Stress-related genomic responses during the course of heat acclimation and its association with ischemic-reperfusion cross-tolerance

Vascular compliance and left ventricular compliance cross talk: Implications for using long-term heat acclimation in cardiac care

1) The first evidence of the beneficial impact of Long-Term-Heat-Acclimation (LTHA) on cardio-vascular compliance was the positive inotropic response and improved left ventricular (LV) compliance noted when isolated hearts from LTHA rats were studied. Human echo study demonstrates that passive HA affects the right ventricle and the atria as well. 2) There is a cross-talk between vascular and cardiac compliance. Vascular compliance per se is defined by central venous pressure-Blood volume relationship-Global Vascular Compliance (GVC). It is determined by the sum of the vascular compliance of the vessels in every organ in any physiological state, varies with LTHA and thus influences cardiac performance. LTHA improves endothelial function, increases NO (nitric oxide) production, in-turn stimulating alterations in ECM (extracellular matrix) via the TGF β1-SMAD pathway. 3) LTHA is associated with transformation from fast to slow myosin, heat acclimation ischemic/hypoxic cross-tolerance and alterations in the extracellular matrix. 4) A human translational study demonstrated improved LV compliance following bypass surgery in LTHA subjects compared to controls. 5) Diastolic dysfunction and the impact of comorbidities with vascular and non- vascular origins are major contributors to the syndrome of heart failure with preserved ejection function (HFPEF). Unfortunately, there is a paucity of treatment modalities that improve diastolic dysfunction. 6) In the current mini-review we suggest that LTHA may be beneficial to HFPEF patients by remodeling cardiac compliance and vascular response.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Heat acclimation mediated cardioprotection is controlled by mitochondrial metabolic remodeling involving HIF-1α

Heat acclimation (HA) induces metabolic plasticity to resist the effects of environmental heat with cross-tolerance to novel stressors such as oxygen supply perturbations, exercise, and alike. Our previous results indicated that hypoxia inducible transcription factor (HIF-1α) contributes to this adaptive process. In the present study, we link functional studies in isolated cardiomyocytes, with molecular and biochemical studies of cardiac mitochondria and demonstrate that HA remodels mitochondrial metabolism and performance. We observed the significant role that HIF-1α plays in the HA heart, as HA reduces oxidative stress during ischemia by shifting mitochondrial substrate preference towards pyruvate, with elevated level and activity of mitochondrial LDH (LDHb), acting a pivotal role. Increased antioxidative capacity to encounter hazards is implicated. These results deepen our understanding of heat acclimation-mediated cross tolerance (HACT), in which adaptive bioenergetic-mechanisms counteract the hazards of oxidative stress.

Circular RNA Expression Profiling and Selection of Key Circular RNAs in the Hypothalamus of Heat-Acclimated Rats

Circular RNAs (circRNAs) have vital roles in great variety of biological processes. However, expression levels and functions of circRNAs related to heat acclimation (HA) are poorly understood. This study is the first time an in-depth circRNA expression profiling were used to investigate circRNA–miRNA interactions in HA rats in order to further comprehend the mechanisms underlying HA. CircRNA expression profile was performed in rats’ hypothalamus of HA and control group with microarray assays and their functions were predicted by using Bioinformatics analysis. Differential circRNAs and their regulated downstream miRNAs and mRNAs were quantitatively validated by means of quantitative polymerase chain reaction in real-time (RT-qPCR). Enzyme-linked immunosorbent assay (ELISA) was then applied to predict the expression of proteins. In total, 53 circRNAs were expressed distinctively between the HA and Control; up- and down-regulation of circRNAs were 28 and 25, respectively, in HA (fold change > 1.5, P < 0.05). Three circRNAs and two miRNAs and three predicted mRNAs were obviously regulated after validated by RT-qPCR in HA rats. Two proteins expression were proportional to their mRNA changes. Further analysis demonstrates that circRNAs closest to HA can be categorized into three signal pathways: including rno_circRNA_014301-vs-rno-miR-3575-vs-Hif-1α, rno_circRNA_014301-vs-rno-miR-3575-vs-Lppr4, and rno_circRNA_010393-vs-rno-miR-20b-3p-vs-Mfap4 in hypoxia response pathways, substance/energy metabolism, and inflammatory response pathways. Our findings implicate that many circRNAs regulate expressions of genes that interact with each other to exert their functions during HA.

Effect of single and repeated heat stress on chemical signals of heat shock response cascade in the rat's heart

Exposure to sublethal heat stress activates a complex cascade of signaling events, such as activators (NO), signal molecules (PKCε), and mediators (HSP70 and COX-2), leading to implementation of heat preconditioning, an adaptive mechanism which makes the organism more tolerant to additional stress. We investigated the time frame in which these chemical signals are triggered after heat stress (41 ± 0.5°С/45 min), single or repeated (24 or 72 h after the first one) in heart tissue of male Wistar rats. The animals were allowed to recover 24, 48 or 72 h at room temperature. Single heat stress caused a significant increase of the concentration of HSP70, NO, and PKC level and decrease of COX-2 level 24 h after the heat stress, which in the next course of recovery gradually normalized. The second heat stress, 24 h after the first one, caused a significant reduction of the HSP70 levels, concentration of NO and PKCɛ, and significant increase of COX-2 concentration. The second exposure, 72 h after the first heat stress, caused more expressive changes of HSP70 and NO in the 24 h-recovery groups. The level of PKCɛ was not significantly changed, but there was significantly increased COX-2 concentration during recovery. Serum activity of AST, ALT, and CK was reduced after single exposure and increased after repeated exposure to heat stress, in both time intervals. In conclusion, a longer period of recovery (72 h) between two consecutive sessions of heat stress is necessary to achieve more expressive changes in mediators (HSP70) and triggers (NO) of heat preconditioning.

Spontaneously hypertensive rats are sensitive to thoracic aorta damage induced by a hot and humid environment

The purpose of this study was to investigate the effect of a hot and humid environment on thoracic aorta damage in spontaneously hypertensive rats (SHRs). Wistar-Kyoto (WKY) rats were randomly divided into three groups (n=8 in each group): Control group (WKY-CN), heat exposure for 8 h group (WKY-8) and heat exposure for 24 h group (WKY-24). The CN group was exposed to room temperature (24°C); WKY-8 and WKY-24 group were exposed to heat (32°C) and 65% relative humidity for 8 and 24 h, respectively. Accordingly, SHRs were randomly divided into three groups (n=8 each group): SHR-CN, SHR-8 and SHR-24. After 7 days of heat exposure, the weight, food consumption and blood pressure of rats was measured. Noradrenaline (NA)-induced contraction of aorta rings was measured using an organ bath system, and vascular morphology was observed. Expression levels of apoptotic genes and proteins in the thoracic aorta were also measured. The experimental results indicated that, in the heat exposure environment, rat food intake was reduced. Rat weight was significantly increased in all groups except SHR-24 (all P<0.01 except SHR-8, P<0.05). Heat exposure significantly increased the blood pressure of rats in the WKY-24 (P<0.01 for systolic; P<0.05 for diastolic), SHR-8 and SHR-24 (all P<0.01) groups. This effect was more notable in SHR compared with WKY. NA-induced contraction of aorta rings significantly increased in the SHR-CN group, compared with the WKY-CN group (P<0.01). Heat exposure significantly elevated the NA-induced contraction in both 8 h groups compared with the CN groups (P<0.01). This effect was accompanied by structural damage to the thoracic aorta and increased expression of apoptotic genes and proteins. In conclusion, thoracic aorta damages in SHRs were more sensitive to heat exposure. The enhanced NA-induced contraction may have partly been due to increased apoptosis in the thoracic aorta.

