Two clearly opposing views exist on the function of alpha-fetoprotein (AFP), a fetal plasma protein that binds estrogens with high affinity, in the sexual differentiation of the rodent brain. AFP has been proposed to either prevent the entry of estrogens or to actively transport estrogens into the developing female brain. The availability of Afp mutant mice (Afp(-/-)) now finally allows us to resolve this longstanding controversy concerning the role of AFP in brain sexual differentiation, and thus to determine whether prenatal estrogens contribute to the development of the female brain. Here we show that the brain and behavior of female Afp(-/-) mice were masculinized and defeminized. However, when estrogen production was blocked by embryonic treatment with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione, the feminine phenotype of these mice was rescued. These results clearly demonstrate that prenatal estrogens masculinize and defeminize the brain and that AFP protects the female brain from these effects of estrogens.
The alpha-fetoprotein gene (Afp) is a member of a multigenic family that comprises the related genes encoding albumin, alphaalbumin, and vitamin D binding protein. The biological role of this major embryonic serum protein is unknown although numerous speculations have been made. We have used gene targeting to show that AFP is not required for embryonic development. AFP null embryos develop normally, and individually transplanted homozygous embryos can develop in an AFP-deficient microenvironment. Whereas mutant homozygous adult males are viable and fertile, AFP null females are infertile. Our analyses of these mice indicate that the defect is caused by a dysfunction of the hypothalamic͞ pituitary system, leading to anovulation.A lpha-fetoprotein (AFP) is a serum glycoprotein produced at high levels during fetal life by the liver and the visceral endoderm of the yolk sac and at lower levels by the developing gastrointestinal tract (1, 2). The protein expressed by the embryo is transferred to the maternal blood circulation, and abnormal levels of embryonic AFP in the maternal serum are indicative of spina bifida or Down's syndrome in the fetus (3, 4). The synthesis of AFP decreases dramatically after birth, and only trace amounts are detected in the adult (2). AFP expression has been shown to be regulated by transcriptional mechanisms involving a relatively large promoter (P1) and three distant enhancers (for reviews see refs. 5 and 6). More recently it has been shown that the first intron of the Afp gene contains an enhancer and an alternative promoter called P2 (7). The genes of the albumin family are linked in the mammalian genomes, and this conserved organization has been proposed to be important for the developmental expression switch of the genes of the family after birth (8). Several hypotheses have been proposed for the physiological function of AFP (for reviews see refs. 6, 9, and 10). Because AFP is synthesized during the cell cycle G 1 and S phases, it has been hypothesized that it affects cell growth (11, 12). The observation that AFP is able to bind estrogen led to the suggestion that AFP plays a role in sexual differentiation of the brain by protecting the fetus from the effects of circulating estrogen (13). In addition to binding estrogen, AFP, like albumin, is able to bind other steroids as well as endogenous and exogenous substances such as fatty acids, bilirubin, and various pharmaceutical agents, suggesting that AFP may play a general transportation role (for review see ref. 14). Moreover, because cellular internalization of the protein has been reported, AFP could also interact with cytoplasmic chaperone proteins that normally escort nuclear receptors or transcription cofactors through the cytoplasm toward organelle interfaces (15, 16). AFP has also been proposed to be one protein that protects the embryo against the maternal immune system, on the basis of the observation that addition of purified AFP into the culture of splenic or lymph node mononuclear cells exerts a suppressive effect on ant...
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