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In spot desmosomes, the cadherins are linked to intracellular intermediate filaments and allow extra- cellular forces to be mechanically communicated (and dissipated) over multiple cells. #152 | | |
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In belt desmosomes, the transmembrane adhesion molecules are associated with intracellular actin microfilaments, by which they can influence cell shape and/or motility. #154 | | |
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In hemidesmosomes, the transmembrane connector proteins are called integrins; like cadherins, these attach to intracellular intermediate filaments, and thus they functionally link the cytoskeleton to the ECM. Focal adhesion complexes are large macromolecu- lar complexes that localize at hemidesmosomes, and include proteins that can generate intracellular signals when cells are subjected to increased shear stress, for example, endothelium in the bloodstream, or cardiac myocytes in a failing heart. #156 | | |
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Communicating junctions (gap junctions) mediate the passage of chemical or electrical signals from one cell to another. The junction consists of a dense planar array of 1.5- to 2-nm pores (called connexons) formed by hex- amers of transmembrane protein connexins. These pores permit the passage of ions, nucleotides, sugars, amino acids, vitamins, and other small molecules; the perme- ability of the junction is rapidly reduced by lowered intracellular pH or increased intracellular calcium. Gap junctions play a critical role in cell–cell communi- cation; in cardiac myocytes, for example, cell-to-cell #158 | | |
calcium fluxes through gap junctions allowing the myocardium to behave as a functional syncytium capable of coordinated waves of contraction—the beating of the heart. #159 | | |
Biosynthetic Machinery: Endoplasmic Reticulum and Golgi Apparatus #160 | | |
The structural proteins and enzymes of the cell are con- stantly renewed by a balance between ongoing synthesis and intracellular degradation. The endoplasmic reticulum (ER) is the site of synthesis of all transmembrane proteins and lipids needed for the assembly of plasma membrane and cellular organelles, including the ER itself. It is also the initial site of synthesis of all molecules destined for export out of the cell. The ER is organized into a mesh- like interconnected maze of branching tubes and flattened lamellae forming a continuous sheet around a single lumen that is topologically contiguous with the extracellular envi- ronment. The ER is composed of distinct domains that are distinguished by the presence or absence of ribosomes (Fig. 1.6). #161 | | |
Rough ER (RER): Membrane-bound ribosomes on the cytosolic face of the RER translate mRNA into proteins that are extruded into the ER lumen or become inte- grated into the ER membrane. This process is directed by specific signal sequences on the N-termini of nascent proteins. Proteins insert into the ER fold and must fold properly in order to assume a functional conformation and assemble into higher order complexes. Proper folding of the extracellular domains of many proteins involves the formation of disulfide bonds. A number of inherited disorders, including many cases of familial hypercholes- terolemia (Chapter 6), are cause by mutations that disrupt disulfide bond formation. In addition, N-linked oligosac- charides (sugar moieties attached to asparagine residues) are added in the ER. Chaperone molecules retain proteins in the ER until these modifications are complete and the proper conformation is achieved. If a protein fails to fold and assemble into complexes appropriately, it is retained and degraded within the ER. Moreover, excess accu- mulation of misfolded proteins—exceeding the capac- ity of the ER to edit and degrade them—leads to the ER stress response (also called the unfolded protein response or the UPR), which triggers cell death through apoptosis (Chapter 2). #162 | | |
As an example of the importance of the ER editing func- tion, the disease cystic fibrosis most commonly results from misfolding of the CFTR membrane transporter protein. In cystic fibrosis, the most common mutation in the CFTR gene results in the loss of a single amino acid residue (phe- nylalanine 508), leading in turn to misfolding, ER retention, and degradation of the CFTR protein. The loss of CFTR function leads to abnormal epithelial chloride transport, hyperviscous bronchial secretions and recurrent airway infections (Chapter 7). #163 | | |
