Effects of mechanical loading on matrix homeostasis and differentiation potential of periodontal ligament cells: A scoping review

Various mechanical loadings, including mechanical stress, orthodontics forces, and masticatory force, affect the functions of periodontal ligament cells. Regulation of periodontal tissue destruction, formation, and differentiation functions are crucial processes for periodontal regeneration therapy. Numerous studies have reported that different types of mechanical loading play a role in maintaining periodontal tissue matrix homeostasis, and osteogenic differentiation of the periodontal ligament cells. This scoping review aims to evaluate the studies regarding the effects of various mechanical loadings on the secretion of extracellular matrix (ECM) components, regulation of the balance between formation and destruction of periodontal tissue matrix, osteogenic differentiation, and multiple differentiation functions of the periodontal ligament. An electronic search for this review has been conducted on two databases; MEDLINE via PubMed and SCOPUS. Study selection criteria included original research written in English that reported the effects of different mechanical loadings on matrix homeostasis and differentiation potential of periodontal ligament cells. The final 204 articles were mainly included in the present scoping review. Mechanical forces of the appropriate magnitude, duration, and pattern have a positive influence on the secretion of ECM components such as collagen, as well as regulate the secretion of matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases. Additionally, these forces regulate a balance between osteoblastic and osteoclast differentiation. Conversely, incorrect mechanical loadings can lead to abnormal formation and destruction of both soft and hard tissue. This review provides additional insight into how mechanical loadings impact ECM homeostasis and multiple differentiation functions of periodontal ligament cells (PDLCs), thus making it valuable for regenerative periodontal treatment. In combination with advancing technologies, the utilization of ECM components, application of different aspects of mechanical force, and differentiation potential of PDLCs could bring potential benefits to future periodontal regeneration therapy.

Matrix stiffness regulates bone repair by modulating 12-lipoxygenase-mediated early inflammation

Lipid metabolism in macrophages has been increasingly emphasized in exerting an anti-inflammatory effect and accelerating fracture healing. 12-lipoxygenase (12-LOX) is expressed in several cell types, including macrophages, and oxidizes polyunsaturated fatty acids (PUFAs) to generate both pro- and anti-inflammatory lipid mediators, of which the n-3 PUFAs play an important part in tissue homeostasis/fibrosis. Although mechanical factor regulates the lipid metabolic axis of inflammatory cells, specifically matrix stiffness influences macrophages metabolic responses, little is known about how matrix stiffness affects the 12-LOX-mediated early inflammation in bone repair. In the present study, demineralized bone matrix (DBM) scaffolds with different matrix stiffness were constructed by controlling the duration of decalcification (0 h (control), 1 h (high), 12 h (medium), and 5 d (low)) to repair the defected rat skull. The expression of inflammatory cytokines and macrophages polarization were analyzed. The lipid metabolites and lipid mediators' biosynthesis by matrix stiffness-regulated were further detected. The results showed that the low matrix stiffness could polarize macrophages into an anti-inflammatory phenotype, promote the expression of anti-inflammatory cytokines and specialized pro-resolving lipid mediators (SPMs) biosynthesis beneficial for the osteogenesis of mesenchymal stem cells (MSCs). After treated with ML355, the expression of anti-inflammatory cytokines/proteins and SPMs biosynthesis in macrophages cultured on low-matrix stiffness scaffolds were repressed, and there were almost no statistical differences among all groups. Findings from this study support that matrix stiffness regulates bone repair by modulating 12-LOX-mediated early inflammation, which suggest a direct mechanical impact of matrix stiffness on macrophages lipid metabolism and provide a new insight into the clinical application of SPMs for bone regeneration.

