Extracellular Matrix

Extracellular Matrix - 1

MEDICAL CELL BIOLOGY

November 17, 2010

Extracellular Matrix

Michael D. Henry, Ph.D.Department of Molecular Physiology and Biophysics

6-510 BSB, [email protected]

READING ASSIGNMENT: Alberts et al., Molecular Biology of the CellChapter 19: pp. 1164-1169, 1178-1195

KEY CONCEPTS:

1. The extracellular matrix (ECM) consists of a complex of secreted macromolecules that lie beneath epithelia and surround muscle, nerve and adipose cells (basal lamina) and the connective tissue matrix.

2. The major components of the extracellular matrix are glycosaminoglycans, proteoglycans, collagen and elastic fibers, and large adhesive glycoproteins.

3. Glycosaminoglycans (GAGs) are unbranched polysaccharide chains consisting of repeating disaccharide units. Proteoglycans consist of a core protein linked to GAGs.

4. Collagens are a family of proteins that consist of a triple helix of chains.

5. Crosslinked elastin molecules form elastic fibers that are coated with fibrillin and have the properties of being able to stretch and recoil.

7. Adhesive glycoproteins such as laminin and fibronectin link the cell to the extracellular matrix through receptors.

8. In many cases, the extracellular matrix is an ordered structure that is assembled through associations between various matrix proteins and the involvement of cell surface receptors.

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KEY WORDS:

extracellular matrix (ECM) collagen fibrilstissue collagen fibersbasal lamina elastinglycosaminoglycans (GAGs) elastic fibersproteoglycans fibrillinhyaluronan, hyaluronic acid Marfan’s syndromechondroitin sulfate dystroglycandermatan sulfate fibronectinheparan sulfate RGDkeratan sulfate integrincore protein basement membranesyndecans muscular dystrophyglypicans laminincollagens connective tissue matrixtriple helix osteogenesis imperfectacollagen polypeptide chain chondrodysplasiahydroxyproline Ehlers-Danlos syndrome Type IVhydroxylysine perlecantype I collagen scurvytype II collagen type IX collagentype III collagen type XII collagentype IV collagen nidogentype VII collagen

Lecture Outline:

I. Extracellular Matrix (ECM) (Fig. 19-53)

definition: a mixture of extracellular proteins and polysaccharides that are secreted locally and surround or lie beneath cells in tissues

confers many of the physical properties on tissues, but also provides information regulating cell behavior in tissues

plays crucial roles in development and disease two broad classifications: basal lamina and connective

tissue matrix

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II. Basal Lamina (Fig. 19-39,40)

definition: a specialized sheet-like ECM that is located on the basal surface of epithelia or surrounding muscle, adipose and Schwann cells, a.k.a. basement membrane

in direct contact with cell surface

A. Structure (Figs. 19-41,42a,b, 43) typically 40-120nm thick core ECM protein components: laminins, type IV collagen,

perlecan, nidogen

these proteins associate with one another to form a sheet-like structure

these proteins interact with receptors on the cell surface and through other collagen molecules this basic structure is connected to the underlying connective tissue matrix

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assembly of core ECM components is determined by their binding properties and interactions with cell surface receptors

laminins are cruciform trimers consisting of an ~400KDa subunit and ~200KDa@ and subunits

multiple isoforms combine to produce at least 11 distinct laminin heterotrimers

can self-assemble into sheets in vitro bind to other ECM proteins and integrin and dystroglycan

receptors on the cell surface receptors facilitate organization of basal lamina

B. Functions (Fig. 19-44)

cell polarity mechanical stability tissue barrier selective filter organize ECM proteins promote cell survival, proliferation and differentiation provide surface for cell migration

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regeneration of muscle tissues

C. Clinical correlation: Muscular dystrophy-The dystrophin-glycoprotein complex (DGC) links the cortical actin cytoskeleton to the extracellular matrix in muscle cells. Mutations in the genes encoding many of these components, or affecting the glycosylation of -dystroglycan lead to different forms of muscular dystrophy, ranging from mild to severe forms of the disease. The DGC is thought to stabilize the muscle cell plasma membrane from contraction-induced damage.

