Cell Biology: The Cell Surface

Differentiations of Cell Surface

The cell surface of certain cells performs various physiological activities such as absorption, secretion, and transportation. To perform such specialized functions, modifications occur in the plasma membrane of such cells. These cell surface differentiations include microvilli, invaginations, basement membrane, and cell-to-cell interconnections or junctions.

Invaginations

The bases (inner ends) of certain cells, such as kidney cells, perform active transportation and contain many invaginations (infoldings) of the plasma membrane. At the base of these folds, septa develop, forming narrow compartments of basal cytoplasm. These infoldings contain many mitochondria which, along with the enzymes of the plasma membrane, provide an energy-rich compound (ATP) to the plasma membrane for the active transportation of solutes.

Microvilli

These are finger-like, slender projections of the plasma membrane found in mesothelial cells, hepatic cells, and epithelial cells of the intestine. Microvilli increase the effective surface area for absorption. These microvilli are 0.6 to 0.8 μm long and 0.1 μm in diameter. Within the cytoplasmic core of a microvillus, fine microfilaments are observed, which form a terminal web in the underlying cytoplasm. The microfilaments contain actin and are attached to the tips of the microvilli by α-actinin; their function is to produce contraction of microvilli.

Basement Membrane

The interface between all epithelia and underlying connective tissue is marked by a non-cellular structure called the basement membrane. This membrane comprises two basic layers:

Basal lamina: In contact with the epithelial basal plasma membrane and composed of a fine feltwork of fibrils of collagen of Type IV embedded in an amorphous matrix. It is secreted by the epithelial cells.
Reticular layer: Exists just beneath the basal lamina and is composed of fine reticular fibers of reticulin protein.

The basement membrane provides structural support for epithelia and may constitute an important barrier to the passage of materials between the epithelial and connective tissue compartments.

Tight Junctions (Zonula Occludens)

The cells of both vertebrate and invertebrate animals display junctions designed to prevent or reduce the flow of even small molecules between the lateral surfaces of adjacent cells. Such junctions are particularly characteristic of epithelial tissues. In higher animals, these are termed tight junctions, and in invertebrates, these are called septate junctions. Tight junctions are situated below the apical border (often below the microvillar surface) of the epithelial cells and act as permeability barriers. All nutrients are absorbed from the intestine into one side of the epithelial cell and then released from the other side into the blood because tight intercellular junctions do not allow small molecules to diffuse directly from the intestine lumen into the blood. In pancreatic acinar tissue, they prevent the leakage of pancreatic secretory proteins, including digestive enzymes, into the blood. Tight junctions are composed of thin bands that completely encircle a cell and are in contact with thin bands of adjacent cells.

The tight junctions appear as a network of ridges on the cytoplasmic half of the membrane, with complementary grooves in the outer half. The ridges appear to be composed of two rows of protein particles, as in a zipper, each one belonging to the adjacent cells. The lines of these particles produce the sealing and for this reason have been named sealing strands. Septate junctions perform functions similar to tight junctions. They differ from tight junctions in that the proteins that straddle the gaps occur in parallel rows or septae. Also, in them, adjacent plasma membrane surfaces are not in direct contact, so that the junctional proteins themselves form the seal.

Desmosomes

Desmosomes are abundantly found in tissues that have to withstand severe mechanical stress, such as skin epithelia, bladder, and cardiac muscle. Their presence in such tissues allows the tissues to function as elastic sheets without the individual cells being torn one from another. Desmosomes are of the following three types:

Belt desmosomes (Zonula adherens): They are generally found at the interface between columnar cells, just below the region of tight junctions. They form a band that forms a girdle around the inner surface of the plasma membrane. At the belt desmosome, the plasma membranes of adjacent cells are parallel, thicker than usual, and 15 to 20 nm apart. The intercellular space between them is filled with an amorphous material.
Spot desmosomes: The spot desmosomes act like rivets to hold epithelial cells together at points of contact. The intercellular core or central stratum between the two membranes consists of specific desmosomal material rich in proteins and mucopolysaccharides. Under each facing plasma membrane of the spot desmosome, there is a discoidal intracellular plaque, having non-glycosylated proteins such as desmoplakins I, II, and III. Numerous 10-nm thick intermediate filaments of keratin protein, called tonofilaments, converge towards the plaque. These filaments form a loop in a wide arc and course back into the cytoplasm. There are thinner filaments that arise from each dense plaque and traverse the plasma membrane to form “trans-membrane linkers” in the intercellular space. These linkers provide mechanical coupling and chemically are made of glycosylated proteins, called desmogleins I and II, with the carbohydrate moiety exposed toward the intercellular space.
Hemidesmosomes: They are half desmosomes which resemble spot desmosomes but join the basal surface of an epithelial cell to a basal lamina. They anchor extracellular proteins such as collagen and other proteins to the cell.

