Endocytosis and Transcytosis of SARS-CoV-2 Across the Intestinal Epithelium and Other Tissue Barriers

Immunologic and inflammatory consequences of SARS-CoV-2 infection and its implications in renal disease

The emergence of the COVID-19 pandemic made it critical to understand the immune and inflammatory responses to the SARS-CoV-2 virus. It became increasingly recognized that the immune response was a key mediator of illness severity and that its mechanisms needed to be better understood. Early infection of both tissue and immune cells, such as macrophages, leading to pyroptosis-mediated inflammasome production in an organ system critical for systemic oxygenation likely plays a central role in the morbidity wrought by SARS-CoV-2. Delayed transcription of Type I and Type III interferons by SARS-CoV-2 may lead to early disinhibition of viral replication. Cytokines such as interleukin-1 (IL-1), IL-6, IL-12, and tumor necrosis factor α (TNFα), some of which may be produced through mechanisms involving nuclear factor kappa B (NF-κB), likely contribute to the hyperinflammatory state in patients with severe COVID-19. Lymphopenia, more apparent among natural killer (NK) cells, CD8+ T-cells, and B-cells, can contribute to disease severity and may reflect direct cytopathic effects of SARS-CoV-2 or end-organ sequestration. Direct infection and immune activation of endothelial cells by SARS-CoV-2 may be a critical mechanism through which end-organ systems are impacted. In this context, endovascular neutrophil extracellular trap (NET) formation and microthrombi development can be seen in the lungs and other critical organs throughout the body, such as the heart, gut, and brain. The kidney may be among the most impacted extrapulmonary organ by SARS-CoV-2 infection owing to a high concentration of ACE2 and exposure to systemic SARS-CoV-2. In the kidney, acute tubular injury, early myofibroblast activation, and collapsing glomerulopathy in select populations likely account for COVID-19-related AKI and CKD development. The development of COVID-19-associated nephropathy (COVAN), in particular, may be mediated through IL-6 and signal transducer and activator of transcription 3 (STAT3) signaling, suggesting a direct connection between the COVID-19-related immune response and the development of chronic disease. Chronic manifestations of COVID-19 also include systemic conditions like Multisystem Inflammatory Syndrome in Children (MIS-C) and Adults (MIS-A) and post-acute sequelae of COVID-19 (PASC), which may reflect a spectrum of clinical presentations of persistent immune dysregulation. The lessons learned and those undergoing continued study likely have broad implications for understanding viral infections' immunologic and inflammatory consequences beyond coronaviruses.

Preferential apical infection of Caco-2 intestinal cell monolayers by SARS-CoV-2 is associated with damage to cellular barrier integrity: Implications for the pathophysiology of COVID-19

SARS-CoV-2 can infect different organs, including the intestine. In an in vitro model of Caco-2 intestinal cell line, we previously found that SARS-CoV-2 modulates the ACE2 receptor expression and affects the expression of molecules involved in intercellular junctions. To further explore the possibility that the intestinal epithelium can serve as an alternative infection route for SARS-CoV-2, we used a model of polarized monolayers of Caco-2 cells (or co-cultures of two intestinal cell lines: Caco-2 and HT29) grown on the polycarbonate membrane of Transwell inserts, inoculated with the virus either in the upper or lower chamber of culture to determine the tropism of the virus for the apical or basolateral pole of these cells. In both polarized Caco-2 cell monolayers and co-culture Caco-2/HT29 cell monolayer, apical SARS-CoV-2 inoculation was found to be much more effective in establishing infection than basolateral inoculation. In addition, apical SARS-CoV-2 infection triggers monolayer degeneration, as shown by histological examination, measurement of trans-epithelial electrical resistance, and cell adhesion molecule expression. During apical infection, the infectious viruses reach the lower chamber, suggesting either a transcytosis mechanism from the apical side to the basolateral side of cells, a paracellular trafficking of the virus after damage to intercellular junctions in the epithelial barrier, or both. Taken together, these data indicate a preferential tropism of SARS-CoV-2 for the apical pole of the human intestinal tract and suggest that infection via the intestinal lumen leads to a systemic infection.

