The adult brain has the remarkable capacity to continuously generate new neurons in the SVZ and OB. However, the functional impact of these adult-born neurons on olfactory behavior is still elusive. Furthermore, signaling mechanisms regulating adult neurogenesis in the SVZ/OB are not well defined. The objective of this study was to investigate the role of ERK5 MAP kinase in the regulation of adult SVZ/OB neurogenesis and associated olfactory behavior.
Although several studies have attempted to address the role of adult neurogenesis in olfactory behavior, findings from different studies are contradictory and the physiological function of adult-born neurons in the OB is still unclear 
. For example, genetic expression of a lethal diphtheria toxin fragment (DTA) in adult-born neurons, which kills these neurons, led to a substantial loss of adult-born granule cells in the OB and a much smaller OB 
. Surprisingly, despite these structural changes in the OB, no deficits in olfactory behavior were observed, casting doubt as to whether adult-born neurons in the OB influence olfactory behavior. However, a subsequent study using lateral ventricle infusion of AraC to ablate adult neurogenesis in the SVZ/OB in mice showed that a reduction in OB adult neurogenesis reduces odor detection sensitivity and impairs short-term (hours) olfactory memory but does not affect odor discrimination or reward-associated long-term memory (days) 
. Although results from this study generated much excitement in the field 
, they contradicted results from other reports 
. For example, left ventricle infusion of AraC in another study implicated OB adult neurogenesis in long-term olfactory memory 
. Furthermore, cranial irradiation of the SVZ, which robustly impaired adult neurogenesis in the OB, showed that only long-term olfactory memory, but not the threshold for odor detection or short-term olfactory memory, is affected 
. In addition, NCAM knockout mice have diminished OB neurogenesis, and are deficient in the odor discrimination task while both the detection threshold for odors and short-term olfactory memory are unaltered 
. Thus, the exact function of adult-born OB neurons is still highly debated.
Several factors may contribute to this controversy including the specificity of methods used to ablate or suppress adult neurogenesis. Although effective at suppressing adult neurogenesis, x-irradiation or anti-mitotic drugs are not specific for adult-born neurons. They target all dividing cells, may alter the neurovascular niche important for adult neurogenesis, and induce neural inflammation. The side effects intrinsic to these methods may be confounding factors contributing to inconsistent behavior results. Indeed, two studies using similar protocols for AraC administration to the lateral ventricle to suppress adult neurogenesis yielded opposite conclusions concerning a role for adult neurogenesis in long-term olfactory memory 
. Studies using traditional knockout of genes important for neurogenesis are also useful, but their interpretations are limited by widespread abnormalities of brain structure or compensatory effects elicited during development. Transgenic expression of a lethal gene, such as diphtheria toxin or thymidine kinase 
, to kill adult-born neurons is more specific to adult neural stem/progenitor cells. However, large amounts of cell death in the SVZ-RMS-OB axis may interfere with normal olfactory function. These technical issues make it difficult to establish a definitive connection between adult neurogenesis and olfactory behavior.
The existing controversy necessitates studies using more specific genetic approaches with less off-target side effects to manipulate adult neurogenesis. Clearly, identification of a signaling molecule that is required for adult neurogenesis which is only expressed in neurogenic regions of the brain, coupled with a transgenic mouse engineered to temporally and spatially delete that gene only in adult neurogenic regions would be a powerful tool to evaluate the relationship between adult neurogenesis and olfaction. This study identifies erk5 as such a candidate gene. Here we report that in the adult mouse brain, ERK5 MAP kinase is selectively expressed in the neurogenic regions including the SVZ, along the RMS, and in the center of the granular cell layer of the OB where adult-born neurons exit the RMS. Furthermore, conditional deletion of the erk5 gene specifically in the neurogenic regions attenuated SVZ/OB neurogenesis in vivo. These data suggest an important role for ERK5 in the regulation of adult neurogenesis along the SVZ/RMS/OB axis.
We demonstrate that erk5
deletion specifically in neurogenic regions of the adult mouse brain did not affect olfactory discrimination in the habituation/dishabituation assay. However, ERK5 icKO mice forget a previously exposed odor much faster and have reduced short-term olfactory memory compared with control littermates. Furthermore, ERK5 icKO mice exhibit reduced detection sensitivity to 1-octanol, a chemically defined odor, at a lower concentration. These results support the findings by Breton-Provencher et al. 
Although adult neurogenesis has been implicated in regulating pheromone-based animal behaviors in mice, such as mating, paternal recognition, and male-male aggression 
, it is not known whether adult neurogenesis influences the detection sensitivity of pheromones. We report here that ERK5 icKO mice exhibit reduced detection sensitivity to 2-heptanone, a mouse pheromone 
, to farnesene, a synthetic, aggression-evoking pheromone 
, and to TMT, an innate fear- and alarm-inducing pheromone 
. Furthermore, unlike control animals, ERK5 icKO male mice do not show preference to urine from normal over ovariectomized females. Following ovariectomy, circulating estradiol in rats is undetectable within 24 hours 
. It is known that urine samples collected from ovariectomized female mice contain lower levels of protein and lipids than those from normal females 
. Furthermore, certain fatty acids, including tridecanoic, palmitic and oleic acids, are present in the urine of normal but not ovariectomized female mice 
. These decreases in circulating estrogen and biochemical constituents in the urine may cause and/or be an indicator of subtle differences in pheromones/odorants present in urine collected from ovariectomized vs. normal female animals. Together, these data suggest that adult neurogenesis in mice plays a critical role in their ability to detect pheromones in low concentrations. Since pheromones are likely present only in low abundance in their normal living environment, adult neurogenesis regulation of pheromone detection may underlie a variety of pheromone-based animal behaviors in mice.
Although several studies have evaluated the impact of adult neurogenesis on long-term memory retention (days) of odor-cued associative olfactory learning 
, adult neurogenesis has not been implicated in the acquisition of this learning task 
. We demonstrate here that ERK5 icKO mice do not learn nearly as well as control mice to associate a specific mouse urine sample with a food reward. These data provide the first evidence that adult neurogenesis in the OB may be critical for the acquisition of odor-cued associative olfactory learning, a process that is independent of the hippocampus 
In summary, our data identify ERK5 MAP kinase as a novel signaling pathway regulating adult neurogenesis in the SVZ/OB. This is the first study utilizing genetic approaches to conditionally delete a specific gene selectively in the neurogenic regions of the adult brain to demonstrate a causal relationship between adult neurogenesis and several distinct forms of olfactory behavior, including detection sensitivity to odorants and pheromones, short-term olfactory memory, as well as acquisition of odor-cued associative olfactory learning.