The Osmolyte TMAO Modulates Protein Folding Cooperativity by Altering Global Protein Stability

Quantitative Assessment of Conformational Heterogeneity in Apolipoprotein E4 Using Hydrogen-Deuterium Exchange Mass Spectrometry

Apolipoprotein E4 (apoE4) is the strongest genetic risk factor for Alzheimer's disease (AD). However, structural differences between apoE4 and the AD-neutral isoform, apoE3, still remain unclear. Recent studies suggest that apoE4 harbors intermediates. However, the biophysical properties and isoform specificity of these intermediates are not known. Here, we use the kinetics of hydrogen-deuterium exchange by mass spectrometry (HDX-MS) to examine the conformational heterogeneities in apoE3 and apoE4. First, we use numerical simulations to compute the HDX-mass spectra of a protein following mixed EX1/EX2 kinetics. The results indicate that in the presence of EX1 kinetics, which is an indicator of conformational heterogeneity, time evolution of the standard deviation (σ(t)) of the spectra exhibits a clear peak, which is dependent on the number of residues (NEX1) and the rate constant of EX1 kinetics (kEX1). Then, we performed experiments with several variants of the apoE proteins and compared them with simulation to estimate NEX1 and kEX1. Kinetics of the mean deuteration is found to be faster for apoE4, consistent with its poorer stability than apoE3. Importantly, in the case of apoE4, σ(t) exhibits a clear peak at t ∼ 60 s, but apoE3 shows only a small peak at 1800 s. Therefore, both NEX1 and kEX1 are larger for apoE4, indicating greater conformational heterogeneity. Notably, the partial EX1 kinetics is observed in both the isolated N-terminal fragment and the full-length form of apoE4, although it is more pronounced in the full-length protein. Moreover, it is enhanced at higher pH and in the presence of bis-ANS. Mutations such as R61T and R112I diminish the EX1 kinetics, making apoE4 behave more like apoE3. Thus, the amino acid substitution at position 112 alters the structural dynamics of the N-terminal domain of apoE4; the changes are further propagated and amplified in the full-length protein. We conclude that HDX-MS is a label-free and robust methodology to characterize structural heterogeneities of proteins even under native conditions. This opens opportunities for screening of the "structure corrector" drug molecules that could convert apoE4 to apoE3-like.

Histidine-rich calcium-binding protein: a molecular integrator of cardiac excitation-contraction coupling

During mammalian cardiomyocyte excitation-contraction coupling, Ca2+ influx through voltage-gated Ca2+ channels triggers Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptor channels. This Ca2+-induced Ca2+ release mechanism controls cardiomyocyte contraction and is exquisitely regulated by SR Ca2+ levels. The histidine-rich calcium-binding protein (HRC) and its aspartic acid-rich paralogue aspolin are high-capacity, low-affinity Ca2+-binding proteins. Aspolin also acts as a trimethylamine N-oxide demethylase. At low intraluminal Ca2+ concentrations, HRC binds to the SR Ca2+-ATPase 2, inhibiting its Ca2+-pumping activity. At high intraluminal Ca2+ levels, HRC interacts with triadin to reduce Ca2+ release through ryanodine receptor channels. This Review analyses the evolution of these Ca2+-regulatory proteins, to gain insights into their roles. It reveals that HRC homologues are present in chordates, annelid worms, molluscs, corals and sea anemones. In contrast, triadin appears to be a chordate innovation. Furthermore, HRC is evolving more rapidly than other cardiac excitation-contraction coupling proteins. This positive selection (or relaxed negative selection) occurs along most of the mammalian HRC protein sequence, with the exception being the C-terminal cysteine-rich region, which is undergoing negative selection. The histidine-rich region of HRC might be involved in pH sensing, as an adaptation to air-breathing, endothermic and terrestrial life. In addition, a cysteine-rich pattern within HRC and aspolin is also found in a wide range of iron-sulfur cluster proteins, suggesting roles in redox reactions and metal binding. The polyaspartic regions of aspolins are likely to underlie their trimethylamine N-oxide demethylase activity, which might be mimicked by the acidic regions of HRCs. These potential roles of HRCs and aspolins await experimental verification.