Neuroglial responses to bacterial, viral, and fungal neuroinfections

Evidence regarding the host's response to peripheral pathogens in humans abound, whereas studies on the pathogenesis of central nervous system-penetrating infections are relatively scarce. However, given the spate of epidemic and pandemic neuroinfections in the 21st century, the field has experienced a renewed interest lately. This chapter discusses a timely and exciting topic on the roles of glial cells, mainly microglia and astrocytes, in neuroinvasive infections. This chapter considered fungal, viral, and bacterial neuroinfections, X-raying their neuroinvasiveness, neurotropism, and neurovirulence before focusing on specific examples notable for each category, including Escherichia coli, Cryptococcus neoformans, and SARS-CoV-2. These infections are renowned worldwide for a high case-fatality rate, leaving many survivors with life-long morbidity and others with a bleak future neurologic prognosis. Importantly, the chapter discusses possible ways microglia and astrocytes are culpable in these infections and provides approaches by which they can be manipulated for therapeutic purposes, identifying viable research gaps in the process. Additionally, it offers a synopsis of ongoing works considering microglial selective targeting to attenuate the pathology, morbidity, and mortality associated with these neuroinfections. Considering that microglia and astrocytes are first responders in the central nervous system, targeting these glial cells could be the game changer in managing existing and emerging neuroinvasive infections.

Improving the Accuracy of Permeability Data to Gain Predictive Power: Assessing Sources of Variability in Assays Using Cell Monolayers

The ability to predict the rate of permeation of new compounds across biological membranes is of high importance for their success as drugs, as it determines their efficacy, pharmacokinetics, and safety profile. In vitro permeability assays using Caco-2 monolayers are commonly employed to assess permeability across the intestinal epithelium, with an extensive number of apparent permeability coefficient (Papp) values available in the literature and a significant fraction collected in databases. The compilation of these Papp values for large datasets allows for the application of artificial intelligence tools for establishing quantitative structure-permeability relationships (QSPRs) to predict the permeability of new compounds from their structural properties. One of the main challenges that hinders the development of accurate predictions is the existence of multiple Papp values for the same compound, mostly caused by differences in the experimental protocols employed. This review addresses the magnitude of the variability within and between laboratories to interpret its impact on QSPR modelling, systematically and quantitatively assessing the most common sources of variability. This review emphasizes the importance of compiling consistent Papp data and suggests strategies that may be used to obtain such data, contributing to the establishment of robust QSPRs with enhanced predictive power.

Difference in Endocytosis Pathways Used by Differentiated Versus Nondifferentiated Epithelial Caco-2 Cells to Internalize Nanosized Particles

Understanding the internalization of nanosized particles by mucosal epithelial cells is essential in a number of areas including viral entry at mucosal surfaces, nanoplastic pollution, as well as design and development of nanotechnology-type medicines. Here, we report our comparative study on pathways of cellular internalization in epithelial Caco-2 cells cultured in vitro as either a polarized, differentiated cell layer or as nonpolarized, nondifferentiated cells. The study reveals a number of differences in the extent that endocytic processes are used by cells, depending on their differentiation status and the nature of applied nanoparticles. In polarized cells, actin-driven and dynamin-independent macropinocytosis plays a prominent role in the internalization of both positively and negatively charged nanoparticles, contrary to its modest contribution in nonpolarized cells. Clathrin-mediated cellular entry plays a prominent role in the endocytosis of positive nanoparticles and cholesterol inhibition in negative nanoparticles. However, in nonpolarized cells, dynamin-dependent endocytosis is a major pathway in the internalization of both positive and negative nanoparticles. Cholesterol depletion affects both nonpolarized and polarized cells' internalization of positive and negative nanoparticles, which, in addition to the effect of cholesterol-binding inhibitors on the internalization of negative nanoparticles, indicates the importance of membrane cholesterol in endocytosis. The data collectively provide a new contribution to understanding endocytic pathways in epithelial cells, particularly pointing to the importance of the cell differentiation stage and the nature of the cargo.

Harnessing immunity: Immunomodulatory therapies in COVID-19

An overly exuberant immune response, characterized by a cytokine storm and uncontrolled inflammation, has been identified as a significant driver of severe coronavirus disease 2019 (COVID-19) cases. Consequently, deciphering the intricacies of immune dysregulation in COVID-19 is imperative to identify specific targets for intervention and modulation. With these delicate dynamics in mind, immunomodulatory therapies have emerged as a promising avenue for mitigating the challenges posed by COVID-19. Precision in manipulating immune pathways presents an opportunity to alter the host response, optimizing antiviral defenses while curbing deleterious inflammation. This review article comprehensively analyzes immunomodulatory interventions in managing COVID-19. We explore diverse approaches to mitigating the hyperactive immune response and its impact, from corticosteroids and non-steroidal drugs to targeted biologics, including anti-viral drugs, cytokine inhibitors, JAK inhibitors, convalescent plasma, monoclonal antibodies (mAbs) to severe acute respiratory syndrome coronavirus 2, cell-based therapies (i.e., CAR T, etc.). By summarizing the current evidence, we aim to provide a clear roadmap for clinicians and researchers navigating the complex landscape of immunomodulation in COVID-19 treatment.

