The hippocampal neuroproteome with aging and cognitive decline: past progress and future directions

Biomarkers of Aging and Relevant Evaluation Techniques: A Comprehensive Review

The risk of developing chronic illnesses and disabilities is increasing with age. To predict and prevent aging, biomarkers relevant to the aging process must be identified. This paper reviews the known molecular, cellular, and physiological biomarkers of aging. Moreover, we discuss the currently available technologies for identifying these biomarkers, and their applications and potential in aging research. We hope that this review will stimulate further research and innovation in this emerging and fast-growing field.

L-arginine metabolism ameliorates age-related cognitive impairment by Amuc_1100-mediated gut homeostasis maintaining

Aging-induced cognitive impairment is associated with a loss of metabolic homeostasis and plasticity. An emerging idea is that targeting key metabolites is sufficient to impact the function of other organisms. Therefore, more metabolism-targeted therapeutic intervention is needed to improve cognitive impairment. We first conducted untargeted metabolomic analyses and 16S rRNA to identify the aging-associated metabolic adaption and intestinal microbiome change. Untargeted metabolomic analyses of plasma revealed L-arginine metabolic homeostasis was altered during the aging process. Impaired L-arginine metabolic homeostasis was associated with low abundance of intestinal Akkermansia muciniphila (AKK) colonization in mice. Long-term supplementation of AKK outer membranes protein-Amuc_1100, rescued the L-arginine level and restored cognitive impairment in aging mice. Mechanically, Amuc_1100 acted directly as a source of L-arginine and enriched the L-arginine-producing bacteria. In aged brain, Amuc_1100 promoted the superoxide dismutase to alleviated oxidation stress, and increased nitric oxide, derivatives of L-arginine, to improve synaptic plasticity. Meanwhile, L-arginine repaired lipopolysaccharide-induced intestinal barrier damage and promoted growth of colon organoid. Our findings indicated that aging-related cognitive impairment was closely associated with the disorders of L-arginine metabolism. AKK-derived Amuc_1100, as a potential postbiotic, targeting the L-arginine metabolism, might provide a promising therapeutic strategy to maintain the intestinal homeostasis and cognitive function in aging.

Cognitive rescue in aging through prior training in rats

Cognitive decline in spatial memory is seen in aging. Understanding affected processes in aging is vital for developing methods to improve wellbeing. Daily memory persistence can be influenced by events around the time of learning or by prior experiences in early life. Fading memories in young can last longer if a novel event is introduced around encoding, a process called behavioral tagging. Based on this principle, we asked what processes are affected in aging and if prior training can rescue them. Two groups of aged rats received training in an appetitive delayed matching-to-place task. One of the groups additionally received prior training of the same task in young and in mid-life, constituting a longitudinal study. The results showed long-term memory decline in late aging without prior training. This would reflect affected encoding and consolidation. On the other hand, short-term memory was preserved and novelty at memory reactivation and reconsolidation enabled memory maintenance in aging. Prior training improved cognition through facilitating task performance, strengthening short-term memory and intermediate memory, and enabling encoding-boosted long-term memory. Implication of these findings in understanding brain mechanisms in cognitive aging and in beneficial effects of prior training is discussed.

Presbyphagia: Dysphagia in the elderly

Dysphagia has been classified as a “geriatric syndrome” and can lead to serious complications that result in a tremendous burden on population health and healthcare resources worldwide. A characteristic age-related change in swallowing is defined as “presbyphagia.” Medical imaging has shown some changes that seriously affect the safety and efficacy of swallowing. However, there is a general lack of awareness of the effects of aging on swallowing function and a belief that these changes are part of normal aging. Our review provides an overview of presbyphagia, which has been a neglected health problem for a long time. Attention and awareness of dysphagia in the elderly population should be strengthened, and targeted intervention measures should be actively implemented.

Blueberry-Mulberry Extract Alleviates Cognitive Impairment, Regulates Gut Metabolites, and Inhibits Inflammation in Aged Mice

Cognitive impairment is associated with aging; however, the underlying mechanism remains unclear. Our previous study found that polyphenol-rich blueberry-mulberry extract (BME) had an antioxidant capability and effectively alleviated cognitive impairment in a mouse model of Alzheimer's disease. Thus, we hypothesized that BME would improve cognitive performance in naturally aging mice and assessed its effects on related signaling pathways. Eighteen-month-old C57BL/6J mice were gavaged with 300 mg/kg/d of BME for 6 weeks. Behavioral phenotypes, cytokine levels, tight junction protein levels, and the histopathology of the brain were assessed, and 16S ribosomal RNA sequencing and targeted metabolome analyses were used for gut microbiota and metabolite measurements. Our results showed that the cognitive performance of aged mice in the Morris water maze test was improved after BME treatment, neuronal loss was reduced, IL-6 and TNF-α levels in the brain and intestine were decreased, and the levels of intestinal tight junction proteins (ZO-1 and occludin) were increased. Further, 16S sequencing showed that BME significantly increased the relative abundance of Lactobacillus, Streptococcus, and Lactococcus and decreased the relative abundance of Blautia, Lachnoclostridium, and Roseburia in the gut. A targeted metabolomic analysis showed that BME significantly increased the levels of 21 metabolites, including α-linolenic acid, vanillic acid, and N-acetylserotonin. In conclusion, BME alters the gut microbiota and regulates gut metabolites in aged mice, which may contribute to the alleviation of cognitive impairment and to inflammation inhibition in both the brain and the gut. Our results provide a basis for future research on natural antioxidant intervention as a treatment strategy for aging-related cognitive impairment.

Environmental enrichment improves declined cognition induced by prenatal inflammatory exposure in aged CD-1 mice: Role of NGPF2 and PSD-95

INTRODUCTION

Research suggests that prenatal inflammatory exposure could accelerate age-related cognitive decline that may be resulted from neuroinflammation and synaptic dysfunction during aging. Environmental enrichment (EE) may mitigate the cognitive and synaptic deficits. Neurite growth-promoting factor 2 (NGPF2) and postsynaptic density protein 95 (PSD-95) play critical roles in neuroinflammation and synaptic function, respectively.

METHODS

We examined whether this adversity and EE exposure can cause alterations in Ngpf2 and Psd-95 expression. In this study, CD-1 mice received intraperitoneal injection of lipopolysaccharide (50 μg/kg) or normal saline from gestational days 15-17. After weaning, half of the male offspring under each treatment were exposed to EE. The Morris water maze was used to assess spatial learning and memory at 3 and 15 months of age, whereas quantitative real-time polymerase chain reaction and Western blotting were used to measure hippocampal mRNA and protein levels of NGPF2 and PSD-95, respectively. Meanwhile, serum levels of IL-6, IL-1β, and TNF-α were determined by enzyme-linked immunosorbent assay.

RESULTS

The results showed that aged mice exhibited poor spatial learning and memory ability, elevated NGPF2 mRNA and protein levels, and decreased PSD-95 mRNA and protein levels relative to their young counterparts during natural aging. Embryonic inflammatory exposure accelerated age-related changes in spatial cognition, and in Ngpf2 and Psd-95 expression. Additionally, the levels of Ngpf2 and Psd-95 products were significantly positively and negatively correlated with cognitive dysfunction, respectively, particularly in prenatal inflammation-exposed aged mice. Changes in serum levels of IL-6, IL-1β, and TNF-α reflective of systemic inflammation and their correlation with cognitive decline during accelerated aging were similar to those of hippocampal NGPF2. EE exposure could partially restore the accelerated decline in age-related cognitive function and in Psd-95 expression, especially in aged mice.

