A Perfect sTORm: The Role of the Mammalian Target of Rapamycin (mTOR) in Cerebrovascular Dysfunction of Alzheimer's Disease: A Mini-Review

The Usnic Acid Analogue 4-FPBUA Enhances the Blood-Brain Barrier Function and Induces Autophagy in Alzheimer's Disease Mouse Models

Preclinical and clinical studies have indicated that compromised blood-brain barrier (BBB) function contributes to Alzheimer's disease (AD) pathology. BBB breakdown ranged from mild disruption of tight junctions (TJs) with increased BBB permeability to chronic integrity loss, affecting transport across the BBB, reducing brain perfusion, and triggering inflammatory responses. We recently developed a high-throughput screening (HTS) assay to identify hit compounds that enhance the function of a cell-based BBB model. The HTS screen identified (S,E)-2-acetyl-6-[3-(4'-fluorobiphenyl-4-yl)acryloyl]-3,7,9-trihydroxy-8,9b-dimethyldibenzo-[b,d]furan-1(9bH)-one (4-FPBUA), a semisynthetic analogue of naturally occurring usnic acid, which protected the in vitro model against Aβ toxicity. Usnic acid is a lichen-derived secondary metabolite with a unique dibenzofuran skeleton that is commonly found in lichenized fungi of the genera Usnea. In this study, we aimed to evaluate the effect of 4-FPBUA in vitro on the cell-based BBB model function and its in vivo ability to rectify BBB function and reduce brain Aβ in two AD mouse models, namely, 5xFAD and TgSwDI. Our findings demonstrated that 4-FPBUA enhanced cell-based BBB function, increased Aβ transport across the monolayer, and reversed BBB breakdown in vivo by enhancing autophagy as an mTOR inhibitor. Induced autophagy was associated with a significant reduction in Aβ accumulation and related pathologies and improved memory function. These results underscore the potential of 4-FPBUA as a candidate for further preclinical exploration to better understand its mechanisms of action and to optimize dosing strategies. Continued research may also elucidate additional pathways through which 4-FPBUA contributed to the amelioration of BBB dysfunction in AD. Collectively, our findings supported the development of 4-FPBUA as a therapeutic agent against AD.

Glucagon-like peptide-1 receptor agonists and sodium-glucose cotransporter 2 inhibitors, anti-diabetic drugs in heart failure and cognitive impairment: potential mechanisms of the protective effects

Heart failure and cognitive impairment emerge as public health problems that need to be addressed due to the aging global population. The conditions that often coexist are strongly related to advancing age and multimorbidity. Epidemiological evidence indicates that cardiovascular disease and neurodegenerative processes shares similar aspects, in term of prevalence, age distribution, and mortality. Type 2 diabetes increasingly represents a risk factor associated not only to cardiometabolic pathologies but also to neurological conditions. The pathophysiological features of type 2 diabetes and its metabolic complications (hyperglycemia, hyperinsulinemia, and insulin resistance) play a crucial role in the development and progression of both heart failure and cognitive dysfunction. This connection has opened to a potential new strategy, in which new classes of anti-diabetic medications, such as glucagon-like peptide-1 receptor (GLP-1R) agonists and sodium-glucose cotransporter 2 (SGLT2) inhibitors, are able to reduce the overall risk of cardiovascular events and neuronal damage, showing additional protective effects beyond glycemic control. The pleiotropic effects of GLP-1R agonists and SGLT2 inhibitors have been extensively investigated. They exert direct and indirect cardioprotective and neuroprotective actions, by reducing inflammation, oxidative stress, ions overload, and restoring insulin signaling. Nonetheless, the specificity of pathways and their contribution has not been fully elucidated, and this underlines the urgency for more comprehensive research.

Evaluating the effect of rapamycin treatment in Alzheimer's disease and aging using in vivo imaging: the ERAP phase IIa clinical study protocol

BACKGROUND

Rapamycin is an inhibitor of the mechanistic target of rapamycin (mTOR) protein kinase, and preclinical data demonstrate that it is a promising candidate for a general gero- and neuroprotective treatment in humans. Results from mouse models of Alzheimer's disease have shown beneficial effects of rapamycin, including preventing or reversing cognitive deficits, reducing amyloid oligomers and tauopathies and normalizing synaptic plasticity and cerebral glucose uptake. The "Evaluating Rapamycin Treatment in Alzheimer's Disease using Positron Emission Tomography" (ERAP) trial aims to test if these results translate to humans through evaluating the change in cerebral glucose uptake following six months of rapamycin treatment in participants with early-stage Alzheimer's disease.

