Scramblases

By Dr. David Clark

8/25/23  Many of you have seen diagrams of the coagulation cascade (blood clotting system) and assumed that we therefore know everything about how blood clotting works.  Not so fast!  The diagrams that you see are only an approximation.  There are many other molecules involved, probably many that haven’t even been identified yet, and many other processes.  The diagrams give us a framework to think about clotting but aren’t the final answer, by far.  We are still trying to understand the complex system that is blood clotting and still finding new components.  Another relatively new group of compounds involved is the TMEM16 family of scramblases.

One of the major findings about clotting happened in the 1980s when it was discovered that the clotting reactions actually take place on surfaces, not in solution out in the bloodstream.  The surfaces are the broken cell walls at the injury site and the cell walls (membranes) of activated platelets.  This makes sense because you want the clotting reactions to be at the site of injury, not floating away in the bloodstream.  Now we’re finding that those surfaces are not just passive but also participate in the clotting process.  To explain this, we need to learn a little about cell membranes.

The cell wall or cell membrane is the covering of the cell.  It is made of molecules called phospholipids.  These are fatty molecules (lipids) that also contain phosphorus groups.  They are long molecules that line up next to each other in a double layer to completely enclose the contents of the cell, as shown in the diagram below.

In this diagram[1], the image on the left shows a cutaway view of a cell surrounded by the cell membrane.  Intracellular refers to inside the cell and extracellular refers to outside the cell.  The blown-up image at the top shows the structure of the cell membrane, which is made of a phospholipid bilayer.  The bilayer consists of two layers of phospholipid molecules lined up with their heads (round circles) facing the outside of the membrane and the tails (long sections) on the inside of the membrane.  Because of the way the different parts of the phospholipid molecules attract each other, this forms a strong but flexible covering for the cell. The cell membrane also regulates which materials can pass in or out of the cell.

There are a number of different types of phospholipid molecules that make up the cell membrane.  One of the most prevalent (about 15% of the total) is called phosphatidylserine (PS).  It is procoagulant (promotes clotting) because it is the molecule to which many of the clotting factors bind during clotting.  In the endothelial cells (ECs) that line the inside of the blood vessels, the PS molecules are all on the inside wall of the cell membrane where the blood can’t see them.  The wall of the ECs that are in contact with the blood has no PS and is therefore anticoagulant – it impedes clotting.

When you have an injury the EC walls are broken open, which exposes the blood to the PS molecules on the inside of the cell wall.  That provides a surface containing PS on which the clotting reactions can proceed.  Tissue factor, which is a protein that is also on the inside wall of ECs, is also exposed and starts the clotting process by activating factor VII, which binds to the PS-containing surface.

Tissue factor and factor VII start the clotting process, but to really get enough clotting activity going requires activating the other clotting pathway, the pathway that includes factors VIII and IX.  That pathway amplifies the clotting signal to produce enough fibrin to actually seal the blood vessel closed.  That requires more PS-containing surface for the clotting reactions.  That’s where the scramblases come in.

The recently-discovered scramblases scramble the arrangement of the phospholipid molecules in cell walls, bringing the PS to the outer surface of the injured cell and its neighboring intact cells.  That provides more sites for the clotting factors to bind to while forming the clot.  It also continues to localize the clotting reactions in the area of the injury.

Is the scramblase rearrangement really important?  Apparently so, because when they looked at mice in which the scramblases had been inhibited, they saw that 50% of the clotting activity had also been inhibited.  They also saw that fibrin (the protein that makes up the clot) did not stick to the EC surface.  That means that the clot doesn’t stick to the injury site.  Thus, this rearrangement that brings PS to the surface of the endothelial cells appears to be very important.

What does this mean for the average hemophilia patient?  Probably not much, at least not right now, but as we continue to learn more about the clotting system, we potentially will be able to find better treatments for clotting and bleeding disorders.  [Prouse T and Majumder R, J Thromb Haemost, online ahead of print 8/25/23]

[1] From Oregon State University at https://open.oregonstate.education/aandp/chapter/3-1-the-cell-membrane/, accessed 1/22/24.