1 Institut de Biologia Molecular de Barcelona (IBMB), CSIC, Jordi Girona 18-26,
08034 Barcelona, Spain
2 IMBB/FORTH, Vassilika Vouton, Iraklio, Crete GR-71110, Greece
3 Department of Biology, University of Crete, Iraklio, Crete GR-71110,
Greece
4 Medical School, University of Crete, Iraklio, Crete GR-71110, Greece
Authors for correspondence (e-mail:
mlcbmc{at}cid.csic.es
and
jcrbmc{at}cid.csic.es)
Accepted 13 October 2003
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SUMMARY |
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Key words: Drosophila, Tracheal system, Septate junctions, Epithelial integrity, Organ size, GPI-linked IgCAM
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Introduction |
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The respiratory organ of Drosophila melanogaster, the tracheae,
has proved to be an excellent model system for the study of organ
morphogenesis and tubulogenesis. The available data indicate that the
mechanisms of tubulogenesis have been highly conserved during evolution
(Affolter et al., 2003;
Hogan and Kolodziej, 2002
;
Lubarsky and Krasnow, 2003
).
Therefore, a detailed understanding of tracheal morphogenesis should
contribute to our understanding of the morphogenesis of other branched tubular
structures.
The tracheal system arises from 20 clusters of ectodermal cells that
invaginate inside the embryo. By a process of directed cell migration, cell
rearrangements and cell shape changes, the tracheal cells then follow a
stereotyped pattern of branching and branch fusion without further cell
division, giving rise to the mature tracheae at the end of embryogenesis
(Manning and Krasnow, 1993;
Samakovlis et al., 1996
).
Genetic analysis has identified many genes and genetic pathways required for
the different steps of tracheal morphogenesis, such as branching and tube
fusion (Affolter and Shilo,
2000
). The cells forming the tracheal tubes remain attached to one
another, and the tissue maintains its epithelial integrity while it completes
the whole branching process, making tracheal development an appropriate model
to study epithelial integrity. In addition, the size and diameter of the tubes
are finely regulated to ensure functionality during animal growth. Although
more than 10 genes have been reported to affect the size and integrity of
tracheal tubes (Beitel and Krasnow,
2000
; Wilk et al.,
2000
), the cellular and molecular mechanisms involved in this
control remain unclear.
The general conclusion from many studies is that the direction of migration of the tracheal cells relies on a set of positional cues provided by nearby cells. Thus, the establishment of interactions between tracheal cells and their substrates is a crucial step in tracheal cell migration, a process ultimately determined by molecules expressed at their surface. In addition, tracheal cells remain clustered as they migrate, and need to communicate to allow concerted growth of the organ, indicating that they establish specific interactions among themselves that might also require cell surface proteins. To gain further insight into the mechanisms involved in tracheal morphogenesis we have begun a search for cell surface molecules with a pattern of expression consistent with a role in this developmental process. As a result of this analysis, we have found that the gene coding for the Lachesin (Lac) cell surface protein is expressed in the tracheal cells.
Our results indicate that Lac accumulates at the Septate Junctions (SJs),
specific invertebrate cell junctions located in the apical part of the lateral
membrane of ectoderm-derived cells. They first appear midway through
embryogenesis and are characterized by a ladder-like arrangement of septa
crossing the intercellular cleft. SJs have been proposed to play a role in the
formation of a trans-epithelial diffusion barrier, establishing and/or
maintaining cell polarity, cell adhesion and cell-cell interactions
(Tepass and Hartenstein, 1994;
Tepass et al., 2001
;
Lane and Skaer, 1980
;
Noirot-Timothée and Noirot,
1980
). Several proteins are known to associate with SJs, and many
of these form supramolecular complexes
(Knust and Bossinger, 2002
;
Tepass et al., 2001
). Until
recently SJs were believed to be unique to invertebrates, where they were
assumed to perform some of the roles of tight junctions in vertebrates.
However, a new scenario is emerging, as many homologues of the
Drosophila SJ proteins, as well as similar structures to the SJs,
have been identified in mammals (Tepass et
al., 2001
).
Here we show that Lac behaves as a homophilic cell adhesion molecule required to provide epithelial integrity to the tracheal tubes and to control tubular epithelium length. Moreover, we can detect enlarged cells in the tracheal tubes of Lac mutant embryos, suggesting that Lac regulates organ size by influencing cell length rather than cell number. In addition, we have found that mutations in genes encoding previously characterized components of the SJs produce a similar tracheal phenotype. Finally, we show that there is an interdependence of Lac and other components of SJs for their proper localization. Thus, this work identifies a new component of the SJs with adhesion properties, and unveils a novel role for the SJs during morphogenesis in the regulation of cell adhesion and cell size.
