Beneficial effects of exendin-4 on experimental polyneuropathy in diabetic mice

Pathological evaluation of the pathogenesis of diabetes mellitus and diabetic peripheral neuropathy

Currently, there are more than 10 million patients with diabetes mellitus in Japan. Therefore, the need to explore the pathogenesis of diabetes and the complications leading to its cure is becoming increasingly urgent. Pathological examination of pancreatic tissues from patients with type 2 diabetes reveals a decrease in the volume of beta cells because of a combination of various stresses. In human type 2 diabetes, islet amyloid deposition is a unique pathological change characterized by proinflammatory macrophage (M1) infiltration into the islets. The pathological changes in the pancreas with islet amyloid were different according to clinical factors, which suggests that type 2 diabetes can be further subclassified based on islet pathology. On the other hand, diabetic peripheral neuropathy is the most frequent diabetic complication. In early diabetic peripheral neuropathy, M1 infiltration in the sciatic nerve evokes oxidative stress or attenuates retrograde axonal transport, as clearly demonstrated by in vitro live imaging. Furthermore, islet parasympathetic nerve density and beta cell volume were inversely correlated in type 2 diabetic Goto-Kakizaki rats, suggesting that diabetic peripheral neuropathy itself may contribute to the decrease in beta cell volume. These findings suggest that the pathogenesis of diabetes mellitus and diabetic peripheral neuropathy may be interrelated.

GLP-1 and Its Derived Peptides Mediate Pain Relief Through Direct TRPV1 Inhibition Without Affecting Thermoregulation

Hormonal regulation during food ingestion and its association with pain prompted the investigation of the impact of glucagon-like peptide-1 (GLP-1) on the transient receptor potential vanilloid 1 (TRPV1). Both endogenous and synthetic GLP-1 and an antagonist of GLP-1, exendin 9-39, reduced heat sensitivity in naïve mice. GLP-1-derived peptides (liraglutide, exendin-4, and exendin 9-39) effectively inhibited capsaicin (CAP)-induced currents and calcium responses in cultured sensory neurons and TRPV1-expressing cell lines. Notably, the exendin 9-39 alleviated CAP-induced acute pain, as well as chronic pain induced by complete Freund's adjuvant (CFA) and spared nerve injury (SNI) in mice, without causing hyperthermia associated with other TRPV1 inhibitors. Electrophysiological analyses revealed that exendin 9-39 binds to the extracellular side of TRPV1, functioning as a noncompetitive inhibitor of CAP. Exendin 9-39 did not affect proton-induced TRPV1 activation, suggesting its selective antagonism. Among exendin 9-39 fragments, exendin 20-29 specifically binds to TRPV1, alleviating pain in both acute and chronic pain models without interfering with GLP-1R function. Our study revealed a novel role for GLP-1 and its derivatives in pain relief, proposing exendin 20-29 as a promising therapeutic candidate.

GLP-1 derivatives with functional sequences transit and migrate through trigeminal neurons

Patients with dementia are increasing with the aging of the population, and dementia has become a disease with high unmet medical needs. Glucagon-like peptide-1 (GLP-1), a neuropeptide, has been reported to improve learning and memory following intracerebroventricular administration. We focused on intranasal administration, which can deliver drugs noninvasively and efficiently to the brain. Although much of the human nasal mucosa is occupied by respiratory epithelium, many capillaries are present in the paracellular route of respiratory epithelium. Therefore, to incorporate GLP-1 into cells, we created a GLP-1 derivative by adding cell-penetrating peptides (CPP) and penetration accelerating sequences (PAS) to GLP-1. We investigated in vitro and in vivo function of PAS-CPP-GLP-1 to enable the translocation of GLP-1 directly from nose to brain. PAS-CPP-GLP-1 enhanced cellular uptake by macropinocytosis with CPP, efficiently escaped from the endosomes due to PAS, and exited the cells. PAS-CPP-GLP-1 also transited trigeminal nerve cells through axon transport and migrated to the adjacent trigeminal nerve cell. Moreover, PAS-CPP-GLP-1 showed significant improvement in learning memory in mice within 20 min of intranasal administration. These results suggested CPP and PAS may be important for the efficient transfer of GLP-1 to the site of action in the brain following intranasal administration.

Glucagon-like peptide-1 receptor agonists for the management of diabetic peripheral neuropathy

Diabetes mellitus is a prevalent chronic disease characterized by hyperglycemia. Diabetic peripheral neuropathy (DPN) is one of the complications of diabetes mellitus and is caused by neuron injury induced by hyperglycemic circumstances. The incidence of DPN varies among different countries and regions, ranging from nearly 20% to over 70%. Patients with DPN may encounter symmetric pain or discomfort of the extremes, leading to reduced quality of life and even amputation. The pharmacological management for painful DPN mainly includes antidepressants due to their analgesic effects. Nevertheless, effective therapies to impact the pathogenesis and progression of DPN are lacking. Glucagon-like peptide-1 receptor (GLP-1R) agonists show efficacy in controlling blood glucose and serve as a treatment modality for diabetes mellitus. In recent years, evidence has been proposed that GLP-1R agonists exert neuroprotective effects through modulating inflammation, oxidative stress, and mitochondrial dysfunction. On the other hand, clinical evidence on the potential of GLP-1R agonists for treating DPN is still controversial and limited. This narrative review summarizes the preclinical and clinical studies investigating the capacity of GLP-1R agonists as therapeutic agents for DPN.

Functional engineering of human iPSC-derived parasympathetic neurons enhances responsiveness to gastrointestinal hormones

Food-derived biological signals are transmitted to the brain via peripheral nerves through the paracrine activity of gastrointestinal (GI) hormones. The signal transduction circuit of the brain-gut axis has been analyzed in animals; however, species-related differences and animal welfare concerns necessitate investigation using in vitro human experimental models. Here, we focused on the receptors of five GI hormones (CCK, GLP1, GLP2, PYY, and serotonin (5-HT)), and established human induced pluripotent stem cell (iPSC) lines that functionally expressed each receptor. Compared to the original iPSCs, iPSCs expressing one of the receptors did not show any differences in global mRNA expression, genomic stability, or differentiation capacities of the three germ layers. We induced parasympathetic neurons from these established iPSC lines to assess vagus nerve activity. We generated GI hormone receptor-expressing neurons (CCKAR, GLP1R, and NPY2R-neuron) and tested their responsiveness to each ligand using Ca2+ imaging and microelectrode array recording. GI hormone receptor-expressing neurons (GLP2R and HTR3A) were generated directly by gene induction into iPSC-derived peripheral nerve progenitors. These receptor-expressing neurons promise to contribute to a better understanding of how the body responds to GI hormones via the brain-gut axis, aid in drug development, and offer an alternative to animal studies.

Diabetic Polyneuropathy: New Strategies to Target Sensory Neurons in Dorsal Root Ganglia

Diabetic polyneuropathy (DPN) is the most common type of diabetic neuropathy, rendering a slowly progressive, symmetrical, and length-dependent dying-back axonopathy with preferential sensory involvement. Although the pathogenesis of DPN is complex, this review emphasizes the concept that hyperglycemia and metabolic stressors directly target sensory neurons in the dorsal root ganglia (DRG), leading to distal axonal degeneration. In this context, we discuss the role for DRG-targeting gene delivery, specifically oligonucleotide therapeutics for DPN. Molecules including insulin, GLP-1, PTEN, HSP27, RAGE, CWC22, and DUSP1 that impact neurotrophic signal transduction (for example, phosphatidylinositol-3 kinase/phosphorylated protein kinase B [PI3/pAkt] signaling) and other cellular networks may promote regeneration. Regenerative strategies may be essential in maintaining axon integrity during ongoing degeneration in diabetes mellitus (DM). We discuss specific new findings that relate to sensory neuron function in DM associated with abnormal dynamics of nuclear bodies such as Cajal bodies and nuclear speckles in which mRNA transcription and post-transcriptional processing occur. Manipulating noncoding RNAs such as microRNA and long-noncoding RNA (specifically MALAT1) that regulate gene expression through post-transcriptional modification are interesting avenues to consider in supporting neurons during DM. Finally, we present therapeutic possibilities around the use of a novel DNA/RNA heteroduplex oligonucleotide that provides more efficient gene knockdown in DRG than the single-stranded antisense oligonucleotide.

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Glucagon-Like Peptide-1 Receptor Agonists as Potential Myelination-Inducible and Anti-Demyelinating Remedies

Glucagon-like peptide-1 receptor agonists (GLP-1RAs) were developed as insulinotropic and anti-hyperglycemic agents for the treatment of type 2 diabetes, but their neurotrophic and neuroprotective activities have been receiving increasing attention. Myelin plays a key role in the functional maintenance of the central and peripheral nervous systems, and recent in vivo and in vitro studies have shed light on the beneficial effects of GLP-1RAs on the formation and protection of myelin. In this article, we describe the potential efficacy of GLP-1RAs for the induction of axonal regeneration and remyelination following nerve lesions and the prevention and alleviation of demyelinating disorders, particularly multiple sclerosis.

