Ana M.M. Oliveira

Rescue of aging-associated decline in Dnmt3a2 expression restores cognitive abilities

Cognitive abilities decline in normal aging, yet the underlying molecular mechanisms are poorly understood. We found that aging was associated with a decrease in the expression of the DNA methyltransferase Dnmt3a2 in the hippocampus and that rescuing Dnmt3a2 levels restored cognitive functions. Moreover, we found that Dnmt3a2 is an activity-regulated immediate early gene that is partly dependent on nuclear calcium signaling and that hippocampal Dnmt3a2 levels determine cognitive abilities in both young adult and aged mice.

Post-training reversible inactivation of the hippocampus enhances novel object recognition memory

Research on the role of the hippocampus in object recognition memory has produced conflicting results. Previous studies have used permanent hippocampal lesions to assess the requirement for the hippocampus in the object recognition task. However, permanent hippocampal lesions may impact performance through effects on processes besides memory consolidation including acquisition, retrieval, and performance. To overcome this limitation, we used an intrahippocampal injection of the GABA agonist muscimol to reversibly inactivate the hippocampus immediately after training mice in two versions of an object recognition task. We found that the inactivation of the dorsal hippocampus after training impairs object-place recognition memory but enhances novel object recognition (NOR) memory. However, inactivation of the dorsal hippocampus after repeated exposure to the training context did not affect object recognition memory. Our findings suggest that object recognition memory formation does not require the hippocampus and, moreover, that activity in the hippocampus can interfere with the consolidation of object recognition memory when object information encoding occurs in an unfamiliar environment.

Subregion-specific p300 conditional knock-out mice exhibit long-term memory impairments

Histone acetylation plays a critical role during long-term memory formation. Several studies have demonstrated that the histone acetyltransferase (HAT) CBP is required during long-term memory formation, but the involvement of other HAT proteins has not been extensively investigated. The HATs CBP and p300 have at least 400 described interacting proteins including transcription factors known to play a role in long-term memory formation. Thus, CBP and p300 constitute likely candidates for transcriptional coactivators in memory formation. In this study, we took a loss-of-function approach to evaluate the role of p300 in long-term memory formation. We used conditional knock-out mice in which the deletion of p300 is restricted to the postnatal phase and to subregions of the forebrain. We found that p300 is required for the formation of long-term recognition memory and long-term contextual fear memory in the CA1 area of the hippocampus and cortical areas.

Nuclear Calcium-VEGFD Signaling Controls Maintenance of Dendrite Arborization Necessary for Memory Formation

The role of neuronal dendrites is to receive and process synaptic inputs. The geometry of the dendritic arbor can undergo neuronal activity-dependent changes that may impact the cognitive abilities of the organism. Here we show that vascular endothelial growth factor D (VEGFD), commonly known as an angiogenic mitogen, controls the total length and complexity of dendrites both in cultured hippocampal neurons and in the adult mouse hippocampus. VEGFD expression is dependent upon basal neuronal activity and requires nuclear calcium-calmodulin-dependent protein kinase IV (CaMKIV) signaling. Suppression of VEGFD expression in the mouse hippocampus by RNA interference causes memory impairments. Thus, nuclear calcium-VEGFD signaling mediates the effect of neuronal activity on the maintenance of dendritic arbors in the adult hippocampus and is required for cognitive functioning. These results suggest that caution be employed in the clinical use of blockers of VEGFD signaling for antiangiogenic cancer therapy.

Calcium signaling in cognition and aging‐dependent cognitive decline

Calcium-dependent signals are key triggers of the molecular mechanisms underlying learning and memory and dysregulation of calcium homeostasis in the aging brain has been proposed to underlie aging-dependent cognitive decline. Mechanisms triggered by calcium in neurons include activity-dependent activation of transcription responsible for the synthesis of molecules underlying the long-term changes of neuronal function. Effectors of calcium signaling with a primordial role in transcription regulation are calcium signal-regulated transcription factors. In this review, we summarize the current knowledge of the contribution of key calcium signal-regulated transcription factors, namely CREB, NFAT, and DREAM, to memory formation. We further describe evidence for dysregulation of the activity of these factors during aging.

Dnmt3a2: a hub for enhancing cognitive functions.

