Mitochondrial dynamics in mammals.

Biologists often think of mitochondria as static, kidney bean-shaped organelles that have the mundane chore of providing energy for the cell. However, the mitochondrial population is in fact dynamic (Bereiter-Hahn and Voth, 1994; Rizzuto et al., 1998), and the hundreds of mitochondria in a cell can have a range of morphologies, including small spheres, long tubules, and interconnected tubules. This morphological plasticity is based on the ability of mitochondria to undergo both organellar fusion and fission. These opposing processes are readily visualized in living fibroblasts, where individual mitochondrial tubules continually migrate back and forth along their long axes on radial tracks (Fig. 1). Frequently, two mitochondrial tubules encounter each other and rapidly fuse, typically end to end. Conversely, there are normally an equal number of fission events that resolve a single mitochondrial tubule into two. By regulating the relative rates of fusion and fission, however, the morphology of the mitochondrial population can be dramatically altered, with important consequences for mitochondrial function. Here we review our current molecular understanding of mitochondrial fusion and fission. Several good reviews are available for yeast mitochondrial dynamics (Hermann and Shaw, 1998; Jensen et al., 2000; Shaw and