1 Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern
University, Evanston, IL 60208, USA
2 Division of Neurosciences, Beckman Research Institute of the City of Hope,
Duarte, CA 91010, USA
Author for correspondence (e-mail:
beitel{at}northwestern.edu)
Accepted 16 June 2003
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SUMMARY |
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Key words: Tracheal system, Na+/K+ ATPase, Na+ pump, Septate junctions, Tight junctions, Epithelial tubes, Epithelial morphogenesis, Tube-size control, Tubulogenesis, Drosophila
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Introduction |
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To identify and study the functions of genes required for epithelial
tube-size control we are using molecular genetic approaches with the
Drosophila tracheal system. The tracheal system is the gas exchange
organ of the fly, but also resembles and performs some of the same functions
as the vertebrate vascular system by directly delivering oxygen through a
ramifying network of tubes (Manning and
Krasnow, 1993). The tracheal system is created from clusters of
invaginated epithelial cells that organize into branches which extend and
interconnect to create a complex tubular network
(Affolter and Shilo, 2000
;
Samakovlis et al., 1996
). The
genetic tools available in Drosophila, coupled with the reproducible
and rapid development of the tracheal system, make it a powerful system for
studying the genetic and molecular basis of tubesize control
(Beitel and Krasnow, 2000
;
Lubarsky and Krasnow,
2003
).
Epithelial tube morphogenesis has been shown to require coordinated cell
shape changes and dynamic adjustments of cell junctions
(Lubarsky and Krasnow, 2003;
O'Brien et al., 2002
;
Hogan and Kolodziej, 2002
).
Adherens junctions have been shown to play an important role in tracheal
morphogenesis (Tanaka-Matakatsu et al.,
1996
; Uemura et al.,
1996
; Oda and Tsukita,
1999
; Chihara et al.,
2003
; Lee and Kolodziej,
2002
) and in vertebrate epithelial tube formation
(Pollack et al., 1997
;
Pollack et al., 1998
).
However, to date, the involvement of Drosophila septate or vertebrate
tight junctions in epithelial tube morphogenesis has not been examined.
Insect septate junctions form the trans-epithelial diffusion barrier that
regulates passage of solutes through the spaces between adjacent cells in an
epithelium (Tepass et al.,
2001). In vertebrates, the paracellular diffusion barrier in
epithelial cells is provided by tight junctions
(Tsukita et al., 2001
;
Anderson, 2001
). Despite their
common barrier function, septate and tight junctions are generally referred to
as analogous structures because they have very different morphologies when
examined using electron microscopy and because septate junctions are located
basolateral to the adherens junction while tight junctions lie apically.
Nonetheless, Drosophila septate and vertebrate tight junctions
contain some similar proteins that are crucial for their function
(Tepass et al., 2001
). For
example, Drosophila discs large encodes a PDZ domain-containing MAGUK
scaffold protein similar to vertebrate ZO-1; coracle is similar to
band 4.1; and scribble to human scribble
(Willott et al., 1993
;
Takahisa et al., 1996
;
Fehon et al., 1994
;
Bilder and Perrimon, 2000
;
Nakagawa and Huibregtse,
2000
). These observations raise the possibility that the
functional similarities of septate and tight junctions may reflect
similarities in molecular architecture.
In previous work, we showed that the tracheal system undergoes highly
regulated tube-size increases during development and identified eight `tube
expansion' genes that are specifically required for remodeling the size of the
tracheal tubes once the initial network has formed
(Beitel and Krasnow, 2000). We
show that one of those genes is the nrv2 locus, which encodes two
isoforms of a ß subunit of the Na+/K+ ATPase, and
that mutations in the ATP
subunit (Atpalpha
FlyBase) locus also cause similar tracheal tube-size defects. Further,
we show nrv2 and ATP
mutations disrupt septate
junction function.
