Laura ToppoziniCAP Congress 2014June 19, 2014

Cholesterol Structure in Lipid Rafts

Cholesterol in Lipid Membranes

Singer and Nicholson, 1972

Simons, 1997

Mosaic model:Lipids, cholesterol, and

proteins homogeneously mixed

Lipid raft model:Cholesterol-rich domains

What is cholesterol’s role in rafts?

Project Details

20,000- molecule simulations

• What is cholesterol’s role in lipid membranes?

• Can we see raft formation in a binary system?

• What can binary system rafts elucidate about rafts in biological membranes?

Neutron experiment at Canadian Neutron

Beam Centre

Coarse-grained MD simulations from collaborators at University of Mainz

Neutron diffraction with stacked,

oriented bilayers

L Toppozini, S Meinhardt, CL Armstrong, Z Yamani, N Kucerka, F Schmid, MC Rheinstadter. The Structure of Cholesterol in Lipid Rafts. Submitted, under review.

~40Å

20 mol%

~100Å

Liquid-ordered phase

Simulation: CG Molecular Dynamics

60 mol%

Liquid-ordered phase

Experiment: Neutron Scattering

supe

rres

olut

ion

{nm

ξ= λ2

∆ λ

Larger λ

Smaller λ

Cholesterol in Lipid Membranes

32.5% cholesterol Liquid–ordered phase

Selective Deuteration

Selectively highlight molecules

Cholesterol Structure

Cholesterol Structure

Liquid-DisorderedCholesterol-lipid

complexes

Umbrella model

Huang & Feigenson. Biophys J. 1999 76(4):2142-57

Cholesterol Structure

Cholesterol Plaques-Cholesterol bilayers/plaques

-Triclinic unit cella=b=12.8 Åα=95˚β=91.9˚γ=98.1˚

Ordered-Complexes-Ordered lipid-cholesterol complexes-Monoclinic unit cella=b= 11Åα= 131˚

Cholesterol Structures in Rafts

The liquid-ordered phase of a binary system is heterogeneous

Domains in binary systems may be nuclei which lead to rafts in biological membranes.

Small, (~10nm) transient (~100ns) structures are observed in lipid-cholesterol system

Observation of three coexisting cholesterol

structures in the lo phase:

Liquid disordered

Ordered lipid/cholesterol

Cholesterol plaques

Acknowledgements

Maikel C.RheinstadterClare ArmstrongNorbert KucerkaZahra YamaniSebastian MeinhardtFriederike Schmid

Lipid Structure

Liquid-disorderedfluid-like,

hexagonal lipid structure

Liquid-Ordered Rafts

Gel-like monoclinic lipid structure of

lipids in the rafts.

alipid-raft

blipid-raft

Simulations: changing chemical potential

Lipid/cholesterol phase diagram

Membranes, Rafts, and Scattering

~3000 bilayers/wafer give larger signals

Hydrated lipid bilayershave diffusive motion: signals are weak

• Typical neutron scattering setupgives large spatial averaging

• Gives bulk structure

• Are small structures being formed but averaged over?

Measurement of Rafts

neutron

Long coherence length

ξ

Short coherence length

ξ

ξ= λ2

∆ λ∝ √𝐸∆𝐸

wave package

ξξ

Condensation and the Umbrella Model

0Phospholipid head-group acts as umbrella, shields hydrophobic regions of cholesterol

0Cholesterol proximity suppresses tail fluctuations

0These complexes may form domains in lo phase at moderate cholesterol concentrations

Experiment: Neutron Scattering

0Change coherence length in-situ by changing wavelength

0Large ξ setup: averaging over large areas.

0Smaller ξ setup: integrates more efficiently over smaller structures