Current and Emerging Pharmacotherapeutic Interventions for the Treatment of Peripheral Nerve Disorders

Peripheral nerve disorders are caused by a range of different aetiologies. The range of causes include metabolic conditions such as diabetes, obesity and chronic kidney disease. Diabetic neuropathy may be associated with severe weakness and the loss of sensation, leading to gangrene and amputation in advanced cases. Recent studies have indicated a high prevalence of neuropathy in patients with chronic kidney disease, also known as uraemic neuropathy. Immune-mediated neuropathies including Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy may cause significant physical disability. As survival rates continue to improve in cancer, the prevalence of treatment complications, such as chemotherapy-induced peripheral neuropathy, has also increased in treated patients and survivors. Notably, peripheral neuropathy associated with these conditions may be chronic and long-lasting, drastically affecting the quality of life of affected individuals, and leading to a large socioeconomic burden. This review article explores some of the major emerging clinical and experimental therapeutic agents that have been investigated for the treatment of peripheral neuropathy due to metabolic, toxic and immune aetiologies.

Using Corneal Confocal Microscopy to Identify Therapeutic Agents for Diabetic Neuropathy

Corneal confocal microscopy (CCM) is emerging as a tool for identifying small fiber neuropathy in both peripheral neuropathies and neurodegenerative disease of the central nervous system (CNS). The value of corneal nerves as biomarkers for efficacy of clinical interventions against small fiber neuropathy and neurodegenerative disease is less clear but may be supported by preclinical studies of investigational agents. We, therefore, used diverse investigational agents to assess concordance of efficacy against corneal nerve loss and peripheral neuropathy in a mouse model of diabetes. Ocular delivery of the peptides ciliary neurotrophic factor (CNTF) or the glucagon-like peptide (GLP) analog exendin-4, both of which prevent diabetic neuropathy when given systemically, restored corneal nerve density within 2 weeks. Similarly, ocular delivery of the muscarinic receptor antagonist cyclopentolate protected corneal nerve density while concurrently reversing indices of systemic peripheral neuropathy. Conversely, systemic delivery of the muscarinic antagonist glycopyrrolate, but not gallamine, prevented multiple indices of systemic peripheral neuropathy and concurrently protected against corneal nerve loss. These data highlight the potential for use of corneal nerve quantification by confocal microscopy as a bridging assay between in vitro and whole animal assays in drug development programs for neuroprotectants and support its use as a biomarker of efficacy against peripheral neuropathy.

Exenatide exerts a neuroprotective effect against diabetic cognitive impairment in rats by inhibiting apoptosis: Role of the JNK/c‑JUN signaling pathway

Exenatide could reduce blood glucose and alleviate cognitive dysfunction induced by diabetes mellitus (DM). In the present study, a diabetic model was established in Sprague‑Dawley rats to further explore the mechanism of exenatide on diabetes‑induced cognitive impairment. Notably, the model rats performed poorly in the Morris water maze test and had more apoptotic neurons compared with the control rats. By contrast, exenatide attenuated cognitive impairment and inhibited neuronal apoptosis in the DM rat model. To explore the neuroprotective mechanisms of exenatide, western blotting was performed to detect the expression levels of markers of endoplasmic reticulum stress, including cytochrome c (Cyt‑c), Caspase‑3, JNK and c‑JUN, in hippocampal tissue. Reverse transcription‑quantitative PCR was also performed to measure the mRNA expression levels of Cyt‑c and Caspase‑3. After 16 weeks of treatment, exenatide treatment downregulated Cyt‑c, Caspase‑3, phosphorylated (p)‑JNK and p‑c‑JUN expression in the hippocampal tissue of diabetic rats. Moreover, Cyt‑c, Caspase‑3, JNK and JUN expression levels were detected following treatment with a specific inhibitor of JNK (SP600125). The results revealed that SP600125 had similar inhibitory effects on the JNK pathway and ERS‑related protein expression (Cyt‑t, Caspase‑3, p‑JNK and p‑c‑JUN). These results suggested that exenatide improved cognitive dysfunction in DM rats and that the underlying mechanism may be associated with inhibiting apoptosis by suppressing the activation of JNK/c‑JUN.

Diabetic Corneal Neuropathy: Pathogenic Mechanisms and Therapeutic Strategies

Diabetes mellitus (DM) is a major global public health problem that can cause complications such as diabetic retinopathy, diabetic neuropathy, and diabetic nephropathy. Besides the reporting of reduction in corneal nerve density and decrease in corneal sensitivity in diabetic patients, there may be a subsequent result in delayed corneal wound healing and increased corneal infections. Despite being a potential cause of blindness, these corneal nerve changes have not gained enough attention. It has been proposed that corneal nerve changes may be an indicator for diabetic neuropathy, which can provide a window for early diagnosis and treatment. In this review, the authors aimed to give an overview of the relationship between corneal nerves and diabetic neuropathy as well as the underlying pathophysiological mechanisms of corneal nerve fiber changes caused by DM for improved prediction and prevention of diabetic neuropathy. In addition, the authors summarized current and novel therapeutic methods for delayed corneal wound healing, nerve protection and regeneration in the diabetic cornea.

Glucagon-Like Peptide 1 Receptor Agonists – Potential Game Changers in the Treatment of Glaucoma?

Glaucoma is a common ocular neurodegenerative disease characterized by the progressive loss of retinal ganglion cells and their axons. It is the most common cause of irreversible blindness. With an increasing number of glaucoma patients and disease progression despite treatment, it is paramount to develop new and effective therapeutics. Emerging new candidates are the receptor agonists of the incretin hormone glucagon-like-peptide-1 (GLP-1), originally used for the treatment of diabetes. GLP-1 receptor (GLP-1R) agonists have shown neuroprotective effects in preclinical and clinical studies on neurodegenerative diseases in both the brain (e.g., Alzheimer’s disease, Parkinson’s disease, stroke and diabetic neuropathy) and the eye (e.g., diabetic retinopathy and AMD). However, there are currently very few studies investigating the protective effects of GLP-1R agonists in the treatment of specifically glaucoma. Based on a literature search on PubMed, the Cochrane Library, and ClinicalTrials.gov, this review aims to summarize current clinical literature on GLP-1 receptor agonists in the treatment of neurodegenerative diseases to elucidate their potential in future anti-glaucomatous treatment strategies.

SARM1 Suppresses Axon Branching Through Attenuation of Axonal Cytoskeletal Dynamics

Axon branching is a fundamental aspect of neuronal morphogenesis, neuronal circuit formation, and response of the nervous system to injury. Sterile alpha and TIR motif containing 1 (SARM1) was initially identified as promoting Wallerian degeneration of axons. We now report a novel function of SARM1 in postnatal sensory neurons; the suppression of axon branching. Axon collateral branches develop from axonal filopodia precursors through the coordination of the actin and microtubule cytoskeleton. In vitro analysis revealed that cultured P0-2 dorsal root ganglion sensory neurons from a SARM1 knockout (KO) mouse exhibit increased numbers of collateral branches and axonal filopodia relative to wild-type neurons. In SARM1 KO mice, cutaneous sensory endings exhibit increased branching in the skin in vivo with normal density of innervation. Transient axonal actin patches serve as cytoskeletal platforms from which axonal filopodia emerge. Live imaging analysis of axonal actin dynamics showed that SARM1 KO neurons exhibit increased rates of axonal actin patch formation and increased probability that individual patches will give rise to a filopodium before dissipating. SARM1 KO axons contain elevated levels of drebrin and cortactin, two actin regulatory proteins that are positive regulators of actin patches, filopodia formation, and branching. Live imaging of microtubule plus tip dynamics revealed an increase in the rate of formation and velocity of polymerizing tips along the axons of SARM1 KO neurons. Stationary mitochondria define sites along the axon where branches may arise, and the axons of SARM1 KO sensory neurons exhibit an increase in stationary mitochondria. These data reveal SARM1 to be a negative regulator of axonal cytoskeletal dynamics and collateral branching.

Neuroprotective efficacy of 4-Hydroxyisoleucine in experimentally induced intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a severe form of brain injury, which is a major cause of mortality in humans. Hydrocephalus and cerebral hematoma lead to severe neurological deficits. A single autologous blood (ALB) injection in rats' brains induces hemorrhage and other conditions that regularly interfere with the standard treatment of several cellular and molecular pathways. Several studies have found that IGF-1/GLP-1 decreases the production of inflammatory markers in peripheral tissues, while some have found that they also have pro-inflammatory functions. Since these receptors are down-regulated in hemorrhagic situations, we looked into the potential neuroprotective effect of 4-hydroxyisoleucine (4-HI); 50 mg/kg and 100 mg/kg, an active compound Trigonellafoenum-graecum, on post-hemorrhagic deficits in rats. Long-term oral administration of 4-HI for 35 days has improved behavioral and neurochemical deficits and severe pathological changes and improved cellular and molecular markers, apoptotic markers in the ALB-induced ICH experimental model.

