High-resolution cryo-EM structures of human SV2A reveal that orthosteric ligands induce an occluded MFS conformation and a secondary allosteric pocket modulates ligand binding
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. Okay, so let's dive into what has been for a long time, a pretty profound mystery in neuroscience.
0:15Epilepsy affects 1000000s of people all over the world. And for decades, we've had these crucial drugs, like Leviterositum, they're lifesavers. Absolutely. And we've known what they target, a protein called SV2A, right, in the brain synaptic machinery.
0:30It's like, uh, we knew the building address for the drug. But the actual architecture of that building, it was a complete black box. I mean, earlier structural studies showed a levitorosidum binding to SV2A, but the protein didn't really seem to change shape.
0:44sat there. It just sat there, and that was the huge contradiction. How can a drug have such a powerful effect if it isn't forcing some kind of major molecular change in its target? That feels deeply unsatisfying, right?
0:55You'd think a drug for something like epilepsy, which is all about hyperactive signaling, would need to be a physical intervention. It should be jamming the mechanism. It has to be. It had to be forcing the protein into some kind of sealed immobile state, but we just, we couldn't see it.
1:10And that's why this is so exciting. Exactly. This new high resolution study we're digging into finally captures this hidden confirmational dance. It shows the drug doesn't just bind. It physically locks the protein down.
1:23And it goes even further. This work has uncovered a, uh, a really sophisticated allosteric modulation mechanism, a hidden secondary pocket. A physical cap, basically. It provides a whole new roadmap for designing the next generation of anti-seizure medications or ASMs.
1:40Before we get into those incredible details. We should really give a shout out to the team that pulled this off. Oh, absolutely. This is a massive effort. We're talking about the work of Shabarish Padafla and a huge collaboration.
1:50It spans St. Jude Children's Research Hospital, the National University of Singapore, the Rockefeller University and UT Southwestern Medical Center. And you can't overstate the technical achievement here.
2:01Getting sub 3 angstrom resolution for a tricky membrane protein, like SE2A, that's monumental. It really pushes our entire understanding forward, not just for this one protein, but for the whole family it's in.
2:15The major facilitator's super family. That's the one. So let's frame this protein SV2A. Where does it fit in the grand scheme of things? Okay, so SV2A is a key member of the major facilitator super family, the MFS.
2:27And this is the biggest group of what are called salute carrier or SLC transporters? And MFS transporters are, they're everywhere, and they're central to life, right? I think they're the 2nd largest family of human membrane proteins.
2:39Second only to G protein coupled receptors. They're basically the cells gatekeepers, moving nutrients, waste, signals, all across the membrane. And to do that, they have to constantly be changing shape.
2:52Which is the crucial context here. It is. In the brain, SV2A is wildly abundant. You find up to a dozen copies in almost every synaptic vesicle in every neuron. So it's got to be vital for neurotransmission.
3:03You'd think so? But here's the kicker. We still don't know for sure what its natural cargo is. We see the carrier, but we don't know what it's carrying. Even with that mystery. It's clinical importance is just, it's undeniable.
3:18SV2A is a huge deal in neuroimaging. A huge deal. A ligin called UCBJ, which we'll get to in a minute, is used as a PE tracer. It literally lights up synaptic density in the brain, which is essential for tracking diseases like Alzheimer's and Parkinson's.
3:35And then, of course, the direct link to epilepsy. The genetic evidence is just overwhelming. If you knock out the SV2A gene and mice. They have severe fatal seizures. And in humans, certain mutations are linked directly to intractable epilepsy.
3:48Making it the perfect target. The perfect high stakes target for all these race tam-based drugs, especially Levitarazidum. So just to sum up the problem they faced. You have this incredibly important dynamic protein that has to change shape to work.
4:00But every time we took a picture of it with the drug bound, it looked static, it looked frozen, it contradicted how we thought this whole family of transporters was supposed to behave. Exactly. And that's what this team set out to fix.
4:11To resolve that discrepancy and to hunt for any hidden, you know, allisteric regulation, a secondary switch that no one had ever seen before for SV2A. So how did they do it? How did they finally get this protein to show its moves?
4:27The main technique was cryoelectron, microscopy cryo EM. That's what got them the sub3 Angstrom resolution. The detail you need to see exactly where the atoms are. But the real game changer was how they prepared the sample.
4:40That was the key to seeing the flexibility. That was the absolute key. Look, if you want to see how a car drives, you can't weld it to a metal stand. Right. And the standard method, using what are called detergent macelles, is kind of like that.
4:51It's a soapy bubble that can sometimes restrict a protein's natural movement. Which is probably why those earlier studies came up empty. It's very likely So what this team did was they reconstituted the protein into something called supposed nanoparticles.
5:03Okay. Think of it as a more natural, flexible sort of fat-based wrapper. It gave the protein room to breathe, room to move like it would in his native environment. That makes so much sense, you trick it into acting normally, so you can finally catch it in the act of opening and closing.
5:19And they didn't just rely on the pictures. They backed it all up with functional assays, radiolic and binding to correlate the structures they were seeing with actual pharmacological function. They tie the image directly to the activity.
5:32Which brings us to the core finding, the solution. What actually happens when the drug binds? We finally saw it. Lygand induced declusion. So the apostate. That's the protein with no drug, is an alumin open state.
5:47It's open towards the inside of the vesicle, ready to go. But when you add leviterositum. The protein shifts dramatically. It snaps shut into an occluded confirmation. This had never been seen before for SV2A, but it is the absolute hallmark of an MFS transporter caught in the middle of its cycle.
