Therapeutic effects of fenofibrate on diabetic peripheral neuropathy by improving endothelial and neural survival in db/db mice

Transcriptomic profiling of sciatic nerves and dorsal root ganglia reveals site-specific effects of prediabetic neuropathy

Peripheral neuropathy (PN) is a severe and frequent complication of obesity, prediabetes, and type 2 diabetes characterized by progressive distal-to-proximal peripheral nerve degeneration. However, a comprehensive understanding of the mechanisms underlying PN, and whether these mechanisms change during PN progression, is currently lacking. Here, gene expression data were obtained from distal (sciatic nerve; SCN) and proximal (dorsal root ganglia; DRG) injury sites of a high-fat diet (HFD)-induced mouse model of obesity/prediabetes at early and late disease stages. Self-organizing map and differentially expressed gene analyses followed by pathway enrichment analysis identified genes and pathways altered across disease stage and injury site. Pathways related to immune response, inflammation, and glucose and lipid metabolism were consistently dysregulated with HFD-induced PN, irrespective of injury site. However, regulation of oxidative stress was unique to the SCN while dysregulated Hippo and Notch signaling were only observed in the DRG. The role of the immune system and inflammation in disease progression was supported by an increase in the percentage of immune cells in the SCN with PN progression. Finally, when comparing these data to transcriptomic signatures from human patients with PN, we observed conserved pathways related to metabolic dysregulation across species, highlighting the translational relevance of our mouse data. Our findings demonstrate that PN is associated with distinct site-specific molecular re-programming in the peripheral nervous system, identifying novel, clinically relevant therapeutic targets.

Potential therapeutic effects of peroxisome proliferator-activated receptors on corneal diseases

The cornea is an avascular tissue in the eye that has multiple functions in the eye to maintain clear vision which can significantly impair one's vision when subjected to damage. Peroxisome proliferator-activated receptors (PPARs), a family of nuclear receptor proteins comprising three different peroxisome proliferator-activated receptor (PPAR) isoforms, namely, PPAR alpha (α), PPAR gamma (γ), and PPAR delta (δ), have emerged as potential therapeutic targets for treating corneal diseases. In this review, we summarised the current literature on the therapeutic effects of PPAR agents on corneal diseases. We discussed the role of PPARs in the modulation of corneal wound healing, suppression of corneal inflammation, neovascularisation, fibrosis, stimulation of corneal nerve regeneration, and amelioration of dry eye by inhibiting oxidative stress within the cornea. We also discussed the underlying mechanisms of these therapeutic effects. Future clinical trials are warranted to further attest to the clinical therapeutic efficacy.

Topical and oral peroxisome proliferator-activated receptor-α agonist ameliorates diabetic corneal neuropathy

Diabetic corneal neuropathy (DCN) is a common diabetic ocular complication with limited treatment options. In this study, we investigated the effects of topical and oral fenofibrate, a peroxisome proliferator-activated receptor-α agonist, on the amelioration of DCN using diabetic mice (n = 120). Ocular surface assessments, corneal nerve and cell imaging analysis, tear proteomics and its associated biological pathways, immuno-histochemistry and western blot on PPARα expression, were studied before and 12 weeks after treatment. At 12 weeks, PPARα expression markedly restored after topical and oral fenofibrate. Topical fenofibrate significantly improved corneal nerve fibre density (CNFD) and tortuosity coefficient. Likewise, oral fenofibrate significantly improved CNFD. Both topical and oral forms significantly improved corneal sensitivity. Additionally, topical and oral fenofibrate significantly alleviated diabetic keratopathy, with fenofibrate eye drops demonstrating earlier therapeutic effects. Both topical and oral fenofibrate significantly increased corneal β-III tubulin expression. Topical fenofibrate reduced neuroinflammation by significantly increasing the levels of nerve growth factor and substance P. It also significantly increased β-III-tubulin and reduced CDC42 mRNA expression in trigeminal ganglions. Proteomic analysis showed that neurotrophin signalling and anti-inflammation reactions were significantly up-regulated after fenofibrate treatment, whether applied topically or orally. This study concluded that both topical and oral fenofibrate ameliorate DCN, while topical fenofibrate significantly reduces neuroinflammation.

Involvement of oxidative stress-AMPK-Cx43-NLRP3 pathway in extracellular matrix remodeling of gastric smooth muscle cells in rats with diabetic gastroparesis

This study aimed to investigate the changes in oxidative stress, adenosine monophosphate-activated protein kinase (AMPK), connexin43 (Cx43), nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) expression, and extracellular matrix (ECM) in the gastric smooth muscle tissues of rats with diabetic gastroparesis (DGP) and high glucose-cultured gastric smooth muscle cells, determine the existence of oxidative stress-AMPK-Cx43-NLRP3 pathway under high glucose condition, and the involvement of this pathway in ECM remodeling in DGP rats. The results showed that with increasing duration of diabetes, oxidation stress levels gradually increased, the AMPK activity decreased first and then increased, NLRP3, CX43 expression, and membrane/cytoplasm ratio of Cx43 expression were increased in the gastric smooth muscle tissues of diabetic rats. Changes in ECM of gastric smooth muscle cells were observed in DGP rats. The DGP group showed higher collagen type I content, increased expression of Caspase-1, transforming growth factor-beta 3 (TGF-β3), and matrix metalloproteinase-2 (MMP-2), decreased tissue inhibitor of metalloproteinase-1 (TIMP-1) expression, and higher interleukin-1 beta content when compared with the control group. For gastric smooth muscle cells cultured under higher glucose, the MMP-2 and TGF-β3 expression was decreased, TGF-β1 and TIMP-1 expression was increased, the interleukin-1 beta content was decreased in cells after inhibition of NLRP3 expression; the NLRP3 and Caspase-1 expression was decreased, and adenosine triphosphate content was lower after inhibition of Cx43; the expression of NLRP3, Caspase-1, P2X7, and the membrane/cytoplasm ratio of CX43 expression was decreased in cells after inhibition of AMPK and oxidative stress, the phospho-AMPK expression was also decreased after suppressing oxidative stress. Our findings suggest that high glucose induced the activation of the AMPK-Cx43-NLRP3 pathway through oxidative stress, and this pathway was involved in the ECM remodeling of gastric smooth muscles in DGP rats by regulating the biological functions of TGF-β3, TGF-β1, MMP-2, and TIMP-1.

