cAMP signaling: a remarkably regional affair

Long AKAP18 isoforms anchor ubiquitin specific proteinases and coordinate calcium reuptake at the sarcoplasmic reticulum

Subcellular targeting of signaling enzymes influences where and when various modes of intracellular communication operate. Macromolecular complexes of signal transduction and signal termination elements favor reversable control of repetitive processes. This includes adrenergic stimulation of excitation-contraction coupling in the heart. Long isoforms of A-kinase anchoring protein 18 (AKAP18γ and δ) modulate this process via regulation of calcium uptake into the sarcoplasmic reticulum through the Ca2+ATPase 2a (SERCA2a). AKAP18 proximity-proteomic screening in cardiomyocytes identifies networks for protein kinase A (PKA) and ubiquitin-specific proteinases (USP's). A 2'phosphoesterase domain on AKAP18 interfaces with the USP4 isoform at the Z bands of sarcomeres. PKA stimulates USP4 activity in the presence of the anchoring protein. AKAP18 anchored PKA phosphorylates serine 829 on USP4, a conserved residue near the active site of this deubiquitinase. Antibodies against the pSer829 motif show that adrenergic stimulation enhances phosphorylation of USP4 in mouse adult cardiomyocytes. In related studies, elevated USP4 phosphorylation at Ser829 is detected in human post-myocardial infraction tissue as compared to healthy tissue. Thus, phosphorylation of sarcoplasmic USP4 may be a cardioprotective response. Pharmacological inhibition of PKA or deletion of the AKAP7/18 gene in mice decreases calcium flux through the exchanger. This suggests that loss of the anchoring protein impacts SERCA2 action. Thus, AKAP18/PKA/USP4 complexes are well positioned to influence the rate and magnitude of calcium reuptake during the cardiac cycle.

A SUMO-interacting motif in the guanine nucleotide exchange factor EPAC1 is required for subcellular targeting and function

Exchange protein directly activated by cAMP (EPAC1), a multifunctional intracellular cAMP receptor, dynamically localizes to various cellular loci, engaging with diverse molecular partners to maintain cellular homeostasis. The study investigates the role of the SUMO interacting motif (SIM) in the subcellular targeting and cellular functions of EPAC1. It reveals that the SIM is a critical structural element for EPAC1's association with RanBP2/Nup358, a nucleoporin of the cytoplasmic filament component of the nuclear pore complex (NPC). Mutational disruption of EPAC1 SIM interferes with EPAC1's ability to activate its canonical effectors, small GTPases, Rap1 and Rap2, and non-canonical functions, such as the formation of nuclear condensates and cellular SUMOylation. Because SIM is also directly involved in cAMP binding, RanBP2's association with EPAC1 with the SIM attenuates EPAC1's cAMP binding affinity to generate an EPAC1 signaling microdomain with reduced cAMP sensitivity around the NPC. The coupling between EPAC1's scaffold association and cAMP binding enables EPAC1 to tune its sensitivity to stress stimuli spatially depending on the cellular locations. These findings provide novel structural insights into EPAC1 signaling, highlighting the importance of SIM in EPAC1's cellular functions and potential novel strategies for therapeutically targeting EPAC1.

The evolution of AKAPs and emergence of PKA isotype selective anchoring determinants

Cyclic AMP is a versatile signaling molecule utilized throughout the eukaryotic domain. A frequent use is to activate protein kinase A (PKA), a serine/threonine kinase that drives many physiological responses. Spatiotemporal organization of PKA occurs though association with A-kinase anchoring proteins (AKAPs). Sequence alignments and phylogenetic analyses trace the evolution of PKA regulatory (R) and catalytic (C) subunits and AKAPs from the emergence of metazoans. AKAPs that preferentially associate with the type I (RI) or type II (RII) regulatory subunits diverged at the advent of the vertebrate clade. Type I PKA anchoring proteins including smAKAP contain an FA motif at positions 1 and 2 of their amphipathic binding helices. Fluorescence recovery after photobleaching measurements indicate smAKAP preferentially associates with RI (T 1/2. 4.37 ± 1.2 s; n = 3) as compared to RII (T 1/2. 2.19 ± 0.5 s; n = 3). Parallel studies measured AKAP79 recovery half times of 8.74 ± 0.3 s (n = 3) for RI and 14.42 ± 2.1 s (n = 3) and for RII, respectively. Introduction of FA and AF motifs at either ends of the AKAP79 helix biases the full-length anchoring protein toward type I PKA signaling to reduce corticosterone release from adrenal cells by 61.5 ± 0.8% (n = 3). Conversely, substitution of the YA motif at the beginning of the smAKAP helix for a pair of leucine's abrogates RI anchoring. Thus, AKAPs have evolved from the base of the metazoan clade into specialized type I and type II PKA anchoring proteins.

