1 European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany
2 Zoology Department, Stockholm University, 10691 Stockholm, Sweden
* Present address: Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid, 28049 Madrid, Spain
Author for correspondence (e-mail: rbarrio{at}cbm.uam.es)
Accepted 30 April 2002
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SUMMARY |
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Key words: Drosophila, Decapentaplegic, Peripheral nervous system, BMP, TGFß, Schnurri, Brinker, Evolution
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INTRODUCTION |
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Early in Xenopus development, the TGFß molecule bone morphogenetic protein 4 [BMP4, which belongs to the BMP2/4/Decapentaplegic (Dpp) subfamily] defines the non-neurogenic region of the embryo, while its inhibitor Chordin (Chd) defines the neurogenic anlage. Similarly, in zebrafish the specification of non-neurogenic and neurogenic regions appears to be governed by BMP2 and CHD, respectively (Hammerschmidt et al., 1996; Holley and Ferguson, 1997
; Kishimoto et al., 1997
; Mullins et al., 1996
; Schulte-Merker et al., 1997
).
Later in vertebrate development, BMP/TGFß signaling is important for specification of dorsal cell fates in the nervous system. It is involved in the formation of the roof plate, neural crest and dorsal sensory interneurons of the neural tube (Lee and Jessell, 1999). In chicken explant assays, the epidermal ectoderm that harbors BMPs, or the BMPs themselves (BMP4 and BMP7), induce neural plate tissue to express dorsal neural markers for the roof plate, neural crest and dorsal interneurons (D1 and D2), while secreted antagonists (Noggin + Follistatin) can inhibit expression of these markers (Dickinson et al., 1995
; Liem et al., 1997
; Liem et al., 1995
). Genetic experiments in zebrafish support the role of BMP signaling in pattern formation within the neural tube. Analysis of swirl/bmp2b, snailhouse/bmp7 and somatibun/smad5 mutants, as well as injection of the BMP antagonist CHD, reveal that BMP signaling is essential for establishing neural crest, dorsal sensory neurons (Rohon Beard neurons) and interneurons of the neural tube (Barth et al., 1999
; Nguyen et al., 2000
). In mouse, a number of candidate TGFß molecules are expressed in the dorsal neural tube and the overlying ectoderm (Lee and Jessell, 1999
; Liem et al., 1997
). These include BMP4 of the BMP2/4/Dpp subfamily; BMP5 and BMP7 of the BMP5-8/60A subfamily; Activin B, which is closely related to Drosophila DmActivin; and growth/differentiation factor 7 (GDF7), which is equally related to Drosophila Dpp, 60A and Screw (Scw) (Newfeld et al., 1999
). After neurulation, the dorsal epidermal ectoderm, which later separates from the neural tube, and the roof plate are sources of these TGFß molecules. Ablation of the roof plate produces defects in dorsal neural fate specification very similar to defects generated by blocking TGFß/BMP signaling (Liem et al., 1997
), strongly suggesting that TGFß signals emanating from the roof plate specify dorsal neural cell types (Lee et al., 2000
; Millonig et al., 2000
). Consistent with this, mice that lack GDF7, which is expressed in the roof plate, lack specific dorsal interneurons (D1A), directly showing a role for this TGFß in the generation of dorsal neural cell types (Lee et al., 1998
). However, this mutation does not reflect all the aspects of TGFß signaling shown in other experiments in which TGFß inhibitors have been added or roofplate has been deleted. It is very likely that GDF7 and other BMP family members expressed by the roofplate hold non-redundant functions (Lee et al., 1998
). Finally, in the ascidian Halocynthia roretzi BMP signaling affects neural plate patterning. At the time of sensory organ fate decision (tailbud stage) BMPb (belonging to the BMP2/4/Dpp subfamily of BMPs) is expressed in the dorsal neural plate (Miya et al., 1997
). Overexpression of BMPb and Chd affect the choice of cell fates between sensory structures forming at the edge of the neural plate, consistent with a role for BMP signaling in specification of cell fates in the dorsal nervous system (Darras and Nishida, 2001
).