DISCUSSION

Overall, the aggravated cognitive disabilities in aged mice may be related to the alterations in Ngpf2 and Psd-95 expression and in systemic state of inflammation due to prenatal inflammatory exposure, and long-term EE exposure may ameliorate this cognitive impairment by upregulating Psd-95 expression.

MS-based glycomics: An analytical tool to assess nervous system diseases

Neurological diseases affect millions of peopleochemistryorldwide and are continuously increasing due to the globe's aging population. Such diseases affect the nervous system and are characterized by a progressive decline in brain function and progressive cognitive impairment, decreasing the quality of life for those with the disease as well as for their families and loved ones. The increased burden of nervous system diseases demands a deeper insight into the biomolecular mechanisms at work during disease development in order to improve clinical diagnosis and drug design. Recently, evidence has related glycosylation to nervous system diseases. Glycosylation is a vital post-translational modification that mediates many biological functions, and aberrant glycosylation has been associated with a variety of diseases. Thus, the investigation of glycosylation in neurological diseases could provide novel biomarkers and information for disease pathology. During the last decades, many techniques have been developed for facilitation of reliable and efficient glycomic analysis. Among these, mass spectrometry (MS) is considered the most powerful tool for glycan analysis due to its high resolution, high sensitivity, and the ability to acquire adequate structural information for glycan identification. Along with MS, a variety of approaches and strategies are employed to enhance the MS-based identification and quantitation of glycans in neurological samples. Here, we review the advanced glycomic tools used in nervous system disease studies, including separation techniques prior to MS, fragmentation techniques in MS, and corresponding strategies. The glycan markers in common clinical nervous system diseases discovered by utilizing such MS-based glycomic tools are also summarized and discussed.

Distribution and Morphological Characteristics of Oligodendrocytes in Selected Areas of the Brain of Male and Female Red Kangaroos (Macropus rufus)

This study was carried out on six adult red kangaroos of both sexes. To determine the location of the oligodendrocytes (OLGs) of the hippocampus (Hip) and corpus callosum (CC), the method of impregnation of the neuroglia with silver salts was applied. The iron distribution in the OLGs was determined by the histochemical method. The Nissl method was used to determine the location of the brain structure and to analyze the number of OLGs. In the Hip, these cells are located one beside another, mainly in blood vessels and neurons; in the neocortex (NC), they are located in layers I–VI; and in the CC, they are arranged in characteristic rows and accompany both nerve fibers and blood vessels. The analysis of the results obtained by the chosen methods in the Hip, NC, and CC in males and females did not show statistically significant differences in the distribution and location of the red kangaroo OLGs. The involvement of these cells is a physiological process that proceeds in a similar manner throughout the life of individuals and actively influences the metabolism of neurons and myelin.

Do spatial and recognition memories have a lateralized processing by the dorsal hippocampus CA3?

The present study evaluated the function of the right and left CA3 of the dorsal hippocampus (dHPC) in the processing of (i) recognition memory, (ii) recent and remote spatial memory, (iii) working memory and (iv) navigation strategy. Wistar rats were divided into four experimental groups: vehicle group (VG), animals received a bilateral injection of phosphate-saline buffer (PBS) in both right and left dorsal CA3; dHPC-R group, animals received an injection of ibotenic acid (IBO) in the right dorsal CA3; dHPC-L group, animals received an IBO injection in left dorsal CA3; and dHPC-Bi group, animals received bilateral injections of IBO in both dorsal CA3. Rats were submitted to a sequence of behavioral tests: Morris water maze (MWM), object recognition test (ORT), forced T-maze and MWM 30 days after the first exposure. The results showed no evidence of functional lateralization and the dorsal CA3 does not seem to be essential for learning and memory (recent and remote) processing and allocentric navigation analyzed in the MWM and T-maze, respectively. However, rats with right or bilateral lesions in the dorsal CA3 failed to recognize the familiar object in the ORT, suggesting a lateralized processing of recognition memory. That result is unprecedented and contributes to the knowledge about the compartmentalization of HPC functions.

IRS1 expression in hippocampus is age-dependent and is required for mature spine maintenance and neuritogenesis

Insulin and insulin-like growth factor type I (IGF-1) play prominent roles in brain activity throughout the lifespan. Insulin/IGF1 signaling starts with the activation of the intracellular insulin receptor substrates (IRS). In this work, we performed a comparative study of IRS1 and IRS2, together with the IGF1 (IGF1R) and insulin (IR) receptor expression in the hippocampus and prefrontal cortex during development. We found that IRS1 and IRS2 expression is prominent during development and declines in the aged hippocampus, contrary to IR, which increases in adulthood and aging. In contrast, IGF1R expression is unaffected by age. Expression patterns are similar in the prefrontal cortex. Neurite development occurs postnatally in the rodent hippocampus and cortex, and it declines in the mature and aged brain and is influenced by trophic factors. In our previous work, we demonstrated that knockdown of IRS1 by shRNA impairs learning and reduces synaptic plasticity in a rat model, as measured by synaptophysin puncta in axons. In this study, we report that shIRS1 alters spine maturation in adult hilar hippocampal neurons. Lastly, to understand the role of IRS1 in neuronal neurite tree, we transfect shIRS1 into primary neuronal cultures and observed that shIRS1 reduced neurite branching and neurite length. Our results demonstrate that IRS1/2 and insulin/IGF1 receptors display different age-dependent expression profiles and that IRS1 is required for spine maturation, demonstrating a novel role for IRS1 in synaptic plasticity.

Effects of Noninvasive Brain Stimulation (NIBS) on Cognitive Impairment in Mild Cognitive Impairment and Alzheimer Disease: A Meta-analysis

OBJECTIVE

The purpose of this meta-analysis was to evaluate the beneficial effects and optimal stimulation protocol of noninvasive brain stimulation (NIBS) including repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) on patients with mild cognitive impairment and Alzheimer disease.

MATERIALS AND METHODS

PubMed, Web of Science, Embase, and the Cochrane Library were searched until March 2020. The cognitive outcomes were extracted and the standardized mean difference with 95% confidence interval was calculated.

RESULTS

Twenty-eight studies were included. The result of NIBS showed significant effect on global cognition (P<0.05). Low-frequency rTMS over right dorsolateral prefrontal cortex (DLPFC), high-frequency rTMS (HF-rTMS) over left DLPFC, and the tDCS over left DLPFC and temporal lobe can significantly improve the memory function (P<0.05). HF-rTMS over left, right, or bilateral DLPFC can significantly improve the language function (P<0.05). Both HF-rTMS and tDCS over left DLPFC can obviously improve the executive function (P<0.05). Multiple sessions of rTMS with 80% to 100% intensity and anode tDCS with 2 mA current density are more suitable for all these functions.

CONCLUSIONS

NIBS has a beneficial effect on cognitive performance in both mild cognitive impairment and Alzheimer disease patients. Distinct optimal stimulation parameters were observed for different cognitive functions.

