In many cases, a concentration and/or electrical gradi- ent between the inside and outside of the cell drives solute movement via passive transport (virtually all plasma mem- branes have an electrical potential difference across them, with the inside negative relative to the outside). In other cases, active transport of certain solutes against a concentra- tion gradient is accomplished by carrier molecules (not channels) using energy released by ATP hydrolysis or a coupled ion gradient. Transporter ATPases include the notorious multidrug resistance (MDR) protein, which pumps polar compounds (e.g., chemotherapeutic drugs) out of cells and may render cancer cells resistant to treatment. #101 | | |
Because membranes are freely permeable to water, it moves into and out of cells by osmosis, depending on rela- tive solute concentrations. Thus, extracellular salt in excess of that in the cytosol (hypertonicity) causes a net movement of water out of cells, whereas hypotonicity causes a net movement of water into cells. The cytosol is rich in charged metabolites and protein species, which attract a large number of counterions that tend to increase the intracel- lular osmolarity. As a consequence, to prevent overhydra- tion cells must constantly pump out small inorganic ions (e.g., Na+)—typically through the activity of membrane ion-exchanging ATPases. Loss of the ability to generate energy (e.g., in a cell injured by toxins or ischemia) there- fore results in osmotic swelling and eventual rupture of cells. Similar transport mechanisms also regulate intracel- lular and intraorganellar pH; most cytosolic enzymes prefer to work at pH 7.4, whereas lysosomal enzymes func- tion best at pH 5 or less. #102 | | |
Receptor-Mediated and #103 | | |
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Uptake of fluids or macromolecules by the cell, called endo- #105 | | |
cytosis, occurs by two fundamental mechanisms (Fig. 1.8). Certain small molecules—including some vitamins—are taken up by invaginations of the plasma membrane called caveolae. For larger molecules, uptake occurs after binding to specific cell-surface receptors; internalization occurs through a membrane invagination process driven by an intracellular matrix of clathrin proteins. Clathrin is a hexamer of proteins that spontaneously assembles into a basketlike lattice to drive the invagination process. We shall come back to these later. #106 | | |
The process by which large molecules are exported from cells is called exocytosis. In this process, proteins synthe- sized and packaged within the RER and Golgi apparatus are concentrated in secretory vesicles, which then fuse with the plasma membrane and expel their contents. #107 | | |
Transcytosis is the movement of endocytosed vesicles between the apical and basolateral compartments of cells; this is a mechanism for transferring large amounts of intact proteins across epithelial barriers (e.g., ingested antibodies in maternal milk across intestinal epithelia) or for the rapid movement of large volumes of solute. In fact, increased transcytosis probably plays a role in the increased vascular permeability seen in tumors. #108 | | |
We now return to the two forms of endocytosis men- tioned earlier. #109 | | |
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Caveolae-mediated endocytosis. Caveolae (“little caves”) are noncoated plasma membrane invaginations associ- ated with GPI-linked molecules, cyclic adenosine monophosphate (cAMP)-binding proteins, SRC-family kinases, and the folate receptor. Caveolin is the major structural protein of caveolae. Internalization of caveo- lae with any bound molecules and associated extracel- lular fluid is denoted potocytosis—literally “cellular #111 | | |
sipping.” Although caveolae likely participate in the transmembrane delivery of some molecules (e.g., folate), they also appear to contribute to the regulation of trans- membrane signaling and/or cellular adhesion via the internalization of receptors and integrins. Mutations in caveolin are associated with muscular dystrophy and electrical abnormalities in the heart. #112 | | |
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Pinocytosis and receptor-mediated endocytosis (Fig. 1.8). Pinocytosis (“cellular drinking”) is a fluid-phase process. The plasma membrane invaginates and is pinched off to form a cytoplasmic vesicle; after delivering their cargo, endocytosed vesicles recycle back to the plasma membrane (exocytosis) for another round of ingestion. Endocytosis and exocytosis are tightly balanced and highly active, as a cell typically pinocytoses 10% to 20% of its own cell volume each hour, or about 1% to 2% of its plasma membrane each minute. Pinocytosis and receptor-mediated endocytosis begin with the formation of a clathrin-coated pit containing the ligand to be inter- nalized (by itself or bound to the receptor), which rapidly invaginates and pinches off to form a clathrin-coated vesicle. Thus, trapped within the vesicle is a gulp of the extracellular milieu, as well as receptor-bound macro- molecules as described below. The vesicles then rapidly uncoat and fuse with an acidic intracellular structure called the early endosome, which progressively matures to late endosomes and ultimately fuses with lysosomes. #114 | | |
Receptor-mediated endocytosis is the major uptake mechanism for certain macromolecules, as exemplified by transferrin and low-density lipoprotein (LDL). These macromolecules bind to receptors that localize to clathrin-coated pits. After binding to their specific recep- tors, LDL and transferrin are endocytosed in vesicles that mature into early and late endosomes. In the acidic environment of the endosome, LDL and transferrin release their bound ligands (cholesterol and iron, respec- tively), which then exit into the cytosol, and the LDL receptor and transferrin receptor subsequently recycle to the plasma membrane. Defects in receptor-mediated transport of LDL are responsible for familial hypercho- lesterolemia, as described in Chapter 7. #115 | | |
