Lipidomics Reveals Mitochondrial Membrane Remodeling Associated with Acute Thermoregulation in a Rodent with a Wide Thermoneutral Zone

Adjustments in energy metabolism of brown adipose tissue in heat-acclimated Kunming mice

The thermogenic capacity of brown adipose tissue (BAT) in rodents decreases with prolonged heat exposure. However, the underlying mechanisms are not well understood. In this study, Kunming mice were acclimated at 23 ± 1 °C and 33 ± 1 °C for four weeks each to examine the body heat balance and BAT alterations. Results showed that heat-acclimated Kunming mice exhibited reduced body mass and elevated body temperature. Additionally, they displayed lower resting metabolic rates, diminished non-shivering thermogenesis, and reduced BAT thermogenic function. Metabolically, there was a significant reduction in several key metabolites involved in energy metabolism in BAT, including thiamine pyrophosphate, citric acid, cis-Aconitate, isocitric acid, oxoglutaric acid, succinate, fumarate, L-Malic acid, oxaloacetate, flavin mononucleotide, nicotinamide adenine dinucleotide, and adenosine 5'-triphosphate. These findings suggest that BAT adapts to heat acclimation by regulating pathways related to pyruvate oxidation, tricarboxylic acid cycle, and oxidative phosphorylation, which may help maintain thermal homeostasis in Kunming mice.

Allyl Isothiocyanate Maintains DHA-Containing Glycerophospholipids and Ameliorates the Cognitive Function Decline in OVX Mice

Low-temperature-induced fatty acid desaturation is highly conserved in animals, plants, and bacteria. Allyl isothiocyanate (AITC) is an agonist of the transient receptor potential ankyrin 1 (TRPA1), which is activated by various chemophysiological stimuli, including low temperature. However, whether AITC induces fatty acid desaturation remains unknown. We showed here that AITC increased levels of glycerophospholipids (GP) esterified with unsaturated fatty acids, especially docosahexaenoic acid (DHA) in TRPA1-expressing HEK cells. Additionally, GP-DHA including phosphatidylcholine (18:0/22:6) and phosphatidylethanolamine (18:0/22:6) was increased in the brain and liver of AITC-administered mice. Moreover, intragastrical injection of AITC in ovariectomized (OVX) female C57BL/6J mice dose-dependently shortened the Δlatency time determined by the Morris water maze test, indicating AITC ameliorated the cognitive function decline in these mice. Thus, the oral administration of AITC maintains GP-DHA in the liver and brain, proving to be a potential strategy for preventing cognitive decline.

The expanding organelle lipidomes: current knowledge and challenges

Lipids in cell membranes and subcellular compartments play essential roles in numerous cellular processes, such as energy production, cell signaling and inflammation. A specific organelle lipidome is characterized by lipid synthesis and metabolism, intracellular trafficking, and lipid homeostasis in the organelle. Over the years, considerable effort has been directed to the identification of the lipid fingerprints of cellular organelles. However, these fingerprints are not fully characterized due to the large variety and structural complexity of lipids and the great variability in the abundance of different lipid species. The process becomes even more challenging when considering that the lipidome differs in health and disease contexts. This review summarizes the information available on the lipid composition of mammalian cell organelles, particularly the lipidome of the nucleus, mitochondrion, endoplasmic reticulum, Golgi apparatus, plasma membrane and organelles in the endocytic pathway. The lipid compositions of extracellular vesicles and lamellar bodies are also described. In addition, several examples of subcellular lipidome dynamics under physiological and pathological conditions are presented. Finally, challenges in mapping organelle lipidomes are discussed.

Rodents Inhabiting the Southeastern Mu Us Desert May Not Have Experienced Prolonged Heat Stress in Summer 2022

Climate change combined with human activities has altered the spatial and temporal patterns of summer extreme heat in the Mu Us Desert. To determine how those rodents living in the desert respond to increased extreme heat in summer, in July 2022, during the hottest month, we examined the rodent species, vegetation coverage, and small-scale heterogeneity in ambient temperature in the southeastern Mu Us Desert. The results showed that Meriones meridianus, Meriones unguiculatus, and Cricetulus longicaudatus were found in the study area, where the vegetation coverage is 33.5-40.8%. Moreover, the maximum temperature of the desert surface was 61.8 °C. The maximum air temperature at 5 cm above the desert surface was 41.3 °C. The maximum temperature in the burrow at a depth of 15 cm was 31 °C. M. unguiculatus might experience 4-9.3 h of heat stress in a day when exposed outside the burrow, whereas M. meridianus would experience 8.5-10.8 h of heat stress. Yet, inside the burrow, both species were barely exposed to heat stress. In conclusion, adjustments in behavioral patterns can be the main way that rodents in the Mu Us Desert adapt to the extreme heat in the summer.

