Identification and characterization of myeloid translocation gene 16b as a novel a kinase anchoring protein in T lymphocytes

An A-Kinase Anchoring Protein (ACBD3) Coordinates Traffic-Induced PKA Activation At The Golgi

KDEL receptor (KDELR) is a key protein that recycles escaped ER resident proteins from the Golgi apparatus back to the ER and maintains a dynamic balance between these two organelles in the early secretory pathway. Studies have shown that this retrograde transport pathway is partly regulated by two KDELR-interacting proteins, Acyl-CoA-binding domain-containing 3 (ACBD3), and cyclic AMP-dependent protein kinase A (PKA). However, whether Golgi-localized ACBD3, which was first discovered as a PKA-anchoring protein in mitochondria, directly interacts with PKA at the Golgi and coordinates its signaling in Golgi-to-ER traffic has remained unclear. In this study, we showed that the GOLD domain of ACBD3 directly interacts with the regulatory subunit II (RII) of PKA and effectively recruits PKA holoenzyme to the Golgi. Forward trafficking of proteins from the ER triggers activation of PKA by releasing the catalytic subunit from RII. Furthermore, we determined that depletion of ACBD3 reduces the Golgi fraction of RII, resulting in moderate, but constitutive activation of PKA and KDELR retrograde transport, independent of cargo influx from the ER. Taken together, these data demonstrate that ACBD3 coordinates the protein secretory pathway at the Golgi by facilitating KDELR/PKA-containing protein complex formation.

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.

CG-NAP/Kinase Interactions Fine-Tune T Cell Functions

CG-NAP, also known as AKAP450, is an anchoring/adaptor protein that streamlines signal transduction in various cell types by localizing signaling proteins and enzymes with their substrates. Great efforts are being devoted to elucidating functional roles of this protein and associated macromolecular signaling complex. Increasing understanding of pathways involved in regulating T lymphocytes suggests that CG-NAP can facilitate dynamic interactions between kinases and their substrates and thus fine-tune T cell motility and effector functions. As a result, new binding partners of CG-NAP are continually being uncovered. Here, we review recent advances in CG-NAP research, focusing on its interactions with kinases in T cells with an emphasis on the possible role of this anchoring protein as a target for therapeutic intervention in immune-mediated diseases.

Genomic and transcriptomic changes complement each other in the pathogenesis of sporadic Burkitt lymphoma

Burkitt lymphoma (BL) is the most common B-cell lymphoma in children. Within the International Cancer Genome Consortium (ICGC), we performed whole genome and transcriptome sequencing of 39 sporadic BL. Here, we unravel interaction of structural, mutational, and transcriptional changes, which contribute to MYC oncogene dysregulation together with the pathognomonic IG-MYC translocation. Moreover, by mapping IGH translocation breakpoints, we provide evidence that the precursor of at least a subset of BL is a B-cell poised to express IGHA. We describe the landscape of mutations, structural variants, and mutational processes, and identified a series of driver genes in the pathogenesis of BL, which can be targeted by various mechanisms, including IG-non MYC translocations, germline and somatic mutations, fusion transcripts, and alternative splicing.

Burkitt lymphoma (BL) is the most common pediatric B-cell lymphoma. Here, within the International Cancer Genome Consortium, the authors performed whole genome and transcriptome sequencing of 39 sporadic BL, describing the landscape of mutations, structural variants, and mutational processes that underpin this disease how alterations on different cellular levels cooperate in deregulating key pathways and complexes.

The Molecular Basis for Specificity at the Level of the Protein Kinase a Catalytic Subunit

Assembly of multi enzyme complexes at subcellular localizations by anchoring- and scaffolding proteins represents a pivotal mechanism for achieving spatiotemporal regulation of cellular signaling after hormone receptor targeting [for review, see (1)]. In the 3' 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) signaling pathway it is generally accepted that specificity is secured at several levels. This includes at the first level stimulation of receptors coupled to heterotrimeric G proteins which through stimulation of adenylyl cyclase (AC) forms the second messenger cAMP. Cyclic AMP has several receptors including PKA. PKA is a tetrameric holoenzyme consisting of a regulatory (R) subunit dimer and two catalytic (C) subunits. The R subunit is the receptor for cAMP and compartmentalizes cAMP signals through binding to cell and tissue-specifically expressed A kinase anchoring proteins (AKAPs). The current dogma tells that in the presence of cAMP, PKA dissociates into an R subunit dimer and two C subunits which are free to phosphorylate relevant substrates in the cytosol and nucleus. The release of the C subunit has raised the question how specificity of the cAMP and PKA signaling pathway is maintained when the C subunit no longer is attached to the R subunit-AKAP complex. An increasing body of evidence points toward a regulatory role of the cAMP and PKA signaling pathway by targeting the C subunits to various C subunit binding proteins in the cytosol and nucleus. Moreover, recent identification of isoform specific amino acid sequences, motifs and three dimensional structures have together provided new insight into how PKA at the level of the C subunit may act in a highly isoform-specific fashion. Here we discuss recent understanding of specificity of the cAMP and PKA signaling pathway based on C subunit subcellular targeting as well as evolution of the C subunit structure that may contribute to the dynamic regulation of C subunit activity.

