Effect of Inhibition of Angiotensin-Converting Enzyme and/or Neutral Endopeptidase on Neuropathy in High-Fat-Fed C57Bl/6J Mice

Reduced hepatic bradykinin degradation accounts for cold-induced BAT thermogenesis and WAT browning in male mice

An important role for liver in the regulation of adipose tissue thermogenesis upon cold exposure has been suggested; however, the underlying mechanisms remain incompletely defined. Here, we identify elevated serum bradykinin levels in response to acute cold exposure in male mice. A bolus of anti-bradykinin antibodies reduces body temperature during acute cold exposure, whereas bradykinin has the opposite effect. We demonstrate that bradykinin induces brown adipose tissue thermogenesis and white adipose tissue browning, and bradykinin increases uncoupling protein 1 (UCP1) expression in adipose tissue. The bradykinin B2 receptor (B2R), adrenergic signaling and nitric oxide signaling are involved in regulating bradykinin-increased UCP1 expression. Moreover, acute cold exposure inhibits hepatic prolyl endopeptidase (PREP) activity, causing reduced liver bradykinin degradation and increased serum bradykinin levels. Finally, by blocking the breakdown of bradykinin, angiotensin-converting enzyme inhibitors (ACEIs) increase serum bradykinin levels and induce brown adipose tissue thermogenesis and white adipose tissue browning via B2R. Collectively, our data provide new insights into the mechanisms underlying organ crosstalk in whole-body physiology control during cold exposure and also suggest bradykinin as a possible anti-obesity target.

Treatment for Diabetic Peripheral Neuropathy: What have we Learned from Animal Models?

INTRODUCTION

Animal models have been widely used to investigate the etiology and potential treatments for diabetic peripheral neuropathy. What we have learned from these studies and the extent to which this information has been adapted for the human condition will be the subject of this review article.

METHODS

A comprehensive search of the PubMed database was performed, and relevant articles on the topic were included in this review.

RESULTS

Extensive study of diabetic animal models has shown that the etiology of diabetic peripheral neuropathy is complex, with multiple mechanisms affecting neurons, Schwann cells, and the microvasculature, which contribute to the phenotypic nature of this most common complication of diabetes. Moreover, animal studies have demonstrated that the mechanisms related to peripheral neuropathy occurring in type 1 and type 2 diabetes are likely different, with hyperglycemia being the primary factor for neuropathology in type 1 diabetes, which contributes to a lesser extent in type 2 diabetes, whereas insulin resistance, hyperlipidemia, and other factors may have a greater role. Two of the earliest mechanisms described from animal studies as a cause for diabetic peripheral neuropathy were the activation of the aldose reductase pathway and increased non-enzymatic glycation. However, continuing research has identified numerous other potential factors that may contribute to diabetic peripheral neuropathy, including oxidative and inflammatory stress, dysregulation of protein kinase C and hexosamine pathways, and decreased neurotrophic support. In addition, recent studies have demonstrated that peripheral neuropathy-like symptoms are present in animal models, representing pre-diabetes in the absence of hyperglycemia.

CONCLUSION

This complexity complicates the successful treatment of diabetic peripheral neuropathy, and results in the poor outcome of translating successful treatments from animal studies to human clinical trials.

Cutaneous innervation in impaired diabetic wound healing

Type 2 diabetes is associated with several potential comorbidities, among them impaired wound healing, chronic ulcerations, and the requirement for lower extremity amputation. Disease-associated abnormal cellular responses, infection, immunological and microvascular dysfunction, and peripheral neuropathy are implicated in the pathogenesis of the wound healing impairment and the diabetic foot ulcer. The skin houses a dense network of sensory nerve afferents and nerve-derived modulators, which communicate with epidermal keratinocytes and dermal fibroblasts bidirectionally to effect normal wound healing after trauma. However, the mechanisms through which cutaneous innervation modulates wound healing are poorly understood, especially in humans. Better understanding of these mechanisms may provide the basis for targeted treatments for chronic diabetic wounds. This review provides an overview of wound healing pathophysiology with a focus on neural involvement in normal and diabetic wound healing, as well as future therapeutic perspectives to address the unmet needs of diabetic patients with chronic wounds.

