[Show abstract][Hide abstract] ABSTRACT: Type I collagen-containing fibrils are major structural components of the extracellular matrix of vertebrate tissues, especially tendon, but how they are formed is not fully understood. MMP14 is a potent pericellular collagenase that can cleave type I collagen in vitro. Here we show that tendon development is arrested in Scleraxis-Cre::Mmp14 lox/lox mice that are unable to release collagen fibrils from plasma membrane fibripositors. In contrast to its role in collagen turnover in adult tissue, MMP14 promotes embryonic tissue formation by releasing collagen fibrils from the cell surface. Notably, tendons grow to normal size and collagen fibril release from fibripositors occurs in Col-r/r mice that have a mutated collagen-I that is uncleavable by MMPs. Furthermore, fibronectin (not collagen-I) accumulates in tendons of Mmp14-null mice. We propose a model for cell-regulated collagen fibril assembly during tendon development in which MMP14 cleaves a molecular bridge tethering collagen fibrils to the plasma membrane of fibripositors.
[Show abstract][Hide abstract] ABSTRACT: eLife digest
Young animals are able to grow in a way that allows them to maintain roughly the same shape until they reach their adult size. The growth of embryos is driven by increases in cell size and number, but it is less clear how the body grows after birth. By this point, many of the cells in the body are part of tendons and other fibrous tissues, where they are surrounded by a mesh of fibres made of collagen and other proteins. These fibres provide strength to the tissue, but may also restrict its ability to grow.
Tendons connect muscles to bones. They contain fibres of collagen that run along their length, which enables them to cope with very strong pulling forces. Kalson et al. used electron microscopy to generate highly detailed three-dimensional models of mouse tendons at three stages: in the embryo, at birth and six weeks later.
The experiments identified two stages in tendon development. During the first stage, the number of cells and fibres across the tendon is determined in the embryo. The fibres also slightly expand in diameter and form regular waves called crimps that are important for the structural strength of the tendon. The second stage happens after birth, during which the number of cells and fibres remains constant, but the tendons continue to grow because the fibres increase in diameter and length. The cells also move to form towers of cells running along the tendon.
From these observations, Kalson et al. propose that the numbers and locations of the cells and collagen fibres that determine the shape and size of tendons are established in the embryo. The collagen fibres create a framework for the continued growth of the tendon after birth. Future challenges are to understand how the number and the arrangement of cells in the tendon is determined before the collagen fibres are made, and how these cells control the number of collagen fibres that form.
[Show abstract][Hide abstract] ABSTRACT: Desmosomes and adherens junctions are intercellular adhesive structures essential for the development and integrity of vertebrate tissue, including the epidermis and heart. Their cell adhesion molecules are cadherins: type 1 cadherins in adherens junctions and desmosomal cadherins in desmosomes. A fundamental difference is that desmosomes have a highly ordered structure in their extracellular region and exhibit calcium-independent hyperadhesion, whereas adherens junctions appear to lack such ordered arrays, and their adhesion is always calcium-dependent. We present here the structure of the entire ectodomain of desmosomal cadherin desmoglein 2 (Dsg2), using a combination of small-angle X-ray scattering, electron microscopy, and solution-based biophysical techniques. This structure reveals that the ectodomain of Dsg2 is flexible even in the calcium-bound state and, on average, is shorter than the type 1 cadherin crystal structures. The Dsg2 structure has an excellent fit with the electron tomography reconstructions of human desmosomes. This fit suggests an arrangement in which desmosomal cadherins form trans interactions but are too far apart to interact in cis, in agreement with previously reported observations. Cadherin flexibility may be key to explaining the plasticity of desmosomes that maintain tissue integrity in their hyperadhesive form, but can adopt a weaker, calcium-dependent adhesion during wound healing and early development.