Furthermore, the findings revealed that 4-HI also improved the levels of other neurotransmitters (Ach, DOPA, GABA, glutamate); inflammatory cytokines (TNF-alpha, IL-1β, IL-17), and oxidative stress markers (MDA, nitrite, LDH, AchE, SOD, CAT, GPx, GSH) in the brain when evaluated after Day 35. There is no proven treatment available for the prevention of post-brain hemorrhage and neurochemical malfunction; available therapy is only for symptomatic relief of the patient. Thus, 4-HI could be a potential clinical approach for treating post-brain haemorrhage and neurochemical changes caused by neurological damage. Furthermore, 4-HI may be linked to other standard therapeutic therapies utilized in ICH as a potential pharmacological intervention.

Sustainable Effects of Human Dental Pulp Stem Cell Transplantation on Diabetic Polyneuropathy in Streptozotocine-Induced Type 1 Diabetes Model Mice

Dental pulp stem cells (DPSCs) are suitable for use in regenerative medicine. Cryopreserved human DPSCs (hDPSCs) ameliorate diabetic polyneuropathy, and the effects of hDPSC transplantation are related to VEGF and NGF secretion. This study evaluated the long-term effects of a single transplantation of hDPSCs on diabetic polyneuropathy. hDPSCs were obtained from human third molars extracted for orthodontic treatment, which were then transplanted into the unilateral hindlimb skeletal muscles 8 weeks after streptozotocin injection in nude mice. The effects of hDPSC transplantation were analyzed at 16 weeks post-transplantation. DPSC transplantation significantly improved delayed nerve conduction velocity, decreased blood flow, and increased sensory perception thresholds. Furthermore, the hDPSC-conditioned medium promoted the neurite outgrowth of dorsal root ganglion neurons. In conclusion, the therapeutic effects of hDPSC transplantation with a single injection last for prolonged periods and may be beneficial in treating long-term diabetic polyneuropathy.

Painful diabetic neuropathy is associated with increased nerve regeneration in patients with type 2 diabetes undergoing intensive glycemic control

AIMS/INTRODUCTION

Painful diabetic peripheral neuropathy (pDPN) is associated with small nerve fiber degeneration and regeneration. This study investigated whether the presence of pDPN might influence nerve regeneration in patients with type 2 diabetes undergoing intensive glycemic control.

MATERIALS AND METHODS

This exploratory substudy of an open-label randomized controlled trial undertook the Douleur Neuropathique en 4 questionnaire and assessment of electrochemical skin conductance, vibration perception threshold and corneal nerve morphology using corneal confocal microscopy in participants with and without pDPN treated with exenatide and pioglitazone or basal-bolus insulin at baseline and 1-year follow up, and 18 controls at baseline only.

RESULTS

Participants with type 2 diabetes, with (n = 13) and without (n = 28) pDPN had comparable corneal nerve fiber measures, electrochemical skin conductance and vibration perception threshold at baseline, and pDPN was not associated with the severity of DPN. There was a significant glycated hemoglobin reduction (P < 0.0001) and weight gain (P < 0.005), irrespective of therapy. Participants with pDPN showed a significant increase in corneal nerve fiber density (P < 0.05), length (P < 0.0001) and branch density (P < 0.005), and a decrease in the Douleur Neuropathique en 4 score (P < 0.01), but no change in electrochemical skin conductance or vibration perception threshold. Participants without pDPN showed a significant increase in corneal nerve branch density (P < 0.01) and no change in any other neuropathy measures. A change in the severity of painful symptoms was not associated with corneal nerve regeneration and medication for pain.

CONCLUSIONS

This study showed that intensive glycemic control is associated with greater corneal nerve regeneration and an improvement in the severity of pain in patients with painful diabetic neuropathy.

Effect of exenatide on peripheral nerve excitability in type 2 diabetes

OBJECTIVE

To assess the effect of exenatide (a GLP-1 receptor agonist), dipeptidyl peptidase-IV (DPP-IV) inhibitors, and sodium-glucose co-transporter 2 (SGLT-2) inhibitors on measures of peripheral nerve excitability in patients with type 2 diabetes.

METHODS

Patients receiving either exenatide (n = 32), a DPP-IV inhibitor (n = 31), or a SGLT-2 inhibitor (n = 27) underwent motor nerve excitability assessments. Groups were similar in age, sex, HbA_1c, diabetes duration, lipids, and neuropathy severity. An additional 10 subjects were assessed prospectively over 3 months while oral anti-hyperglycaemic therapy was kept constant. A cohort of healthy controls (n = 32) were recruited for comparison.

RESULTS

Patients receiving a DPP-IV or SGLT-2 inhibitor demonstrated abnormalities in peak threshold reduction, S2 accommodation, superexcitability, and subexcitability. In contrast, patients treated with exenatide were observed to have normal nerve excitability. In the prospective arm, exenatide therapy was associated with an improvement in nerve function as patients demonstrated corrections in S2 accommodation, superexcitability, and subexcitability at follow-up. These changes were independent of the reductions in HbA_1c following exenatide treatment.

CONCLUSIONS

Exenatide was associated with an improvement in measures of nerve excitability in patients with type 2 diabetes.

SIGNIFICANCE

Exenatide may improve peripheral nerve function in type 2 diabetes.

Diabetic polyneuropathy: Bridging the translational gap

Clinical trials for diabetic polyneuropathy (DPN) have failed to identify therapeutic impacts that have arrested or reversed the disorder, despite a long history. This review considers DPN in the context of a unique neurodegenerative disorder that targets peripheral neurons and their companion glial cells. The approach is to examine what cells, cell substructures, and pathways are implicated in causing DPN and how they might be addressed therapeutically. These include axonopathy, neuronopathy, hyperglycemia, polyol flux, advanced glycation endproduct (AGE)-receptor AGE signaling, growth factor disruption, abnormal insulin signaling, and abnormalities of other intrinsic neuron pathways. Mitochondrial dysfunction and lipid toxicity are largely delegated to the companion review in this issue by Stino and Feldman. Finally, the linkage between axon plasticity of cutaneous nerves, peripheral neuroregenerative pathways, and diabetes are discussed.

Deciphering the Neuroprotective Role of Glucagon-like Peptide-1 Agonists in Diabetic Neuropathy: Current Perspective and Future Directions

Diabetic neuropathy is referred to as a subsequential and debilitating complication belonging to type 1 and type 2 diabetes mellitus. It is a heterogeneous group of disorders with a particularly complex pathophysiology and also includes multiple forms, ranging from normal discomfort to death. The evaluation of diabetic neuropathy is associated with hyperglycemic responses, resulting in an alteration in various metabolic pathways, including protein kinase C pathway, polyol pathway and hexosamine pathway in Schwann and glial cells of neurons. The essential source of neuronal destruction is analogous to these respective metabolic pathways, thus identified as potential therapeutic targets. These pathways regulating therapeutic medications may be used for diabetic neuropathy, however, only target specific drugs could have partial therapeutic activity. Various antidiabetic medications have been approved and marketed, which possess the therapeutic ability to control hyperglycemia and ameliorate the prevalence of diabetic neuropathy. Among all antidiabetic medications, incretin therapy, including Glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors, are the most favorable medications for the management of diabetes mellitus and associated peripheral neuropathic complications. Besides enhancing glucose-evoked insulin release from pancreatic β-cells, these therapeutic agents also play a vital role to facilitate neurite outgrowth and nerve conduction velocity in dorsal root ganglion. Furthermore, incretin therapy also activates cAMP and ERK signalling pathways, resulting in nerve regeneration and repairing. These effects are evidently supported by a series of preclinical data and investigations associated with these medications. However, the literature lacks adequate clinical trial outcomes related to these novel antidiabetic medications. The manuscript emphasizes the pathogenesis, current pharmacological approaches and vivid description of preclinical and clinical data for the effective management of diabetic neuropathy.

Exendin-4 Promotes Schwann Cell Survival/Migration and Myelination In Vitro

Besides its insulinotropic actions on pancreatic β cells, neuroprotective activities of glucagon-like peptide-1 (GLP-1) have attracted attention. The efficacy of a GLP-1 receptor (GLP-1R) agonist exendin-4 (Ex-4) for functional repair after sciatic nerve injury and amelioration of diabetic peripheral neuropathy (DPN) has been reported; however, the underlying mechanisms remain unclear. In this study, the bioactivities of Ex-4 on immortalized adult rat Schwann cells IFRS1 and adult rat dorsal root ganglion (DRG) neuron–IFRS1 co-culture system were investigated. Localization of GLP-1R in both DRG neurons and IFRS1 cells were confirmed using knockout-validated monoclonal Mab7F38 antibody. Treatment with 100 nM Ex-4 significantly enhanced survival/proliferation and migration of IFRS1 cells, as well as stimulated the movement of IFRS1 cells toward neurites emerging from DRG neuron cell bodies in the co-culture with the upregulation of myelin protein 22 and myelin protein zero. Because Ex-4 induced phosphorylation of serine/threonine-specific protein kinase AKT in these cells and its effects on DRG neurons and IFRS1 cells were attenuated by phosphatidyl inositol-3′-phosphate-kinase (PI3K) inhibitor LY294002, Ex-4 might act on both cells to activate PI3K/AKT signaling pathway, thereby promoting myelination in the co-culture. These findings imply the potential efficacy of Ex-4 toward DPN and other peripheral nerve lesions.

