division abnormally delayed and dally-like
Proteoglycans, the molecules of extracellular matrix, carry a highly negative charge due to their glycosaminoglycan (GAG) chains and
large volumes. They were considered to play a secondary role in activities like cell division, adhesion, blood coagulation, etc. until the
importance of their sugar chains in the fibroblast growth factor (FGF) signaling was discovered. Studies of mutations in the genes sugarless (sgl) and sulfateless (sfl) have proved that the proteoglycans involved in Wg signaling
contain heparan sulfate GAG chains. This has led to the attribution of specific functions to these molecules.
The Glypican family of heparan sulfate proteoglycans (HSPGs) is characterized by core proteins with conserved cysteine residues and
attachment to the cell surface by a glycosylphosphatidyl inositol (GPI) anchor. This may lead to endocytic pathways that are different from
other HSPGs, higher lateral mobility and possible apical localization in a cell. Variations in their HS contents may effect binding properties and localization, thus specializing each member for a unique biological function. Glypicans play important roles in morphogenetic pathways, e.g. human glypican 3 (GPC3) is
mutated in Simpson-Golabi-Behmel syndrome making an individual prone to tumors. Dally, the first Drosophila member of the family, is essential for the wingless and decapentaplegic signaling pathways. A new Drosophila glypican, dally-like protein (dlp) is described, having all the features of a glypican (Khare, 2000).
The sequence analysis confirms its position in the glypican family. At the N terminus, there is a hydrophobic stretch of
amino acids indicative of a potential signal sequence. A
second, shorter hydrophobic stretch is found at the C
terminus. Such a stretch is typically observed in GPI
anchored proteins and is necessary for their attachment to
the anchor in endoplasmic reticulum. Closer to the C terminus there is a cluster of seven serine-glycine dipeptides surrounded by acidic residues, a structural feature found in potential GAG attachment sites. All of the 14 cysteines are
conserved. The homology searches suggest that this gene is
closest in sequence to GPC4. However, the evolutionary tree made using program
PROTDIST/NEIGHBOR reveals another lineage closer to Dally. This may be
because the Drosophila genes contain stretches of unique
sequences. The gene has been mapped to chromosomal location 70E (Khare, 2000).
The members of the glypican family show diverse tissue-specific as well as developmental stage-specific expression
patterns. A Northern blot analysis was performed using
suitable cDNAs as probes on extracts from different developmental stages of Drosophila. It showed a 4-kb transcript,
which remains constant during the embryonic stages but
is undetectable during third instar larva. The mRNA is detectable again at the pupal stage and continues in the adult, but the intensity of the band is lower than
in the earlier stages. In situ hybridization analysis was performed to determine
the expression pattern of the gene during embryonic stages.
The expression of dlp appears quite early. At stage 5, there is weak ubiquitous expression in the blastoderm, which concentrates into
two strong anterior and two weak posterior bands at stage
6. At stage 3, the two anterior bands
remain strong in the area which later forms
the procephalic neurogenic region (pNR). This is the region
that gives rise to some of the brain neuroblasts. At stage 10,
a regular pattern of 15 stripes emerges. There is
strong staining at pNR and the anteriormost region of
clypeolabrum. The anterio-ventral region of future gnathal
buds also shows strong expression. The expression pattern
at this stage looks very similar to that of wg. Apart from
this pattern, staining is also seen in the lining of the
midgut, the hindgut and faint staining in the CNS (Khare, 2000).
To see if dlp and wg expression patterns colocalize, a
double staining experiment was performed. Due to low resolution caused by the wg antibody, an engrailed (en)
antibody was used instead. In a wild type embryo, the one-cell wide wg
stripe lies immediately anterior to the two-cell wide en stripe. The dlp stripes are also seen to lie immediately
anterior to the en stripes, but the stripes are wider than a
single cell. It is concluded that dlp does overlap with
the wg expressing cell, but to a great extent, also with the wg
target cells. Yet, dally expression has
been shown to overlap with en expression
and thus, may not overlap with dlp (Khare, 2000).
Recent studies in Drosophila have shown that heparan
sulfate proteoglycans (HSPGs) are required for Wingless
(Wg/Wnt) signaling. In addition, genetic and phenotypic
analyses have implicated the glypican gene dally in this
process. Another Drosophila glypican gene, dally-like (dly) has been identified and it is also involved in Wg signaling. Inhibition of dly gene activity implicates a function for DLY in Wg reception -- overexpression of DLY leads to an accumulation of extracellular Wg. It is proposed that DLY plays a role in the extracellular distribution of Wg. Consistent with this
model, a dramatic decrease of extracellular Wg was
detected in clones of cells that are deficient in proper
glycosaminoglycan biosynthesis. It is concluded that HSPGs play an important role in organizing the extracellular distribution of Wg (Baeg, 2001).
One glypican molecule, Dally, has been implicated in Wg signaling. However, because numerous glypican genes are present
in other animals, a search was carried out of the
Drosophila database for additional glypican family members.
One EST clone showed some sequence similarity
to dally and a full length cDNA was cloned. The sequence of the cDNA revealed
a potential open reading frame of 765 amino acid residues, with 22% and 35% identity to Dally and mouse K-glypican,
respectively. The predicted primary structure of the
molecule exhibits the hallmarks of a glypican protein. The
hydrophilicity plot of the new molecule is similar to those
of the other members of the glypican family, which is
characterized by the presence of NH2- and COOH-terminal
hydrophobic signal sequences. In addition,
this molecule possesses four consensus serine/glycine
dipeptide sequences for glycosaminoglycan (GAG) attachment sites, and a signal
sequence for a GPI-moiety attachment site at the COOH-terminal
region. Moreover, the number and position of
cysteine residues, which are a unique feature of glypican
family members, are almost completely conserved in the
predicted protein. These results led to the identification of Dally-like. Hybridization using
a dly-specific probe to polytene chromosomes from salivary
glands localized the dly gene to the cytological division 70F
on the third chromosome. Finally, Northern blot analysis revealed that dly encodes a single major 3.8 kb transcript (Baeg, 2001).
To discover the function of Dly, its expression in embryos was determined by in situ hybridization. dly transcripts are uniformly expressed at early embryonic stages, but by stage 8 they are enriched in stripes. Double staining
for dly mRNA and Wg protein shows that dly transcripts are
preferentially expressed in three to four cells anterior to the wg-expressing
cells. Interestingly, this expression pattern is similar to that of both dally and frizzled 2, the Wingless receptor (Baeg, 2001).
In an attempt to assess the function of Dly during
embryogenesis, the RNA interference (RNAi) method
was used to perturb Dly protein synthesis. Embryos were injected with a dly double-stranded RNA (dsRNA). These embryos,
referred to as dly dsRNA embryos, show the
absence of naked cuticle. This phenotype is
reminiscent of loss of either wg or hh gene activities. The
segment polarity phenotype is also found when the activity of
dally, which is required for Wg signaling in the embryo, is disrupted by
RNAi, though the effect is less severe in dally dsRNA embryos
than in dly dsRNA embryos. However, when
compared with embryos injected with either dly dsRNA or
dally dsRNA alone, embryos injected with an equimolar
mixture of dly and dally dsRNAs show more severe segment
polarity phenotypes. These embryos are smaller,
particularly in the tail region. They also show an entire
transformation of naked cuticle into cuticle with denticles,
which is observed in wg or hh null mutations. Because dally
does not appear to play a role in Hh signaling, the interaction between
dally and dly observed in the RNAi interference experiment
suggests that Dly and Dally function synergistically in Wg
signaling. Altogether, these results suggest that dly is
a novel segment polarity gene that potentiates Wg signaling (Baeg, 2001).
To further examine the role of Dly in Wg signaling, the function of Dly during wing imaginal disc development was examined. dly transcripts are uniformly expressed in wing discs. In the third instar imaginal disc, wg is
expressed at the DV compartment border and acts over short
and long ranges to pattern the wing disc. Short range Wg
signaling induces the expression of the proneural gene achaete
(ac) in a stripe on each side of the DV boundary, while long
range Wg signaling controls the expression of Distal-less (Dll)
within the wing blade. It was reasoned that overexpression of Dly might
activate Wg signaling because dly dsRNA-injected embryos
resemble those that have lost Wg activity. Interestingly,
overexpression of dly (using the C96-Gal4 driver), which is highly
expressed at the DV boundary of the wing disc, results in
severe wing margin defects and loss of sensory bristles. These phenotypes are reminiscent of the phenotypes seen
when Wg activity is reduced in the wing.
Consistent with the adult wing phenotype, Ac expression is
dramatically decreased in wing discs overexpressing Dly. Furthermore, when dly is overexpressed using the engrailed-Gal4 (en-Gal4) driver, the expression of Dll is reduced in the posterior compartment (Baeg, 2001).
A test was performed to see whether overexpression of transducers of the
Wg signal can rescue the loss-of-function wg-like phenotypes
associated with dly overexpression. Ectopic expression of
either Wg or a gain-of-function Armadillo (Arm) can rescue
the wing margin defects, and induced ectopic bristles that
are characteristic of ectopic expression of the Wg pathway.
Altogether, these results suggest that overexpression of Dly blocks Wg signaling in the wing disc, and that of Dly acts upstream of Arm (Baeg, 2001).
Since Dly is an extracellular GPI-linked molecule, it was
reasoned that patterning defects associated with Dly
overexpression might reflect the ability of Dly to sequester
Wg, and thus prevent it from accessing and activating Frizzled 2. To
visualize the effect of overexpressed Dly on Wg distribution,
Dly was overexpressed using various Gal4 lines and
the wing discs were stained with anti-Wg monoclonal antibodies. Two different staining protocols were used to detect Wg distribution. The
first one, involved fixing the tissue before staining, and thus it
detects mostly cytoplasmic Wg present in either secretory or
internalized vesicles. In the second protocol, the tissue is
incubated with the antibody prior to fixation and mostly detects
extracellular Wg (Baeg, 2001).