Emerging Roles of MTG16 in Cell-Fate Control of Hematopoietic Stem Cells and Cancer

MTG16 (myeloid translocation gene on chromosome 16) and its related proteins, MTG8 and MTGR1, define a small family of transcriptional corepressors. These corepressors share highly conserved domain structures yet have distinct biological functions and tissue specificity. In vivo studies have shown that, of the three MTG corepressors, MTG16 is uniquely important for the regulation of hematopoietic stem/progenitor cell (HSPC) proliferation and differentiation. Apart from this physiological function, MTG16 is also involved in carcinomas and leukemias, acting as the genetic target of loss of heterozygosity (LOH) aberrations in breast cancer and recurrent translocations in leukemia. The frequent involvement of MTG16 in these disease etiologies implies an important developmental role for this transcriptional corepressor. Furthermore, mounting evidence suggests that MTG16 indirectly alters the disease course of several leukemias via its regulatory interactions with a variety of pathologic fusion proteins. For example, a recent study has shown that MTG16 can repress not only wild-type E2A-mediated transcription, but also leukemia fusion protein E2A-Pbx1-mediated transcription, suggesting that MTG16 may serve as a potential therapeutic target in acute lymphoblastic leukemia expressing the E2A-Pbx1 fusion protein. Given that leukemia stem cells share similar regulatory pathways with normal HSPCs, studies to further understand how MTG16 regulates cell proliferation and differentiation could lead to novel therapeutic approaches for leukemia treatment.

Potential immunosuppressive effects of Escherichia coli O157:H7 experimental infection on the bovine host

BACKGROUND

Enterohaemorrhagic Escherichia coli (EHEC), like E. coli O157:H7 are frequently detected in bovine faecal samples at slaughter. Cattle do not show clinical symptoms upon infection, but for humans the consequences after consuming contaminated beef can be severe. The immune response against EHEC in cattle cannot always clear the infection as persistent colonization and shedding in infected animals over a period of months often occurs. In previous infection trials, we observed a primary immune response after infection which was unable to protect cattle from re-infection. These results may reflect a suppression of certain immune pathways, making cattle more prone to persistent colonization after re-infection. To test this, RNA-Seq was used for transcriptome analysis of recto-anal junction tissue and ileal Peyer's patches in nine Holstein-Friesian calves in response to a primary and secondary Escherichia coli O157:H7 infection with the Shiga toxin (Stx) negative NCTC12900 strain. Non-infected calves served as controls.

RESULTS

In tissue of the recto-anal junction, only 15 genes were found to be significantly affected by a first infection compared to 1159 genes in the ileal Peyer's patches. Whereas, re-infection significantly changed the expression of 10 and 17 genes in the recto-anal junction tissue and the Peyer's patches, respectively. A significant downregulation of 69 immunostimulatory genes and a significant upregulation of seven immune suppressing genes was observed.

CONCLUSIONS

Although the recto-anal junction is a major site of colonization, this area does not seem to be modulated upon infection to the same extent as ileal Peyer's patches as the changes in gene expression were remarkably higher in the ileal Peyer's patches than in the recto-anal junction during a primary but not a secondary infection. We can conclude that the main effect on the transcriptome was immunosuppression by E. coli O157:H7 (Stx-) due to an upregulation of immune suppressive effects (7/12 genes) or a downregulation of immunostimulatory effects (69/94 genes) in the ileal Peyer's patches. These data might indicate that a primary infection promotes a re-infection with EHEC by suppressing the immune function.

