Cytoskeleton Cytoskeleton is a three-dimensional network of proteins distributed throughout the cytoplasm of eukaryotic cells. The cytoskeleton has roles in: 1. Cell movement (crawling of blood cells along blood-vessel walls, migration of fibroblasts during wound healing, and movement of cells during embryonic development) 2. Support and strength for the cell

3. Phagocytosis 4. Cytokinesis 5. Cell-cell and cell–extracellular matrix adherence 6. Changes in cell shape The components of the cytoskeleton were originally identified by electron microscopy. These early studies described a system of cytoplasmic “cables” that fell into three size groups, as follows: 1. Microfilaments (7 nm thick) 2. Intermediate filaments (10 nm thick) 3. Microtubules (25 nm in diameter) Biochemical studies, involving the extraction of cytoskeletal proteins from cells with detergents and salts and in vitro translation of specific mRNA, showed that each class of filaments has a unique protein organization. When cytoskeletal proteins were purified, they were used as antigens for the production of antibodies. Antibodies are used as tools for the localization of the various cytoskeletal proteins in the cell. The immunocytochemical localization of cytoskeletal proteins (Figure 1-24) and cell treatment with various chemical agents disrupting the normal organization of the cytoskeleton have been instrumental in understanding the organization and function of the cytoskeleton. Microfilaments The main component of microfilaments is actin. Actin filaments are composed of globular monomers (G-actin, 42 kd), which polymerize to form long helical filaments intertwined in a helix (F-actin). Actin is a versatile and abundant cytoskeletal component forming static and contractile bundles and filamentous networks specified by actin-binding proteins and their distinctive location and function in a cell. F-actin bundles are present in the microvilli of the intestinal (Figure 1-25) and renal epithelial cells (brush border) and the stereocilia from the hair cells of the inner ear. We have already seen that the intracellular portion of the cell adhesion molecules cadherins and integrin `1 interacts with F-actin through linker proteins (see Figures 1-8 and 1-11). As discussed in Chapter 6, Blood and Hematopoiesis, actin, together with spectrin, forms a filamentous network on the inner face of the red blood cell membrane that is crucial for maintaining the shape and integrity of red blood cells. Spectrin is a tetramer consisting of two distinct polypeptide chains (_ and `). Actin filaments are polar. Growth of actin filaments may occur at both ends; however, one end (the “barbed end” or plus end) grows faster than the other end (the “pointed end” or minus end). The names correspond to the arrowhead appearance of myosin head bound at an angle to actin. Actin filaments can branch in the leading edge (lamellipodia) of cells involved in either motility or interaction with other cell types. F-actin branching is initiated from the side of a preexisting actin filament by Arp2/3 (for actin-related protein), an actin nucleating complex of seven proteins (Figure 1-26). Formin regulates the assembly of unbranched actin in cell protrusions such as the intestinal microvilli (see Figure 1-25). Actin monomers have a binding site for adenosine triphosphate (ATP), which is hydrolyzed to adenosine diphosphate (ADP) as polymerization proceeds. Actin polymerization is ATP-dependent (see Box 1-E). The kinetics of actin polymerization involves a mechanism known as treadmilling: G-actin monomers assembled at one end of the filament concurrently disassemble at the other end (see Figure 1-26). Four types of proteins control treadmilling (see Figure 1-26), as follows: 1. Thymosin sequesters pools of G-actin monomers within cells.

