20 Cell Signalling (DIY)

• Main Stages of Cell Signaling (Direct Communication)

  Involves the physical contact between interacting cells
• Main stages of Cell Signalling (Indirect Communication)

  Involves extracellular chemical messengers or signal molecules that bind to receptors
• Define Ligand


  A signal molecule that binds specifically to another molecule, usually a receptor (extracellular chemicals/ first messengers)
• Define Receptor


  Protein that binds and transduces the message of the signal molecule into a cellular response
• Define signal transduction


  A series of steps by which signals are conveyed into the target cell where they are transformed into a cellular response
• Step 1 in Cell Signalling

  • Ligand-receptor Interaction
  • A specific ligand binds to a specific ligand-binding site of a specific protein receptor to form a ligand-receptor complex
  • Complementary in shape and charge to the extracellular ligand-binding site of the receptor
  • Generally causes a receptor protein to undergo a change in conformation (Can directly activate the receptor/ cause o dimerisation of 2 or more receptor subunits)
• Cell Surface Receptors

  • Located on the plasma membrane
  • Binds to large, polar or hydrophilic signal molecules that cannot diffuse readily across the phospholipid bilayer of the plasma membrane
  • Consists of 3 domains:

  1. Extracellular domain: ligand-binding site
  2. Transmembrane domain: embed receptor
  3. Intracellular domain: undergoes conformational change upon ligand binding
• Termination of Cell Signal (Ligand-Receptor Interaction)

  1. Extracellular first messenger can be degraded by enzymes in the extracellular space
  2. Endocytosis of ligand-receptor complex prevents signal transduction from continuing

  IMPORTANCE: ensures signal transduction will not continue permanently which will lead to excessive cellular responses
• Step 2 in Cell Signalling

  • Signal Transduction (Phosphorylation cascade and signal amplification)
  • The series of changes in cellular proteins that converts an extracellular chemical signal to a specific intracellular response
  • Functions to change the behaviour of a cell
• Importance of Signal Transduction

  1. Multiple Steps

  • Activated receptor activates another relay protein, which activates another relay protein molecule until the final protein producing cellular response is activated (specific molecular interactions)

  2. Phosphorylation cascade

  • A sequence of events where one enzyme phosphorylates another, causing a chain reaction leading to the phosphorylation of thousands of proteins
  • Modification of proteins via phosphorylation and dephosphorylation

  3. Signal Amplification

  • The number of activated molecules at each step of the transduction pathway increases
• Termination of Cell Signalling (Signal Transduction)

  1. Increased activity of phosphatases functions to dephosphorylate proteins

  • Inactivation of relay molecules occurs, inhibiting the signal propagation

  2. Production of inhibitors that bind to the ligand-receptor complex and/ or any of the intracellular signal proteins
• Advantages of Signal Transduction

  1. Facilitates signal amplification

  • Only a small number of signal molecules needed to solicit a large cellular response
• Advantages of Signal Transduction (2)

  
  

  2. Multiple responses to 1 signal molecule (ligand) because 1 ligand can trigger multiple signal transduction pathways to elicit different responses

  • E.g. insulin binding to liver cell triggers increase GLUT 4 transporters at cell membrane to increase glucose uptake from blood, resulting in activation of enzymes for increased glycogenesis, glycolysis and decreased gluconeogenesis
• Advantage of Signal Transduction (3)

  
  

  3. Provides multiple checkpoints for regulation

  • Several steps in the signalling pathway can be regulated and controlled
  • Eventually regulates the cellular response of the pathway
• Advantages of Signal Transduction (4)

  
  

  4. Ensures specificity because the specific ligand binds to a specific receptor to elicit specific reaction via specific pathway in each cell type
• Advantages of Signal Transduction (5)

  
  

  5. Ability of a ligand to activate genes in nucleus upon binding to cell surface receptor without the need to move into nucleus
• Step 3 in Cell Signalling

  • Cellular Response (Occurs in cytoplasm/ nucleus)
  • Regulation of gene expression (a gene may be upregulated/ downregulated by the activation of transcription factors)
  • Regulation of metabolic pathways (through the activation/ inhibition of enzymes)
  • Changes in cytoskeleton (assembly of microtubules for movement or vesicles to plasma membrane etc.)
  • Not all signals give rise to the same cellular responses in different cell types (specificity of cell signalling)
• G-protein Linked Receptor (GPCR)

