Collagen metabolism

Detection of Pentosidine Cross-Links in Cell-Secreted Decellularized Matrices Using Time Resolved Fluorescence Spectroscopy

Hyperglycemia-mediated, nonenzymatic collagen cross-links such as pentosidine (PENT) can have deleterious effects on cellular interactions with the extracellular matrix (ECM). Present techniques to quantify PENT are limited, motivating the need for improved methods to study the accumulation and contribution of PENT toward diabetic clinical challenges such as impaired bone healing. Current methods for studying PENT are destructive, laborious, and frequently employ oversimplified collagen films that lack the complexity of the native ECM. The primary goal of this study was to evaluate the capacity of time-resolved fluorescence spectroscopy (TRFS) to detect PENT in cell-secreted ECMs possessing enhanced compositional complexity. To demonstrate an application of this method, we assessed the response of human mesenchymal stem cells (MSCs) to cross-linked substrates to explore the role of detected PENT on osteogenic differentiation. We exposed MSC-secreted decellularized matrices (DMs) to 0.66 M ribose for 2 weeks and used TRFS to detect the accumulation of PENT. Ribose treatment resulted in a 30 nm blue shift in peak fluorescence emission and a significant decrease in average lifetime compared to that of control DMs (4.4 ± 0.3 ns vs 3.5 ± 0.09 ns). Evaluation of samples with high performance liquid chromatography (HPLC) confirmed that changes in observed fluorescence were due to PENT accumulation. A strong correlation was found between TRFS parameters and the HPLC measurement of PENT, validating the use of TRFS as an alternative method of PENT detection. Osteoblastic gene expression was significantly reduced in MSCs seeded on ribose DMs at days 7 and 14. However, no significant differences in calcium deposition were detected between control and ribose DMs. These data demonstrate the efficacy of nondestructive fluorescence spectroscopy to examine the formation of nonenzymatic collagen cross-links within biomimetic culture platforms and showcase one example where an improved biomimetic substrate can be used to probe cell-ECM interactions in the presence of collagen cross-links.

Changes in the Fracture Resistance of Bone with the Progression of Type 2 Diabetes in the ZDSD Rat

Individuals with type 2 diabetes (T2D) have a higher fracture risk compared to non-diabetics, even though their areal bone mineral density is normal to high. Identifying the mechanisms whereby diabetes lowers fracture resistance requires well-characterized rodent models of diabetic bone disease. Toward that end, we hypothesized that bone toughness, more so than bone strength, decreases with the duration of diabetes in ZDSD rats. Bones were harvested from male CD(SD) control rats and male ZDSD rats at 16 weeks (before the onset of hyperglycemia), at 22 weeks (5-6 weeks of hyperglycemia), and at 29 weeks (12-13 weeks of hyperglycemia). There were at least 12 rats per strain per age group. At 16 weeks, there was no difference in either body weight or glucose levels between the two rat groups. Within 2 weeks of switching all rats to a diet with 48 % of kcal from fat, only the ZDSD rats developed hyperglycemia (>250 mg/dL). They also began to lose body weight at 21 weeks. CD(SD) rats remained normoglycemic (<110 mg/dL) on the high-fat diet and became obese (>600 g). From micro-computed tomography (μCT) analysis of a lumbar vertebra and distal femur, trabecular bone volume did not vary with age among the non-diabetic rats but was lower at 29 weeks than at 16 weeks or at 22 weeks for the diabetic rats. Consistent with that finding, μCT-derived intra-cortical porosity (femur diaphysis) was higher for ZDSD following ~12 weeks of hyperglycemia than for age-matched CD(SD) rats. Despite an age-related increase in mineralization in both rat strains (μCT and Raman spectroscopy), material strength of cortical bone (from three-point bending tests) increased with age only in the non-diabetic CD(SD) rats. Moreover, two other material properties, toughness (radius) and fracture toughness (femur), significantly decreased with the duration of T2D in ZDSD rats. This was accompanied by the increase in the levels of the pentosidine (femur). However, pentosidine was not significantly higher in diabetic than in non-diabetic bone at any time point. The ZDSD rat, which has normal leptin signaling and becomes diabetic after skeletal maturity, provides a pre-clinical model of diabetic bone disease, but a decrease in body weight during prolonged diabetes and certain strain-related differences before the onset of hyperglycemia should be taken into consideration when interpreting diabetes-related differences.

