Localization of a novel human A-kinase-anchoring protein, hAKAP220, during spermatogenesis

Interactomes of Glycogen Synthase Kinase-3 Isoforms

Functional differentiation of the two isoforms of the protein-serine/threonine kinase, glycogen synthase kinase-3 (GSK-3), is an unsettled area of research. The isoforms are highly similar in structure and are largely redundant, though there is also evidence for specific roles. Identification of isoform-specific protein interactors may elucidate the differences in function and provide insight into isoform-selective regulation. We therefore sought to identify novel GSK-3 interaction partners and to examine differences in the interactomes of the two isoforms using both affinity purification and proximity-dependent biotinylation (BioID) mass spectrometry methods. While the interactomes of the two isomers are highly similar in HEK293 cells, BioID in HeLa cells yielded a variety of preys that are preferentially associated with one of the two isoforms. DCP1B, which favored GSK-3α, and MISP, which favored GSK-3β, were evaluated for reciprocal interactions. The differences in interactions between isoforms may help in understanding the distinct functions and regulation of the two isoforms as well as offer avenues for the development of isoform-specific strategies.

Homozygous variants in AKAP3 induce asthenoteratozoospermia and male infertility

BACKGROUND

As a common type of asthenoteratozoospermia, multiple morphological abnormalities of the sperm flagella (MMAF) can cause male infertility. Previous studies have revealed genetic factors as a major cause of MMAF. The known MMAF-associated genes are involved in the mitochondrial sheath, outer dense fibre or axoneme of the sperm flagella. These findings indicate the genetic heterogeneity of MMAF.

METHODS AND RESULTS

Here, we conducted genetic analyses using whole-exome sequencing in a cohort of 150 Han Chinese men with asthenoteratozoospermia. Homozygous deleterious variants of AKAP3 (A-kinase anchoring protein 3) were identified in two MMAF-affected men from unrelated families. One AKAP3 variant was a frameshift (c.2286_2287del, p.His762Glnfs*22) and the other variant was a missense mutation (c.44G>A, p.Cys15Tyr), which was predicted to be damaging by multiple bioinformatics tools. Further western blotting and immunofluorescence assays revealed the absence of AKAP3 in the spermatozoa from the man harbouring the homozygous frameshift variant, whereas the expression of AKAP3 was markedly reduced in the spermatozoa of the man with the AKAP3 missense variant p.Cys15Tyr. Notably, the clinical outcomes after intracytoplasmic sperm injection (ICSI) were divergent between these two cases, suggesting a possibility of AKAP3 dosage-dependent prognosis of ICSI treatment.

CONCLUSIONS

Our study revealed AKAP3 as a novel gene involved in human asthenoteratozoospermia.

Modulatory effect of MG-132 proteasomal inhibition on boar sperm motility during in vitro capacitation

A series of biochemical and biophysical changes during sperm capacitation initiates various signaling pathways related to protein phosphorylation leading to sperm hyperactivation, simultaneously with the regulation of proteasomal activity responsible for protein degradation and turnover. Our study aimed to unveil the role of the proteasome in the regulation of boar sperm motility, hyperactivated status, tyrosine phosphorylation, and total protein ubiquitination. The proteolytic activity of the 20S proteasomal core was inhibited by MG-132 in concentrations of 10, 25, 50, and 100 μM; and monitored parameters were analyzed every hour during 3 h of in vitro capacitation (IVC). Sperm motility and kinematic parameters were analyzed by Computer Assisted Sperm Analysis (CASA) during IVC, showing a significant, negative, dose-dependent effect of MG-132 on total and progressive sperm motility (TMOT, PMOT, respectively). Furthermore, proteasomal inhibition by 50 and 100 μM MG-132 had a negative impact on velocity-based kinematic sperm parameters (VSL, VAP, and VCL). Parameters related to the progressivity of sperm movement (LIN, STR) and ALH were the most affected by the highest inhibitor concentration (100 μM). Cluster analysis revealed that the strongest proteasome-inhibiting treatment had a significant effect (p ≤ 0.05) on the hyperactivated sperm subpopulation. The flow cytometric viability results proved that reduced TMOT and PMOT were not caused by disruption of the integrity of the plasma membrane. Neither the protein tyrosine phosphorylation profile changes nor the accumulation of protein ubiquitination was observed during the course of capacitation under proteasome inhibition. In conclusion, inhibition of the proteasome reduced the ability of spermatozoa to undergo hyperactivation; however, there was no significant effect on the level of protein tyrosine phosphorylation and accumulation of ubiquitinated proteins. These effects might be due to the presence of compensatory mechanisms or the alteration of various ubiquitin-proteasome system-regulated pathways.

Refining the genomic location of single nucleotide polymorphism variation affecting Atlantic salmon maturation timing at a key large-effect locus

Efforts to understand the genetic underpinnings of phenotypic variation are becoming more and more frequent in molecular ecology. Such efforts often lead to the identification of candidate regions showing signals of association and/or selection. These regions may contain multiple genes and therefore validation of which genes are actually responsible for the signal is required. In Atlantic salmon (Salmo salar), a large-effect locus for maturation timing, an ecologically important trait, occurs in a genomic region including two genes, vgll3 and akap11, but data for clearly determining which of the genes (or both) contribute to the association have been lacking. Here, we take advantage of natural recombination events detected between the two candidate genes in a salmon broodstock to reduce linkage disequilibrium at the locus, thus enabling delineation of the influence of variation at these two genes on early maturation. By rearing 5,895 males to maturation age, of which 81% had recombinant vgll3/akap11 allelic combinations, we found that vgll3 single nucleotide polymorphism (SNP) variation was strongly associated with early maturation, whereas there was little or no association between akap11 SNP variation and early maturation. These findings provide strong evidence supporting vgll3 as the primary candidate gene in the chromosome 25 locus for influencing early maturation. This will help guide future research for understanding the genetic processes controlling early maturation. This also exemplifies the utility of natural recombinants to more precisely map causal variation underlying ecologically important phenotypic diversity.

Everything you ever wanted to know about PKA regulation and its involvement in mammalian sperm capacitation

The 3', 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) is a tetrameric holoenzyme comprising a set of two regulatory subunits (PKA-R) and two catalytic (PKA-C) subunits. The PKA-R subunits act as sensors of cAMP and allow PKA-C activity. One of the first signaling events observed during mammalian sperm capacitation is PKA activation. Thus, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. It is widely accepted that PKA specificity depends on several levels of regulation. Anchoring proteins play a pivotal role in achieving proper localization signaling, subcellular targeting and cAMP microdomains. These multi-factorial regulation steps are necessary for a precise spatio-temporal activation of PKA. Here we discuss recent understanding of regulatory mechanisms of PKA in mammalian sperm, such as post-translational modifications, in the context of its role as the master orchestrator of molecular events conducive to capacitation.

Isoform-specific GSK3A activity is negatively correlated with human sperm motility

In mouse and bovine sperm, GSK3 activity is inversely proportional to motility. Targeted disruption of the GSK3A gene in testis results in normal spermatogenesis, but mature sperm present a reduced motility, rendering male mice infertile. On the other hand, GSK3B testis-specific KO is fertile. Yet in human sperm, an isoform-specific correlation between GSK3A and sperm motility was never established. In order to analyze GSK3 function in human sperm motility, normospermic and asthenozoospermic samples from adult males were used to correlate GSK3 expression and activity levels with human sperm motility profiles. Moreover, testicular and sperm GSK3 interactomes were identified using a yeast two-hybrid screen and coimmunoprecipitation, respectively. An extensive in-silico analysis of the GSK3 interactome was performed. The results proved that inhibited GSK3A (serine phosphorylated) presents a significant strong positive correlation (r = 0.822, P = 0.023) with the percentage of progressive human sperm, whereas inhibited GSK3B is not significantly correlated with sperm motility (r = 0.577, P = 0.175). The importance of GSK3 in human sperm motility was further reinforced by in-silico analysis of the GSK3 interactome, which revealed a high level of involvement of GSK3 interactors in sperm motility-related functions. The limitation of techniques used for GSK3 interactome identification can be a drawback, since none completely mimics the physiological environment. Our findings prove that human sperm motility relies on isoform-specific functions of GSK3A within this cell. Given the reported relevance of GSK3 protein-protein interactions in sperm motility, we hypothesized that they stand as potential targets for male contraceptive strategies based on sperm motility modulation.

