Effect of diet-induced obesity or type 1 or type 2 diabetes on corneal nerves and peripheral neuropathy in C57Bl/6J mice

Diabetic neuropathy: cutting-edge research and future directions

Diabetic neuropathy (DN) is a prevalent and debilitating complication of diabetes mellitus, significantly impacting patient quality of life and contributing to morbidity and mortality. Affecting approximately 50% of patients with diabetes, DN is predominantly characterized by distal symmetric polyneuropathy, leading to sensory loss, pain, and motor dysfunction, often resulting in diabetic foot ulcers and lower-limb amputations. The pathogenesis of DN is multifaceted, involving hyperglycemia, dyslipidemia, oxidative stress, mitochondrial dysfunction, and inflammation, which collectively damage peripheral nerves. Despite extensive research, disease-modifying treatments remain elusive, with current management primarily focusing on symptom control. This review explores the complex mechanisms underlying DN and highlights recent advances in diagnostic and therapeutic strategies. Emerging insights into the molecular and cellular pathways have unveiled potential targets for intervention, including neuroprotective agents, gene and stem cell therapies, and innovative pharmacological approaches. Additionally, novel diagnostic tools, such as corneal confocal microscopy and biomarker-based tests, have improved early detection and intervention. Lifestyle modifications and multidisciplinary care strategies can enhance patient outcomes. While significant progress has been made, further research is required to develop therapies that can effectively halt or reverse disease progression, ultimately improving the lives of individuals with DN. This review provides a comprehensive overview of current understanding and future directions in DN research and management.

Obesity negatively impacts corneal nerves in patients with diabetes mellitus

BACKGROUND

To investigate the relationship between obesity and corneal nerve metrics in patients with type 2 diabetes mellitus (DM).

METHODS

This cross-sectional study included a total of 385 healthy controls and 663 patients with DM. Metrics for corneal nerve and epithelial cells were evaluated using in-vivo confocal microscopy (IVCM). Corneal nerve and epithelial cell parameters were quantified and compared between patients with and without obesity and across six different body mass index (BMI) categories. Multivariable regression analyses were conducted to determine the association between corneal nerve metrics and BMI in patients with DM.

RESULTS

Of the DM participants, 162 (25.4%) had obesity. Compared to the non-obese group, patients with obesity had significantly lower corneal nerve fiber density (CNFD, P < 0.0001), corneal nerve fiber length (CNFL, P = 0.002), and corneal nerve branch density (CNBD, P = 0.005). Analyses across different BMI categories showed a progressive decline in corneal nerve parameters including CNFD (P < 0.0001), CNFL (P < 0.0001), CNBD (P < 0.0001), corneal nerve fiber total branch density (P = 0.003), corneal nerve fiber area (P = 0.04), and corneal nerve fiber fractal dimension (P = 0.02) with increasing obesity severity. Multivariable regression analyses demonstrated that lower CNFD (β: - 0.21, 95% CI: - 0.29 to - 0.13, P < 0.0001), shorter CNFL (β: - 0.12, 95% CI: - 0.17 to - 0.07, P < 0.0001), and lower CNBD (β: - 0.17, 95% CI: - 0.30 to - 0.04, P = 0.01) were significantly associated with BMI after adjusting for confounders. There were no significant differences in the corneal epithelial parameters between the obese and non-obese groups.

CONCLUSIONS

General obesity, specifically higher BMI, adversely affects corneal nerve health in individuals with DM. Evaluation of corneal nerves and resultant keratopathy should be considered in patients with DM and concomitant obesity.

Green Tea Intake Reduces High-Fat Diet-Induced Sensory Neuropathy in Mice by Upregulating the Antioxidant Defense System in the Spinal Cord

One of the most common complications of obesity is peripheral nerve damage, which progresses to sensory neuropathy. Green tea (GT) intake has been associated with weight loss and metabolic biomarkers modulation due to its antioxidant properties. The present work characterized the effects of GT in high-fat diet (HFD)-induced neuropathy and investigated the mechanisms involved. C57BL/6J male mice were fed an HFD or control diet, associated with GT or vehicle intake for 16 weeks. Weight, blood glucose, and nociceptive thresholds were assessed. Morphological and morphometric analyses of the sciatic nerves were performed. Activation of the cellular antioxidant system in the spinal cord was assessed by real-time PCR. GT intake reduced weight gain, hyperglycemia, and the development of sensory neuropathy. Furthermore, in HFD-fed mice that consumed GT, the morphology of the sciatic nerve was preserved. RT-qPCR analysis showed that HFD-fed mice ingesting GT had higher spinal levels of superoxide dismutase, catalase, glutathione peroxidase, and nuclear factor erythroid 2-related factor 2 (NRF2) mRNA compared to the HFD-fed mice ingesting vehicle, suggesting that the endogenous antioxidant system was more activated in response to GT consumption. In conclusion, the data suggest that GT intake reduces HFD-induced neuropathy, probably by upregulating antioxidant gene expression.

