cAMP-induced morphological changes are counteracted by the activated RhoA small GTPase and the Rho kinase ROKalpha

The role of osteocrin in memory formation during early learning, as revealed by visual imprinting in chicks

Osteocrin (OSTN) is structurally associated with natriuretic peptides. Its expression in the brain, which has only been recognized in anthropoid primates, is induced by sensory stimuli and regulates the activity-dependent dendritic growth of neurons. However, details on the signaling mechanisms of OSTN and its function in plastic changes during learning and memory have yet to be elucidated. We found that OSTN was expressed in the cortical region of the chicken brain. The injection of chicken OSTN (chOSTN) after imprinting training prolonged the memory retention for the imprinting stimulus. Conversely, a reduction in the OSTN receptor chNPR3 inhibited memory retention. The memory retention was positively correlated with a high level of chOSTN and fewer neurites in the cortical region. In conclusion, OSTN-NPR3 signaling promoted memory consolidation and/or retention by regulating neurite branching during childhood.

Focal Adhesion Kinase (FAK) inhibition induces membrane accumulation of aquaporin2 (AQP2) through endocytosis inhibition and actin depolymerization in renal epithelial cells

Cellular trafficking of the water channel aquaporin 2 (AQP2) is regulated by the actin cytoskeleton in collecting duct principal cells (PC) to maintain proper water balance in animals. Critical actin depolymerization/polymerization events are involved in both constitutive AQP2 recycling, and the pathway stimulated by vasopressin receptor signaling. Focal adhesion kinase (FAK) plays an important role in modulating the actin cytoskeleton through inhibiting small GTPases, and multiple studies have shown the involvement of FAK in insulin and cholesterol trafficking through actin regulation. To understand whether FAK contributes to water reabsorption by the kidney, we performed a series of in vitro experiments to examine the involvement of FAK and its signaling in mediating AQP2 trafficking in cultured renal epithelial cells. Our data showed that FAK inhibition by specific inhibitors caused membrane accumulation of AQP2 in AQP2expressing LLCPK1 cells by immunofluorescence staining. AQP2 membrane accumulation induced by FAK inhibition is associated with significantly reduced endocytosis of AQP2 via the clathrin-mediated endocytosis pathway. Moreover, AQP2 membrane accumulation induced by FAK inhibition also occurred in cells expressing the constitutive dephosphorylation mutant of AQP2, S256A. This was confirmed by immunoblotting using a specific antibody against phospho-serine 256 AQP2, supporting a phosphorylation independent mechanism. Finally, we demonstrated that inhibition of FAK caused reduced RhoA signaling and promoted F-actin depolymerization. In conclusion, our study identifies FAK signaling as a pathway that could provide a novel therapeutical avenue for AQP2 trafficking regulation in water balance disorders.

Actomyosin-mediated inhibition of synaptic vesicle release under CB1R activation

Long-term synaptic plasticity is critical for adaptive function of the brain, but presynaptic mechanisms of functional plasticity remain poorly understood. Here, we show that changes in synaptic efficacy induced by activation of the cannabinoid type-1 receptor (CB1R), one of the most widespread G-protein coupled receptors in the brain, requires contractility of the neuronal actomyosin cytoskeleton. Specifically, using a synaptophysin-pHluorin probe (sypH2), we show that inhibitors of non-muscle myosin II (NMII) ATPase as well as one of its upstream effectors Rho-associated kinase (ROCK) prevent the reduction of synaptic vesicle release induced by CB1R activation. Using 3D STORM super-resolution microscopy, we find that activation of CB1R induces a redistribution of synaptic vesicles within presynaptic boutons in an actomyosin dependent manner, leading to vesicle clustering within the bouton and depletion of synaptic vesicles from the active zone. We further show, using sypH2, that inhibitors of NMII and ROCK specifically restore the release of the readily releasable pool of synaptic vesicles from the inhibition induced by CB1R activation. Finally, using slice electrophysiology, we find that activation of both NMII and ROCK is necessary for the long-term, but not the short-term, form of CB1R induced synaptic plasticity at excitatory cortico-striatal synapses. We thus propose a novel mechanism underlying CB1R-induced plasticity, whereby CB1R activation leads to a contraction of the actomyosin cytoskeleton inducing a reorganization of the functional presynaptic vesicle pool, preventing vesicle release and inducing long-term depression.

Regulation of the water channel aquaporin-2 by cullin E3 ubiquitin ligases

Aquaporin 2 (AQP2) is a vasopressin (VP)-regulated water channel in the renal collecting duct. Phosphorylation and ubiquitylation of AQP2 play an essential role in controlling the cellular abundance of AQP2 and its accumulation on the plasma membrane in response to VP. Cullin-RING ubiquitin ligases (CRLs) are multisubunit E3 ligases involved in ubiquitylation and degradation of their target proteins, eight of which are expressed in the collecting duct. Here, we used an established cell model of the collecting duct (mpkCCD14 cells) to study the role of cullins in modulating AQP2. Western blotting identified Cul-1 to Cul-5 in mpkCCD14 cells. Treatment of cells for 4 h with a pan-cullin inhibitor (MLN4924) decreased AQP2 abundance, prevented a VP-induced reduction in AQP2 Ser261 phosphorylation, and attenuated VP-induced plasma membrane accumulation of AQP2 relative to the vehicle. AQP2 ubiquitylation levels were significantly higher after MLN4924 treatment compared with controls, and they remained higher despite VP treatment. Cullin inhibition increased ERK1/2 activity, a kinase that regulates AQP2 Ser261 phosphorylation, and VP-induced reductions in ERK1/2 phosphorylation were absent during MLN4924 treatment. Furthermore, the greater Ser261 phosphorylation and reduction in AQP2 abundance during MLN4924 treatment were attenuated during ERK1/2 inhibition. MLN4924 increased intracellular calcium levels via calcium release-activated calcium channels, inhibition of which abolished MLN4924 effects on Ser261 phosphorylation and AQP2 abundance. In conclusion, CRLs play a vital role in mediating some of the effects of VP to increase AQP2 plasma membrane accumulation and AQP2 abundance. Whether modulation of cullin activity can contribute to body water homeostasis requires further studies.NEW & NOTEWORTHY Aquaporin 2 (AQP2) is essential for body water homeostasis and is regulated by the antidiuretic hormone vasopressin. The posttranslational modification ubiquitylation is a key regulator of AQP2 abundance and plasma membrane localization. Here we demonstrate that cullin-RING E3 ligases play a vital role in mediating some of the effects of vasopressin to increase AQP2 abundance and plasma membrane accumulation. The results suggest that manipulating cullin activity could be a novel strategy to alter kidney water handling.

Pediococcus pentosaceus LAB6- and Lactiplantibacillus plantarum LAB12-Derived Cell Free Supernatant Inhibited RhoA Activation and Reduced Amyloid-Β In Vitro

Alzheimer's disease (AD) is characterized by aggregation of amyloid beta (Aβ) plaque. RhoA may serve as a potential target for prevention against AD given its role in the amyloidogenic pathway. The recent emergence of the gut-brain axis has linked lactic acid bacteria (LAB) to neuroprotection against AD. This study assessed the importance of RhoA inhibition in mediating the neuroprotective potential of LAB. To this end, de Man, Rogosa and Sharpe (MRS) broth fermented by lactobacilli or pediococci were tested against SK-N-SH (a human neuroblastoma cell line) in the presence of RhoA activator II for 24 h after which the RhoA activity was measured using the G-LISA Kit. Fluorescence staining of f-actin stress fibres was performed to validate RhoA inhibition. SK-N-SH was transfected with plasmid expressing amyloid precursor protein (APP) gene. The Aβ concentration in transfected cells exposed to LAB-derived cell free supernatant (CFS) in the presence of RhoA activator II was measured using the ELISA kit. Furthermore, this study measured organic acids in LAB-derived CFS using the gas chromatography. It was found that LAB-derived CFS yielded strain-dependent inhibition of RhoA, with LAB6- and LAB12-derived CFS being the most potent Pediococcal- and Lactiplantibacillus-based RhoA inhibitor, respectively. Lesser stress fibres were formed under treatment with LAB-derived CFS. The LAB-derived CFS also significantly inhibited Aβ in SK-N-SH transfected with APP gene in the presence of RhoA activator II. The LAB-derived CFS was presented with increased lactic acid, acetic acid, butyric acid and propionic acid. The present findings warrant in-depth study using animal models.

