INSL3 in the ruminant: a powerful indicator of gender- and genetic-specific feto-maternal dialogue

Abstract

The hormone Insulin-like peptide 3 (INSL3) is a major secretory product of the Leydig cells from both fetal and adult testes. Consequently, it is a major gender-specific circulating hormone in the male fetus, where it is responsible for the first phase of testicular descent, and in the adult male. In most female mammals, circulating levels are very low, corresponding to only a small production of INSL3 by the mature ovaries. Female ruminants are exceptional in exhibiting high INSL3 gene expression by the thecal cells of antral follicles and by the corpora lutea. We have developed a specific and sensitive immunoassay to measure ruminant INSL3 and show that, corresponding to the high ovarian gene expression, non-pregnant adult female sheep and cows have up to four times the levels observed in other female mammals. Significantly, this level declines during mid-pregnancy in cows carrying a female fetus, in which INSL3 is undetectable. However, in cows carrying a male fetus, circulating maternal INSL3 becomes elevated further, presumably due to the transplacental transfer of fetal INSL3 into the maternal circulation. Within male fetal blood, INSL3 is high in mid-pregnancy (day 153) corresponding to the first transabdominal phase of testicular descent, and shows a marked dependence on paternal genetics, with pure bred or hybrid male fetuses of Bos taurus (Angus) paternal genome having 30% higher INSL3 levels than those of Bos indicus (Brahman) paternity. Thus INSL3 provides the first example of a gender-specific fetal hormone with the potential to influence both placental and maternal physiology.

Figure 1. Circulating INSL3 (A) and progesterone (B) concentrations in gravid (filled bars; N = 6) and non-gravid (open bars; N = 4) sheep (means ± SD).

Statistical significance is shown only where P<0.05. (C) Correlation (r = 0.4; P<0.05) of INSL3 and progesterone values for samples collected on days 5 and 12 only, excluding day 17 (luteolysis).

Figure 2. Circulating maternal concentrations of INSL3 in pregnant Brahman and Angus cows (combined since no significant difference between breeds at any time-point).

Filled bars represent cows carrying a male fetus, open bars those carrying a female fetus. The cross-hatched bar (left) represents cows at the time-point of insemination. N values are indicated in parentheses below the bars. Statistical significance using both t test and Neumann-Keuls post hoc test are as indicated (ns, not significant).

Figure 3. INSL3 concentration in fetal venous blood, fetal weight, fetal testis weight, and calculated total fetal fluids on day 153 of pregnancy, and sorted by fetal genetics, as indicated.

A_maternal indicates an Angus (B. taurus) dam, B_paternal a Brahman (B. indicus) sire, etc.