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Comparative Study
. 2006 Feb 7;16(3):321-7.
doi: 10.1016/j.cub.2005.12.031.

Dynamic anchoring of PKA is essential during oocyte maturation

Kathryn J Newhall  1 , Amy R Criniti, Christine S Cheah, Kimberly C Smith, Katherine E Kafer, Anna D Burkart, G Stanley McKnight
Affiliations
Comparative Study

Dynamic anchoring of PKA is essential during oocyte maturation

Kathryn J Newhall et al. Curr Biol. .

Abstract

In the final stages of ovarian follicular development, the mouse oocyte remains arrested in the first meiotic prophase, and cAMP-stimulated PKA plays an essential role in this arrest. After the LH surge, a decrease in cAMP and PKA activity in the oocyte initiates an irreversible maturation process that culminates in a second arrest at metaphase II prior to fertilization. A-kinase anchoring proteins (AKAPs) mediate the intracellular localization of PKA and control the specificity and kinetics of substrate phosphorylation. Several AKAPs have been identified in oocytes including one at 140 kDa that we now identify as a product of the Akap1 gene. We show that PKA interaction with AKAPs is essential for two sequential steps in the maturation process: the initial maintenance of meiotic arrest and the subsequent irreversible progression to the polar body extruded stage. A peptide inhibitor (HT31) that disrupts AKAP/PKA interactions stimulates oocyte maturation in the continued presence of high cAMP. However, during the early minutes of maturation, type II PKA moves from cytoplasmic sites to the mitochondria, where it associates with AKAP1, and this is shown to be essential for maturation to continue irreversibly.

