hedgehog signaling independent of engrailed and wingless required for post-S1 neuroblast formation in Drosophila CNS

Integrated Patterning Programs During Drosophila Development Generate the Diversity of Neurons and Control Their Mature Properties

During the approximately 5 days of Drosophila neurogenesis (late embryogenesis to the beginning of pupation), a limited number of neural stem cells produce approximately 200,000 neurons comprising hundreds of cell types. To build a functional nervous system, neuronal types need to be produced in the proper places, appropriate numbers, and correct times. We discuss how neural stem cells (neuroblasts) obtain so-called area codes for their positions in the nervous system (spatial patterning) and how they keep time to sequentially produce neurons with unique fates (temporal patterning). We focus on specific examples that demonstrate how a relatively simple patterning system (Notch) can be used reiteratively to generate different neuronal types. We also speculate on how different modes of temporal patterning that operate over short versus long time periods might be linked. We end by discussing how specification programs are integrated and lead to the terminal features of different neuronal types.

Control of nerve cord formation by Engrailed and Gooseberry-Neuro: A multi-step, coordinated process

One way to better understand the molecular mechanisms involved in the construction of a nervous system is to identify the downstream effectors of major regulatory proteins. We previously showed that Engrailed (EN) and Gooseberry-Neuro (GsbN) transcription factors act in partnership to drive the formation of posterior commissures in the central nervous system of Drosophila. In this report, we identified genes regulated by both EN and GsbN through chromatin immunoprecipitation ("ChIP on chip") and transcriptome experiments, combined to a genetic screen relied to the gene dose titration method. The genomic-scale approaches allowed us to define 175 potential targets of EN-GsbN regulation. We chose a subset of these genes to examine ventral nerve cord (VNC) defects and found that half of the mutated targets show clear VNC phenotypes when doubly heterozygous with en or gsbn mutations, or when homozygous. This strategy revealed new groups of genes never described for their implication in the construction of the nerve cord. Their identification suggests that, to construct the nerve cord, EN-GsbN may act at three levels, in: (i) sequential control of the attractive-repulsive signaling that ensures contralateral projection of the commissural axons, (ii) temporal control of the translation of some mRNAs, (iii) regulation of the capability of glial cells to act as commissural guideposts for developing axons. These results illustrate how an early, coordinated transcriptional control may orchestrate the various mechanisms involved in the formation of stereotyped neuronal networks. They also validate the overall strategy to identify genes that play crucial role in axonal pathfinding.

Hedgehog signaling acts with the temporal cascade to promote neuroblast cell cycle exit

During the development of the Drosophila nervous system, the developmentally regulated Hedgehog pathway, together with a series of temporal transcription factors, schedules the end of neurogenesis.

In Drosophila postembryonic neuroblasts, transition in gene expression programs of a cascade of transcription factors (also known as the temporal series) acts together with the asymmetric division machinery to generate diverse neurons with distinct identities and regulate the end of neuroblast proliferation. However, the underlying mechanism of how this “temporal series” acts during development remains unclear. Here, we show that Hh signaling in the postembryonic brain is temporally regulated; excess (earlier onset of) Hh signaling causes premature neuroblast cell cycle exit and under-proliferation, whereas loss of Hh signaling causes delayed cell cycle exit and excess proliferation. Moreover, the Hh pathway functions downstream of Castor but upstream of Grainyhead, two components of the temporal series, to schedule neuroblast cell cycle exit. Interestingly, hh is likely a target of Castor. Hence, Hh signaling provides a link between the temporal series and the asymmetric division machinery in scheduling the end of neurogenesis.

In almost all metazoans, neurons are produced by a group of neural stem cells/progenitors in a precise temporal manner, which is important for generating a functional nervous system. In Drosophila, this “timing” mechanism is mainly governed by the sequential switching of transcription factors in neural stem cells called neuroblasts, such that neuronal fate is associated with its birth order. These temporal factors also coordinate the termination of neuroblast division towards the end of neurogenesis. In this study, we show that Hedgehog (Hh) signaling also regulates the division rate of neuroblasts during their proliferative phase at larval stage, as well as the cessation of proliferation at early pupal stage. Excessive Hh signaling causes premature neuroblast cell cycle exit and early termination of neurogenesis, while loss of Hh signaling results in prolonged proliferation of neuroblasts beyond its physiological window. We also find that Hh signaling acts in concert with the temporal transcription factors, and is itself regulated by these factors. We hypothesize that this mode of interaction (temporal transcription factors with developmentally regulated signals like Hh) during neurogenesis could be widely conserved in other organisms.

