0:00Welcome to Base by Base, the paper cast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. Imagine for a moment that you live near this dense, beautiful forest, and a small brush fire breaks out.
0:13Right, which happens all the time. Exactly. So you call the forestry service, and they send in a controlled burn unit. And, you know, their job is to carefully manage the flames, clear out the deadwood and essentially protect the old growth trees.
0:27Yeah, you'd be pretty relieved to see them show up. Oh, incredibly relieved. But midway through the operation, something goes horribly wrong. The crew suddenly drops all their specialized tools, brings in heavy explosives and just starts indiscriminately blasting the forest apart.
0:43Oh wow. Yeah, they end up destroying the very ecosystem they were sent to save. And, well, in biology, we have a very similar, a very real mystery happening right inside the human brain. We really do. We are talking about the brain's own built in security system.
0:57These are a type of immune cell called microglia. And for decades, I mean, scientists viewed them primarily as the defenders of our neural forest. Right, the good guys. Exactly. But the truth is much more complicated.
1:11When confronted with Alzheimer's disease, these immune cells become entirely unpredictable actors. They really do. a massive problem. Sometimes they protect the brain, acting as this crucial defense force.
1:22But other times they just flip a switch, go into overdrive, and accidentally destroy the neural tissue they are supposed to protect. So the question that has been haunting neuroscientists for years is, how can we predict what these cells will do?
1:34And more importantly, how could we possibly tip the balance to halt Alzheimer's in its tracks? It really is a profound biological paradox. If you think about it from an evolutionary perspective, having a defense mechanism that is just as capable of driving toxic damage as it is of, you know, clearing away danger.
1:52It seems like a massive design flaw. Yeah, a total glitch in the system. Right. So understanding the specific molecular levers that control that behavior. Like the mechanisms that dictate whether a cell is going to act as a protector or destroyer.
2:07That is arguably one of the most critical challenges in modern neuroscience right now. Today we celebrate the work of researchers from the Icon School of Medicine at Mount Sinai, the Max Planck Institute for Biology of Aging, Washington University, and UMass Chan Medical School, who have advanced our understanding of how microglia function in Alzheimer's disease.
2:27Yeah, and to really grasp the magnitude of what this research team uncovered. We kind of have to look closely at the battlefield of Alzheimer's disease itself. Let's set the stage for this deep dive. Right.
2:38So if you were listening to this, you might already know that the primary hallmark of Alzheimer's is the buildup of amyloid plaques. These are the sticky clumps, right? Exactly. They're these misfolded clumps of protein that just accumulate between nerve cells.
2:51And when these plaques start forming, the microglia, which are the innate immune cells, basically the brain's 1st responders, they rush to the scene. Like the forestry crew. Exactly like the crew. They physically migrate through the brain tissue and just gather around these amyloid plaques.
3:06So they are swarming the disaster site, but why do they sometimes help and, you know, sometimes hurt? Because to stick with that force fire analogy, a controlled burn requires intense precision. Com precision, yeah.
3:20If the microglia just start releasing inflammatory chemicals indiscriminately. I mean, they are going to cause massive collateral damage to the surrounding neurons, just like the city's emergency services, bringing in heavy machinery that wrecks the whole neighborhood.
3:35And that collateral damage is exactly what drives the late stages of Alzheimer's. Historically, you know, the field has struggled immensely with this paradox right at the plaque site. Because in some instances, these microglia do their job perfectly.
3:49Like, clean it up. Right. They physically engulf and clear away the amyloid beta. And they even create this tight, protective barrier that shields the surrounding delicate neurons from toxicity. So they're highly neuroprotective.
4:00But then something changes. Yeah, as the disease progresses. These exact same microglia often become dysfunctional. They stop clearing the plaques, and instead they just release a storm of inflammatory cytokines.
4:12The explosive. Yes, the explosives. This drives massive toxic neuro-inflammation, which actually accelerates the destruction of the synaxes and the neurons themselves. Okay, let's unpack this because what exactly flips the switch between these protective and harmful states?
4:29I mean, if you have a cell that can either save the brain or completely destroy it, finding the mechanism that controls that choice, that feels like a global holy grail for Alzheimer's therapy. Oh it absolutely is.
4:41And the researchers didn't just start guessing here. They actually looked at human genetic data for clues first. Okay, where did that point though? Well, that data pointed them toward a very specific protein called PU.one.
4:52Now, PU point one is what we call a pioneer transcription factor. Wait, let's make sure we define that for the listener. Because a transcription factor isn't just, you know, an ordinary protein floating around.