Full-text · Article · Apr 2015 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The small GTPase RhoA is a major regulator of actin reorganization during the formation of stress fibers; thus identifying molecules that regulate Rho activity is necessary for a complete understanding of the mechanisms that determine cell contractility. Here, we have identified Arhgap28 as a Rho GTPase activating protein (RhoGAP) that switches RhoA to its inactive form. We generated an Arhgap28-LacZ reporter mouse that revealed gene expression in soft tissues at E12.5, pre-bone structures of the limb at E15.5, and prominent expression restricted mostly to ribs and limb long bones at E18.5 days of development. Expression of recombinant Arhgap28-V5 in human osteosarcoma SaOS-2 cells caused a reduction in the basal level of RhoA activation and disruption of actin stress fibers. Extracellular matrix assembly studies using a 3-dimensional cell culture system showed that Arhgap28 was upregulated during Rho-dependent assembly of the ECM. Taken together, these observations led to the hypothesis that an Arhgap28 knockout mouse model would show a connective tissue phenotype, perhaps affecting bone. Arhgap28-null mice were viable and appeared normal, suggesting that there could be compensation from other RhoGAPs. Indeed, we showed that expression of Arhgap6 (a closely related RhoGAP) was upregulated in Arhgap28-null bone tissue. An upregulation in RhoA expression was also detected suggesting that Arhgap28 may be able to additionally regulate Rho signaling at a transcriptional level. Microarray analyses revealed that Col2a1, Col9a1, Matn3, and Comp that encode extracellular matrix proteins were downregulated in Arhgap28-null bone. Although mutations in these genes cause bone dysplasias no bone phenotype was detected in the Arhgap-28 null mice. Together, these data suggest that the regulation of Rho by RhoGAPs, including Arhgap28, during the assembly and development of mechanically strong tissues is complex and may involve multiple RhoGAPs.
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Nov 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Collagen fibrils are the major tensile element in vertebrate tissues, in which they occur as ordered bundles in the extracellular matrix. Abnormal fibril assembly and organization results in scarring, fibrosis, poor wound healing and connective tissue diseases. Transmission electron microscopy (TEM) is used to assess the formation of the fibrils, predominantly by measuring fibril diameter. Here we describe a protocol for measuring fibril diameter as well as fibril volume fraction, mean fibril length, fibril cross-sectional shape and fibril 3D organization, all of which are major determinants of tissue function. Serial-section TEM (ssTEM) has been used to visualize fibril 3D organization in vivo. However, serial block face-scanning electron microscopy (SBF-SEM) has emerged as a time-efficient alternative to ssTEM. The protocol described below is suitable for preparing tissues for TEM and SBF-SEM (by 3View). We describe how to use 3View for studying collagen fibril organization in vivo and show how to find and track individual fibrils. The overall time scale is ∼8 d from isolating the tissue to having a 3D image stack.
[Show abstract][Hide abstract] ABSTRACT: Scaling relationships have been formulated to investigate the influence of collagen fibril diameter (D) on age-related variations in the strain energy density of tendon. Transmission electron microscopy was used to quantify D in tail tendon from 1.7- to 35.3-mo-old (C57BL/6) male mice. Frequency histograms of D for all age groups were modeled as two normally distributed subpopulations with smaller (D(D1)) and larger (D(D2)) mean Ds, respectively. Both D(D1) and D(D2) increase from 1.6 to 4.0 mo but decrease thereafter. From tensile tests to rupture, two strain energy densities were calculated: 1) u(E) [from initial loading until the yield stress (σ(Y))], which contributes primarily to tendon resilience, and 2) u(F) [from σ(Y) through the maximum stress (σ(U)) until rupture], which relates primarily to resistance of the tendons to rupture. As measured by the normalized strain energy densities u(E)/σ(Y) and u(F)/σ(U), both the resilience and resistance to rupture increase with increasing age and peak at 23.0 and 4.0 mo, respectively, before decreasing thereafter. Multiple regression analysis reveals that increases in u(E)/σ(Y) (resilience energy) are associated with decreases in D(D1) and increases in D(D2), whereas u(F)/σ(U) (rupture energy) is associated with increases in D(D1) alone. These findings support a model where age-related variations in tendon resilience and resistance to rupture can be directed by subtle changes in the bimodal distribution of Ds.
