Adhesion strength differential of human ligament fibroblasts to collagen types I and III

Mechano Growth Factor Accelerates ACL Repair and Improves Cell Mobility of Mechanically Injured Human ACL Fibroblasts by Targeting Rac1-PAK1/2 and RhoA-ROCK1 Pathways

Exceeded mechanical stress leads to a sublethal injury to anterior cruciate ligament (ACL) fibroblasts, and it will hinder cell mobility and ACL regeneration, and even induce osteoarthritis. The mechano growth factor (MGF) could be responsible for mechanical stress and weakening its negative effects on cell physiological behaviors. In this study, effects of MGF on cell mobility and relevant molecules expression in injured ACL fibroblasts were detected. After an injurious mechanical stretch, the analysis carried out, at 0 and 24 h, respectively, showed that the cell area, roundness, migration, and adhesion of ACL fibroblasts were reduced. MGF (10, 100 ng/mL) treatment could improve cell area, roundness and promote cell migration and adhesion capacity compared with the injured group without MGF. Further study indicated that cell mobility-relevant molecules (PAK1/2, Cdc42, Rac1, RhoA, and ROCK1) expression in ACL fibroblasts was down-regulated at 0 or 24 h after injurious stretch (except Rac1 and RhoA at 0 h). Similarly, MGF improved cell mobility-relevant molecule expression, especially the ROCK1 expression level in ACL fibroblasts at 0 or 24 h after injurious stretch. Protein expression of ROCK1 in injured ACL fibroblasts was also reduced and could be recovered by MGF treatment. In a rabbit partial ACL transection (ACLT) model, ACL exhibited poor regenerative capacity in collagen and extracellular matrix (ECM) synthesis after partial ACLT for 2 or 4 weeks, and MGF remarkably accelerated ACL regeneration and restored its mechanical loading capacity after partial ACLT for four weeks. Our findings suggest that MGF weakens the effects of pathological stress on cell mobility of ACL fibroblasts and accelerates ACL repair, and might be applied as a future treatment approach to ACL rupture in the clinic.

The novel epiligament theory: differences in healing failure between the medial collateral and anterior cruciate ligaments

According to current literature, 90% of knee ligament injuries involve the medial collateral ligament or the anterior cruciate ligament. In contrast to the medial collateral ligament, which regenerates relatively well, the anterior cruciate ligament demonstrates compromised healing. In the past, there were numerous studies in animal models that examined the healing process of these ligaments, and different explanations were established. Although the healing of these ligaments has been largely investigated and different theories exist, unanswered questions persist.

Therefore, the aim of this article is 1) to review the different historical aspects of healing of the medial collateral ligament and present the theories for healing failure of the anterior cruciate ligament; 2) to examine the novel epiligament theory explaining the medial collateral ligament healing process and failure of anterior cruciate ligament healing; and 3) to discuss why the enveloping tissue microstructure of the aforementioned ligaments needs to be examined in future studies.

We believe that knowledge of the novel epiligament theory will lead to a better understanding of the normal healing process for implementing optimal treatments, as well as a more holistic explanation for anterior cruciate ligament healing failure.

The Epiligament: Structure, Postnatal Development and Role in Ligament Healing

While much is known about the ligament, the precise morphology and function of the thin layer of connective tissue lining its surface, termed the epiligament, have not been fully studied yet. Herein, we aimed at reviewing the recent findings on the structural and functional significance of the epiligament in both animal models and human tissue. The epiligament is made up of various connective tissue cells such as fibroblasts, fibrocytes, mast cells, and adipocytes and contains a number of neurovascular bundles. Arrangement of collagen fibers in the epiligament is rather chaotic, in multiple directions, which allows for greater mobility and resistance to stress. Differences in the collagen content and types of enzymes of the group of matrix metalloproteinases between the epiligament and the underlying ligament tissue have been reported and are reviewed herein. While the ligament tissue mainly contains collagen type I, the epiligament is also rich in collagen types III and V. As suggested by a number of studies, the epiligament plays a key role in ligament repair as a donor of cells and matrix metalloproteinases, particularly matrix metalloproteinase-2 and 9, which are essential for scar tissue remodeling. In conclusion, future studies will likely reveal additional functional aspects of the epiligament, which may allow scientists to devise more suitable treatment strategies for damaged ligaments in a world where injuries resulting from sports activities or daily routine have long merited their due attention.

