Signal integration through blending, bolstering and bifurcating of intracellular information

The cAMP-PKA signalling crosstalks with CWI and HOG-MAPK pathways in yeast cell response to osmotic and thermal stress

The yeast Saccharomyces cerevisiae is widely used in food and non-food industries. During industrial fermentation yeast strains are exposed to fluctuations in oxygen concentration, osmotic pressure, pH, ethanol concentration, nutrient availability and temperature. Fermentation performance depends on the ability of the yeast strains to adapt to these changes. Suboptimal conditions trigger responses to the external stimuli to allow homeostasis to be maintained. Stress-specific signalling pathways are activated to coordinate changes in transcription, translation, protein function, and metabolic fluxes while a transient arrest of growth and cell cycle progression occur. cAMP-PKA, HOG-MAPK and CWI signalling pathways are turned on during stress response. Comprehension of the mechanisms involved in the responses and in the adaptation to these stresses during fermentation is key to improving this industrial process. The scope of this review is to outline the advancement of knowledge about the cAMP-PKA signalling and the crosstalk of this pathway with the CWI and HOG-MAPK cascades in response to the environmental challenges heat and hyperosmotic stress.

A Unified Description of Salt Effects on the Liquid-Liquid Phase Separation of Proteins

Protein aggregation via liquid-liquid phase separation (LLPS) is ubiquitous in nature and is intimately connected to many human diseases. Although it is widely known that the addition of salt has crucial impacts on the LLPS of proteins, full understanding of the salt effects remains an outstanding challenge. Here, we develop a molecular theory that systematically incorporates the self-consistent field theory for charged macromolecules into the solution thermodynamics. The electrostatic interaction, hydrophobicity, ion solvation, and translational entropy are included in a unified framework. Our theory fully captures the long-standing puzzles of the nonmonotonic salt concentration dependence and the specific ion effect. We find that proteins show salting-out at low salt concentrations due to ionic screening. The solubility follows the inverse Hofmeister series. In the high salt concentration regime, protein continues salting-out for small ions but turns to salting-in for larger ions, accompanied by the reversal of the Hofmeister series. We reveal that the solubility at high salt concentrations is determined by the competition between the solvation energy and translational entropy of the ion. Furthermore, we derive an analytical criterion for determining the boundary between the salting-in and salting-out regimes, which is in good agreement with experimental results for various proteins and salt ions.

Protein Kinase A in Human Retina: Differential Localization of Cβ, Cα, RIIα, and RIIβ in Photoreceptors Highlights Non-redundancy of Protein Kinase A Subunits

Protein kinase A (PKA) signaling is essential for numerous processes but the subcellular localization of specific PKA regulatory (R) and catalytic (C) subunits has yet to be explored comprehensively. Additionally, the localization of the Cβ subunit has never been spatially mapped in any tissue even though ∼50% of PKA signaling in neuronal tissues is thought to be mediated by Cβ. Here we used human retina with its highly specialized neurons as a window into PKA signaling in the brain and characterized localization of PKA Cα, Cβ, RIIα, and RIIβ subunits. We found that each subunit presented a distinct localization pattern. Cα and Cβ were localized in all cell layers (photoreceptors, interneurons, retinal ganglion cells), while RIIα and RIIβ were selectively enriched in photoreceptor cells where both showed distinct patterns of co-localization with Cα but not Cβ. Only Cα was observed in photoreceptor outer segments and at the base of the connecting cilium. Cβ in turn, was highly enriched in mitochondria and was especially prominent in the ellipsoid of cone cells. Further investigation of Cβ using RNA BaseScope technology showed that two Cβ splice variants (Cβ4 and Cβ4ab) likely code for the mitochondrial Cβ proteins. Overall, our data indicates that PKA Cα, Cβ, RIIα, and RIIβ subunits are differentially localized and are likely functionally non-redundant in the human retina. Furthermore, Cβ is potentially important for mitochondrial-associated neurodegenerative diseases previously linked to PKA dysfunction.

Neural tube closure requires the endocytic receptor Lrp2 and its functional interaction with intracellular scaffolds

Pathogenic mutations in the endocytic receptor LRP2 in humans are associated with severe neural tube closure defects (NTDs) such as anencephaly and spina bifida. Here, we have combined analysis of neural tube closure in mouse and in the African Clawed Frog Xenopus laevis to elucidate the etiology of Lrp2-related NTDs. Lrp2 loss of function impaired neuroepithelial morphogenesis, culminating in NTDs that impeded anterior neural plate folding and neural tube closure in both model organisms. Loss of Lrp2 severely affected apical constriction as well as proper localization of the core planar cell polarity (PCP) protein Vangl2, demonstrating a highly conserved role of the receptor in these processes, which are essential for neural tube formation. In addition, we identified a novel functional interaction of Lrp2 with the intracellular adaptor proteins Shroom3 and Gipc1 in the developing forebrain. Our data suggest that, during neurulation, motifs within the intracellular domain of Lrp2 function as a hub that orchestrates endocytic membrane removal for efficient apical constriction, as well as PCP component trafficking in a temporospatial manner.

Insulin's Discovery: New Insights on Its Hundredth Birthday: From Insulin Action and Clearance to Sweet Networks

In 2021, the 100th anniversary of the isolation of insulin and the rescue of a child with type 1 diabetes from death will be marked. In this review, we highlight advances since the ingenious work of the four discoverers, Frederick Grant Banting, John James Rickard Macleod, James Bertram Collip and Charles Herbert Best. Macleoad closed his Nobel Lecture speech by raising the question of the mechanism of insulin action in the body. This challenge attracted many investigators, and the question remained unanswered until the third part of the 20th century. We summarize what has been learned, from the discovery of cell surface receptors, insulin action, and clearance, to network and precision medicine.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions.

cAMP-PKA signal transduction specificity in Saccharomyces cerevisiae

Living cells have developed a set of complex signaling responses, which allow them to withstand different environmental challenges. Signaling pathways enable the cell to monitor external and internal states and to articulate the appropriate physiological responses. Cellular signal transmission requires the dynamic formation of spatiotemporal controlled molecular interactions. One of the most important signaling circuits in Saccharomyces cerevisiae is the one controlled by cAMP-Protein Kinase A (PKA). In budding yeast, extracellular glucose and a plethora of signals related with growth and stress conditions regulate the intracellular cAMP levels that modulate PKA activity which in turn regulates a broad range of cellular processes. The cAMP-PKA signaling output requires a controlled specificity of the PKA responses. In this review we discuss the molecular mechanisms that are involved in the establishment of the specificity in the cAMP-PKA signaling pathway in S.cerevisiae.

