The role of mitochondrial dysfunction in the development of long covid: a review

The normal microbial occupants of the mammalian intestine are crucial for maintaining gut homeostasis, yet the mechanisms by which intestinal cells perceive and respond to the microbiota are largely unknown. Intestinal epithelial contact with commensal bacteria and/or their products has been shown to activate noninflammatory signaling pathways, such as extracellular signal-related kinase (ERK), thus influencing homeostatic processes. We previously demonstrated that commensal bacteria stimulate ERK pathway activity via interaction with formyl peptide receptors (FPRs). In the current study, we expand on these findings and show that commensal bacteria initiate ERK signaling through rapid FPR-dependent reactive oxygen species (ROS) generation and subsequent modulation of MAP kinase phosphatase redox status. ROS generation induced by the commensal bacteria Lactobacillus rhamnosus GG and the FPR peptide ligand, N-formyl-Met-Leu-Phe, was abolished in the presence of selective inhibitors for G protein-coupled signaling and FPR ligand interaction. In addition, pretreatment of cells with inhibitors of ROS generation attenuated commensal bacteria-induced ERK signaling, indicating that ROS generation is required for ERK pathway activation. Bacterial colonization also led to oxidative inactivation of the redox-sensitive and ERK-specific phosphatase, DUSP3/VHR, and consequent stimulation of ERK pathway signaling. Together, these data demonstrate that commensal bacteria and their products activate ROS signaling in an FPR-dependent manner and define a mechanism by which cellular ROS influences the ERK pathway through a redox-sensitive regulatory circuit.

We have investigated the gastroprotective effect of SEGA (3a), a newly synthesized tryptamine-gallic acid hybrid molecule against non-steroidal anti-inflammatory drug (NSAID)-induced gastropathy with mechanistic details. SEGA (3a) prevents indomethacin (NSAID)-induced mitochondrial oxidative stress (MOS) and dysfunctions in gastric mucosal cells, which play a pathogenic role in inducing gastropathy. SEGA (3a) offers this mitoprotective effect by scavenging of mitochondrial superoxide anion (O(2)(·-)) and intramitochondrial free iron released as a result of MOS. SEGA (3a) in vivo blocks indomethacin-mediated MOS, as is evident from the inhibition of indomethacin-induced mitochondrial protein carbonyl formation, lipid peroxidation, and thiol depletion. SEGA (3a) corrects indomethacin-mediated mitochondrial dysfunction in vivo by restoring defective electron transport chain function, collapse of transmembrane potential, and loss of dehydrogenase activity. SEGA (3a) not only corrects mitochondrial dysfunction but also inhibits the activation of the mitochondrial pathway of apoptosis by indomethacin. SEGA (3a) inhibits indomethacin-induced down-regulation of bcl-2 and up-regulation of bax genes in gastric mucosa. SEGA (3a) also inhibits indometacin-induced activation of caspase-9 and caspase-3 in gastric mucosa. Besides the gastroprotective effect against NSAID, SEGA (3a) also expedites the healing of already damaged gastric mucosa. Radiolabeled ((99m)Tc-labeled SEGA (3a)) tracer studies confirm that SEGA (3a) enters into mitochondria of gastric mucosal cell in vivo, and it is quite stable in serum. Thus, SEGA (3a) bears an immense potential to be a novel gastroprotective agent against NSAID-induced gastropathy.

Hypoxia is known to stimulate reactive oxygen species (ROS) generation. Because reduced glutathione (GSH) is compartmentalized in cytosol and mitochondria, we examined the specific role of mitochondrial GSH (mGSH) in the survival of hepatocytes during hypoxia (5% O2). 5% O2 stimulated ROS in HepG2 cells and cultured rat hepatocytes. Mitochondrial complex I and II inhibitors prevented this effect, whereas inhibition of nitric oxide synthesis with Nomega-nitro-L-arginine methyl ester hydrochloride or the peroxynitrite scavenger uric acid did not. Depletion of GSH stores in both cytosol and mitochondria enhanced the susceptibility of HepG2 cells or primary rat hepatocytes to 5% O2 exposure. However, this sensitization was abrogated by preventing mitochondrial ROS generation by complex I and II inhibition. Moreover, selective mGSH depletion by (R,S)-3-hydroxy-4-pentenoate that spared cytosol GSH levels sensitized rat hepatocytes to hypoxia because of enhanced ROS generation. GSH restoration by GSH ethyl ester or by blocking mitochondrial electron flow at complex I and II rescued (R,S)-3-hydroxy-4-pentenoate-treated hepatocytes to hypoxia-induced cell death. Thus, mGSH controls the survival of hepatocytes during hypoxia through the regulation of mitochondrial generation of oxidative stress.

