A point mutation in the UDP-glucose pyrophosphorylase gene results in decreases of UDP-glucose and inactivation of glycogen synthase

UDP-glucose pyrophosphorylase 2, a regulator of glycogen synthesis and glycosylation, is critical for pancreatic cancer growth

UDP-glucose pyrophosphorylase 2 (UGP2), the enzyme that synthesizes uridine diphosphate (UDP)-glucose, rests at the convergence of multiple metabolic pathways, however, the role of UGP2 in tumor maintenance and cancer metabolism remains unclear. Here, we identify an important role for UGP2 in the maintenance of pancreatic ductal adenocarcinoma (PDAC) growth in both in vitro and in vivo tumor models. We found that transcription of UGP2 is directly regulated by the Yes-associated protein 1 (YAP)-TEA domain transcription factor (TEAD) complex, identifying UGP2 as a bona fide YAP target gene. Loss of UGP2 leads to decreased intracellular glycogen levels and defects in N-glycosylation targets that are important for the survival of PDACs, including the epidermal growth factor receptor (EGFR). These critical roles of UGP2 in cancer maintenance, metabolism, and protein glycosylation may offer insights into therapeutic options for otherwise intractable PDACs.

Defects in Galactose Metabolism and Glycoconjugate Biosynthesis in a UDP-Glucose Pyrophosphorylase-Deficient Cell Line Are Reversed by Adding Galactose to the Growth Medium

UDP-glucose (UDP-Glc) is synthesized by UGP2-encoded UDP-Glc pyrophosphorylase (UGP) and is required for glycoconjugate biosynthesis and galactose metabolism because it is a uridyl donor for galactose-1-P (Gal1P) uridyltransferase. Chinese hamster lung fibroblasts harboring a hypomrphic UGP(G116D) variant display reduced UDP-Glc levels and cannot grow if galactose is the sole carbon source. Here, these cells were cultivated with glucose in either the absence or presence of galactose in order to investigate glycoconjugate biosynthesis and galactose metabolism. The UGP-deficient cells display < 5% control levels of UDP-Glc/UDP-Gal and > 100-fold reduction of [6-³H]galactose incorporation into UDP-[6-³H]galactose, as well as multiple deficits in glycoconjugate biosynthesis. Cultivation of these cells in the presence of galactose leads to partial restoration of UDP-Glc levels, galactose metabolism and glycoconjugate biosynthesis. The V_max for recombinant human UGP(G116D) with Glc1P is 2000-fold less than that of the wild-type protein, and UGP(G116D) displayed a mildly elevated K_m for Glc1P, but no activity of the mutant enzyme towards Gal1P was detectable. To conclude, although the mechanism behind UDP-Glc/Gal production in the UGP-deficient cells remains to be determined, the capacity of this cell line to change its glycosylation status as a function of extracellular galactose makes it a useful, reversible model with which to study different aspects of galactose metabolism and glycoconjugate biosynthesis.

Loss of UGP2 in brain leads to a severe epileptic encephalopathy, emphasizing that bi-allelic isoform-specific start-loss mutations of essential genes can cause genetic diseases

Developmental and/or epileptic encephalopathies (DEEs) are a group of devastating genetic disorders, resulting in early-onset, therapy-resistant seizures and developmental delay. Here we report on 22 individuals from 15 families presenting with a severe form of intractable epilepsy, severe developmental delay, progressive microcephaly, visual disturbance and similar minor dysmorphisms. Whole exome sequencing identified a recurrent, homozygous variant (chr2:64083454A > G) in the essential UDP-glucose pyrophosphorylase (UGP2) gene in all probands. This rare variant results in a tolerable Met12Val missense change of the longer UGP2 protein isoform but causes a disruption of the start codon of the shorter isoform, which is predominant in brain. We show that the absence of the shorter isoform leads to a reduction of functional UGP2 enzyme in neural stem cells, leading to altered glycogen metabolism, upregulated unfolded protein response and premature neuronal differentiation, as modeled during pluripotent stem cell differentiation in vitro. In contrast, the complete lack of all UGP2 isoforms leads to differentiation defects in multiple lineages in human cells. Reduced expression of Ugp2a/Ugp2b in vivo in zebrafish mimics visual disturbance and mutant animals show a behavioral phenotype. Our study identifies a recurrent start codon mutation in UGP2 as a cause of a novel autosomal recessive DEE syndrome. Importantly, it also shows that isoform-specific start-loss mutations causing expression loss of a tissue-relevant isoform of an essential protein can cause a genetic disease, even when an organism-wide protein absence is incompatible with life. We provide additional examples where a similar disease mechanism applies.