During the development of the nervous system in the Drosophila embryo, TGFß signaling is biphasic (Dorfman and Shilo, 2001). In the first phase, the two BMP class molecules Dpp and Scw and the secreted inhibitor Short gastrulation (Sog) have an initial role strikingly similar to the role of homologous proteins in zebrafish and Xenopus (de Robertis and Sasai, 1996
; Holley and Ferguson, 1997
). Through broad dorsoventral patterning of the blastoderm, they define the borders of the ventral neurogenic region (VNE) (Arora et al., 1994
; Biehs et al., 1996
; Neul and Ferguson, 1998
; Nguyen et al., 1998
; Podos and Ferguson, 1999
). The VNE gives rise to the ventral nerve cord (VNC) and some ventral peripheral nervous system (PNS) neurons (Campos-Ortega, 1993
; Schmid et al., 1999
; Schmidt et al., 1997
). Later in development, localized Dpp signaling appears in the dorsal ectoderm where the dorsolateral PNS is forming (Dorfman and Shilo, 2001
; Jackson and Hoffmann, 1994
), raising the possibility of a role for Dpp in specification of dorsal cell fates in PNS development.
We have explored the possibility of an evolutionarily conserved role of TGFß family members in specification of the dorsal nervous system by investigating the function of this second phase of Dpp signaling for PNS development in Drosophila. We report that this signaling indeed promotes formation of the dorsal and lateral PNS neurons, apparently mediated by activation of proneural gene expression. The inhibitor Brinker (Brk) comes into play when Dpp signaling is compromised and is in addition involved in the formation of the ventral PNS neurons. These results indicate that TGFß signaling is important for detailed patterning of the dorsal nervous system in protostomes as well as in deuterostomes.
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MATERIALS AND METHODS |
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Immunohistochemistry
Immunohistochemistry was performed in whole-mount embryos using the following primary antibodies: rabbit anti-ß-galactosidase antiserum (1:1000 dilution; Cappel); anti-Ato rabbit antiserum (1:1000 dilution) (Jarman et al., 1994); anti-p-Mad rabbit antisera (1:50 dilution) (Tanimoto et al., 2000
); and anti-Ac monoclonal antibodies (1:10 dilution) (Skeath et al., 1992
); and 22C10 monoclonal antibody (1:20 dilution) (Fujita et al., 1982
) was obtained from the Developmental Studies Hybridoma Bank at the University of Iowa. Fluorescent Cy2- and Cy3-conjugated secondary antibodies (Jackson Immunoresearch), and Alexa488-conjugated secondary antibodies (Molecular Probes) were used at 1:1000 dilution. HRP-conjugated goat anti-rabbit and goat anti-mouse antibodies (Promega) were used at 1:250 dilution. AP-conjugated goat-anti rabbit antibodies were used at 1:2000 dilution.
Embryos were staged according to Campos Ortega and Hartenstein (Campos-Ortega and Hartenstein, 1997), and were fixed and processed for whole-mount antibody staining using standard techniques (Patel, 1994
). Stained embryos were cleared in 80% glycerol, mounted and examined on a Zeiss Axiophot. Alternatively, fluorescent embryos were analyzed by confocal microscopy using a Zeiss LSM 510 microscope.
Heatshock experiments
Embryos were collected on apple-juice agar plates for 1 hour at 25°C. They were allowed to age for 3 hours at 25°C before administering 1 hour heat shock at 37°C by immersion of the agar plates in a water bath. Embryos were allowed to recover at room temperature for 30 minutes and thereafter kept at 25°C until the correct stage was achieved before fixation and phenotypic analysis.
Cell death staining
Embryos were stained with Acridine Orange to reveal apoptotic cells according to the previously described protocol (Abrams et al., 1993).
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RESULTS |
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The second wave of Dpp signaling is necessary for correct expression of proneural genes
The reduced number of neurons observed in the dorsal and lateral PNS when Dpp signaling is impeded could result from lack of proneural gene expression, which is known to be necessary for PNC and SOP formation (Jan and Jan, 1993). We analyzed the expression of ato and ac to examine the specification of progenitor cell subclasses in mutant backgrounds defective for Dpp signaling. The development of the serially homologous abdominal segments A1 to A7 is similar and very synchronous. Thus, in the wild type, whenever a specific number of PNCs and SOPs appear in one abdominal segment, a similar pattern is observed in the other abdominal segments as well (Fig. 3A,B). This is not true for shnk04412 mutants and for embryos expressing ubiquitous ssog, where the numbers of Ac and Ato positive SOPs and PNCs vary among the abdominal segments (Fig. 3D,F and Fig. 3C,E, respectively). This is consistent with the variably penetrant phenotypes observed in differentiated PNS among abdominal segments (Fig. 2). In embryos expressing Kr-Gal4;UAS-brk, we observed loss of Ato- and Ac-positive PNCs and SOPs specifically in the abdominal segments A1-A3 where brk was misexpressed, when compared with abdominal segments A4-A7 that served as an internal reference. The reduced numbers of Ato- and Ac-positive neuronal progenitors appear to result from failure of PNC formation rather than an increase in cell death ratio: apoptosis does not appear to increase in segments expressing brk compared with the other abdominal segments (Fig. 3I). Taken together, these results suggest that reduction in the number of neurons is produced by failure in proneural gene expression.
