Development of Radiotracers and Fluorescent Probes for Imaging Sigma-2 Receptors In Vitro and In Vivo
Recent studies have suggested that σ2 receptors are localized in lipid ras of the plasma membrane (Torrence-Campbell and Bowen 1996). Lipid ras are microdomains in the cell membrane and are believed to play a key role in both secretory and endocytotic pathways of eukaryotic cells. Lipid ras are enriched with cholesterol, sphingolipids, and glycosylphosphatidylinositol-linked proteins that form specialized structures termed as caveolin on the incorporation of a cholesterol-binding protein. Using sucrose density centrifugation of extracts of rat liver P2 membranes, Gebreselassie and Bowen (2004) demonstrated [3H] DTG-binding in protein fractions containing otillin-2, a molecular marker of lipid ras. Further in vitrobinding studies revealed that [3H]DTG binding was blocked by the σ2-selective ligand CB-64D and not by
the σ1 receptor ligand (+)-pentazocine, conrming that σ2 receptors are colocalized in lipid ras. Other in vitrobinding studies using [3H]DTG in the presence of (+)-pentazocine have shown that σ2 receptor-binding sites are located in the mitochondria (Wang et al. 2003). is diverse distribution of σ2 receptors between the plasma membrane and cell organelles such as the mitochondria and endoplasmic reticulum further emphasized the need for the development of uorescent probes to study the subcellular localization of σ2 receptors using high-resolution imaging techniques such as confocal and two-photon microscopy (Zeng et al. 2007).