Inducible and Conditional Activation of Adult Neurogenesis Rescues Cadmium-Induced Hippocampus-Dependent Memory Deficits in ApoE4-KI Mice.

The apolipoprotein E (ApoE) gene is a genetic risk factor for late-onset Alzheimer's disease, in which ε4 allele carriers have increased risk compared to the common ε3 carriers. Cadmium (Cd) is a toxic heavy metal and a potential neurotoxicant. We previously reported a gene-environment interaction (GxE) effect between ApoE4 and Cd that accelerates or increases the severity of the cognitive decline in ApoE4-knockin (ApoE4-KI) mice exposed to 0.6 mg/L CdCl2 through drinking water compared to control ApoE3-KI mice. However, the mechanisms underlying this GxE effect are not yet defined. Because Cd impairs adult neurogenesis, we investigated whether genetic and conditional stimulation of adult neurogenesis can functionally rescue Cd-induced cognitive impairment in ApoE4-KI mice. We crossed either ApoE4-KI or ApoE3-KI to an inducible Cre mouse strain, Nestin-CreERTM:caMEK5-eGFPloxP/loxP (designated as caMEK5), to generate ApoE4-KI:caMEK5 and ApoE3-KI:caMEK5. Tamoxifen administration in these mice genetically and conditionally induces the expression of caMEK5 in adult neural stem/progenitor cells, enabling the stimulation of adult neurogenesis in the brain. Male ApoE4-KI:caMEK5 and ApoE3-KI:caMEK5 mice were exposed to 0.6 mg/L CdCl2 throughout the experiment, and tamoxifen was administered once Cd-induced impairment in spatial working memory was consistently observed. Cd exposure impaired spatial working memory earlier in ApoE4-KI:caMEK5 than in ApoE3-KI:caMEK5 mice. In both strains, these deficits were rescued after tamoxifen treatment. Consistent with these behavioral findings, tamoxifen treatment enhanced adult neurogenesis by increasing the morphological complexity of adult-born immature neurons. These results provide evidence for a direct link between impaired spatial memory and adult neurogenesis in this GxE model.

Adolescent cadmium exposure impairs cognition and hippocampal neurogenesis in C57BL/6 mice.

Cadmium (Cd) is a toxic heavy metal and a significant public health concern. Epidemiological studies suggest that Cd is a potential neurotoxicant, and its exposure is associated with cognitive deficits in children, adults, and seniors. Our previous study has found that adulthood-only Cd exposure can impair cognition in mice. However, few studies have addressed the effects of Cd exposure during adolescence on cognitive behavior in animals later in life. In the present study, we exposed 4-week-old male C57BL/6 mice to 3 mg/L Cd via drinking water for 28 weeks and assessed their hippocampus-dependent learning and memory. Cd did not affect anxiety or locomotor activity in the open field test. However, Cd exposure impaired short-term spatial memory and contextual fear memory in mice. A separate cohort of 4-week-old mice was similarly exposed to Cd for 13 weeks to investigate the potential mechanism of Cd neurotoxicity on cognition. We observed that Cd-treated mice had fewer adult-born cells, adult-born neurons, and a reduced proportion of adult-born cells that differentiated into mature neurons in the subgranular zone of the dentate gyrus. These results suggest that Cd exposure from adolescence to adulthood is sufficient to cause cognitive deficits and impair key processes of hippocampal neurogenesis in mice.

Inducible and conditional activation of ERK5 MAP kinase rescues mice from cadmium-induced olfactory memory deficits.

