What an immune invasion teaches us about Alzheimer’s disease

Pathologist and computer scientist Julia Belk explains how the brain's immune cells get replaced as we age and what it could mean for neurodegenerative disease
Nicholas Weiler
From Our Neurons to Yours Wu Tsai Neuro Podcast

Today we’re going to be talking about a recent study that is rewriting neuroscience textbooks, potentially changing our understanding of Alzheimer’s disease, our brain’s immune system, and the potential for immunotherapy for the brain.

Today’s guest is Julia Belk, lead author of a new study that discovered that as we age, immune cells from the blood somehow cross the supposedly impenetrable blood–brain barrier, and replace the brain’s native immune cells, the microglia. The findings help us understand growing links between the aging immune system and Alzheimer’s disease and why certain blood mutations – ones traditionally linked to cancer – might actually protect against neurodegeneration.

Julia is an HHMI Hannah Gray postdoctoral fellow in the lab of Sidd Jaiswal at Stanford Medicine. Their research was supported by the Knight Initiative for Brain Resilience at Wu Tsai Neuro.

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Julia Belk of the Jaiswal Lab. Image by Devin Powel.
Devin Powell
Julia Belk at work in the lab.

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Episode credits

This episode was produced by Michael Osborne at 14th Street Studios, with sound design by Mark Bell. Social media strategy is by Julia Diaz, and additional editing by Nathan Collins. Our logo is by Aimee Garza. The show is hosted by Nicholas Weiler at Stanford's Wu Tsai Neurosciences Institute and supported in part by the Knight Initiative for Brain Resilience. 

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Transcript

Nicholas Weiler (00:10):

This is From Our Neurons to Yours, a podcast from the Wu Tsai Neurosciences Institute at Stanford University, bringing you to the frontiers of brain science. I'm your host, Nicholas Weiler.

 (00:24):

Today, we're going to be talking about a recent study that is rewriting neuroscience textbooks as we speak. This is a discovery that may have major implications for our understanding of Alzheimer's disease and our ability to treat brain diseases more broadly. To explain what I mean, let's take a step back. Our brains are supposed to be kind of walled off from the rest of our bodies. We have what's called the blood-brain barrier between our blood and the brain. This is partly to protect the brain from infections, but it's also to protect the brain from our own immune system, which is so powerful it could damage our sensitive brain tissue.

(01:01):

Behind this barrier, the brain essentially has a parallel immune system. Its resident immune cells, the microglia, act a lot like the immune cells in the rest of the body. They gobble up junk, eliminate sick or dying cells, and are the brain's first line of defense against infection. But microglia are also a little different. The rest of your immune system is regularly regenerated from stem cells in your bone marrow, like the rest of your blood. In contrast, the microglia in your brain have to last your whole life, either on their own or by slowly dividing to make new cells. And an increasing body of research is pointing to aging microglia as a driver of inflammation in the brain, which may in turn increase our risk of Alzheimer's and dementia.

(01:43):

At least this was the picture we had until this past July when a paper in the journal Nature by Julia Belk and colleagues in the lab of Sidd Jaiswal here at Stanford Medicine suggested that the story I just told you is wrong and that something a little more bizarre is actually going on.

(02:01):

Julia, Sidd, and their team found that our microglia are actually gradually replaced by immune cells from our blood, the very cells that the blood-brain barrier is supposed to keep out. What's even stranger is that the incoming cells disguise themselves to look exactly like our native microglia, which is maybe why no one had ever noticed this before. As coincidence would have it, a second paper from a different lab came out at almost the same time. This paper was in the journal Science and arrived at basically the same conclusions using a totally different approach. This is not that common in science and really cool because it tells us that this is something that is real and not just a fluke.

(02:42):

Together, these findings change the whole game. Maybe it's not our aging microglia that are responsible for increased risk of Alzheimer's. Maybe it's these replacements which aren't tuned to the right sensitivity for our brains. What's more, if we can discover how these cells are crossing the blood-brain barrier, it could really help develop targeted immunotherapies. In other words, programming immune cells to enter the brain and clean up the junk that leads to dementia and neurodegeneration in the first place.

(03:11):

One of my favorite things about this story is that the Jaiswal lab had never studied the brain before this research started a few years back. They're known for studying this common genetic condition in the blood called CHIP. CHIP raises your risk of cancer and other diseases, but a strange observation that CHIP actually seems to decrease Alzheimer's risk set them up for this wild new discovery. To tell us all about that and talk us through the science and its implications, I'm pleased to welcome Julia Belk to the show.

(03:44):

Julia Belk, welcome to From Our Neurons to Yours. It's great to have you.

Julia Belk (03:48):

Thanks so much for having me, Nick.

