Glycobiology Advance Access originally published online on May 4, 2007
Glycobiology 2007 17(8):828-837; doi:10.1093/glycob/cwm048
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Metabolic homeostasis and tissue renewal are dependent on ß1,6GlcNAc-branched N-glycans
2 Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue R988, Toronto, ON, Canada M5G 1X5
3 Department of Molecular and Medical Genetics
4 Department of Laboratory Medicine and Pathology, University of Toronto, Canada
1 To whom correspondence should be addressed; Tel: +1-416-586-8233; Fax: +1-416-586-8588; e-mail: dennis{at}mshri.on.ca
Received on January 25, 2007; revised on March 22, 2007; accepted on April 19, 2007
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Abstract |
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 Top Abstract IntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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Golgi ß1,6-N-acetylglucosaminyltransferase V (Mgat5) produces ß1,6GlcNAc-branched N-glycans on glycoproteins, which increases their affinity for galectins and opposes loss from the cell surface to constitutive endocytosis. Oncogenic transformation increases Mgat5 expression, increases ß1,6GlcNAc-branched N-glycans on epidermal growth factor and transforming growth factor-ß receptors, and enhances sensitivities to ligands, cell motility, and tumor metastasis. Here, we demonstrate that Mgat5–/– mouse embryonic fibroblasts (MEFs) display reduced sensitivity to anabolic cytokines and reduced glucose uptake and proliferation. Mgat5–/– mice are also hypoglycemic, resistant to weight gain on a calorie-enriched diet, hypersensitive to fasting, and display increased oxidative respiration and reduced fecundity. Serum-dependent activation of the extracellular response kinase (growth) and Smad2/3 (arrest) pathways in Mgat5–/– MEFs and bone marrow cells reveals an imbalance favoring arrest. Mgat5–/– mice have fewer muscle satellite cells, less osteogenic activity in bone marrow, and accelerated loss of muscle and bone mass with aging. Our results suggest that ß1,6GlcNAc-branched N-glycans promote sensitivity to anabolic cytokines, and increase fat stores, tissue renewal, and longevity.
Key words: aging / cytokine signaling / metabolism / Mgat5 / N-glycans / stem cells
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Introduction |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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Glucose is a ligand for a family of signaling receptors in Saccharomyces cerevisiae that couples metabolism with cell replication, filamentous invasive growth, and balanced oxidative and fermentative metabolism (Ozcan et al. 1998
; Rolland et al. 2000). Although downstream intracellular nutrient-sensing pathways (i.e. Akt/Sch9 and Ras-cAMP) are largely conserved in eukaryotes (Sobko 2006), growth signaling in metazoan cells is stimulated by extracellular cytokine and receptor tyrosine kinases (RTKs), as required to specify their complex body plans (Di Paolo and De Camilli 2006). However equally important are signaling pathways that limit growth and are sensitive to extracellular morphogens such as the transforming growth factor-ß (TGF-ß)/bone morphogenic protein (BMP) family (Wakefield and Roberts 2002). TGF-ß receptor (TßR)/Smad and RTK/PI3K/extracellular response kinase (Erk) pathways interact at multiple levels and, ultimately, the former promotes cell cycle arrest by decreasing c-Myc and increasing p21Cip1 and p15INK4a expressions (Siegel and Massague 2003; Seoane et al. 2004). For example, self-renewal and pluripotency of cultured human embryonic stem cells is optimally preserved by precise proportions of basic fibroblast growth factor (bFGF) and Noggin, a BMP/TGF-ß antagonist (Xu et al. 2005). BMP4 alone can shift the equilibrium toward differentiation, as has been shown for tumor-initiating stem cells, thereby inhibiting human glioblastoma growth in mice (Piccirillo et al. 2006). The availability of receptors for insulin and epidermal growth factor (EGF) and platelet derived growth factor (PDGF) cytokines decline with age in primary cells, which may also limit the incidence of cancers while contributing to the loss of renewal with age (Yeo and Park 2002). As such, growth factors promote tissue renewal and metabolism with aging, and their action is balanced by growth-suppressing pathways that limit cancer risk (Campisi 2005; Conboy et al. 2005). Imbalances, such as the loss of the DNA damage checkpoint protein p53, relieve a brake on proliferation and glycolytic metabolism, and increase the risk of cancer but also increase longevity for individuals remaining cancer-free (Tyner et al. 2002; van Heemst et al. 2005; Matoba et al. 2006).
