Regulation of AKAP-Membrane Interactions by Calcium

A-Kinase Anchoring Proteins in Cardiac Myocytes and Their Roles in Regulating Calcium Cycling

The rate of calcium cycling and calcium transient amplitude are critical determinants for the efficient contraction and relaxation of the heart. Calcium-handling proteins in the cardiac myocyte are altered in heart failure, and restoring the proper function of those proteins is an effective potential therapeutic strategy. The calcium-handling proteins or their regulators are phosphorylated by a cAMP-dependent kinase (PKA), and thereby their activity is regulated. A-Kinase Anchoring Proteins (AKAPs) play a seminal role in orchestrating PKA and cAMP regulators in calcium handling and contractile machinery. This cAMP/PKA orchestration is crucial for the increased force and rate of contraction and relaxation of the heart in response to fight-or-flight. Knockout models and the few available preclinical models proved that the efficient targeting of AKAPs offers potential therapies tailor-made for improving defective calcium cycling. In this review, we highlight important studies that identified AKAPs and their regulatory roles in cardiac myocyte calcium cycling in health and disease.

Intrinsically disordered proteins and membranes: a marriage of convenience for cell signalling?

The structure-function paradigm has guided investigations into the molecules involved in cellular signalling for decades. The peripheries of this paradigm, however, start to unravel when considering the co-operation between proteins and the membrane in signalling processes. Intrinsically disordered regions hold distinct advantages over folded domains in terms of their binding promiscuity, sensitivity to their particular environment and their ease of modulation through post-translational modifications. Low sequence complexity and bias towards charged residues are also favourable for the multivalent electrostatic interactions that occur at the surfaces of lipid bilayers. This review looks at the principles behind the successful marriage between protein disorder and membranes in addition to the role of this partnership in modifying and regulating signalling in cellular processes. The HVR (hypervariable region) of small GTPases is highlighted as a well-studied example of the nuanced role a short intrinsically disordered region can play in the fine-tuning of signalling pathways.

AKAP12 Signaling Complex: Impacts of Compartmentalizing cAMP-Dependent Signaling Pathways in the Heart and Various Signaling Systems

Heart failure is a complex clinical syndrome, represented as an impairment in ventricular filling and myocardial blood ejection. As such, heart failure is one of the leading causes of death in the United States. With a mortality rate of 1 per 8 individuals and a prevalence of 6.2 million Americans, it has been projected that heart failure prevalence will increase by 46% by 2030. Cardiac remodeling (a general determinant of heart failure) is regulated by an extensive network of intertwined intracellular signaling pathways. The ability of signalosomes (molecular signaling complexes) to compartmentalize several cellular pathways has been recently established. These signalosome signaling complexes provide an additional level of specificity to general signaling pathways by regulating the association of upstream signals with downstream effector molecules. In cardiac myocytes, the AKAP12 (A-kinase anchoring protein 12) scaffolds a large signalosome that orchestrates spatiotemporal signaling through stabilizing pools of phosphatases and kinases. Predominantly upon β-AR (β2-adrenergic-receptor) stimulation, the AKAP12 signalosome is recruited near the plasma membrane and binds tightly to β-AR. Thus, one major function of AKAP12 is compartmentalizing PKA (protein kinase A) signaling near the plasma membrane. In addition, it is involved in regulating desensitization, downregulation, and recycling of β-AR. In this review, the critical roles of AKAP12 as a scaffold protein in mediating signaling downstream GPCRs (G protein-coupled receptor) are discussed with an emphasis on its reported and potential roles in cardiovascular disease initiation and progression.

Post-surgical adhesions are triggered by calcium-dependent membrane bridges between mesothelial surfaces

Surgical adhesions are bands of scar tissues that abnormally conjoin organ surfaces. Adhesions are a major cause of post-operative and dialysis-related complications, yet their patho-mechanism remains elusive, and prevention agents in clinical trials have thus far failed to achieve efficacy. Here, we uncover the adhesion initiation mechanism by coating beads with human mesothelial cells that normally line organ surfaces, and viewing them under adhesion stimuli. We document expansive membrane protrusions from mesothelia that tether beads with massive accompanying adherence forces. Membrane protrusions precede matrix deposition, and can transmit adhesion stimuli to healthy surfaces. We identify cytoskeletal effectors and calcium signaling as molecular triggers that initiate surgical adhesions. A single, localized dose targeting these early germinal events completely prevented adhesions in a preclinical mouse model, and in human assays. Our findings classifies the adhesion pathology as originating from mesothelial membrane bridges and offer a radically new therapeutic approach to treat adhesions.

Surgical adhesions are organ-joining bands of scar tissue that remain clinically untreatable. Here, the authors show that adhesions are formed through expansive mesothelial membrane bridges, and that blocking these with small molecules prevents formation of adhesions in mice.

BFSP1 C-terminal domains released by post-translational processing events can alter significantly the calcium regulation of AQP0 water permeability

BFSP1 (beaded filament structural protein 1, filensin) is a cytoskeletal protein expressed in the eye lens. It binds AQP0 in vitro and its C-terminal sequences have been suggested to regulate the water channel activity of AQP0. A myristoylated fragment from the C-terminus of BFSP1 was found in AQP0 enriched fractions. Here we identify BFSP1 as a substrate for caspase-mediated cleavage at several C-terminal sites including D433. Cleavage at D433 exposes a cryptic myristoylation sequence (434–440). We confirm that this sequence is an excellent substrate for both NMT1 and 2 (N-myristoyl transferase). Thus caspase cleavage may promote formation of myristoylated fragments derived from the BFSP1 C-terminus (G434-S665). Myristoylation at G434 is not required for membrane association. Biochemical fractionation and immunogold labeling confirmed that C-terminal BFSP1 fragments containing the myristoylation sequence colocalized with AQP0 in the same plasma membrane compartments of lens fibre cells. To determine the functional significance of the association of BFSP1 G434-S665 sequences with AQP0, we measured AQP0 water permeability in Xenopus oocytes co-transfected with transcripts expressing both AQP0 and various C-terminal domain fragments of BFSP1 generated by caspase cleavage. We found that different fragments dramatically alter the response of AQP0 to different concentrations of Ca²⁺. The complete C-terminal fragment (G434-S665) eliminates calcium regulation altogether. Shorter fragments can enhance regulation by elevated calcium or reverse the response, indicative of the regulatory potential of BFSP1 with respect to AQP0. In particular, elimination of the myristoylation site by the mutation G434A reverses the order of water permeability sensitivity to different Ca²⁺ concentrations.

