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Luteinizing hormone reduces the activity of the npr2 guanylyl cyclase in mouse ovarian follicles, contributing to the cyclic gmp decrease that promotes resumption of meiosis in oocytes

Contents lists available at Developmental Biology journal homepage: Luteinizing hormone reduces the activity of the NPR2 guanylyl cyclasein mouse ovarian follicles, contributing to the cyclic GMP decrease thatpromotes resumption of meiosis in oocytes Jerid W. Robinson ,1, Meijia Zhang , Leia C. Shuhaibar , Rachael P. Norris , Andreas Geerts Frank Wunder , John J. Eppig , Lincoln R. Potter nn, Laurinda A. Jaffe n a Department of Pharmacology, University of Minnesota, Minneapolis, MN, USAb State Key Laboratory of Agro-biotechnology, College of Biological Sciences, China Agricultural University, Beijing, People's Republic of Chinac Department of Cell Biology, University of Connecticut Health Center, Farmington, CT, USAd Bayer Pharma AG, Pharma Research Center, Wuppertal, Germanye The Jackson Laboratory, Bar Harbor, ME, USAf Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN, USA In preovulatory ovarian follicles of mice, meiotic prophase arrest in the oocyte is maintained by cyclic Received 8 March 2012 GMP from the surrounding granulosa cells that diffuses into the oocyte through gap junctions. The Received in revised form cGMP is synthesized in the granulosa cells by the transmembrane guanylyl cyclase natriuretic peptide receptor 2 (NPR2) in response to the agonist C-type natriuretic peptide (CNP). In response to luteinizing Accepted 12 April 2012 hormone (LH), cGMP in the granulosa cells decreases, and as a consequence, oocyte cGMP decreases Available online 21 April 2012 and meiosis resumes. Here we report that within 20 min, LH treatment results in decreased guanylyl cyclase activity of NPR2, as determined in the presence of a maximally activating concentration of CNP.
This occurs by a process that does not reduce the amount of NPR2 protein. We also show that by a slower process, first detected at 2 h, LH decreases the amount of CNP available to bind to the receptor.
Mouse ovarian follicle Both of these LH actions contribute to decreasing cGMP in the follicle, thus signaling meiotic Luteinizing hormoneGuanylyl cyclase resumption in the oocyte.
& 2012 Elsevier Inc. All rights reserved.
with the transition from prophase to metaphase, marked by thebreakdown of the nuclear envelope about 2 h after LH exposure.
Mammalian oocytes are maintained in meiotic prophase for However, other events of the prophase-to-metaphase transition prolonged periods. During prophase arrest, the oocyte is located in occur before nuclear envelope breakdown: microtubule organizing a follicle in which it is surrounded by granulosa cells ((A)). As centers assemble (, chromatin condenses the follicle grows to its full size ( 400–500 mm in mice), the oocyte , and cell cycle regulatory proteins acquires the ability to resume meiosis, but due to inhibitory signals undergo changes in activity and localization ( from the granulosa cells, the oocyte remains in prophase Recent studies of the mouse ovary have shown that a key . Then during each reproductive inhibitory substance for maintaining prophase arrest is cGMP, cycle, luteinizing hormone (LH) from the pituitary acts on the which diffuses from the granulosa cells into the oocyte through granulosa cells of the fully grown follicle to cause the oocyte to gap junctions ). In the mature into a fertilizable egg and be ovulated. This process begins oocyte, cGMP inhibits the cAMP phosphodiesterase PDE3A, andthus prevents the degradation of cAMP. Elevated cAMP activatesprotein kinase A, which acts through a complex of mechanisms to n Corresponding author. Fax: þ1 860 679 1269.
nn inhibit the activity of the CDK1-cyclin B kinase and thus to inhibit the prophase-to-metaphase transition ( E-mail addresses: If cGMP in a follicle-enclosed oocyte is experimen- tally decreased, by injection of a cGMP-specific phosphodiester- ase, cAMP is decreased, and as a consequence meiosis resumes Contributed equally to this work.
0012-1606/$ - see front matter & 2012 Elsevier Inc. All rights reserved.

