Evidence of endometrial amino acid metabolism and transport modulation by peri-ovulatory endocrine profiles driving uterine receptivity

Abstract

Background: In beef cattle, changes in the periovulatory endocrine milieu are associated with fertility and conceptus growth. A large preovulatory follicle (POF) and the resulting elevated concentrations of progesterone (P4) during diestrus positively affect pregnancy rates. Amino acids (AA) are important components of maternally derived secretions that are crucial for embryonic survival before implantation. The hypothesis is that the size of the POF and the concentration of P4 in early diestrus modulate the endometrial abundance of SLC transcripts related to AA transport and metabolism and subsequently impact luminal concentrations of AA. The follicle growth of Nelore cows was manipulated to produce two experimental groups: large POF and CL (LF-LCL group) and small POF and CL (SF-SCL group). On Day 4 (D4; Experiment 1) and Day 7 (D7; Experiment 2) after GnRH-induced ovulation (GnRH treatment = D0), the animals were slaughtered and uterine tissues and uterine washings were collected. qRT-PCR was used to evaluate the expression levels of AA transporters in D4 and D7 endometrial tissues. The concentrations of AA were quantified in D4 and D7 uterine washings by HPLC.

Results: Transcript results show that, on D4, SLC6A6, SLC7A4, SLC17A5, SLC38A1, SLC38A7 and SCLY and on D7 SLC1A4, SLC6A1, SLC6A14, SLC7A4, SLC7A7, SLC7A8, SLC17A5, SLC38A1, SLC38A7, SLC43A2 and DDO were more abundant in the endometria of cows from the LF-LCL group (P < 0.05). In addition, concentrations of AA in the uterine lumen were influenced by the endocrine profiles of the mother. In this context, D4 uterine washings revealed that greater concentrations of taurine, alanine and α-aminobutyric acid were present in SF-SCL (P < 0.05). In contrast, lower concentrations of valine and cystathionine were quantified on D7 uterine washings from SF-SCL cows (P < 0.05).

Conclusion: The present study revealed an association between the abundance of transcripts related to AA transport and metabolism in the endometrium and specific periovulatory endocrine profiles related to the receptive status of the mother. Such insights suggest that AAs are involved in uterine function to support embryo development.

Keywords: Amino acids; Beef cattle; Sex steroids; Uterus.

Fig. 1

Experimental model and hormonal treatments. Growth of the pre-ovulatory follicle (POF) of beef cows was programmed to generate two groups of cows, the large follicle-large CL group (LF-LCL; associated with greater receptivity to the embryo and greater fertility) and small follicle-small CL group (SF-SCL). To decrease exposure to P4 and thereby stimulate growth of the POF, animals from LF-LCL group received an injection of PGF at the moment of intravaginal P4-releasing device insertion vs. no injections in the animals from SF-SCL group. Also, removal of the P4-releasing device was 12 h earlier in the LF-LCL group. Follicle size, ovulation and CL size were accessed by ultrasound scanning of the ovaries. Blood samples were collected for P4 assay. Ovulation was induced by GnRH on D0. On D4 (Experiment 1) and D7 (Experiment 2) animal were slaughtered for samples collection. BS, blood sampling; GnRH, 1 μg of busereline acetate im; P4, P4 progesterone-releasing device containing 1 g of P4; +PGF, cows received 0.5 mg of sodium cloprostenol on D-10; -PGF, cows did not receive Cloprostenol on D-10; EB, 2 mg of estradiol benzoate; Slaughter, endpoint for endometrial tissue and uterine washings collection (Adapted from Reference 26)

Fig. 2

Individual and mean concentrations of alanine in uterine washings from D4 and D7 of diestrus. Each gray dot indicates an individual animal. Blue dots indicate mean ± sem. LF-LCL indicates Large Follicle-Large CL group and SF-SCL indicates Small Follicle-Small CL group

Fig. 3

Individual and mean concentrations of taurine in uterine washings from D4 and D7 of diestrus. Each gray dot indicates an individual animal. Blue dots indicate mean ± sem. LF-LCL indicates Large Follicle-Large CL group and SF-SCL indicates Small Follicle-Small CL group

Fig. 4

Individual and mean concentrations of α-aminobutyric acid in uterine washings from D4 and D7 of diestrus. Each gray dot indicates an individual animal. Blue dots indicate mean ± sem. LF-LCL indicates Large Follicle-Large CL group and SF-SCL indicates Small Follicle-Small CL group

Fig. 5

Individual and mean concentrations of valine in uterine washings from D4 and D7 of diestrus. Each gray dot indicates an individual animal. Blue dots indicate mean ± sem. LF-LCL indicates Large Follicle-Large CL group and SF-SCL indicates Small Follicle-Small CL group

Fig. 6

Individual and mean concentrations of cystathionine in uterine washings from D4 and D7 of diestrus. Each gray dot indicates an individual animal. Blue dots indicate mean ± sem. LF-LCL indicates Large Follicle-Large CL group and SF-SCL indicates Small Follicle-Small CL group

Fig. 7

Amino acid transport in the uterus of cows on D4 and D7 of diestrus. This figure shows the comparative abundance of transcripts related to amino acid (AA) transport and metabolism and the luminal concentration of AA between more receptive endometrium (Large Follicle-Large Corpus Luteum group) and less receptive endometrium (Small Follicle-Small Corpus Luteum group) on D4 and D7 after estrus. On D4, the transport of AA seems to occur preferentially from the uterine lumen towards endometrial cells, because despite elevated expression of genes related to AA transporters in endometrium there is lower availability of AA in uterine washings. Such direction of transport benefit events such as cell proliferation, which requires AA. On D7, AA availability in uterine lumen and abundance of genes related to AA transport are both stimulated in the more receptive endometrium. This phenotype is consistent with a greater provision of substrates to support embryonic needs for growth. ↑, up-regulated in LF-LCL group in comparison to SF-SCL group; ↓ down-regulated in LF-LCL group in comparison to SF-SCL group; γ, carriers related to transport of AAs similarly abundant in the lumen of both groups. solid lines connect a transporter with its cognate substrate(s); *, P ≤ 0.05; #, P < 0.1; SLC, Solute carrier protein; SCLY, Selenocysteine Lyase; DDO, D-aspartate Oxidase