Alzheimer's Disease and Diabetes: Insulin Signaling as the Bridge Linking Two Pathologies

A Tale of Two Diseases: Exploring Mechanisms Linking Diabetes Mellitus with Alzheimer's Disease

Dementias, including the type associated with Alzheimer's disease (AD), are on the rise worldwide. Similarly, type 2 diabetes mellitus (T2DM) is one of the most prevalent chronic diseases globally. Although mechanisms and treatments are well-established for T2DM, there remains much to be discovered. Recent research efforts have further investigated factors involved in the etiology of AD. Previously perceived to be unrelated diseases, commonalities between T2DM and AD have more recently been observed. As a result, AD has been labeled as "type 3 diabetes". In this review, we detail the shared processes that contribute to these two diseases. Insulin resistance, the main component of the pathogenesis of T2DM, is also present in AD, causing impaired brain glucose metabolism, neurodegeneration, and cognitive impairment. Dysregulation of insulin receptors and components of the insulin signaling pathway, including protein kinase B, glycogen synthase kinase 3β, and mammalian target of rapamycin are reported in both diseases. T2DM and AD also show evidence of inflammation, oxidative stress, mitochondrial dysfunction, advanced glycation end products, and amyloid deposition. The impact that changes in neurovascular structure and genetics have on the development of these conditions is also being examined. With the discovery of factors contributing to AD, innovative treatment approaches are being explored. Investigators are evaluating the efficacy of various T2DM medications for possible use in AD, including but not limited to glucagon-like peptide-1 receptor agonists, and peroxisome proliferator-activated receptor-gamma agonists. Furthermore, there are 136 active trials involving 121 therapeutic agents targeting novel AD biomarkers. With these efforts, we are one step closer to alleviating the ravaging impact of AD on our communities.

A comprehensive review on RAGE-facilitated pathological pathways connecting Alzheimer’s disease, diabetes mellitus, and cardiovascular diseases

Background

Alzheimer’s disease (AD), cardiovascular disease (CVD), and diabetes are some of the most common causes of morbidity and mortality among the aging populations and cause a heavy burden on the worldwide healthcare system. In this review, we briefly highlighted cellular inflammation-based pathways of diabetes mellitus and CVD through receptor for advanced glycation end products AGEs or RAGE leading to Alzheimer’s disease and interrelation between these vascular and metabolic disorders. The articles were retrieved from Google Scholar, Science Direct, and PubMed databases using the following terms: Alzheimer’s; AGEs; RAGE; RAGE in Alzheimer’s; AGEs in Alzheimer’s; RAGE in diabetes; RAGE related pathways of CVD; RAGE in hypertension; RAGE and RAS system; RAGE and oxidative stress.

Main body of the abstract

AD is a neurodegenerative disease characterized by cognitive dysfunction and neuronal cell death. Vascular complications like hypertension, coronary artery disease, and atherosclerosis as well as metabolic syndromes like obesity and diabetes are related to the pathophysiology of AD. RAGE plays significant role in the onset and progression of AD. Amyloid plaques and neurofibrillary tangles (NFT) are two main markers of AD that regulates via RAGE and other RAGE/ligands interactions which also induces oxidative stress and a cascade of other cellular inflammation pathways leading to AD. Though AD and diabetes are two different disorders but may be inter-linked by AGEs and RAGE. In long-term hyperglycemia, upregulated AGEs interacts with RAGE and produces reactive oxygen species which induces further inflammation and vascular complications. Aging, hypercholesterolemia, atherosclerosis, hypertension, obesity, and inflammation are some of the main risk factors for both diabetes and dementia. Chronic hypertension and coronary artery disease disrupt the functions of the blood-brain barrier and are responsible for the accumulation of senile plaques and NFTs.

Short conclusion

RAGE plays a role in the etiology of Aβ and tau hyperphosphorylation, both of which contribute to cognitive impairment. So far, targeting RAGE may provide a potential sight to develop therapies against some metabolic disorders.

