The nature and location of intramolecular cross-links in collagen

Advances in Noninvasive Molecular Imaging Probes for Liver Fibrosis Diagnosis

Liver fibrosis is a wound-healing response to chronic liver injury, which may lead to cirrhosis and cancer. Early-stage fibrosis is reversible, and it is difficult to precisely diagnose with conventional imaging modalities such as magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, and ultrasound imaging. In contrast, probe-assisted molecular imaging offers a promising noninvasive approach to visualize early fibrosis changes in vivo, thus facilitating early diagnosis and staging liver fibrosis, and even monitoring of the treatment response. Here, the most recent progress in molecular imaging technologies for liver fibrosis is updated. We start by illustrating pathogenesis for liver fibrosis, which includes capillarization of liver sinusoidal endothelial cells, cellular and molecular processes involved in inflammation and fibrogenesis, as well as processes of collagen synthesis, oxidation, and cross-linking. Furthermore, the biological targets used in molecular imaging of liver fibrosis are summarized, which are composed of receptors on hepatic stellate cells, macrophages, and even liver collagen. Notably, the focus is on insights into the advances in imaging modalities developed for liver fibrosis diagnosis and the update in the corresponding contrast agents. In addition, challenges and opportunities for future research and clinical translation of the molecular imaging modalities and the contrast agents are pointed out. We hope that this review would serve as a guide for scientists and students who are interested in liver fibrosis imaging and treatment, and as well expedite the translation of molecular imaging technologies from bench to bedside.

An Allysine-Conjugatable Probe for Fluorogenically Imaging Fibrosis

Allysine, a pivotal biomarker in fibrogenesis, has prompted the development of various radioactive imaging probes. However, fluorogenic probes targeting allysine remain largely unexplored. Herein, by leveraging the equilibrium between the nonfluorescent spirocyclic and the fluorescent zwitterionic forms of rhodamine-cyanine hybrid fluorophores, we systematically fine-tuned the environmental sensitivity of this equilibrium toward the development of fluorogenic probes for fibrosis. The trick lies in modulating the nucleophilicity of the ortho-carboxyl group, which is terminated with a hydrazide group for allysine conjugation. Probe B2 was developed with this strategy, which featured an N-sulfonyl amide group and exhibited superior fibrosis-to-control imaging contrast. Initially presenting as nonfluorescent spirocyclic aggregates in aqueous solutions, B2 displayed a notable fluorogenic response upon conjugation with protein allysine through its hydrazide group, inducing deaggregation and switching to the fluorescent zwitterionic form. Probe B2 outperformed the traditional Masson stain in imaging contrast, achieving an about 260-2600-fold ratio for fibrosis-to-control detection depending on fibrosis severity. Furthermore, it demonstrated efficacy in evaluating antifibrosis drugs. Our results emphasize the potential of this fluorogenic probe as an alternative to conventional fibrosis detection methods. It emerges as a valuable tool for antifibrosis drug evaluation.

Novel tracers for molecular imaging of interstitial lung disease: A state of the art review

Interstitial lung disease is an overarching term for a wide range of disorders characterized by inflammation and/or fibrosis in the lungs. Most prevalent forms, among others, include idiopathic pulmonary fibrosis (IPF) and connective tissue disease associated interstitial lung disease (CTD-ILD). Currently, only disease modifying treatment options are available for IPF and progressive fibrotic CTD-ILD, leading to reduction or stabilization in the rate of lung function decline at best. Management of these patients would greatly advance if we identify new strategies to improve (1) early detection of ILD, (2) predicting ILD progression, (3) predicting response to therapy and (4) understanding pathophysiology. Over the last years, positron emission tomography (PET) and single photon emission computed tomography (SPECT) have emerged as promising molecular imaging techniques to improve ILD management. Both are non-invasive diagnostic tools to assess molecular characteristics of an individual patient with the potential to apply personalized treatment. In this review, we encompass the currently available pre-clinical and clinical studies on molecular imaging with PET and SPECT in IPF and CTD-ILD. We provide recommendations for potential future clinical applications of these tracers and directions for future research.

