New roles for insulin-like hormones in neuronal signalling and protection: new hopes for novel treatments of Alzheimer's disease?

Glucagon-like peptide 1 receptor activation: anti-inflammatory effects in the brain

The glucagon-like peptide 1 is a pleiotropic hormone that has potent insulinotropic effects and is key in treating metabolic diseases such as diabetes and obesity. Glucagon-like peptide 1 exerts its effects by activating a membrane receptor identified in many tissues, including different brain regions. Glucagon-like peptide 1 activates several signaling pathways related to neuroprotection, like the support of cell growth/survival, enhancement promotion of synapse formation, autophagy, and inhibition of the secretion of proinflammatory cytokines, microglial activation, and apoptosis during neural morphogenesis. The glial cells, including astrocytes and microglia, maintain metabolic homeostasis and defense against pathogens in the central nervous system. After brain insult, microglia are the first cells to respond, followed by reactive astrocytosis. These activated cells produce proinflammatory mediators like cytokines or chemokines to react to the insult. Furthermore, under these circumstances, microglia can become chronically inflammatory by losing their homeostatic molecular signature and, consequently, their functions during many diseases. Several processes promote the development of neurological disorders and influence their pathological evolution: like the formation of protein aggregates, the accumulation of abnormally modified cellular constituents, the formation and release by injured neurons or synapses of molecules that can dampen neural function, and, of critical importance, the dysregulation of inflammatory control mechanisms. The glucagon-like peptide 1 receptor agonist emerges as a critical tool in treating brain-related inflammatory pathologies, restoring brain cell homeostasis under inflammatory conditions, modulating microglia activity, and decreasing the inflammatory response. This review summarizes recent advances linked to the anti-inflammatory properties of glucagon-like peptide 1 receptor activation in the brain related to multiple sclerosis, Alzheimer's disease, Parkinson's disease, vascular dementia, or chronic migraine.

Glucagon-like peptide 1 (GLP-1) receptor agonists in experimental Alzheimer's disease models: a systematic review and meta-analysis of preclinical studies

Alzheimer's disease (AD) is a degenerative disease of the nervous system. Glucagon-like peptide-1 receptor agonists (GLP-1 RAs), a drug used to treat type 2 diabetes, have been shown to have neuroprotective effects. This systematic review and meta-analysis evaluated the effects and potential mechanisms of GLP-1 RAs in AD animal models. 26 studies were included by searching relevant studies from seven databases according to a predefined search strategy and inclusion criteria. Methodological quality was assessed using SYRCLE's risk of bias tool, and statistical analysis was performed using ReviewManger 5.3. The results showed that, in terms of behavioral tests, GLP-1 RAs could improve the learning and memory abilities of AD rodents; in terms of pathology, GLP-1 RAs could reduce Aβ deposition and phosphorylated tau levels in the brains of AD rodents. The therapeutic potential of GLP-1 RAs in AD involves a range of mechanisms that work synergistically to enhance the alleviation of various pathological manifestations associated with the condition. A total of five clinical trials were retrieved from ClinicalTrials.gov. More large-scale and high-quality preclinical trials should be conducted to more accurately assess the therapeutic effects of GLP-1 RAs on AD.

Glucagon-like peptide-1 receptor-agonists treatment for cardio-metabolic parameters in schizophrenia patients: a systematic review and meta-analysis

Aims

We performed this meta-analysis to evaluate the efficacy and safety of glucagon-like peptide-1 receptor-agonists (GLP-1RA) treatment on cardio-metabolic parameters among antipsychotic-treated patients with schizophrenia.

Methods

We searched the Web of Science, Cochrane Central Register of Controlled Trials, PubMed, PsycINFO, and Scopus for relevant Randomized Clinical trials (RCTs) from inception until 1 August 2022. Documents were screened for qualified articles, and all concerned outcomes were pooled as risk ratios (RR) or mean difference (MD) in the meta-analysis models using Review Manager (RevMan version 5.4).

Results

Pooling data from 7 RCTs (398 patients) showed that GLP-1 RA was superior to placebo with regard to body weight [MD = - 4.68, 95% CI (-4.90,-4.46), P < 0.00001], waist circumference [MD = - 3.66, 95% CI (-3.89,-3.44), P < 0.00001], body mass index (BMI) [MD = - 1.09, 95% CI (-1.25,-0.93), P < 0.00001], systolic blood pressure (SBP) [MD = - 3.07, 95% CI (-3.61,-2.53), P < 0.00001], and diastolic blood pressure (DBP) [MD = - 2.02, 95% CI (-2.42,-1.62), P < 0.00001]. The total effect did not favor either of the two groups with respect to insulin and respiratory adverse events {[MD = - 0.06, 95% CI (-0.36, 0.24), p = 0.70], [RR = 0.66, 95% CI (0.31, 1.40), p = 0.28]; respectively}.

Conclusion

Our analysis revealed that GLP-1 RA treatment is safe and effective on cardio-metabolic parameters over control in antipsychotic-treated patients with schizophrenia. Nevertheless, the present evidence is not sufficient to confirm the safety and efficacy of GLP-1RA treatment on insulin and respiratory adverse events. Therefore, further studies are recommended.

Systematic review registration

http://www.crd.york.ac.uk/PROSPERO/, identifier: CRD42022333040.

High-fat diet-induced diabetes couples to Alzheimer's disease through inflammation-activated C/EBPβ/AEP pathway

Diabetes is a risk factor for Alzheimer's disease (AD), which is also called type 3 diabetes with insulin reduction and insulin resistance in AD patient brains. However, the molecular mechanism coupling diabetes to AD onset remains incompletely understood. Here we show that inflammation, associated with obesity and diabetes elicited by high-fat diet (HFD), activates neuronal C/EBPβ/AEP signaling that drives AD pathologies and cognitive disorders. HFD stimulates diabetes and insulin resistance in neuronal Thy1-C/EBPβ transgenic (Tg) mice, accompanied with prominent mouse Aβ accumulation and hyperphosphorylated Tau aggregation in the brain, triggering cognitive deficits. These effects are profoundly diminished when AEP is deleted from C/EBPβ Tg mice. Chronic treatment with inflammatory lipopolysaccharide (LPS) facilitates AD pathologies and cognitive disorders in C/EBPβ Tg but not in wild-type mice, and these deleterious effects were substantially alleviated in C/EBPβ Tg/AEP -/- mice. Remarkably, the anti-inflammatory drug aspirin strongly attenuates HFD-induced diabetes and AD pathologies in neuronal C/EBPβ Tg mice. Therefore, our findings demonstrate that inflammation-activated neuronal C/EBPβ/AEP signaling couples diabetes to AD.

Mechanistic Insights for Drug Repurposing and the Design of Hybrid Drugs for Alzheimer's Disease

The heterogeneity and complex nature of Alzheimer's disease (AD) is attributed to several genetic risk factors and molecular culprits. The slow pace and increasing failure rate of conventional drug discovery has led to the exploration of complementary strategies based on repurposing approved drugs to treat AD. Drug repurposing (DR) is a cost-effective, low-risk, and efficient approach for identifying novel therapeutic candidates for AD treatment. Similarly, hybrid drug design through the integration of distinct pharmacophores from known or failed drugs and natural products is an interesting strategy to target the multifactorial nature of AD. In this Perspective, we discuss the potential of DR and highlight promising drug candidates that can be advanced for clinical trials, backed by a detailed discussion on their plausible mechanisms of action. Our article fosters research on the hidden potential of DR and hybrid drug design with the goal of unravelling new drugs and targets to tackle AD.

