探测肝细胞的异质性探测肝细胞的异质性

StephenW

Cell Research (2015) 25:1181–1182. doi:10.1038/cr.2015.117; published online 25 September 2015

        Probing hepatocyte heterogeneity

Ben Z Stanger1

1Departments of Medicine and Cell and Developmental Biology, BRB II/III, Rm. 512, 421 Curie Boulevard, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA 19104

Correspondence: Ben Z Stanger, E-mail: [email protected]

To maintain homeostasis, organs replace cells lost through normal cellular turnover, often through the straightforward replication of existing cells. A recent paper in Nature shows that cells in the liver are not equivalent when it comes to their replicative capacity; rather, a subset of hepatocytes defined by the maintenance of active Wnt signaling bears the brunt of responsibility for maintaining liver mass.

Since at least the 1930s, and possibly well before, the mammalian liver has been recognized for its extraordinary ability to regenerate following injury1. Over the course of the 20th century, a paradigm emerged which posited that the liver uses two distinct mechanisms for regeneration depending on the mode of injury. According to this paradigm, the liver regenerates via the replication of existing cells following removal of a part of the liver (i.e., partial hepatectomy) and via the expansion and differentiation of a specialized stem/progenitor cell pool following exposure to toxins, particularly those toxins that interfere with hepatocyte replication2. Recently, however, several studies have called into question whether stem/progenitor cells contribute substantively to liver regeneration, at least in those injury models previously thought to involve progenitors. Hence, liver regeneration may be driven almost entirely by the replication of existing cells (i.e., hepatocytes), regardless of the mode of injury3. Nevertheless, such findings still leave open the possibility that hepatocytes differ in their replicative potential.

To address the issue of possible heterogeneity, Wang and colleagues4 developed techniques for labeling hepatocytes within different portions of the hepatic lobule to follow their capacity for renewal. The lobule represents the main functional unit of the liver and can be divided into three “zones” (Figure 1). Blood enters the liver through portal tracts located in zone 1, and then percolates between sheets of hepatocytes (“sinusoids”) through zones 2 and 3 until exiting the liver through the central veins. This arrangement facilitates the exchange of small molecules between the bloodstream and the hepatocytes, subserving the liver's roles in detoxification, metabolic regulation, and synthesis of plasma proteins.

Figure 1.Specialized hepatocytes in the pericentral region of the hepatic lobule. The hepatic lobule is represented as a hexagon, with blood entering through portal tracts at the periphery and converging on a central vein (CV) at the epicenter. As blood makes its way inward, it passes through three “zones” of hepatocytes. As shown by Wang et al., central vein endothelial cells release Wnt ligands, resulting in the activation of Wnt signaling in hepatocytes adjacent to the central vein (zone 3). When labeled with a Wnt-responsive reporter (illustrated in green), these cells exhibited a heightened ability to replicate, with label subsequently appearing in hepatocytes in zone 2 (and even zone 1) of the liver.
Full figure and legend (47K)

Zonation within the lobule is known to be under the control of Wnt signaling, with Wnt activity highest in zone 3 hepatocytes near the central vein and lowest in zone 1 hepatocytes near the portal tracts5,6. Based on this, Wang et al.4 hypothesized that the zone 3 cells might constitute a hepatocyte subset with a heightened replicative potential. Taking advantage of the fact that zone 3 hepatocytes have higher Wnt activity (and hence higher expression of the Wnt target gene Axin2), the authors used an Axin2-CreER mouse strain7 to label these cells and follow their fate.

