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Cell communication

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Cell signaLling
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Cell communication

  1. 1. Cell Communication
  2. 2. Cells need to be able to communicate to other cells and respond to environment changes * For multicellular organisms, cell- cell communication is important * External signals are converted into responses within the cell
  3. 3. For unicellular organisms, they need to be able to respond to physical and chemical changes in their Environment Unicellular organisms can communicate and influence one another’s behavior. Many bacteria can respond to chemical signals that are secreted by their neighbors and increase in concentration with increasing population density (quorum sensing), allowing them to coordinate their behavior, including their mobility, antibiotic production, spore formation, and sexual conjugation.
  4. 4. Yeast cells identify their mates by cell signaling
  5. 5. Signal Transduction Pathways Convert signals on a cell’s surface into cellular response
  6. 6. Local and Long-Distance Signaling • Cells in a multicellular organism communicate by chemical messengers • Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells • In local signaling, animal cells may communicate by direct contact, or cell-cell recognition
  7. 7. Animal and Plant Cell
  8. 8. •In local signaling , animal cells •May communicate via direct contact Figure 11.3 (b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces.
  9. 9. •In other cases , animal cells •Communicate using local regulators (a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid. (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell. Local regulator diffuses through extracellular fluid Target cell Secretory vesicle Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse Target cell is stimulated Local signaling Figure 11.4 A B
  10. 10. Long – Distance Signaling Both plants and animals use hormones
  11. 11. An Example of Cell Communication  Insert video here Almazoge
  12. 12. Earl W. Sutherland - Discovered how the hormones epinephrine acts on cells - Suggested that cells receiving signals went through three processes 1. Reception 2. Transduction 3. Response
  13. 13. Step 1 of cell signaling: Reception Usually, two molecules are involved: - Ligand - Receptor
  14. 14. Step two of cell signaling Transduction
  15. 15. Step three of cell signaling: Response
  17. 17. • The cell target d by a particular chemical signal has a receptor protein on or in the target cell that recognizes the signal molecule • Recognition occurs when the signal binds to a specific site on the receptor that is the complementary in shape to the signal. • Ligand binding causes the receptor protein to undergo a change in shape that may activate the receptor so that it can interact with other molecules
  18. 18. Intracellular Receptors • Some ligands are small hydrophobic molecules that can cross cell membranes ( Ex. Testosterone). • Receptors that detect such ligand are intracellular , found in the cytosol or nucleus of target cells. • An activated hormone-receptor complex can act as a transcription factor. Turning on specific genes
  19. 19. Hydrophobic messengers include the steriod and thyroid hormones
  20. 20. Receptors in the plasma membrane • There are three main types of membrane receptors 1. G-protein-linked 2. Tyrosine kinases 3. Ion channel
  21. 21. G- protein –linked receptors
  22. 22. Receptor tyrosine kinase
  23. 23. Ion Channel Receptors
  25. 25. • The transduction stage of signaling is usually a multi step pathway and these pathways amplify the signal and provide more opportunities for coordination and regulation. • If some molecules in a pathway transmit a signal to multiple molecules of the next component in the series, the result can be large numbers of activated molecules at the end of the pathway. • A small number of signal molecules can produce a large cellular response.
  26. 26. • The molecules that relay a signal from receptor to response are mostly proteins. • Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated. • At each step, the signal is transduced into a different form, usually a shape change in a protein. Signal Transduction Pathways
  27. 27. Protein Phosphorylation and Dephosphorylation • In many pathways, the signal is transmitted by a cascade of protein phosphorylation. • Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation (activation) . • Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation (deactivation) .
  28. 28.  This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off or up or down, as required.
  29. 29. Phosphorylation cascade
  30. 30. Small Molecules and Ions as Second Messengers • The extracellular signal molecule (ligand) that binds to the receptor is a pathway’s “first messenger”. • Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion. • Second messengers participate in pathways initiated by GPCRs and RTKs. • Cyclic AMP and calcium ions are common second messengers.
  31. 31. Cyclic AMP • Cyclic AMP (cAMP) is one of the most widely used second messengers • cAMP is made from ATP by Adenylyl cyclase • Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal
  32. 32. Adenylyl cyclase Phosphodiesterase Pyrophosphate AMP H2O ATP P iP cAMP Cyclic AMP – made from ATP
  33. 33. Adenylyl cyclase Pyrophosphate ATP P iP cAMP
  34. 34. Phosphodiesterase AMP H2O cAMP H2O
  35. 35. • Many signal molecules trigger formation of cAMP • Other components of cAMP pathways are G proteins, G protein-coupled receptors, and protein kinases • cAMP usually activates protein kinase A, which phosphorylates various other proteins • Further regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase
  36. 36. Figure 11.12 G protein First messenger (signaling molecule such as epinephrine) G protein-coupled receptor Adenylyl cyclase Second messenger Cellular responses Protein kinase A GTP ATP cAMP
  37. 37. Calcium ions and Inositol Triphosphate (IP3) • Calcium ions (Ca2+) act as a second messenger in many pathways • Calcium is an important second messenger because cells can regulate its concentration
  38. 38. Inositol triphosphate and diacyglycerol • The pathways leading to the release of calcium involve diacylglycerol (DAG) and inositol triphosphate (IP3) as second messengers. • Can trigger an increase in calcium in the cytosol.
  40. 40. • Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities. • The response may occur in the cytoplasm or may involve action in the nucleus. • Many pathways regulate the activity of enzymes Cytoplasmic and Nuclear Responses
  41. 41. Cytoplasmic response
  42. 42. • Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus. • The final activated molecule in the signaling pathway may function as a transcription factor Other Pathways – Nuclear response
  43. 43. Fine-Tuning of the Response  Multistep pathways have two important benefits: I. Amplifying the signal (and thus the response) II. Contributing to the specificity of the response
  44. 44. Signal Amplification  Enzyme cascades amplify the cell’s response to signal.  At each step, the number of activated products is much greater than in the preceding step
  45. 45. The Specificity of Cell Signaling • Different kinds of cells have different collections of proteins • These different proteins allow cells to detect and respond to different signals • Even the same signal can have different effects in cells with different proteins and pathways • Pathway branching and “cross-talk” further help the cell coordinate incoming signals.
  46. 46. Signaling Efficiency: Scaffolding Proteins and Signaling Complexes  Scaffolding proteins are large relay proteins to which other relay proteins are attached  Scaffolding proteins can increase the signal transduction efficiency by grouping together different proteins involved in the same pathway  In some cases, scaffolding proteins may also help activate some of the relay proteins
  47. 47. Termination of Signal  Signal response is terminated quickly by reversal of ligand binding.  Inactivation mechanisms are an essential aspect of cell signaling.  Unbound receptors revert to an inactive state.
  48. 48. References • Power point lectures for Biology seventh Edition by Neil Campbell and Jane Reece. • Mechanisms of cell communication. • Power Point presentation by Yuzhen ye.