The essential role of RI alpha in the maintenance of regulated PKA activity

PKA signaling drives mammary tumorigenesis through Src

Protein kinase A (PKA) hyperactivation causes hereditary endocrine neoplasias; however, its role in sporadic epithelial cancers is unknown. Here, we show that heightened PKA activity in the mammary epithelium generates tumors. Mammary-restricted biallelic ablation of Prkar1a, which encodes for the critical type-I PKA regulatory subunit, induced spontaneous breast tumors characterized by enhanced type-II PKA activity. Downstream of this, Src phosphorylation occurs at residues serine-17 and tyrosine-416 and mammary cell transformation is driven through a mechanism involving Src signaling. The phenotypic consequences of these alterations consisted of increased cell proliferation and, accordingly, expansion of both luminal and basal epithelial cell populations. In human breast cancer, low PRKAR1A/high SRC expression defines basal-like and HER2 breast tumors associated with poor clinical outcome. Together, the results of this study define a novel molecular mechanism altered in breast carcinogenesis and highlight the potential strategy of inhibiting SRC signaling in treating this cancer subtype in humans.

PKA RIα homodimer structure reveals an intermolecular interface with implications for cooperative cAMP binding and Carney complex disease

The regulatory (R) subunit is the cAMP receptor of protein kinase A. Following cAMP binding, the inactive PKA holoenzyme complex separates into two active catalytic (C) subunits and a cAMP-bound R dimer. Thus far, only monomeric R structures have been solved, which fell short in explaining differences of cAMP binding for the full-length protein as compared to the truncated R subunits. Here we solved a full-length R-dimer structure that reflects the biologically relevant conformation, and this structure agrees well with small angle X-ray scattering. An isoform-specific interface is revealed between the protomers. This interface acts as an intermolecular sensor for cAMP and explains the cooperative character of cAMP binding to the RIα dimer. Mutagenesis of residues on this interface not only leads to structural and biochemical changes, but is also linked to Carney complex disease.

Inactivation of the Carney complex gene 1 (PRKAR1A) alters spatiotemporal regulation of cAMP and cAMP-dependent protein kinase: a study using genetically encoded FRET-based reporters

Carney complex (CNC) is a hereditary disease associating cardiac myxoma, spotty skin pigmentation and endocrine overactivity. CNC is caused by inactivating mutations in the PRKAR1A gene encoding PKA type I alpha regulatory subunit (RIα). Although PKA activity is enhanced in CNC, the mechanisms linking PKA dysregulation to endocrine tumorigenesis are poorly understood. In this study, we used Förster resonance energy transfer (FRET)-based sensors for cAMP and PKA activity to define the role of RIα in the spatiotemporal organization of the cAMP/PKA pathway. RIα knockdown in HEK293 cells increased basal as well as forskolin or prostaglandin E1 (PGE1)-stimulated total cellular PKA activity as reported by western blots of endogenous PKA targets and the FRET-based global PKA activity reporter, AKAR3. Using variants of AKAR3 targeted to subcellular compartments, we identified similar increases in the response to PGE1 in the cytoplasm and at the outer mitochondrial membrane. In contrast, at the plasma membrane, the response to PGE1 was decreased along with an increase in basal FRET ratio. These results were confirmed by western blot analysis of basal and PGE1-induced phosphorylation of membrane-associated vasodilator-stimulated phosphoprotein. Similar differences were observed between the cytoplasm and the plasma membrane in human adrenal cells carrying a RIα inactivating mutation. RIα inactivation also increased cAMP in the cytoplasm, at the outer mitochondrial membrane and at the plasma membrane, as reported by targeted versions of the cAMP indicator Epac1-camps. These results show that RIα inactivation leads to multiple, compartment-specific alterations of the cAMP/PKA pathway revealing new aspects of signaling dysregulation in tumorigenesis.

Altered GABAA receptor expression and seizure threshold following acute ethanol challenge in mice lacking the RIIβ subunit of PKA

Ethanol causes pathological changes in GABAA receptor trafficking and function. These changes are mediated in part by ethanol activation of protein kinase A (PKA). The current study investigated the expression of the GABAA α1 and α4 subunits and the kinase anchoring protein AKAP150, as well as bicuculline-induced seizure threshold, at baseline and following acute injection of ethanol (3.5 g/kg IP) in a mouse line lacking the regulatory RIIβ subunit of PKA. Whole cerebral cortices were harvested at baseline, 1 h, or 46 h following injection of ethanol or saline and subjected to fractionation and western blot analysis. Knockout (RIIβ-/-) mice had similar baseline levels of PKA RIIα and GABAA α1 and α4 subunits compared to wild type (RIIβ+/+) littermates, but had deficits in AKAP150. GABAA α1 subunit levels were decreased in the P2 fraction of RIIβ-/-, but not RIIβ+/+, mice following 1 h ethanol, an effect that was driven by decreased α1 expression in the synaptic fraction. GABAA α4 subunits in the P2 fraction were not affected by 1 h ethanol; however, synaptic α4 subunit expression was increased in RIIβ+/+, but not RIIβ-/- mice, while extrasynaptic α4 and δ subunit expression were decreased in RIIβ-/-, but not RIIβ+/+ mice. Finally, RIIβ knockout was protective against bicuculline-induced seizure susceptibility. Overall, the results suggest that PKA has differential roles in regulating GABAA receptor subunits. PKA may protect against ethanol-induced deficits in synaptic α1 and extrasynaptic α4 receptors, but may facilitate the increase of synaptic α4 receptors.

Head and flagella subcompartmental proteomic analysis of human spermatozoa

Subcellular proteomics not only deepens our knowledge of what proteins are present within cells, but also opens our understanding as to where those proteins reside. Given the highly differentiated, cross-linked state of spermatozoa, such studies have proven difficult to perform. In this study we have fractionated spermatozoa into two components, consisting of either the head or flagellar region. Following SDS-PAGE, 1 mm slices were digested and used for LC-MS/MS analysis. In total, 1429 proteins were identified with 721 proteins being exclusively found in the tail and 521 exclusively in the head. Not only is this the largest reported proteomic analysis of human spermatozoa, but also it has provided novel insights into the compartmentalization of proteins, particularly receptors, never previously reported to be present in this cell type.

cAMP/PKA signaling defects in tumors: genetics and tissue-specific pluripotential cell-derived lesions in human and mouse

In the last few years, bench and clinical studies led to significant new insight into how cyclic adenosine monophosphate (cAMP) signaling, the molecular pathway that had been identified in the early 2000s as the one involved in most benign cortisol-producing adrenal hyperplasias, affects adrenocortical growth and development, as well as tumor formation. A major discovery was the identification of tissue-specific pluripotential cells (TSPCs) as the culprit behind tumor formation not only in the adrenal, but also in bone. Discoveries in animal studies complemented a number of clinical observations in patients. Gene identification continued in parallel with mouse and other studies on the cAMP signaling and other pathways.

