Knockout of RNA Binding Protein MSI2 Impairs Follicle Development in the Mouse Ovary: Characterization of MSI1 and MSI2 during Folliculogenesis.

Jessie Sutherland,Eileen Mclaughlin,Victoria Pye,Kara Gunter,Luisa De Andres,Gary Hime,Evan Boon,Barbara Fraser,Alexander Sobinoff,Ilana Bernstein,Janet Holt,Nicole Siddall,Thomas Graf

doi:10.3390/biom5031228

Abstract

Characterizing the mechanisms underlying follicle development in the ovary is crucial to understanding female fertility and is an area of increasing research interest. The RNA binding protein Musashi is essential for post-transcriptional regulation of oocyte maturation in Xenopus and is expressed during ovarian development in Drosophila. In mammals Musashi is important for spermatogenesis and male fertility, but its role in the ovary has yet to be characterized. In this study we determined the expression of mammalian Musashi proteins MSI1 and MSI2 during mouse folliculogenesis, and through the use of a MSI2-specific knockout mouse model we identified that MSI2 is essential for normal follicle development. Time-course characterization of MSI1 and MSI2 revealed distinct differences in steady-state mRNA levels and protein expression/localization at important developmental time-points during folliculogenesis. Using a gene-trap mouse model that inactivates Msi2, we observed a significant decrease in ovarian mass, and change in follicle-stage composition due to developmental blocking of antral stage follicles and pre-antral follicle loss through atresia. We also confirmed that hormonally stimulated Msi2-deficient mice produce significantly fewer MII oocytes (60.9% less than controls, p < 0.05). Furthermore, the majority of these oocytes are of poor viability (62.2% non-viable/apoptotic, p < 0.05), which causes a reduction in female fertility evidenced by decreased litter size in Msi2-deficient animals (33.1% reduction to controls, p < 0.05). Our findings indicate that MSI1 and MSI2 display distinct expression profiles during mammalian folliculogenesis and that MSI2 is required for pre-antral follicle development.

Highlights

The choice to postpone pregnancy until later in life is a growing trend observed among women in developed countries, which is contributing to an increasing incidence of couples seeking fertility treatment [1]
Quantitative PCR was performed on RNA extracted from mouse ovaries and oocytes collected at developmentally important post-natal day (PND) time points [19] in order to observe the steady-state mRNA expression of Msi1 and Msi2
Understanding the mechanisms that control the processes of folliculogenesis and oogenesis is essential to our knowledge of female fertility and reproductive health

Summary

Introduction

The choice to postpone pregnancy until later in life is a growing trend observed among women in developed countries, which is contributing to an increasing incidence of couples seeking fertility treatment [1]. Women reach a peak in fertility during their early twenties, followed by a gradual decline in fertility to the approximate age of 35 [2] Beyond this point infertility increases until menopause is reached at around 50 years of age [3]. The female reproductive lifespan is limited by the finite number of primordial follicles that form within the ovary during late-gestational development in humans, or shortly after birth in the mouse [4]. These follicles undergo continuous programmed cell death, termed “atresia”, with less than 0.1% of the follicles present at birth reaching the ovulatory stage [5].

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