Kanglexin attenuates spinal cord injury by modulating pyroptosis and polarization via the PKA/NF-κB signaling pathway

BACKGROUND

Neuroinflammation is essential for intricate pathophysiologic mechanisms after spinal cord injury (SCI). Increasing evidence suggests that anthraquinones possess anti-inflammatory properties in central nervous system (CNS) disorders. However, the effects of Kanglexin (Klx), a novel synthetic anthraquinone compound, on SCI remain unknown.

METHODS

C57BL/6 mice were utilized to establish a contused SCI model to explore the in vivo neuroprotective and inflammatory modulatory effects of Klx. An inflammation model was also created in vitro using BV2 cells. Neuroprotective effects were assessed by evaluating motor function and neuropathologic alterations. Inflammation modulation was analyzed through markers of polarization and pyroptosis, with further mechanistic insights obtained via transcriptome sequencing.

RESULTS

Klx facilitated the recovery of hindlimb locomotor function and improved neuronal survival after SCI. Both in vitro and in vivo assays revealed that Klx inhibited NLRP3 inflammasome-induced pyroptosis. In addition, Klx promoted the polarization of microglia from the proinflammatory M1 phenotype to the anti-inflammatory M2 phenotype. Mechanistically, Klx enhanced PKA phosphorylation and suppressed NF-κB and IκBα phosphorylation, thereby reducing NF-κB nuclear translocation.

CONCLUSION

Klx demonstrated neuroprotective and inflammation-modulating effects on SCI, suggesting that it might offer a promising therapeutic alternative for SCI.

Autophagosomes anchor an AKAP11-dependent regulatory checkpoint that shapes neuronal PKA signaling

Protein Kinase A (PKA) is regulated spatially and temporally via scaffolding of its catalytic (Cα) and regulatory (RI/RII) subunits by the A-kinase-anchoring proteins (AKAP). By binding to an AKAP11 scaffold, PKA engages in poorly understood interactions with autophagy, a key degradation pathway for neuronal cell homeostasis. Mutations in AKAP11 promote schizophrenia and bipolar disorders (SZ-BP) through unknown mechanisms. Here, through proteomic-based analyses of immunopurified lysosomes, we identify the Cα-RIα-AKAP11 holocomplex as a prominent autophagy-associated protein-kinase complex. AKAP11 scaffolds Cα-RIα interaction with the autophagic machinery via its LC3-interacting region (LIR), enabling both PKA regulation by upstream signals, and its autophagy-dependent degradation. We identify Ser83 on the RIα linker-hinge region as an AKAP11-dependent phospho-residue that modulates RIα-Cα binding to the autophagosome and cAMP-induced PKA activation. Decoupling AKAP11-PKA from autophagy alters downstream phosphorylation events, supporting an autophagy-dependent checkpoint for PKA signaling. Ablating AKAP11 in induced pluripotent stem cell-derived neurons reveals dysregulation of multiple pathways for neuronal homeostasis. Thus, the autophagosome is a platform that modulates PKA signaling, providing a possible mechanistic link to SZ/BP pathophysiology.

Multiple cAMP/PKA complexes at the STIM1 ER/PM junction specified by E-Syt1 and E-Syt2 reciprocally gates ANO1 (TMEM16A) via Ca2

ANO1 plays a crucial role in determining numerous physiological functions, including epithelial secretion, yet its regulatory mechanisms remain incompletely understood. Here, we describe a fundamental dynamic regulation of ANO1 surface expression and Ca2+-dependent gating via the cAMP/PKA pathway at the STIM1 ER/PM junctions. At these junctions, STIM1 assembles AC-AKAP-PKA complexes, while E-Syt1 mediates formation of ANO1-VAPA-IRBIT-E-Syt1-AC8-AKAP5-PKA complex, that phosphorylates ANO1 S673, increasing ANO1 Ca2+ affinity. Within these complexes, the Ca2+ and cAMP pathways act synergistically to enhance ANO1 function. By contrast, E-Syt2 dissociates the ANO1-VAPA interaction, forming ANO1-IRBIT-E-Syt2-AC6-AKAP11-PKA complex that phosphorylates ANO1 S221, which markedly reduces ANO1 Ca2+ affinity. The effects of the E-Syts are primarily mediated by their reciprocal regulation of junctional PI(4)P, PI(4,5)P2 and PtdSer. Accordingly, IRBIT deletion in mice impairs receptor-stimulated activation of ANO1 and fluid secretion. These findings should have broad implications for ANO1 roles and functions across various tissues.