Identifying Novel Clinical Surrogates to Assess Human Bone Fracture Toughness

Fracture risk does not solely depend on strength but also on fracture toughness; ie, the ability of bone material to resist crack initiation and propagation. Because resistance to crack growth largely depends on bone properties at the tissue level, including collagen characteristics, current X-ray based assessment tools may not be suitable to identify age-related, disease-related, or treatment-related changes in fracture toughness. To identify useful clinical surrogates that could improve the assessment of fracture resistance, we investigated the potential of (1)H nuclear magnetic resonance spectroscopy (NMR) and reference point indentation (RPI) to explain age-related variance in fracture toughness. Harvested from cadaveric femurs (62 human donors), single-edge notched beam (SENB) specimens of cortical bone underwent fracture toughness testing (R-curve method). NMR-derived bound water showed the strongest correlation with fracture toughness properties (r = 0.63 for crack initiation, r = 0.35 for crack growth, and r = 0.45 for overall fracture toughness; p < 0.01). Multivariate analyses indicated that the age-related decrease in different fracture toughness properties were best explained by a combination of NMR properties including pore water and RPI-derived tissue stiffness with age as a significant covariate (adjusted R(2)  = 53.3%, 23.9%, and 35.2% for crack initiation, crack growth, and overall toughness, respectively; p < 0.001). These findings reflect the existence of many contributors to fracture toughness and emphasize the utility of a multimodal assessment of fracture resistance. Exploring the mechanistic origin of fracture toughness, glycation-mediated nonenzymatic collagen crosslinks and intracortical porosity are possible determinants of bone fracture toughness and could explain the sensitivity of NMR to changes in fracture toughness. Assuming fracture toughness is clinically important to the ability of bone to resist fracture, our results suggest that improvements in fracture risk assessment could potentially be achieved by accounting for water distribution (quantitative ultrashort echo time magnetic resonance imaging) and by a local measure of tissue resistance to indentation, RPI.

Cortical bone mechanical properties are altered in an animal model of progressive chronic kidney disease

Chronic kidney disease (CKD), which leads tocortical bone loss and increasedporosity,increases therisk of fracture. Animal models have confirmed that these changes compromise whole bone mechanical properties. Estimates from whole bone testing suggest that material properties are negatively affected, though tissue-level assessmentshavenot been conducted. Therefore, the goal of the present study was to examine changes in cortical bone at different length scales using a rat model with theprogressive development of CKD. At 30 weeks of age (∼75% reduction in kidney function), skeletally mature male Cy/+ rats were compared to their normal littermates. Cortical bone material propertieswere assessed with reference point indentation (RPI), atomic force microscopy (AFM), Raman spectroscopy,and high performance liquid chromatography (HPLC). Bones from animals with CKD had higher (+18%) indentation distance increase and first cycle energy dissipation (+8%) as measured by RPI.AFM indentation revealed a broader distribution of elastic modulus values in CKD animals witha greater proportion of both higher and lower modulus values compared to normal controls. Yet, tissue composition, collagen morphology, and collagen cross-linking fail to account for these differences. Though the specific skeletal tissue alterations responsible for these mechanical differences remain unclear, these results indicate that cortical bone material properties are altered in these animals and may contribute to the increased fracture risk associated with CKD.

TGF-beta and wound healing

Wound healing comprises an ordered sequence of events including cell migration and proliferation, synthesis of extracellular matrix, angiogenesis and remodelling. TGF-beta regulates many of these processes. Animal models are used to study healing of simple linear incision wounds and deeper dermal wounds under normal and impaired conditions. TGF-beta increases the rate of healing and the breaking strength of the repaired tissue. It also enhances angiogenesis and consequent blood flow to dermal wounds, partly by stimulating the local release of other growth factors. TGF-beta reverses the adverse affects of glucocorticoids on wound healing and thus may be useful in the treatment of chronic ulcers or wounds in patients whose normal responses have been impaired by therapy with steroids, radiation or other drugs.

