Extracellular matrix and cell-cell interaction Animal systems: Extracellular components – cell matrix adhesion, collagens – types of collagens, elastins, basal lamina and its components, connective tissues, proteoglycans and laminin. – Cell - cell adhesion, cadherins, CAMS (NCAMS), selectins, integrins, desmosomes, hemidesmosomes, – tight and gap junction, Catenins, actins, Tubulins, intermediate filaments, – glycosaminoglycans.

1. Extracellular matrix and cell-cell interaction
Animal systems: Extracellular components
– cell matrix adhesion, collagens
– types of collagens, elastins, basal lamina and its components,
connective tissues, proteoglycans and laminin.
– Cell - cell adhesion, cadherins, CAMS (NCAMS), selectins, integrins,
desmosomes, hemidesmosomes,
– tight and gap junction,
Catenins, actins, Tubulins, intermediate filaments,
– glycosaminoglycans.
Dr. M. THIPPESWAMY

2. Extracellular matrix (ECM). Typical components include
collagen, proteoglycans, fibronectin and laminin.
Interactions between a cell
and its environment or with
other cells are governed by
cell-surface proteins.
EXTRACELLULAR MATRIX (ECM)

3. The extracellular matrix is a network of proteins and carbohydrates that supports and
surrounds the cells in connective tissues.
The matrix proteins are bound with the specific cell surface receptors resulting in the cell–
matrix adhesion, which exerts effect on cell shapes, migration, proliferation, cell survival,
and metabolism.
The extracellular matrix also includes highly specialized structures, such as cartilage,
tendons, basement membranes, and also (with secondary deposition of calcium phosphate
crystals) bones and teeth.
• DEFINITION
– A substance containing collagen, elastin, proteoglycans, glycosaminoglycans, and
fluid, produced by cells and in which the cells are embedded.
EXTRACELLULAR MATRIX (ECM)

4. DEFINITIONS
• Extracellular constituents; All of the constituents of the body outside the
cells; include water, electrolytes, protein, glucose, enzymes, hormones.
• Extracellular fluid; All of the body fluid lying outside the cells. Includes
intravascular fluid or plasma and the interstitial fluid. That part of the
extracellular fluid that is in special cavities which have special characteristics,
e.g. synovial fluid, urine, aqueous humor of eye, are called transcellular fluids.
• Extracellular matrix The network of proteins and carbohydrates that
surround a cell or fill the intercellular spaces.
• Extracellular/Intercellular space The space outside the cell.

5. EXTRACELLULAR MATRIX (ECM)
• Secreted by cells therefore reflect the properties of the particular
tissue
– Bone matrix The intercellular substance of bone, consisting of
collagenous fibers, ground substance, and inorganic salts.
– Cartilage matrix The intercellular substance of cartilage, consisting
of cells and extracellular fibers embedded in an amorphous ground
substance.
– Skin matrix Contains more elastic fibers along with
glycosaminoglycans and ground substance

6. ECM: FUNCTIONS
• Mechanical support for cells and tissues.
• Influences cell shape, movement, development and differentiation.
• Coordinates cellular functions through cell to cell signaling with adhesion
receptors (integrin).
• Reservoir for extracellular signaling molecules
ECM: COMPONENTS
1. Fibrous elements
– Collagen, elastin
2. Link proteins 3. Space filling molecules
– Fibronectin, laminin Proteoglycans and glycosaminoglycans.

7. The Extracellular Matrix I: The Basal Lamina
Three types of molecules are abundant in the extracellular matrix of all tissues.
• Highly viscous proteoglycans, a group of glycoproteins that cushion cells and bind a wide variety
of extracellular molecules
• Collagen fibers, which provide mechanical strength and resilience
• Soluble multiadhesive matrix proteins, which bind to and cross-link cell-surface adhesion
receptors and other ECM components

8. ECM components in the basal lamina are synthesized by the cells that rest on it. Four
ubiquitous protein components are found in basal laminae.
• Type IV collagen, trimeric molecules with both rodlike and globular domains that
form a two-dimensional network
• Laminins, a family of multiadhesive proteins that form a fibrous two-dimensional
network with type IV collagen and that also bind to integrins.
The Extracellular Matrix I: The Basal Lamina
• Entactin (also called nidogen), a rodlike
molecule that cross-links type IV collagen and
laminin and helps incorporate other
components into the ECM
• Perlecan, a large multidomain proteoglycan
that binds to and cross-links many ECM
components and cell-surface molecules

