Potential role of the gut microbiota in synthetic torpor and therapeutic hypothermia

Therapeutic hypothermia is today used in several clinical settings, among them the gut related diseases that are influenced by ischemia/reperfusion injury. This perspective paved the way to the study of hibernation physiology, in natural hibernators, highlighting an unexpected importance of the gut microbial ecosystem in hibernation and torpor. In natural hibernators, intestinal microbes adaptively reorganize their structural configuration during torpor, and maintain a mutualistic configuration regardless of long periods of fasting and cold temperatures. This allows the gut microbiome to provide the host with metabolites, which are essential to keep the host immunological and metabolic homeostasis during hibernation. The emerging role of the gut microbiota in the hibernation process suggests the importance of maintaining a mutualistic gut microbiota configuration in the application of therapeutic hypothermia as well as in the development of new strategy such as the use of synthetic torpor in humans. The possible utilization of tailored probiotics to mold the gut ecosystem during therapeutic hypothermia can also be taken into consideration as new therapeutic strategy.

On the immunological limitations of hibernation and synthetic torpor as a supporting technique for astronauts’ radioprotection in deep space missions

Although human hibernation has been introduced as an effective technique in space exploration,there are concerns regarding the intrinsic risks of the approach(i.e.,synthetic torpor)and other factors involved in this procedure.Besides concerns about the brain changes and the state of consciousness during hibernation,an"Achilles heel"of the hibernation is the negative impact of torpor on factors such as the number of circulating leukocytes,complement levels,response to lipopolysaccharides,phagocytotic capacity,cytokine production,lymphocyte proliferation,and antibody production.Moreover,increased virulence of bacteria in deep space can significantly increase the risk of infection.The increased infection risk during long-term space missions with the combined effects of radiation and microgravity affect the astronauts’immune system.With these additional immune system stressors,torpor-induced extraimmunosuppression can be potentially life threatening for astronauts.

Torpor发生机制最新研究进展

Torpor是指在食物供应减少时期,为维持生存,动物体内代谢活性大幅降低的一种状态,表现为体温、代谢水平和运动水平大幅降低。哺乳动物拥有一套严格的体温调节系统以维持体温恒定,当能量长时间供给不足时,部分哺乳动物会进入冬眠状态,torpor与冬眠状态表现十分类似。研究torpor状态的发生机制在航空航天和军事医学等领域具有重要意义。本文主要从adcyap(adenylate cyclase activating polypeptide)神经元、瘦素、QRFP(pyroglutamylated RFamide peptide)神经元和交感神经系统等4方面就调控torpor状态发生的具体机制进行总结,旨在为torpor发生机制的进一步研究提供思路。

Continuous growth through winter correlates with increased resting metabolic rate but does not affect daily energy budgets due to torpor use

Small mammals that are specialists in homeothermic thermoregulation reduce their self-maintenance costs of normothermy to survive the winter.By contrast,heterothermic ones that are considered generalists in thermoregulation can lower energy expenditure by entering torpor.It is well known that different species vary the use of their strategies to cope with harsh winters in tem-perate zones;however,ittle is still known about the intraspecific variation within populations and the associated external and internal factors.We hypothesized that yellow-necked mice Apodemus flavicollis decrease their resting metabolic rate(RMR)from autumn to winter,and then increase it during spring.However,since the alternative for seasonal reduction of RMR could be the develop-ment of heterothermy,we also considered the use of this strategy.We measured body mass(mp),RMR,and body temperature(Tb)of mice during 2 consecutive years.In the 1st year,mice decreased whole animal RMR in winter,but did not do so in the 2nd year.All mice entered torpor during the 2nd winter,whereas only a few did so during the first one.Mice showed a continuous in-crease of mb,which was steepest during the 2nd year.The relationship between RMR and mb var-ied among seasons and years most likely due to different mouse development stages.The mb gain at the individual level was correlated positively with RMR and heterothermy.This indicates that high metabolism in winter supports the growth of smaller animals,which use torpor as a compen-satory mechanism.Isotope composition of mice hair suggests that in the 1st year they fed mainly on seeds,while in the 2nd,they likely consumed significant amounts of less digestible herbs.The study suggests that the use of specialist or generalist thermoregulatory strategies can differ with environmental variation and associated differences in developmental processes.

Cytokine and Antioxidant Regulation in the Intestine of the Gray Mouse Lemur (Microcebus murinus) During Torpor

During food shortages, the gray mouse lemur（Microcebus murinus） of Madagascar experiences daily torpor thereby reducing energy expenditures. The present study aimed to understand the impacts of torpor on the immune system and antioxidant response in the gut of these animals.This interaction may be of critical importance given the trade-off between the energetically costly immune response and the need to defend against pathogen entry during hypometabolism. The protein levels of cytokines and antioxidants were measured in the small intestine（duodenum, jejunum,and ileum） and large intestine of aroused and torpid lemurs. While there was a significant decrease of some pro-inflammatory cytokines（IL-6 and TNF-a） in the duodenum and jejunum during torpor as compared to aroused animals, there was no change in anti-inflammatory cytokines. We observed decreased levels of cytokines（IL-12p70 and M-CSF）, and several chemokines（MCP-1and MIP-2） but an increase in MIP-1a in the jejunum of the torpid animals. In addition, we evaluated antioxidant response by examining the protein levels of antioxidant enzymes and total antioxidant capacity provided by metabolites such as glutathione（and others）. Our results indicatedthat levels of antioxidant enzymes did not change between torpor and aroused states, although antioxidant capacity was significantly higher in the ileum during torpor. These data suggest a suppression of the immune response, likely as an energy conservation measure, and a limited role of antioxidant defenses in supporting torpor in lemur intestine.

