Although the percentage of older people with dementia is thankfully decreasing, from 13% to 10% in those over 70 from 2011-2019, the absolute number of people with dementia is actually increasing. That’s because the share of the U.S. population that is 65 and over ballooned by 38.6% from 2010-2020, in large part due to baby boomers (shown in dark blue below) crossing the 65-year mark. They’ll keep on doing that until 2031, so the 65+ population will continue to grow quickly until then.
Superimposed 2010 and 2020 age-sex pyramids show how the aging of the baby boomer generation affected the age distribution of the United States
We’re still not very good at all at treating dementia, much less reversing it, but something quite intriguing was established a few years ago in a UCSF-Stanford study:
[E]xposure of an aged animal to young blood can counteract and reverse pre-existing effects of brain aging at the molecular, structural, functional and cognitive level.
And that’s great, but it sounds like something only vampires or Montgomery Burns would arrange. Blood isn’t exactly easy to get, and we already need it for plenty of other things like trauma, blood loss during surgery, kidney failure, etc.
So it’s been really important to try and figure out what exactly in young blood is responsible for these improvements in brain function in older animals and how to apply this to humans. If it’s one or two factors, we could probably use yeast or bacteria to manufacture zillions of pounds of them as pharmaceuticals, learn how to modify them for more stability and efficacy, and even use them in a preventative way.
If you’re at risk for cognitive decline, you could get an infusion or a shot once every 8 weeks or so. And if there’s one thing a pharmaceutical company lo-o-oves, it’s a medicine you have to take regularly and that they can charge the insurance company a lot for, so you bet your sweet bippy it will get developed ASAP the minute we find out what it is.
Well, now there is suddenly a barrage of evidence that indeed pins the effect pretty squarely on just a single blood factor: a protein called platelet factor 4 (PF4). Three independent groups were looking for different ways to extend cognitive function during aging, and they all serendipitously hit on that same factor.
They’ve just released a trio of papers, all open access. Yay! These appear in Nature, Nature Aging, and Nature Communications published August 16 and were led respectively by Dr. Saul Villeda (UCSF), Dr. Dena Dubal (UCSF), and Dr. Tara Walker (University of Queensland).
They committed to releasing their findings at the same time to make the case for PF4 from three different angles.
"When we realized we had independently and serendipitously found the same thing, our jaws dropped," Dubal said. "The fact that three separate interventions converged on platelet factors truly highlights the validity and reproducibility of this biology. The time has come to pursue platelet factors in brain health and cognitive enhancement.”
Well, I guess it has!
It seems a little odd to be talking about blood platelets when we’re trying to improve brain function. When we have a wound, platelets rapidly get activated to help blood coagulate so that the bleeding stops. Here’s what that progression looks like:
Platelet activation stages (maginification): (A) resting (30,000x), (B) activated (13,000x), (C) activated spreading (11,000x), (D) fully spread (9,000x).
One of the most abundant proteins in both human and mouse platelets is PF4, and platelets secrete it after they’ve been activated. PF4 seems to help attract other components of the immune system to promote inflammation. PF4 levels decrease as humans and mice age, but all three studies clearly showed that simply injecting PF4 into aging mice made their cognitive skills match mice half their age.
The three studies all landed on PF4 in different ways. Villeda knew that young blood transfusions restore cognitive function, so he kept testing different blood fractions until he narrowed it down to PF4. Dubal has long been interested in the anti-aging properties of a protein called klotho, and her paper shows that klotho induces PF4 production, and that’s what led her to test PF4 directly. Then Walker was trying to get at why exercise indisputably enhances cognition, and she found that exercise activates platelets, which then secrete — you guessed it — PF4. That’s a remarkable convergence that makes me think this is rock solid.
Before we say, “Oh, this only applies to mice”, the fact is that Stanford researchers have recently shown that blood fractions from young humans have similarly potent restorative effects on cognition in aging mice. That means that the mouse and human systems are analogous enough that the effect has a good chance to hold up in humans. But that’ll have to be in clinical trials. We can’t just go injecting people willy-nilly with PF4. I’ll take some if you’ve got it, though.
All three groups saw beneficial effects in the hippocampus, part of the brain that shares the Greek name for “seahorse” because in humans it sort of looks like one:
Location of hippocampus
Human hippocampus (main part) and fornix (taillike part), compared to a seahorse
The hippocampus is critical for the formation and storage of memories of facts, sensations, directions, etc., although interestingly not for procedural memories (like how to swing a golf club). Part of the hippocampus called the dentate gyrus is one of only two places in the brain where neurons keep forming throughout the lives of rodents and primates. (The other is the granule cell layer of the olfactory bulb, but in humans that process seems to be very limited.)
So if neuron formation slows down in the dentate gyrus as we age, it could cause memory and cognitive decline. That’s why all three teams of researchers were paying special attention to the hippocampus and taking note of all the positive effects PF4 seems to cause there.
But how could blood platelets change anything in the hippocampus? When we take these three papers together, there’s a little fuzziness as to whether PF4 acts directly in the brain, in part because it’s not clear that enough of it can cross the blood-brain barrier to have a direct effect in the brain. Villeda’s group didn’t see any injected PF4 make it into the brain, but Dubal’s group did. Walker’s group favors a direct effect on the brain because they saw positive effects when they administered it directly to the brain, but they didn’t test whether exercise-induced PF4 could make it into the brain on its own.
