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large variety of cellular events. Membrane receptor such as those represented
in the large family of G-protein-coupled receptors can be activated by a variety
of extracellular signals, including neuropeptides, chemokines, biogenic
amines, hormones, lipid-derived mediators, proteases, light, lavours and
odours. Upon ligand binding, the membrane receptors transduce these signals
into a quantity of intracellular responses that regulate cell functions via the
heterotrimeric G-proteins.
46
Actin ilaments provide the basic infrastructure
for maintaining cell morphology and various functions. Cell cytoskeleton
remodelling resulting in morphological changes is often observed after
activation of signalling pathways involving calcium mobilization,
47,48
cAMP
production
49
as well as small G-protein activation.
40,50,51
Activation of
both the P2 purinergic receptors (P2Ys) by its agonist ATP in CHO cells
52
and the bradykinin receptor (B
2
) by its agonist bradykinin in endothelial
cells
53,54
is known to signal via Gq like the AT
1
receptor presented previously.
Their activation is well documented to involve a large variety of biological
processes related to vascular, immunological and intestinal functioning by
the regulation of contraction, chemotaxis, proliferation, gap junction event
in
vitro
. These processes implicitly involve changes in morphological
and mechanical parameters of the cells and are thus good candidate for AFM-
based real-time measurements.
Figure 17.7
presents AFM-based monitoring
of cell activation showing a variation in the AFM signal, consistant with an
initial contraction of the cell body followed by its remodelling. Although,
the AT
1
, P2Ys and the B
2
receptors are known to signal through the Gq
pathway, comparison between the AFM data shows clear differences
that could be quantiied in term of amplitude, kinetic and duration of the
response. These differences most likely have their origin in the eficacy of
a receptor to generate a cell response at a given agonist concentration or
in the contribution of alternative signalling pathways such as Gi/s or the
small G-protein family selectively involved with these receptors. Naturally,
the cell type is also very likely to play a determinant role in the development
of a morphological and/or mechanical response following the activation of
a receptor. Indeed, mechanically competent cells such as cardiomyocytes or
smooth muscle cells are more likely to generate high amplitude mechanical
responses owing to the presence of a functional actomyosin contractile
machinery, when compared with endothelial or epithelial cell lines. However,
as demonstrated for the AT
1
receptor, the contribution of these factors could
be delineated using appropriate tools from pharmacological and molecular
biology toolbox in conjunction with appropriate cell models.
Several other types of receptors either found at the cell membrane or
in the cytosol are involved in morphological, motile or mechanical response
in cells. These receptors include the receptors tyrosine kinase family
and
in vivo
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