Replacement: When Young at Heart is Just Not Enough
Following our introduction to aging research and foray into reprogramming for rejuvenation, we now turn to replacement, the notion that the best way to tackle aging and remain disease-free may be the most audacious approach of all: systematically replacing our old organs, tissues, and cells with young ones. While this may seem outlandish, in theory, it should work. A kidney transplant here, a new liver there, and voila! Good as new. In practice, of course, the human body is not a machine. Replacing a part isn’t as easy as unscrewing a few bolts. Organ transplants are intricate and major operations, and recipients, if they are lucky enough to find a matching donor in the first place, must take immunosuppressive drugs indefinitely. Not to mention that the parts aren’t readily available anyway — there is a huge shortage of human organ donors with no reliable or scalable alternative source. Oh, and there’s the issue that some organs, like the brain, seemingly defy replacement altogether (though some are trying).
Despite the challenge, the prospect that I might help make replacements viable was, in part, what drew me to aging research. Back in 2010, when I first discovered that scientists seriously researched aging and tissue engineering was hot. The recent discovery of the Yamanaka factors had opened seemingly endless possibilities to create new tissues and organs at will from induced pluripotent stem cells (iPSCs; see here for a refresher). This concept of “replacement” led me to a lab exploring the intersection of blood — arguably the easiest “part” to replace — and brain rejuvenation. If you can’t replace the brain itself, I thought, would replacing everything else help rejuvenate the brain? After all, our organs depend on each other for proper function, and blood serves as a major communication highway.
One particular technique caught my eye: heterochronic parabiosis — a Frankenstein-esk method involving the surgical attachment of two mice of different ages side by side. Once sewn, skin and blood vessels fuse, establishing a shared circulatory system in which young blood travels into the old mouse, and vice versa. At the time, parabiosis had already generated intriguing results, regenerating capacity in the liver and muscle, and detrimental effects to the brains of young animals. Researchers would also come to observe cognitive benefits in older brains after parabiosis, plus additional benefits across several more major organs. And, experiments involving young plasma (the cell-free liquid portion of blood) injections suggested that the benefits of parabiosis were mediated at least in part by soluble factors.
While the media tends to sensationalize these findings with comparisons to the fountain of youth, vampires, and “blood boys” (though these are indeed also real), the practical challenges of transfusing young blood for therapeutic purposes are substantial. There’s simply not enough blood to go around. So instead, what if we could identify the blood components responsible for rejuvenation? Could we then mass produce and use those blood factors to prevent aging and disease? A decade ago, many labs had already begun the search, mostly focusing on proteins in plasma. While not the only type of biomolecule in blood — lipids, sugars, small metabolites, and even cell-free DNA can be found — early evidence pointed to proteins, spawned several companies like Alkahest hoping to treat dementia with particular plasma protein fractions (that is, proteins within a specific size range) found in young blood, and Elevian, applying a particular protein called GDF11 to stroke. Others continue to explore new secreted factors, like those that modulate immune responses at Immunis Biomedical, and those derived from stem cells at Juvena Therapeutics. Newer to the area is Retro Biosciences, reportedly focusing on plasmapheresis — a common method used to remove and replace a patient’s plasma.
But what about the source of these proteins? Where were they coming from? Could we identify the old organ that stops producing beneficial factors and stimulate more production, or replace that old organ with a young one to rejuvenate its production? These were the questions I sought to answer when I joined the field. To begin, I collected all major mouse organs across the lifespan and then performed RNA-sequencing, a method that measures the “expression” of all 20,000+ genes in a sample. Remember that genes are specific DNA instructions for how to make proteins. Each different gene makes a different protein. But there is an intermediate step — first, each gene is transcribed into a message, an mRNA molecule, the type that is read with RNA-sequencing. Each gene can be transcribed many times, such that you may have 5,000 mRNA copies of gene A, 400 of gene 4, and so on. These mRNA messages are then read by other cellular machinery that creates a specific protein with a specific role in, on, or outside of the cell, in a process called translation. So, the more mRNAs for a given gene, the more proteins of that gene are synthesized.
