Autophagy: The missing link in diabetic neuropathy?

Leptin deficiency leads to nerve degeneration and impairs axon remyelination by inducing Schwann cell apoptosis and demyelination in type 2 diabetic peripheral neuropathy in rats

Diabetic peripheral neuropathy, characterized by symptoms such as paresthesia, neuropathic pain, and potential lower limb amputation, poses significant clinical management challenges. Recent studies suggest that chronic hyperglycemia-induced Schwann cells (SCs) apoptosis contributes to neurodegeneration and impaired nerve regeneration, but the detailed mechanisms are still unknown. Our study investigated a mixed-sex type 2 diabetes mellitus (T2DM) rat model using leptin knockout (KO) to simulate obesity and diabetes-related conditions. Through extensive assessments, including mechanical allodynia, electrophysiology, and microcirculation analyses, along with myelin degradation studies in KO versus wild-type rats, we focused on apoptosis, autophagy, and SCs dedifferentiation in the sciatic nerve and examined nerve regeneration in KO rats. KO rats exhibited notable reductions in mechanical withdrawal force, prolonged latency, decreased compound muscle action potential (CMAP) amplitude, reduced microcirculation, myelin sheath damage, and increases in apoptosis, autophagy, and SCs dedifferentiation. Moreover, leptin KO was found to impair peripheral nerve regeneration postinjury, as indicated by reduced muscle weight, lower CMAP amplitude, extended latency, and decreased remyelination and SCs density. These findings underscore the effectiveness of the T2DM rat model in clarifying the impact of leptin KO on SCs apoptosis, dedifferentiation, and demyelination, providing valuable insights into new therapeutic avenues for treating T2DM-induced peripheral neuropathy.

Decreased glucagon in diabetic peripheral neuropathy patients with long duration type 2 diabetes

OBJECTIVE

The aim of this study was to investigate the association of fasting C-peptide and glucagon with diabetic peripheral neuropathy (DPN) in patients with type 2 diabetes (T2DM).

METHODS

A comprehensive evaluation was conducted on 797 patients with T2DM to assess the various risk factors affecting DPN. The subjects were categorized into short duration and long duration group according to the duration of diabetes with a threshold of 10 years. Logistic regression analysis was employed to examine the association between DPN and islet function, as well as other parameters. Receiver operating characteristic curve analysis was performed to evaluate the predictive capability of glucagon.

RESULTS

The fasting C-peptide levels were significantly lower in the DPN patients with short duration of diabetes, but lost significance in the long duration group. Conversely, a decreased level of glucagon was only observed in DPN patients with long duration of diabetes. For the group with long duration of diabetes, glucagon was the sole risk factor associated with DPN. The receiver operating characteristic curve analysis revealed that glucagon in the long duration group exhibited a moderate area under the curve of 0.706.

CONCLUSIONS

The serum glucagon levels in T2DM patients with DPN exhibited bidirectional changes based on the duration of diabetes. Decreased glucagon was associated with DPN in T2DM patients with long duration of diabetes.

Roles of Neuronal Protein Kinase Cε on Endoplasmic Reticulum Stress and Autophagic Formation in Diabetic Neuropathy

In chronic diabetic neuropathy (DN), the cellular mechanisms of neuropathic pain remain unclear. Protein kinase C epsilon (PKCε) is an intracellular signaling molecule that mediates chronic pain. This paper addresses the long-term upregulated PKCε in DN associated with endoplasmic reticulum (ER) stress and autophagic formation and correlates to chronic neuropathic pain. We found that thermal hyperalgesia and mechanical allodynia course development were associated with PKCε upregulation after DN but not skin denervation. Pathologically, PKCε upregulation was associated with the expression of inositol-requiring enzyme 1α (IRE1α; ER stress-related molecule) and ubiquitin D (UBD), which are involved in the ubiquitin-proteasome system (UPS)-mediated degradation of misfolded proteins under ER stress. Manders coefficient analyses revealed an approximately 50% colocalized ratio for IRE1α(+):PKCε(+) neurons (0.34-0.48 for M1 and 0.40-0.58 for M2 Manders coefficients). The colocalized coefficients of UBD/PKCε increased (M1: 0.33 ± 0.03 vs. 0.77 ± 0.04, p < 0.001; M2: 0.29 ± 0.05 vs. 0.78 ± 0.04; p < 0.001) in the acute DN stage. In addition, the regulatory subunit p85 of phosphoinositide 3-kinase, which is involved in regulating insulin signaling, exhibited similar expression patterns to those of IRE1α and UBD; for example, it had highly colocalized ratios to PKCε. The ultrastructural examination further confirmed that autophagic formation was associated with PKCε upregulation. Furthermore, PKCεv1-2, a PKCε specific inhibitor, reverses neuropathic pain, ER stress, and autophagic formation in DN. This finding suggests PKCε plays an upstream molecule in DN-associated neuropathic pain and neuropathology and could provide a potential therapeutic target.