6:03So the drug isn't just a gentle tap. It's a wedge that forces the door shut and holds it there. That's a great way to put it. And what's the mechanism? What moves? You see this large inward movement of one of the protein's helices, transmembrane helix one or TM one.
6:16This whole segment shifts. And that causes a key residue, phenolanine 188. T 180. Right. to swing in and act like a seal. It's like a plug that completely collapses that central cavity. There it is. The mechanism of action.
6:29The drug works by locking the transporter in a sealed non-functional state. Precisely. Okay, now let's compare the 2 drugs that bind this main site. the orthosteric site. You have Levituracetam, the classic drug.
6:43And then the PET treacer, UCBJ. And UCBJ binds to the same basic spot, but its affinity is. It's off the charts. It binds nearly a 1000 times stronger than Lovitaracidum. A 1000 times. Why such a massive difference?
6:57It really just comes down to the chemistry. UCBGA has these bulkier chemical groups, these extra bits that stick out and can engage with additional residues deeper inside the pocket. So it has more anchor points.
7:07Exactly. More anchor is giving it that huge boost in affinity. Okay, but hang on. This feels a little counterintuitive. If you see BJ binds so much more tightly, shouldn't it create an even tighter seal, a more complete occlusion?
7:19Ah, and that's where it gets really elegant and it shows you the nuance of drug design. Levitoreasitum, the smaller drug, causes complete occlusion. It totally seals the cavity shut. And the stronger binder.
7:31The bulkier UCBJ. It only causes partial occlusion. That's fascinating. So it's own bulk gets in the way of the protein closing as tightly as it could. seems so. It's anchored in there so tightly, but its own structure sort of props the door open a tiny bit.
7:45It proves that it's not just about binding strength. It's about the exact shape you force the protein into. The geometry. The geometry. And that realization leads us right to the biggest discovery here, the Alisteric site.
7:58The hidden secondary switch. Tell us about this other compound, UCB 124-4283. Right. So this compound was known to be a potentiator. It boosts the binding of other SV2A legends, and the structure showed it binds to a completely separate site, about 13 angstroms away from that main pocket.
8:13So it's not competing. It's helping from a distance. What does it actually do? It acts like a kinetic cap. The functional data showed it dramatically slows down how quickly the main drug falls off the protein.
8:25Because our traps sit there. It traps it. They measure the dissociation half-life for UCBJ. And with this Alisteric modulator present, that time more than doubled. It jumped from 2 minutes to over 5 minutes.
8:37Wow, so it's not about binding tighter initially, but about extending its stay. It keeps the original drug on target, working for longer by physically blocking its escape. And you can see it in the structure.
8:48It sits right on top of the main site, physically holding that occluded state in place and just hysterically hindering the main ligen from getting out. It's holding the door shut from the outside. Which brings us to this other experimental drug, pad 7 all.
9:02The puzzle with that one was that it bound with super high affinity, but the potentiator, that kinetic cap, it couldn't boost its effect at all. Yeah, and no one knew why. But the structure solves it instantly.
9:14Instantly. It's one of those beautiful aha moments. Pat Sevenel is a dual binder. One molecule hits both spots. Yes. A single molecule. It's just large enough that one end slots into the main orthosteric site, and the other end reaches up and occupies the new Alasteric site all at once.
9:31So, it brings its own kinetic cap. It brings its own cap. That's why adding another one had no effect. The space was already taken. It's perfect proof of direct competition at that secondary site. The implications here feel massive and way beyond just epilepsy.
9:46What does this mean for for the whole field of MFS transporters? Oh, first, it puts arrest any idea that SV2A is some kind of weird static exception. It is a fully dynamic MFS transporter, and these drugs work by interrupting its natural cycle.
10:01And for anyone designing drugs, discovering this allosteric site must feel like finding a whole new continent. It's a new regulatory layer, for sure. And maybe the most compelling part is what they call conditional engagement.
10:12Meaning the allosteric binder, UCB 1244283. It didn't bind well on its own. It needed the primary drug, like liviturathidum, to already be sitting in the main pocket. Okay, so the secondary pocket only really forms or becomes stable after the 1st drug is in place.
10:28Exactly. And think what that means. You could design a modulator that is functionally inert that does nothing, unless the primary medication is already on board. That gives you incredible specificity. It's like a drug with a safety switch.
10:42It only engages when and where it's supposed to. That could cut down on so many off target effects. And there's even a roadmap for selectivity between different protein isoforms. They looked at the allosteric site in SV2A versus its cousin SQU to B.
10:57Then they're different. They are. In SV2A, a key residue in that pocket is a Lucine. But in SVDB, the same spot has a bigger amino acid, a glutamine. A small change, but a big difference in the space available.
11:10A huge difference, that bigger glutamine in SV to B might physically block a modulator from ever binding, and that gives you a structural blueprint for designing drugs that only hit SV2A, avoiding any side effects from hitting SV2B or other isoforms.
11:23So if we boil this whole deep dive down. We really have 2 massive takeaways. First, the core mechanism for these anti-seizure drugs is physically forcing the SV2A protein into a sealed lockdown state. And second, there's this previously hidden allasteric pocket that works as a conditional kinetic cap, which can dramatically extend the time that primary drug stays on target and active.
11:47It changes how we should think about efficacy. It's not just about binding strength anymore. It's about binding duration. It really is. And give that MFS transporters are the 2nd largest family of human membrane proteins, and so many are targets for drugs.
12:00It leads you with a pretty provocative question. Could these conditional Al Sarah caps be the key? Could this be a universal principle we can use to make better, more specific drugs for dozens of diseases?
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