key: cord-0958635-0u9dii0j authors: Gopal, Akshita Baiju; Chakraborty, Soumyadeep; Padhan, Pratyush Kumar; Barik, Alok; Dixit, Pragyesh; Chakraborty, Debashish; Poirah, Indrajit; Samal, Supriya; Sarkar, Arup; Bhattacharyya, Asima title: Silent hypoxia in COVID-19: a gut microbiota connection date: 2021-07-06 journal: Curr Opin Physiol DOI: 10.1016/j.cophys.2021.06.010 sha: 9c8a89c5ad9888430dbb884afbc8d58552f06348 doc_id: 958635 cord_uid: 0u9dii0j Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection has triggered the COVID-19 pandemic. Several factors induce hypoxia in COVID-19. Despite being hypoxic, some SARS-CoV-2-infected individuals do not experience any respiratory distress, a phenomenon termed “silent/happy hypoxia”. Prolonged undetected hypoxia is dangerous, sometimes leading to death. A few studies attempted to unravel what causes silent hypoxia, however, the exact mechanisms are still elusive. Here, we aim to understand how SARS-CoV-2 causes silent hypoxia. The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has initiated the current COVID-19 pandemic. COVID-19 symptoms are diverse and extend from mild to severe manifestations of pneumonia, acquired respiratory distress syndrome (ARDS) and multi-organ failure [1] . A prevalent feature associated with COVID-19 is the onset of hypoxemia {low blood oxygen (O2) level}. SARS-CoV-2 replication within the lungs causes an uncontrolled inflammatory response, the "cytokine storm", which impinge on the lung function or perfusion, leading to hypoxemia [2] . This causes a deficiency in tissue oxygenation leading to hypoxia. Compensatory mechanisms like increased ventilation and dyspnea, which are generally initiated in hypoxia, are surprisingly lacking in many COVID-19 patients. This phenomenon is known as "silent/happy hypoxia" or nondyspneic hypoxemia [*3,4]. Since the patient remains unaware of the condition, undetected hypoxia is dangerous. Studies indicate that gut dysbiosis (disruption of the gut microbial homeostasis) is an important manifestation in COVID-19 and can hamper respiratory control [5] . This article explores the J o u r n a l P r e -p r o o f potential role of gut microbiota-brain communication in causing silent hypoxia in COVID-19. The cause of hypoxia in COVID-19 is multifactorial and includes thrombosis, pulmonary infiltration, viral invasion in pneumocytes, profuse cytokine release and inflammatory responses. Sepsis and pulmonary edema-mediated thickening of the alveolar-capillary barrier, viremia and dysregulated renin-angiotensin-aldosterone system (RAAS) also cause systemic hypoxia in COVID-19 [2, 6] . The central chemoreceptors of the respiratory center (RC) (medulla oblongata and pons in the brainstem) and the peripheral chemoreceptors of the carotid body (CB) sense O2 and carbon dioxide (CO2) in the arterial blood [7, 8] . RC is modulated by several metabolites including lactate, and are more sensitive in detecting slight increases in CO2-tension (PaCO2) or a drop in pH than PaO2-decrease. CB evokes peripheral chemoreflexes and ventilatory activity [9] . Although both RC and CB can detect hypoxia, the CB has the main role in O2 homeostasis. Hypoxia depolarizes glomus cells (type I) in the CB, promoting the release of neurotransmitters that signal the nucleus tractus solitarius (NTS) via a small division of the glossopharyngeal nerve (carotid sinus nerve) [10] . These signals are integrated and relayed to the rostral ventrolateral ("pressor") region of the medulla and the hypothalamic paraventricular nucleus that initiate ventilatory output which regulate breathing. Central chemoreceptors communicate (glutaminergic) with the pre-Bötzinger complex (PBC) of the medulla oblongata, the medullary raphe (serotonergic), the fastigial nucleus (glutaminergic) of the cerebellum and the astrocytes of the glial cells [11] . PBC and the retrotrapezoid nucleus/parafacial respiratory group of the brainstem neurons are considered the primary and secondary respiratory rhythm-J o u r n a l P r e -p r o o f regulators, respectively. The RC receives signals from these chemoreceptors, the cerebrum and the hypothalamus to determine the rate or depth of respiration as well as the sensation of dyspnea [12] . Figure 1 provides a schematic representation of the major neural components involved in O2-sensing. Respiratory-responses hugely vary among individuals and are further complicated by respiratory virus infections. SARS-CoV-2 reaches the central nervous system (CNS) by various routes. As discussed later, the neuroinvasive potential of SARS-CoV-2 might directly impair hypoxia-response by targeting the chemosensors [13] [14] [15] . In addition, SARS-CoV-2 