From the Neurobiology Unit, University of Namur
Medical School, 61, rue de Bruxelles, B5000 Namur, Belgium and the
¶ Developmental Genetics Unit, University of Louvain Medical
School, Avenue E. Mounier, B1200 Brussels, Belgium
Received for publication, July 17, 2002, and in revised form, November 4, 2002
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ABSTRACT |
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The Disabled-1 (Dab1)
gene encodes a key regulator of Reelin signaling. Reelin is a large
glycoprotein secreted by neurons of the developing brain, particularly
Cajal-Retzius cells. The DAB1 protein docks to the intracellular part
of the Reelin very low density lipoprotein receptor and apoE receptor
type 2 and becomes tyrosine-phosphorylated following binding of Reelin
to cortical neurons. In mice, mutations of Dab1 and
Reelin generate identical phenotypes. In humans,
Reelin mutations are associated with brain malformations
and mental retardation; mutations in DAB1 have not been
identified. Here, we define the organization of Dab1, which
is similar in human and mouse. The Dab1 gene spreads over
1100 kb of genomic DNA and is composed of 14 exons encoding the major
protein form, some alternative internal exons, and multiple 5'-exons.
Alternative polyadenylation and splicing events generate DAB1
isoforms. Several 5'-untranslated regions (UTRs) correspond to
different promoters. Two 5'-UTRs (1A and 1B) are predominantly used in
the developing brain. 5'-UTR 1B is composed of 10 small exons spread
over 800 kb. With a genomic length of 1.1 Mbp for a coding region of
5.5 kb, Dab1 provides a rare example of genomic complexity,
which will impede the identification of human mutations.
Neuronal migration is a complex process that is affected in a
variety of human disorders such as periventricular heterotopias and
different types of lissencephalies (1-3). The Disabled-1 (Dab1) gene belongs to the Reelin signaling pathway
that plays a key role during brain development in mouse and human
(4-6). Inactivation of Dab1, either by homologous
recombination (7) or by spontaneous mutations in scrambler
or yotari mutant mice (8, 9), generates a phenotype similar
to that of Reelin-deficient mice. This phenotype is characterized by a
poor organization of architectonic patterns at the end of radial
neuronal migration (reviewed in Ref. 10). The neurons that are
the most affected include those of the cortical plate in
the cortex and hippocampus, Purkinje cells, and inferior olivary
neurons. In human, mutations in Reelin result in a specific
lissencephaly with mental retardation and severe abnormalities of the
cerebellum, hippocampus, and brain stem (Norman-Roberts type,
OMIM257320) (11), a phenotype that shows similarity to its mouse
counterpart. Cognitive development is delayed, with little or no
language acquisition and no ability to sit or stand unsupported. Thus
far, no human disease associated with mutations in DAB1 or
other genes in the Reelin pathway has been identified.
Reelin is an extracellular protein secreted by some neurons such as
Cajal-Retzius cells in the marginal zone of the embryonic cerebral
cortex and hippocampus, external granule cells in the cerebellum,
olfactory mitral cells, and ganglion and amacrine cells in the mouse
retina (10, 12) and in the spinal cord (13, 14). The response of target
neurons to Reelin requires the expression of at least one of two
surface receptors that belong to the lipoprotein receptor family,
viz. the very low density lipoprotein receptor
and apoE receptor type 2, as well as the presence of the intracellular
adaptor DAB1. The DAB1 protein contains a 180-amino acid N-terminal
protein interaction/phosphotyrosine-binding (PTB)1 domain that docks to
the short cytoplasmic tail of the very low density lipoprotein receptor
or apoE receptor type 2 at the level of NPXY motifs, with a
preference for unphosphorylated motifs (15-19). Potential tyrosine
phosphorylation sites and a 310-amino acid C-terminal region of unknown
function follow the PTB domain. The binding of Reelin to the
extracellular part of both receptors induces phosphorylation of
tyrosine residues of DAB1, particularly Tyr198 and
Tyr220 (20, 21). Mice expressing a mutant form of DAB1 in
which all the potential tyrosine phosphorylation sites are mutated have a phenotype similar to reeler mice (4), and mice expressing a truncated DAB1 protein missing the C-terminal part have an almost normal phenotype (22). This shows that the PTB domain and tyrosine phosphorylation are both necessary and sufficient to fulfill most of
the DAB1 functions. Cytoplasmic tyrosine kinases of the Src family are
able to phosphorylate DAB1 in vitro, but the kinase(s) involved in DAB1 phosphorylation in vivo remain to be
identified (23). Similarly, the other downstream effectors of the
Reelin signal are not known.
Various isoforms of the mouse DAB1 protein have been described. The
main form contains an open reading frame of 555 amino acids encoding a
80-kDa protein, the predominant form expressed in the brain. Another
form, 555*, contains an additional exon inserted in-frame between
codons 241 and 242. Form 217 results from alternative polyadenylation,
whereas isoform 271 is similar to form 555, except that an additional
exon of 270 bp containing a stop codon is inserted between codons 241 and 242. Upon Northern blotting, three transcripts of 5.5, 4.0, and 1.8 kb have been detected with a probe covering the PTB domain, and protein
isoforms of 36, 45, 60, 80, and 120 kDa have been observed on Western
blots (23).
In this work, we defined the genomic organization of the human and
mouse Dab1 genes. The structure is highly complex and
similar in both species. The gene extends over >1 Mbp of genomic DNA
due to the presence of large introns and the wide dispersion of several alternative transcription initiation sites. The presence of several alternative promoters and alternatively spliced forms points to a fine
regulation of Dab1 expression and further emphasizes the key
position of this gene as a switch in the Reelin signaling pathway. The
complexity of the gene may explain why no human disease associated with
DAB1 mutations could be identified thus far.
