BIO-333 Chapter 19

Adhesion Belt

Adherens junctions in epithelia that form a continuous belt (zonula adherens) just beneath the apical face of the epithelium, encircling each of the interacting cells in the sheet.

Matrix Metalloproteases

Ca2+ or Zn2+ dependent proteolytic enzyme present in the extracellular matrix that degrades matrix proteins. Includes the collagenases.

Describe how cadherins bind to each other (what ion is necessary) and how these attachments break.

Cadherins bind homophilically. Each cadherin domain forms a more-or-less rigid unit, joined to the next cadherin domain by a hinge. Ca2+ ions bind to sites near each hinge and prevent it from flexing, so that the whole string of cadherin domains behaves as a rigid and slightly curved rod. When Ca2+ is removed, the hinges can flex, and the structure becomes floppy.

Are the attachment sites between cadherins strong?

Cadherins typically bind to their partners with relatively low affinity. Strong attachment result from the formation of many such weak bonds in parallel. The strength of this junction is far greater than that of any individual intermolecular bond, and yet regulatory mechanisms can easily disassemble the junction by separating the molecules sequentially.

Tight Junctions

Cell-cell junction that seals adjacent epithelial cell together, preventing the passage of most dissolved molecules from one side of the epithelial sheet to the other.

How and where are fibronectins assembled - what influences this assembly?

Fibronectin molecules assemble into fibrils only on the surface of cells, and only where those cells possess appropriate fibronectin-binding proteins-in particular, integrins. The integrins provide a linkage from the fibronectin outside the cell to the actin cytoskeleton inside it.

Collagens

Fibrous protein rich in glycine and proline that is a major component of the extracellular matrix in animals, conferring tensile strength. Exists in many forms: type I, the most common, is found in skin, tendon, and bone; type II is found in cartilage; type IV is present in basal laminae.

Why do integrins tend to cluster?

Following their activation, integrins cluster together to create a dense plaque in which many integrin molecules are anchored to cytoskeleton filaments.

What can typically move through a gap junction?

Gap junctions have a pore size of 1.4 nm, which allows the exchange of inorganic ions and other small water-soluble molecules, but not macromolecules such as proteins or nucleic acids.

What are the subunits of gap junctions?

Gap-junction connexon is made of six transmembrane connexin subunits.

What bears most of the mechanical stress in tissues?

In connective tissue, the main stress-bearing component is the extracellular matrix. In the epithelial tissue, it is the cytoskeletons of the cells themselves.

Polarized

In epithelia, that the basal end of a cell, adherent to the basal lamina below, differs from the apical end, exposed to the medium above; thus, all epithelia and their individual cells are structurally polarized.

ECM macromolecules are secreted by what type of cell?

In most connective tissues, the matrix macromolecules are secreted by cells call fibroblasts.

Describe why adherens junctions are examples of mechanotransduction?

Adherens junctions seem to sense the forces acting on them and modify local actin and myosin behavior to balance the forces on both sides of the junction. If contractile activity in once cell is increased experimentally, the adherens junctions linking the two cells increase in size, and the contractile activity of the second cell increases to match that of the first-resulting in a balance of forces across the junction. Suggests that adherens junctions are not simply passive sites of protein-protein binding but are dynamic tension sensors that regulate their behavior in response to changing mechanical conditions. This ability to transduce a mechanical signal into a change in junctional behavior is an example of mechanotransduction.

Type IV Collagen

An essential component of mature basal laminae consisting of three long protein chains twisted into a ropelike superhelix with multiple bends. Separate molecules assemble into a flexible, felt-like network that gives the basal lamia tensile strength. Consist of three separately synthesized long protein chain that twist together to form a ropelike superhelix; triple stranded helical structure is interrupted in more than 20 regions, allowing multiple bends. Type IV collagen molecules interact via their terminal domains to assemble extracellularly into flexible, feltlike network that gives the basal lamia tensile strength.

Describe how integrins are activated and inactivated.

In the inactive state, the external segments of the integrin dimer are folded together into a compact structure that cannot bind matrix proteins. In this state, the cytoplasmic tails of the dimer are hooked together, preventing their interaction with cytoskeletal linker proteins. Active state, the two integrin subunits are unhooked at the membrane to expose the intracellular binding sites for cytoplasmic adaptor proteins, and the external domains unfold and extend, like a pair of legs, to expose a high-affinity matrix-binding site at the tips of the subunit. Switch from inactive to active states depends on a major conformational change that simultaneously exposes the external and internal ligand-binding sites at the ends of the integrin molecule.

Selectins

Member of a family of cell-surface carbohydrate-binding proteins that mediate transient, Ca2+-dependent cell-cell adhesion in the bloodstream-for example between white blood cells and the endothelium of the blood vessel wall.