Increased protein kinase A type Iα regulatory subunit expression in parathyroid gland adenomas of patients with primary hyperparathyroidism

Protein kinase A (PKA) regulatory subunit type Iα (RIα) is a major regulatory subunit that functions as an inhibitor of PKA kinase activity. We have previously demonstrated that elevated RIα expression is associated with diffuse-to-nodular transformation of hyperplasia in parathyroid glands of renal hyperparathyroidism. The aim of the current study was to determine whether or not RIα expression is increased in adenomas of primary hyperparathyroidism (PHPT), because monoclonal proliferation has been demonstrated in both adenomas and nodular hyperplasia. Surgical specimens comprising 22 adenomas and 11 normal glands, obtained from 22 patients with PHPT, were analyzed. Western blot and immunohistochemical analyses were employed to evaluate RIα expression. PKA activities were determined in several adenomas highly expressing RIα. RIα expression was also separately evaluated in chief and oxyphilic cells using the "Allred score" system. Expression of proliferating cell nuclear antigen (PCNA), a proliferation marker, was also immunohistochemically examined. Western blot analysis revealed that 5 out of 8 adenomas highly expressed RIα, compared with normal glands. PKA activity in adenomas was significantly less than in normal glands. Immunohistochemical analysis further demonstrated high expression of RIα in 20 out of 22 adenomas. In adenomas, the greater RIα expression and more PCNA positive cells were observed in both chief and oxyphilic cells. The present study suggested that high RIα expression could contribute to monoclonal proliferation of parathyroid cells by impairing the cAMP/PKA signaling pathway.

Mechanism of sperm capacitation and the acrosome reaction: role of protein kinases

Mammalian sperm must undergo a series of biochemical and physiological modifications, collectively called capacitation, in the female reproductive tract prior to the acrosome reaction (AR). The mechanisms of these modifications are not well characterized though protein kinases were shown to be involved in the regulation of intracellular Ca(2+) during both capacitation and the AR. In the present review, we summarize some of the signaling events that are involved in capacitation. During the capacitation process, phosphatidyl-inositol-3-kinase (PI3K) is phosphorylated/activated via a protein kinase A (PKA)-dependent cascade, and downregulated by protein kinase C α (PKCα). PKCα is active at the beginning of capacitation, resulting in PI3K inactivation. During capacitation, PKCα as well as PP1γ2 is degraded by a PKA-dependent mechanism, allowing the activation of PI3K. The activation of PKA during capacitation depends mainly on cyclic adenosine monophosphate (cAMP) produced by the bicarbonate-dependent soluble adenylyl cyclase. This activation of PKA leads to an increase in actin polymerization, an essential process for the development of hyperactivated motility, which is necessary for successful fertilization. Actin polymerization is mediated by PIP(2) in two ways: first, PIP(2) acts as a cofactor for phospholipase D (PLD) activation, and second, as a molecule that binds and inhibits actin-severing proteins such as gelsolin. Tyrosine phosphorylation of gelsolin during capacitation by Src family kinase (SFK) is also important for its inactivation. Prior to the AR, gelsolin is released from PIP(2) and undergoes dephosphorylation/activation, resulting in fast F-actin depolymerization, leading to the AR.

Cotranslational cis-phosphorylation of the COOH-terminal tail is a key priming step in the maturation of cAMP-dependent protein kinase

cAMP-dependent protein kinase A (PKA), ubiquitously expressed in mammalian cells, regulates a plethora of cellular processes through its ability to phosphorylate many protein substrates, including transcription factors, ion channels, apoptotic proteins, transporters, and metabolic enzymes. The PKA catalytic subunit has two phosphorylation sites, a well-studied site in the activation loop (Thr(197)) and another site in the C-terminal tail (Ser(338)) for which the role of phosphorylation is unknown. We show here, using in vitro studies and experiments with S49 lymphoma cells, that cis-autophosphorylation of Ser(338) occurs cotranslationally, when PKA is associated with ribosomes and precedes posttranslational phosphorylation of the activation loop Thr(197). Ser(338) phoshorylation is not required for PKA activity or formation of the holoenzyme complex; however, it is critical for processing and maturation of PKA, and it is a prerequisite for phosphorylation of Thr(197). After Thr(197) and Ser(338) are phosphorylated, both sites are remarkably resistant to phosphatases. Phosphatase resistance of the activation loop, a unique feature of both PKA and PKG, reflects the distinct way that signal transduction dynamics are controlled by cyclic nucleotide-dependent PKs.

Possible counter effect in newborns of 1936A>G (I646V) polymorphism in the AKAP10 gene encoding A-kinase-anchoring protein 10

OBJECTIVE

Cyclic adenosine monophosphate/protein kinase A (PKA) is important in embryonic development. The human AKAP10 gene is polymorphic: 1936A>G results in changes to a PKA-binding domain and increased targeting to mitochondria. Previous studies found G1936 as 'deleterious' in adults, and this study investigates whether this holds true in preterm birth.

STUDY DESIGN

Study group consisted of 80 preterm newborns (PTNs) born before the 38th gestation week. Control group consisted of 123 full-term healthy newborns born after the 37th gestation week with uncomplicated pregnancies. Genomic DNA was extracted from umbilical blood and AKAP10 genotypes were identified by PCR/restriction enzyme.

RESULT

Significant differences in frequencies of 1936A>G genotypes/alleles between both groups were found. PTNs had increased frequency (55%) of AA homozygotes (odds ratio, AA versus AG+GG: 2.63 (95% confidence interval: 1.33 to 5.20), P=0.006) after adjustments: mothers with previous PTNs, smoking, first pregnancy, first delivery and Cesarean section.

CONCLUSION

Results suggest G1936 is preventative factor against preterm birth, in contrast with previously asserted negative effects in adults.

Hyper-activated motility in sperm capacitation is mediated by phospholipase D-dependent actin polymerization

In order to fertilize the oocyte, sperm must undergo a series of biochemical changes in the female reproductive tract, known as capacitation. Once capacitated, spermatozoon can bind to the zona pellucida of the egg and undergo the acrosome reaction (AR), a process that enables its penetration and fertilization of the oocyte. Important processes that characterize sperm capacitation are actin polymerization and the development of hyper-activated motility (HAM). Previously, we showed that Phospholipase D (PLD)-dependent actin polymerization occurs during sperm capacitation, however the role of this process in sperm capacitation is not yet known. In the present study, we showed for the first time the involvement of PLD-dependent actin polymerization in sperm motility during mouse and human capacitation. Sperm incubated under capacitation conditions revealed a time dependent increase in actin polymerization and HAM. Inhibition of Phosphatidic Acid (PA) formation by PLD using butan-1-ol, inhibited actin polymerization and motility, as well as in vitro fertilization (IVF) and the ability of the sperm to undergo the AR. The inhibition of sperm HAM by low concentration of butan-1-ol is completely restored by adding PA, further indicating the involvement of PLD in these processes. Furthermore, exogenous PA enhanced rapid actin polymerization that was followed by a rise in the HAM, as well as an increased in IVF rate. In conclusion, our results demonstrate that PLD-dependent actin polymerization is a critical step needed for the development of HAM during mouse and human sperm capacitation.

