Regulation of cAMP-dependent protein kinases: the human protein kinase X (PrKX) reveals the role of the catalytic subunit alphaH-alphaI loop

Insulin signaling and pharmacology in humans and in corals

Once thought to be a unique capability of the Langerhans islets in the pancreas of mammals, insulin (INS) signaling is now recognized as an evolutionarily ancient function going back to prokaryotes. INS is ubiquitously present not only in humans but also in unicellular eukaryotes, fungi, worms, and Drosophila. Remote homologue identification also supports the presence of INS and INS receptor in corals where the availability of glucose is largely dependent on the photosynthetic activity of the symbiotic algae. The cnidarian animal host of corals operates together with a 20,000-sized microbiome, in direct analogy to the human gut microbiome. In humans, aberrant INS signaling is the hallmark of metabolic disease, and is thought to play a major role in aging, and age-related diseases, such as Alzheimer's disease. We here would like to argue that a broader view of INS beyond its human homeostasis function may help us understand other organisms, and in turn, studying those non-model organisms may enable a novel view of the human INS signaling system. To this end, we here review INS signaling from a new angle, by drawing analogies between humans and corals at the molecular level.

Contribution of membrane-associated oscillators to biological timing at different timescales

Environmental rhythms such as the daily light-dark cycle selected for endogenous clocks. These clocks predict regular environmental changes and provide the basis for well-timed adaptive homeostasis in physiology and behavior of organisms. Endogenous clocks are oscillators that are based on positive feedforward and negative feedback loops. They generate stable rhythms even under constant conditions. Since even weak interactions between oscillators allow for autonomous synchronization, coupling/synchronization of oscillators provides the basis of self-organized physiological timing. Amongst the most thoroughly researched clocks are the endogenous circadian clock neurons in mammals and insects. They comprise nuclear clockworks of transcriptional/translational feedback loops (TTFL) that generate ∼24 h rhythms in clock gene expression entrained to the environmental day-night cycle. It is generally assumed that this TTFL clockwork drives all circadian oscillations within and between clock cells, being the basis of any circadian rhythm in physiology and behavior of organisms. Instead of the current gene-based hierarchical clock model we provide here a systems view of timing. We suggest that a coupled system of autonomous TTFL and posttranslational feedback loop (PTFL) oscillators/clocks that run at multiple timescales governs adaptive, dynamic homeostasis of physiology and behavior. We focus on mammalian and insect neurons as endogenous oscillators at multiple timescales. We suggest that neuronal plasma membrane-associated signalosomes constitute specific autonomous PTFL clocks that generate localized but interlinked oscillations of membrane potential and intracellular messengers with specific endogenous frequencies. In each clock neuron multiscale interactions of TTFL and PTFL oscillators/clocks form a temporally structured oscillatory network with a common complex frequency-band comprising superimposed multiscale oscillations. Coupling between oscillator/clock neurons provides the next level of complexity of an oscillatory network. This systemic dynamic network of molecular and cellular oscillators/clocks is suggested to form the basis of any physiological homeostasis that cycles through dynamic homeostatic setpoints with a characteristic frequency-band as hallmark. We propose that mechanisms of homeostatic plasticity maintain the stability of these dynamic setpoints, whereas Hebbian plasticity enables switching between setpoints via coupling factors, like biogenic amines and/or neuropeptides. They reprogram the network to a new common frequency, a new dynamic setpoint. Our novel hypothesis is up for experimental challenge.

