Measuring collagen degradation

Removal of dentin non-collagenous structures results in the unraveling of microfibril bundles in collagen type I

AIMS

The structural organization of collagen from mineralized tissues, such as dentin and bone, has been a topic of debate in the recent literature. Recent reports have presented novel interpretations of the complexity of collagen type I at different hierarchical levels and in different tissues. Here, we investigate the nanostructural organization of demineralized dentin collagen following the digestion of non-collagenous components with a trypsin enzyme.

MATERIALS AND METHODS

Dentin specimens were obtained from healthy third-molars, cut into small cubes, and polished down to 1 µm roughness. Samples were then demineralized with 10% citric acid for 2 min. Selected specimens were further treated with a solution containing 1 mg/ml trypsin for 48 hours at 37 °C (pH 7.9-9.0). Both untreated and trypsin digested samples were analyzed using SDS-PAGE, Field Emission Scanning Electron Microscopy (FE-SEM), and nanoindentation, where surface hardness and creep properties were compared before and after treatments.

RESULTS

FE-SEM images of demineralized dentin showed the banded morphology of D-periodical collagen type I, which upon enzymatic digestion with trypsin appeared to dissociate longitudinally, consistently unraveling ~20 nm structures (microfibril bundles). Such nanoscale structures, to the best of our knowledge, have not been characterized in dentin previously. Mechanical characterization via nanoindentation showed that the unraveling of such microfibril bundles affected the creep displacement and creep rate of demineralized dentin.

CONCLUSION

In summary, our results provide novel evidence of the organization of collagen type I from dentin, which may have important implications for the interaction of dental materials with the organic dentin matrix and the mechanical properties of mineralized tissues.

Malnutrition, zinc supplementation and catch-up growth: changes in insulin-like growth factor I, its binding proteins, bone formation and collagen turnover

OBJECTIVE

Zinc may be a limiting factor in restricting catch-up growth in severely malnourished children. This study had two aims: (i) to examine the effect of different zinc supplementation regimens on IGF-I, its binding proteins and on markers of bone and collagen turnover in severely malnourished children and (ii) to investigate mechanisms underlying catch-up growth by examining changes in these markers during nutritional rehabilitation, their inter-relationships and their relationships with ponderal and linear growth.

DESIGN

Double-blind randomized intervention study of three regimens of oral zinc supplementation.

PATIENTS

One hundred and forty-one children, aged 6-36 months, mean (SD) age 15.4 (8.7) months, with day 1 weight-for-height SD score (whz) -2.6 (0.93) and height-for-age SD score (haz) -3.79 (1.29).

MEASUREMENTS

Weight, height, lower leg length (by knemometry) at 15-day intervals from day 1 to day 90 of nutritional rehabilitation. Blood collection on days 1, 15 and 30 for IGF-I, IGFBP3, IGFBP2, bone alkaline phosphatase (BAP, osteoblast marker), procollagen type I C-terminal propeptide (PICP, marker of type I collagen synthesis), procollagen type III N-terminal propeptide (P3NP, marker of soft tissue type III collagen synthesis) and type I collagen telopeptide (ICTP, marker of type I collagen breakdown).

RESULTS

There was early rapid weight gain during refeeding, whereas height gain occurred later in the trial. IGF-I, IGFBP3, BAP, PICP and P3NP were low or very low on day 1 compared to well-nourished age-matched European children, and all increased within 15 days (P < 0.001), with PICP and P3NP reaching levels higher than European norms. IGFBP2 and ICTP were high on day 1 and decreased over the same period (P < 0.001). There were no differences in anthropometric outcome or marker responses among zinc regimens. Day 1 whz was correlated with BAP, PICP and P3NP (P < 0.001). Changes in IGF-I, IGFBP3, BAP, PICP and P3NP over 30 days correlated with ponderal growth (whz change) over the same period (all P < 0.01). However, changes in these markers over 30 days correlated better with lower leg growth (all P < 0.01) and linear growth (haz change, P < 0.01 for PICP and P3NP, P < 0.05 for IGFBP3) measured over 90 compared with 30 days. At most time points, there were strong positive correlations (i) among IGF-I, IGFBP3, BAP, PICP and P3NP (P < 0.01) and (ii) between IGFBP2 and ICTP (P < 0.01). Conversely, IGFBP2 was negatively correlated with IGF-I, IGFBP3, BAP, PICP and P3NP at most time points (P < 0.01).

CONCLUSIONS

We found no difference among zinc regimens in growth, IGF-I and its binding proteins or markers of bone and collagen turnover. Severe malnutrition was associated with low rates of bone and collagen synthesis and high rates of collagen degradation, and nutritional rehabilitation was associated with full or partial 'normalization' of the markers studied. Early weight gain and subsequent linear growth were associated with early increments in IGF-I, IGFBP3 and markers of bone and collagen formation. The study of these markers has provided additional insights into the mechanisms of the effects of malnutrition and refeeding on growth.