Liraglutide Reduces Vascular Damage, Neuronal Loss, and Cognitive Impairment in a Mixed Murine Model of Alzheimer's Disease and Type 2 Diabetes

Alzheimer's disease is the most common form of dementia, and epidemiological studies support that type 2 diabetes (T2D) is a major contributor. The relationship between both diseases and the fact that Alzheimer's disease (AD) does not have a successful treatment support the study on antidiabetic drugs limiting or slowing down brain complications in AD. Among these, liraglutide (LRGT), a glucagon-like peptide-1 agonist, is currently being tested in patients with AD in the Evaluating Liraglutide in Alzheimer's Disease (ELAD) clinical trial. However, the effects of LRGT on brain pathology when AD and T2D coexist have not been assessed. We have administered LRGT (500 μg/kg/day) to a mixed murine model of AD and T2D (APP/PS1xdb/db mice) for 20 weeks. We have evaluated metabolic parameters as well as the effects of LRGT on learning and memory. Postmortem analysis included assessment of brain amyloid-β and tau pathologies, microglia activation, spontaneous bleeding and neuronal loss, as well as insulin and insulin-like growth factor 1 receptors. LRGT treatment reduced glucose levels in diabetic mice (db/db and APP/PS1xdb/db) after 4 weeks of treatment. LRGT also helped to maintain insulin levels after 8 weeks of treatment. While we did not detect any effects on cortical insulin or insulin-like growth factor 1 receptor m-RNA levels, LRGT significantly reduced brain atrophy in the db/db and APP/PS1xdb/db mice. LRGT treatment also rescued neuron density in the APP/PS1xdb/db mice in the proximity (p = 0.008) far from amyloid plaques (p < 0.001). LRGT reduced amyloid plaque burden in the APP/PS1 animals (p < 0.001), as well as Aβ aggregates levels (p = 0.046), and tau hyperphosphorylation (p = 0.009) in the APP/PS1xdb/db mice. Spontaneous bleeding was also ameliorated in the APP/PS1xdb/db animals (p = 0.012), and microglia burden was reduced in the proximity of amyloid plaques in the APP/PS1 and APP/PS1xdb/db mice (p < 0.001), while microglia was reduced in areas far from amyloid plaques in the db/db and APP/PS1xdb/db mice (p < 0.001). This overall improvement helped to rescue cognitive impairment in AD-T2D mice in the new object discrimination test (p < 0.001) and Morris water maze (p < 0.001). Altogether, our data support the role of LRGT in reduction of associated brain complications when T2D and AD occur simultaneously, as regularly observed in the clinical arena.

Treating cognitive impairment in schizophrenia with GLP-1RAs: an overview of their therapeutic potential

INTRODUCTION

Schizophrenia is a neuropsychiatric disorder that affects approximately 1% of individuals worldwide. There are no available medications to treat cognitive impairment in this patient population currently. Preclinical evidence suggests that glucagon-like peptide-1 receptor agonists (GLP-1 RAs) improve cognitive function. There is a need to evaluate how GLP-1 RAs alter specific domains of cognition and whether they will be of therapeutic benefit in individuals with schizophrenia.

AREAS COVERED

This paper summarizes the effects of GLP-1 RAs on metabolic processes in the brain and how these mechanisms relate to improved cognitive function. We provide an overview of preclinical studies that demonstrate GLP-1 RAs improve cognition and comment on their potential therapeutic benefit in individuals with schizophrenia.

EXPERT OPINION

To understand the benefits of GLP-1 RAs in individuals with schizophrenia, further preclinical research with rodent models relevant to schizophrenia symptomology are needed. Moreover, preclinical studies must focus on using a wider range of behavioral assays to understand whether important aspects of cognition such as executive function, attention, and goal-directed behavior are improved using GLP-1 RAs. Further research into the specific mechanisms of how GLP-1 RAs affect cognitive function and their interactions with antipsychotic medication commonly prescribed is necessary.

A Comprehensive Analysis of Unique and Recurrent Copy Number Variations in Alzheimer's Disease and its Related Disorders

BACKGROUND

Copy number variations (CNVs) play an important role in the genetic etiology of various neurological disorders, including Alzheimer's disease (AD). Type 2 diabetes mellitus (T2DM) and major depressive disorder (MDD) were shown to have share mechanisms and signaling pathways with AD.

OBJECTIVE

We aimed to assess CNVs regions that may harbor genes contributing to AD, T2DM, and MDD in 67 Saudi familial and sporadic AD patients, with no alterations in the known genes of AD and genotyped previously for APOE.

METHODS

DNA was analyzed using the CytoScan-HD array. Two layers of filtering criteria were applied. All the identified CNVs were checked in the Database of Genomic Variants (DGV).

RESULTS

A total of 1086 CNVs (565 gains and 521 losses) were identified in our study. We found 73 CNVs harboring genes that may be associated with AD, T2DM or MDD. Nineteen CNVs were novel. Most importantly, 42 CNVs were unique in our studied cohort existing only in one patient. Two large gains on chromosomes 1 and 13 harbored genes implicated in the studied disorders. We identified CNVs in genes that encode proteins involved in the metabolism of amyloid-β peptide (AGRN, APBA2, CR1, CR2, IGF2R, KIAA0125, MBP, RER1, RTN4R, VDR and WISPI) or Tau proteins (CACNAIC, CELF2, DUSP22, HTRA1 and SLC2A14).

CONCLUSION

The present work provided information on the presence of CNVs related to AD, T2DM, and MDD in Saudi Alzheimer's patients.

GIP receptor suppresses PAC1receptor-mediated neuronal differentiation via formation of a receptor heterocomplex

Pituitary adenylate cyclase-activating peptide (PACAP) receptor (PAC1R) is a class B Gprotein-coupled receptor (GPCR) that is widely expressed in the human body and is involved in neuronal differentiation. As class B GPCRs are known to form heterocomplexes with family members, we hypothesized that PAC1R mediates neuronal differentiation through interaction with a class B GPCR. We used the BRET assay to identify potential interactions between PAC1R and 11 class B GPCRs. Gastric inhibitory polypeptide receptor (GIPR) and secretin receptor were identified as putative binding partners of PAC1R. The effect of heterocomplex formation by PAC1R on receptor activation was evaluated with the cyclic (c)AMP, luciferase reporter, and calcium signaling assays; and the effects on receptor internalization and subcellular localization were examined by confocal microscopy. The results suggested he PAC1R/GIPR heterocomplex suppressed signaling events downstream of PAC1R, including cAMP production, serum response element and calcium signaling, and β-arrestin recruitment. Protein-protein interaction was analyzed in silico, and induction of neuronal differentiation by the PAC1R heterocomplex was assessed in SH-SY5Y neuronal cells by measure the morphological changes and marker genes expression by real-time quantitative PCR and western blot. Over-expression of GIPR suppressed PACAP/PAC1R-mediated neuronal differentiation and the differentiation markers expression in SH-SY5Y cells. GIPR regulates neuronal differentiation through heterocomplex formation with PAC1R.

GLP-1 mimetics and cognition

Glucagon-like peptide-1 (GLP-1) receptor agonists are a class of antidiabetic drugs that improve the glycaemia via several molecular pathways. Recent evidence suggest that they also have additional effects modulating pathophysiologic pathways included in cognitive disorders. Since some forms of cognitive dysfunction such as Alzheimer's disease are more common among diabetic patients than in the normal population, antidiabetic drugs that have neuroprotective effects affording protection for cognitive disorders would be of benefit. Therefore, we reviewed the pharmacologic effects of GLP-1 analogues and found that they may have the additional benefit of improving cognitive performance via at least eight molecular mechanisms.

A Pilot Study of Exenatide Actions in Alzheimer's Disease

BACKGROUND

Strong preclinical evidence suggests that exenatide, a glucagon-like peptide-1 (GLP- 1) receptor agonist used for treating type 2 diabetes, is neuroprotective and disease-modifying in Alzheimer's Disease (AD).