What are fibrillin?

Microfibrils are composed of a number of distinct glycoproteins, including the large glycoprotein fibrillin, which binds to elastin and is essential for the integrity of elastic fibers. Mutations in the fibrillin gene result in Marfan's syndrome, common human disorder

Pectin

Mixture of polysaccharides rich in galacturonic acid which forms a highly hydrated matrix in which cellulose is embedded in plant cell walls.

How could the loss of anchorage dependence contribute to cancer?

Mutations that disrupt or override this form of control, allowing cells to escape from anchorage dependence, occur in cancer cells and play a major part in their invasive behavior.

Understand the cellulose synthesis can be spatially constrained by cortical microtubules.

Oriented microtubules inside the cell, however, can force a change in the direction in which the rosettes move; they can create boundaries in the plasma membrane that acts like the banks of a canal to constrain rosette movement. Cellulose synthesis can occur independently of microtubules; but it is constrained spatially when cortical microtubules are present to define membrane microdomains within which the enzyme complex can move.

Describe the importance for the ability to rapidly degrade the ECM.

Rapid matrix degradation is required in processes such as tissue repair, and even in the seemingly static ECCM of adult animals there is a slow, continuous turnover, with matrix macromolecules being degraded and resynthesized. This allows bone, for example, to be remodeled so as to adapt to changes in the stresses on it.

Describe the use of scaffold proteins (include the PDZ domain) at the site of tight junctions.

The key organizational proteins at tight junctions are the zonula occludens (ZO) proteins. The three major members of the ZO family-ZO-1, ZO-2, ZO-3- are large scaffold proteins that provide a structural support on which tight junction is built. These intracellular molecules consist of strings of protein-binding domains, typically including several PDZ domains-segments about 80 amino acids long that can recognize and bind the C-terminal tails of specific partner proteins.

Describe the assembly of an adherens junction.

The linkage of cadherins to the cytoskeleton is indirect and depends on adaptor proteins that assemble on the cytoplasmic tail of the cadherin. At adherens junctions, the cadherin tail binds two such proteins: β-catenin and a distant relative called p120-catenin; a third protein called α-catenin interacts with β-catenin and recruits a variety of other proteins to provide a dynamic linkage to actin filaments.

Describe how the talin protein acts as tension sensor.

The long C-terminal tail domain of talin, includes a large number of binding sites for the actin regulatory protein vinculin. Many of these sites are hidden inside folded protein domains but are exposed when those domains are unfolded by stretching the protein. The N-terminal end of talin binds integrin and the C-terminal end binds actin, when actin filaments are pulled by myosin motors inside the cell, the resulting tension stretches the talin rod, thereby exposing vinculin-binding sites. The vinculin molecules then recruit and organize additional actin filaments. Tension thereby increases the strength of the junction.

Generally, describe the extracellular matrix (ECM), how does it vary in form and function?

The matrix is composed of different proteins and polysaccharides that are secreted locally and assembled into an organized meshwork in close association with the surfaces of the cells that produce them. The matrix can become calcified to form rock-hard structures of bone or teeth, or it can form the transparent substance of the cornea, or it can adopt the ropelike organization that gives tendons their enormous tensile strength. The ECM is more than a passive scaffold to provide physical support. It has an active role in regulating the behavior of the cells that touch it, inhabit it, or crawl through its meshes, influencing their survival, development, migration, proliferation, shape, and function.

Describe the basic structure and function of integrins.

There are many varieties of integrins, but they all conform to a common plan. An integrin molecule is composed of two noncovalently associated glycoprotein subunits called α and β. Both subunits span the cell membrane, with short intracellular C-terminal tails and large N-terminal extracellular domains.

Basil lamina

This mat of extracellular matrix that separates epithelial sheets, and many other types of cells such as muscle or fat cells, from connective tissues. Sometimes called basement membrane.

Describe the process of folding an epithelial sheet into an epithelial tube.

Within each cell, a contractile bundle of actin filaments and myosin II lies adjacent to the adhesion belt, oriented parallel to the plasma membrane and tethered to it by the cadherin and their associated intracellular adaptor proteins. The actin-myosin bundles are thus linked, via the cadherins, into an extensive transcellular network. Coordinated contraction of this network provides the motile force for a fundamental process in animal morphogenesis- the folding of epithelial cell sheets into tubes, vesicles, and other related structures.

Proteoglycans

a glycoprotein consisting of a small core protein with many carbohydrate chains attached, found in the extracellular matrix of animal cells.

what groups make up the cadherin superfamily?

1. Classical cadherins 2. Nonclassical cadherins

What are the three classes of ECM macromolecules?

