1 Institute of Molecular Biology and Biotechnology, Foundation for Research and
Technology Hellas, Heraklion, Greece
2 Department of Biology, University of Crete, Heraklion, Greece
* Author for correspondence (e-mail: delidaki{at}imbb.forth.gr)
Accepted 12 July 2005
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SUMMARY |
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Key words: Drosophila, DSL, Notch, mind bomb, neuralized, Lateral inhibition
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Introduction |
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DSL proteins from insects and vertebrates can be classified into two
categories, Delta (Dl) and Serrate/Jagged (Ser/Jag), based on conserved
structural features of their extracellular domains
(Fleming, 1998). These two
families have different expression patterns and consequently function in
distinct Notch-dependent processes. Expression pattern differences, however,
are not the sole distinguishing feature of DSL proteins; the two families
appear to show strong preference for binding to differentially glycosylated
forms of Notch receptors (Haines and
Irvine, 2003
; Okajima et al.,
2003
) - glycosylation of Notch by Fringe stimulates Dl signaling,
whereas it inhibits Ser signaling. In terms of intracellular regulation,
ubiquitin ligases had been described only for Dl proteins, until very recently
(see below); yet, the need for Epsin in order for both Dl and Ser to emit
their signal (Wang and Struhl,
2004
) implicates ubiquitination also in Ser function. Two
different E3 Ub ligases seem to affect Dl function: Neuralized (Neur) has been
characterized in Drosophila (Lai
et al., 2001
; Pavlopoulos et
al., 2001
) and Xenopus
(Deblandre et al., 2001
); and
Mind bomb (Mib) in zebrafish (Itoh et al.,
2003
). Both associate with Dl triggering its endocytosis. Apart
from a catalytic RING domain at their C termini; Neur and Mib display no
further similarity. During the course of 2005 (while this paper was under
review), one Drosophila homolog of Mib, which we call Mib1, was
initially characterized by two groups, who showed that it interacts with both
Dl and Ser and variably affects their activity and endocytosis
(Lai et al., 2005
;
Le Borgne et al., 2005
).
Vertebrate Mib homologs were also shown to associate with both Dl and Jag
family members (Koo et al.,
2005a
; Takeuchi et al.,
2005
). However, these papers did not make it clear whether
different Ub ligases show preference for association with different DSL
proteins, nor whether DSL proteins absolutely require Ub ligases in order to
signal.
The present work and recent work done independently by Wang and Struhl
(Wang and Struhl, 2005) have
addressed both of these issues. Wang and Struhl
(Wang and Struhl, 2005
)
conclusively showed that Mib1 is necessary for signal sending by both Dl and
Ser in wing dorsoventral boundary establishment, a well-characterized instance
of Notch signaling. In that context, absence of Mib1 can be rescued by ectopic
provision of Neur. We have corroborated their findings and have further tested
the role of Dl, Ser, Neur and Mib1 in a different instance of Notch signaling,
lateral inhibition of neural precursors
(Bray, 1998
;
Skeath and Thor, 2003
).
Lateral inhibition was heretofore thought to depend solely on Dl and Neur
(Lai and Rubin, 2001
;
Lehman et al., 1983
), with no
input from Ser or Mib1 (Lai et al.,
2005
; Zeng et al.,
1998
). By contrast, wing DV boundary establishment requires both
Dl and Ser (Irvine and Vogt,
1997
); it also requires Mib1 but not Neur
(Lai et al., 2005
;
Lai and Rubin, 2001
;
Le Borgne et al., 2005
;
Wang and Struhl, 2005
). Lack
of requirement of a factor in any given process may well be a result of its
expression pattern; this seems to be the case for neur, which is not
broadly expressed in wing cells during DV boundary establishment. Similarly,
during embryonic neuroblast lateral inhibition, Ser is not expressed,
making the process solely Dl dependent
(Gu et al., 1995
). We have
focused on adult macrochaete SOP lateral inhibition, which takes place in the
wing disk at the third larval instar, where all Dl, Ser, neur and
mib1 are expressed. Contrary to expectations, we show that both Dl
and Ser participate in this process in a partially redundant fashion, and the
same holds true for Neur and Mib1. More importantly, we show that simultaneous
removal of neur and mib1 results in a complete block of
lateral inhibition. Our results lead us to conclude that (1) Ub ligases are
absolutely required for DSL protein function (at least in the present
contexts) and (2) either Ub ligase can activate either DSL protein. Our work,
taken together with other recent papers
(Wang and Struhl, 2004
;
Lai et al., 2005
;
Le Borgne et al., 2005
;
Wang and Struhl, 2005
), is
strongly in favor of a ubiquitin/epsin-mediated endocytosis pathway playing an
indispensable role in the emission of DSL-Notch signals.
