The site of collagen resorption in the periodontal ligament of the rodent molar

Phagocytosis of Collagen Fibrils by Fibroblasts In Vivo Is Independent of the uPARAP/Endo180 Receptor

As a crucial step in ECM remodeling, collagen degradation occurs through different processes, including both extracellular and intracellular degradation. The extracellular pathways of collagen degradation require secretion of collagenolytic proteases, whereas intracellular collagen degradation occurs in the lysosomal compartment after uptake, involving either pre-cleaved or intact fibrillar collagen. The endocytic collagen receptor uPARAP/Endo180 plays an important role in internalization of large collagen degradation products, whereas its role in the phagocytosis of fibrillar collagen has been debated. In fact, the role of this receptor in regular collagen phagocytosis in vivo has not been established. In this study, we have studied the role of uPARAP in the phagocytosis of collagen fibrils in vivo by analyzing different connective tissues of mice lacking uPARAP. Using transmission electron microscopy (TEM), we found that fibroblasts in the periosteum of tibia and calvaria, as well as in the periodontal ligament of molar and incisor, phagocytosed collagen fibrils independently of uPARAP. Quantification of phagocytosed collagen in the periodontal ligament of uPARAP-deficient mice and controls revealed no difference in the amount of fibrillar collagen taken up by uPARAP-deficient mice. The findings show that under in vivo conditions uPARAP does not play a role in the phagocytic uptake of collagen fibrils by fibroblasts. Consequently, the cellular uptake of collagen fibrils and collagen cleavage products probably occurs through fundamentally different pathways. J. Cell. Biochem. 118: 1590-1595, 2017. © 2016 Wiley Periodicals, Inc.

Benzo[a]pyrene/aryl hydrocarbon receptor signaling inhibits osteoblastic differentiation and collagen synthesis of human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Cigarette smoking has detrimental effects on periodontal tissue, and is known to be a risk factor for periodontal disease, including the loss of alveolar bone and ligament tissue. However, the direct effects of cigarette smoking on periodontal tissue remain unclear. Recently, we demonstrated that benzo[a]pyrene (BaP), which is a prototypic member of polycyclic aryl hydrocarbons and forms part of the content of cigarettes, attenuated the expression of extracellular matrix remodeling-related genes in human periodontal ligament (PDL) cells (HPDLCs). Thus, we aimed to examine the effects of BaP on the osteoblastic differentiation and collagen synthesis of HPDLCs.

MATERIAL AND METHODS

HPDLCs were obtained from healthy molars of three patients, and quantitative reverse transcription-polymerase chain reaction were performed for gene expression analyses of cytochrome P450 1A1 and 1B1, alkaline phosphatase, bone sialoprotein and aryl hydrocarbon receptor (AhR), a receptor for polycyclic aryl hydrocarbons. We have also analyzed the role of the AhR, using 2-methyl-2H-pyrazole-3-carboxylic acid (2-methyl-4-o-tolylazo-phenyl)-amide (CH-223191), which is an AhR antagonist.

RESULTS

The treatment of HPDLCs with BaP reduced mRNA expression of osteogenic genes, alkaline phosphatase activity, mineralization and collagen synthesis. The treatment with CH-223191 subsequently restored the observed suppressive effects of BaP on HPDLCs.

CONCLUSIONS

The present results suggest that BaP exerts inhibitory effects on the maintenance of homeostasis in HPDL tissue, such as osteoblastic differentiation and collagen synthesis of HPDLCs, and that this signaling pathway could be suppressed by preventing the transactivity of AhR. Future studies may unveil a role for the inhibition of AhR as a promising therapeutic agent for periodontal disease caused by cigarette smoking.

Matrix metalloproteinase 14 is required for fibrous tissue expansion

Type I collagen-containing fibrils are major structural components of the extracellular matrix of vertebrate tissues, especially tendon, but how they are formed is not fully understood. MMP14 is a potent pericellular collagenase that can cleave type I collagen in vitro. In this study, we show that tendon development is arrested in Scleraxis-Cre::Mmp14 lox/lox mice that are unable to release collagen fibrils from plasma membrane fibripositors. In contrast to its role in collagen turnover in adult tissue, MMP14 promotes embryonic tissue formation by releasing collagen fibrils from the cell surface. Notably, the tendons grow to normal size and collagen fibril release from fibripositors occurs in Col-r/r mice that have a mutated collagen-I that is uncleavable by MMPs. Furthermore, fibronectin (not collagen-I) accumulates in the tendons of Mmp14-null mice. We propose a model for cell-regulated collagen fibril assembly during tendon development in which MMP14 cleaves a molecular bridge tethering collagen fibrils to the plasma membrane of fibripositors.

DOI:http://dx.doi.org/10.7554/eLife.09345.001

A scaffold of proteins called the extracellular matrix surrounds each of the cells that make up our organs and tissues. This matrix, which contains fibres made of proteins called collagens, provides the physical support needed to hold organs and tissues together. This support is especially important in the tendons—a tough tissue that connects the muscle to bone—and other ‘connective’ tissues.

