progesterone-induced oocyte maturation, concomitant with membranal localization of the protein. Binding studies in mammalian cells expressing XmPRbeta confirmed specific binding of progesterone by the expressed protein. These results suggest that XmPRbeta is a physiological progesterone receptor involved in initiating the resumption of meiosis during maturation of Xenopus oocytes.

Deciphering the H-Ras pathway in Xenopus oocyte

Xenopus oocytes are arrested in prophase of the first meiotic division. In response to progesterone, they re-enter meiosis and arrest again in metaphase of the second meiotic division. This process, called meiotic maturation, is under the control of the Cyclin B-Cdc2 complex, M phase promoting factor (MPF). Injection of a constitutively active Xenopus H-Ras protein activates MPF, suggesting that Ras proteins could be implicated in the progesterone transduction pathway. The aim of this study was (1) to elucidate the pathway triggered by H-Ras leading to MPF activation in Xenopus oocytes and (2) to investigate whether endogenous H-Ras is involved in the physiological process of meiotic maturation. We generated three constitutively active double mutants, each of them recruiting a single effector in mammalian cells, mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K) or RalGDS. Our results show that the activation of a PI3K-related enzyme is crucial for H-Ras-induced MPF activation, whereas the recruitment of either MAPK or RalGDS is not. However, although the H-Ras/PI3K pathway is functional in Xenopus oocytes, it is not the physiological transducer of progesterone responsible for meiotic resumption.

Amyloid precursor proteins anchor CPEB to membranes and promote polyadenylation-induced translation

The cytoplasmic polyadenylation element (CPE) binding factor, CPEB, is a sequence-specific RNA binding protein that controls polyadenylation-induced translation in germ cells and at postsynaptic sites of neurons. A yeast two-hybrid screen with a mouse brain cDNA library identified the transmembrane amyloid precursor-like protein 1 (APLP1) as a CPEB-interacting factor. CPEB binds the small intracellular domain (ICD) of APLP1 and the related proteins APLP2 and APP. These proteins promote polyadenylation and translation by stimulating Aurora A catalyzed CPEB serine 174 phosphorylation. Surprisingly, CPEB, Maskin, CPSF, and several other factors involved in polyadenylation and translation and CPE-containing RNA are all detected on membranes by cell fractionation and immunoelectron microscopy. Moreover, most of the RNA that undergoes polyadenylation does so in membrane-containing fractions. These data demonstrate a link between cytoplasmic polyadenylation and membrane association and implicate APP family member proteins as anchors for localized mRNA polyadenylation and translation.