Gap Junctions

Many cells of the tissues of higher animals are coupled together by interconnecting gap junctions, nexus, or communicating junctions. The presence of gap junctions explains the ionic or electronic connections between adjacent cells. Such electrical coupling is found extensively in embryonic cells. In adult tissues, it is usually found in epithelia, cardiac cells, and liver cells. Gap junctions are found to permit molecules such as inorganic ions, sugars, amino acids, nucleotides, and vitamins to pass with comparative freedom between one cell and another within a tissue, but they prevent larger molecules, such as proteins, nucleic acids, and polysaccharides from being transferred. This observation also explains the phenomenon of metabolic cooperation or metabolic coupling between cells, e.g., cells can transfer to neighboring cells the molecules which cannot be synthesized by the recipient cells.

There are certain other molecules such as AMP, ADP, ATP and cAMP that can pass through gap junctions. A gap junction appears as a plaque-like contact in which the plasma membranes of adjacent cells are in close apposition, separated by a space of only 2 to 4 nm. Gap junctions consist of hollow channels around which a series of six protein subunits are located; a channel has a diameter of about 1.5 to 2 nm. A connexon appears as an annulus of six subunits surrounding the channel, and it is believed that the sliding of the subunits caused the channel to open and close. The permeability of the channel of the gap junction is regulated by Ca^{2+} ions; if the intercellular Ca^{2+} ion level increases, the permeability is reduced or abolished. The gap junctions or connexons of adjacent cells are believed to line up to provide a continuous channel, made up of two connexons opposed end to end.

Septate Junctions

Are the main occluding junctions. They form a continuous band around each epithelial cell. They ensure barrier properties and control paracellular diffusion of solutes across epithelia in invertebrates. Their morphology is distinct because the interacting plasma membranes are joined by proteins that are arranged in parallel rows with a regular periodicity. SJs are also found in mammals, at the nodes of Ranviers where they form the paranodal junction between axons and myelinated glial cells.

Extracellular Matrix

Many of the cells in tissues of multicellular organisms are embedded in an extracellular matrix consisting of secreted proteins and polysaccharides. The extracellular matrix fills the spaces between cells and binds cells and tissues together. The extracellular matrix not only provides structural support to cells and tissues but also plays important roles in regulating the behavior of cells in multicellular organisms. The extracellular matrix is most abundant in connective tissues. For example, the loose connective tissue beneath epithelial cell layers consists predominantly of an extracellular matrix in which fibroblasts are distributed. Other types of connective tissue, such as bone, tendon, and cartilage, similarly consist largely of extracellular matrix, which is principally responsible for their structure and function. Collagen is the most abundant of the proteins. Its fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other.

Cells have protein receptors on the extracellular surfaces of their plasma membranes. When a molecule within the matrix binds to the receptor, it changes the molecular structure of the receptor. The receptor, in turn, changes the conformation of the microfilaments positioned just inside the plasma membrane. These conformational changes induce chemical signals inside the cell that reach the nucleus and turn “on” or “off” the transcription of specific sections of DNA. This affects the production of associated proteins, thus changing the activities within the cell. The role of the extracellular matrix in cell communication can be seen in blood clotting.

When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When a tissue factor binds with another factor in the extracellular matrix, it causes platelets to adhere to the wall of the damaged blood vessel and stimulates the adjacent smooth muscle cells in the blood vessel to contract (thus constricting the blood vessel). Subsequently, a series of steps are initiated which then prompt the platelets to produce clotting factors. Tissue communication is started when a molecule within the matrix binds a receptor; the end results are conformational changes that induce chemical signals that ultimately change activities within the cell.

Plant Cell Walls

The cell walls of eukaryotes (including fungi, algae, and higher plants) are composed principally of polysaccharides. The basic structural polysaccharide of fungal cell walls is chitin (a polymer of N-acetylglucosamine residues), which also forms the exoskeleton of arthropods. The cell walls of most algae and higher plants are composed principally of cellulose, which is the single most abundant polymer on Earth. Cellulose is a linear polymer of glucose residues which are joined by β(1→4) linkages, which allow the polysaccharide to form long straight chains. Several dozen such chains then associate in parallel with one another to form cellulose microfibrils, which can extend for many micrometers in length.

