After passing through the stomach, the next stage of the digestive system occurs in the small intestine, where nutrients and minerals are absorbed. With a host of digestive enzymes, in addition to a diverse microbiome present, the lining of the small intestine is exposed to quite the ‘chemical soup’ that causes the cells much wear and tear. Between the villi of the small intestine, that increase the surface area available to absorb nutrients, are crypts, at the bottom of which reside stem cells. These stem cells are the source for new cells to replace the ones lost at the epithelial lining maintaining the balance of life and death. However, not all the stem cells stay as stem cells. This is important to prevent overgrowth of the intestine which could lead to cancer. The exact molecular mechanisms that underpin this critical balance between stem cell life and death is still incomplete. However, a new paper published in Nature 1 by Koren et al. provides new light on the role of ARTS in this process.
Making the intestine pretty
Goblet cells may not sound very pretty, but they make up one of the six different differentiated cell types in the intestine epithelium. Along with enterocytes covered in microvilli, the goblet cells make up the villus lining. At the other end, at the bottom of the crypt, are the Paneth cells, the crypt-base columnar cells (CBCs) and the +4 cells, so named for their position from the base of the crypt (Figure 1).
Figure 1 – cells in the small intestine crypt
Of these cells, the CBCs are thought to be the true stem cells of the small intestine, replicating to produce more stem cells or cells that will later differentiate to become the other cell types. Since some stem cells will divide to produce more stem cells whilst others divide and differentiate, over time the small intestine crypt becomes dominated by cells originating from one stem cell clone – it becomes monoclonal. A previous study beautifully illustrated this phenomenon using different coloured fluorescent markers as can be seen in Figure 2.
Figure 2 – taken from ref(2), the crypt shows that differentiated cells originate from stem cells at the bottom of the crypt
These results clearly showed how the regeneration of the small intestine occurs through a conveyor belt mechanism 2 . It also confirmed known ideas that the stem cells resided near the bottom of crypts. But how are the stem cells eliminated?
Apoptosis – a type of celldeath
There are many ways thatcells can die – they can get damaged and release all of their contents, cellscan get infected by bacteria or viruses, or cells can trigger their own death.The last case is best known as apoptosis. There are proteins in a cell that promote apoptosis. So, to prevent cells from killing themselves all the time, there are also proteins that prevent cell death. The proteins are referred to as pro-apoptotic or anti-apoptotic respectively. One family of the anti-apoptotic proteins are literally called IAP, for inhibitor of apoptosis. However, these inhibitors can also get inhibited. For example, one IAP, xIAP, can be inhibited by ARTS. ARTS is therefore a pro-apoptotic protein. It’s like how the enemy of an enemy is your friend – xIAP is an enemy of ARTS and apoptosis is an enemy of IAP – so ARTS and apoptosis are friends… Well, I reckon that would be an interesting friendship, but by antagonising xIAP, the inhibitor of apoptosis can no longer inhibit apoptosis.
Where ARTS thou?
By using a tagged antibody that specifically recognises ARTS, the authors were able to stain and locate ARTS within the crypt of the small intestine. ARTS was mainly found both in the stem cells and the neighbouring Paneth cells. When they deleted the ARTS gene from the cells they saw that the crypt increased in size and cell number. This is somewhat expected given ARTS role as a pro-cell-death factor and the phenotype was reversed when xIAP was also genetically removed. By using a stem-cell specific reporter, it became clear that part of this increased cells number was due to an increase in stem cell number. Paneth cells also increased in number.
But is this cell expansion due to less death or more proliferation?
To determine this, the team treated the crypt cells with staurosporine, a chemical that induces apoptosis. Compared to a wild type control, the crypts lacking ARTS showed less expression of a pro-apoptotic protein, cleaved-caspase 3.
But could proliferation alsobe increased?
The Paneth cells, neighbouring the stem cells, promote stem cell proliferation by providing growth factors. The main growth pathway activated in the stem cells is the Wnt/β-catenin. When cells lacked ARTS, the authors saw not only increased levels of a proliferation marker but also increased β-catenin levels in the nucleus – a good indication that the growth pathway has been activated. However, preventing Wnt signalling does not prevent the apoptosis resistance seen without ARTS.
The function of ARTS in theintestine
Koren’s teams work is one of the first to examine the death of stem cells, an important mechanism as left unregulated could result in uncontrollable tissue growth. Since uncontrollable growth is one of the hallmarks of cancer, including colorectal cancer, and that loss of ARTS promotes crypt cell growth, this work provides new theory for rational drug development.
1. Koren, E. et al. ARTSmediates apoptosis and regeneration of the intestinal stem cell niche. Nat.Commun. 1–17 doi:10.1038/s41467-018-06941-4
2. Snippert, H. J. et al. Intestinal crypt homeostasis results from neutral competition between symmetrically dividing Lgr5 stem cells. Cell 143, 134–144 (2010).