Altered Epithelial Cell Proportions in the Fetal Lung of
Glucocorticoid Receptor Null Mice
Timothy J. Cole, Nicola M. Solomon, Rosemary Van Driel, Julie A. Monk, Daniel Bird, Samantha J. Richardson, Rodney J. Dilley,
and Stuart B. Hooper
Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, Victoria; Murdoch Children’s Research Institute,
Royal Children’s Hospital, Parkville, Victoria; Baker Heart Research Institute, Prahran, Victoria; and Department of Physiology,
Monash University, Clayton, Victoria, Australia
Glucocorticoids provide important signals for maturation of the
fetal lung and antenatal glucocorticoids are used to reduce the
respiratory insufficiency suffered by preterm infants. To further
understand the role of glucocorticoids in fetal lung maturation, we
have analyzed mice with a targeted null mutation for the glucocorticoid receptor (GR) gene, which severely retards lung development.
The lungs of fetal GR-null mice have increased lung weight and
DNA content, are condensed and hypercellular, with reduced septal
thinning leading to a 6-fold increase in the airway to capillary diffusion distance. In fetal GR-null mice, mRNA levels of the type II
epithelial cell surfactant protein genes A and C were reduced by
ⵑ 50%. Analysis of epithelial cell types by electron microscopy
revealed that the proportions of type II cells were increased by
ⵑ 30%, whereas the proportions of type-I cells were markedly reduced (by ⵑ 50%). Similarly, we found a 50% reduction in mRNA
levels for T1␣ and aquaporin-5, two type I cell–specific markers,
and a 20% reduction in aquaporin-1 mRNA levels. This demonstrates
that during murine embryonic development, receptor-mediated glucocorticoid signaling facilitates the differentiation of epithelial cells
into type I cells, but is not obligatory for type II cell differentiation.
The development and embryonic growth of the mammalian lung
is a complex and highly organized process involving a combination of intrinsic growth and differentiation factors, and concerted
actions from circulating factors and hormones (1). Glucocorticoids are one of a number of circulating hormones that also
include retinoids, thyroid hormone, and other cAMP-mediated
factors that are important during the final stages of lung maturation. These hormones play important roles during late gestation,
particularly in the differentiation and development of terminal
alveoli and the promotion of lung surfactant production. Synthetic glucocorticoids (particularly betamethasone or dexamethasone) are widely used antenatally to reduce the severity of the
respiratory insufficiency suffered by preterm infants, and act to
accelerate fetal lung maturation and increase lung surfactant
production (2). Antenatal glucocorticoid treatment has had a
major benefit in reducing the incidence of neonatal respiratory
distress syndrome (RDS) and intraventricular hemorrhage, leading to decreased neonatal mortality. However, its use remains
controversial, particularly the administration of multiple doses,
due to the reported side effects of glucocorticoids on lung and
body growth and development of the central nervous system
(Received in original form June 17, 2003 and in revised form October 20, 2003)
Address correspondence to: Dr. Timothy J. Cole, Department of Biochemistry and
Molecular Biology, University of Melbourne, Parkville, 3010, Victoria, Australia.
E-mail: tjcole@unimelb.edu.au
Abbreviations: corticotrophin-releasing hormone, CRH; epithelial cell, EC; epithelial sodium channel, ENaC; glucocorticoid receptor, GR; mineralocorticoid receptor, MR; post coitum, p.c.; surfactant protein, SP; transmission electron microscopy,
TEM; wild type, WT.
Am. J. Respir. Cell Mol. Biol. Vol. 30, pp. 613–619, 2004
Originally Published in Press as DOI: 10.1165/rcmb.2003-0236OC on October 24, 2003
Internet address: www.atsjournals.org
(3, 4). Prenatal glucocorticoid treatment also has the potential
to alter fetal programming, leading to the onset of diseases such
as hypertension when the offspring develop into adults (5). However, it is well established that endogenous glucocorticoids increase
in an exponential-like manner just before birth and participate in
the regulation of biochemical and cytoarchitectural changes in the
developing fetal lung (1), yet little is known of the underlying
molecular and cellular events that underpin these glucocorticoidmediated effects.
The majority of glucocorticoid actions are mediated via the
intracellular glucocorticoid receptor (GR) (6). The GR is a ligandactivated transcriptional regulator and is a member of the steroid
receptor family, a subgroup of the nuclear receptor superfamily
(7). After steroid binding, the GR translocates to the nucleus,
dimerizes, and binds to glucocorticoid-response DNA binding
sites upstream of specific target genes (6). Glucocorticoids via GR
are able to both activate and repress target gene transcription.
Previous studies, in which fetal sheep or rabbits were treated
with synthetic glucocorticoids such as betamethasone and dexamethasone, indicated that glucocorticoids enhanced lung development and type II epithelial cell (EC) differentiation (8).
Numerous biochemical studies in preterm rabbits, rats, mice,
and sheep have shown that glucocorticoids, in part, regulate the
production of lung surfactant, particularly in promoting expression of the type II EC-specific surfactant protein (SP)-A, -B,
and -C genes (1). Glucocorticoids also influence clearance of
lung liquid and promote tissue remodeling before birth, thereby
greatly improving lung tissue compliance and gas diffusion after
birth (9–11).
To investigate the role of glucocorticoid action via GR signaling during embryonic development, the GR gene was ablated
using gene targeting in mice (12), which causes a number of
phenotypic effects. The most striking observation was seen in
the lung, where GR-null mice at birth displayed severe lung
atelectasis with little to no inflation of lung tissue. On a C57Bl6/
129 sv genetic background, ⬎ 90% of GR-null mice die at birth
from respiratory dysfunction, whereas all GR-null mice die at
birth on a 129 sv isogenic genetic background (12). An identical
phenotype of perinatal death and lung dysfunction has been described in another GR-null mouse constructed by gene-targeted
deletion of exon 2 and the proximal promoter of the GR gene
(13).
An almost identical lung phenotype is seen in corticotrophinreleasing hormone (CRH)-deficient mice (of CRH-deficient
mothers), where there is a clear impairment in glucocorticoid
production (14). CRH-null mice have delayed induction of SP-A
and SP-B with reduced pulmonary septal thinning and airway
formation. Interestingly, mice with a gene-targeted point mutation in GR that prevents dimerization and DNA binding develop
normally, with no overt lung defect, indicating that the actions
of glucocorticoids in the developing lung may be mediated by
DNA-binding independent actions (15). A recent study using
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GR-null mice with deletions of exon 2 and the proximal promoter have found a marked reduction in the growth factor midkine, 3 d before birth, that may contribute to the immature lung
phenotype detected at birth (16).
To clarify, in more detail, the role of glucocorticoids during
fetal lung development, we have further analyzed the lungs of
isogenic 129 sv GR-null mice generated previously with a series
of histologic and molecular studies (12). Using both light and
electron microscopy, we show dramatic morphologic differences
in the development of the lung, with lung maturation of GRnull mice severely retarded at birth. Airspace formation in lungs
of GR-null mice is severely reduced compared with wild-type
(WT) controls. Pulmonary surfactant is present in the airway
of GR-null lung tissue, indicating apparent normal surfactant
production and secretion. Lung tissue expansion is markedly
reduced, and upon investigation using electron microscopy, the
lungs of Day 18.5 GR-null mice resemble those of their WT
littermates at Day 16.5 of pregnancy. Finally, careful analysis of
epithelial cell types using electron microscopy and cell-specific
markers reveals a marked reduction in type I ECs at birth, which
would significantly contribute to the respiratory dysfunction due
to a severe reduction in the surface area for gas exchange. In
contrast, the proportion of type II ECs was increased in GRnull mice, indicating that corticosteroid action via GR is unnecessary for type II cell differentiation.
A minimum of 100 ECs were classified for each animal (from six
GR-null and six WT littermate controls) and the number of nuclear
profiles of each type counted (19, 20). Identification of ECs depended
on visualization of the epithelial cell basement membrane, with all
ECs localized to the luminal surface of this membrane. ECs for each
genotype were counted in a blinded analysis and categorized as one of
four phenotypes—stem cells, type I ECs, type II ECs, and intermediate
ECs. Undifferentiated (or stem) epithelial cells were rounded in shape
and contained abundant cytoplasmic glycogen; they did not contain
lamellar bodies or have any evidence of a cytoplasmic extension (see
below). Type I ECs had flattened cytoplasmic extensions, flattened
nuclei, little perinuclear cytoplasm and few cytoplasmic organelles. Cytoplasmic extensions are defined as peripheral regions of cytoplasm
that extend along the luminal surface of the epithelial cell basement
membrane, with both apical and basolateral membranes running in
parallel to each other and separated by ⬍ 0.5 m. Type II ECs were
rounded in shape with a rounded nucleus and had microvilli on their
apical surface and abundant cytoplasmic organelles, including lamellar
bodies. The intermediate cells were a heterogeneous group that displayed characteristics of both type I and type II ECs. Their classification
depended on the presence of a flattened nucleus and marked cytoplasmic extensions, but they also contained lamellar bodies and usually
had microvilli on their apical surface (19). Previous studies have shown
that in sheep, humans, and rats, the nuclear diameters of type-I and
type-II ECs are similar and, therefore, the chances of counting a nuclear
profile of each type are equal using EM (21–23).
Materials and Methods
Lung tissue was dissected from mouse embryos and fixed using the
same procedures as described previously (24). Portions of lung tissue
were embedded in paraffin, cut at 5 m, and stained with hematoxylin
and eosin. Stereologic measurements of lung tissue fraction and volume
and luminal fraction and volume, were made for three GR-null and
three WT littermate controls using standard stereologic techniques and
grids (25). Lung sections were viewed using light microscopy (⫻40)
and projected onto a screen; the final magnification was ⫻720, which
was determined by simultaneous projection of a 0.1-mm graticule.
Mice
GR-null mice were generated by gene targeting as described previously
(12) and produced by mating GR-heterozygous (⫹/⫺) mice. These
mice were generated on an isogenic 129 sv genetic background and
displayed a severe phenotype that resulted in 100% perinatal death. To
study the lung abnormalities of GR-null mice in more detail, pregnant
heterozygous female mice were killed with CO2 at Day 18.5 post coitum
(p.c.), 0.5 d before term. Pups were removed by cesarean section,
weighed, and killed by decapitation. A small piece of tail was used as
a source of genomic DNA for genotyping at the GR locus by PCR as
described previously (17).
Lung Weight and Determination of DNA Content
Total lung tissue was removed intact from Day 18.5 p.c. fetal mice,
blotted to remove excess fluid, and weighed. For determination of DNA
content, lung tissue was digested overnight by proteinase K (0.5 mg/ml)
and then genomic DNA isolated after salt precipitation of protein as
described previously (17). DNA concentrations were determined by
absorbance at 260 nm.
Hormone Assays
Corticosterone levels in plasma from fetal mice were determined by a
corticosterone radioimmunoassay (RIA) using {1, 2, 6, 7- 3H}corticosterone (Amersham, Little Chalfont, UK) and a corticosterone antibody
(B3–163; Endocrine Sciences, Calabas, CA) as described previously
(17). Plasma levels of total thyroxine (T4) was determined with a commercial RIA kit (Diagnostic Products Corp., Los Angeles, CA) using
125
I-labeled T4 as a tracer as described previously (18).
Transmission Electron Microscopy
For transmission electron microscopy (TEM), lung tissue (left major
lobe) was sliced into 2- to 4-mm cubes and immersion fixed in 4%
p-formaldehyde, 2% glutaraldehyde, 4% sucrose in Hepes-buffered
saline, pH 7.4 for 3–4 h at room temperature. The tissue was then
postfixed overnight at 4⬚C in 1.5% OsO4 in veronal acetate, rinsed 3 ⫻
5 min at 4⬚C in Tris/maleate pH 5.2, stained en bloc for 90 min on ice
in 1.5% uranyl acetate in Tris/maleate (pH 7.2), and dehydrated in
cold acetone followed by a final two changes of dry acetone at room
temperature. Fixed tissue blocks were embedded and polymerized in
Epon overnight at 65⬚C. Ultrathin tissue sections (70 nm) were stained
with uranyl acetate and lead citrate and were viewed using a Transmission Electron Microscope (TEM; JEOL100; JEOL Ltd., Akishima, Japan);
random micrographs were taken and used for analysis.
Light Microscopy and Morphometric Tissue Analysis
Isolation of RNA and Northern Blot Analysis
Total RNA was prepared from isolated mouse lung by homogenization
in TRIzol reagent (Gibco/BRL, Rockville, MD), a guanidinium isothiocyanate/phenol-based homogenization solution. After chloroform extraction, RNA was precipitated from the aqueous phase with isopropanol, washed in 70% ethanol and redissolved in nuclease-free sterile
water. Total RNA (10 g) was separated in 1.2% agarose/2.2 M formaldehyde gels and blotted onto Genescreen Plus nylon membranes for
Northern blot analysis by hybridization with 32P-labeled RNA probes.
RNA probes were derived from pBluescript or pBluescribe (Stratagene,
La Jolla, CA) plasmids containing either a 474-nucleotide mouse SP-A
cDNA fragment, a 1,550-nucleotide mouse SP-B cDNA fragment, a
683-nucleotide mouse SP-C cDNA fragment, a 575-nucleotide rat ENaC␣
cDNA fragment, a 520-nucleotide rat ENaC cDNA fragment, a 484nucleotide rat ENaC␥ cDNA fragment, a 310-nucleotide mouse aquaporin-1 (AQP1) cDNA fragment, a 240-nucleotide mouse aquaporin-5
(AQP5) cDNA fragment, or a 751-nucleotide mouse T1␣ cDNA fragment; T1␣ is a type-I AEC-specific marker (26). Each plasmid was
linearized to generate a 32P-labeled antisense riboprobe using a riboprobe kit (Promega, Madison, WI). All filters were hybridized at 68⬚C
in Ultrahyb buffer (Ambion, Austin, TX) and washed at 75⬚C in 0.1⫻
SSC, 0.1% SDS three times for 15 min each. Filters were rehybridized
with a 32P-labeled 110-nucleotide cDNA fragment of the mouse cytochrome oxidase mRNA to control for differences in total RNA loading
on agarose gels.
Statistical Analysis
Hormone levels, tissue and body weights, DNA contents, percent tissue
and airspace volumes as well as differences in AEC proportions were
analyzed by one-way ANOVA followed by a Tukey’s post hoc test,
with statistical significance set at P ⬍ 0.05. For all Northern blot analyses,
RNA levels were quantitated by densitometry analysis on a Typhoon
Phosphoimager from Molecular Dynamics (Sunnyvale, CA) using ImageQuant software, and standardized to the expression of the housekeeping gene cytochrome oxidase (cytoxi).
Cole, Solomon, Van Driel, et al.: Alveolar ECs in GR-Null Mice
Results
Circulating Hormone Levels
As previously observed for C57Bl6/129 sv fetal GR-null mice (12),
we detected in plasma samples from isogenic 129 sv fetal GR-null
mice a significant increase in circulating levels of plasma corticosterone, presumably caused by a defect in normal glucocorticoiddriven negative feedback via the hypothalamus-pituitary-adrenal
axis (Table 1). We found no difference in the levels of circulating
thyroxine (T4), another systemic hormone important in embryonic
lung development (Table 1).
615
TABLE 1. Lung weight, DNA content, and hormone analysis at Day 18.5
pc for GR-null and WT fetal mice
Genotype
n
WT
GR-Null
P Value
Total weight, g
Lung wet weight, mg/g body
weight
Lung DNA content, g/g
body weight
Corticosterone, ng/ml
Thyroxine (total), nmol/liter
6
1.12 ⫾ 0.14
1.15 ⫹ 0.16
N.S.
5
30.8 ⫾ 1.3
38.7 ⫾ 2.5
⬍ 0.01
5
5
6
56.2 ⫾ 3.5
109 ⫾ 3.8
9.22 ⫾ 0.3
89.5 ⫾ 5.4
349 ⫾ 7.4
8.82 ⫾ 0.10
⬍ 0.02
⬍ 0.01
N.S.
Values are shown as ⫾ SEM.
Fetal Body Weights, Lung Weight, and Total Lung DNA Content
Although there was no difference in overall fetal body weight,
lung wet weight and DNA content were significantly higher in
the lungs of GR-null fetal mice, compared with WT mice, which
is indicative of significant hypercellularity (Table 1). This is consistent with the histologic findings (see below) and is similar to
results found in the CRH-null mouse (14).
Histologic Analysis of the Developing Lung in Fetal GR-Null Mice
Light microscopy. At Day 18.5 pc, histologic analysis of the
lung from GR-null mice by light microscopy showed a more
condensed lung architecture with a lack of normal airspace formation and thickened interalveolar septa (Figures 1A and 1B).
The percentage of tissue and airspace for WT, heterozygous,
and the GR-null fetal lung was determined by morphometric
analysis and clearly showed that the lungs from GR-null mice
have a significantly more condensed tissue morphology compared with WT mice, with heterozygous mice having an intermediate level of tissue expansion (Figure 1C).
Electron microscopy. At Day 16.5 pc, 3 d before birth, there
was little difference in the ultrastructure of lung from WT and
GR-null mice; the epithelium of the terminal airways was predominantly comprised of undifferentiated epithelial cells containing large quantities of glycogen (data not shown). However,
by Day 18.5 pc, many of these cells had differentiated (see below)
in the lung of WT fetal mice, whereas the lungs from GR-null
fetal mice remained similar in appearance to that of a Day
16.5 pc WT lung (Figures 2C and 2D). To further assess the
hypercellularity and thicker interairway distances in the lungs
of GR-null fetal mice, we measured the minimum distances
between airways and the nearest adjacent capillaries. Using elec-
tron microscopy, multiple fields of view were assessed from the
lungs of WT (n ⫽ 6) and GR-null fetal mice (n ⫽ 6) at Day
18.5 pc. The lungs of GR-null fetal mice had significantly thicker
air/blood gas diffusion barriers, with multiple cellular compartments between the airways and adjacent capillaries; there was
no evidence of epithelial cell and endothelial cell basement membrane fusion in GR-null mice. As a result, the measured airway
to capillary diffusion distance was increased 6-fold in GR-null
mice. It increased from 0.68 ⫾ 0.14 m in WT mice to 4.02 ⫾
1.00 m in GR-null mice (P ⬍ 0.05; see Figure 3A).
Epithelial Cell Type Proportions in GR-Null Mice
The proportions of epithelial cell phenotypes in the terminal
airways of GR-null fetal mice were compared with those of WT
fetal mice at Day 18.5 pc (n ⫽ 5 for each group; Figure 3B).
Cells were categorized as either type I, type II, stem cells, or an
intermediate cell type, based upon their ultrastructural appearance using electron microscopy (see Materials and Methods
for details). There were significantly more (80% increase, P ⬍
0.05) nuclear profiles counted per unit area of tissue from GRnull fetal mice than were counted in the lungs of WT fetal mice
(Figure 3B). This was largely due to a 50% increase (P ⬍ 0.05)
in type II cells and a 2-fold increase in undifferentiated EC
nuclear profiles (P ⬍ 0.05). On the other hand, the number of
type I epithelial cell nuclear profiles per unit area was reduced
by 40% in GR-null mice compared with WT fetal mice.
Expressed as a proportion of the total number of ECs counted,
the proportion of type I ECs in the lungs of WT fetal mice
(49.6 ⫾ 2.9%) was greater than in GR-null fetal mice (20.0 ⫾
5.1%; P ⬍ 0.05). In contrast, the proportion of type II ECs in
Figure 1. Histologic analysis of lung tissue from Day 18.5 pc fetal GR-null and WT littermate mice by light microscopy. (A ) Lung tissue of Day
18.5 pc WT fetal mice, ⫻200 magnification. (B ) Lung tissue of Day 18.5 pc GR-null fetal mice, ⫻200 magnification. (C ) Morphometric analysis for
percentage (⫾ SD) of tissue (solid bars) and airspace (striped bars) in sections of Day 18.5 pc fetal lung from WT (⫹/⫹), heterozygous (⫹/⫺), and
GR-null (⫺/⫺) mice. Asterisks indicate statistical significance.
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Figure 2. Ultrastructural analysis of lung tissue from
Day 18.5 pc fetal GR-null (⫺/⫺) and WT (⫹/⫹) littermate mice by transmission electron microscopy. (A )
Pulmonary surfactant in the airway of Day 18.5 pc fetal
WT littermate mice. (B ) Pulmonary surfactant in the
airway of Day 18.5 pc GR-null mice. (C ) Lung from
Day 18.5 pc WT fetal mice. (D) Lung from Day 18.5 pc
GR-null fetal mice. RBC, red blood cell; E, endothelial
cell; I, type I epithelial cell (EC); II, type II EC; S,
undifferentiated EC; arrows, type I EC cytoplasmic
extensions; asterisks, areas previously occupied by glycogen. Bar represents 1 m.
the lungs of GR-null fetal mice (44.8 ⫾ 3.9%) was significantly
greater than the proportions of these cell types in the lungs of
WT fetal mice (30.3 ⫾ 3.1%). A close analysis of these lungs
by transmission electron microscopy revealed that the type II
ECs in GR-null fetal mice not only contained multiple lamellar
bodies (Figure 2D), but were also capable of releasing surfactant,
as indicated by the presence of abundant surfactant material
within the future airways (Figures 2A and 2B). Similarly, the
proportion of undifferentiated epithelial stem cells was also significantly elevated in GR-null fetal mice compared with WT fetal
mice (30.4 ⫾ 2.2% versus 13.6 ⫾ 2.5%; Figure 3). Consequently,
we find clear and significant alterations in the proportions of EC
phenotypes in the lungs of GR-null mice. In particular, it appears
that EC differentiation is delayed, as indicated by the higher proportion of undifferentiated stem cells, and that differentiation into a
mature type II cell phenotype is relatively unaffected by ablation
of the GR.
Expression of AEC-Specific Proteins in the Lungs of GR-Null
Fetal Mice
To further assess the differentiated type II epithelial cell phenotype in the lungs of GR-null fetal mice, we analyzed the expression
of the type II cell–specific surfactant protein genes, SP-A, SP-B,
and SP-C. Total RNA from Day 18.5 pc fetal lung tissue of
WT and GR-null mice was analyzed by Northern blot analysis
(Figure 4). We detected a 50% reduction in mRNA levels for
SP-A and SP-C in the lungs of GR-null fetal mice, compared
with WT littermate fetal mice, with no significant change in SPB
mRNA levels. Taken together with the significant increase in
the proportion of type II epithelial cells, this indicates that there
is potentially a large reduction in SP-A and SP-C mRNA per
type II AEC in the lungs of fetal GR-null mice.
To further assess the reduction in type-I epithelial cell proportions we observed in the lungs of GR-null fetal mice, we used
two well-defined type-I epithelial cell markers that are not influenced by glucocorticoids. These were a recently described type-I
cell-specific marker called T1␣, a trans-membrane protein of
currently unknown function (26), and aquaporin-5, a water channel believed to be important in the transcellular movement of
water across the type I epithelial cell (27, 28). The steady-state
mRNA levels of T1␣ and aquaporin-5 were measured by Northern
blot analysis of total lung RNA from GR-null fetal mice versus
WT controls (Figure 5). The levels of T1␣ mRNA were reduced
from 0.4 ⫾ 0.05 arbitrary units in WT fetal mice to 0.23 ⫾
0.07 arbitrary units in GR-null fetal mice, a reduction of ⵑ 50%.
Similarly, the mRNA levels for AQP5 were reduced from 0.18 ⫾
0.02 arbitrary units in WT fetal mice to 0.09 ⫾ 0.03 arbitrary
units in GR-null fetal mice, a reduction of ⵑ 50%. The level
of reduction in expression of T1␣ and AQP5 approximately
corresponded to the reduction in the proportion of type I epithelial cells detected by electron microscopy. These results suggest
that glucocorticoid signaling via GR either directly or indirectly
plays an important role in differentiation of type I epithelial
cells late in fetal development.
Expression of Glucocorticoid-Regulated Genes in the Lungs of
GR-Null Fetal Mice
As glucocorticoids have been reported to play a role in fluid
withdrawal at birth, we also measured the steady-state mRNA
levels in the lungs of GR-null versus WT fetal mice for the epithelial sodium channel (ENaC) subunits ␣, , and ␥ (Figure 4). We
found little change in mRNA levels for the ENaC␣ and ENaC
subunits, but a marked reduction in mRNA levels for the ENaC␥
subunit in lung RNA from GR-null mice. ENaC␥ mRNA levels
were reduced from 0.17 ⫾ 0.03 in WT fetal mice to 0.075 ⫾ 0.02
in GR-null fetal mice. In GR-null fetal mice, we also measured
the steady-state mRNA levels for AQP1, which is an important
water channel in endothelial cells and is a known glucocorticoid-
Cole, Solomon, Van Driel, et al.: Alveolar ECs in GR-Null Mice
617
Figure 3. Airway to capillary distance and the proportions of the different epithelial cell types in the lung of WT (⫹/⫹) and GR-null (⫺/⫺)
fetal mice at Day 18.5 pc. (A ) Comparisons of airway to capillary distance
(m ⫾ SD) in the lungs of Day 18.5 pc WT (⫹/⫹) and GR-null (⫺/⫺)
fetal mice. (B ) Epithelial cell types were identified using the criteria
described in Materials and Methods and were determined from representative electron micrographs of lung tissue from six WT and six GRnull fetal mice. Open bars, type I; striped bars, type II; hatched bars,
undifferentiated EC; solid bars, intermediate. Average numbers are
shown ⫾ SD. Asterisks indicate statistical significance.
induced gene in the developing lung (29) (Figure 5). Levels of
AQP1 mRNA were reduced in GR-null fetal lung to ⵑ 30% of
WT littermates. Abrogated induction of AQP1 in the developing
lung may also contribute to respiratory dysfunction via a reduction in fluid withdrawal at birth.
Discussion
This study has further investigated some key aspects of the
proposed physiologic role of glucocorticoids in embryonic lung
development. GR-null mice at birth have a severe respiratory
defect that has been recently reproduced in a second GR-null
mouse line that lacks the GR promoter and entire second exon
(13). This severe respiratory defect is almost identical to the lung
defect of CRH-null mice that lack production of glucocoticoid
hormones (14). GR-null mice have a reduced capacity to activate
key gluconeogenic enzyme genes in the liver and impaired negative feedback in the hypothalamus-pituitary-adrenal (HPA) axis,
resulting in markedly elevated plasma ACTH and corticosterone
levels (12, 30). The adrenal glands of fetal GR-null mice are
enlarged with significant hypertrophy of the adrenal cortex, and
recent studies show abnormalities in adrenal medullary formation and function (12, 31). Thymic T cells were found to be
resistant to dexamethasone-induced apoptosis, and glucocorticoids have been shown in a number of independent GR-null
mouse lines not to be essential for normal thymic T cell development in both fetal and adult mice (13, 32–34).
The present study addressed the role of glucocorticoids in
the late embryonic development of the mouse lung by investigating in more detail the respiratory defect in GR-null mice. Pharmacologic doses of synthetic glucocorticoids have clearly been
shown to induce lung surfactant production and to accelerate
Figure 4. Northern blot analysis of the surfactant protein genes SP-A,
SP-B, and SP-C; and ENaC subunit ␣, , and ␥ mRNA levels in Day
18.5 pc fetal lung of WT and GR-null mice. (A ) Detection of SP-A,
SP-B, SP-C mRNA, and ENaC subunit mRNAs in three WT (⫹/⫹) and
three GR-null (⫺/⫺) fetal lung samples. Samples (10 g/lane) were
loaded onto a 1.2% agarose gel. RNA were separated by electrophoresis,
transferred to a nylon membrane, and hybridized with riboprobes as
described in Materials and Methods. Filters were rehybridized for
cytochrome oxidase (cytoxi) expression to control for loading. (B ) Quantitation of SP-A, SP-B, SP-C, ENaC␣, ENaC, and ENaC␥ subunit
mRNA levels relative to levels of cytochrome oxidase mRNA. Autoradiographs were scanned and bands quantified using a scanning densitometer from Molecular Dynamics. Asterisks indicate statistical significance.
lung maturation and increase lung compliance (2). The lung
defect in GR-null mice demonstrates the important physiologic
role of endogenous glucocorticoids just before birth. We have
found clear evidence of abrogated remodeling of lung cellular
architecture, which is very similar to changes observed following
surgical removal of the fetal adrenal glands (35). There is a lack
of septal thinning and an alteration in normal epithelial cell
differentiation, primarily causing a reduction in the number of
differentiated type I epithelial cells. This defect of incomplete
epithelial cell differentiation complements the findings from another GR-null mouse line where there is a lack of increased
expression in the lung of midkine, an important cell growth
factor, 3–4 d before birth (16).
Surprisingly, we demonstrate that differentiation into the type
II EC phenotype is accentuated in the absence of GR-mediated
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Figure 5. Northern blot analysis of T1␣, aquaporin-5, and aquaporin-1
mRNA in Day 18.5 pc fetal lung of WT (⫹/⫹) and GR-null (⫺/⫺) mice.
Total RNA samples (7.5 g/lane) were loaded onto a 1.2% agarose gel
and separated by electrophoresis, then transferred to a nylon membrane.
Filters were hybridized with riboprobes as described in Materials and
Methods and rehybridized for cytochrome oxidase (cytoxi) expression
to control for loading. Washed filters were also exposed to a phosphoimaging plate (Molecular Dynamics) and mRNA levels quantitated using
ImageQuant software (Molecular Dynamics). Asterisks indicate statistical significance.
glucocorticoid signaling and provide evidence that pulmonary surfactant can be produced and secreted onto the lung lumen in GRnull mice. This finding is consistent with a previous study in fetal
sheep that demonstrated that fetal hypophysectomy, which abolishes the preparturient increase in circulating fetal cortisol concentrations, greatly increases the number of type II and reduces the
number of type I ECs; this effect was abolished by the reinfusion
of cortisol (36). The mechanisms for these changes in EC proportions are unknown, although an alteration in lung compliance and
expansion may be involved (see below). Nevertheless, our findings,
in combination with those of Crone and coworkers (36), clearly
indicate that differentiation into the type II EC phenotype is not
dependent upon the action of glucocorticoids.
Although expression of the surfactant protein genes in type II
ECs was found to be reduced, it was not abolished in GR-null
mice and this probably reflects the continued action of other
signals such as those activating the cAMP pathway (1). The critical
requirement for cAMP signaling for the SP-D gene is shown in
cAMP response element–binding protein-null mice, where there
is a severe effect on its full induction at birth (37). It is also possible
that glucocorticoids could signal via the mineralocorticoid receptor
(MR), sometimes referred to as the type I GR, but recent studies
indicate that MR is not present in alveolar ECs either in the
adult lung or at birth (38). Finally, non–receptor-mediated rapid
nongenomic actions of glucocorticoids have been reported in immune cells and the cardiovascular system, but have not been
described so far in the developing respiratory system. Therefore,
from our results we conclude that glucocorticoid signaling via
GR is not critical for respiratory surfactant production, although
surfactant composition and phosphatidylcholine content need to
be determined in the GR-null lung to fully assess the normal
functioning of secreted pulmonary surfactant (39).
The secretion of lung liquid plays a critical role in the development of the fetal lung, whereas the withdrawal of this liquid
from the future airways at birth is an important process allowing
the transition to air breathing after birth (40). Membrane chan-
nels that establish transmembranous ionic gradients and promote the unidirectional movement of water, such as the ENaC
and the aquaporins, are very important in the lung and a number
of the components of these channels have been reported as
targets for glucocoticoid induction in the lung before birth
(11, 27). We find evidence for reduced expression of ENaC␥
and aquaporin-1 in the lung of GR-null mice before birth. All
three ENaC subunit genes are rapidly induced by dexamethasone in vitro in the type II epithelial A549 cell line (11), but in
the lung of GR-null mice expression of the ENaC␣ and  subunit
is relatively unaltered compared with the lungs of WT mice.
These results are similar to an initial study on the amilioridesensitive sodium channel in the lung of GR-null mice (11), yet
further analysis of channel activities, unidirectional ion fluxes,
and fluid flow rates in GR-null mice are required to more clearly
define an overall defect in lung fluid flux across the epithelium.
It is well established that the differentiated state of ECs is
closely regulated by the degree of mechanical strain imposed
on them (41, 42), which is predominantly determined by the
degree of lung expansion in vivo (40). Indeed, alterations in fetal
lung expansion, caused by changes in lung luminal volume, have
a profound effect on the differentiated state of ECs (43). Increases in fetal lung expansion induces type II to type I EC
transdifferentiation via an intermediate cell type and can reduce
the proportions of type II cells from ⵑ 30% to ⬍ 2% within
10 d (19). On the other hand, reductions in fetal lung expansion
promote an increase in type II EC proportions, most probably
via transdifferentiation of type I into type II ECs (20); similar
observations have been made in vitro (41). Thus, it is possible
that a reduction in fetal lung expansion is responsible for the
changes in epithelial cell proportions we observed in GR-null
fetal mice. This contention is consistent with our finding of altered tissue structure in the lungs of GR-null fetal mice, as
reductions in lung expansion induce similar structural changes
in the lungs of fetal sheep (43). Such a reduction in lung expansion could have resulted from a defect in the lung liquid secretory
mechanism, due to a reduction in the expression of ion channels
as outlined above, resulting in reduced liquid secretion into the
future airways. Alternatively, the altered lung structure in GRnull fetal mice, particularly the hypercellularity, greater tissue
volumes, and thicker terminal airway walls, may contribute to
a reduction in lung expansion due to a reduction in lung tissue
compliance. Fetal lung expansion is primarily determined by the
ability of the fetal upper airway, particularly the glottis, to retain
liquid within the future airways, which maintains an internal
distending pressure on the lungs of 1–2 mm Hg at rest (40).
Consequently, assuming that adductor activity of the glottis is
not different between WT and GR-null fetal mice, the degree
of lung expansion will primarily be determined by the compliance of lung tissue; a reduction in tissue compliance will reduce
the degree of lung expansion for any given distending pressure.
The finding that type I ECs predominate in the lung of WT
fetal mice at Day 18.5 pc (just before birth) is consistent with
the findings of other studies in which it has been shown that
type I ECs predominate in the lung before birth (19, 20, 23);
this is thought to result from the higher degree of basal lung
expansion in the fetus, compared with the newborn (19). The
higher proportion of undifferentiated epithelial stem cells in
GR-null fetal mice indicates that abolition of signaling via the
GR may delay EC differentiation into both phenotypes. The mechanisms involved are unknown, but it is clear that ultimately, differentiation into the type I or type II cell phenotype is not absolutely
dependent upon GR signaling. Recent evidence indicates that both
type I and type II ECs are capable of transdifferentiation, and that
the mechanical strain experienced by the cells is a critical factor
regulating the pathways that ultimately determine the phenotype
Cole, Solomon, Van Driel, et al.: Alveolar ECs in GR-Null Mice
of each EC (20, 41, 42). Thus, it is possible that corticosteroids
may facilitate the activation of pathways leading to the differentiation of undifferentiated epithelial stem cells, but determination
of the mature phenotype is dependent upon other factors.
In summary, analysis of the lung in the GR-null fetal mouse
indicates that glucocorticoid signling via GR is not essential
for surfactant production. Analysis of epithelial cell types using
electron microscopy and cell-specific markers reveals a marked
reduction in differentiated type I AECs before birth, indicating
that GR signaling either directly or indirectly mediates differentiation of ECs into this phenotype. The absence of GR-mediated
glucocorticoid signaling leads to a profound alteration to the
development of the terminal respiratory units of the lung, which
results in severe respiratory dysfunction at birth.