Hypoxia-inducible factor signaling in vascular calcification in chronic kidney disease patients

Chronic kidney disease (CKD) affects approximately 15% of the adult population in high-income countries and is associated with significant comorbidities, including increased vascular calcifications which is associated with a higher risk for cardiovascular events. Even though the underlying pathophysiology is unclear, hypoxia-inducible factor (HIF) signaling appears to play a central role in inflammation, angiogenesis, fibrosis, cellular proliferation, apoptosis and vascular calcifications which is influenced by multiple variables such as iron deficiency anemia, serum phosphorus and calcium levels, fibroblast growth factor-23 (FGF-23) and Klotho. Along with the growing understanding of the pathology, potential therapeutic alternatives have emerged including HIF stabilizers and SGLT-2 inhibitors. The aim of this review is to discuss the role of HIF signaling in the pathophysiology of vascular calcification in CKD patients and to identify potential therapeutic approaches.

RhoA as a Signaling Hub Controlling Glucagon Secretion From Pancreatic α-Cells

Glucagon hypersecretion from pancreatic islet α-cells exacerbates hyperglycemia in type 1 diabetes (T1D) and type 2 diabetes. Still, the underlying mechanistic pathways that regulate glucagon secretion remain controversial. Among the three complementary main mechanisms (intrinsic, paracrine, and juxtacrine) proposed to regulate glucagon release from α-cells, juxtacrine interactions are the least studied. It is known that tonic stimulation of α-cell EphA receptors by ephrin-A ligands (EphA forward signaling) inhibits glucagon secretion in mouse and human islets and restores glucose inhibition of glucagon secretion in sorted mouse α-cells, and these effects correlate with increased F-actin density. Here, we elucidate the downstream target of EphA signaling in α-cells. We demonstrate that RhoA, a Rho family GTPase, plays a key role in this pathway. Pharmacological inhibition of RhoA disrupts glucose inhibition of glucagon secretion in islets and decreases cortical F-actin density in dispersed α-cells and α-cells in intact islets. Quantitative FRET biosensor imaging shows that increased RhoA activity follows directly from EphA stimulation. We show that in addition to modulating F-actin density, EphA forward signaling and RhoA activity affect α-cell Ca2+ activity in a novel mechanistic pathway. Finally, we show that stimulating EphA forward signaling restores glucose inhibition of glucagon secretion from human T1D donor islets.

Protein Kinase A in cellular migration-Niche signaling of a ubiquitous kinase

Cell migration requires establishment and maintenance of directional polarity, which in turn requires spatial heterogeneity in the regulation of protrusion, retraction, and adhesion. Thus, the signaling proteins that regulate these various structural processes must also be distinctly regulated in subcellular space. Protein Kinase A (PKA) is a ubiquitous serine/threonine kinase involved in innumerable cellular processes. In the context of cell migration, it has a paradoxical role in that global inhibition or activation of PKA inhibits migration. It follows, then, that the subcellular regulation of PKA is key to bringing its proper permissive and restrictive functions to the correct parts of the cell. Proper subcellular regulation of PKA controls not only when and where it is active but also specifies the targets for that activity, allowing the cell to use a single, promiscuous kinase to exert distinct functions within different subcellular niches to facilitate cell movement. In this way, understanding PKA signaling in migration is a study in context and in the elegant coordination of distinct functions of a single protein in a complex cellular process.

Aurora Kinase A Is Involved in Controlling the Localization of Aquaporin-2 in Renal Principal Cells

The cAMP-dependent aquaporin-2 (AQP2) redistribution from intracellular vesicles into the plasma membrane of renal collecting duct principal cells induces water reabsorption and fine-tunes body water homeostasis. However, the mechanisms controlling the localization of AQP2 are not understood in detail. Using immortalized mouse medullary collecting duct (MCD4) and primary rat inner medullary collecting duct (IMCD) cells as model systems, we here discovered a key regulatory role of Aurora kinase A (AURKA) in the control of AQP2. The AURKA-selective inhibitor Aurora-A inhibitor I and novel derivatives as well as a structurally different inhibitor, Alisertib, prevented the cAMP-induced redistribution of AQP2. Aurora-A inhibitor I led to a depolymerization of actin stress fibers, which serve as tracks for the translocation of AQP2-bearing vesicles to the plasma membrane. The phosphorylation of cofilin-1 (CFL1) inactivates the actin-depolymerizing function of CFL1. Aurora-A inhibitor I decreased the CFL1 phosphorylation, accounting for the removal of the actin stress fibers and the inhibition of the redistribution of AQP2. Surprisingly, Alisertib caused an increase in actin stress fibers and did not affect CFL1 phosphorylation, indicating that AURKA exerts its control over AQP2 through different mechanisms. An involvement of AURKA and CFL1 in the control of the localization of AQP2 was hitherto unknown.

The cAMP effector PKA mediates Moody GPCR signaling in Drosophila blood-brain barrier formation and maturation

The blood-brain barrier (BBB) of Drosophila comprises a thin epithelial layer of subperineural glia (SPG), which ensheath the nerve cord and insulate it against the potassium-rich hemolymph by forming intercellular septate junctions (SJs). Previously, we identified a novel Gi/Go protein-coupled receptor (GPCR), Moody, as a key factor in BBB formation at the embryonic stage. However, the molecular and cellular mechanisms of Moody signaling in BBB formation and maturation remain unclear. Here, we identify cAMP-dependent protein kinase A (PKA) as a crucial antagonistic Moody effector that is required for the formation, as well as for the continued SPG growth and BBB maintenance in the larva and adult stage. We show that PKA is enriched at the basal side of the SPG cell and that this polarized activity of the Moody/PKA pathway finely tunes the enormous cell growth and BBB integrity. Moody/PKA signaling precisely regulates the actomyosin contractility, vesicle trafficking, and the proper SJ organization in a highly coordinated spatiotemporal manner. These effects are mediated in part by PKA's molecular targets MLCK and Rho1. Moreover, 3D reconstruction of SJ ultrastructure demonstrates that the continuity of individual SJ segments, and not their total length, is crucial for generating a proper paracellular seal. Based on these findings, we propose that polarized Moody/PKA signaling plays a central role in controlling the cell growth and maintaining BBB integrity during the continuous morphogenesis of the SPG secondary epithelium, which is critical to maintain tissue size and brain homeostasis during organogenesis.

Serotonin receptor 4 regulates hippocampal astrocyte morphology and function

Astrocytes are an important component of the multipartite synapse and crucial for proper neuronal network function. Although small GTPases of the Rho family are powerful regulators of cellular morphology, the signaling modules of Rho-mediated pathways in astrocytes remain enigmatic. Here we demonstrated that the serotonin receptor 4 (5-HT₄ R) is expressed in hippocampal astrocytes, both in vitro and in vivo. Through fluorescence microscopy, we established that 5-HT₄ R activation triggered RhoA activity via Gα₁₃ -mediated signaling, which boosted filamentous actin assembly, leading to morphological changes in hippocampal astrocytes. We investigated the effects of these 5-HT₄ R-mediated changes in mixed cultures and in acute slices, in which 5-HT₄ R was expressed exclusively in astrocytes. In both systems, 5-HT₄ R-RhoA signaling changed glutamatergic synaptic transmission: It increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in mixed cultures and reduced the paired-pulse-ratio (PPR) of field excitatory postsynaptic potentials (fEPSPs) in acute slices. Overall, our present findings demonstrate that astrocytic 5-HT₄ R-Gα₁₃ -RhoA signaling is a previously unrecognized molecular pathway involved in the functional regulation of excitatory synaptic circuits.

