Egg P bodies protect maternal mRNA

Egg P bodies protect maternal mRNA P bodies (processing bodies) are cytoplasmic granules that, in somatic cells, store and degrade mRNAs. But P bodies in the worm egg protect mRNA, according to a study by Boag et al. In a separate study Noble et al. observed that worm eggs have different fl avors of P bodies depending on developmental stage. Boag et al. showed that P bodies in eggs lack an mRNA decapping protein called Pat1 that in somatic cells promotes mRNA degradation. So if egg P bodies aren’t degrading mRNA, what are they doing? A core P body component called CGH-1 holds mRNAs at P-bodies in both somatic cells and egg cells. When the authors removed CGH-1 from eggs, mRNAs were mislocalized and destabilized. “We think CGH-1 acts like a chaperone for a protective mRNA-protein complex,” says PI Keith Blackwell. The oocyte contains large numbers of maternally derived mRNAs, which are all transcribed and packaged at once, but then used in a specifi c temporal pattern for proper development. The protective complex may keep them safe until they are expressed. Noble et al. showed that eggs in fact have a whole range of specialized P bodies. They identifi ed at least three types of P bodies arising at different stages of egg development, and a fourth type in embryos, each with a distinct set of proteins. During early meiosis, “germ granules” associate with germ nuclei, while grP (germline RNP) bodies accumulate in the syncytial (multinucleate) cytoplasm. Neither type carried the RNA decapping enzyme DCAP-2, suggesting they do not degrade mRNA, in line with the observations of Boag et al. As mononucleate oocytes formed and then entered an arrested stage, dcP bodies appeared, which did contain DCAP-2, but, interestingly, didn’t contain measurable amounts of CGH-1. Finally, during early embryogenesis, more canonical P bodies form, carrying CGH-1 and decapping enzymes. Although the different types of P bodies most likely have different functions, they do appear to interact with one another, indicating that they exchange mRNAs. Thus, sorting out which P bodies do what will be a challenging next step. Boag, P.R., et al. 2008. J. Cell Biol. doi:10.1083/jcb.200801183. Noble, S.L., et al. 2008. J. Cell Biol. doi:10.1083/jcb.200802128. Mitochondria spl it with a calcium hit An infl ux of calcium into neurons causes mitochondrial fi ssion, according to Han et al. To respond to changing cellular needs, mitochondria move, fuse together, or undergo fi ssion. Han et al. now reveal some of the proteins and signals that regulate fi ssion in neurons. The authors tested mitochondrial responses to an increase in potassium (K) levels, which mimics an action potential. The K spike brought mitochondria to a halt and prompted their division, making them shorter and rounder. These changes depended on the opening of voltage-dependent calcium channels and the resulting Ca gradient, which in turn activates the Ca/calmodulin-dependent protein kinase (CaMKI ). A CaMKI antagonist blocked K-triggered changes in mitochondrial shape. In their search for CaMKI substrates, the authors turned to Drp1 (dynamin-related protein 1), which promotes mitochondrial fi ssion and whose sequence suggested it could be phosphorylated. And the authors found that it was, in fact, a CaMKI substrate. K treatment not only led to Drp1 phosphorylation, but caused cytoplasmic Drp1 to rapidly relocate to mitochondria and associate with another fi ssion protein, Fis1. Together, these results indicate that a calcium infl ux is an important trigger for mitochondrial fi ssion, and that Drp1 is a fi nal effector of the fi ssion signal. Lead investigator Masayuki Matsushita says there is much more to discover about how calcium controls mitochondrial dynamics, including other roles for CaMKI : “We think it may have other substrates as well, also involved in mitochondrial morphology.” Han, X.-J., et al. 2008. J. Cell Biol. doi:10.1083/jcb.200802164.