Genetic Diversity of a Heat Activated Channel-TRPV1 in Two Desert Gerbil Species with Different Heat Sensitivity

Heat sensation and tolerance are crucial for determining species' survival and distribution range of small mammals. As a member of the transmembrane proteins, transient receptor potential vanniloid 1 (TRPV1) is involved in the sensation and thermoregulation of heat stimuli; however, the associations between animal's heat sensitivity and TRPV1 in wild rodents are less studied. Here, we found that Mongolian gerbils (Meriones unguiculatus), a rodent species living in Mongolia grassland, showed an attenuated sensitivity to heat compared with sympatrically distributed mid-day gerbils (M. meridianus) based on a temperature preference test. To explain this phenotypical difference, we measured the TRPV1 mRNA expression of two gerbil species in the hypothalamus, brown adipose tissue, and liver, and no statistical difference was detected between two species. However, according to the bioinformatics analysis of TRPV1 gene, we identified two single amino acid mutations on two TRPV1 orthologs in these two species. Further Swiss-model analyses of two TRPV1 protein sequences indicated the disparate conformations at amino acid mutation sites. Additionally, we confirmed the haplotype diversity of TRPV1 in both species by expressing TRPV1 genes ectopicly in Escherichia coli system. Taken together, our findings supplemented genetic cues to the association between the discrepancy of heat sensitivity and the functional differentiation of TRPV1 using two wild congener gerbils, promoting the comprehension of the evolutionary mechanisms of the TRPV1 gene for heat sensitivity in small mammals.

Thermo-TRP channels are involved in BAT thermoregulation in cold-acclimated Brandt's voles

Transient receptor potential (TRP) channels, which can sense temperature, pressure and mechanical stimuli, were involved in many physiological and biochemical reactions. Whether thermosensitive TRP channels (Thermo-TRPs) are involved in thermoregulation in small mammals is still not clear. We measured the changes of thermo-TRPs at 4 °C, 23 °C and 30 °C in Brandt's voles (Lasiopodomys brandtii) to test the hypothesis that Thermo-TRPs are involved in cold-induced thermogenesis of brown adipose tissue (BAT) in small mammals. Results showed that air temperatures had no effect on body mass and rectal temperature, but the food intake and basal metabolic rate (BMR) in the 4 °C group were significantly higher than in the 30 °C group. Compared with 30 °C group, the protein contents of uncoupling protein 1(UCP1), TRP vanilloid 2 (TRPV2), TRP ankyrin 1 (TRPA1), TRP melastatin 2 (TRPM2), silent Information Regulator T1 (SIRT1), AMP-activated protein kinase (AMPK) and Calcium/calmodulin-dependent protein kinase II (CaMKII) in BAT increased significantly in 4 °C group, but there was no significant difference in the protein content of Thermo-TRPs in the hypothalamus among groups. Further, the expression of PRDM16 (PR domain containing 16) in inguinal white adipose tissue (iWAT) at 4 °C was significantly higher than that at 30 °C, but no difference was observed in the expression of other browning-related genes or TRPV2. In conclusion, TRP channels may participate in BAT thermoregulation through the CaMKII, AMPK, SIRT1 and UCP1 pathway in cold-acclimated Brandt's voles.

