APOE4, Aging & the Early TMS Path Study to Brain Clearance

This episode explores how the brain’s glymphatic system clears metabolic waste, why that drainage slows with age and APOE4 risk, and how patterned TMS may offer a breakthrough path to restoring flow and memory. Emerging evidence shows that glymphatic function is not fixed—it’s plastic—and can be selectively improved in older adults with mild cognitive impairment. We translate early but promising findings into a roadmap for future prevention.

We begin by outlining glymphatic flow as a glia-driven clearance network, then examine why APOE4 carriers show lower baseline DTI-ALPS—a proxy for reduced fluid transport. You’ll learn how researchers used a theta-burst TMS protocol with sham controls to test whether stimulation could enhance clearance. The results: selective ALPS increases and measurable memory gains, especially in APOE4 carriers.

We discuss the proposed mechanisms linking TMS to improved flow, including effects on sleep architecture, inhibitory interneurons, astrocytic channels, and meningeal lymphatic vessels. We also cover the limitations of DTI-ALPS, the absence of direct biomarkers, and what larger, multimodal trials must address next.

The episode closes with a look at personalized brain tune-ups—how noninvasive stimulation, tailored to genetic risk, may shape the future of dementia prevention.

High-volume keywords used: glymphatic system, APOE4, TMS therapy, mild cognitive impairment, brain clearance, aging brain, memory improvement, neurodegeneration

Listener Takeaways

• How glymphatic flow works and why it declines with age and APOE4
• The theta-burst TMS protocol that increased ALPS and improved memory
• Possible mechanisms linking TMS to better clearance and sleep
• Key limitations of current imaging and biomarker tools
• The emerging path to personalized, noninvasive brain prevention

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This podcast is created by Ai for educational and entertainment purposes only and does not constitute professional medical or health advice. Please talk to your healthcare team for medical advice.

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Transcript

Framing The Brain’s Cleaning Problem

SPEAKER_01 0:00

Welcome back to the deep dive, where we crack open cutting edge research so you can jump straight to the knowledge.

SPEAKER_00 0:06

Today we are talking about something that really does sound like it's straight out of science fiction.

SPEAKER_01 0:11

Aaron Powell It absolutely does. We're talking about literally cleaning your brain and uh doing it non-invasively with magnetic stimulation.

SPEAKER_00 0:19

Yeah, it's pretty wild.

SPEAKER_01 0:20

So the brain is, I mean, it's the body's hungriest organ. It's an engine running on high octane, you know, 24-7.

SPEAKER_00 0:27

Aaron Powell And like any engine, it produces exhaust, a lot of it.

SPEAKER_01 0:31

Aaron Ross Powell Exactly. It generates this massive amount of metabolic waste, especially toxic byproducts like beta-amyloid or a beta.

SPEAKER_00 0:38

Aaron Ross Powell And for decades, scientists had this fundamental puzzle on their hands.

SPEAKER_01 0:43

Aaron Powell How does the central nervous system, which is walled off by the blood-brain barrier, how does it clear out all this trash? It doesn't have the normal lymphatic system like the rest of the body.

SPEAKER_00 0:51

Aaron Powell Right. That was the big biological contradiction. And it wasn't really until around 2012 that the lymphatic system was first characterized.

SPEAKER_01 0:58

Aaron Powell And that discovery gave us the pathway. It was the anatomical answer. So the question is no longer if the brain cleans itself, but how well does it do it as we age?

The Glymphatic System 101

SPEAKER_00 1:08

Aaron Ross Powell And more importantly, could we do anything about it when that system starts to fail?

SPEAKER_01 1:12

Aaron Powell Okay, let's unpack this. We are diving into a study that shows, really for the first time in older adults with mild cognitive impairment, that we can use transcranial magnetic stimulation or TMS to physically boost this brain cleaning process.

SPEAKER_00 1:27

Aaron Powell And the findings are so incredibly specific. The real story here, the anchor for this whole deep dive, is that this stimulation it doesn't just work in general. Its effectiveness is profoundly, I mean radically different depending on one single thing.

