SARS-CoV-2 Invasion and Pathogenesis of COVID-19: A Perspective of Viral Receptors, Bradykinin, and Purinergic System

The present chapter focuses on the mechanisms of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection, pathogenesis, and the possible therapeutic strategies targeted to the viral receptors, purinergic and kallikrein-kinin systems. SARS-CoV-2 spike protein binds with high affinity to the human ACE2 receptor on host cells, but it can also interact with other receptors and enzymes. Following viral infection, a plethora of subsequent molecular and cellular alterations occurs in the host. These alterations, which include the cytokine and bradykinin storms, as well as exacerbated ATP signalling, have been implicated in the genesis and progression of the signs and symptoms observed in COVID-19 patients. These routes and systems provide important targets for developing specific and effective anti-COVID-19 drugs, as well as reveal a novel understanding of pathogenesis and tropism of SARS-CoV-2. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022.

The des-Arg9-bradykinin/B1R axis: Hepatic damage in COVID-19.

Patients infected by the SARS-CoV-2 virus are commonly diagnosed with threatening liver conditions associated with drug-induced therapies and systemic viral action. RNA-Seq data from cells in bronchoalveolar lavage fluid from COVID-19 patients have pointed out dysregulation of kallikrein-kinin and renin-angiotensin systems as a possible mechanism that triggers multi-organ damage away from the leading site of virus infection. Therefore, we measured the plasma concentration of biologically active peptides from the kallikrein-kinin system, bradykinin and des-Arg9-bradykinin, and liver expression of its proinflammatory axis, bradykinin 1 receptor (B1R). We measured the plasma concentration of bradykinin and des-Arg9-bradykinin of 20 virologically confirmed COVID-19 patients using a liquid chromatography-tandem mass spectrometry-based methodology. The expression of B1R was evaluated by immunohistochemistry from post-mortem liver specimens of 27 COVID-19 individuals. We found a significantly higher blood level of des-Arg9-bradykinin and a lower bradykinin concentration in patients with COVID-19 compared to a healthy, uninfected control group. We also observed increased B1R expression levels in hepatic tissues of patients with COVID-19 under all hepatic injuries analyzed (liver congestion, portal vein dilation, steatosis, and ischemic necrosis). Our data indicate that des-Arg9-bradykinin/B1R is associated with the acute hepatic dysfunction induced by the SARS-CoV-2 virus infection in the pathogenesis of COVID-19.

COVID-19 disease and hereditary angioedema

Since December 2019, an outbreak of a novel coronavirus (SARS-CoV-2) infection causing COVID-19 disease has influenced the whole world. Angiotensin converting enzyme 2 (ACE2) receptors on type 2 pneumocytes in humans were determined as the entry for SARSCoV-2. Receptor binding and subsequently endocytosis of ACE2 diminish the cell membrane expression and also the function of ACE2. ACE2 is an enzyme involved in bradykinin metabolism. Lys-des-Arg9-BK occured with enzymatic cleaving of Lys-BK derived from low molecular weight kininogen is inactivated by ACE2 in tissues and it is a vasodilator agent having its own receptor named bradykinin B1. Non-metabolized Lys-des-Arg9-BK can be the reason for tissue vasodilation and increased vascular permeability in the patients with COVID-19. Increased bradykinin levels in patients with hereditary angioedema with C1-INH deficiency (C1-INH-HAE) do not cause increased SARS-CoV-2 infection or more severe disease. Although SARS-CoV-2 infection does not result in increased bradykinin levels, it can increase Lys-des-Arg9-BK levels.Copyright © 2020 Bilimsel Tip Yayinevi. All rights reserved.

The therapeutic relevance of the Kallikrein-Kinin axis in SARS-cov-2-induced vascular pathology.

