Ht31: the first protein kinase A anchoring protein to integrate protein kinase A and Rho signaling

MicroRNAs dysregulated in multiple sclerosis affect the differentiation of CG-4 cells, an oligodendrocyte progenitor cell line

Introduction

Demyelination is one of the hallmarks of multiple sclerosis (MS). While remyelination occurs during the disease, it is incomplete from the start and strongly decreases with its progression, mainly due to the harm to oligodendrocyte progenitor cells (OPCs), causing irreversible neurological deficits and contributing to neurodegeneration. Therapeutic strategies promoting remyelination are still very preliminary and lacking within the current treatment panel for MS.

Methods

In a previous study, we identified 21 microRNAs dysregulated mostly in the CSF of relapsing and/or remitting MS patients. In this study we transfected the mimics/inhibitors of several of these microRNAs separately in an OPC cell line, called CG-4. We aimed (1) to phenotypically characterize their effect on OPC differentiation and (2) to identify corroborating potential mRNA targets via immunocytochemistry, RT-qPCR analysis, RNA sequencing, and Gene Ontology enrichment analysis.

Results

We observed that the majority of 13 transfected microRNA mimics decreased the differentiation of CG-4 cells. We demonstrate, by RNA sequencing and independent RT-qPCR analyses, that miR-33-3p, miR-34c-5p, and miR-124-5p arrest OPC differentiation at a late progenitor stage and miR-145-5p at a premyelinating stage as evidenced by the downregulation of premyelinating oligodendrocyte (OL) [Tcf7l2, Cnp (except for miR-145-5p)] and mature OL (Plp1, Mbp, and Mobp) markers, whereas only miR-214-3p promotes OPC differentiation. We further propose a comprehensive exploration of their change in cell fate through Gene Ontology enrichment analysis. We finally confirm by RT-qPCR analyses the downregulation of several predicted mRNA targets for each microRNA that possibly support their effect on OPC differentiation by very distinctive mechanisms, of which some are still unexplored in OPC/OL physiology.

Conclusion

miR-33-3p, miR-34c-5p, and miR-124-5p arrest OPC differentiation at a late progenitor stage and miR-145-5p at a premyelinating stage, whereas miR-214-3p promotes the differentiation of CG-4 cells. We propose several potential mRNA targets and hypothetical mechanisms by which each microRNA exerts its effect. We hereby open new perspectives in the research on OPC differentiation and the pathophysiology of demyelination/remyelination, and possibly even in the search for new remyelinating therapeutic strategies in the scope of MS.

Disruptors of AKAP-Dependent Protein-Protein Interactions

A-kinase anchoring proteins (AKAPs) are a family of multivalent scaffolding proteins. They engage in direct protein-protein interactions with protein kinases, kinase substrates and further signaling molecules. Each AKAP interacts with a specific set of protein interaction partners and such sets can vary between different cellular compartments and cells. Thus, AKAPs can coordinate signal transduction processes spatially and temporally in defined cellular environments. AKAP-dependent protein-protein interactions are involved in a plethora of physiological processes, including processes in the cardiovascular, nervous, and immune system. Dysregulation of AKAPs and their interactions is associated with or causes widespread diseases, for example, cardiac diseases such as heart failure. However, there are profound shortcomings in understanding functions of specific AKAP-dependent protein-protein interactions. In part, this is due to the lack of agents for specifically targeting defined protein-protein interactions. Peptidic and non-peptidic inhibitors are invaluable molecular tools for elucidating the functions of AKAP-dependent protein-protein interactions. In addition, such interaction disruptors may pave the way to new concepts for the treatment of diseases where AKAP-dependent protein-protein interactions constitute potential drug targets.Here we describe screening approaches for the identification of small molecule disruptors of AKAP-dependent protein-protein interactions. Examples include interactions of AKAP18 and protein kinase A (PKA) and of AKAP-Lbc and RhoA. We discuss a homogenous time-resolved fluorescence (HTRF) and an AlphaScreen^® assay for small molecule library screening and human induced pluripotent stem cell-derived cardiac myocytes (hiPSC-CMs) as a cell system for the characterization of identified hits.

The vasopressin-aquaporin-2 pathway syndromes

Vasopressin is the key hormone involved in water conservation and regulation of water balance, essential for life. In the renal collecting duct, vasopressin binds to the V2 receptor, increasing water permeability through activation of aquaporin-2 redistribution to the luminal membrane. This mechanism promotes rapid water reabsorption, important for immediate survival; however, only recently it has become clear that long-term adverse effects are associated with alterations of the vasopressin-aquaporin-2 pathway, leading to several syndromes associated with water balance disorders. The kidney resistance to the vasopressin action may cause severe dehydration for patients and, conversely, nonosmotic release of vasopressin is associated with water retention and increasing the circulatory blood volume. This chapter discusses the relevance of the altered vasopressin-aquaporin-2 pathway in some diseases associated with water balance disorders, including congenital nephrogenic diabetes insipidus, syndrome of inappropriate secretion of antidiuretic hormone, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The emerging picture suggests that targeting the vasopressin-AQP2 axis can provide therapeutic benefits in those patients.

Local cyclic adenosine monophosphate signalling cascades-Roles and targets in chronic kidney disease

The molecular mechanisms underlying chronic kidney disease (CKD) are poorly understood and treatment options are limited, a situation underpinning the need for elucidating the causative molecular mechanisms and for identifying innovative treatment options. It is emerging that cyclic 3',5'-adenosine monophosphate (cAMP) signalling occurs in defined cellular compartments within nanometre dimensions in processes whose dysregulation is associated with CKD. cAMP compartmentalization is tightly controlled by a specific set of proteins, including A-kinase anchoring proteins (AKAPs) and phosphodiesterases (PDEs). AKAPs such as AKAP18, AKAP220, AKAP-Lbc and STUB1, and PDE4 coordinate arginine-vasopressin (AVP)-induced water reabsorption by collecting duct principal cells. However, hyperactivation of the AVP system is associated with kidney damage and CKD. Podocyte injury involves aberrant AKAP signalling. cAMP signalling in immune cells can be local and slow the progression of inflammatory processes typical for CKD. A major risk factor of CKD is hypertension. cAMP directs the release of the blood pressure regulator, renin, from juxtaglomerular cells, and plays a role in Na⁺ reabsorption through ENaC, NKCC2 and NCC in the kidney. Mutations in the cAMP hydrolysing PDE3A that cause lowering of cAMP lead to hypertension. Another major risk factor of CKD is diabetes mellitus. AKAP18 and AKAP150 and several PDEs are involved in insulin release. Despite the increasing amount of data, an understanding of functions of compartmentalized cAMP signalling with relevance for CKD is fragmentary. Uncovering functions will improve the understanding of physiological processes and identification of disease-relevant aberrations may guide towards new therapeutic concepts for the treatment of CKD.

Drugs That Regulate Local Cell Signaling: AKAP Targeting as a Therapeutic Option

Cells respond to environmental cues by mobilizing signal transduction cascades that engage protein kinases and phosphoprotein phosphatases. Correct organization of these enzymes in space and time enables the efficient and precise transmission of chemical signals. The cyclic AMP-dependent protein kinase A is compartmentalized through its association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of multivalent scaffolds that constrain signaling enzymes and effectors at subcellular locations to drive essential physiological events. More recently, it has been recognized that defective signaling in certain endocrine disorders and cancers proceeds through pathological AKAP complexes. Consequently, pharmacologically targeting these macromolecular complexes unlocks new therapeutic opportunities for a growing number of clinical indications. This review highlights recent findings on AKAP signaling in disease, particularly in certain cancers, and offers an overview of peptides and small molecules that locally regulate AKAP-binding partners.