Mitochondrial quality control in human health and disease

Mitochondria, the most crucial energy-generating organelles in eukaryotic cells, play a pivotal role in regulating energy metabolism. However, their significance extends beyond this, as they are also indispensable in vital life processes such as cell proliferation, differentiation, immune responses, and redox balance. In response to various physiological signals or external stimuli, a sophisticated mitochondrial quality control (MQC) mechanism has evolved, encompassing key processes like mitochondrial biogenesis, mitochondrial dynamics, and mitophagy, which have garnered increasing attention from researchers to unveil their specific molecular mechanisms. In this review, we present a comprehensive summary of the primary mechanisms and functions of key regulators involved in major components of MQC. Furthermore, the critical physiological functions regulated by MQC and its diverse roles in the progression of various systemic diseases have been described in detail. We also discuss agonists or antagonists targeting MQC, aiming to explore potential therapeutic and research prospects by enhancing MQC to stabilize mitochondrial function.

Fibroblast growth factor 21 alleviates diabetes-induced cognitive decline

Diabetes mellitus (DM) causes damage to the central nervous system, resulting in cognitive impairment. Fibroblast growth factor 21 (FGF21) exhibits the potential to alleviate neurodegeneration. However, the therapeutic effect of intracerebroventricular (i.c.v) FGF21 infusion on diabetes-induced cognitive decline (DICD) and its potential mechanisms remain unclear. In this study, the impact of FGF21 on DICD was explored, and 1H nuclear magnetic resonance (NMR)-based metabolomics plus 13C NMR spectroscopy in combine with intravenous [1-13C]-glucose infusion were used to investigate the underlying metabolic mechanism. Results revealed that i.c.v FGF21 infusion effectively improved learning and memory performance of DICD mice; neuron loss and apoptosis in hippocampus and cortex were significantly blocked, suggesting a potential neuroprotective role of FGF21 in DICD. Metabolomics results revealed that FGF21 modulated DICD metabolic alterations related to glucose and neurotransmitter metabolism, which are characterized by distinct recovered enrichment of [3-13C]-lactate, [3-13C]-aspartate, [4-13C]-glutamine, [3-13C]-glutamine, [4-13C]-glutamate, and [4-13C]- γ-aminobutyric acid (GABA) from [1-13C]-glucose. Moreover, diabetes-induced neuron injury and metabolic dysfunctions might be mediated by PI3K/AKT/GSK-3β signaling pathway inactivation in the hippocampus and cortex, which were activated by i.c.v injection of FGF21. These findings indicate that i.c.v FGF21 infusion exerts its neuroprotective effect on DICD by remodeling cerebral glucose and neurotransmitter metabolism by activating the PI3K/AKT/GSK-3β signaling pathway.

Culture of Primary Neurons from Dissociated and Cryopreserved Mouse Trigeminal Ganglion

Corneal nerves originate from the ophthalmic branch of the trigeminal nerve, which enters the cornea at the limbus radially from all directions toward the central cornea. The cell bodies of the sensory neurons of trigeminal nerve are located in the trigeminal ganglion (TG), while the axons are extended into the three divisions, including ophthalmic branch that supplies corneal nerves. Study of primary neuronal cultures established from the TG fibers can therefore provide a knowledge basis for corneal nerve biology and potentially be developed as an in vitro platform for drug testing. However, setting up primary neuron cultures from animal TG has been dubious with inconsistency among laboratories due to a lack of efficient isolation protocol, resulting in low yield and heterogenous cultures. In this study, we used a combined enzymatic digestion with collagenase and TrypLE to dissociate mouse TG while preserving nerve cell viability. A subsequent discontinuous Percoll density gradient followed by mitotic inhibitor treatment effectively diminished the contamination of non-neuronal cells. Using this method, we reproducibly generated high yield and homogenous primary TG neuron cultures. Similar efficiency of nerve cell isolation and culture was further obtained for TG tissue cryopreserved for short (1 week) and long duration (3 months), compared to freshly isolated tissues. In conclusion, this optimized protocol shows a promising potential to standardize TG nerve culture and generate a high-quality corneal nerve model for drug testing and neurotoxicity studies.

