1. Molecular Biology of the Cell
Alberts • Johnson • Lewis • Raff • Roberts • Walter
Molecular Biology of the Cell
Fifth Edition
Chapter 19
Cell Junctions, Cell Adhesion, and the Extracellular Matrix
Copyright © Garland Science 2008

2. Animal cells are bound together through cell-cell adhesion and cell-matrix adhesion

3. Cell-cell adhesions are cell junctions: different types

4. Cell-cell adhesions are cell junctions: different types

5. Cell-cell adhesions are cell junctions: different types

6. Cell-cell adhesions are cell junctions: different types

7. Cell-cell adhesions are cell junctions: different types

9. A typical arrangement of intestinal epithelial cells and the types of junctions between them

10. Cadherins and Integrins in anchoring junctions

11. Cadherins: cell-cell adhesion Integrins: Cell-matrix adhesion

12. Which junction type cadherin and integrins associate with

13. Cadherins mediate calcium dependent cell-cell adhesion in all animals
Removing Calcium from the extracellular medium causes adhesions mediated by cadherins to come adrift. Sometimes, especially for embryonic tissues, this is enough to let the cells be easily separated.
In other cases, a more severe treatment is required, combining Calcium removal with exposure to a protease such as trypsin. The protease loosens additional connections mediated by extracellular matrix and by other cell-cell adhesion molecules that do not depend on Calcium.
In either case, when the dissociated cells are put back into a normal culture medium, they will generally stick together again by reconstructing their adhesions.

14. Studies of the early mouse embryo illustrate the role of cadherins in development.
Up to the eight-cell stage, the mouse embryo cells are only very loosely held together remaining individually more or less spherical; then, rather suddenly, in a process called compaction, they become tightly packed together and joined by cell-cell junctions, so that the outer surface of the embryo becomes smoother

15. The Cadherin superfamily in vertebrate Includes hundreds of different proteins including many with signaling functions

17. The binding between cadherins is generally homophilic: cadherin molecules of a specific subtype on one cell bind to cadherin molecules of the same or closely related subtype on adjacent cells.

18. Cadherin structure and function

19. Cadherins are not like glue, making cell surfaces generally sticky
Cadherins are not like glue, making cell surfaces generally sticky. Rather, they mediate highly selective recognition, enabling cells of a similar type to stick together and to stay segregated from other types of cells
In experiments in which amphibian embryos were dissociated into single cells. These cells were then mixed up and allowed to reassociate. Remarkably, the dissociated cells often reassembled in vitro into structures resembling those of the original embryo

20. Mutations that disrupt the production or function of E-cadherin are in fact often found in cancer cells and are thought to help make them malignant.
This process is called epithelial to mesenchymal transition (the changes that occur in a cell so it is no longer attached, it is free floating and therefore can do metastasis).

21. Catenins link classical Cadherins to the actin cytoskeleton
Figure Molecular Biology of the Cell (© Garland Science 2008)

22. Adherens junctions for adhesion belts: these contract with the help of myosins, providing shape changes in morphogenesis
Figure Molecular Biology of the Cell (© Garland Science 2008)

23. Figure 19-16 Molecular Biology of the Cell (© Garland Science 2008)

24. Desmosomes give epithelial cells strenght
Figure 19-17a Molecular Biology of the Cell (© Garland Science 2008)

25. Proteins that form desmosomes
Figure 19-17b Molecular Biology of the Cell (© Garland Science 2008)

26. Intermediate filaments provide tensile strength in desmosomes
Figure 19-17c, d Molecular Biology of the Cell (© Garland Science 2008)

27. The specific kind of intermediate filaments that bind to desmosomes depends on the cell type; keratin in most epithelial cells
Figure Molecular Biology of the Cell (© Garland Science 2008)

28. The selectins control the binding of white blood cells to the
Selectins are cell-surface carbohydrate-binding proteins (lectins) that mediate a variety of transient, cell-cell adhesion interactions in the bloodstream.
White blood cells lead a nomadic life, roving between the bloodstream and the tissues, and this necessitates special adhesive
behavior.
The selectins control the binding of white blood cells to the
endothelial cells lining blood vessels, thereby enabling the blood cells to migrate out of the bloodstream into a tissue.
Figure 19-19a Molecular Biology of the Cell (© Garland Science 2008)

29. In a lymphoid organ, such as a lymph node or a tonsil, the endothelial cells express oligosaccharides that are recognized by L-selectin on lymphocytes, causing the lymphocytes to loiter and become trapped. At sites of inflammation, the roles are reversed: the endothelial cells switch on expression of selectins that recognize the oligosaccharides on white blood cells and platelets, flagging the cells down to help deal with the local emergency. Selectins do not act alone, however; they collaborate with integrins, which strengthen the binding of the blood cells to the endothelium.
The cell-cell adhesions mediated by both selectins and integrins are heterophilic- that is, the binding is to a molecule of a different type: selectins bind to specific oligosaccharides on glycoproteins and glycolipids, while integrins bind to other specific proteins.