The gut microbe-derived metabolite trimethylamine-N-oxide induces aortic valve fibrosis via PERK/ATF-4 and IRE-1α/XBP-1s signaling in vitro and in vivo

BACKGROUND AND AIMS

The gut microbe-derived metabolite trimethylamine-N-oxide (TMAO) has been implicated in the development of cardiovascular fibrosis. Endoplasmic reticulum (ER) stress occurs after the dysfunction of ER and its structure. The three signals PERK/ATF-4, IRE-1α/XBP-1s and ATF6 are activated upon ER stress. Recent reports have suggested that the activation of PERK/ATF-4 and IRE-1α/XBP-1s signaling contributes to cardiovascular fibrosis. However, whether TMAO mediates aortic valve fibrosis by activating PERK/ATF-4 and IRE-1α/XBP-1s signaling remains unclear.

METHODS

Human aortic valve interstitial cells (AVICs) were isolated from aortic valve leaflets. PERK IRE-1α, ATF-4, XBP-1s and CHOP expression, and production of collagen Ⅰ and TGF-β1 were analyzed following treatment with TMAO. The role of PERK/ATF-4 and IRE-1α/XBP-1s signaling pathways in TMAO-induced fibrotic formation was determined using inhibitors and small interfering RNA.

RESULTS

Diseased valves produced greater levels of ATF-4, XBP-1, collagen Ⅰ and TGF-β1. Interestingly, diseased cells exhibited augmented PERK/ATF-4 and IRE-1α/XBP-1s activation after TMAO stimulation. Inhibition and silencing of PERK/ATF-4 and IRE-1α/XBP-1s each resulted in enhanced suppression of TMAO-induced fibrogenic activity in diseased cells. Mice treated with dietary choline supplementation had substantially increased TMAO levels and aortic valve fibrosis, which were reduced by 3,3-dimethyl-1-butanol (DMB, an inhibitor of trimethylamine formation) treatment. Moreover, a high-choline and high-fat diet remodeled the gut microbiota in mice.

CONCLUSIONS

TMAO promoted aortic valve fibrosis through activation of PERK/ATF-4 and IRE-1α/XBP-1s signaling pathways in vitro and in vivo. Modulation of diet, gut microbiota, TMAO, PERK/ATF-4 and IRE1-α/XBP-1s may be a promising approach to prevent aortic valve fibrosis.

Structurally different chemical chaperones show similar mechanical roles with independent molecular mechanisms

Osmolytes are well known to protect the protein structure against different chemical and physical denaturants. Since their actions with protein surfaces are mechanistically complicated and context dependent, the underlying molecular mechanism is not fully understood. Here, we combined single-molecule magnetic tweezers and molecular dynamics (MD) simulation to explore the mechanical role of osmolytes from two different classes, trimethylamine N-oxide (TMAO) and trehalose, as mechanical stabilizers of protein structure. We observed that these osmolytes increase the protein L mechanical stability by decreasing unfolding kinetics while accelerating the refolding kinetics under force, eventually shifting the energy landscape toward the folded state. These osmolytes mechanically stabilize the protein L and plausibly guide them to more thermodynamically robust states. Finally, we observed that osmolyte-modulated protein folding increases mechanical work output up to twofold, allowing the protein to fold under a higher force regime and providing a significant implication for folding-induced structural stability in proteins.

Understanding the heterogeneity intrinsic to protein folding

Relating the native fold of a protein to its amino acid sequence remains a fundamental problem in biology. While computer algorithms have demonstrated recently their prowess in predicting what structure a particular amino acid sequence will fold to, an understanding of how and why a specific protein fold is achieved remains elusive. A major challenge is to define the role of conformational heterogeneity during protein folding. Recent experimental studies, utilizing time-resolved FRET, hydrogen-exchange coupled to mass spectrometry, and single-molecule force spectroscopy, often in conjunction with simulation, have begun to reveal how conformational heterogeneity evolves during folding, and whether an intermediate ensemble of defined free energy consists of different sub-populations of molecules that may differ significantly in conformation, energy and entropy.

