domeless: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References
Gene name - domeless

Synonyms - master of marelle (mom)

Cytological map position - 18D8

Function - cytokine receptor

Keywords - JAK/STAT pathway, segmentation, trachea

Symbol - dome

FlyBase ID: FBgn0043903

Genetic map position - 1-

Classification - gp130 homolog

Cellular location - surface



NCBI links: Precomputed BLAST | Entrez Gene
Recent literature
Ren, W., Zhang, Y., Li, M., Wu, L., Wang, G., Baeg, G.H., You, J., Li, Z. and Lin, X. (2015). Windpipe controls Drosophila intestinal homeostasis by regulating JAK/STAT pathway via promoting receptor endocytosis and lysosomal degradation. PLoS Genet 11: e1005180. PubMed ID: 25923769
Summary:
The adult intestinal homeostasis is tightly controlled by proper proliferation and differentiation of intestinal stem cells. The JAK/STAT (Janus Kinase/Signal Transducer and Activator of Transcription) signaling pathway is essential for the regulation of adult stem cell activities and maintenance of intestinal homeostasis. Currently, it remains largely unknown how JAK/STAT signaling activities are regulated in these processes. This study has identified windpipe (wdp) as a novel component of the JAK/STAT pathway. Wdp was positively regulated by JAK/STAT signaling in Drosophila adult intestines. Loss of wdp activity resulted in the disruption of midgut homeostasis under normal and regenerative conditions. Conversely, ectopic expression of Wdp inhibited JAK/STAT signaling activity. Importantly, Wdp interacted with the receptor Domeless (Dome), and promoted its internalization for subsequent lysosomal degradation. Together, these data led the study to propose that Wdp acts as a novel negative feedback regulator of the JAK/STAT pathway in regulating intestinal homeostasis.

Fisher, K. H., Stec, W., Brown, S. and Zeidler, M. P. (2015). Mechanisms of JAK/STAT pathway negative regulation by the short coreceptor Eye Transformer / Latran. Mol Biol Cell. PubMed ID: 26658615
Summary:
Transmembrane receptors interact with extracellular ligands to transduce intracellular signaling cascades, modulate target gene expression and regulate processes such as proliferation, apoptosis, differentiation and homeostasis. As a consequence, aberrant signaling events often underlie human disease. While the vertebrate JAK/STAT signaling cascade is transduced via multiple receptor combinations, the Drosophila pathway has only one full-length signaling receptor, Domeless (Dome) and a single negatively acting receptor, Eye Transformer/Latran (Et/Lat). This study investigated the molecular mechanisms underlying Et/Lat activity. Et/Lat was shown to negatively regulates JAK/STAT pathway activity and can bind to Dome so reducing Dome:Dome homo-dimerisation by creating signaling-incompetent Dome:Et/Lat heterodimers. Surprisingly, Et/Lat was found to be able to bind to both JAK and STAT92E, but despite the presence of putative cytokine binding motifs, does not detectably interact with pathway ligands. Et/Lat is trafficked through the endocytic machinery for lysosomal degradation, but at a much slower rate than Dome - a difference that may enhance its ability to sequester Dome into signaling incompetent complexes. These data offer new insights into the molecular mechanism and regulation of Et/Lat in Drosophila that may inform understanding of how short receptors function in other organisms.
Beshel, J., Dubnau, J. and Zhong, Y. (2017). A leptin analog locally produced in the brain acts via a conserved neural circuit to modulate obesity-linked behaviors in Drosophila. Cell Metab 25: 208-217. PubMed ID: 28076762
Summary:
Leptin, a typically adipose-derived "satiety hormone," has a well-established role in weight regulation. This study describes a functionally conserved model of genetically induced obesity in Drosophila by manipulating the fly leptin analog unpaired 1 (upd1). Unexpectedly, cell-type-specific knockdown reveals upd1 in the brain, not the adipose tissue, mediates obesity-related traits. Disrupting brain-derived upd1 in flies leads to all the hallmarks of mammalian obesity: increased attraction to food cues, increased food intake, and increased weight. These effects are mediated by domeless receptors on neurons expressing Drosophila neuropeptide F, the orexigenic mammalian neuropeptide Y homolog. In vivo two-photon imaging reveals upd1 and domeless inhibit this hedonic signal in fed animals. Manipulations along this central circuit also create hypersensitivity to obesogenic conditions, emphasizing the critical interplay between biological predisposition and environment in overweight and obesity prevalence. The study proposes that adipose- and brain-derived upd/leptin may control differing features of weight regulation through distinct neural circuits.


