Introduction

Dental pulp regeneration may be a potential treatment for managing permanent teeth undergoing necrosis [1]. Regenerative endodontic therapy has been assayed with multiple mesenchymal stem cells (MSCs), growth factors, and biological materials [2, 3]. Magnesium-containing scaffolds are considered to be ideal biomaterials for such treatment, with the properties of releasing Mg2+ to enhance dentin regeneration in MSCs [4].

As reported, Mg2+ acts as an intracellular second messenger connecting cell-surface receptor induction and cytosolic effectors [5]. Mg2+ is the fourth most abundant cation in the human body and is critical for ATP-dependent phosphorylation of DNA, RNA, and enzymes, and the average Mg2+ concentration in dentin is about 1% (wt/wt) [6,7,8]. Mg2+ is involved in the bio-mineralization of bones and teeth and directly affects crystallization and pattern generation of the inorganic mineral phase [8,9,10]. Evidence indicates mutations of the Mg2+ transporters TRPM7 and CNNM4 result in deficient dentin mineralization, confirming Mg2+ involvement in tooth development [11, 12]. Animals fed low Mg2+ diets show deficient dentin and enamel mineralization [13]. Our previous study also demonstrates that the Mg2+ transporter Magt1 plays an important role in odontogenic differentiation of bone marrow MSCs by regulating intracellular Mg2+ [14]. Studies have found high extracellular Mg2+ and its transporter regulate odontogenic differentiation in human DPSCs, with involvement in dentin mineralization [4, 15].

However, the regulatory mechanisms of Mg2+ effects on odontogenic differentiation in DPSCs remain undefined. Till now, the altered transcriptome of DPSCs undergoing odontogenic differentiation induced by an Mg2+-enriched microenvironment has not been assessed; thus, we determine the alteration of transcriptome using RNA sequencing. Moreover, we aim to clarify the role of Mg2+-enriched microenvironment in odontogenic differentiation of DPSCs and determine the associated signaling pathways.

Materials and methods

DPSC isolation and identification

Studies involving patients had approval from the Ethics Committee of Sun Yat-Sen University. DPSCs were collected from non-diseased pulp tissues of caries-free third molars of patients. DPSCs were isolated according to a previous report [16] and maintained in α-MEM containing 10% FBS (GIBCO, USA) and 10 mg/mL streptomycin and 10 U/mL penicillin (Sigma, USA). DPSCs at passages 3 to 7 were used in experiments. Flow cytometry analysis was performed to investigate DPSCs for surface markers. DPSCs were incubated with conjugated human antibodies, including CD45-PE, CD73-PE, CD90-APC, and CD166-PE (BD, USA), and assessed flow cytometrically (BD, USA). For confirming the adipogenic differentiation potential of stem cells, DPSCs underwent induction for 28 days with adipogenic medium containing 0.5 μM isobutyl-methylxanthine, 50 μM indomethacin, 0.5 μM dexamethasone, and 5 μg/mL insulin (Sigma, USA). To evaluate osteoblastic differentiation, DPSCs underwent culture for 14 days in osteogenic medium containing 100 nM dexamethasone, 10 mM β-glycerophosphate, and 0.2 mM ascorbic acid (Sigma, USA).

Determination of DPSC proliferation

DPSCs seeded in 96-well plates at 1000 cells/well underwent culture in growth medium after addition of 0, 1, 5, or 10 mM MgCl2 for 1, 2, and 3 days, respectively. Cell Counting Kit-8 (Dojindo, Japan) was applied as directed by the manufacturer.

Odontogenic differentiation of DPSCs induced by the Mg2+-enriched microenvironment

DPSCs seeded in 12-well plates at 3 × 104 cells/well underwent culture in α-MEM containing 10% FBS at 37 °C. At 80% confluency, DPSC culture was performed in odontogenic medium with 100 nM dexamethasone, 10 mM β-glycerophosphate, and 0.2 mM ascorbic acid, with the addition of 0, 1, 5, or 10 mM MgCl2, respectively. After 7 days, the protein amounts of RUNX2, DSP, and DMP-1 were determined by automated immunoblot. After 14 days, mineralization nodules were assessed by Alizarin red S staining.

Alizarin red S staining

Alizarin red S (Sigma, USA) staining was carried out as proposed by the manufacturer. Destaining was performed with 10% cetylpyridinium chloride monohydrate buffer for 30 min, and optical density was obtained at 575 nm. Relative extracellular matrix mineralization was calculated for each sample.