Investigating the role of an immediate early gene FOS as a potential regulator of autophagic response to hypoglycemia in embryonic hypothalamic neurons

Hypoglycemia-associated autonomic failure (HAAF) is a well-established complication of diabetes. Although HAAF has serious outcomes such as recurrent morbidity, coma, and death, the mechanisms of HAAF and its pathological components are largely unknown. Our previous studies have revealed that hypoglycemia is associated with the upregulation of an immediate early gene - FOS. In addition, it is documented that glucose deprivation activates neuronal autophagic activities. Therefore, the present study aimed to identify the role of FOS and one of the core components of the autophagy pathway, Beclin-1 (encoded by the BECN1 gene), in the regulation of autophagic mechanisms in embryonic hypothalamic neurons in response to hypoglycemic conditions. Embryonic Mouse Hypothalamic Cell Line N39 (mHypoE-N39 or N39) was cultured in reduced concentrations of glucose (2000, 900, 500, and 200 mg/L). Gene and protein expression, as well as immunofluorescence studies on autophagy were conducted under different reduced glucose concentrations in N39 hypothalamic neurons with and without FOS and BECN1 gene knockdowns (KD). The outcomes of the present study have demonstrated a significant increase in autophagosome formation and subsequent lysosomal degradation in the hypothalamic neurons in response to reduced glucose concentrations. This hypoglycemic response appears to be lowered to a similar extent in the FOS KD and BECN1 KD cells, albeit insignificantly from the negative control, is indicative of the involvement of FOS in the autophagic response of hypothalamic neurons to hypoglycemia. Moreover, the KD cells exhibited a change in morphology and reduced cell viability compared with the control cells. Our findings suggest that reduced FOS expression could potentially be associated with impaired autophagic activities that are dependent on BECN1, which could lead to decreased or blunted hypothalamic activation in response to hypoglycemia, and this, in turn, may contribute to the development of HAAF.

Glp-1 Mimetics and Autophagy in Diabetic Milieu: State-of-the-Art

The diabetic milieu is associated with cascades of pathophysiological pathways that culminate in diabetic complications and tissue injuries. Autophagy is an essential process mandatory for cell survival and tissue homeostasis by degrading damaged organelles and removing injured cells. However, it may turn into a pathological process in an aberrant mode in the diabetic and/or malignant milieu. Moreover, autophagy could serve as a promising therapeutic target for many complications related to tissue injury. Glp-1 mimetics are a class of newer antidiabetic agents that reduce blood glucose through several pathways. However, some evidence suggests that they can provide extra glycemic benefits by modulating autophagy, although there is no complete understanding of this mechanism and its underlying molecular pathways. Hence, in the current review, we aimed to provide new insights on the possible impact of Glp-1 mimetics on autophagy and consequent benefits as well as mediating pathways.

Arctigenin improves neuropathy via ameliorating apoptosis and modulating autophagy in streptozotocin-induced diabetic mice

BACKGROUND

Oxidative stress mediates the pathophysiology of diabetic neuropathy (DN) with activation of apoptotic pathway and reduction of autophagy. Arctigenin (ARC) is a natural lignan isolated from some plants of the Asteraceae family that shows antioxidant property. The present study aimed to explore the mechanistic neuroprotective effect of ARC on animal model for DN.