can disturb the intricately-balanced gut-brain axis [16] to ultimately impact the functioning of RC. The symbiotic relationship of gut microbes with the host regulates metabolic pathways, immune and neuroendocrine crosstalk [17] . Gut microbes can interact with the brain via the vagus nerve and produce many neuroactive substances such as metabolites, endocrine modulators and neurotransmitters. Since systematic efforts to understand the contribution of SARS-CoV-2-mediated gut dysbiosis towards silent hypoxia have never been made, here we summarize the mechanisms that might be involved. SARS-CoV-2 directly infects enterocytes by binding with ACE2 and causes gut dysbiosis [** 21, 23] . Like many other viruses, SARS-CoV-2 disrupts the intestinal barrier function, causes hematological dissemination of gut microbes and initiate systemic inflammation [23] . High levels of proinflammatory cytokines, interferon γ (IFN-γ), tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6) are found in the blood of COVID-19 patients [24] . These cytokines travel via the systemic circulation and alter the blood-brain barrier (BBB) permeability [16] . Systemic inflammation increases the level of circulating reactive O2 species (ROS) that may further affect the brainstem and the cerebrum [25, 26] . The brain has a limited antioxidant capacity and, therefore, is known to be prone to oxidative stress [27] . Oxidative stress causes neuroinflammation and mitochondrial DNA damage in the NTS [28] . Studies involving germ-free mice also indicate that gut dysbiosis compromises the BBB J o u r n a l P r e -p r o o f integrity, consequently allowing the transmission of proinflammatory cytokines to the brain causing neuroinflammation [29] . α-synuclein is generated in the gut due to SARS-CoV-2-mediated cytokine storm, bacterial endotoxins {mainly, lipopolysaccharide (LPS)} and is subsequently transported to the brain by the vagus nerve causing neuronal damage [30] . LPS may also reach the brain and cause neuroinflammation and BBB disruption [31] . Another major mechanism behind SARS-CoV-2 entry into the brain is the reverse axonal transport from the peripheral nerves [32] . Neurons or glial cells, which express ACE2, get infected by the virus [4] . Study on neurotropic flaviviruses indicate that astrocytes, by virtue of performing aerobic glycolysis, might provide ideal replicative environment for SARS-CoV-2 [13] . The CNS damage can be triggered by neurotropic or neuroimmune effects of SARS-CoV-2 on the brainstem [**33]. The PBC-infection might directly hamper hypoxiasensing [34] . Ventilatory responses and dyspnea are tightly regulated by PaCO2. Prevailing hypotheses explaining the COVID-19-associated silent hypoxia are associated with existing hypocapnia (low PaCO2 in the blood) that prevents brainstem-involvement [35] . During SARS-CoV-2 infection-induced hypoxia, the brain raises metabolic rate and produces lactate but the cerebral blood flow, which is well-maintained, carries away the excess CO2 generated during the process [36] . This hypocapnic hypoxia may hamper the function of central chemoreceptors and cause dyspnea. A study involving a small group of COVID-19 patients show that PaCO2 lower than 39 mm Hg blunts the CNS-response to hypoxia [37] . In contrast, CB detects changes in PaO2 in the arterial blood but it cannot sense O2-saturation. In pyrexia, prevalent in COVID-19 patients, the O2-dissociation curve shifts to the right (i.e. causes hemoglobin-desaturation) rendering CB-chemoreceptors unstimulated J o u r n a l P r e -p r o o f and contributes to silent hypoxia. Poor respiratory control and BBB integrity in the elderly and diabetic COVID-19 patients may explain prevalence of silent hypoxia in these populations. The vagus nerve forms a major neural route connecting the gut to the brain and has innervations in the respiratory tract and the NTS [38] . As dysbiosis modulates the vagal tone, it can perturb the input signaling to the NTS [38, 39] , thereby affecting respiration. Damage to the lung vagal receptors and respiratory muscle mechanoreceptors further explains the absence of dyspnea in COVID-19 [40] . Microbe-released metabolites alter immune-inflammatory responses in the CNS [41] . As inflammatory mediators cause CNS neurodegeneration [16, 42] , gut dysbiosisinduced neuroinflammation damages the RC and might be a potential mechanism behind silent hypoxia [16, 41] . These studies highlight gut-dysbiosis as a critical deregulator of neuronal function. The gut microbiota generates several neurotropic metabolites, neurotransmitters, peptides and gaseous substances, many of which show altered levels in COVID-19 ( alter the brain