The ageing systemic milieu negatively regulates neurogenesis and cognitive function

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Abstract

Summary In the central nervous system (CNS), aging results in a precipitous decline in adult neural stem/progenitor cells (NPCs) and neurogenesis, with concomitant impairments in cognitive functions 1 . Interestingly, such impairments can be ameliorated through systemic perturbations such as exercise 1 . Here, using heterochronic parabiosis we show that blood-borne factors present in the systemic milieu can inhibit or promote adult neurogenesis in an age dependent fashion in mice. Accordingly, exposing a young animal to an old systemic environment, or to plasma from old mice, decreased synaptic plasticity and impaired contextual fear conditioning and spatial learning and memory. We identify chemokines - including CCL11/Eotaxin – whose plasma levels correlate with reduced neurogenesis in heterochronic parabionts and aged mice, and whose levels are increased in plasma and cerebral spinal fluid of healthy aging humans. Finally, increasing peripheral CCL11 chemokine levels in vivo in young mice decreased adult neurogenesis and impaired learning and memory. Together our data indicate that the decline in neurogenesis, and cognitive impairments, observed during aging can be in part attributed to changes in blood-borne factors.

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Mechanisms and functional implications of adult neurogenesis.

Chunmei Zhao, Wei Deng, Fred Gage (2008)

The generation of new neurons is sustained throughout adulthood in the mammalian brain due to the proliferation and differentiation of adult neural stem cells. In this review, we discuss the factors that regulate proliferation and fate determination of adult neural stem cells and describe recent studies concerning the integration of newborn neurons into the existing neural circuitry. We further address the potential significance of adult neurogenesis in memory, depression, and neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

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Local self-renewal can sustain CNS microglia maintenance and function throughout adult life.

Bahareh Ajami, Jami Bennett, Charles Krieger … (2007)

Microgliosis is a common response to multiple types of damage in the CNS. However, the origin of the cells involved in this process is still controversial and the relative importance of local expansion versus recruitment of microglia progenitors from the bloodstream is unclear. Here, we investigated the origin of microglia using chimeric animals obtained by parabiosis. We found no evidence of microglia progenitor recruitment from the circulation in denervation or CNS neurodegenerative disease, suggesting that maintenance and local expansion of microglia are solely dependent on the self-renewal of CNS resident cells in these models.

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Ablation of hippocampal neurogenesis impairs contextual fear conditioning and synaptic plasticity in the dentate gyrus.

Michael Saxe, Fortunato Battaglia, Jing-Wen Wang … (2006)

Although hippocampal neurogenesis has been described in many adult mammals, the functional impact of this process on physiology and behavior remains unclear. In the present study, we used two independent methods to ablate hippocampal neurogenesis and found that each procedure caused a limited behavioral deficit and a loss of synaptic plasticity within the dentate gyrus. Specifically, focal X irradiation of the hippocampus or genetic ablation of glial fibrillary acidic protein-positive neural progenitor cells impaired contextual fear conditioning but not cued conditioning. Hippocampal-dependent spatial learning tasks such as the Morris water maze and Y maze were unaffected. These findings show that adult-born neurons make a distinct contribution to some but not all hippocampal functions. In a parallel set of experiments, we show that long-term potentiation elicited in the dentate gyrus in the absence of GABA blockers requires the presence of new neurons, as it is eliminated by each of our ablation procedures. These data show that new hippocampal neurons can be preferentially recruited over mature granule cells in vitro and may provide a framework for how this small cell population can influence behavior.