METHODS

ERAP is a six-month-long, single-arm, open-label, phase IIa biomarker-driven study evaluating if the drug rapamycin can be repurposed to treat Alzheimer's disease. Fifteen patients will be included and treated with a weekly dose of 7 mg rapamycin for six months. The primary endpoint will be change in cerebral glucose uptake, measured using [18F]FDG positron emission tomography. Secondary endpoints include changes in cognitive measures, markers in cerebrospinal fluid as well as cerebral blood flow measured using magnetic resonance imaging. As exploratory outcomes, the study will assess change in multiple age-related pathological processes, such as periodontal inflammation, retinal degeneration, bone mineral density loss, atherosclerosis and decreased cardiac function.

DISCUSSION

The ERAP study is a clinical trial using in vivo imaging biomarkers to assess the repurposing of rapamycin for the treatment of Alzheimer's disease. If successful, the study would provide a strong rationale for large-scale evaluation of mTOR-inhibitors as a potential disease-modifying treatment in Alzheimer's disease.

TRIAL REGISTRATION

ClinicalTrials.gov ID NCT06022068, date of registration 2023-08-30.

Neurovascular coupling impairment as a mechanism for cognitive deficits in COVID-19

Components that comprise our brain parenchymal and cerebrovascular structures provide a homeostatic environment for proper neuronal function to ensure normal cognition. Cerebral insults (e.g. ischaemia, microbleeds and infection) alter cellular structures and physiologic processes within the neurovascular unit and contribute to cognitive dysfunction. COVID-19 has posed significant complications during acute and convalescent stages in multiple organ systems, including the brain. Cognitive impairment is a prevalent complication in COVID-19 patients, irrespective of severity of acute SARS-CoV-2 infection. Moreover, overwhelming evidence from in vitro, preclinical and clinical studies has reported SARS-CoV-2-induced pathologies in components of the neurovascular unit that are associated with cognitive impairment. Neurovascular unit disruption alters the neurovascular coupling response, a critical mechanism that regulates cerebromicrovascular blood flow to meet the energetic demands of locally active neurons. Normal cognitive processing is achieved through the neurovascular coupling response and involves the coordinated action of brain parenchymal cells (i.e. neurons and glia) and cerebrovascular cell types (i.e. endothelia, smooth muscle cells and pericytes). However, current work on COVID-19-induced cognitive impairment has yet to investigate disruption of neurovascular coupling as a causal factor. Hence, in this review, we aim to describe SARS-CoV-2's effects on the neurovascular unit and how they can impact neurovascular coupling and contribute to cognitive decline in acute and convalescent stages of the disease. Additionally, we explore potential therapeutic interventions to mitigate COVID-19-induced cognitive impairment. Given the great impact of cognitive impairment associated with COVID-19 on both individuals and public health, the necessity for a coordinated effort from fundamental scientific research to clinical application becomes imperative. This integrated endeavour is crucial for mitigating the cognitive deficits induced by COVID-19 and its subsequent burden in this especially vulnerable population.

Risk of Dementia in Patients with Diabetes Using Sodium-Glucose Transporter 2 Inhibitors (SGLT2i): A Systematic Review, Meta-Analysis, and Meta-Regression

INTRODUCTION

Dementia is quite prevalent and among the leading causes of death worldwide. According to earlier research, diabetes may increase the possibility of developing dementia. However, the association between antidiabetic agents and dementia is not yet clear. This investigation examines the association between the use of sodium-glucose transporter 2 inhibitors (SGLT2i) and the risk of dementia in patients with diabetes.

METHODS

Up to April 18, 2023, four databases-Europe PMC, Medline, Scopus, and Cochrane Library-were searched for relevant literature. We included all studies that examine dementia risk in adults with diabetes who use SGLT2i. Random-effect models were used to compute the outcomes in this investigation, producing pooled odds ratios (OR) with 95% confidence intervals (CI).

RESULTS

Pooled data from seven observational studies revealed that SGLT2i use was linked to a lower risk of dementia in people with diabetes (OR 0.45, 95% CI 0.34-0.61; p < 0.00001, I2 = 97%). The reduction in the risk of dementia due to SGLT2i's neuroprotective effect was only significantly affected by dyslipidemia (p = 0.0004), but not by sample size (p = 0.2954), study duration (p = 0.0908), age (p = 0.0805), sex (p = 0.5058), hypertension (p = 0.0609), cardiovascular disease (p = 0.1619), or stroke (p = 0.2734).

CONCLUSIONS

According to this research, taking SGLT2i reduces the incidence of dementia in people with diabetes by having a beneficial neuroprotective impact. Randomized controlled trials (RCTs) are still required in order to verify the findings of our research.