J.W. Robinson et al. / Developmental Biology 366 (2012) 308–316 is located in the mural granulosa cells, mostly within the outerseveral layers of cells, and is absent in the cumulus cells(; ). In response to LH,the permeability of the gap junctions between the granulosa cellsthroughout the follicle is reduced, such that intercellular diffusionwithin the follicle of molecules of the size of cGMP is slowed(). In parallel,cGMP levels in the follicle decrease ; ), from a basal level of  3 mM, to  0.5 mM at 20 min and  0.1 mM at 1 h after applying LH (). CNP levels also decrease ; ), but the earliest of these measure-ments were made at 4 h after LH application, while the cGMPdecrease occurs by 20 min, so their functional significance has notbeen certain. As cGMP in the follicle decreases, cGMP in theinterconnected oocyte falls correspondingly, to a few percent ofthe basal level at 1 h. As a consequence, the inhibition of PDE3A isrelieved, cAMP decreases, and meiosis resumes The decrease in cGMP in the follicle could be caused by a decrease in cGMP synthesis, an increase in cGMP degradation,and/or an increase in cGMP efflux. Here we report that onemechanism by which LH signaling reduces cGMP is by reducingthe activity of the guanylyl cyclase NPR2.
Materials and methods Mice and hormones Fig. 1. In mural granulosa cells, Npr2 mRNA is present at a higher concentrationthan mRNAs for other guanylyl cyclases. (A) Histological section of a mouse ovary,showing an antral follicle, and indicating the mural granulosa cells collected for Ovaries were obtained from prepubertal B6SJLF1 mice (23–25 analysis, as well as other cell types and structures in and around the follicle. (B) day old) from The Jackson Laboratory (Bar Harbor, ME); proce- Relative concentrations of each guanylyl cyclase mRNA in isolated mural granu- dures were approved by the animal care committees of the losa cells. Results for the natriuretic peptide clearance receptor, Npr3, are also University of Connecticut Health Center, China Agricultural Uni- shown. Where no bars are visible, concentrations of mRNAs were o0.1% of Npr2.
The results show the mean 7s.e.m. for 3 RNA preparations.
versity, and The Jackson Laboratory. For granulosa cell collection,cumulus-oocyte complex collection, CNP ELISA assays, and histo-logical analysis, the mice were injected with 5 I.U. equine The generation of the cGMP that maintains meiotic arrest chorionic gonadotropin (eCG) 40–48 h before use, to stimulate requires the function in the granulosa cells of the transmembrane follicle growth and LH receptor expression. Mice for antral follicle guanylyl cyclase natriuretic peptide receptor 2 (NPR2, also known isolation were not injected with eCG; instead the follicles were as guanylyl cyclase-B) and its extracellular agonist C-type exposed to 10 ng/ml follicle stimulating hormone (FSH) in vitro.
natriuretic peptide (CNP, also known as natriuretic peptide C, Ovine LH, human LH, ovine FSH, and eCG, purified from NPPC) (). In ovaries of mice carrying mutations biological sources, were obtained from A.F. Parlow (National in Npr2 or Nppc genes, meiosis resumes precociously ( Hormone and Peptide Program, Torrance, CA). Human recombi- Although there is also evidence for expression of other nant LH was obtained from EMD Serono Research Institute, Inc.
guanylyl cyclases in granulosa cells ) and (Rockland, MA). Human chorionic gonadotropin (hCG) was pur- some evidence that these may contribute to the maintenance of chased from Sigma-Aldrich (St. Louis, MO). Ovine LH was used meiotic arrest (¨ for studies of isolated follicles (10 mg/ml). Because of their and the response of the follicle to LH slower rate of degradation ), human ), CNP-dependent activation of NPR2 is LH or hCG was used for injection into mice (10 mg or 5 I.U., fundamental for generating the inhibitory levels of cGMP.