Hormonal Regulation of Oxidative Phosphorylation in the Brain in Health and Disease

The developing and adult brain is a target organ for the vast majority of hormones produced by the body, which are able to cross the blood-brain barrier and bind to their specific receptors on neurons and glial cells. Hormones ensure proper communication between the brain and the body by activating adaptive mechanisms necessary to withstand and react to changes in internal and external conditions by regulating neuronal and synaptic plasticity, neurogenesis and metabolic activity of the brain. The influence of hormones on energy metabolism and mitochondrial function in the brain has gained much attention since mitochondrial dysfunctions are observed in many different pathological conditions of the central nervous system. Moreover, excess or deficiency of hormones is associated with cell damage and loss of function in mitochondria. This review aims to expound on the impact of hormones (GLP-1, insulin, thyroid hormones, glucocorticoids) on metabolic processes in the brain with special emphasis on oxidative phosphorylation dysregulation, which may contribute to the formation of pathological changes. Since the brain concentrations of sex hormones and neurosteroids decrease with age as well as in neurodegenerative diseases, in parallel with the occurrence of mitochondrial dysfunction and the weakening of cognitive functions, their beneficial effects on oxidative phosphorylation and expression of antioxidant enzymes are also discussed.

Metformin prevents p-tau and amyloid plaque deposition and memory impairment in diabetic mice

Insulin deficiency or resistance can promote dementia and hallmarks of Alzheimer's disease (AD). The formation of neurofibrillary tangles of p-TAU protein, extracellular Aβ plaques, and neuronal loss is related to the switching off insulin signaling in cognition brain areas. Metformin is a biguanide antihyperglycemic drug used worldwide for the treatment of type 2 diabetes. Some studies have demonstrated that metformin exerts neuroprotective, anti-inflammatory, anti-oxidant, and nootropic effects. This study aimed to evaluate metformin's effects on long-term memory and p-Tau and amyloid β modulation, which are hallmarks of AD in diabetic mice. Swiss Webster mice were distributed in the following experimental groups: control; treated with streptozotocin (STZ) that is an agent toxic to the insulin-producing beta cells; STZ + metformin 200 mg/kg (M200). STZ mice showed significant augmentation of time spent to reach the target box in the Barnes maze, while M200 mice showed a significant time reduction. Moreover, the M200 group showed reduced GFAP immunoreactivity in hippocampal dentate gyrus and CA1 compared with the STZ group. STZ mice showed high p-Tau levels, reduced p-CREB, and accumulation of β-amyloid (Aβ) plaque in hippocampal areas and corpus callosum. In contrast, all these changes were reversed in the M200 group. Protein expressions of p-Tau, p-ERK, pGSK3, iNOS, nNOS, PARP, Cytochrome c, caspase 3, and GluN2A were increased in the parietal cortex of STZ mice and significantly counteracted in M200 mice. Moreover, M200 mice also showed significantly high levels of eNOS, AMPK, and p-AKT expression. In conclusion, metformin improved spatial memory in diabetic mice, which can be associated with reducing p-Tau and β-amyloid (Aβ) plaque load and inhibition of neuronal death.

Diabetes Mellitus and Amyotrophic Lateral Sclerosis: A Systematic Review

BACKGROUND

Amyotrophic Lateral Sclerosis (ALS) is a degenerative disorder which affects the motor neurons. Growing evidence suggests that ALS may impact the metabolic system, including the glucose metabolism. Several studies investigated the role of Diabetes Mellitus (DM) as risk and/or prognostic factor. However, a clear correlation between DM and ALS has not been defined. In this review, we focus on the role of DM in ALS, examining the different hypotheses on how perturbations of glucose metabolism may interact with the pathophysiology and the course of ALS.

METHODS

We undertook an independent PubMed literature search, using the following search terms: ((ALS) OR (Amyotrophic Lateral Sclerosis) OR (Motor Neuron Disease)) AND ((Diabetes) OR (Glucose Intolerance) OR (Hyperglycemia)). Review and original articles were considered.

RESULTS

DM appears not to affect ALS severity, progression, and survival. Contrasting data suggested a protective role of DM on the occurrence of ALS in elderly and an opposite effect in younger subjects.

CONCLUSIONS

The actual clinical and pathophysiological correlation between DM and ALS is unclear. Large longitudinal prospective studies are needed. Achieving large sample sizes comparable to those of common complex diseases like DM is a challenge for a rare disease like ALS. Collaborative efforts could overcome this specific issue.