Gα/GSA-1 works upstream of PKA/KIN-1 to regulate calcium signaling and contractility in the Caenorhabditis elegans spermatheca

Correct regulation of cell contractility is critical for the function of many biological systems. The reproductive system of the hermaphroditic nematode C. elegans contains a contractile tube of myoepithelial cells known as the spermatheca, which stores sperm and is the site of oocyte fertilization. Regulated contraction of the spermatheca pushes the embryo into the uterus. Cell contractility in the spermatheca is dependent on actin and myosin and is regulated, in part, by Ca2+ signaling through the phospholipase PLC-1, which mediates Ca2+ release from the endoplasmic reticulum. Here, we describe a novel role for GSA-1/Gαs, and protein kinase A, composed of the catalytic subunit KIN-1/PKA-C and the regulatory subunit KIN-2/PKA-R, in the regulation of Ca2+ release and contractility in the C. elegans spermatheca. Without GSA-1/Gαs or KIN-1/PKA-C, Ca2+ is not released, and oocytes become trapped in the spermatheca. Conversely, when PKA is activated through either a gain of function allele in GSA-1 (GSA-1(GF)) or by depletion of KIN-2/PKA-R, the transit times and total numbers, although not frequencies, of Ca2+ pulses are increased, and Ca2+ propagates across the spermatheca even in the absence of oocyte entry. In the spermathecal-uterine valve, loss of GSA-1/Gαs or KIN-1/PKA-C results in sustained, high levels of Ca2+ and a loss of coordination between the spermathecal bag and sp-ut valve. Additionally, we show that depleting phosphodiesterase PDE-6 levels alters contractility and Ca2+ dynamics in the spermatheca, and that the GPB-1 and GPB-2 Gβ subunits play a central role in regulating spermathecal contractility and Ca2+ signaling. This work identifies a signaling network in which Ca2+ and cAMP pathways work together to coordinate spermathecal contractions for successful ovulations.

RHOA and mDia1 Promotes Apoptosis of Breast Cancer Cells Via a High Dose of Doxorubicin Treatment

Background

Transforming RhoA proteins (RHOA) and their downstream Diaphanous homolog 1 proteins (DIAPH1) or mDia1 participate in the regulation of actin cytoskeleton which plays critical role in cells, i.e., morphologic changes and apoptosis.

Methodology

To determine the cell viability the real time cell analysis (RTCA) and flow cytometry were used. To perform proteomic analysis, the label-free quantitative method and post-translation modification by the nano-HPLC and ESI-MS ion trap mass analyser were used.

Results

The results of the cell viability showed an increase of dead cells (around 30 %) in MCF-7/DOX-1 (i.e., 1μM of doxorubicin was added to MCF-7/WT breast cancer cell line) compared to MCF-7/WT (control) after 24 h doxorubicin (DOX) treatment. The signalling pathway of the Regulation of actin cytoskeleton (p<0.0026) was determined, where RHOA and mDia1 proteins were up-regulated. Also, post-translational modification analysis of these proteins in MCF-7/DOX-1 cells revealed dysregulation of the actin cytoskeleton, specifically the collapse of actin stress fibbers due to phosphorylation of RHOA at serine 188 and mDia1 at serine 22, resulting in their deactivation and cell apoptosis.

Conclusion

These results pointed to an assumed role of DOX to dysregulation of actin cytoskeleton and cell death.

Targeting Adenosine in Cancer Immunotherapy to Enhance T-Cell Function

T cells play a critical role in cancer control, but a range of potent immunosuppressive mechanisms can be upregulated in the tumor microenvironment (TME) to abrogate their activity. While various immunotherapies (IMTs) aiming at re-invigorating the T-cell-mediated anti-tumor response, such as immune checkpoint blockade (ICB), and the adoptive cell transfer (ACT) of natural or gene-engineered ex vivo expanded tumor-specific T cells, have led to unprecedented clinical responses, only a small proportion of cancer patients benefit from these treatments. Important research efforts are thus underway to identify biomarkers of response, as well as to develop personalized combinatorial approaches that can target other inhibitory mechanisms at play in the TME. In recent years, adenosinergic signaling has emerged as a powerful immuno-metabolic checkpoint in tumors. Like several other barriers in the TME, such as the PD-1/PDL-1 axis, CTLA-4, and indoleamine 2,3-dioxygenase (IDO-1), adenosine plays important physiologic roles, but has been co-opted by tumors to promote their growth and impair immunity. Several agents counteracting the adenosine axis have been developed, and pre-clinical studies have demonstrated important anti-tumor activity, alone and in combination with other IMTs including ICB and ACT. Here we review the regulation of adenosine levels and mechanisms by which it promotes tumor growth and broadly suppresses protective immunity, with extra focus on the attenuation of T cell function. Finally, we present an overview of promising pre-clinical and clinical approaches being explored for blocking the adenosine axis for enhanced control of solid tumors.

Endothelial permeability, LDL deposition, and cardiovascular risk factors-a review

Early atherosclerosis features functional and structural changes in the endothelial barrier function that affect the traffic of molecules and solutes between the vessel lumen and the vascular wall. Such changes are mechanistically related to the development of atherosclerosis. Proatherogenic stimuli and cardiovascular risk factors, such as dyslipidaemias, diabetes, obesity, and smoking, all increase endothelial permeability sharing a common signalling denominator: an imbalance in the production/disposal of reactive oxygen species (ROS), broadly termed oxidative stress. Mostly as a consequence of the activation of enzymatic systems leading to ROS overproduction, proatherogenic factors lead to a pro-inflammatory status that translates in changes in gene expression and functional rearrangements, including changes in the transendothelial transport of molecules, leading to the deposition of low-density lipoproteins (LDL) and the subsequent infiltration of circulating leucocytes in the intima. In this review, we focus on such early changes in atherogenesis and on the concept that proatherogenic stimuli and risk factors for cardiovascular disease, by altering the endothelial barrier properties, co-ordinately trigger the accumulation of LDL in the intima and ultimately plaque formation.

Fastigial nucleus electrostimulation promotes axonal regeneration after experimental stroke via cAMP/PKA pathway

Axon regeneration after cerebral ischemia in mammals is inadequate to restore function, illustrating the need to design better strategies for improving outcomes. Improvement of axon regeneration has been achieved through fastigial nucleus electrostimulation (FNS) in animal researches. However, the mechanisms underlying this neuroprotection remain poorly understood. Increasing the levels of the second messenger cyclic AMP (cAMP) enhances axon regeneration, making it an excellent candidate molecule that has therapeutic potential. In the present study, we examined the expression of cAMP signaling in ischemic brain tissues following focal cerebral ischemia. Adult rats were subjected to ischemia induced by middle cerebral artery occlusion (MCAO). A dipolar electrode was placed into the cerebellum to stimulate the cerebellar fastigial nucleus for 1 h after ischemia. Neurological deficits and the expressions of cAMP, PKA (protein kinase A) and ROCK (Rho-kinase) were determined. Axonal regeneration was measured by upregulation of growth-associated protein 43 (GAP43). The data indicated that FNS significantly enhanced axonal regeneration and motor function recovery after cerebral ischemia. FNS also significantly increased cAMP and PKA levels after ischemic brain injury. All the beneficial effects of FNS were blocked by Rp-cAMP, an antagonist of PKA. Our research suggested that the axonal regeneration conferred by FNS was likely achieved via the regulation of cAMP/PKA pathway.

Manganese promotes intracellular accumulation of AQP2 via modulating F-actin polymerization and reduces urinary concentration in mice

Aquaporin-2 (AQP2) is a water channel protein expressed in principal cells (PCs) of the kidney collecting ducts (CDs) and plays a critical role in mediating water reabsorption and urine concentration. AQP2 undergoes both regulated trafficking mediated by vasopressin (VP) and constitutive recycling, which is independent of VP. For both pathways, actin cytoskeletal dynamics is a key determinant of AQP2 trafficking. We report here that manganese chloride (MnCl₂) is a novel and potent regulator of AQP2 trafficking in cultured cells and in the kidney. MnCl₂ treatment promoted internalization and intracellular accumulation of AQP2. The effect of MnCl₂ on the intracellular accumulation of AQP2 was associated with activation of RhoA and actin polymerization without modification of AQP2 phosphorylation. Although the level of total and phosphorylated AQP2 did not change, MnCl₂ treatment impeded VP-induced phosphorylation of AQP2 at its serine-256, -264, and -269 residues and dephosphorylation at serine 261. In addition, MnCl₂ significantly promoted F-actin polymerization along with downregulation of RhoA activity and prevented VP-induced membrane accumulation of AQP2. Finally, MnCl₂ treatment in mice resulted in significant polyuria and reduced urinary concentration, likely due to intracellular relocation of AQP2 in the PCs of kidney CDs. More importantly, the reduced urinary concentration caused by MnCl₂ treatment in animals was not corrected by VP. In summary, our study identified a novel effect of MnCl₂ on AQP2 trafficking through modifying RhoA activity and actin polymerization and uncovered its potent impact on water diuresis in vivo.