The mechanisms responsible for fear memory formation and extinction are far from being understood. Uncovering the molecules and mechanisms regulating these processes is vital for identifying molecular targets for the development of novel therapeutic strategies for anxiety and fear disorders. Cognitive abilities require the activation of gene expression necessary to the consolidation of lasting changes in neuronal function. In this study we established a key role for an epigenetic factor, the de novo DNA methyltransferase, Dnmt3a2, in memory formation and extinction. We found that Dnmt3a2 overexpression in the hippocampus of young adult mice induced memory enhancements in a variety of situations; it converted a weak learning experience into long-term memory, enhanced fear memory formation and facilitated fear memory extinction. Dnmt3a2 overexpression was also associated with the increased expression of plasticity-related genes. Furthermore, the knockdown of Dnmt3a2 expression impaired the animals’ ability to extinguish memories, identifying Dnmt3a2 as a key player in extinction. Thus, Dnmt3a2 is at the core of memory processes and represents a novel target for cognition-enhancing therapies to ameliorate anxiety and fear disorders and boost memory consolidation.

The calmodulin-binding transcription activator CAMTA1 is required for long-term memory formation in mice

The formation of long-term memory requires signaling from the synapse to the nucleus to mediate neuronal activity-dependent gene transcription. Synapse-to-nucleus communication is initiated by influx of calcium ions through synaptic NMDA receptors and/or L-type voltage-gated calcium channels and involves the activation of transcription factors by calcium/calmodulin signaling in the nucleus. Recent studies have drawn attention to a new family of transcriptional regulators, the so-called calmodulin-binding transcription activator (CAMTA) proteins. CAMTAs are expressed at particularly high levels in the mouse and human brain, and we reasoned that, as calmodulin-binding transcription factors, CAMTAs may regulate the formation of long-term memory by coupling synaptic activity and calcium/calmodulin signaling to memory-related transcriptional responses. This hypothesis is supported by genetic studies that reported a correlation between Camta gene polymorphisms or mutations and cognitive capability in humans. Here, we show that acute knockdown of CAMTA1, but not CAMTA2, in the hippocampus of adult mice results in impaired performance in two memory tests, contextual fear conditioning and object-place recognition test. Short-term memory and neuronal morphology were not affected by CAMTA knockdown. Gene expression profiling in the hippocampus of control and CAMTA knockdown mice revealed a number of putative CAMTA1 target genes related to synaptic transmission and neuronal excitability. Patch clamp recordings in organotypic hippocampal slice cultures provided further evidence for CAMTA1-dependent changes in electrophysiological properties. In summary, our study provides experimental evidence that confirms previous human genetic studies and establishes CAMTA1 as a regulator of long-term memory formation.

Dysfunction of neuronal calcium signaling in aging and disease

Calcium has been selected by evolution as a key regulator of cellular functions. Many physiological processes are dependent on intracellular calcium. This has required cells to develop efficient mechanisms for the tight control of calcium ho-meostasis. Basal cytosolic and nuclear calcium levels are low but they can increase dramatically within milliseconds after exposure to external stimuli. The way in which the cell responds to changes in calcium concentration depends on the magnitude, temporal and spatial resolution of the calcium transients. In the central nervous system, calcium signals have numerous functions. These include synaptic transmission, which is central to neuronal communication and several forms of synaptic plasticity, behavioral adaptions and gene transcription. Since many vital physiological processes depend on intracellular calcium, disturbances of calcium homeo-stasis can lead to cellular dysfunction and degeneration. In this special issue, a collection of review articles addresses the dysregulation of calcium homeostasis and associated proteins in aging and various pathological conditions of the nervous system. In addition, the reviews discuss new therapeutic approaches aimed at the normalization of calcium-dependent mechanisms. Calcium plays a critical role in the regulation of neuronal excitability. This can be achieved directly, via the modulation of the activity of ion channels and indirectly, through the activation of signaling pathways. Therefore, unsurprisingly, calcium is viewed as an important factor in neuronal hyperexcitability, seizure generation and epileptogenesis. Steinlein (2014) describes the way that calcium-dependent signaling is involved in epilepsy. The author discusses both genetic epilepsies, in which mutations in calcium channels have been detected and acquired epilepsy, which is the consequence of brain trauma, stroke, infection, or tumors. Insults such as brain trauma and stroke produce an intracellular calcium overload that can render neuronal networks vulnerable to seizures and even trigger cell death. The author acknowledges the fact that, even if the dissection of the calcium signaling pathways involved in epilepsy is a daunting task, it promises to uncover targets for the development of new anti-epileptic drugs. Schaefer et al. (2014) focus on the effects of the cerebral ischemia that is associated with various brain insults and on proteins involved in mitochondrial calcium homeostasis. Ischemia is defined as a decrease in blood and oxygen supply to the brain caused by constriction or obstruction of blood vessels. Oxygen deprivation disrupts oxidative phosphorylation and thus ATP synthesis. Under physiological conditions, calcium influx into mitochondria can stimulate mitochondrial ATP production in order to meet the energy demands …

