Dopaminergic neurodegeneration in a rat model of long-term hyperglycemia: preferential degeneration of the nigrostriatal motor pathway

The Contribution of Type 2 Diabetes to Parkinson's Disease Aetiology

Type 2 diabetes (T2D) and Parkinson's disease (PD) are chronic disorders that have a significant health impact on a global scale. Epidemiological, preclinical, and clinical research underpins the assumption that insulin resistance and chronic inflammation contribute to the overlapping aetiologies of T2D and PD. This narrative review summarises the recent evidence on the contribution of T2D to the initiation and progression of PD brain pathology. It also briefly discusses the rationale and potential of alternative pharmacological interventions for PD treatment.

Evaluation of type 1 diabetes mellitus as a risk factor of Parkinson's disease in a Drosophila model

Diabetes mellitus (DM) is a chronic metabolic disease characterized by high blood glucose levels, resulting from insulin dysregulation. Parkinson's disease (PD) is the most common neurodegenerative motor disorder caused by the selective loss of dopaminergic (DA) neurons in the substantia nigra pars compacta. DM and PD are both age-associated diseases that are turning into epidemics worldwide. Previous studies have indicated that type 2 DM might be a risk factor of developing PD. However, scarce information about the link between type 1 DM (T1DM) and PD does exist. In this work, we have generated a Drosophila model of T1DM based on insulin deficiency to evaluate if T1DM could be a risk factor to trigger PD onset. As expected, model flies exhibited T1DM-related phenotypes such as insulin deficiency, increased content of carbohydrates and glycogen, and reduced activity of insulin signaling. Interestingly, our results also demonstrated that T1DM model flies presented locomotor defects as well as reduced levels of tyrosine hydroxylase (a marker of DA neurons) in brains, which are typical PD-related phenotypes. In addition, T1DM model flies showed elevated oxidative stress levels, which could be causative of DA neurodegeneration. Therefore, our results indicate that T1DM might be a risk factor of developing PD, and encourage further studies to shed light into the exact link between both diseases.

Glucose Metabolism Impairment in Parkinson's Disease

Impairments in systematic and regional glucose metabolism exist in patients with Parkinson's disease (PD) at every stage of the disease course, and such impairments are associated with the incidence, progression, and special phenotypes of PD, which affect each physiological process of glucose metabolism including glucose uptake, glycolysis, tricarboxylic acid cycle, oxidative phosphorylation, and pentose phosphate shunt pathway. These impairments may be attributed to various mechanisms, such as insulin resistance, oxidative stress, abnormal glycated modification, blood-brain-barrier dysfunction, and hyperglycemia-induced damages. These mechanisms could subsequently cause excessive methylglyoxal and reactive oxygen species production, neuroinflammation, abnormal aggregation of protein, mitochondrial dysfunction, and decreased dopamine, and finally result in energy supply insufficiency, neurotransmitter dysregulation, aggregation and phosphorylation of α-synuclein, and dopaminergic neuron loss. This review discusses the glucose metabolism impairment in PD and its pathophysiological mechanisms, and briefly summarized the currently-available therapies targeting glucose metabolism impairment in PD, including glucagon-likepeptide-1 (GLP-1) receptor agonists and dual GLP-1/gastric inhibitory peptide receptor agonists, metformin, and thiazoledinediones.

Effects of diabetes on microglial physiology: a systematic review of in vitro, preclinical and clinical studies

Diabetes mellitus is a heterogeneous chronic metabolic disorder characterized by the presence of hyperglycemia, commonly preceded by a prediabetic state. The excess of blood glucose can damage multiple organs, including the brain. In fact, cognitive decline and dementia are increasingly being recognized as important comorbidities of diabetes. Despite the largely consistent link between diabetes and dementia, the underlying causes of neurodegeneration in diabetic patients remain to be elucidated. A common factor for almost all neurological disorders is neuroinflammation, a complex inflammatory process in the central nervous system for the most part orchestrated by microglial cells, the main representatives of the immune system in the brain. In this context, our research question aimed to understand how diabetes affects brain and/or retinal microglia physiology. We conducted a systematic search in PubMed and Web of Science to identify research items addressing the effects of diabetes on microglial phenotypic modulation, including critical neuroinflammatory mediators and their pathways. The literature search yielded 1327 records, including 18 patents. Based on the title and abstracts, 830 papers were screened from which 250 primary research papers met the eligibility criteria (original research articles with patients or with a strict diabetes model without comorbidities, that included direct data about microglia in the brain or retina), and 17 additional research papers were included through forward and backward citations, resulting in a total of 267 primary research articles included in the scoping systematic review. We reviewed all primary publications investigating the effects of diabetes and/or its main pathophysiological traits on microglia, including in vitro studies, preclinical models of diabetes and clinical studies on diabetic patients. Although a strict classification of microglia remains elusive given their capacity to adapt to the environment and their morphological, ultrastructural and molecular dynamism, diabetes modulates microglial phenotypic states, triggering specific responses that include upregulation of activity markers (such as Iba1, CD11b, CD68, MHC-II and F4/80), morphological shift to amoeboid shape, secretion of a wide variety of cytokines and chemokines, metabolic reprogramming and generalized increase of oxidative stress. Pathways commonly activated by diabetes-related conditions include NF-κB, NLRP3 inflammasome, fractalkine/CX3CR1, MAPKs, AGEs/RAGE and Akt/mTOR. Altogether, the detailed portrait of complex interactions between diabetes and microglia physiology presented here can be regarded as an important starting point for future research focused on the microglia-metabolism interface.

