The glucose metabolite methylglyoxal inhibits expression of the glucose transporter genes by inactivating the cell surface glucose sensors Rgt2 and Snf3 in yeast

Clinical, Neuroimaging, and Metabolic Footprint of the Neurodevelopmental Disorder Caused by Monoallelic HK1 Variants

Background and Objectives

Hexokinase 1 (encoded by HK1) catalyzes the first step of glycolysis, the adenosine triphosphate-dependent phosphorylation of glucose to glucose-6-phosphate. Monoallelic HK1 variants causing a neurodevelopmental disorder (NDD) have been reported in 12 individuals.

Methods

We investigated clinical phenotypes, brain MRIs, and the CSF of 15 previously unpublished individuals with monoallelic HK1 variants and an NDD phenotype.

Results

All individuals had recurrent variants likely causing gain-of-function, representing mutational hot spots. Eight individuals (c.1370C>T) had a developmental and epileptic encephalopathy with infantile onset and virtually no development. Of the other 7 individuals (n = 6: c.1334C>T; n = 1: c.1240G>A), 3 adults showed a biphasic course of disease with a mild static encephalopathy since early childhood and an unanticipated progressive deterioration with, e.g., movement disorder, psychiatric disease, and stroke-like episodes, epilepsy, starting in adulthood. Individuals who clinically presented in the first months of life had (near)-normal initial neuroimaging and severe cerebral atrophy during follow-up. In older children and adults, we noted progressive involvement of basal ganglia including Leigh-like MRI patterns and cerebellar atrophy, with remarkable intraindividual variability. The CSF glucose and the CSF/blood glucose ratio were below the 5th percentile of normal in almost all CSF samples, while blood glucose was unremarkable. This biomarker profile resembles glucose transporter type 1 deficiency syndrome; however, in HK1-related NDD, CSF lactate was significantly increased in all patients resulting in a substantially different biomarker profile.

Discussion

Genotype-phenotype correlations appear to exist for HK1 variants and can aid in counseling. A CSF biomarker profile with low glucose, low CSF/blood glucose, and high CSF lactate may point toward monoallelic HK1 variants causing an NDD. This can help in variant interpretation and may aid in understanding the pathomechanism. We hypothesize that progressive intoxication and/or ongoing energy deficiency lead to the clinical phenotypes and progressive neuroimaging findings.

Multiple roles for the cytoplasmic C-terminal domains of the yeast cell surface receptors Rgt2 and Snf3 in glucose sensing and signaling

The plasma membrane proteins Rgt2 and Snf3 are glucose sensing receptors (GSRs) that generate an intracellular signal for the induction of gene expression in response to high and low extracellular glucose concentrations, respectively. The GSRs consist of a 12-transmembrane glucose recognition domain and a cytoplasmic C-terminal signaling tail. The GSR tails are dissimilar in length and sequence, but their distinct roles in glucose signal transduction are poorly understood. Here, we show that swapping the tails between Rgt2 and Snf3 does not alter the signaling activity of the GSRs, so long as their tails are phosphorylated in a Yck-dependent manner. Attachment of the GSR tails to Hxt1 converts the transporter into a glucose receptor; however, the tails attached to Hxt1 are not phosphorylated by the Ycks, resulting in only partial signaling. Moreover, in response to non-fermentable carbon substrates, Rgt2 and Hxt1-RT (RT, Rgt2-tail) are efficiently endocytosed, whereas Snf3 and Hxt1-ST (ST, Snf3-tail) are endocytosis-impaired. Thus, the tails are important regulatory domains required for the endocytosis of the Rgt2 and Snf3 glucose sensing receptors triggered by different cellular stimuli. Taken together, these results suggest multiple roles for the tail domains in GSR-mediated glucose sensing and signaling.

