Endocrinology Vol. 141, No. 8 2725-2734
Copyright © 2000 by The Endocrine Society
Ovary-Selective Genes I: The Generation and Characterization of an Ovary-Selective Complementary Deoxyribonucleic Acid Library1
Jon D. Hennebold,
Mamoru Tanaka,
Juichiro Saito2,
Berkely R. Hanson3 and
Eli Y. Adashi
Division of Reproductive Sciences, Department of Obstetrics and
Gynecology, University of Utah Health Sciences Center, Salt Lake City,
Utah 84132
Address all correspondence and requests for reprints to: Dr. Eli Y. Adashi, Division of Reproductive Sciences, Department of Obstetrics and Gynecology, University of Utah Health Sciences Center, 546 Chipeta Way, ARUP II, Box 20, Salt Lake City, Utah 84108. E-mail:
eadashi{at}hsc.utah.edu
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Abstract
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The importance of several ovary-selective/specific genes,
i.e. genes preferentially or exclusively expressed in
the ovary, has been established. Indeed, null mutant female mice for
the c-mos, growth and differentiation factor-9,
-inhibin, and zona pellucida-3 genes proved sterile. A loss of
function mutation of the human FSH receptor gene established its
critical role in ovarian function. These data support the hypothesis
that genes expressed selectively or specifically in the ovary are
probably essential for the normal functioning of this organ system. We
have used the differential screening technique suppression subtractive
hybridization to systematically isolate and clone genes that are
expressed in an ovary-selective/specific manner. The resultant target
complementary DNA (cDNA) library has been exhaustively screened to a
point at which additional sequencing was increasingly unlikely (
4%)
to yield additional previously unencountered cDNAs. In
toto, 844 clones were sequenced and analyzed for homology to
known genes using the Basic Local Alignment Tool (BLAST). Of those, 342
were determined to be independent (nonredundant). One hundred and
fifty-nine independent clones proved identical to previously
characterized genes, whereas an additional 100 independent clones
proved significantly homologous (but not identical) to previously
characterized genes. Yet 83 other independent clones did not display
significant homology to previously characterized genes now listed in
the publicly accessible nonredundant databases. As such, these latter
genes were deemed novel. Of these 83 novel genes, a total of 36
displayed ovary-specific/selective expression, as determined by probing
mouse multitissue Northern blots with
32P-labeled/PCR-amplified cDNA inserts. Under these
circumstances, the false positive rate was minimal, as only one novel
clone was expressed at a higher level in nonovarian tissues relative to
ovary. Of the 36 ovary-specific/selective novel genes, 22 proved
subject to hormonal regulation during a simulated estrous cycle. In
this communication we focus on 2 such novel
ovary-specific/hormonally-dependent genes, the full-length sequences of
which were isolated using rapid amplification of 3'-cDNA ends
technology. Taken together, the present study accomplished systematic
identification of those genes that are restricted in their expression
to the ovary. These ovary-selective genes may have significant
implications for the understanding of ovarian function in molecular
terms and for the development of innovative strategies for the
promotion of fertility or its control.
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Introduction
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A HIGHLY SYNCHRONIZED and exquisitely timed
gene expression cascade controls the individual phases of the normal
ovarian life cycle (1, 2). The appropriate transition from one phase of
the cycle to the next requires the timely expression of a specific
gene(s) to ensure the correct continuation of this preprogrammed
process. To date, a major research focus of students of ovarian
function has been the analysis of mostly known genes.
In the last few years several genes have been determined to be
expressed in an ovary-selective manner. Examples include members of the
transforming growth factor-ß superfamily, growth differentiation
factor-9 (GDF-9) and
-inhibin (3, 4); c-mos, a 37-kDa
cytoplasmic serine/threonine kinase (5); as well as the oocyte-derived
zona pellucida (ZP) glycoproteins (ZP 13) (6). Each of the preceding
genes was isolated and studied on a case by case basis with regard to
ovarian function.