An enzyme called MMP14 is able to cut through chains of collagen proteins. It belongs to a family of proteins that are involved in breaking down the extracellular matrix to enable cells to divide and for other important processes in cells. Some cancer cells exploit MMP14 to enable them to leave their tissue of origin and spread around the body. Therefore, when researchers bred mutant mice that lacked MMP14, they expected to see excessive growth of collagen fibres in the connective tissues of the mice. However, these mice actually have extremely thin, fragile connective tissue and die soon after birth.

Earlier in 2015, a group of researchers demonstrated that the first stage of tendon development in mice involves the formation of collagen fibres, which are attached to structures that project from tendon cells called fibripositors. Then, soon after the mice are born, the fibripositors disappear and the collagen fibres are released into the extracellular matrix where they grow longer and become thicker. Now, Taylor, Yeung, Kalson et al.—including some of the researchers from the earlier work—have used electron microscopy to investigate how a lack of MMP14 leads to fragile tendons in young mice.

The experiments show that MMP14 plays a crucial role in the first stage of tendon development by detaching the collagen fibres from the fibripositors. MMP14 also promotes the formation of new collagen fibres; the tendons of mutant mice that lack MMP14 have fewer collagen fibres than normal mice. Further experiments revealed that the release of collagen fibres from fibripositors does not require MMP14 to cleave the chains of collagen proteins themselves. Instead, it appears that MMP14 cleaves another protein that is associated with the fibres, called fibronectin.

Taylor, Yeung, Kalson et al.'s findings show that MMP14 plays an important role in the development of tendons by releasing collagen fibres from fibripositors and promoting the formation of new fibres. The next challenge is to find out how MMP14 regulates the number of collagen fibres in mature tendons and other tissues, and how defects in this enzyme can lead to cancer and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.09345.002

Porphyromonas gingivalis promotes the cell cycle and inflammatory cytokine production in periodontal ligament fibroblasts

OBJECTIVE

The infection of Porphyromonas gingivalis (P. gingivalis) modulates host immune-inflammatory responses and destructs homeostasis of normal cell cycle, thereby leading to periodontal tissue destruction. Human periodontal ligament fibroblasts (PDLFs) are key players in the host immune responses and periodontal tissue regeneration. The aim of the present study was to discover the effects of P. gingivalis infection on the cell cycle and inflammatory cytokine production in PDLFs.

DESIGN

P. gingivalis infection model into PDLFs was established. The effect of P. gingivalis on the cell proliferation and cell cycle were detected by MTT and flow cytometry. The p21, cyclin D1 and cyclin E mRNA expression, p21 protein expression, as well as IL-6 and IL-8 protein levels were analyzed by RT-qPCR, Western blot and ELISA, respectively.

RESULTS

P. gingivalis promoted proliferation and G1 phase of PDLFs. G1 phase promotion was associated with the decreased level of p21 and the up-regulation of cyclin D1 at 6h, and with the increased level of cyclin E at 12h. Simultaneously, the immune-inflammatory response of PDLFs was initiated by P. gingivalis during the initial stage of infection, including the increased expressions of IL-6 and IL-8.

CONCLUSION

We confirmed that the infection of P. gingivalis could modulate the expression of PDLF genes, which control cell cycle and inflammatory cytokine production. Thus, P. gingivalis may contribute to the proliferation and inflammation of periodontal tissue.

Nonmuscle myosin II powered transport of newly formed collagen fibrils at the plasma membrane

Collagen fibrils can exceed thousands of microns in length and are therefore the longest, largest, and most size-pleomorphic protein polymers in vertebrates; thus, knowing how cells transport collagen fibrils is essential for a more complete understanding of protein transport and its role in tissue morphogenesis. Here, we identified newly formed collagen fibrils being transported at the surface of embryonic tendon cells in vivo by using serial block face-scanning electron microscopy of the cell-matrix interface. Newly formed fibrils ranged in length from ~1 to ~30 µm. The shortest (1-10 µm) occurred in intracellular fibricarriers; the longest (~30 µm) occurred in plasma membrane fibripositors. Fibrils and fibripositors were reduced in numbers when collagen secretion was blocked. ImmunoEM showed the absence of lysosomal-associated membrane protein 2 on fibricarriers and fibripositors and there was no effect of leupeptin on fibricarrier or fibripositor number and size, suggesting that fibricarriers and fibripositors are not part of a fibril degradation pathway. Blebbistatin decreased fibricarrier number and increased fibripositor length; thus, nonmuscle myosin II (NMII) powers the transport of these compartments. Inhibition of dynamin-dependent endocytosis with dynasore blocked fibricarrier formation and caused accumulation of fibrils in fibripositors. Data from fluid-phase HRP electron tomography showed that fibricarriers could originate at the plasma membrane. We propose that NMII-powered transport of newly formed collagen fibrils at the plasma membrane is fundamental to the development of collagen fibril-rich tissues. A NMII-dependent cell-force model is presented as the basis for the creation and dynamics of fibripositor structures.