We have recently shown that adaptive changes in N-glycan processing in mammalian cells downstream of metabolic flux to sugar nucleotides regulate surface receptors and cellular sensitivities to cytokines (Lau et al. 2007). Glucose, acetyl-CoA, and glutamine are substrates of the hexosamine pathway to UDP-GlcNAc, a key substrate in the biosynthesis of complex N-glycans found on cell surface receptors and transporters (Sasai et al. 2002; Grigorian et al. 2007). N-acetylglucosaminyltransferases I, II, III, IV, and V (GlcNAc-Ts) (encoded by ß1,6-N-acetylglucosaminyl transferases Mgat1, Mgat2, Mgat3, Mgat4a/b, and Mgat5) act sequentially in the medial Golgi to initiate the N-acetyllactosamine branches on complex N-glycans. End products of the pathway, the ß1,6GlcNAc-branched tetra-antennary N-glycans, are higher affinity ligands for galectins-3 and -9 compared with the less branched structures (Hirabayashi et al. 2002; Lau et al. 2007). Galectins bind and cross-link complex N-glycans on receptors at the cells surface (Demetriou et al. 2001; Brewer et al. 2002) which enhances their surface residency (Partridge et al. 2004). In this regard, Mgat5–/– tumor cells display fewer surface EGF and TGF-ß receptors, proportionately more in the endosomes, and are less responsive to multiple cytokines (Partridge et al. 2004). Oncogenic transformation increases Mgat5 gene expression (Kang et al. 1996; Buckhaults et al. 1997), and ß1,6GlcNAc-branching of N-glycans is associated with a poor prognosis in human patients (Fernandes et al. 1991; Seelentag et al. 1998).
We have recently shown that metabolic flux to UDP-GlcNAc and N-glycan processing cooperates with N-glycan number to regulate cellular sensitivities to cytokines (Lau et al. 2007). The number of N-glycans or multiplicity (n) is an encoded feature of the protein that differs markedly between different surface glycoproteins. The numbers of potential glycoforms increase exponentially with multiplicity, and these larger molecular distributions have higher median affinities for the galectin lattice. Therefore, high multiplicity can enhance avidity for the lattice even when GlcNAc branching is low, whereas glycoproteins with low multiplicity require more GlcNAc branching for stable association with the lattice. Metabolite flux through the hexosamine pathway to UDP-GlcNAc and medial GlcNAc-Ts increases the tri- and tetra-antennary N-glycan content on glycoproteins. Thus, growth factor receptors with high n [e.g. EGFR, PDGFR, FGFR, and insulin-like growth factor receptor IGFR] increase at the cell surface first in response to increasing UDP-GlcNAc, followed by low-n glycoproteins (e.g. TßR) (Lau et al. 2007). Receptor kinases that stimulate cell proliferation and promote oncogenesis have increased numbers and densities of Asn-X-Ser/Thr (X 
Pro) motifs, compared with receptor kinases required for tissue morphogenesis and growth arrest. Computational simulation of N-glycan processing and galectin lattice dynamics and experimental data from cell culture models indicate that metabolic flux to N-glycan processing regulates the proportions of glycoproteins at the surface and thus cellular responses to multiple cytokines (Lau et al. 2007). We report here that primary Mgat5–/– cells display a signaling imbalance (RTK/PI3K/Erk and TGF-ß/Smad) that favors growth arrest. Mgat5-deficient mice are resistant to weight gain on a calorie-enriched diet, and display increased oxidative respiration, reduced osteogenic activity and muscle satellite cells, and an early aging phenotype. Our results suggest that conditional expression of ß1,6GlcNAc-branched N-glycans in postnatal tissues promotes anabolic metabolism and tissue renewal by adapting cellular sensitivities to growth and arrest cues.
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Results |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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Mgat5-modified N-glycans promote glucose uptake and sensitivity to growth factors
If the loss of cytokine sensitivity observed in polyomavirus middle T Mgat5–/– tumor cells (Partridge et al. 2004
) is also present in primary cells from Mgat5–/– mice, we might anticipate a chronic deficiency in growth signaling and suppression of glucose metabolism. Indeed, Erk-p nuclear translocation in response to EGF, PDGF, FGF, and IGF cytokines was suppressed in Mgat5–/– compared with Mgat5+/+ mouse embryonic fibroblasts (MEFs) (Figure 1A). Basal levels of activated Akt and Erk were reduced in Mgat5–/– MEFs, and doubling time increased under normal culture conditions (Figure 1B, data not shown). Glucose uptake rates and reactive oxygen species (ROS) levels were similar in Mgat5–/– and Mgat5+/+ MEFs growing in high-glucose and high-fetal calf serum (FCS) conditions. However, either low-glucose or serum-free medium (SFM) conditions revealed Mgat5–/– MEFs as deficient in both, suggesting that Mgat5-modified N-glycans promote glucose metabolism (Figure 1C and D). Glucose transport is increased by low-glucose conditions and by serum in an additive manner (Figure 1C). Mgat5–/– MEFs proliferate more slowly and are less responsive to increasing serum concentrations in the growth medium, consistent with their deficiency in acute responses to growth factors (Figure 1E). Thus, Mgat5-modified N-glycans sensitize primary non-transformed cells to extrinsic growth cues, increase basal PI3K/Erk activation, and promote glucose uptake and metabolism. Indeed, cell surface ß1,6 GlcNAc-branched N-glycans increase under restricted glucose concentrations compared with excess glucose concentrations (Figure 1F). This indicates that N-glycan branching is sensitive to upregulation by glucose deprivation, which may be the result of altered hexosamine flux, increased expression of Golgi enzymes, or altered trafficking of glycoproteins.