Fibulins and matrilins are novel structural components of the periodontium in the mouse

Periodontitis refers to inflammatory disease of the periodontal structures (the gingiva, dental cementum, periodontal ligament (PDL) and alveolar bone) that ultimately leads to their destruction. Whereas collagens are well-examined main components of the periodontium, little is known about the other structural proteins that make up this tissue. The aim of this study was to identify new extracellular matrix (ECM) components, including fibulins and matrilins, in the periodontium of mice. After sacrificing 14 mice (Sv/129 strain), jaws were prepared. Each tissue sample contained a molar and its surrounding alveolar bone. Immunohistochemistry was carried out on paraffin-embedded sections. Our results show that mice exhibit fibulin-3, -4 and -5 and matrilin-1, -2, -3 and -4 in PDL and in blood vessels of alveolar bone and PDL as well as in the pericellular matrix of osteocytes and cementocytes. In dental cementum, only fibulin-4 is expressed. For the first time, we show that fibulin-3, -4 and -5 and matrilin-1, -2, -3 and -4 are essential components of the periodontal tissues. Our findings indicate an association of these proteins with collagens and oxytalan fibers that might be of future interest in regenerative periodontitis therapy.

Confocal microscopy demonstrates association of LTBP-2 in fibrillin-1 microfibrils and colocalisation with perlecan in the disc cell pericellular matrix

Comparative immunolocalisations of latent transforming growth factor-beta-1 binding protein (LTBP)-2, fibrillin-1, versican and perlecan were undertaken in foetal human and wild type C57BL/6 mouse and Hspg2 exon 3 null HS deficient mouse intervertebral discs (IVDs). LTBP-2 was a prominent pericellular component of annular fibrochondrocytes in the posterior annulus fibrosus (AF), interstitial matrix adjacent to nucleus pulposus (NP) cells and to fibrillar and cell associated material in the anterior AF of the human foetal IVD and also displayed a pericellular localisation pattern in murine IVDs. Perlecan and LTBP-2 displayed strong pericellular colocalisation patterns in the posterior AF and to fibrillar material in the outer anterior AF in the foetal human IVD. Versican was a prominent fibril-associated component in the posterior and anterior AF, localised in close proximity to fibrillin-1 in fibrillar arrangements in the cartilaginous vertebral rudiments around paraspinal blood vessels, to major collagen fibre bundles in the anterior and posterior AF and shorter fibres in the NP. Fibrillin-1 was prominent in the outer anterior AF of the human foetal IVD and in fibres extending from the AF into the cartilaginous vertebral rudiments. LTBP-2 was prominently associated with annular fibrils containing fibrillin-1, versican was localised in close proximity to these but not specifically with LTBP-2. The similar deposition levels of LTBP-2 observed in the AF of the Hspg2 exon 3 null and wild type murine IVDs indicated that perlecan HS was not essential for LTBP-2 deposition but colocalisation of LTBP-2 with perlecan in the foetal human IVD was consistent with HS mediated interactions which have already been demonstrated in-vitro.

Matrix metalloproteinase-2 degrades fibrillin-1 and fibrillin-2 of oxytalan fibers in the human eye and periodontal ligaments in vitro

Oxytalan fibers are distributed in the eye and periodontal ligaments (PDL). The ciliary zonule, known as Zinn’s zonule, in the eye is composed of oxytalan fibers, which are bundles of microfibrils consisting mainly of fibrillin-1 and fibrillin-2. As turnover of oxytalan fibers is slow during life, their degradation mechanism remains unclarified. This study was performed to examine degradation pattern of fibrillin-1 and fibrillin-2 by experimental MMP activation. We cultured human non-pigmented ciliary epithelial cells (HNPCEC) and PDL fibroblasts for 7 days, then treated them with concanavalin A to activate matrix metalloproteinase (MMP)-2, and examined the degradation of fibrillin-1 and fibrillin-2 for 72 hr using immunofluorescence. At 7 days of HNPCEC culture, fibrillin-1-positive fibers were observed, some of which merged with fibrillin-2. After MMP-2 activation, fibrillin-1-positive fibers became thin and disappeared by 72 hr, while fibrillin-2-positive fibers disappeared almost completely within 24 hr. At 7 days of PDL fibroblast culture, fibrillin-1-positive fibers were mostly merged with fibrillin-2. After MMP-2 activation, fibrillin-1-positive fibers became thin by 24 hr and had almost disappeared by 48 hr, while fibrillin-2-positive fibers decreased constantly after 24 hr. A MMP-2 inhibitor completely suppressed these degradations. These results suggest that the patterns of fibrillin-1 and fibrillin-2 degradation differ between the eye and the PDL, possibly reflecting the sensitivity of fibrillin-1 and fibrillin-2 of each type of oxytalan fiber against MMP-2.

