Structure, function and pathophysiology of protease activated receptors

COVID-19-Associated acute respiratory distress syndrome (CARDS): Mechanistic insights on therapeutic intervention and emerging trends

AIMS

The novel Coronavirus disease 2019 (COVID-19) has caused great distress worldwide. Acute respiratory distress syndrome (ARDS) is well familiar but when it happens as part of COVID-19 it has discrete features which are unmanageable. Numerous pharmacological treatments have been evaluated in clinical trials to control the clinical effects of CARDS, but there is no assurance of their effectiveness.

MATERIALS AND METHODS

A systematic review of the literature of the Medline, Scopus, Bentham, PubMed, and EMBASE (Elsevier) databases was examined to understand the novel therapeutic approaches used in COVID-19-Associated Acute Respiratory Distress Syndrome and their outcomes.

KEY FINDINGS

Current therapeutic options may not be enough to manage COVID-19-associated ARDS complications in group of patients and therefore, the current review has discussed the pathophysiological mechanism of COVID-19-associated ARDS, potential pharmacological treatment and the emerging molecular drug targets.

SIGNIFICANCE

The rationale of this review is to talk about the pathophysiology of CARDS, potential pharmacological treatment and the emerging molecular drug targets. Currently accessible treatment focuses on modulating immune responses, rendering antiviral effects, anti-thrombosis or anti-coagulant effects. It is expected that considerable number of studies conducting globally may help to discover effective therapies to decrease mortality and morbidity occurring due to CARDS. Attention should be also given on molecular drug targets that possibly will help to develop efficient cure for COVID-19-associated ARDS.

On the Nonspecific Resistance in Burn Injury: Pathophysiological Aspects (Review)

An analysis of nonspecific resistance in burn patients is conducted. The role of subpopulations of neutrophils and monocytes/ macrophages in severe burn injury is discussed. The significance of blood cells for the burn-induced immune dysfunction, susceptibility to sepsis and multiple organ failure is underscored. The involvement of secondary complications in the development of morbidity and mortality in patients with burn injury is shown. New approaches to identifying individuals with a risk of adverse outcome are considered.

Protease Activated Receptor 1 and Its Ligands as Main Regulators of the Regeneration of Peripheral Nerves

In contrast with the brain and spinal cord, peripheral nerves possess a striking ability to regenerate after damage. This characteristic of the peripheral nervous system is mainly due to a specific population of glial cells, the Schwann cells. Schwann cells promptly activate after nerve injury, dedifferentiate assuming a repair phenotype, and assist axon regrowth. In general, tissue injury determines the release of a variety of proteases which, in parallel with the degradation of their specific targets, also activate plasma membrane receptors known as protease-activated receptors (PARs). PAR1, the prototypical member of the PAR family, is also known as thrombin receptor and is present at the Schwann cell plasma membrane. This receptor is emerging as a possible regulator of the pro-regenerative capacity of Schwann cells. Here, we summarize the most recent literature data describing the possible contribution of PAR1 and PAR1-activating proteases in regulating the regeneration of peripheral nerves.

The Evolving Concept of Neuro-Thromboinflammation for Neurodegenerative Disorders and Neurotrauma: A Rationale for PAR1-Targeting Therapies

Interest in the role of coagulation and fibrinolysis in the nervous system was active in several laboratories dating back before cloning of the functional thrombin receptor in 1991. As one of those, our attention was initially on thrombin and plasminogen activators in synapse formation and elimination in the neuromuscular system, with orientation towards diseases such as amyotrophic lateral sclerosis (ALS) and how clotting and fibrinolytic pathways fit into its pathogenesis. This perspective is on neuro-thromboinflammation, emphasizing this emerging concept from studies and reports over more than three decades. It underscores how it may lead to novel therapeutic approaches to treat the ravages of neurotrauma and neurodegenerative diseases, with a focus on PAR1, ALS, and parmodulins.

Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

Nonenzymatic function of DPP4 in diabetes-associated mitochondrial dysfunction and cognitive impairment

Dipeptidyl peptidase-4 (DPP4) has been proven to exert its functions by both enzymatic and nonenzymatic pathways. The nonenzymatic function of DPP4 in diabetes-associated cognitive impairment remains unexplored. We determined DPP4 protein concentrations or its enzymatic activity in type 2 diabetic patients and db/db mice and tested the impact of the non-enzymatic function of DPP4 on mitochondrial dysfunction and cognitive impairment both in vivo and in vitro. The results show that increased DPP4 activity was an independent risk factor for incident mild cognitive impairment (MCI) in type 2 diabetic patients. In addition, DPP4 was highly expressed in the hippocampus of db/db mice and contributed to mitochondria dysfunction and cognitive impairment. Mechanistically, DPP4 might bind to PAR2 in the hippocampus and trigger GSK-3β activation, which downregulates peroxisome proliferator-activated receptor gamma coactivator 1 alpha expression and leads to mitochondria dysfunction, thereby promoting cognitive impairment in diabetes. Our findings indicate that the nonenzymatic function of DPP4 might promote mitochondrial dysfunction and cognitive impairment in diabetes.

The development of proteinase-activated receptor-2 modulators and the challenges involved

Protease-activated receptor-2 (PAR2) has been extensively studied since its discovery in the mid-1990. Despite the advances in understanding PAR2 pharmacology, it has taken almost 25 years for the first inhibitor to reach clinical trials, and so far, no PAR2 antagonist has been approved for human use. Research has employed classical approaches to develop a wide array of PAR2 agonists and antagonists, consisting of peptides, peptoids and antibodies to name a few, with a surge in patent applications over this period. Recent breakthroughs in PAR2 structure determination has provided a unique insight into proposed PAR2 ligand binding sites. Publication of the first crystal structures of PAR2 resolved in complex with two novel non-peptide small molecule antagonists (AZ8838 and AZ3451) revealed two distinct binding pockets, originally presumed to be allosteric sites, with a PAR2 antibody (Fab3949) used to block tethered ligand engagement with the peptide-binding domain of the receptor. Further studies have proposed orthosteric site occupancy for AZ8838 as a competitive antagonist. One company has taken the first PAR2 antibody (MEDI0618) into phase I clinical trial (NCT04198558). While this first-in-human trial is at the early stages of the assessment of safety, other research into the structural characterisation of PAR2 is still ongoing in an attempt to identify new ways to target receptor activity. This review will focus on the development of novel PAR2 modulators developed to date, with an emphasis placed upon the advances made in the pharmacological targeting of PAR2 activity as a strategy to limit chronic inflammatory disease.

PAR2-Induced Tissue Factor Synthesis by Primary Cultures of Human Kidney Tubular Epithelial Cells Is Modified by Glucose Availability

Coagulopathies common to patients with diabetes and chronic kidney disease (CKD) are not fully understood. Fibrin deposits in the kidney suggest the local presence of clotting factors including tissue factor (TF). In this study, we investigated the effect of glucose availability on the synthesis of TF by cultured human kidney tubular epithelial cells (HTECs) in response to activation of protease-activated receptor 2 (PAR2). PAR2 activation by peptide 2f-LIGRLO-NH₂ (2F, 2 µM) enhanced the synthesis and secretion of active TF (~45 kDa) which was blocked by a PAR2 antagonist (I-191). Treatment with 2F also significantly increased the consumption of glucose from the cell medium and lactate secretion. Culturing HTECs in 25 mM glucose enhanced TF synthesis and secretion over 5 mM glucose, while addition of 5 mM 2-deoxyglucose (2DOG) significantly decreased TF synthesis and reduced its molecular weight (~40 kDa). Blocking glycosylation with tunicamycin also reduced 2F-induced TF synthesis while reducing its molecular weight (~36 kDa). In conclusion, PAR2-induced TF synthesis in HTECs is enhanced by culture in high concentrations of glucose and suppressed by inhibiting either PAR2 activation (I-191), glycolysis (2DOG) or glycosylation (tunicamycin). These results may help explain how elevated concentrations of glucose promote clotting abnormities in diabetic kidney disease. The application of PAR2 antagonists to treat CKD should be investigated further.

