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PLASMA MEMBRANE

Cells are bounded by a distinct plasma membrane, which shares features with the system of internal membranes that compartmentalize the cytoplasm and surround the nucleus. They are all composed of lipids (mainly phospholipids, cholesterol and glycolipids) and proteins, in approximately equal ratios. Plasma membrane lipids form a layer two molecules thick, the lipid bilayer. The hydrophobic ends of each lipid molecule face the interior of the membrane and the hydrophilic ends face outwards. Most proteins are embedded within, or float in, the lipid bilayer as a fluid mosaic. Some proteins, because of extensive hydrophobic regions of their polypeptide chains, span the entire width of the membrane (transmembrane proteins), whereas others are only superficially attached to the bilayer by lipid groups. Both are integral (intrinsic) membrane proteins, as distinct from peripheral (extrinsic) membrane proteins, which are membrane-bound only through their association with other proteins. Carbohydrates in the form of oligosaccharides and polysaccharides are bound either to proteins (glycoproteins) or to lipids (glycolipids), and project mainly into the extracellular domain.

Combinations of biochemical, biophysical and biological techniques have revealed that lipids are not homogenously distributed in membranes, but that some are organized into microdomains in the bilayer, called ‘detergent-resistant membranes’ or lipid ‘rafts’, rich in sphingomyelin and cholesterol (Morris et al 2004). The ability of select subsets of proteins to partition into different lipid microdomains has profound effects on their function, e.g. in T-cell receptor and neurotrophin signalling. The highly organized environment of the domains provides a signalling, trafficking and membrane fusion environment very different from that found in the disorganized fluid mosaic membrane.

In the electron microscope, membranes fixed and contrasted by heavy metals such as osmium appear in section as two densely stained layers separated by an electron-translucent zone – the classic unit membrane (Fig. 1.4). The total thickness is about 5 nm. Freeze-fracture cleavage planes usually pass along the midline of each membrane, where the hydrophobic tails of phospholipids meet. This technique has also demonstrated intramembranous particles embedded in the lipid bilayer; these are in the 5–15 nm range and in most cases represent large transmembrane protein molecules or complexes of molecules. Intramembranous particles are distributed asymmetrically between the two half-membranes, usually adhering more to one face than to the other. In plasma membranes, the inner or protoplasmic (cytoplasmic) half-membrane carries most particles, seen on its surface facing the exterior (the P face). Where they have been identified, particles usually represent channels for the transmembrane passage of ions or molecules.

  

Fig. 1.4  The molecular organization of the plasma membrane, according to the fluid mosaic model of membrane structure. Intrinsic or integral membrane proteins include diffusion or transport channel complexes, receptor proteins and adhesion molecules. These may span the thickness of the membrane (transmembrane proteins) and can have both extracellular and cytoplasmic domains. Transmembrane proteins have hydrophobic zones, which cross the phospholipid bilayer and allow the protein to ‘float’ in the plane of the membrane. Some proteins are restricted in their freedom of movement where their cytoplasmic domains are tethered to the cytoskeleton.

Biophysical measurements show the lipid bilayer to be highly fluid, allowing diffusion in the plane of the membrane. Thus proteins are able to move freely in such planes unless anchored from within the cell. Membranes in general, and the plasma membrane in particular, form boundaries selectively limiting diffusion and creating physiologically distinct compartments. Lipid bilayers are impermeable to hydrophilic solutes and ions and so membranes actively control the passage of ions and small organic molecules such as nutrients, through the activity of membrane transport proteins. However, lipid-soluble substances can pass directly through the membrane so that, for example, steroid hormones enter the cytoplasm freely. Their receptor proteins are either cytosolic or nuclear, rather than being located on the cell surface.

