Drosophila neuroblasts are a model system for studying asymmetric cell division: they divide unequally to produce an apical neuroblast and a basal ganglion mother cell that differ in size, mitotic activity and developmental potential. During neuroblast mitosis, an apical protein complex orients the mitotic spindle and targets determinants of cell fate to the basal cortex, but the mechanism of each process is unknown. Here we show that the tumour-suppressor genes lethal giant larvae (lgl) and discs large (dlg) regulate basal protein targeting, but not apical complex formation or spindle orientation, in both embryonic and larval neuroblasts. Dlg protein is apically enriched and is required for maintaining cortical localization of Lgl protein. Basal protein targeting requires microfilament and myosin function, yet the lgl phenotype is strongly suppressed by reducing levels of myosin II. We conclude that Dlg and Lgl promote, and myosin II inhibits, actomyosin-dependent basal protein targeting in neuroblasts.

New neurons are added to the adult hippocampus throughout life and contribute to cognitive functions including learning and memory. It remains unclear whether ongoing neurogenesis arises from self-renewing neural stem cells (NSC) or from multipotential progenitor cells that cannot self-renew in the hippocampus. This is largely based on observations that neural precursors derived from the subventricular zone (SVZ) can be passaged long-term whereas hippocampal subgranular zone (SGZ) precursors are rapidly depleted by passaging. We demonstrate here that high levels of BMP signaling occur in hippocampal but not SVZ precursors in vitro, and blocking BMP signaling with Noggin is sufficient to foster hippocampal cell self-renewal, proliferation, and multipotentiality using single cell clonal analysis. Moreover, NSC maintenance requires continual Noggin exposure, which implicates BMPs as crucial requlators of NSC aging. In vivo, Noggin is expressed in the adult dentate gyrus and limits BMP signaling in proliferative cells of the SGZ. Transgenic Noggin overexpression in the SGZ increases multiple precursor cell populations, but proportionally increases the glial fibrillary acidic protein (GFAP)+ cell population at the expense of other precursors, suggesting that Noggin acts on NSCs in vivo. To confirm this, we used a dual thymidine analog paradigm to repeatedly label slowly dividing cells over a long duration. We find that small populations of label-retaining cells exist in the SGZ and that Noggin overexpression increases their numbers. Thus, we propose that the adult hippocampus contains a population of NSCs, which can be expanded both in vitro and in vivo by blocking BMP signaling.

The ventral spinal cord generates multiple inhibitory and excitatory interneuron subtypes from four cardinal progenitor domains (p0, p1, p2, p3). Here we show that cell-cell interactions mediated by the Notch receptor play a critical evolutionarily conserved role in the generation of excitatory v2aIN and inhibitory v2bIN interneurons. Lineage-tracing experiments show that the v2aIN and v2bIN develop from genetically identical p2 progenitors. The p2 daughter cell fate is controlled by Delta4 activation of Notch receptors together with MAML factors. Cells receiving Notch signals activate a transcription factor code that specifies the v2bIN fate, whereas cells deprived of Notch signaling express another code for v2aIN formation. Thus, our study provides insight into the cell-extrinsic signaling that controls combinatorial transcription factor profiles involved in regulating the process of interneuron subtype diversification.

The ventral spinal cord consists of interneuron groups arising from distinct, genetically defined, progenitor domains along the dorsoventral axis. Many of these interneuron groups settle in the ventral spinal cord which, in mammals, contains the central pattern generator for locomotion. In order to better understand the locomotor networks, we have used different transgenic mice for anatomical characterization of one of these interneuron groups, called V2 interneurons. Neurons in this group are either V2a interneurons marked by the postmitotic expression of the transcription factor Chx10, or V2b interneurons which express the transcription factors Gata2 and Gata3. We found that all V2a and most V2b interneurons were ipsilaterally projecting in embryos as well as in newborns. V2a interneurons were for the most part glutamatergic while V2b interneurons were mainly GABAergic or glycinergic. Furthermore, we demonstrated that a large proportion of V2 interneurons expressed the axon guidance molecule EphA4, a molecule previously shown to be important for correct organization of locomotor networks. We also showed that V2 interneurons and motor neurons alone did not account for all EphA4-expressing neurons in the spinal cord. Together, these findings enable a better interpretation of neural networks underlying locomotion, and open up the search for as yet unknown components of the mammalian central pattern generator.