Phagocytosis and digestion of collagen by gingival fibroblasts in vivo: a study of serial sections

Overexpression of V-ATPase B2 attenuates lung injury/fibrosis by stabilizing lysosomal membrane permeabilization and increasing collagen degradation

Excessive oxidative stress causes lysosomal membrane permeabilization (LMP), which leads to cell death. Vacuolar ATPase (V-ATPase) is the enzyme responsible for pumping H⁺ into the cytosol and thus maintaining intracellular pH. Previously, we reported that V-ATPase B2 subunit expression is upregulated in the TiO₂-exposed lung epithelium. We investigated the role of the lysosomal V-ATPase B2 subunit in oxidative stress-induced alveolar epithelial cell death and in an experimental lung injury/fibrosis model. Overexpression of V-ATPase B2 increased lysosomal pH and lysosomal activities in the cells. In the presence of H₂O₂, overexpression of V-ATPase B2 increased survival, and silencing of V-ATPase B2 dramatically increased cell death. Overexpression of V-ATPase B2 diminished H₂O₂-triggered LMP, as evidenced by a reduction in acridine orange staining and leakage of cathepsin D from the lysosome to the cytoplasm. In addition, V-ATPase B2-overexpressing macrophages exhibited significantly enhanced uptake and degradation of collagen. V-ATPase B2-overexpressing transgenic mice showed significant inhibition of the bleomycin-induced increases in lung inflammation and fibrosis. We conclude that V-ATPase B2 is critical for maintaining lysosomal activities against excessive oxidative stress by stabilizing LMP. Our findings reveal a previously unknown role of this V-ATPase subunit in a lung injury and fibrosis model.

An enzyme involved in maintaining the correct pH inside the lysosome, an organelle involved in disposal of cellular waste, also plays a critical role in preventing lung injury. Using human lung cells and mouse models of lung fibrosis, a team led by Sung Woo Park from Soonchunhyang University Bucheon Hospital, South Korea, showed that a overexpression of subunit of the vacuolar ATPase enzyme B2, which pumps protons into cellular compartments to create more acidic environments, helps to boost lysosomal activities that lead to prevent oxidative stress-induced cell death and alleviate experimental lung injury/fibrosis. In addition, V-ATPase B2 overexpressed macrophages increased collagen uptake and degradation activities. The findings point to the vacuolar ATPase, and its B2 subunit in particular, as a promising drug target for future treatments of pulmonary fibrosis.

How Tony Melcher advanced our understanding of periodontal biology and regeneration

Tony Melcher, a highly influential and forward-thinking scientist and teacher, focussed on the origins, behaviour and regulation of cells in periodontal tissues. His recent death in April 2020, has motivated us to highlight his multi-level contributions to research in biology and the dental sciences. Tony was particularly adept at recognizing the inherent instructive power of the periodontium, most notably as a model system for studying the inter-relationships between the structure, development and functions of connective tissues. Further, his mentoring of dozens of students who subsequently went on to develop their own careers in research, and his leadership in promoting collaborations in dental sciences world-wide, engendered important advances in the importance and utility of research relating to oral tissues. Here, we reflect upon his development of a large, multi-disciplinary research enterprise, the MRC Group in Periodontal Physiology at the University of Toronto and brief commentaries of those who worked with him there. We examine his early career development and then go on to consider some of his most highly cited publications and their impact on subsequent research trends.

Role of the small GTPase activating protein IQGAP1 in collagen phagocytosis

Many adult connective tissues undergo continuous remodeling to maintain matrix homeostasis. Physiological remodeling involves the degradation of collagen fibers by the intracellular cathepsin-dependent phagocytic pathway. We considered that a multidomain, small GTPase activating protein, IQGAP1, which is involved in the generation of cell extensions, is required for collagen phagocytosis, possibly arising from its interactions with cdc42 and the actin-binding protein Flightless I (FliI). We examined the role of IQGAP1 in collagen phagocytosis by human gingival fibroblasts (HGFs) and by IQGAP1+/+ and IQGAP1-/- mouse embryonic fibroblasts. IQGAP1 was strongly expressed by HGFs, localized to vinculin-stained cell adhesions and sites where cell extensions are initiated, and colocalized with FliI. Immunoprecipitation showed that IQGAP1 associated with FliI. HGFs showed 10-fold increases of collagen binding, 6-fold higher internalization, and 3-fold higher β1 integrin activation between 30 and 180 min after incubation with collagen. Compared with IQGAP1+/+ fibroblasts, deletion of IQGAP1 reduced collagen binding (1.4-fold), collagen internalization (3-fold), β1 integrin activation (2-fold), and collagen degradation (1.8-fold). We conclude that IQGAP1 affects collagen remodeling through its regulation of phagocytic degradation pathways, which may involve the interaction of IQGAP1 with FliI.

Flightless anchors IQGAP1 and R-ras to mediate cell extension formation and matrix remodeling

Tractional remodeling of collagen fibrils by fibroblasts requires long cell extensions that mediate fibril alignment. The formation of these cell extensions involves flightless I (FliI), an actin-binding protein that contains a leucine-rich-repeat (LRR), which binds R-ras and may regulate cdc42. We considered that FliI interacts with small GTPases and their regulators to mediate assembly of cell extensions. Mass spectrometry analyses of FliI immunoprecipitates showed abundant Ras GTPase-activating-like protein (IQGAP1), which in immunostained samples colocalized with FliI at cell adhesions. Knockdown of IQGAP1 reduced the numbers of cell extensions and the alignment of collagen fibrils. In experiments using dominant negative mutants, cdc42 activity was required for the formation of short extensions while R-ras was required for the formation of long extensions. Immunoprecipitation of wild-type and mutant constructs showed that IQGAP1 associated with cdc42 and R-ras; this association required the GAP-related domain (1004–1237 aa) of IQGAP1. In cells transfected with FliI mutants, the LRR of FliI, but not its gelsolin-like domains, mediated association with cdc42, R-ras, and IQGAP1. We conclude that FliI interacts with IQGAP1 and co-ordinates with cdc42 and R-ras to control the formation of cell extensions that enable collagen tractional remodeling.

Vertical ridge augmentation using a porous composite of uncalcined hydroxyapatite and poly-DL-lactide enriched with types 1 and 3 collagen

Background

Previous studies have shown that porous composite blocks containing uncalcined hydroxyapatite (u-HA; 70 wt%) with a scaffold of poly-DL-lactide (PDLLA, 30 wt%) are biodegradable, encourage appropriate bone formation, and are suitable for use as a bone substitute in vertical ridge augmentation. The present study aimed to accelerate osteogenesis in vertical ridge formation by adding types 1 and 3 collagen to the u-HA/PDLLA blocks and assessing the effect.