30. Selectins and integrins act in sequence to let white blood cells leave the bloodstream and enter tissues
Figure 19-19b Molecular Biology of the Cell (© Garland Science 2008)

31. The selectins mediate a weak adhesion because the binding of the lectin domain of the selectin to its carbohydrate ligand is of low affinity. This allows the white blood cell to adhere weakly and reversibly to the endothelium, rolling along the surface of the blood vessel, propelled by the flow of blood. The rolling continues until the blood cell activates its integrins. As we discuss later, these transmembrane molecules can be switched into an adhesive conformation that enables them to latch onto other
molecules external to the cell-in the present case, proteins on the surfaces of the endothelial cells. once it has attached in this way, the white blood cell escapes from the blood stream into the tissue by crawling out of the blood vessel between adjacent endothelial cells.

32. The chief endothelial cell proteins that are recognized by the white blood cell integrins are called ICAMs (intercellular cell adhesion molecules or VCAMs (vascular cell adhesion molecules); these are members of the Ig (immunoglobulin) superfamily
Figure Molecular Biology of the Cell (© Garland Science 2008)

33. Although cadherins and Ig family members are frequently expressed on the same cells, the adhesions mediated by cadherins are much stronger, and they are largely responsible for holding cells together, segregating cell collectives into discrete tissues, and maintaining
tissue integrity. Molecules such as NCAM seem to contribute more to the finetuning of these adhesive interactions during development and regeneration, playing a part in various specialized adhesive phenomena, such as that discussed for blood and endothelial cells. Thus, while mutant mice that lack N-cadherin die early in development, those that lack NCAM develop relatively normally but
show some mild abnormalities in the development of certain specific tissues, including parts of the nervous system.

34. To make a synapse, the pre- and postsynaptic cells have to do more than recognize one another and adhere: they have to assemble a complex system of signal receptors, ion channels, synaptic vesicles, docking proteins, and other components.
This apparatus for synaptic signaling could not exist without cell adhesion molecules to join the pre- and postsynaptic membranes firmly together and to help hold all the components of the signaling machinery in their proper positions. Thus, cadherins are generally present, concentrated at spots around the periphery of the synapse and within it, as well as Ig superfamily members and various other types of adhesion molecules.
But how does the array of adhesion molecules recruit the other components of the synapse and hold them in place? scaffold proteins are thought to have a central role here. These intracellular molecules consist of strings of protein binding domains, typically including several pDZ domains-segments about 70 amino acids long that can recognize and bind the C-terminal intracellular tails of specific transmembrane molecules.

35. A scaffold protein at the synapse
Figure Molecular Biology of the Cell (© Garland Science 2008)

36. Figure 19-22a Molecular Biology of the Cell (© Garland Science 2008)

37. Figure 19-22b Molecular Biology of the Cell (© Garland Science 2008)

38. Protein organization at the synapse
Mutations in synaptic scaffold proteins alter the size and structure of synapses and can have severe consequences for the function of the nervous system. Among other things, such mutations can damage the molecular machinery underlying learning and memory, which depend on the ability of electrical activity to leave a long-lasting trace in the form of alterations of synaptic architecture.
Figure 19-22c Molecular Biology of the Cell (© Garland Science 2008)

39. All epithelia are structurally polarized
The junctions play a key part in organizing and maintaining the polarity of the cells in the sheet
The occluding junctions found in vertebrate epithelia are called tight junctions
Figure Molecular Biology of the Cell (© Garland Science 2008)

40. The sealing function of tight junctions is easy to demonstrate experimentally: a low-molecular-weight tracer added to one side of an epithelium will generally not pass beyond the tight junction
Figure Molecular Biology of the Cell (© Garland Science 2008)

41. When tight junctions are visualized by freeze-fracture electron microscopy, they seem to consist of a branching network of sealing strands that completely encircles the apical end of each cell in the epithelial sheet
Figure Molecular Biology of the Cell (© Garland Science 2008)

42. The extracellular domains of these proteins adhere
Each tight junction sealing strand is composed of a long row of transmembrane adhesion proteins embedded in each of the two interacting plasma membranes
The extracellular domains of these proteins adhere
directly to one another to occlude the intercellular space
Figure 19-26a Molecular Biology of the Cell (© Garland Science 2008)

43. The main transmembrane proteins forming these strands are the claudins, which are essential for tight junction formation and function
Figure 19-26b Molecular Biology of the Cell (© Garland Science 2008)

44. Mice that lack the claudin-1gene, for example, fail to make tight junctions between the cells in the epidermal layer of the skin; as a result, the baby mice lose water rapidly by evaporation through the skin and die within a day after birth.
Conversely, if nonepithelial cells such as fibroblasts are artificially caused to express claudin genes, they will form tight-junctional connections with one another

