Cell cycle re-entry in the aging brain.

The brain is an organ comprised mostly of long-lived, quiescent cells that perform vital functions throughout an animal's life. Due to the brain's limited regenerative ability, these long-lived cells must engage unique mechanisms to cope with accumulated damage over time. We have shown that a subset of differentiated neuronal and glial cells in the fruit fly brain become polyploid during adulthood. Cell cycle re-entry in the brain has previously been associated with neurodegeneration, but there may be a more complex relationship between polyploidy and cell fitness in the brain. Here, we examine how known lifespan modifiers influence the accumulation of polyploidy in the aging fly brain. Flies aged at a low temperature, or with a low protein diet, accumulate polyploid cells in the brain more slowly than expected if this phenotype were solely regulated by lifespan mechanisms. Despite the slower accumulation of polyploid cells, animals under conditions that extend lifespan eventually reach similar levels of polyploidy in the brain as controls. Our work suggests known lifespan modifiers can influence the timing of cell cycle re-entry in the adult brain, indicating there is a flexible window of cell cycle plasticity in the aging brain.