by Monya Baker

Rat pluripotent stem cells could bring knock-out rats, reprogramming insights, and a larger menagerie of stem cells.

A quartet of papers in December describe the production of rat pluripotent stem cells, both from rat embryos and from the genetic manipulation of cultured rat cells.

The rat embryonic stem cells were derived by research teams led by Austin Smith in Cambridge, UK and Qi-Long Ying of the University of Southern California, Los Angeles. They mark the first of many attempts to create ES cells that, when mixed with a normal rat embryo, can contribute to the germline in the resultant rats. Though neither team has yet produced a knockin or knockout rat, Ying believes this could happen in less than a year. The key to success was finding a combination of small molecules capable of inhibiting differentiation, which Ying and Smith described in mouse embryonic stem cells in May. While pathways to promote self-renewal in stem cells may be species specific, Ying and Smith believe that the strategy of preventing differentiation could be applied beyond rodents to other species. (See Off with differentiation.) Ying used three molecules: CHIR99021 (a GSK-3 inhibitor), PD184352 (a MEK inhibitor), and SU5402 (an FGF receptor inhibitor). Smith found that this combination was too harsh on the cells and instead used a two molecule combination that left out SU5402 and used a different MEK inhibitor.

Though the culture conditions were not exactly the same, these are also the types of molecules researchers used alongside viral vectors to reprogram rat liver cells3. All the molecules employed target enzymes within the large class of tyrosine kinases, and researchers are testing for better combinations of molecules.

The liver reprogramming work was led by Sheng Ding of Scripps Institute in La Jolla, California and Hongku Deng of Peking University. Another team of researchers, led by Lei Xiao of the Shanghai Institutes of Biological Sciences, used lentiviruses to reprogram fibroblasts and bone marrow cells collected from adult rats, with an eye to genetically engineering rats4. Ding says a chief goal of his work was to explore different pluripotent states. Last year, researchers led by Roger Pederson of Cambridge University, UK and Ron McKay of the U.S. National Institutes of Health found that embryonic stem cells could be made from very early post-implantation stage embryos, and that these grew well in culture conditions designed for human ES cells. In contrast, the rat iPS and rat ES cells derived in December grew well in the presence of a protein called LIF, which prompts human pluripotent stem cells to differentiate.

Possibly, says Ding, current techniques to derive human embryonic stem cells allow cells to progress to a state that is still pluripotent but also more differentiated than that of mouse embryonic stem cells. Tweaking culture conditions may allow the "capture" of an earlier state. Either way, the creation of these rat pluripotent stem cells should allow techniques developed with mouse ES cells to be more readily transferred to human cells.


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