Green fluorescent protein: its application to the study of intracellular dynamics of steroid receptors

The green fluorescent protein (GFP) comes from the Aequorea victoria jellyfish. In vivo, it acquires the capacity of fluorescence by activation of aequorin. In vitro, it is activated by simple exposure to standard long-wave ultraviolet or blue light. GFP does not require the addition of substrate or cofactors for generation of its green light and it represents a new tool for monitoring the dynamic processes in living cells and organisms. It can produce fluorescence in I various organisms, such as bacteria, plants and mammals. GFP is now applied as a reporter for gene expression, as a marker to study cell lineage and as a tag to localize proteins in living cells. GFP does not require cell fixation or other chemical steps for analysis, contrary to immunocytochemistry. Immunodetection needs fixation and permeabilization steps, which may lead to artefacts in the pattern of localization. This would explain particularly the contradictory results about the localization of the steroid receptors. Several studies have thus been developed using GFP tagged to different steroid receptors: glucocorticoid receptor (GR), mineralocorticoid receptor (MR) and androgen receptor (AR). In absence of hormone, GFP-GR and GFP-AR were described to be cytoplasmic, while the GFP-MR was both cytoplasmic and nuclear. In presence of each specific ligand, GFP-steroid receptors were all nuclear. Formation of nuclear clusters were observed in the presence of agonists for GFP-GR and GFP-MR. GFP permits the analysis of dynamic cellular events and has already been established as an important tool for many branches of biological research.