Adelaide Research and Scholarship
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|Title: ||The role of circadian rhythms in reproduction: development and fertility in the bmal1 null mouse.|
|Author: ||Boden, Michael James|
|Issue Date: ||2008|
|School/Discipline: ||School of Paediatrics and Reproductive Health : Obstetrics and Gynaecology|
|Abstract: ||Circadian rhythms are the endogenous cycling of hormones, activity patterns and gene expression that occur across 24 hours. Disruption of circadian rhythms has been associated with multiple health complications, including reduction of fertility. The bmal1 mouse provides an animal model for central and peripheral loss of rhythmicity. Herein the reproductive function and postnatal development in the bmal1 knockout mouse has been evaluated.
The reproductive capability of the heterozygous breeding colony was investigated, with around 50% of the female breeder mice becoming pregnant within one estrus cycle. The offspring of the colony had a higher than expected level of perinatal mortality while the knockout and heterozygous genotype was under represented among the offspring surviving to weaning, suggesting high knockout embryo or perinatal losses. The circadian phenotype of this mouse model was confirmed, identifying the severe disruption of circadian behavioural rhythms. Further, the growth of the bmal1 knockout mice was retarded compared with their heterozygous and wild type littermates from weaning to 9 months of age.
The reproductive function of the homozygous male bmal1 knockout mouse was evaluated. They showed poor fertility, poorly developed secondary sexual organs, reduced sperm count and reduced sperm motility. Female bmal1 knockout mice had delayed vaginal opening, delayed onset of first estrus, disrupted estrus cyclicity as well as impaired reproductive and mammary tissue development. Steroid hormone synthesis was compromised in both males (testosterone) and females (progesterone) and ovarian morphology revealed reduced corpora lutea formation and structural abnormalities. Female bmal1 knockout mice also evidenced profound infertility, which was caused by a continuum of reproductive insufficiencies including reduced ovulation of oocytes, poorer progression of the preimplantation embryo and failure to successfully implant in the uterus. While the ovaries of bmal1 knockout females were able to respond to exogenous stimulation, the number of ovulated oocytes was reduced, the fertilised oocytes were of reduced quality, progressed poorly to mature blastocyst and once again failed to implant.
A bioinformatical evaluation of a panel of genes closely involved in reproduction and ovarian function was analysed for the presence of circadian enhancer regions (E-box sequences) or RORA response elements (RRE) in their promoter regions. It was revealed that many of the genes investigated contained one or more circadian E-box and RRE sequence, providing a mechanism for the disruption of circadian gene expression within the ovary to cause detrimental changes in gene expression. Further to this, the gene expression profile of these functional genes and clock genes were evaluated in ovarian tissues from wild type and knockout mice across the estrus cycles and across 24 hours. It was shown that the murine ovary rhythmically expressed the genes involved with the molecular clock across 24 hours, as well as several other genes previously associated with rhythmicity in peripheral tissues. Further, the loss of functional bmal1 gene expression resulted in up or down regulation of over 75% of the functional genes investigated, including steroidogenic acute regulatory protein (the rate limiting enzyme for progesterone synthesis).
In conclusion, the bmal1 knockout mouse shows a significant multi-factorial loss in fertility in both males and females. This loss occurs across a range of tissues and results in heavily reduced fertility in the male and complete infertility in the female. Further research could identify in greater detail the precise molecular mechanisms underpinning of this disruption.|
|Advisor: ||Kennaway, David|
|Dissertation Note: ||Thesis (Ph.D.) -- University of Adelaide, School of Paediatrics and Reproductive Health, 2008|
|Subject: ||Circadian rhythms.
|Appears in Collections:||Research Theses|
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