OBJECTIVE

We performed an 18-month double-blind randomized placebo-controlled Phase II clinical trial to assess the safety and tolerability of exenatide and explore treatment responses for clinical, cognitive, and biomarker outcomes in early AD.

METHOD

Eighteen participants with high probability AD based on cerebrospinal fluid (CSF) biomarkers completed the entire study prior to its early termination by the sponsor; partial outcomes were available for twentyone.

RESULTS

Exenatide was safe and well-tolerated, showing an expectedly higher incidence of nausea and decreased appetite compared to placebo and decreasing glucose and GLP-1 during Oral Glucose Tolerance Tests. Exenatide treatment produced no differences or trends compared to placebo for clinical and cognitive measures, MRI cortical thickness and volume, or biomarkers in CSF, plasma, and plasma neuronal extracellular vesicles (EV) except for a reduction of Aβ42 in EVs.

CONCLUSION

The positive finding of lower EV Aβ42 supports emerging evidence that plasma neuronal EVs provide an effective platform for demonstrating biomarker responses in clinical trials in AD. The study was underpowered due to early termination and therefore we cannot draw any firm conclusions. However, the analysis of secondary outcomes shows no trends in support of the hypothesis that exenatide is diseasemodifying in clinical AD, and lowering EV Aβ42 in and of itself may not improve cognitive outcomes in AD.

Comparison between touchscreen operant chambers and water maze to detect early prefrontal dysfunction in mice

The relative lack of sensitive and clinically valid tests of rodent behavior might be one of the reasons for the limited success of the clinical translation of preclinical Alzheimer's disease (AD) research findings. There is a general interest in innovative behavioral methodology, and protocols have been proposed for touchscreen operant chambers that might be superior to existing cognitive assessment methods. We assessed and analyzed touchscreen performance in several novel ways to examine the possible occurrence of early signs of prefrontal (PFC) functional decline in the APP/PS1 mouse model of AD. Touchscreen learning performance was compared between APP/PS1-21 mice and wildtype littermates on a C57BL/6J background at 3, 6 and 12 months of age in parallel to the assessment of spatial learning, memory and cognitive flexibility in the Morris water maze (MWM). We found that older mice generally needed more training sessions to complete the touchscreen protocol than younger ones. Older mice also displayed defects in MWM working memory performance, but touchscreen protocols detected functional changes beginning at 3 months of age. Histological changes in PFC of APP/PS1 mice indeed occurred as early as 3 months. Our results suggest that touchscreen operant protocols are more sensitive to PFC dysfunction, which is of relevance to the use of these tasks and devices in preclinical AD research and experimental pharmacology.

Extending Arms of Insulin Resistance from Diabetes to Alzheimer's Disease: Identification of Potential Therapeutic Targets

BACKGROUND & OBJECTIVE

Type 3 diabetes (T3D) is chronic insulin resistant state of brain which shares pathology with sporadic Alzheimer's disease (sAD). Insulin signaling is a highly conserved pathway in the living systems that orchestrate cell growth, repair, maintenance, energy homeostasis and reproduction. Although insulin is primarily studied as a key molecule in diabetes mellitus, its role has recently been implicated in the development of Alzheimer's disease (AD). Severe complications in brain of diabetic patients and metabolically compromised status is evident in brain of AD patients. Underlying shared pathology of two disorders draws a trajectory from peripheral insulin resistance to insulin unresponsiveness in the central nervous system (CNS). As insulin has a pivotal role in AD, it is not an overreach to address diabetic condition in AD brain as T3D. Insulin signaling is indispensable to nervous system and it is vital for neuronal growth, repair, and maintenance of chemical milieu at synapses. Downstream mediators of insulin signaling pathway work as a regulatory hub for aggregation and clearance of unfolded proteins like Aβ and tau.

CONCLUSION

In this review, we discuss the regulatory roles of insulin as a pivotal molecule in brain with the understanding of defective insulin signaling as a key pathological mechanism in sAD. This article also highlights ongoing trials of targeting insulin signaling as a therapeutic manifestation to treat diabetic condition in brain.

Interlink Between Insulin Resistance and Neurodegeneration with an Update on Current Therapeutic Approaches

The interlink between diabetes mellitus and neurodegenerative diseases such as Alzheimer's Disease (AD) and Parkinson's Disease (PD) has been identified by several researchers. Patients with Type-2 Diabetes Mellitus (T2DM) are found to be affected with cognitive impairments leading to learning and memory deficit, while patients with Type-1 Diabetes Mellitus (T1DM) showed less severe levels of these impairments in the brain. This review aimed to discuss the connection between insulin with the pathophysiology of neurodegenerative diseases (AD and PD) and the current therapeutic approached mediated through insulin for management of neurodegenerative diseases. An extensive literature search was conducted using keywords "insulin"; "insulin resistance"; "Alzheimer's disease"; "Parkinson's disease" in public domains of Google scholar, PubMed, and ScienceDirect. Selected articles were used to construct this review. Studies have shown that impaired insulin signaling contributes to the accumulation of amyloid-β, neurofibrillary tangles, tau proteins and α-synuclein in the brain. Whereas, improvement in insulin signaling slows down the progression of cognitive decline. Various therapeutic approaches for altering the insulin function in the brain have been researched. Besides intranasal insulin, other therapeutics like PPAR-γ agonists, neurotrophins, stem cell therapy and insulin-like growth factor-1 are under investigation. Research has shown that insulin insensitivity in T2DM leads to neurodegeneration through mechanisms involving a variety of extracellular, membrane receptor, and intracellular signaling pathway disruptions. Some therapeutics, such as intranasal administration of insulin and neuroactive substances have shown promise but face problems related to genetic background, accessibility to the brain, and invasiveness of the procedures.

Brain insulin resistance: role in neurodegenerative disease and potential for targeting

Introduction: This review evaluates the novel strategy of treating Alzheimer's and Parkinson's disease (AD and PD) withdrugs that initially have been developed to treat type 2 diabetes. As insulin signalling has been found to be de-sensitized in the brains of patients, drugs that can re-sensitize insulin signalling have been tested to evaluate if this strategy can alter disease progression.Areas covered: The review will give an overview of preclinical and clinical tests in AD and PD of drugs activating insulin receptors, glucagon-like peptide -1 (GLP-1) receptors, and glucose-dependent insulinotropic polypeptide (GIP) receptors.Expert opinion: Insulin, GLP-1 and GIP receptor agonists have shown good effects in preclinical studies. First clinical trials in MCI/AD patients have shown that insulin can improve on key pathological symptoms of AD such as memory impairment, brain activity, neuronal energy utilization, and inflammation markers. A GLP-1 receptor agonist has shown disease-modifying effects in PD patients, and first pilot studies have shown encouraging effects of a GLP-1 receptor agonist in AD patients. Novel dual GLP-1/GIP receptor agonists that cross the blood brain barrier show superior neuroprotective effects compared to single GLP-1 or GIP receptor agonists, and show great promise as novel treatments of AD and PD.

