Ageing bodies need to get rid of decrepit cells

In 1962 Leonard hayflick, then at the Wistar Institute in Philadelphia, now retired, made one of the most famous observations in the science of longevity: in laboratories, non-cancerous mammalian cells can reproduce themselves for only a fixed number of times before cell division ceases and they enter a state called senescence. For human cells, this Hayflick limit is 40-60.

Exceeding the Hayflick limit is not the only route to senescence; it can arise in other ways, too, including as a response to DNA damage. The body normally gets rid of these senescent cells either by triggering a genetic self-destruct sequence called apoptosis or by sending the immune system in to dismantle them. Both forms of housekeeping, though, become less efficient with age, allowing senescent cells to persist in a sort of zombie state that makes things difficult for the healthy cells around them.

James Kirkland of the Mayo Clinic, in Rochester, Minnesota, offers a daunting list of the things that go wrong in tissue where zombie cells accumulate. It includes inflammation, DNA damage, a form of tissue scarring known as fibrosis, disabling aggregations of protein and two of the hallmarks of ageing associated with problems in the MTORC1 nutrient-signalling pathway discussed in the previous article: poor proteostasis and damage to mitochondria. Experiments on mice suggest that senescent cells are involved in Alzheimer’s disease; other work suggests they play roles in diabetes, fibrosis of the lungs, osteoarthritis, osteoporosis and several diseases of the eye.

Dr Kirkland is, however, undaunted. For it was he who, in 2011, pioneered what has become an increasingly studied approach to the problem of cellular senescence. This is the development of drugs known as senolytics designed to kill senescent zombie cells.

His initial research discovered four already characterised molecules that looked promisingly senolytic: dasatinib, fisetin, navitoclax and quercetin. All four stimulate apoptosis, though not all by the same mechanism. They can all be taken orally and extend the lives of laboratory animals. Dasatinib is an anti-leukaemia drug available in America and Europe since 2006; navitoclax is currently in trials as a treatment for myelofibrosis, a bone-marrow cancer; quercetin (which is often added to dasatinib in cancer treatment) and fisetin, meanwhile, are natural substances that are found in fruits.

Dr Kirkland is one of the organisers of what is known as the Translational Geroscience Network. It has 14 centres across America and is running 81 clinical trials on compounds that could become drugs for age-related diseases, with the trials done in ways that seek to add to the understanding of ageing in general. About 30 of them are on potential senolytics, including three of the four on his original list. The idea, he says, is to conduct a lot of small trials in parallel, on different molecules and different target diseases. And, though he thinks there is only about a 25% chance of a successful senolytic drug emerging, he says that when he first started in the field he would have put the chances at 0.001%.

In 2020 a survey of the field by Nature, a journal, identified more than two dozen startups in the senolytic field. They use a broader range of approaches than those initially identified by Dr Kirkland. There are undoubtedly more companies, and more approaches, today. And there have already been disappointments. Unity Biotechnology of San Francisco, funded in early years by Jeff Bezos and Peter Thiel as well as the Longevity Fund, a venture-capital operation, went public in 2018 only to see its share price collapse by two-thirds two years later when early trials of a senolytic aimed at osteoarthritis disappointed.

While Unity and other firms build weapons to fight senescence, another set of companies is looking at the Hayflick limit from the other side. Instead of searching for ways to kill cells that are over the limit, these cellular-rejuvention companies are examining treatments aimed at keeping cells under it and helping them stay in fine fettle while they are there. Their targets are not cells that need to be removed, but organs the cells of which are failing to renew themselves as they should. The two approaches differ in other ways, too. The senolytic approach looks at drugs and supplements already available to try and find those that could help soon. Cellular rejuvenation, made plausible by recent developments in stem-cell science, is more radical and thoroughgoing; it needs, and attracts, those with deeper pockets.

The human body contains hundreds of different types of cell, each with the right properties for a particular sort of job. This differentiation is accomplished by having different sets of genes turned on and off in the different types of cell by means of various “epigenetic” modifications. Some are chemical alterations to the bits of DNA on which specific gene sequences are stored, others affect the proteins around which that DNA is wrapped, still others work in subtler ways. These sorts of epigenetic modification are vital. But the processes which drive and maintain them are another of those bits of life’s workings that get less effective with age. Indeed, the pattern of DNA methylation (a specific type of chemical change to one of the molecule’s genetic letters) can be used to diagnose the age of a cell.

Stem cells are reserves from which new specialised cells of various types can be made. When one divides, one of the two daughter cells sets off down a route of epigenetic specialisation which, a few generations on, will produce a number of cells of specific types. The other daughter will remain a stem cell, ready to produce more daughters when required. In this way the hematopoietic stem cells in blood marrow, for example, can produce progeny from which all the different sorts of white blood cells are derived, as well as the oxygen-carrying red ones. To keep doing so for a lifetime, though, they need to divide a lot more than 40-60 times.

Factor analysis

Allowing stem cells to keep going is the task of an enzyme complex called telomerase. The physical manifestation of a cell’s progress, or descent, towards the Hayflick limit is found in structures at the end of its chromosomes called telomeres. Every time the chromosomes are copied to allow the cell to divide, the telomeres get shorter; after 40-60 divisions they are too short for the chromosome to be copied any more. In stem cells, though, telomerase is used between cell divisions to rebuild the chromosomes’ telomeres, resetting the clock. But it does not do so perfectly. And, as time goes by, stem cells can pick up unwanted epigenetic markers, too. Stem cells diminish in number, in capacity, or both.

