The role of p38 MAPK in the adult muscle stem cell fate and regeneration in ageing

部分由于现代医学的进步，我们中的许多人活得更长，西方世界60岁以上的人口比例比以往任何时候都要高。然而，如果健康状况不佳，这些额外的生命年的好处就会减少;我们想要增加“健康寿命”而不是“寿命”。什么决定健康的老龄化？什么是“老死”而不是死于某种疾病？我们还不了解这些过程。一个强有力的候选人的维护和修复的身体是一个专门的细胞称为成人干细胞。这些细胞不同于媒体上经常报道的胚胎干细胞。虽然胚胎干细胞有能力分化成任何谱系（并成为任何组织），但成体干细胞致力于修复和再生其所在的组织。它通过以下方式实现这一点：i，响应于诸如损伤的线索而激活，ii，增殖，提供一个将修复损伤的细胞池，iii，通过这些细胞分裂之一，替换自身以供将来使用。在老年人中，成体干细胞执行这些功能的能力明显降低，导致组织逐渐恶化。骨骼肌就是一个很好的例子。50岁以后，骨骼肌质量每十年下降约10%;肌肉也变得更弱，含有更多的脂肪。积极的一面是，动物研究已经确定，肌肉成体干细胞（称为“卫星”细胞）在衰老过程中的行为下降不是由于细胞本身的缺陷，而是因为细胞没有从环境中接收正确的信号。该项目的总体目标是研究一种称为p38 MAPK的关键信号分子在解释来自环境的这些信号并决定卫星细胞随后行为的方式方面具有关键作用的理论;因此，它充当了“网关”。p38调节肌肉卫星细胞激活中的许多重要事件，从停止细胞分裂到改变被激活的基因以及细胞可以产生的蛋白质。如果p38被激活得太快，成体干细胞将没有足够的机会形成足够的细胞进行修复和替换。我们将解决以下具体问题：1）增加p38活性是否限制了肌肉卫星细胞的“选择”？如果p38活性被抑制，会发生什么？我们将通过在成年肌肉干细胞中产生缺乏p38的小鼠（敲除小鼠）来回答这个问题。我们预计它们可能会产生更多的前体细胞。2)p38调控基因表达的第一阶段是什么？DNA并不是以裸链的形式存在，而是缠绕在一组称为组蛋白的蛋白质上（整个单位称为染色质）。组蛋白可以通过添加甲基基团进行修饰;现在认为这些修饰最终改变了染色质的形状，使其变得紧凑并且无法接近促进基因表达的分子（转录因子），或者“打开”染色质以便转录因子可以发挥作用。我们将研究两个基本的组蛋白修饰，调节这些事件。3)控制p38激活的信号是什么？有些来自细胞外，有些来自细胞内。我们将增加或减少候选调节剂的活性，并确定p38活性和卫星细胞功能的后果。4)最重要的是，衰老过程中p38活性发生了什么变化？如果我们阻止老年动物肌肉卫星细胞中p38的激活会发生什么？我们将使用第1点中制造的基因敲除小鼠来测试这一点。我们预测，在老年小鼠中发生的卫星细胞活化和增殖的正常减少将在p38基因敲除小鼠中得到改善。这可能会阻止在衰老过程中发生的肌肉质量下降。

In part due to advances in modern medicine, many of us are living longer, and a greater proportion of the population of the Western world is over 60 than ever before. However, there is reduced benefit to these extra years of life if they are overshadowed by poor health; we want to increase 'healthspan' not 'lifespan'. What determines healthy ageing? What does it mean to die 'of old age' rather than some disease? We do not understand these processes yet. One strong candidate for the maintenance and repair of the body is a specialized cell called the adult stem cell. These differ from the embryonic stem cells that are so often in the press. Whereas embryonic stem cells have the ability to differentiate into any lineage (and become any tissue), the adult stem cell is committed to repairing and regenerating the tissue in which it resides. It achieves this by i, activating in response to cues, such as injury ii, proliferating, providing a pool of cells that will repair the damage and iii, through one of these cell divisions, replacing itself for future use. In the elderly, the ability of adult stem cells to perform these functions in markedly reduced, leading to gradual tissue deterioration. A good exemplar of this is skeletal muscle. After the age of 50, skeletal muscle mass declines by ~10% per decade; muscle also becomes weaker and contains more fat. On the positive side, studies in animals have determined that the decline in muscle adult stem cell (called the 'satellite' cell) behavior in ageing is not due to defects within the cell itself but is rather because the cell does not receive the correct signals from its environment. The overall aim of this project is to examine the theory that a key signaling molecule, called p38 MAPK, has a key role in interpreting these signals from the environment and dictating the way in which satellite cells subsequently behave; it therefore acts as a 'gateway'. p38 regulates many of the important events in muscle satellite cell activation, ranging from stopping cells dividing to changing the genes that are activated and therefore the proteins that the cell can make. If p38 is activated too soon, adult stem cells will not have sufficient opportunity to form enough cells for repair and replacement. We will address the following specific questions: 1) Does increased p38 activity restrict the 'choices' of muscle satellite cells? What would happen if p38 activity were suppressed? We will answer this by generating mice that lack p38 in adult muscle stem cells (knockout mice). We expect that they may produce increased numbers of precursor cells. 2) What are the first stages by which p38 regulates gene expression? DNA does not exist as a naked strand but is rather wound around groups of proteins called histones (the whole unit is called chromatin). Histones can be modified by the addition of methyl groups; it is now believed that these modifications ultimately change the shape of chromatin, either making it compact and inaccessible to molecules that will encourage gene expression (transcription factors), or 'opening' the chromatin so that transcription factors can function. We will examine two fundamental histone modifications, which regulate each of these events. 3) What are the signals that control p38 activation? Some of these will come from outside the cell, and some from inside. We will increase or decrease the activity of candidate regulators and determine the consequences for p38 activity and satellite cell function. 4) Most important, what changes occur in p38 activity during ageing? What happens if we prevent p38 activation in muscle satellite cells from old animals? We will test this using the knockout mice made in point 1. We predict that the normal decrease in satellite cell activation and proliferation that occurs in old mice will be ameliorated in the p38 knockout mice. This may prevent part of the decline in muscle mass that occurs in ageing.