EXTRACELLULAR MATRIX (ECM)

William Beresford©

Department of Anatomy, West Virginia University
" By their works ye shall know them." ---- ECM as an expression of cells' character

What cells produce and organize outside themselves allows them to construct huge organisms, with complex repertoires of movements and behaviors, and multiple physiologies based on separate compartments.

Scale of the cell's endeavors - Three images: the farmer's wife on the tractor in the Kansas wheatfield; the conservation scientist trying to maintain an ecosystem; and the drug baron still operating from prison with his phone and products of the prison workshop - " life on the outside".

  context   tasks      sites     means     mechanisms      malfunctions
Cells create and achieve control over their environment by synthesizing combinations of types of macromolecule which interact outside the cells.

Kinds of control

1 Over water - Why you don't slosh as you walk    hyaluronan (a glycosaminoglycan)
                                                  large proteoglycans

                Why you can swallow, wink, spit,  mucin glycoproteins (gps)
                 etc.
2 Creation of barriers - basal laminae            laminin (gp), collagen IV,
                                                  heparan sulfate PG, etc.
                        
                        - connective tissue       small proteoglycans
                          ground substance        hyaluronan
                                                  fibronectin, tenascin (gps)
                                                   
3 Attachment and support                          collagen fibrils & fibers (I & III)
                                                     + associated molecules

4 Elastic recovery of shape, & elastic storage of elastic fibers &   elastin 
  the work of the heart and respiratory muscles   elastic membranes  fibrillin (gp)

5 Resilient firmness in joint, airway,            cartilage collagens II + IX 
  and fetal-skeletal cartilages                   aggregated proteoglycans
                                                  link proteins & hyaluronan
                                                  cartilage glycoproteins

6 Resilient hardness of bone                      collagens I + XII
                                                  bone proteoglycans
                                                  bone glycoproteins
                                                  bone mineral crystals

Molecular structure and diversity - correlations with properties such as tensile strength, water-binding, stickiness, mineral-binding, elasticity, and cell signalling.

Interactions outside the cells -- ECM the rich playground of the cells, and a battleground for intruders.

Extracellular interactions
1 Further assembly of the molecules to make larger and eventually 'visible' structures such as fibrils.

2 Binding between ECM molecules of different types, e.g., type XII collagen to type I, and both to adhesive glycoproteins such as fibronectin, and all to small proteoglycans.

3 Deliberate breakdown of molecules by the forming cells for turnover and renewal, by proteases and other enzymes. Controlled breakdown, with more synthesis and assembly, provides for remodeling and adaptation of ECM, e.g., to heavier load in cartilage or tendon.
Some of these enzymes, e.g. collagenase, include a zinc atom and require calcium ion to work, hence they are termed Matrix MetalloProteinases, e.g. MMP-3. The inhibitors of these enzymes go under the abbreviation TIMPs - Tissue Inhibitors of MMPs and are also made by fibroblasts and other matrix-influencing cells.

4 Unintended degradation by enzymes released from cells engaged in defensive reactions, e.g., white blood cells. ECM is the battleground for defenses initially targetted at microorganisms. " -itises" occur throughout the body and represent very real hazards to life and comfort. For instance, endocarditis weakens and distorts the heart valves.

5 Unwanted degradation by inclusion in cytokine signalling pathways of defensive cells

          
                IL-1
  Macrophage ------------- L   Lymphocytes of inner joint synovium
             \
          IL-1\        Joint cartilage cells also respond to the signal
               \ AC  enzymes up   proteoglycans down   enzyme inhibitors down =
                 AC  inappropriate response causing cartilage-matrix destruction

(AC - articular chondrocytes/cartilage cells)

6 Unwanted degradation by: (a) microbes trying to colonize, e.g., using bacterial hyaluronidase to liquify ground substance; & (b) metastasizing cancer cells breaking through basal laminae & connective tissues.

7 Enzymatic cascades whereby an enzyme is released in an inactive form that has to be cleaved for activation by another enzyme. A similar process occurs with some cytokines which are available in a latent form that requires cleavage before the cytokine is potent.

8 ECM molecules bind and hinder the passage of cytokines - ; & cell-surface proteoglycans bind and concentrate cytokines to increase their effect on nearby receptors + .

9 Unwanted synthesis - the formation of excess collagen clogging organs with delicate blood-cell relations. Cytokines released by activated macrophages trigger synthesis in fibroblasts, causing cirrhosis in the liver and fibrosis in the lung.

10 ECM conveys instructions to cells in its own right, and participates in tissue-tissue interactions, e.g., factors in the basal lamina tell the skeletal muscle cell where in its sarcolemma to insert the acetylcholine receptors and construct a motor endplate.

11 Mechanical conditions can be conveyed to cells by the ECM's support and attachment to cells via integral membrane proteins, e.g. integrins, which convey mechanical stresses to the cytoskeleton and internal signaling pathways.

Proteoglycan:glycoprotein distinction - both join sugar chains to a peptide sequence of amino acids, but what are the differences? Different protein backbones &

Glycoproteins

Questions for a glycoprotein

  1. Ratio of sugars to peptide? Mucin glycoproteins may approach equality by MW. Collagen I is only minimally and temporarily glycosylated.
  2. How many sugar chains are on the protein core?
  3. How are the sugars attached? By -O- links to serine or threonine, or by an -N- link to asparagine?
  4. What core (consensus) sugars start the chains? Types 1-4, and the N-acetylglucosamine and mannose core for -N- linked glycoproteins.
  5. What sugars are in the chains?
  6. Do the chains branch? And how many times? Thus, how many antennae?
  7. With what sugars do the chains end?
  8. How acidic/negatively charged is the glycoprotein, from sulfate groups and/or sialic-/N- acetylneuraminic-acid groups?
  9. Is the protein core designed to be cleaved just outside the cell to release a secreted glycoprotein, or left uncleaved to anchor the glycoprotein to the cell membrane?
  10. What are the physiological roles of the glycoprotein?
  11. What difference does the absence of, or changes in, the sugar/glycosylation pattern make to function?
  12. What role do changes in protein or glyco/sugar components play in disease?
  13. And how much of the information from questions 1-12 plays a part in the pathophysiology? I.e., How much is it helpful to know?

Glyco-babel - the strange language & confusions in Sugarland
sugar ---- glycan, glycation, glycosaminoglycan (GAG)
glyco-, glycoprotein, glycoside, glycosylation, glycocalyx, glycobiology
saccharide, polysaccharide, mucopolysaccharide, oligosaccharide, saccharidase

A proteoglycan is not a glycoprotein. However, a sugar side-chain typical of a glycoprotein can be attached to the protein core of a proteoglycan, additional to the GAGs.

The glycan of a proteoglycan is a glycosaminoglycan, but a glycosaminoglycan can exist separately from a protein. Thus, a GAG is either just a GAG or is part of a PG. However, in cartilage PGs can be linked to a GAG - hyaluronan.

An apomucin is the peptide core of a mucin, not a whole mucin. A mucin is a glycoprotein, but most glycoproteins are not mucins.

Mucin glycoproteins and aggrecan PGs can aggregate with their own kind. The PGs need help from link proteins and hyaluronan to act as the backbone.


William A Beresford --- wberesford@hsc.wvu.edu --- Department of Neurobiology & Anatomy, School of Medicine, West Virginia University, Morgantown WV26506-9128, USA