Application of artificial hibernation technology in acute brain injury

Xiaoni Wang, Shulian Chen, Xiaoyu Wang, Zhen Song, Ziqi Wang, Xiaofei Niu, Xiaochu Chen, Xuyi Chen

Abstract Controlling intracranial pressure, nerve cell regeneration, and microenvironment regulation are the key issues in reducing mortality and disability in acute brain injury.There is currently a lack of effective treatment methods.Hibernation has the characteristics of low temperature, low metabolism, and hibernation rhythm, as well as protective effects on the nervous, cardiovascular, and motor systems.Artificial hibernation technology is a new technology that can effectively treat acute brain injury by altering the body’s metabolism, lowering the body’s core temperature, and allowing the body to enter a state similar to hibernation.This review introduces artificial hibernation technology, including mild hypothermia treatment technology, central nervous system regulation technology, and artificial hibernation-inducer technology.Upon summarizing the relevant research on artificial hibernation technology in acute brain injury, the research results show that artificial hibernation technology has neuroprotective, anti-inflammatory, and oxidative stress-resistance effects, indicating that it has therapeutic significance in acute brain injury.Furthermore, artificial hibernation technology can alleviate the damage of ischemic stroke, traumatic brain injury, cerebral hemorrhage, cerebral infarction, and other diseases, providing new strategies for treating acute brain injury.However,artificial hibernation technology is currently in its infancy and has some complications, such as electrolyte imbalance and coagulation disorders, which limit its use.Further research is needed for its clinical application.

Key Words: acute brain injury; artificial hibernation; hypothermia; low metabolism; mild hypothermia

Introduction

Acute brain injury mainly includes stroke and traumatic brain injury (TBI).Acute brain injury is characterized by high incidence rate, high mortality,high disability rate, and high economic burden (Ferrari et al., 2022).The sharp increase in intracranial pressure (ICP), acute diffuse brain swelling, and intracranial hemorrhage are the main reasons for the high mortality rate of acute brain injury (Chen et al., 2017, 2020; Puy et al., 2023).Acute brain injury can also lead to cognitive, behavioral, and psychological disorders,which may be related to the post-mitotic state of neurons, cortical damage,and changes in vascular microenvironment (Stocchetti and Zanier, 2016;Sharbafshaaer, 2018; Reddi et al., 2022; Zhao et al., 2022).The American Brain Trauma Foundation released its fourth edition of the “Guidelines for the Treatment of Severe Brain Injury” in 2016, which pointed out that current important methods for treating brain injury include bone flap decompression,hypertonic therapy, and cerebrospinal fluid drainage.However, their role is limited and cannot repair multiple functional impairments caused by acute brain injury.The increase in ICP, non-regeneration of neurons, and damage to the microenvironment lead to nerve damage, which is currently a major scientific issue in addressing the treatment of acute brain injury.

In nature, some mammals actively lower their body temperature to save energy and survive in extreme environments.This state is called hibernation,which mainly manifests as decreased body temperature, decreased metabolism, and decreased heart and respiratory rates (Geiser, 2013; Staples,2016).The essence of artificial hibernation technology is to induce the human body to enter an animal-like hibernation state by changing the metabolic mode of the body, resetting the body temperature balance point, and other technologies, reducing core temperature and body metabolism, activating the body’s protective mechanism, and achieving the goals of reducing energy metabolism consumption and preventing damage (Forreider et al., 2017;Shi et al., 2021).At present, artificial hibernation technology mainly includes mild hypothermia treatment technology, central nervous system regulation technology, and artificial hibernation-inducer technology.Recent studies have found that inducing non-hibernating animals to enter a hypothalamic circuit and specific neuronal population similar to hibernating hypothermia can accurately regulate entry into hibernation.Hypothermia is an effective neural therapy method that can effectively reduce mortality and disability rates, greatly reducing brain damage, Therefore, the application of artificial hibernation technology has great potential in the treatment of acute brain injury (Manley et al., 2017; Hrvatin et al., 2020; Takahashi et al., 2020).This review aims to summarize the characteristics of hibernation and discuss the application of artificial hibernation technology in brain injury, focusing on mild hypothermia treatment technology, central nervous system regulation technology, and artificial hibernation-inducer technology.

Search Strategy

We used the PubMed database for online search using the keywords “artificial hibernation” and “acute brain injury.” The search period was from June 2003 to June 2023.Further screening was conducted based on the title and abstract; studies with relatively low relevance to our topic were excluded.We included only those that met the inclusion criteria.