Gut Microbial Community and Host Thermoregulation in Small Mammals

The endotherms, particularly the small mammals living in the polar region and temperate zone, are faced with extreme challenges for maintaining stable core body temperatures in harsh cold winter. The non-hibernating small mammals increase metabolic rate including obligatory thermogenesis (basal/resting metabolic rate, BMR/RMR) and regulatory thermogenesis (mainly nonshivering thermogenesis, NST, in brown adipose tissue and skeletal muscle) to maintain thermal homeostasis in cold conditions. A substantial amount of evidence indicates that the symbiotic gut microbiota are sensitive to air temperature, and play an important function in cold-induced thermoregulation, via bacterial metabolites and byproducts such as short-chain fatty acids and secondary bile acids. Cold signal is sensed by specific thermosensitive transient receptor potential channels (thermo-TRPs), and then norepinephrine (NE) is released from sympathetic nervous system (SNS) and thyroid hormones also increase to induce NST. Meanwhile, these neurotransmitters and hormones can regulate the diversity and compositions of the gut microbiota. Therefore, cold-induced NST is controlled by both Thermo-TRPs—SNS—gut microbiota axis and thyroid—gut microbiota axis. Besides physiological thermoregulation, small mammals also rely on behavioral regulation, such as huddling and coprophagy, to maintain energy and thermal homeostasis, and the gut microbial community is involved in these processes. The present review summarized the recent progress in the gut microbiota and host physiological and behavioral thermoregulation in small mammals for better understanding the evolution and adaption of holobionts (host and symbiotic microorganism). The coevolution of host-microorganism symbionts promotes individual survival, population maintenance, and species coexistence in the ecosystems with complicated, variable environments.

The microbiota-gut-kidney axis mediates host osmoregulation in a small desert mammal

Regulating sodium and water balances is crucial for survival of small, desert mammals. Studies demonstrate that the gut microbiota and their metabolites are involved in host energy homeostasis, but little is known on the interactions among salt loading, gut microbiota, and osmoregulation. The aim of this study was to fill this gap. Mongolian gerbils (Meriones unguiculatus) were offered drinking water (Con) and either water containing moderate (4%, MS) or high NaCl (8%, HS) ad libitum. Intake of HS reduced α diversity of the microbial community and, at the genus level, reduced the relative abundances of Rikenella and Christensenella but increased Atopobium. To confirm the function of gut microbiota in host osmoregulation, we transplanted caecal microbiota in HS gerbils. To cope with salt loading, the gerbils concentrated urine, resulting in negative energy balance and systemic inflammation. The HS gerbils increased hypothalamic arginine vasopressin and intestinal and renal aquaporin 2 to support water retention, and reduced intestinal and renal epithelial sodium channel α to promote sodium excretion. However, HS gerbils with caecal microbiota transplant (CMT) from Con donors maintained energy balance and osmoregulation, and had a much reduced systemic inflammation. Further, CMT from Con donors to HS recipients reshaped the gut microbiota, particularly by reducing Parabacteroides distasonis and Prevotella copri, and increasing Lactobacillus reuteri abundances, with a resulting increase in bacterial metabolites such as butyrate. These findings highlight a vital role of the microbiota-gut-kidney axis in mediating salt-related osmoregulation, allowing small mammals to adapt to high salt loads in a desert habitat.

Limited Physiological Compensation in Response to an Acute Microclimate Change in a Malagasy Bat

Rapid environmental changes are challenging for endothermic species because they have direct and immediate impacts on their physiology by affecting microclimate and fundamental resource availability. Physiological flexibility can compensate for certain ecological perturbations, but our basic understanding of how species function in a given habitat and the extent of their adaptive scope is limited. Here we studied the effect of acute, experimental microclimate change on the thermal physiology of two populations of the widespread Malagasy bat, Macronycteris commersoni. Populations of this species are found roosting under contrasting conditions, i.e., in a constant hot and humid cave or below foliage unprotected from fluctuations in ambient conditions. We exposed free-ranging individuals of each population to the respective opposite condition and thus to novel microclimate within an ecologically realistic scope while measuring metabolic rate and skin temperature. Cave bats in forest setting had a limited capacity to maintain euthermia to the point that two individuals became hypothermic when ambient temperature dropped below their commonly experienced cave temperature. Forest bats on the other hand, had difficulties to dissipate heat in the humid cave set-up. The response to heat, however, was surprisingly uniform and all bats entered torpor combined with hyperthermia at temperatures exceeding their thermoneutral zone. Thus, while we observed potential for flexible compensation of heat through “hot” torpor, both populations showed patterns suggestive of limited potential to cope with acute microclimate changes deviating from their typically occupied roosts. Our study emphasizes that intraspecific variation among populations could be misleading when assessing species’ adaptive scopes, as variation may arise from genetic adaptation, developmental plasticity or phenotypic flexibility, all of which allow for compensatory responses at differing time scales. Disentangling these mechanisms and identifying the basis of variation is vital to make accurate predictions of species’ chances for persisting in ever rapidly changing habitats and climates.