Heat Acclimation-Mediated Cross-Tolerance: Origins in within-Life Epigenetics?

The primary outcome of heat acclimation is increased thermotolerance, which stems from enhancement of innate cytoprotective pathways. These pathways produce “ON CALL” molecules that can combat stressors to which the body has never been exposed, via cross-tolerance mechanisms (heat acclimation-mediated cross-tolerance—HACT). The foundation of HACT lies in the sharing of generic stress signaling, combined with tissue/organ- specific protective responses. HACT becomes apparent when acclimatory homeostasis is achieved, lasts for several weeks, and has a memory. HACT differs from other forms of temporal protective mechanisms activated by exposure to lower “doses” of the stressor, which induce adaptation to higher “doses” of the same/different stressor; e.g., preconditioning, hormesis. These terms have been adopted by biochemists, toxicologists, and physiologists to describe the rapid cellular strategies ensuring homeostasis. HACT employs two major protective avenues: constitutive injury attenuation and abrupt post-insult release of help signals enhanced by acclimation. To date, the injury-attenuating features seen in all organs studied include fast-responding, enlarged cytoprotective reserves with HSPs, anti-oxidative, anti-apoptotic molecules, and HIF-1α nuclear and mitochondrial target gene products. Using cardiac ischemia and brain hypoxia models as a guide to the broader framework of phenotypic plasticity, HACT is enabled by a metabolic shift induced by HIF-1α and there are less injuries caused by Ca⁺² overload, via channel or complex-protein remodeling, or decreased channel abundance. Epigenetic markers such as post-translational histone modification and altered levels of chromatin modifiers during acclimation and its decline suggest that dynamic epigenetic mechanisms controlling gene expression induce HACT and acclimation memory, to enable the rapid return of the protected phenotype. In this review the link between in vivo physiological evidence and the associated cellular and molecular mechanisms leading to HACT and its difference from short-acting cross-tolerance strategies will be discussed.

Pretreatment with indomethacin results in increased heat stroke severity during recovery in a rodent model of heat stroke

It has been suggested that medications can increase heat stroke (HS) susceptibility/severity. We investigated whether the nonsteroidal anti-inflammatory drug (NSAID) indomethacin (INDO) increases HS severity in a rodent model. Core temperature (T_c) of male, C57BL/6J mice (n = 45) was monitored continuously, and mice were given a dose of INDO [low dose (LO) 1 mg/kg or high dose (HI) 5 mg/kg in flavored treat] or vehicle (flavored treat) before heating. HS animals were heated to 42.4°C and euthanized at three time points for histological, molecular, and metabolic analysis: onset of HS [maximal core temperature (T_c,Max)], 3 h of recovery [minimal core temperature or hypothermia depth (HYPO)], and 24 h of recovery (24 h). Nonheated (control) animals underwent identical treatment in the absence of heat. INDO (LO or HI) had no effect on physiological indicators of performance (e.g., time to T_c,Max, thermal area, or cooling time) during heating or recovery. HI INDO resulted in 45% mortality rate by 24 h (HI INDO + HS group). The gut showed dramatic increases in gross morphological hemorrhage in HI INDO + HS in both survivors and nonsurvivors. HI INDO + HS survivors had significantly lower red blood cell counts and hematocrit suggesting significant hemorrhage. In the liver, HS induced cell death at HYPO and increased inflammation at T_c,Max, HYPO, and 24 h; however, there was additional effect with INDO + HS group. Furthermore, the metabolic profile of the liver was disturbed by heat, but there was no additive effect of INDO + HS. This suggests that there is an increase in morbidity risk with INDO + HS, likely resulting from significant gut injury.NEW & NOTEWORTHY This paper suggests that in a translational mouse model, NSAIDs may be counterindicated in situations that put an individual at risk of heat injury. We show here that a small, single dose of the NSAID indomethacin before heat stroke has a dramatic and highly damaging effect on the gut, which ultimately leads to increased systemic morbidity.

Genomic analysis of Ugandan and Rwandan chicken ecotypes using a 600 k genotyping array

Background

Indigenous populations of animals have developed unique adaptations to their local environments, which may include factors such as response to thermal stress, drought, pathogens and suboptimal nutrition. The survival and subsequent evolution within these local environments can be the result of both natural and artificial selection driving the acquisition of favorable traits, which over time leave genomic signatures in a population. This study’s goals are to characterize genomic diversity and identify selection signatures in chickens from equatorial Africa to identify genomic regions that may confer adaptive advantages of these ecotypes to their environments.

Results

Indigenous chickens from Uganda (n = 72) and Rwanda (n = 100), plus Kuroilers (n = 24, an Indian breed imported to Africa), were genotyped using the Axiom® 600 k Chicken Genotyping Array. Indigenous ecotypes were defined based upon location of sampling within Africa. The results revealed the presence of admixture among the Ugandan, Rwandan, and Kuroiler populations. Genes within runs of homozygosity consensus regions are linked to gene ontology (GO) terms related to lipid metabolism, immune functions and stress-mediated responses (FDR < 0.15). The genes within regions of signatures of selection are enriched for GO terms related to health and oxidative stress processes. Key genes in these regions had anti-oxidant, apoptosis, and inflammation functions.

Conclusions

The study suggests that these populations have alleles under selective pressure from their environment, which may aid in adaptation to harsh environments. The correspondence in gene ontology terms connected to stress-mediated processes across the populations could be related to the similarity of environments or an artifact of the detected admixture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2711-5) contains supplementary material, which is available to authorized users.

