New kids on the block: The Popeye domain containing (POPDC) protein family acting as a novel class of cAMP effector proteins in striated muscle

Popeye domain containing proteins modulate the voltage-gated cardiac sodium channel Nav1.5

Popeye domain containing (POPDC) proteins are predominantly expressed in the heart and skeletal muscle, modulating the K2P potassium channel TREK-1 in a cAMP-dependent manner. POPDC1 and POPDC2 variants cause cardiac conduction disorders with or without muscular dystrophy. Searching for POPDC2-modulated ion channels using a functional co-expression screen in Xenopus oocytes, we found POPDC proteins to modulate the cardiac sodium channel Nav1.5. POPDC proteins downregulate Nav1.5 currents in a cAMP-dependent manner by reducing the surface expression of the channel. POPDC2 and Nav1.5 are both expressed in different regions of the murine heart and consistently POPDC2 co-immunoprecipitates with Nav1.5 from native cardiac tissue. Strikingly, the knock-down of popdc2 in embryonic zebrafish caused an increased upstroke velocity and overshoot of cardiac action potentials. The POPDC modulation of Nav1.5 provides a new mechanism to regulate cardiac sodium channel densities under sympathetic stimulation, which is likely to have a functional impact on cardiac physiology and inherited arrhythmias.

Dysregulated Cyclic Nucleotide Metabolism in Alcohol-Associated Steatohepatitis: Implications for Novel Targeted Therapies

BACKGROUND

Cyclic nucleotides are second messengers, which play significant roles in numerous biological processes. Previous work has shown that cAMP and cGMP signaling regulates various pathways in liver cells, including Kupffer cells, hepatocytes, hepatic stellate cells, and cellular components of hepatic sinusoids. Importantly, it has been shown that cAMP levels and enzymes involved in cAMP homeostasis are affected by alcohol. Although the role of cyclic nucleotide signaling is strongly implicated in several pathological pathways in liver diseases, studies describing the changes in genes regulating cyclic nucleotide metabolism in ALD are lacking.

METHODS

Male C57B/6 mice were used in an intragastric model of alcohol-associated steatohepatitis (ASH). Liver injury, inflammation, and fibrogenesis were evaluated by measuring plasma levels of injury markers, liver tissue cytokines, and gene expression analyses. Liver transcriptome analysis was performed to examine the effects of alcohol on regulators of cyclic AMP and GMP levels and signaling. cAMP and cGMP levels were measured in mouse livers as well as in livers from healthy human donors and patients with alcohol-associated hepatitis (AH).

RESULTS

Our results show significant changes in several phosphodiesterases (PDEs) with specificity to degrade cAMP (Pde4a, Pde4d, and Pde8a) and cGMP (Pde5a, Pde6d, and Pde9a), as well as dual-specificity PDEs (Pde1a and Pde10a) in ASH mouse livers. Adenylyl cyclases (ACs) 7 and 9, which are responsible for cAMP generation, were also affected by alcohol. Importantly, adenosine receptor 1, which has been implicated in the pathogenesis of liver diseases, was significantly increased by alcohol. Adrenoceptors 1 and 3 (Adrb), which couple with stimulatory G protein to regulate cAMP and cGMP signaling, were significantly decreased. Additionally, beta arrestin 2, which interacts with cAMP-specific PDE4D to desensitize G-protein-coupled receptor to generate cAMP, was significantly increased by alcohol. Notably, we observed that cAMP levels are much higher than cGMP levels in the livers of humans and mice; however, alcohol affected them differently. Specifically, cGMP levels were higher in patients with AH and ASH mice livers compared with controls. As expected, these changes in liver cyclic nucleotide signaling were associated with increased inflammation, steatosis, apoptosis, and fibrogenesis.

CONCLUSIONS

These data strongly implicate dysregulated cAMP and cGMP signaling in the pathogenesis of ASH. Future studies to identify changes in these regulators in a cell-specific manner could lead to the development of novel targeted therapies for ASH.

High extracellular glucose promotes cell motility by modulating cell deformability and contractility via the cAMP-RhoA-ROCK axis in human breast cancer cells

The mechanical properties, or mechanotypes, of cells are largely determined by their deformability and contractility. The ability of cancer cells to deform and generate contractile force is critical in multiple steps of metastasis. Identifying soluble cues that regulate cancer cell mechanotypes and understanding the underlying molecular mechanisms regulating these cellular mechanotypes could provide novel therapeutic targets to prevent metastasis. Although a strong correlation between high glucose level and cancer metastasis has been demonstrated, the causality has not been elucidated, and the underlying molecular mechanisms remain largely unknown. In this study, using novel high-throughput mechanotyping assays, we show that human breast cancer cells become less deformable and more contractile with increased extracellular glucose levels (>5 mM). These altered cell mechanotypes are due to increased F-actin rearrangement and nonmuscle myosin II (NMII) activity. We identify the cAMP-RhoA-ROCK-NMII axis as playing a major role in regulating cell mechanotypes at high extracellular glucose levels, whereas calcium and myosin light-chain kinase (MLCK) are not required. The altered mechanotypes are also associated with increased cell migration and invasion. Our study identifies key components in breast cancer cells that convert high extracellular glucose levels into changes in cellular mechanotype and behavior relevant in cancer metastasis.

A homozygous loss of function variant in POPDC3: From invalidating exercise intolerance to a limb-girdle muscular dystrophy phenotype

Recessive pathogenic variants in POPDC3 have recently been associated with the rare limb-girdle muscular dystrophy (LGMD) subtype LGMDR26. We studied three siblings and a distantly related individual with a skeletal muscle disorder, harboring the c.486-6T>A splice site variant in POPDC3 in homozygosity. Immunohistochemistry, western blot, and mRNA experiments on patients' skeletal muscle tissue as well as on patients' myoblasts were performed to study the pathogenicity of the predicted loss of function mechanism of the variant. Patients mainly presented with invalidating myalgia and exercise intolerance and limited to no segmentary muscle weakness. CK levels were markedly elevated in all patients. A loss of function mechanism at the RNA level was shown (r.485_486insauag, p.Ile163*). Muscle biopsies performed in three out of four patients showed non-specific myopathic features with a marked type 2 fiber predominance and the presence of a large number of severely atrophic fibers with pyknotic nuclear clumps. We show that skeletal muscle symptoms in LGMDR26 may range from an overt late juvenile to young adult-onset limb-girdle muscular dystrophy phenotype to severe exercise intolerance and myalgia, with consistently highly elevated CK levels. We further prove a clear LOF mechanism of POPDC3 in this rare disorder.

Non-inositol 1,4,5-trisphosphate (IP3) receptor IP3-binding proteins

Conventionally, myo-D-inositol 1, 4,5-trisphosphate (IP₃) is thought to exert its second messenger effects through the gating of IP₃R Ca²⁺ release channels, located in Ca²⁺-storage organelles like the endoplasmic reticulum. However, there is considerable indirect evidence to support the concept that IP₃ might interact with other, non-IP₃R proteins within cells. To explore this possibility further, the Protein Data Bank was searched using the term "IP3". This resulted in the retrieval of 203 protein structures, the majority of which were members of the IP₃R/ryanodine receptor superfamily of channels. Only 49 of these structures were complexed with IP₃. These were inspected for their ability to interact with the carbon-1 phosphate of IP₃, since this is the least accessible phosphate group of its precursor, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). This reduced the number of structures retrieved to 35, of which 9 were IP₃Rs. The remaining 26 structures represent a diverse range of proteins, including inositol-lipid metabolizing enzymes, signal transducers, PH domain containing proteins, cytoskeletal anchor proteins, the TRPV4 ion channel, a retroviral Gag protein and fibroblast growth factor 2. Such proteins may impact on IP₃ signalling and its effects on cell-biology. This represents an area open for exploration in the field of IP₃ signalling.

