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Lack of Dicer causes neurons to degenerate (right).
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Neurons missing their microRNA (miRNA) gradually deteriorate in structure and function, according to Schaefer et al. (page 1553). These bits of RNA prolong neuronal longevity and may keep neurodegenerative diseases at bay.
miRNA—small pieces of RNA generated by the RNA-chopping enzyme called Dicer—strategically shut off genes to allow embryonic cells to differentiate. Because mice that lack Dicer die as embryos, the role of miRNA in vivo has been studied by deleting Dicer only in specific cell types. These studies showed that Dicer—and thus miRNA—is required for both cell differentiation and the survival of some cells. Neurons need miRNA for differentiation, but whether they also need them for survival later was unknown.
Schaefer et al. now find that inactivating Dicer in postmitotic neurons of adult mice eventually kills these cells. The neurons maintained some of their miRNA long after Dicer inactivation and looked and functioned normally. But the reduction in miRNA eventually took its toll. Neuronal signaling malfunctioned, and the cells died off. As cell death progressed, the mice developed symptoms reminiscent of those seen in humans with neurodegenerative disorders. Whether the human disorders are caused by changes in specific miRNA remains to be seen. 
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