Characteristics of Hibernation

When hibernating animals enter hibernation, their basal metabolic rate and/or) core temperature decrease significantly.The state of hibernation and hibernation is rhythmic, which greatly protects the nervous, cardiovascular,muscle, bone, and other system functions.

Body temperature characteristics

(1) Temperature regulation: During hibernation, hibernating animals may experience a slow and controllable decrease in body temperature, with core body temperature even dropping below 0°C.This may be influenced by the levels of sympathetic premotor neurons and the expression of deiodinase in the hypothalamus (Daversa et al., 2018; Frare et al., 2021).When hibernating animals experience hypothermia, they can regulate their body temperature to increase metabolism and achieve the most suitable temperature.(2) Body temperature suitability: Hibernating animals do not infinitely decrease with the decrease of environmental temperature, but have their own most suitable hibernation temperature (Richter et al., 2015).(3) Temperature periodicity:The temperature of hibernating animals is not always at a lower temperature,and there is a periodic awakening and increase in body temperature (Box 1;Oliver et al., 2019).

Box 1: Characteristics of hibernation Body temperature characteristicsThermoregulation Body temperature suitability Temperature periodicity Metabolic characteristicsHypometabolism Metabolic mode replacement Organ differentiation Rhythmic characteristicsRegularity of hibernation intervals Hibernation rhythm Physiological characteristicsNeurological protection Cardiovascular system protection Motor system protection Proteome stability

Metabolic characteristics

(1) Low metabolism: When hibernation occurs, it can decrease to 1–10% of the basal metabolic rate, which may be achieved by temporarily reducing core temperature and important organ function, including reducing brain blood flow, heart rate, and respiratory rate (Carey et al., 2003; Storey and Storey,2010).For example, the smallest mammal, the small stinky shrew, chooses to reduce cortical width and neuronal number to reduce the metabolic rate (Ray et al., 2020).(2) Metabolic mode replacement: The metabolic mode shifts from sugar metabolism to fat metabolism, and the metabolic fuel shifts from dietary carbohydrates to lipids to maintain energy needs during hibernation(Olsen et al., 2021; Ren et al., 2022).(3) Organ differentiation: Although many organs and functions are suppressed during hibernation, the hypothalamus must remain relatively active to regulate body temperature, energy, and rhythm (Mousavi et al., 2023).Metabolism also requires good coordination for hibernating animals to reach a relatively stable state (Box 1).

Rhythmic characteristics

(1) Regularity of hibernation intervals: During hibernation, hibernating animals enter an interbout arousal (IBA) state.During hibernation, animals do not always remain in a low temperature and low metabolism state, but repeatedly enter the IBA state (Mohr et al., 2020).IBA refers to the temporary restoration of core body temperature, animal activity, heart rate, and respiratory rate to a non-hibernating state.However, during the IBA period, the nourishment requirements of hibernating animals suddenly decrease or stop, and the periodicity varies depending on species.During hibernation, 70% of the energy is used for recovery and awakening from the IBA state.However,the specific reason for IBA’s occurrence is currently unclear (Ballinger et al.,2017).(2) Hibernation rhythm: Both facultative and specialized hibernating animals enter hibernation in a rhythmic manner.For facultative hibernating animals, mainly small and medium-sized rodents such as hamsters, enter hibernation rhythmically in response to reduced environmental temperature,resource scarcity, and changes in the photoperiod.The reason for this phenomenon may be induced by the pituitary endocrine system, and the specific mechanism is still unclear.For specialized hibernating animals, even if the environment does not change, including environmental temperature,sufficient resources, and fixed photoperiod, these animals will enter hibernation rhythmically within an expected period of approximately one year, such as squirrels with a rhythmic cycle of 11 months (Box 1; Chayama et al., 2016).The different ways of entering hibernation may be because of different hibernation mechanisms, whose specific mechanisms have not yet been clearly established (Mohr et al., 2020).