Potential for therapeutic targeting of AKAP signaling complexes in nervous system disorders

A common feature of neurological and neuropsychiatric disorders is a breakdown in the integrity of intracellular signal transduction pathways. Dysregulation of ion channels and receptors in the cell membrane and the enzymatic mediators that link them to intracellular effectors can lead to synaptic dysfunction and neuronal death. However, therapeutic targeting of these ubiquitous signaling elements can lead to off-target side effects due to their widespread expression in multiple systems of the body. A-kinase anchoring proteins (AKAPs) are multivalent scaffolding proteins that compartmentalize a diverse range of receptor and effector proteins to streamline signaling within nanodomain signalosomes. A number of essential neurological processes are known to critically depend on AKAP-directed signaling and an understanding of the role AKAPs play in nervous system disorders has emerged in recent years. Selective targeting of AKAP protein-protein interactions may be a means to uncouple pathologically active signaling pathways in neurological disorders with a greater degree of specificity. In this review we will discuss the role of AKAPs in both regulating normal nervous system function and dysfunction associated with disease, and the potential for therapeutic targeting of AKAP signaling complexes.

Molecular basis of AKAP79 regulation by calmodulin

AKAP79/150 is essential for coordinating second messenger-responsive enzymes in processes including synaptic long-term depression. Ca²⁺ directly regulates AKAP79 through its effector calmodulin (CaM), but the molecular basis of this regulation was previously unknown. Here, we report that CaM recognizes a ‘1-4-7-8’ pattern of hydrophobic amino acids starting at Trp79 in AKAP79. Cross-linking coupled to mass spectrometry assisted mapping of the interaction site. Removal of the CaM-binding sequence in AKAP79 prevents formation of a Ca²⁺-sensitive interface between AKAP79 and calcineurin, and increases resting cellular PKA phosphorylation. We determined a crystal structure of CaM bound to a peptide encompassing its binding site in AKAP79. CaM adopts a highly compact conformation in which its open Ca²⁺-activated C-lobe and closed N-lobe cooperate to recognize a mixed α/3₁₀ helix in AKAP79. The structure guided a bioinformatic screen to identify potential sites in other proteins that may employ similar motifs for interaction with CaM.

The A-kinase anchoring protein AKAP79 is regulated by calmodulin (CaM). Here, the authors use crosslinking coupled to mass spectrometry to identify the CaM-binding site in AKAP79 and present the structure of CaM bound to an AKAP79 peptide. The structure shows that CaM adopts a highly compact conformation to interact with a mixed α/3₁₀ helix in AKAP79.

Investigating the human Calcineurin Interaction Network using the πɸLxVP SLiM

Ser/thr phosphorylation is the primary reversible covalent modification of proteins in eukaryotes. As a consequence, it is the reciprocal actions of kinases and phosphatases that act as key molecular switches to fine tune cellular events. It has been well documented that ~400 human ser/thr kinases engage substrates via consensus phosphosite sequences. Strikingly, we know comparatively little about the mechanism by which ~40 human protein ser/thr phosphatases (PSPs) dephosphorylate ~15000 different substrates with high specificity. The identification of substrates of the essential PSP calcineurin (CN) has been exceptionally challenging and only a small fraction has been biochemically confirmed. It is now emerging that CN binds regulators and substrates via two short linear motifs (SLiMs), the well-studied PxIxIT SLiM and the LxVP SLiM, which remains controversial at the molecular level. Here we describe the crystal structure of CN in complex with its substrate NFATc1 and show that the LxVP SLiM is correctly defined as πɸLxVP. Bioinformatics studies using the πɸLxVP SLiM resulted in the identification of 567 potential CN substrates; a small subset was experimentally confirmed. This combined structural-bioinformatics approach provides a powerful method for dissecting the CN interaction network and for elucidating the role of CN in human health and disease.

Identification of striated muscle activator of Rho signaling (STARS) as a novel calmodulin target by a newly developed genome-wide screen

To search for novel target(s) of the Ca(2+)-signaling transducer, calmodulin (CaM), we performed a newly developed genome-wide CaM interaction screening of 19,676 GST-fused proteins expressed in human. We identified striated muscle activator of Rho signaling (STARS) as a novel CaM target and characterized its CaM binding ability and found that the Ca(2+)/CaM complex interacted stoichiometrically with the N-terminal region (Ala13-Gln35) of STARS in vitro as well as in living cells. Mutagenesis studies identified Ile20 and Trp33 as the essential hydrophobic residues in CaM anchoring. Furthermore, the CaM binding deficient mutant (Ile20Ala, Trp33Ala) of STARS further enhanced its stimulatory effect on SRF-dependent transcriptional activation. These results suggest a connection between Ca(2+)-signaling via excitation-contraction coupling and the regulation of STARS-mediated gene expression in muscles.