[Show abstract][Hide abstract] ABSTRACT: The genome sequences of enterohaemorrhagic E. coli O157:H7 strains show multiple open-reading frames with collagen-like sequences that are absent from the common laboratory strain K-12. These putative collagens are included in prophages embedded in O157:H7 genomes. These prophages carry numerous genes related to strain virulence and have been shown to be inducible and capable of disseminating virulence factors by horizontal gene transfer. We have cloned two collagen-like proteins from E. coli O157:H7 into a laboratory strain and analysed the structure and conformation of the recombinant proteins and several of their constituting domains by a variety of spectroscopic, biophysical, and electron microscopy techniques. We show that these molecules exhibit many of the characteristics of vertebrate collagens, including trimer formation and the presence of a collagen triple helical domain. They also contain a C-terminal trimerization domain, and a trimeric α-helical coiled-coil domain with an unusual amino acid sequence almost completely lacking leucine, valine or isoleucine residues. Intriguingly, these molecules show high thermal stability, with the collagen domain being more stable than those of vertebrate fibrillar collagens, which are much longer and post-translationally modified. Under the electron microscope, collagen-like proteins from E. coli O157:H7 show a dumbbell shape, with two globular domains joined by a hinged stalk. This morphology is consistent with their likely role as trimeric phage side-tail proteins that participate in the attachment of phage particles to E. coli target cells, either directly or through assembly with other phage tail proteins. Thus, collagen-like proteins in enterohaemorrhagic E. coli genomes may have a direct role in the dissemination of virulence-related genes through infection of harmless strains by induced bacteriophages.
[Show abstract][Hide abstract] ABSTRACT: A distinctive feature of embryonic tendon development is the steady increase in collagen fibril diameter and associated improvement of tissue mechanical properties. A potential mechanical stimulus for these changes is slow stretching of the tendon during limb growth. Testing this hypothesis in vivo is complicated by the presence of other developmental processes including muscle development and innervation. Here we used a cell culture tendon-like construct to determine if slow stretch can explain the increases in fibril diameter and mechanical properties that are observed in vivo. Non-stretched constructs had an ultrastructural appearance and mechanical properties similar to those of early embryonic tendon. However, slowly stretching during 4 days in culture increased collagen fibril diameter, fibril packing volume, and mechanical stiffness, and thereby mimicked embryonic development. 3D EM showed cells with improved longitudinal alignment and elongated nuclei, which raises the hypothesis that nuclear deformation could be a novel mechanism during tendon development.
Full-text · Article · Nov 2011 · Developmental Dynamics
[Show abstract][Hide abstract] ABSTRACT: Tendons are composed of longitudinally aligned collagen fibrils arranged in bundles with an undulating pattern, called crimp. The crimp structure is established during embryonic development and plays a vital role in the mechanical behaviour of tendon, acting as a shock-absorber during loading. However, the mechanism of crimp formation is unknown, partly because of the difficulties of studying tendon development in vivo. Here, we used a 3D cell culture system in which embryonic tendon fibroblasts synthesise a tendon-like construct comprised of collagen fibrils arranged in parallel bundles. Investigations using polarised light microscopy, scanning electron microscopy and fluorescence microscopy showed that tendon constructs contained a regular pattern of wavy collagen fibrils. Tensile testing indicated that this superstructure was a form of embryonic crimp producing a characteristic toe region in the stress-strain curves. Furthermore, contraction of tendon fibroblasts was the critical factor in the buckling of collagen fibrils during the formation of the crimp structure. Using these biological data, a finite element model was built that mimics the contraction of the tendon fibroblasts and monitors the response of the Extracellular matrix. The results show that the contraction of the fibroblasts is a sufficient mechanical impulse to build a planar wavy pattern. Furthermore, the value of crimp wavelength was determined by the mechanical properties of the collagen fibrils and inter-fibrillar matrix. Increasing fibril stiffness combined with constant matrix stiffness led to an increase in crimp wavelength. The data suggest a novel mechanism of crimp formation, and the finite element model indicates the minimum requirements to generate a crimp structure in embryonic tendon.