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Resistance to obesity in Mgat5-deficient mice
To determine whether Mgat5 is a determinant of metabolic homeostasis, mutant and wild-type mice on a C57BL/6 background were provided with either regular or high-fat and carbohydrate-enriched diet. Mgat5–/– mice are born in the expected Mendelian ratio from heterozygous matings, normal in proportions and weight, and appear healthy as young adults. Time to weaning was similar for all genotypes on either diet; however, 16 weeks after weaning, male and female Mgat5+/+ mice on the enriched diet were approximately 20–30% heavier than Mgat5–/– mice (Figure 2A and B). To assess rates of weight gain in adults, mice raised on the regular diet were transferred to the enriched diet for 6 weeks. Mgat5+/+, Mgat5+/–, and Mgat5–/– male mice on the enriched diet gained weight at rates of 2.1, 1.7, and 0.3% per week, respectively (Figure 2C). Female mice showed similar weight gains for the respective genotypes (Figure 2D). Mgat5+/– and Mgat5+/+ mice born to Mgat5+/– mothers on the enriched food and continued on the same diet were approximately 35% heavier than their Mgat5–/– littermates by approximately 8 months of age (Figure 2E and F). These Mgat5+/– and Mgat5+/+ mice lost approximately 10% of their body weight in 6 weeks when switched from the enriched diet to the regular diet, but regained 15–20% in only 5 weeks upon returning to the enriched diet. The body weights of Mgat5–/– mice were remarkably stable despite changes in their diet. Mgat5+/+ and Mgat5+/– mice on the enriched diet showed large abdominal fat deposits, whereas Mgat5–/– mice were lean with small fat pads in the groin (not shown). However, Mgat5–/– and Mgat5+/+ mice maintained similar daily energy intakes on either the regular or enriched diets (Figure 3A).
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Mgat5+/+ and Mgat5+/– male mice maintained on the enriched diet displayed elevated nonfasting blood glucose, whereas the Mgat5 deficiency blunted the effect of this diet. Nonfasting blood glucose was reduced in male and female Mgat5–/– mice on both diets (Figure 3B). The enriched diet produced a 36 and 66% increase in blood glucose in male Mgat5–/– and Mgat5+/+ mice, respectively. Glucose tolerance tests showed similar kinetics for glucose clearance in Mgat5+/+, Mgat5+/–, and Mgat5–/– mice, with mutant mice returning to lower levels than Mgat5+/+ mice (Figure 3C). Liver glycogen stores were similar for all genotypes, but after an 18-h fast, Mgat5–/– mice experienced a greater depletion and mobilization of fat stores to the liver (Figure 4A–C). Fasting blood glucose levels fell to lower levels in Mgat5–/– and Mgat5+/– mice than Mgat5+/+ mice, suggesting a metabolic abnormality in liver that is enhanced by fasting (Figure 4D).
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Mgat5-modified N-glycans promote fecundity in a nutrient-dependent manner
Fecundity is sensitive to both dietary and genetic determinants of nutrient availability and therefore might be lower in Mgat5–/– mice. We observe that Mgat5–/– females on the C57BL/6 background produced fewer pups per litter than Mgat5+/+ and Mgat5+/– littermates on the regular diet (Figure 4E). Additionally, only 10% of the newborn pups survived to weaning with Mgat5–/– mothers regardless of their genotypes, compared with 80% of pups with Mgat5-expressing mothers (Figure 4F). Mgat5–/– females housed either alone or with a male displayed less time gathering pups and nurturing (Granovsky et al. 2000
). It is possible that the Mgat5 deficiency alters certain sensory functions affecting nurturing, as has been suggested for deficiencies in FosB (Brown et al. 1996) and cAMP response element-binding protein (Jin et al. 2005). Interestingly, when two Mgat5–/– mothers are housed together, the number of pups born per litter is not altered, but rearing efficiency is restored to approximately 80%, suggesting that sharing of material and parenting resources improves Mgat5–/– animals' performance (Figure 4F). Selection pressures to maximize productivity per litter lead to strained parental resources and thus create parent–offspring conflicts (Parker et al. 2002; Hager and Johnstone 2003). Mgat5–/– maternal resources appear to be sufficient for smaller litter sizes as indicated by time to weaning, but this may be at the cost of postnatal culling of pups. Mgat5–/– mothers lost all pups in approximately 25% of litters, which may also arise from a programmed response of reduced commitment to small litters (Kilner et al. 2004).
