title: Systematic colocalization errors between acridine orange and EGFP in astrocyte vesicular organelles.
abstract: Dual-color imaging of acridine orange (AO) and EGFP fused to a vesicular glutamate transporter or the vesicle-associated membrane proteins 2 or 3 has been used to visualize a supposedly well-defined subpopulation of glutamatergic astrocytic secretory vesicles undergoing regulated exocytosis. However, AO metachromasy results in the concomitant emission of green and red fluorescence from AO-stained tissue. Therefore, the question arises whether AO and EGFP fluorescence can be distinguished reliably. We used evanescent-field imaging with spectral fluorescence detection as well as fluorescence lifetime imaging microscopy to demonstrate that green fluorescent AO monomers inevitably coexist with red fluorescing AO dimers, at the level of single astroglial vesicles. The green monomer emission spectrally overlaps with that of EGFP and produces a false apparent colocalization on dual-color images. On fluorophore abundance maps calculated from spectrally resolved and unmixed single-vesicle spectral image stacks, EGFP is obscured by the strong green monomer fluorescence, precluding the detection of EGFP. Hence, extreme caution is required when deriving quantitative colocalization information from images of dim fluorescing EGFP-tagged organelles colabeled with bright and broadly emitting dyes like AO. We finally introduce FM4-64/EGFP dual-color imaging as a remedy for imaging a distinct population of astroglial fusion-competent secretory vesicles.
authors.affiliation: INSERM, U603, Paris, France.
authors.forename: Fabien
authors.initials: F
authors.lastname: Nadrigny
authors.forename: Dongdong
authors.initials: D
authors.lastname: Li
authors.forename: Klaus
authors.initials: K
authors.lastname: Kemnitz
authors.forename: Nicole
authors.initials: N
authors.lastname: Ropert
authors.forename: Annette
authors.initials: A
authors.lastname: Koulakoff
authors.forename: Stephanie
authors.initials: S
authors.lastname: Rudolph
authors.forename: Marco
authors.initials: M
authors.lastname: Vitali
authors.forename: Christian
authors.initials: C
authors.lastname: Giaume
authors.forename: Frank
authors.initials: F
authors.lastname: Kirchhoff
authors.forename: Martin
authors.initials: M
authors.lastname: Oheim
mesh_terms: D000165:Acridine Orange / Q000032:analysis* / Q000737:chemistry*; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000502:physiology* / Q000648:ultrastructure; D002540:Cerebral Cortex / Q000502:physiology; D049452:Green Fluorescent Proteins / Q000737:chemistry*; D007700:Kinetics; D051379:Mice; D008815:Mice, Inbred Strains; D008856:Microscopy, Fluorescence; D015388:Organelles / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D049452:Green Fluorescent Proteins; D000165:Acridine Orange