Within the plant cell wall, cellulose microfibrils are embedded in a matrix consisting of proteins and two other types of polysaccharides: hemicelluloses and pectins. Hemicelluloses are highly branched polysaccharides that are hydrogen-bonded to the surface of cellulose microfibrils. This crosslinks the cellulose microfibrils into a network of tough, fibrous molecules, which is responsible for the mechanical strength of plant cell walls. Pectins are branched polysaccharides containing a large number of negatively charged galacturonic acid residues. Because of these multiple negative charges, pectins bind positively charged ions (such as Ca^{2+}) and trap water molecules to form gels. In the cell wall, the pectins form a gel-like network that is interlocked with the crosslinked cellulose microfibrils. In addition, cell walls contain a variety of glycoproteins that are incorporated into the matrix and are thought to provide further structural support.

The structure and function of cell walls change as plant cells develop. The walls of growing plant cells (called primary cell walls) are relatively thin and flexible, allowing the cell to expand in size. Once cells have ceased growth, they frequently lay down secondary cell walls between the plasma membrane and the primary cell wall. Such secondary cell walls, which are both thicker and more rigid than primary walls, are particularly important in cell types responsible for conducting water and providing mechanical strength to the plant. Primary cell walls contain approximately equal amounts of cellulose, hemicelluloses, and pectins. The more rigid secondary walls generally lack pectin and contain 50 to 80% cellulose. Many secondary walls are further strengthened by lignin, a complex polymer of phenolic residues that is responsible for much of the strength and density of wood. The orientation of cellulose microfibrils also differs in primary and secondary cell walls. The cellulose fibers of primary walls appear to be randomly arranged, whereas those of secondary walls are highly ordered. Secondary walls are frequently laid down in layers in which the cellulose fibers differ in orientation, forming a laminated structure that greatly increases cell wall strength.

One of the critical functions of plant cell walls is to prevent cell swelling as a result of osmotic pressure. In contrast to animal cells, plant cells do not maintain an osmotic balance between their cytosol and extracellular fluids. Osmotic pressure continually drives the flow of water into the cell. This water influx is tolerated by plant cells because their rigid cell walls prevent swelling and bursting. Turgor pressure builds up within the cell, eventually equalizing the osmotic pressure and preventing the further influx of water. It provides the basis for a form of cell growth that is unique to plants. Cell expansion by this mechanism is signaled by plant hormones (auxins) that weaken a region of the cell wall, allowing turgor pressure to drive the expansion of the cell in that direction. The water that flows into the cell accumulates within a large central vacuole, so the cell expands without increasing the volume of its cytosol. Such expansion can result in a 10- to 100-fold increase in the size of plant cells during development.

Intercellular Communication

Communication between cells is called intercellular signaling. Chemical signals are released by a signaling cell and received by a target cell. Target cells have proteins called receptors, which bind to signaling molecules and cause a response. Signaling molecules that bind to receptors are called ligands. Ligands and receptors are specific for each other; a receptor will typically bind only to its specific ligand. There are four categories of chemical signaling found in multicellular organisms: autocrine signaling, paracrine signaling, endocrine signaling, and direct signaling across gap junctions.

Paracrine Signaling

Signals that act locally between cells that are close together are called paracrine signals. Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses that last only a short amount of time. In order to keep the response localized, paracrine ligands are usually quickly degraded by enzymes or removed by neighboring cells. Removing the signals reestablishes the concentration gradient for the signal molecule, allowing them to quickly diffuse through the intracellular space if released again.

The transfer of signals between nerve cells. The tiny space between nerve cells where signal transmission occurs is called a synapse. Signals are propagated along nerve cells by fast-moving electrical impulses. When these impulses reach the end of one nerve cell, chemical ligands called neurotransmitters are released into the synapse by the presynaptic cell. The neurotransmitters diffuse across the synapse. The small distance between nerve cells allows the signal to travel quickly, which enables an immediate response, such as, “take your hand off the stove.” When the neurotransmitter binds the receptor on the surface of the postsynaptic cell, the next electrical impulse is launched. The neurotransmitters are degraded quickly or are reabsorbed by the presynaptic cell so that the recipient nerve cell can recover quickly and be prepared to respond rapidly to the next synaptic signal.