A comparative study of the epiligament of the medial collateral and the anterior cruciate ligament in the human knee. Immunohistochemical analysis of collagen type I and V and procollagen type III

BACKGROUND

Recent reports in rat models have shown that fibroblasts in the epiligament, an enveloping tissue of the ligament, are not static cells and play an important role during the early ligament healing of isolated grade III injury of the collateral ligaments of the knee. Fibroblasts produce collagen types I, III and V and infiltrate within the ligament body via the endoligament. In addition, similarities have been reported between the structure of the epiligament of the medial collateral ligament and anterior cruciate ligament of the knee in rat and in human. In line with the ascribed role of the epiligament tissue and the synthesis of these collagens and their role in ligament healing, the aim of this study was to determine their presence in the normal epiligament of the aforementioned ligaments in humans, to compare their differential expression and to present a novel hypothesis about the failure of healing of the anterior cruciate ligament in contrast to the medial collateral ligament.

MATERIALS AND METHODS

We used samples from the mid-substance of the medial collateral and the anterior cruciate ligament of the knee joint, acquired from 12 fresh knee joints. Routine histological analysis was performed through hematoxylin and eosin stain, Mallory's trichrome stain and Van Gieson's stain. The immunohistochemical analysis was conducted using monoclonal antibodies against collagen type I and V and procollagen type III. The number of cells in the epiligament, endoligament and the ligament tissue was assessed quantitatively through a computerized system for image analysis NIS-Elements Advanced Research and Statistica software.

RESULTS

Our observations revealed certain differences in the morphology of the epiligament, as well as variations in the expression of the investigated molecules. Expression of collagen type I was mostly low-positive (1+) in the epiligament and positive (2+) in the ligament tissue of both ligaments. Expression of procollagen type III was mostly positive (2+) in the epiligament and ligament tissue of the medial collateral ligament, low-positive (1+) in the epiligament and negative (0) in ligament tissue of the anterior cruciate ligament. Expression of collagen type V was predominantly low-positive (1+) in the epiligament and negative (0) in the ligament tissue of both ligaments. The immunoreactivity for all three molecules was always higher in the epiligament of the medial collateral ligament than that of the anterior cruciate ligament.

CONCLUSIONS

The results of our study illustrate for the first time that fibroblasts in the human epiligament are indeed responsible for the synthesis of the main types of collagen participating in the early ligament healing, thus corresponding to previous data of the medial collateral ligament healing in animal models. The differences between the epiligament of the investigated ligaments could add a novel explanation for the failed anterior cruciate ligament healing.

The altered mechanical phenotype of fetal fibroblasts hinders myofibroblast differentiation

During the dermal wound healing process, the mechanical rigidity of the newly deposited extracellular matrix and transforming growth factor‐β1 promote the transition of fibroblasts into myofibroblasts. Myofibroblasts generate large cellular forces that contract and remodel the extracellular matrix leading to scar formation. In contrast, myofibroblasts are not detected in fetal dermal wounds which are more compliant and contain less transforming growth factor‐β1 than adult wounds. Instead, fetal fibroblasts orchestrate scarless healing of dermal wounds resulting in healed tissues that resemble uninjured dermis. While these biomechanical differences suggest that the fetal wound environment promotes smaller cellular forces which enable regeneration, previous studies indicate that fetal fibroblasts have unique contractile properties that may facilitate scarless dermal repair. Therefore, we tested whether physiologic wound rigidities and transforming growth factor‐β1 induce contractile forces and myofibroblast differentiation of fetal dermal fibroblasts. In comparison to their adult dermal counterparts, we found that fetal fibroblasts exhibit a deficient contractile response to rigid extracellular matrix and transforming growth factor‐β1. Our data suggest that the contractile phenotype of fetal dermal fibroblasts limits their cellular force production and prevents their ability to differentiate into myofibroblasts.

The Contractile Phenotype of Dermal Fetal Fibroblasts in Scarless Wound Healing

PURPOSE OF REVIEW

Injured skin in the mammalian fetus can heal regeneratively due to the ability of fetal fibroblasts to effectively reorganize the extracellular matrix (ECM). This process occurs without fetal fibroblasts differentiating into highly contractile myofibroblasts which cause scarring and fibrosis in adult wounds. Here, we provide a brief review of fetal wound healing and the evidence supporting a unique contractile phenotype in fetal fibroblasts. Furthermore, we discuss the biomechanical role of the ECM in driving myofibroblast differentiation in wound healing and the implications for new clinical modalities based on the biophysical properties of fetal fibroblasts.

