Mechanistic study of declining hippocampal neurogenesis in the aging brain

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With an ever-aging population and an estimated prevalence of Alzheimer disease of 5.7 million people in the United States alone, the impetus for more targeted treatments for age-related cognitive disorders is greater now than ever. Neuroplasticity, the ability of neural networks to adapt and remodel given experience, dwindles with age, providing possible mechanistic insights into this decline. An important layer of neuroplasticity, unique to a couple of discrete areas of the adult mammalian brain, is the addition of newly-generated neurons into existing circuits, a process known as neurogenesis. While the existence and importance of adult hippocampal neurogenesis in young adults has been well-established, we know very little about hippocampal neurogenesis in aging brains. Importantly, hippocampal neurogenesis continues into old age although there is a substantial decline in the number of newborn neurons. For example, in 26-month-old rodents, ~1000 proliferating cells could be detected per day, although only half that detected in 5-month-old adult rats. A recent study demonstrated that thousands of new neurons could be detected in the aged adult human dentate gyrus, and further, that in patients with Alzheimer disease, newborn neurons were fewer in number and exhibited delayed maturation. As a starting point, we ask why neurogenesis declines in the aging brain. Based on our preliminary studies, we found that biased circuit activity may regulate hippocampal neurogenesis in the aging brain. During screening of potential molecules biasing circuit activity, we found that one sphingolipid signaling is active in interneurons and becomes less active in the aging brain. We propose to genetically intervene this signaling to study its role in regulating neurogenesis in the aging brain. Lastly, we determine how biased circuit activity regulates hippocampal neurogenesis. Our results will not only provide mechanistic insights into the understanding of neurogenesis in the aging brain it also provides a possible strategy to intervene aging circuit activity to regulate neurogenesis.