Ectothermy and endothermy: evolutionary perspectives of thermoprotection by HSPs

Sex differences in thermoregulation in mammals: Implications for energy homeostasis

Thermal homeostasis is a fundamental process in mammals, which allows the maintenance of a constant internal body temperature to ensure an efficient function of cells despite changes in ambient temperature. Increasing evidence has revealed the great impact of thermoregulation on energy homeostasis. Homeothermy requires a fine regulation of food intake, heat production, conservation and dissipation and energy expenditure. A great interest on this field of research has re-emerged following the discovery of thermogenic brown adipose tissue and browning of white fat in adult humans, with a potential clinical relevance on obesity and metabolic comorbidities. However, most of our knowledge comes from male animal models or men, which introduces unwanted biases on the findings. In this review, we discuss how differences in sex-dependent characteristics (anthropometry, body composition, hormonal regulation, and other sexual factors) influence numerous aspects of thermal regulation, which impact on energy homeostasis. Individuals of both sexes should be used in the experimental paradigms, considering the ovarian cycles and sexual hormonal regulation as influential factors in these studies. Only by collecting data in both sexes on molecular, functional, and clinical aspects, we will be able to establish in a rigorous way the real impact of thermoregulation on energy homeostasis, opening new avenues in the understanding and treatment of obesity and metabolic associated diseases.

Mathematical modeling and analysis of the heat shock protein response during thermal stress in fish and HeLa cells

The climate change has the potential to dramatically affect species' thermal physiology and may change the ecology and evolution of species' lineages. In this work, we investigated the transition of dynamics in the heat shock response when the thermal stress approaches the upper thermal limits of species to understand how the climate change may affect the heat shock responses in ectotherms and endotherms. The heat shock protein 70, HSP70, prevents protein denaturation or misfolding under thermal stresses. When thermal stress increases, the number of misfolded proteins increases, which leads to high levels of HSP70 protein. However, when temperatures approach limits of thermal tolerance (i.e., the critical thermal maximum, CTmax, for ectotherms and the superior critical temperature, SCT, for endotherms), levels of HSP70 protein synthesis start to decrease. Thus, we hypothesized that the temperature at the first reduction of HSP abundance indicates the thermal limits of the species. In this work, we provide a mathematical model to investigate the behavior of the heat shock responses related to HSP70 protein. This model captures the dynamics of HSP70 protein and Hsp70 mRNA, in HeLa cells (i.e., representative for endotherms) and multiple species of fishes (i.e., representative for ectotherms) with different acclimation histories. Based on our hypothesis of the relationship between the HSP70 protein level and CTmax/SCT, our model provides three methods to predict the CTmax of fishes with varying acclimation temperature and the SCT of HeLa cells. The CTmax increases as the acclimation temperature increases in fishes, however the CTmax plateaus when the acclimation temperature is higher than 20°C in brook trout, a representative cool water salmonid. Our model also captures the situation that the heat shock reaction in fish is more sensitive to the heat shock temperature than HeLa cells, when the heat shock temperature is lower than the upper thermal tolerance. However, both fish and HeLa cells are sensitive to the heat shock temperature when the temperature reaches the thermal tolerance limits. Additionally, our sensitive analysis result indicates that the status of some components in the heat shock reaction changes when the temperature reaches the thermal tolerance resulting in failure in protein refolding in fish and speeding up the refolding process in HeLa cells. Mathematical analysis is also applied on a simplified mathematical model to investigate the detailed dynamics of the model, such as the steady states of the substrate, Hsp70 mRNA, and HSP70 protein, at different thermal stresses. The comparison between the original model and simplified model shows that the inhibition on HSP70 protein transcription by thermal stresses leads to the reduction in HSP70 protein at extreme temperatures.