CNP is synthesized by the outer (mural) granulosa cells, and binds to NPR2 throughout the follicle to stimulate cGMP produc- Measurement of relative amounts of guanylyl cyclase mRNAs in tion (; The connection of the cumulus cells to the mural granulosa cells is essential formaintaining meiotic arrest, since when this connection is broken, Mural granulosa cells were collected by puncturing antral leaving the cumulus-oocyte complex free in the antral space, follicles of isolated ovaries with 30 gauge needles. RNA was meiosis resumes ). This supports extracted using TRIzol reagent (Invitrogen Corporation, Carlsbad, the concept that although Npr2 mRNA is most concentrated in the CA). DNAse I digestion was performed to remove residual geno- cumulus cells ), cGMP generated by NPR2 in the mic DNA, and mRNAs were reverse transcribed using random mural layers also provides a critical part of the inhibitory cGMP to Quantitative TaqMan analysis was performed using the Despite this knowledge of how CNP, NPR2, and cGMP function Applied Biosystems PRISM 7900 sequence detection system, to to maintain meiotic arrest, less is known about how signaling by determine the relative concentration of each guanylyl cyclase LH reverses the arrest. LH acts on a G-protein-linked receptor mRNA in granulosa cells. Primer sequences are listed in (LHCGR) ), which in rats and mice, in the supplementary material. Differences in primer efficiency J.W. Robinson et al. / Developmental Biology 366 (2012) 308–316 were determined by measuring the cycle threshold (Ct) values for Samples were prepared by a method modified from each primer pair using 30 ng of genomic DNA. Only small . Two ovaries in 70 ml of 1.0 M acetic acid were heated differences were detected, and these were corrected for by use at 95 1C for 10 min, then lysed with a probe sonicator. 350 ml of of the following formula: MeOH was added to solubilize lipids, and the tube was centri- Ct (corrected)¼Ct (measured)þCt (mean of all probes, genomic fuged at 30,000  g at 4 1C for 15 min. The supernatant (‘‘ovary DNA) Ct (genomic DNA) extract'') contained  250 mg of protein. 50% of the CNP was Normalization was performed using the housekeeping gene Rpl32 recovered in this extract (determined by adding a known amount as a control. The resulting expression is given in arbitrary units.
of CNP to the ovaries before extraction). For each sample, 10, 5,and 2 mg of the extract protein were lyophilized and assayed, 2.3. Measurement of guanylyl cyclase activity in a crude membrane following the manufacturer's instructions. Data were analyzed fraction of follicles using Prism software. Statistical significance of the data wastested using one-way ANOVA with a Dunnett multiple compar- For each experiment, antral follicles from 4 mice were isolated isons post-test. The concentration of CNP in the ovary, if CNP was and cultured for 24–30 h in the presence of FSH to stimulate uniformly distributed, was estimated based on a volume per follicle growth and LH receptor expression ovary of 4 ml ( 4 mg wet weight).
The follicles were divided into 2 equal groups, and halfwere exposed to LH for the indicated time. The 40–50 follicles in Histological analysis of nuclear envelope breakdown kinetics each group were washed in PBS and lysed in phosphataseinhibitor buffer in a 100 ml glass homogeni- Serial sections of mouse ovaries were prepared as previously zer. To obtain a crude membrane fraction, the homogenate described (Follicles with a diameter (200 ml volume) was centrifuged at 10,000  g for 20 min; the of Z350 mm in at least one dimension, as measured in the pellet (  1 ml volume) was resuspended in 50 ml of phosphatase section containing the nucleolus or chromosomes, were analyzed inhibitor buffer and sonicated briefly. Protein content was deter- for the presence of an intact nucleus (see (A)).
mined by solubilizing a 4 ml aliquot in 1% SDS and performing aBCA assay (Pierce, Thermo Fisher Scientific, Rockford, IL). The Results and discussion crude membrane fraction contained  1 mg of protein per follicle.
The samples were frozen in liquid N2 and stored at 80 1C.
In mural granulosa cells, mRNA encoding NPR2 is present at a higher Guanylyl cyclase assays were conducted for each pair of concentration than mRNAs encoding other guanylyl cyclases follicle samples prepared as described above (one sample thathad been treated with LH, one control sample without LH), using Although NPR2 is known to be present in mural granulosa cells methods as previously described ).
and functionally important for maintaining meiotic arrest, there is Assays were performed at 37 1C using 1–2 mg of follicle protein also evidence that NPR1 and soluble guanylyl cyclase subunits per assay tube, in the presence or absence of 1 mM CNP (or ANP), could contribute to the control of meiotic arrest and progression which are maximally activating concentrations for their respec- (see Introduction). Because previous studies did not determine tive receptors (; ).
the relative expression levels of mRNA for NPR2 and other 0.5 mM isobutylmethylxanthine (IBMX) was included to inhibit guanylyl cyclases in mouse granulosa cells, and because not all cGMP phosphodiesterase activity. CNP (or ANP) dependent gua- of the guanylyl cyclases were investigated, we quantitatively nylyl cyclase activity refers to the activity measured in the compared the amounts of mRNA in mural granulosa cells for presence of the natriuretic peptide minus the activity measured each of the mouse guanylyl cyclase genes ( in the absence of the natriuretic peptide. Statistical significance of The mouse genome contains 7 transmembrane and 4 soluble the data was tested using two-way repeated measures ANOVA guanylyl cyclase genes ). We detected mRNA with a Bonferroni post-test; control and LH-treated samples that encoding two transmembrane guanylyl cyclases, NPR1 and had been prepared and assayed together were analyzed as pairs.