Nicholas Weiler (03:49):

So Julia, I'm really excited to talk about this new paper that you have. It's really a landmark study in the field and changes the way we think about a number of things, which we'll get into. I mean, essentially, you've discovered that unbeknownst to anyone, our brains, immune cells, the microglia are slowly and silently getting replaced by doppelgangers as we get older. And this reminds me of Invasion of the Body Snatchers. Or what was that Jordan Peele movie, Us, from a few years ago. Is that how you think of this?

Julia Belk (04:21):

Yes, you're exactly right. So the microglia are these immune cells of our brain, of course. And what's known from mouse studies is that these cells can actually be very long-lived and they're in our brain from birth and then can stay there throughout the entire lifespan of the animal. But we're finding that this is not the same as in humans where these cells are getting replaced gradually as we age. And it's been really surprising because we had no idea that this process was going on in our brain.

Nicholas Weiler (04:50):

And it had always been thought that we have this blood-brain barrier, right? The brain is supposed to be separated from the immune system, from the blood, to protect it from our immune cells, which can be a little aggressive for our sensitive brain tissue. So how does this study change that picture of what exactly the blood brain barrier is doing?

Julia Belk (05:14):

Yeah, you're exactly right. So the brain conventionally is thought of as quite closed off to external inputs, whether that's proteins, immune cells. And our study doesn't necessarily kind of completely change that. The blood brain barrier is still alive and well, but what we're starting to understand is that these peripheral immune cells, in a way, they essentially have the keys to the blood brain barrier. So they have ways to cross the blood brain barrier in certain circumstances, but what's really new here is the idea that this process is happening even in homeostasis continuously. So without any kind of overt probation or pathology within the brain, these cells can still sort of cross and enter the brain parenchyma, so really inside the brain.

Nicholas Weiler (05:59):

We'll get back into this in more detail in a little bit, but what are the implications here for brain aging and Alzheimer's disease? I know microglia have been implicated over the past few years in inflammation and things that happen as we age, potentially being involved in the development of neurodegeneration. We've talked about this with various folks on the show like Katy Andreas in last year. So just at the high level, what do you think are the implications here for how we think about that aging process and the role of the microglia?

Julia Belk (06:30):

Yeah, great question. So we know that microglia in general are important for a variety of functions. Generally as the immune cells in the brain, they are responsible for maintaining homeostasis and cleaning the brain, preventing ideally any type of buildup of protein aggregates, which is really important, for example, like you mentioned in Alzheimer's disease. But this microglia have really been conceptualized as these long-lived self-renewing cells. So now that we know that in humans, these cells are actually getting replaced from the outside, I think we really need to revisit a lot of the conventional wisdom that we have about this microglia because this process that's going on in the human brain really is not captured in most of the mouse models, which are commonly used to study these processes.

(07:16):

Alzheimer's disease is particularly relevant because we know that from human genetics, microglia are really heavily implicated in that specific disease setting. So it's really important to actually understand these cells to try to understand how we can maybe harness them for Alzheimer's disease.

Nicholas Weiler (07:34):

Right. Yeah. The blood-brain barrier has also been a barrier to therapeutics for the brain because just like it's preventing the immune system from getting into the brain, it also often prevents many kinds of medications or therapies, gene therapies and so on. And so there's this interesting idea here that maybe that gives us some therapeutic opportunities as well as trying to understand the basic science of what is happening to our brains as we age, what is happening to our microglia, why are they linked to Alzheimer's disease and so on. 

Julia Belk (08:13):

Yes, exactly. And that's one of the directions I'm really super excited about. And I really think about it on two levels. So first, there's the diseases like Alzheimer's disease where we know microglia are central to disease progression. But exactly like you said, now that we know that these cells are sort of naturally able to transit across the blood-brain barrier, we can think about maybe harnessing that pathway to unlock treatments for a much wider range of brain disorders as well by maybe tethering therapeutic molecules or using those cells to deliver therapeutic molecules into the brain, possibly to other brain cells.

Nicholas Weiler (08:52):

One of the things I find so cool and remarkable about this study is that both you and the Jaiswal Lab more broadly sort of come at this as outsiders to neuroscience and neurodegenerative disease research. I'd love to hear a little bit more about that. I mean, you have a background in electrical engineering and in computer science. The Jaiswal Lab has historically studied cancer and stem cell biology. I'd love to hear a little more about where this finding came from, how you got involved. 

(09:23):

Maybe you could start by telling us a little bit about how you got involved in the research. My understanding is you were an undergrad, you were working on developing technologies to build out power grids and developing countries, and then you ended up coming to Stanford to do a PhD in computer science. So just give us a sense, how did you end up working on the genetics of aging and Alzheimer's disease?