Accelerated wound closure in mice deficient for interleukin-10

The impact of the local inflammatory response on the process of wound healing has been debated for decades. In particular, the question whether infiltrating macrophages and granulocytes promote or impede tissue repair has received much attention. In the present study, we show that wound healing is accelerated in mice deficient for the anti-inflammatory cytokine interleukin (IL)-10. IL-10-/- mice closed excisional wounds significantly earlier compared with IL-10-competent control littermates. This effect was attributable to accelerated epithelialization as well as enhanced contraction of the wound tissue in the mutant animals. Increased alpha-smooth muscle actin expression in IL-10-deficient mice suggests that augmented myofibroblast differentiation is responsible for the enhanced contraction of wounds in mutant mice. The number of macrophages infiltrating the wound tissue was significantly increased in IL-10-/- mice compared with control littermates suggesting that this cell type mediates the accelerated tissue repair. These results show for the first time that IL-10 can impede wound repair.

Integrin α2β1 Is Required for Regulation of Murine Wound Angiogenesis but Is Dispensable for Reepithelialization

The alpha2beta1 integrin functions as the major receptor for collagen type I on a large number of different cell types, including keratinocytes, fibroblasts, endothelial cells, and a variety of inflammatory cells. Recently, we demonstrated that adhesion of keratinocytes to collagen critically depends on alpha2beta1, whereas fibroblasts can partly compensate for loss of alpha2beta1 in simple adhesion to collagen. However, in three-dimensional collagen matrices, alpha2beta1-null fibroblasts are hampered in generating mechanical forces. These data suggested a pivotal role for alpha2beta1 during wound healing in vivo. Unexpectedly, reepithelialization of excisional wounds of alpha2beta1-null mice was not impaired, indicating that keratinocytes do not require adhesion to or migration on collagen for wound closure. Whereas wound contraction and myofibroblast differentiation were similar, wound tensile strain was reduced in alpha2beta1-null mice, suggesting subtle changes in organization of the extracellular matrix. In addition, we observed reduced influx of mast cells into the granulation tissue, whereas infiltration of other inflammatory cells was not impaired. Interestingly, ablation of alpha2beta1 resulted in strong enhancement of neovascularization of granulation tissue and sponge implants. Both ultrastructurally and functionally, these new blood vessels appeared intact. In conclusion, our data show unique and overlapping functions of alpha2beta1 integrin during murine cutaneous wound healing.

Wound healing in the alpha2beta1 integrin-deficient mouse: altered keratinocyte biology and dysregulated matrix metalloproteinase expression

The alpha2beta1 integrin, a collagen/laminin receptor, is expressed at high level in the basal cell layer of the epidermis. To define the role of the alpha2beta1 integrin in wound healing, wound repair was extensively evaluated in wild-type and alpha2-null mice in vivo. In addition, the impact of alpha2beta1 integrin-deficiency on the function of primary murine keratinocytes in vitro was analyzed. Our in vivo findings demonstrate that genetic deletion of the alpha2beta1 integrin does not significantly alter the rate of re-epithelialization, collagen deposition, or tensile strength during wound closure in mice. In marked contrast to the observed similarities in wound healing, deletion of the alpha2beta1 integrin resulted in a dramatic increase in neoangiogenesis in the wound microenvironment. In contrast to in vivo studies, primary keratinocytes from alpha2-null mice adhered poorly and displayed impaired migration on type I collagen in vitro. We demonstrate that alpha2beta1 integrin-ligation negatively regulates expression of genes including matrix metalloproteinases both in vivo and in vitro. Furthermore, the changes in gene expression could potentially account for relatively normal wound healing in the alpha2-deficient mouse and our recent observation that suggests an antiangiogenic role for the alpha2beta1 integrin in vivo.