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Materials and methods |
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pUAST-DlV5His was generated by subcloning an EcoRI/DraI
fragment containing the V5-tagged Dl-coding sequence from pIZ-DlV5His
(Bland et al., 2003) into pUAST
cut with EcoRI-XhoI (filled-in).
Antibodies and immunohistochemistry
Anti-Neur polyclonal antisera
pRSET-neur1050 was generated by cloning a PCR fragment encoding amino acids
11-360 of Neur in frame with the 6xHis tag of the pRSET-C vector
(Invitrogen). The fusion protein was expressed in E. coli and
purified with Ni2+-affinity chromatography under denaturing
conditions (Qiagen). Rabbit antiserum production and affinity purification was
carried out by Davids Biotechnologie.
Other antibodies
Fluorescent and HRP-labeled secondary antibodies were from Molecular Probes
and Jackson Immunoresearch, respectively. Immunohistochemistry was performed
as described by Pavlopoulos et al.
(Pavlopoulos et al.,
2001).
Transient transfections and immunoprecipitation
Transient transfections of S2 cells were carried out with the calcium
phosphate precipitation method. pIZ-DlV5His
(Bland et al., 2003) and
pRMHa3-Sermyc (gift of R. Fleming) were used to express Delta and Serrate,
respectively. pUAST-EGFP-neur and pUAST-neur
R-GFP were used to express
Neur or Neur
R in conjunction with mt-Gal4 (inducible by
Cu2+). Transfected cell lysate was used for immunoprecipitation
with rabbit anti-Neur antiserum and protein A sepharose. One percent of the
total extract was used as control (input). For larval immunoprecipitations,
the lysate was prepared from 30 third-instar disk-CNS complexes.
Drosophila stocks
Gal4 lines
UAS lines
Fly stocks were either obtained via the Bloomington and Szeged Stock Centers or generously provided by colleagues.
Mosaic analysis
All alleles used are described in FlyBase. Mosaics were induced during the
first larval instar using the conventional FLP/FRT technique
(Xu and Rubin, 1993) or the
MARCM system (Lee and Luo,
2001
). For cross details see Table S1 in the supplementary
material.
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Results |
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Both Dl and Ser participate in lateral inhibition of macrochaete SOPs
To address the role of DSL proteins in lateral inhibition, we focused on
the third instar notum, where nine SOPs arise in a well-defined pattern. We
first confirmed that both Dl and Ser are expressed within the proneural
clusters giving rise to these SOPs, although the Dl and Ser patterns are not
entirely identical (see Fig. S1 in the supplementary material). To visualize
SOPs we used the nuclear protein Sens
(Nolo et al., 2000) as a
marker. By counting the number of SOPs per position in different mutant mosaic
clones, we could conclude about the extent of the lateral inhibition defect.
Our first indication that Dl was not solely responsible for lateral inhibition
in these regions was that Dl clones showed a much weaker defect than
either N clones or doubly mutant Dl Ser clones
(Fig. 1A-C;
Table 1); the latter contained
a lot more (typically more than 10) Sens-positive cells per SOP position,
whereas Dl clones usually had two to four SOPs, and some were even
wild-type in appearance (one SOP). Yet, Ser singly mutant clones did
not affect SOP numbers (Fig.
1D). As the difference between our Dl and Dl Ser
clones could conceivably be due to some background mutation(s) other than
Ser, we sought an independent way to assay the role of Ser. One way
to inactivate any Ser contribution in signaling is to overexpress
fringe, as Fringe-modified Notch is refractory to Ser signaling.