Investigating genetic variability in Hsp70 gene-5'-fragment and its association with thermotolerance in Murrah buffalo (Bubalus bubalis) under sub-tropical climate of India

The present study was undertaken to investigate genetic variability in a fragment comprising 5'UTR along with partial coding sequence of Hsp70 gene and its association with thermotolerance traits in Murrah buffalo at ICAR-Research Complex for Eastern Region, Patna (India). The allelic variants were identified from genomic DNA samples using SSCP technique. The PCR products were sequenced and analyzed. Data on different thermotolerance traits recorded in three seasons were analyzed by least squares ANOVA taking the SSCP genotypes as fixed effect. Two allelic variants (A and B), each of 503-bp in size, were documented with frequency of 0.59 and 0.41, respectively, and three genotypes (AA, AB and BB) with corresponding frequency of 0.30, 0.58 and 0.12. The allelic variants were due to single nucleotide substitution at 55th base position leading to a change of threonine (A) to methionine (B) in amino acid sequence. Both the allelic variants had 99.8% similarity in nucleotide sequence. In phylogenetic tree, allele A was in a cluster while allele B and Gangatiri cattle sequence formed a different cluster. The SSCP genotypes had significant effect on different thermotolerance traits in summer with thermo-humidity index of ≥ 84. Buffaloes with AA genotype had the highest (P ˂ 0.05) summer evening rectal temperature, respiration rate and pulse rate, inferring that the buffaloes carrying AA genotype had more stress in summer than those with AB and BB genotype. These SSCP genotypes might have differential role in heat shock protein response to induce thermotolerance of Murrah buffaloes in Gangetic plains.

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.

Effects of acute cold exposure on oxidative balance and total antioxidant capacity in juvenile Chinese soft-shelled turtle, Pelodiscus sinensis

Acute cold exposure may disturb the physiological homeostasis of the body in ectotherms. To date, there has been no information on the effects of cold exposure on homeostasis of reactive oxygen species (ROS) or antioxidant defense response in the Chinese soft-shelled turtle, Pelodiscus sinensis. In this study, P. sinensis juveniles were acclimated at 28 °C, transferred to 8 °C as cold exposure for 12 h, then moved back to 28 °C rewarming for 24 h. We measured the ROS level and total antioxidant capacity (TAC) in the brain, liver, kidney and spleen at 2 and 12 h cold exposure, and at the end of the rewarming period. Malonaldehyde (MDA) and carbonyl protein were used as markers of oxidative damage. Turtles being maintained simultaneously at 28 °C were used as the control group. Cold exposure did not disturb the ROS balance in all 4 tissues, while rewarming raised the ROS level in the brain and kidney of P. sinensis. Cold exposure and rewarming decreased the TAC in the brain, liver and spleen but did not change the TAC in the kidney. MDA and carbonyl protein levels did not increase during the treatment, indicating no oxidative damage in all 4 tissues of P. sinensis. Our results indicated that extreme cold exposure did not impact the inner oxidative balance of P. sinensis, but more ROS was produced during rewarming. P. sinensis showed good tolerance to the harsh temperature change through effective protection of its antioxidant defense system to oxidative damage. This study provides basic data on the stress biology of P. sinensis.

The thermal dependence of Na+ flux in isolated liver cells from ectotherms and endotherms

The thermal dependence (0-40oC) of Na+ flux in isolated liver cells of three endotherms (mice, rat and rabbit) was compared to ectotherms in the form of a thermally tolerant amphibian (cane toad), a cold-water fish (rainbow trout) and a thermophilic reptile (lizard). Mammals were found to share similar high rates of Na+ flux (3.0-3.7 nmoles Na+. mgP−1. min−1) at their normal body temperatures (between 36-39 oC). These Na+ flux rates were significantly greater (p<0.0004-0.0001) than those of the ectotherms that shared similar low rates of Na+ flux (0.7-1.3 nmoles Na+. mgP−1. min−1) at their very different normal acclimated body temperatures (15oC for trout, 25oC for toad and 37oC for the lizard species). Trout, that possess highly unsaturated membranes (similar to those of mammals), showed a Na+ flux with high thermal sensitivity at low temperature similar to that found in mammals at higher temperature. The thermal sensitivity of toad Na+ flux was significantly less (p<0.05-0.01) than that present in rat and rabbit. Trout Na+ flux did not increase with increasing temperature much above 20oC, whereas all other species measured increased their Na+ flux with increasing temperature up to 40oC. In conclusion, at normal operating body temperatures the rate of Na+ flux is much lower in ectotherms.

