Morphology of sheet-like assemblies of pN-collagen, pC-collagen and procollagen studied by scanning transmission electron microscopy mass measurements

Extracellular Targets to Reduce Excessive Scarring in Response to Tissue Injury

Excessive scar formation is a hallmark of localized and systemic fibrotic disorders. Despite extensive studies to define valid anti-fibrotic targets and develop effective therapeutics, progressive fibrosis remains a significant medical problem. Regardless of the injury type or location of wounded tissue, excessive production and accumulation of collagen-rich extracellular matrix is the common denominator of all fibrotic disorders. A long-standing dogma was that anti-fibrotic approaches should focus on overall intracellular processes that drive fibrotic scarring. Because of the poor outcomes of these approaches, scientific efforts now focus on regulating the extracellular components of fibrotic tissues. Crucial extracellular players include cellular receptors of matrix components, macromolecules that form the matrix architecture, auxiliary proteins that facilitate the formation of stiff scar tissue, matricellular proteins, and extracellular vesicles that modulate matrix homeostasis. This review summarizes studies targeting the extracellular aspects of fibrotic tissue synthesis, presents the rationale for these studies, and discusses the progress and limitations of current extracellular approaches to limit fibrotic healing.

Processing and Physicochemical Properties of Magnetite Nanoparticles Coated with Curcuma longa L. Extract

In this work, Curcuma longa L. extract has been used in the synthesis and direct coating of magnetite (Fe₃O₄) nanoparticles ~12 nm, providing a surface layer of polyphenol groups (-OH and -COOH). This contributes to the development of nanocarriers and triggers different bio-applications. Curcuma longa L. is part of the ginger family (Zingiberaceae); the extracts of this plant contain a polyphenol structure compound, and it has an affinity to be linked to Fe ions. The nanoparticles' magnetization obtained corresponded to close hysteresis loop M_s = 8.81 emu/g, coercive field H_c = 26.67 Oe, and low remanence energy as iron oxide superparamagnetic nanoparticles (SPIONs). Furthermore, the synthesized nanoparticles (G-M@T) showed tunable single magnetic domain interactions with uniaxial anisotropy as addressable cores at 90-180°. Surface analysis revealed characteristic peaks of Fe 2p, O 1s, and C 1s. From the last one, it was possible to obtain the C-O, C=O, -OH bonds, achieving an acceptable connection with the HepG2 cell line. The G-M@T nanoparticles do not induce cell toxicity in human peripheral blood mononuclear cells or HepG2 cells in vitro, but they can increase the mitochondrial and lysosomal activity in HepG2 cells, probably related to an apoptotic cell death induction or to a stress response due to the high concentration of iron within the cell.

Sequence-dependent mechanics of collagen reflect its structural and functional organization

Extracellular matrix mechanics influence diverse cellular functions, yet surprisingly little is known about the mechanical properties of their constituent collagen proteins. In particular, network-forming collagen IV, an integral component of basement membranes, has been far less studied than fibril-forming collagens. A key feature of collagen IV is the presence of interruptions in the triple-helix-defining (Gly-X-Y) sequence along its collagenous domain. Here, we used atomic force microscopy to determine the impact of sequence heterogeneity on the local flexibility of collagen IV and of the fibril-forming collagen III. Our extracted flexibility profile of collagen IV reveals that it possesses highly heterogeneous mechanics, ranging from semiflexible regions as found for fibril-forming collagens to a lengthy region of high flexibility toward its N-terminus. A simple model in which flexibility is dictated only by the presence of interruptions fit the extracted profile reasonably well, providing insight into the alignment of chains and demonstrating that interruptions, particularly when coinciding in multiple chains, significantly enhance local flexibility. To a lesser extent, sequence variations within the triple helix lead to variable flexibility, as seen along the continuously triple-helical collagen III. We found this fibril-forming collagen to possess a high-flexibility region around its matrix-metalloprotease binding site, suggesting a unique mechanical fingerprint of this region that is key for matrix remodeling. Surprisingly, proline content did not correlate with local flexibility in either collagen type. We also found that physiologically relevant changes in pH and chloride concentration did not alter the flexibility of collagen IV, indicating such environmental changes are unlikely to control its compaction during secretion. Although extracellular chloride ions play a role in triggering collagen IV network formation, they do not appear to modulate the structure of its collagenous domain.