Heating after intense repeated contractions inhibits glycogen accumulation in mouse EDL muscle: role of phosphorylase in postexercise glycogen metabolism

The effects of heating on glycogen synthesis (incorporation of [¹⁴C]glucose into glycogen) and accumulation after intense repeated contractions were investigated. Isolated mouse extensor digitorum longus muscle (type II) was stimulated electrically to perform intense tetanic contractions at 25°C. After 120 min recovery at 25°C, glycogen accumulated to almost 80% of basal, whereas after recovery at 35°C, glycogen remained low (~25% of basal). Glycogen synthesis averaged 0.97 ± 0.07 µmol·30 min^-1·g wet wt^-1 during recovery at 25°C and 1.48 ± 0.08 during recovery at 35°C ( P < 0.001). There were no differences in phosphorylase and glycogen synthase total activities nor in phosphorylase fractional activity, whereas glycogen synthase fractional activity was increased by ~50% after recovery at 35°C vs. 25°C. Inorganic phosphate (P_i, substrate for phosphorylase) was markedly increased (~300% of basal) following contraction but returned to control levels after 120 min recovery at 25°C. In contrast, P_i remained elevated after recovery at 35°C (>2-fold higher than recovery at 25°C). Estimates of glycogen breakdown indicated that phosphorylase activity (either via inhibition at 25°C or activation at 35°C) was responsible for ~60% of glycogen accumulation during recovery at 25°C and ~45% during recovery at 35°C. These data demonstrate that despite the enhancing effect of heating on glycogen synthesis during recovery from intense contractions, glycogen accumulation is inhibited owing to P_i-mediated activation of phosphorylase. Thus phosphorylase can play a quantitatively important role in glycogen biogenesis during recovery from repeated contractions in isolated type II muscle.

Coordinated movement, neuromuscular synaptogenesis and trans-synaptic signaling defects in Drosophila galactosemia models

The multiple galactosemia disease states manifest long-term neurological symptoms. Galactosemia I results from loss of galactose-1-phosphate uridyltransferase (GALT), which converts galactose-1-phosphate + UDP-glucose to glucose-1-phosphate + UDP-galactose. Galactosemia II results from loss of galactokinase (GALK), phosphorylating galactose to galactose-1-phosphate. Galactosemia III results from the loss of UDP-galactose 4'-epimerase (GALE), which interconverts UDP-galactose and UDP-glucose, as well as UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine. UDP-glucose pyrophosphorylase (UGP) alternatively makes UDP-galactose from uridine triphosphate and galactose-1-phosphate. All four UDP-sugars are essential donors for glycoprotein biosynthesis with critical roles at the developing neuromuscular synapse. Drosophila galactosemia I (dGALT) and II (dGALK) disease models genetically interact; manifesting deficits in coordinated movement, neuromuscular junction (NMJ) development, synaptic glycosylation, and Wnt trans-synaptic signalling. Similarly, dGALE and dUGP mutants display striking locomotor and NMJ formation defects, including expanded synaptic arbours, glycosylation losses, and differential changes in Wnt trans-synaptic signalling. In combination with dGALT loss, both dGALE and dUGP mutants compromise the synaptomatrix glycan environment that regulates Wnt trans-synaptic signalling that drives 1) presynaptic Futsch/MAP1b microtubule dynamics and 2) postsynaptic Frizzled nuclear import (FNI). Taken together, these findings indicate UDP-sugar balance is a key modifier of neurological outcomes in all three interacting galactosemia disease models, suggest that Futsch homolog MAP1B and the Wnt Frizzled receptor may be disease-relevant targets in epimerase and transferase galactosemias, and identify UGP as promising new potential therapeutic target for galactosemia neuropathology.