Dubal’s data on this does look reasonably convincing. Her group wanted to see if PF4 can make it into the brain, so they injected mice with a modified PF4 protein that had a non-natural series of consecutive histidine amino acids (“HIS”) attached to it. There is a fluorescent red dye that will stick to a chain of consecutive histidines, so if you’ve appended that kind of chain to your protein, you’ll be able to detect it by a fluorescent red signal and see where your protein goes.
Dubal’s group already knew that the klotho protein cannot get into the brain, and when they injected mice with “HIS-klotho” and then applied the red dye, they didn’t see any red signal in brain tissue, as expected. But when they injected mice with “HIS-mPF4” (mouse PF4), they looked at hippocampal dentate gyrus tissue and did indeed see red:
Representative hippocampal dentate gyrus images of young mice following injection of 0.5 mg/kg HIS–klotho or HIS–mouse PF4. DAPI is a fluorescent dye that stains DNA blue, and fluorescent lectin stains blood vessels green, so any brain tissue will light up blue and green with those. Those two are sanity checks. But the red dye sticks to “HIS”, and it shows that PF4 appears to have made it from the blood into the dentate gyrus
...so I am leaning towards Dubal’s and Walker’s contention that PF4 acts directly in the brain.
Which is completely frickin’ bizarre.
A protein secreted by blood platelets to help out with immune responses also travels to the brain to maintain cognitive functions? Yoingggg! That’s pretty strange.
Villeda says that the effect might be indirect, because PF4 — a component of the immune system, after all — leads to reduced inflammation in the aging hippocampus, and that could happen indirectly via other factors that can cross the blood-brain barrier very well. And his team did indeed see big changes in the immune systems of mice injected with PF4 that made their function look like those of much younger mice. (Which is exciting in its own right, but a topic for another day.)
But regardless of all that, the presence of more PF4 in the system indisputably resets brain tissue to a more-youthful state, and all three groups clearly saw that. There are numerous ways to represent this, but I’ll try to just show a couple compelling results.
First, Dubal’s team actually showed explicitly that PF4 improves memory formation in the hippocampus by probing it with electrical stimuli.
One way memories are formed seems to be “long-term potentiation”, where the sensitivity of the connection between two neurons can increase for good if those two neurons communicate often. By allowing this, the brain can sort of “rewire” itself on the fly. And that rewiring can be directly observed in the hippocampus.
You can take a slice of the hippocampus where the neurons are bundled in such a way that if you stimulate one group, you’ll see a nice measurable signal in a bunch of downstream neurons because you can get a bunch of synapses to fire at once. So you give a stimulus, and basically provoke the bundle of neurons to yell, “Hey!” at the downstream neurons, and then you detect a certain level of response. But if the bundle of upstream neurons gets pesky enough — “Hey! Hey! Hey! Hey!” like 100 times a second for a while, the downstream neurons say “What?! What, already?! Sheesh!” and thereafter they become more sensitive to the upstream neurons’ requests. Here’s what that basically looks like. We’re graphing the signal from the upstream neurons (the stimulation we are providing on purpose) and the response of the downstream neurons (the EPSP, or excitatory postsynaptic potential) over time. The “tetanus” is our “Hey! Hey! Hey! Hey!” moment that we do for a few minutes, and we call that time zero. Then we go back to regular “Hey!” calls, but the downstream neurons are now more responsive to us. And that’s just the kind of mechanism (called “synaptic plasticity”) that can explain long-term memory, so when we do a test like this, we are literally watching a memory form:
Long-term potentiation: A memory is made
So Dubal’s team took mouse hippocampus slices and tested six each with and without exposing them directly to PF4 for 2 hours, and now that you know what one of these graphs looks like, you’ll appreciate that memory-forming ability got noticeably better when PF4 was added:
“fEPSP” is “field” EPSP because we are measuring the response of not one neuron, but a group of them. The “Veh” (vehicle, meaning everything but PF4 was added) trials had an average jump of about 60%, while the PF4 average was more like 120%
They also saw a jump like this when the mice were simply injected with PF4 (which is also exciting, because that is therapeutically easy!), but the direct treatment shown above implies that PF4 is getting to the brain and acting directly there, or at least that it can.
As far as actual cognitive effects, Villeda’s group did a test called “novel object recognition” (NOR) on aging mice that were treated with saline solution, with or without PF4. Mice got to check out an area with two identical objects for 5 minutes one day, then the next day one of the objects was replaced with a new object, and the mice spent 5 minutes with the two objects. PF4-treated mice spent much more time exploring the new object, presumably because they remembered the old object better from the previous day. And PF4 alone was just as good at eliciting this as whole plasma from young mice. It should be noted that Walker’s group did the same test and got pretty much the same results.
P = 0.0002 basically means there’s a 99.98% chance that this is not just a random effect
Like I said, there are a zillion graphics in the three papers, so you can certainly go feast on those if you want to. It’s a mountain of evidence that should make us pretty hopeful about the chances of this leading to something very good for humans.
The fact that it seems we’ve reduced the vague and complicated “young blood” effect down to just one factor means clinical trials have got to be just around the corner. Hey, by the time PF4 is available as a medication, I might even still remember how to get to the drugstore.
