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.

Hydrogen Availability Is Dependent on the Actions of Both Hydrogen-Producing and Hydrogen-Consuming Microbes

Hydrogen gas (H₂) is produced by H₂-producing microbes in the gut during polysaccharide fermentation. Gut microbiome also includes H₂-consuming microbes utilizing H₂ for metabolism: methanogens producing methane, CH₄, and sulfate-reducing bacteria producing hydrogen sulfide, H₂S. H₂S is not measured in the evaluation of gaseous byproducts of microbial fermentation. We hypothesize that the availability of measured H₂ depends on both hydrogen producers and hydrogen consumers by measuring H₂ in vitro and in vivo. In the in vitro study, groups were Bacteroides thetaiotaomicron (B. theta, H₂ producers), Desulfovibrio vulgaris (D. vulgaris, H₂ consumers), and D. vulgaris + B. theta combined. Gas samples were collected at 2 h and 24 h after incubation and assayed for H₂, CH₄, and H₂S. In the in vivo study Sprague-Dawley rats were gavaged with suspended bacteria in four groups: B. theta, D. vulgaris, combined, and control. Gas was analyzed for H₂ at 60 min. In the in vitro experiment, H₂ concentration was higher in the combined group (188 ± 93.3 ppm) compared with D. vulgaris (27.17 ± 9.6 ppm) and B. theta groups (34.2 ± 29.8 ppm; P < 0.05); H₂S concentration was statistically higher in the combined group (10.32 ± 1.5 ppm) compared with B. theta (0.19 ± 0.03 ppm) and D. vulgaris group (3.46 ± 0.28 ppm; P < 0.05). In the in vivo study, H₂ concentrations were significantly higher in the B. theta group (44.3 ± 6.0 ppm) compared with control (31.8 ± 4.3) and the combined group (34.2 ± 8.7, P < 0.05). This study shows that sulfate-reducing bacteria could convert available H₂ to H₂S, leading to measured hydrogen levels that are dependent on the actions of both H₂ producers and H₂ consumers.

Hydrogen Sulfide Actions in the Vasculature

Hydrogen sulfide (H2 S) is a small, gaseous molecule with poor solubility in water that is generated by multiple pathways in many species including humans. It acts as a signaling molecule in many tissues with both beneficial and pathological effects. This article discusses its many actions in the vascular system and the growing evidence of its role to regulate vascular tone, angiogenesis, endothelial barrier function, redox, and inflammation. Alterations in some disease states are also discussed including potential roles in promoting tumor growth and contributions to the development of metabolic disease. © 2021 American Physiological Society. Compr Physiol 11:1-22, 2021.

Portal Venous Flow Is Increased by Jejunal but Not Colonic Hydrogen Sulfide in a Nitric Oxide-Dependent Fashion in Rats

Hydrogen sulfide (H2S) is a recently discerned endogenous signaling molecule that modulates the vascular system. Endogenous hydrogen sulfide has been shown to dilate both the mesenteric and portal vasculature. Gut microbiome, via sulfur reducing bacteria, is another source of H2S production within the gut lumen; this source of H2S is primarily produced and detoxified in the colon under physiologic conditions. Nitric oxide (NO), a major endogenous vasodilator in the portal circulation, participates in H2S-induced vasodilation in some vascular beds. We hypothesize that jejunal but not colonic H2S increases portal vein flow in a NO-dependent fashion. To evaluate the effects of luminal H2S, venous blood flow, portal venous pressure, and systemic venous pressure were measured in rats after administration of either vehicle or an H2S donor (NaHS) into the jejunum or the colon. We found that portal venous pressure and systemic pressure did not change and were similar between the three study groups. However, portal venous blood flow significantly increased following jejunal administration of NaHS but not in response to colonic NaHS or vehicle administration. To test the contribution of NO production to this response, another group of animals was treated with either an NO synthase inhibitor (N-Ω-nitro-L-arginine, L-NNA) or saline prior to jejunal NaHS infusion. After L-NNA pretreatment, NaHS caused a significant fall rather than increase in portal venous flow compared to saline pretreatment. These data demonstrate that H2S within the small intestine significantly increases portal venous blood flow in a NO-dependent fashion.

Reevaluating our understanding of lactulose breath tests by incorporating hydrogen sulfide measurements

Background and Aim

Breath testing has become a commonly used tool in gastroenterology to evaluate changes in the fermentation pattern of the gut microbiome. Currently, hydrogen and methane gas concentrations are measured in breath testing and evaluated against specific cut‐off values for interpretation as normal or abnormal. However, microbial gas kinetics is a complex process that is not currently fully considered when interpreting breath gas results. Gas exchange between hydrogen producers and hydrogen consumers (methanogens and sulfate‐reducing bacteria) is a process whereby hydrogen availability is determined by both its production and removal. Hydrogen sulfide is a crucial gas involved in this process as it is a major hydrogen‐consumptive pathway involved in energy exchange.

Methods

This is a cross‐sectional study evaluating lactulose breath testing with the inclusion of hydrogen sulfide measurements in patients referred for breath testing for gastrointestinal symptoms of bloating, excessive gas, and/or abdominal pain.

Results

A total of 159 patients were analyzed between October 2016 and June 2017. Mean hydrogen concentrations with a positive trend through a 3‐h period (R² = 0.97), mean methane concentrations with a positive trend (R² = 0.69), and mean hydrogen sulfide concentrations with a negative trend (R² = −0.71) were observed.