Does the Pre-Ovulatory Pig Oviduct Rule Sperm Capacitation In Vivo Mediating Transcriptomics of Catsper Channels?

Spermatozoa need to conduct a series of biochemical changes termed capacitation in order to fertilize. In vivo, capacitation is sequentially achieved during sperm transport and interaction with the female genital tract, by mechanisms yet undisclosed in detail. However, when boar spermatozoa are stored in the tubal reservoir pre-ovulation, most appear to be in a non-capacitated state. This study aimed at deciphering the transcriptomics of capacitation-related genes in the pig pre-ovulatory oviduct, following the entry of semen or of sperm-free seminal plasma (SP). Ex-vivo samples of the utero-tubal junction (UTJ) and isthmus were examined with a microarray chip (GeneChip^® Porcine Gene 1.0 ST Array, Thermo Fisher Scientific) followed by bioinformatics for enriched analysis of functional categories (GO terms) and restrictive statistics. The results confirmed that entry of semen or of relative amounts of sperm-free SP modifies gene expression of these segments, pre-ovulation. It further shows that enriched genes are differentially associated with pathways relating to sperm motility, acrosome reaction, single fertilization, and the regulation of signal transduction GO terms. In particular, the pre-ovulation oviduct stimulates the Catsper channels for sperm Ca²⁺ influx, with AKAPs, CATSPERs, and CABYR genes being positive regulators while PKIs and CRISP1 genes appear to be inhibitors of the process. We postulate that the stimulation of PKIs and CRISP1 genes in the pre-ovulation sperm reservoir/adjacent isthmus, mediated by SP, act to prevent premature massive capacitation prior to ovulation.

Transcription Profiles of Age-at-Maturity-Associated Genes Suggest Cell Fate Commitment Regulation as a Key Factor in the Atlantic Salmon Maturation Process

Despite recent taxonomic diversification in studies linking genotype with phenotype, follow-up studies aimed at understanding the molecular processes of such genotype-phenotype associations remain rare. The age at which an individual reaches sexual maturity is an important fitness trait in many wild species. However, the molecular mechanisms regulating maturation timing processes remain obscure. A recent genome-wide association study in Atlantic salmon (Salmo salar) identified large-effect age-at-maturity-associated chromosomal regions including genes vgll3, akap11 and six6, which have roles in adipogenesis, spermatogenesis and the hypothalamic-pituitary-gonadal (HPG) axis, respectively. Here, we determine expression patterns of these genes during salmon development and their potential molecular partners and pathways. Using Nanostring transcription profiling technology, we show development- and tissue-specific mRNA expression patterns for vgll3, akap11 and six6. Correlated expression levels of vgll3 and akap11, which have adjacent chromosomal location, suggests they may have shared regulation. Further, vgll3 correlating with arhgap6 and yap1, and akap11 with lats1 and yap1 suggests that Vgll3 and Akap11 take part in actin cytoskeleton regulation. Tissue-specific expression results indicate that vgll3 and akap11 paralogs have sex-dependent expression patterns in gonads. Moreover, six6 correlating with slc38a6 and rtn1, and Hippo signaling genes suggests that Six6 could have a broader role in the HPG neuroendrocrine and cell fate commitment regulation, respectively. We conclude that Vgll3, Akap11 and Six6 may influence Atlantic salmon maturation timing via affecting adipogenesis and gametogenesis by regulating cell fate commitment and the HPG axis. These results may help to unravel general molecular mechanisms behind maturation.

A genome-wide single nucleotide polymorphism and copy number variation analysis for number of piglets born alive

Background

In this study we integrated the CNV (copy number variation) and WssGWAS (weighted single-step approach for genome-wide association) analyses to increase the knowledge about number of piglets born alive, an economically important reproductive trait with significant impact on production efficiency of pigs.

Results

A total of 3892 samples were genotyped with the Porcine SNP80 BeadChip. After quality control, a total of 57,962 high-quality SNPs from 3520 Duroc pigs were retained. The PennCNV algorithm identified 46,118 CNVs, which were aggregated by overlapping in 425 CNV regions (CNVRs) ranging from 2.5 Kb to 9718.4 Kb and covering 197 Mb (~ 7.01%) of the pig autosomal genome. The WssGWAS identified 16 genomic regions explaining more than 1% of the additive genetic variance for number of piglets born alive. The overlap between CNVR and WssGWAS analyses identified common regions on SSC2 (4.2–5.2 Mb), SSC3 (3.9–4.9 Mb), SSC12 (56.6–57.6 Mb), and SSC17 (17.3–18.3 Mb). Those regions are known for harboring important causative variants for pig reproductive traits based on their crucial functions in fertilization, development of gametes and embryos. Functional analysis by the Panther software identified 13 gene ontology biological processes significantly represented in this study such as reproduction, developmental process, cellular component organization or biogenesis, and immune system process, which plays relevant roles in swine reproductive traits.

Conclusion

Our research helps to improve the understanding of the genetic architecture of number of piglets born alive, given that the combination of GWAS and CNV analyses allows for a more efficient identification of the genomic regions and biological processes associated with this trait in Duroc pigs. Pig breeding programs could potentially benefit from a more accurate discovery of important genomic regions.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5687-0) contains supplementary material, which is available to authorized users.

Blood Vessel Epicardial Substance (BVES) in junctional signaling and cancer

Blood vessel epicardial substance (BVES) is a tight-junction associated protein that was originally discovered from a cDNA screen of the developing heart. Research over the last decade has shown that not only is BVES is expressed in cardiac and skeletal tissue, but BVES is also is expressed throughout the gastrointestinal epithelium. Mice lacking BVES sustain worse intestinal injury and inflammation. Furthermore, BVES is suppressed in gastrointestinal cancers, and mouse modeling has shown that loss of BVES promotes tumor formation. Recent work from multiple laboratories has revealed that BVES can regulate several molecular pathways, including cAMP, WNT, and promoting the degradation of the oncogene, c-Myc. This review will summarize our current understanding of how BVES regulates the intestinal epithelium and discuss how BVES functions at the molecular level to preserve epithelial phenotypes and suppress tumorigenesis.

SPATC1L maintains the integrity of the sperm head-tail junction

Spermatogenesis is a tightly regulated process involving germ cell-specific and germ cell-predominant genes. Here we investigate a novel germ cell-specific gene, Spatc1l (spermatogenesis and centriole associated 1 like). Expression analyses show that SPATC1L is expressed in mouse and human testes. We find that mouse SPATC1L localizes to the neck region in testicular sperm. Moreover, SPATC1L associates with the regulatory subunit of protein kinase A (PKA). Using CRISPR/Cas9-mediated genome engineering, we generate mice lacking SPATC1L. Disruption of Spatc1l in mice leads to male sterility owing to separation of sperm heads from tails. The lack of SPATC1L is associated with a reduction in PKA activity in testicular sperm, and we identify capping protein muscle Z-line beta as a candidate target of phosphorylation by PKA in testis. Taken together, our results implicate the SPATC1L-PKA complex in maintaining the stability of the sperm head-tail junction, thereby revealing a new molecular basis for sperm head-tail integrity.