Synthesis of alveolar bone Sharpey's fibers during experimental tooth movement in the rat

There is little information concerning the effects of tooth movement on the relative synthesis of bone matrix and Sharpey's fiber collagenous proteins. The purpose of this study was to investigate this situation using radioautographic techniques. The maxillary first molar tooth in rats was tipped toward the midline using an appliance and the animals were injected with 3H-proline after 3 days and sacrificed 24 hr later. Maxillae were sectioned and silver grain proportional areas (grain density/5,000 microm2) evaluated over Sharpey's fibers and adjacent alveolar bone matrix using computerized densitometry and histomorphometric techniques. These data were compared to a group of untreated animals by Fisher's exact test. At depository surfaces of experimental tissues, the silver grain proportional area over bone matrix was significantly greater than over Sharpey's fibers (P<0.05) and control bone matrix (P<0.01). The silver grain proportional area over Sharpey's fibers was not different between the groups. At resorptive surfaces, the silver grain proportional area over both bone matrix and Sharpey's fibers was significantly greater in experimental tissues compared to controls (P<0.01). Thus, movements of adjacent teeth affect both the quantity and ratios of collagenous protein incorporation into Sharpey's fibers and adjacent alveolar bone, which is dependent on the intensity and characteristics of the force.

The effect of ionic products from bioactive glass dissolution on osteoblast proliferation and collagen production

Bioactive ceramics developed during the past few decades have interesting properties from the biological standpoint, but their effects on cellular events remain partially unknown. In the current work, we investigated cellular viability, proliferation, morphology changes and metabolic activity of rat primary culture osteoblasts in contact with the ionic products from the dissolution of a bioactive glass with 60% of silica (BG60S) and a biphasic calcium phosphate (BCP). We observed that although osteoblasts cultured with BG60S showed vacuole formation, cell viability was increased when compared to BCP and control. The vacuole formation was not due to the presence of high calcium concentration in the ionic products from the dissolution of BG60S and was not related to nitric oxide production from the osteoblasts. We did find that high silicon concentration could induce cellular vacuole formation. Additionally, energy dispersive spectroscopy analysis indicated that vacuole contained 75% more silicon than other regions in the cell outside the vacuole. We further found that collagen production was higher in osteoblast cultured in the presence of BG60S compared to BCP and control, while alkaline phosphatase production was similar among cells incubated with BG60S, BCP and control. Together, our results indicate that osteoblast vacuole formation was due to high silicon contents in the dissolution of BG60S and we can suggest that despite the vacuole formation, there is no significant alteration in the bioceramic cell interaction.

The bone lining cell: its role in cleaning Howship's lacunae and initiating bone formation

In this study we investigated the role of bone lining cells in the coordination of bone resorption and formation. Ultrastructural analysis of mouse long bones and calvariae revealed that bone lining cells enwrap and subsequently digest collagen fibrils protruding from Howship's lacunae that are left by osteoclasts. By using selective proteinase inhibitors we show that this digestion depends on matrix metalloproteinases and, to some extent, on serine proteinases. Autoradiography revealed that after the bone lining cells have finished cleaning, they deposit a thin layer of a collagenous matrix along the Howship's lacuna, in close association with an osteopontin-rich cement line. Collagenous matrix deposition was detected only in completely cleaned pits. In bone from pycnodysostotic patients and cathepsin K-deficient mice, conditions in which osteoclastic bone matrix digestion is greatly inhibited, bone matrix leftovers proved to be degraded by bone lining cells, thus indicating that the bone lining cell "rescues" bone remodeling in these anomalies. We conclude that removal of bone collagen left by osteoclasts in Howship's lacunae is an obligatory step in the link between bone resorption and formation, and that bone lining cells and matrix metalloproteinases are essential in this process.

Collagen-phagocytosing ability of periodontal osteoblasts at the bone surface

The collagen-phagocytosing activity of osteoblasts at the alveolar bone-ligament interface of rat mandibular first molars was investigated both histologically and histochemically. Alveolar bones of male Wistar rats (6 months old) were used in this study. Collagen-containing phagosomes appeared in cuboidal osteoblasts aligned on the bone surface. The 5.7% of the osteoblasts exhibiting alkaline phosphatase activity revealed collagen-containing phagosomes, and the collagen fibrils within the phagosomes were at various stages of degradation. In addition, acid phosphatase activity and the immunocytochemical distribution of cathepsin B were found in these collagen-containing phagosomes at similar locations. The presence of both enzymes in the phagosomes suggests that an intracellular degradation of collagen occurs. Therefore, in addition to the osteoblastic functions of synthesizing and secreting bone matrices, osteoblasts are also capable of phagocytosis and the intracellular disintegration of collagen. Our findings suggest that osteoblasts at the alveolar bone-periodontal ligament interface have a collagen-phagocytosing ability and play an important role in the physiological remodeling and metabolic breakdown of collagen fibrils of periodontal ligament without osteoclastic bone remodeling.

