Cell to Cell Communication

• Juxtacrine signaling


  Direct cell signaling. Communication between two adjacent cells in direct contact. Bidirectional and asymmetric.
• Cell to Cell Communication in Development

  An embryo at any stage is held together, organized, and formed by the interactions that occur between cells. The interactions exhibited by cells define their methods of communication.
• Juxtacrine Signaling


  Direct cell signaling, Communication between two adjacent cells in direct contact, Bidirectional and asymmetric

  
• Receptors


  Proteins within a membrane that function to bind either other membrane-associated proteins or signaling proteins
• Homophilic binding


  A receptor in the membrane of one cell that binds the same type of receptor in another cell
• Heterophilic binding


  Binding between different receptor types
• Differential Cell Affinity


  • Reaggregated cells become spatially segregated
• Differential adhesion hypothesis


  Explain patterns of cell sorting based on thermodynamic principles, Interactions among cells forms a hierarchy, Cells rearrange themselves into the most thermodynamically stable pattern
• Receptors


  Proteins within a membrane that function to bind either other membrane-associated proteins or signaling proteins.
• Thermodynamic Model of Cell Interactions

  • If the strength of A-A connections is greater than the strength of A-B or B-B connections, sorting will occur, with the A cells becoming central
  • If the strength of A-A connections is less than or equal to the strength of A-B connections, the aggregate will remain as a random mix of cells
  • If the strength of A-A connections is far greater than the strength of A-B connections, A cells and B cells will form separate aggregates
• Cadherins


  Major cell adhesion molecules, Anchored inside the cell by catenins, External domains adhere cells together, Link and help assemble actin cytoskeleton, Initiate and transduce signals (change gene expression)

  
• Major Types of Cadherin

  • E-cadherin
  • P-cadherin
  • N-cadherin
  • R-cadherin
• Differences in cell surface tension and the tendency of cells to bind together

  Depend on the strength of cadherin interactions

  
• Quantity and Cohesion


  • Quantitative (more cadherins, tighter adhesion)
  • Qualitative
• Cadherin-dependent sorting is correlated with surface tension, cell sorting is dependent on the amount of cadherin interactions between the cells

  
• Type, Timing, and Border Formation

  The timing of particular developmental events can depend on cadherin expression, The type of cadherin matter as well, The formation of boundaries is a critical physical achievement necessary for many morphogenetic events
• Extracellular Matrix


  The environment where cell-to-cell interactions occur, Provides the directions for cell movement or the signal for a developmental event, Made up of: matrix protein collagen, proteoglycans, fibronectin, laminin
• Proteoglycans


  Aid in the delivery of the paracrine factors, Consist of core proteins with attached glycosaminoglycans, Heparan sulfate and chondroitin sulfate

  
• Fibronectin


  Organizes matrix and the cells into an ordered structure, Functions as an adhesive molecule, Has several binding sites and interact with molecules to align cells in the ECM, Plays a role in cell migration

  
• Laminin


  Major component of basal lamina, Assembling ECM, promotes cell adhesion and growth, change cell's shape and allows cell migration

  
• Integrins


  Receptors for extracellular matrix molecules, Integrate the extracellular and intracellular scaffolds, allowing them to work together, Can also signal from the outside of the cell to the inside of the cell, altering gene expression

  
• Epithelial-Mesenchymal Transition


  Orderly series of events whereby epithelial cells are transformed into mesenchymal cells, Initiated when paracrine factors activate gene expression in the target cells, instructing target cells to do a series of reaction

  
• Induction


  Interaction at close range between two or more cells or tissues of different histories and properties, Has 2 components: Inducer (Produces the signal) and Responder (the cell or tissue being induced)
• Competence


  The ability to respond to a specific inductive signal
• Instructive Interaction


  A signal from the inducing cell is necessary for initiating new gene expression in the responding cell
• Permissive Interaction


  The responding tissue has already been specified and needs only an environment that allows the expression of these traits
• Paracrine Factors


  Diffusible molecules that work in a range of about 15 cell diameters, or about 40–200 μm, One cell induces changes in neighboring cells, Autocrine Interaction (same cells that secrete the paracrine factors also respond to them)
• Homophilic binding


  A receptor in the membrane of one cell that binds the same type of receptor in another cell.
• Morphogen


  Diffusable biochemical molecule that can determine the fate of a cell by its concentration, Can be transcription factors produced within a syncytium of nuclei or paracrine factors that are produced in one group of cells and then travel to another population of cells
• Morphogen Gradients


  When the morphogen's concentration drops below a certain threshold, a different cell fate is specified
• The range of a paracrine factor (and thus the shape of its morphogen gradient) depends on several aspects of that factor's synthesis, transport, and degradation
• The induction of numerous organs is affected by a relatively small set of paracrine factors that often function as morphogens
• Many paracrine factors can be grouped into one of four major families on the basis of their structure and function
• Autocrine Interaction


  Same cells that secrete the paracrine factors also respond to them
• Morphogen


  Diffusable biochemical molecule that can determine the fate of a cell by its concentration
• Morphogen


  Can be transcription factors produced within a syncytium of nuclei or paracrine factors that are produced in one group of cells and then travel to another population of cells
• Heterophilic binding


  Binding between different receptor types.
• Paracrine Factors


  • The range of a paracrine factor (and thus the shape of its morphogen gradient) depends on several aspects of that factor's synthesis, transport, and degradation
  • The induction of numerous organs is affected by a relatively small set of paracrine factors that often function as morphogens
• Major families of paracrine factors

  • Fibroblast growth factor (FGF) family
  • Hedgehog family
  • Wnt family
  • TGF-β superfamily
• Signal Transduction Cascades


  Enzymatic reactions that have as their end point either the regulation of transcription factors and/or the regulation of the cytoskeleton