Biophysics and Bleeding Disorders

March is Bleeding Disorders Awareness Month in the US. More than three million Americans who have hemophilia, von Willebrand disease, and other rare bleeding disorders. These conditions prevent blood from clotting the way it should, which can lead to prolonged bleeding after injury, surgery, or physical trauma. We spoke with Biophysical Society member Valerie Tutwiler, an American Heart Association graduate research fellow in the lab of John Weisel at the University of Pennsylvania, about her hemostasis and thrombosis research.

What is the connection between your research and bleeding disorders?

BPJ_112_4.c1.indd

This recent cover of Biophysical Journal shows Tutwiler, Wang, Litvinov, Weisel, and Shenoy’s image of a colorized scanning electron microscope image of a coronary artery thrombus extracted from a heart attack patient.

Blood clotting or hemostasis is the process that stems bleeding. On one hand if you have insufficient clotting this can result in prolonged bleeding, on the other hand a hypercoagulable state can result in thrombosis. Thrombi can result in the obstruction of blood flow, which can cause heart attacks and strokes. My thesis research pertains largely to studying one portion of the coagulation process blood clot contraction, or the volume shrinkage of the clot, which has been implicated to play a role in hemostasis and the restoration of blood flow past otherwise obstructive thrombi.

Why is your research important to those concerned about bleeding disorders?

While there is much known about the various aspects of blood clotting relatively little is known about the process of clot contraction despite the clinical implications of its importance in the formation of a strong hemostatic seal and the restoration of blood flow past otherwise obstructive thrombi. The study of clot contraction is a highly interdisciplinary problem and as a result can be of interest to researchers from many different fields. Platelets are active contractile cells, which interact with an extracellular matrix of fibrin, a naturally occurring polymer with unique mechanical properties. The fibrin matrix can be imbedded with other blood cells, such as red blood cells, as well. From a biophysical standpoint the mechanisms of clot contraction have not been well understood. To better elucidate this process, we performed a systematic study on how the molecular and cellular composition of the blood influences the rate and extent of clot contraction along with the mechanical properties of the contracting clot using a novel application of an optical tracking system.

Additionally, to further explore the mechanical nature of the clot contraction process we developed a mathematical model that couples active platelets with a passive viscoelastic matrix made up of fibrin and red blood cells. The model predicts the process of clot contraction and explains some of the experimental observations of clot size, structure and mechanical forces. Interestingly, we found that clot contraction is altered in thrombotic states such as ischemic stroke patients. Collectively, these findings show that the study of clot contraction has the potential to inform the development of diagnostics and therapeutics.

How did you get into this area of research?

Since beginning research I have been interested in applying engineering techniques to answer biological questions. I became interested in hemostasis and thrombosis research while completing my first co-op experience in undergrad.

How long have you been working on it?

I began doing hemotology research during my undergraduate career. However, I started studying clot contraction specifically when I started my PhD research.

Do you receive public funding for this work? If so, from what agency?

I am currently funded by the American Heart Association as a pre-doctoral fellow, although we also receive funding from the National Institute of Health and National Science Foundation.

Have you had any surprise findings thus far?

We were surprised to find such a striking decrease in the extent of clot contraction in ischemic stroke patients compared to healthy subjects. Correlations with stroke severity suggest that clot contraction may be a potential pathogenic factor in ischemic stroke. These findings have led us to expand our study to other pathological conditions as well.

What is particularly interesting about the work from the perspective of other researchers?

Due to the conservation of the basic principles of contractile proteins and motility, the information learned from the development of a mathematical model of active contractile cells interacting with a viscoelastic matrix can be applied to a variety of different processes.

What is particularly interesting about the work from the perspective of the public?

Bleeding and thrombotic conditions remain leading causes of death and disability worldwide. Gaining a more thorough understanding of the processes involved in hemostasis and thrombosis will lead to the development of more effective diagnostic tools and more targeted therapeutics.

Mechanical Interplay in Clot Contraction

BPJ_112_4.c1.inddBlood clotting, thrombosis, and blood cells all have great biological and clinical significance. Clotting is necessary to stop bleeding yet thrombi can obstruct blood flow, which can cause heart attacks, strokes, venous thrombosis, and pulmonary embolism. Although much is known about various aspects of clotting, much less is known about clot contraction or retraction. Clot contraction is thought to play a role in hemostasis, wound healing and the restoration of flow past otherwise obstructive thrombi.

The cover image for the February 28 issue of the Biophysical Journal shows a colorized scanning electron microscope image of a coronary artery thrombus extracted from a heart attack patient. We chose this image because contraction occurs in such thrombi and all of the elements described in our paper are visualized here: platelets (gray), fibrin (brown) and red blood cells (red). Thus, this image represents a real-world example of the practical significance of our research. Furthermore, we have found that clot contraction is altered in patients with certain thrombotic disorders, such as acute ischemic stroke. Our model provides the fundamental mechanical basis for understanding the contraction of blood clots.

The contraction of blood clots and thrombi is an interdisciplinary problem related to fundamental aspects of cell biology, including cell motility and interaction of cells with an extracellular matrix. The biophysical mechanisms of clot contraction have been poorly understood, although it has been shown that it results from the interaction of actively contracting platelets with the fibrin network, the structural matrix of the clot that has unique mechanical properties. Though many of the same basic principles of motility of other cells are employed in this system, the specialized mechanisms of cellular contractility represent a novel biological application. The consequences of cell-matrix interactions in blood clots are unique and result in massive compaction of the network, rather than motility or alignment of fibers that occur in other cellular contractile environments.

Blood clot contraction is driven by platelet-generated contractile forces that are propagated by the fibrin network and result in clot shrinkage and deformation of red blood cells. We developed a model that combines an active contractile motor element with passive viscoelastic elements consisting of fibrin and red blood cells. This model predicts how clot contraction occurs due to active contractile platelets interacting with a viscoelastic material, and explains the observed dynamics of clot size, ultrastructure, and measured forces.

–¬†Andre E.X. Brown, Chandrasekaran Nagaswami, Valerie Tutwiler, Hailong Wang, Rustem Litvinov, Vivek Shenoy, and John Weisel

Note: This image originally appeared in a different form in Science 325:651, 2009.