Cytokines modulate routes of collagen breakdown. Review with special emphasis on mechanisms of collagen degradation in the periodontium and the burst hypothesis of periodontal disease progression

In this paper, we review recent work on collagen degradation, 2 main routes of breakdown are described and their relevance during healthy and inflammatory conditions of the periodontium is discussed. Special attention is paid to the possible role of cytokines, in particular interleukin 1 (IL-1) and transforming growth factor beta (TGF-beta), on the modulation of collagen phagocytosis and metalloproteinase production. IL-1 has been shown to have a dual function in collagen digestion. It inhibits the intracellular phagocytic pathway, but at the same time, it strongly promotes extracellular digestion by inducing the release of collagenolytic enzymes like collagenase. TGF-beta has an opposite effect on both pathways and antagonizes IL-1. Collagenase is released in an inactive form, and a considerable fraction of the proenzyme may become incorporated in the extracellular matrix. This reservoir of latent enzyme can be activated (for instance by plasmin), leading to a sudden and extensive breakdown of the collagenous fibre meshwork. It is suggested that this phenomenon may also take place during progressive periodontitis and could explain an episodic nature of collagenolysis, clinically resulting in bursts of attachment loss (burst hypothesis).

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

Cytokines modulate phagocytosis and intracellular digestion of collagen fibrils by fibroblasts in rabbit periosteal explants. Inverse effects on procollagenase production and collagen phagocytosis

Degradation of fibrillar collagen may occur in the extracellular space by enzymes, such as the metalloproteinase collagenase, or in the lysosomal apparatus of fibroblasts following phagocytosis. As the mechanisms involved in the regulation of the latter process are unknown, we investigated possible modulating effects of the cytokines epidermal growth factor (EGF), platelet-derived growth factor (PDGF), interleukin-1 alpha (IL-1 alpha) and transforming growth factor-beta (TGF-beta) on both collagen phagocytosis and the release of collagenase in an in vitro model employing periosteal tissue explants. The data demonstrated that the level of intracellular collagen digestion could be influenced by cytokines: IL-1 alpha inhibited and TGF-beta enhanced phagocytosis of fibrillar collagen by periosteal fibroblasts, whereas the cytokines had an opposite effect on the release of procollagenase. In combination, IL-1 alpha and TGF-beta proved to have an antagonizing effect on either parameter. PDGF and EGF had no effect on phagocytosis or collagenase release. The level of phagocytosed collagen correlated positively with the actual breakdown of collagen as assessed by the release of hydroxyproline but negatively with the level of released procollagenase. Our findings demonstrated that cytokines are able to modulate both the phagocytosis of collagen fibrils by fibroblasts and their subsequent intracellular breakdown, as well as the release of procollagenase, an enzyme considered crucial for extracellular collagenolysis. Moreover, our data show a negative correlation between these two parameters. It is concluded that IL-1 alpha, EGF and TGF-beta may be important in modulating the contribution of the intracellular and extracellular route of collagen breakdown.

Expression of the type I collagen gene in rat periodontal ligament during tooth movement as revealed by in situ hybridization

The in situ hybridization technique used digoxigenin-labelled oligodeoxynucleotide. In untreated molars, cells expressing a positive signal for type I collagen mRNA were distributed uniformly in the periodontal ligament space. After experimental tooth movement, the density of cells expressing a positive signal appeared to be much greater in the tension side than the pressure side. In both sides the distribution of the positively hybridizing cells was uniform along the principal fibres of the ligament. This characteristic distribution appeared at 12 h after the initiation of tooth movement, reached a maximum at 1-3 days, and persisted for about 14 days during the treatment. These results indicate that the remodelling of collagen fibres in periodontal ligament occurs in an orderly manner throughout the principal fibres, mainly on the tension side, and that the recovery of gene expression for type I collagen occurs within the first 14 days in response to experimental tooth movement.

Role of cytokines and inflammatory mediators in tissue destruction

Colonization or emergence of microbial pathogens may result in tissue destruction by activation of one or more of five distinct host degradative pathways (matrix metalloproteinase pathway, plasminogen-dependent pathway, phagocytic pathway, PMN-serine proteinase pathway and osteoclastic bone resorption) or by direct cleavage of extracellular matrix constituents by microbial proteinases. Activation of endogenous destructive pathways may be mediated by immune responses resulting in expression of degradative cellular phenotypes among both immigrant and resident cell populations. In addition, expression of degradative phenotypes may be triggered by direct influences on host cells of microbial products (LPS, enzymes, toxins). A body of evidence suggests that each of these mechanisms involves local production of proinflammatory cytokines and growth factors. The matrix metalloproteinase pathway is centrally involved in dissolution of all unmineralized connective tissues and perhaps in resorption of bone as well. The matrix metalloproteinase family consists of nine or more genetically distinct Zn++ endopeptidases which collectively cleave all of the constituents of the extracellular matrix. Recent studies have uncovered many essential elements of a complex, but still incomplete, regulatory network that governs tissue destruction. Proinflammatory cytokines and growth factors induce signalling pathways several of which are dependent on protein kinase C and result in transient expression of the transcription factors c-jun and c-fos. Initiation of transcription of most matrix metalloproteinase genes requires binding of the transcription factor AP-1 (c-jun/c-fos) to a specific promoter sequence but attainment of maximal transcription rates is dependent on interaction with other promoter elements as well. Several matrix metalloproteinases have been detected in crevicular fluids and tissues of inflamed human gingiva as have the proinflammatory cytokines (IL-1 and TNF-alpha) which regulate their transcription. Although the mere presence of enzymes and cytokines does not necessarily impart function per se, these observations suggest that some level of spatial or temporal linkage exists between metalloproteinase/cytokine expression and gingival inflammation.