Cardiac function modulation depends on the A-kinase anchoring protein complex

The A‐kinase anchoring proteins (AKAPs) are a group of structurally diverse proteins identified in various species and tissues. These proteins are able to anchor protein kinase and other signalling proteins to regulate cardiac function. Acting as a scaffold protein, AKAPs ensure specificity in signal transduction by enzymes close to their appropriate effectors and substrates. Over the decades, more than 70 different AKAPs have been discovered. Accumulative evidence indicates that AKAPs play crucial roles in the functional regulation of cardiac diseases, including cardiac hypertrophy, myofibre contractility dysfunction and arrhythmias. By anchoring different partner proteins (PKA, PKC, PKD and LTCCs), AKAPs take part in different regulatory pathways to function as regulators in the heart, and a damaged structure can influence the activities of these complexes. In this review, we highlight recent advances in AKAP‐associated protein complexes, focusing on local signalling events that are perturbed in cardiac diseases and their roles in interacting with ion channels and their regulatory molecules. These new findings suggest that AKAPs might have potential therapeutic value in patients with cardiac diseases, particularly malignant rhythm.

Small molecules for modulating the localisation of the water channel aquaporin-2-disease relevance and perspectives for targeting local cAMP signalling

The tight spatial and temporal organisation of cyclic adenosine monophosphate (cAMP) signalling plays a key role in arginine-vasopressin (AVP)-mediated water reabsorption in renal collecting duct principal cells and in a plethora of other processes such as in the control of cardiac myocyte contractility. This review critically discusses in vitro- and cell-based screening strategies for the identification of small molecules that interfere with AVP/cAMP signalling in renal principal cells; it features phenotypic screening and approaches for targeting protein-protein interactions of A-kinase anchoring proteins (AKAPs), which organise local cAMP signalling hubs. The discovery of novel chemical entities for the modulation of local cAMP will not only provide tools for elucidating molecular mechanisms underlying cAMP signalling. Novel chemical entities can also serve as starting points for the development of novel drugs for the treatment of human diseases. Examples illustrate how screening for small molecules can pave the way to novel approaches for the treatment of certain forms of diabetes insipidus, a disease caused by defects in AVP-mediated water reabsorption.

Roles of A-Kinase Anchoring Proteins and Phosphodiesterases in the Cardiovascular System

A-kinase anchoring proteins (AKAPs) and cyclic nucleotide phosphodiesterases (PDEs) are essential enzymes in the cyclic adenosine 3′-5′ monophosphate (cAMP) signaling cascade. They establish local cAMP pools by controlling the intensity, duration and compartmentalization of cyclic nucleotide-dependent signaling. Various members of the AKAP and PDE families are expressed in the cardiovascular system and direct important processes maintaining homeostatic functioning of the heart and vasculature, e.g., the endothelial barrier function and excitation-contraction coupling. Dysregulation of AKAP and PDE function is associated with pathophysiological conditions in the cardiovascular system including heart failure, hypertension and atherosclerosis. A number of diseases, including autosomal dominant hypertension with brachydactyly (HTNB) and type I long-QT syndrome (LQT1), result from mutations in genes encoding for distinct members of the two classes of enzymes. This review provides an overview over the AKAPs and PDEs relevant for cAMP compartmentalization in the heart and vasculature and discusses their pathophysiological role as well as highlights the potential benefits of targeting these proteins and their protein-protein interactions for the treatment of cardiovascular diseases.

An AKAP-Lbc-RhoA interaction inhibitor promotes the translocation of aquaporin-2 to the plasma membrane of renal collecting duct principal cells

Stimulation of renal collecting duct principal cells with antidiuretic hormone (arginine-vasopressin, AVP) results in inhibition of the small GTPase RhoA and the enrichment of the water channel aquaporin-2 (AQP2) in the plasma membrane. The membrane insertion facilitates water reabsorption from primary urine and fine-tuning of body water homeostasis. Rho guanine nucleotide exchange factors (GEFs) interact with RhoA, catalyze the exchange of GDP for GTP and thereby activate the GTPase. However, GEFs involved in the control of AQP2 in renal principal cells are unknown. The A-kinase anchoring protein, AKAP-Lbc, possesses GEF activity, specifically activates RhoA, and is expressed in primary renal inner medullary collecting duct principal (IMCD) cells. Through screening of 18,431 small molecules and synthesis of a focused library around one of the hits, we identified an inhibitor of the interaction of AKAP-Lbc and RhoA. This molecule, Scaff10-8, bound to RhoA, inhibited the AKAP-Lbc-mediated RhoA activation but did not interfere with RhoA activation through other GEFs or activities of other members of the Rho family of small GTPases, Rac1 and Cdc42. Scaff10-8 promoted the redistribution of AQP2 from intracellular vesicles to the periphery of IMCD cells. Thus, our data demonstrate an involvement of AKAP-Lbc-mediated RhoA activation in the control of AQP2 trafficking.

Suppression of epithelial restitution using an inhibitor against Rho-associated coiled-coil containing protein kinase aggravates colitis through reduced epithelial expression of A-kinase anchor protein 13

In the gastrointestinal tract, the immediate healing response to mucosal damage is critical to sustain mucosal homeostasis. The migration of surrounding epithelial cells to cover the denuded area without proliferation is termed restitution, followed by early reparation of the damage. In this study, we determined the role of A-kinase anchor protein 13 (AKAP13) in mice with dextran sulphate sodium (DSS)-induced colitis upon mucosal injury and restitution, and investigated whether inhibition of Rho-associated coiled-coil containing protein kinase (ROCK), downstream effector of AKAP13, affects these mucosal responses. BALB/c mice were challenged with 4% or 2% DSS in their drinking water for up to 8 or 16days, respectively. During this period, mice received subcutaneous injections of fasudil hydrochloride hydrate (FH, 10mg/kg, twice per day), an inhibitor of phosphorylation of ROCK. In immunohistochemistry, AKAP13 was highly expressed in the mucosal epithelium prior to DSS-induced mucosal injury, and also expressed in ulcer-covering non-proliferative epithelium, which corresponded to restituted epithelial cells. Coadministration of FH increased serum amyloid A levels and histopathological scores for mucosal injury, as compared with the DSS group. The effects were associated with a decrease in gene expression of Akap13 in the mucosal tissue and the inhibition of restitution rata (the length of restituted epithelial cells per ulcer). These results suggested that AKAP13 and ROCK are involved in mucosal response at early injury and restitution during healing in DSS-induced colitis in mice.

AKAP220 manages apical actin networks that coordinate aquaporin-2 location and renal water reabsorption

Filtration through the kidney eliminates toxins, manages electrolyte balance, and controls water homeostasis. Reabsorption of water from the luminal fluid of the nephron occurs through aquaporin-2 (AQP2) water pores in principal cells that line the kidney-collecting duct. This vital process is impeded by formation of an "actin barrier" that obstructs the passive transit of AQP2 to the plasma membrane. Bidirectional control of AQP2 trafficking is managed by hormones and signaling enzymes. We have discovered that vasopressin-independent facets of this homeostatic mechanism are under the control of A-Kinase Anchoring Protein 220 (AKAP220; product of the Akap11 gene). CRISPR/Cas9 gene editing and imaging approaches show that loss of AKAP220 disrupts apical actin networks in organoid cultures. Similar defects are evident in tissue sections from AKAP220-KO mice. Biochemical analysis of AKAP220-null kidney extracts detected reduced levels of active RhoA GTPase, a well-known modulator of the actin cytoskeleton. Fluorescent imaging of kidney sections from these genetically modified mice revealed that RhoA and AQP2 accumulate at the apical surface of the collecting duct. Consequently, these animals are unable to appropriately dilute urine in response to overhydration. We propose that membrane-proximal signaling complexes constrained by AKAP220 impact the actin barrier dynamics and AQP2 trafficking to ensure water homeostasis.

