Module 20 - Integrating Cells into Tissues

Question 1

Define a tissue and an organ.

Answer 1

Tissues are cooperative assemblies of cells, as well as extracelullar matrix.

Organs are cooperative assemblies of tissues.

Question 2

Identify the difference between epithelial and connective tissue.

Answer 2

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.

Question 3

Describe common locations and functions of epithelial tissues

Answer 3

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

1. secretory parts of glands and their ducts
2. receptors for certain sensory organs

Question 4

Relate epithelial functions to different epithelial structure and organisation.

Answer 4

• Simple Squamous: blood vessel
• Simple cuboidal: thyroid
• Simple columnar: intestine
• Stratified squamous: skin
• Transitional: bladder
• Ciliated pseudostratified: respiratory tract

Question 5

Using specific examples, describe cell specialisations that facilitate cell function

Answer 5

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

Question 6

Analyse how genetic mutations can affect cellular function in an epithelium.

Answer 6

• 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

Question 7

Using an example, describe the basis of apico-basal polarity in epithelial cells.

Answer 7

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)

Question 8

Describe the structure and function of tight junctions.

Answer 8

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.

Question 9

Describe the structure and function of adherens junctions.

Answer 9

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.

Question 10

Explain how cadherin play a role in cell-to-cell adhesion.

Answer 10

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.

Question 11

Explain how qualitative and quantitative differences in cadherin expression can affect cell sorting and organogenesis

Answer 11

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.

Question 12

Describe how adherent junctions function in organogenesis and cell migration within an epithelium.

Answer 12

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.

Question 13

Describe the structure and function of desmosomes.

Answer 13

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.

Question 14

How do demsosomes differ from adherent junctions.

Answer 14

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.

Question 15

Explain how genetic defects of junction proteins can result in disease

Answer 15

Mutations to desmosome:

• Pemphigus vulgaris: autoimmune disease that disrupt desmosome; leads to blisters
• Epidermolysis bullosa simplex: mutations in keratin; epidermis separated from dermis

Question 16

Describe the structure and function of focal adhesions

Answer 16

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.

Question 17

Explain “inside-out” and “outside-in” signalling as related to integrin function.

Answer 17

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).

Question 18

Explain how talin functions as a tension sensor at cell matrix junctions and how tension affects cell behaviour.

Answer 18

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)

Question 19

Describe how integrin activation can lead to intracellular signals when they do not have kinase activity themselves.

Answer 19

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.

Question 20

Describe the structure and function of hemidesmosomes.

Answer 20

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)

Question 21

Describe the structure and function of gap junctions.

Answer 21

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.

Question 22

Differ between homotypic and heterotypic gap junctions.

Answer 22

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.

Question 23

Describe the principal components of the extracellular matrix and some of its roles.

Answer 23

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.

Question 24

Describe the biochemical structure of fibrillar collagens.

Answer 24

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)

Question 25

How can a cell produce a large structure, such as a collagen fibre (100s microns) if the average cell is 10 microns in size?

Answer 25

Small collagen subunits are secreted by the biosynthetic pathway and the fibrils and fibres assemble outside the cell.

Question 26

Explain the difference between fibrillar, fibril-associated and sheet-forming collagens.

Answer 26

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

Question 27

Describe the roles and structure of fibril-associated collagens

Answer 27

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)

Question 28

Describe how fibrillary collagens are synthesised and assembled into fibrils and fibres

Answer 28

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.

Question 29

Why is the collagen propeptides retained after translation in the ER?

Answer 29

Collagen propeptides prevent premature assembly of collagen inside cells.

Question 30

Explain how mutations in collagen genes can lead to diseases.

Answer 30

* 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

Question 31

Describe the functions of elastin and fibrillin.

Answer 31

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

Question 32

Describe the structure and function of the adhesive ECM protein fibronectin

Answer 32

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.**

Question 33

What is the function of the basement membrane?

Answer 33

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.

Question 34

Describe the components of the basement membrane.

Answer 34

Type IV collagen, integrins, and other substance adhesion proteins (nidogen, perlecan)

Question 35

Describe the structure and function of proteoglycans

Answer 35

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

Question 36

How do proteoglycans complement the structure/function of structural fibres functionally

Answer 36

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**

Question 37

Describe how the ECM can modulate growth factor activity.

Answer 37

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)

Question 38

Remember **TM Protein, Extracellular Ligand, Cytoskeleton Attachment, and Intracellular Anchoring Proteins** for each Anchoring Cell Junction.

Answer 38

Question 39

Mention the types of cell that produce ECM.

Answer 39

* Fibroblasts: loose connective tissue * Osteoblasts: bone * Chondroblasts: cartilage * Epithelial cells: basement membrane in epithelia

Question 40

Describe the structure and function of the adhesive ECM proteins laminin.

Answer 40

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).