Anchoring Junction

While anchoring junctions enable the association of cells and the basement membrane (hemidesmosomes) or to each other (desmosomes), tight junctions function as a barrier for the paracellular transport of solutes.

From: Comprehensive Biomaterials II , 2017

Cytoskeletal Interactions in the Neuron

A. Adebola , R.K.H. Liem , in Encyclopedia of Neuroscience, 2009

Connections between the Cytoskeleton and Cell Junctional Complexes

Cell-anchoring junctions are cytoskeleton-associated sites of cell–cell and cell–extracellular matrix (ECM) adhesions. In epithelial cells, three types of cell–cell junctions are identified: tight junctions, adherens junctions, and desmosomes ( Figure 2 ). Cell–ECM adhesions include focal adhesions and hemidesmosomes ( Figure 2 ). The formation and stabilization of these adhesion complexes are essential for molecular events that govern how tissues acquire and maintain proper architecture. In general, anchoring junctions are composed of transmembrane adhesion molecules of the cadherin (cell–cell adhesions) or integrin (cell–ECM adhesions) families and an electron-dense submembrane plaque made up of signaling proteins that contribute to the transduction of intracellular signals and adaptor proteins that tether the junctional complex to the cytoskeleton. Several members of the plakin family serve as essential adaptor proteins for desmosomes and hemidesmosomes ( Figure 2 ). Desmosomes are specialized intercellular junctions present in epithelia and cardiac muscle, where IFs are linked to desmosomal cadherins through interactions with desmoplakin. At hemidesmosomes, specialized cell–ECM junctions present in epithelia, BPAG1-e (the epidermal isoform of BPAG1), and plectin provide links between IFs and the transmembrane hemidesmosomal proteins α 6 β 4 integrin and BPAG2, a type XVII collagen. In the absence of these plakins, links between adhesion complexes and IFs are compromised and tissues under mechanical stress are subject to shearing.

Figure 2. Mammalian cell adhesion complexes. The schematic illustrates adhesion complexes present in mammalian epithelia. Epithelial cells are polarized cells with an apical surface that is exposed to the environment and a basal surface that contacts the underlying extracellular matrix (ECM). Cell junctions, sites of cell–cell and cell–ECM contact, are generally composed of transmembrane adhesion proteins and an electron-dense submembrane plaque made up of signaling proteins that contribute to the transduction of intracellular signals and adaptor proteins that tether the junctional complex to the cytoskeleton. There are five different types of cell junctions illustrated in this figure. First, tight junctions or zona occludens (ZO) are selective permeability barriers that seal adjacent cells together. At these junctions, claudins and occludins are adhesive transmembrane proteins that are linked to the actin cytoskeleton through ZO proteins. Second, adherens junctions are cell–cell junctions composed of classical cadherins linked to the actin cytoskeleton through adaptor proteins such as vinculin and catenins. Third, desmosomes are cell–cell junctions that are formed by homotypic interactions between desmosomal cadherins, termed desmocollins and desmogleins, and are linked to intermediate filaments (IFs) through desmoplakin dimers. The armadillo proteins plakoglobin and plakophilin mediate interactions between desmoplakin and the desmosomal cadherins. Desmoplakin can also bind directly to the juxtamembrane region of desmocollin. Fourth, focal adhesions are cell–ECM junctions that are formed by cell adhesion proteins of the integrin family. These junctions are linked to actin filaments by adaptor proteins such as talin, filamin, and vinculin. Fifth, hemidesmosomes are cell–matrix junctions composed of the transmembrane proteins α 6 β 4 integrin and BPAG2 (bullous pemphigoid antigen-2). These transmembrane proteins, which interact with components of the ECM, are linked to cytoplasmic IFs through interactions with the plakin proteins, plectin and BPAG1-e (the epithelial form of bullous pemphigoid antigen-1). Adapted from Jefferson JJ, Leung C, and Liem RKH (2004) Plakins: Goliaths that link cell junctions and the cytoskeleton. Nature Reviews Molecular Cell Biology 5: 542–553, with permission.