NPR2, and two soluble guanylyl cyclase subunits, GUCY1A3 The analysis was performed using Prism software (GraphPad (soluble guanylyl cyclase alpha 1) and GUCY1B3 (soluble guanylyl Software, Inc., La Jolla, CA).
cyclase beta 1). Among these, Npr2 mRNA was expressed at a highlevel, Z14 times higher than any of the other guanylyl cyclases.
Measurement of cGMP in cumulus cells We also tested for mRNA encoding NPR3, which has sequencesimilarity to the extracellular domains of NPR1 and NPR2, but Cumulus-oocyte complexes were isolated at various times lacks the guanylyl cyclase domain and activity ( after hCG injection, and cultured as previously described ( NPR3 is a clearance receptor for natriuretic peptides. Little or no ), with or without 30 nM CNP for 1 h. Cumulus cells Npr3 mRNA was detected.
were then separated for measurement of cGMP using an ELISA Although concentrations of mRNAs are not directly propor- method as previously described ). Statistical tional to the amounts of the proteins they encode, these mea- significance of the data was tested using one-way ANOVA with a surements further support the conclusion that NPR2 is the Dunnett multiple comparisons post-test.
primary guanylyl cyclase that produces cGMP in the follicle.
Measurement of CNP in ovaries LH signaling reduces NPR2 activity in the follicle CNP in ovaries was assayed by an ELISA method (FEK-012-03, One way that LH activation of its receptors in the mural Phoenix Pharmaceuticals Inc., Burlingame, CA) with a primary granulosa cells could decrease cGMP levels within the follicle is antibody made against the 22 amino acid form of CNP. This by reducing NPR2 activity. Two aspects of this question were antibody also recognizes the 53 amino acid form of CNP, and considered: (1) whether LH signaling decreases NPR2 activity in presumably the precursor forms, which include the same 22 the follicle as a whole, of which most of the volume is mural amino acids at their C-termini (; granulosa cells, and (2) whether LH signaling decreases NPR2 activity in the cumulus cells. A decrease in NPR2 activity in either

J.W. Robinson et al. / Developmental Biology 366 (2012) 308–316 or both of these regions could contribute to the measured activity was 0.0770.03 nmole cGMP/mg protein/minute, or 33% of decrease in cGMP in the oocyte. This section describes our studies the CNP-dependent activity (n¼4). However, the ANP-dependent of a crude membrane fraction from whole follicles, and a sub- activity was unchanged by LH (C) and (D)). Some of the ANP- sequent section describes our studies of cumulus cells.
dependent guanylyl cyclase activity that we measured might be due Guanylyl cyclase activity was measured using the particulate to NPR1 expressed in membranes from theca cells and blood vessels fraction obtained by centrifuging a homogenate of follicles. When that were not completely removed from the follicle by microdissec- this crude membrane fraction was incubated without CNP, tion A)). The lack of effect of LH on ANP-dependent cGMP guanylyl cyclase activity was too low to measure accurately, but accumulation serves as a control to indicate that the LH-induced addition of 1 mM CNP increased the activity to 0.2170.02 nmole decrease in CNP-dependent cGMP accumulation is not due to an LH effect on phosphodiesterase activity that could have been present in and (B)). After a 20 min exposure of follicles to LH, the crude membrane fraction despite the presence of IBMX.
