1. 1 Bi/CNS 150 Lecture 1 Monday, September 30, 2013 The ionic basis of neuroscience; Introduction to the course. Henry Lester H2OH2O K + ion carbonyl

2. 2 Who are the Bi/CNS 150 students? 3 graduate students Fields: 2 Bi 1 CNS 4 CCE 1 BE 1 ME Total undergraduate enrollment, 38 12 seniors, 20 juniors, 5 sophomores, 1 freshman Majors: 20 Biology, 3 CNS, 6 BE, 2 Ch, 3 ChE 2 Ph 2 CS Preliminary numbers

3. 3 What is the most abundant molecule in an organism? MoleculeClass VoteComments water

4. 4 Water is the most abundant molecule in an organism H 2 O MW = 18 Density ~ 1 kg/l Therefore the concentration of water in an aqueous solution is ~ (1000 g/liter )/(18 g/mol) = 55 mol/liter or 55 M. All other molecules in the body are at least 100 times less concentrated. Therefore we need to understand the properties of water.

5. 5 Extracellular conc Intracellular (Cytosol) major monovalent Ions Na + 145 mM15 mM K+K+ 4 mM150 mM Cl - 110 mM10 mM divalent cations Ca 2+ 2 mM10 -8 M Mg 2+ 2 mM0.5 mM Other ions P i -2 2 mM40 mM H+H+ 10 -7 M Protein 0.2 mM4 mM Typical extracellular and cytosolic ion concentrations (mammalian cell)

6. 6 One clue to a cell’s ionic concentrations: Sea Water Extracellular conc Intracellular (Cytosol) major monovalent Ions Na + 457 mM145 mM15 mM K+K+ 9.7 mM4 mM150 mM Cl - 536 mM110 mM10 mM divalent cations Ca 2+ 10 mM2 mM10 -8 M Mg 2+ 56 mM2 mM0.5 mM Other ions P i -2 0.7 mM2 mM40 mM H+H+ 10 -7 M Protein 0.2 mM4 mM

7. 7 Membranes provide a barrier to diffusion around cells, forming compartments Alberts 4 th 2-22 © Garland Little Alberts 12-1 © Garland... But specialized proteins (channels and transporters) control the permeation of many molecules natural or synthetic lipid bilayer Alberts 4 th 11-1 © Garland nicotine

8. 8 External Monovalent cations: High Na + Low K + Na + Internal: same as External Na + A Cell that Lacks Concentration Gradients K+K+ K+K+

9. 9 External Monovalent cations: High Na + Low K + Na + K+K+ Internal: Low Na + High K + K+K+ K+K+ K+K+ K+K+ Storing energy in a concentration gradient without osmotic stress: Simply reverse the ratio of Na + and K + K+K+

10. 10 The “Na + pump” splits ATP to make a Na + and K + concentration gradient Alberts 4 th 11-8 © Garland 3 2 From Kandel 6-5

11. 11 Na + K+K+ K+K+ K+K+ K+K+ Converting a concentration gradient to an electrical potential: Create permeability to one ionic species (K + ) Lost positive charge leads to net negative interior potential K + channels K+K+

12. 12 K+K+ K+K+ K+K+ K+K+ K+K+ The Nernst potential: the energy of discharging the concentration gradient for K + ions balances the energy of moving the K + ions through the potential difference

13. Hundreds or thousands of ions flow through a channel protein for each opening Kandel 5-19 A transporter (or pump) protein moves a few ions for each conformational change 13

14. 14 Chem 1 textbook (OGC) Figure 12-10

15. 15 Deriving the Nernst potential (chemistry units) OGN Figure 7-7

16. 16 Deriving the Nernst potential (physics units)

17. 17 Na + What is the selective advantage... that the membrane is permeable at rest to K + rather than to Na + ? [K + ] I = 140 mM; [Na + ] I = 10 mM. A leak of 10 mM: [Na + ] would increase from ~ 10 mM to 20 mM, doubling [Na + ] I and causing a 17 mV change in the Nernst potential. a small inward leak of Na + would change the internal [Na + ] by fractionally more than a small outward leakage of K + would change internal [K + ] But a similar outward leak in K + would decrease [K + ] i from 140 mM to 130 mM, causing a < 2 mV change in the Nernst potential for [K + ]. Conclusion: cell function is more stable when the resting permeability is to K +.

18. 18 Na + What is the selective advantage... that the membrane is permeable at rest to K + rather than to Na + ? Conclusion: cell function is more stable when the resting permeability is to K +. Indeed, there are many dozens of K + channels in the genome, but only ~ 10 Na + channels. K channels are metabolically “free” at rest. Important, because the “Na/K pump” splits ~ 2/3 of the brain’s ATP.