FRET biosensors reveal AKAP-mediated shaping of subcellular PKA activity and a novel mode of Ca(2+)/PKA crosstalk

Scaffold proteins play a critical role in cellular homeostasis by anchoring signaling enzymes in close proximity to downstream effectors. In addition to anchoring static enzyme complexes, some scaffold proteins also form dynamic signalosomes that can traffic to different subcellular compartments upon stimulation. Gravin (AKAP12), a multivalent scaffold, anchors PKA and other enzymes to the plasma membrane under basal conditions, but upon [Ca(2+)]i elevation, is rapidly redistributed to the cytosol. Because gravin redistribution also impacts PKA localization, we postulate that gravin acts as a calcium "switch" that modulates PKA-substrate interactions at the plasma membrane, thus facilitating a novel crosstalk mechanism between Ca(2+) and PKA-dependent pathways. To assess this, we measured the impact of gravin-V5/His expression on compartmentalized PKA activity using the FRET biosensor AKAR3 in cultured cells. Upon treatment with forskolin or isoproterenol, cells expressing gravin-V5/His showed elevated levels of plasma membrane PKA activity, but cytosolic PKA activity levels were reduced compared with control cells lacking gravin. This effect required both gravin interaction with PKA and localization at the plasma membrane. Pretreatment with calcium-elevating agents thapsigargin or ATP caused gravin redistribution away from the plasma membrane and prevented gravin from elevating PKA activity levels at the membrane. Importantly, this mode of Ca(2+)/PKA crosstalk was not observed in cells expressing a gravin mutant that resisted calcium-mediated redistribution from the cell periphery. These results reveal that gravin impacts subcellular PKA activity levels through the spatial targeting of PKA, and that calcium elevation modulates downstream β-adrenergic/PKA signaling through gravin redistribution, thus supporting the hypothesis that gravin mediates crosstalk between Ca(2+) and PKA-dependent signaling pathways. Based on these results, AKAP localization dynamics may represent an important paradigm for the regulation of cellular signaling networks.

The cAMP Signaling Pathway and Direct Protein Kinase A Phosphorylation Regulate Polycystin-2 (TRPP2) Channel Function

Polycystin-2 (PC2) is a TRP-type, Ca(2+)-permeable non-selective cation channel that plays an important role in Ca(2+) signaling in renal and non-renal cells. The effect(s) of the cAMP pathway and kinase mediated phosphorylation of PC2 seem to be relevant to PC2 trafficking and its interaction with polycystin-1. However, the role of PC2 phosphorylation in channel function is still poorly defined. Here we reconstituted apical membranes of term human syncytiotrophoblast (hST), containing endogenous PC2 (PC2hst), and in vitro translated channel protein (PC2iv). Addition of the catalytic subunit of PKA increased by 566% the spontaneous PC2hst channel activity in the presence of ATP. Interestingly, 8-Br-cAMP also stimulated spontaneous PC2hst channel activity in the absence of the exogenous kinase. Either stimulation was inhibited by addition of alkaline phosphatase, which in turn, was reversed by the phosphatase inhibitor vanadate. Neither maneuver modified the single channel conductance but instead increased channel mean open time. PKA directly phosphorylated PC2, which increased the mean open time but not the single channel conductance of the channel. PKA phosphorylation did not modify either R742X truncated or S829A-mutant PC2iv channel function. The data indicate that the cAMP pathway regulates PC2-mediated cation transport in the hST. The relevant PKA site for PC2 channel regulation centers on a single residue serine 829, in the carboxyl terminus.

Minireview: Spatial Programming of G Protein-Coupled Receptor Activity: Decoding Signaling in Health and Disease

Probing the multiplicity of hormone signaling via G protein-coupled receptors (GPCRs) has demonstrated the complex signal pathways that underlie the multiple functions these receptors play in vivo. This is highly pertinent for the GPCRs key in reproduction and pregnancy that are exposed to cyclical and dynamic changes in their extracellular milieu. How such functional pleiotropy in GPCR signaling is translated to specific downstream cellular responses, however, is largely unknown. Emerging data strongly support mechanisms for a central role of receptor location in signal regulation via membrane trafficking. In this review, we discuss current progress in our understanding of the role membrane trafficking plays in location control of GPCR signaling, from organized plasma membrane signaling microdomains, potentially provided by both distinct endocytic and exocytic pathways, to more recent evidence for spatial control within the endomembrane system. Application of these emerging mechanisms in their relevance to GPCR activity in physiological and pathophysiological conditions will also be discussed, and in improving therapeutic strategies that exploits these mechanisms in order to program highly regulated and distinct signaling profiles.

Suppression of chemotaxis by SSeCKS via scaffolding of phosphoinositol phosphates and the recruitment of the Cdc42 GEF, Frabin, to the leading edge

Chemotaxis is controlled by interactions between receptors, Rho-family GTPases, phosphatidylinositol 3-kinases, and cytoskeleton remodeling proteins. We investigated how the metastasis suppressor, SSeCKS, attenuates chemotaxis. Chemotaxis activity inversely correlated with SSeCKS levels in mouse embryo fibroblasts (MEF), DU145 and MDA-MB-231 cancer cells. SSeCKS loss induced chemotactic velocity and linear directionality, correlating with replacement of leading edge lamellipodia with fascin-enriched filopodia-like extensions, the formation of thickened longitudinal F-actin stress fibers reaching to filopodial tips, relative enrichments at the leading edge of phosphatidylinositol (3,4,5)P3 (PIP3), Akt, PKC-ζ, Cdc42-GTP and active Src (Src^poY416), and a loss of Rac1. Leading edge lamellipodia and chemotaxis inhibition in SSeCKS-null MEF could be restored by full-length SSeCKS or SSeCKS deleted of its Src-binding domain (ΔSrc), but not by SSeCKS deleted of its three MARCKS (myristylated alanine-rich C kinase substrate) polybasic domains (ΔPBD), which bind PIP2 and PIP3. The enrichment of activated Cdc42 in SSeCKS-null leading edge filopodia correlated with recruitment of the Cdc42-specific guanine nucleotide exchange factor, Frabin, likely recruited via multiple PIP2/3-binding domains. Frabin knockdown in SSeCKS-null MEF restores leading edge lamellipodia and chemotaxis inhibition. However, SSeCKS failed to co-immunoprecipitate with Rac1, Cdc42 or Frabin. Consistent with the notion that chemotaxis is controlled by SSeCKS-PIP (vs. -Src) scaffolding activity, constitutively-active phosphatidylinositol 3-kinase could override the ability of the Src inhibitor, SKI-606, to suppress chemotaxis and filopodial enrichment of Frabin in SSeCKS-null MEF. Our data suggest a role for SSeCKS in controlling Rac1 vs. Cdc42-induced cellular dynamics at the leading chemotactic edge through the scaffolding of phospholipids and signal mediators, and through the reorganization of the actin cytoskeleton controlling directional movement.

