DNA methylation impacts on learning and memory in aging

Learning and memory are two of the fundamental cognitive functions that confer us the ability to accumulate knowledge from our experiences. Although we use these two mental skills continuously, understanding the molecular basis of learning and memory is very challenging. Methylation modification of DNA is an epigenetic mechanism that plays important roles in regulating gene expression, which is one of the key processes underlying the functions of cells including neurons. Interestingly, a genome-wide decline in DNA methylation occurs in the brain during normal aging, which coincides with a functional decline in learning and memory with age. It has been speculated that DNA methylation in neurons might be involved in memory coding. However, direct evidence supporting the role of DNA methylation in memory formation is still under investigation. This particular function of DNA methylation has not drawn wide attention despite several important studies that have provided supportive evidence for the epigenetic control of memory formation. To facilitate further exploration of the epigenetic basis of memory function, we will review existing studies on DNA methylation that are related to the development and function of the nervous system. We will focus on studies illustrating how DNA methylation regulates neural activities and memory formation via the control of gene expression in neurons, and relate these studies to various age-related neurological disorders that affect cognitive functions.     

胃肠激素在记忆调节中的作用研究进展

脑-肠轴,即脑与胃肠激素之间的相互作用关系已研究多年。脑-肠轴是一种双向的信息交流,它不但调节了胃肠功能的稳态而且与更高级的情绪与认知功能相关。进食时,消化道会分泌很多激素并通过外周途径影响不同的组织器官,包括大脑。而下丘脑在能量代谢、营养平衡以及摄食行为调控中起到重要的作用,它能将内脏功能与海马、杏仁核及皮层等肢体功能进一步整合起来。这些脑区主要参与大脑的高级功能,如认知功能、及情绪的调节。现对近年来不同胃肠激素对学习与记忆的调节功能进行综述。了解这类激素在记忆中的作用,将有助于寻找治疗与学习记忆障碍相关神经疾病的新型治疗策略。     

RET codon 804 mutations in multiple endocrine neoplasia 2: genotype–phenotype correlations and implications in clinical management

The risk of developing neurodegenerative disorders such as Alzheimer's disease or Parkinson's disease is influenced by genetic and environmental factors. Environmental events occurring during development or later in life can be related to disease susceptibility. One way by which the environment may exert its effect is through epigenetic modifications, which might affect the functioning of genes. These include nucleosome positioning, post-translational histone modifications, and DNA methylation. In this review we will focus in the potential role of DNA methylation in neurodegenerative disorders and in the approaches to explore such epigenetic changes. Advances in deciphering the role of epigenetic modifications in phenotype are being uncovered for a variety of diseases, including cancer, autoimmune, neurodevelopmental and cognitive disorders. Epigenetic modifications are now being also associated with cardiovascular and metabolic traits, and they are expected to be especially involved in learning and memory processes, as well as in neurodegenerative disease. The study of the role of methylation and other epigenetic modifications in disease development will provide new insights in the etiopathogenesis of neurodegenerative disorders, and should hopefully shape new avenues in the development of therapeutic strategies.     

miRNA-132: a dynamic regulator of cognitive capacity

Within the central nervous system, microRNAs have emerged as important effectors of an array of developmental, physiological, and cognitive processes. Along these lines, the CREB-regulated microRNA miR-132 has been shown to influence neuronal maturation via its effects on dendritic arborization and spinogenesis. In the mature nervous system, dysregulation of miR-132 has been suggested to play a role in a number of neurocognitive disorders characterized by aberrant synaptogenesis. However, little is known about the inducible expression and function of miR-132 under normal physiological conditions in vivo. Here, we begin to explore this question within the context of learning and memory. Using in situ hybridization, we show that the presentation of a spatial memory task induced a significant ~1.5-fold increase in miR-132 expression within the CA1, CA3, and GCL excitatory cell layers of the hippocampus. To examine the role of miR-132 in hippocampal-dependent learning and memory, we employ a doxycycline-regulated miR-132 transgenic mouse strain to drive varying levels of transgenic miR-132 expression. These studies revealed that relatively low levels of transgenic miR-132 expression, paralleling the level of expression in the hippocampus following a spatial memory task, significantly enhanced cognitive capacity. In contrast, higher (supra-physiological) levels of miR-132 (>3-fold) inhibited learning. Interestingly, both the impaired cognition and elevated levels of dendritic spines resulting from supra-physiological levels of transgenic miR-132 were reversed by doxycycline suppression of transgene expression. Together, these data indicate that miR-132 functions as a key activity-dependent regulator of cognition, and that miR-132 expression must be maintained within a limited range to ensure normal learning and memory formation.     

