Seasonal changes in minor membrane phospholipid classes, sterols and tocopherols in overwintering insect, Pyrrhocoris apterus

Molecular Mechanisms of Lipid-Based Metabolic Adaptation Strategies in Response to Cold

Temperature changes and periods of detrimental cold occur frequently for many organisms in their natural habitats. Homeothermic animals have evolved metabolic adaptation strategies to increase mitochondrial-based energy expenditure and heat production, largely relying on fat as a fuel source. Alternatively, certain species are able to repress their metabolism during cold periods and enter a state of decreased physiological activity known as torpor. By contrast, poikilotherms, which are unable to maintain their internal temperature, predominantly increase membrane fluidity to diminish cold-related damage from low-temperature stress. However, alterations of molecular pathways and the regulation of lipid-metabolic reprogramming during cold exposure are poorly understood. Here, we review organismal responses that adjust fat metabolism during detrimental cold stress. Cold-related changes in membranes are detected by membrane-bound sensors, which signal to downstream transcriptional effectors, including nuclear hormone receptors of the PPAR (peroxisome proliferator-activated receptor) subfamily. PPARs control lipid metabolic processes, such as fatty acid desaturation, lipid catabolism and mitochondrial-based thermogenesis. Elucidating the underlying molecular mechanisms of cold adaptation may improve beneficial therapeutic cold treatments and could have important implications for medical applications of hypothermia in humans. This includes treatment strategies for hemorrhagic shock, stroke, obesity and cancer.

Metabolic Response of Aphid Cinara tujafilina to Cold Stress

Climate changes enable thermophilic insect species to expand their ranges, but also force them to adapt to unfavourable environmental conditions in new habitats. Focusing on Cinara tujafilina, we investigated the metabolic changes in the body of the aphid that enabled it to survive the low temperatures of winter. Using GC–MS analysis, differences in the chemical composition of the aphids in summer and winter were found. The metabolic changes were mainly related to the increased activity of the pathways of carbohydrate metabolism, such as glycolysis and the pentose phosphate pathway; a decrease in tricarboxylic acid cycle (TCA); accumulation of polyols; and increased levels of proline, tyrosine, and fatty acids.

Benefits and Challenges in the Incorporation of Insects in Food Products

Edible insects are being accepted by a growing number of consumers in recent years not only as a snack but also as a side dish or an ingredient to produce other foods. Most of the edible insects belong to one of these groups of insects such as caterpillars, butterflies, moths, wasps, beetles, crickets, grasshoppers, bees, and ants. Insect properties are analyzed and reported in the articles reviewed here, and one common feature is nutrimental content, which is one of the most important characteristics mentioned, especially proteins, lipids, fiber, and minerals. On the other hand, insects can be used as a substitute for flour of cereals for the enrichment of snacks because of their high content of proteins, lipids, and fiber. Technological properties are not altered when these insects-derived ingredients are added and sensorial analysis is satisfactory, and only in some cases, change in color takes place. Insects can be used as substitute ingredients in meat products; the products obtained have higher mineral content than traditional ones, and some texture properties (like elasticity) can be improved. In extruded products, insects are an alternative source of proteins to feed livestock, showing desirable characteristics. Isolates of proteins of insects have demonstrated bioactive activity, and these can be used to improve food formulations. Bioactive compounds, as antioxidant agents, insulin regulators, and anti-inflammatory peptides, are high-value products that can be obtained from insects. Fatty acids that play a significant role in human health and lipids from insects have showed positive impacts on coronary disease, inflammation, and cancer. Insects can be a vector for foodborne microbial contamination, but the application of good manufacturing practices and effective preservation techniques jointly with the development of appropriate safety regulations will decrease the appearance of such risks. However, allergens presented in some insects are a hazard that must be analyzed and taken into account. Despite all the favorable health-promoting characteristics present in insects and insects-derived ingredients, willingness to consume them has yet to be generalized.