Molecular Mechanisms for cAMP-Mediated Immunoregulation in T cells - Role of Anchored Protein Kinase A Signaling Units

The cyclic AMP/protein kinase A (cAMP/PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells. 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. In the immune system, cAMP is a potent negative regulator of T cell receptor-mediated activation of effector T cells (Teff) acting through a proximal PKA/Csk/Lck pathway anchored via a scaffold consisting of the AKAP Ezrin holding PKA, the linker protein EBP50, and the anchoring protein phosphoprotein associated with glycosphingolipid-enriched microdomains holding Csk. As PKA activates Csk and Csk inhibits Lck, this pathway in response to cAMP shuts down proximal T cell activation. This immunomodulating pathway in Teff mediates clinically important responses to regulatory T cell (Treg) suppression and inflammatory mediators, such as prostaglandins (PGs), adrenergic stimuli, adenosine, and a number of other ligands. A major inducer of T cell cAMP levels is PG E2 (PGE2) acting through EP2 and EP4 prostanoid receptors. PGE2 plays a crucial role in the normal physiological control of immune homeostasis as well as in inflammation and cancer immune evasion. Peripherally induced Tregs express cyclooxygenase-2, secrete PGE2, and elicit the immunosuppressive cAMP pathway in Teff as one tumor immune evasion mechanism. Moreover, a cAMP increase can also be induced by indirect mechanisms, such as intercellular transfer between T cells. Indeed, Treg, known to have elevated levels of intracellular cAMP, may mediate their suppressive function by transferring cAMP to Teff through gap junctions, which we speculate could also be regulated by PKA/AKAP complexes. In this review, we present an updated overview on the influence of cAMP-mediated immunoregulatory mechanisms acting through localized cAMP signaling and the therapeutical increasing prospects of AKAPs disruptors in T-cell immune function.

Regulating small G protein signaling to coordinate axon adhesion and repulsion

Small GTPases play critical roles in diverse biological events including regulating both the cytoskeletal and adhesive properties of cells. The importance of small GTPases to these events stems from their ability to be turned on and off, respectively, by specific GEFs and GAPs. In neurons, for example, regulation of small GTPase activity by extracellular guidance cues controls axonal and dendritic process shape, extension and navigation. Here, we discuss recent findings that indicate a specific regulator of small GTPase signaling, the Plexin transmembrane GAP, is differentially controlled by specific extracellular cues to guide growing axons. In particular, Plexins are receptors for one of the largest families of axon guidance cues, Semaphorins and negatively regulate cell morphology and motility by serving as GAPs for Ras/Rap family GTPases. Recent observations reveal that Plexin's GAP activity is controlled by the cAMP-dependent protein kinase (PKA), which phosphorylates Plexin and generates a binding site for the phospho-serine/threonine binding protein 14-3-3ε. This PKA-mediated Plexin-14-3-3ε interaction prevents Plexin from associating with its GTPase substrate, and thus antagonizes Semaphorin signaling. We now further examine these interactions and how they provide a new logic by which axon guidance signaling pathways over-ride one another to steer growing axons. We also further explore how Plexin interacting proteins, including Ras, PKA and 14-3-3 may interact with the Plexin GAP domain. Our observations also further indicate that 14-3-3 proteins may have conserved roles in the regulation of GTPase activity.

14-3-3ε couples protein kinase A to semaphorin signaling and silences plexin RasGAP-mediated axonal repulsion

The biochemical means through which multiple signaling pathways are integrated in navigating axons is poorly understood. Semaphorins are among the largest families of axon guidance cues and utilize Plexin (Plex) receptors to exert repulsive effects on axon extension. However, Semaphorin repulsion can be silenced by other distinct cues and signaling cascades, raising questions of the logic underlying these events. We now uncover a simple biochemical switch that controls Semaphorin/Plexin repulsive guidance. Plexins are Ras/Rap family GTPase activating proteins (GAPs) and we find that the PlexA GAP domain is phosphorylated by the cAMP-dependent protein kinase (PKA). This PlexA phosphorylation generates a specific binding site for 14-3-3ε, a phospho-binding protein that we find to be necessary for axon guidance. These PKA-mediated Plexin-14-3-3ε interactions prevent PlexA from interacting with its Ras family GTPase substrate and antagonize Semaphorin repulsion. Our results indicate that these interactions switch repulsion to adhesion and identify a point of convergence for multiple guidance molecules.

Cyclic AMP-mediated immune regulation--overview of mechanisms of action in T cells

The canonical second messenger cAMP is well established as a potent negative regulator of T cell immune function. Through protein kinase A (PKA) it regulates T cell function at the level of transcription factors, members of the mitogen-activated protein kinase pathway, phospholipases (PLs), Ras homolog (Rho)A and proteins involved in the control of cell cycle progression. Type I PKA is the predominant PKA isoform in T cells. Furthermore, whereas type II PKA is located at the centrosome, type I PKA is anchored close to the T cell receptor (TCR) in lipid rafts by the Ezrin-ERM-binding phosphoprotein of 50 kDa (EBP50)-phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG) scaffold complex. The most TCR-proximal target for type I PKA is C-terminal Src kinase (Csk), which upon activation by raft recruitment and phosphorylation inhibits the Src family tyrosine kinases Lck and Fyn and thus functions to maintain T cell homeostasis. Recently, induction of cAMP levels in responder T cells has emerged as one of the mechanisms by which regulatory T (T(R)) cells execute their suppressive action. Thus, the cAMP-type I PKA-Csk pathway emerges as a putative target for therapeutic intervention in autoimmune disorders as well as in cancer, where T(R) cell-mediated suppression contributes to suboptimal local immune responses.