2. Profilin suppresses nucleation of G-actin and promotes F-actin growth at the barbed end. Profilin can favor the assembly of monomeric G-actin into filaments by facilitating the exchange of bound ADP for ATP. Only ATP-actin monomers can be assembled into filaments. 3. Cofilin (also known as actin depolymerizing factor) triggers depolymerization of ADP-bound actin at the pointed end. Similar to profilin and thymosin, cofilin forms a dimeric complex with G-actin. 4. Gelsolin has a dual role: it is a capping protein and prevents the loss and addition of actin monomers, and it is a severing protein. In the presence of Ca2+, gelsolin fragments actin filaments and remains bound to the barbed end, forming a cap that prevents further filament growth. Figure 1-23. Summary of cell junctions and cell adhesion molecules In the core of the intestinal microvilli, the assembly of G-actin monomers into filaments and the organization of these filaments into thick bundles are controlled by various types of actin-binding or actinrelated proteins. A bundle of parallel nonbranching actin filaments, forming the core of the microvillus, is held together by actin-linking proteins, villin and fimbrin. Side arms of myosin-I and the Ca2+-binding protein calmodulin anchor the bundle to the plasma membrane (see Figure 1-25). Arp2/3 and additional regulatory proteins form a nucleation complex for the assembly of branching actin filaments. Branching actin filaments assemble at the leading edge of a cell during cell motility. In microvilli, formins (proteins with highly conserved formin homology domains, FH1 and FH2), instead of the Arp2/3 complex, seem to regulate the elongation of nonbranching actin filaments, while remaining attached to the barbed end (see Box 1-E). Formins are located at the tip of the microvillus, the cap region (see Figure 1-25). Male patients with defects in proteins that activate the Arp2/3 complex, in particular a protein of the Wiskott-Aldrich syndrome protein (WASP) family, display recurrent respiratory infections because of hereditary immunodeficiency, thrombocytopenia (low platelet count) present from birth on and eczema of the skin after the first month of life (see Box 1-F). The mutation is inherited from the mother, a healthy carrier of the defective gene. Microvilli and stereocilia are comparable structures, although they differ in length and the number of actin filaments: 1. Intestinal microvilli are 1 to 2 +m long, 0.1 +m wide, and consist of 20 to 30 bundled actin filaments. 2. Stereocilia in hair cells of the inner ear have a tapered shape at their base, the length range is 1.5 to 5.5 +m, and each actin bundle contains up to 900 actin filaments. Hair cells are extremely sensitive to mechanical displacement, and a slight movement of the stereocilium is amplified into changes in electric potential transmitted to the brain. We study hair cells of the inner ear in Chapter 9, Sensory Organs: Vision and Hearing. 

Microtubules Microtubules are composed of tubulin dimers (Figure 1-27; see Box 1-G). Each tubulin dimer consists of two tightly bound tubulin molecules: _-tubulin and `-tubulin. Tubulin subunits are arranged in longitudinal rows called protofilaments. Thirteen protofilaments associate side by side with each other to form a cylinder of microtubules with a hollow core. The diameter of a microtubule is 25 nm. Similar to actin filaments, microtubules are structurally polarized. Microtubules have a plus end, which grows more rapidly than the minus end (see Figure 1-27). In contrast to actin filaments, most individual microtubules seem to undergo alternate phases of slow growth and rapid depolymerization. This process, called dynamic instability, consists of three major steps: 1. A polymerization phase, in which GTP-tubulin subunits add to the plus end of the microtubule and a GTP cap is assembled to facilitate further growth. 2. The release of hydrolyzed phosphate (Pi) from tubulin-bound GTP. 3. A depolymerization phase, in which GDP- tubulin subunits are released from the minus end at a fast rate. The polymerization-to-depolymerization transition frequency is known as catastrophe; the depolymerization-to-polymerization transition frequency is known as rescue. The stability of microtubules can be modified by microtubule-associated proteins (MAPs). MAPs are classified into two groups: 1. Classical MAPs, such as MAP1A, MAP1B, MAP2, and tau. 2. Nonclassical MAPs, including Lis1 and DCX family members. MAPs stabilize microtubules by phosphorylation/dephosphorylation. In Chapter 7, Nervous Tissue, we discuss the significance of tau phosphorylation and dephosphorylation in Alzheimer’s disease. A lack of expression of Lis1 causes a sever brain developmental disorder called lissencephaly. Centrosome The centrosome, the major microtubule-organizing center in cells, consist of a pair of centrioles surrounded by the pericentriolar material, an amorphous, electron-dense substance rich in proteins such as pericentrin and a-tubulin. The centrosome has four major functions: 1. It nucleates the polymerization of tubulin subunits into microtubules. 2. It organizes microtubules into functional units, for example, the mitotic spindle. 3. It duplicates once every cell cycle in preparation for cell division. 4. It gives rise to basal body precursors, the originators of multiple or single cilia. Centrosome abnormalities, in particular an increase in their number, are frequent in human tumors and correlate with advanced tumor grade and metastasis. Therefore, centrosome amplification has a lethal effect by preventing cells to assemble normal mitotic spindles but also enhancing the potential of tumorigenesis. Centrosomes are part of the mitotic center, which, together with the mitotic spindle, constitutes the mitotic (or meiotic) apparatus (Figure 1-28). A centriole is a small cylinder (0.2 +m wide and 0.4 +m long) composed of nine microtubule triplets in a helicoid array. In contrast to most cytoplasmic microtubules, which display dynamic instability, the centriolar microtubules are very stable. During interphase, centrioles are oriented at right angles to each other. Before mitosis, centrioles replicate and form two pairs. During mitosis, each pair can be found at opposite poles of the cell, where they direct the formation of the mitotic or meiotic spindle. There are three types of microtubules extending from the centrosomes: 1. Radiating or astral microtubules, anchoring each centrosome to the plasma membrane. 2. Kinetochore microtubules, attaching the chromosome-associated kinetochore to the centrosomes. 3. Polar microtubules, extending from the two poles of the spindle where opposite centrosomes are located (see Figure 1-28). 