The Diapause Lipidomes of Three Closely Related Beetle Species Reveal Mechanisms for Tolerating Energetic and Cold Stress in High-Latitude Seasonal Environments

During winter insects face energetic stress driven by lack of food, and thermal stress due to sub-optimal and even lethal temperatures. To survive, most insects living in seasonal environments such as high latitudes, enter diapause, a deep resting stage characterized by a cessation of development, metabolic suppression and increased stress tolerance. The current study explores physiological adaptations related to diapause in three beetle species at high latitudes in Europe. From an ecological perspective, the comparison is interesting since one species (Leptinotarsa decemlineata) is an invasive pest that has recently expanded its range into northern Europe, where a retardation in range expansion is seen. By comparing its physiological toolkit to that of two closely related native beetles (Agelastica alni and Chrysolina polita) with similar overwintering ecology and collected from similar latitude, we can study if harsh winters might be constraining further expansion. Our results suggest all species suppress metabolism during diapause and build large lipid stores before diapause, which then are used sparingly. In all species diapause is associated with temporal shifts in storage and membrane lipid profiles, mostly in accordance with the homeoviscous adaptation hypothesis, stating that low temperatures necessitate acclimation responses that increase fluidity of storage lipids, allowing their enzymatic hydrolysis, and ensure integral protein functions. Overall, the two native species had similar lipidomic profiles when compared to the invasive species, but all species showed specific shifts in their lipid profiles after entering diapause. Taken together, all three species show adaptations that improve energy saving and storage and membrane lipid fluidity during overwintering diapause. While the three species differed in the specific strategies used to increase lipid viscosity, the two native beetle species showed a more canalized lipidomic response, than the recent invader. Since close relatives with similar winter ecology can have different winter ecophysiology, extrapolations among species should be done with care. Still, range expansion of the recent invader into high latitude habitats might indeed be retarded by lack of physiological tools to manage especially thermal stress during winter, but conversely species adapted to long cold winters may face these stressors as a consequence of ongoing climate warming.

Cold acclimation increases depolarization resistance and tolerance in muscle fibers from a chill-susceptible insect, Locusta migratoria

Cold exposure depolarizes cells in insects due to a reduced electrogenic ion transport and a gradual increase in extracellular K⁺ concentration ([K⁺]). Cold-induced depolarization is linked to cold injury in chill-susceptible insects, and the locust, Locusta migratoria, has been shown to improve cold tolerance following cold acclimation through depolarization resistance. Here we investigate how cold acclimation influences depolarization resistance and how this resistance relates to improved cold tolerance. To address this question, we investigated if cold acclimation affects the electrogenic transport capacity and/or the relative K⁺ permeability during cold exposure by measuring membrane potentials of warm- and cold-acclimated locusts in the presence and absence of ouabain (Na⁺-K⁺ pump blocker) or 4-aminopyridine (4-AP; voltage-gated K⁺ channel blocker). In addition, we compared the membrane lipid composition of muscle tissue from warm- and cold-acclimated locust and the abundance of a range transcripts related to ion transport and cell injury accumulation. We found that cold-acclimated locusts are depolarization resistant due to an elevated K⁺ permeability, facilitated by opening of 4-AP-sensitive K⁺ channels. In accordance, cold acclimation was associated with an increased abundance of Shaker transcripts (gene encoding 4-AP-sensitive voltage-gated K⁺ channels). Furthermore, we found that cold acclimation improved muscle cell viability following exposure to cold and hyperkalemia even when muscles were depolarized substantially. Thus cold acclimation confers resistance to depolarization by altering the relative ion permeability, but cold-acclimated locusts are also more tolerant to depolarization.

Evidence for non-colligative function of small cryoprotectants in a freeze-tolerant insect

Freeze tolerance, the ability to survive internal ice formation, facilitates survival of some insects in cold habitats. Low-molecular-weight cryoprotectants such as sugars, polyols and amino acids are hypothesized to facilitate freeze tolerance, but their in vivo function is poorly understood. Here, we use a combination of metabolomics and manipulative experiments in vivo and ex vivo to examine the function of multiple cryoprotectants in the spring field cricket Gryllus veletis. Cold-acclimated G. veletis are freeze-tolerant and accumulate myo-inositol, proline and trehalose in their haemolymph and fat body. Injecting freeze-tolerant crickets with proline and trehalose increases survival of freezing to lower temperatures or for longer times. Similarly, exogenous myo-inositol and trehalose increase ex vivo freezing survival of fat body cells from freeze-tolerant crickets. No cryoprotectant (alone or in combination) is sufficient to confer freeze tolerance on non-acclimated, freeze-intolerant G. veletis. Given that each cryoprotectant differentially impacts survival in the frozen state, we conclude that small cryoprotectants are not interchangeable and likely function non-colligatively in insect freeze tolerance. Our study is the first to experimentally demonstrate the importance of non-colligative cryoprotectant function for insect freeze tolerance both in vivo and ex vivo, with implications for choosing new molecules for cryopreservation.

