1 Department of Urology, College of Physicians and Surgeons, Columbia
University, New York, NY 10032, USA
2 Department of Pathology, and College of Physicians and Surgeons, Columbia
University, New York, NY 10032, USA
3 Department of Obstetrics and Gynecology, College of Physicians and Surgeons,
Columbia University, New York, NY 10032, USA
* Author for correspondence (e-mail: clm20{at}columbia.edu)
Accepted 25 November 2004
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SUMMARY |
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Key words: Foxd1, Bmp4, Smad1, Fused kidneys, Renal capsule, Mouse
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Introduction |
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Our previous studies suggest that distinct stromal cell populations occupy
the nephrogenic and differentiation zones of the kidney and assist in
maintaining their unique signaling environments
(Levinson and Mendelsohn,
2003). As an example, Foxd1, a member of the forkhead box
(Fox) transcription factor superfamily, is most strongly expressed in
the cortical stroma (Hatini et al.,
1996
). Inactivation of Foxd1 by a genetic `knock-in'
approach leads to severe renal malformations, including impaired branching
morphogenesis and nephron differentiation
(Hatini et al., 1996
). In
addition, the maturing kidneys in the Foxd1 mutant are fused and
remain localized in the pelvis rather than ascending fully to their normal
position in the lumbar region (Hatini et
al., 1996
). The failure of the mutant kidneys to separate from the
midline and leave the pelvic region is reminiscent of fused kidneys that
sometimes occur in human embryogenesis.
The cause of the pelvic kidneys, as well as the defects in UB branching and nephron formation, in the Foxd1-null mutant embryo has remained unclear. For example, it is still unknown if the ureteric or nephrogenic compartments in the mutant kidney are capable of receiving inductive signals, and, if they are, why they do not pattern correctly. We have thus analyzed the Foxd1-null mutant in further detail to explain these key phenotypes and to gain a deeper understanding of the role of the stromal cell population in kidney morphogenesis. We report here that it is defects in the maturation of the renal capsule that is the underlying cause of the ectopic kidneys in the Foxd1-null embryo. In addition, we show that Foxd1 plays a crucial role in forming the correct population of cell types within the renal capsule. Thus, although we show that the ureteric and nephrogenic compartments of the mutant kidney are indeed able to receive inductive signals, it is the ectopic presence of cell types within the mutant capsule, such as Bmp4-expressing cells and endothelial cells, that accounts for the impaired branching and delay in nephron differentiation.
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Materials and methods |
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ß-Galactosidase staining
Frozen sections were post-fixed in 0.2% paraformaldehyde and then placed in
lacZ staining solution (1xPBS, 2 mM MgCl2, 5 mM
potassium ferricyanide, 5 mM potassium ferrocyanide and 1 mg/ml X-gal) at
30°C overnight in the dark for Bmp4lacZ samples. For
Foxd1lacZ samples the incubation times were 3 hours for
mutant tissue and 7 hours for heterozygous tissue to normalize for the number
of Foxd1lacZ alleles.
In situ hybridization and immunohistochemistry
In situ hybridization on frozen thin sections and whole-mount tissue were
performed using DIG-labeled probes as described previously
(Mendelsohn et al., 1999). For
the probes Pax8, Lim1 and Ret, a similar in situ
hybridization protocol was used except that paraffin wax-embedded tissue was
used (after deparaffinization) and [33P]UTP (NEN) was used to label
the probe. Detection of the labeled probe was achieved by immersion of the
slides into emulsion (Kodak), which was then solidified for 3 days at 4°C.
Visualization of the exposed emulsion was performed by both dark- and
light-field microscopy.
Immunohistochemistry for PECAM (PharMingen) was preceded by ß-galactosidase staining as described above. The anti-phospho-Smad1 antibody (Cell Signaling Tech) and the anti-Ki-67 antibody (Zymed) were used according to manufacturers' instructions for paraffin-embedded tissue. ß-Gal immunofluorescence was performed using goat anti-ß-galactosidase antibody (Biogenesis), diluted 1:2000, followed by donkey anti-goat-CY3 (Jackson ImmunoResearch), diluted 1:700.
Lectin histochemistry
Vibratome sections (150 µm) were fixed in 3% paraformaldehyde,
permeabilized with saponin and neurominidase (both from Sigma), and then
labeled with TRITC-PNA lectin and FITC-Lotus lectin (Vector Labs). The samples
were photographed by confocal microscopy.