Julia Belk (09:43):

Yes, yes. So basically for me, I had a great undergraduate research experience. I really liked being in the lab. When I got to Stanford-

Nicholas Weiler (09:51):

You were at MIT?

Julia Belk (09:52):

Yeah, I was at MIT, right. And when I got to graduate school, so I knew I wanted to do research, but I was looking for new research directions to focus on for my PhD, potentially my whole career. And what I got really excited about at that time was the amazing success of immunotherapy in cancer, where right as I was starting graduate school, it seemed like every other week there was another pivotal approval for a clinical trial for one of these therapies that was able to basically harness the immune system to cure some of the most aggressive and deadly cancers.

Nicholas Weiler (10:27):

Completely transformative, right?

Julia Belk (10:29):

Exactly. 

Nicholas Weiler (10:29):

The life expectancy for many different cancers has dropped... I mean has increased dramatically because of these immunotherapies, completely transformed the field.

Julia Belk (10:38):

Exactly. And in particular, what caught my eye is they can really be curative in certain cases, so very long-term responses that are considered cures at this point. So I really wanted to work on the immune system for my PhD. And it's also a really exciting time because we've just had this explosion of technologies to actually measure the immune system and human cells more broadly with incredible resolution at incredible scale. So for me, coming as someone with a more quantitative background, I felt it was really perfect timing where my skillset was actually useful and some of the problems were really, really compelling.

(11:15):

So that's what I did. So I joined a lab that focused on cancer immunotherapy for my PhD. And at the same time, my PhD advisor, Ansu, was really excited about collaborating broadly with many other investigators. And it just so happened that we shared a lab space with another new faculty member, Sidd Jaiswal, and we got to talking and then little by little we got to collaborating on this new direction they had around the brain as well.

(11:42):

The way Sidd got into this, of course, was from his interest in clonal hematopoiesis, of course initially discovering this phenomenon of clonal hematopoiesis where mutations happen in the blood that cause clonal expansions that essentially is almost like a precursor state to a blood cancer. And although Sid's background, Sid's training is really in, as you mentioned, hematopoiesis and stem cell biology, the really fascinating thing is that clonal hematopoiesis has many different disease associations, many of which Sidd has uncovered himself, and that's how we started looking at Alzheimer's.

Nicholas Weiler (12:19):

Well, let's take a step back and talk about that, this idea of clonal hematopoiesis, it's quite a mouthful. Sometimes I think one of the variants of this that you've been looking at is called clonal hematopoiesis of indeterminate potential. We're going to call it CHIP for the purpose of this conversation to talk about CHIP. 

(12:39):

Give us a sense, in layman's terms, what is CHIP? You mentioned that it's something about mutations build up in some of the stem cells in the bone marrow that produce the blood. Is that right?

Julia Belk (12:52):

Yes. I mean, actually it's a very general phenomenon where mutations are accumulating in all of our cells in our body as time passes. It's not even limited to our stem cells or our blood systems. It's a very general phenomenon.

Nicholas Weiler (13:06):

But as we age, mutations, there's... Damage mutations in the DNA, that's normal?

Julia Belk (13:11):

Exactly.

Nicholas Weiler (13:12):

Particularly in dividing cells, right?

Julia Belk (13:13):

Even non-dividing cells.

Nicholas Weiler (13:14):

Okay.

Julia Belk (13:15):

So even in neurons, we now understand they're also accumulating mutations at essentially a linear rate as time passes.

Nicholas Weiler (13:23):

And most of these don't do anything presumably?

Julia Belk (13:25):

Exactly. Exactly. These are totally random mutations at a high level, and so most of them don't do anything. But occasionally just by random chance, a mutation might hit something important, an important gene, and that's what can lead to cancer. And so it's been known, of course, that mutations cause cancer, but then what Sidd found in his postdoctoral work is that these mutations can cause pre-malignant clonal expansions. So clonal expansions that are expanded relative to the other cells, but are not as malignant and as aggressive as a cancer.

Nicholas Weiler (14:02):

And that means that these cells are dividing and their offspring have some competitive advantage, sort of like an evolutionary advantage, we sometimes think of it. So if they're able to produce more offspring and then those offspring spread, they become a larger proportion of the cells overall. Is that what you mean when you say clonal expansion?

Julia Belk (14:21):

Yes, exactly. Exactly. And he first found this in the blood system, but this has now been understood to be... There's an analogous process going on in all of our tissues with these pre-malignant expansions prior to cancer.

Nicholas Weiler (14:35):

Got it. Okay. And so the lab had been interested in studying this CHIP phenomenon, the fact that sometimes there are mutations that give cells... And I think this was specifically looking at stem cells in the bone marrow that produce blood cells and immune cells in the blood, and that these seem to expand as we get older. And they can become a fairly large percentage of our blood, have these mutations. Is that right?