Dnmt3a2 in the nucleus accumbens shell is required for reinstatement of cocaine seeking

Epigenetic mechanisms have gained increasing attention as regulators of synaptic plasticity and responsiveness to drugs of abuse. In particular, it has been shown that the activity of the DNA methyltransferase 3a (Dnmt3a) mediates certain long-lasting effects of cocaine. Here we examined the role of the Dnmt isoforms, Dnmt3a1 and Dnmt3a2, within the nucleus accumbens (NAc) on transcriptional activity of immediate early genes (IEGs) and acute and long-lasting responsiveness to cocaine and cocaine conditioned cues. Using primary striatal cultures, we show that transcription of Dnmt3a2, but not that of Dnmt3a1, is activated by dopamine D1 receptor signaling and that knockdown of Dnmt3a2 using viral vector-mediated expression of Dnmt3a2-specific shRNAs impairs induction of the IEGs, Arc, FosB, and Egr2. Acute cocaine administration increases expression of Dnmt3a2 but not that of Dnmt3a1 in the NAc shell. In contrast, in the NAc core, expression of Dnmt3a1 and Dnmt3a2 was unaffected by cocaine administration. shRNA-mediated knockdown of Dnmt3a2 in vivo impairs the induction of IEGs, including Egr2 and FosB indicating that Dnmt3a2 regulates cocaine-dependent expression of plasticity genes in the rat NAc shell. Cocaine self-administration experiments in rats revealed that Dnmt3a2 regulates drug cue memories that drive reinstatement of cocaine seeking as well as incubation of this phenomenon within the NAc shell. Dnmt3a2 does not influence the primary reinforcing effects of cocaine. Thus, Dnmt3a2 mediates long-lasting cocaine cue memories within the NAc shell. Targeting Dnmt3a2 expression or function may interfere with cocaine craving and relapse. SIGNIFICANCE STATEMENT In humans, drug craving can occur in response to conditioned cues, even after extended periods of abstinence. In rats, cue-induced cocaine seeking has been shown to increase progressively during the first 2 months of abstinence from drug self-administration. This phenomenon, referred to as incubation of cocaine seeking, is consistent with the hypothesis that in humans craving increases over time and remains high following prolonged abstinence. Those long-lasting behavioral changes are likely to be mediated by epigenetic effects and neuroplastic changes within the mesolimbic brain reward system. Here we show that a specific isoform of DNA-methyltransferases in the NAc shell regulates drug cue memories that drive reinstatement of cocaine seeking after both early abstinence and incubation of cocaine craving.

Adult hippocampal MeCP2 preserves the genomic responsiveness to learning required for long-term memory formation

HIGHLIGHTSMeCP2 in the adult hippocampus is required for long‐term memory formation.MeCP2 maintains the chromatin features of mature CA1 neurons.MeCP2 preserves the genomic responsiveness to hippocampus‐dependent learning. ABSTRACT MeCP2 is required both during postnatal neurodevelopment and throughout the adult life for brain function. Although it is well accepted that MeCP2 in the maturing nervous system is critical for establishing normal development, the functions of MeCP2 during adulthood are poorly understood. Particularly, the requirement of hippocampal MeCP2 for cognitive abilities in the adult is not studied. To characterize the role of MeCP2 in adult neuronal function and cognition, we used a temporal and region‐specific disruption of MeCP2 expression in the hippocampus of adult male mice. We found that MeCP2 is required for long‐term memory formation and that it controls the learning‐induced transcriptional response of hippocampal neurons required for memory consolidation. Furthermore, we uncovered MeCP2 functions in the adult hippocampus that may underlie cognitive integrity. We showed that MeCP2 maintains the developmentally established chromatin configuration and epigenetic landscape of CA1 neurons throughout the adulthood, and that it regulates the expression of neuronal and immune‐related genes in the adult hippocampus. Overall, our findings identify MeCP2 as a maintenance factor in the adult hippocampus that preserves signal responsiveness of the genome and allows for integrity of cognitive functions. This study provides new insight into how MeCP2 maintains adult brain functions, but also into the mechanisms underlying the cognitive impairments observed in RTT patients and highlights the understudied role of DNA methylation interpretation in adult cognitive processes.