The effects of age and dietary restriction on the tissue‐specific metabolome of Drosophila

Dietary restriction (DR) is a robust intervention that extends lifespan and slows the onset of age‐related diseases in diverse organisms. While significant progress has been made in attempts to uncover the genetic mechanisms of DR, there are few studies on the effects of DR on the metabolome. In recent years, metabolomic profiling has emerged as a powerful technology to understand the molecular causes and consequences of natural aging and disease‐associated phenotypes. Here, we use high‐resolution mass spectroscopy and novel computational approaches to examine changes in the metabolome from the head, thorax, abdomen, and whole body at multiple ages in Drosophila fed either a nutrient‐rich ad libitum (AL) or nutrient‐restricted (DR) diet. Multivariate analysis clearly separates the metabolome by diet in different tissues and different ages. DR significantly altered the metabolome and, in particular, slowed age‐related changes in the metabolome. Interestingly, we observed interacting metabolites whose correlation coefficients, but not mean levels, differed significantly between AL and DR. The number and magnitude of positively correlated metabolites was greater under a DR diet. Furthermore, there was a decrease in positive metabolite correlations as flies aged on an AL diet. Conversely, DR enhanced these correlations with age. Metabolic set enrichment analysis identified several known (e.g., amino acid and NAD metabolism) and novel metabolic pathways that may affect how DR effects aging. Our results suggest that network structure of metabolites is altered upon DR and may play an important role in preventing the decline of homeostasis with age.     

Host and symbiont genetic determinants of nutritional phenotype in a natural population of the pea aphid

A defining feature of the nutritional ecology of plant sap‐feeding insects is that the dietary deficit of essential amino acids (EAAs) in plant sap is supplemented by EAA‐provisioning microbial symbionts in the insect. Here, we demonstrated substantial variation in the nutritional phenotype of 208 genotypes of the pea aphid Acyrthosiphon pisum collected from a natural population. Specifically, the genotypes varied in performance (larval growth rates) on four test diets lacking the EAAs arginine, histidine and methionine or aromatic EAAs (phenylalanine and tryptophan), relative to the diet containing all EAAs. These data indicate that EAA supply from the symbiotic bacteria Buchnera can meet total aphid nutritional demand for only a subset of the EAA/aphid genotype combinations. We then correlated single nucleotide polymorphisms (SNPs) identified in the aphid and Buchnera genomes by reduced genome sequencing against aphid performance for each EAA deletion diet. This yielded significant associations between performance on the histidine‐free diet and Buchnera SNPs, including metabolism genes predicted to influence histidine biosynthesis. Aphid genetic correlates of performance were obtained for all four deletion diets, with associations on the arginine‐free diet and aromatic‐free diets dominated by genes functioning in the regulation of metabolic and cellular processes. The specific aphid genes associated with performance on different EAA deletion diets are largely nonoverlapping, indicating some independence in the regulatory circuits determining aphid phenotype for the different EAAs. This study demonstrates how variation in the phenotype of associations collected from natural populations can be applied to elucidate the genetic basis of ecologically important traits in systems intractable to traditional forward/reverse genetic techniques.     

Nutritional balance of essential amino acids and carbohydrates of the adult worker honeybee depends on age

Dietary sources of essential amino acids (EAAs) are used for growth, somatic maintenance and reproduction. Eusocial insect workers such as honeybees are sterile, and unlike other animals, their nutritional needs should be largely dictated by somatic demands that arise from their role within the colony. Here, we investigated the extent to which the dietary requirements of adult worker honeybees for EAAs and carbohydrates are affected by behavioural caste using the Geometric Framework for nutrition. The nutritional optimum, or intake target (IT), was determined by confining cohorts of 20 young bees or foragers to liquid diets composed of specific proportions of EAAs and sucrose. The IT of young, queenless bees shifted from a proportion of EAAs-to-carbohydrates (EAA:C) of 1:50 towards 1:75 over a 2-week period, accompanied by a reduced lifespan on diets high in EAAs. Foragers required a diet high in carbohydrates (1:250) and also had low survival on diets high in EAA. Workers exposed to queen mandibular pheromone lived longer on diets high in EAA, even when those diets contained 5× their dietary requirements. Our data show that worker honeybees prioritize their intake of carbohydrates over dietary EAAs, even when overeating EAAs to obtain sufficient carbohydrates results in a shorter lifespan. Thus, our data demonstrate that even when young bees are not nursing brood and foragers are not flying, their nutritional needs shift towards a diet largely composed of carbohydrates when they make the transition from within-hive duties to foraging.     

