Biological Applications of Dynamic Force Spectroscopy - Dynamic Force Spectroscopy and Biomolecular Recognition - page 230

a soluble plasma glycoprotein converted into fibrin by thrombin during blood coag-
ulation. Upon blood clotting, fibrinogen forms a fibrin network with the ability to
entrap erythrocytes and platelets to form clots. An increased fibrinogen concentra-
tion is a factor that increases the risk of cardiovascular diseases. Lee and Marchant
have studied the interaction between the human platelet α IIb β 3 receptor system with
a ligand derived from fibrinogen (Lee and Marchant, 2001). The unbinding force
plotted as a function of the logarithm of the loading rate shows a linear trend. The
corresponding dissociation rate, reported in Table 6.5, results to be much higher than
that measured in bulk solution by flow cytometry (0.2 s −

1 ). They suggested that such
a discrepancy could arise either from some deformation of the platelet membrane or
from conformational changes of the biomolecules during the DFS measurements.
The presence of these changes likely due to the large flexibility displayed by the sys-
tem suggests some caution in the use of the Bell-Evans model to more correctly ana-
lyze the corresponding data. Indeed, they proposed a revisitation of the Bell-Evans
model to include possible deformations of the involved biological systems when the
external force is applied.
Very recently, Carvalho et al. have applied DFS to elucidate the fibronogen-
induced erythrocyte aggregation by studying the interaction of fibrinogen directly
with two kinds of blood components, platelets and eryhrtocytes (Carvalho et al.,
2010). Fibrinogen was immobilized on the AFM tip, while either the blood cells or
the platelets were adsorbed on a glass substrate. They observed distinct peaks in the
histograms of the unbinding force and attributed them to the occurrence of multiple
binding events. Moreover, they find a smaller unbinding force for the fibrinogen-
erythrocyte system with respect to that measured in fibrinogen-platelet interactions.
In addition, for both the fibrinogen-platelet and the fibrinogen-erythrocyte interac-
tions, the presence of two barriers in the energy landscape has been obtained. The
corresponding dissociation rate values indicated the occurrence of a fast and a slow
process for fibrinogen-erythrocyte and of two rather slow processes for fibrinogen-
platelet (see Table 6.5). More specifically, they proposed that both the systems are
characterized by a primary regime with similar dissociation rate and a secondary
one which, however, is significantly different in the two systems. The observation
of a fast process for the fibrinogen-erythrocyte was never observed before, proba-
bly due to the difficulty to follow, in vivo , a process with a lifetime of about a few
milliseconds. On the basis of these results, the authors have hypothesized the exis-
tence of a novel receptor on the human eryhrtocyte membranes that can specifically
bind fibrinogen. They have also ascertained that such a receptor is not significantly
influenced by calcium as instead it occurs for the platelet receptor. As a negative
control, they have tested a sample from a patient with a hereditary disease causing
adeficiency in the fibrinogen receptor. The finding in this case of a drastic decrease
in the unbinding force between fibrinogen and plateles, and also between fibrinogen
and erythrocytes, has confirmed the existence of this new receptor. On such a basis,
the authors have stressed that DFS is a highly sensitive nanotool for diagnostics of
hematological diseases, offering new opportunities even in the functional evaluation
of the disease severity.