Dynamic Nutrient Signaling Networks in Plants.

Nutrients are vital to life through intertwined sensing, signaling, and metabolic processes. Emerging research focuses on how distinct nutrient signaling networks integrate and coordinate gene expression, metabolism, growth, and survival. We review the multifaceted roles of sugars, nitrate, and phosphate as essential plant nutrients in controlling complex molecular and cellular mechanisms of dynamic signaling networks. Key advances in central sugar and energy signaling mechanisms mediated by the evolutionarily conserved master regulators HEXOKINASE1 (HXK1), TARGET OF RAPAMYCIN (TOR), and SNF1-RELATED PROTEIN KINASE1 (SNRK1) are discussed. Significant progress in primary nitrate sensing, calcium signaling, transcriptome analysis, and root-shoot communication to shape plant biomass and architecture are elaborated. Discoveries on intracellular and extracellular phosphate signaling and the intimate connections with nitrate and sugar signaling are examined. This review highlights the dynamic nutrient, energy, growth, and stress signaling networks that orchestrate systemwide transcriptional, translational, and metabolic reprogramming, modulate growth and developmental programs, and respond to environmental cues.

HEXOKINASE1 and glucose-6-phosphate fuel plant growth and development.

As photoautotrophic organisms, plants produce an incredible spectrum of pigments, anti-herbivory compounds, structural materials and energic intermediates. These biosynthetic routes help plants grow, reproduce and mitigate stress. HEXOKINASE1 (HXK1), a metabolic enzyme and glucose sensor, catalyzes the phosphorylation of hexoses, a key introductory step for many of these pathways. However, previous studies have largely focused on the glucose sensing and signaling functions of HXK1, and the importance of the enzyme's catalytic function is only recently being connected to plant development. In this brief Spotlight, we describe the developmental significance of plant HXK1 and its role in plant metabolic pathways, specifically in glucose-6-phosphate production. Furthermore, we describe the emerging connections between metabolism and development and suggest that HXK1 signaling and catalytic activity regulate discrete areas of plant development.

Sugar sensing in C4 source leaves: a gap that needs to be filled.

Plant growth depends on sugar production and export by photosynthesizing source leaves and sugar allocation and import by sink tissues (grains, roots, stems, and young leaves). Photosynthesis and sink demand are tightly coordinated through metabolic (substrate, allosteric) feedback and signalling (sugar, hormones) mechanisms. Sugar signalling integrates sugar production with plant development and environmental cues. In C3 plants (e.g. wheat and rice), it is well documented that sugar accumulation in source leaves, due to source-sink imbalance, negatively feeds back on photosynthesis and plant productivity. However, we have a limited understanding about the molecular mechanisms underlying those feedback regulations, especially in C4 plants (e.g. maize, sorghum, and sugarcane). Recent work with the C4 model plant Setaria viridis suggested that C4 leaves have different sugar sensing thresholds and behaviours relative to C3 counterparts. Addressing this research priority is critical because improving crop yield requires a better understanding of how plants coordinate source activity with sink demand. Here we review the literature, present a model of action for sugar sensing in C4 source leaves, and suggest ways forward.

Spatial expression patterns of genes encoding sugar sensors in leaves of C4 and C3 grasses.

BACKGROUND AND AIMS: The mechanisms of sugar sensing in grasses remain elusive, especially those using C4 photosynthesis even though a large proportion of the world's agricultural crops utilize this pathway. We addressed this gap by comparing the expression of genes encoding components of sugar sensors in C3 and C4 grasses, with a focus on source tissues of C4 grasses. Given C4 plants evolved into a two-cell carbon fixation system, it was hypothesized this may have also changed how sugars were sensed. METHODS: For six C3 and eight C4 grasses, putative sugar sensor genes were identified for target of rapamycin (TOR), SNF1-related kinase 1 (SnRK1), hexokinase (HXK) and those involved in the metabolism of the sugar sensing metabolite trehalose-6-phosphate (T6P) using publicly available RNA deep sequencing data. For several of these grasses, expression was compared in three ways: source (leaf) versus sink (seed), along the gradient of the leaf, and bundle sheath versus mesophyll cells. KEY RESULTS: No positive selection of codons associated with the evolution of C4 photosynthesis was identified in sugar sensor proteins here. Expressions of genes encoding sugar sensors were relatively ubiquitous between source and sink tissues as well as along the leaf gradient of both C4 and C3 grasses. Across C4 grasses, SnRK1ß1 and TPS1 were preferentially expressed in the mesophyll and bundle sheath cells, respectively. Species-specific differences of gene expression between the two cell types were also apparent. CONCLUSIONS: This comprehensive transcriptomic study provides an initial foundation for elucidating sugar-sensing genes within major C4 and C3 crops. This study provides some evidence that C4 and C3 grasses do not differ in how sugars are sensed. While sugar sensor gene expression has a degree of stability along the leaf, there are some contrasts between the mesophyll and bundle sheath cells.

