A DNA nanodevice for mapping sodium at single organelle resolution - raw data

Cellular sodium ion (Na+) homeostasis is integral to organism physiology. Our current understanding of Na+ homeostasis is largely limited to Na+ transport at the plasma membrane. Organelles may also contribute to Na+ homeostasis, however, the direction of Na+ flow across organelle membranes is unknown because Na+ cannot be imaged. Here, we report a pH-independent, organelle-targetable, ratiometric probe that reports lumenal Na+. It is a DNA nanodevice containing a Na+-sensitive fluorophore, a reference dye and an organelle targeting domain. By measuring Na+ at single endosome resolution in mammalian cells and in C. elegans, we discover that lumenal Na+ levels in each stage of the endolysosomal pathway exceed cytosolic levels and decrease as endosomes mature. Further, we find that lysosomal Na+ levels in nematodes are modulated by the Na+/H+ exchanger NHX-5 in response to salt stress. The ability to image sub-cellular Na+ will unveil mechanisms of Na+ homeostasis at an increased level of cellular detail.