◎ Chen Xi, reporter of this newspaper
Relying on "freezing technology", traveling through thousands of years and awakening in the future is a common plot in science fiction. Although it is still far away to freeze and resurrect the entire organism, the scientific and technological progress of frozen organs still brings many surprises to people.
Recently, a study published in the academic journal Nature Communications showed that researchers at the University of Minnesota have achieved long-term freezing and rewarming of kidneys in rats, and these frozen kidneys can restore full kidney function after transplantation. This is the first time scientists have shown that mammalian organs can be successfully transplanted and sustain life after freezing and rewarming. This presents new technical solutions for freezing organs.
Short ex vivo storage time of organs
Organ transplantation is the last "life-saving straw" after human organ failure. Hundreds of thousands of organ transplants are performed worldwide every year, extending hundreds of thousands of lives. In China, 300,000 people stand in line every year for organ transplants, but only more than 10,000 of them can get a new life through organ transplants.
At present, the mainland can carry out six kinds of human organ transplantation operations, including human liver, kidney, heart, lung, pancreas, and small intestine. Among them, the surgical techniques of kidney transplantation and liver transplantation are very mature. However, organ transplantation is not widely available, and one of the main reasons is that the "shelf life" of donor organs is too short.
"Ex vivo preservation of organs is a necessary step in organ transplant surgery, and organ freezing and rewarming are key technologies to extend the ex vivo preservation time of organs, and breakthroughs in this area may reshape the current clinical system of organ transplantation." Dr. Yeqi Guo, attending physician of the Organ Transplantation Center of the First Central Hospital of Nankai University and head of the research group of the Institute of Transplantation Medicine of Nankai University, said.
At present, the traditional organ ex vivo preservation technology is mainly low-temperature static cold preservation, and the "shelf life" of this preservation technology is indeed a bit short. For example, the heart can be stored for 4 hours, the liver for 12 hours, and the kidneys for about 24 hours; Digestive organs such as the pancreas and small intestine are more difficult to preserve and the preservation time will be shorter due to the presence of residual bacteria, digestive enzymes, tissue-resident lymphocytes and other characteristics.
"Therefore, the current organ transplant surgery is generally an emergency surgery, which greatly limits the doctor's treatment of patients in space and time." For example, in order to allow patients to undergo surgery as soon as possible, there are regional considerations when looking for patients with successful matching, so as to avoid the occurrence of prolonged organ storage time, missing the best time for surgery, and even losing the opportunity for surgery. At the same time, the time left for doctors to prepare and examine before surgery is shortened, and only basic examination and preparation can be completed, and there may be a risk that the preoperative assessment is insufficient and the preoperative treatment is not in place.
To this end, researchers have also been exploring new technologies to make ex vivo organs from "short-term preservation" to "long-term preservation". "The technology of 'long-term preservation' of ex vivo organs is mainly composed of two links: 'freezing' and 'melting'." At present, organ freezing and rewarming mainly include the steps of organ cryogen preparation, organ cryogen perfusion, organ cooling, organ continuous cryopreservation and organ rewarming, organ lavage. Among them, the most commonly used method in freezing technology is called "vitrification", that is, the use of high concentration of cryoprotectant and fast freezing speed to prevent the cells of the organ from forming ice crystals in an ultra-low temperature environment, thereby avoiding ice crystal damage to the cells and eventually organ damage. In the process of reheating, it is also necessary to master the rate and let the organ heat up evenly.
Recently, the development of low-temperature mechanical perfusion and normal temperature mechanical perfusion technology has extended the ex vivo preservation time of organs to a certain extent, and won more time for patients and physicians. However, current technology is generally limited to extending the ex vivo preservation time of organs by a few hours to weeks (weeks are limited to preclinical trials), so ex vivo preservation of organs is still within the scope of "short-term preservation".
Organ cryoppreservation is much more difficult than cells
In recent years, with the rapid development of gene and cell technology and products in medical research and application, people will often come into contact with cryopreservation of stem cells, eggs, sperm and embryos. Many people can't help but ask, after so many years of cryopreservation, cells can still be used when needed, and ex vivo organs are also composed of cells, why can't organs be preserved with the same technology?