Reestablishment of Glucose Inhibition of Glucagon Secretion in Small Pseudoislets

Misregulated hormone secretion from the islet of Langerhans is central to the pathophysiology of diabetes. Although insulin plays a key role in glucose regulation, the importance of glucagon is increasingly acknowledged. However, the mechanisms that regulate glucagon secretion from α-cells are still unclear. We used pseudoislets reconstituted from dispersed islet cells to study α-cells with and without various indirect effects from other islet cells. Dispersed islet cells secrete aberrant levels of glucagon and insulin at basal and elevated glucose levels. When cultured, murine islet cells reassociate to form pseudoislets, which recover normal glucose-regulated hormone secretion, and human islet cells follow a similar pattern. We created small (∼40-µm) pseudoislets using all of the islet cells or only some of the cell types, which allowed us to characterize novel aspects of regulated hormone secretion. The recovery of regulated glucagon secretion from α-cells in small pseudoislets depends upon the combined action of paracrine factors, such as insulin and somatostatin, and juxtacrine signals between EphA4/7 on α-cells and ephrins on β-cells. Although these signals modulate different pathways, both appear to be required for proper inhibition of glucagon secretion in response to glucose. This improved understanding of the modulation of glucagon secretion can provide novel therapeutic routes for the treatment of some individuals with diabetes.

Phosphodiesterase Inhibitors as a Therapeutic Approach to Neuroprotection and Repair

A wide diversity of perturbations of the central nervous system (CNS) result in structural damage to the neuroarchitecture and cellular defects, which in turn are accompanied by neurological dysfunction and abortive endogenous neurorepair. Altering intracellular signaling pathways involved in inflammation and immune regulation, neural cell death, axon plasticity and remyelination has shown therapeutic benefit in experimental models of neurological disease and trauma. The second messengers, cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP), are two such intracellular signaling targets, the elevation of which has produced beneficial cellular effects within a range of CNS pathologies. The only known negative regulators of cyclic nucleotides are a family of enzymes called phosphodiesterases (PDEs) that hydrolyze cyclic nucleotides into adenosine monophosphate (AMP) or guanylate monophosphate (GMP). Herein, we discuss the structure and physiological function as well as the roles PDEs play in pathological processes of the diseased or injured CNS. Further we review the approaches that have been employed therapeutically in experimental paradigms to block PDE expression or activity and in turn elevate cyclic nucleotide levels to mediate neuroprotection or neurorepair as well as discuss both the translational pathway and current limitations in moving new PDE-targeted therapies to the clinic.

Cyclic-AMP regulates postnatal development of neural and behavioral responses to NaCl in rats

During postnatal development rats demonstrate an age-dependent increase in NaCl chorda tympani (CT) responses and the number of functional apical amiloride-sensitive epithelial Na+ channels (ENaCs) in salt sensing fungiform (FF) taste receptor cells (TRCs). Currently, the intracellular signals that regulate the postnatal development of salt taste have not been identified. We investigated the effect of cAMP, a downstream signal for arginine vasopressin (AVP) action, on the postnatal development of NaCl responses in 19-23 day old rats. ENaC-dependent NaCl CT responses were monitored after lingual application of 8-chlorophenylthio-cAMP (8-CPT-cAMP) under open-circuit conditions and under ±60 mV lingual voltage clamp. Behavioral responses were tested using 2 bottle/24h NaCl preference tests. The effect of [deamino-Cys1, D-Arg8]-vasopressin (dDAVP, a specific V2R agonist) was investigated on ENaC subunit trafficking in rat FF TRCs and on cAMP generation in cultured adult human FF taste cells (HBO cells). Our results show that in 19-23 day old rats, the ENaC-dependent maximum NaCl CT response was a saturating sigmoidal function of 8-CPT-cAMP concentration. 8-CPT-cAMP increased the voltage-sensitivity of the NaCl CT response and the apical Na+ response conductance. Intravenous injections of dDAVP increased ENaC expression and γ-ENaC trafficking from cytosolic compartment to the apical compartment in rat FF TRCs. In HBO cells dDAVP increased intracellular cAMP and cAMP increased trafficking of γ- and δ-ENaC from cytosolic compartment to the apical compartment 10 min post-cAMP treatment. Control 19-23 day old rats were indifferent to NaCl, but showed clear preference for appetitive NaCl concentrations after 8-CPT-cAMP treatment. Relative to adult rats, 14 day old rats demonstrated significantly less V2R antibody binding in circumvallate TRCs. We conclude that an age-dependent increase in V2R expression produces an AVP-induced incremental increase in cAMP that modulates the postnatal increase in TRC ENaC and the neural and behavioral responses to NaCl.

Inhibiting cortical protein kinase A in spinal cord injured rats enhances efficacy of rehabilitative training

Elevated levels of the second messenger molecule cyclic adenosine monophosphate (cAMP) are often associated with neuron sprouting and neurite extension (i.e., neuroplasticity). Phosphokinase A (PKA) is a prominent downstream target of cAMP that has been associated with neurite outgrowth. We hypothesized that rehabilitative motor training following spinal cord injuries promotes neuroplasticity via PKA activation. However, in two independent experiments, inhibition of cortical PKA using Rp-cAMPS throughout rehabilitative training robustly increased functional recovery and collateral sprouting of injured corticospinal tract axons, an indicator of neuroplasticity. Consistent with these in vivo findings, using cultured STHdh neurons, we found that Rp-cAMPS had no effect on the phosphorylation of CREB (cAMP response element-binding protein), a prominent downstream target of PKA, even with the concomitant application of the adenylate cyclase agonist forskolin to increase cAMP levels. Conversely, when cAMP levels were increased using the phosphodiesterase inhibitor IBMX, Rp-cAMPS potently inhibited CREB phosphorylation. Taken together, our results suggest that an alternate cAMP dependent pathway was involved in increasing CREB phosphorylation and neuroplasticity. This idea was supported by an in vitro neurite outgrowth assay, where inhibiting PKA did enhance neurite outgrowth. However, when PKA inhibition was combined with inhibition of EPAC2 (exchange protein directly activated by cAMP), another downstream target of cAMP in neurons, neurite outgrowth was significantly reduced. In conclusion, blocking PKA in cortical neurons of spinal cord injured rats increases neurite outgrowth of the lesioned corticospinal tract fibres and the efficacy of rehabilitative training, likely via EPAC.

Prostaglandin E2-Dependent Phosphorylation of RAS Inhibition 1 (RIN1) at Ser 291 and 292 Inhibits Transforming Growth Factor-β-Induced RAS Activation Pathway in Human Synovial Fibroblasts: Role in Cell Migration

Prostaglandin E2 (PGE2 )-stimulated G-protein-coupled receptor (GPCR) activation inhibits pro-fibrotic TGFβ-dependent stimulation of human fibroblast to myofibroblast transition (FMT), though the precise molecular mechanisms are not fully understood. In the present study, we describe the PGE2 -dependent suppression and reversal of TGFβ-induced events such as α-sma expression, stress fiber formation, and Ras/Raf/ERK/MAPK pathway-dependent activation of myofibroblast migration. In order to elucidate post-ligand-receptor signaling pathways, we identified a predominant PKA phosphorylation motif profile in human primary fibroblasts after treatment with exogenous PGE2 (EC50 30 nM, Vmax 100 nM), mimicked by the adenyl cyclase activator forskolin (EC50 5 μM, Vmax 10 μM). We used a global phosphoproteomic approach to identify a 2.5-fold difference in PGE2 -induced phosphorylation of proteins containing the PKA motif. Deducing the signaling pathway of our migration data, we identified Ras inhibitor 1 (RIN1) as a substrate, whereby PGE2 induced its phosphorylation at Ser291 and at Ser292 by a 5.4- and 4.8-fold increase, respectively. In a series of transient and stable over expression studies in HEK293T and HeLa cells using wild-type (wt) and mutant RIN1 (Ser291/292Ala) or Ras constructs and siRNA knock-down experiments, we showed that PGE2 -dependent phosphorylation of RIN1 resulted in the abrogation of TGFβ-induced Ras/Raf signaling activation and subsequent downstream blockade of cellular migration, emphasizing the importance of such phosphosites in PGE2 suppression of wound closure. Overexpression experiments in tandem with pull-down assays indicated that specific Ser291/292 phosphorylation of RIN1 favored binding to activated Ras. In principal, understanding PGE2 -GPCR activated signaling pathways mitigating TGFβ-induced fibrosis may lead to more evidence-based treatments against the disease. J. Cell. Physiol. 232: 202-215, 2017. © 2016 Wiley Periodicals, Inc.

