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-Subunit Gene
Department of Biochemistry, Molecular Biology, and Cell Biology, and Center for Reproductive Science, Northwestern University, Evanston, Illinois 60208
Address all correspondence and requests for reprints to: Kelly E. Mayo, Ph.D., Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Evanston, Illinois 60208. E-mail: k-mayo{at}northwestern.edu
Inhibin is an important modulator of reproductive function at both
the endocrine level, through its regulation of pituitary FSH
biosynthesis, and at the paracrine and autocrine levels, as an
intragonadal regulatory factor. To investigate the in
vivo actions of inhibin in FSH regulation and gonadal function,
transgenic mice that overexpress the rat inhibin
-subunit gene were
generated. A transgene that includes the mouse metallothionein-I gene
promoter (MT-
) fused to the rat inhibin
-subunit precursor coding
sequences was used to produce three lines of transgenic mice. Transgene
mRNA is expressed in numerous tissues, including the pituitary, liver,
testis, ovary, and kidney. Inhibin
-subunit protein was also
increased in transgenic pituitary and ovary. Serum inhibin
-subunit
levels are highly increased compared with control mice. Inhibin
ßA- and ßB-subunit protein amounts are
lower in transgenic ovaries compared with wild type, although serum
levels of activin A are not significantly reduced in transgenic female
mice. FSH levels are reduced in both male and female transgenic mice,
whereas LH levels are increased in MT-
female mice. MT-
transgenic females are subfertile and exhibit a 52% reduction in
litter size compared with wild-type females. The smaller litter size of
MT-
female mice was correlated with a reduction in the number of
oocytes ovulated during a normal cycle. Treatment of the transgenic
females with exogenous gonadotropins resulted in an ovulation rate
similar to that of stimulated wild-type animals, suggesting that
altered gonadotropin levels may be responsible for the decreased
ovulation rates. MT-
transgenic male mice are fertile and sire
litters of equivalent size to those sired by wild-type males, despite
an approximately 50% reduction in sperm numbers. These results
indicate that overexpression of the rat inhibin
-subunit gene in
mice leads to a disruption of the normal inhibin-to-activin ratio and
to reproductive deficiencies, and they support the hypothesis that
inhibin and activin act to regulate FSH secretion in
vivo and are essential for normal gonadal function.
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