鸡肉粉完全替代鱼粉饲料中补充包被氨基酸对凡纳滨对虾生长、体成分及组织氨基酸含量的影响

为了探讨鸡肉粉完全替代鱼粉时饲料氨基酸的平衡性以及外源氨基酸的添加方式与凡纳滨对虾生长、体成分、血浆游离氨基酸及肌肉氨基酸含量的关系, 本试验采用26因子试验设计进行了为期56d的饲养试验。2个饲料蛋白质水平分别为40%和32%, 6个饲料处理分别为鱼粉组(对照组)、鸡肉粉组、鸡肉粉+晶体EAA组、鸡肉粉+晶体EAA+晶体NEAA组、鸡肉粉+包被EAA组、鸡肉粉+包被EAA+包被NEAA组, 配制12组饲料。将凡纳滨对虾(0.300.01) g随机分配到36个圆桶(150 L)中, 每桶30尾, 每3个桶为一个处理组, 饲喂一种饲料, 每天饱食投喂三次。在每一饲料蛋白质水平下, 无论是补充晶体氨基酸(CAA)组还是包被氨基酸组对虾的增重率均显著高于鸡肉粉组(P0.05), 且在32%蛋白质水平下, 包被EAA组对虾增重率达到了鱼粉组水平(P0.05); 补充晶体EAA+NEAA组对虾增重率与补充晶体EAA组无差异(P0.05), 但均显著低于补充包被氨基酸组(P0.05); 补充包被EAA组对虾增重率显著高于补充包被EAA+NEAA组(P0.05)。饲料系数的变化正好与增重率变化相反(P0.05)。饲喂高蛋白质水平饲料较之饲喂低蛋白质饲料明显提高对虾增重率、虾体蛋白含量(P0.05), 但降低虾体脂肪含量(P0.05)。包被氨基酸组凡纳滨对虾血浆游离氨基酸含量总体显著低于CAA组(P0.05)。除谷氨酸、甘氨酸以及脯氨酸外, 各组对虾肌肉氨基酸含量无显著差异(P0.05)。结果表明, 在32%饲料蛋白质水平下, 用鸡肉粉完全替代鱼粉时, 饲料中补充包被EAA可明显促进凡纳滨对虾的生长, 且达到了鱼粉组的饲喂效果。     

Dietary fat composition influences glomerular and proximal convoluted tubule cell structure and autophagic processes in kidneys from calorie‐restricted mice

Calorie restriction (CR) has been repeatedly shown to prevent cancer, diabetes, hypertension, and other age‐related diseases in a wide range of animals, including non‐human primates and humans. In rodents, CR also increases lifespan and is a powerful tool for studying the aging process. Recently, it has been reported in mice that dietary fat plays an important role in determining lifespan extension with 40% CR. In these conditions, animals fed lard as dietary fat showed an increased longevity compared with mice fed soybean or fish oils. In this paper, we study the effect of these dietary fats on structural and physiological parameters of kidney from mice maintained on 40% CR for 6 and 18 months. Analyses were performed using quantitative electron microcopy techniques and protein expression in Western blots. CR mitigated most of the analyzed age‐related parameters in kidney, such as glomerular basement membrane thickness, mitochondrial mass in convoluted proximal tubules and autophagic markers in renal homogenates. The lard group showed improved preservation of several renal structures with aging when compared to the other CR diet groups. These results indicate that dietary fat modulates renal structure and function in CR mice and plays an essential role in the determination of health span in rodents.     