Receptor-mediated Ca2+ and PKC signaling triggers the loss of cortical PKA compartmentalization through the redistribution of gravin

A-Kinase Anchoring Proteins (AKAPs) direct the flow of cellular information by positioning multiprotein signaling complexes into proximity with effector proteins. However, certain AKAPs are not stationary but can undergo spatiotemporal redistribution in response to stimuli. Gravin, a 300kD AKAP that intersects with a diverse signaling array, is localized to the plasma membrane but has been shown to translocate to the cytosol following the elevation of intracellular calcium ([Ca(2+)]i). Despite the potential for gravin redistribution to impact multiple signaling pathways, the dynamics of this event remain poorly understood. In this study, quantitative microscopy of cells expressing gravin-EGFP revealed that Ca(2+) elevation caused the complete translocation of gravin from the cell cortex to the cytosol in as little as 60s of treatment with ionomycin or thapsigargin. In addition, receptor mediated signaling was also shown to cause gravin redistribution following ATP treatment, and this event required both [Ca(2+)]i elevation and PKC activation. To understand the mechanism for Ca(2+) mediated gravin dynamics, deletion of calmodulin-binding domains revealed that a fourth putative calmodulin binding domain called CB4 (a.a. 670-694) is critical for targeting gravin to the cell cortex despite its location downstream of gravin's membrane-targeting domains, which include an N-terminal myristoylation site and three polybasic domains. Finally, confocal microscopy of cells co-transfected with gravin-EYFP and PKA RII-ECFP revealed that gravin redistribution mediated by ionomycin, thapsigargin, and ATP each triggered the gravin-dependent loss of PKA localized at the cell cortex. Our results support the hypothesis that gravin redistribution regulates cross-talk between PKA-dependent signaling and receptor-mediated events involving Ca(2+) and PKC.

Mechanism for targeting the A-kinase anchoring protein AKAP18δ to the membrane

A-kinase anchoring proteins (AKAPs) are a family of scaffolding proteins that target PKA and other signaling molecules to cellular compartments and thereby spatiotemporally define cellular signaling events. The AKAP18 family comprises AKAP18α, AKAP18β, AKAP18γ, and AKAP18δ. The δ isoform targets PKA and phosphodiesterase PDE4D to AQP2 (aquaporin-2)-bearing vesicles to orchestrate the acute regulation of body water balance. Therefore, AKAP18δ must adopt a membrane localization that seems at odds with (i) its lack of palmitoylation or myristoylation sites that tailor its isoforms AKAP18α and AKAP18β to membrane compartments and (ii) the high sequence identity to the preferentially cytoplasmic AKAP18γ. Here, we show that the electrostatic attraction of the positively charged amino acids of AKAP18δ to negatively charged lipids explains its membrane targeting. As revealed by fluorescence correlation spectroscopy, the binding constant of purified AKAP18δ fragments to large unilamellar vesicles correlates (i) with the fraction of net negatively charged lipids in the bilayer and (ii) with the total amount of basic residues in the protein. Although distantly located on the sequence, these positively charged residues concentrate in the tertiary structure and form a clear binding surface. Thus, specific recruitment of the AKAP18δ-based signaling module to membranes such as those of AQP2-bearing vesicles must be achieved by additional mechanisms, most likely compartment-specific protein-protein interactions.

A-kinase anchoring proteins as potential drug targets

A-kinase anchoring proteins (AKAPs) crucially contribute to the spatial and temporal control of cellular signalling. They directly interact with a variety of protein binding partners and cellular constituents, thereby directing pools of signalling components to defined locales. In particular, AKAPs mediate compartmentalization of cAMP signalling. Alterations in AKAP expression and their interactions are associated with or cause diseases including chronic heart failure, various cancers and disorders of the immune system such as HIV. A number of cellular dysfunctions result from mutations of specific AKAPs. The link between malfunctions of single AKAP complexes and a disease makes AKAPs and their interactions interesting targets for the development of novel drugs. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

"Shaping" of cell signaling via AKAP-tethered PDE4D: Probing with AKAR2-AKAP5 biosensor

Background

PKA, a key regulator of cell signaling, phosphorylates a diverse and important array of target molecules and is spatially docked to members of the A-kinase Anchoring Protein (AKAP) family. AKAR2 is a biosensor which yields a FRET signal in vivo, when phosphorylated by PKA. AKAP5, a prominent member of the AKAP family, docks several signaling molecules including PKA, PDE4D, as well as GPCRs, and is obligate for the propagation of the activation of the mitogen-activated protein kinase cascade from GPCRs to ERK1,2.

Results

Using an AKAR2-AKAP5 fusion “biosensor”, we investigated the spatial-temporal activation of AKAP5 undergoing phosphorylation by PKA in response to β-adrenergic stimulation. The pattern of PKA activation reported by AKAR2-AKAP5 is a more rapid and spatially distinct from those “sensed” by AKAR2-AKAP12. Spatial-temporal restriction of activated PKA by AKAP5 was found to “shape” the signaling response. Phosphatase PDE4D tethered to AKAP5 also later reverses within 60 s elevated intracellular cyclic AMP levels stimulated by β-adrenergic agonist. AKAP12, however, fails to attenuate the rise in cyclic AMP over this time. Fusion of the AKAP5 PDE4D-binding-domain to AKAP12 was found to accelerate a reversal of accumulation of intracellular cyclic AMP.

Conclusion

AKAPs, which are scaffolds with tethered enzymes, can “shape” the temporal and spatial aspects of cell signaling.