Role of transient receptor potential vanilloid-1 in behavioral thermoregulation of the Mongolian gerbil Meriones unguiculatus

Ambient temperature considerably affects the physiology and behavior of mammals. Thermosensory and thermoregulatory abilities play an important role in the response to changing ambient temperature in endotherms. However, the molecular mechanisms of behavioral thermoregulation remain poorly understood. Transient receptor potential vanilloid-1 (TRPV1) is activated by changes in ambient temperature and is involved in acute thermoregulation. Here, we aimed to determine whether TRPV1 is involved in behavioral thermoregulation in wild rodents by conducting 2 experiments. In the first, 42 adult Mongolian gerbils (Meriones unguiculatus; 14 per treatment) were randomly assigned to 3 housing temperatures (4, 23, and 36°C) for 4 weeks. In the second, 20 gerbils (10 per treatment) were randomly injected with capsaicin (TRPV1 agonist) or AMG517 (TRPV1 antagonist). The results showed a significant decrease in food intake and non-shivering thermogenesis in the gerbils housed at 36°C. Additionally, there was a significant increase in the preference of gerbils housed at 4°C to low temperatures. The expression of TRPV1 protein in the brown adipose tissue (BAT) and liver was significantly positively correlated with that of protein kinase A (PKA). The expression of TRPV1 and PKA proteins in the BAT was positively correlated with the temperature preference of the gerbils. The gerbils injected with capsaicin preferred significantly lower temperatures than the control group gerbils. These findings suggest that TRPV1 and PKA are involved in behavioral thermoregulation in Mongolian gerbils.

Panax ginseng root, not leaf, can enhance thermogenic capacity and mitochondrial function in mice

Context: Panax ginseng C. A. Meyer (Araliaceae) root and leaf have always been considered in the traditional theory as hot and cold properties, respectively.Objective: To clarify the hot and cold properties of ginseng root and leaf from a thermodynamic viewpoint.Materials and methods: Thirty ICR male mice were randomly assigned to control (water), ginseng root group (GRP) and ginseng leaf group (GLP) with a concentration of 0.075 g/mL; the volume was 0.1 mL/10 g (body mass) per day by intragastric administration for 20 days. Ultra-Performance Liquid Chromatography (UPLC) was used to determine quality control through seven ginsenosides contained in ginseng root and leaf. Rest metabolic rate (RMR) and energy expenditure were monitored every 9 days by TSE System. At the 20^th day, serum T3 or T4, liver or brown adipose tissue (BAT) mitochondrial respiration were investigated.Results: The quality control of GRP and GLP were within requirements of 2015 China Pharmacopoeia. The RMR (mLO₂/h) in GLP (47.95 ± 4.20) was significantly lower than control (52.10 ± 4.79) and GRP (55.35 ± 4.48). Mitochondrial protein concentration and respiration were significantly increased in GRP (BAT, 79.12 ± 2 .08 mg/g, 239.89 ± 10.24 nmol O₂/min/g tissue; Liver, 201.02 ± 10.89, 202.44 ± 3.24) and decreased in GLP (BAT, 53.42 ± 3.48, 153.49 ± 5.58; Liver, 138.69 ± 5.69, 104.50 ± 6.25) compared with control.Conclusions: The hot and cold properties of ginseng root and leaf are correlated with thermogenic capacity and mitochondrial function of BAT and liver, which deserve to further research.

Gut Microbiota and Host Thermoregulation in Response to Ambient Temperature Fluctuations