5:01No, far from it. Think of the DNA inside a cell as a massive 1000s of pages long instruction manual. But this manual is usually closed tight, physically schooled around these proteins called his stones, making it unreadable.
5:16A pioneer transcription factor like PU .one is uniquely powerful. It's one of the rare molecules that can physically pry open that dense chromatin. So it opens the manual to a specific page. Exactly. It opens it up so the rest of the cells machinery can actually read the genes and follow the instructions.
5:33It essentially governs the entire identity and behavior of these immune cells. So PU point one is the master architect. It dictates whether the microglia are gonna build protective shields or, you know, weapons of mass destruction.
5:46That is a great way to visualize it. And here is the crucial genetic clue they found from the human data. People who naturally carry a genetic variant that causes them to have slightly lower levels of PU .one expression in their immune cells, they actually show a delayed onset of Alzheimer's disease.
6:02Wait, really? Yeah, and not only that, but they show a reduced severity of the condition overall. That seems counterintuitive. I mean, if P.one is the master architect of the immune response, you would think having less of it would weaken the brain's defenses.
6:15But having less PU .one is actually protective. It is, yeah. The human genetics strongly suggested that dialing down PU .one keeps these myloid cells in a protective state. But, you know, human genetics only gives you a correlation.
6:29Right, it doesn't tell you the why. Exactly. To understand how to actually control the brain's security system. We need to understand the physical mechanism at play. We have to look closely at the cells actively fighting the disease inside the brain to see exactly how PU .one is pulling the strings.
6:46Which means we need to get into the methodology of this deep dive. Because moving from like a broad genetic correlation in humans to tracking individual microscopic cells inside a living brain that is an enormous leap.
6:58Oh, it requires a massive technological effort. To figure this out, the researchers utilized a sparelized Alzheimer's model known as 5 by FAD mice. Okay. them special? Well, these are mice genetically engineered with 5 distinct mutations that cause them to rapidly develop severe amylade plaques, which closely mimics human Alzheimer's pathology.
7:18And importantly, they study these mice alongside actual postmortem human brain tissues from Alzheimer's patients. That's huge. Yeah, they wanted to ensure their findings weren't just some quirk of mouse biology.
7:32I always appreciate when studies bridge that gap between animal models and human tissue, but how do you actually see what a master architect like PU .one is doing inside a single cell. I mean, they're tiny.
7:44They had to layer 3 heavy hitting, cutting edge technologies. The 1st one is single cell and single nucleus RNA sequencing. Oh, I am familiar with this one. If the DNA is the master instruction manual.
7:55The RNA is basically the photocopy of the specific page the cell is reading at that exact moment. Perfect analogy. So by sequencing the RNA, you can look at 1000s of individual microglia and read the exact genetic instructions they are producing right then and there.
8:08Correct. It reveals the exact molecular state of each individual cell. But uh, RNA sequencing has a major flaw when you are studying the brain. What's up? To do it, you have to break down the tissue. You basically have to put the brain tissue into a cellular blender to separate the cells out.
8:26Oh wow. Yeah. And when you do that, you lose all spatial context. You know what the cells are doing, but you have absolutely no idea where they were sitting in the brain. It's like having a list of every phone call made in a city, but no map showing where the callers were standing.
8:40That's a huge problem if we are trying to see how these cells react to amyloid plaques. I mean, you need to know if the cell is touching the plaque or miles away from it. Which brings us to the 2nd technology.
8:51They used a spatial transcriptomics tool called Murfish. Murfish? Yeah, it stands for a multiplexed error robust fluorescence in situ hybridization. That is a mouthful. It really is. But what Murphish does is beautifully solve that geographical problem.
9:05It allows researchers to look at an intact slice of brain tissue and use fluorescent probes to map exactly where these specific RNA instructions are located relative to the amyloid plaques. So Murphish preserves the architecture.
9:20We essentially get the map back. Yes, but there is still one more blind spot. Just because a cell photocopies an RNA instruction doesn't mean it actually follows through and builds the final protein. Right.
9:30Sometimes the RNA just degrades. Exactly. So the research has used a 3rd technology called TRE, or translating ribosome affinity purification. What TRI does is it essentially lists researchers intercept the RNA, right as it's being actively translated into a protein by the cell's ribosomes.
9:47Wow. Yeah, it proves that the genetic instructions are actually being executed. So, what does this all mean? If I'm looking at this like a detective's toolkit, the single cell RNA sequencing is like reading a suspect's diary to know what they are planning.
10:02Murfish is the GPS tracker, showing us exactly where they are standing at the crime scene. Like, are they right next to the amyloid plaque or on the other side of the brain? And T wrap is the wiretap, giving us definitive proof that they are actually carrying out the orders and building the proteins.