Autocrine Signaling

When a cell responds to its own signaling molecule, it is called autocrine signaling (auto = “self”). Autocrine signaling often occurs with other types of signaling. For example, when a paracrine signal is released, the signaling cell may respond to the signal along with its neighbors. Autocrine signaling often occurs during early development of an organism to ensure that cells develop into the correct tissues. Autocrine signaling also regulates pain sensation and inflammatory responses. Further, if a cell is infected with a virus, the cell can signal itself to undergo programmed cell death, killing the virus in the process.

Endocrine Signaling

Signals from distant cells are called endocrine signals, and they originate from endocrine cells. (In the body, many endocrine cells are located in endocrine glands, such as the thyroid gland, the hypothalamus, and the pituitary gland.) These types of signals usually produce a slower response but have a longer-lasting effect. The ligands released in endocrine signaling are called hormones, signaling molecules that are produced in one part of the body but affect other body regions some distance away. Hormones travel large distances between endocrine cells and their target cells via the bloodstream, which is a relatively slow way to move throughout the body. Because of their form of transport, hormones get diluted and are present in low concentrations when they act on their target cells. A cell targets a distant cell through the bloodstream.

Direct Signaling

Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of neighboring cells. These water-filled channels allow small signaling molecules to diffuse between the two cells. Small molecules, such as calcium ions (Ca^{2+}), are able to move between cells, but large molecules like proteins and DNA cannot fit through the channels. The specificity of the channels ensures that the cells remain independent but can quickly and easily transmit signals. Direct signaling allows a group of cells to coordinate their response to a signal that only one of them may have received. In plants, plasmodesmata are ubiquitous, making the entire plant into a giant communication network. A cell targets a neighboring cell through a gap junction.

Types of Receptors

Receptors are protein molecules in the target cell or on its surface that bind to ligands. There are two types of receptors: internal receptors and cell-surface receptors.

Internal Receptors

Also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of target cells and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane. Once inside the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis (transcription) to mediate gene expression.

Gene expression is the cellular process of transforming the information in a cell’s DNA into a sequence of amino acids, which ultimately forms a protein. When the ligand binds to the internal receptor, a conformational change is triggered that exposes a DNA-binding site on the receptor protein. The ligand-receptor complex moves into the nucleus, then binds to specific regulatory regions of the chromosomal DNA and promotes the initiation of transcription. Transcription is the process of copying the information in a cell’s DNA into a special form of RNA called messenger RNA (mRNA); the cell uses information in the mRNA to link specific amino acids in the correct order, producing a protein. Thus, when a ligand binds to an internal receptor, it can directly influence gene expression in the target cell.

Hydrophobic signaling molecules typically diffuse across the plasma membrane and interact with intracellular receptors in the cytoplasm. Many intracellular receptors are transcription factors that interact with DNA in the nucleus and regulate gene expression.

Cell Surface Receptors

Also known as transmembrane receptors, are integral proteins that bind to external signaling molecules. These receptors span the plasma membrane and perform signal transduction, in which an extracellular signal is converted into an intercellular signal. Because cell-surface receptor proteins are fundamental to normal cell functioning, it should come as no surprise that a malfunction in any one of these proteins could have severe consequences. Errors in the protein structures of certain receptor molecules have been shown to play a role in hypertension (high blood pressure), asthma, heart disease, and cancer. Each cell-surface receptor has three main components: an external ligand-binding domain, or extracellular domain; a hydrophobic membrane-spanning region; and an intracellular domain. Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three general categories of cell-surface receptors: enzyme-linked receptors, ion channel-linked receptors, and G-protein-linked receptors.

Hydrophilic signaling molecules typically work by binding to the extracellular portion of a receptor protein and the signal is then transduced across the membrane.

Enzyme-linked Receptors

Are cell-surface receptors with intracellular domains that are associated with an enzyme. In some cases, the intracellular domain of the receptor itself is an enzyme. Other enzyme-linked receptors have a small intracellular domain that interacts directly with an enzyme. Enzyme-linked receptors normally have large extracellular and intracellular domains, but the membrane-spanning region consists of a single alpha-helix in the peptide strand. When a ligand binds to the extracellular domain of an enzyme-linked receptor, a signal is transferred through the membrane, activating the enzyme. Activation of the enzyme sets off a chain of events within the cell that eventually leads to a response.

A receptor tyrosine kinase is an enzyme-linked receptor with a single transmembrane region, and extracellular and intracellular domains. Binding of a signaling molecule to the extracellular domain causes the receptor to dimerize or bond together. Tyrosine residues on the intracellular domain are then auto-phosphorylated, triggering a downstream cellular response. The signal is terminated by a phosphatase that removes the phosphates from the phosphotyrosine residues.