Using the first protocol, in wild-type discs, Wg protein is
found at high levels in a narrow stripe of three to five
cells straddling the DV boundary. Following
overexpression of Dly using either en-Gal4 or ptc-Gal4 a
striking increased accumulation of Wg protein is observed. Using the second protocol, extracellular Wg is organized in a gradient at the basolateral surface of wg-expressing and nearby cells. Following overexpression of Dly,
an increased accumulation of extracellular Wg
protein is detected. This result indicates that Dly can affect extracellular
Wg distribution (Baeg, 2001).
HSPGs are required to increase the local concentration of Wg ligand for its receptors. Considering the roles of Dally and Dly, there are at least two HSPGs at the cell surface of wing disc cells involved in Wg signaling. To
generate mutant cells that lack all GAGs and determine the role
of the HSPGs in Wg signaling, mutant clones of
cells were generated that do not properly synthesize GAGs. sugarless (sgl) mutations cannot be used for this analysis because sgl acts in a cell non-autonomous manner, presumably because the
enzyme synthesizes glucuronic acid that diffuses between
cells. However, sulfateless (sfl) is involved in GAG modification and in its absence, the GAG chains are not synthesized properly. To
determine whether the GAG chains of Dly are modified by
sfl, Dly proteins expressed in wild-type or sfl
mutant pupae were examined by Western blot analysis using anti-Dly
antibodies. The predicted size for Dly is 80 kDa, and
in wild-type pupae, Dly protein appears as a broad band that
migrates to around 80-110 kDa, which presumably results from
addition of GAG chains onto the core protein. In sfl mutant
pupae, the modified form of Dly protein is significantly
reduced and the sharp band of the core protein is increased.
This results indicate that sfl plays a role in Dly modification.
Using the staining protocol that detects cytoplasmic Wg, an alteration in the expression of intracellular Wg could not be detected in sfl mutant clones, indicating that sfl mutant cells
normally transcribe wg and do not accumulate Wg (Baeg, 2001).
However, using the extracellular staining method, a dramatic
decrease of extracellular Wg was detected in sfl mutant cells. Extracellular Wg has been shown to be mainly
associated with the basolateral surface of cells, and GPI-anchored
protein is thought to be primarily attached to the
basal part of the cells. Together, these results suggest that the
HSPGs are involved in extracellular Wg accumulation (Baeg, 2001).
Importantly, high accumulation of extracellular Wg can be
detected on sfl mutant cells located adjacent to wild-type cells, suggesting that HSPGs act locally in a cell non-autonomous
manner. Consistent with this observation, adult
wing patterning in sfl mutant clones show local cell non-autonomy
as well. Clones of sfl mutant cells are associated
with wing margin defects, suggesting that sfl is required for
Wg signaling, however some of the sfl mutant cells located
near wild-type margin cells have a wild-type morphology (Baeg, 2001).
Taken together, these results suggest that HSPGs are involved
in restricting Wg diffusion. Further, the local cell non-autonomy
observed in sfl mutant clones may indicate that
HSPGs may not be absolutely required for the binding of Wg
to its transducing receptor(s) (Baeg, 2001).
Classical genetic screens can be limited by the selectivity of mutational targeting, the complexities of anatomically based phenotypic analysis, or difficulties in subsequent gene identification. Focusing on signaling response to the secreted morphogen Hedgehog (Hh), RNA interference (RNAi) and a quantitative cultured cell assay were used to systematically screen functional roles of all kinases and phosphatases, and subsequently 43% of predicted Drosophila genes. Two gene products reported to function in Wingless (Wg) signaling were identified as Hh pathway components: a cell surface protein (Dally-like protein) required for Hh signal reception, and casein kinase 1alpha, a candidate tumor suppressor that regulates basal activities of both Hh and Wg pathways. This type of cultured cell-based functional genomics approach may be useful in the systematic analysis of other biological processes (Lum, 2002).
The expression of the P1 enhancer trap line, containing a P1 insertion under dally regulation, was examined through the division cycles of lamina precursor cells (LPCs) in third instar larval brains using anti-beta-gal antibody and digoxigenin-labelled DNA probes complementary to lacZ mRNA. The highest levels of beta-gal immunoreactivity are found in LPCs along the anterior segment of the lamina furrow. Cells in this region are in G2 and M phase of the first division. The lacZ mRNA shows a more limited distribution, presumably because of beta-gal protein perdurance and is restricted to the G2 and M phase domains of LPC division one. Cell cycle-dependent expression of enhancer trap insertions in this locus was obtained by staining third instar larval brains with both anti-beta-gal and anti-cyclin B antibodies. There is an overlap of expression of the enhancer trap insertion with cyclin B in several groups of dividing cells, including the inner proliferative center (IPC), suggesting that the cell cycle-restricted expression is not limited to LPCs. However, some cells of the central brain complex express the enhancer trap marker but do not show high levels of cyclin B immunoreactivity (Nakato, 1995).
Larvae homozygous for the P1 insertion which disrupts dally expression shows disorganization of the anterior segment of the outer proliferative center (aOPC)/lamina precursor cell (aOPC/LPC) epithelium in approximately 10% of the CNS preparations examined. Disordering of the eye and reductions or duplications of the antenna, also with low penetrance, are found in P1 homozygous adults. The low penetrance found in the P1 mutant made analysis of the cell division defect difficult. A search was carried out for more severe alleles within existing collections of P-element-induced mutants. One semi-lethal enhancer trap insertion in the region of 66D/E, sl(3)06464, shows a beta-gal-staining pattern in the larval brain like that of line P1. Homozygous sl(3)06464 adults exhibit abnormalities in several adult tissues, including reductions or complete loss of genitalia, disordering and reduction in the number of ommatidia, reductions and duplications of the antenna, and incomplete wing vein V and wing notching. P1 and sl(3)06464 enhancer trap expression patterns in other tissues (antenna, eye, leg and wing discs) and embryonic developmental stages were also coincident. The two P-element alleles are henceforth referred to as dally P1 and dally P2. Genetic complementation tests were performed with dally P1 and dally P2 alleles for the antennal phenotypes since these are easy to score and of fairly high penetrance in dally P1. For antennal defects, dally P1 and dally P2 fail to complement, supporting the conclusion that these two P-element insertions affect the same gene. To confirm that dally P1 and dally P2 are responsible for the phenotypes described above and that these phenotypes are caused by loss-of-function alleles, the P-element insert in dally P1 was mobilized. The imprecise excision class creates small deletions that can potentially effect the removal, either completely or partially, of the normal function of the targeted gene. 23 independent excision alleles failed to complement the adult phenotypes of dally P2 . These alleles, either as homozygotes or in combination with dally P2 show phenotypes with a range of expressivity and penetrance; the more severe mutants affect the same tissues and to the same degree as observed for dally P2 homozygous adults. For example, dally DP-188 shows abnormalities in the eye, antenna, genitalia and wing, similar to dally P2 (Nakato, 1995).
In the wild-type eye disc, the morphogenetic furrow (MF) serves as an anatomical marker for the assembly of the repeated sensory units, the ommatidia. As originally described, the MF is a broad indentation in the eye disc epithelium, which moves from the posterior to the anterior, marking the wave of differentiation that sweeps forward. Cells in different parts of the cell cycle occupy specific positions relative to the MF. Anti-cyclin B and propidium iodide staining were used to identify G2, and mitotic prophase, metaphase, anaphase and telophase cells in the eye disc. Cell division in the developing eye disc epithelium anterior to the MF is asynchronous as seen by the unpatterned cyclin B expression. However, an increase in the level of anti-cyclin B immunoreactivity anterior to the MF is observed, providing evidence of cell cycle synchrony beginning in G2 as cells approach the furrow. Immediately posterior to this G2 domain mitosis occurs. Mitotic cells in metaphase, anaphase and telophase are found immediately ahead of the MF. This mitotic domain reflects the cell cycle synchronization taking place just ahead of the MF. Cells complete mitosis as they enter the furrow, and become synchronized in G1 in the MF. Progression into the subsequent S phase takes place within the furrow, followed by G2. A second coordinate mitosis follows, completing the two division cycles distributed across the MF. In dally mutants, the first zone of cyclin B expression, which normally ends approximately 3-5 cell dimensions anterior to the MF, extends too far posterior, to the beginning of the MF. The M phase of this first division is also displaced toward the posterior in dally mutants. The mitotic cells found at the edge of the MF in dally P2 mutants are in earlier stages of mitosis compared to wild-type (Nakato, 1995).
The second division along the eye disc MF does take place in dally P2 mutants. The cell division defects observed for dally P2 are also found in two other dally alleles, both as homozygotes and in combination with dally P2. As is the case for the lamina, the cell division defects in the eye disc are found without an overall disruption of normal morphology. In the wild type, the neuronal marker Elav is expressed in the assembling ommatidia posterior to the MF. dally mutant discs show this pattern of Elav expression as well, indicating that the cell division defects are not secondary to a gross disruption of eye development. If dally mutations delay cell division, the time required for cells labelled in the S phase of the first division to traverse into the following G1 should be slower in dally mutants, when compared to heterozygous control larvae. In the wild-type eye disc, mitotic cells of the first division are found at the apical surface of the epithelium in front of the MF and, following M phase, nuclei migrate basally into the MF. The position of a nucleus labelled in the previous S phase can therefore be used as a measure of cell cycle progression -- the more basally located cells being further along in their progression through the cell cycle. In dally mutants 4 hours after pulse-labeling, nuclei are delayed in their descent into the furrow, as compared to labelled cells from the heterozygous control. These findings support the conclusion that the first division cycle is delayed as a consequence of compromised dally function (Nakato, 1995).