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Results |
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The thickening of the capsule layer in the mutant kidneys, along with the much higher incidence of cells negative for ß-gal expression and the lack of separation of the kidneys from the dorsal body wall, raise the possibility that the failure in ascent of Foxd1lacZ/lacZ kidneys may be linked to abnormalities in the renal capsule that impede the ability of the kidneys to detach from the body wall.
Loss of Foxd1 expression results in alterations in the cell types of the renal capsule
As loss of Foxd1 expression results in abnormalities in the
morphology of the mutant capsule, we wished to investigate further if this
defect correlated with alterations in the cellular composition and activity of
this structure. We therefore examined the expression of molecular markers that
define the normal renal capsule. In addition to Foxd1, Raldh2 [whose
protein product is a dehydrogenase required for retinoic acid synthesis
(Hsu et al., 2000;
Niederreither et al., 1999
)]
and Sfrp1 [which encodes for a secreted frizzled-related protein that
antagonizes Wnt signaling (Yoshino et al.,
2001
)] are both strongly expressed in the renal capsule (black
arrowheads in Fig. 3A,C,E). In
wild-type embryos, Raldh2 expression was at high levels in the
capsule and the cortical stroma and at lower levels in the nephrogenic
mesenchyme and epithelia (Fig.
3C), whereas Sfrp1 was highly expressed in the capsule
and at barely detectable levels in the cortical stroma that populate the
nephrogenic zone (Fig. 3E). In
Foxd1 mutants, however, Raldh2 expression was undetectable
in the capsule (compare expression underneath black arrowheads in
Fig. 3B with those in
Fig. 3D). Likewise,
Sfrp1 expression was also practically undetectable in the mutant
capsule (Fig. 3F). Thus,
molecular markers normally expressed in the capsule are undetectable in the
capsule of the Foxd1-mutant kidney, suggesting that Foxd1 may be
required for either establishing or maintaining the normal cellular activity
of this structure.
|
Examination of two markers of the medullary stroma, Pod1
(Quaggin et al., 1998) and
p57 (Hiromura et al.,
2001
), revealed that, although there was abundant expression of
these two genes in the Foxd1-mutant kidneys, there was no detectable
expression in the mutant capsule (data not shown). We, therefore, next
examined markers of other cell lineages, such as the nephrogenic mesenchyme
(Gdnf) (Durbec et al.,
1996
) and nephrogenic precursors (Pax2, Wnt4, Lim1, Pax8,
Sfrp2 and Hoxa11) (Dressler
et al., 1990
; Fujii et al.,
1994
; Leimeister et al.,
1998
; Plachov et al.,
1990
; Sariola and Sainio,
1998
; Wellik et al.,
2002
). The expression of these markers were present at similar
levels to wild type in the nephrogenic structures of the mutant. However, they
were not detectable in the capsule of either wild-type or
Foxd1-mutant kidneys (data not shown). These findings suggest that
cells that populate the Foxd1-null capsule are likely to be neither
medullary stroma nor nephron progenitors.
The loss of Sfrp1 expression in the mutant renal capsule led us to
investigate for the presence of endothelial cells in this structure because it
is know that sFRP1 regulates endothelial migration
(Dufourcq et al., 2002).
Normally, endothelial cells are localized to the interior of the kidney at
E16.5 (Fig. 3G), as revealed by
expression of PECAM (Pecam1 - Mouse Genome Informatics), a marker of this cell
type (Sheibani et al., 1999
).
In Foxd1lacZ/lacZ kidneys, however, Pecam1 expression was
also strongly displayed in the capsule, though expression did not overlap with
lacZ (Fig. 3H). This
difference in expression of Pecam1 was also observed at E14.5 and E18.5 (data
not shown). Likewise, the expression of Flk1, another marker of endothelial
cells (Robert et al., 1996
),
revealed an identical expression pattern to Pecam1 at E14.5 in wild-type and
mutant tissue (data not shown). These results indicate that, indeed, a cell
type not normally observed in the renal capsule is abnormally present in the
mutant capsule. Finally, the lectin peanut agglutinin (PNA) displayed ectopic
reactivity with the mutant capsule at E15.5
(Fig. 3I,J), further indicating
an altered composition of the mutant capsule.