Peganum harmala enhanced GLP-1 and restored insulin signaling to alleviate AlCl3-induced Alzheimer-like pathology model

Peganum harmala (P. harmala) is a folk medicinal herb used in the Sinai Peninsula (Egypt) as a remedy for central disorders. The main constituents, harmine and harmaline, have displayed therapeutic efficacy against Alzheimer's disease (AD); however, the P. harmala potential on sensitizing central insulin to combat AD remains to be clarified. An AD-like rat model was induced by aluminum chloride (AlCl3; 50 mg/kg/day for six consecutive weeks; i.p), whereas a methanolic standardized P. harmala seed extract (187.5 mg/kg; p.o) was given to AD rats starting 2 weeks post AlCl3 exposure. Two additional groups of rats were administered either the vehicle to serve as the normal control or the vehicle + P. harmala seed extract to serve as the P. harmala control group. P. harmala enhanced cognition appraised by Y-maze and Morris water maze tests and improved histopathological structures altered by AlCl3. Additionally, it heightened the hippocampal contents of glucagon-like peptide (GLP)-1 and insulin, but abated insulin receptor substrate-1 phosphorylation at serine 307 (pS307-IRS-1). Besides, P. harmala increased phosphorylated Akt at serine 473 (pS473-Akt) and glucose transporter type (GLUT)4. The extract also curtailed the hippocampal content of beta amyloid (Aβ)42, glycogen synthase (GSK)-3β and phosphorylated tau. It also enhanced Nrf2, while reduced lipid peroxides and replenished glutathione. In conclusion, combating insulin resistance by P. harmala is a novel machinery in attenuating the insidious progression of AD by enhancing both insulin and GLP-1 trajectories in the hippocampus favoring GLUT4 production.

Insulin Resistance as a Common Link Between Current Alzheimer's Disease Hypotheses

Almost 115 years ago, Alois Alzheimer described Alzheimer's disease (AD) for the first time. Since then, many hypotheses have been proposed. However, AD remains a severe health public problem. The current medical approaches for AD are limited to symptomatic interventions and the complexity of this disease has led to a failure rate of approximately 99.6%in AD clinical trials. In fact, no new drug has been approved for AD treatment since 2003. These failures indicate that we are failing in mimicking this disease in experimental models. Although most studies have focused on the amyloid cascade hypothesis of AD, the literature has made clear that AD is rather a multifactorial disorder. Therefore, the persistence in a single theory has resulted in lost opportunities. In this review, we aim to present the striking points of the long scientific path followed since the description of the first AD case and the main AD hypotheses discussed over the last decades. We also propose insulin resistance as a common link between many other hypotheses.

Evaluating Disease Similarity Based on Gene Network Reconstruction and Representation

MOTIVATION

Quantifying the associations between diseases is of great significance in increasing our understanding of disease biology, improving disease diagnosis, re-positioning, and developing drugs. Therefore, in recent years, the research of disease similarity has received a lot of attention in the field of bioinformatics. Previous work has shown that the combination of the ontology (such as disease ontology and gene ontology) and disease-gene interactions are worthy to be regarded to elucidate diseases and disease associations. However, most of them are either based on the overlap between disease-related gene sets or distance within the ontology's hierarchy. The diseases in these methods are represented by discrete or sparse feature vectors, which cannot grasp the deep semantic information of diseases. Recently, deep representation learning has been widely studied and gradually applied to various fields of bioinformatics. Based on the hypothesis that disease representation depends on its related gene representations, we propose a disease representation model using two most representative gene resources HumanNet and Gene Ontology to construct a new gene network and learn gene (disease) representations. The similarity between two diseases is computed by the cosine similarity of their corresponding representations.

RESULTS

We propose a novel approach to compute disease similarity, which integrates two important factors disease-related genes and gene ontology hierarchy to learn disease representation based on deep representation learning. Under the same experimental settings, the AUC value of our method is 0.8074, which improves the most competitive baseline method by 10.1%. The quantitative and qualitative experimental results show that our model can learn effective disease representations and improve the accuracy of disease similarity computation significantly.

AVAILABILITY

The research shows that this method has certain applicability in the prediction of gene-related diseases, the migration of disease treatment methods, drug development, and so on.

SUPPLEMENTARY INFORMATION

Supplementary data are available at https://github.com/catly/disease_similarity.