Effect of Nrf2 loss on senescence and cognition of tau-based P301S mice

Cellular senescence may contribute to chronic inflammation involved in the progression of age-related diseases such as Alzheimer's disease (AD), and its removal prevents cognitive impairment in a model of tauopathy. Nrf2, the major transcription factor for damage response pathways and regulators of inflammation, declines with age. Our previous work showed that silencing Nrf2 gives rise to premature senescence in cells and mice. Others have shown that Nrf2 ablation can exacerbate cognitive phenotypes of some AD models. In this study, we aimed to understand the relationship between Nrf2 elimination, senescence, and cognitive impairment in AD, by generating a mouse model expressing a mutant human tau transgene in an Nrf2 knockout (Nrf2KO) background. We assessed senescent cell burden and cognitive decline of P301S mice in the presence and absence of Nrf2. Lastly, we administered 4.5-month-long treatments with two senotherapeutic drugs to analyze their potential to prevent senescent cell burden and cognitive decline: the senolytic drugs dasatinib and quercetin (DQ) and the senomorphic drug rapamycin. Nrf2 loss accelerated the onset of hind-limb paralysis in P301S mice. At 8.5 months of age, P301S mice did not exhibit memory deficits, while P301S mice without Nrf2 were significantly impaired. However, markers of senescence were not elevated by Nrf2 ablation in any of tissues that we examined. Neither drug treatment improved cognitive performance, nor did it reduce expression of senescence markers in brains of P301S mice. Contrarily, rapamycin treatment at the doses used delayed spatial learning and led to a modest decrease in spatial memory. Taken together, our data suggests that the emergence of senescence may be causally associated with onset of cognitive decline in the P301S model, indicate that Nrf2 protects brain function in a model of AD through mechanisms that may include, but do not require the inhibition of senescence, and suggest possible limitations for DQ and rapamycin as therapies for AD.

A Computational Framework to Characterize the Cancer Drug Induced Effect on Aging Using Transcriptomic Data

Cancer treatments such as chemotherapies may change or accelerate aging trajectories in cancer patients. Emerging evidence has shown that “omics” data can be used to study molecular changes of the aging process. Here, we integrated the drug-induced and normal aging transcriptomic data to computationally characterize the potential cancer drug-induced aging process in patients. Our analyses demonstrated that the aging-associated gene expression in the GTEx dataset can recapitulate the well-established aging hallmarks. We next characterized the drug-induced transcriptomic changes of 28 FDA approved cancer drugs in brain, kidney, muscle, and adipose tissues. Further drug-aging interaction analysis identified 34 potential drug regulated aging events. Those events include aging accelerating effects of vandetanib (Caprelsa®) and dasatinib (Sprycel®) in brain and muscle, respectively. Our result also demonstrated aging protective effect of vorinostat (Zolinza®), everolimus (Afinitor®), and bosutinib (Bosulif®) in brain.

The disturbance of protein synthesis/degradation homeostasis is a common trait of age-related neurodegenerative disorders

Protein homeostasis or "proteostasis" represent the process that regulates the balance of the intracellular functional and "healthy" proteins. Proteostasis is fundamental to preserve physiological metabolic processes in the cell and it allow to respond to any given stimulus as the expression of components of the proteostasis network is customized according to the proteomic demands of different cellular environments. In conditions that promote unfolding/misfolding of proteins chaperones act as signaling molecules inducing extreme measures to either fix the problem or destroy unfolded proteins. When the chaperone machinery fails under pathological insults unfolded proteins induce the endoplasmic reticulum (ER) stress activating the unfolded protein response (UPR) machinery. The activation of the UPR restores ER proteostasis primarily through the transcriptional remodeling of ER protein folding, trafficking, and degradation pathways, such as the ubiquitin proteasome system (UPS). If these mechanisms do not manage to clear the aberrant proteins, proteasome overload and become defective, and misfolded proteins may form aggregates thus extending the UPR mechanism. These aggregates are then attempted to be cleared by macroautophagy. Impaired proteostasis promote the accumulation of misfolded proteins that exacerbate the damage to chaperones, surveillance systems and/or degradative activities. Remarkably, the removal of toxic misfolded proteins is critical for all cells, but it is especially significant in neurons since these cannot be readily replaced. In neurons, the maintenance of efficient proteostasis is essential to healthy aging since the dysregulation of the proteostasis network can lead to neurodegenerative disease. Each of these brain pathologies is characterized by the repeated misfolding of one of more peculiar proteins, which evade both the protein folding machinery and cellular degradation mechanisms and begins to form aggregates that nucleate out into large fibrillar aggregates. In this chapter we describe the mechanisms, associated with faulty proteostasis, that promote the formation of protein aggregates, amyloid fibrils, intracellular, and extracellular inclusions in the most common nondegenerative disorders also referred to as protein misfolding disorders.