The Na+/K+ ATPase is an /ß heteromultimer
that creates the essential electrochemical gradient across the plasma membrane
by transporting two K+ into and three Na+ out of the
cell for each ATP hydrolyzed (Blanco and
Mercer, 1998
; Chow and Forte,
1995
). The
subunit is a ten transmembrane-domain protein
containing the Na+ and K+ channels and the
phosphorylation and nucleotide binding sites
(Chow and Forte, 1995
). The
single transmembrane domain ß subunit is required for transport of the
subunit out of the ER (Geering et
al., 1996
; Hasler et al.,
1998
) and is thought to modulate the affinity of the
subunit for Na+ and K+
(Blanco and Mercer, 1998
;
Chow and Forte, 1995
;
Hasler et al., 1998
). In flies
there are three ß subunit loci, nrv1 and nrv3 (CG8663),
each of which produce one isoform, and nrv2 which encodes two
isoforms, Nrv2.1 and Nrv2.2. There are two
subunit loci,
ATP
that produces at least 12
subunit isoforms
(Palladino et al., 2003
), and
CG17923, which appears to be minimally expressed because its transcripts are
poorly represented in existing cDNA libraries. Of these loci, only nrv1,
nrv2 and ATP
have been characterized in detail, primarily
for their roles in the nervous system
(Lebovitz et al., 1989
;
Palladino et al., 2003
;
Sun and Salvaterra, 1995b
;
Sun et al., 1998
). A tracheal
phenotype was recently described for ATP
mutants, although
ß-subunit mutants were not analyzed
(Hemphala et al., 2003
).
In vertebrates, different Na+/K+ ATPase isoforms have
been proposed to have unique functions because they are expressed with tissue
and temporal specificity, and because different isoforms have distinct
biochemical and pharmacological properties in vitro
(Blanco and Mercer, 1998).
However, in vivo experiments have so far failed to provide support for this
hypothesis (Weber et al.,
1998
). Our results show that septate junctions and specific
Na+/K+ ATPase isoforms have previously unidentified
roles in tracheal tube-size control and that the tube-size control and
trans-epithelial barrier functions of septate junctions are distinct.
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Materials and methods |
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Immunohistochemistry, microscopy and morphometric analysis
Antibodies 2A12 and TL1, embryo fixation, staining and staging procedures
are described elsewhere (Samakovlis et
al., 1996). Other antibodies used were AS55
(Reichman-Fried et al., 1994
),
H5F7 (Sun and Salvaterra,
1995a
), anti-
5
(Lebovitz et al., 1989
),
anti-Armadillo (Riggleman et al.,
1990
), anti-Coracle (Fehon et
al., 1994
), anti-DLG (Woods et
al., 1996
), anti-DLT (Bhat et
al., 1999
), anti-Neurexin
(Baumgartner et al., 1996
) and
anti-ß-Galactosidase (Capel). Embryos stained with
5 anti-
were fixed in 4% paraformaldehyde and hand devitellinized. For H5F7
anti-ß, standard formaldehyde/heptane fixation and methanol
devitellinization produced basolateral staining that overlapped with Coracle
and Neurexin, while paraformaldehyde fixation and hand devitellinization
yielded cytoplasmic/perinuclear staining. Using either method, ectodermal
staining was reduced to background levels in nrv2 null mutants. H5F7
staining of embryos expressing UAS-nrvX transgenes revealed that for
the btl-Gal4 and e22c-Gal4 drivers, nrv2.1, nrv2.2 and
nrv3 had equivalent staining levels and subcellular localizations,
but that although detectable, nrv1 staining was significantly
weaker.
Dorsal trunk and transverse connective lengths were measured as described
by Beitel and Krasnow (Beitel and Krasnow,
2000), except Metamorph software (Universal Imaging) was used for
the morphometric analysis. Confocal images were captured using a Leica TCS SP2
maintained and supported by the Northwestern Biological Imaging Facility. To
assess protein levels, heterozygotes and homozygous mutants were imaged at the
same settings on the same slide in the same session. Adjustments performed in
Photoshop were applied equally to all images. All embryos were stage 16 unless
otherwise noted.
Dye exclusion and RNAi
Texas Red-conjugated 10 kDa dextran was injected into embryos as described
by Lamb et al. (Lamb et al.,
1998). We believe that abnormally rapid leakage across tracheal
epithelia indicates septate junction defects and not other tracheal defects as
the disconnected tracheal segments of hnt mutant or homozygous
balancers embryos do not leak dye (data not shown).
Double-stranded RNAs were generated by PCR amplification of templates with
primers containing the T7 promoter sequence, in vitro transcription, and were
injected into early embryos following standard protocols
(Kennerdell and Carthew,
1998). btl-Gal4 UAS-GFP homozygote embryos were used to
visualize the tracheal system. RNAi-injected embryos were allowed to develop
until stage 16-17, injected with 10 kDa dye and viewed with a Zeiss Axioplan2
microscope. Common exon regions were chosen for dsRNAs templates for all genes
tested, except for nrv2.1 and nrv2.2 where unique 5' exons were
targeted. Specificity of isoform-targeted dsRNAs was demonstrated by injecting
nrv2.1 or nrv2.2 dsRNA into btl-GAL4 UAS-nrv2.2 embryos.
nrv2.2 dsRNA injections caused tracheal phenotypes and reduced levels of the
overexpressed Nrv2.2 (5/5 embryos), while nrv2.1 dsRNA injections caused
tracheal morphology defects but did not affect Nrv2.2 levels (9/9
embryos).
Molecular biology
The sequence of nrv3, the predicted Na+/K+
ATPase ß subunit CG8633, was determined from cDNA GH12088 (GenBank
Accession Number AY314744). UAS-nrv1, UAS-nrv2.1,
UAS-nrv2.2 and UAS-nrv3 were constructed by inserting
nrv1, nrv2.1, nrv2.2 (Sun and
Salvaterra, 1995b) and nrv3 cDNAs into the pUAST vector
(Brand and Perrimon, 1993
).
nrv2 alleles were sequenced by PCR amplification of genomic DNA from
heterozygous flies, followed by cycle dye termination sequencing.
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Results |
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To better define the role of nrv2 in epithelial morphogenesis, we
generated a putative nrv2 null allele, nrv2nwu3,
that deletes the first three common nrv2 exons which encode
transmembrane and extracellular domains (see Materials and methods). In
addition, Genova and Fehon (Genova and
Fehon, 2003) provided a second putative null allele,
nrv223B, that removes all nrv2 common exons. The
phenotypes caused by nrv2nwu3 or
nrv223B do not become more severe in trans to a
chromosomal deficiency known to delete nrv2, providing genetic
evidence that these are null alleles.
In nrv2-null embryos, beginning at the time of tracheal tube expansion, multicellular tracheal tubes become increasingly abnormal so that most tube lengths are significantly increased and all tube diameters are irregular with expansions and constrictions along their lengths (Fig. 1B,F-I; Table 1). Defects are also present in regions of single cell tubes formed by autocellular junctions, particularly near the ends of the ganglionic branches where there are lumenal staining discontinuities (Fig. 1G).
|
|
ATP, a Na+/K+ ATPase
subunit locus, is required for tracheal tube size control
To test whether nrv2 functions as part of the
Na+/K+ ATPase to control tracheal tube size, we examined
the embryonic phenotypes of mutations in the major
Na+/K+ ATPase subunit locus, ATP
(Lebovitz et al., 1989
;
Sun et al., 1998
;
Palladino et al., 2003
). The
transposable element insertions ATP
l(3)1278,
ATP
l(3)04694 and
ATP
l(3)07008 caused tracheal defects
similar or identical to nrv2-null mutations, including tube length
increases, diameter expansions and ganglionic branch discontinuities
(Fig. 1M,N). The
ATP
null mutations
ATP
DTS1R1 and
ATP
DTS1R2
(Palladino et al., 2003
) also
caused nrv2-like length and ganglionic branch defects, but caused
only mild diameter defects (Fig.
1O). Although one would normally expect the
ATP
-null mutations to cause more severe phenotypes than
partial loss-of-function mutations, the hypomorphic ATP
mutations might cause strong nrv2-like phenotypes by producing
inactive
subunits that could unproductively interact with Nrv2 and
deplete the pool of Nrv2 available for productive interactions with other
binding partners, such as
subunits expressed from the secondary
Na+/K+ ATPase
subunit locus CG17923. However,
despite some differences between the phenotypic effects of different
ATP
mutations, the observation that both null and partial
loss-of-function mutations cause nrv2-like tracheal tube-size defects
demonstrates that the ATP
locus is required for tracheal
tube-size control and suggests that the nrv2 ß and
ATP
subunits function together in this process.
The Na+/K+ ATPase is required for septate
junction function
During our investigations of the role of the Na+/K+
ATPase in tube-size control, J. Genova and R. Fehon reported that
Na+/K+ ATPase mutants had salivary gland septate
junction defects (personal communication). We therefore tested whether
tracheal septate junction barrier function was defective in
Na+/K+ ATPase mutants using the dye exclusion assay of
Lamb et al. (Lamb et al.,
1998), which tests the ability of an epithelium to exclude a 10
kDa dye. In wild-type embryos, tracheal septate junction barriers become
functional and excluded dye starting at late stage 15 (e.g.
Fig. 2a). However, the tracheal
and salivary gland epithelia in nrv2 and ATP
mutants
do not acquire the ability to regulate paracellular transport and cannot
prevent the dye from inappropriately diffusing into the tracheal and salivary
gland lumens (Fig. 2b,c).
|
|
The Na+/K+ ATPase localizes to septate
junctions independent of Coracle
The function of the Na+/K+ ATPase in the septate
junction could be direct as structural component, indirect through its
generation of the electrochemical gradient, or both direct and indirect. To
assess a possible direct role, we investigated the subcellular localization of
the ATP and Nrv2 proteins. In salivary glands, epidermis and trachea,
ATP
staining predominantly colocalizes with Coracle and Neurexin at the
apicolateral septate junction region, although some variable, low intensity
ATP
staining of the basolateral cell surfaces that did not correlate
with genetic background was also observed
(Fig. 4A-C and data not shown).
In nrv2 null mutants, ATP
levels are significantly reduced in
the trachea and essentially absent in salivary glands
(Fig. 4D). Where ATP
staining is visible, it is no longer localized to the apicolateral septate
junction region, but instead along the entire lateral surface. Thus, the
ATP
localizes to the septate junction, and this localization is Nrv2
dependent.
|
Significantly, although Coracle and Neurexin are mislocalized and their
levels may be reduced in ATP and nrv2 mutants
(Fig. 3B and data not shown),
the localizations and levels of ATP
are unaltered in coracle
mutants (Fig. 4E). Together,
the above results suggest that the Na+/K+ ATPase is a
structural component of the septate junction, and that it is required for
stable formation of a complex containing Coracle and Neurexin.
Tracheal tube size control requires septate junctions but not
paracellular barrier function
To investigate the relationship between septate junctions and tube-size
control, we determined the effects of mutations in other septate junction
components on tracheal morphology. Null mutations in coracle and
neurexin cause tracheal morphology defects that are essentially
identical to those of nrv2 and the tube expansion mutants
varicose, sinuous and convoluted
(Fig. 5C-E,I,K; Table 1)
(Beitel and Krasnow, 2000).
Furthermore, mutations in gliotactin and neuroglian, two
genes required for the blood-brain barrier formed by the septate junction
(Auld et al., 1995
;
Dubreuil et al., 1996
;
Genova and Fehon, 2003
;
Schulte et al., 2003
), also
cause tracheal morphology defects similar to those caused by mutations in the
tube-expansion genes (Fig.
5B,G; Table 1).
Thus, the septate junction complex appears to be required for tracheal
tube-size control.
|
Nrv2.1 and Nrv2.2 both have tracheal tube-size control and junctional
barrier activity
Although the above results demonstrate that the nrv2 locus plays a
crucial role in tracheal tube-size control and septate junction function, they
do not address possible functional differences between the two Nrv2 protein
isoforms, Nrv2.1 and Nrv2.2, because the available nrv2 mutations
affect exons common to both (Fig.
6A; see Materials and methods). Nrv2.1 and Nrv2.2 share the same
predicted transmembrane and extracellular domains, but differ dramatically in
their 49 amino acid intracellular domains in which they are only 29%
identical and 48% similar. To investigate possible functional differences
between the isoforms, we used dsRNAs corresponding to either the nrv2
common exons or to nrv2.1- or nrv2.2-specific exons in RNAi
experiments to `knockdown' either both nrv2 transcripts or the nrv2.1 or
nrv2.2 transcripts specifically (see Material and methods). We found that
injection of any of nrv2 common, nrv2.1 or nrv2.2 dsRNAs caused the same
tracheal tube length and diameter defects as nrv2 null mutations, and
all three dsRNAs caused defects in septate junction barrier function
(Fig. 6D-F). These results
suggest that in normal development, both Nrv2 isoforms are required for
tracheal tube-size control and septate junction function.
|
The above results also demonstrate that both nrv2.1 and
nrv2.2 have tube-size control and paracellular transport activities.
However, given that all combinations of Na+/K+ ATPase
and ß subunits form functional ion pumps
(Lemas et al., 1994
;
Schmalzing et al., 1991
;
Schmalzing et al., 1997
),
these results also raise the possibility that neither nrv2 isoform
has specific functions and that any ß subunit could substitute for Nrv2.