CNP-dependent guanylyl cyclase activity fell to 50% of the activitymeasured in the membrane fraction from follicles without LHexposure, and remained depressed for 2 h after applying LH The LH-induced decrease in NPR2 activity in the follicle occurs (A) and (B)).
without a corresponding decrease in NPR2 protein The decrease in follicle cGMP that will result from a 50% decrease in NPR2 activity depends on the cGMP affinity of the phosphodies- Previous studies have shown that other biological factors that terases present in the granulosa cells ). If the affinity is higher rapidly decrease NPR2 activity in cultured cells do so in a manner (lower Km), the cGMP concentration will fall to a lower level. Much that is independent of NPR2 protein levels (; of the cGMP phosphodiesterase activity in the follicle is sensitive to ). To test if LH decreased the amount of sildenafil and tadalafil, indicating an important PDE5 component NPR2 protein in follicles, we first tried Western blotting, and Based on Km values for PDE5, a 50% reduction immunoprecipitation followed by Western blotting. However, with in NPR2 activity could potentially account for the decrease in follicle the available antibodies, it was not possible to detect endogenous cGMP from 3 mM before LH treatment to  0.5 mM after 20 min levels of the protein using these methods. So instead, we measured guanylyl cyclase activity in follicle membrane fractions after treat- Because a small amount of Npr1 mRNA is also expressed in ment with 1% Triton X-100 and 5 mM MnCl2, a condition known to granulosa cells we also evaluated the effect of LH on maximally activate NPR1 and NPR2 in the absence of natriuretic NPR1 activity, by measuring guanylyl cyclase activity in the pre- peptide and to be indicative of guanylyl cyclase protein levels sence of 1 mM atrial natriuretic peptide (ANP). Studies of human (; ). Guanylyl cyclase NPR1 and NPR2 have shown that 1 mM ANP activates NPR1, but has activity measured in the presence of Triton X-100 and MnCl2 is almost no effect on NPR2 ). In the crude independent of modification of the NPR2 protein by phosphoryla- membrane fraction from follicles, ANP-dependent guanylyl cyclase Fig. 2. LH signaling reduces NPR2 activity in the follicle. (A) Guanylyl cyclase activity of crude membrane fractions prepared from follicles treated with or without LH for20 min was measured with or without 1 mM CNP. Values indicate the mean7range of duplicate measurements for each condition, using one follicle preparation madeafter LH treatment, and another preparation made in parallel but without LH treatment. Where not visible, the error bars are contained within the symbol. (B) Combineddata from 15 experiments like that in A, showing CNP-dependent guanylyl cyclase activity of crude membrane fractions from follicles treated with LH for the indicatedtimes. CNP-dependent activity values were determined by subtracting the basal values measured in the absence of CNP. Activities are expressed as the mean 7s.e.m. for nfollicle preparations. Activities for follicles treated with LH for 20, 60, or 120 min differed significantly from the activity for follicles without LH treatment (np o0.05;nn, po0.01). (C) and (D) Guanylyl cyclase activity of crude membrane fractions prepared from follicles treated with or without LH for 60 min was measured with orwithout 1 mM ANP. Data are presented as described for A and B. LH did not decrease the ANP-dependent guanylyl cyclase activity.

J.W. Robinson et al. / Developmental Biology 366 (2012) 308–316 measured their cGMP content. hCG is often used instead of LH,since both hormones act on the same receptor. 30 nM CNP wasused because this is approximately the minimum concentrationneeded to prevent spontaneous meiotic resumption in isolatedcumulus-oocyte complexes (). Under theseexperimental conditions, measurements of a change in cellularcGMP content in response to LH receptor stimulation couldindicate a change in guanylyl cyclase activity, or a change incGMP phosphodiesterase activity, or a change in cGMP efflux.
However, by measuring the effect of LH receptor stimulation oncGMP content in the presence and absence of CNP, we were ableto distinguish between these possibilities.
