Cell to Cell Interactions

• Cells can adhere to one another (cell-cell adhesion) through cell adhesion molecules (CAMs) that often cluster into specialized cell junctions
• Tight junctions - claudins
• Adhesion belt and desmosome - cadherins
• Gap junction - connexins
• Types of CAMs

  1. Cadherins
  2. Ig-superfamily CAMs
  3. Integrins
  4. Selectins
• Cadherins

  • Mediate Ca2+- dependent cell-cell adhesion
  • Cause compaction during development
• Cadherins mediate cell-cell adhesion by a homophilic mechanisms
• Cadherin-dependent cell-cell adhesion guides the organization of developing tissues sorting out according to their origins
• Cadherin binds preferentially to similar type of cell
• Integrins

  • Strengthen the binding of the blood cells to the endometrium
  • Heterophilic binding to specific proteins
  • Principal cell-surface receptors used by animal cells to bind to the extracellular matrix (ECM)
  • transmembrane linkers between the ECM and cytoskeleton
• Types of Integrins

  1. a5B1
  2. a6B1
  3. aIIbB3
  4. a6B4
• Selectins

  • Cell-surface carbohydrate-binding proteins (lectins) that mediate transient cell-cell adhesions in the bloodstream
  • Heterophilic binding to specific oligosaccharides on glycoproteins and glycolipids
  • Control the binding of WBC to endothelial cells, enabling the blood cells to migrate out of the bloodstream
• Selectins and integrins act in sequence to let WBC leave the bloodstream and enter tissues.
• • Weak adhesion and rolling -> selectin dependent
  • Strong adhesion and emigration -> integrin dependent
• Immunoglobulin (Ig) superfamily of proteins

  • Ca2+ independent cell-cell adhesion
  • Contain one or more Ig-like domains that are characteristic of antibody molecules
• Member of Ig superfamily:

  1. Neural cell adhesion molecule (NCAM): Nerve cells
  2. Nectin: collaborates with cadherin to help build and strengthen adherens junctions in many tissue
  3. Intercellular cell adhesion molecules (ICAM): in blood cells
• Intercellular adhesion molecules (ICAMs): Bind to integrins on blood cells when blood cells migrate out of the bloodstream
• Label the followng:

  A) Cadherin (E-cadherins)

  B) Ig superfamily CAMs (N-CAM)

  C) Mucin-like CAMs

  D) Integrin

  
• Cell-cell anchoring junctions

  1. Adherens junction: classical cadherins
  2. Desmosome: Nonclassical cadherins

  Cell-matrix anchoring junctions

  1. Actin-linked cell-matrix junction: Integrin
  2. Hemidesmosome
• Types of Cell Junctions:

  1. Tight Junctions: interlocking junctional proteins
  2. Anchoring junctions
  3. Gap junctions: Channel between cells
• Label the parts:

  A) tight junction

  B) anchoring junctions

  C) desmosome

  D) hemidesmosome

  E) gap junction

  F) basement membrane

  G) intracellular space

  H) plasma membrane of adjacent cells

  
• Tight junctions: Block the passageways through the gaps between epithelial cells, preventing extracellular molecules from leaking from one side to the other
• Tight junctions (vertebrates) is impermeable to macromolecules, however permeability to small molecules varies in different epithelia
• Tight junctions is for the maintenance of vectorial transport function of cells
• Anchoring junctions

  • Mechanicall attach cells (and their cytoskeletons) to their neighbors or to the extracellular matrix
  • Abundant in tissues that are subjected to severe mechanical stress like heart, muscle and epidermis
• Adherens junctions: Takes in a form of adhesion belt
• Desmosomes: are buttonlike points

  • Rivet (fasten) cells together, connect intermediate filaments of adjoining cells to distribute tensile or shearing forces
  • Mechanical strength
• Pemphigus vulgaris

  • Severe blistering of the skin, with leakage of body fluids into the loosened epithelium
  • Due to antibodies against one of desmosomal cadherin proteins desmogleins 1 and 3.
• Hemidesmosomes

  • half-desmosomes, resemble desmosomes morphologically and in connecting intermediate filaments
  • Connect the basal surface of the epithelial cell to the underlying basal lamina
  • Transmembrane linker proteins are integrins
• Bullous pemphigoid: Autoimmune disease directed against hemidesmosome antigens
• Pemphigus vulgaris: disrupted desmosomes

  Bullous pemphigoid: disrupted hemidesmosomes
• Focal adhesions

  • Cell-matrix junctions
  • Actin filament attachment sites
• Gap junctions

  • Allow cell to cell communication
  • Mediate the passage of chemical or electrical signals from one interacting cell to its partner
• Gap Junctions is spanned by channel-forming proteins (connexons)
• Cells connected by gap junctions share many of their inorganic ions and other small molecules and are therefore chemically and electrically coupled
• Functional significance of Gap junctions

  1. Coordinates activities of electrically active cells
  2. Allow action potentials to spread rapidly from cell to cell, without delay that occurs at chemical synapses
  3. Synchronizes contractions heart and smooth muscle cells
• Plasmodesmata

  • Only intercellular junctions in plants
  • Function like gap juctions
• ANCHORING JUNCTIONS

  Actin filament attachment sites

  1. adherens junctions (CCJ)
  2. actin-linked cell-matrix adhesions (CMJ)

  Intermediate

  1. desmosomes (CMJ)
  2. Hemidesmosomes (CMJ)
• OCCLUDING JUNCTIONS

  1. tight junctions (verta.)
  2. septate junctions (inverta.)
• CHANNEL-FORMING JUNCTIONS

  1. gap junctions (animals)
  2. plasmodesmata (plants)