Dendritic spine remodeling accompanies Alzheimer's disease pathology and genetic susceptibility in cognitively normal aging

Deciphering the role of APOE in cerebral amyloid angiopathy: from genetic insights to therapeutic horizons

Cerebral amyloid angiopathy (CAA), characterized by the deposition of amyloid-β (Aβ) peptides in the walls of medium and small vessels of the brain and leptomeninges, is a major cause of lobar hemorrhage in elderly individuals. Among the genetic risk factors for CAA that continue to be recognized, the apolipoprotein E (APOE) gene is the most significant and prevalent, as its variants have been implicated in more than half of all patients with CAA. While the presence of the APOE ε4 allele markedly increases the risk of CAA, the ε2 allele confers a protective effect relative to the common ε3 allele. These allelic variants encode three APOE isoforms that differ at two amino acid positions. The primary physiological role of APOE is to mediate lipid transport in the brain and periphery; however, it has also been shown to be involved in a wide array of biological functions, particularly those involving Aβ, in which it plays a known role in processing, production, aggregation, and clearance. The challenges posed by the reliance on postmortem histological analyses and the current absence of an effective intervention underscore the urgency for innovative APOE-targeted strategies for diagnosing CAA. This review not only deepens our understanding of the impact of APOE on the pathogenesis of CAA but can also help guide the exploration of targeted therapies, inspiring further research into the therapeutic potential of APOE.

Anesthetic Sevoflurane Induces Enlargement of Dendritic Spine Heads in Mouse Neurons via Tau-Dependent Mechanisms

BACKGROUND

Sevoflurane induces neuronal dysfunction and cognitive impairment. However, the underlying mechanism remains largely to be determined. Tau, cyclophilin D, and dendritic spine contribute to cognitive function. But whether changes in dendritic spines are involved in the effects of sevoflurane and the potential association with tau and cyclophilin D is not clear.

METHODS

We harvested hippocampal neurons from wild-type mice, tau knockout mice, and cyclophilin D knockout mice. We treated these neurons with sevoflurane at day in vitro 7 and measured the diameter of dendritic spine head and the number of dendritic spines. Moreover, we determined the effects of sevoflurane on the expression of excitatory amino acid transporter 3 (EAAT3), extracellular glutamate levels, and miniature excitatory postsynaptic currents (mEPSCs). Finally, we used lithium, cyclosporine A, and overexpression of EAAT3 in the interaction studies.

RESULTS

Sevoflurane-induced tau phosphgorylation increased the diameter of dendritic spine head and decreased the number of dendritic spines in neurons harvested from wild-type and cyclophilin D knockout mice, but not tau knockout mice. Sevoflurane decreased the expression of EAAT3, increased extracellular glutamate levels, and decreased the frequency of mEPSCs in the neurons. Overexpression of EAAT3 mitigated the effects of sevoflurane on dendritic spines. Lithium, but not cyclosporine A, attenuated the effects of sevoflurane on dendritic spines. Lithium also inhibited the effects of sevoflurane on EAAT3 expression and mEPSCs.

CONCLUSIONS

These data suggest that sevoflurane induces a tau phosphorylation-dependent demtrimental effect on dendritic spine via decreasing EAAT3 expression and increasing extracellular glutamate levels, leading to neuronal dysfunction.

Resilience to Alzheimer's disease associates with alterations in perineuronal nets

INTRODUCTION

Some individuals show intact cognition despite the presence of neuropathological hallmarks of Alzheimer's disease (AD). The plasticity of parvalbumin (PV)-containing interneurons might contribute to resilience. Perineuronal nets (PNNs), that is, extracellular matrix structures around neurons, modulate PV neuron function. We hypothesize that PNNs play a role in resilience to AD.

METHODS

PNN amount and morphology were determined in immunolabelled sections of the frontal cortex of control, AD and resilient subjects. Expression levels of genes related to PNNs and microglia signatures were evaluated by bulk RNA sequencing.

RESULTS

The expression of the PNN-component aggrecan around PV neurons is decreased in resilient and AD subjects, whereas PNN-sugar chains are reduced only in resilient subjects. In AD, fewer presynaptic terminals on PV neurons are detected and genes related to PNN degradation are upregulated.

DISCUSSION

These data show distinct PNN changes in individuals resilient to AD, which may contribute to preserved cognition despite the neuropathology.

HIGHLIGHTS

Aggrecan levels are decreased in the frontal cortex of AD and resilient subjects. In resilient subjects, WFA+ PNNs are reduced around neuronal somata. In AD patients, PV neurons show disrupted WFA peridendritic staining and synaptic loss. Expression levels of PNN-degrading enzymes are higher in AD. Excitatory neurons bearing a PNN show low amounts of ptau.

The fate of neuronal synapse homeostasis in aging, infection, and inflammation

Neuroplasticity is the brain's ability to reorganize and modify its neuronal connections in response to environmental stimuli, experiences, learning, and disease processes. This encompasses a variety of mechanisms, including changes in synaptic strength and connectivity, the formation of new synapses, alterations in neuronal structure and function, and the generation of new neurons. Proper functioning of synapses, which facilitate neuron-to-neuron communication, is crucial for brain activity. Neuronal synapse homeostasis, which involves regulating and maintaining synaptic strength and function in the central nervous system (CNS), is vital for this process. Disruptions in synaptic balance, due to factors like inflammation, aging, or infection, can lead to impaired brain function. This review highlights the main aspects and mechanisms underlying synaptic homeostasis, particularly in the context of aging, infection, and inflammation.

Age-related loss of large dendritic spines in the precuneus is statistically mediated by proteins which are predicted targets of existing drugs

Preservation of dendritic spines is a putative mechanism of protection against cognitive impairment despite development of Alzheimer Disease (AD)-related pathologies. Aging, the chief late-onset AD risk factor, is associated with dendritic spine loss in select brain areas. However, no study to our knowledge has observed this effect in precuneus, an area selectively vulnerable to early accumulation of AD-related pathology. We therefore quantified dendritic spine density in precuneus from 98 subjects without evidence of neurocognitive decline, spanning ages 20-96, and found a significant negative correlation between age and large dendritic spine density. In these same subjects, we conducted liquid chromatography-tandem mass spectrometry of >5000 proteins and identified 203 proteins which statistically mediate the effect of age on large dendritic spine density. Using computational pharmacology, we identified ten drugs which are predicted to target these mediators, informing future studies designed to test their effects on age-related dendritic spine loss and cognitive decline.

Gadd45A-mediated autophagy regulation and its impact on Alzheimer's disease pathogenesis: Deciphering the molecular Nexus

BACKGROUND

The growth arrest and DNA damage-inducible 45 (Gadd45) gene has been implicated in various central nervous system (CNS) functions, both normal and pathological, including aging, memory, and neurodegenerative diseases. In this study, we examined whether Gadd45A deletion triggers pathways associated with neurodegenerative diseases including Alzheimer's disease (AD).

METHODS

Utilizing transcriptome data from AD-associated hippocampus samples, we identified Gadd45A as a pivotal regulator of autophagy. Comprehensive analyses, including Gene Ontology enrichment and protein-protein interaction network assessments, highlighted Cdkn1A as a significant downstream target of Gadd45A. Experimental validation confirmed Gadd45A's role in modulating Cdkn1A expression and autophagy levels in hippocampal cells. We also examined the effects of autophagy on hippocampal functions and proinflammatory cytokine secretion. Additionally, a murine model was employed to validate the importance of Gadd45A in neuroinflammation and AD pathology.

RESULTS

Our study identified 20 autophagy regulatory factors associated with AD, with Gadd45A emerging as a critical regulator. Experimental findings demonstrated that Gadd45A influences hippocampal cell fate by reducing Cdkn1A expression and suppressing autophagic activity. Comparisons between wild-type (WT) and Gadd45A knockout (Gadd45A-/-) mice revealed that Gadd45A-/- mice exhibited significant cognitive impairments, including deficits in working and spatial memory, increased Tau hyperphosphorylation, and elevated levels of kinases involved in Tau phosphorylation in the hippocampus. Additionally, Gadd45A-/- mice showed significant increases in pro-inflammatory cytokines and decreases autophagy markers in the brain. Neurotrophin levels and dendritic spine length were also reduced in Gadd45A-/- mice, likely contributing to the observed cognitive deficits.