Peroxisome proliferator-activated receptor-α (PPARα) regulates wound healing and mitochondrial metabolism in the cornea

Diabetes can result in impaired corneal wound healing. Mitochondrial dysfunction plays an important role in diabetic complications. However, the regulation of mitochondria function in the diabetic cornea and its impacts on wound healing remain elusive. The present study aimed to explore the molecular basis for the disturbed mitochondrial metabolism and subsequent wound healing impairment in the diabetic cornea. Seahorse analysis showed that mitochondrial oxidative phosphorylation is a major source of ATP production in human corneal epithelial cells. Live corneal biopsy punches from type 1 and type 2 diabetic mouse models showed impaired mitochondrial functions, correlating with impaired corneal wound healing, compared to nondiabetic controls. To approach the molecular basis for the impaired mitochondrial function, we found that Peroxisome Proliferator-Activated Receptor-α (PPARα) expression was downregulated in diabetic human corneas. Even without diabetes, global PPARα knockout mice and corneal epithelium-specific PPARα conditional knockout mice showed disturbed mitochondrial function and delayed wound healing in the cornea, similar to that in diabetic corneas. In contrast, fenofibrate, a PPARα agonist, ameliorated mitochondrial dysfunction and enhanced wound healing in the corneas of diabetic mice. Similarly, corneal epithelium-specific PPARα transgenic overexpression improved mitochondrial function and enhanced wound healing in the cornea. Furthermore, PPARα agonist ameliorated the mitochondrial dysfunction in primary human corneal epithelial cells exposed to diabetic stressors, which was impeded by siRNA knockdown of PPARα, suggesting a PPARα-dependent mechanism. These findings suggest that downregulation of PPARα plays an important role in the impaired mitochondrial function in the corneal epithelium and delayed corneal wound healing in diabetes.

PIAS1 alleviates diabetic peripheral neuropathy through SUMOlation of PPAR-γ and miR-124-induced downregulation of EZH2/STAT3

Diabetic peripheral neuropathy (DPN) is a frequently occurring chronic complication of diabetes. In this study, we aim to explore the regulatory mechanism of protein inhibitor of activated STAT1 (PIAS1) in DPN in terms of autophagy and apoptosis of Schwann cells. The SUMOlation of PPAR-γ by PIAS1 was examined, and ChIP was performed to verify the binding of PPAR-γ to miR-124 promoter region. Dual-luciferase gene reporter assay was used to validate the binding affinity between miR-124 and EZH2/STAT3. Following loss‐ and gain‐of-function experiments, in vitro assays in high glucose-treated Schwann cells (SC4) and in vivo assays in db/db and ob/ob mice were performed to detect the effects of PIAS1 on autophagy and apoptosis of Schwann cells as well as symptoms of DPN by regulating the PPAR-γ-miR-124-EZH2/STAT3. The expression of PIAS1, PPAR-γ, and miR-124 was downregulated in the sciatic nerve tissue of diabetic mice. PIAS1 enhanced the expression of PPAR-γ through direct binding and SUMOlation of PPAR-γ. PPAR-γ enhanced the expression of miR-124 by enhancing the promoter activity of miR-124. Furthermore, miR-124 targeted and inversely modulated EZH2 and STAT3, promoting the autophagy of Schwann cells and inhibiting their apoptosis. In vivo experiments further substantiated that PIAS1 could promote the autophagy and inhibit the apoptosis of Schwann cells through the PPAR-γ-miR-124-EZH2/STAT3 axis. In conclusion, PIAS1 promoted SUMOlation of PPAR-γ to stabilize PPAR-γ expression, which upregulated miR-124 to inactivate EZH2/STAT3, thereby inhibiting apoptosis and promoting autophagy of Schwann cells to suppress the development of DPN.

Lipids and peripheral neuropathy

PURPOSE OF REVIEW

Hyperlipidaemia is associated with the development of neuropathy. Indeed, a mechanistic link between altered lipid metabolism and peripheral nerve dysfunction has been demonstrated in a number of experimental and clinical studies. Furthermore, post hoc analyses of clinical trials of cholesterol and triglyceride-lowering pharmacotherapy have shown reduced rates of progression of diabetic neuropathy. Given, there are currently no FDA approved disease-modifying therapies for diabetic neuropathy, modulation of lipids may represent a key therapeutic target for the treatment of diabetic nerve damage. This review summarizes the current evidence base on the role of hyperlipidaemia and lipid lowering therapy on the development and progression of peripheral neuropathy.

RECENT FINDINGS

A body of literature supports a detrimental effect of dyslipidaemia on nerve fibres resulting in somatic and autonomic neuropathy. The case for an important modulating role of hypertriglyceridemia is stronger than for low-density lipoprotein cholesterol (LDL-C) in relation to peripheral neuropathy. This is reflected in the outcomes of clinical trials with the different therapeutic agents targeting hyperlipidaemia reporting beneficial or neutral effects with statins and fibrates. The potential concern with the association between proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitor therapy and cognitive decline raised the possibility that extreme LDL-C lowering may result in neurodegeneration. However, studies in murine models and data from small observational studies indicate an association between increased circulating PCSK9 levels and small nerve fibre damage with a protective effect of PCSK9i therapy against small fibre neuropathy. Additionally, weight loss with bariatric surgery leads to an improvement in peripheral neuropathy and regeneration of small nerve fibres measured with corneal confocal microscopy in people with obesity with or without type 2 diabetes. These improvements correlate inversely with changes in triglyceride levels.

SUMMARY

Hyperlipidaemia, particularly hypertriglyceridemia, is associated with the development and progression of neuropathy. Lipid modifying agents may represent a potential therapeutic option for peripheral neuropathy. Post hoc analyses indicate that lipid-lowering therapies may halt the progression of neuropathy or even lead to regeneration of nerve fibres. Well designed randomized controlled trials are needed to establish if intensive targeted lipid lowering therapy as a part of holistic metabolic control leads to nerve fibre regeneration and improvement in neuropathy symptoms.