An investigation to optimize angiogenesis within potential dermal replacements

BACKGROUND

Acute and chronic wounds are costly and invariably expose significant structures. Surgical reconstruction causes donor-site morbidity, scarring, and the need for intensive care. Reconstruction using an artificial dermis avoids donor sites, but available collagen-based solutions are susceptible to poor take. Using an in vitro angiogenic assay, the authors investigated dermal matrices for potential inclusion in a second-generation proangiogenic synthetic dermal replacement.

METHODS

Human placental endothelial cells were cocultured on Cytodex beads (Pharmacia Biotech) and plated in eight different extracellular matrix gels (collagen, fibrin, four glycosaminoglycans, vitronectin, and fibronectin), with or without stimulation from two soluble angiogenic factors. Three different cell lines were used, with 30 beads per condition. Cellular invasion into gels was calculated using Sigma Scan computer software, and statistical comparisons were made.

RESULTS

The authors found that fibrin provided greatest stimulus for endothelial invasion, with invasion in fibrin inhibited by collagen in a concentration-dependent fashion. Invasion by alternative extracellular matrix components and soluble angiogenic factors was far less than that in fibrin.

CONCLUSIONS

The authors identified that extracellular matrices can provide greater angiogenic potential than soluble angiogenic factors. Fibrin provided a better proangiogenic scaffold than collagen. This could well be used to encourage blood vessel ingress and eventual take of a second-generation proangiogenic synthetic dermal replacement.

Vascular endothelial cell regulation of extracellular matrix collagen: role of nitric oxide

Endothelium-derived products have been implicated in the regulation of vascular wall structure through their effects on extracellular matrix metabolism. The purpose of this study was to further understand the paracrine mechanisms underlying endothelial cell regulation of extracellular matrix metabolism by testing the hypothesis that endothelium-derived nitric oxide decreases the concentration of soluble collagens derived from vascular smooth muscle cells (VSMCs). Porcine coronary endothelium and VSMCs were grown under a coculture configuration to assess the paracrine effects of nitric oxide produced by the endothelium on VSMC collagen types I and III. Endogenous endothelial cell nitric oxide production was blocked with N(omega)-nitro-L-arginine methyl ester. Collagen type I and type III were quantitatively measured using an enzyme-linked immunosorbent assay method. The endothelium elicited a time-dependent increase in the concentration of soluble VSMC-derived collagen type I; in contrast, collagen type III was decreased. After inhibition of nitric oxide production, there was a marked increase in both collagen types I and III concentration. These results demonstrated that endothelium-derived nitric oxide differentially alters collagen subtypes produced by VSMCs. The data support the hypothesis that nitric oxide functions via a paracrine mechanism to decrease VSMC collagen types I and III concentration, a finding consistent with an integral role for the endothelium in modulating extracellular matrix metabolism.

Ascorbate differentially regulates elastin and collagen biosynthesis in vascular smooth muscle cells and skin fibroblasts by pretranslational mechanisms

Ascorbate contributes to several metabolic processes including efficient hydroxylation of hydroxyproline in elastin, collagen, and proteins with collagenous domains, yet hydroxyproline in elastin has no known function. Prolyl hydroxylation is essential for efficient collagen production; in contrast, ascorbate has been shown to decrease elastin accumulation in vitro and to alter morphology of elastic tissues in vivo. Ascorbate doses that maximally stimulated collagen production (10-200 microM) antagonized elastin biosynthesis in vascular smooth muscle cells and skin fibroblasts, depending on a combination of dose and exposure time. Diminished elastin production paralleled reduced elastin mRNA levels, while collagen I and III mRNAs levels increased. We compared the stability of mRNAs for elastin and collagen I with a constitutive gene after ascorbate supplementation or withdrawal. Ascorbate decreased elastin mRNA stability, while collagen I mRNA was stabilized to a much greater extent. Ascorbate withdrawal decreased collagen I mRNA stability markedly (4.9-fold), while elastin mRNA became more stable. Transcription of elastin was reduced 72% by ascorbate exposure. Differential effects of ascorbic acid on collagen I and elastin mRNA abundance result from the combined, marked stabilization of collagen mRNA, the lesser stability of elastin mRNA, and the significant repression of elastin gene transcription.

