The use of microfluidic devices has facilitated the detailed study of cellular behavior by providing the ability to tightly control the cellular microenvironment. Microfluidic devices can be used to generate stable or dynamic concentration gradients 227, temperature gradients 228, and dynamically changing media supplies. In this way, cells and organisms can be probed in environments that closely mimic their natural habitats, or in response to defined by dynamic challenges, and valuable information can be revealed that is masked by standard batch culture techniques. Microfluidic devices also have the potential to vastly improve microscopy technology. Combined with sensitive cameras, high-precision automated stages, and powerful computers, researchers have the ability to rapidly acquire and store large arrays of microscopic images, which can provide great detail about a population of living and growing cells 229. Utilizing this technology, researchers can track gene expression dynamics with more precision and higher temporal resolution than possible with standard microscopy. In a recent example of this technology, we have developed a platform that can subject a population of cells to a dynamically varying stimulus (Fig. D3.1a). The device was designed to generate a fluctuating media signal by dynamically combining two media reservoirs according to a time dependent function. We applied this technology to examine a well-studied eukaryotic gene-regulatory network ? the galactose utilization network in S. cerevisiae. By comparing the experimentally measured response of the network to dynamically changing metabolic conditions to computational simulations of an otherwise validated mathematical model of the network we were forced to predict that mRNA half-lifes of two key transcription factors GAL1 and GAL3 must be regulated by glucose 230. This form of post-transcriptional regulation, in which glucose acts to down-regulate GAL protein synthesis, was a previously unknown source of regulation in the galactose utilization network, and was only made possible by experimentally examining the systems emergent properties in response to dynamically regulated environmental conditions.
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