Controlling circuitry underlies the growth optimization of Saccharomyces cerevisiae

Microbial growth emerges from coordinated synthesis of various cellular components from limited resources. In Saccharomyces cerevisiae, cyclic AMP (cAMP)-mediated signaling is shown to orchestrate cellular metabolism; however, it remains unclear quantitatively how the controlling circuit drives resource partition and subsequently shapes biomass growth. Here we combined experiment with mathematical modeling to dissect the signaling-mediated growth optimization of S. cerevisiae. We showed that, through cAMP-mediated control, the organism achieves maximal or nearly maximal steady-state growth during the utilization of multiple tested substrates as well as under perturbations impairing glucose uptake. However, the optimal cAMP concentration varies across cases, suggesting that different modes of resource allocation are adopted for varied conditions. Under settings with nutrient alterations, S. cerevisiae tunes its cAMP level to dynamically reprogram itself to realize rapid adaptation. Moreover, to achieve growth maximization, cells employ additional regulatory systems such as the GCN2-mediated amino acid control. This study establishes a systematic understanding of global resource allocation in S. cerevisiae, providing insights into quantitative yeast physiology as well as metabolic strain engineering for biotechnological applications.

Transcriptional Response of Candida auris to the Mrr1 Inducers Methylglyoxal and Benomyl

Candida auris is an urgent threat to human health due to its rapid spread in health care settings and its repeated development of multidrug resistance. Diseases that increase risk for C. auris infection, such as diabetes, kidney failure, or immunocompromising conditions, are associated with elevated levels of methylglyoxal (MG), a reactive dicarbonyl compound derived from several metabolic processes. In other Candida species, expression of MG reductase enzymes that catabolize and detoxify MG are controlled by Mrr1, a multidrug resistance-associated transcription factor, and MG induces Mrr1 activity. Here, we used transcriptomics and genetic assays to determine that C. auris MRR1a contributes to MG resistance, and that the main Mrr1a targets are an MG reductase and MDR1, which encodes a drug efflux protein. The C. auris Mrr1a regulon is smaller than Mrr1 regulons described in other species. In addition to MG, benomyl (BEN), a known Mrr1 stimulus, induces C. auris Mrr1 activity, and characterization of the MRR1a-dependent and -independent transcriptional responses revealed substantial overlap in genes that were differentially expressed in response to each compound. Additionally, we found that an MRR1 allele specific to one C. auris phylogenetic clade, clade III, encodes a hyperactive Mrr1 variant, and this activity correlated with higher MG resistance. C. auris MRR1a alleles were functional in Candida lusitaniae and were inducible by BEN, but not by MG, suggesting that the two Mrr1 inducers act via different mechanisms. Together, the data presented in this work contribute to the understanding of Mrr1 activity and MG resistance in C. auris. IMPORTANCE Candida auris is a fungal pathogen that has spread since its identification in 2009 and is of concern due to its high incidence of resistance against multiple classes of antifungal drugs. In other Candida species, the transcription factor Mrr1 plays a major role in resistance against azole antifungals and other toxins. More recently, Mrr1 has been recognized to contribute to resistance to methylglyoxal (MG), a toxic metabolic product that is often elevated in different disease states. MG can activate Mrr1 and its induction of Mdr1 which can protect against diverse challenges. The significance of this work lies in showing that MG is also an inducer of Mrr1 in C. auris, and that one of the major pathogenic C. auris lineages has an activating Mrr1 mutation that confers protection against MG.

Casein kinases are required for the stability of the glucose-sensing receptor Rgt2 in yeast

