Mitochondria, cellular organelles, hold a dual role in the generation of energy and reactive oxygen species (ROS), which are known to be involved in many disease processes. “Caloric Restriction” has been recognized to slow down the aging process primarily by lowering ROS production in mitochondria, however the mechanisms involved are still obscure.
My
recent work is focused on cubic membranes that form in the mitochondrial inner
membrane of amoeba (Chaos carolinensis)
upon starvation. Through computer simulation of TEM projections (Deng & Mieczkowski, 1998) (Fig. 2) and EM tomographic reconstruction (Deng et al.,
1999) (Fig. 3) the periodic membrane structures have been shown to be of true
cubic morphologies.
Recent data obtained with amoeba Chaos strongly suggest that cubic transition of mitochondrial cristae plays a protective role against ROS and may thus be a cellular response to oxidative stress. It is also worth noting that many other examples of naturally occurring cubic membranes are associated with systems with elevated ROS, e.g. chloroplasts, spermatozoa, tumor cells, viral-infected cells, aged cells, and highly light-exposed photocytes. The so-called ‘tubuloreticular structure’ observed in virus- infected and tumor cells is frequently used as an ultrastructural marker in pathology, and is of suspicious cubic membrane morphology.
Figure 2:
A typical TEM overview shows several mitochondria in amoeba Chaos carolinensis after 10 days of starvation (lack of supply with food organisms). Bar = 1 µm. The complex patterns in each of the mitochondria labeled a–f are simply different views of the same type of 3-D structure. This is illustrated by the simulated TEM projections (labeled a–f) on the right.From Deng and Mieczkowski (1998).
Figure 3:
Paracrystalline
structure of the mitochondrial inner membrane in starved Chaos
cells. (a) Thin (2.9 nm) slice from
a tomographic reconstruction of an amoeba mitochondrion whose cristae have
adopted the cubic morphology. (b) A surface-rendered model made from tracing the
membrane contours of the cristae. The model is only part of the cubic region.
The model shows that the intracristal space is large interconnected open
compartment. Scale bar is 250 nm. Adopted from Deng et al. (1999).