ReviewDentate gyrus neurogenesis, integration and microRNAs
Highlights
► Neurogenesis is enhanced by activity and correlates to behavioral modification. ► The integration of newborn neurons into the synaptic circuitry is stereotyped and well described. ► We are beginning to understand the mechanisms through which large-scale transcriptional changes regulate the process of neurogenesis and integration.
Introduction
In the adult mammalian brain newborn neurons are born and then are integrated into the functioning synaptic circuitry. There are two major neurogenic regions of the brain – the subgranular layer of the dentate gyrus and the subventricular zone adjacent to the lateral ventricles. Newborn neurons of the subgranular layer become granule cells [1] whereas those derived from the subventricular zone migrate via the rostral migratory stream to become granule and periglomerular cells in the olfactory bulb [2]. In this review we discuss the evidence for dentate gyrus neurogenesis, how newborn neurons become integrated into the dentate gyrus, and some of the molecular mechanisms involved in this process. As newborn neurons integrate an array of cellular changes take place, therefore molecules that can orchestrate large-scale changes to the transcriptome such as transcription factors and possibly microRNAs are of particular interest.
Section snippets
Behavior and dentate gyrus neurogenesis
The proliferation of new neurons in the dentate gyrus and their chances of survival are influenced by environmental and endogenous factors, each of which can have profound effects on behavior. However, whether there is a causal relationship between the generation of new neurons and behaviors such as learning tasks is still debated. Two examples of a positive influence on neurogenesis are enriched environment and voluntary exercise or wheel running. Exposure to tunnels, toys, and running wheels
Integration of newborn neurons into the synaptic circuitry
In the adult rat, it has been estimated that 4000–9000 new cells are born every day [44], [45]. Of those surviving one week, about 75% differentiate into neurons [45], [46]. Of these, about half will die within the first month after birth whereas the other half appear to survive at least 6 months [47]. Thus it has been estimated that between 3.75% and 6% of neurons in the dentate gyrus are less than 1 month old [44], [45].
Although earlier studies focused on precursor cell proliferation,
Activity-dependent transcription
As progenitor cells undergo the transition into differentiated neurons and integrate into the synaptic circuitry, there are large-scale changes in gene expression [62]. Thus, there must be many molecular controls governing the stages of neurogenesis and subsequent circuit integration. One transcription factor governing this process, NeuroD1, is expressed in the dentate gyrus during the transition from precursor cells into neurons [63]. Both overexpression and knockdown of NeuroD1 indicate that
Conclusion
There is increasing evidence linking animal behavior to dentate gyrus neurogenesis in the adult animal. This link is exemplified by the ability of enriched environments, learning, and exercise to enhance adult neurogenesis and data suggesting that the behavioral effects of antidepressants are partially mediated by neurogenesis. However, understanding how adult neurogenesis contributes to learning and behavior is complicated by the difficulty in manipulating neurogenesis in isolation. That the
Acknowledgements
This work was supported by T32NS007381, F32MH079548, and a NARSAD Young Investigator Award (BWL), and MH46613 (GLW).
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