Fight Aging!

Cross-links that join together molecules of the extracellular matrix are necessary for the normal structural properties of tissue. Cross-links can also be formed in a detrimental way by advanced glycation endproducts (AGEs), a form of metabolic waste, and thereby impair structural properties of tissue. Most AGEs are short-lived, but some are quite persistent, hard for our biochemistry to break down, and build up with age. High levels of AGEs are characteristic of an abnormal metabolism, such as in diabetic patients, and can cause long-term harm by promoting inflammation and altered cellular behavior through the receptor for AGEs (RAGE).

Persistent cross-links formed by AGEs, rather than by normal tissue maintenance processes, can reduce elasticity in skin and blood vessels. Of interest to the authors of today's open access research is the question of how AGEs might degrade the strength and resilience of bone or cartilage. Is it via formation of cross-links, or via some other mechanism?

Working with cross-links is challenging. There are many different types of AGE, with quite different characteristics. It is not an area of molecular biochemistry that has received the attention that it deserves, and as a result there are deficiencies in the tools and understanding. Cataloging the amounts and consequences of specific types of AGE in tissues is lagging, and there are uncertainties attached to the present consensus, even the very compelling view that glucosepane is the most important age-related cross-linking AGE, and thus the best target for treatments to break down AGEs.

Given the very different effects resulting from short-lived versus persistent AGEs, or different AGEs with different biochemical interactions, it is rather important to understand the breakdown of AGEs rather than just bulk amounts of various categories of AGEs. Additionally, since AGEs tend to be produced through similar mechanisms, there is no guarantee that any particular AGE is the important damaging mechanism even if when it is unambiguously associated with disease and loss of function. Production of the AGE under consideration may be correlated with the production of many other AGE types, one of which is an important damaging agent. Thus there is a lot of work left to accomplish in this part of the field, and at least some of it will look a lot like today's open access paper.

Mass spectrometric quantitation of AGEs and enzymatic crosslinks in human cancellous bone

Material property of bone is an important determinant of bone strength. The nanoscale structures of bone are formed from collagen fibers surrounded and infiltrated with hydroxyapatite minerals. Collagen fibers provide the material properties such as tensile strength, ductility and toughness, while hydroxyapatite minerals are thought to contribute to stiffness. The functional properties of collagen are influenced by posttranslational modifications (PTMs). Among the modifications, the formation of enzymatic crosslinks between collagen fibrils are essential for physiological bone strength. On the other hand, damaging to bone strength, advanced glycation end-products (AGEs) are the results of non-enzymatic PTMs.

A series of basic and clinical trials have clarified the link between the accumulation of AGEs in bone collagen, and deterioration of bone strength. An in vitro glycation of bovine cortical bone induced pentosidine, an AGE compound, which resulted in reduced stiffness and post-yield strain. This phenomenon was also demonstrated in human cancellous bone. An in vivo study involving spontaneously diabetic rats also revealed that after the onset of diabetes, there was an increase in pentosidine accumulation in the femur with decreased bone strength despite no reduction in bone mineral density. Moreover, the link between AGEs and bone strength has been demonstrated in clinical trials. Urinary excretion of pentosidine, which is used as a surrogate marker for bone AGEs, was shown to be a predictor of vertebral fracture after adjustment for age, bone mineral density, and renal function.

In this study, we established a system that enabled the quantitation of five AGEs (CML, CEL, MG-H1, CMA and pentosidine), as well as two mature and three immature enzymatic crosslinks, in 149 human cancellous bone samples. We examined the patterns of AGEs accumulation to investigate whether pentosidine or total fluorescent AGEs (tfAGEs) more accurately reflects the actual AGEs status in bone collagen. As the clinical manifestations of AGEs accumulation include aging, diabetes and renal failure, we also analyzed the association between AGEs and the clinical parameters such as age, gender, BMI, history of diseases, glycated hemoglobin (HbA1c) as the marker of blood glycemic status over several weeks to months, tartrate-resistant acid phosphatase-5b (TRACP-5b) as the measure of bone resorption, and estimated glomerular filtration rate (eGFR).

The results showed that MG-H1 was the most abundant AGE, whereas pentosidine was 1/200-1/20-fold less abundant than the other four AGEs. The AGEs were significantly and strongly correlated with pentosidine, while showing moderate correlation with tfAGEs. In single and multiple regression analyses, gender was the strongest determinant of the AGEs, followed by age, TRACP-5b, HbA1c, and BMI. The gender difference in oxidative stress and carbonyl stress may explain this. In addition, CML and CEL, the non-crosslinking AGEs, were negatively correlated with the immature crosslinks. This result raises the possibility that non-crosslinking AGEs attribute to the deterioration of bone strength by inhibiting the formation of enzymatic crosslinks.