Maternal recognition of pregnancy and implantation are not associated with an interferon response of the endometrium to the presence of the conceptus in dromedary camel

Abstract

Maternal recognition of pregnancy (MRP) and implantation involve appropriate interactions between the elongating conceptus and the receptive endometrium that will condition development of the feto-placental unit to term. Molecular mechanisms that take place at the conceptus-endometrium interface during early pregnancy have been extensively investigated in domestic ungulates but they are still poorly understood in camelids including the dromedary camel (Camelus dromedarius), a domestic species with important economic and social roles in arid and semi-arid areas. In order to better understand how MRP and implantation take place in the left horn of this species, we investigated expression levels of genes encoding steroid hormones (PGR, ESR1), transcription factors (STAT1, FOXL2), interferon stimulated genes (MX1, MX2, OAS1, RSAD2) including SOCS genes (SOCS1, SOCS2, SOCS3 and CISH), previously identified as conceptus regulated genes in the endometrium of other domestic animals. Using endometrial tissue collected from left and right uterine horns of dromedary camel females that were non pregnant or early pregnant, gene expression of these genes was detected and our results provided first insights on their regulation, showing that (i) conceptus implantation is not associated with an IFN response in the pregnant uterine horn (ii) when regulation of classical interferon-stimulated genes (ISG) occurs, it takes place during the formation of the feto-placental unit, and (iii) gene expression can differ between the left and right uterine horns during implantation and early placentation phase. Additional experiments will be required in dromedary camels to understand the unusual regulation of ISG during implantation as well as to determine the molecular processes that drive the systematic implantation of the elongating conceptus in the left uterine horn.

Introduction

Dromedary camel (Camelus dromedarius) is a multipurpose domestic animal that plays important economic and social roles in countries mostly located in arid and semi-arid areas. Beside their anatomical and physiological differences with other domestic species, camels are unique in their reproductive patterns. They are induced ovulators, with the corpus luteum (CL) displaying a short half-life (8–10 days) [1]. Luteolysis occurs with a rise of PGF₂α pulses between Day 8 and 10 post ovulation (po) [2]. After fertile mating, the camel embryo enters the uterus between Day 6 and Day 6.5 po at early hatched blastocyst stage. The hatched camel blastocyst remains spherical, grows rapidly, and then starts elongating at Day 10 po. Maternal recognition of pregnancy (MRP) takes place before Day 10 po [3] and coincides with the migration of the elongated embryo towards the left uterine horn that always hosts pregnancy [4]. Nevertheless, whether pregnancy in the left horn is associated with specific gene expression has yet to be determined in camelids.

In mammals, establishment then maintenance of pregnancy requires embryonic factors that will prevent luteolysis and will prepare the endometrium for implantation. In ruminants, the expression of numerous endometrial genes has been shown to be first induced by ovarian progesterone (P4) then stimulated by interferon tau (IFNT), prostaglandins and cortisol, these two latter being produced by the conceptus or by the endometrium [5]. In the porcine, MRP is concomitant with the rapid transformation of embryos from a spherical to a tubular shape, and then to filamentous conceptus between Days 10 and 12 po [6]. Also, in the porcine species, estrogens secreted by conceptuses at days 11 and 12 of pregnancy provide the initial signal for MRP and maintenance of a functional CL for continued production of progesterone [7]. In addition, progesterone receptors (PGR) down-regulation is initiated in the uterus by Day 8 and is completed by Day 12 in both cyclic and pregnant animals [8]. As in other mammalian species including ruminants, PGR down-regulation during early pregnancy enables attachment of the conceptus to the uterine endometrium and establishment of pregnancy [9]. In camelids, the embryo signal for MRP has been suggested to be oestradiol 17-beta -as described in pig- and not INFT as reported in cattle and sheep [10], [11]. During pregnancy, oestrogen concentration in peripheral blood increases up to 100 pg/mL [12]. In addition, aromatase, a key steroidogenic enzyme converting cholesterol to oestrogens was found in trophoblast cells of the camel conceptus between days 14 and 30 and is predominantly in trophoblast giant cells by day 30 after ovulation until term [13]. Powell et al. [14] recorded that embryo-produced estradiol may be involved in embryo migration and MRP in the llama and these events may be mediated by differential expression of estrogen receptors (ER) subtypes. Recently, around the time of implantation (Days 8–10 po), expression levels of ER and progesterone receptor (PR) and PTGS2 (previously known as COX-2) were reported to be similar between the two uterine horns of llamas whereas oxytocin receptors (OTR) expression was higher in the right uterine horn [15], [16]. Altogether current data in camelids point out the lack of knowledge of molecular mechanisms that are associated with the reaction of the endometrium to the presence of the embryo when pregnancy initiates. Moreover, the fact about whether systematic pregnancy in the left horn is associated with specific gene patterns needs to be documented. Therefore, the present study aims to investigate gene expression of a selection of endometrial genes in dromedary camels in order to determine (i) how they are regulated by the presence of the conceptus when pregnancy establishes, and (ii) if the development of conceptus in the left uterine horn is associated with a specific regulation of those endometrial genes.