[Thrombin generation assay in autoimmune disease]

Thrombin generation assay (TGA) is a useful tool to evaluate the initiation, propagation and inhibition of coagulation. TGA is a global test that is used to assess hemorrhagic risk in hemophilia patients, but it can also be used to study hypercoagulable states. The interest of TGA is to screen for cardiovascular risk, which is regularly associated with autoimmune disease (AID) such as antiphospholipid syndrome. Indeed, TGA has been used to evaluate hypercoagulability in patients with antiphospholipid syndrome treated with rivaroxaban versus warfarin. In other AIDs without thrombotic events, TGA measurement is elevated, mainly in rheumatoid arthritis (RA), systemic lupus erythematosus and Behçet's disease. These findings in RA are correlated with the inflammatory activity of the disease. In systemic lupus erythematosus and Behçet's disease, TGA appears to reflect disease activity. In conclusion, TGA remains relatively under used in the clinical evaluation of AID, but it could play a greater role in the evaluation of certain potentially thrombogenic treatments in AID. Finally, TGA helps measuring AID activity, due to the clearlink between coagulation and inflammation, despite some limitations of interpretation mainly due to a lack of standardization.

The Role of Protease-Activated Receptor 2 in Hepatocellular Carcinoma after Hepatectomy

Background and Objectives: Protease activated receptor-2 (PAR2) is elevated in a variety of cancers and has been promoted as a potential therapeutic target. However, the clinical and prognostic values of PAR2 in hepatocellular carcinoma (HCC) are poorly characterized. This study aimed to evaluate the expression of PAR2 in HCC tissues and examine the prognostic value of PAR2 after resection in HCC. Materials and Methods: Two hundred and eight resected specimens were collected from HCC patients at Kaohsiung Chang Gung Memorial Hospital. PAR2 protein expression was assessed by western blotting in HCC tissues and matched normal tissues. The correlation between PAR2 expression and clinicopathological parameters was analyzed. Disease-free survival (DFS) and overall survival (OS) were compared using the log-rank test. A Cox regression model was used to identify independent prognostic factors. Results: PAR2 was expressed at higher levels in HCC tissues than the paired adjacent nontumor tissues. High expression of PAR2 was associated with advanced tumor, node, metastasis (TNM )stage and histological grade. Kaplan-Meier analysis indicated high PAR2 expression was associated with poorer DFS and OS compared to low PAR2 expression. Multivariate analyses indicated high PAR2 expression [hazard ratio (HR), 1.779, p = 0.006), α-fetoprotein (AFP) (HR, 1.696, p = 0.003), liver cirrhosis (HR, 1.735, p = 0.002), and advanced TNM stage (HR, 2.061, p < 0.001) were prognostic factors for DFS, and advanced TNM stage (HR, 2.741, p < 0.001) and histological grade (HR, 2.675, p = 0.002) and high PAR2 expression (HR, 1.832, p = 0.012) were significant risk factors for OS. In subgroup analyses, the combination of PAR2 expression and serum AFP provided improved prognostic ability for OS and DFS. Conclusion: Combination PAR2 and AFP predict HCC outcomes after resection. PAR2 represents a potentially clinically relevant biomarker for HCC.

Trypsin-Like Proteases and Their Role in Muco-Obstructive Lung Diseases

Trypsin-like proteases (TLPs) belong to a family of serine enzymes with primary substrate specificities for the basic residues, lysine and arginine, in the P1 position. Whilst initially perceived as soluble enzymes that are extracellularly secreted, a number of novel TLPs that are anchored in the cell membrane have since been discovered. Muco-obstructive lung diseases (MucOLDs) are characterised by the accumulation of hyper-concentrated mucus in the small airways, leading to persistent inflammation, infection and dysregulated protease activity. Although neutrophilic serine proteases, particularly neutrophil elastase, have been implicated in the propagation of inflammation and local tissue destruction, it is likely that the serine TLPs also contribute to various disease-relevant processes given the roles that a number of these enzymes play in the activation of both the epithelial sodium channel (ENaC) and protease-activated receptor 2 (PAR2). More recently, significant attention has focused on the activation of viruses such as SARS-CoV-2 by host TLPs. The purpose of this review was to highlight key TLPs linked to the activation of ENaC and PAR2 and their association with airway dehydration and inflammatory signalling pathways, respectively. The role of TLPs in viral infectivity will also be discussed in the context of the inhibition of TLP activities and the potential of these proteases as therapeutic targets.

Intracellular versus extracellular inhibition of calpain I causes differential effects on pain in a rat model of joint inflammation

Calpain I is a calcium-dependent cysteine protease which has dual effects on tissue inflammation depending on its cellular location. Intracellularly, calpain I has pro-inflammatory properties but becomes anti-inflammatory when exteriorised into the extracellular space. In this study, the effect of calpain I on joint pain was investigated using the kaolin/carrageenan model of acute synovitis. Evoked pain behaviour was determined by von Frey hair algesiometry and non-evoked pain was measured using dynamic hindlimb weight bearing. Local administration of calpain I reduced secondary allodynia in the acute inflammation model and this effect was blocked by the cell impermeable calpain inhibitor E-64c. Calpain I also blocked the algesic effect of the protease activated receptor-2 (PAR-2) cleaving enzyme mast cell tryptase. The cell permeable calpain blocker E-64d also produced analgesia in arthritic joints. These data suggest that calpain I produces disparate effects on joint pain viz. analgesia when present extracellularly by disarming PAR-2, and pro-algesic when the enzyme is inside the cell.

PAR2 promotes high-fat diet-induced hepatic steatosis by inhibiting AMPK-mediated autophagy

Protease-activated receptor 2 (PAR2) is a member of G protein-coupled receptors. There are two types of PAR2 signaling pathways: Canonical G-protein signaling and β-arrestin signaling. Although PAR2 signaling has been reported to aggravate hepatic steatosis, the exact mechanism is still unclear, and the role of PAR2 in autophagy remains unknown. In this study, we investigated the regulatory role of PAR2 in autophagy during high-fat diet (HFD)-induced hepatic steatosis in mice. Increased protein levels of PAR2 and β-arrestin-2 and their interactions were detected after four months of HFD. To further investigate the role of PAR2, male and female wild-type (WT) and PAR2-knockout (PAR2 KO) mice were fed HFD. PAR2 deficiency protected HFD-induced hepatic steatosis in male mice, but not in female mice. Interestingly, PAR2-deficient liver showed increased AMP-activated protein kinase (AMPK) activation with decreased interaction between Ca²⁺/calmodulin-dependent protein kinase kinase β (CAMKKβ) and β-arrestin-2. In addition, PAR2 deficiency up-regulated autophagy in the liver. To elucidate whether PAR2 plays a role in the regulation of autophagy and lipid accumulation in vitro, PAR2 was overexpressed in HepG2 cells. Overexpression of PAR2 decreased AMPK activation with increased interaction of CAMKKβ with β-arrestin-2 and significantly inhibited autophagic responses in HepG2 cells. Inhibition of autophagy by PAR2 overexpression further exacerbated palmitate-induced lipid accumulation in HepG2 cells. Collectively, these findings suggest that the increase in the PAR2-β-arrestin-2-CAMKKβ complex by HFD inhibits AMPK-mediated autophagy, leading to the alleviation of hepatic steatosis.

