The vast diversity of neurons and glia of the central nervous system is generated from a small, heterogeneous population of progenitors that undergo transcriptional changes during development to sequentially specify distinct cell fates. Guided by cell intrinsic and temporal extrinsic cues, invertebrate and mammalian neural progenitors carefully regulate when and how many of each cell type is produced to form functional neural circuits. Emerging evidence indicates that neural progenitors also undergo changes in global chromatin architecture, thereby restricting the duration a particular temporal cue can act. Thus, studies of temporal identity specification and progenitor competence can provide insight into how we may use neural progenitors to more effectively generate specific cell types for brain repair.

Sey/Sey progenitors produce neuroblasts capable of migrating into the OB but fail to generate dopaminergic periglomerular and superficial granule cells. Interestingly, superficial granule neurons also express mRNA for tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. Our data show that SVZ neuroblasts are heterogeneous and that Pax6 is required in a cell-autonomous manner for the production of cells in the dopaminergic lineage.

The subventricular zone (SVZ) of the postnatal brain continuously generates olfactory bulb (OB) interneurons. We show that calretinin ϩ , calbindin ϩ , and dopaminergic (TH

Stem/progenitor cells often generate distinct cell types in a stereotyped birth order, and over time lose competence to specify earlier-born fates by unknown mechanisms. In Drosophila, the Hunchback transcription factor acts in neural progenitors (neuroblasts) to specify early-born neurons, in part by indirectly inducing the neuronal transcription of its target genes, including the hunchback gene. We used in vivo immuno-DNA FISH and found that the hunchback gene moves to the neuroblast nuclear periphery, a repressive subnuclear compartment, precisely when competence to specify early-born fate is lost, and several hours and cell divisions following termination of its transcription. hunchback movement to the lamina correlated with downregulation of the neuroblast nuclear protein, Distal antenna (Dan). Either prolonging Dan expression or disrupting lamina interfered with hunchback repositioning and extended neuroblast competence. We propose that neuroblasts undergo a developmentally-regulated subnuclear genome reorganization to permanently silence Hunchback target genes that results in loss of progenitor competence.

The discovery of neural stem cells in the adult mammalian brain shattered the long-standing belief that neurogenesis is restricted to embryonic and early postnatal periods. The largest germinal region in the adult mammalian brain is the SVZ. The SVZ is classically described as a thin layer of proliferative cells lining the lateral wall of the lateral ventricle (LV) and separated from the ventricular lumen by a layer of ependymal cells (Smart and Leblond 1961;Altman 1969). The existence of NSCs in this region was first suggested by in vitro experiments in which SVZ cells were shown to self-renew and produce neurons, astrocytes, and oligodendrocytes (Reynolds and Weiss 1992;Morshead et al. 1994). In vivo, NSCs generate a large number of young neurons that migrate along the RMS to the OB where they replace multiple types of interneurons (Luskin 1993;Lois and Alvarez-Buylla 1994). NSCs in the SVZ also generate both parenchymal oligodendrocyte progenitors (OPCs) and myelinating oligodendrocytes, most of which migrate into the neighboring corpus callosum (Nait-Oumesmar et al. 1999;Picard-Riera et al. 2002;Menn et al. 2006).Adult neurogenesis leads to the generation and replacement of specific types of neurons in restricted brain regions, including the OB and dentate gyrus of the hippocampus. Many processes of embryonic development are recapitulated during adult neurogenesis, such as neuronal differentiation, migration, maturation, and cell death. However, adult-born neurons confront an environment very different from those born in the developing brain. Adult-born neurons migrate through more complex and frequently extensive territories and must integrate into circuits that are already fully functional. Young neurons in the SVZ and RMS migrate along each other, forming long aggregates of cells called chains (Lois et al. 1996) and are able to migrate long distances in relatively short periods of time (Wichterle et al. 1997). Within 2-5 days from their time of birth in the SVZ, the majority of these young neurons have reached the OB. Once in the OB, young neurons move radially away from the RMS and begin their final differentiation and maturation, a process that takes 5-10 days (Petreanu and Alvarez-Buylla 2002). During this period, new neurons develop dendritic trees and synaptic spines and become functionally integrated into the OB circuitry (Carleton et al. 2003).Initial studies in the neonatal rat brain suggested that new OB neurons originate from a restricted territory in the anterior SVZ, in a region close to the RMS (Luskin 1993; Lois and Alvarez-Buylla 1994). However, subsequent work uncovered an extensive network of chains of young neurons throughout most of the SVZ on the lateral wall of the LV (Doetsch and Alvarez-Buylla 1996), suggesting that migrating cells originate along the length of the lateral ventricular wall. Below, we review new experiments that suggest that the neurogenic SVZ covers regions of the lateral ventricular walls facing the pallium, subpallium, and septum, as well as the RMS. Furthermore, ...