Quantitative phosphoproteomic analysis reveals unique cAMP signaling pools emanating from AC2 and AC6 in human airway smooth muscle cells

Human airway smooth muscle (HASM) is the primary target of ßAR agonists used to control airway hypercontractility in asthma and chronic obstructive pulmonary disease (COPD). ßAR agonists induce the production of cAMP by adenylyl cyclases (ACs), activate PKA and cause bronchodilation. Several other G-protein coupled receptors (GPCR) expressed in human airway smooth muscle cells transduce extracellular signals through cAMP but these receptors elicit different cellular responses. Some G-protein coupled receptors couple to distinct adenylyl cyclases isoforms with different localization, partly explaining this compartmentation, but little is known about the downstream networks that result. We used quantitative phosphoproteomics to define the downstream signaling networks emanating from cAMP produced by two adenylyl cyclases isoforms with contrasting localization in uman airway smooth muscle. After a short stimulus of adenylyl cyclases activity using forskolin, phosphopeptides were analyzed by LC-MS/MS and differences between cells overexpressing AC2 (localized in non-raft membranes) or AC6 (localized in lipid raft membranes) were compared to control human airway smooth muscle. The degree of AC2 and AC6 overexpression was titrated to generate roughly equal forskolin-stimulated cAMP production. 14 Differentially phosphorylated proteins (DPPs) resulted from AC2 activity and 34 differentially phosphorylated proteins resulted from AC6 activity. Analysis of these hits with the STRING protein interaction tool showed that AC2 signaling is more associated with modifications in RNA/DNA binding proteins and microtubule/spindle body proteins while AC6 signaling is associated with proteins regulating autophagy, calcium-calmodulin (Ca2+/CaM) signaling, Rho GTPases and cytoskeletal regulation. One protein, OFD1, was regulated in opposite directions, with serine 899 phosphorylation increased in the AC6 condition 1.5-fold but decreased to 0.46-fold by AC2. In conclusion, quantitative phosphoproteomics is a powerful tool for deciphering the complex signaling networks resulting from discreet signaling events that occur in cAMP compartments. Our data show key differences in the cAMP pools generated from AC2 and AC6 activity and imply that distinct cellular responses are regulated by these two compartments.

Study on differentially expressed genes between stage M and stage MS neuroblastoma

Objective

To search for the DEGs between stage MS NB and stage M NB and speculate the possible mechanism of spontaneous regression of stage MS NB.

Materials and methods

The NB datasets GSE49710 and GSE45547 in the GEO database were selected to screen the DEGs between children with NB stage MS vs. stage M, < 18 months. GO enrichment and KEGG pathway analysis of DEGs was performed using DAVID. The intersecting genes among DEGs and RCD-related genes were selected, and their survival roles and functions were assessed. We then used the collected clinical samples to validate the expression of these genes at the protein level using IHC methods and further analysis to explore their role.

Results

BIRC5, SLCO4A1, POPDC3, and HK2 were found to be downregulated in stage MS NB and related to apoptosis. BIRC5 and HK2 also participate in autophagy. The TF gene is upregulated in stage MS NB and related to ferroptosis. The above five genes are closely related to the survival of children with NB. And the expression levels of all five genes at the protein level were verified by IHC to be consistent with the results of the preliminary screening described above.

Conclusion

BIRC5, SLCO4A1, POPDC3, HK2 and TF are expected to become new important indicators to predict the prognosis of NB and can be used as the basis for further explored the benign prognosis and spontaneous regression mechanism of stage MS NB.

POPDC1 scaffolds a complex of adenylyl cyclase 9 and the potassium channel TREK-1 in heart

The establishment of macromolecular complexes by scaffolding proteins is key to the local production of cAMP by anchored adenylyl cyclase (AC) and the subsequent cAMP signaling necessary for cardiac functions. We identify a novel AC scaffold, the Popeye domain-containing (POPDC) protein. The POPDC family of proteins is important for cardiac pacemaking and conduction, due in part to their cAMP-dependent binding and regulation of TREK-1 potassium channels. We show that TREK-1 binds the AC9:POPDC1 complex and copurifies in a POPDC1-dependent manner with AC9 activity in heart. Although the AC9:POPDC1 interaction is cAMP-independent, TREK-1 association with AC9 and POPDC1 is reduced upon stimulation of the β-adrenergic receptor (βAR). AC9 activity is required for βAR reduction of TREK-1 complex formation with AC9:POPDC1 and in reversing POPDC1 enhancement of TREK-1 currents. Finally, deletion of the gene-encoding AC9 (Adcy9) gives rise to bradycardia at rest and stress-induced heart rate variability, a milder phenotype than the loss of Popdc1 but similar to the loss of Kcnk2 (TREK-1). Thus, POPDC1 represents a novel adaptor for AC9 interactions with TREK-1 to regulate heart rate control.

Bves maintains vascular smooth muscle cell contractile phenotype and protects against transplant vasculopathy via Dusp1-dependent p38MAPK and ERK1/2 signaling

BACKGROUND AND AIMS

Vascular smooth muscle cell (VSMC) plasticity is tightly associated with the pathological process of vasculopathy. Blood vessel epicardial substance (Bves) has emerged as an important regulator of intracardiac vasculogenesis and organ homeostasis. However, the involvement and role of Bves in VSMC plasticity and neointimal lesion development remain unclear.

METHODS

We used an in vivo rat model of graft arteriosclerosis and in vitro PDGF-treated VSMCs and identified the novel VSMC contractile phenotype-related gene Bves using a transcriptomic analysis and literature search. In vitro knockdown and overexpression approaches were used to investigate the mechanisms underlying VSMC phenotypic plasticity. In vivo, VSMC-specific Bves overexpression in rat aortic grafts was generated to assess the physiological function of Bves in neointimal lesion development.

RESULTS

Here, we found that Bves expression was negatively regulated in aortic allografts in vivo and PDGF-treated VSMCs in vitro. The genetic knockdown of Bves dramatically inhibited, whereas Bves overexpression markedly promoted, the VSMC contractile phenotype. Furthermore, RNA sequencing unraveled a positive correlation between Bves and dual-specificity protein phosphatase 1 (Dusp1) expression in VSMCs. We found that Bves knockdown restrained Dusp1 expression, but enhanced p38MAPK and ERK1/2 activation, resulting in the loss of the VSMC contractile phenotype. In vivo, an analysis of a rat graft model confirmed that VSMC-specific Bves and Dusp1 overexpression in aortic allografts significantly attenuated neointimal lesion formation.

CONCLUSIONS

Bves maintains the VSMC contractile phenotype through Dusp1-dependent p38MAPK and ERK1/2 signaling, and protects against neointimal formation, underscoring the important role of Bves in preventing transplant vasculopathy.

