The bioluminescence resonance energy transfer (BRET) method is based on resonance energy transfer between a light-emitting enzyme and a fluorescent acceptor. Since its first description in 1999, several versions of BRET have been described using different substrates and energy donor/acceptor couples. Today, BRET is considered as one of the most versatile techniques for studying the dynamics of protein-protein interactions in living cells. Various studies have applied BRET-based assays to screen new receptor ligands and inhibitors of disease-related-proteases. Inhibitors of protein-protein interactions are likely to become a new major class of therapeutic drugs, and BRET technology is expected to play an important role in the identification of such compounds. This review describes the original BRET-based methodology, more recent variants, and potential applications to drug screening.

Endospanin-1 is a negative regulator of the cell surface expression of leptin receptor (OB-R), and endospanin-2 is a homologue of unknown function. We investigated the mechanism for endospanin-1 action in regulating OB-R cell surface expression. Here we show that endospanin-1 and -2 are small integral membrane proteins that localize in endosomes and the trans-Golgi network. Antibody uptake experiments showed that both endospanins are transported to the plasma membrane and then internalized into early endosomes but do not recycle back to the trans-Golgi network. Overexpression of endospanin-1 or endospanin-2 led to a decrease of OB-R cell surface expression, whereas shRNA-mediated depletion of each protein increased OB-R cell surface expression. This increased cell surface expression was not observed with OB-Ra mutants defective in endocytosis or with transferrin and EGF receptors. Endospanin-1 or endospanin-2 depletion did not change the internalization rate of OB-Ra but slowed down its lysosomal degradation. Thus, both endospanins are regulators of postinternalization membrane traffic of the endocytic pathway of OB-R.Leptin and the leptin receptor are key proteins in pathways regulating energy balance and body weight in both humans and rodents (1-3). The leptin receptor is a single membrane-spanning protein of the class I cytokine receptor family. The leptin receptor gene is transcribed in a number of different messenger RNAs by alternative promoter usage and mRNA splicing (4). Most of these transcripts are splice variants that encode receptor isoforms with different cytoplasmic tails, which are referred to as OB-Ra, OB-Rc, OB-Rd (short isoforms), and OB-Rb (long isoform). The long splice variant OB-Rb mediates the majority of the effects of leptin, mainly via the JAK-STAT signaling pathway (1, 5, 6). The exact function of the short isoforms is still unclear. Short and long isoforms follow very similar intracellular membrane trafficking pathways (7-9).Recently, we identified a negative regulator of leptin receptor function (10). This protein, originally named leptin receptor gene-related protein, or OB-RGRP, is encoded by a transcript expressed from the leptin receptor gene (11). The OB-RGRP mRNA shares two exons that are untranslated in leptin receptor transcripts and contains two additional exons that are absent in leptin receptor transcripts. This transcript is named LEPROT (leptin receptor overlapping transcript). This overlapping gene structure is conserved in humans and rodents (11,12). We named this protein endospanin-1, due to its topology and intracellular localization. We have shown that endospanin-1 down-regulates leptin signaling by lowering the expression of the receptor at the cell surface and thus the sensitivity of the cell to leptin. Furthermore, the physiological relevance of endospanin-1 as a regulator of OB-R function was evaluated in mice. The expression of a lentivirus-delivered short hairpin RNA (shRNA) directed against endospanin-1 in the arcuate nucleus of the hypothalamus prevented the...

Edited by Michael R. BubbKeywords: Bioluminescence resonance energy transfer LEPR Heteromerization Yellow fluorescent Protein for energy transfer Protein-protein interaction Obesity a b s t r a c t Leptin mediates its metabolic effects through several leptin receptor (LEP-R) isoforms. In humans, long (LEPRb) and short (LEPRa,c,d) isoforms are generated by alternative splicing. Most of leptin's effects are believed to be mediated by the OB-Rb isoform. However, the role of short LEPR isoforms and the possible existence of heteromers between different isoforms are poorly understood. Using BRET1 and optimized co-immunoprecipitation, we observed LEPRa/b and LEPRb/c heteromers located at the plasma membrane and stabilized by leptin. Given the widespread coexpression of LEPRa and LEPRb, our results suggest that LEPRa/b heteromers may represent a major receptor species in most tissues.