During fetal development, cells create the gut-associated organs such as the liver, pancreas and intestine in two ways. Efforts to create organs in a dish have focused only on one of these routes, and now researchers at the Novo Nordisk Foundation Center for Stem Cell Medicine (reNEW) in collaboration with the Niels Bohr Institute at the University of Copenhagen have mapped and reproduced the other pathway in the laboratory. This is an important additional step on the way to creating artificial organs.
During fetal development, the miracle of life unfolds as cells develop and step by step become specialised to finally comprise the various organs.
This specialisation of cells needs to be understood to enable the production of artificial organs that can save lives in connection with many diseases and/or old age. Artificial organs could replace a malfunctioning liver, lungs or maybe part of a malfunctioning bowel after cancer therapy. During development, the body creates most of its organs from the embryonic gut.
Until recently, embryonic gut cells were thought to form through a similar route as had been described for most of the other tissues and organs in the body. However, recent discoveries have revealed that the organs associated with the gut such as the lung, liver, pancreas and intestine have an additional origin. Now researchers have mapped this pathway from origin to organ formation, discovered important new landmarks and started to use this route for stem cell development in a dish, bringing them one step closer to realising their vision of being able to create an artificial embryonic gut from stem cells.
The research has been published in Nature Cell Biology.
“For many years, stem cell biologists have attempted to produce organ-specific cells in the laboratory using a map routed in classical developmental biology. What we did is ask how cells progress through this new alternative route in an embryo and whether cells ever follow it when scientists develop them in a dish. We found that largely they do not,” says Joshua Brickman, Professor, Novo Nordisk Foundation Center for Stem Cell Medicine (reNEW), who led the collaborative effort together with Ala Trusina, Associate Professor, Niels Bohr Institute, University of Copenhagen.
"We therefore asked whether there were other ways to go about making organ-like structures in the laboratory."
Two pathways to create the gut and many organs
The gut is formed in two ways during fetal development.
The first is the embryonic pathway in which the cells are programmed very early in development, at the stage that the embryo can be used to make embryonic stem cells. These cells were also believed to develop into all of the gut and gut-associated organs, and embryonic stem cells have therefore been used in all laboratory-based attempts to make organs in a dish.
During fetal development, the cells of the embryo specialise step by step via the embryonic gut into the intestine, liver, pancreas etc. Eventually these cells form highly specialised cells with specific functions, such as for extracting nutrients from the contents of the gut and sending them on through the bloodstream or producing insulin in response to changes in blood sugar.
The second pathway, however, was discovered about 10 years ago based on the advent of modern genetic technologies. This route involves cells that were thought to go on to be support structures that help to provide nutrients to the growing embryo. This requires being in the right place at the right time.
“This research has aimed to understand what happens at the cellular and molecular levels in this transition from cells having one function and acquiring another,” says Martin Proks, PhD student, another researcher behind the study.
Mapping an alternative pathway to establish the gut
To better understand how cells transition from being support cells to being able to comprise the final gut and organs, the researchers conducted many experiments at different levels.
Specific genes contain instructions for how to make each cell in our body. New techniques in single-cell sequencing enable researchers to determine which cells are using which genes and when. The study included sequencing single cells as they developed towards organs from live mouse embryos and from cells cultivated in the laboratory.
Using these data, the researchers were able to map how thousands of genes are turned on and off in connection with support cells gradually turning into embryonic gut or organ cells. Each of these steps has its own unique molecular signature that the researchers mapped in the new study.
Like dots on a piece of paper
The research also involved developing complex computer models to juggle the many data emerging from all the experiments.
“The great thing about this kind of study of the gene expression in single cells is that it creates huge quantities of data. However, this also requires considerable data processing. We therefore had two researchers working to develop new methods of analysing data,” explains Alexander Nielsen, a second PhD student behind this study.
Ala Trusina says that the research has resulted in mapping all the steps a support cell must go through before it can become a gut cell.
“We knew that the cells existed in the gut and where they came from, but we did not know how the support cells became intestinal cells. The different stages are like dots on a piece of paper, and we have joined the dots,” says Martin Proks.
Not possible without an interdisciplinary team
The new knowledge was also used to try and determine how cells in a dish develop. The researchers found that most attempts to develop organ progenitors in the laboratory from embryonic stem cells do not follow the new route, “but that we could nudge a new type of stem cell derived from support cells in the right direction,” says Michaela Rothova, another important contributor to the study.
This suggests that researchers who want to try to make an artificial gut or organ might want to try to use these support stem cells or at least a mixture of support stem cells and embryonic stem cells to push them in the necessary direction step by step.
“This whole field is driven by a desire to eventually make an artificial gut or organ from stem cells, but producing mature functional cell types has been difficult so far. We believe that one reason for this could be that we only had half the starting material,” concludes Josh Brickman.
Ala Trusina adds, “By obtaining deep understanding of both the pathways that lead from either stem cells to gut cells or from support cells to gut cells, we hope to improve the methods of making artificial organs in the future."
The study also involved collaboration with one of the developers of single-cell sequencing technology, Ido Amit’s laboratory at the Weizmann Institute of Science, Rehovot, Israel.
"None of this would have been possible without an interdisciplinary team of researchers, including molecular and developmental biologists, physicists and programmers.”
