Effects of substrate stiffness on dental pulp stromal cells in culture

Substrate stiffness promotes dentinogenesis via LAMB1-FAK-MEK1/2 signaling axis

OBJECTIVES

In vivo, the principal function of mechanosensitive odontoblasts is to synthesize and secrete the matrix which then calcifies and forms reactive dentin after exposure to appropriate stimuli. This study aims to develop the influence of mechanical factors on dentinogenesis based on odontoblasts, which contribute to reparative dentin formation.

METHODS

We fabricated polydimethylsiloxane with different stiffnesses and seeded 17IIA11 odontoblast-like cells on the substrates in different stiffnesses. Cell morphology was detected by scanning electron microscope, and the mineralization phenotype was detected by alkaline phosphatase staining and alizarin red staining, while expression levels of dentinogenesis-related genes (including Runx2, Osx, and Alp) were assayed by qPCR. To explore mechanism, protein distribution and expression levels were detected by immunofluorescent staining, Western blotting, and immunoprecipitation.

RESULTS

In our results, during dentinogenesis, 17IIA11 odontoblast-like cells appeared better extension on stiffer substrates. The binding between LAMB1 and FAK contributed to converting mechanical stimuli into biochemical signaling, thereby controlling mitogen-activated protein kinase kinase 1/2 activity in stiffness-driven dentinogenesis.

CONCLUSION

The present study suggests odontoblast behaviors can be directly regulated by mechanical factors at cell-material interfaces, which offers fundamental mechanism in remodeling cell microenvironment, thereby contributing to physiological phenomena explanation and tissue engineering progress.

Mechanobiology of Dental Pulp Cells

The dental pulp is the inner part of the tooth responsible for properly functioning during its lifespan. Apart from the very big biological heterogeneity of dental cells, tooth microenvironments differ a lot in the context of mechanical properties-ranging from 5.5 kPa for dental pulp to around 100 GPa for dentin and enamel. This physical heterogeneity and complexity plays a key role in tooth physiology and in turn, is a great target for a variety of therapeutic approaches. First of all, physical mechanisms are crucial for the pain propagation process from the tooth surface to the nerves inside the dental pulp. On the other hand, the modulation of the physical environment affects the functioning of dental pulp cells and thus is important for regenerative medicine. In the present review, we describe the physiological significance of biomechanical processes in the physiology and pathology of dental pulp. Moreover, we couple those phenomena with recent advances in the fields of bioengineering and pharmacology aiming to control the functioning of dental pulp cells, reduce pain, and enhance the differentiation of dental cells into desired lineages. The reviewed literature shows great progress in the topic of bioengineering of dental pulp-although mainly in vitro. Apart from a few positions, it leaves a gap for necessary filling with studies providing the mechanisms of the mechanical control of dental pulp functioning in vivo.

Emerging Roles of YAP/TAZ in Tooth and Surrounding: from Development to Regeneration

Yes associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are ubiquitous transcriptional co-activators that control organ development, homeostasis, and tissue regeneration. Current in vivo evidence suggests that YAP/TAZ regulates enamel knot formation during murine tooth development, and is indispensable for dental progenitor cell renewal to support constant incisor growth. Being a critical sensor for cellular mechano-transduction, YAP/TAZ lays at the center of the complex molecular network that integrates mechanical cues from the dental pulp chamber and surrounding periodontal tissue into biochemical signals, dictating in vitro cell proliferation, differentiation, stemness maintenance, and migration of dental stem cells. Moreover, YAP/TAZ-mediated cell-microenvironment interactions also display essential regulatory roles during biomaterial-guided dental tissue repair and engineering in some animal models. Here, we review recent advances in YAP/TAZ functions in tooth development, dental pulp, and periodontal physiology, as well as dental tissue regeneration. We also highlight several promising strategies that harness YAP/TAZ activation for promoting dental tissue regeneration.