A concerted action of Engrailed and Gooseberry-Neuro in neuroblast 6-4 is triggering the formation of embryonic posterior commissure bundles

One challenging question in neurogenesis concerns the identification of cues that trigger axonal growth and pathfinding to form stereotypic neuronal networks during the construction of a nervous system. Here, we show that in Drosophila, Engrailed (EN) and Gooseberry-Neuro (GsbN) act together as cofactors to build the posterior commissures (PCs), which shapes the ventral nerve cord. Indeed, we show that these two proteins are acting together in axon growth and midline crossing, and that this concerted action occurs at early development, in neuroblasts. More precisely, we identified that their expressions in NB 6-4 are necessary and sufficient to trigger the formation of the PCs, demonstrating that segmentation genes such as EN and GsbN play a crucial role in the determination of NB 6-4 in a way that will later influence growth and guidance of all the axons that form the PCs. We also demonstrate a more specific function of GsbN in differentiated neurons, leading to fasciculations between axons, which might be required to obtain PC mature axon bundles.

Generation of cell diversity and segmental pattern in the embryonic central nervous system of Drosophila

Development of the central nervous system (CNS) involves the transformation of a two-dimensional epithelial sheet of uniform ectodermal cells, the neuroectoderm, into a highly complex three-dimensional structure consisting of a huge variety of different neural cell types. Characteristic numbers of each cell type become arranged in reproducible spatial patterns, which is a prerequisite for the establishment of specific functional contacts. The fruitfly Drosophila is a suitable model to approach the mechanisms controlling the generation of cell diversity and pattern in the developing CNS, as it allows linking of gene function to individually identifiable cells. This review addresses aspects of the formation and specification of neural stem cells (neuroblasts) in Drosophila in the light of recent studies on their segmental diversification.

Functions of the segment polarity genes midline and H15 in Drosophila melanogaster neurogenesis

The Drosophila melanogaster ventral nerve cord derives from neural progenitor cells called neuroblasts. Individual neuroblasts have unique gene expression profiles and give rise to distinct clones of neurons and glia. The specification of neuroblast identity provides a cell intrinsic mechanism which ultimately results in the generation of progeny which are different from each other. Segment polarity genes have a dual function in early neurogenesis: within distinct regions of the neuroectoderm, they are required both for neuroblast formation and for the specification of neuroblast identity. Previous studies of segment polarity gene function largely focused on neuroblasts that arise within the posterior part of the segment. Here we show that the segment polarity gene midline is required for neuroblast formation in the anterior-most part of the segment. Moreover, midline contributes to the specification of anterior neuroblast identity by negatively regulating the expression of Wingless and positively regulating the expression of Mirror. In the posterior-most part of the segment, midline and its paralog, H15, have partially redundant functions in the regulation of the NB marker Eagle. Hence, the segment polarity genes midline and H15 play an important role in the development of the ventral nerve cord in the anterior- and posterior-most part of the segment.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Local inhibition of Drosophila homeobox-containing neural dorsoventral patterning genes by Dpp

An important step in Drosophila neurogenesis is to establish the neural dorsoventral (DV) patterning. Here we describe how dpp loss-of- and gain-of-function mutation affects the homeobox-containing neural DV patterning genes expressed in the ventral neuroectoderm. Ventral nervous system defective (vnd), intermediate neuroblast defective (ind), muscle-specific homeobox (msh), and orthodenticle (otd) genes participate in development of the central nervous system and peripheral nervous system, and encode homeodomain proteins. otd and msh genes were ectopically expressed in dpp loss-of-function mutation, but vnd and ind were not affected. However, when dpp was ectopically expressed in the ventral neuroectoderm by rho-GAL4/UAS-dpp system, it caused the repression of vnd, and msh expressions in ventral and dorsal columns of the neuroectoderm, respectively, but not that of ind. The later expression pattern of otd was also restricted by Dpp. The expression pattern of msh, vnd and otd in dpp loss-of-function and gain-of-function mutation indicates that Dpp activity does not reach to the ventral midline and it works locally to establish the dorsal boundary of the ventral neuroectoderm.

