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Developmental and Stem cell biology > Extracellular Matrix > Flashcards

Extracellular Matrix Flashcards

1
Q

What percentage of body mass is comprised of ECM?

A

90%

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2
Q

The cells of the body actually only account for ……% of body mass?

A

10%

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3
Q

What are the 4 overarching processes of development in which the ECM is implicated?

A
  1. Pattern development
  2. Differentiation
  3. Morphogenesis
  4. Maintenance of differentiated phenotype
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4
Q

What are the functions of the ECM?

A
  1. Functions as adhesive substrate
  2. Provides structure
  3. Presents growth factor to their receptor
  4. Sequesters and stores growth factors
  5. Senses and transduces mechanical signals
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5
Q

What achieves the reciprocal behaviour between the cell and its ECM?

A
  1. Anchorage
  2. Biomechanical forces
  3. Non-canonical growth factor presentation
  4. Receptor
  5. Enzymatic modelling
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6
Q

How can the ECM function as an adhesive substrate?

A

Cells can attach to matrix molecules, this may help anchor a cell to particular function.
Once they arrive at their final destination, they can attach and they then help to form the resulting tissue.
Tracks to direct migrating cells, the matrix molecules can act as a substrate/tracks for migratory cells again to each the site at which they are needed.

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7
Q

How can the ECM provide structure?

A

This in an important function of the ECM and is partly due to the orientation and structure of these matrix molecules, e.g. they provide mechanical support, particularly for tssues subjected to huge amounts of mechanical force e.g. bone ad cartilage of the skeleton or heart and blood vessels which experience huge pulsatile forces.
It also provides integrity and elasticity to developing organs.
The orientation and stucture of the matrix molecules can provide mechanical strength, this protects against mecahical perturbations.

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8
Q

What is the benefit of the ECM sequestering growth factors?

A

The ECM molecules can seqeuster these growth factors in the matrix and can determine whether to release them for signalling events or restrict the signalling events by holding them back.
This allows for spatio-temporal regulation of factor release and organises morphogen gradients.

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9
Q

How can the ECM transduce and sense mechanical signals?

A

Via receptors present on the cell surface, the matrix can convert that mechanical force outside the cell into an internal cellular response e.g. changes in gene expression/protein synthesis/activation of enzymes.
The matrix activates intracellular signalling through interaction with cell surface receptors engaging cytoskeletal machinery and synergises with growth factors signalling.

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10
Q

What is meant by the fact that the interaction between cell and matrix is reciprocal?

A

The cell influences the matrix and the matrix influences the cell.
Feedback regulatory mechanisms between cells and the ECM allows cells and tissues to swiftly adapt to their envionmental surrounding.

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11
Q

What are the two main extracellular macromolecules that make up the matrix?

A

Proteoglycans

Fibrous proteins

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12
Q

What are proteoglycans?

A

Often referred to as the “ground substance” proteoglycans for a highly-hydrated gel-like substance in which fibrous proteins are embedded.

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13
Q

What is the function of proteoglycans?

A

Resists compressive force on the matrix whilst permitting rapid diffusion of nutrients, metabolites and hormones between the blood and tissue cells.
The gel-like consistency allows the tissue to withstand mecahnical forces the tissue reverts back to original structure upon removal of the mechanical load.
Because the structure is gel-like, there is fluid flow of metabolites and this allows diffusion of nutrients and metabolites.
Principle role in providing mecahnical support.
Acts as a “space filler” during embryonic development.
Has a major role in “chemical signalling” acting as a depository for growth factors essential for development.
Proteoglycans are capable or restricting/promoting a growth response.
They can sequester growth factors.
They can sequester and regulate activity of other proteins in the matrix as well as growth factors.

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14
Q

Give some examples of fibrous proteins of the ECM?

A

Collagen, elastin, fibronectin and laminin.

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15
Q

How can fibrous proteins be divided?

A

Collagen

Non-collagenous molecules

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16
Q

What is the function of fibrous proteins in the ECM?

A

They provide mechanical strength.