Transferrin decorated-nanostructured lipid carriers (NLCs) are a promising delivery system for rapamycin in Alzheimer's disease: An in vivo study

Alzheimer's disease (AD), the most common neurodegenerative disorder, is characterized by progressive cognitive impairment and memory loss. The mammalian target of rapamycin (mTOR) signaling pathway could regulate learning and memory. The effect of rapamycin (Rapa) on mTOR activity could slow or prevent the progression of AD by affecting various essential cellular processes. Previously, we prepared transferrin (Tf) decorated-nanostructured lipid carriers (NLCs) for rapamycin (150 ± 9 nm) to protect the drug from chemical and enzymatic degradation and for brain targeted delivery of rapamycin. Herein, the effect of Tf-NLCs compared to untargeted anionic-NLCs and free rapamycin, were studied in amyloid beta (Aβ) induced rat model of AD. Behavioral test revealed that the Rapa Tf-NLCs were able to significantly improve the impaired spatial memory induced by Aβ. Histopathological studies of hippocampus also showed neural survival in Rapa Tf-NLCs treated group. The immunosuppressive, and delayed wound healing adverse effects in the rapamycin solution treated group were abolished by incorporating the drug into NLCs. The Aβ induced oxidative stress was also reduced by Rapa Tf-NLCs. Molecular studies on the level of Aβ, autophagy (LC3) and apoptotic (caspase-3) markers, and mTOR activity revealed that the Rapa Tf-NLCs decreased the Aβ level and suppressed the toxic effects of Aβ plaques by modulating the mTOR activity and autophagy, and decreasing the apoptosis level. As a conclusion, the designed Tf-NLCs could be an appropriate and a safe brain delivery system for rapamycin and make this drug more efficient in AD for improving memory and neuroprotection.

Neurovascular coupling mechanisms in health and neurovascular uncoupling in Alzheimer's disease

To match the metabolic demands of the brain, mechanisms have evolved to couple neuronal activity to vasodilation, thus increasing local cerebral blood flow and delivery of oxygen and glucose to active neurons. Rather than relying on metabolic feedback signals such as the consumption of oxygen or glucose, the main signalling pathways rely on the release of vasoactive molecules by neurons and astrocytes, which act on contractile cells. Vascular smooth muscle cells and pericytes are the contractile cells associated with arterioles and capillaries, respectively, which relax and induce vasodilation.

Much progress has been made in understanding the complex signalling pathways of neurovascular coupling, but issues such as the contributions of capillary pericytes and astrocyte calcium signal remain contentious. Study of neurovascular coupling mechanisms is especially important as cerebral blood flow dysregulation is a prominent feature of Alzheimer’s disease. In this article we will discuss developments and controversies in the understanding of neurovascular coupling and finish by discussing current knowledge concerning neurovascular uncoupling in Alzheimer’s disease.

Neuroprotective Effect of SGLT2 Inhibitors

Patients with diabetes are at higher risk of cardiovascular diseases and cognitive impairment. SGLT2 inhibitors (Empagliflozin, Canagliflozin, Dapagliflozin, Ertugliflozin, Sotagliflozin) are newer hypoglycemic agents with many pleiotropic effects. In this review, we discuss their neuroprotective potential. SGLT2 inhibitors (SGLT2i) are lipid-soluble and reach the brain/serum ratio from 0.3 to 0.5. SGLT receptors are present in the central nervous system (CNS). Flozins are not fully SGLT2-selective and have an affinity for the SGLT1 receptor, which is associated with protection against ischemia/reperfusion brain damage. SGLT2i show an anti-inflammatory and anti-atherosclerotic effect, including reduction of proinflammatory cytokines, M2 macrophage polarization, JAK2/STAT1 and NLRP3 inflammasome inhibition, as well as cIMT regression. They also mitigate oxidative stress. SGLT2i improve endothelial function, prevent remodeling and exert a protective effect on the neurovascular unit, blood-brain barrier, pericytes, astrocytes, microglia, and oligodendrocytes. Flozins are also able to inhibit AChE, which contributes to cognitive improvement. Empagliflozin significantly increases the level of cerebral BDNF, which modulates neurotransmission and ensures growth, survival, and plasticity of neurons. Moreover, they may be able to restore the circadian rhythm of mTOR activation, which is quite a novel finding in the field of research on metabolic diseases and cognitive impairment. SGLT2i have a great potential to protect against atherosclerosis and cognitive impairment in patients with type 2 diabetes mellitus.

Pericytes: Intrinsic Transportation Engineers of the CNS Microcirculation

Pericytes in the brain are candidate regulators of microcirculatory blood flow because they are strategically positioned along the microvasculature, contain contractile proteins, respond rapidly to neuronal activation, and synchronize microvascular dynamics and neurovascular coupling within the capillary network. Analyses of mice with defects in pericyte generation demonstrate that pericytes are necessary for the formation of the blood-brain barrier, development of the glymphatic system, immune homeostasis, and white matter function. The development, identity, specialization, and progeny of different subtypes of pericytes, however, remain unclear. Pericytes perform brain-wide 'transportation engineering' functions in the capillary network, instructing, integrating, and coordinating signals within the cellular communicome in the neurovascular unit to efficiently distribute oxygen and nutrients ('goods and services') throughout the microvasculature ('transportation grid'). In this review, we identify emerging challenges in pericyte biology and shed light on potential pericyte-targeted therapeutic strategies.