METHODS

DN was induced using streptozotocin (STZ) at a dose of 45 mg/kg, i.p, for five consecutive days and ARC was administered orally (25 or 50 mg) for 3 weeks. The mechanical sensitivity and thermal latency were determined using von Frey and hotplate, respectively. Beclin, p62, and LC3 were detected as markers for autophagy by western blot. Levels of reduced glutathione, lipid peroxides, and activities of catalase and superoxide dismutase were detected as readout for oxidative stress. Apoptotic parameters and histopathological changes were revealed in all experimental groups.

RESULTS

The present study showed deterioration of the function and structure of neurons as a result of hyperglycemia. Oxidative stress and impaired autophagy were observed in diabetic neurons as well as the activation of apoptotic pathway. ARC improved the behavioral and histopathological changes of diabetic mice. ARC combated oxidative stress through diminishing lipid peroxidation and improving the activity of antioxidant enzymes. This was concomitant by reducing the biomarkers of apoptosis. ARC augmented the expression of Beclin and LC3 while it lessened the expression of p62 indicating the activation of autophagy. These findings suggest that ARC can ameliorate DN by combating apoptosis and oxidative stress and improving autophagy.

Piceatannol promotes neuroprotection by inducing mitophagy and mitobiogenesis in the experimental diabetic peripheral neuropathy and hyperglycemia-induced neurotoxicity

Piceatannol (PCN), a SIRT1 activator, regulates multiple oxidative stress mechanism and has anti-inflammatory potential in various inflammatory conditions. However, its role in Diabetic insulted peripheral neuropathy (DN) remains unknown. Oxidative stress and mitochondrial dysfunction are major contributing factors to DN. Myriad studies have proven that sirtuin1 (SIRT1) stimulation convalesce nerve functions by activating mitochondrial functions like mitochondrial biogenesis and mitophagy. Diabetic neuropathy (DN) was provoked by injecting streptozotocin (STZ) at a dose of 55 mg/kg, i.p to male Sprague Dawley (SD) rats. Mechanical, thermal hyperalgesia was evaluated by using water immersion, Vonfrey Aesthesiometer, and Randall Sellito Calipers. Motor, sensory nerve conduction velocity was measured using Power Lab 4sp system whereas The Laser Doppler system was used to evaluate nerve blood flow. To induce hyperglycemia for the in vitro investigations, high glucose (HG) (30 mM) conditions were applied to Neuro2a cells. At doses of 5 and 10 µM, PCN was examined for its role in SIRT1 and Nrf2 activation. HG-induced N2A cells, reactive oxygen exposure, mitochondrial superoxides and mitochondrial membrane potentials were restored by PCN exposure, and their neurite outgrowth was enhanced. Peroxisome proliferator activated receptor-gamma coactivator-1α (PGC-1α) directed mitochondrial biogenesis was induced by increased SIRT1 activation by piceatannol. SIRT1 activation also enhanced Nrf2-mediated antioxidant signalling. Our study results inferred that PCN administration can counteract the decline in mitochondrial function and antioxidant activity in diabetic rats and HG-exposed N2A cells by increasing the SIRT1 and Nrf2 activities.

Mechanism of Schwann cells in diabetic peripheral neuropathy: A review

Diabetic peripheral neuropathy (DPN) is the most common neuropathy in the world, mainly manifested as bilateral symmetry numbness, pain or paresthesia, with a high rate of disability and mortality. Schwann cells (SCs), derived from neural ridge cells, are the largest number of glial cells in the peripheral nervous system, and play an important role in DPN. Studies have found that SCs are closely related to the pathogenesis of DPN, such as oxidative stress, endoplasmic reticulum stress, inflammation, impaired neurotrophic support and dyslipidemia. This article reviews the mechanism of SCs in DPN.

