Mice without a Functional Relaxin Gene Are Unable to Deliver Milk to Their Pups

Abstract

We have used gene targeting to generate relaxin (rlx)-deficient mice. The majority (15 of 17) of homozygous (rlx Ϫ/Ϫ ) mice are fertile and produce normal litters. However their mammary development is deficient; pups are unable to suckle and die within 24 h of birth unless cross-fostered to a wild-type (rlx ϩ/ϩ ) foster mother. [445][446][447][448][449][450][451][452][453] 1999) R ELAXIN was the name given to a factor that was first shown to cause relaxation and softening of the pubic ligaments of the guinea pig (1). Later, this factor was shown to soften the uterine cervix, inhibit myometrial contraction, and increase the length of the interpubic ligament in estrogen-treated ovariectomized mice and/or rats (2).For many years, there were no pure preparations to establish tests for bioactivity and no sensitive homologous assays for measuring changes in plasma relaxin concentrations during pregnancy. When heterologous or homologous assays did become available for some species, it became clear that the patterns of relaxin secretion differed markedly between species. In the rat and mouse, the relaxin concentration is elevated only during the last half of pregnancy (3), whereas the human circulating concentrations of relaxin rise to a peak toward the end of the first trimester, then decrease to a plateau that is maintained for the rest of pregnancy (4).The molecular structure of relaxin is known for many species, and several important points have emerged (5). The relaxins of all species in which it exists are peptides of between 43-64 amino acids, containing two chains, A and B, covalently linked by two disulfide bonds (the C peptide is trimmed off in the processing of prorelaxin to produce relaxin). There is remarkable heterogeneity in the relaxin family, with, in general, no more than 50% structural homology between relaxins of various species (5). Some ruminants (sheep and cow) do not express a functional relaxin gene (6, 7); however they do express a relaxin-like factor (Leydig cell insulin-like peptide) in ovarian cells.In mice and rats, the corpus luteum is the major source of circulating relaxin during pregnancy (8, 9); however, relaxin expression has also been detected in several other tissues. In the rat, moderate levels of relaxin messenger RNA (mRNA) and protein have been detected in the uterus (10, 11), and relaxin gene expression has also been observed in the brain and prostate gland (10, 12). Binding sites for relaxin have been identified in tissues of the reproductive tract, such as the uterus (13), cervix (14, 15), mammary gland, and nipple (15, 16) as well as in the pubic symphysis (17), cardiac atrium, and brain (13,18,19). In the mammary gland the binding sites are located in the epithelial cells of the lobulo-alveolar elements as well as in smooth muscle and in epithelial cells of the skin of the nipple (15,20).The biological actions of relaxin in the pregnant rat have been characterized extensively. They include inhibition of myometrial contraction and connective tissue remodeling at ...

Reduced expression of the PTH/PTHrP receptor during development of the mammary gland influences the function of the nipple during lactation

“…The nipple and lactation phenotype of the d/d mouse is very similar to those reported in rodents where relaxin levels have been decreased by antibodies or targeted gene disruption (Kuenzi and Sherwood, 1992;Zhao et al, 1999). Relaxin is a member of the insulin family of peptide hormones and is made by the corpus luteum during the second half of pregnancy in the mouse (reviewed in Ivell and Einspanier, 2002).…”

“…Finally, the relaxin-null mouse did not exhibit a pregnancy-induced expansion in the size of smooth muscle bundles in the nipple (Zhao et al, 1999). The failure of the nipple function in the relaxin Ϫ/Ϫ mice was attributed to small size, high tone of the smooth muscle beds, and lack of connective tissue loosening within the structure (Zhao et al, 1999).…”

“…In rodents, the cervix, vagina, pubic symphysis, mammary gland, and the nipple all appear to be targets of the hormone (Ivell and Einspanier, 2002). In the relaxin knockout mouse, milk is produced by the mammary gland but it accumulates in dilated ducts under the nipple and is not delivered to the pups (Zhao et al, 1999). The nipples of the relaxin knockout mouse fail to grow during pregnancy, and histological observations as well as protein extraction studies suggest that connective tissue was not remodeled during pregnancy (Zhao et al, 1999(Zhao et al, , 2000.…”

“…In the relaxin knockout mouse, milk is produced by the mammary gland but it accumulates in dilated ducts under the nipple and is not delivered to the pups (Zhao et al, 1999). The nipples of the relaxin knockout mouse fail to grow during pregnancy, and histological observations as well as protein extraction studies suggest that connective tissue was not remodeled during pregnancy (Zhao et al, 1999(Zhao et al, , 2000. Finally, the relaxin-null mouse did not exhibit a pregnancy-induced expansion in the size of smooth muscle bundles in the nipple (Zhao et al, 1999).…”

“…The nipples of the relaxin knockout mouse fail to grow during pregnancy, and histological observations as well as protein extraction studies suggest that connective tissue was not remodeled during pregnancy (Zhao et al, 1999(Zhao et al, , 2000. Finally, the relaxin-null mouse did not exhibit a pregnancy-induced expansion in the size of smooth muscle bundles in the nipple (Zhao et al, 1999). The failure of the nipple function in the relaxin Ϫ/Ϫ mice was attributed to small size, high tone of the smooth muscle beds, and lack of connective tissue loosening within the structure (Zhao et al, 1999).…”