Cell adhesion and the ECM

Question 1

How is adhesion regulated?

Answer 1

Variable strength and duration of adhesions allows e.g. cell movements during development and tissue repair

Question 2

What does adhesion depend upon?

Answer 2

Strength in numbers - velcro effect. Individual adhesion molecules may have a weak binding to their targets, but are clustered together to cellular junctions

Question 3

Adherens junctions

Answer 3

Anchoring junction. Proteins are classical cadherins - transmembrane proteins with a characteristic repeat structure in their extracellular domain. The cadherin extracellular domains from neighbouring cells bind together to mediate adhesion.

Question 4

What do cadherins require for adhesions?

Answer 4

Calcium ions. In the extracellular domain of the protein, flexible linkers between the cadherin repeats need to be stabilised to facilitate binding between cadherins form adjacent cells, requiring Ca2+ binding

Question 5

How can crucial role of cadherins in adhesion be demonstrate by?

Answer 5

Experiments where E-cadherin (epithelial cells) expression was introduced to cell lines that do not normally express the protein. This resulted in Ca-dependent clustering of cells. Cadherin selectivity can be shown in experiments where E-cadherin expressing cells form separate clusters/structures from N-cadherin expressing cells

Question 6

Connected to actin cytoskeleton

Answer 6

Intracellular adapter proteins mediate binding to actin cytoskeleton. Some variability in the adapters, but invariably, catenin proteins are involved. Loss of adherents junctions affects tissue integrity: invasive carcinomas often have lost E-cadherin expression

Question 7

Desmosomes

Answer 7

Anchoring junction. Responsible for strong cell-cell adhesion mediated by desmosomal cadherins - demogleins and desmocoliins related to classical cadherins

Question 8

What are desmosomes connected to?

Answer 8

Intermediate filaments. Different set of adapter proteins than those used in adherens junctions - form a dense plaque that can be seen in EM pictures

Question 9

What can mutations in desmosomal proteins lead to?

Answer 9

Weak adhesion that can e.g. cause blistering and cell lysis in tissues.

Question 10

Tight junctions

Answer 10

Form a seal between cells - cell membranes pressed close to each other, prevent movement of water and solutes in the space between cells. Therefore, they regulate transport through the epithelia. The binding is close but the adhesion is no strong in terms of contribution to tissue integrity
They restrict movement of membrane lipids in polarised cells from basolateral to apical surface

Question 11

Rows of membrane proteins create a bubble wrap appearance of TJs

Answer 11

The proteins belong to occluding and claudin families that have several transmembrane domains generating extracellular loops that for non-covalent interactions with similar proteins on the adjacent cell. Adapted proteins such as ZO-I help clustering TJ adhesion proteins

Question 12

Gap junctions

Answer 12

Form passageways between adjacent cells - present in most mammalian tissue
Composed of connexin proteins
Humans have 21 different genes for connexins with different sets of connexins expressed in different cell types

Question 13

Heteromceric or homomeric connexin through the cell membrane

Answer 13

6 connexin proteins form

Joins a connexin on the membrane of the adjacent cell to form a gap junction

Question 14

What molecules can pass through a gap junction?

Answer 14

Small molecules, up to about 2 kDa - ions, metabolites, small signalling proteins

Question 15

What affects the permeability of a gap junction?

Answer 15

The diversity of connexins leads to differences in gap junction channel permeability. Permeability is also regulated by e.g. pH, calcium ion and protein phosphorylation

Question 16

In neurons, gap junctions….

Answer 16

can facilitate rapid transmission of electric signals

Question 17

In non-neuronal tissues, gap junctions…

Answer 17

can help integrate metabolic activities e.g. by passing second messenger proteins from one cell to another.

Question 18

In heart muscles, gap junctions…

Answer 18

Help co-ordinate spreading of ionic signals that contribute to contraction of heart muscle

Question 19

What can mutations of connexins cause?

Answer 19

At least 8 human diseases, including a form of deadness, a form of progressive degeneration of peripheral nerves and a condition with heart malformations

Question 20

Plant cell wall adhesion

Answer 20

Do not have anchoring cell-cell junctions. Neighbouring plant cells joined by rigid cell wall. The primary cell wall is deposited after cell division and often reinforced by a secondary deposition. The key component is cellulose, a repeating polysaccharide. Cellulose fibres are embedded in the matrix of another polysaccharide, pectin, and glycoproteins
The cell wall allows diffusion of water and ions but is a barrier to cell-cell communication

Question 21

Plasmodesmatata

Answer 21

Direct communication between adjacent plant cells. Plasmodesmata extend from one cell to another through the wall. Plasma membranes merge to form a continuous cannel – annulus.
Contain within the channel an extension of ER, desmotubule. Also, cytoskeletal filaments can continue through the plasmodesmata