Transcription factor TERF1 promotes seed germination through HEXOKINASE 1 (HXK1)-mediated signaling pathway.

Seed germination, a vital process for plant growth and development, is regulated by ethylene. Previously, we showed that Tomato Ethylene Responsive Factor 1 (TERF1), an ethylene-responsive factor (ERF) transcription factor, could significantly promote seed germination by increasing glucose content. As glucose can function as a signaling molecule to regulate plant growth and development through HEXOKINASE 1 (HXK1), we aim to illustrate how TERF1 promotes seed germination through the HXK1-mediated signaling pathway. We showed that seeds overexpressing TERF1 exhibited more resistance to N-acetylglucosamine (NAG), an inhibitor of the HXK1- mediated signaling pathway. We identified genes regulated by TERF1 through HXK1 based on transcriptome analysis. Gene expression and phenotype analysis demonstrated that TERF1 repressed the ABA signaling pathway through HXK1, which promoted germination through activating the plasma membrane (PM) H+-ATPase. TERF1 also alleviated the endoplasmic reticulum (ER) stress to accelerate germination by maintaining reactive oxygen species (ROS) homeostasis through HXK1. Our findings provide new insights into the mechanism regulated by ethylene through the glucose-HXK1 signaling pathway during seed germination.

Interactome of Arabidopsis ATG5 Suggests Functions beyond Autophagy.

Autophagy is a catabolic pathway capable of degrading cellular components ranging from individual molecules to organelles. Autophagy helps cells cope with stress by removing superfluous or hazardous material. In a previous work, we demonstrated that transcriptional upregulation of two autophagy-related genes, ATG5 and ATG7, in Arabidopsis thaliana positively affected agronomically important traits: biomass, seed yield, tolerance to pathogens and oxidative stress. Although the occurrence of these traits correlated with enhanced autophagic activity, it is possible that autophagy-independent roles of ATG5 and ATG7 also contributed to the phenotypes. In this study, we employed affinity purification and LC-MS/MS to identify the interactome of wild-type ATG5 and its autophagy-inactive substitution mutant, ATG5K128R Here we present the first interactome of plant ATG5, encompassing not only known autophagy regulators but also stress-response factors, components of the ubiquitin-proteasome system, proteins involved in endomembrane trafficking, and potential partners of the nuclear fraction of ATG5. Furthermore, we discovered post-translational modifications, such as phosphorylation and acetylation present on ATG5 complex components that are likely to play regulatory functions. These results strongly indicate that plant ATG5 complex proteins have roles beyond autophagy itself, opening avenues for further investigations on the complex roles of autophagy in plant growth and stress responses.

OsAGO2 controls ROS production and the initiation of tapetal PCD by epigenetically regulating OsHXK1 expression in rice anthers.

Proteins of the ARGONAUTE (AGO) family function in the epigenetic regulation of gene expression. Although the rice (Oryza sativa) genome encodes 19 predicted AGO proteins, few of their functions have thus far been characterized. Here, we show that the AGO protein OsAGO2 regulates anther development in rice. OsAGO2 was highly expressed in anthers. Knockdown of OsAGO2 led to the overaccumulation of reactive oxygen species (ROS) and abnormal anther development, causing premature initiation of tapetal programmed cell death (PCD) and pollen abortion. The expression level of Hexokinase 1 (OsHXK1) increased significantly, and the methylation levels of its promoter decreased, in plants with knocked-down OsAGO2 expression. Overexpression of OsHXK1 also resulted in the overaccumulation of ROS, premature initiation of PCD, and pollen abortion. Moreover, knockdown of OsHXK1 restored pollen fertility in OsAGO2 knockdown plants. Chromatin immunoprecipitation assays demonstrated that OsAGO2 binds directly to the OsHXK1 promoter region, suggesting that OsHXK1 is a target gene of OsAGO2. These results indicate that OsHXK1 controls the appropriate production of ROS and the proper timing of tapetal PCD and is directly regulated by OsAGO2 through epigenetic regulation.