Structure of smAKAP and its regulation by PKA-mediated phosphorylation

The A-kinase anchoring protein (AKAP) smAKAP has three extraordinary features; it is very small, it is anchored directly to membranes by acyl motifs, and it interacts almost exclusively with the type I regulatory subunits (RI) of cAMP-dependent kinase (PKA). Here, we determined the crystal structure of smAKAP's A-kinase binding domain (smAKAP-AKB) in complex with the dimerization/docking (D/D) domain of RIα which reveals an extended hydrophobic interface with unique interaction pockets that drive smAKAP's high specificity for RI subunits. We also identify a conserved PKA phosphorylation site at Ser66 in the AKB domain which we predict would cause steric clashes and disrupt binding. This correlates with in vivo colocalization and fluorescence polarization studies, where Ser66 AKB phosphorylation ablates RI binding. Hydrogen/deuterium exchange studies confirm that the AKB helix is accessible and dynamic. Furthermore, full-length smAKAP as well as the unbound AKB is predicted to contain a break at the phosphorylation site, and circular dichroism measurements confirm that the AKB domain loses its helicity following phosphorylation. As the active site of PKA's catalytic subunit does not accommodate α-helices, we predict that the inherent flexibility of the AKB domain enables its phosphorylation by PKA. This represents a novel mechanism, whereby activation of anchored PKA can terminate its binding to smAKAP affecting the regulation of localized cAMP signaling events.

DATABASE

Structural data are available in the PDB under accession number 5HVZ.

Emerging roles of A-kinase anchoring proteins in cardiovascular pathophysiology

Heart and blood vessels ensure adequate perfusion of peripheral organs with blood and nutrients. Alteration of the homeostatic functions of the cardiovascular system can cause hypertension, atherosclerosis, and coronary artery disease leading to heart injury and failure. A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that are crucially involved in modulating the function of the cardiovascular system both under physiological and pathological conditions. AKAPs assemble multifunctional signaling complexes that ensure correct targeting of the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to precise subcellular compartments. This allows local regulation of specific effector proteins that control the function of vascular and cardiac cells. This review will focus on recent advances illustrating the role of AKAPs in cardiovascular pathophysiology. The accent will be mainly placed on the molecular events linked to the control of vascular integrity and blood pressure as well as on the cardiac remodeling process associated with heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Pharmacological targeting of AKAP-directed compartmentalized cAMP signalling

The second messenger cyclic adenosine monophosphate (cAMP) can bind and activate protein kinase A (PKA). The cAMP/PKA system is ubiquitous and involved in a wide array of biological processes and therefore requires tight spatial and temporal regulation. Important components of the safeguard system are the A-kinase anchoring proteins (AKAPs), a heterogeneous family of scaffolding proteins defined by its ability to directly bind PKA. AKAPs tether PKA to specific subcellular compartments, and they bind further interaction partners to create local signalling hubs. The recent discovery of new AKAPs and advances in the field that shed light on the relevance of these hubs for human disease highlight unique opportunities for pharmacological modulation. This review exemplifies how interference with signalling, particularly cAMP signalling, at such hubs can reshape signalling responses and discusses how this could lead to novel pharmacological concepts for the treatment of disease with an unmet medical need such as cardiovascular disease and cancer.

The alpha1-adrenergic receptors in cardiac hypertrophy: signaling mechanisms and functional implications

Cardiac hypertrophy is a complex remodeling process of the heart induced by physiological or pathological stimuli resulting in increased cardiomyocyte size and myocardial mass. Whereas cardiac hypertrophy can be an adaptive mechanism to stressful conditions of the heart, prolonged hypertrophy can lead to heart failure which represents the primary cause of human morbidity and mortality. Among G protein-coupled receptors, the α1-adrenergic receptors (α1-ARs) play an important role in the development of cardiac hypertrophy as demonstrated by numerous studies in the past decades, both in primary cardiomyocyte cultures and genetically modified mice. The results of these studies have provided evidence of a large variety of α1-AR-induced signaling events contributing to the defining molecular and cellular features of cardiac hypertrophy. Recently, novel signaling mechanisms have been identified and new hypotheses have emerged concerning the functional role of the α1-adrenergic receptors in the heart. This review will summarize the main signaling pathways activated by the α1-AR in the heart and their functional implications in cardiac hypertrophy.

The crystal structure of the RhoA-AKAP-Lbc DH-PH domain complex

The RhoGEF (Rho GTPase guanine-nucleotide-exchange factor) domain of AKAP-Lbc (A-kinase-anchoring protein-Lbc, also known as AKAP13) catalyses nucleotide exchange on RhoA and is involved in the development of cardiac hypertrophy. The RhoGEF activity of AKAP-Lbc has also been implicated in cancer. We have determined the X-ray crystal structure of the complex between RhoA–GDP and the AKAP-Lbc RhoGEF [DH (Dbl-homologous)–PH (pleckstrin homology)] domain to 2.1 Å (1 Å=0.1 nm) resolution. The structure reveals important differences compared with related RhoGEF proteins such as leukaemia-associated RhoGEF. Nucleotide-exchange assays comparing the activity of the DH–PH domain to the DH domain alone showed no role for the PH domain in nucleotide exchange, which is explained by the RhoA–AKAP-Lbc structure. Comparison with a structure of the isolated AKAP-Lbc DH domain revealed a change in conformation of the N-terminal ‘GEF switch’ region upon binding to RhoA. Isothermal titration calorimetry showed that AKAP-Lbc has only micromolar affinity for RhoA, which combined with the presence of potential binding pockets for small molecules on AKAP-Lbc, raises the possibility of targeting AKAP-Lbc with GEF inhibitors.

The crystal structure of the RhoGEF domain of AKAP-Lbc in complex with RhoA combined with nucleotide exchange assays explain differences to related RhoGEF proteins and allow the possibility of targeting the AKAP-Lbc RhoGEF domain with small molecules.

A presynaptic role for PKA in synaptic tagging and memory

Protein kinase A (PKA) and other signaling molecules are spatially restricted within neurons by A-kinase anchoring proteins (AKAPs). Although studies on compartmentalized PKA signaling have focused on postsynaptic mechanisms, presynaptically anchored PKA may contribute to synaptic plasticity and memory because PKA also regulates presynaptic transmitter release. Here, we examine this issue using genetic and pharmacological application of Ht31, a PKA anchoring disrupting peptide. At the hippocampal Schaffer collateral CA3-CA1 synapse, Ht31 treatment elicits a rapid decay of synaptic responses to repetitive stimuli, indicating a fast depletion of the readily releasable pool of synaptic vesicles. The interaction between PKA and proteins involved in producing this pool of synaptic vesicles is supported by biochemical assays showing that synaptic vesicle protein 2 (SV2), Rim1, and SNAP25 are components of a complex that interacts with cAMP. Moreover, acute treatment with Ht31 reduces the levels of SV2. Finally, experiments with transgenic mouse lines, which express Ht31 in excitatory neurons at the Schaffer collateral CA3-CA1 synapse, highlight a requirement for presynaptically anchored PKA in pathway-specific synaptic tagging and long-term contextual fear memory. These results suggest that a presynaptically compartmentalized PKA is critical for synaptic plasticity and memory by regulating the readily releasable pool of synaptic vesicles.

