3 February 2006. The purpose of the prion protein (PrP), other than to beset cows, humans, and other animals with brain-wasting diseases, has been devilishly hard to figure out. In spite of the protein’s widespread expression in many tissues, PrP knockout mice have no dramatic phenotype besides their resistance to infection with pathogenic prions.
Now, work from the labs of Harvey Lodish and Susan Lindquist at the Whitehead Institute and MIT reveals for the first time a positive function of PrP in the long-term maintenance of hematopoietic stem cells (HSCs) in mice. Their study, appearing this week in PNAS Early Edition, shows that PrP is present on the surface of HSCs from adult mice, and marks the cell population that is responsible for successful long-term reconstitution of blood cells after HSC transplantation. The primary role of PrP seems to be to promote the renewal of stem cells—HSCs from PrP-null mice were perfectly functional on a first transplantation, but rapidly lost their ability to repopulate bone marrow after serial transplantation. The results provide a tantalizing glimpse of PrP’s good side, and a way toward a better understanding of why evolution would have saddled us with such a protein.
PrP, the normal form of the pathological prion that causes mad cow disease in bovines and Creutzfeldt-Jakob disease in humans, is a cell surface protein, widely expressed on many cell types. In this study, first author Cheng Cheng Zhang noticed that PrP was abundantly present on adult mouse bone marrow cells. Enrichment for HSCs also enriched for PrP-positive cells, and 85.7 percent of the purified HSC population turned out to bear PrP also.
To directly test the role of PrP in HSC function, the researchers affinity-purified PrP-positive and -negative HSCs and tested which fractions could repopulate the hematopoietic system after transplantation to irradiated mice. Whether they started with whole bone marrow, or more purified stem cells, only the PrP-positive cells gave long-term successful reconstitution.
By these results, PrP appeared to be a bona fide marker for HSCs, but what was the protein’s function? The researchers found that PrP knockout mice had a normal hematopoietic system, with completely adequate levels of progenitors and differentiated cells of all types. Freshly isolated HSCs from PrP knockouts were no less efficient than wild-type cells as donors for bone marrow transplant.
When the investigators pushed stem cells a bit further, however, the lack of PrP began to show up. In serial transplantation experiments, where the bone marrow of reconstituted mice was used as a source of donor cells, the ability of PrP-null stem cells to grow and repopulate the marrow declined dramatically with each successive transplantation. In experiments where normal and PrP-null cell mixtures were transferred, the proportion of blood cells that derived from the PrP-null bone marrow went down with each round. When cells from PrP-null mice were used alone, the result was a dramatic decrease in survival with each iteration. These results were directly attributable to loss of PrP expression, since adding back PrP to null cells isolated after transplantation increased survival in the next round, while the introduction of a mutant PrP did not rescue the mice.
From these experiments, the researchers concluded that while PrP deficiency does not affect HSC activity under normal conditions, it is required for the renewal of stem cells under stress, as in serial transplantation. This idea was backed up by their finding that mice transplanted with PrP-null cells were more sensitive to the bone marrow toxin 5-fluorouracil than mice with hematopoietic systems derived from wild-type stem cells.
The unexpected link between PrP and stem cells provides a first, fascinating look at a physiological role for this infamous protein. The results will no doubt have stem cell mavens from all fields reaching for PrP antibodies to find out how widespread the influence of PrP might be.—Pat McCaffrey.
Zhang CC, Steele AD, Lindquist SL, Lodish HF. Prion protein is expressed on long-term repopulating hematopoietic stem cells and is important for their self-renewal. Proc Natl Acad Sci U S A. 2006 Feb 14;103(7):2184-9. Epub 2006 Feb 7. Abstract