1. Glycosaminoglycans (GAGs) which are large and highly charged polysaccharides that are usually covalently linked to protein in the form of proteoglycans. 2. Fibrous proteins, which are primarily members of the collagen family. 3. Large class of noncollagen glycoproteins, which carry conventional asparagine-linked oligosaccharides.

ICAMS (intracellular cell adhesion molecules)

Chief endothelial cell proteins that are recognized by white blood cells integrins

Describe the variation in collagen organization

Collagen fibrils form structures that resist tensile forces. Mammalian skins are woven in a wickerwork pattern so that they resist tensile stress in multiple directions. Leather consists of this material, suitably preserved. In tendons, collagen fibrils are organized in parallel bundles aligned along the major axis of tension. Mature bond and in the cornea, they are arranged in orderly plywood-like layer, with fibrils in each layer lying parallel to one another but nearly at right angles to the fibrils in the layers on either side.

Gap Junction

Communicating channel-forming cell-cell junction present in most animal tissues that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next.

Laminin

ECM fibrous protein found in basal laminae, where it forms a sheetlike network. Primary organizer of the sheet structure, and, early development, basal laminae consist mainly of laminin molecules. Comprise a large family of proteins, composed of three long polypeptide chains help together by disulfide bonds and arranged in the shape of an asymmetric bouquet.

Fibronectin

ECM protein involved in adhesion of cells to the matrix and guidance of migrating cells during embryogenesis. Integrins on the cell surface or receptors for fibronectin.

Elastin Fibrils

Extensible fiber formed by the protein elastin in many animal connective tissues, such as in skin, blood vessels, and lungs, which gives them their stretchability and resilience. A network of ECM of these tissues gives them the resilience to recoil after transient stretch.

Elastin

Extracellular protein that forms extensible fibers (elastic fibers) in connective tissues. Soluble tropoelastin is secreted into the extracellular space and assembled into elastic fibers close to the plasma membrane, generally in cell-surface infoldings.

Immunoglobulin (Ig) Superfamily

Large and diverse family of proteins that contain immunoglobulin domains or immunoglobulin -like domains. Most are involved in cell-cell interactions or antigen recognition. Member of immunoglobulin superfamily mediate Ca2+ independent cell-cell adhesion.

Turgor Pressure

Large hydrostatic pressure developed inside a plant cell as the result of the intake of water by osmosis; it is the force driving cell expansion in plant growth and it maintains the rigidity of plant stems and leaves.Fluid in the plant cell wall contains more solutes than does the water in the plant's external milieu, it is still hypotonic in comparison with the cell interior. This osmotic imbalance causes the cell to develop a large internal hydrostatic pressure, which pushes outward on the cell wall. The turgor pressure increases just to the point where the cell is in osmotic equilibrium, with no net influx of water despite the salt imbalance.

Glycosaminoglycan (GAG)

Long, linear, highly charged polysaccharide composed of a repeating pair of sugar. Mainly found covalently linked to a protein core in extracellular matrix proteoglycans.

Cellulose

Long, unbranched chains of glucose; major constituent of plant cell walls. Gives the primary cell wall tensile strength.

How do selectins work with integrins with regards to white blood cell adhesion and migration?

Main role, in vertebrates, is in governing the traffic of white blood cells into normal lymphoid organs and any inflamed tissues. White blood cells lead a nomadic life, roving between the bloodstream and the tissues, and this necessitates special adhesive behavior. The selectins control the binding of white blood cells to the endothelial cell lining blood vessels, thereby enabling the blood cells to migrate out of the bloodstream into a tissue.

Describe the concept of anchorage dependence.

Many cells will not grow or proliferate in culture unless they are attached to extracellular matrix; nutrients and soluble growth factors in the culture medium are not enough. When cells lose contact with the extracellular matrix, they undergo apoptosis. This dependence of cell growth, proliferation, and survival on attachment to a substratum is known as anchorage dependence and is mediated mainly by integrins and the intracellular signals they generate.

Fibril-associated collagen

Mediates the interactions of collagen fibrils with one another and with other matrix macromolecules to help determine the organization of the fibrils in the matrix. This collagen has a flexible triple-stranded helical structure and binds to the surface of the fibrils rather than forming aggregates.

Lignin

Network of cross-linked phenolic compounds that forms a supporting network throughout the cell walls of xylem and woody tissue in plants.

Cross-linking glycan

One of a heterogeneous group of branched polysaccharides that help to cross-link cellulose microfibrils into a complex network. Has a long linear backbone of one sugar type with short side chains of other sugars.