The differential diagnosis of familial lentiginosis syndromes

Cutaneous markers of systemic disease are vital for clinicians to recognize. This chapter outlines familial lentiginosis syndromes that include Peutz-Jeghers syndrome, Carney Complex, the PTEN hamartomatous syndromes, and LEOPARD/Noonan syndrome. The inheritance of these syndromes is autosomal dominant; they also share characteristic skin findings that offer a clue to their recognition and treatment. We will discuss the clinical presentation of these disorders, with a focus on the dermatological manifestations, and will provide an update on the molecular mechanisms involved. Recognition of cutaneous markers associated with these rare familial cancer syndromes provides the opportunity to pursue early surveillance for malignancies, as well as genetic counseling.

The testis-specific Cα2 subunit of PKA is kinetically indistinguishable from the common Cα1 subunit of PKA

Background

The two variants of the α-form of the catalytic (C) subunit of protein kinase A (PKA), designated Cα1 and Cα2, are encoded by the PRKACA gene. Whereas Cα1 is ubiquitous, Cα2 expression is restricted to the sperm cell. Cα1 and Cα2 are encoded with different N-terminal domains. In Cα1 but not Cα2 the N-terminal end introduces three sites for posttranslational modifications which include myristylation at Gly1, Asp-specific deamidation at Asn2 and autophosphorylation at Ser10. Previous reports have implicated specific biological features correlating with these modifications on Cα1. Since Cα2 is not modified in the same way as Cα1 we tested if they have distinct biochemical activities that may be reflected in different biological properties.

Results

We show that Cα2 interacts with the two major forms of the regulatory subunit (R) of PKA, RI and RII, to form cAMP-sensitive PKAI and PKAII holoenzymes both in vitro and in vivo as is also the case with Cα1. Moreover, using Surface Plasmon Resonance (SPR), we show that the interaction patterns of the physiological inhibitors RI, RII and PKI were comparable for Cα2 and Cα1. This is also the case for their potency to inhibit catalytic activities of Cα2 and Cα1.

Conclusion

We conclude that the regulatory complexes formed with either Cα1 or Cα2, respectively, are indistinguishable.

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.

Control of PKA stability and signalling by the RING ligase praja2

Activation of G-protein-coupled receptors (GPCRs) mobilizes compartmentalized pulses of cyclic AMP. The main cellular effector of cAMP is protein kinase A (PKA), which is assembled as an inactive holoenzyme consisting of two regulatory (R) and two catalytic (PKAc) subunits. cAMP binding to R subunits dissociates the holoenzyme and releases the catalytic moiety, which phosphorylates a wide array of cellular proteins. Reassociation of PKAc and R components terminates the signal. Here we report that the RING ligase praja2 controls the stability of mammalian R subunits. Praja2 forms a stable complex with, and is phosphorylated by, PKA. Rising cAMP levels promote praja2-mediated ubiquitylation and subsequent proteolysis of compartmentalized R subunits, leading to sustained substrate phosphorylation by the activated kinase. Praja2 is required for efficient nuclear cAMP signalling and for PKA-mediated long-term memory. Thus, praja2 regulates the total concentration of R subunits, tuning the strength and duration of PKA signal output in response to cAMP.

Structure of yeast regulatory subunit: a glimpse into the evolution of PKA signaling

The major cAMP receptors in eukaryotes are the regulatory (R) subunits of PKA, an allosteric enzyme conserved in fungi through mammals. While mammals have four R-subunit genes, Saccharomyces cerevisiae has only one, Bcy1. To achieve a molecular understanding of PKA activation in yeast and to explore the evolution of cyclic-nucleotide binding (CNB) domains, we solved the structure of cAMP-bound Bcy1(168-416). Surprisingly, the relative orientation of the two CNB domains in Bcy1 is very different from mammalian R-subunits. This quaternary structure is defined primarily by a fungi-specific sequence in the hinge between the αB/αC helices of the CNB-A domain. The unique interface between the two CNB domains in Bcy1 defines the allosteric mechanism for cooperative activation of PKA by cAMP. Some interface motifs are isoform-specific while others, although conserved, play surprisingly different roles in each R-subunit. Phylogenetic analysis shows that structural differences in Bcy1 are shared by fungi of the subphylum Saccharomycotina.

Protein kinase A type I activates a CRE-element more efficiently than protein kinase A type II regardless of C subunit isoform

Background

Protein kinase A type I (PKAI) and PKAII are expressed in most of the eukaryotic cells examined. PKA is a major receptor for cAMP and specificity is achieved partly through tissue-dependent expression and subcellular localization of subunits with different biochemical properties. In addition posttranslational modifications help fine tune PKA activity, distribution and interaction in the cell. In spite of this the functional significance of two forms of PKA in one cell has not been fully determined. Here we have tested the ability of PKAI and PKAII formed by expression of the regulatory (R) subunits RIα or RIIα in conjunction with Cα1 or Cβ2 to activate a co-transfected luciferace reporter gene, controlled by the cyclic AMP responsive element-binding protein (CREB) in vivo.

Results

We show that PKAI when expressed at equal levels as PKAII was significantly (p < 0.01) more efficient in inducing Cre-luciferace activity at saturating concentrations of cAMP. This result was obtained regardless of catalytic subunit identity.

Conclusion

We suggest that differential effects of PKAI and PKAII in inducing Cre-luciferace activity depend on R and not C subunit identity.

Rare and unusual endocrine cancer syndromes with mutated genes

The study of a number of rare familial syndromes associated with endocrine tumor development has led to the identification of genes involved in the development of these tumors. Major advances have expanded our understanding of the pathophysiology of these rare endocrine tumors, resulting in the elucidation of causative genes in rare familial diseases and a better understanding of the signaling pathways implicated in endocrine cancers. Recognition of the familial syndrome associated with a particular patient's endocrine tumor has important implications in terms of prognosis, screening of family members, and screening for associated conditions.

Cyclic AMP analog blocks kinase activation by stabilizing inactive conformation: conformational selection highlights a new concept in allosteric inhibitor design

The regulatory (R) subunit of protein kinase A serves to modulate the activity of protein kinase A in a cAMP-dependent manner and exists in two distinct and structurally dissimilar, end point cAMP-bound "B" and C-subunit-bound "H"-conformations. Here we report mechanistic details of cAMP action as yet unknown through a unique approach combining x-ray crystallography with structural proteomics approaches, amide hydrogen/deuterium exchange and ion mobility mass spectrometry, applied to the study of a stereospecific cAMP phosphorothioate analog and antagonist((Rp)-cAMPS). X-ray crystallography shows cAMP-bound R-subunit in the B form but surprisingly the antagonist Rp-cAMPS-bound R-subunit crystallized in the H conformation, which was previously assumed to be induced only by C-subunit-binding. Apo R-subunit crystallized in the B form as well but amide exchange mass spectrometry showed large differences between apo, agonist and antagonist-bound states of the R-subunit. Further ion mobility reveals the apo R-subunit as an ensemble of multiple conformations with collisional cross-sectional areas spanning both the agonist and antagonist-bound states. Thus contrary to earlier studies that explained the basis for cAMP action through "induced fit" alone, we report evidence for conformational selection, where the ligand-free apo form of the R-subunit exists as an ensemble of both B and H conformations. Although cAMP preferentially binds the B conformation, Rp-cAMPS interestingly binds the H conformation. This reveals the unique importance of the equatorial oxygen of the cyclic phosphate in mediating conformational transitions from H to B forms highlighting a novel approach for rational structure-based drug design. Ideal inhibitors such as Rp-cAMPS are those that preferentially "select" inactive conformations of target proteins by satisfying all "binding" constraints alone without inducing conformational changes necessary for activation.