Activation of 20-HETE Synthase Triggers Oxidative Injury and Peripheral Nerve Damage in Type 2 Diabetic Mice

Diabetic Peripheral Neuropathy (DPN), highly prevalent among patients with diabetes, is characterized by peripheral nerve dysfunction. Reactive Oxygen Species (ROS) overproduction has been suggested to orchestrate diabetic complications including DPN. Untargeted antioxidant therapy has exhibited limited efficacy, highlighting a critical need to explore ROS sources altered in a cell-specific manner in DPN. Cytochromes P450 (CYP) enzymes are prominent sources of ROS. Particularly, the 20-HETE synthase, CYP4A, is reported to mediate diabetes-induced renal, retinal, and cardiovascular injuries. This work investigates the role of CYP4A/20-HETE in DPN and their mechanisms of action. Non-obese type 2 Diabetic mice (MKR) were used and treated with a CYP4A-inhibitor (HET0016) or AMPK-activator (Metformin). Peripheral nerves of MKR mice reflect increased CYP4A and 20-HETE levels, concurrent with altered myelin proteins and sensorimotor deficits. This was associated with increased ROS production and altered Beclin-1 and LC3 protein levels, indicative of disrupted autophagic responses in tandem with AMPK inactivation. AMPK activation via Metformin restored nerve integrity, reduced ROS production, and regulated autophagy. Interestingly, similar outcomes were revealed upon HET0016 treatment whereby ROS production, autophagic responses, and AMPK signaling were normalized in diabetic mice. Altogether, the results highlight hyperglycemia-mediated oxidative injury in DPN through a novel CYP4A/20-HETE/AMPK pathological axis. PERSPECTIVE: To our knowledge, this is the first study to highlight the role of CYPs/20-HETE-induced oxidative injury in the pathogenesis of diabetic peripheral neuropathy. Targeting the identified pathological axis CYP4A/20-HETE/AMPK may be of clinical potential in predicting and alleviating peripheral nerve injury in patients with Type 2 Diabetes Mellitus.

Low-intensity low-frequency pulsed ultrasound ameliorates sciatic nerve dysfunction in a rat model of cisplatin-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy is a neurological complication that frequently occurs during chemotherapeutic intervention, resulting in damaged myelin sheath, motor weakness and/or sensory impairment. This study aims to investigate the therapeutic efficiency of low-intensity pulsed low-frequency ultrasound on cisplatin-induced peripheral neuropathy. Rats were randomly divided into five experimental groups as control, cisplatin administration, 10 mg/kg melatonin treatment after cisplatin administration, 1 MHz frequency 0.5 W/cm² pulsed ultrasound treatment after cisplatin administration and 1 MHz frequency 1.5 W/cm² pulsed ultrasound treatment after cisplatin administration. Chemical neuropathy was induced by the injection of 3 mg/kg/week of cisplatin (i.p.) for 5 weeks. Afterwards, melatonin and pulsed ultrasound treatments were applied for 15 consecutive days. Cisplatin administration resulted in a decrease in nociceptive pain perception and nerve conduction velocities together with a decrease in myelin thickness and diameters of axons and myelinated fibers, indicating a dysfunction and degeneration in sciatic nerves. In addition, cisplatin administration led to a decrease, in superoxide dismutase activity, and an increase in malondialdehyde and IL-1β levels together with an increase in caspase-3 protein expression levels and a decrease in Bcl-2 and Parkin levels. The ultrasound treatments resulted in an increase in nociceptive pain perception and sciatic nerve conduction; led to a decrease in oxidative stress and inflammation, restored nerve degeneration and regulated apoptosis and mitophagy. Taken together, low-intensity pulsed low-frequency ultrasound was efficient in restoring the alterations attributable to cisplatin-induced peripheral neuropathy, and warrants further investigations.

Muscone ameliorates diabetic peripheral neuropathy through activating AKT/mTOR signalling pathway

OBJECTIVES

Emerging evidence showed that muscone could improve chronic inflammation after myocardial infarction and protect alcohol-induced osteonecrosis of the femoral head. However, the function of muscone on diabetic peripheral neuropathy (DPN) is obscure.

METHODS

The neuronal Schwann cell RSC 96 cells were treated with 125 mmol/l glucose to simulate the cells in DPN. The RSC 96 cell viability was detected by cell counting kit-8. The RSC 96 cell cycle and apoptosis were determined by flow cytometry. The expression of marker proteins of apoptosis, autophagy and AKT/mTOR signalling pathway was assessed by Western blot.

