Intercellular junctions

Cell junctions-Types-Gap junctions-tight junctions-anchoring junctions- proteins in cell junctions - applied physiology -cell adhesion molecules- integrins, IgG superfamily, cadherins, selectins, functions of CAMs

1. INTERCELLULAR JUNCTIONS Dr. JILSHA

2. OBJECTIVES To get a better understanding on Definition of cell junctions History Classification Types, their functions, proteins involved and applied physiology Cell adhesion molecules (CAMs) Types of CAMs and functions. 2

3. INTRODUCTION The cell membranes of the neighboring cells are connected with one another through the intercellular junctions or the junctional complexes. Also called as membrane junctions. They mediate two types of interactions : Cell to cell Cell to extracellular matrix Abundant in epithelial tissues.

4. On the molecular level, intercellular junctions consists of three components : Transmembrane adhesive protein Cytoplasmic adapter protein Cytoskeletal filament 4

5. HISTORY Cell–cell junctions in the mammary gland were first described in detail by  Dorothy Pitelka   in a series of classic electron micrographs, both transmission and freeze fracture. 5

6. CLASSIFICATION TIGHT JUNCTIONS GAP JUNCTIONS ANCHORING JUNCTIONS CELL TO CELL Adherens junctions Desmosomes CELL TO MATRIX Focal adhesion Hemidesmosomes 6

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8. TIGHT JUNCTIONS Also called as zonula occludens or the occluding zone. Typically located towards the apical region of the cells. The outer layer of the CM of the neighboring cells fuse with each other that obliterates the intercellular space close to their apical margin. Made up of ridges, half of which is contributed by both neighboring cells and each half is bound tightly to each other . 8

9. Form barrier to the movement of ions and other solutes from the lumen into the interstitial space and between cells. Tight junctions permit the passage of some ions and solute in between adjacent cells ( paracellular pathway) by means of leaky ion and water channels and the degree of this “ leakiness” varies, depending in part on the protein makeup of the tight junction. 9

10. Characteristically seen along the apical margins of cells in epithelium such as the intestinal mucosa, the walls of the renal tubules, and the choroid plexus. 10

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12. PROTEINS OF TIGHT JUNCTIONS Two types : 1. Tight junction membrane proteins or integral membrane proteins, such as occludin , claudin and junctional adhesion molecules ( JAMs). 2. Scaffold (framework or platform) proteins or peripheral membrane proteins or cytoplasmic plaque proteins such as cingulin , symplekin and ZO­1, 2, 3. 12

13. Tight junction membrane protein molecules are anchored in the strands of the ridge and attach with their counterparts of neighboring cell, so that both the cells are held together . The scaffold (platform) proteins are attached with the tight junction membrane proteins and strengthen the anchoring in the ridges. 13

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15. FUNCTIONS OF TIGHT JUNCTIONS STRENGTH AND STABILITY The tight junction holds the neighboring cells of the tissues firmly and thus provides strength and stability to the tissues . SELECTIVE PERMEABILITY The tight junction forms a selective barrier for small molecules and a total barrier for large molecules . 15

16. FENCING FUNCTION Tight junction prevents the lateral movement of proteins (integral membrane proteins) and lipids in cell membrane and thus acts as a fence. The fencing function maintains the different composition of proteins and lipids between the apical and basolateral plasma membrane domains. 16

17. MAINTENANCE OF CELL POLARITY Fencing function of the tight junction maintains the cell polarity by keeping the proteins in the apical region of the cell membrane . 17

18. In the brain, tight junctions between the endothelial cells of cerebral blood vessels contribute to the effectiveness of BBB. In the ciliary bodies, they form blood – aqueous barrier between the cells of inner non – pigmented epithelium. 18

19. APPLIED PHYSIOLOGY Diseases caused by mutation of genes encoding proteins of tight junction : Hereditary deafness Ichthyosis Sclerosing cholangitis Hereditary hypomagnesemia Synovial sarcoma 19

20. GAP JUNCTIONS Gap junction is the intercellular junction that allows passage of ions and smaller molecules between the cells. Gap junctions are called nexus between the cells at which the intercellular space is narrowed from 25 nm to 3 nm. 20

21. Made up of transmembrane proteins known as connexons . The connexons from the membrane of two adjacent cells are lined up with one another. Each connexon is formed by six identical protein subunits called connexins . Connexin surrounds an aqueous channel. When connexon of adjacent cells are aligned, the aqueous channels of both cell membranes become a continuous one. 21

22. This allows substances of adjacent two cells to pass through the channel without passing through the ECF . The diameter of each channel is regulated by intracellular Ca 2 + , pH and voltage . As connexons keep the adjacent cell membranes at a fixed gap, the junction is named as “gap junctions”. 22

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25. TIGHT JUNCTION vs GAP JUNCTION 25

26. FUNCTIONS OF GAP JUNCTIONS Diameter of the channel in the gap junction is about 1.5 to 3 nm. So, the channel permits the passage of glucose, amino acids, ions and other substances, which have a molecular weight less than 1,000. It helps in the exchange of chemical messengers and hormones between the cell. 26

27. Electrical synapses: As the pores of gap junction are larger than ligand-gated or voltage-gated channels, passage of substances is easier through them. Gap junctions easily allow ions to pass through and serve as electrical synapses, as they permit rapid propagation of electrical activity from cell to cell. Therefore, tissues with gap junctions between cells behave as physiological syncytium. 27

