bride of sevenless



In the eye imaginal disc BOSS is first detected in cells lying three rows posterior to the morphogenetic furrow. Its expression is restricted to the R8 cell, identifiable by its characteristic shape and position in the ommatidial cluster. BOSS is localized to the apical regions of the monolayer epithelium. In later stages of eye development boss is more widely expressed. The protein is found in all photoreceptor cells, even into adulthood. Expression of boss is also observed in many sensory neurons in the developing antenna and leg discs, and in the ocelli (Krämmer, 1991).

Mutation of roughex perturbs cell fate determination. Many rux mutant clusters contain multiple boss-expressing cells. In some of these clusters, R8 cells are missing. There is also a reduction in the number of cells expressing bar and Seven-up. This may be due to errors in cell fate determination. Alternatively, the reduced number of cells expressing these markers may reflect cell death. Extensive cell death is seen in rux mutants beginning with the MF and extending to the posterior edge of the disc. In rux mutant discs, neuronal differentiation is delayed by approximately 6 hours of development (Thomas, 1994).

Boss/Sev signaling from germline to soma restricts germline-stem-cell-niche formation in the anterior region of Drosophila male gonads

Drosophila germline stem cells are regulated by the somatic microenvironment, or 'niche,' which ensures that the stem cells can both self-renew and produce functional gametes throughout adult life. However, despite its prime importance, little is known about how niche formation is regulated during gonadal development. A receptor tyrosine kinase, Sevenless (Sev), is required to ensure that the niche develops in the anterior region of the male embryonic gonads. Sev is expressed in somatic cells within the posterior region of the gonads. Sev is activated by a ligand, Bride of sevenless (Boss), which is expressed by the germline, to prevent ectopic niche differentiation in the posterior gonadal somatic cells. Thus, it is proposed that signal transduction from germline to soma restricts expansion of the germline-stem-cell niche in the gonads (Kitadate, 2007).

These data show that the posterior somatic gonadal cells (SGCs), as well as the anterior SGCs, have the capacity to contribute to the germline-stem-cell niche within the male embryonic gonads. However, during development, niche differentiation is normally repressed in the posterior SGCs by Sev. In the absence of Sev activity, posterior SGCs are recruited to form an expanded niche. Sev is activated in the SGCs by the Boss ligand emanating from pole cells. This implies that varying the number of pole cells will alter the niche size. A model predicts that a decrease in the number of pole cells should induce ectopic niche formation within the gonads, which consequently increase their chance to recruit pole cells as the stem cells. Thus, it is speculated that the interaction between SGCs and PGCs via the Boss/Sev pathway acts as a key component of a negative-feedback loop to maintain an optimal number of germline stem cells in male gonads. A similar feedback mechanism has been reported in the stem-cell system of plant meristem and in Drosophila larval ovaries (Kitadate, 2007).

However, it was found that overexpression of a constitutively active Sev results in neither hyperactivation of Rl nor repression of hub-cell fate in the anterior SGCs, suggesting that activation of signaling components downstream of Sev is suppressed in the anterior SGCs. It has been widely accepted that the Notch transmembrane receptor and the receptor tyrosine kinase (RTK)/RAS/MAPK pathways antagonize each other in various developmental contexts, and that Notch signaling is involved in stem cell maintenance and differentiation in several stem-cell systems. In Drosophila ovaries, Notch signaling is required for the formation and maintenance of the germline-stem-cell niche. Overexpression of activated Notch induces expansion of the niche, while a reduction of Notch activity results in loss of the niche. In addition, germline cells express ligands for Notch to induce Notch-receptor activity and thereby to promote their own maintenance and function within the niche. Since the Notch receptor is also expressed predominantly in SGCs of male embryonic gonads, it is likely that Notch may antagonize Boss/Sev signaling in the anterior region of the gonads. It is speculated that the negative- and positive-feedback loops between germline and soma through Sev and Notch signaling act antagonistically to regulate proper niche formation during gonadal development. It will be interesting to test this hypothesis in future experiments. This study provides an important step toward understanding the regulatory mechanisms of niche formation in germline development (Kitadate, 2007).