Julia Belk (15:09):

Yes. Yes.

Nicholas Weiler (15:10):

Like people over 50 or 60 sometimes have quite a few of these?

Julia Belk (15:13):

Right. So essentially the way I think about it is that in all of us, there's multiple of these different clonal expansions that are just happening by chance. And then as we age, the fitness advantage of these cells compounds, so the clones get bigger as we age typically. And in older individuals, like individuals above the age of 80, almost all of them will have at least one of these clones that's making up at least 10% of their blood. So it's really a very, very common phenomenon even for large clones in the oldest individuals. And even middle-aged or younger people have these clones, they're just not as expanded yet.

Nicholas Weiler (15:57):

We've taken this detour into some stem cell and cancer biology here, and this is what makes this so interesting. So we know that there's this phenomenon where certain stem cells start producing more and more of your overall blood volume as you get older because they've got these sort of founding mutations. This can lead to cancer, right? Now you've got cells that are expanding. If they get other mutations that might lead to malignancy, that might lead to leukemia, for example. It's also really bad for heart health. It can lead to metabolic syndrome like diabetes. This CHIP thing, generally speaking, is thought to be bad. It's a risk factor for a number of different diseases. But then you said that when you were first talking with Sid, he had just started getting interested in Alzheimer's disease. So help us understand, what's the potential connection there? How do you get from studying blood cancers to being interested in Alzheimer's disease?

Julia Belk (16:53):

Yes. Well, the idea is that CHIP can really shed light into areas of human disease where these peripheral immune clones might have an impact in the disease. And Alzheimer's was a somewhat natural disease to look at because we know that the immune system is so important in Alzheimer's disease. Because as I was mentioning, a huge amount of Alzheimer's disease risk for late onset Alzheimer's disease does concentrate in genes and variants associated with microglia biology. So we knew that at least within the scope of brain disorders, Alzheimer's would be one where it would make sense to potentially ask this question of whether clonal hematopoiesis has a particular association or not with Alzheimer's disease.

Nicholas Weiler (17:42):

And was the idea that the microglia in the brain are sort of related to the kinds of immune cells in the blood that sometimes are affected in CHIP or was it a more general question about are we getting more inflammation in the blood and is that affecting the brain?

Julia Belk (18:01):

Yeah, so at the outset, we really didn't know. And these initial associations of CHIP and Alzheimer's, those were actually done by Sidd in his postdoc. And the way a lot of these studies start out, it's really a human cohort biobank exploration where he just asked the question, "Do people with CHIP, are they more or less likely to get Alzheimer's disease later in life?" And then once he actually identified the association, and it was surprisingly protective association where people with CHIP are less likely to get Alzheimer's, that's when we really started thinking about what might the mechanism of that be. Is it circulating immune cells that are just a little bit different and maybe secreting cytokines that influence the brain? Could there be a more direct connection between the peripheral immune system and the brain? And that's when myself and my PhD advisor also really got involved to help Sidd investigate some of these brain tissue specimens from individuals with CHIP.

Nicholas Weiler (19:01):

So I want to just go back to that because the big surprise there was, I assume that Sidd was initially going into this saying, "Is CHIP also a risk factor for Alzheimer's?" And what he actually found and what you showed very clearly in the paper you published in 2023 is actually CHIP seems to be protective against Alzheimer's, which seems like that must have been a very surprising finding. 

(19:25):

So I want to touch briefly, you had a paper that came out in 2023. I remember working with you at the time on this video for the Knight Initiative for Brain Resilience because this work was one of the first sets of studies that the Knight Initiative supported after it got started. And you all were basically saying, "We've got this bizarre phenomenon where these mutations that arise, this CHIP phenomenon that we've been talking about, seems to be protective against Alzheimer's disease." And this is where you first had this observation that some of the microglia seemed like they actually didn't come from the brain. And that started in this 2023 study, right?

Julia Belk (20:08):

Exactly, exactly. And to set this up a little more, basically, so we gathered brain samples from individuals with and without CHIP, but we were kind of scratching our heads when the results came back because when we looked at the brains of individuals with CHIP and individuals without CHIP, there was really no difference in the cell types that we observed. The only immune cell population in those brains was the resident microglia, which have been thought to stay in the brain throughout life. So we were really scratching our heads as to what was going on because we also could see just from PCR, just from looking at the specific mutations in the brain tissue, that those mutant cells were absolutely in the brain, but we couldn't find an obvious candidate cell type that those mutations could be in. So those were our first kind of clues, but it was really a lot of detective work and a lot of confusion at first to really understand how this might be working.