  Comprises of:

  G protein-coupled receptor (GPCR)

  • Consists of 7 alpha-helices spanning the membrane (7 transmembrane domains)
  • Has an extracellular ligand-binding site that ligand binds to
  • Has an intracellular/ cytoplasmic side that associates with a G-protein

  G protein

  • Found on the cytoplasmic side of the membrane
  • Complexes made up of 3 subunits (alpha, beta and gamma subunits)
  • GTP-binding proteins
  • Alternate between 2 states: Inactive (bound to GDP) and Active (bound to GTP)

  Another protein (usually an enzyme)
• Process of GPCR (RECEPTION - 1)

  INACTIVE G protein

  • In the absence of extracellular signal molecules specific to the receptor
  • Inactive G protein has a GDP molecule bound to it
• Process of GPCR (INITIATION - 2)

  ACTIVE G protein

  • Ligand binds to the receptor and the intracellular domain of the region will change conformation so its binds to an inactive G protein
  • A molecule of GTP displaces GDP on the inactive G protein (G protein becomes activated)
  • Active G protein dissociates from the receptor and moves along the cytoplasmic side of the cell membrane
  • Active G protein binds to and activates the enzyme (usually adenylyl cyclase) which triggers the next step in the pathway leading to cellular response
  • (Adenylyl cyclase catalyses the conversion of ATP to cAMP)
• Process of GPCR (TERMINATION - 3)

  Return to INACTIVE form

  • Intrinsic GTPase activity catalyses the hydrolysis of its GTP to GDP (G protein returns to an inactive state)
  • G protein dissociates from the enzyme and becomes available for reuse
• Receptor Tyrosine Kinase (RTK)

  • A group of cell surface receptors that also function as an enzyme
  • Consists of:

  1. An extracellular ligand-binding site
  2. A transmembrane domain (single alpha-helix spanning the membrane)
  3. An intracellular domain with several tyrosine residues

  • Part of the receptor extends into the cytoplasm as an enzyme - phosphorylate specific tyrosine residues (intrinsic tyrosine kinase activity)
  • Exists as 2 separate subunits/ a linked dimer (Binding does not cause enough of a conformational change to activate the cytoplasmic side of the protein directly)
• Process of RTK (RECEPTION - 1)

  • Dimerisation
  • When ligands bind to each of the 2 receptor subunits, the subunits dimerise, forming a dimer (a protein consisting of 2 polypeptides)
• Process of RTK (INITIATION - 2)

  • Phosphorylation of receptor
  • Dimerisation activates the intrinsic tyrosine-kinase activity of both subunits
  • Each tyrosine kinase catalyses the attachment of a phosphate to specific tyrosine residues on the intracellular domain of the other subunit
  • A phosphorylated dimer is formed via cross-phosphorylation
  • Fully-activated receptor protein dimer will be recognised by specific relay proteins inside the cell
  • Each relay protein binds to a specific phosphorylated tyrosine, resulting in a conformational change and thus activation of the bound relay protein
• Process of RTK (ELIMINATION - 3)

  • Each activated tyrosine kinase dimer may activate 10 or more different intracellular relay proteins simultaneously and trigger many different transduction pathways and cellular responses
  • Tyrosine-kinase receptor system is specialised for triggering more than one signal-transduction pathway at once
  • This allows the cell to regulate and coordinate many aspects of cell growth and reproduction
  • (Abnormal tyrosine-kinase receptors that dimerize even without ligands can cause some kinds of cancer)
• Secondary Messengers

  • small, non-protein, water-soluble molecules/ ions
  • Readily spread throughout the cell by diffusion and are able to function effectively in the cytoplasm
  • Helps to activate cellular proteins
• cAMP

  • In GPCR-initiated pathway
  • produced by breakdown of ATP by activated adenylyl cyclase
  • Signal molecule (first messenger) activates a G-protein-coupled receptor (GPCR) which activates a specific G-protein
  • Specific G-protein binds and activates adenylyl cyclase which catalyses the conversion of ATP to cAMP
  • cAMP concentration is increased and it binds and activates another protein, usually protein kinase A
  • Protein kinase A starts a phosphorylation cascade leading to cellular responses
• Phosphorylation Cascade

  • Protein phosphorylation of signal transduction
  • When initially synthesised, many proteins are inactive and require modification to be activated/ deactivated
  • Protein phosphorylation/ dephosphorylation are types of post-translational modifications that help to regulate protein function

