Coordination of cAMP signaling events through PKA anchoring

Regulation of the renin-angiotensin-aldosterone system by cyclic nucleotides and phosphodiesterases

The renin-angiotensin-aldosterone system (RAAS) is one of the key players in the regulation of blood volume and blood pressure. Dysfunction of this system is connected with cardiovascular and renal diseases. Regulation of RAAS is under the control of multiple intracellular mechanisms. Cyclic nucleotides and phosphodiesterases are the major regulators of this system since they control expression and activity of renin and aldosterone. In this review, we summarize known mechanisms by which cyclic nucleotides and phosphodiesterases regulate renin gene expression, secretion of renin granules from juxtaglomerular cells and aldosterone production from zona glomerulosa cells of adrenal gland. We also discuss several open questions which deserve future attention.

Protein Kinase A (PRKA) Activity Is Regulated by the Proteasome at the Onset of Human Sperm Capacitation

The proteasome increases its activity at the onset of sperm capacitation due to the action of the SACY/PRKACA pathway; this increase is required for capacitation to progress. PRKA activity also increases and remains high during capacitation. However, intracellular levels of cAMP decrease in this process. Our goal was to evaluate the role of the proteasome in regulating PRKA activity once capacitation has started. Viable human sperm were incubated in the presence and absence of epoxomicin or with 0.1% DMSO. The activity of PRKA; the phosphorylation pattern of PRKA substrates (pPRKAs); and the expression of PRKAR1, PRKAR2, and AKAP3 were evaluated by Western blot. The localization of pPRKAs, PRKAR1, PRKAR2, and AKAP3 was evaluated by immunofluorescence. Treatment with epoxomicin changed the localization and phosphorylation pattern and decreased the percentage of pPRKAs-positive sperm. PRKA activity significantly increased at 1 min of capacitation and remained high throughout the incubation. However, epoxomicin treatment significantly decreased PRKA activity after 30 min. In addition, PRKAR1 and AKAP3 were degraded by the proteasome but with a different temporal kinetic. Our results suggest that PRKAR1 is the target of PRKA regulation by the proteasome.

Mitochondrial PKA mediates sperm motility

BACKGROUND

Mitochondria are the major source of ATP to power sperm motility. Phosphorylation of mitochondrial proteins has been proposed as a major regulatory mechanism for mitochondrial bioenergetics.

METHODS

Sperm motility was measured by a computer-assisted analyzer, protein detection by western blotting, membrane potential by tetramethylrhodamine, cellular ATP by luciferase assay and localization of PKA by immuno-electron microscopy.

RESULTS

Bicarbonate is essential for the creation of mitochondrial electro-chemical gradient, ATP synthesis and sperm motility. Bicarbonate stimulates PKA-dependent phosphorylation of two 60kDa proteins identified as Tektin and glucose-6-phosphate isomerase. This phosphorylation was inhibited by respiration inhibition and phosphorylation could be restored by glucose in the presence of bicarbonate. However, this effect of glucose cannot be seen when the mitochondrial ATP/ADP exchanger was inhibited indicating that glycolytic-produced ATP is transported into the mitochondria and allows PKA-dependent protein phosphorylation inside the mitochondria.

CONCLUSIONS

Bicarbonate activates mitochondrial soluble adenylyl cyclase (sAC) which catalyzes cAMP production leading to the activation of mitochondrial PKA. Glucose can overcome the lack of ATP in the absence of bicarbonate but it cannot affect the mitochondrial sAC/PKA system, therefore the PKA-dependent phosphorylation of the 60kDa proteins does not occur in the absence of bicarbonate.

GENERAL SIGNIFICANCE

Production of CO2 in Krebs cycle, which is converted to bicarbonate is essential for sAC/PKA activation leading to mitochondrial membrane potential creation and ATP synthesis.