CONCLUSIONS

These findings support the direct involvement of the Gadd45A gene in AD pathogenesis, and enhancing the expression of Gadd45A may represent a promising therapeutic strategy for the treatment of AD.

How do neurons age? A focused review on the aging of the microtubular cytoskeleton

Aging is the leading risk factor for Alzheimer's disease and other neurodegenerative diseases. We now understand that a breakdown in the neuronal cytoskeleton, mainly underpinned by protein modifications leading to the destabilization of microtubules, is central to the pathogenesis of Alzheimer's disease. This is accompanied by morphological defects across the somatodendritic compartment, axon, and synapse. However, knowledge of what occurs to the microtubule cytoskeleton and morphology of the neuron during physiological aging is comparatively poor. Several recent studies have suggested that there is an age-related increase in the phosphorylation of the key microtubule stabilizing protein tau, a modification, which is known to destabilize the cytoskeleton in Alzheimer's disease. This indicates that the cytoskeleton and potentially other neuronal structures reliant on the cytoskeleton become functionally compromised during normal physiological aging. The current literature shows age-related reductions in synaptic spine density and shifts in synaptic spine conformation which might explain age-related synaptic functional deficits. However, knowledge of what occurs to the microtubular and actin cytoskeleton, with increasing age is extremely limited. When considering the somatodendritic compartment, a regression in dendrites and loss of dendritic length and volume is reported whilst a reduction in soma volume/size is often seen. However, research into cytoskeletal change is limited to a handful of studies demonstrating reductions in and mislocalizations of microtubule-associated proteins with just one study directly exploring the integrity of the microtubules. In the axon, an increase in axonal diameter and age-related appearance of swellings is reported but like the dendrites, just one study investigates the microtubules directly with others reporting loss or mislocalization of microtubule-associated proteins. Though these are the general trends reported, there are clear disparities between model organisms and brain regions that are worthy of further investigation. Additionally, longitudinal studies of neuronal/cytoskeletal aging should also investigate whether these age-related changes contribute not just to vulnerability to disease but also to the decline in nervous system function and behavioral output that all organisms experience. This will highlight the utility, if any, of cytoskeletal fortification for the promotion of healthy neuronal aging and potential protection against age-related neurodegenerative disease. This review seeks to summarize what is currently known about the physiological aging of the neuron and microtubular cytoskeleton in the hope of uncovering mechanisms underpinning age-related risk to disease.

Dendritic spine head diameter predicts episodic memory performance in older adults

Episodic memory in older adults is varied and perceived to rely on numbers of synapses or dendritic spines. We analyzed 2157 neurons among 128 older individuals from the Religious Orders Study and Rush Memory and Aging Project. Analysis of 55,521 individual dendritic spines by least absolute shrinkage and selection operator regression and nested model cross-validation revealed that the dendritic spine head diameter in the temporal cortex, but not the premotor cortex, improved the prediction of episodic memory performance in models containing β amyloid plaque scores, neurofibrillary tangle pathology, and sex. These findings support the emerging hypothesis that, in the temporal cortex, synapse strength is more critical than quantity for memory in old age.

Dendritic spines and their role in the pathogenesis of neurodevelopmental and neurological disorders

Since Cajal introduced dendritic spines in the 19th century, they have attained considerable attention, especially in neuropsychiatric and neurologic disorders. Multiple roles of dendritic spine malfunction and pathology in the progression of various diseases have been reported. Thus, it is inevitable to consider these structures as new therapeutic targets for treating neuropsychiatric and neurologic disorders such as autism spectrum disorders, schizophrenia, dementia, Down syndrome, etc. Therefore, we attempted to prepare a narrative review of the literature regarding the role of dendritic spines in the pathogenesis of aforementioned diseases and to shed new light on their pathophysiology.

Melatonin mitigates cisplatin-induced cognitive impairment in rats and improves hippocampal dendritic spine density

Cisplatin-induced cognitive impairment (chemobrain) affects a considerable percentage of cancer patients and has no established pharmacological treatment. Chemobrain can be associated with neuroinflammation and oxidative stress. Melatonin, a pineal hormone, is known to have antioxidant, anti-inflammatory and neuroprotective potential. In this study, we investigated cisplatin-induced cognitive impairment in rats and whether melatonin can improve or reverse this impairment. Behavioral testing involved measuring working memory using the novel location recognition test (NLRT) under conditions of cisplatin or cisplatin + melatonin treatment, followed by the collection of rats' brains. The brains were subsequently stained with Golgi-Cox stain and then the hippocampus area CA3 of each one was examined, and dendritic spine density was calculated. Treatment with cisplatin resulted in deficits in the rats' performance in the NLRT (P < 0.05). These deficits were prevented by the coadministration of melatonin (P < 0.05). Cisplatin also reduced the density of dendritic spines in the hippocampus (P < 0.0001), specifically CA3 area, while the coadministration of melatonin significantly reversed this reduction (P < 0.001). This study showed that melatonin can ameliorate cisplatin-induced spatial memory deficits and dendritic spines density abnormalities in rats. Given that melatonin is a safe and wildly used supplement, it is feasible to explore its use as a palliative intervention in cancer treatment.

Dendritic spine head diameter is reduced in the prefrontal cortex of progranulin haploinsufficient mice

Loss-of-function mutations in the progranulin (GRN) gene are an autosomal dominant cause of Frontotemporal Dementia (FTD). These mutations typically result in haploinsufficiency of the progranulin protein. Grn+/- mice provide a model for progranulin haploinsufficiency and develop FTD-like behavioral abnormalities by 9-10 months of age. In previous work, we demonstrated that Grn+/- mice develop a low dominance phenotype in the tube test that is associated with reduced dendritic arborization of layer II/III pyramidal neurons in the prelimbic region of the medial prefrontal cortex (mPFC), a region key for social dominance behavior in the tube test assay. In this study, we investigated whether progranulin haploinsufficiency induced changes in dendritic spine density and morphology. Individual layer II/III pyramidal neurons in the prelimbic mPFC of 9-10 month old wild-type or Grn+/- mice were targeted for iontophoretic microinjection of fluorescent dye, followed by high-resolution confocal microscopy and 3D reconstruction for morphometry analysis. Dendritic spine density in Grn+/- mice was comparable to wild-type littermates, but the apical dendrites in Grn+/- mice had a shift in the proportion of spine types, with fewer stubby spines and more thin spines. Additionally, apical dendrites of Grn+/- mice had longer spines and smaller thin spine head diameter in comparison to wild-type littermates. These changes in spine morphology may contribute to altered circuit-level activity and social dominance deficits in Grn+/- mice.

The concept of resilience to Alzheimer's Disease: current definitions and cellular and molecular mechanisms

Some individuals are able to maintain their cognitive abilities despite the presence of significant Alzheimer's Disease (AD) neuropathological changes. This discrepancy between cognition and pathology has been labeled as resilience and has evolved into a widely debated concept. External factors such as cognitive stimulation are associated with resilience to AD, but the exact cellular and molecular underpinnings are not completely understood. In this review, we discuss the current definitions used in the field, highlight the translational approaches used to investigate resilience to AD and summarize the underlying cellular and molecular substrates of resilience that have been derived from human and animal studies, which have received more and more attention in the last few years. From these studies the picture emerges that resilient individuals are different from AD patients in terms of specific pathological species and their cellular reaction to AD pathology, which possibly helps to maintain cognition up to a certain tipping point. Studying these rare resilient individuals can be of great importance as it could pave the way to novel therapeutic avenues for AD.

Alzheimer's therapeutic development: shifting neurodegeneration to neuroregeneration

Alzheimer's disease (AD), similar to AD-related dementias, is characterized by impaired/lost neuronal structures and functions due to a long progression of neurodegeneration. Derailed endogenous signal pathways and disease processes have critical roles in neurodegeneration and are pharmacological targets in inducing neuroregeneration. Pharmacologically switching/shifting the brain status from neurodegeneration to neuroregeneration is emerging as a new therapeutic concept, one that is not only achievable, but also essential for effective therapy for AD. The results of the pharmacological-induced shift from neurodegeneration to neuroregeneration are twofold: arresting cognitive deterioration (and directing the brain toward cognitive recovery) in established AD, and preventing neurodegeneration through building up cognitive resilience in patients with preclinical or probable AD. In this review, we discuss these new developments in AD pharmacology and relevant clinical trials.

