Worm Breeder's Gazette 15(1): 34 (October 1, 1997)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Department of Biological Sciences, Smith College, Northampton, MA 01063
Mitochondria in different cell types have cell type specific morphologies. We are interested in the mechanisms that underlie this morphological differentiation. We are addressing three possible factors that could cause mitochondria to develop specific shapes: 1. cytosolic factors such as inherited maternal proteins or proteins imported from outside the cell; 2. mitochondrial proteins encoded in the nucleus and translated in the cytosol; 3. mitochondrial proteins encoded in mitochondrial DNA (mtDNA) and translated within the matrix of the mitochondrion itself. To begin our study, we assayed for morphological differences among mitochondria in different cell types of C. elegans using epifluorescence and confocal laser scanning microscopy of worms stained with DASPMI (Molecular Probes), a mitochondrion specific fluorescent dye, and then confirmed our results using transmission electron microscopy. The results show that there are indeed morphological differences among the mitochondria of C. elegans, specifically among those in cells of the body wall muscle, gut, and pharynx. Mitochondria in body wall muscle tend to be larger and more stringy than the roundish mitochondria in gut cells; pharyngeal mitochondria are long and thin, more densely packed, and arranged radially with their long axes parallel to the axes of the pharyngeal cells. To begin to distinguish between the effects of nuclear versus mitochondrially encoded proteins, we have studied the effects on the body wall muscle mitochondria of inhibiting both cytosolic and mitochondrial translation. Treatment of the nematodes for two hours with 100mM chloramphenicol, a mitochondrial translation inhibitor, results in an obvious morphological change in the mitochondria as compared to untreated controls: the elongated oval morphology is replaced by small round areas of staining, as if the mitochondria have fragmented into small "daughter" organelles. In contrast, treatment of nematodes for two hours with 10mM cycloheximide, a cytosolic translation inhibitor, causes no observable change in the size or shape of the mitochondria. This result surprised us because most mitochondrial proteins are encoded in the nuclear DNA, translated in the cytosol, and imported into the mitochondrion. mtDNA, on the other hand, encodesonly a few mRNAs, and those are for proteins of the electron transport system and ATP synthase. Thus, the mitochondrial matrix is not the site of translation for a protein that is an obvious candidate for a factor affecting organelle shape. One possibility is that chloramphenicol triggers organelle division. Inhibiting translation may also "kill" the organelle, with fragmentation being a secondary effect. However, the turnover of proteins in the inner mitochondrial membrane, and thus the turnover of mitochondrially translated proteins, is believed to be quite slow relative to the time course of these experiments. Thus, we do not think that the change in mitochondrial shape in response to chloramphenicol is the result of a general failure of electron transport (and the resultant proton gradient across the inner mitochondrial membrane) or a failure of ATP synthesis. Moreover, if failure of these metabolic functions of the mitochondrion were responsible for the mitochondrial shape change that we see, we would expect cycloheximide to have the same effect as chloramphenicol, since every protein complex containing a mitochondrially encoded protein also contains proteins encoded in the nucleus. It may be, however, that mitochondrially encoded proteins are in some way limiting, thus the effects of their loss are apparent earlier. These preliminary results from inhibition experiments are consistent with the third hypothesis, that control of mitochondrial morphology resides within the mitochondrion itself. Nonetheless, the mitochondrion must be detecting some sort of signal that tells it the identity of the cell type in which it resides. We plan to do more controls to confirm the above described results, and to continue experiments to determine both the nature and source of any signals, autonomous or otherwise, received by the mitochondrion. Gillham, N.W. (1994) Organelle Genes and Genomes. New York: Oxford University Press