Tendon hypertrophy is associated with increased hydroxylation of nonhelical lysine residues at two specific cross-linking sites in type I collagen

Review paper: The importance of consideration of collagen cross-links in computational models of collagen-based tissues

Collagen as the main protein in Extra Cellular Matrix (ECM) is the main load-bearing component of fibrous tissues. Nanostructure and architecture of collagen fibrils play an important role in mechanical behavior of these tissues. Extensive experimental and theoretical studies have so far been performed to capture these properties, but none of the current models realistically represent the complexity of network mechanics because still less is known about the collagen's inner structure and its effect on the mechanical properties of tissues. The goal of this review article is to emphasize the significance of cross-links in computational modeling of different collagen-based tissues, and to reveal the need for continuum models to consider cross-links properties to better reflect the mechanical behavior observed in experiments. In addition, this study outlines the limitations of current investigations and provides potential suggestions for the future work.

Collagen Bioinks for Bioprinting: A Systematic Review of Hydrogel Properties, Bioprinting Parameters, Protocols, and Bioprinted Structure Characteristics

Bioprinting is a modern tool suitable for creating cell scaffolds and tissue or organ carriers from polymers that mimic tissue properties and create a natural environment for cell development. A wide range of polymers, both natural and synthetic, are used, including extracellular matrix and collagen-based polymers. Bioprinting technologies, based on syringe deposition or laser technologies, are optimal tools for creating precise constructs precisely from the combination of collagen hydrogel and cells. This review describes the different stages of bioprinting, from the extraction of collagen hydrogels and bioink preparation, over the parameters of the printing itself, to the final testing of the constructs. This study mainly focuses on the use of physically crosslinked high-concentrated collagen hydrogels, which represents the optimal way to create a biocompatible 3D construct with sufficient stiffness. The cell viability in these gels is mainly influenced by the composition of the bioink and the parameters of the bioprinting process itself (temperature, pressure, cell density, etc.). In addition, a detailed table is included that lists the bioprinting parameters and composition of custom bioinks from current studies focusing on printing collagen gels without the addition of other polymers. Last but not least, our work also tries to refute the often-mentioned fact that highly concentrated collagen hydrogel is not suitable for 3D bioprinting and cell growth and development.

Enzymatic and non-enzymatic crosslinks found in collagen and elastin and their chemical synthesis

This review summarized the enzymatic and non-enzymatic crosslinks found in collagen and elastin and their organic synthesis.

Lysine Hydroxylation and Cross-Linking of Collagen

Collagens represent a large family of structurally related proteins containing a unique triple-helical structure. Among them, the fibril-forming collagens are the most abundant in vertebrates providing tissues with form and stability. One of the characteristics of the fibrillar collagens is its sequential posttranslational modifications of specific lysine residues that have major effects on molecular assembly and stability of the fibrils in the extracellular space. Hydroxylation of lysine residues is the first modification catalyzed by lysyl hydroxylases, and is critical for the following glycosylation and in determining the fate of covalent cross-linking. This chapter presents an overview of lysine hydroxylation and cross-linking of collagen, and the analytical methods we have developed.

Lysine post-translational modifications of collagen

Type I collagen is the most abundant structural protein in vertebrates. It is a heterotrimeric molecule composed of two α1 chains and one α2 chain, forming a long uninterrupted triple helical structure with short non-triple helical telopeptides at both the N- and C-termini. During biosynthesis, collagen acquires a number of post-translational modifications, including lysine modifications, that are critical to the structure and biological functions of this protein. Lysine modifications of collagen are highly complicated sequential processes catalysed by several groups of enzymes leading to the final step of biosynthesis, covalent intermolecular cross-linking. In the cell, specific lysine residues are hydroxylated to form hydroxylysine. Then specific hydroxylysine residues located in the helical domain of the molecule are glycosylated by the addition of galactose or glucose-galactose. Outside the cell, lysine and hydroxylysine residues in the N- and C-telopeptides can be oxidatively deaminated to produce reactive aldehydes that undergo a series of non-enzymatic condensation reactions to form covalent intra- and inter-molecular cross-links. Owing to the recent advances in molecular and cellular biology, and analytical technologies, the biological significance and molecular mechanisms of these modifications have been gradually elucidated. This chapter provides an overview on these enzymatic lysine modifications and subsequent cross-linking.