The ectopic presence of Pecam1+/Flk1+ cells in the
mutant capsule was intriguing to us because a recent report has shown that
migration of endothelial cells is promoted by Bmp signaling
(Valdimarsdottir et al.,
2002). Normally, Bmp4+ cells are present in the
interior regions of the maturing kidney
(Dudley and Robertson, 1997
;
Miyazaki et al., 2000
) and,
thus, probably guide the migrating endothelial cells into the future capillary
tufts of the developing glomeruli. The presence of endothelial cells in the
mutant capsule, therefore, suggested to us that cells in the mutant capsule
could be positive for Bmp4 expression. We investigated this
possibility by examining Bmp4 expression (as measured by
lacZ expression from a Bmp4lacZ allele) in tissue
from
Bmp4lacZ/+;Foxd1GFP/+
and Bmp4lacZ/+;Foxd1GFP/GFP
embryos. Indeed, at E14.5 Bmp4 expression was at a high level in the
capsule of the Foxd1-mutant kidney
(Fig. 4B), but not in the
capsule of the wild-type kidney (Fig.
4A). We also investigated the expression pattern of Bmp4
at E12.5: Bmp4lacZ-expressing cells completely surrounded
the mutant kidney (Fig. 4D), an
expression pattern that was not observed in wild-type littermates
(Fig. 4C) at this age. These
results could reflect the difference in location within the pelvis between the
wild-type and mutant kidneys at this age, and their positions relative to the
abundant levels of Bmp4+ cells that exist in the caudal aspects of
the embryo, especially near the cloaca (data not shown).
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The ectopic presence of Bmp4+ cells in the capsule of the mutant kidney results in aberrant signaling to the nephrogenic mesenchyme
As in vitro experiments have shown that Bmp4 can inhibit nephron
differentiation (Raatikainen-Ahokas et
al., 2000), we wished to determine if the ectopic presence of
Bmp4+ cells in the mutant capsule led to abnormal signaling in
Foxd1-null kidneys. Binding of Bmp4 to its cell surface receptors
induces cytoplasmic phosphorylation of Smad1 (phospho-Smad1) in order to
propagate its signal (Dale and Jones,
1999
). We, therefore, performed immunohistochemistry of
phospho-Smad1 on sections of wild-type and Foxd1-mutant embryonic
kidneys in order to determine the targets of Bmp4 signaling. In the wild-type
kidney at E12.5, phospho-Smad1 expression is in the condensed mesenchyme,
particularly in the pre-tubular aggregates that lie on the side of the UB tip
opposite of the forming kidney capsule
(Fig. 4I,I'). However,
there is a distinct lack of phospho-Smad1 expression in the nascent capsule of
the wild-type kidney. In the Foxd1-mutant kidney at this age, there
is also phospho-Smad1 expression in the condensed mesenchyme
(Fig. 4J,J'), and in some
cells in the thick layer of mesenchyme that forms the abnormal capsule of the
mutant kidney.
At E14.5 in the wild-type kidney, phospho-Smad1 expression is in the ureter
and the stalks of the UB, but not in the region of the tips facing the capsule
(Fig. 4K,K'). There is
also expression in the pre-nephrogenic structures of the induced mesenchyme
that exist under the arms of the bifurcated UB tips. These expression results
correspond well with the previously determined expression pattern of the Bmp
receptors in the developing kidney
(Martinez et al., 2001). At
this age in the Foxd1 mutant, phospho-Smad1 expression is present in
the nephron progenitors that lie between the ureteric bud tips and the outside
edge of the kidney (Fig.
4L,L'). Thus, ectopic location of Bmp4+ cells in
the capsule of Foxd1-null kidneys induces ectopic Bmp4 signaling in
the nephrogenic compartment. In addition, there is also phospho-Smad1
expression in the UB but only in the side of the UB that is in direct contact
with the phospho-Smad1+ pre-tubular aggregates
(Fig. 4L,L').
Defects in the proper specification of the mutant capsule result in aberrant patterning of the nephrogenic compartment
The initial study suggested that nephron formation was nearly absent in
Foxd1 mutants (Hatini et al.,
1996). Thus, it is possible that the ectopic formation of the
pre-nephrogenic structures in regions just below the edge of the kidney in the
Foxd1-null embryos may place them in an inappropriate signaling
environment that prevents their further differentiation. To begin to address
this issue and to determine when a possible block in differentiation occurs,
we first wanted to verify that the enlarged condensates that are displayed in
the Foxd1-mutant kidney expressed an appropriate marker of condensed
mesenchyme. Pax2, which is strongly expressed in condensed mesenchyme
(Dressler et al., 1990
), is
localized in nephron progenitors in both wild-type and Foxd1-null
kidneys (Fig. 5A,B). Second, we
also wanted to verify that the subpopulation of condensed mesenchyme that is
induced to form pre-tubular aggregates and renal vesicles in Foxd1
mutants express appropriate markers of these structures. Thus, we examined the
expression of Wnt4 (Sariola and
Sainio, 1998
) and Sfrp2
(Leimeister et al., 1998
),
which in wild-type embryos are expressed in pre-tubular aggregates and renal
vesicles, respectively, and they are exclusively localized beneath ureteric
bud branches in a ratio of one pre-tubular aggregate per bud tip
(Fig. 5C,E and
Fig. 4K'). In
Foxd1 mutants, Wnt4 and Sfrp2 were expressed in
similar structures as wild type, but they were found at the very edge of the
kidney facing the renal capsule (Fig.
5D,F), a location where they were never found in wild-type
littermates. In addition, multiple pre-tubular aggregates (2-4) were observed
per UB ampulla (Fig. 5D;
Fig. 4L). Examination of other
markers of pre-nephrogenic structures, such as Lim1
(Fujii et al., 1994
),
Pax8 (Plachov et al.,
1990
) and Hoxa11
(Patterson et al., 2001
;
Wellik et al., 2002
), revealed
similar results (data not shown). Interestingly, the mislocation of the
pre-nephrogenic structures in the mutant kidney corresponds to the region of
ectopic phospho-Smad1 expression. Thus, abnormal Bmp4 expression in
the capsule may have a directly negative effect on nephron progenitors, as
previously suggested (Raatikainen-Ahokas
et al., 2000
). Third, we wanted to examine the proliferation of
the cells in the pre-nephrogenic structures in the mutant kidney to determine
if lack of growth was the reason for the loss in future nephron formation.
However, immunohistochemistry with the antibody Ki-67, which selectively binds
to proliferating cells (Gerdes et al.,
1991
), revealed that both wild-type and mutant kidneys displayed
similar levels of proliferation in the condensed mesenchyme
(Fig. 5G,H). Similar results
were also observed with phospho-histone H3 and BrdU (data not shown).
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Aberrations in the patterning of the mutant ureteric tree result indirectly from defects in capsule formation
As previously reported, ureteric bud patterning is abnormal in the
Foxd1 mutant, which led to the suggestion that Foxd1-regulated
signals from stromal cells could be required for branching morphogenesis
(Hatini et al., 1996). In the
present study, we observed a threefold difference in ureteric bud branching
(as determined by the number of bud tips) at E13.5 (data not shown), which
increased to
40-fold by E18.5 (Fig.
6A,B). In addition, the overall ureteric patterning in the
Foxd1 mutant was significantly altered. The collecting system of the
mutant kidney was composed of elongated tubules with very few ampulla at the
surface of the kidney, which is in striking contrast to the wild type
(Fig. 6A,B), suggesting that
bifurcation but not elongation was impaired - a phenotype also seen in
Bmp4-treated kidney rudiments (Miyazaki et
al., 2000
). In wild-type embryos, nephron progenitors are
associated only with UB tips, where Ret is selectively localized
(Fig. 6C,E,E'). However,
in Foxd1 mutants the branches of the UB are mostly enclosed by
condensed nephrogenic mesenchyme, and Ret is expressed ectopically
throughout these portions of the enclosed ureteric buds
(Fig. 6D,F,F'). It is
interesting to note, however, that areas of the UB that are not in contact
with condensed mesenchyme (such as the regions indicated by the arrowheads in
Fig. 6F') do not express
Ret, which is analogous to the cleft in the normal UB (yellow
arrowhead in Fig. 6E').
This result suggests that the UB in the Foxd1-mutant is still
competent to respond to signals.
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Discussion |
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Capsule abnormalities in the Foxd1-null embryo may lead to aberrant patterning of the nephrogenic compartment
In the normal kidney, the renal capsule, in addition to Foxd1,
also expresses Sfrp1 and Raldh2. In the Foxd1-null
kidney, however, the maturation of the capsule is aberrant. Instead of a
single, homogeneous layer of cells, there is a thicker, heterogeneous layer
that is histologically abnormal, displays undetectable levels of
Sfrp1 and Raldh2 expression, and contains ectopic cell
types. These include Bmp4-expressing cells and endothelial cells,
though other cell types that we have not identified may also be present.