Genetic and metabolic determinants of nutritional phenotype in an insect-bacterial symbiosis

The pervasive influence of resident microorganisms on the phenotype of their hosts is exemplified by the intracellular bacterium Buchnera aphidicola, which provides its aphid partner with essential amino acids (EAAs). We investigated variation in the dietary requirement for EAAs among four pea aphid (Acyrthosiphon pisum) clones. Buchnera-derived nitrogen contributed to the synthesis of all EAAs for which aphid clones required a dietary supply, and to none of the EAAs for which all four clones had no dietary requirement, suggesting that low total dietary nitrogen may select for reduced synthesis of certain EAAs in some aphid clones. The sequenced Buchnera genomes showed that the EAA nutritional phenotype (i.e. the profile of dietary EAAs required by the aphid) cannot be attributed to sequence variation of Buchnera genes coding EAA biosynthetic enzymes. Metabolic modelling by flux balance analysis demonstrated that EAA output from Buchnera can be determined precisely by the flux of host metabolic precursors to Buchnera. Specifically, the four EAA nutritional phenotypes could be reproduced by metabolic models with unique profiles of host inputs, dominated by variation in supply of aspartate, homocysteine and glutamate. This suggests that the nutritional phenotype of the symbiosis is determined principally by host metabolism and transporter genes that regulate nutrient supply to Buchnera. Intraspecific variation in the nutritional phenotype of symbioses is expected to mediate partitioning of plant resources among aphid genotypes, potentially promoting the genetic subdivision of aphid populations. In this way, microbial symbioses may play an important role in the evolutionary diversification of phytophagous insects.     

Dietary restriction attenuates age-associated muscle atrophy by lowering oxidative stress in mice even in complete absence of CuZnSOD

Age-related loss of muscle mass and function, sarcopenia, has a major impact on the quality of life in the elderly. Among the proposed causes of sarcopenia are mitochondrial dysfunction and accumulated oxidative damage during aging. Dietary restriction (DR), a robust dietary intervention that extends lifespan and modulates age-related pathology in a variety of species, has been shown to protect from sarcopenia in rodents. Although the mechanism(s) by which DR modulates aging are still not defined, one potential mechanism is through modulation of oxidative stress and mitochondrial dysfunction. To directly test the protective effect of DR against oxidative stress-induced muscle atrophy in vivo, we subjected mice lacking a key antioxidant enzyme, CuZnSOD (Sod1) to DR (60% of ad libitum fed diet). We have previously shown that the Sod1(-/-) mice exhibit an acceleration of sarcopenia associated with high oxidative stress, mitochondrial dysfunction, and severe neuromuscular innervation defects. Despite the dramatic atrophy phenotype in the Sod1(-/-) mice, DR led to a reversal or attenuation of reduced muscle function, loss of innervation, and muscle atrophy in these mice. DR improves mitochondrial function as evidenced by enhanced Ca(2+) regulation and reduction of mitochondrial reactive oxygen species (ROS). Furthermore, we show upregulation of SIRT3 and MnSOD in DR animals, consistent with reduced mitochondrial oxidative stress and reduced oxidative damage in muscle tissue measured as F(2) -isoprostanes. Collectively, our results demonstrate that DR is a powerful mediator of mitochondrial function, mitochondrial ROS production, and oxidative damage, providing a solid protection against oxidative stress-induced neuromuscular defects and muscle atrophy in vivo even under conditions of high oxidative stress.     

Metabotyping of long-lived mice using 1H NMR spectroscopy

Significant advances in understanding aging have been achieved through studying model organisms with extended healthy lifespans. Employing 1H NMR spectroscopy, we characterized the plasma metabolic phenotype (metabotype) of three long-lived murine models: 30% dietary restricted (DR), insulin receptor substrate 1 null (Irs1-/-), and Ames dwarf (Prop1df/df). A panel of metabolic differences were generated for each model relative to their controls, and subsequently, the three long-lived models were compared to one another. Concentrations of mobile very low density lipoproteins, trimethylamine, and choline were significantly decreased in the plasma of all three models. Metabolites including glucose, choline, glycerophosphocholine, and various lipids were significantly reduced, while acetoacetate, d-3-hydroxybutyrate and trimethylamine-N-oxide levels were increased in DR compared to ad libitum fed controls. Plasma lipids and glycerophosphocholine were also decreased in Irs1-/- mice compared to controls, as were methionine and citrate. In contrast, high density lipoproteins and glycerophosphocholine were increased in Ames dwarf mice, as were methionine and citrate. Pairwise comparisons indicated that differences existed between the metabotypes of the different long-lived mice models. Irs1-/- mice, for example, had elevated glucose, acetate, acetone, and creatine but lower methionine relative to DR mice and Ames dwarfs. Our study identified several potential candidate biomarkers directionally altered across all three models that may be predictive of longevity but also identified differences in the metabolic signatures. This comparative approach suggests that the metabolic networks underlying lifespan extension may not be exactly the same for each model of longevity and is consistent with multifactorial control of the aging process.     

