The lengthening of dendrites during neuronal development involves the addition of membrane proteins through exocytosis, although how and where membrane is added is unclear. In their study described on p. , Michael Kim, Jay Parrish and colleagues investigate the specific role that exocytosis plays in dendritic morphogenesis. An RNAi-based screen of dendritic arborisation (da) neurons in Drosophila identified Ras opposite (Rop), the Drosophila orthologue of the key exocytosis regulator Munc18-1, as an essential regulator of dendrite growth. In da neurons depleted of Rop, the authors find reduced outgrowth of terminal dendritic branches, followed by primary dendrite degeneration. The authors next show that Rop associates with multiple exocyst subunits, and that Rop and exocyst genes might function together in the same pathway. The authors speculate that the primary dendrites are the predominant site for dendritic exocytosis after finding that Rop–exocyst complexes and a reporter for exocytosis are enriched in this dendritic compartment. They also show diffusion of membrane-associated proteins from primary dendrites into terminal dendritic branches. These proteins do not diffuse in the reverse direction, however, suggesting that the material required to support terminal dendritic growth is supplied by diffusion rather than by targeted exocytosis. This study shows that exocytosis supports dendrite growth by preferentially targeting vesicles to particular regions of a growing dendrite arbor.