Kinetochores are formed by several proteins assembled on centromeric DNA during mitosis and meiosis. The centromere is the chromosomal site where the kinetochore assembles. If kinetochores fail to assemble, chromosomes cannot segregate properly (see Box 1-H). The pericentriolar material contains the a-tubulin ring complex and numerous proteins, including peri-centrin. Each a-tubulin ring complex is the nucleation site or template for the assembly and growth of one microtubule. The centrioles do not have a direct role in the nucleation of microtubules in the centrosome. Tubulin dimers associate to the a-tubulin ring by the _-tubulin subunit. Consequently, the minus end of each microtubule points to the centrosome; the plus end, the growing end, is oriented outward, free in the cytoplasm. The axoneme of cilia and flagella Early in this chapter, we indicate that centrosomes give rise to precursor basal bodies, which are the outgrowth origin of cilia (see Figure 1-6) and flagella. Motile cilia and flagella are cytoplasmic extensions containing a core of microtubules, the axoneme (Figure 1-29). The axoneme consists of nine peripheral microtubule doublets surrounding a central pair of microtubules. This arrangement is known as the 9 + 2 configuration. Each peripheral doublet consists of a complete microtubule (called an A tubule, with 13 protofilaments), sharing its wall with a second, partially completed microtubule (called a B tubule, with 10 to 11 protofilaments). Extending inward from the A tubule are radial spokes that insert into an amorphous inner sheath surrounding the central microtubule pair. Adjacent peripheral doublets are linked by the protein nexin (see Box 1-I). Projecting from the sides of the A tubule are sets of protein arms: the inner and outer arms of dynein, a microtubule-associated adenosine triphosphatase (ATPase). In the presence of ATP, the sliding of peripheral doublets relative to each other bends cilia and flagella. Sliding and bending of microtubules are the basic events of their motility. Ciliopathies can occur when defects occur during: 1. The multiplication and docking of the centrosome-derived precursor basal bodies. An example is the enhanced expression of the protein CP110 that prevents the attachment of basal bodies to the plasma membrane, leading to primary ciliary dyskinesia. 2. The transport of proteins during the assembly of cilia and flagella, resulting in the Bardet-Biedl syndrome (see Box 1-J; see Figure 1-6). Clinical significance: Microtubule-targeted drugs. Sterility Two groups of antimitotic drugs act on microtubules: 1. Microtubule-destabilizing agents, which inhibit microtubule polymerization. 2. Microtubule-stabilizing agents, which affect microtubule function by suppressing dynamic instability. The first group includes colchicine, colcemid, vincristine, and vinblastine, which bind to tubulin and inhibit microtubule polymerization, blocking mitosis. Colchicine is used clinically in the treatment of gout. Vincristine and vinblastine, from Vinca alkaloids isolated from the leaves of the periwinkle plant, have been successfully used in childhood hematologic malignancies (leukemias). Neurotoxicity, resulting from the disruption of the microtubule-dependent axonal flow (loss of microtubules and binding of motor proteins to microtubules), and myelosuppression are two side effects of microtubule-targeted drugs. The second group includes taxol (isolated from the bark of the yew tree) with an opposite effect: It stabilizes microtubules instead of inhibiting their assembly (Figure 1-30). Paclitaxel (taxol) has been used widely to treat breast and ovarian cancers. Similar to Vinca alkaloids, its main side effects are neurotoxicity and suppression of hematopoiesis. Kartagener’s syndrome is an autosomal recessive ciliary dyskinesia frequently associated with bronchiectasis (permanent dilation of bronchi and bronchioles) and sterility in men. Kartagener’s syndrome is the result of structural abnormalities in the axoneme (defective or absent dynein) that prevent mucociliary clearance in the airways (leading to persistent infections) and reduce sperm motility and egg transport in the oviduct (leading to sterility). 