Rapid Amplification of cDNA Ends (RACE)--
Information on
the primers used is provided in Table I. First-strand cDNA
synthesis was performed at 60 °C on 1 µg of DNase I-treated RNA
(embryonic human brain and P0 mouse brain) using Dab1-specific primer in exon 2 (Table
I) and Thermoscript reverse transcriptase
(Invitrogen). After RNase H digestion, the cDNA was purified with a
GlassMAX spin cartridge (Invitrogen) and modified by adding a
polydeoxycytidine sequence to the 5'-end using terminal deoxynucleotidyltransferase (Invitrogen). cDNAs were amplified by
PCR with an abridged anchor primer
(5'-GGCCACGCGTCGACTAGTACGGGIIGGGIIGGGIIG-3'), which hybridized to the
poly(dC) sequence, and antisense Dab1-specific primer 2 using Taq DNA polymerase (Biotools). Nested PCRs were performed with the abridged universal amplification primer
(5'-GGCCACGCGTCGACTAGTAC-3') and antisense
Dab1-specific primer 3. The PCR products were cloned into
the pCRII vector (Invitrogen) and sequenced using the BigDye terminator
cycle sequencing kit (PE Biosystems) and an ABI Prism 377 sequencer.
3'-RACE was carried out on 2 µg of total RNA from E15 mouse brain.
First-strand cDNA synthesis was performed using an adaptor oligo(dT) primer (5'-GGCCACGCGTCGACTAGTACTTTTTTTTTTTTTTTTT-3') with Superscript II reverse transcriptase (Invitrogen). cDNA
was amplified using Dab1-specific primer 4 in exon 15 and
the abridged universal amplification primer. An aliquot of the reaction
was subjected to nested PCR using downstream Dab1-specific
primer 5 and the adaptor primer. The 3'-RACE products were subcloned into the pCRII vector for sequencing.
5'-RNA Ligase-mediated RACE--
5'-RNA ligase-mediated RACE was
performed using the GeneRacer kit (Invitrogen). 250 ng of mRNA from
E17 mouse brain was dephosphorylated using calf intestinal phosphatase
and decapped using tobacco acid pyrophosphatase to target full-length
mRNAs. An RNA oligonucleotide was ligated to the decapped mRNA,
and reverse transcription was performed at 60 °C using random
primers and Thermoscript reverse transcriptase. PCR was done to amplify
the resultant cDNAs using the GeneRacer 5'-primer and
Dab1-specific primer 6 in exon 3 or Dab1-specific primer 7 in exon 1B1. Nested PCR was done
using the GeneRacer 5'-nested primer and Dab-specific primer
8 in exon 2, primer 9 in exon 1B1, primer 10 in exon 1A, and primer 11 in exon 1D. Products were cloned into pCRII and sequenced.
Genomics--
PACs containing parts of the human and mouse
Dab1 genes were obtained by PCR screening of the RZPD
Deutsches Ressourcenzentrum fuer Genomforschung libraries
711 (RPCI21; constructed by Dr. Pieter de Jong, Roswell
Park Cancer Institute) and 709 (RPCI6). Human PACs were isolated as
follows: PACs RPCI6-239D12 and RPCI6-225E22 with primers 12 and 13; PAC
RPCI6-65F20 with primers 14 and 15; and PAC RPCI6-102O10 with primers
16 and 1. Mouse PACs RPCI21-97L11 and RPCI21-31E11 were isolated with
primers 17 and 18. Intron sizes were determined by PCR on genomic or
PAC DNA. Amplification of large fragments was carried out with the
Elongase amplification system (Invitrogen). The sequences of the
exon-intron junctions and of upstream genomic regions were determined
by direct sequencing of PAC DNA or of PCR products. Exon-intron
boundaries were determined by aligning cDNA and genomic sequences.
Results were confirmed by comparison with the human and mouse genome
sequences (human contig, NCBI accession number NT_029223.8 (build 30);
and mouse contig, NCBI accession number NW_000211.1).
Reporter Plasmid Constructs--
Fragments were cloned into the
pGL3-Basic promoterless vector (Promega). A 2.9-kb mouse fragment named
ACD (containing exons 1A-1D) was amplified from genomic DNA by PCR
using primers 19 and 17. The PCR product was cloned into pCRII and
cloned in the forward and reverse orientations into the SacI
site of pGL3-Basic. Fragment AD (821 bp) was amplified using primers 20 and 21, cloned into pCRII, and cloned (SpeI-XbaI)
in the forward and reverse orientations into the NheI site
of pGL3-Basic. Construct A (248 bp) was derived from construct AD by
MluI digestion and ligation. To obtain construct D (574 bp),
fragment AD in pCRII was digested with KpnI and
MluI and cloned into the corresponding sites of pGL3-Basic
for the forward construct and digested with HindIII and
MluI and cloned into the corresponding sites for the reverse construct. Fragment C (1641 bp) was amplified from genomic DNA using
primers 22 and 17, cloned into pCRII, and cloned in the forward and
reverse orientations into the SacI site of pGL3-Basic. Fragment B (1.3 kb) was amplified with primers 23 and 24, cloned into
pCRII, and cloned into the KpnI-XhoI site of
pGL3-Basic. A PmlI-EcoRV digestion/ligation of
this construct was carried out to remove three ATG codons present in
the 5'-UTR.
Cell Culture and Transient Transfections--
Undifferentiated
P19, HepG2, and HEK293 cells were grown in Dulbecco's modified
Eagle's medium (DMEM) (Invitrogen) supplemented with 10% fetal
bovine serum and 100 units/ml penicillin and streptomycin. Confluent
cells (80-90%) were plated into 24-well tissue culture plates 24 h prior to transfection. Recombinant pGL3-Basic reporter constructs
(950 ng/well) containing the firefly luciferase gene and a pRL-TK
vector containing the Renilla luciferase reporter (50 ng/well; to normalize for transfection efficacy) were introduced into
cells using LipofectAMINE 2000 (Invitrogen) at 1 µl/well according to
the manufacturer's recommendations. After 24 h, the cells were
harvested, and luciferase activities were determined using the
Dual-Luciferase Reporter assay system (Promega). Data are expressed as
means ± S.E. of at least three independent measurements.
Culture and Transfection of Primary Neuronal Cultures and P19
Cells--
Primary neuronal cultures were prepared from embryonic
brain hemispheres of 15-day mouse fetuses of the BALB/c strain. The cortex was dissected on ice in 2 ml of phosphate-buffered saline. Tissue fragments (1 mm3) were added to 5 ml of
phosphate-buffered saline containing 0.25% trypsin and incubated for
15 min at 37 °C with occasional shaking. Trypsin digestion was
terminated by three washes in Neurobasal medium (Invitrogen) and
suspension in Neurobasal medium supplemented with 0.4 mg/ml soybean
trypsin inhibitor (Invitrogen) and 0.25 mg/ml DNase I. Cells were
dissociated by gentle pipetting, filtered through Falcon cell strainer
sieves (70 µm), and counted. Following a final centrifugation, cells
were seeded in poly-L-lysine-coated 12-well tissue culture
plates at a density of 8 × 105 cells/well in
Neurobasal medium containing 2% B27 supplement (Invitrogen), 500 µM glutamine, and penicillin/streptomycin.