Liraglutide Suppresses Tau Hyperphosphorylation, Amyloid Beta Accumulation through Regulating Neuronal Insulin Signaling and BACE-1 Activity

Neuronal insulin resistance is a significant feature of Alzheimer's disease (AD). Accumulated evidence has revealed the possible neuroprotective mechanisms of antidiabetic drugs in AD. Liraglutide, a glucagon-like peptide-1 (GLP-1) analog and an antidiabetic agent, has a benefit in improving a peripheral insulin resistance. However, the neuronal effect of liraglutide on the model of neuronal insulin resistance with Alzheimer's formation has not been thoroughly investigated. The present study discovered that liraglutide alleviated neuronal insulin resistance and reduced beta-amyloid formation and tau hyperphosphorylation in a human neuroblostoma cell line, SH-SY5Y. Liraglutide could effectively reverse deleterious effects of insulin overstimulation. In particular, the drug reversed the phosphorylation status of insulin receptors and its major downstream signaling molecules including insulin receptor substrate 1 (IRS-1), protein kinase B (AKT), and glycogen synthase kinase 3 beta (GSK-3β). Moreover, liraglutide reduced the activity of beta secretase 1 (BACE-1) enzyme, which then decreased the formation of beta-amyloid in insulin-resistant cells. This indicated that liraglutide can reverse the defect of phosphorylation status of insulin signal transduction but also inhibit the formation of pathogenic Alzheimer's proteins like Aβ in neuronal cells. We herein provided the possibility that the liraglutide-based therapy may be able to reduce such deleterious effects caused by insulin resistance. In view of the beneficial effects of liraglutide administration, these findings suggest that the use of liraglutide may be a promising therapy for AD with insulin-resistant condition.

Neuroprotective Effects and Treatment Potential of Incretin Mimetics in a Murine Model of Mild Traumatic Brain Injury

Traumatic brain injury (TBI) is a commonly occurring injury in sports, victims of motor vehicle accidents, and falls. TBI has become a pressing public health concern with no specific therapeutic treatment. Mild TBI (mTBI), which accounts for approximately 90% of all TBI cases, may frequently lead to long-lasting cognitive, behavioral, and emotional impairments. The incretins glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gastrointestinal hormones that induce glucose-dependent insulin secretion, promote β-cell proliferation, and enhance resistance to apoptosis. GLP-1 mimetics are marketed as treatments for type 2 diabetes mellitus (T2DM) and are well tolerated. Both GLP-1 and GIP mimetics have shown neuroprotective properties in animal models of Parkinson's and Alzheimer's disease. The aim of this study is to evaluate the potential neuroprotective effects of liraglutide, a GLP-1 analog, and twincretin, a dual GLP-1R/GIPR agonist, in a murine mTBI model. First, we subjected mice to mTBI using a weight-drop device and, thereafter, administered liraglutide or twincretin as a 7-day regimen of subcutaneous (s.c.) injections. We then investigated the effects of these drugs on mTBI-induced cognitive impairments, neurodegeneration, and neuroinflammation. Finally, we assessed their effects on neuroprotective proteins expression that are downstream to GLP-1R/GIPR activation; specifically, PI3K and PKA phosphorylation. Both drugs ameliorated mTBI-induced cognitive impairments evaluated by the novel object recognition (NOR) and the Y-maze paradigms in which neither anxiety nor locomotor activity were confounds, as the latter were unaffected by either mTBI or drugs. Additionally, both drugs significantly mitigated mTBI-induced neurodegeneration and neuroinflammation, as quantified by immunohistochemical staining with Fluoro-Jade/anti-NeuN and anti-Iba-1 antibodies, respectively. mTBI challenge significantly decreased PKA phosphorylation levels in ipsilateral cortex, which was mitigated by both drugs. However, PI3K phosphorylation was not affected by mTBI. These findings offer a new potential therapeutic approach to treat mTBI, and support further investigation of the neuroprotective effects and mechanism of action of incretin-based therapies for neurological disorders.

Upregulation of spinal glucose-dependent insulinotropic polypeptide receptor induces membrane translocation of PKCγ and synaptic target of AMPA receptor GluR1 subunits in dorsal horns in a rat model of incisional pain

It is unclear whether glucose-dependent insulinotropic polypeptide receptor (GIPR) signaling plays an important role in spinal nociception. We hypothesized that the spinal GIPR is implicated in central sensitization of postoperative pain. Our data showed that the cumulative pain scores peaked at 3 h, kept at a high level at 1 d after incision, gradually decreased afterwards and returned to the baseline values at 5 d after incision. Correspondingly, the expression of GIPR in spinal cord dorsal horn peaked at 1 d after incision, and returned to the baseline value at 5 d after incision. The double-labeling immunofluorescence demonstrated that spinal GIPR was expressed in dorsal horn neurons, but not in astrocyte or microglial cells. At 1 d after incision, the effects of intrathecal saline, GIPR antagonist (Pro3)GIP on pain behaviors were investigated. Our data showed that at 30 min and 60 min following intrathecal treatments of 300 ng (Pro3)GIP, the cumulative pain scores were decreased and paw withdrawal thresholds to mechanical stimuli were increased when compared to those immediately before intrathecal treatments. Accordingly, at 30 min after intrathecal injections, the membrane translocation levels of PKCγ and the GluR1 expression in postsynaptic membrane in ipsilateral dorsal horns to the incision were significantly upregulated in rats with intrathecal saline injections, as compared to normal control group. At 30 min after intrathecal treatment, (Pro3)GIP inhibited the membrane translocation levels of PKCγ and the GluR1 expression in postsynaptic membrane in ipsilateral dorsal horns. Our study indicates that upregulation of spinal GIPR may contribute to pain hypersensitivity through inducing membrane translocation level of PKCγ and synaptic target of AMPA receptor GluR1 subunits in ipsilateral dorsal horns of rats with plantar incision.

Current and future clinical utilities of Parkinson's disease and dementia biomarkers: can they help us conquer the disease?

Introduction: Biomarkers for Parkinson's disease and Alzheimer's disease are essential, not only for disease detection, but also provide insight into potential disease relationships leading to better detection and therapy. As metabolic disease is known to increase neurodegeneration risk, such mechanisms may reveal such novel targets for PD and AD. Moreover, metabolic disease, including insulin resistance, offer novel biomarker and therapeutic targets for neurodegeneration, including glucagon-like-peptide-1, dipeptidyl peptidase-4 and adiponectin. Areas covered: The authors reviewed PubMed-listed research articles, including ours, on a number of putative PD, AD and neurodegenerative disease targets of interest, focusing on the relevance of metabolic syndrome and insulin resistance mechanisms, especially type II diabetes, to PD and AD. We highlighted various issues surrounding the current state of knowledge and propose avenues for future development. Expert opinion: Biomarkers for PD and AD are indispensable for disease diagnosis, prognostication and tracking disease severity, especially for clinical therapy trials. Although no validated PD biomarkers exist, their potential utility has generated tremendous interest. Combining insulin-resistance biomarkers with other core biomarkers or using them to predict non-motor symptoms of PD may be clinically useful. Collectively, although still unclear, potential biomarkers and therapies can aid in shedding new light on novel aspects of both PD and AD.

Pathological and cognitive changes in patients with type 2 diabetes mellitus and comorbid MCI and protective hypoglycemic therapies: a narrative review

Type 2 diabetes mellitus (T2DM) is becoming a significant health issue worldwide. Many studies support the hypothesis that patients with T2DM have a higher-than-expected incidence of mild cognitive impairment (MCI) than individuals without diabetes. Based on the results from recent studies, MCI might be associated with the effects of T2DM on glucose metabolism and brain atrophy. As a narrative review, we will illuminate pathological and cognitive changes in patients with T2DM and comorbid MCI and protective hypoglycemic therapies. The early abnormal signs of cognition must be elucidated, and extensive investigations are needed to develop improved therapies for use in the clinic.

Outcomes and clinical implications of intranasal insulin administration to the central nervous system

Insulin signaling in the brain plays a critical role in metabolic control and cognitive function. Targeting insulinergic pathways in the central nervous system via peripheral insulin administration is feasible, but associated with systemic effects that necessitate tight supervision or countermeasures. The intranasal route of insulin administration, which largely bypasses the circulation and thereby greatly reduces these obstacles, has now been repeatedly tested in proof-of-concept studies in humans as well as animals. It is routinely used in experimental settings to investigate the impact on eating behavior, peripheral metabolism, memory function and brain activation of acute or long-term enhancements in central nervous system insulin signaling. Epidemiological and experimental evidence linking deteriorations in metabolic control such as diabetes with neurodegenerative diseases imply pathophysiological relevance of dysfunctional brain insulin signaling or brain insulin resistance, and suggest that targeting insulin in the brain holds some promise as a therapy or adjunct therapy. This short narrative review gives an overview over recent findings on brain insulin signaling as derived from human studies deploying intranasal insulin, and evaluates the potential of therapeutic interventions that target brain insulin resistance.