Nicholas Weiler (21:07):

Right. I remember talking with you at the time for this video, for the Knight Initiative I mentioned, and you all were like, "We've got these really interesting observations. It seems like something bizarre is going on here. There's a mystery. We've got to go figure out what's going on."

(21:30):

So let's talk about what's really going on. This is sort of what you've been doing over the past several years building up to this new study that you published this summer. So let's dive into the details. How did you figure out what exactly is going on? You know that there are cells with CHIP mutations in the brain, but you can't find any cells from the periphery in the brain. So what's the solution?

Julia Belk (21:51):

Yes. Really at that juncture, we're really quite technologically limited in being able to sort of lineage trace these human cells where the idea is we needed some kind of method where we could not only identify those microglia cells, but ask the question, "Are those microglia that we're identifying, have they been in our brain the entire time or have they recently emigrated from the blood?"

Nicholas Weiler (22:18):

So you had this hypothesis, maybe this microglia are actually not coming from the brain, but you needed a way to prove it?

Julia Belk (22:25):

Yes. We needed a way to prove it. And in particular, we also wanted a way to prove it that could be applied to the broader aging human population because that really clearly would be a very interesting result if this phenomenon was going on in the broader human population.

Nicholas Weiler (22:42):

What were the techniques that you had to develop to make this possible? You mentioned lineage tracing, and that's basically like where does this cell come from? Which stem cell niche produced this particular cell?

Julia Belk (22:53):

Exactly, exactly. From the outset, we knew we wanted to try to use mutations that accumulate over life to lineage trace those cells because mutations, naturally occurring mutations, are really the gold standard way to do lineage tracing in humans. The reason for that is that these mutations, once you acquire them, they essentially can't change. So if you see two cells with the same mutation, and these are mutations that are occurring after birth, so-called somatic mutations-

Nicholas Weiler (23:28):

These random mutations we were talking about earlier?

Julia Belk (23:30):

Exactly. If you see two cells with the same mutation, you know that one has to be the offspring of the other compared to other modalities like gene expression or chromatin accessibility, those can change over time. So if you see two cells with the same gene expressed, you can't really conclude much. But with mutations, you can get at this question of directly cell lineage.

(23:53):

So we wanted to use mutations, but then the real kind of technical challenge of this project was trying to get brain samples and a mutation tracking assay that would work together because there were some approaches published to use mutations to lineage trace human cells, but they primarily relied on live cells. They relied on a type of mutation called mitochondria mutations, which have some beneficial properties, but tracking those mutations really requires working with live cells, which in our case would have meant fresh brain tissue, which is very, very hard to procure and work with because most of the available brain tissue are archival tissue specimens.

Nicholas Weiler (24:42):

So they're older, they're not fresh tissue?

Julia Belk (24:45):

Exactly. They're just flash frozen, so you can't isolate live cells anymore. You can think of it as literally a piece of brain has just been put in the freezer directly.

Nicholas Weiler (24:55):

Let's take this back a step. I don't want to get too deep into some of the technical details and miss the big picture here, which is that could you actually start by just telling us what's the big picture of what you concluded here in this paper? How much microglia replacement are you seeing in these brain samples that you looked at?

Julia Belk (25:18):

Yes. So the kind of take home point of our paper was that in all of the older individuals that we looked at, all of them had a quite substantial amount of microglia replacement going on. We actually suspect from our paper that it's all or almost all of the microglia in these individuals.

Nicholas Weiler (25:38):

And they're how old?

Julia Belk (25:41):

Most of the individuals we looked at are over the age of 80, so that's really the kind of oldest age bracket. Using our method, we couldn't fully prove that it's a near 100% replacement, but we did have several lines of evidence kind of supporting that idea. But this contemporaneous paper, which took a completely different approach, provides also orthogonal evidence kind of really supporting the idea that we think it does seem that maybe 100% of microglia can be replaced as we age.

Nicholas Weiler (26:13):

Yeah, you mentioned that. So your paper came out in Nature and almost exactly the same time, another paper came out in Science with very similar findings using a totally different approach. I'd love to hear you talk a little bit more about that. What were some of the ways in which your two papers were complimentary in helping us understand this idea that somehow our microglia are getting replaced almost entirely as we age?

Julia Belk (26:40):

Yes. Yeah. So it was very serendipitous that from two completely different approaches, we really converged on the same findings. And as I mentioned, our approaches really focused on somatic mutations, which are the gold standard to do lineage tracing.

Nicholas Weiler (26:55):

And that's basically the same as what happens with ancestry testing, with 23andMe. They're looking at random mutations that arise in the DNA and you can see those mutations pass through populations, except here we're talking about populations of cells rather than populations of humans. 