Plasma membranes are able to generate electrochemical gradients and potential differences by selective ion transport, and actively take up or export small molecules by energy dependent processes. They also provide surfaces for the attachment of enzymes, sites for the receptors of external signals, including hormones and other ligands, and sites for the recognition and attachment of other cells. Internally, plasma membranes can act as points of attachment for intracellular structures, in particular those concerned with motility and other cytoskeletal functions. Cell membranes are synthesized by the rough endoplasmic reticulum in conjunction with the Golgi apparatus.

The cell coat (glycocalyx)

The plasma membrane differs structurally from internal membranes in that it possesses an external, diffuse, carbohydrate-rich coat, the cell coat or glycocalyx. The cell coat forms an integral part of the plasma membrane, projecting as a diffusely filamentous layer 2–20 nm or more from the lipoprotein surface (see Fig. 1.5). The overall thickness of the plasma membrane is therefore variable, but is typically 8–10 nm. The cell coat is composed of the carbohydrate portions of glycoproteins and glycolipids embedded in the plasma membrane (Fig. 1.4).

  

Fig. 1.5  Intercellular junctions. The positions of the apical junctional complex and other junctional specializations of epithelial cells are shown, along with their electron microscopic appearances (see A–D; B and C are freeze fractured preparations). B shows how, in a gap junction, numerous channels (pores within connexons) are clustered to form a plaque-like junctional region between adjacent plasma membranes. C shows the anastomotic network of contacts between adjacent cell membranes forming a tight junction. D shows the attachments of basal plasma membrane to basal lamina at hemidesmosomes.
(Part B by courtesy of Professor Dieter Hülser, University of Stuttgart. Part C by courtesy of Dr Andrew Kent, King’s College London.) A,D from human tissue.

The precise composition of the glycocalyx varies with cell type: many tissue and cell type-specific antigens are located in the coat, including the major histocompatibility antigen systems and, in the case of erythrocytes, blood group antigens. It also contains adhesion molecules, which enable cells to adhere selectively to other cells or to the extracellular matrix. They have important roles in maintaining the integrity of tissues and in a wide range of dynamic cellular processes, e.g. the formation of intercommunicating neural networks in the developing nervous system and the extravasation of leukocytes. Cells tend to repel each other because of the predominance of negatively charged carbohydrates at cell surfaces. There is consequently a distance of at least 20 nm between the plasma membranes of adjacent cells, other than at specialized junctions.

Cell surface contacts

The plasma membrane is the surface which establishes contact with other cells and with structural components of extracellular matrices. These contacts may have a predominantly adhesive role, or initiate instructive signals within and between cells, or both; they frequently affect the behaviour of cells. Structurally, there are two main classes of contact, both associated with cell adhesion molecules. One class is associated with specializations at discrete regions of the cell surface that are ultrastructurally distinct. These are described on page 6. The second, general, class of adhesive contact has no obvious associated ultrastructural features.

General adhesive contacts

One class of transmembrane or membrane-anchored glycoproteins that project externally from the plasma membrane, and which form adhesive contacts, are the cell adhesion molecules. There are a number of molecular subgroups, which are broadly divisible on the basis of their calcium dependence. Their numerous functions include tissue formation and morphogenesis and there is evidence for coordinated interaction between the subgroups.

Calcium-dependent adhesion molecules

Cadherins, selectins and integrins are calcium-dependent adhesion molecules. Cadherins are transmembrane proteins, with five heavily glycosylated external domains. They are responsible for strong general intercellular adhesion, as well as being components of some specialized adhesive contacts, and are attached by linker proteins (catenins) at their cytoplasmic ends to underlying cytoskeletal fibres (either actin or intermediate filaments). Different cell types possess different members of the cadherin family, e.g. N-cadherins in nervous tissue, E-cadherins in epithelia, and P-cadherins in the placenta. These molecules bind to those of the same type in other cells (homophilic binding), so that cells of the same class adhere to each other preferentially, forming tissue aggregates or layers, as in epithelia. For a review of cadherin-mediated adhesion in morphogenesis, see Gumbiner (2005).