The DPP requirement for cell fate specification and cell cycle synchronization in the developing Drosophila eye was examined by determining whether cells defective for thickveins, saxophone or schnurri show abnormalities in cell division or differentiation. Clones mutant for a null allele of tkv that are anterior or posterior to the morphogenetic furrow have amounts of Cyclin B that are indistinguishable from those in surrounding cells. In contrast, tkv clones that span the MF maintain cyclin B expression in the anterior part of the furrow, even though the surrounding cells arrested in G1 have no detectable Cyclin B. Maintenance of cyclin B expression is thought to indicate a failure of cell cycle progression, as Cyclin B levels decline in M phase. Mitotic figures are not observed in clones in the anterior half of the MF. The phenotype observed in the clones is similar to defects caused by mutations in division abnormally delayed (dally), which is required for G2-M progression ahead of the furrow. Mutations in dally and dpp display genetic interactions in development of the eye, antenna, and genitalia, which suggests that dally augments Dpp function. The behavior of Dpp-receptor mutant clones supports a role for Dpp in controlling progression through G2-M as a means of synchronizing the divisions that accompany differentiation of the eye disc. Cell fate, however, is unaffected by receptor mutation, as revealed by the expression of atonal, a proneural gene required for retinal precursor cell 8 (R8) determination. Because atonal expression is maintained in tkv clones, hh must not act through dpp to induce its expression, and thus dpp mediates a subset of hh functions in the MF (Penton, 1997).
Wingless is a member of the Wnt family of growth factors, secreted proteins that control proliferation and differentiation during development. Studies in Drosophila have shown that responses to Wg require cell-surface heparan sulphate, a glycosaminoglycan component of proteoglycans. These findings suggest that a cell-surface proteoglycan is a component of a Wg/Wnt receptor complex. The protein encoded by the division abnormally delayed (dally) gene is a cell-surface, heparan-sulphate-modified proteoglycan. dally partial loss-of-function mutations compromise Wg-directed events, and disruption of dally function with RNA interference produces phenotypes comparable to those found with RNA interference of wg or frizzled/Dfz2. Ectopic expression of Dally potentiates Wg signalling without altering levels of Wg and can rescue a wg partial loss-of-function mutant. Dally, a regulator of Decapentaplegic (Dpp) signalling during post-embryonic development, has tissue-specific effects on Wg and Dpp signalling. Dally can therefore differentially influence signalling mediated by two growth factors, and may form a regulatory component of both Wg and Dpp receptor complexes (Tsuda, 1999).
In Drosophila, imaginal wing discs, Wg and Dpp, play important roles in the development of sensory organs. These secreted
growth factors govern the positions of sensory bristles by regulating the expression of achaete-scute (ac-sc), genes affecting
neuronal precursor cell identity. Earlier studies have shown that Dally, an integral membrane, heparan sulfate-modified
proteoglycan, affects both Wg and Dpp signaling in a tissue-specific manner. dally is required for the development of specific chemosensory and mechanosensory organs in the wing and notum. dally enhancer trap is expressed at the anteroposterior and dorsoventral boundaries of the wing pouch, under the control of hh and wg, respectively. dally
affects the specification of proneural clusters for dally-sensitive bristles and shows genetic interactions with either wg or
dpp signaling components for distinct sensory bristles. These findings suggest that dally can differentially regulate Wg- or
Dpp-directed patterning during sensory organ assembly. For pSA, a bristle on the lateral notum, dally shows genetic interactions with iroquois complex (IRO-C), a gene complex affecting ac-sc expression. Consistent with this interaction, dally mutants show markedly reduced expression of an iro::lacZ reporter. These findings
establish dally as an important regulator of sensory organ formation via Wg- and Dpp-mediated specification of proneural clusters (Fujise, 2001).
dally enhancer trap expression in the wing
disc was examined. This study revealed expression at the A/P and
D/V boundaries in the wing pouch. At mid- to late-third
larval instar, the expression of dally enhancer trap overlaps
with that of ac-sc at the D/V boundary, indicating that
dally is expressed at high levels in cells flanking the cut- and
wg-expressing edge cells like Delta and Serrate. N signaling in cells of the D/V boundary results in the transcriptional activation of several genes, including vg, wg, ct, and members of the Enhancer of split complex,
E(Spl). Like other genes affecting
assembly of wing margin structures, dally expression of the
D/V boundary is sensitive to N-receptor activation mediated
by Dl and Ser. Blocking Wg signaling at the D/V border
cells can repress dally expression, indicating that dally
expression along the margin is positively regulated by Wg
signaling, which is also required for the expression of
proneural AS-C genes. Further studies are required, however, to determine
whether N also has a direct function in the regulation
of dally expression (Fujise, 2001).
dally cooperates with Wg to form the wing margin structures. Ac expression is severely decreased in dally mutants, supporting the idea that dally serves as a component of the Wg receptor complex to induce AS-C expression at the prospective wing margin. Taken together, these observations indicate that dally is a target gene of Wg signaling pathway, and at the same time, it mediates the same signaling, suggesting that dally is involved in a positive feedback loop of Wg signaling at the D/V boundary of the wing pouch. Frizzled-2, the Wg receptor, is down-regulated by Wg signaling at the D/V boundary. Dally, a putative Wg coreceptor, may also participate in the feedback circuits of Wg signaling, as has been suggested for Fz2 (Fujise, 2001).
dally is required for the development of sensory organs in the adult wing and notum. dally has been also identified as a gene that affects
the development of sensory organs by a gain-of-function screen. dally mutants show reduced numbers of sensory bristles and campaniform sensilla
at the wing margin. Specific macrochaetae on the
notum, DC, pSA, and pPA, are affected in dally mutants. In
all cases, the expression of genes specific for proneural cell
differentiation is compromised in dally-sensitive bristles,
indicating that dally affects sensory organ development at
the step of prepatterning (Fujise, 2001).
Wg and Dpp have been shown to affect prepatterning of
sensory organs by governing the expression of proneural
genes, such as ac-sc. dally has been shown to affect the
signaling levels of either Wg or Dpp. Therefore, an examination was made to determine whether dally affects sensory organ formation via either Wg or Dpp signaling pathways. Genetic experiments provided evidence that, in the prospective
notum region of the wing disc, dally selectively influences
Wg signaling to form the pPA bristle and Dpp signaling to
form the pSA and DC bristles. It is particularly intriguing
that, during development of DC macrochaetae, dally genetically
interacts with only Dpp signaling, while the formation
of these bristles requires both Wg and Dpp activities. It has been indicated that the A/P coordinates of the DC cluster are limited by Dpp signaling. In dally homozygous wing discs, the DC cluster is apparently
shorter in the A/P coordinates compared with wild-type
discs, suggesting that dally regulates Dpp signaling activity
to limit the A/P length of the DC cluster. What are the
mechanisms that can account for the selective interactions
of dally and specific growth factor signaling? One obvious
interpretation of genetic experiments on DC macrochaetae
is that differences in dose effects between dpp and wg are
responsible for the apparent specificity. It is also possible
that the ligand-specificity of Dally is controlled at the
cellular level through modification of heparan sulfate structures (Fujise, 2001).
Dally, Dpp, and IRO-C genetically interact with each other during the formation of the pSA macrochaete. Although interactions between Dpp signaling
and IRO-C have been suggested, evidence is provided that Dpp signaling components interact with the genes of IRO-C. Ectopic Dpp signaling using a
constitutively active type I receptor, tkv, leads to an ectopic
induction of the pSA macrochaete, supporting the idea that Dpp signaling is required for prepatterning for this bristle. Significant
reductions in the expression of iro enhancer trap is observed in
dally mutant wing discs. Expression of the iro at the lateral
notum region is critical for the proneural cluster formation
and bristle development in this region. Taken together, these findings
suggest that dally mediates Dpp signaling to control expression
of the genes of IRO-C during the formation of the pSA bristle (Fujise, 2001).
The signalling molecule Hedgehog (Hh) functions as a morphogen to pattern a field of cells in animal development. Previous studies in Drosophila have demonstrated that Tout-velu (Ttv), a heparan sulphate polymerase, is required for Hh movement across receiving cells. However, the molecular mechanism of Ttv- mediated Hh movement is poorly defined. Dally and Dally-like (Dly), two Drosophila glypican members of the heparan sulphate proteoglycan (HSPG) family, are shown to be the substrates of Ttv and are essential for Hh movement. Embryos lacking dly activity exhibit defects in Hh distribution and its subsequent signalling. However, both Dally and Dly are involved and are functionally redundant in Hh movement during wing development. Hh movement in its receiving cells is regulated by a cell-to-cell mechanism that is independent of dynamin-mediated endocytosis. It is proposed that glypicans transfer Hh along the cell membrane to pattern a field of cells (Han, 2004).
To dissect the molecular mechanism(s) by which HSPG(s) regulates Hh
signalling, attempts were made to identify specific proteoglycan(s) involved in
Hh signalling during embryonic patterning. During embryogenesis, Hh and
Wingless (Wg) are expressed in adjacent cells and are required for patterning
of epidermis. In stage 10 embryos, Hh is expressed in two rows of cells
in the posterior compartment of each parasegment, while Wg is expressed in one
row of cells anterior to Hh expression cells. The expression of Hh is
controlled by Engrailed (En) whose expression is maintained by Wg signalling
through a paracrine regulatory loop. Hh signalling in turn is required for
maintaining the expression of wg whose activity controls the
production of the naked cuticles. Loss of either Hh or Wg signalling leads to a
loss of naked cuticle, which is defined as segment polarity phenotype (Han, 2004 and references therein).
Disruption of dly in embryos by RNA interference (RNAi) leads to a strong segment polarity defect, suggesting that Dly is likely to be involved in Hh and/or Wingless (Wg) signalling in embryonic epidermis. To explore the potential role of Dly in Hh signalling, a number of dly mutant alleles were isolated using EMS mutagenesis. dlyA187 is a null allele and is used for further analyses. Animals zygotically mutant for
dly appears to have normal cuticle patterning and
survive until third instar larvae. However, homozygous mutant embryos derived
from females lacking maternal dly activity (referred to as
dly embryos hereafter) die with a strong segment-polarity phenotype resembling those of mutants of the segment polarity genes hh and wg. In dly embryos, both En expression and wg
transcription fade by stage 10, suggesting further that dly is involved in the Hh and/or Wg pathways (Han, 2004).