Julia Belk (27:12):

Yes. Yes. So the mutation tracking aspect is very similar, but those ancestry tests, they test mutations that we're born with and then are the same in all of our cells versus the mutations that we used for our lineage tracing approaches are mutations that arise after birth and they arise just on one cell initially that then needs to expand to carry on those mutations.

Nicholas Weiler (27:36):

Got it. Okay. So you're doing this genetic lineage tracing approaches, and the other team did a completely different approach to this question?

Julia Belk (27:44):

Yes, yes. And just to mention, the main limitation of our approach is that we're always looking at some cells with these mutations, but then many of the cells in the blood and in the brain will not have those mutations. So the cells without the mutations, our approach can't necessarily conclude anything about. So that's where the contemporaneous approach really was very powerful, where the approach they took was they performed a single cell atlas of the human brain with aging, and they used multiple different technologies in parallel on the same brain pieces. So they used essentially single cell RNA-seq, single cell ATAC-seq to look at epigenetics, and single cell methylation and 3D chromatin, which is a little bit of a mouthful, but let's just simplify it and say on the single cell methylation level, when they looked at the microglia, specifically within the methylation data, something very interesting popped out at them, which is there were really two populations of microglia that they could see only from the methylation data.

Nicholas Weiler (28:52):

And the methylation, these are these tags that get put on DNA. This is part of what we call epigenetics, right? These little chemical tags get put on parts of DNA to regulate how genes are expressed. And what they're seeing is like, "Oh, if we look at that, we can see that actually there are two different groups of normally what would have looked like regular microglia, it turns out they're actually two different groups and they can see that in this epigenetic methylation data."

Julia Belk (29:22):

Exactly, exactly. And people had already looked before with RNA and ATAC-seq at the same question. By RNA, just for example, the microglia really is one cluster. So it was only in methylation that these two populations really appeared and were very clear. And what they found is that all of the younger individuals start with one cluster or one type of microglia. We can just call it micro 1, but then with aging, the second population that they called micro 2 in the paper started showing up. And that process, it really started around age 50. And then by age 80, essentially most of those brain samples or all of those brain samples had almost all of their microglia consisting of this micro 2 population, which, combined with our evidence from the mutation-based lineage tracing, really supports this model where potentially all of our microglia are being replaced with age by these kind of bone marrow cells. 

Nicholas Weiler (30:24):

This is the thing that is so surprising and so bizarre that as we age, it's not only that these immune cells are coming in to the brain as you had originally hypothesized, but they're coming in and essentially disguising themselves, reprogramming themselves to become microglia. They are microglia after they get into the brain. Do you know anything or have we learned anything about their functional differences? Are they acting just like microglia? Are they functioning the same as our regular microglia or are there differences?

Julia Belk (30:57):

Yeah, so that is one of the biggest questions that I think we really need to get to the bottom of. I think the one difference that is the most clear right now is that these cells that are coming in from monocytes are almost certainly shorter lived than the brain resident microglia where we know that-

Nicholas Weiler (31:16):

Monocytes being one of the immune cell types from the periphery?

Julia Belk (31:20):

Yes, yes. So the cells that are being replenished from the blood are almost certainly much shorter-lived than these long-lived, what's called a yolk sac microglia, the microglia that are in the brain from birth. But we definitely need to do a lot more sort of functional interrogation of these cell types, which is really going to require better mouse models to study some of these processes in model systems.

Nicholas Weiler (31:58):

Going back to the question of Alzheimer's disease, we know that the microglia become more pro-inflammatory as we age around the same timeline. So it seems like the hypothesis would be maybe it's this replacement that's happening. We've heard about various theories and evidence for why this might be happening, that the microglia are getting more metabolically exhausted or that they're choking on too much waste products from other cells. There are various different theories about why microglia seem to become more pro-inflammatory, leading to more risk of Alzheimer's disease. Is one of your hypotheses that, like, "Well, maybe actually what's happening is these fresh recruits from the periphery are too aggressive, they're too pro-inflammatory"?

Julia Belk (32:41):

Absolutely. That's definitely one hypothesis. I do think the functional differences are an open question right now. And a big challenge also is that a lot of the microglia replacement studies in mice really rely on transplantation to insert new cells into the brain, which is quite different than what happens in humans where these cells are sort of just naturally repopulating. So I think developing better models where we can really test some of these hypotheses is going to be very important. But we do already have evidence from the people with CHIP that at least some of those CHIP cells appear that they are associated with protection from Alzheimer's disease. So some of them may be able to reduce some of these harmful inflammation as well.

Nicholas Weiler (33:28):

Yeah. I wanted to ask you about that and bring it back to this CHIP phenomenon. Why do you think those mutant cells, which are generally bad for our health, heart disease, metabolic syndrome, cancer, why might those mutations be protective against Alzheimer's?