COX5A Plays a Vital Role in Memory Impairment Associated With Brain Aging via the BDNF/ERK1/2 Signaling Pathway

Cytochrome c oxidase subunit Va (COX5A) is involved in maintaining normal mitochondrial function. However, little is known on the role of COX5A in the development and progress of Alzheimer’s disease (Martinez-Losa et al., 2018). In this study, we established and characterized the genomic profiles of genes expressed in the hippocampus of Senescence-Accelerated Mouse-prone 8 (SAMP8) mice, and revealed differential expression of COX5A among 12-month-aged SAMP8 mice and 2-month-aged SAMP8 mice. Newly established transgenic mice with systemic COX5A overexpression (51% increase) resulted in the improvement of spatial recognition memory and hippocampal synaptic plasticity, recovery of hippocampal CA1 dendrites, and activation of the BDNF/ERK1/2 signaling pathway in vivo. Moreover, mice with both COX5A overexpression and BDNF knockdown showed a poor recovery in spatial recognition memory as well as a decrease in spine density and branching of dendrites in CA1, when compared to mice that only overexpressed COX5A. In vitro studies supported that COX5A affected neuronal growth via BDNF. In summary, this study was the first to show that COX5A in the hippocampus plays a vital role in aging-related cognitive deterioration via BDNF/ERK1/2 regulation, and suggested that COX5A may be a potential target for anti-senescence drugs.

Effects of Prenatal Exposure to Inflammation Coupled With Stress Exposure During Adolescence on Cognition and Synaptic Protein Levels in Aged CD-1 Mice

Age-associated impairment of spatial learning and memory (AISLM) presents substantial challenges to our health and society. Increasing evidence has indicated that embryonic exposure to inflammation accelerates the AISLM, and this can be attributable, at least partly, to changed synaptic plasticity associated with the activities of various proteins. However, it is still uncertain whether social psychological factors affect this AISLM and/or the expression of synaptic protein-associated genes. Synaptotagmin-1 (Syt1) and activity-regulated cytoskeleton-associated protein (Arc) are two synaptic proteins closely related to cognitive functions. In this study, pregnant CD-1 mice received daily intraperitoneal injections of lipopolysaccharide (LPS) (50 μg/kg) or normal saline at days 15-17 of gestation, and half of the offspring of each group were then subjected to stress for 28 days in adolescence. The Morris water maze (MWM) test was used to separately evaluate spatial learning and memory at 3 and 15 months of age, while western blotting and RNAscope assays were used to measure the protein and mRNA levels of Arc and Syt1 in the hippocampus. The results showed that, at 15 months of age, control mice had worse cognitive ability and higher protein and mRNA levels of Arc and Syt1 than their younger counterparts. Embryonic exposure to inflammation or exposure to stress in adolescence aggravated the AISLM, as well as the age-related increase in Arc and Syt1 expression. Moreover, the hippocampal protein and mRNA levels of Arc and Syt1 were significantly correlated with the performance in the learning and memory periods of the MWM test, especially in the mice that had suffered adverse insults in early life. Our findings indicated that prenatal exposure to inflammation or stress exposure in adolescence exacerbated the AISLM and age-related upregulation of Arc and Syt1 expression, and these effects were linked to cognitive impairments in CD-1 mice exposed to adverse factors in early life.

High Frequency Repetitive Transcranial Magnetic Stimulation Alleviates Cognitive Impairment and Modulates Hippocampal Synaptic Structural Plasticity in Aged Mice

Normal aging is accompanied by hippocampus-dependent cognitive impairment, which is a risk factor of Alzheimer’s disease. This study aims to investigate the effect of high frequency-repetitive transcranial magnetic stimulation (HF-rTMS) on hippocampus-dependent learning and memory in aged mice and explore its underlying mechanisms. Forty-five male Kunming mice (15 months old) were randomly divided into three groups: aged sham, 5 Hz rTMS, and 25 Hz rTMS. Two sessions of 5 Hz or 25 Hz rTMS comprising 1,000 pulses in 10 trains were delivered once a day for 14 consecutive days. The aged sham group was treated by the reverse side of the coil. In the adult sham group, 15 male Kunming mice (3 months old) were treated the same way as the aged sham group. A Morris water maze (MWM) was conducted following the stimulation, and synaptic ultrastructure was observed through a transmission electron microscope. HF-rTMS improved spatial learning and memory impairment in the aged mice, and 5 Hz was more significant than 25 Hz. Synaptic plasticity-associated gene profiles were modified by HF-rTMS, especially neurotrophin signaling pathways and cyclic adenosine monophosphate response element binding protein (CREB) cofactors. Compared to the aged sham group, synaptic plasticity-associated proteins, i.e., synaptophysin (SYN) and postsynaptic density (PSD)-95 were increased; brain-derived neurotrophic factor (BDNF) and phosphorylated CREB (pCREB) significantly increased after the 5 Hz HF-rTMS treatment. Collectively, our results suggest that HF-rTMS ameliorated cognitive deficits in naturally aged mice. The 5 Hz rTMS treatment significantly enhanced synaptic structural plasticity and activated the BDNF/CREB pathway in the hippocampus.

Ageing and Cognition

With an increasingly ageing population that is expected to double by 2050 in the U.S., it is paramount that we further understand the neurological changes that occur during ageing. This is relevant not only in the context of "pathological" ageing, where the development of many neurodegenerative disorders is typically a feature of only the older population (and indeed, age is the primary risk factor for many conditions such as Alzheimer's disease), but also for what is considered to be "normal" or "healthy" ageing. Specifically, a significant proportion of the older population are affected by "age-related cognitive decline" (ARCD), which is both independent of dementia and has an incidence 70% higher than dementia alone. However, whilst it is reported that there are pathogenic and phenotypic overlaps between healthy and pathological ageing, it is clear that there is a need to identify the pathways and understand the mechanisms that contribute to this loss of cognitive function with normal ageing, particularly in light of the increasing life expectancy of the global population. Importantly, there is an increasing body of evidence implicating zinc homeostasis as a key player in learning and memory and also potentially ARCD. Further research will ultimately contribute to the development of targeted therapeutics that will promote successful brain ageing. In this chapter we will explore the notion of ARCD, with a perspective on potential key neurochemical pathways that can be targeted for future intervention.

Aging-associated changes in hippocampal glycogen metabolism in mice. Evidence for and against astrocyte-to-neuron lactate shuttle

Lactate derived from astrocytic glycogen has been shown to support memory formation in hippocampi of young animals, inhibiting it in old animals. Here we show, using quantitative mass spectrometry-based proteomics, immunofluorescence, and qPCR that aging is associated with an increase of glycogen metabolism enzymes concentration and shift in their localization from astrocytes to neurons. These changes are accompanied with reorganization of hippocampal energy metabolism which is manifested by elevated capacity of aging neurons to oxidize glucose in glycolysis and mitochondria, and decreased ability for fatty acids utilization. Our observations suggest that astrocyte-to-neuron lactate shuttle may operate in young hippocampi, however, during aging neurons become independent on astrocytic lactate and the metabolic crosstalk between the brain's cells is disrupted.