RPS6 transcriptional modulation in neural tissues of Nauphoeta cinerea during streptozotocin-associated sugar metabolism impairment

The use of insects to model molecular events that characterize degenerative conditions was originally met with scepticism. However, the discovery of insect insulin-like peptides in the 1970's and the demonstration of evolutionary conservation of insulin-related signalling from insects to mammals have highlighted the importance and reduced cost of insect models in biomedical research. Here, we expand on our earlier described modelling of streptozotocin-induced brain glucose metabolic disruption in Nauphoeta cinerea, using RNA-sequencing analysis to study the transcriptional and genetic signatures of degeneration and stress signalling when glucose levels are elevated in the brain of the lobster cockroach. Nymphs were randomly divided into three groups: Control (0.8% NaCl), and two single streptozotocin injection doses (74 nmol and 740 nmol). The transcriptional analyses featured a dysregulation of 226 genes at high dose STZ treatment and 278 genes at the low dose. Our mRNA-sequencing data showed that ribosomal protein genes were the most upregulated genes at both 74 and 740 nmol STZ treatment. We therefore used RT-qPCR and relative transcriptional methods to validate our proposed mechanism of brain glucose toxicity-induced degeneration in Nauphoeta cinerea, which involved the upregulation of ribosomal proteins and rpS6 regulators (mTORC1, protein kinases, casein kinase 1 and Death-associated protein kinase), the upregulation of MAPK cascades (RAS, ERK, P38 and JNK), alongside the downregulation of the PI3K/AKT cascade. Taken together, this study highlights the remarkable opportunity for Nauphoeta cinerea use as an experimental organism in hyperglycaemia, degeneration, and stress signalling.

Type 2 diabetes mellitus augments Parkinson's disease risk or the other way around: Facts, challenges and future possibilities

About 10% of the adult population is living with type 2 diabetes mellitus (T2DM) and 1% of the population over 60 years of age is suffering from Parkinson's disease (PD). A school of thought firmly believes that T2DM, an age-related disease, augments PD risk. Such relationship is reflected from the severity of PD symptoms in drug naive subjects possessing T2DM. Onset of Parkinsonian feature in case controls possessing T2DM corroborates the role of hyperglycemia in PD. A few cohort, meta-analysis and animal studies have shown an increased PD risk owing to insulin resistance. High fat diet and role of insulin signaling in the regulation of sugar metabolism, oxidative stress, α-synuclein aggregation and accumulation, inflammatory response and mitochondrial function in PD models and sporadic PD further connect the two. Although little is reported about the implication of PD in hyperglycemia and T2DM, a few studies have also contradicted. Ameliorative effect of anti-diabetic drugs on Parkinsonian symptoms and vague outcome of anti-PD medications in T2DM patients also suggest a link. The article reviews the literature supporting augmented risk of one by the other, analysis of proof of the concept, facts, challenges, future possibilities and standpoint on the subject.

Metabolism and memory: α-synuclein level in children with obesity and children with type 1 diabetes; relation to glucotoxicity, lipotoxicity and executive functions

Background/Objectives

Children with obesity and those with type 1diabetes (T1D) exhibit subtle neurocognitive deficits, the mechanism of which remains unknown. α-synuclein plays a fundamental role in neurodegeneration. Moreover, its role in glucose and lipids metabolism is emerging. This study aims to assess whether α-synuclein is correlated with the degree of neurodegeneration in children with obesity and those with T1D in comparison to healthy controls and correlate it to various neurocognitive and metabolic parameters.

Subjects/Methods

Forty children with obesity, 40 children with T1D and 40 matched-healthy controls were assessed for anthropometric measurements and blood-pressure. Cognitive evaluation was performed using Stanford–Binet scale and Barkley Deficits in Executive Functioning (EF) Scale-Children and Adolescents. α-synuclein, fasting lipids and glucose were measured with calculation of the homeostatic model of insulin-resistance and estimated-glucose disposal rate.