Hedgehog and Gpr161: Regulating cAMP Signaling in the Primary Cilium

Compartmentalization of diverse types of signaling molecules contributes to the precise coordination of signal propagation. The primary cilium fulfills this function by acting as a spatiotemporally confined sensory signaling platform. For the integrity of ciliary signaling, it is mandatory that the ciliary signaling pathways are constantly attuned by alterations in both oscillating small molecules and the presence or absence of their sensor/effector proteins. In this context, ciliary G protein-coupled receptor (GPCR) pathways participate in coordinating the mobilization of the diffusible second messenger molecule 3',5'-cyclic adenosine monophosphate (cAMP). cAMP fluxes in the cilium are primarily sensed by protein kinase A (PKA) complexes, which are essential for the basal repression of Hedgehog (Hh) signaling. Here, we describe the dynamic properties of underlying signaling circuits, as well as strategies for second messenger compartmentalization. As an example, we summarize how receptor-guided cAMP-effector pathways control the off state of Hh signaling. We discuss the evidence that a macromolecular, ciliary-localized signaling complex, composed of the orphan GPCR Gpr161 and type I PKA holoenzymes, is involved in antagonizing Hh functions. Finally, we outline how ciliary cAMP-linked receptor pathways and cAMP-sensing signalosomes may become targets for more efficient combinatory therapy approaches to counteract dysregulation of Hh signaling.

An Integrative Systems Biology and Experimental Approach Identifies Convergence of Epithelial Plasticity, Metabolism, and Autophagy to Promote Chemoresistance

The evolution of therapeutic resistance is a major cause of death for cancer patients. The development of therapy resistance is shaped by the ecological dynamics within the tumor microenvironment and the selective pressure of the host immune system. These selective forces often lead to evolutionary convergence on pathways or hallmarks that drive progression. Thus, a deeper understanding of the evolutionary convergences that occur could reveal vulnerabilities to treat therapy-resistant cancer. To this end, we combined phylogenetic clustering, systems biology analyses, and molecular experimentation to identify convergences in gene expression data onto common signaling pathways. We applied these methods to derive new insights about the networks at play during transforming growth factor-β (TGF-β)-mediated epithelial–mesenchymal transition in lung cancer. Phylogenetic analyses of gene expression data from TGF-β-treated cells revealed convergence of cells toward amine metabolic pathways and autophagy during TGF-β treatment. Knockdown of the autophagy regulatory, ATG16L1, re-sensitized lung cancer cells to cancer therapies following TGF-β-induced resistance, implicating autophagy as a TGF-β-mediated chemoresistance mechanism. In addition, high ATG16L expression was found to be a poor prognostic marker in multiple cancer types. These analyses reveal the usefulness of combining evolutionary and systems biology methods with experimental validation to illuminate new therapeutic vulnerabilities for cancer.

A New T. gondii Mouse Model of Gene-Environment Interaction Relevant to Psychiatric Disease

Infection with the protozoan parasite, Toxoplasma gondii (T. gondii), was linked to several psychiatric disorders. The exact mechanisms of a hypothesized contribution of T. gondii infection are poorly understood, and it appears that only a subset of seropositive individuals go on to develop a mental illness, suggesting genetic vulnerability. In order to stimulate mechanistic studies of how exposure to T. gondii could interact with genetic predisposition to psychiatric disorders, we have generated and characterized a mouse model of chronic T. gondii infection in BALB/c mice with inducible forebrain neuronal expression of a C-terminus truncated dominant-negative form of disrupted-in-schizophrenia 1 (DN-DISC1). In this gene-environment interaction (GxE) model, exposing control and DN-DISC1 male and female mice to T. gondii produced sex-dependent abnormalities in locomotor activity and prepulse inhibition of the acoustic startle. No genotype- or sex-dependent effects were found on levels of anti-Toxoplasma IgG antibodies or anti-NMDAR or C1q antibodies. Our work demonstrates that a psychiatric genetic risk factor, DN-DISC1, modulates the neurobehavioral effects of chronic T. gondii infection in a sex-dependent manner. The present T. gondii model of GxE provides a valuable experimental system for future mechanistic studies and evaluation of new treatments.

AKAP-Lbc mediates protection against doxorubicin-induced cardiomyocyte toxicity

Doxorubicin (DOX) is a chemotherapic agent that is widely used to treat hematological and solid tumors. Despite its efficacy, DOX displays significant cardiac toxicity associated with cardiomyocytes death and heart failure. Cardiac toxicity is mainly associated with the ability of DOX to alter mitochondrial function. The current lack of treatments to efficiently prevent DOX cardiotoxicity underscores the need of new therapeutic approaches. Our current findings show that stimulation of cardiomyocytes with the α1-adrenergic receptor (AR) agonist phenylephrine (PE) significantly inhibits the apoptotic effect of DOX. Importantly, our results indicate that AKAP-Lbc is critical for transducing protective signals downstream of α1-ARs. In particular, we could show that suppression of AKAP-Lbc expression by infecting primary cultures of ventricular myocytes with lentiviruses encoding AKAP-Lbc specific short hairpin (sh) RNAs strongly impairs the ability of PE to reduce DOX-induced apoptosis. AKAP-Lbc-mediated cardiomyocyte protection requires the activation of anchored protein kinase D1 (PKD1)-dependent prosurvival pathways that promote the expression of the anti-apoptotic protein Bcl2 and inhibit the translocation of the pro-apoptotic protein Bax to mitochondria. In conclusion, AKAP-Lbc emerges as a coordinator of signals that protect cardiomyocytes against the toxic effects of DOX.

The many faces of compartmentalized PKA signalosomes

Cellular signal transmission requires the dynamic formation of spatiotemporally controlled molecular interactions. At the cell surface information is received by receptor complexes and relayed through intracellular signaling platforms which organize the actions of functionally interacting signaling enzymes and substrates. The list of hormone or neurotransmitter pathways that utilize the ubiquitous cAMP-sensing protein kinase A (PKA) system is expansive. This requires that the specificity, duration, and intensity of PKA responses are spatially and temporally restricted. Hereby, scaffolding proteins take the center stage for ensuring proper signal transmission. They unite second messenger sensors, activators, effectors, and kinase substrates within cellular micro-domains to precisely control and route signal propagation. A-kinase anchoring proteins (AKAPs) organize such subcellular signalosomes by tethering the PKA holoenzyme to distinct cell compartments. AKAPs differ in their modular organization showing pathway specific arrangements of interaction motifs or domains. This enables the cell- and compartment- guided assembly of signalosomes with unique enzyme composition and function. The AKAP-mediated clustering of cAMP and other second messenger sensing and interacting signaling components along with functional successive enzymes facilitates the rapid and precise dissemination of incoming signals. This review article delineates examples for different means of PKA regulation and for snapshots of compartmentalized PKA signalosomes.