Cerebrospinal Fluid Concentration of Key Autophagy Protein Lamp2 Changes Little During Normal Aging

Autophagy removes both functional and damaged intracellular macromolecules from cells via lysosomal degradation. Three autophagic mechanisms, namely macroautophagy, chaperone-mediated autophagy (CMA), and microautophagy, have been described in mammals. Studies in experimental systems have found macroautophagy and CMA to decrease with normal aging, despite the fact that oxidative stress, which can activate both processes, increases with normal aging. Whether autophagic mechanisms decrease in the human brain during normal aging is unclear. The primary objective of this study was to examine the association of a major autophagy protein, lysosome-associated membrane glycoprotein (lamp2), with age in cerebrospinal fluid (CSF) samples from healthy subjects. Lamp2 consists of three isoforms, lamp2a, 2b and 2c, all of which participate in autophagy. Lamp2's CSF concentration decreases in Parkinson's disease (PD) and increases in Alzheimer's disease (AD), but whether its CSF concentration changes during normal aging has not been investigated. Our secondary objectives were to examine the associations of lamp2's CSF concentration with CSF levels of the molecular chaperone heat shock 70-kDa protein (HSPA8), which interacts with lamp2a in CMA, and oxidative stress markers 8-hydroxy-2'-deoxyguanosine (8-OHdG), 8-isoprostane (8-ISO) and Total Antioxidant Capacity (TAC) in healthy subjects. We found lamp2's observed associations with these variables to be weak, with all Kendall's tau-b absolute values ≤0.20. These results suggest that CSF lamp2 concentration changes little during normal aging and does not appear to be associated with HSPA8 or oxidative stress. Further studies are indicated to determine the relationship between CSF lamp2 concentration and brain autophagic processes.

Two Behavioral Tests Allow a Better Correlation Between Cognitive Function and Expression of Synaptic Proteins

The molecular substrate of age-associated cognitive decline (AACD) is still elusive. Evidence indicates that AACD is related to synaptic impairment in hippocampus, but different hippocampal regions play different roles, with the dorsal hippocampus (DH) associated to spatial learning, and the ventral hippocampus (VH) crucial for emotionality. If changes in hippocampal function contributes to AACD, this contribution may be reflected in alterations of synaptic protein levels. A commonly used approach to investigate this issue is western blotting. When this technique is applied to the entire hippocampus and the cognitive impairment is evaluated by a single task, changes in expression of a protein might undergo a "dilution effect", as they may occur only in a given hippocampal region. We show that two behavioral tests yield more accurate results than one test in evaluating the function of the whole rat hippocampus by studying the expression of synaptotagmin 1 (SYT1), a vesicular protein whose expression in aged hippocampus is reportedly inconsistent. Analysis of SYT1 levels in the whole hippocampus of rats selected by the Morris water maze (MWM) test only failed to highlight a difference, whereas analysis of SYT1 levels in the whole hippocampus of rats categorized by both the MWM and the step-through passive avoidance (STPA) tests demonstrated a significant increase of SYT1 level in impaired rats. These findings, besides showing that SYT1 increases in impaired aged rats, suggest that using the whole hippocampus in blotting studies may prevent false negative results only if animals are categorized with tests exploring both DH and VH.

Age-Dependent Effects of Acute Alcohol Administration in the Hippocampal Phosphoproteome

Alcohol consumption during adolescence is deleterious to the developing brain and leads to persistent deficits in adulthood. Several results provide strong evidence for ethanol-associated alterations in glutamatergic signaling and impaired synaptic plasticity in the hippocampus. Protein phosphorylation is a well-known and well-documented mechanism in memory processes, but information on phosphoprotein alterations in hippocampus after ethanol exposure is limited. This study focuses on age-related changes in the hippocampal phosphoproteome after acute alcohol administration. We have compared the phosphoprotein expression in the hippocampus of adult and adolescent Wistar rats treated with a single dose of ethanol (5 g/kg i.p.), using a proteomic approach including phosphoprotein enrichment by immobilized metal affinity chromatography (IMAC). Our proteomic analysis revealed that 13 proteins were differentially affected by age, ethanol administration, or both. Most of these proteins are involved in neuroprotection and are expressed less in young rats treated with ethanol. We conclude that acute alcohol induces important changes in the expression of phosphoproteins in the hippocampus that could increase the risk of neurodegenerative disorders, especially when the alcohol exposure begins in adolescence.

Advantages of Structure-Based Drug Design Approaches in Neurological Disorders

OBJECTIVE

The purpose of the review is to portray the theoretical concept on neurological disorders from research data.

BACKGROUND

The freak changes in chemical response of nerve impulse causes neurological disorders. The research evidence of the effort done in the older history suggests that the biological drug targets and their effective feature with responsive drugs could be valuable in promoting the future development of health statistics structure for improved treatment for curing the nervous disorders.

METHODS

In this review, we summarized the most iterative theoretical concept of structure based drug design approaches in various neurological disorders to unfathomable understanding of reported information for future drug design and development.

RESULTS

On the premise of reported information we analyzed the model of theoretical drug designing process for understanding the mechanism and pathology of the neurological diseases which covers the development of potentially effective inhibitors against the biological drug targets. Finally, it also suggests the management and implementation of the current treatment in improving the human health system behaviors.

CONCLUSION

With the survey of reported information we concluded the development strategies of diagnosis and treatment against neurological diseases which leads to supportive progress in the drug discovery.

Developmental Vitamin D Deficiency Affects Spatial Learning in Wistar Rats

Background: Vitamin D deficiency is a global problem. Recent evidence suggests that vitamin D is involved in brain development and function. Vitamin D deficiency has been associated with poor cognitive function in adults, but the effect of developmental vitamin D deficiency (DVDD) on cognitive function and brain development in children has not been well established.Objective: We explored the effects of DVDD on cognitive functions and brain morphology of rat pups.Methods: Wistar rat pups born to control and vitamin D-deficient dams were divided into 4 groups: control (C), deficient during gestation (dG), deficient during lactation (dL), and deficient during gestation and lactation (dGL). Spatial learning and memory were assessed by the Morris water maze test at postnatal day (PND) 24 and PND 45. Cortical thickness at the level of the hippocampus was measured at PND 63, and synapses were counted in specified areas of the hippocampus at PND 32 and PND 63.Results: Repeated-measures ANOVA revealed that at PND 24, learning (escape latency) was impaired (by 42%) in the dGL group, whereas at PND 45, both the dL and the dGL groups showed learning impairment (by 47% and 45%, respectively) compared with their respective C groups (P < 0.05). Short-term or long-term memory was largely unaffected by DVDD either at PND 24 or PND 45. Compared with the C group, all the DVDD groups had fewer synapses in the molecular layer of the hippocampus (P < 0.001). The synapse number decreased by 54% in the dGL group at PND 33 and by 70% in the dL and dGL groups at PND 63. All the DVDD groups at PND 63 showed a reduced cortical thickness (by 22%) compared with the C group (P < 0.05).Conclusion: These results suggest that a combined prenatal and postnatal DVDD for ≥6 wk in rat pups affects learning but not memory.

