MicroRNA indicators of follicular steroidogenesis.

MicroRNAs (miRNAs) can provide useful biomarkers of tissue function. The aim of the present study was to determine, in bovine follicles (n = 66; diameter 4-22 mm), the relationship among several indices of steroidogenesis and levels of 15 miRNAs previously identified to be associated with follicle development. Oestradiol levels, the oestradiol : progesterone (E : P) ratio and cytochrome P450 family 19 subfamily A member 1 (CYP19A1) expression were strongly correlated with each other (ρ > 0.8) and with LH/choriogonadotropin receptor (LHCGR) expression (ρ ≥ 0.6; P < 0.01). Levels of nine different miRNAs in the follicular wall were correlated (P < 0.01) with oestradiol, the E : P ratio and CYP19A1, with miR-873 showing the strongest correlation in each case (ρ > 0.7). Analyses of follicular fluid miRNAs identified miR-202 as correlated with oestradiol, the E : P ratio and CYP19A1 (ρ > 0.5; P < 0.01). When considering all follicle end-points together, we found that using a cut-off value of E : P = 1 overestimated the number of oestrogen-inactive follicles, whereas using CYP19A1 as a classifier provided a clearer separation of follicle samples based on oestrogen activity, in agreement with the E : P ratio, LHCGR expression and levels of miR-873 and miR-202. In conclusion, we identified miR-873 and miR-202 as miRNAs whose levels in follicular tissues can be used as indicators of steroidogenic capacity in bovine. We showed that these or other gene expression parameters, in addition or alternatively to the E : P ratio, should be used to accurately classify follicles based on steroidogenic capacity.