Biomechanical Modulation of Dental Pulp Stem Cell (DPSC) Properties for Soft Tissue Engineering

Dental pulp regeneration strategies frequently result in hard tissue formation and pulp obliteration. The aim of this study was to investigate whether dental pulp stem cells (DPSCs) can be directed toward soft tissue differentiation by extracellular elasticity. STRO-1-positive human dental pulp cells were magnetically enriched and cultured on substrates with elasticities of 1.5, 15, and 28 kPa. The morphology of DPSCs was assessed visually. Proteins relevant in mechanobiology ACTB, ITGB1, FAK, p-FAK, TALIN, VINCULIN, PAXILLIN, ERK 1/2, and p-ERK 1/2 were detected by immunofluorescence imaging. Transcription of the pulp marker genes BMP2, BMP4, MMP2, MMP3, MMP13, FN1, and IGF2 as well as the cytokines ANGPT1, VEGF, CCL2, TGFB1, IL2, ANG, and CSF1 was determined using qPCR. A low stiffness, i.e., 1.5 kPa, resulted in a soft tissue-like phenotype and gene expression, whereas DPSCs on 28 kPa substrates exhibited a differentiation signature resembling hard tissues with a low cytokine expression. Conversely, the highest cytokine expression was observed in cells cultured on intermediate elasticity, i.e., 15 kPa, substrates possibly allowing the cells to act as “trophic mediators”. Our observations highlight the impact of biophysical cues for DPSC fate and enable the design of scaffold materials for clinical pulp regeneration that prevent hard tissue formation.

Enhancement of acellular biomineralization, dental pulp stem cell migration, and differentiation by hybrid fibrin gelatin scaffolds

OBJECTIVE

The current in vitro study aims to evaluate cross-linked hydrogels with and without the addition of fibrin that could potentially be used in endodontic regeneration as a scaffold material.

METHODS

Synthesis of gelatin/fibrin scaffold, and performing nanoscale characterization using cryo-electron microscopy, dynamic rheology, and XRF for structure property relations; plating dental pulp stem cells and determining mineralization, migration, and differentiation using rt-PCR, XRF, and Raman spectroscopy.

RESULTS

Cryo electron imaging shows gelatin and fibrin, when gelled separately to form classical rectangular cross-linked networks, where the modulus scales inversely with the cube root of the mesh size. When gelled together, a network with a fundamentally different structure is formed, which has higher ductility and when placed as a scaffold in osteogenic media, produces twice the mineral content. Immunofluorescence, RT-PCR and Rahman Spectroscopy indicate that the hybrid gel enhances cell migration, induces odontogenic differentiation of dental pulp stem cells, and promotes formation of dentin.

SIGNIFICANCE

The mechanical properties of the hybrid gel scaffold enhance in-migration of stem cells and subsequent differentiation, which are critical for regenerative procedures. Under acellular conditions, placement of the hybrid gel enhances biomineralization, which would strengthen the root if used as a scaffold for endodontic regeneration. Our in vitro findings are consistent with previous in vivo studies which show improved mineralization when bleeding is induced into the canal, given that fibrin is a primary component in blood clotting. Therefore, insertion of the hybrid gelatin-fibrin scaffold could enable more reproducible and consistent outcomes if used for regenerative endodontic treatment (RET).

Biodegradation of an injectable treated dentin matrix hydrogel as a novel pulp capping agent for dentin regeneration

BACKGROUND

A novel injectable mixture termed treated dentin matrix hydrogel (TDMH) has been introduced for restoring dentin defect in DPC. However, no study evaluated its physiological biodegradation. Therefore, the present study aimed to assess scaffold homogeneity, mechanical properties and biodegradability in vitro and in vivo and the regenerated dentin induced by TDMH as a novel pulp capping agent in human permanent teeth.

METHODS

Three TDMH discs were weighted, and dry/wet ratios were calculated in four slices from each disc to evaluate homogeneity. Hydrogel discs were also analyzed in triplicate to measure the compressive strength using a universal testing machine. The in vitro degradation behavior of hydrogel in PBS at 37 °C for 2 months was also investigated by monitoring the percent weight change. Moreover, 20 intact fully erupted premolars were included for assessment of TDMH in vivo biodegradation when used as a novel injectable pulp capping agent. The capped teeth were divided into four equal groups according to extraction interval after 2-, 8-, 12- and 16-weeks, stained with hematoxylin-eosin for histological and histomorphometric evaluation. Statistical analysis was performed using F test (ANOVA) and post hoc test (p = 0.05).