Phosphatidic acid phospholipase A1 mediates ER-Golgi transit of a family of G protein-coupled receptors

The coat protein II (COPII)-coated vesicular system transports newly synthesized secretory and membrane proteins from the endoplasmic reticulum (ER) to the Golgi complex. Recruitment of cargo into COPII vesicles requires an interaction of COPII proteins either with the cargo molecules directly or with cargo receptors for anterograde trafficking. This study shows that cytosolic phosphatidic acid phospholipase A1 (PAPLA1) interacts with COPII protein family members and is required for the transport of Rh1 (rhodopsin 1), an N-glycosylated G protein-coupled receptor (GPCR), from the ER to the Golgi complex. In papla1 mutants, in the absence of transport to the Golgi, Rh1 is aberrantly glycosylated and is mislocalized. These defects lead to decreased levels of the protein and decreased sensitivity of the photoreceptors to light. Several GPCRs, including other rhodopsins and Bride of sevenless, are similarly affected. These findings show that a cytosolic protein is necessary for transit of selective transmembrane receptor cargo by the COPII coat for anterograde trafficking (Kunduri, 2014).

Effects of Mutation or Deletion

In boss mutants, staining for ELAV, a marker for neuronal cells, reveals only seven neuronal cells. The missing photoreceptor precursor is R7 (Hart, 1990).

Some aspect of R7 differentiation is independent of the genetic pathway(s) involving sevenless, boss and sina. An enhancer trap line, H214, has been developed in which beta-galactosidase is primarily expressed in the R7 cell throughout its development. In sevenless mutations, boss and sina expression is initially reduced in H214 although still present in the R7 precursor, and persists in the Equatorial cone cell (the alternative fate of R7 cells in mutants) into which R7 cells develop. The EQC in wild type never expresses lacZ in H214. Thus the presumptive R7 cell receives positional information independent of sevenless (Mlodzik, 1992).

DNA sequences have been determined for three to five alleles of boss in each of four species of Drosophila.The time of divergence between D. melanogaster and D. simulans is estimated as approximately 1 Myr, that between D. teissieri and D. yakuba as approximately 0.75 Myr, and that between the two pairs of sibling species as approximately 2 Myr. (The boss genes themselves have estimated divergence times approximately 50% greater than the species divergence times.) The effective size of the species is estimated as approximately 5 x 10(6), and the average mutation rate is estimated as 1-2 x 10(-9)/nucleotide/generation. The ratio of amino acid polymorphisms within species to fixed differences between species suggests that approximately 25% of all possible single-step amino acid replacements in the boss gene product may be selectively neutral or nearly neutral. The data also imply that random genetic drift has been responsible for virtually all of the observed differences in the portion of the boss gene analyzed among the four species (Ayala, 1994).

A novel method of P-element mutagenesis is described for the isolation of mutants affecting the development of the Drosophila compound eye. It exploits the interaction between the Bride of Sevenless (Boss) ligand and the Sevenless (Sev) receptor tyrosine kinase; acting in concert, they trigger the formation of the UV-sensitive photoreceptor neuron, R7. In live flies, transposition of a boss cDNA transgene, in an otherwise boss mutant background, was used as a "phenotypic trap" to identify enhancers expressed during a narrow time window in eye development. Using a rapid behavioral screen, more than 400,000 flies were tested for restoration of R7. Because R7 is the primary receptor for UV light, flies containing R7 can be easily separated from flies lacking R7 (either boss or sevenless mutant flies). When given a choice between UV and visible light, 90% of boss mutant flies move toward the visible light; in contrast, 90% of the wild-type flies move toward the UV light. UV-tactic behavior can be conferred upon boss mutant flies by expressing boss in the eye disc during a period in which Sev-expressing precursor cells are competent to respond. Using boss in place of lacZ as an enhancer-trap marker enablesa behavioral screening for genes expressed in the eye imaginal disc within the first 30 hours of ommatidial development. Thus, promoter driven expression of boss in a boss mutant background restores R7 function and UV-tactic behavior. Some 1,800 R7-containing revertant flies were identified. Among these, 21 independent insertions with expression of the boss reporter gene in the R8 cell were identified by a external eye morphology and staining with an antibody against Boss. Among 900 lines with expression of the boss reporter gene in multiple cells assessed for homozygous mutant phenotypes, insertions in the marbles, glass, gap1, and fasciclin II genes were isolated. This phenotypic enhancer-trap facilitates (1) the isolation of enhancer-traps with a specific expression pattern, and (2) the recovery of mutants disrupting development of specific tissues. Because the temporal and tissue specificity of the phenotypic trap is dependent on the choice of the marker used, this approach can be extended to other tissues and developmental stages (Pignoni, 1997).