A concerted neuron-astrocyte program declines in ageing and schizophrenia

Human brains vary across people and over time; such variation is not yet understood in cellular terms. Here we describe a relationship between people's cortical neurons and cortical astrocytes. We used single-nucleus RNA sequencing to analyse the prefrontal cortex of 191 human donors aged 22-97 years, including healthy individuals and people with schizophrenia. Latent-factor analysis of these data revealed that, in people whose cortical neurons more strongly expressed genes encoding synaptic components, cortical astrocytes more strongly expressed distinct genes with synaptic functions and genes for synthesizing cholesterol, an astrocyte-supplied component of synaptic membranes. We call this relationship the synaptic neuron and astrocyte program (SNAP). In schizophrenia and ageing-two conditions that involve declines in cognitive flexibility and plasticity1,2-cells divested from SNAP: astrocytes, glutamatergic (excitatory) neurons and GABAergic (inhibitory) neurons all showed reduced SNAP expression to corresponding degrees. The distinct astrocytic and neuronal components of SNAP both involved genes in which genetic risk factors for schizophrenia were strongly concentrated. SNAP, which varies quantitatively even among healthy people of similar age, may underlie many aspects of normal human interindividual differences and may be an important point of convergence for multiple kinds of pathophysiology.

Deletion of Gadd45a Expression in Mice Leads to Cognitive and Synaptic Impairment Associated with Alzheimer's Disease Hallmarks

Gadd45 genes have been implicated in survival mechanisms, including apoptosis, autophagy, cell cycle arrest, and DNA repair, which are processes related to aging and life span. Here, we analyzed if the deletion of Gadd45a activates pathways involved in neurodegenerative disorders such as Alzheimer's Disease (AD). This study used wild-type (WT) and Gadd45a knockout (Gadd45a-/-) mice to evaluate AD progression. Behavioral tests showed that Gadd45a-/- mice presented lower working and spatial memory, pointing out an apparent cognitive impairment compared with WT animals, accompanied by an increase in Tau hyperphosphorylation and the levels of kinases involved in its phosphorylation in the hippocampus. Moreover, Gadd45a-/- animals significantly increased the brain's pro-inflammatory cytokines and modified autophagy markers. Notably, neurotrophins and the dendritic spine length of the neurons were reduced in Gadd45a-/- mice, which could contribute to the cognitive alterations observed in these animals. Overall, these findings demonstrate that the lack of the Gadd45a gene activates several pathways that exacerbate AD pathology, suggesting that promoting this protein's expression or function might be a promising therapeutic strategy to slow down AD progression.

Modulatory Effect of Low-Intensity Transcranial Ultrasound Stimulation on Behaviour and Neural Oscillation in Mouse Models of Alzheimer's Disease

Transcranial ultrasound stimulation (TUS) is a noninvasive brain neuromodulation technique. The application of TUS for Alzheimer's disease (AD) therapy has not been widely studied. In this study, a long-term course (28 days) of TUS was used to stimulate the hippocampus of APP/PS1 mice. We examined the modulatory effect of TUS on behavior and neural oscillation in AD mice. We found that TUS can 1) improve the learning and memory abilities of AD mice; 2) reduce the phase-amplitude coupling of delta-epsilon, delta-gamma and theta-gamma frequency bands of local field potential, and increase the relative power of epsilon frequency bands in AD mice; 3) reduce the spike firing rate of interneurons and inhibit the phase-locked angle deflection between the theta frequency bands and the spikes of the two types of neurons that develops with the progression of the disease in AD mice. In summary, we demonstrate that TUS could effectively improve cognitive behavior and modulate neural oscillation with AD.

Cadherin-13 Maintains Retinotectal Synapses via Transneuronal Interactions

Maintaining precise synaptic contacts between neuronal partners is critical to ensure the proper functioning of the mammalian central nervous system (CNS). Diverse cell recognition molecules, such as classic cadherins (Cdhs), are part of the molecular machinery mediating synaptic choices during development and synaptic maintenance. Yet, the principles governing neuron-neuron wiring across diverse CNS neuron types remain largely unknown. The retinotectal synapses, connections from the retinal ganglion cells (RGCs) to the superior collicular (SC) neurons, offer an ideal experimental system to reveal molecular logic underlying synaptic choices and formation. This is due to the retina's unidirectional and laminar-restricted projections to the SC and the large databases of presynaptic RGC subtypes and postsynaptic SC neuronal types. Here, we focused on determining the role of Type II Cdhs in wiring the retinotectal synapses. We surveyed Cdhs expression patterns at neuronal resolution and revealed that Cdh13 is enriched in the wide-field neurons in the superficial SC (sSC). In either the Cdh13 null mutant or selective adult deletion within the wide-field neurons, there is a significant reduction of spine densities in the distal dendrites of these neurons in both sexes. Additionally, Cdh13 removal from presynaptic RGCs reduced dendritic spines in the postsynaptic wide-field neurons. Cdh13-expressing RGCs use differential mechanisms than αRGCs and On-Off Direction-Selective Ganglion Cells (ooDSGCs) to form specific retinotectal synapses. The results revealed a selective transneuronal interaction mediated by Cdh13 to maintain proper retinotectal synapses in vivo.

CYP46A1-mediated cholesterol turnover induces sex-specific changes in cognition and counteracts memory loss in ovariectomized mice

The brain-specific enzyme CYP46A1 controls cholesterol turnover by converting cholesterol into 24S-hydroxycholesterol (24OH). Dysregulation of brain cholesterol turnover and reduced CYP46A1 levels are observed in Alzheimer's disease (AD). In this study, we report that CYP46A1 overexpression in aged female mice leads to enhanced estrogen signaling in the hippocampus and improved cognitive functions. In contrast, age-matched CYP46A1 overexpressing males show anxiety-like behavior, worsened memory, and elevated levels of 5α-dihydrotestosterone in the hippocampus. We report that, in neurons, 24OH contributes to these divergent effects by activating sex hormone signaling, including estrogen receptors. CYP46A1 overexpression in female mice protects from memory impairments induced by ovariectomy while having no effects in gonadectomized males. Last, we measured cerebrospinal fluid levels of 24OH in a clinical cohort of patients with AD and found that 24OH negatively correlates with neurodegeneration markers only in women. We suggest that CYP46A1 activation is a valuable pharmacological target for enhancing estrogen signaling in women at risk of developing neurodegenerative diseases.

Impact of dendritic spine loss on excitability of hippocampal CA1 pyramidal neurons: a computational study of early Alzheimer disease

Synaptic spine loss is an early pathophysiologic hallmark of Alzheimer disease (AD) that precedes overt loss of dendritic architecture and frank neurodegeneration. While spine loss signifies a decreased engagement of postsynaptic neurons by presynaptic targets, the degree to which loss of spines and their passive components impacts the excitability of postsynaptic neurons and responses to surviving synaptic inputs is unclear. Using passive multicompartmental models of CA1 pyramidal neurons (PNs), implicated in early AD, we find that spine loss alone drives a boosting of remaining inputs to their proximal and distal dendrites, targeted by CA3 and entorhinal cortex (EC), respectively. This boosting effect is higher in distal versus proximal dendrites and can be mediated by spine loss restricted to the distal compartment, enough to impact synaptic input integration and somatodendritic backpropagation. This has particular relevance to very early stages of AD in which pathophysiology extends from EC to CA1.