10:19What's fascinating here is how combining that multi-layered approach allowed the researchers to isolate and compare the microglia sitting right next to the plaques versus those situated farther away. Because geography matters.
10:31Geography, it turns out, dictates everything for these cells. When they layered the Murphish GPS data over the RNA data, the mystery of the switch finally started to make sense. Okay, tell me what they saw at the crime scene.
10:43Did the cells touching the plaques look different? Completely different. They discovered a very specific, highly localized subset of what we can basically call peacekeeper cells right at the surface of the amyloid plaques.
10:53Yeah, the data show that when microglia physically come into contact with the amyloid plaques, a specific subset of them drops their PU .one levels significantly. They transition into a state, the researchers termed PU .1 low.
11:07So the physical contact with the toxic plaque is what actually triggers the cell to turn down its own master architect. Yes. But the big reveal is what happens when that PU .one dial is turned down. Lowering PU .one unlocks a completely unexpected genetic program.
11:24What kind of program? The cells suddenly start expressing immuniregulatory lymphoid receptor proteins, and the absolute star of the show here is a protein called CD 28. Okay, CD 28. You mentioned that with a lot of weight, but for a listener who isn't an immunologist, why is finding CD 28 on these microoglia such a shock?
11:43Because according to classical textbook biology, CD28 has absolutely no business being there. Let's break down the immune system for a second. You have the innate immune system, which is the general, primitive, 1st responder defense.
11:55That's what microchlia are. They are the local beat cops. Then you have the adaptive immune system. This is the highly specialized targeted defense force made up of T cells and B cells. CD 28 is famously known as a surface receptor that is absolutely critical for activating T cells.
12:11It is a cornerstone of the adaptive immune system. Yet here it is, being actively expressed and used by microglia innate immune cells deep inside the brain. Here's where it gets really interesting. It's like the local beat cops, the microblia, suddenly showing up to a riot, carrying highly specialized special ops gear that they somehow just borrowed from the military.
12:32That's exactly what it's like. They are using a T cell tool to do a job in the brain. But wait, you said CD 28 is an activator for T cells? If the goal is to calm down the inflammation in the brain, why would the Peacekeeper cells use an activator?
12:45That is the brilliant biological twist here? In TCLs, CD 28X as an accelerator, driving the immune response forward. But, you know, biology is all about context. Right. When these PU .one low microglia express CD 28 in the brain, they aren't using it to accelerate inflammation, they're using it as a vital piece of a suppressive shield.
13:04A shield? Yeah, it helps them form a tight protective barrier around the plaque that traps the toxicity and cools down the surrounding neuroinflammation. They took a weapon and repurposed it into a shield.
13:15That is incredible. But how do we know for sure that CD 28 is the key to this shield? Do they like test what happens if you take it away? They did. They RAM the ultimate proof of concept experiment. They went into the Alzheimer's mice, and genetically deleted the CD 28 receptors specifically, and only in the microbia.
13:33They left the rest of the immune system completely alone. Oh, wow. So they confiscated the special ops gear from the local cops. What happened to the brain? The shield completely collapsed. completely.
13:43Yeah, without CD 28, the microlia lost their ability to act as peacekeepers. Instead of safely containing the plaques, the cells defaulted into a broad, highly inflammatory state across the whole brain.
13:54Back to the explosive. Exactly. They started releasing those toxic cytokines we talked about earlier. And fundamentally, removing CD 28 significantly increased the overall amyloid plaque load in the brains of these mice.
14:06The disease pathology rapidly worsen. Wow. So by removing just that one borrowed receptor, the microglia defaulted back to that destructive bulldozer state, it definitively proves that these PU part one low CD 28 expressing cells are operating as a specialized suppressive force.
14:26They absolutely need that T cell tool to mitigate the progression of the disease. They do, and the implications of that are just staggering. Knowing that these peacekeepers microglia exist and knowing that they rely on lymphoid receptors like CD 28 to function, it opens up an entirely new playbook for how we might treat Alzheimer's disease in the future.
14:46Because right now, treating Alzheimer's is incredibly difficult. Most drugs and development try to attack and clear the amyloid plaques directly, which, you know, has had very mixed results clinically.
14:56But this research suggests a totally different angle. We could instead target the brain's immune system directly. Absolutely. This discovery points directly toward immunotherapy for the brain. If you think about what has happened in cancer treatment over the last decade, it provides a perfect parallel.
15:11Well, immune checkpoint blockades, absolutely revolutionized oncology. Cancer cells are incredibly tricky, right? They express proteins that bind to receptors on immune cells and essentially turn the immune system off.