Glycine supplementation extends lifespan of male and female mice

Diets low in methionine extend lifespan of rodents, though through unknown mechanisms. Glycine can mitigate methionine toxicity, and a small prior study has suggested that supplemental glycine could extend lifespan of Fischer 344 rats. We therefore evaluated the effects of an 8% glycine diet on lifespan and pathology of genetically heterogeneous mice in the context of the Interventions Testing Program. Elevated glycine led to a small (4%–6%) but statistically significant lifespan increase, as well as an increase in maximum lifespan, in both males (p = 0.002) and females (p < 0.001). Pooling across sex, glycine increased lifespan at each of the three independent sites, with significance at p = 0.01, 0.053, and 0.03, respectively. Glycine‐supplemented females were lighter than controls, but there was no effect on weight in males. End‐of‐life necropsies suggested that glycine‐treated mice were less likely than controls to die of pulmonary adenocarcinoma (p = 0.03). Of the 40 varieties of incidental pathology evaluated in these mice, none were increased to a significant degree by the glycine‐supplemented diet. In parallel analyses of the same cohort, we found no benefits from TM5441 (an inhibitor of PAI‐1, the primary inhibitor of tissue and urokinase plasminogen activators), inulin (a source of soluble fiber), or aspirin at either of two doses. Our glycine results strengthen the idea that modulation of dietary amino acid levels can increase healthy lifespan in mice, and provide a foundation for further investigation of dietary effects on aging and late‐life diseases.     

Dietary restriction and aging, 2009

Dietary restriction (DR) is a robust nongenetic, nonpharmacological intervention that is known to increase active and healthy lifespan in a variety of species. Despite a variety of differences in the protocols and the way DR is carried out in different species, conserved relationships are emerging among multiple species. 2009 saw the field of DR mature with important mechanistic insights from multiple species. A report of lifespan extension in rapamycin‐treated mice suggested that the TOR pathway, a conserved mediator of DR in invertebrates, may also be critical to DR effects in mammals. 2009 also saw exciting discoveries related to DR in various organisms including yeast, worms, flies, mice, monkeys and humans. These studies complement each other and together aim to deliver the promise of postponing aging and age‐related diseases by revealing the underlying mechanisms of the protective effects of DR. Here, we summarize a few of the reports published in 2009 that we believe provide novel directions and an improved understanding of dietary restriction.     

Dietary restriction involves NAD+‐dependent mechanisms and a shift toward oxidative metabolism

Interventions that slow aging and prevent chronic disease may come from an understanding of how dietary restriction (DR) increases lifespan. Mechanisms proposed to mediate DR longevity include reduced mTOR signaling, activation of the NAD⁺‐dependent deacylases known as sirtuins, and increases in NAD⁺ that derive from higher levels of respiration. Here, we explored these hypotheses in Caenorhabditis elegans using a new liquid feeding protocol. DR lifespan extension depended upon a group of regulators that are involved in stress responses and mTOR signaling, and have been implicated in DR by some other regimens [DAF‐16 (FOXO), SKN‐1 (Nrf1/2/3), PHA‐4 (FOXA), AAK‐2 (AMPK)]. Complete DR lifespan extension required the sirtuin SIR‐2.1 (SIRT1), the involvement of which in DR has been debated. The nicotinamidase PNC‐1, a key NAD⁺ salvage pathway component, was largely required for DR to increase lifespan but not two healthspan indicators: movement and stress resistance. Independently of pnc‐1, DR increased the proportion of respiration that is coupled to ATP production but, surprisingly, reduced overall oxygen consumption. We conclude that stress response and NAD⁺‐dependent mechanisms are each critical for DR lifespan extension, although some healthspan benefits do not require NAD⁺ salvage. Under DR conditions, NAD⁺‐dependent processes may be supported by a DR‐induced shift toward oxidative metabolism rather than an increase in total respiration.     

