Targeted disruption of the Akap4 gene causes defects in sperm flagellum and motility

A-kinase anchoring proteins (AKAPs) tether cyclic AMP-dependent protein kinases and thereby localize phosphorylation of target proteins and initiation of signal-transduction processes triggered by cyclic AMP. AKAPs can also be scaffolds for kinases and phosphatases and form macromolecular complexes with other proteins involved in signal transduction. Akap4 is transcribed only in the postmeiotic phase of spermatogenesis and encodes the most abundant protein in the fibrous sheath, a novel cytoskeletal structure present in the principal piece of the sperm flagellum. Previous studies indicated that cyclic AMP-dependent signaling processes are important in the regulation of sperm motility, and gene targeting was used here to test the hypothesis that AKAP4 is a scaffold for protein complexes involved in regulating flagellar function. Sperm numbers were not reduced in male mice lacking AKAP4, but sperm failed to show progressive motility and male mice were infertile. The fibrous sheath anlagen formed, but the definitive fibrous sheath did not develop, the flagellum was shortened, and proteins usually associated with the fibrous sheath were absent or substantially reduced in amount. However, the other cytoskeletal components of the flagellum were present and appeared fully developed. We conclude that AKAP4 is a scaffold protein required for the organization and integrity of the fibrous sheath and that effective sperm motility is lost in the absence of AKAP4 because signal transduction and glycolytic enzymes fail to become associated with the fibrous sheath.     

AKAP3 selectively binds PDE4A isoforms in bovine spermatozoa

Cyclic AMP plays an important role in regulating sperm motility and acrosome reaction through activation of cAMP-dependent protein kinase A (PKA). Phosphodiesterases (PDEs) modulate the levels of cyclic nucleotides by catalyzing their degradation. Although PDE inhibitors specific to PDE1 and PDE4 are known to alter sperm motility and capacitation in humans, little is known about the role or subcellular distribution of PDEs in spermatozoa. The localization of PKA is regulated by A-kinase anchoring proteins (AKAPs), which may also control the intracellular distribution of PDE. The present study was undertaken to investigate the role and localization of PDE4 during sperm capacitation. Addition of Rolipram or RS25344, PDE4-specific inhibitors significantly increased the progressive motility of bovine spermatozoa. Immunolocalization techniques detected both PDE4A and AKAP3 (formerly known as AKAP110) in the principal piece of bovine spermatozoa. The PDE4A5 isoform was detected primarily in the Triton X-100-soluble fraction of caudal epididymal spermatozoa. However, in ejaculated spermatozoa it was seen primarily in the SDS-soluble fraction, indicating a shift in PDE4A5 localization into insoluble organelles during sperm capacitation. AKAP3 was detected only in the SDS-soluble fraction of both caudal and ejaculated sperm. Immunoprecipitation experiments using COS cells cotransfected with AKAP3 and either Pde4a5 or Pde4d provide evidence that PDE4A5 but not PDE4D interacts with AKAP3. Pulldown assays using sperm cell lysates confirm this interaction in vitro. These data suggest that AKAP3 binds both PKA and PDE4A and functions as a scaffolding protein in spermatozoa to regulate local cAMP concentrations and modulate sperm functions.     

An A-kinase anchoring protein is required for protein kinase A regulatory subunit localization and morphology of actin structures during oogenesis in Drosophila

Protein kinase A (PKA) holoenzyme is anchored to specific subcellular regions by interactions between regulatory subunits (Pka-R) and A-kinase anchoring proteins (AKAPs). We examine the functional importance of PKA anchoring during Drosophila oogenesis by analyzing membrane integrity and actin structures in mutants with disruptions in Akap200, an AKAP. In wild-type ovaries, Pka-RII and Akap200 localized to membranes and to the outer rim of ring canals, actin-rich structures that connect germline cells. In Akap200 mutant ovaries, Pka-RII membrane localization decreased, leading to a destabilization of membrane structures and the formation of binucleate nurse cells. Defects in membrane integrity could be mimicked by expressing a constitutively active PKA catalytic subunit (Pka-C) throughout germline cells. Unexpectedly, nurse cells in Akap200 mutant ovaries also had enlarged, thin ring canals. In contrast, overexpressing Akap200 in the germline resulted in thicker, smaller ring canals. To investigate the role of Akap200 in regulating ring canal growth, we examined genetic interactions with other genes that are known to regulate ring canal morphology. Akap200 mutations suppressed the small ring canal phenotype produced by Src64B mutants, linking Akap200 with the non-receptor tyrosine kinase pathway. Together, these results provide the first evidence that PKA localization is required for morphogenesis of actin structures in an intact organism.     

