Abstract

Background

   Upon migrating to the injured sites, bone marrow mesenchymal stem cells
   (BMSCs) play critical roles in the repair of bone lesion caused by
   chronic apical periodontitis. Emerging evidences have shown that
   Enterococcus faecalis is always associated with apical periodontitis,
   especially refractory apical periodontitis. But the mechanism
   underlying how Enterococcus faecalis affects the migration of BMSCs
   remains unclear.

Methods

   The effects of Enterococcus faecalis supernatants on the migration of
   BMSCs were determined by transwell migration assays. miRNA sequencing
   was performed to detect the significantly differentially expressed
   miRNAs of BMSCs. Proteomics analysis was used to detect the protein
   expression alterations of BMSCs. Luciferase report assays were deployed
   to verify the targets of miRNA. Western blot analysis was performed to
   examine the expressions of matrix metalloproteinases-3, matrix
   metalloproteinases-9, Forkhead Box Protein J1 (FOXJ1), and nuclear
   factor kappa B (NFκB). The activations of NFκB were detected by
   luciferase assays with NFκBluc reporter.

Results

   We found that Enterococcus faecalis supernatants could promote the
   migration of BMSCs. The upregulation of miR-200a-3p in this process
   contributed to BMSC migration through downregulating its target
   Forkhead Box Protein J1. Moreover, FOXJ1/ NFκB axis was found to
   regulate matrix metalloproteinases (MMPs) in this process.

Conclusions

   These results above suggest that miR-200a contributes to the migration
   of BMSCs induced by the secretions of E. faecalis via FOXJ1/NFκB/MMPs
   axis.

   Keywords: Bone marrow mesenchymal stem cells, Cell migration, Nuclear
   factor kappa B, Enterococcus faecalis, miRNA, Host-pathogen
   interactions

Background

   Apical periodontitis, which is characterized by the inflammation and
   destruction of the apical periodontium, is always caused by the host
   immune response to microbial infection in the root canal system
   [[35]1]. Those species originated from intestinal flora, such as E.
   faecalis, have been identified as dominant factors causing apical
   periodontitis [[36]2]. E. faecalis is always isolated as a monoculture
   in retreated root canals [[37]3]. It belongs to facultative aerobic
   species and is usually found in secondary infection or post-treatment
   of apical periodontitis, especially in the refractory inflammation
   [[38]4]. E. faecalis is tolerated to antimicrobials and contains the
   ability of surviving in a nutrient-deficient environment. Thus,
   persisting infections in root canal or apical periodontium are always
   associated with E. faecalis [[39]5]. There are various virulence
   factors produced by E. faecalis, such as lipoteichoic acid, aggregation
   substance protein, and surface adhesion [[40]6, [41]7]. During the
   development of pulpitis, E. faecalis can invade and colonize at
   dentinal tubules. Studies on the etiology of refractory apical
   periodontitis have revealed that E. faecalis biofilms in the dentinal
   tubules contribute to the retaining of apical periodontitis [[42]8].

   Since apical periodontitis is characterized by inflammation and bone
   resorption [[43]9], cells associated with this process should be
   carefully taken into consideration. Belonging to multipotent stem cells
   and widely presenting in bone marrow, bone marrow mesenchymal stem
   cells (BMSCs) can differentiate into osteoblasts, chondrocytes, or
   adipocytes [[44]10]. Meanwhile, BMSCs have shown certain ability of
   moving from niche to the peripheral circulation, and further to the
   target tissues [[45]11]. The recruitment of BMSCs is required for the
   repair of bone lesion, and the migration of BMSCs is usually attracted
   by the environmental factors at the site of injury [[46]12]. There are
   various factors gathering at the injury, including infectious factors
   and those produced by injured tissues.

   With bacterial infection, BMSCs contact with bacterial components and
   recognize them through the receptors on the cell membrane. Studies on
   human BMSCs have revealed that lipopolysaccharide (LPS), the cell wall
   component from gram-positive bacteria, can increase their migration
   [[47]13], while the synthetic lipopeptide could inhibit the migration
   of mouse BMSCs [[48]14]. The migration of human dental pulp stem cells
   were also increased with the stimulation of Toll-like receptor 2 (TLR2)
   ligands [[49]15]. Studies above remind us that it depends on the type
   of mesenchymal stem cell in which migration effect would be caused by
   bacterial components. In fact, it is hard to ensure the regeneration of
   the tissue without the efficient migration of BMSCs into the injured
   sites. It requires more acknowledgments in this field to induce BMSCs