Adaptive traits of indigenous cattle breeds: The Mediterranean Baladi as a case study

Generally taken, breeds of Bos taurus ancestry are considered more productive, in comparison with Bos indicus derived breeds that present enhanced hardiness and disease resistance, low nutritional requirements and higher capability of feed utilization. While breeds of B. taurus have been mostly selected for intensive production systems, indigenous cattle, developed mostly from indicine and African taurines, flourish in extensive habitats. Worldwide demographic and economic processes face animal production with new challenges - the increasing demand for animal food products. Intensification of animal husbandry is thus a desired goal in stricken parts of the world. An introduction of productive traits to indigenous breeds might serve to generate improved biological and economic efficiencies. For this to succeed, the genetic merit of traits like efficiency of feed utilization and product quality should be revealed, encouraging the conservation initiatives of indigenous cattle populations, many of which are already extinct and endangered. Moreover, to overcome potential genetic homogeneity, controlled breeding practices should be undertaken. The Baladi cattle are a native local breed found throughout the Mediterranean basin. Purebred Baladi animals are rapidly vanishing, as more European breeds are being introduced or used for backcrosses leading to improved production. The superiority of Baladi over large-framed cattle, in feedlot and on Mediterranean pasture, with respect to adaptability and efficiency, is highlighted in the current review.

HSP70 as a marker of heat and humidity stress in Tarai buffalo

Heat and humidity stress is a constant challenge to buffalo rearing under tropical climatic conditions. Heat shock proteins (HSPs) constitute a ubiquitous class of highly conserved proteins that contribute to cell survival during different conditions of stress. The present study was carried out in Tarai buffaloes to study the expression of HSP70 in their peripheral blood mononuclear cells during different seasons and establish it as a marker of heat and humidity stress in buffaloes. Blood samples were collected from each healthy, non-lactating and non-pregnant buffalo above 2 years of age, once in the month of January (temperature-humidity index (THI) < 72) and in the month of May (THI > 72). Blood samples were also collected during October (THI = 72) to be used as calibrator/control. Real-time PCR was used to profile the HSP70 gene expression in the peripheral blood mononuclear cells (PBMCs). The relative expression values of HSP70 in Tarai buffalo was found to be significantly higher (P < 0.05) during summer season (2.37 ± 0.12) as compared to winter season (0.29 ± 0.04). The expression positively correlated with changes in physiological parameters like respiration rate (RR), pulse rate (PR) and rectal temperature (RT). In conclusion, it can be said that RR and HSP70 may act as characteristic physiological and cellular markers of heat and humidity stress in buffaloes.

Plasma membranes as heat stress sensors: from lipid-controlled molecular switches to therapeutic applications

The classic heat shock (stress) response (HSR) was originally attributed to protein denaturation. However, heat shock protein (Hsp) induction occurs in many circumstances where no protein denaturation is observed. Recently considerable evidence has been accumulated to the favor of the "Membrane Sensor Hypothesis" which predicts that the level of Hsps can be changed as a result of alterations to the plasma membrane. This is especially pertinent to mild heat shock, such as occurs in fever. In this condition the sensitivity of many transient receptor potential (TRP) channels is particularly notable. Small temperature stresses can modulate TRP gating significantly and this is influenced by lipids. In addition, stress hormones often modify plasma membrane structure and function and thus initiate a cascade of events, which may affect HSR. The major transactivator heat shock factor-1 integrates the signals originating from the plasma membrane and orchestrates the expression of individual heat shock genes. We describe how these observations can be tested at the molecular level, for example, with the use of membrane perturbers and through computational calculations. An important fact which now starts to be addressed is that membranes are not homogeneous nor do all cells react identically. Lipidomics and cell profiling are beginning to address the above two points. Finally, we observe that a deregulated HSR is found in a large number of important diseases where more detailed knowledge of the molecular mechanisms involved may offer timely opportunities for clinical interventions and new, innovative drug treatments. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

