Bridging the gap between maleate hydratase, citraconase and isopropylmalate isomerase: Insights into the single broad-specific enzyme.

Developing a microbial chassis with efficient enzymes is key to the synthesis of products by metabolic engineering. The wide distribution of desired pathway enzymes across several species and categories is posing major challenges in screening and selection of the same for pathway reconstruction. One such key enzyme is isopropylmalate isomerase (IPMI) of leucine/isoleucine biosynthetic pathway. The enzymes reported earlier as citraconase and maleate hydratase in Arthrobacter sp. and Pseudomonas sp. respectively, were found to have the characteristics of IPMI. If a systematic study is undertaken to show that these orphan enzymes indeed are part of the aconitase family of enzymes, these reported ones will add to the repertoire of enzymes available for branch-chained amino acid pathway engineering. This work is focused on functional characterisation of the enzymes citraconase and maleate hydratase based on the properties of IPMI. The partially sequenced gene of maleate hydratase reported earlier served as a template to identify the respective genes in these organisms which is found to be that of IPMI with conserved regions in the active site. The native enzymes and the IPMI of A. globiformis and P. pseudoalcaligenes, expressed in E. coli acted upon all the substrates in the forward direction comprising of D-citramalate, citraconate & D-erythro-3-methylmalate. In the reverse direction all the enzymes converted citraconate to D-citramalate with high activity. The estimated equilibrium ratio was same for both the native enzyme and the over-expressed IPMI which is 96:1.5:2.5 for D-citramalate: citraconate: D-erythro-3-methylmalate. The iron requirement for both enzymes which is characteristic of IPMI is ascertained by chelation and reconstitution of the same. Therefore, this work elucidated the broad specificity and the reactions in equilibrium catalysed by these enzymes like that of IPMI, paving way for the integration of these two efficient candidates into aconitase family of enzymes facilitating pathway engineering.

Altered Lysosomal Function Manipulates Cellular Biosynthetic Capacity By Remodeling Intracellular Cholesterol Distribution.

Lysosomes are known to influence cholesterol trafficking into endoplasmic reticulum (ER) membranes. Though intracellular cholesterol levels are known to influence the lipid biosynthetic responses in ER, the specific effects of lysosomal modulation on these outcomes is not known. To demonstrate this, C2C12 cells were treated with chloroquine, a lysosomotropic agent, and its effects on cellular biosynthetic capacity, structural and functional status of ER was determined. In addition to its known effects on autophagy reduction, chloroquine treatment induced accumulation of total cellular lipid and ER-specific cholesterol content. It was also observed that chloroquine caused an increase in smooth-ER content with defects in overall protein turnover. Further, since ER and mitochondria function in close association through ER membrane contact sites, it is likely that lysosomal modulation also brings about associated changes in mitochondria. In this regard, we found that chloroquine reduces mitochondrial membrane potential and mitochondrial dynamics. Collectively, the differential biosynthetic response of rise in lipid content, but not protein content, cannot be accounted by merely considering that chloroquine induced suppression of autophagy causes defects in organelle function. In this defective autophagy scenario, both biosynthetic responses such as lipid and protein synthesis are expected to be reduced rather than only the latter, as observed with chloroquine. These findings suggest that cholesterol trafficking/distribution within cellular organelles could act as an intracellular mediator of differential biosynthetic remodelling in interconnected organelles.

Differential Expression of miRNA-192 is a Potential Biomarker for HIV Associated Immune Recovery Uveitis.

PURPOSE: Notwithstanding well-established clinical features of Immune Recovery Uveitis (IRU), specific diagnostic tools to identify at-risk patients are lacking. Identification of biomarkers for IRU prediction can allow high-risk patients to benefit from specific preventive strategies, development of therapies, and elucidate immune reconstitution associated pathogenesis. METHODS: HIV+ patients were classified into four groups (A, B, C and D) with and without ocular manifestations, with follow-up over a year. Patients' ocular parameters were examined and manifestations like uveitis and IRU noted. Selected miRNAs were investigated in PBMCs by using miRNA PCR assay. Bioinformatic analysis used miRNet to predict the targets of miRNA-192-5p and miRNA-543 and KOBAS for pathways. RESULTS: Hsa-miR-192-5p and hsa-miR-543 levels were measured by qPCR using RNA isolated from PBMCs of HIVinfected patients. Hsa-miR-192-5p and hsa-miR-543 were down regulated in patients exhibiting ocular manifestations. Our results showed hsa-miR-192-5p (Group B vs D p 0.007) and hsa-miR-543 levels in PBMCs reliably distinguish between HIV patients diagnosed with IRU. Both miRNAs target multiple genes involved in inflammatory pathways as predicted by bioinformatic analysis. CONCLUSION: Decreased expression levels of miRNA-192 in patients with ocular manifestations and IRU, could facilitate identification of the status of the disease in HIV patients.