Concerted neuron-astrocyte gene expression declines in aging and schizophrenia

Human brains vary across people and over time; such variation is not yet understood in cellular terms. Here we describe a striking relationship between people's cortical neurons and cortical astrocytes. We used single-nucleus RNA-seq to analyze the prefrontal cortex of 191 human donors ages 22-97 years, including healthy individuals and persons with schizophrenia. Latent-factor analysis of these data revealed that in persons whose cortical neurons more strongly expressed genes for synaptic components, cortical astrocytes more strongly expressed distinct genes with synaptic functions and genes for synthesizing cholesterol, an astrocyte-supplied component of synaptic membranes. We call this relationship the Synaptic Neuron-and-Astrocyte Program (SNAP). In schizophrenia and aging - two conditions that involve declines in cognitive flexibility and plasticity 1,2 - cells had divested from SNAP: astrocytes, glutamatergic (excitatory) neurons, and GABAergic (inhibitory) neurons all reduced SNAP expression to corresponding degrees. The distinct astrocytic and neuronal components of SNAP both involved genes in which genetic risk factors for schizophrenia were strongly concentrated. SNAP, which varies quantitatively even among healthy persons of similar age, may underlie many aspects of normal human interindividual differences and be an important point of convergence for multiple kinds of pathophysiology.

Information encoded in volumes and areas of dendritic spines is nearly maximal across mammalian brains

Many experiments suggest that long-term information associated with neuronal memory resides collectively in dendritic spines. However, spines can have a limited size due to metabolic and neuroanatomical constraints, which should effectively limit the amount of encoded information in excitatory synapses. This study investigates how much information can be stored in the population of sizes of dendritic spines, and whether it is optimal in any sense. It is shown here, using empirical data for several mammalian brains across different regions and physiological conditions, that dendritic spines nearly maximize entropy contained in their volumes and surface areas for a given mean size in cortical and hippocampal regions. Although both short- and heavy-tailed fitting distributions approach [Formula: see text] of maximal entropy in the majority of cases, the best maximization is obtained primarily for short-tailed gamma distribution. We find that most empirical ratios of standard deviation to mean for spine volumes and areas are in the range [Formula: see text], which is close to the theoretical optimal ratios coming from entropy maximization for gamma and lognormal distributions. On average, the highest entropy is contained in spine length ([Formula: see text] bits per spine), and the lowest in spine volume and area ([Formula: see text] bits), although the latter two are closer to optimality. In contrast, we find that entropy density (entropy per spine size) is always suboptimal. Our results suggest that spine sizes are almost as random as possible given the constraint on their size, and moreover the general principle of entropy maximization is applicable and potentially useful to information and memory storing in the population of cortical and hippocampal excitatory synapses, and to predicting their morphological properties.

Synaptic Compensatory Plasticity in Alzheimer's Disease

The loss of excitatory synapses is known to underlie the cognitive deficits in Alzheimer's disease (AD). Although much is known about the mechanisms underlying synaptic loss in AD, how neurons compensate for this loss and whether this provides cognitive benefits remain almost completely unexplored. In this review, we describe two potential compensatory mechanisms implemented following synaptic loss: the enlargement of the surviving neighboring synapses and the regeneration of synapses. Because dendritic spines, the postsynaptic site of excitatory synapses, are easily visualized using light microscopy, we focus on a range of microscopy approaches to monitor synaptic loss and compensation. Here, we stress the importance of longitudinal dendritic spine imaging, as opposed to fixed-tissue imaging, to gain insights into the temporal dynamics of dendritic spine compensation. We believe that understanding the molecular mechanisms behind these and other forms of synaptic compensation and regeneration will be critical for the development of therapeutics aiming at delaying the onset of cognitive deficits in AD.

Licochalcone A: A Potential Multitarget Drug for Alzheimer's Disease Treatment

Licochalcone A (Lico-A) is a flavonoid compound derived from the root of the Glycyrrhiza species, a plant commonly used in traditional Chinese medicine. While the Glycyrrhiza species has shown promise in treating various diseases such as cancer, obesity, and skin diseases due to its active compounds, the investigation of Licochalcone A's effects on the central nervous system and its potential application in Alzheimer's disease (AD) treatment have garnered significant interest. Studies have reported the neuroprotective effects of Lico-A, suggesting its potential as a multitarget compound. Lico-A acts as a PTP1B inhibitor, enhancing cognitive activity through the BDNF-TrkB pathway and exhibiting inhibitory effects on microglia activation, which enables mitigation of neuroinflammation. Moreover, Lico-A inhibits c-Jun N-terminal kinase 1, a key enzyme involved in tau phosphorylation, and modulates the brain insulin receptor, which plays a role in cognitive processes. Lico-A also acts as an acetylcholinesterase inhibitor, leading to increased levels of the neurotransmitter acetylcholine (Ach) in the brain. This mechanism enhances cognitive capacity in individuals with AD. Finally, Lico-A has shown the ability to reduce amyloid plaques, a hallmark of AD, and exhibits antioxidant properties by activating the nuclear factor erythroid 2-related factor 2 (Nrf2), a key regulator of antioxidant defense mechanisms. In the present review, we discuss the available findings analyzing the potential of Lico-A as a neuroprotective agent. Continued research on Lico-A holds promise for the development of novel treatments for cognitive disorders and neurodegenerative diseases, including AD. Further investigations into its multitarget action and elucidation of underlying mechanisms will contribute to our understanding of its therapeutic potential.

More than just risk for Alzheimer's disease: APOE ε4's impact on the aging brain

The apolipoprotein ε4 (APOE ε4) allele is most commonly associated with increased risk for late-onset Alzheimer's disease (AD). However, recent longitudinal studies suggest that these risks are overestimated; most ε4 carriers will not develop dementia in their lifetime. In this article, we review new evidence regarding the impact of APOE ε4 on cognition among healthy older adults. We discuss emerging work from animal models suggesting that ε4 impacts brain structure and function in multiple ways that may lead to age-related cognitive impairment, independent from AD pathology. We discuss the importance of taking an individualized approach in future studies by incorporating biomarkers and neuroimaging methods that may better disentangle the phenotypic influences of APOE ε4 on the aging brain from prodromal AD pathology.

Dietary fatty acids differentially impact phagocytosis, inflammatory gene expression, and mitochondrial respiration in microglial and neuronal cell models

The consumption of diets high in saturated fatty acids and/or refined carbohydrates are associated with neuroinflammation, cognitive dysfunction, and neurodegenerative disease. In contrast, diets high in polyunsaturated fatty acids are associated with anti-inflammatory and neuroprotective effects. We have previously shown that high fat diet (HFD) consumption increases saturated fatty acids and decreases polyunsaturated fatty acids in the hippocampus. We have further shown that HFD elicits exaggerated neuroinflammation and reduced synaptic elements, and results in robust memory deficits in aged rats. Here, we examined the impact of palmitate, an abundant dietary saturated fat, on a variety of cellular responses in BV2 microglia and HippoE-14 neurons, and the extent to which the omega-3 fatty acid, docosahexaenoic acid (DHA), would buffer against these responses. Our data demonstrate that DHA pretreatment prevents or partially attenuates palmitate-induced alterations in proinflammatory, endoplasmic reticulum stress, and mitochondrial damage-associated gene expression in both cell types. Furthermore, we show that synaptoneurosomes isolated from aged, HFD-fed mice are engulfed by BV2 microglia at a faster rate than synaptoneurosomes isolated from aged, chow-fed mice, suggesting HFD alters signaling at synapses to hasten their engulfment by microglia. Consistent with this notion, we found modest increases in complement proteins and a decrease in CD47 protein expression on synaptoneurosomes isolated from the hippocampus of aged, HFD-fed mice. Interestingly, palmitate reduced BV2 microglial phagocytosis, but only of synaptoneurosomes isolated from chow-fed mice, an effect that was prevented by DHA pretreatment. Lastly, we measured the impact of palmitate and DHA on mitochondrial function in both microglial and neuronal cell models using the Seahorse XFe96 Analyzer. These data indicate that DHA pretreatment does not mitigate palmitate-induced reductions in mitochondrial respiration in BV2 microglia and HippoE-14 neurons, suggesting DHA may be acting downstream of mitochondrial function to exert its protective effects. Together, this study provides evidence that DHA can ameliorate the negative impact of palmitate on a variety of cellular functions in microglia- and neuron-like cells.

PINNet: a deep neural network with pathway prior knowledge for Alzheimer's disease

Introduction

Identification of Alzheimer's Disease (AD)-related transcriptomic signatures from blood is important for early diagnosis of the disease. Deep learning techniques are potent classifiers for AD diagnosis, but most have been unable to identify biomarkers because of their lack of interpretability.