Dishevelled C-terminus: prolyl and histidinyl motifs

The phosphoprotein scaffold Dishevelled is an essential component of both Wnt signalling and of the signalsome that constitutes the supermolecular 'punctae' of assembled proteins often observed in fluorescence microscopy. The C-terminal region beyond the DEP domain displays unique and interesting character, exploited herein by careful analysis of the primary structure. Human Dishevelled-1, -2, -3 and fly Dishevelled (Dsh) sequences were downloaded and interrogated in silico. The C-terminus of Dishevelled-3 is revealed by FoldIndex(®) to be rich in ordered structure. It displays primary sequence that is unique and divergent in important ways from vertebrate isoforms as well as from the fly Dsh. The region is amphipathic, high in prolyl content, and harbours polyprolines. Dishevelled-3 displays some regions, where the proline content is >40%. Polyprolyl sequences (2-4 residues) likely constitute important sites of interaction with other Dishevelled isoforms. Several histidine-single amino acid repeats are notable. The 637,638/647,648 repeats of Dvl3 are essential for Wnt non-canonical, but not canonical signalling. Mutagenesis reveals that the C-terminal sequence is essential for the formation of punctae, made visible by fluorescence microscopy. These Dvl3-based signalsomes are very large (25-35 MDa-MW), supermolecular complexes that display dynamic reorganization in response to Wnt stimulation. Dishevelled-3 C-terminus is rich in structure and unique motifs, worthy of detailed analysis with modern molecular tools.

AKAP12 and AKAP5 form higher-order hetero-oligomers

Background

The family of A-kinase-anchoring proteins, AKAPs, constitutes a group of molecular scaffolds that act to catalyze dynamic interactions of protein kinase A, protein kinase C, tyrosine kinases, G-protein-coupled receptors and ion channels. AKAP5 (MW ~47 kDa) and AKAP12 (MW ~191 kDa) homo-oligomerize, but whether or not such AKAPs can hetero-oligomerize into supermolecular scaffolds of increased complexity is unknown.

Results

Affinity chromatography using immobilized AKAPs as "bait" demonstrates unequivocally that AKAP5 and AKAP12 do form minimally hetero-dimers. Steric-exclusion chromatography of AKAP5 and AKAP12 mixtures revealed the existence of very large, supermolecular complexes containing both AKAPs. Docking of AKAP5 to AKAP12 was increased 4-fold by beta-adrenergic agonist stimulation. Overexpression of AKAP12 was found to potentiate AKAP5-mediated Erk1/2 activation in response to stimulation with beta-adrenergic agonist.

Conclusion

AKAP5 and AKAP12 are capable of forming hetero-oligomeric supermolecular complexes that influence AKAP locale and function.

Cross-talk between calcium and protein kinase A in the regulation of cell migration

Calcium (Ca(2+)) and the cAMP-dependent protein kinase (PKA) are pleiotropic cellular regulators and both exert powerful, diverse effects on cytoskeletal dynamics, cell adhesion, and cell migration. Localization, by A-kinase-anchoring proteins (AKAPs), of PKA activity to the protrusive leading edge, integrins, and other regulators of cytoskeletal dynamics has emerged as an important facet of its role in cell migration. Additional recent work has firmly established the importance of Ca(2+) influx through mechanosensitive transient receptor potential (TRP) channels and through store-operated Ca(2+) entry (SOCE) in cell migration. Finally, there is considerable evidence showing that these mechanisms of Ca(2+) influx and PKA regulation intersect--and often interact--and thus may work in concert to translate complex extracellular cues into the intracellular biochemical anisotropy required for directional cell migration.

AKAP5 and AKAP12 Form Homo-oligomers

BACKGROUND

A-kinase-anchoring proteins, AKAPs, constitute a family of scaffolds that play an essential role in catalyzing the spatial-temporal, dynamic interactions of protein kinase A, protein kinase C, tyrosine kinases, G-protein-coupled receptors and ion channels. We studied AKAP5 (AKAP79; MW ~47 kDa) and AKAP12 (gravin, SSECKS; MW ~191 kDa) to probe if these AKAP scaffolds oligomerize.

RESULTS

In gel analysis and sodium-dodecyl sulfate denaturation, AKAP12 behaved with a MW of a homo-dimer. Only in the presence of the chaotropic agent 8 M urea did gel analysis reveal a monomeric form of AKAP12. By separation by steric-exclusion chromatography, AKAP12 migrates with MW of ~840 kDa, suggestive of higher-order complexes such as a tetramer. Interestingly, the N-(1-840) and C-(840-1782) terminal regions of AKAP12 themselves retained the ability to form dimers, suggesting that the structural basis for the dimerization is not restricted to a single "domain" found within the molecule. In either sodium dodecyl sulfate or urea, AKAP5 displayed a relative mobility of a monomer, but by co-immunoprecipitation in native state was shown to oligomerize. When subjected to steric-exclusion chromatography, AKAP5 forms higher-order complexes with MW ~220 kDa, suggestive of tetrameric assemblies.

CONCLUSION

Both AKAP5 and AKAP12 display the capacity to form supermolecular homo-oligomeric structures that likely influence the localization and function of these molecular scaffolds.

Retinoid-induced expression and activity of an immediate early tumor suppressor gene in vascular smooth muscle cells

Retinoids are used clinically to treat a number of hyper-proliferative disorders and have been shown in experimental animals to attenuate vascular occlusive diseases, presumably through nuclear receptors bound to retinoic acid response elements (RARE) located in target genes. Here, we show that natural or synthetic retinoids rapidly induce mRNA and protein expression of a specific isoform of A-Kinase Anchoring Protein 12 (AKAP12β) in cultured smooth muscle cells (SMC) as well as the intact vessel wall. Expression kinetics and actinomycin D studies indicate Akap12β is a retinoid-induced, immediate-early gene. Akap12β promoter analyses reveal a conserved RARE mildly induced with atRA in a region that exhibits hyper-acetylation. Immunofluorescence microscopy and protein kinase A (PKA) regulatory subunit overlay assays in SMC suggest a physical association between AKAP12β and PKA following retinoid treatment. Consistent with its designation as a tumor suppressor, inducible expression of AKAP12β attenuates SMC growth in vitro. Further, immunohistochemistry studies establish marked decreases in AKAP12 expression in experimentally-injured vessels of mice as well as atheromatous lesions in humans. Collectively, these results demonstrate a novel role for retinoids in the induction of an AKAP tumor suppressor that blocks vascular SMC growth thus providing new molecular insight into how retiniods may exert their anti-proliferative effects in the injured vessel wall.

