Ion Homeostasis, Channels, and Transporters An Update on Cellular Mechanisms

2. Ion Homeostasis, Channels, and Transporters An Update on Cellular Mechanisms

3. Ion Homeostasis, Channels, and Transporters An Update on Cellular Mechanisms

4. Basic Concept 1: Compartmentation of Ionic Pools and Electrochemical Driving Forces

5. Basic Concept 2: Categories of Ion Permeability Pathways

6. Basic Concept 3: Disparate Mechanisms for Ion Flux via Channels versus Transporters - The Channel Story Minimal energetic interaction between the transported ion and the channel protein Ionic flux is limited by opening and closing of a single major gate Gating is regulated by conformational changes extrinsic to the permeability barrier or pore

7. Basic Concept 3: Disparate Mechanisms for Ion Flux via Channels versus Transporters - The Transporter Story There are strong and selective energetic interactions between the transported ion(s) and the transporter protein Ionic flux is limited by the alternating opening and closing of two gates Both gates can be simultaneously closed to produce trapping or occlusion of the transported ion(s) within the permeability barrier Movement of each gate is regulated by conformational changes intrinsic to the permeability barrier

8. Basic Concept 3: Disparate Mechanisms for Ion Flux via Channels versus Transporters - The Transporter Story

9. Basic Concept 3: Disparate Mechanisms for Ion Flux via Channels versus Transporters

10. Basic Concept 4: Disparate Mechanisms for Ion Flux via Channels versus Transporters

11. Ion Transport Proteins as Channels versus Carriers: Not as different as we think Concept: Both channel-like activity and transporter-like activity can be accommodated within the basic structures of most transport proteins Model Example: The induction of channel activity in the Na+,K+-ATPase pump upon binding of palytoxin, a marine toxin, that stabilizes both gates of the Na+ pump in the open state

12. Ion Transport Proteins as Channels versus Carriers: Not as different as we think Palytoxin is a lethal toxin from a marine coelenterate (Palythoa coral) Palytoxin binding to the Na+ pump induces appearance of non-selective cation channel activity

13. Ion Transport Proteins as Channels versus Carriers: Not as different as we think Model: guinea pig ventricular myocytes Outside-out membrane patch recording in symmetric [NaCl] Palytoxin (PTX) induces Na+ channel activity independent of ATP However, ATP still acts as a positive allosteric regulator

14. Ion Transport Proteins as Channels versus Carriers: Not as different as we think PTX stabilizes opening of both gates of the Na+ pump, i.e., no occluded state Permeability/ conformation of the non-occluded pore remains allosterically “sensitive” to ATP and K+ K+ acts as a negative allosteric regulator of PTX-induced Na+ “pump-channels”

15. Ion Transport Proteins as Channels versus Carriers: Not as different as we think ClC-family proteins comprise a large family of structurally related membrane proteins that function as Cl- channels in eukaryotic cells The resolved crystal of a prokaryotic member - ClC-ec1 from E. coli - has provided the structural template BUT…….

16. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

17. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

18. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island PDZ (PSD-95, discs large, ZO1) domains are protein-protein interaction sites found in a large number of adapter proteins Such adapter proteins act to localize channels or transporters within large signaling complexes at the sub-membrane cytoskeleton Examples include the PSD-95 (post-synaptic density) adapter that co-assembles neurotransmitter-gated ion channels with cytoskeletal elements, kinases, and small GTPases into signaling complexes at the neuronal synapses NHERFs (Na+/H+ Exchanger Regulatory Factors) comprise another family of PDZ-containing adapters expressed in many epithelia

19. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island Structure of the related NHERF1 and NHERF2 ERM domains bind to cytoskeletal proteins while PDZ domains bind to various transport proteins and signaling proteins Recent studies have shown that NHERFs can also bind the parathyroid hormone receptor (PTH-R) and the type 2 Na-Phosphate Cotransporter (NPT2)

20. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

21. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

22. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

23. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

24. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

25. Interactions of Ion Transport Proteins with Adapter Proteins: No transporter is an island

26. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

27. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

28. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

29. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

30. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

31. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

32. Interactions of Ion Transport Proteins with Local Lipids: The bilayer as more than a low dielectric permeability barrier 

33. Interactions among Ion Transport Proteins and Modulator Proteins: Cell-specific context explains all

34. Interactions among Ion Transport Proteins and Modulator Proteins: Cell-specific context explains all

35. Interactions among Ion Transport Proteins and Modulator Proteins: Cell-specific context explains all

36. Interactions among Ion Transport Proteins and Modulator Proteins: Cell-specific context explains all

37. Interactions among Ion Transport Proteins and Modulator Proteins: Cell-specific context explains all

38. Interactions among Ion Transport Proteins and Modulator Proteins: Cell-specific context explains all

39. Take-Home Lessons Precise homeostasis of the major inorganic cations (Na+, K+, H+) and anions (Cl-, PO43-, HCO3-) is fundamental to all cells However, cell-specific expression of different membrane transport proteins and regulatory factors permits wide variations in the absolute rates of transmembrane flux of these ions These cell-specific differences in ionic flux are exploited for tissue-specific differences in function such as solute flow (e.g. transepithelial movements of metabolites) or information transfer

40. Take-Home Lessons These tissue-specific differences in ionic flux are regulated at multiple levels: via increased/ decreased expression of membrane transport protein genes via changes in the steady-state trafficking of membrane transport protein to and from the plasma membrane via direct post-translational modification (e.g. phosphorylation) of the membrane transport proteins via direct association with tissue-specific adapter or modulator proteins via the local lipid composition of the membrane bilayer

41. References: Original Research Papers Accardi and Miller (2004) Nature 427: 803-807 Artigas and Gadsby (2003) PNAS 100: 501-505 Chuang, Prescott, Kong, Shields, Jordl, Basbaum, Chao, and Julius (2001) Nature 411: 957-962 Dutzler, Campbell, Cadene, Chait, and MacKinnon (2002) Nature 415: 287-294 Mahon, Cole, Lederer, and Segre (2003) Mol Endocrin 17: 2355-2364 Mahon, Donowitz, and Segre (2002) Nature 417: 858-861 Prescott and Julius (2003) Science 300: 1284-1288 Shenolikar, Voltz, Minkoff, Wade, and Weinman (2002) PNAS 99: 11470-11475

42. References: Reviews and Commentaries Channel versus Transporter Mechanisms Hilgemann (2003) PNAS 100: 386-388 Gadsby (2004) Nature 427: 795-796 Adapter/ PDZ Proteins and Channel/ Transporter Regulation Shenolikar and Weinman (2001) Am J Physiol - Renal 280: F389-F395 Noury, Grant, and Borg (2003) Science-STKE 179-RE7: 1-12 PIP2 and Channel/ Transporter Regulation O’Neill and Brown (2003) News Physiol Sci 18: 226-231 Hilgemann, Feng, and Nasuhoglu (2001) Science-STKE 111-RE19: 1-8 Caterina and Julius (2001) Annu Rev Neurosci 24: 487-517 Modulator/ FXYD Proteins and Channel/ Transporter Regulation Cornelius and Mahmmoud (2003) News Physiol Sci 18: 119-124 Crambert and Geering (2003) Science-STKE 166-RE1: 1-9

43. References: Textbooks Boron and Boulpaep (2002) Medical Physiology [Saunders] Alberts et al. (2001) Molecular Biology of the Cell, 4th Edition [Garland] Lodish et al. (2000) Molecular Cell Biology, 4th Edition [W.H. Freeman & Co.]