Methods

To address these challenges, we propose a pathway information-based neural network (PINNet) to predict AD patients and analyze blood and brain transcriptomic signatures using an interpretable deep learning model. PINNet is a deep neural network (DNN) model with pathway prior knowledge from either the Gene Ontology or Kyoto Encyclopedia of Genes and Genomes databases. Then, a backpropagation-based model interpretation method was applied to reveal essential pathways and genes for predicting AD.

Results

The performance of PINNet was compared with a DNN model without a pathway. Performances of PINNet outperformed or were similar to those of DNN without a pathway using blood and brain gene expressions, respectively. Moreover, PINNet considers more AD-related genes as essential features than DNN without a pathway in the learning process. Pathway analysis of protein-protein interaction modules of highly contributed genes showed that AD-related genes in blood were enriched with cell migration, PI3K-Akt, MAPK signaling, and apoptosis in blood. The pathways enriched in the brain module included cell migration, PI3K-Akt, MAPK signaling, apoptosis, protein ubiquitination, and t-cell activation.

Discussion

By integrating prior knowledge about pathways, PINNet can reveal essential pathways related to AD. The source codes are available at https://github.com/DMCB-GIST/PINNet.

An Imbalance in the Pro/mature BDNF Ratio Occurs in Multiple Brain Regions During Normal Ageing in Wild-Type Mice

The early transition to Alzheimer's disease is characterized by a period of accelerated brain atrophy that exceeds normal ageing. Identifying the molecular basis of this atrophy could facilitate the discovery of novel drug targets. The precursor of brain-derived neurotrophic factor, a well characterized neurotrophin, is increased in the hippocampus of aged rodents, while its mature isoform is relatively stable. This imbalance could increase the risk of Alzheimer's disease by precipitating its pathological hallmarks. However, less is known about how relative levels of these isoforms change in middle-aged mice. In addition, the underlying mechanisms that might cause an imbalance are unknown. The main aim of this study was to determine how precursor brain-derived neurotrophic factor changes relative to its mature isoform with normal brain ageing in wild type mice. A secondary aim was to determine if signaling through the neurotrophin receptor, p75 influences this ratio. An increasing ratio was identified in several brain regions, except the hippocampus, suggesting a neurotrophic imbalance occurs as early as middle age. Some changes in receptors that mediate the isoforms effects were also identified, but these did not correspond with trends in the isoforms. Relative amounts of precursor brain-derived neurotrophic factor were mostly unchanged in mutant p75 mice. The lack of changes suggested that signaling through the receptor had no influence on the ratio.

Cross-Platform Synaptic Network Analysis of Human Entorhinal Cortex Identifies TWF2 as a Modulator of Dendritic Spine Length

Proteomic studies using postmortem human brain tissue samples have yielded robust assessments of the aging and neurodegenerative disease(s) proteomes. While these analyses provide lists of molecular alterations in human conditions, like Alzheimer's disease (AD), identifying individual proteins that affect biological processes remains a challenge. To complicate matters, protein targets may be highly understudied and have limited information on their function. To address these hurdles, we sought to establish a blueprint to aid selection and functional validation of targets from proteomic datasets. A cross-platform pipeline was engineered to focus on synaptic processes in the entorhinal cortex (EC) of human patients, including controls, preclinical AD, and AD cases. Label-free quantification mass spectrometry (MS) data (n = 2260 proteins) was generated on synaptosome fractionated tissue from Brodmann area 28 (BA28; n = 58 samples). In parallel, dendritic spine density and morphology was measured in the same individuals. Weighted gene co-expression network analysis was used to construct a network of protein co-expression modules that were correlated with dendritic spine metrics. Module-trait correlations were used to guide unbiased selection of Twinfilin-2 (TWF2), which was the top hub protein of a module that positively correlated with thin spine length. Using CRISPR-dCas9 activation strategies, we demonstrated that boosting endogenous TWF2 protein levels in primary hippocampal neurons increased thin spine length, thus providing experimental validation for the human network analysis. Collectively, this study describes alterations in dendritic spine density and morphology as well as synaptic proteins and phosphorylated tau from the entorhinal cortex of preclinical and advanced stage AD patients.SIGNIFICANCE STATEMENT Proteomic studies can yield vast lists of molecules that are altered under various experimental or disease conditions. Here, we provide a blueprint to facilitate mechanistic validation of protein targets from human brain proteomic datasets. We conducted a proteomic analysis of human entorhinal cortex (EC) samples spanning cognitively normal and Alzheimer's disease (AD) cases with a comparison of dendritic spine morphology in the same samples. Network integration of proteomics with dendritic spine measurements allowed for unbiased discovery of Twinfilin-2 (TWF2) as a regulator of dendritic spine length. A proof-of-concept experiment in cultured neurons demonstrated that altering Twinfilin-2 protein level induced corresponding changes in dendritic spine length, thus providing experimental validation for the computational framework.

Integrated Proteomics to Understand the Role of Neuritin (NRN1) as a Mediator of Cognitive Resilience to Alzheimer's Disease

The molecular mechanisms and pathways enabling certain individuals to remain cognitively normal despite high levels of Alzheimer's disease (AD) pathology remain incompletely understood. These cognitively normal people with AD pathology are described as preclinical or asymptomatic AD (AsymAD) and appear to exhibit cognitive resilience to the clinical manifestations of AD dementia. Here we present a comprehensive network-based approach from cases clinically and pathologically defined as asymptomatic AD to map resilience-associated pathways and extend mechanistic validation. Multiplex tandem mass tag MS (TMT-MS) proteomic data (n = 7787 proteins) was generated on brain tissue from Brodmann area 6 and Brodmann area 37 (n = 109 cases, n = 218 total samples) and evaluated by consensus weighted gene correlation network analysis. Notably, neuritin (NRN1), a neurotrophic factor previously linked to cognitive resilience, was identified as a hub protein in a module associated with synaptic biology. To validate the function of NRN1 with regard to the neurobiology of AD, we conducted microscopy and physiology experiments in a cellular model of AD. NRN1 provided dendritic spine resilience against amyloid-β (Aβ) and blocked Aβ-induced neuronal hyperexcitability in cultured neurons. To better understand the molecular mechanisms of resilience to Aβ provided by NRN1, we assessed how exogenous NRN1 alters the proteome by TMT-MS (n = 8238 proteins) of cultured neurons and integrated the results with the AD brain network. This revealed overlapping synapse-related biology that linked NRN1-induced changes in cultured neurons with human pathways associated with cognitive resilience. Collectively, this highlights the utility of integrating the proteome from the human brain and model systems to advance our understanding of resilience-promoting mechanisms and prioritize therapeutic targets that mediate resilience to AD.

Synapse pathology in Alzheimer's disease

Synapse loss and damage are central features of Alzheimer's disease (AD) and contribute to the onset and progression of its behavioural and physiological features. Here we review the literature describing synapse pathology in AD, from what we have learned from microscopy in terms of its impacts on synapse architecture, to the mechanistic role of Aβ, tau and glial cells, mitochondrial dysfunction, and the link with AD risk genes. We consider the emerging view that synapse pathology may operate at a further level, that of synapse diversity, and discuss the prospects for leveraging new synaptome mapping methods to comprehensively understand the molecular properties of vulnerable and resilient synapses. Uncovering AD impacts on brain synapse diversity should inform therapeutic approaches targeted at preserving or replenishing lost and damaged synapses and aid the interpretation of clinical imaging approaches that aim to measure synapse damage.