9. The “basal lamina” and “basement membrane” are
frequently confused by students and professionals alike.
The basement membrane was discovered first as a very
thin layer of connective proteins just beneath an epithelial
cell layer.
The basal lamina was not discovered until later because it
is not visible by light microscopy (normally only ~50 nm
thick).
Technically, the basal lamina, which consists of multiple
layers itself, is a layer of ECM proteins secreted by the
epithelial layer.
The basal lamina and a thick reticular lamina (ECM
secreted by other cell types) together form what is
considered the basement membrane.
Confusion of “basal lamina” and “basement membrane”
The basal lamina acts as a point of
attachment for cells and function as a
permeability barrier in the glomerulus
(urine production)
Basal lamina
It function as cell-matrix adhesions
through substrate adhesion
molecules (SAMs).
Basal membrane

10. ECM: FIBROUS ELEMENTS - COLLAGEN
Most abundant animal protein
• insoluble tensile fibers
• Basic structure is Gly-X-Y
• Special amino acids hydroxylated
lysine and proline
• 19 types of collagen
• Modified/ different as per the requirements
– Bones
– Cartilage
– Subcutaneous tissue
– Articular capsules
The collagen triple helix.

11. SYNTHESIS OF COLLAGEN

12. TYPES OF COLLAGEN

13. The adverse affect of collagen formation
can lead to serious disease conditions
Form of epidermolysis bullosa (the heritable skin blistering disease) is caused by mutation
in collagen VII which is primarily produced by epidermal keratinocytes and secreted into
the dermal epidermal basement membrane layer.
A variety of chondrodysplasias as well as bone malformations such as osteogenesis
imperfecta (which can be perinatally lethal) have been linked to mutations in various
collagen genes.
Several symptoms of scurvy are due to malformation of collagen in the ECM: weak blood
vessel walls, bleeding gums and loose teeth, and fragile bones.
Scurvy is a disease of ascorbic acid (vitamin C) deficiency, and the effect on ECM is due to
the need for ascorbic acid as a cofactor for enzymes that hydroxylate the prolines and
lysines of collagen.

14. ECM: FIBROUS ELEMENTS ELASTIN
Fibrous protein
• Extensibility and elastic recoil
• Large amount in lungs, large arterial blood
vessels, and some elastic ligaments
• Small amounts in skin and ear cartilage
• Major cross-links are the desmosines, which result from the condensation
of four lysine residues to form a tetra-functional crosslink.
• Highly insoluble, stable and a very low turnover rate.
• Random coil conformations that permit the protein to stretch and
subsequently recoil

15. I. Microfibrils are first formed by linear and
lateral fibrillin-1 interactions.
II. MAGP-1 then associates onto micro fibril bead
surfaces via an interaction with N-terminal
fibrillin-1 domains.
III. Tropoelastin is then deposited on an inter bead
region adjacent to the beads through strong
interactions with the fibrillin-1 central
sequence and subsequently becomes cross-
linked to fibrillin-1. Tropoelastin and MAGP-1
may then interact on micro fibrils.
IV. Further deposition of tropoelastin to micro
fibril-bound tropoelastin and MAGP-1
followed by lysyl oxidase cross-linking
V. Formation of mature elastic fiber
Model of elastic fiber formation
Pathophysiological class of elastin gene mutation
leading to autosomal dominant cutis laxa.

16. S. No. Collagen Elastin
1 Many different genetic types One genetic type
2 Triple helix No triple helix; random coil
conformations permitting stretching
3 (Gly-X-Y)n repeating structure No (Gly-X-Y)n repeating structure
4 Presence of hydroxylysine No hydroxylysine
5 Carbohydrate-containing No carbohydrate
6 Intramolecular aldol cross-links Intramolecular desmosine cross-links
7 Presence of extension peptides
during biosynthesis
No extension peptides present during
biosynthesis
DIFFERENCES BETWEEN COLLAGEN AND ELASTIN