Dl; UAS-fng clones (using the very same Dl chromosome, which
gave a mild phenotype) were generated using the MARCM system, which
inactivates one gene overexpressing another within the same clone. These
clones displayed a significantly higher number of SOPs per cluster than
Dl alone (Fig. 1E,
Table 1, P<0.05).
The control experiment of overexpressing UAS-fng in a wild-type
background produced no defect in SOP numbers. As two independent ways of
blocking Ser activity enhanced the Dl mutant phenotype, we conclude
that in normal tissue Ser contributes to lateral inhibitory Notch
signaling.
|
|
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An interesting observation in this series of experiments was that in contrast to the severe phenotypes of neur Dl or neur Ser clones, neur clones displayed only a weak-moderate defect in lateral inhibition (Fig. 2A), somewhat more severe than Dl clones. We concluded that each ligand has residual activity in the absence of Neur, which we subsequently showed to be dependent on Mib1 (see below).
|
Like Dl, Ser is found both on the apical plasma membrane and in
intracellular vesicles, both endogenously and when overexpressed in wing disk
cells (Fig. 4A,C). This changed
dramatically when a UAS-neur transgene was co-expressed; Ser was
cleared from the apical surface (Fig.
4B,C). The subapical intracellular aggregates were not affected in
the case of endogenous Ser, but were greatly increased in the case of
overexpressed Ser. Using an EGFP tagged neur transgene
(which behaves identically to our untagged UAS-neur; data not shown),
we showed that most of the subapical Ser-overexpressing aggregates also
accumulated Neur, which additionally remained ubiquitously cortical, mostly on
the apical side (Fig. 4B,E).
This cortical localization is what is normally observed for Neur in the
absence of co-overexpressed DSL ligand
(Fig. 4C,D), suggesting that
the large number of Ser-positive/Neur-positive intracellular aggregates
probably appear because of impaired trafficking caused by Ser overexpression.
This response of endogenous and overexpressed Ser to Neur is identical to what
has been previously described for Dl (Lai
et al., 2001; Pavlopoulos et
al., 2001
). Using Dl or Ser mutant backgrounds,
we showed that Neur elicits endocytosis of each DSL protein independently of
the presence of the other (see Fig. S3 in the supplementary material); this
refutes the possibility that the effects of Neur on Ser are due to Dl-Ser
interactions.
Mind bomb1 acts redundantly with Neur in lateral inhibition
Despite its physical and functional association with both DSL proteins
(Lai et al., 2001;
Pavlopoulos et al., 2001
)
(this work), neur loss of function has only a mild lateral inhibition
defect compared with Dl Ser loss of function
(Table 1,
Fig. 2). This led us to
conclude that there is substantial Neur-independent DSL activity. The
characterization of mind bomb as a Dl-targeting Ub ligase in
zebrafish (Chen and Corliss,
2004
; Itoh et al.,
2003
) made us wonder whether a possible Drosophila
ortholog might be responsible for this activity. BLAST search identified two
Drosophila genes with close similarity to zebrafish mib,
CG5841 and CG17492, which we henceforth call mib1 and
mib2, respectively. Of these, Mib1 has a better similarity to
zebrafish Mib. In situ hybridization to embryos revealed a segmentally
repeated stripe pattern of mib1 mRNA at stage 9-10, which disappears
later, whereas third instar wing disks showed low ubiquitous expression (data
not shown) (see also Le Borgne et al.,
2005
). The Drosophila gene disruption project
(Bellen et al., 2004
) has
generated a P-element insertion, EY9780, which disrupts the
mib1 gene in the 5' UTR. EY9780 homozygotes survive to
pupal stage with a good percentage of pharate adult escapers. These have
small, almost non-existent, eyes and wings, and short legs (data not shown)
(Lai et al., 2005
;
Le Borgne et al., 2005
). We
could not detect any mib1 mRNA in EY9780 homozygotes by
RT-PCR (data not shown). Based on this and on the fact that excision of the
P-element reverted the lethality (data not shown), we concluded that this
P-element represents a null allele of mib1 and we designated it as
mib1EY9780 [see also complementary evidence elsewhere
(Lai et al., 2005
;
Le Borgne et al., 2005
;
Wang and Struhl, 2005
)].