A novel splice site variant in the POPDC3 causes autosomal recessive limb-girdle muscular dystrophy type 26

Limb-Girdle muscular dystrophy (LGMD) is a group of muscle disorders with highly heterogeneous genetic patterns and clinical phenotypes, and this group includes multiple subtypes. Different LGMD subtypes have similar phenotypes and clinical overlaps，these subtypes are difficult to distinguish by clinical symptoms alone and can only be accurately diagnosed by analysis in combination with definitive genetic test results. Here, we report a female presenting features of LGMD. After analysis of whole-exome sequencing data, a novel homozygous POPDC3 variant c.486-1G>A (rs113419658) located in the acceptor splice site of intron 2 was identified in the proband. The variant effect on splicing were analyzed by genetic analysis based on cDNA synthesised by the patient's RNA. cDNA analysis indicated that the novel homozygous POPDC3 splice variant disrupted original acceptor splice site, which can cause a frameshift in the mRNA of the POPDC3 gene, thereby producing a truncated POPDC3 protein and ultimately affecting its normal function. POPDC3 variant was recently associated with recessive limb-girdle muscular dystrophy type 26 (LGMDR26). Based on the above results, we hypothesize that this variant is probably a pathogenic variant, and expand the gene variant spectrum of POPDC3. This article is protected by copyright. All rights reserved.

Proteomic and morphological insights and clinical presentation of two young patients with novel mutations of BVES (POPDC1)

Popeye domain containing protein 1 (POPDC1) is a highly conserved transmembrane protein essential for striated muscle function and homeostasis. Pathogenic variants in the gene encoding POPDC1 (BVES, Blood vessel epicardial substance) are causative for limb-girdle muscular dystrophy (LGMDR25), associated with cardiac arrhythmia. We report on four affected children (age 7-19 years) from two consanguineous families with two novel pathogenic variants in BVES c.457C>T(p.Q153X) and c.578T>G (p.I193S). Detailed analyses were performed on muscle biopsies from an affected patient of each family including immunofluorescence, electron microscopy and proteomic profiling. Cardiac abnormalities were present in all patients and serum creatine kinase (CK) values were variably elevated despite lack of overt muscle weakness. Detailed histological analysis of skeletal muscle, however indicated a myopathy with reduced sarcolemmal expression of POPDC1 accompanied by altered sarcolemmal and sarcoplasmatic dysferlin and Xin/XIRP1 abundance. At the electron microscopic level, the muscle fiber membrane was focally disrupted. The proteomic signature showed statistically significant dysregulation of 191 proteins of which 173 were increased and 18 were decreased. Gene ontology-term analysis of affected biological processes revealed - among others - perturbation of muscle fibril assembly, myofilament sliding, and contraction as well as transition between fast and slow fibers. In conclusion, these findings demonstrate that the phenotype of LGMDR25 is highly variable and also includes younger children with conduction abnormalities, no apparent muscular problems, and only mildly elevated CK values. Biochemical studies suggest that BVES mutations causing loss of functional POPDC1 can impede striated muscle function by several mechanisms.

Homozygous missense variant in POPDC3 causes recessive limb girdle muscular dystrophy type 26

BACKGROUND

Limb-girdle muscular dystrophy (LGMD) is a heterogeneous group of diseases, which affects different muscles, predominantly skeletal muscles and cardiac muscles of the body. LGMD is classified into two main sub-types A and B, which are further sub-classified into eight dominant and thirty recessive sub-types. Three genes, mainly POPDC1, POPDC2 and POPDC3, encodes popeye domain-containing protein (POPDC), and the variants of POPDC1 and POPDC3 genes have been associated with LGMD.

METHODS

In this study, we performed whole-exome sequencing (WES) analysis on a single-family to investigate the hallmark features of LGMD. The results of WES were further confirmed by Sanger sequencing and 3D protein modeling was also performed.

RESULTS

WES data analysis and sanger sequencing revealed a homozygous missense variant (c.460A>G; p.Lys154Glu) at a highly conserved amino acid position in the POPDC3. Mutations in the POPDC3 gene have been previously associated with recessive limb-girdle muscular dystrophy type 26. 3D protein modeling further suggested that the identified variant might affect the POPDC3 structure and proper function.

DISCUSSION/CONCLUSIONS

This study confirms the role of POPDC3 in LGMD, and will facilitate in genetic counseling of the family to mitigate the risks of the carrier or affected in future pregnancies.

Phosphodiesterase type 4 anchoring regulates cAMP signaling to Popeye domain-containing proteins

Cyclic AMP is a ubiquitous second messenger used to transduce intracellular signals from a variety of Gs-coupled receptors. Compartmentalisation of protein intermediates within the cAMP signaling pathway underpins receptor-specific responses. The cAMP effector proteins protein-kinase A and EPAC are found in complexes that also contain phosphodiesterases whose presence ensures a coordinated cellular response to receptor activation events. Popeye domain containing (POPDC) proteins are the most recent class of cAMP effectors to be identified and have crucial roles in cardiac pacemaking and conduction. We report the first observation that POPDC proteins exist in complexes with members of the PDE4 family in cardiac myocytes. We show that POPDC1 preferentially binds the PDE4A sub-family via a specificity motif in the PDE4 UCR1 region and that PDE4s bind to the Popeye domain of POPDC1 in a region known to be susceptible to a mutation that causes human disease. Using a cell-permeable disruptor peptide that displaces the POPDC1-PDE4 complex we show that PDE4 activity localized to POPDC1 modulates cycle length of spontaneous Ca²⁺ transients firing in intact mouse sinoatrial nodes.

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POPDC1 forms a complex with type 4 phosphodiesterases (PDE4s) in cardiac myocytes.

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POPDC1 binds PDE4 enzymes in the Upstream Conserved Region 1 (UCR1) domain.

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The PDE4 binding motif within the Popeye domain lies in a region that harbours a mutation, which underpins human disease.

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Disruption of the POPDC1-PDE4 complex modulates the cycle length of spontaneous Ca2+ transients in the sinoatrial node.

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Disruption of the POPDC1-PDE4 complex causes a significant prolongation of the action potential repolarization phase.

Real-Time Measurements of Intracellular cAMP Gradients Using FRET-Based cAMP Nanorulers

3',5'-cyclic adenosine monophosphate (cAMP) is one of the most important and ubiquitous second messengers in cells downstream of G protein-coupled receptors (GPCRs). In a single cell, cAMP can exert innumerous specific cell functions in response to more than one hundred different GPCRs. Cells achieve this extraordinary functional specificity of cAMP signaling by limiting the spread of these signals in space and time. To do so, cells establish nanometer-size cAMP gradients by immobilizing cAMP via cAMP binding proteins and via targeted activity of cAMP-degrading phosphodiesterases (PDEs). As cAMP gradients appear to be essential for cell function, new technologies are needed to accurately measure cAMP gradients in intact cells with nanometer-resolution. Here we describe FRET-based cAMP nanorulers to measure local, nanometer-size cAMP gradients in intact cells in the direct vicinity of PDEs.

The cAMP-signaling cancers: Clinically-divergent disorders with a common central pathway

The cAMP-signaling cancers, which are defined by functionally-significant somatic mutations in one or more elements of the cAMP signaling pathway, have an unexpectedly wide range of cell origins, clinical manifestations, and potential therapeutic options. Mutations in at least 9 cAMP signaling pathway genes (TSHR, GPR101, GNAS, PDE8B, PDE11A, PRKARA1, PRKACA, PRKACB, and CREB) have been identified as driver mutations in human cancer. Although all cAMP-signaling pathway cancers are driven by mutation(s) that impinge on a single signaling pathway, the ultimate tumor phenotype reflects interactions between five critical variables: (1) the precise gene(s) that undergo mutation in each specific tumor type; (2) the effects of specific allele(s) in any given gene; (3) mutations in modifier genes (mutational "context"); (4) the tissue-specific expression of various cAMP signaling pathway elements in the tumor stem cell; and (5) and the precise biochemical regulation of the pathway components in tumor cells. These varying oncogenic mechanisms reveal novel and important targets for drug discovery. There is considerable diversity in the "druggability" of cAMP-signaling components, with some elements (GPCRs, cAMP-specific phosphodiesterases and kinases) appearing to be prime drug candidates, while other elements (transcription factors, protein-protein interactions) are currently refractory to robust drug-development efforts. Further refinement of the precise driver mutations in individual tumors will be essential for directing priorities in drug discovery efforts that target these mutations.