17. • Fibronectin (FN) is a protein encoded by a single gene that, through alternative splicing, generates its two
major forms: cellular FN and plasma FN.
• The cellular FN is an insoluble glycoprotein produced by some epithelial cells, fibroblasts, macrophages and
endothelial cells, among others.
• It is involved in several cellular processes such as embryogenesis, tissue repair, cell migration and adhesion.
• It links the different components of the extracellular matrix (ECM) and helps to organize the cellular
interaction with the matrix.
• Furthermore, it serves as an adhesion molecule, anchoring cells and pathogens. The plasma FN is a soluble
form secreted by hepatocytes.
• The functional form of FN comprises two similar subunits of 220-250 kDa that together form a dimer
maintained by antiparallel disulfide bonds.
• The protein is formed by a sequence of modular structures that are organized to form different binding sites
for integrins, collagen, heparin, FN itself and other extracellular molecules.
ECM: Link protein - Fibronectin

18. Structure of Fibronectin
The classifications are designated fibronectin type I, II, and III repeats, on the basis of
similarities in 2446 amino acid sequence.
One of the type III repeats in the cell-binding region of fibronectin mediates binding
to certain integrins.
The tripeptide sequence Arg-Gly-Asp, usually called the RGD sequence, for
recognition by those integrins.
There are at least 20 different fibronectin isoforms in humans.

19. Fibronectin is essential for embryonic development
• Gene targeting => complete lack of fibronectin
• Embryonic lethal.
• Gross malformations and heart malformation
• Problems in cell adhesion, migration and differentiation

20. • Laminins (at least 15 isoforms identified so far) are cross-shaped trimeric adhesive glycoproteins that have
different domains to specifically bind to cells, type IV collagen, nidogens, and some glycosaminoglycans.
• Laminins, just as type IV collagen and fibronectin, are components of basement membranes.
• Laminins mediate the attachment of parenchymal cells to type IV collagen thereby providing the interaction
between cells and basement membranes.
• Other extracellular matrix glycoproteins are nidogens, tenascins, and fibulins.
• Nidogens (entactins) bind to both laminin and type IV collagen forming the additional connection between
laminins and collagen.
• Tenascin family of proteins (tenascin-C, -X, -R, and -W) can bind fibronectin, tenascins have both cell
adhesive and antiadhesive functions depending on the cell type.
• Fibulins can interact with many matrix components, such as some basement membrane proteins, fibronectin,
fibrillin, and proteoglycans, to form supramolecular structures within the matrix.
ECM: Link protein - Laminin

21. Schematic model showing the general shape,
location of globular domains and coiled-coil
region in which laminin’s three chains are
covalently linked by several disulfide bonds.
Different regions of laminin bind to cell-
surface receptors and various matrix
components.
Electron micrographs of intact laminin
molecule, showing its characteristic cross
appearance (left) and the carbohydrate
binding LG domains near the C-terminus
(right)
Structure of Laminin

22. PROTEOGLYCANS & GLYCOSAMINOGLYCANS GAGS
Proteoglycans are proteins that contain covalently linked
• Core proteins are covalently bound to GAGs.
• Seven types of GAGs: hyaluronic acid, chondroitin sulfate, keratan
sulfates I and II, heparin, heparan sulfate, and dermatan sulfate.
• GAG is an unbranched polysaccharide made up of repeating
disaccharides.
• One component of which is always an amino sugar and the other is
mostly a uronic acid

23. PROTEOGLYCANS & GLYCOSAMINOGLYCANS (GAGS)
• Negatively charged
• Attract and retain water
• Provide medium for ion, nutrient and
mineral exchange

24. The repeating disaccharides of glycosaminoglycans (GAGs), the polysaccharide
components of proteoglycans.

25. Gags Sugars Location
Hyaluronic Acid GIcNAc, GlcUA Synovial fluid, vitreous humor, loose
connective tissue
Chondroitin Sulfate GaINAc, GlcUA associated with HA via link proteins
Cartilage, bone, cornea
Keratan sulfate I GlcNAc, Gal Cornea
Keratan sulfate II GlcNAc, Gal Loose connective tissue
Heparin GlcN, IdUA Mast cells
Heparan sulfate GlcN, GlcUA Skin fibroblasts, aortic wall
Dermatan Sulfate GalNAc, IdUA Wide distribution
PROTEOGLYCANS & GAGS

