Sigma-1 receptors (sigma(1) binding sites) form raft-like microdomains and target lipid droplets on the endoplasmic reticulum: roles in endoplasmic reticulum lipid compartmentalization and export.
The brain sigma-1 receptors can bind neurosteroids and psychotropic drugs, including neuroleptics and cocaine and are implicated in schizophrenia, depression, and drug dependence. In this study, we found that sigma-1 receptors specifically target lipid storage sites (lipid droplets) on the endoplasmic reticulum by forming a distinct class of lipid microdomains. Both endogenously expressing sigma-1 receptors and transfected C-terminally enhanced yellow fluorescent protein (EYFP)-tagged sigma-1 receptors (Sig-1R-EYFP) target unique "ring-like" structures associated with endoplasmic reticulum reticular networks in NG108-15 cells. The ring-like structures contain neutral lipids and are enlarged by the oleate treatment, indicating that they are endoplasmic reticulum-associated lipid droplets (ER-LDs). sigma-1 receptors colocalize with caveolin-2, a cholesterol-binding protein in lipid rafts on the ER-LDs, but not with adipocyte differentiation-related protein (ADRP), a cytosolic lipid droplet (c-LD)-specific protein. When the double-arginine ER retention signal on the N terminus of sigma-1 receptors is truncated, sigma-1 receptors no longer exist on ER-LDs, but predominantly target c-LDs, which contain ADRP. sigma-1 receptors on ER-LDs form detergent-resistant raft-like lipid microdomains, the buoyancy of which is different from that of plasma membrane lipid rafts. (+)-Pentazocine causes sigma-1 receptors to disappear from the microdomains. N-Terminally EYFP-tagged sigma-1 receptors (EYFP-Sig-1R) failed to target ER-LDs. EYFP-Sig-1R-transfected cells showed an unrestricted distribution of neutral lipids all over the endoplasmic reticulum network, decreases in c-LDs and cholesterol in plasma membranes, and the bulbous aggregation of endoplasmic reticulum. Thus, sigma-1 receptors are unique endoplasmic reticulum proteins that regulate the compartmentalization of lipids on the endoplasmic reticulum and their export from the endoplasmic reticulum to plasma membrane and c-LDs.
Cellular Pathobiology Unit, Cellular Neurobiology Research Branch, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Department of Health and Human Services, Baltimore, MD 21224, USA.
Teruo
T
Hayashi
Tsung-Ping
TP
Su
D000818:Animals; D001426:Bacterial Proteins / Q000378:metabolism; D001665:Binding Sites; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006168:Guinea Pigs; D006825:Hybridomas; D007425:Intracellular Membranes; D008164:Luminescent Proteins / Q000378:metabolism; D051379:Mice; D017434:Protein Structure, Tertiary; D051381:Rats; D017480:Receptors, sigma / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D000097605:Sigma-1 Receptor
D001426:Bacterial Proteins; D008164:Luminescent Proteins; D017480:Receptors, sigma; D000097605:Sigma-1 Receptor; C054562:yellow fluorescent protein, Bacteria