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Figures

Figure 1
Figure 1. Generation of AKAP1 Knockout Mice
(A) Targeting of the wild-type allele (top) by homologous recombination led to disruption of exon 2 of AKAP1 and insertion of the neomycin-resistant cassette (bottom). B, BamHI; BsrG, BsrGI; E, EcoRI; P, Pac. (B) Genomic Southern blot of ES cells transfected with the mutant construct was conducted by digesting DNA with BamH1. The wild-type band migrates at 15 kb and the knockout band at 10 kb. (C) Western blot analyses of protein samples of ovaries from AKAP1 wild-type and knockout mice indicated that the 140 kDa isoform is absent in knockout mice. (D) RII overlay on oocytes indicated that AKAP1 is a predominant AKAP in the oocyte. Protein from 100 GV-stage oocytes from either wildtype or knockout females was separated on an SDS-PAGE gel. The protein was transferred to a nitrocellulose membrane and the membrane was incubated in RIIβ protein, followed by incubation with an RIIβ antibody. The red arrow indicates AKAP1. (E) Confocal micrograph images showed immunostaining for αAKAP1 (green) in ovarian sections. Nuclei were stained with DAPI (blue). AKAP1 was predominantly expressed in the oocyte in different stages of follicular development. Preantral and preovulatory follicles are shown. The experiment was conducted three times and representative images are shown (250× magnification). gc, granulosa cells. (F) Within the oocyte (GV stage), AKAP1 (green) showed a punctate distribution that colocalized to the mitochondria (red). Nuclei are stained with ToPro3 (blue). AKAP1 was excluded from cumulus granulosa cells (gc), as shown in the middle panels.
Figure 2
Figure 2. Embryos from AKAP1 Knockout Females Failed to Mature and Were More Sensitive to cAMP
(A) Female AKAP1 knockout or heterozygous mice (22- to 26-days-old) were injected with PMSG, followed by hCG 48 hr later. Immediately after hCG injection, females were housed individually with wild-type males. Embryos were harvested from the oviduct of the females 24 hr later. The embryos were placed in HTF (Irvine Scientific) media supplemented with 0.25 mM EDTA for 72 hr. Embryonic stage was monitored during this time. n = 135 embryos from heterozygous females, n = 97 embryos from knockout females. (B) In vitro GVBD was decreased in oocytes from knockout females. Denuded oocytes from heterozygous and knockout females were collected from ovaries 48 hr after PMSG injection. These oocytes were then assessed for GVBD (meiotic resumption) 3 hr later. Oocytes with a clear nuclear membrane (GV) and nucleoli were classified as being at the GV stage, while those without a visible nuclear structure were classified as GVBD stage. An inhibitor of PKA, Rp-cAMPS (100 μM), reversed the defect in GVBD in knockout oocytes but had no effect in oocytes from heterozygous females. Results are expressed as mean ± SEM for seven experiments (no treatment) and four experiments (Rp-cAMPS). *p < 0.05 by a repeated measures ANOVA with a Newman Kuels post test. (C) In vitro polar body formation was restored to wild-type levels in AKAP1 knockout oocytes treated with 100 μM Rp-cAMPS. Denuded oocytes from wild-type and knockout females were collected from ovaries 48 hr after PMSG injection and incubated at 37ºC with or without Rp-cAMPS (100 μM). After 2 hr of incubation, the Rp-cAMPS was washed out of the media, the oocytes were returned to the incubator, and polar body formation was assessed 18 hr later. Results are expressed as mean ± SEM for six experiments (knockout) and three experiments (wild-type). *p < 0.001 by a one-way ANOVA with a Newman Kuels post test. (D) Oocytes from AKAP1 knockout females were more sensitive to the inhibitory effects of IBMX on GVBD. Oocytes incubated with increasing concentrations of the IBMX were scored for GVBD 3 hr after treatment. Values are expressed as mean ±SEM for four experiments. (E) RI-specific cAMP analogs synergistically inhibit GVBD in oocytes from AKAP1 wild-type mice. Denuded oocytes were incubated with 40 μM of 8cpt-cAMP and increasing concentrations of 8-AHA-cAMP to stimulate RI. Oocytes were scored for GVBD 3 hr after treatment. Values are expressed as mean ± SEM for three experiments. (F) RII-specific cAMP analogs synergistically inhibit GVBD in oocytes from AKAP1 wild-type mice. Denuded oocytes were incubated with 40 μM of 8cpt-cAMP and increasing concentrations of N6mb-cAMP to stimulate RII. Oocytes were scored for GVBD 3 hr after treatment. Values are expressed as mean ± SEM for three experiments.
Figure 3
Figure 3. RIIα and AKAP1 Localization during Oocyte Maturation
(A) ICC was conducted on GV-stage oocytes with primary antibodies to PKA RIIα (green), Mitotracker (red), and ToPro (blue). (B) ICC was conducted in MII-stage oocytes. (C) ICC was conducted in wild-type oocytes collected from ovarian follicles and incubated for 1.5 or 6 hr, respectively. Primary antibodies to RIIα (green) and Mitotracker (red) were used. Oocytes were counterstained with ToPro (blue). This experiment was conducted three times, and representative images are shown. (D) ICC was conducted in MII-stage wild-type oocytes with primary antibodies to AKAP1 (green) or AKAP7γ (green) and Mitotracker (red) or RIIα (red). Oocytes were counter-stained with ToPro (blue). These experiments were conducted three times, and representative images are shown.
Figure 4
Figure 4. Correct PKA Localization Is Required for Meiotic Resumption
(A) Oocytes microinjected with 0.1 mM HT31 or 0.1 mM HT31-P were incubated with or without 0.2 mM IBMX and scored for GVBD 3 hr after treatment. Values are expressed as mean ± SEM for three experiments. (B) Wildtype and knockout GV-stage oocytes were treated with Rp-cAMPS (100 μM) for 1 hr followed by a wash out. IBMX (0.2 mM) was then added, and GVBD was observed 3 hr later. Values are expressed as mean ± SEM for three experiments. (C) Model of AKAP function in the oocyte. In the wild-type GV oocyte, the RIIα subunit of PKA does not interact with AKAP1, but we hypothesize that it is associated with an unidentified AKAP, AKAPx. As the wild-type oocyte begins to mature, RIIα translocates and binds AKAP1 at the mitochondria. MPF, maturation promoting factor, a complex of CDK1 and cyclin B; cdc25b, a dual specificity phosphatase shown to act on CDK1 to activate it; wee1, a tyrosine kinase that phosphorylates and inactivates CDK1. AKAP1 is shown associated with mitochondria at all times.
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