The establishment of multicellular organisms requires the movement of cells and their aggregation into masses, which, during the embryonic development phases, will constitute tissues and organs. These morphogenetic processes imply the ability of cells to recognize each other and to establish stable interactions both with other cells and with the extracellular matrix, forming a cross-linking of fibrous proteins and polymeric carbohydrates. They act as a cementing substance in adult tissues and which allows migration and the movement of cells during the developmental stages of the organism.
In multicellular organisms, the aggregation of cells in tissues involves the acquisition of specialized functions, as occurs, for example, in muscle, nervous, or epithelial tissues; the proliferative capacity of these cells is also strictly controlled. In fact, while in single-celled organisms, a single cell is able to perform all the necessary functions, in multicellular ones, the groups of cells, aggregated into individual tissues, take on specific functions.
The ability of cells belonging to the same tissue to recognize each other was initially suggested by a series of cell aggregation experiments. when cells from different tissues were mixed in an appropriate medium, they aggregated into agglomerations containing homogeneous cell types. It was therefore clear that cells from the same tissue were able to recognize each other and to establish interactions that allowed the formation of aggregates. The cells' ability to recognize and aggregate was lost if isolated cells were treated with proteolytic enzymes, such as trypsin or pronase, which remove proteins present on the cell surface.
The cadherins constitute a family comprising a dozen distinct molecules, with a characteristic tissue distribution. Cadherin E (epithelial), the first molecule of this family to have been described, is expressed very early during embryonic development, at the morula stage, where it is responsible for the compaction reaction, (process by which the cells of the outer layer of the morula form junctions between them, sealing the structure. In this way, the fluids pumped inside the morula by the cells of the outermost layers allow the swelling and the formation of a hollow sphere, the blastula, within which the first real embryonic structures will be organized.
A second important system of adhesive receptors is represented by CAM molecules, which, unlike cadherins, mediate the recognition and adhesion between cells independently of the presence of Ca² + ions (Edelman and Crossin, 1991). The first molecule of this group to be identified was the Neural Cell Adhesion Molecule (N-CAM), expressed mainly on nerve cells, which mediates adhesion with a homophilic type mechanism. It consists of immunoglobulin and types III structural protein modules of fibronectin, a protein of the extracellular matrix. This adhesion molecule is encoded by a unique gene, which, however, can generate different protein forms through an alternative splicing mechanism.
As mentioned previously, the organization of tissues requires, in addition to cell-cell bonds, also the interactions by cells with the extracellular matrix. The latter is particularly abundant in connective tissues where, unlike epithelia, cells are not in close contact with each other. The main proteins that make up this extracellular structure are collagens, fibronectin, and laminins, protein molecules that have a fibrous structure and are capable of mutual interactions, thus guaranteeing the formation of a mechanical support mesh for the tissues.
They are molecules usually made up of three polypeptide chains rich in proline and glycine and wrapped in a helix, so as to form an elongated and rigid molecule.
Fibronectin and laminins - The cross-linked collagen fiber and its interaction with cells are further strengthened by the presence of molecules such as fibronectin and laminins.
Proteoglycans - Another important component of the extracellular matrices is represented by proteic glycans, proteins permanently complex with glycosaminoglycans, long acid polysaccharide molecules. These polymers interact with the specific sites present on fibronectin, laminins, and collagens, thus establishing an association with the other components of the matrix.
The cells anchor themselves to the extracellular matrix using particular receptors known as integrins, glycoproteins that cross the membrane from part to part connecting the extracellular matrix with the cytoskeleton, which represents the intracellular filament system responsible for cellular movement (Hynes, 1987). Integrins were discovered in the 1980s thanks to studies aimed at identifying the fibronectin receptor. At that time, it was known that cultured cells deposit a large amount of fibronectin on the surface of the culture capsule and, by binding to this protein, adhere to the capsule forming a cell monolayer.
The different cell adhesion systems work in a coordinated way determining the fate of the cells and their ability to migrate, interact, and organize themselves in tissues. A classic example of these processes is the formation of the neural tube during embryonic development
Adhesive receptors and generation of signals inside the cell
So far, we have discussed the mechanical role of the extracellular matrix and adhesive receptors in mediating the recognition processes between cells and their aggregation into functionally homogeneous masses. Adhesive receptors, however, also play a crucial role in regulating the processes of differentiation and cell proliferation.
One of the clearest examples of this property is represented by the epidermis, an epithelium whose cells, the keratinocytes, are arranged in overlapping layers. The cadherins are also capable of generating intracellular signals in response to adhesion between cells. In this case, β-catenin plays a very interesting role, even if not yet fully known. This cytoplasmic protein has two important functions: in fact, it does not just guarantee the binding of actin filaments to the membrane, acting as a bridge between the cytoplasmic region of cadherin and the actin filaments themselves, but it has an important role in regulating gene expression. Indeed, it has been shown that β-catenin can bind a transcription factor, the LEF -l protein, and, associated with this, migrate to the nucleus where it acts by regulating the expression of genes. The formation of bonds between cells would favor the association of β-catenin with cadherin, inhibiting its interaction with LEF-l and, therefore, its action at the gene level (Peifer, 1997). Β-catenin can also bind a cytoplasmic protein known by the name of APC (Adenomatous Polyposis Coli, an adenomatous polyp of the colon), which has the function of sequestering it to cadherins and LEF-l and induces degradation.
APC has been discovered as a gene whose loss or inactivation is linked to the development of colonist carcinoma. The lack of APC protein can be hypothesized to lead to the accumulation of an excess of cytoplasmic β-catenin available for interaction with cadherins and LEF-I protein. This could lead to an imbalance of gene expression and cell adhesion at the base of the neoplastic transformation. Cytoplasmic β-catenin levels are also regulated by the Wntl receptor, a secreted protein that plays an important role in both oncogenesis and tissue formation during embryogenesis. By stimulating its receptor, Wntl activates a cascade of reactions involving proteins with curious names such as Disheveled and Shaggy, inducing an increase in cytoplasmic β-catenin levels. Wnt, therefore, produces an effect similar to that induced by the inactivation or absence of the APC protein. By stimulating its receptor, Wntl activates a cascade of reactions involving proteins with curious names such as Disheveled and Shaggy, inducing an increase in cytoplasmic β-catenin levels. Wnt, therefore, produces an effect similar to that induced by the inactivation or absence of the APC protein. By stimulating its receptor, Wntl activates a cascade of reactions involving proteins with curious names such as Disheveled and Shaggy, inducing an increase in cytoplasmic β-catenin levels. Wnt, therefore, produces an effect similar to that induced by the inactivation or absence of the APC protein.