The ability of plakins to connect cell adhesion targets at the plasma membrane with cytoskeletal elements is mediated through interactions with the various protein modules that make up these cytolinkers. Generally, epithelial plakins exhibit a tripartite architecture defined by a central rod domain flanked by globular N- and C-terminal domains, whereby each globular domain is composed of protein modules that can interact with distinct cytoskeletal or cell adhesion targets. At the N-terminus of all plakins (except for an unusual plakin called epiplakin) is a protein module called the plakin domain ( Figure 1 ). This domain has been shown to interact with transmembrane components of desmosomal and hemidesmosomal junctions. The crystal structure of an N-terminal segment of the plakin domain of BPAG1 has been solved. It consists of a pair of three α-helical bundles aligned in an antiparallel array that are connected by a helical linker region. This structure is analogous to the secondary and tertiary structure that defines spectrin repeats found in spectrin family members, although plakin domains are more similar to each other than to spectrin-repeat regions found in spectrin proteins. Although the crystal structure of the entire plakin domain has not been solved, protein domain prediction programs suggest that the complete domain may consist of an SH3 domain flanked by two or more pairs of spectrin repeats. The presence of a putative SH3 domain suggests that plakins may serve as molecular scaffolds that recruit signaling proteins to sites of cell–cell contact, cell–ECM contact, or cytoskeletal activity. In addition to the plakin domain, some plakins also have a calponin-type ABD preceding the plakin domain at its N-terminus. This domain is described in more detail later.

Whereas the plakin domain mediates connections with cell–junctional complexes, it is the C-terminal end of several epithelial plakins that mediates connections with IFs. Connections to the IF network are mediated through a region at the C-terminus composed of plakin repeat motifs. Plakin repeats are described as repeats of 38 residues that form a β-hairpin followed by two antiparallel α-helices. Four and a half of these repeats form a globular PRD. PRDs based on sequence similarity are divided into subgroups termed A, B, and C. The PRDs have been shown to function as IF binding domains. For some plakins, IF binding also requires the presence of a linker (L) subdomain. The number of PRDs varies among different plakins. For example, the desmosomal protein desmoplakin contains three PRDs, whereas hemidesmosomal proteins plectin and BPAG1-e contain six and two, respectively. Although it is thought that the primary function of plakin repeats is to mediate binding to cytoplasmic IFs, there is evidence that they may have additional functions. First, there are plakin isoforms in which the plakin repeat motifs are not arranged as the 4.5-repeat PRD that is seen in mammalian epithelial plakins. Second, plakin homologs present in Drosophila contain plakin repeats, yet the Drosophila genome lacks genes encoding cytoplasmic IFs. The plakin repeats in both Drosophila and Caenorhabditis elegans are not arranged as PRDs. In fact, invertebrate plakins are quite different in structure from the epithelial plakins described so far. Although their N-terminal region contains a plakin domain, the remainder of the invertebrate plakins diverges from the epithelial plakins ( Figure 1 ). In addition to the plakin repeats, some Drosophila and C. elegans plakin isoforms contain a series of spectrin repeats which is thought to confer some flexibility to the molecule as well as serve as a site for protein interaction. These isoforms also contain a C-terminal region that consists of EF-hand motifs and an MT binding region in turn composed of a GAR domain.

The Drosophila plakin homolog is called Shortstop or Shot (also referred to as kakapo). As mentioned previously, the domain architecture of Shot diverges from mammalian epithelial plakins. However, it is quite similar to mammalian plakins that are expressed highly in the nervous system. These plakins include BPAG1-a (neuronal BPAG1 isoform) and MACF1-a (neuronal MACF1 isoform). Insights into the function of Shot came from studies concerning position-specific (PS) integrins. In Drosophila, PS integrins, which are similar to mammalian β 1 integrins, have been shown to be important for integrin-mediated adhesions between dorsal and ventral wing surfaces and between muscle and epidermis (hemiadherens junctions). Mutations of PS integrins result in defects in epidermal integrity and epidermal muscle attachment. A genetic screen for mutants that produce similar phenotypes led to the identification of Shot. Shot behaves similarly to mammalian plakins in that it anchors integrin-mediated adhesion complexes to cytoskeletal elements. However, since Drosophila lack cytosplasmic IFs, Shot anchors the junctional complex to MTs, through interactions with its GAR domain. In Drosophila, MTs seem to have taken on the role of IFs as the filaments that stabilize cells subject to mechanical stress.