Revisiting Viral RNA-Dependent RNA Polymerases: Insights from Recent Structural Studies.

RNA-dependent RNA polymerases (RdRPs) represent a distinctive yet versatile class of nucleic acid polymerases encoded by RNA viruses for the replication and transcription of their genome. The structure of the RdRP is comparable to that of a cupped right hand consisting of fingers, palm, and thumb subdomains. Despite the presence of a common structural core, the RdRPs differ significantly in the mechanistic details of RNA binding and polymerization. The present review aims at exploring these incongruities in light of recent structural studies of RdRP complexes with diverse cofactors, RNA moieties, analogs, and inhibitors.

Sirtuin 6 inhibition protects against glucocorticoid-induced skeletal muscle atrophy by regulating IGF/PI3K/AKT signaling.

Chronic activation of stress hormones such as glucocorticoids leads to skeletal muscle wasting in mammals. However, the molecular events that mediate glucocorticoid-induced muscle wasting are not well understood. Here, we show that SIRT6, a chromatin-associated deacetylase indirectly regulates glucocorticoid-induced muscle wasting by modulating IGF/PI3K/AKT signaling. Our results show that SIRT6 levels are increased during glucocorticoid-induced reduction of myotube size and during skeletal muscle atrophy in mice. Notably, overexpression of SIRT6 spontaneously decreases the size of primary myotubes in a cell-autonomous manner. On the other hand, SIRT6 depletion increases the diameter of myotubes and protects them against glucocorticoid-induced reduction in myotube size, which is associated with enhanced protein synthesis and repression of atrogenes. In line with this, we find that muscle-specific SIRT6 deficient mice are resistant to glucocorticoid-induced muscle wasting. Mechanistically, we find that SIRT6 deficiency hyperactivates IGF/PI3K/AKT signaling through c-Jun transcription factor-mediated increase in IGF2 expression. The increased activation, in turn, leads to nuclear exclusion and transcriptional repression of the FoxO transcription factor, a key activator of muscle atrophy. Further, we find that pharmacological inhibition of SIRT6 protects against glucocorticoid-induced muscle wasting in mice by regulating IGF/PI3K/AKT signaling implicating the role of SIRT6 in glucocorticoid-induced muscle atrophy.

Nitric oxide donor 8-bromo-cGMP suppresses cholesterol depleted RBCs band 3 dysfunction.

Red blood cells (RBCs) carry large cholesterol fractions and imbalance in them leads to several vascular complications. RBCs band 3 protein plays an important role in maintaining membrane integrity and there are many reports on cholesterol and band 3 protein interaction. Yet, RBCs band 3 protein role in regulating cholesterol homeostasis needs to be investigated. In this study, we induced cholesterol-depletion and band 3 inhibition in RBCs; both of which cause stress by decreasing band 3 channel activity with an increase in RBCs adhesion to endothelial cells (EC) by elevating band 3 phosphorylation (Tyr21), methemoglobin level and decreasing nitric oxide level. We hypothesized that nitric oxide (NO), a prominent determinant for RBC structural stability, would protect RBCs from stressors. To estimate this, we used three NO donors (SpNO, Sildenafil citrate and 8-Bromo-cGMP) and found that all 3 NO donors were able to recover, with 8-Bromo-cGMP being the most effective as it not only increased band 3 channel activity but also decreased RBC-EC adhesiveness and methemoglobin level in both stressors. Whereas NO donor's treatment did not display an ameliorative impact when both stresses were combined. Overall, these findings may shed light on the role of 8-bromo-cGMP in regulating RBC cholesterol homeostasis by maintaining band 3 function. Further studies in this direction might help identify targets for the therapeutic use of NO donors in the treatment of blood disorders.

Detrimental effects of fructose on mitochondria in mouse motor neurons and on C. elegans healthspan.

