Subcellular localization of Suppressor of Hairless in Drosophila sense organ cells during Notch signalling

During imaginal development of Drosophila, Suppressor of Hairless [Su(H)], an evolutionarily conserved transcription factor that mediates intracellular signalling by the Notch (N) receptor, controls successive alternative cell fate decisions leading to the differentiation of multicellular sensory organs. We describe here the distribution of the Su(H) protein in the wing disc epithelium throughout development of adult sense organs. Su(H) was found to be evenly distributed in the nuclei of all imaginal disc cells during sensory organ precursor cells selection. Thus differential expression and/or subcellular localization of Su(H) is not essential for its function. Soon after division of the pIIa secondary precursor cell, Su(H) specifically accumulates in the nucleus of the future socket cell. At the onset of differentiation of the socket cell, Su(H) is also detected in the cytoplasm. In this differentiating cell, N and deltex participate in the cytoplasmic retention of Su(H). Still, Su(H) does not colocalize with N at the apical-lateral membranes. These observations suggest that N regulates in an indirect manner the cytoplasmic localization of Su(H) in the socket cell. Finally, the pIIb, shaft and socket cells are found to adopt invariant positions along the anteroposterior axis of the notum. This raises the possibility that tissue-polarity biases these N-mediated cell fate choices.

Control of cell fate choices by lateral signaling in the adult peripheral nervous system of Drosophila melanogaster

The thoracic integument of the adult fruit fly is a relatively simple but highly patterned structure. It is composed of sensory organ cells distributed within a monolayer of epidermal cells. Both cell types are easily detected at the cuticular surface, as each external sense organ forms a sensory bristle and each epidermal cell secretes a small nonsensory hair. Inhibitory cell-cell interactions play a key role in regulating the distribution as well as the formation of the sense organs. This review focuses on the role of these cell-cell interactions in the adoption of alternative cell fates. We also show that Notch, Hairless, and Suppressor of Hairless, three components of this intercellular signaling pathway, exhibit dose-dependent genetic interactions. Finally we address how this intercellular signaling mechanism may be modulated to result in highly reproducible outcomes.

The neurogenic suppressor of hairless DNA-binding protein mediates the transcriptional activation of the enhancer of split complex genes triggered by Notch signaling

The Notch protein (N) acts as a transmembrane receptor for intercellular signals controlling cell fate choices in vertebrates and invertebrates. The signal of N activation may be transduced directly from the cell surface into the nucleus by an evolutionarily conserved transcription factor, Suppressor of Hairless [Su(H)], by its regulated nuclear import. Su(H) is shown here to play a direct role in the immediate response of the genome to N signaling in Drosophila. First, Su(H) mutant embryos derived from mutant germ-line clones exhibited a "neurogenic" phenotype of neural hypertrophy similar to the N phenotype. Second, the lack of N lateral signaling in these Su(H) mutant embryos was associated with a failure to express the m5 and m8 genes from the Enhancer of split Complex [E(spl)-C]. Finally, the Su(H) protein bound to the regulatory sequences of the E(spl)-C m5 and m8 genes, and these binding sites were required for the activation of the m5 and m8 promoters in the ventral neuroectoderm. The expression of the E(spl)-C m8 gene was found to be similarly regulated by Su(H) during wing imaginal disc development. Thus, the transcriptional activation of these E(spl)-C genes by Su(H) appears to be a direct and relatively general response to the activation of N. However, we also present evidence indicating that N signals in an Su(H)-independent manner during mesectoderm formation.

