Figure 1.

Signal compartmentalization by source translocation. Mitochondria (purple), being motile organelles, can create localized signaling environments by clustering to specific cellular compartments. In normoxia, mitochondria in pulmonary arterial smooth muscle cells are distributed throughout the cytoplasm. With hypoxia, they cluster in the central areas of the cell, creating a reactive oxygen species–, ATP-, and calcium-enriched signaling environment (green) in the vicinity of the nucleus (red) and sarcoplasmic reticulum (blue) and a mitochondrial signal–deficient environment near the plasma membrane (brown). Creation of a signaling gradient by movement of the source is a novel paradigm in cellular signaling and may be a mechanism of regulation at the cellular systems biology level.