Efficient production of shinorine, a natural sunscreen material, from glucose and xylose by deleting HXK2 encoding hexokinase in Saccharomyces cerevisiae.

Mycosporine-like amino acids (MAAs), microbial secondary metabolites with ultraviolet (UV) absorption properties, are promising natural sunscreen materials. Due to the low efficiency of extracting MAAs from natural producers, production in heterologous hosts has recently received attention. Shinorine is a well characterized MAA with strong UV-A absorption property. Previous, we developed Saccharomyces cerevisiae strain producing shinorine by introducing four shinorine biosynthetic genes from cyanobacterium Nostoc punctiforme. Shinorine is produced from sedoheptulose 7-phosphate (S7P), an intermediate in the pentose phosphate pathway. Shinorine production was greatly improved by using xylose as a co-substrate, which can increase the S7P pool. However, due to a limited xylose-utilizing capacity of the engineered strain, glucose was used as a co-substrate to support cell growth. In this study, we further improved shinorine production by attenuating glucose catabolism via glycolysis, which can redirect the carbon flux from glucose to the pentose phosphate pathway favoring shinorine production. Of the strategies we examined to reduce glycolytic flux, deletion of HXK2, encoding hexokinase, was most effective in increasing shinorine production. Furthermore, by additional expression of Ava3858 from Anabaena variabilis, encoding a rate-limiting enzyme 2-demethyl 4-deoxygadusol synthase, 68.4 mg/L of shinorine was produced in an optimized medium containing 14 g/L glucose and 6 g/L xylose, achieving a 2.2-fold increase compared with the previous strain.

Sugar sensing responses to low and high light in leaves of the C4 model grass Setaria viridis.

Although sugar regulates photosynthesis, the signalling pathways underlying this process remain elusive, especially for C4 crops. To address this knowledge gap and identify potential candidate genes, we treated Setaria viridis (C4 model) plants acclimated to medium light intensity (ML, 500 µmol m-2 s-1) with low (LL, 50 µmol m-2 s-1) or high (HL, 1000 µmol m-2 s-1) light for 4 d and observed the consequences on carbon metabolism and the transcriptome of source leaves. LL impaired photosynthesis and reduced leaf content of signalling sugars (glucose, sucrose, and trehalose-6-phosphate). In contrast, HL strongly induced sugar accumulation without repressing photosynthesis. LL more profoundly impacted the leaf transcriptome, including photosynthetic genes. LL and HL contrastingly altered the expression of hexokinase (HXK) and sucrose-non-fermenting 1 (Snf1)-related protein kinase 1 (SnRK1) sugar sensors and trehalose pathway genes. The expression of key target genes of HXK and SnRK1 were affected by LL and sugar depletion, while surprisingly HL and strong sugar accumulation only slightly repressed the SnRK1 signalling pathway. In conclusion, we demonstrate that LL profoundly impacted photosynthesis and the transcriptome of S. viridis source leaves, while HL altered sugar levels more than LL. We also present the first evidence that sugar signalling pathways in C4 source leaves may respond to light intensity and sugar accumulation differently from C3 source leaves.

NGT1 Is Essential for N-Acetylglucosamine-Mediated Filamentous Growth Inhibition and HXK1 Functions as a Positive Regulator of Filamentous Growth in Candida tropicalis.

