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This server is maintained by the staff of Dr. Ethan Bier in the Division of Biological Sciences at the University of California, San Diego.
My laboratory is interested in how the Dpp and EGF-R signaling pathways interact to define the neurogenic region of the Drosophila blastoderm embryo and how these two pathways then collaborate to promote wing vein development during early metamorphosis. We use a combination of molecular and genetic approaches to investigate these developmental questions.
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This server is maintained by the staff of Dr. Ethan Bier in the Division of Biological Sciences at the University of California, San Diego.
My laboratory is interested in how the Dpp and EGF-R signaling pathways interact to define the neurogenic region of the Drosophila blastoderm embryo and how these two pathways then collaborate to promote wing vein development during early metamorphosis. We use a combination of molecular and genetic approaches to investigate these developmental questions.
Applications at SuperflyHomophila
Negative Proteome Database
P-screen Batch Search
Stock Deficiencies Search
Research Overview
OverviewResearch in my laboratory focuses on how two signaling pathways collaborate to establish neuroectodermal cell fates in the Drosophila blastoderm embryo. The neuroectoderm, which comprises the lateral region of the blastoderm embryo, gives rise to both neuronal and epidermal cell types. Two early acting genes expressed in the neuroectoderm are short gastrulation (sog) andrhomboid (rho). sog encodes a secreted factor antagonist (Sog) of the TGF-§ family member Dpp (François et al., 1994). Dpp signaling promotes epidermal fates and suppresses the default development pathway of neurogenesis. Sog provides a permissive condition for neurogenesis by acting as an anti-neural inhibitor. The antagonistic relationship between Sog and Dpp has been highly conserved during evolution. Chordin, the vertebrate homologue of Sog (François and Bier, 1995), is an endogenous neural inducer produced by the Spemann organizer which functions by antagonizing the neural suppressive activity of BMP-4 (the vertebrate homologue of Dpp). Furthermore, Sog mimics the activity of Chordin in Xenopus embryos (Schmidt et al., 1995) and vice versa, and Dpp and BMP-4 are functionally interchangeable in frogs and flies.
The rho gene (Bier et al., 1990) encodes an integral membrane protein (Rho) (Sturtevant et al., 1996) which potentiates EGF-R signaling (Sturtevant et al., 1993; Noll et al., 1994). rho is expressed in localized patterns corresponding to cells requiring high levels of EGF-R activity during embryonic (Bier et al., 1990) and adult development (Sturtevant et al., 1993). Since low levels of EGF-R signaling are essential for the viability of nearly all epidermal cells, localized hyperactivation of EGF-R signaling by Rho permits the ubiquitously active EGF-R pathway to be used for discrete developmental purposes. rho and sog mutants interact synergistically indicating that specification of the neuroectoderm depends on a combination of hyperactive EGF-R signaling and attenuated Dpp signaling. Ultimately, the neuroectoderm gives rise to neuronal precursor cells expressing transcription factors such as deadpan (dpn) (Bier et al., 1992) and scratch (scrt) (Roark et al., 1995; Emery and Bier, 1995).
In addition to their roles during early embryogenesis, the Dpp (Yu et al., 1996) and EGF-R (Sturtevant et al. 1993; Noll et al., 1994) pathways function during metamorphosis to make the binary vein versus intervein cell fate choice. The wing is ideal for studying interactions between these two signaling pathways as it is a substantially less complicated structure than the embryo. Additionally, analysis of the many existing vein mutants reveals a variety of cell-cell signaling events required for making and maintaining the vein versus intervein cell fate choice (Sturtevant and Bier, 1995). rho dependent EGF-R signaling promotes vein formation throughout larval and early pupal development. One function of rho is to induce expression of dpp in vein primordia.sogis expressed in intervein cells and functions to block Dpp activity in intervein cells (Yu et al., 1996). While the Dpp and EGF-R pathways collaborate to promote vein formation during wing development, it is noteworthy that these two pathways exert opposing functions during early neurogenesis. Future genetic analyses will exploit the various strengths of the embryo and wing to further characterize interactions between the Dpp and EGF-R signaling and to identify new genes participating in these two pathways.
An important goal of current and future studies in my laboratory is to determine the biochemical mechanisms underlying the genetic phenomena we have characterized. For example, a key question regarding the function of the Sog protein is whether Sog is proteolytically processed from the primary predicted protein precursor and whether such peptides would retain or lose the ability to inactivate components of the Dpp signaling pathway. In the case of rho, one important goal is to identify proteins physically interacting with Rho, and to determine how Rho and other proteins facilitate EGF-R signaling. Ultimately, I seek a synthesis of genetic and biochemical analyses to provide a detailed understanding of how crude positional information is converted into the differentiation of final specialized structures.