Diabetic neuropathy in children and youth: New and emerging risk factors

Pediatric neuropathy attributed to metabolic dysfunction is a well-known complication in children and youth with type 1 diabetes. Moreover, the rise of obesity and in particular of type 2 diabetes may cause an uptick in pediatric neuropathy incidence. However, despite the anticipated increase in neuropathy incidence, pathogenic insights and strategies to prevent or manage neuropathy in the setting of diabetes and obesity in children and youth remain unknown. Data from adult studies and available youth cohort studies are providing an initial understanding of potential diagnostic, management, and preventative measures in early life. This review discusses the current state of knowledge emanating from these efforts, with particular emphasis on the prevalence, clinical presentation, diagnostic approaches and considerations, and risk factors of neuropathy in type 1 and type 2 diabetes in children and youth. Also highlighted are current management strategies and recommendations for neuropathy in children and youth with diabetes. This knowledge, along with continued and sustained emphasis on identifying and eliminating modifiable risk factors, completing randomized controlled trials to assess effectiveness of strategies like weight loss and exercise, and enhancing awareness to support early detection and prevention, are pertinent to addressing the rising incidence of neuropathy associated with diabetes and obesity in children and youth.

Glucagon Prevents Cytotoxicity Induced by Methylglyoxal in a Rat Neuronal Cell Line Model

Although diabetic polyneuropathy (DPN) is a frequent diabetic complication, no effective therapeutic approach has been established. Glucagon is a crucial hormone for glucose homeostasis but has pleiotropic effects, including neuroprotective effects in the central nervous system. However, the importance of glucagon in the peripheral nervous system (PNS) has not been clarified. Here, we hypothesized that glucagon might have a neuroprotective function in the PNS. The immortalized rat dorsal root ganglion (DRG) neuronal cell line 50B11 was treated with methylglyoxal (MG) to mimic an in vitro DPN model. The cells were cultured with or without glucagon or MG. Neurotoxicity, survival, apoptosis, neurite projection, cyclic adenosine monophosphate (cAMP), and protein kinase A (PKA) were examined. Glucagon had no cytotoxicity and rescued the cells from neurotoxicity. Cell survival was increased by glucagon. The ratio of apoptotic cells, which was increased by MG, was reduced by glucagon. Neurite outgrowth was accelerated in glucagon-treated cells. Cyclic AMP and PKA accumulated in the cells after glucagon stimulation. In conclusion, glucagon protected the DRG neuronal cells from MG-induced cellular stress. The cAMP/PKA pathway may have significant roles in those protective effects of glucagon. Glucagon may be a potential target for the treatment of DPN.

Non-glucose risk factors in the pathogenesis of diabetic peripheral neuropathy

In this review, we consider the diverse risk factors in diabetes patients beyond hyperglycemia that are being recognized as contributors to diabetic peripheral neuropathy (DPN). Interest in such alternative mechanisms has been encouraged by the recognition that neuropathy occurs in subjects with metabolic syndrome and pre-diabetes and by the reporting of several large clinical studies that failed to show reduced prevalence of neuropathy after intensive glucose control in patients with type 2 diabetes. Animal models of obesity, dyslipidemia, hypertension, and other disorders common to both pre-diabetes and diabetes have been used to highlight a number of plausible pathogenic mechanisms that may either damage the nerve independent of hyperglycemia or augment the toxic potential of hyperglycemia. While pathogenic mechanisms stemming from hyperglycemia are likely to be significant contributors to DPN, future therapeutic strategies will require a more nuanced approach that considers a range of concurrent insults derived from the complex pathophysiology of diabetes beyond direct hyperglycemia.

Lipopolysaccharide-binding protein is a distinctive biomarker of abnormal pain threshold in the general Japanese population

INTRODUCTION

Small fiber neuropathy (SFN) is an early manifestation in diabetic polyneuropathy (DPN); however, the mechanisms are not fully understood. In diabetes, SFN is presumed to be common in individuals with overt DPN, enhancing activation of polyol pathway, oxidative stress, advanced glycation end products (AGEs), and inflammation. We explored the relationship between clinicohematological factors related to DPN and pain sensation in the Japanese population.

RESEARCH DESIGN AND METHODS

We conducted a population-based study, recruiting 1030 individuals (average age 54.4±0.5 years), in 2017, to participate in our Iwaki project. After initial screening by fasting blood glucose and glycohemoglobin A1c (HbA1c) measurements, the subjects were categorized into control (n=894), type 2 diabetes (n=81), and impaired fasting glucose (n=55) groups. Clinical data were gathered, and relationships between pain threshold from intraepidermal electrical stimulation (PINT) and DPN were examined by analysis of variance, post hoc test, and χ² tests to study correlations among and between groups of the clinical data and DPN.

RESULTS

Univariate linear regression analyses showed significant correlations between PINT and serum lipopolysaccharide-binding protein (LBP) level (ß=0.1025, p=0.001). Adjustments for the clinical measurements confirmed a positive correlation (ß=0.070, p=0.034). Logistic regression analysis revealed high LBP value (>6.7 mg/dL) as a significant risk factor toward abnormal PINT (≥0.35 mA). LBP significantly correlated with the high-sensitivity C reactive protein, inflammation marker, elevated similarly in both pre-diabetic and overt-diabetic groups, compared with controls, but it did not correlate with a decreased Achilles tendon reflex. In contrast, urine 8-hydroxy-2'-deoxyguanosine, oxidative stress marker, and pentosidine, AGEs, markedly increased in individuals with type 2 diabetes with high HbA1c.

CONCLUSIONS

Individuals with high LBP exhibited an elevated PINT in the Japanese population. Low level of inflammation evoked by metabolic endotoxemia is possibly implicated in the pathophysiology of SFN from pre-diabetic stage.

Prevalence and risk factors for diabetic neuropathy and painful diabetic neuropathy in primary and secondary healthcare in Qatar

Aims/Introduction

This study determined the prevalence and risk factors for diabetic peripheral neuropathy (DPN) and painful DPN (pDPN) in patients with type 2 diabetes in primary healthcare (PHC) and secondary healthcare (SHC) in Qatar.

Materials and Methods

This was a cross‐sectional multicenter study. Adults with type 2 diabetes were randomly enrolled from four PHC centers and two diabetes centers in SHC in Qatar. Participants underwent assessment of clinical and metabolic parameters, DPN and pDPN.

Results

A total of 1,386 individuals with type 2 diabetes (297 from PHC and 1,089 from SHC) were recruited. The prevalence of DPN (14.8% vs 23.9%, P = 0.001) and pDPN (18.1% vs 37.5%, P < 0.0001) was significantly lower in PHC compared with SHC, whereas those with DPN at high risk for diabetic foot ulceration (31.8% vs 40.0%, P = 0.3) was comparable. The prevalence of undiagnosed DPN (79.5% vs 82.3%, P = 0.66) was comparably high, but undiagnosed pDPN (24.1% vs 71.5%, P < 0.0001) was lower in PHC compared with SHC. The odds of DPN and pDPN increased with age and diabetes duration, and DPN increased with poor glycemic control, hyperlipidemia and hypertension, whereas pDPN increased with obesity and reduced physical activity.

Conclusions

The prevalence of DPN and pDPN in type 2 diabetes is lower in PHC compared with SHC, and is attributed to overall better control of risk factors and referral bias due to patients with poorly managed complications being referred to SHC. However, approximately 80% of patients had not been previously diagnosed with DPN in PHC and SHC. Furthermore, we identified a number of modifiable risk factors for PDN and pDPN.

Deficiency of glucagon gene-derived peptides induces peripheral polyneuropathy in mice

Although diabetic polyneuropathy (DPN) is the commonest diabetic complication, its pathology remains to be clarified. As previous papers have suggested the neuroprotective effects of glucagon-like peptide-1 in DPN, the current study investigated the physiological indispensability of glucagon gene-derived peptides (GCGDPs) including glucagon-like peptide-1 in the peripheral nervous system (PNS). Neurological functions and neuropathological changes of GCGDP deficient (gcg-/-) mice were examined. The gcg-/- mice showed tactile allodynia and thermal hyperalgesia at 12-18 weeks old, followed by tactile and thermal hypoalgesia at 36 weeks old. Nerve conduction studies revealed a decrease in sensory nerve conduction velocity at 36 weeks old. Pathological findings showed a decrease in intraepidermal nerve fiber densities. Electron microscopy revealed a decrease in circularity and an increase in g-ratio of myelinated fibers and a decrease of unmyelinated fibers in the sural nerves of the gcg-/- mice. Effects of glucagon on neurite outgrowth were examined using an ex vivo culture of dorsal root ganglia. A supraphysiological concentration of glucagon promoted neurite outgrowth. In conclusion, the mice with deficiency of GCGDPs developed peripheral neuropathy with age. Furthermore, glucagon might have neuroprotective effects on the PNS of mice. GCGDPs might be involved in the pathology of DPN.