A quaternary mechanism enables the complex biological functions of octameric human UDP-glucose pyrophosphorylase, a key enzyme in cell metabolism

In mammals, UDP-glucose pyrophosphorylase (UGP) is the only enzyme capable of activating glucose-1-phosphate (Glc-1-P) to UDP-glucose (UDP-Glc), a metabolite located at the intersection of virtually all metabolic pathways in the mammalian cell. Despite the essential role of its product, the molecular basis of UGP function is poorly understood. Here we report the crystal structure of human UGP in complex with its product UDP-Glc. Beyond providing first insight into the active site architecture, we describe the substrate binding mode and intermolecular interactions in the octameric enzyme that are crucial to its activity. Importantly, the quaternary mechanism identified for human UGP in this study may be common for oligomeric sugar-activating nucleotidyltransferases. Elucidating such mechanisms is essential for understanding nucleotide sugar metabolism and opens the perspective for the development of drugs that specifically inhibit simpler organized nucleotidyltransferases in pathogens.

Regulation of UDP-glucose pyrophosphorylase isozyme UGP5 associated with cold-sweetening resistance in potatoes

The regulation of UDP-Glc pyrophosphorylase (UGPase) isozyme, UGP5, was investigated in potato tuber. The cDNA for UGP5 was cloned into the bacterial expression vector pET21d and recombinant (RC) enzyme was expressed in E. coli (BL21 star cells). The RC-UGP5 isozyme was purified to near homogeneity using salt precipitation, hydrophobic interaction, and anion-exchange column chromatography. Kinetic analysis revealed that in the synthesis direction, K(m) values for Glc-1-P (0.83 mM) and UTP (0.22 mM) were similar to those observed previously with the mother tuber (MT)-UGP5. In the pyrophosphorolysis direction, the K(m) values for UDP-Glc (0.68 mM) and PPi (0.56 mM) were slightly higher than those observed previously. Maximum reaction velocities (V(max)) for RC-UGP5 were also elevated. Since the molecular mass, charge, and amino acid sequence of the MT- and RC-UGP5 isozymes were identical, it was assumed that altered kinetic constants may be due to an improper folding of RC-UGP5 polypeptide. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and proteomic analysis demonstrated that the UGP5 isozyme was a single polypeptide with a calculated molecular mass of 51.8kDa consisting of 477 amino acids. Native PAGE and kinetic analysis revealed that this polypeptide was monomeric in nature. Immunoblotting with specific antibodies and LC-MS/MS data indicated that UGP5 did not require any post-translational modification (e.g., phosphorylation, O-glycosylation, oligomerization/de-oligomerization, or the presence of the regulatory 14-3-3 proteins) for its regulation. Additionally, the two closely associated isozymes UGP5 and UGP6 in the cv. Snowden are likely the result of allelic differences of UGPase at a single locus.

General assay for sugar nucleotidyltransferases using electrospray ionization mass spectrometry

An electrospray ionization mass spectrometry-based assay has been developed to study the class of enzymes called sugar nucleotidyltransferases that couple sugar-1-phosphates and nucleotide triphosphates to form Leloir pathway glycosyl donors. The recombinant Escherichia coli and the commercially available yeast uridine-diphosphoglucose pyrophosphorylases were used as model systems. This technique allows the simultaneous and direct detection of the substrates and products without separation and, as described, is as sensitive as traditional coupled techniques. More importantly, the assay is capable of easily measuring kinetic values and inhibition constants for a range of natural and nonnatural substrates. This new assay was used to show for the first time that the reaction of the commercially available yeast uridine-diphosphoglucose pyrophosphorylase preparation is competitively inhibited by adenosine 5'-triphosphate (ATP), an observation that indicates a single active site that accepts both uridine 5'-triphosphate and ATP substrates.