Diet-Induced Rodent Models of Diabetic Peripheral Neuropathy, Retinopathy and Nephropathy

Unhealthy dietary habits are major modifiable risk factors for the development of type 2 diabetes mellitus, a metabolic disease with increasing prevalence and serious consequences. Microvascular complications of diabetes, namely diabetic peripheral neuropathy (DPN), retinopathy (DR), and nephropathy (DN), are associated with high morbidity rates and a heavy social and economic burden. Currently, available therapeutic options to counter the evolution of diabetic microvascular complications are clearly insufficient, which strongly recommends further research. Animal models are essential tools to dissect the molecular mechanisms underlying disease progression, to unravel new therapeutic targets, as well as to evaluate the efficacy of new drugs and/or novel therapeutic approaches. However, choosing the best animal model is challenging due to the large number of factors that need to be considered. This is particularly relevant for models induced by dietary modifications, which vary markedly in terms of macronutrient composition. In this article, we revisit the rodent models of diet-induced DPN, DR, and DN, critically comparing the main features of these microvascular complications in humans and the criteria for their diagnosis with the parameters that have been used in preclinical research using rodent models, considering the possible need for factors which can accelerate or aggravate these conditions.

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.

Neprilysin inhibition: a new therapeutic option for type 2 diabetes?

Neprilysin is a widely expressed peptidase with broad substrate specificity that preferentially hydrolyses oligopeptide substrates, many of which regulate the cardiovascular, nervous and immune systems. Emerging evidence suggests that neprilysin also hydrolyses peptides that play an important role in glucose metabolism. In recent studies in humans, a dual angiotensin receptor-neprilysin inhibitor (ARNi) improved glycaemic control and insulin sensitivity in individuals with type 2 diabetes and/or obesity. Moreover, preclinical studies have also reported that neprilysin inhibition, alone or in combination with renin-angiotensin system blockers, elicits beneficial effects on glucose homeostasis. Since neprilysin inhibitors have been approved for the treatment of heart failure, their repurposing for treating type 2 diabetes would provide a novel therapeutic strategy. In this review, we evaluate existing evidence from preclinical and clinical studies in which neprilysin is deleted/inhibited, we highlight potential mechanisms underlying the beneficial glycaemic effects of neprilysin inhibition, and discuss possible deleterious effects that may limit the efficacy and safety of neprilysin inhibitors in the clinic. We also review the favourable impact neprilysin inhibition can have on diabetic complications, in addition to glucose control. Finally, we conclude that neprilysin inhibitors may be a useful therapeutic option for treating type 2 diabetes; however, their combination with angiotensin II receptor blockers is needed to circumvent deleterious consequences of neprilysin inhibition alone.

Diabetic neuropathy research: from mouse models to targets for treatment

Diabetic neuropathy is one of the most serious complications of diabetes, and its increase shows no sign of stopping. Furthermore, current clinical treatments do not yet approach the best effectiveness. Thus, the development of better strategies for treating diabetic neuropathy is an urgent matter. In this review, we first discuss the advantages and disadvantages of some major mouse models of diabetic neuropathy and then address the targets for mechanism-based treatment that have been studied. We also introduce our studies on each part. Using stem cells as a source of neurotrophic factors to target extrinsic factors of diabetic neuropathy, we found that they present a promising treatment.

Angiotensin-(1-7) contributes to insulin-sensitizing effects of angiotensin-converting enzyme inhibition in obese mice

Angiotensin-converting enzyme (ACE) inhibitors reduce body weight, lower blood pressure (BP), and improve insulin sensitivity in animal models of cardiometabolic syndrome. These effects are generally attributed to reduced angiotensin (ANG) II formation; however, these therapies also increase levels of ANG-(1-7), a beneficial hormone opposing ANG II actions. We hypothesized that this ANG-(1-7) generation contributes to the insulin-sensitizing effects of ACE inhibition in obese mice. Adult male C57BL/6J mice were placed on a 60% high-fat diet for 11 wk. During the last 3 wk of diet, mice received normal water or water containing the ACE inhibitor captopril (50 mg/l) as well as the ANG-(1-7) mas receptor antagonist A779 (400 or 800 ng·kg^-1·min^-1) or saline vehicle via subcutaneous osmotic minipumps. At the end of treatment, arterial BP was measured, and hyperinsulinemic-euglycemic clamps were performed in conscious obese mice receiving vehicle, captopril, captopril plus A779, or A779 ( n = 6-13/group). Captopril reduced body weight (28 ± 2 vs. 41 ± 2 g saline; P = 0.001), lowered systolic BP (109 ± 6 vs. 144 ± 7 mmHg saline; P = 0.041), and improved whole-body insulin sensitivity (steady-state glucose infusion rate: 31 ± 4 vs. 16 ± 2 mg·kg^-1·min^-1 saline; P = 0.001) in obese mice. A779 attenuated captopril-mediated improvements in insulin sensitivity (23 ± 2 mg·kg^-1·min^-1; P = 0.042), with no effect on body weight (32 ± 2 g; P = 0.441) or BP (111 ± 7 mmHg; P = 0.788). There was no effect of A779 alone on cardiometabolic outcomes. These data suggest that insulin-sensitizing effects of ACE inhibition are in part due to activation of ANG-(1-7)/ mas receptor pathways and provide new insight into mechanisms underlying the positive metabolic effects of these therapies.

