Increases in cAMP, MAPK activity, and CREB phosphorylation during REM sleep: implications for REM sleep and memory consolidation.

The cyclic adenosine monophosphate (cAMP), mitogen-activated protein kinase (MAPK), and cAMP response element-binding protein (CREB) transcriptional pathway is required for consolidation of hippocampus-dependent memory. In mice, this pathway undergoes a circadian oscillation required for memory persistence that reaches a peak during the daytime. Because mice exhibit polyphasic sleep patterns during the day, this suggested the interesting possibility that cAMP, MAPK activity, and CREB phosphorylation may be elevated during sleep. Here, we report that cAMP, phospho-p44/42 MAPK, and phospho-CREB are higher in rapid eye movement (REM) sleep compared with awake mice but are not elevated in non-REM sleep. This peak of activity during REM sleep does not occur in mice lacking calmodulin-stimulated adenylyl cyclases, a mouse strain that learns but cannot consolidate hippocampus-dependent memory. We conclude that a preferential increase in cAMP, MAPK activity, and CREB phosphorylation during REM sleep may contribute to hippocampus-dependent memory consolidation.

Inducible and conditional deletion of extracellular signal-regulated kinase 5 disrupts adult hippocampal neurogenesis.

Recent studies have led to the exciting idea that adult-born neurons in the dentate gyrus of the hippocampus may play a role in hippocampus-dependent memory formation. However, signaling mechanisms that regulate adult hippocampal neurogenesis are not well defined. Here we report that extracellular signal-regulated kinase 5 (ERK5), a member of the mitogen-activated protein kinase family, is selectively expressed in the neurogenic regions of the adult mouse brain. We present evidence that shRNA suppression of ERK5 in adult hippocampal neural stem/progenitor cells (aNPCs) reduces the number of neurons while increasing the number of cells expressing markers for stem/progenitor cells or proliferation. Furthermore, shERK5 attenuates both transcription and neuronal differentiation mediated by Neurogenin 2, a transcription factor expressed in adult hippocampal neural progenitor cells. By contrast, ectopic activation of endogenous ERK5 signaling via expression of constitutive active MEK5, an upstream activating kinase for ERK5, promotes neurogenesis in cultured aNPCs and in the dentate gyrus of the mouse brain. Moreover, neurotrophins including NT3 activate ERK5 and stimulate neuronal differentiation in aNPCs in an ERK5-dependent manner. Finally, inducible and conditional deletion of ERK5 specifically in the neurogenic regions of the adult mouse brain delays the normal progression of neuronal differentiation and attenuates adult neurogenesis in vivo. These data suggest ERK5 signaling as a critical regulator of adult hippocampal neurogenesis.

Overexpression of type I adenylyl cyclase in the forebrain impairs spatial memory in aged but not young mice.

Hippocampus-dependent memory requires a cAMP signal that is generated by Ca2+-stimulated adenylyl cyclases (AC1, AC8). Young transgenic mice overexpressing AC1 in the forebrain (AC1+ mice) have enhanced hippocampal long-term potentiation, superior memory for novel object recognition and more persistent remote contextual memory. To determine whether increasing AC1 expression improves memory when older mice are trained, we analyzed fear, recognition, and spatial memory in mice aged to 25 months. Here we report that young adult AC1+ mice have enhanced social recognition memory, and normal fear and spatial memory. Surprisingly, aged AC1+ mice had poorer spatial memory than age-matched wild-type littermates. These data suggest that the decrease in Ca2+-stimulated adenylyl cyclase activity during aging of wild-type mice may be an adaptive mechanism required to maintain spatial memory function.

Neuronal activity rapidly induces transcription of the CREB-regulated microRNA-132, in vivo.

Activity-dependent changes in gene-expression are believed to underlie the molecular representation of memory. In this study, we report that in vivo activation of neurons rapidly induces the CREB-regulated microRNA miR-132. To determine if production of miR-132 is regulated by neuronal activity its expression in mouse brain was monitored by quantitative RT-PCR (RT-qPCR). Pilocarpine-induced seizures led to a robust, rapid, and transient increase in the primary transcript of miR-132 (pri-miR-132) followed by a subsequent rise in mature microRNA (miR-132). Activation of neurons in the hippocampus, olfactory bulb, and striatum by contextual fear conditioning, odor-exposure, and cocaine-injection, respectively, also increased pri-miR-132. Induction kinetics of pri-miR-132 were monitored and found to parallel those of immediate early genes, peaking at 45 min and returning to basal levels within 2 h of stimulation. Expression levels of primary and mature-miR-132 increased significantly between postnatal Days 10 and 24. We conclude that miR-132 is an activity-dependent microRNA in vivo, and may contribute to the long-lasting proteomic changes required for experience-dependent neuronal plasticity.

PI3 kinase signaling is required for retrieval and extinction of contextual memory.

