Peroxisomes play important roles in cellular metabolism by oxidizing fatty acids, bile salts and cholesterol and by converting hydrogen peroxide to nontoxic forms, but where peroxisomes originate from has been unclear. The long-standing view has been that peroxisomes are semiautonomous oranelles, like mitochondria, which multiply strictly by growth and division. That most peroxisomal proteins are synthesized on free ribosomes and are imported directly into peroxisomes from the cytoplasm supports this view. However, peroxisomes can disappear from a cell and then be regenerated de novo, unlike mitochondria. This regenerative capacity has led to an alternative view in which other organelles- such as ER- participate in the formation and maintenance of peroxisomal membranes.
Our work with peroxisomes has been aimed at addressing whether the ER plays a role in peroxisomal biogenesis in mammalian cells, and if so, how this is regulated. Towards this goal, we have used diverse live cell fluorescent labeling strategies, including photoactivation, to pulse-label peroxisomal components (including the early event peroxin, PEX16) and to follow their targeting to peroxisomes. Evidence favoring an ER origin of peroxisomal membranes came from our finding that when the ER pool of PEX16-PAGFP was photoactivated and followed over time, the photoactivated molecules redistributed to peroxisomes (Kim et al., JCB, 2006). To test what role this ER-to-peroxisome pathway plays in the normal proliferation of peroxisomes during the cell cycle, we employed a photo-labeling, pulse-chase strategy for distinguishing newly synthesized from previously synthesized peroxisomal protein components and for visualizing both old and new peroxisomes. We found that old peroxisomes contained both newly synthesized and previously synthesized protein components, whereas new peroxisomes contained only newly synthesized peroxisomal protein components (Kim et al., JCB, 2006). This argued against fission being the predominant mechanism for mammalian peroxisome formation and indicated that de novo biogenesis of peroxisomes from the ER was important for maintenance of peroxisomes under normal conditions.
These results have helped solidify the view that peroxisomes are derived from the ER and have provided insight into how peroxisomes proliferate and are maintained within mammalian cells. Ongoing work in the lab is aimed at using the new live cell imaging strategies to investigate how peroxisome proliferation is regulated in response to drugs and other physiological conditions. We are also investigating how peroxisomes are turned over within cells and the mechanism(s) for uptake of soluble proteins into these organelles.
Figure: COS 7 cell expressing an ER marker (red: ssRFPKDEL) and a peroxisomal marker (green: mGFP-SKL). The image is a projection of three z-stacks at maximum projection giving a stack size of 2 microns in Z. Bar = 10 microns.
Studies conducted by Peter Kim.