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Subfertility in male and female mice with mutations in Cecr2
- Author / Creator
- Norton, Kacie A
In both mice and humans, defects in spermatogenesis, oogenesis, and pregnancy can all affect fertility. Spermatogenesis requires the constant renewal of spermatogonial stem cells, completion of meiosis, and an intricate remodeling process to form functional spermatozoa. Oogenesis similarly requires an amplification of germ cells followed by meiosis and oocyte maturation, but occurs largely during embryogenesis to establish the pool of oocytes available for a female’s entire reproductive life. As females must additionally support the developing embryo, female infertility can also be caused by defects affecting pregnancy such as those disrupting implantation and placentation. Chromatin remodelers have been shown to have diverse functions in these processes, including roles in meiosis and regulation of gene expression.
CECR2 is part of a chromatin remodeling complex with known roles in neural tube closure, stereocilia organization within the inner ear, and kidney development. It has also been shown that male mice homozygous for the hypomorphic Cecr2GT allele are subfertile. The overall goal of this work was to investigate the role of Cecr2 in reproduction in both sexes. Since complete loss of Cecr2 results in perinatal death due to a neural tube defect, this study was accomplished by using mice of two Cecr2 genotypes: previously studied Cecr2GT/GT mice and compound heterozygote Cecr2GT/Del mice, which have a lower level of CECR2 due to the presence of one copy of the presumptive null Cecr2Del allele.
This work showed that in males, CECR2 is localized to gonocytes in embryonic day 18.5 testes and spermatogonia in adult testes. Fertility testing experiments revealed that both Cecr2GT/GT and Cecr2GT/Del subfertility is age-dependent. Mutant males sired few pups just after sexual maturity and their litter size improved significantly with age, although never reaching normal levels. Cecr2GT/Del males also have corresponding defects in testis histology and size, sperm concentration and motility, and the frequency at which they fertilize oocytes in vivo, phenotypes that all improve with age. Histological abnormalities in the testis first became apparent in 24 day old Cecr2GT/Del mice, but Cecr2Del/Del testes have fewer seminiferous cords at embryonic day 18.5. An RNA-seq analysis comparing Cecr2+/+ and Cecr2GT/Del testes at P24 revealed 387 differentially expressed genes. Strikingly, while overall only 149 (39%) genes had higher transcript levels in Cecr2GT/Del testes, sex chromosome genes were exclusively found at increased levels in comparison to Cecr2+/+ testes. This suggested a possible defect in meiotic sex chromosome inactivation, which was supported by the detection of increased autosomal asynapsis in mutant spermatocytes of 24-28 day old but not 142-149 day old males.
In females, Cecr2 is expressed in the ovary, oviduct, and uterus at embryonic day 18.5. Both Cecr2GT/GT and Cecr2GT/Del females are subfertile with an approximately 50% reduction in litter size, but no change in litter size was observed with age. Cecr2GT/GT females ovulated a normal number of oocytes after superovulation, and there was no difference from wild-type in the percentage of those oocytes that were fertilized in vivo. In addition, they had a normal number of implantation sites at embryonic day 5.5, but embryo death occurred around day 9.5-10.5 and was accompanied by increased vaginal blood. In contrast, Cecr2GT/Del females had fewer implantation sites at embryonic day 5.5 despite normal ovarian histology. This is unlikely due to a defect in oocyte transport through the oviduct, as cilia function appeared normal in Cecr2GT/Del females. However, decidual tissue was prematurely lost in some Cecr2GT/Del females after artificially induced decidualization. As defects in decidualization can result in both the failure of implantation and embryonic loss later in gestation, abnormal decidualization in Cecr2 mutants could play a role in both of these phenotypes.
Given that male Cecr2 mutants have defects in spermatogenesis but female subfertility is likely due to uterine defects, Cecr2 appears to have a sexually dimorphic role in reproduction. Additionally, Cecr2 likely has a particularly important function in prepubertal spermatogenesis, as mutants are most severely subfertile just after sexual maturity. These intriguing phenotypes merit further investigation, as do changes in the transcriptome of Cecr2GT/Del testes that may help explain Cecr2’s function in spermatogenesis. Overall, this work has strengthened our understanding of the role Cecr2 plays in reproduction in both males and females and has provided the foundation for future studies.
- Graduation date
- Spring 2020
- Type of Item
- Doctor of Philosophy
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