Collectively, the fibrous proteins strengthen and coordinate the organisation of the matrix.

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17
Q

Describe the structure of proteoglycans?

A

There is a core protein covalently linked to glycosaminoglycans to form one proteoglycan.
These monomers come together to form aggregates 80-100million Daltons in weight.
The monomers have a G1 globular domain, an interglobular domain and then another globular domain G2 and in aggrecan, towards the N-terminal end, keratan sulphate chains predominate (approx 50) and spanning the whole molecule are 100 chondroitin sulphate chains, these monomers are approx 2.5 million Daltons in weight.
30-50 aggrecan monomers come together and bind a central filament of hyaluoran stabilised by a 1:1 interaction with link proteins - the monomers are bound to the hyaluoranon non-covalently, electrophilic attractions.

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18
Q

What is the main proteoglycan?

A

Aggrecan

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19
Q

What is a glycosaminoglycan?

A

Glycosaminoglycans are unbranched polysaccharide chains composed of repeating dissacharide units.

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20
Q

Describe the structure of a glycosaminoglycan and how this confers functionality?

A

Glycosaminoglycans are composed of repeating dissacharides which contain an amino sugar, etiher N-acetylglucosamine or N-acetylgalactosamine and a uronic acid either glucuronic or iduronic acid.
The GAGs are highly negatively charged due to SO42- and COO- groups, they recruit positively charged cations such as Ca2+ and Mg2+ and draw in water to stabilise the osmotic pressure.
This confers the viscoelasticity to the proteoglycans, this gives them the ability to resist mechanical forces.

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21
Q

What are the 4 main glycosaminoglycans and how are they distinguished?

A
  1. Hyaluronan
  2. Chondroitin sulphate and dermatan sulphate.
  3. Heparan sulphate
  4. Keratan sulphate
    They are distinguished according the sugars that comprise the dissacharides, the type of linkage between the sugars (alpha or beta), the number of sulphate groups and their location.
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22
Q

What is unusual about hyaluronan?

A

It has no sulphate groups.

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23
Q

How are the 4 main glycosaminoglycans distinguished from one another?

A

The sugars of which it is composed.
The type of linkage between the monosaccharides.
The number of sulphate groups.
The location of the sulphate groups.

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24
Q

Which glycosaminoglycan doesn’t have any sulphate groups?

A

Hyaluronan.

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25
Q

Why is hyaluronan important during embryonic development?

A

It acts as a space filler, it gets hydrated with water and therefore can act as a space filler packing out space and then cells migrate in and lay down matrix and form the tissues and organs of the body.

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26
Q

What are syndecans? Structure? Function?

A

Syndecans are a family of 4 (currently) proteoglycans that traverse the plasma membrane.
They have an internal component facilitating interaction with the intracellular actin cytoskeleton.
The N-terminal extracellular component projects out into the matrix and has a number of GAG side chains e.g. heparan sulphate chains that recruit growth factors and cytokines etc.

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27
Q

What is important about the N-terminal domain of syndecans?

A

The extracellular N-terminal domain has GAG side chains, particularly heparan sulphate chains which enable the syndecan to recruit and attract grwoth factors, morphogens, cytokines etc.
The external domain also has binding sites for receptors found on the cell, particularly integrins.

28
Q

What is the role of syndecan 2 in specification of left/right asymmetry?

A

A dominant negative mutant of syndecan-2 generated through targetted morpholinos results in reverse heart gut looping providing a clue that syndecan-2 is important for specification of the left/right asymmetry in xenopus.
Through rescue assays to see what could rescue the phenotype it was determined that it wasn’t syndecan-2 itself that was important but the fact that it was sequestering and presenting the growth factor Vg1 (discovered because Vg1 could rescue the phenotype).

29
Q

How can collagens be divided?

A

Fibrillar and non-fibillar.

30
Q

How many types of collage have been discovered to date?

A

28.

31
Q

What is the difference between a collagen homotrimer and a collagen heterotrimer?

A

In a homotrimer, all 3 alpha chains are encoded by the same gene and in a heterotrimer, there are different alpha chains, enocoded by different genes.