Acknowledgments: The authors thank Prof. Richard Harding for helpful discussions. S.J.R. is supported by an Australian Research Fellowship from the Australian Research Council (ARC) of Australia. T.J.C. is the Biochemistry Fund Fellow
at the University of Melbourne, Australia. S.B.H. is a Senior Research Fellow of
the National Health and Medical Research Council (NH&MRC) of Australia.
This work was supported by grants from the ARC and NH&MRC of Australia.
References
1. Mendelson, C. R. 2000. Role of transcription factors in fetal lung development
and surfactant protein gene expression. Annu. Rev. Physiol. 62:875–915.
2. Lyons, C. A., and T. J. Garite. 2002. Corticosteroids and fetal pulmonary
maturity. Clin. Obstet. Gynecol. 45:35–41.
3. Newnham, J. P., T. J. Moss, I. Nitsos, and D. M. Sloboda. 2002. Antenatal
corticosteroids: the good, the bad and the unknown. Curr. Opin. Obstet.
Gynecol. 14:607–612.
4. Hanson, M. 2002. Birth weight and the fetal origins of adult disease. Pediatr.
Res. 52:473–474.
5. Dodic, M., V. Hantzis, J. Duncan, S. Rees, I. Koukoulas, K. Johnson, E. M.
Wintour, and K. Moritz. 2002. Programming effects of short prenatal exposure to cortisol. FASEB J. 16:1017–1026.
6. McKenna, N. J., and B. W. O’Malley. 2002. Combinatorial control of gene
expression by nuclear receptors and coregulators. Cell 108:465–474.
7. Robinson-Rechavi, M., H. E. Garcia, and V. Laudet. 2003. The nuclear receptor
superfamily. J. Cell Sci. 116:585–586.
8. Snyder, J. M., H. F. Rodgers, J. A. O’Brien, N. Mahli, S. A. Magliato, and
P. L. Durham. 1992. Glucocorticoid effects on rabbit fetal lung maturation
in vivo: an ultrastructural morphometric study. Anat. Rec. 232:133–140.
9. Wallace, M. J., S. B. Hooper, and R. Harding. 1995. Effects of elevated fetal
cortisol concentrations on the volume, secretion, and reabsorption of lung
liquid. Am. J. Physiol. 269:R881–R887.
10. Wallace, M. J., S. B. Hooper, and R. Harding. 1996. Role of the adrenal glands
in the maturation of lung liquid secretory mechanisms in fetal sheep. Am.
J. Physiol. 270:R33–R40.
11. Itani, O. A., S. D. Auerbach, R. F. Husted, K. A. Volk, S. Ageloff, M. A.
Knepper, J. B. Stokes, and C. P. Thomas. 2002. Glucocorticoid-stimulated
lung epithelial Na(⫹) transport is associated with regulated ENaC and sgk1
expression. Am. J. Physiol. Lung Cell. Mol. Physiol. 282:L631–L641.
12. Cole, T. J., J. A. Blendy, A. P. Monaghan, K. Krieglstein, W. Schmid, A.
Aguzzi, G. Fantuzzi, E. Hummler, K. Unsicker, and G. Schutz. 1995. Targeted disruption of the glucocorticoid receptor gene blocks adrenergic chromaffin cell development and severely retards lung maturation. Genes Dev.
9:1608–1621.
13. Brewer, J. A., O. Kanagawa, B. P. Sleckman, and L. J. Muglia. 2002. Thymocyte
apoptosis induced by T cell activation is mediated by glucocorticoids in
vivo. J. Immunol. 169:1837–1843.
14. Muglia, L. J., D. S. Bae, T. T. Brown, S. K. Vogt, J. G. Alvarez, M. E. Sunday,
and J. A. Majzoub. 1999. Proliferation and differentiation defects during
lung development in corticotropin-releasing hormone-deficient mice. Am.
J. Respir. Cell Mol. Biol. 20:181–188.
15. Reichardt, H. M., K. H. Kaestner, J. Tuckermann, O. Kretz, O. Wessely, R.
Bock, P. Gass, W. Schmid, P. Herrlich, P. Angel, and G. Schutz. 1998.
DNA binding of the glucocorticoid receptor is not essential for survival.
Cell 93:531–541.
16. Kaplan, F., J. Comber, R. Sladek, T. J. Hudson, L. J. Muglia, T. Macrae, S.
Gagnon, M. Asada, J. A. Brewer, and N. B. Sweezey. 2003. The growth
factor midkine is modulated by both glucocorticoid and retinoid in fetal
lung development. Am. J. Respir. Cell Mol. Biol. 28:33–41.
17. Cole, T. J., H. J. Harris, I. Hoong, N. Solomon, R. Smith, Z. Krozowski, and
M. J. Fullerton. 1999. The glucocorticoid receptor is essential for maintaining
basal and dexamethasone-induced repression of the murine corticosteroidbinding globulin gene. Mol. Cell Endocrinol. 154:29–36.
619
18. Shepherdley, C. A., C. B. Daniels, S. Orgeig, S. J. Richardson, B. K. Evans, and
V. M. Darras. 2002. Glucocorticoids, thyroid hormones, and iodothyronine
deiodinases in embryonic saltwater crocodiles. Am. J. Physiol. Regul. Integr.
Comp. Physiol. 283:R1155–R1163.
19. Flecknoe, S., R. Harding, G. Maritz, and S. B. Hooper. 2000. Increased lung
expansion alters the proportions of type I and type II alveolar epithelial cells
in fetal sheep. Am. J. Physiol. Lung Cell. Mol. Physiol. 278:L1180–L1185.
20. Flecknoe, S. J., M. J. Wallace, R. Harding, and S. B. Hooper. 2002. Determination of alveolar epithelial cell phenotypes in fetal sheep: evidence for the
involvement of basal lung expansion. J. Physiol. 542:245–253.
