Gene expression and accumulation of fibrillin-1, fibrillin-2, and tropoelastin in cultured periodontal fibroblasts

Fibrillin protein, a candidate for creating a suitable scaffold in PDL regeneration while avoiding ankylosis

The tooth is stabilized by fiber-rich tissue called the periodontal ligament (PDL). The narrow space of the PDL does not calcify in the physiological state even thought it exists between two calcified tissues, namely, the cementum of the root and alveolar bone. Two situations that require PDL regeneration are periodontitis and dental trauma. Periodontitis induces the loss of PDL and alveolar bone due to inflammation related to infection. Conversely, in PDLs damaged by dental trauma, accelerating bone formation as an overreaction of the healing process is induced, thereby inducing dentoalveolar ankylosis at the tooth root surface. PDL regeneration following dental trauma must therefore be considered separately from periodontitis. Therefore, PDL regeneration in dental trauma must be considered separately from periodontitis. This review focuses on the components involved in avoiding dentoalveolar ankylosis, including oxytalan fibers, aggregated microfibrils, epithelial cell rests of Malassez (ERM), and TGF-β signaling. During root development, oxytalan fibers produced by PDL cells work in collaboration with the epithelial components in the PDL (e.g., Hertwig's root sheath [HERS] and ERM). We herein describe the functions of oxytalan fibers, ERM, and TGF-β signals which are involved in the avoidance of bone formation.

Fibrillin-2 Degradation by Matrix Metalloproteinase-2 in Periodontium

Elastic system fibers, comprised of microfibrils and tropoelastin, are extracellular components of periodontal tissue. During development, the microfibrils act as a template on which tropoelastin is deposited. However, the process of elastic system fiber remodeling is not fully understood. Therefore, we examined whether matrix metalloproteinases (MMPs) are involved in the remodeling of fibrillins (major components of microfibrils) by human gingival fibroblasts and periodontal ligament (PDL) fibroblasts. Gingival and PDL fibroblasts were cultured for 6 weeks. In some cultures, MMP inhibitor or tissue inhibitor of matrix metalloproteinsase-2 (TIMP-2) was added to the medium for an additional 2 weeks. Active MMP-2 (62 kDa) appeared as cell-membrane-associated or in extracellular matrix only in PDL fibroblast cell layers. The addition of MMP inhibitor or TIMP-2 significantly increased fibrillin-2 accumulation in PDL fibroblast cell layers, and decreased the amount of fibrillin-2 fragments, suggesting that active MMP-2 may degrade fibrillin-2, and that MMPs may play a role in the remodeling of elastic system fibers in PDL.

Immunohistochemical expression of fibrillin-1 and fibrillin-2 during tooth development

BACKGROUND AND OBJECTIVE

Oxytalan fibers are categorized as a microfibril assembly without elastin deposition, and are unique components in the periodontal ligament (PDL). However, little is known about their formation during PDL development. To clarify the mechanisms of oxytalan fiber formation in developing PDL, we performed immunohistochemical analysis to detect the direct expression of fibrillin-1 and fibrillin-2, which are major components of microfibrils.

MATERIAL AND METHODS

Frozen sections of lower molars from mice at several stages of growth were prepared without chemical fixation and decalcification using the film transfer method. Immunostaining was performed with anti-fibrillin-1 and -2, and anticytokeratin antibodies.

RESULTS

Fibrillin-1 was not expressed in the dental follicle during the crown forming stage. At postneonatal day 9, fibrillin-1 expression started with meshwork appearance between the epithelial cells from Hertwig's epithelial root sheath at the root dentin surface. Fibirillin-2 was detected much earlier than fibrillin-1 expression. Fibrillin-2 was expressed with a liner appearance, running parallel to the root axis in PDL, and was partially co-expressed with cytokeratin 14 expression in Hertwig's epithelial root sheath. Furthermore, we detected both fibrillin-1 and fibrillin-2 expression in human PDL. Fibrillin-1 was detected in fibers with a vertically oriented root axis in PDL. Fibrillin-2 was widely expressed in PDL, including around the epithelial cell rests of Malassez. Fibrillin-1 and fibrillin-2 were clearly co-expressed in thick fiber structures in human PDL.