Epigenetics and cytoprotection with heat acclimation

Studying “phenotypic plasticity” involves comparison of traits expressed in response to environmental fluctuations and aims to understand tolerance and survival in new settings. Reversible phenotypic changes that enable individuals to match their phenotype to environmental demands throughout life can be artificially induced, i.e., acclimation or occur naturally, i.e., acclimatization. The onset and achievement of acclimatory homeostasis are determined by molecular programs that induce the acclimated transcriptome. In heat acclimation, much evidence suggests that epigenetic mechanisms are powerful players in these processes. Epigenetic mechanisms affect the accessibility of the DNA to transcription factors, thereby regulating gene expression and controlling the phenotype. The heat-acclimated phenotype confers cytoprotection against novel stressors via cross-tolerance mechanisms, by attenuation of the initial damage and/or by accelerating spontaneous recovery through the release of help signals. This indispensable acclimatory feature has a memory and can be rapidly reestablished after the loss of acclimation and the return to the physiological preacclimated phenotype. The transcriptional landscape of the deacclimated phenotype includes constitutive transcriptional activation of epigenetic bookmarks. Heat shock protein (HSP) 70/HSP90/heat shock factor 1 memory protocol demonstrated constitutive histone H4 acetylation on hsp70 and hsp90 promotors. Novel players in the heat acclimation setup are poly(ADP-ribose)ribose polymerase 1 affecting chromatin condensation, DNA linker histones from the histone H1 cluster, and transcription factors associated with the P38 pathway. We suggest that these orchestrated responses maintain euchromatin and proteostasis during deacclimation and predispose to rapid reacclimation and cytoprotection. These mechanisms represent within-life epigenetic adaptations and cytoprotective memory.

Cross Acclimation between Heat and Hypoxia: Heat Acclimation Improves Cellular Tolerance and Exercise Performance in Acute Normobaric Hypoxia

BACKGROUND

The potential for cross acclimation between environmental stressors is not well understood. Thus, the aim of this investigation was to determine the effect of fixed-workload heat or hypoxic acclimation on cellular, physiological, and performance responses during post acclimation hypoxic exercise in humans.

METHOD

Twenty-one males (age 22 ± 5 years; stature 1.76 ± 0.07 m; mass 71.8 ± 7.9 kg; [Formula: see text]O2 peak 51 ± 7 mL(.)kg(-1.)min(-1)) completed a cycling hypoxic stress test (HST) and self-paced 16.1 km time trial (TT) before (HST1, TT1), and after (HST2, TT2) a series of 10 daily 60 min training sessions (50% N [Formula: see text]O2 peak) in control (CON, n = 7; 18°C, 35% RH), hypoxic (HYP, n = 7; fraction of inspired oxygen = 0.14, 18°C, 35% RH), or hot (HOT, n = 7; 40°C, 25% RH) conditions.

RESULTS

TT performance in hypoxia was improved following both acclimation treatments, HYP (-3:16 ± 3:10 min:s; p = 0.0006) and HOT (-2:02 ± 1:02 min:s; p = 0.005), but unchanged after CON (+0:31 ± 1:42 min:s). Resting monocyte heat shock protein 72 (mHSP72) increased prior to HST2 in HOT (62 ± 46%) and HYP (58 ± 52%), but was unchanged after CON (9 ± 46%), leading to an attenuated mHSP72 response to hypoxic exercise in HOT and HYP HST2 compared to HST1 (p < 0.01). Changes in extracellular hypoxia-inducible factor 1-α followed a similar pattern to those of mHSP72. Physiological strain index (PSI) was attenuated in HOT (HST1 = 4.12 ± 0.58, HST2 = 3.60 ± 0.42; p = 0.007) as a result of a reduced HR (HST1 = 140 ± 14 b.min(-1); HST2 131 ± 9 b.min(-1) p = 0.0006) and Trectal (HST1 = 37.55 ± 0.18°C; HST2 37.45 ± 0.14°C; p = 0.018) during exercise. Whereas PSI did not change in HYP (HST1 = 4.82 ± 0.64, HST2 4.83 ± 0.63).

CONCLUSION

Heat acclimation improved cellular and systemic physiological tolerance to steady state exercise in moderate hypoxia. Additionally we show, for the first time, that heat acclimation improved cycling time trial performance to a magnitude similar to that achieved by hypoxic acclimation.

Insights into the role of heat shock protein 72 to whole-body heat acclimation in humans

Heat acclimation results in systemic and cellular adaptions that reduce the negative effect of heat and, consequently, the risk of heat illness. Although the classical changes observed with heat acclimation lead to increased tolerance to exercise in the heat by reducing heat storage (reflected in reduced core and skin temperatures) and increasing whole-body capacity for heat dissipation (greater plasma volume, sweat output, and skin blood flow), it appears that heat acclimation also induces changes at the cellular level that might increase tolerance of the whole organism to a higher core temperature for the development of fatigue. Thermotolerance is a process that involves increased resilience to an otherwise lethal heat stress that follows a sublethal exposure to heat. Thermotolerance is believed to be the result of increased content of heat shock proteins (Hsp), specially a member of the 70 kDa family, Hsp72 kDa. In humans, we and others have reported that heat acclimation increases intracellular Hsp72 levels. This increase in intracellular Hsp72 could improve whole-body organism thermotolerance by maintaining intestinal epithelial tight junction barriers, by increasing resistance to gut-associated endotoxin translocation, or by reducing the inflammatory response. In this review, we will initially provide an overview of the physiological adaptations induced by heat acclimation and emphasize the main cellular changes that occur with heat acclimation associated with intracellular accumulation of Hsp72. Finally, we will present an argument for a role of whole-body heat acclimation in augmenting cellular thermotolerance, which may protect vital organs from deleterious effects of heat stress in humans.

Modulatory effect of betaine on expression dynamics of HSPs during heat stress acclimation in goat (Capra hircus)

Changing climatic scenario with expected global rise in surface temperature compelled more focus of research over decoding heat stress response mechanism of animals and mitigation of heat stress. Recently betaine, a trimethyl form of glycine has been found to ameliorate heat stress in some species of animals. To overcome deleterious effect of heat stress, an attempt was taken to investigate the effect of betaine supplementation on heat stress mitigation in goats. Eighteen female Barbari goats were taken and randomly divided into 3 groups (n=6) such as control, HS (Heat stressed), HS+B (Heat stressed administered with betaine). Except for the control group, other groups were exposed to repeated heat stress (42 °C) for 6 h for sixteen consecutive days. Blood samples were collected at the end of heat exposure on day 1 (Initial heat stress acclimation - IHSA), day 6 (Short term heat stress acclimation - STHSA) and day 16 (Long term heat stress acclimation - LTHSA). When the groups were compared between different heat stress acclimatory phases, expression of all HSPs (HSP60, HSP70, HSP90 and HSP105/110) showed a similar pattern with a first peak on IHSA, reaching a basal level on STHSA followed by second peak on LTHSA. The messenger RNA (mRNA) and protein expression of HSPs was observed to be higher (P<0.05) in HS group than HS+B group except HSP90 on IHSA and HSP60 on STHSA. HSP105/110 expression was highest (P<0.05) on LTHSA. Immunocytochemical analysis revealed that HSPs were mainly localized both in nucleus and cytoplasm of PBMCs. In conclusion, heat stress increases HSPs expression and betaine administration was shown to have a dwindling effect on expression of HSPs, suggesting a possible role of this chemical chaperone on heat stress amelioration.

