title: Molecular rheometry: direct determination of viscosity in Lo and Ld lipid phases via fluorescence lifetime imaging.
abstract: Understanding of cellular regulatory pathways that involve lipid membranes requires the detailed knowledge of their physical state and structure. However, mapping the viscosity and diffusion in the membranes of complex composition is currently a non-trivial technical challenge. We report fluorescence lifetime spectroscopy and imaging (FLIM) of a meso-substituted BODIPY molecular rotor localised in the leaflet of model membranes of various lipid compositions. We prepare large and giant unilamellar vesicles (LUVs and GUVs) containing phosphatidylcholine (PC) lipids and demonstrate that recording the fluorescence lifetime of the rotor allows us to directly detect the viscosity of the membrane leaflet and to monitor the influence of cholesterol on membrane viscosity in binary and ternary lipid mixtures. In phase-separated 1,2-dioleoyl-sn-glycero-3-phosphocholine-cholesterol-sphingomyelin GUVs we visualise individual liquid ordered (Lo) and liquid disordered (Ld) domains using FLIM and assign specific microscopic viscosities to each domain. Our study showcases the power of FLIM with molecular rotors to image microviscosity of heterogeneous microenvironments in complex biological systems, including membrane-localised lipid rafts.
authors.affiliation: Department of Chemistry, Imperial College London, London, UK.
authors.forename: Yilei
authors.initials: Y
authors.lastname: Wu
authors.forename: Martin
authors.initials: M
authors.lastname: Stefl
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Olzyńska
authors.forename: Martin
authors.initials: M
authors.lastname: Hof
authors.forename: Gokhan
authors.initials: G
authors.lastname: Yahioglu
authors.forename: Philip
authors.initials: P
authors.lastname: Yip
authors.forename: Duncan R
authors.initials: DR
authors.lastname: Casey
authors.forename: Oscar
authors.initials: O
authors.lastname: Ces
authors.forename: Jana
authors.initials: J
authors.lastname: Humpolíčková
authors.forename: Marina K
authors.initials: MK
authors.lastname: Kuimova
mesh_terms: D008055:Lipids / Q000737:chemistry*; D015394:Molecular Structure; D012212:Rheology; D013050:Spectrometry, Fluorescence; D014783:Viscosity
chemical_list: D008055:Lipids