BIOLOGICAL OVERVIEW

The JAK/STAT signaling pathway plays important roles in vertebrate development and the regulation of complex cellular processes. Components of the pathway are conserved in Dictyostelium, Caenorhabditis, and Drosophila, yet the complete sequencing and annotation of the D. melanogaster and C. elegans genomes has failed to identify a receptor, raising the possibility that an alternative type of receptor exists for the invertebrate JAK/STAT pathway. domeless (dome) codes for a transmembrane protein required for all JAK/STAT functions in the Drosophila embryo. This includes its known requirement for embryonic segmentation and a newly discovered function in trachea specification. The Dome protein has an extracellular structure similar to the vertebrate cytokine class I receptors, although its sequence has greatly diverged. Like many interleukin receptors, Dome has a cytokine binding homology module (CBM) and three extracellular fibronectin-type-III domains (FnIII). Despite its low degree of overall similarity, key amino acids required for signaling in the vertebrate cytokine class I receptors are conserved in the CBM region. Dome is a signal-transducing receptor with most similarity to the IL-6 receptor family, but it also has characteristics found in the IL-3 receptor family. This suggests that the vertebrate families evolved from a single ancestral receptor which also gave rise to dome (Brown, 2001).

JAK/STAT signaling was first identified in vertebrates as mediating the response to some cytokines and growth factors. Ligand binding induces receptor homo- or hetero-dimerization and subsequent signal transduction. The receptors lack a tyrosine kinase domain but associate with cytoplasmic tyrosine kinases of the JAK family. After receptor dimerization, JAK phosphorylates a tyrosine residue on the receptor, and cytoplasmic STAT is recruited to the complex. JAK then phosphorylates STAT, which dimerizes, translocates to the nucleus, and induces gene transcription. In Drosophila, one JAK encoded by hopscotch (hop), one STAT encoded by STAT92E, and one ligand encoded by outstretched/unpaired (upd) have been identified, but no receptor has been found. Mutations for either hop, STAT92E, or upd result in an identical, characteristic segmentation phenotype (Brown, 2001).

Mutations in STAT92E affect the posterior spiracles, part of the respiratory apparatus of the larva. domeless was identified in a screen for P element insertion mutations that give a phenotype similar to STAT92E. The six alleles, three strong (dome217, dome441, and dome468) and three weak (dome321, dome405, and dome367), all affect the shape of the posterior spiracles, with the strongest leading to a loss of the characteristic dome shape. Mobilization of the P element reverts both the lethality and the phenotype, confirming that the insertions cause the observed defects (Brown, 2001).

A database sequence search using DNA flanking the P elements identified an expressed-sequence tag (EST) encoding a putative transmembrane protein. The finding that expression of this cDNA rescues the dome spiracle phenotype confirms that this cDNA encodes the dome gene (Brown, 2001).

A second study identified domeless using a screen for suppressors of an eye phenotype caused by overexpression of unpaired. Overexpression of upd using a UAS-upd and GMR-Gal4 driver causes compound eye dramatic overgrowth in the adult eye because of an increase in the number of ommatidia. The average number of ommatidia in the compound eye of UAS-upd/GMR-Gal4 female flies is 978 ± 10 compared with 745 ± 7 in wild-type flies. Histological sections through the overgrown eyes reveal that most ommatidia have normal photoreceptor cells and regular cell size, indicating that Upd activity mainly regulates cell proliferation in the compound eye. However, the ommatidia look more crowded and have irregular space and arrangement, and several big vacuoles are integrated into the ommatidia lattice. The severity of eye morphology appears proportional to the strength of the Hop/Stat92E-mediated signaling, because removing one copy of hop partially suppresses the big eye phenotype; the average number of ommatidia is 854 ± 9). The advantage of this sensitized system lies in the possibility of conducting a screen for mutations that reduce (suppressors) or increase (enhancers) the degree of eye size. It was reasoned that a twofold reduction in the dose of a gene (by mutating one of its two copies) that functions downstream of Upd should dominantly alter signaling strength, which, in turn, should visibly modify the eye size. Based on this assumption, available X-chromosome P-element insertion mutations were screened and one complementation group of suppressors with four alleles was identified at the cytological location 18E. Based on its presumed role in the Hop/Stat92E signal transduction pathway, this novel gene was named master of marelle (mom). The relative strength of four mom alleles in suppressing the UAS-Upd/GMR-Gal4 fly big eye phenotype is mom1 > mom2 >  mom3 = mom4, and mom1 is the strongest allele. mom is indeed the same gene as domeless (Chen, 2002).