Physiological Roles of Mammalian Transmembrane Adenylyl Cyclase Isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors. The transmembrane ACs display varying expression patterns across tissues, giving potential for them having a wide array of physiologic roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gα_s so it was long assumed that all ACs are activated by Gα_s-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

Electrophysiological Remodeling: Cardiac T-Tubules and ß-Adrenoceptors

Beta-adrenoceptors (βAR) are often viewed as archetypal G-protein coupled receptors. Over the past fifteen years, investigations in cardiovascular biology have provided remarkable insights into this receptor family. These studies have shifted pharmacological dogma, from one which centralized the receptor to a new focus on structural micro-domains such as caveolae and t-tubules. Important studies have examined, separately, the structural compartmentation of ion channels and βAR. Despite links being assumed, relatively few studies have specifically examined the direct link between structural remodeling and electrical remodeling with a focus on βAR. In this review, we will examine the nature of receptor and ion channel dysfunction on a substrate of cardiomyocyte microdomain remodeling, as well as the likely ramifications for cardiac electrophysiology. We will then discuss the advances in methodologies in this area with a specific focus on super-resolution microscopy, fluorescent imaging, and new approaches involving microdomain specific, polymer-based agonists. The advent of powerful computational modelling approaches has allowed the science to shift from purely empirical work, and may allow future investigations based on prediction. Issues such as the cross-reactivity of receptors and cellular heterogeneity will also be discussed. Finally, we will speculate as to the potential developments within this field over the next ten years.

Mechanisms of cAMP compartmentation in cardiac myocytes: experimental and computational approaches to understanding

The small diffusible second messenger 3',5'-cyclic adenosine monophosphate (cAMP) is found in virtually every cell in our bodies, where it mediates responses to a variety of different G protein coupled receptors (GPCRs). In the heart, cAMP plays a critical role in regulating many different aspects of cardiac myocyte function, including gene transcription, cell metabolism, and excitation-contraction coupling. Yet, not all GPCRs that stimulate cAMP production elicit the same responses. Subcellular compartmentation of cAMP is essential to explain how different receptors can utilize the same diffusible second messenger to elicit unique functional responses. However, the mechanisms contributing to this behaviour and its significance in producing physiological and pathological responses are incompletely understood. Mathematical modelling has played an essential role in gaining insight into these questions. This review discusses what we currently know about cAMP compartmentation in cardiac myocytes and questions that are yet to be answered.

Key radioresistance regulation models and marker genes identified by integrated transcriptome analysis in nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a malignancy that is endemic to China and Southeast Asia. Radiotherapy is the usual treatment, however, radioresistance remains a major reason for failure. This study aimed to find key radioresistance regulation models and marker genes of NPC and clarify the mechanism of NPC radioresistance by RNA sequencing and bioinformatics analysis of the differences in gene expression profiles between radioresistant and radiosensitive NPC tissues. A total of 21 NPC biopsy specimens with different radiosensitivity were analyzed by RNA sequencing. Differentially expressed genes in RNA sequencing data were identified using R software. The differentially expressed gene data derived from RNA sequencing as well as prior knowledge in the form of pathway databases were integrated to find sub‐networks of related genes. The data of RNA sequencing with the GSE48501 data from the GEO database were combined to further search for more reliable genes associated with radioresistance of NPC. Survival analyses using the Kaplan–Meier method based on the expression of the genes were conducted to facilitate the understanding of the clinical significance of the differentially expressed genes. RT‐qPCR was performed to validate the expression levels of the differentially expressed genes. We identified 1182 differentially expressed genes between radioresistant and radiosensitive NPC tissue samples. Compared to the radiosensitive group, 22 genes were significantly upregulated and 1160 genes were downregulated in the radioresistant group. In addition, 10 major NPC radiation resistance network models were identified through integration analysis with known NPC radiation resistance‐associated genes and mechanisms. Furthermore, we identified three core genes, DOCK4, MCM9, and POPDC3 among 12 common downregulated genes in the two datasets, which were validated by RT‐qPCR. The findings of this study provide new clues for clarifying the mechanism of NPC radioresistance, and further experimental studies of these core genes are warranted.

The Transition from Gastric Intestinal Metaplasia to Gastric Cancer Involves POPDC1 and POPDC3 Downregulation

Intestinal metaplasia (IM) is an intermediate step in the progression from premalignant to malignant stages of gastric cancer (GC). The Popeye domain containing (POPDC) gene family encodes three transmembrane proteins, POPDC1, POPDC2, and POPDC3, initially described in muscles and later in epithelial and other cells, where they function in cell–cell interaction, and cell migration. POPDC1 and POPDC3 downregulation was described in several tumors, including colon and gastric cancers. We questioned whether IM-to-GC transition involves POPDC gene dysregulation. Gastric endoscopic biopsies of normal, IM, and GC patients were examined for expression levels of POPDC1-3 and several suggested IM biomarkers, using immunohistochemistry and qPCR. Immunostaining indicated lower POPDC1 and POPDC3 labeling in IM compared with normal tissues. Significantly lower POPDC1 and POPDC3 mRNA levels were measured in IM and GC biopsies and in GC-derived cell lines. The reduction in focal IM was smaller than in extensive IM that resembled GC tissues. POPDC1 and POPDC3 transcript levels were highly correlated with each other and inversely correlated with LGR5, OLFM4, CDX2, and several mucin transcripts. The association of POPDC1 and POPDC3 downregulation with IM-to-GC transition implicates a role in tumor suppression and highlights them as potential biomarkers for GC progression and prospective treatment targets.

Local cyclic adenosine monophosphate signalling cascades-Roles and targets in chronic kidney disease

The molecular mechanisms underlying chronic kidney disease (CKD) are poorly understood and treatment options are limited, a situation underpinning the need for elucidating the causative molecular mechanisms and for identifying innovative treatment options. It is emerging that cyclic 3',5'-adenosine monophosphate (cAMP) signalling occurs in defined cellular compartments within nanometre dimensions in processes whose dysregulation is associated with CKD. cAMP compartmentalization is tightly controlled by a specific set of proteins, including A-kinase anchoring proteins (AKAPs) and phosphodiesterases (PDEs). AKAPs such as AKAP18, AKAP220, AKAP-Lbc and STUB1, and PDE4 coordinate arginine-vasopressin (AVP)-induced water reabsorption by collecting duct principal cells. However, hyperactivation of the AVP system is associated with kidney damage and CKD. Podocyte injury involves aberrant AKAP signalling. cAMP signalling in immune cells can be local and slow the progression of inflammatory processes typical for CKD. A major risk factor of CKD is hypertension. cAMP directs the release of the blood pressure regulator, renin, from juxtaglomerular cells, and plays a role in Na⁺ reabsorption through ENaC, NKCC2 and NCC in the kidney. Mutations in the cAMP hydrolysing PDE3A that cause lowering of cAMP lead to hypertension. Another major risk factor of CKD is diabetes mellitus. AKAP18 and AKAP150 and several PDEs are involved in insulin release. Despite the increasing amount of data, an understanding of functions of compartmentalized cAMP signalling with relevance for CKD is fragmentary. Uncovering functions will improve the understanding of physiological processes and identification of disease-relevant aberrations may guide towards new therapeutic concepts for the treatment of CKD.