Comparative proteomic profiling reveals a role for Cisd2 in skeletal muscle aging

Skeletal muscle has emerged as one of the most important tissues involved in regulating systemic metabolism. The gastrocnemius is a powerful skeletal muscle composed of predominantly glycolytic fast‐twitch fibers that are preferentially lost among old age. This decrease in gastrocnemius muscle mass is remarkable during aging; however, the underlying molecular mechanism is not fully understood. Strikingly, there is a ~70% decrease in Cisd2 protein, a key regulator of lifespan in mice and the disease gene for Wolfram syndrome 2 in humans, within the gastrocnemius after middle age among mice. A proteomics approach was used to investigate the gastrocnemius of naturally aged mice, and this was compared to the autonomous effect of Cisd2 on gastrocnemius aging using muscle‐specific Cisd2 knockout (mKO) mice as a premature aging model. Intriguingly, dysregulation of calcium signaling and activation of UPR/ER stress stand out as the top two pathways. Additionally, the activity of Serca1 was significantly impaired and this impairment is mainly attributable to irreversibly oxidative modifications of Serca. Our results reveal that the overall characteristics of the gastrocnemius are very similar when naturally aged mice and the Cisd2 mKO mice are compared in terms of pathological alterations, ultrastructural abnormalities, and proteomics profiling. This suggests that Cisd2 mKO mouse is a unique model for understanding the aging mechanism of skeletal muscle. Furthermore, this work substantiates the hypothesis that Cisd2 is crucial to the gastrocnemius muscle and suggests that Cisd2 is a potential therapeutic target for muscle aging.     

Aging alters contractile properties and fiber morphology in pigeon skeletal muscle

In this study, we tested the hypothesis that skeletal muscle from pigeons would display age-related alterations in isometric force and contractile parameters as well as a shift of the single muscle fiber cross-sectional area (CSA) distribution toward smaller fiber sizes. Maximal force output, twitch contraction durations and the force–frequency relationship were determined in tensor propatagialis pars biceps muscle from young 3-year-old pigeons, middle-aged 18-year-old pigeons, and aged 30-year-old pigeons. The fiber CSA distribution was determined by planimetry from muscle sections stained with hematoxylin and eosin. Maximal force output of twitch and tetanic contractions was greatest in muscles from young pigeons, while the time to peak force of twitch contractions was longest in muscles from aged pigeons. There were no changes in the force–frequency relationship between the age groups. Interestingly, the fiber CSA distribution in aged muscles revealed a greater number of larger sized muscle fibers, which was verified visually in histological images. Middle-aged and aged muscles also displayed a greater amount of slow myosin containing muscle fibers. These data demonstrate that muscles from middle-aged and aged pigeons are susceptible to alterations in contractile properties that are consistent with aging, including lower force production and longer contraction durations. These functional changes were supported by the appearance of slow myosin containing muscle fibers in muscles from middle-aged and aged pigeons. Therefore, the pigeon may represent an appropriate animal model for the study of aging-related alterations in skeletal muscle function and structure.     

