Maternal Insulin-like Growth Factor-II Promotes Placental Functional Development Via the Type 2 IGF Receptor in the Guinea Pig

Abstract

In guinea pigs, maternal insulin-like growth factor (IGF) infusion in early-pregnancy enhances placental transport near-term, increasing fetal growth and survival. The effects of IGF-II, but not IGF-I, appear due to enhanced placental labyrinthine (exchange) development. To determine if the type-2 IGF receptor (IGF2R) mediates these distinct actions of exogenous IGF-II in the mother, we compared the impact of IGF-II with an IGF-II analogue, Leu²⁷-IGF-II, which only binds the IGF2R. IGF-II, Leu²⁷-IGF-II (1 mg/kg per day.sc) or vehicle were infused from days 20–38 of pregnancy (term = 67 days) and placental structure and uptake and transfer of [³H]-methyl-d-glucose (MG) and [¹⁴C]-amino-isobutyric acid (AIB) and fetal growth and plasma metabolites, were measured on day 62. Both IGF-II and Leu²⁷-IGF-II increased the volume of placental labyrinth, trophoblast and maternal blood space within the labyrinth and total surface area of trophoblast for exchange, compared to vehicle. Leu²⁷-IGF-II also reduced the barrier to diffusion (trophoblast thickness) compared to vehicle and IGF-II. Both IGF-II and Leu²⁷-IGF-II increased fetal plasma amino acid concentrations and placental transfer of MG to the fetus compared to vehicle, with Leu²⁷-IGF-II also increasing AIB transport compared with vehicle and IGF-II. In addition, Leu²⁷-IGF-II increased fetal weight compared to vehicle. In conclusion, maternal treatment with IGF-II or Leu²⁷-IGF-II in early gestation, induce similar placental and fetal outcomes near term. This suggests that maternal IGF-II in early gestation acts in part via the IGF2R to persistently enhance placental functional development and nutrient delivery and promote fetal growth.

Introduction

Fetal growth is dependent on the placental supply of nutrients. This is determined by the mother's ability to acquire nutrients and the capacity of the placenta to transfer these to the fetus. Impaired placental development leads to failure of the materno-placental supply and results in intrauterine growth restriction (IUGR) [1] and other major pregnancy complications, such as miscarriage [2], preeclampsia [1] and preterm birth [3] which are significant causes of maternal and fetal mortality and morbidity. In addition, IUGR is associated with cardiovascular disease, obesity and diabetes in adult offspring [4]. Understanding the molecular regulation of placental development may identify potential causes of, and possible therapeutic targets for, impaired placentation and these adverse consequences.

The insulin-like growth factors (IGFs) are anabolic polypeptides which appear to have a major role in placentation. Both IGF-I and IGF-II are major determinants of fetal growth [5], [6], [7] and are highly expressed by the mother and fetus in most species [8]. IGF-II is also abundantly produced by the placenta [9]. Maternal plasma IGF-I and IGF-II concentrations increase during early pregnancy in several species (as reviewed by ref. [10]) including humans [11]. These may play an endocrine role in placentation as both reduced plasma IGF-I [12] and increased circulating concentrations of inactive pro-IGF-II peptide [13] are associated with placental dysfunction and fetal growth restriction in women. Furthermore, reductions in maternal plasma IGFs in guinea pigs exposed to food restriction correlate with markers of delayed structural and functional maturation of the placenta and reduced fetal growth [14], [15], [16].

Furthermore, we have also shown that increasing maternal systemic concentrations of IGF in the guinea pig, by infusing IGF-I or IGF-II during early to mid pregnancy, enhances fetal growth and survival near term [17]. Early pregnancy infusion of either IGF also increased placental glucose transport and the concentration of fetal circulating amino acids [17], [18]. Despite similar outcomes for fetal growth and survival and placental function, each IGF exerted distinct effects on placental growth and maternal body composition, suggesting that maternal IGFs may play complementary roles in placentation and pregnancy to ensure fetal growth and survival.

In the guinea pig, maternal IGF-I treatment in early to mid pregnancy did not alter placental development but maternal adipose stores were reduced in late pregnancy, which may reflect increased diversion of substrates to the conceptus [17]. In contrast, maternal IGF-II treatment did not affect maternal body composition but enhanced development of the placental exchange region (labyrinth) into late gestation [17]. This altered placental development correlated with improved placental transport capacity [18] and suggests that IGF-II effects on fetal growth were likely to be mediated via an impact on placental development.

IGF-II can alter trophoblast phenotype and function in vitro and such actions may underlie these reported consequences of exogenous IGF-II administration in the pregnant guinea pig. IGF-II promotes murine trophoblast differentiation by transforming ectoplacental cone cells (trophoblast stem cells) into trophoblast giant cells in vitro [19]. IGF-II also stimulates human placental trophoblast proliferation [20], migration [21], invasion [22], matrix metalloproteinase synthesis and activity [20] and nutrient uptake [23] in vitro.

The distinct effects of exogenous IGF-I and IGF-II during pregnancy in the guinea pig may arise from different interactions of the IGFs with various receptors. All three receptors, insulin receptor (InsR) [24] and the type 1 and 2 IGF receptors (IGF1R and IGF2R, respectively) [17], are expressed by the guinea pig placenta and are ubiquitously expressed by maternal tissues. IGF-I has negligible affinity for IGF2R and InsR [25], [26], thus its actions are likely to be mediated by the IGF1R. IGF-II, on the other hand, can bind all three receptors, making it difficult to decipher which one is responsible for mediating the effects of exogenous maternal IGF-II. It is possible that IGF-II actions on the placenta may be mediated by the InsR which has been implicated in mediating IGF-II actions on fetal growth in mice [27] or by the IGF2R, which it binds with much greater affinity than the IGF1R [25]. Although, IGF2R initially appeared to function only as a clearance receptor for IGF-II, as Igf2r gene ablation in mice leads to increased serum and tissue levels of IGF-II [28], more recent lines of evidence suggest that IGF-II can signal via this receptor and affect a range of cellular functions such as differentiation, migration and apoptosis in a variety of cell types [29], [30], [31], [32], [33], [34], [35], [36]. Furthermore, IGF-II acting via IGF2R promotes human placental trophoblast migration in vitro [21].

This investigation aimed to determine if the IGF2R mediates the actions of exogenous IGF-II in the mother by comparing the impact of IGF-II with that of an IGF-II analogue, Leu²⁷-IGF-II, which only binds the IGF2R. Leu²⁷-IGF-II has a significantly reduced binding capacity for the IGF1R and InsR compared to IGF-II (80 and 220-fold lower affinity, respectively), while its affinity for IGF2R is unaltered [37] with only slightly reduced affinity for IGF binding proteins [38].