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.

Control of Mitochondrial Morphology

Marsha Crawley, Dany Adams

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
        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