The transcription factor FOXM1c is activated by protein kinase CK2, protein kinase A (PKA), c-Src and Raf-1

The transcription factor FOXM1c possesses a very strong C-terminal TAD (transactivation domain), but full-length FOXM1c is only a weak transactivator because the TAD is completely inhibited by the auto-inhibitory N-terminus. The N-terminus blocks the TAD by directly binding to the TAD. Accordingly, FOXM1c deletion mutants without N-terminus are strong transactivators. Therefore, the question arises whether signals exist, which activate full-length FOXM1c by releasing the FOXM1c-TAD from its inhibition by the N-terminus. Indeed, full-length FOXM1c is strongly activated by protein kinase CK2 and PKA (protein kinase A). Both CK2 and PKA do not activate a FOXM1c deletion mutant without N-terminus demonstrating that the activation of FOXM1c by CK2 and PKA depends on the presence of the N-terminus. Consequently, CK2 and PKA activate FOXM1c by alleviating the inhibition of FOXM1c by its N-terminus. The presence of two potential CK2 phosphorylation sites and two potential PKA phosphorylation sites in the N-terminus of FOXM1c suggests that CK2 and PKA may activate FOXM1c through phosphorylation of the FOXM1c N-terminus. Thus, CK2 and PKA strongly activate full-length FOXM1c because they alleviate the repression of FOXM1c by its own auto-inhibitory N-terminus. Also c-Src activates full-length FOXM1c by relieving the inhibition of FOXM1c by its N-terminus. In contrast, Raf-1 activates FOXM1c independently of the FOXM1c N-terminus. In summary, this study shows for the first time that FOXM1c is activated by the four kinases CK2, PKA, c-Src and Raf-1.

Role of protein kinase C-dependent A-kinase anchoring proteins in lysophosphatidic acid-induced cAMP signaling in human diploid fibroblasts

Previously, we reported that lysophosphatidic acid (LPA)-induced adenosine 3',5'-cyclic monophosphate (cAMP) production by human diploid fibroblasts depends on the age of the fibroblasts. In this study, we examined the role of A-kinase anchoring proteins (AKAP) in the regulation of LPA-stimulated cAMP production in senescent fibroblasts. We found that levels of protein kinase C (PKC)-dependent AKAPs, such as Gravin and AKAP79, were elevated in senescent cells. Co-immunoprecipitation experiments revealed that Gravin and AKAP79 do not associate with adenylyl cyclase type 2 (AC2) but bind to AC4/6, which interacts with calcium-dependent PKCs alpha/beta both in young and senescent fibroblasts. When the expression of Gravin and AKAP79 was blocked by small interference RNA transfection, the basal level of cAMP was greatly reduced and the cAMP status after LPA treatment was also reversed. Protein kinase A showed a similar pattern in terms of its basal activity and LPA-dependent modulation. These data suggest that Gravin and to a lesser extent, AKAP79, may play important roles in maintaining the basal AC activity and in coupling the AC systems to inhibitory signals such as Gialpha in young cells, and to stimulatory signals such as PKCs in senescent cells. This study also demonstrates that Gravin is especially important for the long-term activation of PKC by LPA in senescent cells. We conclude that LPA-dependent increased level of cAMP in senescent human diploid fibroblasts is associated with increases in Gravin levels resulting in its increased binding with and activation of calcium-dependent PKC alpha/beta and AC4/6.

Mast cell function: regulation of degranulation by serine/threonine phosphatases

Mast cells play both effector and modulatory roles in a range of allergic and immune responses. The principal function of these cells is the release of inflammatory mediators from mast cells by degranulation, which involves a complex interplay of signalling molecules. Understanding the molecular architecture underlying mast cell signalling has attracted renewed interest as the capacity for therapeutic intervention through controlling mast cell degranulation is now accepted as a viable proposition. The dynamic regulation of signalling by protein phosphorylation is a well-established phenomenon and many of the early events involved in mast cell activation are well understood. Less well understood however are the events further downstream of receptor activation that allow movement of granules through the cytoskeletal barrier and docking and fusion of granules with the plasma membrane. Whilst a potential role for the protein phosphatase family of signalling enzymes in mast cell function has been accepted for some time, the evidence has largely been derived from the use of broad specificity pharmacological inhibitors and results often depend upon the experimental conditions, leading to conflicting views. In this review, we present and discuss the pharmacological and recent molecular evidence that protein phosphatases, and in particular the protein phosphatase serine/threonine phosphatase type 2A (PP2A), have major regulatory roles to play and may be potential targets for the design of new therapeutic agents.