Microtubules: Cargo transport and motor proteins The transport of vesicles and nonvesicle cargos occurs along microtubules and F-actin. Specific molecular motors associate to microtubules and F-actin to mobilize cargos to specific intracellular sites. Microtubule-based molecular motors include kinesin and cytoplasmic dynein for the long-range transport of cargos. F-actin–based molecular motors include unconventional myosin Va and VIIa for the short-range transport of cargos. We discuss additional aspects of the F-actin–based cargo transport mechanism during the transport of melanosomes in Chapter 11, Integumentary System. Three examples of microtubule-based cargo transport in mammalian systems are as follows (see Box 1-K): 1. Axonemal transport, including flagella (intraflagellar transport) and cilia (intraciliary transport) (Figure 1-31). During axonemal transport, particles are mobilized by kinesin and cytoplasmic dynein along the microtubule doublets of the axoneme. Defective axonemal transport results in the abnormal assembly of cilia and flagella, including polycystic kidney disease, retinal degeneration, respiratory ciliary dysfunction, and lack of sperm tail development. As indicated before (see Box 1-J), the Bardet-Biedl syndrome is a disorder caused by basal body/ciliary dysfunction secondary to a defective microtubulebased transport function. 2. Axonal transport, along the axon of neurons (see Figure 1-31). 3. Intramanchette transport, along microtubules of the manchette, a transient structure assembled during the elongation of the spermatid head (see Chapter 20, Spermatogenesis). Microtubules: Axonal transport Axons are cytoplasmic extensions of neurons responsible for the conduction of neuronal impulses. Membrane-bound vesicles containing neurotransmitters produced in the cell body of the neuron travel to the terminal portion of the axon, where the content of the vesicle is released at the synapse. Bundles of microtubules form tracks within the axon to carry these vesicles. Vesicles are transported by two motor proteins (see Figure 1-31): 1. Kinesin 2. Cytoplasmic dynein Kinesins and cytoplasmic dyneins participate in two types of intracellular transport movements: 1. Saltatory movement, defined by the continuous and random movement of mitochondria and vesicles. 2. Axonal transport, a more direct intracellular movement of membrane-bound structures. Kinesins and cytoplasmic dyneins have two ATP-binding heads and a tail. Energy derives from continuous ATP hydrolysis by ATPases present in the heads. The head domains interact with microtubules, and the tail binds to specific receptor binding sites on the surface of vesicles and organelles. Kinesin uses energy from ATP hydrolysis to move vesicles from the cell body of the neuron toward the end portion of the axon (anterograde transport). Cytoplasmic dynein also uses ATP to move vesicles in the opposite direction (retrograde transport). Myosin family of proteins Members of the myosin family of proteins bind and hydrolyze ATP to provide energy for their movement along actin filaments from the pointed (minus) end to the barbed (plus) end. Myosin I and myosin II are the predominant members of the myosin family (Figure 1-32; see Box 1-L). Myosin I, regarded as an unconventional myosin, is found in all cell types and has only one head domain and a tail. The head is associated with a single light chain. The head interacts with actin filaments and contains ATPase, which enables myosin I to move along the filaments by binding, detaching, and rebinding. The tail binds to vesicles or organelles. When myosin I moves along an actin filament, the vesicle or organelle is transported. Myosin I molecules are smaller than myosin II molecules, lack a long tail, and do not form dimers. Myosin II, a conventional myosin, is present in muscle and nonmuscle cells. Myosin II consists of a pair of identical molecules. Each molecule consists of an ATPase-containing head domain and a long rodlike tail. The tails of the dimer link to each other along their entire length to form a two-stranded coiled rod. The tail of myosin II self-assembles into dimers, tetramers, and a bipolar filament with the heads pointing away from the midline. The two heads, linked together but pointing in opposite directions, bind to adjacent actin filaments of opposite polarity. Each myosin head bound to Factin moves toward the barbed (positive) end. Consequently, the two actin filaments are moved against each other, and contraction occurs (see Figure 1-32). Heads and tails of myosin II can be cleaved by enzymes (trypsin or papain) into light meromyosin (LMM) and heavy meromyosin (HMM). LMM forms filaments, but lacks ATPase activity and