To further determine whether Dly activity is required for Hh signalling in
embryogenesis, Hh signalling activity was examined in dly embryos
during mesoderm development. Hh and Wg signalling have distinct roles in
patterning embryonic mesoderm. Hh signalling activates the expression of a
mesodermal specific gene bagpipe (bap) in the anterior
region of each parasegment, whereas Wg signalling inhibits bap
expression in the posterior region. bap expression is diminished in the
hh mutant, but is expanded to the posterior parasegment in the
wg mutant. Consistent with a role of Dly in Hh signalling, it was found
that bap expression was strikingly reduced in dly embryos. Together with the segment polarity phenotype, these results strongly argue that Dly is required for Hh signalling during embryogenesis (Han, 2004).
The role of Dly in Hh signalling was further examined during wing
development in which Hh and Wg signalling function independently of each
other. In the wing disc, Hh signalling induces the expression of its target
genes in a narrow stripe of tissue in the A compartment abutting the AP
boundary. Hh signalling patterns the central domain of wing blade and controls
the positioning of longitudinal veins L3 and L4. The roles of Dly in Hh signalling were examined by analyzing adult wing
defects using 'directed mosaic' technique. Surprisingly, no
detectable phenotypes were observed in adult wings bearing dly mutant clones. It was reasoned that Hh signalling may be mediated by other HSPGs in
the wing. One candidate is the glypican dally that has
been shown to be involved in Wg and Dpp
signalling. Because available dally alleles used previously
were hypomorphic, several dally null alleles were generated by
P-element mediated mutagenesis. dally80 is a null allele
and was used for analysis. However, similar to other dally alleles, homozygous dally80animals are viable. The wing bearing
dally80 clones exhibits a partial loss of the L5 vein with
a high penetrance, but no detectable defects in the central domain of wing
blade. To determine whether dally and dly have overlapping roles in Hh
signalling in wing development, clones mutant for both
dally80 and dlyA187 (referred as
dally-dly hereafter) were generated. Interestingly, the adult wings bearing clones
mutant for dally-dly show L3-L4 fusion. This phenotype is
typical of loss of Hh function, suggesting that Dally and Dly play redundant
roles in Hh signalling in wing development (Han, 2004).
This study demonstrates that Dly is the main HSPG involved in Hh
signalling during embryogenesis, at least in epidermis and mesoderm, the two
tissues that were carefully examined. Three lines of evidence strongly support
this conclusion. (1) Embryos lacking both maternal and zygotic dly
activities develop a strong segment polarity defect and exhibit diminished
expression of En and Wg. (2) Hh can be detected as punctate
particles at least one cell diameter from its producing cells and these
punctate particles are absent in dly-null embryos. (3)
A reduced expression of bap was observed in dly mutant embryos, a phenotype specifically attributed to the Hh signalling rather than Wg signalling defect. Previously, it was shown that the punctate particles of Hh staining are absent in ttv null embryos. The formation of such
Hh staining particles, referred to as large punctate structures (LPS), requires
cholesterol modification, and movement of these large
punctate structures across cells is dependent on Ttv activity. The current
results are consistent with these observations and suggest that Dly is the
main HSPG involved in the movement of these LPS across cells. It is
conceivable that the punctate particles of Hh staining that were observed may
represent Hh-Dly complexes. In this regard, Dly may either prevent secreted Hh
from being degraded and/or facilitate Hh movement from its expression cells to
adjacent receiving cells. These two mechanisms are not mutually exclusive. In
the absence of Dly function, secreted Hh is either degraded or fails to move
to the adjacent cells (Han, 2004).
In addition to dly, three other HSPGs, including Dally, Dsyndecan
and Trol, are also expressed in various tissues during embryogenesis. In
particular, dally is expressed in epidermis and has been shown to be
involved in Wg signalling. Removal of Dally activity in embryos either by
dally hypomorphic mutants or by RNA interference (RNAi) generates
denticle fusions. Further studies demonstrate that the cuticle defect
associated with dally embryos by RNAi is weaker than that of
dly. The results in this study suggest that Dly plays more
profound roles in embryonic patterning than Dally. It remains to be determined
whether Dally and other two Drosophila HSPGs are involved in Hh
signalling in other developmental processes during embryogenesis (Han, 2004).
Dally and Dly are involved and are redundant in
Hh signalling in the wing disc. Consistent with this, the
GAG chains of Dally and Dly are shown to be altered in the absence of Ttv activity, suggesting that both Dally and Dly are indeed the substrates for Ttv. Redundant roles of cell membrane proteins have been demonstrated in many other signalling systems. For example, both Frizzled (Fz) and Drosophila Frizzled 2 (Fz2) are redundant receptors for Wg,
although Fz2 has relative high affinity in binding to Wg protein. Dly
protein is distributed throughout the entire wing disc.
Previous studies have demonstrated that dally is highly expressed at the
AP border. Interestingly, Dally expression at the AP border is
overlapped with the ptc expression domain and is under the control of
Hh signalling. It is likely that both Dally and Dly are capable of binding to Hh and facilitating the movement of the Hh protein. In the absence of one of them, another member is probably sufficient to facilitate Hh movement (Han, 2004).
dally-dly double mutant clones have relatively
weaker defects in Hh signalling in the wing disc than those of the
ttv and sfl mutants. One possible explanation is the
perdurance of Dally and Dly proteins. Alternatively, two other HSPGs,
Dsyndecan and Trol, may also participate in Hh signalling in the absence of
Dally and Dly in the wing disc. These issues remain to be examined using both
dsyndecan and trol null mutants (Han, 2004).
Do HSPGs act as co-receptors in Hh signal transduction? Hh is a
heparin-binding protein and is likely to interact with HSPGs through their HS
GAG chains. In support of this, Dly was shown to colocalize with Hh punctate
particles. It is
conceivable that Dally and Dly could either transfer Hh to its receptor Ptc or
form a Hh-Dally/Dly-Ptc ternary complex in which Dally and Dly may function to
facilitate Hh-Ptc interaction or stabilize a Hh-Ptc complex. In this regard,
Dally and Dly may function both in transporting Hh protein and acting as
co-receptors in Hh signalling. Consistent with this view, a recent report using
RNAi in tissue culture based assays identified Dly as a new component of the
Hh pathway (Lum, 2003).
It was shown that Dly plays a cell-autonomous role upstream or at the level of
Ptc in activating the expression of Hh responsive-reporter, suggesting a role
of Dly in the delivery of Hh to Ptc (Han, 2004).
It is important to note that some of results obtained from tissue culture
based assays (Lum, 2003) are not consistent with in vivo results reported in this study as well as previous studies on Ttv. Cl-8 cells were originally derived from the wing disc. However, it was found that removal of dly activity alone has no detectable effect on Hh signalling in the wing disc. This apparent discrepancy
may due to several factors: (1) Hh-N, instead of Hh-Np was used as a source
for Hh in their work; (2) Cl-8 cell may have altered the proteoglycan
expression pattern, which can be significantly different from Hh-responding
wing cells in which Dally expression is upregulated by Hh signalling; (3) it is possible that Dly may have a higher capacity than Dally to bind
Hh, as in the case for Wg. In this regard, removal of Dly will probably lead to more profound effects than removal of other HSPGs on binding of Hh-N to the cell
surface, perhaps in the delivery of Hh-N to Ptc (Han, 2004).
Within sfl, or ttv or dally-dly
mutant clones, the posterior-most cells adjacent to wild-type cells are still
capable of transducing Hh signalling. It is most likely that Hh proteins bound by Dally and Dly in wild-type cells can directly interact with Ptc located on the cell surface of the adjacent mutant cells to transduce its signalling. In support of this
view, a Hh-CD2 membrane fusion
protein has the ability to activate Hh signalling in its adjacent cells.
Furthermore, studies on Dispatched (Disp), a novel sterol-sensing domain protein dedicated to the release of cholesterol-modified hedgehog from signaling cells, have shown that the first row of anterior cells
adjacent to posterior Hh-producing cells have significant Hh signalling
activity in disp mutant wing discx, in which Hh is retained on the
cell surface of Hh producing cells. Interestingly, Hh punctate particles were observed in the posterior-most HSPG mutant cells adjacent to wild-type cells. These Hh punctate particles are most likely intracellular Hh proteins internalized
through Ptc mediated endocytosis process. In this regard, HSPGs may not be
required for Ptc-mediated Hh internalization (Han, 2004).
Recent biochemical studies from vertebrate cells have shown that Shh-Np is
secreted from cells and can be readily detected in conditioned culture medium. It was
also shown that overexpression of Disp can increase the yield of Hh protein in
the culture medium. These experiments suggest that Hh can be directly secreted
from Hh expressing cells. Can secreted Hh proteins freely diffuse to Hh
receiving cells through extracellular spaces? To address this issue, detailed analyses for Hh signalling have been carried out in the complete absence of HS
GAG using sfl and ttv or absence of glypicans using
dally-dly. A narrow strip (one cell diameter in width)
of sulfateless (sfl) or ttv, or dally-dly mutant cells prevents
the transpassing of the Hh signal. Hh staining disappears in sfl
mutant clones, except at a residual level in the posterior-most row of cells. Based on these
observations, a model is favored in which Hh movement is regulated by a
cell-to-cell mechanism rather than by free diffusion (Han, 2004).
The results of this study further suggest that Hh movement is independent of
dynamin-mediated endocytosis, which has been shown to be involved in the
transportation of morphogen molecules such as Dpp and
Wg. A blockage of dynamin function does not
eliminate Hh movement and its subsequent signalling; instead, it leads to a
striking reduction of punctate particles of Hh staining and an accumulation of
cell-surface Hh protein. Expanded Ptc expression domain is observed when
dynamin-mediated endocytosis is blocked. These new findings
provide compelling evidence that dynamin-mediated endocytosis is not required
for Hh movement and its subsequent signalling, but is involved in Ptc-mediated
internalization of the Hh protein (Han, 2004).