Cadmium (Cd) is a heavy metal that is one of the most toxic environmental pollutants throughout the world. We previously reported that Cd exposure impairs olfactory memory in mice. However, the underlying mechanisms for its neurotoxicity for olfactory function are not well understood. Since adult Subventricular zone (SVZ) and Olfactory Bulb (OB) neurogenesis contributes to olfaction, olfactory memory defects caused by Cd may be due to inhibition of neurogenesis. In this study, using bromodeoxyuridine (BrdU) labeling and immunohistochemistry, we found that 0.6 mg/L Cd exposure through drinking water impaired adult SVZ/OB neurogenesis in C57BL/6 mice. To determine if the inhibition of olfactory memory by Cd can be reversed by stimulating adult neurogenesis, we utilized the transgenic caMEK5 mouse strain to conditional stimulate of adult neurogenesis by activating the endogenous ERK5 MAP kinase signaling pathway. This was accomplished by conditionally induced expression of active MEK5 (caMEK5) in adult neural stem/progenitor cells. The caMEK5 mice were exposed to 0.6 mg/L Cd for 38 weeks, and tamoxifen was administered to induce caMEK5 expression and stimulate adult SVZ/OB neurogenesis during Cd exposure. Short-term olfactory memory test and sand-digging based, odor-cued olfactory learning and memory test were conducted after Cd and tamoxifen treatments to examine their effects on olfaction. Here we report that Cd exposure impaired short-term olfactory memory and odor-cued associative learning and memory in mice. Furthermore, the Cd-impaired olfactory memory deficits were rescued by the tamoxifen-induction of caMEK5 expression. This suggests that Cd exposure impairs olfactory function by affecting adult SVZ/OB neurogenesis in mice.

Inducible and Conditional Stimulation of Adult Hippocampal Neurogenesis Rescues Cadmium-Induced Impairments of Adult Hippocampal Neurogenesis and Hippocampus-Dependent Memory in Mice.

Cadmium (Cd) is a heavy metal and an environmental pollutant. However, the full spectrum of its neurotoxicity and the underlying mechanisms are not completely understood. Our previous studies demonstrated that Cd exposure impairs adult hippocampal neurogenesis and hippocampus-dependent memory in mice. This study aims to determine if these adverse effects of Cd exposure can be mitigated by genetically and conditionally enhancing adult neurogenesis. To address this issue, we utilized the transgenic constitutive active MEK5 (caMEK5) mouse strain we previously developed and characterized. This mouse strain enables us to genetically and conditionally activate adult neurogenesis by administering tamoxifen to induce expression of a caMEK5 in adult neural stem/progenitor cells, which stimulates adult neurogenesis through activation of the endogenous extracellular signal-regulated kinase 5 mitogen-activated protein kinase pathway. The caMEK5 mice were exposed to 0.6 mg/l Cd through drinking water for 38 weeks. Once impairment of memory was confirmed, tamoxifen was administered to induce caMEK5 expression and to activate adult neurogenesis. Behavior tests were conducted at various time points to monitor hippocampus-dependent memory. Upon completion of the behavior tests, brain tissues were collected for cellular studies of adult hippocampal neurogenesis. We report here that Cd impaired hippocampus-dependent spatial memory and contextual fear memory in mice. These deficits were rescued by the tamoxifen induction of caMEK5 expression. Furthermore, Cd inhibition of adult hippocampal neurogenesis was also reversed. This rescue experiment provides strong evidence for a direct link between Cd-induced impairments of adult hippocampal neurogenesis and hippocampus-dependent memory.

The Effects of Gene-Environment Interactions Between Cadmium Exposure and Apolipoprotein E4 on Memory in a Mouse Model of Alzheimer's Disease.

Cadmium (Cd) is a heavy metal of great public health concern. Recent studies suggested a link between Cd exposure and cognitive decline in humans. The ε4 allele, compared with the common ε3 allele, of the human apolipoprotein E gene (ApoE) is associated with accelerated cognitive decline and increased risks for Alzheimer's disease (AD). To investigate the gene-environment interactions (GxE) between ApoE-ε4 and Cd exposure on cognition, we used a mouse model of AD that expresses human ApoE-ε3 (ApoE3-KI [knock-in]) or ApoE-ε4 (ApoE4-KI). Mice were exposed to 0.6 mg/l CdCl2 through drinking water for 14 weeks and assessed for hippocampus-dependent memory. A separate cohort was sacrificed immediately after exposure and used for Cd measurements and immunostaining. The peak blood Cd was 0.3-0.4 µg/l, within levels found in the U.S. general population. All Cd-treated animals exhibited spatial working memory deficits in the novel object location test. This deficit manifested earlier in ApoE4-KI mice than in ApoE3-KI within the same sex and earlier in males than females within the same genotype. ApoE4-KI but not ApoE3-KI mice exhibited reduced spontaneous alternation later in life in the T-maze test. Finally, Cd exposure impaired neuronal differentiation of adult-born neurons in the hippocampus of male ApoE4-KI mice. These data suggest that a GxE between ApoE4 and Cd exposure leads to accelerated cognitive impairment and that impaired adult hippocampal neurogenesis may be one of the underlying mechanisms. Furthermore, male mice were more susceptible than female mice to this GxE effect when animals were young.