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.     

Identification of sperm-specific proteins that interact with A-kinase anchoring proteins in a manner similar to the type II regulatory subunit of PKA

The cAMP-dependent protein kinase (PKA) is targeted to specific subcellular compartments through its interaction with A-kinase anchoring proteins (AKAPs). AKAPs contain an amphipathic helix domain that binds to the type II regulatory subunit of PKA (RII). Synthetic peptides containing this amphipathic helix domain bind to RII with high affinity and competitively inhibit the binding of PKA with AKAPs. Addition of these anchoring inhibitor peptides to spermatozoa inhibits motility (Vijayaraghavan, S., Goueli, S. A., Davey, M. P., and Carr, D. W. (1997) J. Biol. Chem. 272, 4747-4752). However, inhibition of the PKA catalytic activity does not mimic these peptides, suggesting that the peptides are disrupting the interaction of AKAP(s) with proteins other than PKA. Using the yeast two-hybrid system, we have now identified two sperm-specific human proteins that interact with the amphipathic helix region of AKAP110. These proteins, ropporin (a protein previously shown to interact with the Rho signaling pathway) and AKAP-associated sperm protein, are 39% identical to each other and share a strong sequence similarity with the conserved domain on the N terminus of RII that is involved in dimerization and AKAP binding. Mutation of conserved residues in ropporin or RII prevents binding to AKAP110. These data suggest that sperm contains several proteins that bind to AKAPs in a manner similar to RII and imply that AKAPs may have additional and perhaps unique functions in spermatozoa.     

Mutations in AKAP5 Disrupt Dendritic Signaling Complexes and Lead to Electrophysiological and Behavioral Phenotypes in Mice

AKAP5 (also referred to as AKAP150 in rodents and AKAP79 in humans) is a scaffolding protein that is highly expressed in neurons and targets a variety of signaling molecules to dendritic membranes. AKAP5 interacts with PKA holoenzymes containing RIIα or RIIβ as well as calcineurin (PP2B), PKC, calmodulin, adenylyl cyclase type V/VI, L-type calcium channels, and β-adrenergic receptors. AKAP5 has also been shown to interact with members of the MAGUK family of PSD-scaffolding proteins including PSD95 and SAP97 and target signaling molecules to receptors and ion channels in the postsynaptic density (PSD). We created two lines of AKAP5 mutant mice: a knockout of AKAP5 (KO) and a mutant that lacks the PKA binding domain of AKAP5 (D36). We find that PKA is delocalized in both the hippocampus and striatum of KO and D36 mice indicating that other neural AKAPs cannot compensate for the loss of PKA binding to AKAP5. In AKAP5 mutant mice, a significant fraction of PKA becomes localized to dendritic shafts and this correlates with increased binding to microtubule associated protein-2 (MAP2). Electrophysiological and behavioral analysis demonstrated more severe deficits in both synaptic plasticity and operant learning in the D36 mice compared with the complete KO animals. Our results indicate that the targeting of calcineurin or other binding partners of AKAP5 in the absence of the balancing kinase, PKA, leads to a disruption of synaptic plasticity and results in learning and memory defects.     

Conservation and function of a bovine sperm A-kinase anchor protein homologous to mouse AKAP82.

Protein kinase A regulates sperm motility through the cAMP-dependent phosphorylation of proteins. One mechanism to direct the activity of the kinase is to localize it near its protein substrates through the use of anchoring proteins. A-Kinase anchoring proteins (AKAPs) act by binding the type II regulatory subunit of protein kinase A and tethering it to a cellular organelle or cytoskeletal element. We showed previously that mAKAP82, the major protein of the fibrous sheath of the mouse sperm flagellum, is an AKAP. The available evidence indicates that protein kinase A is compartmentalized to the fibrous sheath by binding mAKAP82. To characterize AKAP82 in bovine sperm, a testicular cDNA library was constructed and used to isolate a clone encoding bAKAP82, the bovine homologue. Sequence analysis showed that the primary structure of bAKAP82 was highly conserved. In particular, the amino acid sequence corresponding to the region of mAKAP82 responsible for binding the regulatory subunit of protein kinase A was identical in the bull. Bovine AKAP82 was present in both epididymal and ejaculated sperm and was localized to the entire principal piece of the flagellum, the region in which the fibrous sheath is located. Finally, bAKAP82 bound the regulatory subunit of protein kinase A. These data support the idea that bAKAP82 functions as an anchoring protein for the subcellular localization of protein kinase A in the flagellum.     