Areas of Research
Embryonic dorsal-ventral patterning
dpp-sog signaling pathway
rho-EgfR signaling pathway
Wing vein development in the fruit fly
Analysis of Human Diseases with Drosphila Genome - unavailable: error on page
OverviewResearch in my laboratory focuses on how two signaling pathways collaborate to establish neuroectodermal cell fates in the Drosophila blastoderm embryo. The neuroectoderm, which comprises the lateral region of the blastoderm embryo, gives rise to both neuronal and epidermal cell types. Two early acting genes expressed in the neuroectoderm are short gastrulation (sog) andrhomboid (rho). sog encodes a secreted factor antagonist (Sog) of the TGF-§ family member Dpp (François et al., 1994). Dpp signaling promotes epidermal fates and suppresses the default development pathway of neurogenesis. Sog provides a permissive condition for neurogenesis by acting as an anti-neural inhibitor. The antagonistic relationship between Sog and Dpp has been highly conserved during evolution. Chordin, the vertebrate homologue of Sog (François and Bier, 1995), is an endogenous neural inducer produced by the Spemann organizer which functions by antagonizing the neural suppressive activity of BMP-4 (the vertebrate homologue of Dpp). Furthermore, Sog mimics the activity of Chordin in Xenopus embryos (Schmidt et al., 1995) and vice versa, and Dpp and BMP-4 are functionally interchangeable in frogs and flies.
The rho gene (Bier et al., 1990) encodes an integral membrane protein (Rho) (Sturtevant et al., 1996) which potentiates EGF-R signaling (Sturtevant et al., 1993; Noll et al., 1994). rho is expressed in localized patterns corresponding to cells requiring high levels of EGF-R activity during embryonic (Bier et al., 1990) and adult development (Sturtevant et al., 1993). Since low levels of EGF-R signaling are essential for the viability of nearly all epidermal cells, localized hyperactivation of EGF-R signaling by Rho permits the ubiquitously active EGF-R pathway to be used for discrete developmental purposes. rho and sog mutants interact synergistically indicating that specification of the neuroectoderm depends on a combination of hyperactive EGF-R signaling and attenuated Dpp signaling. Ultimately, the neuroectoderm gives rise to neuronal precursor cells expressing transcription factors such as deadpan (dpn) (Bier et al., 1992) and scratch (scrt) (Roark et al., 1995; Emery and Bier, 1995).
In addition to their roles during early embryogenesis, the Dpp (Yu et al., 1996) and EGF-R (Sturtevant et al. 1993; Noll et al., 1994) pathways function during metamorphosis to make the binary vein versus intervein cell fate choice. The wing is ideal for studying interactions between these two signaling pathways as it is a substantially less complicated structure than the embryo. Additionally, analysis of the many existing vein mutants reveals a variety of cell-cell signaling events required for making and maintaining the vein versus intervein cell fate choice (Sturtevant and Bier, 1995). rho dependent EGF-R signaling promotes vein formation throughout larval and early pupal development. One function of rho is to induce expression of dpp in vein primordia.sogis expressed in intervein cells and functions to block Dpp activity in intervein cells (Yu et al., 1996). While the Dpp and EGF-R pathways collaborate to promote vein formation during wing development, it is noteworthy that these two pathways exert opposing functions during early neurogenesis. Future genetic analyses will exploit the various strengths of the embryo and wing to further characterize interactions between the Dpp and EGF-R signaling and to identify new genes participating in these two pathways.
An important goal of current and future studies in my laboratory is to determine the biochemical mechanisms underlying the genetic phenomena we have characterized. For example, a key question regarding the function of the Sog protein is whether Sog is proteolytically processed from the primary predicted protein precursor and whether such peptides would retain or lose the ability to inactivate components of the Dpp signaling pathway. In the case of rho, one important goal is to identify proteins physically interacting with Rho, and to determine how Rho and other proteins facilitate EGF-R signaling. Ultimately, I seek a synthesis of genetic and biochemical analyses to provide a detailed understanding of how crude positional information is converted into the differentiation of final specialized structures.
Areas of Research
Embryonic dorsal-ventral patterning
dpp-sog signaling pathway
rho-EgfR signaling pathway
Wing vein development in the fruit fly
Analysis of Human Diseases with Drosphila Genome - unavailable: error on page
Embryonic dorsal-ventral patterning
Positional information provided solely by the mother is sufficient to subdivide the Drosophilaembryo into three domains (ventral, lateral, and dorsal) corresponding to three basic tissue types (mesoderm, neuroectoderm, and non-neural ectoderm). Ovarian follicle cells collaborate with the oocyte to activate an extracellular complement-related protease cascade culminating in the production of a ventral signal called Spätzle. The Spätzle signal generates a nuclear concentration gradient of the NFk-B-related Dorsal transcription factor in the responding embryo. Nuclear Dorsal levels are high in ventral presumptive mesodermal cells, lower in lateral cells comprising the neuroectoderm, and absent in cells giving rise to non-neural ectoderm (Fig. 1).
