Studying Cell–Surface Interactions In Vitro: A Survey of Experimental Approaches and Techniques - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 37

layer forms within the first few seconds of contact between the surface and the
biological environment and determines the compliance of the surface with

subsequent cell attachment and spreading.
After the adsorption of proteins, cell attachment and spreading are initiated
by nonspecific interactions between the cells and the protein-decorated surface
(Fig. 1 b). These comprise electrostatic, electrodynamic, steric and entropic inter-
actions. The first two are predominantly attractive in nature and based on the
presence of fixed charges and dipoles on both the cell surface and the substrate
surface. On the other hand, close adhesion between cell and surface requires the
compression of the glycocalix decorating the cell membrane and the surface-
attached protein layer which gives rise to steric and entropic repulsion. Once the
balance of nonspecific interactions has provided sufficiently close proximity,
specific interactions between cell-surface receptors and the surface immobilized
proteins are established and provide mechanically stable substrate anchorage of the
cells [ 7 , 8 ]. The most prominent class of cell-surface receptors involved in cell
adhesion and spreading is the integrin family, which will be discussed in more
detail in the subsequent section.
Whether or not stable substrate anchorage occurs depends on (1) the expression
of integrins with affinity for the extracellular proteins pre-adsorbed on the surface
and (2) the composition of the surface attached protein layer and the conformation
of the adsorbed proteins. These two conditions eventually determine the fate of
cells settling upon an in vitro surface: the cells will start to attach firmly and
spread, maximizing their interface with an adhesive surface (Fig. 1 b, left panel) or
they will stay in a rounded morphology, loosely attached and unable to spread,
when specific interactions cannot be formed (Fig. 1 b, right panel). The latter will
drive anchorage-dependent cells towards apoptosis. Generally speaking, while
hydrophilic surfaces promote cell adhesion due to their coating with a native
protein layer, hydrophobic surfaces covered with a layer of unfolded protein often
counteract cell adhesion since specific recognition sequences within the extra-
cellular proteins are not accessible to the cell-surface receptors.

3 Molecular Architecture of Specific Cell-Surface
Interactions

The most prominent type of transmembrane receptors responsible for specific
cell-substrate interactions are the integrins [ 9 ]. Integrins are a family of non-cova-
lently associated, a,b-heterodimeric transmembrane glycoproteins that project from
the cell membrane by roughly 20 nm [ 10 ]. To date, 24 different integrins have been
identified, resulting from different combinations of 18 a- and 8 b-subunits [ 11 ].
Both subunits exhibit some structural similarity: each is composed of a long
stalk-like extracellular segment with a globular domain at the N-terminus. The
N-terminal domains of both subunits combine to form the specific ligand binding site
that interacts with ECM proteins in the presence of divalent cations (Mg 2+ or Ca 2+ ).