Dementia and Adherence to Anti-Diabetic Medications: A Meta-Analysis

Introduction Diabetes mellitus (DM) and dementia (DN) are common morbid disorders with high mortality, the two disorders shared the pathogenesis of proinflammation and insulin resistance. Polypharmacy is expected when DM and DN co-exist and medication adherence is essential to an effective self-care and management plan. This meta-analysis aimed to assess medication persistence among patients with diabetes and cognitive impairment (CogImp). Methods We systematically searched the literature through PubMed, Medline, Cochrane library, and the first 100 articles published in Google Scholar. We included articles publishes in English and conducted on humans, no limitation was set to the date of publication, all the articles were approached from the first published up to March 15, 2021. The keywords used were Dementia, cognitive impairment, cognitive decline, cognitive dysfunction, diabetes self-care, compliance to anti-diabetic drugs, and medication adherence. One hundred-seventy-six were identified, the 12 full texts screened, only four fulfilled the inclusion and exclusion criteria. Results The studies were published in Europe, the United States, and Asia (all were observational). The results showed no effects of dementia on medication adherence, P-value of 0.41, odd ratio: 1.09, 95% CI: 0.89-1.32, Chi-square for heterogeneity: 12.15, I² = 75%, and standard difference = 3. The P-value for heterogeneity was 0.007. The studies included 2,556 patients and 1,854 events. Conclusion No association was found between dementia and compliance to anti-diabetic medications. Further prospective studies are needed to solve the issue.

Impact of Gut Microbiome Manipulation in 5xFAD Mice on Alzheimer's Disease-Like Pathology

The gut brain axis seems to modulate various psychiatric and neurological disorders such as Alzheimer's disease (AD). Growing evidence has led to the assumption that the gut microbiome might contribute to or even present the nucleus of origin for these diseases. In this regard, modifiers of the microbial composition might provide attractive new therapeutics. Aim of our study was to elucidate the effect of a rigorously changed gut microbiome on pathological hallmarks of AD. 5xFAD model mice were treated by antibiotics or probiotics (L. acidophilus and L. rhamnosus) for 14 weeks. Pathogenesis was measured by nest building capability and plaque deposition. The gut microbiome was affected as expected: antibiotics significantly reduced viable commensals, while probiotics transiently increased Lactobacillaceae. Nesting score, however, was only improved in antibiotics-treated mice. These animals additionally displayed reduced plaque load in the hippocampus. While various physiological parameters were not affected, blood sugar was reduced and serum glucagon level significantly elevated in the antibiotics-treated animals together with a reduction in the receptor for advanced glycation end products RAGE-the inward transporter of Aβ peptides of the brain. Assumedly, the beneficial effect of the antibiotics was based on their anti-diabetic potential.

Dexamethasone Protects Against Ischaemic Brain Injury via Inhibiting the pAkt Signalling Pathway Through Increasing Hap1

Dexamethasone has been reported to reduce the infarct volume and protect neurological function after ischaemic injury, but the mechanism of Dex in brain injury is not clear. We aimed to study the mechanism by which dexamethasone protects against ischaemic brain injury. Western blotting was to detected the expression of Hap1,TrkB, Akt and Erk; TTC staining to analyse ischemic volume; neurological deficit evaluation to test degree of ischemic injury; immunofluorescence staining to analyse the distribution of Hap1; and the MCAO model was used to study these processes. All data are expressed as the means ± SEM and were analysed by GraphPad Prism 6. P < 0.05 was considered statistically significant. After dexamethasone (Dex) treatment, Hap1 levels were increased and peaked at 2 days; then, we found that body weight was decreased in Hap1-/+ mice. Further study showed that Dex treatment reduced the ischaemic volume and improved neurological function. Finally, we showed that Hap1 regulated the levels of pTrkB, pAkt and pErk 1/2 in ischaemic injury after Dex treatment. Our data suggest that dexamethasone protects against ischaemic brain injury by inhibiting the pAkt signalling pathway through increasing Hap1.