Selectins are found on leukocytes, platelets and vascular endothelial cells. They are transmembrane lectin glycoproteins that can bind with low affinity to the carbohydrate groups on other cell surfaces to permit movement between the two, e.g. the rolling adhesion of leukocytes on the walls of blood vessels (p. 136). They function cooperatively in sequence with integrins, which strengthen the selectin adhesion.

Integrins are glycoproteins that typically mediate adhesion between cells and extracellular matrix components such as fibronectin, collagen, laminin. They integrate interactions between the matrix and the cell cytoskeleton to which they are linked, and so facilitate cell migration within the matrix. An integrin molecule is formed of two subunits (α and β), each of which has several subtypes. Combinations of alternative subunits provide some 24 known integrin heterodimers, each one directed to a particular extracellular molecule, although there is considerable overlap in specificity. Some integrins depend for their binding on magnesium, rather than calcium.

Calcium-independent adhesion molecules

The best known calcium-independent adhesion molecules are glycoproteins that have external domains related to immunoglobulin molecules. Most are transmembrane proteins. Some are entirely external, either attached to the plasma membrane by a glycosylphosphatidylinositol anchor, or secreted as soluble components of the extracellular matrix. Different types are expressed in different tissues. Neural cell adhesion molecules (N-CAMs) are found on a number of cell types, but are expressed widely by neural cells. Intercellular adhesion molecules (I-CAMs) are expressed on vascular endothelial cells. Cell adhesion molecule binding is predominantly homophilic, although some use a heterophilic mechanism, e.g. vascular intercellular adhesion molecule (VCAM), which can bind to integrins.

For further information on all aspects of cell adhesion molecules and intercellular contacts, see Pollard & Earnshaw (2007).

Specialized adhesive contacts

Specialized adhesive contacts, some of which mediate activities other than simple mechanical cohesion, are localized regions of the cell surface with particular ultrastructural characteristics. Three major classes exist: occluding, adhesive and communicating junctions (Fig. 1.5).

Occluding junctions (tight junctions, zonula occludens)

Occluding junctions create diffusion barriers in continuous layers of cells, including epithelia, mesothelia and endothelia, and prevent the passage of materials across the cellular layer through intercellular spaces. They form a continuous belt (zonula) around the cell perimeter, near the apical surface in cuboidal or columnar epithelial cells. At a tight junction, the membranes of the adjacent cells come into contact, so that the gap between them is obliterated. Freeze-fracture electron microscopy shows that the contacts between the membranes lie along branching and anastomosing ridges formed by the incorporation of chains of intramembranous protein particles on the P face of the lipid bilayer (Fig. 1.5C).

This arrangement ensures that substances can only pass through the layer of cells by diffusion or transport through their apical membranes and cytoplasm. The cells thus selectively modify the environment on either side of the layer. Occluding junctions also create regional differences in the plasma membranes of the cells in which they are found. For example, in epithelia, the composition of the apical plasma membrane differs from that of the basolateral regions (see Fig. 1.3), and this allows these regions to engage in functions such as directional transport of ions and uptake of macromolecules. Because tight junctions have high concentrations of fixed transmembrane proteins, they act as barriers to lateral diffusion of lipid and protein within membranes. The integrity of tight junctions is calcium-dependent. Cells can transiently alter the permeability of their tight junctions to increase passive paracellular transport in some circumstances.

Adhesive junctions

Adhesive junctions include intercellular and cell–extracellular matrix contacts, where cells adhere strongly to each other or to adjacent matrix components. Intercellular contacts can be subdivided according to the extent and location of the contact. They all display a high concentration of cell adhesion molecules, which externally bind adjacent cells, and internally link to the cytoskeleton via intermediary proteins.