We therefore tested whether either of the two other Drosophila
Na+/K+ ATPase ß subunits, nrv1
(Sun and Salvaterra, 1995b
) or
nrv3 (CG8663), had tube-size or barrier activities. Driving
UAS-nrv1 or UAS-nrv3 constructs with either
btl-Gal4 or the e22c-Gal4 driver did not rescue any of the tracheal
tube-size or septate junction barrier defects of nrv2 mutants (e.g.
Fig. 6I,J;
Fig. 2, legend), despite the
fact that expression from the UAS-nrv2.1, UAS-nrv2.2 and
UAS-nrv3 transgenes appeared equivalent as assessed by
immunohistochemical staining (see Materials and methods). Consistent with
these results, injection of dsRNA corresponding to nrv1 or
nrv3 into wild-type embryos did not cause either the characteristic
tube-expansion defects or septate junction barrier defects
(Fig. 6G and data not shown).
We therefore conclude that Nrv2.1 and Nrv2.2 both have specific tube-size and
septate junction barrier activities not present in other
Na+/K+ ATPase ß subunits.
Genetic interactions define multiple pathways for tracheal tube size
control
The nearly identical tracheal morphological defects in nrv2, coracle,
varicose and convoluted (Fig.
5D,E,I,K; Table 1)
suggested that these genes may act in a single linear genetic pathway. If so,
then double mutant combinations of nrv2-null alleles and mutations in
other tube-expansion genes should have the same tracheal phenotypes as a
nrv2-null single mutant. This prediction is true for coracle
and gliotactin (Fig.
5F,H), and is consistent with Coracle mislocalization in
nrv2 mutants (Fig.
3B). However, the double mutant combination of a
nrv2-null mutation and a convoluted mutation that does not
cause septate junction barrier defects
(Fig. 2h) causes more severe
phenotypes than nrv2-null mutations
(Fig. 5J). Similarly, the
double mutant combination of a nrv2-null mutation and a
varicose mutation that causes septate junction barrier defects
(Fig. 2, legend) causes more
severe phenotypes than nrv2-null mutants
(Fig. 5L), indicating that
these genes are unlikely to act in a simple linear pathway. The nrv2
cystic double mutant phenotype was indistinguishable from cystic
(Fig. 5M,N), suggesting that
cystic acts in parallel or downstream of nrv2.
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Discussion |
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Septate junction control of tube size is not mediated by paracellular
barrier function
A simple explanation for the abnormal sizes of tracheal tubes in mutants
having septate junction defects would be that ionic or hydrostatic
disequilibria across the tracheal epithelium disrupts tracheal cell
morphogenesis. If so, then all mutants with similar paracellular barrier
defects should have equivalent tracheal morphologies. However, we found that
barrier mutants had tracheal morphologies ranging from near wild type in the
case of cor14* to diameter- or length-specific defects in
cystic and megatrachea. These results support the conclusion
that septate junction control of tube size is not dependent on regulation of
paracellular diffusion.
Multiple pathways of septate junction tube size control and
paracellular transport
The mutant phenotypes and genetic interactions among tracheal
tube-expansion and septate junction mutants suggest there are at least two
genetic pathways by which septate junctions regulate tracheal tube size
(Fig. 7A). For example,
nrv2 and coracle appear to act in the same genetic pathway
as nrv2 and coracle null mutants have the same tracheal
phenotypes as each other and as nrv2 coracle double null mutants
(Fig. 7A, middle column). This
genetic evidence is supported by our observations that the localization of
Coracle to septate junctions is disrupted in nrv2 and
ATP mutants. By contrast, although nrv2-null and
varicose mutants have the same tracheal phenotypes, nrv2 and
varicose are unlikely to act in the same linear genetic pathway
because nrv2 varicose double mutants have more severe tracheal
phenotypes than nrv2-null mutants
(Fig. 7A, left column). This
result suggests that either varicose and nrv2 function in
separate pathways to control tube size, or there is redundancy between the
functions of these genes. We present the separate pathway model as a formal
logic diagram in Fig. 7A and as
a molecular model in Fig.
7B.
|
Possible mechanisms for Na+/K+ ATPase and
septate junction regulation of tube size
A central issue raised by our findings is the nature of the molecular
functions(s) of the Na+/K+ ATPase and septate junctions
in tube-size control. Although the Na+/K+ ATPase has
been studied intensively for more than 40 years for its function as an ion
pump (Chow and Forte, 1995;
Blanco and Mercer, 1998
), our
data indicate that the tracheal tube-size function of the
Na+/K+ ATPase is intimately associated with its role in
septate junction function. Furthermore, as described above, the paracellular
barrier and tube-size control functions of the septate junction are
separable.