Effects of the entomopathogenic fungus Metarhizium flavoviride on the fat body lipid composition of Zophobas morio larvae (Coleoptera: Tenebrionidae)

Insects employ different defense strategies against fungal infections and chemicals. We aimed to identify the lipid compositions of the fat body of Zophobas morio larvae before and after fungal infection with the entomopathogenic fungus Metarhizium flavoviride. We used gas chromatography-mass spectrometry to analyze lipid extracts of the fat body isolated of Z. morio 2, 5, and 7 days after fungal infection (treatment group) and compared it with the lipid extracts in a control group injected with physiological isotonic saline. In all the samples, fatty acids were the most abundant compound found in the fat body extracts, with hexadecanoic acid/C16:0 being the most abundant lipid. However, the types and concentrations of lipids changed after fungal infection, likely as an immune response. The most considerable changes occurred in the concentrations of long-chain fatty acids, i.e., hexadecanoic acid/C16:0, octadecenoic acid/C18:1, and octadecanoic acid/C18:0. Contents of methyl ester increased significantly after infection, but that of other esters, especially octanoic acid decyl ester/OADE, decreased on the 5th day after infection. To the best of our knowledge, this is the first detailed analysis of the changes in the lipid composition of the fat body of Z. morio larvae as a result of fungal infection. Our results suggest that entomopathogenic fungal infection affects the internal lipid composition of insects, potentially as a way of adjusting to such infection. These results can help understand infection processes and defense strategies of insects against fungal infection. Ultimately, they can contribute to the creation of more effective chemicals against pest insects.

Fat body disintegration after freezing stress is a consequence rather than a cause of freezing injury in larvae of Drosophila melanogaster

Extracellular freezing of insect body water may cause lethal injury either by direct mechanical stress exerted by growing ice crystals on cells and tissues or, indirectly, by deleterious physico-chemical effects linked to freeze-induced cell dehydration. Here we present results showing that the macroscopic damage (cell ruptures, tissue disintegration) to fat body of Drosophila melanogaster is not directly caused by mechanical forces linked to growth of ice crystals but rather represents a secondary consequence of other primary freeze injuries occurring at subcellular or microscopic levels. Larvae of D. melanogaster were acclimated to produce variants ranging from freeze susceptible to freeze tolerant. Then, larvae were exposed to supercooling and freezing stresses at different subzero temperatures. The larval survival and macroscopic damage to fat body tissue was scored in 1632 larvae exposed to cold stress. In most cases, fat body damage was not evident immediately following cold stress but developed later. This suggests that the fat body disintegration is a consequence rather than a cause of cold injury. Analysis of fat body membrane phospholipids revealed that increased freeze tolerance was associated with increased relative proportion of phosphatidylethanolamines (PEs) at the expense of phosphatidylcholines (PCs). The PE/PC ratio increased from 1.08 in freeze-susceptible larvae to 2.10 in freeze-tolerant larvae. The potential effects of changing PE/PC ratio on phospholipid bilayer stability upon supercooling and freezing stress are discussed.

How crickets become freeze tolerant: The transcriptomic underpinnings of acclimation in Gryllus veletis