Modeling the mechanics of cells in the cell-spreading process driven by traction forces

Mechanical properties of cells and their mechanical interaction with the extracellular environments are main factors influencing cellular function, thus indicating the progression of cells in different disease states. By considering the mechanical interactions between cell adhesion molecules and the extracellular environment, we developed a cell mechanical model that can characterize the mechanical changes in cells during cell spreading. A cell model was established that consisted of various main subcellular components, including cortical cytoskeleton, nuclear envelope, actin filaments, intermediate filaments, and microtubules. We demonstrated the structural changes in subcellular components and the changes in spreading areas during cell spreading driven by traction forces. The simulation of nanoindentation tests was conducted by integrating the indenting force to the cell model. The force-indentation curve of the cells at different spreading states was simulated, and the results showed that cell stiffness increased with increasing traction forces, which were consistent with the experimental results. The proposed cell mechanical model provides a strategy to investigate the mechanical interactions of cells with the extracellular environments through the adhesion molecules and to reveal the cell mechanical properties at the subcellular level as cells shift from the suspended state to the adherent state.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

Cyclic AMP-elevating Agents Promote Cumulus Cell Survival and Hyaluronan Matrix Stability, Thereby Prolonging the Time of Mouse Oocyte Fertilizability

Cumulus cells sustain the development and fertilization of the mammalian oocyte. These cells are retained around the oocyte by a hyaluronan-rich extracellular matrix synthesized before ovulation, a process called cumulus cell-oocyte complex (COC) expansion. Hyaluronan release and dispersion of the cumulus cells progressively occur after ovulation, paralleling the decline of oocyte fertilization. We show here that, in mice, postovulatory changes of matrix are temporally correlated to cumulus cell death. Cumulus cell apoptosis and matrix disassembly also occurred in ovulated COCs cultured in vitro. COCs expanded in vitro with FSH or EGF underwent the same changes, whereas those expanded with 8-bromo-adenosine-3',5'-cyclic monophosphate (8-Br-cAMP) maintained integrity for a longer time. It is noteworthy that 8-Br-cAMP treatment was also effective on ovulated COCs cultured in vitro, prolonging the vitality of the cumulus cells and the stability of the matrix from a few hours to >2 days. Stimulation of endogenous adenylate cyclase with forskolin or inhibition of phosphodiesterase with rolipram produced similar effects. The treatment with selective cAMP analogues suggests that the effects of cAMP elevation are exerted through an EPAC-independent, PKA type II-dependent signaling pathway, probably acting at the post-transcriptional level. Finally, overnight culture of ovulated COCs with 8-Br-cAMP significantly counteracted the decrease of fertilization rate, doubling the number of fertilized oocytes compared with control conditions. In conclusion, these studies suggest that cAMP-elevating agents prevent cumulus cell senescence and allow them to continue to exert beneficial effects on oocyte and sperm, thereby extending in vitro the time frame of oocyte fertilizability.

Parathyroid hormone regulation of hypoxia-inducible factor signaling in osteoblastic cells

Osteoblasts perceive and respond to changes in their pericellular environment, including biophysical signals and oxygen availability, to elicit an anabolic or catabolic response. Parathyroid hormone (PTH) affects each arm of skeletal remodeling, with net anabolic or catabolic effects dependent upon duration of exposure. Similarly, the capacity of osteoblastic cells to perceive pericellular oxygen has a profound effect on skeletal mass and architecture, as mice expressing stable hypoxia-inducible factor (HIF)-1α and -2α demonstrate age-dependent increases in bone volume per tissue volume and osteoblast number. Further, HIF levels and signaling can be influenced in an oxygen-independent manner. Because the cellular mechanisms involved in PTH regulation of the skeleton remain vague, we sought whether PTH could influence HIF-1α expression and HIF-α-driven luciferase activity independently of altered oxygen availability. Using UMR106.01 mature osteoblasts, we observed that 100nM hPTH(1-34) decreased HIF-1α and HIF-responsive luciferase activity in a process involving heat shock protein 90 (Hsp90) and cyclic AMP but not intracellular calcium. Altering activity of the small GTPase RhoA and its effector kinase ROCK altered HIF-α-driven luciferase activity in the absence and presence of PTH. Taken together, these data introduce PTH as a regulator of oxygen-independent HIF-1α levels through a mechanism involving cyclic AMP, Hsp90, and the cytoskeleton.

Prostaglandin E(2)-dependent blockade of actomyosin and stress fibre formation is mediated through S1379 phosphorylation of ROCK2

Prostaglandin E2 is a pleiotropic bioactive lipid that controls cytoskeletal alterations, although the precise G-protein coupled EP receptor signalling mechanisms remain ill defined. We adopted a phosphoproteomic approach to characterize post-receptor downstream signalling substrates using antibodies that selectively recognize and immunoprecipitate phosphorylated substrates of a number of kinases. Using human synovial fibroblasts in monolayer cell culture, PGE2 induced rapid and sustained changes in cellular morphology and reduction in cytoplasmic volume that were associated with disassembly of the phalloidin-stained stress fibres as judged by light and confocal microscopy. Furthermore, PGE2 induced a rapid dephosphorylation of myosin light chain II (MLC) at S19 under basal or cytokine-induced conditions that was linked to an activation of myosin light chain phosphatase. The use of specific synthetic EP agonists suggested that the response was mediated by EP2 receptors, as other EP agonists did not manifest the same effect on MLC phosphorylation. In addition, PGE2 induced sustained Y118 dephosphorylation of phospho-paxillin and loss of focal adhesions as observed by confocal microscopy and Western analysis. Phosphoproteomic analysis of PGE2 /GPCR/PKA phosphosubstrates identified a unique, non-redundant, phosphorylated (>30-fold) site on rho-associated coiled coil-containing kinase 2 (ROCK2) at S1379. Analysis of ROCK2 mutant behaviour (e.g. S1379A) in overexpression studies revealed that PGE2 -dependent phosphorylation of ROCK2 resulted in the inhibition of the kinase, since induced MLC phosphorylation was no longer blocked by PGE2 nor could PGE2 induce disassembly of stress fibres. Thus, PGE2 -dependent blockade of actomyosin fibre formation, characteristic of myofibroblasts, may be mediated through specific ROCK2 S1379 phosphorylation.

cAMP Promotes Cell Migration Through Cell Junctional Complex Dynamics and Actin Cytoskeleton Remodeling: Implications in Skin Wound Healing

Stem cells have attracted great interest for their therapeutic capacity in tissue regeneration. Cyclic adenosine 3',5'-monophosphate (cAMP), existing in high concentration at wound sites, mediated various signaling pathways such as cytoskeleton dynamics, cell adhesion, and cell migration in stem cells, which suggest the critical roles of cAMP in the wound healing process through functional regulation of stem cells. However, the mechanisms behind the effect of cAMP on mouse embryonic stem cell (mESC) motility and its roles on skin wound healing remain to be fully elucidated. In the present study, 8-Bromo cAMP-treated mESCs showed significant wound closure and improved neovascularization. Moreover, 8-Bromo cAMP stimulated mESC migration into the wound bed. 8-Bromo cAMP also increased ESC motility in in vitro migration assay. 8-Bromo cAMP induced myosin light chain phosphorylation through Rac1 and Cdc42 signaling, which were involved in 8-Bromo cAMP-induced decrease in expression of junction proteins (connexin 43, E-cadherin, and occludin) at the plasma membrane. Subsequently, 8-Bromo cAMP induced the disruption of cell junctions (including gap junctions, adherens junctions, and tight junctions), which reduced the function of the gap junctions and cell adhesion. In addition, 8-Bromo cAMP-induced Rac1 and Cdc42 activation increased Arp3, TOCA, PAK, and N-WASP expression, but decreased cofilin phosphorylation level, which elicited actin cytoskeleton remodeling. In contrast to the control, 8-Bromo cAMP evoked a substantial migration of cells into the denuded area, which was blocked by the small interfering RNAs of the signaling pathway-related molecules or by inhibitors. In conclusion, cAMP enhanced the migration of mESCs through effective coordination of junctional disruption and actin cytoskeleton remodeling, which increased the wound healing capacity of ESCs.

