3.7 - 3.9 Integrating cells into tissues Flashcards
What are tissues?
Cells in most multicellualr organisms are arranged into tissues. Tissues are cooperative assemblies of cells and the extracellular matrix
What are organs?
Cooperative assemblies of tissues
What are the 5 major types of tissue?
- Epithelial
- Connective
- Nervous
- Muscle
- Blood and lymphoid tissues
What are the key characteristics of epithelial tissue?
- Cells intimately connected to each other (junctions)
- Apico-basal polarity
- Little extracellular matrix (basement membrane)
What are the key characteristics of connective tissue?
- Cells have few contacts with each other
- No apico-basal polarity
- Large amount of extra cellular matrix
What is nervous tissue?
Specialised, electrochemical signalling
What is muscle tissue?
Specialised, contractile
How are all tissue types seen in an organ such as the gut?
What do the epithelia line?
- External body surfaces
- Internal body cavities
- Tube organs that communicate with exterior (alimentary, genito-urinary and respiratory tracts)
What does the epithelia form?
- Secretory parts of glands and their ducts
- Receptors for centain sensory organs
- Even the brain arises from an epithelium (neuroectoderm) in the embryo
Name these epithelia
What are the functions of the epithelia?
- Protection from mechanical and environmental insults
- Lining of internal tube organs (oviduct and respiratory tracts)
- Example: respiratory epithelium line air conducting tubes to lungs (infection risk)
- Secrete mucus
- Cell specialisation
- Cilia
- Goblet cells
How is a thick keratinised layer produced in the epidermis?
- Epidermis starts as a stem cell on the basal laminar
- Proliferates and differentiate to form keratinocytes that produce keratin and intermediate filaments
- Modified cell death where they lose their nucleus and are just bags of keratin protein
What happens in the respiratory epithelium if there is a genetic defect of cilia?
- Genetic defect of cilia in dynein genes stops them from beating, as dynein are responsible for movement
- Results in primary ciliary dyskinesia
- Situs inversus, male infertility
- Recurrent respiratory disease in children
What happens to the respiratory epithelium if there is a genetic defect of chloride channel?
- Abnormal export from the ER
- Causes cystic fibrosis
- Mucus so thick that cilia can’t move it
What is the function of epithelia in the organs (intestine)?
- Absorption from lumen of organs (intestinal tract and kidney tubules)
- In the intestine there are villi
- Further membrane specialisations in the microvilli (brush border)
- Protect epithelium from acids by secreting mucus
What is the polarity of the gut epithelium which absorbs nutrient molecules?
- It has apico-basal polarity and apical and basal membrane have different properties
- Apical has active transport channel where there is increased glucose concentration in the cell
- Basal has passive channel and transporter protein for diffusion of glucose to the blood
- There are special cell-cell junctions that prevent backflow of molecules
How do the apical and basal membranes differ?
- Apical is for absorption, secretion, specialisations (microvilli and cilia)
- Basal is for adhesion to extracellular matrix and secretion into sub mucosa
What are the four different types of junctions for epithelial cells?
- Tight junctions
- Adherens junctions (desmosomes)
- Gap juncitons
- Focal contacts (hemidesmosomes)
How are cell junctions in epithelial cells usually aranged?
- Cells may have more than one type of junction
- Arranged in junctional complexes
What are tight junctions?
- Usually near the apical surface
- They are bands of plasma membrane proteins encircling the cell
- Prevent leakage of molecules across the epithelium
- Separate different membrane domains of epithelium, essential to maintain cell apico-basal polarity
What is the structure of tight junctions?
- Bands of membrane proteins, claudin and occludin, in adjoining cells
- Proteins form very strong links, non covalent hydrogen bonds
- More bands means more impermeability
How permeable are tight junctions?
- Experiments with tracer molecules show how effective tight junctions are at preventing movement of molecules between cells. Permeability varies in different cell types
What is the role of adherens junctions?
- They link epithelial cells to each other
- Link with cell cytoskeleton rather than intermediate filaments through actin
- They involve homophillic interactions between cell adhesion molecules called cadherins
- E, P, N cadherins (classical)
What are the interactions between adherens junctions?
- Cadherins are transmembrane proteins, made up of flexible extracellular domain
- The flexibility is stabilised if you have Ca2+ so it extends into the extra-cellular space and adheres to each other via N-terminal cadherin repeat
What other proteins are there in adherens junctions?
- Links to the actin cytoskeleton via linker/adaptor proteins
- beta-catenin
- alpha-catenin
- p120 catenin
- gamma-catenin
- vinculin
- Beta catenin also functions in growth factor signalling
What is the funciton of adherens junctions in movement?
What is a prominent example of epithelial folding in development?