Toward a blueprint for UDP-glucose pyrophosphorylase structure/function properties: homology-modeling analyses

UDP-glucose pyrophosphorylase (UGPase) is an important enzyme of synthesis of sucrose, cellulose, and several other polysaccharides in all plants. The protein is evolutionarily conserved among eukaryotes, but has little relation, aside from its catalytic reaction, to UGPases of prokaryotic origin. Using protein homology modeling strategy, 3D structures for barley, poplar, and Arabidopsis UGPases have been derived, based on recently published crystal structure of human UDP-N-acetylglucosamine pyrophosphorylase. The derived 3D structures correspond to a bowl-shaped protein with the active site at a central groove, and a C-terminal domain that includes a loop (I-loop) possibly involved in dimerization. Data on a plethora of earlier described UGPase mutants from a variety of eukaryotic organisms have been revisited, and we have, in most cases, verified the role of each mutation in enzyme catalysis/regulation/structural integrity. We have also found that one of two alternatively spliced forms of poplar UGPase has a very short I-loop, suggesting differences in oligomerization ability of the two isozymes. The derivation of the structural model for plant UGPase should serve as a useful blueprint for further function/structure studies on this protein.

Glucose starvation results in UDP-glucose deficiency and inactivation of glycogen synthase

The effects of glucose starvation on glycogen synthase (GS) activity and protein expression were investigated. Fibroblasts were cultured in medium supplemented with either glucose or pyruvate. Pyruvate-cultured cells exhibited UDP-glucose contents that amounted to approximately 10% of those in cells cultured with glucose. GS activity, protein and mRNA amounts in pyruvate-cultured cells were decreased to approximately 35, 60, and 60%, respectively, of values in glucose-cultured cells. Incubation of extracts from glucose-cultured cells with radioactive UDP-glucose resulted in substantial binding of ligand to immunoprecipitated GS. However, binding in immunoprecipitates from pyruvate-cultured cells was decreased to approximately 25% of values in glucose-cultured cells. These data indicate that glucose starvation and the subsequent depletion of UDP-glucose result in: (1) inactivation of GS, owing to a decrease in its ability to bind UDP-glucose, and (2) decreased amount of GS protein, owing to a decrease in the levels of GS mRNA.

A cellular UDP-glucose deficiency causes overexpression of glucose/oxygen-regulated proteins independent of the endoplasmic reticulum stress elements

A low level of UDP-Glc occurs in cells exposed to hypoxia or glucose starvation. This work reveals that a 65% reduction in the cellular UDP-Glc level causes up-regulation of the mitochondrial chaperone GRP75 and the endoplasmic reticulum (ER) resident chaperones GRP58, ERp72, GRP78, GRP94, GRP170, and calreticulin. Conditions that cause misfolding of proteins within the ER activate the transcription factors ATF6alpha/beta and induce translation of the transcription factors XBP-1/TREB5 and ATF4/CREB2. These transcription factors induce the overexpression of ER chaperones and CHOP/GADD153. However, the 65% decrease in the cellular UDP-Glc level does not cause activation of ATF6alpha, splicing of XBP-1/TREB5, induction of ATF4/CREB2, or expression of CHOP/GADD153. The activity of the promoters of the ER chaperones is increased in UDP-Glc-deficient cells, but the activity of the CHOP/GADD153 promoter is not affected, in comparison with their respective activities in cells having compensated for the UDP-Glc deficiency. The results demonstrate that the unfolded protein response remains functionally intact in cells with a 65% decrease in the cellular UDP-Glc level and provide evidence that this decrease is a stress signal in mammalian cells, which triggers the coordinate overexpression of mitochondrial and ER chaperones, independently of the ER stress elements.