Without injection of the mice with hCG, addition of 30 nM CNP to cumulus-oocyte complexes elevated the cGMP content of thecumulus cells by 4.1 70.9 times (n¼6). When cumulus-oocytecomplexes were isolated from mice at 1 h after hormone injec-tion, and then incubated in the presence of CNP for an additionalhour, the cGMP content of the cumulus cells was the same as thatin cumulus cells from mice without hormone injection However, when the cumulus-oocyte complexes were isolated at2 h after hormone injection, and incubated with CNP until 3 h,cGMP had decreased to 70% of values obtained without hormonetreatment ). With isolation of the complexes at 3 h,followed by a CNP incubation and measurement at 4 h, cGMP Fig. 3. The LH-induced decrease in NPR2 activity occurs without a corresponding had decreased to 42% decrease in NPR2 protein. (A) Guanylyl cyclase activity of crude membrane In the absence of CNP, the cGMP content of the cumulus cells fractions prepared from follicles treated with or without LH for 20 min wasmeasured with or without 1% Triton X-100 and 5 mM MnCl2, to maximally was low, as expected for this in vitro condition in which NPR2 activate guanylyl cyclase. Values indicate the mean 7range of duplicate measure- would not be activated. Under this condition, the cGMP content ments for each condition, using one follicle preparation made after LH treatment, was not decreased by LH receptor stimulation ), indicating and another preparation made in parallel but without LH treatment. (B) Combined that LH receptor signaling does not increase cGMP phosphodies- data from 3 experiments like that in A, showing Mn/triton-dependent guanylylcyclase activity of crude membrane fractions from follicles treated with or without terase activity in these cells, or cause an increase in cGMP efflux.
LH for 20 min (mean 7s.e.m.). LH did not decrease the Mn/triton-dependent Thus the LH receptor-induced decrease in cumulus cell cGMP guanylyl cyclase activity.
seen in the presence of CNP can be attributed to a decrease incGMP production. These findings indicate that LH signalingdecreases NPR2 activity in the cumulus cells, but only after 2– Detergent-dependent guanylyl cyclase activity was the same 3 h, versus 20 min in the mural cells. This delay is likely to be a in samples from follicles with or without LH treatment for 20 min consequence of the localization of the LH receptors in separate (Since NPR1 and NPR2 are the only detectable membrane cells (the mural granulosa). As will be discussed below, this guanylyl cyclases in granulosa cells, and since NPR1 is a relatively intercellular signaling is most likely mediated by the release of minor component, these detergent measurements indicate that at EGF-like growth factors from the mural granulosa cells.
20 min, LH does not decrease the amount of NPR2 protein. A The decrease in cGMP production in the cumulus cells could possible cause of the rapid LH-induced decrease in NPR2 activity result from a decrease in the amount of NPR2 protein, or from a is dephosphorylation, which can result from elevation of intra-cellular Ca2 þ and/or activation of protein kinase C ; In the cumulus cells, CNP-dependent cGMP production decreases inresponse to LH receptor stimulation, but more slowly than in themural granulosa cells Because of the direct connection between the cumulus cells and the oocyte, and because of the higher level of Npr2 mRNA in thecumulus cells compared with the mural cells itwas of particular interest to investigate whether LH signals that areinitiated in the mural granulosa cells regulate NPR2 activity in thecumulus cells. As described above, LH receptors are not present inthe cumulus cells, so such regulation would have to involvesignaling between different regions of the follicle.
Due to the small amount of protein that could be obtained, we could not analyze guanylyl cyclase activity in a cumulus cell Fig. 4. In the cumulus cells, CNP-dependent cGMP production decreases in membrane fraction as we did for the more abundant material response to LH receptor stimulation, but more slowly than in the mural granulosa from whole follicles. Instead, we isolated cumulus-oocyte com- cells. Cumulus-oocyte complexes were collected at various times after hCG plexes from ovaries at various times after injection of mice with injection into mice and incubated for one hour with or without 30 nM CNP. Thecumulus cells were separated and cGMP content was measured. Values indicate human chorionic gonadotropin (hCG) to stimulate the LH recep- the mean 7s.e.m. for the number of experiments shown in parentheses. Values for tor, incubated the complexes in the presence or absence of 30 nM 2–3 and 3–4 h post hCG were significantly different from the no hCG value CNP for an additional hour, then isolated the cumulus cells and (nnp o0.01).

J.W. Robinson et al. / Developmental Biology 366 (2012) 308–316 modification of the NPR2 protein such as dephosphorylation. In 37% (A)). The decrease in CNP at 2 h corresponds to the time support of the first possibility, the amount of Npr2 mRNA in the at which nuclear envelope breakdown in the oocyte is beginning, cumulus cells decreases to 50% of the basal level at 3 h after LH as determined from histological sections of ovaries of similarly receptor stimulation (However, it is unknown treated mice ((B)). Thus, the CNP decrease occurs well after how rapidly NPR2 protein would decrease as a consequence.