A dual targeted β-defensin and exome sequencing approach to identify, validate and functionally characterise genes associated with bull fertility

Bovine fertility remains a critical issue underpinning the sustainability of the agricultural sector. Phenotypic records collected on >7,000 bulls used in artificial insemination (AI) were used to identify 160 reliable and divergently fertile bulls for a dual strategy of targeted sequencing (TS) of fertility-related β-defensin genes and whole exome sequencing (WES). A haplotype spanning multiple β-defensin genes and containing 94 SNPs was significantly associated with fertility and functional analysis confirmed that sperm from bulls possessing the haplotype showed significantly enhanced binding to oviductal epithelium. WES of all exons in the genome in 24 bulls of high and low fertility identified 484 additional SNPs significantly associated with fertility. After validation, the most significantly associated SNP was located in the FOXJ3 gene, a transcription factor which regulates sperm function in mice. This study represents the first comprehensive characterisation of genetic variation in bovine β-defensin genes and functional analysis supports a role for β-defensins in regulating bull sperm function. This first application of WES in AI bulls with divergent fertility phenotypes has identified a novel role for the transcription factor FOXJ3 in the regulation of bull fertility. Validated genetic variants associated with bull fertility could prove useful for improving reproductive outcomes in cattle.

The genetic basis and fitness consequences of sperm midpiece size in deer mice

An extensive array of reproductive traits varies among species, yet the genetic mechanisms that enable divergence, often over short evolutionary timescales, remain elusive. Here we examine two sister-species of Peromyscus mice with divergent mating systems. We find that the promiscuous species produces sperm with longer midpiece than the monogamous species, and midpiece size correlates positively with competitive ability and swimming performance. Using forward genetics, we identify a gene associated with midpiece length: Prkar1a, which encodes the R1α regulatory subunit of PKA. R1α localizes to midpiece in Peromyscus and is differentially expressed in mature sperm of the two species yet is similarly abundant in the testis. We also show that genetic variation at this locus accurately predicts male reproductive success. Our findings suggest that rapid evolution of reproductive traits can occur through cell type-specific changes to ubiquitously expressed genes and have an important effect on fitness.

Across species there are large changes in sperm morphology, yet how and why is unclear. Here, the authors show in Peromyscus mice, that the sperm midpiece length influences swimming speed and male fertility, and is regulated by cell-type specific expression of a ubiquitously expressed gene, Prkar1a.

Gene expression analysis at multiple time-points identifies key genes for nerve regeneration

INTRODUCTION

The purpose of this study was to provide a comprehensive understanding of gene expression during Wallerian degeneration and axon regeneration after peripheral nerve injury.

METHODS

A microarray was used to detect gene expression in the distal nerve 0, 3, 7, and 14 days after sciatic nerve crush. Bioinformatic analysis was used to predict function of the differentially expressed mRNAs. Microarray results and the key pathways were validated by quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

Differentially expressed mRNAs at different time-points (3, 7, and 14 days) after injury were identified and compared with a control group (0 day). Nine general trends of changes in gene expression were identified. Key signal pathways and 9 biological processes closely associated with nerve regeneration were identified and verified.

CONCLUSIONS

Differentially expressed genes and biological processes and pathways associated with axonal regeneration may elucidate the molecular-biological mechanisms underlying peripheral nerve regeneration. Muscle Nerve 55: 373-383, 2017.

The vgll3 Locus Controls Age at Maturity in Wild and Domesticated Atlantic Salmon (Salmo salar L.) Males

Wild and domesticated Atlantic salmon males display large variation for sea age at sexual maturation, which varies between 1-5 years. Previous studies have uncovered a genetic predisposition for variation of age at maturity with moderate heritability, thus suggesting a polygenic or complex nature of this trait. The aim of this study was to identify associated genetic loci, genes and ultimately specific sequence variants conferring sea age at maturity in salmon. We performed a genome wide association study (GWAS) using a pool sequencing approach (20 individuals per river and phenotype) of male salmon returning to rivers as sexually mature either after one sea winter (2009) or three sea winters (2011) in six rivers in Norway. The study revealed one major selective sweep, which covered 76 significant SNPs in which 74 were found in a 370 kb region of chromosome 25. Genotyping other smolt year classes of wild and domesticated salmon confirmed this finding. Genotyping domesticated fish narrowed the haplotype region to four SNPs covering 2386 bp, containing the vgll3 gene, including two missense mutations explaining 33-36% phenotypic variation. A single locus was found to have a highly significant role in governing sea age at maturation in this species. The SNPs identified may be both used as markers to guide breeding for late maturity in salmon aquaculture and in monitoring programs of wild salmon. Interestingly, a SNP in proximity of the VGLL3 gene in humans (Homo sapiens), has previously been linked to age at puberty suggesting a conserved mechanism for timing of puberty in vertebrates.

AKAP3 degradation in sperm capacitation is regulated by its tyrosine phosphorylation

BACKGROUND

The A-kinase anchoring protein (AKAP) family is essential for sperm motility, capacitation and the acrosome reaction. PKA-dependent protein tyrosine phosphorylation occurs in mammalian sperm capacitation including AKAP3. In a recent study, we showed that AKAP3 undergoes degradation under capacitation conditions. Thus, we tested here whether AKAP3 degradation might be regulated by its tyrosine phosphorylation.

METHODS

The intracellular levels of AKAP3 were determined by western blot (WB) analysis using specific anti-AKAP3 antibodies. Tyrosine phosphorylation of AKAP3 was tested by immunoprecipitation and WB analysis. Acrosome reaction was examined using FITC-pisum sativum agglutinin.

RESULTS

AKAP3 is degraded and undergoes tyrosine-dephosphorylation during sperm capacitation and the degradation was reduced by inhibition of tyrosine phosphatase and enhanced by inhibition of tyrosine kinase. Sperm starvation or inhibition of mitochondrial respiration, which reduce cellular ATP levels, significantly accelerated AKAP3 degradation. Treatment with vanadate, or Na(+) or bicarbonate depletion, reduced AKAP3-degradation and the AR rate, while antimycin A or NH4Cl elevated both AKAP3-degradation and the AR degree. Treatment of sperm with NH4Cl enhanced PKA-dependent phosphorylation of four proteins, further supporting the involvement of AKAP3-degradation in capacitation. To demonstrate more specifically that sperm capacitation requires AKAP3-degradation, we inhibited AKAP3-degradation using anti-AKAP3 antibody in permeabilized cells. The anti-AKAP3-antibody induced significant inhibition of AKAP3-degradation and of the AR rate.

CONCLUSION

Sperm capacitation process requires AKAP3-degradation, and the degradation degree is regulated by the level of AKAP3 tyrosine phosphorylation.

GENERAL SIGNIFICANCE

Better understanding of the molecular mechanisms that mediate sperm capacitation can be used for infertility diagnosis, treatment and the developing of male contraceptives.

Neurochondrin is an atypical RIIα-specific A-kinase anchoring protein

Protein kinase activity is regulated not only by direct strategies affecting activity but also by spatial and temporal regulatory mechanisms. Kinase signaling pathways are coordinated by scaffolding proteins that orchestrate the assembly of multi-protein complexes. One family of such scaffolding proteins are the A-kinase anchoring proteins (AKAPs). AKAPs share the commonality of binding cAMP-dependent protein kinase (PKA). In addition, they bind further signaling proteins and kinase substrates and tether such multi-protein complexes to subcellular locations. The A-kinase binding (AKB) domain of AKAPs typically contains a conserved helical motif that interacts directly with the dimerization/docking (D/D) domain of the regulatory subunits of PKA. Based on a pull-down proteomics approach, we identified neurochondrin (neurite-outgrowth promoting protein) as a previously unidentified AKAP. Here, we show that neurochondrin interacts directly with PKA through a novel mechanism that involves two distinct binding regions. In addition, we demonstrate that neurochondrin has strong isoform selectivity towards the RIIα subunit of PKA with nanomolar affinity. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases.