Role of the lysyl oxidase enzyme family in cardiac function and disease

Heart diseases are a major cause of morbidity and mortality world-wide. Lysyl oxidase (LOX) and related LOX-like (LOXL) isoforms play a vital role in remodelling the extracellular matrix (ECM). The LOX family controls ECM formation by cross-linking collagen and elastin chains. LOX/LOXL proteins are copper-dependent amine oxidases that catalyse the oxidation of lysine, causing cross-linking between the lysine moieties of lysine-rich proteins. Dynamic changes in LOX and LOXL protein-expression occur in a variety of cardiac pathologies; these changes are believed to be central to the associated tissue-fibrosis. An awareness of the potential pathophysiological importance of LOX has led to the evaluation of interventions that target LOX/LOXL proteins for heart-disease therapy. The purposes of this review article are: (i) to summarize the basic biochemistry and enzyme function of LOX and LOXL proteins; (ii) to consider their tissue and species distribution; and (iii) to review the results of experimental studies of the roles of LOX and LOXL proteins in heart disease, addressing involvement in the mechanisms, pathophysiology and therapeutic responses based on observations in patient samples and relevant animal models. Therapeutic targeting of LOX family enzymes has shown promising results in animal models, but small-molecule approaches have been limited by non-specificity and off-target effects. Biological approaches show potential promise but are in their infancy. While there is strong evidence for LOX-family protein participation in heart failure, myocardial infarction, cardiac hypertrophy, dilated cardiomyopathy, atrial fibrillation and hypertension, as well as potential interest as therapeutic targets, the precise involvement of LOX-family proteins in heart disease requires further investigation.

Targeting the lysyl oxidases in tumour desmoplasia

The extracellular matrix (ECM) is a fundamental component of tissue microenvironments and its dysregulation has been implicated in a number of diseases, in particular cancer. Tumour desmoplasia (fibrosis) accompanies the progression of many solid cancers, and is also often induced as a result of many frontline chemotherapies. This has recently led to an increased interest in targeting the underlying processes. The major structural components of the ECM contributing to desmoplasia are the fibrillar collagens, whose key assembly mechanism is the enzymatic stabilisation of procollagen monomers by the lysyl oxidases. The lysyl oxidase family of copper-dependent amine oxidase enzymes are required for covalent cross-linking of collagen (as well as elastin) molecules into the mature ECM. This key step in the assembly of collagens is of particular interest in the cancer field since it is essential to the tumour desmoplastic response. LOX family members are dysregulated in many cancers and consequently the development of small molecule inhibitors targeting their enzymatic activity has been initiated by many groups. Development of specific small molecule inhibitors however has been hindered by the lack of crystal structures of the active sites, and therefore alternate indirect approaches to target LOX have also been explored. In this review, we introduce the importance of, and assembly steps of the ECM in the tumour desmoplastic response focussing on the role of the lysyl oxidases. We also discuss recent progress in targeting this family of enzymes as a potential therapeutic approach.

Analyses of lysine aldehyde cross-linking in collagen reveal that the mature cross-link histidinohydroxylysinonorleucine is an artifact

Lysyl oxidase-generated intermolecular cross-links are essential for the tensile strength of collagen fibrils. Two cross-linking pathways can be defined, one based on telopeptide lysine aldehydes and another on telopeptide hydroxylysine aldehydes. Since the 1970s it has been accepted that the mature cross-linking structures on the lysine aldehyde pathway, which dominates in skin and cornea, incorporate histidine residues. Here, using a range of MS-based methods, we re-examined this conclusion and found that telopeptide aldol dimerization is the primary mechanism for stable cross-link formation. The C-telopeptide aldol dimers formed labile addition products with glucosylgalactosyl hydroxylysine at α1(I)K87 in adjacent collagen molecules that resisted borohydride reduction and after acid hydrolysis produced histidinohydroxylysinonorleucine (HHL), but only from species with a histidine in their α1(I) C-telopeptide sequence. Peptide MS analyses and the lack of HHL formation in rat and mouse skin, species that lack an α1(I) C-telopeptide histidine, revealed that HHL is a laboratory artifact rather than a natural cross-linking structure. Our experimental results also establish that histidinohydroxymerodesmosine is produced by borohydride reduction of N-telopeptide allysine aldol dimers in aldimine intermolecular linkage to nonglycosylated α1(I) K930. Borohydride reduction of the aldimine promotes an accompanying base-catalyzed Michael addition of α1(I) H932 imidazole to the α,β-unsaturated aldol. These aldehydes are intramolecular at the N terminus but at the C terminus they can be both intramolecular and intermolecular according to present and earlier findings.

Molecular Probes for Imaging Fibrosis and Fibrogenesis

Fibrosis, or the accumulation of extracellular matrix molecules that make up scar tissue, is a common result of chronic tissue injury. Advances in the clinical management of fibrotic diseases have been hampered by the low sensitivity and specificity of noninvasive early diagnostic options, lack of surrogate end points for use in clinical trials, and a paucity of noninvasive tools to assess fibrotic disease activity longitudinally. Hence, the development of new methods to image fibrosis and fibrogenesis is a large unmet clinical need. Herein, an overview of recent and selected molecular probes for imaging of fibrosis and fibrogenesis by magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography is provided.

