Below, I’ll paste highlights of a Nature paper showing that entire mouse prostates can be regenerated from a single cell: nice evidence of stem cells’ power plus the ability to find these rare cells. Also, in the race to create pluripotent cells without embryos or genetic engineering, some cell types seem easier than others. A paper in Nature Biotechnology shows that an easily reprogrammable cell type is also the easiest to get in a biopsy.
I saw reports on the news wires of a new technology from Austin Smith’s lab for efficiently reprogramming cells, which has reportedly been published in PLoS and licensed by Stem Cell Sciences. I can’t find the actual paper, but it was blogged here. I’m betting it’s an extension of work reported in Nature a few months ago which used small molecules to inhibit differentiation in mouse embryonic stem cells and which was licensed by the same company.
Also, if you’ve encountered me in the past month or so, I’ve probably sidled up and asked you to tell me the most problematic terms in the stem cell field. You told me terms including: reprogramming, function, commitment, population, purity. But the most common term I heard “stemness” or “stem cell”, which is what I used for my write-up in the disputed definitions in Nature this week. I talked to over a dozen people for this, and so collected far more wisdom than will fit in 500 words. Please send me an email and tell me what you think!
And finally, articles going live on Nature Reports this week include an interview with Tom Graf, who showed how changing transcription factor networks change transform one cell type to another. Also, two separate papers on how organisms trade regeneration potential to decrease risk of cancer. One, in Drosophila, from Yukiko Yamashita. Another in mice from Sean Morrison. Anyone remember how Lgr5 pinpointed intestinal stem cells? It’s shining light on surprising stem cell activity in the hair follicle too.
Grow your own prostate
By Elie Dolgin
Over the past decade, scientists have discovered a number of cell-surface markers that might identify prostate stem cells. But all these markers are also expressed in other stem cell types. Now, researchers have found a new marker for a rare adult mouse prostate stem cell population, and showed that a single cell from this population can generate a new prostate after transplantation in vivo. The ability to recreate this organ should help researchers better understand how prostate cancer starts and possibly even help them to grow replacement prostate tissue in the laboratory.
To pinpoint definitive prostate stem (PS) cells, Wei-Qiang Gao of Genentech in South San Francisco, California, and his colleagues noticed that many of the previously described PS cell markers were preferentially expressed in the region of the prostate nearest to the urethra. They then discovered that another marker, the cytokine receptor CD117, or c-Kit, had a similar expression profile. Treating prostates from young mice with anti-CD117 antibodies in culture for several days prevented the formation of prostate internal structure, indicating that CD117 is required for normal prostate development1.
Gao’s team then identified a multipotent, self-renewing PS cell population defined by CD117 expression in combination with three other surface markers — Sca-1, CD133 and CD44 — which constituted 0.12% of a population of possible PS cells. The researchers transplanted single mouse PS cells together with rat urogenital mesenchymal stromal cells into 97 immunocompromised mice, successfully generating 14 functional, secretion-producing prostates.
The prostate now joins the mammary gland, blood and skin as organs that can be successfully regenerated from single murine stem cells. “It generalizes to another organ with a complex structure that a single cell can do this from scratch,” says Connie Eaves at the British Columbia Cancer Research Centre in Vancouver, Canada, whose team was one of the first to reconstitute an entire mammary gland from single stem cells2. Eaves also notes that the purity of the PS cells isolated by Gao’s team was much greater than in most other studies — around 10% as judged by the fraction of cells following serial dilutions that gave rise to prostates on transplantation in vivo. “That’s impressive,” she says. “That means that you can start to do something with these populations and draw inferences from them.”
“It’s a great technical breakthrough,” says David Hudson, of the University of York, UK,who recently characterized a population of putative human PS cells expressing CD133 (ref. 3). In Gao’s mouse PS cells, CD133 was expressed alongside CD117, so Hudson thinks he and others were on the right track to finding human PS cells; they just hadn’t found the most definitive marker. “Given the conservation of other markers, including CD44 and CD133, between species,” he says, “there’s not really any reason for us to think that there’s going to be a huge difference” between mice and humans. In addition to the mouse experiments, Gao’s team showed that CD117 is expressed in human prostate tissue. It now remains to be seen whether CD117-marked human cells are true PS cells, and if single human cells can generate new prostates, Hudson notes.
1. Leong, K. G. et al. Generation of a prostate from a single adult stem cell. Nature advance online publication 22 October 2008. doi: 10.1038/nature07427. | Article |
2. Stingl, J. et al. Purification and unique properties of mammary epithelial stem cells. Nature 439, 993–997 (2006). | Article |
3. Shepherd, C. J. et al. Expression profiling of CD133+ and CD133- epithelial cells from human prostate. Prostate 68, 1007–1024 (2008). | Article |
Elie Dolgin is a science writer based in Vancouver, Canada.
Embryonic stem cells from a single hair
Skin biopsies to obtain fibroblasts for reprogramming into human embryonic-like stem cells could soon become a thing of the past. It has now proved possible to induce the epidermal skin cells that cling to a plucked human hair to pluripotency — that is, to become capable of forming any sort of cell in the body.