PKA compartmentalization via AKAP220 and AKAP12 contributes to endothelial barrier regulation

cAMP-mediated PKA signaling is the main known pathway involved in maintenance of the endothelial barrier. Tight regulation of PKA function can be achieved by discrete compartmentalization of the enzyme via physical interaction with A-kinase anchoring proteins (AKAPs). Here, we investigated the role of AKAPs 220 and 12 in endothelial barrier regulation. Analysis of human and mouse microvascular endothelial cells as well as isolated rat mesenteric microvessels was performed using TAT-Ahx-AKAPis peptide, designed to competitively inhibit PKA-AKAP interaction. In vivo microvessel hydraulic conductivity and in vitro transendothelial electrical resistance measurements showed that this peptide destabilized endothelial barrier properties, and dampened the cAMP-mediated endothelial barrier stabilization induced by forskolin and rolipram. Immunofluorescence analysis revealed that TAT-Ahx-AKAPis led to both adherens junctions and actin cytoskeleton reorganization. Those effects were paralleled by redistribution of PKA and Rac1 from endothelial junctions and by Rac1 inactivation. Similarly, membrane localization of AKAP220 was also reduced. In addition, depletion of either AKAP12 or AKAP220 significantly impaired endothelial barrier function and AKAP12 was also shown to interfere with cAMP-mediated barrier enhancement. Furthermore, immunoprecipitation analysis demonstrated that AKAP220 interacts not only with PKA but also with VE-cadherin and ß-catenin. Taken together, these results indicate that AKAP-mediated PKA subcellular compartmentalization is involved in endothelial barrier regulation. More specifically, AKAP220 and AKAP12 contribute to endothelial barrier function and AKAP12 is required for cAMP-mediated barrier stabilization.

AKAP3 synthesis is mediated by RNA binding proteins and PKA signaling during mouse spermiogenesis

Mammalian spermatogenesis is regulated by coordinated gene expression in a spatiotemporal manner. The spatiotemporal regulation of major sperm proteins plays important roles during normal development of the male gamete, of which the underlying molecular mechanisms are poorly understood. A-kinase anchoring protein 3 (AKAP3) is one of the major components of the fibrous sheath of the sperm tail that is formed during spermiogenesis. In the present study, we analyzed the expression of sperm-specific Akap3 and the potential regulatory factors of its protein synthesis during mouse spermiogenesis. Results showed that the transcription of Akap3 precedes its protein synthesis by about 2 wk. Nascent AKAP3 was found to form protein complex with PKA and RNA binding proteins (RBPs), including PIWIL1, PABPC1, and NONO, as revealed by coimmunoprecipitation and protein mass spectrometry. RNA electrophoretic gel mobility shift assay showed that these RBPs bind sperm-specific mRNAs, of which proteins are synthesized during the elongating stage of spermiogenesis. Biochemical and cell biological experiments demonstrated that PIWIL1, PABPC1, and NONO interact with each other and colocalize in spermatids' RNA granule, the chromatoid body. In addition, NONO was found in extracytoplasmic granules in round spermatids, whereas PIWIL1 and PABPC1 were diffusely localized in cytoplasm of elongating spermatids, indicating their participation at different steps of mRNA metabolism during spermatogenesis. Interestingly, type I PKA subunits colocalize with PIWIL1 and PABPC1 in the cytoplasm of elongating spermatids and cosediment with the RBPs in polysomal fractions on sucrose gradients. Further biochemical analyses revealed that activation of PKA positively regulates AKAP3 protein synthesis without changing its mRNA level in elongating spermatids. Taken together, these results indicate that PKA signaling directly participates in the regulation of protein translation in postmeiotic male germ cells, underscoring molecular mechanisms that regulate protein synthesis during mouse spermiogenesis.

Dissociation between AKAP3 and PKARII promotes AKAP3 degradation in sperm capacitation

Ejaculated spermatozoa must undergo a series of biochemical modifications called capacitation, prior to fertilization. Protein-kinase A (PKA) mediates sperm capacitation, although its regulation is not fully understood. Sperm contain several A-kinase anchoring proteins (AKAPs), which are scaffold proteins that anchor PKA. In this study, we show that AKAP3 is degraded in bovine sperm incubated under capacitation conditions. The degradation rate is variable in sperm from different bulls and is correlated with the capacitation ability. The degradation of AKAP3 was significantly inhibited by MG-132, a proteasome inhibitor, indicating that AKAP3 degradation occurs via the proteasomal machinery. Treatment with Ca²⁺-ionophore induced further degradation of AKAP3; however, this effect was found to be enhanced in the absence of Ca²⁺ in the medium or when intracellular Ca²⁺ was chelated the degradation rate of AKAP3 was significantly enhanced when intracellular space was alkalized using NH₄Cl, or when sperm were treated with Ht31, a peptide that contains the PKA-binding domain of AKAPs. Moreover, inhibition of PKA activity by H89, or its activation using 8Br-cAMP, increased AKAP3 degradation rate. This apparent contradiction could be explained by assuming that binding of PKA to AKAP3 protects AKAP3 from degradation. We conclude that AKAP3 degradation is regulated by intracellular alkalization and PKA_RII anchoring during sperm capacitation.

AKAP9 is essential for spermatogenesis and sertoli cell maturation in mice

Mammalian male fertility relies on complex inter- and intracellular signaling during spermatogenesis. Here we describe three alleles of the widely expressed A-kinase anchoring protein 9 (Akap9) gene, all of which cause gametogenic failure and infertility in the absence of marked somatic phenotypes. Akap9 disruption does not affect spindle nucleation or progression of prophase I of meiosis but does inhibit maturation of Sertoli cells, which continue to express the immaturity markers anti-Mullerian hormone and thyroid hormone receptor alpha in adults and fail to express the maturation marker p27(Kip1). Furthermore, gap and tight junctions essential for blood-testis barrier (BTB) organization are disrupted. Connexin43 (Cx43) and zona occludens-1 are improperly localized in Akap9 mutant testes, and Cx43 fails to compartmentalize germ cells near the BTB. These results identify and support a novel reproductive tissue-specific role for Akap9 in the coordinated regulation of Sertoli cells in the testis.

Analysis of the key elements of FFAT-like motifs identifies new proteins that potentially bind VAP on the ER, including two AKAPs and FAPP2

Background

Two phenylalanines (FF) in an acidic tract (FFAT)-motifs were originally described as having seven elements: an acidic flanking region followed by 6 residues (EFFDA–E). Such motifs are found in several lipid transfer protein (LTP) families, and they interact with a protein on the cytosolic face of the ER called vesicle-associated membrane protein-associated protein (VAP). Mutation of which causes ER stress and motor neuron disease, making it important to determine which proteins bind VAP. Among other proteins that bind VAP, some contain FFAT-like motifs that are missing one or more of the seven elements. Defining how much variation is tolerated in FFAT-like motifs is a preliminary step prior to the identification of the full range of VAP interactors.

Results

We used a quantifiable in vivo system that measured ER targeting in a reporter yeast strain that over-expressed VAP to study the effect of substituting different elements of FFAT-like motifs in turn. By defining FFAT-like motifs more widely than before, we found them in novel proteins the functions of which had not previously been directly linked to the ER, including: two PKA anchoring proteins, AKAP220 and AKAP110; a family of plant LTPs; and the glycolipid LTP phosphatidylinositol-four-phosphate adaptor-protein-2 (FAPP-2).

Conclusion

All of the seven essential elements of a FFAT motif tolerate variation, and weak targeting to the ER via VAP is still detected if two elements are substituted. In addition to the strong FFAT motifs already known, there are additional proteins with weaker FFAT-like motifs, which might be functionally important VAP interactors.