CONCLUSION

Our results suggest that both fibrillin-1 and fibrillin-2 expression is required to form thick oxytalan fibers in PDL. Based on the expression patterns for fibrillin-1 and fibrillin-2, they have different functions during tooth root and PDL development. Early expression of fibrillin-2 may regulate dental epithelial cell behavior during root and PDL development.

Microfibril-associated glycoprotein-1 controls human ciliary zonule development in vitro

The ciliary zonule in the eye, also known as Zinn’s zonule, is composed of oxytalan fibers, which are bundles of microfibrils consisting mainly of fibrillin-1. However, it is still unclear which of the microfibril-associated molecules present in the ciliary zonule controls oxytalan fibers. Microfibril-associated glycoprotein-1 (MAGP-1) is the only microfibril-associated molecule identified in the human ciliary zonule. In the present study, we used siRNA against MAGP-1 in cultures of human non-pigmented ciliary epithelial cells to examine the extracellular deposition and appearance of fibrillin-1 employing Western blotting and immunofluorescence. MAGP-1 suppression led to a reduction of fibrillin-1 deposition. Immunofluorescence also confirmed that RNAi-mediated down-regulation of MAGP-1 led to suppression of fiber development. These results suggest that MAGP-1 plays a crucial role in the extracellular deposition of fibrillin-1 during formation of the human ciliary zonule.

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.

ADAMTSL6β protein rescues fibrillin-1 microfibril disorder in a Marfan syndrome mouse model through the promotion of fibrillin-1 assembly

Marfan syndrome (MFS) is a systemic disorder of the connective tissues caused by insufficient fibrillin-1 microfibril formation and can cause cardiac complications, emphysema, ocular lens dislocation, and severe periodontal disease. ADAMTSL6β (A disintegrin-like metalloprotease domain with thrombospondin type I motifs-like 6β) is a microfibril-associated extracellular matrix protein expressed in various connective tissues that has been implicated in fibrillin-1 microfibril assembly. We here report that ADAMTSL6β plays an essential role in the development and regeneration of connective tissues. ADAMTSL6β expression rescues microfibril disorder after periodontal ligament injury in an MFS mouse model through the promotion of fibrillin-1 microfibril assembly. In addition, improved fibrillin-1 assembly in MFS mice following the administration of ADAMTSL6β attenuates the overactivation of TGF-β signals associated with the increased release of active TGF-β from disrupted fibrillin-1 microfibrils within periodontal ligaments. Our current data thus demonstrate the essential contribution of ADAMTSL6β to fibrillin-1 microfibril formation. These findings also suggest a new therapeutic strategy for the treatment of MFS through ADAMTSL6β-mediated fibrillin-1 microfibril assembly.

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.

Fibulin-5 contributes to microfibril assembly in human periodontal ligament cells

The elastic system fibers comprise oxytalan, elaunin and elastic fibers, which differ in their relative microfibril and elastin content. Human periodontal ligaments (PDL) contain only oxytalan fibers (pure microfibrils) among them. Since fibulin-5 regulates the organization of elastic fibers to link the fibers to cells, we hypothesized that fibulin-5 may contribute to the formation of oxytalan fibers. We used siRNA for fibulin-5 in PDL cell culture to examine the extracellular deposition of fibrillin-1 and -2, which are the major components of microfibrils. Fibulin-5 was labeled on microfibrils positive for fibrillin-1 and -2. Fibulin-5 suppression reduced the level of fibrillin-1 and -2 deposition to 60% of the control level. These results suggest that fibulin-5 may control the formation of oxytalan fibers, and play a role in the homeostasis of oxytalan fibers.