While coronavirus disease 2019 (COVID-19) begins as a respiratory infection, it progresses as a systemic disease involving multiorgan microthromboses that underly the pathology. SARS-CoV-2 enters host cells via attachment to the angiotensin-converting enzyme 2 (ACE2) receptor. ACE2 is widely expressed in a multitude of tissues, including the lung (alveolar cells), heart, intestine, kidney, testis, gallbladder, vasculature (endothelial cells), and immune cells. Interference in ACE2 signaling could drive the aforementioned systemic pathologies, such as endothelial dysfunction, microthromboses, and systemic inflammation, that are typically seen in patients with severe COVID-19. ACE2 is a component of the renin-angiotensin system (RAS) and is intimately associated with the plasma kallikrein-kinin system (KKS). As many papers are published on the role of ACE and ACE2 in COVID-19, we will review the role of bradykinin, and more broadly the KSS, in SARS-CoV-2-induced vascular dysfunction. Furthermore, we will discuss the possible therapeutic interventions that are approved and in development for the following targets: coagulation factor XII (FXII), tissue kallikrein (KLK1), plasma kallikrein (KLKB1), bradykinin (BK), plasminogen activator inhibitor (PAI-1), bradykinin B1 receptor (BKB1R), bradykinin B2 receptor (BKB2R), ACE, furin, and the NLRP3 inflammasome. Understanding these targets may prove of value in the treatment of COVID-19 as well as in other virus-induced coagulopathies in the future.

Mass spectrometric study of variation in kinin peptide profiles in nasal fluids and plasma of adult healthy individuals.

BACKGROUND: The kallikrein-kinin system is assumed to have a multifunctional role in health and disease, but its in vivo role in humans currently remains unclear owing to the divergence of plasma kinin level data published ranging from the low picomolar to high nanomolar range, even in healthy volunteers. Moreover, existing data are often restricted on reporting levels of single kinins, thus neglecting the distinct effects of active kinins on bradykinin (BK) receptors considering diverse metabolic pathways. A well-characterized and comprehensively evaluated healthy cohort is imperative for a better understanding of the biological variability of kinin profiles to enable reliable differentiation concerning disease-specific kinin profiles. METHODS: To study biological levels and variability of kinin profiles comprehensively, 28 healthy adult volunteers were enrolled. Nasal lavage fluid and plasma were sampled in customized protease inhibitor prespiked tubes using standardized protocols, proven to limit inter-day and interindividual variability significantly. Nine kinins were quantitatively assessed using validated LC-MS/MS platforms: kallidin (KD), Hyp4-KD, KD1-9, BK, Hyp3-BK, BK1-8, BK1-7, BK1-5, and BK2-9. Kinin concentrations in nasal epithelial lining fluid were estimated by correlation using urea. RESULTS: Circulating plasma kinin levels were confirmed in the very low picomolar range with levels below 4.2 pM for BK and even lower levels for the other kinins. Endogenous kinin levels in nasal epithelial lining fluids were substantially higher, including median levels of 80.0 pM for KD and 139.1 pM for BK. Hydroxylated BK levels were higher than mean BK concentrations (Hyp3-BK/BK = 1.6), but hydroxylated KD levels were substantially lower than KD (Hyp4-KD/KD = 0.37). No gender-specific differences on endogenous kinin levels were found. CONCLUSIONS: This well-characterized healthy cohort enables investigation of the potential of kinins as biomarkers and would provide a valid control group to study alterations of kinin profiles in diseases, such as angioedema, sepsis, stroke, Alzheimer's disease, and COVID-19.