Targeting Peroxisome Proliferator-Activated Receptor-α (PPAR- α) to reduce paclitaxel-induced peripheral neuropathy

BACKGROUND AND PURPOSE

Paclitaxel, a widely used anti-cancer drug, is frequently associated with prolonged and severe peripheral neuropathies (PIPN), associated with neuroinflammation. Currently, PIPN effective treatments are lacking. Peroxisome Proliferator-Activated Receptor-α (PPAR-⍺) can modulate inflammatory responses. Thus, the use of PPAR-⍺ agonists, such as fibrates (fenofibrate and choline-fenofibrate), currently used in dyslipidemia treatment, could represent an interesting therapeutic approach in PIPN.

EXPERIMENTAL APPROACH

Our studies tested the efficacy of fenofibrate (150 mg/kg, daily, i.p.) and choline fenofibrate (60 mg/kg daily, p.o.) in reversing and preventing the development of PIPN (paclitaxel: 8 mg/kg, i.p., every other day for 4 days) in male and female C57BL/6J mice. Mechanical and cold hypersensitivity, conditioned place preference, sensory nerve action potential (SNAP), as well as the expression of PPAR-⍺, TNF-⍺, IL-1β and IL-6 mRNA were evaluated.

KEY RESULTS

While fenofibrate treatment partially reversed and prevented the development of mechanical hypersensitivity, this was completely reversed and prevented by choline-fenofibrate. Both fibrates were able to completely reverse and prevent cold hypersensitivity induced by paclitaxel. The reduction of SNAP amplitude induced by paclitaxel was also reversed by both fenofibrate and choline-fenofibrate. Our results indicate that suppression of paclitaxel-induced hypersensitivity by fibrates involves the regulation of PPAR-⍺ expression and decrease neuroinflammation in DRG. Finally, the co-treatment of Paclitaxel and fenofibric acid (fibrates active metabolite) was tested on different cancer cell lines, no decrease in the antitumoral effect of paclitaxel was observed.

CONCLUSIONS AND IMPLICATIONS

Taken together, our results show for the first time the therapeutic potential (prevention and reversal) of fibrates in PIPN and opens to a potential pharmacological repurposing of these drugs.

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.

Maintaining the balance of TDP-43, mitochondria, and autophagy: a promising therapeutic strategy for neurodegenerative diseases

Mitochondria are the energy center of cell operations and are involved in physiological functions and maintenance of metabolic balance and homeostasis in the body. Alterations of mitochondrial function are associated with a variety of degenerative and acute diseases. As mitochondria age in cells, they gradually become inefficient and potentially toxic. Acute injury can trigger the permeability of mitochondrial membranes, which can lead to apoptosis or necrosis. Transactive response DNA-binding protein 43 kDa (TDP-43) is a protein widely present in cells. It can bind to RNA, regulate a variety of RNA processes, and play a role in the formation of multi-protein/RNA complexes. Thus, the normal physiological functions of TDP-43 are particularly important for cell survival. Normal TDP-43 is located in various subcellular structures including mitochondria, mitochondrial-associated membrane, RNA particles and stress granules to regulate the endoplasmic reticulum–mitochondrial binding, mitochondrial protein translation, and mRNA transport and translation. Importantly, TDP-43 is associated with a variety of neurodegenerative diseases, including amyotrophic lateral sclerosis, frontotemporal dementia and Alzheimer's disease, which are characterized by abnormal phosphorylation, ubiquitination, lysis or nuclear depletion of TDP-43 in neurons and glial cells. Although the pathogenesis of TDP-43 proteinopathy remains unknown, the presence of pathological TDP-43 inside or outside of mitochondria and the functional involvement of TDP-43 in the regulation of mitochondrial morphology, transport, and function suggest that mitochondria are associated with TDP-43-related diseases. Autophagy is a basic physiological process that maintains the homeostasis of cells, including targeted clearance of abnormally aggregated proteins and damaged organelles in the cytoplasm; therefore, it is considered protective against neurodegenerative diseases. However, the combination of abnormal TDP-43 aggregation, mitochondrial dysfunction, and insufficient autophagy can lead to a variety of aging-related pathologies. In this review, we describe the current knowledge on the associations of mitochondria with TDP-43 and the role of autophagy in the clearance of abnormally aggregated TDP-43 and dysfunctional mitochondria. Finally, we discuss a novel approach for neurodegenerative treatment based on the knowledge.

Effects of Dual Peroxisome Proliferator-Activated Receptors α and γ Activation in Two Rat Models of Neuropathic Pain