Insulin-Like Growth Factor Binding Protein 2 Is Associated With Biomarkers of Alzheimer's Disease Pathology and Shows Differential Expression in Transgenic Mice

There is increasing evidence that metabolic dysfunction plays an important role in Alzheimer's disease (AD). Brain insulin resistance and subsequent impairment of insulin and insulin-like growth factor (IGF) signaling are associated with the neurodegenerative and clinical features of AD. Nevertheless, how the brain insulin/IGF signaling system is altered in AD and the effects of these changes on AD pathobiology are not well understood. IGF binding protein 2 (IGFBP-2) is an abundant cerebral IGF signaling protein and there is early evidence suggesting it associates with AD biomarkers. We evaluated the relationship between protein levels of IGFBP-2 with cerebrospinal fluid (CSF) biomarkers and neuroimaging markers of AD progression in 300 individuals from across the AD spectrum. CSF IGFBP-2 levels were correlated with CSF tau levels and brain atrophy in non-hippocampal regions. To further explore the role of IGFBP2 in tau pathobiology, we evaluated the expression of IGFBP2 in different human and mouse brain cell types and brain tissue from two transgenic mouse models: the P301L-tau model of tauopathy and TASTPM model of AD. We observed significant differential expression of IGFBP2 in both transgenic mouse models relative to wild-type mice in cortex but not in hippocampus. In both humans and mice, IGFBP2 is most highly expressed in astrocytes. Taken together, our findings suggest that IGFBP-2 may be linked to tau pathology and provides further evidence for a relationship between metabolic dysregulation and neurodegeneration. Our results also raise the possibility that this relationship may extend beyond neurons.

Engineered commensal bacteria prevent systemic inflammation-induced memory impairment and amyloidogenesis via producing GLP-1

The anti-obesity drug GLP-1 has been proven to have an impact on central nervous system, while its extremely short half-life greatly limited its use. In this study, our group constructed two engineering strains MG1363-pMG36e-GLP-1 and VNP20009-pLIVE-GLP-1 to continuously express GLP-1, and supplementation of these strains, especially MG1363-pMG36e-GLP-1, had significantly restored the spatial learning and memory impairment of mice caused by LPS (p < 0.05), suppressed glia activation and Aβ accumulation, and downregulated inflammatory expressions of COX-2, TLR-4, TNF-a, and IL-1β. In addition, MG1363-pMG36e-GLP-1 had significantly blocked the translocation of NF-κB signal and inhibited the phosphorylation of redox-sensitive cytoplasmic signalings of MAPKs and PI3K/AKT. These data suggest that MG1363-pMG36e-GLP-1 could be used as a safe and effective nonabsorbed oral treatment for neuroinflammation-related diseases such as Alzheimer's disease (AD).

Targeting glucose-dependent insulinotropic polypeptide receptor for neurodegenerative disorders

INTRODUCTION

Incretin hormones, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1) exert pleiotropic effects on endocrine pancreas and nervous system. Expression of GIP and GIP receptor (GIPR) in neurons, their roles in neurogenesis, synaptic plasticity, neurotransmission, and neuromodulation uniquely position GIPR for therapeutic applications in neurodegenerative disorders. GIP analogs acting as GIPR agonists attenuate neurobehavioral and neuropathological sequelae of neurodegenerative disorders in preclinical models, e.g. Alzheimer's disease (AD), Parkinson's disease (PD), and cerebrovascular disorders. Modulation of GIPR signaling offers an unprecedented approach for disease modification by arresting neuronal viability decline, enabling neuronal regeneration, and reducing neuroinflammation. Growth-promoting effects of GIP signaling and broad-based neuroprotection highlight the therapeutic potential of GIPR agonists. Areas covered: This review focuses on the role of GIPR-mediated signaling in the central nervous system in neurophysiological and neuropathological conditions. In context of neurodegeneration, the article summarizes potential of targeting GIPR signaling for neurodegenerative conditions such as AD, PD, traumatic brain injury, and cerebrovascular disorders. Expert opinion: GIPR represents a validated therapeutic target for neurodegenerative disorders. GIPR agonists impart symptomatic improvements, slowed neurodegeneration, and enhanced neuronal regenerative capacity in preclinical models. Modulation of GIPR signaling is potentially a viable therapeutic approach for disease modification in neurodegenerative disorders.

Diagnostic Biomarkers of Alzheimer's Disease as Identified in Saliva using 1H NMR-Based Metabolomics

Using 1H NMR metabolomics, we biochemically profiled saliva samples collected from healthy-controls (n = 12), mild cognitive impairment (MCI) sufferers (n = 8), and Alzheimer's disease (AD) patients (n = 9). We accurately identified significant concentration changes in 22 metabolites in the saliva of MCI and AD patients compared to controls. This pilot study demonstrates the potential for using metabolomics and saliva for the early diagnosis of AD. Given the ease and convenience of collecting saliva, the development of accurate and sensitive salivary biomarkers would be ideal for screening those at greatest risk of developing AD.

Central Nervous System Delivery of Intranasal Insulin: Mechanisms of Uptake and Effects on Cognition

Intranasal insulin has shown efficacy in patients with Alzheimer's disease (AD), but there are no preclinical studies determining whether or how it reaches the brain. Here, we showed that insulin applied at the level of the cribriform plate via the nasal route quickly distributed throughout the brain and reversed learning and memory deficits in an AD mouse model. Intranasal insulin entered the blood stream poorly and had no peripheral metabolic effects. Uptake into the brain from the cribriform plate was saturable, stimulated by PKC inhibition, and responded differently to cellular pathway inhibitors than did insulin transport at the blood-brain barrier. In summary, these results show intranasal delivery to be an effective way to deliver insulin to the brain.

The GLP-1 Receptor Agonist Liraglutide Improves Memory Function and Increases Hippocampal CA1 Neuronal Numbers in a Senescence-Accelerated Mouse Model of Alzheimer's Disease

Recent studies indicate that glucagon-like peptide 1 (GLP-1) receptor agonists, currently used in the management of type 2 diabetes, exhibit neurotrophic and neuroprotective effects in amyloid-β (Aβ) toxicity models of Alzheimer’s disease (AD). We investigated the potential pro-cognitive and neuroprotective effects of the once-daily GLP-1 receptor agonist liraglutide in senescence-accelerated mouse prone 8 (SAMP8) mice, a model of age-related sporadic AD not dominated by amyloid plaques. Six-month-old SAMP8 mice received liraglutide (100 or 500 μg/kg/day, s.c.) or vehicle once daily for 4 months. Vehicle-dosed age-matched 50% back-crossed as well as untreated young (4-month-old) SAMP8 mice were used as control groups for normal memory function. Vehicle-dosed 10-month-old SAMP8 mice showed significant learning and memory retention deficits in an active-avoidance T-maze, as compared to both control groups. Also, 10-month-old SAMP8 mice displayed no immunohistological signatures of amyloid-β plaques or hyperphosphorylated tau, indicating the onset of cognitive deficits prior to deposition of amyloid plaques and neurofibrillary tangles in this AD model. Liraglutide significantly increased memory retention and total hippocampal CA1 pyramidal neuron numbers in SAMP8 mice, as compared to age-matched vehicle-dosed SAMP8 mice. In conclusion, liraglutide delayed or partially halted the progressive decline in memory function associated with hippocampal neuronal loss in a mouse model of pathological aging with characteristics of neurobehavioral and neuropathological impairments observed in early-stage sporadic AD.