Impact of anticoagulation status on recanalization and outcome of cerebral venous thrombosis

Effective anticoagulation status may determine the recanalization and outcome of cerebral venous thrombosis (CVT). We report impact of anticoagulation status on recanalization and outcome of CVT. This is a retrospective study on 126 patients with CVT diagnosed on magnetic resonance venography (MRV). Their clinical features and risk factors were noted. The data were retrieved from a prospectively maintained registry, and international normalized ratio (INR) was noted after discharge till 3 months. All the patients were on acenocoumarol. Based on INR value, patients were categorized as Group A (effective anticoagulation INR within the therapeutic range or above) and Group B (ineffective anticoagulation INR > 50% below the therapeutic range). A repeat MRV at 3 months was done for recanalization. Outcome at 3 months was evaluated using modified Rankin Scale (mRS), and categorized as good (mRS ≤ 2) and poor (mRS 2 or more) 101(80.2%) patients were in group A and 25(19.8%) in group B. Their demographic, risk factors, magnetic resonance imaging (MRI) and MRV findings were comparable. On repeat MRV, recanalization occurred in 22/24(91.7%); 15(88%) in group A and 7(100%) in group B. Recanalization was independent of coagulation status. Seven (5.6%) patients died and 107(84.9%) had good outcome; 85(84.2%) in group A and 22(88%) in group B. Kaplan Meier analysis also did not reveal survival or good outcome benefits between the groups. In CVT, outcome and recanalization at 3 months are not dependent on coagulation status. Further prospective studies are needed regarding duration of anticoagulant and its impact on recanalization and outcome.

Mechanisms of thrombosis and research progress on targeted antithrombotic drugs

Globally, the incidence of thromboembolic diseases has increased in recent years, accompanied by an increase in patient mortality. Currently, several targeting delivery strategies have been developed to treat thromboembolic diseases. In this review, we discuss the mechanisms of thrombolysis and current anticoagulant drugs, particularly those with targeting capability, highlighting advances in the accurate treatment of thrombolysis with fewer adverse effects. Such approaches include magnetic drug-loading systems combined with molecular imaging to recanalize blood vessels and systems based on chimeric Arg-Gly-Asp (RGD) sequences that can target platelet glycoprotein receptor. With such progress in targeted antithrombotic drugs, targeted thrombolysis treatment shows significant potential benefit for patients.

Expression of protease activated receptor-2 is reduced in renal cell carcinoma biopsies and cell lines

Expression of the protease sensing receptor, protease activated receptor-2 (PAR2), is elevated in a variety of cancers and has been promoted as a potential therapeutic target. With the development of potent antagonists for this receptor, we hypothesised that they could be used to treat renal cell carcinoma (RCC). The expression of PAR2 was, therefore, examined in human RCC tissues and selected RCC cell lines. Histologically confirmed cases of RCC, together with paired non-involved kidney tissue, were used to produce a tissue microarray (TMA) and to extract total tissue RNA. Immunohistochemistry and qPCR were then used to assess PAR2 expression. In culture, RCC cell lines versus primary human kidney tubular epithelial cells (HTEC) were used to assess PAR2 expression by qPCR, immunocytochemistry and an intracellular calcium mobilization assay. The TMA revealed an 85% decrease in PAR2 expression in tumour tissue compared with normal kidney tissue. Likewise, qPCR showed a striking reduction in PAR2 mRNA in RCC compared with normal kidney. All RCC cell lines showed lower levels of PAR2 expression than HTEC. In conclusion, we found that PAR2 was reduced in RCC compared with normal kidney and is unlikely to be a target of interest in the treatment of this type of cancer.

Disruption of Epithelial Barrier of Caco-2 Cell Monolayers by Excretory Secretory Products of Trichinella spiralis Might Be Related to Serine Protease

The physical barrier is composed of epithelial cells which are joined together through intercellular connections. It serves to prevent pathogenic microorganisms from departing the intestinal lumen to invade the host. The excretory secretory (ES) products of Trichinella spiralis are critical for invasion. However, whether ES products of T. spiralis can act on the intestinal barrier is still unknown. In this study, the role of ES products of T. spiralis muscle larvae (Ts-ML-ES) in host invasion was studied by establishing an in vitro cell monolayers model. Barrier integrity analysis by a transmembrane resistance test and a paracellular permeability assay revealed that the Ts-ML-ES was able to destroy barrier function. It occurred via a reduction in the expression of tight junction (TJ) proteins, which was induced by serine protease. Furthermore, Western bolt analysis indicated that Ts-ML-ES reduced the expression of TJ proteins via the MAPK signaling pathway. Based on these data, we conclude that serine protease are likely the main factors from Ts-ML-ES that affect host intestinal barrier integrity by reducing the expression of TJs via the P38-MAPK signaling pathway. Serine protease in Ts-ML-ES might be a key invasion factor in T. spiralis.

PAR2 Activation on Human Kidney Tubular Epithelial Cells Induces Tissue Factor Synthesis, That Enhances Blood Clotting

Coagulation abnormalities and increased risk of atherothrombosis are common in patients with chronic kidney diseases (CKD). Mechanisms that alter renal hemostasis and lead to thrombotic events are not fully understood. Here we show that activation of protease activated receptor-2 (PAR2) on human kidney tubular epithelial cells (HTECs), induces tissue factor (TF) synthesis and secretion that enhances blood clotting. PAR-activating coagulation-associated protease (thrombin), as well as specific PAR2 activators (matriptase, trypsin, or synthetic agonist 2f-LIGRLO-NH2 (2F), induced TF synthesis and secretion that were potently inhibited by PAR2 antagonist, I-191. Thrombin-induced TF was also inhibited by a PAR1 antagonist, Vorapaxar. Peptide activators of PAR1, PAR3, and PAR4 failed to induce TF synthesis. Differential centrifugation of the 2F-conditoned medium sedimented the secreted TF, together with the exosome marker ALG-2 interacting protein X (ALIX), indicating that secreted TF was associated with extracellular vesicles. 2F-treated HTEC conditioned medium significantly enhanced blood clotting, which was prevented by pre-incubating this medium with an antibody for TF. In summary, activation of PAR2 on HTEC stimulates synthesis and secretion of TF that induces blood clotting, and this is attenuated by PAR2 antagonism. Thrombin-induced TF synthesis is at least partly mediated by PAR1 transactivation of PAR2. These findings reveal how underlying hemostatic imbalances might increase thrombosis risk and subsequent chronic fibrin deposition in the kidneys of patients with CKD and suggest PAR2 antagonism as a potential therapeutic strategy for intervening in CKD progression.

Proteinase-Mediated Macrophage Signaling in Psoriatic Arthritis

Objective

Multiple proteinases are present in the synovial fluid (SF) of an arthritic joint. We aimed to identify inflammatory cell populations present in psoriatic arthritis (PsA) SF compared to osteoarthritis (OA) and rheumatoid arthritis (RA), identify their proteinase-activated receptor 2 (PAR2) signaling function and characterize potentially active SF serine proteinases that may be PAR2 activators.

Methods

Flow cytometry was used to characterize SF cells from PsA, RA, OA patients; PsA SF cells were further characterized by single cell 3’-RNA-sequencing. Active serine proteinases were identified through cleavage of fluorogenic trypsin- and chymotrypsin-like substrates, activity-based probe analysis and proteomics. Fluo-4 AM was used to monitor intracellular calcium cell signaling. Cytokine expression was evaluated using a multiplex Luminex panel.

Results

PsA SF cells were dominated by monocytes/macrophages, which consisted of three populations representing classical, non-classical and intermediate cells. The classical monocytes/macrophages were reduced in PsA compared to OA/RA, whilst the intermediate population was increased. PAR2 was elevated in OA vs. PsA/RA SF monocytes/macrophages, particularly in the intermediate population. PAR2 expression and signaling in primary PsA monocytes/macrophages significantly impacted the production of monocyte chemoattractant protein-1 (MCP-1). Trypsin-like serine proteinase activity was elevated in PsA and RA SF compared to OA, while chymotrypsin-like activity was elevated in RA compared to PsA. Tryptase-6 was identified as an active serine proteinase in SF that could trigger calcium signaling partially via PAR2.

Conclusion

PAR2 and its activating proteinases, including tryptase-6, can be important mediators of inflammation in PsA. Components within this proteinase-receptor axis may represent novel therapeutic targets.