32
Q

Where are types I-V collagen found?

A

Type I = skin, tendons, vascular ligatre, organs, bone.
Type II = cartilage and intervertebral discs.
Type III = reticulate e.g. in bone marrow.
Type IV = basal lamina
Type V = cell surfaces, hair and placenta.

33
Q

How are collagen fibrils typically arranged?

A

Collagen fibrils are laid down at right angles to each other, and this important for their function.
This is particularly important for tissues exposed to large mechanical forces/stresses e.g. skin.

34
Q

What is the function of collagen?

A

Fibrils form structures that resist tensile forces.

This contributes to correct alignment of cells for proliferation and differentiation.

35
Q

Describe the first collagen mutant mouse?

A

The first mouse mutagenesis model was looking at the importance of type I collagen in 1983.
There was an insertional mutation in alpha 1 collagen gene (COL1A1) and this was embryonic lethal.
The mice lacking type I collagen reached a late stage of development later dying of aortic rupture.
This makes sense because the aorta has to withstand regular pulsatile mechanical force.

36
Q

What are laminins?

A

Laminins are heterotrimeric proteins containing an alpha beta and gamma chain.
They are a non-collagenous glycoprotein.

37
Q

What is the function of laminin?

A

Laminins can bind to receptors on the cell surface (integrins) but can also bind other molecules in the ECM thus acting as a bridge between the cell and the ECM.
This enables laminins to facilitate communication between cells and the ECM.
Laminins are a major component of the basal lamina.
Laminins are important for migration because the basement membrane can act as a substrate for cells to crawl/migrate along.
Laminins bind cell membrane through laminin receptors e.g. dystroglycan and perlecan.

38
Q

What are the components of the basement membrane?

A

Laminin, perlecan, nidogen, type IV collagen.

39
Q

List 4 types of non-collagenous glycoproteins?

A
  1. Laminin.
  2. Elastin.
  3. Tenascin-C.
  4. Fibronectin
40
Q

Describe the structure and function of elastin?

A

Elastin gives elasticity to the tissue.
There is high elastin in tissues required to be elastic in structure, e.g. those that need to deform under pressure and then revert back to their original shape.

41
Q

What % of the aorta dry weight is elastin?

A

50%

42
Q

Describe the structure and function of fibronectin?

A

Fibronectin is a glycoprotein that dimerises and has multiple domains e.g. cell attachment sites so can bind integrins and may have binding domains with collagen.
Fibronectin helps form a continuum between the ECM and cells within the ECM.
Fibronectin plays an important role in cell adhesion to the matrix and guides cell migration in vertebrate embryos.

43
Q

What kind of molecule is laminin?

A

Non-collagenous glycoprotein.

44
Q

What kind of molecule is fibronectin?

A

Non-collagenous glycoprotein.

45
Q

What kind of molecule is tenascin-C?

A

Non-collagenous glycoprotein.

46
Q

What kind of molecule is elastin?

A

Non-collagenous glycoprotein.

47
Q

Describe the function and structure of tenascin?

A

Tenascin is an oligomeric glycoprotein expressed by various cells and interacts with fibronectin to modify cell adhesion.
Different isoforms of tenascin exist.
Tenascin-C is good at binding growth factors e.g. FGFs and morphogens like BMPs.

48
Q

Which isoform of tenascin predominates during embyrogenesis?

A

Tenasin-C is highly expressed and synthesised by migratig cells, e.g. the neural crest.

49
Q

Does the ECM composition change throughout development? Give one example?

A

Yes, the ECM composition changes during development
There may be continued expression of existing/new ECM and equally switching off/restriction of certain molecules, this gives potential for tissue-specific regulation.
During branching morphogenesis, there is an increased expression of fibronectin and switching off of E-cadherin enabling cleft/invaginations to form.
Fibronectin is essential for cleft formation during the initiation of epithelial branching.

50
Q

What are the 2 models for the assembly of the ECM during development?