Hypothalamic opsin 3 suppresses MC4R signaling and potentiates Kir7.1 to promote food consumption

Mammalian opsin 3 (OPN3) is a member of the opsin family of G-protein-coupled receptors with ambiguous light sensitivity. OPN3 was first identified in the brain (and named encephalopsin) and subsequently found to be expressed in other tissues. In adipocytes, OPN3 is necessary for light responses that modulate lipolysis and glucose uptake, while OPN3 in human skin melanocytes regulates pigmentation in a light-independent manner. Despite its initial discovery in the brain, OPN3 functional mechanisms in the brain remain elusive. Here, we investigated the molecular mechanism of OPN3 function in the paraventricular nucleus (PVN) of the hypothalamus. We show that Opn3 is coexpressed with the melanocortin 4 receptor (Mc4r) in a population of PVN neurons, where it negatively regulates MC4R-mediated cAMP signaling in a specific and Gαi/o-dependent manner. Under baseline conditions, OPN3 via Gαi/o potentiates the activity of the inward rectifying Kir7.1 channel, previously shown to be closed in response to agonist-mediated activation of MC4R in a Gαs-independent manner. In mice, we found that Opn3 in Mc4r-expressing neurons regulates food consumption. Our results reveal the first mechanistic insight into OPN3 function in the hypothalamus, uncovering a unique mechanism by which OPN3 functions to potentiate Kir7.1 activity and negatively regulate MC4R-mediated cAMP signaling, thereby promoting food intake.

Extensive location bias of the GPCR-dependent translatome via site-selective activation of mTOR

G protein-coupled receptors (GPCRs) modulate various physiological functions by rewiring cellular gene expression in response to extracellular signals. Control of gene expression by GPCRs has been studied almost exclusively at the transcriptional level, neglecting an extensive amount of regulation that takes place translationally. Hence, little is known about the nature and mechanisms of gene-specific posttranscriptional regulation downstream of receptor activation. Here, we apply an unbiased multiomics approach to delineate an extensive translational regulatory program initiated by the prototypical beta2-adrenergic receptor (β2-AR) and provide mechanistic insights into how these processes are orchestrated. Using ribosome profiling (Ribo-seq), we identify nearly 120 gene targets of adrenergic receptor activity for which expression is exclusively regulated at the level of translation. We next show that all translational changes are induced selectively by endosomal β2-ARs and report that this proceeds through activation of the mammalian target of rapamycin (mTOR) pathway. Specifically, within the set of translational GPCR targets, we find significant enrichment of genes with 5' terminal oligopyrimidine (TOP) motifs, a gene class classically known to be translationally regulated by mTOR. We then demonstrate that endosomal β2-ARs are required for mTOR activation and subsequent mTOR-dependent TOP mRNA translation. This site-selective crosstalk between the pathways is observed in multiple cell models with native β2-ARs, across a range of endogenous and synthetic adrenergic agonists, and for other GPCRs with intracellular activity. Together, this comprehensive analysis of drug-induced translational regulation establishes a critical role for location-biased GPCR signaling in fine-tuning the cellular protein landscape.

A signal transduction blind spot: the function of adenylyl cyclase transmembrane domains

Signal transduction of external primary signals into intracellular elevations of the second messenger cyclic AMP is an ancient and universal regulatory mechanism in biology. In mammals, 9 of the 10 adenylyl cyclases (ACs) share a common topology that includes a large transmembrane (TM) domain assembled from two clusters of six helices. This domain accounts for ~ 35% of the coding sequence but, remarkably, its function is still an open question. In this viewpoint, we consider how the first primary AC sequences spurred ideas for the purpose of AC TM domains, including voltage-sensing and transporter functions. In the original conceptions of second messenger signalling, ACs were put forward as potential receptors, and we discuss emerging evidence in support of this function. We also consider growing evidence that cyclase TM helical bundles help to organise multiprotein signalling complexes by engaging in interactions with other membrane-embedded proteins. Cyclase TM regions are more diverse between isoforms than the catalytic domain-we conclude by considering how this might be exploited in therapeutic strategies targeting specific cyclase isoforms.