In yeast, glucose induction of HXT (glucose transporter gene) expression is achieved via the Rgt2 and Snf3 glucose sensing receptor (GSR)-mediated signal transduction pathway. The membrane-associated casein kinases Yck1 and Yck2 (Ycks) are involved in this pathway, but their exact role remains unclear. Previous work suggests that the Ycks are activated by the glucose-bound GSRs and transmit the glucose signal from the plasma membrane to the nucleus. However, here we provide evidence that the YCks are constitutively active and required for the stability of the Rgt2 receptor. Cell surface levels of Rgt2 are significantly decreased in a yck1Δyck2ts mutant, but this is not due to endocytosis-mediated vacuolar degradation of the receptor. Similar observations are made in an akr1Δ mutant, where the Ycks are no longer associated with the membrane, and in a sod1Δ mutant in which the kinases are unstable. Of note, in an akr1Δ mutant, both the Ycks and Rgt2 are mislocalized to the cytoplasm, where Rgt2 is stable and functions as an effective receptor for glucose signaling. We also demonstrate that Rgt2 is phosphorylated on the putative Yck consensus phosphorylation sites in its C-terminal domain (CTD) in a Yck-dependent manner and that this glucose-induced modification is critical for its stability and function. Thus, these results indicate a role for the Ycks in stabilizing Rgt2 and suggest that Rgt2 may use glucose binding as a molecular switch not to activate the Ycks but to promote Yck-dependent interaction and phosphorylation of the CTD that increases its stability.

mTOR-dependent dysregulation of autophagy contributes to the retinal ganglion cell loss in streptozotocin-induced diabetic retinopathy

BACKGROUND

Neurodegeneration, an early event in the pathogenesis of diabetic retinopathy (DR), precedes clinically detectable microvascular damage. Autophagy dysregulation is considered a potential cause of neuronal cell loss, however underlying mechanisms remain unclear. The mechanistic target of rapamycin (mTOR) integrates diverse environmental signals to coordinate biological processes, including autophagy. Here, we investigated the role of mTOR signaling in neuronal cell death in DR.

METHODS

Diabetes was induced by a single intraperitoneal injection of streptozotocin and tissue samples were harvested at 1, 2, 3, 4, and 6 months of diabetes. Early-stage of DR was investigated in 1-month-diabetic mice treated with phlorizin (two daily subcutaneous injections at a dose of 200 mg/kg of body weight during the last 7 full days of the experiment and the morning of the 8th day, 3 h before sacrifice) or rapamycin (daily intraperitoneal injections, at a dose of 3 mg/kg for the same period as for phlorizin treatment). The effect of autophagy modulation on retinal ganglion cells was investigated in 3-months-diabetic mice treated with phlorizin (two daily subcutaneous injections during the last 10 full days of the experiment and the morning of the 11th day, 3 h before sacrifice) or MHY1485 (daily i.p. injections, at a dose of 10 mg/kg for the same period as for phlorizin treatment). Tissue samples obtained from treated/untreated diabetic mice and age-matched controls were used for Western blot and histologic analysis.

RESULTS

mTOR-related proteins and glucose transporter 1 (GLUT1) was upregulated at 1 month and downregulated in the following period up to 6 months. Diabetes-induced neurodegeneration was characterized by an increase of apoptotic marker-cleaved caspase 3, a decrease of the total number of cells, and NeuN immunoreactivity in the ganglion cell layer, as well as an increase of autophagic protein. Insulin-independent glycemic control restored the mTOR pathway activity and GLUT1 expression, along with a decrease of autophagic and apoptotic proteins in 3-months-diabetic mice neuroretina. However, blockade of autophagy using MHY1485 resulted in a more protective effect on ganglion cells compared with phlorizin treatment.

CONCLUSION

Collectively, our study describes the mechanisms of neurodegeneration through the hyperglycemia/ mTOR/ autophagy/ apoptosis pathway. Video Abstract.

Ethanol tolerance assessment in recombinant E. coli of ethanol responsive genes from Lactobacillus buchneri NRRL B-30929

