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The Journal of Neuroscience, March 1, 2002, 22(5):1640-1647
The Mouse Crx 5'-Upstream Transgene Sequence Directs
Cell-Specific and Developmentally Regulated Expression in Retinal
Photoreceptor Cells
Akiko
Furukawa1, 3,
Chieko
Koike1,
Pia
Lippincott1,
Constance L.
Cepko2, and
Takahisa
Furukawa1
1 Center for Developmental Biology and Kent Waldrep
Foundation Center for Basic Research on Nerve Growth and Regeneration,
University of Texas Southwestern Medical Center, Dallas, Texas
75390-9133, 2 Department of Genetics and Howard Hughes
Medical Institute, Harvard Medical School, Boston, Massachusetts 02115, and 3 Department of Ophthalmology, Osaka University Medical
School, Osaka 565-0871, Japan
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ABSTRACT |
Crx, an Otx-like homeobox gene, is
expressed primarily in the photoreceptors of the retina and in the
pinealocytes of the pineal gland. The CRX homeodomain protein is a
transactivator of many photoreceptor/pineal-specific genes in
vivo, such as rhodopsin and the cone opsins. Mutations in
Crx are associated with the retinal diseases, cone-rod
dystrophy-2, retinitis pigmentosa, and Leber's congenital amaurosis,
which lead to loss of vision. We have generated transgenic mice, using
5'- and/or 3'-flanking sequences from the mouse Crx
homeobox gene fused to the -galactosidase (lacZ)
reporter gene, and we have investigated the promoter function of the
cell-specific and developmentally regulated expression of
Crx. All of the independent transgenic lines commonly
showed lacZ expression in the photoreceptor cells of the
retina and in the pinealocytes of the pineal gland. We characterized
the transgenic lines in detail for cell-specific lacZ
expression patterns by 5-bromo-4-chloro-3-indolyl
-D-galactoside staining and lacZ
immunostaining. The lacZ expression was observed in
developing and developed photoreceptor cells. This observation was
confirmed by coimmunostaining of dissociated retinal cells with the
lacZ and opsin antibodies. The ontogeny analysis
indicated that the lacZ expression completely agrees with a temporal expression pattern of Crx during retinal
development. This study demonstrates that the mouse Crx
5'-upstream genomic sequence is capable of directing a cell-specific
and developmentally regulated expression of Crx in
photoreceptor cells.
Key words:
photoreceptor; Crx; retina; transgenic mouse; transcriptional regulation; promoter
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INTRODUCTION |
The retina is a very sensitive light
detector, and the photoreceptor cells in the retina are essential for
vision formation. The vertebrate retina contains two types of
photoreceptors, rods and cones. Cones are responsible for daylight and
color vision. Rods mediate dim light vision. Both rod and cone
photoreceptors elaborate a specialized structure, the outer segment, to
catch the light. The outer segments of rods and cones are filled with light-absorbing visual pigments, rhodopsin and cone opsins, respectively.
Considerable progress has been made toward elucidating the
physiological and biochemical basis of the phototransduction pathway in
photoreceptors. In contrast, the molecular basis of photoreceptor differentiation has been poorly understood. Most of the molecules involved in the phototransduction pathway are expressed
specifically in the photoreceptors of the retina and in the
pinealocytes of the pineal gland. Because defining the mechanisms of
regulation of photoreceptor-specific genes is important to
understanding the regulation of cell type specificity, mechanisms of
transcriptional regulations of these photoreceptor-specific molecules
have been studied extensively. However, the molecular basis, which
confers photoreceptor specificity, is not well understood. CRX, an
OTX-like homeoprotein, has been proposed to be a regulator of various
photoreceptor-specific genes (Furukawa et al., 1997b ).
Regulation by CRX of many photoreceptor-specific genes has been
proposed on the basis of CRX binding sites, their regulatory regions
identified by protein-DNA binding assays and transient transfection
assays (Chen et al., 1997 ; Furukawa et al., 1997b , 1999 ; Livesey
et al., 2000 ). Interestingly, it recently has been reported that many
photoreceptor-specific genes also are expressed in the pineal gland in
which Crx is expressed (Blackshaw and Snyder, 1997 ). The
pineal gland is related evolutionarily to the photoreceptors in the
retina. In mammals the major function of the pineal gland is the
secretion of melatonin. Melatonin is synthesized from serotonin by two
key enzymes, N-acetyltransferase (NAT) and
hydroxyindole-O-methyltransfererase (HIOMT). The
regulatory regions of these genes contain multiple CRX binding sites
(Li et al., 1998 ). Crx is a candidate factor to
transactivate the expression of these two key enzymes in the melatonin
synthesis pathway. Via the Crx knock-out mouse study we
actually demonstrated that Crx is essential for the
expression of NAT and other photoreceptor-specific genes in the pineal
gland (Furukawa et al., 1999 ).