Extracellular Matrix Compositon of the Equine Superficial Digital Flexor Tendon: Relationship with Age and Anatomical Site

The objectives of the present study were to test the hypotheses that: (1) the composition of the extracellular matrix of the equine superficial digital flexor tendon (SDFT) shows great functional similarities with articular cartilage, i.e. that significant differences exist in biochemical composition of differently loaded areas (which in the case of tendons may be more apparent as tendon shows more obvious differences than cartilage); and (2) that, as in articular cartilage, no substantial alterations in biochemical composition take place during ageing once adulthood has been attained. Tendon samples were taken from 60 adult slaughter horses from a central area at cross-section in the mid-metacarpal region and at the height of the proximal sesamoid bones (sesamoid region) of the SDFT. Contents of collagenous and non-collagenous components were determined. None of the parameters were correlated with age in either region, except for a significant increase in pentosidine cross-links with age in the sesamoid region. Between the two anatomical regions, there were significant differences in all variables, except for hydroxylysylpyridinoline cross-links. It was concluded that in the equine SDFT, similar to articular cartilage, most molecular parameters are not influenced by age in mature horses, indicating a low remodelling rate. Tendon composition is clearly different between regions, apparently reflecting different specific modes of biomechanical loading at the points sampled.

Heterogeneous basis of the type VIB form of Ehlers-Danlos syndrome (EDS VIB) that is unrelated to decreased collagen lysyl hydroxylation

Skin fibroblasts from the majority of patients with the clinical diagnosis of Ehlers-Danlos syndrome type VI (EDS VI; kyphoscoliosis type), have significantly decreased lysyl hydroxylase (LH) activity due to mutations in the LH1 gene (classified as EDS VIA: OMIM no. 225400). A rare condition exists in which patients are clinically similar but have normal levels of LH activity (designated EDS VIB: OMIM no. 229200). To define the biochemical defect, we have examined cultured fibroblasts from four EDS VIB patients for changes in the levels of the mRNAs for LH1, LH2, and LH3, collagen cross-linking patterns, and the extent of lysine hydroxylation of type I collagen alpha chains. Although normal levels of LH1 mRNA were observed in all four patients, in two patients the levels of LH2 mRNA were decreased by >50%, and a similar decrease was observed in LH3 mRNA in the other two patients. A distinct pattern of collagen cross-links, indicative of decreased lysyl hydroxylation, could be identified in EDS VIA patients, but there was no clear correlation between collagen cross-link pattern and changes in the individual LH mRNAs in EDS VIB patients. Linkage to tenascin-X was excluded in these patients. This study suggests that the basis for this form of EDS VI is genetically heterogeneous, and that alternative pathways in addition to lysine hydroxylation of collagen may be affected.

Lysine hydroxylation of collagen in a fibroblast cell culture system

The lysine (Lys) hydroxylation pattern of type I collagen produced by human fibroblasts in culture was analyzed and compared. Fibroblasts were cultured from normal human skin (NSF), keloid (KDF), fetal skin (FDF), and skin tissues of Ehlers-Danlos syndrome type VIA and VIB patients (EDS-VIA and -VIB). The type I collagen alpha chains with or without non-helical telopeptides were purified from the insoluble matrix and analyzed. In comparison with NSFs, KDF and FDF showed significantly higher Lys hydroxylation, particularly in the telopeptide domains of both alpha chains. Both EDS-VIA and -VIB showed markedly lower Lys hydroxylation in the helical domains of both alpha chains whereas that in the telopeptides was comparable with those of NSFs. A similar profile was observed in the tissue sample of the EDS-VIB patient. These results demonstrate that the Lys hydroxylation pattern is domain-specific within the collagen molecule and that this method is useful to characterize the cell phenotypes in normal/pathological connective tissues.