LTBP-2 competes with tropoelastin for binding to fibulin-5 and heparin, and is a negative modulator of elastinogenesis

Latent transforming growth factor-beta-1 binding protein-2 (LTBP-2) is a protein of ill-defined function associated with elastic fibers during elastinogenesis. Although LTBP-2 binds fibrillin-1, fibulin-5, and heparin/heparan sulfate, molecules critical for normal elastic fiber assembly, it does not interact directly with elastin or its precursor, tropoelastin. We investigated the modulating effect of LTBP-2 on two key interactions of tropoelastin during elastinogenesis a) with fibulin-5 and b) with heparan sulfate (using heparin). Firstly, using solid phase assays we showed that LTBP-2 bound fibulin-5 (Kd=26.47±5.68 nM) with an affinity similar to that of the tropoelastin-fibulin-5 interaction (Kd=24.66±5.64 nM). Then using a competitive binding assay we showed that LTBP-2 inhibited the tropoelastin-fibulin-5 interaction in a dose dependent manner with almost complete inhibition obtained with 5-fold molar excess of LTBP-2. Interestingly, a fragment of LTBP-2 containing the fibulin-5 binding sequence only partially inhibited the tropoelasin-fibulin-5 interaction suggesting that LTBP-2 was directly blocking only the C-terminal tropoelastin binding site on fibulin-5 and indirectly blocking tropoelastin binding to the N-terminal region. In parallel experiments heparin was shown to have minor inhibitory effects on fibulin-5 interactions with tropoelastin and LTBP-2. However, LTBP-2 was shown to significantly inhibit the binding of heparin to tropoelastin with 50% inhibition achieved with 10 fold molar excess of LTBP-2. Confocal microscopy of fibroblast matrix showed strong co-distribution of LTBP-2 with fibulin-5 and fibrillin-1 and partial co-distribution with heparan sulfate proteoglycans, perlecan and syndecan-4. Also addition of exogenous LTBP-2 to ear cartilage chondrocyte cultures blocked elastinogenesis in a concentration-dependent manner. Overall the results indicate that LTBP-2 may have a negative regulatory role during elastic fiber assembly, perhaps in displacing elastin microassemblies from complexes with fibulin-5 and/or cell surface heparan sulfate proteoglycans.

Gravity loading induces adenosine triphosphate release and phosphorylation of extracellular signal-regulated kinases in human periodontal ligament cells

AIM

The periodontal ligament (PDL) receives mechanical stress (MS) from dental occlusion or orthodontic tooth movement. Mechanical stress is thought to be a trigger for remodeling of the PDL and alveolar bone, although its signaling mechanism is still unclear. So we investigated the effect of MS on adenosine triphosphate (ATP) release and extracellular signal-regulated kinases (ERK) phosphorylation in PDL cells.

METHODS

Mechanical stress was applied to human PDL cells as centrifugation-mediated gravity loading. Apyrase, Ca(2+)-free medium and purinergic receptor agonists and antagonists were utilized to analyze the contribution of purinergic receptors to ERK phosphorylation.

RESULTS

Gravity loading and ATP increased ERK phosphorylation by 5 and 2.5 times, respectively. Gravity loading induced ATP release from PDL cells by tenfold. Apyrase and suramin diminished ERK phosphorylation induced by both gravity loading and ATP. Under Ca(2+)-free conditions the phosphorylation by gravity loading was partially decreased, whereas ATP-induced phosphorylation was unaffected. Receptors P2Y4 and P2Y6 were prominently expressed in the PDL cells.

