Scientists have found that existing memories may be erased in our brain by a process that involves the generation of new neurons. This clearance might be important to “make room” for the acquisition of new memories.
The results are reported in today's issue of Neuron.
The research team, led by Joe Tsien of the Department of Molecular Biology at Princeton University, generated mice that lack a protein called presenilin-1 throughout much of the brain. Mutations in presenilin-1 are responsible for the majority of cases of early-onset Alzheimer's disease, but the function of the protein in the context of the normal CNS is poorly understood.
These mice were viable and grew normally, but the researchers observed that after spending time in a rich, stimulating environment filled with diversions and mouse toys, the brains of presenilin-1 mutant mice generated fewer new neurons than the brains of normal mice.
Tsien and colleagues initially thought that this reduced neurogenesis might cause learning deficits, but, after months of testing, none could be detected. The researchers did observe, however, that time spent in an enriched environment generally enhanced the retention of recent learning.
To their surprise, they also found that some newly formed memories were harder to erase in the mice lacking presenilin-1 than in the control mice.
This suggested to the authors that generation of new neurons is important for the memory-clearance process.
Memory retention ordinarily seems like a good thing. However, as Tsien points out, “adding new neurons to existing networks may potentially disrupt, rather than improve, the function of these networks”.
These findings raise a possibility that chronic abnormalities in this clearance process may contribute to the devastating memory disorder associated with Alzheimer's disease.
In addition, they raise a potential cautionary note about the therapeutic use of neural stem cell transplantation for neurodegenerative disorders.
[Contact: Joe Z. Tsien]