Fission and fusion of the neuronal endoplasmic reticulum.

The endoplasmic reticulum (ER) is central for protein synthesis and is the largest intracellular Ca2+ store in neurons. The neuronal ER is classically described to have a continuous lumen spanning all cellular compartments. This allows neuronal ER to integrate spatially separate events in the cell. Recent in vitro as well as in vivo findings, however, demonstrate that the neuronal ER is a structurally dynamic entity, capable of rapid fragmentation, i.e., ER fission. The ER fragments can fuse back together and reinstate ER continuity. This reversible phenomenon can be induced repeatedly within the same cell, is temperature-dependent, and compatible with cell survival. The key trigger for dendritic ER fission is N-methyl D-aspartate (NMDA) receptor stimulation in the presence of extracellular Ca2+. However, the exact molecular machinery responsible for the fission and fusion of neuronal ER remains unknown. Reversible ER fission represents a new cell biological event downstream of NMDA receptor-gated Ca2+ influx and may thus influence many aspects of neuronal function in physiology and disease. Hence, it constitutes a new field for exploration in neuroscience that will benefit greatly from recent advances in light microscopy imaging techniques allowing dynamic characterization of cellular events in vitro and in vivo.