Full-text · Article · Jul 2011 · Biomechanics and Modeling in Mechanobiology
[Show abstract][Hide abstract] ABSTRACT: Tendon-like tissue generated from stem cells in vitro has the potential to replace tendons and ligaments lost through injury and disease. However, thus far, no information has been available on the mechanism of tendon formation in vitro and how to accelerate the process. We show here that human mesenchymal stem cells (MSCs) and bone marrow-derived mononuclear cells (BM-MNCs) can generate tendon-like tissue in 7days mediated by transforming growth factor (TGF) β3. MSCs cultured in fixed-length fibrin gels spontaneously synthesized narrow-diameter collagen fibrils and exhibited fibripositors (actin-rich, collagen fibril-containing plasma membrane protrusions) identical to those that occur in embryonic tendon. In contrast, BM-MNCs did not synthesize tendon-like tissue under these conditions. We performed real-time PCR analysis of MSCs and BM-MNCs. MSCs upregulated genes encoding type I collagen, TGFβ3, and Smad2 at the time of maximum contraction of the tendon-like tissue (7days). Western blot analysis showed phosphorylation of Smad2 at maximum contraction. The TGFβ inhibitor SB-431542, blocked the phosphorylation of Smad2 and stopped the formation of tendon-like tissue. Quantitative PCR showed that BM-MNCs expressed very low levels of TGFβ3 compared to MSCs. Therefore we added exogenous TGFβ3 protein to BM-MNCs in fibrin gels, which resulted in phosphorylation of Smad2, synthesis of collagen fibrils, the appearance of fibripositors at the plasma membrane, and the formation of tendon-like tissue. In conclusion, MSCs that self-generate TGFβ signaling or the addition of TGFβ3 protein to BM-MNCs in fixed-length fibrin gels spontaneously make embryonic tendon-like tissue in vitro within 7days.
Full-text · Article · Oct 2010 · Matrix biology: journal of the International Society for Matrix Biology
[Show abstract][Hide abstract] ABSTRACT: Tendons attach muscles to bone and thereby transmit tensile forces during joint movement. However, a detailed understanding of the mechanisms that establish the mechanical properties of tendon has remained elusive because of the practical difficulties of studying tissue mechanics in vivo. Here we have performed a study of tendon-like constructs made by culturing embryonic tendon cells in fixed-length fibrin gels. The constructs display mechanical properties (toe-linear-fail stress-strain curve, stiffness, ultimate tensile strength, and failure strain) as well as collagen fibril volume fraction and extracellular matrix (ECM)/cell ratio that are statistically similar to those of embryonic chick metatarsal tendons. The development of mechanical properties during time in culture was abolished when the constructs were treated separately with Triton X-100 (to solubilise membranes), cytochalasin (to disassemble the actin cytoskeleton) and blebbistatin (a small molecule inhibitor of non-muscle myosin II). Importantly, these treatments had no effect on the mechanical properties of the constructs that existed prior to treatment. Live-cell imaging and (14)C-proline metabolic labeling showed that blebbistatin inhibited the contraction of the constructs without affecting cell viability, procollagen synthesis, or conversion of procollagen to collagen. In conclusion, the mechanical properties per se of the tendon constructs are attributable to the ECM generated by the cells but the improvement of mechanical properties during time in culture was dependent on non-muscle myosin II-derived forces.
