Periodontitis & preeclampsia: were outer membrane vesicles a potential connection?

Abstract Introduction Preeclampsia (PE) is gestation-specific hypertension coupled by systemic multiple organ damages, remaining the leading causes of maternal and infant death worldwide. Materials and methods Though numerous pathogenetic mechanisms have been engaged in this disorder and several methods have been undertaken to treat PE, few clinical strategies are effective in PE management, suggesting more studies from novel perspective being in great need to decipher the underlying mechanisms of PE. Results Growing evidence shows that women with periodontitis, an oral microflora-induced chronic inflammation of the periodontal tissues, are more inclined to suffer PE, which may be tightly associated with microflora-derived outer membrane vesicles (OMVs). Latest studies reveal that OMVs are spherical membrane-enclosed entities released by bacteria and can gain free access to the circulation of the host and therefore reach the remote tissue of the host, participating the interaction among oral bacterial with the host and contribute to some systemic disease with carried bioactive materials. Conclusions OMVs may be the underlying mechanism linking oral flora-induced periodontitis with dysfunction trophoblast and finally contributes to the pathogenesis of in PE. Here we provide evidence to support the potential roles of OMVs linking periodontal disease between PE.


Background
Preeclampsia (PE) is characterized by de novo hypertension after 20 weeks of gestation and systemic multiple organ damages, which is one of the leading causes of maternal death and the delivery of placental tissue remains the only definitive therapy [1]. Although the related therapeutic measures can improve the acute symptoms of the patient, they cannot fundamentally reverse the progression of the disease and ultimately prevent the occurrence of PE, suggesting that there may be other unknown factors involved in the occurrence and development of PE.
Periodontitis is a chronic inflammation of the periodontal tissues such as gums, periodontal ligament, alveolar bone, or cementum, with clinical manifestations of red and swollen gums, periodontal pocket pus discharge and tooth loosening, which is the main cause of tooth loss in adults [2]. Despite that oral microflora, the second largest microbial community, remains a steady-state balance in normal cases, periodontitis is mainly induced by the changes in the characteristics of oral microbial flora including Actinomyces actinomycetemcomitans (Aa), Porphyromonas gingivalis (Pg), Fusobacterium nucleatum, Prevotella intermedia and Treponema denticulatum. A study conducted by Duke University showed that pregnant women with periodontal disease had significantly higher probability of PE during delivery than their counterparts with good oral condition, and that periodontal disease progressed easily during pregnancy [3]. However, it is not clear how the remote oral flora engages in the occurrence and development of PE.
Latest research reported that all life-forms secrete cellular components across their plasma membrane to interact with their outside environments and one way for microbial flora accomplishes this is by secreting outer membrane vesicles (OMVs) [4]. A variety of bioactive substances such as lipids, protein, and nucleic acids are carried and transported by OMVs, which could penetrate the blood-brain barrier and the blood-fetal barrier [5]. With the increasing understanding of OMVs, researchers find that OMVs emerge as distinct cellular entities and help bacteria adapt to diverse niches, compete to survive, and more surprisingly play a significant role in the interaction between host and pathogen and are associated with a range of diseases including myocardial infarction, Alzheimer's disease, diabetes, and other systemic diseases [6]. In this article, we will provide evidence to support the potential roles of OMVs linking periodontal disease between PE.

Enigmatic pathogenesis of PE
Though major advances have been achieved in various medical puzzles in numerous fields, PE is still the tough problem in obstetrics and gynecology and remains the leading cause of maternal and infant death, with placenta delivery being the only definitive therapy. Nowadays, most researchers hold the view that there were two subtypes of PE: early and late onset forms and that they arise owing to quite different pathogenesis. Early onset PE may center around compromised placentation, whist the late form is associated with the imbalance between uterine perfusion and progressive fetoplacental demands, coupled with a maternal genetic predisposition to cardiovascular and metabolic disease [7,8]. In addition, placental ischemia and hypoxia, oxidative stress, endoplasmic reticulum stress, changes in cytokines and complement levels, autoantibody production, and neurohumoral imbalance also play important roles in the pathogenesis of PE [9]. A large number of scholars from different entry points to use aspirin [10], magnesium sulfate [11], vitamin D [12], acetylcholinesterase inhibitor [13], 17-alpha-hydroxyprogesterone caproate [14][15][16] and Mito Q [17] for the treatment of PE. However, these methods could not solve PE, suggesting other unknown factors engaged in the pathogenesis of PE. More studies from novel perspective are in great need to decipher the underlying mechanisms of oral flora imbalance in the pathogenesis of PE.

Inclination of perinatal periodontitis patients to PE
The oral microflora is the second largest microbial community in the human body after the intestinal flora, and it maintains a steady-state balance with the body under normal conditions [2]. However, in some cases, the imbalance of oral flora composition can cause chronic inflammation of periodontal tissues (periodontitis), which includes inflammation of gums, periodontal ligament, alveolar bone, and cementum [18]. Its risk factors include female, environmental factors, smoking and HLA-DRB1 SE phenotype [3]. In recent years, with the increasing emphasis on oral health, researchers have gradually found that the imbalance of oral flora not only affects the local oral cavity, but is also related to myocardial infarction, Alzheimer's disease, diabetes, and other systemic diseases [6].
Subsequent studies revealed a significant association between periodontal disease and premature birth, low birth weight, abortion, and PE in women of childbearing age. Some researchers hold the view that periodontitis and PE may be presented as symptoms of an underlying pathophysiologic mechanism, or maybe even a reverse mechanism: the maternal gestational (immunologic) changes as potential inductor of periodontitis. Other scholars believe that inflammatory factors such as TNF-a and IL-6, released by local periodontal inflammation, may be the key to the correlation between periodontitis and PE. To test this hypothesis, Professor Politano GT's team included 58 parturient with PE and 58 healthy controls in the study and showed that there was a correlation between periodontitis and PE (OR 3.73; 95%CI 1.32-10.58), but the levels of circulating TNF-a and IL-6 levels in PE patients had no significant correlation with the occurrence of periodontitis [19]. Besides, clinical observation studies have shown that approximately 63.8% of patients with PE have periodontal disease, compared with 36.6% of healthy controls in the same period and that the occurrence of periodontal disease is significantly related to the occurrence of PE (p < 0.001) [20,21]. The underlying mechanisms linking periodontitis and beforementioned systemic disease as well as PE remain poor understood.