The absence of Brk counteracts the neuronal loss produced by compromised Dpp signaling
The transcriptional activation of target genes mediated by Dpp signaling is known to be regulated in different ways. While some target genes appear to be activated directly by Mad, Shn and the mediator Medea (Med), probably working as a complex, other target genes are regulated indirectly by relieving the repressive activity of Brk (Affolter et al., 2001). Dpp signaling and brk expression are mutually inhibitory. In particular, Dpp signaling represses brk expression through the action of Shn. In shn mutant embryos, brk expression expands dorsally during the germ band elongation stage, when the PNS forms (Marty et al., 2000
; Torres-Vazquez et al., 2001
). Conversely, in brk mutant embryos, dpp expression expands ventrally, as does expression of Dpp target genes like pnr and dad (Jazwinska et al., 1999b
; Torres-Vazquez et al., 2001
). The question arises as to whether the reduced number of neurons, in the experiments documented in Fig. 2, is a direct effect of the loss of Dpp signaling via Shn or, alternatively, is the result of the dorsally expanded Brk expression in shn mutant embryos. This question was addressed by comparing the numbers of neurons in wild type, single mutant and double mutant embryos (Fig. 4). The numbers of dorsal and lateral neurons are comparable in wild type and brk mutant embryos, suggesting that Brk is normally not involved in the formation of these neurons. By contrast, these numbers are substantially reduced in shn mutants, confirming that Dpp signaling is indeed involved in this process. Importantly, the number of neurons observed in brk;shn double mutant embryos is higher than in shn single mutants, approaching the number seen in the wild type. However, the recovery in brkm68;shn1 embryos is incomplete, suggesting an additional role of Shn independent of Brk, as shown previously (Torres-Vazquez et al., 2001
). These conclusions are consistent with a putative regulatory diagram presented on the left in Fig. 5A (see Discussion).
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In summary, apparently two populations of neurons exit. The dorsal and lateral clusters form one population that is mainly dependent on Dpp signaling, while the ventral cluster forms another population largely independent of Dpp signaling.
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DISCUSSION |
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It is known from previous studies that Dpp signaling and Brk are mutually antagonistic. For example, in the second phase of Dpp signaling, brk is negatively regulated by Dpp signaling itself via Shn (Marty et al., 2000; Torres-Vazquez et al., 2001
). Conversely, in the first phase of Dpp signaling Brk directly or indirectly counteracts Dpp signaling: in brk mutant embryos, the domain of dpp expression expands while the VNE (from which the ventral nerve cord develops) is reduced (Jazwinska et al., 1999b
; Skeath et al., 1992
). Cell lineage tracing studies have shown that some ventral PNS neurons originate from the VNE (Brewster and Bodmer, 1996
; Schmid et al., 1999
; Schmidt et al., 1997
). The similar genetic requirements for brk in the ventral PNS population and in the ventral nerve cord arising from the VNE strongly suggest that this population represents the part of the PNS that originates within the VNE.
Both Dpp signaling and Brk are thought to act by transcriptional regulation. For simplicity, in the diagram of Fig. 5A, we show them as acting antagonistically on the same target genes in the dorsolateral population of neurons. However, in the ventral population, no antagonistic effect is observed in brk;shn double mutants: Brk promotes the formation of neurons and Shn also has a positive albeit secondary effect. This suggests that the major double negative Brk-dependent mechanism and the minor positive Shn-dependent mechanism work in parallel on this population. Two distinct, dorsolateral and ventral mechanisms are feasible, because specific proneural clusters can be regulated by different proneural genes (Jarman et al., 1993; Ruiz-Gomez and Ghysen, 1993
) or by different enhancers of the same gene. The diagrams in Fig. 5A are not unique explanations, alternative circuitries may be identified in further work. The diagrams serve to illustrate our main conclusion, that different genetic mechanisms, which are driven predominantly by Dpp or Brk, are regulating PNS neuronal formation in the two populations.
Whatever the detailed mechanism of PNS formation, it is now clear that TGFß signaling is important for two independent phases in Drosophila embryonic nervous system development. First, it is necessary for defining the border of the VNE. In the second phase, TGFß signaling by Dpp is necessary for development of the dorsalmost nervous system, the dorsal and lateral PNS, while the ventral PNS population is regulated primarily by the inhibitor Brk. This biphasic role has parallels in vertebrates, as summarized in the Introduction, and has important implications about the evolutionary history of the development of the nervous system.
