Today we’re going to talk science ( Stem Cells) for once, not opinions. This particular post was inspired by a recent ARUK event I attended where almost everyone was working on clinical cohorts and everyone else, with the exception of myself, was working on iPSCs. And I mean everyone. I felt quite out of fashion. So today we’re going to talk about induced pluripotent stem cells; what they are, why people use them and what they could be used for in dementia research. And in the interests of honesty, I will admit that I am mostly writing this so that I can learn about them. Because I’m selfish.
We’ll start with some basics of cell biology. Multicellular organisms, like you, are made up of a bunch of different types of cells. For the sake of this piece we’ll ignore some of them and instead focus on somatic cells and stem cells.
Somatic cells are differentiated; they’re things like your skin and your hair, your brain cells and your gut cells. The majority of these cells are capable of essentially cloning themselves to make more, this is the reason you don’t end up looking like something from a horror movie every time you get sunburned. Neurons are one of the exceptions, they are not capable of dividing.
Stem cells are undifferentiated, or slightly differentiated, cells which can essentially go on forever as they are, or they can be induced to be, differentiated into a variety of different cell types. They can be referred to as pluripotent – cells which can turn into pretty much anything, or multipotent – cells which can turn into a finite bunch of things. An example of this multipotency is the stem cells in the bone marrow and in the fat – the stem cells in the bone marrow are not designed to make fat cells and vice versa.
So why do we care?
Somatic cells contain all the genetic information from the individual they are taken from. If they have a hereditary condition, or a particular mutation of interest, those cells will have the genes which encode it. So if we could take one of those cells and make it pluripotent, then we would have a cell that we could turn into almost anything that also contained all of the relevant (and possibly disease-causing) mutations.
Shinya Yamanaka was the first researcher to establish that it was possible to essentially send cells backwards in time by introducing genes crucial for pluripotency. He started with a panel of twenty-four genes, which he introduced into some fibroblasts using retroviruses, and found that the fibroblasts began expressing markers of pluripotency. He then started to take away the factors one at a time until he ended up with a panel of four markers – Oct4, Sox2, cMyc, and Klf4 – which were found to be key to inducing pluripotency. In the following years Yamanaka’s group and a number of others came up with alternative strategies, including anchored proteins and small molecules, which produced the same effect as retroviral introduction of these genes. (Side bar – Yamanaka called them iPSCs with a small I because the iPod had been around for a few years and was ridiculously popular (look it up young people, it was how you listened to things before Spotify but after cassette tapes) and the iPhone was about to come out so obviously iPSCs with a small I made the most sense.)
The main issue with developing iPSC lines is time and efficiency. Induction of pluripotency only happens in around 0.1-1% of cells if we’re being generous, and originally it was much lower than that. So if you want a line with a particular patient mutation you’re going to have to work at it for an age before you can actually say that’s what you’ve got. And they’re needy things, they need really regular feeding and checking and so forth. I have friends who worked on these and didn’t get a free weekend for months on end.
But if we assume for now you don’t care about mutations, you just want something more human and potentially more realistic than a cancerous cell line, what can you do with them once you have them?
Well, if you take an ordinary fibroblast and make it pluripotent what you have is a cell that can potentially turn into anything you like; a neuron, an astrocyte, an endothelial cell.
If you don’t have time to do this yourself a lot of iPSC banks are now available where cell lines derived from (usually) fibroblasts have been generated and their SNP (single nucleotide polymorphism) profile has been generated and stored.
And the genetic profile of the cells remains important. Original iPSC experiments studying disease models used donations from healthy family members as controls, but researchers discovered that even within families the genetic differences generated made comparing results extremely challenging. To circumvent this, they introduced the isogenic approach. Here, gene editing using things like CRISPR/Cas9 are used to either correct a mutation in cells from a patient with a known mutation, or to introduce said mutation in healthy control cells. It is then possible to compare your cells of interest to their isogenic controls.
Already we can begin to see two major issues with the use of iPSCs to model things like dementia.
First, the whole idea of generating a pluripotent cell is to revert it from its present state, into a more embryo-like state. This is the antithesis of an aging-state where cells begin to undergo senescence and, crucially, show reduced proliferative capacity. Some researchers have gotten around this by introducing factors known to be associated with aging. For example, mutations in lamin A (a nuclear envelope protein) result in an unstable nucleus which limits the cells capacity to divide and repair DNA damage. People with this mutation exhibit premature aging and by introducing it into iPSCs it is possible to replicate some of the important facets of aging which might contribute to disease progression.
Second, if we are interested in diseases of aging then we should be taking our original cell populations from diseased or older people. By using punch biopsies of the skin from these populations we also run the risk of clouding the genetic landscape, for want of a less flowery phrase. I’ll explain. Older people have been around for longer and exposed to the sun for longer. UV light is a natural mutagen and likely to introduce random mutations into skin cells – something which can cause melanoma if it occurs in excess. In sporadic diseases, such as Alzheimer’s, where the underlying causes are largely hypothetical, introduction of lots of genetic variability in cell lines is likely to make pinning down a specific cause quite difficult. But it can be swings and roundabouts, if something sticks out through all this noise then that can only be a good thing.
There is one final problem with iPSCs which is not dementia specific but rather research environment specific, and I alluded to it above. Time. To turn an iPSC into a neural progenitor cell takes between three and four weeks. To turn it from a neural progenitor cell into a neuron takes another two to four weeks. Assuming everything goes absolutely perfectly that means that an experiment will take around 2 months just to make the cells. As an in vivo researcher I already suffer from the slow turnaround time for data, and the publish or perish attitude towards science may mean that iPSCs are only bound for the rich and populous labs of this world.
Despite these challenges, using iPSCs to generate disease specific models of things like the blood brain barrier, combining them with cutting edge organoid or organ-on-chip techniques helps us directly link our results to patients and reduces our need for animal research. Or to take it even further, by combining them with animal models of disease we might be able to refute or confirm the disease models we’re currently using. Either way I suspect they’re here to stay so if you’re not on the bandwagon yet, go chat with someone who is and see what iPSCs can offer you.
Dr Yvonne Couch is an Alzheimer’s Research UK Fellow at the University of Oxford. Yvonne studies the role of extracellular vesicles and their role in changing the function of the vasculature after stroke, aiming to discover why the prevalence of dementia after stroke is three times higher than the average. It is her passion for problem solving and love of science that drives her, in advancing our knowledge of disease. Yvonne shares her opinions, talks about science and explores different careers topics in her monthly blogs – she does a great job of narrating too.