In heterochronic parabiosis, the circulatory systems of an old mouse and young mouse are surgically joined. It is now well established that this accelerates measures of aging in the young mouse, and reverses measures of aging in the old mouse. This observation has given rise to a rapidly shifting area of research that has evolved in a number of directions over the past twenty years. Competing hypotheses regarding the mechanisms by which sharing blood in this way can impact aging have fallen in and out of favor, but are largely still present in some form – and still competing.
Initially, the focus was on factors present to a larger degree in young blood that may encourage favorable changes in the behavior of aged cells and tissues. GDF-11 was an initial discovery, and remains under clinical development as a mode of therapy. Other factors identified as potentially beneficial include oxytocin. Early approaches to build therapies based on transfusion of plasma from young individuals into old individuals produced poor results in both animal studies and human clinical trials, however.
Over time, the view shifted to harmful factors present to a larger degree in aged blood. Dilution of those factors was seen as the critical point, giving rise to experiments in which diluting blood with saline and albumin in aged individuals appeared to produce benefits in animal studies. Human clinical trials remain a work in progress, and it is yet to be settled as to how useful this approach to therapy will turn out to be – not to mention optimal treatment protocols and duration of benefits from a single treatment. At the end of the day, while clinical applications are under development, it remains to be settled as to exactly why heterochronic parabiosis works to improve function in the older mouse. Which mechanisms are valid, and what is the relative importance of those mechanisms? Research moves slowly.
Aging insights from heterochronic parabiosis models
Heterochronic parabiosis has proven to be a valuable tool to decipher some key circulating molecules involved in the aging process, both promoting and delaying it. In general, circulating factors exchanged during parabiosis may promote or delay cellular senescence and help eliminate senescent cells. Parabiosis also appears to rejuvenate mitochondrial function in several contexts. Parabiosis has also been shown to regulate inflammatory processes, either by promoting them during accelerated aging or by preventing them during induced rejuvenation. Specifically, in the brain, accelerated aging leads to altered intercellular communication and increased DNA damage culminating in genetic instability. In contrast, induced rejuvenation enhances proteostasis and epigenetic modifications. In bone marrow, muscle and liver, stem cell depletion is mitigated during induced rejuvenation. Mitochondrial function is improved in brain, hematopoietic and immune cells, vascular endothelium, and muscle. In addition, macroautophagy plays a crucial role in muscle and kidney rejuvenation. Accumulation of senescent cells in the brain, pancreas, hematopoietic and immune cells, muscle, and VAT is prevented during induced rejuvenation. Chronic inflammation is favored during accelerated aging by parabiosis in the brain, bones, muscles, liver, vascular endothelium, kidneys, eyes, and VAT. In contrast, induced rejuvenation reduces inflammation in these tissues and organs.
Proteins identified as relevant in the aging process through this strategy are poised to be prominent targets, first to better understand this intricate process and then to elucidate strategies to delay the harmful effects of aging, such as various age-related diseases, thus improving our quality of life. Researchers around the world can search for inhibitors for targets that promote aging and activators for those that delay it.
It is important to note that one of the main challenges faced by studies on heterochronic parabiosis is that, in most cases, the conditions used vary significantly. This includes variations in factors such as the sex and age of the animals, their location and cross-linked blood vessels, the length of the cross-linking period, surgical procedures, diet, and exercise capacity, among other variables. Therefore, it would be appropriate to establish a convention where the conditions are the same or as similar as possible in order to enhance reproducibility.
Comparing the benefits of induced rejuvenation with the deleterious effects of accelerated aging is complex, as the changes do not usually occur in opposite directions within the same processes. On the contrary, they often involve changes in the same direction, possibly indicating repair, compensatory mechanisms, or alterations in entirely different processes. Despite this complexity, it would be valuable to find a method to evaluate these effects, possibly using statistical and computational models. Additionally, studies aimed at determining whether the age difference between animals and the duration of their cross-linking lead to significant variations in heterochronic parabiosis outcomes would be particularly insightful.
Also, it should also be noted that most studies focus mainly on circulating proteins, while information on other circulating biomolecules such as DNA, non-coding RNAs, extracellular vesicles, lipids, carbohydrates, and their metabolites is rather limited. Similarly, most research only considers blood cells, despite the fact that other cell types can be transferred. This disparity underscores the need for further research to better understand the roles and mechanisms of these less studied biomolecules and cells in various biological processes.
Exploring the combined dataset of transcriptomic, epigenomic, proteomic, and metabolomic information derived from various tissues and cells in parabiosis experiments has the potential to provide a holistic understanding of the aging process. This integrated approach could unveil intricate molecular mechanisms underlying aging-related changes, providing a comprehensive and structured view of how different biological pathways interact and contribute to aging. However, the analysis of these multidimensional data sets presents significant challenges due to their complexity and the large amount of information they encompass. This complexity is due to the interaction of various molecular processes and the need to integrate data from different omics levels.
There are still many challenges and opportunities to be explored with heterochronic parabiosis. Among them, standardizing protocols to obtain as much information as possible and ensure reproducibility, identifying more specific factors with pharmaceutical potential, defining how transferable the findings are to humans, among many other things. It would be interesting if similar experiments could be carried out in long-lived rodents, such as naked mole rats or blind mole rats, as well as in other mammalian models of aging, such as bats, or in animals that experience a rapid decline in health leading to death, such as boreal quolls during the breeding season. These studies could provide valuable information, especially when compared to findings in mice.
