Chapter 15- Cell Junctions 1

1. What are the three common types of cell junctions in animals?

Adhesive junctions, tight junctions, and gap junctions.

2. What is the primary role of adhesive junctions?

They anchor the cytoskeleton to the cell surface, providing tissue strength and integrity.

3. Which adhesion receptors mediate many cell–cell attachments in adherens junctions?

Cadherins (calcium-dependent adhesion proteins).

4. In adherens junctions, cadherins connect to which cytoskeletal element?

They connect to actin filaments via intracellular linker proteins.

5. What type of interaction is typical of cadherins: homophilic or heterophilic?

Homophilic interactions (cadherins on one cell bind identical cadherins on the neighboring cell).

6. Which cadherin is abundant in epithelial tissues and 'zips' cells together in a Ca2+-dependent manner?

E-cadherin.

7. How does the epithelial–mesenchymal transition (EMT) affect cadherin expression?

EMT involves loss or change of cadherin expression, reducing cell–cell adhesion and enabling motility/metastasis.

8. What are desmosomes and where are they abundant?

Button-like spots of strong adhesion abundant in tissues under mechanical stress (e.g., skin, heart muscle, uterus).

9. Desmosomes connect to which cytoskeletal filaments?

Intermediate filaments (not actin).

10. Name the desmosomal cadherins.

Desmogleins and desmocollins.

11. Define homophilic vs. heterophilic adhesion receptor interactions.

Homophilic: identical receptors on adjacent cells bind. Heterophilic: different receptors on adjacent cells bind.

12. What are lectins and how do they aid transient cell–cell adhesion?

Carbohydrate-binding proteins that link cells by binding specific sugars on cell surfaces.

13. What are CAMs and to which superfamily do they belong?

Cell adhesion molecules; they belong to the immunoglobulin superfamily (IgSF).

14. Give two examples of IgSF CAMs involved in axon outgrowth/bundling in development.

N-CAM and L1-CAM.

15. Which selectins are expressed on leukocytes, platelets, and endothelial cells, respectively?

L-selectin (leukocytes), P-selectin (platelets), E-selectin (endothelial cells).

16. Stable leukocyte adhesion at inflammation sites involves which receptor pair?

Integrins on leukocytes binding to ICAMs on endothelium.

17. What is the main function of tight junctions?

To form seals between epithelial cells that prevent paracellular movement of molecules.

18. Name two core transmembrane proteins of tight junction strands.

Claudins and occludin (plus JAMs from the IgSF).

19. What is paracellular transport and which tight junction proteins help define it?

Ion/molecule movement between cells through tight junction pores formed by claudins’ extracellular loops.

20. How do tight junctions influence membrane protein mobility?

They block lateral diffusion of many integral proteins and outer-leaflet lipids, maintaining membrane domain polarity.

21. Where are tight junctions especially prominent?

Intestinal epithelium, gland ducts (liver, pancreas), urinary bladder, and brain endothelium (blood–brain barrier).

22. What is a gap junction and its basic structural unit?

A cell–cell channel for small molecules/ions; the unit is the connexon.

23. What proteins assemble connexons and how wide is the pore?

Connexins; each connexon has six connexins forming a ~3 nm pore.

24. Do gap junctions allow passage of proteins or nucleic acids?

No—pores are too small; only small molecules and ions pass.

25. Compare adherens junctions to desmosomes in cytoskeletal linkage.

Adherens: link to actin; Desmosomes: link to intermediate filaments—stronger spot adhesions.

26. How do adhesion receptors connect to the cytoskeleton inside the cell?

Via intracellular attachment/linker proteins that bind cytoskeletal elements.

27. Why are adhesive junctions considered dynamic structures?

Adhesion proteins recycle through endo/exocytosis and serve as scaffolds for signaling and cytoskeletal assembly.

28. What coordinates with cell adhesion to regulate tissue behavior?

Cell signaling, movement, proliferation, and survival.

29. What is the extracellular portion of cadherins responsible for?

Mediating Ca2+-dependent binding to identical cadherins on neighboring cells (homophilic adhesion).

30. How does tissue specificity of cadherins contribute to development?

Differential cadherin expression helps segregate cells into specific tissues and layers.

31. Which junction type forms a continuous belt near the apical surface of epithelial cells?

Tight junctions.

32. What is the role of JAMs in tight junctions?

They are IgSF proteins that participate in tight junction sealing and signaling.

33. How can different claudins influence epithelial permeability?

Different claudin isoforms form ion-selective pores, setting tissue-specific paracellular permeability.

34. What is meant by 'adhesion is coordinated with signaling'?

Adhesion complexes recruit kinases/adaptors, influencing pathways that control growth, polarity, and motility.

35. What is the desmosome core?

The extracellular space where desmosomal cadherins from adjacent cells interact.

36. Why are desmosomes crucial in heart tissue?

They provide mechanical coupling between cardiomyocytes to withstand contraction forces.

37. What happens to membrane lipid movement at tight junctions?

Lipid movement is restricted in the outer leaflet near TJs, preserving apical/basolateral domains.

38. Describe heterophilic adhesion with an example.

Different receptors on opposing cells bind; e.g., selectins binding specific glycans on leukocytes.

39. What is the function of lectins in cell–cell interactions?

They transiently bridge cells by binding sugar moieties on glycoproteins.

40. Explain how gap junctions support electrical coupling.

They allow ion flow between cells, synchronizing activity in tissues like heart and smooth muscle.

41. Do all tissues use the same connexins?

No; different tissues express different connexin isoforms but channels function similarly.

42. What cytoskeletal element do adherens junctions organize besides actin filaments?

They can organize cortical actin belts that support epithelial shape and tension.

43. What cellular processes can disrupt tight junction integrity?

Inflammation, cytokines, pathogens, and Ca2+ depletion can open TJs and increase leak.

44. How do integrins participate in leukocyte adhesion cascade?

Integrins switch to high-affinity states, binding ICAMs to form firm adhesion after rolling.

45. What is EMT and why is it clinically relevant?

Cell state change from epithelial to mesenchymal, linked to development, wound healing, and cancer metastasis.