In this study, we extracted total exosomes and placenta-derived exosomes from the plasma of GDM pregnant women and healthy pregnant women in early and mid-gestation. Characteristic identification, database matching, and bioinformation analysis confirmed that exosomes were successfully extracted. On this basis, a comparative proteomics study was carried out to obtain DEPs of exosomes between cases and controls (Fig. 12).
At 12–16 weeks of gestation, the DEPs of T-EXO and PLAP-EXO were mainly involved in the complement and coagulation cascade, immune system process, inflammatory response, and platelet degranulation. These processes or pathways associated with mechanisms have been reported to be closely related to the pathogenesis of GDM [91–93] and diabetic complications [94]. The complement-related proteins include complement proteins or complement regulatory proteins or receptor proteins, and are involved in complement activation (classical and / or alternative pathways) and regulation. Most of them were up-regulated, including C4BPB, CLU, CFHR4, and CRP in total exosomes, and C3, C7, C9, and CHF in placenta-derived exosomes. Among them, C3 plays a central role in the activation of the complement system [95]. Elevated C3 levels have been reported to be associated with obesity, dyslipidemia, inflammation, insulin resistance and liver dysfunction [95] [96] as well as diabetes and GDM [97–99]. C7 levels has been observed to be elevated in the peripheral blood of GDM patients [100]. CFH is an alternative complement pathway inhibitor. Its levels are negatively correlated with insulin sensitivity [101] and may be elevated in GDM patients with insulin resistance [40]. CFHR4 (CFH-related proteins 4) is a complement in the form of CFH cofactor-enhancing activity that modulates activity. It binds to the central complement component C3b [102, 103]. Besides, the expression of CLU was decreased in total exosomes and increased in placenta-derived exosomes. Clustering precursor has been observed to be decreased in the serum of pregnant women with GDM [104]. CLU has various functions and is a complement inhibitor, it and vitronectin inhibit the C5b-8 complex insertion into membrane attack complex [105, 106]. Another complement-related protein, CD5L, is a key regulator of lipid synthesis and regulates inflammatory response, which has been observed to be decreased in the plasma of type 2 diabetes mellitus (T2DM) patients [107, 108]. Together, these results suggested that complement may play a critical role in the pathogenesis of GDM.
However, the involvement of the complement system appears to be complex. Up- or down-regulation of complement-related proteins, leading to over-activation or deficiency of complement, may lead to the development of disease. At the same time, complement regulatory proteins have a regulatory role in the complement activation process. For example, in this study, complement proteins C3, C7 and C9 were increased in placental exosomes, while two complement regulatory proteins that inhibit complement activation, CFH and CLU, were increased and down-regulated, respectively. In contrast, in our previous study, serum levels of C7, C9 and CFH were down-regulated in pregnant women who subsequently developed GDM [40] [39]. Therefore, further studies are needed to elucidate the function of complement proteins in pregnancy and GDM. The results imply that under-involvement of the complement system as well as over-involvement of the complement system leads to pathophysiology and that an appropriate balance is important [105].
GDM is not only a metabolic disease but is also a low-grade inflammatory response [109–111]. In the present study, at 12–16 weeks, five DEPs (CRP, LBP, ORM1, ORM2, SERPINC1) related to inflammatory response were found to be altered in T-EXO. SERPINC1 is common among T-EXO and PLAP-EXO. In particular, elevated levels of CRP was observed in the T-EXO. CRP is an inflammatory marker released by the liver in response to cytokine stimulation, are associated with increased risk of type Ⅱ diabetes, myocardial infarction, stroke, and peripheral vascular disease [112] [113]. Elevated maternal CRP during pregnancy is associated with pregnancy complications [114], including GDM [39, 111, 115, 116].