Golgi apparatus: From the RER, proteins and lipids des- tined for other organelles or for extracellular export are shuttled into the Golgi apparatus. This organelle consists of stacked cisternae that progressively modify proteins in an orderly fashion from cis (near the ER) to trans (near the plasma membrane); macromolecules are shuttled between the various cisternae within membrane-bound vesicles. As molecules move from cis to trans, the N-linked oligosac- charides originally added to proteins in the ER are pruned and further modified in a stepwise fashion; O-linked oli- gosaccharides (sugar moieties linked to serine or threo- nine) are also appended. Some of this glycosylation is important in directing molecules to lysosomes (via the mannose-6-phosphate receptor); other glycosylation adducts may be important for cell–cell or cell–matrix inter- actions, or for clearing senescent cells (e.g., platelets and red cells). In addition to the stepwise glycosylation of lipids and proteins, the cis Golgi network is where proteins are recycled back to the ER, and the trans Golgi network is where proteins and lipids are dispatched to other organ- elles (including the plasma membrane), or to secretory vesicles destined for extracellular release. The Golgi complex is especially prominent in cells specialized for secretion, including goblet cells of the intestine, bronchial epithelium (secreting large amounts of polysaccharide-rich mucus), and plasma cells (secreting large quantities of antibodies). #164 | | |
Smooth ER (SER): The SER in most cells is relatively sparse, forming the transition zone from RER to transport vesicles moving to the Golgi. However, in cells that syn- thesize steroid hormones (e.g., in the gonads or adrenals), or that catabolize lipid-soluble molecules (e.g., in the liver), the SER may be particularly conspicuous. Indeed, repeated exposure to compounds that are metabolized by the SER (e.g., phenobarbital, which is catabolized by the cyto- chrome P-450 system) leads to a reactive hyperplasia of the SER. The SER also is responsible for sequestering intracel- lular calcium; subsequent release from the SER into the cytosol can mediate a number of responses to extracellular signals. In addition, in muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for the cycli- cal release and sequestration of calcium ions that regulate muscle contraction and relaxation, respectively. #165 | | |
Waste Disposal: Lysosomes and Proteasomes #166 | | |
As already mentioned briefly, cellular waste disposal depends on the activities of lysosomes and proteasomes (Fig. 1.10). #167 | | |
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Lysosomes are membrane-bound organelles containing roughly 40 different acid hydrolases (i.e., enzymes that function best in acidic pH ≤5), including proteases, nucleases, lipases, glycosidases, phosphatases, and sul- fatases. Lysosomal enzymes are initially synthesized in the ER lumen and then tagged with a mannose-6-phos- phate (M6P) residue within the Golgi apparatus. Such M6P-modified proteins are subsequently delivered to lysosomes through trans-Golgi vesicles that express M6P receptors. The other macromolecules destined for catabolism in the lysosomes arrive by one of three other pathways (Fig. 1.10): #169 | | |
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Material internalized by fluid-phase pinocytosis or receptor-mediated endocytosis passes from plasma membrane to early endosome to late endosome, and ultimately into the lysosome, becoming progres- sively more acidic in the process. The early endosome is the first acidic compartment encountered, whereas proteolytic enzymes only begin significant digestion #171 | | |
in the late endosome; late endosomes mature into lysosomes. #172 | | |
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Senescent organelles and large protein complexes can be shuttled into lysosomes by a process called autophagy. Through poorly understood mechanisms, obsolete organelles are corralled by a double mem- brane derived from the ER; the membrane progres- sively expands to encircle a collection of structures and forms an autophagosome, which then fuses with lysosomes where the contents are catabolized. In addition to facilitating the turnover of aged and defunct cellular constituents, autophagy also is used to preserve cell viability during nutrient depletion. The significance of autophagy in cell biology was recognized by the award of the 2016 Nobel Prize to Yoshinori Ohsumi for his discoveries relating to the mechanism of autophagy. This topic is discussed in more detail in Chapter 2. #174 | | |