Ambient temperature (T_a ) is an important factor in shaping phenotypic plasticity. Plasticity is generally beneficial for animals in adapting to their environments. Gut microbiota are crucial in regulating host physiological and behavioral processes. However, whether the gut microbiota play a role in regulating host phenotypic plasticity under the conditions of repeated fluctuations in environmental factors has rarely been examined. We used intermittent T_a acclimations to test the hypothesis that the plasticity of gut microbiota confers on the host a metabolic adaptation to T_a fluctuations. Mongolian gerbils (Meriones unguiculatus) were acclimated to intermittent 5°C to 23°C, 37°C to 23°C or 23°C to 23°C conditions for 3 cycles (totally 3 months). Intermittent T_a acclimations induced variations in resting metabolic rate (RMR), serum thyroid hormones, and core body temperature (T_b ). We further identified that the β-diversity of the microbial community varied with T_a and showed diverse responses during the 3 cycles. Some specific bacteria were more sensitive to T_a and were associated with host dynamic metabolic plasticity during T_a acclimations. In addition, depletion of gut microbiota in antibiotic-treated gerbils impaired metabolic plasticity, particularly at low T_a , whereas supplementation with propionate as an energy resource improved the inhibited thermogenic capacity and increased the survival rate in the cold. These findings demonstrate that both gut microbiota and their host were more adaptive after repeated acclimations, and dynamic gut microbiota and their metabolites may confer host plasticity in thermoregulation in response to T_a fluctuations. It also implies that low T_a is a crucial cue in driving symbiosis between mammals and their gut microbiota during evolution.IMPORTANCE Whether gut microbiota play a role in regulating host phenotypic plasticity in small mammals living in seasonal environments has rarely been examined. The present study, through an intermittent temperature acclimation model, indicates that both gut microbiota and their host were more adaptive after repeated acclimations. It also demonstrates that dynamic gut microbiota confer host plasticity in thermoregulation in response to intermittent temperature fluctuations. Furthermore, low temperature seems to be a crucial cue in driving the symbiosis between mammals and their gut microbiota during evolution.

The physiological and molecular mechanisms to maintain water and salt homeostasis in response to high salt intake in Mongolian gerbils (Meriones unguiculatus)

Desert rodents are faced with many challenges such as high dietary salt in their natural habitats and they have evolved abilities to conserve water and tolerate salt. However, the physiological and molecular mechanisms involved in water and salt balances in desert rodents are unknown. We hypothesized that desert rodents regulated water and salt balances by altering the expression of AQP2 and α-ENaC in the kidney. Mongolian gerbils (Meriones unguiculatus), a desert species, were acclimated to drinking water with different salt contents: (0, control; 4% NaCl, moderate salt, MS; 8% NaCl, high salt, HS) for 4 weeks. The gerbils drinking salty water had lower body mass, food intake, water intake, metabolic water production and urine volume. The HS gerbils increased the expression of arginine vasopressin (AVP) in the hypothalamus, and also enhanced the expression of AQP2 and cAMP/PKA/CREB signaling pathway in the kidney. In addition, these gerbils reduced serum aldosterone levels and α-ENaC expression in the kidney. Creatinine clearance was lower in the HS group than that in the control group, but serum and urine creatinine levels did not change. These data indicate that desert rodents rely on AVP-dependent upregulation of AQP2 and aldosterone-dependent downregulation of α-ENaC in the kidney to promote water reabsorption and sodium excretion under high salt intake.

Aquaporins, evaporative water loss and thermoregulation in heat-acclimated Mongolian gerbils (Meriones unguiculatus)

Evaporative water loss is an essential strategy to maintain stable body temperature in heat-exposed rodents. However, the thermoregulatory role and adjustment of evaporative heat loss capacity is unclear during prolonged heat exposure. Here, we studied the role of evaporative water loss in thermoregulation in Mongolian gerbils during heat acclimation. After 3 weeks of heat acclimation, gerbils exhibited a lower body temperature than the controls, and no difference in evaporative losses of water from the lung or saliva spreading compared with the controls. Heat acclimation did not alter the expression of aquaporin-1 and aquaporin-5 in the lungs and the expression of aquaporin-5 in the salivary glands. The expression of aquaporin-2 in the kidneys was kept stable, while the expression of aquaporin-1 in the kidneys was down-regulated. In addition, resting metabolic rate and non-shivering thermogenesis of heat-acclimated gerbils were reduced to 51% and 55% of the control group, respectively. Taken together, heat-acclimated Mongolian gerbils can reduce the metabolic thermogenesis without enhancing the evaporative water loss capacity for thermoregulation.

Mouse Thermoregulation: Introducing the Concept of the Thermoneutral Point

Human and mouse thermal physiology differ due to dissimilar body sizes. Unexpectedly, in mice we found no ambient temperature zone where both metabolic rate and body temperature were constant. Body temperature began increasing once cold-induced thermogenesis was no longer required. This result reproduced in male, female, C57BL/6J, 129, chow-fed, diet-induced obese, and ob/ob mice as well as Trpv1-/-;Trpm8-/-;Trpa1-/- mice lacking thermal sensory channels. During the resting-light phase, the energy expenditure minimum spanned ∼4°C of ambient temperature, whereas in the active-dark phase it approximated a point. We propose the concept of a thermoneutral point (TNP), a discrete ambient temperature below which energy expenditure increases and above which body temperature increases. Humans do not have a TNP. As studied, the mouse TNP is ∼29°C in light phase and ∼33°C in dark phase. These observations inform how thermoneutrality is defined and how mice are used to model human energy physiology and drug development.