CROSS-LINKS MULTISCALE EFFECTS ON BONE ULTRASTRUCTURE BIOMECHANICAL BEHAVIOR

Bone is a multiscale combination of collagen molecules merged with mineral crystals. Its high rigidity and stability stem amply from its polymeric organic matrix and secondly from the connections established between interdifferent and intradifferent scale components through cross-links. Several studies have shown that the cross-links inhibition results in a reduction in strength of bone but they do not quantify the degree to which these connections contribute to the bone rigidity and toughness. This report is classified among the few works that measure the cross-links multiscale impact on the ultrastructure bone mechanical behavior.

This work aims firstly to study the effect of cross-links at the molecule scale and secondly to gather from literature studies results handling with cross-links effects on the other bone ultrastructure scales in order to reveal the multiscale effect of cross-links. This study proves that cross-links increasing number improves the mechanical performance of each scale of bone ultrastructure. On the other hand, cross-links have a multiscale contribution that depends on its rank related to existing cross-links connecting the same geometries and it depends on mechanical characteristics of geometries connected.

Effects of maturation and advanced glycation on tensile mechanics of collagen fibrils from rat tail and Achilles tendons

Connective tissues are ubiquitous throughout the body and consequently affect the function of many organs. In load bearing connective tissues like tendon, the mechanical functionality is provided almost exclusively by collagen fibrils that in turn are stabilized by covalent cross-links. Functionally distinct tendons display different cross-link patterns, which also change with maturation, but these differences have not been studied in detail at the fibril level. In the present study, a custom built nanomechanical test platform was designed and fabricated to measure tensile mechanics of individual fibrils from rat tendons. The influence of animal maturity (4 vs. 16 week old rats) and functionally different tendons (tail vs. Achilles tendons) were examined. Additionally the effect of methylglyoxal (MG) treatment in vitro to form advanced glycation end products (AGEs) was investigated. Age and tissue type had no significant effect on fibril mechanics, but MG treatment increased strength and stiffness without inducing brittleness and gave rise to a distinct three-phase mechanical response corroborating that previously reported in human patellar tendon fibrils. That age and tissue had little mechanical effect, tentatively suggest that variations in enzymatic cross-links may play a minor role after initial tissue formation.

STATEMENT OF SIGNIFICANCE

Tendons are connective tissues that connect muscle to bone and carry some of the greatest mechanical loads in the body, which makes them common sites of injury. A tendon is essentially a biological rope formed by thin strands called fibrils made of the protein collagen. Tendon function relies on the strength of these fibrils, which in turn depends on naturally occurring cross-links between collagen molecules, but the mechanical influence of these cross-links have not been measured before. It is believed that beneficial cross-linking occurs with maturation while additional cross-linking with aging may lead to brittleness, but this study provides evidence that maturation has little effect on mechanical function and that age-related cross-linking does not result in brittle collagen fibrils.

Differential expression of extracellular matrix and integrin genes in the longissimus thoracis between bulls and steers and their association with intramuscular fat contents

This study was performed to compare expression of genes for extracellular matrix (ECM) components, ECM degrading factors, and integrin subunits in the longissimus thoracis (LT) between bulls and steers. Steers had lower (P<0.05) ECM component collagen type 1 α1 and collagen type 3 α1 mRNA levels than did bulls, but they had higher (P<0.05) thrombospondin 1 mRNA and protein levels. Steers had higher (P<0.01) matrix metalloproteinase (MMP) 9 mRNA levels than did bulls. Steers had higher (P<0.05) integrin α5 mRNA levels but lower (P<0.05) integrin β6 mRNA and protein levels; however, expression levels of several other integrin subunits were not different between steers and bulls. MMP9 mRNA levels were positively correlated (P<0.05) with intramuscular fat content in bull group. In conclusion, these results demonstrate that castration has moderate effects on expression of ECM components, ECM degrading factors, and integrin subunit genes in the LT.

Molecular Magnetic Resonance Imaging of Lung Fibrogenesis with an Oxyamine-Based Probe

Fibrogenesis is the active production of extracellular matrix in response to tissue injury. In many chronic diseases persistent fibrogenesis results in the accumulation of scar tissue, which can lead to organ failure and death. However, no non-invasive technique exists to assess this key biological process. All tissue fibrogenesis results in the formation of allysine, which enables collagen cross-linking and leads to tissue stiffening and scar formation. We report herein a novel allysine-binding gadolinium chelate (GdOA), that can non-invasively detect and quantify the extent of fibrogenesis using magnetic resonance imaging (MRI). We demonstrate that GdOA signal enhancement correlates with the extent of the disease and is sensitive to a therapeutic response.