Candida tropicalis is a pathogenic fungus that can cause opportunistic infections in humans. The ability of Candida species to transition between yeast and filamentous growth forms is essential to their ability to undergo environmental adaptation and to maintain virulence. In other fungal species, such as Candida albicans, N-acetylglucosamine (GlcNAc) can induce filamentous growth, whereas it suppresses such growth in C. tropicalis. In the present study, we found that knocking out the GlcNA-specific transporter gene NGT1 was sufficient to enhance C. tropicalis filamentous growth on Lee's plus GlcNAc medium. This suggests that GlcNAc uptake into C. tropicalis cells is essential to the disruption of mycelial growth. As such, we further studied how GlcNAc catabolism-related genes were able to influence C. tropicalis filamentation. We found that HXK1 overexpression drove filamentous growth on Lee's media containing glucose and GlcNAc, whereas the deletion of the same gene disrupted this filamentous growth. Interestingly, the deletion of the DAC1 or NAG1 genes impaired C. tropicalis growth on Lee's plus GlcNAc plates. Overall, these results indicate that HXK1 can serve as a positive regulator of filamentous growth, with excess GlcNAc-6-PO4 accumulation being toxic to C. tropicalis. These findings may highlight novel therapeutic targets worthy of future investigation.

Biochemical properties and subcellular localization of six members of the HXK family in maize and its metabolic contribution to embryo germination.

BACKGROUND: Seed germination is a crucial process in the plant life cycle when a dramatic variation of type and sugar content occurs just as the seed is hydrated. The production of hexose 6 phosphate is a key node in different pathways that are required for a successful germination. Hexokinase (HXK) is the only plant enzyme that phosphorylates glucose (Glc), so it is key to fueling several metabolic pathways depending on their substrate specificity, metabolite regulatory responses and subcellular localization. In maize, the HXK family is composed of nine genes, but only six of them (ZmHXK4-9) putatively encode catalytically active enzymes. Here, we cloned and functionally characterized putative catalytic enzymes to analyze their metabolic contribution during germination process. RESULTS: From the six HXKs analyzed here, only ZmHXK9 has minimal hexose phosphorylating activity even though enzymatic function of all isoforms (ZmHXK4-9) was confirmed using a yeast complementation approach. The kinetic parameters of recombinant proteins showed that ZmHXK4-7 have high catalytic efficiency for Glc, fructose (Fru) and mannose (Man), ZmHXK7 has a lower Km for ATP, and together with ZmHXK8 they have lower sensitivity to inhibition by ADP, G6P and N-acetylglucosamine than ZmHXK4-6 and ZmHXK9. Additionally, we demonstrated that ZmHXK4-6 and ZmHXK9 are located in the mitochondria and their location relies on the first 30 amino acids of the N-terminal domain. Otherwise, ZmHXK7-8 are constitutively located in the cytosol. HXK activity was detected in cytosolic and mitochondrial fractions and high Glc and Fru phosphorylating activities were found in imbibed embryos. CONCLUSIONS: Considering the biochemical characteristics, location and the expression of ZmHXK4 at onset of germination, we suggest that it is the main contributor to mitochondrial activity at early germination times, at 24 h other ZmHXKs also contribute to the total activity. While in the cytosol, ZmHXK7 could be responsible for the activity at the onset of germination, although later, ZmHXK8 also contributes to the total HXK activity. Our observations suggest that the HXKs may be redundant proteins with specific roles depending on carbon and ATP availability, metabolic needs, or sensor requirements. Further investigation is necessary to understand their specific or redundant physiological roles.

Vma3p protects cells from programmed cell death through the regulation of Hxk2p expression.

In yeast, the vacuolar proton-pumping ATPase (V-ATPase) acidifies vacuoles to maintain pH of cytoplasm. Yeast cells lacking V-ATPase activity, due to a disruption of any VMA (vacuolar membrane ATPase) gene, remain viable but demonstrate growth defects. Although it has been suggested that VMA genes are critical for phospholipid biosynthesis, the link between VMA genes and phospholipid biosynthesis is still uncertain. Here, we found that cells lacking Vma3p, one of the major V-ATPase assembly genes, had a growth defect in the absence of inositol, suggesting that Vma3p is important in phospholipid biosynthesis. Through real-time PCR, we found that cells lacking Vma3p down-regulated HXK2 expression. Furthermore, acetic acid sensitivity assay showed that cells lacking Vma3p were more sensitive to acetic acid than WT cells. HXK2 encodes hexokinase 2 which can phosphorylate glucose during phospholipid biosynthesis. Since cells lacking HXK2 are sensitive to acetic acid and this is an indicator of programmed cell death, our observations suggest that Vma3p plays an important role in programmed cell death. Taken together, we have proposed a working model to describe how Vma3p protects cells against apoptosis through the regulation of HXK2 expression.