RECENT FINDINGS

We and others have found that fetal fibroblasts are refractory to the environmental stimuli necessary for myofibroblast differentiation in adult wound healing including mechanical stress.

SUMMARY

Understanding the biomechanical mechanisms that regulate the contractile phenotype of fetal fibroblasts may unlock new avenues for anti-scarring therapies that target myofibroblast differentiation of adult fibroblasts.

Collagen-cellulose composite thin films that mimic soft-tissue and allow stem-cell orientation

Mechanical properties of collagen films are less than ideal for biomaterial development towards musculoskeletal repair or cardiovascular applications. Herein, we present a collagen-cellulose composite film (CCCF) compared against swine small intestine submucosa in regards to mechanical properties, cell growth, and histological analysis. CCCF was additionally characterized by FE-SEM, NMR, mass spectrometry, and Raman Microscopy to elucidate its physical structure, collagen-cellulose composition, and structure activity relationships. Mechanical properties of the CCCF were tested in both wet and dry environments, with anisotropic stress-strain curves that mimicked soft-tissue. Mesenchymal stem cells, human umbilical vein endothelial cells, and human coronary artery smooth muscle cells were able to proliferate on the collagen films with specific cell orientation. Mesenchymal stem cells had a higher proliferation index and were able to infiltrate CCCF to a higher degree than small intestine submucosa. With the underlying biological properties, we present a collagen-cellulose composite film towards forthcoming biomaterial-related applications.

Differential expressions of lysyl oxidase family in ACL and MCL fibroblasts after mechanical injury

Lysyl oxidase (LOX) family has the capacity to catalyse the cross-linking of collagen and elastin, implicating its important fundamental roles in tissue development and injury healing. However, the variations in expression of the LOX family in the normal and injured anterior cruciate ligament (ACL) are not fully known. To better understand the role of LOX family in the self-healing inability mechanism of injured ACL, this study is to measure the LOX family's differential expressions in ACL and medial collateral ligament (MCL) fibroblasts after mechanical injury induced by using an equi-biaxial stretching chamber. The cells received various degrees of mechanical stretch 0% (resting state), 6% (physiological state) and 12% (injurious state), respectively. The gene profile and protein expressions were analysed by semi-quantitative PCR, quantitative real-time PCR and Western blotting. At physiological state, gene expression showed LOX in ACL was 2.6-5.2 folds higher than that in MCL in all culture time periods, LOXL-4 1.2-3.6 folds, but LOXL-3 in MCL showed 1.1-4.8 folds higher than that in ACL. In injurious state, MCL gene expressions were 2.8-29.6 folds higher than ACL in LOX, LOXL-2, LOXL-3 and LOXL-4 at 2, 6 and 12h periods. These differential expression profiles of the LOX family in the two ligament tissues were further used to explain the intrinsic differences between ACL and MCL, and why injured ACL could not be amenable to repair itself, whereas MCL could.

Differential response to CoCl2-stimulated hypoxia on HIF-1α, VEGF, and MMP-2 expression in ligament cells

The adult human anterior cruciate ligament (ACL) has a poor functional healing response, whereas the medial collateral ligament (MCL) does not. The difference in intrinsic properties of these ligament cells can be due to their different response to their located microenvironment. Hypoxia is a key environmental regulator after ligament injury. In this study, we investigated the differential response of ACL and MCL fibroblasts to hypoxia on hypoxia-inducible factor-1α, vascular endothelial growth factor, and matrix metalloproteinase-2 (MMP-2) expression. Our results show that ACL cells responded to hypoxia by up-regulating the HIF-1α expression significantly as compared to MCL cells. We also observed that in MCL fibroblasts response to hypoxia resulted in increase in expression of VEGF as compared to ACL fibroblasts. After hypoxia treatment, mRNA and protein levels of MMP-2 increased in both ACL and MCL. Furthermore we found in ACL pro-MMP-2 was converted more into active form. However, hypoxia decreased the percentage of wound closure for both ligament cells and had a greater effect on ACL fibroblasts. These results demonstrate that ACL and MCL fibroblasts respond differently under the hypoxic conditions suggesting that these differences in intrinsic properties may contribute to their different healing responses and abilities.