Results

Children with obesity and those with T1D had significantly higher α-synuclein (p < 0.001) and total EF percentile (p = 0.001) than controls. α-synuclein was negatively correlated to total IQ (p < 0.001 and p = 0.001), and positively correlated with total EF percentile (p = 0.009 and p = 0.001) and EF symptom count percentile (p = 0.005 and p < 0.001) in children with T1D and obesity, respectively. Multivariate-regression revealed that α-synuclein was independently related to age (p = 0.028), diabetes-duration (p = 0.006), HbA1C% (p = 0.034), total IQ (p = 0.013) and EF symptom count percentile (p = 0.003) among children with T1D, and to diastolic blood-pressure percentile (p = 0.013), waist/hip ratio SDS (p = 0.007), total EF percentile (P = 0.033) and EF symptom count percentile (p < 0.001) in children with obesity.

Conclusion

α-synuclein could have a mechanistic role in neurocognitive deficit among children with obesity and T1D.

Exploring the link between Parkinson's disease and type 2 diabetes mellitus in Drosophila

Parkinson's disease (PD) is the second most common neurodegenerative disease. Diabetes mellitus (DM) is a metabolic disease characterized by high levels of glucose in blood. Recent epidemiological studies have highlighted the link between both diseases; it is even considered that DM might be a risk factor for PD. To further investigate the likely relation of these diseases, we have used a Drosophila PD model based on inactivation of the DJ-1β gene (ortholog of human DJ-1), and diet-induced Drosophila and mouse type 2 DM (T2DM) models, together with human neuron-like cells. T2DM models were obtained by feeding flies with a high sugar-containing medium, and mice with a high fat diet. Our results showed that both fly models exhibit common phenotypes such as alterations in carbohydrate homeostasis, mitochondrial dysfunction or motor defects, among others. In addition, we demonstrated that T2DM might be a risk factor of developing PD since our diet-induced fly and mouse T2DM models present DA neuron dysfunction, a hallmark of PD. We also confirmed that neurodegeneration is caused by increased glucose levels, which has detrimental effects in human neuron-like cells by triggering apoptosis and leading to cell death. Besides, the observed phenotypes were exacerbated in DJ-1β mutants cultured in the high sugar medium, indicating that DJ-1 might have a role in carbohydrate homeostasis. Finally, we have confirmed that metformin, an antidiabetic drug, is a potential candidate for PD treatment and that it could prevent PD onset in T2DM model flies. This result supports antidiabetic compounds as promising PD therapeutics.

Sesamin protects against neurotoxicity via inhibition of microglial activation under high glucose circumstances through modulating p38 and JNK signaling pathways

Diabetes mellitus (DM), one of the principal causes of morbidity and mortality worldwide, is implicated in the progression of age-related neurodegenerative diseases (NDDs), in which microglial activation is a crucial mediator. Sesamin, a kind of phytochemical, shows inhibitory effects on microglial activation. The present study studied whether sesamin protects against neurotoxicity triggered by high glucose-induced microglial activation. We firstly demonstrated that high doses of glucose, which mimics hyperglycemia in DM, did induce the activation of murine BV2 microglial cells, increasing inflammatory responses such as the production of ROS or inflammatory mediators like IL-1β, TNF-⍺, and nitric oxide, through activation of p38 and JNK signaling pathways. Next, conditioned medium (CM) collected from high glucose-activated BV2 cell culture was used to show aggravated neurotoxicity in differentiated PC12 cells, indicating that high glucose-activated microglia could induce neurotoxicity. Interestingly, pretreatment of BV2 cells with sesamin diminished high glucose-induced microglia activation and inflammatory responses. Moreover, neurotoxicity in PC12 cells was found to be decreased in the group treated with CM from the sesamin-pretreated BV2 cell culture, suggesting sesamin inhibited microglial activation, thereby protecting neurons from activated microglia-mediated neurotoxicity. Thus, sesamin might be a potential compound to use in the prevention of diabetic-induced NDDs.

Long-term hyperglycemia aggravates α-synuclein aggregation and dopaminergic neuronal loss in a Parkinson’s disease mouse model

Background

Growing evidence suggests an association between Parkinson’s disease (PD) and diabetes mellitus (DM). At the cellular level, long-term elevated levels of glucose have been shown to lead to nigrostriatal degeneration in PD models. However, the underlying mechanism is still unclear. Previously, we have elucidated the potential of type 2 diabetes mellitus (T2DM) in facilitating PD progression, involving aggregation of both alpha-synuclein (α-syn) and islet amyloid polypeptide in the pancreatic and brain tissues. However, due to the complicated effect of insulin resistance on PD onset, the actual mechanism of hyperglycemia-induced dopaminergic degeneration remains unknown.