Physiological characteristics

(1) Neurological protection: Hibernating animals can reduce the activity of brain cells, reduce blood flow, and also maintain brain function through the structural plasticity of neurons, avoiding ischemic and hypoxic damage caused by hibernation (von der Ohe et al., 2006; Cogut et al., 2018; Wolf et al., 2018; Singhal et al., 2020).(2) Cardiovascular system protection: The cardiovascular system of hibernating animals is characterized by resistance to low temperature, hypoxia, and arrhythmia, which effectively maintains the homeostasis of the cardiovascular system during hibernation (Bonis et al.,2019; Childers et al., 2019; Xie et al., 2021).(3) Exercise system protection:Hibernation can maintain animal bone mass, such as in bears, which maintain calcium homeostasis and their skeletal structure and strength despite experiencing insufficient physical activity and low nutrient intake (Zhang et al., 2019, 2021a).Moreover, hibernating animals exhibit less muscle atrophy after awakening, which may be related to a decrease in protein synthesis and degradation rates during hibernation, while maintaining balance (Arfat et al.,2020; Miyazaki et al., 2022).(4) Proteome stability: During hibernation, when faced with metabolic fluctuations, organs respond in a unique and tissuespecific manner, with few cross organ proteomic adjustments occurring (Box 1;Grabek et al., 2015).

Overview of Artificial Hibernation Technology and Brain Protection Mechanisms

Overview of artificial hibernation technology

Artificial hibernation technology mainly includes mild hypothermia treatment technology, central nervous system regulation technology, and artificial hibernation-inducer technology.(1) Mild hypothermia treatment technology mainly involvesin vitroorin vivocooling methods, or a combination of sedative drugs that can inhibit the central nervous system, inhibit physiological functions, and weaken the response to external pathological stimuli (Lewis et al., 2017; Kurisu et al., 2019).Low temperatures are mainly divided into four categories: ultra-deep low temperatures (4–16°C), deep low temperatures(17–27°C), moderate low temperatures (28–32°C), and mild low temperatures(33–35°C), among which moderate to mild low temperature (32–35°C) is called sub low temperature.Mild hypothermia treatment technology achieves the goal of treating diseases by controlling the body temperature within the range of mild hypothermia (Cerebral Protection In Cardiac Intensive Care Group Neural Regeneration And Repair Committee Chinese Research Hospital Association, Neural Intensive Nursing And Rehabilitation Group Neural Regeneration And Repair Committee Chinese Research Hospital Association,2020).Currently, the technology is relatively mature (Polderman, 2008).

(2) The regulatory techniques of the central nervous system mainly include chemical genetics and brain computer interfaces.Chemical genetics technology uses a combination of chemical drugs and genetic technology to modify some biological macromolecules and enables them to interact with specific cell populations.It mainly comprises two structural components,including receptors activated by specific drugs and artificially designed drugs.The combination of the two can specifically activate or inhibit neuronal discharge (Gomez et al., 2017; Roth, 2016).Brain computer interface refers to the direct connection pathway established between the culture of human or animal brain or brain cells and external devices.There is a distinction between unidirectional brain area interfaces and bidirectional brain area interfaces.The former can only perform unidirectional information exchange, while the latter can perform bidirectional information exchange (Luo et al., 2022).

(3) The artificial hibernation inducer technology originated from the plasma of ground squirrels.Dawe and Spurrier (1969) infused the plasma of summer active ground squirrels, causing them to enter hibernation.The substance in the plasma that can induce hibernation is the hibernation inducer.Hydrogen sulfide (H2S) and adenosine-related inducers are currently common artificial hibernation inducers that can induce the body to produce low-temperature,metabolic, and other hibernation-like states by artificially administering a certain concentration of inducers, achieving the goal of treating diseases(Wolf et al., 2018; Olah et al., 2021; Cai et al., 2023).All three technologies have been applied in clinical or basic research, and have achieved satisfactory results, laying the foundation for the treatment of acute brain injury.

Brain protection mechanism of artificial hibernation technology Neuroprotection and cold shock proteins