The 'omics revolution and our understanding of sperm cell biology

The foundations of proteomics are to study gene products and their regulatory roles within cells. Paradoxically, the only evidence that sperm cells make new proteins is through mitochondrial protein synthesis. Yet despite this, spermatozoa are the perfect candidates for mass spectrometry and hence, proteomic analysis. These enterprising cells use a plethora of post-translational modifications in order to gain functionality following their production within the testis. By using a combination of two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), and more recently liquid chromatography-mass spectrometry (LC-MS)/MS, recent advances in sperm cell biology, through the use of proteomics, is making unparalleled progress. The protein inventory lists being generated have shed light on transmembrane proteins, kinases and chaperones never previously recognized. In addition, the ability to isolate either phosphopeptides or glycopeptides and quantify the differences between cells of two different populations make proteomic analysis of spermatozoa a real chance to finally answer some age old questions.

Phosphodiesterases catalyze hydrolysis of cAMP-bound to regulatory subunit of protein kinase A and mediate signal termination

Although extensive structural and biochemical studies have provided molecular insights into the mechanism of cAMP-dependent activation of protein kinase A (PKA), little is known about signal termination and the role of phosphodiesterases (PDEs) in regulatory feedback. In this study we describe a novel mode of protein kinase A-anchoring protein (AKAP)-independent feedback regulation between a specific PDE, RegA and the PKA regulatory (RIα) subunit, where RIα functions as an activator of PDE catalysis. Our results indicate that RegA, in addition to its well-known role as a PDE for bulk cAMP in solution, is also capable of hydrolyzing cAMP-bound to RIα. Furthermore our results indicate that binding of RIα activates PDE catalysis several fold demonstrating a dual function of RIα, both as an inhibitor of the PKA catalytic (C) subunit and as an activator for PDEs. Deletion mutagenesis has localized the sites of interaction to one of the cAMP-binding domains of RIα and the catalytic PDE domain of RegA whereas amide hydrogen/deuterium exchange mass spectrometry has revealed that the cAMP-binding site (phosphate binding cassette) along with proximal regions important for relaying allosteric changes mediated by cAMP, are important for interactions with the PDE catalytic domain of RegA. These sites of interactions together with measurements of cAMP dissociation rates demonstrate that binding of RegA facilitates dissociation of cAMP followed by hydrolysis of the released cAMP to 5'AMP. cAMP-free RIα generated as an end product remains bound to RegA. The PKA C-subunit then displaces RegA and reassociates with cAMP-free RIα to regenerate the inactive PKA holoenzyme thereby completing the termination step of cAMP signaling. These results reveal a novel mode of regulatory feedback between PDEs and RIα that has important consequences for PKA regulation and cAMP signal termination.

Mutations and polymorphisms in the gene encoding regulatory subunit type 1-alpha of protein kinase A (PRKAR1A): an update

PRKAR1A encodes the regulatory subunit type 1-alpha (RIalpha) of the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA). Inactivating PRKAR1A mutations are known to be responsible for the multiple neoplasia and lentiginosis syndrome Carney complex (CNC). To date, at least 117 pathogenic variants in PRKAR1A have been identified (online database: http://prkar1a.nichd.nih.gov). The majority are subject to nonsense mediated mRNA decay (NMD), leading to RIalpha haploinsufficiency and, as a result, activated cAMP signaling. Recently, it became apparent that CNC may be caused not only by RIalpha haploinsufficiency, but also by the expression of altered RIalpha protein, as proven by analysis of expressed mutations in the gene, consisting of amino acid substitutions and in-frame genetic alterations. In addition, a new subgroup of mutations that potentially escape NMD and result in CNC through altered (rather than missing) protein has been analyzed-these are frame-shifts in the 3' end of the coding sequence that shift the stop codon downstream of the normal one. The mutation detection rate in CNC patients is recently estimated at above 60%; PRKAR1A mutation-negative CNC patients are characterized by significant phenotypic heterogeneity. In this report, we present a comprehensive analysis of all presently known PRKAR1A sequence variations and discuss their molecular context and clinical phenotype.

Not lost in space: trafficking in the hedgehog signaling pathway

Compartmentalization within cells provides spatial organization of signaling pathways and ensures the specificity of signaling. In vertebrates, the primary cilium, a tiny microtubule-based protrusion present on most cells, is essential for organizing events during Hedgehog signal transduction. When cells are stimulated with Hedgehog ligands, proteins in the pathway move in and out of the cilia. Protein kinase A (PKA), which is implicated in diverse cellular processes including protein trafficking, is a component of the Hedgehog signaling pathway. PKA has been localized near primary cilia, at a location suitable for regulating the localization of other proteins in the pathway.

Sensing domain dynamics in protein kinase A-I{alpha} complexes by solution X-ray scattering

The catalytic (C) and regulatory (R) subunits of protein kinase A are exceptionally dynamic proteins. Interactions between the R- and C-subunits are regulated by cAMP binding to the two cyclic nucleotide-binding domains in the R-subunit. Mammalian cells express four different isoforms of the R-subunit (RIα, RIβ, RIIα, and RIIβ) that all interact with the C-subunit in different ways. Here, we investigate the dynamic behavior of protein complexes between RIα and C-subunits using small angle x-ray scattering. We show that a single point mutation in RIα, R333K (which alters the cAMP-binding properties of Domain B) results in a compact shape compared with the extended shape of the wild-type R·C complex. A double mutant complex that disrupts the interaction site between the C-subunit and Domain B in RIα, RIα_ABR333K·C(K285P), results in a broader P(r) curve that more closely resembles the P(r) profiles of wild-type complexes. These results together suggest that interactions between RIα Domain B and the C-subunit in the RIα·C complex involve large scale dynamics that can be disrupted by single point mutations in both proteins. In contrast to RIα·C complexes. Domain B in the RIIβ·C heterodimer is not dynamic and is critical for both inhibition and complex formation. Our study highlights the functional differences of domain dynamics between protein kinase A isoforms, providing a framework for elucidating the global organization of each holoenzyme and the cross-talk between the R- and C-subunits.

Radial spoke protein 3 is a mammalian protein kinase A-anchoring protein that binds ERK1/2

Initially identified in Chlamydomonas, RSP3 (radial spoke protein 3) is 1 of more than 20 identified radial spoke structural components of motile cilia and is required for axonemal sliding and flagellar motility. The mammalian orthologs for this and other radial spoke proteins, however, remain to be characterized. We found mammalian RSP3 to bind to the MAPK ERK2 through a yeast two-hybrid screen designed to identify interacting proteins that have a higher affinity for the phosphorylated, active form of the protein kinase. Consistent with the screening result, the human homolog, RSPH3, interacts with and is a substrate for ERK1/2. Moreover, RSPH3 is a protein kinase A-anchoring protein (AKAP) that scaffolds the cAMP-dependent protein kinase holoenzyme. The binding of RSPH3 to the regulatory subunits of cAMP-dependent protein kinase, RIIalpha and RIIbeta, is regulated by ERK1/2 activity and phosphorylation. Here we describe an ERK1/2-interacting AKAP and suggest a mechanism by which cAMP-dependent protein kinase-AKAP binding can be modulated by the activity of other enzymes.