Once in the nucleus, maternal Dorsal determines D/V domains of zygotic gene expression in a concentration dependent fashion (Fig. 1). Dorsal functions as an activator of genes expressed in ventral and lateral regions of the embryo, but as a repressor of genes expressed in dorsal cells. In ventral cells, the mesoderm determining genes twist and snail are activated by peak levels of Dorsal. Lower levels of Dorsal activate expression of rhomboid (rho) in lateral presumptive neuroectodermal cells. Similarly, lateral expression of short gastrulation (sog) is dependent on Dorsal, although it remains to determined whether this effect is due to direct activation. A cluster of related bHLH encoding genes comprising the achaete-scute Complex (AS-C) are also expressed in lateral cells. Dorsal, however, is not an essential activator of these genes. Cells of the dorsal non-neural ectoderm have undetectable levels of nuclear Dorsal, which permits the expression of several genes including decapentaplegic (dpp), zerknüllt (zen), and tolloid (tld). In ventral and lateral cells, Dorsal represses expression of these target genes.
While the mother is responsible for determining the position of lateral neuroectoderm versus dorsal non-neural ectoderm in Drosophila, zygotic genes expressed in each of these domains are required to maintain that initial subdivision. Two genes which play a pivotal role in the neural versus non-neural subdivision of the ectoderm are sog and dpp. dpp is expressed in dorsal non-neural cells and encodes a secreted TGF-ß family member (Dpp) most related to the vertebrate bone morphogenetic protein-4 (BMP-4), while sog is expressed in the neuroectoderm and encodes a predicted extracellular protein (Sog) similar to vertebrate Chordin (Chd) (François et al., 1994;François and Bier, 1995). Both Dpp and Sog are likely to diffuse from their sites of production into adjacent territories. As discussed on the Dpp-Sog page, Dpp and Sog are expressed in the same relative patterns as BMP-4 and Chd respectively, and these homologous pairs of molecules are functionally interchangeable between flies and frogs (François and Bier, 1995; Schmidt et al., 1995).
sog and dpp function antagonistically several times during Drosophila embryogenesis (François et al., 1994; Biehs et al., 1996) and pupal development (Yu et al., 1996). In the early blastoderm embryo, sog opposes dpp in two different contexts: 1) Sog prevents Dpp signaling from invading the neuroectoderm, and 2) Sog is required for subdividing the dorsal region into amnioserosa (the dorsal-most domain) and dorsal non-neural ectoderm, most likely by creating a Dpp activity gradient (Biehs et al., 1996).
In the early embryo, Dpp signaling functions both to maintain expression of dorsally-acting genes and to suppress expression of neuroectodermal genes (Biehs et al., 1996). Several dorsally-acting genes are transcriptionally activated by Dpp signaling including zen and dpp itself. The positive feedback loop through which Dpp signaling activates its own expression is referred to as autoactivation (Fig. 2). Among the genes repressed by Dpp signaling are those of the AS-Complex, which provide a necessary pre-condition for neural development. Because AS-C genes can be expressed dorsally in the absence of Dpp and nuclear Dorsal, Dorsal plays little, if any, role in restricting AS-C expression to lateral cells. Expression of AS-C in the absence of Dorsal is consistent with the formation of cuticle having partial neuroectodermal character in dpp dl double mutant embryos. Thus, Dpp signaling simultaneously promotes dorsal non-neural ectodermal cell fates while it suppresses neuroectodermal fates.
As the mother initially restricts dpp expression to dorsal cells through the repressive action of Dorsal, one could ask why it should be necessary to have a Dpp antagonist such as Sog in the neuroectoderm? The reason is schematically represented in Figs. 1. and 2. Dpp protein produced in dorsal cells diffuses down into the neuroectoderm where it can autoactivate to induce de novo dpp expression. This results in an invasive positive feedback loop (i.e. Dpp diffuses into the non-dpp expressing domain and activates dpp expression in those cells). Dpp diffusion and autoactivation are useful properties for assuring that all cells within the dorsal domain assume a non-neural fate. However, active opposition of Dpp signaling within the neuroectoderm is necessary to prevent dpp expression from spreading throughout the entire ectoderm.
Positional information provided solely by the mother is sufficient to subdivide the Drosophilaembryo into three domains (ventral, lateral, and dorsal) corresponding to three basic tissue types (mesoderm, neuroectoderm, and non-neural ectoderm). Ovarian follicle cells collaborate with the oocyte to activate an extracellular complement-related protease cascade culminating in the production of a ventral signal called Spätzle. The Spätzle signal generates a nuclear concentration gradient of the NFk-B-related Dorsal transcription factor in the responding embryo. Nuclear Dorsal levels are high in ventral presumptive mesodermal cells, lower in lateral cells comprising the neuroectoderm, and absent in cells giving rise to non-neural ectoderm (Fig. 1).
