BIOL 331 - Module 1 - Interactions between cells and the environment

epidermis

Has closely packed cells of epithelial tissue

dermis

a type of connective tissue in the epidermis

fibroblasts

of the dermis, have receptors that mediate interactions and transmit messages, secrete to ECM

produce collagen, found in connective tissues in the dermis below the basement membrane, in smooth muscle cells and epithelial cells

basement membrane

an extracellular matrix (not a lipid membrane)

integral protein families that mediate cell-cell adhesion

Selections - have a CHO ligand

Immunoglobulin super family (IgSF) - homotypic interactions

Members of the Integrin family - heterotypic interactions with IgSFs

Cadherins - project from the plasma membrane

cadherins

• Glycoproteins, calcium dependent cell-cell adhesions

• Adhesion or transmit signals from the ECM to cytoplasm 

• Bind a similar cadherin on a neighbouring cell, eg. E-cadherins bind to E-cadherin (E stands for epithelial)  

• Possibly the single most important factor in molding cells into cohesive tissues in the embryo and holding them together in the adult - maintain tissue integrity 

• loss associated with malignancy/cancer spread

immunoglobulin super family (IGSF)

• Contain Ig domains can connect to the integrin family, or connect to another IgSF (homotypic interactions)

• Mediate calcium-independent adhesion - no Ca²⁺ involved 

• Many IgSF proteins are ICAMs

  • intercellular adhesion molecules (ICAMs) - eg. VCAMs are a type of ICAM

• Integrins are some of the proteins that act as receptors for ICAMs

selectins

• Calcium-dependent 

• a family of membrane glycoproteins that bind to specific oligosaccharide (carbohydrate moiety)

• have a small cytoplasmic segment, a single membrane-spanning domain, and a large extracellular portion 

lectin

a term for a compound that binds (non-covalently) to specific carbohydrate group

leukocyte rolling

1. Inflammation activates endothelial cells, which upregulated the selectins and they become more adhesive to the neutrophils (bind and roll along)

2. Selectins bind to the carbohydrate residues (Psgl-1) on neutrophil, a phagocytic leukocyte 

3. Platelet activating factor or IL-8 on the surface of endothelial cells activates G-protein coupled receptors on the neutrophil and this leads to activation of integrin on neutrophil 

4. Integrins bind to ICAMs on endothelial surface and a cascade of events results in cytoskeletal rearrangement such that the cell can extravasate (migrate through)

5. Transendothelial migration - through endothelial cells to enter tissue where infection is

found on endothelial cells

Selectin, ICAMs, platelet activating factor (IL-8)

found on neutrophils

Carbohydrates, glycoproteins, integrin, G protein coupled receptor

E-cadherin

epithelial cadherin, can protect against migration of cancer cells away from the primary tumor

keratin pairs

K10 & K1 and K14 & K5

heterodimers that form intermediate filaments in epithelial cells

junctions of the epithelium

• Tight Junction - Lateral Membrane

• Adherens Junction - Lateral Membrane

• Gap Junction - Lateral Membrane

• Desmosome - Lateral Membrane

• Hemidesmosome - Basal Membrane

• Focal Adhesion - Basal Membrane

electron microscopy

uses electrons rather than light, very high resolution/visualization of cellular structures, samples imaged under a vacuum, so live cells can’t be imaged

tight junctions

aka zonula occluden

• At the top of the cell (apical membrane)

• Occur between neighboring epithelial cells

• Prevent solute distribution where different solute concentrations are in adjacent compartments 

• Proteins → occludins, claudins

  • Interact at contact points between two opposing membranes

• Form close contacts between cells

• gate and fence functions

gate function

of tight junctions, controls the passage of ions, proteins, and water between cells (paracellular pathway)

eg. blood brain barrier - ions or water can’t pass, but cells of the immune system can pass 

fence function

of tight junctions, block diffusion of integral membrane proteins between apical and basolateral membranes of one cell, connect to the actin cytoskeleton and microtubules 

Contribute to polarity, only some proteins on either end of the cell

adherens junctions

• Increase tissue strength

• Connect the external environment to the actin cytoskeleton

• Provides a pathway for signals to be transmitted from the exterior to the cytoplasm and nucleus 

  • B-catenin

• __________ form a belt (zonula adherens) that encircles the cells near their apical surface in epithelial cells 

desmosomes

aka macula adherens

• Also increase tissue strength 

• Primarily adhesive

• Also contain cadherins (desmosomal cadherins)

  • Cadherins interact with multiple proteins to form a cytoplasmic plaque on the inner surface of the plasma membranes

  • Keratin intermediate filament cytoskeleton anchors two cells together - provides strength to a sheet of cells

• Each cell contributes one cytoplasmic plaque to the desmosome 

gap junctions

• Basolateral membrane - intracellular (hydrophilic) communication channels between two cytoplasms, do not attach to cytoskeleton

• Transmit small soluble signaling molecules directly through the membrane 

• Made of connexin proteins

  • 6 identical connexins from each cell form a transmembrane channel with a central pore called a connexon  