An entirely specific type I A-kinase anchoring protein that can sequester two molecules of protein kinase A at mitochondria

A-kinase anchoring proteins (AKAPs) tether the cAMP-dependent protein kinase (PKA) to intracellular sites where they preferentially phosphorylate target substrates. Most AKAPs exhibit nanomolar affinity for the regulatory (RII) subunit of the type II PKA holoenzyme, whereas dual-specificity anchoring proteins also bind the type I (RI) regulatory subunit of PKA with 10-100-fold lower affinity. A range of cellular, biochemical, biophysical, and genetic approaches comprehensively establish that sphingosine kinase interacting protein (SKIP) is a truly type I-specific AKAP. Mapping studies located anchoring sites between residues 925-949 and 1,140-1,175 of SKIP that bind RI with dissociation constants of 73 and 774 nM, respectively. Molecular modeling and site-directed mutagenesis approaches identify Phe 929 and Tyr 1,151 as RI-selective binding determinants in each anchoring site. SKIP complexes exist in different states of RI-occupancy as single-molecule pull-down photobleaching experiments show that 41 ± 10% of SKIP sequesters two YFP-RI dimers, whereas 59 ± 10% of the anchoring protein binds a single YFP-RI dimer. Imaging, proteomic analysis, and subcellular fractionation experiments reveal that SKIP is enriched at the inner mitochondrial membrane where it associates with a prominent PKA substrate, the coiled-coil helix protein ChChd3.

Isoform-specific targeting of PKA to multivesicular bodies

PKA RIα subunit is localized to MVBs by the A-kinase–anchoring protein AKAP11 when disassociated from the PKA catalytic subunit.

Although RII protein kinase A (PKA) regulatory subunits are constitutively localized to discrete cellular compartments through binding to A-kinase–anchoring proteins (AKAPs), RI subunits are primarily diffuse in the cytoplasm. In this paper, we report a novel AKAP-dependent localization of RIα to distinct organelles, specifically, multivesicular bodies (MVBs). This localization depends on binding to AKAP11, which binds tightly to free RIα or RIα in complex with catalytic subunit (holoenzyme). However, recruitment to MVBs occurs only with the release of PKA catalytic subunit (PKAc). This recruitment is reversed by reassociation with PKAc, and it is disrupted by the presence of AKAP peptides, mutations in the RIα AKAP-binding site, or knockdown of AKAP11. Cyclic adenosine monophosphate binding not only unleashes active PKAc but also leads to the targeting of AKAP11:RIα to MVBs. Therefore, we show that the RIα holoenzyme is part of a signaling complex with AKAP11, in which AKAP11 may direct RIα functionality after disassociation from PKAc. This model defines a new paradigm for PKA signaling.

Anchored protein kinase A recruitment of active Rac GTPase

Protein kinase A-anchoring proteins (AKAPs) influence fundamental cellular processes by directing the cAMP-dependent protein kinase (PKA) toward its intended substrates. In this report we describe the identification and characterization of a ternary complex of AKAP220, the PKA holoenzyme, and the IQ domain GTPase-activating protein 2 isoform (IQGAP2) that is enriched at cortical regions of the cell. Formation of an IQGAP2-AKAP220 core complex initiates a subsequent phase of protein recruitment that includes the small GTPase Rac. Biochemical and molecular biology approaches reveal that PKA phosphorylation of Thr-716 on IQGAP2 enhances association with the active form of the Rac GTPase. Cell-based experiments indicate that overexpression of an IQGAP2 phosphomimetic mutant (IQGAP2 T716D) enhances the formation of actin-rich membrane ruffles at the periphery of HEK 293 cells. In contrast, expression of a nonphosphorylatable IQGAP2 T716A mutant or gene silencing of AKAP220 suppresses formation of membrane ruffles. These findings imply that IQGAP2 and AKAP220 act synergistically to sustain PKA-mediated recruitment of effectors such as Rac GTPases that impact the actin cytoskeleton.

Protein phosphatase 1 complexes modulate sperm motility and present novel targets for male infertility

Infertility is a growing concern in modern society, with 30% of cases being due to male factors, namely reduced sperm concentration, decreased motility and abnormal morphology. Sperm cells are highly compartmentalized, almost devoid of transcription and translation consequently processes such as protein phosphorylation provide a key general mechanism for regulating vital cellular functions, more so than for undifferentiated cells. Reversible protein phosphorylation is the principal mechanism regulating most physiological processes in eukaryotic cells. To date, hundreds of protein kinases have been identified, but significantly fewer phosphatases (PPs) are responsible for counteracting their action. This discrepancy can be explained in part by the mechanism used to control phosphatase activity, which is based on regulatory interacting proteins. This is particularly true for PP1, a major serine/threonine-PP, for which >200 interactors (PP1 interacting proteins-PIPs) have been indentified that control its activity, subcellular location and substrate specificity. For PP1, several isoforms have been described, among them PP1γ2, a testis/sperm-enriched PP1 isoform. Recent findings support our hypothesis that PP1γ2 is involved in the regulation of sperm motility. This review summarizes the known sperm-specific PP1-PIPs, involved in the acquisition of mammalian sperm motility. The complexes that PP1 routinely forms with different proteins are addressed and the role of PP1/A-kinase anchoring protein complexes in sperm motility is considered. Furthermore, the potential relevance of targeting PP1-PIPs complexes to infertility diagnostics and therapeutics as well as to male contraception is also discussed.

The identification of novel cyclic AMP-dependent protein kinase anchoring proteins using bioinformatic filters and peptide arrays

A-kinase anchoring proteins (AKAPs) localize cyclic AMP-dependent protein kinase (PKA) to specific regions in the cell and place PKA in proximity to its phosphorylation targets. A computational model was created to identify AKAPs that bind to the docking/dimerization domain of the RII alpha isoform of the regulatory subunit of PKA. The model was used to search the entire human proteome, and the top candidates were tested for an interaction using peptide array experiments. Verified interactions include sphingosine kinase interacting protein and retinoic acid-induced protein 16. These interactions highlight new signaling pathways mediated by PKA.

Networking with AKAPs: context-dependent regulation of anchored enzymes

A-Kinase Anchoring Proteins (AKAPs) orchestrate and synchronize cellular events by tethering the cAMP-dependent protein kinase (PKA) and other signaling enzymes to organelles and membranes. The control of kinases and phosphatases that are held in proximity to activators, effectors, and substrates favors the rapid dissemination of information from one cellular location to the next. This article charts the inception of the PKA-anchoring hypothesis, the characterization of AKAPs and their nomenclature, and the physiological roles of context-specific AKAP signaling complexes.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 3: developmental changes in spermatid flagellum and cytoplasmic droplet and interaction of sperm with the zona pellucida and egg plasma membrane

Spermiogenesis constitutes the steps involved in the metamorphosis of spermatids into spermatozoa. It involves modification of several organelles in addition to the formation of several structures including the flagellum and cytoplasmic droplet. The flagellum is composed of a neck region and middle, principal, and end pieces. The axoneme composed of nine outer microtubular doublets circularly arranged to form a cylinder around a central pair of microtubules is present throughout the flagellum. The middle and principal pieces each contain specific components such as the mitochondrial sheath and fibrous sheath, respectively, while outer dense fibers are common to both. A plethora of proteins are constituents of each of these structures, with each playing key roles in functions related to the fertility of spermatozoa. At the end of spermiogenesis, a portion of spermatid cytoplasm remains associated with the released spermatozoa, referred to as the cytoplasmic droplet. The latter has as its main feature Golgi saccules, which appear to modify the plasma membrane of spermatozoa as they move down the epididymal duct and hence may be partly involved in male gamete maturation. The end product of spermatogenesis is highly streamlined and motile spermatozoa having a condensed nucleus equipped with an acrosome. Spermatozoa move through the female reproductive tract and eventually penetrate the zona pellucida and bind to the egg plasma membrane. Many proteins have been implicated in the process of fertilization as well as a plethora of proteins involved in the development of spermatids and sperm, and these are high lighted in this review.