does not bind to actin. HMM binds to actin, is capable of ATP hydrolysis, and does not form filaments. HMM is responsible for generating force during muscle contraction. HMM can be cleaved further into two subfragments called S1. Each S1 fragment contains ATPase and light chains and binds actin. Myosin V, an unconventional myosin, is doubleheaded with a coiled double tail. The head region binds to F-actin; the distal globular ends of the tails bind to Rab27a, a receptor on vesicle membranes. Myosin Va mediates vesicular transport along F-actin tracks. A specific example is the transport of melanosomes from melanocytes to keratinocytes, first along microtubules and later along F-actin (see Chapter 11, Integumentary System). Mutations in the Rab27a and myosin Va genes disrupt the F-actin transport of melanosomes. An example in humans is Griscelli syndrome, a rare autosomal recessive disorder characterized by pigment dilution of the hair caused by defects in melanosome transport and associated with disrupted T cell cytotoxic activity and neurologic complications. Figure 1-33 summarizes the relevant structural and functional characteristics of motor proteins. 

Myosin light-chain kinase The self-assembly of myosin II and interaction with actin filaments in nonmuscle cells takes place in certain sites according to functional needs. These events are controlled by the enzyme myosin lightchain kinase (MLCK), which phosphorylates one of the myosin light chains (called the regulatory light chain) present on the myosin head. The activity of MLCK is regulated by the Ca2+-binding protein calmodulin (Figure 1-34). MLCK has a catalytic domain and a regulatory domain. When calmodulin and Ca2+ bind to the regulatory domain, the catalytic activity of the kinase is released. The MLCK–calmodulin–Ca2+ complex catalyzes the transfer of a phosphate group from ATP to the myosin light chain, and myosin cycles along F-actin to generate force and muscle contraction. Phosphorylation of one of the myosin light chains results in two effects: 1. It exposes the actin-binding site on the myosin head. This step is essential for an interaction of the myosin head with the F-actin bundle. 2. It releases the myosin tail from its sticky attachment site near the myosin head. This step also is critical because only myosin II stretched tails can self-assemble and generate bipolar filaments, a requirement for muscle contraction (see Figure 1-33). In smooth muscle cells, a phosphatase removes the phosphate group from myosin light chains. Skeletal muscle contraction does not require phosphorylation of the myosin light chains. We discuss additional details of muscle contraction when we study the muscle tissue (see Chapter 7, Muscle Tissue). Intermediate filaments Intermediate filaments (Figure 1-35) represent a heterogeneous group of structures so named because their diameter (10 nm) is intermediate between those of microtubules (25 nm) and microfilaments (7 nm). Intermediate filaments are the most stable cytoskeletal structures. Detergent and salt treatments extract microfilament and microtubule components and leave intermediate filaments insoluble. The structure of the intermediate filament does not fluctuate between assembly and disassembly states similar to microtubules and microfilaments. Note that in contrast to microtubules and actin filaments, which are assembled from globular proteins with nucleotide-binding and hydrolyzing activity, intermediate filaments consists of filamentous monomers lacking enzymatic activity. In contrast to actin and tubulin, the assembly and disassembly of intermediate filament monomers are regulated by phosphorylation and dephosphorylation, respectively. Intermediate filament protein monomers consist of three domains (see Figure 1-35): A central _-helical rod domain is flanked by a nonhelical N-terminal head domain and a C-terminal tail domain. The assembly of intermediate filaments occurs in four steps: 1. A pair of filamentous monomers of variable length and amino acid sequence of the head and tail domains, form a parallel dimer through their central rod domain coiled around each other. 2. A tetrameric unit is then assembled by two antiparallel half staggered coiled dimers. Therefore, in contrast to microtubules and actin filaments, the antiparallel alignment of the initial tetramers determines a lack of structural polarity of intermediate filament (absence of plus and minus ends). One end of an intermediate filament cannot be distinguished from another. If molecular motors associate to an intermediate filament, they would find it difficult to identify one direction from another. 3. Eight tetramers associate laterally to form a 16 nm-thick unit length filament (ULF). 