Suppressor of Hairless is required for signal reception during lateral inhibition in the Drosophila pupal notum

Suppressor of Hairless (Su(H)) activity is zygotically required in larval imaginal discs for the singling out of adult sense organ precursor (SOP) cells: loss of Su(H) function results in too many proneural cluster cells adopting the SOP fate, while overexpression of the Su(H) protein prevents SOP specification. Su(H) null mutant alleles are recessive lethal at the late larval and early pupal stages. The development of Su(H) mutant cells in pupae was therefore studied in somatic clones. Clonal analysis first showed that Su(H) is required for the regular spacing of microchaete precursor cells, as clusters of mutant SOPs were detected at positions where singled out sense organ cells are normally found. Second, Su(H) mutant SOPs produced neuron-like cells, consistent with a late defect in Notch (N) signalling. Third, a careful cell-by-cell analysis of clone borders showed that Su(H) mutant cells may adopt the SOP fate even when directly adjacent to wild-type cells. Finally, quantitative clone border analysis indicates that the relative level of Su(H) gene dosage appears to bias the selection of the future SOP: cells with a higher level of Su(H) activity are more likely to adopt the epidermal fate. These results show that notum cells strictly require Su(H) activity for receiving the lateral inhibitory signal. Thus, the DNA-binding protein encoded by the Su(H) gene may act downstream of the N receptor to implement the epidermal, non-SOP fate.

Evolutionarily conserved positive and negative cis-acting elements control the blastoderm-specific expression of the Drosophila serendipity alpha cellularisation gene

The serendipity alpha (sry alpha) cellularisation gene is only transcribed at the blastoderm stage, when this morphogenetic event takes place. We show that a 95 bp sry alpha upstream region is sufficient for blastoderm-specific expression of a lacZ reporter gene. This region encompasses four nucleotide motifs (I-IV, 5' to 3') conserved at similar relative positions in several Drosophila species. Removal of motif I leads to ectopic expression of lacZ in precursor cells of the PNS. Deletion of motif IV decreases the level of lacZ transcripts and modifies their banded pattern of accumulation late in cycle 14, whereas deletion of motifs II and III abolishes the sry alpha promoter activity. Motif III includes a consensus recognition site for b-HLH proteins. A point mutation in this E-box both severely reduces lacZ expression at blastoderm and prevents its ectopic expression in the PNS upon deleting motif I. These two effects depend upon da+ activity, suggesting that daughterless may positively control sry alpha transcription.

The sequence similarity of the Drosophila suppressor of hairless protein to the integrase domain has no functional significance in vivo

Suppressor of Hairless [Su(H)] plays an essential role in neurogenesis in Drosophila by controlling successive alternative cell fate decisions in the developing adult epidermis. Analysis of the predicted amino acid sequence of the Su(H) protein revealed a weak similarity to the catalytic domain of a family of phage integrases and yeast recombinases. We present here the results of a site-directed mutagenesis of the integrase-related region of Su(H), which indicate that this sequence similarity has no functional significance in vivo. We suggest that the JK-RBP protein, encoded by the mouse homologue of Su(H), does not act as a recombinase, as originally proposed.

Inhibition of the DNA-binding activity of Drosophila suppressor of hairless and of its human homolog, KBF2/RBP-J kappa, by direct protein-protein interaction with Drosophila hairless

We have purified the sequence-specific DNA-binding protein KBF2 and cloned the corresponding cDNA, which is derived from the previously described RBP-J kappa gene, the human homolog of the Drosophila Suppressor of Hairless [Su(H)] gene. Deletion studies of the RBP-J kappa and Su(H) proteins allowed us to define a DNA-binding domain conserved during evolution. Because Su(H) mutant alleles exhibit dose-sensitive interactions with Hairless (H) loss-of-function mutations, we have investigated whether the RBP-J kappa or Su(H) proteins directly interact with the H protein in vitro. We show here that H can inhibit the DNA binding of both Su(H) and RBP-J kappa through direct protein-protein interactions. Consistent with this in vitro inhibitory effect, transcriptional activation driven by Su(H) in transfected Drosophila S2 cells is inhibited by H. These results support a model in which H acts, at least in part, as a negative regulator of Su(H) activity. This model offers a molecular view to the antagonistic activities encoded by the H and Su(H) genes for the control of sensory organ cell fates in Drosophila. We further propose that a similar mechanism might occur in mammals.