AKAR2-AKAP12 fusion protein "biosenses" dynamic phosphorylation and localization of a GPCR-based scaffold

Background

The cAMP-dependent protein kinase A (PKA) plays a pivotal role in virtually all cells, there being a multitude of important target molecules that are substrates for PKA in cell signaling. The spatial-temporal dynamics of PKA activation in living cells has been made accessible by the development of clever biosensors that yield a FRET signal in response to the phosphorylation by PKA. AKAR2 is genetically encoded fluorescent probe that acts as a biosensor for PKA activation. AKAP12 is a scaffold that docks PKA, G-protein-coupled receptors, cell membrane negatively-charged phospholipids, and catalyzes receptor resensitization and recycling. In the current work, the AKAR2 biosensor was fused to the N-terminus of AKAP12 to evaluate its ability to function and report on dynamic phosphorylation of the AKAP12 scaffold.

Results

AKAR2-AKAP12 can be expressed in mammalian cells, is fully functional, and reveals the spatial-temporal activation of AKAP12 undergoing phosphorylation by PKA in response to beta-adrenergic activation in human epidermoid carcinoma A431 cells.

Conclusion

The dynamic phosphorylation of AKAP12 "biosensed" by AKAR2-AKAP12 reveals the scaffold in association with the cell membrane, undergoing rapid phosphorylation by PKA. The perinuclear, cytoplasmic accumulation of phosphorylated scaffold reflects the phosphorylated, PKA-activated form of AKAP12, which catalyzes the resensitization and recycling of desensitized, internalized G-protein-coupled receptors.

Decreased expression of A-kinase anchoring protein 150 in GT1 neurons decreases neuron excitability and frequency of intrinsic gonadotropin-releasing hormone pulses

The frequency of intrinsic pulsatile GnRH secretion from endogenous GnRH neurons and GT1 GnRH cell lines is stimulated by increased intracellular cAMP levels. The downstream molecules comprising the cAMP signaling pathway are organized in microdomains by a family of scaffolding proteins, A-kinase anchoring proteins (AKAPs). These molecules tether protein kinase A, cAMP-specific phosphodiesterases, phosphatases to known substrates. In neurons AKAP150 organizes many of the signaling molecules known to regulate the excitability and intrinsic pulsatile activity of GnRH neurons. AKAP150 was expressed in both the GT1-1 and GT1-7 cells. We determined the role of AKAP150 in coordinating GT1-1 cell excitability and intrinsic GnRH pulsatile secretion by lowering AKAP150 levels with a small interfering RNA (siRNA) adenovirus construct to AKAP150 (Ad-AKAP150-siRNA). Infection with Ad-AKAP150-siRNA specifically decreased AKAP150 mRNA levels by 74% and protein levels by 53% relative to uninfected cells or cells infected with a luciferase control adenovirus siRNA vector. In GT1 cells, spontaneous Ca(2+) oscillations, an index of neuron excitability, are stimulated by increased levels of intracellular cAMP and lowered by decreased levels. The frequency of spontaneous Ca(2+) oscillations in Ad-AKAP150-siRNA-treated GT1-1 cells decreased by 47.2% relative to controls. A dramatic decrease in the number of spontaneous GnRH pulses was also observed after infection with Ad-AKAP150-siRNA. The interpulse interval increased to 143 +/- 20.25 min in Ad-AKAP150-siRNA infected cells from 32.2 +/- 7.3 min in luciferase control adenovirus siRNA vector-infected cells. These data demonstrate an important role of AKAP150 in coordinating signaling events regulating the frequency of intrinsic pulsatile GnRH secretion.

Adenylyl cyclase--A-kinase anchoring protein complexes: the next dimension in cAMP signaling

The formation of multiprotein complexes is a repeated theme in biology ranging from the regulation of the extracellular signal-regulated kinase and cAMP signaling pathways to the formation of postsynaptic density complexes or tight junctions. A-kinase anchoring proteins (AKAPs) are well known for their ability to scaffold protein kinase A and components upstream and downstream of cAMP production, including G protein-coupled receptors, cAMP-dependent Rap-exchange factors, and phosphodiesterases. Specific adenylyl cyclase (AC) isoforms have also been identified as components of AKAP complexes, namely AKAP79, Yotiao, and mAKAP. In this review, we summarize recent evidence for AC-AKAP complexes and requirements for compartmentalization of cAMP signaling. The ability of AKAPs to assemble intricate feedback loops to control spatiotemporal aspects of cAMP signaling adds yet another dimension to the classic cAMP pathway.

Supramolecular assemblies and localized regulation of voltage-gated ion channels

This review addresses the localized regulation of voltage-gated ion channels by phosphorylation. Comprehensive data on channel regulation by associated protein kinases, phosphatases, and related regulatory proteins are mainly available for voltage-gated Ca2+ channels, which form the main focus of this review. Other voltage-gated ion channels and especially Kv7.1-3 (KCNQ1-3), the large- and small-conductance Ca2+-activated K+ channels BK and SK2, and the inward-rectifying K+ channels Kir3 have also been studied to quite some extent and will be included. Regulation of the L-type Ca2+ channel Cav1.2 by PKA has been studied most thoroughly as it underlies the cardiac fight-or-flight response. A prototypical Cav1.2 signaling complex containing the beta2 adrenergic receptor, the heterotrimeric G protein Gs, adenylyl cyclase, and PKA has been identified that supports highly localized via cAMP. The type 2 ryanodine receptor as well as AMPA- and NMDA-type glutamate receptors are in close proximity to Cav1.2 in cardiomyocytes and neurons, respectively, yet independently anchor PKA, CaMKII, and the serine/threonine phosphatases PP1, PP2A, and PP2B, as is discussed in detail. Descriptions of the structural and functional aspects of the interactions of PKA, PKC, CaMKII, Src, and various phosphatases with Cav1.2 will include comparisons with analogous interactions with other channels such as the ryanodine receptor or ionotropic glutamate receptors. Regulation of Na+ and K+ channel phosphorylation complexes will be discussed in separate papers. This review is thus intended for readers interested in ion channel regulation or in localization of kinases, phosphatases, and their upstream regulators.

