cell bio module 6

• The objectives of this lecture are to introduce the topic of cell communication by: illustrating signaling in unicellular Dictyostelium slime mold cells and in human neutrophil white blood cells. We will also identify the general principles of cell signaling and we will describe the mechanisms by which cells in multicellular organisms are able to communicate using signals.
• Dictyostelium discoideum slime mold

  • Transitions from a collection of unicellular amoebae into a multicellular slug and then into a fruiting body
  • Aggregated amoebae work together to form a multicellular slug that can migrate towards heat, light, and humidity in a search for potential food
  • Anterior end of the slug forms the stalk and the posterior end forms the spores of the fruiting body
• Dictyostelium life cycle


  1. Vegetative growth phase (abundant food)

  2. Starvation initiates aggregation in response to cAMP signaling

  3. Aggregated cells form migrating slug

  4. Slug stops and cells differentiate (anterior cells form stalk, posterior cells form fruiting body)

  5. Fruiting body contains spores that can remain dormant until food becomes available
• Dictyostelium feed on bacteria, such as E.coli.
• Cyclic AMP (cAMP)


  Signaling molecule produced by starved Dictyostelium cells that initiates aggregation
• 1. protein coupled receptor (GPCR)


  Receptor on Dictyostelium cells that binds cAMP, activating the receptor
• Dictyostelium cells perceive cAMP signal


  Cells reorganize their intracellular actin cytoskeleton network to move towards the source of the signal
• In the absence of clathrin-mediated protein transport, Dictyostelium cells are unable to transport the cAMP receptor to the cell surface, so they cannot respond to the cAMP signal.
• Neutrophils


  • White blood cells that can respond to chemical signals produced by bacteria
  • Have a cell-surface receptor that specifically recognizes the fMLP peptide produced by bacteria
• Neutrophil responds to fMLP peptide signal


  Moves towards the source of the signal and engulfs the bacterium
• Cell signaling


  The transmission of information from one cell to another that induces a change in behavior
• Cell signaling process


  1. Signaling cell produces and releases signaling molecules

  2. Target cell has a receptor that binds the signal

  3. Binding activates a signal transduction pathway inside the target cell

  4. Signal transduction pathway interprets and transduces the signal

  5. Culminates in a change in target cell behavior

  6. Signal must be removed to terminate the response
• Signal-receptor interactions


  • Specific and high-affinity, determined by molecular complementarity between interacting surfaces
  • A single amino acid change can disrupt signal binding and signaling
• Receptor binding to signal

  Causes a conformational change in the intracellular domain of the receptor, activating the signal transduction pathway
• Specificity of cell signaling

  • Specificity of ligand for receptor binding
  • Specificity of intracellular signal transduction pathway
• Types of cellular responses to signals

  • Fast response (activation of cytosolic enzyme)
  • Slow response (change in protein levels via transcription and translation)
• Receptor-signal affinity


  Measured by Kd (dissociation constant), the concentration of ligand required for half-maximal binding
• Cell signaling


  1. Signal production by signaling cell

  2. Signal reception by target cell

  3. Activation of internal signal transduction pathways

  4. Interpretation of signal

  5. Cellular response
• Cell signaling


  • It is the process by which a cell receives and responds to signal in its local environment
• Major classes of cell surface receptors

  • Cytokine receptors
  • Receptor-tyrosine kinases (RTKs)
  • G-protein coupled receptors (GPCRs)
• Cytokine receptor and JAK-STAT pathway


  1. Erythropoietin (Epo) signal

  2. Erythropoietin receptor

  3. JAK-STAT signal transduction pathway

  4. Cellular response in erythrocyte progenitor cells
• Erythropoietin receptor


  Monomeric, single-pass transmembrane protein that is normally inactive
• Activation of erythropoietin receptor

  1. Epo binding

  2. Receptor dimerization

  3. JAK kinase autophosphorylation

  4. JAK kinase activation

  5. Phosphorylation of receptor intracellular domain
• STAT activation in JAK-STAT pathway

  1. STAT monomers bind to phosphorylated receptor docking sites

  2. STAT phosphorylation by JAK kinase

  3. STAT dimerization

  4. STAT nuclear translocation

  5. Transcription of target genes
• SH2 domain


  Protein-protein interaction domain that binds to phosphorylated tyrosine residues
• SH2 domain binding is reversible, dependent on tyrosine phosphorylation state
• Protein-protein interaction domains


  • SH2
  • PTB
  • 14-3-3
• Protein-protein interaction domains link together two proteins, with binding often dependent on reversible modifications to the target peptide
• Pro-Asn-pTyr-Glu-Glu-Ile-Pro sequence


  When the target protein has this sequence, SH2 domains will bind with high affinity and specificity when the tyrosine is phosphorylated, but binds with low affinity when the tyrosine is unphosphorylated
• The P-tyrosine and isoleucine fit precisely within the SH2 binding pocket, allowing for reversible binding of the SH2 domain to the target peptide
• Protein-protein interaction domains


  • They are responsible for linking together two proteins
  • Binding can be dependent upon reversible modifications to the target peptide
• SH2, PTB, and 14-3-3 domains bind peptides containing phosphorylated tyrosine with high affinity, but not the corresponding unphosphorylated peptide
• PDZ domains bind hydrophobic residues at the C-terminus of a protein, and SH3 and WW domains bind proline-rich domains on proteins
• STAT5 transcription factor


  Activated by the erythropoietin receptor, regulates genes necessary for the differentiation of erythroid progenitor cells into mature red blood cells
• Bcl-xL protein


  An inhibitor of apoptosis, allows erythroid progenitor cells to persist and differentiate
• Red blood cells are formed in the fetal liver during development, providing a visual assay for detecting the activation of the cytokine/JAK-STAT pathway
• Lack of erythropoietin receptor


  No red blood cell formation in the fetal liver
• Mutations in genes coding for proteins required in the cytokine pathway can cause the same phenotype of no red blood cell formation
• Continual activation of the cytokine pathway

  Overproduction of red blood cells, leading to increased blood viscosity and risk of stroke or heart attack
• Epo doping


  Athletes intentionally increase their haematocrit by taking exogenous erythropoietin, to increase oxygen carrying capacity and endurance