BIOL2056 Lectures 17-20: From Cells to Tissue - ECM (PDF)

BIOL2056: Lectures 17-20 From cells to tissue: ECM 17 From cells to tissue: ECM 18 From cells to tissue: Cell Adhesion & communication 19 From cells to tissue: Cell Adhesion & communication 20 Model systems to study Cell Biology Dr Nicole Prior...

BIOL2056: Lectures 17-20 From cells to tissue: ECM 17 From cells to tissue: ECM 18 From cells to tissue: Cell Adhesion & communication 19 From cells to tissue: Cell Adhesion & communication 20 Model systems to study Cell Biology Dr Nicole Prior [email protected] Reading list: Lecturers 17-20 Part IV: Integrating Cells into Tissues 921-975 Chapter 19: Cell-Cell junctions 1035 -1085  Learning outcomes: Lecture 17 From cells to tissue: Cell Adhesion & ECM By the end of this session you should be able to: Describe the need for cell adhesion and how it is achieved Describe the function on the ECM and how it differs in specific tissues Explain the main components of the ECM and the properties they confer  The need for cell adhesion Some cells exist alone – unicellular organisms Bacteria can be ‘planktonic’ – freely existing in bulk solution or ‘sessile’ – attached to a surface or within a biofilm No permanent connections with other cells but they can adhere to surfaces, food etc Initial attachment may be via pili (fimbrae) Considered virulence factors in virulent bacteria Allow bacterial cells to adhere and resist immune attack  The need for cell adhesion Transition from a unicellular to a multicellular form presents challenges Some faces in contact with environment which may change Different sides of the cell have differing roles and interactions How to “stick” cells together How to have communication and transport between cells How to create specialised domains  The need for cell adhesion A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. Cells may be linked by direct interactions, or they may be held together within the extracellular matrix. Animal tissues fall into 2 broad categories Connective tissues Low cell density, abundant extracellular matrix (bones, tendons) Cell- cell contacts are rare ECM is load bearing Cell attachments to the ECM allowing force transmission Epithelia tissues Cells closely bound together into epithelia sheets (gut lining, skin epidermis) Thin extracellular matrix on one face - basal lamina  What is the extracellular matrix? The materials lying outside the cell are known collectively as the extracellular matrix (ECM). Definition Any material produced by cells and secreted into the surrounding medium. Usually applied to the non-cellular portion of animal tissue. Plants and fungi produce an extracellular matrix or walls Arthropods produce chitin Chitin and cellulose are by far the most abundant biopolymers on earth.  The Extracellular matrix Different tissues having their own specific ECM. The ECM has roles in normal tissue development, function and disease. The ECM is a complex network of proteins and polysaccharide chains that are manufactured by cells, secreted and modified outside the cell by several different enzymes. Functions of the extracellular matrix Mechanical: tensile and compressive strength and elasticity. Protection: buffering against extracellular change and retention of water. Organisation: control of cell behaviour by binding of growth factors and interaction with cell-surface receptors. The classes of macromolecules constituting the ECM in different animal tissues are broadly similar; Yet variations in the relative amounts of these different classes of molecules and how they are organised give rise to an amazing diversity of materials. Specialisations of the ECM Matrix of bone and teeth is highly mineralised to withstand compression Cornea of the eye has a transparent ECM The ECM of tendons is highly elastic Blood plasma is a liquid form of ECM Components of the extracellular matrix In most connective tissues, the matrix macromolecules are secreted by cells called fibroblasts Osteoblasts form bone / Chondroblasts form cartilage Main macromolecular components of the ECM Glycosaminoglycans (GAGs) - acidic polysaccharide derivatives, proteoglycans) Fibrous proteins – includes members of the collagen family Non-collagen glycoproteins - e.g. fibronectin and laminin Others - e.g. elastin  Glycosaminoglycans (GAGs) GAGs are unbranched polymers of repeated disaccharide derivatives, including amino sugars, sulfated acetylamino sugars and uronic acids. Galactose Galactosamine Galacturonic acid N-acetyl galactosamine-4-sulfate The equivalent glucose derivatives are also common components of GAGs.  Properties of GAGs Acidic and negatively charged Attract positive ions (eg Na+) which attracts water causing gel formation Comprise 10% of ECM mass but 90% of volume GAGs (especially hyaluronan) provide compressive strength Metabolically cheap bulking agent  Types of GAGs - Hyaluronan Hyaluronan is spun out from the cell membrane Enormous (107 kDa - much larger than other GAGs) Not sulfated Not attached covalently to protein – ‘stand alone’ N-acetyl-glucosamine- glucuronic acid Often added to the ECM to hold open areas that would otherwise fill up with cells; it is then removed by hyaluronidase after appropriate cell migration.  