BACKGROUND: Fructose-common sweetener, consumed in large quantities, is now known to be associated with various metabolic diseases. Recent reports suggest fructose's involvement in neurodegeneration, neurotoxicity, and neuroinflammation. But, its impact at cellular and subcellular level and on energy metabolism, especially, mitochondrial bioenergetics, in neurons is not known. OBJECTIVES: To study the adverse effects of high fructose in general, and on the mitochondria in a spinal cord motor neuron cell line, NSC-34, in vitro, and Caenorhabditis elegans in vivo. METHODS: NSC-34 was treated with 0.5%-5% of fructose for different time periods. Fructose's effect on cell viability (MTT assay), metabolic activity (XF24 Seahorse assays) and C. elegans, chronically fed with 5% fructose and alteration in healthspan/mitochondria was monitored. RESULTS: In NSC-34: Fructose at 4-5% elicits 60% cell death. Unlike 1%, 5% fructose (F5%) decreased mitochondrial membrane potential by 29%. Shockingly, 6hours F5% treatment almost abolished mitochondrial respiration - basal-respiration (∨123%), maximal-respiration (∨ 95%) and spare-respiratory-capacity (∨ 83%) and ATP production (∨98%) as revealed by XF 24- Seahorse assays. But non - mitochondrial respiration was spared. F5% treatment for 48hrs resulted in the total shutdown of respiratory machinery including glycolysis. Chronic feeding of wildtype C.elegans to F5% throughout, shortened lifespan by ~3 days (∨ 17%), progressively reduced movement (day-2 -∨10.25%, day-5 -∨25% and day-10 -∨56%) and food intake with age (day-5-∨9% and day-10 -∨48%) and instigated mitochondrial swelling and disarray in their arrangement in adult worms body-wall muscle cells. CONCLUSION: Chronic exposure to high fructose negatively impacts cell viability, mitochondrial function, basal glycolysis, and healthspan.

Does mTORC1 inhibit autophagy at dual stages?: A possible role of mTORC1 in late-stage autophagy inhibition in addition to its known early-stage autophagy inhibition.

Extensive studies have attributed the lysosomal localization of the mechanistic target of rapamycin complex 1 (mTORC1) during its activation. However, the exact biological significance of this lysosomal localization of mTORC1 remains ill-defined. Interestingly, findings have shown that localization of the lysosome itself is altered under conditions influencing mTORC1 activity. In this perspective, we hypothesize that the localization of mTORC1 and lysosome could be interconnected in a way that manifests regulation of autophagy that is already under progression at the time of mTORC1 activation. This provides a new possibility for autophagy regulation whose complete mechanistic insights remain to be determined.

Distinct utilization of biotin in and between adipose and brain during aging is associated with a lipogenic shift in Wistar rat brain.

Tissue-specific metabolism determines their functions that collectively sense and respond to numerous stress cues to achieve systemic homeostasis. Chronic stress skews such metabolic profiles and leads to failure of organs as evidenced by a bias towards lipid synthesis and storage in the aging brain, muscle, and liver under Alzheimer's disease, sarcopenia, and non-alcoholic fatty liver disease, respectively. In contrast, the tissue destined for lipid synthesis and storage, such as adipose, limits its threshold and develops diabetes mellitus. However, the underlying factors that contribute to this lipogenic shift between organs are unknown. From this perspective, differential biotin utilization between lipid-rich tissues such as adipose and brain during aging was hypothesized owing to the established role of biotin in lipogenesis. The same was tested using young and aged Wistar rats. We found that adipose-specific biotin content was much higher than the brain irrespective of aging status, as well as its associated cues. However, within tissues, the adipose fails to maintain its biotinylation levels during aging whereas the brain seizes more biotin and exhibits lipid accumulation. Furthermore, mimicking the age-related stress cues in vitro such as high glucose and endoplasmic reticulum stress deprive the astroglial biotin content, but not that of adipocytes. Lipid accumulation in the aging brain was also correlated with increased S-adenosylmethionine levels and biotin utilization by astrocytes. In summary, differential biotin utilization between adipose and brain under aging and their respective cell types like adipocytes and astrocytes under age-associated stress cues connects well with the lipogenic shift in rat brain.

Astrocytes resolve ER stress through mitochondrial fusion facilitated by biotin availability.