Material and methods

The bone substitute in the present study comprised porous composite blocks of u-HA (70 wt%) with a PDLLA (27–29 wt%) scaffold and enriched with types 1 and 3 collagen (1.7 ~ 3.4 wt%). The control blocks were composed of u-HA (70 wt%) and PDLLA (30 wt%). The materials were formed into 8-mm diameter, 2-mm high discs and implanted onto the cranial bones of six rabbits. The animals were sacrificed 4 weeks after implantation, and histological and histomorphometrical analyses were performed to quantitatively evaluate newly formed bone.

Results

New bone formation occurred with both block types, showing direct contact with the original bone. Mean ± standard deviation bone formation was significantly greater in the experimental blocks (25.6% ± 4.8%) than in the control blocks (17.0% ± 4.7%).

Conclusions

Histological and histomorphometrical observations indicated that new bone was formed with both block types. The u-HA/PDLLA block with types 1 and 3 collagen is a more promising candidate for vertical ridge augmentation than the u-HA/PDLLA alone block.

Novel proteins that regulate cell extension formation in fibroblasts

Cell extensions are critical structures that enable matrix remodeling in wound healing and cancer invasion but the regulation of their formation is not well-defined. We searched for new proteins that mediated cell extension formation over collagen by tandem mass tagged mass spectrometry analysis of purified extensions in 3T3 fibroblasts. Unexpectedly, importin-5, ENH isoform 1b (PDLIM5) and 26 S protease regulatory subunit 6B (PSMC4) were more abundant (> 10-fold) in membrane-penetrating cell extensions than cell bodies, which was confirmed by immunostaining and immunoblotting and also observed in human gingival fibroblasts. After siRNA knockdown of these proteins and plating cells on grid-supported floating collagen gels for 6 h, formation of cell extensions and collagen remodeling were examined. Knockdown of importin-5 reduced collagen compaction (1.9-fold), pericellular collagen degradation (~ 1.8-fold) and number of cell extensions (~ 69%). Knockdown of PSMC4 reduced collagen compaction (~ 1.5-fold), pericellular collagen degradation (~ 1.7-fold) and number of cell extensions (~ 42%). Knockdown of PDLIM5 reduced collagen compaction (~ 1.6-fold) and number of cell extensions (~ 21%). Inhibition of the TGF-β RI kinase, Smad3 or ROCK-II signaling pathways reduced the abundance of PDLIM5 in cell extensions but PSMC4 and importin-5 were reduced only by Smad3 or ROCK-II inhibitors. We conclude that these novel proteins are required for cell extension formation and their recruitment into extensions involves the Smad3 and ROCK signaling pathways.

TRPV4 mediates the Ca2+ influx required for the interaction between flightless-1 and non-muscle myosin, and collagen remodeling

Fibroblasts remodel extracellular matrix collagen, in part, through phagocytosis. This process requires formation of cell extensions, which in turn involves interaction of the actin-binding protein flightless-1 (FliI) with non-muscle myosin IIA (NMMIIA; heavy chain encoded by MYH9) at cell-matrix adhesion sites. As Ca²⁺ plays a central role in controlling actomyosin-dependent functions, we examined how Ca²⁺ controls the generation of cell extensions and collagen remodeling. Ratio fluorimetry demonstrated localized Ca²⁺ influx at the extensions of fibroblasts. Western blotting and quantitative (q)PCR showed high expression levels of the Ca²⁺-permeable transient receptor potential vanilloid-4 (TRPV4) channel, which co-immunoprecipitated with β1 integrin and localized to adhesions. Treatment with α2β1-integrin-blocking antibody or the TRPV4-specific antagonist AB159908, as well as reduction of TRPV4 expression through means of siRNA, blocked Ca²⁺ influx. These treatments also inhibited the interaction of FliI with NMMIIA, reduced the number and length of cell extensions, and blocked collagen remodeling. Pulldown assays showed that Ca²⁺ depletion inhibited the interaction of purified FliI with NMMIIA filaments. Fluorescence resonance energy transfer experiments showed that FliI-NMMIIA interactions require Ca²⁺ influx. We conclude that Ca²⁺ influx through the TRPV4 channel regulates FliI-NMMIIA interaction, which in turn enables generation of the cell extensions essential for collagen remodeling.

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.

Ultrastructural investigation on fibroblast interaction with collagen scaffold

Collagen-based scaffolds are used as temporary or permanent coverings to help wound healing. Under natural conditions, wound healing is affected by such factors as cell types, growth factors and several components of the extracellular matrix. Due to the complexity of the cell-to-matrix interaction, many cell based mechanisms regulating wound healing in vivo are not yet properly understood. However, the whole process can be partially simulated in vitro to determine how cells interact with the collagen scaffold in relation to such features as physico-chemical properties, matrix architecture and fiber stability. Under these conditions, cell migration into the collagen matrix can be easily assessed and causally correlated with these features. In this study, we aimed at providing a structural analysis of how NIH3T3 fibroblasts migrate and proliferate in vitro when seeded on a native type-I collagen scaffold. To this end, samples were collected at regular time intervals and analyzed by light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Through this experimental approach we demonstrate that collagen is gradually frayed into progressively thinner fibrils as fibroblasts migrate into the matrix, embrace the collagen fibers with long filopodia and form large intracellular vacuoles. A key role in this process is also played by microvesicles shed from the fibroblast plasma membrane and spread over long distances inside the collagen matrix. These observations indicate that a native type-I equine collagen provides favorable conditions for simulating collagen processing in vitro and eventually for unraveling the mechanisms controlling cell uptake and intracellular degradation.

Flightless I interacts with NMMIIA to promote cell extension formation, which enables collagen remodeling

The role of the actin-capping protein flightless I in collagen remodeling by mouse fibroblasts is examined. Flightless and nonmuscle myosin IIA cooperate to enable collagen phagocytosis.