Gravin dynamics regulates the subcellular distribution of PKA

Gravin, a multivalent A-kinase anchoring protein (AKAP), localizes to the cell periphery in several cell types and is postulated to target PKA and other binding partners to the plasma membrane. An N-terminal myristoylation sequence and three regions rich in basic amino acids are proposed to mediate this localization. Reports indicating that phorbol ester affects the distribution of SSeCKS, the rat orthologue of gravin, further suggest that PKC may also regulate the subcellular distribution of gravin, which in turn may affect PKA distribution. In this study, quantitative confocal microscopy of cells expressing full-length and mutant gravin-EGFP constructs lacking the proposed targeting domains revealed that either the N-myristoylation site or the polybasic regions were sufficient to target gravin to the cell periphery. Moreover, phorbol ester treatment induced redistribution of gravin-EGFP from the cell periphery to a juxtanuclear vesicular compartment, but this required the presence of the N-myristoylation site. Confocal microscopy further revealed that not only did gravin-EGFP target a PKA RII-ECFP construct to the cell periphery, but PKC activation resulted in redistribution of the gravin and PKA constructs to the same subcellular site. It is postulated that this dynamic response by gravin to PKC activity may mediate PKC dependent control of PKA activity.

Cell shape regulation by Gravin requires N-terminal membrane effector domains

Gravin (AKAP12, SSeCKS) is a scaffolding protein that acts as a potent inhibitor of tumor metastasis in vivo and in vitro, and regulates morphogenesis during vertebrate gastrulation. Despite being implicated in many cellular processes, surprisingly little is known about the mechanism by which Gravin elicits cell shape changes. In this work, we use in vitro cell spreading assays to demonstrate that the Gravin N-terminus containing the three MARCKS-like basic regions (BRs) is necessary and sufficient to regulate cell shape in vitro. We show that the conserved phosphorylation sites in the BRs are essential for their function in these assays. We further demonstrate that the Gravin BRs are necessary for in vivo function during gastrulation in zebrafish. Together, these results provide an important step forward in understanding the mechanism of Gravin function in cell shape regulation and provide valuable insight into how Gravin acts as a cytoskeletal regulator.

Localization of SSeCKS in unmyelinated primary sensory neurons

Background

SSeCKS (Src SupprEssed C Kinase Substrate) is a proposed protein kinase C substrate/A kinase anchoring protein (AKAP) that has recently been characterized in the rat peripheral nervous system. It has been shown that approximately 40% of small primary sensory neurons contain SSeCKS-immunoreactivity in a population largely separate from substance P (95.2%), calcitonin gene related peptide (95.3%), or fluoride resistant acid phosphatase (55.0%) labeled cells. In the spinal cord, it was found that SSeCKS-immunoreactive axon collaterals terminate in the dorsal third of lamina II outer in a region similar to that of unmyelinated C-, or small diameter myelinated Aδ-, fibers. However, the precise characterization of the anatomical profile of the primary sensory neurons containing SSeCKS remains to be determined. Here, immunohistochemical labeling at the light and ultrastructural level is used to clarify the myelination status of SSeCKS-containing sensory neuron axons and to further clarify the morphometric, and provide insight into the functional, classification of SSeCKS-IR sensory neurons.

Methods

Colocalization studies of SSeCKS with myelination markers, ultrastructural localization of SSeCKS labeling and ablation of largely unmyelinated sensory fibers by neonatal capsaicin administration were all used to establish whether SSeCKS containing sensory neurons represent a subpopulation of unmyelinated primary sensory C-fibers.

Results

Double labeling studies of SSeCKS with CNPase in the dorsal horn and Pzero in the periphery showed that SSeCKS immunoreactivity was observed predominantly in association with unmyelinated primary sensory fibers. At the ultrastructural level, SSeCKS immunoreactivity was most commonly associated with axonal membrane margins of unmyelinated fibers. In capsaicin treated rats, SSeCKS immunoreactivity was essentially obliterated in the dorsal horn while in dorsal root ganglia quantitative analysis revealed a 43% reduction in the number of SSeCKS-labeled cells. This attenuation is concomitant with a decrease in fluoride-resistant acid phosphatase labeled fibers in the spinal cord dorsal horn and small neuronal somata in sensory ganglia.

Conclusion

These results demonstrate that SSeCKS is primarily localized within a distinct subpopulation of small diameter, largely unmyelinated C-fiber primary sensory neurons putatively involved in nociception.

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.