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Discussion |
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Ubiquitin ligases and DSL protein function
Although Neur was known to affect Dl localization and function in some
instances (Lai et al., 2001;
Lai and Rubin, 2001
;
Le Borgne and Schweisguth,
2003
; Pavlopoulos et al.,
2001
; Yeh et al.,
2000
), ubiquitin ligases were not considered as essential
components of Notch signaling. The characterization of Mib1 described here and
in recent papers (Lai et al.,
2005
; Le Borgne et al.,
2005
; Wang and Struhl,
2005
) points to a much more prominent role of these factors.
mib1 appears to be required in a large number of Notch-dependent
processes where neur is not expressed, e.g. the wing DV boundary. The
fact that mib1 neur double mutants appear to lose all ability to
perform lateral inhibition (Fig.
5) strongly supports the hypothesis that Ub ligases may always be
required for Dl/Ser signaling. A comprehensive survey of Notch-dependent
events with respect to neur and mib1 will test this
hypothesis and may uncover additional E3 ligases with this activity; Mib2
represents a potential candidate.
The intimate relation between Neur/Mib1 and DSL proteins is generally
assayed in three ways: (1) physical association, (2) effects on Dl/Ser
endocytosis and (3) effects on Dl/Ser signaling. All of these had been well
documented for the Neur-Dl combination
(Lai et al., 2001;
Pavlopoulos et al., 2001
) and,
more recently, for the Mib1-Dl and Mib1-Ser combinations
(Lai et al., 2005
;
Le Borgne et al., 2005
;
Wang and Struhl, 2005
) (this
work). In the present work we have added the final pair, Neur-Ser, using all
of the above assays. The conclusion, stated simply, is that both Neur and Mib1
associate with and affect the endocytosis and function of both Dl and Ser.
Mechanism of Dl/Ser signaling
Ubiquitination of transmembrane proteins tags them for endocytosis, using a
complex of adaptors, including epsin, which carry ubiquitin recognition
domains (Haglund et al.,
2003). The simplest scenario for the role of Neur/Mib1 in Dl/Ser
signaling would be that they attach ubiquitin to Dl/Ser to trigger
endocytosis. Signaling would ensue, either as a consequence of
recruiting/clustering ubiquitinated DSL cargo to specialized plasma membrane
domains conducive to signaling, or by more elaborate routes involving DSL
protein recycling through the endocytic pathway as a prerequisite for their
modification/activation (Wang and Struhl,
2004
).
Alternatively, Neur/Mib1 need not ubiquitinate the DSL proteins directly.
In the ubiquitin-dependent endocytosis pathway, many of the adaptor proteins
are themselves ubiquitinated, possibly favoring the formation of
interconnected cargo-adaptor complexes
(Polo et al., 2002); Neur/Mib1
could have one or more of the adaptors, including themselves, as substrates.
DSL protein chimaeras become Mib1 independent if their intracellular domains
are substituted with ones bearing alternative internalization motifs
(Wang and Struhl, 2005
). Of
two such artificial Mib1-independent versions of Dl, one is
ubiquitination/epsin-independent (Dl-LDL-receptor fusion), whereas the other
(Dl-random-peptide-R fusion) still curiously requires ubiquitination/epsin for
activity (Wang and Struhl,
2004
). Nothing is yet known about the native Dl/Ser intracellular
domains, other than the puzzling fact that they are neither similar nor
evolutionarily conserved, despite apparent conservation of recognition by
Neur/Mib.
An even more puzzling observation in the light of our model is that some
DSL proteins in C. elegans appear to be secreted
(Chen and Greenwald, 2004).