Perinatal Brain Docosahexaenoic Acid Concentration Has a Lasting Impact on Cognition in Mice

Background: Premature infants are deprived of prenatal accumulation of brain docosahexaenoic acid [DHA (22:6n-3)], an omega-3 fatty acid [ω-3 FA (n-3 FA)] important for proper development of cognitive function. The resulting brain DHA deficit can be reversed by ω-3 FA supplementation.Objective: The objective was to test whether there is a critical period for providing ω-3 FA to correct cognitive deficits caused by developmental ω-3 FA deprivation in mice.Methods: Twelve timed-pregnant mice [embryonic day 14 (E14), C57/BL6NCr] were fed an ω-3 FA-deficient diet containing 0.04% α-linolenic acid [ALA (18:3n-3)], and their offspring were fed the same deficient diet (Def group) or changed to an ω-3 FA-adequate diet containing 3.1% ALA at 3 wk, 2 mo, or 4 mo of age. In parallel, 3 E14 pregnant mice were fed the adequate diet and their offspring were fed the same diet (Adeq group) throughout the experiment. Brain FA composition, learning and memory, and hippocampal synaptic protein expression were evaluated at 6 mo by gas chromatography, the Morris water maze test, and western blot analysis, respectively.Results: Maternal dietary ω-3 FA deprivation decreased DHA by >50% in the brain of their offspring at 3 wk of age. The Def group showed significantly worse learning and memory at 6 mo than those groups fed the adequate diet. These pups also had decreased hippocampal expression of postsynaptic density protein 95 (43% of Adeq group), Homer protein homolog 1 (21% of Adeq group), and synaptosome-associated protein of 25 kDa (64% of Adeq group). Changing mice to the adequate diet at 3 wk, 2 mo, or 4 mo of age restored brain DHA to the age-matched adequate concentration. However, deficits in hippocampal synaptic protein expression and spatial learning and memory were normalized only when the diet was changed at 3 wk.Conclusion: Developmental deprivation of brain DHA by dietary ω-3 FA depletion in mice may have a lasting impact on cognitive function if not corrected at an early age.

Comparative Studies on Behavioral, Cognitive and Biomolecular Profiling of ICR, C57BL/6 and Its Sub-Strains Suitable for Scopolamine-Induced Amnesic Models

Cognitive impairment and behavioral disparities are the distinctive baseline features to investigate in most animal models of neurodegenerative disease. However, neuronal complications are multifactorial and demand a suitable animal model to investigate their underlying basal mechanisms. By contrast, the numerous existing neurodegenerative studies have utilized various animal strains, leading to factual disparity. Choosing an optimal mouse strain for preliminary assessment of neuronal complications is therefore imperative. In this study, we systematically compared the behavioral, cognitive, cholinergic, and inflammatory impairments of outbred ICR and inbred C57BL/6 mice strains subject to scopolamine-induced amnesia. We then extended this study to the sub-strains C57BL/6N and C57BL/6J, where in addition to the above-mentioned parameters, their endogenous antioxidant levels and cAMP response-element binding protein (CREB)/brain-derived neurotrophic factor (BDNF) protein expression were also evaluated. Compared with the ICR strain, the scopolamine-inflicted C57BL/6 strains exhibited a substantial reduction of spontaneous alternation and an approximately two-fold increase in inflammatory protein expression, compared to the control group. Among the sub-strains, scopolamine-treated C57BL/6N strains exhibited declined step-through latency, elevated acetylcholinesterase (AChE) activity and inflammatory protein expression, associated with reduced endogenous antioxidant levels and p-CREB/BDNF expression, compared to the control and tacrine-treated groups. This indicates that the C57BL/6N strains exhibit significantly enhanced scopolamine-induced neuronal impairment compared to the other evaluated strains.

The effects of aging in the hippocampus and cognitive decline

Aging is a natural process that is associated with cognitive decline as well as functional and social impairments. One structure of particular interest when considering aging and cognitive decline is the hippocampus, a brain region known to play an important role in learning and memory consolidation as well as in affective behaviours and mood regulation, and where both functional and structural plasticity (e.g., neurogenesis) occur well into adulthood. Neurobiological alterations seen in the aging hippocampus including increased oxidative stress and neuroinflammation, altered intracellular signalling and gene expression, as well as reduced neurogenesis and synaptic plasticity, are thought to be associated with age-related cognitive decline. Non-invasive strategies such as caloric restriction, physical exercise, and environmental enrichment have been shown to counteract many of the age-induced alterations in hippocampal signalling, structure, and function. Thus, such approaches may have therapeutic value in counteracting the deleterious effects of aging and protecting the brain against age-associated neurodegenerative processes.

Hippocampal proteomics defines pathways associated with memory decline and resilience in normal aging and Alzheimer's disease mouse models

Alzheimer's disease (AD), the most common form of dementia in the elderly, has no cure. Thus, the identification of key molecular mediators of cognitive decline in AD remains a top priority. As aging is the most significant risk factor for AD, the goal of this study was to identify altered proteins and pathways associated with the development of normal aging and AD memory deficits, and identify unique proteins and pathways that may contribute to AD-specific symptoms. We used contextual fear conditioning to diagnose 8-month-old 5XFAD and non-transgenic (Ntg) mice as having either intact or impaired memory, followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) to quantify hippocampal membrane proteins across groups. Subsequent analysis detected 113 proteins differentially expressed relative to memory status (intact vs impaired) in Ntg mice and 103 proteins in 5XFAD mice. Thirty-six proteins, including several involved in neuronal excitability and synaptic plasticity (e.g., GRIA1, GRM3, and SYN1), were altered in both normal aging and AD. Pathway analysis highlighted HDAC4 as a regulator of observed protein changes in both genotypes and identified the REST epigenetic regulatory pathway and Gi intracellular signaling as AD-specific pathways involved in regulating the onset of memory deficits. Comparing the hippocampal membrane proteome of Ntg versus AD, regardless of cognitive status, identified 138 differentially expressed proteins, including confirmatory proteins APOE and CLU. Overall, we provide a novel list of putative targets and pathways with therapeutic potential, including a set of proteins associated with cognitive status in normal aging mice or gene mutations that cause AD.

Cognitive deficit in hippocampal-dependent tasks in Werner syndrome mouse model

Mammalian aging is often characterized by metabolic disturbances, cognitive declines and DNA repairs deficiency, but the underlying molecular mechanisms are still not well understood. Alterations in DNA repair can significantly exacerbate aging. Mammalian neuronal cells which accumulate unrepaired DNA damage over time could potentially lead to brain functions disorders. Focusing on the ATP-dependent RecQ-type DNA helicase, an enzyme involved in repair of double strand DNA, a mouse model of Werner syndrome (WS) had been proposed as a model of accelerated aging. Until now, no study has investigated the impact of this premature aging syndrome on learning and memory. Spatial memory and cognitive flexibility are particularly affected by the aging process in both men and rodents. Studies have shown that aged mice exhibited similar performance than young adult mice on non-hippocampus dependent memory whereas their performances were decreased in hippocampus-dependent tasks. In this study, we have submitted 3, 5 and 8 month-old WS mice to several behavioral paradigms to evaluate hippocampus-dependent (spatial object location, Morris water maze and fear conditioning) and non hippocampus-dependent (object recognition) memories. No effect on the locomotion activity and anxiety level has been observed in adult WS mice. Interestingly, the 8 month-old WS mice exhibit long-term memory impairment similar to aged mice, suggesting that adult WS mice do develop some aspects of cognitive aging.

