One of the first steps in neurogenesis is the diversification of cells along the dorsoventral axis. In Drosophila the central nervous system develops from three longitudinal columns of cells: ventral cells that express theThe Drosophila central nervous system (CNS) develops from a bilateral neuroectoderm that lies to each side of a narrow strip of ventral midline cells. Single neuroectodermal cells delaminate from the surface epithelium and move into the interior of the embryo to form neural precursor cells called neuroblasts. The early neuroblasts form an orthogonal grid of four rows (1, 3, 5, and 7) along the anterior-posterior (AP) axis and three columns (ventral, intermediate, and dorsal) along the dorsoventral (DV) axis. Subsequently, each neuroblast expresses a characteristic combination of genes and contributes a stereotyped family of neurons and glia to the CNS. Thus the earliest steps in patterning the CNS are the formation and specification of neuroblasts. Neuroblast formation is regulated by two phenotypically opposite classes of genes: Proneural genes promote neuroblast formation, whereas the neurogenic genes inhibit neuroblast formation. Proneural genes encode a family of basic helix-loop-helix transcription factors that are expressed in 4-6 cell clusters at specific positions within the neuroectoderm. Embryos lacking the proneural genes achaete/scute or lethal of scute have a reduced number of neuroblasts (for review, see Skeath and Carroll 1994). Conversely, neurogenic genes are expressed uniformly in the neuroectoderm, and embryos that lack any one neurogenic gene function, such as Notch or Delta, develop an excess number of neuroblasts (for review, see Campos-Ortega 1995).The generation of neuronal diversity begins with the specification of unique neuroblast identities along both the AP and DV axes. The wingless, hedgehog, gooseberry, and engrailed genes are expressed in stripes of neuroectoderm that subdivide the AP axis. They are required for establishing AP row identity within the neu- Cold Spring Harbor Laboratory Press on May 9, 2018 -Published by genesdev.cshlp.org Downloaded from

The Drosophila embryonic central nervous system (CNS) develops from a bilateral neuroectoderm that forms adjacent to the specialized cells of the ventral midline. Neuroectoderm on each side of the ventral midline can be subdivided, on the basis of patterns of gene expression and neuroblast formation, into an orthogonal grid of four rows (1, 3, 5, 7) along the anteroposterior (AP) axis and three columns (ventral, intermediate, and dorsal) along the DV axis. The earliest neuroblast array has four neuroblasts in the ventral column, two in the intermediate column, and four in the dorsal column. Neuroblasts divide repeatedly to produce a series of smaller ganglion mother cells (GMCs), each of which produce two postmitotic neurons or glia. Every neuroblast is uniquely identifiable on the basis of its AP and DV position, and each generates a characteristic family of neurons and glia.Neuroblast formation is regulated by the proneural genes achaete, scute, and lethal of scute (for review, see Campos-Ortega 1993). Each of these proneural genes is expressed in clusters of 4-6 cells at different positions within the neuroectoderm (e.g., achaete is expressed in four clusters, in the ventral and dorsal columns of rows 3 and 7). Proneural genes promote the formation of neuroblasts, whereas Delta-Notch signaling inhibits neuroblast formation; the balance of proneural and Notch activity results in the formation of a single neuroblast from each cluster (for review, see Campos-Ortega 1993).What are the cues that specify correct neuroblast identity along the AP and DV axes? The segment polarity genes wingless, hedgehog, gooseberry, and engrailed are expressed in stripes in the neuroectoderm and specify the AP row identity of neuroblasts (Chu-LaGraff and Doe 1993;Zhang et al. 1994;Skeath et al. 1995;Bhat 1996;Matsuzaki and Saigo 1996;Bhat and Schedl 1997). Conditional inactivation (for wingless; Chu-LaGraff and Doe 1993) or misexpression (for gooseberry; Skeath et al. 1995) experiments show that segment polarity gene function is required in the neuroectoderm, prior to neuroblast delamination, for the proper specification of neuroblast identity.Less is known about how neuroectoderm and neuro-

At the molecular level, members of the NKx2.2 family of transcription factors establish neural compartment boundaries by repressing the expression of homeobox genes specific for adjacent domains [Muhr et al. (2001) Cell, 104, 861–873; Weiss et al. (1998) Genes Dev., 12, 3591–3602]. The Drosophila homologue, vnd, interacts genetically with the high-mobility group protein, Dichaete, in a manner suggesting co-operative activation [Zhao and Skeath (2002) Development, 129, 1165–1174]. However, evidence for direct interactions and transcriptional activation is lacking. Here, we present molecular evidence for the interaction of Vnd and Dichaete that leads to the activation of target gene expression. Two-hybrid interaction assays indicate that Dichaete binds the Vnd homeodomain, and additional Vnd sequences stabilize this interaction. In addition, Vnd has two activation domains that are typically masked in the intact protein. Whether vnd can activate or repress transcription is context-dependent. Full-length Vnd, when expressed as a Gal4 fusion protein, acts as a repressor containing multiple repression domains. A divergent domain in the N-terminus, not found in vertebrate Vnd-like proteins, causes the strongest repression. The co-repressor, Groucho, enhances Vnd repression, and these two proteins physically interact. The data presented indicate that the activation and repression domains of Vnd are complex, and whether Vnd functions as a transcriptional repressor or activator depends on both intra- and inter-molecular interactions.

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