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Materials and methods |
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Immunostaining, in situ hybridization and permeabilization assays
In situ hybridization was performed according to standard protocols, with
Lac RNA probes generated using the whole cDNA as template, and
produced with the Megascript kit (Ambion).
To generate an anti-Lac antibody, mice were immunized and boosted with a
recombinant Lachesin-HIS fusion protein (corresponding to roughly the first
250 amino acids, i.e. the first and second Ig domains). Specificity of the
antibody was tested by staining Lac1 and Df(2R)CB21
homozygous embryos, and by detection of Lac protein overexpressed by the Gal4
system. The sera from all three mice recognize Lac protein, both in
immunostaining experiments (1:1000) and western blots (1:5000-10000). In
double staining protocols, where we needed to use a mouse antibody directed
against a protein other than Lac, we made use of a LacGFP line
(Morin et al., 2001) that
reproduces the same expression pattern as the endogenous Lac protein.
Immunostaining was performed on embryos fixed in 4% formaldehyde for 20-30
minutes according to standard protocols, except for Arm staining, for which we
used a heat fixation protocol (Peifer,
1993). The following antibodies were used: anti-GFP (Molecular
Probes), mAb2A12 (Developmental Studies Hybridoma Bank; DSHB), anti-Crb (Cq4,
DSHB), anti-Dlg (from A. Müller), anti-Arm (N27A1, DSHB), anti-Cora (from
R. G. Fehon), anti-Nrx (from S. Baumgartner), anti-FasIII (DA15 from D.
Brower), anti-Nrg (1B7 from C. S. Goodman), and anti-ßGal (Cappel).
Biotinylated or Cy3-, FITC- and Cy5-secondary antibodies (Jackson
ImmunoResearch) were used at a dilution of 1/300. For HRP histochemistry, the
signal was amplified using the Vectastain-ABC kit (Vector Laboratories). For
fluorescent staining, the signal was amplified using TSA (NEN Life Sciences)
when required. Confocal images were obtained with a Leica SP1 microscope.
Permeabilization assays were performed by injecting rhodamine-labeled
dextran (Mr 10,000; Molecular Probes, Eugene, OR) into the
hemocoel of embryos, as described by Lamb et al.
(Lamb et al., 1998)
Electron microscopy
Dechorionated Drosophila embryos were cryofixed by high pressure
freezing using a EMPact (Leica). Freeze-substitution was performed in an
`Automatic Freeze substitution System' (AFS; Leica), using acetone containing
0.5% of uranyl acetate, for 3 days at 90°C. On the fourth day, the
temperature was slowly increased, by 5°C/hour, to 50°C. At this
temperature, samples were rinsed in acetone, and then infiltrated and embedded
in Lowicryl HM20 for 10 days. Ultrathin sections were picked up on
Formvar-coated gold grids. For immunogold localization, samples were blocked
with 5% BSA in PBS for 20 minutes, and incubated at room temperature for 2
hours with anti-Lac antibody diluted 1/25 in PBS. Washes were performed with
0.25% Tween 20 in PBS, prior to adding goat anti-mouse conjugated to 10 nm
colloidal gold for 1 hour at room temperature. Finally, samples were washed
and contrasted with 2% uranyl acetate for 20 minutes, then observed in a Jeol
1010 electron microscope with a SIS Mega View III CCD. Control samples were
treated equally but were not incubated with the primary antibody.
Bead aggregation assays
Chimaeric Lac-Fc protein was made by cloning a Lac fragment,
corresponding to amino acids 1-327, in frame with the Fc fragment of the human
IgG in the pcDNA3.1-Fc vector (a gift from Katja Bruckner). The chimaeric
protein was recovered from the conditioned medium of transiently transfected
HEK293 cells, and was concentrated to approximately 3 µg/ml using Centricon
filters (Amicon). Polystyrene beads (Fluospheres; Molecular Probes), 1 µm
in diameter, were coated with rabbit anti-human Fc (Jackson) and coupled to
Lac-Fc. Control beads were generated by incubating anti-Fc coated beads with
conditioned medium from mock-transfected cells. The bead aggregation assay was
performed according to Pavlou et al.