Modulation of Gene Expression in Key Survival Pathways During Daily Torpor in the Gray Mouse Lemur, Microcebus murinus

A variety of mammals employ torpor as an energy-saving strategy in environments of marginal or severe stress either on a daily basis during their inactive period or on a seasonal basis during prolonged multi-day hibernation. Recently, a few Madagascar lemur species have been identified as the only primates that exhibit torpor; one of these is the gray mouse lemur（Microcebus murinus）. To explore the regulatory mechanisms that underlie daily torpor in a primate, we analyzed the expression of 28 selected genes that represent crucial survival pathways known to be involved in squirrel and bat hibernation. Array-based real-time PCR was used to compare gene expression in control（aroused） versus torpid lemurs in five tissues including the liver, kidney,skeletal muscle, heart, and brown adipose tissue. Significant differences in gene expression during torpor were revealed among genes involved in glycolysis, fatty acid metabolism, antioxidant defense, apoptosis, hypoxia signaling, and protein protection. The results showed upregulation of select genes primarily in liver and brown adipose tissue. For instance, both tissues showed elevated gene expression of peroxisome proliferator activated receptor gamma（ppargc）, ferritin（fth1）, and protein chaperones during torpor. Overall, the data show that the expression of only a few genes changed during lemur daily torpor, as compared with the broader expression changes reported for hibernation in ground squirrels. These results provide an indication that the alterations in gene expression required for torpor in lemurs are not as extensive as those needed for winter hibernation in squirrel models. However, identification of crucial genes with altered expression that support lemur torpor provides key targets to be explored and manipulated toward a goal of translational applications of inducible torpor as a treatment option in human biomedicine.

The future of artificial hibernation medicine:protection of nerves and organs after spinal cord injury

Spinal cord injury is a serious disease of the central nervous system involving irreversible nerve injury and various organ system injuries.At present,no effective clinical treatment exists.As one of the artificial hibernation techniques,mild hypothermia has preliminarily confirmed its clinical effect on spinal cord injury.However,its technical defects and barriers,along with serious clinical side effects,restrict its clinical application for spinal cord injury.Artificial hibernation is a futureoriented disruptive technology for human life support.It involves endogenous hibernation inducers and hibernation-related central neuromodulation that activate particular neurons,reduce the central constant temperature setting point,disrupt the normal constant body temperature,make the body adapt"to the external cold environment,and reduce the physiological resistance to cold stimulation.Thus,studying the artificial hibernation mechanism may help develop new treatment strategies more suitable for clinical use than the cooling method of mild hypothermia technology.This review introduces artificial hibernation technologies,including mild hypothermia technology,hibernation inducers,and hibernation-related central neuromodulation technology.It summarizes the relevant research on hypothermia and hibernation for organ and nerve protection.These studies show that artificial hibernation technologies have therapeutic significance on nerve injury after spinal co rd injury through inflammatory inhibition,immunosuppression,oxidative defense,and possible central protection.It also promotes the repair and protection of res pirato ry and digestive,cardiovascular,locomoto r,urinary,and endocrine systems.This review provides new insights for the clinical treatment of nerve and multiple organ protection after spinal cord injury thanks to artificial hibernation.At present,artificial hibernation technology is not mature,and research fa ces various challenges.Neve rtheless,the effort is wo rthwhile for the future development of medicine.

哺乳动物的蛰眠:类型、物种分布与模式

哺乳动物的蛰眠(包括冬眠、夏眠和日蛰眠等)是最具吸引力的生命现象之一,是动物应对寒冷、食物短缺、干旱等不良环境条件的适应策略之一。冬眠生理学(生态学)研究具有重要的理论和实际意义。国际学术界在该领域发展比较迅速,国内发展相对缓慢。本文从哺乳动物蛰眠的季节和持续时间、蛰眠期间所利用能量的来源和贮存方式、启动蛰眠的信号来源等方面综述了哺乳动物蛰眠的类型;介绍了蛰眠的哺乳动物物种的系统学分布;并对温带或北极动物的冬眠和冬眠阵及其各阶段的体温和代谢率变化特征、日温剧烈波动环境下的冬眠特征以及日眠和日眠阵等方面进行了概括介绍,以期能促进国内相关领域的发展。