Molecular programs induced by heat acclimation confer neuroprotection against TBI and hypoxic insults via cross-tolerance mechanisms

Neuroprotection following prolonged exposure to high ambient temperatures (heat acclimation HA) develops via altered molecular programs such as cross-tolerance Heat Acclimation-Neuroprotection Cross-Tolerance (HANCT). The mechanisms underlying cross-tolerance depend on enhanced "on-demand" protective pathways evolving during acclimation. The protection achieved is long lasting and limits the need for de novo recruitment of cytoprotective pathways upon exposure to novel stressors. Using mouse and rat acclimated phenotypes, we will focus on the impact of heat acclimation on Angiotensin II-AT2 receptors in neurogenesis and on HIF-1 as key mediators in spontaneous recovery and HANCT after traumatic brain injury (TBI). The neuroprotective consequences of heat acclimation on NMDA and AMPA receptors will be discussed using the global hypoxia model. A behavioral-molecular link will be crystallized. The differences between HANCT and consensus preconditioning will be reviewed.

Patterns of gene expression associated with recovery and injury in heat-stressed rats

Background

The in vivo gene response associated with hyperthermia is poorly understood. Here, we perform a global, multiorgan characterization of the gene response to heat stress using an in vivo conscious rat model.

Results

We heated rats until implanted thermal probes indicated a maximal core temperature of 41.8°C (T_c,Max). We then compared transcriptomic profiles of liver, lung, kidney, and heart tissues harvested from groups of experimental animals at T_c,Max, 24 hours, and 48 hours after heat stress to time-matched controls kept at an ambient temperature. Cardiac histopathology at 48 hours supported persistent cardiac injury in three out of six animals. Microarray analysis identified 78 differentially expressed genes common to all four organs at T_c,Max. Self-organizing maps identified gene-specific signatures corresponding to protein-folding disorders in heat-stressed rats with histopathological evidence of cardiac injury at 48 hours. Quantitative proteomics analysis by iTRAQ (isobaric tag for relative and absolute quantitation) demonstrated that differential protein expression most closely matched the transcriptomic profile in heat-injured animals at 48 hours. Calculation of protein supersaturation scores supported an increased propensity of proteins to aggregate for proteins that were found to be changing in abundance at 24 hours and in animals with cardiac injury at 48 hours, suggesting a mechanistic association between protein misfolding and the heat-stress response.

Conclusions

Pathway analyses at both the transcript and protein levels supported catastrophic deficits in energetics and cellular metabolism and activation of the unfolded protein response in heat-stressed rats with histopathological evidence of persistent heat injury, providing the basis for a systems-level physiological model of heat illness and recovery.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1058) contains supplementary material, which is available to authorized users.

Heat acclimation memory: do the kinetics of the deacclimated transcriptome predispose to rapid reacclimation and cytoprotection?

Faster reinduction of heat acclimation (AC) after its decline indicates "AC memory." Our previous results revealed involvement of epigenetic mechanisms of transcriptional regulation. We hypothesized that the decline of AC (DeAC) is a period of "dormant memory" during which many processes are alerted to enable rapid reacclimation (ReAC). Using a genomewide approach we studied the AC, DeAC, and ReAC transcriptomes, to uncover hallmark pathways linked to "molecular memory" in the cardioacclimatome. Fifty rats subjected to heat acclimation [34°C for 2d (AC2d) or 30d (AC30)], DeAC (24°C, 30 days), ReAC (34°C, 2 days), and untreated controls were used. The GeneChip Rat Gene 1.0 ST Array was employed for left ventricular (cardiac) mRNA hybridization. Three independent bioinformatic analyses showed that 1) during AC2d enrichment of DNA impair/repair-linked genes is seen, and this is the molecular on-switch of acclimation; 2) genes activated in AC30 underlie the qualitative physiological adaptations of cardiac performance; 3) particular molecular programs encompassing constitutive upregulation of p38 MAPK, Jak/Stat, and Akt pathways and targets are specifically activated during DeAC and ReAC; and 4) epigenetic markers such as linker histones (histones H1 cluster), associated with nucleosome spacing, transcriptional chromatin modifiers, poly-(ADP-ribose) polymerase-1 (PARP1) linked to chromatin compaction, and microRNAs are only altered during DeAC/ReAC. The latter are newcomers to the AC/DeAC puzzle. We suggest that these transcriptional responses maintain euchromatin and proteostasis and enable faster physiological recovery upon ReAC by rapidly reestablishing the protected acclimated cardiophenotype. We propose that the cardiac AC model can be applied to acclimation processes in general.

Heat acclimation and cross tolerance to hypoxia: Bridging the gap between cellular and systemic responses

Recent research has suggested a potential for some of the physiological and cellular responses to heat acclimation to carry over to improved tolerance of the novel stresses of another environment. This cross-tolerance is evident in heat-acclimated animals that exhibit enhanced tolerance to either hypoxic or ischemic stress, and is primarily attributed to shared cellular stress response pathways. These pathways include Hypoxia-Inducible Factor-1 (HIF-1) and Heat Shock Proteins (HSP). Whether these shared cellular stress response pathways translate to systemic cross-tolerance (improved exercise tolerance, reduced risk of environment-associated illness) has not been clearly shown, particularly in humans. This review highlights the HIF-1 and HSP pathways and their relationship with systemic acclimation responses, and further examines the potential cellular and systemic adaptations that may result in cross-tolerance between hot and hypoxic environments.

The protective effect of heat acclimation from hypoxic damage in the brain involves changes in the expression of glutamate receptors

Long-term heat acclimation (34 °C, 30d) alters the physiological responses and the metabolic state of organisms. It also improves ability to cope with hypoxic stress via a cross-tolerance mechanism. Within the brain, the hippocampal and frontal cortex neurons are the most sensitive to hypoxia and cell death is mainly caused by calcium influx via glutamate-gated ion channels, specifically NMDA and AMPA receptors. GluN1 subunit levels of NMDA-R correspond to NMDA-R levels. GluN2B/GluN2A subunit ratio is a qualitative index of channel activity; a higher ratio implies lower calcium permeability. The GluA2 subunit of AMPA-R controls channel permeability by inhibiting calcium penetration. Here, in rats model we (i)used behavioral-assessment tests to evaluate heat acclimation mediated hypoxic (15' 4.5 ± 0.5% O2) neuroprotection, (ii) measured protein and transcript levels of NMDA-R and AMPA-R subunits before and after hypoxia in the hippocampus and the frontal cortex, to evaluate the role of Ca(2+) in neuro-protection/cross-tolerance. Behavioral tests confirmed hypoxic tolerance in long-term (30d) but not in short-term (2d) heat acclimated rats. Hypoxic tolerance in the long-term acclimated phenotype was accompanied by a significant decrease in basal NMDA receptor GluN1 protein and an increase in its mRNA. The long-term acclimated rats also showed post ischemic increases in the GluN2B/GluN2A subunit ratio and GluA2 subunit of the AMPA receptor, supporting the hypothesis that reduced calcium permeability contributes to heat acclimation mediated hypoxia cross-tolerance. Abrupt post ischemic change in GluN2B/GluN2A subunit ratio with no change in NMDA-R subunits transcript levels implies that post-translational processes are inseparable acclimatory cross-tolerance mechanism.