Some ectotherms can survive internal ice formation. In temperate regions, freeze tolerance is often induced by decreasing temperature and/or photoperiod during autumn. However, we have limited understanding of how seasonal changes in physiology contribute to freeze tolerance, and how these changes are regulated. During a six week autumn-like acclimation, late-instar juveniles of the spring field cricket Gryllus veletis (Orthoptera: Gryllidae) become freeze tolerant, which is correlated with accumulation of low molecular weight cryoprotectants, elevation of the temperature at which freezing begins, and metabolic rate suppression. We used RNA-Seq to assemble a de novo transcriptome of this emerging laboratory model for freeze tolerance research. We then focused on gene expression during acclimation in fat body tissue due to its role in cryoprotectant production and regulation of energetics. Acclimated G. veletis differentially expressed >3000 transcripts in fat body. This differential expression may contribute to metabolic suppression in acclimated G. veletis, but we did not detect changes in expression that would support cryoprotectant accumulation or enhanced control of ice formation, suggesting that these latter processes are regulated post-transcriptionally. Acclimated G. veletis differentially regulated transcripts that likely coordinate additional freeze tolerance mechanisms, including upregulation of enzymes that may promote membrane and cytoskeletal remodelling, cryoprotectant transporters, cytoprotective proteins, and antioxidants. Thus, while accumulation of cryoprotectants and controlling ice formation are commonly associated with insect freeze tolerance, our results support the hypothesis that many other systems contribute to surviving internal ice formation. Together, this information suggests new avenues for understanding the mechanisms underlying insect freeze tolerance.

Diapause Induced by Temperature and Photoperiod Affects Fatty Acid Compositions and Cold Tolerance of Phthorimaea Operculella (Lepidoptera: Gelechiidae)

To find out the potential condition for diapause induction in the potato tuber moth Phthorimaea operculella (Zeller), combination of constant temperatures (15, 20, and 30°C) and photoperiods (8, 9, 10, 11, 12, 14, and 16 h) were employed from egg to adult emergence. In addition, changes in the total phospholipid fatty acid content and cold tolerance of non-diapausing and diapausing pupae were examined. The critical daylength for diapause induction were 12.43 h at 20°C and lower temperatures that can induce in 50% of population. Moreover, the composition of total phospholipid fatty acids in the pupae revealed seven major fatty acids in both non-diapausing and diapausing pupae: oleic (26-32%), palmitic (21-29%), linoleic (18-21%), palmitoleic (4-10%), stearic (9%), linolenic (7-8%), and pentadecanoic acids (3-5%) with an increase in proportion of unsaturated fatty acids in diapause state. In contrast to increase of oleic acid (C18:1) from 26 to 32% in non-diapausing to diapausing pupae, a decrement trend from 29 to 21% in palmitic acid (C16:0) was observed at the same state. Additionally, supercooling point was observed to be significantly lower in diapausing (-22.6°C) than in non-diapausing pupae (-18.5°C) and the fresh weight of diapausing pupae was found to be significantly higher than non-diapausing ones. The significance of these findings would allow us a better understanding of interrelationship between diapause and cold tolerance.

Physiological basis for low-temperature survival and storage of quiescent larvae of the fruit fly Drosophila melanogaster

The cryopreservation techniques proposed for embryos of the fruit fly Drosophila melanogaster are not yet ready for practical use. Alternative methods for long-term storage of D. melanogaster strains, although urgently needed, do not exist. Herein, we describe a narrow interval of low temperatures under which the larvae of D. melanogaster can be stored in quiescence for up to two months. The development of larvae was arrested at the pre-wandering stage under fluctuating thermal regime (FTR), which simultaneously resulted in diminishing the accumulation of indirect chill injuries. Our physiological, metabolomic, and transcriptomic analyses revealed that compared to larvae stored at constant low temperatures, the larvae stored under FTR conditions were able to decrease the rates of depletion of energy substrates, exploited brief warm episodes of FTR for homeostatic control of metabolite levels, and more efficiently exerted protection against oxidative damage.

GC-MS and LC-MS approaches for determination of tocopherols and tocotrienols in biological and food matrices

Tocopherols and tocotrienols, widely described as vitamin E derivatives, have been proven to take part in a number of important biological functions. Among them, antioxidant properties had been investigated and documented in the literature. Since tocochromanols have revealed their plausible beneficial impact on several pathological processes, such as cancerogenesis or cognitive impairment diseases, there is a growing interest in quantitative determination of these compounds in biological fluids, tissues and plant organs. However, due to vitamin E chemical features, such as lipophilic and non-polar characteristics, quantitative determination of the compounds seems to be problematic. In this paper we present current analytical approaches in tocopherols and tocotrienols determination in biological and food matrices with the use of chromatographic techniques, especially gas chromatography (GC) and high performance liquid chromatography (HPLC) coupled with mass spectrometry. Derivatization techniques applied for GC-MS analysis in the case of tocol derivatives, especially silylation and acylation, are described. Significant attention is paid to ionization process of tocopherols and tocotrienols.

