Feature: Skin deep

One of the most fundamental questions of mammalian cell biology is how stem cells decide precisely when they’re going to start growing and dividing into a particular type of mature cell.

Another is how this process is used for good or ill. Associated Professor Fiona Watt from the University of Cambridge’s Department of Oncology has spent much of her career wrestling with just these questions.

“My big research interest is stem cells in adult tissues,” says Watt. “I am not talking about embryonic stem cells or induced pluripotent cell populations – I am only interested in the cells that are normally responsible for maintaining tissues in the adult body.”

And as one of the most highly respected and successful stem cell scientists in the world, Watt has certainly taken this interest a long way.

In her latest career move, Watt will be moving from her already prestigious leadership positions in Cambridge later this year to head a new stem cell research centre in London, where she is very much looking forward to seeing the cell biology and clinical sides of the research she will oversee starting to make a real difference in medicine.

The long-standing tissue of trade for Watt’s own research is mammalian skin. “Skin is a very interesting tissue to work with because we know that the stem cells are there and needed throughout adult life. Normal skin works to protect the body from its day-to-day environment using a thick outer layer of dead cells that are continuously shed and therefore have to be replaced by the proliferation of stem cells deeper in the tissue.”

This makes skin a fabulous tissue for studying what activates and regulates the proliferation and differentiation of stem cells as they leave their home on the extracellular membrane at the base of the multilayered skin structure and move towards a very specific role at the tissue surface or epidermis.

Skin is also a highly tractable experimental model, readily available and contains a veritable microcosm of environments and cell types to study all in the one, very active layer.

Of course, understanding more about these processes in normal adult tissue will also help to inform scientists about what happens in diseases such as skin cancer with the hope of identifying novel therapeutic and preventative strategies for tumours of skin and other stratified epithelia in the body.

“Stem cells are of central importance in the genesis of such tumours since as permanent tissue residents they have the potential to accumulate oncogenic mutations over many years.”

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A coat of many colours

Epidermal stem cells can give rise to many different lineages of cells needed to re-populate the different components of skin, including the interfollicular epidermis, sebaceous glands and hair follicles.

Watt’s research looks at what makes the stem cells decide to suddenly change where and when they are to differentiate, what determines the sort of cell they will become, how do the stem cells and their surroundings control where in the skin they should grow and divide, and what stem cell factors contribute to the development of cancer?

She employs a variety of in vitro and in vivo approaches in addressing these questions. “Many of these are based on the finding that you can culture epidermal cells from biopsies of normal human skin,” says Watt.

“In fact, grafting patients with cultured human epidermis is an established stem cell therapy that has been around since the beginning of the 1980s. The fact that stem cells survive in culture enables us to design experiments in which we can really hone in on the properties of these human cells.”

At the Hunter Meeting in March, Watt discussed some of her group’s latest work using single human epidermal cells, from normal skin or from tumours, to analyse how single cells interact with and respond to signals received from their environment.

These signals could come from components of the extracellular matrix or from growth factors and other molecules secreted by nearby cells. How such interplay between stem cells and their microenvironment, or niche, could influence stem cell regulation is a long-standing interest for Watt and her group.

They are investigating it using complementary systems: an in vivo approach to observe the stem cells in situ with all the factors operating; and an in vitro approach that deconstructs the stem cell niche such that individual factors can be evaluated.

“We are currently collaborating with chemists and bioengineers to make artificial stem cell environments – a microniche – and then asking what happens if we genetically alter the cell. How will that affect the way in which it responds to its environment?

“We have developed micropatterned glass substrates coated with extracellular matrix components such as collagen or laminin that can selectively capture single human epidermal stem cells from groups of cells and test the effects of systematically and quantitatively altering individual parameters of the microenvironment.

“We can then do a whole range of analyses on these substrates including global single-cell gene expression profiling, single-molecule RNA FISH [fluorescence in situ hybridisation] and live-cell imaging … and we are getting some really interesting results.”