The critical role that ovary-selective genes may play in ovarian
physiology was subsequently established through the generation of null
mutant mice. For example, the ovaries of null mutants for GDF-9
possessed normal numbers of primordial and primary follicles but were
devoid of all downstream follicular stages (7). Female null mutants for
c-mos proved infertile in that meiotically active oocytes
failed to arrest at metaphase II and, in fact, attempted a third round
of meiosis (8, 9). Female null mutants for ZP-3 displayed markedly
reduced (90%) ovulatory efficiency and consequent infertility, the
complete absence of the ZP, and disorganization of the surrounding
cumulus granulosa cells (10, 11). These studies, through the generation
of null mutants, established that the above-mentioned ovary-selective
genes are critical to murine ovarian function.
Taken together, these and other studies serve to demonstrate that
several genes, whether restricted in their expression to the ovary or
preferentially expressed in the ovary, constitute critical molecular
determinants of ovarian function. It was, therefore, the purpose of
this communication to systematically isolate genes that are expressed
in an ovary-selective/specific manner.
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Materials and Methods
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In vivo protocols
Female C57BL/6 mice, 19 days of age upon arrival, were purchased
from The Jackson Laboratory (Bar Harbor, ME). Mice were
initially quarantined for 3 days at the University of Utah Animal
Resources Center. The latter adheres to the guidelines outlined by the
Animal Welfare Act and by institutional animal care and use committee
protocols. At 25 days of age, one group of mice (n = 8) was killed
by halothane-induced asphyxiation (Halocarbon, River Edge, NJ), thereby
providing unstimulated ovarian material as well as nonovarian tissues.
A second group of mice (n = 37) was injected with 10 IU each of
PMSG. Forty-eight hours after PMSG injection, a group of mice was
killed (n = 7) to secure ovaries at the preovulatory phase of a
reproductive cycle. The remaining mice (n = 30) were injected with
10 IU each of hCG. Subgroups (n = 5/subgroup) of the latter were
killed at 3, 6, 9, 12, 24, and 48 h post-hCG injection. Ovaries
removed at 3, 6, 9, and 12 h post-hCG treatment encompass the
interval preceding follicular rupture, whereas the 24 and 48 h
post-hCG treatment groups encompass the luteal phase of the ovarian
cycle.
RNA isolation
Total RNA was isolated from the following nonovarian tissues of
immature 25-day-old female C57BL/6 mice: brain, heart, kidney, liver,
spleen, and lung. Total RNA was also isolated from the ovaries of
25-day-old female C57BL/6 mice undergoing superovulation as described
above. The isolation of total RNA was performed using the RNAeasy Kit
(QIAGEN, Santa Clara, CA) according to the manufacturers
directions. Polyadenylated [poly(A)+] RNA was
subsequently isolated by the use of an oligo(deoxythymidine) magnetic
sphere-based separation system (RNAatract, Promega Corp.,
Madison, WI).
Suppression subtractive hybridization (SSH)
SSH was performed with the PCR-Select Kit (CLONTECH Laboratories, Inc. San Diego, CA) according to the
manufacturers directions. Briefly, an equal amount of
poly(A)+ RNA isolated from each of the nonovarian
tissues was combined to generate a total of 1 µg
poly(A)+ RNA. This messenger RNA (mRNA) was used
to generate the driver complementary DNA (cDNA) through the use of the
SMART cDNA synthesis kit (CLONTECH Laboratories, Inc.)
according to the manufacturers instructions. Ovarian
poly(A)+ RNA (1 µg) isolated from mice
undergoing the above-described superovulation protocol was used to
construct the tester cDNA. Thirty primary and 12 secondary PCR cycles
were used in amplifying the target (subtracted) ovary-selective
cDNAs.
Cloning and sequencing of cDNAs
The PCR products generated by SSH were digested with
RsaI to generate blunt ends and to remove the adapters
previously ligated to both ends of the target cDNAs. These cDNAs were
subsequently purified using QIAGENs PCR purification
system, ligated into the vector pCR-SCRIPT (Stratagene,
San Diego, CA) and transformed into Escherichia coli strain
XL-1' Blue (Stratagene). The individual cDNA inserts were
isolated by PCR amplification using flanking T3 and T7 primer sites.
The plasmid template used in the PCR reaction was obtained by the
Wizard mini-prep kit (Promega Corp., Madison, WI) or by
directly using the bacterial cultures lysed in
ddH20 at a dilution of 1:50.
