Researchers swap in human brain cells for a mouse’s cortex

研究人员将小鼠大脑皮层替换为人类脑细胞

In recent years, there has been a lot of excitement about the potential for studying human diseases in what are called “organoids.” These small patches of tissue, formed using stem cells, appear to produce many of the same cell types and at least some of the structures normally formed in actual organs, and thus can provide a better model for diseases that rely on the sometimes complex interactions among multiple specialized cell types that are a feature of the human body. 近年来，利用所谓的“类器官”（organoids）研究人类疾病的潜力引起了广泛关注。这些由干细胞形成的小块组织，似乎能产生许多与真实器官相同的细胞类型，并形成部分正常器官结构。因此，对于那些依赖人体多种特化细胞间复杂相互作用的疾病，类器官能提供更好的研究模型。

But even the most sophisticated organoids lack a lot of the features of a real human body. This is especially true for brain organoids, which don’t form any of the connections with specialized brain structures needed to behave “normally.” On Wednesday, a research group at Stanford University described a possible way to study brain organoids in a somewhat more natural context: They genetically wiped out a large portion of the mouse brain and replaced it with human brain organoid cells. 然而，即使是最先进的类器官也缺乏真实人体的大量特征。对于大脑类器官而言尤其如此，它们无法形成“正常”行为所需的、与大脑特化结构之间的任何连接。周三，斯坦福大学的一个研究小组描述了一种在更自然的环境中研究大脑类器官的可能方法：他们通过基因手段清除了小鼠大脑的很大一部分，并用人类大脑类器官细胞取而代之。

The replacements: Organoids, because they adopt a three-dimensional tissue structure and consist of various specialized cells, provide a much better model for an intact tissue than simply having a bunch of disassociated cells lying flat on a culture dish. But they still have a lot of limitations—they’re not hooked up to a circulatory system that allows the liver to process chemicals the organoid produces and don’t have immune cells moving through them, to give just a couple of examples. 替换方案：类器官因其采用三维组织结构并由多种特化细胞组成，相比于仅仅将一堆离散细胞平铺在培养皿中，它能为完整组织提供更好的模型。但它们仍有许多局限性——例如，它们没有连接到循环系统（无法让肝脏处理类器官产生的化学物质），也没有免疫细胞在其中穿行，这仅是其中的两个例子。

This is especially limiting for studies of the brain, where any specialized structures are surrounded by structures that may exchange information with them, and often have long-range connections. An organoid is better than nothing, but it may not be a lot better if you’re interested in a disease that impacts communication among multiple brain regions. 这对于大脑研究尤其具有局限性，因为大脑中的任何特化结构都被可能与其交换信息的结构所包围，且通常具有长距离连接。类器官聊胜于无，但如果你关注的是影响多个大脑区域间通讯的疾病，它的作用可能非常有限。

One alternative has been to implant human neural stem cells into the brains of another species, where they’ll generally integrate into the nervous system and actively signal to their neighbors. But, given that those human cells are surrounded by the normally functioning neurons of their hosts, it’s not clear how much you can learn from this. The obvious solution there is to get rid of the host cells and try to have the human cells take over their functions. 一种替代方案是将人类神经干细胞植入其他物种的大脑中，它们通常会整合到神经系统中，并主动向邻近细胞发送信号。然而，考虑到这些人类细胞被宿主正常运作的神经元所包围，从中能学到多少东西尚不明确。显而易见的解决方案是清除宿主细胞，并尝试让这些人类细胞接管其功能。

But that option runs into all sorts of problems, largely related to the fact that the organism you’re implanting them into (generally a mouse) actually needs its brain cells. Human neurons mature much more slowly than those of mice and may not form the connections that are needed quickly enough for an animal that only requires 21 days of gestation. In the absence of normal mouse tissue, nothing would provide the human cells with the signals that help organize them into functional units. 但该方案会遇到各种问题，主要原因在于你植入的目标生物（通常是小鼠）实际上需要它自己的脑细胞。人类神经元的成熟速度远慢于小鼠，对于仅需21天孕期的小鼠而言，人类神经元可能无法足够快地形成所需的连接。在缺乏正常小鼠组织的情况下，没有任何东西能为人类细胞提供将其组织成功能单元所需的信号。

The Stanford team decided to test a compromise and delete a portion of the mouse’s brain and put human brain organoids in its place. But they took a bold step and chose the cortex as the portion they would delete. The cortex handles many of the complex features of the nervous system, such as decision-making and memory, and its disruption would be expected to have dramatic consequences. 斯坦福大学团队决定尝试一种折中方案：删除小鼠大脑的一部分，并用人类大脑类器官取而代之。他们采取了一个大胆的举措，选择了大脑皮层作为删除区域。大脑皮层负责神经系统的许多复杂功能，如决策和记忆，其受损预计会产生严重的后果。