(Pavlou et al., 2002), with a
1 hour incubation at room temperature. Diluted aliquots were mounted on
microscope slides and examined for the formation of bead clusters, using a
confocal microscope (Biorad Radiance 2100).
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Results |
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|
Lachesin is required for tracheal morphogenesis
To assess the role of the Lac protein in tracheal morphogenesis we have
analyzed the BG1462 line, which has a P-element inserted into the 5'UTR
of the Lac gene (Fig.
1B). BG1462 is late embryonic lethal, both when homozygous and
over two deficiencies that uncover the Lac gene
(Fig. 1A). In addition, its
lethality can be reverted by precise excision of the P-element. Thus, we have
renamed BG1462 as Lac1.
We have also generated the Lac2 mutation by imprecise excision of BG1462. Lac2 is a deletion that removes the Lac ATG without affecting nearby genes (see Materials and methods). Comparison of the tracheal phenotypes in homozygous conditions, and over deficiencies that uncover the gene, indicate that both Lac1 and Lac2 behave as strong loss-of-function alleles or null alleles. In situ hybridization with a Lac probe showed no, or very low, signal in homozygous Lac1 and Lac2 embryos (not shown). Immunostaining with a specific anti-Lac antibody that we generated (see Materials and methods, and below) produced a similar result: we did not detect protein expression in homozygous Lac2 embryos, although residual levels of signal were detected in Lac1 homozygous embryos in tissues other than the trachea (not shown). Consistent with these results, Lac1 and Lac2 mutants display the same tracheal phenotype.
The early events of tracheal development, such as guidance and primary branching, occur normally in Lac mutant embryos. In addition, branch fusion and extension of terminal branches also show a normal pattern, indicating that Lac is not required for these processes. However, from stage 15, Lac1 and Lac2 mutants start to display several defects that become more apparent by stage 16. In particular, most branches become more sinuous or convoluted than in the wild-type; this is especially conspicuous in the dorsal trunk. The lumen shows an uneven appearance, with expansions and constrictions along the tubes, and in addition we detect an abnormal accumulation of lumen components when monitoring the lumenal antigen 2A12. We also observe numerous lumenal breaks and discontinuities, mainly in the dorsal and lateral branches (Fig. 2B-D). Moreover, tracheal tubes do not inflate at the end of embryogenesis indicating that they do not become functional (not shown).
|
A group of mutants, previously reported by Beitel and Krasnow, display a
tracheal phenotype similar to that of Lac
(Beitel and Krasnow, 2000). One
of these mutations, bulb, maps to the same region as the Lac
gene. The following observations suggest that bulb is a mutation of
the Lac gene. First, the bulb mutation fails to complement
both Lac1 and Lac2:
bulb/Lac1 embryos (Fig.
2G) show the same kind of tracheal phenotype as bulb and
Lac1 homozygotes, and they do not survive. Second,
although in bulb embryos Lac is expressed in several tissues in a
similar way to the wild type, we do not detect Lac expression in tracheal
cells (not shown). Thus, bulb appears to be a regulatory mutation of
Lac.
Lachesin controls the size and epithelial integrity of tracheal tubes
To analyze in more detail the nature of the Lac mutant defects we
have used a tauGFP construct to visualize the tracheal cells. We have
found that the convoluted shape of the tracheal tubes, mainly the dorsal
trunk, is a consequence of them having lengthened too much. However, this
extra growth does not appear to derive from an increase in the number of
tracheal cells, but rather from an increase in the length of the cells. For
example, we have scored the same number of cells in the dorsal trunk between
metameres 7 and 8 (18±2 cells on average in this interval in the
wild-type, n=14; and 18.3±1.7 in Lac1,
n=16), but the total length of the branch in this interval in
Lac mutants is 110% of that of wild-type embryos (n=10
intervals measured for Lac1 and for wild-type embryos)
(Fig. 2J,K). Accordingly we do
not detect extra tracheal cell proliferation in Lac mutants, as
assessed by the absence of -phosphorylated histone H3 staining. Similar
observations were reported for bulb
(Beitel and Krasnow, 2000
).
Moreover, we do not detect an enlargement of the nuclei of the mutant cells,
as compared with the wild type. These results reveal that Lac plays a
role in regulating organ size, probably by affecting the shape of the
cells.
