Adhesion molecules and adhesion receptors are essential for tissue structural organization. Differential expression of such molecules is implicit in the processes of cell growth and differentiation, such as wound repair and embryogenesis. There are four major families of cell adhesion molecules: cadherins, integrins, the immunoglobulin superfamily and selectins.

Cadherins

The cadherins establish molecular links between adjacent cells. They form zipper-like structures at ‘adherens junctions’, areas of the plasma membrane where cells make contact with other cells. Through these junctions, bundles of actin filaments run from cell to cell. Related molecules such as desmogleins form the main constituents of desmosomes, the intercellular contacts found abundantly between epithelial cells. Desmosomes serve as anchoring sites for intermediate filaments of the cytoskeleton. When dissociated embryonic cells are grown in a dish, they tend to cluster according to their tissue of origin. The homophilic (like with like) interaction of cadherins is the basis of this separation, and has a key role in segregating embryonic tissues. The expression of specific adhesion molecules in the embryo is crucial for the migration of cells and the differentiation of tissues. For example, when neural crest cells stop producing N-CAM (see below) and N-cadherin and start to display integrin receptors they can separate, and begin to migrate on the extracellular matrix. Changes in cadherin expression are often associated with tumour metastatic potential.

Integrins

These are membrane glycoproteins with α and β subunits which exist in active and inactive forms. The integrins principally bind to extracellular matrix components such as fibrinogen, elastase and laminin. The amino acid sequence arginine-glycine-aspartic acid (RGD) is a potent recognition sequence for integrin binding, and integrins replace cadherins in the focal membrane anchorage of hemidesmosomes and focal adhesion junctions (Fig. 3.6). A feature of integrins is that the active form can come about as a result of a cytoplasmic signal that causes a conformational change in the extracellular domain, increasing affinity for its ligand. This ‘inside-out’ signalling occurs when leucocytes are stimulated by bacterial peptides, rapidly increasing leukocyte integrin affinity for immunoglobulin superfamilies structures such as the Fc portion of immunoglobulin. The ‘outside-in’ signalling follows the binding of the ligand to the integrin and stimulates secondary signals resulting in diverse events such as endocytosis, proliferation and apoptosis. Defective integrins are associated with many immunological and clotting disorders such as Bernard-Soulier syndrome and Glanzmann’s thrombasthenia.

Immunoglobulin superfamily cell adhesion molecules (CAMs)

These molecules contain domain sequences which are immunoglobulin-like structures. The neural-cell adhesion molecule (N-CAM) is found predominantly in the nervous system. It mediates a homophilic (like with like) adhesion. When bound to an identical molecule on another cell, N-CAM can also associate laterally with a fibroblast growth factor receptor and stimulate the tyrosine kinase activity of that receptor to induce the growth of neurites. Thus adhesion molecules can trigger cellular responses by indirect activation of other types of receptors. The placenta and gastrointestinal tract also express immunoglobulin superfamily members, but their function is not completely understood.

Selectins

Unlike most adhesion molecules (which bind to other proteins), the selectins interact with carbohydrate-ligands or mucin complexes on leucocytes and endothelial cells (vascular and haematological systems). Selectins were named after the tissues in which they were first identified. L-selectin is found on leucocytes and mediates the homing of lymphocytes to lymph nodes. E-selectin appears on endothelial cells after they have been activated by inflammatory cytokines; the small basal amount of E-selectin in many vascular beds appears to be necessary for the migration of leucocytes. P-selectin is stored in the alpha granules of platelets and the Weibel-Palade bodies of endothelial cells, but it moves rapidly to the plasma membrane upon stimulation of these cells. All three selectins play a part in leucocyte rolling.

The cytoskeleton and plasma membrane interconnect, and extracellular domains form junctions between cells to form tissues. There are three types of junction between cells: tight junctions, adherent junctions and gap junctions.

