MULTICELLULAR ORGANISMS

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MULTICELLULAR ORGANISMS

• Cell-Cell Adhesion
• Cell-Matrix Adhesion
• The Extracellular Matrix, ECM

Http//www.plab.ku.dk/bock/index.htm Link
Multicellular organisms 2003
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MULTICELLULAR ORGANISMS

• The appearance of multicellular organisms allows
  specialization of cells and formation of organs
• Vertebrates have more than 100 specialized cell
  types (plants have more than 15)
• A special matrix, the extracellular matrix, ECM,
  fills out the space between cells
• ECM also binds cells together, acts as reservoir
  for growth factors and hormones, and creates an
  environment in which molecules and cells can
  migrate

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MULTICELLULAR ORGANISMS

• By means of cell adhesion molecules, CAMs, cells
  are capable of recognizing each other
• Plasma membrane receptors take care of cell-ECM
  interactions

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CELL JUNCTIONS

• Adherens junctions
• Gap junctions
• Tight junctions
• Desmosomes/Hemidesmosomes
• Focal adhesions

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CELL-CELL ADHESION MOLECULES

• Cadherins
• Ig superfamily CAMs
• Selectins
• Integrins
• Connexins
• Occludin and claudin proteins

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NEURONAL CELL ADHESION MOLECULESLEARNING AND
MEMORY

• Male humans with L1-mutations develop
• Mental retardation
• Hydrocephalos
• Adducted thumbs
• NCAM knock-out animals develop
• Morphological changes in bulbus olfactorius and
  hippocampus
• Impaired learning
• Emotional disturbances
• Modulation of NCAM and L1-function interferes
  with LTP and learning and memory

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CADHERINS

• A family of Ca2-dependent CAMs
• Ca2 causes dimerization of Cadherins
• The binding is homophilic

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CELL ADHESION MOLECULES AND DISEASES

• Auto antibody against the cadherin desmoglein
  induce the skin disease Pemphigus vulgaris
• Mutations of claudin 16 affect paracellular flow
  of Mg2 in the kidney
• Mutations of claudin 14 lead to deafness due to
  an altered transport around hair cells in the
  cochlea
• Several bacterial toxins affect tight junctions
  leading to increased paracellular transport

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SELECTINS

• Selectins are involved in extravasation
• Inflammatory signals activate endothelial cells
  making P-Selectin undergo exocytosis
• P-Selectin on the surface of endothelial cells
  binds a specific carbohydrate ligand (Sialyl
  Lewis -x) on leukocytes
• The leukocytes attach to the endothelial wall and
  roll slowly on it
• PAF and integrins are then activated and the
  leukocytes start to extravasate

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GAP JUNCTIONS 1

• A cluster of channels between two plasma
  membranes
• Each membrane contain a hemichannel called a
  connexon made of 6 subunits - connexins
• There are 12 different connexin genes
• Usually connexons are hetero-oligomeric and the
  composition determines permeability

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GAP JUNCTIONS 2

• Allow particles of lt 1.2 nm in diameter to pass
• Ions, ATP, cAMP can pass I.e. hormonal
  stimulation of one cell can spread to connected
  cells, and thereby organize coordinated functions
  such as secretion, contraction, movement of cilia
• The channels close at increased Ca2
  concentrations allowing regulation of the degree
  of coupling to surrounding cells

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CONNEXIN DISEASES

• Mutations in several connexin genes are
• accompanied by
• Deafness
• Cataract
• Heart malformations
• Charcot-Marie-Tooth (degeneration of peripheral
  nerves)

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CELL-MATRIX ADHESION

• Integrins
• Collagens
• Laminin and Fibronectin
• Proteoglycans and Glucosaminoglycans

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CELL MATRIX ADHESION

• Integrins on the cell surface mediate cell-ECM
  binding
• Integrins are composed of an??- and a ?-chain
• There are 3 different ?-chains and more than 10
  types of ?-chains
• The chain composition determines the ligand
  specificity
• The affinity is generally low (Kd 10-6 -10-8)

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INTEGRINS

• Integrins can be activated through a signal from
  the interior of the cell
• Activation involves conformational changes of
  the integrin
• Various integrins recognize specific sequences in
  their ligands. E.g. ?4?1 recognizes EILDV (in
  VCAM-1 and in fibronectin) and ?5?1 recognizes
  RGD in many ECM proteins

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INTEGRIN CONTAINING JUNCTIONS