Mammalian/mechanistic target of rapamycin (mTOR) complexes in neurodegeneration

Novel targets to arrest neurodegeneration in several dementing conditions involving misfolded protein accumulations may be found in the diverse signaling pathways of the Mammalian/mechanistic target of rapamycin (mTOR). As a nutrient sensor, mTOR has important homeostatic functions to regulate energy metabolism and support neuronal growth and plasticity. However, in Alzheimer's disease (AD), mTOR alternately plays important pathogenic roles by inhibiting both insulin signaling and autophagic removal of β-amyloid (Aβ) and phospho-tau (ptau) aggregates. It also plays a role in the cerebrovascular dysfunction of AD. mTOR is a serine/threonine kinase residing at the core in either of two multiprotein complexes termed mTORC1 and mTORC2. Recent data suggest that their balanced actions also have implications for Parkinson's disease (PD) and Huntington's disease (HD), Frontotemporal dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). Beyond rapamycin; an mTOR inhibitor, there are rapalogs having greater tolerability and micro delivery modes, that hold promise in arresting these age dependent conditions.

Contrasting Metabolic Insufficiency in Aging and Dementia

Metabolic insufficiency and neuronal dysfunction occur in normal aging but is exaggerated in dementia and Alzheimer's disease (AD). Metabolic insufficiency includes factors important for both substrate supply and utilization in the brain. Metabolic insufficiency occurs through a number of serial mechanisms, particularly changes in cerebrovascular supply through blood vessel abnormalities (ie, small and large vessel vasculopathy, stroke), alterations in neurovascular coupling providing dynamic blood flow supply in relation to neuronal demand, abnormalities in blood brain barrier including decreased glucose and amino acid transport, altered glymphatic flow in terms of substrate supply across the extracellular space to cells and drainage into CSF of metabolites, impaired transport into cells, and abnormal intracellular metabolism with more reliance on glycolysis and less on mitochondrial function. Recent studies have confirmed abnormal neurovascular coupling in a mouse model of AD in response to metabolic challenges, but the supply chain from the vascular system into neurons is disrupted much earlier in dementia than in equivalently aged individuals, contributing to the progressive neuronal degeneration and cognitive dysfunction associated with dementia. We discuss several metabolic treatment approaches, but these depend on characterizing patients as to who would benefit the most. Surrogate biomarkers of metabolism are being developed to include dynamic estimates of neuronal demand, sufficiency of neurovascular coupling, and glymphatic flow to supplement traditional static measurements. These surrogate biomarkers could be used to gauge efficacy of metabolic treatments in slowing down or modifying dementia time course.

Endothelial-specific insulin receptor substrate-1 overexpression worsens neonatal hypoxic-ischemic brain injury via mTOR-mediated tight junction disassembly

Hypoxic-ischemic (HI) encephalopathy is the major cause of mortality and disability in newborns. The neurovascular unit is a major target of acute and chronic brain injury, and therapies that protect simultaneously both neurons and vascular endothelial cells from neonatal HI injury are in demand. Insulin receptors and its key downstream molecule-insulin receptor substrate −1 (IRS-1) are potential neuroprotective targets and expressed both in neuron and endothelial cells. To investigate whether IRS-1 can act similarly in neurons and vascular endothelial cells in protecting neurovascular units and brain form HI injury, we found that neuron-specific IRS-1 transgenic rats showed reduced neurovascular injury and infarct volumes, whereas endothelial-specific IRS-1 transgenic rats showed increased blood-brain barrier (BBB) disruption and exaggerated neurovascular injury after neonatal HI brain injury. Endothelial-specific IRS-1 overexpression increased vascular permeability and disassembled the tight junction protein (zonula occludens-1) complex. Inhibition of mammalian target of rapamycin (mTOR) by rapamycin preserved tight junction proteins and attenuated BBB leakage and neuronal apoptosis after HI in the endothelial-specific IRS-1 transgenic pups. Together, our findings suggested that neuronal and endothelial IRS-1 had opposite effects on the neurovascular integrity and damage after neonatal HI brain injury and that endothelial IRS-1 worsens neurovascular integrity after HI via mTOR-mediated tight junction protein disassembly.

mTOR in Alzheimer disease and its earlier stages: Links to oxidative damage in the progression of this dementing disorder