Identifying the key factors affecting the trimethylamine N-oxide content of teleost fishes collected from the marginal seas of China and the epipelagic zone of the northwest Pacific Ocean

To identify the key factors influencing the trimethylamine N-oxide (TMAO) content of teleost fishes living in shallow seas and the epipelagic zone of the deep ocean, the muscle TMAO content was measured in 152 teleost fishes (21 species) collected from the marginal seas of China and the epipelagic zone of the northwest Pacific Ocean (NWPO) during May-July 2021. The results showed that the TMAO content in all fishes varied from 4.99 to 82.97 mmol kg-1, and it varied notably among different species, e.g., the highest average content (72.71 ± 8.22 mmol kg-1 in Argyrosomus argentatus) was 1 order of magnitude higher than the lowest one (Scomber japonicus), but the ratios of the highest content to the lowest content in each species varied from 1.29 to 3.28, suggesting that the interspecific variations in TMAO content were obviously greater than the intraspecific variations. Moreover, no correlation was observed between the TMAO content of the 152 fishes and the corresponding environmental factors of seawater depth, salinity and temperature, indicating that species played a more important role than environmental factors in driving TMAO accumulation. To exclude the influence of species, intraspecies correlations between TMAO content and environmental factors were analyzed. In the marginal seas of China, only ∼8 % of the TMAO content of teleost fishes (1 species) showed a positive correlation with salinity and depth, but ∼50 % of the TMAO content (5 species) was negatively correlated with temperature. Moreover, the TMAO content of the fish increased by 4.66 ± 1.38 % compared with their corresponding intraspecific average values for every 1 °C of temperature decrease. A similar phenomenon was also found in the TMAO content of pelagic teleost fishes in the NWPO, suggesting that temperature was a key environmental factor affecting the TMAO content of teleost fishes in shallow seas and the epipelagic zone of the deep ocean.

Hydrogen Deuterium Exchange and other Mass Spectrometry-based Approaches for Epitope Mapping

Antigen-antibody interactions are a fundamental subset of protein-protein interactions responsible for the "survival of the fittest". Determining the interacting interface of the antigen, called an epitope, and that on the antibody, called a paratope, is crucial to antibody development. Because each antigen presents multiple epitopes (unique footprints), sophisticated approaches are required to determine the target region for a given antibody. Although X-ray crystallography, Cryo-EM, and nuclear magnetic resonance can provide atomic details of an epitope, they are often laborious, poor in throughput, and insensitive. Mass spectrometry-based approaches offer rapid turnaround, intermediate structural resolution, and virtually no size limit for the antigen, making them a vital approach for epitope mapping. In this review, we describe in detail the principles of hydrogen deuterium exchange mass spectrometry in application to epitope mapping. We also show that a combination of MS-based approaches can assist or complement epitope mapping and push the limit of structural resolution to the residue level. We describe in detail the MS methods used in epitope mapping, provide our perspective about the approaches, and focus on elucidating the role that HDX-MS is playing now and in the future by organizing a discussion centered around several improvements in prototype instrument/applications used for epitope mapping. At the end, we provide a tabular summary of the current literature on HDX-MS-based epitope mapping.

Age and APOE affect L-carnitine system metabolites in the brain in the APOE-TR model

With age the apolipoprotein E (APOE) E4 allele (involved in lipid homeostasis) is associated with perturbation of bioenergetics pathways in Alzheimer's disease (AD). We therefore hypothesized that in aging mice APOE genotype would affect the L-carnitine system (central to lipid bioenergetics), in the brain and in the periphery. Using liquid chromatography-mass spectrometry, levels of L-carnitine and associated metabolites: γ-butyrobetaine (GBB), crotonobetaine, as well as acylcarnitines, were evaluated at 10-, 25-, and 50-weeks, in the brain and the periphery, in a targeted replacement mouse model of human APOE (APOE-TR). Aged APOE-TR mice were also orally administered 125 mg/kg of L-carnitine daily for 7 days followed by evaluation of brain, liver, and plasma L-carnitine system metabolites. Compared to E4-TR, an age-dependent increase among E2- and E3-TR mice was detected for medium- and long-chain acylcarnitines (MCA and LCA, respectively) within the cerebrovasculature and brain parenchyma. While following L-carnitine oral challenge, E4-TR mice had higher increases in the L-carnitine metabolites, GBB and crotonobetaine in the brain and a reduction of plasma to brain total acylcarnitine ratios compared to other genotypes. These studies suggest that with aging, the presence of the E4 allele may contribute to alterations in the L-carnitine bioenergetic system and to the generation of L-carnitine metabolites that could have detrimental effects on the vascular system. Collectively the E4 allele and aging may therefore contribute to AD pathogenesis through aging-related lipid bioenergetics as well as cerebrovascular dysfunctions.