Neuropathic pain is a growing healthcare problem causing a global burden. Currently used analgesics such as opioids are associated with adverse effects; urging the need for safer alternatives. Here we aimed to investigate the potential analgesic effects of tesaglitazar; dual peroxisome proliferator-activated receptors α and γ (PPARα and γ) agonist in rat models of neuropathic pain. This study also aimed to investigate the modulation of the transient receptor potential vanilloid 1 (TRPV1) receptor activity by tesaglitazar which could provide a potential mechanism that underlie tesaglitazar antinociceptive effects. Von Frey filaments were used to determine the paw withdrawal threshold (PWT) in adult male Sprague Dawley rats (180-250g) following i.p. injection of streptozotocin (STZ) or cisplatin, which were used as models of neuropathic pain. Antinociceptive effects of tesaglitazar were determined 6 hours after drug administration. Cobalt influx assays in cultured dorsal root ganglia (DRG) neurons were used to study the effects of tesaglitazar preincubation on capsaicin-evoked cobalt influx. Both cisplatin and STZ produced a significant decrease in PWT. The higher dose of tesaglitazar (20μg/kg) significantly restored PWT in both neuropathic pain models (P<0.05). 10μM capsaicin produced a robust cobalt response in DRG neurons. Preincubation of DRG neurones with tesaglitazar 6 hours prior to stimulation with capsaicin significantly reduce capsaicin-evoked cobalt responses in a PPARα and PPARγ dependent fashion (P<0.05). In conclusion, tesaglitazar produced significant analgesic effects in STZ and cisplatin-induced neuropathy, possibly by modulating TRPV1 receptor activity. This may be of potential benefit in clinical practice dealing with peripheral neuropathy.

Calcimimetic restores diabetic peripheral neuropathy by ameliorating apoptosis and improving autophagy

Decreased AMPK-eNOS bioavailability mediates the development of diabetic peripheral neuropathy (DPN) through increased apoptosis and decreased autophagy activity in relation to oxidative stress. Schwann cells are responsible for maintaining structural and functional integrity of neurons and for repairing damaged nerves. We evaluated the neuro-protective effect of cinacalcet on DPN by activating the AMPK-eNOS pathway using db/db mice and human Schwann cells (HSCs). Sciatic nerve of db/db mice was characterized by disorganized myelin, axonal shrinkage, and degeneration that were accompanied by marked fibrosis, inflammation, and apoptosis. These phenotypical alterations were significantly improved by cinacalcet treatment along with improvement in sensorimotor functional parameters. Cinacalcet demonstrated favorable effects through increased expression and activation of calcium-sensing receptor (CaSR)-CaMKKβ and phosphorylation of AMPK-eNOS signaling in diabetic sciatic nerve. Cinacalcet decreased apoptosis and increased autophagy activity in relation to decreased oxidative stress in HSCs cultured in high-glucose medium as well. This was accompanied by increased expression of the CaSR, intracellular Ca⁺⁺ ([Ca⁺⁺]i) levels, and CaMKKβ-LKB1-AMPK signaling pathway, resulting in the net effect of increased eNOS phosphorylation, NOx concentration, Bcl-2/Bax ratio, beclin 1, and LC3-II/LC3-I ratio. These results demonstrated that cinacalcet treatment ameliorates inflammation, apoptosis, and autophagy through increased expression of the CaSR, [Ca⁺⁺]i levels and subsequent activation of CaMKKβ-LKB-1-AMPK-eNOS pathway in the sciatic nerve and HSCs under diabetic condition. Therefore, cinacalcet may play an important role in the restoration and amelioration of DPN by ameliorating apoptosis and improving autophagy.

The Potential Role of Fatty Acids in Treating Diabetic Neuropathy

PURPOSE OF REVIEW

This review will summarize recent findings of the effect of supplemental fatty acids, with an emphasis on omega-3 polyunsaturated fatty acids, as a treatment for diabetic peripheral neuropathy.

RECENT FINDINGS

Pre-clinical studies have provided evidence that treating diabetic rodents with δ linolenic acid (omega-6 18:3) and to a greater extent with eicosapentaenoic and docosahexaenoic acids (omega-3 20:5 and 22:6, respectively) improve and even reverse vascular and neural deficits. Additional studies have shown resolvins, metabolites of eicosapentaenoic and docosahexaenoic acids, can induce neurite outgrowth in neuron cultures and that treating type 1 or type 2 diabetic mice with resolvin D1 or E1 provides benefit for peripheral neuropathy similar to fish oil. Omega-3 polyunsaturated fatty acids derived from fish oil and their derivatives have anti-inflammatory properties and could provide benefit for diabetic peripheral neuropathy. However, clinical trials are needed to determine whether this statement is true.

Gene therapy with the angiogenic neuropeptide secretoneurin ameliorates experimental diabetic neuropathy

The pathogenesis of diabetic neuropathy remains enigmatic. Damage to the vasa nervorum may be responsible for this disorder. Recently, we showed that secretoneurin (SN) induces angiogenesis in hindlimb and myocardial ischemia. Moreover, beneficial effects were observed in wound healing. We therefore hypothesized that SN therapy may ameliorate diabetic neuropathy. We used db/db mice as animal model for neuropathy. Gene therapy was accomplished by intramuscular injection of SN plasmid along the sciatic nerve. Sciatic nerve motor and sensory conduction velocities were then measured for 9 wk. Nerve conduction velocities showed normal values in heterozygous mice for the observational period, but were severely reduced in homozygous mice in which velocities were significantly improved by SN, but not by control plasmid gene therapy. The reaction time in the tail-flick test improved significantly in SN-treated animals. The induction of growth of vasa nervorum seems to be part of the underlying mechanism. In addition, SN positively affected Schwann cell function in vitro and induced activation of important signaling pathways. Our observations suggest that SN exerts beneficial effects on nerve function in vivo and on Schwann cells in vitro. It therefore may be a promising treatment option for diabetic neuropathy.-Theurl, M., Lener, D., Albrecht-Schgoer, K., Beer, A., Schgoer, W., Liu, Y., Stanzl, U., Fischer-Colbrie, R., Kirchmair, R. Gene therapy with the angiogenic neuropeptide secretoneurin ameliorates experimental diabetic neuropathy.