P19 embryonic carcinoma cells were cultivated at a density of 1 × 106 cells/10 ml of medium in non-adhesive 10-cm diameter
dishes in DMEM with 5% fetal bovine serum and
all-trans-retinoic acid (RA) (Sigma R 2625) at a final
concentration of 5 × 10 In Situ Hybridization and Northern Blotting--
The probe for
exon 1A (130 bp) was amplified with primers 25 and 26. The probe for
exon 1B was amplified by RT-PCR from E15 mouse brain using primers 27 and 28 and cloned into pBluescript. This 713-bp fragment contains four
exons of 5'-UTR 1B (see Fig. 1): 1B1, 1B2, 1B4, and 1B8. The probe for
exon 1D (343 bp) was amplified with primers 29 and 30. The probe for
alternative exon 555* was a 189-bp fragment amplified with primers 31 and 32 and cloned into pCRII. It contains both exons 555* plus
27 bp of exon 9 and 47 bp of exon 10. A 480-bp probe covering most of
the PTB region was amplified with primers 16 and 33 and cloned into
pCRII. In situ hybridization was carried out as explained in
the legend to Fig. 5. The probe for the mouse Dab1
3'-UTR was a 2-kb fragment amplified with primers 34 and 35.
The published physical map of the mouse Dab1
gene contains inconsistencies (8, 9). We therefore reconsidered
the physical map and genomic
organizations of the human and mouse
Dab1 genes as summarized in Fig.
1 and Table
II.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
Primers used in this study
7 M. After 2 days, the medium was replaced, and fresh RA was added. At day 4, aggregates were harvested, washed once in DMEM without serum, and
suspended in 2 ml of 0.25% trypsin/EDTA and DNase I (50 µg/ml).
Cells were collected by centrifugation and suspended in 5 ml of DMEM
and 10% fetal bovine serum. They were triturated to yield a single
cell suspension and counted. Dissociated cells were plated at a density
of 6 × 105 cells in 1 ml of DMEM and 10% fetal
bovine serum in 12-well poly-L-lysine-coated plates. The
day following plating, arabinosylcytosine (Sigma C 1768) was added at a
final concentration of 5 µg/ml. Both primary cortical neurons and P19
cells were transfected on day 4 after plating. 1 day prior to
transfection, one-half of the medium was replaced. For transfection, 4 µg of LipofectAMINE 2000 (Invitrogen)/well in 50 µl of Opti-MEM
(Invitrogen) was mixed with 1 µg of DNA in 50 µl of Opti-MEM and
incubated for 20 min at room temperature prior to addition to cells in
400 µl of culture medium. Following 5 h of incubation at
37 °C, the supernatant was removed and replaced with 1 ml of medium.
Assay for luciferase activity was done 24 h after transfection.
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
Nucleotide sequences of exon-intron boundaries of the human and mouse
Dab1 genes
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Fig. 1.
Organization of the human and mouse
Dab1 genes. A, the human and mouse
Dab1 genes map in a large block of cosynteny between
human chromosome 1 (HSA1) and mouse chromosome 4 (MMU4). The
human DAB1 gene maps to 1p32-p31 at 57.39 Mbp from
the centromere, and the mouse Dab1 gene maps to chromosome 4 at 102.19 Mbp from the centromere (left, Ensembl
program). In both species (right), Dab1
is flanked by C8B and hypothetical XM_055482 gene (similar
to mouse AK008020); the centromere is represented by a filled
oval. D4MitNN microsatellites previously used to clone
the mouse Dab1 gene (9) are represented. LEPR,
leptin receptor; JUN, jun oncogene;
AK2, adenylate kinase-2 pseudogene; TACST2,
tumor-associated calcium signal transducer-2; C8A and
C8B, complement factor 8A and 8B genes,
respectively; Ak3, adenylate kinase-3. B, shown
is an overview of the mouse Dab1 gene (the human gene is
similar). Dab1 is shaded in gray, and neighboring
genes in are black; Dab1 exons are represented by
vertical lines. The 5'-UTR is not to scale. PAC and YAC
clones used to define the exon-intron organization are schematized by
horizontal lines (solid lines for human clones
and dashed lines for mouse clones). IAP is the
locus of insertion of an IAP retrotransposon in scrambler
mutant mice. C, shown are the exons and introns of the human
and mouse Dab1 genes. The organization and sequence of
coding exons 2-15 are highly conserved in human and mouse. The complex
human and mouse 5'-UTRs are schematized separately. Conserved 5'-UTR
sequences in human and mouse are shown as gray boxes. Note
the complexity of UTR 1B. Tyrosine residues important for DAB1
function are indicated.
Physical Maps-- The mouse Dab1 gene maps to chromosome 4 at 52.7 centimorgans, whereas human DAB1 maps to chromosome 1p32-p31 (24). In both species, the 5'-end is located on the centromeric side. The gene is flanked by the mouse AK008020 locus (identical to human XM_055482 or LOC115209) on the centromeric side and by the complement factor 8B (C8B) gene on the telomeric side. As shown in Fig. 1A, Dab1 is located between microsatellites D4Mit118 and D4Mit75, but is at least 3 Mbp away from marker D4Mit176, confirming the data published by Ware et al. (9). D4Mit331 is located between Dab1 exons 1B1 and 1B2 (described below), 565 kb upstream of the Dab1 ATG codon and 700 kb centromeric to D4Mit29, in agreement with the reported genetic distance of 0.6 centimorgans between D4Mit29 and D4Mit331 (9). D4Mit29 is located in intron 4, 1.5 kb from exon 4 of Dab1 (Fig. 1B). D4Mit75 maps 440 kb distal to the ATG codon and 300 kb telomeric to D4Mit29.