Regulatory DNA required for vnd/NK-2 homeobox gene expression pattern in neuroblasts

Vnd/NK-2 protein was detected in 11 neuroblasts per hemisegment in Drosophila embryos, 9 medial and 2 intermediate neuroblasts. Fragments of DNA from the 5'-flanking region of the vnd/NK-2 gene were inserted upstream of an enhancerless betagalactosidase gene in a P-element and used to generate transgenic fly lines. Antibodies directed against Vnd/NK-2 and beta-galactosidase proteins then were used in double-label experiments to correlate the expression of beta-galactosidase and Vnd/NK-2 proteins in identified neuroblasts. DNA region A, which corresponds to the -4.0 to -2.8-kb fragment of DNA from the 5'-flanking region of the vnd/NK-2 gene was shown to contain one or more strong enhancers required for expression of the vnd/NK-2 gene in ten neuroblasts. DNA region B (-5.3 to -4.0 kb) contains moderately strong enhancers for vnd/NK-2 gene expression in four neuroblasts. Hypothesized DNA region C, whose location was not identified, contains one or more enhancers that activate vnd/NK-2 gene expression only in one neuroblast. These results show that nucleotide sequences in at least three regions of DNA regulate the expression of the vnd/NK-2 gene, that the vnd/NK-2 gene can be activated in different ways in different neuroblasts, and that the pattern of vnd/NK-2 gene expression in neuroblasts of the ventral nerve cord is the sum of partial patterns.

On the range of hedgehog signaling

Hedgehog (Hh) is a secreted signaling protein that regulates the development of many organ systems. It can travel from its site of synthesis, a process that involves covalent attachment of cholesterol to its carboxyl terminus, proteins with putative sterol sensing domains in both sending and receiving cells, and glycosaminoglycans. Understanding how the movement of Hh is controlled and propelled will be key to understanding how it carries out its essential roles.

Sloppy paired acts as the downstream target of wingless in the Drosophila CNS and interaction between sloppy paired and gooseberry inhibits sloppy paired during neurogenesis

Wingless (Wg) and other Wnt proteins play a crucial role in a number of developmental decisions in a variety of organisms. In the ventral nerve cord of the Drosophila embryo, Wg is non-autonomously required for the formation and specification of a neuronal precursor cell, NB4-2. NB4-2 gives rise to a well-studied neuronal lineage, the RP2/sib lineage. While the various components of the Wg-signaling pathway are also required for generating NB4-2, the target gene(s) of this pathway in the signal-receiving cell is not known. In this paper, we show that sloppy paired 1 and sloppy paired 2 function as the downstream targets of the Wg signaling to generate the NB4-2 cell. Thus, while the loss-of-function mutations in wg and slp have the same NB4-2 formation and specification defects, these defects in wg mutants can be rescued by expressing slp genes from a heterologous promoter. That slp genes function downstream of the Wg signaling is also indicated by the result that expression of slp genes is lost from the neuroectoderm in wg mutants and that ectopic expression of wg induces ectopic expression of slp. Finally, previous results show that Gooseberry (Gsb) prevents Wg from specifying NB4-2 identity to the wg-expressing NB5-3. In this paper, we also show that gsb interacts with slp and prevents Slp from specifying NB4-2 identity. Overexpression of slp overcomes this antagonistic interaction and respecifies NB5-3 as NB4-2. This respecification, however, can be suppressed by a simultaneous overexpression of gsb at high levels. This mechanism appears to be responsible for specifying NB5-3 identity to a row 5 neuroblast and preventing Wg from specifying NB4-2 identity to that cell.