Inhibition of Mg2+ entry by 2-aminoethoxydiphenyl borate (2-APB)

Given that TRPM7 was reported to transport Mg2+ across the cell membrane, we inhibited Mg2+-entry by treatment with 100 μM of the TRPM7 inhibitor 2-APB (Aladdin, China) [17].

Automated immunoblot

Total protein from DPSCs was obtained with RIPA (Cell Signaling, USA). Automated immunoblot was carried out with Simple Wes (Protein Simple, USA) as directed by the manufacturer. In brief, 2.5 μg total protein samples were mixed with the standard fluorescent master mix and added to prefilled Wes assay plates, alongside antibody diluent (Protein Simple); primary antibodies targeting BMP2 (Affinity, USA), BMPR1 (Affinity), p-Smad1/5/9 (Affinity), RUNX2 (Affinity), DSP (Santa Cruz, USA), DMP-1 (Genetex, USA), ERK, p-ERK, JNK, p-JNK, p38, p-p38 (Cell Signaling), and β-Tublin (Affinity, USA) were probed; anti-rabbit and anti-mouse secondary antibodies conjugated to Streptavidin-HRP (Protein Simple) were used for detection. The compass software (Protein Simple) was utilized for analysis.

Flow cytometry evaluation of intracellular magnesium

Intracellular magnesium amounts in DPSCs were assessed with Mag-Fluo-4-AM (Invitrogen, USA) as directed by the manufacturer. Briefly, Mag-Fluo-4-AM was added at 2 μM and incubated for 30 min. This was followed by three cell washes. Fluorescence intensities were obtained by flow cytometry (BD, USA) in triplicate assays.

RNA sequencing (RNA-seq)

RNA-seq was carried out according to our previous work [14]. Briefly, DNase I treated total RNA specimens were enriched for mRNA using Oligo (dT) magnetic beads. Random-hexamer primers were employed for reverse transcription, and second-strand cDNA synthesis was carried out with Second Strand cDNA Synthesis Kit (Beyotime, China) as directed by the manufacturer. Short fragment purification used the QiaQuick PCR extraction kit (Qiagen, the Netherlands). Sequencing adapters were next added to these short fragments. Suitable fragments were amplified, and an Illumina HiSeq™ 2000 (Illumina Inc., USA) was employed for the cDNA sequencing. False discovery rate (FDR) adjusted p < 0.05 was obtained by the Benjamini-Hochberg method.

GO and KEGG pathway analyses

Gene Ontology (GO, http://www.geneontology.org) and Kyoto Encyclopedia of Genes and Genomes pathway (KEGG, http://www.genome.jp/kegg/pathway.html) analyses were carried out for determining biological functions and enriched metabolic or signal transduction pathways associated with the differentially expressed genes (DEGs), respectively.

MAPK signaling assessment

To assess MAPK signaling involvement, DPSCs underwent culture in 12-well plates in odontogenic medium with the addition of 0, 1, 5, or 10 mM MgCl2 for 7 days, respectively. Then, total protein from DPSCs was isolated to measure the protein amounts of ERK, p-ERK, JNK, p-JNK, p38, and p-p38 by automated immunoblot. During odontogenic differentiation of DPSCs administered 5 mM MgCl2 for 7 days, ERK/MAPK signaling was suppressed with 50 μM U0126 (AbMole BioScience, USA) for 2 h every 2 days.

Statistical analysis

Experiments were performed three times. Data were expressed as mean ± SD and were assessed by ANOVA test with SPSS 17.0 (SPSS, USA); p < 0.05 indicated statistical significance.

Results

Effects of the Mg2+-enriched microenvironment on proliferation and differentiation in DPSCs

Passage 3 DPSCs were examined. As shown in Fig. 1a, DPSCs could differentiate into osteoblasts and adipocytes, suggesting their multi-lineage differentiation potential. Flow cytometry analysis showed DPSCs had low CD45 (hematopoietic cell marker) amounts (0.60%), but elevated amounts of the mesenchymal stem cell markers CD73 (99.02%), CD90 (99.09%), and CD166 (98.13%) (Fig. 1b).

Fig. 1
figure 1

Identification of DPSCs. a Mineral deposits stained by Alizarin Red S in DPSCs following osteogenic differentiation for 2 weeks. Adipocyte staining by Oil Red O upon adipoinduction for 4 weeks. b The expression rates of CD45, CD73, CD90, and CD166 are shown

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To evaluate the impact of high extracellular Mg2+ on their proliferation, DPSCs underwent incubation in Mg2+-enriched media at different concentrations (0 mM, 1 mM, 5 mM, and 10 mM) for 1, 2, and 3 days, respectively. Cell Counting Kit-8 analysis showed that high extracellular Mg2+ did not affect DPSC proliferation (Fig. 2).