Optimal nutrition: calcium, magnesium and phosphorus

In the past, a major challenge for nutrition research was in defining indicators of nutritional adequacy. More recently, the research base related to the role of nutrition in chronic disease has expanded sufficiently to permit moving beyond deficiency indicators to other indicators with broader functional significance. Thus, nutrition research is faced with the new challenge of defining 'optimal nutrition'. One definition of optimal nutrition with respect to any particular nutrient could be when a functional marker reaches an 'optimal value' or plateau beyond which it is not longer affected by intake or stores of the nutrient. A functional marker of nutrient status could be defined as a physiological or biochemical factor which (1) is related to function or effect of the nutrient in target tissue(s) and (2) is affected by dietary intake or stores of the nutrient (which may include markers of disease risk). Examples of such indicators or markers are those related to risk of chronic diseases such as osteoporosis, CHD, or hypertension. The present review focuses on the concept of optimal nutrition with respect to three nutrients, Ca, Mg and P. However, for P and Mg there are as yet no functional indicators which respond to dietary intake, and in such cases nutrient requirements are established using more traditional approaches, e.g. balance data. For Ca, there has been interest in using maximal Ca retention, which is based on balance data, bone mass measurements and biomarkers of bone turnover as useful functional indicators of the adequacy of Ca intake.

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.

Changes in (markers of) bone metabolism during high dose corticosteroid pulse treatment in patients with rheumatoid arthritis

OBJECTIVE

To examine the effect of high dose corticosteroid pulse treatment (three times 200 mg dexamethasone intravenously in eight days) on calcium and bone metabolism in 17 consecutive patients with active rheumatoid arthritis (RA).

METHODS

Bone formation was quantified by measurement of serum alkaline phosphatase, osteocalcin, and carboxyterminal propeptide of type I procollagen (pro-I-CPP) concentrations. Bone resorption was measured by urinary excretion of calcium, hydroxyproline, (free and total) deoxypyridinoline (Dpyr), (free and total) pyridinoline (Pyr), and serum concentrations of the carboxyterminal cross linked telopeptide of type I collagen (I-CTP). Disease activity of RA was measured by erythrocyte sedimentation rate, C reactive protein, and Ritchie and Thompson joint scores.

RESULTS

Disease activity was initially high, and decreased during corticosteroid pulse treatment and the following five weeks. Osteocalcin, alkaline phosphatase, and pro-I-CPP concentrations were initially within normal limits, while I-CTP, Dpyr, and Pyr were increased. Osteocalcin and pro-I-CPP concentrations decreased (p < 0.01) during corticosteroid pulse treatment, but rapidly returned to baseline after the treatment. No changes were observed in alkaline phosphatase and urinary excretion of calcium and hydroxyproline. Bone resorption measured by serum I-CTP and urinary excretion of Pyr and Dpyr was unchanged or decreased (p < 0.05-0.01), depending on the time of measurement and the parameter measured.

CONCLUSIONS

In these patients with active RA, bone resorption was increased, while bone formation was within normal limits. During high dose corticosteroid pulse treatment, bone formation was only transiently decreased, while markers of bone resorption were unchanged or decreased. Because corticosteroid pulse treatment has only a short term negative effect on bone formation, and because it probably reduces bone resorption, at least partly as a result of the decreased disease activity, the effect of corticosteroid pulse treatment on bone may be assumed to be relatively mild.

Serum ionized calcium, intact PTH and novel markers of bone turnover in bedridden elderly patients

Chronic immobilization could markedly affect calcium and bone metabolism in elderly people. To investigate this, and to test the theory of 'type II' osteoporosis in bedridden elderly patients with low vitamin D status, 55 such subjects were examined. Serum concentrations of ionized calcium (Ca++), intact parathyrin (PTH) and two novel markers of bone collagen formation (carboxyterminal propeptide of type I procollagen; PICP) and resorption (carboxyterminal crosslinked telopeptide of type I collagen; ICTP) were measured. The effects on these parameters after 40 weeks of supplementation with vitamin D (1000 IU d-1) and/or calcium (1 g d-1) were subsequently prospectively evaluated. Despite low (mean 11.6 nmoll-1) serum 25-hydroxyvitamin D levels (25-OHD), those of 1,25-dihydroxy-vitamin D (1,25-(OH)2D) were mostly normal. Neither correlated with Ca++ or PTH. PTH correlated negatively not only with Ca++ (r = -0.328, P < 0.05) but also with ICTP (r = -0.306, P < 0.05). Mean PICP was normal but ICTP was elevated and tended to correlate positively with Ca++ (r = 0.268, P = 0.06). Vitamin D supplementation did not change PICP or ICTP considerably, despite slightly increased 1,25-(OH)2D and slightly decreased PTH. Ca++ values were normal and remained stable. In conclusion, Ca++ and PTH are poor indicators of vitamin D status in chronically immobilized elderly subjects. Furthermore, the results suggest that the increased bone resorption is not due to 'type II' secondary hyperparathyroidism; rather the resorption is primarily increased. Correction of vitamin D deficiency does not seem to benefit ageing bones unless adequate mechanical loading is provided.