Signature precursor and mature microRNAs in cervical ripening during gestational diabetes mellitus lead to pre-term labor and other impediments in future

Abstract

Gestational diabetes mellitus (GDM) is a pathological condition in which the placenta releases a hormone called human placental lactogen that prevents maternal insulin uptake. GDM is characterised by varying degrees of carbohydrate intolerance and is first identified during pregnancy. Around 5-17% of pregnancies are GDM pregnancies. Older or obese women have a higher risk of developing GDM during gestation. Hyperglycemia is a classic manifestation of GDM and leads to alterations in eNOS and iNOS expression and subsequently causes ROS and RNS overproduction. ROS and RNS play an important role in maintaining normal physiology, when present in low concentrations. Increased concentrations of ROS is harmful and can cause cellular and tissue damage. Oxidative stress is defined as an imbalance between pro-oxidant and antioxidant molecules that manifests due to hyperglycemia. miRNAs are short, non-coding RNAs that play a critical role in regulating gene expression. Studies have shown that the placenta expresses more than 500 miRNAs, which play a crucial role in trophoblast division, movement, and apoptosis. Latest research has revealed that hyperglycemic conditions and increased oxidative stress, characteristic of GDM, can lead to the dysregulation of miRNAs. The placenta also releases miRNAs into the maternal circulation. The secreted miRNAs are encapsulated in exosomes or vesicles. These exosomes interact with tissues and organs at distant sites, releasing their cargo intracellularly. This crosstalk between hyperglycemia, ROS and miRNA expression in GDM has detrimental effects on both foetal and maternal health. One of the complications of GDM is preterm labour. GDM induced iNOS expression has been implicated in cervical ripening, which in turn causes preterm birth. This article focuses on the speculations of oxidative and nitrative stress markers that lead to detrimental effects in GDM. We have also envisaged the role of non-coding miRNA interactions in regulating gene expression for oxidative damage.

Graphical abstract: Holistic view of miRNA in GDM. I)(A) Placenta as a metabolic organ that provides the foetus with nutrients, oxygen and hormones to maintain pregnancy. Human placental lactogen (hPL) is one such hormone that is released into maternal circulation. hPL is known to induce insulin resistance. (B) ß-cell dysfunction leads to reduced glucose sensing and insulin production. Insulin resistance, a characteristic of GDM, exacerbates insulin ß cell dysfunction leading to maternal hyperglycemia. Hyperglycemia leads to increased ROS and RNS production through several mechanisms. Consequently, GDM is characterised by increased oxidative and nitrative stress.II)Exposure to maternal hyperglycemia causes increased ROS and RNS production in trophoblast cells. Oxidative stress caused by hyperglycemia may lead to eNOS uncoupling, causing eNOS to behave as a superoxide producing enzyme. iNOS expression in trophoblast cells leads to increased NO production. iNOS-derived NO reacts with ROS to produce RNS, thereby increasing nitrosative stress. Expression of antioxidant defences are reduced. Hyperglycemia and oxidative stress may alter the expression of some miRNAs. Some miRNAs are upregulated while others are downregulated. Some miRNAs are secreted into maternal circulation in the form of exosomes. Oxidative stress markers, nitrative stress markers and circulating miRNAs are found to be increased in maternal circulation.

Keywords: Gestational diabetes Mellitus; Preterm labour; ROS; iNOS; miRNA.

Fig. 1

Precursor and Mature miRNA biogenesis miRNAs are encoded by the introns of a gene. These genes are transcribed into primary RNA (pri-miRNA) by RNA polymerase II/III. Pri-miRNAs are further processed into precursor miRNA (pre-miRNA) and are transported to the cytoplasm of the cell where they are further processed to form the miRNA duplex. The AGO protein binds to one of the strands of the miRNA duplex, referred to as the guide strand. The strand complementary to the guide strand undergoes degradation. AGO protein and the guide strand together form the RNA-inducing silencing (RISC) complex. The mature miRNA involved in mRNA degradation, translational repression and mRNA deadenylation. Some miRNAs are packed into exosomes and secreted into maternal circulation in the form of circulating miRNAs. The expression of miRNAs may be upregulated or down regulated by placental pathologies such as GDM. Circulating miRNAs may be used as potential diagnostic biomarkers for screening of GDM.

Fig. 2

Oxidative and Nitrosative implications during GDM There are a variety of mechanisms by which hyperglycemia induces ROS and RNS production in trophoblast cells A) iNOS expression is increased in trophoblasts, resulting in aberrant NO production B) eNOS expression is upregulated by H 2 O 2 derived from NOX-derived superoxide. eNOS- derived NO reacts with NOX-derived superoxide resulting in the formation of peroxynitrite which causes BH 4 oxidation. Decrease in BH 4 causes eNOS uncoupling, causing eNOS to function as a superoxide producing enzyme C) The PKC pathway is activated by maternal hyperglycemia. This in turn activates the NOX enzyme. NOX catalyses the conversion of molecular oxygen into superoxide in an NADPH-dependent manner D) Xanthine oxidase catalyses the conversion of molecular oxygen to superoxide, alongside the conversion of hypoxanthine to xanthine E) Mitochondrial ROS is yet another source of ROS in the trophoblast cells. It is produced by incorrectly coupled electron transport during oxidative phosphorylation Superoxide and hydrogen peroxide contribute to increased oxidative stress while peroxynitrite and NO causes an increase in nitrosative stress in the placental trophoblast cells

Fig. 3

Pro-Inflammatory mediators during GDM GDM is associated with a pro-inflammatory environment. Cytokines IL-6 and TNF-alpha induce the production of iNOS. Additionally, trophoblasts exposed to hyperglycemia contribute to an increase in iNOS-derived NO as well as ROS. NO induces apoptosis and causes softening of the cervix. NO is also known to induce the expression of MMP-1 in human cervical fibroblasts. MMP-1 degrades collagen that aids in cervical ripening. ROS produced by trophoblast cells Cervical ripening in GDM leads to preterm labour