Real-time mechanisms of exacerbated synaptic remodeling by microglia in acute models of systemic inflammation and tauopathy

Remodeling of synapses by microglia is essential for synaptic plasticity in the brain. However, during neuroinflammation and neurodegenerative diseases, microglia can induce excessive synaptic loss, although the precise underlying mechanisms are unknown. To directly observe microglia-synapse interactions under inflammatory conditions, we performed in vivo two-photon time-lapse imaging of microglia-synapse interactions after bacterial lipopolysaccharide administration to model systemic inflammation, or after inoculation of Alzheimer's disease (AD) brain extracts to model disease-associated neuroinflammatory microglial response. Both treatments prolonged microglia-neuron contacts, decreased basal surveillance of synapses and promoted synaptic remodeling in response to synaptic stress induced by focal single-synapse photodamage. Spine elimination correlated with the expression of microglial complement system/phagocytic proteins and the occurrence of synaptic filopodia. Microglia were observed contacting spines, then stretching and phagocytosing spine head filopodia. Thus, in response to inflammatory stimuli microglia exacerbated spine remodeling through prolonged microglial contact and elimination of spines 'tagged' by synaptic filopodia.

From cradle to grave: neurogenesis, neuroregeneration and neurodegeneration in Alzheimer's and Parkinson's diseases

Two of the most common neurodegenerative disorders - Alzheimer's and Parkinson's diseases - are characterized by synaptic dysfunction and degeneration that culminate in neuronal loss due to abnormal protein accumulation. The intracellular aggregation of hyper-phosphorylated tau and the extracellular aggregation of amyloid beta plaques form the basis of Alzheimer's disease pathology. The major hallmark of Parkinson's disease is the loss of dopaminergic neurons in the substantia nigra pars compacta, following the formation of Lewy bodies, which consists primarily of alpha-synuclein aggregates. However, the discrete mechanisms that contribute to neurodegeneration in these disorders are still poorly understood. Both neuronal loss and impaired adult neurogenesis have been reported in animal models of these disorders. Yet these findings remain subject to frequent debate due to a lack of conclusive evidence in post mortem brain tissue from human patients. While some publications provide significant findings related to axonal regeneration in Alzheimer's and Parkinson's diseases, they also highlight the limitations and obstacles to the development of neuroregenerative therapies. In this review, we summarize in vitro and in vivo findings related to neurogenesis, neuroregeneration and neurodegeneration in the context of Alzheimer's and Parkinson's diseases.

Comparison of Golgi-Cox and Intracellular Loading of Lucifer Yellow for Dendritic Spine Density and Morphology Analysis in the Mouse Brain

Dendritic spines are small protrusions on dendrites that serve as the postsynaptic site of the majority of excitatory synapses. These structures are important for normal synaptic transmission, and alterations in their density and morphology have been documented in various disease states. Over 130 years ago, Ramón y Cajal used Golgi-stained tissue sections to study dendritic morphology. Despite the array of technological advances, including iontophoretic microinjection of Lucifer yellow (LY) fluorescent dye, Golgi staining continues to be one of the most popular approaches to visualize dendritic spines. Here, we compared dendritic spine density and morphology among pyramidal neurons in layers 2/3 of the mouse medial prefrontal cortex (mPFC) and pyramidal neurons in hippocampal CA1 using three-dimensional digital reconstructions of (1) brightfield microscopy z-stacks of Golgi-impregnated dendrites and (2) confocal microscopy z-stacks of LY-filled dendrites. Analysis of spine density revealed that the LY microinjection approach enabled detection of approximately three times as many spines as the Golgi staining approach in both brain regions. Spine volume measurements were larger using Golgi staining compared to LY microinjection in both mPFC and CA1. Spine length was mostly comparable between techniques in both regions. In the mPFC, head diameter was similar for Golgi staining and LY microinjection. However, in CA1, head diameter was approximately 50% smaller on LY-filled dendrites compared to Golgi staining. These results indicate that Golgi staining and LY microinjection yield different spine density and morphology measurements, with Golgi staining failing to detect dendritic spines and overestimating spine size.

Structural Plasticity of the Hippocampus in Neurodegenerative Diseases

Neuroplasticity is the capacity of neural networks in the brain to alter through development and rearrangement. It can be classified as structural and functional plasticity. The hippocampus is more susceptible to neuroplasticity as compared to other brain regions. Structural modifications in the hippocampus underpin several neurodegenerative diseases that exhibit cognitive and emotional dysregulation. This article reviews the findings of several preclinical and clinical studies about the role of structural plasticity in the hippocampus in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. In this study, literature was surveyed using Google Scholar, PubMed, Web of Science, and Scopus, to review the mechanisms that underlie the alterations in the structural plasticity of the hippocampus in neurodegenerative diseases. This review summarizes the role of structural plasticity in the hippocampus for the etiopathogenesis of neurodegenerative diseases and identifies the current focus and gaps in knowledge about hippocampal dysfunctions. Ultimately, this information will be useful to propel future mechanistic and therapeutic research in neurodegenerative diseases.

Acetylcholine deficiency disrupts extratelencephalic projection neurons in the prefrontal cortex in a mouse model of Alzheimer's disease

Short-term memory deficits have been associated with prefrontal cortex (PFC) dysfunction in Alzheimer’s disease (AD) and AD mouse models. Extratelencephalic projection (ET) neurons in the PFC play a key role in short-term working memory, but the mechanism between ET neuronal dysfunction in the PFC and short-term memory impairment in AD is not well understood. Here, using fiber photometry and optogenetics, we found reduced neural activity in the ET neurons in the medial prefrontal cortex (mPFC) of the 5×FAD mouse model led to object recognition memory (ORM) deficits. Activation of ET neurons in the mPFC of 5×FAD mice rescued ORM impairment, and inhibition of ET neurons in the mPFC of wild type mice impaired ORM expression. ET neurons in the mPFC that project to supramammillary nucleus were necessary for ORM expression. Viral tracing and in vivo recording revealed that mPFC ET neurons received fewer cholinergic inputs from the basal forebrain in 5×FAD mice. Furthermore, activation of cholinergic fibers in the mPFC rescued ORM deficits in 5×FAD mice, while acetylcholine deficiency reduced the response of ET neurons in the mPFC to familiar objects. Taken together, our results revealed a neural mechanism behind ORM impairment in 5×FAD mice.

Short-term memory deficits are associated with prefrontal cortex dysfunction in Alzheimer’s disease. Here, the authors assessed extratelencephalic projection (ET) neurons and found reduced ET neural activity in the medial prefrontal cortex (mPFC) and showed ET neurons received fewer cholinergic inputs from the basal forebrain in 5×FAD mice which led to object recognition memory deficits.

A53T α-synuclein induces neurogenesis impairment and cognitive dysfunction in line M83 transgenic mice and reduces the proliferation of embryonic neural stem cells

Dementia with Lewy body (DLB) is the second most common degenerative dementia after Alzheimer's disease. There is no therapeutic drug for DLB currently. It's urgent for us to understand the pathological mechanism of dementia mediated by α-synuclein, as the main component of Lewy body. Here, we found that the A53T α-synuclein transgenic mice showed decreased nesting behavior starting from the age of 1 month. The results in Morris water maze test suggested that the 6-month-old mice had learning memory deficits. Golgi staining indicated that the apical neuronal dendritic spines of hippocampal CA1 neurons were significantly reduced in 6-month-old homozygotes and heterozygotes, although MAP2 protein expression revealed no significant difference in the hippocampus among wild-type mice, homozygotes and heterozygotes. In vitro, we proved mutant A53T α-synuclein decreased the dendritic branches and dendrite spines on the embryonic mice hippocampal neurons. Furthermore, Ki67 immunofluorescence staining identified that the Ki67-positive cells of the hippocampal dentate gyrus and subventricular zone were significantly reduced in 6-month-old homozygotes and heterozygotes, compared with age-matched wild-type mice. Similarly, when 6-month-old mice were injected with BrdU for one day, the immunostaining results also confirmed that BrdU-positive cells were significantly reduced in homozygous and heterozygous mice. Lastly, we transfected primary embryonic hippocampal neural stem cells with lentivirus vector expressing A53T α-synuclein in vitro. Both BrdU staining and Western blotting showed that A53T α-synuclein significantly decreased the proliferation of embryonic neural stem cells. Taken together, these data suggest that A53T α-synuclein can induce adult neurogenesis impairment and cognitive dysfunction. The A53T α-synuclein transgenic mice may be used as an animal model for DLB. Promoting adult neurogenesis may be a promising approach to treat DLB pathogenesis.