The influence of age, anthropometric and metabolic variables on LDIFLARE and corneal confocal microscopy in healthy individuals

Introduction

The laser Doppler imaging (LDI) _FLARE and corneal confocal microscopy (CCM) are reliable markers of small fibre function (SFF) and structure (SFS), respectively, but the impact of potential confounding variables needs to be defined. The objective of this study was to determine the influence of age, anthropometric and biochemical variables on LDI and CCM.

Methods

80 healthy volunteers (43 males) (age: 39.7±15.2 yrs.) underwent LDI_FLARE and CCM assessment and the effect of age, anthropometric and biochemical variables was determined using multivariate analysis.

Results

There was an age-related decline in LDI_FLARE (0.07cm²/yr; R² = 0.669; p = <0.0001) and CCM parameters (CNFD: 0.05 fibres/mm² /yr; R² = 0.590; p = <0.0001, CNBD: 0.06 branches/mm²/yr; R² = 0.549; p = 0.001and CNFL 0.07 mm/mm²/yr; R² = 0.369; p = 0.009). BMI did not influence SFF (p = 0.08) but had a significant independent association with CNFD (p = 0.01). Fasting triglycerides (TG) independently influenced the LDI_FLARE (β_c:-0.204; p = 0.008) and all CCM indices (β_c:-0.191 to -0.243; p = <0.05). HbA_1c was significantly associated with CNFD only (p = 0.001) but not with LDI_FLARE, CNBD or CNFL (p = ≥0.05). Blood pressure and total cholesterol were not associated with LDI_FLARE or any CCM parameters. There was a significant correlation between LDI_FLARE and all CCM parameters (p = ≤0.01).

Conclusions

This study shows that in healthy controls, both SFF measured by LDI_FLARE and SFS assessed by CCM showed a significant inverse correlation with age and triglycerides, perhaps suggesting the use of age-specific normative values when interpreting these outcomes. Furthermore, this study shows that in healthy controls, despite measuring different neural parameters, both methods correlated significantly with each other.

Therapeutic Effects of a Novel Agonist of Peroxisome Proliferator-Activated Receptor Alpha for the Treatment of Diabetic Retinopathy

Purpose

Clinical studies have shown that peroxisome proliferator-activated receptor alpha (PPARα) agonist fenofibrate has therapeutic effects on diabetic retinopathy (DR). The purpose of this study was to identify a novel PPARα agonist and to evaluate its beneficial effects on DR.

Methods

The transcriptional activity of PPARα was measured by a luciferase-based promoter assay. TUNEL was used to evaluate apoptosis in retinal precursor cells (R28). Diabetes was induced in rats by injection of streptozotocin. Retinal inflammation was examined using leukostasis assay, and retinal vascular leakage was measured using permeability assay. Retinal function was measured using electroretinogram (ERG) recording, and retinal apoptosis was quantified using the cell death ELISA. The anti-angiogenic effect was evaluated in the oxygen-induced retinopathy (OIR) model.

Results

A compound, 7-chloro-8-methyl-2-phenylquinoline-4-carboxylic acid (Y-0452), with a chemical structure distinct from existing PPARα agonists, activated PPARα transcriptional activity and upregulated PPARα expression. Y-0452 significantly inhibited human retinal capillary endothelial cell migration and tube formation. The compound also protected R28 cells against apoptosis and inhibited NF-κB signaling in R28 cells exposed to palmitate. In diabetic rats, Y-0452 ameliorated leukostasis and vascular leakage in the retina. In addition, Y-0452 preserved the retinal function and reduced retinal cell death in diabetic rats. Y-0452 also alleviated retinal neovascularization in the OIR model.

Conclusions

Y-0452 is a novel PPARα agonist and has therapeutic potential for DR.

AMPK orchestrates an elaborate cascade protecting tissue from fibrosis and aging

Fibrosis is a common process characterized by excessive extracellular matrix (ECM) accumulation after inflammatory injury, which is also a crucial cause of aging. The process of fibrosis is involved in the pathogenesis of most diseases of the heart, liver, kidney, lung, and other organs/tissues. However, there are no effective therapies for this pathological alteration. Annually, fibrosis represents a huge financial burden for the USA and the world. 5'-AMP-activated protein kinase (AMPK) is a pivotal energy sensor that alleviates or delays the process of fibrogenesis. In this review, we first present basic background information on AMPK and fibrogenesis and describe the protective roles of AMPK in three fibrogenic phases. Second, we analyze the protective action of AMPK during fibrosis in myocardial, hepatic, renal, pulmonary, and other organs/tissues. Third, we present a comprehensive discussion of AMPK during fibrosis and draw a conclusion. This review highlights recent advances, vital for basic research and clinical drug design, in the regulation of AMPK during fibrosis.