In C. elegans, muscle cells are mechanically linked to the cuticle through electron-dense structures similar to mammalian hemidesmosomes called fibrous organelles (FOs). FOs consist of an IF array anchored at the basal surface to the underlying basement membrane and associated muscle and at the apical surface to the cuticle. VAB-10, the plakin homolog in C. elegans, functions similarly to hemidesmosomal plakins. However, unlike hemidesmosomes, FOs do not contain integrins or a BPAG2 homolog; instead, other transmembrane proteins (myotactin, MUA-3, and MUP-4) that might function analogously are present. Two VAB-10 isoforms have been identified – VAB-10A, which localizes to FOs, and VAB-10B, which localizes to bands between FOs. Loss of both VAB-10 isoforms results in epidermal detachment from the cuticle and muscle and perturbation of IFs and circumferential actin bundles. The role of VAB-10 in the nervous system has not been elucidated.

Thus far, our discussion on the role of plakins as key integrators of plasma membrane components and the cytoskeleton has been restricted to epithelia and muscle, tissues that are subject to substantial mechanical stress. Are plakins involved in the formation and/or maintenance of adhesive connections between neurons or between neurons and the underlying ECM? Such connections would have important implications for development and maintenance of neural structures. Several reports hint that indeed plakins may be involved in events that require neural adhesion. For example, mutations in Drosophila Shot affect sensory and motor axon growth, terminal arborization at the neuromuscular junction (NMJ), the assembly of presynaptic specializations at the NMJ, and the formation of neuronal dendrites in the central nervous system (CNS) – all cellular events that require cell adhesion.

In mammals, the plakin domain of BPAG1 interacts with the LAP protein erbin. LAP proteins, which include erbin, scribble, and densin-180, are PDZ binding proteins that are involved in the clustering of receptors at synaptic junctions. Erbin interacts with ErbB2, a mammalian epidermal growth factor receptor that in turn interacts with neuregulin. Neuregulin is a signaling protein that is essential for the formation and maintenance of the NMJ. Erbin has been found to localize to postsynaptic sites at the NMJ and sites of ErbB2 localization in the CNS. Thus, BPAG1 in conjunction with erbin may play an important role in the regulation of neuregulin signaling and subcellular organization of ErbB proteins, which in turn could play an important role in the formation and maintenance of NMJs. Despite these connections to NMJ integrity, there is no direct evidence that shows that plakins link the cytoskeleton to junctional complexes in the nervous system.

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Volume 1

Alan S.L. Yu , ... János Peti-Peterdi , in Seldin and Giebisch's The Kidney (Fifth Edition), 2013

Structure and Localization of Desmosomes

Desmosomes are specialized anchoring junctions that serve as tethers for cytoplasmic intermediate filaments. 207 While they are particularly important for maintaining the integrity of tissues subject to mechanical stress, such as the epidermis and myocardium, they are also present in all epithelia and are thought to play roles in tissue morphogenesis. Desmosomes appear as pairs of dense, disc-shaped plaques on opposing lateral membranes of adjacent cells (Figure 12.2a). The desmosomal cadherins, desmoglein and desmocollin, mediate cell–cell adhesion via their extracellular domains (Figure 12.10b). The armadillo proteins, plakoglobin/γ-catenin, and plakophilin act as adaptor proteins that link the cytoplasmic domain of the desmosomal cadherins to desmoplakin, which in turn links the desmosomal plaque to the intermediate filament cytoskeleton.

