ACE-2, TMPRSS2, and Neuropilin-1 Receptor Expression on Human Brain Astrocytes and Pericytes and SARS-CoV-2 Infection Kinetics

Tackling persistent neurological symptoms in patients following acute COVID-19 infection: an update of the literature

INTRODUCTION

The COVID-19 pandemic has taught myriad lessons and left several questions we are yet to comprehend. Initially, the scientific community was concerned with the management of acute disease and immunization. Once the peak of the pandemic receded, it became clear that a proportion of patients were far from fully recovered. Researchers started to recognize those persisting symptoms as a new entity termed 'Long COVID,' where neurological symptoms are evident and have a major impact on quality of life.

AREAS COVERED

The main purpose of this narrative review is to analyze and synthesize the current literature regarding Long COVID, its relation to the nervous system, and to explore the evidence on treatments for persistent neurological symptoms. The most common reported and observed neurologic manifestations include fatigue, cognitive impairment, pain, polyneuropathy, and neuropsychiatric disorders. A variety of pharmacologic and non-pharmacologic therapies have been evaluated and yielded mixed results. Many of them focused on immunomodulation and none currently have U.S. FDA approval.

EXPERT OPINION

Challenges remain in terms of clinical characterization and prognosis of Long COVID, besides understanding its pathophysiology. Standardization of biomarkers and diagnostic criteria will allow the use of common nomenclature and data elements in the design of future clinical studies.

Neurological Impact of Respiratory Viruses: Insights into Glial Cell Responses in the Central Nervous System

Respiratory viral infections pose a significant public health threat, particularly in children and older adults, with high mortality rates. Some of these pathogens are the human respiratory syncytial virus (hRSV), severe acute respiratory coronavirus-2 (SARS-CoV-2), influenza viruses (IV), human parvovirus B19 (B19V), and human bocavirus 1 (HBoV1). These viruses cause various respiratory symptoms, including cough, fever, bronchiolitis, and pneumonia. Notably, these viruses can also impact the central nervous system (CNS), leading to acute manifestations such as seizures, encephalopathies, encephalitis, neurological sequelae, and long-term complications. The precise mechanisms by which these viruses affect the CNS are not fully understood. Glial cells, specifically microglia and astrocytes within the CNS, play pivotal roles in maintaining brain homeostasis and regulating immune responses. Exploring how these cells interact with viral pathogens, such as hRSV, SARS-CoV-2, IVs, B19V, and HBoV1, offers crucial insights into the significant impact of respiratory viruses on the CNS. This review article examines hRSV, SARS-CoV-2, IV, B19V, and HBoV1 interactions with microglia and astrocytes, shedding light on potential neurological consequences.

Exploring the Complexities of Long COVID

Since the emergence of the SARS-CoV-2 virus in 2019, nearly 700 million COVID-19 cases and 7 million deaths have been reported globally. Despite most individuals recovering within four weeks, the Center for Disease Control (CDC) estimates that 7.5% to 41% develop post-acute infection syndrome (PAIS), known as 'Long COVID'. This review provides current statistics on Long COVID's prevalence, explores hypotheses concerning epidemiological factors, such as age, gender, comorbidities, initial COVID-19 severity, and vaccine interactions, and delves into potential mechanisms, including immune responses, viral persistence, and gut dysbiosis. Moreover, we conclude that women, advanced age, comorbidities, non-vaccination, and low socioeconomic status all appear to be risk factors. The reasons for these differences are still not fully understood and likely involve a complex relationship between social, genetic, hormonal, and other factors. Furthermore, individuals with Long COVID-19 seem more likely to endure economic hardship due to persistent symptoms. In summary, our findings further illustrate the multifaceted nature of Long COVID and underscore the importance of understanding the epidemiological factors and potential mechanisms needed to develop effective therapeutic strategies and interventions.