Once in the nucleus, maternal Dorsal determines D/V domains of zygotic gene expression in a concentration dependent fashion (Fig. 1). Dorsal functions as an activator of genes expressed in ventral and lateral regions of the embryo, but as a repressor of genes expressed in dorsal cells. In ventral cells, the mesoderm determining genes twist and snail are activated by peak levels of Dorsal. Lower levels of Dorsal activate expression of rhomboid (rho) in lateral presumptive neuroectodermal cells. Similarly, lateral expression of short gastrulation (sog) is dependent on Dorsal, although it remains to determined whether this effect is due to direct activation. A cluster of related bHLH encoding genes comprising the achaete-scute Complex (AS-C) are also expressed in lateral cells. Dorsal, however, is not an essential activator of these genes. Cells of the dorsal non-neural ectoderm have undetectable levels of nuclear Dorsal, which permits the expression of several genes including decapentaplegic (dpp), zerknüllt (zen), and tolloid (tld). In ventral and lateral cells, Dorsal represses expression of these target genes.
While the mother is responsible for determining the position of lateral neuroectoderm versus dorsal non-neural ectoderm in Drosophila, zygotic genes expressed in each of these domains are required to maintain that initial subdivision. Two genes which play a pivotal role in the neural versus non-neural subdivision of the ectoderm are sog and dpp. dpp is expressed in dorsal non-neural cells and encodes a secreted TGF-ß family member (Dpp) most related to the vertebrate bone morphogenetic protein-4 (BMP-4), while sog is expressed in the neuroectoderm and encodes a predicted extracellular protein (Sog) similar to vertebrate Chordin (Chd) (François et al., 1994;François and Bier, 1995). Both Dpp and Sog are likely to diffuse from their sites of production into adjacent territories. As discussed on the Dpp-Sog page, Dpp and Sog are expressed in the same relative patterns as BMP-4 and Chd respectively, and these homologous pairs of molecules are functionally interchangeable between flies and frogs (François and Bier, 1995; Schmidt et al., 1995).
sog and dpp function antagonistically several times during Drosophila embryogenesis (François et al., 1994; Biehs et al., 1996) and pupal development (Yu et al., 1996). In the early blastoderm embryo, sog opposes dpp in two different contexts: 1) Sog prevents Dpp signaling from invading the neuroectoderm, and 2) Sog is required for subdividing the dorsal region into amnioserosa (the dorsal-most domain) and dorsal non-neural ectoderm, most likely by creating a Dpp activity gradient (Biehs et al., 1996).
In the early embryo, Dpp signaling functions both to maintain expression of dorsally-acting genes and to suppress expression of neuroectodermal genes (Biehs et al., 1996). Several dorsally-acting genes are transcriptionally activated by Dpp signaling including zen and dpp itself. The positive feedback loop through which Dpp signaling activates its own expression is referred to as autoactivation (Fig. 2). Among the genes repressed by Dpp signaling are those of the AS-Complex, which provide a necessary pre-condition for neural development. Because AS-C genes can be expressed dorsally in the absence of Dpp and nuclear Dorsal, Dorsal plays little, if any, role in restricting AS-C expression to lateral cells. Expression of AS-C in the absence of Dorsal is consistent with the formation of cuticle having partial neuroectodermal character in dpp dl double mutant embryos. Thus, Dpp signaling simultaneously promotes dorsal non-neural ectodermal cell fates while it suppresses neuroectodermal fates.
As the mother initially restricts dpp expression to dorsal cells through the repressive action of Dorsal, one could ask why it should be necessary to have a Dpp antagonist such as Sog in the neuroectoderm? The reason is schematically represented in Figs. 1. and 2. Dpp protein produced in dorsal cells diffuses down into the neuroectoderm where it can autoactivate to induce de novo dpp expression. This results in an invasive positive feedback loop (i.e. Dpp diffuses into the non-dpp expressing domain and activates dpp expression in those cells). Dpp diffusion and autoactivation are useful properties for assuring that all cells within the dorsal domain assume a non-neural fate. However, active opposition of Dpp signaling within the neuroectoderm is necessary to prevent dpp expression from spreading throughout the entire ectoderm.
dpp-sog signaling pathway
Sog antagonizes Dpp Signaling
The Drosophila decapentaplegic (dpp) gene encodes a secreted protein homologous to vertebrate Bone Morphogenetic Protein 4 (BMP-4) in the TGF-ß superfamily. dpp expression is restricted to the dorsal region of the blastoderm embryo and is required for development of all dorsal cell types (Fig. 3). Several years ago we isolated another gene required for patterning the dorsal region of the blastoderm embryo called short gastrulation (sog) (François et al., 1994; Fig. 3). Although sog exerts an effect on dorsal cells in a dosage dependent fashion, it is expressed in lateral neuroectodermal cells. sog encodes a predicted secreted protein (Sog) consistent with Sog functioning to influence dorsal cell fates at a distance. Sog appears to be a dedicated Dpp antagonist (François et al., 1994). For example, sog- and dpp- mutants have opposite phenotypes with respect to expression of extreme dorsal markers, and the phenotype of sog-;dpp- double mutant embryos is the same as that of dpp- single mutants (Biehs et al, 1996). Furthermore, sogmutants suppress the haplo-insufficient lethality of dpp null mutants. While dpp-/+ individuals rarely survive to adulthood, sog-/+; dpp-/+ trans-heterozygous animals have nearly wild type viability (François et al., 1994).