The interplay between lipid and Aβ amyloid homeostasis in Alzheimer's Disease: risk factors and therapeutic opportunities

Alzheimer's Diseases (AD) is characterized by the accumulation of amyloid deposits of Aβ peptide in the brain. Besides genetic background, the presence of other diseases and an unhealthy lifestyle are known risk factors for AD development. Albeit accumulating clinical evidence suggests that an impaired lipid metabolism is related to Aβ deposition, mechanistic insights on the link between amyloid fibril formation/clearance and aberrant lipid interactions are still unavailable. Recently, many studies have described the key role played by membrane bound Aβ assemblies in neurotoxicity. Moreover, it has been suggested that a derangement of the ubiquitin proteasome pathway and autophagy is significantly correlated with toxic Aβ aggregation and dysregulation of lipid levels. Thus, studies focusing on the role played by lipids in Aβ aggregation and proteostasis could represent a promising area of investigation for the design of valuable treatments. In this review we examine current knowledge concerning the effects of lipids in Aβ aggregation and degradation processes, focusing on the therapeutic opportunities that a comprehensive understanding of all biophysical, biochemical, and biological processes involved may disclose.

Ibrutinib modulates Aβ/tau pathology, neuroinflammation, and cognitive function in mouse models of Alzheimer's disease

We previously demonstrated that ibrutinib modulates LPS‐induced neuroinflammation in vitro and in vivo, but its effects on the pathology of Alzheimer's disease (AD) and cognitive function have not been investigated. Here, we investigated the effects of ibrutinib in two mouse models of AD. In 5xFAD mice, ibrutinib injection significantly reduced Aβ plaque levels by promoting the non‐amyloidogenic pathway of APP cleavage, decreased Aβ‐induced neuroinflammatory responses, and significantly downregulated phosphorylation of tau by reducing levels of phosphorylated cyclin‐dependent kinase‐5 (p‐CDK5). Importantly, tau‐mediated neuroinflammation and tau phosphorylation were also alleviated by ibrutinib injection in PS19 mice. In 5xFAD mice, ibrutinib improved long‐term memory and dendritic spine number, whereas in PS19 mice, ibrutinib did not alter short‐ and long‐term memory but promoted dendritic spinogenesis. Interestingly, the induction of dendritic spinogenesis by ibrutinib was dependent on the phosphorylation of phosphoinositide 3‐kinase (PI3K). Overall, our results suggest that ibrutinib modulates AD‐associated pathology and cognitive function and may be a potential therapy for AD.

Two cases of type 2 diabetes mellitus successfully treated with probiotics

The gut microbiota, and particularly probiotic bacteria, has emerged as a promising and novel intervention to fight the looming worldwide diabetes epidemic when combined with the appropriate medication. Herein, we report two cases of patient with type 2 diabetes refractory to conventional therapy that showed notable improvement after probiotic intervention.

Huntingtin-associated protein 1 plays an essential role in the pathogenesis of type 2 diabetes by regulating the translocation of GLUT4 in mouse adipocytes

OBJECTIVE

Glucose disposal by insulin-responsive tissues maintains the body glucose homeostasis and insulin resistance leads to a risk of developing type 2 diabetes (T2DM). Insulin stimulates the translocation of glucose transporter isoform 4 (GLUT4) vesicles from intracellular compartments to the plasma membrane to facilitate glucose uptake. However, the underlying mechanisms of GLUT4 vesicle translocation are not well defined. Here we show the role of huntingtin-associated protein 1 (HAP1) in GLUT4 translocation in adipocytes and the pathogenesis of T2DM.

RESEARCH DESIGN AND METHODS

The parameters for glucose metabolism including body weight, glucose tolerance and insulin tolerance were assessed in wild-type (WT) and Hap1^+/- mice. HAP1 protein expression was verified in adipose tissue. Hap1 mRNA and protein expression was monitored in adipose tissue of high-fat diet (HFD)-induced diabetic mice. Insulin-stimulated GLUT4 vesicle translocation and glucose uptake were detected using immunofluorescence techniques and quantified in primary adipocytes from Hap1^-/- mice. The interaction between HAP1 and GLUT4 was assessed by immunofluorescence colocalization and co-immunoprecipitation in HEK293 cells and adipose tissue. The role of sortilin in HAP1 and GLUT4 interaction was approved by co-immunoprecipitation and RNA interference.