  1. Phosphorylation (by Kinase - which adds phosphate groups from ATP to the protein) [Most relay molecules]
  2. Dephosphorylation (by phosphatase - which removes phosphate groups from proteins by hydrolysis) [In the absence of the extracellular signal]
• Signal Amplification

  Produces a large number of an intracellular mediator from a relatively small number of extracellular signals

  • Number of activated proteins become much greater than the previous step
  • One cell surface receptor can trigger the production of 10^8 copies of the final product from 1 signalling molecule (small extracellular signal molecule can activate large amount of intracellular molecule to produce a large cellular response)
  • Protein Kinases (when enzymes are activated)
  • Protein Phosphatases (when response wants to be terminated)
• Importance of Signal Amplification

  • Allows signal transduction to proceed even with very little amount of signal molecules/ receptors at the start
  • Necessary as there may not be enough receptors to yield the appropriate responses - helps to convert the signals to appropriate cellular responses
• Insulin (RTK)

  • From the Beta Cells of the Islets of Langerhans of the pancreas
  •  decreases blood glucose concentration (Only hormone that does so)
  • Deficiency leads to Type I diabetes mellitus
  • Hyperglycemia stimulates, Hypoglycemia inhibits
• Glucagon (GPCR)

  • From the alpha cells of the Islets of Langerhans of the pancreas
  •  increases blood glucose concentration
  • Hyperglycemia inhibits, Hypoglycemia stimulates
• Importance of regulating blood glucose levels

  • Glucose is the ideal substrate for cellular respiration
  • Cellular respiration yields energy
  • excess energy must be stored for use during fasting periods between meal
  • A drastic decrease in blood glucose level could lead to fainting, convulsion, coma and finally death.
• Glucagon GPCR process

  Glucagon acts on liver cells to promote glycogen breakdown and encourage glucose synthesis
• Glucagon GPCR process

  1. Glucagon binds to the extracellular ligand-binding site of the glucagon GPCR receptor of liver cells

  2. Induced conformational changes in the intracellular domain of the transmembrane receptor

  3. Inactive G protein binds to the receptor, followed by the displacement of a GDP molecule with a GTP molecule on the G protein (now activated)

  4. Activated G protein binds to and activates the next enzyme in the cascade, adenylyl cyclase

  5. Increase in cAMP levels which serves to amplify the glucagon signal

  6. cAMP binds to and activates protein kinase A (PKA) which is a cAMP-dependent protein kinase

  7. Activated PKA phosphorylates and activates phosphorylase kinase, which in turn phosphorylates glycogen phosphorylase, converting it into the active form

  8. Also phosphorylate and inactivates glycogen synthase enzyme
• Insulin (RTK)

  Functions to increase the permeability of muscle and liver cells to increase glucose uptake
• Insulin function

  Stimulates glycogenesis, lipid and protein metabolism, as well as many other cellular metabolic activities
• Insulin binding

  Binds to the extracellular ligand-binding site of insulin receptor on liver cell (exists as a linked dimer), causing a conformational change in the intracellular domain
• Insulin receptor activation

  1. Change in conformation of the intracellular domain leads to the cross-phosphorylation of the specific tyrosine residues on the receptor subunits

  2. Different intracellular relay proteins will bind to specific phosphorylated tyrosine residues on receptor subunits

  3. Intrinsic kinase activity of receptor can phosphorylate these relay proteins, which can activate different intracellular pathways
• Cellular response to insulin

  1. Induction of glycogen synthesis via activated glycogen synthase

  2. Stimulation of glucose uptake in muscle and adipose tissue via translocation of vesicles containing GLUT4 transporters to the plasma membrane

  3. Rate of glycogenesis increases - conversion of glucose to glycogen

  4. Stimulates glycogen storage in liver and muscle

  5. Inhibits glycogenolysis - the breakdown of glycogen to glucose in liver and muscle

  6. Inhibits gluconeogenesis (glucose synthesis) - the breakdown of fats and proteins and the conversion of non-carbohydrate sources to glucose in liver cells

  7. Increases the use of glucose as a substrate for cellular respiration

  8. Increases rate of glucose oxidation and synthesis of ATP

  9. Increases rate of protein synthesis

  10. Increases rate of conversion of glucose to fats and the rate of fat deposition in adipose tissues