HUMAN RHINOVIRUS INFECTION CAUSING ARDS IN AN IMMUNOCOMPETENT HOST

SESSION TITLE: Critical Care in Chest Infections Case Report Posters 1 SESSION TYPE: Case Report Posters PRESENTED ON: 10/17/2022 12:15 pm - 01:15 pm INTRODUCTION: The "common cold” is a syndrome defined by upper respiratory symptoms in addition to: rhinorrhea, fever, chills, headache, and/or malaise. Classically "colds” are thought of as a mild, self-limiting disease;however, they can cause severe respiratory symptoms in immunocompetent individuals. We present a case of severe acute respiratory distress syndrome (ARDS) caused by the Human Rhinovirus in an immunocompetent host. CASE PRESENTATION: 61-year-old gentleman with a past medical history significant for hypertension presented to an outside hospital for worsening shortness of breath, fatigue, and cough with production x 3 weeks. Social history is notable that he had a 12-pack-year history and quit smoking tobacco approximately 10 years ago. On arrival, the patient was noted to be hypoxic with percent saturation of 88% on 2 L nasal cannula. He rapidly deteriorated and required intubation 5 days after admission. The patient subsequently transferred to a tertiary care intensive care unit for further workup and management. Upon arrival at the tertiary care center, he was found to have a PaO2/FiO2 ratio of 71 and ARDS protocol was initiated. Despite pronation, paralyzation, dexamethasone, and nitric oxide, the patient continued to deteriorate. Three COVID-19 PCR's and COVID-19 antibody resulted negative. Extensive work-up including fungal, autoimmune, viral, and bacterial were negative with the exception of a positive rhinovirus PCR. MRI brain was completed due to patient's unequal pupils which demonstrated numerous recent infarcts of the bilateral cerebral and cerebellar hemispheres with mass-effect with mild leftward shift. The family ultimately decided to pursue comfort measures and the patient died. DISCUSSION: Human Rhinovirus is responsible for ? to ½ of common colds in adults making it the most common cause of "colds.” Due to its more than 100 serotypes, an average adult has approximately 2-3 Rhinovirus infections per year. Rhinovirus infections are classically thought to be self-resolving and mild, particularly in the immunocompetent. However, several recent studies have shown coinfection of the rhinovirus in patients with community acquired pneumonia;although these studies have been unable to tease out how clinically significant the rhinovirus infection was in these patients. The patient case above is an example that the Rhinovirus may be a more important culprit in community-acquired pneumonia than previously suspected. In addition to its possible respiratory conditions, studies have demonstrated an increase in risk of stroke. Currently, there are no FDA-approved antivirals for the Human Rhinovirus, treatment largely aimed to reduce symptomatology. CONCLUSIONS: The medical community, in large, thinks of the Rhinovirus as a relatively benign disease process. Though this may be the case in most patients, even immunocompetent individuals can suffer from serious complications of the virus. Reference #1: Chu HY;Englund JA;Strelitz B;Lacombe K;Jones C;Follmer K;Martin EK;Bradford M;Qin X;Kuypers J;Klein EJ;"Rhinovirus Disease in Children Seeking Care in a Tertiary Pediatric Emergency Department.” Journal of the Pediatric Infectious Diseases Society, U.S. National Library of Medicine, https://pubmed.ncbi.nlm.nih.gov/26908489/. Reference #2: JO;, Proud D;Naclerio RM;Gwaltney JM;Hendley. "Kinins Are Generated in Nasal Secretions during Natural Rhinovirus Colds.” The Journal of Infectious Diseases, U.S. National Library of Medicine, https://pubmed.ncbi.nlm.nih.gov/2295843/. Reference #3: Subramanian, A., et al. "Stroke Following Positive Biomarker for Viral Respiratory Illnesses.” B47. CRITICAL CARE: NON-PULMONARY CRITICAL CARE, 2020, https://doi.org/10.1164/ajrccm-conference.2020.201.1_meetings.a3566. DISCLOSURES: No relevant relationships by Philip Forys No relevant relationships by Brandon Pearce

The rationale for the treatment of long-Covid symptoms - A cardiologist's view.

The ongoing coronavirus disease 2019 pandemic left us with thousands of patients suffering from neurological, cardiovascular, and psychiatric disorders named post-acute sequelae of COVID-19 or just long-Covid. In parallel, the vaccination campaigns against SARS-CoV-2 spike protein saved millions of lives worldwide but long-Covid symptoms also appeared rarely following vaccination with a strong overlap to the "canonical" long-Covid symptoms. A therapeutic strategy targeting both, post-VAC and post-SARS-CoV-2 long-Covid symptoms is warranted since exposure to the S-protein either by vaccination or SARS-CoV-2 infection may trigger identical immuno-inflammatory cascades resulting in long-Covid symptoms.

Targeting thromboinflammation in COVID-19 - A narrative review of the potential of C1 inhibitor to prevent disease progression.

Coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 is associated with a clinical spectrum ranging from asymptomatic carriers to critically ill patients with complications including thromboembolic events, myocardial injury, multisystemic inflammatory syndromes and death. Since the beginning of the pandemic several therapeutic options emerged, with a multitude of randomized trials, changing the medical landscape of COVID-19. The effect of various monoclonal antibodies, antiviral, anti-inflammatory and anticoagulation drugs have been studied, and to some extent, implemented into clinical practice. In addition, a multitude of trials improved the understanding of the disease and emerging evidence points towards a significant role of the complement system, kallikrein-kinin, and contact activation system as drivers of disease in severe COVID-19. Despite their involvement in COVID-19, treatments targeting these plasmatic cascades have neither been systematically studied nor introduced into clinical practice, and randomized studies with regards to these treatments are scarce. Given the multiple-action, multiple-target nature of C1 inhibitor (C1-INH), the natural inhibitor of these cascades, this drug may be an interesting candidate to prevent disease progression and combat thromboinflammation in COVID-19. This narrative review will discuss the current evidence with regards to the involvement of these plasmatic cascades as well as endothelial cells in COVID-19. Furthermore, we summarize the evidence of C1-INH in COVID-19 and potential benefits and pitfalls of C1-INH treatment in COVID-19.

A comprehensive review on current understanding of bradykinin in COVID-19 and inflammatory diseases.

Bradykinin, a member of the kallikrein-kinin system (KKS), is a potent, short-lived vasoactive peptide that acts as a vasodilator and an inflammatory mediator in a number of signaling mechanisms. Bradykinin induced signaling is mediated through kinin B1 (BDKRB1) and B2 (BDKRB2) transmembrane receptors coupled with different subunits of G proteins (Gαi/Gα0, Gαq and Gß1γ2). The bradykinin-mediated signaling mechanism activates excessive pro-inflammatory cytokines, including IL-6, IL-1ß, IL-8 and IL-2. Upregulation of these cytokines has implications in a wide range of clinical conditions such as inflammation leading to fibrosis, cardiovascular diseases, and most recently, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In SARS-CoV-2 infection, bradykinin is found to be at raised levels and is reported to trigger a diverse array of symptoms. All of this brings bradykinin to the core point as a molecule of immense therapeutic value. Our understanding of its involvement in various pathways has expanded with time. Therefore, there is a need to look at the overall picture that emerges from the developments made by deciphering the bradykinin mediated signaling mechanisms involved in the pathological conditions. It will help devise strategies for developing better treatment modalities in the implicated diseases. This review summarizes the current state of knowledge on bradykinin mediated signaling in the diverse conditions described above, with a marked emphasis on the therapeutic potential of targeting the bradykinin receptor.

Management of Patients with Hereditary Angioedema During the COVID-19 Pandemic

Objective: The aim of this study was to determine the clinical course and treatment outcomes of patients with hereditary angioedema (HAE) after infection with coronavirus disease 2019 (COVID-19). Materials and Methods: Thirty-nine patients with HAE were included in this study. These patients were regularly followed up over phone calls since the first COVID-19 case was seen in our country. Patients were asked to visit the hospital if there was a history of contact with a confirmed COVID-19 patient or if the patient developed clinical symptoms of COVID-19. Results: There were 21 (54%) patients with type I HAE, and 18 (46%) with type II HAE. All patients received treatment for angioedema attacks (C1-inhibitor [C1-INH], icatibant), and seven (20%) received long-term prophylaxis (danazol). Treatment for attacks was continued for all patients during the pandemic. Patients taking danazol were switched to long-term prophylaxis using the C1-INH concentrate. Eleven (28%) patients with HAE developed COVID-19 during this study. Only one patient had severe COVID-19. Six patients (54.5%) were diagnosed with type II HAE, and five (45.5%) were diagnosed with type I HAE. The most common COVID-19 symptoms were fever (7/11;64%) and myalgia (6/11;55%). Mild angioedema attacks were experienced by 36% (4/11) of the HAE patients diagnosed with COVID-19. Icatibant was used in all patients. Conclusion: Agents used for HAE block the kallikrein-kinin system and may be useful in the treatment of COVID-19. Considering their beneficial effects on COVID-19, it is recommended that HAE patients should continue the use of agents blocking the kallikrein-kinin system.