Zonula adherens (intermediate junction)

A zonula adherens is a continuous, belt-like zone of adhesion around the apical perimeters of epithelial, mesothelial and endothelial cells, parallel and just basal to the tight junction in epithelia. High concentrations of cadherins occur in this zone; their cytoplasmic ends are anchored via the proteins vinculin and α-actinin to a layer of actin microfilaments. These junctions help to reinforce the intercellular attachment of the tight junction and prevent its mechanical disruption. The gap between cell surfaces is about 20 nm. Usually, no electron-dense material is observed within this intercellular space.

Fascia adherens

A fascia adherens is similar to a zonula adherens, but is more limited in extent and forms a strip or patch of adhesion, e.g. between smooth muscle cells, in the intercalated discs of cardiac muscle cells and between glial cells and neurones. The junctions involve cadherins attached indirectly to actin filaments on the inner side of the membrane.

Desmosomes (maculae adherentes)

Desmosomes are limited, plaque-like areas of particularly strong intercellular contact. They can be located anywhere on the cell surface. In epithelial cells, there may be a circumferential row of desmosomes parallel to the tight and intermediate junctional zones, an arrangement that forms the third, most basally situated, component of the epithelial apical junctional complex (Fig. 1.5). The intercellular gap is approximately 25 nm, is filled with electron-dense filamentous material running transversely across it and is also marked by a series of densely staining bands running parallel to the cell surfaces. Adhesion is mediated by calcium-dependent cadherins, desmoglein and desmocollin. Within the cells on either side, a cytoplasmic density underlies the plasma membrane and includes the anchor proteins desmoplakin and plakoglobin, into which the ends of intermediate filaments are inserted. The type of intermediate filament depends on cell type, e.g. keratins are found in epithelia and desmin filaments in cardiac muscle cells. Desmosomes form strong anchorage points, likened to spot-welds, between cells subject to mechanical stress, e.g. in the prickle cell layer of the epidermis, where they are extremely numerous and large.

Hemidesmosomes

Hemidesmosomes are best known as anchoring junctions between the bases of epithelial cells and the basal lamina. Ultrastructurally, they resemble a single-sided desmosome, anchored on one side to the plasma membrane, and on the other to the basal lamina and adjacent collagen fibrils (Fig. 1.5D and see Fig. 7.5). On the cytoplasmic side of the membrane there is a dense plaque into which keratin filaments are inserted. Hemidesmosomes use integrins as their adhesion molecules, whereas desmosomes use cadherins.

Less highly structured attachments with a similar arrangement exist between many other cell types and their surrounding matrix, e.g. between smooth muscle cells and their matrix fibrils, and between the ends of skeletal muscle cells and tendon fibres. The smaller, punctate adhesions resemble focal adhesion plaques.

Focal adhesion plaques

Focal adhesion plaques are regions of local attachment between cells and the extracellular matrix. They are typically situated at or near the ends of actin filament bundles (stress fibres), which are anchored through intermediary proteins to the cytoplasmic domains of integrins. In turn, these are attached at their external ends to collagen or other filamentous structures in the extracellular matrix. They are usually short-lived: their formation and subsequent disruption are part of the motile behaviour of migratory cells.

Gap junctions (communicating junctions)

Gap junctions resemble tight junctions in transverse section, but the two apposed lipid bilayers are separated by an apparent gap of 3 nm which is bridged by numerous transmembrane channels (connexons). Connexons are formed by a ring of six connexin proteins in each membrane. Their external surfaces meet those of the adjacent cell in the middle. A minute central pore links one cell to the next (Fig. 1.5). These channels may exist in small numbers, and this makes them difficult to detect structurally. However, they lower the transcellular electrical resistance and so can be detected by microelectrodes. Larger assemblies of many thousands of channels are often packed in hexagonal arrays (Fig. 1.5B). Such junctions form limited attachment plaques rather than continuous zones, and so allow free passage of substances within the adjacent intercellular space, unlike tight junctions. They occur in numerous tissues including the liver, epidermis, pancreatic islet cells, connective tissues, cardiac muscle and smooth muscle, and are also common in embryonic tissues. In the central nervous system, they are found in the ependyma and between neuroglial cells, and they form electrical synapses between some types of neurone, although this is rare in humans. Very recently, a second family of gap junctional proteins has been discovered, the pannexins. In humans, expression of pannexins has been most extensively studied in the nervous system (reviewed in Litvin et al 2006).