19. 19 Under what circumstances do neurons use Cl - fluxes? Apparently it’s not straightforward to make a permeability pathway that distinguishes among anions using protein side chains. Therefore there is no “anion pair” corresponding to K + / Na +. Few cells use anions to set the resting potential. But most postsynaptic inhibitory channels do use anion (mainly Cl - ) fluxes. Could neurons utilize plasma membrane H + fluxes? Probably not. There are not enough protons to make a bulk flow, required for robustly maintaining the ion concentration gradients. (but some very small organelles (~ 0.1  m) and bacteria do indeed store energy as H + gradients). Other monovalent ions

20. 20 What is the selective advantage that cells maintain Ca 2+ at such low levels? Cells made a commitment, more than a billion yr ago, to use high-energy phosphate bonds for energy storage. Therefore cells contain a high internal phosphate concentration. But Ca phosphate is insoluble near neutral pH. Therefore cells cannot have appreciable concentration of Ca 2+ ; they typically maintain Ca 2+ at < 10 –8 M. What is the selective advantage that cells don’t use Mg 2+ fluxes? The answer derives from considering the atomic-scale structure of a K + - selective channel (next slide), which received the 2003 Nobel Chemistry Prize: http://www.its.caltech.edu/~lester/Bi-150/kcsa.pdb (A suitable molecular graphics program, such as Swiss-prot viewer, must be installed on your computer) Divalent Cations

21. 21 In the “selectivity filter” of most K + channels, K + ions lose their waters of hydration and are co-ordinated by backbone carbonyl groups H2OH2O K + ion carbonyl (Like Kandel Figure 5-15)

22. 22 Atomic-scale structure of (bacterial) Na + channels (2011, 2012) shows that here, too, partial loss of water is important for permeation Views from the extracellular solution (As in Kandel Figure 5-1, Na + channels select with their side chains) Views from the membrane plane The entire water-like pathway Payandeh et al, Nature 2011; Zhang et al, Nature 2012 PDB files 4EKW, 4DXW

23. Na +, K + 1 ns (~ 10 9 /s) Na +, K +, and Ca 2+ can flow through single channels at rates > 1000-fold greater than Mg 2+ Ca 2+ 5 ns (2 x 10 8 /s) Mg 2+ 10  s (10 5 /s) As the most charge-dense cation, Mg 2+ holds its waters of hydration most tightly. Time required to exchange waters of hydration The “surface / volume” principle: We know of several Mg 2 transporters, but Mg 2+ channels apparently exist only in mitochondria & bacteria. Moomaw & Maguire, Physiologist, 200823

24. Zigmond et al. (Eds.) Fundamental Neuroscience, © Sinauer (1999)... this is crucial for learning and memory Indeed, Mg 2+ remains in the NMDA receptor channel so long... that it becomes a voltage-dependent blocker 24

25. Kandel 6-5 Primary (ATP-coupled) vs secondary (ion-coupled) pumps / transporters 25

26. 26 These gradients can be used in two ways: 1. The gradients are used for uphill “exchange” to control the concentrations of other small molecules. 2. Transient, local increases in intracellular Ca 2+ and Na + concentrations can now be used for signaling inside cells! Next image Cells have evolved elaborate processes for pumping out intracellular Na + and Ca 2+

27. 27 Ion-coupled transporters in the plasma membrane also control the levels of neurotransmitters Antidepressants (“SSRIs” = serotonin-selective reuptake inhibitors): Prozac, Zoloft, Paxil, Celexa, Luvox Drugs of abuse: MDMA Attention-deficit disorder medications: Ritalin, Dexedrine, Adderall, Strattera (?) Drugs of abuse: cocaine amphetamine Na + -coupled cell membrane serotonin transporter Na + -coupled cell membrane dopamine transporter cytosol outside Presynaptic terminals Trademarks: Marks material that won’t appear on an exam

28. 28 The “alternating access” mechanism explains both ATP-driven (primary) and ion-coupled (secondary) transport Based on structure (Ca 2+ pump) Based on biochemistry

29. 29 These proteins have evolved in a natural—perhaps necessary--way to provide that The resting potential arises via selective permeability to K + This selective permeability also leads to the Nernst potential. Transient breakdowns in membrane potential are used as nerve signals. Neuronal and non-neuronal cells also signal via transient influxes of Na + and Ca 2+. 3 classes of proteins that transport ions across membranes: modified from Alberts 4 th 11-4 © Garland Ion channels that flux many ions per event Ion-coupled transporters “Active” transporters (pumps) that split ATP (transporter)

30. 30 Transport proteins (transporters, pumps, and channels) are 5% of the human genome... ~ 1250 genes

31. 31 http://www.cns.caltech.edu/bi150/ The Bi / CNS 150 Home Page

32. 32 Come to class, please. Quizzes occur randomly, During ~ 1/3 of the lectures, And count for 10% of your grade. Exams will cover material in the lectures and the required readings in Kandel. Don’t consult previous problem sets or exams.

33. 33 https://www.coursera.org/#course/drugsandbrain Coursera Drugs and the Brain 7 weeks of lectures Partial overlap with Bi/CNS 150. Extra credit for Bi/CNS 150 students (~ 1/3 grade).. Credit will be assigned **after** we make the Bi/BNS 150 curve; Therefore you won’t be penalized for not taking the MOOC. You must complete all MOOC work by 19 December 2013

34. 34 If you drop the course, or if you register late, please email Teagan (in addition to the Registrar’s cards). Also, if you want to change sections, please email Teagan

35. 35 End of Lecture 1 Henry Lester’s office hours occur at an unusual time today: 12:30 -1:15 PM. At the usual place: Outside the Red Door