LEVELS OF ENDOTHELIN-1 AND HIGH MOLECULAR WEIGHT KININOGEN IN BLOD OF BREAST CANCER PATIENTS AFTER SARS-COV-2 INFECTION

The aim. To analyze the blood levels of endothelin-1 (ET - 1) and high molecular weight kininogen (HMWK) in patients with breast cancer (BC) previously infected with the new coronavirus. Material and methods. The study group included 20 patients with stage II - IV BC (invasive carcinoma). All patients were receiving chemotherapy at the time of their SA RS-CoV-2 infection. The comparison group included 19 women without breast cancer, who were matched for age. All women of both groups had an RT-PC R confirmed SA RS-Cov-2 infection. Blood levels of ET - 1 and HMWK were measured by ELISA 3-10 weeks after the positive antigen test results. The control group included 10 women of the same age without cancer and without CO VID - 19 symptoms and anti-SA RS-CoV-2 antibodies. Results. The ET - 1 levels in the comparison group were within the reference range, while HMWK levels were significantly higher than those in breast cancer patients. In BC patients with lung metastases, the ET - 1 levels were higher than those in the comparison group patients, while in others (no history of lung metastases, with mild infection course or pneumonia), the ET - 1 levels were similar to those in the comparison and control groups. The HMWK levels in the study and comparison groups were significantly higher than those in controls. Among BC patients, there were women who had significantly higher ET - 1 and HMWK levels compared to the reference levels, and the majority of these patients had lung metastases and previous CO VID - 19 pneumonia. Conclusion. The measurement of HMWK blood levels demonstrated that the plasma contact activation system and the kallikrein-kinin system were active for a long period after the infection both in BC patients and in women without cancer. A high level of ET - 1, the endothelial dysfunction marker, persisted for a long time in some BC patients. Our results were consistent with results of other studies supporting the hypothesis that SA RS-CoV-2 virus infection is a systemic vascular disease with long-term consequences, and its mechanisms require further study.

Interactions amongst inflammation, renin-angiotensin-aldosterone and kallikrein-kinin systems: suggestive approaches for COVID-19 therapy.

Coronavirus disease 2019 (COVID-19) is a rapid-spread infectious disease caused by the SARS-CoV-2 virus, which can culminate in the renin-angiotensin-aldosterone (RAAS) and kallikrein-kinin (KKS) systems imbalance, and in serious consequences for infected patients. This scoping review of published research exploring the RAAS and KKS was undertaken in order to trace the history of the discovery of both systems and their multiple interactions, discuss some aspects of the viral-cell interaction, including inflammation and the system imbalance triggered by SARS-CoV-2 infection, and their consequent disorders. Furthermore, we correlate the effects of continued use of the RAAS blockers in chronic diseases therapies with the virulence and physiopathology of COVID-19. We also approach the RAAS and KKS-related proposed potential therapies for treatment of COVID-19. In this way, we reinforce the importance of exploring both systems and the application of their components or their blockers in the treatment of coronavirus disease.

Early reduction of SARS-CoV-2-replication in bronchial epithelium by kinin B2 receptor antagonism.