Systematic Quantification of GPCR/cAMP-Controlled Protein Kinase A Interactions

The diffusible second messenger cyclic AMP (cAMP) originates from multiple G protein-coupled receptor (GPCR) cascades activating the intracellular key effector protein kinase A (PKA). Spatially and temporally restricted cAMP-fluxes are directly sensed by macromolecular PKA complexes. The consequences are alterations of molecular interactions, which lead to activation of compartmentalized PKA phosphotransferase activities, regulating a vast array of cellular functions. To decode cell-type and cell-compartment specific PKA functions, the spatio-temporal dynamics of small molecule:protein interactions, protein:protein interactions (PPIs), cAMP-mobilization, and phosphotransferase activities need to be determined directly in the appropriate cellular context. A collection of cell-based reporters has been developed to either visualize or quantitatively measure kinase activities or PKA complex formation/dissociation. In this review, we list a collection of unimolecular and bimolecular PKA biosensors, followed by the specification of the modular design of a Renilla luciferase based protein-fragment complementation assay (PCA) platform for measuring PKA network interactions. We discuss the application spectrum of the PCA reporter to identify, quantify, and dissect dynamic and transient PKA complexes downstream of specific GPCR activities. We specify the implementation of a PCA PKA platform to systematically quantify the concurrent involvement of receptor-cAMP signaling, post-translational modifications, and kinase subunit mutations/perturbations in PKA activation. The systematic quantification of transient PKA network interactions will contribute to a better understanding how GPCR-recognized input signals are streamlined through the compartmentalized and cAMP-interacting PKA signalosome.

Cooperation and crosstalk in axon guidance cue integration: Additivity, synergy, and fine-tuning in combinatorial signaling

Neural circuit development involves the coordinated growth and guidance of axons to their targets. Following the identification of many guidance cue molecules, recent experiments have focused on the interactions of their signaling cascades, which can be generally classified as additive or non-additive depending on the signal convergence point. While additive (parallel) signaling suggests limited molecular interaction between the pathways, non-additive signaling involves crosstalk between pathways and includes more complex synergistic, hierarchical, and permissive guidance cue relationships. Here the authors have attempted to classify recent studies that describe axon guidance signal integration according to these divisions. They also discuss the mechanistic implications of such interactions, as well as general ideas relating signal integration to the generation of diversity of axon guidance responses. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 891-904, 2017.

Liquid-liquid phase separation in cellular signaling systems

Liquid-liquid demixing or phase separation of protein with RNA is now recognized to be a key part of the mechanism for assembly of ribonucleoprotein granules. Cellular signaling also appears to employ phase separation as a mechanism for amplification or control of signal transduction both within the cytoplasm and at the membrane. The concept of receptor clustering, identified more than 3 decades ago, is now being examined through the lens of phase separation leading to new insights. Intrinsically disordered proteins or regions are central to these processes owing to their flexibility and accessibility for dynamic protein-protein interactions and post-translational modifications. We review some recent examples, examine the mechanisms driving phase separation and delineate the implications for signal transduction systems.

eQTL networks unveil enriched mRNA master integrators downstream of complex disease-associated SNPs

The causal and interplay mechanisms of Single Nucleotide Polymorphisms (SNPs) associated with complex diseases (complex disease SNPs) investigated in genome-wide association studies (GWAS) at the transcriptional level (mRNA) are poorly understood despite recent advancements such as discoveries reported in the Encyclopedia of DNA Elements (ENCODE) and Genotype-Tissue Expression (GTex). Protein interaction network analyses have successfully improved our understanding of both single gene diseases (Mendelian diseases) and complex diseases. Whether the mRNAs downstream of complex disease genes are central or peripheral in the genetic information flow relating DNA to mRNA remains unclear and may be disease-specific. Using expression Quantitative Trait Loci (eQTL) that provide DNA to mRNA associations and network centrality metrics, we hypothesize that we can unveil the systems properties of information flow between SNPs and the transcriptomes of complex diseases. We compare different conditions such as naïve SNP assignments and stringent linkage disequilibrium (LD) free assignments for transcripts to remove confounders from LD. Additionally, we compare the results from eQTL networks between lymphoblastoid cell lines and liver tissue. Empirical permutation resampling (p<0.001) and theoretic Mann-Whitney U test (p<10(-30)) statistics indicate that mRNAs corresponding to complex disease SNPs via eQTL associations are likely to be regulated by a larger number of SNPs than expected. We name this novel property mRNA hubness in eQTL networks, and further term mRNAs with high hubness as master integrators. mRNA master integrators receive and coordinate the perturbation signals from large numbers of polymorphisms and respond to the personal genetic architecture integratively. This genetic signal integration contrasts with the mechanism underlying some Mendelian diseases, where a genetic polymorphism affecting a single protein hub produces a divergent signal that affects a large number of downstream proteins. Indeed, we verify that this property is independent of the hubness in protein networks for which these mRNAs are transcribed. Our findings provide novel insights into the pleiotropy of mRNAs targeted by complex disease polymorphisms and the architecture of the information flow between the genetic polymorphisms and transcriptomes of complex diseases.

Analysis of AKAP7γ Dimerization

A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that contribute to spatiotemporal regulation of PKA-mediated phosphorylation events. In particular, AKAP7 is a family of alternatively spliced proteins that participates in cardiac calcium dynamics. Here, we demonstrate via pull-down from transfected cells and by direct protein-protein association that AKAP7γ self-associates. Self-association appears to be an isoform specific phenomenon, as AKAP7α did not associate with itself or with AKAP7γ. However, AKAP7γ did associate with AKAP7δ, suggesting the long isoforms of the AKAP can form heterodimers. Surface plasmon resonance found that the AKAP7γ self-association occurs via two high affinity binding sites with K D values in the low nanomolar range. Mapping of the binding sites by peptide array reveals that AKAP7γ interacts with itself through multiple regions. Photon counting histogram analysis (PCH) of AKAP7γ-EGFP expressed in HEK-293 cells confirmed that AKAP7γ-EGFP self-associates in a cellular context. Lastly, computational modeling of PKA dynamics within AKAP7γ complexes suggests that oligomerization may augment phosphorylation of scaffolded PKA substrates. In conclusion, our study reveals that AKAP7γ forms both homo- and heterodimers with the long isoforms of the AKAP and that this phenomenon could be an important step in mediating effective substrate phosphorylation in cellular microdomains.