In the developing compound eye of Drosophila, neuronal differentiation of the R7 photoreceptor cell is induced by the interaction of the receptor tyrosine kinase Sevenless with its ligand, Bride of sevenless (Boss), which is expressed on the surface of the neighboring R8 cell. Boss is an unusual ligand for a receptor tyrosine kinase: it is composed of a large extracellular domain, a transmembrane domain with seven membrane-spanning segments and a cytoplasmic tail. Expression of a monomeric, secreted form of the extracellular domain of Boss is not sufficient for Sevenless activation, and instead acts as a weak antagonist. Because oligomerization appears to be a critical step in the activation of receptor tyrosine kinases, oligomerized forms of the Boss extracellular domain were used to test their ability to bind to Sevenless in vivo and restore R7 induction in vivo. Oligomerization is achieved by fusion to the leucine zipper of the yeast transcription factor GCN4 or to the tetramerization helix of the Lac repressor. Binding of these multivalent proteins to Sevenless can be detected in vitro by immunoprecipitation of cross-linked ligand/receptor complexes and in vivo by receptor-dependent ligand localization. However, neither R8-specific nor ubiquitous expression of multivalent extracellular Boss (Exboss) ligands rescues the boss phenotype. Instead, these ligands acted as competitive inhibitors for wild-type Boss protein and thereby suppressed R7 induction. Therefore the role of the transmembrane or cytoplasmic domains of Boss in the activation of the Sev receptor cannot be replaced by oligomerization (Sevrioukov, 1998).

Why do the oligomeric forms of Exboss not activate Sev? Given that oligomerization actually enhances receptor binding, it does not appear likely that the tight association of the leucine zippers occludes the interface of Exboss, which is required for Sev binding. A potential problem could be the specific spatial arrangement of the Exboss subunits in the Exboss-GCN dimers or Exboss-Lac oligomers. To address this issue a test was performed on three versions of Exboss-GCN, which differ only in the orientation of their subunits relative to one another. Given the similarity of the effects of the three rotated Exboss-GCN forms and of Exboss-Lac, which uses a different oligomerization unit, it seems unlikely that a specific spatial arrangement of the Exboss subunits is the cause of their dominant-negative effects. The most straightforward explanation for the data is that the 7TM and cytoplasmic domains of Boss have an additional, different function in R7 induction. The structure of the Boss protein is reminiscent of seven transmembrane receptors, although no sequence homology has yet been identified. This similarity raises the possibility that Boss may function not only as a ligand but also as a receptor. One possible function for Boss as a receptor would be to act indirectly in R7 induction. However, constitutively active Sevenless receptor results in R7 induction even in the absence of Boss, arguing against a role for Boss as a receptor in the induction of R7 cell fate. A second possible function for the Boss protein as a receptor is to affect R8 development. An R8-specific Rhodopsin, Rh5, is expressed in a subset of R8 cells that is paired with R7 cells expressing Rh3. Interestingly, in eye discs lacking R7 cells, R8 cells no longer express the Rh5 opsin (Chou, 1996 and Papatsenko, 1997). These recent findings constitute the first indication that a signal from R7 influences gene expression in R8, and provide an assay to test the possibility that Boss acts as a receptor as well as a ligand (Sevrioukov, 1998).

In Drosophila, Src oncogene 1 was considered a unique ortholog of the vertebrate c-src; however, more recent evidence has been shown to the contrary. The closest relative of vertebrate c-src found to date in Drosophila is not Dsrc64, but Dsrc41, a gene identified for the first time in this paper. In contrast to Src64, overexpression of wild-type Src41 causes little or no appreciable phenotypic change in Drosophila. Both gain-of-function and dominant-negative mutations of Src41 cause the formation of supernumerary R7-type neurons, suppressible by one-dose reduction of boss, sevenless, Ras1, or other genes involved in the Sev pathway. Dominant-negative mutant phenotypes are suppressed and enhanced, respectively, by increasing and decreasing the copy number of wild-type Src41. The colocalization of Src41 protein, actin fibers and DE-cadherin, as well as the Src41-dependent disorganization of actin fibers and putative adherens junctions in precluster cells, suggest that Src41 may be involved in the regulation of cytoskeleton organization and cell-cell contacts in developing ommatidia (Takahashi, 1996).


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bride of sevenless: Biological Overview | Regulation | Developmental Biology | Effects of Mutation

date revised: 15 February 2015

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