We have found that many lumenal breaks are the result of an anomalous behaviour of the tracheal cells, which detach from one another in some of the tracheal branches (Fig. 2I). We observed cell detachments that break the tracheal tubes, particularly in those branches where cells elongate more and are thought to be subjected to stronger pulling forces. Indeed, these are the cells where a lessening in cell adhesion should be more readily detected. This phenotype points to a role for the Lac protein in cell adhesion.
On the other hand, the tracheal cells of Lac mutants retain a
normal epithelial polarity, as judged by the distribution of an apical marker,
such as Crumbs (Crb) (Fig.
2L,M) (Wodarz et al.,
1995), and a normal polarization of the cytoskeleton, as judged by
scoring the minus end of the microtubules with a nodGFP transgene
(Bolivar et al., 2001
)
(Fig. 2N,O).
Lachesin functions as a homophilic adhesion molecule in a bead aggregation assay
Lac shows closest similarity to members of the Ig superfamily belonging to
the neuronal Ig cell adhesion molecule (IgCAM) class, which includes
vertebrate L1, NCAM, TAG1, Contactin and IgLONs, and Drosophila
Neuroglian, Wrapper and Klingon (reviewed by
Karagogeos, 2003;
Tessier-Lavigne and Goodman,
1996
). Members of this family have been shown to engage in
homophilic and/or heterophilic interactions to mediate cell adhesion, raising
the possibility that the molecular mechanism by which Lac contributes to
tracheal development could be based on its adhesive properties. In order to
establish whether Lac also works as a homophilic cell adhesion molecule, we
tested its activity in a bead aggregation assay
(Faivre-Sarrailh et al., 1999
;
Pavlou et al., 2002
). In this
assay, chimaeric molecules consisting of the protein of interest and the Fc
fragment of human IgG are coupled to polystyrene beads, and assayed for their
ability to form aggregates (see Materials and methods). Whereas control beads
do not aggregate, Lac-Fc coated beads form aggregates
(Fig. 3), showing that Lac can
work as a homophilic cell adhesion molecule. This in vitro assay confirms the
adhesion properties of the Lac protein suggested by the cell detachment
phenotype we observed in mutant tracheae.
|
|
To more finely determine the localization of Lac protein along the apicolateral membrane, we analyzed embryos by immunoelectron microscopy. In order to preserve the structure and antigenicity of the sample, we carried out electron microscopy analysis following cryofixation and freeze-substitution techniques (see Materials and methods). The results show that Lac distribution is restricted to the membrane region, where the septae are observed (Fig. 5), further confirming that Lac is associated with SJs.
|
Lachesin is required to maintain the trans-epithelial diffusion barrier
SJs have been proposed to play a prominent role in the formation of
trans-epithelial diffusion barriers
(Baumgartner et al., 1996;
Lamb et al., 1998
). As our
subcellular localization experiments showed that Lac is a component of the
SJs, we tested whether Lac is also involved in the formation of such
a barrier in the tracheal tubes. We performed a dye permeability assay, by
injecting a 10 kDa rodhamine-labeled dextran into the hemocoel
(Lamb et al., 1998
) of
Lac mutant embryos at the end of embryogenesis. In wild-type embryos,
the dextran did not show any diffusion into the tracheal lumen, even more than
one hour after injection (Fig.
6A). However, in Lac mutant embryos, the dye diffused
very quickly and completely filled the tracheal lumen
(Fig. 6B), showing that these
mutants are unable to establish or maintain the tracheal diffusion barrier. In
addition, we found that the dye was internalized in the salivary glands,
indicating that they are also affected. In agreement with this, we observed
necrotic tissue in the salivary gland region of Lac mutant embryos at
later stages (data not shown). As we have shown earlier, Lac mutants
display defects in the accumulation of tracheal lumen components such as 2A12.
The fact that Lac is necessary to maintain the trans-epithelial
diffusion barrier suggests that the improper accumulation of lumen components
in Lac mutants could be due to leakage of the tracheal tubes rather
than to a defect in secretion.
|
We analyzed the expression of several SJs proteins in Lac2 mutants. We could not detect differences in the levels of expression of these proteins in the tissues analyzed. However, we detected some changes in the subcellular localization of the SJs proteins we tested, as is the case for Cora, Dlg, Scrib, Nrx, Fas3 and Nrg (Fig. 6C-H and not shown). These proteins were no longer tightly localized to the SJs region, but rather were spread into more basolateral positions, although they were still found in the membrane and an apically concentrated distribution was still observed. These differences were particularly conspicuous in the salivary glands. The salivary glands consist of large columnar cells, where the SJs are restricted to a relatively small region in the lateral membrane, making it easier to observe a mislocalization of the SJs proteins. By contrast, these defects were not so easily detectable in other tissues, including the trachea, because of the wider distribution of the SJs on the lateral cell surface (Fig. 6E,F; and not shown). These results show that Lac plays a role in the proper recruitment or accumulation of several SJs proteins into the complexes.
