Label-Free Imaging of Heme Dynamics in Living Organisms by Transient Absorption Microscopy

Heme, a hydrophobic and cytotoxic macrocycle, is an essential cofactor in a large number of proteins and is important for cell signaling. This must mean that heme is mobilized from its place of synthesis or entry into the cell to other parts of the cell where hemoproteins reside. However, the cellular dynamics of heme movement is not well understood, in large part due to the inability to image heme noninvasively in live biological systems. Here, using high-resolution transient absorption microscopy, we showed that heme storage and distribution is dynamic in Caenorhabditis elegans. Intracellular heme exists in concentrated granular puncta which localizes to lysosomal-related organelles. These granules are dynamic, and their breaking down into smaller granules provides a mechanism by which heme stores can be mobilized. Collectively, these direct and noninvasive dynamic imaging techniques provide new insights into heme storage and transport and open a new avenue for label-free investigation of heme function and regulation in living systems.