Purified/PCR-amplified cDNAs were sequenced using T7 primers at the DNA
sequencing core facility of the Huntsman Cancer Institute at the
University of Utah Health Sciences Center using Perkin-Elmer Corp. ABI 377 automated sequencers. The sequence data obtained
were analyzed for homology to previously characterized genes deposited
in the National Center for Biotechnology Informatics (NCBI) database,
which includes entries from GenBank, European Molecular Biology
Laboratory (EMBL), and DNA Database of Japan (DDBJ) databases using the
Basic Local Alignment Tool (BLAST) program.
Analysis of subtraction efficiency
An equal amount of cDNA from the (presubtraction) tester pool
and final SSH-subtracted product were used as a template for the
amplification of the housekeeping gene glyceraldehyde-3-phosphate
dehydrogenase (G3PDH). The forward (5'-TGAAGGTCGGTGTGAACGGATTTGGC-3')
and reverse (5'-CATGTAGGCCATGAGGTCCACCAC-3') G3PDH primers
(CLONTECH Laboratories, Inc.) were used to amplify a
983-bp product within the following PCR parameters: denaturation, 94 C
for 45 sec; annealing, 60 C for 45 sec; and extension, 72 C for 1 min
and 30 sec. Samples were removed following the completion of 14, 18,
22, 26, and 30 cycles. The resultant amplicon was resolved on a 2%
agarose gel stained with ethidium bromide.
Verification of ovary-selective expression (reverse Northern blot
analysis)
Poly(A)+ RNA from the previously described
nonovarian and ovarian tissues was used to generate double stranded
cDNA using the SMART cDNA synthesis system according to the
manufacturers directions (CLONTECH Laboratories, Inc.).
The nonovarian and ovarian cDNAs generated were purified, quantified,
separated on a 1% agarose gel, and blotted onto a Nylon membrane (MSI,
Westborough, MA). The membranes were then probed with
32P-labeled/PCR-amplified cDNA inserts according
to the method of Sambrook et al. (12). The blots were
imaged, and band density was determined by a phosphorimager
(Bio-Rad Laboratories, Inc., Hercules, CA). Equal cDNA
loading was confirmed by probing with a
32P-labeled/PCR-amplified 983-bp G3PDH
product.
Northern blot analysis
Total RNA (20 µg) isolated from ovaries at different stages of
the superovulation protocol was separated on denaturing 1%
agarose-formaldehyde gels and transferred to nylon membranes according
to the protocol of Sambrook et al. (12). Before transfer,
RNA quality and concentration were assessed by ethidium bromide
staining and visualization under UV light. Nonovarian tissue blots (20
µg total RNA) were purchased from Origene Technologies (Rockville,
MD). Nylon membranes were prehybridized for 26 h at 42 C in 5 x
SSPE (sodium chloride-sodium phosphate-EDTA), 50% formamide, 5 x
Denhardts solution, and 0.25% SDS. Probes were generated by
radiolabeling individual PCR-amplified cDNA inserts with
[32P]deoxy-CTP using the random priming method
(Amersham Pharmacia Biotech, Piscataway, NJ). The probes
were denatured in a boiling water bath for 5 min before quenching with
ice. Membranes were hybridized with the relevant probe overnight at 42
C in the same (above-mentioned) solution used for prehybridization. The
membranes were washed in 2 x SSC (standard saline citrate) and
0.1% SDS at room temperature three times for 5 min each time, followed
by two washes in 0.125 x SSC and 0.25% SDS for 15 min at 60 C.
The blots were rinsed in 4 x SSC and imaged through the use of a
phosphorimager (Bio-Rad Laboratories, Inc.). The intensity
of signals was quantified using Molecular Analyst software
(Bio-Rad Laboratories, Inc.). Equivalent RNA loading was
verified by probing the same (stripped) blots with radiolabeled PCR
products of the housekeeping genes ß-actin (for the ovarian blots)
and G3PDH (for the nonovarian tissue blots).
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Results and Discussion
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The technique of SSH has been used in a variety of experimental
settings for the analysis of differentially expressed genes (13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23).
In this study SSH was employed to identify genes that are expressed in
an ovary-selective manner. The efficiency of the SSH procedure, as
assessed by depletion of the housekeeping gene G3PDH, was determined by
PCR amplification over a graded increase in the number of cycles. In
the subtracted (target) ovarian cDNA population, the amount of G3PDH
was significantly reduced relative to that of the unsubtracted ovarian
cDNA (Fig. 1A
). An additional 812 PCR
cycles were required for the subtracted (target) cDNA to achieve the
same level of G3PDH amplification as in the unsubtracted ovarian cDNA.