Fig. 2
figure 2

Cell Counting Kit-8 analysis showing that Mg2+-enriched microenvironment has no effect on DPSC proliferation

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To analyze the impact of high extracellular Mg2+ on their differentiation, DPSCs were cultured in Mg2+-enriched odontogenic medium at different concentrations (0 mM, 1 mM, 5 mM, and 10 mM). After 7 days, RUNX2, DMP-1, and DSP protein amounts were markedly elevated in DPSCs treated with 1 mM, 5 mM, and 10 mM Mg2+, respectively, compared with 0 mM Mg2+ (Fig. 3a). Consistently, mineralization nodules were also markedly increased (Fig. 3b). However, there was no difference between the 5 and 10 mM Mg2+ groups (Fig. 3). These results showed that the Mg2+-enriched microenvironment promoted the odontogenic differentiation of DPSCs.

Fig. 3
figure 3

Effects of Mg2+-enriched microenvironment on DPSC differentiation. a After 7 days, RUNX2, DMP-1, and DSP protein amounts were markedly elevated in DPSCs treated with 1 mM, 5 mM, and 10 mM Mg2+, compared with the 0 mM Mg2+ group. b Consistently, mineralization nodules were also markedly increased after 14 days. However, there was no difference between the 5 and 10 mM Mg2+ groups

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Mg2+-enriched microenvironment promotes odontogenesis in DPSCs by increasing intracellular Mg2+

Intracellular magnesium levels in DPSCs treated with Mg2+-enriched odontogenic media at different concentrations (0 mM, 1 mM, 5 mM, and 10 mM) were assessed flow cytometrically using Mag-Fluo-4-AM. Figure 4a suggests that intracellular Mg2+ was starkly increased in DPSCs treated with 1 mM, 5 mM, and 10 mM Mg2+, compared with the 0 mM Mg2+ group. However, there was no difference in intracellular magnesium of DPSCs treated with 5 mM and 10 mM Mg2+, which is consistent with their comparable effects on odontogenic differentiation.

Fig. 4
figure 4

Mg2+-enriched microenvironment promotes odontogenic differentiation in DPSCs by increasing intracellular Mg2+. a Intracellular Mg2+ was significantly increased in DPSCs exposed to high extracellular Mg2+ as assessed flow cytometrically using Mag-Fluo-4-AM; however, there was no difference between the 5 and 10 mM Mg2+ groups. b Mg2+-entry was significantly inhibited by the TRPM7 inhibitor 2-APB. c The positive effect of Mg2+-enriched microenvironment on odontogenic differentiation in DPSCs was also blocked by 2-APB

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Given that TRPM7 was reported to transport Mg2+ across the cell membrane [17], we next examined the effects of its blocker 2-APB. The results showed that Mg2+ entry was significantly blunted by 2-APB (100 μM, inhibiting TRPM7 [17]) (Fig. 4b). The positive effect of high extracellular Mg2+ (5 mM Mg2+) on odontogenic differentiation of DPSCs was also blocked by 2-APB (Fig. 4c). These results indicated that high extracellular Mg2+ promoted odontogenic differentiation in DPSCs via intracellular Mg2+ increase.

High extracellular Mg2+ alters the transcriptome of DPSCs

Compared with control cells (0 mM Mg2+), exposure to high extracellular Mg2+ (5 mM Mg2+) for 7 days resulted in 734 DEGs in DPSCs, including 250 upregulated and 484 downregulated (Fig. 5a, b).

Fig. 5
figure 5

RNA-Seq of DPSCs undergoing odontogenic differentiation upon induction by high extracellular Mg2+. a, b There were734 genes significantly altered, with 250 upregulated and 484 downregulated. c GO analysis of differentially expressed genes (DEGs). d KEGG pathway analysis revealed that the DEGs were involved in many signaling pathways, including MAPK and TGF-β pathways

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GO and KEGG pathway analyses of DEGs

In GO analysis, DEGS were mostly involved in developmental process, anatomical structure development, organism development, system development, channel activity, metal ion binding, integral component of plasma membrane, etc. (Fig. 5c). KEGG pathway analysis indicated that the DEGs retrieved participated in many pathways, e.g., MAPK and TGF-β pathways (Fig. 5d).

