Pulse labeling and long-term tracing of newborn neurons in the adult subgranular zone.

Research over the past decades has demonstrated that adult brain produces neural progenitor cells which proliferate and differentiate to newborn neurons that integrate into the existing circuit. However, detailed differentiation processes and underlying mechanisms of newly generated neurons are largely unknown due to the limitation of available methods for labeling and manipulating neural progenitor cells and newborn neurons. In this study, we designed a tightly controlled, noninvasive system based on Cre/loxP recombination to achieve long-term tracing and genetic manipulation of adult neurons in vivo. In this system, tamoxifen-inducible recombinase, CreER(T2), was driven by BAC-based promoter of doublecortin (DCX, a marker of newborn neurons). By crossing this Cre line with reporter mouse, we found that newborn neurons in the dentate gyrus (DG) could be selectively pulse-labeled by tamoxifen-induced expression of yellow fluorescent protein (YFP). YFP-positive neurons were identified by coimmunostaining with cell type-specific markers and characterized by electrophysiological recording. Furthermore, analysis of the migration of these neurons showed that the majority of these labeled neurons migrated to the inner part of granule cell layer. Moreover, spine growth of inner molecular layer of newborn granule neurons takes a dynamic pattern of invert U-shape, in contrast to the wedge-shaped change in the outer molecular layer. Our transgenic tool provides an efficient way to selectively label and manipulate newborn neuron in adult mouse DG.