Artistic representation of quiescent neural stem cells arranged in a circle (yellow) with one of the cells exiting quiescence (green). Image credit: Blomfield et al. (CC BY 4.0)
Stem cells in embryos give rise to all the tissues in the body. Adults also have stem cells, but they are fewer in number and they are usually dedicated to repairing and regenerating specific tissues. A region of the brain called the hippocampus, which is involved in learning, memory and mood, has a pool of neural stem cells. These cells can produce new brain cells long into adulthood, but maintaining their regenerative potential is a balancing act. Enough new brain cells need to be made to keep up with the brain’s demands, but if every stem cell matured into a brain cell, the brain’s capacity for repair would be lost. So, some neural stem cells hit a metaphorical snooze button to enter a resting state known as quiescence.
Stem cells in the hippocampus make a protein called Ascl1 that interacts with DNA to switch on quiescent cells so they will divide and mature. Left unchecked, Ascl1 could deplete the stem cell supply, so resting stem cells must have a way to turn Ascl1 off, but it was previously unknown how. Clues point to the E proteins, which interact with Ascl1 to allow it to bind to DNA. If the E proteins are not present, Ascl1 cannot work as a genetic switch. E proteins can also interact with inhibitor of DNA binding/differentiation proteins, known as Id proteins for short. To find out whether Id proteins affect Ascl1 activity, Blomfield et al. looked at stem cells in the hippocampus of adult mice, and at quiescent stem cells grown in the laboratory.
Blomfield et al. showed that all stem cells in the hippocampus make Ascl1, but its levels are much lower when stem cells are resting. This difference was down to an Id protein called Id4. In resting stem cells, Id4 interacted with E proteins, preventing them from binding to Ascl1, and stopping Ascl1 from ‘waking up’ the cells. This not only left Ascl1 unable to activate its target genes, it also made it vulnerable to destruction by the cell's protein recycling system. Mice with no Id4 in their hippocampus stem cells had higher levels of Ascl1, and their stem cells were more active.
The number of stem cells in a resting state increases as we age, and in illnesses like depression, limiting brain cell replacement. Uncovering the signals that switch Id4 on or off could reveal why stem cells rest more with age and illness. This could help us find ways to kick-start the production of new brain cells in adulthood.