MECHANISMS OF DEFICIENT TYPE I PROTEIN KINASE A ACTIVITY IN LUPUS T LYMPHOCYTES

Systemic lupus erythematosus (SLE) is an autoimmune disease in which the immune response to antigen results in exaggerated CD4(+) T helper and diminished CD8(+) T cytotoxic responses. To determine the mechanisms underlying impaired T cell effector functions, we have investigated the cAMP/protein kinase A (cAMP/PKA) signaling pathway. The results demonstrate that diminished PKA-catalyzed protein phosphorylation is the result of deficient type I (PKA-I) and type II (PKA-II) isozyme-specific activities. The prevalence of deficient PKA-I and PKA-II activities in SLE T cells is approximately 80% and 40%, respectively. Diminished PKA-I activities are not associated with disease activity and appear to be stable over time. Two disparate mechanisms account for these low PKA-I and PKA-II isozyme activities. Moreover, novel transcript mutations of the RI alpha gene have been identified that are characterized by deletions, transitions, and transversions. Most mutations are clustered adjacent to GAGAG motifs and CT repeats. In conclusion, aberrant signaling via the cAMP/PKA pathway occurs in SLE T cells, and this is proposed to contribute to abnormal T cell effector functions.

Phosphorylation and consequent stimulation of the tyrosine kinase c-Abl by PKA in mouse spermatozoa; its implications during capacitation

Upon ejaculation, spermatozoa undergo a series of post-translational modifications in a process known as capacitation in order to prepare for fertilization. In the absence of capacitation, fertilization cannot occur. Spermatozoa are unusual in that one of the hallmarks of capacitation is a global up-regulation in phosphotyrosine expression, which is known to be mediated upstream by PKA. Little is known about the signaling events downstream of PKA apart from the involvement of SRC, as a key mediator of PKA-induced tyrosine phosphorylation in the sperm tail. Here we describe the presence of c-Abl in mouse spermatozoa. In vitro analysis confirmed that PKA can up-regulate c-Abl kinase activity. In vivo, this tyrosine kinase was found to associate, and become threonine phosphorylated by PKA in the sperm flagellum. By treating spermatozoa with hemolysin we could demonstrate that a significant proportion of the tyrosine phosphorylation associated with capacitation could be suppressed by the c-Abl inhibitor, Gleevac. This is the first report of c-Abl being up-regulated by PKA for any cell type. We present a model, whereby these kinases may operate together with SRC to ensure optimal levels of tyrosine phosphorylation in the sperm flagellum during the attainment of a capacitated state.

Mutations in regulatory subunit type 1A of cyclic adenosine 5'-monophosphate-dependent protein kinase (PRKAR1A): phenotype analysis in 353 patients and 80 different genotypes

BACKGROUND

The "complex of myxomas, spotty skin pigmentation, and endocrine overactivity," or "Carney complex" (CNC), is caused by inactivating mutations of the regulatory subunit type 1A of the cAMP-dependent protein kinase (PRKAR1A) gene and as yet unknown defect(s) in other gene(s). Delineation of a genotype-phenotype correlation for CNC patients is essential for understanding PRKAR1A function and providing counseling and preventive care.

METHODS

A transatlantic consortium studied the molecular genotype and clinical phenotype of 353 patients (221 females and 132 males, age 34 +/- 19 yr) who carried a germline PRKAR1A mutation or were diagnosed with CNC and/or primary pigmented nodular adrenocortical disease.

RESULTS

A total of 258 patients (73%) carried 80 different PRKAR1A mutations; 114 (62%) of the index cases had a PRKAR1A mutation. Most PRKAR1A mutations (82%) led to lack of detectable mutant protein (nonexpressed mutations) because of nonsense mRNA mediated decay. Patients with a PRKAR1A mutation were more likely to have pigmented skin lesions, myxomas, and thyroid and gonadal tumors; they also presented earlier with these tumors. Primary pigmented nodular adrenocortical disease occurred earlier, was more frequent in females, and was the only manifestation of CNC with a gender predilection. Mutations located in exons were more often associated with acromegaly, myxomas, lentigines, and schwannomas, whereas the frequent c.491-492delTG mutation was commonly associated with lentigines, cardiac myxomas, and thyroid tumors. Overall, nonexpressed PRKAR1A mutations were associated with less severe disease.

CONCLUSION

CNC is genetically and clinically heterogeneous. Certain tumors are more frequent, with specific mutations providing some genotype-phenotype correlation for PRKAR1A mutations.

Regulatory subunit I-controlled protein kinase A activity is required for apical bile canalicular lumen development in hepatocytes

Signaling via cAMP plays an important role in apical cell surface dynamics in epithelial cells. In hepatocytes, elevated levels of cAMP as well as extracellular oncostatin M stimulate apical lumen development in a manner that depends on protein kinase A (PKA) activity. However, neither the identity of PKA isoforms involved nor the mechanisms of the cross-talk between oncostatin M and cAMP/PKA signaling pathways have been elucidated. Here we demonstrate that oncostatin M and PKA signaling converge at the level of the PKA holoenzyme downstream of oncostatin M-stimulated MAPK activation. Experiments were performed with chemically modified cAMP analogues that preferentially target regulatory subunit (R) I or RII holoenzymes, respectively, in hepatocytes. The data suggest that the dissociation of RI- but not RII-containing holoenzymes, as well as catalytic activity of PKA, is required for apical lumen development in response to elevated levels of cAMP and oncostatin M. However, oncostatin M signaling does not stimulate PKA holoenzyme dissociation in living cells. Based on pharmacological and cell biological studies, it is concluded that RI-controlled PKA activity is essential for cAMP- and oncostatin M-stimulated development of apical bile canalicular lumens.

New genes and/or molecular pathways associated with adrenal hyperplasias and related adrenocortical tumors

Over the course of the last 10 years, we have studied the genetic and molecular mechanisms leading to disorders that affect the adrenal cortex, with emphasis on those that are developmental, hereditary and associated with adrenal hypoplasia or hyperplasia, multiple tumors and abnormalities in other endocrine glands. On the basis of this work, we propose an hypothesis on how adrenocortical tumors form and the importance of the cyclic AMP-dependent signaling pathway in this process. The regulatory subunit type 1-alpha (RIalpha) of protein kinase A (PKA) (the PRKAR1A gene) is mutated in most patients with Carney complex and primary pigmented nodular adrenocortical disease (PPNAD). Phosphodiesterase-11A (the PDE11A gene) and -8B (the PDE8B gene) mutations were found in patients with isolated adrenal hyperplasia and Cushing syndrome, as well in patients with PPNAD. PKA effects on tumor suppression and/or development and the cell cycle are becoming clear: PKA and/or cAMP act as a coordinator of growth and proliferation in the adrenal cortex. Mouse models in which the respective genes have been knocked out see m to support this notion. Genome-wide searches for other genes responsible for adrenal tumors and related diseases are ongoing; recent evidece of the involvement of the mitochondrial oxidation pathway in adrenocortical tumorigenesis is derived from our study of rare associations such as those of disorders predisposing to adrenomedullary and related tumors (Carney triad, the dyad of paragangliomas and gastric stromal sarcomas or Carney-Stratakis syndrome, hereditary leiomyomatosis and renal cancer syndrome) which appear to be associated with adrenocortical lesions.