Regio- and Stereocontrolled Dieckmann Approach to Treprostinil-Inspired, Polycyclic Scaffold For Building Macrocyclic Diversity

We developed a regio- and stereocontrolled Dieckmann cyclization approach to the synthesis of a novel, natural-product-like scaffold that was inspired from treprostinil (UT-15). This was further utilized in a diversity-based, 15-membered macrocyclic synthesis of two different sets of hybrid compounds. The amino acid moiety embedded in the macrocyclic skeleton allow exploring various chiral side chain groups within the ring.

Cyclic nucleotide phosphodiesterases (PDEs): coincidence detectors acting to spatially and temporally integrate cyclic nucleotide and non-cyclic nucleotide signals

The cyclic nucleotide second messengers cAMP and cGMP each affect virtually all cellular processes. Although these hydrophilic small molecules readily diffuse throughout cells, it is remarkable that their ability to activate their multiple intracellular effectors is spatially and temporally selective. Studies have identified a critical role for compartmentation of the enzymes which hydrolyse and metabolically inactivate these second messengers, the PDEs (cyclic nucleotide phosphodiesterases), in this specificity. In the present article, we describe several examples from our work in which compartmentation of selected cAMP- or cGMP-hydrolysing PDEs co-ordinate selective activation of cyclic nucleotide effectors, and, as a result, selectively affect cellular functions. It is our belief that therapeutic strategies aimed at targeting PDEs within these compartments will allow greater selectivity than those directed at inhibiting these enzymes throughout the cells.

CD44 is a multidomain signaling platform that integrates extracellular matrix cues with growth factor and cytokine signals

The reception and integration of the plethora of signals a cell receives from its microenvironment is decisive in determining cell behavior. Perturbation of extracellular cues, or an inappropriate response to or integration of these signals lies at the root of many diseases such as cancer. The transmembrane protein CD44 contributes to the reception of a broad variety of microenvironmental components, including extracellular matrix constituents such as hyaluronic acid, as well as growth factors and cytokines. In this chapter, we review the range of extracellular cues that are recognized by CD44, and show how CD44 serves to integrate this information at several levels through the mechanisms by which it contributes to transduction of these various microenvironmental signals.

Differential roles of signal transduction mechanisms in long-term potentiation of excitatory synaptic transmission induced by activation of group I mGluRs in the spinal trigeminal subnucleus oralis

Group I metabotropic glutamate receptors (mGluR1 and 5) have been implicated in long-term potentiation (LTP), a persistent increase of synaptic efficiency, in the central nervous system including the spinal trigeminal nucleus (Vsp). In the ascending pathway from the caudalis (Vc) to the oralis (Vo) subnuclus in Vsp, it has been shown that the activation of group I mGluRs (mGluR1 and 5) with their agonist (S)-3,5-dihydroxyphenylglycine (DHPG) produces a delayed type of LTP of excitatory synaptic transmission and this LTP was mediated by mGluR1. Further, this study attempts to pharmacologically characterize essential signaling components for the expression of DHPG-induced LTP. As a result, it is found that the group I mGluRs essentially use G protein-mediated activation of the phospholipase C (PLC) pathway to express the LTP. However, recruited signaling molecules following the activation of PLC are differentially involved in the expression of LTP: i.e. IP3 receptor, intracellular Ca(2+) rise, CaMKII and ERK function as positive regulators, whereas PKC as a negative regulator. Furthermore, both L-type voltage-dependent Ca(2+) channel and canonical transient receptor potential channel positively contribute to the expression of LTP. Taken together, these results suggest that signaling molecules recruited by the activation of group I mGluRs collaboratively or oppositely control the optimal expression of synaptic plasticity at excitatory synapses in the Vo.

Advances in targeting cyclic nucleotide phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) catalyse the hydrolysis of cyclic AMP and cyclic GMP, thereby regulating the intracellular concentrations of these cyclic nucleotides, their signalling pathways and, consequently, myriad biological responses in health and disease. Currently, a small number of PDE inhibitors are used clinically for treating the pathophysiological dysregulation of cyclic nucleotide signalling in several disorders, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication and chronic obstructive pulmonary disease. However, pharmaceutical interest in PDEs has been reignited by the increasing understanding of the roles of individual PDEs in regulating the subcellular compartmentalization of specific cyclic nucleotide signalling pathways, by the structure-based design of novel specific inhibitors and by the development of more sophisticated strategies to target individual PDE variants.

G-protein stimulatory subunit alpha and Gq/11α G-proteins are both required to maintain quiescent stem-like chondrocytes

Round chondrocytes in the resting zone of the growth plate provide precursors for columnar chondrocytes and have stem-like properties. Here we demonstrate that these stem-like chondrocytes undergo apoptosis in the absence of the receptor (PPR) for parathyroid hormone-related protein. We examine the possible roles of heterotrimeric G-proteins activated by the PPR. Inactivation of the G-protein stimulatory α-subunit (G(s)α) leads to accelerated differentiation of columnar chondrocytes, as seen in the PPR knockout, but a remnant of growth cartilage remains, in contrast to disappearance of the growth cartilage in the PPR knockout. Stem-like chondrocytes lose their quiescence and proliferate upon G(s)α ablation. Inactivation of G(s)α in mice with a mutant PPR that cannot activate G proteins, Gq and G11, leads to a PPR knockout-like phenotype. Thus, G(s)α is the major mediator of the anti-differentiation action of the PPR, while activation of both G(s)α and Gq/11α is required for quiescence of stem-like chondrocytes.

Clinical and molecular genetics of the phosphodiesterases (PDEs)

Cyclic nucleotide phosphodiesterases (PDEs) are enzymes that have the unique function of terminating cyclic nucleotide signaling by catalyzing the hydrolysis of cAMP and GMP. They are critical regulators of the intracellular concentrations of cAMP and cGMP as well as of their signaling pathways and downstream biological effects. PDEs have been exploited pharmacologically for more than half a century, and some of the most successful drugs worldwide today affect PDE function. Recently, mutations in PDE genes have been identified as causative of certain human genetic diseases; even more recently, functional variants of PDE genes have been suggested to play a potential role in predisposition to tumors and/or cancer, especially in cAMP-sensitive tissues. Mouse models have been developed that point to wide developmental effects of PDEs from heart function to reproduction, to tumors, and beyond. This review brings together knowledge from a variety of disciplines (biochemistry and pharmacology, oncology, endocrinology, and reproductive sciences) with emphasis on recent research on PDEs, how PDEs affect cAMP and cGMP signaling in health and disease, and what pharmacological exploitations of PDEs may be useful in modulating cyclic nucleotide signaling in a way that prevents or treats certain human diseases.

