Primary Source: Al-Attar R, Storey KB. Lessons from nature: Leveraging the freeze-tolerant wood frog as a model to improve organ cryopreservation and biobanking. Comp Biochem Physiol B Biochem Mol Biol. 2022 Aug-Sep;261:110747. doi: 10.1016/j.cbpb.2022.110747. Epub 2022 Apr 20. PMID: 35460874. https://pubmed.ncbi.nlm.nih.gov/35460874/
While modern medicine has made great strides in treating those with illnesses leading to failing or limited organ capabilities, there is a consistent and largely unanswered need for transplantable organs to ultimately save dying patients more permanently. The World Health Organization estimates that less than 10% of the global need for organ transplant is met per year. While the number of organ donors and the varying criteria for organ donation in different countries is certainly a contributing factor, limitations on preserving transplantable organs also play a major role in this issue. Improving the preservation of transplantable organs would allow for transportation over greater distances, as well as additional compatibility testing or immunomodulatory intervention to avoid transplant rejection.
Currently, organs meant for transplant are kept at low temperatures above freezing to slow down cellular processes that would put stress on the organ, while avoiding damage caused by freezing temperatures. Looking to the natural world, however, several animals survive the freezing cold of winter every year and are no worse for it. Researchers Dr. Rasha Al-attar and Dr. Kenneth Storey take inspiration from just one of the creatures that take the winter’s cold head-on, the wood frog, Rana sylvatica, in a recent publication summarizing applications the medical industry can implement to preserve transplantable organs at lower temperatures, and subsequently extend their viability.
How to Freeze Like a Wood Frog
Surviving while being almost completely frozen through is no easy feat, and as such is only possible through the combination of a variety of efforts inside these amphibians. All frogs have permeable skin—and, in the case of the wood frog, this permeable skin is what allows ice crystals forming on the outside of the frog to form inside of the frog as well. When this happens, the frog undergoes a rapid increase in heart rate as glycogen the frog has stored throughout the summer and fall are converted to glucose to be transported throughout the body. More crucial structures such as abdominal organs and the brain uptake more glucose than skin and muscle, assuring these tissues will not lose the entirety of their cellular fluid to the growing ice crystals, as the addition of solutes like glucose lower the freezing point of water.
At a cellular level, transcription changes in response to freezing in various ways. In the circulatory system, proteins meant to bind to ice crystals and prevent re-crystallization into larger ice crystals are released, along with enzymes that promote the fluidity of cell membranes. As ice takes up a larger volume than water, this prevents capillaries from rupturing during the freezing process. Metabolism slows to a halt as cells throughout the frog undergo a coordinated drop in mitochondrial activity, as well as production of most proteins and enzymes associated with long-term rather than immediate survivability, such as those focused on cell growth. This serves to reduce production of reactive oxygen in the cell, which can damage DNA and other cellular machinery, as well as reduce overall energy needs as the cell switches to internal fuels for survival. The cell accomplishes these transcription changes through several different means, such as making DNA encoding these components temporarily unreadable (a process known as epigenetic regulation) or releasing microRNA meant to bind to messenger RNA encoding these components on its way to be transcribed, rendering it useless for the time being.
Adapting From Amphibians
While these complex systems help the wood frog survive through the winter, not all of these evolutionary tools are directly applicable to humans. Regardless, understanding these adaptations can help in the development of analogous medical procedures for use in organ transplants. For example, while glucose may work in frogs to lower the freezing point of water inside the cell, its usage can result in proteins called Advanced Glycation End-products (AGE’s), which have been associated with diabetes, Alzheimer’s disease, chronic kidney disease, and other degenerative diseases. Frogs use glucose nonetheless because it is readily taken up in all types of cells in the body through very general transport channels. To maximize the benefits of this frog model while avoiding the AGE’s it may produce, testing has been done with glucose-derivative sugars in mammalian cell-lineages. One such test, published in 2014, increased cryopreservation of rat livers by up to 96 hours. Other adaptations, such as the decrease in metabolism and reduced production of reactive oxygen, are possible through already developed drug treatments, though are likely to vary in applicability depending on the tissue being treated. Such treatment has already shown increased viability of thawed tissue in several mammalian cell lineages, such as mice, owl-monkey, and human tissue samples.
Through understanding of the wood frog’s evolutionary abilities, the medical industry is one step closer to efficiently filling the all-too-often unmet need for organ transplants. For many suffering from organ failure, with no options other than treatment while sitting on a wait-list, further advancement in cryopreservation and similar technologies may act as a light at the end of the tunnel.