Thermal sensation and cell adaptability

Whole person adaptive comfort is discussed with reference to recent findings in molecular scale systems biology. The observations are upscaled to hypotheses relating to less traditional interpretations of thermal processes, which have new implications for indoor climate management and design. Arguments are presented for a revision of current focus, model and paradigm. The issue is seen as a problem of integrating theoretical development, conceptual modeling and as an investigation of the extent to which environments and acclimatization can be used to achieve individual fitness and health, not only at the subjective comfort level, as hitherto promoted. It is argued that there are many questions yet to be asked about adaptability before celebrating a particular adaptive state.

Heat acclimation affects circulating levels of prostaglandin E2, COX-2 and orexin in humans

We examined serum levels of prostaglandin E2 (PGE2), cyclooxygenase (COX)-2 and orexin before and after heat acclimation (HA) to test the hypothesis that decreased basal body temperature due to HA correlate with circulating levels of these key thermoregulatory molecules. Nine healthy human male volunteers were recruited (age, 21.9±2.7 years). The subjects were exposed to half-body immersion in hot water (42±0.5°C) at the same time of day (2-5p.m.) on alternate days for 3 weeks. The HA protocol included 10 bouts of 30min immersion. All experiments were performed in an automated climate chamber (temperature, 26.0±0.5°C; relative humidity, 60±3.0%; air velocity, <1m/s). Tympanic and skin temperatures were measured, and mean body temperature was calculated. The difference in body weight was used to estimate total sweat loss. Serum levels of PGE2, COX-2 and orexin were analyzed before and after HA. Body temperature decreased significantly (P<0.05) after HA, whereas sweat volume increased significantly (P<0.01). Serum PGE2, COX-2 and orexin concentrations decreased significantly compared to those at pre-acclimation (P<0.001, P<0.01, P<0.01, respectively). Our data suggest that decreased basal body temperature after HA is associated with decreases in thermoregulatory molecules, such as PGE2, COX-2 and orexin.

Heat acclimation decreased oxidative DNA damage resulting from exposure to high heat in an occupational setting

Heat acclimation is a physiologically and biochemically adapted process when species transition from one environmental temperature to one of the increased temperature. There is very limited epidemiological evidence on the heat-related impacts during exposure to extremely high heat in an occupational environment. This study sought to identify a potential biomarker of heat acclimation and the burden of heat on the body. The aim of this study was to elucidate oxidative DNA damage and heat acclimation through a self-comparison study design in navy boiler tenders, subjects exposed to extremely high heat in an occupational setting. Fifty-eight male soldiers who work in a boiler room were recruited for this study. The subjects were initially assessed with a health examination and body composition assessment before sailing. In order to compare the within-subject differences before and after heat exposure, the index-related heat exposure was collected before and after a routine 5-h work shift and 7-day sailing. Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG), a useful marker of oxidative DNA damage was the measurement by liquid chromatography/tandem mass spectrometry. The median of the change in urinary 8-OHdG was 0.78 μg/g creatinine, as the urinary 8-OHdG after sailing was significantly higher than before sailing (p < 0.01). The urinary 8-OHdG was significantly decreased in heat-acclimated boiler tenders. Oxidative DNA damage was significantly decreased in heat-acclimated subjects. Urinary 8-OHdG can be used as a biomarker to assess the effect of heat stress as a result of occupational exposure to extremely high heat conditions.

Heat acclimation and exercise training interact when combined in an overriding and trade-off manner: physiologic-genomic linkage

Combined heat acclimation (AC) and exercise training (EX) enhance exercise performance in the heat while meeting thermoregulatory demands. We tested the hypothesis that different stress-specific adaptations evoked by each stressor individually trigger similar cardiac alterations, but when combined, overriding/trade-off interactions take place. We used echocardiography, isolated cardiomyocyte imaging and cDNA microarray techniques to assay in situ cardiac performance, excitation-contraction (EC) coupling features, and transcriptional programs associated with cardiac contractility. Rat groups studied were controls (sedentary 24°C); AC (sedentary, 34°C, 1 mo); normothermic EX (treadmill at 24°C, 1 mo); and heat-acclimated, exercise-trained (EXAC; treadmill at 34°C, 1 mo). Prolonged heat exposure decreased heart rate and contractile velocity and increased end ventricular diastolic diameter. Compared with controls, AC/EXAC cardiomyocytes demonstrated lower l-type Ca(2+) current (I(CaL)) amplitude, higher Ca(2+) transient (Ca(2+)T), and a greater Ca(2+)T-to-I(CaL) ratio; EX alone enhanced I(CaL) and Ca(2+)T, whereas aerobic training in general induced cardiac hypertrophy and action potential elongation in EX/EXAC animals. At the genomic level, the transcriptome profile indicated that the interaction between AC and EX yields an EXAC-specific molecular program. Genes affected by chronic heat were linked with the EC coupling cascade, whereas aerobic training upregulated genes involved with Ca(2+) turnover via an adrenergic/metabolic-driven positive inotropic response. In the EXAC cardiac phenotype, the impact of chronic heat overrides that of EX on EC coupling components and heart rate, whereas EX regulates cardiac morphometry. We suggest that concerted adjustments induced by AC and EX lead to enhanced metabolic and mechanical performance of the EXAC heart.

Influence of climate on emergency department visits for syncope: role of air temperature variability

Background

Syncope is a clinical event characterized by a transient loss of consciousness, estimated to affect 6.2/1000 person-years, resulting in remarkable health care and social costs. Human pathophysiology suggests that heat may promote syncope during standing. We tested the hypothesis that the increase of air temperatures from January to July would be accompanied by an increased rate of syncope resulting in a higher frequency of Emergency Department (ED) visits. We also evaluated the role of maximal temperature variability in affecting ED visits for syncope.

Methodology/Principal Findings

We included 770 of 2775 consecutive subjects who were seen for syncope at four EDs between January and July 2004. This period was subdivided into three epochs of similar length: 23 January–31 March, 1 April–31 May and 1 June–31 July. Spectral techniques were used to analyze oscillatory components of day by day maximal temperature and syncope variability and assess their linear relationship.

There was no correlation between daily maximum temperatures and number of syncope. ED visits for syncope were lower in June and July when maximal temperature variability declined although the maximal temperatures themselves were higher. Frequency analysis of day by day maximal temperature variability showed a major non-random fluctuation characterized by a ∼23-day period and two minor oscillations with ∼3- and ∼7-day periods. This latter oscillation was correlated with a similar ∼7-day fluctuation in ED visits for syncope.

Conclusions/Significance

We conclude that ED visits for syncope were not predicted by daily maximal temperature but were associated with increased temperature variability. A ∼7-day rhythm characterized both maximal temperatures and ED visits for syncope variability suggesting that climate changes may have a significant effect on the mode of syncope occurrence.