Isoform-specific regulation of immune cell reactivity by the catalytic subunit of protein kinase A (PKA)

There are two major genes encoding the catalytic subunits of protein kinase A, Calpha and Cbeta. The functional significance of these isoforms is enigmatic. Lymphoid cells of the immune system express both Calpha and Cbeta. In this study we tested the role of Calpha and Cbeta in regulating immune cell reactivity to antigens using mice carrying a targeted disruption of the Calpha and Cbeta gene respectively. Calpha and Cbeta ablation both resulted in a 50% reduction in PKA-specific kinase activity and the level of PKA type I but not PKA type II. Moreover, despite that C subunit ablation did not affect immune cell development and homeostasis, Calpha but not Cbeta ablation augmented expression of the activation marker CD69 on lymphocytes. CD69 induction coincided with immune cell hyperresponsiveness and was associated with reduced sensitivity to cAMP-mediated inhibition of anti-CD3 induced T cell proliferation. Our results imply that Calpha is required for normal immune cell reactivity and demonstrates isoform-specific effects and non-redundant functions of C subunit isoforms expressed in the same cell.

Protein kinase A effects of an expressed PRKAR1A mutation associated with aggressive tumors

Most PRKAR1A tumorigenic mutations lead to nonsense mRNA that is decayed; tumor formation has been associated with an increase in type II protein kinase A (PKA) subunits. The IVS6+1G>T PRKAR1A mutation leads to a protein lacking exon 6 sequences [R1 alpha Delta 184-236 (R1 alpha Delta 6)]. We compared in vitro R1 alpha Delta 6 with wild-type (wt) R1 alpha. We assessed PKA activity and subunit expression, phosphorylation of target molecules, and properties of wt-R1 alpha and mutant (mt) R1 alpha; we observed by confocal microscopy R1 alpha tagged with green fluorescent protein and its interactions with Cerulean-tagged catalytic subunit (C alpha). Introduction of the R1 alpha Delta 6 led to aberrant cellular morphology and higher PKA activity but no increase in type II PKA subunits. There was diffuse, cytoplasmic localization of R1 alpha protein in wt-R1 alpha- and R1 alpha Delta 6-transfected cells but the former also exhibited discrete aggregates of R1 alpha that bound C alpha; these were absent in R1 alpha Delta 6-transfected cells and did not bind C alpha at baseline or in response to cyclic AMP. Other changes induced by R1 alpha Delta 6 included decreased nuclear C alpha. We conclude that R1 alpha Delta 6 leads to increased PKA activity through the mt-R1 alpha decreased binding to C alpha and does not involve changes in other PKA subunits, suggesting that a switch to type II PKA activity is not necessary for increased kinase activity or tumorigenesis.

A phase I safety and dose escalation trial of docetaxel combined with GEM231, a second generation antisense oligonucleotide targeting protein kinase A R1alpha in patients with advanced solid cancers

PURPOSE

GEM231 is a second-generation antisense oligonucleotide targeting the mRNA of the R1alpha regulatory subunit of cAMP dependent protein kinase A. Preclinical studies have demonstrated synergistic antitumor activity when GEM231 is combined with docetaxel. This trial assesses the safety of this combination.

EXPERIMENTAL DESIGN

Docetaxel was administered once every three weeks (one-cycle) at doses between 50-75 mg/m2. GEM231 was administered twice weekly at 220 mg/m2 for 3 (schedule-A), or 2 (schedule-B) weeks.

RESULTS

Twenty patients with chemotherapy-refractory advanced cancer received a total of 39 cycles of therapy. Six patients in schedule-A received docetaxel 50 mg/m2, and 14 patients in schedule-B received docetaxel 50-75 mg/m2. In schedule-A, 2 of 6 patients developed cycle-1 dose limiting toxicity (DLT)-grade-3 fatigue or grade-3 serum transaminase elevation. In schedule-B, 1 of 4 patients developed cycle-1 DLT at the highest dose of docetaxel tested (75 mg/m2)--grade-3 febrile neutropenia. Subsequent dose escalations were not pursued since the overall incidence of grade-3 toxicities (including those that occurred after cycle 1) was 75%, and this dose was close to the single agent MTD of docetaxel. Grade-3 toxicities included fatigue (2 patients), transaminase elevation (4 patients), and altered mentation (1 patient). The mean post-infusion aPTT was significantly higher than the pre-infusion value [14.8 seconds; p<0.001]; however, there were no hemorrhagic episodes.

CONCLUSIONS

The recommended dose for further development of the combination of docetaxel and GEM231 is 75 mg/m2 and 220 mg/m2, respectively. It is important to administer GEM231 twice weekly for 2 consecutive weeks followed by a one-week break.

PKA type IIalpha holoenzyme reveals a combinatorial strategy for isoform diversity

The catalytic (C) subunit of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is inhibited by two classes of regulatory subunits, RI and RII. The RII subunits are substrates as well as inhibitors and do not require adenosine triphosphate (ATP) to form holoenzyme, which distinguishes them from RI subunits. To understand the molecular basis for isoform diversity, we solved the crystal structure of an RIIalpha holoenzyme and compared it to the RIalpha holoenzyme. Unphosphorylated RIIalpha(90-400), a deletion mutant, undergoes major conformational changes as both of the cAMP-binding domains wrap around the C subunit's large lobe. The hallmark of this conformational reorganization is the helix switch in domain A. The C subunit is in an open conformation, and its carboxyl-terminal tail is disordered. This structure demonstrates the conserved and isoform-specific features of RI and RII and the importance of ATP, and also provides a new paradigm for designing isoform-specific activators or antagonists for PKA.

R-subunit isoform specificity in protein kinase A: distinct features of protein interfaces in PKA types I and II by amide H/2H exchange mass spectrometry

The two isoforms (RI and RII) of the regulatory (R) subunit of cAMP-dependent protein kinase or protein kinase A (PKA) are similar in sequence yet have different biochemical properties and physiological functions. To further understand the molecular basis for R-isoform-specificity, the interactions of the RIIbeta isoform with the PKA catalytic (C) subunit were analyzed by amide H/(2)H exchange mass spectrometry to compare solvent accessibility of RIIbeta and the C subunit in their free and complexed states. Direct mapping of the RIIbeta-C interface revealed important differences between the intersubunit interfaces in the type I and type II holoenzyme complexes. These differences are seen in both the R-subunits as well as the C-subunit. Unlike the type I isoform, the type II isoform complexes require both cAMP-binding domains, and ATP is not obligatory for high affinity interactions with the C-subunit. Surprisingly, the C-subunit mediates distinct, overlapping surfaces of interaction with the two R-isoforms despite a strong homology in sequence and similarity in domain organization. Identification of a remote allosteric site on the C-subunit that is essential for interactions with RII, but not RI subunits, further highlights the considerable diversity in interfaces found in higher order protein complexes mediated by the C-subunit of PKA.