In scrambler mutant mice, a portion of an intracisternal A particle (IAP) sequence is inserted in the antisense orientation in the Dab1 mRNA by aberrant splicing. The mutation results in production of an enlarged transcript of ~7 kb, with the introduction of multiple stop codons. The defect results from the use of a cryptic splice acceptor site in intron 4 coupled with a cryptic donor site in the IAP element. In the scrambler mRNA, 28 bases unrelated to Dab1 or to the IAP are inserted between Dab1 exon 4 and the IAP sequence. BLAST alignment against the mouse genome localized these 28 bases in intron 4, 11 kb distal to exon 4. No IAP sequence is present in this region in the C57BL/6 DNA. Although we cannot exclude that the IAP element was present in the DC/le strain in which the scrambler mutation arose, it appears more likely that an IAP insertion caused the mutation. In yotari mutant mice, 357 nucleotides corresponding to exons 5-8 are missing from the Dab1 mRNA, and the open reading frame is maintained. At the genomic level, this deletion in the mRNA is due to the insertion of a 962-bp L1 element. This insertion starts at the junction of exon 5-intron 5 and ends in the middle of exon 8 (25).
Genomic Organization-- The mouse genomic organization was derived from direct sequencing on PACs and sequencing of PCR products derived from YACs and genomic DNA. Data from the NCBI mouse genome sequence (accession number NW_000211.1) confirmed the structure of the gene. The human genomic organization was defined by direct sequencing on PAC clones and confirmed by the NCBI human genome sequence (accession number NT_029223.8). As explained below, several alternative first exons named A-F were identified in both the mouse and human genes and are dispersed over large genomic regions. Genomic YAC clones covering the mouse Dab1 region were described previously (9) and are shown in Fig. 1B. YAC 37G4 (500-800 kb) contains exons 2-15, and YAC 175A2 (1220-1500 kb) extends from the complex 5'-UTR 1B to exon 9. Mouse PACs RPCI21-97L11 and RPCI21-31E11 contain upstream exons 1A-1D and 1B8. The sequence-tagged site content of PACS and YACs was not analyzed in detail. However, the fact that the mouse and human genomic organization is similar suggests that the clones were not rearranged.
The entire human DAB1 ORF (Fig. 1B) is contained in four PAC clones covering ~300 kb of genomic DNA. Clone RPCI6-102O10 (132 kb; accession number AL390243) contains 120 kb of intron 1, exon 2, and 15 kb of intron 2; clone RPCI6-65F20 (107 kb; accession number AL138779) contains exons 3-6; clone RPCI6-239D12 (175 kb; accession number AL161740) contains exons 10-15 and the C8B gene; and clone RPCI6-225E22 (not sequenced) contains exon 5 up to at least exon 15. The human PACs have been characterized by the Sanger Center using fluorescent in situ hybridization and sequence determination and are not chimeric.
The Dab1 coding regions (from exon 2 containing the ATG codon to exon 14 containing the stop codon) are spread over 254 kb of genomic DNA for the mouse gene and over 294 kb for the human gene. The size of the major DAB1 protein is 555 amino acids, which corresponds to a coding capacity of 0.2% compared with a mean genomic coding density of ~10%. The organization of the mouse and human genes is conserved, and all exon-intron splice junctions conform to the GT/AG rule (Fig. 1C and Table II). With the exception of exons 12 and 15 (549 and >3300 bp, respectively), exons are relatively small, ranging in size from 39 to 140 bp. Introns in the ORF region range in size from 89 bp to 146 kb. The PTB domain is encoded in exons 3-6. Important tyrosine residues are encoded in exons 6 (Tyr185), 7 (Tyr198), 8 (Tyr200 and Tyr220), and 9 (Tyr232) (4, 21).
The 3'-end of the mouse Dab1 mRNA was determined using
3'-RACE. Using a primer in exon 15, we found a Dab1
3'-untranslated segment that extends 1252 bp downstream from the stop
codon. This sequence contains several putative polyadenylation signals
and aligns with several ESTs. Another set of ESTs align with genomic sequences 1 kb farther downstream (Fig.
2A). RT-PCR with primer 34 defined in this downstream EST set and other primers in exons 14 and 15 showed that the 3'-UTR extends at least until another polyadenylation
signal located 3325 bp from the stop codon. To verify that the
Dab1 3'-UTR is >3 kb long, we performed Northern blot
analysis of mouse brain mRNA using a 2-kb probe corresponding to
this 3'-sequence (Fig. 2B). This probe revealed a single
band of 5.5 kb, whereas probes corresponding to the
Dab1 PTB coding region revealed a band of similar
size plus two additional bands of lower size (see Fig. 4). In human,
four EST clones (accession numbers AA541650, AI799728, R52905,
and R67274) contain a polyadenylation signal (followed by a poly(A)
tail) localized 3344 bp downstream from the DAB1 stop
codon.
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Alternative First Exons and Organization of the 5'-Region-- Comparison of the different Dab1 sequences revealed extensive variation in the 5'-regions. In mouse, the Dab1 cDNA sequence initially described (accession number Y08379) (23) contains 263 nucleotides of 5'-UTR encoded by exon 1A and 136 bp of 5'-UTR encoded by exon 2, which contains the ATG codon. The macaque AB05528 and human AK095513 DAB1 cDNAs contain another 5'-UTR (1B) of 497 and 532 bp, respectively. The human AF263547 DAB1 cDNA contains a different 5'-UTR region of 629 bp, and the human XM_010707 cDNA sequence contains yet another different first exon of 508 bp.
As these data suggest the presence of alternative first exons, we performed 5'-RACE and RNA ligase-mediated RACE on embryonic human and mouse brain RNAs. Using embryonic mouse brain RNA, four different products named 1A-1D were obtained (Fig. 1C and Table II). Mouse fragment 1A corresponds to Dab1 exon 1 in sequence Y083379 (23) and is found in 10 mouse and two rat ESTs. RACE product 1B is similar to human DAB1 cDNA sequences AF263547, AB05528, and AK095513 and is present in three mouse ESTs and one human EST. In mouse, RT-PCR and RACE experiments with primers specific for this product 1B revealed that it does not correspond to a single exon, but is composed of combinations of 10 different exons, 1B1-1B10 (Fig. 1C). Exon 1C does not correspond to any published sequence, but could be amplified by RT-PCR, whereas exon 1D is novel and is present in one mouse and one rat EST sequence; it is conserved in human and mouse and was amplified from human brain RNA by RT-PCR. Attempts to map the transcription initiation sites by primer extension were unsuccessful, possibly because of high GC content and secondary structures of the alternative first exons. Using repeated RNA ligase-mediated RACE reactions on poly(A) RNA from E17 mouse brain, we were unable to extend the Dab1 UTR sequences farther and therefore considered them close to full-length. All the sequences obtained by RNA ligase-mediated RACE were also obtained using classical RACE reactions, and all RACE products were shown to be connected to Dab1 by RT-PCR.