Moving to the beat: a review of mammalian sperm motility regulation

Because it is generally accepted that a high percentage of poorly motile or immotile sperm will adversely affect male fertility, analysis of sperm motility is a central part of the evaluation of male fertility. In spite of its importance to fertility, poor sperm motility remains only a description of a pathology whose underlying cause is typically poorly understood. The present review is designed to bring the clinician up to date with the most current understanding of the mechanisms that regulate sperm motility and to raise questions about how aberrations in these mechanisms could be the underlying causes of this pathology.

Facilitation of L-type Ca2+ channels in dendritic spines by activation of beta2 adrenergic receptors

We studied the contribution of L-type Ca2+ channels to action potential-evoked Ca2+ influx in dendritic spines of CA1 pyramidal neurons and the modulation of these channels by the beta2 adrenergic receptor. Backpropagating action potentials (bAPs) (three at 50 Hz) were evoked by brief somatic current injections, and Ca2+ transients were recorded in proximal basal dendrites and associated spines. The R- and T-type Ca2+ channel blocker NiCl2 (100 microm) significantly reduced Ca2+ transients in both spines and their parent dendrites (approximately 50%), suggesting that these channels are the major source of bAP-evoked Ca2+ influx in these structures. The L-type Ca2+ channel blockers nimodipine and nifedipine (both 10 microm) reduced spine Ca2+ transients by approximately 10%, whereas the L-type Ca2+ channel activators FPL 64176 (2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methylester) and Bay K 8644 ((+/-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester) (both 10 microm) significantly enhanced the spine Ca2+ transients by 40-50%. Activation of beta2 adrenergic receptors with salbutamol (40 microm) or formoterol (5 microm) resulted in significant enhancements of the spine (40-50%) but not dendritic Ca2+ transients. This increase was prevented when L-type Ca2+ channels were blocked with nimodipine (10 microm) or when cAMP-dependent protein kinase A (PKA) was inhibited with KT5720 (3 microm), Rp-cAMPS (Rp-adenosine cyclic 3',5'-phosphorothioate) (100 microm), or PKI (100 microm). The above data suggest that L-type Ca2+ channels are functionally present in dendritic spines of CA1 pyramidal neurons, contribute to spine Ca2+ influx, and can be modulated by the beta2 adrenergic receptor through PKA in a highly compartmentalized manner.

The role of serine/threonine protein phosphatases in exocytosis

Modulation of exocytosis is integral to the regulation of cellular signalling, and a variety of disorders (such as epilepsy, hypertension, diabetes and asthma) are closely associated with pathological modulation of exocytosis. Emerging evidence points to protein phosphatases as key regulators of exocytosis in many cells and, therefore, as potential targets for the design of novel therapies to treat these diseases. Diverse yet exquisite regulatory mechanisms have evolved to direct the specificity of these enzymes in controlling particular cell processes, and functionally driven studies have demonstrated differential regulation of exocytosis by individual protein phosphatases. This Review discusses the evidence for the regulation of exocytosis by protein phosphatases in three major secretory systems, (1) mast cells, in which the regulation of exocytosis of inflammatory mediators plays a major role in the respiratory response to antigens, (2) insulin-secreting cells in which regulation of exocytosis is essential for metabolic control, and (3) neurons, in which regulation of exocytosis is perhaps the most complex and is essential for effective neurotransmission.

cAMP-independent role of PKA in tonicity-induced transactivation of tonicity-responsive enhancer/ osmotic response element-binding protein

Hypertonicity-induced increase in activity of the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP) protects renal cells by increasing transcription of genes, including those involved in increased accumulation of organic osmolytes. We previously showed that hypertonicity increases transactivating activity of TonEBP/OREBP. Assay with a binary GAL4 transactivation system showed that the 984 C-terminal amino acids of TonEBP/OREBP (amino acids 548-1531) contain a tonicity-dependent transactivation domain (TAD). Also, amino acids 548-1531 undergo tonicity-dependent phosphorylation, and some inhibitors of protein kinases reduce the tonicity-dependent transactivation. In the present studies we examined the role of protein kinase A (PKA).