At 12–16 weeks, immune system processes were also significantly enriched. In the DEPs of T-EXO, they were associated with the innate immune system, whereas in PLAP-EXO DEPs, they are associated with innate and adaptive immune responses. Indeed, complement, immune, and inflammation are interrelated and contribute to the pathophysiological mechanism of GDM [70, 96, 117–119] and T2DM [120]. As a regulator of both the innate and the adaptive immune system, complement system represents an important part of this inflammatory response. In this study, some of the DEPs related to them were overlapped. For example, CRP, CLU and CD5l were associated with all three. LBP, an acute-phase glycoprotein associated immune and inflammatory response, was found to be up-regulated in the T-EXO. Likewise, one of the hallmarks of the inflammatory response is increased activation and recruitment of immune cells. In the DEPs of T-EXO, some DEPs were related to leukocyte migration and leukocyte mediated immunity, while in the DEPs of PLAP-EXO, some were associated with neutrophil degranulation, leukocyte mediated immunity, and myeloid leukocyte activation. Interestingly, significant neutrophil infiltration has been observed in the placenta of GDM patients [121, 122]. Therefore, our results support that impaired immune and inflammatory homeostasis may contribute to GDM, which can be achieved by affecting insulin resistance (IR) and ß-cell function [123].
Compared to normal pregnancy, the hypercoagulable state of GDM is further enhanced [40, 124–126]. Here, several DEPs were associated with coagulation. Of which, SERPINC1 was found to be increased in the T-EXO, and FGA, FGB, FGG, and SERPIND1 were up-regulated in PLAP-EXO in early pregnancy. Interestingly, FGA, FGB, and FGG were significantly up-regulated in the serum of pregnant women who subsequently developed GDM or with GDM in our previous studies [39, 40]. Another study observed that fibrinogen concentrations were significantly elevated in the pregnant women with GDM [127]. The hypercoagulable state may be primarily caused by hyperglycemia. GDM may lead to blood coagulation due to increased platelet activation, increased coagulation factor synthesis (including fibrinogen), and decreased fibrinolytic activist [127]. On the other hand, some proteins that inhibit blood coagulation have also changed, such as SERPINC1, SERPING1, and PROS1. SERPING1 is a plasma proteinase C1 inhibitor. SERPINC1 is the most important serine protease inhibitor and regulates the coagulation cascade and inhibits thrombin activity. They have been reported to be associated with diabetes [128, 129]. PROS1 is an anticoagulant plasma protein. SERPINC1 and PROS1 was up-regulated in the T-EXO and PLAP-EXO, while SERPIND1 was up-regulated in the T-EXO of women who subsequently developed GDM. Similar to our previous observation [39], it may be a feedback inhibition reaction to the complement and coagulation cascade, and play a role in regulating the complement and cascade process.
Studies have shown that patients with T2DM have abnormal platelet activation [130] and their coagulation system is in a hypercoagulable state [131]. Increased fibrinogen leads to platelet aggregation, a step-in platelet activation [132, 133]. We found that some DEPs at 12–16 weeks in the T-EXO and PLAP-EXO were associated with platelet activation/degranulation. Platelet degranulation is related to its activation state, usually an increase in mean platelet volume (MPV), indicating that it is more active. Therefore, higher MPV indicates higher prothrombotic status [134]. Interestingly, an increase in MPV has been related to diabetes and GDM [135–139].
At 24–28 weeks, the DEPs were also mainly enriched in extracellular exosome, and related to complement, immunity and inflammation. The most DEPs associated with complement, immunity and inflammation were immunoglobulins, and most of them were down-regulated in the T-EXO and PLAP-EXO. This similar to T2DM [108], the role of immunity in GDM is bimodal: immunity may contribute to the pathology of T2DM, by contrast, the suppression of immunity is one of the major consequences of T2DM. Besides, several other DEPs are also of interest. CD14 was observed to be increased in the T-EXO. It is a monocyte and macrophage marker, has been shown to be increased in placental and omental adipose tissue during inflammation [140]. GDM pregnancies had an increased percentage of circulating CD14 + cells and higher levels of soluble CD14+ (sCD14+) in serum [141]. Moreover, CFH was up-regulated in the T-EXO of pregnant with GDM, CD5L was down-regulated and C8B was up-regulated in the PLAP-EXO of pregnant with GDM.