Matrix metalloproteinases: a review

Matrix metalloproteinases (MMPs) are a family of nine or more highly homologous Zn(++)-endopeptidases that collectively cleave most if not all of the constituents of the extracellular matrix. The present review discusses in detail the primary structures and the overlapping yet distinct substrate specificities of MMPs as well as the mode of activation of the unique MMP precursors. The regulation of MMP activity at the transcriptional level and at the extracellular level (precursor activation, inhibition of activated, mature enzymes) is also discussed. A final segment of the review details the current knowledge of the involvement of MMP in specific developmental or pathological conditions, including human periodontal diseases.

Multinucleated fibroblastic cells in the periodontal ligaments of aged rats

Using 12- to 18-month-old rats, we examined the ultrastructural and cytochemical features of multinucleated fibroblastic cells (MFCs) in the periodontal ligament (PDL) of molars. In aged rats, the MFCs were distributed randomly in the PDL and exhibited cytoplasmic structural variations which were not dependent on the number of nuclei. There was a tendency for the MFCs to cluster in the PDL. The MFCs, rich in cytoplasmic organelles involved with procollagen synthesis such as rough endoplasmic reticulum and the Golgi apparatus, incorporated and secreted 3H-proline-labelled products. The MFCs also possessed many phagosomes containing intact collagen fibrils. These MFCs were apparently involved in phagocytosis and intracellular degradation of incorporated collagen fibrils. Phagosome-rich MFCs contain acid phosphatase activity in primary and secondary lysosomes, similar or stronger in intensity to that which can be demonstrated in mononuclear fibroblasts. However, unlike mononuclear fibroblasts, the MFCs did not exhibit alkaline phosphatase activity along their plasma membranes. These results suggest that MFCs demonstrate a range of fibroblastic cellular activity, including collagen phagocytosis, and that they may lack certain plasma membrane glycoproteins, which might explain the occurrence of multinucleation in these cells.

Enhanced collagen phagocytosis by rat molar periodontal fibroblasts after topical application of lipopolysaccharide--ultrastructural observations and morphometric analysis

To investigate the effect of lipopolysaccharide (LPS) on phagocytic activity of collagen fibrils by periodontal fibroblasts, we studied rat molar gingival connective tissue and periodontal ligament under light and electron microscopy after topical application of LPS (5 mg/ml in physiological salt solution (PS)) on the gingival sulcus. Phagocytic activity of collagen fibrils by fibroblasts was evaluated by counting the number of collagen-containing vacuoles inside fibroblasts that were present within a defined area (1200 microns2). Values obtained from fibroblasts in the subepithelial connective tissue, the region near the alveolar crest, and the middle region of periodontal tissue were compared. Periodontal ligament fibroblasts showed increased phagocytosis of the collagen fibrils from 3 hours to 1 day after topical LPS application, but no differences were observed in the gingival tissue. The intracytoplasmic vacuoles containing collagen fibrils were of various sizes and shapes, showing positive for acid phosphatase and/or alkaline phosphatase reaction. Collagen phagocytic activity of the fibroblasts in the middle region of the periodontal ligament also increased after PS treatment. However, this was significantly less than that observed in LPS-treated animals (p less than 0.01). This study indicates that LPS may enhance the degradation of collagen by stimulating the phagocytic activity of the periodontal ligament fibroblasts.

Selective inhibition of cysteine proteinases by Z-Phe-AlaCH2F suppresses digestion of collagen by fibroblasts and osteoclasts

Effects of the selective inhibitor of cathepsins B and L, Z-Phe-AlaCH2F were studied on the degradation of fibrillar collagen by fibroblasts and osteoclasts in cultured rabbit calvariae at the electron microscopic level. Periosteal fibroblasts from inhibitor-treated explants showed a dose-dependent increase of the volume fraction of vacuoles containing cross-banded collagen fibrils. This was a 7-fold increase over control fibroblasts and the ratio of intracellular and extracellular collagen increased from 2 to 43. The presence of collagen-containing vacuoles was also found in some osteoclasts from inhibitor-treated explants (1 microM or more). The inhibitor appeared to have cytotoxic effects at a concentration of 100 microM. It was concluded that this selective inhibitor exerts its effects intralysosomally in living cells, indicating possibilities for in vivo inhibition of protein degradation.