Modulation of Paracellular Permeability in SARS-CoV-2 Blood-to-Brain Transcytosis

SARS-CoV-2 primarily infects the lungs via the ACE2 receptor but also other organs including the kidneys, the gastrointestinal tract, the heart, and the skin. SARS-CoV-2 also infects the brain, but the hematogenous route of viral entry to the brain is still not fully characterized. Understanding how SARS-CoV-2 traverses the blood-brain barrier (BBB) as well as how it affects the molecular functions of the BBB are unclear. In this study, we investigated the roles of the receptors ACE2 and DPP4 in the SARS-CoV-2 infection of the discrete cellular components of a transwell BBB model comprising HUVECs, astrocytes, and pericytes. Our results demonstrate that direct infection on the BBB model does not modulate paracellular permeability. Also, our results show that SARS-CoV-2 utilizes clathrin and caveolin-mediated endocytosis to traverse the BBB, resulting in the direct infection of the brain side of the BBB model with a minimal endothelial infection. In conclusion, the BBB is susceptible to SARS-CoV-2 infection in multiple ways, including the direct infection of endothelium, astrocytes, and pericytes involving ACE2 and/or DPP4 and the blood-to-brain transcytosis, which is an event that does not require the presence of host receptors.

Mathematical model explains differences in Omicron and Delta SARS-CoV-2 dynamics in Caco-2 and Calu-3 cells

Within-host infection dynamics of Omicron dramatically differs from previous variants of SARS-CoV-2. However, little is still known about which parameters of virus-cell interplay contribute to the observed attenuated replication and pathogenicity of Omicron. Mathematical models, often expressed as systems of differential equations, are frequently employed to study the infection dynamics of various viruses. Adopting such models for results of in vitro experiments can be beneficial in a number of aspects, such as model simplification (e.g., the absence of adaptive immune response and innate immunity cells), better measurement accuracy, and the possibility to measure additional data types in comparison with in vivo case. In this study, we consider a refinement of our previously developed and validated model based on a system of integro-differential equations. We fit the model to the experimental data of Omicron and Delta infections in Caco-2 (human intestinal epithelium model) and Calu-3 (lung epithelium model) cell lines. The data include known information on initial conditions, infectious virus titers, and intracellular viral RNA measurements at several time points post-infection. The model accurately explains the experimental data for both variants in both cell lines using only three variant- and cell-line-specific parameters. Namely, the cell entry rate is significantly lower for Omicron, and Omicron triggers a stronger cytokine production rate (i.e., innate immune response) in infected cells, ultimately making uninfected cells resistant to the virus. Notably, differences in only a single parameter (e.g., cell entry rate) are insufficient to obtain a reliable model fit for the experimental data.

SARS-CoV-2 mechanisms of cell tropism in various organs considering host factors

A critical step in the drug design for SARS-CoV-2 is to discover its molecular targets. This study comprehensively reviewed the molecular mechanisms of SARS-CoV-2, exploring host cell tropism and interaction targets crucial for cell entry. The findings revealed that beyond ACE2 as the primary entry receptor, alternative receptors, co-receptors, and several proteases such as TMPRSS2, Furin, Cathepsin L, and ADAM play critical roles in virus entry and subsequent pathogenesis. Additionally, SARS-CoV-2 displays tropism in various human organs due to its diverse receptors. This review delves into the intricate details of receptors, host proteases, and the involvement of each organ. Polymorphisms in the ACE2 receptor and mutations in the spike or its RBD region contribute to the emergence of variants like Alpha, Beta, Gamma, Delta, and Omicron, impacting the pathogenicity of SARS-CoV-2. The challenge posed by mutations raises questions about the effectiveness of existing vaccines and drugs, necessitating consideration for updates in their formulations. In the urgency of these critical situations, repurposed drugs such as Camostat Mesylate and Nafamostat Mesylate emerge as viable pharmaceutical options. Numerous drugs are involved in inhibiting receptors and host factors crucial for SARS-CoV-2 entry, with most discussed in this review. In conclusion, this study may provide valuable insights to inform decisions in therapeutic approaches.

Addo S, Harris RA, Lucas R, Csányi G. SARS-CoV-2 Spike Protein Stimulates Macropinocytosis in Murine and Human Macrophages via PKC-NADPH Oxidase Signaling