Differential expression of human lysyl hydroxylase genes, lysine hydroxylation, and cross-linking of type I collagen during osteoblastic differentiation in vitro

The pattern of lysyl hydroxylation in the nontriple helical domains of collagen is critical in determining the cross-linking pathways that are tissue specific. We hypothesized that the tissue specificity of type I collagen cross-linking is, in part, due to the differential expression of lysyl hydroxylase genes (Procollagen-lysine,2-oxyglutarate,5-dioxygenase 1, 2, and 3 [PLOD1, PLOD2, and PLOD3]). In this study, we have examined the expression patterns of these three genes during the course of in vitro differentiation of human osteoprogenitor cells (bone marrow stromal cells [BMSCs]) and normal skin fibroblasts (NSFs). In addition, using the medium and cell layer/matrix fractions in these cultures, lysine hydroxylation of type I collagen alpha chains and collagen cross-linking chemistries have been characterized. High levels of PLOD1 and PLOD3 genes were expressed in both BMSCs and NSFs, and the expression levels did not change in the course of differentiation. In contrast to the PLOD1 and PLOD3 genes, both cell types showed low PLOD2 gene expression in undifferentiated and early differentiated conditions. However, fully differentiated BMSCs, but not NSFs, exhibited a significantly elevated level (6-fold increase) of PLOD2 mRNA. This increase coincided with the onset of matrix mineralization and with the increase in lysyl hydroxylation in the nontriple helical domains of alpha chains of type I collagen molecule. Furthermore, the collagen cross-links that are derived from the nontriple helical hydroxylysine-aldehyde were found only in fully differentiated BMSC cultures. The data suggests that PLOD2 expression is associated with lysine hydroxylation in the nontriple helical domains of collagen and, thus, could be partially responsible for the tissue-specific collagen cross-linking pattern.

Tissue specificity of a new splice form of the human lysyl hydroxylase 2 gene

In this study we present the first report of alternative RNA splicing in a gene for lysyl hydroxylase (LH) in a normal population. This splicing event, which we have observed in the LH2 gene, appears to be tissue specific. The LH2 isoform was recently cloned and sequenced from a human kidney cDNA library and predicted to encode a 737 amino acid protein. In the present study, we have isolated a cDNA for LH2 from human skin fibroblasts that codes for a protein of 758 amino acids, of which 21 amino acids are encoded by a new exon. This 63-bp exon, designated exon 13A, is located between exons 13 and 14 of the originally-described LH2 gene. Amplification of cDNAs by PCR, using primers from exons 13 and 14, showed the presence of two distinct LH2 mRNA populations. A 209-bp transcript was expressed in mRNAs isolated from all tissues examined and was the only transcript expressed in skin, lung, aorta and dura, whereas in mRNAs from spleen, cartilage, liver, kidney, frontal lobe and placenta, an additional shorter 146-bp transcript was amplified. DNA sequence analysis showed that these two mRNAs resulted from the alternative splicing of exon 13A. The transcript containing exon 13A is expressed as the major LH2 form in all tissues except kidney and spleen. Analysis of genomic DNA from skin, placenta and spleen showed that both transcripts were generated from the same LH2 gene. Both upstream (intron 13) and downstream (intron 13A) sequences bordering exon 13A had normal consensus sequences for the acceptor (ag) and donor (gt) splice sites. Preliminary studies indicated that only single transcripts which included exon 13A were amplified from normal fetal skin at different stages of gestation. This suggests that although exon 13A is variably expressed in different tissues, this alternative splicing event is not developmentally regulated.