Antagonistic activities of Suppressor of Hairless and Hairless control alternative cell fates in the Drosophila adult epidermis

Successive alternative cell fate choices in the imaginal disc epithelium lead to the differentiation of a relatively invariant pattern of multicellular adult sensory organs in Drosophila. We show here that the activity of Suppressor of Hairless is required for both the sensory organ precursor (SOP) versus epidermal cell fate decision, and for the trichogen (shaft) versus tormogen (socket) cell fate choice. Complete loss of Suppressor of Hairless function causes most proneural cluster cells to accumulate high levels of the achaete and Delta proteins and to adopt the SOP fate. Late or partial reduction in Suppressor of Hairless activity leads to the apparent transformation of the tormogen (socket) cell into a second trichogen (shaft) cell, producing a ‘double shaft’ phenotype. We find that overexpression of Suppressor of Hairless has the opposite phenotypic effects. SOP determination is prevented by an early excess of Suppressor of Hairless activity, while at a later stage, the trichogen (shaft) cell is transformed into a second tormogen (socket) cell, resulting in ‘double socket’ bristles. We conclude that, for two different cell fate decisions in adult sensory organ development, decreasing or increasing the level of Suppressor of Hairless function confers mutant phenotypes that closely resemble those associated with gain and loss of Hairless activity, respectively. These results, along with the intermediate SOP phenotype observed in Suppressor of Hairless; Hairless double mutant imaginal discs, suggest that the two genes act antagonistically to commit imaginal disc cells stably to alternative fates.

Relationship between expression of serendipity alpha and cellularisation of the Drosophila embryo as revealed by interspecific transformation

A dramatic reorganization of the cytoskeleton underlies the cellularisation of the syncytial Drosophila embryo. Formation of a regular network of acto-myosin filaments, providing a structural framework, and possibly a contractile force as well, appears essential for the synchronous invagination of the plasma membrane between adjacent nuclei. The serendipity alpha (sry alpha) gene is required for this complete reorganization of the microfilaments at the onset of membrane invagination. We compare here the structure and expression of sry alpha between D. pseudoobscura, D. subobscura and D. melanogaster. Interspersion of evolutionarily highly conserved and divergent regions is observed in the protein. One such highly conserved region shows sequence similarities to a motif found in proteins of the ezrin-radixin-moesin (ERM) family. Four 7–13 bp motifs are conserved in the 5′ promoter region; two of these are also found, and at the same position relative to the TATA box, in nullo, another zygotic gene recently shown to be involved in cellularisation. The compared patterns of expression of D. melanogaster sry alpha and nullo, and D. pseudoobscura sry alpha reveal a complex regulation of the spatiotemporal accumulation of their transcripts. The D. pseudoobscura sry alpha gene is able to rescue the cellularisation defects associated with a complete loss of sry alpha function in D. melanogaster embryos, even though species-specific aspects of its expression are maintained. Despite their functional homologies, the D. melanogaster and D. pseudoobscura sry alpha RNAs have different subcellular localisations, suggesting that this specific localization has no conserved role in targeting the sry alpha protein to the apical membranes.

Suppressor of Hairless, the Drosophila homolog of the mouse recombination signal-binding protein gene, controls sensory organ cell fates

Suppressor of Hairless (Su(H)) is required at two stages of adult sensory organ development in Drosophila. Complete loss of Su(H) function results in a "neurogenic" phenotype in imaginal discs, in which too many cells adopt the sensory organ precursor cell fate. Su(H) is also involved in controlling the fates of sensillum accessory cells and is specifically expressed in two of these cells. Su(H) is the Drosophila homolog of the mouse J kappa RBP gene, whose product binds specifically to the recombination signal sequence of immunoglobulin J kappa segments. The Su(H) and J kappa RBP proteins are 82% identical over most of their length, and share with bacteriophage integrates and yeast recombinases a motif that includes residues directly involved in catalyzing recombination.