Memory retrieval is a dynamic aspect of memory formation that can be studied separately from other stages of memory processing. Although several signal transduction pathways including ERK/MAP kinase have been implicated in memory retrieval, the underlying signaling events are poorly defined. Here we report that re-exposure of mice to context after contextual training stimulates the activity of phosphatidylinositol 3 kinase (PI3K) in the hippocampus. Inhibition of PI3K activity in the hippocampus in vivo blocked contextual memory retrieval and extinction. Inhibitors of PI3K signaling also blocked increases in ERK/MAP kinase activity associated with memory retrieval. This suggests that PI3K activation in the hippocampus is critical for memory retrieval and is required for activation of ERK/MAP kinase during retrieval.

Chronic rapamycin treatment or lack of S6K1 does not reduce ribosome activity in vivo.

Reducing activity of the mTORC1/S6K1 pathway has been shown to extend lifespan in both vertebrate and invertebrate models. For instance, both pharmacological inhibition of mTORC1 with the drug rapamycin or S6K1 knockout extends lifespan in mice. Since studies with invertebrate models suggest that reducing translational activity can increase lifespan, we reasoned that the benefits of decreased mTORC1 or S6K1 activity might be due, at least in part, to a reduction of general translational activity. Here, we report that mice given a single dose of rapamycin have reduced translational activity, while mice receiving multiple injections of rapamycin over 4 weeks show no difference in translational activity compared with vehicle-injected controls. Furthermore, mice lacking S6K1 have no difference in global translational activity compared with wild-type littermates as measured by the percentage of ribosomes that are active in multiple tissues. Translational activity is reduced in S6K1-knockout mice following single injection of rapamycin, demonstrating that rapamycin's effects on translation can occur independently of S6K1. Taken together, these data suggest that benefits of chronic rapamycin treatment or lack of S6K1 are dissociable from potential benefits of reduced translational activity, instead pointing to a model whereby changes in translation of specific subsets of mRNAs and/or translation-independent effects of reduced mTOR signaling underlie the longevity benefits.

Rapamycin reverses elevated mTORC1 signaling in lamin A/C-deficient mice, rescues cardiac and skeletal muscle function, and extends survival.

Mutations in LMNA, the gene that encodes A-type lamins, cause multiple diseases including dystrophies of the skeletal muscle and fat, dilated cardiomyopathy, and progeria-like syndromes (collectively termed laminopathies). Reduced A-type lamin function, however, is most commonly associated with skeletal muscle dystrophy and dilated cardiomyopathy rather than lipodystrophy or progeria. The mechanisms underlying these diseases are only beginning to be unraveled. We report that mice deficient in Lmna, which corresponds to the human gene LMNA, have enhanced mTORC1 (mammalian target of rapamycin complex 1) signaling specifically in tissues linked to pathology, namely, cardiac and skeletal muscle. Pharmacologic reversal of elevated mTORC1 signaling by rapamycin improves cardiac and skeletal muscle function and enhances survival in mice lacking A-type lamins. At the cellular level, rapamycin decreases the number of myocytes with abnormal desmin accumulation and decreases the amount of desmin in both muscle and cardiac tissue of Lmna(-/-) mice. In addition, inhibition of mTORC1 signaling with rapamycin improves defective autophagic-mediated degradation in Lmna(-/-) mice. Together, these findings point to aberrant mTORC1 signaling as a mechanistic component of laminopathies associated with reduced A-type lamin function and offer a potential therapeutic approach, namely, the use of rapamycin-related mTORC1 inhibitors.

TOR on the brain.

Signaling by target of rapamycin (mTOR in mammals) has been shown to modulate lifespan in several model organisms ranging from yeast to mice. In mice, reduced mTOR signaling by chronic rapamycin treatment leads to life span extension, raising the possibility that rapamycin and its analogs may benefit the aging brain and serve as effective treatments of age-related neurodegenerative diseases. Here, we review mTOR signaling and how neurons utilize mTOR to regulate brain function, including regulation of feeding, synaptic plasticity and memory formation. Additionally, we discuss recent findings that evaluate the mechanisms by which reduced mTOR activity might benefit the aging brain in normal and pathological states. We will focus on recent studies investigating mTOR and Alzheimer's disease, Parkinson's disease, and polyglutamine expansion syndromes such as Huntington's disease.

Activation of dopamine neurons is critical for aversive conditioning and prevention of generalized anxiety.

Generalized anxiety is thought to result, in part, from impairments in contingency awareness during conditioning to cues that predict aversive or fearful outcomes. Dopamine neurons of the ventral midbrain exhibit heterogeneous responses to aversive stimuli that are thought to provide a critical modulatory signal to facilitate orientation to environmental changes and assignment of motivational value to unexpected events. Here we describe a mouse model in which activation of dopamine neurons in response to an aversive stimulus is attenuated by conditional genetic inactivation of functional NMDA receptors on dopamine neurons. We discovered that altering the magnitude of excitatory responses by dopamine neurons in response to an aversive stimulus was associated with impaired conditioning to a cue that predicts an aversive outcome. Impaired conditioning by these mice was associated with the development of a persistent, generalized anxiety-like phenotype. These data are consistent with a role for dopamine in facilitating contingency awareness that is critical for the prevention of generalized anxiety.