Altered function of neuronal L-type calcium channels in ageing and neuroinflammation: Implications in age-related synaptic dysfunction and cognitive decline

The rapid developments in science have led to an increase in human life expectancy and thus, ageing and age-related disorders/diseases have become one of the greatest concerns in the 21st century. Cognitive abilities tend to decline as we get older. This age-related cognitive decline is mainly attributed to aberrant changes in synaptic plasticity and neuronal connections. Recent studies show that alterations in Ca²⁺ homeostasis underlie the increased vulnerability of neurons to age-related processes like cognitive decline and synaptic dysfunctions. Dysregulation of Ca²⁺ can lead to dramatic changes in neuronal functions. We discuss in this review, the recent advances on the potential role of dysregulated Ca²⁺ homeostasis through altered function of L-type voltage gated Ca²⁺ channels (LTCC) in ageing, with an emphasis on cognitive decline. This review therefore focuses on age-related changes mainly in the hippocampus, and with mention of other brain areas, that are important for learning and memory. This review also highlights age-related memory deficits via synaptic alterations and neuroinflammation. An understanding of these mechanisms will help us formulate strategies to reverse or ameliorate age-related disorders like cognitive decline.     

Cholinergic receptors: functional role of nicotinic ACh receptors in brain circuits and disease

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability throughout the nervous system by acting on both the cys-loop ligand-gated nicotinic ACh receptor channels (nAChRs) and the G protein-coupled muscarinic ACh receptors (mAChRs). The hippocampus is an important area in the brain for learning and memory, where both nAChRs and mAChRs are expressed. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca, the activation of which can activate both nAChRs and mAChRs in the hippocampus and regulate synaptic communication and induce oscillations that are thought to be important for cognitive function. Dysfunction in the hippocampal cholinergic system has been linked with cognitive deficits and a variety of neurological disorders and diseases, including Alzheimer’s disease and schizophrenia. My lab has focused on the role of the nAChRs in regulating hippocampal function, from understanding the expression and functional properties of the various subtypes of nAChRs, and what role these receptors may be playing in regulating synaptic plasticity. Here, I will briefly review this work, and where we are going in our attempts to further understand the role of these receptors in learning and memory, as well as in disease and neuroprotection.     

Could decitabine treatment impair memory functions in humans?

The role of epigenetic mechanisms in cognitive functions and neurological/psychiatric disorders has been studied in a number of studies recently. One of these mechanisms is DNA methylation, for which DNA methyltransferases (DNMT) are responsible. Decitabine, or 5-aza-2′-deoxycytidine, is a cytosine-analog DNMT inhibitor and is used in the treatment of certain myelodysplastic syndromes (MDS) subsets. Several studies address the role of DNA methylation and negative effects of decitabine on memory formation and consolidation in animals. We, therefore, hypothesize that standard decitabine treatment for MDS in patients without dementia might cause learning and memory deficits. A clinical trial is proposed to test the hypothesis which could support the role of DNA methylation in cognitive abilities of humans.     

Dissociating the long-term effects of fetal/neonatal iron deficiency on three types of learning in the rat

Iron deficiency (ID) is a common nutrient deficiency worldwide. This condition is linked to changes in myelin formation, dopaminergic function, and energy metabolism. Early ID results in persistent long-term cognitive and behavioral disturbances in children, despite a return to normal iron status. The present study assesses formerly ID adult rats on maze learning tasks that depend on specific brain regions related to learning, specifically the hippocampus, striatum, and amygdala. Rat dams were fed ID chow starting on gestational Day 2 through postnatal Day 7, and behavioral testing began at postnatal Day 65--following a return to normal iron status. Formerly ID rats exhibited delayed acquisition of the hippocampus-dependant task and no differences from controls on the striatum- and amygdala-dependent tasks. These findings likely reflect long-term reduction in but not abolition of hippocampus-dependent learning and preserved function in other brain structures (e.g., striatum and amygdala).     