光照黑暗循环条件下达乌尔黄鼠的冬眠模式和能量消耗

为研究冬眠季节的光照条件对贮脂类冬眠动物入眠的影响,在达乌尔黄鼠腹腔埋植体温记录元件i Button,在体重高峰后的下降阶段置于5℃和12L∶12D的光照条件下,观察测定其冬眠模式和能量消耗。达乌尔黄鼠冬眠模式出现深冬眠型、少冬眠型和不冬眠型,蛰眠阵包括深冬眠阵、短冬眠阵和日眠阵。不同冬眠阵中最低体温、冬眠阵的持续时间、阵间产热的持续时间、冷却速率和复温速率差异显著;阵间产热的最高体温基本相同。平均每日能量消耗在日眠阵中最高、短冬眠阵中居中、深冬眠阵中最低。入眠时间多集中于黑暗时相,觉醒时间多集中于光照时相。本研究结果提示,在冬眠季节施加光照黑暗循环条件可减少达乌尔黄鼠冬眠的时间,升高阵间最低体温,缩短冬眠阵与阵间产热的持续时间,降低复温速率;增加冬眠期间能量消耗。入眠与觉醒受光照条件影响,具有明显的光暗节律。

Responding to the weather:energy budgeting by a small mammal in the wild

Energy conservation is paramount for small mammals because of their small size,large surface area to volume ratio,and the resultant high heat loss to the environment.To survive on limited food resources and to fuel their expensive metabolism during activity,many small mammals employ daily torpor to reduce energy expenditure during the rest phase.We hypothesized that a small terrestrial semelparous marsupial,the brown antechinus Antechinus stuartii,would maximize activity when foraging conditions were favorable to gain fat reserves before their intense breeding period,but would increase torpor use when conditions were poor to conserve these fat reserves.Female antechinus were trapped and implanted with small temperature-sensitive radio transmitters to record body temperature and to quantify torpor expression and activity patterns in the wild.Most antechinus used torpor at least once per day over the entire study period.Total daily torpor use increased and mean daily body temperature decreased significantly with a reduction in minimum ambient temperature.Interestingly,antechinus employed less torpor on days with more rain and decreasing barometric pressure.In contrast to torpor expression,activity was directly related to ambient temperature and inversely related to barometric pressure.Our results reveal that antechinus use a flexible combination of physiology and behavior that can be adjusted to manage their energy budget according to weather variables.

日蛰眠的哺乳动物及其蛰眠条件与生理特征

日蛰眠是哺乳动物中某些类群应对寒冷、食物短缺等不利环境条件,而在一天中的某个时段降低体温和代谢的一种重要生存适应策略。本文分析了文献报道的近百种已知日蛰眠哺乳动物的系统分类、地理分布、生活环境、体型及食性、体温和代谢率等特征。日蛰眠物种在系统分类上隶属于有袋类(Marsupialia)和胎盘类(Placentalia);这些物种大多分布于温带地区(纬度20°S^30°S和30°N^40°N),环境条件存在明显的季节变化及昼夜变化。日蛰眠动物体型相对较小,主要摄食营养丰富的食物,食草类动物很少日蛰眠。日蛰眠动物的平均蛰眠体温为19.75℃,蛰眠代谢率为基础代谢率的34%。动物通过日蛰眠节约能量,度过不良环境条件。日蛰眠的低代谢和低体温机制对人类健康和生物医学研究具有重要的理论和现实意义。

黑线毛足鼠能量策略的个体差异与权衡

为了应对由冬季低温与食物匮乏所带来的挑战,动物进化出了不同的能量策略,如迁徙、换毛、贮食、储脂和蛰眠等,但个体间可能存在差异,且不同策略之间可能存在一定的权衡。有研究者认为,黑线毛足鼠(Phodopus sungorus)的贮食和日蛰眠等能量策略之间可能存在权衡。本研究测定了低温短光[(10±2)℃,光照与黑暗之比L︰D为8︰16]及自由取食或限食条件下黑线毛足鼠的体重、摄食量、贮食量、食物摄取总量、基础代谢率、体核温度与活动性等7项指标的个体差异,并探讨摄食(包括贮食)、基础代谢率与蛰眠之间是否存在权衡,以推测黑线毛足鼠对低温与食物匮乏能量响应策略的个体差异产生的原因。结果显示,在低温短光、自由取食条件下,摄食量无性别差异,雄性个体的体重大于雌性个体,体核温度及活动性低于雌性个体。在限食阶段且低温短光情况下,出现了蛰眠个体,蛰眠个体在自由取食阶段的活动性高于非蛰眠个体,即活动性高的个体更可能产生蛰眠。蛰眠个体与非蛰眠个体的基础代谢率无差异。但是,在蛰眠个体中,基础代谢率与蛰眠频率负相关,活动性与蛰眠持续时间负相关。在非蛰眠个体中,限食程度升高后活动量显著增加。此外,虽然与贮食和蛰眠可能存在负相关关系的预测相符,贮食个体均不蛰眠,但是动物是否贮食、贮食量以及食物摄取总量(包括贮食量)均与蛰眠无显著相关。本研究表明,能量对策之间的权衡并非发生于贮食和蛰眠之间,而是存在于基础代谢率、活动性与蛰眠之间,不蛰眠的个体倾向于降低活动性以节约能量,而基础代谢率和自由取食阶段活动性高的蛰眠个体则拥有更低的蛰眠表达。