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly population and has worldwide impact. The etiology of the disease is complex and results from the confluence of multiple mechanisms ultimately leading to neuronal loss and cognitive decline. Among risk factors, aging is the most relevant and accounts for several pathogenic events that contribute to disease-specific toxic mechanisms. Accumulating evidence linked the alterations of the mammalian target of rapamycin (mTOR), a serine/threonine protein kinase playing a key role in the regulation of protein synthesis and degradation, to age-dependent cognitive decline and pathogenesis of AD. To date, growing studies demonstrated that aberrant mTOR signaling in the brain affects several pathways involved in energy metabolism, cell growth, mitochondrial function and proteostasis. Recent advances associated alterations of the mTOR pathway with the increased oxidative stress. Disruption of all these events strongly contribute to age-related cognitive decline including AD. The current review discusses the main regulatory roles of mTOR signaling network in the brain, focusing on its role in autophagy, oxidative stress and energy metabolism. Collectively, experimental data suggest that targeting mTOR in the CNS can be a valuable strategy to prevent/slow the progression of AD.

mTOR Attenuation with Rapamycin Reverses Neurovascular Uncoupling and Memory Deficits in Mice Modeling Alzheimer's Disease

Vascular dysfunction is a universal feature of aging and decreased cerebral blood flow has been identified as an early event in the pathogenesis of Alzheimer's disease (AD). Cerebrovascular dysfunction in AD includes deficits in neurovascular coupling (NVC), a mechanism that ensures rapid delivery of energy substrates to active neurons through the blood supply. The mechanisms underlying NVC impairment in AD, however, are not well understood. We have previously shown that mechanistic/mammalian target of rapamycin (mTOR) drives cerebrovascular dysfunction in models of AD by reducing the activity of endothelial nitric oxide synthase (eNOS), and that attenuation of mTOR activity with rapamycin is sufficient to restore eNOS-dependent cerebrovascular function. Here we show mTOR drives NVC impairments in an AD model through the inhibition of neuronal NOS (nNOS)- and non-NOS-dependent components of NVC, and that mTOR attenuation with rapamycin is sufficient to restore NVC and even enhance it above WT responses. Restoration of NVC and concomitant reduction of cortical amyloid-β levels effectively treated memory deficits in 12-month-old hAPP(J20) mice. These data indicate that mTOR is a critical driver of NVC dysfunction and underlies cognitive impairment in an AD model. Together with our previous findings, the present studies suggest that mTOR promotes cerebrovascular dysfunction in AD, which is associated with early disruption of nNOS activation, through its broad negative impact on nNOS as well as on non-NOS components of NVC. Our studies highlight the potential of mTOR attenuation as an efficacious treatment for AD and potentially other neurologic diseases of aging.SIGNIFICANCE STATEMENT Failure of the blood flow response to neuronal activation [neurovascular coupling (NVC)] in a model of AD precedes the onset of AD-like cognitive symptoms and is driven, to a large extent, by mammalian/mechanistic target of rapamycin (mTOR)-dependent inhibition of nitric oxide synthase activity. Our studies show that mTOR also drives AD-like failure of non-nitric oxide (NO)-mediated components of NVC. Thus, mTOR attenuation may serve to treat AD, where we find that neuronal NO synthase is profoundly reduced early in disease progression, and potentially other neurologic diseases of aging with cerebrovascular dysfunction as part of their etiology.

Mitophagy pathways and Alzheimer's disease: From pathogenesis to treatment

Alzheimer's disease (AD) is an age-dependent, incurable mental illness that is associated with the accumulation of aggregates of amyloid-beta (Aβ) and hyperphosphorylated tau fragments (p-tau). Detailed studies on postmortem AD brains, cell lines, and mouse models of AD have shown that numerous cellular alterations, including mitochondrial deficits, synaptic disruption and glial/astrocytic activation, are involved in the disease process. Mitophagy is a cellular process by which damaged/weakened mitochondria are selectively eliminated from the cell. In AD, impairments in mitophagy trigger the gradual accumulation of defective mitochondria. This review will focus on the recent progress in understanding the molecular mechanisms and pathological role of mitophagy and its implications for AD pathogenesis. We will also discuss the novel concept of the regulation of mitophagy as a therapeutic avenue for the prevention and treatment of AD.

Blood biomarkers indicate that the preclinical stages of Alzheimer's disease present overlapping molecular features

It is still debated whether non-specific preclinical symptoms of Alzheimer's disease (AD) can have diagnostic relevance. We followed the evolution from cognitively normal to AD by NMR-based metabolomics of blood sera. Multivariate statistical analysis of the NMR profiles yielded models that discriminated subjective memory decline (SMD), mild cognitive impairment (MCI) and AD. We validated a panel of six statistically significant metabolites that predicted SMD, MCI and AD in a blind cohort with sensitivity values ranging from 88 to 95% and receiver operating characteristic values from 0.88 to 0.99. However, lower values of specificity, accuracy and precision were observed for the models involving SMD and MCI, which is in line with the pathological heterogeneity indicated by clinical data. This excludes a "linear" molecular evolution of the pathology, pointing to the presence of overlapping "gray-zones" due to the reciprocal interference of the intermediate stages. Yet, the clear difference observed in the metabolic pathways of each model suggests that pathway dysregulations could be investigated for diagnostic purposes.