Tests were performed to see whether dome and STAT92E interact genetically. Zygotic STAT92E homozygotes have a very mild spiracle phenotype due to the persistence of maternally expressed RNA. Despite this, the weak dome367 phenotype is strongly enhanced by STAT92E mutants, suggesting that both genes are in the same genetic pathway (Brown, 2001).

To determine the phenotype caused by eliminating domeless maternal and zygotic products, germ line clones were induced. Maternal and zygotic dome embryos have segmentation defects identical to those reported for mutations in the STAT92E and hop (JAK) mutants. Defects include the deletion of the A5 and most of the A4 denticle belts, partial or total fusion of A6 to A7, and a variable reduction of the thoracic and the A8 segments. These phenotypes are also observed in Df(1)osUE69, which deletes the ligand, upd. The segmentation defects in STAT92E, upd, and hop have been shown to be due to the abnormal expression of pair rule genes. In dome germ line clones, the expression of even-skipped is affected in stripes 3 and 5, as described for the other members of this pathway (Brown, 2001).

To further investigate whether dome has the genetic characteristics expected of the JAK/STAT receptor, dome interactions with upd, the known JAK/STAT ligand, were tested. To do this, advantage was taken of the fact that when the h-GAL4 line is used for ectopic expression of upd in the embryo, the result is abnormal head formation in 81% of the embryos. When upd is expressed ectopically in dome zygotic mutant embryos, this proportion is reduced to 16%. This result is consistent with dome being necessary to transduce the upd signal (Brown, 2001).

To find out if the intracellular domain of Dome is required for its function, UAS constructs were made in which the putative intracellular domain was deleted. One construct, UAS-domeDeltaCYT, contains the extracellular and transmembrane portion of the protein and should be membrane bound. The other, UAS-domeDeltaTMCYT, contains only the extracellular part and might be secreted. Neither of these proteins is able to rescue the dome zygotic phenotype, proving that the Dome intracellular domain is required for signal transduction. Because both forms still contain the cytokine binding domain, they must have the potential to titrate the ligand and act as signaling antagonists. This has been shown to be the case for the soluble form of gp130, the signal-transducing subunit of many vertebrate cytokine class I receptors. Consistent with this, when a maternal GAL4 was used for expression of UAS-domeDeltaTMCYT or UAS-domeDeltaCYT at early stages of development, approximately 50% of the larvae acquired segmentation defects. The most frequent defects were deletions and fusions of A4 and A5 segments, the segments more sensitive to loss of JAK/STAT function, but stronger defects were also observed. These phenotypes are increased if the mothers are also heterozygous for a hop allele, further proving the central role of dome in JAK/STAT signaling (Brown, 2001).

In mammals, the gp130 subfamily of receptors includes gp130, leukemia inhibitory factor receptor (LIFR), oncostatin M receptor (OSMR), ciliary neurotrophic factor receptor (CTNFR), cardiotrophin-1 receptor (CT-1R), granulocyte colony-stimulating factor receptor (G-CSFR), and the obesity/leptin gene receptor (OBR). The functional gp130 subfamily of receptors generally comprises multiple components of membrane proteins, and these multichain receptor complexes often share the common signal gp130 as a component critical for signal transduction (Hirano, 1998). Cytokines in the IL-6 family initiate intracellular signaling by binding to a specific ligand-binding subunit and then subsequently form a complex with the signal-transducing receptor component, gp130. IL-6 binds to IL-6R, and the IL-6/IL-6R complex then associates with gp130, allowing it to homodimerize. However, OSMR and G-CSFR are unique among the gp130 subfamily of receptors. In the case of OSMR, OSM binds directly to gp130 with low affinity. Upon binding by OSM, gp130 generates a high-affinity receptor complex by forming a heterodimer with either the LIFR- or the OSM-specific receptor subunit (LIFRß and OSMRß, respectively). The G-CSFR shows high homology with gp130; G-CSFR forms homodimers by itself in response to G-CSF (Chen, 2002 and references therein).

Upd has a very limited similarity to the IL-6 family of cytokines. It is difficult to predict whether Upd signaling is more like IL-6 or G-CSF. If it is more like IL-6, there should be another receptor subunit that would be functionally similar to IL-6R. Upd may first bind to the missing receptor and then form a functional complex with Mom. On the other hand, if Upd is more like G-CSF, Mom may be the only receptor. Upd binding should promote Mom to form a homodimer and start the signal transduction process. However, Upd may interact with its receptor in a different way. Upd structure is less consistent with a cytokine-type molecule. The Upd protein is extremely basic, with a predicted pI of nearly 12. In contrast to many soluble cytokines, Upd is associated with ECM (extracellular matrix), which may help it bind to the receptor and limit the range of activity of the ligand (Chen, 2002).