Respiratory quotient
Weight gain requires that more calories be allotted to anabolic metabolism and storage than utilized in basic maintenance, physical activity, and adaptive thermogenesis. On the regular diet, the respiratory quotient (RQ) (VCO2/VO2) was reduced in Mgat5–/– mice during the light cycle (P < 0.001), but was not significantly lower in the dark cycle (Figure 5A), suggesting that fatty acid oxidation is increased in Mgat5–/– mice, particularly during the sleep cycle. Locomotion and total movement of Mgat5+/+ and Mgat5–/– mice throughout the day were similar under controlled nonstressful conditions (Figure 5B). The anorexigenic hormone leptin was 1.81 ± 0.26 (n = 19) and 3.25 ± 0.77 ng/mL (n = 14) (P < 0.057) in mutant and wild-type mice on the regular diet, respectively. Serum insulin was similar at 0.89 ± 0.12 versus 0.91 ± 0.13 ng/mL, but glucagon levels were 41.2 ± 10.7 and 106.7 + 5.0 pM in Mgat5+/+ and Mgat5–/– mice, respectively. Basal and poststress serum corticosterone levels were also not different in Mgat5–/– and Mgat5+/+ mice (data not shown). Stearoyl-CoA desaturase mutant mice display a lean phenotype similar to that of Mgat5–/– mice with reduced leptin levels and increased fatty acid oxidation (Dobrzyn et al. 2004). Calorie-restricted animals also display reduced leptin and blood glucose levels, and increased plasma fatty acids and resting oxidative metabolism as measured by RQ. Taken together, metabolic homeostasis in Mgat5-deficient mice is characterized by reduced serum glucose and increased catabolic metabolism, suggesting that Mgat5-modified N-glycans provide positive feedback to glucose metabolism and fat storage.
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Mgat5 promotes renewal in muscle and bone
Mgat5-modified N-glycans sensitize MEFs to growth factors and enhance basal glucose uptake in an additive or synergistic manner. If reduced cellular glucose uptake is a characteristic of homeostasis in Mgat5–/– mice, skeletal muscle may display features of chronic starvation, notably increased fast-twitch myofibers, a glycolytic cell type with fewer mitochondria than slow-twitch myofibers (Ryder et al. 2003
). Indeed, muscle from 3-month-old Mgat5–/– mice on the regular diet display twice as many fast-twitch myofibers compared with Mgat5+/+ muscle (Figure 6A and B). Soft tissue (i.e. largely muscle) and adipose tissue increase in Mgat5+/+ mice between 3 and approximately 16 months of age, but remain unchanged in Mgat5–/– littermates, suggesting an accelerated loss of tissue repair and growth with aging (Figure 6C). Serum levels of Na+, K+, Ca++, Cl–, lactate, glutamine, and urea were normal, whereas creatine was 41% lower in Mgat5–/– mice at around 1 year of age, which may reflect a reduction in muscle growth and renewal (Table I).
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Satellite cells, positioned between basal lamina and the membrane of the myofibers, provide most of the regenerative potential of adult muscles. In response to injury, satellite cells are stimulated to proliferate and differentiate to form MyoD+ myocytes and myotubes (Conboy et al. 2003
). Fewer satellite cells were isolated from newborn Mgat5–/– mice compared with wild-type littermates, suggesting that either insufficient satellite stem cells are generated during embryogenesis or their renewal is deficient in postnatal life, or both (Figure 6D and E). The Pax7 + to MyoD + ratio in Mgat5–/– and Mgat5+/+ satellite cell cultures was 0.5:1 and 1:1, respectively, indicating a higher rate of differentiation in mutant cells (Figure 6F). At higher cell densities, Mgat5+/+ satellite cells fuse at higher frequency to form MyoD+ myotubules, but fusion was impaired in Mgat5–/– cultures even though the cells were largely MyoD+ (Figure 6D). Mgat5–/– satellite cells were less sensitive to serum-dependent activation of Erk, consistent with a tendency to favor myodifferentiation over self-renewal (Figure 6G).