A Curcumin Analog Exhibits Multiple Biologic Effects on the Pathogenesis of Alzheimer's Disease and Improves Behavior, Inflammation, and β-Amyloid Accumulation in a Mouse Model

Drugs for the treatment of Alzheimer’s disease (AD) are in urgent demand due to the unmet need and the social burden associated with the disease. Curcumin has been historically considered as a beneficial product for anti-aging and AD. However, many efforts to develop curcumin for clinical use are hindered mainly due to its poor bioavailability. Recent development in drug delivery and structural design has resolved these issues. In this study, we identified a small molecule, TML-6, as a potential drug candidate for AD through screening a panel of curcumin derivatives using six biomarker platforms related to aging biology and AD pathogenesis. The structural modification of TML-6 is designed to improve the stability and metabolism of curcumin. Cell biological studies demonstrated that TML-6 could inhibit the synthesis of the β-amyloid precursor protein and β-amyloid (Aβ), upregulate Apo E, suppress NF-κB and mTOR, and increase the activity of the anti-oxidative Nrf2 gene. In the 3x-Tg AD animal model, TML-6 treatment resulted in significant improvement in learning, suppression of the microglial activation marker Iba-1, and reduction in Aβ in the brain. Although TML-6 exhibited a greater improvement in bioavailability as compared to curcumin, formulation optimization and toxicological studies are under development to assure its druggability. Taken together, TML-6 meets the current strategy to develop therapeutics for AD, targeting the combination of the Aβ cascade and aging-related biology processes.

Multifarious roles of mTOR signaling in cognitive aging and cerebrovascular dysfunction of Alzheimer's disease

Age-related cognitive failure is a main devastating incident affecting even healthy people. Alzheimer's disease (AD) is the utmost common form of dementia among the geriatric community. In the pathogenesis of AD, cerebrovascular dysfunction is revealed before the beginning of the cognitive decline. Mounting proof shows a precarious impact of cerebrovascular dysregulation in the development of AD pathology. Recent studies document that the mammalian target of rapamycin (mTOR) acts as a crucial effector of cerebrovascular dysregulation in AD. The mTOR contributes to brain vascular dysfunction and subsequence cerebral blood flow deficits as well as cognitive impairment. Furthermore, mTOR causes the blood-brain barrier (BBB) breakdown in AD models. Inhibition of mTOR hyperactivity protects the BBB integrity in AD. Furthermore, mTOR drives cognitive defect and cerebrovascular dysfunction, which are greatly prevalent in AD, but the central molecular mechanisms underlying these alterations are obscure. This review represents the crucial and current research findings regarding the role of mTOR signaling in cognitive aging and cerebrovascular dysfunction in the pathogenesis of AD.

Mechanisms of Vascular Aging

Aging of the vasculature plays a central role in morbidity and mortality of older people. To develop novel treatments for amelioration of unsuccessful vascular aging and prevention of age-related vascular pathologies, it is essential to understand the cellular and functional changes that occur in the vasculature during aging. In this review, the pathophysiological roles of fundamental cellular and molecular mechanisms of aging, including oxidative stress, mitochondrial dysfunction, impaired resistance to molecular stressors, chronic low-grade inflammation, genomic instability, cellular senescence, epigenetic alterations, loss of protein homeostasis, deregulated nutrient sensing, and stem cell dysfunction in the vascular system are considered in terms of their contribution to the pathogenesis of both microvascular and macrovascular diseases associated with old age. The importance of progeronic and antigeronic circulating factors in relation to development of vascular aging phenotypes are discussed. Finally, future directions and opportunities to develop novel interventions to prevent/delay age-related vascular pathologies by targeting fundamental cellular and molecular aging processes are presented.