Several mechanisms have been proposed to explain morphogen transport across
a field of cells. These mechanisms include (1) free diffusion, (2) active
transport by planar transcytosis, (3) cytonemes, (4) argosomes. The results of this study suggest that Hh moves through a cell-to-cell
mechanism rather than free diffusion. Furthermore,
dynamin-mediated endocytosis is unlikely to be involved in Hh movement. On the
basis of these findings, the following model is proposed by which the HSPGs Dally
and Dly may regulate the cell-to-cell movement of the Hh protein across a
field of cells. In this model, Hh is released by Disp from its producing cells and is
immediately captured by the GAG chains of glypicans on the cell surface. The
differential concentration of Hh proteins on the surface of producing cells
and receiving cells drives the unidirectional displacement of Hh from one GAG
chain to another towards more distant receiving cells. Within the same cell,
the transport of Hh may be facilitated by the lateral movement of glypicans on
the cell membrane. On the receiving cells, glypicans may present Hh to Ptc,
which then mediates the internalization of Hh. Glypican mutant cells can not
relay Hh proteins further because they lack HS GAG on the surface. However, they
are able to respond to the Hh signal because Ptc may contact the Hh on the
membrane of the adjacent wild-type cells. Further studies are needed to
determine whether other mechanism(s) including cytonemes and argosomes are
also involved in Hh movement (Han, 2004).
Heparan sulfate proteoglycans (HSPGs), a class of glycosaminoglycan-modified proteins, control diverse patterning events via their regulation of growth-factor signaling and morphogen distribution. In C. elegans, zebrafish, and the mouse, heparan sulfate (HS) biosynthesis is required for normal axon guidance, and mutations affecting Syndecan (Sdc), a transmembrane HSPG, disrupt axon guidance in Drosophila embryos. Glypicans, a family of glycosylphosphatidylinositol (GPI)-linked HSPGs, are expressed on axons and growth cones in vertebrates, but their role in axon guidance has not been determined. This study demonstrates that the Drosophila glypican Dally-like protein (Dlp) is required for proper axon guidance and visual-system function. Mosaic studies reveal that Dlp is necessary in both the retina and the brain for different aspects of visual-system assembly. Sdc mutants also show axon guidance and visual-system defects, some that overlap with dlp and others that are unique. dlp+ transgenes are able to rescue some sdc visual-system phenotypes, but sdc+ transgenes are ineffective in rescuing dlp abnormalities. Together, these findings suggest that in some contexts HS chains provide the biologically critical component, whereas in others the structure of the protein core is also essential (Rawson, 2005).
The distribution of Dlp was examined in the developing visual system by using a monoclonal antibody that specifically recognizes Dlp in tissues. In Drosophila, the adult eye is comprised of approximately 800 sensory units, or ommatidia, each with eight distinct photoreceptors, R1-R8. In the eye imaginal disc, Dlp was found on photoreceptor cell bodies and on cells within the morphogenetic furrow. Axons from R1-R6 terminate in the lamina, the first optic ganglion, whereas those from R7 and R8 project to the medulla. In the optic lobe, Dlp is present on photoreceptor axons at the boundary between the lamina and adjacent tissues and along the lamina plexus. Dlp is also observed in the medulla neuropil, medulla glia, medulla neuropil glia, neuroblasts of the proliferative centers, and in the mushroom body neuropil (Rawson, 2005).
To evaluate the function of Dlp in axon guidance, a photoreceptor-specific monoclonal antibody (24B10) was used to visualize photoreceptor projections in dlp mutants. In 50% of dlp mutant hemispheres, the lamina plexus was irregular and thickened. Additionally, 80% of dlp mutant larvae had fibers that aberrantly crossed between ommatidial bundles and/or photoreceptor process expansions outside the normal termination zone of the lamina plexus. Examination of dlp mutant pupae revealed that 80% of optic lobes contain irregularities in the R7 and R8 medulla termini. Crossover of R7 axons to neighboring medulla cartridges was observed (~50%) and misrouting of R7/R8 axons (~20%) (Rawson, 2005).
Visual-system function was assessed in dlp mutants by recording electroretinogram (ERG) profiles in adult flies. A wild-type ERG is composed of the photoreceptor response generated by a light-induced depolarization of the photoreceptor neurons and of the two transient voltage changes, the 'on-transient' and the 'off-transient', resulting from currents related to synaptic transmission during the initiation and termination of the light stimulus. dlp mutants show statistically significant defects in the photoreceptor response and in both on- and off-transients, suggesting that Dlp is required for proper photoreceptor currents and synaptic transmission (Rawson, 2005).
Axon guidance in the visual system depends on photoreceptor specification, as well as on glial-cell migration and lamina-neuron differentiation. Although dlp mutants have reduced and roughened eyes, thin sections of dlp eyes demonstrate that all photoreceptors are present and retain proper polarity in each ommatidium. Staining with several photoreceptor-specific markers confirm that dlp mutant photoreceptors differentiate properly. Likewise, glial cells and lamina neurons were found in the correct number and location in dlp mutants, indicating that patterning defects of these critical cells cannot account for the observed axon-guidance defects (Rawson, 2005).
Because Dlp is expressed in several visual-system elements and cell types, somatic mosaic studies were conducted to determine which cells require dlp for visual-system assembly. Using a method that generates clones encompassing a majority of cells in the retina, crossover of axons between ommatidial bundles and photoreceptor process expansions outside the lamina plexus was observed in 67% of animals with dlp mutant photoreceptors projecting to a heterozygous brain. Conversely, R7/R8 termination defects were absent from the medulla of 40 hr pupae with dlp mutant retinas and dlp/+ optic lobes. These results indicate that Dlp is required in the eye to specify proper axon guidance to the lamina, but not to the medulla (Rawson, 2005).
The photoreceptor-specific requirement for Dlp was further evaluated via the mosaic analysis with a repressible cell marker (MARCM) technique to visualize axons from small dlp mutant clones. dlp mutant photoreceptors display ectopic axon outgrowths directed away from their proper targets in the lamina (37% of dlp/dlp axons). Ectopic axon processes were four times more prevalent on axon bundles near the boundaries of the lamina than on those in more-central regions. This suggests that expression of Dlp on photoreceptors may be important for the detection of repellant cues that prevent aberrant axon outgrowth (Rawson, 2005).
Finally, the tissue-specific requirement was evaluated for Dlp in physiological function of the visual system. Using a mosaic strategy that generated eyes composed solely of dlp mutant photoreceptors, no see statistically significant defects were seen in ERG recordings compared to controls. These results demonstrate that the ERG defects of dlp mutants are produced by loss-of-function in the optic lobe, not in the eye (Rawson, 2005).
Drosophila Syndecan (Sdc) represents another class of HSPG, and it is required for normal axon guidance in the embryo. Antibody specific for Sdc revealed that this proteoglycan is present on photoreceptors in the retina and on photoreceptor projections to the optic lobe. Like Dlp, Sdc is enriched throughout the lamina plexus and at the boundary between the lamina and adjacent tissues. Sdc immunoreactivity was also detectable at a low level on cells just medial to the lamina plexus, the medulla glia, but was absent from some Dlp-expressing cells such as the mushroom body and the medulla neuropil glia (Rawson, 2005).
Analysis of sdc null mutant larvae revealed that 50% of optic lobes had photoreceptor-projection abnormalities and/or lamina-plexus defects including gaps (29%) and gross disorganization (21%, n = 28). Compared to dlp mutants, sdc mutants showed a low penetrance of the lamina-thickening (4%) and lamina-axon-crossover phenotypes. Additionally, some sdc mutants had R7/R8-axon misrouting in the larval stage. sdc mutant pupae showed crossover of R7 axons between medullary cartridges (100%) and defective axon pathfinding to the medulla (86%) but a low penetrance of R7/R8-termini disruption (~10%), a phenotype common in dlp mutants (80%-100%). As is the case for dlp, sdc mutants do not show defects in the specification of photoreceptors, glia, or lamina neurons, confirming that the above phenotypes are not secondary to other overt patterning deficiencies. Overall, dlp and sdc mutants share similar axon-guidance phenotypes, but each has a largely distinct level of penetrance for a given defect (Rawson, 2005).
Electrophysiological analysis revealed that sdc mutants have grossly abnormal ERGs, with defective photoreceptor depolarization and complete absence of on- and off-transients. These ERG abnormalities -- particularly the virtual loss of photoreceptor depolarization -- are distinct from those found in dlp animals. In addition, whereas dlp null mutants have a reduced and roughened eye, sdc mutants do not. These findings are consistent with distinct molecular functions for Dlp and Sdc during visual-system assembly (Rawson, 2005).
Sdc and Dlp are both heparan sulfate-modified proteoglycans, and Dlp expression is capable of limited rescue of sdc axon-guidance abnormalities in the embryo. To determine whether these two cell-surface molecules have overlapping functions in the visual system, a series of rescue experiments was performed. With or without a GAL4 driver, dlp mutant animals bearing a UAS-dlp+ construct show complete rescue of pupal axon-pathfinding defects and ERG abnormalities, suggesting that low levels of Dlp expression are sufficient to provide the function necessary for normal axon guidance and visual-system assembly. Despite this, ubiquitous expression of Sdc was unable to rescue the axon-guidance abnormalities of dlp mutants, demonstrating that Dlp and Sdc are not functionally interchangeable during photoreceptor axon guidance (Rawson, 2005).
The ability of UAS-dlp+ transgenes to rescue axon-guidance phenotypes of sdc mutant larvae and pupae was tested. Pupal sdc mutant phenotypes, including misrouting of R7/R8 photoreceptors to the medulla and crossover of R7 to neighboring medulla cartridges, are largely rescued with neuronal-directed expression of Sdc. Conversely, neuron-specific expression of Sdc in sdc mutants is not sufficient to rescue photoreceptor projection defects to the larval lamina or the ERG abnormalities in adults, suggesting that expression of Sdc in additional cell types is critical for complete restoration of axonal patterning and visual-system function. In contrast to the inability of Sdc expression to rescue dlp mutant phenotypes, neuron-specific expression of Dlp restores proper R7/R8 photoreceptor projection to the medulla of sdc mutant pupae. This rescue demonstrates that a structurally distinct HSPG can provide some of the functions normally served by Sdc (Rawson, 2005).