Although gap junctions form diffusion channels between cells, the size of their apertures limits diffusion to small molecules and ions (up to a molecular weight of about 1000 kDa). Thus they admit sodium, potassium and calcium ions, various second messenger components, and a number of metabolites, but they exclude messenger RNA and other macromolecules. In some excitable tissues (e.g. cardiac and smooth muscle), one cell can activate another electrically by current flow through gap junctions. Communicating junctions probably permit metabolic cooperation between groups of adjacent cells; the significance of this activity in embryogenesis, normal tissue function, homeostasis and repair is only beginning to be understood.

Other types of junction

Chemical synapses and neuromuscular junctions are specialized areas of intercellular adhesion where neurotransmitters secreted from a neuronal terminal gain access to specialized receptor molecules on a recipient cell surface. They are described on pages 44 and 62, respectively.

Cell signalling

Cellular systems in the body communicate with each other to coordinate and integrate their functions. This occurs through a variety of processes known collectively as cell signalling, in which a signalling molecule produced by one cell is detected by another, almost always by means of a specific receptor protein molecule. The recipient cell transduces the signal, which it most usually detects at the plasma membrane, into intracellular chemical messages that change cell behaviour.

The signal may act over a long distance, e.g. endocrine signalling through the release of hormones into the bloodstream, or neuronal synaptic signalling via electrical impulse transmission along axons and subsequent release of chemical transmitters of the signal at synapses (p. 44) or neuromuscular junctions (p. 62). A specialized variation of endocrine signalling (neurocrine or neuroendocrine signalling) occurs when neurones or paraneurones (e.g. chromaffin cells of the suprarenal medulla) secrete a hormone into interstitial fluid and the bloodstream.

Alternatively, signalling may occur at short range through a paracrine mechanism, in which cells of one type release molecules into the interstitial fluid of the local environment, to be detected by nearby cells of a different type that express the specific receptor protein. Neurocrine cell signalling uses chemical messengers found also in the central nervous system which may act in a paracrine manner via interstitial fluid or reach more distant target tissues in the bloodstream. Cells may generate and respond to the same signal. This is autocrine signalling, a phenomenon that reinforces the coordinated activities of a group of like cells, which respond together to a high concentration of a local signalling molecule. The most extreme form of short-distance signalling is contact-dependent (juxtacrine) signalling, where one cell responds to transmembrane proteins of an adjacent cell that bind to surface receptors in the responding cell membrane. Contact-dependent signalling also includes cellular responses to integrins on its surface binding to elements of the extracellular matrix. Juxtacrine signalling is important during development and in immune responses.

These different types of intercellular signalling mechanism are illustrated in Figure 1.6. For further reading, see Alberts et al (2002) and Pollard & Earnshaw (2007).

  

Fig. 1.6  The different modes of cell–cell signalling.