45. to form the tight-junctional network of sealing strands
The claudins and occludins have to be held in the right position in the cell, so as
to form the tight-junctional network of sealing strands
This network usually lies
just apical to the adherens and desmosome junctions that bond the cells together mechanically, and the whole assembly is called a junctional complex
The parts of this junctional complex depend on each other for their formation For example, anti-cadherin antibodies that block the formation
of adherens junctions also block the formation of tight junctions.
Figure Molecular Biology of the Cell (© Garland Science 2008)

46. In the case of epithelial cells, the fundamental generators of cell polarity have to establish the difference between the apical and basal poles, and they have to do so in a properly oriented way, in accordance with the cell's surroundings.
Figure Molecular Biology of the Cell (© Garland Science 2008)

47. In a cultured line of epithelial cells, called MDCK cells
In a cultured line of epithelial cells, called MDCK cells. These can be separated from one another and cultured in suspension in a collagen gel. A single isolated cell in these circumstances does not show any obvious polarity, but if it is allowed to divide to form a small colony of cells, these cells will organize themselves into
a hollow epithelial vesicle where the polarity of each cell is clearly apparent. The vesicle becomes surrounded by a basal lamina, and all the cells orient themselves in the same way, with apex-specific marker molecules facing the lumen.
Evidently, the MDCK cells have a spontaneous tendency to become polarized, but the mechanism is cooperative and depends on contacts with neighbors.
B)W hen Rac functioni s blocked, the cells show inverted polarity fail to form a cyst with a central cavity, and cease to deposit laminin in the normal manner around the periphery of the cell cluster.
(C) When the cyst is embedded in a matrix rich in exogenous laminin, near-normal polarity is restored even though Rac
Function is still blocked.

48. GAP junctions: the connections allow neighboring cells to
exchange small molecules but not macromolecules
The channels formed by the gap-junction proteins allow inorganic ions and other small water-soluble molecules to pass directly from the cytoplasm of one cell to the cytoplasm of the other
Figure Molecular Biology of the Cell (© Garland Science 2008)

49. Connexins are four-pass transmembrane proteins, six of which assemble to form a hemichannel, or connexon.
Figure 19-34a Molecular Biology of the Cell (© Garland Science 2008)

50. Gap junctions as seen in the electron microscope
Figure Molecular Biology of the Cell (© Garland Science 2008)

51. Individual gap-junction channels do not remain continuously open; instead, they flip between open and closed states. Moreover, the permeability of gap junctions is rapidly (within seconds) and reversibly reduced by experimental manipulations that increase the cytosolic concentration of free Calcium to very high levels.
The purpose of Calcium control seems clear. When a cell is damaged, its plasma membrane can become leaky. Ions present at high concentration in the extracellular fluid, such as Calcium and Sodium, then move into the cell, and valuable metabolites leak out. If the cell were to remain coupled to its healthy neighbors, these too would suffer a dangerous disturbance of their internal chemistry. But the large influx of Calcium into the damaged cell causes its gap-junction channels to close immediately, effectively isolating the cell and
preventing the damage from spreading to other cells.

52. The basal lamina is a thin, tough, flexible sheet of matrix molecules under all epithelia
Figure (part 1 of 3) Molecular Biology of the Cell (© Garland Science 2008)

53. Figure 19-39 (part 2 of 3) Molecular Biology of the Cell (© Garland Science 2008)

54. Figure 19-39 (part 3 of 3) Molecular Biology of the Cell (© Garland Science 2008)

55. Basal lamina roles:
Determine cell polarity
Influence cell metabolism
Organize the proteins in adjacent plasma membranes
Promote cell survival and proliferation
Serve as roads for cell migration

56. The basal lamina is synthesized by the cells on each side of it:
Figure Molecular Biology of the Cell (© Garland Science 2008)

57. Like other extracellular matrices in animal tissues, the basal
lamina consists of two main classes of extracellular macromolecules: (l) fibrous proteins (usually glycoproteins, which have short oligosaccharide side chains attached) and (2) polysaccharide chains of the type called glycosaminoglycans (GAGI), which are usually found covalently linked to specific core proteins to form proteoglycans
Figure Molecular Biology of the Cell (© Garland Science 2008)

58. Laminin is a Primary Component of the Basal Lamina
Mice lacking it die during embryogenesis because they are unable to make basal lamina.
Figure 19-42a Molecular Biology of the Cell (© Garland Science 2008)

59. Figure 19-42b Molecular Biology of the Cell (© Garland Science 2008)

60. Type IV collagen is a second essential component of mature basal laminae
Figure Molecular Biology of the Cell (© Garland Science 2008)

61. The basal lamina can act as a selective barrier to the movement of cells, as well as a filter for molecules. The lamina beneath an epithelium, for example, usually prevents fibroblasts in the underlying connective tissue from making contact with the epithelial cells. It does not, however, stop macrophages, lymphocytes,
or nerve processes from passing through it, using specialized protease enzymes to cut a hole for their transit. The basal lamina is also important in tissue regeneration after injury. When cells in tissues such as muscles, nerves, and epithelia are damaged or killed, the basal lamina often survives and provides a scaffold along which regenerating cells can migrate. In this way, the original tissue
architecture is readily reconstructed.