A conserved role for late TGFß signaling in nervous system formation
In a current hypothesis, the insect ventral side corresponds to the dorsal side of vertebrates, because of an inversion of the dorsoventral axis during evolution (Arendt and Nubler-Jung, 1994; de Robertis and Sasai, 1996
; Holley and Ferguson, 1997
). Furthermore, the central nervous system (CNS), which consists of the brain and nerve cord, although located ventrally in insects and dorsally in chordates, presents striking similarities and has been suggested to be evolutionarily conserved among Bilateria (Arendt and Nubler-Jung, 1999
). In both insects and vertebrates the anteroposterior regionalization of the neuroectoderm is regulated by orthologous homeotic genes, the cephalic gap genes, engrailed and eyeless/Pax6 genes (Arendt and Nubler-Jung, 1996
; Reichert and Simeone, 1999
). Both the ventral midline cells of Drosophila and the floor plate of vertebrates function as centers for mediolateral regionalization. One difference is that in vertebrates, the main morphogen emanating from the midline is Sonic hedgehog (Shh), while in Drosophila it is a TGF
-related ligand (Spitz) signaling through the dEGF-receptor (Arendt and Nubler-Jung, 1999
). Despite the fact that non-homologous molecules serve to pattern the mediolateral axis, striking similarities exists in the pattern of genes that specify a series of longitudinal columns in the neuroectoderm (Arendt and Nubler-Jung, 1999
; Jurata et al., 2000
). In both Drosophila and vertebrates, homologous genes specify the proneural columns from the midline outwards: vnd/Nkx-2.2, ind/Gsh-1 and msh/Msx1/2/3 (Fig. 5B), indicating that the mediolateral regionalization of the neural axis is evolutionary conserved (Weiss et al., 1998
).
Several lines of evidence suggest that the Drosophila embryonic PNS also has a direct equivalent in vertebrate embryos: the sensory neurons in, or emerging from, the dorsal neural tube. The first evidence concerns the expression patterns of lateral column genes. In vertebrates, the most lateral part of the Msx-positive neural plate gives rise to the roof plate of the dorsal neural tube, from which the neural crest originates before it migrates out and gives rise to the sensory neurons of the PNS and other tissues (Fig. 5) (Arendt and Nubler-Jung, 1999; Echelard et al., 1994
; Selleck and Bronner-Fraser, 1995
). The adjacent Msx expression domain that remains in the neural tube includes precursors of dorsal sensory interneurons. In several extant vertebrates, some primary sensory neurons do not migrate out, but remain in the dorsal spinal cord, where they are identified as Rohon-Beard neurons in zebrafish and Xenopus and dorsal cells in Amphioxus (Baker and Bronner-Fraser, 1997
; Clarke et al., 1984
; Martin and Wickelgren, 1971
). In Drosophila, the msh gene (the ortholog of Msx) is expressed in the lateral columns of the Drosophila VNE, and slightly later in the developing PNS (DAlessio and Frasch, 1996
). Thus, both in vertebrates and in Drosophila the Msx/Msh-positive lateral (later dorsal) nervous system includes precursors of the sensory PNS and interneuron populations.
The second evidence concerns the expression patterns of proneural genes. These genes can be grouped as Achaete-Scute Complex (AS-C) related genes and ato-related genes (Hassan and Bellen, 2000). The lateral and ventral motor region of the vertebrate neural tube express only the AS-C homolog, Mash1, while the dorsolateral interneuron and sensory neuron regions of the neural tube and the neural crest express both the AS-C like Mash1 and Math1, the protein most related to Ato and Amo (Gowan et al., 2001
; Hassan and Bellen, 2000
). Similarly, in Drosophila the ventral regions from which the CNS develops express only AS-C genes, whereas the PNS develops from a more dorsolateral region where both AS-C and Ato/Amo proteins are expressed.
A final parallel is the expression pattern of TGFß molecules. By early neurulation in chordates (the ascidian H. roretzi, Xenopus, zebrafish, chicken and mouse) BMPs are strongly localized at the edges of the neural plate (Darras and Nishida, 2001; Miya et al., 1997
; Streit and Stern, 1999
). In Drosophila, the active Dpp signaling covers the dorsal part of the epidermis, including the forming dorsal PNS (Dorfman and Shilo, 2001
; Jackson and Hoffmann, 1994
). Thus, in all these organisms, BMP2/4/Dpp orthologs are expressed at the lateral (later dorsal) parts of the forming nervous system. Taken together, these data suggest that the dorsalmost neural tube of vertebrates, including the neural crest, is evolutionarily homologous to the most dorsal nervous tissue in Drosophila, the embryonic PNS (Fig. 5).