Compartmentalised cAMP signalling in the primary cilium

cAMP is a universal second messenger that relies on precise spatio-temporal regulation to control varied, and often opposing, cellular functions. This is achieved via selective activation of effectors embedded in multiprotein complexes, or signalosomes, that reside at distinct subcellular locations. cAMP is also one of many pathways known to operate within the primary cilium. Dysfunction of ciliary signaling leads to a class of diseases known as ciliopathies. In Autosomal Dominant Polycystic Kidney Disease (ADPKD), a ciliopathy characterized by the formation of fluid-filled kidney cysts, upregulation of cAMP signaling is known to drive cystogenesis. For decades it has been debated whether the primary cilium is an independent cAMP sub-compartment, or whether it shares a diffusible pool of cAMP with the cell body. Recent studies now suggest it is a specific pool of cAMP generated in the cilium that propels cyst formation in ADPKD, supporting the notion that this antenna-like organelle is a compartment within which cAMP signaling occurs independently from cAMP signaling in the bulk cytosol. Here we present examples of cAMP function in the cilium which suggest this mysterious organelle is home to more than one cAMP signalosome. We review evidence that ciliary membrane localization of G-Protein Coupled Receptors (GPCRs) determines their downstream function and discuss how optogenetic tools have contributed to establish that cAMP generated in the primary cilium can drive cystogenesis.

Differential changes in cyclic adenosine 3′‐5′ monophosphate ( cAMP ) effectors and major Ca 2+ handling proteins during diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is associated with differential and time-specific regulation of β-adrenergic receptors and cardiac cyclic nucleotide phosphodiesterases with consequences for total cyclic adenosine 3'-5' monophosphate (cAMP) levels. We aimed to investigate whether these changes are associated with downstream impairments in cAMP and Ca²⁺ signalling in a type 1 diabetes (T1D)-induced DCM model. T1D was induced in adult male rats by streptozotocin (65 mg/kg) injection. DCM was assessed by cardiac structural and molecular remodelling. We delineated sequential changes affecting the exchange protein (Epac1/2), cAMP-dependent protein kinase A (PKA) and Ca²⁺ /Calmodulin-dependent kinase II (CaMKII) at 4, 8 and 12 weeks following diabetes, by real-time quantitative PCR and western blot. Expression of Ca²⁺ ATPase pump (SERCA2a), phospholamban (PLB) and Troponin I (TnI) was also examined. Early upregulation of Epac1 transcripts was noted in diabetic hearts at Week 4, followed by increases in Epac2 mRNA, but not protein levels, at Week 12. Expression of PKA subunits (RI, RIIα and Cα) remained unchanged regardless of the disease stage, whereas CaMKII increased at Week 12 in DCM. Moreover, PLB transcripts were upregulated in diabetic hearts, whereas SERCA2a and TnI gene expression was unchanged irrespective of the disease evolution. PLB phosphorylation at threonine-17 was increased in DCM, whereas phosphorylation of both PLB at serine-16 and TnI at serine-23/24 was unchanged. We show for the first time differential and time-specific regulations in cardiac cAMP effectors and Ca²⁺ handling proteins, data that may prove useful in proposing new therapeutic approaches in T1D-induced DCM.

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. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Alterations in Protein Kinase A Substrate Specificity as a Potential Cause of Cushing Syndrome

Cushing syndrome is a severe endocrine disorder of cortisol excess associated with major metabolic and cardiovascular sequelae. We recently identified somatic mutations in PRKACA, the gene encoding the catalytic (C) α subunit of protein kinase A (PKA), as being responsible for cortisol-producing adrenocortical adenomas (CPAs), which are a major cause of Cushing syndrome. In spite of previous studies on the two initially identified mutations (L206R, 199_200insW), the mechanisms of action of the clinically highly relevant PRKACA mutations remain poorly understood. Here, by investigating a large panel of PRKACA mutations, including all those identified so far in Cushing syndrome, we unexpectedly found that not all mutations interfere with the binding of regulatory (R) subunits as previously hypothesized. Because several mutations lie in a region of PKA Cα involved in substrate recognition, we investigated their consequences on substrate specificity by quantitative phosphoproteomics. We found that all three mutations analyzed (L206R, 200_201insV, and d244-248+E249Q) cause major changes in the preference of PKA for its targets, leading to hyperphosphorylation of several PKA substrates, most notably including histone H1.4 at Ser36, which is required for and promotes mitosis. This is reflected by a ninefold hyperphosphorylation of H1.4 in CPAs carrying the L206R mutation. Thus, our findings suggest that in addition to hampering binding to R subunits, PRKACA mutations act by altering PKA substrate specificity. These findings shed light on the molecular events leading to Cushing syndrome and illustrate how mutations altering substrate specificity of a protein kinase may cause human disease.