RESULTS

The expression of Hap1 mRNA and protein was detected in WT mouse adipose tissue and downregulated in adipose tissue of HFD-induced diabetic mice. Hap1^+/- mice exhibited increased body weight, pronounced glucose tolerance and significant insulin intolerance compared with the WT mice. HAP1 colocalized with GLUT4 in mouse adipocytes and cotransfected HEK293 cells. Furthermore, the insulin-stimulated GLUT4 vesicle translocation and glucose uptake were defective in Hap1^-/- adipocytes. Finally, sortilin mediated the interaction of HAP1 and GLUT4.

CONCLUSIONS

Our study showed that HAP1 formed a protein complex with GLUT4 and sortilin, and played a critical role in insulin-stimulated GLUT4 translocation in adipocytes. Its downregulation may contribute to the pathogenesis of diabetes.

Mens Sana in Corpore Sano: Does the Glycemic Index Have a Role to Play?

Although diet interventions are mostly related to metabolic disorders, nowadays they are used in a wide variety of pathologies. From diabetes and obesity to cardiovascular diseases, to cancer or neurological disorders and stroke, nutritional recommendations are applied to almost all diseases. Among such disorders, metabolic disturbances and brain function and/or diseases have recently been shown to be linked. Indeed, numerous neurological functions are often associated with perturbations of whole-body energy homeostasis. In this regard, specific diets are used in various neurological conditions, such as epilepsy, stroke, or seizure recovery. In addition, Alzheimer’s disease and Autism Spectrum Disorders are also considered to be putatively improved by diet interventions. Glycemic index diets are a novel developed indicator expected to anticipate the changes in blood glucose induced by specific foods and how they can affect various physiological functions. Several results have provided indications of the efficiency of low-glycemic index diets in weight management and insulin sensitivity, but also cognitive function, epilepsy treatment, stroke, and neurodegenerative diseases. Overall, studies involving the glycemic index can provide new insights into the relationship between energy homeostasis regulation and brain function or related disorders. Therefore, in this review, we will summarize the main evidence on glycemic index involvement in brain mechanisms of energy homeostasis regulation.

Insulin resistance and bioenergetic manifestations: Targets and approaches in Alzheimer's disease

AIM

Insulin has a well-established role in cognition, neuronal detoxification and synaptic plasticity. Insulin transduction affect neurotransmitter functions, influence bioenergetics and regulate neuronal survival through regulating glucose energy metabolism and downward pathways.

METHODS

A systematic literature review of PubMed, Medline, Bentham, Scopus and EMBASE (Elsevier) databases was carried out with the help of the keywords like "Alzheimer's disease; Hypometabolism; Oxidative stress; energy failure in AD, Insulin; Insulin resistance; Bioenergetics" till June 2020. The review was conducted using the above keywords to collect the latest articles and to understand the nature of the extensive work carried out on insulin resistance and bioenergetic manifestations in Alzheimer's disease.

KEY FINDINGS

The article sheds light on insulin resistance mediated hypometabolic state on pathological progression of AD. The disrupted insulin signaling has pathological outcome in form of disturbed glucose homeostasis, altered bioenergetic state which increases build-up of senile plaques (Aβ), neurofibrillary tangles (τ), decline in transportation of glucose and activation of inflammatory pathways. The mechanistic link of insulin resistant state with therapeutically explorable potential transduction pathways is the focus of the reviewed work.

SIGNIFICANCE

The present work opines that the mechanism by which the insulin resistance mediates dysregulation of bioenergetics and progresses to neurodegenerative state holds the tangible potential to succeed in the development of novel dementia therapies. Further, hypometabolic complications and altered insulin signaling may be explored as a mechanistic relation between bioenergetic deficits and AD.