Differentiating between the sequence of structural events on alternative pathways of folding of a heterodimeric protein

Distinguishing between competing pathways of folding of a protein, on the basis of how they differ in their progress of structure acquisition, remains an important challenge in protein folding studies. A previous study had shown that the heterodimeric protein, double chain monellin (dcMN) switches between alternative folding pathways upon a change in guanidine hydrochloride (GdnHCl) concentration. In the current study, the folding of dcMN has been characterized by the pulsed hydrogen exchange (HX) labeling methodology used in conjunction with mass spectrometry. Quantification of the extent to which folding intermediates accumulate and then disappear with time of folding at both low and high GdnHCl concentrations, where the folding pathways are known to be different, shows that the folding mechanism is describable by a triangular three-state mechanism. Structural characterization of the productive folding intermediates populated on the alternative pathways has enabled the pathways to be differentiated on the basis of the progress of structure acquisition that occurs on them. The intermediates on the two pathways differ in the extent to which the α-helix and the rest of the β-sheet have acquired structure that is protective against HX. The major difference is, however, that β2 has not acquired any protective structure in the intermediate formed on one pathway, but it has acquired significant protective structure in the intermediate formed on the alternative pathway. Hence, the sequence of structural events is different on the two alternative pathways.

Salt stress resilience in plants mediated through osmolyte accumulation and its crosstalk mechanism with phytohormones

Salinity stress is one of the significant abiotic stresses that influence critical metabolic processes in the plant. Salinity stress limits plant growth and development by adversely affecting various physiological and biochemical processes. Enhanced generation of reactive oxygen species (ROS) induced via salinity stress subsequently alters macromolecules such as lipids, proteins, and nucleic acids, and thus constrains crop productivity. Due to which, a decreasing trend in cultivable land and a rising world population raises a question of global food security. In response to salt stress signals, plants adapt defensive mechanisms by orchestrating the synthesis, signaling, and regulation of various osmolytes and phytohormones. Under salinity stress, osmolytes have been investigated to stabilize the osmotic differences between the surrounding of cells and cytosol. They also help in the regulation of protein folding to facilitate protein functioning and stress signaling. Phytohormones play critical roles in eliciting a salinity stress adaptation response in plants. These responses enable the plants to acclimatize to adverse soil conditions. Phytohormones and osmolytes are helpful in minimizing salinity stress-related detrimental effects on plants. These phytohormones modulate the level of osmolytes through alteration in the gene expression pattern of key biosynthetic enzymes and antioxidative enzymes along with their role as signaling molecules. Thus, it becomes vital to understand the roles of these phytohormones on osmolyte accumulation and regulation to conclude the adaptive roles played by plants to avoid salinity stress.

Trimethylamine N-oxide promotes atherosclerosis via regulating the enriched abundant transcript 1/miR-370-3p/signal transducer and activator of transcription 3/flavin-containing monooxygenase-3 axis

Atherosclerosis (AS) is one of the main causes of cardiovascular diseases (CVDs). Trimethylamine N-oxide (TMAO) exacerbates the development of AS. This study aimed to investigate the roles of TMAO in AS. In this study, mice were fed with high fat food (HF) and/or injected with TMAO. Oil red O staining was applied for histological analysis. ELISA, qRT-PCR, and Western blot were conducted to determine the TMAO, serum, mRNA, and protein levels. CCK-8, colony formation assay, and flow cytometry assays were performed to detect the functions of human aortic endothelial cells (HUVECs). The results showed that TMAO induced thick internal and external walls and intimal plaques in vivo, and HUVEC dysfunction in vitro. TMAO and lncRNA enriched abundant transcript 1 (NEAT1) were increased in AS clinical samples and TMAO-HUVECs. Downregulated NEAT1 inhibited proliferation and promoted the apoptosis of HUVECs. NEAT1 regulated the expression of signal transducer and activator of transcription 3 (STAT3) via sponging miR-370-3p. Overexpression of miR-370-3p facilitated the effects of NEAT1 on the cellular functions of HUVECs, while STAT3 exerted opposing effects. The activation of STAT3 promoted the expression of flavin-containing monooxygenase-3 (FMO3). Taken together, our results show that TMAO-NEAT1/miR-370-3p/STAT3/FMO3 forms a positive feedback loop to exacerbate the development of AS. This novel feedback loop may be a promising therapeutic target for AS.

