Module 20 - Integrating Cells into Tissues Flashcards
Define a tissue and an organ.
Tissues are cooperative assemblies of cells, as well as extracelullar matrix.
Organs are cooperative assemblies of tissues.
Identify the difference between epithelial and connective tissue.
Epithelial cells are intimately connected to each other (junction). It also possesses apico-basal polarity. Little ECM is present (located in the basement membrane).
Connective tissues have cells that are in a few contacts with each other. It has no apico-basal polarity. It also sits along a large amount of ECM.
Describe common locations and functions of epithelial tissues
Epithelial tissues are located mostly along the external body surfaces, internal body cavities, as well as tube organs that communicate with the exterior (alimentary, gento-urinary, and respiratory tracts).
Its main functions include but not limited to
- secretory parts of glands and their ducts
- receptors for certain sensory organs
Relate epithelial functions to different epithelial structure and organisation.
- Simple Squamous: blood vessel
- Simple cuboidal: thyroid
- Simple columnar: intestine
- Stratified squamous: skin
- Transitional: bladder
- Ciliated pseudostratified: respiratory tract
Using specific examples, describe cell specialisations that facilitate cell function
Skin (Epidermis):
- keratinized-squamous epithelial cells
- multilayered
- protection from mechanical, abrasion stresses; water-proof; thermoregulation
Respiratory epithelium
- Mucus-secreting (Goblet cells)
- Coordinated beating of cilia
- line air-conducting tubes to lungs (reduce infection risks)
Intestine lining/Kidney tubules
- Villi - increased surface area
- Membrane specialization (microvilli)
- Secrete mucus
- Absorption from the lumen of organs
Analyse how genetic mutations can affect cellular function in an epithelium.
- Genetic defect of dynein genes can lead to immotile cilia (Kartagener’s Syndrome); causes male infertility, recurrent respiratory disease in children
- Genetic defect of chloride ion channel (CFTR gene) causes a change in mucus property; sticky and thick mucus
Using an example, describe the basis of apico-basal polarity in epithelial cells.
The gut epithelium has the main function of absorbing nutrient molecules from the lumen. These epithelial cells have apico-basal polarity, where the membranes must have different structures and properties(as well as asymmetric localization of organelles) as they are exposed to different environments.
Apical: active transport channels (moving glucose into the epithelial cell)
Basal: passive transport channel (diffusing glucose into the bloodstream)
Describe the structure and function of tight junctions.
Tight junctions are made up of bands of plasma membrane proteins (occludin, claudin) which encircles the cell. These proteins form very strong links with adjacent proteins; creating an impermeable gap between the epithelial cells.
Describe the structure and function of adherens junctions.
Adherens junction main function is to link epithelial cells to each other. They connect actin filament bundle in one cell with that in the next cell; involving homophilic interactions between cell adhesion molucles, cadherins.
Explain how cadherin play a role in cell-to-cell adhesion.
Cadherin is a transmembrane protein, with repeats in the extracellular domain. In the presence of Ca2+, it stabilizes the cadherin repeat. Homophilic interactions between the N-terminal of adjacent cadherin forms the junction.
Explain how qualitative and quantitative differences in cadherin expression can affect cell sorting and organogenesis
Cadherins can control which cells will associate with each other and how they are arranged based on qualitative and quantitative differences in cadherin expression.
Qualitative: Two groups of cells that expresses different types of cadherin will sort each other out and congregate with like-cells
Quantitative: Two groups of cells that expresses different levels of the same cadherin type will sort each other out; cells with high cadherin levels will congregrate first before the other cells binds around them.
These differential of cadherin cell expression forms the basic mechanism of cell sorting and organogenesis.
Describe how adherent junctions function in organogenesis and cell migration within an epithelium.
While the N-terminus of cadherins bind to form the junction, the C-terminal, with linker/adapter proteins, binds to the actin cytoskeleton. In epthelial cells, this leads to the formation of a belt framework of actin filaments.
During tissue morphogenesis, cells change patterns of cadherin expression leading to the rearrangement of the actin frameworks (epithelial folding). Both organogenesis and epithelial cell migration can arise from this mechanism.
Describe the structure and function of desmosomes.
Desmosomes connects intermediate filaments in one cell to those in the next cell.
Desmosomes “spot-weld” cells together through cadherin (desmoglein, desmocolin) proteins which is attached to a dense plaque of anchor proteins in the cell. Intermediate filaments then would bind in the other side of the plaque via adaptor proteins (plakoglobin, plakophilin, desmoplakin).
These structures are found in tissues that are subejcted to high stress (e,g, Heart) as it is able to distribute the tensile force.
How do demsosomes differ from adherent junctions.
