January 10, 2017

Story and photo by David Edwards

December's featured paper is titled, "Single cell lineage tracing reveals a role for TgfβR2 in intestinal stem cell dynamics and differentiation." It was published in the Proceedings of the National Academy of Sciences as a collaboration from multiple universities. The contributors from OHSU were first author Jared M. Fischer, Ph.D., a postdoctoral fellow; Ashleigh J. Miller, Ph.D., a former graduate student; and last author R. Michael Liskay, Ph.D., professor of molecular and medical genetics, OHSU School of Medicine.

Every four or five days, the intestine completely renews itself, dividing and proliferating to create millions of new cells while simultaneously sloughing off the old ones. This process of renewal is highly regulated and involves a series of tightly controlled biological steps that depends on the spatial arrangement of cells within the intestine. The inner surface of the intestine itself isn't smooth but contains countless folds, with sections that protrude from the surface – villi – and sections that are recessed – crypts. The renewal process begins with the division of intestinal stem cells (ISCs), which are found at the bottom of the crypts.

There are two major types of ISCs. The first, called proliferating ISCs, are constantly dividing, either producing other types of intestinal cells or more copies of themselves. These cells provide the majority of the intestinal self-renewal. The second type, called quiescent (or dormant) ISCs, are not actively dividing. These two ISC populations are in constant communication, maintaining a balance between the newly created ISC cells and other intestinal cells types.

When the proportion of cell types in the intestine is disrupted, it can have dramatic consequences, and this disruption can cause various inflammatory diseases like ulcerative colitis and even cancer. Understanding how ISCs communicate – and what happens when cells can no longer effectively communicate – gives us important insight into how these diseases begin and perhaps how best to treat them.

Communication breakdown

There are many ways in which these ISC populations communicate, tapping into common forms of cellular communication that occur throughout the body, such as through WNT, BMP, and/or Notch signaling. However, there is one common form of communication – called Tgfβ signaling – which remains understudied in the context of ISCs.

In Tgfβ signaling, the communicating cell secretes a small chemical called a ligand (Tgfβ1, 2, or 3), which binds to a receptor on the recipient cell (TgfβR1 or R2) and activates the proteins necessary to perform various functions. It is known that Tgfβ signaling, which is altered in many types of cancer (including intestinal cancer) and is generally important for cell division and regulation.

Previous research showed that a broad deletion of Tgfβ signaling within the intestine didn't show any obvious malfunctions. However, nobody had looked at the precise communication between individual Tgfβ-positive and -negative cells, which better reflects the actual biology occurring in the intestine. An imbalanced communication between ISC populations could give certain cells a growth advantage, thereby disrupting the proportion of intestinal cell subtypes and causing disease.

The importance of Tgfβ

A team of researchers led by Dr. R. Michael Liskay, set out to investigate the role of Tgfβ signaling in ISC communication. They used a mouse model that would randomly generate a low number of ISCs with mutations in TgfβR2 (a Tgfβ receptor). Those mutated cells would be labeled with dyes, allowing them to track them and their daughter cells. To more easily monitor these cells over time, they isolated ISCs from mice and placed them into special plastic dishes, where they would automatically grow into artificial crypt-like structures called enteroids.

They found that, compared to non-mutated cells, cells with mutated TgfβR2 had (1) a greater survival of clonal ISCs, which is a result of increased competition;and (2) increased time required for the daughter cells to fully occupy the entire crypt surface. These results suggest that Tgfβ signaling is indeed important in ISC development, controlling both the extinction and expansion of ISC clones. It supports the idea that Tgfβ signaling in important in the normal maintenance of the intestinal stem cell environment.

In addition, they also saw in mice that loss of TgfβR2 reduced the ability for crypts to regenerate after injury and their ability to create a certain type of intestinal cell called Paneth cells, which help secrete certain anti-microbial compounds. These results might indicate why Tgfβ signaling is important for inflammation, which can result from an inadequate protection of the intestine against bacteria.

"I was intrigued by this work because of the seeming overlap of the role of this receptor in both cancer and tissue regeneration," said Mary Heinricher, Ph.D., associate dean for basic research, OHSU School of Medicine.

These findings are just the beginning, however. "In the future, we hope to further examine the role of Tgfβ signaling in other disease states, such as Crohn's disease or colitis," said Dr. Jared M. Fischer, a postdoctoral fellow in Liskay's laboratory and lead author on the paper. The researchers are also interested in looking directly at the impact of Tgfβ signaling in quiescent ISCs, which are important for helping the intestine to recover after injury or, in the case of intestinal cancer, recover after treatment with chemotherapy.

Resources

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Citation

Single cell lineage tracing reveals a role for TgfβR2 in intestinal stem cell dynamics and differentiation. Proc Natl Acad Sci USA 2016 Oct;113(43):12192-7. Jared M Fischer, Peter P Calabrese, Ashleigh J Miller, Nina M Muñoz, William M Grady, Darryl Shibata, and R Michael Liskay.

More Published Papers

• Previous School of Medicine Papers of the Month
• Recent OHSU published papers

Pictured above: Drs. Jared Fischer (left), Michael Liskay