Thermoresponsivity of poly(N-isopropylacrylamide) microgels in water-trehalose solution and its relation to protein behavior

HYPOTHESES

Additives are commonly used to tune macromolecular conformational transitions. Among additives, trehalose is an excellent bioprotectant and among responsive polymers, PNIPAM is the most studied material. Nevertheless, their interaction mechanism so far has only been hinted without direct investigation, and, crucially, never elucidated in comparison to proteins. Detailed insights would help understand to what extent PNIPAM microgels can effectively be used as synthetic biomimetic materials, to reproduce and study, at the colloidal scale, isolated protein behavior and its sensitivity to interactions with specific cosolvents or cosolutes.

EXPERIMENTS

The effect of trehalose on the swelling behavior of PNIPAM microgels was monitored by dynamic light scattering; Raman spectroscopy and molecular dynamics simulations were used to explore changes of solvation and dynamics across the swelling-deswelling transition at the molecular scale.

FINDINGS

Strongly hydrated trehalose molecules develop water-mediated interactions with PNIPAM microgels, thereby preserving polymer hydration below and above the transition while drastically inhibiting local motions of the polymer and of its hydration shell. Our study, for the first time, demonstrates that slowdown of dynamics and preferential exclusion are the principal mechanisms governing trehalose effect on PNIPAM microgels, at odds with preferential adsorption of alcohols, but in full analogy with the behavior observed in trehalose-protein systems.

Structural Characterization of the Cooperativity of Unfolding of a Heterodimeric Protein using Hydrogen Exchange-Mass Spectrometry

Little is known about how the sequence of structural changes in one chain of a heterodimeric protein is coupled to those in the other chain during protein folding and unfolding reactions, and whether individual secondary structural changes in the two chains occur in one or many coordinated steps. Here, the unfolding mechanism of a small heterodimeric protein, double chain monellin, has been characterized using hydrogen exchange-mass spectrometry. Transient structure opening, which enables HX, was found to be describable by a five state N ↔ I₁ ↔ I₂ ↔ I₃ ↔ U mechanism. Structural changes occur gradually in the first three steps, and cooperatively in the last step. β strands 2, 4 and 5, as well as the α-helix undergo transient unfolding during all three non-cooperative steps, while β1 and the two loops on both sides of the helix undergo transient unfolding during the first two steps. In the absence of GdnHCl, only β3 in chain A of the protein unfolds during the last cooperative step, while in the presence of 1 M GdnHCl, not only β3, but also β2 in chain B unfolds cooperatively. Hence, the extent of cooperative structural change and size of the cooperative unfolding unit increase when the protein is destabilized by denaturant. The naturally evolved two-chain variant of monellin folds and unfolds in a more cooperative manner than does a single chain variant created artificially, suggesting that increasing folding cooperativity, even at the cost of decreasing stability, may be a driving force in the evolution of proteins.

Trimethylamine N-oxide is a new plant molecule that promotes abiotic stress tolerance

Trimethylamine N-oxide (TMAO) is a well-known naturally occurring osmolyte in animals that counteracts the effect of different denaturants related to environmental stress and has recently been associated with severe human chronic diseases. In plants, however, the presence of TMAO has not yet been reported. In this study, we demonstrate that plants contain endogenous levels of TMAO, that it is synthesized by flavin-containing monooxygenases, and that its levels increase in response to abiotic stress conditions. In addition, our results reveal that TMAO operates as a protective osmolyte in plants, promoting appropriate protein folding and as an activator of abiotic stress-induced gene expression. Consistent with these functions, we show that TMAO enhances plant adaptation to low temperatures, drought, and high salt. We have thus uncovered a previously unidentified plant molecule that positively regulates abiotic stress tolerance.

Protecting thermodynamic stability of protein: The basic paradigm against stress and unfolded protein response by osmolytes

Organic osmolytes are known to play important role in stress protection by stabilizing macromolecules and suppressing harmful effects on functional activity. There is existence of several reports in the literature regarding their effects on structural, functional and thermodynamic aspects of many enzymes and the interaction parameters with proteins have been explored. Osmolytes are compatible with enzyme function and therefore, can be accumulated up to several millimolar concentrations. From the thermodynamic point of view, osmolyte raises mid-point of thermal denaturation (T_m) of proteins while having no significant effect on ΔG_D° (free energy change at physiological condition). Unfavorable interaction with the peptide backbone due to preferential hydration is the major driving force for folding of unfolded polypeptide in presence of osmolyte. However, the thermodynamic basis of stress protection and origin of compatibility paradigm has been a debatable issue. In the present manuscript, we attempt to elaborate the origin of stress protection and compatibility paradigm of osmolytes based on the effect on thermodynamic stability of proteins. We also infer that protective effects of osmolytes on ΔG_D° (of proteins) could also indicate its potential involvement in unfolded protein response and overall stress biology on macromolecular level.

