Collagen cross-linking

Potential skin involvement in ALS: revisiting Charcot's observation - a review of skin abnormalities in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease affecting motor neurons of the brain and spinal cord, leading to progressive paralysis and death. Interestingly, many skin changes have been reported in ALS patients, but never as yet fully explained. These observations could be due to the common embryonic origin of the skin and neural tissue known as the ectodermal germ layer. Following the first observation in ALS patients' skin by Dr Charcot in the 19th century, in the absence of bedsores unlike other bedridden patients, other morphological and molecular changes have been observed. Thus, the skin could be of interest in the study of ALS and other neurodegenerative diseases. This review summarizes skin changes reported in the literature over the years and discusses about a novel in vitro ALS tissue-engineered skin model, derived from patients, for the study of ALS.

An increase of mature type skin collagen cross-link, histidinohydroxylysinonorleucine, in the sclerotic skin of morphea

Histidinohydroxylysinonorleucine (HHL) is a trivalent cross-link of type I collagen that is stable and has physiologic properties. In this study, we conducted a biochemical quantification of HHL in five patients with morphea. We compared the quantity of HHL contained in the lesional skin and the adjacent normal skin. The molar ratio of HHL to collagen in the sclerotic lesion was significantly higher than that of the adjacent normal skin (0.36 +/- 0.09 vs. 0.28 +/- 0.10, P<0.01). We could find no relationship between an increase of HHL content and the duration of the disease. The results indicate that an increased amount of HHL may play a certain role to inducing the sclerotic skin change characteristic of morphea.

Collagen cross-linking of skin in patients with amyotrophic lateral sclerosis

Collagen cross-links of skin tissue (left upper arm) from 11 patients with amyotrophic lateral sclerosis (ALS) and 9 age-matched control subjects were quantified. It was found that patients with ALS had a significant reduction in the content of an age-related, stable cross-link, histidinohydroxylysinonorleucine, that was negatively correlated with the duration of illness. The contents of sodium borohydride-reducible labile cross-links, dehydro-hydroxylysinonorleucine and dehydro-histidinohydroxymerodesmosine, were significantly increased and were positively associated with the duration of illness (r = 0.703, p less than 0.05 and r = 0.684, p less than 0.05, respectively). The results clearly indicate that during the course of ALS, the cross-linking pathway of skin collagen runs counter to its normal aging, resulting in a "rejuvenation" phenomenon of skin collagen. Thus, cross-linking of skin collagen is affected in ALS.

Increased glycation and pigmentation of collagen in aged and young parabiotic rats and mice

Changes of non-enzymatic collagen glycosylation were followed in 2- and 24-month-old rats and mice and in parabiotic animals of the same age. With advancing age increased glycation of collagen was observed in both old male Wistar rats and white mice. Further it was demonstrated that both aortal and skin collagen of young animals is rapidly non-enzymatically glycosylated in the common milieu created by parabiotic animals and the proportion of non-enzymatically incorporated glucose approaches in the young counterparts the level found in old individuals. Similar trends as with non-enzymatic glycosylation were found with a fluorescent (370/440 nm) product present in both categories of collagen preparations. This fluorescence was higher in old animals and was considerably increased in the young counterpart of the parabiotic couple 6 weeks after operation. The nature of the fluorescent product appears different from pyridinoline and remains to be elucidated.

Chemistry of the collagen cross-links. Origin and partial characterization of a putative mature cross-link of collagen

The conversion of the reducible divalent cross-links in collagen to non-reducible multivalent cross-links in mature collagen has resulted in the identification of several new amino acids as the putative mature cross-link. None of these compounds has completely satisfied the necessary criteria. We have now isolated an amino acid of high Mr, derived from lysine, that is only present in high-Mr peptides derived from mature collagen. Its increase with age of the tissue correlates with the decrease in the reducible cross-links, and it is present both in mature skin and bone, which are initially cross-linked through the aldimine and oxo-imine divalent cross-link respectively. We propose that this amino acid, as yet incompletely characterized and designated compound M, is a major cross-link of mature collagen.

Biosynthesis of collagen crosslinks: in vivo labelling of neonatal skin, tendon, and bone in rats

Collagen crosslinks in neonatal rats were labelled in vivo by a single intraperitoneal injection of 200 microCi of [14C]lysine. Rats were killed at times ranging from 30 minutes to 10 weeks after injection. Whole skin, tendon, and bone were analyzed, after reduction and hydrolysis, for collagen crosslink content by HPLC. Crosslinks and amino acids were visualized by their incorporation of radioactivity from [14C]lysine and also fluorometrically by post-column derivatization with o-phthalaldehyde. The incorporation of 14C from labelled lysine into the principal difunctional reducible crosslinks, N6.6'-dehydro-5,5'-dihydroxylysinonorleucine and N6.6'-dehydro-5-hydroxylysinonorleucine, increased most rapidly between 4 and 12 hours after injection, results similar to those observed by others studying crosslink biosynthesis in vitro. Incorporation of 14C into the tetrafunctional crosslink histidinohydroxymerodesmosine proceeded more slowly than it did for the difunctional crosslinks. Values for the amount of radioactivity incorporated into the various crosslinks reached an apparent constant value between 3 and 5 days after injection for all three tissues studied. These values remained approximately constant for the duration of the experiment except for HHMD in tendon, which showed an increase in incorporated radioactivity at 8 and 10 weeks after injection. Direct chemical quantification of these same crosslinks by determination of the fluorescence of their o-phthalaldehyde adducts was also performed. We conclude that in vivo labelling of collagen crosslinks can be studied, at least in rapidly growing neonates, after a single injection of radioactive lysine. The results of such studies support previous suggestions by others about the rate of formation of difunctional crosslinks based upon studies using in vitro systems. Our results further suggest that formation of the tetrafunctional reducible crosslink histidinohydroxymerodesmosine proceeds relatively rapidly in vivo. Finally, we conclude that such labelled crosslinks are apparently quite stable after biosynthesis, suggesting the possibility of studies of the metabolic fate of collagen crosslinks over appreciable fractions of the lifetime of a rat.