In vertebrates, glypicans are expressed on axons and growth cones in the developing nervous system. Whereas previous findings have established a function for glypicans in growth-factor signaling, their role in axon guidance has not been reported. Consistent with the expression pattern of Dlp on axons and glial cells of the developing visual system, dlp mutants show defects in photoreceptor projections to the lamina and medulla. Mosaic analysis demonstrates that Dlp is required on photoreceptors for some, but not all, aspects of axon guidance, serving functions in both peripheral and central components of the visual system. dlp is not required in the retina for normal electrophysiological responses to light, indicating that the synaptic-transmission defects in dlp mutant adults are due to loss of Dlp activity in the optic lobes (Rawson, 2005).
Comparison of sdc and dlp mutants as well as transgene rescue experiments demonstrates that Sdc and Dlp have some overlapping functions in visual-system assembly but others that are unique. For example, photoreceptor misrouting to the medulla in sdc mutants can be rescued by neuron-specific expression of dlp+, consistent with the capacity of dlp+ to rescue midline-crossover defects in sdc mutant embryos. In contrast to the ability of dlp+ transgenes to rescue sdc mutants, UAS-sdc+ constructs are unable to rescue any of the dlp mutant phenotypes, even under conditions where very modest levels of Dlp rescues completely. These findings show that Dlp functions cannot be readily provided by another unrelated HSPG, and they argue that the conserved sequence elements of the Dlp core protein are critical for these unique functions (Rawson, 2005).
Effects of HSPGs on axon pathfinding have been considered principally in the context of classical axon-guidance pathways, Slit-Robo signaling in particular. Although there is genetic evidence from mouse, Drosophila, and C. elegans that HSPGs affect Slit-Robo signaling, there are many other possibilities. HSPGs have been extensively characterized as molecules that affect both morphogen signaling and distributions. Recent studies demonstrating that the classical morphogens Wnt, Hh, and BMP also play a bona fide role in axon guidance suggest the possibility that HSPGs govern axon guidance by affecting morphogen function or distributions during this process (Rawson, 2005).
The two glypicans Dally and Dally-like have been implicated in modulating the activity of Wingless, a member of the Wnt family of secreted glycoprotein. So far, the lack of null mutants has prevented a rigorous assessment of their roles. A small deletion was created in the two loci. Analysis of single and double mutant embryos suggests that both glypicans participate in normal Wingless function, although embryos lacking maternal and zygotic activity of both genes are still capable of transducing the signal from overexpressed Wingless. Genetic analysis of dally-like in wing imaginal discs leads to a model whereby, at the surface of any given cell of the epithelium, Dally-like captures Wingless but instead of presenting it to signalling receptors expressed in this cell, it passes it on to neighbouring cells, either for paracrine signalling or for further transport. In the absence of dally-like, short-range signalling is increased at the expense of long-range signalling (reported by the expression of the target gene distalless) while the reverse is caused by Dally-like overexpression. Thus, Dally-like acts as a gatekeeper, ensuring the sharing of Wingless among cells along the dorsoventral axis. This analysis suggests that the other glypican, Dally, could act as a classical co-receptor (Franch-Marro, 2005).
The fact that mutations in dally and dlp cause different phenotypes suggests that, although they both underpin Wingless function, these two glypicans could perform distinct activities. It is likely that both Dally and Dlp are able to capture Wingless at the surface of imaginal disc cells. From the point of view of a given cell in vivo, Wingless captured by Dally would be mostly destined for 'internal consumption', while Dlp-bound Wingless would be for export only. Subsequent long-range transport would occur by hopping from Dlp on one cell to Dlp on the next. Both glypicans would contribute to increasing the concentration of Wingless at the cell surface (Dally in cis and Dlp in trans). It is suggested that in the embryo too, Dlp and Dally help in the presentation and reception of Wingless, respectively. However, in this system, little Wingless transport takes place, maybe because release of Wingless from Dlp is not allowed. It is interesting that, in embryos, dlp is highly expressed in cells that secrete Wingless. Therefore, the role of Dlp would mainly be to ensure that plenty of Wingless is retained at the surface of Wingless-expressing cells thus allowing sustained short-range signalling. In both the embryonic and disc systems, the genetic redundancy between dally and dlp could be viewed as follows: reduction of capturing activity in dally mutants would be compensated by the 'presentation activity' of Dlp and vice versa. Further cell biological work will be needed to fully explore the specific activities of Dally and Dlp and also to discover how Wingless is transferred from one cell to another during transport, perhaps with the help of enzymes such as Notum/Wingful (Franch-Marro, 2005).
The Drosophila ovary is an excellent system with which to study germline stem cell (GSC) biology. Two or three female GSCs are maintained in a structure called a niche at the anterior tip of the ovary. The somatic niche cells surrounding the GSCs include terminal filament cells, cap cells and escort stem cells. Mounting evidence has demonstrated that BMP-like morphogens are the immediate upstream signals to promote GSC fate by preventing the expression of Bam, a key differentiation factor. In contrast to their morphogenic long-range action in imaginal epithelia, BMP molecules in the ovarian niche specify GSC fate at single-cell resolution. How this steep gradient of BMP response is achieved remains elusive. In this study, it was found that the glypican Dally is essential for maintaining GSC identity. Dally is highly expressed in cap cells. Cell-specific Dally-RNAi, mutant clonal analysis and cell-specific rescue of the GSC-loss phenotype suggest that Dally acts in the cap cells adjacent to the GSCs. It was confirmed that Dally facilitated BMP signaling in GSCs by examining its downstream targets in various dally mutants. Conversely, when Dally was overexpressed in somatic cells outside the niche, the number of GSC-like cells was increased apparently by expanding the pro-GSC microenvironment. Furthermore, in a genetic setting a BMP-sensitivity distinction was revealed between germline and somatic cells, namely that Dally is required for short-range BMP signaling in germline but not in somatic cells. It is proposed that Dally ensures high-level BMP signaling in the ovarian niche and thus female GSC determination (Guo, 2009).
To understand how a steep gradient of BMP response is established and thus determines cell fate at single-cell resolution in the GSC niche of Drosophila ovary, genetic approaches were taken to examine the role of the glypican Dally in the process. Based on the current data, a model is proposed of how the expression pattern of Dally shapes the BMP-signaling range and consequently determines distinct cell fates in the ovarian niche (Guo, 2009).
Female GSC fate requires high BMP signaling, which is provided in the ovarian niche. Dally is highly expressed in the cap cells that contact the GSCs. Cap-cell-localized and membrane-bound Dally either stabilizes/concentrates BMP molecules, or enhances BMP sensitivity to ensure that only the germ cells in contact with cap cells become GSC. Upon removal of Dally, BMP concentration at the niche or BMP sensitivity in the germ cells adjacent to cap cells dissipates, and GSCs cannot be maintained and subsequently differentiate. Conversely, when Dally is ectopically overexpressed in the escort cells posterior to the niche, BMP signaling or sensitivity increases in all germ cells encapsulated by these escort cells, and GSC-like cells accumulate in the germarium (Guo, 2009).
Consistent with this model, the secreted form of Dally expressed from cap cells caused GSC loss, possibly by competing with endogenous membrane-anchored Dally for binding with BMP molecules or by interfering with BMP signaling. Because secreted Dally expressed from somatic cells in addition to cap cells did not cause GSC expansion as the membrane-anchored Dally did, the evidence further supports the idea that Dally's function in the GSC niche depends on the cap-cell-specific expression and membrane anchoring (Guo, 2009).
When somatic cells displace the differentiating germ cells in the niche and become close to or in contact with cap cells where the BMP morphogen is localized, these somatic cells are able to respond to BMP when Dally is lacking. Whether a cellular BMP response is Dally dependent or not distinguishes the germline and somatic cells (Guo, 2009).
It was noticed that it took 15 days for Dally overexpression in somatic cells to make all germ cells GSC-like in the germarium, although C587Gal4 was active since stage larval 3 at the latest. One possible explanation is that Dpp is limited and Dally stabilizes Dpp. This possibility is supported by a recent report demonstrating that Dally and Dpp physically interact with each other in the cultured S2 cells and that Dally stabilizes Dpp on the cell surface in the wing imaginal epithelia. Consistent with their theory of cell-surface-associated stabilization, the secreted Dally, although retaining the ability to bind Dpp, did not have the activity that the full-length Dally possesses in terms of enhancing GSC proliferation. It suggests that Dally can only stabilize Dpp at the cell surface. Additionally, secreted Dally expressed in the same cells in which the endogenous Dally is produced had a weak dominant-negative effect. By contrast, the secreted Dally expressed elsewhere did not have any detectable effect on GSC, suggesting that it did not compete with the endogenous Dally expressed from the cap cells. These results imply that the anterior tip of the germarium contains the main source of BMP molecules, which the secreted Dally from cap cells has a better chance to catch than that from elsewhere (Guo, 2009).
In the imaginal epithelia, glypican Dally and Dlp are essential for Dpp gradient formation but not for short-range Dpp signaling because one to two rows of cells in the glypican double mutant clone are able to respond to the nearby Dpp signals. Similarly, in dally mutant ovary, in which the germarium was emptied due to GSC loss, BMP response was observed in the escort cells getting close to the cap cells, where the BMP source is supposed to be. However, the germ cell surrounded by the BMP-responsive somatic cells was refractory to BMP morphogen in exactly the same circumstances. It appears that when Dally is compromised, the germline is less sensitive to BMP signaling and Dally either recruits more ligands to the adjacent germ cell or somehow enhances its response to BMP. Contrarily, the somatic cells do not require Dally to sense and respond to BMP morphogen in short range. Whether Dlp is essential for germarial somatic cells in BMP response is unclear. What accounts for the distinction in BMP sensitivity between germline and somatic cells remains to be investigated (Guo, 2009).