Involvement of Neuro-Immune Interactions in Pruritus With Special Focus on Receptor Expressions

Pruritus is a common, but very challenging symptom with a wide diversity of underlying causes like dermatological, systemic, neurological and psychiatric diseases. In dermatology, pruritus is the most frequent symptom both in its acute and chronic form (over 6 weeks in duration). Treatment of chronic pruritus often remains challenging. Affected patients who suffer from moderate to severe pruritus have a significantly reduced quality of life. The underlying physiology of pruritus is very complex, involving a diverse network of components in the skin including resident cells such as keratinocytes and sensory neurons as well as transiently infiltrating cells such as certain immune cells. Previous research has established that there is a significant crosstalk among the stratum corneum, nerve fibers and various immune cells, such as keratinocytes, T cells, basophils, eosinophils and mast cells. In this regard, interactions between receptors on cutaneous and spinal neurons or on different immune cells play an important role in the processing of signals which are important for the transmission of pruritus. In this review, we discuss the role of various receptors involved in pruritus and inflammation, such as TRPV1 and TRPA1, IL-31RA and OSMR, TSLPR, PAR-2, NK1R, H1R and H4R, MRGPRs as well as TrkA, with a focus on interaction between nerve fibers and different immune cells. Emerging evidence shows that neuro-immune interactions play a pivotal role in mediating pruritus-associated inflammatory skin diseases such as atopic dermatitis, psoriasis or chronic spontaneous urticaria. Targeting these bidirectional neuro-immune interactions and the involved pruritus-specific receptors is likely to contribute to novel insights into the underlying pathogenesis and targeted treatment options of pruritus.

PAR2 induces ovarian cancer cell motility by merging three signalling pathways to transactivate EGFR

BACKGROUND AND PURPOSE

Specific cellular functions mediated by GPCRs are often associated with signalling through a particular G protein or β-arrestin. Here, we examine signalling through a GPCR, protease-activated receptor 2 (PAR2), in a high-grade serous ovarian cancer cell line (OV90).

EXPERIMENTAL APPROACH

Human ovarian cancer tissues (n = 1,200) and nine human ovarian cancer cell lines were assessed for PAR2 expression. PAR2 signalling mechanisms leading to cell migration and invasion were dissected using cellular assays, western blots, CRISPR-Cas9 gene knockouts, pharmacological inhibitors of PAR2 and downstream signalling proteins in OV90 cancer cells.

KEY RESULTS

PAR2 was significantly overexpressed in clinical ovarian cancer tissues and in OV90 ovarian cancer cells. PAR2 agonists, an endogenous protease (trypsin) and a synthetic peptide (2f-LIGRL-NH₂ ), induced migration and invasion of OV90 ovarian cancer cells through activating a combination of Gα_q/11 , Gα_12/13 and β-arrestin1/2, but not Gα_s or Gα_i . This novel cooperative rather than parallel signalling resulted in downstream serial activation of Src kinases, then transactivation of epidermal growth factor receptor (EGFR), followed by downstream MEK-ERK1/2-FOS/MYC/STAT3-COX2 signalling. Either a PAR2 antagonist (I-191), CRISPR-Cas9 gene knockouts (PAR2 or Gα proteins or β-arrestin1/2), or inhibitors of each downstream protein attenuated human ovarian cancer cell motility.

CONCLUSION AND IMPLICATIONS

This study highlights a novel shared signalling cascade, requiring each of Gα_q/11 , Gα_12/13 and β-arrestin1/2 for PAR2-induced ovarian cancer cell migration and invasion. This mechanism controlling a cellular function is unusual in not being linked to a specific individual G protein or β-arrestin-mediated signalling pathway.

Allosteric binding sites at the receptor-lipid bilayer interface: novel targets for GPCR drug discovery

As a superfamily of membrane receptors, G-protein-coupled receptors (GPCRs) have significant roles in human physiological processes, including cell proliferation, metabolism, and neuromodulation. GPCRs are vital targets of therapeutic drugs, and their allosteric regulation represents a novel direction for drug discovery. Given the numerous breakthroughs in structural biology, diverse allosteric sites on GPCRs have been identified within the extracellular and intracellular loops, and the seven core transmembrane helices. However, a unique type of allosteric site has also been discovered at the interface of the receptor-lipid bilayer, similar to the β₂-adrenergic receptor. Here, we review recent identifications of these allosteric sites and the detailed modulator-target interactions within the interface for each modulator to highlight the role of lipids in GPCR allosteric drug discovery.

The PAR2 inhibitor I-287 selectively targets Gαq and Gα12/13 signaling and has anti-inflammatory effects

Protease-activated receptor-2 (PAR2) is involved in inflammatory responses and pain, therefore representing a promising therapeutic target for the treatment of immune-mediated inflammatory diseases. However, as for other GPCRs, PAR2 can activate multiple signaling pathways and those involved in inflammatory responses remain poorly defined. Here, we describe a new selective and potent PAR2 inhibitor (I-287) that shows functional selectivity by acting as a negative allosteric regulator on Gα_q and Gα_12/13 activity and their downstream effectors, while having no effect on G_i/o signaling and βarrestin2 engagement. Such selective inhibition of only a subset of the pathways engaged by PAR2 was found to be sufficient to block inflammation in vivo. In addition to unraveling the PAR2 signaling pathways involved in the pro-inflammatory response, our study opens the path toward the development of new functionally selective drugs with reduced liabilities that could arise from blocking all the signaling activities controlled by the receptor.

Avet et al. characterize I-287, an inhibitor to protease-activated receptor 2 using BRET-assays. They find that I-287 selectively inhibits Gα_q and Gα_12/13 without affecting the activation of G_i/o or the recruitment of βarrestin2 and that it blocks inflammation in vitro and in vivo.

Identification of a Quorum Sensing-Dependent Communication Pathway Mediating Bacteria-Gut-Brain Cross Talk

Despite recently established contributions of the intestinal microbiome to human health and disease, our understanding of bacteria-host communication pathways with regard to the gut-brain axis remains limited. Here we provide evidence that intestinal neurons are able to "sense" bacteria independently of the host immune system. Using supernatants from cultures of the opportunistic pathogen Staphylococcus aureus (S. aureus) we demonstrate the release of mediators with neuromodulatory properties at high population density. These mediators induced a biphasic response in extrinsic sensory afferent nerves, increased membrane permeability in cultured sensory neurons, and altered intestinal motility and secretion. Genetic manipulation of S. aureus revealed two key quorum sensing-regulated classes of pore forming toxins that mediate excitation and inhibition of extrinsic sensory nerves, respectively. As such, bacterial mediators have the potential to directly modulate gut-brain communication to influence intestinal symptoms and reflex function in vivo, contributing to homeostatic, behavioral, and sensory consequences of infection.

Protease-activated receptor 4 plays a role in lipopolysaccharide-induced inflammatory mechanisms in murine macrophages

The role of protease-activated receptor (PAR)4 in thrombin-induced platelet aggregation has been studied, and PAR4 blockade is thought to be useful as a new and promising approach in antiplatelet therapy in humans. In recent years, studies have been conducted to clarify the role of PAR4 in the host defense against invading microorganisms and pathogen-induced inflammation; however, to date, the role of PAR4 in mediating the LPS-induced inflammatory repertoire in macrophages remains to be elucidated. Here, we investigated the effects of the synthetic PAR4 agonist peptide (PAR4-AP) AYPGKF-NH₂ on the phagocytosis of zymosan-FITC particles; NO, ROS, and iNOS expression; and cytokine production in C57/BL6 macrophages cocultured with PAR4-AP/LPS. The PAR4-AP impaired LPS-induced and basal phagocytosis, which was restored by pharmacological PAR4 blockade. Coincubation with the PAR4-AP/LPS enhanced NO and ROS production and iNOS expression; decreased IL-10, but not TNF-α, in the culture supernatant; and increased translocation of the p65 subunit of the proinflammatory gene transcription factor NF-κ-B. Our results provide evidence for a complex mechanism and new approach by which PAR4 mediates the macrophage response triggered by LPS through counter-regulating the phagocytic activity of macrophages and innate response mechanisms implicated in the killing of invading pathogens.