AKAP13 Rho-GEF and PKD-binding domain deficient mice develop normally but have an abnormal response to β-adrenergic-induced cardiac hypertrophy

BACKGROUND

A-kinase anchoring proteins (AKAPs) are scaffolding molecules that coordinate and integrate G-protein signaling events to regulate development, physiology, and disease. One family member, AKAP13, encodes for multiple protein isoforms that contain binding sites for protein kinase A (PKA) and D (PKD) and an active Rho-guanine nucleotide exchange factor (Rho-GEF) domain. In mice, AKAP13 is required for development as null embryos die by embryonic day 10.5 with cardiovascular phenotypes. Additionally, the AKAP13 Rho-GEF and PKD-binding domains mediate cardiomyocyte hypertrophy in cell culture. However, the requirements for the Rho-GEF and PKD-binding domains during development and cardiac hypertrophy are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To determine if these AKAP13 protein domains are required for development, we used gene-trap events to create mutant mice that lacked the Rho-GEF and/or the protein kinase D-binding domains. Surprisingly, heterozygous matings produced mutant mice at Mendelian ratios that had normal viability and fertility. The adult mutant mice also had normal cardiac structure and electrocardiograms. To determine the role of these domains during β-adrenergic-induced cardiac hypertrophy, we stressed the mice with isoproterenol. We found that heart size was increased similarly in mice lacking the Rho-GEF and PKD-binding domains and wild-type controls. However, the mutant hearts had abnormal cardiac contractility as measured by fractional shortening and ejection fraction.

CONCLUSIONS

These results indicate that the Rho-GEF and PKD-binding domains of AKAP13 are not required for mouse development, normal cardiac architecture, or β-adrenergic-induced cardiac hypertrophic remodeling. However, these domains regulate aspects of β-adrenergic-induced cardiac hypertrophy.

cAMP-stimulated phosphorylation of diaphanous 1 regulates protein stability and interaction with binding partners in adrenocortical cells

DIAPH1, the RhoA effector protein, forms a complex in adrenocortical cells and is phosphorylated by a cAMP/PKA-dependent pathway. Phosphorylation differentially modulates protein–protein interactions, regulates the stability of the protein, and facilitates sumoylation.

Diaphanous homologue 1 (DIAPH1) is a Rho effector protein that coordinates cellular dynamics by regulating microfilament and microtubule function. We previously showed that DIAPH1 plays an integral role in regulating the production of cortisol by controlling the rate of mitochondrial movement, by which activation of the adrenocorticotropin (ACTH)/cAMP signaling pathway stimulates mitochondrial trafficking and promotes the interaction between RhoA and DIAPH1. In the present study we use mass spectrometry to identify DIAPH1 binding partners and find that DIAPH1 interacts with several proteins, including RhoA, dynamin-1, kinesin, β-tubulin, β-actin, oxysterol-binding protein (OSBP)–related protein 2 (ORP2), and ORP10. Moreover, DIAPH1 is phosphorylated in response to dibutyryl cAMP (Bt₂cAMP) at Thr-759 via a pathway that requires extracellular signal-related kinase (ERK). Alanine substitution of Thr-759 renders DIAPH1 more stable and attenuates the interaction between DIAPH1 and kinesin, ORP2, and actin but has no effect on the ability of the protein to interact with RhoA or β-tubulin. Finally, overexpression of a DIAPH1 T759A mutant significantly decreases the rate of Bt₂cAMP-stimulated mitochondrial movement. Taken together, our findings establish a key role for phosphorylation in regulating the stability and function of DIAPH1.

PKA and PDE4D3 anchoring to AKAP9 provides distinct regulation of cAMP signals at the centrosome

Control of cell cycle progression relies on unique regulation of centrosomal cAMP/PKA signals through PKA and PDE4D3 interaction with the A kinase anchoring protein AKAP9.

Previous work has shown that the protein kinase A (PKA)–regulated phosphodiesterase (PDE) 4D3 binds to A kinase–anchoring proteins (AKAPs). One such protein, AKAP9, localizes to the centrosome. In this paper, we investigate whether a PKA–PDE4D3–AKAP9 complex can generate spatial compartmentalization of cyclic adenosine monophosphate (cAMP) signaling at the centrosome. Real-time imaging of fluorescence resonance energy transfer reporters shows that centrosomal PDE4D3 modulated a dynamic microdomain within which cAMP concentration selectively changed over the cell cycle. AKAP9-anchored, centrosomal PKA showed a reduced activation threshold as a consequence of increased autophosphorylation of its regulatory subunit at S114. Finally, disruption of the centrosomal cAMP microdomain by local displacement of PDE4D3 impaired cell cycle progression as a result of accumulation of cells in prophase. Our findings describe a novel mechanism of PKA activity regulation that relies on binding to AKAPs and consequent modulation of the enzyme activation threshold rather than on overall changes in cAMP levels. Further, we provide for the first time direct evidence that control of cell cycle progression relies on unique regulation of centrosomal cAMP/PKA signals.

A-kinase anchoring proteins: scaffolding proteins in the heart

The pleiotropic cyclic nucleotide cAMP is the primary second messenger responsible for autonomic regulation of cardiac inotropy, chronotropy, and lusitropy. Under conditions of prolonged catecholaminergic stimulation, cAMP also contributes to the induction of both cardiac myocyte hypertrophy and apoptosis. The formation of localized, multiprotein complexes that contain different combinations of cAMP effectors and regulatory enzymes provides the architectural infrastructure for the specialization of the cAMP signaling network. Scaffolds that bind protein kinase A are called "A-kinase anchoring proteins" (AKAPs). In this review, we discuss recent advances in our understanding of how PKA is compartmentalized within the cardiac myocyte by AKAPs and how AKAP complexes modulate cardiac function in both health and disease.

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

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

The Lbc Rho guanine nucleotide exchange factor α-catulin axis functions in serotonin-induced vascular smooth muscle cell mitogenesis and RhoA/ROCK activation

Serotonin (5-hydroxytryptamine, 5-HT) is mitogenic for several cell types including pulmonary arterial smooth muscle cells (PASMC), and is associated with the abnormal vascular smooth muscle remodeling that occurs in pulmonary arterial hypertension. RhoA/Rho kinase (ROCK) function is required for 5-HT-induced PASMC mitogenesis, and 5-HT activates RhoA; however, the signaling steps are poorly defined. Rho guanine nucleotide exchange factors (Rho GEFs) transduce extracellular signals to Rho, and we found that 5-HT treatment of PASMC led to increased membrane-associated Lbc Rho GEF, suggesting modulation by 5-HT. Lbc knockdown by siRNA attenuated 5-HT-induced thymidine uptake in PASMC, indicating a role in PASMC mitogenesis. 5-HT triggered Rho-dependent serum response factor-mediated reporter activation in PASMC, and this was reduced by Lbc depletion. Lbc knockdown reduced 5-HT-induced RhoA/ROCK activation, but not p42/44 ERK MAP kinase activation, suggesting that Lbc is an intermediary between 5-HT and RhoA/ROCK, but not ERK. 5-HT stimulation of PASMC led to increased association between Lbc, RhoA, and the α-catulin scaffold. Furthermore, α-catulin knockdown attenuated 5-HT-induced PASMC thymidine uptake. 5-HT-induced PASMC mitogenesis was reduced by dominant-negative G(q) protein, suggesting cooperation with Lbc/α-catulin. These results for the first time define a Rho GEF involved in vascular smooth muscle cell growth and serotonin signaling, and suggest that Lbc Rho GEF family members play distinct roles. Thus, the Lbc/α-catulin axis participates in 5-HT-induced PASMC mitogenesis and RhoA/ROCK signaling, and may be an interventional target in diseases involving vascular smooth muscle remodeling.