cAMP and protein kinase A in endocrine (and other) tumors

Studies of the biological role of cAMP have indicated dual and often opposing effects on proliferation and differentiation. Elevation of the intracellular cAMP in normal and transformed cells may lead to cell proliferation; in other cells, it induces changes in morphology, apoptosis and/or differentiation. The best known mediator of cAMP action in the cell is cAMP-dependent protein kinase or protein kinase A (PKA). PKA exists as two different isozymes, designated type I (PKA-I) and type II (PKA-II); the two isoforms are essentially distinct in their physicochemical properties. The relative ratio of PKA-I and PKA-II varies throughout the cell cycle in cells of the same type, it changes significantly during development and follows different patterns in the various tissues. Disruption of the apparently fine balance between the main two PKA isozymes is strongly associated with tumorigenesis and tumor growth, and vice versa. The enormous variety of cAMP/PKA functions and the net effect of this signaling system on cellular growth, proliferation and differentiation have been the subject of debate for more than 30 years among investigators in the field. The relatively recent identification of PRKAR1A mutations and PKA-I deficiency as a cause of endocrine and other tumors in human and mice was instrumental in advancing our understanding of how cAMP and PKA work in regulating the cell cycle. This article reviews the current state of knowledge in the field; the use of pharmacologic modulation of the cAMP/PKA system with the goal of treating certain tumors appears to be near, although very little has been accomplished so far, at least in terms of studies on humans.

Compartmentalized phosphorylation of IAP by protein kinase A regulates cytoprotection

Cell death pathways are likely regulated in specialized subcellular microdomains, but how this occurs is not understood. Here, we show that cyclic AMP-dependent protein kinase A (PKA) phosphorylates the inhibitor of apoptosis (IAP) protein survivin on Ser20 in the cytosol, but not in mitochondria. This phosphorylation event disrupts the binding interface between survivin and its antiapoptotic cofactor, XIAP. Conversely, mitochondrial survivin or a non-PKA phosphorylatable survivin mutant binds XIAP avidly, enhances XIAP stability, synergistically inhibits apoptosis, and accelerates tumor growth, in vivo. Therefore, differential phosphorylation of survivin by PKA in subcellular microdomains regulates tumor cell apoptosis via its interaction with XIAP.

Identification and mapping of protein kinase A binding sites in the costameric protein myospryn

Recently we identified a novel target gene of MEF2A named myospryn that encodes a large, muscle-specific, costamere-restricted alpha-actinin binding protein. Myospryn belongs to the tripartite motif (TRIM) superfamily of proteins and was independently identified as a dysbindin-interacting protein. Dysbindin is associated with alpha-dystrobrevin, a component of the dystrophin-glycoprotein complex (DGC) in muscle. Apart from these initial findings little else is known regarding the potential function of myospryn in striated muscle. Here we reveal that myospryn is an anchoring protein for protein kinase A (PKA) (or AKAP) whose closest homolog is AKAP12, also known as gravin/AKAP250/SSeCKS. We demonstrate that myospryn co-localizes with RII alpha, a type II regulatory subunit of PKA, at the peripheral Z-disc/costameric region in striated muscle. Myospryn interacts with RII alpha and this scaffolding function has been evolutionarily conserved as the zebrafish ortholog also interacts with PKA. Moreover, myospryn serves as a substrate for PKA. These findings point to localized PKA signaling at the muscle costamere.

Identification, cloning and characterization of a novel 47 kDa murine PKA C subunit homologous to human and bovine Cbeta2

Background

Two main genes encoding the catalytic subunits Cα and Cβ of cyclic AMP dependent protein kinase (PKA) have been identified in all vertebrates examined. The murine, bovine and human Cβ genes encode several splice variants, including the splice variant Cβ2. In mouse Cβ2 has a relative molecular mass of 38 kDa and is only expressed in the brain. In human and bovine Cβ2 has a relative molecular mass of 47 kDa and is mainly expressed in lymphoid tissues.

Results

We identified a novel 47 kDa splice variant encoded by the mouse Cβ gene that is highly expressed in lymphoid cells. Cloning, expression, and production of a sequence-specific antiserum and characterization of PKA catalytic subunit activities demonstrated the 47 kDa protein to be a catalytically active murine homologue of human and bovine Cβ2. Based on the present results and the existence of a human brain-specifically expressed Cβ splice variant designated Cβ4 that is identical to the former mouse Cβ2 splice variant, the mouse splice variant has now been renamed mouse Cβ4.

Conclusion

Murine lymphoid tissues express a protein that is a homologue of human and bovine Cβ2. The murine Cβ gene encodes the splice variants Cβ1, Cβ2, Cβ3 and Cβ4, as is the case with the human Cβ gene.

Selective modulation of protein kinase A I and II reveals distinct roles in thyroid cell gene expression and growth

A global gene expression profiling of TSH stimulation on differentiated (FRTL5) and partially dedifferentiated [FRT/TSHR (TSH receptor)] rat thyroid cells was performed. A total of 123 TSH-regulated genes (95 newly described) were identified in FRTL5, whereas no significant transcriptional modifications were seen in FRT/TSHR cells. Because regulatory subunit IIbeta (RIIbeta) of protein kinase A (PKA), a key element downstream of cAMP, was expressed in FRTL5 but not in cAMP-refractory FRT/TSHR cells, we hypothesized that this gene may play an important role in TSH signaling. We therefore performed a series of experiments to investigate the involvement of RIIbeta and the different PKA isoforms. A positive effect of PKA II- but not of PKA I-selective activation on gene transcription and proliferation in FRTL5 cells, as well as an impairment of TSH nuclear effects after RIIbeta silencing were observed, suggesting that PKA II plays an essential role in TSH signaling. This view was supported by the restoration of TSH nuclear effects after reexpression of RIIbeta in FRT/TSHR cells. Because PKA I stimulation could increase iodide uptake in FRTL5 cells without affecting gene transcription, PKA I may mediate TSH actions at posttranscriptional levels. Analyses on three human cancer cell lines confirmed the possible loss of RIIbeta expression and antiproliferative activity of PKA I-selective cAMP analogs ( approximately 60% at 200 microm in BRAF-mutated cells). The inhibitory effect of PKA I apparently required constitutive MAPK activation and was associated with an inhibition of ERK phosphorylation. These findings may open new therapeutic perspectives in patients with thyroid cancer.

Identification of SRC as a key PKA-stimulated tyrosine kinase involved in the capacitation-associated hyperactivation of murine spermatozoa

Fertilization of the mammalian oocyte depends on the ability of spermatozoa to undergo a process known as capacitation as they ascend the female reproductive tract. A fundamental feature of this process is a marked increase in tyrosine phosphorylation by an unusual protein kinase A (PKA)-mediated pathway. To date, the identity of the intermediate PKA-activated tyrosine kinase driving capacitation is still unresolved. In this study, we have identified SRC as a candidate intermediate kinase centrally involved in the control of sperm capacitation. Consistent with this conclusion, the SRC kinase inhibitor SU6656 was shown to suppress both tyrosine phosphorylation and hyperactivation in murine spermatozoa. Moreover, SRC co-immunoprecipitated with PKA and this interaction was found to lead to an activating phosphorylation of SRC at position Y416. We have also used difference-in-2D-gel-electrophoresis (DIGE) in combination with mass spectrometry to identify a number of SRC substrates that become phosphorylated during capacitation including enolase, HSP90 and tubulin. Our data further suggest that the activation of SRC during capacitation is negatively controlled by C-terminal SRC kinase. The latter was localized to the acrosome and flagellum of murine spermatozoa by immunocytochemistry, whereas capacitation was associated with an inactivating serine phosphosphorylation of this inhibitory kinase.

