Lipid Membranes: The Strange Mechanics of Form and Function

1. Lipid Membranes: The Strange Mechanics of Form and Function Tristan Ursell Applied Physics, Caltech December 2nd, 2008

2. What to Walk Away With Lipids play crucial roles in molecular biology on par with proteins and DNA. Membranes are a strange material with no macroscopic analog, whose properties directly relate their function. Rough overview of membrane function, and its connection to mechanical (‘mesoscopic’) models. Knowledge of two cases studies in lipid mechanics: lateral lipid organization and membrane-protein interactions.

3. Unique Role of Lipids What is the modern conception of molecular biology? What makes these molecules unique? - endogenously produced and regulated - modular design - critical in actively maintaining homeostasis Information flow diagram: but is this the only kind of relevant information? - lacks any notion of ‘where’ - lacks any notion of ‘who’ / self and non-self

4. Unique Role of Lipids What is the modern conception of molecular biology?

5. A Strange Molecule Like DNA base-pairs, structure relates to function when lipids are many. polar / hydrophilic 2-3 nm apolar / hydrophobic ~ 1 kilo Dalton

6. Strange Molecules Like DNA base-pairs: lipids self-organize and structure relates to function when they are many liquid crystal enthalpy entropy

7. Like Other Materials Mesoscopically, bilayers form an elastic material ... stretch bend compress

8. Not Like Other Materials … with one very unique property. In-plane the bilayer acts like a 2D fluid.

9. Not Like Other Materials Bilayers ‘self-heal’

10. Not Like Other Materials Bilayers ‘self-heal’ alleviate stress by fracture alleviate stress by adding material

11. Form and Function Bilayers relieve internal stresses - Can easily perform a number of useful functions.

12. Form and Function Bilayers relieve internal stresses - Can take on amazing shapes like no other material.

13. What’s in a membrane? Creative Commons Lic. ‘Lipocentric’ view: How are lipids laterally organized? What are the resultant membrane morphologies?

14. A Dynamic Relationship entropy enthalpy T

15. A Dynamic Relationship entropy enthalpy dimpled domains T

16. Surprising Organization Elastic interactions lead to spatial order. Is this a fluid? orientational order translational order 10 um ~ 4 um 2.25um

17. Many Shapes Mechanics predict two (non-flat) morphologies. buds dimples 10 um 10 um

18. Changing Shapes New forms of transport observed in vitro.

19. Acknowledgements Thanks to: Rob Phillips Kerwyn Huang Sarah Keller Funding: NSF CIMMS NIH Director’s Pioneer Award

20. Surprising Organization Elastic interactions lead to spatial ordering. Crowds are liquid crystals. This crowd has about 3000 people. How big a crowd would we need to be analogous? 30,000,000 people

21. The Generic Barrier Together lipids form the cell’s generic chemical barrier.

22. The ‘Algebra’ of Morphology • Unique, stable morphologies exist with well-defined free energies and precise transition rules.

23. Membrane-Protein Interactions

24. Key Questions • Are there forces which spatially organize lipid domains and affect phase separation? • If so, what is their origin, and how do they depend on bilayer mechanical properties? • Can we make quantitative measurements of the connection between bilayer mechanics and morphology?

25. Mechanical Properties of Domains • If morphology is important, what affects it? • Bending – prefers flat (kBT) • Membrane Tension – prefers flat (kBT/nm2) • Line Tension – prefers non-flat (kBT/nm) • Domain Area (nm2 ) • In vesicles – surface area / volume constraint Experiment: Dietrich et al 2001; Veatch and Keller 2003; Baumgart et al 2003; Bacia et al 2005; Yanagisawa et al 2007 Theory: Lipowsky 1992; Julicher and Lipowsky 1996, Taniguchi 1996; Gozdz and Gompper 2001 ; Harden et al 2005; Sens and Turner 2006;

26. The ‘Dimpling’ Transition • Flat domains ‘pop’ into dimples • An energetic competition between bending, line tension, and domain size. Boundary Slope 3 um Dimensionless Area

27. The ‘Dimpling’ Transition • Flat domains ‘pop’ into dimples • An energetic competition between bending, line tension, and domain size. Dimensionless Line Tension 3 um Dimensionless Area

28. Dimpled Domains Interact • How does dimpling explain the observed behavior? • Dimpled domains in proximity interact elastically. • Mechanical models can predict the form of these interactions. 3 um 3 um

29. Dimpled Domains Interact • Statistics of domain motion encode information about the interaction, and compare well with theory. 10 um

30. Effects on Coalescence • Elastic interactions maintain heterogeneity on long time-scales and short length-scales. High tension – domains lie flat, do not interact, and coalesce rapidly Low tension – domains dimple, interact, and coalesce very slowly

31. The ‘Budding’ Transition • Experimental observations revealed another morphology.

32. The ‘Budding’ Transition • Experimental observations revealed another morphology. • - Why do only certain domains bud?

33. The ‘Budding’ Transition • We can calculate the budding phase-diagram, which shows three, size-selective regions. Dimensionless Line Tension Dimensionless Area

34. Domain Budding • Domain budding is size-selective, and measures elastic parameters Previous Experiment Baumgart et al 2003; Tian et al 2007

35. Mapping Out ‘Reality’ • Using FEM analysis, leave all the approximation behind… • Where are the exact phase boundaries between morphologies? • What are their precise shapes and energies? • Are there other morphologies? (yes)

36. Experimental Setup • Giant unilamellar vesicles are electroformed above the transition temperature (~50C). • Mixture of DOPC (18 carbon tail, Ld, rhodamine labeled), DPPC (16 carbon tail, Lo, perylene labeled) and cholesterol • Carefully control osmotic pressure to regulate surface tension. Veatch et al (2005); Veatch and Keller (2003); Baumgart et al (2003)

37. At first glance … • What is ‘wrong’ with this picture? Phase kinetics did not look like a simple binary fluid. 10 um Alexander Wagner NDSU

38. A Hint of Morphology • Observed domain shapes alluded to the importance of morphology. 10 um 10 um 10 um Dietrich et al 2001; Baumgart et al 2003; Veatch and Keller 2003; Bacia et al 2005; Yanagisawa et al 2007

39. A Hint of Morphology • Observed domain shapes alluded to the importance of morphology. 10 um 10 um 10 um Baumgart et al (2003) Dietrich et al 2001; Veatch and Keller 2003; Bacia et al 2005; Yanagisawa et al 2007, Baumgart et al 2003

40. Morphological Transitions • Most of these transitions are observable in vitro

41. Lipids in vivo: Macrophages • Cells labeled with DPPE-Rhod / DOPE-NBD show that only hydrophobicity determines phases

42. Chemical Structures • DPPC:DOPC:Chol:DPPE-Rhod:Perylene(80:35:30:2:3.5)

43. The Dimpling Transition

44. Boundary Corrections • Motion constrained to a circle is subject to a ‘fictitious’ potential. • Also assume surface metric is flat (in our focal plane).

45. Estimating Line Tension • Budding Condition • Using which onesdid and did not bud

46. What is a lipid domain? • Size 50-500nm • Membrane-protein organization, signaling, endo- and exo-cytosis, virus uptake, etc…