Application prospects of glucagon-like peptide-1 receptor agonists in treatment of metabolic diseases

Metabolic syndrome [overweight, obesity, type 2 diabetes mellitus (T2DM), and lipid metabolism disorder] is directly related to cardiovascular diseases, non-alcoholic fatty liver disease (NAFLD), tumors, and other diseases, and its prevalence has been increasing sharply, bringing an increasingly heavy burden on society. Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are a novel class of glucose-dependent hypoglycemic drugs modified from glucagon-like peptide-1. Since their mechanism of action is different from that of traditional insulin and oral secretagogues, they overcome the adverse reactions associated with traditional oral medications and insulin, can improve the success rate of T2DM control, and has been widely used in the treatment of T2DM. Recent studies have found that, in addition to hypoglycemic effect, GLP-1RAs can improve metabolic diseases, such as diabetes-related complications, NAFLD, and cardiovascular diseases, and therefore have broad application prospects. This paper reviews the potential role of GLP-1RAs in these diseases.

Effect of treatment with exenatide and pioglitazone or basal-bolus insulin on diabetic neuropathy: a substudy of the Qatar Study

INTRODUCTION

To assess the effect of exenatide and pioglitazone or basal-bolus insulin on diabetic peripheral neuropathy (DPN) in patients with poorly controlled type 2 diabetes (T2D).

RESEARCH DESIGN AND METHODS

This is a substudy of the Qatar Study, an open-label, randomized controlled trial. 38 subjects with poorly controlled T2D were studied at baseline and 1-year follow-up and 18 control subjects were assessed at baseline only. A combination of exenatide (2 mg/week) and pioglitazone (30 mg/day) or glargine with aspart insulin were randomly assigned to patients to achieve an HbA1c <53 mmol/mol (<7%). DPN was assessed with corneal confocal microscopy (CCM), DN4, vibration perception and sudomotor function.

RESULTS

Subjects with T2D had reduced corneal nerves, but other DPN measures were comparable with the control group. In the combination treatment arm (n=21), HbA1c decreased by 35.2 mmol/mol (3.8 %) (p<0.0001), body weight increased by 5.6 kg (p<0.0001), corneal nerve branch density increased (p<0.05), vibration perception worsened (p<0.05), and DN4 and sudomotor function showed no change. In the insulin treatment arm, HbA1c decreased by 28.7 mmol/mol (2.7 %) (p<0.0001), body weight increased by 4.6 kg (p<0.01), corneal nerve branch density and fiber length increased (p≤0.01), vibration perception improved (p<0.01), and DN4 and sudomotor function showed no change. There was no association between the change in CCM measures with change in HbA1c, weight or lipids.

CONCLUSIONS

Treatment with exenatide and pioglitazone or basal-bolus insulin results in corneal nerve regeneration, but no change in neuropathic symptoms or sudomotor function over 1 year.

The challenges of diabetic polyneuropathy: a brief update

PURPOSE OF REVIEW

The current review addresses one of the most common neurological disorders, diabetic polyneuropathy (DPN). DPN is debilitating, irreversible and dwarfs the prevalence of most other chronic disorders of the nervous system. Its complications include foot ulceration, amputation, falling and intractable neuropathic pain. Moreover, tight control of hyperglycemia reduces the incidence of DPN in type 1 diabetes mellitus but its role in type 2 diabetes mellitus is less clear.

RECENT FINDINGS

New therapeutic options to reverse the development of DPN or its associated pain have been proposed but none have significantly changed the clinical approach. The cause of DPN remains controversial traditionally focused on the impact of metabolic abnormalities, polyol flux, microvascular changes, mitochondria, oxidative stress, lipid biology and others. In particular, there has been less attention toward how this chronic disorder alters peripheral neurobiology. It is now clear that in chronic models of diabetes mellitus there exists a unique form of neurodegeneration with a range of protein, mRNA and microRNA alterations to consider. How to reconcile these molecular and structural alterations with metabolic mechanisms is a challenge. In sensory neurons alone, a primary target of DPN, both central perikaryal cytoplasmic and nuclear changes and altered distal sensory axon terminal plasticity may be involved.

SUMMARY

In this review, the current therapeutic status of DPN is described with greater emphasis on some new but selected thoughts on its neurobiology. New mechanistic understanding will be essential to developing precision therapeutics for DPN.

Lipoic Acid Decreases the Expression of Poly ADP-Ribose Polymerase and Inhibits Apoptosis in Diabetic Rats

PURPOSE

To study the effects of lipoic acid on poly ADP-ribose polymerase (PARP) expression and apoptosis in diabetic rats.

MATERIALS AND METHODS

Sprague-Dawley rats (n=30) with high-fat diet- and streptozotocin-induced diabetes were randomly divided into two groups: diabetic model (DM) group and lipoic acid (LA) treatment group; another 10 rats were selected as normal controls (NC). The serum levels of 8-hydroxy-2'-deoxyguanosine, nitrotyrosine, and 8-isoprostane; sciatic nerve cell apoptosis index; and PARP expression were detected in the rats, and morphological changes in the sciatic nerve were recorded.

RESULTS

The blood glucose level in the DM and LA groups was significantly higher than that of the NC group (P<0.01). Compared to the NC group, the DM group showed demyelinating changes to sciatic nerve fibers. PARP expression; serum levels of 8-hydroxy-2-deoxyguanosine, nitrotyrosine, and 8-isoprostane; and the apoptosis index of sciatic nerve cells were significantly higher than those of the NC group (P<0. 01). Following LA treatment, the above indices showed significant improvement (P<0.01).

CONCLUSION

Lipoic acid may improve the symptoms of diabetic neuropathy by reducing PARP activity and inhibiting apoptosis.

Secreted factors from cultured dental pulp stem cells promoted neurite outgrowth of dorsal root ganglion neurons and ameliorated neural functions in streptozotocin-induced diabetic mice

AIMS/INTRODUCTION

Transplantation of stem cells promotes axonal regeneration and angiogenesis in a paracrine manner. In the present study, we examined whether the secreted factors in conditioned medium of stem cells from human exfoliated deciduous teeth (SHED-CM) had beneficial effects on diabetic polyneuropathy in mice.

MATERIALS AND METHODS

Conditioned medium of stem cells from human exfoliated deciduous teeth was collected 48 h after culturing in serum-free Dulbecco's modified Eagle's medium (DMEM), and separated into four fractions according to molecular weight. Dorsal root ganglion neurons from C57BL/6J mice were cultured with SHED-CM or DMEM to evaluate the effect on neurite outgrowth. Streptozotocin-induced diabetic mice were injected with 100 μL of SHED-CM or DMEM into the unilateral hindlimb muscles twice a week over a period of 4 weeks. Peripheral nerve functions were evaluated by the plantar test, and motor and sensory nerve conduction velocities. Intraepidermal nerve fiber densities, capillary number-to-muscle fiber ratio, capillary blood flow and morphometry of sural nerves were also evaluated.

RESULTS

Conditioned medium of stem cells from human exfoliated deciduous teeth significantly promoted neurite outgrowth of dorsal root ganglion neurons compared with DMEM. Among four fractions of SHED-CM, the only fraction of <6 kDa promoted the neurite outgrowth of dorsal root ganglion neurons. In addition, SHED-CM significantly prevented decline in sensory nerve conduction velocities compared with DMEM in diabetic mice. Although SHED-CM did not improve intraepidermal nerve fiber densities or morphometry of sural nerves, SHED-CM ameliorated the capillary number-to-muscle fiber ratio and capillary blood flow.

CONCLUSIONS

These results suggested that SHED-CM might have a therapeutic effect on diabetic polyneuropathy through promoting neurite outgrowth, and the increase in capillaries might contribute to the improvement of neural function.

Liraglutide treatment reduced interleukin-6 in adults with type 1 diabetes but did not improve established autonomic or polyneuropathy

AIMS

Type 1 diabetes can be complicated with neuropathy that involves immune-mediated and inflammatory pathways. Glucagon-like peptide-1 receptor agonists such as liraglutide, have shown anti-inflammatory properties, and thus we hypothesized that long-term treatment with liraglutide induced diminished inflammation and thus improved neuronal function.

METHODS

The study was a randomized, double-blinded, placebo-controlled trial of adults with type 1 diabetes and confirmed symmetrical polyneuropathy. They were randomly assigned (1:1) to receive either liraglutide or placebo. Titration was 6 weeks to 1.2-1.8 mg/d, continuing for 26 weeks. The primary endpoint was change in latency of early brain evoked potentials. Secondary endpoints were changes in proinflammatory cytokines, cortical evoked potential, autonomic function and peripheral neurophysiological testing.