ERK signaling is activated by high extracellular Mg2+ during odontogenic differentiation of DPSCs

As shown above, KEGG pathway analysis revealed MAPK signaling activation by high extracellular Mg2+during odontogenic differentiation of DPSCs. To verify this finding, DPSCs were incubated in odontogenic medium with the addition of 0, 1, 5, and 10 mM MgCl2 for 7 days, respectively. The results showed that ERK phosphorylation was markedly enhanced in DPSCs treated with 1 mM, 5 mM, and 10 mM Mg2+ compared with the 0 mM Mg2+ group, but p38 and JNK phosphorylation showed no change (Fig. 6a). In accordance, the positive effect of high extracellular Mg2+ on ERK phosphorylation was blocked by 2-APB (100 μM, inhibiting TRPM7) (Fig. 6b), which reduced intracellular Mg2+amounts (Fig. 4b).

Fig. 6
figure 6

The ERK and BMP2 signaling pathway is activated by high extracellular Mg2+ in DPSCs during odontogenic differentiation. a ERK phosphorylation was significantly enhanced in DPSCs treated with 1 mM, 5 mM, and 10 mM Mg2+ compared with the 0 mM Mg2+ group, but p38 and JNK phosphorylation amounts were unchanged. b ERK phosphorylation was reduced by 2-APB. c Consistently, the protein levels of BMP2, BMPR1, and phosphorylated Smad1/5/9 were significantly increased in DPSCs exposed to high extracellular Mg2+. d BMP2, BMPR1, and phosphorylated Smad1/5/9 protein amounts were decreased by 2-APB

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BMP2/Smads signaling is activated by high extracellular Mg2+during odontogenic differentiation of DPSCs

As reported previously, BMP2 is a TGF-β family member controlling odontogenic differentiation of stem cells [18, 19]. To verify the above KEGG pathway data, we performed Western blot to determine whether BMP2/Smads signaling is activated by high extracellular Mg2+. The results showed that BMP2, BMPR1, and phosphorylated Smad1/5/9 were significantly increased in DPSCs cultured in odontogenic medium containing high extracellular Mg2+(Fig. 6c). In accordance, the positive effects of high extracellular Mg2+ on BMP2, BMPR1, and phosphorylated Smad1/5/9 were blocked by 2-APB (100 μM, inhibiting TRPM7) (Fig. 6d).

Mg2+-enriched microenvironment promotes odontogenic differentiation in DPSCs via ERK/BMP2/Smads signaling

We inhibited ERK signaling with U0126, and mineralization nodules were markedly decreased, as well as RUNX2, DSP, and DMP-1 protein amounts in DPSCs treated with Mg2+-enriched odontogenic medium (5 mM Mg2+) for 7 days (Fig. 7a,b). Interestingly, BMP2, BMPR1, and phosphorylated Smad1/5/9 amounts were also significantly decreased by U0126, indicating that BMP2/Smads acted as downstream of ERK (Fig. 7c).

Fig. 7
figure 7

Mg2+-enriched microenvironment promotes odontogenic differentiation in DPSCs via ERK/BMP2/Smads signaling. a After treatment with U0126, the protein amounts of RUNX2, DSP, and DMP-1 were markedly decreased in DPSCs treated with high extracellular Mg2+. b Consistently, mineralization nodules were also reduced. c After treatment with U0126, BMP2, BMPR1, and phosphorylated Smad1/5/9 protein amounts were significantly decreased, indicating that BMP2/Smads signaling acted as downstream of ERK

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Taken together, these findings suggested that Mg2+-enriched microenvironment promoted odontogenic differentiation in DPSCs by activating ERK/BMP2/Smads signaling via intracellular Mg2+ increase (Fig. 8).

Fig. 8
figure 8

Summary diagram. Mg2+-enriched microenvironment promotes odontogenic differentiation in DPSCs by activating ERK/BMP2/Smads signaling via intracellular Mg2+ increase

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Discussion

Regenerative endodontic therapy involves biological procedures for replacing damaged dental structures such as dentin and root, and cells of the dentin/pulp complex [20]. DPSCs are suitable for such treatment, and their differentiation into odontogenic stem cells associated with biomaterials and growth factors is critical for dental pulp regeneration [1]. It is known that magnesium-containing scaffolds are ideal tools for regenerative endodontic therapy, releasing high extracellular Mg2+ to enhance dentin regeneration in DPSCs [4]. Our previous studies found the Mg2+ transporters TRPM7 and MagT1 regulate odontogenic differentiation in stem cells [14, 15]. Nakano and collaborators [12] reported that TRPM7 silencing in mice results in marked tooth hypomineralization, suggesting TRPM7 to be involved in enamel and dentin biomineralization by supplying enough Mg2+ for ALPL activity. Luder and colleagues [11] demonstrated that mutation of CNNM4, a Mg2+ transporter, caused mineralization abnormalities in both enamel and dentin, in association with starkly reduced magnesium amounts. Consistent with previous studies, the present work determined that Mg2+-enriched microenvironment enhanced odontogenic differentiation in DPSCs by altering the formation of mineralization nodules as well as RUNX2, DMP-1, and DSP protein amounts, through intracellular Mg2+ concentration increase.