Novel Anti-fibrotic Therapies

Fibrosis is a major player in cardiovascular disease, both as a contributor to the development of disease, as well as a post-injury response that drives progression. Despite the identification of many mechanisms responsible for cardiovascular fibrosis, to date no treatments have emerged that have effectively reduced the excess deposition of extracellular matrix associated with fibrotic conditions. Novel treatments have recently been identified that hold promise as potential therapeutic agents for cardiovascular diseases associated with fibrosis, as well as other fibrotic conditions. The purpose of this review is to provide an overview of emerging antifibrotic agents that have shown encouraging results in preclinical or early clinical studies, but have not yet been approved for use in human disease. One of these agents is bone morphogenetic protein-7 (BMP7), which has beneficial effects in multiple models of fibrotic disease. Another approach discussed involves altering the levels of micro-RNA (miR) species, including miR-29 and miR-101, which regulate the expression of fibrosis-related gene targets. Further, the antifibrotic potential of agonists of the peroxisome proliferator-activated receptors will be discussed. Finally, evidence will be reviewed in support of the polypeptide hormone relaxin. Relaxin is long known for its extracellular remodeling properties in pregnancy, and is rapidly emerging as an effective antifibrotic agent in a number of organ systems. Moreover, relaxin has potent vascular and renal effects that make it a particularly attractive approach for the treatment of cardiovascular diseases. In each case, the mechanism of action and the applicability to various fibrotic diseases will be discussed.

Lipids: A Suitable Therapeutic Target in Diabetic Neuropathy?

Diabetic polyneuropathy (DPN) encompasses multiple syndromes with a common pathogenesis. Glycemic control shows a limited correlation with DPN, arguing in favor of major involvement of other factors, one of which is alterations of lipid and lipoprotein metabolism. Consistent associations have been found between plasma triglycerides/remnant lipoproteins and the risk of DPN. Studies in cultured nerve tissue or in murine models of diabetes have unveiled mechanisms linking lipid metabolism to DPN. Deficient insulin action increases fatty acids flux to nerve cells, inducing mitochondrial dysfunction, anomalous protein kinase C signaling, and perturbations in the physicochemical properties of the plasma membrane. Oxidized low-density lipoproteins bind to cellular receptors and promote generation of reactive oxygen species, worsening mitochondrial function and altering the electrical properties of neurons. Supplementation with specific fatty acids has led to prevention or reversal of different modalities of DPN in animal models. Post hoc and secondary analyses of clinical trials have found benefits of cholesterol reducing (statins and ezetimibe), triglyceride-reducing (fibrates), or lipid antioxidant (thioctic acid) therapies over the progression and severity of DPN. However, these findings are mostly hypothesis-generating. Randomized trials are warranted in which the impact of intensive plasma lipids normalization on DPN outcomes is specifically evaluated.

Nuclear Countermeasure Activity of TP508 Linked to Restoration of Endothelial Function and Acceleration of DNA Repair

There is increasing evidence that radiation-induced damage to endothelial cells and loss of endothelial function may contribute to both acute radiation syndromes and long-term effects of whole-body nuclear irradiation. Therefore, several drugs are being developed to mitigate the effects of nuclear radiation, most of these drugs will target and protect or regenerate leukocytes and platelets. Our laboratory has demonstrated that TP508, a 23-amino acid thrombin peptide, activates endothelial cells and stem cells to revascularize and regenerate tissues. We now show that TP508 can mitigate radiation-induced damage to endothelial cells in vitro and in vivo. Our in vitro results demonstrate that human endothelial cells irradiation attenuates nitric oxide (NO) signaling, disrupts tube formation and induces DNA double-strand breaks (DSB). TP508 treatment reverses radiation effects on NO signaling, restores tube formation and accelerates the repair of radiation-induced DSB. The radiation-mitigating effects of TP508 on endothelial cells were also seen in CD-1 mice where systemic injection of TP508 stimulated endothelial cell sprouting from aortic explants after 8 Gy irradiation. Systemic doses of TP508 that mitigated radiation-induced endothelial cell damage, also significantly increased survival of CD-1 mice when injected 24 h after 8.5 Gy exposure. These data suggest that increased survival observed with TP508 treatment may be due to its effects on vascular and microvascular endothelial cells. Our study supports the usage of a regenerative drug such as TP508 to activate endothelial cells as a countermeasure for mitigating the effects of nuclear radiation.

Alternatives to the Streptozotocin-Diabetic Rodent

The study of diabetic neuropathy has relied primarily on the use of streptozotocin-treated rat and mouse models of type 1 diabetes. This chapter will review the creation and use of other rodent models that have been developed in order to investigate the contribution of factors besides insulin deficiency to the development and progression of diabetic neuropathy as it occurs in obesity, type 1 or type 2 diabetes. Diabetic peripheral neuropathy is a complex disorder with multiple mechanisms contributing to its development and progression. Even though many animal models have been developed and investigated, no single model can mimic diabetic peripheral neuropathy as it occurs in humans. Nonetheless, animal models can play an important role in improving our understanding of the etiology of diabetic peripheral neuropathy and in performing preclinical screening of potential new treatments. To date treatments found to be effective for diabetic peripheral neuropathy in rodent models have failed in clinical trials. However, with the identification of new endpoints for the early detection of diabetic peripheral neuropathy and the understanding that a successful treatment may require a combination therapeutic approach there is hope that an effective treatment will be found.

Fenofibrate increases cardiac autophagy via FGF21/SIRT1 and prevents fibrosis and inflammation in the hearts of Type 1 diabetic mice

Fenofibrate (FF) as a commonly-used lipid-lowering medicine in clinics was examined for its potentially repurposing to prevent the cardiac abnormalities in patients with type 1 diabetes. We demonstrated here that fenofibrate significantly prevented diabetes-induced cardiac dysfunction and remodeling in fibroblast growth factor 21 (FGF21)-dependent manner.