Inhibitory effect of Ligustrum vulgare leaf extract on the development of neuropathic pain in a streptozotocin-induced rat model of diabetes

BACKGROUND

Chronic hyperalgesia and allodynia associated with progressive damage of peripheral neurons are the most prevalent complications of diabetes mellitus. Plants belonging to the family of Oleaceae were traditionally used in folk medicine for the management of diabetes.

HYPOTHESIS/PURPOSE

The aim of this study was to investigate whether an aqueous extract from the leaves of Ligustrum vulgare (common privet) could be useful to target neuropathic pain in a rat streptozotocin (STZ) model of diabetes.

METHODS

The chemical composition of the aqueous extract from privet leaf was characterized with the UHPLC-DAD-MS method and the analytical quantification of its constituents was performed with HPLC-DAD. Mechanical hyperalgesia and allodynia were evaluated with the Randall-Selitto and von Frey tests.

RESULTS

Our investigation revealed the presence of secoiridoids: oleacein (23.48 ± 0.87 mg/g), oleocanthal (8.44 ± 0.08 mg/g), oleuropein (1.50 ± 0.01 mg/g), as well as phenylpropanoids: echinacoside (6.46 ± 0.07 mg/g), verbascoside (4.03 ± 0.04 mg/g) and p-coumaroyl glucarates in the dried aqueous extract of privet leaves. Behavioral data indicated that chronic intraperitoneal administration of the extract (50-200 mg/kg) for 21 days resulted in a decrease in diabetes-induced hyperalgesia and allodynia. Blood glucose levels remained unaltered, while body weight and water intake decreased significantly.

CONCLUSION

The aqueous privet leaf extract could serve useful in facilitating treatment of painful diabetic neuropathy. Additionally, the study showed that the antihyperalgesic activity of Ligustrum vulgare leaf extract is not likely related to its antihyperglycemic properties.

Improved glycaemia in high-fat-fed neprilysin-deficient mice is associated with reduced DPP-4 activity and increased active GLP-1 levels

AIM/HYPOTHESIS

Neprilysin, a widely expressed peptidase, is upregulated in metabolically altered states such as obesity and type 2 diabetes. Like dipeptidyl peptidase-4 (DPP-4), neprilysin can degrade and inactivate the insulinotropic peptide glucagon-like peptide-1 (GLP-1). Thus, we investigated whether neprilysin deficiency enhances active GLP-1 levels and improves glycaemia in a mouse model of high fat feeding.

METHODS

Nep ^+/+ and Nep ^-/- mice were fed a 60% fat diet for 16 weeks, after which active GLP-1 and DPP-4 activity levels were measured, as were glucose, insulin and C-peptide levels during an OGTT. Insulin sensitivity was assessed using an insulin tolerance test.

RESULTS

High-fat-fed Nep ^-/- mice exhibited elevated active GLP-1 levels (5.8 ± 1.1 vs 3.5 ± 0.8 pmol/l, p < 0.05) in association with improved glucose tolerance, insulin sensitivity and beta cell function compared with high-fat-fed Nep ^+/+ mice. In addition, plasma DPP-4 activity was lower in high-fat-fed Nep ^-/- mice (7.4 ± 1.0 vs 10.7 ± 1.3 nmol ml^-1 min^-1, p < 0.05). No difference in insulin:C-peptide ratio was observed between Nep ^-/- and Nep ^+/+ mice, suggesting that improved glycaemia does not result from changes in insulin clearance.