Tight junctions

Tight junctions (zonula occludens) hold cells together. They are situated at the ends of margins adjacent to epithelial cells (e.g. intestinal and renal cells) and form a barrier to the movement of ions and solutes across the epithelium, although they can be variably ‘leaky’ to certain solutes. The proteins responsible for the intercellular tight junction closure are called claudins. They show selective expression within tissue and regulate what small ions may pass through the gaps between cells. For example, the kidney displays a differential expression of these claudin proteins. Mutations of claudin-16 (which is expressed in the distal convoluted tubule in the kidney, where magnesium is reabsorbed) are responsible for some forms of Gitelman’s syndrome. Since magnesium reabsorption is paracellular, tight junctions (which contain claudin-16) prevent these divalent ions rapidly diffusing back between the cells into renal tubules.

Adherent junctions

Adherent junctions (zonula adherens) are continuous on the basal side of cells. They contain cadherins and are the major site of attachment of intracellular microfilaments. Intermediate filaments attach to desmosomes, which are apposed areas of thickened membranes of two adjacent cells. Hemidesmosomes attach cells to the basal lamina and are also connected to intermediate filaments. Transmembrane integrins link the extracellular matrix to microfilaments at focal areas where cells also attach to their basal laminae. In blistering dermatological disorders autoantibodies cause damage by attacking tight junction desmosomal proteins such as desmoglein-3 in pemphigus vulgaris and desmoglein-1 in pemphigus foliaceus.

Gap junctions

Gap junctions allow substances to pass directly between cells without entering the extracellular fluids. Protein channels (connexons) are lined up between two adjacent cells and allow the passage of solutes up to molecular weight 1000 kDa (e.g. amino acids and sugars), as well as ions, chemical messengers and other factors. The diameter of these channels is regulated by intracellular Ca2+, pH and voltage. Connexons are made up of six subunits surrounding a channel and their isoforms in tissues are encoded by different genes. Mutant connexons can cause disorders, such as the X-linked form of Charcot-Marie-Tooth disease.

This is the fluid component inside the cell membrane and contains many specialized organelles. It contains a scaffolding or cytoskeleton that regulates the passage and direction in which the interior solutes and storage granules flow. The cytoplasm contains:

• Endoplasmic reticulum (ER). This consists of interconnecting tubules or flattened sacs (cisternae) of lipid bilayer membrane. It may contain ribosomes on the surface (termed rough endoplasmic reticulum (RER) when present, or smooth endoplasmic reticulum (SER) when absent). The ER is involved in the processing of proteins: the ribosomes translate mRNA into a primary sequence of amino acids of a protein peptide chain. This chain is synthesized into the ER where it is first folded and modified into mature peptides. ER is the major site of drug metabolism.
• Golgi apparatus. This consists of flattened cisternae similar to the ER. It is characterized as a stack of cisternae from which vesicles bud off from the thickened ends. The primary processed peptides of the ER are exported to the Golgi apparatus for maturation into functional proteins (e.g. glycosylation of proteins which are to be excreted occurs here) before packaging into secretory granules and cellular vesicles that bud off the end.
• Lysosomes. These are dense cellular vesicles containing acidic digestive enzymes. They fuse with phagocytotic vesicles from the outer cell membrane, digesting the contents into small biomolecules that can cross the lysosomal lipid bilayer into the cell cytoplasm. Lysosomal enzymes can also be released outside the cell by fusion of the lysosome with the plasma membrane. Lysosomal action is crucial to the function of macrophages and polymorphs in killing and digesting infective agents, tissue remodelling during development and osteoclast remodelling of bone. Not surprisingly, many metabolic disorders result from impaired lysosomal function.
• Peroxisomes. These are dense cellular vesicles so named because they contain enzymes that catalyse the breakdown of hydrogen peroxide. They are involved in the metabolism of bile and fatty acids, and are primarily concerned with detoxification, e.g. d-amino acid oxidase and H₂O₂ catalase. The inability of the peroxisomes to function correctly can lead to rare metabolic disorders such as Zellweger’s syndrome and rhizomelic dwarfism.
• Mitochondria. These organelles are the powerhouse of the cell. Each mitochondrion comprises two lipid bilayer membranes and a central matrix. It also possesses several copies of its own DNA in a circular genome. The outer membrane contains many gated receptors responsible for the import of raw materials like pyruvate and ADP, and the export of products such as oxaloacetate (precursor of amino acids and sugars) and ATP. An interesting caveat to our symbiotic relationship is that proteins of the Bcl-2/Bax family are incorporated in this outer membrane and can release mitochondrial enzymes that trigger apoptosis. The inner membrane is often highly infolded to form cristae to increase its effective surface area. It contains transmembrane enzyme complexes of the electron transport chain, which generate an H⁺ ion gradient. This gradient then drives the adjacent transmembrane ATPase complex to form ATP from ADP and P_i. The inner matrix contains the enzymes of the Krebs cycle that generate the substrates of both the electron transport chain (FADH₂ and NADH) and central metabolism.