Networking with AKAPs: context-dependent regulation of anchored enzymes

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

Molecular modeling of BAD complex resided in a mitochondrion integrating glycolysis and apoptosis

BAD (Bcl-2 antagonist of cell death) and GK (glucokinase) reside in a mitochondrial complex together with PKA and PP1 catalytic units (PKAc and PP1c) and WAVE-1 that integrates glycolysis and apoptosis. Our research results reveal that BAD is phosphorylated and inactivated on Ser 75 in a BAD-Bcl-xL complex by PKA (targeted to mitochondria through association with WAVE1), resulting in the dissociation of BAD and its binding to GK. Moreover, GK can interact with PP1c and also distinguish WAVE1. On the other hand, BAD is dephosphorylated and activated on Ser75 by PP1c, leading to the separation of PKAc and its binding to the regulatory (R) subunit of PKA which by the dimerization domain of its R subunit connects with WAVE1 linked with GK of the complex. This may be the reason of the complex existing in liver mitochondria, regardless of phosphorylated and dephosphorylated BAD. Additionally, GK like PKA may also prevent Bcl-xL from rebinding to BAD by phosphorylating BAD at Ser 118. The BAD complex model reveals that BAD and GK play key roles because of BAD as a substrate for the PKA-PP1 pair and by BH3 domain directly interacting with GK. This is helpful for our development and research of the molecular mechanism of BAD integrating glycolysis and apoptosis.

Mechanisms of protein kinase A anchoring

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

AKAP-scaffolding proteins and regulation of cardiac physiology

A kinase anchoring proteins (AKAPs) compose a growing list of diverse but functionally related proteins defined by their ability to bind to the regulatory subunit of protein kinase A. AKAPs perform an integral role in the spatiotemporal modulation of a multitude of cellular signaling pathways. This review highlights the extensive role of AKAPs in cardiac excitation/contraction coupling and cardiac physiology. The literature shows that particular AKAPs are involved in cardiac Ca(2+) influx, release, reuptake, and myocyte repolarization. Studies have also suggested roles for AKAPs in cardiac remodeling. Transgenic studies show functional effects of AKAPs, not only in the cardiovascular system but in other organ systems as well.

Phosphorylation events and the modulation of aquaporin 2 cell surface expression

PURPOSE OF REVIEW

This review highlights the role of phosphorylation in the trafficking and targeting of aquaporin 2. Current knowledge will be put into the context of modulating the cell surface expression of aquaporin 2 by vasopressin in renal epithelial cells, which is critical for regulation of urinary concentration and control of fluid and electrolyte homeostasis.

RECENT FINDINGS

In addition to previously identified phosphorylation sites on aquaporin 2, new data have revealed three other serine residues in the C-terminus whose phosphorylation is altered by vasopressin. Several steps in aquaporin 2 recycling, including exocytosis and endocytosis, are coordinated by phosphorylation and dephosphorylation to regulate cell surface accumulation. Aquaporin 2 phosphorylation on serine 256 regulates aquaporin 2 association with proteins that are involved in trafficking, including hsc/hsp70 and myelin and lymphocyte-associated protein.

SUMMARY

Aquaporin 2 trafficking is regulated by phosphorylation of serine 256 and other amino acid residues in its cytoplasmic domain. These events increase or decrease interaction of aquaporin 2 with key regulatory proteins to determine the cellular distribution and fate of aquaporin 2, both after vasopressin addition and under baseline conditions. Better understanding of these mechanisms may provide new therapeutic avenues for patients with X-linked nephrogenic diabetes insipidus, as well as providing basic cell biological information relevant to membrane trafficking processes in general.

A multi-angular mass spectrometric view at cyclic nucleotide dependent protein kinases: in vivo characterization and structure/function relationships

Mass spectrometry has evolved in recent years to a well-accepted and increasingly important complementary technique in molecular and structural biology. Here we review the many contributions mass spectrometry based studies have made in recent years in our understanding of the important cyclic nucleotide activated protein kinase A (PKA) and protein kinase G (PKG). We both describe the characterization of kinase isozymes, substrate phosphorylation, binding partners and post-translational modifications by proteomics based methodologies as well as their structural and functional properties as revealed by native mass spectrometry, H/D exchange MS and ion mobility. Combining all these mass spectrometry based data with other biophysical and biochemical data has been of great help to unravel the intricate regulation of kinase function in the cell in all its magnificent complexity.

Genetic disruption of protein kinase A anchoring reveals a role for compartmentalized kinase signaling in theta-burst long-term potentiation and spatial memory

Studies of hippocampal long-term potentiation (LTP), a cellular model of memory storage, implicate cAMP-dependent protein kinase (PKA) in presynaptic and postsynaptic mechanisms of LTP. The anchoring of PKA to AKAPs (A kinase-anchoring proteins) creates compartmentalized pools of PKA, but the roles of presynaptically and postsynaptically anchored forms of PKA in late-phase LTP are unclear. In this study, we have created genetically modified mice that conditionally express Ht31, an inhibitor of PKA anchoring, to probe the roles of anchored PKA in hippocampal LTP and spatial memory. Our findings show that at hippocampal Schaffer collateral CA3-CA1 synapses, theta-burst LTP requires presynaptically anchored PKA. In addition, a pool of anchored PKA in hippocampal area CA3 is required for spatial memory. These findings reveal a novel and significant role for anchored PKA signaling in cellular mechanisms underlying memory storage.

Identification and characterization of RHOA-interacting proteins in bovine spermatozoa

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

Regulation of g protein-coupled receptor signaling by a-kinase anchoring proteins

Specificity of transduction events is controlled at the molecular level by scaffold, anchoring, and adaptor proteins, which position signaling enzymes at proper subcellular localization. This allows their efficient catalytic activation and accurate substrate selection. A-kinase anchoring proteins (AKAPs) are group of functionally related proteins that compartmentalize the cAMP-dependent protein kinase (PKA) and other signaling enyzmes at precise subcellular sites in close proximity to their physiological substrate(s) and favor specific phosphorylation events. Recent evidence suggests that AKAP transduction complexes play a key role in regulating G protein-coupled receptor (GPCR) signaling. Regulation can occur at multiple levels because AKAPs have been shown both to directly modulate GPCR function and to act as downstream effectors of GPCR signaling. In this minireview, we focus on the molecular mechanisms through which AKAP-signaling complexes modulate GPCR transduction cascades.