Increased phosphorylation of a distinct subcellular pool of protein phosphatase, PP1gamma2, during epididymal sperm maturation

The enzyme PP1gamma2 is a testis- and sperm-specific isoform of type 1 protein phosphatase (PP1), and it is the only isoform of PP1 in spermatozoa. The enzyme PP1gamma2 is essential for spermatogenesis and is also a key enzyme in the development and regulation of sperm motility. The carboxy terminus of the enzyme contains a consensus amino acid sequence for phosphorylation by cyclin-dependent kinases. Using antibodies specific to this phosphorylated amino acid sequence domain, we found that phosphorylated PP1gamma2 is present in bovine epididymal spermatozoa. The level of phosphorylated PP1gamma2 is significantly higher in motile caudal compared to immotile caput epididymal spermatozoa. A number of treatments, such as 2-chloro adenosine, cAMP analogues, cAMP phosphodiesterase inhibitors, and calcium, which stimulate sperm motility, did not alter the level of phosphorylated PP1gamma2. However, calyculin A, which is an inhibitor of protein phosphatase subtypes PP1 and PP2A, significantly increases the level of phosphorylated PP1gamma2 in both caput and caudal epididymal spermatozoa. Partial purification by column chromatography showed that phosphorylated PP1gamma2 is catalytically active. Phosphorylated PP1gamma2 is the only spontaneously catalytically active form of the enzyme in caudal sperm extracts. Western blot analysis shows that the enzyme cyclin-dependent kinase 2, one of the enzymes that phosphorylates the consensus domain at the carboxy terminus in PP1 isoforms, is present in spermatozoa. Western blot analysis of proteins extracted from purified head and tail fragments of spermatozoa showed that phosphorylated PP1gamma2 is present predominantly in the sperm head. Fluorescence immunocytochemistry also showed that phosphorylated PP1gamma2 is present predominantly in the posterior region of the sperm head. The distinct subcellular localization and changes in its level during sperm maturation suggest a possible role for sperm phosphorylated PP1gamma2 in signaling events during fertilization.     

Identification and characterization of D-AKAP1 as a major adipocyte PKA and PP1 binding protein

Protein kinase A (PKA) plays an important role in the regulation of lipid metabolism in adipocytes. The activity of PKA is known to be modulated by its specific location in the cell, a process mediated by A-kinase anchoring proteins (AKAPs). In order to examine the subcellular localization of PKA in this tissue we performed a search for AKAP proteins in adipocytes. We purified a 120 kDa protein which can bind both the regulatory subunit of PKA as well as the catalytic subunit of protein phosphatase 1 (PP1). This protein was found to be enriched in the lipid droplet fraction of primary adipocytes and was identified as D-AKAP1. This protein may play an important role in the regulation of PKA in adipocytes.     

Binding and inactivation of the germ cell-specific protein phosphatase PP1gamma2 by sds22 during epididymal sperm maturation

Testis- and sperm-specific protein phosphatase, PP1gamma2, is a key enzyme regulating sperm function. Its activity decreases during sperm maturation in the epididymis. Inhibition of PP1gamma2 leads to motility initiation and stimulation. Our laboratory is focused on identifying mechanisms responsible for the decline in PP1gamma2 activity during sperm motility initiation in the epididymis. Previously, using immuno-affinity chromatography, we showed that a mammalian homologue of yeast sds22 is bound to PP1gamma2 in motile caudal spermatozoa (Huang Z, et al. Biol Reprod 2002; 67:1936-1942). The objectives of this study were to determine: 1) stoichiometry of PP1gamma2-sds22 binding and 2) whether PP1gamma2 in immotile caput epididymal spermatozoa is bound to sds22. The enzyme from caudal and caput sperm extracts was purified by column chromatography. Immunoreactive PP1gamma2 and sds22 from both caudal and caput spermatozoa were found in the flow-through fraction of a DEAE-cellulose column. However, PP1gamma2 from caudal spermatozoa was inactive, whereas in caput spermatozoa it was active. The DEAE-cellulose flow-through fractions were next passed through a SP-sepharose column. Caudal sperm sds22 and PP1gamma2 coeluted in the gradient fraction. In contrast, caput sperm sds22 and PP1gamma2 were separated in the flow-through and gradient fractions, respectively. Further purification through a Superose 6 column showed that PP1gamma2-sds22 complex from caudal sperm was 88 kDa in size. Caput sperm sds22 and PP1gamma2 eluted at 60 kDa and 39 kDa, respectively. SDS-PAGE of these purified fractions revealed that in caudal sperm, the 88-kDa species is composed of sds22 (43 kDa) and PP1gamma2 (39 kDa), suggesting a 1:1 complex between these two proteins. PP1gamma2 bound to sds22 in this complex was inactive. Caput sperm sds22 eluting as a 60-kDa species was found to be associated with a 17-kDa protein (p17). This suggests that dissociation of sds22 from p17 or some other posttranslational modification of sds22 is required for its binding and inactivation of PP1gamma2. Studies are currently underway to determine the mechanisms responsible for development of sds22 binding to PP1gamma2 during epididymal sperm maturation.     