Glucagon-like peptide 1 receptor activation regulates cocaine actions and dopamine homeostasis in the lateral septum by decreasing arachidonic acid levels

Agonism of the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) has been effective at treating aspects of addictive behavior for a number of abused substances, including cocaine. However, the molecular mechanisms and brain circuits underlying the therapeutic effects of GLP-1R signaling on cocaine actions remain elusive. Recent evidence has revealed that endogenous signaling at the GLP-1R within the forebrain lateral septum (LS) acts to reduce cocaine-induced locomotion and cocaine conditioned place preference, both considered dopamine (DA)-associated behaviors. DA terminals project from the ventral tegmental area to the LS and express the DA transporter (DAT). Cocaine acts by altering DA bioavailability by targeting the DAT. Therefore, GLP-1R signaling might exert effects on DAT to account for its regulation of cocaine-induced behaviors. We show that the GLP-1R is highly expressed within the LS. GLP-1, in LS slices, significantly enhances DAT surface expression and DAT function. Exenatide (Ex-4), a long-lasting synthetic analog of GLP-1 abolished cocaine-induced elevation of DA. Interestingly, acute administration of Ex-4 reduces septal expression of the retrograde messenger 2-arachidonylglycerol (2-AG), as well as a product of its presynaptic degradation, arachidonic acid (AA). Notably, AA reduces septal DAT function pointing to AA as a novel regulator of central DA homeostasis. We further show that AA oxidation product γ-ketoaldehyde (γ-KA) forms adducts with the DAT and reduces DAT plasma membrane expression and function. These results support a mechanism in which postsynaptic septal GLP-1R activation regulates 2-AG levels to alter presynaptic DA homeostasis and cocaine actions through AA.

Subcutaneous administration of liraglutide ameliorates learning and memory impairment by modulating tau hyperphosphorylation via the glycogen synthase kinase-3β pathway in an amyloid β protein induced alzheimer disease mouse model

Type 2 diabetes mellitus is a risk factor for Alzheimer's disease (AD). The glucagon-like peptide-1 analog liraglutide, a novel long-lasting incretin hormone, has been used to treat type 2 diabetes mellitus. In addition, liraglutide has been shown to be neurotrophic and neuroprotective. Here, we investigated the effects of liraglutide on amyloid β protein (Aβ)-induced AD in mice and explored its mechanism of action. The results showed that subcutaneous administration of liraglutide (25nmol/day), once daily for 8 weeks, prevented memory impairments in the Y Maze and Morris Water Maze following Aβ1-42 intracerebroventricular injection, and alleviated the ultra-structural changes of pyramidal neurons and chemical synapses in the hippocampal CA1 region. Furthermore, liraglutide reduced Aβ1-42-induced tau phosphorylation via the protein kinase B and glycogen synthase kinase-3β pathways. Thus liraglutide may alleviate cognitive impairment in AD by at least decreasing the phosphorylation of tau.

Novel GLP-1 (Glucagon-Like Peptide-1) Analogues and Insulin in the Treatment for Alzheimer's Disease and Other Neurodegenerative Diseases

The link between diabetes mellitus and Alzheimer's disease (AD) has been known for the last few decades. Since insulin and insulin receptors are known to be present in the brain, the downstream signalling as well as the effect of hyperinsulinemia have been extensively studied in both AD and Parkinson's disease. Glucagon-like peptide-1 (GLP-1) is a hormone belonging to the incretin family, and its receptors (GLP-1Rs) can be found in pancreatic cells and in vascular endothelium. Interestingly, GLP-1Rs are found in the neuronal cell body and dendrites in the central nervous system (CNS), in particular in the hypothalamus, hippocampus, cerebral cortex and olfactory bulb. Several studies have shown the importance of both insulin and GLP-1 signalling on cognitive function, and many preclinical studies have been performed to evaluate the potential protective role of GLP-1 on the brain. Here we review the underlying mechanism of insulin and GLP-1 signalling in the CNS, as well as the preclinical data for the use of GLP-1 analogues such as liraglutide, exenatide and lixisenatide in neurodegenerative diseases.

Amyloid β: one of three danger-associated molecules that are secondary inducers of the proinflammatory cytokines that mediate Alzheimer's disease

This review concerns how the primary inflammation preceding the generation of certain key damage-associated molecular patterns (DAMPs) arises in Alzheimer's disease (AD). In doing so, it places soluble amyloid β (Aβ), a protein hitherto considered as a primary initiator of AD, in a novel perspective. We note here that increased soluble Aβ is one of the proinflammatory cytokine-induced DAMPs recognized by at least one of the toll-like receptors on and in various cell types. Moreover, Aβ is best regarded as belonging to a class of DAMPs, as do the S100 proteins and HMBG1, that further exacerbate production of these same proinflammatory cytokines, which are already enhanced, and induces them further. Moreover, variation in levels of other DAMPs of this same class in AD may explain why normal elderly patients can exhibit high Aβ plaque levels, and why removing Aβ or its plaque does not retard disease progression. It may also explain why mouse transgenic models, having been designed to generate high Aβ, can be treated successfully by this approach.

Aberrant insulin signaling in Alzheimer's disease: current knowledge

Alzheimer's disease (AD) is the most common form of dementia affecting elderly people. AD is a multifaceted pathology characterized by accumulation of extracellular neuritic plaques, intracellular neurofibrillary tangles (NFTs) and neuronal loss mainly in the cortex and hippocampus. AD etiology appears to be linked to a multitude of mechanisms that have not been yet completely elucidated. For long time, it was considered that insulin signaling has only peripheral actions but now it is widely accepted that insulin has neuromodulatory actions in the brain. Insulin signaling is involved in numerous brain functions including cognition and memory that are impaired in AD. Recent studies suggest that AD may be linked to brain insulin resistance and patients with diabetes have an increased risk of developing AD compared to healthy individuals. Indeed insulin resistance, increased inflammation and impaired metabolism are key pathological features of both AD and diabetes. However, the precise mechanisms involved in the development of AD in patients with diabetes are not yet fully understood. In this review we will discuss the role played by aberrant brain insulin signaling in AD. In detail, we will focus on the role of insulin signaling in the deposition of neuritic plaques and intracellular NFTs. Considering that insulin mitigates beta-amyloid deposition and phosphorylation of tau, pharmacological strategies restoring brain insulin signaling, such as intranasal delivery of insulin, could have significant therapeutic potential in AD treatment.

Role of anti-diabetic drugs as therapeutic agents in Alzheimer's disease

Recent data have suggested a strong possible link between Type 2 Diabetes Mellitus and Alzheimer's disease (AD), although exact mechanisms linking the two are still a matter of research and debate. Interestingly, both are diseases with high incidence and prevalence in later years of life. The link appears so strong that some scientists use Alzheimer's and Type 3 Diabetes interchangeably. In depth study of recent data suggests that the anti diabetic drugs not only have possible role in treatment of Alzheimer's but may also arrest the declining cognitive functions associated with it. The present review gives an insight into the possible links, existing therapeutics and clinical trials of anti diabetic drugs in patients suffering from AD primarily or as co-morbidity. It may be concluded that the possible beneficial effects and usefulness of the current anti diabetic drugs in AD cannot be neglected and further research is required to achieve positive results. Currently, several drug trials are in progress to give conclusive evidence based data.