Metalloproteinases are not involved in the phagocytosis of collagen fibrils by fibroblasts

The effect of various metalloproteinase-inhibiting compounds on collagen phagocytosis by fibroblasts was studied in cultured periosteal tissue. Evidence is presented indicating that neither anti-collagenase nor anti-stromelysin interfere with the uptake of collagen fibrils from the extracellular space and their intracellular digestion. Similar results were obtained with tissue inhibitor of metalloproteinases (TIMP). In the presence of the proteinase inhibitor leupeptin, a compound which strongly inhibits the intracellular degradation of phagocytosed collagen, a time-dependent increase in the amount of internalized collagen was found. This increase proved to be similar in explants treated as well as in those not treated with the metalloproteinase-inhibiting compounds. It is concluded that enzymes, such as collagenase and stromelysin, do not play a crucial role in the phagocytosis and intracellular digestion of collagen fibrils by fibroblasts. If these enzymes are involved it must be prior to these events. Based on the morphometric data the intralysosomal degradation time of collagen was calculated to be about 30 minutes. A comparison with findings in the literature on collagen metabolism in the periodontal ligament of the rat molar suggests that all collagen degraded may pass through the phagolysome pathway during physiological turnover and remodelling.

Collagen remodeling in wound healing by gingival fibroblasts in vitro

Collagen degradation by fibroblasts was studied in the absence of other cell types to improve our understanding of the mechanisms by which fibroblasts digest collagen. Human gingival fibroblasts were cultured in α -MEM medium for eight weeks. Incisional wounds were made in the fibroblast cultures, and the cells were fixed by different procedures at two days post-wounding. Collagen remodeling has been investigated by tracer experiments and by cytochemical demonstration of acid and alkaline phosphatase activity at the ultrastructural level and stereological analysis in experimental wound-healing in vitro.

The results showed that fibroblasts in the wounded zone exhibited high collagen phagocytic activity, and indicate that fibroblasts have a fundamental role to play in collagen remodeling in wound repair in vitro. This in vitro experimental system is also suggested as a useful model for the analysis of collagen remodeling in wound-healing by fibroblasts.

Collagen degradation in the rabbit skin during short-term tissue culture

Full thickness rabbit skin explants were cultured on plastic dish for 1 week and the sequential morphological changes were examined daily by light and electron microscopy. During the cultured period, bundles of dermal collagen fibres gradually loosened and were removed from the upper dermis and from the cut margin of the explant, which was covered by a sheet of migrating epidermal cells. In these areas, cells containing phagocytosed collagen fibrils were observed from the 3rd day to the end of the culture period. These cells containing phagocytosed collagen fibrils included dermal fibroblasts and macrophages, epidermal keratinocytes and endothelial cells lining blood vessels. The presence of acid phosphatase activity in vacuoles containing the collagen fibrils suggested that intracellular degradation of collagen was occurring. In addition, extracellular collagen degradation was recognized around fibroblasts and beneath the migrating epidermis by the high collagenolytic activity at these sites. These findings suggest that both intra- and extracellular collagen degradation may participate in collagen removal from dermal connective tissue in cultured skin explants.

Localization of acid phosphatase activity in collagen-secreting and collagen-resorbing fibroblasts

The ultrastructural localization of acid phosphatase (ACPase) activity was examined in cultured human gingival fibroblasts in the formative and resorptive phases. In the collagen-secreting fibroblasts, weak ACPase activity was demonstrated in the lysosomes, inner Golgi cisternae, and condensing vacuoles, and none was found in the Golgi-associated endoplasmic reticulum-lysosome system (GERL), presecretory granules, or secretory granules. On the contrary, collagen phagocytosis induced strong ACPase activity in the GERL, which was in addition to the weaker activity found in the same sites as those in the collagen-secreting cells. At the same time, collagen secretion was suppressed, and dense elongated secretory bodies associated with ACPase activity accumulated within the cells. When collagen fibrils had been interiorized in whole or in part within the phagosomes, primary lysosomes derived from the Golgi-GERL complex then fused with them to form phagolysosomes. Collagen degradation occurred within these bodies. The observations indicate significant differences in ACPase activity used as a marker for lysosomal enzyme activities in the different functional phases of fibroblasts. These results suggest that fibroblasts work only one way at a given time, viz., collagen synthesis or collagen degradation.

Collagen phagocytosis by fibroblasts in the periodontal ligament of the mouse molar during the initial phase of hypofunction

This study was undertaken in order to determine whether hypofunction of teeth is associated with changes in collagen phagocytosis by fibroblasts of the periodontal ligament. In mice, the lower right molars were extracted and the animals killed one, two, three, four, or seven days later. The maxillary first molars with their surrounding periodontium were processed for electron microscopy and their periodontal ligament subjected to morphometric analysis. It was observed that, whereas the volume density of extracellular collagen in the ligament of the hypofunctional molars decreased from 50% to 30% during the course of the experiment, the fraction of fibrillar collagen ingested by the cells increased over two-fold. This increase was already manifest very shortly after the onset of the experiment and offers an explanation for the net loss of collagen fibrils from the extracellular space.