SPEAKER_01 1:42

Aaron Powell And that's the person's genetics.

SPEAKER_00 1:44

Aaron Powell That's it. We're zeroing in on the APOE A4 gene. You probably know it as the strongest genetic risk factor for sporadic Alzheimer's disease. Right. The data suggests that this specific group, the one most genetically vulnerable, is the exact group that benefits the most from this non-invasive cleaning boost.

SPEAKER_01 2:00

Aaron Powell Which is that's just fascinating because it points to a future of really personalized preventative treatment. But okay, we need to start at the beginning. Before we talk about boosting the system, we need to get a clear picture of what the glymphatic system actually is and you know how on earth researchers measured it inside a living person.

SPEAKER_00 2:22

That's the critical first step. So the lymphatic system, it's best to think of it less as a vessel and more as a really sophisticated fluid exchange network.

SPEAKER_01 2:31

Okay.

SPEAKER_00 2:32

It's gria-dependent, which just means it relies on the brain's support cells. The way it works is uh you have cerebrospinal fluid, or CSF, that flows really rapidly into these things called perivascular spaces.

SPEAKER_01 2:44

Like little channels running alongside the arteries.

SPEAKER_00 2:46

Exactly, fluid-filled channels. And once the CSF is in there, it starts to exchange with the fluid that's actually surrounding the neurons, the interstitial fluid.

SPEAKER_01 2:53

And that's what carries the waste out.

SPEAKER_00 2:55

Yes. And this whole process is powerfully driven by these specialized support cells called astrocytes.

SPEAKER_01 3:01

So the astrocytes, the brain's support team, are basically wrapping their feet around the blood vessels and squeezing the pump.

SPEAKER_00 3:07

That's a great way to think about it. They are the control mechanism. Their little end feet are just packed with this critical hardware, specifically aquaporn 4 water channels, AQP4.

SPEAKER_01 3:19

And those channels are the key.

SPEAKER_00 3:20

Absolutely fundamental. They push the fluid transport along, and then once the waste-filled fluid gets to the skull base, it drains out through the meningeal lymphatic vessels.

SPEAKER_01 3:30

So what happens when this, I mean, this incredibly intricate plumbing system starts to fail?

SPEAKER_00 3:36

Well, that's when you get the lymphatic crisis. With age, and especially with diseases like Alzheimer's, the efficiency just plummets.

SPEAKER_01 3:43

And the clearance slows down.

SPEAKER_00 3:44

That deficient clearance lets toxic proteins like a beta start to accumulate. And the scary part is that this buildup is already happening, maybe 15 years before a person shows any clinical signs of memory loss.

SPEAKER_01 3:55

Wow. So the key would be to intervene in that window when it's just starting to slow down. But the system isn't always on, right? It changes throughout the day.

SPEAKER_00 4:03

Aaron Powell It does. It has this powerful diurnal pattern. It really maxes out during deep slow wave sleep, SWS.

SPEAKER_01 4:10

Okay.

SPEAKER_00 4:11

And that confirms the system has a high degree of plasticity. It's really responsive to our physiological state, which is exactly why the researchers thought, you know, maybe we can influence it with something like TMS.

SPEAKER_01 4:23

Aaron Powell, which brings us to the measurement problem. If this is all happening microscopically deep in the brain, how do you even track it without doing something really invasive?

Measuring Flow With DTI-ALPS

SPEAKER_00 4:32

Aaron Powell I mean, for a long time, the gold standard was incredibly invasive. It involved a lumbar puncture, injecting a contrast agent directly into the spine, and then tracking it with an MRI.

SPEAKER_01 4:41

Aaron Powell Right. Not something you can do for a 10-day study.

SPEAKER_00 4:43

Not at all. So they use this non-invasive contrast-free MRI technique. It's called diffusion tensor imaging analysis along the perivascular space.

SPEAKER_01 4:52

Aaron Powell Thankfully shortened to DTI ALPS.