Question 22

The extracellular matrix

Answer 22

A complex arrangement of molecules filling spaces in between the cells, forming an organised structure. Mostly found in CT such as tendons, cartilage, bone or skin dermis. Diverse structures maybe created by different amounts and organisation of ECM components. Cells secrete ECM that is finally assume;ed outside the cell

Question 23

ECM receptors

Answer 23

Cells have these on their plasma membrane, with the integral family of most importance

Question 24

Function of the ECM

Answer 24

Provides physical strength and structural support and influences cell shape and behaviour e.g. proliferation and migration
ECM can participate in storage of some growth factors and other biologically stable active molecules
Involved in tissue repair mechanism and guidance of tissue interactions during embryonic development

Question 25

Major types of MHC molecules

Answer 25

Glycosamionglycans and fibrillar proteins such as collages.
Some proteins interact to form networks in the matrix and some have copies of distinctive repeated small globular domains to allow multiple interactions

Question 26

Glycosaminoglycans

Answer 26

Polysaccharide chains usually found covalently attached to proteins to form proteoglycans. 4 main types of polysaccharide GAGs that vary in sugar residues, linkage and modifications. Made of repeating disaccharide units. One sugar residue is always an amino sugar. GAGs are negatively charged

Question 27

Hydrated gels

Answer 27

Fill most volume of ECM and resist compression. Made of glycosaminoglycans and proteoglycans

Question 28

Proteoglycans

Answer 28

Interact with proteins and participate in storage and activity of growth factors. Some found at cell surfaces as membrane proteins

Question 29

Collagens

Answer 29

Most abundant protein family - comprise of 25% protein mass

Characterised by a triple helical domain where 3 polypeptide chains wound around each other in a superhelix

Question 30

Why is glycine essential in collagens?

Answer 30

The smallest aa and fits in facing the centre of the triple helix.

Question 31

Why are hydroxyproline residues important in collagens?

Answer 31

Stabilise the triple-helix structure

Form additional hydrogen bonds and hold the proline ring structure in a conformation that helps stabilise the helix

Question 32

Main different functional types of collagens

Answer 32

Fibrillar, fibril-associates, network/sheet-forming collagens

Question 33

Collagen biosynthesis - intracellular

Answer 33

Intracellularly, post-translational modifications take place in the ER and golgi, after translation of mRNA to polypeptide and cleavage of a signal peptide - proline hydroxylation, lysine hydroxylation and glycosylation of some hydroxy-lys and asp residues
Polypeptide chains making up fibrillar collagens have N- and C- terminal properties that facilitate association of the three polypeptides that form the triple-helix. During helix folding, chaperone proteins prevent micfoldng

Question 34

Collagen biosynthesis - extracellular

Answer 34

Fibrillar collagens secreted as triple helixes
Polypeptides cleaved out and individual collagen molecules self-assemble to long fibrils. In tissues, collagen appears banded or striated in EM pictures due to overlapping assembly of triple helixes.

Question 35

Within the fibril…

Answer 35

Individual collagen molecules cross linked by covalent bonds. Short segments at each end of the collagen are not in the triple helical conformation and contain the aa hydroxylysine. Lysol oxidase catalyses the formation of an intermolecular crosslink between the 2 collagen molecules

Question 36

Vitamin C

Answer 36

Required by the formation of stable collagen triple helix.

Is a co-factor for the enzyme prolyl hydroxylase that converts proline resides in Gly-X-Y repeats to hydroxyproline.

Question 37

Absence of vitamin C

Answer 37

Scurvy
Defective alpha chains without hydroxyproline and hydroxylysine do not form stable triple helices and are rapidly degraded in cell symptoms of scurvy arise from gradual loss of existing collagen

Question 38

Association of collagen fibrils with other proteins

Answer 38

e.g. non-fibrillar collagens and proteoglycans regulated orientation and size of fibrils.

Question 39

Each type of collagen tissue has its own distinctive matrix

Answer 39

e.g. in cartilage, association of fibrillar type II collagen molecules bind with type IX collagen to create space for water-binding proteoglycans whereas type I fibrils in skin dermis are more highly packed and in bone the layers of type I collagen generate a plywood-like lamellar structure

Question 40

Basal lamina

Answer 40

A special case of thin sheet-like network of ECM proteins associated with epithelial, endothelial muscle and Schwann cells. Provides physical support, developmental control, filtering functions attached to epithelial cells via hemidesmosomes.

Question 41

Major constituents of basal lamina

Answer 41

Laminins - comprise of 3 different pp chains from late cruciform complexes
, collagen IV (form networks), nidogens and heparin sulphate proteoglycans