Julia Belk (33:46):

Yeah, great question. And the pathway that we have the most sort of evidence for so far is this pathway called phagocytosis. So this is a process by which microglia can directly engulf and potentially clear harmful aggregates like amyloid and tau out of the brain. And so from some of our preliminary evidence, and this is still a hypothesis at this point, but preliminary evidence both from mouse models and now from some human patients has demonstrated that some of these mutant CHIP cells are actually better at phagocytosis than their wild-type counterparts. So if we think about this as a model, it's possible that these mutant cells can enter the brain in middle age and then essentially steward the brain or help preemptively keep the brain clean via maybe enhanced phagocytosis that could lead to reduced buildup of some of these plaques and tangles that we know are incredibly harmful as the brain ages.

(34:55):

And I want to emphasize that's a hypothesis, but that's sort of our model for how this might work.

Nicholas Weiler (35:03):

Got it. Okay. And I just want to recap really quickly here because I know we've covered a lot of ground. So we started with the observation that people with this CHIP phenomenon where certain stem cells in the blood are producing a larger and larger proportion of our blood because of certain mutations that give them a competitive advantage. Surprisingly, this seems to be a bit protective against Alzheimer's disease. And this led you down this path of trying to understand how does something that's happening in the blood affect the brain. How do we get from this expansion of certain blood cells with certain mutations to reduced risk of Alzheimer's disease? And this led to this path of realizing, "Oh, we're not seeing any blood cells in the brain because they are reprogramming themselves fundamentally to replace our microglia."

(35:55):

And even without this CHIP phenomenon, this seems to happen in everyone by the time you get to 80 or 90 years old. It's a significant fraction of your microglia, if not all of your microglia, actually were born in the bone marrow and came from the peripheral blood into the brain. And so that is just a completely different model of how the immune system interacts with the brain than we've ever had before. So this completely changes the picture of how we think of the immune system of the brain as we age. And so the question I want to ask now is why do we think this happens? When does this start and what might be some of the triggers that either allow the immune system of the body to invade the brain or is the brain requesting reinforcements from the body? Which direction does this go? Is it both? What are your thoughts on that? How are you thinking about it?

Julia Belk (36:53):

Yes, yes. So essentially what we know, and this is substantially from some very elegant mouse models, is that the number one thing that lets immune cells from the blood enter the brain in a setting without disease is space in the microglia niche. So what that means is essentially if the original yolk sac microglia are gone or are depleted, then that will trigger an influx of cells from the periphery into the brain, even in young animals. So even without any overt blood-brain barrier disruption or aging or any other probation, the cells can naturally cross right through the blood-brain barrier if there's space in the microglia niche.

(37:37):

So we think, our hypothesis, is that's what might be going on in human aging, that for whatever reason that we don't know yet, the pool of yolk sac microglia might essentially peter out. It might not be able to self-renew anymore and space might open up within the niche that then recruits cells from the outside.

Nicholas Weiler (37:59):

I heard you talking with Eric Topol on the Ground Truths podcast a couple of months ago after this paper came out about this. And you were talking a little bit about ways that maybe you could trigger this replacement of microglia intentionally. First, why would you want to do that? And second, how could it be done?

Julia Belk (38:20):

Yes, great question. So there has been interest in being able to trigger microglia replacement, especially for treating certain rare diseases where microglia are defective. So there are certain genetic diseases that individuals have where it would be very beneficial to be able to replace potentially their defective microglia with cells from outside the brain. And so different conditioning regimens have been developed to enable this. So that means essentially drug treatments or other treatments that essentially get rid of the microglia in the brain and enable this replacement. And actually one of the pioneers in this space is right here at Stanford, Marius Wernig, who we were very fortunate to collaborate with on some of our studies. And essentially, there's combinations of drugs that you can give to mice, or possibly to humans as well, to get rid of those yoke sac microglia and enable replacement. And you can also combine it, I should say for therapeutic purposes, you can combine these conditioning regimens with, for example, bone marrow transplantation that enables wild type healthy bone marrow cells to then replace these defective microglia in the brain.

Nicholas Weiler (39:35):

But this also seems like there would be a potential now that we have a sense of how this is happening. Now we have a way of getting cells into the brain. You started off talking about your interest in cancer immunotherapy, this thing that has been completely transformative for many cancers, curative in some cases for the first time. Do we now have a way of thinking about immunotherapy for the brain that we didn't have before?

Julia Belk (39:59):

Exactly. So what our work really shows is that microglia replacement happens naturally. It doesn't require these very complex and potentially invasive conditioning regimens in older individuals because microglia will naturally be kind of replenished from the peripheral immune system. So that really opens up a much broader set of opportunities potentially for immunotherapy because I kind of alluded to it, but this microglia replacement regimens that have been published, they are quite intensive. So you would never do these regimens for common diseases like Alzheimer's disease. They've really been used in very severe, rare genetic diseases. So our findings really open the possibility that maybe this pathway of microglia replacement can be harnessed more broadly for age-related brain disorders.

