In contrast, the cell population identified by Wang and Nusse in the mice is diploid, with a normal complement of DNA. They can divide to make others like themselves, or to make cells that start as diploid but then acquire additional copies of their genome as they move outward from the central vein into the main body of the liver.
“People in the field have always thought of hepatocytes as a single cell type,” said Wang. “And yet the cell we identified is clearly different from others in the liver. Maybe we should accept that there may be several subtypes of hepatocytes, potentially with different functions.”
Wang and Nusse identified the liver stem cells around the central vein by looking in mice for cells anywhere in the organ that expressed a protein called Axin2. The protein is produced by cells in response to the presence of members of the Wnt signaling protein family. Years of research in the Nusse laboratory have shown that the Wnt proteins play a critical role in embryonic development, and also in the growth and maintenance of stem cells throughout the body.
Wang and Nusse further found that, in the liver, the endothelial cells that line the interior surface of the central vein make Wnt2 and Wnt9b. These Wnt proteins, in turn, confer stem cell properties on the neighboring hepatocytes.
Finally, the researchers learned that a portion of the descendants of the Axin2-expressing cells move outward from the central vein over time. These cells become polyploid and begin to express other, hepatocyte-specific genes. After one year, these descendants had replaced about 30 percent of the entire mouse liver, and made up about 40 percent of all hepatocytes in the organ.
The most common reason that promising new drugs for any type of condition fail is that they are found to be toxic to liver.
The newly identified liver stem cell also expresses genes associated with very early embryonic development, which may give a clue as to when and where they arise.
“Perhaps these stem cells in the adult liver actually arise very early in development,” said Nusse, “when the embryo sets aside a certain population of cells to maintain the organ during adult life.”
Potential aid for drug testing
Although the current research was conducted in mice, the possibility that there is more than one kind of hepatocyte in humans could transform the study of liver biology, the researchers said. For example, hepatocytes have proven notoriously difficult to grow in laboratory culture for study or for use in drug testing.
“The most common reason that promising new drugs for any type of condition fail is that they are found to be toxic to liver,” said Wang. “Researchers have been trying for decades to find a way to maintain hepatocytes in the laboratory on which to test the effects of potential medications before trying them in humans. Perhaps we haven’t been culturing the right subtype. These stem cells might be more likely to fare well in culture.”
There’s also an opportunity to better understand human disease.
“Does liver cancer arise from a specific subtype of cells?” said Wang. “This model also gives us a way to understand how chromosome number is controlled. Does the presence of the Wnt proteins keep the stem cells in a diploid state? These are fundamental biological questions we can now begin to address.”
Other Stanford authors of the paper include MD/PhD student Ludan Zhao and research associates Matt Fish and Catriona Logan, PhD.
The research was supported by the Howard Hughes Medical Institute and the Reed-Stinehart Foundation.
Information about Stanford’s Department of Developmental Biology, which also supported the work, is available at http://devbio.stanford.edu.