In today's open access paper, the authors report on a successful effort to restore youthful behavior in neural stem cell populations in aged mice. A lentiviral gene therapy to upregulate plagl2 and downregulate dyrk1a resulted in increased production of new neurons, to a pace normally seen in young animals, and cognitive function improved as a consequence. Like all stem cell populations, the activity of neural stem cells declines with age. A supply of new neurons that will integrate with existing neural circuits is necessary to maintain the function of the brain, particularly learning and recovery from the sort of small-scale damage that is inflicted on brain tissue over time, such as through the rupture of capillaries.
Thus the research community is very interested in finding ways to safely increase neurogenesis, the production of new neurons, in the aging brain. Beyond functional gains and resilience, it is hoped that increased neurogenesis can help in the recovery from serious brain injury, such as that caused by stroke. Another approach to this challenge is the reprogramming of supporting glial cells in the brain, turning them into neurons, though recent work in that part of the field has proven disappointing. It remains to be seen as to which of the various approaches will reach the clinic and use in human patients, and how long it will take to achieve that goal.
Aged neural stem cells (NSCs) are mostly dormant, and even when they are activated, they primarily produce astrocytes. Thus, aged NSCs lose their proliferative and neurogenic potential, leading to the cessation of neurogenesis. In this study, we showed that the iPaD (inducing Plagl2 and anti-Dyrk1a) lentivirus substantially rejuvenated the proliferative and neurogenic potential of NSCs in the aged brain. Clonal analysis by a sparse labeling approach as well as transcriptome analysis indicated that iPaD can rejuvenate aged NSCs (19-21 mo of age) to a level comparable with those at 1 or 2 months of age and successfully improved cognition of aged mice.
Once rejuvenated and activated by iPaD, aged dormant NSCs can generate, on average, 4.9 neurons but very few astrocytes in 3-week tracing. Furthermore, these activated NSCs were maintained for as long as 3 months in the aged brain, suggesting that active neurogenesis continues for an extended period of time after iPaD treatment. Nevertheless, iPaD-activated neurogenesis gradually declined. Furthermore, clonal analyses showed that 78.1% (9-month-old) and 81.7% (19-month-old) of iPaD-activated clones maintained RGL cells 4 week after activation, suggesting that ∼20% of activated NSCs are exhausted during this period. However, it is unknown whether this exhaustion is due to limitation of iPaD or loss of iPaD activity, and further analyses are requited to answer this question.
A recent study showed that resting NSCs, those once proliferated but returned to quiescence, are the major origin of active NSCs in the aged brain. This population comprises only 3%-5% of the total NSCs, while the other major population is dormant NSCs, which have never proliferated. Because the iPaD is able to activate 70%-80% of NSCs in the aged brain, it is likely that it mostly targets dormant NSCs, raising the possibility that the higher the infection efficiency of the iPaD virus, the more NSCs are activated to produce new neurons.
During aging, the gene expression and accessible chromatin landscapes change dynamically in NSCs. Transcriptome analysis showed that there are eight clusters of genes showing different expression patterns. Among them, clusters 2, 3, and 6 are of particular interest: In clusters 2 and 3, gene expression is down-regulated in aged NSCs compared with embryonic NSCs but up-regulated by iPaD, while in cluster 6, gene expression is up-regulated in aged NSCs but repressed by iPaD. Genes involved in cell proliferation are enriched in clusters 2 and 3, while genes involved in aging are enriched in cluster 6. In addition, chromatin-modifying genes are also enriched in clusters 2 and 3. These results suggest that iPaD can rejuvenate aged NSCs by up-regulating embryonic-high genes and repressing age-associated genes via modulation of chromatin accessibility. The detailed mechanism by which iPaD differentially regulates the chromatin structures remains to be analyzed.