CONCLUSION

Gravity loading induced ATP release and ERK phosphorylation in PDL fibroblasts, and ATP signaling via P2Y receptors was partially involved in this phosphorylation, which in turn would enhance gene expression for the remodeling of PDL tissue during orthodontic tooth movement.

Latent transforming growth factor-β binding protein 2 negatively regulates coalescence of oxytalan fibers induced by stretching stress

Oxytalan fibers are extracellular matrix components consisting of pure microfibrils. However, the mechanism whereby oxytalan fibers develop is not fully understood. We have previously reported that in human periodontal ligament (PDL) fibroblasts subjected to stretching stress, bundles of oxytalan fibers coalesce under the control of fibulin-5. Latent transforming growth factor-β binding protein 2 (LTBP-2) is known to bind to fibulin-5. The purpose of this study was to clarify the role of LTBP-2 in the coalescence of oxytalan fibers. We subjected PDL fibroblasts to stretching in order to examine the effects of LTBP-2 on the coalescence of oxytalan fibers in cell/matrix layers. Interaction of LTBP-2 with fibulin-5 was examined by immunoprecipitation assay, and changes in LTBP-2 deposition upon stretching were investigated by Western blotting and immunofluorescence assays. We used small interfering RNA against LTBP-2 in PDL cell culture and examined the appearance of oxytalan fibers on the basis of immunofluorescence. Stretching induced coalescence of oxytalan fibers, but did not affect LTBP-2 expression. The amount of extracellularly deposited LTBP-2 was decreased by about 70% as a result of stretching, compared with the control. LTBP-2 interacted with fibulin-5 on the fibers, and stretching decreased the amount of the LTBP-2 interacted with fibulin-5 by about 60%. Oxytalan fiber coalescence did not occur when LTBP-2 was suppressed by about 95%, whereas it occurred when LTBP-2 was suppressed by about 40%, fibulin-5 being colocalized with oxytalan fibers. These results suggest that LTBP-2, in response to tension stress, may negatively control the function of fibulin-5, thereby modulating the mechanism of oxytalan fiber coalescence.

The oxytalan fibre network in the periodontium and its possible mechanical function

The biomechanical character of the periodontal ligament (PDL) is crucial in its response to functional and orthodontic forces. Collagen has been the primary subject of investigations in this field. Several studies, however, indicate that oxytalan fibres, which belong to the elastic fibre family, also contribute to the biomechanical character and behaviour of the PDL. In order to elucidate this, we have evaluated the available literature on the oxytalan fibre network within the PDL and supra-alveolar tissues with respect to development, morphology and distribution, and response to mechanical stimulation. To this end, we have combined the classical histological studies with more recent in vitro studies. Oxytalan fibres develop simultaneously with the root and the vascular system within the PDL. A close association between oxytalan fibres and the vascular system also remains later in life, suggesting a role in vascular support. Mechanical loading of the PDL, through orthodontic force application, appears to induce an increase in the number, size, and length of oxytalan fibres. In line with this, in vitro stretching of PDL fibroblasts (PDLFs) results in an increased production of fibrillin, a major structural component of the microfibrils that make up oxytalan fibres. The available data suggest a mechanical function for oxytalan, but to date experimental data are limited. Further research is required to clarify their exact mechanical function and possible role in orthodontic tooth movement.

Development of the oxytalan fiber system in the rat molar periodontal ligament evaluated by light- and electron-microscopic analyses

In the elastic fiber system of the periodontal ligaments only oxytalan fibers can be identified, whereas all three types of fibers, oxytalan, elaunin and elastic fibers, are present in the gingiva. However, little information is available concerning their organization in the developing periodontal ligament. In the present study, growth and distribution of the oxytalan fiber system were examined in the developing periodontal ligament of rat molars using the specific staining for oxytalan, elastic and collagen fibers, and electron-microscopic analyses. Oxytalan staining clearly confirmed the earliest oxytalan fibers in a bell-staged tooth germ at embryonic day 18, which were tiny violet-colored fibers in the dental follicle. Their cross images were made up of dot-like microfibrils of 10-15nm in diameter close to fibroblasts in the dental follicle of the rat molars aged 1 day. These microfibrils appeared to be linked to one another through delicate filaments in 3-nm-diameter. At the beginning of root formation, the cross figures of oxytalan fibers were found as dot-like structures around the root sheath as well as in areas very close to blood vessels. As development proceeded, longer oxytalan fibers were produced in the apico-occlusal direction along with blood vessels. In addition, the immunoreactive products to anti-amyloid β protein on the surface of blood vessels suggest that this molecule might be involved in the adhesion of oxytalan fibers to vascular basement membranes. Thus, the oxytalan fiber system might regulate periodontal ligament function through tensional variations registered on the walls of the vascular structures.