Researchers are chipping away on two fronts towards the goal of generating pluripotent human stem cells without involving embryos or genetic engineering. In one approach, they are looking for ways to activate pluripotency genes without permanently changing the cell’s genome. Recently, a team led by Doug Melton at Harvard University showed that one of the four genes normally required for reprogramming could be replaced by treatment with a small molecule1.
On the other front, stem-cell researchers are searching for cell types that might be more easily ‘reprogrammed’ to pluripotency than the skin fibroblasts currently used. Some evidence suggests that liver cells and neural stem cells meet this requirement, but these aren’t readily obtainable from human volunteers.
Reporting in Nature Biotechnology, Juan Carlos Izpisúa Belmonte at the Salk Institute in La Jolla, California, reports that keratinocytes, a skin cell type attached to hairs simply plucked from the scalp, can be readily reprogrammed2.
The researchers used the formula pioneered by Shinya Yamanaka to originally reprogram fibroblasts. This required inserting multiple copies of four pluripotency genes for into the keratinocytes. The rates of successful reprogramming were more than 100-fold higher than those typically reported for fibroblasts, and reprogramming the keratinocytes took only about 10 days, as opposed to three weeks or more.
Whereas fibroblasts are found in the middle layer of the skin, keratinocytes occur in the upper layer, where they produce the protein that forms hair and fingernails. Out of several cell types that Belmonte’s team investigated, keratinocytes were the easiest to reprogram. Izpisúa Belmonte and his colleagues created KiPS (keratinocyte-derived induced pluripotent cells) from five adults. They fully characterized one line, from a 30-year-old woman, to show it could be differentiated into cardiomyocytes and dopamine-producing neurons.
It’s not entirely clear why keratinocytes are easier to reprogram, but compared with fibroblasts the pattern of genes they express (before reprogramming) is more similar to that of genuine human embryonic stem cells.
Reprogrammed cell lines derived from individual patients could be very useful for drug screening or regenerative therapies, but robust, reliable techniques for generating the cells must be established. An accessible, easily reprogrammable cell type could be a fruitful starting place.
[[Author Affiliation]] Monya Baker is editor of Nature Reports Stem Cells
1. Huangfu, D. Induction of pluripotent stem cells from primary human fibroblasts with only Oct4 and Sox2. Nature Biotechnol. advance online publication, doi:10.1038/nbt.1502 (12 October 2008).
2. Aasen, T. et al. Efficient and rapid generation of induced pluripotent stem cells from human keratinocytes. Nature Biotechnol. advance online publication, doi:10.1038/nbt.1503 (17 October 2008).
What’s a stem cell?
Ask a group of stem-cell biologists to define stem cell, and they’ll say roughly the same thing: a cell that can, long term, divide to make more copies of itself as well as cells with more specialized identities. Ask the same scientists to list the most disputed terms in the field, however, and ‘stem cell’ will be top of that list.
The problem here is an operational one: reasonable people disagree on which cells qualify under the definition. “It’s not unusual to pick up a paper and see someone call something a stem cell and the evidence that it is, is just not there,” says Lawrence Goldstein, who directs the stem-cell research programme at the University of California, San Diego.
Alleged ‘stem cells’ can fail to meet the definition on many counts. Stem cells should persist long term, yet many ‘stem cells’ exist only in the fetus. Multipotency — the ability to generate multiple cell types — is a criterion for a haematopoietic, or blood-forming, stem cell, but spermatagonial stem cells only produce sperm. Stem cells specific to tissue such as cartilage, the kidney and the cornea have been reported, with varying degrees of acceptance. The quest for a ‘stemness signature’, a collection of markers common to all stem cells, has been met with frustration.
Debate erupts most commonly over whether a particular cell should be considered a stem cell, which can divide indefinitely, or a progenitor cell, which can differentiate into fewer cell types and is thought to burn itself out after a certain number of divisions.
The only way to be really sure of what a cell can, and cannot do, is to observe it, but it is difficult to study cells in vivo, and putting them in a dish might change their behaviour. Haematopoietic stem cells were the first to be identified and have, to some extent, set default standards. Putative stem cells are isolated, then placed into animals whose own haematopoietic stem cells have been destroyed by radiation. If the blood-forming system is restored, the transplant is assumed to have contained stem cells. But such an assay is impossible when working with other cell types, such as neural stem cells, which are harder to transplant and assess in disease models. And it is difficult to pin the label to one cell type, when studies commonly involve a mixed population. “It is perhaps not realistic to come up with a generally applicable definition of an adult stem cell,” says Thomas Graf of the Centre for Genomic Regulation in Barcelona.
Some researchers are side-stepping the debate by referring in their papers to ‘stem/progenitor cells’. Fully understanding what each cell can do is more important than knowing what to call them all, says Goldstein. “Some of this just breaks down,” he says. “That’s biology. It wasn’t designed to fit the language.”