We previously identified specific proteins associated with ethanol stress response in a Lactobacillus buchneri strain capable of growing in 10% ethanol. In the current study, the exceptional roles of ethanol responsive genes are examined to determine if they can increase ethanol tolerance in E. coli host cells. The recombinant strains carrying ethanol responsive genes were subjected to growth analyses in media with and without 4% ethanol. Among the expression of these genes and growth analyses of the recombinant strains in ethanol, six genes Lbuc_0522 (NADPH-dependent methylglyoxal reductase), Lbuc_0354 (succinate semialdehyde dehydrogenase), Lbuc_1211(threonyl_tRNA synthetase), Lbuc_2051 (nitroreductase), Lbuc_0707 (branched chain amino acid aminotransferase) and Lbuc_1852 (proline-specific peptidase) conferred host cells tolerance to 4% ethanol. Six genes Lbuc_1523 (phage major capsid protein, HK 97 family), Lbuc_1319 (phosphoglycerate kinase), Lbuc_0787 encoding fumarylacetoacetate hydrolase, Lbuc_1219 encoding UDP-N-acetylmuramate-L-alanine ligase, Lbuc_0466 encoding ornithine carbamoyltransferase and Lbuc_0858 encoding glycine hydroxymethyltransferase showed no impact on growth in media with 4% ethanol with IPTG induction when compared with E. coli carrying control pET28b plasmid. The expression of two genes Lbuc_1557 (S-layer glycoprotein) and Lbuc_2157 (6-phosphogluconate dehydrogenase) resulted ethanol sensitivity phenotype.

Reactive Carbonyls Induce TOR- and Carbohydrate-Dependent Hormetic Response in Yeast

The induction of the beneficial and detrimental effects by reactive carbonyl species in yeast has been investigated. In this study, we have presented evidence that glyoxal and methylglyoxal at low concentrations were able to induce a hormetic adaptive response in glucose-grown but not fructose-grown yeast. The hormetic effect was also TOR-dependent. The mutation in genes encoding either TOR1 or TOR2 protein makes yeast highly sensitive to both α-dicarbonyls studied. Simultaneous disruption of TOR1 and TOR2 resulted in higher yeast sensitivity to the α-dicarbonyls as compared to parental cells, but double mutant survived better under carbonyl stress than its single mutant counterparts. The data obtained are consistent with the previous works which reported high toxicity of the α-dicarbonyls and extend them with the report on the beneficial TOR-dependent hormetic effect of glyoxal and methylglyoxal.

Phenylpyrrole fungicides act on triosephosphate isomerase to induce methylglyoxal stress and alter hybrid histidine kinase activity

Fludioxonil, a natural product of pyrrolnitrin, is a potent fungicide used on crops worldwide. Drug action requires the presence of a group III hybrid histidine kinase (HHK) and the high osmolarity glycerol (HOG) pathway. We have reported that the drug does not act directly on HHK, but triggers the conversion of the kinase to a phosphatase, which dephosphorylates Ypd1 to constitutively activate HOG signaling. Still, the direct drug target remains unknown and mode of action ill defined. Here, we heterologously expressed a group III HHK, dimorphism-regulating kinase 1 (Drk1) in Saccharomyces cerevisae to delineate fludioxonil’s target and action. We show that the drug interferes with triosephosphate isomerase (TPI) causing release of methylglyoxal (MG). MG activates the group III HHK and thus the HOG pathway. Drug action involved Drk1 cysteine 392, as a C392S substitution increased drug resistance in vivo. Drug sensitivity was reversed by dimedone treatment, indicating Drk1 responds in vivo to an aldehydic stress. Fludioxonil treatment triggered elevated cytosolic methylglyoxal. Likewise, methylglyoxal treatment of Drk1-expressing yeast phenocopied treatment with fludioxonil. Fludioxonil directly inhibited TPI and also caused it to release methylglyoxal in vitro. Thus, TPI is a drug target of the phenylpyrrole class of fungicides, inducing elevated MG which alters HHK activity, likely converting the kinase to a phosphatase that acts on Ypd1 to trigger HOG pathway activation and fungal cell death.

Trees, fungi and bacteria: tripartite metatranscriptomics of a root microbiome responding to soil contamination

BACKGROUND

One method for rejuvenating land polluted with anthropogenic contaminants is through phytoremediation, the reclamation of land through the cultivation of specific crops. The capacity for phytoremediation crops, such as Salix spp., to tolerate and even flourish in contaminated soils relies on a highly complex and predominantly cryptic interacting community of microbial life.