RACE reactions on human brain RNA yielded four different products. One is similar to mouse exon 1A, with no match in human EST data bases. A second RACE product is similar to the highly complex mouse fragment 1B and is present in human cDNA clones AF263547, AB055282, and AK095513. RACE and RT-PCR experiments revealed that 5'-UTR 1B is composed of combinations of a least seven different exons. We were not able to clone the 5'-end of the reconstituted RNA sequence XM_010707 using RACE or to connect it to DAB1 exon 2 using RT-PCR on embryonic human brain RNA. Exon E is not present in EST data bases, but can be connected to DAB1 exon 2 by RT-PCR. Exon F is similar to bases 608-701 of sequence XM_060465, the rest of which is unrelated to DAB1.
Among the novel 5'-UTRs, three are conserved between human and mouse (Fig. 1C), viz. exons 1A (90% identity), 1B (mouse 1B1/human 1B1, 65% identity; mouse 1B2/human 1B4, 67% identity; and mouse 1B4/human 1B7, 79% identity), and 1D (55% identity). The sequences are present in EST data bases and have been isolated as cDNA by others, thus confirming their expression.
Genomic Organization of the 5'-Region-- The Dab1 5'-UTR spreads over 850 kb in mouse and 961 kb in human (Fig. 1, B and C). Whereas exons 1A, 1C, and 1D in mouse and exons 1A and 1D-1F in human are clustered in a 1.5-kb fragment, the complex 5'-UTR 1B has a highly unusual structure. It is composed of 10 exons (1B1-1B10) in mouse and seven exons (1B1-1B7) in human, with sizes ranging from 63 to >675 nucleotides, separated by introns with sizes between a few hundred nucleotides and >300 kb. The sequence of UTR 1B is dispersed over >800 kb of genomic DNA, which is consistent with physical mapping data. YAC 175A2, which is 1500 kb in length, contains the region between exon 1B8 and coding exon 9, but does not contain exons 1B1 and 1B2 (Fig. 1C). Exons 1B1-1B10 are flanked with consensus splice sites and obey the GT/AG rule. Both in mouse and man, the alternative exons that compose UTR 1B contain numerous ATG codons and upstream ORFs (uORFs) (Table II). For example, exon 1B1 contains one uORF of 87 codons in human and of 101 codons in mouse, and the first 55 encoded amino acids are highly conserved. uORFs are common in certain genes that are involved in the control of cellular growth and differentiation. This may have implications for the control of DAB1 mRNA translation, as many examples have been described in which ORFs present in the 5'-UTR influence expression levels (26, 27).
Expression of Alternative First Exons--
To assess whether the
different 5'-exons have different expression patterns, PCRs were
carried out on mouse brain cDNA at different stages from E11 to E18
and at postnatal stages from P0 to adult. We used forward primers in
the alternative first exons and reverse primers in exons 2 and 5 (Fig.
3A). As shown in Fig.
3B, exons 1A and 1D were expressed in all stages tested. Amplification of exon 1C was weak, indicating low level expression in
the tissues examined (data not shown). The complex UTR 1B was barely
detectable at E11 and E12, whereas two main bands were amplified in RNA
isolated from brain at E15 and later, including adult (Fig.
3B). We tested the expression of the alternative first exons
in P19 cells, which differentiate into neurons in the presence of RA
(Fig. 3C). Exons 1A and 1D were detected in undifferentiated and differentiated P19 cells. In contrast, the expression of UTR 1B was
complex. Multiple bands were amplified in undifferentiated cells and up
to 4 days after RA induction. When neural induction was complete, only
two bands were visible. This developmental regulation was confirmed
in vivo. As shown in Fig. 3 (C and D), a pattern of multiple bands amplified from E11 mouse RNA becomes restricted to two main amplicons at P0 and adult. Sequencing of the two
main amplicons showed that they are formed of fragments 1B1, 1B2, and
1B4, with alternative inclusion of fragment 1B8. Interestingly, the
sequences of fragments 1B1, 1B2, and 1B4 are conserved in mouse, man,
and macaque. Sequencing of the other amplicons confirmed that they
correspond to different combinations of exons 1B1-1B10, as shown in
Fig. 3E. These fragments contain variable numbers of ATG
codons, suggesting that some uORFs are excluded from segment 1B in
parallel to neuronal differentiation (Fig. 3E and Table II).
The ability of uORFs to down-regulate translation is documented in
mammals, as mentioned above (26, 27). Although the brain is the major
site of Dab1 expression, low mRNA levels are also
detected in the kidney, liver, and uterus (23). By RT-PCR, exons 1A and
1D were amplified from the brain, testis, kidney, and liver, but not
from the spleen, heart, or thymus. By contrast, the mature forms of
exon 1B were solely amplified from brain cDNA, confirming its
neuron-specific expression in the adult.
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Northern blot analysis was performed using probes corresponding to the
different alternative first exons and a probe covering the PTB
domain-encoding region as a control. With the PTB domain probe, three
bands of ~5.5, 4, and 1.3 kb were revealed in poly(A) RNAs from E17
and P0 brain and correspond to the pattern described previously (23,
9). Satisfactory results were achieved with the exon 1A probe, which
revealed three bands with similar sizes and an additional band of
~1.8 kb (Fig. 4). No signal could be detected with a probe for exon 1D. A probe for UTR 1B revealed a major
5.5-kb band and several smaller and fainter bands, all of which are
absent in RNA extracted from scrambler mouse brain (data not
shown). No clear correspondence between the bands revealed on
Northern blots and the alternative forms of the Dab1 RNA
could be established.