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.

Structure of the AML1-ETO NHR3-PKA(RIIα) complex and its contribution to AML1-ETO activity

AML1-ETO is the chimeric protein product of t(8;21) in acute myeloid leukemia. The ETO portion of the fusion protein includes the nervy homology region (NHR) 3 domain, which shares homology with A-kinase anchoring proteins and interacts with the regulatory subunit of type II cAMP-dependent protein kinase A (PKA(RIIα)). We determined the solution structure of a complex between the AML1-ETO NHR3 domain and PKA(RIIα). Based on this structure, a key residue in AML1-ETO for PKA(RIIα) association was mutated. This mutation did not disrupt AML1-ETO's ability to enhance the clonogenic capacity of primary mouse bone marrow cells or its ability to repress proliferation or granulocyte differentiation. Introduction of the mutation into AML1-ETO had minimal impact on in vivo leukemogenesis. Therefore, the NHR3-PKA(RIIα) protein interaction does not appear to significantly contribute to AML1-ETO's ability to induce leukemia.

Spatiotemporal control of cyclic AMP immunomodulation through the PKA-Csk inhibitory pathway is achieved by anchoring to an Ezrin-EBP50-PAG scaffold in effector T cells

A variety of immunoregulatory signals to effector T cells from monocytes, macrophages and regulatory T cells act through cyclic adenosine monophosphate. In the effector T cell, the protein kinase A (PKA) type I isoenzyme localizes to lipid rafts during T cell activation and modulates directly the proximal events that take place after engagement of the T cell receptor. The most proximal target for PKA phosphorylation is C-terminal Src kinase (Csk), which initiates a negative signal pathway that fine-tunes the T cell activation process. The A kinase anchoring protein Ezrin colocalizes PKA and Csk by forming a supramolecular signaling complex consisting of PKA, Ezrin, Ezrin/radixin/moesin (ERM) binding protein of 50 kDa (EBP50), phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (GEMs) (PAG) and Csk.

Myeloid translocation gene 16b is a dual A-kinase anchoring protein that interacts selectively with plexins in a phospho-regulated manner

The myeloid translocation gene (MTG) homologue Nervy associates with PlexinA on the plasma membrane, where it functions as an A-kinase anchoring protein (AKAP) to modulate plexin-mediated semaphorin signaling in Drosophila. Mammalian MTG16b is an AKAP found in immune cells where plexin-mediated semaphorin signaling regulates immune responses. This study provides the first evidence that MTG16b is a dual AKAP capable of binding plexins. These interactions are selective (PlexinA1 and A3 bind MTG, while PlexinB1 does not) and can be regulated by PKA-phosphorylation. Collectively, these data suggest a possible mechanism for the targeting and integration of adenosine 3',5'-cyclic monophosphate (cAMP) and semaphorin signaling in immune cells.

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.

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

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

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.

ABCD of the phosphodiesterase family: interaction and differential activity in COPD

Phosphodiesterases (PDEs) are important enzymes that hydrolyze the cyclic nucleotides adenosine 3'5'-cyclic monophosphate (cAMP) and guanosine 3'5'-cyclic monophosphate (cGMP) to their inactive 5' monophosphates. They are highly conserved across species and as well as their role in signal termination, they also have a vital role in intra-cellular localization of cyclic nucleotide signaling and integration of the cyclic nucleotide pathways with other signaling pathways. Because of their pivotal role in intracellular signaling, they are now of considerable interest as therapeutic targets in a wide variety diseases, including COPD where PDE inhibitors may have bronchodilator, anti-inflammatory and pulmonary vasodilator actions. This review examines the diversity and cellular localization of the isoforms of PDE, the known and speculative relevance of this to the treatment of COPD, and the range of PDE inhibitors in development together with a discussion of their possible role in treating COPD.