RESULTS

(i) An inhibitor of PKA (H89) reduces tonicity-dependent increases in transactivation, ORE/TonE reporter activity, and induction of aldose reductase and betaine transporter mRNAs. (ii) Overexpression of the catalytic subunit of PKA (PKAc) increases transactivation activity of amino acids 548-1531 and activity of an ORE/TonE reporter. The increases are much greater under isotonic than under hypertonic conditions. (iii) A dominant-negative PKAc reduces activity of an ORE/TonE reporter. (iv) PKAc activity increases with tonicity but cAMP does not. (v) TonEBP/OREBP and PKAc coimmunoprecipitate. (vi) amino acids 872-1271, including N- and C-terminal polyglutamine stretches, demonstrate tonicity-dependent transactivation, albeit less than amino acids 548-1531, and a similar role for PKA.

CONCLUSIONS

(i) PKA plays an important role in TonEBP/OREBP activation of tonicity-dependent gene expression; (ii) PKA activation of TonEBP/OREBP appears to be cAMP-independent; and (iii) amino acids 872-1271 are sufficient for tonicity-dependent transactivation of TonEBP/OREBP.

A 15-residue bifunctional element in D-AKAP1 is required for both endoplasmic reticulum and mitochondrial targeting

The cAMP-dependent protein kinase anchoring protein, d-AKAP1, has two N-terminal splice variants. The shorter forms (N0, d-AKAP1a, and -1c) target to mitochondria, and the longer forms (N1, d-AKAP1b, and -1d) with 33 additional residues N-terminal to N0 target to the endoplasmic reticulum (ER) (Huang, L. J., Wang, L., Ma, Y., Durick, K., Perkins, G., Deerinck, T. J., Ellisman, M. H., and Taylor, S. S. (1999) J. Cell Biol. 145, 951-959). In d-AKAP1a, translation may initiate from both Met-34 or Met-49 producing two molecules both targeted to mitochondria. The shorter molecule contains the 15-residue targeting motif, homologous to the N-terminal mitochondrial targeting motif of hexokinase I. Extensive mutagenesis showed that one hydrophobic surface of the 15-residue hexokinase-homologous segment contained the key elements for mitochondrial targeting. The same 15 residues are also part of the ER-targeting signal, but for ER targeting multiple hydrophobic residues are required that encompass both surfaces of the helix. The different involvement of the same helical motif for targeting to the two organelles appears to reflect different modes of interaction with the two organelles. This is the first example of a bifunctional helical element that is required for both ER and mitochondrion targeting.

Clues to calcineurin function in mammalian fast-twitch muscle

It is believed that brief, high amplitude Ca2+ transients, as found in fast-twitch muscles, are not sufficient to activate the calcineurin (Cn)-dependent signaling pathway involved in regulation of slow myosin and slow sarcoplasmic reticulum Ca2+-ATPase genes (Olson and Williams, Cell 101: 689-692, 2000). The results reported here try to fill the gap between this molecular knowledge, and the still fragmentary pieces of information on a possible different role of calcineurin in the same type of muscles. In the present work calcineurin was determined immunocytochemically by labeling fast- and slow-twitch fibers of representative rabbit muscles with anti-CnB antibodies, and was assessed by western blotting of isolated subcellular fractions. Calcineurin was found to be largely soluble and to be constitutively overexpressed in fast muscle as CnAalpha and CnAbeta isoforms, the latter appearing to be predominant. Particulate calcineurin was not only associated with myofibrils but also with membranes of various origins. Fluorescence microscopy showed that calcineurin was distributed in the same pattern with respect to sarcomeres in both types of fibers, and formed punctate dots spanning the I-Z-I region, rather than being exclusively located at the Z-line, a disposition described for cardiomyocytes (Frey et al., Proc Natl Acad Sci USA 97: 14,632-14,637, 2000). From knowledge that, in mammalian skeletal muscle fibers, junctional triads are located at the A-I band boundary, we explored the distribution of calcineurin between triadic components, after having verified that it was present in very low amounts in dystrophin-enriched sarcolemmal membranes. Our results demonstrate that a small but significant proportion of calcineurin coenriched with transverse tubules (TT), and copurified with the DHPR and with DHPR-associated PKA-AKAP15/18, thus suggesting that it is assembled as a multiprotein complex in the junctional membrane domain of TT. The membrane specificity of this association is further corroborated by the negative evidence for the presence of calcineurin in SR terminal cisternae. Calcineurin was separated from the DHPR and isolated as a AKAP15/18 subcomplex, including beta2 adrenergic receptor, in addition to PKA and calcineurin, following equilibrium centrifugation of detergent extracts on a linear sucrose gradient. We show that the alpha1 subunit skeletal isoform of the DHPR, is a substrate for calcineurin dephosphorylation, after previous phosphorylation by endogenous PKA.