Using this approach, Wang et al. made a number of important observations about the dynamics of cellular turnover in the liver. First, they found that pericentral hepatocytes divide at roughly twice the rate as other hepatocytes, with an estimated doubling time of roughly 14 days, confirming suggestive results from a previous study8. Second, they found that the labeled cells made up an increased percentage of liver mass over time, expanding from roughly 5% of liver area a week following labeling to roughly 30% of liver area a year later. Third, the authors discovered that central vein endothelial cells produce high levels of the soluble Wnt ligands Wnt2 and Wnt9b. Wnt signaling in the pericentral cells was required for their enhanced proliferation, suggesting that central vein-derived Wnts might define a “niche” for fast-replicating pericentral cells. Finally, Wang et al. showed that in contrast to most hepatocytes, which are polyploid and contain a 4N or 8N complement of DNA, the majority of Axin2+ hepatocytes are diploid. This latter finding was interpreted as an explanation for the enhanced replicative activity of pericentral hepatocytes; however, other studies have shown that diploid and polyploid hepatocytes have equivalent growth rates in vivo9.

In light of these findings, it is enticing to think that this population of pericentral hepatocytes might be the long sought-after “liver stem cells”. Indeed, other work has shown that in addition to robust self-renewal properties, hepatocytes have the capacity to differentiate into cholangiocytes, a distinct cell type10. According to the formal definition of a stem cell — a cell which has the capacity to both self-renew and give rise to another lineage — pericentral hepatocytes would thus appear to fit the bill. Paradoxically, however, pericentral hepatocytes are uniquely incompetent incompetent in terms of their ability to become cholangiocytes11, suggesting that there is a tradeoff in a cell's ability to replicate vs (trans)-differentiate.

Because lineage tracing studies, particularly those that rely on Cre recombinase, can be misleading, the results of Wang et al. will need to be confirmed using independent approaches. Nevertheless, the findings raise a number of stimulating questions. How does Wnt signaling govern hepatocyte replication and other features of hepatocyte behavior? Why are pericentral hepatocytes enriched for diploid cells if euploidy does not result in a growth advantage for hepatocytes, and what are the factors that determine replicative capacity? And finally, how do pericentral hepatocytes factor in during liver regeneration, when the liver must sense, and return to, an appropriate size? Regardless of the answers to these questions, this study points to the importance of considering heterogeneity within the pool of differentiated cells — and not just stem cells — when evaluating mechanisms of tissue homeostasis and regeneration.

StephenW

细胞研究（2015）25：1181年至1182年。 DOI：10.1038 / cr.2015.117;网上公布的25月2015年
探测肝细胞的异质性

本žStanger1

医学和细胞和发育生物学，BRB II / III，Rm的1Departments。 512，421居里大道，医学佩雷尔曼学院宾夕法尼亚大学，费城，PA 19104

函授：本ž斯坦格，电子信箱：[email protected]

保持动态平衡，器官替换细胞通过正常的细胞周转丢失，往往通过现有的细胞的直接的复制。最近的一篇论文在Nature表明细胞在肝脏中是不等价的，当谈到自己的复制能力;相反，通过积极的Wnt信号维持定义肝细胞的一个子集负有维护肝脏质量的责任首当其冲。

至少从20世纪30年代，并有可能好之前，在哺乳动物的肝脏已被确认为再生以下injury1其非凡的能力。在20世纪的过程中，一范例出现其中假定，肝脏使用两种不同的机制用于再生取决于损伤的模式。根据这种模式，肝脏经由以下除去肝脏（即，部分切除）的一部分的现有的细胞的复制，并通过一个专门的干细胞/祖细胞池的暴露于毒素，特别是那些毒素的扩张和分化再生该干扰肝细胞replication2。然而，最近一些研究质疑干/祖细胞是否有助于实质性肝再生，至少在以前被认为涉及祖细胞的损伤模型。因此，肝再生可被驱动而不管injury3的模式几乎完全由现有的细胞（即，肝细胞）的复制。然而，这样的结果仍保持开放其复制能力的肝细胞不同的可能性。