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For example, a study from Watt’s lab published in 2010 in Nature Cell Biology looked at some of the triggers that convince human epidermal stem cells to initiate terminal differentiation. Various experiments showed that even changing the size of the cell platform or ‘island’ could affect the cells’ to-be-or-not-to-be differentiation choice.

That is, epidermal stem cells seeded on collagen-coated islands became less rounded and started to terminally differentiate at a higher frequency when placed on relatively larger circular areas. They also found that such differentiation could be regulated based on how the stem cell rearranged its own actin cytoskeleton in response to specific growth factors and other signals applied to the system.

These sorts of studies have “highlighted the importance of defining the physical parameters of the stem cell niche. Engineered microenvironments are valuable tools for dissecting how extrinsic stimuli interact with core transcriptional networks to control cell fate decisions,” the authors wrote.

Following on directly from this study, the team is now investigating how stem cells and their specific signalling pathways respond to further factors such as cell topology, and whether these environment-dependent responses are altered in tumour stem cells.

Taking it to the source

Complementing the in vitro studies, Watt’s group is also comparing the responses of different stem cell populations to the activation of specific signalling pathways using in vivo models of genetically modified mice.

“I am also very interested in the cell environment itself and how that might be changed when stem cells are activated, whether our findings reflect intrinsic differences between the cells or differences in their local microenvironments. It seems like you get this kind of reciprocal communication and we are trying to get a handle on that in mice.” As an example of this focus, Watt published in Cell last year that stem cells in the hair follicle deposit a protein called nephronectin into the extracellular basement membrane, thus changing the intrinsic nature of its own niche.

The nephronectin then mediated the recruitment of other specific cells in the skin to that site and induced them to upregulate particular signalling pathways. In short, it seems that the hair follicle stem cells are providing a very specialised cell niche by inducing local variation in the extracellular matrix composition and also directly affecting specific skin functions via surrounding cells.

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“We also use genetically modified mice for lineage tracing, where we mark a single cell and monitor what happens to its daughter cells over time,” says Watt. “Lineage tracing is a powerful tool for studying stem cell properties in adult mammalian tissues, and thus improve our understanding of tissue development, homeostasis, and disease, especially when it is combined with our in vitro systems to examine signals that determine cell-fate decisions,” she says.

“We started working on our in vitro niches about four years ago, but I feel that we are really still just at the beginning – there is so much more that we need to do. It is always important to understand the basics of the starting population of stem cells you are studying, for instance, and work with homogeneous populations, so a lot has to be known first before setting up each system.”

However, according to Watt, the beauty of these in vitro cell environments is that you can test one parameter at a time, or potentially setup a high throughput automated environment with up to 2000 potential different environments available and able to be tested all at once.

“For example, you could change concentrations of multiple factors simultaneously or tweak different signalling pathways and the extent to which they are activated. The next step for us now is to do exactly that: scale up the systems so we can start to look at more niche parameters simultaneously.”

Watt is also keen to extend the technology to other types of stem cells. “It is possible using bioengineering techniques to generate in vitro assays that would potentially work for any cell type.

“So having set up the assays we can easily make them available for other people to analyse their favourite stem cells or cancer cells, so this becomes a very nice platform for collaboration, which is a very important aspect of what we do and one that I really enjoy.”

Later this year, Watt will head to the big smoke to establish the new Centre for Stem Cells and Regenerative Medicine at University College London. The focus of the work and the groups recruited will pretty much mirror her own: the interactions between stem cells and their niche.

“A very important part of that will come from our connections with a wide cross section of the clinical research community. We will also be specially setting up a lab to provide space and facilities for collaborators interested in looking at their own stem cell niche.

“The venture presents a great opportunity to collaborate with clinicians on problems that are of great interest to cell biologists, but also clinically very important, because there is so much to be gained at that interface.”