Molecular imaging of oxidized collagen quantifies pulmonary and hepatic fibrogenesis

Fibrosis results from the dysregulation of tissue repair mechanisms affecting major organ systems, leading to chronic extracellular matrix buildup, and progressive, often fatal, organ failure. Current diagnosis relies on invasive biopsies. Noninvasive methods today cannot distinguish actively progressive fibrogenesis from stable scar, and thus are insensitive for monitoring disease activity or therapeutic responses. Collagen oxidation is a universal signature of active fibrogenesis that precedes collagen crosslinking. Biochemically targeting oxidized lysine residues formed by the action of lysyl oxidase on collagen with a small-molecule gadolinium chelate enables targeted molecular magnetic resonance imaging. This noninvasive direct biochemical elucidation of the fibrotic microenvironment specifically and robustly detected and staged pulmonary and hepatic fibrosis progression, and monitored therapeutic response in animal models. Furthermore, this paradigm is translatable and generally applicable to diverse fibroproliferative disorders.

Analysis of bimodal thermally-induced denaturation of type I collagen extracted from calfskin

DSC tracks of collagen in solution revealing a bimodal behaviour during its heat-induced denaturation.

Chemical structure, biosynthesis and synthesis of free and glycosylated pyridinolines formed by cross-link of bone and synovium collagen

This review focuses on the chemical structure, biosynthesis and synthesis of free and glycosylated pyridinolines (Pyds), fluorescent collagen cross-links, with a pyridinium salt structure. Pyds derive from the degradation of bone collagen and have attracted attention for their use as biochemical markers of bone resorption and to assess fracture risk prediction in persons suffering from osteoporosis, bone cancer and other bone or collagen diseases. We consider and critically discuss all reported syntheses of free and glycosylated Pyds evidencing an unrevised chemistry, original and of general utility, analysis of which allows us to also support a previously suggested non-enzymatic formation of Pyds in collagen better rationalizing and justifying the chemical events.

Influence of copper, iron, zinc and fe (3) (+) haemoglobin levels on the etiopathogenesis of chronic calcific pancreatitis--a study in patients with pancreatitis

Chronic pancreatitis is a serious condition associated with severe abdominal pain, and a significant percentage of patients progresses to irreversible calcification in pancreas. The present study evaluates the degree to which the levels of trace elements, copper, iron, selenium, zinc and haemoglobin-Fe(3+), in blood, serum and pancreas have any role to play in the calcification process associated with fibrosis in pancreas. Twenty-seven calcific (CCP) and 23 non-calcific chronic pancreatitis (CP) patients and equal number of age- and sex-matched normal volunteers (50) were enrolled in the study. Surgically removed pancreatic tissue and blood samples were analysed for copper, iron, selenium, zinc, protein, collagen and lipid peroxidation products in terms of malondialdehyde, protein carbonyls, glutathione, methemoglobin, methemoglobin reductase and ceruloplasmin activity levels. We could find that the pancreatic tissue levels of copper, iron, protein and collagen contents were significantly elevated in CCP patients when compared to CP patients. Serum levels of copper, free ionic copper and iron were also elevated in CCP patients. The serum and the pancreatic tissue level of zinc and selenium showed a significant decrease in CCP patients. The level of methemoglobin was elevated more significantly with the concomitant decline in the activity of methemoglobin reductase. There was a positive correlation between the pancreatic level of copper and iron with the collagen and protein levels. The results of the present study revealed that the levels of copper and iron, the pro-oxidants and zinc and selenium may influence calcification process in CCP patients. Hypoxia-related tissue injury due to the formation of oxidised haemoglobin may also contribute to the pathogenesis of calcification in pancreas.

Phasor approach to fluorescence lifetime microscopy distinguishes different metabolic states of germ cells in a live tissue

We describe a label-free imaging method to monitor stem-cell metabolism that discriminates different states of stem cells as they differentiate in living tissues. In this method we use intrinsic fluorescence biomarkers and the phasor approach to fluorescence lifetime imaging microscopy in conjunction with image segmentation, which we use to introduce the concept of the cell phasor. In live tissues we are able to identify intrinsic fluorophores, such as collagen, retinol, retinoic acid, porphyrin, flavins, and free and bound NADH. We have exploited the cell phasor approach to detect a trend in metabolite concentrations along the main axis of the Caenorhabditis elegans germ line. This trend is consistent with known changes in metabolic states during differentiation. The cell phasor approach to lifetime imaging provides a label-free, fit-free, and sensitive method to identify different metabolic states of cells during differentiation, to sense small changes in the redox state of cells, and may identify symmetric and asymmetric divisions and predict cell fate. Our method is a promising noninvasive optical tool for monitoring metabolic pathways during differentiation or disease progression, and for cell sorting in unlabeled tissues.