Diabetic Polyneuropathy in Type 2 Diabetes Mellitus: Inflammation, Oxidative Stress, and Mitochondrial Function

Diabetic polyneuropathy (DPN) is defined as peripheral nerve dysfunction. There are three main alterations involved in the pathologic changes of DPN: inflammation, oxidative stress, and mitochondrial dysfunction. Inflammation induces activation of nuclear factor kappa B, activator protein 1, and mitogen-activated protein kinases. Oxidative stress induced by hyperglycemia is mediated by several identified pathways: polyol, hexosamine, protein kinase C, advanced glycosylation end-products, and glycolysis. In addition, mitochondrial dysfunction accounts for most of the production of reactive oxygen and nitrosative species. These free radicals cause lipid peroxidation, protein modification, and nucleic acid damage, to finally induce axonal degeneration and segmental demyelination. The prevalence of DPN ranges from 2.4% to 78.8% worldwide, depending on the diagnostic method and the population assessed (hospital-based or outpatients). Risk factors include age, male gender, duration of diabetes, uncontrolled glycaemia, height, overweight and obesity, and insulin treatment. Several diagnostic methods have been developed, and composite scores combined with nerve conduction studies are the most reliable to identify early DPN. Treatment should be directed to improve etiologic factors besides reducing symptoms; several approaches have been evaluated to reduce neuropathic impairments and improve nerve conduction, such as oral antidiabetics, statins, and antioxidants (alpha-lipoic acid, ubiquinone, and flavonoids).

Fibrates inhibit the apoptosis of Batten disease lymphoblast cells via autophagy recovery and regulation of mitochondrial membrane potential

Batten disease (BD; also known as juvenile neuronal ceroid lipofuscinosis) is a genetic disorder inherited as an autosomal recessive trait and is characterized by blindness, seizures, cognitive decline, and early death resulting from the inherited mutation of the CLN3 gene. Mitochondrial oxidative stress, endoplasmic reticulum (ER) stress, disrupted autophagy, and enhanced apoptosis have been suggested to play a role in BD pathogenesis. Fibrates, a class of lipid-lowering drugs that induce peroxisome proliferator-activated receptor-α (PPAR-α) activation, are the most commonly used PPAR agonists. Assuming that fibrates have a neuroprotective effect, we studied the effects of fibrates, fenofibrate, bezafibrate, and gemfibrozil on apoptosis, depolarization of mitochondrial membrane, and defective autophagy in BD lymphoblast cells. The viability of fibrate-treated BD lymphoblast cells increased to levels of normal lymphoblast cells. In addition, treatment with fibrates inhibited depolarization of mitochondrial membrane potential in BD lymphoblast cells. Defective autophagy in BD lymphoblast cells was normalized when treated with fibrates as indicated by increased acridine orange staining. The recovery of autophagy in BD lymphoblast cells is most likely attributed to the upregulation of autophagy proteins, lysosomal-associated membrane protein 1 (LAMP1), and LC3 I/II, after treatment with fibrates. This study therefore suggests that fibrates may have a therapeutic potential against BD.

Is fenofibrate a reasonable treatment for diabetic microvascular disease?

Type 2 diabetes is a pandemic disease, and its prevalence is increasing mainly due to an increase in obesity and life expectancy. Diabetic complications and their comorbidities constitute the most important economic cost of the disease and represent a significant economic burden for the healthcare systems of developed countries. Despite improving standards of care, people with diabetes remain at risk of the development and progression of microvascular diabetic complications. Therefore, the identification of novel therapeutic approaches is necessary. The aim of this article is to provide an overview of the clinical benefits of fenofibrate on microvascular diabetic complications, with special emphasis on diabetic retinopathy. In addition, the potential mechanisms of action will be briefly discussed.

Therapeutic Effects of PPAR α on Neuronal Death and Microvascular Impairment

Peroxisome-proliferator activated receptor-alpha (PPARα) is a broadly expressed nuclear hormone receptor and is a transcription factor for diverse target genes possessing a PPAR response element (PPRE) in the promoter region. The PPRE is highly conserved, and PPARs thus regulate transcription of an extensive array of target genes involved in energy metabolism, vascular function, oxidative stress, inflammation, and many other biological processes. PPARα has potent protective effects against neuronal cell death and microvascular impairment, which have been attributed in part to its antioxidant and anti-inflammatory properties. Here we discuss PPARα's effects in neurodegenerative and microvascular diseases and also recent clinical findings that identified therapeutic effects of a PPARα agonist in diabetic microvascular complications.

Evaluating the Phase II drugs currently under investigation for diabetic neuropathy

Introduction: The worldwide number of patients suffering from diabetes mellitus (DM) is projected to approach 552 million by the year 2030. As diabetic neuropathy (DN) is present in 8% of new diabetic patients at the time of diagnosis and occurs in ∼ 50% of all patients with established DM, the number of patients who will develop painful DN will also increase. The suboptimal efficacies of currently approved drugs have prompted investigators to develop new therapeutic agents for the management of painful DN. Areas covered: In this review, the authors present and elucidate the current status of drugs under investigation for the treatment of painful DN. A short synopsis of currently approved drugs is also given. Literature information and data analysis were retrieved from PubMed, the American Diabetes and Neurological Associations Websites and ClinicalTrials.gov. The keywords used in the search included: DM, DN, painful diabetic neuropathy. Expert opinion: In addition to treating the pain associated with DN, the actual causes of the disease should also be targeted for improved management. It is hoped that drugs which improve vascular blood flow, induce neural regeneration, reduce hyperglycemia, oxidative stress and inflammation can be more effective for the overall treatment of painful DN.