In mice, desmoplakin and plakoglobin are expressed at highest levels in the distal tubule, connecting tubule, and collecting duct, 211 whereas in humans, plakoglobin, like other catenins, is expressed along the entire nephron. 213

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International Review of Cell and Molecular Biology

Qi Wei , Hayden Huang , in International Review of Cell and Molecular Biology, 2013

2.3 Desmosomes

As another group of cadherin-based anchoring junctions, desmosomes are considered particularly important for maintaining the overall integrity of tissues, including epidermal and myocardial tissues ( Green and Simpson, 2007; Stokes, 2007). Morphologically, desmosomes have a characteristic and highly organized structure. They form a symmetrical, disc-shaped, electron-dense plaque consisting of an outer dense plaque and an inner dense plaque (North et al., 1999). Just like adherens junctions, desmosomes can be broken down into three major components: (nonclassical) cadherins (desmocollin and desmoglein), armadillo proteins (plakoglobin and plakophilin), and a (cytoskeletal-linking) plakin (desmoplakin) (Getsios et al., 2004). Of these five major desmosomal proteins, heterophilic interactions between desmocollin and desmoglein mediate direct intercellular communication (Runswick et al., 2001; Tselepis et al., 1998). Plakoglobin, which will be referenced repeatedly in this chapter, is notable for being the only known constituent common to desmosomes and adherens junctions (Cowin et al., 1986).

One key function of desmosomes is regulating the development and maintenance of strong intercellular adhesion. The linkage of the desmosome-intermediate filament network contributes to the formation of a "scaffolding" effect, which distributes mechanical stresses throughout tissues (Huen et al., 2002). It is thus expected that mutations in genes encoding desmosomal proteins can lead to diseases with altered intercellular adhesion and subsequently, diminished tissue integrity. In support of this, many desmosomal-related conditions appear in cardiac and dermal tissues, which are subjected to constant mechanical stress (Brooke et al., 2012). Some recent evidence attributes the characteristic "superglue" function of desmosomes to its ability to adopt a strongly adhesive state known as "hyperadhesion" (Cirillo et al., 2010; Garrod and Kimura, 2008). Other than intercellular adhesion, recent work has emphasized the multiple critical roles that desmosomal proteins play in regulating the various facets of development, life, and disease, some of which will be covered later.

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Liver Physiology and Energy Metabolism

Namita Roy-Chowdhury , Jayanta Roy-Chowdhury , in Sleisenger and Fordtran's Gastrointestinal and Liver Disease (Ninth Edition), 2010

Plasma Membranes

The plasma membranes consist of lipid bilayers composed of glycerophospholipids, cholesterol, and sphingolipids that provide barrier to water and most polar substances. 3,4 The inner and outer leaflets of the plasma membrane differ in lipid, protein, and carbohydrate composition, reflecting their functional differences. Protein molecules within the leaflets mediate transport of specific molecules and serve as a link with cytoskeletal structures and the extracellular matrix. Hepatocyte plasma membranes consist of 36% lipid, 54% protein, and 10% carbohydrate by dry weight. Outer leaflets of hepatocyte plasma membranes are enriched in carbohydrates.

Lipid rafts are microdomains (∼50 nm diameter) of the outer leaflets of the plasma membrane that are highly enriched in cholesterol and sphingolipids. 5 These are coupled to cholesterol-rich microdomains in the inner leaflet by an unknown mechanism. Raft lipids and associated proteins diffuse together laterally on the membrane surface. Some surface receptors become associated with the rafts on ligand binding, or they can lead to "clustering" of smaller rafts into larger ones. Lipid rafts are important in signal transduction, apoptosis, cell adhesion and migration, cytoskeletal organization, and protein sorting during both exocytosis and endocytosis (see later). Certain viruses enter cells via the lipid rafts.

Membrane proteins perform receptor, enzyme, and transport functions. 6 Integral membrane proteins traverse the lipid bilayer once or multiple times or are buried in the lipid. Additional "extrinsic" protein molecules are associated with plasma membrane. Membrane proteins can rotate or diffuse laterally but usually do not flip-flop from one leaflet to another. Concentration of specific membrane proteins is maintained by a balance between their synthesis and degradation by shedding of membrane vesicles, proteolytic digestion within the membrane, or internalization into the cell. Receptor proteins internalized into the cell may be degraded or recycled to the cell surface.