Role of the N-acetylglucosamine kinase (Hxk1) in the regulation of white-gray-opaque tristable phenotypic transitions in C. albicans.

The amino sugar N-acetylglucosamine (GlcNAc) is a host-related environmental cue and a potent inducer of morphological transitions in the human fungal pathogen Candida albicans. It has been well established that GlcNAc promotes white-to-opaque switching and yeast-to-hyphal growth transition primarily through the Ras-cAMP signaling pathway. As a commensal yeast of humans, C. albicans can efficiently use GlcNAc as the carbon source. In this study, we sought to investigate whether the catabolic pathway of GlcNAc is involved in the regulation of white-gray-opaque tristable transitions in C. albicans. Phenotypic switching assays demonstrated that deletion of the GlcNAc kinase gene, HXK1, induced the gray and opaque phenotypes in a SC5314 background strain, which is heterozygous at the mating type locus (a/α) and is unable to switch to the gray or opaque phenotype under standard culture conditions. Cell type-enriched genes were exclusively expressed in the white, gray, and opaque cells of the hxk1/hxk1 mutant. Mating assays demonstrated that, similar to the counterparts of BJ1097 (a natural white-gray-opaque switchable strain), opaque cells of the hxk1/hxk1 mutant (Δ/α) mated more efficiently than white and gray cells. The transcription factors, Wor1 and Efg1, are required for the development of the opaque and white cell types in the hxk1/hxk1 mutant, respectively. However, deletion of the GlcNAc-specific transporter gene (NGT1), GlcNAc-6-phosphate deacetylase gene (DAC1), and glucosamine-6-phosphate deaminase gene (NAG1) in the same background strain had no obvious effect on white-gray-opaque transitions. Our findings suggest that the GlcNAc kinase, Hxk1, may function as a morphological regulator independent on its catabolic role in C. albicans.

Involvement of Aif1 in apoptosis triggered by lack of Hxk2 in the yeast Saccharomyces cerevisiae.

We recently showed that in hxk2Δ cells, showing constitutive localization of active Ras at the mitochondria, addition of acetic acid caused an increase of both apoptotic and necrotic cells compared with the wild-type strain, providing a new role for hexokinase 2 (EC 2.7.1.1) as an anti-apoptotic factor, besides its known role as a glycolytic enzyme and as a regulator of gene transcription of several Mig1-regulated genes. We also demonstrated that apoptosis induced by lack of Hxk2 may not require the activation of Yca1. Here, we show that deletion of HXK2 causes hypersensitivity to H2O2 and that addition of this well-known apoptotic stimulus to hxk2Δ cells causes an increase in the level ROS, apoptosis and mitochondrial membrane potential. We also show that deletion of AIF1 in hxk2Δ cells enhances survival after induction of apoptosis with both H2O2 and acetic acid, rescues the reduction of both growth rate and cell size, abrogates both H2O2 and acetic acid-induced ROS accumulation and decreases cell death, suggesting that Aif1 might be involved in both H2O2 and acetic acid-induced cell death in hxk2Δ cells. Moreover, we show that active Ras proteins relocalize to the plasma membrane and to the nucleus in hxk2Δ aif1Δ cells.

Co-transcriptional degradation by the 5'-3' exonuclease Rat1p mediates quality control of HXK1 mRNP biogenesis in S. cerevisiae.