The heat shock response in congeneric land snails (Sphincterochila) from different habitats

Land snails are subject to daily and seasonal variations in temperature and in water availability, and use heat shock proteins (HSPs) as part of their survival strategy. We used experimental heat stress to test whether adaptation to different habitats affects HSP expression in two closely related Sphincterochila snail species, a desert species, Sphincterochila zonata, and a Mediterranean-type species, Sphincterochila cariosa. Our findings show that in S. cariosa, heat stress caused rapid induction of Hsp70 proteins and Hsp90 in the foot and kidney tissues, whereas the desert-inhabiting species S. zonata displayed delayed induction of Hsp70 proteins in the foot and upregulation of Hsp90 alone in the kidney. Our study suggests that Sphincterochila species use HSPs as part of their survival strategy following heat stress and that adaptation to different habitats results in the development of distinct strategies of HSP expression in response to heat, namely the reduced induction of HSPs in the desert-dwelling species. We suggest that the desert species S. zonata relies on mechanisms and adaptations other than HSP induction, thus avoiding the fitness consequences of continuous HSP upregulation.

Heat shock proteins and resistance to desiccation in congeneric land snails

Land snails are subject to daily and seasonal variations in temperature and in water availability and depend on a range of behavioral and physiological adaptations for coping with problems of maintaining water, ionic, and thermal balance. Heat shock proteins (HSPs) are a multigene family of proteins whose expression is induced by a variety of stress agents. We used experimental desiccation to test whether adaptation to different habitats affects HSP expression in two closely related Sphincterochila snail species, a desiccation-resistant, desert species Sphincterochila zonata, and a Mediterranean-type, desiccation-sensitive species Sphincterochila cariosa. We examined the HSP response in the foot, hepatopancreas, and kidney tissues of snails exposed to normothermic desiccation. Our findings show variations in the HSP response in both timing and magnitude between the two species. The levels of endogenous Hsp72 in S. cariosa were higher in all the examined tissues, and the induction of Hsp72, Hsp74, and Hsp90 developed earlier than in S. zonata. In contrary, the induction of sHSPs (Hsp25 and Hsp30) was more pronounced in S. zonata compared to S. cariosa. Our results suggest that land snails use HSPs as part of their survival strategy during desiccation and as important components of the aestivation mechanism in the transition from activity to dormancy. Our study underscores the distinct strategy of HSP expression in response to desiccation, namely the delayed induction of Hsp70 and Hsp90 together with enhanced induction of sHSPs in the desert-dwelling species, and suggests that evolution in harsh environments will result in selection for reduced Hsp70 expression.

Agonistic encounters and cellular angst: social interactions induce heat shock proteins in juvenile salmonid fish

Juvenile salmonid fish readily form dominance hierarchies when faced with limited resources. While these social interactions may result in profound behavioural and physiological stress, it is unknown if this social stress is evident at the level of the cellular stress response--specifically, the induction of stress or heat shock proteins (Hsps). Thus, the goal of our study was to determine if Hsps are induced during hierarchy formation in juvenile rainbow trout (Oncorhynchus mykiss). To this end, we measured levels of three Hsps, Hsp70, Hsc (heat shock cognate)70 and Hsp90 in the white muscle, liver and brain of trout that had been interacting for 36 h, 72 h or 6 days. Our data indicate that Hsps are induced in both dominant and subordinate fish in a time- and tissue-specific manner. In further mechanistic experiments on fasted and cortisol-treated fish, we demonstrated that high plasma cortisol does not affect Hsp induction in trout white muscle or liver, but both conditions may be part of the mechanism for Hsp induction with social stress in the brain. We conclude that the behavioural and physiological stress experienced by juvenile rainbow trout in dominance hierarchies can be extended to the induction of Hsps.