Cell Junctions

Hepatocytes are organized into sheets (seen as chords in two-dimensional sections) by occluding ("tight"), communicating ("gap"), and anchoring junctions ( Fig. 72-1). Tight junctions or desmosomes form gasket-like seals around the bile canaliculi, thereby permitting a concentration difference of solutes between the cytoplasm and bile canaliculus. Desmosomes are specialized membrane structures that anchor intermediate filaments to the plasma membrane and link cells together. Gap junctions are subdomains of contiguous membranes of hepatocytes that comprise ~3% of the total surface membrane. They consist of hexagonal particles with hollow cores, termed connexons, made up of six connexin molecules. 7 Connexons of one cell are joined to those of an adjacent cell to form a radially symmetrical cylinder that can open or close the central channel. Gap junctions are involved in nutrient exchange, synchronization of cellular activities, and conduction of electrical impulses.

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BASAL CELLS

M.J. Evans , in Encyclopedia of Respiratory Medicine, 2006

Junctional Adhesion

The structural role of basal cells in the airways is for attachment of columnar epithelium to the BMZ. Epithelial cells are attached to the BMZ by hemidesmosomes and cell adhesion molecules. Cytokeratins 5 and 14 link anchoring junctions of basal cells with the cytoskeletons of adjacent cells through desmosomes and to the BMZ with hemidesmosomes. This arrangement of junctional adhesion provides mechanical stability to a group of cells or tissue. In airway epithelium, basal cells are the only cells that form hemidesmosome junctions with the BMZ. Columnar cells are attached to the BMZ via desmosome attachment with basal cells. The significance of basal cells in junctional adhesion can be demonstrated by treating the tissue with ethylenediaminetetraacetic acid (EDTA). Desmosome junctions are dependent on calcium. When the tissue is treated with EDTA, the calcium is removed from the desmosome, and the columnar epithelium is released leaving the basal cells attached to the BMZ ( Figure 1(b)).

The number of basal cells present at a particular airway level and their morphology is related to their role in junctional adhesion. When the columnar epithelium increases in height, there is an increase in the size and shape of basal cells along with a corresponding increase in desmosome attachment with the columnar epithelium and hemidesmosome attachment with the BMZ. These changes maintain a constant amount of junctional adhesion between the columnar epithelium and the BMZ. Thus, the relationship between basal cell junctional adhesions and height of the epithelium is constant and not related to airway level or animal species.

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The Cell

Manisha Lalan , ... Ambikanandan Misra , in Challenges in Delivery of Therapeutic Genomics and Proteomics, 2011

1.7 Cell–Cell Adhesion and Extracellular Matrix

Cells in a multicellular organism are organized into specific tissues, and the individual cells in the tissues are associated with each other as well as the extracellular matrix. The linkages between the individual cells, called cell junctions , are classified into three major categories based on their physiological role: occluding junctions, anchoring junctions, and communicating junctions. Occluding junctions include tight junctions that serve to create an impermeable or semipermeable barrier between the adjoining epithelial cells. They are barriers to the transportation of material and control the movement of membrane transport proteins between the apical and basal layers of epithelia. The second category, anchoring junctions, connect the cytoskeletal network of the cell to the adjoining cells and/or the extracellular matrix, helping the tissue to survive any inflicted mechanical stress. These types of junctions are quite abundant and are further classified as adherens junctions, desmosomes, and hemidesmosomes. The adherens junctions connect the intracellular actin filaments to other cells or extracellular matrix through cadherin and integrin proteins, respectively. Similarly, desmosomes connect the intermediate filaments from one cell to the next, and hemidesmosomes are the connection site for the intermediate filaments to the basal lamina. Gap junctions constitute the communicating type of cell junctions. Their presence allows for selective transport between the cells of material having molecular weight less than 1000   Da. Molecularly a channel-like structure made up of proteins, they permit movement of inorganic ions and are crucial in regulating the electrical signals at neural and neuromuscular junctions [77].

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Chromosome Scaffold

J.M. Pluth , D.M. Sridharan , in Brenner's Encyclopedia of Genetics (Second Edition), 2013

The Scaffold–Loop Model

According to the scaffold–loop model, the extended chromosome scaffold in interphase cells tethers a few kilobases of compacted 30   nm chromatin fiber into long helical loops in an organized, nonrandom fashion that is further organized into specific domains ( Figure 1 ). During metaphase, association of the scaffolding proteins at the loop anchoring junctions results in tertiary folding of the chromatin and further compaction of the flexible scaffold, forming the basis for the higher-order condensed structure of the metaphase chromosome. The exact geometry that governs this tertiary folding is still an unresolved mystery. The scaffold–loop model is supported by in situ hybridization of interphase nuclei with a mixture of fluorescent-labeled probes specific for randomly chosen sequences along the full length of one chromosome. When the linear piece of DNA for this region is probed, the sequences are located far apart, whereas when this same chromosome region is probed in different cells, the probe regions are observed close to each other.

