Using long-term in vivo imaging combined with computational modeling, a multinational team of researchers has gained new insights into what makes potential stem cells able to fulfill their role functionally. 

They found that only a subset of cells expressing the intestinal stem cell marker LGR5 were able to behave as stem cells, that is, to both self-renew and give rise to more differentiated daughter cells in their divisions. 

Furthermore, they showed that the proportion of LGR5-expressing cells that were functional stem cells differed between the small and large intestine. 

"The biomarkers are basically the same for small and large intestines but functionally, they are very different," Edouard Hannezo told BioWorld. To confidently identify a stem cell, "you have to consider what cells do, and not just which markers they express." 

Hannezo is a professor at the Institute of Science and Technology Austria and the co-corresponding author of the paper reporting the findings, which appeared in the July 14, 2022, issue of Nature. 

The intestines are not just a smooth tube – structures called intestinal villi project from the intestinal wall, maximizing the surface area which can absorb nutrients as food passes through the gut. 

The villi are covered with a single layer of epithelial cells. That intestinal epithelial layer is among the tissues with the strongest ability to regenerate itself, which it does, on the average, about weekly. The intestinal stem cells driving that regeneration are at the basis of the villi in structures called crypts. Hannezo and his colleagues were curious about how stem cell function was regulated. They first used intravital imaging, a method to track the fate of single cells in vivo, to follow individual cells over a long period of time. 

"It's been over 10 years that we can do that for the short term," Hannezo said. But for the work now published in Nature, co-corresponding author Jacco van Rheenen and his team at The Netherlands Cancer Institute "improved their techniques so that they could image their mice over several months." 

Wherefore art thou stem-like? 

Using blood vessels as landmarks, the researchers were able to follow the movements of LGR5-expressing cells, and discovered that whether they acted as stem cells depended on their location, which – unsurprisingly – depended on their movement, which – surprisingly – differed between small and large intestines. 

First author Bernat Corominas-Murtra explained in a prepared statement that "cells in the epithelium are not just pushed outwards from the crypt by the cell divisions below them – like on a conveyor belt – but there is another kind of motion involved." 

Cells that moved against the flow could return to the crypt, where they once again acted as stem cells because they had access to the microenvironment of the stem cell niche. 

"You could imagine that cell movement and stem cell niche are two different things, Hannezo said, but if you are competing for the niche, those two things are intrinsically linked [because] cell movements help you to get to the right place." 

The researchers also compared stem cell behaviors in the small and large intestines, and showed that "the number of functional stem cells is much smaller in large intestine," because there is less contrarian movement. 

Hannezo said that the difference between the small and large intestine was unexpected, because "there are tons of things that look very similar" between the two LGR5-expressing populations. "It shows that cells which have a very similar molecular profile are going to behave very differently depending on their environmental context." 

The team also developed a theoretical model to predict which cells would behave as stem cells, based on the geometry of the tissue. 

Hannezo said that his team wants to look at other organs, as well as to cancer stem cells, to see whether it can predict which cells would be functional stem cells .