PKA-dependent phosphorylation of LIMK1 and Cofilin is essential for mouse sperm acrosomal exocytosis

Mammalian sperm must acquire their fertilizing ability after a series of biochemical modifications in the female reproductive tract collectively called capacitation to undergo acrosomal exocytosis, a process that is essential for fertilization. Actin dynamics play a central role in controlling the process of exocytosis in somatic cells as well as in sperm from several mammalian species. In somatic cells, small GTPases of the Rho family are widely known as master regulators of actin dynamics. However, the role of these proteins in sperm has not been studied in detail. In the present work we characterized the participation of small GTPases of the Rho family in the signaling pathway that leads to actin polymerization during mouse sperm capacitation. We observed that most of the proteins of this signaling cascade and their effector proteins are expressed in mouse sperm. The activation of the signaling pathways of cAMP/PKA, RhoA/C and Rac1 is essential for LIMK1 activation by phosphorylation on Threonine 508. Serine 3 of Cofilin is phosphorylated by LIMK1 during capacitation in a transiently manner. Inhibition of LIMK1 by specific inhibitors (BMS-3) resulted in lower levels of actin polymerization during capacitation and a dramatic decrease in the percentage of sperm that undergo acrosomal exocytosis. Thus, we demonstrated for the first time that the master regulators of actin dynamics in somatic cells are present and active in mouse sperm. Combining the results of our present study with other results from the literature, we have proposed a working model regarding how LIMK1 and Cofilin control acrosomal exocytosis in mouse sperm.

Pseudorabies virus US3 triggers RhoA phosphorylation to reorganize the actin cytoskeleton

The conserved alphaherpesvirus serine/threonine kinase US3 causes dramatic changes in the actin cytoskeleton, consisting of actin stress fibre breakdown and protrusion formation, associated with increased virus spread. Here, we showed that US3 expression led to RhoA phosphorylation at serine 188 (S188), one of the hallmarks of suppressed RhoA signalling, and that expression of a non-phosphorylatable RhoA variant interfered with the ability of US3 to induce actin rearrangements. Furthermore, inhibition of cellular protein kinase A (PKA) eliminated the ability of US3 to induce S188 RhoA phosphorylation, pointing to a role for PKA in US3-induced RhoA phosphorylation. Hence, the US3 kinase leads to PKA-dependent S188 RhoA phosphorylation, which contributes to US3-mediated actin rearrangements. Our data suggest that US3 efficiently usurps the antagonistic RhoA and Cdc42/Rac1/p21-activated kinase signalling branches to rearrange the actin cytoskeleton.

Enhanced neurite outgrowth of human model (NT2) neurons by small-molecule inhibitors of Rho/ROCK signaling

Axonal injury in the adult human central nervous system often results in loss of sensation and motor functions. Promoting regeneration of severed axons requires the inactivation of growth inhibitory influences from the tissue environment and stimulation of the neuron intrinsic growth potential. Especially glial cell derived factors, such as chondroitin sulfate proteoglycans, Nogo-A, myelin-associated glycoprotein, and myelin in general, prevent axon regeneration. Most of the glial growth inhibiting factors converge onto the Rho/ROCK signaling pathway in neurons. Although conditions in the injured nervous system are clearly different from those during neurite outgrowth in vitro, here we use a chemical approach to manipulate Rho/ROCK signalling with small-molecule agents to encourage neurite outgrowth in cell culture. The development of therapeutic treatments requires drug testing not only on neurons of experimental animals, but also on human neurons. Using human NT2 model neurons, we demonstrate that the pain reliever Ibuprofen decreases RhoA (Ras homolog gene family, member A GTPase) activation and promotes neurite growth. Inhibition of the downstream effector Rho kinase by the drug Y-27632 results in a strong increase in neurite outgrowth. Conversely, activation of the Rho pathway by lysophosphatidic acid results in growth cone collapse and eventually to neurite retraction. Finally, we show that blocking of Rho kinase, but not RhoA results in an increase in neurons bearing neurites. Due to its anti-inflammatory and neurite growth promoting action, the use of a pharmacological treatment of damaged neural tissue with Ibuprofen should be explored.

RhoGTPases as key players in mammalian cell adaptation to microgravity

A growing number of studies are revealing that cells reorganize their cytoskeleton when exposed to conditions of microgravity. Most, if not all, of the structural changes observed on flown cells can be explained by modulation of RhoGTPases, which are mechanosensitive switches responsible for cytoskeletal dynamics control. This review identifies general principles defining cell sensitivity to gravitational stresses. We discuss what is known about changes in cell shape, nucleus, and focal adhesions and try to establish the relationship with specific RhoGTPase activities. We conclude by considering the potential relevance of live imaging of RhoGTPase activity or cytoskeletal structures in order to enhance our understanding of cell adaptation to microgravity-related conditions.

A novel therapeutic effect of statins on nephrogenic diabetes insipidus

Statins competitively inhibit hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase, resulting in reduced plasma total and low-density lipoprotein cholesterol levels. Recently, it has been shown that statins exert additional ‘pleiotropic’ effects by increasing expression levels of the membrane water channels aquaporin 2 (AQP2). AQP2 is localized mainly in the kidney and plays a critical role in determining cellular water content. This additional effect is independent of cholesterol homoeostasis, and depends on depletion of mevalonate-derived intermediates of sterol synthetic pathways, i.e. farnesylpyrophosphate and geranylgeranylpyrophosphate. By up-regulating the expression levels of AQP2, statins increase water reabsorption by the kidney, thus opening up a new avenue in treating patients with nephrogenic diabetes insipidus (NDI), a hereditary disease that yet lacks high-powered and limited side effects therapy. Aspects related to water balance determined by AQP2 in the kidney, as well as standard and novel therapeutic strategies of NDI are discussed.

Amelioration of cisplatin-induced experimental peripheral neuropathy by a small molecule targeting p75 NTR

Cisplatin is an effective and widely used first-line chemotherapeutic drug for treating cancers. However, many patients sustain cisplatin-induced peripheral neuropathy (CIPN), often leading to a reduction in drug dosages or complete cessation of treatment altogether. Therefore, it is important to understand cisplatin mechanisms in peripheral nerve tissue mediating its toxicity and identify signaling pathways for potential intervention. Rho GTPase activation is increased following trauma in several models of neuronal injury. Thus, we investigated whether components of the Rho signaling pathway represent important neuroprotective targets with the potential to ameliorate CIPN and thereby optimize current chemotherapy treatment regimens. We have developed a novel CIPN model in the mouse. Using this model and primary neuronal culture, we determined whether LM11A-31, a small-molecule, orally bioavailable ligand of the p75 neurotrophin receptor (p75(NTR)), can modulate Rho GTPase signaling and reduce CIPN. Von Frey filament analysis of sural nerve function showed that LM11A-31 treatment prevented decreases in peripheral nerve sensation seen with cisplatin treatment. Morphometric analysis of harvested sural nerves revealed that cisplatin-induced abnormal nerve fiber morphology and the decreases in fiber area were alleviated with concurrent LM11A-31 treatment. Cisplatin treatment increased RhoA activity accompanied by the reduced tyrosine phosphorylation of SHP2, which was reversed by LM11A-31. LM11A-31 also countered the effects of calpeptin, which activated RhoA by inhibiting SHP2 tyrosine phosphatase. Therefore, suppression of RhoA signaling by LM11A-31 that modulates p75(NTR) or activates SHP2 tyrosine phosphatase downstream of the NGF receptor enhances neuroprotection in experimental CIPN in mouse model.

Role of cAMP-dependent protein kinase A activity in low-dose endothelial monocyte-activating polypeptide-II-induced opening of blood-tumor barrier

Our previous studies demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) can selectively increase the permeability of blood-tumor barrier (BTB). In addition, low-dose EMAP-II significantly decreases the cyclic adenosine monophosphate (cAMP) concentration and the protein kinase A (PKA) expression level in tumor tissues in the rat C6 glioma model. In this study, an in vitro BTB model was used to investigate the potential role of cAMP/PKA signaling cascade in EMAP-II-induced BTB hyperpermeability. Our data revealed that low-dose EMAP-II (0.05 nM) induced a significant decrease in total intracellular cAMP concentration and PKA activity in rat brain microvascular endothelial cells (RBMECs). Pretreatment with forskolin to increase intracellular cAMP nearly completely blocked the EMAP-II-induced decrease in transendothelial electric resistance and increase in horseradish peroxidase flux across the BTB. Similar pretreatment completely prevented the EMAP-II-induced changes in RhoA/Rho kinase activity, expression and distribution of tight junction-associated protein ZO-1, and myosin light chain phosphorylation, as well as actin cytoskeleton arrangement in RBMECs. Pretreatment with 6Bnz-cAMP to activate PKA significantly attenuated these EMAP-II-induced alterations in RBMECs. In summary, our present study demonstrates that the cAMP/PKA signaling cascade works as a crucial signaling pathway in EMAP-II-induced BTB hyperpermeability.