cGMP decreases in the follicle (detected at 20 min after LH, Protein turnover rates for NPR2 have not been investigated, but ), but early enough to potentially contribute to turnover of the closely related protein NPR1 in cultured cells is a stimulating nuclear envelope breakdown. After nuclear envelope slow process, with a half-time of Z8 h breakdown, CNP continued to decrease, reaching 15% at 4 h afterLH, and 7% at 8 h The amount of CNP in the ovary decreases in response to LH, A likely cause of the CNP decrease is that Nppc mRNA preceding nuclear envelope breakdown decreases to about half of the basal level by 2 h after LH receptorstimulation Thus LH signaling might Another factor that could contribute to the LH-induced reduction reduce Nppc mRNA synthesis or increase its degradation. Because in cGMP levels in the follicle is a decrease in CNP. CNP decreases the turnover of CNP is very rapid, with a half-life of about 3 min in have been reported previously for rat, mouse, and human plasma a decrease in Nppc mRNA could rapidly , but the earliest of decrease the amount of CNP. Other possible factors that could these measurements were made at 4 h after LH receptor stimula- contribute to the decrease in CNP are an increase in the natriure- tion, so it was unclear if the CNP decrease occurred early enough to tic peptide clearance receptor NPR3, and an increase in the contribute to causing nuclear envelope breakdown and the events activity of proteases that degrade CNP that precede it, vs later events leading to ovulation. To investigatethe time course of the decrease in CNP, we injected mice with LH,and at various times afterwards, collected their ovaries for analysis The amount of CNP in the ovary increases as follicles develop to the of CNP content using an ELISA based on an antibody that should preovulatory stage recognize all forms of CNP and its precursor NPPC.
Without LH injection, there were 150 fmoles of CNP per mg of We also examined the effect of equine chorionic gonadotropin ovary extract protein ), corresponding to an overall con- (eCG, also called PMSG) on CNP levels. Unlike human chorionic centration of  10 nM. However, the immunoreactive material gonadotropin (hCG), which binds to the LH receptor, eCG binds to detected by the ELISA contains both extracellular peptide and the follicle stimulating hormone receptor, which stimulates antral intracellular precursor protein, and only the peptide that has been follicle growth and LH receptor expression. eCG is often used secreted into the extracellular space can activate NPR2. Thus the experimentally to cause follicles to grow and to progress to the concentration of peptide that could function to regulate NPR2 is preovulatory stage; it was used for this purpose for the CNP measurements described above.
No change from the pre-LH level of CNP was seen at 1 h after We found that eCG injection of the mice, 44 h before collecting injection of LH, but by 2 h, the amount of CNP had decreased to the ovaries, increased their CNP content ). The increase in CNPin response to eCG is consistent with findings that mRNA encodingthe CNP precursor (NPPC) increases in mouse ovaries in response toeCG ), and that CNP and cGMP increasebetween the days of diestrus and proestrus, in rats and hamsters(). Thus ourfindings add to the accumulating evidence that during folliclegrowth to the preovulatory stage, CNP and cGMP content of theovary increases. At the preantral stage, cyclic nucleotide regulationis not needed to maintain meiotic arrest ). Then with follicle growth, as the oocyte accumulates moreCDK1 and other factors that result in meiotic competence ( Fig. 5. The amount of CNP in the ovary decreases in response to LH, precedingnuclear envelope breakdown. (A) Time course of the decrease in the CNP content Fig. 6. The amount of CNP in the ovary increases in response to activation of of ovary extracts, following LH injection into mice. Values indicate the mean 7 follicle stimulating hormone receptors. The mice to be used for the CNP s.e.m. for the number of mice shown in parentheses. Values for 2, 4, and 8 h LH measurements shown in (A) were injected 44 h previously with eCG to treatments were significantly different from the no LH value (np o0.05; stimulate follicle growth. In response to eCG, the amount of CNP per mg of ovary nnpo0.01). (B) Time course of nuclear envelope breakdown, following LH injection protein increased  4 times. The þ eCG data shown in are the same as the no into mice. Values indicate the percentage of fully grown follicles ( Z350 mm in LH (0 h) data shown in (A), so the statistical significance of these data was diameter) in which the oocyte contained a prophase-arrested nucleus; the number tested together. Values for ovaries from mice with or without eCG treatment were of follicles and the number of mice counted are indicated.