Retrovirally mediated overexpression of glycosaminoglycan-deficient biglycan in arterial smooth muscle cells induces tropoelastin synthesis and elastic fiber formation in vitro and in neointimae after vascular injury

Galactosamine-containing glycosaminoglycans (GAGs), such as the chondroitin sulfate chains of the proteoglycan versican, have been shown to inhibit elastogenesis. Another proteoglycan that may influence elastogenesis is biglycan, which possesses two GAG chains. To assess the importance of these chains on elastogenesis in blood vessels, rat aortic smooth muscle cells were transduced with a GAG-deficient biglycan cDNA-containing retroviral vector (LmBSN). Control cells were transduced with either biglycan or empty vector. Transduced cells were characterized in vitro and then seeded into balloon-injured rat carotid arteries to determine the effects on neointimal structure. Cultured cells overexpressing LmBSN showed marked up-regulation of tropoelastin and fibulin-5 mRNAs, increased amounts of desmosine and insoluble elastin, and increased deposition of elastic fibers as compared with empty vector- and biglycan-transduced cells. Conversely, collagen alpha(1) synthesis and the deposition of collagen fibers were both markedly decreased in LmBSN cultures. In vivo, neointimae formed from cells that overexpressed LmBSN and showed increased deposits of elastin that aggregated into parallel nascent fibers, generally arranged circumferentially. Neointimae that formed from cells with biglycan or empty vector contained fewer and less aggregated deposits of elastin. These findings suggest that the GAG chains of biglycan serve as inhibitors of elastin synthesis and assembly, and that biglycan can act as an important modulator of the composition of the extracellular matrix of blood vessels.

Integrin alphavbeta3 regulates microfibril assembly in human periodontal ligament cells

Fibrillin-1 is the major structural component of extracellular microfibrils. However, the mechanism by which extracellular fibrillin-1 assembles into microfibrils is not fully understood. Fibrillin-1 contains the Arg-Gly-Asp (RGD) motif, which may allow binding to RGD-recognizing integrins. We hypothesized that integrin alphavbeta3 on the cell surface of human periodontal ligament (PDL) fibroblasts may influence fibrillin-1 assembly into cell/matrix layers. We treated PDL fibroblasts with an integrin alphavbeta3-specific antagonist to examine fibrillin-1 assembly. Western blotting and immunofluorescence analysis showed that treatment with the integrin alphavbeta3 antagonist at 5 muM clearly abolished fibrillin-1 deposition. These results provide for the first time evidence that integrin alphavbeta3 regulates extracellular assembly of fibrillin-1, thereby modulating cell-mediated homeostasis of microfibrils.

Microfibril-associated glycoprotein-1 and fibrillin-2 are associated with tropoelastin deposition in vitro

Elastic system fibers consist of microfibrils and tropoelastin. During development, microfibrils act as a template on which tropoelastin is deposited. Microfibril-associated glycoprotein-1 (MAGP-1) and fibrillin-2, the major components of microfibrils, provide the likely template for tropoelastin deposition. In this study, we used the RNA interference (RNAi) technique to establish MAGP-1 and fibrillin-2 gene-specific knock-downs individually in elastin-producing cells (human gingival fibroblasts). We then examined the extracellular deposition of tropoelastin by western blotting. These two genes were specifically suppressed to < 30% of the control level, and this was responsible for the diminution of tropoelastin deposition. An immunofluorescence study also confirmed that RNAi-mediated down-regulation of MAGP-1 or fibrillin-2 led to the loss of tropoelastin immunoreactivity. These results suggest that MAGP-1 and fibrillin-2 are, directly or indirectly, associated with the extracellular deposition of tropoelastin during elastic fiber formation in human gingival fibroblasts in vitro.

Induction of fibulin-5 gene is regulated by tropoelastin gene, and correlated with tropoelastin accumulation in vitro

Fibulin-5 (also known as DANCE) is an elastin-binding protein that is thought to play a role in elastogenesis. We examined the relationship between the gene expression of fibulin-5 and the gene expression and accumulation of tropoelastin by comparing elastin-producing cells (human gingival fibroblasts) with non-elastin-producing cells (human periodontal ligament fibroblasts) by Northern blot analysis. Fibulin-5 gene induction was found only in elastin-producing cells. Induction of the fibulin-5 gene in elastin-producing cells occurred after induction of the tropoelastin gene, and the fibulin-5 level was reduced upon RNA interference-mediated down-regulation of tropoelastin. Fibulin-5 gene induction was also correlated with a rapid increase of tropoelastin accumulation within the cell layer. These results may suggest that the fibulin-5 gene induction is directly or indirectly regulated by tropoelastin gene expression and plays a role in the accumulation of elastic fibers within matrices.