|
We analyzed the expression of the different 5'-first exons by in
situ hybridization with 33P-labeled riboprobes. The
short sequence of the exons and their high GC content (~75% for
exons 1A and 1D) made it difficult to obtain a signal, and satisfactory
results could be obtained only with the complex UTR 1B. As shown in
Fig. 5 (D-F), the UTR 1B expression pattern at E14 and in the newborn (P0) brain was very similar to that observed with the PTB probe (A-C), with the
exception that the ventricular zone appeared to be very weakly labeled
at both stages. Both signals were also similar in other parts of the
brain. This suggests that 5'-UTR 1B and the form containing exon 1A are
the two major Dab1 forms in the brain. This was
confirmed using multiplex RT-PCR on adult brain RNA reactions, which
yielded approximate proportions of messages containing exons 1A, 1B,
and 1D in the brain of 47, 33, and 20%, respectively.
|
Mouse Dab1 Promoter Activity in Neurons and Cell Lines--
The
presence of multiple 5'-exons suggests that the transcription of
Dab1 is regulated by different promoters. Promoter activity was assessed by transient transfection of HEK293 and HepG2 cells, which
do not express Dab1, as well as undifferentiated P19
embryonic carcinoma cells and embryonic mouse primary neuronal
cultures, which express Dab1 (Fig.
6) (23). A 150-bp fragment upstream of
exon 1A (construct A+ in Fig. 6A) was active in all cells
tested (25-fold in neurons, 8-fold in P19, 12-fold in HEK293, and
7-fold in HepG2). A comparable or higher activity was observed when
this sequence was cloned in the reverse orientation (construct A in Fig. 6B), suggesting that this region may function as a
bidirectional promoter, at least in vitro. This may be
related to its high GC content (75% in mouse with 32 CpG
dinucleotides and 80% in human with 38 CpG dinucleotides), with
three SP1-binding sites conserved between mouse and human coupled with
the absence of TATA and CAAT sequences. Construct C+,
containing the region upstream of exon 1C, showed weak promoter
activity in all cell lines tested. This segment has a lower GC content
of ~56%. The promoter prediction programs Promoter Inspector, TSSW,
and TSSG detected a promoter, a degenerate TATA box, and a
transcription initiation site in this region. Construct D+,
corresponding to the 500-bp region upstream of exon 1D, was 6-fold more
active than the promoterless vector in HEK293 cells and neurons and
3-fold more active in HepG2 and P19 cells. This region had no promoter
activity when cloned in the reverse orientation. The programs also
predicted a promoter and a degenerate TATA box in this segment.
Construct AD+, which contains both regions upstream of exons 1A and 1D,
showed promoter activity comparable to that of fragment A+. Construct
ACD+, which contains exons 1A, 1C, and 1D, showed promoter activity
comparable to that of fragment C+. There was no activity of this
segment when cloned in the reverse orientation. Two constructs were
used to assay the promoter activity of regions upstream of exon 1B1 (data not shown). A 1.3-kb construct that includes 350 nucleotides of
exon 1B1 and three ATG codons was inactive in primary cortical neurons.
To avoid possible interference of the ATG codons with translation of
the luciferase reporter, another construct was derived by deleting
these ATG triplets. However, no promoter activity was detected in
primary cortical neurons, indicating that the promoter of form 1B may
be located farther upstream in the 260-kb genomic interval between exon
1B1 and the AK008020 gene.
|
Internal Alternative Splicing Events--
Using PCR on human brain
cDNA and alignment of genomic and EST sequences, we were unable to
identify exons corresponding to mouse Dab1 forms 217 and 271 in human. Using RT-PCR, the presence of fragment 555* was confirmed in
mouse and man. In both species, this sequence corresponds to two exons
of 51 and 48 bp separated by an intron of 91 bp in mouse and of 89 bp
in human (Fig. 7A). Both exons
were consistently co-amplified. Interestingly, an alternatively spliced
product of 57 bp was detected in the corresponding location in the
Dab1 cDNA in lizard and chick (Fig. 7B). As
shown in Fig. 7C, the peptide sequences encoded by the two
small exons that form fragment 555* in mouse and man and by the single
57-nucleotide exon in lizard and chick are conserved, suggesting a
duplication event during evolution. Upon Northern blotting using a
probe that includes exons 555* and some adjacent sequences, a major RNA
species of ~5.5 kb, presumably corresponding to the longest form of
the Dab1 mRNA, was detected in poly(A) RNA from E17
mouse brain (data not shown). In undifferentiated P19 cells, the
Dab1 cDNA did include fragment 555*. However, when
differentiation of P19 cells was induced with RA, a proportion of
Dab1 cDNA without fragment 555* appeared at day 2 and
increased progressively to become the major form at day 9 (Fig.
7D). In early embryonic mouse brain (E11 and E12), the
Dab1 isoform with fragment 555* was predominant, but RNAs
from later developmental stages (E12 and later) and from primary
neuronal cultures did not contain this fragment (Fig. 7B).
In non-neural tissues such as liver and kidney, the Dab1 mRNA contained fragments 555* (data not shown). A similar
pattern was found in chick, with inclusion of the small 57-nucleotide exon in RNA from E6 or adult eye, but exclusion of that exon from brain
RNA at E20 (Fig. 7B).
|
Using in situ hybridization with a cDNA probe covering
fragments 555* and adjacent segments (Figs. 5 (G-I) and
7A), a strong signal was detected in ventricular zones of
precursor proliferation; the moderate labeling of post-migratory fields
could be related to the parts of the probe adjacent to exons 555* (Fig.
5, G-I). The predominance of the exon 555*-related signal
in ventricular zones was also noted in other parts of the brain.
Altogether, these observations suggest that the exclusion of exons 555*
parallels neural differentiation.
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DISCUSSION |
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Both in man and mouse, the Dab1 gene reveals an unusual complexity that leaves ample room for subtle regulation of its expression and function. Examples of such highly complex genomic organization are few and include the metabotropic glutamate receptor GRM8 gene, which spans >800 kb of genomic DNA for a coding length of 2.3 kb (28), and the human neurotrophin receptor genes NTRK2 and NTRK3 (29), which extend over >350 and 380 kb for coding lengths of 3.7 and 2.8 kb, respectively. Intriguingly, the Dab1 paralogous gene Dab2 (named DAB2 or DOC2 in human) is much simpler than Dab1, with an ORF extending over <50 kb of genomic sequence compared with 300 kb for Dab1 (30, 31). Apparently, this situation is not unusual. For example, the NTRK1 gene, closely related to NTRK2 and NTRK3, spreads over only 20 kb (29). A similar feature is found in the two mouse paralogous phospholipase D genes Pld1 and Pld2. Whereas Pld1 contains 28 exons and spans ~147 kb, the whole Pld2 gene is contained in 17 kb of genomic DNA (32). From an evolutionary standpoint, it would be interesting to know whether such huge differences in the genomic complexity of paralogous genes result from extension or contraction of the set of introns in one of the genes after duplication. Like most Drosophila genes, the fly Disabled gene has small introns and extends over 12 kb of genomic DNA (33), suggesting that the large size of Dab1 might result from intron extension.
Our results also reveal a remarkable diversity in the 5'-UTR of both the human and mouse Dab1 genes. We have identified six alternative 5'-UTRs in human and four in mouse, three of which are conserved. Fragments with promoter activity were defined for two of them, but we were unable to clone the promoter for 5'-UTR 1B. This 5'-UTR is unusually complex and spreads over 1 Mbp of genomic DNA. It is composed of seven exons in human and 10 exons in mouse, with three exons conserved between both species and always included together in the mRNA. This results in a 5'-UTR of 1 kb or more (with the inclusion of alternative exons), which is much larger than the average size of 210 bp (34, 35). In situ hybridization using a probe specific for UTR 1B and RT-PCRs clearly showed that it is part of the Dab1 mRNA. The long 5'-UTR of Dab1 contains small uORFs and numerous upstream ATG codons that precede the major translation initiation site, some of which are conserved between human and mouse. The ability of uORFs to down-regulate translation of mRNA in mammals is well documented. For example, in mice, this phenomenon is implicated in the 50-fold increase in the concentration of the cyclin-dependent kinase inhibitor p18INK4c during differentiation of skeletal muscle cells. In proliferating myoblasts, this gene is abundantly transcribed, but not detectably translated, because the mRNA carries a 1115-nucleotide-long 5'-UTR with five upstream ATG codons. During differentiation, a downstream promoter produces a second form of mRNA with a much shorter 5'-UTR that efficiently supports translation (36). It would be interesting to test whether mutations in the long 5'-UTR of Dab1 affect the expression of the major ORF, resulting in altered regulation of gene expression in vivo.
Large gene size and complexity may be important for the production and
processing of the transcripts. Based on a transcription rate of ~1.4
kb/min (37) and data on the dystrophin gene (38), transcription of
Dab1 would require at least 13 h. This is close to or
larger than the estimated division time of neuronal precursors (39),
suggesting that the promoter associated with form 1B could not be
utilized in proliferating cells. In summary, our data show that
Dab1 is far more complex than expected and that further work is needed to understand better the control of Dab1
expression and the molecular machinery by which it exerts its powerful
activity. The detailed genomic structure reported here should
facilitate the study of human DAB1 mutations, which
are predicted to yield abnormal brain phenotypes similar to those
related to Reelin deficiency.
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FOOTNOTES |
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* This work was supported in part by Grant 3.4533.95 from the Fonds de la Recherche Scientifique et Médicale, Grants 186 and 248 from the Actions de Recherches Concertées, and by the Fondation Médicale Reine Elisabeth (Belgium).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EBI Data Bank with accession number(s) AF525763, AY131331, AY172736, and AY174214-AY174232.
§ Chargée de Recherche of the Fonds National de la Recherche Scientifique. To whom correspondence should be addressed. Tel.: 32-81-724-274; Fax: 32-81-724-280; E-mail: Isabelle.Bar@fundp.ac.be.
Postdoctoral Fellow supported by European Union Program
CONCORDE Grant QLG3-CT-2000-00158.
Published, JBC Papers in Press, November 21, 2002, DOI 10.1074/jbc.M207178200
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ABBREVIATIONS |
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The abbreviations used are: PTB, phosphotyrosine-binding; RACE, rapid amplification of cDNA ends; P, postnatal day; E, embryonic day; PAC, P1 artificial chromosome; YAC, yeast artificial chromosome; contig, group of overlapping clones; UTR, untranslated region; DMEM, Dulbecco's modified Eagle's medium; RA, all-trans-retinoic acid; RT, reverse transcription; IAP, intracisternal A particle; ORF, open reading frame; uORF, upstream open reading frame; EST, expressed sequence tag.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1. | Gupta, A., Tsai, L. H., and Wynshaw-Boris, A. (2002) Nat. Rev. Genet. 3, 342-355[CrossRef][Medline] [Order article via Infotrieve] |
2. | Monuki, E. S., and Walsh, C. A. (2001) Nat. Neurosci. 4 (suppl.), 1199-1206 |
3. | Lambert de Rouvroit, C., and Goffinet, A. M. (2001) Mech. Dev. 105, 47-56[CrossRef][Medline] [Order article via Infotrieve] |
4. | Howell, B. W., Herrick, T. M., Hildebrand, J. D., Zhang, Y., and Cooper, J. A. (2000) Curr. Biol. 10, 877-885[CrossRef][Medline] [Order article via Infotrieve] |
5. | Rice, D. S., and Curran, T. (2001) Annu. Rev. Neurosci. 24, 1005-1039[CrossRef][Medline] [Order article via Infotrieve] |
6. | Feng, Y., and Walsh, C. A. (2001) Nat. Rev. Neurosci. 2, 408-416[CrossRef][Medline] [Order article via Infotrieve] |
7. | Howell, B. W., Hawkes, R., Soriano, P., and Cooper, J. A. (1997) Nature 389, 733-737[CrossRef][Medline] [Order article via Infotrieve] |
8. | Sheldon, M., Rice, D. S., D'Arcangelo, G., Yoneshima, H., Nakajima, K., Mikoshiba, K., Howell, B. W., Cooper, J. A., Goldowitz, D., and Curran, T. (1997) Nature 389, 730-733[CrossRef][Medline] [Order article via Infotrieve] |
9. | Ware, M. L., Fox, J. W., Gonzalez, J. L., Davis, N. M., Lambert de Rouvroit, C., Russo, C. J., Chua, S. C., Jr., Goffinet, A. M., and Walsh, C. A. (1997) Neuron 19, 239-249[Medline] [Order article via Infotrieve] |
10. | Lambert de Rouvroit, C., and Goffinet, A. M. (1998) Adv. Anat. Embryol. Cell Biol. 150, 1-106[Medline] [Order article via Infotrieve] |
11. | Hong, S. E., Shugart, Y. Y., Huang, D. T., Shahwan, S. A., Grant, P. E., Hourihane, J. O., Martin, N. D., and Walsh, C. A. (2000) Nat. Genet. 26, 93-96[CrossRef][Medline] [Order article via Infotrieve] |
12. | Rice, D. S., and Curran, T. (2000) J. Comp. Neurol. 424, 327-338[CrossRef][Medline] [Order article via Infotrieve] |
13. | Carroll, P., Gayet, O., Feuillet, C., Kallenbach, S., de Bovis, B., Dudley, K., and Alonso, S. (2001) Mol. Cell. Neurosci. 17, 611-623[CrossRef][Medline] [Order article via Infotrieve] |
14. |
Yip, J. W.,
Yip, Y. P.,
Nakajima, K.,
and Capriotti, C.
(2000)
Proc. Natl. Acad. Sci. U. S. A.
97,
8612-8616 |
15. | D'Arcangelo, G., Homayouni, R., Keshvara, L., Rice, D. S., Sheldon, M., and Curran, T. (1999) Neuron 24, 471-479[Medline] [Order article via Infotrieve] |
16. | Trommsdorff, M., Gotthardt, M., Hiesberger, T., Shelton, J., Stockinger, W., Nimpf, J., Hammer, R. E., Richardson, J. A., and Herz, J. (1999) Cell 97, 689-701[Medline] [Order article via Infotrieve] |
17. | Hiesberger, T., Trommsdorff, M., Howell, B. W., Goffinet, A., Mumby, M. C., Cooper, J. A., and Herz, J. (1999) Neuron 24, 481-489[Medline] [Order article via Infotrieve] |
18. |
Gotthardt, M.,
Trommsdorff, M.,
Nevitt, M. F.,
Shelton, J.,
Richardson, J. A.,
Stockinger, W.,
Nimpf, J.,
and Herz, J.
(2000)
J. Biol. Chem.
275,
25616-25624 |
19. |
Howell, B. W.,
Lanier, L. M.,
Frank, R.,
Gertler, F. B.,
and Cooper, J. A.
(1999)
Mol. Cell. Biol.
19,
5179-5188 |
20. |
Howell, B. W.,
Herrick, T. M.,
and Cooper, J. A.
(1999)
Genes Dev.
13,
643-648 |
21. |
Keshvara, L.,
Benhayon, D.,
Magdaleno, S.,
and Curran, T.
(2001)
J. Biol. Chem.
276,
16008-16014 |
22. |
Herrick, T. M.,
and Cooper, J. A.
(2002)
Development
129,
787-796 |
23. |
Howell, B. W.,
Gertler, F. B.,
and Cooper, J. A.
(1997)
EMBO J.
16,
121-132 |
24. | Lambert de Rouvroit, C., and Goffinet, A. M. (1998) Genomics 53, 246-247[CrossRef][Medline] [Order article via Infotrieve] |
25. | Kojima, T., Nakajima, K., and Mikoshiba, K. (2000) Brain Res. Mol. Brain Res. 75, 121-127[CrossRef][Medline] [Order article via Infotrieve] |
26. | van der Velden, A. W., and Thomas, A. A. (1999) Int. J. Biochem. Cell Biol. 31, 87-106[CrossRef][Medline] [Order article via Infotrieve] |
27. |
Morris, D. R.,
and Geballe, A. P.
(2000)
Mol. Cell. Biol.
20,
8635-8642 |
28. | Scherer, S. W., Soder, S., Duvoisin, R. M., Huizenga, J. J., and Tsui, L. C. (1997) Genomics 44, 232-236[CrossRef][Medline] [Order article via Infotrieve] |
29. | Valent, A., Danglot, G., and Bernheim, A. (1997) Eur. J. Hum. Genet. 5, 102-104[Medline] [Order article via Infotrieve] |
30. | Sheng, Z., He, J., Tuppen, J. A., Sun, W., Fazili, Z., Smith, E. R., Dong, F. B., and Xu, X. X. (2000) Genomics 70, 381-386[CrossRef][Medline] [Order article via Infotrieve] |
31. | Sheng, Z., He, J., Tuppen, J. A., Martin, W. D., Dong, F. B., and Xu, X. X. (2001) Gene 268, 31-39[CrossRef][Medline] [Order article via Infotrieve] |
32. | Redina, O. E., and Frohman, M. A. (1998) Gene 222, 53-60[CrossRef][Medline] [Order article via Infotrieve] |
33. | Gertler, F. B., Hill, K. K., Clark, M. J., and Hoffmann, F. M. (1993) Genes Dev. 7, 441-453[Abstract] |
34. |
Pesole, G.,
Liuni, S.,
Grillo, G.,
Licciulli, F.,
Mignone, F.,
Gissi, C.,
and Saccone, C.
(2002)
Nucleic Acids Res.
30,
335-340 |
35. | Mignone, F., Gissi, C., Liuni, S., and Pesole, G. (2002) Genome Biol. 3, S0004 |
36. |
Phelps, D. E.,
Hsiao, K. M., Li, Y., Hu, N.,
Franklin, D. S.,
Westphal, E.,
Lee, E. Y.,
and Xiong, Y.
(1998)
Mol. Cell. Biol.
18,
2334-2343 |
37. | Shermoen, A. W., and O'Farrell, P. H. (1991) Cell 67, 303-310[Medline] [Order article via Infotrieve] |
38. | Tennyson, C. N., Klamut, H. J., and Worton, R. G. (1995) Nat. Genet. 9, 184-190[Medline] [Order article via Infotrieve] |
39. | Takahashi, T., Nowakowski, R. S., and Caviness, V. S., Jr. (1995) J. Neurosci. 15, 6046-6057[Abstract] |
40. | Bernier, B., Bar, I., Pieau, C., Lambert de Rouvroit, C., and Goffinet, A. M. (1999) J. Comp. Neurol. 413, 463-479[CrossRef][Medline] [Order article via Infotrieve] |
41. | Simmons, D., Arriza, J. L., and Swanson, L. W. (1989) J. Histotechnol. 12, 169-181 |