RESULTS

No statistical differences among hydrogel slices were detected with (p = 0.192) according to homogeneity. TDMH compression modulus was (30.45 ± 1.11 kPa). Hydrogel retained its shape well up to 4 weeks and after 8 weeks completely degraded. Histological analysis after 16 weeks showed a significant reduction in TDMH area and a simultaneous significant increase in the new dentin area. The mean values of TDMH were 58.8% ± 5.9 and 9.8% ± 3.3 at 2 and 16 weeks, while the new dentin occupied 9.5% ± 2.8 at 2 weeks and 82.9% ± 3.8 at 16 weeks.

CONCLUSIONS

TDMH was homogenous and exhibited significant stability and almost completely recovered after excessive compression. TDMH generally maintained their bulk geometry throughout 7 weeks. The in vivo response to TDMH was characterized by extensive degradation of the hydrogel and dentin matrix particles and abundant formation of new dentin. The degradation rate of TDMH matched the rate of new dentin formation.

TRIAL REGISTRATION

PACTR201901866476410: 30/1/2019.

Paxillin tunes the relationship between cell-matrix and cell-cell adhesions to regulate stiffness-dependent dentinogenesis

Mechanical stiffness is recognized as a key physical factor and directs cell function via a mechanotransduction process, from extracellular physical cues to intracellular signaling cascades that affect transcriptional activity. Cells continually receive mechanical signals from both the surrounding matrix and adjacent cells. However, how mechanical stiffness cue at cell-substrate interfaces coordinates cell-cell junctions in guiding mesenchymal stem cell behaviors is poorly understood. Here, polydimethylsiloxane substrates with different stiffnesses were used to study mechanosensation/transduction mechanisms in controlling odontogenic differentiation of dental papilla cells (DPCs). DPC phenotypes (morphology and differentiation) changed in response to the applied force derived from stiff substrates. Significantly, higher expression of paxillin on stiffer substrates promoted DPC dentinogenesis. Upon treatment with siRNA to knockdown paxillin, N-cadherin increased mainly in the cytomembrane at the area of cell-cell contacts, whereas β-catenin decreased in the nuclei. The result of a double luciferase reporter assay showed that stiffness promoted β-catenin binding to TCF, which could coactivate the target genes associated with odontogenic differentiation, as evidenced by bioinformatics analysis. Finally, we determined that the addition of a β-catenin inhibitor suppressed DPC mineralization in all the stiffness groups. Thus, our results indicated that a mechanotransduction process from cell-substrate interactions to cell-cell adhesions was required for DPC odontogenic differentiation under the stimulation of substrate stiffness. This finding suggests that stem cell fate specification under the stimulus of stiffness at the substrates is based on crosstalk between substrate interactions and adherens junctions, which provides an essential mechanism for cell-based tissue engineering.

Engineering Hydrogels for Modulation of Material-Cell Interactions

Hydrogels are a recurrent platform for Tissue Engineering (TE) strategies. Their versatility and the variety of available methods for tuning their properties highly contribute to hydrogels' success. As a result, the design of advanced hydrogels has been thoroughly studied, in the quest for better solutions not only for drugs- and cell-based therapies but also for more fundamental studies. The wide variety of sources, crosslinking strategies, and functionalization methods, and mostly the resemblance of hydrogels to the natural extracellular matrix, make this 3D hydrated structures an excellent tool for TE approaches. The state-of-the-art information regarding hydrogel design, processing methods, and the influence of different hydrogel formulations on the final cell-biomaterial interactions are overviewed herein. This article is protected by copyright. All rights reserved.