Methods

We employed the type 1 diabetes mellitus (T1DM) model induced by streptozotocin (STZ) injection in a transgenic mouse line (BAC-α-syn-GFP) overexpressing human α-syn, to investigate the direct effect of elevated blood glucose on nigrostriatal degeneration.

Results

STZ treatment induced more severe pathological alterations in the pancreatic islets and T1DM symptoms in α-syn-overexpressing mice than in wild-type mice, at one month and three months after STZ injections. Behavioral tests evaluating motor performance confirmed the nigrostriatal degeneration. Furthermore, there was a marked decrease in dopaminergic profiles and an increase of α-syn accumulation and Serine 129 (S129) phosphorylation in STZ-treated α-syn mice compared with the vehicle-treated mice. In addition, more severe neuroinflammation was observed in the brains of the STZ-treated α-syn mice.

Conclusion

Our results solidify the potential link between DM and PD, providing insights into how hyperglycemia induces nigrostriatal degeneration and contributes to pathogenic mechanisms in PD.

Single-Cell Sequencing Analysis of the db/db Mouse Hippocampus Reveals Cell-Type-Specific Insights Into the Pathobiology of Diabetes-Associated Cognitive Dysfunction

Diabetes-associated cognitive decline (DCD), is one of the complications of diabetes, which is characterized by a series of neurophysiological and pathological abnormalities. However, the exact pathogenesis of DCD is still unknown. Single-cell RNA sequencing (scRNA-seq) could discover unusual subpopulations, explore functional heterogeneity and identify signaling pathways and potential markers. The aim of this research was to provide deeper opinion into molecular and cellular changes underlying DCD, identify different cellular types of the diabetic mice hippocampus at single-cell level, and elucidate the factors mediating the pathogenesis of DCD. To elucidate cell specific gene expression changes in the hippocampus of diabetic encephalopathy. Single-cell RNA sequencing of hippocampus from db/m and db/db mice was carried out. Subclustering analysis was performed to further describe microglial cell subpopulations. Interestingly using immunohistochemistry, these findings were confirmed at the protein level. Single cell analysis yielded transcriptome data for 14621 hippocampal cells and defined 11 different cell types. Analysis of differentially expressed genes in the microglia compartments indicated that infection- and immune system process- associated terms, oxidative stress and inflammation play vital roles in the progression of DCD. Compared with db/m mouse, experiments at the protein level supported the activation of microglia, increased expression of inflammatory factors and oxidative stress damage in the hippocampus of db/db mouse. In addition, a major finding of our research was the subpopulation of microglia that express genes related to pro-inflammatory disease-associated microglia (DAM). Our research reveals pathological alterations of inflammation and oxidative stress mediated hippocampal damage in the db/db mice, and may provide potential diagnostic biomarkers and therapeutic interventions for DCD.

Fyn kinase regulates dopaminergic neuronal apoptosis in animal and cell models of high glucose (HG) treatment

Background

High glucose (HG) is linked to dopaminergic neuron loss and related Parkinson’s disease (PD), but the mechanism is unclear.

Results

Rats and differentiated SH-SY5Y cells were used to investigate the effect of HG on dopaminergic neuronal apoptotic death. We found that a 40-day HG diet elevated cleaved caspase 3 levels and activated Fyn and mTOR/S6K signaling in the substantia nigra of rats. In vitro, 6 days of HG treatment activated Fyn, enhanced binding between Fyn and mTOR, activated mTOR/S6K signaling, and induced neuronal apoptotic death. The proapoptotic effect of HG was rescued by either the Fyn inhibitor PP1 or the mTOR inhibitor rapamycin. PP1 inhibited mTOR/S6K signaling, but rapamycin was unable to modulate Fyn activation.

Conclusions

HG induces dopaminergic neuronal apoptotic death via the Fyn/mTOR/S6K pathway.