Structures of cellular organelles are intertwined with their functions that undergo alterations once the organelles are stressed. Since organelle functions are dependent on each other, an organelle-specific stress possibly influences the structure and function of its associated organelles. In this perspective, our study demonstrated that endoplasmic reticulum (ER)-specific stress induced by tunicamycin in primary astroglial culture is associated with altered mitochondrial dynamics and matched with the changes as observed in the aging rat brain. However, the exogenous addition of biotin, a highly lipogenic and mitochondrial vitamin, ameliorates ER stress even though its direct targets are not known within ER. Alternatively, the increased biotinylation of mitochondrial carboxylases preserves its basal respiratory capacity by upregulating mitofusin 2 (Mfn2) and, possibly, its associated role on mitochondrial fusion. Furthermore, the Mfn2 increase by biotin augments physical interaction between ER and functional mitochondria to exchange biomolecules as a part of ER stress resolution. This suggests an increased demand for micronutrient biotin under ER stress resolves the same by undergoing appropriate structural and metabolic contacts between ER and mitochondria. These findings provide a paradigm to resolve stress in one organelle by sustaining the metabolic commitments of another interdependent organelle. The findings also highlight a novel role of biotin in inducing Mfn2 expression and localization under ER stress in addition to its known role as a co-enzyme.

Astroglial biotin deprivation under endoplasmic reticulum stress uncouples BCAA-mTORC1 role in lipid synthesis to prolong autophagy inhibition in the aging brain.

Autophagy delays the onset of endoplasmic reticulum (ER) stress by recycling cellular debris. However, the cues that elicit autophagy under the emergence of ER stress and their dysregulation during aging remains obscure. Amino acids, notably branched-chain amino acids (BCAA), get accumulated in the cells once protein synthesis is inhibited by ER stress. The BCAA mimic satiety to inhibit autophagy via mechanistic targets of rapamycin complex 1 (mTORC1) activation and, in contrast, their catabolism supplements de novo lipogenesis for the formation of autophagosome membranes. Thus promoting BCAA utilization is hypothesized to induce autophagy to alleviate ER stress. Nevertheless, except protein synthesis, the rest of BCAA utilization and lipogenesis depends on the co-enzyme biotin. Hence, the levels of biotinylated carboxylases and lipids were assessed in the aging brain of Wistar rats. Despite the increased levels of biotinylated carboxylases and lipids, the aging brain accumulates BCAA. Since astrocytes are the primary site of BCAA and lipid metabolism and the increased expression of glial fibrillary acidic protein (GFAP) denotes astroglial ER stress, co-localization studies were performed to determine the extent of biotinylation in GFAP positive cells. Although total biotin intensity was higher in aged brain slices, the astrocytes specific decrease in biotinylation is attributed to BCAA accumulation, mTORC1 overactivation, autophagy inhibition, and ER stress in the aging brain. The ER stress in primary astrocytes using tunicamycin also mimic the in vivo phenotype. Biotin supplementation ameliorated the changes observed in vitro, corroborating the significance of astrocytes biotin availability to promote autophagy under ER stress in aging.

Autophagy inhibition by biotin elicits endoplasmic reticulum stress to differentially regulate adipocyte lipid and protein synthesis.

Biotin is an indispensable adipogenic agent, and its ability to coordinate carbohydrate, lipid, and amino acid metabolism sensitizes insulin signaling in adipocytes. This enables the organism to adapt and survive under nutrient stress by synthesis and storage of lipids. Biotin deficiency mimics insulin resistance with alterations in cellular intermediary metabolism. Though the mechanism of lipogenesis is well established across cell types, considering its predisposition to accumulate only lipids, it is necessary to elucidate the mechanism that minimizes the effects of biotin on adipocyte protein synthesis. In order to determine the differential metabolic phenotype by biotin, the primary cultures of adipocytes were induced to differentiate in the presence and absence of excess biotin. Serum pre-incubated with avidin was used to limit biotin availability in cultured cells. Biotin restricts cellular signaling associated with protein synthesis without altering total protein content. The decline in autophagy elicits endoplasmic reticulum stress to inhibit protein synthesis by eIF2α phosphorylation possibly via accumulation of misfolded/long-lived proteins. Furthermore, the compensatory increase in Unc51 like autophagy activating kinase 1 possibly competes with eukaryotic initiation factor 4E-binding protein 1 and ribosomal p70 S6kinase phosphorylation by mechanistic targets of rapamycin complex 1 to uncouple its effect on protein synthesis. In conclusion, autophagy inhibition by biotin uncouples protein synthesis to promote lipogenesis by eliciting endoplasmic reticulum stress and differential phosphorylation of mechanistic targets of rapamycin complex 1 substrates.

Ischemia/reperfusion injury in male guinea pigs: An efficient model to investigate myocardial damage in cardiovascular complications.