In Mgat5–/– bone marrow cultures, nuclear Erk-p was decreased and nuclear Smad2/3 increased upon stimulation with serum, consistent with reduced replicative potential (Figure 7A). TGF-ß/Smad signaling is required for the later stages of osteoblast differentiation and mineralization, but elevated TGF-ß signaling limits the early expansion of osteoprogenitor colonies (Binkert et al. 1999). Male and female Mgat5–/– mice display multiple features characteristic of early aging, including loss of bone mineral density (BMD) in vertebrae and femur and in whole-body measurements (Figure 7B). Bone mass content and BMD in whole-body scans were reduced by approximately 20% in Mgat5–/– mice over 1 year of age compared with their Mgat5+/+ littermates (Supplementary Figures S1 and S2). Bone marrow stromal cells from 3-month-old mice were cultured under conditions that induce osteogenesis, and the Mgat5–/– cells displayed reduced calcium phosphate deposition compared with stromal cells from Mgat5+/+ littermates, suggesting a deficiency in either osteoblast numbers or differentiation (Figure 7C). Longevity is reduced in Mgat5–/– mice (Figure 7D), and other age-related pathologies observed in 129/sv and C57Bl/6 Mgat5–/– mice include anal prolapse, excessive leanness, spinal compression or kyphosis, and excessive anxiety (Supplementary Table I). Expression of surface ß1,6GlcNAc-branched N-glycans declines with aging in wild-type 129/sv mice but not in Mgat5+/– mice, as indicated by a lower L-PHA (Phaseolus vulgaris lectin L) reactivity of lymphocytes at 12 months of age compared with 3-month-old animals (Figure 7E).
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Finally, a similar imbalance of Erk and Smad2/3 activation following serum stimulation of MEFs was observed in Mgat5–/– MEFs, with Mgat5+/– MEFs displaying an intermediate phenotype (Figure 8A–F). Low-glucose growth conditions further suppressed Erk activation in Mgat5–/– MEFs and prolonged Smad2/3 activation (Figure 8A–D). Our results suggest that ß1,6GlcNAc-branched N-glycans promote self-renewal and pluripotency of stem cells by balancing signaling, including RTK/PI3K/Erk (growth and glycoytic metabolism) and TGF-ß/Smad (arrest and differentiation) (Figure 9).
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Discussion |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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Here, we report that Mgat5–/– mice are resistant to weight gain on an enriched diet, hypersensitive to fasting, mildly hypoglycemic, less fecund, and display reduced RQ while calorie intake and physical activity are normal. Tonic levels of activated Akt and Erk in Mgat5–/– MEFs under normal culture conditions as well as acute response to growth factors are reduced. Glucose uptake is impaired in Mgat5–/– MEFs under low-serum or low-glucose growth conditions, suggesting that Mgat5–/– MEFs are less resilient or adaptable to growth conditions. This suggests that Mgat5 N-glycan products enhance adoption to changes in nutrient conditions, which may include short-term shifts between anabolic and catabolic metabolism, as well as long-term adaptation.
Mgat5–/– MEFs display enhanced sensitivity to TGF-ß and an apparent insufficiency in growth signaling, particularly in low-glucose conditions. TßRI/II complexes are rapidly recycled between the surface and endosomes, independent of their activation by ligand (Di Guglielmo et al. 2003). Hence, TGF-ß signaling is particularly sensitive to membrane remodeling, which is stimulated by the growth-promoting RTKs. As such, surface levels of TßR can be increased to levels that are functionally dominant for growth arrest by slowing endocytosis and/or increasing the hexosamine/Golgi/galectin lattice (Figure 9). In this regard, low-glucose conditions increased ß1,6GlcNAc-branched N-glycans and UDP-GlcNAc levels (Figure 1F, data not shown). In low glucose, reduced protein synthesis and slower glycoprotein transit through the Golgi may spare UDP-GlcNAc and increase the fraction of ß1,6GlcNAc-branched N-glycans. Although surface TßR and other arrest mediators may be functionally dominant under limiting conditions, the levels of growth-promoting high-n RTKs are also maintained by the lattice, and may serve to prime cells for a shift to more favorable growth conditions (Figure 9).
In vivo, Mgat5–/– mice displayed a significantly lower RQ during the rest period (light cycle) when serum glucose levels are generally lower. The Mgat5 deficiency did not impair ß-cell function and insulin sensitivity, as suggested by normal glucose tolerance in Mgat5–/– mice. Skeletal muscle in adult Mgat5–/– mice displays characteristics of reduced glucose metabolism and, with aging, muscle and bone mass declined more rapidly compared with wild-type mice. The activation of Erk and Smad2/3 in Mgat5–/– MEFs and bone marrow stem cells by serum favored Smad2/3, suggesting that the Mgat5 deficiency increases the probability of growth arrest. In vivo, Mgat5–/– muscle displays pathology associated with reduced glycolytic metabolism consistent with a lower RQ, suggesting a shift toward increased metabolism of other carbon sources such as fatty acids and protein. Moreover, young Mgat5–/– mice are deficient in bone marrow osteogenesis and muscle satellite cells compared with age-matched wild-type littermates. In phosphatase and tensin homologue (Pten+/–) mice in which PI3K/Akt growth signaling is increased, early overproliferation of stem cells is followed by their loss, which could be partially rescued by inhibiting downstream mTor kinase with rapamycin (Yilmaz et al. 2006). The deletion of the Rho GTPase Rac1 in epidermis, a pleiotropic regulator of growth signaling downstream of RTKs, stimulated stem cells to divide and then undergo terminal differentiation, thus a failure of maintenance and repair (Benitah et al. 2005). Taken with the present results, balancing growth and arrest signaling in stem cells and tissue renewal is dependent on conditional regulation of ß1,6GlcNAc-branched N-glycans on glycoprotein receptors.