Protease-Activated Receptor-2 Is Associated With Adverse Outcomes in Canine Mammary Carcinoma

Protease-activated receptor-2 (PAR2) is a G protein-coupled receptor that is activated by serine proteases. In humans, PAR2 is highly expressed in various cancers, including breast cancer, and is associated with cancer progression and metastasis. However, the expression and roles of PAR2 in canine mammary carcinoma remain unclear. The purpose of this study was to examine the expression of PAR2 in canine mammary carcinoma, the association between PAR2 expression and clinical characteristics, and the role of PAR2 in the metastatic phenotypes of tumor cells. Mammary carcinoma from 31 dogs and 10 normal mammary glands were included in this study, and used for immunohistochemical analysis of PAR2 expression. Normal mammary glands did not express PAR2. In contrast, mammary carcinomas showed PAR2 immunoreactivity in the cytoplasm, and its expression level varied between specimens from negative to strongly positive. The overall survival of dogs with high PAR2 expression was shorter than that of dogs with low PAR2 expression. Moreover, PAR2 expression level was associated with the presence of lymph node involvement, advanced clinical stage, and high histopathological grade. In vitro analyses revealed that a PAR2 agonist accelerated cell migration and invasion in a canine mammary carcinoma cell line. In addition, the PAR2 agonist induced epithelial-mesenchymal transition and actin polymerization. These results suggest that PAR2 expression plays a role in tumor progression and clinical outcomes in canine mammary carcinoma.

Granzymes in cardiovascular injury and disease

Chronic inflammation and impaired wound healing play important roles in the pathophysiology of cardiovascular diseases. Moreover, the aberrant secretion of proteases plays a critical role in pathological tissue remodeling in chronic inflammatory conditions. Human Granzymes (Granule secreted enzymes - Gzms) comprise a family of five (GzmA, B, H, K, M) cell-secreted serine proteases. Although each unique in function and substrate specificities, Gzms were originally thought to share redundant, intracellular roles in cytotoxic lymphocyte-induced cell death. However, an abundance of evidence has challenged this dogma. It is now recognized, that individual Gzms exhibit unique substrate repertoires and functions both intracellularly and extracellularly. In the extracellular milieu, Gzms contribute to inflammation, vascular dysfunction and permeability, reduced cell adhesion, release of matrix-sequestered growth factors, receptor activation, and extracellular matrix cleavage. Despite these recent findings, the non-cytotoxic functions of Gzms in the context of cardiovascular disease pathogenesis remain poorly understood. Minimally detected in tissues and bodily fluids of normal individuals, GzmB is elevated in patients with acute coronary syndromes, coronary artery disease, and myocardial infarction. Pre-clinical animal models have exemplified the importance of GzmB in atherosclerosis, aortic aneurysm, and cardiac fibrosis as animals deficient in GzmB exhibit reduced tissue remodeling, improved disease phenotypes and increased survival. Although a role for GzmB in cardiovascular disease is described, further work to elucidate the mechanisms that underpin the remaining human Gzms activity in cardiovascular disease is necessary. The present review provides a summary of the pre-clinical and clinical evidence, as well as emerging areas of research pertaining to Gzms in tissue remodeling and cardiovascular disease.

Cathepsin G and its Dichotomous Role in Modulating Levels of MHC Class I Molecules

Cathepsin G (CatG) is involved in controlling numerous processes of the innate and adaptive immune system. These features include the proteolytic activity of CatG and play a pivotal role in alteration of chemokines as well as cytokines, clearance of exogenous and internalized pathogens, platelet activation, apoptosis, and antigen processing. This is in contrast to the capability of CatG acting in a proteolytic-independent manner due to the net charge of arginine residues in the CatG sequence which interferes with bacteria. CatG is a double-edged sword; CatG is also responsible in pathophysiological conditions, such as autoimmunity, chronic pulmonary diseases, HIV infection, tumor progression and metastasis, photo-aged human skin, Papillon-Lefèvre syndrome, and chronic inflammatory pain. Here, we summarize the latest findings for functional responsibilities of CatG in immunity, including bivalent regulation of major histocompatibility complex class I molecules, which underscore an additional novel role of CatG within the immune system.

Role of the Helix-8 and C-Terminal Tail in Regulating Proteinase Activated Receptor 2 Signaling

The C-terminal tail of G-protein-coupled receptors (GPCR) contain important regulatory sites that enable interaction with intracellular signaling effectors. Here we examine the relative contribution of the C-tail serine/threonine phosphorylation sites (Ser^383-385, Ser³⁸⁷-Thr³⁹²) and the helix-8 palmitoylation site (Cys³⁶¹) in signaling regulation downstream of the proteolytically activated GPCR, PAR2. We examined Gα_q/11-coupled calcium signaling, β-arrestin-1/-2 recruitment, and MAPK activation (p44/42 phosphorylation) by wild-type and mutant receptors expressed in a CRISPR/Cas9 PAR2-knockout HEK-293 cell background with both peptide stimulation of the receptor (SLIGRL-NH₂) as well as activation with its endogenous trypsin revealed a tethered ligand. We find that alanine substitution of the membrane proximal serine residues (Ser^383-385Ala) had no effect on SLIGRL-NH₂- or trypsin-stimulated β-arrestin recruitment. In contrast, alanine substitutions in the Ser³⁸⁷-Thr³⁹² cluster resulted in a large (∼50%) decrease in β-arrestin-1/-2 recruitment triggered by the activating peptide, SLIGRL-NH₂, but was without an effect on trypsin-activated β-arrestin-1/-2 recruitment. Additionally, we find that alanine substitution of the helix-8 cysteine residue (Cys³⁶¹Ala) led to a large decrease in both Gα_q/11 coupling and β-arrestin-1/-2 recruitment to PAR2. Furthermore, we show that Gα_q/11 inhibition with YM254890, inhibited ERK phosphorylation by PAR2 agonists, while genetic deletion of β-arrestin-1/-2 by CRISPR/Cas9 enhanced MAPK activation. Knockout of β-arrestins also enhanced Gα_q/11-mediated calcium signaling. In line with these findings, a C-tail serine/threonine mutant that has decreased β-arrestin recruitment also showed enhanced ERK activation. Thus, our studies point to multiple mechanisms that regulate β-arrestin interaction with PAR2 and highlight differences in regulation of tethered-ligand- and peptide-mediated activation of this receptor.

The multiple roles of mite allergens in allergic diseases

PURPOSE OF REVIEW

Mites are the most worldwide spread allergens and relevant causative of respiratory allergies. Life cycle, component allergens, biological activity and immunogenicity are discussed in depth.

RECENT FINDINGS

It is now known that mite allergens are able to stimulate the innate immune system through different receptors, for example, TLRs and PARs. The activation of the cells in the airway mucosa is followed by type 2 polarizing cytokine production in predisposed individuals. This complex network plays a pivotal role into the promotion of Th2 differentiation.

SUMMARY

This is a comprehensive review regarding all the mite allergens known so far, including their location within dust mites, composition, biological activities and binding receptors relevant to the fate of the immunological response.

Increased protease activated receptors in the colon of patients with Hirschsprung's disease

PURPOSE

The pathophysiology of Hirschsprung's associated enterocolitis (HAEC) is not understood. Abnormal intestinal motility and altered intestinal epithelial barrier function have been suggested to play a key role in the causation of HAEC. Protease-activated receptors (PARs) 1 and 2, have been implicated in inflammatory reactions, intestinal permeability and modulation of motility in the gut.

METHODS

We investigated PAR-1 and PAR-2 protein expression in aganglionic and ganglionic regions of patients with Hirschsprung's Disease (HSCR) (n = 10) versus normal control colon (n = 10). Protein distribution was assessed by using immunofluorescence and confocal microscopy. Gene and protein expression were quantified using quantitative real-time polymerase chain reaction (qPCR), western blot analysis, and densitometry.