KEY FINDINGS

We observed that after high glucose (HG) treatment, the number of cell apoptosis was increased, cell proliferation was decreased, as well as the expression of apoptosis-related proteins and autophagy-related proteins were changed. However, this phenomenon can be reversed by muscone. Meanwhile, the expression of phosphorylated AKT and mammalian target of rapamycin (mTOR) was down-regulated with HG treatment, while the expression quantity was up-regulated after disposed with muscone.

CONCLUSIONS

Our outcomes demonstrated that autophagy and apoptosis of RSC 96 cells induced by HG can be alleviated by muscone through modulating AKT/mTOR signalling pathway, suggesting that muscone might be a potential molecule with influence in connection to DPN.

STAT3 phosphorylation mediates high glucose-impaired cell autophagy in an HDAC1-dependent and -independent manner in Schwann cells of diabetic peripheral neuropathy

Schwann cells are the main supportive cells of the peripheral nerves. Schwann cells suffer inhibition of autophagy under hyperglycemia treatment in diabetic peripheral neuropathy (DPN). However, the exact mechanism is still not fully elucidated. We first observed the decrease of autophagy markers (LC3-II/LC3-I, P62) in the sciatic nerves of diabetic mice vs. normal mice, accompanied with the loss of myelinated nerve fibers and abnormal myelin sheath. In line with this, LC3-II/LC3-I and P62 were also significantly reduced in high glucose-treated rat Schwann cell 96 (RSC96) cells compared with normal glucose-treated cells. Furthermore, we found that trichostatin A [an inhibitor of histone deacetylase (HDAC)] evidently improved LC3-II/LC3-I in high glucose-treated RSC96 cells, without an effect on P62 expression. Again, HDAC1 and HDAC5 were revealed to be increased in RSC96 cells stimulated with high glucose. Inhibition of HDAC1 but not HDAC5 by small hairpin RNA vector enhanced LC3-II/LC3-I in high glucose-cultured RSC96 cells. In addition, LC3-II conversion regulators [autophagy-related protein (Atg)3, Atg5, and Atg7] were detected in high glucose-treated and HDAC1-knockdown RSC96 cells, and Atg3 was proven to be the key target of HDAC1. The presuppression of Atg3 offset the improvement of LC3-II/LC3-I resulting from HDAC1 inhibition in high glucose-treated RSC96 cells. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway was activated in RSC96 cells treated with high glucose, which was indicated by increased STAT3 phosphorylation. Blocking STAT3 phosphorylation by chemical inhibitor AG490 induced HDAC1 down-regulation followed by increases in Atg3 and LC3-II/LC3-I. Interestingly, we also found that AG490 treatment enhanced P62 expression in high glucose-stimulated RSC96 cells. Taken together, our findings demonstrate that hyperglycemia inhibits LC3-II/LC3-I in an HDAC1-Atg3-dependent manner and decreases P62 expression in an HDAC-independent manner via the JAK-STAT3 signaling pathway in the Schwann cells of DPN.-Du, W., Wang, N., Li, F. Jia, K., An, J., Liu, Y., Wang, Y., Zhu, L., Zhao, S. Hao, J. STAT3 phosphorylation mediates high glucose-impaired cell autophagy in an HDAC1-dependent and -independent manner in Schwann cells of diabetic 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.

Deregulation of autophagy under hyperglycemic conditions is dependent on increased lysine 63 ubiquitination: a candidate mechanism in the progression of diabetic nephropathy

Diabetic nephropathy patients (DN) are characterized by increased lysine63 ubiquitination (Lys63-Ub) at the tubular level. Autophagy is deregulated under diabetic conditions, even though the molecular mechanisms and the consequences of this alteration need to be elucidated. The aim of this study was to investigate the link between Lys63-Ub and autophagy in DN and the involvement of these two processes in tubular cell fate. Immunohistochemistry of beclin-1, LC3, and p62 on kidney biopsies highlighted increased protein expression of all these autophagic factors at the tubular level in DN compared to other nephritis. Transmission electron microscopy confirmed the presence of diffuse vacuolization and autophago(lyso)somal structures in proximal tubular cells in DN. Accumulation of Lys63-Ub proteins in DN increased in accordance with the tubular damage and was associated to increased LC3 expression both in vivo and in vitro. Hyperglycemia (HG) induced LC3 and p62 protein expression in HK2 cells together with Lys63-ubiquitinated proteins, and the inhibition of HG-induced Lys63-Ub by NSC697923 inhibitor, significantly reduced both LC3 and p62 expression. Moreover, in DN, those tubules expressing LC3 showed increased caspase-3 expression, supporting the hypothesis that deregulated autophagy induces apoptosis of tubular cells. In vitro, we confirmed a tight association between impaired autophagy, Lys63-Ub, and apoptosis since Lys63-Ub inhibition by NSC697923 abrogated HG-induced cell death and LC3 silencing also blocked hyperglycemia-induced caspase-3 activation. Our data suggested that prolonged hyperglycemia in diabetic patients can impair autophagy as a consequence of Lys63-Ub protein accumulation, thus promoting intracellular autophagic vesicles increase, finally leading to tubular cell death in DN.

