Cell adhesion is the interaction of a cell with a neighboring cell or with the underlying extracellular matrix, via specialized multi-protein adhesive structures:
Cells interact with each other, and their substrate, throughout their lifetime. These interactions can be transient, such as at the immunological synapse, or they can be long-lived, such as at a neuromuscular junction. These complex cellular structures involve many proteins; from receptor molecules to structural scaffolding proteins. Significant differences in composition exist between an adhesion complex that interacts with the cellular substrate, or extracellular matrix, and one that interacts with another cell. Despite the differences however their fundamental function remains the same; to enable cellular communication through the generation and transduction of mechanical signals. While cell-cell adhesions serve as cellular ‘handshakes’, cell-matrix adhesions allow a cell to pull against its substrate to either measure the substrate rigidity, or to pull the cell forward.
Cell adhesions can be described as a functional extension of the actin cytoskeleton. Indeed, all adhesion types are linked physically to the actin filament network, and the dynamic processes of actin filament polymerization and disassembly are intertwined with the turnover and function of the adhesions complexes. Cell adhesions are mediated by either transmembrane cell-adhesion molecules (CAMs), which binding similar partner proteins on opposing cells, or adhesion receptors, which bind various ligands. These proteins are integral to the formation of adhesions and essentially link the intracellular space to the extracellular space to help relay information to the cell interior about the surroundings.
Cells adhere to the ECM, or to other cells, via complexes that can collectively be called anchoring junctions (reviewed in ). These multiprotein complexes are found in all cell types where they they stabilize the cells position, provide stability and rigidity, and support tissue integrity by holding cell sheets together. Anchoring junctions also form a tight seal between neighboring cells to restrict the flow of molecules between cells and from one side of the tissue to the other. Lastly, anchoring junctions regulate the motility of both single cells and cellular masses through their substrates. These anchor points are highly dynamic, primarily associated with actin filaments, and come in many different forms.
Features of anchoring junctions
There are three main features of anchoring junctions:
There are four main types of anchoring junctions:
Several types of anchoring junctions have been identified with each involved in distinct types of adhesion.
Ligand-receptor binding is followed by the rapid association of other proteins to the intracellular portion of the receptor; this reinforcement of the adhesion domain is controlled by adhesion receptor mobility in the membrane . Such change in forces can affect mechanosensory molecules to activate intracellular signal transduction cascades (e.g. the Rho family of GTPases) and mechanotransduction events that mediate a number of diverse processes such as cell proliferation, fate, migration, shape and polarization  (reviewed in ).
Various types of cell-matrix receptors exist. These include:
Anchoring junctions are multiprotein complexes. Crucial to the formation of these junctions are cell adhesion molecules (CAMs).
CAMs have many distinct domains that allow them to mediate cell-cell contacts by binding to specific partner proteins; when these interactions occur between apposed cells they are described as either homophilic (i.e. binding to the same kind of CAM molecule) or heterophilic (binding to a different kind of CAM molecule). Furthermore, CAMs can mediate interactions between cells of the same type (aka homotypic adhesion) or between different cell types (aka heterotypic adhesion).
CAMs are grouped into four main families:
Many other proteins are recruited to the site of cell adhesion, where they play a role connecting cell adhesion molecules to the internal components of the cell, or provide structural support to the larger adhesion complexes.
These proteins include: