Proteomic analysis of porcine endometrial tissue during peri-implantation period reveals altered protein abundance
Graphical abstract
Introduction
The establishment of pregnancy requires the participation of a receptive endometrium and the development of the embryo to the implantation competent stage [1], [2]. In mammalian species, including pigs, the endometrium undergoes a transformation in response to the physiological changes triggered by ovarian hormones in different stages of the cycle to prepare for embryo attachment and implantation [3]. These transformations involve changes to the endometrial structure and spatiotemporal alterations in molecular profiles. The endometrium also secretes a wide array of growth factors, proteins and cytokines, which constitute the histotroph, an important source of energy and nutrition to a growing embryo [4], [5]. In both cycling and pregnant animals, similar molecular changes occur in the endometrium up to the initiation of conceptus elongation [6], suggesting that the changes responsible for the formation of a receptive endometrium are initially under maternal control. The endometrial receptivity triggered by ovarian steroids is later modified by the molecular signaling between the embryo and the endometrium. In pigs, this signaling between the embryo and endometrium starts around Days 11 to 12 of pregnancy when there is a surge of estrogens secreted by the conceptuses in the uterine lumen [7]. This period is also known as the maternal recognition of pregnancy in pigs. It has been hypothesized that an increase in estrogen concentration stimulates the secretion of endometrial proteins, promoting trophoblast growth, which are critical for implantation [8].
Using the candidate gene approach, many studies have evaluated gene and protein expressions in the porcine endometrium to elucidate the molecular mechanisms responsible for the establishment of pregnancy in pigs [3], [9], [10]. These efforts, along with global gene expression studies, have tremendously advanced our understanding of porcine pregnancy and endometrial biology. In recent years, while many global gene expression studies in the porcine endometrium have been conducted on differential gene expression during pregnancy [11], [12], [13], [14], there are very few reports regarding global changes in porcine endometrial proteome during pregnancy [6], [15]. Kayser et al. [6] reported changes in uterine fluid proteome during early porcine pregnancy and some proteins reported by them are common to this study. However, as uterine fluid does not contain high abundance cellular proteins, it is expected to have a less complex proteome than that of endometrial tissue. Given the complexity of the endometrium, the uterine fluid and endometrial proteome would not be expected to be quantitatively or qualitatively identical, though common proteins could be identified.
In our study, 2D-DIGE was used to construct a 2D map of the cycling and pregnant porcine endometrium to identify the proteins that are differentially expressed with the progression of the estrous cycle and pregnancy. We observed both estrous cycle and pregnancy-dependent changes and also identified a number of proteins that have not previously been identified in the porcine endometrium.
Section snippets
Animals
All procedures involving the use of animals were approved by the Animal Ethics Committee, University of Warmia and Mazury in Olsztyn, Poland, and were conducted in accordance with the national guidelines for agricultural animal care. Estrous induction and synchronization were achieved in 16 crossbred gilts (Sus scrofa domesticus) weighing ~ 100 kg by hormonal treatment with administration of an intramuscular injection of 750 IU of equine chorionic gonadotropin followed by 500 IU of human chorionic
DIGE-based profiling of endometrial proteins on Days 9 and 12 of the estrous cycle
A DIGE-based proteomic approach was used to identify proteins that were differentially expressed with the progression of the estrous cycle from Day 9 to Day 12. These factors might be responsible for endometrial preparation for the possible presence of an embryo in the uterus. Proteins from Day 9 and 12 endometrial lysates were separated using IEF. As the protein components of endometrial lysate were better resolved in the pH range of 4–7 as compared to 3–11, we used IPG strips in the pH ranges
Discussion
In mammals, the endometrium has a critical role in the development and survival of the conceptus. Consequently, there has been extensive study of the molecular processes associated with the progression of the estrous cycle, endometrial receptivity and embryonic-maternal cross-talk [17], [18], [19], [20], [21]. However, a qualitative proteomic study of the protein profile of the porcine endometrium defining endometrial function is lacking in pigs. In our current study, we have presented 1)
Conflict of interest
The authors have declared no conflict of interest.
Acknowledgment
This work was supported by a Project 2011/01/B/NZ4/03542 from the National Science Centre, funds appropriated to the Institute of Animal Reproduction and Food Research, Olsztyn, Poland.
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