Another important function of sog is to antagonize Dpp signaling within the neuroectoderm itself (Biehs et al, 1996). sog- mutants lack ventral cuticle and a subset of neuroblasts derived from the neuroectodermal region of the embryo. These sog- mutant phenotypes are enhanced by increasing the number of dpp copies. In the absence of sog, dorsally produced Dpp diffuses down into the lateral neuroectodermal region of the embryo where it activates its own expression through a positive autoregulatory loop (autoactivation) and suppress neurogenesis (Biehs et al, 1996). Thus, sog normally provides a permissive condition for the default pathway of neurogenesis by preventing Dpp autoactivation.
As mentioned on the Introductory Page, Sog and Dpp have been highly conserved during evolution (François and Bier, 1995). The Xenopus homologue of sog called chordin is expressed in the neurogenic Spemann organizer and has a potent dorsalizing activity which can mimic transplantation of the Spemann organizer by inducing a secondary axis (Schmidt et al., 1995).sog and chordin are expressed in corresponding regions of fly and frog embryos which abut domains of dpp and BMP-4 expression respectively. Most critically, Sog and Chordin or Dpp and BMP-4 can substitute for each other functionally in flies and frogs (Schmidt et al., 1995). A unifying theme in flies, frogs, and fish is that the default cell fate of ectoderm is neural and that this intrinsic tendency is actively suppressed by Dpp/BMP-4 signaling in non-neural regions of the embryo. Sog and Chordin function as anti-neural inhibitors to protect the neuroectoderm from the invasive positive feedback loop created by Dpp/BMP-4 diffusion and autoactivation (Bier, 1997).
sog and dpp also play antagonistic roles during wing vein development (Yu et al., 1996). Reduction of dpp function compromises vein formation whereas ectopic dpp supplied during pupal stages generates excess veins. Conversely, reducing sog activity promotes ectopic vein formation while sog mis-expression leads to vein-loss phenotypes. Sog appears to function by preventing Dpp from autoactivating in vein cells. Our current model is that intervein expression of Sog defines straight channels within which Dpp can diffuse and autoactivate to promote vein continuity (see wing vein section below). Interestingly, sog is only capable of blocking one of the two known actions of Dpp during vein development (i.e. vein promotion but not lateral inhibition).
The Sog product, which is a predicted secreted factor (François et al., 1994), is likely to diffuse from its lateral site of production into the dorsal region and influence cell fates (Biehs et al, 1996). Sog has a large extracellular domain containing four repeats of a 10 cysteine motif and several dibasic amino acids that could serve as potential processing sites for serine proteases. As it has been shown that vertebrate Chordin binds to BMP-4 with high affinity, Sog is likely to bind to Dpp and sequester it in an inactive form. Our data suggest that the story is not quite this simple, however, since mis-expression of sog falls short of producing phenotypes as strong as those indpp- loss-of-function mutants (Biehs et al, 1996). In addition, there are situations in which sogcannot block Dpp activity (Yu et al., 1996).
We are currently investigating the basis for the specificity of Sog function. One possible explanation for this apparent specificity of Sog function is that Sog can block signaling through the Saxophone type-I Dpp receptor but cannot interfere with signaling through the Thick Veins type-I receptor. We also are testing whether putative secreted Sog peptides might diffuse dorsally to bind to Dpp and/or other components of the Dpp signaling pathway. For this analysis we are raising antisera to various domains of Sog as well as constructing tagged versions of Sog and components of the Dpp signaling pathway to test for direct protein-protein interactions. We also are mutagenizing each of the four Sog CR repeats to determine which in any of these domains is required for Sog activity.
Sog antagonizes Dpp Signaling
The Drosophila decapentaplegic (dpp) gene encodes a secreted protein homologous to vertebrate Bone Morphogenetic Protein 4 (BMP-4) in the TGF-ß superfamily. dpp expression is restricted to the dorsal region of the blastoderm embryo and is required for development of all dorsal cell types (Fig. 3). Several years ago we isolated another gene required for patterning the dorsal region of the blastoderm embryo called short gastrulation (sog) (François et al., 1994; Fig. 3). Although sog exerts an effect on dorsal cells in a dosage dependent fashion, it is expressed in lateral neuroectodermal cells. sog encodes a predicted secreted protein (Sog) consistent with Sog functioning to influence dorsal cell fates at a distance. Sog appears to be a dedicated Dpp antagonist (François et al., 1994). For example, sog- and dpp- mutants have opposite phenotypes with respect to expression of extreme dorsal markers, and the phenotype of sog-;dpp- double mutant embryos is the same as that of dpp- single mutants (Biehs et al, 1996). Furthermore, sogmutants suppress the haplo-insufficient lethality of dpp null mutants. While dpp-/+ individuals rarely survive to adulthood, sog-/+; dpp-/+ trans-heterozygous animals have nearly wild type viability (François et al., 1994).