Cadmium Exposure Impairs Adult Hippocampal Neurogenesis.

Cadmium (Cd) is an environmental pollutant of considerable interest throughout the world and potentially a neurotoxicant. Our recent data indicate that Cd exposure induces impairment of hippocampus-dependent learning and memory in mice. However, the underlying mechanisms for this defect are not known. The goal of this study was to determine if Cd inhibits adult neurogenesis and to identify underlying signaling pathways responsible for this impairment. Adult hippocampal neurogenesis is a process in which adult neural progenitor/stem cells (aNPCs) in the subgranular zone (SGZ) of the dentate gyrus (DG) generate functional new neurons in the hippocampus which contributes to hippocampus-dependent learning and memory. However, studies concerning the effects of neurotoxicants on adult hippocampal neurogenesis and the underlying signaling mechanisms are limited. Here, we report that Cd significantly induces apoptosis, inhibits proliferation, and impairs neuronal differentiation in primary cultured aNPCs derived from the SGZ. In addition, the c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase signaling pathways are activated by Cd and contribute to its toxicity. Furthermore, we exposed 8-week-old male C57BL/6 mice to Cd through drinking water for 13 weeks to assess the effects of Cd on adult hippocampal neurogenesis in vivo. Cd treatment reduced the number of 5-week-old adult-born cells in the DG and impaired the differentiation of adult-born hippocampal neurons. These results suggest that Cd exposure impairs adult hippocampal neurogenesis both in vitro and in vivo. This may contribute to Cd-mediated inhibition of hippocampus-dependent learning and memory.

Cadmium Exposure Impairs Cognition and Olfactory Memory in Male C57BL/6 Mice.

Cadmium (Cd) is a heavy metal of high interest to the superfund initiative. Recent epidemiology studies have suggested a possible association between Cd exposure and cognitive as well as olfactory impairments in humans. However, studies in animal models are needed to establish a direct causal relationship between Cd exposure and impairments in cognition and olfaction. This study aims to investigate the toxic effect of Cd on cognition and olfactory function in mice. One group of 8-week-old C57BL/6 male mice was exposed to 3 mg/l Cd (in the form of CdCl2) through drinking water for 20 weeks for behavior tests and final blood Cd concentration analysis. The behavior tests were conducted before, during, and after Cd exposure to analyze the effects of Cd on cognition and olfactory function. Upon completion of behavior tests, blood was collected to measure final blood Cd concentration. Two additional groups of mice were similarly exposed to Cd for 5 or 13 weeks for peak blood Cd concentration measurement. The peak blood Cd concentration was 2.125-2.25 µg/l whereas the final blood Cd concentration was 0.18 µg/l. At this exposure level, Cd impaired hippocampus-dependent learning and memory in novel object location test, T-maze test, and contextual fear memory test. It also caused deficits in short-term olfactory memory and odor-cued olfactory learning and memory. Results in this study demonstrate a direct relationship between Cd exposure and cognitive as well as olfactory impairments in an animal model.

Conditional Inhibition of Adult Neurogenesis by Inducible and Targeted Deletion of ERK5 MAP Kinase Is Not Associated with Anxiety/Depression-Like Behaviors

Although there is evidence that adult neurogenesis contributes to the therapeutic efficacy of chronic antidepressant treatment for anxiety and depression disorders, the role of adult neurogenesis in the onset of depression-related symptoms is still open to question. To address this issue, we utilized a transgenic mouse strain in which adult neurogenesis was specifically and conditionally impaired by Nestin-CreER-driven, inducible knockout (icKO) of erk5 MAP kinase in Nestin-expressing neural progenitors of the adult mouse brain upon tamoxifen administration. Here, we report that inhibition of adult neurogenesis by this mechanism is not associated with an increase of the baseline anxiety or depression in non-stressed animals, nor does it increase the animal's susceptibility to depression after chronic unpredictable stress treatment. Our findings indicate that impaired adult neurogenesis does not lead to anxiety or depression.