Combined effects of TNF-α, IL-1β, and HIF-1α on MMP-2 production in ACL fibroblasts under mechanical stretch: an in vitro study

The dynamics between inflammatory factors, mechanical stress, and healing factors, in an intra-articular joint, are very complex after injury. Injury to intra-articular tissue [anterior cruciate ligament (ACL), synovium] results in hypoxia, accumulation of various pro-inflammatory factors, cytokines, and metalloproteases. Although the presence of increased amounts of matrix-metalloproteinases (MMP) in the joint fluid after knee injury is considered the key factor for ACL poor healing ability; however, the exact role of collective participants of the joint fluid on MMP-2 activity and production has not been fully studied yet. To investigate the combined effects of mechanical injury, inflammation and hypoxia induced factor-1α (HIF-1α) on induction of MMP-2; we mimicked the microenvironment of joint cavity after ACL injury. The results show that TNF-α and IL-1β elevate the activity of MMP-2 in a dose- and time-dependent manner. In addition, mechanical stretch further enhances the MMP-2 protein levels with TNF-α, IL-1β, and their mixture. CoCl(2) -induced HIF-1α (100 and 500 µM) also increases the levels and activity of MMP-2. Mechanical stretch has a strong additional effect on MMP-2 production with HIF-1α. Our results conclude that mechanical injury, HIF-1α and inflammatory factors collectively induce increased MMP-2 production in ACL fibroblasts, which was inhibited by NF-κB pathway inhibitor (Bay-11-7082).

GDF-5/7 and bFGF activate integrin alpha2-mediated cellular migration in rabbit ligament fibroblasts

Cellular activities responding to growth factors are important in ligament healing. The anterior cruciate ligament (ACL) has poor healing potential compared to the medial collateral ligament (MCL). To assess the differences, we investigated the proliferation, migration, adhesion, and matrix synthesis responding to growth factors in rabbit ACL and MCL fibroblasts. ACL cell proliferation to basic fibroblast growth factor (bFGF), bone morphogenetic protein-2, growth and differentiation factor (GDF)-5, and GDF-7 treatment was similar to that of MCL cells. GDF-5 enhanced Col1a1 expression in ACL and MCL fibroblasts up to 4.7- and 17-fold levels of control, respectively. MCL fibroblasts showed stronger migration activities in response to bFGF and GDF-5 than ACL cells. GDF-5/7 and bFGF also changed the stress fiber formation and cellular adhesion by modulating the distribution of integrin alpha2. Functional blocking analyses using anti-integrin alpha2 antibodies revealed that cellular migration responding to growth factors depended on the integrin alpha2-mediated adhesion on type I collagen. The expression of integrin alpha2 was also increased by growth factors in both cells. Our results demonstrate that GDF-5/7 and bFGF stimulate cellular migration by modulating integrin alpha2 expression and integrin alpha2-dependent adhesion, especially in MCL fibroblasts. These findings suggest that the different healing potential between ACL and MCL may be caused by different cellular behavior in the integrin alpha2-mediated cellular migration in response to growth factors.

Temporal effect of functional blocking of beta1 integrin on cell adhesion strength under serum depletion

Cell adhesion is generally concomitant to the formation of focal adhesion. Although it is well-known that focal adhesion plays an important role in the functional regulations of anchorage dependent cells, previous experimental studies have not provided quantitative description of the relation between focal adhesion and biophysical responses of cells. Furthermore, there is lack of knowledge on the importance of the beta1 integrin subunit to the dynamic responses of cells during initial cell seeding. In this study, we attempt to bridge the quantitative connection between focal adhesion density and cell-substrate interactions and evaluate the influence on functional blocking of beta1 integrin on adhesion strength. Total internal reflection fluorescence microscopy (TIRFM), fluorescence microscopy, and phase contrast microscopy was employed to study the time-dependent evolvement of vinculin pattern, distribution of actin filament, and morphological change, respectively, during 4 h of culture for porcine esophageal fibroblasts (non-blocked and beta1-blocked) on a fibronectin-coated surface. Micropipet aspiration technique was used to study the change of mechanotransduction through the determination of adhesion force and strength. It is shown in our experimental results that spread area, adhesion force, and adhesion strength increases over time on the two types of cells. Throughout the culture period, the two key mechanotransduction parameters of non-blocked cells is higher than those of beta1-blocked cells. Interestingly, adhesion strength initially ascends, then begins to diminish at a critical time point, and finally resumes increasing linearly against the increase of focal adhesion density. This variation as mentioned above can be explained by peeling and fracture models based on the dissimilar vinculin pattern of cells after being cultured for different time periods. Moreover, the averaged focal adhesion strength and non-focal adhesion strength of beta1-blocked cells are significantly less than those of non-blocked of cells. The weaker adhesion strength on beta(1)-blocked cells is directly caused by lower focal and non-focal adhesion strength, as well as by smaller focal adhesion density.