Acute brain injury can cause cell death, decreased ATP synthesis, dysfunction of ion channels, increase in intracellular sodium, and cytotoxic edema.It can also cause the production of excitatory neurotransmitter glutamate that triggers lysate, promotes free radical production, and calcium influx,exacerbating neuronal damage (Sun et al., 2019; Kleuskens et al., 2021;Imataka et al., 2023).During hibernation, both transcripts and protein levels in the cerebral cortex increase with the increase of s-humanin levels (s-humanin has strong structural and sequence similarities with its human analogue,human, which is a powerful neuroprotective mitochondrial peptide), which can protect neural function by stimulating the expression of anti-apoptotic proteins or antioxidant enzymes (Szereszewski and Storey, 2019; Logan and Storey, 2020).RNA binding motif protein 3 (RBM3) and RNA binding protein (CIRP) are cold shock proteins involved in neuroprotection, and their expression levels are affected by low temperature (Figure 1 and Box 2).When mice undergo low-temperature pre cooling, the expression level of RBM3 increases, providing continuous synaptic protection to prion mice,preventing behavioral defects and neuronal loss, and significantly prolonging survival (Perettiet al., 2015; Preu ß ner et al., 2023).Overexpression of RBM3 can reverse the dendritic density, spatial learning, and memory deficits of neurons after TBI, as well as the expression of pT231, pS396, and Tau5 in the hippocampus.RBM3 can also stimulate neuronal differentiation and inhibit cell apoptosis in the subventricular and subgranular regions caused by ischemia and hypoxia, reduce neuronal damage and apoptosis one week after cardiac arrest, and promote neuronal repair (Danladi and Sabir, 2021; Zhang et al., 2021b).Low temperature can significantly increase the expression of CIRP by about 30 times, while overexpressed CIRP can inhibit H2O2and HIF-1-αmediated cell apoptosis (Li et al., 2012; Chen et al., 2019).It can also activate the Akt and ERK pathways, block the mitochondrial apoptosis pathway, and thereby inhibit neuronal apoptosis (Zhang et al., 2015).Low temperature can reduce neuronal damage and apoptosis, promote neuronal production, and provide a theoretical basis for the treatment of acute brain injury.

Inhibition of inflammatory response

After acute brain injury occurs, damaged cells can activate microglia and astrocytes, secrete various inflammatory mediators, exacerbate neuronal damage, increase the probability of cerebral vasospasm, activate or worsen the blood-brain barrier (BBB), brain edema, and oxidative stress (Roth et al.,2014; Xu et al., 2015; Schneider et al., 2018).The microdialysis probe was inserted into the brain tissues of hibernating and non-hibernating Arctic hamsters, and the results showed that microglia were clearly visible in the brain tissues of non-hibernating animals, while no significant microglia were found in hibernating animals (Zhou et al., 2001).Current research suggests that hypothermia can reduce the number of microglia activated by inhibiting autophagy and promoting cell apoptosis, inhibit the MyD88-dependent TLR4 signaling pathway to reduce the pro-inflammatory function of microglia, and weaken the neuroinflammatory response induced by microglia by reducing CD68 and myeloperoxidase (Xu et al., 2015; Zhang et al., 2018; Guo et al.,2021; He et al., 2022).Low temperatures can weaken the infiltration of cortical macrophages after TBI, further weakening two key chemokines that regulate the activation and infiltration of microglia, reduce the ratio of proinflammatory (M1) to anti-inflammatory (M2) macrophages, tumor necrosis factor-α, interleukin-1β, The expression levels of interleukin-12, interleukin-23 proteins, and mRNA significantly increased M2-related genes, including interleukin-10, Fizz1, Ym1, arginase, and CD163 (Lee et al., 2016; Truettner et al., 2017), indicating that anti-inflammatory effects are an important mechanism for low-temperature treatment of acute brain injury (Figure 1 and Box 2).

Figure 1 ｜Brain protection mechanism of artificial hibernation technology.Acute brain injury can cause dysfunction of ion channels, increase intracellular sodium, promote the production of free radicals and calcium influx, and aggravate neuronal damage.Low temperature can increase the expression levels of RNA binding motif protein 3 (RBM3) and RNA binding protein (CIRP).RBM3 can reduce neuronal loss, protect synapses, reduce cell apoptosis, and promote cell recovery.Overexpressed CIRP can inhibit H2O2 and hypoxia-inducible factor 1α (HIF-1α)-mediated activation of Akt and ERK pathways and inhibit neuronal apoptosis.Acute brain injury can activate microglia and astrocytes, secrete a variety of inflammatory mediators, aggravate neuronal damage, and increase the probability of cerebral vasospasm.Artificial hibernation technology can reduce MyD88-dependent Toll-like receptor (TLR4) signaling pathways, weaken the activation of microglia, and reduce the proinflammatory factors tumor necrosis factor (TNF)-α and interleukin (IL)-1β, and the expression levels of IL-12, IL-23 protein and mRNA, increase the anti-inflammatory factors IL-10, arginase, and CD163.After acute brain injury, the probability of increased reactive oxygen species increases, leading to neuroinflammation, secondary neuronal damage, and even death.Artificial hibernation technology activates Nrf2-ARE signaling pathway, superoxide dismutase, and glutathione peroxidase activities, and reduces oxidative damage.Created using Microsoft PowerPoint.