Actin filament ends in linker proteins in adherens junction, but intermediate filament don’t (they bind to other desmosomes).
Type of cadherin: classical for adherens junction (E, N, P Cadherin), non-classical for desmosomes (desmoglein, desmocolin)
Linker proteins differ for both of them.
Explain how genetic defects of junction proteins can result in disease
Mutations to desmosome:
- Pemphigus vulgaris: autoimmune disease that disrupt desmosome; leads to blisters
- Epidermolysis bullosa simplex: mutations in keratin; epidermis separated from dermis
Describe the structure and function of focal adhesions
Focal adhesion anchors actin filaments in cell to extracellular matrix.
These is achieved through integrins (TM adhesion proteins) which binds to the ECM proteins. The β subunit of integrin is linked to the actin cytoskeleton via adaptor proteins (vinculin, talin). It is also involved in cell movement (via generation of cell traction forces) and attachement.
Explain “inside-out” and “outside-in” signalling as related to integrin function.
Cells regulate the activity of cell-surface integrins, where integrins will not engage ECM ligand unless activated by an intracellular signal, which leads to strong adaptor-protein binding (inside-out signalling).
ECM binding then leads to the recruitment of intracellular proteins (vinculin, talin, filamin, etc), which permits ECM signals to be transmitted and lead to the activation of other kinases, such as FAK (Focal Adhesion Kinase).
Explain how talin functions as a tension sensor at cell matrix junctions and how tension affects cell behaviour.
Talin consists of multiple cryptic binding sites for vinculin binding. When tension is applied, by actin (tension stress) these sites are revealed allowing vinculin binding (tension sensor), which leads to stabilization of vinculin and integrins in focal adhesions.
This promotes signalling at focal adhesions and growth factor release (due to ECM stretching)
Describe how integrin activation can lead to intracellular signals when they do not have kinase activity themselves.
Integrin activation allows the binding to ECM ligand. This in turn lead to the recruitment of intracellular proteins (vinculin, talin, filamin, etc) which recruits other kinases (sucah FAK). They then can lead to intracellular signals to a wide range of pathways and responses.
Describe the structure and function of hemidesmosomes.
Hemidesmosomes anchors intermediate filaments in a cell to extracellular matrix, which involves integrins α6β4 (TM adhesion protein). The integrin links intermediate filaments via adaptor protein (plectin, dystonin). Thay are involved in high tensile strength adhesion to matrix (eg. skin)
Describe the structure and function of gap junctions.
Gap junctions are composed of two structures called a connexon (each having six connexins subunits). They align between each other, forming an open channel between the adjacent cells (2-4 nm apart).
Gap junctions allow small molecules to pass from cell-to-cell (diameter 1.5 nm). They have diverse functions, playing a role in neural transmssion, electrical conduction, and coordinated sharing of ions and metabolites.
Differ between homotypic and heterotypic gap junctions.
There are different types of connexins (membrane spanning proteins). Connexons that are made up of the same type of connexins are called homomeric connexons and if not it is called heteromeric.
Homotypic gap junctions consists of two homomeric connexons of the same type of connexins. Any other combination is called heterotypic gap junctions.
Describe the principal components of the extracellular matrix and some of its roles.
ECM refers to the spaces between cells that are composed of :
- Proteins
- fibrous/structural (collagen, elastin)
- adhesive (laminin)
- Polysaccharide gel-like material (ground material) -> proteoglycans
ECM plays many roles, including for structural support and regulation of cellular activities.
Describe the biochemical structure of fibrillar collagens.
Collagens are rich in proline and glycine arranged in repeats (Gly-X-Y triplets) as an α-helix chain, where the X and y represent any aa (usually proline or lysine). Collagen molecules consist of three α chains arranged in a superhelix. Note: the helix is left-handed due to the fixed dihedral angle of proline. Many collagen molecules assemble together through covalent cross-linking to form collagen fibrils.
This structure provides the tensile strength properties of collagen (withstand stretching)
How can a cell produce a large structure, such as a collagen fibre (100s microns) if the average cell is 10 microns in size?
Small collagen subunits are secreted by the biosynthetic pathway and the fibrils and fibres assemble outside the cell.
Explain the difference between fibrillar, fibril-associated and sheet-forming collagens.
Not all collagens form fibres. Here are the examples:
Fibrillar Collagens only form fibril structures (Collagen I, II, III, V).
Sheet-Forming Collagens (Collagen IV)
Fibril-Associated Collagens (Collagen VI, IX, XII): they ‘decorate’ fibrillar collagens and mediate fibril interactions
Describe the roles and structure of fibril-associated collagens
They ‘decorate’ fibrillar collagens and mediate fibril interactions. Different arrangements are present for different tissues.