As PCR amplification is an exponential process, this differential in
cycle number translates into a 256- to 4096-fold depletion of G3PDH
cDNA in the subtracted ovarian material (Fig. 1B
).

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Figure 1. SSH subtraction efficiency. Subtraction efficiency
was determined by analyzing the amount of G3PDH present in both the
unsubtracted starting cDNA and the subtracted target cDNA through the
use of increasing number of PCR cycles (A). The signal intensity for
G3PDH was determined for both the unsubtracted starting material as
well as for the subtracted target material (B).
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After the cloning of the individual SSH-generated cDNA products into a
plasmid vector and transformation of the latter into the appropriate
bacterial host, 844 independent clones were isolated. The individual
cDNA inserts were amplified using primers corresponding to plasmid
sequences flanking the multiple cloning site. The individual PCR
products were subsequently sequenced.
To establish an appropriate conclusion for the screening process of the
subtracted ovary-selective library, the number of new sequences
obtained was plotted against the total number of clones sequenced (Fig. 2
). Only 2 new clones were obtained upon
sequencing the last batch of 50 clones. These results suggested that
the screening of the subtracted ovarian cDNA library was all but
complete and that few new sequences were probably derived by further
screening.

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Figure 2. Ovary-selective library screening saturation. The
number of new independent clones identified is plotted against the
number of clones sequenced.
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Each sequenced clone was analyzed using the BLAST program and
classified as known, homologous, or novel after its evaluation against
entries within the publicly accessible nonredundant database of NCBI
(Table 1
). Known genes were classified as
such if the homology was complete (>98% identity) between the
subtracted SSH-generated clone and the database match. The complete
list of known genes isolated by SSH was deposited in the Ovarian
Kaleidoscope Gene Database (http://ovary.stanford.edu). Clones were
classified as homologous if they did not completely match (<98%) but
exhibited an E probability value of <1-5
relative to entries in the nonredundant database of NCBI. Novel clones
were designated as such if no significant match (E probability value
>1-5) was obtained in the nonredundant
database. Clones not matching entries within the nonredundant database
were still designated as novel even if a complete sequence match was
uncovered within the NCBI EST database. Such EST entries represent
incomplete sequence information (
300600 bp) of an uncharacterized
gene and thus do not detract from the novelty designation. cDNAs
possessing significant homology to genomic clones (i.e.
cosmid, BAC, and PAC) were also classified as novel, as these entries
are comprised of large uncharacterized DNA fragments from a variety of
different species. Of the 844 clones analyzed, 342 were determined to
be nonredundant (independent) sequences. It follows that different
clones containing the same cDNA insert were sequenced multiple times.
Of the 342 nonredundant genes sequenced, 159 corresponded to known
genes, whereas 100 proved partially homologous to entries in the
nonredundant NCBI database. Eighty-three clones were classified as
novel, of which 66 shared homology with entries within the NCBI EST
database.
The ovarian cDNAs isolated from the (subtracted/SSH-generated) library
that corresponded to known genes, included several that have previously
been reported to be expressed, specifically or selectively, in the
ovary (Table 2
). Examples include
proteins encoded by genes involved in ovarian steroidogenesis [StAR,
3ß-hydroxysteroid dehydrogenase I (3ßHSD), 17ßHSD I, and
aromatase) and in cell to cell communication (connexin-43) (24, 25, 26, 27, 28).
Also noted were genes corresponding to secreted ovarian regulators
(
-inhibin, ßB-inhibin, and GDF-9),and to the oocytic ZP-3
protein (3, 4, 6). Interestingly, among the clones sequenced, the Mti10
retrotransposon was highly represented in the ovary-selective cDNA
library (28 of 844 clones, or 3.3% of the total). The Mti10
retrotransposon, a mobile genetic element that relies on a RNA
intermediate to move throughout the genome, belongs to the MaLR
(mammalian apparent long terminal repeat-retrotransposons) family of
transposons (29). It has been estimated that most mammalian genomes
contain between 40,000100,000 copies of family members of the MaLR
retrotransposon, thereby accounting for a significant proportion of the
genome (29).
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Table 2. Examples of known and homologous genes isolated from
the SSH-derived (target) ovary-selective cDNA library
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Although the utilization of SSH in the current study successfully
yielded previously characterized ovary-specific genes, several expected
genes were not present within the target cDNA library. For example, the
oocyte-specific genes bone morphogenetic protein-15 (30), factor in the
germline-
(31), germ cell nuclear factor (32), and c-mos
(5) were not found within the subtracted ovarian library. Likewise, the
granulosa cell-restricted FSH receptor cDNA (33) was not present within
the final subtracted target ovarian cDNAs. The absence of these genes
from the library may be due to an incomplete representation of the
relevant mRNA in the tester cDNA pool that was used in the subtraction
process. This hypothesis is supported by the observation that the genes
in question could not be detected by PCR in the unsubtracted ovarian
cDNAs used as the starting material for the SSH procedure (data not
shown).
Both the tester and driver cDNA pools were generated by the use of the
SMART (Switching Mechanism At 5'-end of RNA Transcript) cDNA synthesis
kit (CLONTECH Laboratories, Inc.). This process relies on
the addition of unique adapter oligonucleotides to the first strand
cDNA. The unique adapters can then be used to prime the PCR
amplification and the generation of double stranded cDNA. The advantage
of this procedure is that it allows for the generation of large amounts
of cDNA from limited quantities of RNA. Due to the utilization of PCR,
however, some of the cDNAs may not be amplified as efficiently as
others and may thus be lost from the SSH starting material. A similar
inability to identify all of the expected known genes after a
differential screening of retinal candidate genes was recently reported
by others and ascribed to the incomplete representation of the total
cDNA repertoire (34).
A significant number of subtracted/SSH-generated cDNAs
exhibited partial homology to entries in the nonredundant NCBI
database (Table 2
). Most of the cDNAs in this category probably
represent mouse homologues of genes identified in other species
(e.g. rat P450scc, rabbit 17
-hydroxylase/1720-lyase,
etc.). A large proportion (n = 104) of the clones
isolated from the subtracted/SSH-generated cDNA library exhibited
significant homology to rat P450scc, representing in all likelihood the
mouse counterpart. Only the 5'-region of the mouse P450scc gene has
previously been deposited in the NCBI database. The latter was mirrored
by 29 clones isolated from the subtracted/SSH-generated cDNA
library.
The degree of ovary-selective expression for 20 randomly chosen
(SSH-derived) cDNAs was verified by reverse Northern analysis. Of the
20 clones analyzed, only 1 exhibited a higher level of expression in
the nonovarian (driver) cDNA compared with the ovarian (tester) cDNA,
demonstrating the low false positive rate associated with SSH in this
experimental circumstance. Figure 3
depicts the ovary-selective expression of 3 cDNAs corresponding to
known genes. Genes previously reported to be ovary-selective (GDF-9 and
3ßHSD) (4, 26) were confirmed as such. A member of the MT transposon
family (Mti10) was also determined to be ovary selective by reverse
Northern blot analysis. Ovary-selective expression of Mti10 was further
supported by the observation that EST (expressed sequence tag)s
corresponding to this transposable element have been isolated
predominantly from unfertilized eggs and 2-cell embryo cDNA libraries.
Confirmation of equivalent cDNA loading was accomplished by probing for
the housekeeping gene G3PDH.

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Figure 3. Verification of ovary-selective expression by
reverse Northern blot analysis. PCR-amplified cDNA inserts from
randomly picked clones were radiolabeled and used to probe blots
containing cDNAs generated from nonovarian (driver) or ovarian (tester)
mRNA. After the reverse Northern blot analysis, the cDNA inserts were
sequenced to ascertain their identities. Equivalent cDNA loading of the
blots was verified by probing with a radiolabeled G3PDH PCR product.
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The novel SSH-generated cDNAs were subjected to further determination
of ovarian and phase specificity by Northern blot analysis. Nylon
membranes containing RNA from 12 different mouse tissues as well as RNA
from individual phases (i.e. unstimulated, follicular,
ovulatory, and luteal) of a simulated estrous cycle were probed with
radiolabeled/PCR-amplified inserts corresponding to all 83 novel cDNA
clones (Table 3
). From this analysis, 17
clones were determined to be ovary specific and phase specific. Only 4
ovary-specific clones were constitutively expressed throughout a
simulated cycle. Fifteen of the 83 novel clones were classified as
ovary selective, in that their ovarian expression proved highest, with
limited expression in 1 or more of the nonovarian tissues. Of the 15
ovary-selective clones isolated, 5 were expressed in a phase-specific
manner, with 10 being expressed constitutively. The expression of 46 of
the novel cDNAs could not be detected by the Northern blot technique.
This negative outcome may reflect the low level of sensitivity of the
Northern blot methodology employed. Verification of ovary-selective or
-specific expression of these 46 negative clones will require the use
of a more sensitive methodology, such as RT-PCR, which is currently
underway. Only 1 of the novel clones was expressed at a higher level in
the nonovarian tissues relative to the ovarian RNA, demonstrating a low
false positive rate (2.1%) for the SSH technique as used under this
experimental circumstance.
From the group of novel genes determined to be ovary selective, two
examples are presented in Fig. 4
. The
expression of the two novel clones (M3-D2 and M7-A10) could not be
detected in any of the nonovarian RNA samples. The presence of RNA on
the membranes was verified by successfully reprobing for G3PDH
transcripts.

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Figure 4. Verification of ovary-specific expression of the
novel clones M3-D2 and M7-A10 by Northern blot analysis. PCR products
corresponding to the novel clones M3-D2 and M7-A10 were radiolabeled
and used to probe membranes containing total RNA (20 µg/lane)
isolated from 12 different nonovarian tissues. The presence of RNA was
verified by reprobing the membranes with a radiolabeled G3PDH PCR
product.
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The phase-specific expression pattern of clones M3-D2 and M7-A10 is
shown in Fig. 5
. The expression of the
M3-D2 gene increased approximately 4.5-fold relative to that in
unstimulated ovaries 48 h after treatment with PMSG (Fig. 5
, A and
C). Increases of 10- and 9.6-fold (relative to unstimulated ovaries)
were observed in the ovarian expression of the novel gene M3-D2 at 24
and 48 h after the injection of hCG to PMSG-primed mice
(P < 0.05), respectively. Another novel gene (M7-A10),
exhibited a 50% reduction in ovarian expression (relative to
unstimulated ovaries) 6 h after the administration of hCG to
PMSG-primed mice (P < 0.05; Fig. 5
, B and D). However,
24 and 48 h post-hCG, the expression of M7-A10 was increased
significantly (P < 0.05) relative to the control value
(1.5- and 1.8-fold). The transcript sizes of the M3-D2 and M7-A10
genes, as calculated from the Northern blot analysis, were
approximately 1.3 and 1.5 kbp, respectively. Equivalent RNA loading was
verified by reprobing the same membrane for ß-actin transcripts.

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Figure 5. Phase-specific expression of the novel clones
M3-D2 and M7-A10 by Northern blot analysis. PCR products corresponding
to the novel clones M3-D2 (A) and M7-A10 (B) were radiolabeled and used
to probe membranes containing total ovarian RNA (20 µg/lane) isolated
from mice undergoing a simulated estrous cycle. Equivalent RNA loading
was verified by reprobing the membranes with radiolabeled/PCR-amplified
ß-actin. The signal intensities were determined by densitometry. The
level of M3-D2/ß-actin (C) and M7-A10/ß-actin (D) ratios were
calculated and compared with expression in the unstimulated control
(CTRL) ovaries. The data are represented as the mean ±
SEM of three independent experiments. *, Statistical
significance (determined by ANOVA followed by Fishers least
significant difference post-hoc analysis, StatView 5.0)
of P < 0.05 compared with the CTRL samples.
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As SSH yields partial cDNA sequences, the full-length cDNA sequences of
clones M3-D2 and M7-A10 were subsequently determined by rapid
amplification of 3'-cDNA ends. The putative protein encoded by clones
M3-D2 proved to be a 283-amino acid protein, the molecular mass of
which was 29.9 kDa (Fig. 6A
). The latter proved
highly acidic (theoretical pI of 3.9) due to the large number of
glutamic acid residues (49 of a total of 283). Based on its restricted
expression profile and highly acidic nature, we have termed this
protein ovary-specific acidic protein (OSAP). According to the PSORT
protein analysis tools (http://psort.nibb.ac.jp:8000), OSAP does not
possess a signal peptide, endoplasmic reticulum localization
motifs, peroxisomal targeting sequences, or transmembrane-spanning
domains. The PSORT cellular localization prediction algorithm suggested
that OSAP may be directed to reside in the mitochondria. By using
NCBIs BLASTp program, low but significant homology (E value of
10-12) was observed between OSAP and the sea
lamprey neurofilament protein (NF-180). The acidic regions of
neurofilament proteins from other species were also found to possess
limited but significant homology to OSAP (data not shown).