The co-transcriptional biogenesis of export-competent messenger ribonucleoprotein particles (mRNPs) in yeast is under the surveillance of quality control (QC) steps. Aberrant mRNPs resulting from inappropriate or inefficient processing and packaging reactions are detected by the QC system and retained in the nucleus with ensuing elimination of their mRNA component by a mechanism that requires the catalytic activity of Rrp6p, a 3'-5' exonuclease associated with the RNA exosome. In previous studies, we implemented a new experimental approach in which the production of aberrant mRNPs is massively increased upon perturbation of mRNP biogenesis by the RNA-dependent helicase/translocase activity of the bacterial Rho factor expressed in S. cerevisiae. The analyses of a subset of transcripts such as PMA1 led us to substantiate the essential role of Rrp6p in the nuclear mRNP QC and to reveal a functional coordination of the process by Nrd1p. Here, we extended those results by showing that, in contrast to PMA1, Rho-induced aberrant HXK1 mRNPs are targeted for destruction by an Nrd1p- and Rrp6p-independent alternative QC pathway that relies on the 5'-3' exonuclease activity of Rat1p. We show that the degradation of aberrant HXK1 mRNPs by Rat1p occurs co-transcriptionally following decapping by Dcp2p and leads to premature transcription termination. We discuss the possibility that this alternative QC pathway might be linked to the well-known specific features of the HXK1 gene transcription such as its localization at the nuclear periphery and gene loop formation.

Ubiquitination of the glycosomal matrix protein receptor PEX5 in Trypanosoma brucei by PEX4 displays novel features.

Trypanosomatids contain peroxisome-like organelles called glycosomes. Peroxisomal biogenesis involves a cytosolic receptor, PEX5, which, after its insertion into the organellar membrane, delivers proteins to the matrix. In yeasts and mammalian cells, transient PEX5 monoubiquitination at the membrane serves as the signal for its retrieval from the organelle for re-use. When its recycling is impaired, PEX5 is polyubiquitinated for proteasomal degradation. Stably monoubiquitinated TbPEX5 was detected in cytosolic fractions of Trypanosoma brucei, indicative for its role as physiological intermediate in receptor recycling. This modification's resistance to dithiothreitol suggests ubiquitin conjugation of a lysine residue. T. brucei PEX4, the functional homologue of the ubiquitin-conjugating (UBC) enzyme responsible for PEX5 monoubiquitination in yeast, was identified. It is associated with the cytosolic face of the glycosomal membrane, probably anchored by an identified putative TbPEX22. The involvement of TbPEX4 in TbPEX5 ubiquitination was demonstrated using procyclic ∆PEX4 trypanosomes. Surprisingly, glycosomal matrix protein import was only mildly affected in this mutant. Since other UBC homologues were upregulated, it might be possible that these have partially rescued PEX4's function in PEX5 ubiquitination. In addition, the altered expression of UBCs, notably of candidates involved in cell-cycle control, could be responsible for observed morphological and motility defects of the ∆PEX4 mutant.

N-acetylglucosamine kinase, HXK1 contributes to white-opaque morphological transition in Candida albicans.

Morphological transition (yeast-hyphal and white-opaque) is an important biological process in the life cycle of pathogenic yeast, Candida albicans and is a major determinant of virulence. Earlier reports show that the amino sugar, N-acetylglucosamine (GlcNAc) induces white to opaque switching in this pathogen. We report here a new contributor to this switching phenomenon, namely N-acetylglucosamine kinase or HXK1, the first enzyme of the GlcNAc catabolic cascade. Microarray profile analysis of wild type vs. hxk1 mutant cells grown under switching inducing condition showed upregulation of opaque specific and cell wall specific genes along genes involved in the oxidative metabolism. Further, our qRT-PCR and immunoblot analysis revealed that the expression levels of Wor1, a master regulator of the white-opaque switching phenomenon remained unaltered during this HXK1 mediated transition. Thus the derepression of opaque specific gene expression observed in hxk1 mutant could be uncoupled to the expression of WOR1. Moreover, this regulation via HXK1 is independent of Ras1, a major regulator of morphogenetic transition and probably independent of MTL locus too. These results extend our understanding of multifarious roles of metabolic enzymes like Hxk1 and suggest an adaptive mechanism during host-pathogen interactions.

Fibroblast growth factor 7 alleviates myocardial infarction by improving oxidative stress via PI3Kα/AKT-mediated regulation of Nrf2 and HXK2.