We examined the role of the actin-capping protein flightless I (FliI) in collagen remodeling by mouse fibroblasts. FliI-overexpressing cells exhibited reduced spreading on collagen but formed elongated protrusions that stained for myosin10 and fascin and penetrated pores of collagen-coated membranes. Inhibition of Cdc42 blocked formation of cell protrusions. In FliI-knockdown cells, transfection with constitutively active Cdc42 did not enable protrusion formation. FliI-overexpressing cells displayed increased uptake and degradation of exogenous collagen and strongly compacted collagen fibrils, which was blocked by blebbistatin. Mass spectrometry analysis of FliI immunoprecipitates showed that FliI associated with nonmuscle myosin IIA (NMMIIA), which was confirmed by immunoprecipitation. GFP-FliI colocalized with NMMIIA at cell protrusions. Purified FliI containing gelsolin-like domains (GLDs) 1–6 capped actin filaments efficiently, whereas FliI GLD 2–6 did not. Binding assays showed strong interaction of purified FliI protein (GLD 1–6) with the rod domain of NMMIIA (k_D = 0.146 μM), whereas FliI GLD 2–6 showed lower binding affinity (k_D = 0.8584 μM). Cells expressing FliI GLD 2–6 exhibited fewer cell extensions, did not colocalize with NMMIIA, and showed reduced collagen uptake compared with cells expressing FliI GLD 1–6. We conclude that FliI interacts with NMMIIA to promote cell extension formation, which enables collagen remodeling in fibroblasts.

Cell adhesion proteins: roles in periodontal physiology and discovery by proteomics

Adhesion molecules expressed by periodontal connective tissue cells are involved in cell migration, matrix remodeling and inflammatory responses to infection. Currently, the processes by which the biologic activity of these molecules are appropriately regulated in time and space to preserve tissue homeostasis, and to control inflammatory responses and tissue regeneration, are not defined. As cell adhesions are heterogeneous, dynamic, contain a complex group of interacting molecules and are strongly influenced by the type of substrate to which they adhere, we focus on how cell adhesions in periodontal connective tissues contribute to information generation and processing that regulate periodontal structure and function. We also consider how proteomic methods can be applied to discover novel cell-adhesion proteins that could potentially contribute to the form and function of periodontal tissues.

Role of discoidin domain receptor 1 in dysregulation of collagen remodeling by cyclosporin A

The anti-transplant rejection drug cyclosporin A (CsA) causes loss of collagen homeostasis in rapidly remodeling connective tissues, such as human gingiva. As a result of CsA treatment, collagen degradation by fibroblasts is inhibited, which leads to a net increase of tissue collagen and gingival overgrowth. Since fibrillar collagen is the primary ligand for the discoidin domain receptor 1 (DDR1), we hypothesized that CsA perturbs DDR1-associated functions that affect collagen homeostasis. For these experiments, human fibroblasts obtained from gingival explants or mouse 3T3 fibroblasts (wild type, over-expressing DDR1 or DDR1 knockdown) or mouse GD25 cells (expressing DDR1 but null for β1 integrin), were treated with vehicle (dimethyl sulfoxide) or with CsA. The effect of CsA on cell binding to collagen was examined by flow cytometry; cell-mediated collagen remodeling was analyzed with contraction, compaction and migration assays. We found that CsA inhibited cell binding to collagen, internalization of collagen, contraction of collagen gels and cell migration over collagen in a DDR1-dependent manner. CsA also enhanced collagen compaction around cell extensions. Treatment with CsA strongly reduced surface levels of β1 integrins in wild type and DDR1 over-expressing 3T3 cells but did not affect β1 integrin activation or focal adhesion formation. We conclude that CsA inhibition of collagen remodeling is mediated through its effects on both DDR1 and cell surface levels of the β1 integrin.

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.

Collagen remodeling by phagocytosis is determined by collagen substrate topology and calcium-dependent interactions of gelsolin with nonmuscle myosin IIA in cell adhesions

Cell adhesion to collagen presented on beads activates Ca²⁺ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca²⁺-dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.

We examine how collagen substrate topography, free intracellular calcium ion concentration ([Ca²⁺]_i, and the association of gelsolin with nonmuscle myosin IIA (NMMIIA) at collagen adhesions are regulated to enable collagen phagocytosis. Fibroblasts plated on planar, collagen-coated substrates show minimal increase of [Ca²⁺]_i, minimal colocalization of gelsolin and NMMIIA in focal adhesions, and minimal intracellular collagen degradation. In fibroblasts plated on collagen-coated latex beads there are large increases of [Ca²⁺]_i, time- and Ca²⁺-dependent enrichment of NMMIIA and gelsolin at collagen adhesions, and abundant intracellular collagen degradation. NMMIIA knockdown retards gelsolin recruitment to adhesions and blocks collagen phagocytosis. Gelsolin exhibits tight, Ca²⁺-dependent binding to full-length NMMIIA. Gelsolin domains G4–G6 selectively require Ca²⁺ to interact with NMMIIA, which is restricted to residues 1339–1899 of NMMIIA. We conclude that cell adhesion to collagen presented on beads activates Ca²⁺ entry and promotes the formation of phagosomes enriched with NMMIIA and gelsolin. The Ca²⁺ -dependent interaction of gelsolin and NMMIIA in turn enables actin remodeling and enhances collagen degradation by phagocytosis.

Gelsolin and non-muscle myosin IIA interact to mediate calcium-regulated collagen phagocytosis

The formation of adhesion complexes is the rate-limiting step for collagen phagocytosis by fibroblasts, but the role of Ca(2+) and the potential interactions of actin-binding proteins in regulating collagen phagocytosis are not well defined. We found that the binding of collagen beads to fibroblasts was temporally and spatially associated with actin assembly at nascent phagosomes, which was absent in gelsolin null cells. Analysis of tryptic digests isolated from gelsolin immunoprecipitates indicated that non-muscle (NM) myosin IIA may bind to gelsolin. Immunostaining and immunoprecipitation showed that gelsolin and NM myosin IIA associated at collagen adhesion sites. Gelsolin and NM myosin IIA were both required for collagen binding and internalization. Collagen binding to cells initiated a prolonged increase of [Ca(2+)](i), which was absent in cells null for gelsolin or NM myosin IIA. Collagen bead-induced increases of [Ca(2+)](i) were associated with phosphorylation of the myosin light chain, which was dependent on gelsolin. NM myosin IIA filament assembly, which was dependent on myosin light chain phosphorylation and increased [Ca(2+)](i), also required gelsolin. Ionomycin-induced increases of [Ca(2+)](i) overcame the block of myosin filament assembly in gelsolin null cells. We conclude that gelsolin and NM myosin IIA interact at collagen adhesion sites to enable NM myosin IIA filament assembly and localized, Ca(2+)-dependent remodeling of actin at the nascent phagosome and that these steps are required for collagen phagocytosis.

Mfge8 diminishes the severity of tissue fibrosis in mice by binding and targeting collagen for uptake by macrophages

Milk fat globule epidermal growth factor 8 (Mfge8) is a soluble glycoprotein known to regulate inflammation and immunity by mediating apoptotic cell clearance. Since fibrosis can occur as a result of exaggerated apoptosis and inflammation, we set out to investigate the hypothesis that Mfge8 might negatively regulate tissue fibrosis. We report here that Mfge8 does decrease the severity of tissue fibrosis in a mouse model of pulmonary fibrosis; however, it does so not through effects on inflammation and apoptotic cell clearance, but by binding and targeting collagen for cellular uptake through its discoidin domains. Initial analysis revealed that Mfge8-/- mice exhibited enhanced pulmonary fibrosis after bleomycin-induced lung injury. However, they did not have increased inflammation or impaired apoptotic cell clearance after lung injury compared with Mfge8+/+ mice; rather, they had a defect in collagen turnover. Further experiments indicated that Mfge8 directly bound collagen and that Mfge8-/- macrophages exhibited defective collagen uptake that could be rescued by recombinant Mfge8 containing at least one discoidin domain. These data demonstrate a critical role for Mfge8 in decreasing the severity of murine tissue fibrosis by facilitating the removal of accumulated collagen.

Integrin-dependent phagocytosis: spreading from microadhesion to new concepts

By linking actin dynamics to extracellular components, integrins are involved in a wide range of cellular processes that are associated with or require cytoskeletal remodelling and cell-shape changes. One such function is integrin-dependent phagocytosis, a process that several integrins are capable of mediating and that allows the binding and clearance of particles. Integrin-dependent phagocytosis is involved in a wide range of physiological processes, from the clearance of microorganisms and apoptotic-cell removal to extracellular-matrix remodelling. Integrin signalling is also exploited by microbial pathogens for entry into host cells. Far from being a particular property of specific integrins and specialised cells, integrin-dependent uptake is emerging as a general, intrinsic ability of most integrins that is associated with their capacity to signal to the actin cytoskeleton. Integrin-mediated phagocytosis can therefore be used as a robust model in which to study integrin regulation and signalling.

Actinobacillus actinomycetemcomitans lipopolysaccharide stimulates collagen phagocytosis by human gingival fibroblasts

INTRODUCTION

Collagen phagocytosis by fibroblasts is involved in the intracellular pathway related to collagen breakdown in soft connective tissues. The possible role of lipopolysaccharide (LPS) in regulating this fibroblast function has not been elucidated so we investigated the effect of LPS from Actinobacillus actinomycetemcomitans, a periodontopathic bacterium, on collagen phagocytic activity in human gingival fibroblasts and associated regulatory mechanisms.

METHODS

LPS pretreatment stimulated binding of collagen-coated beads to cells and, subsequently, their internalization.

RESULTS

The LPS-activated collagen phagocytic process was enhanced in the presence of the soluble form of CD14 (sCD14) or LPS-binding protein (LBP), while the LPS/LBP treatment activated Akt and induced actin reorganization. Furthermore, these LPS/LBP-induced effects were partially suppressed by adding phosphatidyl-inositol-3 kinase (PI3K) inhibitors.

CONCLUSION

These results suggest that A. actinomycetemcomitans LPS disturbs the homeostasis of collagen metabolism within gingival tissue by facilitating collagen phagocytosis by gingival fibroblasts, and serum sCD14 and LBP positively regulate the action of LPS. In addition, the PI3K/Akt signaling is thought to partially mediate the LPS/LBP-stimulated collagen phagocytic pathway, which may be dependent on actin cytoskeletal rearrangement.

Phagocytosis of collagen by fibroblasts and invasive cancer cells is mediated by MT1-MMP

Degradation of collagen is required for the physiological remodelling of connective tissues during growth and development, as well as in wound healing, inflammatory diseases, and cancer cell invasion. In remodelling adult tissues, degradation of collagen occurs primarily through a phagocytic pathway. While various steps in this pathway have been characterized, the enzyme required to fragment collagen fibrils for phagocytosis has not been identified. Laser confocal microscopy, transmission electron microscopy and biochemical assays were used to show that degradation of collagen substrates by fibroblasts correlated with the expression of the membrane-bound metalloproteinase MT1-MMP (membrane-type 1 matrix metalloproteinase). The MT1-MMP was localized to sites of collagen cleavage on the cell surface and also within the cells. In contrast with MT1-MMP, the gelatinase MMP-2 was not required for collagen phagocytosis. Similar analyses of several ovarian cancer, breast cancer and fibrosarcoma cells indicated that highly metastatic cells also degrade collagen through a phagocytic pathway that is mediated by MT1-MMP. Collectively, these studies demonstrate a pivotal role for catalytically active MT1-MMP in preparing collagen fibrils for phagocytic degradation by normal and transformed cells.

Cyclosporin inhibition of collagen remodeling is mediated by gelsolin

Cyclosporin A (CsA) inhibits collagen remodeling by interfering with the collagen-binding step of phagocytosis. In rapidly remodeling connective tissues such as human periodontium this interference manifests as marked tissue overgrowth and loss of function. Previous data have shown that CsA inhibits integrin-induced release of Ca(2+) from internal stores, which is required for the binding step of collagen phagocytosis. Because gelsolin is a Ca(2+)-dependent actin-severing protein that mediates collagen phagocytosis, we determined whether gelsolin is a CsA target. Compared with vehicle controls, CsA treatment of wild-type mice increased collagen accumulation by 60% in periodontal tissues; equivalent increases were seen in vehicle-treated gelsolin-null mice. Collagen degradation by phagocytosis in cultured gelsolin wild-type fibroblasts was blocked by CsA, comparable to levels of vehicle-treated gelsolin-null fibroblasts. In wild-type cells treated with CsA, collagen binding was similar to that of gelsolin-null fibroblasts transfected with a gelsolin-severing mutant and treated with vehicle. CsA blocked collagen-induced Ca(2+) fluxes subjacent to bound collagen beads, gelsolin recruitment, and actin assembly at bead sites. CsA reduced gelsolin-dependent severing of actin in wild-type cells to levels similar to those in gelsolin-null fibroblasts. We conclude that CsA-induced accumulation of collagen in the extracellular matrix involves disruption of the actin-severing properties of gelsolin, thereby inhibiting the binding step of collagen phagocytosis.

A critical role for the membrane-type 1 matrix metalloproteinase in collagen phagocytosis

Degradation of collagen is important for the physiological remodeling of connective tissues during growth and development as well as in wound healing, inflammatory diseases, and cancer cell invasion. In remodeling adult tissues, degradation of collagen occurs primarily through a phagocytic pathway. However, although various steps in the phagocytic pathway have been characterized, the enzyme required to initially fragment collagen fibrils for subsequent phagocytosis has not been identified. We have used laser confocal microscopy, transmission electron microscopy, and biochemical assays to show that human fibroblasts initiate degradation of collagen through the collagenase activity of the membrane-bound metalloproteinase MT1-MMP. Degradation of natural and reconstituted collagen substrates correlated with the expression of MT1-MMP, which was localized at sites of collagen cleavage at the surface of the cells and also within the cells, whereas collagen degradation was abrogated when MT1-MMP expression was blocked by small interfering RNA treatment. In contrast to MT1-MMP, the gelatinolytic activity of MMP-2 was not required for collagen phagocytosis. These studies demonstrate a pivotal role of catalytically active MT1-MMP in preparing collagen fibrils for phagocytic degradation.

Separate functions of gelsolin mediate sequential steps of collagen phagocytosis

Collagen phagocytosis is a critical mediator of extracellular matrix remodeling. Whereas the binding step of collagen phagocytosis is facilitated by Ca2+-dependent, gelsolin-mediated severing of actin filaments, the regulation of the collagen internalization step is not defined. We determined here whether phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] regulation of gelsolin is required for collagen internalization. In gelsolin null fibroblasts transfected with gelsolin severing mutants, actin severing and collagen binding were strongly impaired but internalization and actin monomer addition at collagen bead sites were much less affected. PI(4,5)P2 accumulated around collagen during internalization and was associated with gelsolin. Cell-permeable peptides mimicking the PI(4,5)P2 binding site of gelsolin blocked actin monomer addition, the association of gelsolin with actin at phagosomes, and collagen internalization but did not affect collagen binding. Collagen beads induced recruitment of type 1 gamma phosphatidylinositol phosphate kinase (PIPK1gamma661) to internalization sites. Dominant negative constructs and RNA interference demonstrated a requirement for catalytically active PIPK1gamma661 for collagen internalization. We conclude that separate functions of gelsolin mediate sequential stages of collagen phagocytosis: Ca2+-dependent actin severing facilitates collagen binding, whereas PI(4,5)P2-dependent regulation of gelsolin promotes the actin assembly required for internalization of collagen fibrils.

Collagen Phagocytosis by Fibroblasts Is Regulated by Decorin

Decorin is a small, leucine-rich proteoglycan that binds to collagen and regulates fibrillogenesis. We hypothesized that decorin binding to collagen inhibits phagocytosis of collagen fibrils. To determine the effects of decorin on collagen degradation, we analyzed phagocytosis of collagen and collagen/decorin-coated fluorescent beads by Rat-2 and gingival fibroblasts. Collagen beads bound to gingival cells by alpha2beta1 integrins. Binding and internalization of decorin/collagen-coated beads decreased dose-dependently with increasing decorin concentration (p < 0.001). Inhibition of binding was sustained over 5 h (p < 0.001) and was attributed to interactions between decorin and collagen and not to decorin-collagen receptor interactions. Both the non-glycosylated decorin core protein and the thermally denatured decorin significantly inhibited collagen bead binding (approximately 50 and 89%, respectively; p < 0.05). Mimetic peptides corresponding to leucine-rich repeats 1-3, encompassed by a collagen-binding approximately 11-kDa cyanogen bromide fragment of decorin and leucine-rich repeats 4 and 5, previously shown to bind to collagen, were tested for their ability to inhibit collagen bead binding. Although the synthetic peptide 3 alone exhibited saturable binding to collagen, neither peptides 3 nor 1 and 2 markedly inhibited phagocytosis. Leucine-rich repeat 3 bound to a triple helical peptide containing the alpha2 integrin-binding site of collagen. When collagen beads were co-incubated with peptides 3 and 4, inhibition of collagen phagocytosis (55%) was equivalent to intact native/recombinant core protein. Thus a novel collagen binding domain in decorin acts cooperatively with leucine-rich repeat 4 to mask the alpha2beta1 integrin-binding site on collagen, an important sequence for the phagocytosis of collagen fibrils.

Degradation of extracellular matrix by extrahepatic stellate cells in the intestine of the lamprey,Lampetra japonica

To investigate the mechanisms involved in the atrophy of the intestines in lampreys (Lampetra japonica) during the spawning migration stage, we examined by morphological methods their intestines with special reference to degradation of extracellular matrix (ECM) components. Stellate cells are known to be distributed not only in the liver (hepatic stellate cells) but also in other organs, such as the pancreas, intestine, lung, and kidney (extrahepatic stellate cells). Hepatic stellate cells are well known to be able to biosynthesize, secrete, and degrade ECM. Therefore, we investigated the cellular and molecular mechanisms involved in the atrophy of the intestines by focusing on these intestinal extrahepatic stellate cells. The cells were found to contain phagocytosed and degraded collagen fibrils, which are one of the ECM components. A positive reaction for trimetaphosphatase (TMPase, a cytochemical marker of lysosomes) was preferentially detected in round or elongated vesicles in the intestinal extrahepatic stellate cells and the deposits of the reaction products coexisted with the degraded collagen fibrils. However, the basement membrane of the intestine, which membrane is also an ECM component, was preserved throughout the spawning migration stage of the lamprey and accumulated as a mass of thick membrane, suggesting the existence of a special mechanism for selective digestion of ECM components. These results indicate that the intestinal extrahepatic stellate cells in Lampetra japonica during its spawning migration stage might play an important mechanistic role in the atrophy of lamprey intestines by phagocytizing collagen fibrils and digesting the phagocytized collagen fibrils in their lysosomes.

Differential binding to dorsal and ventral cell surfaces of fibroblasts: effect on collagen phagocytosis

Matrix remodeling by phagocytic fibroblasts is essential for growth and development but the regulatory processes are undefined. We evaluated the impact of spreading on the binding step of collagen phagocytosis with a novel culture system that more closely replicates phagocytosis in vivo than previous models. 3T3 cells were plated on collagen-coated beads, thereby loading only ventral surfaces (adhesion with spreading), or were allowed to spread on collagen films and then loaded with beads on their dorsal surfaces (adhesion without spreading). Ventral surfaces bound three-fold more beads than dorsal surfaces which was accompanied by accelerated phagosomal maturation. Arp3 and cortactin, markers of the actin-associated spreading machinery, strongly accumulated around ventrally but not dorsally loaded beads, suggesting that spreading contributes to enhanced binding of ventral surfaces. Further, ventral surfaces exhibited two-fold more free alpha2beta1 integrins, the major collagen receptors. Notably, compared to cells spread on collagen substrates, spreading cells exhibited a three-fold higher alpha2beta1 mobile fraction which was correlated with limited engagement of ventral receptors by actin filaments. Thus integrin ligation by actin filaments regulates the mobility of collagen receptors which in turn mediates the enhanced binding of collagen beads on spreading surfaces.

Decreased capacity of asthmatic bronchial fibroblasts to degrade collagen

The mechanisms of fibrillar collagen accumulation in asthmatic bronchi remain unclear, an imbalance between synthesis and degradation of collagen may be implicated in this process. The aim of this study was to compare the capacities of normal (BNF) and asthmatic (BAF) bronchial fibroblasts to degrade collagen. Metalloproteinases and their inhibitors were measured by ELISA, types I and III procollagen synthesis was determined by liquid RIA and, finally, zymography was used to assess the presence of active and latent forms of MMPs. The capacity of fibroblasts to degrade collagen coated onto latex beads was evaluated by flow cytometry. Our results showed that MMP-2 secretion was significantly higher in BNF when compared to BAF and this was confirmed by gelatin zymography. In BNF culture, TIMP-1 and MMP-1 secretions positively correlated with types I and III procollagen synthesis. However, in BAF, this correlation was negative. This suggests that a balance exists between collagen synthesis and degradation in BNF and that this balance is compromised in BAF. On the other hand, BAF did show significantly reduced capacity to degrade collagen when compared to that of BNF. This reduced phagocytic activity was not associated with a decrease in collagen receptor expression. This study establishes for the first time that a relationship exists between metalloproteinases enzyme dysregulation and the reduced capacity of asthmatic bronchial fibroblast to degrade collagen. These events may shed light on why accumulation of collagen can be observed in asthmatic airways.

MT1-MMP-deficient mice develop dwarfism, osteopenia, arthritis, and connective tissue disease due to inadequate collagen turnover

MT1-MMP is a membrane-bound matrix metalloproteinase (MT-MMP) capable of mediating pericellular proteolysis of extracellular matrix components. MT1-MMP is therefore thought to be an important molecular tool for cellular remodeling of the surrounding matrix. To establish the biological role of this membrane proteinase we generated MT1-MMP-deficient mice by gene targeting. MT1-MMP deficiency causes craniofacial dysmorphism, arthritis, osteopenia, dwarfism, and fibrosis of soft tissues due to ablation of a collagenolytic activity that is essential for modeling of skeletal and extraskeletal connective tissues. Our findings demonstrate the pivotal function of MT1-MMP in connective tissue metabolism, and illustrate that modeling of the soft connective tissue matrix by resident cells is essential for the development and maintenance of the hard tissues of the skeleton.

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.

Deficiency in collagen and fibronectin phagocytosis by human buccal mucosa fibroblasts in vitro as a possible mechanism for oral submucous fibrosis

Oral submucous fibrosis (OSF), a chronic oral mucosal condition commonly found in south Asians, is a disorder characterized by a quantitative as well as a qualitative alteration of collagen deposition within the subepithelial layer of the oral mucosa. Since degradation of collagen by fibroblast phagocytosis is an important pathway for physiological remodelling of soft connective tissues, we have investigated phagocytosis of collagen- and fibronectin-coated latex beads by fibroblast cultures with an in vitro model system. Coated fluorescent latex beads were incubated with human oral mucosa fibroblasts and the fluorescence associated with internalized beads was measured by flow cytometry. Cells from normal tissues that had been incubated with beads for 16 h contained a mean of 75% collagen phagocytic cells and 70% fibronectin phagocytic cells; however, about 15% and 10% of phagocytic cells individually contained more than twice the mean number of beads per cell. In contrast, cells from OSF tissues exhibited a 40% reduction of the proportions of collagen phagocytic cells (mean=35%) and a 48% decrease of the proportions of fibronectin phagocytic cells (mean=22%), none of the cells having a high number of beads as compared to normal fibroblasts. OSF lesions appear to contain fibroblasts with marked deficiencies in collagen and fibronectin phagocytosis. To investigate if inhibition of phagocytosis could be demonstrated in vitro, normal fibroblast cultures were incubated with areca nut alkaloids (arecoline, arecaidine). The cultures had a dose-dependent reduction in the proportions of phagocytic cells. On the other hand, corticosteroid used in the treatment of OSF exhibited a dose-dependent enhancement in the proportion of phagocytic cells. Therefore, our hypothesis for OSF, although oversimplified, is that betel nut alkaloids (arecoline, arecaidine) inhibit fibroblast phagocytosis and this provides a mechanism for the development of OSF. The benefit of a local intralesional injection of corticosteroid is also possibly, at least in part, through an enhancement of fibroblast collagen phagocytosis.

Involvement of V-ATPases in the digestion of soft connective tissue collagen

The contribution of vacuolar H+-ATPases (V-ATPases) to collagen degradation was investigated in soft connective tissue explants (periosteum). Immunolocalisation showed faint to intense staining of cells throughout the periosteum. The V-ATPase inhibitors, bafilomycin A1 and folimycin, decreased overall collagen degradation by 40 and 50% after 24 and 48 h, respectively. The participation of V-ATPases in intracellular degradation of collagen was demonstrated by the decrease of the amount of phagocytosed collagen in fibroblasts upon inhibition of pump activity. The inhibition of degradation was not due to a reduction in activity of gelatinase A, an enzyme previously found to mediate collagen degradation, as assessed by zymographic analysis of tissue and conditioned medium. Bafilomycin A1 even induced an increase of gelatinase A and B levels in both fractions. In conclusion, acidification by V-ATPases may represent an important mechanism in extracellular and intracellular collagen degradation in soft connective tissue.

Continuity between wound macrophage and fibroblast phenotype: analysis of wound fibroblast phagocytosis

Analysis of phagocytic activity in wound fibroblasts was chosen as a means to assess the possible continuity between macrophage and fibroblast phenotypes. Fibroblast phagocytosis of uncoated, IgG-coated, or collagen-coated fluorescent beads was analyzed by flow cytometry in vivo and in vitro. Phagocytosis of fluorescent beads by procollagen I-positive cells (fibroblasts) was evaluated in vivo by injecting beads into subcutaneously implanted sponge wounds in anesthetized Fisher rats. Phagocytic activity of a purified population of wound fibroblasts was measured in vitro and correlated with oxidation state using hydroethidium. In the wound environment, 50-60% of the cells that engulfed uncoated, IgG-coated, or collagen-coated beads were procollagen I-positive cells (i.e., fibroblasts). Procollagen I-positive cells engulfed uncoated and IgG-coated beads in preference to collagen-coated beads in vivo. Cultured wound fibroblasts engulfed uncoated, IgG-coated, and collagen-coated particles. The majority of fibroblasts that engulfed beads were in an elevated oxidation state. We conclude that substantial fibroblast phagocytosis occurs in the wound, but scavenger receptor-mediated fibroblast phagocytosis is different from that of macrophages. Additional markers will be helpful in defining the macrophage fibroblast continuum.

Participation of intracellular cysteine proteinases, in particular cathepsin B, in degradation of collagen in periosteal tissue explants

The involvement of cysteine proteinases in the degradation of soft connective tissue collagen was studied in cultured periosteal explants. Using cysteine proteinase inhibitors that were active intracellularly or extracellularly (Ep453 and Ep475, respectively), it was shown that over-all collagen degradation, as measured by the release of hydroxyproline, decreased significantly on inhibition of the intracellular pool of cysteine proteinases by Ep453. This inhibitor also induced an accumulation of intracellular fibrillar collagen in fibroblasts, indicating a decreased degradation of phagocytosed collagen. The extracellular inhibitor, Ep475, had minor or no effects. Histochemical analysis using a substrate for the cysteine proteinases cathepsins B and L revealed a high level of enzyme activity, which was completely blocked in explants preincubated with a selective intracellular inhibitor of cathepsin B, Ca074-Me. Moreover, the cathepsin B inhibitor strongly affected collagen degradation, decreasing the release of hydroxyproline and increasing the accumulation of phagocytosed collagen. These effects were comparable or slightly stronger than those found with the general intracellular inhibitor (Ep453). Taken together, these data strongly suggest that intracellular cysteine proteinases, in particular cathepsin B, play an important role in the digestion of soft connective tissue collagen.

Deregulation of collagen phagocytosis in aging human fibroblasts: effects of integrin expression and cell cycle

Intracellular degradation of collagen by phagocytosis in fibroblasts is essential for physiological remodeling of the extracellular matrix in a wide variety of connective tissues but imbalances between degradation and synthesis can lead to loss of tissue collagen. As aging is associated with loss of dermal and periodontal collagen and with increased lysomomal enzyme content in fibroblasts, we examined the regulation of collagen phagocytosis by integrin expression and the cell cycle in an in vitro fibroblast aging model. Two different fibroblast lines (CL1; CL2) at the fourth subculture were passaged up to replicative senescence to model aging processes in vitro. Cells were incubated with collagen-coated or BSA-coated green fluorescent beads for 3 h to assess alpha 2 beta 1-integrin-mediated or nonspecific phagocytosis, respectively. Single-cell suspensions were stained with DAPI and sulforhodamine 101 to separate cycling G1 and noncycling G0 cells. Staining for alpha 2-integrin, bead internalization, and bivariate analyses of DNA/protein content were measured by three-color flow cytometry. Serum deprivation was used to induce increases in the proportion of G0 cells. For G1 cells, the proportion of collagen phagocytic cells was > 50% for all passages and collagen beads were internalized > 5-fold more frequently than BSA beads. In contrast, G0 cells with diploid DNA content but low protein content exhibited greatly reduced phagocytic capacity (< 10% of cells internalized collagen or BSA beads), the number of beads per cells was 4-fold less, and alpha 2 integrin expression was very low compared to G1 cells. The proportion of collagen phagocytic cells and the proportion of alpha 2-integrin-positive cells increased with transit through the cell cycle. At higher passage numbers mean cell volume and cytoplasmic granularity were reduced approximately 30% but at replicative senescence cells with large surface area and subdiploid DNA predominated. The proportion of collagen and BSA phagocytic G1 cells increased 1.5- and 5-fold, respectively, and the number of beads per cell increased < 3-fold. However, surface alpha 2-integrin staining remained unchanged. These data indicate that the collagen and nonspecific internalization pathways were greatly unregulated, independent of cell cycle phase, and that cellular aging in vitro strongly influences the specificity and rate of phagocytic processes in fibroblasts. We suggest that age-related loss of collagen in connective tissues undergoing turnover may be a manifestation of a deregulated increase of collagen phagocytosis in which the net loss of degraded collagen exceeds new synthesis.

Collagen phagocytosis and apoptosis are induced by high level alkaline phosphatase expression in rat fibroblasts

Study of fibroblast origins and lineages is complicated by the lack of unambiguous markers that could be used to identify discrete subpopulations on the basis of functional attributes. We have studied the role of the membrane-anchored hydrolytic enzyme tissue-nonspecific alkaline phosphatase (TN-AP) and the placental alkaline phosphatase (PL-AP) in collagen phagocytosis and in the deletion of cells by apoptosis. Rat-2 cells, which do not constitutively express AP, were transfected with full-length rat TN-AP or PL-AP cDNAs to determine the impact of the TN-AP collagen-binding domain on cell function. Various levels of expression were driven by early (strong) or late (weak) SV40 promoters in the plasmid construct. Controls were transfected with plasmids that did not contain AP cDNA. AP expression in transfected cells was confirmed by Northern blotting, histochemical analysis, and SDS-PAGE analysis of membrane-anchored enzyme released by phosphatidyl inositol phospholipase C. Low levels of TN-AP expression increased cell spreading slightly, nearly doubled the percentage of collagen phagocytic cells (up to 80%), and increased the number of internalized collagen-coated fluorescence beads per cell. In cells transfected with PL-AP (i.e., no collagen-binding domain), collagen phagocytosis was not affected. Internalization of BSA beads was also not affected by either AP isozyme, indicating that AP was selective for integrin-mediated phagocytosis. In single cells, histochemically demonstrable TN-AP activity on cell membranes was colocalized with the binding of collagen beads, but this colocalization was not detected in cells transfected with PL-AP. Phagocytosis was inhibited by antibodies to the alpha 2 integrin and to AP but not by levamisole, an inhibitor of AP phosphohydrolytic activity. High-level TN-AP expression caused a fivefold reduction of cell proliferation and was associated with the development of cells with sub-G1 DNA content, nuclear condensation, and nuclear budding. In AP-positive cultures, there was a greatly increased number of floating cells; nick-labeling of DNA by terminal transferase and biotinylated dUTP showed a 15-fold increase of stained cells. These data indicate that low-level TN-AP expression enhances collagen phagocytosis, presumably through the TN-AP collagen-binding domain. High-level AP expression promotes cell deletion by apoptosis. We suggest that the expression of AP by fibroblasts indicates a novel role for this enzyme in collagen degradation by phagocytosis.