The promise of cyclic AMP modulation to restore cognitive function in neurodevelopmental disorders

Cyclic AMP (cAMP) is a key regulator of synaptic function and is dysregulated in both neurodevelopmental (NDD) and neurodegenerative disorders. Due to the ease of diffusion and promiscuity of downstream effectors, cAMP signaling is restricted within spatiotemporal domains to localize activation. Among the best-studied mechanisms is the feedback inhibition of cAMP-dependent protein kinase (PKA) activity by phosphodiesterases 4 (PDE4s) at synapses controlling neuronal plasticity, which is largely regulated by PDE4D. In fact, genetic variants in genes for multiple PKA subunits and PDE4D lead to NDDs. Here, we discuss the rationale for choosing PDE4D as a candidate for the design of selective allosteric inhibitors and the recent advances in clinical trials. These new compounds improve cognitive function in preclinical animal models due to improved selectivity and more physiological inhibition of the active enzyme. We also discuss opportunities for better understanding of PDE4D function in general, and for the development of next-generation inhibitors.

Research progress on Chaihu Shugan San in treating perimenopausal syndrome: A review

Perimenopausal syndrome (PMS) refers to a series of physical and psychological symptoms caused by fluctuating or decreasing sexual hormone levels during the pre- and postmenopausal periods. With the rapid development of society, more and more women suffer from menstrual disorders and insomnia caused by PMS. Chaihu Shugan San (CSS) is a famous traditional Chinese medicine prescription known for soothing the liver, relieving depression, and regulating qi and blood. Numerous clinical experiments and pharmacological studies have confirmed that CSS has a significant effect on PMS treatment. However, the composition of CSS is complex, its pharmacological effects are diverse, and its therapeutic mechanism for PMS has not been clearly explained. Therefore, this article reviews the classical literature, mechanism, pharmacological effects and clinical research of CSS in the treatment of PMS, so as to provide a reference for clinical application and further research.

Targeted protein degradation of PDE4 shortforms by a novel proteolysis targeting chimera

Cyclic AMP (cAMP) has a crucial role in many vital cellular processes and there has been much effort expended in the discovery of inhibitors against the enzyme superfamily that degrades this second messenger, namely phosphodiesterases (PDEs). The journey of competitive PDE inhibitors to the clinic has been hampered by side effects profiles that have resulted from a lack of selectivity for subfamilies and individual isoforms because of high conservation of catalytic site sequences and structures. Here we introduce a proteolysis targeting chimera (PROTAC) that can specifically target a small subset of isoforms from the PDE4 family to send the enzyme for degradation at the proteasome by recruiting a ubiquitin E3 ligase into proximity with the PDE. We constructed our PDE4 PROTAC (KTX207) using a previously characterized PDE4 inhibitor, and we show that evolution of the compound into a PROTAC improves selectivity, potency and enables a long-lasting effect even after the compound is removed from cells after a short treatment duration. Functionally, KTX207 is more effective at increasing cAMP, is 100 times more anti-inflammatory, and is significantly better at reducing the growth in cancer cell models than the PDE4 inhibitor alone. Our study highlights the advantages of targeted degradation versus active-site occupancy for PDE4 inhibition and discusses the potential of this novel pharmacological approach to improve the safety profile of PDE4 inhibition in the future.

Protein kinase A and local signaling in cancer

Protein kinase A (PKA) is a basophilic kinase implicated in the modulation of many cell-signaling and physiological processes. PKA also contributes to cancer-relevant events such as growth factor action, cell cycle control, cell migration and tumor metabolism. Germline and somatic mutations in PKA, gene amplifications, and chromosome rearrangements that encode kinase fusions, are linked to a growing number of malignant neoplasms. Mislocalization of PKA by exclusion from A-Kinase Anchoring Protein (AKAP) signaling islands further underlies cancer progression. This article highlights the influence of AKAP signaling and local kinase action in selected hallmarks of cancer. We also feature the utility of kinase inhibitor drugs as frontline and future anti-cancer therapies.