PP2B/calcineurin-mediated desensitization of TRPV1 does not require AKAP150

Activation of protein kinases and phosphatases at the plasma membrane often initiates agonist-dependent signalling events. In sensory neurons, AKAP150 (A-kinase-anchoring protein 150) orientates PKA (protein kinase A), PKC (protein kinase C) and the Ca2+/calmodulin-dependent PP2B (protein phosphatase 2B, also known as calcineurin) towards membrane-associated substrates. Recent evidence indicates that AKAP150-anchored PKA and PKC phosphorylate and sensitize the TRPV1 (transient receptor potential subfamily V type 1 channel, also known as the capsaicin receptor). In the present study, we explore the hypothesis that an AKAP150-associated pool of PP2B catalyses the dephosphorylation and desensitization of TRPV1. Biochemical, electrophysiological and cell-based experiments indicate that PP2B associates with AKAP150 and TRPV1 in cultured TG (trigeminal ganglia) neurons. Gene silencing of AKAP150 reduces basal phosphorylation of TRPV1. However, functional studies in neurons isolated from AKAP150-/- mice indicate that the anchoring protein is not required for pharmacological desensitization of TRPV1. Behavioural analysis of AKAP150-/- mice further support this notion, demonstrating that agonist-stimulated desensitization of TRPV1 is sensitive to PP2B inhibition and does not rely on AKAP150. These findings allow us to conclude that pharmacological desensitization of TRPV1 by PP2B may involve additional regulatory components.     

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.     

Interaction between fidgetin and protein kinase A-anchoring protein AKAP95 is critical for palatogenesis in the mouse

The gene defective in fidget mice encodes fidgetin, a member of the AAA (ATPases associated with diverse cellular activities) family of ATPases. Using a yeast two-hybrid screen, we identified cAMP-dependent protein kinase A anchoring protein 95 kDa (AKAP95) as a potential fidgetin-binding protein. Epitope-tagged fidgetin co-localized with endogenous AKAP95 in the nuclear matrix, and the physical interaction between fidgetin and AKAP95 was further confirmed by reciprocal immunoprecipitation. To evaluate the biological significance of the fidgetin-AKAP95 binding, we created AKAP95 mutant mice through a gene trap strategy. Akap95 mutant mice are surprisingly viable with no overt phenotype. However, a significant number of mice carrying both Akap95 and fidget mutations die soon after birth due to cleft palate, consistent with the overlapping expression of AKAP95 and fidgetin in the branchial arches during mouse embryogenesis. These results expand the spectrum of the pleiotropic phenotypes of fidget mice and provide new leads on the in vivo function of AKAP95.     

Sperm phosphoproteome profiling by ultra performance liquid chromatography followed by data independent analysis (LC-MS(E)) reveals altered proteomic signatures in asthenozoospermia

Sperm motility is an important prerequisite for successful fertilization and is regulated by cyclic AMP activated protein kinase A which phosphorylates flagella proteins like axonemal dynein and initiates motility. Increase in calcium influx reverses this process by dephosphorylation that is mediated by calcineurin. Analyzing the phosphoenriched fractions of spermatozoa lysates from eight normozoospermic-, and asthenozoospermic-samples, respectively, by Nano UPLC-MS(E), the present study investigates the phosphoproteins involved in sperm motility in an attempt to identify the key pathways regulating sperm motility and likely to be altered in spermatozoa of asthenozoospermic individuals. 66 phosphoproteins were differentially regulated in asthenozoospermia. The deregulated proteins comprised predominantly the HSPs, cytoskeletal proteins, proteins associated with the fibrous sheath, and those associated with energy metabolism. EM analysis of these spermatozoa demonstrated significant defects in mitochondria, and fibrous sheath and these defects could be correlated with the altered proteome. Pathway analysis revealed that carbohydrate and energy metabolism, cAMP mediated PKA signaling, PI3K/AKT signaling and pathway regulating actin based motility by Rho were significantly altered indicating that motility in spermatozoa is regulated through the concerted effort of these pathways. The data identified signature molecules that have the potential as biomarkers for diagnosing etiology of asthenozoospermia.     