Defective collagen crosslinking in bone, but not in ligament or cartilage, in Bruck syndrome: indications for a bone-specific telopeptide lysyl hydroxylase on chromosome 17

Bruck syndrome is characterized by the presence of osteoporosis, joint contractures, fragile bones, and short stature. We report that lysine residues within the telopeptides of collagen type I in bone are underhydroxylated, leading to aberrant crosslinking, but that the lysine residues in the triple helix are normally modified. In contrast to bone, cartilage and ligament show unaltered telopeptide hydroxylation as evidenced by normal patterns of crosslinking. The results provide compelling evidence that collagen crosslinking is regulated primarily by tissue-specific enzymes that hydroxylate only telopeptide lysine residues and not those destined for the helical portion of the molecule. This new family of enzymes appears to provide the primary regulation for controlling the different pathways of collagen crosslinking and explains why crosslink patterns are tissue specific and not related to a genetic collagen type. A genome screen identified only a single region on chromosome 17p12 where all affected sibs shared a cluster of haplotypes identical by descent; this might be the BS (Bruck syndrome) locus and consequently the region where bone telopeptidyl lysyl hydroxylase is located. Further knowledge of this enzyme has important implications for conditions where aberrant expression of telopeptide lysyl hydroxylase occurs, such as fibrosis and scar formation.

Mechanisms of maturation and ageing of collagen

The deleterious age-related changes in collagen that manifest in the stiffening of the joints, the vascular system and the renal and retinal capillaries are primarily due to the intermolecular cross-linking of the collagen molecules within the tissues. The formation of cross-links was elegantly demonstrated by Verzar over 40 years ago but the nature and mechanisms are only now being unravelled. Cross-linking involves two different mechanisms, one a precise enzymically controlled cross-linking during development and maturation and the other an adventitious non-enzymic mechanism following maturation of the tissue. It is this additional non-enzymic cross-linking, known as glycation, involving reaction with glucose and subsequent oxidation products of the complex, that is the major cause of dysfunction of collagenous tissues in old age. The process is accelerated in diabetic subjects due to the higher levels of glucose. The effect of glycation on cell-matrix interactions is now being studied and may be shown to be an equally important aspect of ageing of collagen. An understanding of these mechanisms is now leading to the development of inhibitors of glycation and compounds capable of cleaving the cross-links, thus alleviating the devastating effects of ageing.

Collagen cross-links in mineralizing tissues: a review of their chemistry, function, and clinical relevance

Bone collagen cross-links are now widely used to assess bone resorption levels in many metabolic bone diseases. The post-translational modifications of bone and other mineralizing collagens are significantly different from those of other type I collagen matrices, a fact that has been exploited during recent advances in the development of biochemical markers of bone resorption. The enzymatic collagen cross-linking mechanism is based upon aldehyde formation from specific telopeptide lysine or hydroxylysine residues. The immature ketoimine cross-links in bone form via the condensation of a telopeptide aldehyde with a helical lysine or hydroxylysine. Subsequent maturation to the pyridinoline and pyrrole cross-links occur by further reaction of the ketoimines with telopeptide aldehydes. In mineralizing tissues, a relatively low level of lysyl hydroxylation results in low levels of hydroxylysyl pyridinoline, and the occurrence of the largely bone specific lysyl pyridinoline and pyrrolic cross-links. The collagen post-translational modifications appear to play an integral role in matrix mineralization. The matrix of the turkey tendon only mineralizes after a remodeling of the collagen and the subsequent formation of a modified matrix more typical of bone than tendon. Further, disturbances in the post-translational modification of collagen can also affect the mineralization density and crystal structure of the tissue. In addition to their use as a convenient measure of matrix degradation, collagen cross-links are of significant importance for the biomechanical integrity of bone. Recent studies of osteoporotic bone, for example, have demonstrated that subtle perturbations in the pattern of lysine hydroxylation result in changes in the cross-link profile. These alterations, specifically changes in the level of the pyrrolic cross-link, also correlate with the strength of the bone. Further research into the biochemistry of bone collagen cross-links may expand current understanding and their clinical application in metabolic bone disease. This review also demonstrates the potential for further study into this area to provide more subtle information into the mechanisms and etiology of disease and aging of mineralizing tissues.

