Why is heat stroke deadly? Chinese scientists reveal the mechanism of death from heat stroke

On May 5, The team of Professor Lü Ben of the Third Xiangya Hospital of Central South University published a research paper entitled “Z-DNA binding protein 1 promotes heatstroke-induced cell death” online in Science, revealing the important lethal mechanism of heat stroke – high body temperature induces excessive programmed cell death through Z-DNA-binding protein-1 (ZBP1). This, in turn, results in life-threatening disseminated intravascular coagulation (DIC) and multi-organ injury. The first unit of the thesis was the Third Xiangya Hospital of Central South University.

After completing his doctorate in clinical medicine at Central South University, Lü Ben went to the Feinstein Institute of Medicine in the United States to study for a second doctorate. In 2013, he returned to his homeland. “Post-80s” he is now the director of the medical management office of Central South University and the vice president of Xiangya Medical College.

Ten years to sharpen a sword. In the past 10 years, Lü Ben has led the team to carry out a series of studies around the pathogenesis of critical illness. After revealing the important pathophysiology of sepsis shock and multi-organ failure, deciphering the mechanism of bacterial infection inducing coagulation reactions, and revealing the mechanism of sepsis inducing disseminated intravascular coagulation, he finally led the team to find an important mechanism for the death of heat stroke.

Previous treatment of heat stroke was not ideal

Heat stroke, also known as “severe heat stroke”, is a fatal emergency with a high case fatality rate. Over the past 20 years, record-breaking heat waves have led to an increasing number of heat-related deaths, including heat stroke, across the globe. As the global climate continues to warm, the number of deaths from heat stroke is increasing year by year. Data from a review published in Nature in February showed that the mortality rate of classic severe heat stroke was as high as 63.2%.

However, the mechanism by which high body temperature leads to organ damage and body death was not well understood in the past.

At present, the treatment of patients with heat stroke is generally to cool the patient first, and if the patient has organ failure after cooling, the patient will be immediately treated with corresponding organ function protection treatment. Although the equipment and technology for the treatment of organ functions such as kidney and liver are currently very advanced in the industry, the cure rate of heat stroke is still low.

Why has the treatment of heat stroke been unsatisfactory? This may be related to the fact that the academic community does not know much about the cause of death of heat stroke. Previous academic beliefs have been that high body temperature mainly leads to tissue cell death and inflammatory response through physical damage. Guided by this view, the industry generally adopts a method of cooling and corresponding organ support to treat heat stroke.

“Some countries have even higher rates of heat stroke mortality.” Lü Ben, the corresponding author of the paper, believes that to some extent, the previous academic views are not comprehensive enough, which has caused certain limitations in the treatment of heat stroke. “The previous view is that heat stroke is a physical damage process, high heat produces cytotoxic effects, when the temperature in the human body continues to rise to a certain threshold, it will directly lead to irreversible damage to the cells, and then cause inflammatory reactions, thrombosis and other symptoms, followed by systemic multi-organ failure.” Despite aggressive cooling and organ support management, the mortality rate of heat stroke remains high due to the lack of specific and effective therapeutic drugs and means. Therefore, our team further explored new mechanisms and made important progress, finding that programmed cell death plays an important role in the occurrence and development of heat stroke, which may find potential intervention targets for future clinical treatment and provide precise treatment ideas. Lü Ben said.

“Cooling treatment is the first key point in the treatment of severe heat stroke patients. Many patients with severe heat stroke have high body temperature when they are onset, and we immediately treat the patient symptomatically to make their body temperature drop rapidly to normal, but the follow-up is still secondary to the functional damage of various organs, and even the patient dies in the end. This suggests that even if the doctor can quickly cool down the patient with high body temperature for the first time, he still cannot block the course of heat stroke. Yuan Fangfang, the paper’s first author and a postdoctoral fellow at Central South University, said, “We think there must be some other mechanism that causes heat stroke death, rather than simple physical factors.” ”

In short, the current treatment of heat stroke has not found the most critical crux. This has led to a high mortality rate from heat stroke and has also made the treatment of heat stroke unsatisfactory for a long time.

Find key genes to reveal the mechanism of death from heat stroke

Critical illness is a severe and life-threatening syndrome that is often induced by infection, trauma, and high body temperature. Among them, the acute illness induced by infection is sepsis, which causes 11 million deaths worldwide every year. Sepsis and heat stroke often present with a systemic inflammatory response, DIC, and multi-organ failure.

In the early stage of the study of sepsis, Lü Ben’s team found an interesting clue. “When we prepared a mouse model of heat stroke using high ambient temperature (39 °C) and high humidity (60%), we found that high body temperature induces multiple programmed cell deaths through the RIPK3 pathway instead of the HMGB1-caspase-11 pathway, which leads to DIC and multi-organ injury. Hyperthermia induces phosphorylation of RIPK3 and MLKL in organ tissues such as liver, lungs and intestines, as well as the shearing of apoptotic-related proteins such as caspase-8; knocking out the RIPK3 gene prevents hyperthermia-induced inflammatory responses, DIC, multi-organ injury and death. Lü Ben said.

