The Drosophila patched gene encodes a putative membrane protein required for segmental patterning

Apical localization of pair-rule transcripts requires 3' sequences and limits protein diffusion in the Drosophila blastoderm embryo

The peripheral cytoplasm (periplasm) of the Drosophila blastoderm embryo is subdivided into apical and basal compartments by a layer of nuclei. We have demonstrated three classes of periplasmic transcript localization: apical, basal, and unlocalized (apical and basal), each of which depends on 3' sequences. We define 3' apical localization signals within the even-skipped, fushi tarazu, and hairy pair-rule segmentation genes and the alpha 1-tubulin and bicoid genes. 3' human alpha-globin sequences direct transcripts basally. Transcript destination depends on transcript structure, not on transcript stability or chromosomal location. Apical transcripts direct apical compartmentalization of cytoplasmic protein. We propose that apical localization of pair-rule transcripts restricts lateral protein diffusion, thereby allowing pair-rule proteins to define sharp boundaries and precise spatial domains.

Unrestricted expression of the Drosophila gene patched allows a normal segment polarity

In the Drosophila embryo, mutations in the segment polarity gene patched (ptc) cause the replacement of the middle region of each segment by a mirror-image duplication of the remaining structures, including the parasegmental border. This gene, which encodes a transmembrane protein, is initially expressed in a generalized way at blastoderm, but later stops being transcribed in cells expressing the engrailed gene, and even later in cells in the middle of the parasegment. The genes engrailed (en) and wingless (wg) are also segment-polarity genes, and they are expressed in adjacent stripes flanking the parasegment borders in the embryo; in ptc mutants wg expression extends anteriorly and an ectopic stripe of en expression is induced. The suggestion has been made that ptc must be transcribed in a specific subset of cells to prevent en expression anterior to the wg-expressing stripe. Here we report that unrestricted expression of ptc from a heat-shock promoter has no adverse effect on development of Drosophila embryos. The heat-shock construct can also rescue ptc mutants, restoring wg expression to its normal narrow stripe. The ectopic en stripe fails to appear, but the normal one remains unaffected. The results imply that, despite its localized requirement, the restricted expression of ptc does not itself allocate positional information.

Role of the Drosophila patched gene in positional signalling

After cellularization of the Drosophila embryo, positional differences within each primordial segment are maintained and elaborated by processes that require cell interactions. The best-documented examples of such intercellular signalling are the mutual interactions between neighbouring cells expressing the homeodomain protein engrailed and the secreted glycoprotein encoded by wingless, the Drosophila homologue of the murine Wnt-1 gene. Little is known about the molecular basis of these signalling mechanisms but the activities of several other genes, notably patched and hedgehog, have been implicated in the process. Here we show that the role of patched in positional signalling is permissive rather than instructive, its activity being required to suppress wingless transcription in cells predisposed to express the latter. According to this view, expression of wingless is normally maintained only in those cells receiving an extrinsic signal, encoded by hedgehog, that antagonizes the repressive activity of patched. We suggest that the patched protein may itself be the receptor for this signal, implying that this is an unusual mechanism of ligand-dependent receptor inactivation.

Interactions between segment polarity genes and the generation of the segmental pattern in Drosophila

Although mutations in the segment polarity genes wingless, engrailed, hedgehog, gooseberry and cubitus-interruptusD all affect the region of naked cuticle within each segment of the Drosophila larva, subtle phenotypic differences suggest that these genes play different roles in segmental patterning. In this paper, the regulative interactions between these genes are analysed. They have revealed that the products of most of these genes accomplish more than one function during embryogenesis. Whereas early on a positive feed-back loop involving wg, en and hh maintains the expression of wg and en in the extremes of each parasegment, later on wg and en become independent from each other. en appears to regulate the expression of hh and ptc, while wg depends on gsb and ciD.

Segment polarity genes and cell patterning within the Drosophila body segment

The formation of complex structures in any multicellular organism requires cooperative behaviour within cell populations. Genetic and molecular analysis of pattern formation in the Drosophila embryo is providing us with new insights into the cellular basis of this process, implicating a diversity of molecules, some familiar, others novel, in intercellular communication.