Engineered secreted T-cell receptor alpha beta heterodimers

We have produced a soluble form of a mouse alpha beta T-cell antigen receptor (TCR) by shuffling its variable (V) and constant (C) domains to the C region of an immunoglobulin kappa light chain. These chimeric molecules composed of V alpha C alpha C kappa and V beta C beta C kappa chains were efficiently secreted (up to 1 micrograms/ml) by transfected myeloma cells as noncovalent heterodimers of about 95-kDa molecular mass. In the absence of direct binding measurement, we have refined the epitopic analysis of the soluble V alpha C alpha C kappa-V beta C beta C kappa dimers and shown that they react with an anti-clonotypic antibody and two antibodies directed to the C domain of the TCR alpha and beta chains. Conversely, we have raised three distinct monoclonal antibodies against the soluble TCR heterodimers and shown that they recognize surface-expressed TCRs. Two of these antibodies were found to react specifically with the products of the V alpha 2 (V delta 8) and V beta 2 gene segments, respectively. When considered together, these data suggest that these soluble TCR molecules are folded in a conformation indistinguishable from that which they assume at the cell surface.

Genetic analysis of the cellularization of the Drosophila embryo

The synchronous cellularization of the Drosophila embryo at the blastoderm stage provides a unique system for studying the molecular mechanisms involved in cytokinesis, using genetical and biochemical approaches. The cellularization process requires the major components of the embryonic cytoskeleton that are deposited into the egg during oogenesis. Genetical analysis indicates that it requires also the products of additional maternally-acting genes, as well as that of a limited set of zygotically-acting genes. The cellularization defective phenotypes associated with small deficiencies uncovering these latter loci reveal specific steps within this complex process. The molecular analysis of these genes will ultimately provide meaningful insights into the normal process of cellularization. Among them, the serendipity alpha gene encodes a membrane-associated protein, which is exclusively accumulated during cellularization, and is required for the reorganization of the microfilaments as the onset of cellularization.

The serendipity alpha gene encodes a membrane-associated protein required for the cellularization of the Drosophila embryo

The Drosophila serendipity alpha (sry alpha) gene is specifically transcribed at the blastoderm stage, from nuclear cycle 11 to the onset of gastrulation, in all somatic nuclei. This pattern of transcription and a zygotic cellularization defect observed in embryos homozygous for Df(3R)X3F, a deficiency covering the sry locus, suggest that sry alpha plays a role in the cellularization of the syncytial blastoderm embryo. P-element rescue experiments show that one copy of the sry alpha gene rescues the defective cellularization phenotype associated with Df(3R)X3F. Lack of sry alpha activity results in erratic disruptions of the cytoskeleton at the beginning of the interphase of mitotic cycle 14. Multinucleate cells form during plasma membrane invagination. Immunodetection of the sry alpha protein using anti-sry alpha polyclonal antibodies indicates that the 58-kD sry alpha protein accumulates transiently at the blastoderm stage. The sry alpha protein is associated with the invaginating plasma membrane and colocalizes with F-actin. We propose that sry alpha is involved in the localization of membrane furrows within the syncytial blastoderm.

cis-regulatory elements of the Drosophila blastoderm-specific serendipity alpha gene: ectopic activation in the embryonic PNS promoted by the deletion of an upstream region

The Drosophila serendipity alpha gene (sry alpha) is specifically expressed at the blastoderm stage in all somatic cells. By deletion analysis of sry alpha-lacZ fusion genes, the sry alpha cis-acting regulatory elements have been restricted to the [-311, +130] 5'-region of the gene and separated in two domains. The [-118, +130] domain is sufficient for transcriptional activation at the blastoderm stage. The [-311, -118] domain is required for a full level of expression. Deletion of this upstream domain leads to a secondary pattern of lacZ expression in precursor cells of the peripheral nervous system (PNS). The sry alpha gene is not itself secondarily expressed in the PNS, as shown by in situ hybridization. The patterns of expression of the different sry alpha-lacZ fusion genes suggest a combinatorial mode of regulation of sry alpha expression at blastoderm.