The gp130 subfamily of receptors has no intrinsic tyrosine kinase domain, but constitutively associates with tyrosine kinase JAKs. The family members possess conserved motifs in the cytoplasmic region, such as box 1, box 2, and box 3 in the membrane proximal-to-distal order. These conserved boxes are important for receptor-mediated signal transduction (Hirano, 1998). The box 1 motif in the cytoplasmic region of gp130 is important for the association with JAK kinases; the box 3 motif provides a docking site for the SH2 domain of STAT3 protein, which recruited to gp130 now serves as a substrate for JAKs. The activation of STAT3 is dependent on the phosphorylation of any one of the four tyrosines (Y767, Y814, Y905, Y915) in the C terminus of the gp130 that have a glutamine residue at the third position behind tyrosine (Y-X-X-Q). Mom has no visible box 1 and 2 motifs but has one tyrosine residue (Y966) fitting a YXXQ consensus motif. Mom binds Hop and Stat92E in biochemical experiments (Chen, 2002).

In addition to the JAK/STAT pathway, multiple signaling molecules are tyrosine-phosphorylated in response to the IL-6 family of cytokines. CNTF, LIF, OSM, and IL-6 induce tyrosine phosphorylation of phospholipase Cgamma and SHP-2 (a phosphotyrosine phosphatase, also called PTP1-D, SHPTP-2, PTP2C, and Syp), which is a mammalian homolog of Drosophila corkscrew (csw). The Ras-MAPK pathway is activated by the IL-6 cytokine family. The activation of the Ras-MAPK pathway is possibly mediated by SHP-2 and/or Shc, which bind a Grb2-SOS complex (Hirano, 1998). Tyrosine 759 of gp130 is required for the tyrosine phosphorylation of SHP-2 and its binding to gp130, the association between SHP-2 and Grb2, and the full activation of MAPK (Chen, 2002).

In flies, the phenotypes caused by loss of function of the Hop/Stat92E pathway and of the Ras-MAPK pathway are very distinct. In the embryo, loss-of-function mutations in the Hop/Stat92E pathway cause segmentation defects. The absence of additional phenotypes, such as terminal defects or poorly differentiated cuticle in germ-line clone-derived embryos, indicates that this pathway does not cross-talk with the Ras-MAPK pathway in either the Torso or Egfr RTK signaling pathways. In the eye, the Ras-MAPK pathway regulates photoreceptor formation. Changing the activity of the Hop/Stat92E pathway does not affect photoreceptor fate rather than affect cell proliferation (Chen, 2002).

However, there are some similarities in phenotypes between the Hop/Stat92E pathway and the Drosophila Ras-MAPK pathway. For example, overexpression of hop in the wing disc results in vein phenotypes similar to those seen in animals that express activated forms of D-raf. Loss-of-function D-raf mutations result in larval/pupal lethality with underproliferation of diploid tissues, similar to hop mutations. However, these similarities are based on overexpression phenotypes as well as most of the mammalian evidence for cross-talk. These results should be taken with caution just in case an artifactual phenomenon was created by these overexpressions (Chen, 2002).

The mammalian gp130 subfamily of receptors interacts with the receptors' respective ligands and has pleiotropic functions. G-CSF is the principal hematopoietic growth factor regulating the production of neutrophils, and it is widely used to treat neutropenia in a variety of clinical settings. The mouse obesity gene encodes a soluble protein (OB or leptin) that produces weight-reducing effects in mice when administrated in vivo. The structure of the receptor for this factor (OBR or leptin-R) is highly homologous to that of gp130. The IL-6 family of cytokines shares gp130 as a component critical for signal transduction in the cytokine receptor complexes and has redundant functions. Some of these biological activities of IL-6 are also often exerted by other cytokines, namely, IL-11, LIF, OSM, CNTF, and CT-1. The biological activity by which each cytokine was initially identified was growth promotion of myeloma and plasmacytoma for IL-11, growth inhibition of a mouse myeloid leukemia cell line for LIF, growth inhibition of a human melanoma cell line for OSM, promotion of survival of rat-cultured ciliary neurons for CNTF, and induction of cardiac hypertrophy in vitro for CT-1. These cytokines show structural similarity and, more importantly, have biological functions that overlap with those of IL-6 (Chen, 2002).

Mom is the only member of this family of receptors so far identified in Drosophila. The fly has also only one JAK (Hop) and one STAT (Stat92E). It remains to be seen whether this simple Mom/Hop/Stat92E pathway has the pleiotropic functions that are accomplished by a great complex of a homologous receptor family. This simple genetic model system will greatly enhance understanding of the gp130 subfamily of receptors' biological functions and mediated signal transductions (Chen, 2002).