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Figure 6. The full-length cDNA sequence and putative amino
acid sequence for the novel clones M3-D2 and M7-A10. The full-length
cDNA sequence for clone M3-D2 (A) and M7-A10 (B) was obtained using
rapid amplification of 3'-cDNA ends. The putative amino acid sequence
is shown below the corresponding cDNA sequence. The
underlined region indicates the polyadenylation signal
sequence.
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The open reading frame, derived from the full-length cDNA sequence of
M7-A10 encodes a 235-amino acid protein, the molecular mass of which
proved to be 26.8 kDa (Fig. 6B
). PSORT analysis suggested that the
M7-A10 protein lacks a signal peptide, ER localization motifs,
peroxisomal targeting sequences, or transmembrane-spanning domains. The
PSORT cellular localization prediction algorithm suggested that M7-A10
localizes to the cytoplasm. By using NCBIs BLASTp program, M7-A10 was
found to possess significant homology only at the amino acid level to 2
yeast proteins, termed MOB1 and MOB2 (Fig. 7
). MOB1 (Mps One Binder) was originally
identified via a yeast two-hybrid system as a binding partner for the
yeast protein kinase MPS1 (35). MPS1, in turn, is necessary for proper
mitotic checkpoint control and spindle pole body duplication (36).
Conditional (temperature-sensitive) alleles for the yeast MOB1 gene
gave rise to a late nuclear division arrest at the restrictive
temperature (35). Another MPS1 binding protein was subsequently cloned
and named MOB2 (35). The MOB2 protein also exhibited significant
homology to the ovary-specific clone M7-A10. Based on the homology
among M7-A10, MOB1, and MOB2, we propose that this new putative protein
be named mammalian MOB homolog.

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Figure 7. MOB1, MOB2, and M7-A10 amino acid homology.
The CLUSTAL-W alignment program was used to determine the homology that
may exist between the yeast mitotic regulators MOB1/MOB2 and
M7-A10.
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The mitotic defects associated with MOB1 and MOB2 yeast mutants may be
related to their interaction with another protein, termed MPS1. MPS1
encodes an essential dual specificity kinase whose activity is required
for both spindle body duplication, a process necessary for proper
chromosomal segregation and the maintenance of the correct complement
of chromosomes (36, 37, 38). Additionally, MPS1 is responsible for arrest
at a mitotic checkpoint. Mitotic arrest of cells is an important
process that allows for correction of spindle body defects and repair
of DNA damage (39). Taken together, these yeast studies suggest a role
for the ovary-specific mammalian MOB homolog in the maintenance of
chromosomal integrity in the ovary, possibly during meiosis.
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Acknowledgments
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The authors express their gratitude to Dr. Raymond F. Gesteland
(Department of Human Genetics, University of Utah Health Sciences
Center), Dr. William L. Carroll (Department of Pediatrics and the
Huntsman Cancer Institute, University of Utah Health Sciences Center),
and Dr. John H. Weis (Department of Pathology, University of Utah
Health Sciences Center) for discussions on differential screening
strategies. We also thank Linda Gracie-Elder and Andy Raposa for
providing expert secretarial support.
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Footnotes
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1 This work was supported in part by NIH Grants HD-30288 and HD-37845
(to E.Y.A.), a Lalor Foundation Fellowship Award (to J.D.H.), and a
Society for Gynecologic Investigation Medical Student Research Award
(to B.H.). 
2 Current address: Department of Obstetrics and Gynecology, St.
Marianna University School of Medicine, 216-1 Sugao, Miyamae-ku,
Kawasaki, Kanagawa, 216, Japan. 
3 Current address: George Washington University School of Medicine,
2300 I Street, Washington, D.C. 20037. 
Received March 13, 2000.
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