KEY MESSAGES

In vivo autophagy is deregulated in diabetic patients with renal disease (DN). Accumulation of Lys63 ubiquitinated proteins is associated to autophagy deregulation. Accumulation of Lys63 ubiquitinated proteins correlated with apoptosis activation. Lys63 ubiquitination inhibition abrogated hyperglycemia-induced autophagy and apoptosis.

Longitudinal study of neuropathy, microangiopathy, and autophagy in sural nerve: Implications for diabetic neuropathy

OBJECTIVES

The progression and pathophysiology of neuropathy in impaired glucose tolerance (IGT) and type 2 diabetes (T2DM) is poorly understood, especially in relation to autophagy. This study was designed to assess whether the presence of autophagy-related structures was associated with sural nerve fiber pathology, and to investigate if endoneurial capillary pathology could predict the development of T2DM and neuropathy.

PATIENTS AND METHODS

Sural nerve physiology and ultrastructural morphology were studied at baseline and 11 years later in subjects with normal glucose tolerance (NGT), IGT, and T2DM.

RESULTS

Subjects with T2DM had significantly lower sural nerve amplitude compared to subjects with NGT and IGT at baseline. Myelinated and unmyelinated fiber, endoneurial capillary morphology, and the presence and distribution of autophagy structures were comparable between groups at baseline, except for a smaller myelinated axon diameter in subjects with T2DM and IGT compared to NGT. The baseline values of the subjects with NGT and IGT who converted to T2DM 11 years later demonstrated healthy smaller endoneurial capillary and higher g-ratio versus subjects who remained NGT. At follow-up, T2DM showed a reduction in nerve conduction, amplitude, myelinated fiber density, unmyelinated axon diameter, and autophagy structures in myelinated axons. Endothelial cell area and total diffusion barrier was increased versus baseline.

CONCLUSIONS

We conclude that small healthy endoneurial capillary may presage the development of T2DM and neuropathy. Autophagy occurs in human sural nerves and can be affected by T2DM. Further studies are warranted to understand the role of autophagy in diabetic neuropathy.

HSPB8 Promotes the Fusion of Autophagosome and Lysosome during Autophagy in Diabetic Neurons

Although autophagy has been proposed to play an emerging role in diabetic neuropathy, autophagy and its possible role remains unclear. Moreover, only few studies about diabetes have explored the autophagy mediated by heat shock protein beta-8 (HSPB8) and Bcl-2 associated athanogene 3 (BAG3). In the present study, we examined the autophagy induced by high glucose levels in an in vivo rat model of diabetes induced by streptozotocin (STZ) and an in vitro model of retinal ganglion cell-5 (RGC5) cells under high glucose conditions. In the spinal cord tissues of the STZ-induced diabetic rats, the levels of light chain 3 (LC3) and Beclin-1-marked autophagy rose with increasing HSPB8 and BAG3 levels. By confocal immunofluorescence, HSPB8 and LC3 were observed to be co-localized in the spinal cord tissues. In the RGC5 cells, high-glucose stimulation upregulated the expression of LC3-Ⅱ, Beclin-1, and HSPB8 in a dose-dependent manner. When the RGC5 cells were subjected to high-glucose conditions, HSPB8 overexpression, along with upregulated LC3-Ⅱ and Beclin-1 expression, increased the autophagic rate, whereas siRNA-silenced HSPB8 decreased the autophagic rate. Furthermore, in GFP-mRFP-LC3 probe experiments, HSPB8 overexpression promoted autophagosome-lysosome fusion, whereas HSPB8 silencing disrupted this process. In the cells treated with HSPB8 and siRNA, the fusion was impaired, as indicated by the elevated p62 expression. HSPB8 overexpression can partly rescue the blocking of the autophagy flux with chloroquine through the reduction of p62 expression level. Our study demonstrated that HSPB8 is involved in the high glucose-induced autophagy under the in vivo and in vitro conditions and critically participated in the autophagosome-lysosome fusion during the autophagy flux.