Microflora-derived OMVs and its detrimental effects in diseases
In 1965, Bishop and his collaborators first observed that OMV was released by auxotrophic Escherichia coli strain under a limited lysine condition [22]. Later, Knox et al. employed electron microscopy to identified that these secreted cellular components were spherical, nano-sized entities and from the outer membrane of gram-negative bacteria and of endocytic origin [23]. OMVs are rich in toxins, bioactive proteins, and virulence factors and are engaged in bacteria-bacteria and even bacteria-host interactions [24]. Remote tissues and cells can ingest circulating OMVs and lead to phenotypic and functional changes [25][26][27][28][29][30].
Ozaki's team found that OMVs released by Pg were equipped with Pg-derived proteases gingipains and these OMVs could transferred to mice liver, which inhibited the insulin-induced Akt-GSK-3b signaling cascade and contributed to the development of diabetes mellitus [31]. With the expanding knowledge of OMVs, researchers find that OMVs could penetrate the bloodbrain barrier and the blood-fetal barrier [5]. Nonpathogenic E. coli-derived OMVs contained LPS and was able to induce systemic inflammation and even bacterial sepsis in mice [32]. Han et al. reported that oral Aggregatibacter actinomycetemcomitansderived OMVs crossed the blood-brain barrier in mice and promoted the production of TNF-a in macrophages, linking periodontal disease to neuroinflammatory disease [33]. Jose et al. found that enterotoxigenic Escherichia coli-derived OMVs could pass the placenta barrier and changed the pregnancy outcomes [34]. With the increasing understanding of OMVs, researchers are aware that OMVs emerge as distinct cellular entities and help bacteria adapt to diverse niches, compete to survive, and more surprisingly play a significant role in the interaction between host and pathogen and are associated with a range of diseases including myocardial infarction, Alzheimer's disease, diabetes, and other systemic diseases [6].

Is placental trophoblast a potential target of OMVs?
Trophoblast is the basic functional cell of placenta, which plays a vital role in embryonic development and pregnancy maintenance. The abnormal function of trophoblast is related to the abnormal placentation in the early stage of gestation and is thought to trigger PE. Compromised trophoblast cells may release a large number of toxic substances such as inflammatory cytokines, anti-angiogenic factors, and free fetal DNA into the maternal circulation and induce extensive endothelial damage, the typical hallmark of PE [7]. Moreover, most of the symptoms in severe PE patients can be relieved after the delivery of placental tissue, which once again proves the importance of placenta with trophoblast as the main structure in the pathogenesis of PE. Importantly, a large number of studies have shown that vesicles derived from endothelial, trophoblast, macrophage and other different sources could be uptaken by placental trophoblast and resulted in defective angiogenesis, inflammation and vascular mechanism, which is closely related to the pathogenesis of PE [35][36][37][38][39][40][41][42][43].
A Japanese study found that the level of Aa was not increased in the circulation of PE women in comparison with normal control, which has been the most recognized pathogenic bacteria in the development of periodontitis. This study suggested that pathogenic bacteria didn't induce PE via the direct invasion into maternal circulation [44]. In our unpublished work as well as others', OMVs have been reported as being stable and capable of passing through the blood-brain barrier and placental barrier, giving rising to the possibility that OMVs from oral flora could act as an information transmitter to remotely regulate and mediate the occurrence and development of systemic diseases [34]. Given that recent blockbuster studies pointed out that OMVs released by intestinal or oral flora were not only ingested by distant organs such as liver, spleen, and heart, but also ingested by neurons after crossing the blood-brain barrier, OMVs might be the breakthrough point for research on the damage to distant organs caused by the imbalance of flora, including placenta. Some OMVs released by some pathogenic bacteria after oral flora imbalance might participate in the cross talk between oral flora and placenta, which finally contributes to the occurrence and development of PE.

Conclusion
Preeclampsia is a gestation-specific syndrome characterized by de novo hypertension and evidence of target organ dysfunction and remains one of the important causes of maternal and infant death. Though numerous pathogenetic mechanisms have been engaged in this disorder and several methods have been undertaken to treat PE, few clinical strategies are effective in PE management, suggesting more studies from novel perspective being in great need to decipher the underlying mechanisms of PE. Previous studies reported a strong relationship between PE and oral microflora-induced periodontitis. Further studies added that microflora-derived OMVs are rich in toxins, bioactive proteins, and virulence factors and are engaged in bacteria-bacteria and even bacteria-host interactions. Given the potential of OVMs shuttling from oral cave to placenta with bioactive materials, it is suggested that OMVs may be the underlying mechanism linking oral flora-induced periodontitis with dysfunction trophoblast and finally contributes to the pathogenesis of in PE (Figure 1).
Deciphering the role of OMVs in PE is expected to deepen current understanding of this life-threatening disease, provide new theoretical basis and for drug intervention and clinical management of PE.