Secreted mutants of Drosophila Dl and Ser act as Notch antagonists
(Mishra-Gorur et al., 2002
;
Sun and Artavanis-Tsakonas,
1997
), consistent with a requirement for endocytosis in DSL
signaling. Even C. elegans LAG-2 (a transmembrane DSL) needs EPN-1
(epsin ortholog), in order to signal to GLP-1 (Notch-like) during germline
differentiation (Tian et al.,
2004
), which is hard to reconcile with secreted DSL proteins.
Apparently, ubiquitination/endocytosis can be bypassed in some contexts,
allowing secreted DSL proteins to signal via a yet unknown process.
Whatever the molecular details and variations turn out to be, we are
quickly coming to realize that ubiquination plays a prominent role in Notch
signaling, in both sending and receiving cells. In the latter, Ub ligases
downregulate Notch activity either at the membrane
(Qiu et al., 2000;
Sakata et al., 2004
;
Wilkin et al., 2004
) or in the
nucleus (Gupta-Rossi et al.,
2001
; Oberg et al.,
2001
; Wu et al.,
2001
). Besides downregulation, however, Notch ubiquitination is
also needed for activation: ubiquitination apparently targets Notch to a
compartment where it can be activated by
-secretase cleavage
(Gupta-Rossi et al., 2004
).
How two ubiquitination/trafficking events, activating DSL proteins in one cell
and Notch in another, might be coordinated across the extracellular space is a
mystery worth investigating in the future.
Note added in proof
Koo et al. (Koo et al.,
2005b) have studied murine Mib1 and have come to a similar
conclusion, namely that Mib1 associates with all Notch ligands (Dll1, Dll3,
Dll4, Jag1 and Jag2) and is necessary for their activation.
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ACKNOWLEDGMENTS |
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Footnotes |
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Supplementary material for this article is available at http://dev.biologists.org/cgi/content/full/132/18/4041/DC1
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Bellen, H. J., Levis, R. W., Liao, G., He, Y., Carlson, J. W.,
Tsang, G., Evans-Holm, M., Hiesinger, P. R., Schulze, K. L., Rubin, G. M. et
al. (2004). The BDGP gene disruption project: single
transposon insertions associated with 40% of Drosophila genes.
Genetics 167,761
-781.
Bland, C. E., Kimberly, P. and Rand, M. D.
(2003). Notch-induced proteolysis and nuclear localization of the
Delta ligand. J. Biol. Chem.
278,13607
-13610.
Blochlinger, K., Bodmer, R., Jan, L. Y. and Jan, Y. N. (1990). Patterns of expression of Cut, a protein required for external sensory organ development in wild-type and cut mutant Drosophila embryos. Genes Dev. 4,1322 -1331.[Abstract]
Bray, S. (1998). Notch signalling in Drosophila: three ways to use a pathway. Semin. Cell Dev. Biol. 9,591 -597.[CrossRef][Medline]
Chen, N. and Greenwald, I. (2004). The lateral signal for LIN-12/Notch in C. elegans vulval development comprises redundant secreted and transmembrane DSL proteins. Dev. Cell 6, 183-192.[CrossRef][Medline]
Chen, W. and Corliss, D. C. (2004). Three modules of zebrafish Mind bomb work cooperatively to promote Delta ubiquitination and endocytosis. Dev. Biol. 267,361 -373.[CrossRef][Medline]
de Celis, J. F. and Bray, S. (1997). Feed-back
mechanisms affecting Notch activation at the dorsoventral boundary in the
Drosophila wing. Development
124,3241
-3251.
Deblandre, G. A., Lai, E. C. and Kintner, C. (2001). Xenopus Neuralized is a ubiquitin ligase that interacts with XDelta1 and regulates Notch signaling. Dev. Cell 1, 795-806.[CrossRef][Medline]
Fleming, R. J. (1998). Structural conservation of Notch receptors and ligands. Semin. Cell Dev. Biol. 9, 599-607.[CrossRef][Medline]
Gu, Y., Hukriede, N. A. and Fleming, R. J.
(1995). Serrate expression can functionally replace
Delta activity during neuroblast segregation in the
Drosophila embryo. Development
121,855
-865.
