The initial asymmetry that specifies the oral-aboral (OA) axis of the sea urchin embryo has long been a mystery. It was shown previously that OA polarity can be entrained in embryos by imposing a respiratory asymmetry, with the most oxidizing side of the embryo tending to develop as the oral pole. This suggests that one of the earliest observable asymmetries along the incipient OA axis, a redox gradient established by a higher density and/or activity of mitochondria on the prospective oral side of the embryo, might play a causal role in establishing the axis. Here, we examine the origin and functional significance of this early redox gradient. Using MitoTracker Green, we show that mitochondria are asymmetrically distributed in the unfertilized egg of Strongylocentrotus purpuratus, and that the polarity of the maternal asymmetry is maintained in the zygote. Vital staining indicates that the side of the embryo that inherits the highest density of mitochondria tends to develop into the oral pole. This correlation holds when mitochondria are redistributed by centrifugation of eggs or by transfer of purified mitochondria into zygotes, indicating that an asymmetric mitochondrial distribution can entrain OA polarity, possibly through effects on intracellular redox state. In support of this possibility, we find that specification of oral ectoderm is suppressed when embryos are cultured under hypoxic conditions that enforce a relatively reducing redox state. This effect is reversed by overexpression of nodal, an early zygotic marker of oral specification whose localized expression suffices to organize the entire OA axis, indicating that redox state is upstream of nodal expression. We therefore propose that a threshold level of intracellular oxidation is required to effectively activate nodal, and that precocious attainment of this threshold within the blastomeres containing the highest density of mitochondria results in asymmetric nodal activity and consequent specification of the OA axis.
|Number of pages||12|
|State||Published - Sep 1 2004|
Bibliographical noteFunding Information:
We thank Drs. Bruce Brandhorst and Bryan Crawford for sharing with one of us (JAC) at a sea urchin meeting several years ago their unpublished observation that asymmetries in mitochondrial distribution exist in unfertilized S. purpuratus eggs. We also thank Dr. Andy Ransick for the idea of centrifuging eggs and advice on how to do it; Drs. Andy Cameron and Ochan Otim for assistance with preliminary experiments in the early stages of this project; Dr. John Morrill for Nile blue, a collection of classic papers, and many stimulating discussions; and Dr. Eric Davidson for useful comments and continuing encouragement in support of these studies. This work was funded by the Stowers Institute for Medical Research.
- Metabolic gradient
- Redox state
- Sea urchin
ASJC Scopus subject areas
- Molecular Biology
- Developmental Biology
- Cell Biology