Disinhibition of the prefrontal cortex leads to brain-wide increases in neuronal activation that are modified by spatial learning

Deficient prefrontal cortex (PFC) GABA function is hypothesized to play a role in schizophrenia and other psychiatric disorders. In rodents, PFC GABA_A receptor antagonism produces cognitive and behavioral changes relevant to these disorders, including impaired spatial memory assessed with the traditional working/reference memory radial maze task. This aspect of spatial memory does not depend on PFC, suggesting that deficient PFC GABAergic transmission may interfere with non-PFC-dependent cognitive functions via aberrant increases in PFC output. To test this, we assessed whether PFC GABA_A antagonism (50 ng bicuculline methbromide) alters neuronal activation in PFC terminal regions, including the striatum, thalamus, hippocampus, amygdala, and cortical regions, of adult male rats using the immediate early gene, c-Fos, as an activity marker. A subset of these animals were also trained and/or tested on the working/reference memory radial maze task. These treatments caused widespread increases in neuronal activation in animals under baseline conditions, with notable exception of the hippocampus. Furthermore, PFC GABA_A antagonism impaired task performance. In most instances, training and/or testing on the radial maze had no additional effects on neuronal activation. However, in both the hippocampus and rhomboid thalamic nucleus, PFC GABA_A antagonism caused a selective increase in neuronal activation in animals trained on the maze. These results indicate that deficiencies in PFC GABAergic transmission may have widespread impacts on neuronal activity that may interfere with certain PFC-independent cognitive functions. Furthermore, these alterations in activity are modulated by plasticity induced by spatial learning in the hippocampus and rhomboid thalamic nucleus.

     

The serotonergic system in ageing and Alzheimer's disease

Alzheimer's disease (AD) is one of the major neurodegenerative diseases that deteriorates cognitive functions and primarily affects associated brain regions involved in learning and memory, such as the neocortex and the hippocampus. Following the discovery and establishment of its role as a neurotransmitter, serotonin (5-HT), was found to be involved in a multitude of neurophysiological processes including mnesic function, through its dedicated pathways and interaction with cholinergic, glutamatergic, GABAergic and dopaminergic transmission systems. Abnormal 5-HT neurotransmission contributes to the deterioration of cognitive processes in ageing, AD and other neuropathologies, including schizophrenia, stress, mood disorders and depression. Numerous studies have confirmed the pathophysiological role of the 5-HT system in AD and that several drugs enhancing 5-HT neurotransmission are effective in treating the AD-related cognitive and behavioural deficits. Here we present a comprehensive overview of the role of serotonergic neurotransmission in brain development, maturation and ageing, discuss its role in higher brain function and provide an in depth account of pathological modifications of serotonergic transmission in neurological diseases and AD.     

DNA methylation and memory formation

Memory formation and storage require long-lasting changes in memory-related neuronal circuits. Recent evidence indicates that DNA methylation may serve as a contributing mechanism in memory formation and storage. These emerging findings suggest a role for an epigenetic mechanism in learning and long-term memory maintenance and raise apparent conundrums and questions. For example, it is unclear how DNA methylation might be reversed during the formation of a memory, how changes in DNA methylation alter neuronal function to promote memory formation, and how DNA methylation patterns differ between neuronal structures to enable both consolidation and storage of memories. Here we evaluate the existing evidence supporting a role for DNA methylation in memory, discuss how DNA methylation may affect genetic and neuronal function to contribute to behavior, propose several future directions for the emerging subfield of neuroepigenetics, and begin to address some of the broader implications of this work.     

