08 CELL COMMUNICATION

key words

• gap junction

• local signaling

• long distance signaling

• paracrine signaling

• synaptic signaling

• endocrine signaling

• hormones

• signal transduction pathway

• reception

• transduction

• response

• ligand

• protein kinase

• protein phosphatase

• phosphorylation

• dephosphorylation

• second messenger

• cyclic AMP (cAMP)

• adenylyl cyclase

• epinephrine

• protein kinase A

• g protein coupled receptor (GPCR)

• receptor tyrosine kinase (RTK)

• ligand-gated ion channel

• intracellular receptor

class notes

• most signaling molecules bind to receptors on membrane, but some receptors are intracellular

• signaling transduction pathway
  i. ligand binds to receptor
  ii. activates G protein
  iii. G protein activates adenylyl cyclase
  iv. cyclase onverts ATP to cyclic AMP (2nd messenger)
  v. AMP activates series of 2-protein kinases
  vi. final kinase activates enzyme → effect occurs

• amplifies effect: ligand activates 100 GCRs, GCRs activate 100 cyclases, etc.

• GPCRs (G protein coupled receptors)
  i. starts with inactive receptor (unbound) and inactive G protein (bound to GDP, not GTP)
  ii. ligand binds to receptor → receptor activated and changes shape, allowing G protein to bind → G protein changes shape, allowing GTP to bind to it (replacing GDP) → G protein activated
  iii. GTP binding detaches G protein from receptor and G protein can move through membrane again → random movement: runs into inactive adenylyl cyclase and binds → adenylyl cyclase activated → causes cellular response (via phosphorylation cascade)
      a. G protein can only bind to adenylyl cyclase because of its new shape with the bound GTP

• must be able to deactivate in order to stop signal/be able to reset and receive new signals
  i. binding to enzyme causes G protein to change shape → dephosphorylates GTP, turning it into GDP and deactivating G protein
      a. G protein hydrolyzes GTP to turn it into GDP
  ii. inactive G protein changes shape and is now unable to bind to adenylyl cylase → G protein detaches, deactivating adenylyl cyclase and changing its shape

• GPCRs abundant: half of all cell surface receptors

• receptor tyrosine kinases (RTKs)
  i. starts with two inactive monomers: RTK proteins, each with tyrosines and a ligand-binding site
  ii. a ligand binds to each receptor, causing them to come together and form a dimer
  iii. dimerization activates tyrosine kinase region of each monomer → adds phosphate from ATP to tyrosine tail of other monomer until dimer is fully phosphorylated
  iv. fully active RTK now recognized by specific, inactive relay proteins in cell → relay proteins bind and are activated → trigger cellular response
      a. one RTK can activate different relays at once → several different cellular responses simultaneously

• ligand-gated ion channels
  i. ligand binds → receptor changes shape, allowing specific ions into cell → ion concentrations change, which may affect cell activity somehow
      a. ligand can also change to close and block ions

• intracellular steroid hormone receptors
  i. signaling molecule (steroid hormones) generally small/nonpolar enough to simply diffuse through target cell membrane → ligand binds to intracellular receptors, forming hormone-receptor complex
  ii. HRC may enter nucleus and bind to specific genes, turning them on/off
  iii. note: hormones can be steroids, proteins, etc. but these are specifically steroid hormones

textbook notes

• signal transduction pathway: pathway transducing (changing) signals into form that results in response

• local signaling
  i. direct contact via cell junctions: signaling substances dissolved in cytosol passes freely between adjacent cells
  a. animal cells: gap junctions
  b. plant cells: plasmodesmata
  c. cell-cell recognition: communicate via direct contact between membrane-bound cell-surface molecules
  ii. paracrine signaling: signaling cell secrets messenger molecules (local regulators) to influence nearby cells
  a. ex: growth factors: compounds that stimulate neary target cells to grow/divide
  iii. synaptic signaling: electrical signal along nerve cell → secretes neurotransmitters (chemical signals) that diffuse across synapse (space between nerve/target)  to target cell → target cell response

• long-distance signaling: hormones
  i. endocrine signaling: specialized cells release hormones → travel through circulatory system to target cells
  ii. plant: travel in vessels/move through cells/diffuse through air as gas to reach target

• cell encounters secreted signaling molecule → must have specific receptor molecule to bind to signal → signal transduced inside cell → cell responds
  i. reception: target cell detects signaling molecule from outside cell by binding signaling molecule to receptor protein on cell surface
  ii. transduction: binding changes receptor protein → converts signal to form that can bring about specific response
  a. sometimes single step, more often series of molecules (signal transduction pathway)
  b. relay molecules: molecules in pathway
  iii. response: transduced signal triggers specific response