Neuronal stress signaling and eIF2α phosphorylation as molecular links between Alzheimer's disease and diabetes

Mounting evidence from clinical, epidemiological, neuropathology and preclinical studies indicates that mechanisms similar to those leading to peripheral metabolic deregulation in metabolic disorders, such as diabetes and obesity, take place in the brains of Alzheimer's disease (AD) patients. These include pro-inflammatory mechanisms, brain metabolic stress and neuronal insulin resistance. From a molecular and cellular perspective, recent progress has been made in unveiling novel pathways that act in an orchestrated way to cause neuronal damage and cognitive decline in AD. These pathways converge to the activation of neuronal stress-related protein kinases and excessive phosphorylation of eukaryotic translation initiation factor 2α (eIF2α-P), which plays a key role in control of protein translation, culminating in synapse dysfunction and memory loss. eIF2α-P signaling thus links multiple neuronal stress pathways to impaired neuronal function and neurodegeneration. Here, we present a critical analysis of recently discovered molecular mechanisms underlying impaired brain insulin signaling and metabolic stress, with emphasis on the role of stress kinase/eIF2α-P signaling as a hub that promotes brain and behavioral impairments in AD. Because very similar mechanisms appear to operate in peripheral metabolic deregulation in T2D and in brain defects in AD, we discuss the concept that targeting defective brain insulin signaling and neuronal stress mechanisms with anti-diabetes agents may be an attractive approach to fight memory decline in AD. We conclude by raising core questions that remain to be addressed toward the development of much needed therapeutic approaches for AD.

Sitagliptin, a dipeptidyl peptidase-4 inhibitor, improves recognition memory, oxidative stress and hippocampal neurogenesis and upregulates key genes involved in cognitive decline

AIM

To examine whether prolonged dipeptidyl peptidase-4 (DPP-4) inhibition can reverse learning and memory impairment in high-fat-fed mice.

METHODS

High-fat-fed mice received oral sitagliptin (50 mg/kg body weight) once daily or saline vehicle over 21 days. An additional group of mice on standard chow received saline vehicle. Energy intake, body weight, glucose and insulin concentrations were measured at regular intervals. Glucose tolerance, insulin sensitivity, novel object recognition, DPP-4 activity, hormone analysis, hippocampal gene expression and histology were performed.

RESULTS

Sitagliptin decreased circulating DPP-4 activity and improved glucose tolerance, glucose-stimulated insulin secretion and insulin sensitivity, and reduced plasma triglycerides and cholesterol levels. DPP-4 inhibition improved recognition memory (1.2-fold increase) without affecting hypermoteric activity or anxiety levels. Improvement in memory and learning was linked to reduced immunostaining for 8-oxoguanine and increased doublecortin staining in the hippocampus, which were indicative of reduced brain oxidative stress and increased hippocampal neurogenesis, respectively. These effects were associated with significant upregulation of hippocampal gene expression of glucagon-like peptide-1 (GLP-1) receptor, glucose-dependent insulinotropic polypeptide receptor, synaptophysin, sirtuin 1, glycogen synthase kinase 3β, superdioxide mutase 2, nuclear factor (erythroid-derived 2)-like 2 and vascular endothelial growth factor. Total plasma and brain GLP-1 concentrations were significantly increased after sitagliptin therapy, whereas DPP-4 activity in brain tissue was not altered.

CONCLUSION

These studies show that sitagliptin can reverse memory impairment in high-fat-fed mice and is also associated with improved insulin sensitivity, enhanced hippocampal neurogenesis and reduced oxidative stress. DPP-4 inhibitors may therefore exhibit dual benefits by improving metabolic control and reducing the decline in cognitive function.

Extra-pancreatic effects of incretin-based therapies

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion and inhibits glucagon secretion in the pancreatic islets of Langerhans under hyperglycaemia. In type 2 diabetes (T2DM), GLP-1 improves glycaemic control without a hypoglycaemia risk. GLP-1 receptors have also been found in extra-pancreatic tissues, e.g., the cardiovascular system, the gastrointestinal system, and the central nervous system. Since cardiovascular comorbidities and degenerative neurological changes are associated with T2DM, the interest in the extrapancreatic effects of GLP-1 has increased. GLP-1-based therapies with either GLP-1 receptor agonists (GLP-1 RA) or DPP-4 inhibitors (that delay the degradation of endogenous GLP-1) have become widely used therapeutic options in T2DM. In clinical studies, GLP-1 RA have demonstrated a significant lowering of blood pressure that is independent of body weight changes. Preclinical data and small short-term studies with GLP-1 and GLP-1 RA have shown cardioprotective effects in ischaemia models. GLP-1 as well as a treatment with GLP-1 RA also induces a stable body weight loss by affecting GLP-1 signaling in the hypothalamus and by slowing gastric emptying. Regarding neuroprotective actions in degenerative neurological disease models for Parkinson's- or Alzheimer's disease or neurovascular complications like stroke, animal studies have shown positive results. In this article, a summary of the extrapancreatic effects of GLP-1 and GLP-1-based therapies is presented.

The extra-pancreatic effects of GLP-1 receptor agonists: a focus on the cardiovascular, gastrointestinal and central nervous systems

The glucagon-like peptide-1 receptor agonists (GLP-1RAs) exenatide, liraglutide and lixisenatide have been shown to improve glycaemic control and beta-cell function with a low risk of hypoglycaemia in people with type 2 diabetes. GLP-1 receptors are also expressed in extra-pancreatic tissues and trial data suggest that GLP-1RAs also have effects beyond their glycaemic actions. Preclinical studies using native GLP-1 or GLP-1RAs provide substantial evidence for cardioprotective effects, while clinical trial data have shown beneficial actions on hypertension and dyslipidaemia in people with type 2 diabetes. Significant weight loss has been reported with GLP-1RAs in both people with type 2 diabetes and obese people without diabetes. GLP-1RAs also slow down gastric emptying, but preclinical data suggest that the main mechanism behind GLP-1RA-induced weight loss is more likely to involve their effects on appetite signalling in the brain. GLP-1RAs have also been shown to exert a neuroprotective role in rodent models of stroke, Alzheimer's disease and Parkinson's disease. These extra-pancreatic effects of GLP-1RAs could provide multi-factorial benefits to people with type 2 diabetes. Potential adverse effects of GLP-1RA treatment are usually manageable but may include gastrointestinal effects, increased heart rate and renal injury. While extensive further research is still required, early data suggest that GLP-1RAs may also have the potential to favourably impact cardiovascular disease, obesity or neurological disorders in people without diabetes in the future.

Central effects of GLP-1: new opportunities for treatments of neurodegenerative diseases

The incretin hormone glucagon-like peptide 1 (GLP-1) has many effects in the body. It is best known for the 'incretin effect', facilitating insulin release from the pancreas under hyperglycaemic conditions. Building on this, GLP-1 mimetics have been developed as a treatment for type 2 diabetes. In the course of monitoring of patients, it has become apparent that GLP-1 mimetics have a range of other physiological effects in the body. In preclinical trials, a substantial body of evidence has been built that these mimetics have neuroprotective and anti-inflammatory effects. GLP-1 also has very similar growth-factor-like properties to insulin, which is presumably the underlying basis of the neuroprotective effects. In preclinical studies of Alzheimer's disease (AD), Parkinson's disease (PD), stroke and other neurodegenerative disorders, it has been shown that most GLP-1 mimetics cross the blood-brain barrier and show impressive neuroprotective effects in numerous studies. In animal models of AD, GLP-1 mimetics such as exendin-4, liraglutide and lixisenatide have shown protective effects in the CNS by reducing β-amyloid plaques, preventing loss of synapses and memory impairments, and reducing oxidative stress and the chronic inflammatory response in the brain. In animal models of PD, exendin-4 showed protection of dopaminergic neurons in the substantia nigra and prevention of dopamine loss in the basal ganglia while preserving motor control. These encouraging findings have spawned several clinical trials, some of which have shown encouraging initial results. Therefore, GLP-1 mimetics show great promise as a novel treatment for neurodegenerative conditions.