Western diet consumption and cognitive impairment: links to hippocampal dysfunction and obesity

Intake of saturated fats and simple carbohydrates, two of the primary components of a modern Western diet, is linked with the development of obesity and Alzheimer's Disease. The present paper summarizes research showing that Western diet intake is associated with cognitive impairment, with a specific emphasis on learning and memory functions that are dependent on the integrity of the hippocampus. The paper then considers evidence that saturated fat and simple carbohydrate intake is correlated with neurobiological changes in the hippocampus that may be related to the ability of these dietary components to impair cognitive function. Finally, a model is described proposing that Western diet consumption contributes to the development of excessive food intake and obesity, in part, by interfering with a type of hippocampal-dependent memory inhibition that is critical in the ability of animals to refrain from responding to environmental cues associated with food, and ultimately from consuming energy intake in excess of that driven solely by caloric need.     

Dimensions of the hippocampus,memory, and learning in the ontogenesis of rats

Damage to the hippocampus in 20-, 50-, and 110-day-old rats impairs the processes of learning and short-term memory in them. In 50-day-old rats, hippocampectomy has less of an influence on the process of learning and memory than for 20- and 110-day-old animals. The anatomic and physiological characteristics of the hippocampus in 20-day-old rats may be evidence of a special importance of this formation at the early stages of ontogenesis, when the cerebral cortex is still insufficiently mature and its associations with other structures have not been entirely formed. The nonlinear nature of the dependence of the disruption of learning in rats of different ages after hippocampectomy suggests that the function of the rat hippocampus undergoes changes during the process of individual development of the animal.Translated from Zhurnal Évolyutsionnoi Biokhimii i Fiziologii, Vol. 12, No. 3, pp. 250–255, May–June, 1976.     

Brain-derived neurotrophic factor modulates the severity of cognitive alterations induced by mutant huntingtin: Involvement of phospholipaseCγ activity and glutamate receptor expression

The involvement of brain-derived neurotrophic factor (BDNF) in cognitive processes and the decrease in its expression in Huntington's disease suggest that this neurotrophin may play a role in learning impairment during the disease progression. We therefore analyzed the onset and severity of cognitive deficits in two different mouse models with the same mutant huntingtin but with different levels of BDNF (R6/1 and R6/1:BDNF+/− mice). We observed that BDNF modulates cognitive function in different learning tasks, even before the onset of motor symptoms. R6/1:BDNF+/− mice showed earlier and more accentuated cognitive impairment than R6/1 mice at 5 weeks of age in discrimination learning; at 5 weeks of age in procedural learning; and at 9 weeks of age in alternation learning. At the earliest age at which cognitive impairment was detected, electrophysiological analysis was performed in the hippocampus. All mutant genotypes showed reduced hippocampal long term potentiation (LTP) with respect to wild type but did not show differences between them. Thus, we evaluated the involvement of BDNF-trkB signaling and glutamate receptor expression in the hippocampus of these mice. We observed a decrease in phospholipaseCγ activity, but not ERK, in R61, BDNF+/− and R6/1:BDNF+/− hippocampus at the age when LTP was altered. However, a specific decrease in the expression of glutamate receptors NR1, NR2A and GluR1 was detected only in R6/1:BDNF+/− hippocampus. Therefore, these results show that BDNF modulates the learning and memory alterations induced by mutant huntingtin. This interaction leads to intracellular changes, such as specific changes in glutamate receptors and in BDNF-trkB signaling through phospholipaseCγ.     

Valproic acid reduces spatial working memory and cell proliferation in the hippocampus

Valproic acid (VPA) is widely used clinically, as an anticonvulsant and mood stabilizer but is, however, also known to block cell proliferation through its ability to inhibit histone deacetylase enzymes. There have been a number of reports of cognitive impairments in patients taking VPA. In this investigation we examined the relationship between cognition and changes in cell proliferation within the hippocampus, a brain region where continued formation of new neurons is associated with learning and memory. Treatment of rats by i.p. injection of VPA, reduced cell proliferation in the sub granular zone of the dentate gyrus within the hippocampus. This was linked to a significant impairment in their ability to perform a hippocampus-dependent spatial memory test (novel object location). In addition, drug treatment caused a significant reduction in brain-derived neurotrophic factor (BDNF) and Notch 1 but not doublecortin levels within the hippocampus. These results support the idea that VPA may cause cognitive impairment and provide a possible mechanism for this by reducing neurogenesis within the hippocampus.     

