A Better Model
Using data from the experiments, Webb and Oztekin and their doctoral students simulate how vWF behaves or changes conformation in different flow patterns with unprecedented quantitative accuracy. Oztekin is a fluid mechanist who specializes in computational fluid dynamics. He works to understand the blood flow and how vWF reacts to and interacts with it. Webb does atomistic and molecular modeling. The group is using coarse-grain molecular modeling as they seek to answer the question: “How simple can we make this molecular model to get the correct physical behavior that we’re trying to probe but still be able to actually study it computationally?”
Using parameters from experimental results as well as from previous simulations, doctoral student Chuqiao Dong runs explicit atom simulations to get a reasonable model of the A2 domain. Dong’s explicit atom work, says Webb, has inspired the team’s coarse-grain model work.
“[The team is] sort of taking inspiration from Chuqiao [Dong] to then say, ‘Alright, we want to go back to this simpler model, but we want to make it a little less simple based on things we observed in the atomic scale simulations,’” he explains.
Whereas other research teams have utilized a single bead to represent a vWF monomer and its domains, the Lehigh team’s base chain model consists of two beads (the A1 and A3 domains) connected by one spring (the A2 domain) representing one monomer of vWF.
Says doctoral student Michael Morabito: “All of these beads are able to interact with each other through various forces, and they’re also able to interact with each other through the solvent [blood]. So on this level, something that’s very important in our research is understanding what are called hydrodynamic interactions: If one bead moves in the flow field, that displaces the solvent. And when that solvent is displaced, it moves other beads. This one moves, causes this one to move, only through the solvent … The way that all these beads interact is in a very complex nature, which allows us to model these unfolding events and these unraveling events.”
Representing the vWF monomer as one bead as others have previously, says doctoral student Sagar Kania, doesn’t allow for the study of how A2 unfolds because that single bead represents the entire vWF monomer and does not distinguish A2 as a sub-monomer entity. This approach misses “an important piece of the puzzle.”
The Lehigh team, says Kania, is making their model more complicated for more accurate observation of the A2 domain behavior.
“In the old model, they were considering the force [of blood flow] on the bead, but not considering force on the spring [A2],” he says. “The model was such that we couldn’t consider the force on the spring which represents the monomeric entity [A2 domain] in the model. [We now] consider that drag force, or the force by the fluid on the spring that is connected between the beads, to get the critical shear rate.”
The critical shear rate, or the rate of blood flow at which the A2 domain unfolds, is essential to understanding the mechanical properties of vWF. Adopting their model from the experimental data gets the simulation team closer to vWF’s actual architecture, but it then increases the critical shear rate.
“The strength of the blood flow that we had to employ [in our model] in order to make the A2 domain unfold was huge—like orders of magnitude larger than what would really happen in the blood flow,” says Morabito.
Webb explains: “We bring observations like these to our colleagues doing experiments and the ensuing discussions help us better understand the simulation conditions needed to most accurately compare to experiment. In work currently in review, Yi [Wang] and Mike [Morabito] have advanced companion experimental and simulation data that are in remarkable agreement.”
Kania and Morabito are working to maintain in their model the correct molecular architecture of vWF while also incorporating drag on the protein to correctly model its effect.
Says Morabito: “Trying to preserve the architecture of vWF while also correctly capturing its dynamics at the correct blood flow rates has been the most surprising and difficult challenge for me here.”
Inspired Drug Delivery
The team seeks to not only design drugs to replace vWF for those who lack it, but also to utilize vWF’s flow-sensing mechanism to design other types of drugs or drug carriers that can be activated and release drugs in situ when there’s a change in flow pattern.
Zhang and Cheng and their teams are considering whether they can make polymer systems that mimic the behavior of vWF in the in vitro system that can eventually be used in vivo as both a therapeutic drug for bleeding disorders and also as a drug-release carrier. In other words, they hope to engineer polymer systems to mimic the function or behavior of vWF.
“A drug [might be] encapsulated in the polymer until it unfolds, and then the drug is exposed and released at a high-shear stress environment or abnormal flow environment,” she explains. “That’s the general idea.”
In that case, in the event of a stroke, instead of relying on the injection of a critical drug at the hospital, a patient prone to blood clots could already have the drug in his body, dormant until it is activated by changes in the blood-flow environment.
“You can have the drug encapsulated by the carrier and injected,” explains Zhang. “But the carrier, just like the vWF, is inactive—it just protects the drug. When the flow pattern changes, when a stroke happens or the blockade in the vessel happens, this carrier, when it passes by those locations, the drug can be released in situ, right in that location.”
This type of drug delivery might provide an alternative to the risk of circulating a blood-thinning drug through the body of a patient with a bleeding disorder, which could cause problems in areas beyond a blood clot.
“You want to just release drugs where you have the stroke or you have the vessel blockage,” says Zhang.
More knowledge about vWF’s functions can help with other medical problems as well. In certain conditions, vWF can activate where it shouldn’t, creating problems such as thrombosis, or undesired clotting.
“Sometimes in the arteries or in the heart when the patient doesn’t have the heart valve closed completely … vWF is activated by this high shear force and this generates trouble because you don’t want this molecule to be activated,” Zhang explains. “There’s a problem in that scenario. So if we can design drugs or drug carriers mimicking the vWF, it’s a huge advancement.”
Additionally, recent studies have indicated that implanted medical devices can change blood flow conditions and inappropriately activate vWF.
“Flow fields near implants might create enough of the shear forces, as they call it, to open the molecule up and cause thrombosis,” says Webb, “so that can be really bad. There are a lot of unknowns about this, even though there is a lot that’s been learned over the past decade.”