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).

Role of integrins in regulation of collagen phagocytosis by human fibroblasts

Phagocytosis of collagen fibrils by fibroblasts is an important pathway for degradation of extracellular matrix in mature connective tissues. To study regulatory mechanisms in phagocytosis, 2-microns fluorescent beads coated with either collagen (COL) or bovine serum albumin (BSA) were incubated with human gingival fibroblasts in vitro. For these studies single cell suspensions were prepared by trypsinization, and bead internalization and collagen receptor expression were assessed by flow cytometry. After 3-h incubations, up to 8-fold more cells internalized COL beads than BSA-coated beads. Increased collagen coating concentration was associated with elevated proportions of cells that internalized COL beads, and was observed also in the presence of competing fibronectin-coated beads. The number of beads per cell and the percent of phagocytic cells increased proportionally with higher bead loadings. At > 4 beads per cell a maximum of approximately 80% of cells were phagocytic. Cells reacted with mAbs against the alpha 1, alpha 2, and alpha 3 integrin subunits were, respectively, 5%, 98% and 93% positively stained above background controls. All cells that internalized COL beads exhibited alpha 2 staining but there were large proportions of phagocytic cells that were not stained for alpha 1. In unfixed cells, bead internalization caused an immediate reduction of surface staining of membrane-bound alpha 2 by approximately 55% which returned to control levels within 3 h, indicating that cell-surface alpha 2 was internalized by phagocytosis. Preincubation of cells with up to 8 COL beads per cell reduced the proportion of phagocytic cells and the number of internalized beads after a second COL bead incubation 4 h later. To assess the relationship between the percent of phagocytic cells and alpha 2 integrin levels, serum starvation and cycloheximide experiments were conducted. Compared to controls, serum starvation for 24 h induced a 3.2-fold increase of cells internalizing COL beads but did not alter alpha 2 staining levels. In contrast, 3 h cycloheximide treatment reduced alpha 2 staining to 60% of control levels and this treatment also inhibited COL bead internalization. GRGDTP peptide as well as mAbs against the alpha 1 and alpha 2 subunits significantly reduced internalization of COL beads by 1.8 to 2.6-fold, whereas GRGESP peptide and alpha 3 mAb exerted no effect. Internalization of BSA beads was not affected by any of these treatments. Collectively, these data indicate that the alpha 2 integrin, along with other, as yet unidentified components, is likely involved in COL bead internalization. The alpha 2 integrin subunit is rapidly recycled or synthesized following a phagocytic load. In contrast, the alpha 1 integrin is not directly required for phagocytosis but may regulate the internalization step.

Periodontal ligament cell population: the central role of fibroblasts in creating a unique tissue

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament (PL) and have important roles in the development, function, and regeneration of the tooth support apparatus. Biological processes initiated during the formation of the PL contribute to the long-lasting homeostasic properties exhibited by PL fibroblast populations.

DEVELOPMENT

The formation of the PL is likely controlled by epithelial-mesenchymal and epithelial hard tissue interactions, but the actual mechanisms that contribute to the development of cellular lineages in the PL are unknown. Fibroblasts in the normally functioning PL migrate through the tissue along collagen fibres to cementum and bone and in an apico-coronal direction during tooth eruption. ADULT TISSUE: Cell kinetic experiments have shown that PL fibroblasts comprise a renewal cell system in steady-state and the progenitors can generate multiple types of more differentiated, specialized cells. Progenitor cell populations of the PL are enriched in locations adjacent to blood vessels and in contiguous endosteal spaces. In normally functioning periodontal tissues, there is a relatively modest turnover of cells in which apoptotic cell death balances proliferation. Large increases of cell formation and cell differentiation occur after application of orthodontic forces or wounding. As PL cells comprise multiple cellular phenotypes, it has been postulated that after wounding, the separate phenotypes repopulating the site will ultimately dictate the tissue form and type.

CONCLUSIONS

PL fibroblasts play an essential role in responses to mechanical force loading of the tooth by remodelling and repairing effete or damaged matrix components. In consideration of the important roles played by fibroblasts in PL homeostasis, they could be described as "the architect, builder, and caretaker" of the periodontal ligament.

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.

Type VI collagen is phagocytosed by fibroblasts and digested in the lysosomal apparatus: involvement of collagenase, serine proteinases and lysosomal enzymes

Type VI collagen is present in most connective tissues, where it is considered to play a crucial role in the attachment of cells to the extracellular matrix and/or in the three-dimensional organization of the collagen meshwork. Although some information is available on its formation, the mechanisms involved in its degradation are not understood. Here, we present evidence for lysosomal digestion of type VI collagen by fibroblasts of periosteal explants. In the lysosomal apparatus of these cells, broad-banded filamentous aggregates characterized by 100-nm periodicity were found, which proved to consist of type VI collagen as indicated by their stainability with anti-type VI collagen antibodies. By interfering with synthesis (ascorbate or alpha, alpha-dipyridyl), intracellular translocation of collagen-containing vesicles (colchicine) as well as phagocytosis (cytochalasin B), it was shown that the intracellular broad-banded type VI collagen represented phagocytosed material. In the presence of acidotropic agents (NH4Cl and methylamine) the amount of intracellular type VI collagen increased significantly (5- to 10-fold), suggesting that a rise of pH in the endosomal/lysosomal apparatus causes inhibition of its degradation. By using a variety of proteinase inhibitors, it was found that inhibition of collagenase (when used in combination with NH4Cl), or inhibition of cysteine proteinases (both with and without NH4Cl), resulted in an increased amount of intracellular type VI collagen, whereas inhibition of serine proteinases significantly lowered the level of intracellular type VI collagen. The data presented are the first to indicate a pathway by which type VI collagen degradation may occur: fibroblasts phagocytose type VI collagen and subsequently digest this collagen in their lysosomal apparatus. Degradation depends on the activity of several enzymes, among them collagenase and serine proteinases, probably exerting their activity in the extracellular space just before the actual internalization. After uptake, digestion involves pH-sensitive lysosomal enzymes, including those belonging to the class of cysteine proteinases.

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.