Figure 1. Simplistic view of chromosome scaffold containing MARs anchoring the loops of DNA chromatin.

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Cellular Mechanisms and Embryonic Tissues

Patricia Collins , in Fetal Medicine (Third Edition), 2020

Cell–Cell junctions

Juxtaposed cells usually do not touch. For contact to be established, the cells produce specific molecules which promote the development of a cell–cell junction between them (Fig. 2.4). Junctional complexes allow sheets of epithelial cells to act in concert in maintaining a barrier or in producing alterations in the overall epithelial morphology; they also permit cell–cell communication and are in this respect especially important in development. Cell junctions are classified into three main groups: (i) tight junctions, which prevent leakage of molecules between cells from one side of a sheet of cells to the other 8 ; (ii) anchoring junctions , where the neighbouring cell membranes attach and are supported by cytoskeletal elements within the cells, either actin or intermediate filaments (this type of junction also anchors epithelial cells to the ECM); and (iii) communicating junctions, which mediate the passage of electrical or chemical signals from one cell to another.

The formation of these junctional complexes is dependent on a range of cell adhesion molecules (CAMs) (see Fig. 2.4). In cell–cell anchoring junctions (adhesion belts and desmosomes), the CAMs involved are termed cadherins; they are attached intracellularly to intermediate or actin filaments in the cell cortex. 9 The latter run parallel to the plasma membrane; thus the actin bundles of adjacent cells are linked. Concomitant contraction of the actin bundles results in narrowing of the apices of the epithelial cells and rolling of the epithelial layer into a deep groove or a tube.

Epithelial cells contact the underlying basal lamina they synthesise by different types of anchoring junctions (hemidesmosomes and focal contacts). In these cases, the transmembrane linker proteins belong to the integrin family of ECM receptors. 10 Cytoskeletal filaments support the connection of the integrin within the cell membrane to the ECM.

Communication between adjacent epithelial cells is mediated by gap junctions. In forming a gap junction, each cell contributes six identical protein subunits (called connexins) which form a structure, similar to an old-fashioned cotton reel, termed a connexon. This is situated across the bilaminar membrane with the thicker rims extending into the extracellular and intracellular spaces. Each connexon is capable of opening and closing, thus controlling the gap. When two connexons from adjacent cells are aligned, a tubular connection is made between the cells. Each gap junction is really a cluster of apposed connexons which each permit molecules smaller than 1000 daltons to pass through them. In early embryos, most cells are electrically coupled to one another by gap junctions. Later in development, epithelial cells synthesise gap junctions at particular stages when it is inferred that information is passing from cell to cell. When gap junctions are removed, there is often a difference in differentiation in the cellular progeny. Gap junctions are seen in adult tissues (e.g., connecting cardiac myocytes to permit transmission of the electrical signals of the cardiac cycle). (For further information on CAMs, see Alberts et al. 5 )

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Functional Cell Biology

B. Geiger , ... M. Vaman Rao , in Encyclopedia of Cell Biology, 2016

Structural and Functional Diversity of Cell–Cell Junctions

Cell–cell adhesions (=junctions) differ structurally, molecularly, and functionally, according to characteristic subcellular positions and functions. Based on their physiological role, cell–cell junctions can be broadly divided into three types: occluding, anchoring, and signaling (Figure 2).

Figure 2. Diversity of cell–cell junctions in mouse epithelial cells. Transmission electron micrographs of mouse small intestine (a and a′, presented at two orientations) and tongue (b), showing different cell–cell junctions, including tight junctions (TJ), adherens junctions (AJ), and desmosomes (Des). (b) Highly developed desmosomes in stratified epithelium of the mouse tongue, displaying the laminated structure of these junctions and the associated cytokeratins (CK). (c–c″): Cultured YAMC epithelial cells derived from mouse intestine, triple-labeled for actin (c), showing the adherens junctions and (out of focus) the microfilament bundles associated with the basal matrix adhesions of the cells; cingulin-labeled tight junctions (c′) and the two labels together (actin – red; cingulin – green), plus DAPI staining of the nuclei (blue), in the triple-colored image (c″). Scale bar: 20   µm.