Trimethylamine But Not Trimethylamine Oxide Increases With Age in Rat Plasma and Affects Smooth Muscle Cells Viability

It has been suggested that trimethylamine oxide (TMAO), a liver oxygenation product of gut bacteria-produced trimethylamine (TMA), is a marker of cardiovascular risk. However, mechanisms of the increase and biological effects of TMAO are obscure. Furthermore, the potential role of TMAO precursor, that is TMA, has not been investigated. We evaluated the effect of age, a cardiovascular risk factor, on plasma levels of TMA and TMAO, gut bacteria composition, gut-to-blood penetration of TMA, histological and hemodynamic parameters in 3-month-old and 18-month-old, male, Sprague-Dawley and Wistar-Kyoto rats. Cytotoxicity of TMA and TMAO was studied in human vascular smooth muscle cells. Older rats showed significantly different gut bacteria composition, a significantly higher gut-to-blood TMA penetration, and morphological and hemodynamic alterations in intestines. In vitro, TMA at concentration of 500 µmol/L (2-fold higher than in portal blood) decreased human vascular smooth muscle cells viability. In contrast, TMAO at 1,000-fold higher concentration than physiological one had no effect on human vascular smooth muscle cells viability. In conclusion, older rats show higher plasma level of TMA due to a "leaky gut". TMA but not TMAO affects human vascular smooth muscle cells viability. We propose that TMA but not TMAO may be a marker and mediator of cardiovascular risk.

TMA, A Forgotten Uremic Toxin, but Not TMAO, Is Involved in Cardiovascular Pathology

Trimethylamine-N-oxide (TMAO) has been suggested as a marker and mediator of cardiovascular diseases. However, data are contradictory, and the mechanisms are obscure. Strikingly, the role of the TMAO precursor trimethylamine (TMA) has not drawn attention in cardiovascular studies even though toxic effects of TMA were proposed several decades ago. We assessed plasma TMA and TMAO levels in healthy humans (HH) and cardiovascular patients qualified for aortic valve replacement (CP). The cytotoxicity of TMA and TMAO in rat cardiomyocytes was evaluated using an MTT test. The effects of TMA and TMAO on albumin and lactate dehydrogenase (LDH) were assessed using fluorescence correlation spectroscopy. In comparison to HH, CP had a two-fold higher plasma TMA (p < 0.001) and a trend towards higher plasma TMAO (p = 0.07). In CP plasma, TMA was inversely correlated with an estimated glomerular filtration rate (eGFR, p = 0.002). TMA but not TMAO reduced cardiomyocytes viability. Incubation with TMA but not TMAO resulted in the degradation of the protein structure of LDH and albumin. In conclusion, CP show increased plasma TMA, which is inversely correlated with eGFR. TMA but not TMAO exerts negative effects on cardiomyocytes, likely due to its disturbing effect on proteins. Therefore, TMA but not TMAO may be a toxin and a marker of cardiovascular risk.

The Microbial Metabolite Trimethylamine N-Oxide Links Vascular Dysfunctions and the Autoimmune Disease Rheumatoid Arthritis

Diet and microbiota each have a direct impact on many chronic, inflammatory, and metabolic diseases. As the field develops, a new perspective is emerging. The effects of diet may depend on the microbiota composition of the intestine. A diet that is rich in choline, red meat, dairy, or egg may promote the growth, or change the composition, of microbial species. The microbiota, in turn, may produce metabolites that increase the risk of cardiovascular disease. This article reviews our current understanding of the effects of the molecule trimethylamine-N-oxide (TMAO) obtained from food or produced by the microbiota. We review the mechanisms of actions of TMAO, and studies that associate it with cardiovascular and chronic kidney diseases. We introduce a novel concept: TMAO is one among a group of selective uremic toxins that may rise to high levels in the circulation or accumulate in various organs. Based on this information, we evaluate how TMAO may harm, by exacerbating inflammation, or may protect, by attenuating amyloid formation, in autoimmune diseases such as rheumatoid arthritis.

Is increased plasma TMAO a compensatory response to hydrostatic and osmotic stress in cardiovascular diseases?