Another important function of sog is to antagonize Dpp signaling within the neuroectoderm itself (Biehs et al, 1996). sog- mutants lack ventral cuticle and a subset of neuroblasts derived from the neuroectodermal region of the embryo. These sog- mutant phenotypes are enhanced by increasing the number of dpp copies. In the absence of sog, dorsally produced Dpp diffuses down into the lateral neuroectodermal region of the embryo where it activates its own expression through a positive autoregulatory loop (autoactivation) and suppress neurogenesis (Biehs et al, 1996). Thus, sog normally provides a permissive condition for the default pathway of neurogenesis by preventing Dpp autoactivation.
As mentioned on the Introductory Page, Sog and Dpp have been highly conserved during evolution (François and Bier, 1995). The Xenopus homologue of sog called chordin is expressed in the neurogenic Spemann organizer and has a potent dorsalizing activity which can mimic transplantation of the Spemann organizer by inducing a secondary axis (Schmidt et al., 1995).sog and chordin are expressed in corresponding regions of fly and frog embryos which abut domains of dpp and BMP-4 expression respectively. Most critically, Sog and Chordin or Dpp and BMP-4 can substitute for each other functionally in flies and frogs (Schmidt et al., 1995). A unifying theme in flies, frogs, and fish is that the default cell fate of ectoderm is neural and that this intrinsic tendency is actively suppressed by Dpp/BMP-4 signaling in non-neural regions of the embryo. Sog and Chordin function as anti-neural inhibitors to protect the neuroectoderm from the invasive positive feedback loop created by Dpp/BMP-4 diffusion and autoactivation (Bier, 1997).
sog and dpp also play antagonistic roles during wing vein development (Yu et al., 1996). Reduction of dpp function compromises vein formation whereas ectopic dpp supplied during pupal stages generates excess veins. Conversely, reducing sog activity promotes ectopic vein formation while sog mis-expression leads to vein-loss phenotypes. Sog appears to function by preventing Dpp from autoactivating in vein cells. Our current model is that intervein expression of Sog defines straight channels within which Dpp can diffuse and autoactivate to promote vein continuity (see wing vein section below). Interestingly, sog is only capable of blocking one of the two known actions of Dpp during vein development (i.e. vein promotion but not lateral inhibition).
The Sog product, which is a predicted secreted factor (François et al., 1994), is likely to diffuse from its lateral site of production into the dorsal region and influence cell fates (Biehs et al, 1996). Sog has a large extracellular domain containing four repeats of a 10 cysteine motif and several dibasic amino acids that could serve as potential processing sites for serine proteases. As it has been shown that vertebrate Chordin binds to BMP-4 with high affinity, Sog is likely to bind to Dpp and sequester it in an inactive form. Our data suggest that the story is not quite this simple, however, since mis-expression of sog falls short of producing phenotypes as strong as those indpp- loss-of-function mutants (Biehs et al, 1996). In addition, there are situations in which sogcannot block Dpp activity (Yu et al., 1996).
We are currently investigating the basis for the specificity of Sog function. One possible explanation for this apparent specificity of Sog function is that Sog can block signaling through the Saxophone type-I Dpp receptor but cannot interfere with signaling through the Thick Veins type-I receptor. We also are testing whether putative secreted Sog peptides might diffuse dorsally to bind to Dpp and/or other components of the Dpp signaling pathway. For this analysis we are raising antisera to various domains of Sog as well as constructing tagged versions of Sog and components of the Dpp signaling pathway to test for direct protein-protein interactions. We also are mutagenizing each of the four Sog CR repeats to determine which in any of these domains is required for Sog activity.
rho-EgfR signaling pathway
Rhomboid hyperactivates EGF-R signaling
The rho gene (Bier et al., 1990) encodes a novel integral membrane protein (Rho) (Sturtevant et al., 1996) that potentiates EGF-Receptor (EGF-R) signaling (Fig. 4) in localized patterns during embryogenesis and adult development (Sturtevant et al., 1993; Noll et al., 1994). For example, rho is expressed in broad lateral stripes in the neuroectoderm of blastoderm embryos and, in rho- mutants, epithelial cells derived from the neuroectoderm fail to differentiate(Bier et al., 1990). Similarly, rho is expressed in wing vein primordia during wing development and is required for vein formation (Sturtevant et al., 1993). In contrast, genes encoding other components of the EGF-R pathway are ubiquitously expressed, consistent with a low basal requirement for EGF-R signaling in all epithelial cells. When rho is mis-expressed, EGF-R signaling is hyperactivated correspondingly and cell fates are transformed accordingly (Sturtevant et al., 1993; Noll et al., 1994).