Gupta-Rossi, N., Le Bail, O., Gonen, H., Brou, C., Logeat, F.,
Six, E., Ciechanover, A. and Israel, A. (2001). Functional
interaction between SEL-10, an F-box protein, and the nuclear form of
activated Notch1 receptor. J. Biol. Chem.
276,34371
-34378.
Gupta-Rossi, N., Six, E., LeBail, O., Logeat, F., Chastagner,
P., Olry, A., Israel, A. and Brou, C. (2004).
Monoubiquitination and endocytosis direct gamma-secretase cleavage of
activated Notch receptor. J. Cell Biol.
166, 73-83.
Haglund, K., Di Fiore, P. P. and Dikic, I. (2003). Distinct monoubiquitin signals in receptor endocytosis. Trends Biochem. Sci. 28,598 -603.[CrossRef][Medline]
Haines, N. and Irvine, K. D. (2003). Glycosylation regulates Notch signalling. Nat. Rev. Mol. Cell Biol. 4,786 -797.[Medline]
Heitzler, P. and Simpson, P. (1991). The choice of cell fate in the epidermis of Drosophila. Cell 64,1083 -1092.[CrossRef][Medline]
Heitzler, P., Bourouis, M., Ruel, L., Carteret, C. and Simpson,
P. (1996). Genes of the Enhancer of split and
achaete-scute complexes are required for a regulatory loop between
Notch and Delta during lateral signalling in Drosophila.Development 122,161
-171.
Irvine, K. D. and Vogt, T. F. (1997). Dorsal-ventral signaling in limb development. Curr. Opin. Cell Biol. 9,867 -876.[CrossRef][Medline]
Itoh, M., Kim, C. H., Palardy, G., Oda, T., Jiang, Y. J., Maust, D., Yeo, S. Y., Lorick, K., Wright, G. J., Ariza-McNaughton, L. et al. (2003). Mind bomb is a ubiquitin ligase that is essential for efficient activation of Notch signaling by Delta. Dev. Cell 4,67 -82.[CrossRef][Medline]
Klueg, K. M. and Muskavitch, M. A. (1999).
Ligand-receptor interactions and trans-endocytosis of Delta, Serrate and
Notch: members of the Notch signalling pathway in Drosophila. J.
Cell Sci. 112,3289
-3297.
Koelzer, S. and Klein, T. (2003). A
Notch-independent function of Suppressor of Hairless during the development of
the bristle sensory organ precursor cell of Drosophila.Development 130,1973
-1988.
Koo, B. K., Yoon, K. J., Yoo, K. W., Lim, H. S., Song, R., So,
J. H., Kim, C. H. and Kong, Y. Y. (2005a). Mind bomb-2 is an
E3 ligase for Notch ligand. J. Biol. Chem.
280,22335
-22342.
Koo, B.-K., Lim, H.-S., Song, R., Yoon, M.-J., Yoon, K.-J.,
Moon, J.-S., Kim, Y.-W., Kwon, M. c., Yoo, K. W., Kong, M.-P. et al.
(2005b). Mind bomb 1 is essential for generating functional Notch
ligands to activate Notch. Development
132,3459
-3470.
Lai, E. C. (2004). Notch signaling: control of
cell communication and cell fate. Development
131,965
-973.
Lai, E. C. and Rubin, G. M. (2001). neuralized functions cell-autonomously to regulate a subset of Notch-dependent processes during adult Drosophila development. Dev. Biol. 231,217 -233.[CrossRef][Medline]
Lai, E. C., Deblandre, G. A., Kintner, C. and Rubin, G. M. (2001). Drosophila Neuralized is a ubiquitin ligase that promotes the internalization and degradation of Delta. Dev. Cell 1,783 -794.[CrossRef][Medline]
Lai, E. C., Roegiers, F., Qin, X., Jan, Y. N. and Rubin, G.
M. (2005). The ubiquitin ligase Drosophila Mind bomb promotes
Notch signaling by regulating the localization and activity of Serrate and
Delta. Development 132,2319
-2332.