Inhibition of collagen fibril formation

Background

The overall aim of presented study is to test the inhibition of the formation of collagen fibrils as the novel approach to reduce accumulation of pathological fibrotic deposits. The main hypothesis is that by interfering with the initial steps of the extracellular process of collagen fibril formation, it is possible to reduce the formation of fibrotic tissue.

Methods

The experimental model includes antibody-based inhibitors that specifically bind to the sites that participate in the collagen/collagen interaction.

Results

Employed antibody-based inhibitors effectively limit the amount of collagen fibrils formed in vitro and in engineered tissue models of localized fibrosis.

Conclusions

(i) Inhibition of collagen formation is an attractive target to reduce excessive formation of fibrotic tissue.

(ii) Antibody-based inhibitors of collagen fibril formation are promising therapeutic agents with a potential to limit localized fibrosis in a number of tissues.

Collagen self-assembly and the development of tendon mechanical properties

The development of the musculoskeleton and the ability to locomote requires controlled cell division as well as spatial control over deposition of extracellular matrix. Self-assembly of procollagen and its final processing into collagen fibrils occurs extracellularly. The formation of crosslinked collagen fibers results in the conversion of weak liquid-like embryonic tissues to tough elastic solids that can store energy and do work. Collagen fibers in the form of fascicles are the major structural units found in tendon. The purpose of this paper is to review the literature on collagen self-assembly and tendon development and to relate this information to the development of elastic energy storage in non-mineralizing and mineralizing tendons. Of particular interest is the mechanism by which energy is stored in tendons during locomotion. In vivo, collagen self-assembly occurs by the deposition of thin fibrils in recesses within the cell membrane. These thin fibrils later grow in length and width by lateral fusion of intermediates. In vitro, collagen self-assembly occurs by both linear and lateral growth steps with parallel events seen in vivo; however, in the absence of cellular control and enzymatic cleavage of the propeptides, the growth mechanism is altered, and the fibrils are irregular in cross section. Results of mechanical studies suggest that prior to locomotion the mechanical response of tendon to loading is dominated by the viscous sliding of collagen fibrils. In contrast, after birth when locomotion begins, the mechanical response is dominated by elastic stretching of crosslinked collagen molecules.

Prospects and limitations of the rational engineering of fibrillar collagens

Recombinant collagens are attractive proteins for a number of biomedical applications. To date, significant progress was made in the large-scale production of nonmodified recombinant collagens; however, engineering of novel collagen-like proteins according to customized specifications has not been addressed. Herein we investigated the possibility of rational engineering of collagen-like proteins with specifically assigned characteristics. We have genetically engineered two DNA constructs encoding multi-D4 collagens defined as collagen-like proteins, consisting primarily of a tandem of the collagen II D4 periods that correspond to the biologically active region. We have also attempted to decrease enzymatic degradation of novel collagen by mutating a matrix metalloproteinase 1 cleavage site present in the D4 period. We demonstrated that the recombinant collagen alpha-chains consisting predominantly of the D4 period but lacking most of the other D periods found in native collagen fold into a typical collagen triple helix, and the novel procollagens are correctly processed by procollagen N-proteinase and procollagen C-proteinase. The nonmutated multi-D4 collagen had a normal melting point of 41 degrees C and a similar carbohydrate content as that of control. In contrast, the mutant multi-D4 collagen had a markedly lower thermostability of 36 degrees C and a significantly higher carbohydrate content. Both collagens were cleaved at multiple sites by matrix metalloproteinase 1, but the rate of hydrolysis of the mutant multi-D4 collagen was lower. These results provide a basis for the rational engineering of collagenous proteins and identifying any undesirable consequences of altering the collagenous amino acid sequences.