Quantitative protein profiling of hippocampus during human aging

The hippocampus appears commonly affected by aging and various neurologic disorders in humans, whereas little is known about age-related change in overall protein expression in this brain structure. Using the 4-plex tandem mass tag labeling, we carried out a quantitative proteomic study of the hippocampus during normal aging using postmortem brains from Chinese subjects. Hippocampal samples from 16 subjects died of non-neurological/psychiatric diseases were divided into 4 age groups: 22-49, 50-69, 70-89, and >90. Among 4582 proteins analyzed, 35 proteins were significantly elevated, whereas 25 proteins were downregulated, along with increasing age. Several upregulated proteins, including transgelin, vimentin, myosin regulatory light polypeptide 9, and calcyphosin, were further verified by quantitative Western blot analysis of hippocampal tissues from additional normal subjects. Bioinformatic analysis showed that the upregulated and downregulated proteins were largely involved in several important protein-protein interaction networks. Proteins in the electron transport chain and synaptic vesicle fusion pathway were consistently downregulated with aging, whereas proteins associated with Alzheimer's disease showed little change. Our study demonstrates substantial protein profile changes in the human hippocampus during aging, which could be of relevance to age-related loss of hippocampal functions.

Daily Carnosine and Anserine Supplementation Alters Verbal Episodic Memory and Resting State Network Connectivity in Healthy Elderly Adults

Carnosine and anserine are strong antioxidants, previously demonstrated to reduce cognitive decline in animal studies. We aimed to investigate their cognitive and neurophysiological effects, using functional MRI, on humans. Thirty-one healthy participants (age 40–78, 10 male/21 female) were recruited to a double-blind placebo-controlled study. Participants were assigned to twice-daily doses of imidazole dipeptide formula (n = 14), containing 500 mg (carnosine/anserine, ratio 1/3) or an identical placebo (n = 17). Functional MRI and neuropsychological assessments were carried out at baseline and after 3 months of supplementation. We analyzed resting state functional connectivity with the FSL fMRI analysis package. There were no differences in neuropsychological scores between the groups at baseline. After 3 months of supplementation, the carnosine/anserine group had better verbal episodic memory performance and decreased connectivity in the default mode network, the posterior cingulate cortex and the right fronto parietal network, as compared with the placebo group. Furthermore, there was a correlation between the extents of cognitive and neuroimaging changes. These results suggest that daily carnosine/anserine supplementation can impact cognitive function and that network connectivity changes are associated with its effects.

The Proteome Profiles of the Cerebellum of Juvenile, Adult and Aged Rats--An Ontogenetic Study

In this study, we searched for proteins that change their expression in the cerebellum (Ce) of rats during ontogenesis. This study focuses on the question of whether specific proteins exist which are differentially expressed with regard to postnatal stages of development. A better characterization of the microenvironment and its development may result from these study findings. A differential two-dimensional polyacrylamide gel electrophoresis (2DE) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis of the samples revealed that the number of proteins of the functional classes differed depending on the developmental stages. Especially members of the functional classes of biosynthesis, regulatory proteins, chaperones and structural proteins show the highest differential expression within the analyzed stages of development. Therefore, members of these functional protein groups seem to be involved in the development and differentiation of the Ce within the analyzed development stages. In this study, changes in the expression of proteins in the Ce at different postnatal developmental stages (postnatal days (P) 7, 90, and 637) could be observed. At the same time, an identification of proteins which are involved in cell migration and differentiation was possible. Especially proteins involved in processes of the biosynthesis and regulation, the dynamic organization of the cytoskeleton as well as chaperones showed a high amount of differentially expressed proteins between the analyzed dates.

The proteome profiles of the olfactory bulb of juvenile, adult and aged rats - an ontogenetic study

Background

In this study, we searched for proteins that change their expression in the olfactory bulb (oB) of rats during ontogenesis. Up to now, protein expression differences in the developing animal are not fully understood. Our investigation focused on the question whether specific proteins exist which are only expressed during different development stages. This might lead to a better characterization of the microenvironment and to a better determination of factors and candidates that influence the differentiation of neuronal progenitor cells.

Results

After analyzing the samples by two-dimensional polyacrylamide gel electrophoresis (2DE) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), it could be shown that the number of expressed proteins differs depending on the developmental stages. Especially members of the functional classes, like proteins of biosynthesis, regulatory proteins and structural proteins, show the highest differential expression in the stages of development analyzed.

Conclusion

In this study, quantitative changes in the expression of proteins in the oB at different developmental stages (postnatal days (P) 7, 90 and 637) could be observed. Furthermore, the expression of many proteins was found at specific developmental stages. It was possible to identify these proteins which are involved in processes like support of cell migration and differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-014-0058-x) contains supplementary material, which is available to authorized users.

Decreased mitochondrial bioenergetics and calcium buffering capacity in the basal ganglia correlates with motor deficits in a nonhuman primate model of aging

Altered mitochondrial function in the basal ganglia has been hypothesized to underlie cellular senescence and promote age-related motor decline. We tested this hypothesis in a nonhuman primate model of human aging. Six young (6-8 years old) and 6 aged (20-25 years old) female Rhesus monkeys (Macaca mulatta) were behaviorally characterized from standardized video records. Additionally, we measured mitochondrial bioenergetics along with calcium buffering capacity in the substantia nigra and putamen (PUT) from both age groups. Our results demonstrate that the aged animals had significantly reduced locomotor activity and movement speed compared with younger animals. Moreover, aged monkeys had significantly reduced ATP synthesis capacity (in substantia nigra and PUT), reduced pyruvate dehydrogenase activity (in PUT), and reduced calcium buffering capacity (in PUT) compared with younger animals. Furthermore, this age-related decline in mitochondrial function in the basal ganglia correlated with decline in motor function. Overall, our results suggest that drug therapies designed to enhance altered mitochondrial function may help improve motor deficits in the elderly.

BioAge: toward a multi-determined, mechanistic account of cognitive aging

The search for reliable early indicators of age-related cognitive decline represents a critical avenue for progress in aging research. Chronological age is a commonly used developmental index; however, it offers little insight into the mechanisms underlying cognitive decline. In contrast, biological age (BioAge), reflecting the vitality of essential biological systems, represents a promising operationalization of developmental time. Current BioAge models have successfully predicted age-related cognitive deficits. Research on aging-related cognitive function indicates that the interaction of multiple risk and protective factors across the human lifespan confers individual risk for late-life cognitive decline, implicating a multi-causal explanation. In this review, we explore current BioAge models, describe three broad yet pathologically relevant biological processes linked to cognitive decline, and propose a novel operationalization of BioAge accounting for both moderating and causal mechanisms of cognitive decline and dementia. We argue that a multivariate and mechanistic BioAge approach will lead to a greater understanding of disease pathology as well as more accurate prediction and early identification of late-life cognitive decline.

Hippocampal subregions exhibit both distinct and shared transcriptomic responses to aging and nonneurodegenerative cognitive decline

Impairment of hippocampal-dependent spatial learning and memory with aging affects a large segment of the aged population. Hippocampal subregions (CA1, CA3, and DG) have been previously reported to express both common and specific morphological, functional, and gene/protein alterations with aging and cognitive decline. To comprehensively assess gene expression with aging and cognitive decline, transcriptomic analysis of CA1, CA3, and DG was conducted using Adult (12M) and Aged (26M) F344xBN rats behaviorally characterized by Morris water maze performance. Each subregion demonstrated a specific pattern of responses with aging and with cognitive performance. The CA1 and CA3 demonstrating the greatest degree of shared gene expression changes. Analysis of the pathways, processes, and regulators of these transcriptomic changes also exhibit a similar pattern of commonalities and differences across subregions. Gene expression changes between Aged cognitively Intact and Aged cognitively Impaired rats often showed an inversion of the changes between Adult and Aged rats. This failure to adapt rather than an exacerbation of the aging phenotype questions a conventional view that cognitive decline is exaggerated aging. These results are a resource for investigators studying cognitive decline and also demonstrate the need to individually examine hippocampal subregions in molecular analyses of aging and cognitive decline.