Differential gene expression in human granulosa cells from recombinant FSH versus human menopausal gonadotropin ovarian stimulation protocols

BACKGROUND

The study was designed to test the hypothesis that granulosa cell (GC) gene expression response differs between recombinant FSH and human menopausal gonadotropin (hMG) stimulation regimens.

METHODS

Females < 35 years-old undergoing IVF for tubal or male factor infertility were prospectively randomized to one of two stimulation protocols, GnRH agonist long protocol plus individualized dosages of (1) recombinant (r)FSH (Gonal-F) or (2) purified human menopausal gonadotropin (hMG; Menopur). Oocytes were retrieved 35 h post-hCG, and GC were collected. Total RNA was extracted from each GC sample, biotinylated cRNA was synthesized, and each sample was run on Human Genome Bioarrays (Applied Microarrays). Unnamed genes and genes with <2-fold difference in expression were excluded.

RESULTS

After exclusions, 1736 genes exhibited differential expression between groups. Over 400 were categorized as signal transduction genes, approximately 180 as transcriptional regulators, and approximately 175 as enzymes/metabolic genes. Expression of selected genes was confirmed by RT-PCR. Differentially expressed genes included A kinase anchor protein 11 (AKAP11), bone morphogenetic protein receptor II (BMPR2), epidermal growth factor (EGF), insulin-like growth factor binding protein (IGFBP)-4, IGFBP-5, and hypoxia-inducible factor (HIF)-1 alpha.

CONCLUSIONS

Results suggest that major differences exist in the mechanism by which pure FSH alone versus FSH/LH regulate gene expression in preovulatory GC that could impact oocyte maturity and developmental competence.

Protein kinases A and C and phosphatidylinositol 3 kinase regulate glycogen synthase kinase-3A serine 21 phosphorylation in boar spermatozoa

The cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) pathways control most relevant functions in male germ cells including motility. Recently we demonstrated that phosphorylation state of glycogen synthase kinase-3alpha (GSK3A) is also a key event in the control of boar spermatozoa motility. However, the upstream regulators of GSK3A serine phosphorylation (inhibition) in male germ cells remain largely unknown. This work investigates the involvement of PKA, PKC and PI3K pathways in GSK3A phosphorylation in boar spermatozoa. A capacitating medium (TCM) or the phosphodiesterase-resistant cell permeable cAMP analogue 8Br-cAMP cause a significant increase in Ser21 GSK3A phosphorylation associated with a simultaneous significant increase in boar spermatozoa motility. These effects are blocked after preincubation of spermatozoa with PKA inhibitor H89 or PKC inhibitor Ro-32-0432. The PI3K inhibitor LY294002 increases both spermatozoa motility parameters and the basal GSK3A phosphorylation, but does not affect either TCM- or 8Br-cAMP-stimulated GSK3A phosphorylation. PI3K inhibition effects are mediated by an increase in intracellular cAMP levels in boar spermatozoa and are suppressed by PKA inhibitor H89. In summary, we demonstrate that PKA, PKC and PI3K pathways crosstalk in porcine male germ cells to crucially regulate GSK3A phosphorylation which subsequently controls cell motility. In addition, our results suggest that PI3K is upstream of PKA which lies upstream of PKC in this regulatory cascade(s). Our findings contribute to elucidate the molecular mechanisms underlying the regulation of one of the most relevant male germ cell functions, motility.

Structure of D-AKAP2:PKA RI complex: insights into AKAP specificity and selectivity

A-kinase anchoring proteins (AKAPs) regulate cyclic AMP-dependent protein kinase (PKA) signaling in space and time. Dual-specific AKAP 2 (D-AKAP2) binds to the dimerization/docking (D/D) domain of both RI and RII regulatory subunits of PKA with high affinity. Here we have determined the structures of the RIalpha D/D domain alone and in complex with D-AKAP2. The D/D domain presents an extensive surface for binding through a well-formed N-terminal helix, and this surface restricts the diversity of AKAPs that can interact. The structures also underscore the importance of a redox-sensitive disulfide in affecting AKAP binding. An unexpected shift in the helical register of D-AKAP2 compared to the RIIalpha:D-AKAP2 complex structure makes the mode of binding to RIalpha novel. Finally, the comparison allows us to deduce a molecular explanation for the sequence and spatial determinants of AKAP specificity.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Splicing factor arginine/serine-rich 17A (SFRS17A) is an A-kinase anchoring protein that targets protein kinase A to splicing factor compartments

Protein kinase A (PKA) is targeted to distinct subcellular localizations by specific protein kinase A anchoring proteins (AKAPs). AKAPs are divided into subclasses based on their ability to bind type I or type II PKA or both. Dual-specificity AKAPs were recently reported to have an additional PKA binding determinant called the RI specifier region. A bioinformatic search with the consensus RI specifier region identified a novel AKAP, the splicing factor arginine/serine-rich 17A (SFRS17A). Here, we show by a variety of protein interaction assays that SFRS17A binds both type I and type II PKA in vitro and inside cells, demonstrating that SFRS17A is a dual-specific AKAP. Moreover, immunofluorescence experiments show that SFRS17A colocalizes with the catalytic subunit of PKA as well as the splicing factor SC35 in splicing factor compartments. Using the E1A minigene splicing assay, we found that expression of wild type SFRS17A conferred regulation of E1A alternative splicing, whereas the mutant SFRS17A, which is unable to bind PKA, did not. Our data suggest that SFRS17A is an AKAP involved in regulation of pre-mRNA splicing possibly by docking a pool of PKA in splicing factor compartments.

Calyculin A-sensitive protein phosphatases are involved in maintenance of progressive movement in mouse spermatozoa in vitro by suppression of autophosphorylation of protein kinase A

Protein serine/threonine phosphorylation in mammalian sperm flagella has been considered to play important roles in regulation of motility. Protein phosphorylation state reflects balance of enzymatic activities between protein phosphatases and protein kinases [predominantly protein kinase A (PKA)]. The aims of this study were to disclose roles of protein phosphatases in the regulation of sperm motility and to provide evidence for suppression of PKA full activation by protein phosphatases in sperm flagella. Mouse epididymal spermatozoa were incubated with a cell-permeable protein phosphatase 1 (PP1)/protein phosphatase 2A (PP2A) inhibitor (calyculin A: 25-125 nM) at 37.5 C. After incubation, they were used for immunodetection of phosphorylated proteins, PKA and PP1 gamma2, assessment for motility and co-immunoprecipitation of PP1gamma2 with PKA. Incubation with calyculin A enhanced the phosphorylation states of several proteins (>250 kDa, 170 kDa, 155 kDa, 140 kDa and 42 kDa for serine/threonine phosphorylation and 70 kDa for tyrosine phosphorylation) and PKA catalytic subunits [at the autophosphorylation residue (Thr-197) for its full enzymatic activation] in the flagella. Coincidently, this incubation induced changes of sperm flagellar movement from the progressive type to the hyperactivation-like type. Indirect immunofluorescence and co-immunoprecipitation showed that PKA was co-localized with PP1 gamma2 in the principal pieces of sperm flagella. These findings suggest that calyculin A-sensitive protein phosphatases (PP1/PP2A) suppress full activation of PKA as well as enhancement of the phosphorylation states of other flagellar proteins in sperm flagella in order to prevent precocious changes of flagellar movement from the progressive type to hyperactivation.

