# RXR Expression Profiles in Yak Reproductive Tissues During Follicular, Luteal, and Pregnancy Phases

## TL;DR

This study explores how RXR genes are expressed in yak reproductive tissues during different reproductive phases, suggesting their role in regulating fertility in high-altitude environments.

## Contribution

The study provides the first systematic analysis of RXR gene expression in yak reproductive tissues across follicular, luteal, and pregnancy phases.

## Key findings

- RXR expression peaks in ovarian granulosa cells during the follicular phase, in oviductal epithelium during the luteal phase, and in uterine endometrial glands during pregnancy.
- RXRs show dynamic nuclear–cytoplasmic localization shifts in yak reproductive tissues, indicating potential regulatory roles in reproduction.
- The findings suggest RXRs are involved in follicular development, fertilization, and early embryonic development in yaks.

## Abstract

Retinol X receptors (RXRs), including α, β, and γ subtypes, play a key role in the reproductive process and are involved in regulating various physiological processes, including gonadal development, sexual differentiation, reproductive behavior, and reproductive-related diseases. Yaks (Bos grunniens), adapted to the high-altitude Qinghai–Tibet Plateau, exhibit low fertility due to environmental stressors, impacting livestock farming. In this study, we investigated the RXR gene expression profiles in the uterine, ovarian, and oviductal tissues of yaks during distinct reproductive phases. The results indicated that there are significant differences in the expression of RXRs in the ovaries, oviducts, and uterus of yaks during various reproductive phases. Our results indicate that RXRs may be involved in a wide range of yak reproductive stages, namely, follicular development, fertilization, and early embryonic development. This observation provides a preliminary framework for investigating the influence of these receptors on the reproductive physiology of yaks; although, it is important to note that this framework is not definitive.

The yak is a large ruminant that lives in the high-altitude and hypoxic environment of the Qinghai–Tibet Plateau in China and typically exhibits limited reproductive capacity, posing a significant challenge to the advancement of animal husbandry in the region. Retinoid X receptors (RXRs), as an important member of the NR superfamily, play a key role in the regulation of reproductive hormone synthesis, follicular development, and embryo implantation. However, there is still a lack of systematic research on the expression characteristics and potential functions of RXRs in the yak’s reproductive system. This study characterized RXR expression in ovarian, uterine, and oviductal tissues from three yaks per reproductive phase (follicular, luteal, and pregnancy). Using Quantitative Real-Time PCR Experiments (RT-qPCR), Western blot (WB), immunohistochemistry (IHC), and immunofluorescence (IF), we analyzed RXR mRNA and protein expression and localization. RXR expression varied significantly (p ≤ 0.05), peaking in ovaries during the follicular phase, oviducts during the luteal phase, and uteri during pregnancy. RXRs were localized in ovarian granulosa and theca cells, oviductal epithelium, and uterine endometrial glands, with dynamic nuclear–cytoplasmic shifts. These findings suggest RXRs regulate key reproductive processes in yaks, offering insights on improving fertility in high-altitude environments.