Mutations of various photoreceptor-specific genes have been shown to be
responsible for the human genetic retinal disease retinitis pigmentosa
(for review, see Dryja and Li, 1995 ). Mutations of human CRX, which
also is expressed in photoreceptor cells specifically in the retina,
have been demonstrated to be associated with three types of
photoreceptor diseases: autosomal dominant cone-rod dystrophy-2 (adCRD2; Freund et al., 1997 ; Swain et al., 1997 ; Sohocki et al., 1998 ), retinitis pigmentosa (Sohocki et al., 1998 ), and Leber's congenital amaurosis (LCA; Freund et al., 1998 ; Sohocki et al., 1998 ).
Patients of adCRD2 and retinitis pigmentosa undergo slow degeneration
of photoreceptors, leading to blindness in later stages of their lives.
LCA is a very severe type of photoreceptor disease that usually causes
congenital blindness.
To understand the mechanisms of regulation of Crx expression
in the development of rods and cones in the retina, we generated transgenic mice by using the 5'- and 3'-flanking mouse Crx
(mCrx) sequence fused to the -galactosidase ( -gal;
lacZ) reporter gene. The present studies have indicated that
the upstream mCrx genomic sequence directs, temporally and
spatially, the lacZ expression in retinal photoreceptor
cells corresponding to the mCrx expression pattern. In
addition, we demonstrated that CRX transactivates itself to maintain
its expression in vivo by positive feedback.
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MATERIALS AND METHODS |
Transgene vector and generation of transgenic mice.
We obtained the Crx genomic clone from a 129SVJ mouse
library (Stratagene, La Jolla, CA) by using a mouse Crx cDNA
probe. We ligated and subcloned a 10 kb XhoI (partial
digestion)-EcoRI fragment and a PCR-amplified 2 kb
EcoRI-SmaI fragment containing exon 1 into a
p -gal-Basic vector (Clontech, Palo Alto, CA) to make the
Pcrx12k-lacZ construct (see Fig. 1A).
Sequencing verified the 2 kb EcoRI-SmaI fragment. The Pcrx2k-lacZ vector contains the 2 kb
EcoRI-exon 1 fragment and a 10 kb
SmaI-EcoRI fragment that contains the first intron.
We extracted the Pcrx2k-lacZ and the
Pcrx12k-lacZ from the recombinant plasmids by a
NotI and SalI digestion. We fractionated the
NotI-SalI fragments by electrophoresis on a
0.8% agarose gel and purified them by electroelution in dialysis
tubes. We microinjected the DNA fragment into pronuclei of B6SJL/F2
C57BL/6 × SJL F2 hybrids. Then Southern blot
hybridization of a HindIII-ClaI 954 bp fragment of the -galactosidase or PCR verified the integration of the lacZ gene. In the PCR analysis we detected the transgene by
using a sense primer (5'-TGCCGGTCTGGGAGGCATTGGTCTGGACACCAG-3') and an anti-sense primer (5'-AGTTTGAGGGGACGACGACAGTATCGGCCTCAG-3').
Antibodies. We acquired the following primary antibodies:
mouse monoclonal antibodies against lacZ from Chemicon
(Temecula, CA), against calbindin D-28k and
against syntaxin (HPC-1) from Sigma (St. Louis, MO), and against
vimentin from Zymed (San Francisco, CA); the Rho4D2 bovine monoclonal
antibody against rhodopsin, a generous gift from Dr. R. S. Molday
(University of British Columbia); rabbit polyclonal antibodies against
lacZ from Cortex Biochem (San Leandro, CA), chx10, a gift
from Dr. R. McInnes (The Research Institute, Hospital for Sick
Children, Toronto, Ontario), and against cone opsin blue and
red/green, a generous gift from Dr. Y. Takada (Jikei Medical School of
Tokyo, Tokyo, Japan). We used these antibodies at a 1:400-1:1000 dilution.
We purchased the following secondary antibodies: Cy3-conjugated donkey
IgG against mouse IgG, Cy3-conjugated goat IgG against rabbit IgG,
fluorescein isothiocyanate (FITC)-conjugated donkey IgG against rabbit
IgG, and FITC-conjugated goat IgG against mouse IgG from Jackson
ImmunoResearch Laboratories (West Grove, PA). We used these antibodies
at a 1:2000 dilution.
Histochemistry and immunostaining. We used 3- to 4-week-old
transgenic mice. While they were anesthetized with ketamine and xylazine (10:1), we enucleated the eyes and harvested the brains and
the pineal glands. We performed 5-bromo-4-chloro-3-indolyl -D-galactoside (X-Gal) staining in a solution of
1 mg/ml X-Gal, 35 mM
KFe3(CN)6, 35 mM
K4Fe(CN)6-H2O,
2 mM MgCl2, 0.02% NP-40, and 0.01%
Na-deoxycholate in PBS. We dissected the eyes at the limbus and fixed
them with 0.5% glutaraldehyde in PBS for 12-24 hr and
then removed the vitreous matter and fixed them for an additional 1 hr;
meanwhile, the brains and pineal glands were immersed in the same
fixative solution for 12-24 hr before X-Gal staining. We incubated the
eyes, brains, and pineal glands in the X-Gal reaction solution for 48 hr at 37°C. We cryoprotected the samples after fixation with 30%
sucrose in PBS, embedded them in optimal cutting temperature
compound (Miles, Elkhart, IN), and froze them on dry ice. The samples
were sectioned into 30-µm-thick sections with a cryostat at 27°C
and were mounted on glass slides.
We dissected the eye at the limbus, isolated the retinas mechanically,
and fixed them with 4% formaldehyde in PBS for immunofluorescence staining. The samples were cryoprotected, embedded, and frozen as above
and sectioned into 18-µm-thick sections. We fixed the mounted
sections with 4% formaldehyde in PBS and rinsed and preincubated them
with blocking solution [2% normal goat serum, 2% normal donkey serum
(Jackson ImmunoResearch), and 0.02% Triton X-100 (Sigma) in PBS]
overnight. We incubated the sections with the primary antibodies at
room temperature for 1 hr, rinsed them with blocking solution, and
incubated them with the secondary antibodies for 30 min. We
coverslipped the sections with Aqua Poly/Mount (Polysciences, Warrington, PA) after rinsing them with 0.02% Triton X-100 in PBS.
Cell dissociation and immunostaining. We dissected retinas
from postnatal day 10 (P10) transgenic mice (2kB and 12kA) free from
other tissues and incubated them in HBSS lacking
Ca2+/Mg2+
(Invitrogen, San Diego, CA), with trypsin (Worthington,
Freehold, NJ) added to a final concentration of 1 mg/ml, at room
temperature for 10 min. After trypsinization we added a soybean trypsin
inhibitor (Sigma) to the final concentration of 2 mg/ml. We pelleted
the cells by 2000 rpm centrifugation for 2 min, resuspended them, and
triturated them into a single-cell suspension in HBSS with 100 µg/ml
DNase I (Sigma). The cells were plated on
poly-D-lysine-coated (Sigma) eight-well glass slides
(Cel-Line Associates, Newfield, NJ) before fixation. We
immunofluorescence-stained them as above.
-gal assay. We dissected retinas from P10 transgenic mice
(2kB) and dissociated the cells as described above. The cells were resuspended in 0.25 M Tris-HCl, pH 7.8, and alternately
frozen in liquid nitrogen and thawed in a 37°C water bath, 3 min
each, for a total of three freeze/thaw cycles. Both supernatant and cell extract were saved after a 14,000 rpm centrifugation for 15 min.
We incubated the cell extracts at 37°C with 15 mM
chlorophenol red- -D-galactopyranoside (CPRG; Roche
Bioscience, Palo Alto, CA) and measured the absorbance at 574 nm.
Northern blot analysis. We prepared total RNA from P21
retinas, brain cortices, cerebella, and brainstems from 2kA and 12kA transgenic mice. We used 10 µg of total RNA for electrophoresis and a
HindIII-ClaI 954 bp fragment of
-galactosidase as a labeled probe. The electrophoresis, transfer,
and hybridization were done as described previously (Furukawa et al.,
1997a ).
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RESULTS |
Generation of the mCrx-lacZ transgenic mice
To address whether the Crx upstream sequence is capable
of directing its expression in a cell-specific and developmentally regulated manner, we fused the 5'-flanking region and/or the first intron of the mouse Crx gene to the lacZ reporter
gene (Fig. 1A).

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Figure 1.
The Crx
promoter-lacZ transgene structure, the mouse
Crx genomic sequence around the transcription initiation
site, and Northern blot analysis of transgenic mice. A,
Diagrammatic representation of the genomic structure of the mouse
Crx gene and the Crx-lacZ
fusion constructs used for injection. Two transgenes that were used for
the generation of transgenic mice are shown beneath the map of the
mouse Crx genomic region. The mouse Crx
gene is composed of four exons indicated by boxes on the
genomic map. The homeodomain is indicated by the black
box. Data are presented as the number of mice expressing the
transgene in the photoreceptor-specific pattern through development per
the number of transgenic mice that showed germline transmission.
B, The nucleotide sequence around the transcription
initiation site of the mouse Crx gene fused with
-galactosidase. An asterisk marks the transcription
initiation site of the mouse Crx. The translation
initiation site of -galactosidase is underlined, and
the connected site of the mouse Crx and the
-galactosidase is indicated by an arrow. The possible
CRX binding sites are indicated by boxes.
C, Northern blot analysis of total RNA (10 µg)
isolated from cortex (lane 1), brainstem (lane
2), cerebellum (lane 3), and retina (lane
4) of Pcrx2k-lacZ (2kA) transgenic mice.
The HindIII-ClaI 954 bp fragment of
-galactosidase was used as a labeled probe. 18S and
28S are ribosomal RNAs used as marker RNA.
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We first isolated phage clones encoding the mouse Crx locus
as part of our effort to make a mouse knock-out of Crx
(Furukawa et al., 1999 ). We have mapped the 5'-end of the
Crx mRNA by using the RNA ligase-mediated rapid
amplification of 5'-cDNA ends (RLM-RACE) method (Maruyama and Sugano,
1994 ; Volloch et al., 1994 ), and we determined the transcription
initiation site (Fig. 1B). The mouse Crx
upstream sequence lacks typical consensus TATA, GC, and CAT
boxes. A sequence of the region from the murine Crx
locus is shown in Figure 1B. The transcription
initiation site is indicated by an asterisk. We sequenced the entire 2 kb region and registered it in the GenBank database (accession number
AF301006). In this 2 kb region we found three CRX binding consensus
sequences. Two of them are located proximal to the transcription
initiation sites of the mouse Crx gene (Fig.
1B).
For the Pcrx12k-lacZ construct the fusion gene consists of
a 12 kb mCrx upstream genomic fragment, starting from 34 bp
upstream of the mCrx translation initiation site; a 131 bp
fragment containing the translation initiation site of the
Drosophila melanogaster alcohol dehydrogenase gene; the
lacZ gene; a small t antigen intron; and a polyadenylation
site derived from the SV40 gene (Fig. 1A). For the
Pcrx2k-lacZ construct, we used a 2 kb mCrx
upstream genomic fragment and the 10 kb first intron fragment.
We have generated transgenic mice, using these two constructs, with the
aim of identifying a region (or regions) that transactivates Crx transcription specifically in photoreceptor cells. On
blot hybridization analysis of tail DNAs, we identified six and four independent transgenic lines that passed their transgenes onto their
offspring for the Pcrx12k-lacZ and the
Pcrx2k-lacZ, respectively. Three of six
Pcrx12k-lacZ lines (named 12kA, B, and F) and four of four
Pcrx2k-lacZ lines (named 2kA, B, E, and G) exhibited an X-Gal-positive staining in the photoreceptor layers of the retina. Blot
hybridization analysis of tail DNAs indicated that 12kA and 12kF each
possess approximately five to six copies of the lacZ gene in
the genomic DNA. 12kB has approximately seven to eight copies, 2kA and
2kG each have approximately one to two copies of the lacZ
gene, 2kB has ~10-15 copies, and 2kE has ~40-50 copies of the
lacZ gene in the genomic DNA (data not shown).
We first analyzed tissue specificity of the lacZ expression
by Northern blot hybridization of total RNA of the retina, cerebral cortex, cerebellum, and brainstem isolated from these transgenic animals (Fig. 1C). This analysis revealed that
in these lines the lacZ mRNA was expressed specifically in
the retina and pineal gland, but not in other parts of the brain
(except in the pineal gland).
All Pcrx12K and Pcrx2K transgenic mice showing lacZ
expression exhibited lacZ expression specifically in
photoreceptor cells and in the pineal gland. Although we also examined
tissue specificity of the lacZ expression by X-Gal staining
of whole-mount embryos at embryonic day 12.5 (E12.5) and sections of
various adult tissues, we did not observe any significant
staining except in the retina and the pineal gland (data
not shown). We therefore conclude that an overlapping 2 kb upstream
region is responsible for the specific expression of mCrx in
photoreceptors and pineal gland.
LacZ expression pattern in the retina
We examined an X-Gal staining pattern in the retinal section of
the three Pcrx12k-lacZ and four Pcrx2k-lacZ
transgenic lines at adult stage (3-4 weeks old). The densities of the
X-Gal reaction product were not identical among these transgenic mice.
The 12kA and 2kB retinal sections showed relatively strong and dense
staining. All of these transgenic mice, however, exhibited the same
spatial and developmental expression in the photoreceptor cell layer of the retina (Fig. 2). For additional
analysis we focused our analysis on the 2kB line, which showed slightly
stronger X-Gal staining. The retina consists of several different cell
layers: the outer nuclear layer (ONL), composed of photoreceptor cell
bodies; the inner nuclear layer (INL), consisting of bipolar,
horizontal, and amacrine cells; and the ganglion cell layer (GCL),
containing primarily ganglion cells. Photoreceptors, bipolar cells, and
horizontal cells make synaptic connections in the outer
plexiform layer (OPL). A differentiated mature photoreceptor is
composed of the outer segment (OS), inner segment (IS), cell body, and
synaptic terminus (ST). In all transgenic lines X-Gal staining was
observed in the inner segment and synaptic terminus of photoreceptor
cells (Fig. 2). Although a cytoplasmic -galactosidase was used in
this study, X-Gal-stained products were localized specifically in inner
segments and synaptic termini. The mechanisms underlying this specific localization of the -galactosidase are not yet clear.

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Figure 2.
Transverse retinal sections of the three
Pcrx12k-lacZ (12kA, 12kB, and 2kF), the four
Pcrx2k-lacZ (2kA, 2kB, 2kE, and 2kG) transgenic mice,
and a nontransgenic mouse (NC) stained with X-Gal. Shown is the X-Gal
staining pattern in lower magnification (A, C, E, G, I, K, M,
O) and in higher magnification (B, D, F, H, J, L, N,
P) from 12 kA (A, B), 12kB (C,
D), 12kF (E, F), 2kA (G,
H), 2kB (I, J), 2kE (K,
L), 2kG (M, N), and nontransgenic
(O, P) mice. The intensities of the X-Gal stain are
different among the seven transgenic lines, but the staining is
observed commonly in the inner segment and the axon terminals of the
photoreceptor cells. X-Gal staining was observed all through the region
of the retina. For negative control the retina of a nontransgenic
littermate was stained with X-Gal. The X-Gal stain was observed
slightly in the ganglion cell layer. PE, Pigment
epithelium. Scale bars, 100 µm.
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To investigate whether lacZ is expressed in rods under the
control of the mCrx flanking sequences, we performed
double-immunofluorescence staining with anti-rhodopsin antibody and
anti-lacZ antibody on retinal transverse sections and examined the
immunostaining pattern of lacZ expression via fluorescent
microscopy. The immunofluorescent pattern of the lacZ
expression was photoreceptor-specific, which was identical with the
X-Gal staining patterns shown above (Fig. 3). In addition, we also performed
double-immunofluorescence staining with cell type-specific markers and
anti-lacZ antibody to examine whether lacZ is expressed in
nonphotoreceptors. Double-immunofluorescence staining of the
lacZ and cell type-specific markers was performed by two
different combinations of primary antibodies against rhodopsin (rod),
chx10 (bipolar cell), calbindin (horizontal cell), vimentin (Müller glia), HPC-1 (amacrine cell), and lacZ. A
polyclonal lacZ rabbit antibody and monoclonal cell type-specific mouse
antibodies were used, except for chx10. For the staining of bipolar
cells, a polyclonal chx10 antibody and a monoclonal lacZ mouse antibody were used. Rhodopsin-positive immunoreactivity and
lacZ-positive immunoreactivity are overlapped. None of other
markers overlap with lacZ-positive immunoreactivity. In
summary, mCrx promoter-driven lacZ is expressed
specifically in the photoreceptor layer of the retina.

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Figure 3.
Double-immunofluorescence staining of
lacZ and cell type-specific markers in transverse
retinal sections of the 2kB transgenic mice. Double-immunofluorescence
staining of the lacZ and cell type-specific markers was
performed by two different combinations of primary antibodies against
rhodopsin (rod; A), chx10 (bipolar cell;
D), calbindin (horizontal cell; G),
vimentin (Müller glia; J), HPC-1 (amacrine
cell; M), and lacZ (B, E,
H, K, N). Overlaid images are displayed in C, F,
I, L, O. A polyclonal lacZ rabbit antibody and monoclonal cell
type-specific mouse antibodies were used, except for chx10. We used a
polyclonal chx10 antibody and a monoclonal lacZ mouse antibody for the
staining of bipolar cells. Rhodopsin-positive immunoreactivity and
lacZ-positive immunoreactivity are overlapped. None of
the other markers overlaps with lacZ-positive
immunoreactivity. Scale bars, 50 µm.
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Double-immunofluorescence staining in dissociated
retinal cells
To characterize cell types expressing lacZ further, we
performed double immunostaining of dissociated adult retinal cells (Fig. 4). By this method we can examine
the lacZ expression at the single-cell level in greater
detail than by immunofluorescence staining of sections. The retina of
the 2kB mouse was dissociated by trypsin treatment, and dissociated
cells were double immunostained with anti-lacZ and anti-rhodopsin
antibodies. In a dissociated cell preparation there were many
lacZ-positive/anti-rhodopsin-positive cells (Fig.
4A-C). We then examined the expression of
lacZ in other cell types for which the cell bodies are
located in the inner nuclear layer, including horizontal cells (Fig.
4D-F), Müller glia (Fig.
4G-I), amacrine cells (Fig.
4J-L), and bipolar cells (Fig.
4M-O). We found that lacZ-expressing
cells are negative for anti-calbindin, -vimentin, -HPC-1, and -chx10
antibodies (Fig. 4D-O). This result indicates that
lacZ is not expressed in these cell types.

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Figure 4.
Double-immunofluorescence staining of dissociated
retinal cells of the 2kB transgenic mice by anti-lacZ and cell
type-specific markers. Double-immunofluorescence staining of the
lacZ and cell type-specific markers was performed by two
different combinations of primary antibodies against rhodopsin (rod;
A), calbindin (horizontal cell; D),
vimentin (Müller glia; G), HPC-1 (amacrine cell;
J), chx10 (bipolar cell;
M), and lacZ (B, E, H, K,
N, Q, T). Overlaid images are displayed in C, F,
I, L, O, R, U. Combinations of polyclonal anti-lacZ rabbit
antibody (B, E, H, K) and monoclonal
anti-cell-specific marker mouse antibodies (A, D, G,
J), or of polyclonal anti-cell-specific marker rabbit
antibodies (M, P, S) and monoclonal mouse anti-lacZ
(N, Q, T) antibody were used for staining. There
are many numbers of lacZ/rhodopsin-positive
(C) and some lacZ/cone
opsin-positive cells (R, U) seen in a dispersed
preparation. However, lacZ-expressing cells are
anti-calbindin, anti-vimentin, and anti-HPC-1, and chx10-negative
(F, I, L, O), indicating that lacZ is not
expressed in other cell types.
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We also performed double immunostaining with the anti-lacZ and
anti-cone opsin (blue or green/red) antibodies (Fig.
4P-U). There were some
anti-lacZ-positive/anti-cone opsin-positive cells. These results
demonstrated that lacZ is expressed in both rods and cones.
This completely agrees with the expression pattern of Crx,
which is expressed both in rods and cones (Chen et al., 1997 ; Furukawa
et al., 1997b ).
LacZ expression during retinal development
The expression of mCrx initiates and develops during
the embryonic and neonatal period in accordance with the development of
photoreceptor cells in the retina (Furukawa et al., 1997b ). We
investigated the ontogeny of lacZ expression during retinal development in 12kA, 2kA, and 2kB transgenic mice by X-Gal staining. Because the lacZ expression patterns during development are
very similar among these three lines (data not shown), we will present the lacZ expression pattern of 2kB (Figs.
5, 6). The X-Gal reaction product was not
detected at E11.5 (data not shown) and was detected first at E12.5
(Fig. 5A,B). This completely agrees with the observation that the mouse Crx mRNA first was detected by in
situ hybridization in the developing mouse retina at E12.5
(Furukawa et al., 1997b ). At E15.5 and E17.5 the X-Gal staining
became denser and more uniform in the presumptive photoreceptor layer
(Fig. 5E-H). In summary, the expression pattern of
lacZ in the embryonic stages completely agrees with the
expression pattern of Crx detected by in situ hybridization.

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Figure 5.
Embryonic retinas stained with X-Gal. X-Gal
staining was performed with transverse sections of embryonic retinas of
the 2kB mice at E12.5 (A, B), E13.5 (C,
D), E17.5 (E, F), and at E13.5 of
nontransgenic mouse (NC, negative control; G,
H). A red arrow indicates developing
photoreceptor cells in the presumptive photoreceptor layer. A
red arrowhead indicates nonspecific staining at the
ganglion cell layer of the retina from nontransgenic mouse. Scale bars,
100 µm.
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After birth, X-Gal staining was detected in the developing
photoreceptor layer and weakly in the inner nuclear layer and ganglion cell layer (Fig.
6A-D). Because the
X-Gal-stained retina from nontransgenic mice also showed weak signal in
the inner nuclear layer and the ganglion cell layer (Fig.
6G,H), all of or at least most of the X-Gal reaction
products outside of the photoreceptor layer are considered to come from
endogenous galactosidase activity in the mouse retina.

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Figure 6.
Postnatal retinas stained with X-Gal. X-Gal
staining was performed with transverse sections of postnatal retinas of
the Pcrx2kB mice. A red arrow indicates developing
photoreceptor cells in the photoreceptor layer. A red
arrowhead indicates nonspecific staining at the ganglion cell
layer. A slight nonspecific staining also is observed in the inner
nuclear layer of the nontransgenic retina. PE, Pigment
epithelium; IPL, inner plexiform layer. Scale
bars, 100 µm.
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LacZ expression in pineal gland
Many photoreceptor-specific genes are known to be expressed in the
pineal gland in which Crx also is expressed (Blackshaw and
Snyder, 1997 ). The pineal gland is a small organ located on the midline
in the dorsal cranium, which is related evolutionarily with
photoreceptor cells in the retina. We also examined lacZ expression of the Crx promoter lacZ transgenic
mice, 12kA and 2kB, in the pineal gland. The X-Gal reaction product was
detected in the pinealocytes of the adult pineal gland (Fig.
7). This result also agrees with the
expression of Crx mRNA in the pineal gland (Chen et al.,
1997 ; Furukawa et al., 1999 ).

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Figure 7.
Adult pineal gland stained with X-Gal.
X-Gal staining was performed with transverse sections of the12kA
(A) 2kB (B), and
nontransgenic (C) mouse adult pineal gland.
Intense X-Gal staining is seen in the pineal glands of the transgenic
mice. Scale bars, 100 µm.
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Crx itself is required for maintenance of the
transgene expression
The existence of two CRX binding consensus motifs near the
transcription initiation site suggests that CRX protein itself can bind
and transactivate the Crx transcription (Fig.
1B). To address the requirement for CRX function in
the activity of the Crx regulatory sequences that have been
characterized here, we examined the expression of a
Crx-lacZ transgene in the Crx mutant background. The 2kB transgenic line was crossed with the Crx
mutant mice (Furukawa et al., 1999 ). Animals hemizygous for the
Crx mutant allele were crossed with Crx
promoter-lacZ heterozygous transgenic mice. The retinas of
P10 pups were harvested and lysed; lacZ activity was
measured by liquid assay. We chose to harvest P10 retinas because
retinal degeneration of the Crx null mouse has not started at this stage. Compared with the mice in the wild-type background, the
retinas of transgenic mice in the Crx null background
exhibited only 31% of the lacZ activity (Fig.
8A). Interestingly,
retinas of transgenic mice in the Crx heterozygous mutant
background also exhibited only 57% of the lacZ activity.
This reduced lacZ activity in the Crx
heterozygous mutants may reflect a Crx gene dosage effect
for the phenotypes observed in the Crx heterozygous mutant mice (Furukawa et al., 1999 ). However, retinas of nontransgenic mouse
in the Crx null background, the Crx heterozygous
mutant background, and wild type exhibited 3.4, 3.4, and 3.7% of the lacZ activity, respectively. These activities are lower by
far than those of transgenic mice. These data support a role of CRX protein in autoregulation.

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|
Figure 8.
Requirement of the CRX function for the
maintenance of Crx promoter-lacZ
transcription. A, Relative -galactosidase activity of
retinas of 2kB transgenic mice in the wild-type,
Crx+/ , or
Crx / mutant
background. Error bar indicates SEM. B, A cryosection of
the retina from a 2kB transgenic mouse in
Crx+/+ background at E12.5.
C, A cryosection of the retina from a 2kB transgenic
mouse in the Crx /
background showing lacZ expression in the presumptive
photoreceptor layer at E12.5. D, A cryosection of the
retina from a nontransgenic wild-type mouse at E12.5. E,
A cryosection of the retina from a nontransgenic
Crx / mouse at
E12.5.
|
|
We then examined to see whether CRX is required for the induction of
Crx transcription at E12.5, when Crx
transcription first is detected during development. We performed X-Gal
staining on the retina of the 2kB line in the Crx null
background (Fig. 8C). We observed X-Gal staining similar to
the wild-type background shown in Figure 8B. We
observed no X-Gal staining in the photoreceptor layer of the retina of
nontransgenic mouse either in the
Crx / background or in wild
type (Fig. 8D,E). This result showed that Crx itself is not essential for the initial induction of the
Crx transcription.
 |
DISCUSSION |
Expression of Crx is restricted in large part in
postmitotic differentiating photoreceptor cells, and its expression
also is maintained in mature differentiated photoreceptor cells.
Crx is the earliest known marker of photoreceptor identity
in the developing retina. Crx mutant mice do not elaborate
photoreceptor outer segments. There was a complete absence of rod and
cone activity as assayed by electroretinogram. Expression of many
photoreceptor and pineal-specific genes was found to be reduced in
Crx mutants. Therefore, Crx is essential for
proper differentiation of photoreceptor cells. Identification of the
mechanisms of how Crx transcription is induced is a crucial
step toward identifying the signaling pathways that control early cell
fate determination of photoreceptor cells.
We have investigated the spatial and temporal expression patterns of
the lacZ reporter gene under the regulation of flanking sequences of the mouse Crx gene. The results of this study
have demonstrated that the 5'-flanking region of the mouse
Crx encodes the sequence determinant that is necessary and
sufficient for the photoreceptor-specific and developmentally regulated
expression of the mouse Crx gene. First, all of the
transgenic lines that were positive in the lacZ expression
consistently showed the lacZ expression in the photoreceptor
cells of the retina and in the pinealocytes of the pineal gland.