To determine the requirement of known SJs proteins in Lac localization, we assayed Lac distribution in cora (Fig. 6I,J) and zygotic lgl mutants (not shown). We could not detect defects in the levels of Lac protein, but we detected subtle changes in the localization of Lac, which was not so sharply enhanced at the most apical part of the apicolateral membrane. Again, this mild change in Lac localization was more obvious in the salivary glands than in other tissues (Fig. 6I-L). These results indicate that the impairment of SJ integrity affects Lac accumulation.
SJs are required for tracheal morphogenesis
All together, our results suggest Lac as a new component of the SJs.
Therefore, we reasoned that SJs could be more generally involved in ensuring
proper cell length and cell adhesion during tracheal morphology, and thus we
have examined mutants for genes encoding other SJ proteins. In particular, we
examined cora and Nrx mutant embryos, and found that they
both display tracheal phenotypes very similar to those of Lac mutant
embryos (Fig. 6M,N). In
particular, both mutants show overgrown tubes with unusual expansions, defects
in the accumulation of lumen antigens and lumen breaks. These results
demonstrate that there is a general role for SJ in tracheal morphogenesis, and
that Lac, as a new component, contributes to this function.
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Discussion |
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The tracheal tubes seem particularly sensitive to defects in SJs; in
particular, there is a loss of epithelial integrity not reported for other
ectodermal tissues. In this regard, the specific and strong expression of
Lac in tracheal cells from stage 13 might hint at a strong
requirement for Lac, and possibly SJs, during tracheal morphogenesis. It is
important to note that SJs start to form late in embryogenesis, only after
most of the morphogenetic events have taken place; however, at these stages,
the tracheal tree has not yet completed its morphogenesis and could require
fully functional SJs. The integrity of tracheal tubes may be dependent on the
ability of their cells to adhere to one another, as they are subjected to
stronger pulling forces than most other ectodermal tissues. Interestingly,
differences at the level of the cellular junctions have been reported between
epidermal and tracheal cells (Tepass and
Hartenstein, 1994).
Lac protein and the SJs
Co-localization studies with confocal microscopy, immunoelectron microscopy
analysis, functional studies on the permeability of the trans-epithelial
barrier, and the mutual requirement of Lac and other SJ proteins for their
correct subcellular localization indicate that Lac is a new component of the
SJs. As mentioned before, localization or stabilization of some (but not all)
SJ proteins has been found to be dependent upon that of other components. For
example, Cora has a very faint staining and a diffuse localization in
Nrx mutants (Baumgartner et al.,
1996), whereas localization of Dlg or Fas3 shows no obvious defect
in those same mutants (Genova and Fehon,
2003
; Ward et al.,
1998
). SJ components whose proper localization is interdependent
are thought to physically interact, and to belong to the same supramolecular
complexes in the SJs (Baumgartner et al.,
1996
; Bilder et al.,
2003
). These results led to the proposal that SJs are formed by
the recruitment of distinct components into different protein complexes.
However, it still remains unknown which protein/s would initially be required
to target these protein complexes to the SJs
(Tepass et al., 2001
). Our
results indicate that several SJ proteins are mislocalized in Lac
mutants to a certain degree. One possibility is that Lac interacts with one of
the components of the SJs, and that in the absence of this interaction SJs are
not properly assembled. Lac is a cell surface protein, associated with the
outer leaflet of the membrane bilayer via its GPI-tail, and could potentially
interact, in cis and/or in trans, with transmembrane proteins belonging to SJ
complexes. It has been shown that Contactin and TAG1, vertebrate GPI-linked
proteins of the Ig superfamily, bind to vertebrate Nrx-like proteins in the
context of axo-glial interactions in the region of the node of Ranvier
(Bhat et al., 2001
;
Boyle et al., 2001
;
Traka et al., 2003
). We have
tested a similar hypothesis for Lac and Nrx with the S2 cell aggregation assay
(Bieber, 1994
). Although
Lac-expressing transfected S2 cells form aggregates, consistent with the
results of the bead-aggregation assay (Fig.