Molecular signals that shape the integrative responses of the heat-acclimated phenotype

The introduction of molecular biology to thermoregulation was delayed compared with its application in other research fields pertinent to human health and disease. Using principles from molecular biology, we revisited fundamental problems in integrative and environmental physiology and were able to explore new research horizons. Global genomic responses in tandem with an appropriate physiological experimental model are a good experimental design strategy that can unravel the molecular mechanisms underlying integrative thermoregulatory responses. In this way, dynamic adaptation models, with accentuated or diminished regulatory circuits, triggered by superimposition of novel stressors sharing similar protective pathways, have significant benefits. On the basis of this approach, we will discuss the molecular physiological linkage of heat acclimation alone or combined with exercise training and decipher stress-specific genes in the thermoregulatory circuits in the heart and skeletal muscles. Opposing/competing adaptive features are required for each of the above-mentioned physiological conditions. Aerobic training increases the capacity to store/use ATP. In contrast, the acclimated phenotype attempts to counteract excessive heat production. Nevertheless, both treatments augment muscle force generation. These changes are tissue-specific; in the exercise-trained rat heart, there is up-regulation of Ca2+-induced Ca2+ release mechanism genes, whereas in the skeletal muscle (soleus), the enrichment is found in genes involved in metabolism. The final issue discussed in this review is the possibility that heat shock proteins serve as consensus markers of heat stress. The role of the autonomic nervous system in their induction during heat stress and how they affect integrative body systems are described.

Response of mice to continuous 5-day passive hyperthermia resembles human heat acclimation

Chronic repeated exposure to hyperthermia in humans results in heat acclimation (HA), an adaptive process that is attained in humans by repeated exposure to hyperthermia and is characterized by improved heat elimination and increased exercise capacity, and acquired thermal tolerance (ATT), a cellular response characterized by increased baseline heat shock protein (HSP) expression and blunting of the acute increase in HSP expression stimulated by re-exposure to thermal stress. Epidemiologic studies in military personnel operating in hot environments and elite athletes suggest that repeated exposure to hyperthermia may also exert long-term health effects. Animal models demonstrate that coincident exposure to mild hyperthermia or prior exposure to severe hyperthermia can profoundly affect the course of experimental infection and injury, but these models do not represent HA. In this study, we demonstrate that CD-1 mice continuously exposed to mild hyperthermia (ambient temperature ~37°C causing ~2°C increase in core temperature) for 5 days and then exposed to a thermal stress (42°C ambient temperature for 40 min) exhibited some of the salient features of human HA, including (1) slower warming during thermal stress and more rapid cooling during recovery and (2) increased activity during thermal stress, as well as some of the features of ATT, including (1) increased baseline expression of HSP72 and HSP90 in lung, heart, spleen, liver, and brain; and (2) blunted incremental increase in HSP72 expression following acute thermal stress. This study suggests that continuous 5-day exposure of CD-1 mice to mild hyperthermia induces a state that resembles the physiologic and cellular responses of human HA. This model may be useful for analyzing the molecular mechanisms of HA and its consequences on host responsiveness to subsequent stresses.

Posttranslational modifications in histones underlie heat acclimation-mediated cytoprotective memory

We have demonstrated that heat acclimation (AC) causes selective, long-lasting, transcriptional changes in cytoprotective and chromatin remodeling-associated genes, which maintain their AC transcriptome profile, despite the loss of the AC phenotype (Tetievsky et al. Physiol Genomics 34: 78-87, 2008). We postulated that AC memory involves upstream epigenetic information, which predisposes to rapid reacclimation (ReAC) and cytoprotective memory. Here we tested the hypothesis that posttranslational histone modifications are linked to this process. Rats subjected to AC (34°C for 2 or 30 days), deacclimation (DeAC; 24°C, 30 days), and ReAC (34°C, 2 days), and untreated controls were used. Histone H4 lysine acetylation and histone H3 acetylation and phosphorylation in the heat shock element (HSE) of the promoters of heat shock protein-70 (hsp70) and -90 (hsp90) genes were examined. Histone acetyltransferase recruitment of TIP60 (60-kDa histone acetyltransferase-interactive protein), the catalytic subunit of NuH4, was used to validate acetylation. Heat shock factor-1 (HSF-1)-HSE binding to the hsp70 and hsp90 genes was measured to confirm HSF-1 binding to euchromatin. Our results indicate that, while histone H3Ser10 phosphorylation occurred during the AC 2-day phase, AC constitutively elevated histone H4 acetylation in the HSE of hsp70 and hsp90 promoters. HSF-1-HSE binding was detected in the hsp70 gene throughout AC-DeAC-ReAC. The hsp90 gene lacked HSF-1 binding during DeAC, but resumed a high binding level upon ReAC. HSP-90 is a critical cytoprotective protein, and the HSF-1-hsp90 binding profile matched levels of this protein. We conclude that, while early histone H3 phosphorylation is probably required for subsequent histone H4 acetylation, the constitutively acetylated histone H4 and the preserved euchromatin state throughout AC-DeAC-ReAC predispose to rapid cytoprotective acclimatory memory.

Long- but not short-term heat acclimation produces an apoptosis-resistant cardiac phenotype: a lesson from heat stress and ischemic/reperfusion insults

Long-term heat acclimation (AC, 30d/34 degrees C) is a phenotypic adaptation leading to increased thermotolerance during heat stress (HS, 2 h 41 degrees C). AC also renders protection against ischemic/reperfusion (I/R, 30' global ischemia/40' reperfusion) insult via cross-tolerance mechanisms. In contrast to the protected AC phenotype, the onset of acclimation (34 degrees C, AC2d) is characterized by cellular perturbations, suggesting increased susceptibility to HS and I/R insults. In this investigation, we tested the hypothesis that apoptosis resistance is part of the AC repertoire and that, at the initial phase of acclimation (AC2d), cytoprotection is impaired. TUNEL staining and caspase 3 levels in HS and I/R insulted hearts affirmed this hypothesis. To examine the role of the mitochondria in life/death decision in AC2d and 30d AC settings vs. control hearts, we studied the Bcl-2 apoptotic cascade and found increased levels of the anti-apoptotic Bcl-X(L) and decreased levels of the pro-apoptotic death promoter Bad in hearts from AC2d and AC animals. In these groups, cytochrome c (cyt c) was elevated in the mitochondria and remained unchanged in the cytosol. This adaptation was insufficient to negate apoptosis in AC2d rats. At this early acclimation phase (and in controls), increased caspase 8 activity confirmed activation of the extrinsic (Fas ligand) apoptosis pathway. In conclusion, the elevated Bcl-X(L)/Bad ratio and decreased cyt c leakage to the cytosol are insufficient to protect the heart and interactions with additional cytoprotective pathways involved in acclimation (elevated HSP70, ROS, and sarcolemmal adaptations to abolish extrinsic apoptosis pathways) are required to induce the apoptosis-resistant AC phenotype.