Inducible activation of ERK5 MAP kinase enhances adult neurogenesis in the olfactory bulb and improves olfactory function.

Recent discoveries have suggested that adult neurogenesis in the subventricular zone (SVZ) and olfactory bulb (OB) may be required for at least some forms of olfactory behavior in mice. However, it is unclear whether conditional and selective enhancement of adult neurogenesis by genetic approaches is sufficient to improve olfactory function under physiological conditions or after injury. Furthermore, specific signaling mechanisms regulating adult neurogenesis in the SVZ/OB are not fully defined. We previously reported that ERK5, a MAP kinase selectively expressed in the neurogenic regions of the adult brain, plays a critical role in adult neurogenesis in the SVZ/OB. Using a site-specific knock-in mouse model, we report here that inducible and targeted activation of the endogenous ERK5 in adult neural stem/progenitor cells enhances adult neurogenesis in the OB by increasing cell survival and neuronal differentiation. This conditional ERK5 activation also improves short-term olfactory memory and odor-cued associative olfactory learning under normal physiological conditions. Furthermore, these mice show enhanced recovery of olfactory function and have more adult-born neurons after a zinc sulfate-induced lesion of the main olfactory epithelium. We conclude that ERK5 MAP kinase is an important endogenous signaling pathway regulating adult neurogenesis in the SVZ/OB, and that conditional activation of endogenous ERK5 is sufficient to enhance adult neurogenesis in the OB thereby improving olfactory function both under normal conditions and after injury.

Genetic activation of ERK5 MAP kinase enhances adult neurogenesis and extends hippocampus-dependent long-term memory.

Recent studies have shown that inhibition of adult neurogenesis impairs the formation of hippocampus-dependent memory. However, it is not known whether increasing adult neurogenesis affects the persistence of hippocampus-dependent long-term memory. Furthermore, signaling mechanisms that regulate adult neurogenesis are not fully defined. We recently reported that the conditional and targeted knock-out of ERK5 MAP kinase in adult neurogenic regions of the mouse brain attenuates adult neurogenesis in the hippocampus and disrupts several forms of hippocampus-dependent memory. Here, we developed a gain-of-function knock-in mouse model to specifically activate endogenous ERK5 in the neurogenic regions of the adult brain. We report that the selective and targeted activation of ERK5 increases adult neurogenesis in the dentate gyrus by enhancing cell survival, neuronal differentiation, and dendritic complexity. Conditional ERK5 activation also improves the performance of challenging forms of spatial learning and memory and extends hippocampus-dependent long-term memory. We conclude that enhancing signal transduction of a single signaling pathway within adult neural stem/progenitor cells is sufficient to increase adult neurogenesis and improve the persistence of hippocampus-dependent memory. Furthermore, activation of ERK5 may provide a novel therapeutic target to improve long-term memory.

Extracellular signal-regulated kinase 5 (ERK5) mediates prolactin-stimulated adult neurogenesis in the subventricular zone and olfactory bulb.

Prolactin-stimulated adult neurogenesis in the subventricular zone (SVZ) and olfactory bulb (OB) mediates several reproductive behaviors including mating/pregnancy, dominant male pheromone preference in females, and paternal recognition of offspring. However, downstream signaling mechanisms underlying prolactin-induced adult neurogenesis are completely unknown. We report here for the first time that prolactin activates extracellular signal-regulated kinase 5 (ERK5), a MAP kinase that is specifically expressed in the neurogenic regions of the adult mouse brain. Knockdown of ERK5 by retroviral infection of shRNA attenuates prolactin-stimulated neurogenesis in SVZ-derived adult neural stem/progenitor cells (aNPCs). Inducible erk5 deletion in adult neural stem cells of transgenic mice inhibits neurogenesis in the SVZ and OB following prolactin infusion or mating/pregnancy. These results identify ERK5 as a novel and critical signaling mechanism underlying prolactin-induced adult neurogenesis.