Signalling molecules and their receptors

The majority of signalling molecules (ligands) are hydrophilic. They cannot cross the plasma membrane of a recipient cell to effect changes intracellularly unless they first bind to a plasma membrane receptor protein. Ligands are mainly proteins (usually glycoproteins), polypeptides or highly charged biogenic amines. They include: classic peptide hormones of the endocrine system; cytokines, which are mainly of haemopoietic cell origin and involved in inflammatory responses and tissue remodelling (e.g. the interferons, interleukins, tumour necrosis factor, leukaemia inhibitory factor); polypeptide growth factors (e.g. the epidermal growth factor superfamily, nerve growth factor, platelet-derived growth factor, the fibroblast growth factor family, transforming growth factor beta and the insulin-like growth factors). Polypeptide growth factors are multifunctional molecules with more widespread actions and cellular sources than their names suggest. They and their receptors are commonly mutated or aberrantly expressed in certain cancers. The cancer-causing gene variant is termed a transforming oncogene and the normal (wild-type) version of the gene is a cellular oncogene or proto-oncogene. The activated receptor acts as a transducer to generate intracellular signals, which are either small diffusible second messengers (e.g. calcium, cyclic adenosine monophosphate or the plasma membrane lipid-soluble diacylglycerol), or larger protein complexes that amplify and relay the signal to target control systems. For further reading on growth factors and other signalling molecules, see Epstein (2003).

Some signals are hydrophobic and able to cross the plasma membrane freely. Classic examples are the steroid hormones, thyroid hormones, retinoids and vitamin D. Steroids, for instance, enter cells non-selectively, but elicit a specific response only in those target cells which express specific cytoplasmic or nuclear receptors. Light stimuli also cross the plasma membranes of photoreceptor cells and interact intracellularly, at least in rod cells, with membrane-bound photosensitive receptor proteins. Hydrophobic ligands are transported in the bloodstream or interstitial fluids, generally bound to carrier proteins, and they often have a longer half-life and longer-lasting effects on their targets than do water-soluble ligands.

A separate group of signalling molecules that are able to cross the plasma membrane freely is typified by the gas, nitric oxide. The principal target of short-range nitric oxide signalling is smooth muscle, which relaxes in response. Nitric oxide is released from vascular endothelium as a result of the action of autonomic nerves that supply the vessel wall. It causes local relaxation of smooth muscle and dilation of vessels. In the penis, this mechanism is responsible for penile erection. Nitric oxide is unusual among signalling molecules in having no specific receptor protein; instead, it acts directly on intracellular enzymes of the response pathway.

Receptor proteins

There are some 20 different families of receptor proteins, each with several isoforms responding to different ligands. The great majority of these receptors are transmembrane proteins. Members of each family share structural features that indicate either shared ligand-binding characteristics in the extracellular domain or shared signal transduction properties in the cytoplasmic domain, or both. There is little relationship either between the nature of a ligand and the family of receptor proteins to which it binds and activates, or the signal transduction strategies by which an intracellular response is achieved. The same ligand may activate fundamentally different types of receptor in different cell types.

Cell surface receptor proteins are generally grouped according to their linkage to one of three intracellular systems: ion channel-linked receptors; G-protein-coupled receptors; receptors that link to enzyme systems. Other receptors do not fit neatly into any of these categories. All the known G-protein-coupled receptors belong to a structural group of proteins that pass through the membrane seven times in a series of serpentine loops. These receptors are thus known as seven-pass transmembrane receptors or, because the transmembrane regions are formed from α-helical domains, as seven-helix receptors. The most well-known of this large group of phylogenetically ancient receptors are the odorant-binding proteins of the olfactory system, the light-sensitive receptor protein, rhodopsin, and many of the receptors for clinically useful drugs. A comprehensive list of receptor proteins, their activating ligands and examples of the resultant biological function, is given in Pollard & Earnshaw (2007).

Intracellular signalling

A wide variety of small molecules carry signals within cells, conveying the signal from its source (e.g. activated plasma membrane receptor) to its target (e.g. the nucleus). These second messengers convey signals as fluctuations in local concentration, according to rates of synthesis and degradation by specific enzymes (e.g. cyclases involved in cyclic nucleotide (cAMP, cGMP) synthesis), or, in the case of calcium, according to the activities of calcium channels and pumps. Other, lipidic, second messengers such as phosphatidylinositol, derive from membranes and may act within the membrane to generate downstream effects. For further consideration of the complexity of intracellular signalling pathways, see Pollard & Earnshaw (2007).