Acute myocardial infarction (MI) triggers oxidative stress, which worsen cardiac function, eventually leads to remodeling and heart failure. Unfortunately, effective therapeutic approaches are lacking. Fibroblast growth factor 7 (FGF7) is proved with respect to its proliferative effects and high expression level during embryonic heart development. However, the regulatory role of FGF7 in cardiovascular disease, especially MI, remains unclear. FGF7 expression was significantly decreased in a mouse model at 7 days after MI. Further experiments suggested that FGF7 alleviated MI-induced cell apoptosis and improved cardiac function. Mechanistic studies revealed that FGF7 attenuated MI by inhibiting oxidative stress. Overexpression of FGF7 actives nuclear factor erythroid 2-related factor 2 (Nrf2) and scavenging of reactive oxygen species (ROS), and thereby improved oxidative stress, mainly controlled by the phosphatidylinositol-3-kinase α (PI3Kα)/AKT signaling pathway. The effects of FGF7 were partly abrogated in Nrf2 deficiency mice. In addition, overexpression of FGF7 promoted hexokinase2 (HXK2) and mitochondrial membrane translocation and suppressed mitochondrial superoxide production to decrease oxidative stress. The role of HXK2 in FGF7-mediated improvement of mitochondrial superoxide production and protection against MI was verified using a HXK2 inhibitor (3-BrPA) and a HXKII VDAC binding domain (HXK2VBD) peptide, which competitively inhibits localization of HXK2 on mitochondria. Furthermore, inhibition of PI3Kα/AKT signaling abolished regulation of Nrf2 and HXK2 by FGF7 upon MI. Together, these results indicate that the cardio protection of FGF7 under MI injury is mostly attributable to its role in maintaining redox homeostasis via Nrf2 and HXK2, which is mediated by PI3Kα/AKT signaling.

N-acetylglucosamine kinase, Hxk1 is a multifaceted metabolic enzyme in model pathogenic yeast Candida albicans.

The sensing of environmental conditions such as nutrient availability and the ability to adapt and respond to changing conditions are crucial for the survival of living organisms. Evidence from several organisms have revealed that some metabolic enzymes act as sensors of nutrient status and regulate the expression of sets of genes required for nutrients utilization and condition specific environmental adaptation. Thus metabolic enzymes regulate the signaling pathway by acting as transcriptional regulators and providing required metabolites. The commensal yeast, Candida albicans has recently emerged as a model system for understanding the N-acetylglucosamine (GlcNAc) signaling pathway in eukaryotes. GlcNAc kinase (Hxk1), the first enzyme of the catabolic cascade, has been shown to perform several functions such as regulation of gene expression and regulation of the metabolic status of the cell thereby resulting in a change in cell morphology (yeast-hyphal transition, white-opaque switching), metabolic gene expression, synthesis of metabolic precursors, induction of glycolytic flux rate and biofilm formation. Here, in this review we have discussed various roles of Hxk1that have not been reported in other organisms previously. The enzyme exhibits dynamic changes in subcellular localization consistent with its expanded functions inside the cell. Thus Hxk1 in C. albicans orchestrates several dynamic cellular processes and this signaling system can act as a paradigm to understand the cell fate and metabolic specialization in other eukaryotes too. Still, the molecular cues involved in Hxk1 mediating functions are yet to be unveiled; the relationship between Hxk1 sensing and its signaling effects is also not understood yet.

N-Acetylglucosamine Sensing and Metabolic Engineering for Attenuating Human and Plant Pathogens.

During evolution, both human and plant pathogens have evolved to utilize a diverse range of carbon sources. N-acetylglucosamine (GlcNAc), an amino sugar, is one of the major carbon sources utilized by several human and phytopathogens. GlcNAc regulates the expression of many virulence genes of pathogens. In fact, GlcNAc catabolism is also involved in the regulation of virulence and pathogenesis of various human pathogens, including Candida albicans, Vibrio cholerae, Leishmania donovani, Mycobacterium, and phytopathogens such as Magnaporthe oryzae. Moreover, GlcNAc is also a well-known structural component of many bacterial and fungal pathogen cell walls, suggesting its possible role in cell signaling. Over the last few decades, many studies have been performed to study GlcNAc sensing, signaling, and metabolism to better understand the GlcNAc roles in pathogenesis in order to identify new drug targets. In this review, we provide recent insights into GlcNAc-mediated cell signaling and pathogenesis. Further, we describe how the GlcNAc metabolic pathway can be targeted to reduce the pathogens' virulence in order to control the disease prevalence and crop productivity.