NB: A kinase is an enzyme that transfers phosphate groups from ATP to another protein. The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules. The phosphorylated residues can then transmit the signal to the next messenger within the cytoplasm. Epidermal growth factor receptors are an example of receptor tyrosine kinases that follows this mode of signaling. Defects in ErbB signaling in this family can lead to neuromuscular diseases such as multiple sclerosis and Alzheimer’s disease.

Ion Channel-linked Receptors

Bind to a ligand and open a channel through the membrane that allows specific ions to pass through. This type of cell-surface receptor has an extensive membrane-spanning region with hydrophobic amino acids. Conversely, the amino acids that line the inside of the channel are hydrophilic to allow for the passage of ions. When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein’s structure that allows ions such as sodium, calcium, magnesium, or hydrogen to pass through.

Ion channel-linked receptors open and allow ions to enter a cell. An example of an ion channel-linked receptor is found on neurons. When neurotransmitters bind to these receptors, a conformational change allows sodium ions to flow across the cell membrane, causing a change in the membrane potential.

G-protein-linked Receptors

Bind to a ligand and activate an associated G-protein. The activated G- protein then interacts with a nearby membrane protein, which may be an ion channel or an enzyme. All G-protein-linked receptors have seven transmembrane domains, but each receptor has a specific extracellular domain and G-protein-binding site. Cell signaling using G-protein-linked receptors occurs as a cycle. Once the ligand binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP. The subunits of the G-protein then split into α and βγ subunits. One or both of these G-protein fragments may be able to activate other proteins in the cell. After a while, the GTP on the active α subunit of the G-protein is hydrolyzed to GDP and the βγ subunit is deactivated. The subunits re-associate to form the inactive G-protein and the cycle begins again. G-protein-linked receptors are used in many physiological processes including those for vision transduction, taste, and regulation of the immune system and inflammation.

Methods of Intracellular Signaling

The induction of a signaling pathway depends on the modification of a cellular component by an enzyme. There are numerous enzymatic modifications that can occur to activate the next component of the pathway. The following are some of the more common events in intracellular signaling:

Phosphorylation

Is the addition of a phosphate group to a molecule in a process called phosphorylation. The phosphate can be added to a nucleotide such as GMP to form GDP or GTP. Phosphates are also often added to serine, threonine, and tyrosine residues of proteins, where they replace the hydroxyl group of the amino acid. The transfer of the phosphate is catalyzed by an enzyme called a kinase. Phosphorylation may activate or inactivate enzymes, and the reversal of phosphorylation, dephosphorylation, will reverse the effect.

In protein phosphorylation, a phosphate group (PO_4^{-3}) is added to residues of the amino acids serine, threonine, or tyrosine. The phosphate group is added by a kinase. ATP is often used as the substrate to add the phosphate group to these amino acids. The phosphate group often results in a shape change in the protein that can activate or turns off the function of the protein.

Second Messengers

Are small molecules that propagate a signal after it has been initiated by the binding of the signaling molecule to the receptor. These molecules help to spread a signal through the cytoplasm by altering the behavior of certain cellular proteins. A second messenger utilized by many different cell types is cyclic AMP (cAMP). Cyclic AMP is synthesized by the enzyme adenylyl cyclase from ATP. The main role of cAMP in cells is to bind to and activate an enzyme called cAMP-dependent kinase (A-kinase). A-kinase regulates many vital metabolic pathways: It phosphorylates serine and threonine residues of its target proteins, activating them in the process. A-kinase is found in many different types of cells, and the target proteins in each kind of cell are different. Another secondary messenger is Ca^{2+} which can be released to flood the cell.

Formation of cyclic AMP (cAMP). cAMP serves as a second messenger in many cell types. Termination of the signal occurs when an enzyme called phosphodiesterase converts cAMP into AMP. Different cells react differently to cAMP.

The alpha subunit from a G-protein receptor is shown activating two different types of signaling. In the first image, cAMP is produced by the enzyme adenylate cyclase when activated by the alpha subunit. cAMP then activates other proteins that affect gene transcription. In the second image, the alpha subunit from the G-protein triggers a cascade that releases Ca^{2+} from the smooth endoplasmic reticulum. In this case, Ca^{2+} is the secondary messenger that causes the cellular response. The G-protein α subunit causes different responses.

Activated α subunit associates with adenylyl cyclase to produce cAMP, triggering a phosphorylation cascade that ultimately alters gene expression.
Activated α subunit activates phospholipase C, ultimately leading to a flood of calcium ions.