Myocardial ischemia/reperfusion (I/R) injury is the major problem that aggravates cardiac damage. Several established animal models fail to explain the similarity in disease mechanism and progression as seen in humans; whereas guinea pig shows high similarity in cardiovascular parameters. Hence, current study is aimed to develop an animal model using guinea pigs that may best correlate with disease mechanism of human myocardial I/R injury. Male guinea pigs were randomized into three groups: normal diet (ND), high fat diet (HFD) and sham; fed with respective diets for 90 days. Myocardial infarction (MI) was induced by ligating left anterior descending artery (LAD) for 30 min followed by 24 h and 7 days of reperfusion in ND and HFD groups. Electrocardiogram (ECG) showed the alterations in electrical conduction during myocardial I/R injury. Elevated levels of lactate dehydrogenase (LDH) and creatine kinase-MB ((CK-MB)) were higher in HFD compared to ND. Inflammatory markers such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) were up-regulated in I/R injury animals compared to sham. Fold change of these protein expression levels were higher in HFD compared to ND. Elevated lipid profile and increased aortic wall thickness in HFD animals depicts the risk of developing cardiovascular complications. ECG analysis strongly confirmed MI through changes in sinus rhythm that are reflected in infarcted tissue as verified through TTC staining. Thus the combination of HFD followed by I/R injury proved to be an efficient model to study pathophysiology of myocardial I/R injury with minimal tissue damage and surgical mortality.

5-hydroxytryptamine receptor stimulation of mitochondrial biogenesis.

Mitochondrial dysfunction is both a cause and target of reactive oxygen species during ischemia-reperfusion, drug, and toxicant injury. After injury, renal proximal tubular cells (RPTC) recover mitochondrial function by increasing the expression of the master regulator of mitochondrial biogenesis, peroxisome-proliferator-activated-receptor-gamma-coactivator-1alpha (PGC-1alpha). The goal of this study was to determine whether 5-hydroxytryptamine (5-HT) receptor agonists increase mitochondrial biogenesis and accelerate the recovery of mitochondrial function. Reverse transcription-polymerase chain reaction analysis confirmed the presence of 5-HT2A, 5-HT2B, and 5-HT2C receptor mRNA in RPTC. The 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI; 3-10 microM) increased PGC-1alpha levels, expression of mitochondrial proteins ATP synthase beta and NADH dehydrogenase (ubiquinone) 1beta subcomplex 8 (NDUFB8), MitoTracker Red staining intensity, cellular respiration, and ATP levels through a 5-HT receptor and PGC-1alpha-dependent pathway. Similar effects were observed with the 5-HT2 agonist m-chlorophenylpiperazine and were blocked by the 5-HT2 antagonist 8-[3-(4-fluorophenoxy) propyl]-1-phenyl-1,3,8-triazaspiro[4,5]decan-4-one (AMI-193). In addition, DOI accelerated the recovery of mitochondrial function after oxidant-induced injury in RPTC. This is the first report to demonstrate 5-HT receptor-mediated mitochondrial biogenesis, and we suggest that 5-HT-agonists may be effective in the treatment of mitochondrial and cell injury.

Protein oxidative damage in arsenic induced rat brain: influence of DL-alpha-lipoic acid.

A body of evidence has accumulated implicating free radical generation and reaction of arsenic with protein thiols in the biochemical and molecular mechanisms of arsenic toxicity. Brain readily undergoes oxidative damage, so it is important to determine whether arsenic-induced changes in rat brain may be associated with oxidative events. An increase in oxidative stress may contribute to the development of protein damage in rat brain. Present experiments were performed to study the effect of arsenic (sodium arsenite, 100 ppm mixed in drinking water) on protein oxidation and further to demonstrate the potential of dl-alpha-lipoic acid (70 mg/kg body weight) against arsenic-induced changes in different anatomic regions (cortex, striatum, cerebellum, hypothalamus and hippocampus) of the brain of male Wistar rats. We report here that arsenic treated rats had a significantly higher level of oxidised protein as assessed by increased carbonyl residues and decreased protein thiols (protein sulfhydryls) as compared to control rats in all five regions studied, with the most notable changes occurring in the cortex, striatum and hippocampus. Coadministration of lipoic acid along with arsenic resulted in reversal of the arsenic induced trends in carbonyl and sulfhydryl concentrations. The results of the study showed, lipoic acid treatment reduces oxidative protein damage in arsenic intoxicated rat brain regions, which is associated with its antioxidant activity that combines free radical scavenging and metal chelating properties.