Occluding junctions are only found in epithelial sheets, such as the lining of the gut and kidney, where formation of a barrier between the two sides of the monolayer is essential. Proteins of the claudin and occludin family mediate occluding adhesions in invertebrate tight junctions (Furuse et al., 1998), and closely related proteins function similarly in invertebrate septate junctions. Tight junctions in invertebrates are located at the most apical position along the lateral membrane of neighboring cells. In addition to sealing the space between cells, tight junctions also create a 'fence' within the cell's membrane that helps to separate the apical and baso-lateral membrane domains (Farquhar and Palade, 1963).

The most ancient anchoring junction found in all animal cells is the adherens junction (AJ), which is based on cadherin adhesion receptors linked to the actin cytoskeleton ( Farquhar and Palade, 1963). AJs provide a physical connection between cells of the body by reinforcing their cell–cell adhesions with the actomyosin cytoskeleton (Yonemura, 2011). In the following sections, we will focus on AJs, their receptors, their connection with the actin cytoskeleton, and regulation of their function by an ensemble of proteins we refer to as the 'cadherin adhesome' (Zaidel-Bar, 2013).

Unique to vertebrates is a second type of anchoring junction: the desmosome. Cell–cell adhesion in desmosomes is based on specialized cadherins known as desmogleins and desmocollins (Chitaev and Troyanovsky, 1997). Unlike AJs, these adhesion receptors are connected within the cell to intermediate filaments. Desmosomes provide additional mechanical resilience to vertebrate epithelial tissues, such as skin (Garrod and Chidgey, 2008).

Signaling or communication junctions include the channel-forming GAP junctions and neuronal synapses, as well as the more transient stimulatory immune synapse, involved in T-cell activation (Rozental et al., 2000). In contrast to tight junctions that block the passage of molecules in the space between neighboring cells, GAP junctions bridge this gap, by forming channels through which small molecules such as ions, amino acids, and secondary messengers can freely pass from one cell to another (Mese et al., 2007). GAP junctions, formed by four-pass transmembrane proteins from the connexin family, are found in many animal tissues, where they serve to coordinate the activity of cells, such as those in the beating heart muscle (Mese et al., 2007).

Neuronal and immune synapses are specialized junctions that combine adhesion and signaling proteins for the localized transfer of information between particular pairs of cells. Though they will not be discussed here, the interested reader is directed to the following reviews (Rozental et al., 2000; Shimizu and Stopfer, 2013).

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Male Reproduction

Nathália de Lima e Martins Lara , ... Luiz Renato de França , in Encyclopedia of Reproduction (Second Edition), 2018

Sertoli–Germ Cell

The interactions between the Sertoli and germ cells in the seminiferous epithelium must be tightly regulated, because Sertoli cells provide the main control over spermatogenesis, providing nutritional and structural support for all developing germ cells. This Sertoli/germ cell interaction occurs through different types of junctions, such as adherens junctions, desmosomes, and gap junctions, depending on the stage of the seminiferous epithelium cycle and germ cell-specific requirements. For example, adherens junctions are usually detected between Sertoli and spermatogonia located in the stem cell niche, being important for stem cell homing and colonization. Desmosomes, in turn, are the major anchoring junction between spermatocytes/early spermatids and the Sertoli cells. Additionally, gap junctions provide crucial communications between adjacent germ cells, which is essential for the coordination of germ cell development throughout the different stages of differentiation ( Fig. 2).

Fig. 2

Fig. 2. (A, B) Desmosome-like junctions between Sertoli cell (SC) and germ cell (GC). The insert represents a higher magnification of the area depicted by the small box. Opposing red arrowheads, Sertoli cell barrier; TP, Tunica propria.