Gene delivery of Homer1c rescues spatial learning in a rodent model of cognitive aging

Homer1c has been shown to play a role in learning and memory. Overexpression of Homer1c in the hippocampus can improve memory in normal rats and can also rescue spatial learning deficits in Homer1 knockout mice. In a previous study, we found that Homer1c mRNA is upregulated after a spatial learning paradigm in aged rats that successfully learn the task, when compared to aged rats that are learning-impaired (AI). This study was designed to validate the role of Homer1c in successful cognitive aging. In this article, we report that gene delivery of Homer1c into the hippocampus of aged learning-impaired rats significantly improves individual performance on an object location memory task. The learning ability of these rats on the Morris Water Maze was also superior to that of AI control rats. In summary, using 2 independent spatial memory tasks, we demonstrate that Homer1c is sufficient to improve the spatial learning deficits in a rodent model of cognitive aging. These results point to Homer1c as a potential therapeutic target for improving age-related cognitive impairment.     

Deep brain stimulation of fornix for memory improvement in Alzheimer’s disease: A critical review

Memory reflects the brain function in encoding, storage and retrieval of the data or information, which is a fundamental ability for any live organism. The development of approaches to improve memory attracts much attention due to the underlying mechanistic insight and therapeutic potential to treat neurodegenerative diseases with memory loss, such as Alzheimer’s disease (AD). Deep brain stimulation (DBS), a reversible, adjustable, and non-ablative therapy, has been shown to be safe and effective in many clinical trials for neurodegenerative and neuropsychiatric disorders. Among all potential regions with access to invasive electrodes, fornix is considered as it is the major afferent and efferent connection of the hippocampus known to be closely associated with learning and memory. Indeed, clinical trials have demonstrated that fornix DBS globally improved cognitive function in a subset of patients with AD, indicating fornix can serve as a potential target for neurosurgical intervention in treating memory impairment in AD. The present review aims to provide a better understanding of recent progresses in the application of fornix DBS for ameliorating memory impairments in AD patients.     

Forebrain acetylcholine regulates adult hippocampal neurogenesis and learning

Hippocampus-mediated learning enhances neurogenesis in the adult dentate gyrus (DG), and this process has been suggested to be involved in memory formation. The hippocampus receives abundant cholinergic innervation and acetylcholine (ACh) plays an important role in learning and Alzheimer's disease (AD) pathophysiology. Here, we show that a selective neurotoxic lesion of forebrain cholinergic input with 192 IgG-saporin reduces DG neurogenesis with a concurrent impairment in spatial memory. Conversely, systemic administration of the cholinergic agonist physostigmine increases DG neurogenesis. We find that changes of forebrain ACh levels primarily influence the proliferation and/or the short-term survival rather than the long-term survival or differentiation of the new neurons. We further demonstrate that these newly born cells express the muscarinic receptor subtypes M1 and M4. Our data provide evidence that forebrain ACh promotes neurogenesis, and suggest that the impaired cholinergic function in AD may in part contribute to deficits in learning and memory through reductions in the formation of new hippocampal neurons.     

Changes in hippocampal capillaries in transgenic type 2 diabetic mice: A stereological investigation

The cognitive dysfunction associated with type 2 diabetes mellitus (T2DM) has been widely studied, and many structures in the hippocampus, such as neurons and synapses, have been shown to play a crucial role in the cognitive decline. However, the mechanism of these changes remains unknown. To further explore this issue, we investigated the changes in the blood supply of the hippocampus in transgenic T2DM mice. In the current study, histochemistry, immunohistochemistry, and unbiased stereological methods were utilized to research the effects of T2DM on hippocampal capillaries of transgenic db/db mice. Twenty (Lepr^db) mut/mut mice and twenty (Lepr^db) wt/wt mice were used in this study. The learning and memory ability was appraised by Morris water maze test.