Other GAGs GAG Dissacharide unit Chondroitin-4-sulphate D-glucuronic acid N-acetyl-D-galactosamine Chondroitin-6-sulphate D-glucuronic acid N-acetyl-D-galactosamine Dermatin sulphate D-glucuronic acid N-acetyl-D-galactosamine Heparan sulphate D-glucuronic acid N-acetyl-D-glucosamine Heparin variable dissacharide unit e.g. IdoA(2S)-GlcNS(6S ) Keratan sulphate D-galactose N-acetyl-D-glucosamine Usually covalently bound into proteoglycans 20-200 sugars residues long Chondroitin  Other GAGs - Proteoglycans A proteoglycan is a serine-rich protein decorated with hundreds of O-linked (usually via serine), acidic, sulfated GAGs. A specific link tetrasaccharide is first assembled on a serine side chain. The rest of the GAG chain, consisting mainly of a repeating disaccharide unit, is then synthesized, with one sugar being added at a time.  Proteoglycans - Aggrecan Aggrecan is a common proteoglycan in the ECM. Its core protein is decorated with around 100 chondroitin and 30 keratan chains. Aggrecan then binds to hyaluronan (100:1) via adaptor proteins. Aggrecan-hyaluronan aggregates can be as big as bacteria (5 µm long). Hyaluronan – GAG D-glucuronic acid and D –N-acetylgucosamine  Heparan sulphate proteoglycans Important role in cell growth. Bind chemokines at inflammatory sites, prolonging white-cell attracting activity. Bind and block certain proteases. Oligomerises FGF (fibroblast growth factor), giving easier binding to its tyrosine-kinase receptor. Heparan sulphate is a polymer of trisulphated GlcNAc and iduronic acid Collagen and other ECM proteins Collagen - fibrous protein consisting of three α-chains forming a triple helix. Provides tensile strength to the ECM. Collagen chains consist of GXY repeats G – Glycine X – commonly proline Y – commonly hydroxyproline Hydrogen bonding between the -OH groups of HP stabilises the triple helix Lysines can be hydroxylated and subsequently glycosylated  Collagen synthesis Synthesis as pro-α-chain on RER Assembly of procollagen Procollagen is secreted from vesicles into extracellular space Terminal propeptides are cleaved to form 100nm long collagen chains Hydroxylation of selected prolines and lysines Glycosylation of selected hydroxylysines  Collagen synthesis Collagen molecules are crosslinked to form fibrils Oxidative deamination of hydroxylysine and lysine forms reactive aldehyde groups, which link molecules together (and also link α-chains together too). Collagen fibrils then self-assemble into fibres, Collagen fibrils are highly stable and last around 10 years (compare to most enzymes which turn-over in about an hour). A defect in collagen underlies the basis of scurvy James Lind c.1747 showed that citrus fruits prevented and cured scurvy Vitamin C Primates and guinea pigs get scurvy… …most other animals make their own vitamin C Hydroxyproline is formed post-translationally by the action of proline hydroxylase. Proline hydroxylase requires vitamin C as a cofactor In the absence of vitamin C tissues containing collagen (gums, skin, capillaries) are weakened because unhydroxylated collagen is destroyed prior to secretion  Types of Collagen Type I – Most common fibrillar form – found in skin bones and tendons Type II – Similar tensile strength to cartilage Type IX and XII don’t form fibers and are fibril-associated. Link type I or II fibrils. More flexible than type I or II due to more frequent substitution of the GXY by other aas Type IV and VII – form a mesh structure in the basal lamina Other components of the extracellular matrix Collagen gives the ECM its tensile strength Elastin characteristics Elastin provides the ECM with elasticity ~750 aa long Highly hydrophobic Rich in proline and glycine Non glycosylated Alternating stretches of hydrophobic residues and alanine / lysine rich α helices Crosslinking via α helical regions. Hydrophobic domains are extensible due to loose random coil conformation The role of fibrillin in elastin deposition Elastic fibers comprise an elastin core coated with a sheath of microfibrils Microfibrils made of a number of glycoproteins including fibrillin. Microfibrils may act as a scaffold guiding subsequent elastin formation Defects in the FIBRILLIN 1 gene result in Marfan syndrome (weak elastic tissue) Abraham Lincoln - long fingers, weak aorta, pigeon chest  The basal lamina The basal lamina plays several important roles GAGs Heparan sulfate Structural. Proteins Laminin. Determines cell polarity. Type IV collagen. Organises and binds cells. Nidogen (entactin). Forms a barrier to certain cells. Perlecan. Forms highways for cell migration. The basal lamina – interactions between proteins  Lecture Summary A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. ECM Any material produced by cells and secreted into the surrounding medium. The structure of the extracellular matrix differs in composition between tissue types – but is essentially made up of collagen fibers, proteoglycans and polysaccharides. The basal lamina is a specialised ECM found underlying epithelial cells – also referred to as the basement membrane.