STEM/TEM studies of collagen fibril assembly

Quantitative scanning transmission electron microscopy (STEM), implemented on a conventional transmission electron microscope with STEM-attachment, has been a primary tool in our laboratory for the quantitative analysis of collagen fibril assembly in vivo and in vitro. Using this technique, a precise measurement of mass per unit length can be made at regular intervals along a fibril to generate an axial mass distribution (AMD). This in turn allows the number of collagen molecules to be calculated for every transverse section of the fibril along its entire length. All fibrils show a near-linear AMD in their tip regions. Only fibrils formed in tissue environments, however, show a characteristic abrupt change in mass slope along their tips. It appears that this tip growth characteristic is common to fibrils from evolutionarily diverse systems including vertebrate tendon and the mutable tissues of the echinoderms. Computer models of collagen fibril assembly have now been developed based on interpretation of the STEM data. Two alternative models have so far been generated for fibril growth by accretion; one is based on diffusion limited aggregation (DLA) and the other based on an interface-limited growth mechanism. Inter-fibrillar fusion can also contribute to the growth of fibrils in vertebrate tissues and STEM data indicates the presence of a tight regulation in this process. These models are fundamental for the hypotheses regarding how cells synthesise and spatially organise an extracellular matrix (ECM), rich in collagen fibrils.

Calcium determines the supramolecular organization of fibrillin-rich microfibrils

Microfibrils are ubiquitous fibrillin-rich polymers that are thought to provide long-range elasticity to extracellular matrices, including the zonular filaments of mammalian eyes. X-ray diffraction of hydrated bovine zonular filaments demonstrated meridional diffraction peaks indexing on a fundamental axial periodicity (D) of approximately 56 nm. A Ca2+-induced reversible change in the intensities of the meridional Bragg peaks indicated that supramolecular rearrangements occurred in response to altered concentrations of free Ca2+. In the presence of Ca2+, the dominant diffracting subspecies were microfibrils aligned in an axial 0.33-D stagger. The removal of Ca2+ caused an enhanced regularity in molecular spacing of individual microfibrils, and the contribution from microfibrils not involved in staggered arrays became more dominant. Scanning transmission electron microscopy of isolated microfibrils revealed that Ca2+ removal or addition caused significant, reversible changes in microfibril mass distribution and periodicity. These results were consistent with evidence from x-ray diffraction. Simulated meridional x-ray diffraction profiles and analyses of isolated Ca2+-containing, staggered microfibrillar arrays were used to interpret the effects of Ca2+. These observations highlight the importance of Ca2+ to microfibrils and microfibrillar arrays in vivo.

Surface located procollagen N-propeptides on dermatosparactic collagen fibrils are not cleaved by procollagen N-proteinase and do not inhibit binding of decorin to the fibril surface

Dermatosparaxis is a recessive disorder of animals (including man) which is caused by mutations in the gene for the enzyme procollagen N-proteinase and is characterised by extreme skin fragility. Partial loss of enzyme activity results in accumulation of pNcollagen (collagen with N-propeptides) and abnormal collagen fibrils in the fragile skin. How the N-propeptides persist in the tissue and how abnormal fibril morphology results in fragile skin is poorly understood. Using biochemical and quantitative mass mapping electron microscopy we showed that the collagen fibrils in the skin of a dermatosparactic calf contained 57% type I pNcollagen and 43% type I collagen and the fibrils were irregularly arranged in bundles and hieroglyphic in cross-section. Image analysis of the fibril cross-sections suggested that the deviation from circularity of dermatosparactic fibrils was caused by N-propeptides of pNcollagen being located at the fibril surface. Comparison of experimental and theoretical axial mass distributions of the fibrils showed that the N-propeptides were located to the overlap zone of the fibril D-period (where D=67 nm, the characteristic axial periodicity of collagen fibrils). Treatment of the dermatosparactic fibrils with N-proteinase did not remove the N-propeptides from the fibrils, although the N-propeptides were efficiently removed by trypsin and chymotrypsin. However, the N-propeptides were efficiently cleaved by the N-proteinase when the pNcollagen molecules were extracted from the fibrils. These results are consistent with close packing of N-propeptides at the fibril surface which prevented cleavage by the N-proteinase. Long-range axial mass determination along the fibril length showed gross non-uniformity with multiple mass bulges. Of note is the skin fragility in dermatosparaxis, and also the appearance of mass bulges along the fibril long axis symptomatic of the fragile skin of mice which lack decorin. Western blot analysis showed that the dermatosparactic fibrils bound elevated levels of the proteoglycan, compared with normal skin fibrils. The results showed that N-propeptides can distort the morphology of fibrils, that they do not inhibit binding of gap-associated macromolecules (such as decorin) and that the normal mechanical properties of skin are strongly dependent on the close association of near-cylindrical fibrils, thereby enabling maximal fibril-fibril interactions.