Transport across cell membranes

Lipid bilayers are increasingly impermeable to molecules as they increase in size or hydrophilicity. Transport mechanisms are therefore required to carry essential polar molecules, including ions, nutrients, nucleotides and metabolites of various kinds, across the plasma membrane and into or out of membrane-bound intracellular compartments. Transport is facilitated by a variety of membrane transport proteins, each with specificity for a particular class of molecule, e.g. sugars. Transport proteins fall mainly into two major classes, channel proteins and carrier proteins.

Channel proteins form aqueous pores in the membrane, which open and close under the regulation of intracellular signals, e.g. G-proteins, to allow the flux of solutes (usually inorganic ions) of specific size and charge. Transport through ion channels is always passive and ion flow through an open channel depends only on the ion concentration gradient and its electronic charge, and the potential difference across the membrane. These factors combine to produce an electrochemical gradient, which governs ion flux. Channel proteins are utilized most effectively by the excitable plasma membranes of nerve cells, where the resting membrane potential can change transiently from about −80 mV (negative inside the cell) to +40 mV (positive inside the cell) when stimulated by a neurotransmitter (as a result of the opening and subsequent closure of channels selectively permeable to sodium and potassium).

Carrier proteins bind their specific solutes, such as amino-acids, and transport them across the membrane through a series of conformational changes. This latter process is slower than ion transport through membrane channels. Transport by carrier proteins can occur either passively by simple diffusion, or actively against the electrochemical gradient of the solute. Active transport must therefore be coupled to a source of energy, such as ATP generation, or energy released by the coordinate movement of an ion down its electrochemical gradient. Linked transport can be in the same direction as the solute, in which case the carrier protein is described as a symporter, or in the opposite direction, when the carrier acts as an antiporter.

Translocation of proteins across intracellular membranes

Proteins are generally synthesized on ribosomes in the cytosol or on the rough endoplasmic reticulum. A few are made on mitochondrial ribosomes. Once synthesized, many proteins remain in the cytosol, where they carry out their functions. Others, such as integral membrane proteins or proteins for secretion, are translocated across intracellular membranes for post-translational modification and targeting to their destinations. This is achieved by the signal sequence, an addressing system contained within the protein sequence of amino-acids, which is recognized by receptors or translocators in the appropriate membrane. Proteins are thus sorted by their signal sequence (or set of sequences that become spatially grouped as a signal patch when the protein folds into its tertiary configuration), so that they are recognized by and enter the correct intracellular membrane compartment.

Exocytosis and endocytosis

Secreted proteins, lipids, mucins, small molecules such as amines and other cellular products destined for export from the cell are transported to the plasma membrane in small vesicles released from the trans face of the Golgi apparatus. This pathway is either constitutive, in which transport and secretion occur more or less continuously, or it is regulated by external signals, as in the control of salivary secretion by autonomic neural stimulation. In regulated secretion, the secretory product is stored temporarily in membrane-bound secretory granules or vesicles. Exocytosis is achieved by fusion of the secretory vesicular membrane with the plasma membrane and release of the vesicle contents into the extracellular domain.

In polarized cells, e.g. most epithelia, exocytosis occurs at the apical plasma membrane and the cells secrete into a duct lumen or onto a free surface such as the lining of the stomach. In hepatocytes, bile is secreted across a very small area of plasma membrane forming the wall of the bile canaliculus. This region is defined as the apical plasma membrane, and is the site of exocrine secretion, whereas secretion of hepatocyte plasma proteins into the bloodstream is targeted to the basolateral surfaces facing the sinusoids. Packaging of different secretory products into appropriate vesicles takes place in the trans-Golgi network. Delivery of secretory vesicles to their correct plasma membrane domains is achieved by sorting sequences in the cytoplasmic tails of vesicular membrane proteins.