26. • Proteoglycans consist of membrane-associated or secreted core proteins covalently
linked to one or more glycosaminoglycan (GAG) chains, which are linear polymers of
sulfated disaccharides.
• Hyaluronan, a highly hydrated GAG, is a major component of the ECM of migrating
and proliferating cells. Certain cell-surface adhesion receptors bind hyaluronan to cells.
• Large proteoglycan aggregates containing a central hyaluronan molecule noncovalently
bound to the core protein of multiple proteoglycan molecules (e.g., aggrecan)
contribute to the distinctive mechanical properties of the matrix.
PROTEOGLYCANS & GLYCOSAMINOGLYCANS (GAGS)

27. Cell–Cell and Cell–Matrix Adhesion: An Overview

28. 1. The apical surface of these cells is packed with fingerlike microvilli projected intestinal lumen.
2. The basal surface rests on extracellular matrix (ECM).
3. The ECM associated with epithelial cells is usually organized into various interconnected layers.
4. Cell-adhesion molecules (CAMs) bind to CAMs on other cells, mediating cell–cell adhesions.
5. Both types of cell-surface adhesion molecules are usually integral membrane proteins bind to
multiple intracellular adapter proteins. Specific aggregates of CAMs or adhesion receptors form
various types of cell junctions that play important role for cell communication between cells.
6. Tight junctions, lying just under the microvilli, prevent the diffusion substaces between the cells.
7. Gap junctions allow the movement through connexon channels of small molecules and ions
between the cytosols of adjacent cells.
8. The remaining three types of junctions, adherens junctions.
9. Spot desmosomes and hemidesmosomes link the cytoskeleton of a cell to other cells or the ECM.
Cell–Cell and Cell–Matrix Adhesion: An Overview

29. • Cell-adhesion molecules (CAMs): Proteins in
the plasma membrane of cells that bind similar
proteins on other cells, thereby mediating cell-
cell adhesion.
• Four classes of CAMs include cadherins,
IgCAMs, integrins and selectins
Cell-adhesion molecules (CAMs) and adhesion receptors
• Lateral interactions between cell-adhesion molecules (CAMs) within the plasma membrane of a cell
form dimers and larger oligomers.
• The parts of the molecules that participate in these cis interactions vary among the different CAMs.
Subsequent trans interactions between distal domains of CAMs on adjacent cells generate a
zipperlike strong adhesion between the cells.

30. Dimeric E-cadherins most commonly form
homophilic cross-bridges with E-cadherins on
adjacent cells.
Members of the immunoglobulin (Ig) superfamily of
CAMs (NCAMs) can form both homophilic linkages
and heterophilic (nonself) linkages.
Selectins, shown as dimers, contain a carbohydrate-
binding lectin domain that recognizes specialized
sugar structures on glycoproteins and glycolipids on
adjacent cells.
Heterodimeric integrins function as CAMs or as
adhesion receptors that bind to very large,
multiadhesive matrix proteins such as fibronectin.
The cytoplasmic domains of these proteins are often
associated with adapter proteins that link them to the
cytoskeleton or to signalling pathways.
Cell-adhesion molecules (CAMs) and adhesion receptors

31. The cadherins are homophilic Ca2+ -dependent glycoproteins.
The classic cadherins (E-, N- and P-) are concentrated at the
intermediate cell junctions, which link to the actin filament
network through specific linking proteins called catenins.
Each cadherin exhibits a unique pattern of tissue distribution,
such as epithelial (E-cadherins), placental (P-cadherins),
neural (N-cadherins), retinal (R-cadherins), brain (B-
cadherins and T-cadherins), and muscle (M-cadherins).
Many cell types express combinations of cadherin types. The
extracellular domain has major repeats called extracellular
cadherin domains (ECD). Sequences involved in Ca2+
binding between the ECDs are necessary for cell adhesion.
The cytoplasmic domain has specific regions where catenin
proteins bind.
Adherens Junction. This type of cell-cell adhesion is
based on interaction of cadherins, which are
connected intracellularly to the actin cytoskeleton
through the linker proteins a- and b- catenin. Also
depicted here is the actin bundling protein a-actinin.
Cadherins