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Phagocytosis of microorganisms or large fragments of matrix or debris occur primarily in professional phagocytes (macrophages and neutrophils). The material is engulfed to form a phagosome that subse- quently fuses with a lysosome. #176 | | |
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Proteasomes play an important role in degrading cytosolic proteins (Fig. 1.10); these include denatured or misfolded proteins (akin to what occurs within the ER), as well as other proteins whose levels and half-life need to be tightly regulated (e.g., transcription factors). Many (but not all) proteins destined for proteasome destruction are targeted after covalent addition of a protein called ubiq- uitin. Polyubiquitinated molecules are progressively unfolded and funneled into the polymeric proteasome complex, a cylinder containing multiple different prote- ase activities, each with its active site pointed at the hollow core. Proteasomes digest proteins into small (6–12 amino acids) fragments that can subsequently be further degraded to their constituent amino acids and recycled, or presented to immune cells in the context of major histocompatibility complex class I molecules, an important component of host immune surveillance. #178 | | |
CELLULAR METABOLISM AND MITOCHONDRIAL FUNCTION #179 | | |
Mitochondria evolved from ancestral prokaryotes that were engulfed by primitive eukaryotes about 1.5 billion years ago. Their origin explains why mitochondria contain their own DNA genome (circularized, about 1% of the total cellular DNA), which encodes roughly 1% of the total cel- lular proteins and approximately 20% of the proteins involved in oxidative phosphorylation. Although their genomes are small, mitochondria can nevertheless perform all the steps of DNA replication, transcription, and transla- tion. Interestingly, the mitochondrial machinery is similar to present-day bacteria; for example, mitochondria initiate protein synthesis with N-formylmethionine and are sensi- tive to anti-bacterial antibiotics. #180 | | |
Mitochondria are dynamic, constantly undergoing fission and fusion with other mitochondria; in this way, mitochondria can undergo regular renewal to stave off degenerative changes that might occur because of genetic disorders or oxygen free radical damage. Mitochondria turn over rapidly, with estimated half-lives ranging from 1 to 10 days, depending on the tissue, nutritional status, meta- bolic demands, and intercurrent injury. Because the ovum #181 | | |
contributes the vast majority of cytoplasmic organelles to the fertilized zygote, mitochondrial DNA is virtually entirely maternally inherited. However, because the protein constituents of mitochondria derive from both nuclear and mitochondrial genetic transcription, mitochondrial disor- ders may be X-linked, autosomal, or maternally inherited. Mitochondria provide the enzymatic machinery for oxi- dative phosphorylation (and thus the relatively efficient generation of energy from glucose and fatty acid sub- strates). They also have central roles in anabolic metabo- lism and the regulation programmed cell death, so-called “apoptosis” (Fig. 1.11). #182 | | |
Energy Generation. Each mitochondrion has two sepa- rate and specialized membranes. The inner membrane con- tains the enzymes of the respiratory chain folded into cristae. This encloses a core matrix space that harbors the bulk of certain metabolic enzymes, such as the enzymes of the citric acid cycle. Outside the inner membrane is the intermembrane space, site of ATP synthesis, which is, in turn, enclosed by the outer membrane; the latter is studded with porin proteins that form aqueous channels permeable to small (<5000 daltons) molecules. Larger mol- ecules (and even some smaller polar species) require spe- cific transporters. #183 | | |
The major source of the energy needed to run all basic cellular functions derives from oxidative metabolism. Mitochondria oxidize substrates to CO2, and in the process transfer high-energy electrons from the original molecule (e.g., gluocse) to molecular oxygen to water. The oxidation of various metabolites drives hydrogen ion (proton) pumps that transfer H+ ions from the core matrix into the inter- membrane space. As these H+ ions flow back down their electrochemical gradient, the energy released is used to synthesize ATP. #184 | | |
It should be noted that the electron transport chain need not necessarily be coupled to ATP generation. Thus, an inner membrane protein enriched in brown fat called ther- mogenin (or UCP-1 = uncoupling protein 1) is a proton #185 | | |