METHODS

Here, Illumina HiSeq 2500 sequencing and de novo transcriptome assembly were used to observe gene expression in washed Salix purpurea cv. 'Fish Creek' roots from trees pot grown in petroleum hydrocarbon-contaminated or non-contaminated soil. All 189,849 assembled contigs were annotated without a priori assumption as to sequence origin and differential expression was assessed.

RESULTS

The 839 contigs differentially expressed (DE) and annotated from S. purpurea revealed substantial increases in transcripts encoding abiotic stress response equipment, such as glutathione S-transferases, in roots of contaminated trees as well as the hallmarks of fungal interaction, such as SWEET2 (Sugars Will Eventually Be Exported Transporter). A total of 8252 DE transcripts were fungal in origin, with contamination conditions resulting in a community shift from Ascomycota to Basidiomycota genera. In response to contamination, 1745 Basidiomycota transcripts increased in abundance (the majority uniquely expressed in contaminated soil) including major monosaccharide transporter MST1, primary cell wall and lamella CAZy enzymes, and an ectomycorrhiza-upregulated exo-β-1,3-glucanase (GH5). Additionally, 639 DE polycistronic transcripts from an uncharacterised Enterobacteriaceae species were uniformly in higher abundance in contamination conditions and comprised a wide spectrum of genes cryptic under laboratory conditions but considered putatively involved in eukaryotic interaction, biofilm formation and dioxygenase hydrocarbon degradation.

CONCLUSIONS

Fungal gene expression, representing the majority of contigs assembled, suggests out-competition of white rot Ascomycota genera (dominated by Pyronema), a sometimes ectomycorrhizal (ECM) Ascomycota (Tuber) and ECM Basidiomycota (Hebeloma) by a poorly characterised putative ECM Basidiomycota due to contamination. Root and fungal expression involved transcripts encoding carbohydrate/amino acid (C/N) dialogue whereas bacterial gene expression included the apparatus necessary for biofilm interaction and direct reduction of contamination stress, a potential bacterial currency for a role in tripartite mutualism. Unmistakable within the metatranscriptome is the degree to which the landscape of rhizospheric biology, particularly the important but predominantly uncharacterised fungal genetics, is yet to be discovered.

The apple polyphenol phloretin inhibits breast cancer cell migration and proliferation via inhibition of signals by type 2 glucose transporter

Human triple-negative breast cancer (TNBC) is the most aggressive and poorly understood subclass of breast cancer. Glucose transporters (GLUTs) are required for glucose uptake in malignant cancer cells and are ideal targets for cancer therapy. To determine whether the inhibition of GLUTs could be used in TNBC cell therapy, the apple polyphenol phloretin (Ph) was used as a specific antagonist of GLUT2 protein function in human TNBC cells. Interestingly, we found that Ph (10–150 μM, for 24 h) inhibited cell growth and arrested the cell cycle in MDA-MB-231 cells in a p53 mutant-dependent manner, which was confirmed by pre-treatment of the cells with a p53-specific dominant-negative expression vector. We also found that Ph treatment (10–150 μM, for 24 h) significantly decreased the migratory activity of the MDA-MB-231 cells through the inhibition of paxillin/FAK, Src, and alpha smooth muscle actin (α-sMA) and through the activation of E-cadherin. Furthermore, the anti-tumorigenic effect of Ph (10, 50 mg/kg or DMSO twice a week for six weeks) was demonstrated in vivo using BALB/c nude mice bearing MDA-MB-231 tumor xenografts. A decrease in N-cadherin, vimentin and an increase in p53, p21 and E-cadherin were detected in the tumor tissues. In conclusion, inhibition of GLUT2 by the apple polyphenol Ph could potentially suppress TNBC tumor cell growth and metastasis.

