Solvatochromic reporter to image plasma membrane order leaflet by leaflet reveals a highly asymmetric bilayer locally modulated by transbilayer interactions

The plasma membrane (PM) of cells is an asymmetric bilayer composed of a complex mixture of lipids and proteins. The emergent biophysical properties of its leaflets and their consequence on cellular function remain unexplored owing to the limitation in available probes. In this manuscript, we report on the design and characterisation of a PM-localised solvatochromic probe, C3L, based on the established reporter Laurdan. C3L is retained at the inner-leaflet due to flippase and scramblase activity, as determined by Bovine Serum Albumin-based back exchange experiments. By measuring the Generalized Polarization of C3L we are able to determine membrane order, leaflet-by-leaflet at the PM. Our results reveal that the PM is made of a tightly-packed outer/exofacial-leaflet in contrast to a disordered inner/cytoplasmic-leaflet. This differential packing is established by the maintenance of lipid asymmetry, the presence of specific lipids, Sphingomyelin in the outer-leaflet and Phosphatidylserine at the inner-leaflet, and exaggerated during mesenchymal to epithelial transition. We find spatial signatures of local modulation of the inner leaflet by protein scaffolds which promote trans-bilayer coupling of lipids. C3L thus informs on steady state lipid organisation at the asymmetric membrane while allowing further exploration of spatial regulation of lateral heterogeneity at each leaflet.