In a signal transduction diagram, arrows are generally used to link molecules to show enzymatic reactions and intermolecular interactions. To obtain an exhaustive understanding of a signal transduction system, however, the diagram must contain three axes in the space and the time base, because all events are regulated ingeniously in space and time. The scale over time and space is ignored in biochemical approaches in which electrophoresis is applied to a specimen prepared by grinding millions of cells. A farseeing article entitled, “Fluorescence Imaging Creates a Window on the Cell,” was written by Roger Tsien in 1994, which appeared in Chemical & Engineering News. He advocated employing the so-called real-time and single-cell imaging technique to fully appreciate cell-to-cell heterogeneity. He also had steadfastly pursued the creation of a reliable gate that would enable researchers to better understand the “feelings” of individual cells.
Over the past two decades, various genetically encoded probes have been generated principally using fluorescent proteins, and are used to investigate the function of specific signaling mechanisms in synaptic transmission, integration, and plasticity. I will discuss how the probes have advanced our understanding of the spatio-temporal regulation of biological functions inside cells, neurons, embryos, and brains. I will speculate on how these approaches will continue to improve due to the various features of fluorescent proteins that serve as the interface between light and life.
Due to recent remarkable progress in gene transfer techniques, including electroporation, virus-mediated gene transfer, and germline transmission of transgenes, the experimental animals to be studied are not limited to mice but extended to primates. Newly emerging genetically encoded tools will surely stimulate the imagination of many neuroscientists, and this is expected to spark an upsurge in the demand for them.