Mechanisms of Aberrant PKA Activation by Cα Subunit Mutations

Somatic mutations in PRKACA, coding for the catalytic α subunit of protein kinase A (PKA), have been recently identified as the most frequent genetic alteration in cortisol-secreting adrenocortical adenomas, which are responsible for adrenal Cushing's syndrome. The mutations identified so far lie at the interface between the catalytic (C) and regulatory (R) subunit of PKA. Detailed functional studies of the most frequent of these mutations (L206R) as well as of another one in the same region of the C subunit (199_200insW) have revealed that these mutations cause constitutive activation of PKA and lack of regulation by cAMP. This is due to interference with the binding of the R subunit, which keeps the C subunit inactive in the absence of cyclic AMP. Here, we review these recent findings, with a particular focus on the mechanisms of action of PRKACA mutations.

Differential expression of the protein kinase A subunits in normal adrenal glands and adrenocortical adenomas

Somatic mutations in protein kinase A catalytic α subunit (PRKACA) were found to be causative for 30-40% of cortisol-producing adenomas (CPA) of the adrenal gland, rendering PKA signalling constitutively active. In its resting state, PKA is a stable and inactive heterotetramer, consisting of two catalytic and two regulatory subunits with the latter inhibiting PKA activity. The human genome encodes three different PKA catalytic subunits and four different regulatory subunits that are preferentially expressed in different organs. In normal adrenal glands all regulatory subunits are expressed, while CPA exhibit reduced protein levels of the regulatory subunit IIβ. In this study, we linked for the first time the loss of RIIβ protein levels to the PRKACA mutation status and found the down-regulation of RIIβ to arise post-transcriptionally. We further found the PKA subunit expression pattern of different tumours is also present in the zones of the normal adrenal cortex and demonstrate that the different PKA subunits have a differential expression pattern in each zone of the normal adrenal gland, indicating potential specific roles of these subunits in the regulation of different hormones secretion.

PRKX, a Novel cAMP-Dependent Protein Kinase Member, Plays an Important Role in Development

The human protein kinase X gene (PRKX) and cAMP-dependent protein kinase (PKA) are both c-AMP-dependent serine/threonine protein kinases within the protein kinase AGC subgroup. Of all the protein kinases in this group, PRKX is the least studied. PRKX has been isolated from patients with chondrodysplasia punctate and is involved in numerous processes, including sexual differentiation and fertilization, normal kidney development and autosomal dominant polycystic kidney disease (ADPKD), blood maturation, neural development, and angiogenesis in vitro. Although the role of PRKX in development and disease has been reported recently, the underlying mechanism of PRKX activity is largely unknown. In addition, based on the expression pattern of PRKX and the extensive role of PKA in disease and development, PRKX might have additional crucial functions that have not been addressed in the literature. In this review, we summarize the characteristics and developmental functions of PRKX that have been reported by recent studies. In particular, we elucidate the structural and functional differences between PRKX and PKA, as well as the possible roles of PRKX in development and related diseases. Finally, we propose future studies that could lead to important discoveries of more PRKX functions and the underlying mechanisms involved.