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). While recent studies have demonstrated that SARS-CoV-2 may enter kidney and colon epithelial cells by inducing receptor-independent macropinocytosis, it remains unknown whether this process also occurs in cell types directly relevant to SARS-CoV-2-associated lung pneumonia, such as alveolar epithelial cells and macrophages. The goal of our study was to investigate the ability of SARS-CoV-2 spike protein subunits to stimulate macropinocytosis in human alveolar epithelial cells and primary human and murine macrophages. Flow cytometry analysis of fluid-phase marker internalization demonstrated that SARS-CoV-2 spike protein subunits S1, the receptor-binding domain (RBD) of S1, and S2 stimulate macropinocytosis in both human and murine macrophages in an angiotensin-converting enzyme 2 (ACE2)-independent manner. Pharmacological and genetic inhibition of macropinocytosis substantially decreased spike-protein-induced fluid-phase marker internalization in macrophages both in vitro and in vivo. High-resolution scanning electron microscopy (SEM) imaging confirmed that spike protein subunits promote the formation of membrane ruffles on the dorsal surface of macrophages. Mechanistic studies demonstrated that SARS-CoV-2 spike protein stimulated macropinocytosis via NADPH oxidase 2 (Nox2)-derived reactive oxygen species (ROS) generation. In addition, inhibition of protein kinase C (PKC) and phosphoinositide 3-kinase (PI3K) in macrophages blocked SARS-CoV-2 spike-protein-induced macropinocytosis. To our knowledge, these results demonstrate for the first time that SARS-CoV-2 spike protein subunits stimulate macropinocytosis in macrophages. These results may contribute to a better understanding of SARS-CoV-2 infection and COVID-19 pathogenesis.

Immunology of SARS-CoV-2 Infection

According to the data from the World Health Organization, about 800 million of the world population had contracted coronavirus infection caused by SARS-CoV-2 by mid-2023. Properties of this virus have allowed it to circulate in the human population for a long time, evolving defense mechanisms against the host immune system. Severity of the disease depends largely on the degree of activation of the systemic immune response, including overstimulation of macrophages and monocytes, cytokine production, and triggering of adaptive T- and B-cell responses, while SARS-CoV-2 evades the immune system actions. In this review, we discuss immune responses triggered in response to the SARS-CoV-2 virus entry into the cell and malfunctions of the immune system that lead to the development of severe disease.

COVID-19 on Oral Health: A New Bilateral Connection for the Pandemic

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and transmission are generally known to be produced by respiratory droplets and aerosols from the oral cavity (O.C.) of infected subjects, as stated by the World Health Organization. Saliva also retains the viral particles and aids in the spread of COVID-19. Angiotensin-converting enzyme Type 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) are two of the numerous factors that promote SARS-CoV-2 infection, expressed by O.C. structures, various mucosa types, and the epithelia of salivary glands. A systemic SARS-CoV-2 infection might result from viral replication in O.C. cells. On the other hand, cellular damage of different subtypes in the O.C. might be associated with various clinical signs and symptoms. Factors interfering with SARS-CoV-2 infection potential might represent fertile ground for possible local pharmacotherapeutic interventions, which may confine SARS-CoV-2 virus entry and transmission in the O.C., finally representing a way to reduce COVID-19 incidence and severity.

Balance between maternal antiviral response and placental transfer of protection in gestational SARS-CoV-2 infection

The intricate interplay between maternal immune response to SARS-CoV-2 and the transfer of protective factors to the fetus remains unclear. By analyzing mother-neonate dyads from second and third trimester SARS-CoV-2 infections, our study shows that neutralizing antibodies (NAbs) are infrequently detected in cord blood. We uncovered that this is due to impaired IgG-NAb placental transfer in symptomatic infection and to the predominance of maternal SARS-CoV-2 NAbs of the IgA and IgM isotypes, which are prevented from crossing the placenta. Crucially, the balance between maternal antiviral response and transplacental transfer of IgG-NAbs appears to hinge on IL-6 and IL-10 produced in response to SARS-CoV-2 infection. In addition, asymptomatic maternal infection was associated with expansion of anti-SARS-CoV-2 IgM and NK cell frequency. Our findings identify a protective role for IgA/IgM-NAbs in gestational SARS-CoV-2 infection and open the possibility that the maternal immune response to SARS-CoV-2 infection might benefit the neonate in 2 ways, first by skewing maternal immune response toward immediate viral clearance, and second by endowing the neonate with protective mechanisms to curtail horizontal viral transmission in the critical postnatal period, via the priming of IgA/IgM-NAbs to be transferred by the breast milk and via NK cell expansion in the neonate.

KDM5 Family Demethylase Inhibitor KDOAM-25 Reduces Entry of SARS-CoV-2 Pseudotyped Viral Particles into Cells

We studied the effect of KDM5 family demethylase inhibitors (JIB-04, PBIT, and KDOAM-25) on the penetration of SARS-CoV-2 pseudotyped viruses into differentiated Caco-2 cells and HEK293T cells with ACE2 hyperexpression. The above drugs were not cytotoxic. Only KDOAM-25 significantly reduced virus entry into the cells. The expression of ACE2 mRNA in Caco-2 significantly increased, while TMPRSS2 expression did not significantly change under these conditions. In differentiated Caco-2 cells, KDOAM-25 did not affect the expression of BRCA1, CDH1, TP53, SNAI1, VIM, and UGCG genes, for which an association with knockdown or overexpression of KDM5 demethylases or with the action of demethylase inhibitors had previously been shown. In undifferentiated Caco-2 cells, the expression of BRCA1, SNAI1, VIM, and CDH1 was significantly increased under the action of KDOAM-25.