Two Birds One Stone: The Neuroprotective Effect of Antidiabetic Agents on Parkinson Disease-Focus on Sodium-Glucose Cotransporter 2 (SGLT2) Inhibitors

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease affecting more than 1% of the population over 65 years old. The etiology of the disease is unknown and there are only symptomatic managements available with no known disease-modifying treatment. Aging, genes, and environmental factors contribute to PD development and key players involved in the pathophysiology of the disease include oxidative stress, mitochondrial dysfunction, autophagic-lysosomal imbalance, and neuroinflammation. Recent epidemiology studies have shown that type-2 diabetes (T2DM) not only increased the risk for PD, but also is associated with PD clinical severity. A higher rate of insulin resistance has been reported in PD patients and is suggested to be a pathologic driver in this disease. Oral diabetic drugs including sodium-glucose cotransporter 2 (SGLT2) inhibitors, glucagon-like peptide-1 (GLP-1) receptor agonists, and dipeptidyl peptidase-4 (DPP-4) inhibitors have been shown to provide neuroprotective effects in both PD patients and experimental models; additionally, antidiabetic drugs have been demonstrated to lower incidence rates of PD in DM patients. Among these, the most recently developed drugs, SGLT2 inhibitors may provide neuroprotective effects through improving mitochondrial function and antioxidative effects. In this article, we will discuss the involvement of mitochondrial-related oxidative stress in the development of PD and potential benefits provided by antidiabetic agents especially focusing on sglt2 inhibitors.

Diabetes and Cognitive Impairment: A Role for Glucotoxicity and Dopaminergic Dysfunction

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by hyperglycemia, responsible for the onset of several long-term complications. Recent evidence suggests that cognitive dysfunction represents an emerging complication of DM, but the underlying molecular mechanisms are still obscure. Dopamine (DA), a neurotransmitter essentially known for its relevance in the regulation of behavior and movement, modulates cognitive function, too. Interestingly, alterations of the dopaminergic system have been observed in DM. This review aims to offer a comprehensive overview of the most relevant experimental results assessing DA’s role in cognitive function, highlighting the presence of dopaminergic dysfunction in DM and supporting a role for glucotoxicity in DM-associated dopaminergic dysfunction and cognitive impairment. Several studies confirm a role for DA in cognition both in animal models and in humans. Similarly, significant alterations of the dopaminergic system have been observed in animal models of experimental diabetes and in diabetic patients, too. Evidence is accumulating that advanced glycation end products (AGEs) and their precursor methylglyoxal (MGO) are associated with cognitive impairment and alterations of the dopaminergic system. Further research is needed to clarify the molecular mechanisms linking DM-associated dopaminergic dysfunction and cognitive impairment and to assess the deleterious impact of glucotoxicity.

Investigation of the Possible Correlation between Idiopathic Parkinson's Disease and Diabetes Mellitus in Egyptian Patients: A Pilot Study

OBJECTIVES

To study the diabetes-Parkinson's disease (PD) linkage.

METHODS

The investigators recorded the rapid eye movement sleep behavior disorder screening questionnaire (RBDSQ) score for 60 diabetic patients: 30 patients were treated with metformin-inclusive sulfonylurea and 30 patients were treated with sulphonylurea(s) monotherapy and matched with 30 controls. We evaluated blood glucose kinetics during a 75 g oral glucose tolerance test for (22) nondiabetic parkinsonian patients and (10) controls. The motor complications scores were recorded for all parkinsonian patients using the relevant parts of the Unified Parkinson's Disease Rating Scale (UPDRS) part IV.

RESULTS

Diabetics recorded higher scores of RBDSQ than controls (p < 0.001), with no differences related to antidiabetic therapy. In nondiabetic PD patients, after oral glucose, blood glucose was significantly higher at T1 (p < 0.001) than controls. Moreover, the total area under the time curve for blood glucose levels was significantly higher in PD compared to controls (281.22 ± 52.25 vs. 245.65 ± 48.63 mg.hr./dL; p=0.013). Higher blood glucose levels were associated with motor abnormalities. Diabetic PD patients recorded higher scores of UPDRS (p < 0.001).

CONCLUSION

Diabetes mellitus and Parkinson's disease are linked, which raises concerns about either of them, probably increasing the risk of the other. This trial is registered with NCT03685357.

Diabetes Mellitus and Parkinson's Disease: A Systematic Review and Meta-Analyses

BACKGROUND

A link between diabetes mellitus (DM) and Parkinson's disease (PD) have been proposed but evidence are sparse and inconsistent.

OBJECTIVE

Perform a systematic review of all evidence that link DM and PD characterising the prevalence of DM in PD patients, the risk of developing PD in DM patients and the influence of DM on PD severity and progression.

METHODS

MEDLINE, Scopus, and Cochrane Library from inception to June 30, 2021 were searched. Studies reporting prevalence, incidence, severity and disease progression of DM and PD were included. Prevalence of DM in PD and incidence of PD in DM patients, and characteristics of PD.