The triple helix structure is interrupted by non-helical domains which leads to the flexibility of fibril-associated collagens. It is not cleaved after secretion (retaining propeptides) and HENCE does not aggregate to form fibrils.
Rather, it binds periodically to other collagen fibrils (IX->II, XII->I)
Describe how fibrillary collagens are synthesised and assembled into fibrils and fibres
Collagen precursors (procollagens) are synthesized into the ER lumen by the ribosomes. Both the N and C terminal retain their propeptides.
Intrachain disulphide bonds between the N and C terminal propeptide sequences align to form triple helix structure in ER. Procollagen is then modified in ER and Golgi (hydroxylated and glycosylated), and secreted by exocytosis.
After that, type I procollagen is then cleaved by procollagen peptidases which removes the propeptides -> collagen self-polymerises into fibrils, which may pack together to form fibres which occur due to the cross-linking between the non-helical ends of the collagen molecules.
Why is the collagen propeptides retained after translation in the ER?
Collagen propeptides prevent premature assembly of collagen inside cells.
Explain how mutations in collagen genes can lead to diseases.
- Osteogenesis imperfecta (Collagen I): glycine substitutions lead to failure to form triple helices -> brittle bones
- Achondrogenesis (Col. II): failure in collagen synthesis or transport leading to abnormal cartilage and bone and joint formation
- Ehlers Danlos Syndrome (Col. III): fragile skin, hypermobile joints, elastic skin
- Scurvy (Vit C deficient): failure to hydroxylate prolines and lysines in fibrillar collagens
Describe the functions of elastin and fibrillin.
Elastic fibres:
- made up of elastin and fibrillin
- provides elasticity to tissues (major proteins in arteries)
- forms covalently cross-linked network of elastin molecules and stabilized by fibrilin
Describe the structure and function of the adhesive ECM protein fibronectin
These ECM proteins mediate the adhesion of cells to the ECM. The proteins have multiple binding domains to bind to cell (via integrin receptor) and other ECM components.
Fibronectin comes in two forms: a soluble dimer (plasma) and insoluble cell-associated dimer (disulphide bonds), which requires integrins (RGD)/actin to form fibrillar fibronectin and tension to reveal cryptic binding sites to permit association between the molecules to form the insoluble fibrils at adhesions. Fibronectin is essential for cell migration and cell attachment.
What is the function of the basement membrane?
The primary function of the basement membrane is to anchor down the epithelium to its loose connective tissue (the dermis) underneath. This is achieved by cell-matrix adhesions through substrate adhesion molecules (SAMs).
These specialized ECM are synthesized by the epithelial cells that rest upon it. It also contains growth factors and may modulate cell behaviours.
Describe the components of the basement membrane.
Type IV collagen, integrins, and other substance adhesion proteins (nidogen, perlecan)
Describe the structure and function of proteoglycans
Proteoglycans have the following functions:
- form gel-like ‘ground substance
- resist compressive forces
- permit diffusion of nutrients, metabolites, hormones and other factors
They are made up of a large protein core linked to negatively charged chains (due to carboxyl and sulphate groups) of polysaccharides (glycosaminoglycans - GAG), which consists of disaccharide repeats (N-acetylglucosamine and glucuronic acid). These GAG chains are linked to the core (serine residue) via a linker tetrasaccharides
How do proteoglycans complement the structure/function of structural fibres functionally
Negative charges from the GAG chains attracts cations (such as Na+), which lead to the formation of gels due to osmosis of water following the increased ion concentration.
The resulting osmotic pressure contributes to resistance to compression which balances the tensile strength of structural fibres, such as collagen.
Note: found mostly in cartilage
Describe how the ECM can modulate growth factor activity.
Proteoglycans of the ECM can bind and regulate the activity of growth factors
- sequester from cells (inhibit signalling)
- present to cells (enhance signalling)
- increase diffusion capacity of GF (enhance signalling)
Remember TM Protein, Extracellular Ligand, Cytoskeleton Attachment, and Intracellular Anchoring Proteins for each Anchoring Cell Junction.
Mention the types of cell that produce ECM.
- Fibroblasts: loose connective tissue
- Osteoblasts: bone
- Chondroblasts: cartilage
- Epithelial cells: basement membrane in epithelia
Describe the structure and function of the adhesive ECM proteins laminin.
Laminin is composed of three chains (α, β, γ) held by disulphide bonds, which multiple forms of the chains. It also has multiple binding domains (like fibronectin) to bind to the cell and other ECM component.
They are common in basement membranes (interacting with collagens, etc). and interacts with integrins in cell surface (adhesion).