Inhibition of T cell activation by cyclic adenosine 5'-monophosphate requires lipid raft targeting of protein kinase A type I by the A-kinase anchoring protein ezrin

cAMP negatively regulates T cell immune responses by activation of type I protein kinase A (PKA), which in turn phosphorylates and activates C-terminal Src kinase (Csk) in T cell lipid rafts. Using yeast two-hybrid screening, far-Western blot, immunoprecipitation and immunofluorescense analyses, and small interfering RNA-mediated knockdown, we identified Ezrin as the A-kinase anchoring protein that targets PKA type I to lipid rafts. Furthermore, Ezrin brings PKA in proximity to its downstream substrate Csk in lipid rafts by forming a multiprotein complex consisting of PKA/Ezrin/Ezrin-binding protein 50, Csk, and Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains. The complex is initially present in immunological synapses when T cells contact APCs and subsequently exits to the distal pole. Introduction of an anchoring disruptor peptide (Ht31) into T cells competes with Ezrin binding to PKA and thereby releases the cAMP/PKA type I-mediated inhibition of T cell proliferation. Finally, small interfering RNA-mediated knockdown of Ezrin abrogates cAMP regulation of IL-2. We propose that Ezrin is essential in the assembly of the cAMP-mediated regulatory pathway that modulates T cell immune responses.

The scaffolding protein CG-NAP/AKAP450 is a critical integrating component of the LFA-1-induced signaling complex in migratory T cells

T cell migration represents a complex highly coordinated process involving participation of surface receptor/ligand interactions, cytoskeletal rearrangements, and phosphorylation-dependent signaling cascades. Members of the A-kinase anchoring protein (AKAP) family of giant scaffolding proteins can assemble and compartmentalize multiple signaling and structural molecules thereby providing a platform for their targeted positioning and efficient interactions. We characterize here the expression, intracellular distribution, and functional role of the scaffolding protein CG-NAP (centrosome and Golgi localized protein kinase N-associated protein)/AKAP450 in the process of active T cell motility induced via LFA-1 integrins. This protein is predominantly localized at the centrosome and Golgi complex. T cell locomotion triggered by LFA-1 ligation induces redistribution of CG-NAP/AKAP450 along microtubules in trailing cell extensions. Using an original in situ immunoprecipitation approach, we show that CG-NAP/AKAP450 is physically associated with LFA-1 in the multimolecular signaling complex also including tubulin and the protein kinase C beta and delta isoenzymes. CG-NAP/AKAP450 recruitment to this complex was specific for the T cells migrating on LFA-1 ligands, but not on the beta(1) integrin ligand fibronectin. Using the GFP-tagged C-terminal CG-NAP/AKAP450 construct, we demonstrate that expression of the intact CG-NAP/AKAP450 and its recruitment to the LFA-1-associated multimolecular complex is critically important for polarization and migration of T cells induced by this integrin.

The Role of Protein Kinase A Anchoring via the RIIα Regulatory Subunit in the Murine Immune System

Intracellular cAMP may inhibit T cell activation and proliferation via activation of the cAMP-dependent protein kinase, PKA. PKA signaling is maintained through interactions of the regulatory subunit with A-kinase anchoring proteins (AKAPs). We demonstrated that T cells contain AKAPs and now ask whether PKA anchoring to AKAPs via the RIIalpha regulatory subunit is necessary for cAMP-mediated inhibition of T cell activation. We studied the immune systems of mice lacking the RIIalpha regulatory subunit of PKA (-/-) and the ability of cells isolated from these mice to respond to cAMP. Dissection of spleen and thymus from wild-type (WT) and -/- mice, single cell suspensions generated from these organs, and flow cytometry analysis illustrate that the gross morphology, cell numbers, and cell populations in the spleen and thymus of the -/- mice are similar to WT controls. In vitro, splenocytes from -/- mice respond to anti-CD3/anti-CD28 and PMA/ionomycin stimulation and produce IL-2 similar to WT. Cytokine analysis revealed no significant difference in Th1 or Th2 differentiation. Finally, equivalent frequencies of CD8(+) IFN-gamma producing effector cells were stimulated upon infection of WT or -/- mice with Listeria monocytogenes. These data represent the first study of the role of RIIalpha in the immune system in vivo and provide evidence that T cell development, homeostasis, and the generation of a cell-mediated immune response are not altered in the RIIalpha -/- mice, suggesting either that RIIalpha is not required for normal immune function or that other proteins are able to compensate for RIIalpha function.

The MTG proteins: chromatin repression players with a passion for networking

The human myeloid translocation genes (MTGs) encode a family of proteins with a modular structure that can be traced to the Drosophila protein nervy. The nuclear MTGs can mediate the formation of complex protein networks among nuclear corepressors (Sin3a, N-CoR, SMRT), chromatin-modifying enzymes (histone deacetylases), and DNA-binding transcription factors. Hierarchical modulation of repression at target genes by MTG protein complexes is likely required for fine spatial and temporal gene regulation during development and differentiation. Genomic changes can disrupt these sophisticated protein networks and underlie novel pathogenic causes of cancer and neurodegeneration.

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.