21. Crapo, J. D., S. L. Young, E. K. Fram, K. E. Pinkerton, B. E. Barry, and R. O.
Crapo. 1983. Morphometric characteristics of cells in the alveolar region
of mammalian lungs. Am. Rev. Respir. Dis. 128:S42–S46.
22. Crapo, J. D., B. E. Barry, P. Gehr, M. Bachofen, and E. R. Weibel. 1982. Cell
number and cell characteristics of the normal human lung. Am. Rev. Respir.
Dis. 126:332–337.
23. Alcorn, D. G., T. M. Adamson, J. E. Maloney, and P. M. Robinson. 1981. A
morphologic and morphometric analysis of fetal lung development in the
sheep. Anat. Rec. 201:655–667.
24. Williams, M. C. 1977. Conversion of lamellar body membranes into tubular
myelin in alveoli of fetal rat lungs. J. Cell Biol. 72:260–277.
25. Nardo, L., G. Maritz, R. Harding, and S. B. Hooper. 2000. Changes in lung
structure and cellular division induced by tracheal obstruction in fetal sheep.
Exp. Lung Res. 26:105–119.
26. Rishi, A. K., M. Joyce-Brady, J. Fisher, L. G. Dobbs, J. Floros, J. VanderSpek,
J. S. Brody, and M. C. Williams. 1995. Cloning, characterization, and development expression of a rat lung alveolar type I cell gene in embryonic
endodermal and neural derivatives. Dev. Biol. 167:294–306.
27. Verkman, A. S., M. A. Matthay, and Y. Song. 2000. Aquaporin water channels
and lung physiology. Am. J. Physiol. Lung Cell. Mol. Physiol. 278:L867–
L879.
28. Williams, M. C. 2003. Alveolar type I cells: molecular phenotype and development. Annu. Rev. Physiol. 65:669–695.
29. King, L. S., S. Nielsen, and P. Agre. 1996. Aquaporin-1 water channel protein
in lung: ontogeny, steroid-induced expression, and distribution in rat.
J. Clin. Invest. 97:2183–2191.
30. Christoffels, V. M., T. Grange, K. H. Kaestner, T. J. Cole, G. J. Darlington,
C. M. Croniger, and W. H. Lamers. 1998. Glucocorticoid receptor, C/
EBP, HNF3, and protein kinase A coordinately activate the glucocorticoid
response unit of the carbamoylphosphate synthetase I gene. Mol. Cell. Biol.
18:6305–6315.
31. Finotto, S., K. Krieglstein, A. Schober, F. Deimling, K. Lindner, B. Bruhl, K.
Beier, J. Metz, J. E. Garcia-Arraras, J. L. Roig-Lopez, P. Monaghan, W.
Schmid, T. J. Cole, C. Kellendonk, F. Tronche, G. Schutz, and K. Unsicker.
1999. Analysis of mice carrying targeted mutations of the glucocorticoid
receptor gene argues against an essential role of glucocorticoid signalling
for generating adrenal chromaffin cells. Development 126:2935–2944.
32. Purton, J. F., R. L. Boyd, T. J. Cole, and D. I. Godfrey. 2000. Intrathymic
T cell development and selection proceeds normally in the absence of
glucocorticoid receptor signaling. Immunity 13:179–186.
33. Godfrey, D. I., J. F. Purton, R. L. Boyd, and T. J. Cole. 2000. Stress-free Tcell development: glucocorticoids are not obligatory. Immunol. Today 21:
606–611.
34. Purton, J. F., Y. Zhan, D. R. Liddicoat, C. L. Hardy, A. M. Lew, T. J. Cole,
and D. I. Godfrey. 2002. Glucocorticoid receptor deficient thymic and
peripheral T cells develop normally in adult mice. Eur. J. Immunol. 32:3546–
3555.
35. Liggins, G. C. 1976. Adrenocortical-related maturational events in the fetus.
Am. J. Obstet. Gynecol. 126:931–941.
36. Crone, R. K., P. Davies, G. C. Liggins, and L. Reid. 1983. The effects of
hypophysectomy, thyroidectomy, and postoperative infusion of cortisol or
adrenocorticotrophin on the structure of the ovine fetal lung. J. Dev. Physiol. 5:281–288.
37. Mantamadiotis, T., T. Lemberger, S. C. Bleckmann, H. Kern, O. Kretz, A.
Martin Villalba, F. Tronche, C. Kellendonk, D. Gau, J. Kapfhammer, C.
Otto, W. Schmid, and G. Schutz. 2002. Disruption of CREB function in
brain leads to neurodegeneration. Nat. Genet. 31:47–54.
38. Suzuki, T., H. Sasano, S. Suzuki, G. Hirasawa, J. Takeyama, Y. Muramatsu,
F. Date, H. Nagura, and Z. S. Krozowski. 1998. 11Beta-hydroxysteroid
dehydrogenase type 2 in human lung: possible regulator of mineralocorticoid action. J. Clin. Endocrinol. Metab. 83:4022–4025.
39. Nanjundan, M., and F. Possmayer. 2001. Pulmonary lipid phosphate phosphohydrolase in plasma membrane signalling platforms. Biochem. J. 358:637–
646.
40. Harding, R., and S. B. Hooper. 1996. Regulation of lung expansion and lung
growth before birth. J. Appl. Physiol. 81:209–224.
41. Danto, S. I., J. M. Shannon, Z. Borok, S. M. Zabski, and E. D. Crandall. 1995.
Reversible transdifferentiation of alveolar epithelial cells. Am. J. Respir.
Cell Mol. Biol. 12:497–502.
42. Gutierrez, J. A., R. F. Gonzalez, and L. G. Dobbs. 1998. Mechanical distension
modulates pulmonary alveolar epithelial phenotypic expression in vitro.
Am. J. Physiol. 274:L196–L202.
43. Alcorn, D., T. M. Adamson, T. F. Lambert, J. E. Maloney, B. C. Ritchie, and
P. M. Robinson. 1977. Morphological effects of chronic tracheal ligation
and drainage in the fetal lamb lung. J. Anat. 123:649–660.