The Rho protein is highly concentrated in plaque-like structures at the apical cell surface (Sturtevant et al., 1996). Electron microscopy reveals that Rho protein is found in all compartments of the secretory pathway as well as in large structures at the cell surface. These cell surface structures, which most likely correspond to the large plaque-like structures visible by light microscopy, are comprised of patches of plasma membrane and masses of material with a vesicular appearance directly underlying the membrane patches. Rho plaques may define specialized sites at which EGF-R signaling is potentiated (Fig. 4). One mechanism for potentiation of EGF-R signaling could be cleavage of the membrane bound precursor form of the EGF-R ligand to generate a more active secreted form of ligand.
Major experimental goals for rho are to identify proteins interacting physically with Rho, to investigate the significance of the unusual subcellular distribution of the Rho protein in superficial plaque-like structures, and to define functional domains of the Rho protein through a combination of site-directed and in vivo mutagenesis.
Rhomboid hyperactivates EGF-R signaling
The rho gene (Bier et al., 1990) encodes a novel integral membrane protein (Rho) (Sturtevant et al., 1996) that potentiates EGF-Receptor (EGF-R) signaling (Fig. 4) in localized patterns during embryogenesis and adult development (Sturtevant et al., 1993; Noll et al., 1994). For example, rho is expressed in broad lateral stripes in the neuroectoderm of blastoderm embryos and, in rho- mutants, epithelial cells derived from the neuroectoderm fail to differentiate(Bier et al., 1990). Similarly, rho is expressed in wing vein primordia during wing development and is required for vein formation (Sturtevant et al., 1993). In contrast, genes encoding other components of the EGF-R pathway are ubiquitously expressed, consistent with a low basal requirement for EGF-R signaling in all epithelial cells. When rho is mis-expressed, EGF-R signaling is hyperactivated correspondingly and cell fates are transformed accordingly (Sturtevant et al., 1993; Noll et al., 1994).
The Rho protein is highly concentrated in plaque-like structures at the apical cell surface (Sturtevant et al., 1996). Electron microscopy reveals that Rho protein is found in all compartments of the secretory pathway as well as in large structures at the cell surface. These cell surface structures, which most likely correspond to the large plaque-like structures visible by light microscopy, are comprised of patches of plasma membrane and masses of material with a vesicular appearance directly underlying the membrane patches. Rho plaques may define specialized sites at which EGF-R signaling is potentiated (Fig. 4). One mechanism for potentiation of EGF-R signaling could be cleavage of the membrane bound precursor form of the EGF-R ligand to generate a more active secreted form of ligand.
Major experimental goals for rho are to identify proteins interacting physically with Rho, to investigate the significance of the unusual subcellular distribution of the Rho protein in superficial plaque-like structures, and to define functional domains of the Rho protein through a combination of site-directed and in vivo mutagenesis.
Wing vein development in the fruit fly
We have initiated an extensive analysis of wing vein development as a model system for studying a binary cell fate choice (Sturtevant and Bier, 1995). Based on these studies, we proposed a sequential model of vein formation. The first step, which takes place during embryogenesis and early larval development, is subdivision of the anterior-posterior (A/P) axis of the wing segment into a series of alternating sectors. The boundaries between these discrete sectors define discontinuities which induce the formation of vein primordia. Vein development is initiated independently on both the dorsal and ventral surfaces of the wing disc through the action of opposing vein promoting genes (e.g. mutants lack one or more veins) and vein suppression genes (e.g. mutants have ectopic veins).
In the case of the second longitudinal vein (L2), we have established a link between signals emanating from the A/P compartment boundary and induction of the vein developmental program (Sturtevant et al., 1997; Fig. 5). Thus, the Dpp protein, which is produced in a narrow stripe of cells along the A/P compartment boundary, diffuses in both anterior and posterior directions to activate expression of the transcription factor spalt (sal) in a broad central domain. The anterior boundary of the sal expression domain then induces expression of rhomboid (rho) in the L2 primordium (Sturtevant et al., 1997). This localized expression of rho promotes EGF-R signaling (see Rho-EGF-R page) and causes cells to differentiate as vein cells rather than intervein cells (Sturtevant et al., 1993; Noll et al., 1994).
Once vein development is initiated, at least three different types of cell-cell communication contribute to the differentiation of continuous and straight veins: 1) lateral inhibitory signal(s) elaborated by presumptive vein cells restrict vein formation to the center of broad vein competent domains, 2) dorsal-to-ventral signal(s) maintain vein fates in cells on the ventral surface of the wing, and 3) vein continuity signal(s) promote vein formation in straight lines along the axis of vein extension. These various signals presumably collaborate to insure that the dorsal and ventral components of veins are strictly aligned and uninterrupted. It is likely that Dpp, which is expressed in veins, functions as a vein continuity signal and that Sog expression in intervein cells constrains Dpp autoactivation to narrow straight channels (Yu et al., 1996).