Phasor approach to fluorescence lifetime microscopy distinguishes different metabolic states of germ cells in a live tissue

We describe a label-free imaging method to monitor stem-cell metabolism that discriminates different states of stem cells as they differentiate in living tissues. In this method we use intrinsic fluorescence biomarkers and the phasor approach to fluorescence lifetime imaging microscopy in conjunction with image segmentation, which we use to introduce the concept of the cell phasor. In live tissues we are able to identify intrinsic fluorophores, such as collagen, retinol, retinoic acid, porphyrin, flavins, and free and bound NADH. We have exploited the cell phasor approach to detect a trend in metabolite concentrations along the main axis of the Caenorhabditis elegans germ line. This trend is consistent with known changes in metabolic states during differentiation. The cell phasor approach to lifetime imaging provides a label-free, fit-free, and sensitive method to identify different metabolic states of cells during differentiation, to sense small changes in the redox state of cells, and may identify symmetric and asymmetric divisions and predict cell fate. Our method is a promising noninvasive optical tool for monitoring metabolic pathways during differentiation or disease progression, and for cell sorting in unlabeled tissues.

Collagen: relatively invariant (helical) and variable (nonhelical) regions

The structural identity of certain helical regions of collagen from human and rat skin equals or exceeds that of other homologous proteins. In contrast, the short nonhelical sequences in the two proteins, although homologous, differ appreciably in structure. The requirements of the collagen helix and the numerous intermolecular interactions characteristic of collagen may restrict the number of functionally acceptable amino acid replacements occurring during evolution.

Advances in collagen cross-link analysis

The combined application of ion-trap mass spectrometry and peptide-specific antibodies for the isolation and structural analysis of collagen cross-linking domains is illustrated with examples of results from various types of collagen with the emphasis on bone and cartilage. We highlight the potential of such methods to advance knowledge on the importance of post-translational modifications (e.g., degrees of lysine hydroxylation and glycosylation) and preferred intermolecular binding partners for telopeptide and helical cross-linking domains in regulating cross-link type and placement.

Enzymic and non-enzymic cross-linking mechanisms in relation to turnover of collagen: relevance to aging and exercise

The molecular mechanisms involved in the aging of collagen and consequent increase in mechanical strength and stiffness occur in a series of enzymic and non-enzymic intermolecular cross-links. The enzymic mechanism involves divalent aldimine intermolecular cross-links derived from the reaction of aldehydes which then mature to trivalent cross-links and further stabilize the collagen fiber and is now well known. Recent studies have demonstrated that the rate of turnover and level of telopeptide lysyl hydroxylation modifies the nature of the cross-link and hence the mechanical strength of the fiber. The slow turnover of mature collagen subsequently allows accumulation of the products of the adventitious non-enzymic reaction of glucose with the lysines in the triple helix to form glucosyl lysine and its Amadori product, that is, the Maillard reaction. These products are subsequently oxidized to a complex series of advanced glycation end-products, some of which are intermolecular cross-links between the triple helices rendering the fiber too stiff for optimal functioning of the collagen fibers, and consequently of the particular tissue involved. The glycation reactions following maturation are true aging processes, and attempts at their specific inhibition involve competitive inhibition of the Maillard reaction and chemical cleavage of the glycation cross-links. It is clear that the nature of the age-related cross-links and hence tissue strength depends on the rate of turnover of the collagen. An examination of the particular effect of strenuous exercise on the rate of turnover of collagen and hence cross-linking in different tissues could lead to a better understanding of optimal sports training regimes.

Biochemical analysis of collagen in adhesive tissues formed after digital flexor tendon injuries

Restrictive adhesion is a major problem following flexor tendon surgery. We investigated crosslinks and types of collagen in two adhesive tissue models in chickens, characterized by loose adhesion and dense adhesion. The flexor digitorum profundus tendon was cut and sutured in two ways: (1) the severed tendon was sutured outside the tendon sheath (loose adhesion model), and (2) the tendon sheath was excised and the severed tendon was sutured on the damaged bony floor (dense adhesion model). Biochemical analysis of collagen was done for each group after tenorrhaphy. In the outside-sheath group, the ratio of the collagen crosslinks, dihydroxylysinonorleucine/hydroxylysinonorleucine, was lower than that in the sheath-excised group at 12 weeks, while the collagen crosslinks ratio remained considerably high in the sheath-excised group. The ratio of type III collagen to type I collagen in the outside-sheath group was higher than that in the sheath-excised group at 12 weeks and approached an almost normal value at 24 weeks. The collagen type III/I ratio remained low in the sheath-excised group. The current study shows that biochemical properties are different between loose and dense adhesions. In dense adhesive scars, pathological collagen turnover occurs even at 24 weeks after tenorrhaphy.