RESULTS

A total of 21 studies (n = 11,396) included data on DM prevalence in PD patients, 12 studies (n = 17,797,221) included data on incidence of PD in DM patients, and 10 studies (n = 2,482) included data on DM impact on PD severity and disease progression. The prevalence of DM in PD patients was 10.02 %, (95%C.I. 7.88 -12.16), DM patients showed a higher risk of developing PD (OR: 1.34 95%CI 1.26-1.43 p < 0.0001) compared to non-DM, and PD patients with DM showed a greater severity of motor symptoms, with higher Hoehn and Yahr stage (SMD: 0.36 95%CI 0.12-0.60; p < 0.001) and higher UPDRS (SMD 0.60 95%CI 0.28-0.92; p < 0.001) compared with PD patients without DM.

CONCLUSION

Although the prevalence of DM in PD patients is similar to the general population, patients with DM have a higher risk of developing PD, and the presence of DM is associated with greater PD severity and faster progression, which suggests that DM may be a facilitating factor of neurodegeneration.

Distribution in the brain and possible neuroprotective effects of intranasally delivered multi-walled carbon nanotubes

Carbon nanotubes (CNTs) are currently under active investigation for their use in several biomedical applications, especially in neurological diseases and nervous system injury due to their electrochemical properties. Nowadays, no CNT-based therapeutic products for internal use appear to be close to the market, due to the still limited knowledge on their fate after delivery to living organisms and, in particular, on their toxicological profile. The purpose of the present work was to address the distribution in the brain parenchyma of two intranasally delivered MWCNTs (MWCNTs 1 and a-MWCNTs 2), different from each other, the first being non electroconductive while the second results in being electroconductive. After intranasal delivery, the presence of CNTs was investigated in several brain areas, discriminating the specific cell types involved in the CNT uptake. We also aimed to verify the neuroprotective potential of the two types of CNTs, delivering them in rats affected by early diabetic encephalopathy and analysing the modulation of nerve growth factor metabolism and the effects of CNTs on the neuronal and glial phenotypes. Our findings showed that both CNT types, when intranasally delivered, reached numerous brain areas and, in particular, the limbic area that plays a crucial role in the development and progression of major neurodegenerative diseases. Furthermore, we demonstrated that electroconductive MWCNTs were able to exert neuroprotective effects through the modulation of a key neurotrophic factor and probably the improvement of neurodegeneration-related gliosis.

Nmnat1 Modulates Mitochondrial Oxidative Stress by Inhibiting Caspase-3 Signaling in Alzheimer's Disease

Nigrostriatal pathway disturbance is one of the major pathogenic factors in Alzheimer's disease (AD). Dopaminergic neuron dysfunction results in bradykinesia and akinesia (inability to initiate movement), indicating a significant risk factor for substantia nigra pars compacta lesions. Furthermore, the nicotinamide adenine dinucleotide (NAD⁺) is associated with Aβ toxicity decline in AD therapy. Nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1) is an essential enzyme that preserves normal neuronal function and protects neurons from insult. This study aimed to investigate the potential therapeutic effects of Nmnat1 and its underlying mechanisms in a triple-transgenic mouse model of AD (3xTgAD). Results showed that Nmnat1 improved the substantial behavioral measures of cognitive impairments compared with the 3xTgAD control. Additionally, Nmnat1 overexpression significantly increased tyrosine hydroxylase-positive neurons and anti-apoptotic protein Bcl2 and caspase-3 expression levels in 3xTgAD mice. Nmnat1 also effectively controlled SOD1 activation. In conclusion, Nmnat1 substantially decreases multiple AD-associated pathological characteristics at least partially by the increase of caspase-3 activation.

Potential relationship between dietary long-chain saturated fatty acids and hypothalamic dysfunction in obesity

Diet-induced hypothalamic inflammation, which leads to hypothalamic dysfunction and a loss of regulation of energy balance, is emerging as a potential driver of obesity. Excessive intake of long-chain saturated fatty acids is held to be the causative dietary component in hypothalamic inflammation. This review summarizes current evidence on the role of long-chain saturated fatty acids in promoting hypothalamic inflammation and the related induction of central insulin and leptin insensitivity. Particularly, the present review focuses on the molecular mechanisms linking long-chain saturated fatty acids and hypothalamic inflammation, emphasizing the metabolic fate of fatty acids and the resulting lipotoxicity, which is a key driver of hypothalamic dysfunction. In conclusion, long-chain saturated fatty acids are key nutrients that promote hypothalamic inflammation and dysfunction by fostering the build-up of lipotoxic lipid species, such as ceramide. Furthermore, when long-chain saturated fatty acids are consumed in combination with high levels of refined carbohydrates, the proinflammatory effects are exacerbated via a mechanism that relies on the formation of advanced glycation end products.

