Cell Communication

1. Cell Communication Chapter 11

2. Recent Nobel Prizes in Medicine • 2001 – Hartwell, Hunt, & Nurse – cyclinpathways and cell cycle regulation (yeast) • 2002 – Brenner, Sulston, Horvitz – used C. elegans to elucidate the mechanism of apoptosis • 2006 – Fire & Mello - used C.elegans to discover the pathway of RNA interference

3. Simon Sez… • Signaling is a lot like the game “Simon Says…” • The signal is received: the game players hear: “Simon says take a step forward” • The signal is transduced: players must decide whether to step forward or not • The signal elicits a response: players step forward if the command was preceded by “Simon says”

4. 3 Stages of Signaling • Thesis: external signals are received & converted to responses within the cell 1. Reception • Signaling molecule binds to receptor protein = Shape Change 2. Transduction • Cascades of molecular interactions relay signals from receptor proteins to target molecules in the cell 3. Response • Regulation of transcription or cytoplasmic activities

5. Cell-to-Cell Communication • Critical for multicellular organisms • Trillions of cells must communicate in order to coordinate their activities • Recently, research is indicating how this communication can be corrupted, and cancer may occur • Although fairly unimportant for the AP Exam, crucial for understanding the breakthroughs in cancer and genetics that are occurring almost monthly now

6. Commonality? • The same set of cell signaling mechanisms show up: 1. Embryonic development 2. Hormone action • Viagra’s effects are mediated through a STP (signal transduction pathway) called “cGMP Pathway” 3. Physiology • Cascades are responsible for blood vessel dilation & inflammatory response in allergic rxns 4. Oncogenesis (Onco- means cancer) • Cyclin pathway & Oncoproteins

7. 11.1 Introduction • Cell Signaling was probably 1st for mating between microorganisms • We can look at bacteria (Saccharomycescerevisiae) • Responsible for bread, wine, & beer • 2 Mating Types (a and α) • When their mating factors bind to an external receptor on the other mating type, the cells will grow closer together • Eventually, the cells fuse together (mating) • Results in an new cell that is genetic different from either mating type

9. Mechanism? • The signal at the cell surface gets transformed into a cellular response (mating), but how? • Signal Transduction Pathway (STP) • The mechanism of the STP in prokaryotes and complex mammals is strikingly identical

10. Local vs. Long-Distance • Sometimes there is direct contact • Other times, signals travel a short distance • Called Local Regulators • Influence cells in the vicinity • Example, growth factors – compounds that stimulate nearby target cells to grow & divide • Examples include Paracrine & Synaptic signaling

13. Long Distance Signaling • Hormones (Endocrine Signals) • Endocrinologists study?? • Hormones travel by circulatory system vessels • Hormones vary widely in size • Another example, signal transmission along the length of a neuron

14. Long-Distance Signaling

15. Overview of Cell Signaling

16. 3 Stages of Cell Signaling 1. Reception • Target cell detects a signaling molecule coming from outside the cell • Cell surface receptor protein binds the signal molecule 2. Transduction • Signal molecule binds and causes conformational change in the surface receptor protein, which initiates the signal transduction pathway • Sometimes STP is a single step, but mostly multiple steps compose a STP

17. 3 Steps of Cell Signaling 3. Response • Transduced signal triggers a specific cellular response • This response can be almost any cellular activity • Catalysis by an enzyme • Rearrangement of the cytoskeleton • Activation of particular genes in the nucleus • Such as oncogene activation

18. 11.2 - Reception • Reception involves getting the signal to the correct receiver • Think of talking on a bus, others may hear, but you only want your listener to get the full message • Signaling molecules are complementary in shape to the receptor molecule • Signaling molecule sometimes called ligand • Molecule that specifically binds to another larger molecule

19. Reception (Page 2) • Usually receptor-ligand binding causes a conformational change in the receptor molecule • The conformational change activates the receptor & initiates a cascade event or STP • Can be likened to allosteric regulation of an enzyme • Shape change at one site = promotion/inhibition of enzyme activity

20. 2 Types of Signal Receptors • 1. Plasma Membrane Receptors • G Protein-Coupled Receptors • Receptor Tyrosine Kinase • Ion Channel Receptors • 2. Intracellular Receptors • Chemical messengers that pass through membrane • Steroid & thyroid hormones • Tend to carry out complete signal transduction by themselves

21. G Protein-Coupled Receptor (GPCR) • Def. – Plasma membrane receptor that works with the help of a G protein • G protein – protein that binds GTP • Common receptor • Found in yeast mating factors, epinephrine, & assorted other hormones & neurotransmitters • Also responsible for the symptoms of many bacterial diseases, such as cholera & botulism • Similarity between prokaryotes and eukaryotes evidences early evolution of G protein receptor molecules • 60% of all meds exert their influence by G protein pathways

22. G Protein-Coupled Receptors (Page 2) • All G Proteins have same basic structure • 7 α-helices spanning the membrane • Loops on either face of the membrane for binding sites

23. G Protein-Coupled Receptor Signaling molecule binds, causing conformational change in receptor. Inactive G protein binds & is activated by GTP formation. G Protein also functions as an GTPase, so makes itself inactive in the absence of continuing signals