In vivo screening for substrates of protein kinase A using a combination of proteomic approaches and pharmacological modulation of kinase activity

Protein kinase A (PKA) is a serine/threonine kinase whose activity depends on the levels of cyclic AMP (cAMP). PKA plays essential roles in numerous cell types such as myocytes and neurons. Numerous substrate screens have been attempted to clarify the entire scope of the PKA signaling cascade, but it is still underway. Here, we performed a comprehensive screen that consisted of immunoprecipitation and mass spectrometry, with a focus on the identification of PKA substrates. The lysate of HeLa cells treated with Forskolin (FSK)/3-isobutyl methyl xanthine (IBMX) and/or H-89 was subjected to immunoprecipitation using anti-phospho-PKA substrate antibody. The identity of the phosophoproteins and phosphorylation sites in the precipitants was determined using liquid chromatography tandem mass spectrometry (LC/MS/MS). We obtained 112 proteins as candidate substrates and 65 candidate sites overall. Among the candidate substrates, Rho-kinase/ROCK2 was confirmed to be a novel substrate of PKA both in vitro and in vivo. In addition to Rho-kinase, we found more than a hundred of novel candidate substrates of PKA using this screen, and these discoveries provide us with new insights into PKA signaling.

Quantitative phosphoproteomic analysis of signaling downstream of the prostaglandin e2/g-protein coupled receptor in human synovial fibroblasts: potential antifibrotic networks

The Prostaglandin E2 (PGE2) signaling mechanism within fibroblasts is of growing interest as it has been shown to prevent numerous fibrotic features of fibroblast activation with limited evidence of downstream pathways. To understand the mechanisms of fibroblasts producing tremendous amounts of PGE2 with autocrine effects, we apply a strategy of combining a wide-screening of PGE2-induced kinases with quantitative phosphoproteomics. Our large-scale proteomic approach identified a PKA signal transmitted through phosphorylation of its substrates harboring the R(R/X)X(S*/T*) motif. We documented 115 substrates, of which 72 had 89 sites with a 2.5-fold phosphorylation difference in PGE2-treated cells than in untreated cells, where approximately half of such sites were defined as being novel. They were compiled by networking software to focus on highlighted activities and to associate them with a functional readout of fibroblasts. The substrates were associated with a variety of cellular functions including cytoskeletal structures (migration/motility), regulators of G-protein coupled receptor function, protein kinases, and transcriptional/translational regulators. For the first time, we extended the PGE2 pathway into an elaborate network of interconnecting phosphoproteins, providing vital information to a once restricted signalosome. These data provide new insights into eicosanoid-initiated cell signaling with regards to the regulation of fibroblast activation and the identification of new targets for evidenced-based pharmacotherapy against fibrosis.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

cAMP signaling regulates platelet myosin light chain (MLC) phosphorylation and shape change through targeting the RhoA-Rho kinase-MLC phosphatase signaling pathway

Cyclic adenosine monophosphate (cAMP)-dependent signaling modulates platelet shape change through unknown mechanisms. We examined the effects of cAMP signaling on platelet contractile machinery. Prostaglandin E1 (PGE1)-mediated inhibition of thrombin-stimulated shape change was accompanied by diminished phosphorylation of myosin light chain (MLC). Since thrombin stimulates phospho-MLC through RhoA/Rho-associated, coiled-coil containing protein kinase (ROCK)-dependent inhibition of MLC phosphatase (MLCP), we examined the effects of cAMP on this pathway. Thrombin stimulated the membrane localization of RhoA and the formation of a signaling complex of RhoA/ROCK2/myosin phosphatase-targeting subunit 1 (MYPT1). This resulted in ROCK-mediated phosphorylation of MYPT1 on threonine 853 (thr(853)), the disassociation of the catalytic subunit protein phosphatase 1δ (PP1δ) from MYPT1 and inhibition of basal MLCP activity. Treatment of platelets with PGE1 prevented thrombin-induced phospho-MYPT1-thr(853) in a protein kinase A (PKA)-dependent manner. Examination of the molecular mechanisms revealed that PGE1 induced the phosphorylation of RhoA on serine(188) through a pathway requiring cAMP and PKA. This event inhibited the membrane relocalization of RhoA, prevented the association of RhoA with ROCK2 and MYPT1, attenuated the dissociation of PP1δ from MYPT1, and thereby restored basal MLCP activity leading to a decrease in phospho-MLC. These data reveal a new mechanism by which the cAMP-PKA signaling pathway regulates platelet function.

Automated method for the rapid and precise estimation of adherent cell culture characteristics from phase contrast microscopy images

The quantitative determination of key adherent cell culture characteristics such as confluency, morphology, and cell density is necessary for the evaluation of experimental outcomes and to provide a suitable basis for the establishment of robust cell culture protocols. Automated processing of images acquired using phase contrast microscopy (PCM), an imaging modality widely used for the visual inspection of adherent cell cultures, could enable the non-invasive determination of these characteristics. We present an image-processing approach that accurately detects cellular objects in PCM images through a combination of local contrast thresholding and post hoc correction of halo artifacts. The method was thoroughly validated using a variety of cell lines, microscope models and imaging conditions, demonstrating consistently high segmentation performance in all cases and very short processing times (<1 s per 1,208 × 960 pixels image). Based on the high segmentation performance, it was possible to precisely determine culture confluency, cell density, and the morphology of cellular objects, demonstrating the wide applicability of our algorithm for typical microscopy image processing pipelines. Furthermore, PCM image segmentation was used to facilitate the interpretation and analysis of fluorescence microscopy data, enabling the determination of temporal and spatial expression patterns of a fluorescent reporter. We created a software toolbox (PHANTAST) that bundles all the algorithms and provides an easy to use graphical user interface. Source-code for MATLAB and ImageJ is freely available under a permissive open-source license. Biotechnol. Bioeng. 2014;111: 504–517. © 2013 Wiley Periodicals, Inc.

The glycerophosphoinositols: from lipid metabolites to modulators of T-cell signaling

Glycerophosphoinositols (GPIs) are bioactive, diffusible phosphoinositide metabolites of phospholipase A₂ that act both intracellularly and in a paracrine fashion following their uptake by specific transporters. The most representative compound, glycerophosphoinositol (GroPIns), is a ubiquitous component of eukaryotic cells that participates in central processes, including cell proliferation and survival. Moreover, glycerophosphoinositol 4-phosphate (GroPIns4P) controls actin dynamics in several cell systems by regulating Rho GTPases. Recently, immune cells have emerged as targets of the biological activities of the GPIs. We have shown that exogenous GroPIns4P enhances CXCL12-induced T-cell chemotaxis through activation of the kinase Lck in a cAMP/PKA-dependent manner. While highlighting the potential of GroPIns4P as an immunomodulator, this finding raises questions on the role of endogenously produced GroPIns4P as well as of other GPIs in the regulation of the adaptive immune responses under homeostatic and pathological settings. Here we will summarize our current understanding of the biological activities of the GPIs, with a focus on lymphocytes, highlighting open questions and potential developments in this promising new area.

Regulation of RhoA signaling by the cAMP-dependent phosphorylation of RhoGDIα

RhoA plays a pivotal role in regulating cell shape and movement. Protein kinase A (PKA) inhibits RhoA signaling and thereby induces a characteristic morphological change, cell rounding. This has been considered to result from cAMP-induced phosphorylation of RhoA at Ser-188, which induces a stable RhoA-GTP-RhoGDIα complex and sequesters RhoA to the cytosol. However, few groups have shown RhoA phosphorylation in intact cells. Here we show that phosphorylation of RhoGDIα but not RhoA plays an essential role in the PKA-induced inhibition of RhoA signaling and in the morphological changes using cardiac fibroblasts. The knockdown of RhoGDIα by siRNA blocks cAMP-induced cell rounding, which is recovered by RhoGDIα-WT expression but not when a RhoGDIα-S174A mutant is expressed. PKA phosphorylates RhoGDIα at Ser-174 and the phosphorylation of RhoGDIα is likely to induce the formation of a active RhoA-RhoGDIα complex. Our present results thus reveal a principal molecular mechanism underlying G(s)/cAMP-induced cross-talk with G(q)/G(13)/RhoA signaling.