Protective Effect of Psoralea corylifolia L. Seed Extract against Palmitate-Induced Neuronal Apoptosis in PC12 Cells

The extract of Psoralea corylifolia seeds (PCE) has been widely used as a herbal medicine because of its beneficial effect on human health. In this study, we investigated the protective effects and molecular mechanisms of PCE on palmitate- (PA-) induced toxicity in PC12 cells, a neuron-like cell line. PCE significantly increased cell viability in PA-treated PC12 cells and showed antiapoptotic effects, as evidenced by decreased expression of cleaved caspase-3, cleaved poly(ADP-ribose) polymerase, and bax protein as well as increased expression of bcl-2 protein. In addition, PCE treatment reduced PA-induced reactive oxygen species production and upregulated mRNA levels of antioxidant genes such as nuclear factor (erythroid-derived 2)-like 2 and heme oxygenase 1. Moreover, PCE treatment recovered the expression of autophagy marker genes such as beclin-1 and p62, which was decreased by PA treatment. Treatment with isopsoralen, one of the major components of PCE extract, also recovered the expression of autophagy marker genes and reduced PA-induced apoptosis. In conclusion, PCE exerts protective effects against lipotoxicity via its antioxidant function, and this effect is mediated by activation of autophagy. PCE might be a potential pharmacological agent to protect against neuronal cell injury caused by oxidative stress or lipotoxicity.

Adenosine Monophosphate-Activated Protein Kinase Abates Hyperglycaemia-Induced Neuronal Injury in Experimental Models of Diabetic Neuropathy: Effects on Mitochondrial Biogenesis, Autophagy and Neuroinflammation

Impaired adenosine monophosphate kinase (AMPK) signalling under hyperglycaemic conditions is known to cause mitochondrial dysfunction in diabetic sensory neurons. Facilitation of AMPK signalling is previously reported to ameliorate inflammation and induce autophagic response in various complications related to diabetes. The present study assesses the role of AMPK activation on mitochondrial biogenesis, autophagy and neuroinflammation in experimental diabetic neuropathy (DN) using an AMPK activator (A769662). A769662 (15 and 30 mg/kg, i.p) was administered to Sprague-Dawley rats (250-270 g) for 2 weeks after 6 weeks of streptozotocin (STZ) injection (55 mg/kg, i.p.). Behavioural parameters (mechanical/thermal hyperalgesia) and functional characteristics (motor/sensory nerve conduction velocities (MNCV and SNCV) and sciatic nerve blood flow (NBF)) were assessed. For in vitro studies, Neuro2a (N2A) cells were incubated with 25 mM glucose to simulate high glucose condition and then studied for mitochondrial dysfunction and protein expression changes. STZ administration resulted in significant hyperglycaemia (>250 mg/dl) in rats. A769662 treatment significantly improved mechanical/thermal hyperalgesia threshold and enhanced MNCV, SNCV and NBF in diabetic animals. A769662 exposure normalised the mitochondrial superoxide production, membrane depolarisation and markedly increased neurite outgrowth of N2A cells. Further, AMPK activation also abolished the NF-κB-mediated neuroinflammation. A769662 treatment increased Thr-172 phosphorylation of AMPK results in stimulated PGC-1α-directed mitochondrial biogenesis and autophagy induction. Our study supports that compromised AMPK signalling in hyperglycaemic conditions causes defective mitochondrial biogenesis ultimately leading to neuronal dysfunction and associated deficits in DN and activation of AMPK can be developed as an attractive therapeutic strategy for the management of DN.