Lung collagen cross-links in rats with experimentally induced pulmonary fibrosis

Rats were intratracheally instilled with bleomycin or with silica (quartz) dust to induce lung fibrosis. Several weeks later, purified collagen chains (or collagen digests) were isolated from the lungs of these animals and from age-matched controls instilled intratracheally with saline solution, and the ratios of hydroxylysine to lysine and of the dysfunctional cross-links DHLNL to HLNL were quantified. Collagen from fibrotic lungs had significantly higher ratios of DHLNL:HLNL than did control lungs, 15.5 +/- 4.8 and 17.1 +/- 4.8 vs. 2.3 +/- 0.5 for the silica-instilled and the bleomycin-instilled animals, respectively. The hydroxylysine:lysine ratio was significantly increased for the alpha 1(I) chain, to a value 170% of that of lung collagen from control animals, and for several of its constituent CNBr peptides. Lung tissue was exhaustively digested with collagenase and specific cross-linked peptides were isolated and characterized. The cross-linked alpha 1(I) x alpha 1(I) peptide linked by the residues 87 x 16C, with a ratio of DHLNL:HLNL of 17:1, demonstrated that the increased hydroxylation of the dysfunctional cross-links in fibrotic lung collagen could be accounted for in part by increased hydroxylation of the lysine residue at position 16C of the C-terminal telopeptide of the collagen alpha 1(I) chain. It proved impossible to locate the corresponding N-terminal cross-linked fragment from alpha 1(I) x alpha 1(I) chains, 9N x 930, possibly due to further reactions of this material to form the material referred to as poly(CB6). Isolated poly (CB6) accounted for more than half of the total alpha 1(I)CB6 peptide expected in lung collagen, and had a hydroxylysine:lysine content 2.8 times greater in bleomycin-treated animals than in their age-matched controls. Evidence was also found for a cross-linked alpha 1(III) x alpha 1(I) peptide linking residue 87 from the alpha 1(III) chain with residue 16C from the alpha 1(I) chain; it also had an increased ratio of DHLNL:HLNL. We conclude that the increased hydroxylation of lysine observed in two different animal models of lung fibrosis occurs preferentially at the N- and C-terminal nonhelical extension peptides of the alpha 1(I) collagen chains, and that this apparent specificity of overhydroxylation of fibrotic collagen may have important structural and pathological consequences.

Regional and age variations in growing tendon

Gastrocnemius tendons of 10 White Leghorn chickens at 6, 8, and 12 weeks of age were divided into proximal, middle, and distal portions to assess regional variability in composition and growth. Body weight increases approximately 150% during the period examined, whereas the lateral gastrocnemius muscle and tendon increase approximately 193% and 227%, respectively. No significant changes in cellularity (DNA concentration) or hydroxypyridinium (OHP) crosslinks occur with increasing age. Hydroxyproline (HYP) concentration increases by 12 weeks of age, as hexuronate, glucosamine, and galactosamine decrease. Composition shows some regional variation: the distal region of the tendon has a lower HYP concentration and increased GAGs and OHP crosslinks compared to either the proximal or middle regions, which do not differ from each other. The mean collagen fibril diameter increases with age, but the oldest tendons also contain more small diameter fibrils (< 40 nm). There is a unimodal fibril distribution at all three ages, although this has broadened by 12 weeks. The data from this study suggest that rapid tendon growth occurs throughout the time period examined and that changes characteristic of mature tendon, such as increased OHP crosslink concentration, have not yet developed in hatchlings because of the large amount of new tissue being produced. Whereas all three regions of the tendon are similar in size, composition of the distal region differs from that of the proximal and middle regions, suggesting that this portion of the tendon should be avoided when sampling a tendon.