SPEAKER_00 4:56

Yes, thankfully. DTI ALPS basically measures the direction and the speed of water movement in the deep white matter.

SPEAKER_01 5:03

Okay, so if I'm getting this right, DTI LPS is a proxy. You're not measuring the actual waste, like a beta, leaving the brain. You're measuring the water. It's like checking the pressure and the flow rate in the plumbing to see if it's working.

SPEAKER_00 5:14

Aaron Powell That's a perfect analogy. A higher DTI ALPS index suggests faster fluid movement, so you know better lymphatic flow. It's not the whole picture, but we know a lower ALPS index correlates with more a beta and worse cognitive scores. It's a really useful snapshot of the pipes.

SPEAKER_01 5:33

Aaron Powell Excellent. So we know what they are measuring. Let's get to the intervention itself. They use TMS, specifically this really potent patterned form called theta burst stimulation or TBS.

SPEAKER_00 5:44

Right. TBS delivers these rapid repeating bursts of magnetic pulses. Its big advantage is that you can get similar therapeutic results to conventional TMS, but in a much shorter treatment time.

SPEAKER_01 5:55

And this wasn't just a shot in the dark. There was already strong preclinical data.

SPEAKER_00 5:59

Oh yeah, very robust. In mouse models of Alzheimer's, RTMS had already been shown to physically boost fluid transport, clear out of beta, and even restore those critical AQP4 channels.

SPEAKER_01 6:09

So this study took that promising mechanism and brought it into humans. They got 36 older adults, all with mild cognitive impairment.

TMS/TBS Protocols In Humans

SPEAKER_00 6:16

And the protocol was really meticulous. It was designed to separate the effect of the real treatment from a placebo.

SPEAKER_01 6:23

Right.

SPEAKER_00 6:23

So each person went through three separate 10-day blocks, all randomized with a washout period in between: continuous TBS, intermittent TBS, and a sham, or placebo TBS.

SPEAKER_01 6:34

Aaron Powell And they measured DTI, ALPS, and face name associative memory, the FNEME score, before and after each block.

SPEAKER_00 6:42

Exactly.

SPEAKER_01 6:42

This brings us back to that genetic X factor, APOE, HO4. We know it's a huge player in Alzheimer's risk. What did the study find in the people carrying this gene?

SPEAKER_00 6:53

Aaron Powell What's so fascinating here is, well, it's the massive baseline deficit.

SPEAKER_01 6:57

Okay.

SPEAKER_00 6:57

The study identified 13 people carrying that APOED4 gene and 23 who weren't. Right. And even before any of the active stimulation, the APO fee carriers, they already showed a significantly lower DTI ALPS index.

SPEAKER_01 7:09

So their system was already slower to begin with.

SPEAKER_00 7:11

Exactly. The drain was just visibly slower in that genetically high risk group. It's this beautiful confirmation in living humans with MCI that this known genetic risk actually shows up as reduced lymphatic flow.

SPEAKER_01 7:23

Which gives a structural reason for their heightened risk. So the main event. Did turning on the magnets actually speed up the drain? And did it help their memory?

SPEAKER_00 7:32

It did, but with extreme selectivity. So when they looked at the results and they combined the two active TBS protocols since they were pretty similar, they saw a significant increase in DTI-ALPS. But this boost was profoundly moderated by their genetic status.

SPEAKER_01 7:48

So it was the APOE4 carriers who benefited?

SPEAKER_00 7:52

Yes. They showed a large, significant pre-to-post increase in their bilateral DTI-ALPS. The non-carriers, on the other hand, they showed almost no significant change at all.

SPEAKER_01 8:03

Wow. So the interventions selectively worked on the group that was starting from a lower baseline and had the highest genetic risk.

SPEAKER_00 8:09

Precisely.

SPEAKER_01 8:09

Wait, how big was this effect? I know in these kinds of human studies, a strong effect is, you know, hard to come by.

SPEAKER_00 8:14

It was unusually large. The interaction between time and APOE A4 status had an effect size, a Cohen's D of 1.71.