SARS-CoV-2 has evolved to enter the host via the ACE2 receptor which is part of the kinin-kallikrein pathway. This complex pathway is only poorly understood in context of immune regulation but critical to control infection. This study examines SARS-CoV-2-infection and epithelial mechanisms of the kinin-kallikrein-system at the kinin B2 receptor level in SARS-CoV-2-infection that is of direct translational relevance. From acute SARS-CoV-2-positive study participants and -negative controls, transcriptomes of nasal curettages were analyzed. Primary airway epithelial cells (NHBEs) were infected with SARS-CoV-2 and treated with the approved B2R-antagonist icatibant. SARS-CoV-2 RNA RT-qPCR, cytotoxicity assays, plaque assays, and transcriptome analyses were performed. The treatment effect was further studied in a murine airway inflammation model in vivo. Here, we report a broad and strong upregulation of kallikreins and the kinin B2 receptor (B2R) in the nasal mucosa of acutely symptomatic SARS-CoV-2-positive study participants. A B2R-antagonist impeded SARS-CoV-2 replication and spread in NHBEs, as determined in plaque assays on Vero-E6 cells. B2R-antagonism reduced the expression of SARS-CoV-2 entry receptor ACE2, G protein-coupled receptor signaling, and ion transport in vitro and in a murine airway inflammation in vivo model. In summary, this study provides evidence that treatment with B2R-antagonists protects airway epithelial cells from SARS-CoV-2 by inhibiting its replication and spread, through the reduction of ACE2 levels and the interference with several cellular signaling processes. Future clinical studies need to shed light on the airway protection potential of approved B2R-antagonists, like icatibant, in the treatment of early-stage COVID-19. KEY MESSAGES: Induction of kinin B2 receptor in the nose of SARS-CoV-2-positive patients. Treatment with B2R-antagonist protects airway epithelial cells from SARS-CoV-2. B2R-antagonist reduces ACE2 levels in vivo and ex vivo. Protection by B2R-antagonist is mediated by inhibiting viral replication and spread.

RAAS INHIBITORS THERAPY – COVID-19 PANDEMIC

Acute respiratory syndrome with various signs and outcomes caused by the SARSCoV-2 virus is the biggest challenge facing health systems worldwide today. The renin-angiotensin-aldosterone and kinin-kallikrein systems and within these two endopeptidases (ACE and ACE2) play a crucial role in the developing clinical feature of COVID-19. Adverse effects of the ACE-stimulated Ang II/AT1R axis (oxidant, pro-inflammatory effect, vasoconstriction) are counterbalanced by the ACE2-induced AT2R and MasR activities (antioxidant, anti-inflammatory effect, vasodilation). The severity of SARS-CoV-2 pneumonia and systemic inflammation explains the impairment of ACE2 (as an important defence factor of the lungs) caused by the biding spike protein of the SARS-CoV-2, which decreases the ACE2 levels. In parallel, bradykinin production also increases and intensifies the SARS-CoV-2-induced cytokine storm through the BKB1 and BKB2 receptors. Since the RAAS inhibitors (ACEI, ARB) affect the two regulatory systems and enzymes at different sites and to different degrees, their role must urgently have been clarified in the COVID-19 since their use is essential and general of many population-wide diseases (hypertension, cardiovascular, renal and metabolic conditions). Based on pathophysiological and experimental data, it is reasonable to hypothesize that in COVID-19 with comorbidities, especially in the elderly, the decreased ACE2 expression may be restored by RAAS inhibitors and the missed or reduced protective effect may be revitalised. This protective effect applies to both RAAS inhibitors. Clinical trials clearly support the declared opinion of many international societies that the use of RAAS inhibitors does not increase the risk of the occurrence of SARS-CoV-2 in itself let alone the severe and critical cases. Accordingly, initiated RAAS inhibitor therapy not only may rather must be continued during the development of COVID-19. © 2021 Literatura Medica Publishing House. All rights reserved.

Reliable measurement of plasma kinin peptides: Importance of preanalytical variables.

