The contribution of gut-brain axis to development of neurological symptoms in COVID-19 recovered patients: A hypothesis and review of literature

Microbiota-brain axis: Exploring the role of gut microbiota in psychiatric disorders - A comprehensive review

Mental illness is a hidden epidemic in modern science that has gradually spread worldwide. According to estimates from the World Health Organization (WHO), approximately 10% of the world's population suffers from various mental diseases each year. Worldwide, financial and health burdens on society are increasing annually. Therefore, understanding the different factors that can influence mental illness is required to formulate novel and effective treatments and interventions to combat mental illness. Gut microbiota, consisting of diverse microbial communities residing in the gastrointestinal tract, exert profound effects on the central nervous system through the gut-brain axis. The gut-brain axis serves as a conduit for bidirectional communication between the two systems, enabling the gut microbiota to affect emotional and cognitive functions. Dysbiosis, or an imbalance in the gut microbiota, is associated with an increased susceptibility to mental health disorders and psychiatric illnesses. Gut microbiota is one of the most diverse and abundant groups of microbes that have been found to interact with the central nervous system and play important physiological functions in the human gut, thus greatly affecting the development of mental illnesses. The interaction between gut microbiota and mental health-related illnesses is a multifaceted and promising field of study. This review explores the mechanisms by which gut microbiota influences mental health, encompassing the modulation of neurotransmitter production, neuroinflammation, and integrity of the gut barrier. In addition, it emphasizes a thorough understanding of how the gut microbiome affects various psychiatric conditions.

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.

The bidirectional links between coronavirus disease 2019 and Alzheimer's disease

Coronavirus disease 2019 (COVID-19) can be a critical disease, particularly in the elderly and those with comorbidities. Patients with Alzheimer's disease are more vulnerable to COVID-19 consequences. The latest results have indicated some common risk factors for both diseases. An understanding of the pathological link between COVID-19 and Alzheimer's disease will help develop timely strategies to treat both diseases. This review explores the bidirectional links between COVID-19 and Alzheimer's disease.

Gastrointestinal Manifestations and Their Association with Neurologic and Sleep Problems in Long COVID-19 Minority Patients: A Prospective Follow-Up Study

BACKGROUND

Long-COVID is a condition post SARS-CoV-2 infection with persistent or recurring symptoms affecting multiple organs, and may involve viral persistence, changes to the microbiome, coagulopathies, and alterations to neuro-immune interactions. These factors can disrupt the Gut-Brain Axis, which is a complex system involving bidirectional communication between the central nervous system and the gastrointestinal (GI) system. As a result of these disruptions, individuals with long-COVID may develop post-infectious functional GI disorders, which can cause a range of symptoms affecting the digestive system.

AIM

To understand frequency of GI manifestations of Long-COVID and to determine association with sleep or neurological symptoms in a predominantly minority population.

METHODS

We included patients with positive SARS-CoV-2 PCR (n = 747) who were hospitalized from Feb. 2020 to May 2021 at Howard University Hospital and followed between 6 and 12 months from discharge. GI, sleep, and neurological symptoms (via the Montreal Cognitive Assessment (MoCA) scoring system) were assessed using a standardized questionnaire. Linear regression analysis, χ2 and Fisher's exact test were utilized to determine the statistical significance of correlations of GI/Neuro/COVID.

RESULTS

The mean age of patients was 58, with 51.6% females and a predominant African American ethnicity (73.6%, n = 550). A total of 108 patients died during their initial hospital stay, with the remaining 639 patients followed-up. Three hundred fifty (350) patients responded to the questionnaire (57 patients died during the follow-up period). Overall, 39 (13.3%) patients reported GI-related symptoms, out of which 19 (6.4%) had persistent symptoms and 20 (6.8%) developed new onset GI symptoms. Nausea and vomiting were the most common 24/39 (61.5%), followed by abdominal pain 7/39 (18%), diarrhea 5/39 (12.8%), and others 3/39 (7.6%). Patients who presented with vomiting during acute SARS-CoV-2 infection were more likely to have Long-COVID GI manifestations (P = 0.023). Use of ACE inhibitors, abnormal lymphocyte count and elevated ferritin are other variables that showed significant associations with Long-COVID GI manifestations (P = 0.03, 0.006 and 0.03, respectively). During follow-up, a total of 28 (9.5%) patients reported difficulty with sleep and 79 (27%) patients had abnormal MoCA assessment. With further analysis, there was a trend between presentation of GI symptoms on admission with abnormal MoCA assessment, and an association between abnormal LFTs and history of liver disease during hospitalization with subsequent sleep problems. Baseline characteristics, clinical comorbidities, other laboratory values, hospital length of stay, mechanical ventilation, medications during hospitalization, re-admission and Flu or COVID-19 vaccination have not shown any association with Long-COVID GI symptoms in our cohort.