Le Borgne, R. and Schweisguth, F. (2003). Unequal segregation of Neuralized biases Notch activation during asymmetric cell division. Dev. Cell 5, 139-148.[CrossRef][Medline]
Le Borgne, R., Remaud, S., Hamel, S. and Schweisguth, F. (2005). Two distinct E3 ubiquitin ligases have complementary functions in the regulation of delta and serrate signaling in Drosophila. PLoS Biol. 3,e96 .[CrossRef][Medline]
Lee, T. and Luo, L. (2001). Mosaic analysis with a repressible cell marker (MARCM) for Drosophila neural development. Trends Neurosci. 24,251 -254.[CrossRef][Medline]
Lehman, R., Jimenez, F., Dietrich, U. and Campos-Ortega, J. A. (1983). On the phenotype and development of mutants of early neurogenesis in Drosophila melanogaster. Roux Arch. Dev. Biol. 192,62 -74.[CrossRef]
Li, Y. and Baker, N. E. (2004). The roles of cis-inactivation by Notch ligands and of neuralized during eye and bristle patterning in Drosophila. BMC Dev. Biol. 4, 5.[CrossRef][Medline]
Micchelli, C. A., Rulifson, E. J. and Blair, S. S.
(1997). The function and regulation of cut expression on
the wing margin of Drosophila: Notch, Wingless and a dominant
negative role for Delta and Serrate. Development
124,1485
-1495.
Mishra-Gorur, K., Rand, M. D., Perez-Villamil, B. and
Artavanis-Tsakonas, S. (2002). Down-regulation of Delta by
proteolytic processing. J. Cell Biol.
159,313
-324.
Nolo, R., Abbott, L. A. and Bellen, H. J. (2000). Senseless, a Zn finger transcription factor, is necessary and sufficient for sensory organ development in Drosophila.Cell 102,349 -362.[CrossRef][Medline]
Oberg, C., Li, J., Pauley, A., Wolf, E., Gurney, M. and Lendahl,
U. (2001). The Notch intracellular domain is ubiquitinated
and negatively regulated by the mammalian Sel-10 homolog. J. Biol.
Chem. 276,35847
-35853.
Okajima, T., Xu, A. and Irvine, K. D. (2003).
Modulation of Notch-ligand binding by protein O-fucosyltransferase 1 and
Fringe. J. Biol. Chem.
278,42340
-42345.
Overstreet, E., Chen, X., Wendland, B. and Fischer, J. A. (2003). Either part of a Drosophila epsin protein, divided after the ENTH domain, functions in endocytosis of Delta in the developing eye. Curr. Biol. 13,854 -860.[CrossRef][Medline]
Overstreet, E., Fitch, E. and Fischer, J. A.
(2004). Fat facets and Liquid facets promote Delta endocytosis
and Delta signaling in the signaling cells.
Development 131,5355
-5366.
Pavlopoulos, E., Pitsouli, C., Klueg, K. M., Muskavitch, M. A., Moschonas, N. K. and Delidakis, C. (2001). neuralized encodes a peripheral membrane protein involved in Delta signaling and endocytosis. Dev. Cell 1, 807-816.[CrossRef][Medline]
Polo, S., Sigismund, S., Faretta, M., Guidi, M., Capua, M. R., Bossi, G., Chen, H., De Camilli, P. and Di Fiore, P. P. (2002). A single motif responsible for ubiquitin recognition and monoubiquitination in endocytic proteins. Nature 416,451 -455.[CrossRef][Medline]
Qi, H., Rand, M. D., Wu, X., Sestan, N., Wang, W., Rakic, P.,
Xu, T. and Artavanis-Tsakonas, S. (1999). Processing of the
Notch ligand Delta by the metalloprotease Kuzbanian.
Science 283,91
-94.
Qiu, L., Joazeiro, C., Fang, N., Wang, H. Y., Elly, C., Altman,
Y., Fang, D., Hunter, T. and Liu, Y. C. (2000). Recognition
and ubiquitination of Notch by Itch, a hect-type E3 ubiquitin ligase.
J. Biol. Chem. 275,35734
-35737.