Oxidative stress in aging: advances in proteomic approaches

Aging is a gradual, complex process in which cells, tissues, organs, and the whole organism itself deteriorate in a progressive and irreversible manner that, in the majority of cases, implies pathological conditions that affect the individual's Quality of Life (QOL). Although extensive research efforts in recent years have been made, the anticipation of aging and prophylactic or treatment strategies continue to experience major limitations. In this review, the focus is essentially on the compilation of the advances generated by cellular expression profile analysis through proteomics studies (two-dimensional [2D] electrophoresis and mass spectrometry [MS]), which are currently used as an integral approach to study the aging process. Additionally, the relevance of the oxidative stress factors is discussed. Emphasis is placed on postmitotic tissues, such as neuronal, muscular, and red blood cells, which appear to be those most frequently studied with respect to aging. Additionally, models for the study of aging are discussed in a number of organisms, such as Caenorhabditis elegans, senescence-accelerated probe-8 mice (SAMP8), naked mole-rat (Heterocephalus glaber), and the beagle canine. Proteomic studies in specific tissues and organisms have revealed the extensive involvement of reactive oxygen species (ROS) and oxidative stress in aging.

Aging-induced proteostatic changes in the rat hippocampus identify ARP3, NEB2 and BRAG2 as a molecular circuitry for cognitive impairment

Disturbed proteostasis as a particular phenotype of the aging organism has been advanced in C. elegans experiments and is also conceived to underlie neurodegenerative diseases in humans. Here, we investigated whether particular changes in non-disease related proteostasis can be identified in the aged mammalian brain, and whether a particular signature of aberrant proteostasis is related to behavioral performance of learning and memory. Young (adult, n = 30) and aged (2 years, n = 50) Wistar rats were tested in the Morris Water Maze (MWM) to distinguish superior and inferior performers. For both young and old rats, the best and worst performers in the MWM were selected and the insoluble proteome, termed aggregome, was purified from the hippocampus as evidence for aberrant proteostasis. Quantitative proteomics (iTRAQ) was performed. The aged inferior performers were considered as a model for spontaneous, age-associated cognitive impairment. Whereas variability of the insoluble proteome increased with age, absolute changes in the levels of insoluble proteins were small compared to the findings in the whole C. elegans insoluble proteome. However, we identified proteins with aberrant proteostasis in aging. For the cognitively impaired rats, we identified a changed molecular circuitry of proteins selectively involved in F-actin remodeling, synapse building and long-term depression: actin related protein 3 (ARP3), neurabin II (NEB2) and IQ motif and SEC7 domain-containing protein 1 (BRAG2). We demonstrate that aberrant proteostasis is a specific phenotype of brain aging in mammals. We identify a distinct molecular circuitry where changes in proteostasis are characteristic for poor learning and memory performance in the wild type, aged rat. Our findings 1. establish the search for aberrant proteostasis as a successful strategy to identify neuronal dysfunction in deficient cognitive behavior, 2. reveal a previously unknown functional network of proteins (ARP3, NEB2, BRAG2) involved in age-associated cognitive dysfunction.

A high-salt diet further impairs age-associated declines in cognitive, behavioral, and cardiovascular functions in male Fischer brown Norway rats

Aging-associated declines in cognitive, emotional, and cardiovascular function are well known. Environmental stress triggers critical changes in the brain, further compromising cardiovascular and behavioral health during aging. Excessive dietary salt intake is one such stressor. Here, we tested the effect of high salt (HS) on anxiety, learning-memory function, and blood pressure (BP) in male Fischer brown Norway (FBN) rats. Adult (A; 2 mo) and old (O; 20 mo) male rats were fed normal-salt (NS; 0.4% NaCl) or HS (8% NaCl) diets for 4 wk after being implanted with telemeter probes for conscious BP measurement. Thereafter, tests to assess anxiety-like behavior and learning-memory were conducted. The rats were then killed, and samples of plasma, urine, and brain tissue were collected. We found that systolic BP was higher in O-NS (117 ± 1.2 mm Hg) than in A-NS (105 ± 0.8 mm Hg) rats (P < 0.05). Furthermore, BP was higher in O-HS (124 ± 1.4 mm Hg) than in O-NS (117 ± 1.2 mm Hg) rats (P < 0.05). Moreover, anxiety-like behavior (light-dark and open-field tests) was not different between A-NS and O-NS rats but was greater in O-HS rats than in A-NS, O-NS, or A-HS rats (P < 0.05). Short-term memory (radial arm water maze test) was similar in A-NS and O-NS rats but was significantly impaired in O-HS rats compared with A-NS, O-NS, or A-HS rats (P < 0.05). Furthermore, oxidative stress variables (in plasma, urine, and brain) as well as corticosterone (plasma) were greater in O-HS rats when compared with A-NS, O-NS, or A-HS rats (P < 0.05). The antioxidant enzyme glyoxalase-1 expression was selectively reduced in the hippocampus and amygdala of O-HS rats compared with A-NS, O-NS, or A-HS rats (P < 0.05), whereas other antioxidant enzymes, glutathione reductase 1, manganese superoxide dismutase (SOD), and Cu/Zn SOD remained unchanged. We suggest that salt-sensitive hypertension and behavioral derangement are associated with a redox imbalance in the brain of aged FBN rats.

Insulin-like growth factor-1 in CNS and cerebrovascular aging

Insulin-like growth factor-1 (IGF-1) is an important anabolic hormone that decreases with age. In the past two decades, extensive research has determined that the reduction in IGF-1 is an important component of the age-related decline in cognitive function in multiple species including humans. Deficiency in circulating IGF-1 results in impairment in processing speed and deficiencies in both spatial and working memory. Replacement of IGF-1 or factors that increase IGF-1 to old animals and humans reverses many of these cognitive deficits. Despite the overwhelming evidence for IGF-1 as an important neurotrophic agent, the specific mechanisms through which IGF-1 acts have remained elusive. Recent evidence indicates that IGF-1 is both produced by and has important actions on the cerebrovasculature as well as neurons and glia. Nevertheless, the specific regulation and actions of brain- and vascular-derived IGF-1 is poorly understood. The diverse effects of IGF-1 discovered thus far reveal a complex endocrine and paracrine system essential for integrating many of the functions necessary for brain health. Identification of the mechanisms of IGF-1 actions will undoubtedly provide critical insight into regulation of brain function in general and the causes of cognitive decline with age.