The Tight skin mouse: demonstration of mutant fibrillin-1 production and assembly into abnormal microfibrils

Mice carrying the Tight skin (Tsk) mutation harbor a genomic duplication within the fibrillin-1 (Fbn 1) gene that results in a larger than normal in-frame Fbn 1 transcript. In this study, the consequences of the Tsk mutation for fibrillin-containing microfibrils have been examined. Dermal fibroblasts from Tsk/+ mice synthesized and secreted both normal fibrillin (approximately 330 kD) and the mutant oversized Tsk fibrillin-1 (approximately 450 kD) in comparable amounts, and Tsk fibrillin-1 was stably incorporated into cell layers. Immunohistochemical and ultrastructural analyses of normal and Tsk/+ mouse skin highlighted differences in the gross organization and distribution of microfibrillar arrays. Rotary shadowing of high Mr preparations from Tsk/+ skin demonstrated the presence of abundant beaded microfibrils. Some of these had normal morphology and periodicity, but others were distinguished by diffuse interbeads, longer periodicity, and tendency to aggregate. The presence of a structurally abnormal population of microfibrils in Tsk/+ skin was unequivocally demonstrated after calcium chelation and in denaturating conditions. Scanning transmission electron microscopy highlighted the presence of more mass in Tsk/+ skin microfibrils than in normal mice skin microfibrils. These data indicate that Tsk fibrillin-1 polymerizes and becomes incorporated into a discrete population of beaded microfibrils with altered molecular organization.

Scanning transmission electron microscopy mass analysis of fibrillin-containing microfibrils from foetal elastic tissues

We have applied scanning transmission electron microscopy to intact native fibrillin-containing microfibrils isolated from foetal bovine elastic tissues in order to derive new insights into microfibril organisation. This technique provides quantitative data on the mass per unit length and axial mass distribution of unstained, unshadowed macromolecules. Scanning transmission electron microscopy of microfibrils from aorta, skin and nuchal ligament revealed that the beads corresponded to peaks of mass and the interbead regions to troughs of mass. These major features of axial mass distribution were characteristic of all microfibrils examined. Tissue-specific and age-dependent variations in mass were identified in microfibrils that were structurally comparable by rotary shadowing electron microscopy. Increased microfibril mass correlated with increasing gestational age. The additional mass was associated predominantly at, or close to, the bead. Some microfibril populations exhibited pronounced assymetry in their axial mass distribution. These data indicate that intact native microfibrillar assemblies from developing elastic tissues are heterogeneous in composition. Loss of mass following chondroitinase ABC or AC lyase treatment confirmed the presence of chondroitin sulphate in nuchal ligament microfibrillar assemblies.

Targeted disruption of decorin leads to abnormal collagen fibril morphology and skin fragility

Decorin is a member of the expanding group of widely distributed small leucine-rich proteoglycans that are expected to play important functions in tissue assembly. We report that mice harboring a targeted disruption of the decorin gene are viable but have fragile skin with markedly reduced tensile strength. Ultrastructural analysis revealed abnormal collagen morphology in skin and tendon, with coarser and irregular fiber outlines. Quantitative scanning transmission EM of individual collagen fibrils showed abrupt increases and decreases in mass along their axes. thereby accounting for the irregular outlines and size variability observed in cross-sections. The data indicate uncontrolled lateral fusion of collagen fibrils in the decorindeficient mice and provide an explanation for the reduced tensile strength of the skin. These findings demonstrate a fundamental role for decorin in regulating collagen fiber formation in vivo.