The prospero gene specifies cell fates in the Drosophila central nervous system

The molecular mechanisms used to generate neuronal diversity are largely unknown. To identify genes controlling cell fate in the Drosophila central nervous system, we screened for mutations that alter expression of homeobox genes in the developing central nervous system (indicating changes in cell fates). We also screened "enhancer trap" lines to identify genes expressed in neuronal stem cells (neuroblasts). The prospero gene was discovered in both screens. prospero is expressed in a subset of neuroblasts, sensory neuron precursors, and identified glial precursors. It is not expressed in neurons. Neuroblasts lacking prospero function generate abnormal cell lineages, producing incorrectly specified progeny that differentiate into neurons showing axon pathfinding defects. prospero is therefore a novel type of gene expressed in neuroblasts and known to specify neuronal fate.

Transcriptional regulation of a pair-rule stripe in Drosophila

The periodic, seven-stripe pattern of the primary pair-rule gene even-skipped (eve) is initiated by crude, overlapping gradients of maternal and gap gene proteins in the early Drosophila embryo. Previous genetic studies suggest that one of the stripes, stripe 2, is initiated by the maternal morphogen bicoid (bcd) and the gap protein hunchback (hb), while the borders of the stripe are formed by selective repression, involving the gap protein giant (gt) in anterior regions and the Krüppel (Kr) protein in posterior regions. Here, we present several lines of evidence that are consistent with this model for stripe 2 expression, including in vitro DNA-binding experiments and transient cotransfection assays in cultured cells. These experiments suggest that repression involves a competition or short-range quenching mechanism, whereby the binding of gt and Kr interferes with the binding or activity of bcd and hb activators at overlapping or neighboring sites within the eve stripe 2 promoter element. Such short-range repression could reflect a general property of promoters composed of multiple, but autonomous regulatory elements.

The segment polarity gene armadillo encodes a functionally modular protein that is the Drosophila homolog of human plakoglobin

The Drosophila segment polarity gene armadillo is required for pattern formation within embryonic segments and imaginal discs. We have found that armadillo is highly conserved during evolution; it is 63% identical to human plakoglobin, a protein found in adhesive junctions joining epithelial and other cells. We have examined arm protein localization in a number of larval tissues and found that arm protein accumulation within cells shares many features with the accumulation of plakoglobin. We have compared the phenotype and molecular lesions responsible for the different arm mutations. Surprisingly, severely truncated proteins retain some function; the degree of function is strictly correlated with the length of the truncated protein, suggesting that the internally repetitive arm protein is modular in function. We present a possible model for the cellular role of arm.

Serine/threonine protein kinases in Drosophila

The study of serine/threonine kinases in Drosophila is coming of age. Recently several kinases have been identified and their role in cell determination has been established. This review discusses these recent findings and describes the potential for genetic analyses of kinase activity and signal transduction.

A putative serine/threonine protein kinase encoded by the segment-polarity fused gene of Drosophila

The segmented pattern of the Drosophila embryo depends on a regulatory cascade involving three main classes of genes. An early regulatory programme, set up before cellularization, involves direct transcriptional regulation mediated by gap and pair-rule genes. In a second phase occurring after cellularization, interactions between segment-polarity genes are involved in cell communication. Segment-polarity genes are required for pattern formation in different domains of each metamere and act to define and maintain positional information in each segment. The segment-polarity gene fused is maternally required for correct patterning in the posterior part of each embryonic metamere. It is also necessary later in development, as fused mutations lead to anomalies of adult cuticular structures and tumorous ovaries. Here we provide molecular evidence that this gene encodes a putative serine/threonine protein kinase, a new function for the product of a segmentation gene. This result provides further insight into segment-polarity interactions and their role in pattern formation.

An early embryonic product of the gene shaggy encodes a serine/threonine protein kinase related to the CDC28/cdc2+ subfamily