Conclusion

By incorporating energy exchange in the interpretation of the lactulose breath test, we reevaluated specific breath gas profiles, including those commonly described as “hydrogen nonproducers” and the “double‐peak” phenomenon.

Malignant biliary obstruction: From palliation to treatment

Malignant obstruction of the bile duct from cholangiocarcinoma, pancreatic adenocarcinoma, or other tumors is a common problem which may cause debilitating symptoms and increase the risk of subsequent surgery. The optimal treatment - including the decision whether to treat prior to resection - depends on the type of malignancy, as well as the stage of disease. Preoperative biliary drainage is generally discouraged due to the risk of infectious complications, though some situations may benefit. Patients who require neoadjuvant therapy will require decompression for the prolonged period until attempted surgical cure. For pancreatic cancer patients, self-expanding metallic stents are superior to plastic stents for achieving lasting decompression without stent occlusion. For cholangiocarcinoma patients, treatment with percutaneous methods or nasobiliary drainage may be superior to endoscopic stent placement, with less risk of infectious complications or failure. For patients of either malignancy who have advanced disease with palliative goals only, the choice of stent for endoscopic decompression depends on estimated survival, with plastic stents favored for survival of < 4 mo. New endoscopic techniques may actually extend stent patency and patient survival for these patients by achieving local control of the obstructing tumor. Both photodynamic therapy and radiofrequency ablation may play a role in extending survival of patients with malignant biliary obstruction.

Amelioration of endotoxin-induced acute lung injury in pigs by HWA 138 and A 80 2715: new analogs of pentoxifylline

We have evaluated the potential therapeutic effects of the xanthine derivatives HWA 138 (1-(5 hydroxy-5-methylhexyl)-3-methylxanthine) and A 80,2715 (1-(5 hydroxy-5-methylhexyl)-3-methyl-7-propylxanthine) on acute lung injury in endotoxemic pigs when administered following the septic insult. Salmonella abortus equi endotoxin (LPS) was given as a continuous intravenous infusion of 2 micrograms/kg/h over a period of 8 h. 1 h after the start of the LPS infusion the animals received a bolus injection followed by continuous infusion of A 80,2715 (3 mg/kg + 1.5 mg/kg/h; n = 6), HWA 138 (3 mg/kg + 1.5 mg/kg/h; n = 6), or saline (LPS control; n = 6). Treatment with A 80,2715 or HWA 138 inhibited the LPS-induced increases in the pulmonary artery pressure (p < .05; ANOVA) and in lung wet/dry weight ratio (p < .05), and ameliorated the LPS-induced deterioration in lung mechanics (decreased lung dynamic compliance (p < .05); increased peak airway pressure (p < .05)). Xanthine treatment, however, failed to significantly improve arterial PO2 and did not affect peripheral leukopenia. The results of this study suggest a potential therapeutic role for HWA 138 and A 80,2715 in attenuating endotoxin-induced acute lung injury.

Sensitive detection of the activation state of blood coagulation in porcine DIC models by a new fibrin immunoassay

Elevated levels of soluble fibrin in plasma indicate that thrombin converts fibrinogen to fibrin without sufficient inhibitory control. Therefore, measurement of soluble fibrin (SF) in plasma may be considered as a laboratory test for intravascular coagulation. We have demonstrated that a new immunoassay for detection of SF in human plasma (Lill et al., Blood Coag Fibrinol 1993; 4: 97-102), based on a fibrin specific monoclonal antibody, also detects porcine SF with high sensitivity. Thrombin-dependent generation of SF in porcine plasma in vitro resulted in increased reactivity of the assay system, which was time and dose dependent. Dextran sulphate (DXS) and bacterial lipopolysaccharide (LPS) were used as stimuli in in vivo experiments in pigs. Plasma levels of SF increased steadily after intravenous administration of DXS (5 mg/kg for 1 h) to 38 +/- 7.8 micrograms/ml (mean +/- SEM) at 2 h, whereas LPS (2 micrograms/kg/h for 6 h) markedly increased plasma SF levels to over 120 micrograms/ml (at 6 h) after a lag phase of 2 h. In conclusion, this new immunoassay for human fibrin allows specific and sensitive detection of soluble fibrin in porcine plasma.

Development of a chromogenic substrate assay for the determination of hirudin in plasma

Hirudin is a potent and specific thrombin inhibitor. Since recombinant hirudin is being considered for anticoagulant and antithrombotic therapy we developed a fast and sensitive chromogenic substrate assay for its determination in plasma. The plasma samples (20 microliters) were incubated with 1 ml reagent mixture (0.2 M Tris buffer, 0.025 M NaCl, pH 8.1, containing 0.833 M urea, 0.7 trypsin inhibitor U/ml aprotinin, 100 ng/ml Polybrene and 0.31 NIH U/ml bovine thrombin) for 1 min. Thereafter 100 microliters Chromozym TH (Tos-Gly-Pro-Arg-pNA, 1.9 mM) was added. The change in absorbance/min (delta A/min) was recorded at 405 nm. delta A/min was linear for at least 3 min. The calibration curve was linear at least up to 800 ng hirudin/ml plasma. Intra-assay and inter-assay coefficients of variation were 2.8-3.1% and 5.3-5.8% respectively. The influence of progressive thrombin inhibitors can be neglected because of the short incubation time. Plasma samples can be assayed directly if aprotinin, polybrene and urea are added to the reagent mixture.