3), they do not aggregate with untransfected S2 cells that
endogenously express Nrx (Baumgartner et
al., 1996
) (M.L., M.S., M.K., D.K. and J.C., unpublished). This
negative result suggests that Lac does not interact, at least in trans, with
Nrx. Ig superfamily members often show heterophilic interactions with other
family members (reviewed by Karagogeos,
2003
), raising the possibility that Lac interacts with components
of SJs carrying Ig domains.
However, our results also indicate that the mislocalization of the analyzed
SJ proteins in Lac mutants is not as severe as the one described for
proteins that are thought to physically interact, as the levels of the
proteins analyzed seemed normal. Therefore, our results do not necessarily
point to a direct interaction of Lac with those proteins, and could suggest,
instead, that SJs are not perfectly assembled in Lac mutants and, as
a consequence, their components are less well localized or stabilized. In that
scenario, Lac could recognize a preformed structure of the SJs in order to be
properly localized; Lac localization appears to require functional SJs because
it is indeed abnormal in mutants that affect SJs structure, such as
cora (Lamb et al.,
1998). It has been suggested that SJ multiprotein complexes
present in adjacent cells have to interact with each other to ensure proper
assembly of the junction (Genova and
Fehon, 2003
). The nature of Lac as a homophilic cell adhesion
protein suggests that it could mediate such intercellular interactions, and
could have a role as a component of SJs specifically ensuring or reinforcing
the SJs-mediated adhesion between neighbouring cells.
Cell adhesion, cell shape and organ size
Our results suggest a new role for SJs in morphogenesis, by the control of
cell length and adhesion. Organ and body size often depend on control of cell
number (Conlon and Raff,
1999), and SJs have been previously suggested to control cell
proliferation (Lamb et al.,
1998
; Tepass et al.,
2001
). Our results suggest that, in the case of the tracheal
system, SJs regulate organ size by influencing cell length rather than cell
number. We want to point out that these cells do not need to be necessarily
bigger to generate an increase in tracheal branch length. A major contribution
to the increase in organ size could lie in the fact that cells become more
elongated. Indeed, Lac mutant tracheal cells appear more elongated,
identifying the control of cell shape as one of the regulatory mechanisms of
tracheal tube size. It remains an open question whether, in this case, the
control of cell length is a direct consequence of the role in cell adhesion.
Thus, for example, in a pure mechanical model, the lessening of the tight
contact between cells could abrogate a constraint for their elongation.
Alternative explanations are also possible; for instance, tight contact among
the tracheal epithelial cells would be required for coordinated signalling,
allowing polarized conduction of information. Moreover, some of the SJ
components themselves could control cell and tissue size via intercellular and
intracellular cell signalling, as has been hypothesized, for example, for Cora
and Nrg (Genova and Fehon,
2003
; Lamb et al.,
1998
). An unexpected nuclear function for the vertebrate tight
junction component ZO2, structurally and functionally related to
Drosophila Dlg, has been recently suggested
(Islas et al., 2002
), possibly
linking the permeability barrier and signalling functions of these structures.
In this respect, other features of the Lac protein not necessarily related to
its adhesion properties could be relevant to the control of tube size. For
instance, Lac could also have a role in signalling, as is known for other
GPI-linked Ig proteins (Stefanova et al.,
1991
; Walsh and Doherty,
1997
).
Although the available data indicate that the mechanisms of tubulogenesis
share many basic strategies across diverse animal groups
(Affolter et al., 2003;
Hogan and Kolodziej, 2002
;
Lubarsky and Krasnow, 2003
),
the generality of our results could be hampered by the fact that SJs have not
been reported in vertebrates. However, the localization of the Lac protein at
the SJs, and the function of SJs in morphogenesis, could be of functional
significance to other systems for several reasons. First, some of the roles of
SJs are assumed to be performed by tight junctions in vertebrates. Second,
some tight junction components are structurally related to, and others are
homologues of, SJs proteins. Third, many homologues of the Drosophila
SJ proteins, as well as structures that are similar to the SJs, have been
identified in mammals (Bellen et al.,
1998
; Girault and Peles,
2002
; Tepass et al.,
2001
). In this context, SJs-like proteins could be similarly
involved in defining proper organ size and morphogenesis in these other
systems, by influencing cell adhesion properties and cell shape.
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ACKNOWLEDGMENTS |
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![]() |
Footnotes |
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