Brian G. Reid

The green fluorescent protein (GFP) from the jellyfish Aequorea Victoria forms an intrinsic chromophore through cyclization and oxidation of an internal tripeptide motif [Prasher, D. C., et al. (1992) Gene 111, 229-233; Cody, C. E., et al. (1993) Biochemistry 32, 1212-1218]. We monitored the formation of the chromophore in vitro using the S65T-GFP chromophore mutant. S65T-GFP recovered from inclusion bodies in Escherichia coli lacks the mature chromophore, suggesting that protein destined for inclusion bodies aggregated prior to productive folding. This material was used to follow the steps leading to chromophore formation. The process of chromophore formation in S65T-GFP was determined to be an ordered reaction consisting of three distinct kinetic steps. Protein folding occurs fairly slowly (k(f) = 2.44 x 10(-3) s(-1)) and prior to any chromophore modification. Next, an intermediate step occurs that includes, but is not necessarily limited to, cyclization of the tripeptide chromophore motif (k(c) = 3.8 x 10(-3) s(-1)). The final and slow step (k(ox) = 1.51 x 10(-4) s(-1)) in chromophore formation involves oxidation of the cyclized chromophore. Since the chromophore forms de novo from purified denatured protein and is a first-order process, we conclude that GFP chromophore formation is an autocatalytic process.

INTRODUCTION: For the past 30 years 2D-cell-based assay models have dominated preclinical cancer drug discovery efforts. 2D-cell-based models fail to predict in vivo efficacy, contributing to a lower success rate and higher cost required to translate an investigational new drug to clinical approval. Technological advances in 3D-cell culture models bridge the gap between 2D and in vivo models to improve upon the current success rates of cancer drug discovery. AREAS COVERED: This review focuses on the multicellular tumor spheroid (MCTS), particularly how this model can be utilized for HTS drug discovery. We discuss the current technologies for uniform culture of MCTS suitable for HTS and detection methods utilized for assay development and drug screening. EXPERT OPINION: Substantial hurdles remain before we reach the ultimate goal of robust HTS of large compound libraries with MCTS models. Specifically, we can group these challenges into three categories: MCTS growth, data collection, and data analysis. The MCTS model should be utilized with fluorescent readouts and high-content imaging with a systems biology approach to model human tumors in vitro. Such models will be more predictive of in vivo efficacy, improving on the current success rates of cancer drug discovery from bench to bedside.