The biological relevance of nitric oxide (NO) in cells to processes of signaling, metabolic regulation, and disease treatment has become abundantly clear. NO or reactive oxygen species (ROS) can oxidize myoglobin to the met state (metMb; the Fe3+ state of myoglobin), a change accompanied with an altered absorbance profile in the visible region. Recent studies show that metMb has a broad functional role in metabolic pathways, oxidative/nitrative regulation and gene networks of many cells. Thus, real-time monitoring of the different charge states of myoglobin is a promising field of research. We previously introduced a Förster resonance energy transfer (FRET) sensor, EYFP-Myoglobin-mCherry, to measure the deoxygenation, oxygenation and met states of myoglobin, creating a simultaneous oxygen (O2) and NO sensor. In this sandwich probe, the mCherry binary chimera lifetime responds to oxygenated vs. deoxygenated myoglobin, while the yellow fluorescent protein (YFP) lifetime selectively responds to metMb (while indifferent to O2 concentration). We now use Citrine, a more robust YFP, in place of EYFP and append a mitochondrial targeting peptide sequence to specifically target mitochondria. We use fluorescence lifetime imaging (FLIM) of this mtCitrine-Myoglobin-mCherry sandwich probe while monitoring both oxygenation level and NO-induced met formation in mitochondria of mouse embryonic fibroblasts. We also test the NO response of Citrine alone to verify that the met sensitivity is specific to the Mb sandwich probe and not Citrine alone.
Oxygen (O2) is one of the most important biometabolites. In abundance, it serves as the limiting terminus of aerobic respiratory chains in the mitochondria of higher organisms; in deficit, it is a potent determinant of development and regulation of other physiological and therapeutic processes. Most knowledge on intracellular and interstitial concentration ([O2]) is derived from mitochondria isolated from cells or tissue biopsies, providing detailed but nonnative insight into respiratory chain function. The possible loss of essential metabolites during isolation and disruption of the normal interactions of the organelle with the cytoskeleton may cause these data to misrepresent intact cells. Several optical methodologies were also developed, but they are often unable to detect heterogeneity of metabolic characteristics among different individual cells in the same culture, and most cannot detect heterogeneous consumption within different areas of a single cell. Here, we propose a noninvasive and highly sensitive fluorescence lifetime microscopy probe, myoglobin-mCherry, appropriate to intracellular targeting. Using our probe, we monitor mitochondrial contributions to O2 consumption in A549 nonsmall cell lung cancer cells and we reveal heterogeneous [O2] within the intracellular environments. The mitochondrial [O2] at a single-cell level is also mapped by adding a peptide to target the probe to the mitochondria.
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