15:23So researchers develop revolutionary drugs that target these specific receptors, like PD1 and PDL1, to release the brakes, essentially waking the immune system back up to attack the tumor. Right. That won the Nobel Prize, it fundamentally changed how we view cancer treatment.
15:39But how does that connect back to Alzheimers? Well, what the researchers found in this study is that these protective PU .1 low microglia aren't just expressing CD28. They're also expressing other famous immune checkpoints, including those exact same targets, PD1 and PDL1.
15:55No way. Yes, it is the exact same specialized machinery. This implies that just as we revolutionize cancer treatment by manipulating these receptors to unleash the immune system, we might be able to target CD 28, or PD1, to promote these protective suppressive microglial functions in Alzheimer's patients.
16:12We could potentially use drugs to artificially lock the microglia into that peacekeeper state, forcing them to build shields rather than causing all that collateral damage. That is an incredibly hopeful horizon.
16:24But I do have to push back a little here because we are making a massive leap. We are talking about mice specifically, 5 by FAD mice, engineered to get plaques at an unnatural rate, and we are talking about cancer drugs that we know can have intense, sometimes dangerous side effects in the body, like triggering severe autoimmune responses.
16:45Can we really just copy paste cancer immunotherapy strategies directly into the human brain? I mean, the brain is a tightly regulated, incredibly delicate environment. This raises an important question, and it is the exact right nuance to bring to this discussion.
17:00The researchers are very transparent about the limitations of their study. The 5FAD mouse model is an aggressive model of amyloid buildup. It's excellent for studying plaques, but it does not capture every single nuance of sporadic human Alzheimer's disease.
17:15Right, which takes decades to develop. Exactly. It develops slowly over decades and involves other toxic factors, like towel protein tangles, which just weren't the focus here. Yeah, mice are not just tiny humans.
17:26We've cured Alzheimer's and mice countless times only for the drugs to fail in human trials. That is a very fair critique of the field. However, it's crucial to remember that the researchers did not stop at mice here.
17:39They validated these findings in human tissue. They did. Yes, they looked at postmortem, frontal cortex tissues from individuals who had Alzheimer's, and they confirmed the presence of these exact same PU .1 low, lymphoid expressing microglia right next to the human plaques.
17:56So the fundamental biology absolutely translates. But your point about the safety of immunotherapy. That is the key hurdle moving forward. Right, because if you stimulate CD 28 systemic, like, if you just put a drug into a patient's bloodstream, you risk triggering unchecked inflammation or severe autoimmune reactions everywhere else in their body.
18:13Exactly. The next logical steps for this entire field involve rigorous testing to figure out how to safely manipulate these lymphoid co-stimulatory and co-inhibitory receptors specifically in the brain.
18:27The goal will be precision. Precision delivery. Delivering the therapy, so it only tips the balance of the microglia at the exact site of the plaques without overactivating the rest of the immune system.
18:39It's like trying to radio the special ops gear only to the cops standing exactly at the riot line without accidentally arming every single civilian in the entire city. It requires a delivery system that is practically science fiction right now.
18:53It is an immense challenge drug delivery and highly specific targeting for sure, but you cannot build a targeting system until you know what the target actually is. And that is what this paper provides.
19:03The foundational map of what to target in the Alzheimer's brain has now been fundamentally altered. It changes the whole paradigm from kill the plaque to equip the peacekeepers. We are no longer just looking at the debris.
19:14We are looking at the heavy machinery that the brain is using to respond to it. If we connect this to the bigger picture. The protective function of the brain's immune cells against Alzheimer's disease relies entirely on lowering a master transcription factor called PU .one, which unexpectedly allows these cells to adopt lymphoid receptors like CD 28.
19:33These specialize microglia act as vital suppressors of neuroinflammation, potentially opening the door to revolutionary brain immunotherapies that mirror the breakthroughs we've seen in cancer. It makes you wonder how many other secrets the innate immune system is hiding.
19:49If you are listening to this, think about your own biology. What does this mean for the future of treating not just Alzheimer's, but other neurodegenerative diseases where the brain's immune system loses its way?
19:59If the brain can borrow specialized tools from T cells just by flipping a genetic switch, what other biological rules are waiting to be rewritten? It's fascinating thought. This episode was based on an open access article under the CCBY 4.0 license.
20:14You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow, or subscribe in your podcast app, and leave a 5 star rating. If you'd like to support our work, use the donation link in the description.
20:27Now stay with us for an original track created especially for this episode and inspired by the article you've just heard about. Thanks for listening and join us next time as we explore more science base by base.