Increased strength in the Col-Tgel induces apoptosis in the human dental pulp stem cells: 3D culturing of human dental pulp stem cells at different strengths of collagen

Human dental pulp stem cells (HDPSCs) have great potential to be used in regenerative medicine. To use these stem cells effectively for this purpose, they should be grown in a 3D cell culture that mimics their natural niches instead of a 2D conventional cell culture. The aim of this study was to grow the HDPSCs in the 3D cell culture created by Transglutaminase-crosslinked collagen hydrogels (Col-Tgel) in two different strengths to find a suitable 3D cell culture environment for these stem cells. Two stiffness of the 3D Col-Tgel were used to grow the HDPSCs: soft and medium matrix with strength of 0.9-1.5 kPa and 14-20 kPa, respectively. HDPSCs express markers similar to MSCs, therefore seven such markers were analyzed in the HDPSCs during their growth in the 2D and in the 3D soft and medium Col-Tgel. The CD105 and CD90 markers were significantly (p < 0.05) downregulated in HDPSCs cultured in both 3D cell culture conditions compared with HDPSCs in 2D cell culture. Furthermore, CD34 marker, a negative marker, expressed by a few cells in HDPSCs culture was upregulated (p < 0.05) in HDPSCs cultured in medium 3D Col-Tgel, indicating cells that expressing the marker grow better in medium 3D Col-Tgel. The apoptosis results revealed that HDPSCs in medium 3D Col-Tgel had the least number of live cells and a significantly (p < 0.05) higher early apoptosis rate compared to HDPSCs in 2D and 3D Col-Tgel medium. MTT analysis also showed a significant difference among the three cell culture conditions. We conclude that HDPSCs cultured on 3D soft Col-Tgel showed better proliferation than cells cultured in 3D medium gel. These results demonstrate that the ideal environment to grow HDPSCs in 3D is the soft Col-Tgel not medium Col-Tgel.

Soft surfaces promote astrocytic differentiation of mouse embryonic neural stem cells via dephosphorylation of MRLC in the absence of serum

Astrocytes, which can be obtained from neural stem cells (NSCs) by adding serum and/or recombinant proteins in culture media or by passaging NSCs repeatedly, are expected to be applicable in regenerative medicine for the treatment of neurodegenerative diseases. However, astrocytes obtained using existing methods are costly and have poor quality. The stiffness of culture surfaces has been reported to affect astrocytic differentiation of adult NSCs. However, the influence of surface stiffness on astrocytic differentiation of embryonic NSCs has not yet been reported. In this study, we showed that astrocytic differentiation of embryonic NSCs was increased on soft surfaces (1 kPa and 12 kPa) compared with the NSCs on stiff surfaces (2.8 GPa) in serum-free condition. Furthermore, di-phosphorylated myosin regulatory light chain (PP-MRLC) was decreased in embryonic NSCs cultured on the soft surfaces than the cells on the stiff surfaces. Additionally, astrocytic differentiation of embryonic NSCs was induced by a Ras homolog associated kinase (ROCK) inhibitor, which decreased PP-MRLC in NSCs. These results suggest that decreasing the PP-MRLC of embryonic NSCs on soft surfaces or treating NSCs with a ROCK inhibitor is a good method to prepare astrocytes for application in regenerative medicine.

Stiffness Regulates the Morphology, Adhesion, Proliferation, and Osteogenic Differentiation of Maxillary Schneiderian Sinus Membrane-Derived Stem Cells

Recent studies, which aim to optimize maxillary sinus augmentation, have paid significant attention exploring osteogenic potential of maxillary Schneiderian sinus membrane-derived cells (MSSM-derived cells). However, it remains unclear that how MSSM-derived cells could respond to niche's biomechanical properties. Herein, this study investigated the possible effects of substrate stiffness on rMSSM-derived stem cell fate. Initially, rMSSM-derived stem cells with multiple differentiation potential were successfully obtained. We then fabricated polyacrylamide substrates with varied stiffness ranging from 13 to 68 kPa to modulate the mechanical environment of rMSSM-derived stem cells. A larger cell spreading area and increased proliferation of rMSSM-derived stem cells were found on the stiffer substrates. Similarly, cells became more adhesive as their stiffness increased. Furthermore, the higher stiffness facilitated osteogenic differentiation of rMSSM-derived stem cells. Overall, our results indicated that increase in stiffness could mediate behaviors of rMSSM-derived stem cells, which may serve as a guide in future research to design novel biomaterials for maxillary sinus augmentation.