Normally, the exclusion of these non-capsular cell types from the capsule may
be based on expression of inhibitory signals or on differential adhesive
properties. For example, one defect in the mutant capsule may be a change in
the composition of the extracellular matrix (ECM), as there is an ectopic
reactivity of PNA and lectins adhere to sugar moieties of glycoproteins that
are often expressed in the ECM (Faraggiana
et al., 1982). Indeed, a change in lectin histochemistry is a
valid criterion of disease in the study of pathology
(Danguy et al., 1998
;
Silva et al., 1993
).
The early alterations in capsule cellular composition and morphology
suggest that this defect may be the cause of the abnormal nephrogenic and
ureteric patterning that occurs later. However, as renal patterning is
reciprocal, it is also possible that loss of Foxd1 expression in
stroma generates signals that act on non-stromal cell types, which in turn
regulate normal capsule differentiation. Nonetheless, the ectopic presence of
Bmp4-expressing cells in the capsule of the mutant kidney is of particular
importance. The normal localization of Bmp4-expressing cells is consistent
with its proposed role in promoting nephron differentiation
(Raatikainen-Ahokas et al.,
2000) and maturation of the collecting duct system
(Miyazaki et al., 2000
).
Indeed, our current studies support the conclusions of these in vitro
experiments by showing that the targets of Bmp4 signaling, as judged by
phopho-Smad1 expression, are the pre-tubular aggregates of the induced
mesenchyme and the pre-nephrogenic epithelia that they form, as well as the UB
stalks. In Foxd1-null kidneys, however, the establishment of unique
zones of induction (the nephrogenic zone) and differentiation (the
juxtamedullary region) never occurs. Instead, there is an absence of a clearly
defined nephrogenic zone, nephron development is delayed and when glomeruli do
appear, they occupy all regions of the kidney, including the edges. Our
findings suggest that this aberrant patterning is due to the inappropriate
presence of Bmp4-secreting cells in the mutant renal capsule. This ectopic
localization results in an alteration in the normal pattern of Bmp4 signaling,
such that signaling to both peripheral and interior nephron progenitors
occurs. Thus, it appears likely that in wild-type embryonic kidneys all
nephrogenic mesenchyme can respond to Bmp4 signals, but the establishment of
unique zones of nephron induction and differentiation is highly dependent on
the appropriate positioning of Bmp4-secreting cells. Therefore, defects in the
formation of the renal capsule in the Foxd1-null mouse, which allows
inappropriate accumulation of Bmp4-expressing cells in an important signaling
environment, could disrupt the normal patterning of the nephrogenic
compartment.
Foxd1-signaling in cortical stroma is not required directly for nephron differentiation
In spite of this mislocalization of pre-nephrogenic structures in the
mutant, the fact that Bmp4 could still signal to the pre-tubular aggregates
and the pre-nephrogenic epithelia reveals that the defect in the
Foxd1 mutant is not an inability of the condensed mesenchyme to
respond to inductive signals. Indeed, in addition to displaying phospho-Smad1
expression, there is also expression of Wnt4, Lim1, Pax8, Sfrp2 and
Hoxa11 in the mutant. Furthermore, although the initial report
describing the Foxd1-null mouse mentioned a severe reduction in the
differentiation of the condensed mesenchyme into nephrons
(Hatini et al., 1996), our
studies revealed only a delay in nephrogenesis up to E15.5, which was overcome
by E16.5. In fact, near the time of birth (E18.5) the mutant kidney displayed
the presence of nephrons (on a per-volume basis) at levels nearly approaching
wild type. This result is surprising as the number of UB tips is drastically
reduced in the mutant kidney and it has previously been thought that nephron
number is determined by the number of UB tips
(Clark and Bertram, 1999
;
Oliver, 1968
). This paradox
may be explained by the observation that, unlike the situation in wild-type
embryos, where each UB tip is associated with one pre-tubular aggregate, UB
ampullae in Foxd1-null kidneys are associated with two to four
pre-tubular aggregates each. Presently, it is still unclear whether this
change in the UB:pre-tubular aggregate ratio in the mutant is due to a general
defect in the patterning of the nephrogenic compartment or, rather, due to a
misregulation of signals in a potential Foxd1-regulated mesenchyme-stroma-UB
signaling axis that dictates this ratio. What is clear, however, is that
nephron differentiation in the Foxd1-null mutant is independent of
the defects in UB branching and that the defect in the Foxd1-mutant
kidney is not so much a reduction in its nephrogenic capacity, but, rather, in
its ability to correctly organize this compartment.