- Formation of neural tube
- Notochord releases factors that stimulate infolding in neural tube
- Change in cadherin as cells change their fate
- Shows how cells change patterns of cadherin expression during tissue morphogenesis
How do cadherins influence cell sorting?
How is cadherin used in embryogenesis?
- Cells from different layers of an early amphibian embryo will sort according to their origins,
- mesoderm (green), neural plate (blue), and epidermis (red) sort into a structure that resembles and embryo with a neural tube in the centre
What do desmosomes do?
- they spot weld cells together
- Distribute tensile forces
- Inter-connect intermediate filaments of adjacent cells
- Found in tissues subject to high mechanical stress such as the heart, muscle and epidermis
What molecules interact in desmosomes?
- Link epithelial cells to each other via homophillic interactions between cell adhesion molecules of the cadherin family (desmoglein, desmocollin)
- Non classical cadherins
- Link with cytoskeleton (intermediate filaments) via adaptor proteins (plakoglobin, plakophillin, desmoplakin)
How do desmosomes compare to adherens junctions with reference to the actin filaments?
- Actin filaments terminates while intermediate continutes to pass through
What are the characteristics of intermediate filaments?
- Criss cross cell cytoplasm
- Confer tensile strength
- Intermediate filaments are often cells specific (desmin in the heart, keratin in the skin and other epithelia)
How do desmosomes relate to autoimmune disease?
- Pemphigus vulgaris due to antibodies to desdemonal proteins which distrupt the desmosome
- Causes skin and mucosal blistering as epidermis separates from dermis
- Potentially fatal
How do desmosomes relate to congenital disease?
- Epidermolysis bullosa simplex due to mutations in keratins
- Cuases skin and mucosal blistering
- Potentially fatal
What linkages are involved in a focal linkage?
- Transmembrane adhesion proteins called integrins which bind extracellular matrix proteins
- Link to actin cytoskeleton via adaptor proteins (vinculin, talin)
- Involved in cell movements and attachment (myotendinous junction)
How do focal adhesions generate cell traction forces?
- Interaction of integrins with their substrates plays an important role in cell motility via generation of cell traction forces (CTF) in combination with actin assembly and disassembly
- Myosin interactions at the rear of the cell
What are focal adhesions sites of anchorage for?
- Focal adhesions are sites of anchorage for intracellular actin filaments and extracellular ECM molecules
- Also concentrate other signalling molecules, where phosphotyrosines at the integrin junction is phosphorylated for cell signalling
How is integrins an example of inside out signalling?
- Cells regulate the activity of cell surface integrins
- Integrins will not engage extracellular matrix ligand unless activated
- Example : growth factor receptor signal activates integrins via intracellular signalling
- Talin curls up on itself, vinculin binding sites are open with permits actin to engage with integrins, talin also bind to beta subunit
How does outside-in signalling occur in integrins?
- Activation by inside out signals allows binding to extra cellular matrix ligand
- Extra cellular matrix binding recruits intracellular protein kinases, vinculin, talin, filamin, alpha-actin
- Multiple integrins recruited to focal adhesions gives signals into the cell
How is talin a tension sensor at cell-matrix junctions?
- It has multiple binding sites (vinculin, actin, integrins)
- Tension stress from actin filaments reveals cryptic binding sites
- stabilises vinculin and integrins in focal adhesions
- Promotes signalling at focal adhesions
- Tension stretches the extra cellular matrix causing growth factor release
What does this experiment show?
- It has an actin binding domain and vinculin binding domain
- Fix talin to a glass slide via N terminal domain
- Magnetic bead at the C terminal which stretches it out
- Vinculin binding site exposed and vinculin binds
- Putting tension on actin filaments
How is cell proliferation dependent on traction and distribution of growth factors?
- Integrins may act as mechanoreceptors
- Tension on the cytoskeleton may combine with growth factor signalling to promote G1 progression
- Cell with one spot dies as it cannot spread out and establish focal adhesions to set up signalling centre
How does outside in signalling with integrins require other kinases?
- Clusters of intergrins permit extracellular matrix signals to be transmitted but integrins have no kinase domains
- Signalling events require other kinases such as focal adhesion kinase (FAK)
- FAK activates the Ras Map kinase pathway
- Integrins don’t have kinase activity but by forming focal adhesion you can can activate signalling
What are hemidesmosomes?
What integrins are involved in the skin in hemidesmosomes?
- alpha6beta4 integrins in hemidesmosomes adhere cells to the basement membrane
- Link to intermediate filaments
What happens if there is gene knockout of hemidesmosome integrins in mice?
- Deletion of beta4 integrins means there are skin blisters
- Failure of keratinocytes to adhere to basement membrane
- Skin not attached to the dermis and separates with tension
- Fatal resulting in neonatal death
Summary table
What can pass through a gap junction?