High-affinity AKAP7delta-protein kinase A interaction yields novel protein kinase A-anchoring disruptor peptides

PKA (protein kinase A) is tethered to subcellular compartments by direct interaction of its regulatory subunits (RI or RII) with AKAPs (A kinase-anchoring proteins). AKAPs preferentially bind RII subunits via their RII-binding domains. RII-binding domains form structurally conserved amphipathic helices with unrelated sequences. Their binding affinities for RII subunits differ greatly within the AKAP family. Amongst the AKAPs that bind RIIalpha subunits with high affinity is AKAP7delta [AKAP18delta; K(d) (equilibrium dissociation constant) value of 31 nM]. An N-terminally truncated AKAP7delta mutant binds RIIalpha subunits with higher affinity than the full-length protein presumably due to loss of an inhibitory region [Henn, Edemir, Stefan, Wiesner, Lorenz, Theilig, Schmidtt, Vossebein, Tamma, Beyermann et al. (2004) J. Biol. Chem. 279, 26654-26665]. In the present study, we demonstrate that peptides (25 amino acid residues) derived from the RII-binding domain of AKAP7delta bind RIIalpha subunits with higher affinity (K(d)=0.4+/-0.3 nM) than either full-length or N-terminally truncated AKAP7delta, or peptides derived from other RII binding domains. The AKAP7delta-derived peptides and stearate-coupled membrane-permeable mutants effectively disrupt AKAP-RII subunit interactions in vitro and in cell-based assays. Thus they are valuable novel tools for studying anchored PKA signalling. Molecular modelling indicated that the high affinity binding of the amphipathic helix, which forms the RII-binding domain of AKAP7delta, with RII subunits involves both the hydrophobic and the hydrophilic faces of the helix. Alanine scanning (25 amino acid peptides, SPOT technology, combined with RII overlay assays) of the RII binding domain revealed that hydrophobic amino acid residues form the backbone of the interaction and that hydrogen bond- and salt-bridge-forming amino acid residues increase the affinity of the interaction.

The where's and when's of kinase anchoring

Kinase anchoring has gained acceptance as a means to synchronize spatial and temporal aspects of cell signaling. A-kinase anchoring proteins (AKAPs) are a diverse group of functionally related proteins that target protein kinase A and other enzymes to coordinate a range of signaling events. Recent advances in this field have shown that incorporating phosphodiesterases into AKAP signaling complexes exerts local control of cAMP metabolism, that phosphorylation of some AKAPs potentiates downstream signaling events, that anchoring of distinct enzyme combinations functions as a mechanism to expand the repertoire of cellular events controlled by a single AKAP, and that fluorescent biosensors can be used to visualize dynamic aspects of localized cAMP signaling.

Estrogen effects on the expression of Brx in the brain and pituitary of the mouse

A member of the Dbl family of oncoproteins was discovered in breast cancer tissue extracts. This novel protein, designated Brx, contains an estrogen-receptor binding motif and is highly expressed in hormone-responsive breast tissue. Due to its ability to augment ligand-dependent activation of estrogen receptors, we analyzed the expression of Brx in the adult mouse brain and pituitary. Results indicated that Brx was expressed in specific regions of the brain and pituitary. Furthermore, the results indicate that differences exist in both brain and pituitary tissue of male and female mice with greater expression in the female. However, estrogen did not influence Brx expression in ovariectomized mice. The anatomical studies support a role for Brx in its association with the estrogen receptor and that Brx may be involved in neuronal and pituitary function in a sexually dimorphic manner.

AKAP-Lbc: a molecular scaffold for the integration of cyclic AMP and Rho transduction pathways

A Kinase-anchoring proteins (AKAPs) are a family of functionally related proteins involved in the targeting of the PKA holoenzyme towards specific physiological substrates. We have recently identified a novel anchoring protein expressed in cardiomyocytes, called AKAP-Lbc, that functions as a PKA-targeting protein as well as a guanine nucleotide exchange factor (GEF) that activates the GTPase RhoA. Here, we discuss the most recent findings elucidating the molecular mechanisms and the transduction pathways involved in the regulation of the AKAP-Lbc signaling complex inside cells. We could show that AKAP-Lbc is regulated in a bi-directional manner by signals that activate or deactivate its Rho-GEF activity. Activation of AKAP-Lbc occurs in response to agonists that stimulate G proteins coupled receptors linked to the heterotrimeric G protein G12, whereas inactivation occurs through mechanisms that require phosphorylation of AKAP-Lbc by anchored PKA and subsequent recruitment of the regulatory protein 14-3-3. Interestingly, we could demonstrate that AKAP-Lbc can form homo-oligomers inside cells and that 14-3-3 can inhibit the Rho-GEF activity of AKAP-Lbc only when the anchoring protein adopts an oligomeric conformation. These findings reveal the molecular architecture of the AKAP-Lbc transduction complex and provide a mechanistic explanation of how upstream signaling pathways can be integrated within the AKAP-Lbc transduction complex to precisely modulate the activation of Rho.

Expression of the proto-oncoprotein breast cancer nuclear receptor auxiliary factor (Brx) is altered in eutopic endometrium of women with endometriosis

OBJECTIVE

To evaluate the expression of estrogen receptor alpha (ERalpha), estrogen receptor beta (ERbeta), and breast cancer nuclear receptor auxiliary factor (Brx) in eutopic endometrium of normal women and women with endometriosis.

DESIGN

Prospective observational study.

SETTING

Tertiary care and research center.

PATIENT(S)

Twenty-nine women with endometriosis and 35 healthy ovulatory volunteers of similar ages.

INTERVENTION(S)

Endometrial biopsy.

MAIN OUTCOME MEASURE(S)

Expression of immunohistochemical staining intensity and localization of ERalpha, ERbeta, and Brx proteins in eutopic endometrium during the menstrual cycle.

RESULT(S)

Expression of ERalpha and ERbeta was highest in the proliferative phase and was similar in both groups. Brx expression differed between healthy volunteers and those with endometriosis. During the proliferative phase, immunostaining intensity of Brx was greater in both the glandular and the stromal compartments of biopsies from patients with endometriosis compared to healthy volunteers; nuclear stromal Brx staining was more common in patients with endometriosis.

CONCLUSION(S)

The spatiotemporal expression of Brx was altered in eutopic endometrium of women with endometriosis. These findings suggest a fundamental alteration in the endometrium of women who have endometriosis. The role of Brx in ectopic implantation of endometrium deserves further study.

Minireview: aquaporin 2 trafficking

In the kidney aquaporin-2 (AQP2) provides a target for hormonal regulation of water transport by vasopressin. Short-term control of water permeability occurs via vesicular trafficking of AQP2 and long-term control through changes in the abundance of AQP2 and AQP3 water channels. Defective AQP2 trafficking causes nephrogenic diabetes insipidus, a condition characterized by the kidney inability to produce concentrated urine because of the insensitivity of the distal nephron to vasopressin. AQP2 is redistributed to the apical membrane of collecting duct cells through activation of a cAMP signaling cascade initiated by the binding of vasopressin to its V2-receptor. Protein kinase A-mediated phosphorylation of AQP2 has been proposed to be essential in regulating AQP2-containing vesicle exocytosis. Cessation of the stimulus is followed by endocytosis of the AQP2 proteins exposed on the plasma membrane and their recycling to the original stores, in which they are retained. Soluble N-ethylmaleimide sensitive fusion factor attachment protein receptors (SNARE) and actin cytoskeleton organization regulated by small GTPase of the Rho family were also proved to be essential for AQP2 trafficking. Data for functional involvement of the SNARE vesicle-associated membrane protein 2 in AQP2 targeting has recently been provided. Changes in AQP2 expression/trafficking are of particular importance in pathological conditions characterized by both dilutional and concentrating defects. One of these conditions, hypercalciuria, has shown to be associated with alteration of AQP2 urinary excretion. More precisely, recent data support the hypothesis that, in vivo external calcium, through activation of calcium-sensing receptors, modulates the expression/trafficking of AQP2. Together these findings underscore the importance of AQP2 in kidney pathophysiology.

