1. CELL RECEPTORS AND SIGNALLING By Phil and Alex

2. Basics Signalling controls all aspects of cell behaviour: Growth Differentiation Metabolism 3 main types of signalling: Paracrine Endocrine

3. Cell signalling 3 types of effects: 1. Altered gene transcription 2. Alter ion balance across the plasma membrane 3. Alter activity level of existing enzymes (by phosphorylation)

4. Types of receptors Receptor Tyrosine Kinases: Transmembranal – bit outside the cell binds the ligand Upon binding  dimerization (2 separate parts make 1)  activation of kinase domain  phosphorylation of intracellular proteins  signalling cascade Example: Erythropoietin  acts on Jak-Stat

5. Types of receptors Intracellular Receptors Ligand crosses plasma membrane  binds to receptor Small, uncharged, lipophilic molecules E.g. Steroids Receptors: Transcription factors or enzymes When activated they turn gene transcription on or off Some examples of steroids… Progesterone Testosterone Cortisol Aldosterone

6. Types of receptors GPCR Alpha, beta and gamma subunits `ON` state = GTP bound `OFF state = GDP bound When ligand binds to the receptor  GDP released, GTP binds  downstream signalling via stimulation of enzymes and release of secondary messengers

7. Enzymes affecting second messengers levels Sometimes these happen after a ligand binds to a GPCR: Examples: Active subunitProtein affectedActionExample Gα sAdenylate cyclase stimulated ↑ cAMP  smooth muscle relaxation Salbutamol Gα iAdenylate cyclase inhibited ↓ cAMP Adrenaline in some tissues Gα qPhospholipase C stimulated PIP2 to IP3+DAG  ↑Ca2+  smooth muscle contraction Vasopressin

8. Types of receptors Ion channel receptors A channel which will open or close depending on whether a ligand is/isn’t bound to it, or is sensitive to voltage. E.g. GLUT 4 is insulin dependent in skeletal muscle and adipose tissue

9. MEMBRANE TRANSPORT 9

10. SIMPLE DIFFUSION When things are so small and/(or) hydrophobic that they just whizz straight through the cell membrane. Always goes DOWN their concentration gradient. What does hydrophobic mean? It means things that don’t like (phobic) water (hydro) (or actually any charged things). 10 Cell membrane

11. FACILITATED DIFFUSION To help stuff go through the membrane quicker than it normally would if it was just ‘simple diffusion’. (Especially if it can’t go through by simple diffusion in the first place). Needs a protein channel/carrier/transportery thing to help it on its way 11 Cell membrane

12. ACTIVE TRANSPORT Needs a protein to help KEY POINT: goes against the concentration gradient. Therefore it needs ENERGY 12 Cell membrane

13. TYPES OF TRANSPORTER Channels Stuff just smashes straight through – therefore they’re pretty quick Carriers The protein needs to re-jig itself – which takes slightly longer Ligand-gated A ‘ligand’ needs to bind before it can open (a ligand can be any sort of small molecule) Voltage-gated Only opens if the charge across the membrane is right 13

14. THE ONES LADDSY WANTS YOU TO KNOW Passive transporters (i.e. driven by concentration gradients). 1. Glucose transporters GLUT1-4 and SGLT1(1 - intestine) +2(2 - kidneys) are the main ones The allow glucose into cells. GLUCOSE CAN KEEP GOING DOWN A CONCENTRATION GRADIENT BECAUSE AS SOON AS IT ENTERS THE CELL IT GETS TURNED INTO GLUCOSE-6-PHOSPHATE. 2. Anion exchanger (Cl - /HCO 3 - ) Regulates cell pH (by pumping HCO 3 - out). Does what it says on the tin 14

15. Active transporters (uses energy). 3. Plasma membrane Ca 2+ -ATPase. Maintains low cytoplasmic Ca 2+ concentration 4. Sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase. Same again 5. Plasma membrane Na + :K + -ATPase. 3 sodium in 2 potassium out. Helps maintain concentration gradient 6. Plasma membrane H + :K + -ATPase. 1 proton out 1 potassium in. Maintains acidic envirnment int the stomach 7. V-type and F-type H + -ATPases. V is vacuolar (creates an acidic environment). F is in the mitochondria – is SUPER IMPORTANT because is the only one which instead of USING ATP it MAKES ATP for everyone else. 8. ABC transporters. ABC = ATP Binding Cassette (basically a section where ATP can bind which a lot of proteins use). These do loads of random jobs. 15

16. Co-transport systems 9. Na + :glucose, Na + :AA, Na + :Cl - and Na + :HCO 3 - symporters. In the intestinal epithelium 10. Na + :H + and Na + :Ca 2+ antiporters. Regulate cell volume and pH. Important for action potentials 16

17. MEMBRANE POTENTIAL What is membrane potential? It’s the difference in charge across the membrane In human cells, the INSIDE of the membrane is always MORE NEGATIVE than the outside. Therefore, membrane potentials are always negative numbers DEPOLARISATION The inside of the membrane gets LESS NEGATIVE (more positive). HYPERPOLARISATION Cytoplasmic side becomes MORE NEGATIVE 17

18. What goes where? Voltage gated Na channels open at the arrival of the action potential Na enters the cell and so depolarises the membrane even more (cause it’s a positive ion going on to the negative side) Once you get past a threshold there’s an ‘explosion’ of Na ions entering the cell It all slows down once Na ions get close to their equilibrium (and the channels close) When the membrane depolarises enough the K channels open. (Positive stuff going out means the membrane potential Repolarises). Back to square one. Na + influx into cell K + efflux out of cell 12340 Time (ms) +20 0 -20 +40 -40 -60 Potential (mV) Resting potential Action potential