Researchers at the RIKEN Center for Developmental Biology have unraveled the mystery of why human embryonic stem (ES) cells and induced pluripotent stem (iPS) cells undergo programmed cell death (apoptosis) when cultured in isolation. By unlocking the potential of cell therapy techniques, the discovery promises new hope to sufferers of debilitating degenerative diseases.
Cell dissociation, a technique for isolating cells in procedures such as subcloning, poses one of the greatest obstacles to effective stem cell research due to its damaging effects on human ES cells. 99% of human ES cells cultured in this way are destroyed by an extensive apoptotic response that is curiously absent in mouse ES cells. Earlier research by the researchers uncovered that inhibition of a protein known as the Rho-associated kinase (ROCK) reduced this rate of cell death by 30%, yet fundamental questions remained about the mechanisms involved.
To answer these questions, the researchers applied live-cell imaging to the earliest phase of dissociation in human and mouse ES cells. Results revealed a striking contrast: whereas the mouse ES cells hardly moved, the human ES cells skittered about in a so-called "death dance," immediately sprouting finger-shaped bulges, known as blebs, which grew until the cells burst and died. The researchers traced this early-onset blebbing, whose duration and severity exceeded anything ever before observed, to the hyperactivation of myosin, a type of protein responsible for cell motility.
Contrary to expectation, the researchers went on demonstrate that it is this myosin hyperactivation, mediated by activation of the ROCK kinase, which is the direct cause of apoptosis in dissociated human ES cells, and not the blebbing itself. Further implicated in this process is the inhibition of another protein known for its role in cell motility, Rac, which together with ROCK activation strongly promotes myosin hyperactivation leading to cell death.
Reported in the journal Cell Stem Cell, these results provide a first ever comprehensive elucidation of mechanisms underlying dissociation-induced apoptosis in human ES cells. While vastly improving the efficiency of colony formation in dissociation culture, the findings also promise safer and more effective cellular therapy treatments for a range of debilitating degenerative diseases.
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