Compartmentalized cAMP signalling regulates vasopressin-mediated water reabsorption by controlling aquaporin-2

The cAMP/PKA (protein kinase A) signalling pathway is activated by a plethora of stimuli. To facilitate the specificity of a cellular response, signal transduction complexes are formed and segregated to discrete sites (compartmentalization). cAMP/PKA signalling compartments are maintained by AKAPs (A-kinase anchoring proteins) which bind PKA and other signalling proteins, and by PDEs (phosphodiesterases). The latter hydrolyse cAMP and thus limit its diffusion and terminate PKA activity. An example of a cAMP-dependent process requiring compartmentalization of cAMP/PKA signals is arginine-vasopressin-regulated water reabsorption in renal principal cells. A detailed understanding of the protein interactions within a signal transduction complex offers the possibility to design agents influencing PKA binding to a specific AKAP, the targeting of an AKAP or the interactions of AKAPs with other signalling molecules. The ability to specifically modulate selected branches of a signal transduction pathway would greatly advance basic research, and may lead to new drugs suitable for the treatment of diseases caused by dysregulation of anchored PKA signalling (e.g. renal and cardiovascular diseases).

RNA silencing identifies PDE4D5 as the functionally relevant cAMP phosphodiesterase interacting with beta arrestin to control the protein kinase A/AKAP79-mediated switching of the beta2-adrenergic receptor to activation of ERK in HEK293B2 cells

PDE4B and PDE4D provide >90% of PDE4 cAMP phosphodiesterase activity in human embryonic kidney (HEK293B2) cells. Their selective small interference RNA (siRNA)-mediated knockdown potentiates isoprenaline-stimulated protein kinase A (PKA) activation. Whereas endogenous PDE4D co-immunoprecipitates with beta arrestin, endogenous PDE4B does not, even upon PDE4D knockdown. Ectopic overexpression of PDE4B2 confers co-immunoprecipitation with beta arrestin. Knockdown of PDE4D, but not PDE4B, amplifies isoprenaline-stimulated phosphorylation of the beta2-adrenergic receptor (beta2-AR) by PKA and activation of extracellular signal-regulated kinase (ERK) through G(i). Isoform-selective knockdown identifies PDE4D5 as the functionally important species regulating isoprenaline stimulation of both these processes. Ht31-mediated disruption of the tethering of PKA to AKAP scaffold proteins attenuates isoprenaline activation of ERK, even upon PDE4D knockdown. Selective siRNA-mediated knockdown identifies AKAP79, which is constitutively associated with the beta2-AR, rather than isoprenaline-recruited gravin, as being the functionally relevant AKAP in this process. Isoprenaline-stimulated membrane recruitment of PDE4D is ablated upon beta arrestin knockdown. A mutation that compromises interactions with beta arrestin prevents catalytically inactive PDE4D5 from performing a dominant negative role in potentiating isoprenaline-stimulated ERK activation. Beta arrestin-recruited PDE4D5 desensitizes isoprenaline-stimulated PKA phosphorylation of the beta2-AR and the consequential switching of its signaling to ERK. The ability to observe a cellular phenotype upon PDE4D5 knockdown demonstrates that other PDE4 isoforms, expressed at endogenous levels, are unable to afford rescue in HEK293B2 cells.

Impaired agonist-dependent myosin phosphorylation and decreased RhoA in rat portal hypertensive mesenteric vasculature

The purpose of the present study was to examine the effects of portal hypertension on agonist-induced myosin phosphorylation and RhoA expression in vascular smooth muscle. A possible link to cAMP-dependent events was also examined. Portal hypertension was produced by stenosis of the portal vein. Vessel segments were treated with or without 50 microM of the PKA inhibitor Rp-cAMPS for 30 min and subsequently stimulated with 10(-4) M phenylephrine. Myosin regulatory light-chain phosphorylation was detected by immunoblotting. Total RNA from first-order mesenteric arteries and portal veins was isolated and amplified by RT-PCR using RhoA and GAPDH primers. RhoA protein expression was also measured in first-order mesenteric arteries using Western blot analysis. Myosin phosphorylation in maximally stimulated first-order mesenteric arteries was significantly lower in portal hypertensive animals (19.9 +/- 2.86%) when compared with sham-operated control (43.8 +/- 3.53%). Inhibition of PKA selectively increased myosin phosphorylation to 34.7 +/- 4.18%. Rp-cAMPS did not affect the phosphorylation of the portal veins or superior mesenteric arteries. RhoA mRNA and membrane-associated RhoA protein expression in portal hypertensive first-order mesenteric arteries were significantly lower when compared with controls. Acute inhibition of PKA had no effect on RhoA mRNA expression. However, it restored membrane-associated RhoA protein expression in portal hypertensive vessels to control levels. The results suggest that reductions in membrane-associated RhoA expression, which appear to be regulated by cAMP-dependent events, lead to reduced myosin phosphorylation and may underlie the reduced vasoconstrictor effectiveness in the resistance vasculature of portal hypertensive intestine.

Regulation of actin-based cell migration by cAMP/PKA

A wide variety of soluble signaling substances utilize the cyclic AMP-dependent protein kinase (PKA) pathway to regulate cellular behaviors including intermediary metabolism, ion channel conductivity, and transcription. A growing literature suggests that integrin-mediated cell adhesion may also utilize PKA to modulate adhesion-associated events such as actin cytoskeletal dynamics and migration. PKA is dynamically regulated by integrin-mediated cell adhesion to extracellular matrix (ECM). Furthermore, while some hallmarks of cell migration and cytoskeletal organization require PKA activity (e.g. activation of Rac and Cdc42; actin filament assembly), others are inhibited by it (e.g. activation of Rho and PAK; interaction of VASP with the c-Abl tyrosine kinase). Also, cell migration and invasion can be impeded by either inhibition or hyper-activation of PKA. Finally, a number of A-kinase anchoring proteins (AKAPs) serve to associate PKA with various components of the actin cytoskeleton, thereby enhancing and/or specifying cAMP/PKA signaling in those regions. This review discusses the growing literature that supports the hypothesis that PKA plays a central role in cytoskeletal regulation and cell migration.

AKAP signalling complexes: focal points in space and time

Multiprotein signalling networks create focal points of enzyme activity that disseminate the intracellular action of many hormones and neurotransmitters. Accordingly, the spatio-temporal activation of protein kinases and phosphatases is an important factor in controlling where and when phosphorylation events occur. Anchoring proteins provide a molecular framework that orients these enzymes towards selected substrates. A-kinase anchoring proteins (AKAPs) are signal-organizing molecules that compartmentalize various enzymes that are regulated by second messengers.

Lbc proto-oncogene product binds to and could be negatively regulated by metastasis suppressor nm23-H2

Lbc was identified as transforming gene from human leukemic cells and encodes Rho type guanine nucleotide exchange factor with 47kDa molecular weight. We isolated overlapping cDNAs of Lbc from human lung tissue. Full-length Lbc cDNA encodes 309kDa huge protein with Ht31 PKA anchoring motif, Dof domain, C1 domain, and coiled-coil structure. In order to analyze the regulatory mechanism of its activity, we searched for binding proteins. By yeast two-hybrid screening, we identified metastasis suppressor nm23-H2 as binding protein, which interacts with amino-terminal region of Lbc containing Dof domain. nm23 gene family encodes nucleoside diphosphate kinase, however, the binding of nm23-H2 to Lbc was independent of kinase activity. nm23-H1, which binds to Rac-specific GEF Tiam1, could not bind to Lbc suggesting nm23-H2 would be specific regulator for Lbc. Expression of nm23-H2 in cells leads to decrease the amount of GTP-bound Rho and suppress stress fiber formation stimulated by expression of Lbc. Our data suggest that metastasis suppressor nm23-H2 could regulate Lbc negatively by binding to amino-terminal region of Lbc proto-oncogene product.