III. Components of Connective Tissue Matrix (Fig. 19-54)

bulk constituents of many tissues, biophysical properties contribute to tissue function

produced (secreted) by cells 2 main constituents: glycosaminoglycans and fibrous

proteins

A. Glycosaminoglycans (GAGs) (Figures 19-55,56)

unbranched polysaccharide chains composed of repeating disaccharide unit

usually sulfated; highly negatively charged

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4 main groups: 1) hyaluronan 2) chondroitin/dermatan sulfate 3) heparan sulfate 4) keratin sulfate

form a gel that attracts sodium and water and swells resists compression

hyaluronan (hyaluronic acid) is the only GAG that is not sulfated, it expands with water to occupy a large volume

hyaluronic acid resists compressive forces, important constituent of joint fluid

hyaluronic acid is important in developing tissues where it provides a cell-free space for cells to migrate during organogenesis

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B. Proteoglycans (Figs. 19-58,59,60a,b)

consist of a core protein, to which GAGs (except hyaluronic acid) are covalently linked in the Golgi apparatus to serine

~95% carbohydrate by weight, can be very large core proteins are variable in size and sequence, likewise the number of

attached GAGs is variable, and degree of sulfation can be variable most are secreted, but there are some membrane bound proteoglycans

aggrecan, a major component of cartilage, aggregates with hyaluronic acid to form molecules that are as large as a bacterium

proteoglycans are associated with basal lamina (perlecan) and fibrous matrix proteins

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proteoglycans can form gels of varying pore size and charge density, acting in selective filtration such as in the kidney glomerular basal lamina

proteoglycans regulate cell signaling by binding and controlling the diffusion of growth factors through the extracellular matrix and/or acting as co-receptors with transmembrane signaling receptors (e.g. syndecans or glypicans)

C. Collagens (Figs. 19-62,63,66; Table 19-7) most abundant proteins in mammals

(25% total protein mass), synthesized and secreted primarily by connective tissue cells

primary feature of most collagens is long, stiff triple helix structure, resists shear force

three collagen polypeptide chains wound around one another

rich in proline and glycine residues (Gly-X-Y) repeats (proline is commonly X and hydroxyproline is commonly Y-form stable interchain hydrogen bonds)

42 genes encoding different chains although many combinations are possible, fewer than 40

types of collagen have been described

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several broad classes: fibrillar, fibril-associated, network forming, transmembrane, and proteoglycan

mutations in collagen genes lead to many different diseases fibrillar collagen synthesis begins intracellularly and final

assembly is completed outside of the cell

enzymatic crosslinking between hydroxylysine residues increases tensile strength

fibril-associated proteins organize collagen fibrils into parallel and anti-parallel bundles-collagen fibers

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cells exert tension on collagens which also contributes to organization of extracellular matrix

D. Clinical Correlations:1) Genetic diseases involving collagen mutations:type I collagen - osteogenesis imperfecta, characterized by weak bonestype II collagen - chondrodysplasia, results in defective cartilage, bone and joint deformitiestype III collagen - Ehlers-Danlos syndrome Type IV, characterized by weak blood vessels and organs, rupture of aorta or intestine

2) Scurvy: Vitamin C deficiency results in reduced activity of proline hydroxylase enzyme, failure of proline hydroxylation results in impaired triple-helix formation and collagens are degraded intracellularly, results in gradual loss of collagensblood vessels become fragile and teeth become loose in sockets.

E. Elastin (Fig. 19-71)

a network of elastic fibers in tissues such as skin, blood vessels and lungs provide ability to recoil after stretching

main component is elastin, but long collagen fibrils limit extent of stretching to prevent tearing

elastin protein is rich in glycine and proline but little hydroxyproline and no hydroxylysine and is not glycosylated

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elastin molecules are cross-linked through lysines to form fibrils

this network can adopt a loose random-coil configuration which provides elasticity

elastin fibril assembly facilitated by associated proteins such as fibrillin

aorta is 50% elastin by weight

F. Clinical Correlation: Marfan’s syndrome: defect in the fibrillin gene leads to defects in elastin. Affected individuals are tall, have hyperextensible limbs and their aorta is prone to rupture. Abraham Lincoln may have been affected.

G. Fibronectin (Figs. 19-72b,c,73,74)

composed of a disulfide-linked dimer of fibronectin monomers

contains numerous type III fibronectin repeats which bind to integrins on cell surfaces through RGD (Arg-Gly-Asp) sequence

tension exerted by cells through integrins regulated extracellular fibronectin assembly

fibronectin important for cell attachment to the extracellular matrix, cell migration, development and wound healing

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Extracellular Matrix