Unlocking the Memory Component of Alzheimer’s Disease:Biological Processes and Pathways across Brain Regions

Alzheimer’s Disease (AD) is a neurodegenerative disorder characterized by a progressive loss of memory and a general cognitive decline leading to dementia. AD is characterized by changes in the behavior of the genome and can be traced across multiple brain regions and cell types. It is mainly associated with β-amyloid deposits and tau protein misfolding, leading to neurofibrillary tangles. In recent years, however, research has shown that there is a high complexity of mechanisms involved in AD neurophysiology and functional decline enabling its diverse presentation and allowing more questions to arise. In this study, we present a computational approach to facilitate brain region-specific analysis of genes and biological processes involved in the memory process in AD. Utilizing current genetic knowledge we provide a gene set of 265 memory-associated genes in AD, combinations of which can be found co-expressed in 11 different brain regions along with their functional role. The identified genes participate in a spectrum of biological processes ranging from structural and neuronal communication to epigenetic alterations and immune system responses. These findings provide new insights into the molecular background of AD and can be used to bridge the genotype–phenotype gap and allow for new therapeutic hypotheses.

Comparative neuropathology in aging primates: A perspective

While humans exhibit a significant degree of neuropathological changes associated with deficits in cognitive and memory functions during aging, non-human primates (NHP) present with more variable expressions of pathological alterations among individuals and species. As such, NHP with long life expectancy in captivity offer an opportunity to study brain senescence in the absence of the typical cellular pathology caused by age-related neurodegenerative illnesses commonly seen in humans. Age-related changes at neuronal population, single cell, and synaptic levels have been well documented in macaques and marmosets, while age-related and Alzheimer's disease-like neuropathology has been characterized in additional species including lemurs as well as great apes. We present a comparative overview of existing neuropathologic observations across the primate order, including classic age-related changes such as cell loss, amyloid deposition, amyloid angiopathy, and tau accumulation. We also review existing cellular and ultrastructural data on neuronal changes, such as dendritic attrition and spine alterations, synaptic loss and pathology, and axonal and myelin pathology, and discuss their repercussions on cellular and systems function and cognition.

Dendritic Spines: Mediators of Cognitive Resilience in Aging and Alzheimer's Disease

Cognitive resilience is often defined as the ability to remain cognitively normal in the face of insults to the brain. These insults can include disease pathology, such as plaques and tangles associated with Alzheimer's disease, stroke, traumatic brain injury, or other lesions. Factors such as physical or mental activity and genetics may contribute to cognitive resilience, but the neurobiological underpinnings remain ill-defined. Emerging evidence suggests that dendritic spine structural plasticity is one plausible mechanism. In this review, we highlight the basic structure and function of dendritic spines and discuss how spine density and morphology change in aging and Alzheimer's disease. We note evidence that spine plasticity mediates resilience to stress, and we tackle dendritic spines in the context of cognitive resilience to Alzheimer's disease. Finally, we examine how lifestyle and genetic factors may influence dendritic spine plasticity to promote cognitive resilience before discussing evidence for actin regulatory kinases as therapeutic targets for Alzheimer's disease.

Metformin a Potential Pharmacological Strategy in Late Onset Alzheimer's Disease Treatment

Alzheimer's disease (AD) is one of the most devastating brain disorders. Currently, there are no effective treatments to stop the disease progression and it is becoming a major public health concern. Several risk factors are involved in the progression of AD, modifying neuronal circuits and brain cognition, and eventually leading to neuronal death. Among them, obesity and type 2 diabetes mellitus (T2DM) have attracted increasing attention, since brain insulin resistance can contribute to neurodegeneration. Consequently, AD has been referred to "type 3 diabetes" and antidiabetic medications such as intranasal insulin, glitazones, metformin or liraglutide are being tested as possible alternatives. Metformin, a first line antihyperglycemic medication, is a 5'-adenosine monophosphate (AMP)-activated protein kinase (AMPK) activator hypothesized to act as a geroprotective agent. However, studies on its association with age-related cognitive decline have shown controversial results with positive and negative findings. In spite of this, metformin shows positive benefits such as anti-inflammatory effects, accelerated neurogenesis, strengthened memory, and prolonged life expectancy. Moreover, it has been recently demonstrated that metformin enhances synaptophysin, sirtuin-1, AMPK, and brain-derived neuronal factor (BDNF) immunoreactivity, which are essential markers of plasticity. The present review discusses the numerous studies which have explored (1) the neuropathological hallmarks of AD, (2) association of type 2 diabetes with AD, and (3) the potential therapeutic effects of metformin on AD and preclinical models.

Masitinib for the treatment of Alzheimer's disease

The actual standard treatment for mild-to-moderately severe Alzheimer's disease only attacks its symptoms. Masitinib is a potent and selective phenylaminothiazole-type tyrosine kinase inhibitor which is currently in Phase III studies for the treatment of Alzheimer's disease (AD) with the aim of modifying its evolution and with multiple pharmacological targets such as inhibition of mast cells activity, inhibition of microglia activation, modulation of Aβ and Tau protein signaling pathway and prevention of synaptic damage. Here, we review the preclinical and clinical studies that investigated the administration of masitinib treatment in monotherapy in AD. All research studies revealed positive effects concerning the cognitive functions in AD and generally with good safety and tolerability.

Quantification of Dendritic Spines Remodeling under Physiological Stimuli and in Pathological Conditions

Numerous brain diseases are associated with abnormalities in morphology and density of dendritic spines, small membranous protrusions whose structural geometry correlates with the strength of synaptic connections. Thus, the quantitative analysis of dendritic spines remodeling in microscopic images is one of the key elements towards understanding mechanisms of structural neuronal plasticity and bases of brain pathology. In the following article, we review experimental approaches designed to assess quantitative features of dendritic spines under physiological stimuli and in pathological conditions. We compare various methodological pipelines of biological models, sample preparation, data analysis, image acquisition, sample size, and statistical analysis. The methodology and results of relevant experiments are systematically summarized in a tabular form. In particular, we focus on quantitative data regarding the number of animals, cells, dendritic spines, types of studied parameters, size of observed changes, and their statistical significance.

System-Level Analysis of Alzheimer's Disease Prioritizes Candidate Genes for Neurodegeneration

Alzheimer’s disease (AD) is a debilitating neurodegenerative disorder. Since the advent of the genome-wide association study (GWAS) we have come to understand much about the genes involved in AD heritability and pathophysiology. Large case-control meta-GWAS studies have increased our ability to prioritize weaker effect alleles, while the recent development of network-based functional prediction has provided a mechanism by which we can use machine learning to reprioritize GWAS hits in the functional context of relevant brain tissues like the hippocampus and amygdala. In parallel with these developments, groups like the Alzheimer’s Disease Neuroimaging Initiative (ADNI) have compiled rich compendia of AD patient data including genotype and biomarker information, including derived volume measures for relevant structures like the hippocampus and the amygdala. In this study we wanted to identify genes involved in AD-related atrophy of these two structures, which are often critically impaired over the course of the disease. To do this we developed a combined score prioritization method which uses the cumulative distribution function of a gene’s functional and positional score, to prioritize top genes that not only segregate with disease status, but also with hippocampal and amygdalar atrophy. Our method identified a mix of genes that had previously been identified in AD GWAS including APOE, TOMM40, and NECTIN2(PVRL2) and several others that have not been identified in AD genetic studies, but play integral roles in AD-effected functional pathways including IQSEC1, PFN1, and PAK2. Our findings support the viability of our novel combined score as a method for prioritizing region- and even cell-specific AD risk genes.

Perspectives on ROCK2 as a Therapeutic Target for Alzheimer's Disease

Rho-associated coiled-coil containing kinase isoform 2 (ROCK2) is a member of the AGC family of serine/threonine kinases and an extensively studied regulator of actin-mediated cytoskeleton contractility. Over the past decade, new evidence has emerged that suggests ROCK2 regulates autophagy. Recent studies indicate that dysregulation of autophagy contributes to the development of misfolded tau aggregates among entorhinal cortex (EC) excitatory neurons in early Alzheimer's disease (AD). While the accumulation of tau oligomers and fibrils is toxic to neurons, autophagy facilitates the degradation of these pathologic species and represents a major cellular pathway for tau disposal in neurons. ROCK2 is expressed in excitatory neurons and pharmacologic inhibition of ROCK2 can induce autophagy pathways. In this mini-review, we explore potential mechanisms by which ROCK2 mediates autophagy and actin dynamics and discuss how these pathways represent therapeutic avenues for Alzheimer's disease.