Bmp4 signaling determines UB patterning indirectly via nephron progenitors
Previous reports have suggested that Bmp4 inhibits budding of the UB while
promoting its elongation (Bush et al.,
2004; Miyazaki et al.,
2000
). Likewise, Bush et al.
(Bush et al., 2004
) have also
suggested that the tips of the growing UB are not exposed to high
concentrations of Bmps, but rather the region of the UB destined to become the
stalk sees high concentrations of these factors and is thus inhibited from
branching. Our results reveal that phospho-Smad1 expression in the wild-type
kidney is in the stalks of the UB with no detectable expression in the region
of the UB tips facing the capsule. This result correlates well with this
earlier suggestion by Bush et al. as Bmp4 signaling is in the appropriate
region of the UB to promote its elongation (that is, in the stalks) without
interfering with its ability to branch (that is, lack of signaling in the UB
tips facing the capsule).
In the Foxd1 mutant, there is Bmp4 signaling to the UB, though
only in those regions that are in contact with mesenchyme and pre-nephrogenic
structures that also display Bmp4 signaling (see
Fig. 4L,L'). Nonetheless,
the branching pattern of the ureteric tree is aberrant. After E12.5, it does
not display `Y-shaped' branches typical of the wild-type ureteric tree but
instead amorphous branches, with thickened ampullae, and they are few in
number. In addition, unlike the wild-type UB, which is enclosed by nephron
progenitors only at its tips, many of the UB branches of the mutant kidney are
enclosed in a thick layer of nephron progenitors, which we hypothesize is a
consequence of ectopic Bmp4 signaling in the capsule. Thus, in the
Foxd1-null kidney the UB cleft and stalk are not distinguished from
the tips and Ret expression remains at a high level throughout this
region of the UB, unlike the wild-type UB in which Ret expression
becomes selectively localized in the tips when they become capped by
condensation of the nephrogenic mesenchyme. This ectopic domain of
Ret expression in the UB of the mutant may be a result of abnormal
contact with nephron progenitors or it may be due to failure of these domains
to access signals that control branching and elongation. Consistent with these
possibilities, the regions of the mutant UB that are not in contact with
nephron progenitors do not display Ret expression, indicating that
the mutant UB may still be competent to respond to signals but is prevented
from doing so by the presence of the enlarged condensates. Indeed, the
eventual differentiation in the mutant of the nephron progenitors into mature
nephrons at E16.5, probably allows further UB branching because by E18.5 there
are tips at the surface of the kidney, though 40-fold less than wild
type. Thus, unlike what was previously thought
(Hatini et al., 1996
), it
appears that the architecture of the mature ureteric tree in the mutant kidney
- long spindly branches with very few tips at the surface of the kidney - is a
consequence of the defects in the patterning of the nephrogenic compartment,
rather than from a direct defect in signaling from the stroma. This result in
vivo correlates very well with in vitro organ cultures treated with exogenous
Bmp4 that also display defects in ureter patterning (and also without a loss
of Ret expression) but are indirectly due to interference in the
differentiation of the nephrogenic mesenchyme
(Raatikainen-Ahokas et al.,
2000
).
The capsule provide critical functions required for the normal patterning of the developing kidney
Although additional studies will be necessary to determine further signals
affected by the loss of Foxd1 expression, the unique expression of
Foxd1 in the capsular stroma and the severe affects that occur upon
its absence highlight the importance of this cell type in the overall, normal
patterning of the kidney. In particular, the results in our study suggest that
in the wild-type kidney a very careful balance of signals emanating from the
capsular stroma, the cortical stroma, and the interior patterns the
nephrogenic and ureteric compartments of the nephrogenic zone. Signals from
the capsule, such as sFRP1, may keep the mesenchyme from differentiating too
quickly, as well as indirectly promoting UB branching while orientating the
growth of the ampullae toward the kidney surface
(Schumacher et al., 2002).
Meanwhile, signals from the interior of the kidney, such as Bmp4, result in
the induction of the mesenchyme and elongation of the UB stalks, followed by
signals from the stroma that influence the balance between glomerulogenesis
and tubulogenesis (Yang et al.,
2002
). In this way, a controlled radial expansion of the kidney
occurs, while maintaining its proper compartmentalization. In the
Foxd1 mutant, this careful balance of signals is significantly
disturbed because of an aberration in the maturation of the kidney capsule;
thus, illustrating both the capacity and the importance of renal capsule
signaling that is necessary for the proper positioning and pattering of the
developing kidney.
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ACKNOWLEDGMENTS |
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