Akiyama, T., et al. (2008). Dally regulates Dpp morphogen gradient formation by stabilizing Dpp on the cell surface. Dev. Biol. 313(1): 408-19. PubMed Citation: 18054902
Al-Haideri, M., et al. (1997). Heparan sulfate proteoglycan-mediated uptake of apolipoprotein E-triglyceride-rich lipoprotein particles: a major pathway at physiological particle concentrations. Biochemistry 36(42): 12766-12772. PubMed Citation: 9335533
Asundi, V. K., et al. (1997). Developmental and cell-type-specific expression of cell surface heparan sulfate proteoglycans in the rat heart. Exp. Cell Res. 230(1): 145-153. PubMed Citation: 9013716
Aviezer, D., et al. (1994). Differential structural requirements of heparin and heparan sulfate proteoglycans that promote binding of basic fibroblast growth factor to its receptor. J. Biol. Chem. 269(1): 114-121. PubMed Citation: 8276782
Ayers, K. L., and Gallet, A. (2010). The long-range activity of Hedgehog is regulated in the apical extracellular space by the glypican Dally and the hydrolase Notum. Dev. Cell 18: 605-620. PubMed Citation: 20412775
Baeg, G.-H., et al. (2001). Heparan sulfate proteoglycans are critical for the organization of the extracellular distribution of Wingless. Development 128: 87-94. PubMed Citation: 11092814
Baeg, G. H., Selva, E. M., Goodman, R. M., Dasgupta, R. and Perrimon, N. (2004). The Wingless morphogen gradient is established by the cooperative action of Frizzled and Heparan Sulfate Proteoglycan receptors. Dev. Biol. 276(1): 89-100. 15531366
Belenkaya, T. Y., Han, C., Yan, D., Opoka, R. J., Khodoun, M., Liu, H. and Lin, X. (2004). Drosophila Dpp morphogen movement is independent of dynamin-mediated endocytosis but regulated by the glypican members of heparan sulfate proteoglycans. Cell 119(2): 231-44. 15479640
Bellaiche, Y., The, I., Perrimon, N. (1998). Tout-velu is a Drosophila homologue of the putative tumour suppressor
EXT-1 and is needed for Hh diffusion. Nature 394(6688): 85-88. PubMed Citation: 9665133
Binari, R. C., et al. (1997). Genetic evidence that heparin-like glycosaminoglycans are involved in wingless signaling. Development 124(13): 2623-2632. PubMed Citation: 9217004
Bonneh-Barkay, D., et al. (1997). Identification of glypican as a dual modulator of the biological activity of fibroblast growth factors. J. Biol. Chem. 272(19): 12415-12421. PubMed Citation: 9139688
Brandan, E., et al. (1996). Synthesis and processing of glypican during differentiation of
skeletal muscle cells. Eur. J. Cell Biol. 71(2): 170-176. PubMed Citation: 8905294
Bülow, H. E., et al. (2008). Extracellular sugar modifications provide instructive and cell-specific information for axon-guidance choices. Curr. Biol. 18(24): 1978-85. PubMed Citation: 19062279
Caneparo, L., et al. (2007). Dickkopf-1 regulates gastrulation movements by coordinated modulation of Wnt/beta catenin and Wnt/PCP activities, through interaction with the Dally-like homolog Knypek. Genes Dev. 21(4): 465-80. Medline abstract: 17322405
Capurro, M. I. et al. (2005). Glypican-3 promotes the growth of hepatocellular carcinoma by stimulating canonical Wnt signaling. Cancer Res. 65: 6245-6254. PubMed Citation: 16024626
Capurro, M. I., et al. (2008), Glypican-3 inhibits Hedgehog signaling during development by competing with Patched for Hedgehog binding. Dev. Cell 14: 700-711. PubMed Citation: 18477453
Crickmore, M. A. and Mann, R. S. (2007). Hox control of morphogen mobility and organ development through regulation of glypican expression. Development 134(2): 327-34. Medline abstract: 17166918
Crisona, N. J., Allen, K. D. and Strohman, R. C. (1998). Muscle satellite cells from dystrophic (mdx) mice have elevated levels of heparan sulphate proteoglycan receptors for fibroblast growth factor. J. Muscle Res. Cell Motil. 19(1): 43-51. PubMed Citation: 9477376
Desbordes, S. C. and Sanson, B. (2003). The glypican Dally-like is required for Hedgehog signalling in the embryonic epidermis of Drosophila. Development 130: 6245-6255. 14602684
Edgren, G., et al. (1997). Glypican (heparan sulfate proteoglycan) is palmitoylated,
deglycanated and reglycanated during recycling in skin
fibroblasts. Glycobiology 7(1): 103-112. PubMed Citation: 9061369
Eugster, C., Panáková. D., Mahmoud, A. and Eaton, S. (2007). Lipoprotein-heparan sulfate interactions in the Hh pathway. Dev. Cell 13: 57-71. Medline abstract: 17609110
Franch-Marro, et al. (2005). Glypicans shunt the Wingless signal between local signalling and further transport. Development 132(4): 659-66. 15647318
Fujise, M., Izumi, I., Selleck, S. B. and Nakato, H. (2001). Regulation of dally, an integral membrane proteoglycan, and its function during adult sensory organ formation of Drosophila. Developmental Biology 235: 433-448. 11437449
Fujise, M., et al. (2003). Dally regulates Dpp morphogen gradient formation in the Drosophila wing. Development 130: 1515-1522. 12620978
Gall, A., et al. (2003). Glypican 4 modulates FGF signalling and regulates dorsoventral forebrain patterning in Xenopus embryos. Development 130: 4919-4929. 12930779
Gallet, A., Staccini-Lavenant, L. and Thérond, P. P. (2008). Cellular trafficking of the glypican Dally-like is required for full-strength Hedgehog signaling and Wingless transcytosis. Dev. Cell 14: 712-725. PubMed Citation: 18477454
Gerlitz, O. and Basler, K. (2002). Wingful, an extracellular feedback inhibitor of Wingless. Genes Dev. 16: 1055-1059. 12000788
Giraldez, A. J., Copley, R. R. and Cohen, S. M. (2002). HSPG modification by the secreted enzyme Notum shapes the Wingless morphogen gradient. Dev. Cell 2: 667-676. 12015973
Grisaru, S., et al. (2001). Glypican-3 modulates BMP- and FGF-mediated
effects during renal branching morphogenesis. Dev. Bio. 231: 31-46. 11322381
Guo, Z. and Wang, Z. (2009). The glypican Dally is required in the niche for the maintenance of germline stem cells and short-range BMP signaling in the Drosophila ovary. Development 136(21): 3627-35. PubMed Citation: 19793889
Hacker, U., Lin, X. and Perrimon, N. (1997). The Drosophila sugarless gene modulates Wingless signaling and encodes an enzyme involved in polysaccharide biosynthesis. Development 124: 3565-3573. PubMed Citation: 9342049
Haerry, T. E., et al. (1997). Defects in glucuronate biosynthesis disrupt Wingless signaling in Drosophila. Development 124(16): 3055-3064. PubMed Citation: 9272947
Han, C., Belenkaya, T. Y., Wang, B. and Lin, X. (2004). Drosophila glypicans control the cell-to-cell movement of Hedgehog by a dynamin-independent process. Development 131: 601-611. 14729575
Han, C., et al. (2005). Drosophila glypicans Dally and Dally-like shape the extracellular Wingless morphogen gradient in the wing disc. Development 132: 667-679. 15647319
Hartmann, G., et al. (1998). Engineered mutants of HGF/SF with reduced binding to heparan sulphate proteoglycans, decreased clearance and
enhanced activity in vivo. Curr. Biol. 8(3): 125-134. PubMed Citation: 9443912
Hou, S., et al. (2007). The secreted serine protease xHtrA1 stimulates long-range FGF signaling in the early Xenopus embryo. Dev. Cell 13(2): 226-41. PubMed citation: 17681134
Huang, Z. and Kunes, S. (1996). Hedgehog, transmitted along retinal axons, triggers neurogenesis in the developing visual centers of the Drosophila brain. Cell 86: 411-422. PubMed Citation: 8756723
Jackson, S. M., et al. (1997). dally, a Drosophila glypican, controls cellular responses to the TGF-beta-related morphogen, Dpp. Development 124(20): 4113-4120. PubMed Citation: 9374407
Ji, Z. S., et al. (1997). Heparan sulfate proteoglycans participate in hepatic lipase and apolipoprotein E-mediated binding and uptake of plasma lipoproteins, including high density lipoproteins. J. Biol. Chem. 272(50): 31285-31292. PubMed Citation: 9395455
Johnson, K. G., et al. (2006). The HSPGs Syndecan and Dallylike bind the receptor phosphatase LAR and exert distinct effects on synaptic development. Neuron 49(4): 517-31. 16476662
Kamimura, K., et al. (2006). Specific and flexible roles of heparan sulfate modifications in Drosophila FGF signaling. J. Cell. Biol. 174(6): 773-8. Medline abstract: 16966419
Kaname, S. and Ruoslahti, E. (1996). Betaglycan has multiple binding sites for transforming growth factor-beta 1. Biochem J. 315( Pt 3): 815-820
Khare, N. and Baumgartner, S. (2000). Dally-like protein, a new Drosophila glypican with expression overlapping with wingless. Mech. Dev. 99: 199-202.
Kim, M. S., et al. (2011). Structure of the protein core of the glypican Dally-like and localization of a region important for hedgehog signaling. Proc. Natl. Acad. Sci. 108(32): 13112-7. PubMed Citation: 21828006
Kirkpatrick, C. A., et al. (2004). Spatial regulation of Wingless morphogen distribution and signaling by Dally-like protein. Dev. Cell 7: 513-523. 15469840
Koziel, L., Kunath, M., Kelly, O. G. and Vortkamp, A. (2004). Ext1-dependent heparan sulfate regulates the range of Ihh signaling during endochondral ossification. Dev. Cell 6: 801-813. 15177029
Kreuger, J., Perez, L., Giraldez, A. J. and Cohen, S. M. (2004). Opposing activities of Dally-like glypican at high and low levels of Wingless morphogen activity. Dev. Cell 7: 503-512. 15469839
Li, M., et al. (1997). Expression of OCI-5/glypican 3 during intestinal morphogenesis: regulation by cell shape in intestinal epithelial cells. Exp. Cell Res. 235(1): 3-12
Liang, Y., et al. (1997). Glypican and biglycan in the nuclei of neurons and glioma cells: presence of functional nuclear localization signals and dynamic changes in glypican during the cell cycle. J. Cell Biol. 139(4): 851-864
Liang, Z. and Biggin, M. D. (1998). Eve and Ftz regulate a wide array of genes in blastoderm embryos: the selector homeoproteins directly or indirectly regulate most genes in Drosophila. Development 125(22): 4471-4482
Lindahl, U., Kusche-Gullberg, M. and Kjellen, L. (1998). Regulated diversity of heparan sulfate. J. Biol. Chem. 273: 24979-24982. 9737951
Litwack, E. D., et al. (1994). Neuronal expression of glypican, a cell-surface glycosylphosphatidylinositol-anchored heparan sulfate
proteoglycan, in the adult rat nervous system. J. Neurosci. 14(6): 3713-3724.