The Post-amyloid Era in Alzheimer's Disease: Trust Your Gut Feeling

The amyloid hypothesis, the assumption that beta-amyloid toxicity is the primary cause of neuronal and synaptic loss, has been the mainstream research concept in Alzheimer's disease for the past two decades. Currently, this model is quietly being replaced by a more holistic, “systemic disease” paradigm which, like the aging process, affects multiple body tissues and organs, including the gut microbiota. It is well-established that inflammation is a hallmark of cellular senescence; however, the infection-senescence link has been less explored. Microbiota-induced senescence is a gradually emerging concept promoted by the discovery of pathogens and their products in Alzheimer's disease brains associated with senescent neurons, glia, and endothelial cells. Infectious agents have previously been associated with Alzheimer's disease, but the cause vs. effect issue could not be resolved. A recent study may have settled this debate as it shows that gingipain, a Porphyromonas gingivalis toxin, can be detected not only in Alzheimer's disease but also in the brains of older individuals deceased prior to developing the illness. In this review, we take the position that gut and other microbes from the body periphery reach the brain by triggering intestinal and blood-brain barrier senescence and disruption. We also surmise that novel Alzheimer's disease findings, including neuronal somatic mosaicism, iron dyshomeostasis, aggressive glial phenotypes, and loss of aerobic glycolysis, can be explained by the infection-senescence model. In addition, we discuss potential cellular senescence targets and therapeutic strategies, including iron chelators, inflammasome inhibitors, senolytic antibiotics, mitophagy inducers, and epigenetic metabolic reprograming.

Role of age-related alterations of the cerebral venous circulation in the pathogenesis of vascular cognitive impairment

There has been an increasing appreciation of the role of vascular contributions to cognitive impairment and dementia (VCID) associated with old age. Strong preclinical and translational evidence links age-related dysfunction and structural alterations of the cerebral arteries, arterioles, and capillaries to the pathogenesis of many types of dementia in the elderly, including Alzheimer's disease. The low-pressure, low-velocity, and large-volume venous circulation of the brain also plays critical roles in the maintenance of homeostasis in the central nervous system. Despite its physiological importance, the role of age-related alterations of the brain venous circulation in the pathogenesis of vascular cognitive impairment and dementia is much less understood. This overview discusses the role of cerebral veins in the pathogenesis of VCID. Pathophysiological consequences of age-related dysregulation of the cerebral venous circulation are explored, including blood-brain barrier disruption, neuroinflammation, exacerbation of neurodegeneration, development of cerebral microhemorrhages of venous origin, altered production of cerebrospinal fluid, impaired function of the glymphatics system, dysregulation of cerebral blood flow, and ischemic neuronal dysfunction and damage. Understanding the age-related functional and phenotypic alterations of the cerebral venous circulation is critical for developing new preventive, diagnostic, and therapeutic approaches to preserve brain health in older individuals.

Rapamycin and Alzheimer's disease: Time for a clinical trial?

The drug rapamycin has beneficial effects in a number of animal models of neurodegeneration and aging including mouse models of Alzheimer's disease. Despite its compelling preclinical record, no clinical trials have tested rapamycin or other mTOR inhibitors in patients with Alzheimer's disease. We argue that such clinical trials should be undertaken.

Tau and mTOR: The Hotspots for Multifarious Diseases in Alzheimer's Development

The hyperphosphorylation of tau protein and the overexpression of mTOR are considered to be the driving force behind Aβ plaques and Neurofibrillay Tangles (NFT's), hallmarks of Alzheimer's disease (AD). It is now evident that miscellaneous diseases such as Diabetes, Autoimmune diseases, Cancer, etc. are correlated with AD. Therefore, we reviewed the literature on the causes of AD and investigated the association of tau and mTOR with other diseases. We have discussed the role of insulin deficiency in diabetes, activated microglial cells, and dysfunction of blood-brain barrier (BBB) in Autoimmune diseases, Presenilin 1 in skin cancer, increased reactive species in mitochondrial dysfunction and deregulated Cyclins/CDKs in promoting AD pathogenesis. We have also discussed the possible therapeutics for AD such as GSK3 inactivation therapy, Rechaperoning therapy, Immunotherapy, Hormonal therapy, Metal chelators, Cell cycle therapy, γ-secretase modulators, and Cholinesterase and BACE 1-inhibitors which are thought to serve a major role in combating pathological changes coupled with AD. Recent research about the relationship between mTOR and aging and hepatic Aβ degradation offers possible targets to effectively target AD. Future prospects of AD aims at developing novel drugs and modulators that can potentially improve cell to cell signaling, prevent Aβ plaques formation, promote better release of neurotransmitters and prevent hyperphosphorylation of tau.

Prospects of Pharmacological Interventions to Organismal Aging

Intense research in the areas of cellular and organismal aging using diverse laboratory model systems has enriched our knowledge in the processes and the signalling pathways involved in normal and pathological conditions. The field finds itself in a position to take decisive steps towards clinical applications and interventions not only for targeted age-related diseases such as cardiovascular conditions and neurodegeneration but also for the modulation of health span and lifespan of a whole organism. Beyond nutritional interventions such as dietary restriction without malnutrition and various regimes of intermittent fasting, accumulating evidence provides promise for pharmacological interventions. The latter, mimic caloric or dietary restriction, tune cellular and organismal stress responses, affect the metabolism of microbiome with subsequent effects on the host or modulate repair pathways, among others. In this mini review, we summarise some of the evidence on drugs that can alter organismal lifespan and the prospects they might offer for promoting healthspan and delaying age-related diseases.