CONCLUSION

Dyspeptic symptoms were common GI manifestations in the acute and post COVID periods. GI symptoms, abnormal LFTs and a history of liver disease during the acute infectious phase associates with abnormal MoCA and sleep problems during follow-up. Further large population studies are needed to determine if COVID-19 leads to a GI symptoms-associated Long-COVID phenotypes and other symptoms through the Gut-Brain-Axis.

Sodium Butyrate Induced Neural Stem/Progenitor Cell Death in an Experimental Model of Japanese Encephalitis

The anti-inflammatory and neuroprotective effects of short chain fatty acid (SCFA) butyrate have been explored in a wide array of neurological pathologies. It is a 4-carbon SCFA produced from the fermentation of dietary fibers by the gut-microbiota. As evident from previous literature, butyrate plays a wide array of functions in CNS and interestingly enhances the differentiation potential of Neural stem/Progenitor Cells (NSPCs). Japanese encephalitis virus (JEV) is a well-known member of the Flaviviridae family and has been shown to alter neural stem cell pool of the brain, causing devastating consequences. In this study, we administered sodium butyrate (NaB) post JEV infection in BALB/c mouse model to examine any possible amelioration of the viral infection in NSPCs. In addition, ex vivo neurospheres and in vitro model of NSPCs were also used to study the effect of sodium butyrate in JEV infection. As an unprecedented finding, butyrate treated infected animals presented early onset of symptoms, as compared to their respective JEV infected groups. Alongside, we observed an increased viral load in NSPCs isolated from these animals as well as in cell culture models upon sodium butyrate treatment. Cytometric bead array analysis also revealed an increase in inflammatory cytokines, particularly, MCP-1 and IL-6. Further, increased expression of the key members of the canonical NF-κB pathway, viz-a-viz p-NF-κB, p-Iκ-Bα and p-IKK was observed. Overall, the increased inflammation and cell death caused early symptom progression in NaB-treated JEV infected animal model, which is contradictory to the well documented protective nature of NaB and therefore a better understanding of SCFA-based modulation of the gut-brain axis in viral infections is required.

The Gut-Liver-Brain Axis: From the Head to the Feet

The gut-liver-brain axis, a multifaceted network of communication, intricately connects the enteric, hepatic, and central nervous systems [...].

Gut-brain pathogenesis of post-acute COVID-19 neurocognitive symptoms

Approximately one third of non-hospitalized coronavirus disease of 2019 (COVID-19) patients report chronic symptoms after recovering from the acute stage of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Some of the most persistent and common complaints of this post-acute COVID-19 syndrome (PACS) are cognitive in nature, described subjectively as "brain fog" and also objectively measured as deficits in executive function, working memory, attention, and processing speed. The mechanisms of these chronic cognitive sequelae are currently not understood. SARS-CoV-2 inflicts damage to cerebral blood vessels and the intestinal wall by binding to angiotensin-converting enzyme 2 (ACE2) receptors and also by evoking production of high levels of systemic cytokines, compromising the brain's neurovascular unit, degrading the intestinal barrier, and potentially increasing the permeability of both to harmful substances. Such substances are hypothesized to be produced in the gut by pathogenic microbiota that, given the profound effects COVID-19 has on the gastrointestinal system, may fourish as a result of intestinal post-COVID-19 dysbiosis. COVID-19 may therefore create a scenario in which neurotoxic and neuroinflammatory substances readily proliferate from the gut lumen and encounter a weakened neurovascular unit, gaining access to the brain and subsequently producing cognitive deficits. Here, we review this proposed PACS pathogenesis along the gut-brain axis, while also identifying specific methodologies that are currently available to experimentally measure each individual component of the model.

The microbiome in post-acute infection syndrome (PAIS)

Post-Acute Infection Syndrome (PAIS) is a relatively new medical terminology that represents prolonged sequelae symptoms after acute infection by numerous pathogenic agents. Imposing a substantial public health burden worldwide, PASC (post-acute sequelae of COVID-19 infection) and ME/CFS (myalgic encephalomyelitis/chronic fatigue syndrome) are two of the most recognized and prevalent PAIS conditions. The presences of prior infections and similar symptom profiles in PAIS reflect a plausible common etiopathogenesis. The human microbiome is known to play an essential role in health and disease. In this review, we reviewed and summarized available research on oral and gut microbiota alterations in patients with different infections or PAIS conditions. We discussed key theories about the associations between microbiome dysbiosis and PAIS disease development, aiming to explore the mechanistic roles and potential functions the microbiome may have in the process. Additionally, we discuss the areas of knowledge gaps and propose the potential clinical applications of the microbiome for prevention and treatment of PAIS conditions.