Collagen fibril formation

Collagen is most abundant in animal tissues as very long fibrils with a characteristic axial periodic structure. The fibrils provide the major biomechanical scaffold for cell attachment and anchorage of macromolecules, allowing the shape and form of tissues to be defined and maintained. How the fibrils are formed from their monomeric precursors is the primary concern of this review. Collagen fibril formation is basically a self-assembly process (i.e. one which is to a large extent determined by the intrinsic properties of the collagen molecules themselves) but it is also sensitive to cell-mediated regulation, particularly in young or healing tissues. Recent attention has been focused on "early fibrils' or "fibril segments' of approximately 10 microns in length which appear to be intermediates in the formation of mature fibrils that can grow to be hundreds of micrometers in length. Data from several laboratories indicate that these early fibrils can be unipolar (with all molecules pointing in the same direction) or bipolar (in which the orientation of collagen molecules reverses at a single location along the fibril). The occurrence of such early fibrils has major implications for tissue morphogenesis and repair. In this article we review the current understanding of the origin of unipolar and bipolar fibrils, and how mature fibrils are assembled from early fibrils. We include preliminary evidence from invertebrates which suggests that the principles for bipolar fibril assembly were established at least 500 million years ago.

Further evidence that the failure to cleave the aminopropeptide of type I procollagen is the cause of Ehlers-Danlos syndrome type VII

Dermal fibroblasts from a Chinese Ehlers-Danlos syndrome type VII patient synthesized approximately equal amounts of normal pro-alpha 2(I) chains of type I procollagen and abnormal ones with electrophoretic mobility of pN alpha 2(I) chains, in which the amino-propeptide (N-propeptide) was retained. Reverse-transcriptase PCR analysis of the proband's RNA showed outsplicing of the 54 base exon 6 in half of the pro-alpha 2(I) mRNAs. Exon 6 encodes 18 amino acids of the N-telopeptide which contains the procollagen N-proteinase cleavage site and a cross-link precursor lysine. Loss of these sequences would result in failure to cleave the amino-propeptide of pro-alpha 2(I) and the accumulation of pN-alpha 2(I) chains. Nucleotide sequencing analyses of the proband's COL1A2 gene showed the presence of a T to C transition at position +2 of intron 6 in one allele and the proband is heterozygous for the defect. This mutation which destroyed the consensus GT dinucleotide at the 5' splice donor site of the intron is responsible for the loss of exon 6 by exon skipping. Electron microscopic analysis of the patient's dermis showed the presence of abnormal collagen I fibrils of irregular diameter and circularity. This mutation in COL1A2 in an EDS VII patient is the first reported case in the Chinese population and is identical to one reported for another EDS-VII (Libyan) patient. The occurrence of an identical mutation in two probands of different ethnic origin is direct evidence that the mutant genotype is the cause of the EDS VII phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Growing tips of type I collagen fibrils formed in vitro are near-paraboloidal in shape, implying a reciprocal relationship between accretion and diameter

Collagen fibrils generated in vitro at 37 degrees C by enzymic removal of C-terminal propeptides from type I pC-collagen (an intermediate in the normal processing of type I procollagen to collagen containing the C-terminal propeptides but not the N-terminal propeptides) display shape polarity, with one tip fine tapered and the other coarse tapered. Mass measurements by scanning transmission electron microscopy show that the mass per unit length along both kinds of tip increases roughly linearly over distances of approximately 100 D periods from the fibril end [D (axial periodicity) = 67 nm]. The fine tips of fibrils of widely differing lengths exhibit near-identical mass distributions, the mass in all cases increasing at the rate of approximately 17 molecules per D period, irrespective of fibril length. Coarse tips display less regular behavior. These results show that (i) the shape of a fine tip is not conical but resembles more closely a paraboloid of revolution, and (ii) for this shape to be maintained throughout growth, accretion (rate of mass uptake per unit area) cannot everywhere be the same on the surface of the tip but must decrease as the diameter increases. To a first approximation, accretion alpha (diameter)-1.