Epithelial Junctions and Extracellular matrix

What are epithelia?

• avascular tissues composed of cells, usually organised into sheets or tubules attached to an underlying ECM basement membrane

• cover booth the external and internal surfaces of the body

• some are modified to form glandular structures (exocrine glands)

How many types of Epithelia are there?

• simple

• stratified

• columnar

• cuboidal

• squamos

Functions of Epithelia

• mechanical protection e.g skin

• permeability barrier e.g small intestine

• absorption e.g small intestine

• filtration e.g epithelium of renal corpuscle

• secretion e.g sweat glands

• diffusion of gases or fluids e.g lung alveoli

• sensory e.g retina

Epithelial are polarised

• have functionally distinct sides or surfaces

• the apical and basolateral surface

• relevant for directional transport, specialised functions and tissue organisation

How is physical integrity of the epithelium maintained?

• epithelia are held together by cell junctions

• cell junctions are a specialised site on a cell at which it is attached to another cell or extracellular matrix

4 Functional Groups of Cell Junctions

• anchoring junctions - linking cells together or to the extracellular matrix

• occluding junctions - seal the gaps between cells

• channel-forming junctions - create passageways linking the cytoplasm of adjacent cells

• signal relaying junctions - allow signals to be communicated from cell to cell

How do anchoring/strengthening junctions link cells together or to the extracellular matrix?

• Adherens junction: actin filaments, cadherins, A-catenin b-catenin

• Desmosome: intermediate filaments, cadherins, plakoglobin desmoplakin

• Focal adhesion: actin filaments, integrin, focal adhesion kinase

• Hemidesmosome: intermediate filaments, integrin, dystonin

Adherens Junction

• link adjacent epithelial cells to each other

• Main proteins is Cadherin

• link to cytoskeleton: attach to actin filaments inside the cell

• help cells stick side by side, provide mechanical strength, plays a role in cell shape change and tissue morphogenesis

Desmosomes

• spot welds that connect epithelial cells to each other at specific points

• main proteins: desmogleins and desmocollins (part of the cadherin family)

• link to the cytoskeleton is attaching to intermediate filaments like keratin

• Role: provide tensile strength, especially in tissues exposed to mechincal stress

• prevent cells from pulling apart

Hemidesmosomes

• function: anchor epithelial cells to the basement membrane (underlying connective tissue)

• main proteins: integrins (bind to laminin in basement membrane)

• cytoskeleton link: attach to intermediate filaments like keratin

• role: provide stable anchorage of cells to the extracellular matrix

Focal adhesions

• function: connect epithelial cells to the extracellular matrix

• main proteins: integrins, along with talin, vincullin, paxillin and other

• cytoskeleton link: attach to actin filaments

• role: involved in cell movement, signalling and wound healing

How do epithelial sheets bend to form a tube or vesicle

• relies on changes in cell shape, cell-cell adhesion and cytoskeletal dynamics

Apical Constriction in Epithelial sheet bending

1. Apical Constriction

• cells at a specific site in the epithelial sheet constrict at their apical surface (the side facing the lumen or external space)

• the apical surface becomes smaller, while the basal side remains wider, making the cells wedge-shaped

KEY PLAYERS

• actomyosin contraction

• regulated by proteins like RhoA, Shroom and myosin II

• sheet bends inwards

Basal relaxation in Epithelial sheet bending

• cells relax or expand their basal surfaces enhancing wedge shape from the opposite side

Epithelial Tube Formation

• a narrow strip of epithelial cells undergoes apical constriction

• the sheet invaginates into a groove - the groove deepens . eventually pinches off into a tube

• e.g neural tube formation

Epithelial vesicle formation

• a localised patch of cells constricts apically

• the epithelium invaginates into a cup like shape, deepens, and pinches off to form a hollow sphere or vesicle

• e.g lens vesicle in eye development

How do desmosomes link to intermediate filaments of the cytoskeleton?

• IFs are structural components of the cytoskeleton, providing mechanical strength to cells

• filaments are made of keratin, desmin and other IF proteins

• desmosomal cadherins extend across the plasma membrane of adjacent cells and bind to each other through their extracellular domains, helping to adhere to cells together

• on the cytoplasmic side, the intracellular domain of these cadherins binds to plakoglobin and plakophillins

• these proteins serve as linkers, connecting the cadherins to the cytoskeleton

• the plakoglobin and plakophillin interact with desmoplakin, which in turn directly binds to the intermediate fialments

• desmoplakin has a dual role: stabilises desmosomes structure and transmits mechanical stress

How do integrins mediate cell-matrix contacts

1. Binding to ECM components

• integrins interact with specific ECM ligands through the extracellular domain which has specific sequences that integrins recognise and bind to

• the binding of integrins to the ECM components typically happens in specialised adhesive sites called focal adhesions

2. Conformational Change

• when integrins bind to ECM ligands they undergo a conformational change that activates integrin, transitioning from a low affinity state to a high affinity state

• this conformational change allows integrins to form stable contacts with ECM and interact with the cytoskeleton

3. Linking to the cytoskeleton

• activated integrins recruit various cytoplasmic proteins to form complexes called focal adhesion complexes

• hemisdesmosomes anchor epithelial cells to the basal lamina

Defective Desmosomes

• Pemphigus vulgaris

• autoimmune destruction of desmosomal protein

• severe blistering

• dehydration and infection

• death

How do occluding junctions seal gaps between epithelial cells?