Cyclic nucleotide phosphodiesterases

Cyclic nucleotide second messengers (cAMP and cGMP) play a central role in signal transduction and regulation of physiologic responses. Their intracellular levels are controlled by the complex superfamily of cyclic nucleotide phosphodiesterase (PDE) enzymes. Continuing advances in our understanding of the molecular pharmacology of these enzymes has led to the development of selective inhibitors as therapeutic agents for disease states ranging from cancer and heart failure to depression and sexual dysfunction. Several PDE types have been identified as therapeutic targets for immune/inflammatory diseases. This article briefly reviews the available in vitro, preclinical, and clinical data supporting the potential for selective PDE inhibitors as immunomodulatory agents.

Role and identification of protein kinase A anchoring proteins in vasopressin-mediated aquaporin-2 translocation

The antidiuretic hormone arginine vasopressin (AVP) regulates water reabsorption in renal principal cells by inducing a cAMP/protein kinase A-dependent translocation of water channels [aquaporin-2 (AQP2)] from intracellular vesicles into the apical cell membranes. Using primary cultured rat inner medullary collecting duct (IMCD) cells, it has been shown that AQP2 translocation in response to AVP stimulation occurs only if protein kinase A (PKA) is anchored to PKA anchoring proteins (AKAPs), which are present in various subcellular compartments. The identity of the AKAPs involved has not yet been elucidated. One potential candidate is a new splice variant of AKAP18, namely AKAP18 delta.

Domain architecture of a Caenorhabditis elegans AKAP suggests a novel AKAP function

A-kinase anchoring proteins (AKAPs) are adapter proteins that are involved in directing cAMP-dependent protein kinase and some other signaling enzymes to certain intracellular locations. In this study, we investigate the domain architecture of an AKAP from Caenorhabditis elegans (AKAP(CE)). We show that AKAP(CE) shares two domains with the Smad anchor for receptor activation, a FYVE-finger and a transforming growth factor beta (TGFbeta) receptor binding domain, suggesting that AKAP(CE) may interact with a receptor belonging to the TGFbeta receptor family. This predicted novel AKAP function supports the recent view of AKAPs as adapter proteins that can be involved in various signaling pathways.

The transcriptional signature of dioxin in human hepatoma HepG2 cells

We have used a high density microarray hybridization approach to characterize the transcriptional response of human hepatoma HepG2 cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We find that exposure to 10 nM TCDD for 8 hr alters by at least a factor of 2.1 the expression of 310 known genes and of an equivalent number of expressed sequence tags. Treatment with TCDD in the presence of 20 microg/mL of cycloheximide blocked the effect on 202 of these genes, allowing us to distinguish between primary effects of TCDD exposure, which take place whether cycloheximide is present or not, and secondary effects, which are blocked by inhibition of protein synthesis. Of the 310 known genes affected by TCDD, 30 are up-regulated and 78 are down-regulated regardless of cycloheximide treatment, and 84 are up-regulated and 118 are down-regulated only when protein synthesis is not inhibited. Functional clustering of genes regulated by TCDD reveals many potential physiological interactions that might shed light on the multiple biological effects of this compound. Our results, however, suggest that arriving at a sound understanding of the molecular mechanisms governing the biological outcome of TCDD exposure promises to be orders of magnitude more complicated than might have been previously imagined.