The product(s) of the gene shaggy (sgg) is required for seemingly unrelated events during the development of Drosophila melanogaster. In embryos, maternal and zygotically derived sgg products are required initially to construct a normal syncytial blastoderm and later for normal segmentation. Furthermore, in mutant animals a process of intercellular communication that is required for the segregation of the neural and epidermal lineage during the formation of the central nervous system and the adult peripheral nervous system is disrupted. Here we describe a transcription unit of approximately 40 kb lying within the cloned chromosomal interval 3B1, and provide evidence that it encodes the sgg+ function. Of seven developmentally regulated transcripts that are partially generated by alternative splicing, two seem to be responsible for early sgg activity. Sequence analysis of corresponding cDNA(s) predicts a protein of 514 amino acids with a canonical catalytic domain found in serine/threonine specific protein kinases, linked to an unusual region rich in Gly, Ala and Ser. A search for homologies as well as a comparative study of the kinase catalytic domain with that of other proteins, revealed that the protein kinase domain of sgg is distantly related to the members of the CDC28/cdc2+ subfamily of protein kinases, all of which play cardinal roles in the regulation of the yeast and mammalian cell cycles. Ubiquitous expression of sgg transcripts was found during embryonic stages. A possible role of the sgg protein in a signal transduction pathway necessary for intercellular communication at different stages of development is discussed.

Regulation of the pituitary-specific homeobox gene GHF1 by cell-autonomous and environmental cues

Homeodomain proteins function in determination of mating type in yeast, segmentation in fruit flies and cell-type specific gene expression in mammals. In Drosophila, expression of homeobox genes is controlled by cell-autonomous interactions between regulatory proteins and environmental clues. Similar controls may operate during mammalian limb development and frog embryogenesis. But, the exact way in which expression of homeodomain proteins is regulated in these systems is not clear and requires biochemical analysis of homeobox gene transcription. We now describe such an analysis of the GHF1 gene, which encodes a mammalian homeodomain protein specifying expression of the growth hormone (GH) gene in anterior pituitary somatotrophs. GHF1 is transcribed in a highly restricted manner and the presence of GHF1 protein is correlated both temporally and spatially with activation of the GH gene during pituitary development. Analysis of the GHF1 promoter indicates that transcription is also controlled by cell-autonomous interactions involving positive autoregulation by GHF1, and environmental cues that modulate the intracellular level of cyclic AMP and thereby the activity of cAMP response element binding protein (CREB), a ubiquitous transactivator that binds to the GHF1 promoter.

Putative protein kinase product of the Drosophila segment-polarity gene zeste-white3

The metameric pattern of the Drosophila embryo is regulated by a combination of maternal and zygotic genes. The segment-polarity class of genes are required for the correct patterning within each segmental unit. Mutations in any one of these genes results in deletions and duplications of parts of each segment. The segment-polarity genes act coordinately by means of local cellular interactions to assign and maintain an identity for each cell in the segment, and to establish segment boundaries. Here we describe the molecular characterization of a novel segment-polarity gene, zeste-white3 (zw3). Embryos derived from germ lines that are homozygous for zw3 mutations (zw3 embryos) have phenotypes similar to embryos that are mutant for the segment-polarity gene naked (nkd). These embryos lack most of the ventral denticles, which are differentiated structures derived from the most anterior region of each segment. We have isolated the zw3 gene and compared the structure of one maternal and one zygotic transcript encoded by the gene. The zw3 gene is unique among the segment-polarity genes so far characterized, in that it encodes proteins that have homology to serine-threonine protein kinases. This indicates that zw3 may play a part in a signal transduction pathway involved in the establishment of cell identity within each embryonic segment.

Biochemical characterization of the Drosophila dpp protein, a member of the transforming growth factor beta family of growth factors

The decapentaplegic (dpp) gene of Drosophila melanogaster is required for pattern formation in the embryo and for viability of the epithelial cells in the imaginal disks. The dpp protein product predicted from the DNA sequence is similar to members of a family of growth factors that includes transforming growth factor beta (TGF-beta). We have produced polyclonal antibodies to a recombinant dpp protein made in bacteria and used a metallothionein promoter to express a dpp cDNA in Drosophila S2 cells. Similar to other proteins in the TGF-beta family, the dpp protein produced by the Drosophila cells was proteolytically cleaved, and both portions of the protein were secreted from the cells. The amino-terminal 47-kilodalton (kDa) peptide was found in the medium and in the proteins adhering to the plastic petri dish. The carboxy-terminal peptide, the region with sequence similarity to the active ligand portion of TGF-beta, was found extracellularly as a 30-kDa homodimer. Most of the 30-kDa homodimer was in the S2 cell protein adsorbed onto the surface of the plastic dish. The dpp protein could be released into solution by increased salt concentration and nonionic detergent. Under these conditions, the amino-terminal and carboxy-terminal portions of dpp were not associated in a stable complex.