A-kinase anchoring proteins interact with phosphodiesterases in T lymphocyte cell lines

The cAMP protein kinase A (PKA) pathway in T cells conveys an inhibitory signal to suppress inflammation. This study was performed to understand the mechanisms involved in cAMP-mediated signaling in T lymphocytes. A-kinase anchoring proteins (AKAPs) bind and target PKA to various subcellular locations. AKAPs also bind other signaling molecules such as cyclic nucleotide phosphodiesterases (PDEs) that hydrolyze cAMP in the cell. PDE4 and PDE7 have important roles in T cell activation. Based on this information, we hypothesized that AKAPs associate with PDEs in T lymphocytes. Immunoprecipitation of Jurkat cell lysates with Abs against both the regulatory subunit of PKA (RIIalpha) and specific AKAPs resulted in increased PDE activity associated with RIIalpha and AKAP95, AKAP149, and myeloid translocation gene (MTG) compared with control (IgG). Immunoprecipitation and pull-down analyses demonstrate that PDE4A binds to AKAP149, AKAP95, and MTG, but not AKAP79, whereas PDE7A was found to bind only MTG. Further analysis of MTG/PDE association illustrated that PDE4A and PDE7A bind residues 1-344 of MTG16b. Confocal analysis of HuT 78 cells stained with anti-PDE7A showed overlapping staining patterns with the Golgi marker GM130, suggesting that PDE7A is located in the Golgi. The staining pattern of PDE7A also showed similarity to the staining pattern of MTG, supporting the immunoprecipitation data and suggesting that MTG may interact with PDE7A in the Golgi. In summary, these data suggest that AKAPs interact with both PKA and PDE in T lymphocytes and thus are a key component of the signaling complex regulating T cell activation.

WAVE1, an A-kinase anchoring protein, during mammalian spermatogenesis

BACKGROUND

Proper compartmentalization of signalling cascades is paramount to many intracellular activities during spermatogenesis and sperm function. In the present study we focus on the A-kinase-anchoring protein (AKAP) WAVE1, a member of the Wiskott-Aldrich syndrome (WASP) family of adaptor proteins, to study its localization throughout mammalian spermatogenesis.

METHODS

Using transmission electron microscopy, immunocytochemistry and western blotting, we examined the distribution of WAVE1 and putative partners during mammalian spermatogenesis. The localization and association of PKA RII, the regulatory subunit II of protein kinase A, tyrosine kinase Abl, and small GTPase RAC1 were also explored.

RESULTS

WAVE1 localization in spermatocytes and round spermatids coincided with Golgi apparatus distribution, whereas in elongated spermatids and testicular sperm WAVE1 localized to the mitochondrial sheath. Following epididymal passage, WAVE1 was found exclusively on the mitochondrial sheath, suggesting that the protein may function in this region. WAVE1 and PKA RII co-localized along the mitochondrial sheath, PKA RII concentrates in the mid-piece, and RAC1 associated with the post-acrosomal region and the connecting piece. The distribution of WAVE1, PKA RII and RAC1 is conserved in mature mouse, bull, baboon and human sperm.

CONCLUSIONS

The data support the possibility of a functional signalling unit established by WAVE1 and its associated proteins in the mid-piece of maturing sperm.

Aberrant CBFA2T3B gene promoter methylation in breast tumors

Background

The CBFA2T3 locus located on the human chromosome region 16q24.3 is frequently deleted in breast tumors. CBFA2T3 gene expression levels are aberrant in breast tumor cell lines and the CBFA2T3B isoform is a potential tumor suppressor gene. In the absence of identified mutations to further support a role for this gene in tumorigenesis, we explored whether the CBFA2T3B promoter region is aberrantly methylated and whether this correlates with expression.

Results

Aberrant hypo and hypermethylation of the CBFA2T3B promoter was detected in breast tumor cell lines and primary breast tumor samples relative to methylation index interquartile ranges in normal breast counterpart and normal whole blood samples. A statistically significant inverse correlation between aberrant CBFA2T3B promoter methylation and gene expression was established.

Conclusion

CBFA2T3B is a potential breast tumor suppressor gene affected by aberrant promoter methylation and gene expression. The methylation levels were quantitated using a second-round real-time methylation-specific PCR assay. The detection of both hypo and hypermethylation is a technicality regarding the methylation methodology.

Association of sperm protein 17 with A-kinase anchoring protein 3 in flagella

BACKGROUND

Sperm protein 17 (Sp17) is a three-domain protein that contains: 1) a highly conserved N-terminal domain that is 45% identical to the human type II alpha regulatory subunit (RII alpha) of protein kinase A (PKA); 2) a central sulphated carbohydrate-binding domain; and 3) a C-terminal Ca++/calmodulin (CaM) binding domain. Although Sp17 was originally discovered and characterized in spermatozoa, its mRNA has now been found in a variety of normal mouse and human tissues. However, Sp17 protein is found predominantly in spermatozoa, cilia and human neoplastic cell lines. This study demonstrates that Sp17 from spermatozoa binds A-kinase anchoring protein 3 (AKAP3), confirming the functionality of the N-terminal domain.