Sakata, T., Sakaguchi, H., Tsuda, L., Higashitani, A., Aigaki, T., Matsuno, K. and Hayashi, S. (2004). Drosophila Nedd4 regulates endocytosis of notch and suppresses its ligand-independent activation. Curr. Biol. 14,2228 -2236.[CrossRef][Medline]
Schweisguth, F. (2004). Notch signaling activity. Curr. Biol. 14,R129 -R138.[CrossRef][Medline]
Schweisguth, F. and Posakony, J. W. (1994).
Antagonistic activities of Suppressor of Hairless and
Hairless control alternative cell fates in the Drosophila
adult epidermis. Development
120,1433
-1441.
Seugnet, L., Simpson, P. and Haenlin, M. (1997a). Requirement for dynamin during Notch signaling in Drosophila neurogenesis. Dev. Biol. 192,585 -598.[CrossRef][Medline]
Seugnet, L., Simpson, P. and Haenlin, M.
(1997b). Transcriptional regulation of Notch and
Delta: requirement for neuroblast segregation in Drosophila.Development 124,2015
-2025.
Skeath, J. B. and Thor, S. (2003). Genetic control of Drosophila nerve cord development. Curr. Opin. Neurobiol. 13,8 -15.[CrossRef][Medline]
Sun, X. and Artavanis-Tsakonas, S. (1997).
Secreted forms of DELTA and SERRATE define antagonists of Notch signaling in
Drosophila. Development
124,3439
-3448.
Takeuchi, T., Adachi, Y. and Ohtsuki, Y.
(2005). Skeletrophin, a novel ubiquitin ligase to the
intracellular region of Jagged-2, is aberrantly expressed in multiple myeloma.
Am. J. Pathol. 166,1817
-1826.
Tian, X., Hansen, D., Schedl, T. and Skeath, J. B.
(2004). Epsin potentiates Notch pathway activity in
Drosophila and C. elegans. Development
131,5807
-5815.
Wang, W. and Struhl, G. (2004). Drosophila
Epsin mediates a select endocytic pathway that DSL ligands must enter to
activate Notch. Development
131,5367
-5380.
Wang, W. and Struhl, G. (2005). Distinct roles
for Mind bomb, Neuralized and Epsin in mediating DSL endocytosis and signaling
in Drosophila. Development
132,2883
-2894.
Wilkin, M. B., Carbery, A. M., Fostier, M., Aslam, H., Mazaleyrat, S. L., Higgs, J., Myat, A., Evans, D. A., Cornell, M. and Baron, M. (2004). Regulation of notch endosomal sorting and signaling by Drosophila Nedd4 family proteins. Curr. Biol. 14,2237 -2244.[CrossRef][Medline]
Wu, G., Lyapina, S., Das, I., Li, J., Gurney, M., Pauley, A.,
Chui, I., Deshaies, R. J. and Kitajewski, J. (2001). SEL-10
is an inhibitor of notch signaling that targets notch for ubiquitin-mediated
protein degradation. Mol. Cell. Biol.
21,7403
-7415.
Xu, T. and Rubin, G. M. (1993). Analysis of
genetic mosaics in developing and adult Drosophila tissues.
Development 117,1223
-1237.
Yeh, E., Zhou, L., Rudzik, N. and Boulianne, G. L.
(2000). neuralized functions cell autonomously to
regulate Drosophila sense organ development. EMBO
J. 19,4827
-4837.
Yeh, E., Dermer, M., Commisso, C., Zhou, L., McGlade, C. J. and Boulianne, G. L. (2001). Neuralized functions as an E3 ubiquitin ligase during Drosophila development. Curr. Biol. 11,1675 -1679.[CrossRef][Medline]
Yoon, K. and Gaiano, N. (2005). Notch signaling in the mammalian central nervous system: insights from mouse mutants. Nat. Neurosci. 8,709 -715.[CrossRef][Medline]
Zeng, C., Younger-Shepherd, S., Jan, L. Y. and Jan, Y. N.
(1998). Delta and Serrate are redundant Notch ligands required
for asymmetric cell divisions within the Drosophila sensory organ
lineage. Genes Dev. 12,1086
-1091.