RESULTS

qPCR and Western blot analysis revealed that PAR-1 and PAR-2 expression was significantly increased in ganglionic and aganglionic bowel in HSCR compared to controls (p < 0.003). Confocal microscopy revealed strong PAR-1 and PAR-2 expression in smooth muscles, interstitial cells of Cajal (ICCs), platelet-derived growth factor-alpha receptor-positive (PDGFRα⁺) cells, enteric neurons and epithelium in the ganglionic and aganglionic bowel compared to controls.

CONCLUSION

Increased PAR-1 and PAR-2 expression in the colon of patients with HSCR suggests that excessive local release of PAR activating proteases may trigger inflammatory responses leading to HAEC.

Role of PAR1-Egr1 in the Initiation of Thoracic Aortic Aneurysm in Fbln4-Deficient Mice

Objective:

Remodeling of the extracellular matrix plays a vital role in cardiovascular diseases. Using a mouse model of postnatal ascending aortic aneurysms (termed Fbln4 SMKO ), we have reported that abnormal mechanosensing led to aneurysm formation in Fbln4 SMKO with an upregulation of the mechanosensitive transcription factor, Egr1 (Early growth response 1). However, the role of Egr1 and its upstream regulator(s) in the initiation of aneurysm development and their relationship to an aneurysmal microenvironment are unknown.

Approach and Results:

To investigate the contribution of Egr1 in the aneurysm development, we deleted Egr1 in Fbln4 SMKO mice and generated double knockout mice ( DKO , Fbln4 SMKO ; Egr1 −/− ). Aneurysms were prevented in DKO mice (42.8%) and Fbln4 SMKO ; Egr1 +/− mice (26%). Ingenuity Pathway Analysis identified PAR1 (protease-activated receptor 1) as a potential Egr1 upstream gene. Protein and transcript levels of PAR1 were highly increased in Fbln4 SMKO aortas at postnatal day 1 before aneurysm formed, together with active thrombin and MMP (matrix metalloproteinase)-9, both of which serve as a PAR1 activator. Concordantly, protein levels of PAR1, Egr1, and thrombin were significantly increased in human thoracic aortic aneurysms. In vitro cyclic stretch assays (1.0 Hz, 20% strain, 8 hours) using mouse primary vascular smooth muscle cells induced marked expression of PAR1 and secretion of prothrombin in response to mechanical stretch. Thrombin was sufficient to induce Egr1 expression in a PAR1-dependent manner.

Conclusions:

We propose that thrombin, MMP-9, and mechanical stimuli in the Fbln4 SMKO aorta activate PAR1, leading to the upregulation of Egr1 and initiation of ascending aortic aneurysms.

Synthesis and initial pharmacology of dual-targeting ligands for putative complexes of integrin αVβ3 and PAR2

Unpublished data from our labs led us to hypothesize that activated protein C (aPC) may initiate an anti-inflammatory signal in endothelial cells by modulating both the integrin αVβ3 and protease-activated receptor 2 (PAR2), which may exist in close proximity on the cellular surface. To test this hypothesis and to probe the possible inflammation-related pathway, we designed and synthesized dual-targeting ligands composed of modified versions of two αVβ3 ligands and two agonists of PAR2. These novel ligands were connected via copper-catalyzed alkyne-azide cycloadditions with polyethylene glycol (PEG) spacers of variable length. Initial in vitro pharmacology with EA.hy926 and HUVEC endothelial cells indicated that these ligands are effective binders of αVβ3 and potent agonists of PAR2. These were also used in preliminary studies investigating their effects on PAR2 signaling in the presence of inflammatory agents, and represent the first examples of ligands targeting both PARs and integrins, though concurrent binding to αVβ3 and PAR2 has not yet been demonstrated.

Punicalagin Ameliorates Lupus Nephritis via Inhibition of PAR2

Lupus nephritis (LN) is the most frequent phenotype in patients with systemic lupus erythematosus (SLE) and has a high rate of progression to end-stage renal disease, in spite of intensive treatment and maintenance therapies. Recent evidence suggests that protease-activated receptor-2 (PAR2) is a therapeutic target for glomerulonephritis. In this study, we performed a cell-based high-throughput screening and identified a novel potent PAR2 antagonist, punicalagin (PCG, a major polyphenol enriched in pomegranate), and evaluated the effects of PCG on LN. The effect of PCG on PAR2 inhibition was observed in the human podocyte cell line and its effect on LN was evaluated in NZB/W F1 mice. In the human podocyte cell line, PCG potently inhibited PAR2 (IC₅₀ = 1.5 ± 0.03 µM) and significantly reduced the PAR2-mediated activation of ERK1/2 and NF-κB signaling pathway. In addition, PCG significantly decreased PAR2-induced increases in ICAM-1 and VCAM-1 as well as in IL-8, IFN-γ, and TNF-α expression. Notably, the intraperitoneal administration of PCG significantly alleviated kidney injury and splenomegaly and reduced proteinuria and renal ICAM-1 and VCAM-1 expression in NZB/W F1 mice. Our results suggest that PCG has beneficial effects on LN via inhibition of PAR2, and PCG is a potential therapeutic agent for LN.

Advances in Understanding the Initial Steps of Pruritoceptive Itch: How the Itch Hits the Switch

Pruritoceptive (dermal) itch was long considered an accompanying symptom of diseases, a side effect of drug applications, or a temporary sensation induced by invading pruritogens, as produced by the stinging nettle. Due to extensive research in recent years, it was possible to provide detailed insights into the mechanism of itch mediation and modulation. Hence, it became apparent that pruritus is a complex symptom or disease in itself, which requires particular attention to improve patients’ health. Here, we summarize recent findings in pruritoceptive itch, including how this sensation is triggered and modulated by diverse endogenous and exogenous pruritogens and their receptors. A differentiation between mediating pruritogen and modulating pruritogen seems to be of great advantage to understand and decipher the molecular mechanism of itch perception. Only a comprehensive view on itch sensation will provide a solid basis for targeting this long-neglected adverse sensation accompanying numerous diseases and many drug side effects. Finally, we identify critical aspects of itch perception that require future investigation.

Characterization and pro-inflammatory potential of indoor mold particles

A number of epidemiological studies find an association between indoor air dampness and respiratory health effects. This is often suggested to be linked to enhanced mold growth. However, the role of mold is obviously difficult to disentangle from other dampness-related exposure including microbes as well as non-biological particles and chemical pollutants. The association may partly be due to visible mycelial growth and a characteristic musty smell of mold. Thus, the potential role of mold exposure should be further explored by evaluating information from experimental studies elucidating possible mechanistic links. Such studies show that exposure to spores and hyphal fragments may act as allergens and pro-inflammatory mediators and that they may damage airways by the production of toxins, enzymes, and volatile organic compounds. In the present review, we hypothesize that continuous exposure to mold particles may result in chronic low-grade pro-inflammatory responses contributing to respiratory diseases. We summarize some of the main methods for detection and characterization of fungal aerosols and highlight in vitro research elucidating how molds may induce toxicity and pro-inflammatory reactions in human cell models relevant for airway exposure. Data suggest that the fraction of fungal hyphal fragments in indoor air is much higher than that of airborne spores, and the hyphal fragments often have a higher pro-inflammatory potential. Thus, hyphal fragments of prevalent mold species with strong pro-inflammatory potential may be particularly relevant candidates for respiratory diseases associated with damp/mold-contaminated indoor air. Future studies linking of indoor air dampness with health effects should assess the toxicity and pro-inflammatory potential of indoor air particulate matter and combined this information with a better characterization of biological components including hyphal fragments from both pathogenic and non-pathogenic mold species. Such studies may increase our understanding of the potential role of mold exposure.

