MULTICELLULAR ORGANISMS Nov 2003

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


       Http://www.plab.ku.dk/bock/index.htm
         Link: Multicellular organisms 2003
        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
       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
        CELL JUNCTIONS

•   Adherens junctions
•   Gap junctions
•   Tight junctions
•   Desmosomes/Hemidesmosomes
•   Focal adhesions
        CELL-CELL
    ADHESION MOLECULES
•   Cadherins
•   Ig superfamily CAMs
•   Selectins
•   Integrins
•   Connexins
•   Occludin and claudin proteins
NEURONAL CELL ADHESION MOLECULES
      LEARNING 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
            CADHERINS

• A family of Ca2+-dependent CAMs
• Ca2+ causes dimerization of Cadherins
• The binding is homophilic
  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
               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
        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
       GAP JUNCTIONS 2

• Allow particles of < 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
     CONNEXIN DISEASES
Mutations in several connexin genes are
accompanied by:
– Deafness
– Cataract
– Heart malformations
– Charcot-Marie-Tooth (degeneration of
  peripheral nerves)
           CELL-MATRIX
            ADHESION

•   Integrins
•   Collagens
•   Laminin and Fibronectin
•   Proteoglycans and Glucosaminoglycans
 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)
               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
   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)
      INTEGRIN DISEASES

• Genetic defects in integrin 2 lead to
  leucocyte-adhesion deficiency. The patient
  becomes susceptible to bacterial infections
             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
             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
           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
          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
                LAMININ
•Laminin is a key component of the basal lamina
DISEASES OF THE BASAL LAMINA

• Alport’s 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
  (Goodpasture’s syndrome)
          FIBRONECTIN

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