HSP70 plays a role in the defense of acute and chronic heat stress in Mongolian gerbils (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) show a wide thermal neutral zone (TNZ, 26.5-38.9 °C). Whether heat shock proteins (HSPs) are involved in thermal tolerance for gerbils has still been unknown. We investigated the effects of acute and chronic high temperature within and above TNZ on the expressions of HSP70 and HSP90 and oxidative status in Mongolian gerbils, to test the hypothesis that the gerbils need increase the expression in HSPs to defense the acute and chronic heat stress. In experiment I, 50 Mongolian gerbils were exposed to 23 °C, 27 °C, 37 °C, 40 °C and 43.5 °C for 80 min respectively, and then sacrificed 12 h after treatment. HSP70 expression in the liver increased at 40 °C compared to that at 23 °C, but did not change after 27 °C, 37 °C or 43.5 °C exposure. There were no differences in HSP90 expression, oxidative stress parameters such as malonaldehyde (MDA) and hydrogen peroxide (H₂O₂), or antioxidant parameters such as superoxide dismutase (SOD) and total antioxidant capacity (T-AOC) in the liver. HSP70 and HSP90 expression both in the heart and brain showed no differences among groups. In experiment II, another set of 30 gerbils were acclimated to 23 °C, 27 °C and 37 °C for 21 days, respectively. During chronic acclimation, HSP70 expression increased and H₂O₂ level decreased in the liver in 37 °C group compared to other two groups. Both H₂O₂ and SOD in the brain decreased in 37 °C group, but there were no differences in HSP70, MDA or T-AOC in the brain. These data indicate that Mongolian gerbils can maintain basal levels of HSPs after acute exposure to temperatures within the wide TNZ, but rely on increased HSP70 in the liver to protect from heat damage at temperatures above TNZ and during chronic heat acclimation. The increased HSP70 expression in the liver may contribute to keeping from heat damage in desert rodents.

Brown adipose tissue plays thermoregulatory role within the thermoneutral zone in Mongolian gerbils (Meriones unguiculatus)

Brown adipose tissue (BAT) plays an important role in thermoregulation and many metabolic processes in small mammals, especially in cold adaptation. However, in warm adaptation, ambient temperature cannot directly activate BAT by sympathetic nervous system. Mongolian gerbils exhibit a wider thermoneutral zone (26.5-38.9 °C). We hypothesized that BAT atrophied near the lower critical temperature and further atrophied near the upper critical temperature. Male gerbils were acclimated to 23 °C, 27 °C or 37 °C, respectively, for 3 weeks. Results showed that regulatory non-shivering thermogenesis did not change in gerbils acclimated to 27 °C compared with 23 °C group, whereas it was reduced by 43.5% in gerbils acclimated to 37 °C. Bigger lipid droplet in BAT was observed in gerbils acclimated to 27 °C and 37 °C compared with 23 °C group, while the expression of uncoupling protein 1 and tyrosine hydroxylase was only reduced in gerbils acclimated to 37 °C. Further, thermoneutral acclimation did not change BAT thermogenesis by down-regulation of peroxisome proliferator-activated receptor gamma coactivator-1α, PR domain containing 16, peroxisome proliferator-activated receptor-α or peroxisome proliferator activated receptor-γ gene expression in BAT. In addition, body temperature was reduced in gerbils acclimated to 37 °C compared with 23 °C group, which was associated with a decreased resting metabolic rate and regulatory non-shivering thermogenesis. In conclusion, BAT does not atrophy near the lower critical temperature, whereas it atrophies near the upper critical temperature, suggesting that BAT may play thermoregulatory role within the TNZ in Mongolian gerbils.