• A junction consists of an exterior ligand, a
  transmem-brane protein, a linker, and a
  cytoskeletal component
• An adherence junction connects an ECM component
  with an integrin linked to an adapter (e.g.
  vinculin) and F-actin
• A hemidesmosome connects an ECM-component to
  integrin and via an adapter (e.g. plectin) to
  intermediate filaments (keratins)

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INTEGRIN DISEASES

• Genetic defects in integrin ?2 lead to
  leucocyte-adhesion deficiency. The patient
  becomes susceptible to bacterial infections

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DISINTEGRINS

• Disintegrins contain the RGD sequence and
  interfere with integrin-ECM adhesion allowing
  deadhesion and cell migration
• The ADAMs (A Disintegrin And a Metalloprotease)
  remodel surface proteins f.x. at the fusion of
  sperm and egg, the fusion of myoblasts during
  myogenesis, release of TNF? from the surface

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COLLAGENS

• The most abundant animal protein
• At least 16 types exist
• The structural unit is composed of three 300 nm
  long coiled subunits in a triple helix
• The helical structure depends on the abundant
  presence of glycin, proline (and hydroxyproline)
  making a motif gly-pro-x, which is necessary for
  twisting together the three strands

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COLLAGENS 2

• Collagens are synthesized as precursors called
  procollagens
• They are glycosylated in ER and Golgi adding Gal
  and Gly to hydroxy-lysine residues and long
  oligosaccharides to selected asparagine residues
• Proline and lysine are hydroxylated
• Disulphide bonds are made between the N- and
  C-terminal parts of the propeptides
• After exocytosis the N- and C-terminals are
  trimmed, only then can the fibrils be formed

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COLLAGENS 3

• Lack of vitamin C prevents hydroxylation ?
  impaired fibrils
• Mutations or deletions of ?-chains in Collagen I
  can lead to the disease Osteogenesis imperfecta

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LAMININ

• Laminin is a key component of the basal lamina

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DISEASES OF THE BASAL LAMINA

• Alports syndrome appears as impaired
  ultrafiltration in the kidney resulting in renal
  failure and hearing loss. Mutations in collagen
  IV ?-chains result in this syndrome.
• Antibodies against ?3-chains of collagen IV lead
  to pulmonary hemorrhage and renal failure
  (Goodpastures syndrome)

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FIBRONECTIN

• Fibronectins attach cells to collagens
• Fibronectins are dimers
• Fibronectins express the RGD sequence recognized
  by integrins

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PROTEOGLYCANS 1

• The Polysaccharides in proteoglycans are long
  repeating polymers of dissacharides called
  Glucosaminoglycans (GAGs)
• One sugar of the dissacharides is a uronic acid
  and the other is an aminosugar (e.g.
  N-acetylglucosamine)
• One or both sugars contain one or two sulphate
  residues

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PROTEOGLYCANS 2

• Heparin sulphate and chondroitin sulphate are
  added to a 3-sugar linker (Xyl-Gal-Gal) added
  to a Serine in the core protein
• Proteoglycans are found both in ECM and attached
  to the plasma membrane

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PROTEOGLYCANS IN THE ECM

• In cartilage the key proteoglycan is aggrecan
• The central component of aggrecan is a
  carbohydrate, hyaluronan
• At 40 nm intervals aggrecan core proteins are
  attached (assisted by a linker protein) to a
  decasaccharide sequence in hyaluronan
• Attached to the aggrecan core protein are
  multiple GAGs (via the trisaccharide linker)
• The GAGs in aggrecan are chondroitinsulphate and
  keratin sulphate
• MW of an aggrecan 2 x 108

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PROTEOGLYCANS ON THE CELL SURFACE

• A typical example is syndecan
• The core protein spans the membrane with a short
  cytosolic domain
• The GAGs are attached via the trisaccharide
  linker to serine residues
• The GAGs in syndecan are heparan sulphate chains
• Syndecan binds extracellularly to collagens and
  fibronectin and intracellularly to the
  cytoskeleton

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HYALURONAN (HA)

• HA is a GAG found in ECM
• HA is also a key component of complex
  proteoglycans
• HA consists of approx. 50,000 disaccharides in a
  random coil. It can be bound to the surface
  receptor CD44
• HA gives strength, flexibility and smoothness to
  the ECM and forms a viscous hydrated gel in which
  cells can migrate
• HA makes the ECM able to resist compression

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DISEASES OF GAG
Rare genetic defects in enzymes required for the
synthesis of Dermatan sulfate lead to defects in
bones, joints, muscles, and skin. The individuals
do not grow to normal hight and appear
prematurely aged.