Members of the postsynaptic density-95 (PSD95)/synapse-associated protein-90 (SAP90) family of scaffolding proteins contain a common set of modular protein interaction motifs including PDZ (postsynaptic density-95/Discs large/zona occludens-1), Src homology 3, and guanylate kinase domains, which regulate signaling and plasticity at excitatory synapses. We report that N-terminal alternative splicing of PSD95 generates an isoform, PSD95beta that contains an additional "L27" motif, which is also present in SAP97. Using yeast two hybrid and coimmunoprecipitation assays, we demonstrate that this N-terminal L27 domain of PSD-95beta, binds to an L27 domain in the membrane-associated guanylate kinase calcium/calmodulin-dependent serine kinase, and to Hrs, an endosomal ATPase that regulates vesicular trafficking. By transfecting heterologous cells and hippocampal neurons, we find that interactions with the L27 domain regulate synaptic clustering of PSD95beta. Disrupting Hrs-regulated early endosomal sorting in hippocampal neurons selectively blocks synaptic clustering of PSD95beta but does not interfere with trafficking of the palmitoylated isoform, PSD95alpha. These studies identify molecular and functional heterogeneity in synaptic PSD95 complexes and reveal critical roles for L27 domain interactions and Hrs regulated vesicular trafficking in postsynaptic protein clustering.

Here we describe the embryonic CNS expression of 5,000 GAL4 lines made using molecularly defined cis-regulatory DNA inserted into a single attP genomic location. We document and annotate the patterns in early embryos when neurogenesis is at its peak, and in older embryos where there is maximal neuronal diversity and the first neural circuits are established. We note expression in other tissues such as the lateral body wall (muscle, sensory neurons, trachea) and viscera. Companion papers report on the adult brain and larval imaginal discs, and the integrated datasets are available online (www.janelia.org/flylight/gal4-gen1). This collection of embryonically-expressed GAL4 lines will be valuable for determining neuronal morphology and function; the 1862 lines expressed in small subsets of neurons (<20/segment) will be especially valuable for characterizing interneuronal diversity and function, as interneurons comprise the majority of all CNS neurons, yet their gene expression profile and function remain virtually unexplored.

SUMMARYA fundamental question in brain development is how precursor cells generate a diverse group of neural progeny in an ordered manner. Drosophila neuroblasts sequentially express the transcription factors Hunchback (Hb), Krüppel (Kr), Pdm1/Pdm2 (Pdm) and Castor (Cas). Hb is necessary and sufficient to specify early-born temporal identity and, thus, Hb downregulation is essential for specification of later-born progeny. Here, we show that distal antenna (dan) and distal antenna-related (danr), encoding pipsqueak motif DNA-binding domain protein family members, are detected in all neuroblasts during the Hb-to-Cas expression window. Dan and Danr are required for timely downregulation of Hb in neuroblasts and for limiting the number of early-born neurons. Dan and Danr function independently of Seven-up (Svp), an orphan nuclear receptor known to repress Hb expression in neuroblasts, because Dan, Danr and Svp do not regulate each other and dan danr svp triple mutants have increased early-born neurons compared with either dan danr or svp mutants. Interestingly, misexpression of Hb can induce Dan and Svp expression in neuroblasts, suggesting that Hb might activate a negative feedback loop to limit its own expression. We conclude that Dan/Danr and Svp act in parallel pathways to limit Hb expression and allow neuroblasts to transition from making early-born neurons to late-born neurons at the proper time.