AKAP-Lbc Nucleates a Protein Kinase D Activation Scaffold

The transmission of cellular signals often proceeds through multiprotein complexes where enzymes are positioned in proximity to their upstream activators and downstream substrates. In this report we demonstrate that the A-kinase anchoring protein AKAP-Lbc assembles an activation complex for the lipid-dependent enzyme protein kinase D (PKD). Using a combination of biochemical, enzymatic, and immunofluorescence techniques, we show that the anchoring protein contributes to PKD activation in two ways: it recruits an upstream kinase PKCeta and coordinates PKA phosphorylation events that release activated protein kinase D. Thus, AKAP-Lbc synchronizes PKA and PKC activities in a manner that leads to the activation of a third kinase. This configuration illustrates the utility of kinase anchoring as a mechanism to constrain the action of broad-spectrum enzymes.

Role of Lbc RhoGEF in Galpha12/13-induced signals to Rho GTPase

Heterotrimeric Galpha12/13 signals induce cellular responses such as serum response element (SRE)-mediated gene transcription via Rho GTPase. Guanine nucleotide exchange factors (GEFs) are strong candidates for linking Galpha signals to Rho. For example, p115 RhoGEF transduces Galpha13 signals to Rho and inhibits Galpha12/13 signals via the RhoGEF LH domain which links to Galpha subunits. Here, we have evaluated the signaling capacity of Lbc RhoGEF in the context of Galpha12/13 signals. In vitro GEF assays indicate that baculoviral-expressed proto-Lbc has minimal exchange activity, implying that a stimulus is required for Lbc activity in vivo. Expression of a catalytically inactive proto-Lbc mutant in HEK293T cells attenuates Galpha12- and thrombin-induced activation of an SRE transcriptional reporter, and the levels of inhibition observed is similar to that obtained with an analogous p115 RhoGEF mutant. proto-Lbc mutant expression also led to decreased levels of Galpha12-induced RhoA activation in vivo. Complex formation between Galpha12 and Lbc forms was detected. Analysis of the Lbc peptide sequence reveals a previously undetected region which may link to Galpha subunit signals. These findings support a role for Lbc in Galpha12-induced signaling pathways to Rho.

Localized effects of cAMP mediated by distinct routes of protein kinase A

More than 20% of the human genome encodes proteins involved in transmembrane and intracellular signaling pathways. The cAMP-protein kinase A (PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells and is involved in regulation of cellular functions in almost all tissues in mammals. Various extracellular signals converge on this signal pathway through ligand binding to G protein-coupled receptors, and the cAMP-PKA pathway is therefore tightly regulated at several levels to maintain specificity in the multitude of signal inputs. Ligand-induced changes in cAMP concentration vary in duration, amplitude, and extension into the cell, and cAMP microdomains are shaped by adenylyl cyclases that form cAMP as well as phosphodiesterases that degrade cAMP. Different PKA isozymes with distinct biochemical properties and cell-specific expression contribute to cell and organ specificity. A kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP-PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphatases and cAMP-specific phosphodiesterases as well as components of other signaling pathways into multiprotein signaling complexes that serve as crossroads for different paths of cell signaling. Targeting of PKA and integration of a wide repertoire of proteins involved in signal transduction into complex signal networks further increase the specificity required for the precise regulation of numerous cellular and physiological processes.

The minor histocompatibility antigen HA-3 arises from differential proteasome-mediated cleavage of the lymphoid blast crisis (Lbc) oncoprotein

Minor histocompatibility (H) antigens crucially affect the outcome of human leukocyte antigen (HLA)-identical allogeneic stem cell transplantation (SCT). To understand the basis of alloimmune responses against minor H antigens, identification of minor H peptides and their antigenicity-determining mechanisms is essential. Here we report the identification of HA-3 and its encoding gene. The HA-3 peptide, VTEPGTAQY (HA-3T), is encoded by the lymphoid blast crisis (Lbc) oncogene. We thus show for the first time that a leukemia-associated oncogene can give rise to immunogenic T-cell epitopes that may have participated in antihost and antileukemic alloimmune responses. Genotypic analysis of HA-3- individuals revealed the allelic counterpart VMEPGTAQY (HA-3M). Despite the lack of T-cell recognition of HA-3- cells, the Thr-->Met substitution had only a modest effect on peptide binding to HLA-A1 and a minimal impact on recognition by T cells when added exogenously to target cells. This substitution did not influence transporter associated with antigen processing (TAP) transport, but, in contrast to the HA-3T peptide, HA-3M is destroyed by proteasome-mediated digestion. Thus, the immunogenicity of minor H antigens can result from proteasome-mediated destruction of the negative allelic peptide.

Osmolality and solute composition are strong regulators of AQP2 expression in renal principal cells

The water permeability of the renal collecting duct is regulated by the insertion of aquaporin-2 (AQP2) into the apical plasma membrane of epithelial (principal) cells. Using primary cultured epithelial cells from the inner medulla of rat kidney (IMCD cells), we show that osmolality and solute composition are potent regulators of AQP2 mRNA and protein synthesis, as well as the classical cAMP-dependent pathway, but do not affect the arginine vasopressin-induced AQP2 shuttle. In the presence of the cAMP analog dibutyryl cAMP (DBcAMP, 500 microM), NaCl and sorbitol, but not urea, evoked a robust increase of AQP2 expression in IMCD cells, with NaCl being far more potent than sorbitol. cAMP-responsive element-binding protein phosphorylation increased with DBcAMP concentrations but was not altered by changes in osmolality. In the rat and human AQP2 promoter, we identified a putative tonicity-responsive element. We conclude that, in addition to the arginine vasopressin/cAMP-signaling cascade, a further pathway activated by elevated effective osmolality (tonicity) is crucial for the expression of AQP2 in IMCD cells, and we suggest that the effect is mediated via the tonicity-responsive element.

Association of Lbc Rho guanine nucleotide exchange factor with alpha-catenin-related protein, alpha-catulin/CTNNAL1, supports serum response factor activation

The Rho GTPase signaling pathway is required for actin cytoskeletal organization and serum response factor-dependent gene transcription. Lbc is a Rho-specific guanine nucleotide exchange factor that contains a modulatory C-terminal region. To elucidate Lbc regulatory mechanism(s), a yeast two-hybrid screen for proteins that interact with the Lbc C-terminal region was carried out, resulting in multiple isolation of cDNAs encoding the same 734-amino acid Lbc interacting protein. The Lbc interacting protein has homology with the alpha-catenin cell adhesion component and is identical to the alpha-catenin-like alpha-catulin protein of unknown function. The human alpha-catulin gene (CTNNAL1) maps to 9q31-32. Here we identify the predicted endogenous alpha-catulin product, document alpha-catulin and Lbc co-expression in multiple human cell lines, and show alpha-catulin and Lbc subcellular co-fractionation and intracellular localization. The required regions for Lbc and alpha-catulin interaction were mapped, and complex formation between Lbc and alpha-catulin in mammalian cells was detected. Functionally, alpha-catulin co-expression leads to increased Lbc-induced serum response factor activation in vivo as measured by a transcriptional reporter assay. Furthermore, alpha-catulin co-expression enhances Lbc-induced GTP-Rho formation in vivo. These results support the concept that the recently identified alpha-catulin protein may modulate Rho pathway signaling in vivo by providing a scaffold for the Lbc Rho guanine nucleotide exchange factor.