New insights into the pathogenesis of SARS-CoV-2 during and after the COVID-19 pandemic

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the respiratory distress condition known as COVID-19. This disease broadly affects several physiological systems, including the gastrointestinal, renal, and central nervous (CNS) systems, significantly influencing the patient's overall quality of life. Additionally, numerous risk factors have been suggested, including gender, body weight, age, metabolic status, renal health, preexisting cardiomyopathies, and inflammatory conditions. Despite advances in understanding the genome and pathophysiological ramifications of COVID-19, its precise origins remain elusive. SARS-CoV-2 interacts with a receptor-binding domain within angiotensin-converting enzyme 2 (ACE2). This receptor is expressed in various organs of different species, including humans, with different abundance. Although COVID-19 has multiorgan manifestations, the main pathologies occur in the lung, including pulmonary fibrosis, respiratory failure, pulmonary embolism, and secondary bacterial pneumonia. In the post-COVID-19 period, different sequelae may occur, which may have various causes, including the direct action of the virus, alteration of the immune response, and metabolic alterations during infection, among others. Recognizing the serious adverse health effects associated with COVID-19, it becomes imperative to comprehensively elucidate and discuss the existing evidence surrounding this viral infection, including those related to the pathophysiological effects of the disease and the subsequent consequences. This review aims to contribute to a comprehensive understanding of the impact of COVID-19 and its long-term effects on human health.

Mapping ACE2 and TMPRSS2 co-expression in human brain tissue: implications for SARS-CoV-2 neurological manifestations

The Coronavirus Disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) primarily targets respiratory cells, but emerging evidence shows neurological involvement, with the virus directly affecting neurons and glia. SARS-CoV-2 entry into a target cell requires co-expression of ACE2 (Angiotensin-converting enzyme-2) and TMPRSS2 (Trans membrane serine protease-2). Relevant literature on human neurological tissue is sparse and mostly focused on the olfactory areas. This prompted our study to map brain-wide expression of these entry proteins and assess age-related changes. The normal brain tissue samples were collected from cerebral cortex, hippocampus, basal ganglia, thalamus, hypothalamus, brain stem and cerebellum; and were divided into two groups - up to 40 years (n = 10) and above 40 years (n = 10). ACE2 and TMPRSS2 gene expression analysis was done using qRT-PCR and protein co-expression was seen by immunofluorescence. The ACE2 and TMPRSS2 gene expression was observed to be highest in hypothalamus and thalamus regions, respectively. Immunoreactivity for both ACE-2 and TMPRSS2 was observed in all examined brain regions, confirming the presence of these viral entry receptors. Co-localisation was maximum in hypothalamus. Our study did not find any trend related to different age groups. The expression of both these viral entry receptors suggests that normal human brain is susceptibility to SARS-CoV-2, perhaps which could be related to the cognitive and neurological impairment that occur in patients.

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.

In-vitro and in-vivo assessment of nirmatrelvir penetration into CSF, central nervous system cells, tissues, and peripheral blood mononuclear cells

Three years after SARS-CoV-2 emerged as a global infectious threat, the virus has become endemic. The neurological complications such as depression, anxiety, and other CNS complications after COVID-19 disease are increasing. The brain, and CSF have been shown as viral reservoirs for SARS-CoV-2, yielding a potential hypothesis for CNS effects. Thus, we investigated the CNS pharmacology of orally dosed nirmatrelvir/ritonavir (NMR/RTV). Using both an in vitro and an in vivo rodent model, we investigated CNS penetration and potential pharmacodynamic activity of NMR. Through pharmacokinetic modeling, we estimated the median CSF penetration of NMR to be low at 18.11% of plasma with very low accumulation in rodent brain tissue. Based on the multiples of the 90% maximal effective concentration (EC90) for SARS-CoV-2, NMR concentrations in the CSF and brain do not achieve an exposure level similar to that of plasma. A median of only 16% of all the predicted CSF concentrations in rats were > 3xEC90 (unadjusted for protein binding). This may have implications for viral persistence and neurologic post-acute sequelae of COVID-19 if increased NMR penetration in the CNS leads to decreased CNS viral loads and decreased CNS inflammation.

Mechanisms by Which SARS-CoV-2 Invades and Damages the Central Nervous System: Apart from the Immune Response and Inflammatory Storm, What Else Do We Know?