Box 2: Brain protection mechanism of artificial hibernation technology Neuroprotection and cold shock proteins Increase the expression levels of RNA binding motif protein 3 and RNA binding protein Reduce neuronal loss Protect synapses Reduce cell apoptosis Promote cell recovery Inhibition of inflammatory response Weaken the activation of microglia Reduce the proinflammatory factor tumor necrosis factor-α, interleukin (IL)-1β , IL-12, IL-23 levels Increased the anti-inflammatory factors IL-10, arginase and CD163 Oxidative stress resistanceActivation of superoxide dismutase and glutathione peroxidase Reduce reactive oxygen species Protecting neurons

Oxidative stress resistance

After acute brain injury, the increase in oxygen consumption increases the probability of an increase in reactive oxygen species (Li et al., 2023).Oxidative stress triggers various cascade reactions, which may lead to increased permeability, leading to neuroinflammation, secondary neuronal damage, and even death, as well as motor and cognitive disorders (Fesharaki-Zadeh, 2022).During hibernation, hibernators can increase their levels of antioxidants(including ascorbic acid and glutathione); upregulate the expression of some enzymes (e.g., glutathione reductase, peroxidase, and superoxide dismutase);and effectively clear free radicals and enhance antioxidant defense ability (Yin et al., 2016; Wei et al., 2019; Patnaik and Sahoo, 2021).Low temperature can activate the Nrf2-ARE signaling pathway, enhance the activity of antioxidant enzymes superoxide dismutase and glutathione peroxidase after TBI, and reduce the content of lipid peroxidation malondialdehyde in damaged brain tissue (Yan et al., 2022).Hypothermia can also regulate the normalization of nitrite/nitrate levels after cerebral edema, and significantly reduce the expression of heme oxygenase-1, endothelial nitric oxide synthase, and inducible nitric oxide synthase (Jiang et al., 2009).It can also reduce the protein carbonyl level of oxidative damage markers, increase the activity of S-transferase, reduce the activity of serum paraoxygenase, and reduce oxidative damage in patients with cardiac arrest (Hackenhaar et al., 2017).From this, it can be seen that low temperature can resist oxidative stress reactions through the antioxidant enzyme system (Figure 1 and Box 2).

Application of Artificial Hibernation Technology in Acute Brain Injury

Mild hypothermia treatment techniques

Basic and clinical studies have shown that mild hypothermia treatment technology can be used for the treatment of ischemic stroke, TBI,and cerebral hemorrhage (CH; Figure 2).(1) Ischemic stroke and mild hypothermia can reduce brain metabolism, reduce cerebral blood flow,and inhibit the release of excitatory toxicity during the acute phase of stroke (Yenari et al., 2008; Yenari and Han, 2012).In the subacute phase of stroke, brain damage is reduced by reducing cell apoptosis, increasing cell survival rate, inhibiting inflammation, and protecting the blood-brain barrier(Liu et al., 2018; Li et al., 2019; Duan et al., 2020).For the chronic stage,mild hypothermia can promote the occurrence of nerves and synapses,and enhance angiogenesis (Kuo et al., 2010; Li et al., 2010; Yenari and Han, 2012).The probability of death and disability reduction in newborns receiving mild hypothermia treatment 6–24 hours after birth is 76% (Laptook et al., 2017).(2) In clinical practice, TBI, is controlled at a core temperature of 32–35°C through intravascular cooling, local cooling of the brain, and systemic cooling.Previous studies showed that hypothermia can reduce ICP;lower brain metabolism; alleviate oxidative stress reactions and brain edema;improve learning, memory, and motor dysfunction; and reduce the mortality rate of TBI patients (Bayir et al., 2009; O’Leary et al., 2016; Feng et al.,2017; Andrews et al., 2018).(3) Mild hypothermia treatment may increase cerebral blood flow in the post CH region, improve the Glasgow Coma Scale score, reduce the risk of delayed cerebral ischemia, reduce the incidence of vasospasm-related infarction, and reduce mortality (Su et al., 2015; Choi et al., 2017; Kobata et al., 2022).Although many studies have shown that mild hypothermia therapy is beneficial in treating acute brain injury, there is still a lack of high-quality evidence to prove it.Furthermore, the results of previous and on-going experiments have not yet demonstrated the benefits of mild hypothermia therapy technology(Table 1).Olah et al.(2018) stated that systemic cooling can reduce the risk of death in patients with severe acute brain injury.However, Watson et al.(2018) pointed out that the use of mild hypothermia therapy in randomized controlled trials with low bias risk can lead to higher mortality rates in TBI.Suehiro et al.(2015) also stated that patients with diffuse brain injury grade III who received mild hypothermia treatment had a relatively higher mortality rate than those who received fever control treatment, and it was also difficult to control the ICP.However, mild hypothermia treatment for young patients who have undergone surgical excision of hematoma showed a more favorable effect.The controversial research results may be fundamentally related to differences in research methodology, statistics, and clinical design.Moreover,mild hypothermia treatment techniques can also lead to various complications such as circulatory restriction, tremor, increased risk of infection, electrolyte imbalance, and coagulation dysfunction.When the rewarming time is rapid, it can also cause an increase in ICP rebound (Safar and Kochanek, 2001; Shiozaki et al., 2001; O’Phelan et al., 2015; O’Leary et al., 2016).Therefore, targeted mild hypothermia treatment techniques should be developed for the patient population and subtypes to control complications and improve efficacy.

