Fluorescent and Photocaged Lipids to Probe the Ceramide-mediated Reorganization of Biological Membranes

Abstract

This thesis describes the development of novel fluorescent and photocaged lipids, and their application as tools to probe the morphological effects of ceramide (Cer)-mediated membrane reorganization in supported lipid bilayers. Cer is a sphingolipid found in eukaryotic cells that plays a key role in regulating biological processes such as apoptosis, cell-to-cell communication, differentiation and some types of pathogenesis. Sphingolipid and cholesterol-rich lipid rafts in the plasma membrane are thought to be the point of origin for many of this lipid second messenger’s effects. Cer is formed in the exoplasmic leaflet of the plasma membrane via the enzymatic hydrolysis of sphingomyelin. The compositional complexity of biological membranes has prompted the adoption of simpler model systems to study the effects of Cer generation. When it is directly incorporated into model membranes, Cer segregates into highly ordered domains with physical properties that are distinct from those of the surrounding fluid environments. However, enzymatic generation of Cer induces complex and dynamic membrane heterogeneity that is difficult to interpret and reconcile with its direct incorporation. Here I describe the synthesis of 4-nitrobenzo-2-oxa-1,3-diazol-7-yl (NBD)-labelled cholesterol (Chol) and Cer analogs, and their use as probes in model membranes exhibiting liquid-disordered (Ld) and liquid-ordered (Lo) phase coexistence. The Chol probes reproduce the modest enrichment of Chol in Lo membrane domains as well as the Cer-induced displacement of cholesterol. One of the NBD Chol probes is used to provide direct visualization of Chol redistribution during enzymatic Cer generation, and assists in identifying new features as Cer-rich regions. The NBD-labelled Cer quantifies membrane order using orientational order parameter measurements derived from polarized total internal reflection fluorescence microscopy (pTIRFM) images. The probe reports on changes in membrane order upon enzymatic generation of Cer, and indicates a significant increase in the molecular order of Ld membrane regions that is consistent with the redistribution of Chol into these areas. The probe also identifies de novo Cer-rich domains as areas of particularly high molecular order. In the final project area, 6-Bromo-7-hydroxycoumarin-4-ylmethyl (Bhc)-caged Cers are shown to release Cer rapidly and efficiently upon irradiation with near-visible UV light. The caged lipids are then incorporated into supported membranes and photolyzed to release Cer with a high degree of spatial and temporal control. Controlled Cer generation is then used to drive protein-ganglioside clustering in lipid bilayers.