Cell-based coagulation Welcome to another Chalk Talk episode. Here
we'll be continuing our series of episodes on secondary hemostasis and will present the
cell-based model of coagulation. As seen in the first episode: the starting point of coagulation
is vascular injury, with platelets rapidly accumulating at the site of injury. At the
same time, blood comes into contact with smooth muscle cells and fibroblasts present in the
vessel wall. These cells express factor three, a membrane protein, on the cell surface. Because
factor three is present in the tissue surrounding the vessel, it’s also termed tissue factor.
In an intact blood vessel, tissue factor is separated from other clotting factors by the
endothelium to prevent coagulation. When the endothelium is injured, tissue factor is exposed
and forms a complex with factor seven, resulting in activated factor seven. The complex of
factor three and activated factor seven then activates factor ten.
Subsequently, factor ten cleaves factor two, also called prothrombin, to factor two a,
also called thrombin. So, as a comparison, let’s go back to the
coagulation cascade. The events described so far are part of the extrinsic pathway.
However, in contrast to the cascade model, cell-based coagulation doesn’t lead to fibrin
cleavage at this stage. Until this point, thrombin forms slowly; however,
the small amount of thrombin remains insufficient to produce adequate amounts of fibrin. Yet,
this small amount of thrombin is very important and can be used to activate factors in the
intrinsic pathway. Accordingly, the generated thrombin is referred to as the ‘spark’.
The goal of this thrombin spark is to initiate a feedback loop between the extrinsic and
intrinsic pathways, which we’ll take a closer look at in a moment. But first, let’s discuss
the special features of the cell-based model. In contrast to the cascade model, clotting
factors don’t only react with one another in blood, but specifically on the cell membrane
surface, which is the central location of coagulation in the cell-based model. Factors
three, seven, ten, and two are present on the surface of smooth muscle cells and fibroblasts.
The question that arises at this stage is how can coagulation shift from the surface
of these cells to the surface of platelets? Factor ten a can’t be involved, because
it’s rapidly inactivated when away from the cell membrane. Therefore, thrombin adopts
the role of the mediator, as it’s able to reach the platelet surface. Let’s explore
this in more detail. Thrombin has several functions on the platelet
surface; however, we won’t be presenting all of them in this episode. For the time
being, it’s important to just remember the two following functions:
Thrombin can bind to platelets via the receptor for von Willebrand factor. That’s right!
It’s the same receptor that mediates platelet adhesion. By binding to the receptor for von
Willebrand factor, thrombin supports platelet activation. Therefore, it prepares the platelet
membrane for coagulation by initiating the flip of membrane lipids from the inner to
the outer layer of the membrane. The second important task of thrombin is the
activation of other clotting factors. Thrombin initially activates factors eleven, eight,
and five, therefore establishing a link to the intrinsic pathway. Before we take a look
at how coagulation further proceeds, let’s have a closer look at factor eight.
In plasma, factor eight is bound to von Willebrand factor. Once factor eight is activated by
thrombin, it results in the release of von Willebrand factor, which, in turn, binds to
the receptor for von Willebrand factor and facilitates platelet adhesion. As shown here,
both processes occur in the immediate vicinity of one another on the platelet surface. This
is a very good example that demonstrates how primary and secondary hemostasis are closely
linked. Now, back to the coagulation process. Up until
this stage, we’ve mentioned that a small amount of thrombin is generated, which activates
factors eleven, eight, and five. Activated factor eleven subsequently activates factor
nine, which then forms a complex with factor eight. This complex, comprising factors nine
and eight, activates factor ten. This brings us back to the next phase of the
cell-based model, which is comparable to the common pathway model of the coagulation cascade.
The following aspect is very important for understanding in vivo coagulation. Factor
ten can be activated by two different complexes, either by factors three and seven or by factors
nine and eight. These complexes are termed tenases, which stems from ‘ten’ for the
substrate factor they activate and the suffix ‘ase’, which is generally used for enzymes.
In reference to the pathways of the cascade model, they are also termed extrinsic and
intrinsic tenases. Don’t worry if things appear a little complicated; it’s actually
quite easy. Extrinsic tenase contains factors three and seven and is present on smooth muscle
cells and fibroblasts of the vessel wall. Intrinsic tenase contains factors nine and
eight and is present on the platelet surface. So, why is this absolutely crucial? Intrinsic
tenase is approximately 50 times more effective in activating factor ten than extrinsic tenase.