28. APPLIED PHYSIOLOGY In humans, there are about 20 different genes that code for connexin proteins. Mutation of these genes results in diseases that are specific for tissues. Few examples are : Deafness Keratoderma Cataract Charcot­ Marie­Tooth disease 28

29. ANCHORING JUNCTIONS Anchoring junctions are the junctions, which provide strength to the cells by acting like mechanical attachments. Anchoring junctions are responsible for the structural integrity of the tissues. P resent in the tissues like heart muscle and epidermis of skin, which are subjected to severe mechanical stress. 29

30. The firm attachment between two cells or between a cell and the extracellular matrix is provided by either actin filaments or the intermediate filaments. Depending upon this, anchoring junctions are classified into four types: 1 . Actin filament attachment Adherens junction (cell to cell) Focal adhesion (cell to matrix) 2 . Intermediate filament attachment Desmosome (cell to cell) Hemidesmosome (cell to matrix) 30

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32. ADHERENS JUNCTION Also called as zonula adherens . Cell to cell junction, which connects the actin filaments of one cell to those of another cell. T he membranes of the adjacent cells are held together by transmembrane proteins called cadherins . Adherens junction is present in the intercalated disks between the branches of cardiac muscles. The adherens junction present in epidermis helps the skin to withstand the mechanical stress. 32

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34. FOCAL ADHESION Focal adhesion is the cell to matrix junctions, which connects the actin filaments of the cell to the extracellular matrix. In epithelia of various organs, this junction connects the cells with their basal lamina. The transmembrane proteins, which hold the cell membrane and the matrix are called integrins . 34

35. DESMOSOME Desmosome is a cell to cell junction, where the intermediate filaments connect two adjacent cells. Also called as macula adherens . Characterized by focal thickening of two adjacent cell membranes. The thickened area is the presence of dense layer of proteins on the cytoplasmic surface of the membrane. Thickened area of two sides is separated by a gap of 25 nm . 35

36. Intermediate filaments from cytosol are attached to the thickened areas . The transmembrane proteins involved in desmosome are mainly cadherins . 36

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38. HEMIDESMOSOME Hemidesmosome is a cell to matrix junction, which connects the intermediate filaments of the cell to the extracellular matrix. In appearance, they look like half of desmosomes, and therefore are called hemidesmosome . Thickening of membrane occurs in only one cell. The proteins involved in this are integrins . 38

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40. APPLIED PHYSIOLOGY Dysfunction of adherens junction and focal junction in colon due to mutation of proteins results in colon cancer. Dysfunction of desmosome causes bullous pemphigoid . Dysfunction of hemidesmosome also causes bullous pemphigoid . The patients develop antibodies against integrins . 40

41. SUMMARY 41

42. CELL ADHESION MOLECULES Cells are attached to each other and to the basal lamina by cell adhesion molecules (CAMs) that are present abundantly at intercellular connections. By their property of adhesions between the cells, they provide stability to the tissue. Involved in stimulating signals that regulate cell differentiation, cell cycle, cell migration and cell survival. 42

43. SYSTEMS OF BINDING Extracellular and intracellular bindings. EXTRACELLULAR : Many of CAMs bind to membrane proteins called laminins . Laminins are cross-shaped large membrane molecules that have multiple receptor domains on the extracellular matrix. CAMs bind to these extracellular receptor domains. 43

44. INTRACELLULAR Cell adhesion molecules (CAMs) pass through the cell membrane to expose into the interior of the cell and attach with the cytoskeleton inside the cell . This intracellular binding of CAMs with cytoskeletal structures enhances strength of cell adhesions 44

45. NATURE OF BINDING Homophilic and Heterophilic bindings. HOMOPHILIC BINDING In homophilic bindings, CAMs attach with similar molecules present on the other cells. HETEROPHILIC BINDING In heterophilic bindings, CAMs attach with different molecules of other cells. 45

46. TYPES OF CAMs Broadly categorized into four varieties: Integrins IgG superfamily proteins cadherins Selectins 46

47. INTEGRINS Heterodimeric glycoproteins that bind to various receptors . Two types : alpha and beta. Forms focal adhesion and hemidesmosomes thus participating in cell to cell adhesion. 47

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49. IgG SUPERFAMILY Calcium – independent transmembrane glycoproteins. Members : ICAM, VCAM – 1, PECAM -1, NCAM. Both homophilic and heterophilic binding. Involved in recognition, binding or adhesion process of cells. 49

50. CADHERINS Calcium-dependent adhesion molecules. Exhibit homophilic binding. These CAMs form adherens junction and desomosome . Subclasses include : Neural, Placental and Epithelial cadherin. Failure of cadherin mediated cell to cell adhesion – breast cancer. 50

51. SELECTINS Divalent cation dependent glycoproteins. They have carbohydrate binding domains. Members : Endothelial selectin , Leukocycte selectin , Platelet selectin . Plays important role in host defense mechanisms. 51

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53. FUNCTIONS OF CAMs They zip cell to cell. Cell adherence is strengthened by CAMs . For their attachment with cytoskeleton, they play role in cell movement . Cellular signals are transmitted out of the cell or extracellular signals are transmitted into the cell via CAMs. 53

54. They play significant role in inflammation and wound healing. CAMs prevent apoptosis. Loss of cellular contact from extracellular matrix due to defects in CAMs hastens the process of apoptosis. 54

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