L2: Signal transduction

• Cells usually communicate with one another through “extracellular messenger molecules”
• Cell Communication.

  A) Autocrine

  B) Paracrine

  C) Endocrine

  
• Convergence, Divergence, Cross-talk:

  • Convergence : Different receptors activate a common effector (e.g., Ras).
  • Divergence : Same ligand activates multiple pathways.
  • Cross-talk : Interactions between different signaling pathways.
• Protein kinases --- Add phosphate group to other proteins

  Protein phosphatases --- Remove phosphate group from other proteins
• Most protein kinase transfer phosphate group to serine, threonine, or especially tyrosine residues of their protein substrate.
• Abnormal PYP is associated with uncontrolled cell proliferation, differentiation, and disease.
• Extracellular carriers • Amino acids • Gases: NO and CO • Steroids • Eicosanoids • Polypeptides and proteins
• Receptors evolved to mediate signal transduction

  • G protein-coupled receptors (GPCRs)

  • Receptor protein -tyrosine kinases (RTKs)

  • Ligand -gated channels

  • Steroid hormone receptors

  • B- and T-cell receptors
• Autocrine - a signal that is released by the cell itself.
• Paracrine - secreted by one cell and affects another cell
• Endocrine - hormones are released into the bloodstream and travel to target organs
• Transmits a signal to a nearby 4 enzyme called “effector” which generate a “second messenger ”
• Protein Tyrosine Phosphorylation (PYP)

  A) PTPs remove

  B) PTKs add

  
• > Ligand binds to the receptors

  > Gα subunit release its GDP, which is replaced by GTP

  > Gα subunit dissociates from the Gβɤ complex and binds to an effector (e.g. adenylyl cyclase)

  > Activated adenylyl cyclase produces cyclic AMP (cAMP)
• Cholera toxin (from Vibrio cholerae) อหิวาตกโรค

  • Modify Gα subunits and inhibit their GTPase activity in the intestinal epithelium cells

  • Adenylyl cyclase remain in an activated mode, churning out cAMP

  • Causes the epithelial cells to secrete large volume of fluid into the intestinal lumen
• Pertussis toxin (from Bordetella pertussis) ไอกรน

  • Inactivates Gα subunits

  • Interfere the signaling pathway related the host defensive response against the bacterial infection

  • Causes “whooping cough”
• Retinitis pigmentosa (RA) : A mutation that leads to a loss of function of the encoded receptor.

  Mutation leads to

  • Premature termination or improper folding of the rhodopsin protein

  • The synthesized rhodopsin cannot activate its G protein and thus cannot pass the signal downstream to the effector

  
• Thyroid adenomas A mutation that leads to a gain of function of the encoded receptor.

  The TSH receptor was mutated at site 5 or 6 :

  • The TSH receptor constitutively activate a G Protein on its inner surface

  • Sending a continual signal through the pathway that leads to

  • Excessive thyroid secretion

  • Excessive cell proliferation

  
• PKA : Protein Kinase A
• PKA anchoring proteins (AKAPs) – a signaling hub - AKAPs provide a scaffold for coordinating protein-protein interactions by sequestering PKA to specific locations within the cell

  Different cells express different AKAPs, resulting in localization of PKA in the presence of different substrates and consequently phosphorylation of different substrates in response in cAMP level
• Heterotrimeric G Proteins:

  A) inhibiting

  B) receptors

  
• Second Messenger

  1. Cyclic AMP (cAMP)
  2. Phosphatidylinositol (PI)
• Phosphatidylinositol (PI) :

  Effector : PI-PLCβ : PI-specific phospholipase C-β

  secondary messenger : DAG and IP3
• Phosphatidylinositol (PI)

  A) DAG

  B) PKC

  C) Ca2+

  D) SER

  E) IP3

  
• Activation of RTK = Dimerization of RTK

  1. Ligand-Mediated Dimerization
  2. Receptor-Mediated Dimerization
• Insulin Receptor Substrate (IRS) Phosphorylated and serve as binding sites for other proteins
• Insulin receptor: One of a Protein-Tyrosine kinase

  PKB (known as AKT) Plays a role in mediating the response to insulin and other extracellular signal

  PDK1 Phosphorylate and activate the kinase activity of PKB

  mTOR A second kinase, phosphorylate and activate PKB
• fill the blank

  
• Signals from a variety of unrelated receptors, each binding to its own ligand, can converge to activate a common effector, such as Ras or Raf
• Signals from the same ligand, such as EGF or insulin, can diverge to activate a variety of different effectors and pathways, leading to diverse cellular responses
• Signals can be passed back and forth between different pathways, a phenomenon known as cross-talk
• NO as an activator of guanylyl cyclase

  1. Binding of acetylcholine to the outer surface of endothelial cell
  2. Rising of cytosolic Ca2+
  3. Nitric oxide synthase was activated
  4. NO diffuses into smooth muscle cell
  5. NO binds and stimulates GTP which synthesizes cGMP
  6. cGMP binds to PKG (cGMP dependent protein kinase) which phosphorylates substrates causing relaxation of muscle cell and dilation of the blood vessel
• effector : an organ or cell that acts in response to a stimulus : enzyme
• what is involve G protein?

  G Protein-Coupled Receptors (GPCRs)

  
• The activation of the GPCR Rhodopsin.

  Receptor : Rhodopsin

  Effector : cGMP phosphodiesterase

  Physiologic response : Visual excitation
• Heterotrimeric G protein - transducin

  
• Termination of the GPCR responses (3,4 = Desensitization The cell stops responding to the stimulus)

  1. The GTPase activity of Gα hydrolyzes the bound GTP, deactivating Gα
  2. Gα reassociates with Gβɤ, reforming the trimeric G protein, and the effector ceases the activity
  3. The receptor has been phosphorylated by a G protein-coupled receptor kinase (GRK)
  4. The phosphorylated receptor has been bound by an “ arrestin ” which inhibits the ligand-bound receptor from activating additional G proteins
• Resensitized : The arrestin bound GPCRs may be dephosphorylated and returned to the plasma membrane.
• cAMP : Secondary signaling
• Bacterial toxins

  • Cholera toxin (from Vibrio cholerae) : Modify Gα subunits and inhibit their GTPase(none stop) activity in the intestinal epithelium cells
  • Pertussis toxin (from Bordetella pertussis) : Inactivates Gα subunits