Full-text · Article · Oct 2010 · Matrix biology: journal of the International Society for Matrix Biology
[Show abstract][Hide abstract] ABSTRACT: Collagen fibrils are the principal tensile element of vertebrate tissues where they occur in the extracellular matrix as spatially organised arrays. A major challenge is to understand how the mechanisms of nucleation, growth and remodelling yield fibrils of tissue-specific diameter and length. Here we have developed a seeding system whereby collagen fibrils were isolated from avian embryonic tendon and added to purified collagen solution, in order to characterise fibril surface nucleation and growth mechanisms. Fragmentation of tendon in liquid nitrogen followed by Dounce homogenisation generated fibril length fragments. Most (>94%) of the fractured ends of fibrils, which show an abrupt square profile, were found to act as nucleation sites for further growth by molecular accretion. The mechanism of this nucleation and growth process was investigated by transmission electron microscopy, atomic force microscopy and scanning transmission electron microscopy mass mapping. Typically, a single growth spur occurred on the N-terminal end of seed fibrils whilst twin spurs frequently formed on the C-terminal end before merging into a single tip projection. The surface nucleation and growth process generated a smoothly tapered tip that achieved maximum diameter when the axial extension reached approximately 13 mum. Lateral growth also occurred along the entire length of all seed fibrils that contained tip projections. The data support a model of collagen fibril growth in which the broken ends of fibrils are nucleation sites for propagation in opposite axial directions. The observed fibril growth behaviour has direct relevance to tendon matrix remodelling and repair processes that might involve rupture of collagen fibrils.
[Show abstract][Hide abstract] ABSTRACT: The inflammation-associated long pentraxin PTX3 plays key roles in innate immunity, female fertility, and vascular biology (e.g. it inhibits FGF2 (fibroblast growth factor 2)-mediated angiogenesis). PTX3 is composed of multiple protomers, each composed of distinct N- and C-terminal domains; however, it is not known how these are organized or contribute to its functional properties. Here, biophysical analyses reveal that PTX3 is composed of eight identical protomers, associated through disulfide bonds, forming an elongated and asymmetric, molecule with two differently sized domains interconnected by a stalk. The N-terminal region of the protomer provides the main structural determinant underlying this quaternary organization, supporting formation of a disulfide-linked tetramer and a dimer of dimers (a non-covalent tetramer), giving rise to the asymmetry of the molecule. Furthermore, the PTX3 octamer is shown to contain two FGF2 binding sites, where it is the tetramers that act as the functional units in ligand recognition. Thus, these studies provide a unifying model of the PTX3 oligomer, explaining both its quaternary organization and how this is required for its antiangiogenic function.
[Show abstract][Hide abstract] ABSTRACT: The ability of tendon to withstand tensile forces is largely attributable to an extracellular matrix containing parallel collagen fibrils organized into fascicles. A major belief is that force is transmitted between collagen fibrils via interactions of molecules at the fibril surface. However, there is existing evidence (reviewed here) for persistent connections between fibrils formed by interfibrillar fusion. Furthermore, in vitro studies have shown the ability of the ends of fibrils to fuse together. In this study, we show using serial section electron microscopy of embryonic mouse-tail tendon further evidence for interfibril fusion in vivo. We showed: (1) fibrils fused via Y-shaped branches without disruption of the 67 nm D-periodicity, (2) the frequency of the branches was approximately 1:20 000 D-periods, and (3) the small angle of the Y ranged from 4 degrees to 10 degrees, indicating a structure-based mechanism of branch formation. The regular occurrence of Y-shaped branches between collagen fibrils suggests direct force transmission between fibrils. Furthermore, the formation of the Y-shaped branches by tip-to-shaft fusion would explain the paucity of fibril tips in vivo.
No preview · Article · May 2009 · Scandinavian Journal of Medicine and Science in Sports
[Show abstract][Hide abstract] ABSTRACT: Conventional preparation techniques for electron microscopy employ contrast enhancing heavy metal stains in solution to visualize isolated macromolecules. In rotary shadowing electron microscopy, the heavy metal is evaporated onto surface adsorbed molecules and macromolecular assemblies. High resolution shadowing remains a valuable method for the visualization and characterization of extracellular matrix macromolecules including fibrillar collagens, microfibrillar elements, and glycoproteins.