Phosphoproteomic Analysis as an Approach for Understanding Molecular Mechanisms of cAMP-Dependent Actions

In recent years, highly sensitive mass spectrometry-based phosphoproteomic analysis is beginning to be applied to identification of protein kinase substrates altered downstream of increased cAMP. Such studies identify a very large number of phosphorylation sites regulated in response to increased cAMP. Therefore, we now are tasked with the challenge of determining how many of these altered phosphorylation sites are relevant to regulation of function in the cell. This minireview describes the use of phosphoproteomic analysis to monitor the effects of cyclic nucleotide phosphodiesterase (PDE) inhibitors on cAMP-dependent phosphorylation events. More specifically, it describes two examples of this approach carried out in the authors' laboratories using the selective PDE inhibitor approach. After a short discussion of several likely conclusions suggested by these analyses of cAMP function in steroid hormone-producing cells and also in T-cells, it expands into a discussion about some newer and more speculative interpretations of the data. These include the idea that multiple phosphorylation sites and not a single rate-limiting step likely regulate these and, by analogy, many other cAMP-dependent pathways. In addition, the idea that meaningful regulation requires a high stoichiometry of phosphorylation to be important is discussed and suggested to be untrue in many instances. These new interpretations have important implications for drug design, especially for targeting pathway agonists. SIGNIFICANCE STATEMENT: Phosphoproteomic analyses identify thousands of altered phosphorylation sites upon drug treatment, providing many possible regulatory targets but also highlighting questions about which phosphosites are functionally important. These data imply that multistep processes are regulated by phosphorylation at not one but rather many sites. Most previous studies assumed a single step or very few rate-limiting steps were changed by phosphorylation. This concept should be changed. Previous interpretations also assumed substoichiometric phosphorylation was not of regulatory importance. This assumption also should be changed.

Potential therapeutic applications of AKAP disrupting peptides

The 3'-5'-cyclic adenosine monophosphate (cAMP)/PKA pathway represents a major target for pharmacological intervention in multiple disease conditions. Although the last decade saw the concept of highly compartmentalized cAMP/PKA signaling consolidating, current means for the manipulation of this pathway still do not allow to specifically intervene on discrete cAMP/PKA microdomains. Since compartmentalization is crucial for action specificity, identifying new tools that allow local modulation of cAMP/PKA responses is an urgent need. Among key players of cAMP/PKA signaling compartmentalization, a major role is played by A-kinase anchoring proteins (AKAPs) that, by definition, anchor PKA, its substrates and its regulators within multiprotein complexes in well-confined subcellular compartments. Different tools have been conceived to interfere with AKAP-based protein-protein interactions (PPIs), and these primarily include peptides and peptidomimetics that disrupt AKAP-directed multiprotein complexes. While these molecules have been extensively used to understand the molecular mechanisms behind AKAP function in pathophysiological processes, less attention has been devoted to their potential application for therapy. In this review, we will discuss how AKAP-based PPIs can be pharmacologically targeted by synthetic peptides and peptidomimetics.

The role of Popeye domain-containing protein 1 (POPDC1) in the progression of the malignant phenotype

The Popeye domain-containing protein 1 (POPDC1), a tight junction-associated transmembrane protein with a unique binding site for cAMP, has been shown to act as a tumour suppressor in cancer cells. Through interaction with many downstream effectors and signalling pathways, POPDC1 promotes cell adhesion and inhibits uncontrolled cell proliferation, epithelial-to-mesenchymal transition and metastasis. However, POPDC1 expression is down-regulated in many types of cancer, thereby reducing its tumour-suppressive actions. This review discusses the role of POPDC1 in the progression of the malignant phenotype and highlights the broad range of benefits POPDC1 stabilisation may achieve therapeutically. Cancer stem cells (CSCs) are a key hallmark of malignancies and commonly promote treatment resistance. This article provides a comprehensive overview of CSC signalling mechanisms, many of which have been shown to be regulated by POPDC1 in other cell types, thus suggesting an additional therapeutic benefit for POPDC1-stabilising anti-cancer drugs.

The therapeutic potential of targeting exchange protein directly activated by cyclic adenosine 3',5'-monophosphate (Epac) for central nervous system trauma

Millions of people worldwide are affected by traumatic spinal cord injury, which usually results in permanent sensorimotor disability. Damage to the spinal cord leads to a series of detrimental events including ischaemia, haemorrhage and neuroinflammation, which over time result in further neural tissue loss. Eventually, at chronic stages of traumatic spinal cord injury, the formation of a glial scar, cystic cavitation and the presence of numerous inhibitory molecules act as physical and chemical barriers to axonal regrowth. This is further hindered by a lack of intrinsic regrowth ability of adult neurons in the central nervous system. The intracellular signalling molecule, cyclic adenosine 3′,5′-monophosphate (cAMP), is known to play many important roles in the central nervous system, and elevating its levels as shown to improve axonal regeneration outcomes following traumatic spinal cord injury in animal models. However, therapies directly targeting cAMP have not found their way into the clinic, as cAMP is ubiquitously present in all cell types and its manipulation may have additional deleterious effects. A downstream effector of cAMP, exchange protein directly activated by cAMP 2 (Epac2), is mainly expressed in the adult central nervous system, and its activation has been shown to mediate the positive effects of cAMP on axonal guidance and regeneration. Recently, using ex vivo modelling of traumatic spinal cord injury, Epac2 activation was found to profoundly modulate the post-lesion environment, such as decreasing the activation of astrocytes and microglia. Pilot data with Epac2 activation also suggested functional improvement assessed by in vivo models of traumatic spinal cord injury. Therefore, targeting Epac2 in traumatic spinal cord injury could represent a novel strategy in traumatic spinal cord injury repair, and future work is needed to fully establish its therapeutic potential.

Advances, Perspectives and Potential Engineering Strategies of Light-Gated Phosphodiesterases for Optogenetic Applications

The second messengers, cyclic adenosine 3'-5'-monophosphate (cAMP) and cyclic guanosine 3'-5'-monophosphate (cGMP), play important roles in many animal cells by regulating intracellular signaling pathways and modulating cell physiology. Environmental cues like temperature, light, and chemical compounds can stimulate cell surface receptors and trigger the generation of second messengers and the following regulations. The spread of cAMP and cGMP is further shaped by cyclic nucleotide phosphodiesterases (PDEs) for orchestration of intracellular microdomain signaling. However, localized intracellular cAMP and cGMP signaling requires further investigation. Optogenetic manipulation of cAMP and cGMP offers new opportunities for spatio-temporally precise study of their signaling mechanism. Light-gated nucleotide cyclases are well developed and applied for cAMP/cGMP manipulation. Recently discovered rhodopsin phosphodiesterase genes from protists established a new and direct biological connection between light and PDEs. Light-regulated PDEs are under development, and of demand to complete the toolkit for cAMP/cGMP manipulation. In this review, we summarize the state of the art, pros and cons of artificial and natural light-regulated PDEs, and discuss potential new strategies of developing light-gated PDEs for optogenetic manipulation.