In chordates, after the neural anlage is specified, TGFß signaling is necessary for induction of dorsal neuronal cell populations and migration of neural crest cells (Darras and Nishida, 2001; Lee and Jessell, 1999
). An important contribution of the present work is the demonstration that TGFß signaling is necessary for induction of the most dorsal nervous system in Drosophila (the dorsolateral PNS) as it is in vertebrates. We suggest that the induction of dorsal neuronal subtypes by TGFß signaling is an evolutionarily conserved step in nervous system patterning in all Bilateria.
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ACKNOWLEDGMENTS |
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REFERENCES |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Abrams, J. M., White, K., Fessler, L. I. and Steller, H. (1993). Programmed cell death during Drosophila embryogenesis. Development 117, 29-43.
Affolter, M., Marty, T., Vigano, M. A. and Jazwinska, A. (2001). Nuclear interpretation of Dpp signaling in Drosophila. EMBO J. 20, 3298-3305.
Arendt, D. and Nubler-Jung, K. (1994). Inversion of dorsoventral axis? Nature 371, 26.[Medline]
Arendt, D. and Nubler-Jung, K. (1996). Common ground plans in early brain development in mice and flies. BioEssays 18, 255-259.[Medline]
Arendt, D. and Nubler-Jung, K. (1999). Comparison of early nerve cord development in insects and vertebrates. Development 126, 2309-2325.
Arora, K., Dai, H., Kazuko, S. G., Jamal, J., OConnor, M. B., Letsou, A. and Warrior, R. (1995). The Drosophila schnurri gene acts in the Dpp/TGF-ß signaling pathway and encodes a transcription factor homologous to the human BMP family. Cell 81, 781-790.[Medline]
Arora, K., Levine, M. S. and OConnor, M. B. (1994). The screw gene encodes a ubiquitously expressed member of the TGF-ß family required for specification of dorsal cell fates in the Drosophila embryo. Genes Dev. 8, 2588-2601.[Abstract]
Baker, C. V. and Bronner-Fraser, M. (1997). The origins of the neural crest. Part II: an evolutionary perspective. Mech. Dev. 69, 13-29.[Medline]
Barth, K. A., Kishimoto, Y., Rohr, K. B., Seydler, C., Schulte-Merker, S. and Wilson, S. W. (1999). BMP activity establishes a gradient of positional information throughout the entire neural plate. Development 126, 4977-4987.
Biehs, B., Francois, V. and Bier, E. (1996). The Drosophila short gastrulation gene prevents Dpp from autoactivating and suppressing neurogenesis in the neuroectoderm. Genes Dev. 10, 2922-2934.[Abstract]
Brewster, R. and Bodmer, R. (1996). Cell lineage analysis of the Drosophila peripheral nervous system. Dev. Genet. 18, 50-63.[Medline]
Campbell, G. and Tomlinson, A. (1999). Transducing the Dpp morphogen gradient in the wing of Drosophila: regulation of Dpp targets by brinker. Cell 96, 553-562.[Medline]
Campos-Ortega, J. A. (1993). Early neurogenesis in Drosophila melanogaster: Cold Spring Harbour Laboratory Press.
Campos-Ortega, J. A. and Hartenstein, V. (1997). The embryonic development of Drosophila melanogaster. Berlin: Springer-Verlag.
Castelli-Gair, J., Greig, S., Micklem, G. and Akam, M. (1994). Dissecting the temporal requirements for homeotic gene function. Development 120, 1983-1995.
Clarke, J. D., Hayes, B. P., Hunt, S. P. and Roberts, A. (1984). Sensory physiology, anatomy and immunohistochemistry of Rohon-Beard neurones in embryos of Xenopus laevis. J. Physiol. 348, 511-525.[Abstract]
DAlessio, M. and Frasch, M. (1996). msh may play a conserved role in dorsoventral patterning of the neuroectoderm and mesoderm. Mech. Dev. 58, 217-231.[Medline]
Dai, H., Hogan, C., Gopalakrishnan, B., Torres-Vazquez, J., Nguyen, M., Park, S., Raftery, L. A., Warrior, R. and Arora, K. (2000). The zinc finger protein Schnurri acts as a Smad partner in mediating the transcriptional response to Decapentaplegic. Dev. Biol. 227, 373-387.[Medline]
Darras, S. and Nishida, H. (2001). The BMP/Chordin antagonism controls sensory pigment cell specification and differentiation in the ascidian embryo. Dev. Biol. 236, 271-288.[Medline]
de Robertis, E. M. and Sasai, Y. (1996). A common plan for dorsoventral patterning in Bilateria. Nature 380, 37-40.[Medline]
Dickinson, M. E., Selleck, M. A., McMahon, A. P. and Bronner-Fraser, M. (1995). Dorsalization of the neural tube by the non-neural ectoderm. Development 121, 2099-2106.