为了应对可能的异质性的问题，王及其同事开发的技术用于标记肝小叶的不同部分肝细胞内按照他们的更新能力。小叶表示肝的主要功能单元，并且可以分成三个“区域”（图1）。血液进入通过位于区域1肝门束肝脏，然后肝片之间渗透过区域2和3（“正弦曲线”），直到通过中央静脉排出肝脏。这个布置便于血浆蛋白的血液和肝细胞，在解毒，代谢调节subserving肝脏的角色，和合成之间的小分子交换。
图1。
图1 - 很遗憾，我们无法为此提供方便的替代文本。如果您需要帮助来访问此图片，请与[email protected]或作者

专业的肝细胞在肝小叶中心周围区域。肝小叶被表示为六边形，通过肝门束血液进入在周边和会聚在中央静脉（CV）处的中心。由于血使内向的方式，它通过肝细胞的三个“区”通行证。如图王等人，中央静脉内皮细胞释放的Wnt配体，导致Wnt信号在邻近中心静脉（区域3）的肝细胞的活化。当与Wnt信号响应记者（绿色图示）标记，这些细胞表现出复制一个升高的能力，带有标签随后出现在肝细胞中区2肝脏的（甚至是1区）。
全图和传说（47K）

小叶内区划被称为是Wnt信号的控制下，具有最高的区域3肝靠近中央静脉和最低的区域1的肝细胞附近的门户tracts5,6 Wnt信号活性。在此基础上，Wang等al.4推测区3细胞可能构成具有提高的复制能力肝细胞子集。考虑一个事实，即区域3的肝细胞有较高的Wnt活性（与Wnt靶基因的的Axin2因此，高表达）的优点，作者使用了的Axin2-CreER鼠标strain7标记这些细胞，并按照他们的命运。

使用这种方法，Wang等人。提出了一些关于在肝细胞更新的动力很重要的意见。第一，他们发现，中心周围肝细胞分裂以大约两倍的速率作为其他的肝细胞，与大约7-14天估计倍增时间，确认从以前study8暗示的结果。其次，他们发现该标记的细胞由肝脏质量随时间增加的百分比，以下标记到大约30％的肝区的一年后从肝面积大约为5％，每周扩大。第三，笔者发现，中央静脉内皮细胞产生高水平的可溶性Wnt信号配体WNT2和Wnt9b的。在中心周围细胞的Wnt信号被要求为他们增强扩散，这表明中央静脉源性Wnts可以定义一个“小众”的快速复制中心周围细胞。最后，王等人。表明，在对比大部分肝细胞，这是多倍体和含有DNA的4N或8N补体，多数的Axin2 +肝细胞是二倍体。后者发现被解释为中心周围肝细胞的增强复制活动的说明;然而，其他研究显示，二倍体和多倍体的肝细胞在vivo9等效增长率。

根据这些研究结果，它是诱人认为，中心周围肝细胞的这部分人口可能是长抢手的“肝干细胞”。事实上，其它的工作表明，除了健壮自我更新属性，肝细胞具有分化成胆管，一个不同的细胞类型10的容量。根据干细胞的正式定义 - 它有能力既是细胞的自我更新和产生其它谱系 - 中心周围的肝细胞将因此出现符合这个要求。矛盾的是，然而，中心周围肝细胞具有独特的无能无能在其成为cholangiocytes11能力而言，这表明有在细胞中的复制VS（反式）-differentiate能力的折衷。

因为谱系追踪研究，特别是那些依赖于Cre重组酶，可能会产生误导，王等人的结果​​。将需要使用独立的方法来确认。然而，调查结果提出了一些刺激的问题。如何Wnt信号执政肝细胞的复制和肝细胞行为的其他功能？为什么中心周围肝细胞富集二倍体细胞如果整倍体不会导致生长优势为肝细胞，和什么是决定复制能力的因素是什么？最后，该怎么办中心周围的肝细胞在肝脏再生的因素，当肝脏必须检测，并返回，合适的大小？无论这些问题的答案中，这项研究指出考虑分化的细胞池中的异质性的重要性 - 而不仅仅是干细胞 - 评估组织稳态和再生机制时。