Initially reported as pneumonia of unknown origin, COVID-19 is increasingly being recognized for its impact on the nervous system, despite nervous system invasions being extremely rare. As a result, numerous studies have been conducted to elucidate the mechanisms of nervous system damage and propose appropriate coping strategies. This review summarizes the mechanisms by which SARS-CoV-2 invades and damages the central nervous system, with a specific focus on aspects apart from the immune response and inflammatory storm. The latest research findings on these mechanisms are presented, providing new insights for further in-depth research.

Understanding the Neurotrophic Virus Mechanisms and Their Potential Effect on Systemic Lupus Erythematosus Development

Central nervous system (CNS) pathologies are a public health concern, with viral infections one of their principal causes. These viruses are known as neurotropic pathogens, characterized by their ability to infiltrate the CNS and thus interact with various cell populations, inducing several diseases. The immune response elicited by neurotropic viruses in the CNS is commanded mainly by microglia, which, together with other local cells, can secrete inflammatory cytokines to fight the infection. The most relevant neurotropic viruses are adenovirus (AdV), cytomegalovirus (CMV), enterovirus (EV), Epstein-Barr Virus (EBV), herpes simplex virus type 1 (HSV-1), and herpes simplex virus type 2 (HSV-2), lymphocytic choriomeningitis virus (LCMV), and the newly discovered SARS-CoV-2. Several studies have associated a viral infection with systemic lupus erythematosus (SLE) and neuropsychiatric lupus (NPSLE) manifestations. This article will review the knowledge about viral infections, CNS pathologies, and the immune response against them. Also, it allows us to understand the relevance of the different viral proteins in developing neuronal pathologies, SLE and NPSLE.

Exploration of the Noncoding Genome for Human-Specific Therapeutic Targets-Recent Insights at Molecular and Cellular Level

While it is well known that 98-99% of the human genome does not encode proteins, but are nevertheless transcriptionally active and give rise to a broad spectrum of noncoding RNAs [ncRNAs] with complex regulatory and structural functions, specific functions have so far been assigned to only a tiny fraction of all known transcripts. On the other hand, the striking observation of an overwhelmingly growing fraction of ncRNAs, in contrast to an only modest increase in the number of protein-coding genes, during evolution from simple organisms to humans, strongly suggests critical but so far essentially unexplored roles of the noncoding genome for human health and disease pathogenesis. Research into the vast realm of the noncoding genome during the past decades thus lead to a profoundly enhanced appreciation of the multi-level complexity of the human genome. Here, we address a few of the many huge remaining knowledge gaps and consider some newly emerging questions and concepts of research. We attempt to provide an up-to-date assessment of recent insights obtained by molecular and cell biological methods, and by the application of systems biology approaches. Specifically, we discuss current data regarding two topics of high current interest: (1) By which mechanisms could evolutionary recent ncRNAs with critical regulatory functions in a broad spectrum of cell types (neural, immune, cardiovascular) constitute novel therapeutic targets in human diseases? (2) Since noncoding genome evolution is causally linked to brain evolution, and given the profound interactions between brain and immune system, could human-specific brain-expressed ncRNAs play a direct or indirect (immune-mediated) role in human diseases? Synergistic with remarkable recent progress regarding delivery, efficacy, and safety of nucleic acid-based therapies, the ongoing large-scale exploration of the noncoding genome for human-specific therapeutic targets is encouraging to proceed with the development and clinical evaluation of novel therapeutic pathways suggested by these research fields.

Neuroproteomic Analysis after SARS-CoV-2 Infection Reveals Overrepresented Neurodegeneration Pathways and Disrupted Metabolic Pathways

