NRG-1 talks back to neurons

n page 1133, Bao et al. demonstrate that a neuronal growth factor signals to neighboring cells while also communicating back to the nucleus of its own cell. This two-way signaling is necessary to maintain neuronal survival. In the forward direction , either a soluble or membrane-bound form of the Nrg-1 growth factor activates erbB receptor tyrosine kinases on a variety of adjacent cells. The Nrg-1/erbB partnership is well known to control responses in the erbB-expressing cells, such as epithelial cell motility and proliferation, through MAP kinase pathways. Now, the target cells are shown to talk back to neurons that express membrane-bound Nrg-1 via the same erbB–Nrg-1 complex. Bao et al. show that treatment with soluble erbB protects Nrg-1–expressing neurons from cell death in vitro. The protective activity appears to stem from Nrg-1's ability to regulate transcription of apoptotic genes, including BAK and RIP, whose expression levels were repressed by treatment with erbB. Membrane-bound Nrg-1 was cleaved by a ␥-secretase–like activity in response to erbB treatment, thus releasing the Nrg-1 intracellular domain, which moved to the nucleus. Although Nrg-1 has not been shown to bind to DNA, the authors demonstrate that it does has trans-activating activity on a reporter construct, and preliminary evidence indicates it may interact with zinc finger–containing transcription factors. ᭿ O The intracellular domain of Nrg-1 (red) moves to the nucleus (blue) upon erbB treatment (right). Golgi united are also divided olgi bodies do not need to disassemble to divide, according to results from Uchiyama et al. on page 1067. In mammalian cells, the Golgi apparatus disassembles into vesicles and short tubules before mitosis. Golgi breakdown requires that two membrane fusion pathways necessary for Golgi reassembly after mitosis be temporarily inhibited. For one, the NSF pathway, this inhibition is achieved by Cdc2-mediated phosphorylation of an NSF-associated factor at early mitosis. But how the cell cycle regulates the second, the p47/p97 pathway, was not known. Uchiyama et al. have determined that Cdc2-mediated phosphorylation also inhibits the p47/p97 pathway. p97 is an ATPase that binds to the Golgi through its associated factor, p47. Uchiyama et al. find that p47 is phosphorylated by Cdc2 upon entry into mitosis. Phosphorylated p47 bound to p97 in mitotic cells and had a decreased binding affinity for Golgi membranes, thus releasing the complex from the Golgi. Addition of a phosphorylation-insensitive version of p47 to mitotic cells blocked Golgi breakdown. However, the mutant did not impair …