CONCLUSIONS/INTERPRETATION

Under conditions of increased dietary fat, an improved glycaemic status in neprilysin-deficient mice is associated with elevated active GLP-1 levels, reduced plasma DPP-4 activity and improved beta cell function. Thus, neprilysin inhibition may be a novel treatment strategy for type 2 diabetes.

Loss of Sodium/Hydrogen Exchanger NHA2 Exacerbates Obesity- and Aging-Induced Glucose Intolerance in Mice

We previously demonstrated that the sodium/hydrogen exchanger NHA2, also known as NHEDC2 or SLC9B2, is critical for insulin secretion by β–cells. To gain more insights into the role of NHA2 on systemic glucose homeostasis, we studied the impact of loss of NHA2 during the physiological aging process and in the setting of diet-induced obesity. While glucose tolerance was normal at 2 months of age, NHA2 KO mice displayed a significant glucose intolerance at 5 and 12 months of age, respectively. An obesogenic high fat diet further exacerbated the glucose intolerance of NHA2 KO mice. Insulin levels remained similar in NHA2 KO and WT mice during aging and high fat diet, but fasting insulin/glucose ratios were significantly lower in NHA2 KO mice. Peripheral insulin sensitivity, measured by insulin tolerance tests and hyperinsulinemic euglycemic clamps, was unaffected by loss of NHA2 during aging and high fat diet. High fat diet diminished insulin secretion capacity in both WT and NHA2 KO islets and reduced expression of NHA2 in WT islets. In contrast, aging was characterized by a gradual increase of NHA2 expression in islets, paralleled by an increasing difference in insulin secretion between WT and NHA2 KO islets. In summary, our results demonstrate that loss of the sodium/hydrogen exchanger NHA2 exacerbates obesity- and aging-induced glucose intolerance in mice. Furthermore, our data reveal a close link between NHA2 expression and insulin secretion capacity in islets.

Effect of Inhibition or Deletion of Neutral Endopeptidase on Neuropathic Endpoints in High Fat Fed/Low Dose Streptozotocin-Treated Mice

Previously we demonstrated that a vasopeptidase inhibitor of angiotensin converting enzyme and neutral endopeptidase (NEP), a protease that degrades vaso- and neuro-active peptides, improves neural function in diabetic rodent models. The purpose of this study was to determine whether inhibition or deletion of NEP provides protection from neuropathy caused by diabetes with an emphasis on morphology of corneal nerves as a primary endpoint. Diabetes, modeling type 2, was induced in C57Bl/6J and NEP deficient mice through a combination of a high fat diet and streptozotocin. To inhibit NEP activity, diabetic C57Bl/6J mice were treated with candoxatril using a prevention or intervention protocol. Twelve weeks after the induction of diabetes in C57Bl/6J mice, the existence of diabetic neuropathy was determined through multiple endpoints including decrease in corneal nerves in the epithelium and sub-epithelium layer. Treatment of diabetic C57Bl/6J mice with candoxatril improved diabetic peripheral neuropathy and protected corneal nerve morphology with the prevention protocol being more efficacious than intervention. Unlike C57Bl/6J, mice deficient in NEP were protected from the development of neuropathologic alterations and loss of corneal nerves upon induction of diabetes. These studies suggest that NEP contributes to the development of diabetic neuropathy and may be a treatable target.

Mouse models of diabetic neuropathy

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes and is associated with significant morbidity and mortality. DPN is characterized by progressive, distal-to-proximal degeneration of peripheral nerves that leads to pain, weakness, and eventual loss of sensation. The mechanisms underlying DPN pathogenesis are uncertain, and other than tight glycemic control in type 1 patients, there is no effective treatment. Mouse models of type 1 (T1DM) and type 2 diabetes (T2DM) are critical to improving our understanding of DPN pathophysiology and developing novel treatment strategies. In this review, we discuss the most widely used T1DM and T2DM mouse models for DPN research, with emphasis on the main neurologic phenotype of each model. We also discuss important considerations for selecting appropriate models for T1DM and T2DM DPN studies and describe the promise of novel emerging diabetic mouse models for DPN research. The development, characterization, and comprehensive neurologic phenotyping of clinically relevant mouse models for T1DM and T2DM will provide valuable resources for future studies examining DPN pathogenesis and novel therapeutic strategies.