BACKGROUND: The kallikrein-kinin system is involved in many (patho)physiological processes and kinin peptides are considered potential clinical biomarkers. Variance in blood specimen collection and processing, artificial ex vivo bradykinin formation, and rapid degradation of kinins have contributed to divergence in published plasma levels, therefore limiting their significance. Thus, reliable preanalytical settings are highly required. OBJECTIVES: This study aimed to develop and evaluate a standardized preanalytical procedure for reliable kinin quantification. The procedure was based on identification of the most impactful variables on ex vivo plasma level alterations. METHODS: Suitable protease inhibitors and blood specimen collection and handling conditions were systematically investigated. Their influence on plasma levels of seven kinins was monitored using an established in-house liquid chromatography-tandem mass spectrometry platform. RESULTS: In nonstandardized settings, ex vivo rise of bradykinin was found to already occur 30 seconds after blood sampling with high interindividual variation. The screening of 17 protease inhibitors resulted in a customized seven-component protease inhibitor, which efficiently stabilized ex vivo kinin levels. The reliability of kinin levels was substantially jeopardized by prolonged rest time until centrifugation, phlebotomy methodology (eg, straight needles, catheters), vacuum sampling technique, or any time delays during venipuncture. The subsequently developed standardized procedure was applied to healthy volunteers and proved it significantly limited interday and interindividual kinin level variability. CONCLUSION: The developed procedure for blood specimen collection and handling is feasible in clinical settings and allows for determination of reliable kinin levels. It may contribute to further elucidating the role of the kallikrein-kinin system in diseases like angioedema, sepsis, or coronavirus disease 2019.

High Molecular Weight Kininogen: A Review of the Structural Literature.

Kininogens are multidomain glycoproteins found in the blood of most vertebrates. High molecular weight kininogen demonstrate both carrier and co-factor activity as part of the intrinsic pathway of coagulation, leading to thrombin generation. Kininogens are the source of the vasoactive nonapeptide bradykinin. To date, attempts to crystallize kininogen have failed, and very little is known about the shape of kininogen at an atomic level. New advancements in the field of cryo-electron microscopy (cryoEM) have enabled researchers to crack the structure of proteins that has been refractory to traditional crystallography techniques. High molecular weight kininogen is a good candidate for structural investigation by cryoEM. The goal of this review is to summarize the findings of kininogen structural studies.

SARS-CoV-2 Exacerbates COVID-19 Pathology Through Activation of the Complement and Kinin Systems.

Infection with SARS-CoV-2 triggers the simultaneous activation of innate inflammatory pathways including the complement system and the kallikrein-kinin system (KKS) generating in the process potent vasoactive peptides that contribute to severe acute respiratory syndrome (SARS) and multi-organ failure. The genome of SARS-CoV-2 encodes four major structural proteins - the spike (S) protein, nucleocapsid (N) protein, membrane (M) protein, and the envelope (E) protein. However, the role of these proteins in either binding to or activation of the complement system and/or the KKS is still incompletely understood. In these studies, we used: solid phase ELISA, hemolytic assay and surface plasmon resonance (SPR) techniques to examine if recombinant proteins corresponding to S1, N, M and E: (a) bind to C1q, gC1qR, FXII and high molecular weight kininogen (HK), and (b) activate complement and/or the KKS. Our data show that the viral proteins: (a) bind C1q and activate the classical pathway of complement, (b) bind FXII and HK, and activate the KKS in normal human plasma to generate bradykinin and (c) bind to gC1qR, the receptor for the globular heads of C1q (gC1q) which in turn could serve as a platform for the activation of both the complement system and KKS. Collectively, our data indicate that the SARS-CoV-2 viral particle can independently activate major innate inflammatory pathways for maximal damage and efficiency. Therefore, if efficient therapeutic modalities for the treatment of COVID-19 are to be designed, a strategy that includes blockade of the four major structural proteins may provide the best option.

Angiotensin-Converting Enzyme 2 in the Pathogenesis of Renal Abnormalities Observed in COVID-19 Patients.