Discovery of Novel Nonpeptidic PAR2 Ligands

Proteinase-activated receptor 2 (PAR2) is a class A G protein-coupled receptor whose activation has been associated with inflammatory diseases and cancer, thus representing a valuable therapeutic target. Pathophysiological roles of PAR2 are often characterized using peptidic PAR2 agonists. Peptidic ligands are frequently unstable in vivo and show poor bioavailability, and only a few approaches toward drug-like nonpeptidic PAR2 ligands have been described. The herein-described ligand 5a (IK187) is a nonpeptidic PAR2 agonist with submicromolar potency in a functional assay reflecting G protein activation. The ligand also showed substantial β-arrestin recruitment. The development of the compound was guided by the crystal structure of PAR2, when the C-terminal end of peptidic agonists was replaced by a small molecule based on a disubstituted phenylene scaffold. IK187 shows preferable metabolic stability and may serve as a lead compound for the development of nonpeptidic drugs addressing PAR2.

Loss of Protease-Activated Receptor 4 Prevents Inflammation Resolution and Predisposes the Heart to Cardiac Rupture After Myocardial Infarction

BACKGROUND

Cardiac rupture is a major lethal complication of acute myocardial infarction (MI). Despite significant advances in reperfusion strategies, mortality from cardiac rupture remains high. Studies suggest that cardiac rupture can be accelerated by thrombolytic therapy, but the relevance of this risk factor remains controversial.

METHODS

We analyzed protease-activated receptor 4 (Par4) expression in mouse hearts with MI and investigated the effects of Par4 deletion on cardiac remodeling and function after MI by echocardiography, quantitative immunohistochemistry, and flow cytometry.

RESULTS

Par4 mRNA and protein levels were increased in mouse hearts after MI and in isolated cardiomyocytes in response to hypertrophic and inflammatory stimuli. Par4-deficient mice showed less myocyte apoptosis, reduced infarct size, and improved functional recovery after acute MI relative to wild-type (WT). Conversely, Par4^-/- mice showed impaired cardiac function, greater rates of myocardial rupture, and increased mortality after chronic MI relative to WT. Pathological evaluation of hearts from Par4^-/- mice demonstrated a greater infarct expansion, increased cardiac hemorrhage, and delayed neutrophil accumulation, which resulted in impaired post-MI healing compared with WT. Par4 deficiency also attenuated neutrophil apoptosis in vitro and after MI in vivo and impaired inflammation resolution in infarcted myocardium. Transfer of Par4^-/- neutrophils, but not of Par4^-/- platelets, in WT recipient mice delayed inflammation resolution, increased cardiac hemorrhage, and enhanced cardiac dysfunction. In parallel, adoptive transfer of WT neutrophils into Par4^-/- mice restored inflammation resolution, reduced cardiac rupture incidence, and improved cardiac function after MI.

CONCLUSIONS

These findings reveal essential roles of Par4 in neutrophil apoptosis and inflammation resolution during myocardial healing and point to Par4 inhibition as a potential therapy that should be limited to the acute phases of ischemic insult and avoided for long-term treatment after MI.

Membrane-anchored serine proteases as regulators of epithelial function

Cleavage of proteins in the extracellular milieu, including hormones, growth factors and their receptors, ion channels, and various cell adhesion and extracellular matrix molecules, plays a key role in the regulation of cell behavior. Among more than 500 proteolytic enzymes encoded by mammalian genomes, membrane-anchored serine proteases (MASPs), which are expressed on the surface of epithelial cells of all major organs, are excellently suited to mediate signal transduction across the epithelia and are increasingly being recognized as important regulators of epithelial development, function, and disease [ 1-3]. In this minireview, we summarize current knowledge of the in vivo roles of MASPs in acquisition and maintenance of some of the defining functions of epithelial tissues, such as barrier formation, ion transport, and sensory perception.

Synaptic Potentiation at Basal and Apical Dendrites of Hippocampal Pyramidal Neurons Involves Activation of a Distinct Set of Extracellular and Intracellular Molecular Cues

In the central nervous system, several forms of experience-dependent plasticity, learning and memory require the activity-dependent control of synaptic efficacy. Despite substantial progress in describing synaptic plasticity, mechanisms related to heterogeneity of synaptic functions at local circuits remain elusive. Here we studied the functional and molecular aspects of hippocampal circuit plasticity by analyzing excitatory synapses at basal and apical dendrites of mouse hippocampal pyramidal cells (CA1 region) in acute brain slices. In the past decade, activity of metalloproteinases (MMPs) has been implicated as a widespread and critical factor in plasticity mechanisms at various projections in the CNS. However, in the present study we discovered that in striking contrast to apical dendrites, synapses located within basal dendrites undergo MMP-independent synaptic potentiation. We demonstrate that synapse-specific molecular pathway allowing MMPs to rapidly upregulate function of NMDARs in stratum radiatum involved protease activated receptor 1 and intracellular kinases and GTPases activity. In contrast, MMP-independent scaling of synaptic strength in stratum oriens involved dopamine D1/D5 receptors and Src kinases. Results of this study reveal that 2 neighboring synaptic systems differ significantly in extracellular and intracellular cascades that control synaptic gain and provide long-searched transduction pathways relevant for MMP-dependent synaptic plasticity.

Molecular basis for activation and biased signaling at the thrombin-activated GPCR proteinase activated receptor-4 (PAR4)

Proteinase-activated receptor (PAR)-4 is a member of the proteolytically-activated PAR family of G-protein-coupled receptors (GPCR) that represents an important target in the development of anti-platelet therapeutics. PARs are activated by proteolytic cleavage of their receptor N terminus by enzymes such as thrombin, trypsin, and cathepsin-G. This reveals the receptor-activating motif, termed the tethered ligand that binds intramolecularly to the receptor and triggers signaling. However, PARs are also activated by exogenous application of synthetic peptides derived from the tethered-ligand sequence. To better understand the molecular basis for PAR4-dependent signaling, we examined PAR4-signaling responses to a peptide library derived from the canonical PAR4-agonist peptide, AYPGKF-NH2, and we monitored activation of the Gαq/11-coupled calcium-signaling pathway, β-arrestin recruitment, and mitogen-activated protein kinase (MAPK) pathway activation. We identified peptides that are poor activators of PAR4-dependent calcium signaling but were fully competent in recruiting β-arrestin-1 and -2. Peptides that were unable to stimulate PAR4-dependent calcium signaling could not trigger MAPK activation. Using in silico docking and site-directed mutagenesis, we identified Asp230 in the extracellular loop-2 as being critical for PAR4 activation by both agonist peptide and the tethered ligand. Probing the consequence of biased signaling on platelet activation, we found that a peptide that cannot activate calcium signaling fails to cause platelet aggregation, whereas a peptide that is able to stimulate calcium signaling and is more potent for β-arrestin recruitment triggered greater levels of platelet aggregation compared with the canonical PAR4 agonist peptide. These findings uncover molecular determinants critical for agonist binding and biased signaling through PAR4.

Innate immunity in diabetic kidney disease

Increasing evidence suggests that renal inflammation contributes to the pathogenesis and progression of diabetic kidney disease (DKD) and that anti-inflammatory therapies might have renoprotective effects in DKD. Immune cells and resident renal cells that activate innate immunity have critical roles in triggering and sustaining inflammation in this setting. Evidence from clinical and experimental studies suggests that several innate immune pathways have potential roles in the pathogenesis and progression of DKD. Toll-like receptors detect endogenous danger-associated molecular patterns generated during diabetes and induce a sterile tubulointerstitial inflammatory response via the NF-κB signalling pathway. The NLRP3 inflammasome links sensing of metabolic stress in the diabetic kidney to activation of pro-inflammatory cascades via the induction of IL-1β and IL-18. The kallikrein-kinin system promotes inflammatory processes via the generation of bradykinins and the activation of bradykinin receptors, and activation of protease-activated receptors on kidney cells by coagulation enzymes contributes to renal inflammation and fibrosis in DKD. In addition, hyperglycaemia leads to protein glycation and activation of the complement cascade via recognition of glycated proteins by mannan-binding lectin and/or dysfunction of glycated complement regulatory proteins. Data from preclinical studies suggest that targeting these innate immune pathways could lead to novel therapies for DKD.