Besides respiratory illness, SARS-CoV-2, the causative agent of COVID-19, leads to neurological symptoms. The molecular mechanisms leading to neuropathology after SARS-CoV-2 infection are sparsely explored. SARS-CoV-2 enters human cells via different receptors, including ACE-2, TMPRSS2, and TMEM106B. In this study, we used a human-induced pluripotent stem cell-derived neuronal model, which expresses ACE-2, TMPRSS2, TMEM106B, and other possible SARS-CoV-2 receptors, to evaluate its susceptibility to SARS-CoV-2 infection. The neurons were exposed to SARS-CoV-2, followed by RT-qPCR, immunocytochemistry, and proteomic analyses of the infected neurons. Our findings showed that SARS-CoV-2 infects neurons at a lower rate than other human cells; however, the virus could not replicate or produce infectious virions in this neuronal model. Despite the aborted SARS-CoV-2 replication, the infected neuronal nuclei showed irregular morphology compared to other human cells. Since cytokine storm is a significant effect of SARS-CoV-2 infection in COVID-19 patients, in addition to the direct neuronal infection, the neurons were treated with pre-conditioned media from SARS-CoV-2-infected lung cells, and the neuroproteomic changes were investigated. The limited SARS-CoV-2 infection in the neurons and the neurons treated with the pre-conditioned media showed changes in the neuroproteomic profile, particularly affecting mitochondrial proteins and apoptotic and metabolic pathways, which may lead to the development of neurological complications. The findings from our study uncover a possible mechanism behind SARS-CoV-2-mediated neuropathology that might contribute to the lingering effects of the virus on the human brain.

SARS-CoV-2 RNA Persists in the Central Nervous System of Non-Human Primates Despite Clinical Recovery

Adverse neurological and psychiatric outcomes, collectively termed the post-acute sequelae of SARS-CoV-2 infection (PASC), persist in adults clinically recovered from COVID-19. Effective therapeutic interventions are fundamental to reducing the burden of PASC, necessitating an investigation of the pathophysiology underlying the debilitating neurological symptoms associated with the condition. Herein, eight non-human primates (Wild-Caught African Green Monkeys, n =4; Indian Rhesus Macaques, n =4) were inoculated with the SARS-CoV-2 isolate USA-WA1/2020 by either small particle aerosol or via multiple routes. At necropsy, tissue from the olfactory epithelium and pyriform cortex/amygdala of SARS-CoV-2 infected non-human primates were collected for ribonucleic acid in situ hybridization (i.e., RNAscope). First, angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPRSS2) mRNA are downregulated in the pyriform cortex/amygdala of non-human primates clinically recovered from SARS-CoV-2 inoculation relative to wildtype controls. Second, abundant SARS-CoV-2 mRNA was detected in clinically recovered non-human primates; mRNA which is predominantly harbored in pericytes. Collectively, examination of post-mortem pyriform cortex/amygdala brain tissue of non-human primates clinically recovered from SARS-CoV-2 infection revealed two early pathophysiological mechanisms potentially underlying PASC. Indeed, therapeutic interventions targeting the downregulation of ACE2, decreased expression of TMPRSS2, and/or persistent infection of pericytes in the central nervous system may effectively mitigate the debilitating symptoms of PASC.

Understanding the Immune-Endocrine Effects of Vitamin D in SARS-CoV-2 Infection: A Role in Protecting against Neurodamage

Calcitriol and hydroxyderivatives of lumisterol and tachisterol are secosteroid hormones with immunomodulatory and anti-inflammatory properties. Since the beginning of the COVID-19 pandemic, several studies have correlated deficient serum concentrations of vitamin D3 (calcifediol) with increased severity of the course of SARS-CoV-2 infection. Among systemic complications, subjective (anosmia, ageusia, depression, dizziness) and objective (ischemic stroke, meningoencephalitis, myelitis, seizures, Guillain-Barré syndrome) neurological symptoms have been reported in up to 80% of severe COVID-19 patients. In this narrative review, we will resume the pathophysiology of SARS-CoV-2 infection and the mechanisms of acute and chronic neurological damage. SARS-CoV-2 can disrupt the integrity of the endothelial cells of the blood-brain barrier (BBB) to enter the nervous central system. Invasion of pro-inflammatory cytokines and polarization of astrocytes and microglia cells always in a pro-inflammatory sense together with the pro-coagulative phenotype of cerebral endothelial cells in response to both SARS-CoV-2 and immune cells invasion (immunothrombosis) are the major drivers of neurodamage. Calcitriol and hydroxyderivatives of lumisterol and tachisterol could play an adjuvant role in neuroprotection through mitigation of neuroinflammation and protection of endothelial integrity of the BBB. Dedicated studies on this topic are currently lacking and are desirable to confirm the link between vitamin D3 and neuroprotection in COVID-19 patients.