Moderate calorie restriction promotes longevity in S. cerevisiae, Caenorhabditis elegans, and mammals (Guarente and Picard 2005), and in yeast, minimizes storage and balances glycolytic and oxidative respiration to promote high-energy charge and low ROS (Lin et al. 2002). Mice with a single copy of the insulin receptor gene (IGF-1R) display normal energy metabolism and slightly longer lifespan (Holzenberger et al. 2003). Although the apparent reduction in glucose utilization in Mgat5–/– mice might be expected to extend longevity, the accompanying increase in oxidative respiration and reduced soft tissue mass with aging suggest a limiting defect in anabolic metabolism. Akt1-deficient mice also display a slow-growth phenotype but manifested at birth, whereas in Mgat5–/– mice, renewal and growth in adult mice is impaired (Cho et al. 2001). Pten phosphatase opposes intracellular PI3K/Akt signaling and is a potent tumor suppressor. The Mgat5-deficiency increases survival time by 8–40% in Pten+/– mice, suggesting that Mgat5–/– exerts a brake on hyperactivation of PI3K/Akt signaling and slows tumor development in the Pten+/– background (Cheung and Dennis 2007). Increased p53 activity also suppresses tumor development, and favors arrest signaling, oxidative metabolism, loss of stem cells, and early mortality (Tyner et al. 2002; van Heemst et al. 2005), features common with the Mgat5–/– phenotype. N-glycan processing, PI3K/Pten, and p53 appear to be highly integrated with signaling networks and basic metabolism.
The allosteric feedback regulation of enzymes by metabolites occurs on a timescale of seconds, whereas changes in N-glycan biosynthesis and turnover of surface glycoproteins are slower. Mgat5 and N-glycan processing may adapt cellular sensitivity to cytokines by functioning both upstream and downstream of metabolism, thereby impacting body weight, fecundity, stem cell renewal, and aging. The efficient utilization and storage of nutrients when food is abundant increases the probability of survival in the capricious environments that characterize most of natural history. However, the rise in obesity and type-2 diabetes with the Western lifestyle indicates a strong interaction between genes and the nutrient environment (Diamond 2003; O'Rahilly et al. 2005). Further analysis of metabolite flux to sugar-nucleotide and N-glycan branching may suggest new avenues to enhance productive lifespan through stem cell maintenance and suppression of obesity.
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Materials and methods |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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Mice and cell lines
The Mgat5 mutation was maintained on C57BL/6 and 129/sv backgrounds, backcrossed 10 generations (Granovsky et al. 2000
). The C57BL/6 background is hypersensitive to obesity and insulin resistance on a high-fat diet, and was adopted for the diet experiments in Figures 2 and 3. The 129/sv strain was used for the generation of MEFs and for all other experiments in this report. The Mgat5 mutation had similar effects on reducing fecundity and early mortality on both strain backgrounds. Mice were fed the regular diet unless stated otherwise, which was gram percent of 38.6% starch, 22.5% protein, 9% fat, with a metabolizable energy of 3.11 kcal/g (LabDiet® 5012, Purina Mills, St. Louis, MO). The enriched diet comprised 38% starch, 20.5% protein, 18.5% fat for a metabolizable energy of 3.82 kcal/g (PicoLab® mouse diet 20, Purina Mills) with an additional 6% dry supplement of sucrose. Daily food intake per animal was determined every 24 h over 10 days and expressed per body mass corrected by power function (g baseline weight)0.568, as described previously (Conti et al. 2006). MEFs were isolated from E14.5 embryos from Mgat5+/–x Mgat5+/– matings using standard protocols and cultured in Dulbecco's modified Eagle's medium (DMEM) containing 10% FCS (serum) (Invitrogen, Carlsbad, CA). After two passages, the MEFs were cryopreserved in 10% dimethyl sulfoixde (DMSO)–20% FCS in DMEM and used in experiments after no more than two further passages or 2 weeks in culture.