Dorfman, R. and Shilo, B. Z. (2001). Biphasic activation of the BMP pathway patterns the Drosophila embryonic dorsal region. Development 128, 965-972.
Echelard, Y., Vassileva, G. and McMahon, A. P. (1994). Cis-acting regulatory sequences governing Wnt-1 expression in the developing mouse CNS. Development 120, 2213-2224.
Fujita, S. C., Zipursky, S. L., Benzer, S., Ferrus, A. and Shotwell, S. L. (1982). Monoclonal antibodies against the Drosophila nervous system. Proc. Natl. Acad. Sci. USA 79, 7929-7933.[Abstract]
Ghysen, A., Dambly-Chaudière, C., Aceves, E., Jan, L. Y. and Jan, Y. N. (1986). Sensory neurons and peripheral pathways in Drosophila embryos. Rouxs Arch. Dev. Biol. 195, 281-289.
Gowan, K., Helms, A. W., Hunsaker, T. L., Collisson, T., Ebert, P. J., Odom, R. and Johnson, J. E. (2001). Crossinhibitory activities of Ngn1 and Math1 allow specification of distinct dorsal interneurons. Neuron 31, 219-232.[Medline]
Grieder, N. C., Nellen, D., Burke, R., Basler, K. and Affolter, M. (1995). Schnurri is required for Drosophila Dpp signaling and encodes a zinc finger protein similar to the mammalian transcription factor PRDII-BF1. Cell 81, 791-800.[Medline]
Hammerschmidt, M., Serbedzija, G. N. and McMahon, A. P. (1996). Genetic analysis of dorsoventral pattern formation in the zebrafish: requirement of a BMP-like ventralizing activity and its dorsal repressor. Genes Dev. 10, 2452-2461.[Abstract]
Hassan, B. A. and Bellen, H. J. (2000). Doing the MATH: is the mouse a good model for fly development? Genes Dev. 14, 1852-1865.
Holley, S. A. and Ferguson, E. L. (1997). Fish are like flies are like frogs: conservation of dorsal-ventral patterning mechanisms. BioEssays 19, 281-284.[Medline]
Huang, M. L., Hsu, C. H. and Chien, C. T. (2000). The proneural gene amos promotes multiple dendritic neuron formation in the Drosophila peripheral nervous system. Neuron 25, 57-67.[Medline]
Jackson, P. D. and Hoffmann, F. M. (1994). Embryonic expression patterns of the Drosophila decapentaplegic gene: separate regulatory elements control blastoderm expression and lateral ectodermal expression. Dev. Dyn. 199, 28-44.[Medline]
Jan, Y. N. and Jan, L. Y. (1993). The Peripheral Nervous System. Cold Spring Harbour Laboratory Press.
Jarman, A. P., Grau, Y., Jan, L. Y. and Jan, Y. N. (1993). atonal is a proneural gene that directs chordotonal organ formation in the Drosophila peripheral nervous system. Cell 73, 1307-1321.[Medline]
Jarman, A. P., Grell, E. H., Ackerman, L., Jan, L. Y. and Jan, Y. N. (1994). atonal is the proneural gene for Drosophila photoreceptors. Nature 369, 398-400.[Medline]
Jazwinska, A., Kirov, N., Wieschaus, E., Roth, S. and Rushlow, C. (1999a). The Drosophila gene brinker reveals a novel mechanism of Dpp target gene regulation. Cell 96, 563-573.[Medline]
Jazwinska, A., Rushlow, C. and Roth, S. (1999b). The role of Brinker in mediating the graded response to Dpp in early Drosophila embryos. Development 126, 3323-3334.
Jurata, L. W., Thomas, J. B. and Pfaff, S. L. (2000). Transcriptional mechanisms in the development of motor control. Curr. Opin. Neurobiol. 10, 72-79.[Medline]
Kania, A., Salzberg, A., Bhat, M., DEvelyn, D., He, Y., Kiss, I. and Bellen, H. J. (1995). P-element mutations affecting embryonic peripheral nervous system development in Drosophila melanogaster. Genetics 139, 1663-1678.