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The ability of cells to adopt a particular shape, maintain polarity, organize the intracellular organelles, and move about depends on the intracellular scaffolding of pro- teins called the cytoskeleton (Fig. 1.9). In eukaryotic cells, there are three major classes of cytoskeletal proteins: #117 | | |
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Actin microfilaments are fibrils 5- to 9-nm in diameter formed from the globular protein actin (G-actin), the most abundant cytosolic protein in cells. G-actin mono- mers noncovalently polymerize into long filaments (F-actin) that intertwine to form double-stranded #119 | | |
helices. In muscle cells, the filamentous protein myosin binds to actin and moves along it, driven by ATP hydro- lysis (the basis of muscle contraction). In non-muscle cells, F-actin assembles via an assortment of actin- binding proteins into well-organized bundles and net- works that control cell shape and movement. #120 | | |
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Intermediate filaments are fibrils 10-nm in diameter that comprise a large and heterogeneous family. Members include lamins A, B, and C, which contribute to the struc- ture of nuclear lamina. Individual types of intermediate filaments have characteristic tissue-specific patterns of expression that are useful for identifying the cellular origin of poorly differentiated tumors. #122 | | |
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Vimentin: Mesenchymal cells (fibroblasts, endothe- lium) anchoring intracellular organelles #124 | | |
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Desmin: Muscle cells, forming the scaffold on which actin and myosin contract #126 | | |
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Neurofilaments: Axons of neurons, imparting strength and rigidity #128 | | |
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Glial fibrillary acidic protein: Glial cells that support neurons #130 | | |
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Cytokeratins: Epithelial cells express more than 30 dis- #132 | | |
tinct varieties with distinct patterns of expression in different lineages (e.g., lung versus gastrointestinal epithelia). These can serve as histochemical markers for various epithelia #133 | | |
Intermediate filaments are found predominantly in a ropelike polymerized form and primarily serve to impart tensile strength and allow cells to bear mechani- cal stress. The nuclear membrane lamins are important not only for maintaining nuclear morphology but also for regulating nuclear gene transcription. The critical roles of lamins is emphasized by rare but fascinating disorders caused by lamin mutations, which range from certain forms of muscular dystrophy to progeria, a disease of premature aging. Intermediate filaments also form the major structural proteins of epidermis and hair. #134 | | |
Microtubules are 25-nm-thick fibrils composed of nonco- valently polymerized dimers of α- and β-tubulin arrayed in constantly elongating or shrinking hollow tubes with a defined polarity; the ends are designated “+” or “−.” The “−” end is typically embedded in a microtubule orga- nizing center (MTOC or centrosome) near the nucleus where it is associated with paired centrioles, while the “+” end elongates or recedes in response to various stimuli by the addition or subtraction of tubulin dimers. Microtubules are involved in several important cellular functions: #135 | | |
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Support cables for “molecular motor” proteins that allow the movement of vesicles and organelles around cells. Kinesins are the motors for anterograde (− to +) transport, whereas dyneins move cargo in a retrograde direction (+ to −). #138 | | |
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The mechanical support for sister chromatid separa- tion during mitosis #140 | | |
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The core of primary cilia, single nonmotile projections on nucleated cells that help regulate proliferation and differentiation #142 | | |
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The core of motile cilia (e.g., in bronchial epithelium) or flagella (in sperm) #144 | | |
Cell-Cell Interactions #145 | | |
Cells interact and communicate with one another by forming junctions that provide mechanical links and enable surface receptors to recognize ligands on other cells. Cell junctions are organized into three basic types (Fig. 1.9): #146 | | |
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Occluding junctions (tight junctions) seal adjacent cells together to create a continuous barrier that restricts the paracellular (between cells) movement of ions and other molecules. Viewed en face, occluding junctions form a tight meshlike network of macromolecular contacts between neighboring cells. The complexes that mediate these cell–cell interactions are composed of multiple proteins, including occludin and claudin. In addition to being a high-resistance barrier to solute movement, occluding junctions also maintain cellular polarity by forming the boundary between apical and basolateral domains of cells. Significantly, these junctions (as well as the desmosomes described later) are dynamic struc- tures that can dissociate and reform as required to facili- tate epithelial proliferation or inflammatory cell migration. #148 | | |
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Anchoring junctions (desmosomes) mechanically attach cells—and their intracellular cytoskeletons—to other cells or to the ECM. When the adhesion focus is between cells, and is small and rivetlike, it is designated a spot desmosome. When such a focus attaches the cell to the ECM, it is called a hemidesmosome. Similar adhesion domains can also occur as broad bands between cells, where they are denoted as belt desmosomes. Cell–cell des- mosomal junctions are formed by the homotypic asso- ciation of transmembrane glycoproteins called cadherins. #150 | | |