Secreted Chaperones in Neurodegeneration

Protein homeostasis, or proteostasis, is a combination of cellular processes that govern protein quality control, namely, protein translation, folding, processing, and degradation. Disruptions in these processes can lead to protein misfolding and aggregation. Proteostatic disruption can lead to cellular changes such as endoplasmic reticulum or oxidative stress; organelle dysfunction; and, if continued, to cell death. A majority of neurodegenerative diseases involve the pathologic aggregation of proteins that subverts normal neuronal function. While prior reviews of neuronal proteostasis in neurodegenerative processes have focused on cytoplasmic chaperones, there is increasing evidence that chaperones secreted both by neurons and other brain cells in the extracellular - including transsynaptic - space play important roles in neuronal proteostasis. In this review, we will introduce various secreted chaperones involved in neurodegeneration. We begin with clusterin and discuss its identification in various protein aggregates, and the use of increased cerebrospinal fluid (CSF) clusterin as a potential biomarker and as a potential therapeutic. Our next secreted chaperone is progranulin; polymorphisms in this gene represent a known genetic risk factor for frontotemporal lobar degeneration, and progranulin overexpression has been found to be effective in reducing Alzheimer's- and Parkinson's-like neurodegenerative phenotypes in mouse models. We move on to BRICHOS domain-containing proteins, a family of proteins containing highly potent anti-amyloidogenic activity; we summarize studies describing the biochemical mechanisms by which recombinant BRICHOS protein might serve as a therapeutic agent. The next section of the review is devoted to the secreted chaperones 7B2 and proSAAS, small neuronal proteins which are packaged together with neuropeptides and released during synaptic activity. Since proteins can be secreted by both classical secretory and non-classical mechanisms, we also review the small heat shock proteins (sHsps) that can be secreted from the cytoplasm to the extracellular environment and provide evidence for their involvement in extracellular proteostasis and neuroprotection. Our goal in this review focusing on extracellular chaperones in neurodegenerative disease is to summarize the most recent literature relating to neurodegeneration for each secreted chaperone; to identify any common mechanisms; and to point out areas of similarity as well as differences between the secreted chaperones identified to date.

Antidiabetic drugs for Alzheimer's and Parkinson's diseases: Repurposing insulin, metformin, and thiazolidinediones

Medical and scientific communities have been striving to disentangle the complexity of neurodegenerative diseases, particularly Alzheimer's disease (AD) and Parkinson's disease (PD), in order to develop a cure or effective treatment for these diseases. Along this journey, it has become important to identify the early events occurring in the prodromal phases of these diseases and the disorders that increase the risk of neurodegeneration highlighting common pathological features. This strategy has led to a wealth of evidence identifying diabetes, mainly type 2 diabetes mellitus (T2DM) as a main risk factor for the onset and progression of AD and PD. Impaired glucose metabolism, insulin resistance, and mitochondrial dysfunction are features common to both type 2 diabetes mellitus (T2DM), and AD and PD, and they appear before clinical diagnosis of the two neurodegenerative diseases. These could represent the strategic nodes of therapeutic intervention. Following this line of thought, a conceivable approach is to repurpose antidiabetic drugs as valuable agents that may prevent or reduce the risk of cognitive decline and neurodegeneration. This review summarizes the past and current findings that link AD and PD with T2DM, emphasizing the common pathological mechanisms. The efficacy of antidiabetic drugs, namely intranasal insulin, metformin, and thiazolidinediones, in the prevention and/or treatment of AD and PD is also discussed.

Upregulation of AMPK Ameliorates Alzheimer's Disease-Like Tau Pathology and Memory Impairment

The studies have shown that 5'-adenosine monophosphate (AMP)-activated protein kinase (AMPK) is involved in Alzheimer's disease (AD) pathology, but the effects of AMPK on AD-like Tau abnormal phosphorylation and its underlying mechanism remains unclear. Herein, we found that the mRNA expression and activity of AMPK are significantly decreased in the brains of the aging C57 mice and 3 × Tg AD mice when compared with their respective control. Moreover, when downregulation of AMPK with AAV-siAMPK-eGFP in the hippocampus CA3 of 3-month-old C57 mice, the mice display AD-like Tau hyperphosphorylation, fear memory impairment, and glycogen synthase kinase-3β (GSK3β) activity increased. On the other hand, there are also AD-like Tau hyperphosphorylation, impairment of fear memory, and AMPK activity decreased in streptozotocin (STZ) mice. Interestingly, AMPK overexpression could efficiently rescue AD-like Tau phosphorylation and brain impairment in STZ mice. Moreover, the activity of GSK3β and the level of Tau phosphorylation (Ser396 and Thr231 sites) were significantly decreased in HEK293 Tau cells transfected by AMPK plasmid or treated with agonists salicylate (SS), but GSK3β agonists Wortmannin (Wort) could ablate AMPK-mediated Tau dephosphorylation. Taken together, the study indicated that AMPK reduces Tau phosphorylation and improves brain function and inhibits GSK3β in AD-like model. These findings proved that AMPK might be a new target for AD in the future.