In the developing spinal cord, signals from the roof plate are required for the development of three classes of dorsal interneuron: D1, D2, and D3, listed from dorsal to ventral. Here, we demonstrate that absence of Wnt1 and Wnt3a, normally expressed in the roof plate, leads to diminished development of D1 and D2 neurons and a compensatory increase in D3 neuron populations. This occurs without significantly altered expression of BMP and related genes in the roof plate. Moreover, Wnt3a protein induces expression of D1 and D2 markers in the isolated medial region of the chick neural plate, and Noggin does not interfere with this induction. Thus, Wnt signaling plays a critical role in the specification of cell types for dorsal interneurons. Received August 13, 2001; revised version accepted January 10, 2002. During development of the vertebrate central nervous system (CNS), highly proliferative cells in the ventricular zone of the neural tube serve as progenitors of the various types of neurons, such as interneurons and motor neurons. In the ventral half of the spinal cord, the secreted signaling molecule Sonic hedgehog (Shh) functions as a gradient signal for the generation of five distinct classes of neurons along the dorsoventral axis. Shh secreted from the notochord and floor plate controls the specification of ventral cell types in a dose-dependent manner Ericson et al. 1996;Tanabe and Jessell 1996;Jessell 2000).Three subclasses of interneuron, called D1, D2, and D3 positioned from the dorsal side, are indicated by the expression of homeodomain proteins LH2, Islet1, and Lim1/2 in the dorsal half of the spinal cord (Liem et al. 1997). These dorsal interneurons are derived from progenitors in the ventricular zone. These progenitors are also subdivided by expression of the basic helix-loophelix (bHLH) proteins Math1, Neurogenin1 (Ngn1), and Mash1 (Lee et al. 1998(Lee et al. , 2000. It has been established that Math1-expressing cells give rise to LH2 + neurons (Helms and Johnson 1998). Recent studies indicate that the roof plate is the major source of inductive signals controlling the generation of the D1 and D2 classes of dorsal interneuron (Lee and Jessell 1999). Experiments using cultured chick neural plate tissue have demonstrated that signals from the roof plate are sufficient to promote dorsal interneuron differentiation in vitro (Liem et al. 1997). Moreover, absence of D1/D2 class neurons in mouse embryos lacking the roof plate, either by homozygosity of the dreher (dr) allele carrying a loss-of-function mutation in the LIM-homeobox gene Lmx1, or by genetic roof plate ablation, has provided compelling evidence for the roof plate as the determinant source of dorsal interneurons (Lee et al. 2000;Millonig et al. 2000).Around the time when dorsal interneurons are generated, cells at the dorsal end of the neural tube express secretory proteins belonging to BMP, FGF, and Wnt families (Lee and Jessell 1999). It has been shown in the chick that BMP family proteins mimic the roof plate in the induction of dorsal...

In the developing central nervous system, cellular diversity depends in part on organising signals that establish regionally restricted progenitor domains, each of which produces distinct types of differentiated neurons. However, the mechanisms of neuronal subtype specification within each progenitor domain remain poorly understood. The p2 progenitor domain in the ventral spinal cord gives rise to two interneuron (IN) subtypes, V2a and V2b, which integrate into local neuronal networks that control motor activity and locomotion. Foxn4, a forkhead transcription factor, is expressed in the common progenitors of V2a and V2b INs and is required directly for V2b but not for V2a development. We show here in experiments conducted using mouse and chick that Foxn4 induces expression of delta-like 4 (Dll4) and Mash1 (Ascl1). Dll4 then signals through Notch1 to subdivide the p2 progenitor pool. Foxn4, Mash1 and activated Notch1 trigger the genetic cascade leading to V2b INs, whereas the complementary set of progenitors, without active Notch1, generates V2a INs. Thus, Foxn4 plays a dual role in V2 IN development: (1) by initiating Notch-Delta signalling, it introduces the asymmetry required for development of V2a and V2b INs from their common progenitors; (2) it simultaneously activates the V2b genetic programme.

Astrocytes are the most abundant and functionally diverse glial population in the vertebrate central nervous system (CNS). However, the mechanisms underlying astrocyte specification are poorly understood. It is well established that cellular diversification of neurons in the embryo is generated by position-dependent extrinsic signals and combinatorial interactions of transcription factors that direct specific cell fates by suppressing alternative fates. It is unknown whether a comparable process determines embryonic astrocyte identity. Indeed, astrocyte development is generally thought to take place in a position-independent manner. Here we show multiple functions of Stem cell leukaemia (Scl, also known as Tal1), which encodes a basic helix-loop-helix (bHLH) transcription factor, in the regulation of both astrocyte versus oligodendrocyte cell fate acquisition and V2b versus V2a interneuron cell fate acquisition in the p2 domain of the developing vertebrate spinal cord. Our findings demonstrate a regionally restricted transcriptional programme necessary for astrocyte and V2b interneuron development, with striking parallels to the involvement of SCL in haematopoiesis. They further indicate that acquisition of embryonic glial subtype identity might be regulated by genetic interactions between SCL and the transcription factor Olig2 in the ventral neural tube.

During spinal cord development, oligodendrocytes are generated from a restricted region of the ventral ventricular zone and then spread out into the entire spinal cord. These events are controlled by graded inductive and repressive signals derived from a local organizing center. Sonic hedgehog was identified as an essential ventral factor for oligodendrocyte lineage specification, whereas the dorsal cue was less clear. In this study, Wnt proteins were identified as the dorsal factors that directly inhibit oligodendrocyte development. Wnt signaling through a canonical beta-catenin pathway prevents its differentiation from progenitor to an immature state. Addition of rmFz-8/Fc, a Wnt antagonist, increased the number of immature oligodendrocytes in the spinal cord explant culture, demonstrating that endogenous Wnt signaling controls oligodendrocyte development.