Cloning and characterization of the segment polarity gene cubitus interruptus Dominant of Drosophila

The segment polarity mutation, cubitus interruptus Dominant (ciD), of Drosophila melanogaster causes defects in the posterior half of every embryonic segment. We cloned sequences from the ciD region on the proximal fourth chromosome by "tagging" the gene with the transposable element P. Genetic and molecular evidence indicates that the P-element insertions, which all occurred within the same restriction fragment, are in 5'-regulatory regions of the ciD gene within 3 kb of the first exon of its transcript. The putative ciD transcript was identified on the basis of its absence in homozygous ciD embryos. Its spatial pattern of expression during development is unusual in that, unlike most other segmentation genes, it exhibits uniform expression throughout cellular blastoderm and gastrulation and does not resolve into a periodic pattern until the end of the fast phase of germ-band elongation when it is present in 15 broad segmentally repeating stripes along the anterior-posterior axis of the embryo. Registration of the ciD stripes of expression relative to the stripes of other segment polarity genes shows that ciD is expressed in the anterior three-quarters of every segment. This registration does not correlate with the pattern defects observed in ciD mutants. Sequence analysis indicates that the protein encoded by the ciD transcript contains a domain of five tandem amino acid repeats that have sequence similarity to the zinc-finger repeats of the Xenopus transcription factor TFIIIA and that share the highest degree of identity with the human zinc-finger protein GLI, which has been found to be amplified in several human glioblastomas.

Positional cues and differential gene expression in somatic embryos of higher plants

Much of the organization of higher vascular plants is determined during the formation of the embryo. In addition to the zygotic embryo which results from sexual fertilization in the ovule, many plants are capable of producing embryos from somatic cells. Of particular interest to plant developmental biologists is the phenomenon of somatic embryogenesis in cultures of the domesticated carrot which, because of its tractable nature in experimental manipulations, is presently regarded as a suitable model for studying pattern formation in plants. This short review considers the state of our knowledge concerning the origin and perception of positional information in plant embryos, and the temporal and spatial expression of genes. The available data provide a number of promising leads for cell-cell interactions in embryos, and there are some clear indications that the spatial distribution of certain gene products is correlated with changes in morphology. However, there is, as yet, insufficient evidence with which to forge a link between positional cues and the expression of genes which influence developmental transitions in embryos.

Biochemical characterization of the Drosophila dpp protein, a member of the transforming growth factor beta family of growth factors

The decapentaplegic (dpp) gene of Drosophila melanogaster is required for pattern formation in the embryo and for viability of the epithelial cells in the imaginal disks. The dpp protein product predicted from the DNA sequence is similar to members of a family of growth factors that includes transforming growth factor beta (TGF-beta). We have produced polyclonal antibodies to a recombinant dpp protein made in bacteria and used a metallothionein promoter to express a dpp cDNA in Drosophila S2 cells. Similar to other proteins in the TGF-beta family, the dpp protein produced by the Drosophila cells was proteolytically cleaved, and both portions of the protein were secreted from the cells. The amino-terminal 47-kilodalton (kDa) peptide was found in the medium and in the proteins adhering to the plastic petri dish. The carboxy-terminal peptide, the region with sequence similarity to the active ligand portion of TGF-beta, was found extracellularly as a 30-kDa homodimer. Most of the 30-kDa homodimer was in the S2 cell protein adsorbed onto the surface of the plastic dish. The dpp protein could be released into solution by increased salt concentration and nonionic detergent. Under these conditions, the amino-terminal and carboxy-terminal portions of dpp were not associated in a stable complex.

Spatial and temporal control elements of the Drosophila engrailed gene

engrailed (en) is a segmentation gene expressed in a series of stripes throughout embryonic development. Here, I show that regulatory sequences for striped expression are present within the first intron of en. The 1-kb intron is able to confer striped expression early, but not late, in development. This shows that different regulatory sequences are required for en stripes at different times in development. Furthermore, stripes generated by the intron are coincident with en stripes in a wild-type background but behave differently from endogenous engrailed stripes in some segmentation mutant backgrounds. Thus, although the intron can induce apparently normal stripes, it lacks some of the regulatory sequences present within the endogenous gene. These experiments suggest that multiple regulatory programs control an expression in stripes, and each may be able to confer "normal" spatial regulation independently.