We are currently conducting genetic screens to identify new wing vein mutants. Goals of future studies of vein formation are to identify genes functioning upstream of the Dpp and EGF-R signaling pathways through suppressor/enhancer mutant screens, to assess the contribution of different Dpp-Receptor subunits to promoting vein continuity versus lateral inhibition, to isolate new genes involved in initiating vein formation at sector boundaries, and to identify components of the dorsal-to-ventral signaling pathway.
We have initiated an extensive analysis of wing vein development as a model system for studying a binary cell fate choice (Sturtevant and Bier, 1995). Based on these studies, we proposed a sequential model of vein formation. The first step, which takes place during embryogenesis and early larval development, is subdivision of the anterior-posterior (A/P) axis of the wing segment into a series of alternating sectors. The boundaries between these discrete sectors define discontinuities which induce the formation of vein primordia. Vein development is initiated independently on both the dorsal and ventral surfaces of the wing disc through the action of opposing vein promoting genes (e.g. mutants lack one or more veins) and vein suppression genes (e.g. mutants have ectopic veins).
In the case of the second longitudinal vein (L2), we have established a link between signals emanating from the A/P compartment boundary and induction of the vein developmental program (Sturtevant et al., 1997; Fig. 5). Thus, the Dpp protein, which is produced in a narrow stripe of cells along the A/P compartment boundary, diffuses in both anterior and posterior directions to activate expression of the transcription factor spalt (sal) in a broad central domain. The anterior boundary of the sal expression domain then induces expression of rhomboid (rho) in the L2 primordium (Sturtevant et al., 1997). This localized expression of rho promotes EGF-R signaling (see Rho-EGF-R page) and causes cells to differentiate as vein cells rather than intervein cells (Sturtevant et al., 1993; Noll et al., 1994).
Once vein development is initiated, at least three different types of cell-cell communication contribute to the differentiation of continuous and straight veins: 1) lateral inhibitory signal(s) elaborated by presumptive vein cells restrict vein formation to the center of broad vein competent domains, 2) dorsal-to-ventral signal(s) maintain vein fates in cells on the ventral surface of the wing, and 3) vein continuity signal(s) promote vein formation in straight lines along the axis of vein extension. These various signals presumably collaborate to insure that the dorsal and ventral components of veins are strictly aligned and uninterrupted. It is likely that Dpp, which is expressed in veins, functions as a vein continuity signal and that Sog expression in intervein cells constrains Dpp autoactivation to narrow straight channels (Yu et al., 1996).
We are currently conducting genetic screens to identify new wing vein mutants. Goals of future studies of vein formation are to identify genes functioning upstream of the Dpp and EGF-R signaling pathways through suppressor/enhancer mutant screens, to assess the contribution of different Dpp-Receptor subunits to promoting vein continuity versus lateral inhibition, to isolate new genes involved in initiating vein formation at sector boundaries, and to identify components of the dorsal-to-ventral signaling pathway.
Bier Lab Protocols - under Research
APPLICATIONS
Applications at Superfly
Homophila - proxy error
Negative Proteome Database proxy error
P-screen Batch Search - http://superfly.ucsd.edu/Pscreen/
Stock Deficiencies Search
The Booleome DatabaseDrosophila P-elements Search
Applications at Superfly
Homophila - proxy error
Negative Proteome Database proxy error
P-screen Batch Search - http://superfly.ucsd.edu/Pscreen/
Stock Deficiencies Search
The Booleome DatabaseDrosophila P-elements Search
PEOPLE
Principal InvestigatorDr. Ethan Bier - [email protected]
ResearcherAnnabel Guichard - [email protected]
Beatriz Cruz Moreno - [email protected]
Post-Doctoral FellowsDr. Orna Cook - [email protected]
Dr. Tamar Grossman - [email protected]
Dr. Kyung Hwa Kang - [email protected]
Dr. Claudia Mieko Mizutani - [email protected]
Dr. Margery Smelkinson - [email protected]
Graduate StudentsLong Do - [email protected]
Francisco Esteves - [email protected]
Lab ManagerHeather Elledge - [email protected]
Undergraduate Students
Principal InvestigatorDr. Ethan Bier - [email protected]
ResearcherAnnabel Guichard - [email protected]
Beatriz Cruz Moreno - [email protected]
Post-Doctoral FellowsDr. Orna Cook - [email protected]
Dr. Tamar Grossman - [email protected]
Dr. Kyung Hwa Kang - [email protected]
Dr. Claudia Mieko Mizutani - [email protected]
Dr. Margery Smelkinson - [email protected]
Graduate StudentsLong Do - [email protected]
Francisco Esteves - [email protected]
Lab ManagerHeather Elledge - [email protected]
Undergraduate Students