Understanding the impact of nuclear-localized GPCRs on cellular signalling

G protein-coupled receptors (GPCRs) have historically been associated with signalling events driven from the plasma membrane. More recently, signalling from endosomes has been recognized as a feature of internalizing receptors. However, there was little consideration given to the notion that GPCRs can be targeted to distinct subcellular locations that did not involve an initial trafficking to the cell surface. Here, we focus on the evidence for and the potential impact of GPCR signalling specifically initiated from the nuclear membrane. We also discuss the possibilities for selectively targeting this and other internal pools of receptors as novel venues for drug discovery.

Disruptions to protein kinase A localization in adrenal pathology

Cell signaling fidelity requires specificity in protein-protein interactions and precise subcellular localization of signaling molecules. In the case of protein phosphorylation, many kinases and phosphatases exhibit promiscuous substrate pairing and therefore require targeting interactions to modify the appropriate substrates and avoid cross-talk among different pathways. In the past 10 years, researchers have discovered and investigated how loss of specific interactions and subcellular targeting for the protein kinase A catalytic subunit (PKAc) lead to cortisol-producing adenomas and the debilitating stress disorder adrenal Cushing's syndrome. This article reviews classical studies regarding PKA localization in glucocorticoid-producing adrenal cells and synthesizes recent evidence of disrupted PKA localization and selective regulatory interactions in adrenal pathology.

Anchored PKA synchronizes adrenergic phosphoregulation of cardiac Cav1.2 channels

Adrenergic modulation of voltage gated Ca2+ currents is a context specific process. In the heart Cav1.2 channels initiate excitation-contraction coupling. This requires PKA phosphorylation of the small GTPase Rad (Ras associated with diabetes) and involves direct phosphorylation of the Cav1.2 α1 subunit at Ser1700. A contributing factor is the proximity of PKA to the channel through association with A-kinase anchoring proteins (AKAPs). Disruption of PKA anchoring by the disruptor peptide AKAP-IS prevents upregulation of Cav1.2 currents in tsA-201 cells. Biochemical analyses demonstrate that Rad does not function as an AKAP. Electrophysiological recording shows that channel mutants lacking phosphorylation sites (Cav1.2 STAA) lose responsivity to the second messenger cAMP. Measurements in cardiomyocytes isolated from Rad-/- mice show that adrenergic activation of Cav1.2 is attenuated but not completely abolished. Whole animal electrocardiography studies reveal that cardiac selective Rad KO mice exhibited higher baseline left ventricular ejection fraction, greater fractional shortening, and increased heart rate as compared to control animals. Yet, each parameter of cardiac function was slightly elevated when Rad-/- mice were treated with the adrenergic agonist isoproterenol. Thus, phosphorylation of Cav1.2 and dissociation of phospho-Rad from the channel are local cAMP responsive events that act in concert to enhance L-type calcium currents. This convergence of local PKA regulatory events at the cardiac L-type calcium channel may permit maximal β-adrenergic influence on the fight-or-flight response.

Autophagosomes coordinate an AKAP11-dependent regulatory checkpoint that shapes neuronal PKA signaling

Protein Kinase A (PKA) is regulated spatially and temporally via scaffolding of its catalytic (Cα/β) and regulatory (RI/RII) subunits by the A-kinase-anchoring proteins (AKAP). PKA engages in poorly understood interactions with autophagy, a key degradation pathway for neuronal cell homeostasis, partly via its AKAP11 scaffold. Mutations in AKAP11 drive schizophrenia and bipolar disorders (SZ-BP) through unknown mechanisms. Through proteomic-based analysis of immunopurified lysosomes, we identify the Cα-RIα-AKAP11 holocomplex as a prominent autophagy-associated protein kinase complex. AKAP11 scaffolds Cα-RIα to the autophagic machinery via its LC3-interacting region (LIR), enabling both PKA regulation by upstream signals, and its autophagy-dependent degradation. We identify Ser83 on the RIα linker-hinge region as an AKAP11-dependent phospho-residue that modulates RIα-Cα binding and cAMP-induced PKA activation. Decoupling AKAP11-PKA from autophagy alters Ser83 phosphorylation, supporting an autophagy-dependent checkpoint for PKA signaling. Ablating AKAP11 in induced pluripotent stem cell-derived neurons reveals dysregulation of multiple pathways for neuronal homeostasis. Thus, the autophagosome is a novel platform that modulate PKA signaling, providing a possible mechanistic link to SZ/BP pathophysiology.