Establishing a sensitive fluorescence-based quantification method for cyclic nucleotides

BACKGROUND

Approximately 40% of prescribed drugs exert their activity via GTP-binding protein-coupled receptors (GPCRs). Once activated, these receptors cause transient changes in the concentration of second messengers, e.g., cyclic adenosine 3',5'-monophosphate (cAMP). Specific and efficacious genetically encoded biosensors have been developed to monitor cAMP fluctuations with high spatial and temporal resolution in living cells or tissue. A well characterized biosensor for cAMP is the Förster resonance energy transfer (FRET)-based Epac1-camps protein. Pharmacological characterization of newly developed ligands acting at GPCRs often includes numerical quantification of the second messenger amount that was produced.

RESULTS

To quantify cellular cAMP concentrations, we bacterially over-expressed and purified Epac1-camps and applied the purified protein in a cell-free detection assay for cAMP in a multi-well format. We found that the biosensor can detect as little as 0.15 pmol of cAMP, and that the sensitivity is not impaired by non-physiological salt concentrations or pH values. Notably, the assay tolerated desiccation and storage of the protein without affecting Epac1-camps cyclic nucleotide sensitivity.

CONCLUSIONS

We found that determination cAMP in lysates obtained from cell assays or tissue samples by purified Epac1-camps is a robust, fast, and sensitive assay suitable for routine and high throughput analyses.

Sinigrin Enhanced Antiasthmatic Effects of Beta Adrenergic Receptors Agonists by Regulating cAMP-Mediated Pathways

Millions of patients suffer from asthma worldwide. However, the first-line drugs used to treat asthma, namely, the beta-adrenergic receptors agonists (β-agonists), are not recommended for use as monotherapy because of their severe dose-related side effects. This limitation has prompted the search for new therapies, which can be used in conjunction with β--agonists so that lower doses can be administered. Sinigrin is a major compound found in many antiasthmatic medicinal plants. In this study, we explored the antiasthmatic activity of sinigrin when used in combination with β-agonists and its underlying mechanism. Sinigrin enhanced the asthma-relieving effects of isoproterenol and reduced the effective isoproterenol dose in an acute-asthma model in guinea pigs. Mechanistically, sinigrin enhanced the cAMP levels induced by β-agonists by inhibiting PDE4. The resulting increase in cAMP levels stimulated the activity of the downstream effector protein kinase A, which would be expected to ultimately induce the relaxation of airway smooth muscle. In conclusion, sinigrin enhances the asthma-relieving effects of β-agonists by regulating the cAMP signaling pathway and represents a potential add-on drug to β-agonists for the treatment of asthma.

Reshaping cAMP nanodomains through targeted disruption of compartmentalised phosphodiesterase signalosomes

Spatio-temporal regulation of localised cAMP nanodomains is highly dependent upon the compartmentalised activity of phosphodiesterase (PDE) cyclic nucleotide degrading enzymes. Strategically positioned PDE-protein complexes are pivotal to the homeostatic control of cAMP-effector protein activity that in turn orchestrate a wide range of cellular signalling cascades in a variety of cells and tissue types. Unsurprisingly, dysregulated PDE activity is central to the pathophysiology of many diseases warranting the need for effective therapies that target PDEs selectively. This short review focuses on the importance of activating compartmentalised cAMP signalling by displacing the PDE component of signalling complexes using cell-permeable peptide disrupters.

POPDC proteins and cardiac function

The Popeye domain-containing gene family encodes a novel class of cAMP effector proteins in striated muscle tissue. In this short review, we first introduce the protein family and discuss their structure and function with an emphasis on their role in cyclic AMP signalling. Another focus of this review is the recently discovered role of POPDC genes as striated muscle disease genes, which have been associated with cardiac arrhythmia and muscular dystrophy. The pathological phenotypes observed in patients will be compared with phenotypes present in null and knockin mutations in zebrafish and mouse. A number of protein-protein interaction partners have been discovered and the potential role of POPDC proteins to control the subcellular localization and function of these interacting proteins will be discussed. Finally, we outline several areas, where research is urgently needed.

The Role of the Popeye Domain Containing Gene Family in Organ Homeostasis

The Popeye domain containing (POPDC) gene family consists of POPDC1 (also known as BVES), POPDC2 and POPDC3 and encodes a novel class of cyclic adenosine monophosphate (cAMP) effector proteins. Despite first reports of their isolation and initial characterization at the protein level dating back 20 years, only recently major advances in defining their biological functions and disease association have been made. Loss-of-function experiments in mice and zebrafish established an important role in skeletal muscle regeneration, heart rhythm control and stress signaling. Patients suffering from muscular dystrophy and atrioventricular block were found to carry missense and nonsense mutations in either of the three POPDC genes, which suggests an important function in the control of striated muscle homeostasis. However, POPDC genes are also expressed in a number of epithelial cells and function as tumor suppressor genes involved in the control of epithelial structure, tight junction formation and signaling. Suppression of POPDC genes enhances tumor cell proliferation, migration, invasion and metastasis in a variety of human cancers, thus promoting a malignant phenotype. Moreover, downregulation of POPDC1 and POPDC3 expression in different cancer types has been associated with poor prognosis. However, high POPDC3 expression has also been correlated to poor clinical prognosis in head and neck squamous cell carcinoma, suggesting that POPDC3 potentially plays different roles in the progression of different types of cancer. Interestingly, a gain of POPDC1 function in tumor cells inhibits cell proliferation, migration and invasion thereby reducing malignancy. Furthermore, POPDC proteins have been implicated in the control of cell cycle genes and epidermal growth factor and Wnt signaling. Work in tumor cell lines suggest that cyclic nucleotide binding may also be important in epithelial cells. Thus, POPDC proteins have a prominent role in tissue homeostasis and cellular signaling in both epithelia and striated muscle.

The Epac1 Protein: Pharmacological Modulators, Cardiac Signalosome and Pathophysiology

The second messenger 3′,5′-cyclic adenosine monophosphate (cAMP) is one of the most important signalling molecules in the heart as it regulates many physiological and pathophysiological processes. In addition to the classical protein kinase A (PKA) signalling route, the exchange proteins directly activated by cAMP (Epac) mediate the intracellular functions of cAMP and are now emerging as a new key cAMP effector in cardiac pathophysiology. In this review, we provide a perspective on recent advances in the discovery of new chemical entities targeting the Epac1 isoform and illustrate their use to study the Epac1 signalosome and functional characterisation in cardiac cells. We summarize the role of Epac1 in different subcompartments of the cardiomyocyte and discuss how cAMP–Epac1 specific signalling networks may contribute to the development of cardiac diseases. We also highlight ongoing work on the therapeutic potential of Epac1-selective small molecules for the treatment of cardiac disorders.

Selective small-molecule EPAC activators

The cellular signalling enzymes, EPAC1 and EPAC2, have emerged as key intracellular sensors of the secondary messenger cyclic 3',5'-adenosine monophosphate (cyclic adenosine monophosphate) alongside protein kinase A. Interest has been galvanised in recent years thanks to the emergence of these species as potential targets for new cardiovascular disease therapies, including vascular inflammation and insulin resistance in vascular endothelial cells. We herein summarise the current state-of-the-art in small-molecule EPAC activity modulators, including cyclic nucleotides, sulphonylureas, and N-acylsulphonamides.

POPDC3 Gene Variants Associate with a New Form of Limb Girdle Muscular Dystrophy

OBJECTIVE

The Popeye domain containing 3 (POPDC3) gene encodes a membrane protein involved in cyclic adenosine monophosphate (cAMP) signaling. Besides gastric cancer, no disease association has been described. We describe a new muscular dystrophy associated with this gene.