N-myristoylation regulates the SnRK1 pathway in Arabidopsis

Cotranslational and posttranslational modifications are increasingly recognized as important in the regulation of numerous essential cellular functions. N-myristoylation is a lipid modification ensuring the proper function and intracellular trafficking of proteins involved in many signaling pathways. Arabidopsis thaliana, like human, has two tightly regulated N-myristoyltransferase (NMT) genes, NMT1 and NMT2. Characterization of knockout mutants showed that NMT1 was strictly required for plant viability, whereas NMT2 accelerated flowering. NMT1 impairment induced extremely severe defects in the shoot apical meristem during embryonic development, causing growth arrest after germination. A transgenic plant line with an inducible NMT1 gene demonstrated that NMT1 expression had further effects at later stages. NMT2 did not compensate for NMT1 in the nmt1-1 mutant, but NMT2 overexpression resulted in shoot and root meristem abnormalities. Various data from complementation experiments in the nmt1-1 background, using either yeast or human NMTs, demonstrated a functional link between the developmental arrest of nmt1-1 mutants and the myristoylation state of an extremely small set of protein targets. We show here that protein N-myristoylation is systematically associated with shoot meristem development and that SnRK1 (for SNF1-related kinase) is one of its essential primary targets.

v-Src-mediated Down-regulation of SSeCKS Metastasis Suppressor Gene Promoter by the Recruitment of HDAC1 into a USF1-Sp1-Sp3 Complex

SSeCKS (Src-suppressed C kinase substrate), also called gravin/AKAP12, is a large scaffolding protein with metastasis suppressor activity. Two major isoforms of SSeCKS are expressed in most cell and tissue types under the control of two independent promoters, designated alpha and beta, separated by 68 kb. SSeCKS transcript and protein levels are severely decreased in Src- and Ras-transformed fibroblasts and in many epithelial tumors. By dissecting its promoters with progressive deletion analysis, we identified the sequence between -106 and -49 in the alpha proximal promoter as the minimal v-Src-responsive element, which contains E- and GC-boxes bound by USF1 and Sp1/Sp3, respectively. Both E- and GC-boxes are crucial for v-Src-responsive and basal promoter activities. v-Src does not alter USF1 binding levels at the E-box, but it increases Sp1/Sp3 binding to the GC-box despite no change in their cellular protein abundance. SSeCKS alpha and beta transcript levels in v-Src/3T3 cells can be restored by treatment with the histone deacetylase inhibitor, trichostatin A, but not with the DNA demethylation agent, 5-azacytidine. Chromatin changes are found only on the alpha promoter even though the beta proximal promoter contains a similar E- and GC-box arrangement. Recruitment of HDAC1 is necessary and sufficient to cause repression of alpha proximal promoter activity, and the addition of Sp1 and/or Sp3 potentiates the repression. Our data suggest that suppression of the beta promoter is facilitated by Src-induced changes in the alpha promoter chromatinization mediated by a USF1-Sp1-Sp3 complex.

Mechanism of specific membrane targeting by C2 domains: localized pools of target lipids enhance Ca2+ affinity

The C2 domain is a ubiquitous, conserved protein signaling motif widely found in eukaryotic signaling proteins. Although considerable functional diversity exists, most C2 domains are activated by Ca2+ binding and then dock to a specific cellular membrane. The C2 domains of protein kinase Calpha (PKCalpha) and cytosolic phospholipase A2alpha (cPLA2alpha), for example, are known to dock to different membrane surfaces during an intracellular Ca2+ signal. Ca2+ activation targets the PKCalpha C2 domain to the plasma membrane and the cPLA2alpha C2 domain to the internal membranes, with no detectable spatial overlap. It is crucial to determine how such targeting specificity is achieved at physiological bulk Ca2+ concentrations that during a typical signaling event rarely exceed 1 muM. For the isolated PKCalpha C2 domain in the presence of physiological Ca2+ levels, the target lipids phosphatidylserine (PS) and phosphatidylinositol-4,5-bisphosphate (PIP2) are together sufficient to recruit the PKCalpha C2 domain to a lipid mixture mimicking the plasma membrane inner leaflet. For the cPLA2alpha C2 domain, the target lipid phosphatidylcholine (PC) appears to be sufficient to drive membrane targeting to an internal membrane mimic at physiological Ca2+ levels, although the results do not rule out a second, unknown target molecule. Stopped-flow kinetic studies provide additional information about the fundamental molecular events that occur during Ca2+-activated membrane docking. In principle, C2 domain-directed intracellular targeting, which requires coincidence detection of multiple signals (Ca2+ and one or more target lipids), can exhibit two different mechanisms: messenger-activated target affinity (MATA) and target-activated messenger affinity (TAMA). The C2 domains studied here both utilize the TAMA mechanism, in which the C2 domain Ca2+ affinity is too low to be activated by physiological Ca2+ signals in most regions of the cell. Only when the C2 domain nears its target membrane, which provides a high local concentration of target lipid, is the effective Ca2+ affinity increased by the coupled binding equilibrium to a level that enables substantial Ca2+ activation and target docking. Overall, the findings emphasize the importance of using physiological ligand concentrations in targeting studies because super-physiological concentrations can drive docking interactions even when an important targeting molecule is missing.

Src Docks to A-kinase Anchoring Protein Gravin, Regulating β2-Adrenergic Receptor Resensitization and Recycling

Gravin (AKAP12) is a membrane-associated scaffold that provides docking for protein kinases, phosphatases, and adaptor molecules obligate for resensitization and recycling of beta(2)-adrenergic receptors. Gravin binds to the cell membrane in a Ca(2+)-sensitive manner and to receptors through well characterized protein-protein interactions. Although the interaction of serine/threonine, cyclic AMP-dependent protein kinase with protein kinase A-anchoring proteins is well described and involves a kinase regulatory subunit binding domain in the C terminus of these proteins, far less is known about tyrosine kinase docking to members of this family of scaffolds. The non-receptor tyrosine kinase Src regulates resensitization of beta(2)-adrenergic receptors and docks to gravin. Gravin displays nine proline-rich domains distributed throughout the molecule. One class I ligand for Src homology domain 3 docking, found in the N terminus ((10)RXPXXP(15)) of gravin, is shown to bind Src. Binding of Src to gravin activates the intrinsic tyrosine kinase of Src. Mutagenesis/deletion of the class I ligand (P15A,P16A) on the N terminus of gravin abolishes both the docking of Src to gravin as well as the receptor resensitization and recycling catalyzed by gravin. The Src-binding peptide-(1-51) of gravin behaves as a dominant-negative for AKAP gravin regulation of receptor resensitization/recycling. The tyrosine kinase Src plays an essential role in the AKAP gravin-mediated receptor resensitization and recycling, an essential aspect of receptor biology.