METHODS

In this study in vitro precipitation and immunolocalization demonstrate that Sp17 binds to AKAP3 (AKAP110) in spermatozoa.

RESULTS

Sp17 is present in the head and tail of spermatozoa, in the tail it is in the fibrous sheath, which contains AKAP3 and AKAP4. Recombinant AKAP3 and AKAP4 RII binding domains were synthesized as glutathione S-transferase (GST) fusion proteins immobilized on glutathione-agarose resin and added to CHAPS extracts of human spermatozoa. Western blots of bound and eluted proteins probed with anti-Sp17 revealed that AKAP3 bound and precipitated a significant level of Sp17 while AKAP4 did not. AKAP4 binds AKAP3 and expression of AKAP3 is reduced in AKAP4 knockout sperm, therefore we tested AKAP4 knockout spermatozoa for Sp17 and found that there was a reduction in the amount of Sp17 expressed when compared to wild type spermatozoa. Co-localization of AKAP3 and Sp17 by immunofluorescence was demonstrated along the length of the principal piece of the flagella.

CONCLUSIONS

As predicted by its N-terminal domain that is 45% identical to the human RIIalpha of PKA, Sp17 from spermatozoa binds the RII binding domain of AKAP3 along the length of the flagella.

Neuronal microtubule-associated protein 2D is a dual a-kinase anchoring protein expressed in rat ovarian granulosa cells

A-kinase anchoring proteins (AKAPs) function to target protein kinase A (PKA) to specific locations within the cell. AKAPs are functionally identified by their ability to bind the type II regulatory subunits (RII) of PKA in an in vitro overlay assay. We previously showed that follicle-stimulating hormone (FSH) induces the expression of an 80-kDa AKAP (AKAP 80) in ovarian granulosa cells as they mature from a preantral to a preovulatory phenotype. In this report, we identify AKAP 80 as microtubule-associated protein 2D (MAP2D), a low molecular weight splice variant of the neuronal MAP2 protein. MAP2D is induced in granulosa cells by dexamethasone and by FSH in a time-dependent manner that mimics that of AKAP 80, and immunoprecipitation of MAP2D depletes extracts of AKAP 80. MAP2D is the only MAP2 protein present in ovaries and is localized to granulosa cells of preovulatory follicles and to luteal cells. MAP2D is concentrated at the Golgi apparatus along with RI and RII and, based on coimmunoprecipitation results, appears to bind both RI and RII in granulosa cells. Reduced expression of MAP2D resulting from treatment of granulosa cells with antisense oligonucleotides to MAP2 inhibited the phosphorylation of cAMP-response element-binding protein. These results suggest that this classic neuronal RII AKAP is a dual RI/RII AKAP that performs unique functions in ovarian granulosa cells that contribute to the preovulatory phenotype.

Nervy links protein kinase a to plexin-mediated semaphorin repulsion

Cyclic nucleotides regulate axonal responses to a number of guidance cues through unknown molecular events. We report here that Drosophila nervy, a member of the myeloid translocation gene family of A kinase anchoring proteins (AKAPs), regulates repulsive axon guidance by linking the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) to the Semaphorin 1a (Sema-1a) receptor Plexin A (PlexA). Nervy and PKA antagonize Sema-1a-PlexA-mediated repulsion, and the AKAP binding region of Nervy is critical for this effect. Thus, Nervy couples cAMP-PKA signaling to PlexA to regulate Sema-1a-mediated axonal repulsion, revealing a simple molecular mechanism that allows growing axons to integrate inputs from multiple guidance cues.

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.