Seasonal changes in the composition of storage and membrane lipids in overwintering larvae of the codling moth, Cydia pomonella

The codling moth (Cydia pomonella) is a major insect pest of apples worldwide. It overwinters as a diapausing fifth instar larva. The overwintering is often a critical part of the insect life-cycle in temperate zone. This study brings detailed analysis of seasonal changes in lipid composition and fluidity in overwintering larvae sampled in the field. Fatty acid composition of triacylglycerol (TG) depots in the fat body and relative proportions of phospholipid (PL) molecular species in biological membranes were analyzed. In addition, temperature of melting (Tm) in TG depots was assessed by using differential scanning calorimetry and the conformational order (fluidity) of PL membranes was analyzed by measuring the anisotropy of fluorescence polarization of diphenylhexatriene probe in membrane vesicles. We observed a significant increase of relative proportion of linoleic acid (C18:2n6) at the expense of palmitic acid (C16:0) in TG depots during the larval transition to diapause accompanied with decreasing melting temperature of total lipids, which might increase the accessibility of depot fats for enzymatic breakdown during overwintering. The fluidity of membranes was maintained very high irrespective of developmental mode or seasonally changing acclimation status of larvae. The seasonal changes in PL composition were relatively small. We discuss these results in light of alternative survival strategies of codling moth larvae (supercooling vs. freezing), variability and low predictability of environmental conditions, and other cold tolerance mechanisms such as extending the supercooling capacity and massive accumulation of cryoprotective metabolites.

Molecular basis of chill resistance adaptations in poikilothermic animals

Chill and freeze represent very different components of low temperature stress. Whilst the principal mechanisms of tissue damage and of acquired protection from freeze-induced effects are reasonably well established, those for chill damage and protection are not. Non-freeze cold exposure (i.e. chill) can lead to serious disruption to normal life processes, including disruption to energy metabolism, loss of membrane perm-selectivity and collapse of ion gradients, as well as loss of neuromuscular coordination. If the primary lesions are not relieved then the progressive functional debilitation can lead to death. Thus, identifying the underpinning molecular lesions can point to the means of building resistance to subsequent chill exposures. Researchers have focused on four specific lesions: (i) failure of neuromuscular coordination, (ii) perturbation of bio-membrane structure and adaptations due to altered lipid composition, (iii) protein unfolding, which might be mitigated by the induced expression of compatible osmolytes acting as 'chemical chaperones', (iv) or the induced expression of protein chaperones along with the suppression of general protein synthesis. Progress in all these potential mechanisms has been ongoing but not substantial, due in part to an over-reliance on straightforward correlative approaches. Also, few studies have intervened by adoption of single gene ablation, which provides much more direct and compelling evidence for the role of specific genes, and thus processes, in adaptive phenotypes. Another difficulty is the existence of multiple mechanisms, which often act together, thus resulting in compensatory responses to gene manipulations, which may potentially mask disruptive effects on the chill tolerance phenotype. Consequently, there is little direct evidence of the underpinning regulatory mechanisms leading to induced resistance to chill injury. Here, we review recent advances mainly in lower vertebrates and in arthropods, but increasingly in genetic model species from a broader range of taxa.

Adipokinetic hormone induces changes in the fat body lipid composition of the beetle Zophobas atratus

In insects, neuropeptide adipokinetic hormone (AKH) released from the corpora cardiaca mobilizes lipids and carbohydrates in the fat body. We examined the developmental differences in the action of Tenmo-AKH, a bioanalogue belonging to the adipokinetic/hypertrahelosemic family (AKH/HrTH), on the lipid composition of larval and pupal fat bodies in the beetle Zophobas atratus. Tenmo-AKH was administered to the beetle larvae and pupae either as a single dose or as two doses of 20 pmol during a 24h interval. Extracts of fat bodies were used to analyse the lipid composition by gas chromatography (GC) combined with mass spectrometry (GC-MS). Control extracts were analyzed using the same method. Fatty acids (FA) and fatty acid methyl esters (FAME) were the most abundant compounds in the fat bodies from both developmental stages. We observed significant differences in their concentrations following hormonal treatment. Tenmo-AKH also induced a distinct increase in larval sterols, fatty alcohols and benzoic acid.