Resin bonding to Er: YAG laser-irradiated dentin: combined effects of pre-treatments with citric acid and glutaraldehyde

The purpose of this study was to evaluate the combined effects of citric acid and glutaraldehyde (GA) on the resin bonding to Er: YAG laser-irradiated dentin. Bovine dentin was prepared with 180- to 600-grit SiC paper and then uniformly irradiated with an Er: YAG laser (laser-irradiated group) or immersed in water at 60 degrees C for 15 min (heated group). The samples were then acid-conditioned with 10% citric acid (10-0) or 10% citric acid/3% ferric chloride (10-3) for 15 s and treated with GA for 10 min before bonding to an acrylic rod with 4-META/MMA-TBB resin. These samples were trimmed to prepare miniaturized dumbbell-shaped specimens. After storage in water at 37C for 1 d, the tensile bond strength was measured, and the fractured surface was evaluated using a scanning electron microscope (SEM). In the laser-irradiated and heated groups, the 10-3+GA-treated specimen had higher bond strength than that of 10-0+GA. On the other hand, the tensile bond strength of 10-3 +GA in the non-irradiated group was lower that that of 10-0+GA. In conclusion, the combination of 10-3 and GA for bonding with 4-META/MMA-TBB resin was the most effective for Er: YAG laser-irradiated dentin and heated dentin, but it was not effective for the non-irradiated dentin.

Effects of lathyrogens on the mechanical strength of the periodontal ligament in the rat mandibular first molar

Effects of lathyrogens such as aminoacetonitrile (AAN), beta-aminopropionitrile (BAPN) and cysteamine--known inhibitors of cross-linking of collagen--on the mechanical strength of the periodontal ligament of the rat mandibular first molar were examined by measuring the ultimate load required to extract the tooth from its socket in the dissected jaw. Single injections of AAN (40 approximately 100 mg/100 g body weight) or of BAPN (100 mg/100 g body weight) caused significant decreases of the mechanical strength 24 h after administration of the drugs but that of cysteamine (30 mg/100 g body weight) did not. Significant correlations between the dose of AAN or of BAPN and the mechanical strength were found following daily administrations of the drugs for 5 days. The relative potency of AAN to BAPN was estimated to be 4.5 by a slope ratio assay. The rapid appearance and disappearance of the effect of lathyrogens on the mechanical strength of the periodontal ligament of the rat mandibular first molar provide further evidence that the turnover of the collagen in the tissue is fast. The half-time of collagen synthesis was estimated to be approximately 3 d.

Binding of lysyl oxidase to fibrils of type I collagen

The binding of highly purified bovine aortic lysyl oxidase to native fibrils of type I collagen has been measured by assay of unbound lysyl oxidase activity in the supernatants of enzyme-collagen mixtures after centrifugation. The apparent binding affinity of lysyl oxidase for native fibrils is quite similar to that for fibrils prepared from pepsin- or chymotrypsin-digested type I collagen, demonstrating that the enzyme binds to the triple-helical portion of collagen molecules. The data also indicate that the enzyme binds predominantly to the fibrillar surface. The results suggest that lysyl oxidase initiates crosslink formation at an early stage in collagen fibrillogenesis.

Transformation of the structure of collagen. A time-resolved analysis of mechanochemical processes using synchrotron radiation

Mechanochemically induced molecular transformations of collagen fibres were analysed using time-resolved small-angle diffraction spectra and histomechanical measurements. In particular, the influence of aqueous and methanolic perchlorate solutions was examined. According to a transformation continuing from the periphery towards the centre, the macroscopic contraction that is completed less than five minutes after incubation with perchlorate is caused by peripherally transformed fibrils only, whereas the centrally situated fibrils first undergo an accordion-like folding, but after more than 20 minutes are transformed similarly. The triple-helical transformation is preceded by a structure-breaking effect on structural water that can be monitored in time-resolved diffraction spectra. The combined loss of meridional low-angle reflections and cross-striated fibrils in micrographs is irreversible. By dialysis of colloidally dissolved collagen against a solution of ATP, however, segment-long spacing aggregates are obtained. Under isometric conditions, an instantaneous transformation of intermittent regions leads to an increase in the long period of adjacent, still structured regions of the same fibril that is correlated with a delayed increase in tension in the fibre. Increase of tension under isometric conditions as well as the flow-properties of a fibre relaxed in perchlorate are interpreted in terms of the parallel sliding of subunits of varying lengths, which has been demonstrated by diffraction analysis.

Morphologic and phenotypic analysis of an outcross line of blotchy mouse

Blotchy is an X-linked recessive mutation at the "Mottled" locus in the mouse. The affected blotchy male (Blo/Y) mouse from an inbred genetic background demonstrates morphologic and physiologic abnormalities consistent with emphysema in adult life. Breeding of Blo/Y mice has been difficult because the inbred Blo/Y males are sterile. We report the successful development of a line of outbred Blo/Y male and Blo/Blo female nice by the controlled outcross mating of the inbred heterozygous Blo/+ female with the Argonne hybrid B6CF1 male mouse. The subsequent outcross Blo/Y progeny breed vigorously with the outcrossed Blo/+ female. The lungs of the outbred Blo/Blo female and inbred Blo/Y male mice demonstrate mild to moderate panacinar emphysema with a significant decrease in internal surface area (p less than 0.005) and an increase in mean linear intercept (p less than 0.005). In contrast, the lungs of the outbred Blo/Y is structurally normal. Despite the absence of emphysema-like changes in the outbred Blo/Y males, there were phenotypic features that suggest inherited abnormalities in connective tissue proteins including 1) high incidence of aortitis leading to premature death from aneurysmal rupture, and 2) significant decrease in the morphometrically determined parenchymal elastic fiber length in the lung (p less than 0.01). The outbred blotchy strain may be a useful experimental animal model in determining the pathogenesis of emphysema.