Daily hypoxia increases basal monocyte HSP72 expression in healthy human subjects

Heat shock protein 72 (HSP72) performs vital roles within the body at rest and during periods of stress. In vitro, research demonstrates HSP72 induction in response to hypoxia. Recently, in vivo, an acute hypoxic exposure (75 min at 2,980 m) was sufficient to induce significant increases in monocyte expressed HSP72 (mHSP72) and a marker of oxidative stress in healthy human subjects. The purpose of the current study was to identify the impact of 10 consecutive days of hypoxic exposures (75 min at 2,980 m) on mHSP72 and erythropoietin (EPO) expression, markers of oxidative stress, and maximal oxygen consumption in graded incremental aerobic exercise. Eight male subjects were exposed to daily normobaric hypoxic exposures for 75 min at 2,980 m for 10 consecutive days, commencing and ceasing at 0930 and 1045, respectively. This stressor was sufficient to induce significant increases in mHSP72, which was significantly elevated from day 2 of the hypoxic exposures until 48 h post-final exposure. Notably, this increase had an initial rapid (30% day on day compared to baseline) and final slow phase (16% day on day compared to baseline) of expression. The authors postulate that 7-day hypoxic exposure in this manner would be sufficient to induce near maximum hypoxia-mediated basal mHSP72 expression. Elevated levels of mHSP72 are associated with acquired thermotolerance and provide cross tolerance to non-related stressors in vivo, the protocol used here may provide a useful tool for elevating mHSP72 in vivo. Aside from these major findings, significant transient daily elevations were seen in a marker of oxidative stress, alongside sustained increases in EPO expression. However, no physiologically significant changes were seen in maximal oxygen consumption or time to exhaustion.

Changes in ambient temperature and oxygenation during the proestrus do not affect duration, regularity and repeatability of the estrus cycle in female rats

There are a lot of factors affecting the release of hormones from the anterior part of pituitary gland and their interactions with other parts of the endocrine, nervous and immune systems. The special significance of the proestrus phase of the estrous cycle of the rat, during which LH and FSH levels increase, followed by ovulation is known. The short length of the estrous cycle and the well recognized sequence of vaginal lavage cytology make it useful for investigating the influence of a stressful environment on the reproductive function. Short duration and mild changes in environmental conditions is considered as a factor analogous to psychological stress. The study was undertaken to determine the effects of a short duration change in the ambient temperature and oxygenation (30 minutes) on the proestrus phase of reproductive cycle and on the repeatability and regularity of phases of the reproductive cycle of Wistar strain rats. The animals were kept under standard conditions and had food and water available ad libitum. The climatic chamber with automatically adjustable and monitored internal parameters (temperature, oxygenation, humidity) was used to develop stress conditions. An estimation of the vaginal lavage using the microscope was done to determine the estrous cycle. The animals were divided into 6 groups. On the day of experiment: the control group (CG) stayed in the climatic chamber for 30 minutes (ambient temperature 21 degrees C, normoxia - 21% O(2)), the five test groups (TG - I - V) remained in the climatic chamber for 30 minutes, in the established environmental conditions (I - 21 degrees C, 10% O(2); II - 10 degrees C, 21% O(2); III - 10 degrees C, 10% O(2); IV - 35 degrees C, 21% O(2); V - 35 degrees C, 10% O(2)). During the following days after the experiment, a microscopic estimation of vaginal lavage was collected over again. There were no changes of duration and sequence of the present estrous cycle and repeatability of the next cycles. Our results indicate that short duration change in the ambient conditions do not cause a disturbance in the hypothalamo - pituitary - gonadal axis, although it can activate adaptation mechanisms of the organism.

Physiological and molecular evidence of heat acclimation memory: a lesson from thermal responses and ischemic cross-tolerance in the heart

Sporadic findings in humans suggest that reinduction of heat acclimation (AC) after its loss occurs markedly faster than that during the initial AC session. Animal studies substantiated that the underlying acclimatory processes are molecular. Here we test the hypothesis that faster reinduction of AC (ReAC) implicates "molecular memory." In vivo measurements of colonic temperature profiles during heat stress and ex vivo assessment of cross-tolerance to ischemia-reperfusion or anoxia insults in the heart demonstrated that ReAC only needs 2 days vs. the 30 days required for the initial development of AC. Stress gene profiling in the experimental groups highlighted clusters of transcriptionally activated genes (37%), which included heat shock protein (HSP) genes, antiapoptotic genes, and chromatin remodeling genes. Despite a return of the physiological phenotype to its preacclimation state, after a 1 mo deacclimation (DeAC) period, the gene transcripts did not resume their preacclimation levels, suggesting a dichotomy between genotype and phenotype in this system. Individual detection of hsp70 and hsf1 transcripts agreed with these findings. HSP72, HSF1/P-HSF1, and Bcl-xL protein profiles followed the observed dichotomized genomic response. In contrast, HSP90, an essential cytoprotective component mismatched transcriptional activation upon DeAC. The uniform activation of the similarly responding gene clusters upon De-/ReAC implies that reacclimatory phenotypic plasticity is associated with upstream denominators. During AC, DeAC, and ReAC, the maintenance of elevated/phosphorylated HSF1 protein levels and transcriptionally active chromatin remodeling genes implies that chromatin remodeling plays a pivotal role in the transcriptome profile and in preconditioning to rapid cytoprotective acclimatory memory.

Differential neuroprotective properties of endogenous and exogenous erythropoietin in a mouse model of traumatic brain injury

Both heat acclimation (HA) and post-injury treatment with recombinant human erythropoietin (Epo, rhEpo, exogenous Epo) are neuroprotective against traumatic brain injury (TBI). Our previous data demonstrated that HA-induced neuroprotection includes improved functional recovery and reduced cerebral edema formation. Additionally, in earlier Western-blot analyses, we found that HA mice display increased expression of the specific erythropoietin receptor (EpoR) and of hypoxia-inducible factor-1 alpha (HIF-1 alpha), the inducible subunit of the transcription factor, which regulates Epo gene expression, but not of Epo itself. In light of this, the aim of the current study was threefold: (1) to assess Epo expression in the trauma area and hippocampus following HA, rhEpo administration, or combined HA-rhEpo treatment, using immunohistochemical methods that offer enhanced anatomical resolution; (2) to examine the effects of endogenous and exogenous Epo on edema formation in normothermic (NT) mice; and (3) to evaluate the effects of exogenous Epo administration on neuroprotective outcome measures in HA animals. HA induced enhanced expression of endogenous Epo in the trauma area and the hippocampus. Treatment with anti-Epo antibody given to NT mice increased edema formation, whereas rhEpo induced no beneficial effect. Cognitive performance testing and immunohistochemical findings reinforced HA and rhEpo as separate protective interventions but showed no advantage to combining the two strategies. We therefore suggest that HA-induced neuroprotection is shaped by pre-existing mediators but cannot be modified by post-injury treatment aimed at increasing the levels of neuroprotective agents.