cAMP signaling in cortisol-producing adrenal adenoma

The cAMP signaling pathway is one of the major players in the regulation of growth and hormonal secretion in adrenocortical cells. Although its role in the pathogenesis of adrenocortical hyperplasia associated with Cushing's syndrome has been clarified, a clear involvement of the cAMP signaling pathway and of one of its major downstream effectors, the protein kinase A (PKA), in sporadic adrenocortical adenomas remained elusive until recently. During the last year, a report by our group and three additional independent groups showed that somatic mutations of PRKACA, the gene coding for the catalytic subunit α of PKA, are a common genetic alteration in patients with Cushing's syndrome due to adrenal adenomas, occurring in 35-65% of the patients. In vitro studies revealed that those mutations are able to disrupt the association between catalytic and regulatory subunits of PKA, leading to a cAMP-independent activity of the enzyme. Despite somatic PRKACA mutations being a common finding in patients with clinically manifest Cushing's syndrome, the pathogenesis of adrenocortical adenomas associated with subclinical hypercortisolism seems to rely on a different molecular background. In this review, the role of cAMP/PKA signaling in the regulation of adrenocortical cell function and its alterations in cortisol-producing adrenocortical adenomas will be summarized, with particular focus on recent developments.

Human heart failure is accompanied by altered protein kinase A subunit expression and post-translational state

β-Adrenergic receptor blockade reduces total mortality and all-cause hospitalizations in patients with heart failure (HF). Nonetheless, β-blockade does not halt disease progression, suggesting that cAMP-dependent protein kinase (PKA) signaling downstream of β-adrenergic receptor activation may persist through unique post-translational states. In this study, human myocardial tissue was used to examine the state of PKA subunits. As expected, total myosin binding protein-C phosphorylation and Ser23/24 troponin I phosphorylation significantly decreased in HF. Examination of PKA subunits demonstrated no change in type II regulatory (RIIα) or catalytic (Cα) subunit expression, although site specific RIIα (Ser96) and Cα (Thr197) phosphorylation were increased in HF. Further, the expression of type I regulatory subunit (RI) was increased in HF. Isoelectric focusing of RIα demonstrated up to three variants, consistent with reports that Ser77 and Ser83 are in vivo phosphorylation sites. Western blots with site-specific monoclonal antibodies showed increased Ser83 phosphorylation in HF. 8-fluo-cAMP binding by wild type and phosphomimic Ser77 and Ser83 mutant RIα proteins demonstrated reduced Kd for the double mutant as compared to WT RIα. Therefore, failing myocardium displays altered expression and post-translational modification of PKA subunits that may impact downstream signaling.

Stimulation of proglucagon gene expression by human GPR119 in enteroendocrine L-cell line GLUTag

GPR119 is a G protein-coupled receptor expressed on enteroendocrine L-cells that synthesize and secrete the incretin hormone glucagon-like peptide-1 (GLP-1). Although GPR119 agonists stimulate L-cell GLP-1 secretion, there is uncertainty concerning whether GLP-1 biosynthesis is under the control of GPR119. Here we report that GPR119 is functionally coupled to increased proglucagon (PG) gene expression that constitutes an essential first step in GLP-1 biosynthesis. Using a mouse L-cell line (GLUTag) that expresses endogenous GPR119, we demonstrate that PG gene promoter activity is stimulated by GPR119 agonist AS1269574. Surprisingly, transfection of GLUTag cells with recombinant human GPR119 (hGPR119) results in a constitutive and apparently ligand-independent increase of PG gene promoter activity and PG mRNA content. These constitutive actions of hGPR119 are mediated by cAMP-dependent protein kinase (PKA) but not cAMP sensor Epac2. Thus, the constitutive action of hGPR119 to stimulate PG gene promoter activity is diminished by: 1) a dominant-negative Gαs protein, 2) a dominant-negative PKA regulatory subunit, and 3) a dominant-negative A-CREB. Interestingly, PG gene promoter activity is stimulated by 6-Bn-cAMP-AM, a cAMP analog that selectively activates α and β isoforms of type II, but not type I PKA regulatory subunits expressed in GLUTag cells. Finally, our analysis reveals that a specific inhibitor of Epac2 activation (ESI-05) fails to block the stimulatory action of 6-Bn-cAMP-AM at the PG gene promoter, nor is PG gene promoter activity stimulated by: 1) a constitutively active Epac2, or 2) cAMP analogs that selectively activate Epac proteins. Such findings are discussed within the context of ongoing controversies concerning the relative contributions of PKA and Epac2 to the control of PG gene expression.