Capturing cooperative interactions with the PSI-MI format

The complex biological processes that control cellular function are mediated by intricate networks of molecular interactions. Accumulating evidence indicates that these interactions are often interdependent, thus acting cooperatively. Cooperative interactions are prevalent in and indispensible for reliable and robust control of cell regulation, as they underlie the conditional decision-making capability of large regulatory complexes. Despite an increased focus on experimental elucidation of the molecular details of cooperative binding events, as evidenced by their growing occurrence in literature, they are currently lacking from the main bioinformatics resources. One of the contributing factors to this deficiency is the lack of a computer-readable standard representation and exchange format for cooperative interaction data. To tackle this shortcoming, we added functionality to the widely used PSI-MI interchange format for molecular interaction data by defining new controlled vocabulary terms that allow annotation of different aspects of cooperativity without making structural changes to the underlying XML schema. As a result, we are able to capture cooperative interaction data in a structured format that is backward compatible with PSI-MI–based data and applications. This will facilitate the storage, exchange and analysis of cooperative interaction data, which in turn will advance experimental research on this fundamental principle in biology.

Database URL:http://psi-mi-cooperativeinteractions.embl.de/

A-kinase anchoring protein Lbc coordinates a p38 activating signaling complex controlling compensatory cardiac hypertrophy

In response to stress, the heart undergoes a remodeling process associated with cardiac hypertrophy that eventually leads to heart failure. A-kinase anchoring proteins (AKAPs) have been shown to coordinate numerous prohypertrophic signaling pathways in cultured cardiomyocytes. However, it remains to be established whether AKAP-based signaling complexes control cardiac hypertrophy and remodeling in vivo. In the current study, we show that AKAP-Lbc assembles a signaling complex composed of the kinases PKN, MLTK, MKK3, and p38α that mediates the activation of p38 in cardiomyocytes in response to stress signals. To address the role of this complex in cardiac remodeling, we generated transgenic mice displaying cardiomyocyte-specific overexpression of a molecular inhibitor of the interaction between AKAP-Lbc and the p38-activating module. Our results indicate that disruption of the AKAP-Lbc/p38 signaling complex inhibits compensatory cardiomyocyte hypertrophy in response to aortic banding-induced pressure overload and promotes early cardiac dysfunction associated with increased myocardial apoptosis, stress gene activation, and ventricular dilation. Attenuation of hypertrophy results from a reduced protein synthesis capacity, as indicated by decreased phosphorylation of 4E-binding protein 1 and ribosomal protein S6. These results indicate that AKAP-Lbc enhances p38-mediated hypertrophic signaling in the heart in response to abrupt increases in the afterload.

Engineering A-kinase anchoring protein (AKAP)-selective regulatory subunits of protein kinase A (PKA) through structure-based phage selection

PKA is retained within distinct subcellular environments by the association of its regulatory type II (RII) subunits with A-kinase anchoring proteins (AKAPs). Conventional reagents that universally disrupt PKA anchoring are patterned after a conserved AKAP motif. We introduce a phage selection procedure that exploits high-resolution structural information to engineer RII mutants that are selective for a particular AKAP. Selective RII (R_Select) sequences were obtained for eight AKAPs following competitive selection screening. Biochemical and cell-based experiments validated the efficacy of R_Select proteins for AKAP2 and AKAP18. These engineered proteins represent a new class of reagents that can be used to dissect the contributions of different AKAP-targeted pools of PKA. Molecular modeling and high-throughput sequencing analyses revealed the molecular basis of AKAP-selective interactions and shed new light on native RII-AKAP interactions. We propose that this structure-directed evolution strategy might be generally applicable for the investigation of other protein interaction surfaces.

The switches.ELM resource: a compendium of conditional regulatory interaction interfaces

Short linear motifs (SLiMs) are protein interaction sites that play an important role in cell regulation by controlling protein activity, localization, and local abundance. The functionality of a SLiM can be modulated in a context-dependent manner to induce a gain, loss, or exchange of binding partners, which will affect the function of the SLiM-containing protein. As such, these conditional interactions underlie molecular decision-making in cell signaling. We identified multiple types of pre- and posttranslational switch mechanisms that can regulate the function of a SLiM and thereby control its interactions. The collected examples of experimentally characterized SLiM-based switch mechanisms were curated in the freely accessible switches.ELM resource (http://switches.elm.eu.org). On the basis of these examples, we defined and integrated rules to analyze SLiMs for putative regulatory switch mechanisms. We applied these rules to known validated SLiMs, providing evidence that more than half of these are likely to be pre- or posttranslationally regulated. In addition, we showed that posttranslationally modified sites are enriched around SLiMs, which enables cooperative and integrative regulation of protein interaction interfaces. We foresee switches.ELM complementing available resources to extend our knowledge of the molecular mechanisms underlying cell signaling.

STAT3-mediated coincidence detection regulates noncanonical immediate early gene induction

Signaling pathways interact with one another to form dynamic networks in which the cellular response to one stimulus may depend on the presence, intensity, timing, or localization of other signals. In rare cases, two stimuli may be simultaneously required for cells to elicit a significant biological output. This phenomenon, generally termed "coincidence detection," requires a downstream signaling node that functions as a Boolean AND gate to restrict biological output from a network unless multiple stimuli are received within a specific window of time. Simultaneous activation of the EGF receptor (EGFR) and a thrombin receptor (protease-activated receptor-1, PAR-1) increases the expression of multiple immediate early genes (IEGs) associated with growth and angiogenesis. Using a bioinformatic comparison of IEG promoter regions, we identified STAT3 as a critical transcription factor for the detection of coincident EGFR/PAR-1 activation. EGFR activation induces classical STAT3 Tyr(705) phosphorylation but also initiates an inhibitory signal through the PI3K-AKT signaling axis that prevents STAT3 Ser(727) phosphorylation. Coincident PAR-1 signaling resolves these conflicting EGF-activated pathways by blocking AKT activation and permitting GSK-3α/β-dependent STAT3 Ser(727) phosphorylation and STAT3-dependent gene expression. Functionally, combinatorial EGFR/PAR-1 signaling suppresses EGF-induced proliferation and thrombin-induced leukocyte adhesion and triggers a STAT3-dependent increase in endothelial cell migration. This study reveals a novel signaling role for STAT3 in which the simultaneous presence of extracellular EGF and thrombin is detected at the level of STAT3 post-translational modifications. Collectively, our results describe a novel regulatory mechanism in which combinatorial EGFR/PAR-1 signaling regulates STAT3-dependent IEG induction and endothelial cell migration.