Protein kinase A targeting and activation as seen by small-angle solution scattering

We have studied the solution structures of the multi-functional protein kinase A using small-angle X-ray and neutron scattering and have found a remarkable structural diversity in the different isoforms of this multi-subunit enzyme, in spite of its having high sequence homology and a common domain organization within its sequences. The available high-resolution crystal and NMR structural data for the protein kinase A components have aided in the interpretation of the solution scattering data and enabled us to develop models that bring insights into protein kinase A activation and targeting mechanisms, such as the opening and closing of the catalytic cleft to facilitate substrate binding or inhibition, respectively, and the role of sequence segments that join functional domains in the R subunit in providing a structurally flexible scaffold for interactions with the C subunit and A kinase-anchoring proteins (AKAPs).

Structural themes and variations in protein kinase A as seen by small-angle scattering and neutron contrast variation

Using small-angle solution scattering and neutron contrast variation, we have studied the structure of the multi-subunit protein kinase A. We have gained insights into how nature can take a set of common structural domains (or themes) and modulate their interactions via sequence variations and second messenger mediated signaling to affect enzyme activity and receptor binding important for targeting this multi-function enzyme to specific sub-cellular locations. These studies demonstrate the power of neutron contrast variation to expand our knowledge of the dynamic supra-molecular structures that carry out biological function.

Inhibition of protein kinase A in murine enteric neurons causes lethal intestinal pseudo-obstruction

A number of in vitro studies suggest that many important developmental and functional events in the enteric nervous system are regulated by the intracellular signaling enzyme cAMP protein kinase A (PKA). To evaluate the in vivo significance of these observations, a Cre-inducible, dominant-negative, mutant regulatory subunit (RIalphaB) of PKA was activated in enteric neurons by either a Proteolipid protein-Cre transgene or a Hox11L1-Cre "knock-in" allele. In both models, RIalphaB activation resulted consistently in profound distension of the proximal small intestine within 2 weeks after birth. Intestinal transit of radio-opaque tracers was severely retarded in the double-transgenic animals, which died shortly after weaning. In the enteric nervous system, recombination was restricted to neurons as demonstrated by histochemical analysis and confocal microscopic colocalization of a Cre recombinase-dependent reporter gene with the neuronal marker Hu(C/D), in contrast with the glial marker S100. Histochemical analysis of beta-galactosidase expression and acetylcholinesterase activity, as well as neuronal counts, demonstrated that intestinal dysmotility was not associated with obvious malformation of the myenteric plexus. However, inhibition of PKA activity in enteric neurons disrupted the major motor complexes of isolated intestinal segments in vitro. These results provide strong evidence that PKA activity plays a critical role in enteric neurotransmission in vivo, and highlight neuronal PKA or related signaling molecules as potential therapeutic targets in gastrointestinal motility disorders.

Solution scattering reveals large differences in the global structures of type II protein kinase A isoforms

Isoform diversity within the protein kinase A (PKA) family is achieved by catalytic (C) subunits binding to different isoforms of regulatory subunit homodimers (R2). In a previous small-angle X-ray scattering study, we showed that the type Ialpha R2 homodimer has a distinctive Y-shaped structure, while the IIalpha and IIbeta homodimers are highly flexible and extended in solution. Here we present the results of X-ray scattering experiments on different isoforms of the PKA holoenzyme (R2C2) and show that the type IIbeta R2 homodimer undergoes a dramatic compaction upon binding C subunits that involves a 10A reduction in radius of gyration (from 56 to 46 A) and a 35 A shortening of the maximum linear dimension (from 180-145 A). In contrast, the type IIalpha R2 homodimer shows very little change in these structural parameters and remains extended upon C-subunit binding. This large difference is surprising given the highly conserved sequence and domain organization for the different R isoforms. A mutant RIIbeta holoenzyme and an RIIalpha/RIIbeta chimera were used to explore the role of the sequence linking different functional domains within RIIbeta in the observed C subunit-induced compaction. Structural modeling was used to aid in interpreting the scattering results in terms of the role of inter-domain and inter-subunit contacts in determining the global conformations of the different isoforms. The results provide an important structural foundation for understanding isoform-specific PKA localization and signaling.

Growth hormone-secreting tumors: genetic aspects and data from animal models

Hereditary cases of growth hormone (GH)-secreting tumors have been classified into three clinical entities: the multiple endocrine neoplasia type 1 (MEN1) syndrome, the Carney complex (CNC) and the isolated familial somatotropinomas (IFS). The genomic defects associated with MEN1 are all linked to various mutations of the MEN1 gene, which is located at chromosome 11q13 and codes for menin, a nuclear protein expressed in multiple tissues. Inactivation of the MEN1 gene appears to be only rarely associated with sporadic pituitary tumor development. A CNC-associated gene, the type 1 alpha regulatory subunit (R1alpha) of cAMP-dependent protein kinase A (PRKAR1A), is located at 17q23-24. A second CNC candidate gene is located at chromosome 2p15-16, with characteristics of inheritance consistent with an oncogene; however, this gene has not been identified yet. PRKAR1A mutations are infrequently associated with sporadic GH-secreting adenomas. A candidate IFS gene is located at 11q13, in proximity to the MEN1 gene, at a locus narrowed down to a 2.21-Mb area, with approximately 50 genes, that does not appear to include the MEN1 gene. Apart from the linkage of IFS to 11q13, a possible linkage to 2p16 has also been raised, although data are still inconclusive. This manuscript reviews genetic aspects of hereditary GH-secreting tumors, data from animal models resulting from the inactivation of the MEN1 and PRKAR1A tumor suppressor genes and available in vitro data regarding possible functions of menin, the product of the MEN1 gene.

PKA-mediated phosphorylation regulates the function of activation-induced deaminase (AID) in B cells

During humoral immune responses, two distinct genetic modification events diversify the Ig genes in germinal center (GC) B cells: somatic hypermutation and class switch recombination (CSR). Both processes require the activity of activation-induced cytidine deaminase (AID), an enzyme expressed specifically in GC B cells. However, the mechanisms that regulate AID activity are largely unknown. Here we report that protein kinase A (PKA) phosphorylates AID and regulates its activity in GC B cells. AID physically interacts with the PKA holoenzyme in the cytoplasm and is phosphorylated by the PKA catalytic subunit at specific residues. AID phosphorylation is required for CSR, because substitution of the two phosphorylation targets impairs its ability to rescue CSR in AID-deficient B cells. Pharmacologic inhibition of PKA prevents isotype class switching in a murine B-cell lymphoma cell line; conversely, B cells from mice where PKA activity is made constitutive by conditional deletion of the PKA regulatory subunit gene display enhanced CSR. These findings implicate PKA in the regulation of AID function and suggest that the control of T cell-dependent immune responses may be modulated, via AID, by signals that activate PKA.