Nicholas Weiler (40:55):

And so you could imagine maybe, as we've said a few times, that these CHIP mutations that are protective against Alzheimer's disease are generally bad news in the body, or generally bad news for your heart, for metabolic disorders and so on. But maybe you could now imagine doing some targeted genetic modifications to those bone marrow cells so that when you get this microglia replacement, you have microglia that are programmed to be better at protecting against Alzheimer's disease. Is that sort of the ultimate dream here?

Julia Belk (41:31):

Exactly, exactly. And like you just said, we don't necessarily want to induce CHIP because of its negative impacts on some of the other tissues within the body, but by working to understand how those CHIP cells potentially mediate a protective effect on the brain, that can lead us to new targets that could be good engineering targets to install within the brain.

(41:55):

And there's others as well. So you can think about, for example, installing receptors in this microglia that make them better at clearing amyloid or tau. You could also think about installing protective genetic alleles, like for example, the APOE2 allele is one of the most validated protective targets in Alzheimer's disease. So maybe we should just install that into all of our microglia. But there's really a lot of work to be done to think about what engineering targets are going to be the best for each therapeutic context that could then harness microglia replacement to do useful things in the brain.

Nicholas Weiler (42:32):

Fantastic. Well, what are one or two of the questions that you are excited to address in the coming years? What's next for you to take us down this path that we've just been sketching?

Julia Belk (42:45):

Yeah, so there's really two sides to the kind of future direction. So firstly on the human side, there's still a lot we don't know about how this process is happening within the human brain. So for example, exactly where does it start within the brain? We've looked at a few different brain regions that's giving us the hypothesis that this is a brain-wide phenomenon, but we really don't know exactly where it might start first in the brain, which could give us clues as to what's triggering it. And then second, on the engineering side, we really need to think about what targets we want to install in these peripheral immune cells to let them mediate protective functions on the brain.

(43:28):

And then also it would be interesting to try to understand exactly how they cross the blood-brain barrier to see if we can develop some generalizable rules for how that process happens and how we can harness that therapeutically as well.

Nicholas Weiler (43:44):

Fantastic. Yeah, this whole idea of the blood-brain barrier as an impermeable and permanent boundary between the body and the brain seems like it needs a pretty wholesale reevaluation. I've heard several people talking about actually if you give cells the right keys, they can make it through that lock. And so understanding what those regulatory processes are, what is the code that determines whether it's a barrier or not. I remember hearing someone talk about, "Well, there are a lot of cells that can get in through the blood-brain barrier, but the brain actually just kills them off once they're in there.'

Julia Belk (44:18):

Oh, interesting.

Nicholas Weiler (44:18):

There are many different mechanisms that the brain uses to protect itself against the body, and those are all things that are not absolutes. They're things that are regulated that have their own biological codes. And so it's so exciting to be learning some of those secrets for the first time.

Julia Belk (44:35):

Yeah.

Nicholas Weiler (44:35):

Well, Julia, thank you so much for coming on From Our Neurons to Yours. This paper is just... It's a wild ride, a really different way of thinking about brain aging and really exciting for the future.

Julia Belk (44:45):

Yeah. Thank you so much, Nick, for having me. I've been a long time listener of the pod, and so it's great to get to chat with you here as well.

Nicholas Weiler (44:53):

Now you know all of our secrets.

Julia Belk (44:55):

Yes, exactly. Exactly.

Nicholas Weiler (44:58):

Thanks so much again to our guest, Julia Belk. She's a Hanna Gray HHMI postdoctoral scholar in the lab of Sidd Jaiswal at Stanford Medicine. To read more about this new study and the rest of this work, check out the links in the show notes.

(45:12):

If you enjoyed this episode, be sure to subscribe for more conversations from the Frontiers of Brain Science. We also love hearing from you. If you have thoughts about the show or questions about the brain you'd like to hear us discuss in a future episode, send us an email. We're at neuronspodcast@stanford.edu. You can also leave us a comment on your favorite podcast platform. While you're at it, please give us a rating and share the show with your friends. It may seem like a small thing, but it is tremendously valuable for us to be able to bring more listeners to the frontiers of neuroscience.

(45:42):

From our Neurons to Yours is produced by Michael Osborne at 14th Street Studios with sound design by Mark Bell. Our social media strategy is by Julia Diaz. Additional editing by Nathan Collins. Our logo was designed by Amy Garza. I'm Nicholas Weiler. Until next time.