Selectivity in enrichment of cAMP-dependent protein kinase regulatory subunits type I and type II and their interactors using modified cAMP affinity resins

cAMP regulates cellular functions primarily by activating PKA. The involvement of PKAs in various signaling pathways occurring simultaneously in different cellular compartments necessitates stringent spatial and temporal regulation. This specificity is largely achieved by binding of PKA to protein scaffolds, whereby a distinct group of proteins called A kinase anchoring proteins (AKAPs) play a dominant role. AKAPs are a diverse family of proteins that all bind via a small PKA binding domain to the regulatory subunits of PKA. The binding affinities between PKA and several AKAPs can be different for different isoforms of the regulatory subunits of PKA. Here we employ a combination of affinity chromatography and mass spectrometry-based quantitative proteomics to investigate specificity in PKA-AKAP interactions. Three different immobilized cAMP analogs were used to enrich for PKA and its interacting proteins from several systems; HEK293 and RCC10 cells and rat lung and testis tissues. Stable isotope labeling was used to confidently identify and differentially quantify target proteins and their preferential binding affinity for the three different cAMP analogs. We were able to enrich all four isoforms of the regulatory subunits of PKA and concomitantly identify more than 10 AKAPs. A selective enrichment of the PKA RI isoforms could be achieved; which allowed us to unravel which AKAPs bind preferentially to the RI or RII regulatory domains of PKA. Of the twelve AKAPs detected, seven preferentially bound to RII, whereas the remaining five displayed at least dual specificity with a potential preference for RI. For some of these AKAPs our data provide the first insights into their specificity.

Cell-permeable peptide-based disruption of endogenous PKA-AKAP complexes: a tool for studying the molecular roles of AKAP-mediated PKA subcellular anchoring

Stimulation of numerous G protein-coupled receptors leads to the elevation of intracellular concentrations of cAMP, which subsequently activates the PKA pathway. Specificity of the PKA signaling module is determined by a sophisticated subcellular targeting network that directs the spatiotemporal activation of the kinase. This specific compartmentalization mechanism occurs through high-affinity interactions of PKA with A-kinase anchoring proteins (AKAPs), the role of which is to target the kinase to discrete subcellular microdomains. Recently, a peptide designated "AKAPis" has been proposed to competitively inhibit PKA-AKAP interactions in vitro. We therefore sought to characterize a cell-permeable construct of the AKAPis inhibitor and use it as a tool to characterize the impact of PKA compartmentalization by AKAPs. Using insulin-secreting pancreatic beta-cells (INS-1 cells), we showed that TAT-AKAPis (at a micromolar range) dose dependently disrupted a significant fraction of endogenous PKA-AKAP interactions. Immunoflurescent analysis also indicated that TAT-AKAPis significantly affected PKA subcellular localization. Furthermore, TAT-AKAPis markedly attenuated glucagon-induced phosphorylations of p44/p42 MAPKs and cAMP response element binding protein, which are downstream effectors of PKA. In parallel, TAT-AKAPis dose dependently inhibited the glucagon-induced potentiation of insulin release. Therefore, AKAP-mediated subcellular compartmentalization of PKA represents a key mechanism for PKA-dependent phosphorylation events and potentiation of insulin secretion in intact pancreatic beta-cells. More interestingly, our data highlight the effectiveness of the cell-permeable peptide-mediated approach to monitoring in cellulo PKA-AKAP interactions and delineating PKA-dependent phosphorylation events underlying specific cellular responses.

Dual specificity A-kinase anchoring proteins (AKAPs) contain an additional binding region that enhances targeting of protein kinase A type I

A-kinase anchoring proteins (AKAPs) target protein kinase A (PKA) to a variety of subcellular locations. Conventional AKAPs contain a 14-18-amino acid sequence that forms an amphipathic helix that binds with high affinity to the regulatory (R) subunit of PKA type II. More recently, a group of dual specificity AKAPs has been classified on the basis of their ability to bind the PKA type I and the PKA type II isozymes. In this study we show that dual specificity AKAPs contain an additional PKA binding determinant called the RI Specifier Region (RISR). A variety of protein interaction assays and immunoprecipitation and immunolocalization experiments indicates that the RISR augments RI binding in vitro and inside cells. Cellular delivery of the RISR peptide uncouples RI anchoring to Ezrin leading to release of T cell inhibition by cAMP. Likewise, expression of mutant Ezrin forms where RI binding has been abrogated by substitution of the RISR sequence prevents cAMP-mediated inhibition of T cell function. Thus, we propose that the RISR acts in synergy with the amphipathic helix in dual specificity anchoring proteins to enhance anchoring of PKA type I.

AKAP220 colocalizes with AQP2 in the inner medullary collecting ducts

During dehydration, protein kinase A phosphorylates aquaporin 2 (AQP2) at serine 256 and this is essential for apical membrane sorting of AQP2 in the collecting ducts. A-kinase anchoring proteins (AKAPs) bind protein kinase A and protein phosphatases conferring substrate specificity to these enzymes and localize them to the appropriate intracellular compartment. We found that AKAP220 bound to AQP2 in a yeast two-hybrid screen. Further, it was highly localized to the papilla compared to other regions of the kidney. Using double immunofluorescence and immunoelectron microscopy we found that AKAP220 co-localized with AQP2 in the cytosol of the inner medullary collecting ducts. Forskolin-mediated phosphorylation of AQP2, transiently expressed in COS cells, was increased by AKAP220 co-expression. Our results suggest that AKAP220 may be involved in the phosphorylation of AQP2 by recruiting protein kinase A.

Bicarbonate-induced phosphorylation of p270 protein in mouse sperm by cAMP-dependent protein kinase

Signaling by cAMP-dependent protein kinase (PKA) plays an important role in the regulation of mammalian sperm motility. However, it has not been determined how PKA signaling leads to changes in motility, and specific proteins responsible for these changes have not yet been identified as PKA substrates. Anti-phospho-(Ser/Thr) PKA substrate antibodies detected a sperm protein with a relative molecular weight of 270,000 (p270), which was phosphorylated within 1 min after incubation in a medium supporting capacitation. Phosphorylation of p270 was induced by bicarbonate or a cAMP analog, but was blocked by the PKA inhibitor H-89, indicating that p270 is likely a PKA substrate in sperm. In addition, phosphorylation of p270 was inhibited by stearated peptide st-Ht31, suggesting that p270 is phosphorylated by PKA associated with an A-kinase anchoring protein (AKAP). AKAP4 is the major fibrous sheath protein of mammalian sperm and tethers regulatory subunits of PKA to localize phosphorylation events. Phosphorylation of p270 occurred in sperm lacking AKAP4, suggesting that AKAP4 is not involved directly in the phosphorylation event. Phosphorylated p270 was enriched in fractionated sperm tails and appeared to be present in multiple compartments including a detergent-resistant membrane fraction. PKA phosphorylation of p270 within 1 min of incubation under capacitation conditions suggests that this protein may have an important role in the initial signaling events that lead to the activation and subsequent hyperactivation of sperm motility.

GABAC-receptor stimulation activates cAMP-dependent protein kinase via A-kinase anchoring protein 220

In our previous study, anti-apoptotic effects of GABA(C)-receptor stimulation was suppressed by inhibitors of cAMP-dependent protein kinase (PKA), implying GABA(C) receptor-mediated PKA activation. The present study showed that GABA(C)-receptor stimulation with its agonist, cis-4-aminocrotonic acid (CACA), protected cultured hippocampal neurons from amyloid beta 25 - 35 (Abeta25 - 35) peptide-enhanced glutamate neurotoxicity. This protective effect of CACA was blocked by PKA inhibitors, KT 5720 and H-89, as well as a specific GABA(C)-receptor antagonist, (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA). To test the possibility of GABA(C) receptor-mediated PKA activation, association of GABA(C) receptor with A-kinase anchoring proteins (AKAPs) and effect of an AKAP antisense oligonucleotide on the PKA activation were examined in primary cultured rat hippocampal neurons. Stimulation of the cells with CACA-activated PKA was assessed by the phosphorylated PKA substrate (135 kDa) level. Specific antibodies raised against GABA(C)-receptor rho subunits precipitated each rho subunit, AKAP220, and PKA regulatory and catalytic subunits from rat brain lysates, suggesting that rho is associated with the AKAP220/PKA complex. Furthermore, antisense oligonucleotide of AKAP220 suppressed such GABA(C) stimulation-induced PKA activation, suggesting that GABA(C)-receptor stimulation activates PKA via AKAP220.