Long COVID: Is there a kidney link?

Metabolic causes such as altered bioenergetics and amino acid metabolism may play a major role in Long COVID. Renal-metabolic regulation is an integral part of these pathways but has not been systematically or routinely investigated in Long COVID. Here we discuss the biochemistry of renal tubular injury as it may contribute to Long COVID symptoms. We propose three potential mechanisms that could be involved in Long COVID namely creatine phosphate metabolism, un-reclaimed glomerular filtrate and COVID specific proximal tubule cells (PTC) injury-a tryptophan paradigm. This approach is intended to allow for improved diagnostics and therapy for the long-haul sufferers.

Immune evasion of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2); molecular approaches

In December 2019, a new betacoronavirus, known as severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), caused an outbreak at the Wuhan seafood market in China. The disease was further named coronavirus disease 2019 (COVID-19). In March 2020, the World Health Organization (WHO) announced the disease to be a pandemic, as more cases were reported globally. SARS-CoV-2, like many other viruses, employs diverse strategies to elude the host immune response and/or counter immune responses. The infection outcome mainly depends on interactions between the virus and the host immune system. Inhibiting IFN production, blocking IFN signaling, enhancing IFN resistance, and hijacking the host's translation machinery to expedite the production of viral proteins are among the main immune evasion mechanisms of SARS-CoV-2. SARS-CoV-2 also downregulates the expression of MHC-I on infected cells, which is an additional immune-evasion mechanism of this virus. Moreover, antigenic modifications to the spike (S) protein, such as deletions, insertions, and also substitutions are essential for resistance to SARS-CoV-2 neutralizing antibodies. This review assesses the interaction between SARS-CoV-2 and host immune response and cellular and molecular approaches used by SARS-CoV-2 for immune evasion. Understanding the mechanisms of SARS-CoV-2 immune evasion is essential since it can improve the development of novel antiviral treatment options as well as vaccination methods.

Intestinal Damage, Inflammation and Microbiota Alteration during COVID-19 Infection

BACKGROUND

The virus SARS-CoV-2 is responsible for respiratory disorders due to the fact that it mainly infects the respiratory tract using the Angiotensin-converting enzyme 2 (ACE2) receptors. ACE2 receptors are also highly expressed on intestinal cells, representing an important site of entry for the virus in the gut. Literature studies underlined that the virus infects and replicates in the gut epithelial cells, causing gastrointestinal symptoms such as diarrhea, abdominal pain, nausea/vomiting and anorexia. Moreover, the SARS-CoV-2 virus settles into the bloodstream, hyperactivating the platelets and cytokine storms and causing gut-blood barrier damage with an alteration of the gut microbiota, intestinal cell injury, intestinal vessel thrombosis leading to malabsorption, malnutrition, an increasing disease severity and mortality with short and long-period sequelae.

CONCLUSION

This review summarizes the data on how SARS-CoV-2 effects on the gastrointestinal systems, including the mechanisms of inflammation, relationship with the gut microbiota, endoscopic patterns, and the role of fecal calprotectin, confirming the importance of the digestive system in clinical practice for the diagnosis and follow-up of SARS-CoV-2 infection.

Development of a novel mathematical model that explains SARS-CoV-2 infection dynamics in Caco-2 cells

Mathematical modeling is widely used to study within-host viral dynamics. However, to the best of our knowledge, for the case of SARS-CoV-2 such analyses were mainly conducted with the use of viral load data and for the wild type (WT) variant of the virus. In addition, only few studies analyzed models for in vitro data, which are less noisy and more reproducible. In this work we collected multiple data types for SARS-CoV-2-infected Caco-2 cell lines, including infectious virus titers, measurements of intracellular viral RNA, cell viability data and percentage of infected cells for the WT and Delta variants. We showed that standard models cannot explain some key observations given the absence of cytopathic effect in human cell lines. We propose a novel mathematical model for in vitro SARS-CoV-2 dynamics, which included explicit modeling of intracellular events such as exhaustion of cellular resources required for virus production. The model also explicitly considers innate immune response. The proposed model accurately explained experimental data. Attenuated replication of the Delta variant in Caco-2 cells could be explained by our model on the basis of just two parameters: decreased cell entry rate and increased cytokine production rate.

