A Review of Collagen Cross-Linking in Aging

Many types of metabolic waste and byproduct molecules are generated by the normal operation of cellular metabolism. You can't run an engine without exhaust or a factory without waste generation. The majority of these unwanted outputs are swept away to be broken down and recycled by a panoply of varied housekeeping mechanisms, but unfortunately this is not not the case for all of them. Some hardy forms of waste linger, accumulating throughout life, and this problem becomes worse in later years as all of the systems in our biology lose their effectiveness due to damage. The damage of aging in living beings is an accelerating downward spiral because it also degrades the very mechanisms that are in place to repair it on an ongoing basis.

Cross-links in the complex structures of the extracellular matrix are a consequence of some classes of metabolic waste, such as advanced glycation end-products (AGEs). The properties of any given tissue are determined by the nanoscale structure and arrangement of proteins in the extracellular matrix, but cross-links degrade that structure by chaining these proteins together. A growing level of cross-linking can reduce elasticity in softer tissues such as skin or blood vessel walls, for example, and that loss of elasticity has serious consequences for health. Similar loss of structural properties can occur for tissues where strength or ability to bear load are the important factors. There are a lot of different types of AGE, many of which are short-lived because a healthy biochemistry is quite capable of removing them. Some are long-lived and resilient, however, such as glucosepane that accounts for the overwhelming majority of AGEs in old human skin.

Despite breaking down AGEs being an obvious potential target for therapies, and this research meshing well with the strengths of the pharmaceutical industry, it is actually the case that very little work takes place on ways to safely remove persistent AGEs from tissues. It can be argued that this is in part due to a high profile failure in AGE-breaker drug development not so many years ago, but also because there are few tools and laboratories capable of working with glucosepane in any meaningful way. It is an odd oversight, one of those scientific blank spots that perpetuates itself because every group than might choose to work on this area looks at the absence of basic tools and then moves on to something easier and more likely to return a profit. The SENS Research Foundation is at present funding a research program to fix this situation by building the tools needed to work with glucosepane.

Here I'll point out an open access review on the topic of cross-links in the collagen structure of the extracellular matrix, with a focus on tendons in particular:

The role of collagen crosslinks in ageing and diabetes - the good, the bad, and the ugly

The non-enzymatic reaction of proteins with glucose (glycation) is a topic of rapidly growing importance in human health and medicine. There is increasing evidence that this reaction plays a central role in ageing and disease of connective tissues. Of particular interest are changes in type-I collagens, long-lived proteins that form the mechanical backbone of connective tissues in nearly every human organ. Despite considerable correlative evidence relating extracellular matrix (ECM) glycation to disease, little is known of how ECM modification by glucose impacts matrix mechanics and damage, cell-matrix interactions, and matrix turnover during aging. More daunting is to understand how these factors interact to cumulatively affect local repair of matrix damage, progression of tissue disease, or systemic health and longevity.

Various approaches have been taken to prevent formation of AGEs (for an excellent review see "Characteristics, formation, and pathophysiology of glucosepane: a major protein cross-link"). For instance, a reduced alimentary glucose uptake has been shown to be beneficial, as have approaches seeking to breakdown or block intermediate molecular interactions. Further efforts have shown potential benefit in "protecting" amino acid residues by agents that competitively bind aldehydes. Complementing these preventative approaches, some therapeutic approaches have sought to break existing AGE crosslinks.

Contrary to the mentioned preventative approaches, crosslink breaking can reverse AGE crosslinking and its deleterious effects on tissue mechanics and matrix remodeling. Since AGE crosslinks in tendon are only secondary complications of diabetes, most anti-AGE work has been done in other tissues (such as skin and arteries). However, their potential effectiveness was first demonstrated using rat tail tendon. In any case, as far as we are aware there is no study testing the ability of crosslink breaking therapies to ameliorate the predisposition of tendon to mechanical damage, or promote "healthy" tissue remodeling at a repair site.

Comments

I'm not convinced AGE-breakers are the only way to go in addressing cross-links.

As the article itself notes, every tissue eventually hits its turnover time. Even if it is on a 10-year scale, I intend to live a lot longer than that. The problem lies in the homeostasis of blood serum AGE levels at the time. If your AGE levels are high, you'll randomly form just as many crosslinks in the new tissue as in the old one being replaced, if not more. By contast, if you can keep serum AGE levels low over the long term, the formation rate should stay at or below the clearance rate and your body will resolve the problem on its own.

