Visualization of the spatial and temporal dynamics of intracellular signaling

Enzyme activity – it's all about image

Unraveling the functional roles of proteins is a major challenge facing the postgenome researcher. Advances towards this goal have been made through the development of both chemical and biochemical tools for monitoring protein activity. Recently, a myriad of fluorescence-based imaging tools have emerged for in vitro, in vivo and whole animal applications. These tools have provided methods to monitor the spatial and temporal distribution of proteins and bioorganic molecules dynamically. Here, recent advances in chemical and biochemical techniques that allow the detection of enzymatic activity within intact cells and in vivo are reviewed. Such technologies have the potential to be integrated into drug-development programs to facilitate both the functional validation of pharmaceutical targets and the treatment of human disease.

Mechanism of Csk-mediated down-regulation of Src family tyrosine kinases in epidermal growth factor signaling

The Src family tyrosine kinases (SFKs) play pivotal roles as molecular switches that link a variety of extracellular cues to intracellular signaling pathway. The function of SFK is regulated by phosphorylation at the C-terminal regulatory site mediated by Csk. Recently a novel SFK target Cbp (or PAG) was identified as a membrane-anchored scaffold protein for Csk. To establish the mechanism of Csk/Cbp-mediated regulation of SFK in vivo, we observed dynamic changes in the interaction of Csk with Cbp by utilizing fusion proteins with modified green fluorescent proteins: cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP). Upon SFK activation induced by epidermal growth factor stimulation, fluorescent resonance energy transfer (FRET) response was detected transiently at membrane ruffles in COS1 cells co-expressing CFP-Csk and Cbp-YFP and in cells expressing a single-molecule FRET indicator consisting of CskSH2 and Cbp. Suppression of SFK by PP2 or use of a mutant Cbp that lacks the Csk binding site abolished the FRET response, although a dominant-negative form of Csk enhanced and sustained the FRET response, demonstrating that the FRET response is dependent upon the SFK activity. These observations show that Csk/Cbp-mediated down-regulation of SFK takes place at membrane ruffles in an early stage of epidermal growth factor signaling and suggest that the Csk/Cbp-based FRET indicators are useful for monitoring the status of SFK in living cells.

Fluorescence imaging of physiological activity in complex systems using GFP-based probes

Genetically encoded probes for the optical imaging of excitable cell activity have been constructed by fusing fluorescent proteins to functional proteins that are involved in physiological signaling systems, such as those that control membrane potential, free calcium and cyclic nucleotide concentrations and pH. Using specific promoters and targeting signals, the probes are introduced into an intact organism and directed to specific tissue regions, cell types, and subcellular compartments, thereby extracting specific signals more efficiently and in a more relevant physiological context than before. Optical imaging using genetically encoded probes has enabled us to decipher spatio-temporal information coded in complex tissues.

Oscillatory expression of Hes1, p53, and NF-kappaB driven by transcriptional time delays

Feedback inhibition of gene expression is a widespread phenomenon in which the expression of a gene is downregulated by its protein product. Feedback in eukaryotic cells involves time delays resulting from transcription, transcript splicing and processing, and protein synthesis. In principle, such delays can result in oscillatory mRNA and protein expression. However, experimental evidence of such delay-driven oscillations has been lacking. Using mathematical modeling informed by recent data, I show that the observed oscillatory expression and activity of three proteins is most likely to be driven by transcriptional delays. Each protein (Hes1, p53, and NF-kappaB) is a component of a short feedback inhibition loop. The oscillatory period is determined by the delay and the protein and mRNA half-lives, but it is robust to changes in the rates of transcription and protein synthesis. In contrast to nondelayed models, delayed models do not require additional components in the feedback loop. These results provide direct evidence that transcriptional delays can drive oscillatory gene activity and highlight the importance of considering delays when analyzing genetic regulatory networks, particularly in processes such as developmental pattern formation, where short half-lives and feedback inhibition are common.