A-kinase-anchoring protein-Lbc anchors IκB kinase β to support interleukin-6-mediated cardiomyocyte hypertrophy

In response to stress, the heart undergoes a pathological remodeling process associated with hypertrophy and the reexpression of a fetal gene program that ultimately causes cardiac dysfunction and heart failure. In this study, we show that A-kinase-anchoring protein (AKAP)-Lbc and the inhibitor of NF-κB kinase subunit β (IKKβ) form a transduction complex in cardiomyocytes that controls the production of proinflammatory cytokines mediating cardiomyocyte hypertrophy. In particular, we can show that activation of IKKβ within the AKAP-Lbc complex promotes NF-κB-dependent production of interleukin-6 (IL-6), which in turn enhances fetal gene expression and cardiomyocyte growth. These findings provide a new mechanistic hypothesis explaining how hypertrophic signals are coordinated and conveyed to interleukin-mediated transcriptional reprogramming events in cardiomyocytes.

A-kinase anchoring proteins: molecular regulators of the cardiac stress response

In response to stress or injury the heart undergoes a pathological remodeling process, associated with hypertrophy, cardiomyocyte death and fibrosis, that ultimately causes cardiac dysfunction and heart failure. It has become increasingly clear that signaling events associated with these pathological cardiac remodeling events are regulated by scaffolding and anchoring proteins, which allow coordination of pathological signals in space and time. A-kinase anchoring proteins (AKAPs) constitute a family of functionally related proteins that organize multiprotein signaling complexes that tether the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to ensure integration and processing of multiple signaling pathways. This review will discuss the role of AKAPs in the cardiac response to stress. Particular emphasis will be given to the adaptative process associated with cardiac hypoxia as well as the remodeling events linked to cardiac hypertrophy and heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Regulation of neuronal mRNA translation by CaM-kinase I phosphorylation of eIF4GII

Ca²⁺/calmodulin-dependent kinases (CaMKs) are essential for neuronal development and plasticity, processes requiring de novo protein synthesis. Roles for CaMKs in modulating gene transcription are well established, but their involvement in mRNA translation is evolving. Here we report that activity-dependent translational initiation in cultured rat hippocampal neurons is enhanced by CaMKI-mediated phosphorylation of Ser1156 in eukaryotic initiation factor eIF4GII (4GII). Treatment with bicuculline or gabazine to enhance neuronal activity promotes recruitment of wild-type 4GII, but not the 4GII S1156A mutant or 4GI, to the heterotrimeric eIF4F (4F) complex that assembles at the 5' cap structure (m⁷GTP) of mRNA to initiate ribosomal scanning. Recruitment of 4GII to 4F is suppressed by pharmacological inhibition (STO-609) of CaM kinase kinase, the upstream activator of CaMKI. Post hoc in vitro CaMKI phosphorylation assays confirm that activity promotes phosphorylation of S1156 in transfected 4GII in neurons. Changes in cap-dependent and cap-independent translation were assessed using a bicistronic luciferase reporter transfected into neurons. Activity upregulates cap-dependent translation, and RNAi knockdown of CaMKIβ and γ isoforms, but not α or δ, led to its attenuation as did blockade of NMDA receptors. Furthermore, RNAi knockdown of 4GII attenuates cap-dependent translation and reduces density of dendritic filopodia and spine formation without effect on dendritic arborization. Together, our results provide a mechanistic link between Ca²⁺ influx due to neuronal activity and regulation of cap-dependent RNA translation via CaMKI activation and selective recruitment of phosphorylated 4GII to the 4F complex, which may function to regulate activity-dependent changes in spine density.

Beyond 'furballs' and 'dumpling soups' - towards a molecular architecture of signaling complexes and networks

The molecular architectures of intracellular signaling networks are largely unknown. Understanding their design principles and mechanisms of processing information is essential to grasp the molecular basis of virtually all biological processes. This is particularly challenging for human pathologies like cancers, as essentially each tumor is a unique disease with vastly deranged signaling networks. However, even in normal cells we know almost nothing. A few 'signalosomes', like the COP9 and the TCR signaling complexes have been described, but detailed structural information on their architectures is largely lacking. Similarly, many growth factor receptors, for example EGF receptor, insulin receptor and c-Met, signal via huge protein complexes built on large platform proteins (Gab, Irs/Dok, p130Cas[BCAR1], Frs families etc.), which are structurally not well understood. Subsequent higher order processing events remain even more enigmatic. We discuss here methods that can be employed to study signaling architectures, and the importance of too often neglected features like macromolecular crowding, intrinsic disorder in proteins and the sophisticated cellular infrastructures, which need to be carefully considered in order to develop a more mature understanding of cellular signal processing.

Motif switches: decision-making in cell regulation

Tight regulation of gene products from transcription to protein degradation is required for reliable and robust control of eukaryotic cell physiology. Many of the mechanisms directing cell regulation rely on proteins detecting the state of the cell through context-dependent, tuneable interactions. These interactions underlie the ability of proteins to make decisions by combining regulatory information encoded in a protein's expression level, localisation and modification state. This raises the question, how do proteins integrate available information to correctly make decisions? Over the past decade pioneering work on the nature and function of intrinsically disordered protein regions has revealed many elegant switching mechanisms that underlie cell signalling and regulation, prompting a reevaluation of their role in cooperative decision-making.

Suppression of survival signalling pathways by the phosphatase PHLPP

The recently discovered pleckstrin homology (PH) domain leucine-rich repeat protein phosphatase (PHLPP) family is emerging as a central component in suppressing cell survival pathways. Originally discovered in a rational search for a phosphatase that directly dephosphorylates and inactivates Akt, PHLPP is now known to potently suppress cell survival both by inhibiting proliferative pathways and by promoting apoptotic pathways. In the first instance, PHLPP directly dephosphorylates a conserved regulatory site (termed the hydrophobic motif) on Akt, protein kinase C and S6 kinase, thereby terminating signalling by these pro-survival kinases. In the second instance, PHLPP dephosphorylates and thus activates the pro-apoptotic kinase Mst1, thereby promoting apoptosis. PHLPP is deleted in a large number of cancers and the genetic deletion of one isozyme in a PTEN (phosphatase and tensin homologue located on chromosome 1) +/- (or heterozygous) prostate cancer model results in increased tumourigenesis, underscoring the role of PHLPP as a tumour suppressor. This review summarizes the targets and cellular actions of PHLPP, with emphasis on its role as a tumour suppressor in the oncogenic phosphoinositide 3-kinase (PI3K)/Akt signalling cascade.