Mechanisms and strategies to enhance penetration during intravesical drug therapy for bladder cancer

Bladder cancer (BCa) is one of the most prevalent cancers worldwide. The effectiveness of intravesical therapy for bladder cancer, however, is limited due to the short dwell time and the presence of permeation barriers. Considering the histopathological features of BCa, the permeation barriers for drugs to transport across consist of a mucus layer and a nether tumor physiological barrier. Mucoadhesive delivery systems or mucus-penetrating delivery systems are developed to enhance their retention in or penetration across the mucus layer, but delivery systems that are capable of mucoadhesion-to-mucopenetration transition are more efficient to deliver drugs across the mucus layer. For the tumor physiological barrier, delivery systems mainly rely on four types of penetration mechanisms to cross it. This review summarizes the classical and latest approaches to intravesical drug delivery systems to penetrate BCa.

S Protein, ACE2 and Host Cell Proteases in SARS-CoV-2 Cell Entry and Infectivity; Is Soluble ACE2 a Two Blade Sword? A Narrative Review

Since the spread of the deadly virus SARS-CoV-2 in late 2019, researchers have restlessly sought to unravel how the virus enters the host cells. Some proteins on each side of the interaction between the virus and the host cells are involved as the major contributors to this process: (1) the nano-machine spike protein on behalf of the virus, (2) angiotensin converting enzyme II, the mono-carboxypeptidase and the key component of renin angiotensin system on behalf of the host cell, (3) some host proteases and proteins exploited by SARS-CoV-2. In this review, the complex process of SARS-CoV-2 entrance into the host cells with the contribution of the involved host proteins as well as the sequential conformational changes in the spike protein tending to increase the probability of complexification of the latter with angiotensin converting enzyme II, the receptor of the virus on the host cells, are discussed. Moreover, the release of the catalytic ectodomain of angiotensin converting enzyme II as its soluble form in the extracellular space and its positive or negative impact on the infectivity of the virus are considered.

Insight into the role of clathrin-mediated endocytosis inhibitors in SARS-CoV-2 infection

Emergence of SARS-CoV-2 variants warrants sustainable efforts to upgrade both the diagnostic and therapeutic protocols. Understanding the details of cellular and molecular basis of the virus-host cell interaction is essential for developing variant-independent therapeutic options. The internalization of SARS-CoV-2, into lung epithelial cells, is mediated by endocytosis, especially clathrin-mediated endocytosis (CME). Although vaccination is the gold standard strategy against viral infection, selective inhibition of endocytic proteins, complexes, and associated adaptor proteins may present a variant-independent therapeutic strategy. Although clathrin and/or dynamins are the most important proteins involved in CME, other endocytic mechanisms are clathrin and/or dynamin independent and rely on other proteins. Moreover, endocytosis implicates some subcellular structures, like plasma membrane, actin and lysosomes. Also, physiological conditions, such as pH and ion concentrations, represent an additional factor that mediates these events. Accordingly, endocytosis related proteins are potential targets for small molecules that inhibit endocytosis-mediated viral entry. This review summarizes the potential of using small molecules, targeting key proteins, participating in clathrin-dependent and -independent endocytosis, as variant-independent antiviral drugs against SARS-CoV-2 infection. The review takes two approaches. The first outlines the potential role of endocytic inhibitors in preventing endocytosis-mediated viral entry and its mechanism of action, whereas in the second computational analysis was implemented to investigate the selectivity of common inhibitors against endocytic proteins in SARS-CoV-2 endocytosis. The analysis revealed that remdesivir, methyl-β-cyclodextrin, rottlerin, and Bis-T can effectively inhibit clathrin, HMG-CoA reductase, actin, and dynamin I GTPase and are more potent in inhibiting SARS-CoV-2 than chloroquine. CME inhibitors for SARS-CoV-2 infection remain understudied.

Autoimmunity and Immunodeficiency in Severe SARS-CoV-2 Infection and Prolonged COVID-19

SARS-CoV-2 causes the complex and heterogeneous illness known as COVID-19. The disease primarily affects the respiratory system but can quickly become systemic, harming multiple organs and leading to long-lasting sequelae in some patients. Most infected individuals are asymptomatic or present mild symptoms. Antibodies, complement, and immune cells can efficiently eliminate the virus. However, 20% of individuals develop severe respiratory illness and multiple organ failure. Virus replication has been described in several organs in patients who died from COVID-19, suggesting a compromised immune response. Immunodeficiency and autoimmunity are responsible for this impairment and facilitate viral escape. Mutations in IFN signal transduction and T cell activation are responsible for the inadequate response in young individuals. Autoantibodies are accountable for secondary immunodeficiency in patients with severe infection or prolonged COVID-19. Antibodies against cytokines (interferons α, γ and ω, IL1β, IL6, IL10, IL-17, IL21), chemokines, complement, nuclear proteins and DNA, anticardiolipin, and several extracellular proteins have been reported. The type and titer of autoantibodies depend on age and gender. Organ-specific autoantibodies have been described in prolonged COVID-19. Their role in the disease is under study. Autoimmunity and immunodeficiency should be screened as risk factors for severe or prolonged COVID-19.