4. Individual ULFs join end-to-end to form a short filaments that continue growing longitudinally by annealing to other ULFs and existing intermediate filaments. The elongation of the filament is followed by internal compaction to achieve the 10 nm-thick intermediate filament. The tight association of dimers, tetramers and ULFs provide intermediate filaments with high tensile strength and resistance to stretching, compression, twisting and bending forces. Intermediate filaments provide structural strength or scaffolding for the attachment of other structures. Intermediate filaments form extensive cytoplasmic networks extending from cage-like perinuclear arrangements to the cell surface. Intermediate filaments of different molecular classes are characteristic of particular tissues or states of differentiation (for example, in the epidermis of skin). Five major types of intermediate filament proteins have been identified on the basis of sequence similarities in the _-helical rod domain. They are referred to as types I through V (see Box 1-M). About 50 intermediate filament proteins have been reported so far.

Type I (acidic keratins) and type II (neutral to basic keratins). This class of proteins forms the intermediate filament cytoskeleton of epithelial cells (called cytokeratins to distinguish them from the keratins of hair and nails). Equal amounts of acidic (40 to 60 kd) and neutral-basic (50 to 70 kd) cytokeratins combine to form this type of intermediate filament protein. Type I and type II intermediate filament keratins form tonofilaments associated with molecules present in the cytoplasmic plaques of desmosomes and hemidesmosomes (see Figures 1-18 and 1-19). We come back to intermediate filament–binding proteins, such as filaggrins, when we discuss the differentiation of keratinocytes in the epidermis of the skin (Chapter 11, Integumentary System), and plectin, when we analyze the cytoskeletal protective network of skeletal muscle cells (Chapter 7, Muscle Tissue). In the epidermis, the basal cells express keratins K5 and K14. The upper differentiating cells express keratins K1 and K10. In some regions of the epidermis, such as in the palmoplantar region, keratin K9 is found. Mutations in K5 and K14 cause hereditary blistering skin diseases belonging to the clinical type epidermolysis bullosa simplex (see later, Clinical significance: Intermediate filaments and skin blistering diseases). Type III. This group includes the following intermediate filament proteins: Vimentin (54 kd) is generally found in cells of mesenchymal origin. Desmin (53 kd) is a component of skeletal muscle cells and is localized to the Z disk of the sarcomere (see Chapter 7, Muscle Tissue). This intermediate filament protein keeps individual contractile elements of the sarcomeres attached to the Z disk and plays a role in coordinating muscle cell contraction. Desmin is also found in smooth muscle cells. Glial fibrillary acidic protein (GFAP) (51 kd) is observed in astrocytes and some Schwann cells (see Chapter 8, Nervous Tissue). Peripherin (57 kd) is a component of neurons of the peripheral nervous system and is coexpressed with neurofilament proteins (see Chapter 8, Nervous Tissue). Type IV. This group includes neurofilaments, nestin, syncoilin and _-internexin. Neurofilaments are the main components. Neurofilaments (NFs) are found in axons and dendrites of neurons. Three types of proteins can be found in a neurofilament: NF-L (60 to 70 kd), NF-M (105 to 110 kd), and NF-H (135 to 150 kd), for low-molecular-weight, middle-molecular-weight, and high-molecular-weight neurofilaments. Abnormal accumulations of neurofilaments (neurofibrillary tangles) are a characteristic feature of a number of neuropathologic conditions. _-Internexin (66 kd) is found predominantly in the central nervous system (particularly in the spinal cord and optic nerve). Type V. Proteins of this group, the nuclear lamins, are encoded by three genes: LMNA, LMNB1, and LMNB2. Lamin A and lamin C arise from the alternative splicing of transcripts encoded by the LMNA gene. The LMNB1 gene encodes lamin B1 expressed in all somatic cells. The LMNB2 gene encodes lamin B2, expressed in all somatic cells, and lamin B3, that is specific for spermatogenic cells. Nuclear lamins (60 to 75 kd) differ from the other intermediate filament proteins in that they organize an orthogonal meshwork, the nuclear lamina, in association with the inner membrane of the nuclear envelope. Lamins provide mechanical support for the nuclear envelope and bind chromatin. Because of their clinical relevance, we come back to nuclear lamins and associated proteins when we discuss the organization of the nuclear envelope. 