Subcellular signaling in the endothelium: cyclic nucleotides take their place

When lecturing on the topic of cellular signaling I have had occasion to ask the class for examples of cellular processes NOT impacted by cyclic AMP (cAMP) and am struck by how few examples exist. Indeed, studies spanning the past 60 years have detailed how this ubiquitous second messenger impacts virtually all cellular processes, including intermediary metabolism, contractility, motility, proliferation, and gene expression in most mammalian cells. Since the hydrophobic cAMP could in principle diffuse rapidly throughout the cell once formed, the remarkable spatial and temporal specificity of its numerous actions in cells is truly impressive. Herein I introduce the main players involved in coordinating actions of cAMP in vascular endothelial cells (VECs), and focus on the increasing awareness of the dominant role that cyclic nucleotide phosphodiesterases (PDEs), the sole cellular enzymes capable of hydrolytically inactivating cAMP, play in fostering this specificity.

Protein kinase Cα promotes cell migration through a PDZ-dependent interaction with its novel substrate discs large homolog 1 (DLG1)

Protein scaffolds maintain precision in kinase signaling by coordinating kinases with components of specific signaling pathways. Such spatial segregation is particularly important in allowing specificity of signaling mediated by the 10-member family of protein kinase C (PKC) isozymes. Here we identified a novel interaction between PKCα and the Discs large homolog (DLG) family of scaffolds that is mediated by a class I C-terminal PDZ (PSD-95, disheveled, and ZO1) ligand unique to this PKC isozyme. Specifically, use of a proteomic array containing 96 purified PDZ domains identified the third PDZ domains of DLG1/SAP97 and DLG4/PSD95 as interaction partners for the PDZ binding motif of PKCα. Co-immunoprecipitation experiments verified that PKCα and DLG1 interact in cells by a mechanism dependent on an intact PDZ ligand. Functional assays revealed that the interaction of PKCα with DLG1 promotes wound healing; scratch assays using cells depleted of PKCα and/or DLG1 have impaired cellular migration that is no longer sensitive to PKC inhibition, and the ability of exogenous PKCα to rescue cellular migration is dependent on the presence of its PDZ ligand. Furthermore, we identified Thr-656 as a novel phosphorylation site in the SH3-Hook region of DLG1 that acts as a marker for PKCα activity at this scaffold. Increased phosphorylation of Thr-656 is correlated with increased invasiveness in non-small cell lung cancer lines from the NCI-60, consistent with this phosphorylation site serving as a marker of PKCα-mediated invasion. Taken together, these data establish the requirement of scaffolding to DLG1 for PKCα to promote cellular migration.

Ethanol-induced increase in Fyn kinase activity in the dorsomedial striatum is associated with subcellular redistribution of protein tyrosine phosphatase α

In vivo exposure of rodents to ethanol leads to a long-lasting increase in Fyn kinase activity in the dorsomedial striatum (DMS). In this study, we set out to identify a molecular mechanism that contributes to the enhancement of Fyn activity in response to ethanol in the DMS. Protein tyrosine phosphatase α (PTPα) positively regulates the activity of Fyn, and we found that repeated systemic administration or binge drinking of ethanol results in an increase in the synaptic localization of PTPα in the DMS, the same site where Fyn resides. We also demonstrate that binge drinking of ethanol leads to an increase in Fyn activity and to the co-localization of Fyn and PTPα in lipid rafts in the DMS. Finally, we show that the level of tyrosine phosphorylated (and thus active) PTPα in the synaptic fractions is increased in response to contingent or non-contingent exposure of rats to ethanol. Together, our results suggest that the redistribution of PTPα in the DMS into compartments where Fyn resides is a potential mechanism by which the activity of the kinase is increased upon ethanol exposure. Such neuroadaptations could be part of a mechanism that leads to the development of excessive ethanol consumption.

Ligand-directed functional selectivity at the mu opioid receptor revealed by label-free integrative pharmacology on-target

Development of new opioid drugs that provide analgesia without producing dependence is important for pain treatment. Opioid agonist drugs exert their analgesia effects primarily by acting at the mu opioid receptor (MOR) sites. High-resolution differentiation of opioid ligands is crucial for the development of new lead drug candidates with better tolerance profiles. Here, we use a label-free integrative pharmacology on-target (iPOT) approach to characterize the functional selectivity of a library of known opioid ligands for the MOR. This approach is based on the ability to detect dynamic mass redistribution (DMR) arising from the activation of the MOR in living cells. DMR assays were performed in HEK-MOR cells with and without preconditioning with probe molecules using label-free resonant waveguide grating biosensors, wherein the probe molecules were used to modify the activity of specific signaling proteins downstream the MOR. DMR signals obtained were then translated into high resolution heat maps using similarity analysis based on a numerical matrix of DMR parameters. Our data indicate that the iPOT approach clearly differentiates functional selectivity for distinct MOR signaling pathways among different opioid ligands, thus opening new avenues to discover and quantify the functional selectivity of currently used and novel opioid receptor drugs.

Active regulation of receptor ratios controls integration of quorum-sensing signals in Vibrio harveyi

Single-cell quantification of the input–output relation of the quorum-sensing circuit reveals how Vibrio harveyi employs multiple feedback loops to simultaneously control quorum-sensing signal integration and to ensure signal transmission fidelity.

We identify the role of multiple feedback loops in the quorum-sensing circuit of the model bacterium, Vibrio harveyi. Single-cell microscopy and genetic analysis demonstrate that a novel feedback loop regulates receptor ratios to control the integration of multiple signals.

Quantitative investigation of cells with all feedback loops present as well as mutants with specific feedback loops disrupted reveals that the multiple feedback loops expand the input dynamic range and compress the output dynamic range of signal transmission, and also control the noise level of the output.

Our experimental observations can be interpreted in terms of a simple model of the quorum-sensing network. Plotting output after reparameterizing the input variables directly reveals how feedback controls receptors ratios.

Organisms detect multiple environmental cues simultaneously and use the information to coordinate their behaviors. Correctly integrating signals generally requires complex signal transduction pathways (Pawson and Scott, 2010). In addition to accurately integrating signals, regulatory circuits must ensure signal transmission fidelity. Information can be lost or corrupted by internal or external perturbations, so circuits must be designed to function robustly in the presence of such fluctuations. For example, the circadian clock in Neurospora (Virshup and Forger, 2009) and the chemotaxis network in Escherichia coli (Oleksiuk et al, 2011) accurately compensate for temperature variation. However, while signal integration and signal transmission have been addressed separately, little is known about mechanisms cells use to solve both tasks simultaneously. In this study, we report how the model bacterium Vibrio harveyi simultaneously integrates and faithfully transmits multiple chemical signals.