neurochemistry [46] . SCFA, especially butyrate, maintain the intestinal J o u r n a l P r e -p r o o f tight junctions, BBB integrity, show neuroprotective effects [47] and even is capable of ACE2 downregulation in the colonic organoids of rats [48] . Murine RC and CB are responsive to SCFA by the mediation of Olfr78, a Gs-coupled receptor involved in mild-moderate hypoxia-sensing [49] . These evidences implicate that SARS-CoV-2mediated depletion of SCFA would impair hypoxia-sensing. Inflammatory bowel diseases (IBD), which include ulcerative colitis (UC) and Crohn's disease (CD), show striking-similarities with COVID-19 in their pathophysiological mechanisms. IBD are associated with immune dysregulation, damaged intestinal barrier and gut dysbiosis [50] . Eventually, inflammatory processes spread extraintestinally and affect other organs including the respiratory organs and the brain. IBD patients display "pathological hypoxia" frequently but some patients remain nondyspneic [51] and asymptomatic unless assessed by lung function test [52] . The gut microbiome of IBD patients, has less SCFA producers Roseburia and F. prausnitzii, depletion of beneficial Faecalibacterium sp., Ruminococcus and increased Clostridium sp. abundance [50] . As in COVID-19, SCFA, specifically butyrate, is consistently low in the gut of individuals with IBD. Interestingly, ACE2 receptors are induced in IBD [53] and possibly correlates with SCFA downregulation. These reports signify the need of future studies to unravel the relationship of gut metabolites with respiratory controls dependent-and independent of SARS-CoV-2 infection. The molecular mechanism of hypoxia-sensing is still elusive; however, the role of hypoxia-adaptive hypoxia-inducible factor-1 (HIF-1) and HIF-2 are wellknown. Hypoxia stabilizes the α-subunit of HIF. HIF-1α deficiency and HIF-2α accumulation contribute towards a blunted hypoxic response by the CB [54] . J o u r n a l P r e -p r o o f CB [15] . In contrast to SARS-CoV-2, other viruses attacking the respiratory system such as influenza virus and respiratory syncytial virus, which do not have an association with silent hypoxia, increase SCFA and valerate [55] . SCFA increase HIF-1α stability in enterocytes which contribute in improving the intestinal barrier function [56] . It will be interesting to know whether SCFA downregulation in COVID-19 contributes towards HIF-1α downregulation in CB and blunting of hypoxiaresponse. Gut microbiota produces various neuromodulators [*43,57]. Among the neuronal compounds detected in the rat glomus cells are NO, enkephalins, neurotensins, neuropeptide Y, substance P, dopamine, GABA, vasoactive intestinal peptide and tyrosine hydroxylase [58] . The major catecholamine functional in the CB is dopamine which exerts inhibitory signals to both hypoxia-sensing and ventilatory efforts [9] . Pathogenic Clostridium sp., positively correlated with COVID-19 in elderly people, can synthesize dopamine and thus, possibly impairs hypoxia-sensing. The chemoreceptors present at the cardiorespiratory center of the NTS, the medulla oblongata and the cerebellum are glutaminergic and inhibited by GABA [59] . Enrichment of GABA synthesizing Bacteroides population in COVID-19 might inhibit these neurons impacting O2-sensing [20, 60] . In summary, we theorize that SARS-CoV-2 modulates gut microbes which fine-tune gut-derived metabolites, potentially altering hypoxia-sensing ( Figure 2 ). J o u r n a l P r e -p r o o f COVID-19-research is still in its nascent stage. The problem associated with silent hypoxia in COVID-19 is the lack of dyspnea which also deters the opportunity to study the gut microbiota-brain axis during this stage. Increase in testing can help in identifying infected individuals even if they do not show any respiratory distress and bring them under medical surveillance. Early detection of circulating metabolites in asymptomatic individuals would help in the prediction of silent hypoxia. Focus should be on exploring the reversal of gut dysbiosis in COVID-19 through microbiotamodification therapy (food, prebiotic/ probiotic and fecal material transplant) [65] which look promising in reversing gut dysbiosis in several diseases. Akshita, Soumyadeep and Asima prepared the original draft; reviewing and editing were done by Pratyush, Pragyesh, Debashish, Supriya, Indrajit, Arup and Asima; artworks were done by Soumyadeep, Pratyush and Alok. The entire work was planned and supervised by Asima. All authors approved this version of the manuscript to be published. No conflicts of interest exist. ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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