Adult neurogenesis is absent in a cyclin D mutant (top) but normal in wild-type progeny (bottom).

In recent years, scientists have found evidence that new neurons arise in the brains of adult mammals, but little is known about the underlying mechanisms. Now, on page 209, Kowalczyk et al. find that in neurogenic regions of the adult brain only one cyclin D protein, cyclin D2, is required for neural precursors to enter the cell cycle.

To determine which cyclins were required for adult neurogenesis, the team analyzed mice lacking either the cyclin D1 or D2 gene. Neuronal proliferation in the hippocampus, which is the region required for memory formation, was completely inhibited in cyclin D2 mutants, but was unaffected in cyclin D1 mutants. Astrocytes continued to proliferate in mice lacking D2, albeit to a limited extent, suggesting that the gene is absolutely required for neurogenesis but not for glial cell proliferation.

By contrast, dividing neural precursors isolated from mouse pups contain all three cyclin D proteins, 1, 2, and 3. Thus, there is a mechanistic distinction between adult and developmental neurogenesis. The researchers hope to use cyclin D2's newly defined role in adult neurogenesis to test how neural proliferation relates to the formation or extinction of memories in adult animals.