GENE STRUCTURE

cDNA clone length - 4871

Bases in 5' UTR - 544

Exons - 3

Bases in 3' UTR - 478


PROTEIN STRUCTURE

Amino Acids - 1282

Structural Domains

The amino acid sequence analysis of Mom/Domeless identified two hydrophobic segments: the first one near the N terminus is a probable signal peptide; the second one in the middle may identify the transmembrane domain. In the extracellular domain, the most remarkable feature is the presence of four fibronectin type III (FN3) repeats of ~90 amino acids each. Fibronectin type III repeats are typically 90-100 amino acids long with the location of aromatic amino acids and nearby hydrophobic residues being conserved. Many cytokine receptors have FN3 repeats in their extracellular domains (Chen, 2002).

Computer-assisted homology searches in the GenBank and EMBL databases have identified several receptor-linked protein tyrosine phosphatases (PTPs) and cytokine receptors. However, the homology between Mom and PTPs is only limited to the FN3 repeats; Mom does not have the protein tyrosine phosphatase (PTPase) domains. Mom is distantly related to the cytokine receptor family, particularly the gp130-subfamily proteins. All these receptors contain, in their extracellular region, a domain of ~200 amino acid residues, which is conserved in the family. This domain is characterized by four positionally conserved cysteine residues in its N-terminal half and a WSXWS motif in the C-terminal end. It is suggested that this domain consists of seven ß-strands positioned antiparallel to form a barrel-like shape so that a trough formed between two barrel-like modules functions as a ligand-binding pocket. The predicted primary sequence of Mom includes a cytokine receptor family domain containing four conserved cysteine residues and a WSEPM sequence that resembles the WSXWS motif in the cytokine receptor domain of the gp130-subfamily receptors, followed by four fibronectin type III (FN3) repeats. The Mom cytoplasmic domain is more diversified from the gp130-subfamily receptors. The potential box 1 and 2 motifs are less visible in the Mom sequence. Nevertheless, Mom has one tyrosine residue (Y966) fitting a YXXQ consensus motif for STAT binding in the gp130 protein and may bind Stat92E protein (Chen, 2002).

In conclusion, Mom and the gp130-subfamily receptors exhibit certain homologies in domain structures rather than in primary amino acid sequence. Mom may be a distantly related member of the gp130-subfamily receptors (Chen, 2002).

The 4.8 kb dome encodes a 1282 amino acid protein with a putative signal peptide of 23 amino acids and a transmembrane domain. The extracellular region contains five fibronectin-type-III (FnIII) domains, of which two have similarity to the cytokine binding module (CBM) found in the vertebrate cytokine receptor class I family. No invertebrate receptors of this family have been described, despite the completion of the sequencing projects in Drosophila and C. elegans. The vertebrate cytokine receptor family comprises more than 20 different receptors that signal through the JAK/STAT pathway. The CBM present in the vertebrate interleukin receptors is typically composed of two FnIII domains that contain a set of four conserved cysteine residues in the N-terminal domain and a WSXWS motif in the C-terminal domain. Dome contains these features, but the C-terminal domain of Dome has an incomplete WSXWS motif (NTXWS). Dome has 18% identity to LIFR and 26% identity to CNTFR and is within the typical range of sequence similarity limits for all cytokine receptors. Interestingly, Dome also has some characteristics of the IL-3 receptor family. These characteristics include an alternating region of hydrophobic residues (YXLXVRVR) in the CBM-C domain and the incomplete WSXWS motif, present only in IL-3Ralpha. The intracellular region of Dome is rich in both serine and threonine (16%) as well as proline (11%) and has an acidic region; features shared by the IL-2Rß receptor and GM-CSFR. Like other receptors of this class, Dome lacks a kinase domain. Although the sequence similarity of Dome with the vertebrate cytokine class I receptors is low, the shared characteristics suggest that domeless may encode the elusive Drosophila JAK/STAT receptor (Brown, 2001).


EVOLUTIONARY HOMOLOGS

Heinrich, P. C., Behrmann, I., Muller-Newen, G., Schaper, F. and Graeve, L. (1998). Interleukin-6-type cytokine signalling through the gp130/Jak/STAT pathway. Biochem J. 334: 297-314. Online version - requires Acrobat reader

Hirano, T. (1998). Interleukin 6 and its receptor: ten years later. Int Rev Immunol. 16(3-4): 249-84. Online version


domeless: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 5 March 2002

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