SPEAKER_01 8:21

1.71?

APOE4 Baseline Deficits Revealed

SPEAKER_00 8:22

Yeah. I mean, for context, in neuropsychiatry research, anything over 0.8 is considered a large effect. A 1.71 is just it's enormous. It suggests this near perfect split where one group responds dramatically and the other barely at all.

SPEAKER_01 8:38

That is a huge data point. It's not just statistical noise, it's a specific target being hit in a specific population. But was this just a physical measure? Did the increased flow actually improve brain function?

SPEAKER_00 8:50

It absolutely did. And this is the key. Within the APOE A4 carriers only, a greater increase in that DTI A L P S index correlated really strongly with larger improvements in their memory scores.

SPEAKER_01 9:02

You're kidding.

SPEAKER_00 9:03

Now the correlation was quite robust between 0.42 and 0.46.

SPEAKER_01 9:06

That closes the loop perfectly. So the TBS helps the population most in need, and that physical cleaning is directly tied to a measurable cognitive improvement.

SPEAKER_00 9:15

It provides this incredible novel evidence that glymphatic plasticity exists in humans. And then we can actually engage it to improve cognition, specifically in these vulnerable groups.

SPEAKER_01 9:23

Here's where it gets really interesting. How does this even work? How does zapping the brain with magnetism cause this physical effect? And why is it so incredibly sensitive to that one gene?

SPEAKER_00 9:35

The researchers laid out four really compelling interconnected mechanisms. The first one, and maybe the most straightforward, is sleep augmentation.

SPEAKER_01 9:43

Okay.

SPEAKER_00 9:44

Since we know glymphatic function is maximized during that deep slow wave sleep, TBS might simply be working by increasing SWS activity. And we've seen that before in other studies with older adults.

SPEAKER_01 9:55

And that makes perfect sense because previous work shows the age-related loss of that deep sleep is way faster and ho carriers. They have the most to gain.

SPEAKER_00 10:04

Exactly. Their system is already struggling because that natural sleep boost is diminished. And that leads to the second idea, which focuses on neural control.

SPEAKER_01 10:13

Right.

Selective Gains And Effect Size

SPEAKER_00 10:13

The synchronized neural activity you need for deep sleep and for good clearance. It's facilitated by inhibitory cells called gabergic interneurons.

SPEAKER_01 10:21

This is where that great quote comes from, right? Neurons that fire together, shower together.

SPEAKER_00 10:26

That's the one. So you need that synchronized firing to open up the spaces and get the fluid moving.

SPEAKER_01 10:31

And the inner neurons control that.

SPEAKER_00 10:33

Yes. And these inner neurons are particularly sensitive to TBS. And critically, they're also known to be dysfunctional, specifically in APOE E4 carriers.

SPEAKER_01 10:42

So TBS might be fixing the broken conductor of the brain's symphony.

SPEAKER_00 10:47

That's a great way to put it. It might be restoring the function of these vulnerable inhibitory cells, which then boosts the synchronized sleep activity needed for clearance.

SPEAKER_01 10:56

Okay, so that handles the electrical side, but what about the physical structures, the pump itself?

SPEAKER_00 11:00

Aaron Powell That brings us to the third mechanism, astrocytic remodeling. Remember, the astrocytes manage the pump with those AQP4 water channels. Right. Well, the APOEO4 gene actually promotes the creation of a toxic type of astrocyte called the A1 phenotype. And when astrocytes turn into this A1 type, you get AQP4 mislocalization.

SPEAKER_01 11:21

Meaning the essential water channels move away from where they're supposed to be, right by the vessels.

SPEAKER_00 11:26

It's exactly right. AT4 fundamentally messes up the physical structure of the pump. But in animal models, TBS has been shown to encourage a shift away from that toxic A1 type toward the protective A2 type.

SPEAKER_01 11:38

So it puts the plumbing back in the right place.

SPEAKER_00 11:40

It seems to restore that AQP4 polarization. It's a really potent cellular explanation for both the low baseline incurriors and their huge response to the stimulation.