Figure 2 ｜Application of artificial hibernation technology in acute brain injury.Artificial hibernation has the following applications in acute brain injury: (a) Artificial hibernation technology can enhance learning and memory and motor function after traumatic brain injury, slow down tau hyperphosphorylation, reduce cell apoptosis,inhibit oxidative stress, and reduce brain edema.(b) Artificial hibernation technology can promote synaptogenesis and angiogenesis after ischemic stroke, inhibit inflammatory reactions, cell apoptosis, and axonal rupture.(c) Artificial hibernation technology can reduce the incidence of vasospasm and infarction caused by vasospasm after cerebral hemorrhage, and reduce mortality.(d) Artificial hibernation technology can reduce cell apoptosis, Toll-like receptor (TLR) protein, interleukin-1 receptor (IL-1R), tumor necrosis factor receptor (TNFR), matrix metalloprotein 9 (MMP-9), and cerebral infarction area after cerebral infarction.Created using Microsoft PowerPoint.

Table 1 ｜The advantages and disadvantages of artificial hibernation technology in acute brain injury

Central nervous system regulation technology Chemical genetics technology

Chemical genetics technology can induce a decrease in core body temperature and achieve the goal of treating acute brain injury.In the cerebral ischemia (CI) model, Zhang et al.(2022) injected DREADD-fused adeno-associated virus (AAV-hSyn-DIOGq-mCherry) into the preoptic area of the bilateral hypothalamus in advance, followed by the injection of chemical activator CNO to activate thermosensitive neurons, which can quickly induce hypothermia within 30 minutes, lasting for 12 hours, and can repeatedly induce hypothermia.It can reduce cerebral edema and cerebral infarction area in CI mouse models, improve motor function defects, and protect brain neurons.Additionally, there will be a decrease in oxygen consumption, heat production, and respiratory rate, without excess uncontrollable shivering.Liang et al.(2017) used 8-OHDPAT (5-HT1a receptor agonist) to stimulate the 5-HT1a receptor, which in turn stimulated thermosensitive neurons in the anterior hypothalamus of the preoptic area to cause a decrease in temperature, improve cognitive impairment and motor function in CI mice,reduce axonal breakage, and protect the BBB, with good neuroprotective effects.Chemical genetics technology can accurately regulate neurons with high specificity; it is also possible to better analyze the pathological mechanisms of acute brain injury based on the tracking of tool viruses (Atasoy and Sternson, 2018).However, the time accuracy of chemical genetics is poor, and even minute level precision control is difficult to achieve, and it continues to function for a certain period of time, causing neurons to remain in an abnormal state (Urban and Roth, 2015).The chemical genetics system is relatively complex and belongs to a new type of technology.Currently, there is limited research on acute brain injury, making it difficult to understand its advantages and disadvantages.The brain tissue is complex, and it is difficult to estimate whether there will be adverse effects using tools such as viruses(Table 1; Claes et al., 2022).

Brain-computer interface

Similarly, in the CI model, the electrode stimulation system was implanted into the bilateral medial preoptic nucleus, and high-frequency stimulation was used for deep brain stimulation.By activating thermosensitive neurons,the core temperature was rapidly reduced, resulting in a decrease in oxygen consumption, heat production, and respiratory rate.This significantly reduced the brain edema and cerebral infarction volume of CI mice, effectively protecting their motor and neurological functions (Zhang et al., 2022).At present, the technology of brain computer interface is not yet mature, and there are relatively few basic and clinical studies.However, with the rapid development of medical and computer technology, there will be enormous research potential in both the acute stage of acute brain injury and the rehabilitation stage.Intrusive brain computer interfaces are mostly achieved through implantable electrodes to establish connections with external devices, with high spatiotemporal resolution and signal quality (Orban et al., 2022).However, this requires invasive surgical implantation, but its application range is narrow and signal transmission is blocked by scar tissue over time (Nicolas-Alonso and Gomez-Gil, 2012; Abiri et al., 2019; Orban et al., 2022).Non-invasive brain computer interfaces have negligible risk and are portable and convenient for data collection, but their electrical signals are poor and their accuracy is low (Table 1; Ramadan and Vasilakos, 2017; Orban et al., 2022).It requires collaboration of multiple disciplines such as computer science, materials science, and mechanics to obtain more accurate and practical data to guide clinical treatment and rehabilitation.