Nuclear translocation of Erk-p and Smad2/3
Cells plated in 96-well plates at 1000 cells/well were serum-starved for 24 h and then stimulated with TGF-ß1, EGF (R&D Systems, Minneapolis, MN), or serum. Cells were fixed after 40 min of TGF-ß stimulation to measure Smad2/3 nuclear translocation, or after 5 min of EGF stimulation to measure Erk-p nuclear translocation. Cells were fixed for 10 min with 3.7% formaldehyde at room temperature, washed with phosphate-buffered saline (PBS) plus 1% serum, and permeabilized using 100% MeOH for 2 min. The cells were washed 3 times and blocked in PBS plus 10% serum overnight at 4 °C. Mouse anti-Smad2/3 (Transduction Laboratories S66220
[GenBank]
, BD Biosciences, San Jose, CA) or mouse phospho-Erk1/2 (Thr202/Tyr204) (Sigma M-8159) or antiphospho Smad2 (Ser465/467) (Cell Signaling 3101S, Technology, Danvers, Sigma-Aldrich, St. Louis, MO) was added at 1/1000 in PBS plus 10% serum for 2 h at 20 °C. The cells were washed 3 times with PBS plus 1% serum. AlexaFluor 488-labeled antimouse Ig secondary antibody (Molecular Probes) was added at 1/1000 with Hoechst 33342 (1/2000) (Molecular Probes, Invitrogen) for 1 h at 20 °C. After washing 3 times, the plates were scanned using the ArrayScan II fluorescence microscope (Cellomics, Pittsburgh). The nuclear and cytoplasmic staining intensities were determined individually for 200 cells/well and cytoplasmic staining intensity subtracted from nuclear staining intensity for each cell. The mean differential signal ± SE (n = 200) was generally < 4% at each assay point.
Spleen cells from 129/sv were stained with 10 ng/mL of fluorescein isothiocyanate (FITC) L-PHA (E-Y Labs, San Mateo, CA) and tetramethylrhodamine isothiocyanate (TRITC)-anti-CD8 (eBioscience, San Diego, CA) at room temperature for 1 h and then fixed without permeabilization. Cells were analyzed by fluorescence activated cell sorting (FACS), and L-PHA mean fluorescence intensity in CD8 + cells was quantified.
Glucose transport and ROS measurements
Cells were seeded in triplicate at 2.5 x 105 cells/well in six-well plates. Following incubation at 37 °C, the cells were washed twice with assay buffer (Krebs–Ringer solution: 116 mM NaCl, 5.4 mM KCl, 0.8 mM MgSO4, 1.0 mM CaCl2, 25 mM Tris–HCl, 0.2% bovine serum albumin (BSA), 1.0 mM NaH2PO4, and 2.0 mM Na2PO4, pH 7.45). Cells were then incubated with assay buffer (1 mL/well) at 37 °C for 30 min, with the addition of 1 mL of [3H]2-deoxy-D-glucose (2 µCi/mL) (NEN) in assay buffer for a further 20 min. The cells were washed 4 times with ice-cold assay buffer, and 0.5 mL of ice-cold 5% trichloroacetic acid (TCA) was added to each well. After 30 min on ice, 0.4 mL of TCA supernatant was removed and [3H]2-deoxy-D-glucose was quantified by scintillation counting.
To measure ROS, cells were seeded in triplicate at a density of 1000 cells/well in 96-well plates (Corning 96 Flat Opaque, Corning, NY) and cultured for 24 h. Cells were washed with PBS, followed by the addition of 100 µl of 1 µM H2DCFDA (Molecular Probes, Invitrogen) in PBS. The plates were incubated at 37 °C for 30 min, and oxidized probe at the bottom of each well was determined by excitation at 485–20 nm and emission was measured at 530–25 nm using an Analyst HT plate reader (Molecular Devices) and the Criterion Host software (LJL Biosystems, Molecular Devices Corporation, Sunnyvale, CA). Background fluorescence in the absence of probe was subtracted, and a standard curve for oxidation was generated using hydrogen peroxide. Results are plotted as the mean ± SD of triplicate determinations, and experiments were repeated 3 times.
Respiratory quotient
Oxygen consumption and carbon dioxide production rates were recorded every 15 min for 24 h with the use of a mouse indirect calorimeter (Oxymax; Columbus Instruments, Columbus, OH). Water and food (regular diet) were available ad libitum in the chambers. Measurements were taken in an airtight Oxymax chamber with an airflow rate of 0.5 L/min. RQ was calculated as the molar ratio of Vco2 toVo2 for 10 mice per genotype, averaging the 15 min measurements for the light and dark cycle CO2 and O2. Locomotion and total movement were measured continuously during the 24 h. Locomotion is a measure of lateral movement, whereas total movement includes body movements such as grooming as well as lateral movement.