METHODS

We screened 1,500 patients with unclassified limb girdle weakness or hyperCKemia for pathogenic POPDC3 variants. Five patients carrying POPDC3 variants were examined by muscle magnetic resonance imaging (MRI), muscle biopsy, and cardiac examination. We performed functional analyses in a zebrafish popdc3 knockdown model and heterologous expression of the mutant proteins in Xenopus laevis oocytes to measure TREK-1 current.

RESULTS

We identified homozygous POPDC3 missense variants (p.Leu155His, p.Leu217Phe, and p.Arg261Gln) in 5 patients from 3 ethnically distinct families. Variants affected highly conserved residues in the Popeye (p.Leu155 and p.Leu217) and carboxy-terminal (p.Arg261) domains. The variants were almost absent from control populations. Probands' muscle biopsies were dystrophic, and serum creatine kinase levels were 1,050 to 9,200U/l. Muscle weakness was proximal with adulthood onset in most patients and affected lower earlier than upper limbs. Muscle MRI revealed fat replacement of paraspinal and proximal leg muscles; cardiac investigations were unremarkable. Knockdown of popdc3 in zebrafish, using 2 different splice-site blocking morpholinos, resulted in larvae with tail curling and dystrophic muscle features. All 3 mutants cloned in Xenopus oocytes caused an aberrant modulation of the mechano-gated potassium channel, TREK-1.

INTERPRETATION

Our findings point to an important role of POPDC3 for skeletal muscle function and suggest that pathogenic variants in POPDC3 are responsible for a novel type of autosomal recessive limb girdle muscular dystrophy. ANN NEUROL 2019;86:832-843.

POPDC3 is a potential biomarker for prognosis and radioresistance in patients with head and neck squamous cell carcinoma

Radiotherapy is the primary means of treatment for patients with head and neck squamous cell carcinoma (HNSCC); however, radioresistance-induced recurrence is the primary cause of HNSCC treatment failure. Therefore, identifying specific predictive biomarkers of the response to radiotherapy may improve prognosis. In the present study, to identify the potential candidate genes associated with radioresistance in patients with HNSCC, the microarray datasets GSE9716, GSE61772 and GSE20549 were downloaded from the Gene Expression Omnibus database. The original CEL files were preprocessed using the Affymetrix package and quantile normalization and background correction were conducted using the Core package in Bioconductor. The GSE9716 dataset, consisting of 18 irradiated and 16 non-irradiated samples, was divided into two groups according to their exposure to irradiation: i) Non-irradiation group, which included 8 radioresistant samples and 8 radiosensitive samples; and ii) post-irradiation group, which included 9 radioresistant samples and 9 radiosensitive samples. The two groups were treated as separate datasets and screened. A total of 86 differentially expressed genes (DEGs) were identified in the non-irradiation group and 405 DEGs in the post-irradiation group. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analysis detected several significant functions associated with the DEGs. In the co-expression analysis, 76 hub genes in the light green module and 917 hub genes with a high connectivity were selected for further analysis. Finally, overlapping the DEGs and hub genes from the two groups yielded a map of 13 shared differentially expressed genes. The 13 genes showed significantly different expression in radioresistant samples compared with the radiosensitive samples before and after irradiation. Out of these genes, popeye domain-containing protein 3 (POPDC3) was highly expressed in the post-irradiation group compared with the non-irradiation group. In survival analysis, high POPDC3 expression correlated with poor a prognosis for patients with HNSCC. The independent prognostic factors were identified using univariate and multivariate Cox analyses based on The Cancer Genome Atlas database. These were incorporated into a nomogram to predict 3- and 5-year overall survival. Receiver operating characteristic curves were used to estimate the accuracy of the nomogram. Together these studies suggest that POPDC3 may serve as a potential predictive biomarker for prognosis and radioresistance of patients with HNSCC as well as clinical diagnosis and treatment of patients.

Small molecules for modulating the localisation of the water channel aquaporin-2-disease relevance and perspectives for targeting local cAMP signalling

The tight spatial and temporal organisation of cyclic adenosine monophosphate (cAMP) signalling plays a key role in arginine-vasopressin (AVP)-mediated water reabsorption in renal collecting duct principal cells and in a plethora of other processes such as in the control of cardiac myocyte contractility. This review critically discusses in vitro- and cell-based screening strategies for the identification of small molecules that interfere with AVP/cAMP signalling in renal principal cells; it features phenotypic screening and approaches for targeting protein-protein interactions of A-kinase anchoring proteins (AKAPs), which organise local cAMP signalling hubs. The discovery of novel chemical entities for the modulation of local cAMP will not only provide tools for elucidating molecular mechanisms underlying cAMP signalling. Novel chemical entities can also serve as starting points for the development of novel drugs for the treatment of human diseases. Examples illustrate how screening for small molecules can pave the way to novel approaches for the treatment of certain forms of diabetes insipidus, a disease caused by defects in AVP-mediated water reabsorption.

The Popeye domain containing gene family encoding a family of cAMP-effector proteins with important functions in striated muscle and beyond

The Popeye domain containing (POPDC) gene family encodes a novel class of membrane-bound cyclic AMP effector proteins. POPDC proteins are abundantly expressed in cardiac and skeletal muscle. Consistent with its predominant expression in striated muscle, Popdc1 and Popdc2 null mutants in mouse and zebrafish develop cardiac arrhythmia and muscular dystrophy. Likewise, mutations in POPDC genes in patients have been associated with cardiac arrhythmia and muscular dystrophy phenotypes. A membrane trafficking function has been identified in this context. POPDC proteins have also been linked to tumour formation. Here, POPDC1 plays a role as a tumour suppressor by limiting c-Myc and WNT signalling. Currently, a common functional link between POPDC’s role in striated muscle and as a tumour suppressor is lacking. We also discuss several alternative working models to better understand POPDC protein function.

cAMP/PKA signaling compartmentalization in cardiomyocytes: Lessons from FRET-based biosensors

3',5'-cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger produced in response to the stimulation of G protein-coupled receptors (GPCRs). It regulates a plethora of pathophysiological processes in different organs, including the cardiovascular system. It is now clear that cAMP is not uniformly distributed within cardiac myocytes but confined in specific subcellular compartments where it modulates key players of the excitation-contraction coupling as well as other processes including gene transcription, mitochondrial homeostasis and cell death. This review will cover the major cAMP microdomains in cardiac myocytes. We will describe recent work using pioneering tools developed for investigating the organization and the function of the major cAMP microdomains in cardiomyocytes, including the plasma membrane, the sarcoplasmic reticulum, the myofilaments, the nucleus and the mitochondria.

Muscular dystrophy with arrhythmia caused by loss-of-function mutations in BVES

Objective

To study the genetic and phenotypic spectrum of patients harboring recessive mutations in BVES.

Methods

We performed whole-exome sequencing in a multicenter cohort of 1929 patients with a suspected hereditary myopathy, showing unexplained limb-girdle muscular weakness and/or elevated creatine kinase levels. Immunohistochemistry and mRNA experiments on patients' skeletal muscle tissue were performed to study the pathogenicity of identified loss-of-function (LOF) variants in BVES.

Results

We identified 4 individuals from 3 families harboring homozygous LOF variants in BVES, the gene that encodes for Popeye domain containing protein 1 (POPDC1). Patients showed skeletal muscle involvement and cardiac conduction abnormalities of varying nature and severity, but all exhibited at least subclinical signs of both skeletal muscle and cardiac disease. All identified mutations lead to a partial or complete loss of function of BVES through nonsense-mediated decay or through functional changes to the POPDC1 protein.