32. • N-CAM 180 and N-CAM 140 are anchored in the membrane by a single hydrophobic α helix and differ in the
length of their cytoplasmic domains.
• N-CAM 120 is attached to the membrane by a glycosylphosphatidylinositol (GPI) anchor.
• Each of these three N-CAMs can also vary in the length of the poly α (2→8) sialic acid chain, whose
attachment site is indicated
• N-CAMs, a group of Ca2+-independent cell-cell
adhesion proteins belong to the Ig superfamily
of CAMs.
• Their full name - nerve-cell adhesion molecule
reflects their particular importance in nervous
tissue.
• Like N-cadherin, N-CAMs during
morphogenesis, playing an important role in
differentiation of muscle, glial, and nerve cells.
(Ig) superfamily of CAMs (NCAMs)

33. Integrin receptors are composed
of two polypeptides that each
pass through the membrane once
Integrin
• Integrins are transmembrane receptors that facilitate cell-extracellular matrix
(ECM) adhesion.
• Upon ligand binding, integrins activate signal transduction pathways that
mediate cellular signals such as regulation of the cell cycle, organization of
the intracellular cytoskeleton, and movement of new receptors to the cell
membrane.
• The presence of integrins allow rapid and flexible responses to events at the
cell surface
• Integrins work alongside other receptors such as cadherins, the immunoglobulin superfamily cell adhesion
molecules, selectins and syndecans to mediate cell–cell and cell–matrix interaction. Ligands for integrins
include fibronectin, vitronectin, collagen and laminin.
• Integrins are obligate heterodimers, meaning that they have two subunits: α (alpha) and β (beta).
• Extracellular side, there is a metal ion coordination site usually occupied by Mg++, that is necessary for ligand
binding.

34. Selectins
• Selectins bind heterophilically to oligosaccharide
moieties on glycoproteins.
• In fact the name of the family is based on lectin, a
generic term for proteins that bind sugars.
• From C-terminus to N-terminus, a cytoplasmic
domain, a transmembrane domain, a series of CR
(consensus repeat) or structural domains (from 2 in
L-selectin to 9 in P-selectin), an EGF (epidermal
growth factor) -like domain, and a lectin-like
domain.
• The lectin-like domain binds a specific oligosaccharide composed of sialic acid, galactose, GlcNAc, and
fucose, of which the sialic acid and fucose are most important for recognition.
• Selectin-mediated adhesion is a Ca2+ -dependent process.
• L-selectin, which is found on leukocytes, E-selectin found on endothelial cells and P-selectin found in platelets.

35. Principal types of cell junctions
• The basal surface of the cells rests on a basal
lamina, and the apical surface is packed with finger
like microvilli that project into the intestinal
lumen.
• Tight junctions, lying just under the microvilli,
prevent the diffusion of many substances between
the intestinal lumen and the blood through the
extracellular space between cells.
• Gap junctions allow the movement of small
molecules and ions between the cytosols of
adjacent cells.
• The remaining three types of junctions-adherens
junctions, spot desmosomes, and
hemidesmosomes-are critical to cell-cell and cell-
matrix adhesion and signaling.

36. Cell junctions
JUNCTION Adhesion
type
Principlal CAMs or
Adhesion receptors
Cytoskeletal Attachment Function
Anchoring junctions
1. Adherens Junctions Cell - Cell Cadherins Actin filaments Shape, tension,
signalling
2. Desmosomes Cell – Cell Desmosomal
cadherins
Intermediate filaments Strength, durability,
signalling
3. Hemidesmosomes Cell - matrix Integrin Intermediate filaments Shape, rigidity,
signalling
Tight junctions Cell - Cell Occludin, claudin,
JAMs
Actin filaments Controlling solute flow,
signalling
Gap Junctions Cell - Cell Connexins, innexins,
pannexins
Possible indirect
connections to
cytoskeleton through
adapters to other junctions
Communication; small-
molecule transport
between cells

37. • Adherens junctions (zonula adherens, intermediate junction, or "belt
desmosome") are protein complexes whose cytoplasmic face is
linked to the actin cytoskeleton.
• Principal interactions of structural proteins at cadherin-based
adherens junction.
• Actin filaments are associated with adherens junctions in addition to
several other actin-binding proteins such as vinculin.
•
• The head domain of vinculin associates to E-cadherin via α-, β - and
γ -catenins.
• The tail domain of vinculin binds to membrane lipids and to actin
filaments.
Adherens junctions

