Something in elderly blood is bad for brains. Plasma from old mice or humans worsens cognition and biological indicators of brain health, when infused into young mice. Conversely, plasma from young mice (or humans) rejuvenates old brains.
Much of this research has come from neurobiologist Tony Wyss-Coray’s group at Stanford University, which is pursuing what constituents of blood might be responsible. One previous study identified a protein, which declines with age, that has powerful beneficial effects. That protein can cross from the blood into the brain, but Wyss-Coray wondered how certain molecules contained in blood typically “talk” to the brain. Must they interact with brain cells directly, or can they communicate indirectly, through the gateway to the brain, the blood-brain barrier?
To investigate, Wyss-Coray’s team tried a new approach in their latest study, published May 13 in Nature Medicine. “We reasoned that the most obvious way plasma would interact with the brain is through blood vessels,” Wyss-Coray says. “So, we looked at proteins that change with age and had something to do with the vasculature.” One protein that becomes more abundant with age, VCAM1, stood out, and the team showed that it appears to play a pivotal role in the effects of aged blood on the brain. Biological and cognitive measures alike indicated that blocking VCAM1 not only prevents old plasma from damaging young mouse brains but can even reverse deficits in old mice. The work has important implications for age-related cognitive decline and brain diseases. “Cognitive dysfunction in aging is one of our biggest biomedical challenges, and we have no effective medical therapies. None,” says neuroscientist Dena Dubal, of the University of California, San Francisco, who was not involved in the study. “It’s such an important line of investigation; it has tremendous implications.”
VCAM1 (Vascular Cell Adhesion Molecule–1) is a protein that protrudes from the endothelial cells lining the walls of blood vessels and latches on to circulating immune cells (white blood cells, or “leukocytes”). It responds to injury or infection by increasing in number and triggering immune responses. An enzyme shears VCAM1 off endothelial cells at roughly the same rate it is produced, so the total amount in cells stays fairly stable and the amount in circulation is a good proxy for this.
The researchers first checked whether the increase in circulating VCAM1 with age was also accompanied by more of the protein bound to the cells, which they found to be the case for about 5 percent of brain endothelial cells.
They then used cutting-edge “single cell” genetic sequencing technology to inspect these rare cells, finding that they contain many receptors for pro-inflammatory proteins, known as cytokines. “It’s like these cells that express VCAM1 are a type of sensor of the blood environment,” Wyss-Coray says.
The researchers wanted to know whether this increase in VCAM1 attached to cells merely accompanies signs of brain aging, or whether it actually helps cause the damage. One sign that a brain is getting older is widespread activation of its immune cells, called microglia. These cellular housekeepers, which normally perform routine housekeeping functions, enter an inflammatory state, releasing cytokines and free radicals. “So, they’re not cleaning the house, they’re messing it up,” Wyss-Coray says. “They really trash the place.”
Another indicator is a decline in activity related to the formation of new brain cells in the hippocampus, a brain region involved in memory and one of few regions thought to produce new cells in adulthood. The team used two techniques to block VCAM1: One of them genetically deleted the protein from the mice’s brains. Another injected an antibody that binds to it to stop anything else attaching. Both methods prevented signs of brain aging in young mice infused with old plasma and reversed existing markers in elderly mice brains. The researchers then gave the mice learning and memory tests. In one, which involves remembering which of several holes is safe to drop through, treated elderly mice performed as well as youngsters once fully trained. “The aged mice looked like they were young again in terms of their ability to learn and remember,” Dubal says. “It’s remarkable.”
The researchers’ working theory for what happens, is that cytokines in aged blood first trigger brain endothelial cells to produce more VCAM1. When leukocytes then attach to the protein, the cells signal the brain to activate microglia. This creates an inflamed environment that puts dampers on the stem cells involved in new neuron formation. “What they’re showing here, is the blood-brain barrier’s not static, and can sense changes in the blood, then relay those signals to the brain, telling it to become more inflamed, explains Richard Daneman a neuropharmacologist specializing in the blood-brain barrier at the University of California, San Diego.
Stopping leukocytes from interacting with VCAM1 prevents this signaling and thus protects against or even reverses the effects of old blood. “One really has the feeling reading through this, that a major leap has been made [not only] in basic science discovery but also [in pointing to] a new therapeutic pathway for one of our most devastating problems,” Dubal contends. The precise molecular details of this pathway remain to be determined, Wyss-Coray says. “Is VCAM1 signalling into the cell, or are immune cells releasing toxic factors?” he asks. “We need to understand, at the molecular level, how this works.”
Treatments based on these findings would not necessarily have to cross the blood-brain barrier. “One of our biggest challenges is how do we get treatments into the brain given this fortress wall?” Dubal says. But VCAM1 is on the blood side of that wall. A downside is that blocking a component of the immune system could have side effects. A drug, called Tysabri that binds to leukocytes, stopping them attaching to VCAM1, is already used for treating multiple sclerosis. Problems arose shortly after its approval as some patients harbored a virus before treatment that then ran rampant. Patients are now screened for this virus. “It’s not without risk or caution that we use immunosuppressive therapies,” Dubal says “But they’ve proven very effective in certain conditions.”
One possibility would be to reduce VCAM1 activity to healthy, youthful levels, rather than block it completely. “We’re not directly blocking immune cells, we’re regulating the target of immune cells, so maybe that allows us to be subtle and not completely block immune activation in cases of injury,” Wyss-Coray says. “That needs to be shown.” Once more is known, there may also be other ways to intervene, such as stopping the signals that tell the brain to become inflamed or prevent VCAM1 from increasing in the first place, Daneman says. “Understanding the whole pathway will potentially enable us to limit those side effects.”
The main caveat, of course, is that whether the findings in mice lead to effective human therapies remains to be seen, but there are reasons for optimism. Human plasma was also used in the mice. “That improves the relevance to humans,” Dubal says, ”And soluble VCAM, in humans, like in mice, increases with aging. We won’t know until we test it, but it’s really promising.” The team is planning to test a VCAM1 antibody in people whose cognition declines after stroke, perhaps because of an immune response. “I’m hoping we can recover or prevent some of these cognitive deficits and recover function after stroke,” Wyss-Coray says.
Numerous antibodies already exist. “VCAM1 antibodies have been developed by many pharma companies,” Wyss-Coray says. “They didn’t pursue them once [Tysabri] got approved, but they could be resurrected and tested. We could translate this relatively quickly, because it’s a target that’s easily accessible and there’s precedent for targeting this pathway.”