However, how high extracellular Mg2+ affects odontogenic differentiation remains unknown. Therefore, RNA-Seq was carried out to assess transcriptome alterations in DPSCs exposed to Mg2+-enriched microenvironment. Interestingly, GO and KEGG pathway analyses revealed 734 DEGs contributing to odontogenic differentiation in DPSCs induced by high extracellular Mg2+. The obtained DEGs participated in many pathways, such as MAPK and TGF-β pathways.

MAPK signaling regulates stem cell proliferation, migration, and differentiation [21,22,23]. Yu and colleagues [24] demonstrated that dentine matrix promoted odontogenic differentiation in bone marrow MSCs by inducing ERK and P38 MAPK pathways. Huang and collaborators [25] revealed that dental pulp cell-derived exosomes promote odontogenic differentiation in bone marrow MSCs via P38/MAPK signaling. As reported in previous studies, MAPK signaling also contributes to odontogenic differentiation in DPSCs [26]. In the present study, ERK phosphorylation was significantly increased in DPSCs treated with high extracellular Mg2+, but p38 and JNK phosphorylation levels remained unchanged, indicating ERK/MAPK signaling is induced by high extracellular Mg2+. Consistently, blunting ERK/MAPK signaling with U0126 blocked the effects of high extracellular Mg2+ on mineralization and odontogenic differentiation in DPSCs.

According to previous studies, intracellular Mg2+ plays a pivotal role in ERK/MAPK signaling induction [27,28,29,30]. For example, Mg2+ deprivation downregulates p-ERK1/2 in MDCK cells, and Mg2+ re-supplementation reverses this effect [29, 30]. Meanwhile, TRPM7 suppression inhibits ERK phosphorylation by reducing intracellular Mg2+ amounts [27, 28]. Our previous studies also revealed that suppression of MagT1 inactivates the ERK/MAPK pathway by decreasing intracellular Mg2+, which results in inhibited odontogenic differentiation of bone marrow MSCs [14]. In the present study, intracellular magnesium in DPSCs exposed to high extracellular Mg2+ was assessed flow cytometrically with Mag-Fluo-4-AM. As shown above, intracellular Mg2+ amounts were starkly increased in DPSCs treated with high extracellular Mg2+. In accordance, the positive effect of high extracellular Mg2+ on ERK phosphorylation was blocked by 2-APB, which reduces intracellular Mg2+ amounts, indicating that high extracellular Mg2+ activates ERK/MAPK signaling by increasing intracellular Mg2+.

BMP2, a member of the TGF-β family of proteins, contributes to odontogenic differentiation in DPSCs [18, 19]. Consistent with KEGG pathway data, BMP2, BMPR1, and phosphorylated Smad1/5/9 amounts were significantly increased in DPSCs incubated in Mg2+-enriched odontogenic media. As expected, the positive effects of high extracellular Mg2+ on BMP2, BMPR1, and phosphorylated Smad1/5/9 were blocked by the Mg2+transporter inhibitor 2-APB, indicating that BMP2/Smads signaling is activated during odontogenic differentiation induced by Mg2+-enriched microenvironment. This finding corroborated Cheng et al. [31] showing that high-purity magnesium scaffolds promote bone regeneration in rabbits via activation of BMP2 signaling by releasing high amounts of extracellular Mg2+.

Interestingly, ERK signaling inhibition by U0126 resulted in significantly decreased BMP2, BMPR1, and phosphorylated Smad1/5/9 amounts, indicating that BMP2/Smads is a downstream signaling of ERK. Taken together, these findings demonstrated that high extracellular Mg2+ promotes odontogenic differentiation in DPSCs by activating ERK/BMP2/Smads signaling via intracellular Mg2+increase.

Conclusions

Overall, significant transcriptome alterations were detected in DPSCs during odontogenic differentiation associated with high extracellular Mg2+. In addition, Mg2+-enriched microenvironment enhanced odontogenic differentiation in DPSCs by activating ERK/BMP2/Smads signaling via intracellular Mg2+ increase.