The influence of the estrous cycle on the volume density and appearance of collagen containing vacuoles in fibroblasts of the rat uterus

The influence of the estrous cycle on the number of intracellular collagen-containing vacuoles (CCVs) has been studied in the uterus of the virgin Wistar rat. CCVs seem to be involved in two processes. Translucent CCVs dark CCVs and dark residual bodies appear to be involved in collagen uptake and degradation, whilst the so-called filamentous bodies (FBs), which are large structures filled with amorphous filaments may be concerned with collagen synthesis. The volume density of these FBs is influenced by the estrous cycle and peak values are accompanied by the highest values of extracellular collagen.

The role of microtubules in the phagocytosis of collagen by fibroblasts

The effects of the anti-microtubular agents, colchicine and vinblastine, on the phagocytosis of collagen by fibroblasts were assessed quantitatively in cultured mouse bone explants. It was found that in the absence of the microtubular system the volume density of lysosomal vacuoles containing cross-banded collagen fibrils in periosteal cells did not differ from that seen in controls. In contrast, cytochalasin B which interferes with the microfilament system prevented the accumulation of collagen-containing vacuoles in the cytoplasm. The data indicate that the phagocytosis of collagen fibrils by fibroblasts does not depend on the integrity of the microtubular apparatus, but seems to require an intact microfilament system.

The effect of tension on collagen remodelling by fibroblasts: a stereological ultrastructural study

Peak levels of collagen phagocytic activity by fibroblasts during scar maturation have been shown to coincide with a reorientation of major fibre bundles in a process analogous to Wolff's Law. This activity has been shown to increase when greater tension exists across the scar. In this study, the influences of reduced tension on collagen remodeling was studied. In two groups of five rats the right achilles tendon was either: transected at its muscle insertion or was exposed in a similar fashion but left intact. The tendon samples were retrieved five days following initial surgery with a third group of five rats serving as a source of control tendon. Relative volume of collagen phagocytosis was measured at an ultrastructural level using stereological methods. No significant influence of reduced tension on collagen phagocytosis was observed. The average relative volume of fibroblast cytoplasm was significantly increased in the transected tendon. This may represent an inductive influence of as yet undetermined nature.

Immunocytochemical and biochemical characterization of the connective tissue component of fibrous papular lesions of oral mucosa

In an attempt to define the fibroblastic components of fibrous papular lesions of oral mucosa, 21 cases representing these lesions were selected for study. Paraffin or frozen sections of the lesions were stained, using the immunoperoxidase technique, with antibodies to: alpha-l-antitrypsin, alpha-l-antichymotrypsin, lysozyme, fibronectin, actin, myosin, and for Clq binding. Three of these cases were biochemically analyzed for collagen Types I, III, and V. This study demonstrated the presence of cytochemical markers in fibrous papules that were similar to those observed for the compartment of circumvascular fibroblasts in control normal mucosa. Analysis of the collagens present indicated that in addition to histologic similarities, the gene products of these extracellular matrices were similar to those reported for angiofibromas and Shagreen patches in tuberous sclerosis. The cells in these lesions appear to be distinct from myofibroblasts and the high affinity complement binding fibroblasts characterized by Bordin et al.(1).

The digestion of phagocytosed collagen is inhibited by the proteinase inhibitors leupeptin and E-64

Using morphometric methods the effects of the thiol-proteinase inhibitors leupeptin and E-64 on the digestion of intracytoplasmic collagen fibrils were studied in cultured mouse bone explants. Both drugs caused a dose-dependent increase of lysosomal structures containing cross-banded collagen fibrils (CCV) in periosteal fibroblasts. After an incubation period of 48 hours, leupeptin (in a concentration of 65 microM) caused a thirty-fold increase in the volume fraction of CCV. This effect proved to be reversible following upon the withdrawal of the drug. Since the leupeptin-related accumulation of intracellular collagen fibrils was not significantly inhibited by alpha, alpha dipyridyl (a drug that interferes with collagen fibril formation), it is thought unlikely that the fibrils represented newly synthesized collagen. This view is further substantiated by data obtained from explants incubated in the presence of the phagocytosis-inhibiting agent cytochalasin B. This compound completely inhibited the leupeptin-related accumulation of CCV. The data strongly suggest that collagen fibrils found in cytoplasmic vacuoles of periosteal fibroblasts represent collagen taken up by phagocytosis, the integrity of cytoplasmic actin filament systems is a prerequisite for phagocytosis of collagen to occur, and thiol-proteinases, such as cathepsin B, L, and/or N, play an essential role in the digestion of internalized collagen.

Intracellular collagen fibers in the capsule around silicone expanders in guinea pigs

Ultrastructural studies of fibrous capsules surrounding silicone tissue expanders in guinea pigs revealed a number of fibroblasts containing collagen fibers inside cytoplasm with typical periodicity. These fibers were single or multiple, appeared straight, coiled, or bent, and lay in narrow, undulating membrane spaces. These intracellular collagen fibers were found in as many as 15% of the cells in capsules between 7 and 12 weeks of expansion. These observations suggest that during capsule development there is some imbalance between the synthesis of collagen fibers and their degradation. It is possible that increased synthesis of collagen fibers as well as their phagocytosis by fibroblasts may exist simultaneously.