Second, in a detailed characterization of the lacZ
expression in retinal sections of the seven transgenic lines, the
lacZ expression always was observed in photoreceptor cells.
Furthermore, in a dissociated retinal cell preparation the
opsin-immunoreactive cells were lacZ-positive. Third, the temporospatial pattern of the lacZ expression completely
agreed with that of the mouse Crx expression during retinal development.
Although Crx expression is restricted primarily to the
photoreceptor cells in the retina, it also has been suggested that bipolar cells may express Crx weakly in mammals. A recent
report about the strong expression of a zebrafish Crx in the
bipolar cells might support this (Liu et al., 2001 ). However, we did
not detect any significant expression of the lacZ reporter
in bipolar cells. Our transgenic mice may express lacZ in
bipolar cells, but its expression level might be too low to be
detected. Another possibility is that the Crx genomic
regions used in this study may lack an enhancer element to drive
Crx in bipolar cells.
Crx is an essential key transcription factor that governs
development of the phototransduction pathway and outer segment
formation. At P10 the expression of rhodopsin, cone opsins (blue and
green/red), rod transducin -subunit, cone arrestin, and recoverin
clearly were reduced in Crx null mice. The levels of cone
transducin, phosphodiesterase (PDE) (rod and cone), rod cGMP-gated
channel (RNG), rod arrestin, and peripherin RNAs were reduced weakly. In contrast, cone cGMP-gated channel (CNG) was upregulated in Crx null mutant (Furukawa et al., 1999 ). Crx
regulates these photoreceptor-specific genes directly and/or
indirectly. Therefore, the precise control of Crx expression
is essential for proper photoreceptor differentiation. Many studies on
gene regulation of photoreceptor-specific genes have been reported,
including rhodopsin, cone opsins, interphotoreceptor retinoid-binding protein (IRBP), PDE, and rod arrestin (Wang et al.,
1992 ; Kikuchi et al., 1993 ; Bobola et al., 1995 ; Chen and Zack, 1996 ;
Di Polo et al., 1996 ; Kimura et al., 2000 ). We compared the sequences
of the promoter regions of these genes with the 2 kb region of mouse
Crx, but we did not find any significant homologous region.
Therefore, this may suggest that cis-regulatory mechanisms
of induction of Crx transcription are different from those
of other photoreceptor-specific genes such as opsins.
We demonstrated that Crx itself plays an important role in
maintaining its own expression in vivo. The
-galactosidase activity of the Pcrx2k transgenic mice is reduced to
31% in the Crx /
background. However, Crx begins to be expressed at E12.5 in
the Crx / background,
showing that Crx is not essential for its initial induction.
This leads us to hypothesize that there is an unknown factor (or
factors) binding to the 2 kb region to induce Crx
transcription. Once Crx transcription is induced, the
high-level expression of Crx is achieved by an autopositive
feedback mechanism (Fig. 9). Then CRX
upregulates various photoreceptor-specific genes, including opsins and
transducins.

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|
Figure 9.
Transcriptional regulation of mouse
Crx in photoreceptor cells. Crx is not
expressed in retinal progenitors, but Crx begins to be
expressed after they become postmitotic and start differentiation. An
unknown factor (or factors) X initially induces Crx
transcription by interacting with the 2 kb region. Then
Crx itself contributes to upregulating and maintaining
the expression of Crx by autopositive feedback.
|
|
The identification of the tissue-specific factor (or factors)
interacting with the cis-acting element that we have
described will contribute substantially toward understanding the
mechanism of regulation of the Crx gene and, quite likely,
the regulation of photoreceptor cell fate. Because Crx is
the earliest known marker of photoreceptor identity in the developing
retina, the mouse Crx promoter fragment that we have
characterized in this study also will be a very useful tool, such as in
the generation of developing and developed photoreceptor-specific
transgenic mice, for various studies of development and function of
photoreceptor cells.
 |
FOOTNOTES |
Received Jan. 20, 2001; revised Oct. 17, 2001; accepted Nov. 27, 2001.
This work was supported by the Howard Hughes Medical Institute and the
National Institutes of Health. We thank Dr. R. S. Molday for
Rho4D2 antibody, Dr. R. McInnes for chx10 antibody, and Dr. Y. Takada
for antibodies against cone opsins.
Correspondence should be addressed to Takahisa Furukawa,
Osaka Bioscience Institute, 6-2-4 Furuedai, Suita-city, Osaka 565-0874, Japan. E-mail:furukawa{at}obi.or.jp.
 |
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