The plasma membrane is freely permeable to gases such as O2, CO2 and N2, and to small uncharged molecules such as H2O (not H+ and OH-) and urea. Whilst larger hydrophobic lipid-soluble molecules – like steroids – also pass freely through the membrane, large uncharged molecules (glucose, amino acids and nucleotides) and small charged ions (K+, Na+, Ca2+, Cl-, Mg2+ and HCO3-) cannot pass unless via a specific transport protein embedded in the plasma membrane. Two structural types of transport molecules/complexes exist.

- Channel proteins literally open a channel in the lipid membrane to allow a specific solute to pass through.
- Carrier proteins are slower in action, shuttling the solute across and either facilitating diffusion down a gradient across the membrane, or actively pumping solutes against the gradient using ATP as an energy source.

Active carrier pumps and gated ion channels work together in neural transmission. These carrier proteins pump Na+ and K+ across the neuronal cell membrane to create a differential gradient, but ion channels open in response to stimuli to cause a rapid depolarization, allowing the ions to flow back. At synaptic junctions these ion channels open in response to chemical signals such as the release of glutamate, epinephrine (adrenaline) or acetylcholine.

ATP-dependent transport molecules (ATPases) belong to a superfamily called the ‘ABC transporter superfamily’. These include the multidrug-resistance protein (MDR), which pumps out hydrophobic drugs and is overexpressed by tumour cells, and the chloride ion pump coded by the cystic fibrosis gene. All share a common structure of six transmembrane domains interrupted by a cytoplasmic ATPase domain, followed by a further six transmembrane domains and another cytoplasmic ATPase. The cystic fibrosis chloride ion channel is unusual in that it requires the binding and hydrolysis of both ATP and cAMP for activation.

Specialized cells such as macrophages and neutrophils can engulf about 20% of their surface area in the pursuit of large particles such as bacteria. Lysosomes rapidly fuse with phagosomes, giving equally rapid digestion of the contents, and recycling as much of the internalized membrane as possible. Phagocytosis is only triggered when specific cell surface receptors – such as the macrophage Fc receptor – are occupied by their ligand. Pinocytosis is a much smaller-scale model of phagocytosis and is continually occurring in all cells. In contrast to phagocytosis, receptors for smaller molecular complexes, such as low-density lipoprotein (LDL) result in surface clumping and the internal accumulation of a protein called clathrin. Clathrin-coated pits pinch inwards as clathrin-coated vesicles. Clathrin prevents fusion of lysosomes, and thus its removal will result in lysosomal fusion and degradation of the contents. Maintenance of a clathrin coat can result in transcellular transit of the contents and their exocytosis at another side of the plasma membrane, i.e. apical-to-basal surface transcytosis. Similarly, cell organelles bud off vesicles coated in clathrin to prevent lysosomal fusion and degradation. Some of these vesicles rapidly fuse with the plasma membrane and exocytose their contents. Other vesicles do not immediately fuse with the plasma membrane (or indeed any other organelle). The clathrin-coated vesicles have additional lipid bilayer-embedded proteins called v-SNAREs (signal and response elements), which interact with target organelle membrane proteins called t-SNAREs. Vesicle fusion is therefore specific, comprising fusion in the correct place (at a particular organelle or part of the plasma membrane) at the correct time (e.g. the fusion of neuronal transmitter vesicles and release of the transmitter at the synaptic membrane when stimulated).