While not a perfect correlate to protein abundance (sometimes mRNA levels and protein levels vastly differ), this expression does often predict the amount of the corresponding protein in the sample, and because proteins have known roles in the cell, one can infer what has changed with age. In this way, early experiments indicated immune-related changes were among the largest contributors to aging throughout the body. Although my research took a different direction, other members of the lab pursued a similar concept in humans, leading to the newly founded Teal Omics. The goal: predict aging of internal organs using minimally invasive blood draws.
These early discoveries of immune changes with age sparked an idea. Rather than replace each component of blood, why not replace the source of blood itself: the bone marrow? Bone marrow not only generates our red blood cells and platelets, but it also produces white blood cells that guard against infection. As immune function diminishes with age, vaccines lose efficacy and the elderly become drastically more susceptible to communicable diseases. There’s no clearer example in recent history than COVID-19, but this vulnerability to infection underlies more deaths than commonly appreciated. For instance, falls lead to broken hips and surgery, which can result in infection that leads to death. If we could just replace the bone marrow, we could rejuvenate the body’s entire immune AND communication system.
However, akin to whole organ transplants, bone marrow transplants are reserved for life-and-death situations. This necessity arises from the highly toxic conditioning regimens employed to eradicate the recipient’s native bone marrow, creating space for the transplanted cells. These treatments, while assisting to destroy the cancer, subject patients to extreme discomfort and leave them without an immune system, subjecting them to excessive risk of infection for months. In an experimental setting, the situation is similar: whole-body irradiation is standard for bone marrow transplants in mice. Unfortunately, this makes experiments transplanting young bone marrow to aged mice a bit difficult to interpret, though even so, several labs have reported benefits to multiple organ systems, including the brain, and younger human donors correlate with improved survival in patients with at least some types of blood cancer (unpublished data).
Actually, my optimism about heterochronic bone marrow transplantation arose from work of a neighboring lab that developed a novel method of bypassing harsh conditioning regimens. Instead, these researchers specifically targeted and eliminated hematopoietic stem cells (HSCs) — the cells from which the entire immune system arises. They would then transplant not whole bone marrow, but just HSCs, which can be collected from blood after coaxing them to leave the marrow (this means donating “marrow” is as easy as donating blood — sign up at Be The Match).
Not only is this research critical for blood cancer patients, but unlocking safe conditioning regimens would open the door for HSC transplantation to address various maladies, if not aging itself. After all, immune cells are systemic, traversing and living throughout the entire body, not only to search for pathogens but helping to maintain normal organ function. Unsurprisingly, the immune system and inflammation are either the direct causes of or associated with nearly every disease. Replacing this every so often could have untold benefits for maintaining health and preventing chronic diseases from emerging in the first place. The new conditioning regimen specific for hematopoietic stem cells is now undergoing development at Jasper Therapeutics, and could help make this a reality. And, though I failed to achieve sufficient engraftment of young HSCs in my experiments, others succeeded and demonstrated lifespan extension in mice. Currently, the LEV Foundation is attempting to reproduce these results, with lifespan experiments underway.
The immune system is so central to aging and disease that scientists are attempting a variety of rejuvenation approaches. For example, ImmuneAge Bio aims to reboot the immune system by rejuvenating and expanding HSCs ex vivo (outside of the body) before reinfusing them into patients. Mogling Bio utilizes a Cdc42 inhibitor which appears to restore order to the inner scaffolding of old cells, a treatment that extended mouse lifespan even when administered transiently after the mice had already reached old age. Minova Therapeutics has developed a method of transplanting placenta-derived mitochondria into HSCs, both as a treatment for rare mitochondrial diseases and also blood cancers. The potential transfer of those healthy mitochondria from the injected HSCs to other cells in the body means the therapeutic benefit could extend beyond the immune system. And of course, there are companies reprogramming HSCs like Retro Biosciences. In addition, both Retro and NewLimit, target T cells — specific immune cells used in certain cancer therapies.
