Cellular adhesion molecules and the extracellular matrix

Cellular adhesion molecules fall into four major families

Depending on the family, binding can be homophilic or heterophilic

Cadherins

Cadherins are responsible for Ca++-dependent cell-cell adhesion Extracellular domains – homophilic binding Homophilic binding allows cells that express the same type of cadherin to recognize and attach to each other Intracellular domains – attach to actin cytoskeleton through catenins – so extracellular connections can affect intracellular organization Mutation or downregulation of cadherin is associated with metastasis in cancer. Mechanical agitation in Ca++-free solution is a useful experimental technique to separate tissues into individual cells

Cadherins attach to the actin-based cytoskeketon through anchor proteins including catenins

Cadherins connect cells at a desmosome

Cadherins join adjacent epithelial cells to one another

Role of cadherins in organizing a synapse The next slide shows the events of synaptogenesis A target cell puts out outgrowths called filopodia – as one of them approaches a potential presynaptic neuron’s axon, the two cells become linked by cadherins. In the presynaptic cell, catenins organize a release zone for synaptic vesicles with docking site proteins In the postsynaptic cell, catenins form a corral in which receptors collect

Ig-like cellular adhesion molecules

Immunoglobulin-like cellular adhesion proteins Extracellular domain contains multiple subdomains that resemble those of antibody molecules or immunoglobulins Structure of the subdomains is stabilized by disulfide bonds rather than Ca++, as in cadherins.

The recognition proteins of the immune system arose from cell adhesion proteins A true specific immune system is largely a vertebrate invention – but metazoans have had cell adhesion molecules for a long time antibodies and T-cell receptors evolved from Ig-cell adhesion molecules through splice variations that lacked a membrane anchor – antibodies Highly variable extracellular domains – antibodies and T-cell receptors

Selectins

Selectins recognize the carbohydrate portion of cell surface and matrix glycoproteins Binding is Ca++-dependent Selectins have a major role in inflammation and hemostasis

Integrins Integrins are dimers consisting of non-covalently bound α and β subunits. In mammals, there are 18 different α and 8 different β subunits that combine to form 24 different dimers with different ligand-binding properties. Integrins mediate both ‘outside-in’ and ‘inside-out’ signaling: Outside-in: integrin binding to extracellular matrix activates 2nd messenger pathways Inside-out: integrins can change between low-affinity and high affinity in response to binding of regulatory proteins (vinculin, talin, paxillin, many others) to their short cytoplasmic domains. For some integrins, this can occur on a second-to-second time scale.

Integrin activation involves a conformational change

Integrins mediate focal contacts -temporary attachments to the substrate

The Extracellular Matrix A complex mixture of non-living material that fills the spaces between cells

Functions of the extracellular matrix Tissue integrity Tissue differentiation Intercellular communication

What is the extracellular matrix? Extracellular space is filled with a mesh of proteins and polysaccharides organized and assembled by the cells. Two main classes of molecules Glucoseaminoglycans associated with protein to form proteoglycans fibrous proteins - the major fibrous proteins are collagens – there are 24 of them Ultimately, matrix molecules make up such gross structures as ligaments and tendons

Fibroblasts are specialized for collagen production Notice that the collagen strands are laid out in a criss-cross fashion that confers maximum strength

The ECM has a complex role in regulating behavior, survival, shape and function of the cells associated with it.

An example – cultured cells Immortalized epithelial cells – for example mammary tumor cells - can be raised in primary cell culture If they are kept suspended in medium, they look and act just like amoebas If they are allowed to settle on a glass or plastic surface, they spread and multiply until a confluent layer is formed, but this layer doesn’t look like or function as an epithelium If they are allowed to settle on a collagen surface, they form an artificial epithelium with tight junctions and apical-basal polarity, and begin to indulge in ion and fluid transport.

Typically, cells have and use multiple types of adhesion molecules

Example 1 – Leukocyte migration in response to chemokines

Events from previous slide Step 2. Inflammatory mediators cause vascular endothelial cells at site of infection to express selectins that adhere to leukocyte cell surface (a similar interaction causes platelets to adhere to a damaged endothelium to initiate blood clotting) Step 3. Binding of selectins to the leukocyte causes it to activate integrins – these attach it to Ig family adhesion molecules on endothelial surface Step 4. Leukocyte behavior changes – it begins to creep like an amoeba through the junctions between endothelial cells Once in the extracellular space, leukocyte migrates toward source of chemokines –using focal contacts to attach to the extracellular matrix

Example 2: Onset, maintenance and termination of mammary gland function

First, some definitions Basal lamina, secreted by epithelial cells Reticular lamina, formed by fibroblasts Together make up the basement membrane, the form of extracellular matrix associated with epithelial and endothelial tissues

Prolactin sustains lactation by manipulating the interaction of alveolar cells and basement membrane During pregnancy, steroid hormones estradiol and progesterone from the placenta, stimulate proliferation of mammary alveolar and duct cells After delivery, prolactin, a protein hormone from the pituitary, initiates milk secretion: integrin-laminin interaction stimulates expression of milk proteins by alveolar cells. The same interaction also promotes cell survival and proliferation. End of lactation is initiated by drop in prolactin secretion. Matrix metalloproteases (MMPs) disassemble the matrix, removing survival signals. Most mammary alveolar cells undergo apoptosis, and milk production stops.