Diabetes Causes Dysfunctional Dopamine Neurotransmission Favoring Nigrostriatal Degeneration in Mice

BACKGROUND

Numerous studies indicate an association between neurodegenerative and metabolic diseases. Although still a matter of debate, growing evidence from epidemiological and animal studies indicate that preexisting diabetes increases the risk to develop Parkinson's disease. However, the mechanisms of such an association are unknown.

OBJECTIVES

We investigated whether diabetes alters striatal dopamine neurotransmission and assessed the vulnerability of nigrostriatal neurons to neurodegeneration.

METHODS

We used streptozotocin-treated and genetically diabetic db/db mice. Expression of oxidative stress and nigrostriatal neuronal markers and levels of dopamine and its metabolites were monitored. Dopamine release and uptake were assessed using fast-scan cyclic voltammetry. 6-Hydroxydopamine was unilaterally injected into the striatum using stereotaxic surgery. Motor performance was scored using specific tests.

RESULTS

Diabetes resulted in oxidative stress and decreased levels of dopamine and its metabolites in the striatum. Levels of proteins regulating dopamine release and uptake, including the dopamine transporter, the Girk2 potassium channel, the vesicular monoamine transporter 2, and the presynaptic vesicle protein synaptobrevin-2, were decreased in diabetic mice. Electrically evoked levels of extracellular dopamine in the striatum were enhanced, and altered dopamine uptake was observed. Striatal microinjections of a subthreshold dose of the neurotoxin 6-hydroxydopamine in diabetic mice, insufficient to cause motor alterations in nondiabetic animals, resulted in motor impairment, higher loss of striatal dopaminergic axons, and decreased neuronal cell bodies in the substantia nigra.

CONCLUSIONS

Our results indicate that diabetes promotes striatal oxidative stress, alters dopamine neurotransmission, and increases vulnerability to neurodegenerative damage leading to motor impairment. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Elucidating the Relationship Between Diabetes Mellitus and Parkinson's Disease Using 18F-FP-(+)-DTBZ, a Positron-Emission Tomography Probe for Vesicular Monoamine Transporter 2

Diabetes mellitus (DM) and Parkinson’s disease (PD) have been and will continue to be two common chronic diseases globally that are difficult to diagnose during the prodromal phase. Current molecular genetics, cell biological, and epidemiological evidences have shown the correlation between PD and DM. PD shares the same pathogenesis pathways and pathological factors with DM. In addition, β-cell reduction, which can cause hyperglycemia, is a striking feature of DM. Recent studies indicated that hyperglycemia is highly relevant to the pathologic changes in PD. However, further correlation between DM and PD remains to be investigated. Intriguingly, polycystic monoamine transporter 2 (VMAT2), which is co-expressed in dopaminergic neurons and β cells, is responsible for taking up dopamine into the presynaptic vesicles and can specifically bind to the β cells. Furthermore, we have summarized the specific molecular and diagnostic functions of VMAT2 for the two diseases reported in this review. Therefore, VMAT2 can be applied as a target probe for positron emission tomography (PET) imaging to detect β-cell and dopamine level changes, which can contribute to the diagnosis of DM and PD during the prodromal phase. Targeting VMAT2 with the molecular probe ¹⁸F-FP-(+)-DTBZ can be an entry point for the β cell mass (BCM) changes in DM at the molecular level, to clarify the potential relationship between DM and PD. VMAT2 has promising clinical significance in investigating the pathogenesis, early diagnosis, and treatment evaluation of the two diseases.

Dipeptidyl peptidase-4 inhibitors and sulfonylureas prevent the progressive impairment of the nigrostriatal dopaminergic system induced by diabetes during aging

The nigrostriatal dopaminergic system (NDS) controls motor activity, and its impairment during type 2 diabetes (T2D) progression could increase Parkinson's disease risk in diabetics. If so, whether glycemia regulation prevents this impairment needs to be addressed. We investigated whether T2D impairs the NDS and whether dipeptidyl peptidase-4 inhibition (DPP-4i; a clinical strategy against T2D but also neuroprotective in animal models) prevents this effect, in middle-aged mice. Neither T2D (induced by 12 months of high-fat diet) nor aging (14 months) changed striatal dopamine content assessed by high-performance liquid chromatography. However, T2D reduced basal and amphetamine-stimulated striatal extracellular dopamine, assessed by microdialysis. Both the DPP-4i linagliptin and the sulfonylurea glimepiride (an antidiabetic comparator unrelated to DPP-4i) counteracted these effects. The functional T2D-induced effects did not correlate with NDS neuronal/glial alterations. However, aging itself affected striatal neurons/glia, and the glia effects were counteracted mainly by DPP-4i. These findings show NDS functional pathophysiology in T2D and suggest the preventive use of two unrelated anti-T2D drugs. Moreover, DPP-4i counteracted striatal age-related glial alterations suggesting striatal rejuvenation properties.

