Probing proteasome activity in living cells

The idea that nerve cells cannot be replenished was long regarded as fact, but experiments in the last few decades have overturned this misconception. Integral to this turnaround was a report that cells isolated from the adult mammalian central nervous system could, through cell division, produce large clonal spheres composed of daughter cells capable of differentiating into neurons, astrocytes and oligodendrocytes. This observation resulted in the development of the aptly named and widely used ‘neurosphere assay’ to detect stem cells. Now, one of the originators of this method cautions that, though useful, it has several shortcomings, which must be appreciated before applying and interpreting the assay. Perspective p333