Comparison of early nerve cord development in insects and vertebrates

It is widely held that the insect and vertebrate CNS evolved independently. This view is now challenged by the concept of dorsoventral axis inversion, which holds that ventral in insects corresponds to dorsal in vertebrates. Here, insect and vertebrate CNS development is compared involving embryological and molecular data. In insects and vertebrates, neurons differentiate towards the body cavity. At early stages of neurogenesis, neural progenitor cells are arranged in three longitudinal columns on either side of the midline, and NK-2/NK-2.2, ind/Gsh and msh/Msx homologs specify the medial, intermediate and lateral columns, respectively. Other pairs of regional specification genes are, however, expressed in transverse stripes in insects, and in longitudinal stripes in the vertebrates. There are differences in the regional distribution of cell types in the developing neuroectoderm. However, within a given neurogenic column in insects and vertebrates some of the emerging cell types are remarkably similar and may thus be phylogenetically old: NK-2/NK-2.2-expressing medial column neuroblasts give rise to interneurons that pioneer the medial longitudinal fascicles, and to motoneurons that exit via lateral nerve roots to then project peripherally. Lateral column neuroblasts produce, among other cell types, nerve root glia and peripheral glia. Midline precursors give rise to glial cells that enwrap outgrowing commissural axons. The midline glia also express netrin homologs to attract commissural axons from a distance.

frizzled and frizzled 2 play a partially redundant role in wingless signaling and have similar requirements to wingless in neurogenesis

The Drosophila Frizzled (Fz) and Frizzled2 (DFz2) proteins function as receptors for Wingless (Wg) in tissue culture cells. While previous results indicate that loss of function for fz has tissue polarity defects, loss-of-function effects of Dfz2 are not known. Here, we have examined the requirements of fz and Dfz2 during neurogenesis. Our results indicate that both Fz and DFz2 function in Wg signaling, and loss of either of the two affects the same subset of neuroblasts as those affected by loss of wg. While these defects are partially penetrant in embryos lacking either fz or Dfz2, the penetrance is significantly enhanced in embryos lacking both. Since the penetrance of the CNS phenotypes is not complete in double mutants, additional components that allow some degree of Wg signaling must exist in vivo.

Dorsoventral patterning in the Drosophila central nervous system: the intermediate neuroblasts defective homeobox gene specifies intermediate column identity

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 the vnd/nk2 homeobox gene, intermediate cells, and dorsal cells that express the msh homeobox gene. Here we describe a new Drosophila homeobox gene, intermediate neuroblasts defective (ind), which is expressed specifically in the intermediate column cells. ind is essential for intermediate column development: Null mutants have a transformation of intermediate to dorsal column neuroectoderm fate, and only 10% of the intermediate column neuroblasts develop. The establishment of dorsoventral column identity involves negative regulation: Vnd represses ind in the ventral column, whereas ind represses msh in the intermediate column. Vertebrate genes closely related to vnd (Nkx2.1 and Nkx2.2), ind (Gsh1 and Gsh2), and msh (Msx1 and Msx3) are expressed in corresponding ventral, intermediate, and dorsal domains during vertebrate neurogenesis, raising the possibility that dorsoventral patterning within the central nervous system is evolutionarily conserved.

Formation and specification of ventral neuroblasts is controlled by vnd in Drosophila neurogenesis

During Drosophila neural development, neuroblasts delaminate from the neuroectoderm of each hemisegment in a stereotypic orthogonal array of five rows and three columns (ventral, intermediate, and dorsal). Prevailing evidence indicates that the individual neuroblast fate is determined by the domain-specific expression of genes along the dorsoventral and anteroposterior axis. Here, we analyze the role of Vnd, a NK-2 homeodomain protein, expressed initially in the ventral neuroectoderm adjacent to the ventral midline, in the dorsoventral patterning of the neuroectoderm and the neuroblasts. We show that in vnd null mutants most ventral neuroblasts do not form and the few that form do not develop ventral fates, but instead develop intermediate-like fates. Furthermore, we demonstrate that Vnd influences the gene expression patterns in the ventral proneural clusters and neuroectoderm, and that its action in neuroblast formation includes, but is not exclusive to the activation of proneural AS-C genes. Through the use of GAL4/UAS gene-expression system we show that ectopic Vnd expression can promote ventral-like fates in intermediate and dorsal neuroblasts and can suppress certain normal characteristics of the intermediate and dorsal neuroectoderm. Our results are discussed in the context of the current evidence in dorsoventral patterning in the Drosophila neuroectoderm.