Catalase overexpression attenuates angiotensinogen expression and apoptosis in diabetic mice

UVB Radiation Induces Apoptosis in Keratinocytes by Activating a Pathway Linked to “BLT2-Reactive Oxygen Species”

The finding that upon neuronal activation glutamate is transported postsynaptically from synaptic clefts and increased lactate availability for neurons suggest that brain mitochondria (BM) utilize a mixture of substrates, namely pyruvate, glutamate, and the tricarboxylic acid cycle metabolites. We studied how glutamate affected oxidative phosphorylation and reactive oxygen species (ROS) production in rat BM oxidizing pyruvate + malate or succinate. Simultaneous oxidation of glutamate + pyruvate + malate increased state 3 and uncoupled respiration by 52 and 71%, respectively. The state 4 ROS generation increased 100% over BM oxidizing pyruvate + malate and 900% over that of BM oxidizing glutamate + malate. Up to 70% of ROS generation was associated with reverse electron transport. These effects of pyruvate + glutamate + malate were observed only with BM and not with liver or heart mitochondria. The effects of glutamate + pyruvate on succinate-supported respiration and ROS generation were not organ-specific and depended only on whether mitochondria were isolated with or without bovine serum albumin. With the non-bovine serum albumin brain and heart mitochondria oxidizing succinate, the addition of pyruvate and glutamate abrogated inhibition of Complex II by oxaloacetate. We conclude that (i) during neuronal activation, simultaneous oxidation of glutamate + pyruvate temporarily enhances neuronal mitochondrial ATP production, and (ii) intrinsic inhibition of Complex II by oxaloacetate is an inherent mechanism that protects against ROS generation during reverse electron transport.

SOD2 in platelets: with age comes responsibility

A HIF-1 Target, ATIA, Protects Cells from Apoptosis by Modulating the Mitochondrial Thioredoxin, TRX2

Mitochondria were first described in 1840 as bioblasts, elementary organisms responsible for vital cellular functions, but were subsequently named mitochondria, from the Greek names mitos (thread) and chondros (granule), which describes their appearance during spermatogenesis.1 Their discovery generated substantial interest given their structure resembling bacteria, which led in subsequent years to important scientific discoveries positioning mitochondria as the energy powerhouse of the cell. The unique architecture of mitochondria, consisting of 2 membranes (outer and inner) and compartments (intermembrane space and matrix), is crucial for their vital functions. Mitochondria serve not only as primary sources of cellular energy, but also modulate several cellular processes, including oxidative phosphorylation, calcium homeostasis, thermogenesis, oxygen sensing, proliferation, and apoptosis.2 Therefore, mitochondrial injury and dysfunction might be implicated in the pathogenesis of several diseases. Hypertension accounts for nearly 30% of patients reaching end-stage renal disease.3 Renal injury secondary to hypertension or to ischemia associated with renovascular hypertension (distal to renal artery stenosis) may have significant and detrimental effect on health outcomes. Studies have highlighted several deleterious pathways, including inflammation, oxidative stress, and fibrosis that are activated in the hypertensive kidney, eliciting functional decline.4,5 However, the precise molecular mechanisms responsible for renal injury have not been fully elucidated. Over the past few years, increasing evidence has established the experimental foundations linking mitochondrial alterations to hypertensive renal injury (Table). Mitochondriopathies, abnormalities of energy metabolism secondary to sporadic or inherited mutations in nuclear or mitochondrial DNA (mtDNA) genes, may contribute to the development and progression of hypertension and its complications. In addition, several studies have reported mitochondrial damage and dysfunction consequent to hypertensive renal disease. View this table: Table. Evidence of Renal Mitochondrial Damage in Models of Hypertension and Antihypertensive Treatment Importantly, hypertensive-induced renal injury is characterized by activation of several deleterious pathways, including oxidative stress, renin–angiotensin–aldosterone …

Prognostic Value of Hyperlactatemia and Lactate Clearance After Mitral Valve Surgery

From human and animal studies, estrogen is known to protect the myocardium from an ischemic insult. However, there is limited knowledge regarding mechanisms by which estrogen directly protects cardiomyocytes. In this report, we employed an in vitro model, in which cultured rat cardiomyocytes underwent prolonged hypoxia followed by reoxygenation (H/R), to study the cardioprotective mechanism of estrogen. 17-beta-estradiol (E2) acting via estrogen receptors inhibited H/R-induced apoptosis of cardiomyocytes. Mitochondrial reactive oxygen species (ROS) generated from H/R activated p38alpha MAPK, and inhibition of p38alpha with SB203580 significantly prevented H/R-induced cell death. E2 suppressed ROS formation and p38alpha activation by H/R and concomitantly augmented the activity of p38beta. Unlike p38alpha, p38beta was little affected by H/R. Dominant negative p38beta protein expression decreased E2-mediated cardiomyocyte survival and ROS suppression during H/R stress. The prosurvival signaling molecule, phosphoinositol-3 kinase (PI3K), has previously been linked to cell survival following ischemia-reperfusion injury. Here, E2-activated PI3K was found to inhibit ROS generated from H/R injury, leading to inhibition of downstream p38alpha. We further linked these signaling pathways in that p38beta was activated by E2 stimulation of PI3K. Thus, E2 differentially modulated two major isoforms of p38, leading to cardiomyocyte survival. This was achieved by signaling through PI3K, integrating cell survival mediators.