transporter that can dissipate the proton gradient (uncou- ple it from oxidative phosphorylation) in the form of heat (nonshivering thermogenesis). As a natural (albeit usually low-level) byproduct of substrate oxidation and electron transport, mitochondria also are an important source of reactive oxygen species (e.g., oxygen free radicals, hydro- gen peroxide); importantly, hypoxia, toxic injury, or even mitochondrial aging can lead to significantly increased levels of intracellular oxidative stress. #186 | | |
Intermediate Metabolism. Pure oxidative phosphoryla- tion produces abundant ATP, but also “burns” glucose to CO2 and H2O, leaving no carbon moieties for use as build- ing blocks for lipids or proteins. For this reason, rapidly growing cells (both benign and malignant) increase glucose and glutamine uptake and decrease their production of ATP per glucose molecule—forming lactic acid in the pres- ence of adequate oxygen—a phenomenon called the Warburg effect (or aerobic glycolysis). Both glucose and glutamine provide carbon moieties that prime the mito- chondrial tricarboxylic acid (TCA) cycle, but instead of being used to make ATP, intermediates are “spun off” to make lipids, nucleic acids, and proteins. Thus, depending on the growth state of the cell, mitochondrial metabolism can be modulated to support either cellular maintenance or cellular growth. Ultimately, growth factors, nutrient supplies, and oxygen availability, as well as cellular signal- ing pathways and sensors that respond to these exogenous factors, govern these metabolic decisions. #187 | | |
Cell Death. Mitochondria are like the proverbial Dr. Jekyll and Mr. Hyde. On the one hand, they are factories of energy production in the form of ATP that allow the cells to survive; on the other hand, they participate in driving cell death when the cells are exposed to noxious stimuli #188 | | |
that the cells cannot adapt to. The role of mitochondria in the two principle forms of cell death, necrosis and apopto- sis, are discussed in Chapter 2. In addition to providing ATP and metabolites that enable the bulk of cellular activ- ity, mitochondria also regulate the balance of cell survival and death. #189 | | |
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Cell communication is critical in multicellular organisms. At the most basic level, extracellular signals determine whether a cell lives or dies, whether it remains quiescent, or whether it is stimulated to perform a specific function. Intercellular signaling is important in the developing embryo, in maintaining tissue organization, and in ensur- ing that tissues respond in an adaptive and effective fashion to various threats, such as local tissue trauma or a systemic infection. Loss of cellular communication and the “social controls” that maintain normal relationships of cells can variously lead to unregulated growth (cancer) or an inef- fective response to an extrinsic stress (as in shock). #191 | | |
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An individual cell is constantly exposed to a remarkable cacophony of signals, which must be “interpeted” and inte- grated into responses that benefit the organism as a whole. Some signals may induce a given cell type to differentiate, others may stimulate proliferation, and yet others may direct the cell to perform a specialized function. Multiple signals received in combination may trigger yet another totally unique response. Many cells require certain inputs just to continue living; in the absence of appropriate exog- enous signals, they die by apoptosis. #193 | | |
The sources of the signals that most cells respond to can be classified into several groups: #194 | | |
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Pathogens and damage to neighboring cells. Many cells have an innate capacity to sense and respond to damaged cells (danger signals), as well as foreign invaders such as microbes. The receptors that generate these danger signals are discussed in Chapters 3 and 5. #196 | | |
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Cell-cell contacts, mediated through adhesion molecules and/or gap junctions. As mentioned previously, gap junction signaling is accomplished between adjacent cells via hydrophilic connexons that permit the movement of small ions (e.g., calcium), various metabolites, and potential second messenger molecules such as cAMP. #198 | | |
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Cell-ECM contacts, mediated through integrins, which are discussed in Chapter 3 in the context of leukocyte attachment to other cells during inflammation. #200 | | |