Methionine metabolism and methyltransferases in the regulation of aging and lifespan extension across species

Methionine restriction (MetR) extends lifespan across different species and exerts beneficial effects on metabolic health and inflammatory responses. In contrast, certain cancer cells exhibit methionine auxotrophy that can be exploited for therapeutic treatment, as decreasing dietary methionine selectively suppresses tumor growth. Thus, MetR represents an intervention that can extend lifespan with a complementary effect of delaying tumor growth. Beyond its function in protein synthesis, methionine feeds into complex metabolic pathways including the methionine cycle, the transsulfuration pathway, and polyamine biosynthesis. Manipulation of each of these branches extends lifespan; however, the interplay between MetR and these branches during regulation of lifespan is not well understood. In addition, a potential mechanism linking the activity of methionine metabolism and lifespan is regulation of production of the methyl donor S‐adenosylmethionine, which, after transferring its methyl group, is converted to S‐adenosylhomocysteine. Methylation regulates a wide range of processes, including those thought to be responsible for lifespan extension by MetR. Although the exact mechanisms of lifespan extension by MetR or methionine metabolism reprogramming are unknown, it may act via reducing the rate of translation, modifying gene expression, inducing a hormetic response, modulating autophagy, or inducing mitochondrial function, antioxidant defense, or other metabolic processes. Here, we review the mechanisms of lifespan extension by MetR and different branches of methionine metabolism in different species and the potential for exploiting the regulation of methyltransferases to delay aging.     

Biomarkers of aging in Drosophila

Low environmental temperature and dietary restriction (DR) extend lifespan in diverse organisms. In the fruit fly Drosophila, switching flies between temperatures alters the rate at which mortality subsequently increases with age but does not reverse mortality rate. In contrast, DR acts acutely to lower mortality risk; flies switched between control feeding and DR show a rapid reversal of mortality rate. Dietary restriction thus does not slow accumulation of aging‐related damage. Molecular species that track the effects of temperatures on mortality but are unaltered with switches in diet are therefore potential biomarkers of aging‐related damage. However, molecular species that switch upon instigation or withdrawal of DR are thus potential biomarkers of mechanisms underlying risk of mortality, but not of aging‐related damage. Using this approach, we assessed several commonly used biomarkers of aging‐related damage. Accumulation of fluorescent advanced glycation end products (AGEs) correlated strongly with mortality rate of flies at different temperatures but was independent of diet. Hence, fluorescent AGEs are biomarkers of aging‐related damage in flies. In contrast, five oxidized and glycated protein adducts accumulated with age, but were reversible with both temperature and diet, and are therefore not markers either of acute risk of dying or of aging‐related damage. Our approach provides a powerful method for identification of biomarkers of aging.     

Dietary restriction affects lifespan but not cognitive aging in Drosophila melanogaster

Dietary restriction extends lifespan in a wide variety of animals, including Drosophila, but its relationship to functional and cognitive aging is unclear. Here, we study the effects of dietary yeast content on fly performance in an aversive learning task (association between odor and mechanical shock). Learning performance declined at old age, but 50‐day‐old dietary‐restricted flies learned as poorly as equal‐aged flies maintained on yeast‐rich diet, even though the former lived on average 9 days (14%) longer. Furthermore, at the middle age of 21 days, flies on low‐yeast diets showed poorer short‐term (5 min) memory than flies on rich diet. In contrast, dietary restriction enhanced 60‐min memory of young (5 days old) flies. Thus, while dietary restriction had complex effects on learning performance in young to middle‐aged flies, it did not attenuate aging‐related decline of aversive learning performance. These results are consistent with the hypothesis that, in Drosophila, dietary restriction reduces mortality and thus leads to lifespan extension, but does not affect the rate with which somatic damage relevant for cognitive performance accumulates with age.     

Changes in the expression of four heat shock proteins during the aging process in Brachionus calyciflorus (rotifera)

Heat shock proteins (HSPs) are molecular chaperones and have an important role in the refolding and degradation of misfolded proteins, and these functions are related to aging. Rotifer is a useful model organism in aging research, owing to small body size (0.1–1 mm), short lifespan (6–14 days), and senescence phenotypes that can be measured relatively easily. Therefore, we used rotifer as a model to determine the role of four typical hsp genes on the aging process in order to provide a better understanding of rotifer aging. We cloned cDNA encoding hsp genes (hsp40, hsp60, hsp70, and hsp90) from the rotifer Brachionus calyciflorus Pallas, analyzed their molecular characteristics, determined its modulatory response under different temperatures and H₂O₂ concentrations and investigated the changes in expression of these genes during the aging process. We found that Bchsp70 mRNA expression significantly decreased with aging. In addition, we also studied the effects of dietary restriction (DR) and vitamin E on rotifer lifespan and reproduction and analyzed the changes in expression of these four Bchsp genes in rotifers treated with DR and vitamin E. The results showed that DR extended the lifespan of rotifers and reduced their fecundity, whereas vitamin E had no significant effect on rotifer lifespan or reproduction. Real-time PCR indicated that DR increased the expression of these four Bchsps. However, vitamin E only improved the expression of Bchsp60, and reduced the expression of Bchsp40, Bchsp70, and Bchsp90. DR pretreatment also increased rotifer survival rate under paraquat-induced oxidative stress. These results indicated that hsp genes had an important role in the anti-aging process.     

