Cell adhesion

The study of cell adhesion is part of cell biology. Cells are often not found in isolation, rather they tend to stick to other cells or non-cellular components of their environment. A fundamental question is: what makes cells sticky? Cell adhesion generally involves protein molecules at the surface of cells, so the study of cell adhesion involves cell adhesion proteins and the molecules that they bind to.

Cell adhesion proteins (or Cell adhesion molecules, CAMs)
Proteins are often transmembrane receptors. Transmembrane cell adhesion proteins extend across the cell surface membrane and typically have domains that extend into both the extracellular space and the intracellular space. The extracellular domain of a cell adhesion protein can bind to other molecules that might be either on the surface of an adjacent cell (cell-to-cell adhesion) or part of the extracellular matrix (cell-to-ECM adhesion). The molecule that a cell adhesion protein binds to is called its ligand. There are families of cell adhesion proteins that can be characterized in terms of the structure of the adhesion proteins and their ligands. Adhesion between two copies of the same adhesion protein is called "homophilic" binding. Adhesion between an adhesion protein and some other molecule is "heterophilic" binding.

Cytoskeletal interactions
For a cell adhesion protein like the one shown in the diagram, the intracellular domain binds to protein components of the cell's cytoskeleton. This allows for very tight adhesion. Without attachment to the cytoskeleton, a cell adhesion protein that is tightly bound to a ligand would be in danger of being hydrolyzed by extracellular hydrolytic enzymes. This will rip out the adhesion protein from the fragile cell membrane. Often the connection between the cell adhesion proteins and the cytoskeleton is not as direct as shown in the diagram. For example, cadherin cell adhesion proteins are typically coupled to the cytoskeleton by way of special linking proteins called "catenins".

Importance of cell adhesion
Cell adhesion proteins are important for the normal functioning of living organisms. Cell adhesion proteins hold together the components of solid tissues. They are also important for the function of migratory cells like white blood cells. Regulation of cell adhesion proteins is important during embryonic development for the process of morphogenesis. Some people have "blistering diseases" that result from inherited molecular defects in genes for adhesion proteins. Some cancers involve mutations in genes for adhesion proteins that result in abnormal cell-to-cell interactions and tumor growth. Cell adhesion proteins are also important for interactions that allow viruses and bacteria to cause damage to humans. Cell adhesion proteins hold synapses together and the regulation of synaptic adhesion is involved in learning and memory. In Alzheimer's disease there is abnormal regulation of synaptic cell adhesion.