Question

1. The extracellular matrix (ECM) was regarded decades ago as an uninteresting set of proteins and unusual carbohydrates that were required for cell adhesion and tissue integrity. Now, however, there is much more of an interest in the ECM given its importance in cancer and metastasis - a concept that is embodied in the phrase, "tumor microenvironment".

a. What is the importance of E-cadherin?

b. Collagen synthesis is unique given its final assembly stage. Explain this statement.

c. Challenge: You are working with a neuronal cell line isolated from a human brain tumor. It appears to have a neuronal phenotype when grown in culture on laminin coated dishes but appears to have more of a typical round, small (3 to 5mm) phenotype when grown on a standard, collagen-only coated dish or a poly-lysine coated (positive charge only - no ECM component) dish. Under all 3 cases the cells attach to the substrate. You suspect that the reason the neuronal phenotype is supported by the laminin coating is due to the ability of laminin, but not collagen or poly-lysine, to bind to the integrin receptors RGD binding domain thus triggering a molecular cascade that leads to the expression of the neuronal phenotype. How might you test this hypothesis?

d. Why are glycosaminoglycans important to cell differentiation, tissue generation and embryogenesis?

Answer 1

(a) E-cadherin tumor suppressor genes are particularly active area of research in development and tumorigenesis. The calcium-dependent interactions among E-cadherin molecules are critical for the formation and maintenance of adherent junctions in areas of epithelial cell-cell contact.

CAMs such as the cadherin glycoproteins normally function as the glue that holds cells together and act as important mediators of cell to cell interactions. E-cadherins, on the surface of all epithelial cells, are linked to the actin cytoskeleton through interactions with catenins in the cytoplasm

(b) Collagen synthesis is unique given its final assembly stage. Explain this statement. The last phase comes from the formation of specific amino acids such as Glycine, Proline and Hydroxproline. Collagen also has two uncommon derivative amino acids that are not directly inserted during translation.

Collagen is composed of 3 chains. The chains are wound together to form a triple helix. The process of collagen synthesis occurs mainly in the cells of fibroblasts which are specialized cells with the main function of synthesizing collagen and stroma. Collagen synthesis occurs both intracellularly and extracellularly.

(d) Glycosaminoglycans (GAGs) are important ECM components, because they are highly negatively charged polymers that strongly bind water and ions, creating turgor in ECM spaces, and because they bind and sequester physiologically important proteins.

Proteoglycans help regulate pathways that control stem cell fate, and therefore represent an excellent tool to manipulate these pathways. Despite their importance, there is a dearth of data linking glycosaminoglycan structure within proteoglycans with stem cell differentiation.