  • Each cell contributes 1 connexon 

  • 2 connexons form a gap junction

• Molecular pipelines that pass through the adjoining plasma membranes and open into the cytoplasm of the adjoining cells 

• Size restriction is 1000 Da

• Large numbers of connexons are found in a gap junction plaque - concentrated in one area

hemidesmosomes

• When cells are a part of a tissue, cell doesn’t move, anchored to the ECM

• Connects to keratin intermediate filaments

• Involves the cell substratum 

• Don’t share the same proteins as desmosomes (except keratin)

• Cell-matrix attachment in vivo is seen at the basal surface of epithelial cells, anchored to the underlying basement membrane

• Contain a dense cytoplasmic plaque with keratin filaments

  • Keratin filaments are linked to the ECM by integrins

focal adhesions

• Also involves the cell substratum 

• Connects to actin (motility), cell locomotion

• Discrete sites of cell attachment, dynamic structures (made and broken down)

• Cultured cells are anchored to the surface of the dish only at scattered, discrete sites called focal adhesions 

• Attachment of integrin to a protein outside the cell activates focal adhesion kinase (FAK) and SRC kinase (for cell survival and proliferation) 

  • Chain reaction to transmit signals to the nucleus

glycocalyx/cell coat

Carbohydrate projections extending from integral membrane proteins in the membrane

• More prominent in some cell types such as the intestinal epithelial cells 

Mediate cell-cell and cell-substratum interactions, provide mechanical protection to the cells, serve as a barrier, right outside cell plasma membrane 

basement membrane (basal lamina)

• Extends past the plasma membrane and consists of secreted proteins

• Epithelial tissues (skin, parts of the mammary gland, kidney, intestine, etc.)

• Also surrounds muscle and fat cells, and lines the digestive and respiratory tracts and inner endothelial lining of blood vessels 

• Eg. chondrocytes (cartilage cells) - proteins in ECM for cushions

integrins

interact with ECM material, focal adhesions connect to ECM proteins via _______ proteins

integrin activation

• Talin binds to the beta subunit of integrin ⇒ induces separation of the two subunits (alpha and beta) and conversion to an active conformation

• Active integrins become clustered due to they cytoplasmic domains interacting with the cytoskeleton

  • Inside-Out Activation (via talin) 

inside-out signalling of integrins

depends on talin which helps activate the ligand binding function of integrins and stabilizes the activated state, strengthens adhesive contacts and force needed for migration and invasion

outside-in signalling of integrins

binding of substrate by integrin alters its conformation, which activates focal adhesion kinase (FAK) which starts a signaling cascade to the nucleus, important for survival, cell spreading, and proliferation, transfers integrin-mediated external signals to the inside of the cell

RGD binding site

how integrins contact other proteins

not all integrins have one

site binds arginine-glycine-aspartic acid

extracellular matrix

sends critical signals for survival, orientation/polarity, and differentiation

important for growth and organization

ECM proteins

collagen, fibronectin, proteoglycans, laminin

collagen

A fibrous glycoproteins present only in ECMs, very strong

produced by fibroblasts

• Structure - triple helix of three helical alpha chains for strength, resilience (3 helical chains = 1 subunit)

  • Tissue mechanical properties are correlated with 3D organization of collagen (eg. tendons, cornea)

  • Corneal Stroma

corneal stroma

layers of collagen fibrils of uniform diameter and spacing arranged at right angles

fibronectin

• consists of two similar polypeptides joined by a pair of disulfide bonds 

• Each polypeptide is composed of a linear series of distinct modules

• Bind to numerous ECM components and cell surface receptors

• Eg. cells migrating on _____ coated glass, and die if they move off, integrin can’t interact with anything and cell undergoes apoptosis

proteoglycan

• Bind huge number of cations, which bind a large number of water molecules → porous hydrated gel

  • Allows for increased resistance to crushing or compression - good for our joints

  • Collagens resist pulling forces and form a scaffold for proteoglycans, together providing ECMs strength and resistance 

• Consists of a core protein, carrying covalently attached glycosaminoglycans 

  • protein-polysaccharide carrying glycosaminoglycans which are negatively charged

  • Interacts with water molecules for cushioning

laminin

• Glycoproteins made of 3 different proteins linked by disulfide bonds (trimers)

  • Trimers organized into a cross-like shape

• Involved in migration growth and differentiation (specialization)

• Bind cell-surface receptors, other laminins, to proteoglycans and other BM proteins

• Laminin and collagen form separate by interconnected networks

• Eg. primordial germ cells migrate to the gonad on highways of laminin

matrix metalloproteinases

Based on the extracellular matrix (ECM) specificity of the enzymes, 3 subtypes - collagenases, gelatinases, stromelysins

• MMPs clear paths through the ECM so cells can migrate through

• Can greatly influence a cell’s potential for migration, growth, and differentiation

• Role in development of neuronal outgrowth

• Strengthen basement membrane 

collagenases

substrates - connective-tissue-collagen

gelatinases

substrates - basement-membrane-collagen, gelatin

stromelysins

substrates - proteoglycan, laminin, fibronectin, gelatin