Stem cell metabolomics for bone therapies and tissue engineering

就像《夺宝奇兵》（Indiana Jones）电影中的场景一样，想象一下这样一个场景：一位人类学家在探索失落的玛雅文明时，发现了一些头骨，上面似乎有一副几乎完整的假牙。这些牙齿是在他们死后被植入的，还是在他们活着的时候就被植入的？更仔细的检查表明，它们确实是一块块贝壳，被塑造成单独的牙齿，插入颌骨，以取代因自然原因失去的牙齿。值得注意的是，详细的分析表明这些壳状假牙已经完全融入颌骨。换句话说，颌骨很高兴地接受了外壳植入物——这种特征被称为骨整合。进一步的研究表明，这种外壳不仅与骨骼结合在一起，而且还具有骨诱导作用，即促进骨骼形成。而且没有明显的排斥反应。更有趣的是，这里我们有一个无脊椎动物系统（软体动物外壳），可以在脊椎动物（人类）中形成骨骼。两者之间的联系在哪里？解开这种联系是这个项目的一个重要方面。不仅仅是为了确定它是如何以及为什么发生的，而是为了解开和利用骨诱导的机制，并用它来提供骨植入物的材料。玛雅人使用的贝壳和大多数无脊椎动物一样，是由碳酸钙或白垩组成的。通常情况下，碳酸钙以两种被称为多晶体的形式存在：方解石和文石。文石也被称为珍珠石或珍珠之母。虽然这些多晶体具有相同的化学成分，但它们具有不同的结构。我们的初步数据证实，这些无脊椎多形体之一，珠质，确实能诱导骨形成。另一种多晶型，方解石，似乎具有不同的功能，因为它鼓励细胞避免特化为特定的细胞类型，如脂肪或骨生成细胞。我们的目标是了解这些反应，以便我们可以设计和构建可用于引发理想和可预测的细胞反应的材料。这一点很重要，原因如下：到2031年，英国超过50%的人口将超过65岁。随着髋关节和膝关节置换术需求的增加，这种人口状况将给骨科医生带来极大的压力。同时，人口寿命更长，因此，在生命的后期可能需要进一步的干预。目前，越来越多的运动活跃的年轻人现在呈现出运动损伤，随后的骨关节炎的可能性很高。问题是，在更年轻、更活跃的人群中，植入物往往会更快失效，而修复手术也不如最初的替代手术成功。在创伤诊所，面部重建手术需要大面积完整的骨骼；通常痛苦地远离便利的场所。这些例子对临床医生、科学家和工程师提出了重大挑战，他们需要迅速推动新技术的大规模应用，以提供关节植入的新材料来满足这些需求。在另一个临床问题上，向临床提供高质量的非特化细胞将促进直接再生治疗和帮助组织工程。目前的问题是，在培养基中培养的细胞几乎会立即特化。方解石促使细胞保持非特化，这一事实为组织治疗提供了足够数量的细胞。时机再好不过了。我们现在才能够理解和利用骨的形成和细胞的供应，随着人口的迅速老龄化，迫切需要这种变革的方法。

Like a scene from an Indiana Jones movie, imagine the scenario where an anthropologist, exploring the lost civilisation of the Mayans, comes across skulls that appear to have an almost full set of false teeth. Were these teeth inserted post mortem to embellish their way to the after-life, or, had they been inserted during their time on Earth? Closer inspection showed that they were indeed pieces of shell, fashioned into individual teeth that had been inserted into the jaw bone to replace teeth that had been lost through natural causes. Remarkably, detailed analysis showed that these shell dentures had totally integrated into the jaw bone. In other words, the jaw bone had happily accepted the shell implants - this feature is called osteo-integration. Further work showed that, not only was the shell integrated into the bone - it was also osteo-inductive, i.e. encouraged bone formation. It also did this without apparent rejection of the implant. What is even more intriguing is that here we have an invertebrate system (mollusc shell) enabling bone formation in a vertebrate (human). Where does the connection lie? Unravelling this connection is an important aspect of this project. Not just the quest to determine how and why it happens, but to unlock and exploit the mechanism of osteo-induction and use this to provide material for bone implants.The shell used by the Mayans, as in most invertebrates, is composed of calcium carbonate, or chalk. Normally, calcium carbonate exists in one of two forms, called polymorphs: calcite and aragonite. Aragonite is also called nacre or Mother of Pearl. Although these polymorphs have the same chemical composition they have different structures.Our preliminary data confirm that one of these invertebrate polymorphs, nacre, does indeed induce bone formation. The other polymorph, calcite, appears to hold a different function in that it encourages cells to avoid specialisation into specific cell types such as fat- or bone-producing cells. We aim to understand these responses so that we can design and construct materials that can be used to elicit desirable and predictable cellular responses. This is important for the following reasons:By 2031 more than 50% of people in the UK will be over 65. This demographic scenario will place extreme pressure on orthopaedic surgeons with a rise in the required number of hip and knee replacements. Simultaneously, the population is living longer and thus, further intervention may be required later in life. Currently, a more sports active younger population are now presenting with sport injuries with a high potential for subsequent osteoarthritis. The problem is that in younger and more active people, implants tend to fail more rapidly and revision operations are less successful than the original replacement surgery. In trauma clinics, facial reconstructive surgery demands large areas of intact bone; usually painfully sequestered from convenient sites. These examples present major challenges for clinicians, and also for scientists and engineers who need to rapidly drive new technologies on a suitably large scale in order to meet these demands by supplying novel materials for joint implantation. In a separate clinical issue, the supply of high-quality non-specialised cells to clinic would advance direct regenerative therapy and aid tissue engineering. The current problem is that cells grown in culture tend to become specialised almost immediately. The fact that calcite encourages cells to remain non-specialised offers the possibility of providing sufficient quantities of cells for tissue therapies.The timing could not be better. We are only now in a position to understand and exploit bone formation and supply of cells, with such a rapidly ageing population, there is urgent need for such transformative approaches.