There are other mechanisms in which initial delivery of secretory products is less selective, but is followed by selective retention (or degradation) or reprocessing and redistribution by endosomes. Ultimately, secretory vesicles undergo docking, priming (to prepare the vesicle for a regulatory signal, where secretion is regulation-dependent) and fusion with the plasma membrane to release their contents. The process of exocytosis also delivers integral membrane components to the cell surface in the normal turnover and recycling of the plasma membrane. However, excess plasma membrane generated by vesicle fusion during exocytosis is rapidly removed by concurrent endocytosis.

The process of endocytosis involves the internalization of vesicles derived from the plasma membrane. The vesicles may contain: engulfed fluids and solutes from the extracellular interstitial fluid (pinocytosis); larger macromolecules, often bound to surface receptors (receptor-mediated endocytosis); particulate matter, including microorganisms or cellular debris (phagocytosis). Pinocytosis generally involves small fluid-filled vesicles and is a marked property of capillary endothelium, e.g. where vesicles containing nutrients and oxygen dissolved in blood plasma are transported from the vascular lumen to the endothelial basal plasma membrane (see Fig. 6.11). Interstitial fluid containing dissolved carbon dioxide is also taken up by pinocytosis for simultaneous transportation across the endothelial cell wall in the opposite direction, for release into the bloodstream by exocytosis. This shuttling of pinocytotic vesicles is also termed transcytosis. Larger volumes of fluid are engulfed by dendritic cells, e.g. in the process of sampling interstitial fluids by macropinocytosis in immune surveillance for antigens (p. 79). Interstitial fluid is inevitably taken up during receptor-mediated endocytosis when ligands are internalized.

Receptor-mediated endocytosis, also known as clathrin-dependent endocytosis, is initiated at specialized regions of the plasma membrane known as clathrin-coated pits. Clathrin is a protein that cross-links adjacent adaptor protein (adaptin) complexes to form a basket-like structure, bending the membrane inwards into a hemisphere. Much, but not all, fluid-phase pinocytosis also utilizes clathrin-coated pits. Ligands such as the iron-transporting protein, transferrin, and the cholesterol-transporting low-density lipoprotein bind to their receptors, which cluster in clathrin-coated pits through an interaction with adaptins. The pits then invaginate and pinch off from the plasma membrane, internalizing both receptor and ligand. The processing of endocytic vesicles and their contents is described on p. 12. For further details of the molecular mechanisms of endocytosis, see Alberts et al (2002) or Pollard & Earnshaw (2007).

Phagocytosis

Phagocytosis is a property of many cell types, but is most efficient in cells specialized for this activity. The professional phagocytes of the body belong to the monocyte lineage of haemopoietic cells, in particular the tissue macrophages (p. 78). Other effective phagocytes are neutrophil granulocytes and most dendritic cells (p. 79), which are also of haemopoietic origin. Phagocytosis plays an important part in the immune defence system of the body, in which the amoeboid process of ingestion of organisms for nutrition has evolved into a mechanism for the clearance of microorganisms invading the body. Macrophages also ingest particulate material including inorganic matter, such as inhaled dust particles, in addition to debris from dead cells and protein aggregates such as immune complexes in the blood, airways, interstitial spaces and connective tissue matrices.

Phagocytosis is a triggered process, initiated when a phagocytic cell binds to a particle or organism, often through a process of molecular recognition. Typically, a pathogenic microorganism may first be coated by antibodies, which are bound in turn by receptors for the Fc portion of the antibody molecule expressed by macrophages and neutrophils; in this way the microorganism is attached to the cell. This triggers the production of large pseudopodia, which engulf the organism and internalize it, as their pseudopod tips fuse together. The process appears to depend on actin–myosin-based cellular motility and, unlike receptor-mediated endocytosis, it is energy dependent. Phagosomes thus formed are as large as the body they engulf and can be a considerable proportion of the volume of the phagocytic cell. Inside the cell, the phagosome fuses with lysosomes, which degrade its contents.

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