Blood-brain barrier (BBB)-on-a-chip: a promising breakthrough in brain disease research

The blood-brain barrier (BBB) represents a key challenge in developing brain-penetrating therapeutic molecules. BBB dysfunction is also associated with the onset and progression of various brain diseases. The BBB-on-a-chip (μBBB), an organ-on-chip technology, has emerged as a powerful in vitro platform that closely mimics the human BBB microenvironments. While the μBBB technology has seen wide application in the study of brain cancer, its utility in other brain disease models ("μBBB+") is less appreciated. Based on the advances of the μBBB technology and the evolution of in vitro models for brain diseases over the last decade, we propose the concept of a "μBBB+" system and summarize its major promising applications in pathological studies, personalized medical research, drug development, and multi-organ-on-chip approaches. We believe that such a sophisticated "μBBB+" system is a highly tunable and promising in vitro platform for further advancement of the understanding of brain diseases.

Sars-Cov-2 Spike Protein-Induced Damage of hiPSC-Derived Cardiomyocytes

Sars-Cov-2 may trigger molecular and functional alterations of cardiomyocytes (CMs) of the heart due to the presence of receptor angiotensin-converting enzyme 2 (ACE2) of the host cells. While the endocytic itinerary of the virus via cleavage of the spike protein of Sars-Cov-2 is well understood, the role of the remaining part of the spike protein subunit and ACE2 complex is still elusive. Herein, the possible effects of this complex are investigated by using synthetic spike proteins of Sars-Cov-2, human-induced pluripotent stem cells (hiPSC), and a culture device made of an arrayed monolayer of cross-linked nanofibers. hiPSCs are first differentiated into CMs that form cardiac tissue-like constructs with regular beating and expression of both ACE2 and gap junction protein Connexin 43. When incubated with the spike proteins, the hiPSC-CMs undergo a rhythmic fluctuation with overstretched sarcomere structures and dispersed gap junction proteins. When incubated with the spike proteins and supplementary angiotensin II, the damage of the spike protein on hiPSC-CMs is enhanced due to downregulated ACE2, chromatin margination, altered Connexin 43 expression, sarcomere disruption, and beating break. This discovery may imply latent effects of the spike proteins on the heart.

Effects of spike protein and toxin-like peptides found in COVID-19 patients on human 3D neuronal/glial model undergoing differentiation: possible implications for SARS-CoV-2 impact on brain development

The possible neurodevelopmental consequences of SARS-CoV-2 infection are presently unknown. In utero exposure to SARS-CoV-2 has been hypothesized to affect the developing brain, possibly disrupting neurodevelopment of children. Spike protein interactors, such as ACE2, have been found expressed in the fetal brain, and could play a role in potential SARS-CoV-2 fetal brain pathogenesis. Apart from the possible direct involvement of SARS-CoV-2 or its specific viral components in the occurrence of neurological and neurodevelopmental manifestations, we recently reported the presence of toxin-like peptides in plasma, urine and fecal samples specifically from COVID-19 patients. In this study, we investigated the possible neurotoxic effects elicited upon 72-hour exposure to human relevant levels of recombinant spike protein, toxin-like peptides found in COVID-19 patients, as well as a combination of both in 3D human iPSC-derived neural stem cells differentiated for either 2 weeks (short-term) or 8 weeks (long-term, 2 weeks in suspension + 6 weeks on MEA) towards neurons/glia. Whole transcriptome and qPCR analysis revealed that spike protein and toxin-like peptides at non-cytotoxic concentrations differentially perturb the expression of SPHK1, ELN, GASK1B, HEY1, UTS2, ACE2 and some neuronal-, glia- and NSC-related genes critical during brain development. Additionally, exposure to spike protein caused a decrease of spontaneous electrical activity after two days in long-term differentiated cultures. The perturbations of these neurodevelopmental endpoints are discussed in the context of recent knowledge about the key events described in Adverse Outcome Pathways relevant to COVID-19, gathered in the context of the CIAO project (https://www.ciao-covid.net/).