- Adjacent cell membranes are 2-4 nm apart
- Allow small molecules such as ions, sugars, nucleotides, vitamins, singalling mediators (Ca2+, cAMP, IP3) to pass through cells with pore size 1.5 nm
- Excludes macromolecules > 1000 daltons (proteins, nucleic acids and polysaccharides)
What are the channels between a gap junction?
- Channels (connexons) consist of 6 membrane spanning proteins (connexins)
- Channels/connexons in register between two cells form the gap junction
What are the functions of gap juncitons in these tissues?
What do connexin mutations lead to?
- Cx26 mutations commonest cause of congenital deafness
Where is the connective tissue here?
What is the extracellular matrix and its components?
- spaces between cells composed of a complex array of molecules
- composed of tough fibrous proteins embedded in a polysaccharide gel-like material (ground substance)
How does the extracellular matrix vary in conenctive tissues?
- Variety in connective tissues due to differences in composition and arrangement of extracellular matrix
- Tendon has numberous fibrous proteins, little ground substance
- Bone has calcified ground substance and fibrils
- Cartilage has large amount of polysaccharide gel
What are the roles of the extracellular matrix?
- Structural support
- Regulation of cellular activities
- Cell survival
- Cell migration
- Cell proliferation
- cell shape
- cell function
What produces the extra cellular matrix?
- Extracellular matrix is produced by cells within it
- Cells produce and secrete organic components of the extracellular matrix
- Cells organise the extracellular matrix
How do cells organise collagen in the extracellular matrix?
- Collagen fibres must be correctly aligned within tissues
- Cells deposit and organise collagen matrices
- Alternate layers of longitudinal and transversly sections fibres
- Fibroblast organise and arrange that extracellular matrix
Which cells produce extracellular matrix and in what tissues?
- Fibroblasts (loose connective tissue)
- Osteoblasts(bone)
- Chondroblasts(cartilage)
- Epithelial cells (basement membrane in epithelia)
What is the extracellular matrix broadly composed of?
- Proteins
- Fibrous/structural such as collagen and elastin
- Adhesive such as laminin
- Proteoglycans which are carbohydrate modified proteins, very large molecules which attract water
What are the characteristics of collagen in the extracellular matrix?
- About 25% of total protein mass in mammals
- 42 different collagen genes with different properties
- Provides tensile strength, can withstand stretching
- Rich in proline and glycine arranged in repeats as an alpha helical polypeptide chain
What is the structure of collagen?
- Collagen alpha chains composed of a series of Gly-X-Y triples where X is any amino acid, usually proline or lysine and Y is any amino acid usually hydroxy-proline or hydroxy-lysine
- Collagen molecules consist of three alpha chains arranged in a super helix
- Alpha chain is a left handed helix due to dihedral angle of prolines
How are collagens arranged in fibrils?
- Many collagen molecules assemble together (covalent cross linking) to form collagen fibrils (10-300 nm in diameter)
- Regular packing of collagen molecules leads to cross striations (67 nm periodicity)
How do collagen fibres arrange themselves into fibrils?
Which collagens form fibres?
- Only collagens I, II, III, V form fibrilar collagens
- Collagen IV forms sheets
- Collagens VI, IX, XII are fibril associated collagens which decorate fibrilar collagens and mediate fibril interactions
What is the structure of fibril associated collagens?
- Triple helix is interrupted by non helical domains giving flexibility
- They are not cleaved after secretion so they retain polypeptides
- They do no aggregate to form fibrils
- Bind periodicially to other collagen fibrils
- Type IX binds to II
- Type XII to type I
What are the steps of collagen synthesis before secretion?
- Collagen precursors (procollagens) are synthesised into ER lumen
- N and C terminal ends have propeptide
- Intrachain disulfide bonds between N and C terminal propeptide sequences align chains to form triple helix in ER
- Procollagen is modified in ER and Golgi (hydroxylated and glycosylated) and secreted by exocytosis
What are the steps of collagen synthesis after secretion?
- After processing and assembly of 3 pro-alpha-chains, type I procollagen is secreted into the extracellular space
- Extracellular enzymes (procollagen peptidases) remove N and C terminal propeptides so collagen self polymerises into fibrils
- Collagen pro-peptides prevent premature assembly of collagen in side cells
How does cross linking occur in collagen synthesis?
- After secretion covalent bonds (lysine residues) cross-link the collagen molecules, particularly in non-helical ends
- Extent of cross-linking affects tensile strength.
- Inhibition of cross-linking reduces tensile strength
- Highest level of cross-linking occurs in tendon collagen (high tensile strength)
What happens if there is a defect in collagen I?
- Osteogenesis imperfect (brittle bones)
- Variety of mutations (col1A1, col1A2)
- Glycine substitutions failure to form triple helices
What happens if there is a defect in collagen II?