Beta-adrenergic stimulation enhances the heat-shock response in fish

We have taken advantage of the unique properties of nucleated rainbow trout (Oncorhynchus mykiss) red blood cells (rbcs) to demonstrate that beta-adrenergic stimulation with the agonist, isoproterenol, significantly enhanced the heat-induced induction of heat-shock proteins (Hsps) in trout rbcs without affecting hsp expression on its own. Furthermore, this beta-adrenergic potentiation of hsp expression occurred only at physiologically relevant concentrations of adrenergic stimulation. In further experiments, we found that adrenaline increased 100-fold and noradrenaline increased 50-fold in trout after a 1-h heat shock at 25 degrees C, approximately 12 degrees C above acclimation temperature. This is the first time the adrenergic heat-shock response has been described for a temperate fish species. We conclude that beta-adrenergic stimulation enhances hsp expression in trout rbcs following heat stress, indicating physiological regulation of the cellular stress response in fish.

Trimethylamine oxide suppresses stress-induced alteration of organic anion transport in choroid plexus

The effect of physicochemical stress on organic anion transport across the vertebrate blood–cerebrospinal fluid (CSF) barrier in the presence and absence of an endogenous cytoprotectant, trimethylamine oxide (TMAO), was investigated in isolated IVth choroid plexus (CP) of spiny dogfish shark(Squalus acanthias), an animal with naturally high levels of TMAO(∼70 mmol l–1). Active transepithelial absorption of the organic anion, 2,4-dichlorophenoxyacetic acid (2,4-D), by IVth CP mounted in Ussing chambers was measured after in vitro stress, and a marker for the cellular stress response, inducible heat shock protein 70 (Hsp70), was assayed by immunoblot analysis. Transient heat stress (a shift from the normal 13.5°C to 23.5°C for 1 h) decreased 2,4-D transport by ∼66%;however, the same stress minus TMAO (isosmotic replacement with urea) had no effect on transport rate. In the absence of TMAO, stress-induced Hsp70 accumulation was more than double that seen in the presence of TMAO. Likewise,exposure to 50 μmol l–1 Zn for 6 h induced a twofold greater Hsp70 accumulation in the absence of TMAO than in its presence, and the higher Hsp70 level was associated with a higher 2,4-D transport rate. Heat stress and 50 μmol l–1 Zn also induced more pronounced increases in Hsp70 mRNA in the absence of TMAO. Thus, the cellular stress response can significantly alter CP organic anion transport capacity, and an endogenous osmolyte can suppress that response.

Reciprocal activation of HSF1 and HSF3 in brain and blood tissues: is redundancy developmentally related?

Transcriptional induction of heat-shock genes in response to temperature elevation and other stresses is mediated by heat-shock transcription factors (HSFs). Avian cells express two redundant heat-shock responsive factors, HSF1 and HSF3, which differ in their activation kinetics and threshold induction temperature. Unlike the ubiquitous activation of HSF1, the DNA-binding activity of HSF3 is restricted to undifferentiated avian cells and embryonic tissues. Herein, we report a reciprocal activation of HSF1 and HSF3 in vivo. Whereas HSF1 mediates transcriptional activity only in the brain upon severe heat shock, HSF3 is exclusively activated in blood cells upon light, moderate, and severe heat shock, promoting induction of heat-shock genes. Although not activated, HSF1 is expressed in blood cell nuclei in a granular appearance, suggesting regulation of genes other than heat-shock genes. Intraspecific comparison of heat-sensitive and heat-resistant fowl strains indicates that the unique activation pattern of HSF3 in blood tissue is a general phenomenon, not related to thermal history. Taken together, HSF1 and HSF3 mediate transcriptional activity of adult tissues and differentiated cells in a nonredundant manner. Instead, an exclusive, tissue-specific activation is observed, implying that redundancy may be developmentally related. The physiological and developmental implications are discussed.