Inflammatory Bowel Disease and COVID-19: How Microbiomics and Metabolomics Depict Two Sides of the Same Coin

The integrity of the gastrointestinal tract structure and function is seriously compromised by two pathological conditions sharing, at least in part, several pathogenetic mechanisms: inflammatory bowel diseases (IBD) and coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. IBD and COVID-19 are marked by gut inflammation, intestinal barrier breakdown, resulting in mucosal hyperpermeability, gut bacterial overgrowth, and dysbiosis together with perturbations in microbial and human metabolic pathways originating changes in the blood and fecal metabolome. This review compared the most relevant metabolic and microbial alterations reported from the literature in patients with IBD with those in patients with COVID-19. In both diseases, gut dysbiosis is marked by the prevalence of pro-inflammatory bacterial species and the shortfall of anti-inflammatory species; most studies reported the decrease in Firmicutes, with a specific decrease in obligately anaerobic producers short-chain fatty acids (SCFAs), such as Faecalibacterium prausnitzii. In addition, Escherichia coli overgrowth has been observed in IBD and COVID-19, while Akkermansia muciniphila is depleted in IBD and overexpressed in COVID-19. In patients with COVID-19, gut dysbiosis continues after the clearance of the viral RNA from the upper respiratory tract and the resolution of clinical symptoms. Finally, we presented and discussed the impact of gut dysbiosis, inflammation, oxidative stress, and increased energy demand on metabolic pathways involving key metabolites, such as tryptophan, phenylalanine, histidine, glutamine, succinate, citrate, and lipids.

The molecular mechanism of SARS-CoV-2 evading host antiviral innate immunity

The newly identified Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has resulted in a global health emergency (COVID-19) because of its rapid spread and high mortality. Since the virus epidemic, many pathogenic mechanisms have been revealed, and virus-related vaccines have been successfully developed and applied in clinical practice. However, the pandemic is still developing, and new mutations are still emerging. Virus pathogenicity is closely related to the immune status of the host. As innate immunity is the body's first defense against viruses, understanding the inhibitory effect of SARS-CoV-2 on innate immunity is of great significance for determining the target of antiviral intervention. This review summarizes the molecular mechanism by which SARS-CoV-2 escapes the host immune system, including suppressing innate immune production and blocking adaptive immune priming. Here, on the one hand, we devoted ourselves to summarizing the combined action of innate immune cells, cytokines, and chemokines to fine-tune the outcome of SARS-CoV-2 infection and the related immunopathogenesis. On the other hand, we focused on the effects of the SARS-CoV-2 on innate immunity, including enhancing viral adhesion, increasing the rate of virus invasion, inhibiting the transcription and translation of immune-related mRNA, increasing cellular mRNA degradation, and inhibiting protein transmembrane transport. This review on the underlying mechanism should provide theoretical support for developing future molecular targeted drugs against SARS-CoV-2. Nevertheless, SARS-CoV-2 is a completely new virus, and people's understanding of it is in the process of rapid growth, and various new studies are also being carried out. Although we strive to make our review as inclusive as possible, there may still be incompleteness.

Penetration of the SARS-CoV-2 Spike Protein across the Blood–Brain Barrier, as Revealed by a Combination of a Human Cell Culture Model System and Optical Biosensing

Since the outbreak of the global pandemic caused by severe acute respiratory coronavirus 2 (SARS-CoV-2), several clinical aspects of the disease have come into attention. Besides its primary route of infection through the respiratory system, SARS-CoV-2 is known to have neuroinvasive capacity, causing multiple neurological symptoms with increased neuroinflammation and blood–brain barrier (BBB) damage. The viral spike protein disseminates via circulation during infection, and when reaching the brain could possibly cross the BBB, which was demonstrated in mice. Therefore, its medical relevance is of high importance. The aim of this study was to evaluate the barrier penetration of the S1 subunit of spike protein in model systems of human organs highly exposed to the infection. For this purpose, in vitro human BBB and intestinal barrier cell–culture systems were investigated by an optical biosensing method. We found that spike protein crossed the human brain endothelial cell barrier effectively. Additionally, spike protein passage was found in a lower amount for the intestinal barrier cell layer. These observations were corroborated with parallel specific ELISAs. The findings on the BBB model could provide a further basis for studies focusing on the mechanism and consequences of spike protein penetration across the BBB to the brain.

Induction of Hypoxic Response in Caco-2 Cells Promote the Expression of Genes Involved in SARS-CoV-2 Endocytosis and Transcytosis

In the present manuscript we analyzed the influence of hypoxic response in Caco-2 cells on the expression of genes and miRNAs involved in the mechanisms of intracellular transport of SARS-CoV-2 viral particles, especially endocytosis and transcytosis. With the use of RNA sequencing of Caco-2 cells treated with hypoxia-inducing oxyquinoline derivative, we showed two-fold increase in the expression of the main SARS-CoV-2 receptor ACE2. Expression of the non-canonical receptor TFRC was also elevated. We also observed a significant increase in the expression levels of genes from the low-density lipoprotein (LDL) receptor family, which play a crucial role in the transcytosis: LDLR, LRP1, LRP4, and LRP5. Upregulation of LDLR was coupled with the downregulation of hsa-miR-148a-3p, which can directly bind to LDLR mRNA. Thus, the hypoxic response in Caco-2 cells includes upregulation of genes involved in the mechanisms of endocytosis and transcytosis of SARS-CoV-2 viral particles.