In one class of model that accounts for these observations, the role of the
Na+/K+ ATPase is to organize septate junctions, which
control tube size by an undetermined mechanism. The many functions of
vertebrate tight junctions provide possible examples of non-barrier mechanisms
by which septate junctions could control tube size. In particular, tight
junctions organize polarized apical secretion mediated by the exocyst
(O'Brien et al., 2002), bind
cytoskeletal components such as ankyrin and fodrin
(Fanning et al., 1999
),
contain potential signaling molecules such as the tyrosine kinases Src, Yes
and protein kinase C (Fanning et al.,
1999
), and have recently been shown to regulate the activity of a
Y-box transcription factor (Balda et al.,
2003
). In addition, both septate and tight junctions complexes
contain proteins that organize epithelial cell apical/basal domains
(Tepass et al., 2001
).
Of the tube-size control models that do not invoke ion-transport functions
of the Na+/K+ ATPase, models involving apicobasal domain
organization are particularly attractive. Apical surface regulation is a
common theme in tubulogenesis (Lubarsky
and Krasnow, 2003; Buechner et
al., 1999
; O'Brien et al.,
2002
), and has been shown to play an important role in tube-size
control in the Drosophila salivary gland
(Myat and Andrew, 2002
).
Several observations support the possibility that septate junctions control
tracheal tube size through the apical cell surface. First, the differential
regulation of tracheal apical and basal cell surfaces suggests that tracheal
tube size control is mediated at the apical cell surface
(Beitel and Krasnow, 2000
).
Second, the increased tracheal tube lengths and diameters present in
tube-expansion mutants necessitate an increased apical cell surface area.
Given that the Dlg/Scrib/Lgl complex normally present in septate junctions has
an early embryonic function to negatively regulate the extent of the apical
membrane domain (Bilder et al.,
2003
; Tanentzapf and Tepass,
2003
), this complex could also act later to negatively regulate
tracheal apical surface area.
In an alternative class of models that are not exclusive of the above
possibilities, the ion-pump activity of the Na+/K+
ATPase may directly or indirectly mediate tube-size control. For example,
pharmacologically blocking Na+/K+ ATPase ion-transport
activity leads to increased intracellular Ca2+ levels in some cell
types (Ravens and Himmel,
1999), and Ca2+ signaling abnormalities may be the
molecular defect that causes the enlarged tubules of polycystic kidney disease
(PKD) (Calvet, 2002
;
Hou et al., 2002
;
Yoder et al., 2002a
;
Yoder et al., 2002b
). Another
example is that the low intracellular Na+ level maintained by the
Na+/K+ ATPase is required for formation of tight
junctions and stress fibers in Madin-Darby canine kidney (MDCK) cells, an
epithelial cell line that can form tubules in response to hepatocyte growth
factor (Rajasekaran et al.,
2001
). Septate junction formation might also require low
intracellular Na+ levels. Finally, disruption of the cellular
Na+/K+ electrochemical gradient could impact secondary
active transport of other solutes that may be important for proper tube-size
regulation.
Although the exact biochemical roles of the Na+/K+ ATPase and septate junctions in tube-size control are unclear, identification of these complexes as parts of a tube-size control mechanism is an important step towards further understanding these mechanisms at the molecular level.
Na+/K+ ATPase in vertebrate epithelial
tube-size disorders
The Na+/K+ ATPase has been implicated in vertebrate
tube-size control by the abnormal subcellular localization of the
Na+/K+ ATPase in the inappropriately expanded tubules in
individuals with PKD and in several animal models of cystic kidney diseases
(Avner et al., 1992;
Carone et al., 1995
;
Ogborn et al., 1995
;
Wilson et al., 2000
;
Wilson et al., 1991
). However,
it has not yet been determined whether this mislocalization contributes to the
progression of cystic diseases or whether it is merely a secondary effect of
other cellular defects. Our finding that the Na+/K+
ATPase is required for normal tube-size control in the Drosophila
tracheal system suggests that the vertebrate Na+/K+
ATPase may play an important role in maintaining the normal size of kidney and
other epithelial and endothelial tubes. Ultimately, a molecular understanding
of the tube-size control mechanisms should allow development of new strategies
for preventing and treating PKD and other diseases resulting from epithelial
and endothelial tube defects.
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ACKNOWLEDGMENTS |
---|
![]() |
Footnotes |
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