The auxetic nucleus: nuclear mechanotransduction and its role in regulating stem cell differentiation

胚胎干细胞(ESCs)在多能性状态下可以自我更新，这意味着它们可以分化出各种组织类型；因此，它们在再生医学中非常有前途。我们最近发现它们有一个非常有趣和令人惊讶的特性。就像胚胎干细胞开始离开这种多能性状态--也就是当它分化时--它们的细胞核--细胞中容纳所有遗传物质的大型结构--变得“生长”。伸长性是指材料在机械应力作用下的响应特性。想想看，在机械压力下，张紧的橡皮筋变薄，当球被压缩时，它变得更胖：这是大多数材料所做的。然而，与橡皮筋不同的是，伸展材料拉伸时会变得更胖，压缩时会变薄。这一属性即使与其他细胞核相比也是非常独特的，但我们发现另一种细胞类型在其细胞核中表现出同样的属性。这种细胞类型是少突胶质前体细胞(OPC)，它类似于ESCs，因为它们是一种自我更新的干细胞。在发育过程中，OPC产生少突胶质细胞(中枢神经系统的髓鞘细胞)，在成人中负责在脱髓鞘后产生新的少突胶质细胞(一种独特的、临床上重要的神经再生过程，称为重新髓鞘形成)。这两个干细胞都是再生的关键，我们相信我们的研究将为它们的工作原理提供新的线索。伸展对ESC/OPC核有两个重要的影响。首先，它对结构有影响，因为伸缩性是由独特的结构特征引起的(例如伸展蜂窝：见https://www.youtube.com/watch?v=vdkYuLsT7Sc).我们将结合生物技术、生物学和物理技术来了解什么核结构属性对生长性负责；通过发现这一点，我们将更好地了解核结构在分化过程中是如何变化的，以及这些变化可能如何促进分化。第二个影响是，机械应力会产生巨大的体积波动。考虑到伸展的原子核在拉伸时变得更胖，在压缩时变得更薄，显然，与大多数材料不同的是，它的体积随着机械应力的变化而变化很大。这反过来又会导致大量的可溶性分子在机械应力作用下穿过原子核。考虑到，尤其是在组织中，干细胞经历了频繁和显著的机械应力，我们认为这对如何调节分化很重要。其原因是，在ESCs或OPC分化之前，有许多信号分子是分化所必需的，它们被保持在细胞核外。当受到机械压力的原子核显著膨胀时(我们看到原子核中的体积增加了50%，作用力相对较小)，这将迫使这些分子中的一些进入原子核，在那里它们可以找到他们的目标。我们认为，在这种方式下，伸长性使原子核就像一个泵，推动分子穿过它的膜。我们将使用我们开发的生物技术对细胞施加机械应力来观察这些体积波动和分子在核膜上的同步运动。我们还将使用生物技术来分析这些分子的靶标，以确定这种延伸效应是否导致ESCs/OPC的功能变化。这项研究将对生物技术、再生医学和干细胞生物学产生影响。它将为干细胞如何工作以及我们如何研究它们带来新的见解。利用我们在干细胞、生物物理学和生物技术方面的联系，我们将广泛传播我们的成果，在几个学术学科产生影响。鉴于其影响的高潜力和高度跨学科的性质，拟议的研究非常适合BBSRC的投资组合。

Embryonic stem cells (ESCs) self-renew in a state of pluripotency, meaning they can give rise to all tissue types; therefore, they are very promising for regenerative medicine. We recently discovered they have a very interesting and surprising property. Just as an ESC begins to leave behind this state of pluripotency - i.e. as it differentiates - their nucleus, the large structure in the cell that houses all the genetic material, becomes 'auxetic'. Auxeticity is a property that refers to the response of a material under mechanical stress. Consider that, under mechanical stress, a tensed rubber band becomes thinner, and when a ball is compressed it becomes fatter: this is what most materials do. However, an auxetic material, in contrast to a rubber band, becomes fatter when stretched, and thinner when compressed. This property is highly unique even compared to other cell nuclei, but we found one other cell type that manifests this same property in its nucleus. That cell type is the oligodendrocyte progenitor cell (OPC), which is similar to ESCs in that they are a self-renewing stem cell. In development, OPCs give rise to oligodendrocytes (the myelinating cell of the central nervous system) and in the adult is responsible for generating new oligodendrocytes following demyelination (a unique and clinically important neural regenerative process called remyelination). Both of these stem cells are keys to regeneration, and we believe our studies will shed new light on how they work. Auxeticity has two important repercussions for the ESC/OPC nucleus. First, it has implications for structure because auxeticity arises from unique structural characteristics (such as the auxetic honeycomb: see https://www.youtube.com/watch?v=vdkYuLsT7Sc). We will use a combination of biotechnology, biological and physics techniques to understand what nuclear structural properties are responsible for auxeticity; in finding this, we will better understand how nuclear structure changes during differentiation, and how these changes might facilitate differentiation. The second repercussion is that auxeticity yields massive volume fluctuations with mechanical stress. Consider that an auxetic nucleus gets fatter when stretched, and thinner when compressed, and it is clear that, unlike most materials, it changes volume considerably with mechanical stress. This, in turn, will cause a large flux of soluble molecules across the nucleus with mechanical stress. Given that, particularly in tissue, stem cells undergo frequent and significant mechanical stress, we believe this is important for how differentiation is regulated. The reason for this is that there are a number of signaling molecules that are necessary for differentiation that are kept outside the nucleus before ESCs or OPCs differentiate. When the mechanically-stressed nucleus significantly swells (we see volume increases in the nucleus of up to 50% with relatively small forces), that will force some of these molecules into the nucleus where they can find their targets. We propose that in this way, auxeticity causes the nucleus to be like a pump for moving molecules across its membrane. We will use the biotechnology we develop to apply mechanical stress to the cells to observe these volume fluctuations and concurrent movement of molecules across the nuclear membrane. We will also use biological techniques to analyse the targets of these molecules to determine if this auxetic effect is causing functional changes in ESCs/OPCs. The research will impact biotechnology, regenerative medicine and stem cell biology. It will bring to bear new insight into how stem cells work, and how we can investigate them. Using our connections in stem cells, biophysics, and biotechnology, we will widely circulate our results, generating impact in several academic disciplines. Given its high potential for impact and its highly cross-disciplinary nature, the proposed research is highly suited for the portfolio of the BBSRC.