In a process called quorum sensing, bacteria communicate by synthesizing, releasing, and detecting signal molecules called autoinducers (AIs). To study the integration of such signals, we studied a strain of V. harveyi that integrates two AI signals into its quorum-sensing circuit: AI-1, an intra-species signal, and AI-2, a ‘universal' inter-species signal. Each signal is detected by a cognate receptor AI-1 by LuxN, and AI-2 by LuxPQ (Figure 4A). The information encoded in the two AIs is transduced through a shared signaling pathway into the master quorum-sensing regulator LuxR. In this study, the AIs serve as inputs and LuxR serves as the output of the quorum-sensing circuit. Interestingly, there are five distinct feedback loops in the V. harveyi quorum-sensing circuit (Figure 4A). How does the circuit use shared components to distinguish between the two AI inputs and what role does each feedback loop have in signal integration and transmission?

Using single-cell microscopy, we assayed the activity of the quorum-sensing circuit with a focus on defining the functions of the feedback loops. We quantitatively investigated the signaling input–output relation both in cells with all feedback loops present (Figure 4A) as well as in mutants with specific feedback loops disrupted (Figure 4E, I, M, and Q). We compared the mean LuxR level (Figure 4B, F, J, N, and R) and noise level (Figure 4C, G, K, O, and S) for the input–output relation of five strains. We discovered that the LuxN feedback loop regulates receptor ratios (LuxN to LuxPQ) to control the integration of two signals. We also found that the multiple feedback loops expand the input dynamic range and compress the output dynamic range of signal transmission, and also control the noise in the output.

In summary, we used single-cell microscopy to quantify the integration of quorum-sensing signals in V. harveyi. Multiple feedback loops in the quorum-sensing circuit actively regulate receptor ratios to control signal integration, sculpt the input–output dynamic range, and regulate the noise level. This system presents a paradigm for how complex circuitry allows cells to appropriately detect and respond to multiple signals in a dynamically changing environment.

Quorum sensing is a chemical signaling mechanism used by bacteria to communicate and orchestrate group behaviors. Multiple feedback loops exist in the quorum-sensing circuit of the model bacterium Vibrio harveyi. Using fluorescence microscopy of individual cells, we assayed the activity of the quorum-sensing circuit, with a focus on defining the functions of the feedback loops. We quantitatively investigated the signaling input–output relation both in cells with all feedback loops present as well as in mutants with specific feedback loops disrupted. We found that one of the feedback loops regulates receptor ratios to control the integration of multiple signals. Together, the feedback loops affect the input–output dynamic range of signal transmission and the noise in the output. We conclude that V. harveyi employs multiple feedback loops to simultaneously control quorum-sensing signal integration and to ensure signal transmission fidelity.

A-kinase anchoring proteins: scaffolding proteins in the heart

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

Characterization of a central Ca2+/calmodulin-dependent protein kinase IIalpha/beta binding domain in densin that selectively modulates glutamate receptor subunit phosphorylation

The densin C-terminal domain can target Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) in cells. Although the C-terminal domain selectively binds CaMKIIα in vitro, full-length densin associates with CaMKIIα or CaMKIIβ in brain extracts and in transfected HEK293 cells. This interaction requires a second central CaMKII binding site, the densin-IN domain, and an "open" activated CaMKII conformation caused by Ca(2+)/calmodulin binding, autophosphorylation at Thr-286/287, or mutation of Thr-286/287 to Asp. Mutations in the densin-IN domain (L815E) or in the CaMKIIα/β catalytic domain (I205/206K) disrupt the interaction. The amino acid sequence of the densin-IN domain is similar to the CaMKII inhibitor protein, CaMKIIN, and a CaMKIIN peptide competitively blocks CaMKII binding to densin. CaMKII is inhibited by both CaMKIIN and the densin-IN domain, but the inhibition by densin is substrate-selective. Phosphorylation of a model peptide substrate, syntide-2, or of Ser-831 in AMPA receptor GluA1 subunits is fully inhibited by densin. However, CaMKII phosphorylation of Ser-1303 in NMDA receptor GluN2B subunits is not effectively inhibited by densin in vitro or in intact cells. Thus, densin can target multiple CaMKII isoforms to differentially modulate phosphorylation of physiologically relevant downstream targets.

RIM proteins activate vesicle priming by reversing autoinhibitory homodimerization of Munc13

At a synapse, the presynaptic active zone mediates synaptic vesicle exocytosis. RIM proteins are active zone scaffolding molecules that--among others--mediate vesicle priming and directly or indirectly interact with most other essential presynaptic proteins. In particular, the Zn²+ finger domain of RIMs binds to the C₂A domain of the priming factor Munc13, which forms a homodimer in the absence of RIM but a heterodimer with it. Here, we show that RIMs mediate vesicle priming not by coupling Munc13 to other active zone proteins as thought but by directly activating Munc13. Specifically, we found that the isolated Zn²+ finger domain of RIMs autonomously promoted vesicle priming by binding to Munc13, thereby relieving Munc13 homodimerization. Strikingly, constitutively monomeric mutants of Munc13 rescued priming in RIM-deficient synapses, whereas wild-type Munc13 did not. Both mutant and wild-type Munc13, however, rescued priming in Munc13-deficient synapses. Thus, homodimerization of Munc13 inhibits its priming function, and RIMs activate priming by disrupting Munc13 homodimerization.

A phosphodiesterase 3B-based signaling complex integrates exchange protein activated by cAMP 1 and phosphatidylinositol 3-kinase signals in human arterial endothelial cells

Enzymes of the phosphodiesterase 3 (PDE3) and PDE4 families each regulate the activities of both protein kinases A (PKAs) and exchange proteins activated by cAMP (EPACs) in cells of the cardiovascular system. At present, the mechanisms that allow selected PDEs to individually regulate the activities of these two effectors are ill understood. The objective of this study was to determine how a specific PDE3 variant, namely PDE3B, interacts with and regulates EPAC1-based signaling in human arterial endothelial cells (HAECs). Using several biochemical approaches, we show that PDE3B and EPAC1 bind directly through protein-protein interactions. By knocking down PDE3B expression or by antagonizing EPAC1 binding with PDE3B, we show that PDE3B regulates cAMP binding by its tethered EPAC1. Interestingly, we also show that PDE3B binds directly to p84, a PI3Kγ regulatory subunit, and that this interaction allows PI3Kγ recruitment to the PDE3B-EPAC1 complex. Of potential cardiovascular importance, we demonstrate that PDE3B-tethered EPAC1 regulates HAEC PI3Kγ activity and that this allows dynamic cAMP-dependent regulation of HAEC adhesion, spreading, and tubule formation. We identify and molecularly characterize a PDE3B-based "signalosome" that integrates cAMP- and PI3Kγ-encoded signals and show how this signal integration regulates HAEC functions of importance in angiogenesis.