A role for collagen phagocytosis by fibroblasts in scar remodeling: an ultrastructural stereologic study

A role for collagen phagocytosis and intracellular degradation by fibroblasts during remodeling activity has been suggested by studies on several connective tissues characterized by high rates of collagen turnover and remodeling. The possible importance of such activity in the normal remodeling of scar tissue has been studied by a quantitative ultrastructural stereologic measure of collagen phagocytosis by fibroblasts at various post-wounding intervals in mouse skin scars. The results demonstrate a correlation between the peak periods of such phagocytic activity and the interval during which collagen fiber reorientation across the scar appears to take place.

Cell density and cell generation in the periodontal ligament of mice

The number of cell nuclei per mm2 and the volume density of cell nuclei and blood vessels in the periodontal ligament at different levels of the mesial root of the first mandibular molar of the adult mouse and in different areas of the ligament at each level have been examined in the light microscope. Significantly higher numbers of cell nuclei per mm2 were observed adjacent to bone, cementum, and blood vessels than in the avascular body of the ligament at all levels and on all aspects of the root. This distribution of number of cell nuclei per mm2 was constant over 4 1/2 months of aging and a doubling of body weight. The volume density of cell nuclei was significantly higher in cells adjacent to bone and cementum and in gingival connective tissue than in both the vicinity of blood vessels and the body of the ligament. The blood vessels, which were present predominantly in the bone-related half of the ligament, were absent from the zone immediately adjacent to cementum. The labeling indexes of periodontal ligament cells were determined from autoradiographs of the mesial root of the first mandibular molar of the mouse after pulse-labeling with 3H-Tdr. Labeling indexes were highest in zones adjacent to blood vessels, and the labeling index was significantly higher in the middle of the ligament than in zones adjacent to bone and cementum, and consequently was inversely related to cell density.

Collagen fibril diameter and its relation to collagen turnover in three soft connective tissues in the rat

Collagen fibril diameters were measured in electron micrographs of rat skin, gingiva and periodontal ligament. Gingiva was divided into two zones, termed elastin-containing gingiva and attached gingiva, depending on the presence or absence of elastic fibrils. The results revealed that skin had the largest fibrils, followed by elastin-containing gingiva, attached gingiva and periodontal ligament respectively. These differences in fibril diameter were highly significant. The observed trend in fibril diameter was the inverse of that documented for collagen turnover and collagen phagocytosis in the same tissues. A link between fibril diameter and collagen turnover is discussed.

Crosslink analysis as an indicator of collagen turnover in periodontal ligament from functioning and non-functioning teeth in the dog

The assay of transiently-occurring reducible collagen crosslinks was used as an indication of collagen turnover in the periodontium from individual teeth. A tooth was removed on one side of the jaw to relieve the antagonist tooth of all occlusal forces. This did not influence the very high rate of collagen turnover observed in periodontal ligaments. It is concluded that this high rate of turnover is an intrinsic property of the tissue and is uninfluenced by occlusal or eruptive forces.

A stereologic analysis of collagen phagocytosis by fibroblasts in three soft connective tissues with differing rates of collagen turnover

The purpose of this study was to assess the importance of the phagocytic mechanism of collagen resorption in the normal turnover and remodelling of soft connective tissues. Collagen phagocytosis by fibroblasts in rat skin, attached gingiva, and periodontal ligament was quantitated using the methodology of electron microscopic stereology. Periodontal ligament contained five and 15 times as much phagocytosed collagen as attached gingiva and skin respectively. Also, for each tissue examined, a positive correlation was observed between the amount of collagen phagocytosed and the known rate of mature collagen turnover.

Collagen resorption by periodontal ligament fibroblasts at the hard tissue-ligament interfaces of the mouse periodontium

Electron microscopic examination of the periodontal ligament of the mouse molar at the ligament-cementum and ligament-bone interfaces revealed fibroblasts apparently engaged in the severing and phagocytosis of ligament collagen fibrils originally anchored to bone or cementum as "Sharpey's fibers".

Unipolarity of fibroblasts in rodent periodontal ligament

In order to investigate whether fibroblasts in rodent periodontal ligament have a structural polarity, the position of the Golgi apparatus or the centriolar region in the cells was studied using light and electron microscopy. It appeared that, in the periodontal ligament of continuously erupting mouse and rat incisors, centrioles in fibroblasts on the tooth side of the ligament are preferably located in the anterior (occlusally directed) part of the cytoplasm. Polarity of fibroblasts in a single direction was less pronounced or absent on the bone side of the tissue. In the mouse, fibroblasts in the connective tissue adjacent to the incisor also contained an extensive system of cytoplasmic microtubules, whereas in the fibroblasts on the bone side of the ligament microtubules were less frequent. Unipolarity of fibroblasts was also observed in the periodontal ligament of the rat maxillary first molar, which is characterized by a limited eruption. Here, the Golgi region was usually situated in that pole of the cells that was directed towards the alveolar wall and the occlusal plane. It is suggested that structural polarity of fibroblasts in the periodontal ligament of rodent teeth is associated with orientation of functional activities of the cells, such as unidirectional movement or unidirectional deposition or phagocytosis of collagen.