Muscle and satellite cells
Satellite cells were extracted from the muscle of the limbs by incubating with collagenase–dispase solution (Hoffmann–La Roche, Basal, Switzerland) for 30 min with agitation. Fibroblasts were removed by adherence to tissue culture plates in Ham's F-10 (Invitrogen) incubated for 60 min at 37 °C. The non-adhering satellite cells were transferred to collagen-coated dishes and cultured in Ham's F-10 medium, 20% serum, penicillin/streptomycin, Fungizone (Invitrogen), and 20 nM bFGF (Sigma).
For immunohistochemistry, satellite cells were plated onto collagen-coated glass coverslips and cultured for 1–4 days in the earlier-mentioned medium. Slides were fixed with 3.7% formaldehyde in PBS and permeabilized with cold 100% methanol for 2 min, and blocked in PBS, 10% serum overnight at 4 °C. Cells were stained with a 1 : 50 dilution of mouse anti-Pax7 (Developmental Studies Hybridoma Bank, Iowa City, IA) and 1 : 50 rabbit anti-MyoD (Santa Cruz Biotechnology, Santa Cruz, CA) diluted in PBS, 10% serum for 2 h at room temperature. Slides were washed with PBS, 10% serum and stained with 1 : 100 AlexaFluor 488 goat antimouse Ig, and Cy5-labeled goat antirabbit Ig (Jackson Labs, The Jackson Laboratory, Bar Harbor, ME) and 1: 500 Hoechst for 30 min at room temperature. Slides were washed, mounted, and imaged. Cryosections of quadriceps muscle (8 µm) were mounted, postfixed with 4% paraformaldehyde, and stained for alkaline ATPase, pH 9.4, which stained fast-twitch myofibers darkly, whereas slow-twitch myofibers stained lightly.
Bone phenotype and osteogenesis in bone marrow cultures
Dual-energy X-ray absorptiometry (Pixi mus, Fitchburg, WI; GE Healthcare Bio-Sciences, Piscataway, NJ) was used to measure bone mineral content, bone area, and BMD on whole animals. Femurs were removed under aseptic conditions from mice, cleaned of adherent soft tissues, and washed extensively in antibiotics. The distal ends were removed and the marrow contents were flushed out with 10 mL of culture medium. Cells were dispersed by repeated passage through a 20-gauge needle and incubated in alpha-MEM supplemented with 15% serum, ascorbic acid (50 µg/mL), antibiotics [penicillin G 100 µg/mL, gentamicin 50 µg/mL, Fungizone (Invitrogen) 0.3 µg/mL], 10 mM ß-glycerophosphate, and vitamin C. Culture media was supplemented further with dexamethasone (10–8 M). Following 6 days of culture, the cells were replated at a density of 1 x 102 cells/mm2 in 96-well plates and grown for another 12–14 days, with changes of the same medium at 48 h intervals. At the end of culture, the cells were fixed with 10% buffered formalin and stained for calcium with Alizarin red-S to identify mineralized bone nodules. To quantify mineralized tissue formation in the cultures, the absorbance at 525 nm was measured using a 96-well plate reader. Duplicate wells were stained for lipid content. Cells were fixed with 70% ethanol and stained with Sudan IV in acetone:ethanol for 20 min and washed with 70% ethanol for photographic documentation.
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Supplementary data |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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Supplementary data are available at Glycobiology online http://glycob.oxfordjournals.org/).
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Conflict of interest statement |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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None declared.
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Acknowledgments |
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TopAbstractIntroductionResultsDiscussionMaterials and methodsSupplementary dataConflict of interest statementAcknowledgmentsReferences |
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This research was supported by grants to J.W.D. from the Canadian Institute for Health Research. The authors thank Susan Wang, Dr Luisa Moreno, Lois Kelsey, and Laili Soleimani for technical assistance.
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Abbreviations |
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bFGF, fibroblast growth factor; BMD, bone mineral density; BMP, bone morphogenic protein; DMEM, Dulbecco's modified Eagle's medium; EGFR, epidermal growth factor (EGF) receptor; Erk, extracellular response kinase; FCS, fetal calf serum; FITC, fluorescein isothiocyanate; GlcNAc-T, N-acetylglucosaminyltransferase; IGF, insulin-like growth factor; L-PHA, Phaseolus vulgaris lectin L; MEF, mouse embryonic fibroblast; Mgat5, ß1,6-N-acetylglucosaminyltransferase V gene; PBS, phosphate-buffered saline; PDGF, platelet-derived growth factor; PI3K, phosphoinositide 3-kinase; Pten, phosphatase and tensin homolog; ROS, reactive oxygen species; RQ, respiratory quotient; RTK, receptor tyrosine kinase; SFM, serum-free medium; TßR, transforming growth factor-ß receptor; UDP-GlcNAc, UDP-N-acetylglucosamine
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