After the generation of small amounts of thrombin and subsequent activation of intrinsic tenase,
a large amount of factor ten is activated. Factor ten then binds to activated factor
five. By the way, factor five has been transported to the site by platelets, is released upon
their activation, and functions as a cofactor for the conversion of prothrombin to thrombin.
The binding of factor five to factor ten results in increased factor ten activity by 250 to
1000-fold. This shows the importance of initial activation of factors such as factor five
by the thrombin spark, which helps to get coagulation going. Now, prothrombin is rapidly
converted to active thrombin. To relate this to the cascade model, the factors that are
assigned to the intrinsic pathway are responsible for generating most of the thrombin, with
over 95% of the total amount of thrombin. This leads to a decisive step in the coagulation
process: Fibrinogen is cleaved to fibrin, known as activated factor one. The fibrin
molecules then assemble into fibrin strands. In the final step, thrombin also activates
factor thirteen, which crosslinks individual fibrin strands into a stable fibrin network.
Red blood cells are also caught in the fibrin network, which is why the newly formed thrombus
is also termed red thrombus. Secondary hemostasis, according to the cell-based model, is now
complete. Please note that we haven’t mentioned all
of the existing interactions between clotting factors. It’s easy to see that coagulation
is a very complex process with sometimes opposing demands. On the one hand, it must always be
ready to rapidly stop bleeding. On the other hand, false activation needs
to be prevented in the absence of a wound by negative interactions. So, in order to
find the right balance between complexity and comprehensibility, we’ve concentrated
on the most important steps in this description and tried to introduce them as straightforward
as possible. Yet, coagulation doesn’t follow a linear pattern, as many processes run in
parallel and can be stimulated or inhibited by feedback loops.
At this stage, we’d like to supplement one of the links. Factor seven not only activates
factor ten and initiates the generation of small amounts of thrombin, but also directly
activates factor nine. This occurs to a lesser extent than factor ten activation, but through
the link between factors seven and nine, the body creates a link between the intrinsic
and extrinsic systems and ensures that coagulation starts promptly. This reinforcement loop is
a positive feedback loop and is also termed the Josso loop after its discoverer.
So, were you able to understand everything until now? If yes, well done! If you haven’t,
don’t worry! We’ll be summarizing everything now and things will become much clearer.
OK, let’s go. Coagulation is classically divided into the extrinsic and intrinsic pathways,
which both run into a common pathway. The newer cell-based model attempts to describe
how the sequence of events occur under physiological conditions. The main difference is that the
cell-based model of coagulation doesn’t separate the two pathways, but describes several
stages in which coagulation proceeds. After endothelial injury, blood comes into
contact with tissue factor and coagulation is activated. Extrinsic tenases activate factor
ten and initiate the generation of small amounts of thrombin. The thrombin generated plays
a key role, because it activates platelets and factors eleven, eight, and five on the
platelet surface. Intrinsic tenases are subsequently formed
on the platelet surface, which strongly activate factor ten. Factor ten forms prothrombinase
complexes with factor five. The name of this complex comes from its ability to cleave prothrombin
to thrombin. To understand this model, it’s important to note that most of the thrombin
is generated by the prothrombinase complex. In the final step of the cascade, fibrin is
formed, which is crosslinked by factor thirteen. Maybe all of these steps can be better remembered
if you refer to them by their three phases. These are termed initiation, amplification,
and propagation. Initiation is similar to the extrinsic pathway
and occurs on the cells of the vascular wall. Amplification takes place when small amounts
of thrombin initiate the intrinsic system on platelets. During propagation, numerous
tenase and prothrombin complexes assemble on the platelet surface. When these complexes
accumulate, large amounts of thrombin and fibrin form, resulting in a thrombin burst.
Thrombin, therefore, plays a double role in this coagulation model, acting as a spark
in the initiation phase and a burst in the propagation phase. This double role is made
possible by the fact that coagulation occurs on the surface of various cells. Therefore,
this aspect is worth focusing on in our next Chalk Talk episode.
We’ll take a detailed look there at how clotting factors bind to the cell surface
and examine how this mechanism can be used in coagulation tests. But before that, let’s