Coronavirus disease 2019 (COVID-19) was first reported in late December 2019 in Wuhan, China. The etiological agent of this disease is severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and the high transmissibility of the virus led to its rapid global spread and a major pandemic (ongoing at the time of writing this review). The clinical manifestations of COVID-19 can vary widely from non-evident or minor symptoms to severe acute respiratory syndrome and multi-organ damage, causing death. Acute kidney injury (AKI) has been recognized as a common complication of COVID-19 and in many cases, kidney replacement therapy (KRT) is required. The presence of kidney abnormalities on hospital admission and the development of AKI are related to a more severe presentation of COVID-19 with higher mortality rate. The high transmissibility and the broad spectrum of clinical manifestations of COVID-19 are in part due to the high affinity of SARS-CoV-2 for its receptor, angiotensin (Ang)-converting enzyme 2 (ACE2), which is widely expressed in human organs and is especially abundant in the kidneys. A debate on the role of ACE2 in the infectivity and pathogenesis of COVID-19 has emerged: Does the high expression of ACE2 promotes higher infectivity and more severe clinical manifestations or does the interaction of SARS-CoV-2 with ACE2 reduce the bioavailability of the enzyme, depleting its biological activity, which is closely related to two important physiological systems, the renin-angiotensin system (RAS) and the kallikrein-kinin system (KKS), thereby further contributing to pathogenesis. In this review, we discuss the dual role of ACE2 in the infectivity and pathogenesis of COVID-19, highlighting the effects of COVID-19-induced ACE2 depletion in the renal physiology and how it may lead to kidney injury. The ACE2 downstream regulation of KKS, that usually receives less attention, is discussed. Also, a detailed discussion on how the triad of symptoms (respiratory, inflammatory, and coagulation symptoms) of COVID-19 can indirectly promote renal injury is primary aborded.

Kallikrein-kinin blockade in patients with COVID-19 to prevent acute respiratory distress syndrome.

COVID-19 patients can present with pulmonary edema early in disease. We propose that this is due to a local vascular problem because of activation of bradykinin 1 receptor (B1R) and B2R on endothelial cells in the lungs. SARS-CoV-2 enters the cell via ACE2 that next to its role in RAAS is needed to inactivate des-Arg9 bradykinin, the potent ligand of the B1R. Without ACE2 acting as a guardian to inactivate the ligands of B1R, the lung environment is prone for local vascular leakage leading to angioedema. Here, we hypothesize that a kinin-dependent local lung angioedema via B1R and eventually B2R is an important feature of COVID-19. We propose that blocking the B2R and inhibiting plasma kallikrein activity might have an ameliorating effect on early disease caused by COVID-19 and might prevent acute respiratory distress syndrome (ARDS). In addition, this pathway might indirectly be responsive to anti-inflammatory agents.

The COVID-19 pandemic represents an unprecedented threat to global health. Millions of cases have been confirmed around the world, and hundreds of thousands of people have lost their lives. Common symptoms include a fever and persistent cough and COVID-19 patients also often experience an excess of fluid in the lungs, which makes it difficult to breathe. In some cases, this develops into a life-threatening condition whereby the lungs cannot provide the body's vital organs with enough oxygen. The SARS-CoV-2 virus, which causes COVID-19, enters the lining of the lungs via an enzyme called the ACE2 receptor, which is present on the outer surface of the lungs' cells. The related coronavirus that was responsible for the SARS outbreak in the early 2000s also needs the ACE2 receptor to enter the cells of the lungs. In SARS, the levels of ACE2 in the lung decline during the infection. Studies with mice have previously revealed that a shortage of ACE2 leads to increased levels of a hormone called angiotensin II, which regulates blood pressure. As a result, much attention has turned to the potential link between this hormone system in relation to COVID-19. However, other mouse studies have shown that ACE2 protects against a build-up of fluid in the lungs caused by a different molecule made by the body. This molecule, which is actually a small fragment of a protein, lowers blood pressure and causes fluid to leak out of blood vessels. It belongs to a family of molecules known as kinins, and ACE2 is known to inactivate certain kinins. This led van de Veerdonk et al. to propose that the excess of fluid in the lungs seen in COVID-19 patients may be because kinins are not being neutralized due to the shortage of the ACE2 receptor. This had not been hypothesized before, even though the mechanism could be the same in SARS which has been researched for the past 17 years. If this hypothesis is correct, it would mean that directly inhibiting the receptor for the kinins (or the proteins that they come from) may be the only way to stop fluid leaking into the lungs of COVID-19 patients in the early stage of disease. This hypothesis is unproven, and more work is needed to see if it is clinically relevant. If that work provides a proof of concept, it means that existing treatments and registered drugs could potentially help patients with COVID-19, by preventing the need for mechanical ventilation and saving many lives.