Recent clinical studies show a positive correlation between elevated plasma TMAO and increased cardiovascular risk. However, the mechanism of the increase and biological effects of TMAO in the circulatory system are obscure. Plasma TMAO level depends mostly on the following three factors. First, the liver produces TMAO from TMA, a gut bacteria metabolite of dietary choline and carnitine. Second, plasma TMAO increases after ingestion of dietary TMAO from fish and seafood. Finally, plasma TMAO depends on TMAO and TMA excretion by the kidneys. Ample evidence highlights protective functions of TMAO, including the stabilization of proteins and cells exposed to hydrostatic and osmotic stresses, for example in fish exposed to hydrostatic stress (deep water) and osmotic stress (salty water). Osmotic stress and hydrostatic stresses are augmented in cardiovascular diseases such as hypertension. In hypertensive subjects a diastole-systole change in hydrostatic pressure in the heart may exceed 220 mmHg with a frequency of 60-220/min. This produces environment in which hydrostatic pressure changes over 100,000 times per 24 h. Furthermore, cardiovascular diseases are associated with disturbances in water-electrolyte balance which produce changes in plasma osmolarity. Perhaps, the increase in plasma TMAO in cardiovascular diseases is analogous to increased level of plasma natriuretic peptide B, which is both a cardiovascular risk marker and a compensatory response producing beneficial effects for pressure/volume overloaded heart. In this regard, there is some evidence that a moderate increase in plasma TMAO due to TMAO supplementation may be beneficial in animal model of hypertension-related heart failure. Finally, increased plasma TMAO is present in humans consuming seafood-rich diet which is thought to be health-beneficial. We hypothesize that increased plasma TMAO serves as a compensatory response mechanism which protects cells from hydrostatic and osmotic stresses.

Protective Effect of Tunisian Flaxseed Oil against Bleomycin-Induced Pulmonary Fibrosis in Rats

The aims of this study are to investigate the preventive effect of flaxseed oil (FO) on bleomycin (BLM)-induced pulmonary fibrosis (PF). Thirty adult male Wistar rats (180-220 g) were randomly divided into three groups. The control group (G1) received no treatment, the group (G2) received only intratracheally BLM, and the group (G3) received FO (2 mL/kg body weight) once a day for 60 days + BLM (4 mg/kg body weight "bw"). Our results demonstrated that FO protected against BLM-induced PF, by increasing proline, fructose, glucose, glyceride, choline, lactate, and malate metabolites in bronchoalveolar lavage fluid (Balf) which are involved in anti-inflammatory reactions. Also, FO-treatment reduced the score of fibrosis and the inflammatory index and revealed a decrease in tumor growth factor beta (TGFβ) density in alveoli, inflammatory infiltrate and fibrocytes, comparatively to the BLM group. As well, our data demonstrated that acute BLM-induced fibrosis was accompanied by an oxidative stress in lung tissue as assessed by an increase of lipid peroxidation as well as antioxidant enzyme activities depletion such as superoxide dismutase (SOD) and catalase (CAT). The FO treatment reversed all disturbances of BLM-induced oxidative stress parameters, and increased fatty acids levels promoting anti-inflammatory reactions especially in erythrocytes (linoleic, α-linolenic, docosapentaenoic acids).

Unfoldness of the denatured state of proteins determines urea: Methylamine counteraction in terms of Gibbs free energy of stabilization

In many tissues and organisms, large amount of urea gets accumulated to maintain osmotic balance. To evade the threatening impact of urea, living organisms accumulate methylamines, a class of osmolytes, in proportion of 2:1 (urea:methylamine). To understand underlying cause(s) for protein-specific counteraction behavior, thermodynamic stability (ΔG_D^o) of three disulfide free proteins (myoglobin, bovine cytochrome c and barstar) in the mixture of urea and methylamine has been estimated from guanidinium chloride-(GdmCl) driven denaturation curves. Using the experimentally measured values of ΔG_D^o obtained in the presence of individual methylamines and urea, we predicted the molar ratio of urea and a methylamine required for perfect compensation for each of the proteins. Interestingly, for all proteins studied, a similar ratio has been observed for perfect compensation. The predicted ratio for perfect compensation in terms of thermodynamic parameters was about 2:1 M ratio of urea to methylamine. Furthermore, a partial counteraction was observed in the myoglobin and barstar. However, for bovine cytochrome c, perfect compensation was observed in both GdmCl- and heat-driven denaturations. Our observations clearly suggest that the counteraction phenomenon depends on the extent of the unfolding of the denatured states of proteins.