The testis forms a unique type of adherens junction, called ectoplasmic specialization (ES), which is not found in any other epithelium and is usually considered as one of the strongest anchoring junctions in the mammalian body (Cheng and Mruk, 2015). Despite participating in the SCB (basal ES), apical ectoplasmic specialization is also found at the Sertoli cell-elongated spermatid interface, conferring spermatid adhesion and polarity. Spermatid polarity involves the elongating heads pointing perpendicularly toward the basement membrane of the seminiferous tubule, allowing for maximum packing of developing spermatids in the seminiferous epithelium. This polarity and the very well-coordinated spermatogenic process allows the usually very high sperm output observed during spermatogenesis. Loss of spermatid adhesion and early release of the spermatids to the lumen of the seminiferous tubule is often caused by the disruption of cell polarity. In this context, it has also been shown that DICER (involved in the miRNA biosynthesis pathway) is an important player in the regulation of these Sertoli/germ cell junctions (Korhonen et al., 2015).

Structurally, the ectoplasmic specializations consist on bundles of actin microfilaments sandwiched between flattened cisternae of the Sertoli cell endoplasmic reticulum, and are present in the Sertoli cell membrane that faces the spermatid. This structure is closely associated with the microtubule network of the Sertoli cell, and is probably responsible for the movement of elongating spermatids across the seminiferous epithelium during the stages of the cycle. Injection of a microtubule stabilizer (taxol) lead to the retention of spermatids deep within the Sertoli cell crypts, while disruption of both ES and microtubules trapped the spermatids inside the epithelium, blocking their release at spermiation, until they were phagocytosed by the Sertoli cells. The molecular composition of the ES is constituted by nectin-, cadherin- and integrin-based protein complexes. A polarity protein complex, composed of partitioning-defective3/partitioning-defective6/atypical protein kinase C (Par3/Par6/aPKC), is also an integrated component of the apical ectoplasmic specialization.

The apical ectoplasmic specialization appears in step 8 spermatids, replacing gap junctions and desmosomes as the only anchoring device between the spermatid and the Sertoli cell. Around the time of spermiation (steps 18–19 of spermiogenesis in rats, and steps 15–16 in mice), the apical ES undergoes degeneration, through the formation of structures similar to endocytic vesicles, which initiates the elimination of cytoplasmic debris resulting from the release of the spermatids. These vesicular structures are called tubulobulbar complexes. They form narrow tubular projections of the plasma membrane of the elongating spermatid head, penetrating an invagination of the Sertoli cell plasma membrane and forming a bulbous end. The tubular portion of the complex is surrounded by actin filaments, while the bulb is more associated with cisternae of the endoplasmic reticulum. The tubulobulbar complex facilitates the internalization of the disassembled apical ES junctions, in preparation for the release of sperm cells during spermiation (Vogl et al., 2013). Steroids, especially androgens, may be involved with the regulation of tubulobulbar complex formation, as it occurs concomitant with the occurrence of androgen-dependent stages of the seminiferous epithelium cycle (Fig. 3).

Fig. 3

Fig. 3. Ectoplasmic specialization (ES) and tubulobulbar complex in elongating spermatids. (A) Schematic representation of the ectoplasmic specialization (left) and the tubulobulbar complex (right) and the relationship between the elongating spermatids and the Sertoli cell. (B) Electron microscopy of an elongating spermatid showing the ES structure, composed by actin filaments (yellow asterisks) sandwiched between smooth endoplasmic reticulum cisternae (arrows) and the apposing plasma membranes of the Sertoli and the germ cell (opposing white arrowheads). (C, D) Cross-section of mouse seminiferous tubules illustrating the intimate relationship between the Sertoli cell and the elongating spermatids delimited by the rectangle. (C) Stage I tubule showing the elongating spermatids deep within the cripts of the Sertoli cell. (D) Stage VII tubule, showing elongated spermatids at the edge of the seminiferous epithelium, close to the lumen, in preparation for spermiation. ES, Ectoplasmic specialization; SC, Sertoli cell; Ac, Acrosome; M, Mitochondria; PL, Preleptotene spermatocytes; P, Pachytene spermatocytes; RS, Round spermatids; E, Elongating spermatids; RB, Residual body; PMC, Peritubular myoid cell.

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https://www.sciencedirect.com/science/article/pii/B9780128012383645646