cAMP/PKA signaling defects in tumors: genetics and tissue-specific pluripotential cell-derived lesions in human and mouse

In the last few years, bench and clinical studies led to significant new insight into how cyclic adenosine monophosphate (cAMP) signaling, the molecular pathway that had been identified in the early 2000s as the one involved in most benign cortisol-producing adrenal hyperplasias, affects adrenocortical growth and development, as well as tumor formation. A major discovery was the identification of tissue-specific pluripotential cells (TSPCs) as the culprit behind tumor formation not only in the adrenal, but also in bone. Discoveries in animal studies complemented a number of clinical observations in patients. Gene identification continued in parallel with mouse and other studies on the cAMP signaling and other pathways.

Exploring the Plasmodium falciparum cyclic-adenosine monophosphate (cAMP)-dependent protein kinase (PfPKA) as a therapeutic target

One of the prototype mammalian kinases is PKA and various roles have been defined for PKA in malaria pathogenesis. The recently described phospho-proteomes of Plasmodium falciparum introduced a great volume of phospho-peptide data for both basic research and identification of new anti-malaria therapeutic targets. We discuss the importance of phosphorylations detected in vivo at different sites in the parasite R and C subunits of PKA and highlight the inhibitor sites in the parasite R subunit. The N-terminus of the parasite R subunit is predicted to be very flexible and we propose that phosphorylation at multiple sites in this region likely represent docking sites for interactions with other proteins, such as 14-3-3. The most significant observation when the P. falciparum C subunit is compared to mammalian C isoforms is lack of phosphorylation at a key site tail implying that parasite kinase activity is not regulated so tightly as mammalian PKA. Phosphorylation at sites in the activation loop could be mediating a number of processes from regulating parasite kinase activity, to mediating docking of other proteins. The important differences between Plasmodium and mammalian PKA isoforms that indicate the parasite kinase is a valid anti-malaria therapeutic target.

A conserved Glu-Arg salt bridge connects coevolved motifs that define the eukaryotic protein kinase fold

Eukaryotic protein kinases (EPKs) feature two coevolved structural segments, the Activation segment, which starts with the Asp-Phe-Gly (DFG) and ends with the Ala-Pro-Glu (APE) motifs, and the helical GHI subdomain that comprises αG-αH-αI helices. Eukaryotic-like kinases have a much shorter Activation segment and lack the GHI subdomain. They thus lack the conserved salt bridge interaction between the APE Glu and an Arg from the GHI subdomain, a hallmark signature of EPKs. Although the conservation of this salt bridge in EPKs is well known and its implication in diseases has been illustrated by polymorphism analysis, its function has not been carefully studied. In this work, we use murine cAMP-dependent protein kinase (protein kinase A) as the model enzyme (Glu208 and Arg280) to examine the role of these two residues. We showed that Ala replacement of either residue caused a 40- to 120-fold decrease in catalytic efficiency of the enzyme due to an increase in K(m)(ATP) and a decrease in k(cat). Crystal structures, as well as solution studies, also demonstrate that this ion pair contributes to the hydrophobic network and stability of the enzyme. We show that mutation of either Glu or Arg to Ala renders both mutant proteins less effective substrates for upstream kinase phosphoinositide-dependent kinase 1. We propose that the Glu208-Arg280 pair serves as a center hub of connectivity between these two structurally conserved elements in EPKs. Mutations of either residue disrupt communication not only between the two segments but also within the rest of the molecule, leading to altered catalytic activity and enzyme regulation.