Soluble amyloid precursor protein-α rescues age-linked decline in neural progenitor cell proliferation.

Neurogenesis is thought to play a role in cognitive function and hippocampal plasticity. Previous studies suggest that neurogenesis declines with aging. However, the onset and mechanism of declined neurogenesis are not fully elucidated. Here we show that the major decline in neurogenesis takes place during adulthood, before aging. Decline in neurogenesis takes place in the subgranular layer of the dentate gyrus and in the subventricular zone, and is primarily due to a reduced number of fast-proliferating neural progenitor cells. Importantly, this decline can be rescued by intraventricular injection of recombinant soluble amyloid precursor protein (sAPPα), which regulates neural progenitor cell proliferation in the adult brain. The counterpart, sAPPß, a product of the amyloidogenic cleavage pathway of amyloid precursor protein, fails to exhibit a proliferative effect in vitro and in vivo, in equimolar concentrations to sAPPα. These observations suggest that adulthood is an appropriate time window for an intervention that upregulates neurogenesis, such as enhancement of sAPPα levels, for the prevention of declining brain plasticity and cognitive function.

Impaired survival of neural progenitor cells in dentate gyrus of adult mice lacking fMRP.

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability in humans. Individuals affected with the disorder exhibit a deficiency of the fragile X mental retardation protein (FMRP), due to transcriptional silencing of the Fmr1 gene. It is widely accepted that learning deficits in FXS result from impaired synaptic function and/or plasticity in the brain. Interestingly, recent evidence suggests that conditional knockout of Fmr1 in neural progenitor cells in mice impairs hippocampal neurogenesis, which in turn contributes to learning impairments. To examine the nature of the neurogenic impairments and determine whether they impact the morphology of the dentate gyrus, we assessed the extent of neural progenitor cell proliferation, survival, and differentiation in older adult Fmr1 knockout mice. Here, we show that the number of fast-proliferating cells in the subgranular layer of the dentate gyrus, as well as the subsequent survival of these cells, are dramatically reduced in Fmr1 knockout mice. In addition, the number of mature neurons in the granule layer of the dentate gyrus of these mice is significantly smaller than in wild type littermate controls, suggesting that impaired proliferation and survival of neural progenitor cells compromises the structure of the dentate gyrus. Impaired adult neurogenesis may underlie, at least in part, the learning deficits that characterize fragile X syndrome.