Secondary cell wall

Permanent rigid cell wall that is laid down underneath the thin primary cell wall in certain plant cells that have completed their growth

Plasmodesma

Plant equivalent of a gap junction. Communicating cell-cell junction in plants in which a channel of cytoplasm lined by plasma membrane connects two adjacent cells through a small pore in their cell walls.

Connexins

Protein components of gap junctions, a four-pass transmembrane protein. Six connexins assemble in the plasma membrane to form a connexon, or "hemichannel".

Scaffold Proteins

Protein that binds groups of intracellular signaling proteins into a signaling complex, often anchoring the complex, at a specific location in the cell.

PDZ Domains

Protein-binding domain present in many scaffold proteins, and often used as a docking site for intracellular tails of transmembrane proteins.

What accounts for the variation in collagen fibers?

Secreted on large quantities by connective tissue cells, constitutes 25% of the total protein mass.

Discuss the specificity of cadherins.

Selective cell-cell recognition systems make cells of the same differentiated tissue preferentially adhere to one another.

What cells synthesize the basal lamina?

The basal lamina is synthesized by the cells on each side of it: the epithelial cells contribute one set of basal lamina components, while cells on the underlying bed of connective tissue contribute another set.

Primary cell walls

The first cell wall produced by a developing plant cell; it is thin and flexible, allowing room for cell growth.

What are the functions of metalloproteases and serine proteases?

Together, metalloproteases and serine proteases cooperate to degrade matrix proteins such as collagen, laminin, and fibronectin.

RDG Sequence

Tripeptide sequence of arginine-glycine-aspartic acid that forms a binding site for integrins, present in fibronectin and some other extracellular proteins.

Describe how the orientation of cellulose microfibrils influences the direction of cell elongation.

Turgor pressure inside the cell drives the expansion, but behavior of the cell wall governs its direction and extent. For the cell wall to stretch and deform, the microfibrils must either slide past one another or become more widely separated, or both. The orientation of the microfibrils in the innermost layers or the wall governs the direction in which the cell expands. Cells in plants therefore anticipate their future morphology by controlling the orientation of the cellulose microfibrils that they deposit in the wall.

Hyaluronan

Type of nonsulfated glycosaminoglycan with a regular repeating sequence of up to 25,000 identical disaccharide units, not linked to a core protein. Found in the fluid lubricating joints and in many other tissues.

Serine Protease

Type of protease that has a reactive serine in the active site.

Are GAGs hydrophilic or hydrophobic (hint: do they have a net charge)?

Unbranched polysaccharide chains composed of repeating disaccharide units. One of the two sugars in the repeating disaccharide is always an amino sugar, which in most cases is sulfated. The second sugar is usually a uronic acid. B/c they are sulfate or carboxyl groups on most of their sugars, GAGs are highly negatively charged. Strongly hydrophilic.

Connective tissues

any support tissue that lies between other tissues and consists of cells embedded in a relatively large amount of extracellular matrix. Includes bones, cartilage, and loose connective tissue.

Epithelial cells of the small intestine

are indirectly connected to one another through desmosomes, and to the basal lamina through hemidesmosomes.

Homophilic binding

binding between molecules of the same kind, especially those involved in cell-cell adhesion.

Desmosomes

contains specialized cadherins that link to intermediate filaments. Main function is to provide mechanical strength. Present in most mature vertebrate epithelia and are particularly plentiful in tissues that are subject to high levels of mechanical stress, such as the heart muscle and the epidermis.

Claudins

essential for tight-junction formation and function.

Classical cadherins

family of cadherin proteins, including the E-cadherin, N-cadherin, and P-cadherin, that are closely related in sequence throughout their extracellular and intracellular domains.

Occludin

important for limiting junctional permeability.

Nonclassical cadherins

large family of cadherins that are more distantly related in sequence than classical cadherins and include proteins involved in adhesion and signaling.

Catenins

link classical cadherins to the actin cytoskeleton.

NCAM (neural cell adhesion molecule)

mediate homophilic binding.

Cadherin

member of the large cadherin superfamily of transmembrane adhesion proteins. Mediates homophilic calcium-dependent cell-cell adhesion in animal tissues.

Vinculin

promotes the recruitment of more actin to the junction.

aggrecan

proteoglycan found abundantly in cartilage; binds hyaluronic acid to form giant aggregates

Decorin

secreted by fibroblasts and has a single GAG chain, bind to collagen fibrils and regulates fibril assembly and fibril diameter.

Epithelial tissues

tissues, such as lining of the gut or the epidermal covering of the skin, cells are tightly bound together into sheets called epithelia.

List the diverse functions of the basil lamina.

• Act as a selective barrier to the movement of cells, as well as a filter for molecules. • Prevents fibroblasts in the underlying connective tissue from making contact with the epithelial cells. • Important in tissue regeneration after injury.