Kishimoto, Y., Lee, K. H., Zon, L., Hammerschmidt, M. and Schulte-Merker, S. (1997). The molecular nature of zebrafish swirl: BMP2 function is essential during early dorsoventral patterning. Development 124, 4457-4466.
Lage, P., Jan, Y. N. and Jarman, A. P. (1997). Requirement for EGF receptor signaling in neural recruitment during formation of Drosophila chordotonal sense organ clusters. Curr. Biol. 7, 166-175.[Medline]
Lee, K. J., Dietrich, P. and Jessell, T. M. (2000). Genetic ablation reveals that the roof plate is essential for dorsal interneuron specification. Nature 403, 734-740.[Medline]
Lee, K. J. and Jessell, T. M. (1999). The specification of dorsal cell fates in the vertebrate central nervous system. Annu. Rev. Neurosci. 22, 261-294.[Medline]
Lee, K. J., Mendelsohn, M. and Jessell, T. M. (1998). Neuronal patterning by BMPs: a requirement for GDF7 in the generation of a discrete class of commissural interneurons in the mouse spinal cord. Genes Dev. 12, 3394-3407.
Liem, K. F., Jr, Tremml, G., Roelink, H. and Jessell, T. M. (1995). Dorsal differentiation of neural plate cells induced by BMP-mediated signals from epidermal ectoderm. Cell 82, 969-979.[Medline]
Liem, K. F., Jr, Tremml, G. and Jessell, T. M. (1997). A role for the roof plate and its resident TGF-ß-related proteins in neuronal patterning in the dorsal spinal cord. Cell 91, 127-138.[Medline]
Lindsley, D. and Zimm, G. G. (1992). The Genome of Drosophila melanogaster. San Diego, USA: Academic Press.
Martin, A. R. and Wickelgren, W. P. (1971). Sensory cells in the spinal cord of the sea lamprey. J. Physiol. 212, 65-83.[Medline]
Marty, T., Muller, B., Basler, K. and Affolter, M. (2000). Schnurri mediates Dpp-dependent repression of brinker transcription. Nat. Cell Biol. 2, 745-749.[Medline]
Massagué, J. and Chen, Y. G. (2000). Controlling TGF-ß signaling. Genes Dev. 14, 627-644.
Millonig, J. H., Millen, K. J. and Hatten, M. E. (2000). The mouse Dreher gene Lmx1a controls formation of the roof plate in the vertebrate CNS. Nature 403, 764-769.[Medline]
Minami, M., Kinoshita, N., Kamoshida, Y., Tanimoto, H. and Tabata, T. (1999). brinker is a target of Dpp in Drosophila that negatively regulates Dpp- dependent genes. Nature 398, 242-246.[Medline]
Miya, T., Morita, K., Suzuki, A., Ueno, N. and Satoh, N. (1997). Functional analysis of an ascidian homologue of vertebrate BMP-2/BMP-4 suggests its role in the inhibition of neural fate specification. Development 124, 5149-5159.
Mullins, M. C., Hammerschmidt, M., Kane, D. A., Odenthal, J., Brand, M., van Eeden, F. J., Furutani-Seiki, M., Granato, M., Haffter, P., Heisenberg, C. P. et al. (1996). Genes establishing dorsoventral pattern formation in the zebrafish embryo: the ventral specifying genes. Development 123, 81-93.
Neul, J. L. and Ferguson, E. L. (1998). Spatially restricted activation of the SAX receptor by SCW modulates DPP/TKV signaling in Drosophila dorsal-ventral patterning. Cell 95, 483-494.[Medline]
Newfeld, S. J., Wisotzkey, R. G. and Kumar, S. (1999) Molecular evolution of a developmental pathway: phylogenetic analyses of transforming growth factor-beta family ligands, receptors and Smad signal transducers. Genetics 152, 783-795.
Nguyen, M., Park, S., Marques, G. and Arora, K. (1998). Interpretation of a BMP activity gradient in Drosophila embryos depends on synergistic signaling by two type I receptors, SAX and TKV. Cell 95, 495-506.[Medline]
Nguyen, V. H., Trout, J., Connors, S. A., Andermann, P., Weinberg, E. and Mullins, M. C. (2000). Dorsal and intermediate neuronal cell types of the spinal cord are established by a BMP signaling pathway. Development 127, 1209-1220.