Artificial hibernation inducer technology

Artificial hibernation inducers such as H2S and adenosine-related inducers are commonly used in the treatment of brain injury.H2S is a substance produced endogenously by mammals and acts as a neuromodulator in the brain.Inhaling H2S can significantly reduce body temperature and metabolic rate(Blackstone et al., 2005).In 2005, researchers in Seattle found that exposing non-hibernating rodents to 80 ppm H2S resulted in normal CO2output and O2consumption of 10%, while the respiratory rate decreased from 120 to 10 breaths per minute, and body temperature decreased from 37°C to 15°C.This induced a decrease in metabolism and core hypothermia in rodents, leading to a type of hibernation state that is easily reversible and does not cause any harm to the animals.This indicates the possibility of H2S-induced hibernation in medical applications (Blackstone et al., 2005).H2S is a reversible inhibitor of brain cytochrome c oxidase.Inhibiting brain cytochrome c oxidase can inhibit animals’ demand for oxygen and weaken their dependence on supply.Treatment with H2S can reduce the production of reactive oxygen species, thereby weakening cell damage.In the CA rat model, exposure to a mixture of air and H2S for 2 days can induce sustained hypothermia (30.8± 0.7°C) in elderly stroke rats.Hypothermia can reduce the infarct area by 50% and the number of phagocytes.It can reduce the inflammatory markers caspase 12 and NF-κB and grp78 in the surrounding area of the infarction,indicating that H2S reduces transcription activity related to inflammation and apoptosis (Florian et al., 2008).Sandu et al.(2016) also found that exposure to H2S can achieve long-term and controlled systemic hypothermia in CA rats, increase neovascular density in the cortex around the infarct site, and improve vestibular motor function and asymmetric sensory motor deficits.H2S has a significant metabolic reducing effect in rodents, but this is not applicable in large mammals (Li et al., 2008; Stein et al., 2012).Moreover,not only does it not have a metabolic reducing effect but also plays a role in hemodynamics and metabolic stimulants (Li et al., 2008).The reasons may be related to the ratio of surface area to mass, resistance to core temperature changes, differences in body weight, differences in H2S dosage, and the use of anesthesia (Table 1; Wang, 2012).

Adenosine-related inducers in acute brain injury mainly include 5-adenosine monophosphate (5′-AMP), which can activate the central adenosine A1 receptor, induce a low metabolic state, and lower core body temperature.This may be related to the uptake of 5′-AMP by red blood cells, thereby reducing oxygen transport.The low-temperature reduction induced by 5′-AMP accelerates the activation of microglia by whole body carbon ion irradiation,activated microglia by Iba-1 positive cells in the brain, and apoptotic cells in the liver, protecting multiple organs and alleviating brain injury (Puspitasari et al., 2022).(1) In a rat model of cerebral infarction, hypothermia can significantly reduce the infiltration of neutrophil elastase into neuronal cells;reduce matrix metalloproteinase-9, interleukin-1 receptor, tumor necrosis factor receptor, and Toll-like receptor protein expression in the infarcted area of middle cerebral artery occlusion in rats; reduce apoptotic cells and infarct area; and protect neural function (Miao et al., 2015).(2) In a TBI mouse model, hypothermia could improve chronic brain injury induced by TBI by improving cognition, reducing spinous process loss, and preventing tau phosphorylation.The treatment results may be related to the upregulation of cold shock protein RNA binding motif protein 3 expression by hypothermia(Figure 2; Liu et al., 2020).The degree of temperature reduction can depend on the dosage of 5′-AMP, which allows for better application of 5′-AMP due to synchronous changes in body temperature depending on the dosage administered (Miao et al., 2015).And after the body cools down, it can be naturally rewarming without any treatment, without obvious side effects.However, the induction of deep hypothermia by 5′-AMP may be influenced by environmental temperature.When the environmental temperature decreases, the body temperature can quickly decrease and be maintained for several hours.For example, when the temperature is 15°C, the animal body temperature can decrease to 16–17°C and be maintained for several hours.However, when the temperature is at ambient room temperature,5′-AMP does not induce deep hypothermia (Table 1; Miao et al., 2015).This also provides a new approach for 5′-AMP to induce and maintain deep hypothermia.