Dorsoventral patterning in the Drosophila central nervous system: the vnd homeobox gene specifies ventral column identity

The Drosophila CNS develops from three columns of neuroectodermal cells along the dorsoventral (DV) axis: ventral, intermediate, and dorsal. In this and the accompanying paper, we investigate the role of two homeobox genes, vnd and ind, in establishing ventral and intermediate cell fates within the Drosophila CNS. During early neurogenesis, Vnd protein is restricted to ventral column neuroectoderm and neuroblasts; later it is detected in a complex pattern of neurons. We use molecular markers that distinguish ventral, intermediate, and dorsal column neuroectoderm and neuroblasts, and a cell lineage marker for selected neuroblasts, to show that loss of vnd transforms ventral into intermediate column identity and that specific ventral neuroblasts fail to form. Conversely, ectopic vnd produces an intermediate to ventral column transformation. Thus, vnd is necessary and sufficient to induce ventral fates and repress intermediate fates within the Drosophila CNS. Vertebrate homologs of vnd (Nkx2.1 and 2.2) are similarly expressed in the ventral CNS, raising the possibility that DV patterning within the CNS is evolutionarily conserved.

Differential effects of EGF receptor signalling on neuroblast lineages along the dorsoventral axis of the Drosophila CNS

The Drosophila ventral nerve cord derives from a stereotype population of about 30 neural stem cells, the neuroblasts, per hemineuromere. Previous experiments provided indications for inductive signals at ventral sites of the neuroectoderm that confer neuroblast identities. Using cell lineage analysis, molecular markers and cell transplantation, we show here that EGF receptor signalling plays an instructive role in CNS patterning and exerts differential effects on dorsoventral subpopulations of neuroblasts. The Drosophila EGF receptor (DER) is capable of cell autonomously specifiying medial and intermediate neuroblast cell fates. DER signalling appears to be most critical for proper development of intermediate neuroblasts and less important for medial neuroblasts. It is not required for lateral neuroblast lineages or for cells to adopt CNS midline cell fate. Thus, dorsoventral patterning of the CNS involves both DER-dependent and -independent regulatory pathways. Furthermore, we discuss the possibility that different phases of DER activation exist during neuroectodermal patterning with an early phase independent of midline-derived signals.

eagle is required for the specification of serotonin neurons and other neuroblast 7–3 progeny in the Drosophila CNS

During development of the Drosophila nerve cord, neuroblast 7–3 gives rise to a pair of mitotic sister serotonin neurons in each hemisegment. Here we show that the zinc finger gene eagle, which is expressed in neuroblast 7–3, is essential for specifying the fate of serotonin neurons. We find that loss-of-function eagle mutations produce an unusual differential phenotype with respect to the sister serotonin cells and that eagle is necessary for the maintenance of engrailed and zfh-2 expression in the serotonin neurons. We present a model that uniquely identifies all progeny neurons in the neuroblast 7–3 lineage based on the expression of specific molecular markers, position within the nerve cord and the effect of eagle loss-of-function mutations. Although serotonin is an important neurotransmitter conserved throughout the animal kingdom, we show that hypomorphic alleles of eagle can produce viable adults that have a dramatic reduction in the number of serotonin-producing neurons.

The embryonic central nervous system lineages of Drosophila melanogaster. II. Neuroblast lineages derived from the dorsal part of the neuroectoderm

In Drosophila, central nervous system (CNS) formation starts with the delamination from the neuroectoderm of about 30 neuroblasts (NBs) per hemisegment. They give rise to approximately 350 neurons and 30 glial cells during embryonic development. Understanding the mechanisms leading to cell fate specification and differentiation in the CNS requires the identification of the NB lineages. The embryonic lineages derived from 17 NBs of the ventral part of the neuroectoderm have previously been described (Bossing et al., 1996). Here we present 13 lineages derived from the dorsal part of the neuroectoderm and we assign 12 of them to identified NBs. Together, the 13 lineages comprise approximately 120 neurons and 22 to 27 glial cells which we include in a systematic terminology. Therefore, NBs from the dorsal neuroectoderm produce about 90% of the glial cells in the embryonic ventral ganglion. Two of the NBs give rise to glial progeny exclusively (NB 6-4A, GP) and five to glia as well as neurons (NBs 1-3, 2-5, 5-6, 6-4T, 7-4). These seven NBs are arranged as a group in the most lateral region of the NB layer. The other lineages (NBs 2-4, 3-3, 3-5, 4-3, 4-4, 5-4, clone y) are composed exclusively of neurons (interneurons, motoneurons, or both). Additionally, it has been possible to link the lateral cluster of even-skipped expressing cells (EL) to the lineage of NB 3-3. Along with the previously described clones, the vast majority (more than 90%) of cell lineages in the embryonic ventral nerve cord (thorax, abdomen) are now known. Moreover, previously identified neurons and most glial cells are now linked to certain lineages and, thus, to particular NBs. This complete set of data provides a foundation for the interpretation of mutant phenotypes and for future investigations on cell fate specification and differentiation.