Thyroid hormones mediate metabolic rate and oxidative, anti-oxidative balance at different temperatures in Mongolian gerbils (Meriones unguiculatus)

Oxidative damage is a potential physiological cost of thermoregulation during seasonal adjustments to air temperature (T_a) in small mammals. Here, we hypothesized that T_a affects serum thyroid hormone levels and these hormones can mediate the changes in metabolic rate and oxidative damage. Mongolian gerbils (Meriones unguiculatus) were acclimated at different T_as (5 °C, 23 °C and 37 °C) for 3 weeks. Serum tri-iodothyronine (T3) levels increased at 5 °C but decreased at 37 °C compared to the control (23 °C). Protein carbonyls increased in liver at 37 °C compared with control, however, lipid damage (malonaldehyde, MDA) in both serum and liver was unrelated to T_a. After the effects of different T_as on thyroid hormone levels and oxidative damage markers were determined, we further investigate whether thyroid hormones mediated metabolic rate and oxidative damage. Another set of gerbils received 0.0036% L-thyroxin (hyperthyroid), 0.04% Methylimazol (hypothyroid) or water (control). Hypothyroid group showed a 34% reduction in resting metabolic rate (RMR) also 42% and 26% increases in MDA and liver protein carbonyl respectively, whereas hyperthyroid group had higher RMR, liver mass and superoxide dismutase (SOD) compared to control. Serum T3 or T3/T4 levels were correlated positively with RMR, liver mass, and SOD, but negatively with MDA and uncoupling protein 2 (UCP2). We concluded that high T_a induced hypothyroidism, decreased RMR and increased oxidative damage, whereas low T_a induced hyperthyroidism, increased RMR and unchanged oxidative damage. These data supported our hypothesis that thyroid hormones can be a cue to mediate metabolic rate and different aspects of oxidative and antioxidant activities at different T_as.

Implication of metabolomic profiles to wide thermoneutral zone in Mongolian gerbils (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) have evolved a wide thermoneutral zone (26.5-38.9 °C) and high upper critical temperature, and appear to have a high tolerance for heat exposure. Here, we use a metabolomic approach to measure global metabolite profiles for gerbils between lower (27 °C) and upper critical temperatures (38 °C) to investigate the role of metabolomic characterization in maintaining basal metabolic rates within a wide thermoneutral zone. We found that in serum and liver, 14 and 19 metabolites were significantly altered, respectively. In the aerobic respiration-related tricarboxylic cycle (TCA), 5 intermediates (isocitric acid, cis-aconitic acid, α-ketoglutaric acid, fumaric acid and malic acid) were increased in serum in 38 °C animals; however, no such increase was found in the liver. A stable level of hepatic TCA cycle intermediates may be related to the steady state of aerobic respiration at 38 °C. Metabolomic results also revealed that acute heat exposure caused increased oxidative stress and low molecular weight antioxidants in Mongolian gerbils. Increased methionine and 2-hydroxybutyrate suggest an accelerated synthesis of glutathione. Increased urate and its precursors, inosine and hypoxanthine, were detected at 38 °C. Glucuronate, threonate and oxalate involved in ascorbate synthesis and degradation were increased in serum at 38 °C. In conclusion, although dramatic metabolomic variation was found, a stable hepatic TCA cycle may contribute to maintaining a constant basal metabolic rate within a wide thermoneutral zone in Mongolian gerbils.

Adipocyte lipopolysaccharide binding protein (LBP) is linked to a specific lipidomic signature

OBJECTIVE

Lipopolysaccharide binding protein (LBP) is part of a family of structurally and functionally related lipid transfer proteins. This study aimed to investigate the associations of LBP with the lipid composition of human adipose tissue.

METHODS

Lipidomic analysis was performed in whole adipose tissue. To validate the associations found, LBP was knocked down (KD) in 3T3-L1 adipocytes, and lipidomic profile was evaluated by nontargeted lipidomics.

RESULTS

LBP gene expression was negatively associated with phosphatidylcholine, phosphatidylserine, and sphingomyelin relative abundance in vivo. LBP expression was also decreased in those samples with the highest docosahexanoic content, implicated in the resolution of inflammation. The KD of LBP (by ∼70%) led to sharp changes in the lipidome of adipocytes. Of note, specific plasmalogen species PE(O-16:0/22:5), PE(38:2), odd chain glycerolipids, and cholesteryl linoleate were upregulated by LBP KD. In contrast, GM3 gangliosides, several ceramides, a triacylglycerol potentially containing arachidonate, and cholesteryl palmitate were downregulated by LBP KD.

CONCLUSIONS

LBP seems to play a role in the regulation of lipid composition of adipose tissue linked to resilience to inflammation.