J.W. Robinson et al. / Developmental Biology 366 (2012) 308–316 , CNP is synthesized in order to prevent premature elevation and protein kinase C activation can lead to dephosphor- ylation and inactivation of NPR2 (). Calciumelevation could also increase the activity of the PDE1 family of cGMPphosphodiesterases ).
Pathways by which LH signaling in the mural granulosa cells causes Since the mRNA encoding the precursor protein of CNP is cGMP to decrease in the oocyte expressed in the same cells as the LH receptor exposure to LH could result in a reduction in the amount of CNP The connections between LH-induced activation of G-proteins by signaling within the same cells. Likewise, since the LH receptor in the outer layers of the mural granulosa cells and the ensuing is expressed in the outer several layers of the mural granulosa events in the follicle that lead to the decrease in cGMP and cells the LH-induced decrease in NPR2 resumption of meiosis in the oocyte are only partially understood activity in these cells could result from signaling within the same cell. However, the LH-induced decrease in NPR2 activity in the These connections include not only the path- cumulus cells, or in the inner layers of the mural cell epithelium, ways leading to a decrease in granulosa cell guanylyl cyclase both of which regions lack LH receptors ( activity, but also pathways that reduce gap junction permeability ), must involve signaling between cells.
through MAP kinase-dependent phosphorylation of connexin 43 Based on evidence that the cGMP decrease in the follicle is partially dependent on EGF receptor signaling diffusion of cGMP into the oocyte. Both the decrease in gap junction permeability and the decrease in granulosa cell guanylyl growth factors released from the outer layers of mural granulosa cyclase activity contribute to the decrease in oocyte cGMP.
cells in response to LH are mediators of the paracrine signals, and At the level of the mural granulosa cells, LH receptor signaling could also contribute to autocrine signaling in the outer layers of the activates Gs and adenylyl cyclase mural granulosa cells. EGF receptor signaling is essential for LH- thus elevating cAMP. LH receptor signaling also activates Gi, Gq, and induced nuclear envelope breakdown (), and EGF phospholipase Cb ; ¨ receptor activation, as indicated by increased phosphorylation of the thus elevating calcium (via IP3) in granulosa receptor protein, occurs as early as 30 min after LH treatment cells in culture ; ). However, ). However, it remains unknown how LH activation of protein kinase C by the diacylglycerol that is generated receptor signaling triggers the synthesis and/or release of the EGF- by phospholipase C has, to our knowledge, not been detected so far like growth factors epiregulin and amphiregulin. RNA encoding . Further studies of these signaling events precursors of these growth factors increases by 2 h after LH receptor using intact follicles will be informative, because both calcium stimulation but in addition, LH signaling mightactivate the proteases that release epiregulin and amphiregulin frompre-existing precursors EGF receptor signaling is required for much of the increase in MAP kinase activity in response to LH (andthus contributes to phosphorylation of connexin 43 and theresulting decrease in gap junction permeability (; ). EGF receptor signaling also activatesphospholipase Cg ), and could thuselevate calcium and protein kinase C activity, amplifying the LHreceptor signaling that may occur through through phospholipaseCb. As discussed above, these signaling events could decreaseNPR2 activity, and possibly increase PDE1 activity, thus loweringcGMP in the granulosa cells and oocyte.
By 20 min after applying LH to ovarian follicles, the guanylyl cyclase activity of NPR2 elicited by a saturating concentration ofCNP is decreased by half. This correlates with a similarly rapiddecrease in follicle cGMP. There is then a slower decrease in NPR2responsiveness to CNP in the cumulus cells, first seen at 2–3 h. By2 h, LH signaling also induces a decrease in the amount of CNP inthe ovary. Together, these 3 factors that decrease guanylyl cyclaseactivity contribute to the decrease in cGMP in the follicle. Becausethe mural granulosa cells, cumulus cells, and oocyte are con-nected by gap junctions to form a syncitium with respect tocGMP, cGMP in the oocyte equilibrates with that in the surround-ing somatic cell compartment, and the resulting decrease inoocyte cGMP promotes meiotic resumption.
Fig. 7. Signaling pathways connecting LH binding to its receptors in the outerlayers of the mural granulosa cells to resumption of meiosis in a mammalian We thank Tracy Uliasz, Amber Selko, and Marilyn O'Brien for oocyte. The green box indicates the findings of this study in the context of otheraspects of the signaling network.
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