Protein 14-3-3zeta binds to protein phosphatase PP1gamma2 in bovine epididymal spermatozoa

The protein phosphatase PP1gamma2 is critical in the regulation of sperm motility and fertility. Its activity is regulated by its binding proteins and by phosphorylation. We have recently shown that PP1gamma2 is phosphorylated and that the amount of phosphorylated PP1gamma2 increases during sperm epididymal maturation (Huang et al., Biol Reprod 2004; 70:439-447). Microsequencing revealed that protein 14-3-3 coeluted with phosphorylated PP1gamma2 during column chromatography of bovine sperm extracts. Western blot analyses confirmed the presence of protein 14-3-3 not only in bovine spermatozoa but also in spermatozoa of diverse species-bull, hamster, horseshoe crab, monkey, rat, turkey, and Xenopus. The binding between PP1gamma2 and protein 14-3-3 was confirmed by coimmunoprecipitation experiments and in pull-down assays with recombinant GST-14-3-3. Western blot analysis and protein 14-3-3 immunoprecipitates with antibodies against the consensus binding domain of protein 14-3-3 reveal that, in addition to PP1gamma2, at least two other protein 14-3-3 binding partners are present in spermatozoa. Fluorescence immunocytochemistry results indicate that phosphorylated PP1gamma2 and protein 14-3-3 both localize to the postacrosomal region of the head and principal piece of bovine spermatozoa. Together, these results provide conclusive evidence that protein 14-3-3 is present in mature spermatozoa and that PP1gamma2 is one of its binding partners.     

Essential role of A-kinase anchor protein 121 for cAMP signaling to mitochondria

A-Kinase anchor proteins (AKAPs) immobilize and concentrate protein kinase A (PKA) isoforms at specific subcellular compartments. Intracellular targeting of PKA holoenzyme elicits rapid and efficient phosphorylation of target proteins, thereby increasing sensitivity of downstream effectors to cAMP action. AKAP121 targets PKA to the cytoplasmic surface of mitochondria. Here we show that conditional expression of AKAP121 in PC12 cells selectively enhances cAMP.PKA signaling to mitochondria. AKAP121 induction stimulates PKA-dependent phosphorylation of the proapoptotic protein BAD at Ser(155), inhibits release of cytochrome c from mitochondria, and protects cells from apoptosis. An AKAP121 derivative mutant that localizes on mitochondria but does not bind PKA down-regulates PKA signaling to the mitochondria and promotes apoptosis. These findings indicate that PKA anchored by AKAP121 transduces cAMP signals to the mitochondria, and it may play an important role in mitochondrial physiology.     

Association of the type 1 protein phosphatase PP1 with the A-kinase anchoring protein AKAP220

The cyclic AMP (cAMP)-dependent protein kinase (PKA) and the type 1 protein phosphatase (PP1) are broad-specificity signaling enzymes with opposing actions that catalyze changes in the phosphorylation state of cellular proteins. Subcellular targeting to the vicinity of preferred substrates is a means of restricting the specificity of each enzyme [1] [2]. Compartmentalization of the PKA holoenzyme is mediated through association of the regulatory subunits with A-kinase anchoring proteins (AKAPs), whereas a diverse family of phosphatase-targeting subunits directs the location of the PP1 catalytic subunit (PP1c) [3] [4]. Here, we demonstrate that the PKA-anchoring protein, AKAP220, binds PP1c with a dissociation constant (KD) of 12.1 +/- 4 nM in vitro. Immunoprecipitation of PP1 from cell extracts resulted in a 10.4 +/- 3.8-fold enrichment of PKA activity. AKAP220 co-purified with PP1c by affinity chromatography on microcystin sepharos Immunocytochemical analysis demonstrated that the kinase, the phosphatase and the anchoring protein had distinct but overlapping staining patterns in rat hippocampal neurons. Collectively, these results provide the first evidence that AKAP220 is a multivalent anchoring protein that maintains a signaling scaffold of PP1 and the PKA holoenzyme.