A group of human diseases, known as laminopathies, are linked to defects in proteins of the nuclear envelope, including lamins (see Box 1-N). Numerous laminopathies affect cardiac and skeletal muscle, adipose tissue (lipodystrophies), and motor and sensory peripheral nerves. Two hypotheses concerning the pathogenic mechanism of laminopathies have been considered: 1. The gene expression hypothesis regards lamin A and lamin C as essential for the correct tissue-specific expression of certain genes. 2. The mechanical stress hypothesis proposes that a defect in lamin A and lamin C weakens the structural integrity of the nuclear envelope. During mitosis, the phosphorylation of lamin serine residues causes a transient disassembly of the meshwork, followed by a breakdown of the nuclear envelope into small fragments. At the end of mitosis, lamins are dephosphorylated, and the lamin meshwork and the nuclear envelope reorganize. See the cell nucleus section concerning the mechanism of phosphorylation and dephosphorylation of lamins during the cell cycle. Hemidesmosomes and intermediate filaments Hemidesmosomes are specialized junctions observed in basal cells of the stratified squamous epithelium attaching to the basement membrane (Figure 1-36). Inside the cell, the proteins BPAG1 (for bullous pemphigoid antigen 1) and plectin (members of the plakin family of cross-linker proteins) are associated to intermediate filaments (also called tonofilaments). Plectin connects intermediate filaments to the integrin subunit `4 . On the extracellular side, integrin _6 `4 , BPAG2 (for bullous pemphigoid antigen 2) and laminin 5, a protein present in specialized structures called anchoring filaments, link hemidesmosomes to the basal lamina. 

The plakin-related protein BPAG1 associates to BPAG2, a transmembrane protein with an extracellular collagenous domain. Putting all things together, BPAG1 constitutes a bridge between the transmembrane protein BPAG2 and intermediate filaments. If this bridge is disrupted, as in bullous pemphigoid, the epidermis becomes detached from the basal lamina anchoring sites. BPAG1 and BPAG2 were discovered in patients with bullous pemphigoid, an autoimmune disease. 

Clinical significance: Skin blistering diseases Bullous pemphigoid is an autoimmune blistering disease similar to pemphigus vulgaris (called “pemphigoid”, similar to pemphigus). Blisters or bullae develop at the epidermis-dermis junction when circulating immunoglobulin G (IgG) cross-reacts with bullous pemphigoid antigen 1 or 2. IgG-antigen complexes lead to the formation of complement complexes (C3, C5b, and C9), which damage the attachment of hemidesmosomes and perturb the synthesis of anchoring proteins by basal cells (Figure 1-37). The production of local toxins causes the degranulation of mast cells and release of chemotactic factors attracting eosinophils. Enzymes released by eosinophils cause blisters or bullae.

Intermediate filaments strengthen the cellular cytoskeleton. The expression of mutant keratin genes results in the abnormal assembly of keratin filaments, which weakens the mechanical strength of cells and causes inherited skin diseases, as shown in Figure 1-38: 1. Epidermolysis bullosa simplex (EBS), characterized by skin blisters after minor trauma. EBS is determined by keratin 5 and 14 mutant genes. 2. Epidermolytic hyperkeratosis (EH), in which patients have excessive keratinization of the epidermis owing to mutations of keratin 1 and 10 genes. 3. Epidermolytic plantopalmar keratoderma (EPPK), a skin disease producing fragmentation of the epidermis of the palms and soles, caused by a mutation of the keratin 9 gene.