Myostatin in the placentae of pregnancies complicated with gestational diabetes mellitus

Highlights

• Plasma concentrations of myostatin were evaluated in NGT and pre-symptomatic GDM women.

• Myostatin protein levels were evaluated in placentae of women with GDM and NGT pregnancies.

• No difference was seen in myostatin concentrations in plasma of NGT or pre-symptomatic GDM women.

• Myostatin protein expression is altered in placentae of women with GDM compared to NGT placentae.

• Placentae of diet and insulin treated women with GDM differentially expressed myostatin protein.

Abstract

Introduction

Gestational diabetes mellitus (GDM) is characterised by maternal glucose intolerance and insulin resistance during pregnancy. Myostatin, initially identified as a negative regulator of muscle development may also function in the regulation of placental development and glucose uptake. Myostatin expression in placentae of GDM complicated pregnancies is unknown. However, higher myostatin levels occur in placentae of pregnancies complicated with preeclampsia. We hypothesise that myostatin will be differentially expressed in GDM complicated pregnancies.

Methods

Myostatin concentrations (ELISA) were evaluated in plasma of presymptomatic women who later developed GDM and compared to plasma of normal glucose tolerant (NGT) women. Furthermore, myostatin protein expression (Western blot) was studied in placentae of pregnant women with GDM (treated with diet or insulin) compared to placentae of NGT women.

Results

No significant difference in myostatin concentration was seen in plasma of pre-symptomatic GDM women compared to NGT women. In placenta significant differences in myostatin protein expressions (higher precursor; p < 0.05and lower dimer: p < 0.005) were observed in GDM complicated compared to NGT pregnancies. Furthermore, placentae of GDM women treated with insulin compared to diet have higher dimer (p < 0.005) and lower precursor (p < 0.05). Compared to lean women, placentae of obese NGT women were lower in myostatin dimer expression (p < 0.05).

Discussion

Myostatin expression in placental tissue is altered under stress conditions (e.g. obesity and abnormal glucose metabolism) found in pregnancies complicated with GDM. We hypothesise that myostatin is active in these placentae and could affect glucose homoeostasis and/or cytokine production thereby altering the function of the placenta.

Introduction

Gestational diabetes mellitus (GDM) is characterised by maternal glucose intolerance leading to hyperglycaemia, β-cell dysfunction and insulin resistance [1]. GDM affects ∼5% of all pregnancies globally [1], [2], [3]. GDM is associated with an increased short and long term risk of adverse outcomes for mother and infant, including a higher risk of induced labour, Caesarean section delivery, preterm birth, hypertension, preeclampsia stillbirth, macrosomia and infant respiratory distress syndrome, developing metabolic disorders, diabetes and cardiovascular disease [1], [4], [5], [6], [7].

Myostatin is a member of the Transforming Growth Factor-Beta (TGF-β) super family and members of this family function in the development of the placenta [8], [9]. Comprehensive reviews are available on myostatin biosynthesis, signalling and function [8], [10], [11]. Myostatin is initially synthesised as a ∼52 kDa precursor protein. Two proteolytic cleavages of the precursor release a ∼42 kDa N-terminal pro-peptide and a ∼12 kDa mature myostatin protein. The active form of myostatin is a homodimer of the mature protein. Myostatin circulating in blood is found mainly in a latent complex [10], [12], [13], [14] composed of the active dimer bound non-covalently to two monomeric pro-peptides [10], [13], [15]. As a secreted protein in the human circulation [16], myostatin is thought to function in both an autocrine and paracrine manner [8], [17].

Myostatin is best known as a negative regulator of muscle development [8]. Increased muscle, decreased adipose tissues and an improved metabolic status were observed in myostatin knockout mice (suppression of hyperglycaemia) and in mice with a loss of function mutation(s) to the myostatin gene (less insulin resistance) [8], [18], [19]. Specific inhibition of myostatin in muscle has been identified to be most effective in improving glucose metabolism and insulin sensitivity [20].

Expression of myostatin in the human placenta has been identified to be negatively correlated with gestational age, as lower protein levels in term compared to preterm human placentae [21]. Myostatin is localised to cytotrophoblast, syncytiotrophoblast and extravillous trophoblast cells of first trimester and term human placentae [14], [22]. Following myostatin treatment alterations to glucose uptake of placental explants and BeWo cell (placental cell line) [21], [23], and increased proliferations of primary isolated extravillous trophoblast cells [22] have been observed.

In complicated pregnancies, myostatin is known to be higher (mRNA and protein) in placentae of pregnancies complicated with preeclampsia [24]. A study by Hu et al. of serum myostatin concentrations of women with overt GDM found no significant difference [25]. However, no reports are available on the expression of myostatin in placentae of GDM. We hypothesise that myostatin will be differentially expressed in GDM complicated pregnancies. The current observational study evaluated myostatin expression in plasma of pre-symptomatic women who later developed GDM, to ascertain whether a difference in myostatin concentration can be seen earlier in pregnancy. Moreover, myostatin protein expression was evaluated in placentae of women with GDM and/or obesity, as myostatin expression in placentae GDM complicated pregnancies is currently unknown.