RESULTS

Thirty-nine patients completed the study, of whom 19 received liraglutide. In comparison to placebo, liraglutide reduced interleukin-6 (-22.6%; 95% confidence interval [CI]: -38.1, -3.2; P = .025) with concomitant numerical reductions in other proinflammatory cytokines. However neuronal function was unaltered at the central, autonomic or peripheral level. Treatment was associated with -3.38 kg (95% CI: -5.29, -1.48; P < .001] weight loss and a decrease in urine albumin/creatinine ratio (-40.2%; 95% CI: -60.6, -9.5; P = .02).

CONCLUSION

Hitherto, diabetic neuropathy has no cure. Speculations can be raised whether mechanism targeted treatment, e.g. lowering the systemic level of proinflammatory cytokines may lead to prevention or treatment of the neuroinflammatory component in early stages of diabetic neuropathy. If ever successful, this would serve as an example of how fundamental mechanistic principles are translated into clinical practice similar to those applied in the cardiovascular and nephrological clinic.

GLP-1's role in neuroprotection: a systematic review

Glucagon-like peptide 1 (GLP-1) is a target for treatment of diabetes; however, its function in the brain is not well studied. In this systematic review, we aimed to analyze the neuroprotective role of GLP-1 and its defined mechanisms. Methods: We searched 'Web of Science' and 'Pubmed' to identify relevant studies using GLP-1 as the keyword. Two hundred and eighty-nine clinical and preclinical studies have been included. Data have been presented by grouping neurodegenerative, neurovascular and specific cell culture models. Results: Recent literature shows that GLP-1 and its agonists, DPP-4 inhibitors and combined GLP-1/GIP molecules are effective in partially or fully reversing the effects of neurotoxic compounds, neurovascular complications of diabetes, neuropathological changes related with Alzheimer's disease, Parkinson's disease or vascular occlusion. Possible mechanisms that provide neuroprotection are enhancing the viability of the neurons and restoring neurite outgrowth by increased neurotrophic factors, increasing subventricular zone progenitor cells, decreasing apoptosis, decreasing the level of pro-inflammatory factors, and strengthening blood-brain barrier. Conclusion: Based on the preclinical studies, GLP-1 modifying agents are promising targets for neuroprotection. On the other hand, the number of clinical studies that investigate GLP-1 as a treatment is low and further clinical trials are needed for a benchside to bedside translation of recent findings.

Glucagon-Like Peptide-1 Receptor Agonist Protects Dorsal Root Ganglion Neurons against Oxidative Insult

OBJECTIVE

Diabetic polyneuropathy (DPN) is one of the most prevalent diabetic complications. We previously demonstrated that exendin-4 (Ex4), a glucagon-like peptide-1 receptor agonist (GLP-1RA), has beneficial effects in animal models of DPN. We hypothesized that GLP-1 signaling would protect neurons of the peripheral nervous system from oxidative insult in DPN. Here, the therapeutic potential of GLP-1RAs on DPN was investigated in depth using the cellular oxidative insult model applied to the dorsal root ganglion (DRG) neuronal cell line.

RESEARCH DESIGN AND METHODS

Immortalized DRG neuronal 50B11 cells were cultured with and without hydrogen peroxide in the presence or absence of Ex4 or GLP-1(7-37). Cytotoxicity and viability were determined using a lactate dehydrogenase assay and MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium inner salt), respectively. Antioxidant enzyme activity was evaluated using a superoxide dismutase assay. Alteration of neuronal characteristics of 50B11 cells induced by GLP-1RAs was evaluated with immunocytochemistry utilizing antibodies for transient receptor potential vanilloid subfamily member 1, substance P, and calcitonin gene-related peptide. Cell proliferation and apoptosis were also examined by ethynyl deoxyuridine incorporation assay and APOPercentage dye, respectively. The neurite projection ratio induced by treatment with GLP-1RAs was counted. Intracellular activation of adenylate cyclase/cyclic adenosine monophosphate (cAMP) signaling was also quantified after treatment with GLP-1RAs.

RESULTS

Neither Ex4 nor GLP-1(7-37) demonstrated cytotoxicity in the cells. An MTS assay revealed that GLP-1RAs amended impaired cell viability induced by oxidative insult in 50B11 cells. GLP-1RAs activated superoxide dismutase. GLP-1RAs induced no alteration of the distribution pattern in neuronal markers. Ex4 rescued the cells from oxidative insult-induced apoptosis. GLP-1RAs suppressed proliferation and promoted neurite projections. No GLP-1RAs induced an accumulation of cAMP.

CONCLUSIONS

Our findings indicate that GLP-1RAs have neuroprotective potential which is achieved by their direct actions on DRG neurons. Beneficial effects of GLP-1RAs on DPN could be related to these direct actions on DRG neurons.

Exendin-4 promotes actin cytoskeleton rearrangement and protects cells from Nogo-A-Δ20 mediated spreading inhibition and growth cone collapse by down-regulating RhoA expression and activation via the PI3K pathway

Exendin-4 is a protein of the GLP-1 family currently used to treat diabetes. Recently, a greater number of biological properties have been associated with the GLP-1 family. Our data shows that exendin-4 treatment significantly increases the cytoskeleton rearrangement, which leads to an increasingly differentiated phenotype and reduced cell migration. We also found that exendin-4 could prevent SH-SY5Y and PC12 cells from Nogo-A-Δ20 mediated spreading inhibition and neurite collapse. Western blot analysis indicated that exendin-4 treatment both reduced the expression and activation of RhoA via the PI3K signaling pathway. These data suggest that exendin-4 may protect nerve regeneration by preventing the inhibition of Nogo-A via down-regulating RhoA expression and activation.

Diabetic neuropathy and the sensory neuron: New aspects of pathogenesis and their treatment implications

Diabetic polyneuropathy (DPN) continues to be generally considered as a "microvascular" complication of diabetes mellitus alongside nephropathy and retinopathy. The microvascular hypothesis, however, might be tempered by the concept that diabetes directly targets dorsal root ganglion sensory neurons. This neuron-specific concept, supported by accumulating evidence, might account for important features of DPN, such as its early sensory neuron degeneration. Diabetic sensory neurons develop neuronal atrophy alongside a series of messenger ribonucleic acid (RNA) changes related to declines in structural proteins, increases in heat shock protein, increases in the receptor for advanced glycation end-products, declines in growth factor signaling and other changes. Insulin is recognized as a potent neurotrophic factor, and insulin ligation enhances neurite outgrowth through activation of the phosphoinositide 3-kinase-protein kinase B pathway within sensory neurons and attenuates phenotypic features of experimental DPN. Several interventions, including glucagon-like peptide-1 agonism, and phosphatase and tensin homolog inhibition to activate growth signals in sensory neurons, or heat shock protein overexpression, prevent or reverse neuropathic abnormalities in experimental DPN. Diabetic sensory neurons show a unique pattern of microRNA alterations, a key element of messenger RNA silencing. For example, let-7i is widely expressed in sensory neurons, supports their growth and is depleted in experimental DPN; its replenishment improves features of DPN models. Finally, impairment of pre-messenger RNA splicing in diabetic sensory neurons including abnormal nuclear RNA metabolism and structure with loss of survival motor neuron protein, a neuron survival molecule, and overexpression of CWC22, a splicing factor, offer further novel insights. The present review addresses these new aspects of DPN sensory neurodegeneration.

Association Between Diabetic Microangiopathies and Glycemic Variability Assessed by Continuous Glucose Monitoring

The aim of this retrospective study was to elucidate the association between glucose profile using the continuous glucose monitoring system (CGMS) and microvascular complications in patients with type 2 diabetes mellitus (T2DM). The subjects were 160 inpatients with T2DM. The mean blood glucose (MBG) level, percentage of time in a 24-hour period spent with blood glucose level higher than 180 mg/dl (time at >180 mg/dl), standard deviation (SD), and mean amplitude of glycemic excursions (MAGE) were measured continuously over 48 hours using the CGMS. The primary outcome was the association between microvascular complications and glycemic variability. The secondary outcome was the association between microangiopathies and MBG. The SD and MAGE were not associated with presence of microangiopathies or number of complications. There were also no associations between abnormal vibratory sensation in the bilateral lower extremities, coefficient of variation of the R-R interval (CVRR), retinopathy stage, nephropathy stage, or microalbuminuria. MBG was associated, however, with retinopathy, retinopathy stage, and number of complications. Time at >180 mg/dl correlated with abnormal vibratory sensation in the bilateral lower extremities and presence or stage of retinopathy. MBG and time at >180 mg/dl were not associated with presence or stage of nephropathy. Our findings suggest that broad glycemic variability was not associated with microvascular complications, the number of which increased in patients with a high mean glucose level and long time spent with hyperglycemia. It is important, therefore, to reduce the mean glucose level and time spent with hyperglycemia to prevent future microangiopathies.