The role of the msh homeobox gene during Drosophila neurogenesis: implication for the dorsoventral specification of the neuroectoderm

Development of the Drosophila central nervous system begins with the delamination of neural and glial precursors, called neuroblasts, from the neuroectoderm. An early and important step in the generation of neural diversity is the specification of individual neuroblasts according to their position. In this study, we describe the genetic analysis of the msh gene which is likely to play a role in this process. The msh/Msx genes are one of the most highly conserved families of homeobox genes. During vertebrate spinal cord development, Msx genes (Msx1-3) are regionally expressed in the dorsal portion of the developing neuroectoderm. Similarly in Drosophila, msh is expressed in two longitudinal bands that correspond to the dorsal half of the neuroectoderm, and subsequently in many dorsal neuroblasts and their progeny. We showed that Drosophila msh loss-of-function mutations led to cell fate alterations of neuroblasts formed in the dorsal aspect of the neuroectoderm, including a possible dorsal-to-ventral fate switch. Conversely, ectopic expression of msh in the entire neuroectoderm severely disrupted the proper development of the midline and ventral neuroblasts. The results provide the first in vivo evidence for the role of the msh/Msx genes in neural development, and support the notion that they may perform phylogenetically conserved functions in the dorsoventral patterning of the neuroectoderm.

The differentiation of the serotonergic neurons in the Drosophila ventral nerve cord depends on the combined function of the zinc finger proteins Eagle and Huckebein

The Drosophila ventral nerve cord (vNC) derives from a stereotyped population of neural stem cells, neuroblasts (NBs), each of which gives rise to a characteristic cell lineage. The mechanisms leading to the specification and differentiation of these lineages are largely unknown. Here we analyse mechanisms leading to cell differentiation within the NB 7–3 lineage. Analogous to the grasshopper, NB 7–3 is the progenitor of the Drosophila vNC serotonergic neurons. The zinc finger protein Eagle (Eg) is expressed in NB 7–3 just after delamination and is present in all NB 7–3 progeny until late stage 17. DiI cell lineage tracing and immunocytochemistry reveal that eg is required for normal pathfinding of interneuronal projections and for restricting the cell number in the thoracic NB 7–3 lineage. Moreover, eg is required for serotonin expression. Ectopic expression of Eg protein forces specific additional CNS cells to enter the serotonergic differentiation pathway. Like NB 7–3, the progenitor(s) of these ectopic cells express Huckebein (Hkb), another zinc finger protein. However, their progenitors do not express engrailed (en) as opposed to the NB 7–3 lineage, where en acts upstream of eg. We conclude that eg and hkb act in concert to determine serotonergic cell fate, while en is more distantly involved in this process by activating eg expression. Thus, we provide the first functional evidence for a combinatorial code of transcription factors acting early but downstream of segment polarity genes to specify a unique neuronal cell fate.

Requirement for engrailed and invected genes reveals novel regulatory interactions between engrailed/invected, patched, gooseberry and wingless during Drosophila neurogenesis

During neurogenesis, the transmembrane protein Patched (Ptc) promotes a wingless (wg)-mediated specification of a neuronal precursor cell, NB4-2, by repressing gooseberry (gsb). In this study, novel interactions of these genes with engrailed (en) and invected (inv) during neurogenesis have been uncovered. While in row 4 cells Ptc represses gsb and wg, in row 5 cells en/inv relieve Ptc repression of gsb by a non-autonomous mechanism that does not involve hedgehog (hh). This differential regulation of gsb leads to the specification of NB5-3 and NB4-2 identities to two distinct neuroblasts. The uncoupling of the ptc-gsb regulatory circuit also enables gsb to promote Wg expression in row 5 cells. Our results suggest that the en/inv-->ptc-->gsb-->wg pathway uncovered here and the hh-->wg are distinct pathways that function to maintain wild-type level of Wg. Our results also indicate that Hh is not the only ligand for Ptc and similarly Ptc is not the only receptor for Hh.