Conclusions

We report the identification of homozygous LOF mutations in BVES, causal in a young adult-onset myopathy with concomitant cardiac conduction disorders in the absence of structural heart disease. These findings underline the role of POPDC1, and by extension, other members of this protein family, in striated muscle physiology and disease. This disorder appears to have a low prevalence, although it is probably underdiagnosed because of its striking phenotypic variability and often subtle yet clinically relevant manifestations, particularly concerning the cardiac conduction abnormalities.

Multiple phosphorylation sites on the RegA phosphodiesterase regulate Dictyostelium development

In Dictyostelium, the intracellular cAMP-specific phosphodiesterase RegA is a negative regulator of cAMP-dependent protein kinase (PKA), a key determinant in the timing of developmental morphogenesis and spore formation. To assess the role of protein kinases in the regulation of RegA function, this study identified phosphorylation sites on RegA and characterized the role of these modifications through the analysis of phospho-mimetic and phospho-ablative mutations. Mutations affecting residue T676 of RegA, a presumed target of the atypical MAP kinase Erk2, altered the rate of development and impacted cell distribution in chimeric organisms suggesting that phosphorylation of this residue reduces RegA function and regulates cell localization during multicellular development. Mutations affecting the residue S142 of RegA also impacted the rate developmental morphogenesis but in a manner opposite of changes at T676 suggesting the phosphorylation of the S142 residue increases RegA function. Mutations affecting residue S413 residue altered aggregate sizes and delayed developmental progression suggesting that PKA operates in a negative feedback mechanism to increase RegA function. These results suggest that the phosphorylation of different residues on RegA can lead to increased or decreased RegA function and therefore in turn regulate developmental processes such as aggregate formation, cell distribution, and the kinetics of developmental morphogenesis.

Ten years of research on the role of BVES/ POPDC1 in human disease: a review

Since the blood vessel epicardial substance or Popeye domain-containing protein 1 (BVES/POPDC1) was first identified in the developing heart by two independent laboratories in 1999, an increasing number of studies have investigated the structure, function, and related diseases of BVES/POPDC1. During the first 10 years following the discovery of BVES/POPDC1, studies focused mainly on its structure, expression patterns, and functions. Based on these studies, further investigations conducted over the previous decade examined the role of BVES/POPDC1 in human diseases, such as colitis, heart diseases, and human cancers. This review provides an overview of the structure and expression of BVES/POPDC1, mainly focusing on its potential role and mechanism through which it is involved in human cancers.

Exchange Protein Directly Activated by cAMP Modulates Ebola Virus Uptake into Vascular Endothelial Cells

Members of the family Filoviridae, including Ebola virus (EBOV) and Marburg virus (MARV), cause severe hemorrhagic fever in humans and nonhuman primates. Given their high lethality, a comprehensive understanding of filoviral pathogenesis is urgently needed. In the present studies, we revealed that the exchange protein directly activated by cAMP 1 (EPAC1) gene deletion protects vasculature in ex vivo explants from EBOV infection. Importantly, pharmacological inhibition of EPAC1 using EPAC-specific inhibitors (ESIs) mimicked the EPAC1 knockout phenotype in the ex vivo model. ESI treatment dramatically decreased EBOV infectivity in both ex vivo vasculature and in vitro vascular endothelial cells (ECs). Furthermore, postexposure protection of ECs against EBOV infection was conferred using ESIs. Protective efficacy of ESIs in ECs was observed also in MARV infection. Additional studies using a vesicular stomatitis virus pseudotype that expresses EBOV glycoprotein (EGP-VSV) confirmed that ESIs reduced infection in ECs. Ultrastructural studies suggested that ESIs blocked EGP-VSV internalization via inhibition of macropinocytosis. The inactivation of EPAC1 affects the early stage of viral entry after viral binding to the cell surface, but before early endosome formation, in a phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)-dependent manner. Our study delineated a new critical role of EPAC1 during EBOV uptake into ECs.

Protein kinase A activation inhibits DUX4 gene expression in myotubes from patients with facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is among the most prevalent of the adult-onset muscular dystrophies. FSHD causes a loss of muscle mass and function, resulting in severe debilitation and reduction in quality of life. Currently, only the symptoms of FSHD can be treated, and such treatments have minimal benefit. The available options are not curative, and none of the treatments address the underlying cause of FSHD. The genetic, epigenetic, and molecular mechanisms triggering FSHD are now quite well-understood, and it has been shown that expression of the transcriptional regulator double homeobox 4 (DUX4) is necessary for disease onset and is largely thought to be the causative factor in FSHD. Therefore, we sought to identify compounds suppressing DUX4 expression in a phenotypic screen using FSHD patient-derived muscle cells, a zinc finger and SCAN domain-containing 4 (ZSCAN4)-based reporter gene assay for measuring DUX4 activity, and ∼3,000 small molecules. This effort identified molecules that reduce DUX4 gene expression and hence DUX4 activity. Among those, β₂-adrenergic receptor agonists and phosphodiesterase inhibitors, both leading to increased cellular cAMP, effectively decreased DUX4 expression by >75% in cells from individuals with FSHD. Of note, we found that cAMP production reduces DUX4 expression through a protein kinase A-dependent mode of action in FSHD patient myotubes. These findings increase our understanding of how DUX4 expression is regulated in FSHD and point to potential areas of therapeutic intervention.

The Popeye Domain Containing Genes and Their Function as cAMP Effector Proteins in Striated Muscle

The Popeye domain containing (POPDC) genes encode transmembrane proteins, which are abundantly expressed in striated muscle cells. Hallmarks of the POPDC proteins are the presence of three transmembrane domains and the Popeye domain, which makes up a large part of the cytoplasmic portion of the protein and functions as a cAMP-binding domain. Interestingly, despite the prediction of structural similarity between the Popeye domain and other cAMP binding domains, at the protein sequence level they strongly differ from each other suggesting an independent evolutionary origin of POPDC proteins. Loss-of-function experiments in zebrafish and mouse established an important role of POPDC proteins for cardiac conduction and heart rate adaptation after stress. Loss-of function mutations in patients have been associated with limb-girdle muscular dystrophy and AV-block. These data suggest an important role of these proteins in the maintenance of structure and function of striated muscle cells.

Roles of A-Kinase Anchoring Proteins and Phosphodiesterases in the Cardiovascular System

A-kinase anchoring proteins (AKAPs) and cyclic nucleotide phosphodiesterases (PDEs) are essential enzymes in the cyclic adenosine 3′-5′ monophosphate (cAMP) signaling cascade. They establish local cAMP pools by controlling the intensity, duration and compartmentalization of cyclic nucleotide-dependent signaling. Various members of the AKAP and PDE families are expressed in the cardiovascular system and direct important processes maintaining homeostatic functioning of the heart and vasculature, e.g., the endothelial barrier function and excitation-contraction coupling. Dysregulation of AKAP and PDE function is associated with pathophysiological conditions in the cardiovascular system including heart failure, hypertension and atherosclerosis. A number of diseases, including autosomal dominant hypertension with brachydactyly (HTNB) and type I long-QT syndrome (LQT1), result from mutations in genes encoding for distinct members of the two classes of enzymes. This review provides an overview over the AKAPs and PDEs relevant for cAMP compartmentalization in the heart and vasculature and discusses their pathophysiological role as well as highlights the potential benefits of targeting these proteins and their protein-protein interactions for the treatment of cardiovascular diseases.