Many studies indicate that soluble RAGE proteins (sRAGE and esRAGE) are decoy ligands for AGE's, which bind to the AGE's for standard excretion clearance. This is probably why a correlation study on healthy centenarians vs. young people and young people with myocardial infarctions fingered sRAGE levels as a relevant marker. Increasing soluble RAGE levels might be a simpler method to addressing crosslinks than AGE-breakers.

Statins have been shown to increase soluble RAGE levels in vitro as well in vivo. And the effect carries through to reducing AGE levels in vivo as well.

Statins have also have been on the market long enough that most side effects (rare rhabdomyolisis for instance) are well known, too. I'm a healthy 30-something and I'm considering going on statins for primary prevention for this reason. Any thoughts on that approach?

Posted by: Aryeh at December 18th, 2014 12:35 PM

@Aryeh - there are loads of people on Statins, and if it had an effect on lifespan I think this would have been noticed by now, even if only in a sub group who don't suffer or die from heart attacks. So that is so pretty good evidence against your hypothesis.

You'd very probably just be better off donating the money you would have spent on Statins to the SENS Research Foundation or the SENS RF Cambridge Lab (who are looking into AGE breaking drugs and Enzymes).

Maybe even start your own fundraiser for this lab?

I know it is tempting to look around at existing drugs and products for a solution that is available now. But you'll almost certainly be wasting that money. There is a way forward, but it is through the correct research, not through the undiscovered property of some already available drug sitting in your medical cabinet.

Posted by: Jim at December 19th, 2014 5:10 AM

Jim,

The target population being prescribed statins tend to already have health issues or risk factors which could mask some of the benefits of the statin therapy if it was applied to a younger, healthier population.

I'm also unsure if typical statin dosages are sufficient to restore AGE levels to the levels found in younger subjects: even in studies with atorvastatin (which is most effective at increasing sRAGE in vitro of the statins tested), at typical prescribed levels (10mg), they found perhaps a 20% increase in soluble RAGE levels. While that's helpful, it's probably not enough to have a significant impact.

Overall, I agree that if statins are a real solution to AGE's, something should have shown up earlier in population studies. I'm also interested in seeing if there are any sRAGE promoters known out there which may be more targeted and effective than statins.

Unfortunately, under our currently regulatory regime, in which it costs well over a billion dollars and 10+ years to bring a new drug to market, finding off-label uses for existing drugs, perhaps at higher dosage levels, is an avenue that's kind of tempting.

Wish I had the money to make a dent in the SENS budget, but can definitely give something extra for the year.

Posted by: Aryeh at December 20th, 2014 11:12 PM

Are those papers describing AGEs in serum, intra-cellular AGEs, or collagen ECM AGEs?

Posted by: Jim at December 21st, 2014 4:18 PM

We have made progress in identifying by Ramen Spectroscopy the major age cross link signatures in diabetic skin and it may be possible to break them, but with funding as it is and few glycation researchers remaining or entering the field, any significant progress will be far off in my opinion. The enzymes mentioned are not able to deglycate large molecules. Engineered or directed evolutionary approaches were not successful for many reasons. The grave yard species of bacteria, would be one logical source for new enzymes, but if I were a bacterium, I'd take the highway rather than chew through the brush and that's probably why progress had been slow. These enzymes will require more appropriate or relevant substrates to matter for the human animal.

Posted by: Dr. Obrenovich at December 21st, 2014 5:23 PM

Jim,

All over the place. Some are on serum AGE levels, others look at AGE levels in the glomerular basement membrane in the kidney, others at levels in atheroscleratic plaques. In same cases effect does seem to be more pronounced than in others. Also depends what baseline levels they're measuring from. The lack of standardized units for measuring AGE's makes it a little hard to compare across studies.

---

Dr. Obrenovich,

I would agree that finding enzymes to break existing bonds is not a very viable route. That's why I felt using soluble RAGE (or something like it) to bring down the AGE-protein formation rate, and then wait for matrix and cellular turnover to take its course, is a better strategy.

Posted by: Aryeh at December 21st, 2014 10:32 PM

I am curious to hear your views on Redox Signaling in relationship to slowing down the aging process. Redox Signaling is a byproduct of the cells reproducing healthy cells. Are you familiar with this science? Does this relate to AGE?

Thank you

Posted by: Jeannette at January 4th, 2015 9:37 PM

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