How do cells know who/what to adhere to? 11/7-9

1. How do cells know who/what to adhere to? 11/7-9 Extracellular matrix: Collagen, Elastin and Glycoprotein Importance of Cell-Cell adhesion N-CAMS: Bind to other N-CAMS Cadherins: Bind to other cadherins Selectins: Specialized to bind target glycoproteins Intergrins: bind laminin of the basal lamina or fibronectin Fibronectin Tight junctions vs. Gap junctions

3. Tendons consist of extracellular proteins. They are composed of collagen fibers that are produced by fibroblasts prior to secretion (exocytosis). Collagen “fibers” are arranged in a staggered overlapping pattern of “fibrils”.

4. Disulfide bonds are critical for the cross-links that strengthen collagen! Vitamin C (ascorbate) promotes oxidation of the sulfhydryl groups! Collagen forms a matrix around which other cells and bone may organize.

5. Rubbery elastin stretches and snaps back into the original shape! It is rich in negative charges that repel each other to help prevent “excess” stretching and to help it snap “back”.

6. Glycocalyx provides a critical carbohydrate matrix that sits between a cell’s plasma membrane and the external environment. It also contains a series of glycoproteins whose protein segments mediate enzymatic digestion.

7. Cartilage is a proteoglycan (modified ground substance) that contains varying amounts of collagen and elastin, depending on its location and function. The function of cartilage in a joint is to provide a watery surface across which joints can move across with minimal friction.The function of cartilage in your ear is to be able to return to its original shape.The function of cartilage in a vertebral disc is to resist rupture.

8. Why is cell-cell adhesion so important to understand? • Migrating cells must reach, recognize, bind and stay with their target cells, tissues and organs! • Important implications in embryogenesis, cancer, infection, transplantation potentials (stem cells) and disease! • Tissue Development in the Embryo/Adult: • Epithelial cell to basal lamina: post burn skin • Neural crest cell in embryo: amputee nervous system • Endothelium and blood vessels: cancer • Pathway Guidance to Lead Cells Home: • Chemotaxis vs. Chemo-avoidance: • Ligands/Receptors/Second Messengers • N-CAMS and transmembrane adhesion molecules help mediate binding of cells to the ECM and each other!

9. Embryogenesis is the development of tissues and organs. This is a review of embryonic development in a chick, similar process also occur in human embryos.

10. N-CAMS: Ca+Independent Bind to other N-CAMS Cadherins: Ca-Dependent Bind to other cadherins Selectins: don’t bind each other. Specialized to bind target glycoproteins Intergrins: bind laminin of the basal lamina or fibronectin in the ECM or to other cells. There are a variety of options available to cells when they seek to bind to other cells or know if they are in the correct location. Lets review some of the options available. Know one item that makes each unique.

11. Cadherins are a class of Ca++Dependent cell-cell adhesion molecules which are important in embryogenesis. Their loss permits cells to migrate within the chick embryo to become muscle. Dimer anchored by tx-membrane sequences Localization of sub-types: E-epithelial N-neuronal P-placental VE-endothelial For instance: N-cadherin binds to other N-cadherins, but not to E, P or VE-cadherins This keeps neurons attached to other neurons! Important to vertebrates and invertebrate animals Classic exrracellular Cadherin binds to actin on intracellular side. Non-Classic binds tonofilament on inside: for extra strength If potential neurons don’t lose N-cadherins, they can’t migrate! About 85% of cancers are epithelial in nature (due to high replication rate), metastasis is linked to a loss of E-cadherins

12. White blood cells tend to “Roll” along the surface of blood vessels looking for trouble (a signal of damage outside vessel). When damage occurs endothelial cells move P-selectin from vesicle to the cell surface and this binds the leukocyte glycoproteins! Then integrin binds its ICAM!

13. What amino acids would you expect at the locations where the glycosylations occur on glycophorin? Why is the carbohydrate sialic acid (-charged) important for the extracellular side on an RBC? When the erythrocyte has circulated for about 60-120 days, the sialic acids and glycosylations will have rubbed off to expose the receptor, which mediate lysis in the spleen or liver.