Sympathetic nerve terminals in the tunica media of human superficial temporal and middle cerebral arteries: wet histofluorescence

In specimens from the superficial temporal artery (STA) and middle cerebral artery (MCA), obtained during STA-MCA anastomosis, green fluorescent varicose fibers of sympathetic nerves were clearly visible with both formaldehyde-glutaraldehyde and sucrose-potassium phosphate-glyoxylic acid wet-histofluorescent techniques. These fibers were fairly thick, were densely packed and had a meshwork-like arrangement. Fluorescent terminals were seen both in the adventitia and in the outer muscular layer of the media in both STA and MCA specimens. They were more often observed in patients with prominent atherosclerosis in these vessels. The present study suggests the possible role of sympathetic nerve terminals in the development of vasospasm and occlusive lesions in cerebral vessels. It may also help to explain the marked constriction and transient occlusion following a STA-MCA bypass procedure.

The mechanism for the promotion of tenderness in meat during the post-mortem process: a review

The post-mortem changes in the chemical composition and structure of striated muscle have been reviewed on the basis of various concepts that emerged from the studies of different investigators to explain the course of tenderization of meat during aging. These concepts include the changes in the sarcoplasmic proteins, myofibrillar proteins (such as complete dissociation of actomyosin, partial dissociation of actomyosin, cleavage of disulfide linkages, depolymerization of F-actin filaments, cleavage of myosin filaments, disorganization of Z-bands and the troponin-tropomyosin complex), sarcolemma, connective tissue elements (collagen fibrils, ground substance), and the protein-ion relationship of the muscle cells (more strictly, syncytia). The experimental evidence for and against each of the views is discussed critically in the light of certain fundamentals of biophysical chemistry and biochemistry. Finally, an alternative hypothesis has been presented based on the differential effect of the post-mortem formation of lactic acid (H+ ion concentration) on the intra- and extracellular components of muscle and the possible role of lysosomal cathepsins. Consequently, a series of biophysical, biochemical, and ultrastructural changes seem to account for the mechanism by which meat becomes tender during the aging process.

Biochemical and immunochemical study of lysyl oxidase in experimental hepatic fibrosis in the rat

Lysyl oxidase catalyzes the crosslinking of collagen and elastin. Lysyl oxidase activity was measured and localized in rat liver during the evolution of hepatic fibrosis induced by CCl4. Enzyme activity measured with DL-[6-3H]-lysine-labeled collagen substrates in liver and plasma increased sharply after approximately 3 wk of injection, reached a maximum at 6 wk, and then decreased. The increase in activity correlated histologically with early connective tissue septa formation, and the magnitude of increase was significantly greater than that found for the intracellular collagen biosynthetic enzymes protocollagen prolyl hydroxylase and lysyl hydroxylase. Indirect immunofluorescence studies showed that lysyl oxidase was present in association with collagen in the extracellular space. However, it was not possible to correlate the distribution pattern with a particular liver cell type. These observations suggest that serial measurements of lysyl oxidase activity in liver or plasma may be useful for correlating changes in connective tissue formation with histologic connective tissue deposition.

Reduction of blood pressure and vascular collagen in hypertensive rats by beta-aminopropionitrile

beta-Aminopropionitrile, a specific inhibitor of lysyl oxidase prevented the rise in blood pressure induced by deoxycorticosterone-salt in rats. In addition, after the onset of hypertension, administration of beta-aminopropionitrile lowered the blood pressure. Concomitant with the lowering of blood pressure, there was a reduction in the more highly crosslinked form of vascular collagen. These findings would indicate that increases in vascular connective tissue are not only sequelae of hypertension, but may also contribute to the maintenance of elevated blood pressure.