Effects of applied DC electric field on ligament fibroblast migration and wound healing

Applied electric fields (static and pulsing) are widely used in orthopedic practices to treat nonunions and spine fusions and have been shown to improve ligament healing in vivo. Few studies, however, have addressed the effect of electric fields (EFs) on ligament fibroblast migration and biosynthesis. In the current study, we applied static and pulsing direct current (DC) EFs to calf anterior cruciate ligament (ACL) fibroblasts. ACL fibroblasts demonstrated enhanced migration speed and perpendicular alignment to the applied EFs. The motility of ligament fibroblasts was further modulated on type I collagen. In addition, type I collagen expression increased in ACL fibroblasts after exposure to pulsing EFs. In vitro wound-healing studies showed inhibitory effects of static EFs, which were alleviated with a pulsing EF. Our results demonstrate that applied EFs augment ACL fibroblast migration and biosynthesis and provide potential mechanisms by which EFs may be used for enhancing ligament healing and repair.

Differential MMP-2 activity of ligament cells under mechanical stretch injury: an in vitro study on human ACL and MCL fibroblasts

Recent studies have revealed that following injuries, ligament tissues such as anterior cruciate ligaments (ACL), release large amounts of matrix metalloproteinases (MMPs). These enzymes have a devastating effect on the healing process of the injured ligaments. Although these enzymes are produced following ligament injuries, because of different healing capacities seen between the medial collateral ligament (MCL) and ACL, we were curious to find if the MMP activity was expressed and modulated differently in these tissues. For this purpose ACL and MCL fibroblasts were seeded on equi-biaxial stretch chambers and were stretched in different levels. The stretched cells were assayed using Zymography, Western Blot and global MMP activity assays. The results showed that within 72 h after injurious stretch, production of 72 kD pro-MMP-2 increased in both ACL and MCL. However, the ACL fibroblasts generated significantly more pro-MMP-2 than the MCL fibroblasts. Furthermore we found in ACL pro-MMP-2 was converted more into active form. With 4-aminophenyl mercuric acetate (APMA) treatment, large amounts of pro-MMP-2 were converted into active form in both. This indicates that there is no significant difference between ACL and MCL fibroblasts in post-translational modification of MMP-2. The fluorescent MMP activity assays revealed that the MMP family activities were higher in the injured ACL fibroblasts than the MCL. Since the MMPs are critically involved in extracellular matrix (ECM) turnover, these findings may explain one of the reasons why the injured ACL hardly repairs. The higher levels of active MMP-2 seen in the ACL injuries may disrupt the delicate balance of ECM remodeling process. These results suggest that the generation and modulation of MMP-2 may be directly involved in the different responses seen in ACL and MCL injuries.

Tissue Engineering of Ligaments: A Comparison of Bone Marrow Stromal Cells, Anterior Cruciate Ligament, and Skin Fibroblasts as Cell Source

Anterior cruciate ligament (ACL) reconstruction surgery still has important problems to overcome, such as "donor site morbidity" and the limited choice of grafts in revision surgery. Tissue engineering of ligaments may provide a solution for these problems. Little is known about the optimal cell source for tissue engineering of ligaments. The aim of this study is to determine the optimal cell source for tissue engineering of the anterior cruciate ligament. Bone marrow stromal cells (BMSCs), ACL, and skin fibroblasts were seeded onto a resorbable suture material [poly(L-lactide/glycolide) multifilaments] at five different seeding densities, and cultured for up to 12 days. All cell types tested attached to the suture material, proliferated, and synthesized extracellular matrix rich in collagen type I. On day 12 the scaffolds seeded with BMSCs showed the highest DNA content (p < 0.01) and the highest collagen production (p < 0.05 for the two highest seeding densities). Scaffolds seeded with ACL fibroblasts showed the lowest DNA content and collagen production. Accordingly, BMSCs appear to be the most suitable cells for further study and development of tissue-engineered ligament.