• only certain target cells detect/react to signaling molecule
  i. receptor protein
  a. signaling molecule shape complements specific site on receptor and binds there
  b. signaling molecule acts as ligand: molecule specifically binding to another molecule
  ii. most signal receptors are plasma membrane proteins

• ligand binding → receptor protein changes shape → activates receptor
  i. able to interact with other molecules or causes aggregation of multiple receptor molecules (leads to further responses)

• three major types of cell-surface transmembrane receptors
  i. GPCR (G protein-coupled receptors): works with G protein (binds energy-rich molecule GTP)
  a. 3 parts: receptor, G protein, and enzyme
       b. ligand binds and receptor changes shape to allow G protein to bind → GDP displaced by GTP and G protein activates, detaching from receptor → diffuses along membrane to reach adenylyl cyclase, binding and activating it/changing its shape → phosphorylation cascade → G protein hydrolyzes GTP to GDP → G protein back to inactive
  b. GPCRs only activate one transduction pathway
  ii. receptor tyrosine kinases (RTK): membrane receptors attaching phosphates to tyrosines
  a. kinase: any enzyme catalyzing phosphate group transfers
  b. tyrosine kinase: dephosphorylates ATP to phosphorylate amino acid tyrosine on substrate protein (transfers phosphate from ATP to tyrosine kinase)
  c. can trigger multiple signal transduction pathways at once
  d. initially exist as 2 separate monomer units → ligand binds and receptor monomers dimerize, forming dimer complex → activates tyrosine kinase region on each monomer → each TK adds phosphate from ATP to tyrosine on tail of other monomer → receptor activated and relay proteins inside cells recognize → each protein binds to specific phosphorylated tyrosine → protein structure changes, activating protein and triggering transduction pathway
  iii. ion channel receptors: signaling molecule binds to receptor protein and receptor changes shape, allowing “gate” to open/close and allow/block ion movement
  a. changing ion concentration inside cell affects activity
  c. voltage-gated ion channels: controlled by electrical signals instead of ligands

• binding of signaling molecules is reversible: bind/dissociate many times
  i. ligand concentration outside of cell determines how many times ligand is bound/causes signaling

• intracellular receptor proteins: in cytoplasm/nucleus of target cells
  i. signaling molecule hydrophobic/small enough → passes through target’s plasma membrane
  ii. binding changes receptor into hormone-receptor complex → response (usually turning genes on/off)
  a. cell DNA genes function by being transcribed/processed into mRNA to be translated into proteins by ribosomes
  b. transcription factors: special proteins controlling which genes turned on in specific cell at specific time
  c. receptor may act as transcription factor, carrying out transduction itself

• transduction stage usually multistep pathway involving protein activation via addition/removal of phosphate or release of other small messenger molecules/ions
  i. multiple steps can greatly amplify signal and provide more opportunities for coordination/control than simpler systems
  ii. original signaling molecule not physically passed along pathway (usually never even enters cell)
      a. signal transduced into different form in every step (usually different form = shape change via phosphorylation)

• phosphorylation cascade: relay molecules generally kinases that phosphorylate each other sequentially until final response-generating protein activated

• protein phosphatases: dephosphorylating enzymes to turn off signal transduction pathway when initial signal no longer present
  i. kinases reusable (cell can respond again to extracellular signal)

• second messengers: small, non-protein, water-soluble molecules/ions that transmit message from initial extracellular ligand to intracellular
  i. first messenger: extracellular signaling molecule
  ii. small/water-soluble → easily diffuse through cell

• cyclic AMP/cAMP: broadcasts signal to cytoplasm
  i. enzyme adenylyl cyclase converts to ATP to cAMP in response to signal
      a. epinephrine binds to receptor → activates adenylyl cyclase → catalyzes synthesis of cAMP
  ii. phosphodiesterase: enzyme converting cAMP to AMP
      a. signal not infinite and more epinephrine needed to boost signal

• calcium used even more widely than cAMP as second messenger
  i. increasing cytosolic concentration of Ca²⁺ triggers various signaling pathways

• many signaling pathways ultimately regulate protein synthesis by turning specific genes on/off
  i. final activated molecule may function as transcription factor

• may regulate protein activity instead of synthesis by activating gene expression
  i. ex: signal may cause opening/closing of ion channel or change in cell metabolism