The Role of Gut Dysbiosis in the Loss of Intestinal Immune Cell Functions and Viral Pathogenesis

The gut microbiome plays a critical role in maintaining overall health and immune function. However, dysbiosis, an imbalance in microbiome composition, can have profound effects on various aspects of human health, including susceptibility to viral infections. Despite numerous studies investigating the influence of viral infections on gut microbiome, the impact of gut dysbiosis on viral infection and pathogenesis remains relatively understudied. The clinical variability observed in SARS-CoV-2 and seasonal influenza infections, and the presence of natural HIV suppressors, suggests that host-intrinsic factors, including the gut microbiome, may contribute to viral pathogenesis. The gut microbiome has been shown to influence the host immune system by regulating intestinal homeostasis through interactions with immune cells. This review aims to enhance our understanding of how viral infections perturb the gut microbiome and mucosal immune cells, affecting host susceptibility and response to viral infections. Specifically, we focus on exploring the interactions between gamma delta (γδ) T cells and gut microbes in the context of inflammatory viral pathogenesis and examine studies highlighting the role of the gut microbiome in viral disease outcomes. Furthermore, we discuss emerging evidence and potential future directions for microbiome modulation therapy in the context of viral pathogenesis.

Neurodegenerative and Neurodevelopmental Diseases and the Gut-Brain Axis: The Potential of Therapeutic Targeting of the Microbiome

The human gut microbiome contains the largest number of bacteria in the body and has the potential to greatly influence metabolism, not only locally but also systemically. There is an established link between a healthy, balanced, and diverse microbiome and overall health. When the gut microbiome becomes unbalanced (dysbiosis) through dietary changes, medication use, lifestyle choices, environmental factors, and ageing, this has a profound effect on our health and is linked to many diseases, including lifestyle diseases, metabolic diseases, inflammatory diseases, and neurological diseases. While this link in humans is largely an association of dysbiosis with disease, in animal models, a causative link can be demonstrated. The link between the gut and the brain is particularly important in maintaining brain health, with a strong association between dysbiosis in the gut and neurodegenerative and neurodevelopmental diseases. This link suggests not only that the gut microbiota composition can be used to make an early diagnosis of neurodegenerative and neurodevelopmental diseases but also that modifying the gut microbiome to influence the microbiome-gut-brain axis might present a therapeutic target for diseases that have proved intractable, with the aim of altering the trajectory of neurodegenerative and neurodevelopmental diseases such as Alzheimer's disease, Parkinson's disease, multiple sclerosis, autism spectrum disorder, and attention-deficit hyperactivity disorder, among others. There is also a microbiome-gut-brain link to other potentially reversible neurological diseases, such as migraine, post-operative cognitive dysfunction, and long COVID, which might be considered models of therapy for neurodegenerative disease. The role of traditional methods in altering the microbiome, as well as newer, more novel treatments such as faecal microbiome transplants and photobiomodulation, are discussed.

COVID-19 Patients with Early Gastrointestinal Symptoms Show Persistent Deficits in Specific Attention Subdomains

Previous studies have shown that COVID-19 inpatients exhibited significant attentional deficits on the day of discharge. However, the presence of gastrointestinal symptoms (GIS) has not been evaluated. Here, we aimed to verify: (1) whether COVID-19 patients with GIS exhibited specific attention deficits; (2) which attention subdomain deficits discriminated patients with GIS and without gastrointestinal symptoms (NGIS) from healthy controls. On admission, the presence of GIS was recorded. Seventy-four physically functional COVID-19 inpatients at discharge and sixty-eight controls underwent a Go/No-go computerized visual attentional test (CVAT). A Multivariate Analysis of Covariance (MANCOVA) was performed to examine group differences in attentional performance. To discriminate which attention subdomain deficits discriminated GIS and NGIS COVID-19 patients from healthy controls, a discriminant analysis was applied using the CVAT variables. The MANCOVA showed a significant overall effect of COVID-19 with GIS on attention performance. The discriminant analysis indicated that the GIS group could be differentiated from the controls by variability of reaction time and omissions errors. The NGIS group could be differentiated from controls by reaction time. Late attention deficits in COVID-19 patients with GIS may reflect a primary problem in the sustained and focused attention subsystems, whereas in NGIS patients the attention problems are related to the intrinsic-alertness subsystem.

COVID-19 and Multiple Sclerosis: A Complex Relationship Possibly Aggravated by Low Vitamin D Levels

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an exceptionally transmissible and pathogenic coronavirus that appeared at the end of 2019 and triggered a pandemic of acute respiratory disease, known as coronavirus disease 2019 (COVID-19). COVID-19 can evolve into a severe disease associated with immediate and delayed sequelae in different organs, including the central nervous system (CNS). A topic that deserves attention in this context is the complex relationship between SARS-CoV-2 infection and multiple sclerosis (MS). Here, we initially described the clinical and immunopathogenic characteristics of these two illnesses, accentuating the fact that COVID-19 can, in defined patients, reach the CNS, the target tissue of the MS autoimmune process. The well-known contribution of viral agents such as the Epstein-Barr virus and the postulated participation of SARS-CoV-2 as a risk factor for the triggering or worsening of MS are then described. We emphasize the contribution of vitamin D in this scenario, considering its relevance in the susceptibility, severity and control of both pathologies. Finally, we discuss the experimental animal models that could be explored to better understand the complex interplay of these two diseases, including the possible use of vitamin D as an adjunct immunomodulator to treat them.