Others also work on tools that may aid in the pursuit of replacement. Ossium Health is establishing a bone marrow bank while exploring ways to reduce transplant rejection, and they also provide bone matrix grafts from organ donors. Gamida Cell’s nicotinamide-based Omisirge may help with recovery from transplantation by accelerating neutrophil recovery, reducing infection risk. In addition, Videregen is working to regenerate the thymus, a small organ that sits above the heart where T cell development takes place. With age, the thymus undergoes drastic atrophy, practically disappearing by middle age. Thymmune Therapeutics — making headlines recently after receiving $37M from the Advanced Research Projects Agency for Health (ARPA-H) — also works on this historically neglected area.
One particularly exciting concept, while not targeting aging per se, is the idea of manufacturing blood components, like red blood cells (RBCs), outside of the body, thus eliminating the need for human donors. Working to manufacture universal-donor RBCs at industrial scale, RedC Biotech could help tackle a huge unmet clinical need. Others are performing similar work, though Scarlet Therapeutics intends to use RBCs as carriers for therapeutics.
With the ease of access to blood and its components, efforts in replacement and rejuvenation are expanding rapidly. But beyond this, the idea of replacement has taken off as well, and perhaps for good reason. A growing number of well-respected researchers believe that our best bet for keeping people healthy longer is to bypass traditional approaches that attempt to manipulate the immense complexity of molecular biology, and instead replace worn organs with young ones. The argument is that there are just so many processes that go wrong, and so many types of cellular and molecular damage to reverse, that it would take immense technological advances and concurrent therapies to fix them all. Generating and transplanting a fresh organ, many would argue, is actually far easier to accomplish.
Though voicing this opinion seriously remains somewhat new, the transplantation field is over half a century old. In fact, one of its pioneers, Roy Calne, recently passed away. But clearly, human donors are insufficient even to meet our current needs. So how could we imagine a future where everyone receives a new liver, and kidney, and heart?
One clever approach, taken by LyGenesis, could help alleviate the organ shortage by enabling each liver donation to benefit multiple patients. They accomplish this by isolating hepatocytes — the central cell type of liver — from the donated tissue and then injecting those into the patient’s lymph node with an endoscope. The lymph node provides an environment that naturally promotes cell growth, and the donated cells expand to form a mini-liver. For patients with end-stage liver disease, this could be lifesaving. Satellite Bio employs a similar concept by implanting satellite adaptive tissues (SATs), palm-sized disks of cells that become vascularized once inside the body. Another unique approach is that taken by Morphoceuticals, who champion instigating innate regeneration, arguing that the body already contains instructions for how to generate organs and limbs — it just needs the right signals. Utilizing a combination of factors delivered via a wearable bioreactor, they have successfully regenerated frog limbs.
Others take the engineering perspective, like Iviva Medical with 3D-printed cell-free microfluidic scaffolds seeded with kidney cells. With the goal of eliminating the need for dialysis, the similar Trestle Biotherapeutics generates implantable kidney tissues. And indeed, implantables receive much focus, as these pair therapeutic benefits with the ease of manufacturing. For example, Dimension Inx creates implantable bio-ink seeded bone and ovary scaffolds, and Humacyte generates blood vessels by seeding an absorbable scaffold with cells that subsequently deposit their own scaffold. Once the cells are removed, what remains is an acellular vessel useful for vascular disease or trauma without fear of rejection. Similarly, Verigraft decellularizes donated veins, arteries, valves, and nerves before reseeding the resulting scaffolds with a patient’s own cells.
Another major category of investigation is xenotransplantation — transplanting organs from non-human organisms, typically pigs due to their similar size to humans. United Therapeutics, through its subsidiary Revivicor and recent acquisition of Miromatrix, is advancing programs for a number of major organs, including decellularized pig organs reseeded with human cells to enable rejection-free transplantation. Revivicor uses a different approach to tackle this goal, genetically engineering porcine cells to be resistant to immune rejection. After combining these cells with pig egg cells in a technique called somatic cell nuclear transfer (see here for a primer), all cells of the resulting embryo inherit the engineered genome. Once an adult, the organs are suitable for xenotransplantation. This research appears full steam ahead as Revivicor expands with a $100 million pig-organ facility. And they are not alone. Qihan biotech similarly edits pig genomes to inactivate the endogenous retroviruses that pose a risk to humans, and Makana Therapeutics utilizes a triple knock-out to rid the pig of three specific cell-surface molecules that provoke immune rejection. eGenesis pursues similar goals, successfully performing 69 gene edits on kidneys transplanted into monkeys. When combined with immunosuppressants, these kidneys survived for up to two years.