A
  1. ECM component made locally by cells and rapidly becomes insoluble and “laid down” direction in the region in which it was secreted.
  2. ECM molecule is secreted, diffuses away and assembles into matrices at specific sites controlled by restricted expression a receptor or a locally synthesised ECM component.
    Both are probable but one might be preferentially used in a particular circumstance.
51
Q

What do many ECM molecules have in common?

A

They harbour a Arg-Gly-Asp sequence motif/structural motif - called the RGD motif.

52
Q

What is the RGD sequence?

A

Many ECM glycoproteins share a particular sequence/structual motif, Arg-Gly-Asp, the nature of the Arg and the Asp amino acids in particular make this an adhesive motif because Arg and Asp are charged and so rside in hydrophilic regions of matrix molecules.
This motif can be recognised by specific receptors called integrins which are cell-surface receptors, the primary means by which a cell communicates with its ECM.

53
Q

What receptor binds the RGD motif?

A

Integrins.

54
Q

What motif to the integrin cell surface receptors bind?

A

Arg-Gly-Asp, the RGD structural motif.

55
Q

Describe the structure of an integrin?

A

Integrins are a large family of heterodimeric glycoprotein receptors.
Like fibronectin, they come together an exist as a heterodimer which is the functional integrin receptor.
It exists as two non-covalently bound alpha and beta subunits, in mammals 24 alpha and 9 beta subunits have been identified thus far.
The alpha and beta subunit come together to form a dimer, the integrins also have a tail region which sits in the cytoplasm of a cell and binds and interacts with the actin cytoskeleton.
The N-terminal extracellular region projects out into the matrix and binds matrix molecules e.g. collagen or fibronectin.

56
Q

How many alpha subunits and beta subunits have been identified in integrins in mammals?

A

24 alpha.

9 beta.

57
Q

What are the 3 principle functions of integrins?

A
  1. Attaching cell to ECM.
  2. Cell migration.
  3. Tranducing signals from ECM to cell.
58
Q

How to integrins transduce signals from ECM to cell?

A

Any perturbations/events that occur outside the cell in the matrix will cause conformation change in the shape of the integrin, this will activate the integrin and there will be a tug on the intracellular cytoplasmic tail which initiates a cascade of events inside the cell mediated by the actin cytoskeleton involving a number of RTKs.

59
Q

What are some functions of integrins?

A
  1. Integrins couple the ECM outside a cell to the cytoskeleton inside the cell.
  2. Connection between the cell and ECM enables cells to endure pulling forces without being torn away from the matrix, critical during development.
  3. Utilising signalling cues in the local environment, integris enforce cellular decisions, e.g. attachment, migration, differentiation, or apoptosis.
  4. Play a role in cell signalling modulating the pathways of transmembrane protien kinases e.g. RTKs to effect a cellular response.
60
Q

What happens when you knockout the beta-1 integrin subunit?

A

Embryonic lethality - b1 integrin subunit is responsible for growth and survival of the inner cell mass.

61
Q

What is the function of the integrin alpha-5 subunit?

A

This is a mechanosensor.

62
Q

List some cell junctions?

A

Tight junction - seals gaps between epithelial cells.
Adherens junction - connects actin filament bundle in one cell with that in the next cell.
Desmosome - connects intermediate filaments in one cell to those in the next cell.
Gap junction - allows the passage of small water-soluble molecules from cell to cell.
Hemidesmosome - anchors intermediate filaments in a cell to the ECM.
Actin-linked cell-matrix junction - anchors actin filament in cell to extracellular matrix.

63
Q

What are cadherins?

A

Cadherins are a large family of proteins involving in forming adherens junctions.
Cadherins are calcium dependent and have an extracellular domain that facilitates cell-cell cross-talk.
They have an intracellular domain that binds a number of signalling and adaptor proteins.

64
Q

What are cadherins involved in?

A

Positioning of cells
Responsible for separation of the different tissue layers
Cell migration

65
Q

What are the 3 proteins that mediate the connection of a desmosome?

A

Desmoglein
Desmoplaquin
Desmocollin

Developmental and Stem cell biology (5 decks)

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