38. desmosomes
Cell adhesion in desmosomes
Desmosomes
• Cells will form adhesive interactions with other cells as well as with ECM.
• Most of these interactions utilize a different set of proteins, although
integrins have been found to interact with some cell adhesion proteins.
• An example of a cell-cell interaction with many similarities to a cell-ECM
interaction, but using different adhesion molecules, is the desmosome.
• A desmosome ("binding body"), also known as a macula adhaerens, is a
cell structure specialized for cell-to-cell adhesion. A type of junctional
complex, they are localized spot-like adhesions randomly arranged on the
lateral sides of plasma membranes.
• Like its basal-lamina-attached couterpart, the hemidesmosome, the
desmosomes are necessary for the structural integrity of epithelial layers,
and are the most common cell-cell junction in such tissues.

39. Hemidesmosomes are
integrin-containing
anchoring junctions that
attach cells to elements of
the underlying extra
cellular matrix.
Hemidesmosomes consist
of cytosolic keratin, non-
covalently bonded to a
cytosolic plectin plaque,
which is bonded to a
single-pass transmembrane
adhesion molecule such as
the integrin.
The integrin might then attach to one of many multi-adhesive proteins such as laminin, resident within the
extracellular matrix, thereby forming one of many potential adhesions between cell and matrix.
Hemidesmosomes

40. Tight junctions
• Tight junctions block the diffusion of proteins and some lipids in
the plane of the plasma membrane. They also limit and regulate
the extracellular flow of water and solutes from one side of the
epithelium to the other.
• The tight junction might be formed by the linkage of rows of
protein particles in adjacent cells.
• In the inset micrograph of an ultrathin sectional view of a tight
junction, the adjacent cells can be seen in close contact where the
rows of proteins interact.
• The major proteins in tight junctions, both occludin and claudin-
1 contain four transmembrane helices, whereas the junction
adhesion molecule (JAM) has a single transmembrane domain
and a large extracellular region.

41. Gap junctions
• Gap junctions are constructed of multiple copies of connexion proteins,
assembled into a transmembrane channel that interconnects the cytoplasms
of two adjacent cells.
• Small molecules and ions can pass through gap junctions, permitting
metabolic and electrical coupling of adjacent cells.
• The gap junction comprises a cluster of channels between two plasma
membranes separated by a gap of about 2–3 nm.
• Both membranes contain connexion hemichannels, cylinders of six
dumbbell shaped connexin molecules.
• Two connexons join in the gap between the cells to form a gap-junction
channel, 1.5–2.0 nm in diameter, that connects the cytosols of the two cells.
• Electron density of a recombinant gap-junction channel determined by
electron crystallography. M-membrane bilayer; E-extracellular gap; C-
cytosol.

42. • Abundant structural protein in eukaryotic cells that
interacts with many other proteins. The monomeric
globular (G-actin) polymerizes to form actin filaments
(F-actin). In muscle cells F-actin interacts with myosin
during contraction.
• Cell adhesions link cells to the extracellular matrix
(ECM) and to each other and depend on interactions
with the actin cytoskeleton.
Intermediate filaments of ECM and cell junctions
• β-catenin is a dual function protein, involved in regulation and
coordination of cell–cell adhesion and gene transcription.
• Beta-catenin is widely expressed in many tissues. In cardiac muscle,
beta-catenin localizes to adherens junctions funtions as electrical and
mechanical coupling between adjacent cardiomyocytes.
• Mutations and overexpression of β-catenin are associated with many
cancers, including hepatocellular carcinoma, colorectal carcinoma,
lung cancer, malignant breast tumors, ovarian and endometrial cancer.
Actin

43. • Cell shape is determined by micro scaffolding
created by microtubules and microfilaments.
• Internally, these tubules and filaments make up
the cytoskeleton.
• Externally they are the components of the
extracellular matrix
• Microtubules have a diameter of about 24 nm and are usually found in groups of 13 protofilaments.
• The basic structural subunit is a dimer of two similar proteins, and tubulin, each with a molecular weight of
55,000 daltons.
• Tubulin and several larger microtubule-associated proteins (MAPs) interact in the presence of GTP and Mg to
form hollow tubes known as protofilaments which can stretch for long distances across the cytoplasm.
Intermediate filaments of ECM and cell junctions Tubulin

44. THANK YOU