No preview · Article · Feb 2009 · Methods in Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: In the scanning transmission electron microscope, the degree of electron scattering induced by biological specimens, such as ECM macromolecules, is dependent on the molecular mass. By calibrating the ratio of scattered to non-scattered electrons against a known mass standard, such as tobacco mosaic virus, it is possible to quantify absolute changes in both mass and mass distribution. These mass mapping approaches can provide important information on ECM assembly, organisation, and interactions which is not obtainable by other means.
Full-text · Article · Feb 2009 · Methods in Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: Tissue development in multicellular animals relies on the ability of cells to synthesise an extracellular matrix (ECM) containing spatially-organised fibrous assemblies, the most widespread of which is based on collagen fibrils whose length greatly exceeds that of individual cells. The importance of the correct regulation of fibril deposition is exemplified in diseases such as osteogenesis imperfecta (caused by mutations in collagen genes), fibrosis (caused by ectopic accumulation of collagen) and cardiovascular disease (which involves cells and macromolecules binding to collagen in the vessel wall). Much is known about the molecular biology of collagens but less is known about collagen fibril structure and how the fibrils are formed (fibrillogenesis). This is explained in part by the fact that the fibrils are non-crystalline, extensively cross-linked, and very large, which makes them refractory to study by conventional biochemical and high-resolution structure-determination techniques. Electron microscopy has become established as the method of choice for studying collagen fibril structure and assembly, and this article describes the electron microscope methods most often used.
[Show abstract][Hide abstract] ABSTRACT: Embryonic tendon cells (ETCs) have actin-rich fibripositors that accompany parallel bundles of collagen fibrils in the extracellular matrix. To study fibripositor function, we have developed a three-dimensional cell culture system that promotes and maintains fibripositors. We show that ETCs cultured in fixed-length fibrin gels replace the fibrin during ~6 days in culture with parallel bundles of narrow-diameter collagen fibrils that are uniaxially aligned with fibripositors, thereby generating a tendon-like construct. Fibripositors occurred simultaneously with onset of parallel collagen fibrils. Interestingly, the constructs have a tendon-like crimp. In initial experiments to study the effects of tension, we showed that cutting the constructs resulted in loss of tension, loss of fibripositors and the appearance of immature fibrils with no preferred orientation.
[Show abstract][Hide abstract] ABSTRACT: Connective tissues are biological composites comprising of collagen fibrils embedded in (and reinforcing) the hydrated proteoglycan-rich (PG) gel within the extracellular matrices (ECMs). Age-related changes to the mechanical properties of tissues are often associated with changes to the structure of the ECM, namely, fibril diameter. However, quantitative attempts to correlate fibril diameter to mechanical properties have yielded inconclusive evidence. Here, we described a novel approach that was based on the rule of mixtures for fiber composites to evaluate the dependence of age-related changes in tendon tensile strength (sigma) and stiffness (E) on the collagen fibril cross-sectional area fraction (rho), which is related to the fibril volume fraction. Tail tendons from C57BL6 mice from age groups 1.6-35.3 months old were stretched to failure to determine sigma and E. Parallel measurements of rho as a function of age were made using transmission electron microscopy. Mathematical models (rule of mixtures) of fibrils reinforcing a PG gel in tendons were used to investigate the influence of rho on ageing changes in sigma and E. The magnitudes of sigma, E, and rho increased rapidly from 1.6 months to 4.0 months (P-values <0.05) before reaching a constant (age independent) from 4.0 months to 29.0 months (P-values >0.05); this trend continued for E and rho (P-values >0.05) from 29.0 months to 35.3 months, but not for sigma, which decreased gradually (P-values <0.05). Linear regression analysis revealed that age-related changes in sigma and E correlated positively to rho (P-values <0.05). Collagen fibril cross-sectional area fraction rho is a significant predictor of ageing changes in sigma and E in the tail tendons of C57BL6 mice.
Full-text · Article · May 2008 · Journal of Biomechanical Engineering