While xenotransplantation remains hot, driven by the advances in Nobel prize-winning CRISPR gene editing technologies, room for improvement remains, as current gene-edited xenotransplants still appear to elicit rejection in humans. Alternatively, the oft-overlooked gut microbiome may provide benefits without the hassle of immune rejection — some evidence suggests benefits to old animals receiving a young fecal microbiota transplantation (FMT), including lifespan extension in fish, and improvements in the gut, brain, and eye. And, FMT is already used in the clinic to treat recurrent C. difficile infections, though here too, opportunities for progress abound.
If there is one organ that cannot be transplanted, it is the brain. Although other organs are complex to replace for other reasons — just think of our 600+ muscles or our two square meters of skin — the brain is you. You can’t just get a new one like you can a new kidney. But perhaps, some would argue, it might just be possible to replace the brain little by little, leaving you intact all the while. A number of companies pursue cell therapies — using stem cells to generate brain cell types for injection to treat neurodegenerative diseases. A few pursue more unique methods, like NeuExcel’s in vivo conversion of astrocytes to neurons. In theory, such a method could supply new neurons to the aging brain to reverse neurodegeneration. And then there is Glionics, a company aspiring to microglia replacement in humans. This technique is based on experiments demonstrating the complete ablation of microglia in the adult mouse brain followed by reseeding the brain with young microglia, which soon populate regions far from their site of administration.
And finally, there are the more speculative approaches that make global headlines — wild experiments transplanting an entire head, for example. This is, according to Wikipedia, distinct from brain transplantation, or looked at from the reverse perspective, whole-body transplantation. These experiments face not only extremely challenging technical hurdles, like how to attach nerves and blood vessels and prevent immune rejection, but serious ethical considerations, should technology develop enough to make this a reality.
But, many bet on just this future, cryopreserving their heads in the hopes that our future society has the tools to reanimate them with new, young bodies. I know what your impression must be, but cryopreservation for medical purposes is serious business. Methods to better preserve donor organs would have a huge impact on the thousands relegated to waitlists, preventing waste, and granting critical flexibility to donate across long distances, which is often a huge logistical challenge if not an impossibility when live organs are required. Lately, several cryopreservation efforts have been launched by aging researchers, including Oxford Cryotechnology and Lorentz Bio. While some might argue that cryopreservation is Plan B, more people seem to be converting to cryo as Plan A.
But not everyone. To close, we visit Renewal Bio, whose work, if successful, could potentially supplant at least some of the above replacement approaches. Renewal is at the forefront of ex utero human embryogenesis. While it is tempting to theorize this could lead to personalized clones that could supply patient-specific organs, it’s important to remember that 1) the technology is not in place to advance embryos this far, and 2) there are stringent rules limiting the duration of embryonic development. In the future, it may be that such development is allowed to progress (or not, depending on the country or state), so long as brain development is fully prevented — something possible already in mice, for example, by turning off a specific gene called Lim1. Even considering these limitations, within only a few weeks, multiple cell types of huge therapeutic potential arise, including eggs to treat infertility, pancreatic beta cells for diabetes, and hematopoietic stem cells for cancer and anemia. And, using iPSCs, all of these could be patient-specific! Why go to the trouble and complexity of engineered tissues and pig organs when you can generate therapeutics directly from human cells?
From replacing blood to bone marrow, to solid organs and our brains themselves, the borders between reality and science fiction are more blurred than ever. Each replacement approach above has its own merits and drawbacks in relation to traditional approaches like drug development. But ask yourself, do those pursuing replacement as a solution to aging and disease have a point? Is moving beyond the hallmarks of aging, by solving them all at once with young organs, actually the most promising way to stave off aging and help prevent dementia, diabetes, heart disease, and cancer? Only time will tell — hopefully for us, not too much of it.
Prime Movers Lab invests in breakthrough scientific startups founded by Prime Movers, the inventors who transform billions of lives. We invest in seed-stage companies reinventing energy, transportation, infrastructure, manufacturing, human augmentation, and agriculture.