What if the exhausted cells could be pepped up, or replaced? In 2006 Yamanaka Shinya, of Kyoto University, and others showed that by administering a specific set of four “transcription factors”, proteins that affect which genes are used as blueprints for proteins, they could strip the cell-type-specific epigenetic markers off the DNA in a cell and return it to the state of a “pluripotent” stem cell. That opened the prospect that these induced pluripotent stem cells, or iPSCs, if prodded with other chemicals or implanted in the relevant type of tissue, could be used to generate specific cell types on demand. More recently it has been shown that using a subset of those four proteins offers a way to rejuvenate cells to a lesser degree, not stripping them right back to the stem-cell state but nevertheless removing some of the apparently excessive epigenetic markings that come with age.

The biggest player in the cellular de-ageing business is a company called Altos Labs. It was founded in 2022 with $3bn of initial capital from various sources, including Yuri Milner, a Russian-born billionaire. It has three campuses, two in California, the other in England, just south of Cambridge. To work in these the company has recruited some luminaries of the field. One such is Steven Horvath, who when at UCLA developed a way of predicting an animal’s age based on the amount of methylation it has endured in parts of its DNA. Horvath’s clock, as it is known, can be used to see if the age of cells is tracking the age of the animal they find themselves in, lagging it or leading it: in other words, whether the animal is ageing well or badly.

Altos plays its cards close to its chest; it is hard to say quite what approaches it is taking. But the field as a whole is faced with two options. One is to combat stem-cell exhaustion by making fresh stem cells in the lab and transplanting them in. The other is to try to rejuvenate tissues and organs in situ, by turning back the Horvath clocks in their cells a little way.

Dr Church at Harvard likes this latter approach, sometimes called transient or partial reprogramming. Researchers at Rejuvenate Bio, a firm in which he has an interest, have described using modified viruses to carry genes for three of the Yamanaka proteins into cells to be rejuvenated. In mice this reprogramming gives old tissues a fresh capacity for self-repair; subsequent damage is set right as efficiently as in young individuals. This has been shown to be true for skeletal muscle, nerve fibres, eyes, skin, hearts, livers and pancreases. It can even ameliorate loss of long-term memory. Rejuvenate’s researchers have, however, gone further than that. Their experiment (admittedly not yet peer reviewed) showed that OSK treatment (so-called from the initials of the three Yamanaka proteins involved) can actually extend life in laboratory mice.

The trial in question, posted on a site called bioRXiv, which exists to facilitate the early release of such un-peer-reviewed papers, reported a doubling of the remaining life expectancy of elderly mice given the treatment. Instead of living for less than nine further weeks they soldiered on for more than 18 (the mice were 124 weeks old when treated, an age equivalent to that of a human in their late 70s).

Nothing to offer but blood, nerves and T-cells

Others exploring in-situ rejuvenation include Life Biosciences, in Boston, a recent venture of David Sinclair, the prophet of sirtuins. Its first project is an attempt to use partial reprogramming as a way to repair the damage glaucoma does to the neurons of the optic nerve. The firm’s researchers have shown that the approach works on mice. AgeX Therapeutics of Alameda, California, uses a different set of rejuvenating transcription factors, identified by Michael West, one of its founders.

According to Joe Betts-LaCroix, boss of Retro Biosciences, the firm’s researchers are looking into rejuvenating the immune system by reprogramming the stem cells which differentiate into blood cells, including the white blood cells which form one branch of that system, and those that turn into the T-cells found in another branch of it.

Some other firms, though, prefer the idea of stem-cell transplants, a branch of a field known as cell therapy. Application of the Yanamaka factors means it is now possible to make stem cells to order—including from a patient’s own differentiated tissue, which will thus be recognised as friendly by the immune system and avoid the problem of rejection.

One of the leaders in this field is BlueRock Therapeutics, a subsidiary of Bayer that is based in Cambridge, Massachusetts. It says it has developed a way of making pluripotent human stem cells at scale and then tweaking them with further transcription factors to set them off on various paths that lead, eventually, to nerve cells, cardiac cells, immune-system cells and so on.

Its initial target is Parkinson’s disease, a condition caused by a loss of nerve cells in a region of the brain called the substantia nigra. This specificity and localisation makes Parkinson’s an attractive target for cell therapy, and the firm has embarked on a phase-I clinical trial involving 12 people. If that and subsequent, larger trials work, BlueRock hopes the range of targets can be widened.

Bayer’s involvement is a sign that big pharma has hopes for such approaches. So is a collaboration between Lineage Cell Therapeutics, of Carlsbad, California, and Genentech, a subsidiary of Roche, to develop a treatment for dry age-related macular degeneration, a cause of blindness. It is one of the places where the outsiderish field of lifespan and healthspan extension blurs with the medical mainstream; cell therapy is also a coming thing in cancer treatment, and pharma companies such as AstraZeneca are looking into it for tissue-restoration, too. Success in those fields could feed success in work on ageing—and vice versa. ■