Pericytes regulate vascular basement membrane remodeling and govern neutrophil extravasation during inflammation

During inflammation polymorphonuclear neutrophils (PMNs) traverse venular walls, composed of the endothelium, pericyte sheath and vascular basement membrane. Compared to PMN transendothelial migration, little is known about how PMNs penetrate the latter barriers. Using mouse models and intravital microscopy, we show that migrating PMNs expand and use the low expression regions (LERs) of matrix proteins in the vascular basement membrane (BM) for their transmigration. Importantly, we demonstrate that this remodeling of LERs is accompanied by the opening of gaps between pericytes, a response that depends on PMN engagement with pericytes. Exploring how PMNs modulate pericyte behavior, we discovered that direct PMN-pericyte contacts induce relaxation rather than contraction of pericyte cytoskeletons, an unexpected response that is mediated by inhibition of the RhoA/ROCK signaling pathway in pericytes. Taking our in vitro results back into mouse models, we present evidence that pericyte relaxation contributes to the opening of the gaps between pericytes and to the enlargement of the LERs in the vascular BM, facilitating PMN extravasation. Our study demonstrates that pericytes can regulate PMN extravasation by controlling the size of pericyte gaps and thickness of LERs in venular walls. This raises the possibility that pericytes may be targeted in therapies aimed at regulating inflammation.

Cannabidiolic acid, a major cannabinoid in fiber-type cannabis, is an inhibitor of MDA-MB-231 breast cancer cell migration

Cannabidiol (CBD), a major non-psychotropic constituent of fiber-type cannabis plant, has been reported to possess diverse biological activities, including anti-proliferative effect on cancer cells. Although CBD is obtained from non-enzymatic decarboxylation of its parent molecule, cannabidiolic acid (CBDA), few studies have investigated whether CBDA itself is biologically active. Results of the current investigation revealed that CBDA inhibits migration of the highly invasive MDA-MB-231 human breast cancer cells, apparently through a mechanism involving inhibition of cAMP-dependent protein kinase A, coupled with an activation of the small GTPase, RhoA. It is established that activation of the RhoA signaling pathway leads to inhibition of the mobility of various cancer cells, including MDA-MB-231 cells. The data presented in this report suggest for the first time that as an active component in the cannabis plant, CBDA offers potential therapeutic modality in the abrogation of cancer cell migration, including aggressive breast cancers.

Cyclic AMP dysregulates intestinal epithelial cell restitution through PKA and RhoA

BACKGROUND

Mucosal homeostasis is dependent on the establishment and maintenance of the cell-cell contacts that comprise the physiological barrier. Breaks in the barrier are linked to multiple diseases such as inflammatory bowel disease. While increased cyclic adenosine monophosphate (cAMP) levels limit inflammation by decreasing leukocyte infiltration, the effects of elevated cAMP on intestinal epithelial repair are unknown.

METHODS

Restitution in animals administered rolipram was monitored by microscopic examination after laser wounding of the intestinal epithelium or in mice treated with dextran sodium sulfate (DSS). In vitro analysis was conducted using IEC6 and T84 cells to determine the role of elevated cAMP in altering Rho-dependent cellular migration signaling pathways.

RESULTS

We show that treatment with rolipram, forskolin, and cAMP analogs decrease intestinal epithelial cell migration in vitro. In vivo cell imaging revealed that increased cAMP resulted in a decreased cellular migration rate, with cells at the edge displaying the highest activity. As expected, elevated cAMP elicited increased protein kinase A (PKA) activity, in turn resulting in the inactivation and sequestration of RhoA and decreased actin reorganization. The ablation of restitution by cAMP was not restricted to cell culture, as forskolin and rolipram treatment significantly decreased epithelial microwound closure induced by the two photon confocal injury model.

CONCLUSIONS

Together, these data suggest that administration of cAMP-elevating agents paradoxically decrease infiltration of damage-causing leukocytes while also preventing epithelial repair and barrier maintenance. We propose that treatment with cAMP-elevating agents severely limits mucosal reepithelialization and should be contraindicated for use in chronic inflammatory bowel disorders.

Acute activation of β2-adrenergic receptor regulates focal adhesions through βArrestin2- and p115RhoGEF protein-mediated activation of RhoA

β(2)-Adrenergic receptors (β(2)ARs) regulate cellular functions through G protein-transduced and βArrestin-transduced signals. β(2)ARs have been shown to regulate cancer cell migration, but the underlying mechanisms are not well understood. Here, we report that β(2)AR regulates formation of focal adhesions, whose dynamic remodeling is critical for directed cell migration. β(2)ARs induce activation of RhoA, which is dependent on βArrestin2 but not G(s). βArrestin2 forms a complex with p115RhoGEF, a guanine nucleotide exchange factor for RhoA that is well known to be activated by G(12/13)-coupled receptors. Our results show that βArrestin2 forms a complex with p115RhoGEF in the cytosol in resting cells. Upon β(2)AR activation, both βArrestin2 and p115RhoGEF translocate to the plasma membrane, with concomitant activation of RhoA and formation of focal adhesions and stress fibers. Activation of RhoA and focal adhesion remodeling may explain, at least in part, the role of β(2)ARs in cell migration. These results suggest that βArrestin2 may serve as a convergence point for non-G(12/13) and non-G(q) protein-coupled receptors to activate RhoA.

Coxiella burnetii alters cyclic AMP-dependent protein kinase signaling during growth in macrophages

Coxiella burnetii is the bacterial agent of human Q fever, an acute, flu-like illness that can present as chronic endocarditis in immunocompromised individuals. Following aerosol-mediated transmission, C. burnetii replicates in alveolar macrophages in a unique phagolysosome-like parasitophorous vacuole (PV) required for survival. The mechanisms of C. burnetii intracellular survival are poorly defined and a recent Q fever outbreak in the Netherlands emphasizes the need for better understanding this unique host-pathogen interaction. We recently demonstrated that inhibition of host cyclic AMP-dependent protein kinase (PKA) activity negatively impacts PV formation. In the current study, we confirmed PKA involvement in PV biogenesis and probed the role of PKA signaling during C. burnetii infection of macrophages. Using PKA-specific inhibitors, we found the kinase was needed for biogenesis of prototypical PV and C. burnetii replication. PKA and downstream targets were differentially phosphorylated throughout infection, suggesting prolonged regulation of the pathway. Importantly, the pathogen actively triggered PKA activation, which was also required for PV formation by virulent C. burnetii isolates during infection of primary human alveolar macrophages. A subset of PKA-specific substrates were differentially phosphorylated during C. burnetii infection, suggesting the pathogen uses PKA signaling to control distinct host cell responses. Collectively, the current results suggest a versatile role for PKA in C. burnetii infection and indicate virulent organisms usurp host kinase cascades for efficient intracellular growth.

Protein kinase A-mediated phosphorylation of RhoA on serine 188 triggers the rapid induction of a neuroendocrine-like phenotype in prostate cancer epithelial cells

Whilst androgen ablation therapy is used to treat locally advanced or metastatic forms of prostate cancer, side-effects can include the emergence of an androgen-independent neuroendocrine cell population which is associated with poor prognosis. Here we have examined how cyclic AMP elevation regulates early events in the neuroendocrine differentiation process. We demonstrate that selective activation of protein kinase A is necessary and sufficient for cyclic AMP (cAMP) elevation to rapidly promote a neuroendocrine phenotype in LNCaP cells independent of de novo protein synthesis. Furthermore, the effects of cAMP could be recapitulated by inhibition of RhoA signalling or pharmacological inhibition of Rho kinase. Conversely, expression of constitutively active Gln63Leu-mutated RhoA acted as a dominant-negative inhibitor of cAMP-mediated NE phenotype formation. Consistent with these observations, cAMP elevation triggered the PKA-dependent phosphorylation of RhoA on serine 188, and a non-phosphorylatable Ser188Ala RhoA mutant functioned as a dominant-negative inhibitor of cAMP-mediated neuroendocrine phenotype formation. These results suggest that PKA-mediated inhibition of RhoA via its phosphorylation on serine 188 and the subsequent inhibition of ROCK activity plays a key role in determining initial changes in cellular morphology during LNCaP cell differentiation to a neuroendocrine phenotype. It also raises the possibility that targeted suppression of this pathway to inhibit neuroendocrine cell expansion might be a useful adjuvant to conventional prostate cancer therapy.