Water deprivation up-regulates urine osmolality and renal aquaporin 2 in Mongolian gerbils (Meriones unguiculatus)

To better understand how desert rodents adapt to water scarcity, we examined urine osmolality, renal distribution and expression of aquaporins (AQPs) in Mongolian gerbils (Meriones unguiculatus) during 7 days of water deprivation (WD). Urine osmolality of the gerbils during WD averaged 7503 mOsm kg(-1). Renal distributions of AQP1, AQP2, and AQP3 were similar to that described in other rodents. After the 7 day WD, renal AQP2 was up-regulated, while resting metabolic rate and total evaporative water loss decreased by 43% and 36%, respectively. Our data demonstrated that Mongolian gerbils showed high urine concentration, renal AQPs expression and body water conservation to cope with limited water availability, which may be critical for their survival during dry seasons in cold deserts.

Phosphatidylethanolamine Is a Key Regulator of Membrane Fluidity in Eukaryotic Cells

Adequate membrane fluidity is required for a variety of key cellular processes and in particular for proper function of membrane proteins. In most eukaryotic cells, membrane fluidity is known to be regulated by fatty acid desaturation and cholesterol, although some cells, such as insect cells, are almost devoid of sterol synthesis. We show here that insect and mammalian cells present similar microviscosity at their respective physiological temperature. To investigate how both sterols and phospholipids control fluidity homeostasis, we quantified the lipidic composition of insect SF9 and mammalian HEK 293T cells under normal or sterol-modified condition. As expected, insect cells show minimal sterols compared with mammalian cells. A major difference is also observed in phospholipid content as the ratio of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) is inverted (4 times higher in SF9 cells). In vitro studies in liposomes confirm that both cholesterol and PE can increase rigidity of the bilayer, suggesting that both can be used by cells to maintain membrane fluidity. We then show that exogenously increasing the cholesterol amount in SF9 membranes leads to a significant decrease in PE:PC ratio whereas decreasing cholesterol in HEK 293T cells using statin treatment leads to an increase in the PE:PC ratio. In all cases, the membrane fluidity is maintained, indicating that both cell types combine regulation by sterols and phospholipids to control proper membrane fluidity.

Environmental metabolomics reveal geographic variation in aerobic metabolism and metabolic substrates in Mongolian gerbils (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) have a large-scale distribution in northern China. Geographic physiological variations which related to energy and water metabolism are critical to animals' local adaptation and distribution. However, the underlying biochemical mechanism of such variation and its role in adaptation remains largely unknown. We used GC-MS metabolomics approach to investigate the biochemical adaptation of Mongolian gerbils from xeric (desert), transition (desert steppe) and mesic (typical steppe) environments. Gerbils in desert population had lower resting metabolic rate (RMR) and total evaporative water loss (TEWL) than mesic population. Serum metabolomics revealed that concentrations of five tricarboxylic acid cycle intermediates (citrate, cis-aconitate, α-ketoglutarate, fumarate and malate) were lower in desert population than mesic population. Gastrocnemius metabolomics and citrate synthase activity analysis showed a lower concentration of citrate and lower citrate synthase activity in desert population. These findings suggest that desert dwelling gerbils decrease RMR and TEWL via down-regulation of aerobic respiration. Gastrocnemius metabolomics also revealed that there were higher concentrations of glucose and glycolytic intermediates, but lower concentrations of lipids, amino acids and urea in desert population than mesic population. This geographic variation in metabolic substrates may enhance metabolic water production per oxygen molecule for desert population while constraining aerobic respiration to reduce RMR and TEWL.

High incubation temperatures enhance mitochondrial energy metabolism in reptile embryos

Developmental rate increases exponentially with increasing temperature in ectothermic animals, but the biochemical basis underlying this thermal dependence is largely unexplored. We measured mitochondrial respiration and metabolic enzyme activities of turtle embryos (Pelodiscus sinensis) incubated at different temperatures to identify the metabolic basis of the rapid development occurring at high temperatures in reptile embryos. Developmental rate increased with increasing incubation temperatures in the embryos of P. sinensis. Correspondingly, in addition to the thermal dependence of mitochondrial respiration and metabolic enzyme activities, high-temperature incubation further enhanced mitochondrial respiration and COX activities in the embryos. This suggests that embryos may adjust mitochondrial respiration and metabolic enzyme activities in response to developmental temperature to achieve high developmental rates at high temperatures. Our study highlights the importance of biochemical investigations in understanding the proximate mechanisms by which temperature affects embryonic development.