Peroxiredoxin 6 (Prdx6), a bifunctional enzyme with glutathione peroxidase and phospholipase A2 (PLA(2)) activities, participates in the activation of NADPH oxidase 2 (NOX2) in neutrophils, but the mechanism for this effect is not known. We now demonstrate that Prdx6 is required for agonist-induced NOX2 activation in pulmonary microvascular endothelial cells (PMVEC) and that the effect requires the PLA(2) activity of Prdx6. Generation of reactive oxygen species (ROS) in response to angiotensin II (Ang II) or phorbol 12-myristate 13-acetate was markedly reduced in perfused lungs and isolated PMVEC from Prdx6 null mice. Rac1 and p47(phox), cytosolic components of NOX2, translocated to the endothelial cell membrane after Ang II treatment in wild-type but not Prdx6 null PMVEC. MJ33, an inhibitor of Prdx6 PLA(2) activity, blocked agonist-induced PLA(2) activity and ROS generation in PMVEC by >80%, whereas inhibitors of other PLA(2)s were ineffective. Transfection of Prx6 null cells with wild-type and C47S mutant Prdx6, but not with mutants of the PLA(2) active site (S32A, H26A, and D140A), "rescued" Ang II-induced PLA(2) activity and ROS generation. Ang II treatment of wild-type cells resulted in phosphorylation of Prdx6 and its subsequent translocation from the cytosol to the cell membrane. Phosphorylation as well as PLA(2) activity and ROS generation were markedly reduced by the MAPK inhibitor, U0126. Thus, agonist-induced MAPK activation leads to Prdx6 phosphorylation and translocation to the cell membrane, where its PLA(2) activity facilitates assembly of the NOX2 complex and activation of the oxidase.

Cadmium triggers apoptosis of LLC-PK1 cells through induction of endoplasmic reticulum (ER) stress. We found that cadmium caused generation of reactive oxygen species (ROS) and that cadmium-induced ER stress was inhibited by antioxidants. In contrast, suppression of ER stress did not attenuate cadmium-triggered oxidative stress, suggesting that ER stress occurs downstream of oxidative stress. Exposure of the cells to either O(2)(*), H(2)O(2), or ONOO(-) caused apoptosis, whereas ER stress was induced only by O(2)(*) or ONOO(-). Transfection with manganese superoxide dismutase significantly attenuated cadmium-induced ER stress and apoptosis, whereas pharmacological inhibition of ONOO(-) was ineffective. Interestingly, transfection with catalase attenuated cadmium-induced apoptosis without affecting the level of ER stress. O(2)(*) caused activation of the activating transcription factor 6-CCAAT/enhancer-binding protein-homologous protein (CHOP) and the inositol-requiring ER-to-nucleus signal kinase 1-X-box-binding protein 1 (XBP1) proapoptotic cascades, and overexpression of manganese superoxide dismutase attenuated cadmium-triggered induction of both pathways. Furthermore, phosphorylation of proapoptotic c-Jun N-terminal kinase by O(2)(*) or cadmium was suppressed by dominant-negative inhibition of XBP1. These data elucidated 1) cadmium caused ER stress via generation of ROS, 2) O(2)(*) was selectively involved in cadmium-triggered, ER stress-mediated apoptosis through activation of the activating transcription factor 6-CHOP and inositol-requiring ER-to-nucleus signal kinase 1-XBP1 pathways, and 3) phosphorylation of JNK was caused by O(2)(*)-triggered activation of XBP1.

Bacterial uracil modulates Drosophila DUOX-dependent gut immunity via Hedgehog-induced signaling endosomes.

Overexpression of the mammalian proapoptotic protein Bax induces cell death in plant and yeast cells. The Bax inihibitor-1 (BI-1) gene rescues yeast and plant from Bax-mediated lethality. Using the Arabidopsis BI-1 (AtBI-1) gene controlled by the GAL1 promoter as a cell death suppressor in yeast, Cdf1 (cell growth defect factor-1) was isolated from Arabidopsis cDNA library. Overexpression of Cdf1 caused cell death in yeast, whereas such an effect was suppressed by co-expression of AtBI-1. The Cdf1 protein fused with a green fluorescent protein was localized in the mitochondria and resulted in the loss of mitochondrial membrane potential in yeast. The Bax-resistant mutant BRM1 demonstrated tolerance against Cdf1-mediated lethality, whereas the Deltaatp4 strain was sensitive to Cdf1. Our results suggest that Cdf1 and Bax cause mitochondria-mediated yeast lethality through partially overlapped pathways.