Label-free imaging of cholesterol and lipid distributions in model membranes

TL;DR

This paper introduces a label-free Raman imaging technique combined with chemometrics to visualize and analyze cholesterol and lipid distributions in model membranes, overcoming limitations of fluorescence methods.

Contribution

It presents a novel, non-invasive method for detailed, chemical-specific imaging of lipid membranes, including cholesterol, at the single-membrane level.

Findings

Successful visualization of lipid and cholesterol domains without labels

Confirmation of features observed in fluorescence microscopy

Unprecedented analysis of cholesterol distribution in membranes

Abstract

Over recent decades, lipid membranes have become standard models for examining the biophysics and biochemistry of cell membranes. Interrogation of lipid domains within biomembranes is generally done with fluorescence microscopy via exogenous chemical probes. However, most fluorophores have limited partitioning tunability, with the majority segregating in the least biologically relevant domains (i.e., low-density liquid domains). Therefore, a molecular-level picture of the majority of non-labeled lipids forming the membrane is still elusive. Here, we present simple, label-free imaging of domain formation in lipid monolayers, with chemical selectivity in unraveling lipid and cholesterol composition in all domain types. Exploiting conventional vibrational contrast in spontaneous Raman imaging, combined with chemometrics analysis, allows for examination of ternary systems containing saturated lipids, unsaturated lipids, and cholesterol. We confirm features commonly observed by fluorescence microscopy, and provide an unprecedented analysis of cholesterol distribution at the single-membrane level.