14. How does the body know when to destroy a red blood cell that is 120 days old or older? Solution: selective removal of RBC sialic acids. What does sialic acid,also called N-acetylneuraminic acid (NANA), look like? What is it’s net charge? NANA attached to glycoprotein Neuraminase removes NANA No NANA: N-CAM Exposed! Cell binds to macrophage Removal at spleen/liver 120 day life-span 1 trillion RBCs in the body 1 million cells removed/second 1 pint/30-60 days!

16. Fibronectin is an important mediator between cells and the contents of the matrix. Generally found as a extracellular dimer crosslinked by a pair of dissulfides (-S-S-) • Each subunit (dimer) contains different domains, each domain has a potentially different function. • Sample domains: • Fibrin binding Form clot • Heparin binding Clot lysis • Collagen binding bind where tissues exposed • Cell surface receptorshold cells down, this is why tissue culture plates are typically coated with a thin layer of collagen or fibronectin! • Collagenase is used when you want to remove cells from a culture dish.

17. There are several discrete structures that are observed at the locations where the adherence molecules bind cells together.

18. Two other cell surface receptors: Basal Lamina: found underneath all epithelial cells in the body and creates a matrix for cellular attachment/protection • Laminin protein: primary constituent • Large peptide (alpha, beta, gamma subunits) • Dense packing • Cross-shape • Many potential binding domains • Integrin: helps create a link to intracellular proteins • Large dimer (alpha and beta subunits) • Extracellular aspect- binds matrix or ICAMs • Intracellular aspect- indirectly binds actin (via tallin and vinculin) • Also an integral part of hemidesmosome • Extracellular binding is often linked to changes in activity of protein kinases in the cytosol

19. Cell-Cell adhesions can occur at specifics locations. What are some of the classic locations? • Desmosome: button of contact specialized for stress! • Dense deposit of tonofiliments! • Narrow intercellular space! • Hemidesmosome: attachment to lamina • Contains tonofilaments,,, sort of ½ of a desmosome! • Adherence Junctions: Belt-like or Focal • Bands of attachement • Continuity with actin in muscle actin/myosin complex • Tight Junction: Protein belt for molecular impermeability • Important where body meets external environment: gut, skin, kidney • Gap Junction: connexins to allow small molecule diffusion between cells. • Especially important in the heart!

20. Once inside pathogen can disable cell! Disabled cell is “hijacked” by the pathogen and protected by the cell! Examples: HIV-Virus, Anthrax, Listeria, Shigella and Yersinia Most Effective Targets: Immune Cells! Common Receptor Targets: Integrin and E-cadherin Often times bacteria and viral particles have special receptors that fill the receptors on the cell, causing the host to pull cell inside!

21. Tight Junctions are critical for maintaining water impermeability at places where cells meet the external environment! • Why are TJ critical in the gut, kidney and skin? • Where would the water go? • There is no measurable space between adjacent cells bound by TJ! • TJ Blocks: • 1) ….movement of ions, water and proteins between these adjacent cell • 2) ….Movement of proteins between the outside (apical) and inside (basolateral) aspects of the plasma membrane. • TJ consists of a mesh-like band of thin proteins that wrap entirely around cells many many times. The proteins (claudins and occludin) of the two cells bond to each other pulling the two cells very tightly together forming an impermeable barrier.

22. The claudin and occludin subunits of the two adjacent cells form a belt around the lateral aspect of the cell, typically on the apical side.

23. Tight Junctions mean that the only way materials can enter a cell is to pass through the transport enzymes of the apical membrane, pass though the cytoplasm, and use a different enzyme on the baso-lateral membrane. (2 Diff. Tx Enzymes) This gives a precise way for a cell to regulate transport!

24. Pores are critical for permitting the movement of ions during AP propagation between heart cells! Pores create a functional “syncitium” in the heart! Connexons may also be important for the passage of water, sugars and amino acids. Gap Junctions are specialized pores consisting of two connexons that link the intracellular contents of adjacent cells. This arrangement allows small molecules to physically pass between cell and is especially important in the heart!