Alvarez-Borda B, Haripal B, Nottebohm F.
Timing of brain-derived neurotrophic factor exposure affects life expectancy of new neurons.
Proc Natl Acad Sci U S A. 2004 Mar 16;101(11):3957-61.
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The discoveries that neurons that mediate birdsong in canaries are replaced by new neurons produced from stem cells, and that this turnover of the neurons is regulated by testosterone in a seasonal manner, have provided important insight into the control of neurogenesis by environmental factors. In their new study, Alvarez-Borda et al. provide evidence that BDNF promotes the survival of newly generated neurons in the high vocal center of the canaries. Remarkably, there is only a very tight time window of approximately two weeks following neurogenesis in the spring when BDNF is capable of promoting the long-term survival of the newly generated neurons. These findings have important implications for the regulation of adult neurogenesis in mammals as well as for the importance of neurogenesis in learning and memory processes.
Recent studies of neurogenesis in the hippocampus and forebrain of mice suggest that the continued production of new neurons is required for at least some aspects of learning and memory (Shors et al., 2002; Feng et al., 2001). Presumably, newly generated neurons must integrate into neuronal circuits in order to function in learning and memory, and this requires that they migrate to the appropriate site, differentiate into the relevant phenotype, and form functional synapses. Of course, functional neurogenesis also requires that the neurons survive, which in many instances is not the case, as indeed many newly generated neurons undergo apoptosis. As in the canary, it has been shown that BDNF promotes the differentiation (Cheng et al., 2003) and survival (Lee et al., 2002) of newly generated neurons in the mammalian brain.
Might there be a role for altered neurogenesis in the cognitive dysfunction that occurs in Alzheimer's disease [AD]? Neurogenesis is decreased during aging (Kuhn et al., 1996), and aging is a major risk factor for AD. Studies of transgenic mice with amyloid deposits in their brains, and of cultured human neural stem cells exposed to amyloid-β peptide, suggest that increased levels of Aβ can impair neurogenesis (Haughey et al., 2006). However, analyses of brain tissue from AD patients suggested that neurogenesis is not reduced and even might be increased in AD (Jin et al., 2004). It therefore remains to be determined whether impairment of neurogenesis contributes to cognitive dysfunction in mouse models or AD patients. Nevertheless, levels of BDNF (Connor et al., 1997), and its high-affinity receptor TrkB (Allen et al., 1999), are decreased in affected brain regions in AD patients, and there are indications that polymorphisms in the BDNF gene can affect the risk of AD (Kunugi et al., 2001). A deficit in BDNF signaling would be expected to impair synaptic plasticity and might also suppress neurogenesis in AD.