• response doesn’t simply turn on/off: regulated in multiple ways
  i. signaling pathways amplify cell’s response to single signaling event
  a. degree of amplification depends on function of specific pathway molecules
  b. amplification effect stems from fact that proteins are active long enough to process multiple molecules of substrate before going inactive
  ii. many steps in pathway provide control points where response can be further regulated (very specific response, allows coordination with other signaling pathways)
  iii. overall response efficiency enhanced by scaffolding proteins
  iv. crucial regulation point is signal termination

• different kinds of cells turn on different gene sets → different kinds of cells have different protein collections (signal receptor/relay/response proteins)
  i. response of cell to a signal depends on its collection
  ii. means that two cells responding differently to same signal differ in proteins handling/responding to signal
  iii. different pathways may have some molecules in common
  iv. multiple pathway possibilities
  a. pathway leads to single response
  b. pathway branches → two responses
  c. cross-talk (interaction) between 2 pathways (ex: activation/inhibition)

• STP efficiency increased by scaffolding proteins: large relay proteins to which several other relay proteins are attached
  i. increases efficiency because rate of protein-protein interaction not limited by diffusion rates (doesn’t have to diffuse throughout cell to get to next protein)
  ii. scaffolding proteins may also directly activate relay proteins

• inactivation mechanisms also necessary for cell to remain capable of responding to signals
  i. cell ability to receive new signals depends on reversibility of changes produced by prior signals
  i. ex: binding signaling molecules to receptors is reversible
  a. external concentration of signaling molecules decreases → fewer receptors bound → unbound receptors revert to inactive form
  b. response only occurs at certain concentration of bound receptors; if not, relay molecules return to inactive forms and cell ready to respond to new signal

study questions

1. describe and give examples of intracellular signaling, cell surface signaling, local signaling, and hormone signaling
   intracellular signaling: the signaling molecule is small/nonpolar, and is thus able to simply diffuse through the cell membrane to reach a receptor in the cytoplasm. once bound, a hormone-receptor complex is formed, triggering a response (sometimes by going into the nucleus to turn genes on/off). one example is testosterone
   cell surface signaling: the signaling molecule binds to a receptor on the surface of the cell membrane, triggering a signal transduction pathway inside the cell. one example is how epinephrine uses the G-protein coupled receptor to increase glucose levels in the blood
   local signaling: molecules secrete signaling molecules to nearby cells. one example is contact-mediated, where the two cells are physically touching and are able to communicate via gap junctions
   hormone signaling: molecules secrete signaling molecules to travel through the bloodstream, traveling long distances to other parts of the body to reach their target cell(s). one example is insulin

2. name and describe the three steps of signaling
   reception: the ligand binds to the receptor
   transduction: the signal is transduced; a phosphorylation cascade occurs, where a pathway of proteins are phosphorylated successively, eventually activating a final protein that will carry out the response
   response: the final cellular response as a result of the signal

3. describe and give examples of GPCRs, RTKs, ligand-gated ion channels, and intracellular receptors
   GPCRs: ligand binds to receptor. activates receptor. receptor changes shape, allowing G-protein to bind. G-protein does bind. G-protein activates. GTP replaces GDP. G-protein detaches from receptor and travels through membrane until it reaches adenylyl cyclase. it binds. adenylyl cylclase is activated. adenylyl cyclase turns ATP into cAMP, which amplifies the signal, triggering a phosphorylation cascade. G-protein hydrolyzes GTP into GDP again. G-protein inactive and can no longer bind to adenylyl cyclase, so it detaches. adenylyl cyclase inactive. G-protein inactive. system reset. (e.g. epinephrine causing blood glucose levels to rise)
   RTKs: two ligands bind to two monomers (RTK proteins). binding causes them to dimerize. dimerization causes ATP to bind. tyrosine kinases phosphorylate each other until the dimer is fully phosphorylated. relay proteins recognize active RTK and phosphorylation cascade begins (e.g. epithelial growth factor)
   ligand-gated ion channels: ligand binds, changing protein shape to either block/allow in ions. changing ion concentrations trigger cellular response (e.g. calcium)
   intracellular receptors: signaling molecule small/nonpolar enough to diffuse through membrane. finds intracellular receptor. binds. forms hormone-receptor complex, which carries out response (usually going into nucleus to turn genes on/off) (e.g. steroid hormones)

1. explain how cell signaling pathways proceed via the steps of reception, transduction, and response

2. explain the steps of the epinephrine-GPCR pathway

3. predict what might happen to cellular pathways if they are interrupted or given more substrate

questions

1. are ligands and signaling molecules the same thing?