• claudins on adjacent cells bind to one another in a tight parallel arrangement, weaving and creating a seal which is done through extracellular loops of claudins restricting the passage of small molecules

• occludin further stabilises and regulates permeability

  • interacting with claudins and intracellular scaffold proteins promoting the assembly of the junction and tightening the seal

• the ZO proteins link the transmembrane proteins to the actin cytoskeleton

• this allows water and ion regulation, selective permeability, tissue compartmentalisation and a barrier to toxins and pathogens

Occluding/Tight Junctions help maintain cell polarity

• separating the apical and basolateral domains of the plasma membrane, organising the cytoskeleton and regulating the selective movement of proteins and lipids

Loss of function of tight junctions

• disease

• barrier function loss

  • Crohns disease: inflammation of the bowels

• Fence function

  • cancer - loss of cell polarity and cell contact which increases motility and metasis

Channel forming junctions purpose

• allow ions and small molecules to move between cells

• ions can pass through for electron coupling enabling synchronous muscle contractions

• small molecules can pass to coordinate intracellular signalling between cells

• facilitates calcium signalling in various tissues

• provides rapid electrical communication in tissues like the heart

• relevant for embryonic development, tissue repair, electrical synchronisation and metabolic and cellular coordination

• connexons composed of connections form the gap junction channel between adjacent cells

• junctions regulated by calcium levels, pH and phosphorylation

• disruptions in gap junctions are linked to various disease e.g cataracrs

Plasmodesmata

• microscopic channels that connect adjacent plant cells

• facilitate the passage of small molecules

• enable the symplastic transport of materials across the plant for growth regulation

• regulate cell growth and differentiation by enabling communication between cells during plant development

Purpose of signal relaying

• facilitate the direct transfer of signals between adjacent cells

• enables the cells to exchange small molecules and signalling molecules for cellular communication

• e.g calcium ions, cyclic AMP, and IP3 propogate

What happens when signal relaying junctions are lost

Myasthenia Gravis

• autoimmune destruction of neuromuscular junction

• droopy eye

• muscle weakness

What is the extracellular matrix?

• any substance produced by cells and secreted into the extracellular space within the tissues

1. Structural importance: physical support for cells and a linkage between different cells or tissues

2. Cell motility - forms a substrate on which cells can move and furthermore it provides cues that guide the direction of movement

Connective tissues ECM

• prodominant ECM: scattered cells

• Composition of ECM: consists of collagen fibers crosslinked by accessory proteins in a matrix of proteoglycans

Epithelial Cells ECM

• scant ECM: consists of layers of cells closely bound to one another to form protective sheets

• ECM concentrated under epithelia provides a base for the cells to sit on and acts as a molecular sieve and substrate for migration cells

ECM in plants

• cell wall

• cellulose fibres crosslinked with hemicellulose in a matrix of polysaccharides

• analogous with animals

Collagen

• 25% of total protein mass of body

• 42 different collagen genes in mammals

• different tissues often contain different types of collagen

Single collagen polypeptide chain

• cable like structures are collagen fibres

• each strand is a collagen fibril

• dots are collagen fibres that are perpendicular

• accessory proteins are often other types of collagen

Pro-collagen termini

• prevent fibrillation assembly

• hydroxylation of certain proteins and lysine residues

• glycolysis ions

• ends up with modified version of a collagen monomer

• self assembles into a 3 strand polymer

• at the end of peptide - propeptides

  • propeptide regions cannot form triple helicies due to not having a core structure

• those limited assembles are put into secretory vesicles to outside of cells leading to cleaving of propeptides to triple helical bits being freely available for rapid cell assembly into fibres

• secretory vesicles end up forming lines controlling the orientation of the molecules and self assembly in cell

• cleaved after secretion

Elastin

• provides elasticity to tissues

• composed of large filaments with lots of random coils that can be compared to springs

• connected by crosslinked lysine or hydroxylysines

• stretching of coils

Similarities between plant and extra cellular

• most of matrix consists of polysaccharides

• plant have pectin

• aniamls has glycoaminoglycans

Glycoaminoglycans

• highly negatively charged

• repeating disaccharide

• hydrophillic

• linked to non fiberous proteins - form proteoglycans

• proteoglycans are associated with the hylaronan through a set of link proteins

• role is to absorb a lot of water and occupy a lot of space

Why do many type of connective tissue exist?

• the relative proportion of fibres to cells within ECM

• the number and proportion of different cell types within the ECM

• the proportion and arrangement of the fibres in the ECM

• the compositions of the non fibrous components of the ECM

Why does stretchy skin exist

• failure of conversions of lysine to hydroxylysine by lysyl hydroylase or failure to cleave off propeptide termini

• fibrils and fibres don’t form

Areolar connective tissue

• wraps around internal organs to cushion organs

• loosely structure ECM

• a lot of collagen fibres with lots of space

• plenty of elastin

• scattered fibroblast nuclei

Adipose tissue

• adipocytes become dominant tissue (contrast to other idea that cell is dominated by ECM)

• loose and embedded

Tendons and ligaments

• used to allow a muscle to pull on a bone

• dominant component is collagen fibres

• secrete collagen which remembers resisting tension

• all of collagen fibres assembled along that plane to allow resistance against muscle

Dermis

• lots of collagen

• sparsely distributed

• collagen is not organised in parallel, organised randomly

Elastic Cartilage

• outside ear

• lots of elastin fibres allowing it to regain shape

• combination of cells for secretion

Bone

• dominant component is the matrix component of ECM

Scurvy

• caused by absence of Vit C during long journeys

• gum bleeding

• teeth falling out

• Vit C deficiency results in defective collagen synthesis

• Vit C is a cofactor for prolyl hydroxlase

• Hydroxylproline stabilises triple stranded collagen through formation of cross links

Fibrosysplasia ossificans progressiva

• muscle and connective tissue like tendons are gradually replaced by bone

• caused by activating mutation in the ACVR1/ALK2 encoding activin A receptor type 1/activin-like kinase 2