EMILIN-1 regulates the amount of oxytalan fiber formation in periodontal ligaments in vitro

The elastic system fibers comprise oxytalan, elaunin, and elastic fibers, differing in their relative microfibril and elastin contents. Among them, human periodontal ligament (PDL) contains only oxytalan fibers (pure microfibrils). Elastin microfibril interface-located protein-1 (EMILIN-1) is localized at the interface between microfibrils and elastin. We hypothesized that EMILIN-1 may contribute to the formation of oxytalan fibers. We used a small interfering RNA (siRNA) for EMILIN-1 in PDL cell culture to examine the extracellular deposition of fibrillin-1 (the major component of microfibrils). EMILIN-1 was labeled on microfibrils positive for fibrillin-1 and was colocalized with fibrillin-1 upon immunoprecipitation assay. EMILIN-1 suppression reduced the level of fibrillin-1 deposition to 23% of the control, and this was responsible for the diminution of fibrillin-1 deposition revealed by immunofluorescence. These results suggest that EMILIN-1 may regulate the formation of oxytalan fibers and play a role in their homeostasis.

The Roles of Angiotensin II in Stretched Periodontal Ligament Cells

The loading caused by occlusion and mastication plays an important role in maintaining periodontal ligament (PDL) tissues. We hypothesized that a loading magnitude would be involved in the production of biological factors that function in the maintenance of PDL tissues. Here, we identified up-regulated gene expressions of transforming growth factor-β1 (TGF-β1), alkaline phosphatase (ALP), and angiotensinogen in human PDL fibroblastic cells (HPLFs) that were exposed to 8% stretch loading. Immunolocalization of angiotensin I/II (Ang I/II), which was converted from angiotensinogen, was detected in rat PDL tissues. HPLFs that were stimulated by Ang II also increased their gene expressions of TGF-β1 and ALP. Furthermore, the antagonist for Ang II type 2 receptor, rather than for type 1, significantly inhibited gene expressions induced by the stretch loading. Analysis of these data suggests that Ang II mediates the loading signal in stretched HPLFs to induce expressions of TGF-β1 and ALP.

Fibulin-4 and -5, but not Fibulin-2, are Associated with Tropoelastin Deposition in Elastin-Producing Cell Culture

Elastic system fibers consist of microfibrils and tropoelastin. During development, microfibrils act as a template on which tropoelastin is deposited. Fibrillin-1 is the major component of microfibrils. It is not clear whether elastic fiber-associated molecules, such as fibulins, contribute to tropoelastin deposition. Among the fibulin family, fibulin-2, -4 and -5 are capable of binding to tropoelastin and fibrillin-1. In the present study, we used the RNA interference (RNAi) technique to establish individual gene-specific knockdown of fibulin-2, -4 and -5 in elastin-producing cells (human gingival fibroblasts; HGF). We then examined the extracellular deposition of tropoelastin using immunofluorescence. RNAi-mediated down-regulation of fibulin-4 and -5 was responsible for the diminution of tropoelastin deposition. Suppression of fibulin-5 appeared to inhibit the formation of fibrillin-1 microfibrils, while that of fibulin-4 did not. Similar results to those for HGF were obtained with human dermal fibroblasts. These results suggest that fibulin-4 and -5 may be associated in different ways with the extracellular deposition of tropoelastin during elastic fiber formation in elastin-producing cells in culture.