Exercise-heat acclimation in humans alters baseline levels and ex vivo heat inducibility of HSP72 and HSP90 in peripheral blood mononuclear cells

The induction of cellular acquired thermal tolerance (ATT) during heat acclimation (HA) in humans is not well described. This study determined whether exercise-HA modifies the human heat shock protein (HSP)72 and HSP90 responses and whether changes are correlated with physiological adaptations to HA. Using a 10-day HA protocol comprising daily exercise (treadmill walking) in a hot environment ( Ta = 49°C, 20% RH), we analyzed baseline and ex vivo heat-induced expression of HSP72 and HSP90 in peripheral blood mononuclear cells (PBMCs) isolated prior to exercise from eight subjects on day 1 and 10 of the HA protocol. Classical physiological responses to HA were observed, including significantly reduced heart rate and core body temperature, and significantly increased sweating rate. Baseline levels of HSP72 and HSP90 were significantly increased following acclimation by 17.7 ± 6.1% and 21.1 ± 6.5%, respectively. Ex vivo induction of HSP72 in PBMCs exposed to heat shock (43°C) was blunted on day 10 compared with day 1. A correlation was identified ( r2 = 0.89) between changes in core temperature elevation and ex vivo HSP90 responses to heat shock between days 1 and 10, indicating that volunteers demonstrating the greatest physiological HA tended to exhibit the greatest blunting of ex vivo HSP induction in response to heat shock. In summary, 1) exercise-HA resulted in increased baseline levels of HSP72 and HSP90, 2) ex vivo heat inducibility of HSP72 was blunted after HA, and 3) volunteers demonstrating the greatest physiological HA tended to exhibit the greatest blunting of ex vivo HSP induction in response to heat shock. These data demonstrate that physiological adaptations in humans undergoing HA are accompanied by both increases in baseline levels and changes in regulation of cytoprotective HSPs.

Large scale analysis of genes contributing to the herbal preparation dependent hippocampal plasticity in postischemic rehabilitation

Herbal preparations can affect the expression of many genes involved in the ischemic process. These genes have been providing insights into the molecular basis of brain plasticity in stroke rehabilitation. However, the extent of plasticity has not been investigated using a chemogenomic approach. A herbal preparation (270 mg/kg) used to treat ischemic mice for 45 days after global ischemia resulted in a significant decrease in infarct volume and neurological score compared with that of vehicle. This effect was characterized by investigating chemical genomic profiles of the mouse hippocampus with a cDNA microarray containing 1176 known genes. Treatment with the herbal preparation reversed the expression of 46 genes out of 100 genes altered in untreated ischemic mouse hippocampus. These data indicated that more genes were upregulated (60.78%) than downregulated (30.61%), and only 46 genes (46%) appear to be prime targets for therapeutic intervention in ischemia. The altered genes can be classified into seven groups, including signal transduction (12 genes, 27%), oncogene (8 genes, 17%), and transcriptional regulation (7 genes, 15%). Such multiple plasticity of expression could be considered as the beneficial role of this herbal preparation in stroke rehabilitation. Changes in gene expression of nuclear factor of activated T cells, 14-3-3 eta, and beta-arrestin suggest a potential role for the immune system in this plasticity. Brain plasticity originates from a balance of up and downregulated genes (Yin and Yang), and reversal of gene expression in multiple pathways indicates that a complex signaling network may be constructed and investigated further.

Akt phosphorylation is required for heat acclimation-induced neuroprotection

Long-term heat exposure, known as heat acclimation (HA; 30 days at 34 +/- 1 degrees C) is neuroprotective against traumatic brain injury. Acclimated mice were previously found to display improved functional recovery as well as an increase in the levels of the specific erythropoietin receptor. As the activation of this receptor is known to facilitate functional recovery on one hand and the phosphorylation and activation of Akt, an intracellular kinase which regulates anti-apoptotic pathways on the other, in this study we investigated whether HA affects Akt phosphorylation prior to and following injury and whether this step is required for development of HA-induced neuroprotection. Akt phosphorylation was blocked using Triciribine (TCN), a compound shown to block the phosphorylation process without affecting upstream effectors of this kinase, and several post-injury functional end-point measures were subsequently evaluated. Acclimation led to a post-injury increase in the levels of phosphorylated Akt, resulting in higher levels when compared with normothermic controls at 4 h post-injury (63.6 +/- 5.2% and 42.7 +/- 3.7%, respectively, p </= 0.05). This increase was diminished following TCN administration. Post-injury TCN treatment abolished the HA-induced functional benefits, including effects on motor and cognitive functions as well as the attenuation of edema formation. We therefore suggest that Akt phosphorylation is essential for HA-induced neuroprotection after traumatic brain injury.

Heat acclimation and cross-tolerance against novel stressors: genomic–physiological linkage

Heat acclimation (AC) is a "within lifetime" reversible phenotypic adaptation, enhancing thermotolerance and heat endurance via a transition to "efficient" cellular performance when acclimatory homeostasis is reached. An inseparable outcome of AC is the development of cross-tolerance (C-T) against novel stressors. This chapter focuses on central plasticity and the molecular-physiological linkage of acclimatory and C-T responses. A drop in temperature thresholds (T-Tsh) for activation of heat-dissipation mechanisms and an elevated T-Tsh for thermal injury development imply autonomic nervous system (ANS) and cytoprotective network involvement in these processes. During acclimation, the changes in T-Tsh for heat dissipation are biphasic. Initially T-Tsh drops, signifying the early autonomic response, and is associated with perturbed peripheral effector cellular performance. Pre-acclimation values return when acclimatory homeostasis is achieved. The changes in the ANS suggest that acclimatory plasticity involves molecular and cellular changes. These changes are manifested by the activation of central peripheral molecular networks and post-translational modifications. Sympathetic induction of elevated HSP 72 reservoirs, with faster heat shock response, is only one example of this. The global genomic response, detected using gene-chips and cluster analyses imply upregulation of genes encoding ion channels, pumps, and transporters (markers for neuronal excitability) in the hypothalamus at the onset of AC and down regulation of metabotrophic genes upon long term AC. Peripherally, the transcriptional program indicates a two-tier defense strategy. The immediate transient response is associated with the maintenance of DNA and cellular integrity. The sustained response correlates with long-lasting cytoprotective-signaling networks. C-T is recorded against cerebral hypoxia, hyperoxia, and traumatic brain injury. Using the highly developed ischemic/reperfused heart model as a baseline, it is evident that C-T stems via protective shared pathways developed with AC. These comprise constitutive elevation of HIF 1alpha and associated target pathways, HSPs, anti-apoptosis, and antioxidative pathways. Collectively the master regulators of AC and C-T are still enigmatic; however, cutting-edge investigative techniques, using a broad molecular approach, challenge current ideas, and the data accumulated will pinpoint novel pathways and provide new perspectives.