A tissue‐specific screen of ceramide expression in aged mice identifies ceramide synthase‐1 and ceramide synthase‐5 as potential regulators of fiber size and strength in skeletal muscle

Loss of skeletal muscle mass is one of the most widespread and deleterious processes in aging humans. However, the mechanistic metabolic principles remain poorly understood. In the framework of a multi‐organ investigation of age‐associated changes of ceramide species, a unique and distinctive change pattern of C_16:0 and C_18:0 ceramide species was detected in aged skeletal muscle. Consistently, the expression of CerS1 and CerS5 mRNA, encoding the ceramide synthases (CerS) with substrate preference for C_16:0 and C_18:0 acyl chains, respectively, was down‐regulated in skeletal muscle of aged mice. Similarly, an age‐dependent decline of both CerS1 and CerS5 mRNA expression was observed in skeletal muscle biopsies of humans. Moreover, CerS1 and CerS5 mRNA expression was also reduced in muscle biopsies from patients in advanced stage of chronic heart failure (CHF) suffering from muscle wasting and frailty. The possible impact of CerS1 and CerS5 on muscle function was addressed by reversed genetic analysis using CerS1^Δ/Δ and CerS5^Δ/Δ knockout mice. Skeletal muscle from mice deficient of either CerS1 or CerS5 showed reduced caliber sizes of both slow (type 1) and fast (type 2) muscle fibers, fiber grouping, and fiber switch to type 1 fibers. Moreover, CerS1‐ and CerS5‐deficient mice exhibited reduced twitch and tetanus forces of musculus extensor digitorum longus. The findings of this study link CerS1 and CerS5 to histopathological changes and functional impairment of skeletal muscle in mice that might also play a functional role for the aging skeletal muscle and for age‐related muscle wasting disorders in humans.     

Importance of protein quality versus quantity in alternative host plants for a leaf-feeding insect

The nutritional value of alternative host plants for leaf-feeding insects such as caterpillars is commonly measured in terms of protein quantity. However, nutritional value might also depend on the quality of the foliar protein [i.e., the composition of essential amino acids (EAAs)]. A lack of comparative work on the EAA compositions of herbivores and their host plants has hampered the testing of this hypothesis. We tested the “protein quality hypothesis” using the tree-feeding caterpillars of Lymantria dispar (gypsy moth) and two taxonomically unrelated host plants, red oak (Quercus rubra) and sugar maple (Acer saccharum). Because L. dispar has higher fitness on oak than on maple, support for the hypothesis would be found if protein were of higher quality from oak than from maple. The whole-body EAA composition of L. dispar larvae was measured to estimate its optimum dietary protein composition, which was compared with the EAA compositions of oak and maple leaves. Contrary to the protein quality hypothesis, the EAA compositions of oak and maple were not significantly different in the spring. The growth-limiting EAAs in both tree species were histidine and methionine. Similar results were observed in the summer, with the exception that the histidine composition of oak was between 10 and 15 % greater than in maple leaves. The two main factors that affected the nutritional value of protein from the tree species were the quantities of EAAs, which were consistently higher in oak, and the efficiency of EAA utilization, which decreased from 80 % in May to <50 % in August. We conclude that the relative nutritional value of red oak and sugar maple for L. dispar is more strongly affected by protein quantity than quality. Surveys of many wild herbaceous species also suggest that leaf-feeding insects would be unlikely to specialize on plants based on protein quality.