- Achondrogenesis (col2A1, GMAP210)
- Failure in collagen synthesis or transport
- Abnormal cartilage gives abnormal bone and joint formation which is severe
What happens if there is a defect in collagen III?
- Ehlers Danlos syndrome
- Fragile skin, blood vessels and hypermobile joints, elastic skin
How is scurvy a defect in collagen synthesis?
Failure to hydroxylate prolines and lysines in fibrilar collagen due to reduced levels of ascorbate (vitamin C)
What does elastin do?
- It provides elasticity to tissues (lungs, blood vessels)
- It is the major protein in arteries
- Forms covalently corss linked network of elastin molecules
- With fibrillin it forms elastic fibres
What happens in Williams-Beuren syndrome?
- Elastin is mutated giving supravalvular arotic stenosis, mental retardation, facial dysmorphoa
How does the artery have arrangements of elastin and collagen?
- Elastic arteries such as the aorta undergo high pressure fluctuations
- Need elastic fibres (elastin and fibrillin) for recoil of vessel wall
- Collagen provides tensile strength and eleastin elasticity
What happens if you have a detect in fibrillin I?
- Marfan syndrome
- Large blood vessels with dilation of pulmonary artery and aorta resulting in aneurysms
- Skeletal defects causing scoliosis from elastic fibres in elastic cartilage and growth plates
- Dislocated lens as fibrils that hold lens are rich in elastin
- Heart has mitral valve insufficiency (valves usually rich in elastic fibres)
What are the two ways that cells can attach to the extra cellular matrix?
- Transiently and weakly. (Eg. migration)
- Irreversibly and strongly. (Eg. Muscle and tendons).
What are the 4 extracellular matrix proteins that mediate adhesion?
- Laminin
- Fibronectin
- Tenascin
- Collagen
What are the two forms of fibronectin?
- It is either a soluble dimer in plasma
- Or insoluble cell-associated dimer with disulfide bonds. It requires integrins (RGD) or actin to form fibrillar fibronectin
- Tension reveals cryptic binding sites to permit association between fibronectin molecules to form fibrils at adhesions
What happens if you lose fibronectin via gene knockout?
- Embryonic lethal, no formation of blood vessels as endothelial cells fail to migrate or attach to basement membranes
- Antibodies to RGD domains or RGD peptides inhibit cell attachmetn and migration
- If you lose RGP peptides you inhibit cell from attaching as it will compete with RGD in fibronectin
What is laminin?
- An adhesive extracellular matrix protein
- Has 3 chains (alpha, beta, gamma) held together by disulphide bonds
- Multiple different forms (5alpha, 3beta, 3 gamma)
- Multiple binding domains to bind to cell (integrins) or to bind ot other extracellular matrix components
Where are laminins found?
- Common in basement membranes
- Interact with collagens, nidogen and perlecan
- Interact with integrins on cell surfac
What happens if you knockout laminin gamma1 chain/ other mutations?
- Knockout of laminin gamma1 chain is embryonic lethal as you fail to form a basement membrane
- Mutations include;
- Epidermolysis bullosa (LAMC2, LAMA3)
- Muscular dystrophy (LAMA2)
- Neuromuscular disorder (LAMB2)
What are basement membranes?
- They are specialised extracellular matrix
- Underlies epithelium cells and tubular structures, separting epithelia and connective tissue
- Synthesised by cells that rest upon it
Is the basement membrane inert?
- Not inert
- Contains growth factors
- Can modulate cell behaviours
What is the structure of proteoglycans?
- large protein core linked to negatively charged chains of polysaccharides (due to carboxyl and sulfate groups)
- Glycosaminoglycans
- Consist of disaccharide repeats
What do proteoglycans do in the extracellular matrix?
- Serve complimentary function to collagens
- Form gel-like ground substance
- Resist compressive forces
- Permit diffusion of nutrients, metabolites, hormones and growth factors
How are proteoglycans linked?
- Multiple GAG chains are linked to a core protein (serine residues) via linker tetrasaccharides.
- Need to connect to polypeptide chain of core protein
- Link tetrasaccharide links to disaccharide repeat via serine amino acid
What is the size of proteoglycans and how does it compare to glycoprotein?
What are the very large aggregates of proteoglycans known as?
How do proteoglycans form gels?
- negative charge attracts cations such as Na+
- Forms gels due to water following ion concentration
- Osmotic pressure gives resistance to compression with balances tensile strength of colalgen
- Form <10% of dry weight of fibrous protein but may occupy 90% of space.
What is the role of proteoglycans in signalling?
- Can bind and regulate activity of secreted proteins such as growth factors
- Sequester from cells (inhibit signalling)
- Present to cells (enhance signalling)
- Increase diffusion capacity of growth factors (enhance signalling)