24. Receptor Tyrosine Kinase (RTK) • Enzymatic activity • Kinase – enzyme that catalyzes transfer of phosphate group • Here, Phosphate is transferred from ATP to Tyrosines (type of amino acid) • Rapid amplification • 1 tyrosine kinase complex can activate 10 or more STPs or cellular responses

25. Receptor Tyrosine Kinase Dimerization

26. Ion Channel Receptor • Membrane receptor with a region that acts as a “gate” when the receptor changes shape. • When the signal molecule binds, gate opens or closes • Allows or blocks flow of specific ions • Such as Na+ or Ca2+ • Important in the nervous system • Neurotransmitters & synaptic transmission • Could be controlled by electrical signals instead of chemicals (Voltage-gated ion channels)

27. Ion Channel Receptor

28. Intracellular Receptor Testosterone -- Example of steroid Most intracellular receptor signals do the entire transduction on their own Testosterone behaving as a transcription factor -- controls which genes (DNA) are transcribed into mRNA

29. 11.3 Transduction • Multistep Pathway • Signal amplification • One molecule in the series, passes signal to multiple molecules of the next component • Think of dominoes • Small Signal amount  Large cellular response

30. Amplification & Pathways

31. 11.3 Transduction (Page 2) • Incoming signal is NOT physically passed along a signaling pathway • Certain information is passed on • By transduction (change form) into a different form of each protein in the cascade • Typically transduction brought about by phosphorylation (remember from CH8?)

32. 11.3 Transduction (Page 3) • Signal Transduction Pathways often involve a phosphorylation cascade • A series of different molecules in a pathway are phosphorylated in turn, each molecule adding a phosphate group to the next one in line • As each molecule is phosphorylated, it becomes active • As each molecule transfers the phosphate, it becomes inactive again • Involves enzymes called protein kinases • General term for an enzyme that transfers phosphate from ATP to a protein

33. Phosphorylation Cascade

34. Protein Kinases • Phosphorylation & dephosphorylation of proteins is the primary mechanism of cellular activity regulation • 2% of genomic content codes for protein kinases • Abnormal kinase activity leads to cancer • Protein Phosphatases • Enzymes that remove phosphate from proteins • Recycles pathway components

35. GF = Growth factor RTK = Receptor Tyrosine Kinase Ras = G Protein -- Associated with tumor growth Rho = G protein Traf = Tumor necrosis factor

36. RTK = Receptor Tyrosine Kinase GPCR – G-protein coupled receptor PDK1 = Protein dehydrogenase kinase Akt = Protein kinase -- Involved in apoptosis Apoptosis = Programmed cell death

37. Second Messengers • Capable of initiating phosphorylation cascade on their own • Signal pathway members that are nonprotein, small, and water-soluble • Cyclic AMP or Calcium ions • Called 2nd since the membrane receptor is called 1st messenger • 1st messenger can only be RTK or GPCR

38. Cyclic AMP Epinephrine (First Messenger) 1. Binds to GPCR 2. Activates G protein 3. Adenylylcyclase converts ATP  cyclicAMP -- Since 1 GPCR can affect multiple G proteins, the epinephrine signal is amplified through this pathway Why would epinephrine want its signal amplified?

39. Zimbabwe’s Cholera Epidemic • We can explain how (vibrio cholera) actually creates its symptoms • Cholera bacteria (ingested via infected H2O) secretes an enzyme (toxin) that modifies G protein • This G protein regulates cell tonicity and water secretion • Toxin interferes with GTP conversion (deactivation) • So GTP keeps activating adenylylcyclase cyclicAMP • Water and salt secretion continues when it should have shut off long ago • Amplification? • Patients require significant rehydration to compensate for fluid and electrolyte loss

40. Calcium Ions • More common 2nd messenger than cyclicAMP • Effective since intracellular concentration is so low, that any absolute change in numbers = a significant change in percentage terms • Muscle cell contractions, secretion, cell division

41. 11.4 Response • STP usually leads to a regulation of cellular activities • Response may occur in the cytoplasm or nucleus • 2 Types of typical response: 1. Enzyme activity is regulated (turned on or off) 2. Synthesis of enzymes is regulated (promoted or inhibited)

42. Transcription factors • DNA  RNA  Protein (Central Dogma) • DNA  mRNA = Transcription • mRNA  Protein = Translation • Transcription factors (TF) control which genes are transcribed • Which genes are converted into mRNA • Which genes are turned on or off • Usually TFs affect multiple genes • Typically, the final molecule in a STP is a transcription factor

44. Signal Amplification • 2 Reasons for multistep pathways: 1. Signal Amplification 2. Specificity of Cell Signaling • Signal Amplification • Intermediates in the pathway exist in active form long enough to activate numerous other molecules before becoming inactive

45. Signal Specificity • Different cells have different proteins • Different receptor proteins • Different relay proteins or molecules • Allows different cells to have a different response to the same signal • In Liver cells: Epinephrine = glycogen breakdown  glucose production • In cardiac muscle: Epinephrine = rapid & enhanced contractions

47. Scaffolding Proteins • Definition - large relay proteins that physically hold several other relay proteins • Physical proximity significantly increases efficiency of signal transfer • Scaffolding in brain cells hold together networks of signaling-pathway proteins at synapses

48. Signal Termination • Just as certain molecules are activated, they too can be inactivated • Signal molecule leaves the receptor = inactivation • Protein phosphatases inactivate phosphorylated protein kinases • Soon the target cell is returned to its inactive form, and ready to be activated again.