Of note, at 24–28 weeks, the DEPs related to metabolism and GDM and/or diabetes were enriched, including adiponectin, afamin, CETP, IGFALS, RYR2, and TTR in the T-EXO, and apolipoprotein, PKM, and SHBG in the PLAP-EXO. Adiponectin is an important adipokine that controls fat metabolism and insulin sensitivity [142, 143], and is associated with GDM [144], even extending beyond pregnancy [142]. Similar to the previous study [145], it was decreased in the T-EXO of patients with GDM in this study. Afamin belongs to the albumin family and acts to bind and transport vitamin E and may be involved in oxidative stress and anti-apoptosis. Serum levels of afamin were significantly elevated in GDM patients before and during pregnancy [146]. IGFALS, an acid-labile subunit of insulin-like growth factor that binds to insulin-like growth factor-binding proteins, thereby controlling insulin-like growth factors essential for placental development and growth bioavailability [147]. It was found to be increased in the serum of pregnant with GDM in our previous study [39]. TTR is a thyroid hormone-binding protein that, in addition to transporting thyroxine, plays a role in glucose and lipid metabolism, and insulin resistance [148]. The serum TTR concentration in GDM women is significantly higher than that in non-GDM women [149–151]. Similarly, in this study, afamin and IGFALS, and TTR were up-regulated in the T-EXO. The insulin secretion-related protein RYR2 play a key role in regulating insulin secretion and glucose homeostasis [152]. Here, it was down-regulated in the T-EXO.
In addition, impaired lipid transport and homeostasis are associated with GDM [40]. Lipid metabolism of the placenta is also disturbed in GDM [153]. Our results showed that cholesteryl ester transfer protein (CETP) was down-regulated in the T-EXO, while apolipoprotein was up-regulated in the PLAP-EXO of pregnant women with GDM at 24–28 weeks. In the PLAP-EXO, two other proteins also deserve attention, i.e., up-regulated PKM and down-regulated SHBG. PKM is an important enzyme of glycolysis and any alteration in the enzyme activity will severely affect the glucose utilization [154], which controls signal strength in the insulin secretory pathway [155]. Its expression is increased in the preeclampsia placenta at delivery [156]. SHBG plays an important role in regulating and transferring sex hormones. Its production is controlled by insulin and inversely related to insulin resistance [157]. The studies have shown that the level of SHBG is significantly lower in GDM pregnant women than that in healthy women [157].
We also noticed changes in the expression of several pregnancy-related proteins, including PZP, PSG2, PSG5, PSG3, and PSG9. Altered levels of PZP and PSG have been reported in the blood of patients with GDM [49, 54, 158, 159]. PSGs are involved in immune regulation [160]. Our results suggest that they may play an important role in maintaining healthy pregnancy.
Collectively, to the best of our knowledge, this study is the first to apply a proteomic approach to study plasma and placental exosomal proteins in patients with GDM in early and mid-pregnancy. The results suggest that the mechanisms involved in DEPs of T-EXO and PLAP-EXO are not exactly the same in pregnant women with GDM in early and mid-gestation, which summarized in Fig. 12. The protein dysregulation found in early gestation suggests that they are involved in the pathogenesis of GDM rather than being the result of subsequent metabolic changes [153]. GDM is a progressive process and mechanisms that emerge early promote disease progression, which is consistent with previous studies [161]. In addition, the mechanisms associated with T-EXO and PLAP-EXO suggest that maternal factors contribute to disease onset or affect the placenta. Maternal metabolic and immune status may alter early placental metabolism and function. The inflammatory environment regulates maternal glucose metabolism, and maternal inflammation is associated with a high susceptibility to GDM [2]. On the other hand, in agreement with previous studies [39], the placenta plays a key role in the pathophysiology of GDM. PLAP-EXO may lead to a pro-inflammatory state associated with GDM [40], while the placenta itself is in an inflammatory state. Moreover, as the mechanisms involved in these DEPs are consistent with the disease mechanisms of GDM and most of them are blood or exosomal proteins reported in previous studies to be associated with GDM, they have the potential to be used as predictive or diagnostic markers, especially hub protein. However, further large sample size studies and validation are still needed. Furthermore, the number of DEPs identified in the two periods was also different, and more were identified in the early stage, suggesting that the search for biomarkers from an exosomal perspective is not only feasible, but also early prediction is significant for GDM prevention and intervention.