Multilayered control of peroxisomal activity upon salt stress in Saccharomyces cerevisiae

Peroxisomes are dynamic organelles and the sole location for fatty acid β-oxidation in yeast cells. Here, we report that peroxisomal function is crucial for the adaptation to salt stress, especially upon sugar limitation. Upon stress, multiple layers of control regulate the activity and the number of peroxisomes. Activated Hog1 MAP kinase triggers the induction of genes encoding enzymes for fatty acid activation, peroxisomal import and β-oxidation through the Adr1 transcriptional activator, which transiently associates with genes encoding fatty acid metabolic enzymes in a stress- and Hog1-dependent manner. Moreover, Na⁺ and Li⁺ stress increases the number of peroxisomes per cell in a Hog1-independent manner, which depends instead of the retrograde pathway and the dynamin related GTPases Dnm1 and Vps1. The strong activation of the Faa1 fatty acyl-CoA synthetase, which specifically localizes to lipid particles and peroxisomes, indicates that adaptation to salt stress requires the enhanced mobilization of fatty acids from internal lipid stores. Furthermore, the activation of mitochondrial respiration during stress depends on peroxisomes, mitochondrial acetyl-carnitine uptake is essential for salt resistance and the number of peroxisomes attached to the mitochondrial network increases during salt adaptation, which altogether indicates that stress-induced peroxisomal β-oxidation triggers enhanced respiration upon salt shock.

Improvement of ethanol production from crystalline cellulose via optimizing cellulase ratios in cellulolytic Saccharomyces cerevisiae

Crystalline cellulose is one of the major contributors to the recalcitrance of lignocellulose to degradation, necessitating high dosages of cellulase to digest, thereby impeding the economic feasibility of cellulosic biofuels. Several recombinant cellulolytic yeast strains have been developed to reduce the cost of enzyme addition, but few of these strains are able to efficiently degrade crystalline cellulose due to their low cellulolytic activities. Here, by combining the cellulase ratio optimization with a novel screening strategy, we successfully improved the cellulolytic activity of a Saccharomyces cerevisiae strain displaying four different synergistic cellulases on the cell surface. The optimized strain exhibited an ethanol yield from Avicel of 57% of the theoretical maximum, and a 60% increase of ethanol titer from rice straw. To our knowledge, this work is the first optimization of the degradation of crystalline cellulose by tuning the cellulase ratio in a cellulase cell-surface display system. This work provides key insights in engineering the cellulase cocktail in a consolidated bioprocessing yeast strain. Biotechnol. Bioeng. 2017;114: 1201-1207. © 2017 Wiley Periodicals, Inc.

Apple Polyphenol Phloretin Inhibits Colorectal Cancer Cell Growth via Inhibition of the Type 2 Glucose Transporter and Activation of p53-Mediated Signaling

Glucose transporters (GLUTs) are required for glucose uptake in malignant cells, and they can be used as molecular targets for cancer therapy. An RT-PCR analysis was performed to investigate the mRNA levels of 14 subtypes of GLUTs in human colorectal cancer (COLO 205 and HT-29) and normal (FHC) cells. RT-PCR (n = 27) was used to assess the differences in paired tissue samples (tumor vs normal) isolated from colorectal cancer patients. GLUT2 was detected in all tested cells. The average GLUT2 mRNA level in 12 of 27 (44.4%) cases was 2.4-fold higher in tumor compared to normal tissues (*, p = 0.027). Higher GLUT2 mRNA expression was preferentially detected in advanced-stage tumors (stage 0 vs 3 = 16.38-fold, 95% CI = 9.22-26.54-fold; *, p = 0.029). The apple polyphenol phloretin (Ph) and siRNA methods were used to inhibit GLUT2 protein expression. Ph (0-100 μM, for 24 h) induced COLO 205 cell growth cycle arrest in a p53-dependent manner, which was confirmed by pretreatment of the cells with a p53-specific dominant negative expression vector. Hepatocyte nuclear factor 6 (HNF6), which was previously reported to be a transcription factor that activates GLUT2 and p53, was also induced by Ph (0-100 μM, for 24 h). The antitumor effect of Ph (25 mg/kg or DMSO twice a week for 6 weeks) was demonstrated in vivo using BALB/c nude mice bearing COLO 205 tumor xenografts. In conclusion, targeting GLUT2 could potentially suppress colorectal tumor cell invasiveness.