Li Xiangguo, vice president of Tianjin Stem Cell Development and Application Association, introduced that the current stem cell cryopreservation technology is to store cells in a deep and low temperature environment to reduce cell metabolism and make cells in a dormant state, which is a technology to achieve long-term storage. The specific method is to use gradient cooling method, and the prepared cells are programmed to be cooled and transferred to a liquid nitrogen tank at -196 °C for long-term storage.
"This technique can be used to preserve various types of cells, such as embryonic cells." Li Xiangguo said that in theory, cell cryopreservation can achieve permanent storage. Doctors and patients have readily available cryopreserved cells for therapeutic, research, or regenerative medicine applications.
However, stem cells, embryos, etc. belong to the category of cell freezing and rewarming, if the technology of freezing cells is used to freeze organs, this is obviously not a simple quantitative process, and it will face more problems when applied to tissues of the entire size of the organ.
"First, cryoprotectants do not penetrate evenly into larger tissues, and when organs are larger, the time it takes for their central parts to solidify is also extended, which will lead to the formation of ice crystals." Mu Yeqi also introduced that the refrigerant used for cell freezing is not cytotoxic, and the refrigerant of the frozen organ also needs to specifically consider its toxicity to the organ. To make matters worse, each organ is made up of a variety of cells, each with different characteristics and functions, and the same cryogen may be toxic to different types of cells. No matter what type of cell is damaged, it affects the use of the organ.
Second, the freezing state of a single cell or a tiny embryo with only about 100 cells is easier to evaluate, and the cell damage during rewarming is easier to evaluate. As a whole with tissue structure, the organ needs to be evenly heated during the rewarming process, and the heating can not lead to the expansion or contraction force between different tissue structures of the organ, so that the organ causes physical damage.
"At present, 'vitrification' only partially solves the problem of organ 'freezing', but problems such as ice crystal formation during freezing, temperature control during freezing and rewarming, uniform thawing of tissues and organs as a whole during rewarming, and ischemia-reperfusion damage still exist." Sticky Yeqi thought.
Or will reshape the clinical organ transplant system
The study published in Nature Communications solved the problem of rewarming larger organs, and researchers developed a "nano rewarming" technology: during the freezing process, when perfusing the organ with a protective agent, iron oxide nanoparticles are added to it, and when rewarming, the frozen organ is placed in a radio frequency coil, and the current will generate an induced magnetic field, which generates heat through the iron particles in the organ; The protective agent is evenly perfused into the organ through the capillaries, and because the radio frequency electric field penetrates the tissue without attenuation, the heating rate is guaranteed and the heating is uniform.
"The 'nano rewarming' technology of the University of Minnesota research team also includes replacing propylene glycol in the original protectant with ethylene glycol, and developed a new refrigerant VMP, which reduces the toxicity of the refrigerant." According to Mu Yeqi, the study showed that rat kidneys stored for 100 days could remain viable after thawing, and rats that received kidney transplants successfully survived the 30-day study period.
Because the problem of rewarming immediately following organ "vitrification" technology has been difficult to solve, some scientists have begun to find new ways to bypass these difficulties for new exploration.
Professor Taylor of Harvard University designed a synthetic sugar to protect the liver according to the freezing and rewarming principle of Arctic forest frogs in nature, and stored the human liver for 27 hours at -4 °C; In addition, by combining synthetic sugars with biological ice core Snomax, the team was able to store tissue at -15°C for 5 days with less tissue damage after thawing than in the control group.
In addition, some scientists work the "freezing" link. For example, the UC Berkeley team freezes organs at higher pressures without causing damage, thereby limiting the formation of ice crystals. Using this strategy, the team stored a pig heart at -4°C for 21 hours and then transplanted it into a healthy pig, where the heart beat smoothly after the transplant. And this strategy does not require the use of a large number of cryoprotectants, reducing the toxic side effects on organs.
Talking about the impact of the development of organ and cryorewarming technology on future medical treatment, Mu Yeqi said that this will reshape the current clinical organ transplant system. "After this technology matures and is applied to the clinic, organ transplantation will no longer be just an emergency surgery, and will have a longer time to improve preoperative preparation." The distribution system also allows more time for organ allocation and optimal tissue matching, while patients in remote areas have more opportunities for transplantation. Organs that are currently less utilized, such as the heart, lungs, and pancreas, can be better preserved and transported. Sticky Yeqi said expectantly.