Still, the researchers found a gene that is active in almost all cortical cells and used it to drive the deletion of a key gene that’s needed to separate chromosomes during cell division. Amazingly, despite killing off most of the cells that should go on to form the mature cortex and cutting the brain’s volume in half, it was possible for the mice to survive this. 尽管如此，研究人员发现了一个在几乎所有皮层细胞中都活跃的基因，并利用它来驱动一个关键基因的删除，而该基因是细胞分裂过程中分离染色体所必需的。令人惊讶的是，尽管杀死了大部分本应形成成熟皮层的细胞，并将大脑体积减少了一半，这些小鼠依然能够存活。

The researchers had to eliminate most of the other pups to ensure the cortex-free mice got enough nursing. They left them with their mothers to nurse longer and then provided them with very high-calorie food. But these steps allowed nearly full survival of mice without much in the way of a cortex. (The mice were also immunocompromised to avoid an immune reaction to human cells, but this is less of an issue in a sterile mouse care facility.) With that in place, the researchers then started implanting human cortex organoids into the area where the mouse’s cortex was no longer developing. 研究人员不得不淘汰掉大部分其他幼崽，以确保这些缺失皮层的小鼠能获得充足的哺乳。他们让这些小鼠与母亲待在一起更长时间以进行哺乳，随后为它们提供高热量食物。这些措施使得这些几乎没有皮层的小鼠得以存活。（这些小鼠还经过免疫缺陷处理，以避免对人类细胞产生免疫反应，但在无菌小鼠饲养设施中，这并不是大问题。）在此基础上，研究人员开始将人类皮层类器官植入到小鼠皮层不再发育的区域。

Partial recovery: A bit over 85 percent of the animals that were implanted with a human organoid successfully incorporated the graft tissues. Those went on to contribute 92 percent of the cells found in the cortex of these animals. So, it wasn’t a complete replacement, but the human cells largely took over the space normally occupied by the cortex. Once there, they formed all the major types of neurons that are known to be present in the cortex. So, all that is good. 部分恢复：超过85%植入人类类器官的动物成功整合了移植组织。这些组织贡献了这些动物皮层中92%的细胞。因此，这虽然不是完全的替换，但人类细胞在很大程度上接管了通常由皮层占据的空间。一旦进入该区域，它们形成了皮层中已知的所有主要神经元类型。所以，这一切都是积极的。

The human cells also formed some long-distance connections, as evidenced by the fact that processes from human cells were detected as far away as the spinal cord. And the neurons did engage in synchronized activity spikes, suggesting a degree of coordination. The less good is a general lack of structure. There is some indication of local organization, in that specific cell types that form within distinct layers of a normal cortex tended to be near each other in the cortex of these animals. But there was no sign of those distinct layers being formed, suggesting that larger-scale organization is lacking. 人类细胞还形成了一些长距离连接，证据是研究人员在远至脊髓的地方检测到了来自人类细胞的突起。此外，这些神经元确实参与了同步活动尖峰，表明了一定程度的协调性。不足之处在于缺乏整体结构。有迹象表明存在局部组织，即在正常皮层不同层中形成的特定细胞类型，在这些动物的皮层中往往彼此靠近。但没有迹象表明形成了那些独特的层级结构，这表明缺乏更大规模的组织。

What does this mean for the mice? Overall, it appears that having a disorganized human replacement cortex was better than having no cortex at all, but not as good as having a normally structured one. The researchers set them loose in an area with video monitoring and then used a machine-learning classifier to group similar behavior patterns. Normal mice and the mice that had their cortex eliminated showed distinct patterns of behavior. The mice with a humanized cortex formed a third cluster, distinct from the other two. Similar things were true with body weight: mice that lacked a cortex were much lighter than normal mice, but the ones with the humanized… 这对小鼠意味着什么？总的来说，拥有一个结构混乱的人类替代皮层似乎比完全没有皮层要好，但不如拥有正常结构的皮层。研究人员将它们放入一个带有视频监控的区域，然后使用机器学习分类器对相似的行为模式进行分组。正常小鼠和被切除皮层的小鼠表现出截然不同的行为模式。拥有人源化皮层的小鼠形成了第三个聚类，与前两者不同。体重方面的情况也类似：缺乏皮层的小鼠比正常小鼠轻得多，但那些拥有人源化皮层的小鼠……