For anyone who follows crime dramas, you’ve probably come across the token episode where the resident computer genius pulls up a double helix to unravel some genetic clue about an elusive villain. Of course, most people know that the information in DNA requires more than a few clicks of the “zoom” button. But my point is that the double helix is one of the few molecules in science that is recognized by the public at large. Double helix = genetics = inheritance. Our twisted up fate… (or part thereof).
Heard of collagen? It’s one of the most important components of the extracellular matrix – the scaffold that gives our cells a framework and enables us to have structure and mobility. Without it, we wouldn’t…be. It is the most abundant protein in the human body.
Not surprisingly, collagen’s structure is intrinsically related to its function. Instead of being composed of a double helix like DNA (conducive to data storage), fibrillar collagen is composed of many triple helices that ultimately become cross-linked to each other. The enzyme that accomplishes this cross-linking, lysyl oxidase (LOX), resides outside the cell. Thus, it is only after the cell exports its triple-helix building blocks that they bridge to each other to form an incredibly strong and complex matrix.
If you now jump into the mindset of a tissue engineer, you might realize that since collagen is so critical to all tissues – especially connective tissues – it would make a useful material as a scaffold. However, most engineered connective tissues don’t attain the tensile properties of native tissue – a major challenge to clinical translation!
One contributing factor may be that the new extracellular matrix being formed doesn’t have enough collagen cross-links. While various methods of forcing cross-linking have been attempted in the lab (dehydration, gluteraldehyde, ultraviolet radiation etc.), one has to wonder: if we want to best capture native collagen’s architecture, shouldn’t we just somehow take advantage of LOX?
The authors of a paper just published in the Proceedings of the National Academy of Science seem to think so.
In the study, which came out of UC-Davis, the authors investigated the effect of LOX on the mechanical properties of two different kinds of cultured collagen-based tissues: both explanted connective tissues (ligaments, tendon, and cartilage, taken live and maintained in culture) and tissue-engineered neocartilage (literally, new cartilage). Furthermore, two different strategies of LOX administration were studied. The first involved exposing the tissues to hypoxia (oxygen deprivation), which was hypothesized to increase the cells’ own production of LOX (a HIF-1 mediated endogenous mechanism). The second method was to administer LOX from an outside source (exogenously).
As predicted, hypoxia led to greater collagen cross-linking in tissue explants and a subsequent improvement in the mechanical properties. Adding a LOX inhibitor blocked the effect, supporting the notion that LOX plays an important role in this pathway. When LOX was instead added exogenously, it was also able to promote cross-linking in explanted tissues as well as neocartilage. Importantly, for the neocartilage, the authors identified both a dose and time dependent effect for encouraging cross-linking (better to use higher dose LOX and earlier). They then demonstrated that if LOX-pretreated neocartilage was implanted into mice, the mechanical properties continued to improve.
This work is exciting because, as the authors point out, it can help with two different strategies towards ameliorating connective tissue injuries. For example, if explanted native tissues were being used for repair, they have to be stored without losing their integrity. By cross-linking collagen, LOX can help prevent the loss of tensile properties and may even help slow enzymatic degradation. If, instead, tissue engineered cartilage were to be used for repair, then administering LOX early in the maturation process can help the new cartilage better mimic that which is naturally formed by our bodies.
Ultimately, any work on collagen also contributes to the field of tissue engineering at large. It is simply a critical protein for our survival and everyday experience. A little respect for our [cross-linked] triple helix.
Makris E.A., N. K. Paschos, J. C. Hu & K. A. Athanasiou (2014). Developing functional musculoskeletal tissues through hypoxia and lysyl oxidase-induced collagen cross-linking, Proceedings of the National Academy of Sciences, 111 (45) E4832-E4841. DOI: http://dx.doi.org/10.1073/pnas.1414271111
Glowacki J. (2008). Collagen scaffolds for tissue engineering, Biopolymers, 89 (5) 338-344. DOI: http://dx.doi.org/10.1002/bip.20871
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