Analysis of the Relationship between Type II Diabetes Mellitus and Parkinson's Disease: A Systematic Review

In the early sixties, a discussion started regarding the association between Parkinson's disease (PD) and type II diabetes mellitus (T2DM). Today, this potential relationship is still a matter of debate. This review aims to analyze both diseases concerning causal relationships and treatments. A total of 104 articles were found, and studies on animal and “in vitro” models showed that T2DM causes neurological alterations that may be associated with PD, such as deregulation of the dopaminergic system, a decrease in the expression of peroxisome proliferator-activated receptor-gamma coactivator-1α (PGC-1α), an increase in the expression of phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes 15 (PED/PEA-15), and neuroinflammation, as well as acceleration of the formation of alpha-synuclein amyloid fibrils. In addition, clinical studies described that Parkinson's symptoms were notably worse after the onset of T2DM, and seven deregulated genes were identified in the DNA of T2DM and PD patients. Regarding treatment, the action of antidiabetic drugs, especially incretin mimetic agents, seems to confer certain degree of neuroprotection to PD patients. In conclusion, the available evidence on the interaction between T2DM and PD justifies more robust clinical trials exploring this interaction especially the clinical management of patients with both conditions.

Diabetes, a Contemporary Risk for Parkinson's Disease: Epidemiological and Cellular Evidences

Diabetes mellitus (DM), a group of diseases characterized by defective glucose metabolism, is the most widespread metabolic disorder affecting over 400 million adults worldwide. This pathological condition has been implicated in the pathogenesis of a number of central encephalopathies and peripheral neuropathies. In further support of this notion, recent epidemiological evidence suggests a link between DM and Parkinson’s disease (PD), with hyperglycemia emerging as one of the culprits in neurodegeneration involving the nigrostriatal pathway, the neuroanatomical substrate of the motor symptoms affecting parkinsonian patients. Indeed, dopaminergic neurons located in the mesencephalic substantia nigra appear to be particularly vulnerable to oxidative stress and degeneration, likely because of their intrinsic susceptibility to mitochondrial dysfunction, which may represent a direct consequence of hyperglycemia and hyperglycemia-induced oxidative stress. Other pathological pathways induced by increased intracellular glucose levels, including the polyol and the hexosamine pathway as well as the formation of advanced glycation end-products, may all play a pivotal role in mediating the detrimental effects of hyperglycemia on nigral dopaminergic neurons. In this review article, we will examine the epidemiological as well as the molecular and cellular clues supporting the potential susceptibility of nigrostriatal dopaminergic neurons to hyperglycemia.

Hydrogen Sulfide Inhibits High Glucose-Induced Neuronal Senescence by Improving Autophagic Flux via Up-regulation of SIRT1

Hyperglycemia, a key characteristic and risk factor for diabetes mellitus (DM), causes neuronal senescence. Hydrogen sulfide (H₂S) is a novel neuroprotectant. The present work was to investigate the potential effect of H₂S on hyperglycemia-induced neuronal senescence and the underlying mechanisms. We found that NaHS, a donor of H₂S, inhibited high glucose (HG)-induced cellular senescence in HT22 cells (an immortalized mouse hippocampal cell line), as evidenced by a decrease in the number of senescence associated-β-galactosidase (SA-β-gal) positive cells, increase in the growth of cells, and down-regulations of senescence mark proteins, p16^INK4a and p21^CIP1. NaHS improved the autophagic flux, which is judged by a decrease in the amount of intracellular autophagosome as well as up-regulations of LC3II/I and P62 in HG-exposed HT22 cells. Furthermore, blocked autophagic flux by chloroquine (CQ) significantly abolished NaHS-exerted improvement in the autophagic flux and suppression in the cellular senescence of GH-exposed HT22 cells, which indicated that H₂S antagonizes HG-induced neuronal senescence by promoting autophagic flux. We also found that NaHS up-regulated the expression of silent mating type information regulation 2 homolog 1 (SIRT1), an important anti-aging protein, in HG-exposed HT22 cells. Furthermore, inhibition of SIRT1 by sirtinol reversed the protection of H₂S against HG-induced autophagic flux blockade and cellular senescence in HT22 cells. These data indicated that H₂S protects HT22 cells against HG-induced neuronal senescence by improving autophagic flux via up-regulation of SIRT1, suggesting H₂S as a potential treatment strategy for hyperglycemia-induced neuronal senescence and neurotoxicity.