The relationship between sudomotor function and skin microvascular reactivity in individuals with type 1 diabetes of long duration

AIM

The aim of this study was to assess the relationship between sudomotor function and microvascular perfusion in patients with type 1 diabetes (DM1).

METHODS

We evaluated 415 patients (206 women), with DM1, median age of 41 (IQR: 33-53) years, disease duration of 25 (IQR: 20-32) years. We assessed metabolic control of diabetes and the presence of peripheral and cardiac autonomic neuropathy. Sudomotor function was assessed using Sudoscan device by electrochemical skin conductance (ESC). Microvascular function was measured by laser-Doppler flowmetry with basal perfusion, the peak flow after occlusion (PORHpeak) and THmax which is the percentage change between basal perfusion and the peak flow during thermal hyperemia (TH). The accumulation of advanced glycation end products in the skin was assessed by skin autofluorescence (AF) measurement using AGE Reader. We subdivided patients based on the presence of diabetic peripheral neuropathy (DPN), cardiac autonomic neuropathy (CAN) and according to normal value of ESC.

RESULTS

Patients with abnormal ESC had higher skin AF [2.5 (2.1-2.9) vs 2.1 (1.9-2.5) AU, p < 0.001], lower eGFR [83 (72-96) vs 98 (86-108) ml/min/1.73 m2, p < 0.001], higher basal perfusion [25 (12-81) vs 14 (7-43) PU, p < 0.001], lower THmax [664 (137-1461) vs 1115 (346-1933) %, p = 0.002], higher PORHpeak [104 (59-167) vs 70 (48-135) PU, p < 0.001] as compared to subjects with normal ESC results. We found negative correlation between THmax and TG level (Rs = -0.14, p < 0.005), AF (Rs = -0.19, p = 0.001), vibration perception threshold - VPT (Rs = -0.24, p < 0.001) and positive correlation with HDL level (Rs = 0.14, p = 0.005), Feet ESC (Rs = 0.21, p < 0.001) and Hands ESC (Rs = 0.14, p = 0.004). We found positive correlation between PORHpeak and TG level (Rs = 0.14, p = 0.003), skin AF (Rs = 0.29, p < 0.001), VPT (0.27, p < 0.001) and negative correlation with eGFR (Rs = -0.2, p < 0.001), HDL (Rs = -0.12, p = 0.01), Feet ESC (Rs = -0.27, p < 0.001) and Hand ESC (Rs = -0.16, p = 0.002).

CONCLUSION

Impaired microvascular reactivity is associated with sudomotor dysfunction in patients with type 1 diabetes.

Glucagon-like peptide 1 receptor (GLP-1R) expression by nerve fibres in inflammatory bowel disease and functional effects in cultured neurons

INTRODUCTION

Glucagon like-peptide 1 receptor (GLP-1R) agonists diminish appetite and may contribute to the weight loss in inflammatory bowel disease (IBD).

OBJECTIVES

The aim of this study was to determine, for the first time, the expression of GLP-1R by colon nerve fibres in patients with IBD, and functional effects of its agonists in cultured rat and human sensory neurons.

METHODS

GLP-1R and other nerve markers were studied by immunohistochemistry in colon biopsies from patients with IBD (n = 16) and controls (n = 8), human dorsal root ganglia (DRG) tissue, and in GLP-1R transfected HEK293 cells. The morphological effects of incretin hormones oxyntomodulin, exendin-4 and glucagon were studied on neurite extension in cultured DRG neurons, and their functional effects on capsaicin and ATP signalling, using calcium imaging.

RESULTS

Significantly increased numbers of colonic mucosal nerve fibres were observed in IBD biopsies expressing GLP-1R (p = 0.0013), the pan-neuronal marker PGP9.5 (p = 0.0008), and sensory neuropeptide CGRP (p = 0.0014). An increase of GLP-1R positive nerve fibres in IBD colon was confirmed with a different antibody to GLP-1R (p = 0.016). GLP-1R immunostaining was intensely positive in small and medium-sized neurons in human DRG, and in human and rat DRG cultured neurons. Co-localization of GLP-1R expression with neuronal markers in colon and DRG confirmed the neural expression of GLP-1R, and antibody specificity was confirmed in HEK293 cells transfected with the GLP-1R. Treatment with oxyntomodulin, exendin-4 and GLP-1 increased neurite length in cultured neurons compared with controls, but did not stimulate calcium influx directly, or affect capsaicin responses. However, exendin-4 significantly enhanced ATP responses in human DRG neurons.

CONCLUSION

Our results show that increased GLP-1R innervation in IBD bowel could mediate enhanced visceral afferent signalling, and provide a peripheral target for therapeutic intervention. The differential effect of GLP-1R agonists on capsaicin and ATP responses in neurons suggest they may not affect pain mechanisms mediated by the capsaicin receptor TRPV1, but may enhance the effects of purinergic agonists.

Transplantation of dental pulp stem cells improves long-term diabetic polyneuropathy together with improvement of nerve morphometrical evaluation

Background

Although previous reports have revealed the therapeutic potential of stem cell transplantation in diabetic polyneuropathy, the effects of cell transplantation on long-term diabetic polyneuropathy have not been investigated. In this study, we investigated whether the transplantation of dental pulp stem cells (DPSCs) ameliorated long-term diabetic polyneuropathy in streptozotocin (STZ)-induced diabetic rats.

Methods

Forty-eight weeks after STZ injection, we transplanted DPSCs into the unilateral hindlimb skeletal muscles. Four weeks after DPSC transplantation (i.e., 52 weeks after STZ injection) the effects of DPSC transplantation on diabetic polyneuropathy were assessed.

Results

STZ-induced diabetic rats showed significant reductions in the sciatic motor/sensory nerve conduction velocity, increases in the current perception threshold, and decreases in capillary density in skeletal muscles and intra-epidermal nerve fiber density compared with normal rats, all of which were ameliorated by DPSC transplantation. Furthermore, sural nerve morphometrical analysis revealed that the transplantation of DPSCs significantly increased the myelin thickness and area. DPSC-conditioned media promoted the neurite outgrowth of dorsal root ganglion neurons and increased the viability and myelin-related protein expression of Schwann cells.

Conclusions

These results indicated that the transplantation of DPSCs contributed to the neurophysiological and neuropathological recovery from a long duration of diabetic polyneuropathy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0729-5) contains supplementary material, which is available to authorized users.

Activation of Glucagon-Like Peptide-1 Receptor Promotes Neuroprotection in Experimental Autoimmune Encephalomyelitis by Reducing Neuroinflammatory Responses

The signaling axis of glucagon-like peptide-1 (GLP-1)/GLP-1 receptor (GLP-1R) has been an important component in overcoming diabetes, and recent reports have uncovered novel beneficial roles of this signaling axis in central nervous system (CNS) disorders, such as Alzheimer's disease, Parkinson's disease, and cerebral ischemia, accelerating processes for exendin-4 repositioning. Here, we studied whether multiple sclerosis (MS) could be a complement to the CNS disorders that are associated with the GLP-1/GLP-1R signaling axis. Both components of the signaling axis, GLP-1 and GLP-1R proteins, are expressed in neurons, astrocytes, and microglia in the spinal cord of normal mice. In particular, they are abundant in Iba1-positive microglia. Upon challenge by experimental autoimmune encephalomyelitis (EAE), an animal model of MS, the mRNA expression of both GLP-1 and GLP-1R was markedly downregulated in EAE-symptomatic spinal cords, indicating attenuated activity of GLP-1/GLP-1R signaling in EAE. Such a downregulation obviously occurred in LPS-stimulated rat primary microglia, a main cell type to express both GLP-1 and GLP-1R, further indicating attenuated activity of GLP-1/GLP-1R signaling in activated microglia. To investigate whether increased activity of GLP-1R has a therapeutic benefit, exendin-4 (5 μg/kg, i.p.), a GLP-1R agonist, was administered daily to EAE-symptomatic mice. Exendin-4 administration to symptomatic EAE mice significantly improved the clinical signs of the disease, along with the reversal of histopathological sequelae such as cell accumulation, demyelination, astrogliosis, microglial activation, and morphological transformation of activated microglia in the injured spinal cord. Such an improvement by exendin-4 was comparable to that by FTY720 (3 mg/kg, i.p.), a drug for MS. The neuroprotective effects of exendin-4 against EAE were also associated with decreased mRNA expression of proinflammatory cytokines, such as interleukin (IL)-17, IL-1β, IL-6, and tumor necrosis factor (TNF)-α, all of which are usually upregulated in injured sites of the EAE spinal cord. Interestingly, exendin-4 exposure similarly reduced mRNA levels of IL-1β and TNF-α in LPS-stimulated microglia. Furthermore, exendin-4 administration significantly attenuated activation of NF-κB signaling in EAE spinal cord and LPS-stimulated microglia. Collectively, the current study demonstrates the therapeutic potential of exendin-4 for MS by reducing immune responses in the CNS, highlighting the importance of the GLP-1/GLP-1R signaling axis in the development of a novel therapeutic strategy for MS.