Increased hippocampal NgR1 signaling machinery in aged rats with deficits of spatial cognition

Myelin-associated inhibitor/NgR1 signaling has important roles in modulation of synaptic plasticity, with demonstrated effects on cognitive function. We have previously demonstrated that NgR1 and its ligands are upregulated in the hippocampus of aged rats with impaired spatial learning and memory, but it is unknown whether increased expression of these proteins indicates a potential increase in pathway signaling because NgR1 requires co-receptors for signal transduction through RhoA. Two co-receptor complexes have been identified to date, comprised of NgR1 and LINGO-1, and either p75 or TROY. In this study, we assessed the expression of LINGO-1, p75 and TROY, and the downstream effector RhoA in mature adult (12 months) and aged (26 months) male Fischer 344/Brown Norway hybrid rats classified as cognitively impaired or cognitively intact by Morris water maze testing. The hippocampal distribution of NgR1 and its co-receptors was assessed to determine whether receptor/co-receptor interaction, and therefore signaling through this pathway, is possible. Protein expression of LINGO-1, p75, TROY and RhoA was significantly elevated in cognitively impaired, but not intact, aged rats compared with mature adults, and expression levels correlated significantly with water maze performance. Co-localization of NgR1 with LINGO-1, p75 and TROY was observed in hippocampal neurons of aged, cognitively impaired rats. Further, expression profiles of NgR1 pathway components were demonstrated to classify rats as cognitively intact or cognitively impaired with high accuracy. Together, this suggests that hippocampal induction of this pathway is a conserved phenomenon in cognitive decline that may impair learning and memory by suppressing neuronal plasticity.

Aging, synaptic dysfunction, and insulin-like growth factor (IGF)-1

Insulin-like growth factor (IGF)-1 is an important neurotrophic hormone. Deficiency of this hormone has been reported to influence the genesis of cognitive impairment and dementia in the elderly patients. Nevertheless, there are studies indicating that cognitive function can be maintained into old age even in the absence of circulating IGF-1 and studies that link IGF-1 to an acceleration of neurological diseases. Although IGF-1 has a complex role in brain function, synaptic effects appear to be central to the IGF-1-induced improvement in learning and memory. In this review, synaptic mechanisms of learning and memory and the effects of IGF-1 on synaptic communication are discussed. The emerging data indicate that synaptic function decreases with age and that IGF-1 contributes to information processing in the brain. Further studies that detail the specific actions of this important neurotrophic hormone will likely lead to therapies that result in improved cognitive function for the elderly patients.

Neuroglial expression of the MHCI pathway and PirB receptor is upregulated in the hippocampus with advanced aging

The hippocampus undergoes changes with aging that impact neuronal function, such as synapse loss and altered neurotransmitter release. Nearly half of the aged population also develops deficits in spatial learning and memory. To identify age-related hippocampal changes that may contribute to cognitive decline, transcriptomic analysis of synaptosome preparations from adult (12 months) and aged (28 months) Fischer 344-Brown Norway rats assessed for spatial learning and memory was performed. Bioinformatic analysis identified the MHCI pathway as significantly upregulated with aging. Age-related increases in mRNAs encoding the MHCI genes RT1-A1, RT1-A2, and RT1-A3 were confirmed by qPCR in synaptosomes and in CA1 and CA3 dissections. Elevated levels of the MHCI cofactor (B2m), antigen-loading components (Tap1, Tap2, Tapbp), and two known MHCI receptors (PirB, Klra2) were also confirmed. Protein expression of MHCI was elevated with aging in synaptosomes, CA1, and DG, while PirB protein expression was induced in both CA1 and DG. MHCI expression was localized to microglia and neuronal excitatory postsynaptic densities, and PirB was localized to neuronal somata, axons, and dendrites. Induction of the MHCI antigen processing and presentation pathway in hippocampal neurons and glia may contribute to age-related hippocampal dysfunction by increasing neuroimmune signaling or altering synaptic homeostasis.

Hippocampal expression of myelin-associated inhibitors is induced with age-related cognitive decline and correlates with deficits of spatial learning and memory

Impairment of cognitive functions including hippocampus-dependent spatial learning and memory affects nearly half of the aged population. Age-related cognitive decline is associated with synaptic dysfunction that occurs in the absence of neuronal cell loss, suggesting that impaired neuronal signaling and plasticity may underlie age-related deficits of cognitive function. Expression of myelin-associated inhibitors (MAIs) of synaptic plasticity, including the ligands myelin-associated glycoprotein, neurite outgrowth inhibitor A, and oligodendrocyte myelin glycoprotein, and their common receptor, Nogo-66 receptor, was examined in hippocampal synaptosomes and Cornu ammonis area (CA)1, CA3 and dentate gyrus subregions derived from adult (12-13 months) and aged (26-28 months) Fischer 344 × Brown Norway rats. Rats were behaviorally phenotyped by Morris water maze testing and classified as aged cognitively intact (n = 7-8) or aged cognitively impaired (n = 7-10) relative to adults (n = 5-7). MAI protein expression was induced in cognitively impaired, but not cognitively intact, aged rats and correlated with cognitive performance in individual rats. Immunohistochemical experiments demonstrated that up-regulation of MAIs occurs, in part, in hippocampal neuronal axons and somata. While a number of pathways and processes are altered with brain aging, we report a coordinated induction of myelin-associated inhibitors of functional and structural plasticity only in cognitively impaired aged rats. Induction of MAIs may decrease stimulus-induced synaptic strengthening and structural remodeling, ultimately impairing synaptic mechanisms of spatial learning and memory and resulting in cognitive decline.

Concurrent hippocampal induction of MHC II pathway components and glial activation with advanced aging is not correlated with cognitive impairment

BACKGROUND

Age-related cognitive dysfunction, including impairment of hippocampus-dependent spatial learning and memory, affects approximately half of the aged population. Induction of a variety of neuroinflammatory measures has been reported with brain aging but the relationship between neuroinflammation and cognitive decline with non-neurodegenerative, normative aging remains largely unexplored. This study sought to comprehensively investigate expression of the MHC II immune response pathway and glial activation in the hippocampus in the context of both aging and age-related cognitive decline.

METHODS

Three independent cohorts of adult (12-13 months) and aged (26-28 months) F344xBN rats were behaviorally characterized by Morris water maze testing. Expression of MHC II pathway-associated genes identified by transcriptomic analysis as upregulated with advanced aging was quantified by qPCR in synaptosomal fractions derived from whole hippocampus and in hippocampal subregion dissections (CA1, CA3, and DG). Activation of astrocytes and microglia was assessed by GFAP and Iba1 protein expression, and by immunohistochemical visualization of GFAP and both CD74 (Ox6) and Iba1.

RESULTS

We report a marked age-related induction of neuroinflammatory signaling transcripts (i.e., MHC II components, toll-like receptors, complement, and downstream signaling factors) throughout the hippocampus in all aged rats regardless of cognitive status. Astrocyte and microglial activation was evident in CA1, CA3 and DG of intact and impaired aged rat groups, in the absence of differences in total numbers of GFAP+ astrocytes or Iba1+ microglia. Both mild and moderate microglial activation was significantly increased in all three hippocampal subregions in aged cognitively intact and cognitively impaired rats compared to adults. Neither induction of MHCII pathway gene expression nor glial activation correlated to cognitive performance.

CONCLUSIONS

These data demonstrate a novel, coordinated age-related induction of the MHC II immune response pathway and glial activation in the hippocampus, indicating an allostatic shift toward a para-inflammatory phenotype with advancing age. Our findings demonstrate that age-related induction of these aspects of hippocampal neuroinflammation, while a potential contributing factor, is not sufficient by itself to elicit impairment of spatial learning and memory in models of normative aging. Future efforts are needed to understand how neuroinflammation may act synergistically with cognitive-decline specific alterations to cause cognitive impairment.