Intranasal insulin as a treatment for Alzheimer's disease: a review of basic research and clinical evidence

Research in animals and humans has associated Alzheimer's disease (AD) with decreased cerebrospinal fluid levels of insulin in combination with decreased insulin sensitivity (insulin resistance) in the brain. This phenomenon is accompanied by attenuated receptor expression of insulin and insulin-like growth factor, enhanced serine phosphorylation of insulin receptor substrate-1, and impaired transport of insulin across the blood-brain barrier. Moreover, clinical trials have demonstrated that intranasal insulin improves both memory performance and metabolic integrity of the brain in patients suffering from AD or its prodrome, mild cognitive impairment. These results, in conjunction with the finding that insulin mitigates hippocampal synapse vulnerability to beta amyloid, a peptide thought to be causative in the development of AD, provide a strong rationale for hypothesizing that pharmacological strategies bolstering brain insulin signaling, such as intranasal administration of insulin, could have significant potential in the treatment and prevention of AD. With this view in mind, the review at hand will present molecular mechanisms potentially underlying the memory-enhancing and neuroprotective effects of intranasal insulin. Then, we will discuss the results of intranasal insulin studies that have demonstrated that enhancing brain insulin signaling improves memory and learning processes in both cognitively healthy and impaired humans. Finally, we will provide an overview of neuroimaging studies indicating that disturbances in insulin metabolism--such as insulin resistance in obesity, type 2 diabetes and AD--and altered brain responses to insulin are linked to decreased cerebral volume and especially to hippocampal atrophy.

Incretin mimetics as pharmacologic tools to elucidate and as a new drug strategy to treat traumatic brain injury

Traumatic brain injury (TBI), either as an isolated injury or in conjunction with other injuries, is an increasingly common event. An estimated 1.7 million injuries occur within the USA each year and 10 million people are affected annually worldwide. Indeed, nearly one third (30.5%) of all injury-related deaths in the USA are associated with TBI, which will soon outpace many common diseases as the major cause of death and disability. Associated with a high morbidity and mortality and no specific therapeutic treatment, TBI has become a pressing public health and medical problem. The highest incidence of TBI occurs in young adults (15-24 years age) and in the elderly (≥75 years of age). Older individuals are particularly vulnerable to these types of injury, often associated with falls, and have shown increased mortality and worse functional outcome after lower initial injury severity. In addition, a new and growing form of TBI, blast injury, associated with the detonation of improvised explosive devices in the war theaters of Iraq and Afghanistan, are inflicting a wave of unique casualties of immediate impact to both military personnel and civilians, for which long-term consequences remain unknown and may potentially be catastrophic. The neuropathology underpinning head injury is becoming increasingly better understood. Depending on severity, TBI induces immediate neuropathologic effects that, for the mildest form, may be transient; however, with increasing severity, these injuries cause cumulative neural damage and degeneration. Even with mild TBI, which represents the majority of cases, a broad spectrum of neurologic deficits, including cognitive impairments, can manifest that may significantly influence quality of life. Further, TBI can act as a conduit to longer term neurodegenerative disorders. Prior studies of glucagon-like peptide-1 (GLP-1) and long-acting GLP-1 receptor agonists have demonstrated neurotrophic/neuroprotective activities across a broad spectrum of cellular and animal models of chronic neurodegenerative (Alzheimer's and Parkinson's diseases) and acute cerebrovascular (stroke) disorders. In view of the mechanisms underpinning these disorders as well as TBI, we review the literature and recent studies assessing GLP-1 receptor agonists as a potential treatment strategy for mild to moderate TBI.

Decreased neuronal bursting and phase synchrony in the hippocampus of streptozotocin diabetic rats

Diabetic encephalopathy is one of the complications of diabetes. Cognitive dysfunction is the main consequence. Previous findings from neuroanatomical and in vitro electrophysiological studies showed that the structure and function of the hippocampus is impaired in diabetes, which may underlie the cognitive dysfunction induced by diabetes. However the study of electrophysiological abnormality of hippocampal neurons in intact networks is sparse. In the current study, we recorded the spontaneous firing of neurons in hippocampal CA1 area in anesthetized streptozotozin (STZ)-diabetic and age-matched control rats. Profound reduction in burst activity was found in diabetic rats. Compared to control rats, the intra-burst inter-spike intervals were prolonged significantly in diabetic rats, while the burst ratio and the mean number of spikes within a burst decreased significantly. Treatment with APP 17-mer peptide retarded the effects of diabetes on these parameters. In addition, the average PLV of diabetic rats was lower than that of control rats. These findings provide in vivo electrophysiological evidence for the impairment of hippocampal function in STZ-diabetic rats, and may have some implications in the mechanisms associated with cognitive deficits in diabetes.

β-endorphin degradation and the individual reactivity to traumatic stress

Reactivity to traumatic stress varies between individuals and only a minority of those exposed to trauma develops stress-induced psychopathologies. Currently extensive effort is made to unravel the specific mechanisms predisposing to vulnerability vs. resilience to stress. We investigated in rats the role of β-endorphin metabolism in vulnerability to acute traumatic stress. Responders (showing extreme anxiety; n=7) and resilient non-responders (not differing from the non-stressed individuals; n=8) to traumatic foot-shock stress were compared for their blood levels of stress hormones as well as brain levels and activity of two opioid-degrading enzymes. β-endorphin is a substrate to insulin degrading enzyme, which also degrades insulin. Therefore, the effects of insulin application on behavioral and hormonal responses and on β-endorphin degradation were tested. Pre- and post-stress levels of serum corticosterone, and post-stress plasma β-endorphin concentration differentiated between the responders and the non-responders. In brain, responders showed enhanced degradation rates of β-endorphin, assessed by Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS), in hippocampal and amygdalar slices as compared to non-responders. Application of insulin to the amygdala, prior to exposure to traumatic stress, reduced post-stress anxiety and serum corticosterone levels only in the responders. In parallel, amygdalar β-endorphin degradation rate was also reduced by insulin. These results suggest that slowing down β-endorphin degradation rate may constitute an integral part of the normal stress-response, upon a failure of which an extreme anxiety develops. Modulation of opioid degradation may thus present a potential novel target for interference with extreme anxiety.

Intranasal insulin as a treatment for Alzheimer's disease: a review of basic research and clinical evidence

Research in animals and humans has associated Alzheimer's disease (AD) with decreased cerebrospinal fluid levels of insulin in combination with decreased insulin sensitivity (insulin resistance) in the brain. This phenomenon is accompanied by attenuated receptor expression of insulin and insulin-like growth factor, enhanced serine phosphorylation of insulin receptor substrate-1, and impaired transport of insulin across the blood-brain barrier. Moreover, clinical trials have demonstrated that intranasal insulin improves both memory performance and metabolic integrity of the brain in patients suffering from AD or its prodrome, mild cognitive impairment. These results, in conjunction with the finding that insulin mitigates hippocampal synapse vulnerability to beta amyloid, a peptide thought to be causative in the development of AD, provide a strong rationale for hypothesizing that pharmacological strategies bolstering brain insulin signaling, such as intranasal administration of insulin, could have significant potential in the treatment and prevention of AD. With this view in mind, the review at hand will present molecular mechanisms potentially underlying the memory-enhancing and neuroprotective effects of intranasal insulin. Then, we will discuss the results of intranasal insulin studies that have demonstrated that enhancing brain insulin signaling improves memory and learning processes in both cognitively healthy and impaired humans. Finally, we will provide an overview of neuroimaging studies indicating that disturbances in insulin metabolism--such as insulin resistance in obesity, type 2 diabetes and AD--and altered brain responses to insulin are linked to decreased cerebral volume and especially to hippocampal atrophy.