Strengths and Limitations

Each artificial hibernation technology has its own advantages and disadvantages.There are various methods of mild hypothermia treatment,which are relatively established and easy to operate, and are currently widely used.However, treatment equipment is often relatively large, heavy, and has a small movable range, making its application further restricted for patients with limited mobility, especially in non-clinical settings.There are also many complications that can cause damage to other organs and even the whole body.The central nervous system regulation technology has multiple targets,precise effects, and strong artificial controllability.However, owing to the diversity of the central nervous system, the potential side effects of activating brain regions on the body are currently unclear, and invasive techniques can also have a certain impact on the body (Orban et al., 2022).Artificial hibernation inducer technology has a relatively wide range of applications,and animals can self-rewarm after cooling with minimal side effects, making it more feasible for small animals (Wang, 2012).However, the effect is not significant in large animals, and it is greatly affected by environmental temperature and has a short development time.The target of action is not yet clear, which limits its clinical application (Wang, 2012; Cai et al., 2023).

This review has some limitations.We reviewed the characteristics of hibernation, the overview of artificial hibernation technology, brain protection mechanisms, and its application in acute brain injury.However,there has been no comprehensive review of the research progress on artificial hibernation technology and brain protection mechanisms.Moreover, artificial hibernation technology is currently in its infancy, and this article mainly discusses basic research.The research subjects are mostly rodents, and clinical data is insufficient.For wider clinical application, a large amount of research is needed.

Conclusion and Outlook

Artificial hibernation technology has good neuroprotective, anti-inflammatory,antioxidant stress, and other effects, providing a good theoretical basis for the application of acute brain injury.Artificial hibernation technology can alleviate the damage of ischemic stroke, TBI, CH, cerebral infarction, and other diseases, demonstrating its promising therapeutic potential for acute brain injury, and providing new hope and therapeutic strategies for acute brain injury.

At present, artificial hibernation technology is generally in the early stage of research, with most data being generated from basic research.Artificial hibernation technology has good neuroprotective, anti-inflammatory, and oxidative stress-resistance effects, providing a good theoretical basis for the application of acute brain injury.Reduce the complications of the application of mild hypothermia treatment technology, improve mild hypothermia treatment instruments, expand the scope and efficiency of use scenarios,improve efficacy, and reduce adverse effects on patients.Improve the time precision of chemical genetics technology, explore the mechanism, and improve its therapeutic efficiency.Further, a non-invasive brain-computer interface can be developed to reduce harm to the human body, improve accuracy, and expand the use of disease types.Research also needs to increase basic research on artificial hibernation inducers, innovatively develop inducer types, and reduce the impact of the environment on inducers.

Combining the body temperature, metabolism, biological rhythm, and physiological characteristics under artificial hibernation can not only be applied in medical treatment but also in major national technological innovation.(1) Deep ground: Artificial hibernation technology reduces the harm caused by radiation entering the body, and reducing radiation can weaken personnel’s infrared characteristics and better applies it in combat.(2)Deep space: With the vigorous development of China’s space industry, human spaceflight technology has made a major breakthrough.Artificial hibernation technology can be developed to fully simulate hibernation, as it can enable astronauts to enter a “deep sleep” state, reducing their metabolism, and maintaining a balanced physiological state, it will promote a significant step forward in the aerospace industry.(3) Deep sea: When deep-sea divers engage in deep-sea diving, they face a sudden drop in air pressure, which can easily lead to bone necrosis, among other conditions.Hibernation can stabilize bone mass, and if artificial hibernation technology can achieve this physiological characteristic, it can greatly assist the development of deepsea technology.In the future, artificial hibernation life rest technology has the potential to overturn the existing life support system and greatly expand the range of human capabilities; hence, it is important to ensure personnel efficiency and develop new life support.

Author contributions:Manuscript design: XyC, XcC, ZS, ZW, XN.Manuscript writing and literature retrieval: XnW, SC, XyW.Data collation and analysis:XnW, SC, XyW.Illustration of the manuscript: XnW, SC.Manuscript editing:XnW, XyW.All authors approved the final version of the manuscript.

Conflicts of interest:None declared.

Data availability statement:Not applicable.

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