Patel, N. H. (1994). Imaging neuronal subsets and other cell types in whole-mount Drosophila embryos and larvae using antibody probes. Methods Cell Biol. 44, 445-487.[Medline]
Podos, S. D. and Ferguson, E. L. (1999). Morphogen gradients: new insights from DPP. Trends Genet. 15, 396-402.[Medline]
Reichert, H. and Simeone, A. (1999). Conserved usage of gap and homeotic genes in patterning the CNS. Curr. Opin. Neurobiol. 9, 589-595.[Medline]
Ruiz-Gomez, M. and Ghysen, A. (1993). The expression and role of a proneural gene, achaete, in the development of the larval nervous system of Drosophila. EMBO J. 12, 1121-1130.[Abstract]
Rusten, T. E., Cantera, R., Urban, J., Technau, G., Kafatos, F. C. and Barrio, R. (2001). Spalt modifies EGFR-mediated induction of chordotonal precursors in the embryonic PNS of Drosophila promoting the development of oenocytes. Development 128, 711-722.
Schmid, A., Chiba, A. and Doe, C. Q. (1999). Clonal analysis of Drosophila embryonic neuroblasts: neural cell types, axon projections and muscle targets. Development 126, 4653-4689.
Schmidt, H., Rickert, C., Bossing, T., Vef, O., Urban, J. and Technau, G. M. (1997). The embryonic central nervous system lineages of Drosophila melanogaster. II. Neuroblast lineages derived from the dorsal part of the neuroectoderm. Dev. Biol. 189, 186-204.[Medline]
Schulte-Merker, S., Lee, K. J., McMahon, A. P. and Hammerschmidt, M. (1997). The zebrafish organizer requires Chordino. Nature 387, 862-863.[Medline]
Selleck, M. A. and Bronner-Fraser, M. (1995). Origins of the avian neural crest: the role of neural plate-epidermal interactions. Development 121, 525-538.
Skeath, J. B., Panganiban, G., Selague, M. and Carroll, S. B. (1992). Gene regulaton in two dimensions: the proneural achaetae and scute genes are controlled by combinations of axis-patterning genes through a common intergenic control region. Genes Dev. 6, 2606-2619.[Abstract]
Staehling-Hampton, K., Laughon, A. S. and Hoffmann, F. M. (1995). A Drosophila protein related to the human zinc finger transcription factor PRDII/MBPI/HIV-EP1 is required for Dpp signaling. Development 121, 3393-3403.
Streit, A. and Stern, C. D. (1999). Neural induction. A birds eye view. Trends Genet. 15, 20-24.[Medline]
Tanabe, Y. and Jessell, T. M. (1996). Diversity and pattern in the developing spinal cord. Science 274, 1115-1123.
Tanimoto, H., Itoh, S.., ten Dijke, P. M. and Tabata, T. (2000). Hedgehog creates a gradient of Dpp activity in Drosophila wing imaginal discs. Mol. Cell 5, 59-71.[Medline]
Torres-Vazquez, J., Park, S., Warrior, R. and Arora, K. (2001). The transcription factor Schnurri plays a dual role in mediating Dpp signaling during embryogenesis. Development 128, 1657-1670.
Tsuneizumi, K., Nakayama, T., Kamoshida, Y., Koernberg, T., Christian, J. and Tabata, T. (1997). Daughters against dpp modulates Dpp organizing activity in Drosophila wing development. Nature 389, 627-631.[Medline]
Udagawa, Y., Hanai, J., Tada, K., Grieder, N. C., Momoeda, M., Taketani, Y., Affolter, M., Kawabata, M. and Miyazono, K. (2000). Schnurri interacts with Mad in a Dpp-dependent manner. Genes Cells 5, 359-369.
Weiss, J. B., von Ohlen, T., Mellerick, D. M., Dressler, G., Doe, C. Q. and Scott, M. P. (1998). Dorsoventral patterning in the Drosophila central nervous system: the intermediate neuroblasts defective homeobox gene specifies intermediate column identity. Genes Dev. 12, 3591-3602.
Younossi-Hartenstein, A. and Hartenstein, V. (1997). Pattern, time of birth, and morphogenesis of sensillum progenitors in Drosophila. Microsc. Res. Tech. 39, 479-491.[Medline]
Yu, K., Srinivasan, S., Shimmi, O., Biehs, B., Rashka, K. E., Kimelman, D., OConnor, M. B. and Bier, E. (2000). Processing of the Drosophila Sog protein creates a novel BMP inhibitory activity. Development 127, 2143-2154.