Modulation of transforming growth factor-beta 1 stimulated elastin and collagen production and proliferation in porcine vascular smooth muscle cells and skin fibroblasts by basic fibroblast growth factor, transforming growth factor-alpha, and insulin-like growth factor-I

During tissue repair and development, matrix accumulation is modulated as multiple signals impinge on target cells. We have investigated the effects of combinations of the mitogenic cytokines, basic fibroblast growth factor (bFGF), transforming growth factor alpha (TGF-alpha), and insulin-like growth factor-1 (IGF-1) with transforming growth factor-beta 1 (TGF-beta 1) with respect to the production of two matrix components, elastin and type I collagen. Using specific enzyme-linked immunoassays for detection of secreted precursors in both vascular smooth muscle cells and skin fibroblasts from the domestic pig, production of these two fibrous proteins was shown to be strongly stimulated by TGF-beta 1. In the smooth muscle cell, both bFGF and TGF-alpha were potent antagonists of TGF-beta 1-mediated matrix production, whereas IGF-1 was only weakly additive with respect to elastin production. Antagonism was also evident to a lesser extent in skin fibroblasts. Reduced responsiveness to TGF-beta 1 did not appear to be due to a switch to a proliferative state, since TGF-beta 1 itself acted as a mitogen in confluent SMC, and TGF-alpha was only a weak mitogen in confluent fibroblast cultures. Although a predominant effect of TGF-beta is matrix accumulation, these findings suggest that this property will be significantly modified by the cytokine context.

Hyaluronate derivatives and their application to wound healing: preliminary observations

Hyaluronic acid and its derivatives show promise as biomaterials in wound healing applications. Studies of cutaneous wound repair were carried out in two animal models to compare the biological effects of hyaluronic acid and hyaluronic acid ethyl ester, a new semisynthetic derivative. The two compounds were tested in partial-thickness excisional wounds in 40-kg pigs and full-thickness excisional wounds in the rabbit ear as 0.2% (w/w) formulations in a neutral Na alginate vehicle. All compounds were administered daily under an occlusive, polyurethane dressing. Neither hyaluronic acid nor the hyaluronic acid ethyl ester showed toxic or inflammatory influences over the observation period of about 2 weeks. Morphometric analysis of porcine wounds revealed small differences among treatment groups which may have been masked by the effect of the vehicle. The rabbit ear model data suggested a very slight inhibition of wound closure. Biochemical analysis of ear wounds showed this injury model to be a sensitive system for evaluation of vulnerary agents. The hyaluronate-treated wounds tended to accumulate collagen more slowly, which may reflect the capacity of these compounds to modify the scarring process. Given the ability to fabricate hyaluronate esters into films and fibers, these data suggest that such biomaterials will not, by themselves, exert a negative influence on the repair process and may improve healing, either alone or in combination with other soluble agents.

Accelerated healing of ulcer wounds in the rabbit ear by recombinant human transforming growth factor-beta 1

A dermal ulcer wound-healing model was established in rabbit ear to examine the effects of recombinant human transforming growth factor-beta 1 (rhTGF-beta 1) in wound healing. Histomorphometric examination of the wounds indicate a biphasic healing response 7 days after a single application of rhTGF-beta 1 at the time of wounding. Statistically significant healing occurred at 5-100 ng but not at higher doses of 500 or 1000 ng rhTGF-beta 1/wound. Enhanced collagen synthesis as determined by [3H]proline incorporation occurred at 15 and 25 ng and was significantly depressed at 500 ng rhTGF-beta 1/wound. Multiple doses of 100 ng rhTGF-beta 1 applied to the wound at the time of wounding and for 3 days after wounding provided results comparable to the single application of growth factor. Delaying treatment 24 hr after wounding did not enhance wound healing compared with vehicle. Our findings suggest that rhTGF-beta 1 can be a valuable growth factor to improve the healing of ulcer wounds.