The team then used RIPK3 kinase inactivated mice, MLKL knockout mice, and MLKL-Caspase-8 dual gene knockout mice to confirm that the lethal effects induced by high body temperature are mainly dependent on LIPK3 kinase and its phosphorylated substrate MLKL-mediated programmed cell necrosis, and a small part rely on caspase-8-mediated programmed cell death.

“That is, we found that high body temperature did not quickly and directly ‘kill’ the cell, causing physical damage to the cell, but rather activated the cell’s programmed death pathway.” Thermal stress can directly induce LIPK3-dependent programmed cell death. The RIPK3 gene is key to high body temperature-induced inflammatory responses, multi-organ injuries, and even death. Lü Ben said.

On this basis, the team further identified ZBP1 as a key molecule for heat stress-induced RIPK3 phosphorylation and programmed cell death.

Previous studies have shown that ZBP1 is an intracellular pattern recognition receptor that can be activated by viruses or host-derived Z nucleic acids and plays an important role in fighting viral infections. Through further research, Lü Ben’s team found for the first time that ZBP1 is a key molecule for thermal activation of RIPK3 and cellular programmed death, and mediates high body temperature-induced DIC, multi-organ injury and death.

“Interestingly, thermal stress can induce the expression of ZBP1. Bioinformatics analysis suggests the presence of a binding element of heat shock factor 1 on the promoter of the ZBP1 gene. Through mutation analysis, we confirmed that thermal stress enhances ZBP1 expression by inducing the binding of HSF1 to the promoter of the ZBP1 gene. Lü Ben said that these findings have completely overturned the previous academic view of heat stroke disease that “high body temperature leads to tissue cell death and inflammatory response through physical damage”.

Discovering “thermoreceptors” within cells

The 2021 Nobel Prize in Physiology or Medicine honors “the discovery of temperature and haptic receptors.” Among them, the team of American scholar David Julius discovered the capsaicin receptor TRPV1 as a membrane ion channel that can be activated by heat, revealing the mechanism of the body’s perception of high temperature.

Lü Ben’s team found that the expression of ZBP1 alone was not enough to lead to the activation of RIPK3 and the programmed death of cells. In the process of exploring how thermal stress activates ZBP1, they found that high-temperature-induced ZBP1 activation does not depend on its recognition of Z-type nucleic acids. The ZBP1 protein contains the Za domain and the RHIM domain. Among them, the main function of the Za domain is to identify Z-type nucleic acids of viral or host origin.

Previous academic views have held that the binding of the Za domain to Z-type nucleic acids is a necessary part of ZBP1 activation. However, by constructing a series of ZBP1 mutants, the team found that heat-stress-induced ZBP1 activation relies on the RHIM domain and not on the Za domain. Although ZBP1, which is missing from the Za domain, is completely inactive to viral infection, it can still mediate heat-stress-induced RIPK3 activation and programmed cell death. This discovery shattered the academic understanding of ZBP1 and revealed a new mechanism for ZBP1 activation.

“David Julius’s team discovered the cell membrane ion channel ‘thermoreceptors’, and on this basis, our study clearly suggests that there may also be additional ‘thermoreceptors’ within cells that can induce programmed cell death that ZBP1 and RIPK3-dependent when subjected to thermal stress.” Although this life phenomenon may play a protective role in anti-infective immunity, it can induce excessive programmed cell death under the action of persistent hyperthermia, eventually causing DIC, inflammatory response, multi-organ injury and even death. Lü Ben said.

There were four reviewers of the paper after submission, which was highly recognized by editors and reviewers, one of whom proposed zero revisions to the paper and directly suggested receiving the article, and the other two reviewers also “strongly recommended that Science publish immediately”. “This study is very important and the data is clear,” they say. “This is a new and very important discovery that reveals a new mechanism that causes the course of heat stroke.”

“In this study, we revealed a new theory of the pathogenesis of heat stroke, and innovatively combined clinical and scientific research depths, based on multidisciplinary intersectional research methods and new attempts on technical routes.” Lü Ben said that the theoretical scientific significance of this study is not only to reveal the lethal mechanism of heat stroke, but also to suggest that there are also other “temperature receptors” in the cell, showing the internal relationship between cell program death and “temperature receptors”, suggesting potential drug intervention targets, and providing important ideas for the prevention and treatment of critical diseases such as heat stroke.

Lü Ben said that in the next step, the team will be committed to revealing the specific components and activation mechanisms of “temperature receptors” in cells, looking for potential drug intervention targets for heat stroke, and exploring future heat stroke prevention and control strategies.

Lü Ben (left) and Yuan Fangfang (right) perform cellular immunofluorescence observations. Photographed by Wang Haohao

Lü Ben (right) and Yuan Fangfang (left) conduct cell death testing experiments. Photographed by Wang Haohao

The research has been funded by the National Science Foundation for Outstanding Young Scholars, the National Natural Science Foundation of China, the China Postdoctoral Science Foundation, and the Key R&D Project of Hunan Province. (Source: China Science Daily, Wang Haohao, Li Shan)

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