Insensitivity of dental pulp stem cells migration to substrate stiffness

Dental pulp stem cells (DPSCs) are a promising cell source for regeneration of dental pulp. Migration is a key event but influence of the microenvironment rigidity (5 kPa at the center of dental pulp to 20 GPa for the dentin) is largely unknown. Mechanical signals are transmitted from the extracellular matrix to the cytoskeleton, to the nuclei, and to the chromatin, potentially regulating gene expression. To identify the microenvironmental influence on migration, we analyzed motility on PDMS substrates with stiffness increasing from 1.5 kPa up to 2.5 MPa. We found that migration speed slightly increases as substrate stiffness decreases in correlation with decreasing focal adhesion size. Motility is relatively insensitive to substrate stiffness, even on a bi-rigidity PDMS substrate where DPSCs migrate without preferential direction. Migration is independent of both myosin II activity and YAP translocation after myosin II inhibition. Additionally, inhibition of Arp2/3 complex leads to significant speed decrease for all rigidities, suggesting contribution of the lamellipodia in the migration. Interestingly, the chromatin architecture remains stable after a 7-days exposure on the PDMS substrates for all rigidity. To design scaffold mimicking dental pulp environment, similar DPSCs migration for all rigidity, leaves field open to choose this mechanical parameter.

The effect of hyaluronic acid hydrogels on dental pulp stem cells behavior

Dental caries and trauma, particularly in childhood, are among the most prevalent teeth problems, which result in the creation of cavities and probably tooth loss. Thus, novel regenerative approaches with high efficiency and less toxicity are required. Stem cell therapy along with the implementation of scaffolds has provided excellent opportunities in the regeneration of teeth structure. Hyaluronic acid (HA) hydrogels have enticed great attention in the field of regenerative medicine. The unique chemical and structural properties of HA and its derivatives have enabled their application in tissue engineering. Several factors such as the location and type of the lesion, teeth age, the type of capping materials determine the success rate of pulp therapy. HA hydrogels have been considered as biocompatible and safe scaffold supports in human dental cell therapies.

Microenvironmental Stiffness Regulates Dental Papilla Cell Differentiation: Implications for the Importance of Fibronectin-Paxillin-β-Catenin Axis

The mechanical stiffness of substrates is recognized to be an important physical cue in the microenvironment of local cellular residents in mammalian species due to their great capacity in regulating cell behavior. Dental papilla cells (DPCs) play an important role in the field of dental tissue engineering for their stem cell-like properties. Therefore, it is essential to provide the suitable microenvironment by combining with the physical cues of biomaterials for DPCs to carry out the function of effective tissue regeneration. However, how the substrate stiffness influences the odontogenic differentiation of DPCs is still unclear. Thus, we fabricated poly(dimethylsiloxane) substrates with varied stiffness for cell behavior. Both cell morphology and focal adhesion were shown to have significant changes in response to varied stiffness. Paxillin, an important protein adapter of focal adhesion kinase protein, was shown to interact with both ectoplasmic fibronectin and cytoplasmic β-catenin by coimmunoprecipitation. The resultant changes of β-catenin by varied stiffness were confirmed by immunofluorescent stain and western blotting. Further, the higher quantity nuclear translocation of β-catenin and the less phospho-β-catenin on the stiff substrate were detected. This nuclear translocation in the stiff substrate finally led to an increased mineralization of DPCs relative to the soft substrate detected by Von Kossa and Alizarin Red stain. Taken together, this work not only points out that the substrate stiffness can regulate the odontogenic differentiation potential of DPCs via fibronectin/paxillin/β-catenin pathway but also provides significant consequence for biomechanical control of cell behavior in cell-based tooth tissue regeneration.