Litwack, E. D., et al. (1998). Expression of the heparan sulfate proteoglycan glypican-1 in the developing rodent. Dev. Dyn. 211(1): 72-87
Lin, H., Huber, R., Schlessinger, D. and Morin, P. J. (1999). Frequent silencing of the GPC3 gene in ovarian cancer cell lines. Cancer Res. 59: 807-810. 10029067
Liu, S., et al. (1997). Heparin/heparan sulfate (HP/HS) interacting protein (HIP) supports cell attachment and selective, high affinity binding of HP/HS. J. Biol. Chem. 272(41): 25856-25862
Lum, L., Yao, S., Mozer, B., Rovescalli, A., von Kessler, D., Nirenberg, M. and Beachy, P. A. (2003). Identification of Hedgehog pathway components by RNAi in Drosophila cultured cells. Science 299: 2039-2045. 12663920
Lyon, M., Rushton, G. and Gallagher, J. T. (1997). The interaction of the transforming growth factor-betas with heparin/heparan sulfate is isoform-specific. J. Biol. Chem. 272(29): 18000-18006
Marois, E., Mahmoud, A. and Eaton, S. (2006). The endocytic pathway and formation of the Wingless morphogen gradient. Development 133(2): 307-17. 16354714
McBride, P. A., et al. (1998). Heparan sulfate proteoglycan is associated with amyloid plaques and neuroanatomically targeted PrP pathology throughout the incubation period of scrapie-infected mice. Exp. Neurol. 149(2): 447-454
McCormick, C., et al. (1998). The putative tumour suppressor EXT1 alters the expression of cell-surface heparan sulfate. Nat. Genet. 19(2): 158-161.
McLellan, J. S., et al. (2006). Structure of a heparin-dependent complex of Hedgehog and Ihog. Proc. Natl. Acad. Sci. 103: 17208-17213. Medline abstract: 17077139
Mertens, G., et al. (1996). Heparan sulfate expression in polarized epithelial cells: the apical sorting of glypican (GPI-anchored proteoglycan) is inversely related to its heparan sulfate content. J. Cell Biol. 132(3): 487-497
Moline, M. M., Dierick, H. A., Southern, C. and Bejsovec, A. (2000). Non-equivalent roles of Drosophila Frizzled and Dfrizzled2 in embryonic Wingless signal transduction. Curr. Biol. 10: 1127-1130. 10996794
Murthy, S.S., Shen, T., De Rienzo, A., Lee, W.C., Ferriola, P.C., Jhanwar, S.C., Mossman, B.T., Filmus, J. and Testa, J.R. (2000). Expression of GPC3, an X-linked recessive overgrowth gene, is silenced in malignant mesothelioma. Oncogene 19: 410-416. 10656689
Nakato, H., Futch. T. A. and Selleck, S. B. (1995). The division abnormally delayed (dally) gene: a putative integral
membrane proteoglycan required for cell division patterning during
postembryonic development of the nervous system in Drosophila. Development 121(11): 3687-3702
Neri, G., Gurrieri, F., Zanni, G. and Lin, A. (1998). Clinical and molecular aspects of the Simpson-Golabi-Behmel syndrome. Am. J. Med. Genet. 79: 279-283. 9781908
Niu S., et al. (1996). Expression of avian glypican is developmentally regulated. Dev. Dyn. 207(1): 25-34
Niu S., et al. (1998). Structure, regulation and function of avian glypican. J. Mol. Cell. Cardiol. 30(3): 537-550
Ohkawara, B., et al. (2003). Role of glypican 4 in the regulation of convergent extension movements during gastrulation in Xenopus laevis. Development 130: 2129-2138. 12668627
Olivares, G. H. et al. (2009). Syndecan-1 regulates BMP signaling and dorso-ventral patterning of the ectoderm during early Xenopus development. Dev. Biol. 329: 338-349. PubMed Citation: 19303002
Panáková, D., Sprong, H., Marois, E., Thiele, C. and Eaton, S. (2005). Lipoprotein particles are required for Hedgehog and Wingless signalling. Nature 435: 58-65. Medline abstract: 15875013
Penton, A., Selleck, S. B. and Hoffmann, F. M. (1997). Regulation of cell cycle synchronization by decapentaplegic during Drosophila eye development. Science 275(5297): 203-206
Pilia, G., Hughes-Benzie, R. M., MacKenzie, A., Baybayan, P., Chen, E. Y., Huber, R., Neri, G., Cao, A., Forabosco, A. and Schlessinger, D. (1996). Mutations in GPC3, a glypican gene, cause the Simpson-Golabi-Behmel overgrowth syndrome. Nat. Genet. 12: 241-247. 8589713
Rawson, J. M., et al. (2005). The heparan sulfate proteoglycans Dally-like and Syndecan have distinct functions in axon guidance and visual-system assembly in Drosophila. Curr. Biol. 15(9): 833-8. 15886101
Reichsman, F., et al. (1996). Glycosaminoglycans can modulate extracellular localization of the wingless protein and promote signal transduction. J. Cell Biol. 135(3): 819-827
Saunders, S., Paine-Saunders, S. and Lander, A. D. (1997). Expression of the cell surface proteoglycan glypican-5 is developmentally regulated in kidney, limb, and brain. Dev. Biol. 190(1): 78-93
Selleck, S. B. and Steller, H. (1991). The influence of retinal innervation on neurogenesis in the first optic ganglion of Drosophila. Neuron 6: 83-99
Selleck, S. B., Gonzalez, C., Glover, D. M. and White, K. (1992). Regulation of the G1-S transition in post-embryonic neuronal precursors by axon ingrowth. Nature 355: 253-255
Serpe, M., et al. (2008). The BMP-binding protein Crossveinless 2 is a short-range, concentration-dependent, biphasic modulator of BMP signaling in Drosophila. Dev. Cell 14: 940-953. PubMed Citation: 18539121
Shao, M., et al. (2009). Down syndrome critical region protein 5 regulates membrane localization of Wnt receptors, Dishevelled stability and convergent extension in vertebrate embryos. Development 136: 2121-2131. PubMed Citation: 19465602
Song, H. H., Shi, W. and Filmus, J. (1997). OCI-5/rat glypican-3 binds to fibroblast growth factor-2 but
not to insulin-like growth factor-2. J. Biol. Chem. 272(12): 7574-7577
Steinfeld, R., Van Den Berghe, H. and David, G. (1996). Stimulation of fibroblast growth factor receptor-1 occupancy and signaling by cell surface-associated syndecans and glypican. J. Cell Biol. 133(2): 405-416
Stipp, C. S., Litwack, E. D. and Lander, A. D. (1994). Cerebroglycan: an integral membrane heparan sulfate proteoglycan that is unique to the developing nervous system and expressed specifically during neuronal differentiation. J. Cell Biol. 124(1-2): 149-160
Summerford, C. and Samulski, R. J. (1998). Membrane-associated heparan sulfate proteoglycan is a receptor for adeno-associated virus type 2 virions. J. Virol. 72(2): 1438-1445
Traister, A., Shi, W. and Filmus, J. (2008). Mammalian Notum induces the release of glypicans and other GPI-anchored proteins from the cell surface. Biochem. J. 410: 503-511. PubMed Citation: 17967162
Tsuda, M., Kamimura, K., Nakato, H., Archer, M., Staatz, W., Fox, B., Humphrey, M., Olson, S., Futch, T., Kaluza, V. et al. (1999). The cell-surface proteoglycan Dally regulates Wingless signalling in Drosophila. Nature 400: 276-280. 10421371
Veugelers, M., et al. (1999). Glypican-6, a new member of the glypican family of cell surface heparan sulfate proteoglycans. J. Biol. Chem. 274(38): 26968-77
Williamson, T. G., et al. (1996). Secreted glypican binds to the amyloid precursor protein of Alzheimer's disease (APP) and inhibits APP-induced neurite outgrowth. J. Biol. Chem. 271(49): 31215-31221
Yao, S., Lum. L. and Beachy, P. (2006). The ihog cell-surface proteins bind Hedgehog and mediate pathway activation, Cell 125: 343-357. Medline abstract: 16630821
Yan, D. and Lin, X. (2007). Drosophila glypican Dally-like acts in FGF-receiving cells to modulate FGF signaling during tracheal morphogenesis. Dev. Biol. 312(1): 203-16. PubMed Citation: 17959166
Yan, D., et al. (2010). The cell-surface proteins Dally-like and Ihog differentially regulate Hedgehog signaling strength and range during development. Development 137(12): 2033-44. PubMed Citation: 20501592
Xiang, Y. Y., Ladeda, V. and Filmus, J. (2001). Glypican-3 expression is silenced in human breast cancer. Oncogene 20: 7408-7412. 11704870
Yan, D., et al. (2009). The core protein of glypican Dally-like determines its biphasic activity in Wingless morphogen signaling. Dev. Cell 17: 470-481. PubMed Citation: 19853561
Yao, S. Lum, L. and Beachy, P. (2006). The Ihog cell-surface proteins bind Hedgehog and mediate pathway activation. Cell 125: 343-357. PubMed Citation: 16630821
Zheng, X., Mann R. K., Sever N. and Beachy P. A. (2010). Genetic and biochemical definition of the Hedgehog receptor. Genes Dev. 24: 57-71. PubMed Citation: 20048000
division abnormally delayed and dally-like:
Biological Overview
| Evolutionary Homologs
| Regulation
| Developmental Biology
| Effects of Mutation
date revised: 15 October 2011
Home page: The Interactive Fly © 1997 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.