Impact of cAMP on the T-cell response to type II collagen

There is considerable interest in the possible use of cAMP-elevating agents in the treatment of autoimmune diseases such as rheumatoid arthritis. The objective of this study was to evaluate the impact of different cAMP-elevating agents on the T-cell response to type II collagen within the context of collagen-induced arthritis, a murine model of rheumatoid arthritis. Spleen cells or lymph node cells from type-II-collagen-immunized DBA/1 mice were cultured in the presence of type II collagen plus one of five different cAMP-elevating agents: rolipram, forskolin, prostaglandin E2, 8-bromo-cAMP, or cholera toxin. Levels of interferon-gamma (IFN-gamma), interleukin-4 (IL-4) and IL-5 were measured in culture supernatants by enzyme-linked immunosorbent assay. All of the cAMP-elevating agents tested were found to profoundly suppress IFN-gamma production in a dose-dependent manner. IL-4 and IL-5 production was slightly up-regulated at low concentrations of the cAMP-elevating agents and was modestly suppressed at the highest concentrations of cAMP-elevating agents. Experiments were then carried out to determine whether T cells were directly affected by cAMP-elevating agents or whether the immunomodulatory effects were mediated via antigen-presenting cells. Pulsing T cells alone for a brief period with cholera toxin produced an almost identical effect to pulsing antigen-presenting cells alone, i.e. down-regulation of proliferation, down-regulation of IFN-gamma production with little effect on IL-5 production. It was concluded that cAMP-elevating agents suppressed T helper type 1 responses to type II collagen to a greater extent than T helper type 2 responses. The cAMP-elevating agents could directly influence the activity of T cells but, in addition, influenced the ability of antigen-presenting cells to support T helper type 1 responses.

A-kinase anchoring protein 3 messenger RNA expression in ovarian cancer and its implication on prognosis

A-kinase anchoring protein 3 (AKAP3) is a sperm protein and its expression appears to be restricted to the testis in normal adult tissues. We investigated AKAP3 mRNA expression in 20 normal ovaries and 54 ovarian cancers of different histological types, grades and stages by reverse transcription-polymerase chain reaction (RT-PCR). The PCR products were analyzed by conventional agarose gel electrophoresis and capillary electrophoresis on a microtip device to determine the expression semiquantitatively. Little or no expression was observed in the 20 normal ovarian specimens. High AKAP3 mRNA expression was observed in 15 ovarian cancer specimens (28 %). The expression was correlated with the histological grade and clinical stage. AKAP3 mRNA was observed at a significantly higher frequency in poorly differentiated (p = 0.009) and advanced stage (III and IV, p = 0.014) tumors. No correlation was found between AKAP3 mRNA expression and other variables. In Cox multivariate analysis, AKAP3 mRNA expression was found to be a significant predictor of both overall and progression-free survival in patients with poorly differentiated tumors.

Protein kinase A-anchoring (AKAP) domains in brefeldin A-inhibited guanine nucleotide-exchange protein 2 (BIG2)

Like other guanine nucleotide-exchange proteins (GEPs) that activate ADP-ribosylation factor (ARF) GTPases, brefeldin A-inhibited GEP2, BIG2, contains an approximately 200-aa Sec7 domain that is responsible for this catalytic activity and its inhibition by brefeldin A. The Sec7 domain is located near the center of the molecule and serves to accelerate replacement of GDP bound to ARF with GTP. To explore possible functions of the N-terminal region of BIG2 (1-832), we used three coding-region constructs as bait to screen a human heart cDNA library in a yeast two-hybrid system, retrieving two unique clones that encode a type I protein kinase A (PKA) regulatory subunit, RI alpha. Coimmunoprecipitation experiments confirmed interaction of in vitro translated BIG2 and RI alpha, as well as of the endogenous proteins in cytosol of cultured HepG2 cells. Using 28 deletion mutants, we found three regions of BIG2 that interacted with R subunits of PKA. Residues 27-48 (domain A) interacted with RI alpha and RI beta, 284-301 (domain B) interacted with RII alpha and RII beta, and 517-538 (domain C) interacted with RI alpha, RII alpha, and RII beta. Sequence analysis and helical wheel projection of amino acids in the three domains revealed potential amphipathic wheel structures characteristic for binding of PKA R subunits. Western blot analysis of subcellular fractions demonstrated translocation of BIG2 (and BIG1) from cytosol to the Golgi and other membrane structures after incubation of cells with 8-Br-cAMP or forskolin. All findings are consistent with a role for BIG2 as an A kinase-anchoring protein (or AKAP) that could coordinate cAMP and ARF regulatory pathways.

Cutting edge: A-kinase anchor proteins are involved in maintaining resting T cells in an inactive state

A-kinase anchor proteins (AKAPs) target protein kinase A (PKA) to different subcellular locations and are thought to play important roles in the cAMP signaling pathway. The aims of this study were to determine whether T cells express AKAPs and, if so, to establish their physiological significance. CD4(+) T cells were found to express eight AKAPs. Disruption of the AKAP-PKA interaction caused high levels of IL-2, IL-4, IL-5, and IFN-gamma production in the absence of stimulation via CD3epsilon and CD28 molecules. Disruption of the AKAP-PKA interaction acted synergistically with suboptimal doses of Ag in boosting proliferative responses of T cells. Finally, disruption of the AKAP-PKA interaction rendered T cells insensitive to cAMP-elevating agents. It was concluded that AKAPs, through their association with PKA, are involved in maintaining T cell homeostasis and in regulating the sensitivity of T cells to incoming cAMP signals.