Impaired Axonal Regeneration in Diabetes. Perspective on the Underlying Mechanism from In Vivo and In Vitro Experimental Studies

Axonal regeneration after peripheral nerve injury is impaired in diabetes, but its precise mechanisms have not been elucidated. In this paper, we summarize the progress of research on altered axonal regeneration in animal models of diabetes and cultured nerve tissues exposed to hyperglycemia. Impaired nerve regeneration in animal diabetes can be attributed to dysfunction of neurons and Schwann cells, unfavorable stromal environment supportive of regenerating axons, and alterations of target tissues receptive to reinnervation. In particular, there are a number of factors such as enhanced activity of the negative regulators of axonal regeneration (e.g., phosphatase and tensin homolog deleted on chromosome 10 and Rho/Rho kinase), delayed Wallerian degeneration, alterations of the extracellular matrix components, enhanced binding of advanced glycation endproducts (AGEs) with the receptor for AGE, and delayed muscle reinnervation that can be obstacles to functional recovery after an axonal injury. It is also noteworthy that we and others have observed excessive neurite outgrowth from peripheral sensory ganglion explants from streptozotocin (STZ)-diabetic mice in culture and enhanced regeneration of small nerve fibers after sciatic nerve injury in STZ-induced diabetic rats. The excess of abortive neurite outgrowth may lead to misconnections of axons and target organs, which may interfere with appropriate target reinnervation and functional repair. Amelioration of perturbed nerve regeneration may be crucial for the future management of diabetic neuropathy.

Incretin-Based Therapies for Diabetic Complications: Basic Mechanisms and Clinical Evidence

An increase in the rates of morbidity and mortality associated with diabetic complications is a global concern. Glycemic control is important to prevent the development and progression of diabetic complications. Various classes of anti-diabetic agents are currently available, and their pleiotropic effects on diabetic complications have been investigated. Incretin-based therapies such as dipeptidyl peptidase (DPP)-4 inhibitors and glucagon-like peptide-1 receptor agonists (GLP-1RA) are now widely used in the treatment of patients with type 2 diabetes. A series of experimental studies showed that incretin-based therapies have beneficial effects on diabetic complications, independent of their glucose-lowering abilities, which are mediated by anti-inflammatory and anti-oxidative stress properties. Based on these findings, clinical studies to assess the effects of DPP-4 inhibitors and GLP-1RA on diabetic microvascular and macrovascular complications have been performed. Several but not all studies have provided evidence to support the beneficial effects of incretin-based therapies on diabetic complications in patients with type 2 diabetes. We herein discuss the experimental and clinical evidence of incretin-based therapy for diabetic complications.

Effect of Exendin-4 on Autophagy Clearance in Beta Cell of Rats with Tacrolimus-induced Diabetes Mellitus

Growing evidence suggests that GLP-1 protects beta cells against various cellular injuries by modulating autophagy. In this study, we examined whether exendin-4 (Ex-4), a GLP-1 analog, had preventive effects on tacrolimus (Tac)-induced beta cell injury by improving autophagy clearance. Rats with Tac-induced diabetes mellitus exhibited increased autophagy-associated protein expression, light chain 3B levels, and autophagic vacuole numbers in pancreatic beta cells. Additionally, Tac increased autophagy in a dose- and time-dependent manner in vitro, and inhibition of autophagosome using 3-methyladenine reduced Tac-induced islet cell injury by decreasing reactive oxygen species production and apoptosis. Ex-4 treatment decreased Tac-induced hyperglycaemia, oxidative stress, and apoptosis, accompanied by decreased autophagy-associated protein expression and autophagosome numbers. In vivo and in vitro studies showed that Tac treatment impaired lysosomal function and autophagosome-lysosome fusion; these processes were improve by Ex-4 treatment. Moreover, addition of bafilomycin A1, an inhibitor of lysosomal function, abolished the protective effects of Ex-4. Our findings reveal that Tac-induced diabetes mellitus was a state of excessive burden of autophagosomes and impairment of autophagy clearance and that Ex-4 protected against Tac-induced pancreatic islet injury by reducing the burden of autophagosomes via activation of autophagosome clearance. Thus, Ex-4 had therapeutic effects on Tac-induced pancreatic beta cell injury.

Sensory Neurodegeneration in Diabetes: Beyond Glucotoxicity

Diabetic polyneuropathy in humans is of gradual, sometimes insidious onset, and is more likely to occur if glucose control is poor. Arguments that the disorder arises chiefly from glucose toxicity however ignore the greater complexity of a unique neurodegenerative disorder. For example, sensory neurons regularly thrive in media with levels of glucose at or exceeding those of poorly controlled diabetic persons. Also, all of the linkages between hyperglycemia and neuropathy develop in the setting of altered insulin availability or sensitivity. Insulin itself is recognized as a potent growth, or trophic factor for adult sensory neurons. Low doses of insulin, insufficient to alter blood glucose levels, reverse features of diabetic neurodegeneration in animal models. Insulin resistance, as occurs in diabetic adipose tissue, liver, and muscle, also develops in sensory neurons, offering a mechanism for neurodegeneration in the setting of normal or elevated insulin levels. Other interventions that "shore up" sensory neurons prevent features of diabetic polyneuropathy from developing despite persistent hyperglycemia. More recently evidence has emerged that a series of subtle molecular changes in sensory neurons can be linked to neurodegeneration including epigenetic changes in the control of gene expression. Understanding the new complexity of sensory neuron degeneration may give rise to therapeutic strategies that have a higher chance of success in the clinical trial arena.

The dipeptidyl peptidase IV inhibitor vildagliptin suppresses development of neuropathy in diabetic rodents: effects on peripheral sensory nerve function, structure and molecular changes

Incretin-related therapy was found to be beneficial for experimental diabetic neuropathy, but its mechanism is obscure. The purpose of this study is to explore the mechanism through which dipeptidyl peptidase IV inhibitor, vildagliptin (VG), influences neuropathy in diabetic rodents. To this end, non-obese type 2 diabetic Goto-Kakizaki rats (GK) and streptozotocin (STZ)-induced diabetic mice were treated with VG orally. Neuropathy was evaluated by nerve conduction velocity (NCV) in both GK and STZ-diabetic mice, whereas calcitonin-gene-related peptide expressions, neuronal cell size of dorsal root ganglion (DRG) and intraepidermal nerve fiber density were examined in GK. DRG from GK and STZ-diabetic mice served for the analyses of GLP-1 and insulin signaling. As results, VG treatment improved glucose intolerance and increased serum insulin and GLP-1 in GK accompanied by the amelioration of delayed NCV and neuronal atrophy, reduced calcitonin-gene-related peptide expressions and intraepidermal nerve fiber density. Diet restriction alone did not significantly influence these measures. Impaired GLP-1 signals such as cAMP response element binding protein, protein kinase B/Akt (PKB/Akt) and S6RP in DRG of GK were restored in VG-treated group, but the effect was equivocal in diet-treated GK. Concurrently, decreased phosphorylation of insulin receptor substrate 2 in GK was corrected by VG treatment. Consistent with the effect on GK, VG treatment improved NCV in diabetic mice without influence on hyperglycemia. DRG of VG-treated diabetic mice were characterized by correction of GLP-1 signals and insulin receptor substrate 2 phosphorylation without effects on insulin receptor β expression. The results suggest close association of neuropathy development with impaired signaling of insulin and GLP-1 in diabetic rodents. Diabetic neurons are resistant to insulin and such insulin resistance may contribute to development of neuropathy. DPP-IV inhibitor, vildagliptin, corrected insulin resistance and improved neuropathy irrespective of blood glucose via augmented action of GLP-1.

Incretin-Based Therapy for Prevention of Diabetic Vascular Complications

Diabetic vascular complications are the most common cause of mortality and morbidity worldwide, with numbers of affected individuals steadily increasing. Diabetic vascular complications can be divided into two categories: macrovascular andmicrovascular complications. Macrovascular complications include coronary artery diseaseand cerebrovascular disease, while microvascular complications include retinopathy and chronic kidney disease. These complications result from metabolic abnormalities, including hyperglycemia, elevated levels of free fatty acids, and insulin resistance. Multiple mechanisms have been proposed to mediate the adverse effects of these metabolic disorders on vascular tissues, including stimulation of protein kinase C signaling and activation of the polyol pathway by oxidative stress and inflammation. Additionally, the loss of tissue-specific insulin signaling induced by hyperglycemia and toxic metabolites can induce cellular dysfunction and both macro- and microvascular complications characteristic of diabetes. Despite these insights, few therapeutic methods are available for the management of diabetic complications. Recently, incretin-based therapeutic agents, such as glucagon-like peptide-1 and dipeptidyl peptidase-4 inhibitors, have been reported to elicit vasotropic actions, suggesting a potential for effecting an actual reduction in diabetic vascular complications. The present review will summarize the relationship between multiple adverse biological mechanisms in diabetes and putative incretin-based therapeutic interventions intended to prevent diabetic vascular complications.