Inhibition of T cell activation by cyclic adenosine 5'-monophosphate requires lipid raft targeting of protein kinase A type I by the A-kinase anchoring protein ezrin

cAMP negatively regulates T cell immune responses by activation of type I protein kinase A (PKA), which in turn phosphorylates and activates C-terminal Src kinase (Csk) in T cell lipid rafts. Using yeast two-hybrid screening, far-Western blot, immunoprecipitation and immunofluorescense analyses, and small interfering RNA-mediated knockdown, we identified Ezrin as the A-kinase anchoring protein that targets PKA type I to lipid rafts. Furthermore, Ezrin brings PKA in proximity to its downstream substrate Csk in lipid rafts by forming a multiprotein complex consisting of PKA/Ezrin/Ezrin-binding protein 50, Csk, and Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains. The complex is initially present in immunological synapses when T cells contact APCs and subsequently exits to the distal pole. Introduction of an anchoring disruptor peptide (Ht31) into T cells competes with Ezrin binding to PKA and thereby releases the cAMP/PKA type I-mediated inhibition of T cell proliferation. Finally, small interfering RNA-mediated knockdown of Ezrin abrogates cAMP regulation of IL-2. We propose that Ezrin is essential in the assembly of the cAMP-mediated regulatory pathway that modulates T cell immune responses.

MyRIP anchors protein kinase A to the exocyst complex

The movement of signal transduction enzymes in and out of multi-protein complexes coordinates the spatial and temporal resolution of cellular events. Anchoring and scaffolding proteins are key to this process because they sequester protein kinases and phosphatases with a subset of their preferred substrates. The protein kinase A-anchoring family of proteins (AKAPs), which target the cAMP-dependent protein kinase (PKA) and other enzymes to defined subcellular microenvironments, represent a well studied group of these signal-organizing molecules. In this report we demonstrate that the Rab27a GTPase effector protein MyRIP is a member of the AKAP family. The zebrafish homolog of MyRIP (Ze-AKAP2) was initially detected in a two-hybrid screen for AKAPs. A combination of biochemical, cell-based, and immunofluorescence approaches demonstrate that the mouse MyRIP ortholog targets the type II PKA holoenzyme via an atypical mechanism to a specific perinuclear region of insulin-secreting cells. Similar approaches show that MyRIP interacts with the Sec6 and Sec8 components of the exocyst complex, an evolutionarily conserved protein unit that controls protein trafficking and exocytosis. These data indicate that MyRIP functions as a scaffolding protein that links PKA to components of the exocytosis machinery.

Identification and characterization of RHOA-interacting proteins in bovine spermatozoa

In somatic cells, RHOA mediates actin dynamics through a GNA13-mediated signaling cascade involving RHO kinase (ROCK), LIM kinase (LIMK), and cofilin. RHOA can be negatively regulated by protein kinase A (PRKA), and it interacts with members of the A-kinase anchoring (AKAP) family via intermediary proteins. In spermatozoa, actin polymerization precedes the acrosome reaction, which is necessary for normal fertility. The present study was undertaken to determine whether the GNA13-mediated RHOA signaling pathway may be involved in acrosome reaction in bovine caudal sperm, and whether AKAPs may be involved in its targeting and regulation. GNA13, RHOA, ROCK2, LIMK2, and cofilin were all detected by Western blot in bovine caudal sperm. Overlay, immunoprecipitation, and subsequent mass spectrometry analysis identified several RHOA-interacting proteins, including proacrosin, angiotensin-converting enzyme, tubulin, aldolase C, and AKAP4. Using overlay and pulldown techniques, we demonstrate that phosphorylation of AKAP3 increases its interaction with the RHOA-interacting proteins PRKAR2 (the type II regulatory subunit of PRKA, formerly RII) and ropporin (ROPN1, a PRKAR2-like protein, or R2D2). Varying calcium concentrations in pulldown assays did not significantly alter binding to R2D2 proteins. These data suggest that the actin-regulating GNA13-mediated RHOA-ROCK-LIMK-cofilin pathway is present in bovine spermatozoa, that RHOA interacts with proteins involved in capacitation and the acrosome reaction, and that RHOA signaling in sperm may be targeted by AKAPs. Finally, AKAP3 binding to PRKAR2 and ROPN1 is regulated by phosphorylation in vitro.

Diversity of cAMP-Dependent Protein Kinase Isoforms and Their Anchoring Proteins in Mouse Ventricular Tissue

Using a chemical proteomics approach, we efficiently enriched for the generally low abundant cAMP signaling proteins, and their interactors, directly from mouse ventricular tissue. The presence of undesired contaminating (noncyclic) nucleotide-binding proteins was diminished using a tailored sequential elution protocol. Through further optimization of this affinity purification and elution protocol, we were able to detect all known protein kinase A regulatory isoforms (PKA-R). Furthermore, 11 different A-kinase anchoring proteins (AKAPs) were detected. A proposed fusion protein of paralemmin 2 and AKAP2 could be decisively established as a novel AKAP at the protein level in ventricular tissue. When comparing this dataset of cAMP-affinity purified proteins with earlier data obtained with immobilized cGMP from rat ventricular tissue, we observe a large overlap in the retained proteins but also some clear differences. Furthermore, implementation of an in-depth analysis of in vivo phosphorylation sites on PKA-R revealed the presence of several differentially phosphorylated PKA-R isoforms. This illustrates yet another layer of functional regulation in cyclic nucleotide signaling. In general, our improved chemical proteomics screen offers a broad, but detailed, view on nature's complex diversity in cyclic nucleotide signaling mechanisms. Possibly different AKAP-isoforms may direct differentially phosphorylated PKA-R isoforms to different cellular compartments, providing a multifaceted platform for just this kinase.

PKA, Germ Cells, and Fertility

Temporal and spatial regulation of PKA activity are essential for vigorous sperm motility and for the resumption of meiosis in oocytes, two events required for successful fertilization. Genetic mutations in mice that affect PKA signaling in germ cells lead to infertility and illustrate the importance of this pathway in mammalian reproduction.

Characterization of A-kinase-anchoring disruptors using a solution-based assay

Subcellular localization of PKA (cAMP-dependent protein kinase or protein kinase A) is determined by protein-protein interactions between its R (regulatory) subunits and AKAPs (A-kinase-anchoring proteins). In the present paper, we report the development of the Amplified Luminescent Proximity Homogeneous Assay (AlphaScreen) as a means to characterize AKAP-based peptide competitors of PKA anchoring. In this assay, the prototypic anchoring disruptor Ht31 efficiently competed in RIIalpha isoform binding with RII-specific and dual-specificity AKAPs (IC50 values of 1.4+/-0.2 nM and 6+/-1 nM respectively). In contrast, RIalpha isoform binding to a dual-specific AKAP was less efficiently competed (IC50 of 156+/-10 nM). Characterization of two RI-selective anchoring disruptors, RIAD (RI-anchoring disruptor) and PV-38 revealed that RIAD (IC50 of 13+/-1 nM) was 20-fold more potent than PV-38 (IC50 of 304+/-17 nM) and did not compete in the RIIalpha-AKAP interaction. We also observed that the kinetics of RII displacement from pre-formed PKA-AKAP complexes and competition of RII-AKAP complex formation by Ht31 differed by an order of magnitude when the component parts were mixed in vitro. No such difference in potency was seen for RIalpha-AKAP complexes. Thus the AlphaScreen assay may prove to be a valuable tool for detailed characterization of a variety of PKA-AKAP complexes.