Activated clotting factor X mediates mitochondrial alterations and inflammatory responses via protease-activated receptor signaling in alveolar epithelial cells

There is growing evidence for the contribution of the activated coagulation factor X (FXa) in the development of chronic inflammatory lung diseases. Therefore, we aimed to investigate effects of exogenous FXa on mitochondrial and metabolic function as well as the induction of inflammatory molecules in type II alveolar epithelial cells. Effects of FXa on epithelial cells were investigated in A549 cell line. Activation of extracellular signal-regulated kinase (ERK) and induction of inflammatory molecules were examined by immunoblot and gene expression analysis. Mitochondrial function was assessed by the measurement of oxygen consumption during maximal oxidative phosphorylation and quantitative determination of cardiolipin oxidation. Apoptosis was tested using a caspase 3 antibody. Metabolic activity and lactate dehydrogenase assay were applied for the detection of cellular viability. FXa activated ERK1/2 and induced an increase in the expression of pro-inflammatory cytokines, which was prevented by an inhibitor of FXa, edoxaban, or an inhibitor of protease-activated receptor 1, vorapaxar. Exposure to FXa caused mitochondrial alteration with restricted capacity for ATP generation, which was effectively prevented by edoxaban, vorapaxar and GB83 (inhibitor of protease-activated receptor 2). Of note, exposure to FXa did not initiate apoptosis in epithelial cells. FXa-dependent pro-inflammatory state and impairment of mitochondria did not reach the level of significance in lung epithelial cells. However, these effects might limit regenerative potency of lung epithelial cells, particular under clinical circumstances where lung injury causes exposure to clotting factors.

Faecal Proteases from Pouchitis Patients Activate Protease Activating Receptor-2 to Disrupt the Epithelial Barrier

BACKGROUND AND AIMS

The pathogenesis of pouch inflammation may involve epithelial barrier disruption. We investigated whether faecal proteolytic activity is increased during pouchitis and results in epithelial barrier dysfunction through protease activating receptor [PAR] activation, and assessed whether the intestinal microbiome may be the source of the proteases.

METHODS

Faecal samples were measured for protease activity using a fluorescein isothiocyanate [FITC]-casein florescence assay. Caco-2 cell monolayers were exposed to faecal supernatants to assess permeability to FITC-dextran. Tight junction protein integrity and PAR activation were assessed by immunoblot and immunofluorescence. A truncated PAR2 protein in Caco-2 cells was achieved by stable transfection using CRISPR/Cas9 plasmid. PAR2 activation in pouch biopsies was examined using antibodies directed to the N-terminus of the protein. Microbial composition was analysed based on 16S rRNA gene sequence analysis.

RESULTS

Ten pouchitis patients, six normal pouch [NP] patients and nine healthy controls [HC] were recruited. The pouchitis patients exhibited a 5.19- and 5.35-fold higher faecal protease [FP] activity [p ≤ 0.05] compared to the NP and HC participants, respectively. The genus Haemophilus was positively associated with FP activity [R = 0.718, false discovery rate < 0.1]. Faecal supernatants from pouchitis patients activated PAR2 on Caco-2 monolayers, disrupted tight junction proteins and increased epithelial permeability. PAR2 truncation in Caco-2 abrogated faecal protease-mediated permeability. Pouch biopsies obtained from pouchitis patients, but not from NP patients, displayed PAR2 activation.

CONCLUSIONS

Protease-producing bacteria may increase faecal proteolytic activity that results in pouch inflammation through disruption of tight junction proteins and increased epithelial permeability in a PAR2-dependent manner. This mechanism may initiate or propagate pouch inflammation.

The repertoire of family A-peptide GPCRs in archaic hominins

Given the importance of G-protein coupled receptors in the regulation of many physiological functions, deciphering the relationships between genotype and phenotype in past and present hominin GPCRs is of main interest to understand the evolutionary process that contributed to the present-day variability in human traits and health. Here, we carefully examined the publicly available genomic and protein sequence databases of the archaic hominins (Neanderthal and Denisova) to draw up the catalog of coding variations in GPCRs for peptide ligands, in comparison with living humans. We then searched in the literature the functional changes, phenotypes and risk of disease possibly associated with the detected variants. Our survey suggests that Neanderthal and Denisovan hominins were likely prone to lower risk of obesity, to enhanced platelet aggregation in response to thrombin, to better response to infection, to less anxiety and aggressiveness and to favorable sociability. While some archaic variants were likely advantageous in the past, they might be responsible for maladaptive disorders today in the context of modern life and/or specific regional distribution. For example, an archaic haplotype in the neuromedin receptor 2 is susceptible to confer risk of diabetic nephropathy in type 1 diabetes in present-day Europeans. Paying attention to the pharmacological properties of some of the archaic variants described in this study may be helpful to understand the variability of therapeutic efficacy between individuals or ethnic groups.

Integration of TGF-β-induced Smad signaling in the insulin-induced transcriptional response in endothelial cells

Insulin signaling governs many processes including glucose homeostasis and metabolism, and is therapeutically used to treat hyperglycemia in diabetes. We demonstrated that insulin-induced Akt activation enhances the sensitivity to TGF-β by directing an increase in cell surface TGF-β receptors from a pool of intracellular TGF-β receptors. Consequently, increased autocrine TGF-β signaling in response to insulin participates in insulin-induced angiogenic responses of endothelial cells. With TGF-β signaling controlling many cell responses, including differentiation and extracellular matrix deposition, and pathologically promoting fibrosis and cancer cell dissemination, we addressed to which extent autocrine TGF-β signaling participates in insulin-induced gene responses of human endothelial cells. Transcriptome analyses of the insulin response, in the absence or presence of a TGF-β receptor kinase inhibitor, revealed substantial positive and negative contributions of autocrine TGF-β signaling in insulin-responsive gene responses. Furthermore, insulin-induced responses of many genes depended on or resulted from autocrine TGF-β signaling. Our analyses also highlight extensive contributions of autocrine TGF-β signaling to basal gene expression in the absence of insulin, and identified many novel TGF-β-responsive genes. This data resource may aid in the appreciation of the roles of autocrine TGF-β signaling in normal physiological responses to insulin, and implications of therapeutic insulin usage.

Does the Lipid Bilayer Orchestrate Access and Binding of Ligands to Transmembrane Orthosteric/Allosteric Sites of G Protein-Coupled Receptors?

The ligand-binding sites of many G protein-coupled receptors (GPCRs) are situated around and deeply embedded within the central pocket formed by their seven transmembrane-spanning α-helical domains. Generally, these binding sites are assumed accessible to endogenous ligands from the aqueous phase. Recent advances in the structural biology of GPCRs, along with biophysical and computational studies, suggest that amphiphilic and lipophilic molecules may gain access to these receptors by first partitioning into the membrane and then reaching the binding site via lateral diffusion through the lipid bilayer. In addition, several crystal structures of class A and class B GPCRs bound to their ligands offer unprecedented details on the existence of lipid-facing allosteric binding sites outside the transmembrane helices that can only be reached via lipid pathways. The highly organized structure of the lipid bilayer may direct lipophilic or amphiphilic drugs to a specific depth within the bilayer, changing local concentration of the drug near the binding site and affecting its binding kinetics. Additionally, the constraints of the lipid bilayer, including its composition and biophysical properties, may play a critical role in "pre-organizing" ligand molecules in an optimal orientation and conformation to facilitate receptor binding. Despite its clear involvement in molecular recognition processes, the critical role of the membrane in binding ligands to lipid-exposed transmembrane binding sites remains poorly understood and warrants comprehensive investigation. Understanding the mechanistic basis of the structure-membrane interaction relationship of drugs will not only provide useful insights about receptor binding kinetics but will also enhance our ability to take advantage of the apparent membrane contributions when designing drugs that target transmembrane proteins with improved efficacy and safety. In this minireview, we summarize recent structural and computational studies on membrane contributions to binding processes, elucidating both lipid pathways of ligand access and binding mechanisms for several orthosteric and allosteric ligands of class A and class B GPCRs.