SPEAKER_01 11:51

Okay, we fixed the synchrony and the pump, but if the final drain is backed up, it doesn't matter. What about the pathways that lead out of the brain?

Flow Gains Link To Memory Gains

SPEAKER_00 11:58

That's the fourth mechanism, the vascular pathways. TMS has been shown to upregulate something called VGAFC.

SPEAKER_01 12:04

And why is that important?

SPEAKER_00 12:06

Because VEGSC dilates the meningeal lymphatic vessels, the MLVs. That's the final exit ramp for all the waste. And since we know those MLVs are also progressively dysfunctional in AP4 carriers, TBS might be opening up the final drainage pipes that are so often constricted in this population.

SPEAKER_01 12:22

That is just an incredibly complete biological story. It goes from water channels to sleep cycles to the final drain pipes. It makes that huge effect size seem less random and much more targeted.

SPEAKER_00 12:33

It really does.

SPEAKER_01 12:34

But as with any preliminary human study like this, we have to be critical. We have to look at the limitations.

SPEAKER_00 12:40

We must maintain scientific rigor, absolutely. First, we have to remember the main methodological limitation. DTI ALPS is a proxy.

SPEAKER_01 12:49

Right.

SPEAKER_00 12:50

It's constrained to deep white matter. It doesn't directly measure whole brain clearance. And it's not measuring big toxic molecules like a beta. We are inferring clearance from fluid flow.

SPEAKER_01 13:01

And what other data was missing? What would make these results truly unassailable?

SPEAKER_00 13:06

Well, there were three critical missing pieces. One, no direct sleep data, no EEG to measure SWS architecture. So we're hypothesizing that TBS is boosting deep sleep, but we can't empirically confirm that link.

SPEAKER_01 13:18

Okay, that's a big one.

How TBS Might Work

SPEAKER_00 13:19

Two, they didn't have direct Alzheimer's biomarkers. So no A beta or astrocytic markers like GFA measured in the blood or CSF. Without that, we can't directly connect the DTIALPS increase to a physical reduction in disease pathology.

SPEAKER_01 13:33

We have a proxy linked to a memory score, not a direct confirmation of toxin removal.

SPEAKER_00 13:38

Exactly. And third, you know, the sample size was modest, just 36 people. And they were pretty homogeneous, mostly female, right-handed, highly educated. That limits how much we can generalize from this.

SPEAKER_01 13:50

So future work is pretty clearly defined.

SPEAKER_00 13:52

Very clearly. This study needs to be replicated in much larger, more diverse populations. And those follow-up studies absolutely have to include biofluid measures and more comprehensive cognitive testing to really validate the DTI ALPS proxy and confirm these mechanisms.

SPEAKER_01 14:08

So what does this all mean?

SPEAKER_00 14:10

I think this deep dive gives us really compelling evidence that non-invasive neuromodulation TBS can physically target the brain's waste clearance system. And crucially, the people who showed the biggest, most functional benefit were those with the highest genetic risk.

SPEAKER_01 14:24

The APOE4 carriers.

SPEAKER_00 14:25

The ones who already had that measurable baseline deficit. This is just novel evidence that the lymphatic system has plasticity in humans, right? Where we need therapeutic tools the most.

SPEAKER_01 14:34

It feels like we've moved beyond just treating the symptoms of cognitive decline. We're now finding evidence that we might be able to physically tune the brain's own cleaning cycle to improve cognition.

SPEAKER_00 14:45

It suggests the damage from the APOEE fees foods might not be some immutable destiny. It could be a structural deficit that we can temporarily boost or even repair using targeted energy.

SPEAKER_01 14:56

So if we can non-invasively manipulate the brain's cleaning cycle with electrical stimulation based on someone's genetic risk and their measured metabolic deficits, does this suggest a future where targeted, personalized brain stimulation becomes a core preventative measure, like a metabolic tune up for your brain to fight off aid related cognitive decline?

SPEAKER_00 15:16

That is definitely something for you to mull over until our next deep dive.