Isolation of a crosslinked cyanogen-bromide peptide from insoluble rabbit collagen. Tissue differences in hydroxylation and glycosylation of the crosslink

A radioactive peptide has been isolated from cyanogen bromide digests of sodium boro[3H]-hydride-reduced collagen from rabbit bone, tendon and skin. It was identified as a crosslinked peptide linking the short C-terminal cyanogen bromide peptide alpha1-CB6B (17 amino acid residues) to alpha1-CB5 (37 residues) from the helical part of the chain of an adjacent molecule. Both peptides could be separated after cleaving the crosslink with periodate. Thus the crosslinked peptide alpha1-CB5 X alpha1-CB6B originates from an intermolecular crosslink between quarter-staggered molecules within collagen fibrils previously assigned as 'head-to-tail' link. The chemical nature of the reduced crosslinking component was identified and was shown to differ between peptides derived from different tissues: alpha1-CB6B X alpha1 CB5 from bone contains hydroxylysinohydroxynorleucine [o5Lys(o5omegaNle)] whereas the skin peptide contains hydroxylysinonorleucine [o5Lys(omegaNle)]. The peptide derived from tendon contains both components. The relation of o5Lys(omegaNle) to o5Lys(o5omegaNle) in the peptides corresponds to that of the original tissue. On the other hand, histidino-hydroxymerodesmosine which is a major reduced cross-linking component in skin and tendon, is completely absent in the isolated peptides. The crosslinking component in the skin peptide is completely glycosylated, mainly by glucosylgalactosyl residues and to a smaller extent by galactosyl residues. o5Lys(o5omegaNle) from the bone peptide is only partly glycosylated, containing equal amounts of the disaccharide and monosaccharide. Only slight glycosylation was found in the tendon peptide.

Collagen development in granulation tissue as compared with collagen of skin and aorta from injured and non-injured rats

Granulation tissue in rats was produced by subcutaneous implantation of viscose cellulose sponges. Granulomas, aortae, and skin samples were taken 4, 8, 14, 22, 33, and 42 days after the sponge implantation and compared with age-matched non-operated rats. 14C-proline was given 4 hours before death to animals killed on day 0, 14, and 42. The 14C-OH-proline activity in salt insoluble collagen was higher in granulation tissue and aorta than in skin. This indicates a faster formation, or an increased stability of the intermolecular cross-links in granulation tissue and aorta, than in skin. The percentage of free OH-proline was than in skin, reflecting a relatively increased collagen degradation in granulation tissue. An increased collagen degradation may also, in part, explain a registered higher alpha/beta ratio in collagen from granulation tissue than from skin, as well as the increase in alpha/beta ratio in the older granulomas. The sponge implantation did not affect the collagen of aorta and skin, but caused a decrease in the dry weight of aorta and skin, and an increase in the number of granulocytes in the blood.

Modified collagen membrane as a skin substitute: preliminary studies

Modified collagen membrane when compared to autograft, homograft, heterograft and silicone polymer membrane, demonstrated a superior adherence on split and full-thickness surfaces, but proved to be an inferior covering for granulating surfaces. Water vapor transport was found to be adequate and adaptable to modification. It was found to be permeable to most commonly used topical antibiotics, and no significant antigenicity could be demonstrated. In a controlled animal burn study, modified collagen membrane proved superior to homograft and heterograft in regard to mortality, time to grafting, bacteriology and autograft take. These studies provide a background for further laboratory and clinical studies now in progress.

Collagen and mechanical strength in various organs of rats treated with D-penicillamine or amino-acetonitrile

After oral treatment with D-penicillamine (D-Pc) or with aminoacetonitrile (AAn) for 10 days, mechanical and chemical parameters were studied simultaneously in various organs of Sprague Dawley rats. Tensile strength of skin strips and of tail tendons, breaking strength of femur bones and tensile strength of granuloma tissue (induced by implanted glass rods) were measured and calculated. In the same tissue the soluble collagen fractions and the insoluble collagen were determined. Total collagen and the ratio insoluble vs. soluble collagen were calculated. Tensile strength of skin, tendon and granuloma tissue were greatly reduced by D-Pc treatment but only minimally influenced by AAN treatment. On the other hand only AAN significantly reduced the breaking strength of bone. All these changes were closely correlated with the content of insoluble collagen in the respective tissues. The correlation coefficients to total collagen were similar but lower. The correlation coefficients between strength and the ratio insoluble vs. soluble collagen were generally still lower. Earlier findings in aged and corticoid treated rats, proving that insoluble collagen content determines mechanical strength of connnective and supporting tissue thus could be confirmed.

Biosynthesis of collagen crosslinks: increased activity of purified lysyl oxidase with reconstituted collagen fibrils

Lysyl oxidase catalyzes the formation of crosslinking aldehydes in collagen and elastin. This report demonstrates that the enzyme has high activity with collagen precipitated as native fibrils, an apparent K(m) of 0.95 muM, and low activity toward either soluble forms such as denatured collagen, isolated alpha chain, or isolated alpha1-CBl peptide, or precipitated collagen fibrils after pepsin treatment. These results indicate that the biosynthesis of the aldehyde crosslink intermediate probably occurs primarily after the onset of fibril formation in vivo. Biosynthesis of aldehydes and subsequent crosslinks may be related to the rate of fibril formation as well as to the concentration of lysyl oxidase in vivo.