Pontine Arteriolosclerosis and Locus Coeruleus Oxidative Stress Differentiate Resilience from Mild Cognitive Impairment in a Clinical Pathologic Cohort

Locus coeruleus (LC) neurodegeneration is associated with cognitive deterioration during the transition from normal cognition to mild cognitive impairment (MCI) and Alzheimer disease (AD). However, the extent to which LC degenerative processes differentiate cognitively normal, "resilient" subjects bearing a high AD pathological burden from those with MCI or AD remains unclear. We approached this problem by quantifying the number of LC neurons and the percentage of LC neurons bearing AT8 tau pathology, TDP-43 pathology, or a marker for DNA/RNA oxidative damage, in well-characterized subjects diagnosed as normal cognition-low AD pathology (NC-LP), NC-high AD pathology (NC-HP), MCI, or mild/moderate AD. In addition, the severity of pontine arteriolosclerosis in each subject was compared across the groups. There was a trend for a step-wise ∼20% loss of LC neuron number between the NC-LP, NC-HP and MCI subjects despite a successive, significant ∼80%-100% increase in tau pathology between these groups. In contrast, increasing pontine arteriolosclerosis severity scores and LC oxidative stress burden significantly separated the NC-LP/HP and MCI/AD groups via comparative, correlation, and regression analysis. Pontine perfusion, as well as LC neuronal metabolic and redox function, may impact noradrenergic LC modulation of cognition during the preclinical and prodromal stages of AD.

Ghrelin treatment leads to dendritic spine remodeling in hippocampal neurons and increases the expression of specific BDNF-mRNA species

Ghrelin (Gr) is an orexigenic peptide that acts via its specific receptor, GHSR-1a distributed throughout the brain, being mainly enriched in pituitary, cortex and hippocampus (Hp) modulating a variety of brain functions. Behavioral, electrophysiological and biochemical evidence indicated that Gr modulates the excitability and the synaptic plasticity in Hp. The present experiments were designed in order to extend the knowledge about the Gr effect upon structural synaptic plasticity since morphological and quantitative changes in spine density after Gr administration were analyzed "in vitro" and "in vivo". The results show that Gr administered to hippocampal cultures or stereotactically injected in vivo to Thy-1 mice increases the density of dendritic spines (DS) being the mushroom type highly increased in secondary and tertiary extensions. Spines classified as thin type were increased particularly in primary extensions. Furthermore, we show that Gr enhances selectively the expression of BDNF-mRNA species.

Dendritic Spine Remodeling and Synaptic Tau Levels in PS19 Tauopathy Mice

Synapse or dendritic spine loss is the strongest correlate of cognitive decline in Alzheimer's disease (AD), and neurofibrillary tangles (NFTs), but not amyloid-β plaques, associate more closely with transition to mild cognitive impairment. Yet, how dendritic spine architecture is affected by hyperphosphorylated tau is still an ongoing question. To address this, we combined cell and biochemical analyses of the Tau P301S mouse line (PS19). Individual pyramidal neurons in the hippocampus and medial prefrontal cortex (mPFC) were targeted for iontophoretic microinjection of fluorescent dye, followed by high-resolution confocal microscopy and 3D morphometry analysis. In the hippocampus, PS19 mice and non-transgenic (NTG) littermates displayed equivalent spine density at 6 and 9 months, but both genotypes exhibited age-related thin spine loss. PS19 mice exhibited significant increases in synaptic tau protein levels and mean dendritic spine head diameter with age. This suggests that CA1 pyramidal neurons in PS19 mice may undergo spine remodeling in response to tau accumulation and age. In the mPFC, spine density was similar among PS19 mice and NTG littermates at 6 and 9 months, but age-related reductions in synaptic tau levels were observed among PS19 mice. Collectively, these studies reveal brain region-specific changes in dendritic spine density and morphology in response to age and the presence of hyperphosphorylated tau in the PS19 mouse line.

ADAP1/Centaurin-α1 Negatively Regulates Dendritic Spine Function and Memory Formation in the Hippocampus

ADAP1/Centaurin-α1 (CentA1) functions as an Arf6 GTPase-activating protein highly enriched in the brain. Previous studies demonstrated the involvement of CentA1 in brain function as a regulator of dendritic differentiation and a potential mediator of Alzheimer’s disease (AD) pathogenesis. To better understand the neurobiological functions of CentA1 signaling in the brain, we developed Centa1 knock-out (KO) mice. The KO animals showed neither brain development nor synaptic ultrastructure deficits in the hippocampus. However, they exhibited significantly higher density and enhanced structural plasticity of dendritic spines in the CA1 region of the hippocampus compared with non-transgenic (NTG) littermates. Moreover, the deletion of Centa1 improved performance in the object-in-place (OIP) spatial memory task. These results suggest that CentA1 functions as a negative regulator of spine density and plasticity, and of hippocampus-dependent memory formation. Thus, CentA1 and its downstream signaling may serve as a potential therapeutic target to prevent memory decline associated with aging and brain disorders.

Hippocampal insulin resistance and the Sirtuin 1 signaling pathway in diabetes-induced cognitive dysfunction

In the peripheral nervous system, the activation of Sirtuin 1 can improve insulin resistance; however, the role played by Sirtuin 1 in the central nervous system remains unknown. In this study, rat models of diabetes mellitus were generated by a single injection of streptozotocin. At 8 weeks after streptozotocin injection, the Morris water maze test and western blot assays confirmed that the diabetic model rats had learning and memory deficits, insulin resistance, and Sirtuin 1 expression could be detected in the hippocampus. Insulin and the insulin receptor inhibitor S961 were intranasally administered to investigate the regulatory effects of insulin signaling on Sirtuin 1. The results showed that insulin administration improved the impaired cognitive function of diabetic model rats and increased the expression levels of phosphorylated insulin receptor, phosphorylated insulin receptor substrate 1, and Sirtuin 1 in the hippocampus. Conversely, S961 administration resulted in more severe cognitive dysfunction and reduced the expression levels of phosphorylated insulin receptor, phosphorylated insulin receptor substrate 1, and Sirtuin 1. The Sirtuin 1 activator SRT2104 and the inhibitor Sirtinol were injected into the lateral ventricle, which revealed that the activation of Sirtuin 1 increased the expression levels of target of rapamycin complex 1, phosphorylated cAMP-response element-binding protein, and brain-derived neurotrophic factor. Hippocampal dendritic length and spine density also increased in response to Sirtuin 1 activation. In contrast, Sirtinol decreased the expression levels of target of rapamycin complex 1, phosphorylated cAMP-response element-binding protein, and brain-derived neurotrophic factor and damaged the dendritic structure. These findings suggest that the Sirtuin 1 signaling pathway plays an important role in the development of insulin resistance-related cognitive deficits in diabetic rats. This study was approved by the Animal Ethics Welfare Committee of the First Affiliated Hospital of Hunan University of Chinese Medicine (approval No. ZYFY201811207) in November 2018.

Emerging role of AMPA receptor subunit GluA1 in synaptic plasticity: Implications for Alzheimer's disease

It is well established that GluA1 mediated synaptic plasticity plays a central role in the early development of AD. The complex cellular and molecular mechanisms that enable GluA1‐related synaptic regulation remain to fully understood. Particularly, understanding the mechanisms that disrupt GluA1 related synaptic plasticity is central to the development of disease‐modifying therapies which are sorely needed as the incidence of AD rises. We surmise that the published evidence establishes deficits in synaptic plasticity as a central factor of AD aetiology. We additionally highlight potential therapeutic strategies for the treatment of AD, and we delve into the roles of GluA1 in learning and memory. Particularly, we review the current understanding of the molecular interactions that confer the actions of this ubiquitous excitatory receptor subunit including post‐translational modification and accessory protein recruitment of the GluA1 subunit. These are proposed to regulate receptor trafficking, recycling, channel conductance and synaptic transmission and plasticity.