Novel three-dimensional analysis method for accurate evaluation of cutaneous small sensory nerve fibers in mice

Intraepidermal nerve fiber (IENF) density is commonly evaluated to diagnose peripheral neuropathy. However, conventional two-dimensional (2D) analysis using rodent models shows high interstudy variability. Three-dimensional (3D) IENF analysis has been proposed for human skin biopsies because the spatial location of each nerve can be easily determined. However, no studies have compared 2D and 3D analyses of mouse cutaneous nerve fibers under the same conditions. We aimed to establish a more accurate analysis method for mouse cutaneous nerve fibers. We used the glabrous plantar metatarsal skin of male C57BL/6J mice. The middle area of the plantar skin was used for 2D and 3D analyses, and the marginal area was also investigated in the 3D analysis. Tissue transparency, nerve fiber-specific antibodies, confocal microscopy, and IMARIS software were used for the 3D analysis. The 3D analysis clearly defined branching points and continuity, allowing accurate IENF density measurement. Conversely, the 2D analysis could not accurately determine IENF density because it could not detect the continuity of the nerve from the dermis to epidermis. Thus, the actual IENF density from the 3D analysis was significantly less than that from the 2D analysis. In addition, the density and length of IENFs in the middle area were significantly higher than those in the marginal area. This 3D approach enables the precise capture of IENF trajectories with various parameters, establishing a standard method for evaluating peripheral neuropathy models. Furthermore, our findings indicate that comparative studies aiming to analyze mouse IENF need to consider the site of skin sampling.

Skin keratinocyte-derived SIRT1 and BDNF modulate mechanical allodynia in mouse models of diabetic neuropathy

Diabetic neuropathy is a debilitating disorder characterized by spontaneous and mechanical allodynia. The role of skin mechanoreceptors in the development of mechanical allodynia is unclear. We discovered that mice with diabetic neuropathy had decreased sirtuin 1 (SIRT1) deacetylase activity in foot skin, leading to reduced expression of brain-derived neurotrophic factor (BDNF) and subsequent loss of innervation in Meissner corpuscles, a mechanoreceptor expressing the BDNF receptor TrkB. When SIRT1 was depleted from skin, the mechanical allodynia worsened in diabetic neuropathy mice, likely due to retrograde degeneration of the Meissner-corpuscle innervating Aβ axons and aberrant formation of Meissner corpuscles which may have increased the mechanosensitivity. The same phenomenon was also noted in skin-keratinocyte specific BDNF knockout mice. Furthermore, overexpression of SIRT1 in skin induced Meissner corpuscle reinnervation and regeneration, resulting in significant improvement of diabetic mechanical allodynia. Overall, the findings suggested that skin-derived SIRT1 and BDNF function in the same pathway in skin sensory apparatus regeneration and highlighted the potential of developing topical SIRT1-activating compounds as a novel treatment for diabetic mechanical allodynia.

Correlation between thyroid hormone sensitivity and diabetic peripheral neuropathy in euthyroid patients with type 2 diabetes mellitus

Previous studies have revealed that thyroid hormone (TH) levels are associated with the risk of diabetic peripheral neuropathy (DPN) in euthyroid patients with type 2 diabetes mellitus (T2DM). However, the relationship between TH sensitivity, a complementary method for assessing thyroid function, and DPN remains unclear. This study aimed to investigate the correlation between DPN and TH sensitivity in euthyroid patients with T2DM. Exactly 708 euthyroid adults with T2DM were retrospectively enrolled. The FT3/FT4 ratio was used to estimate peripheral TH sensitivity. Central TH sensitivity was assessed using the Thyrotroph T4 Resistance Index (TT4RI), Thyroid-Stimulating Hormone Index (TSHI), Thyroid Feedback Quantile-based Index (TFQI), and Parametric TFQI (PTFQI). DPN was assessed using neurologic symptoms, signs, and nerve conduction velocity tests. The relationship between DPN and TH sensitivity was examined using logistic regression analysis. We observed that an elevated FT3/FT4 ratio was associated with DPN (OR = 1.36, 95%CI: 1.13-1.63, p = 0.0012). For each standard deviation (SD) increase in the TT4RI, TSHI, TFQI, and PTFQI, the OR of DPN was 0.80 (95%CI: 0.68-0.94, p = 0.0078), 0.72 (95%CI: 0.60-0.86, p = 0.0002), 0.69 (95%CI: 0.58-0.83, p < 0.0001), and 0.69 (95%CI: 0.58-0.82, p < 0.0001), respectively. These results suggested that reduced central and peripheral TH sensitivity is associated with a decreased risk of developing DPN.

Skin keratinocyte-derived SIRT1 and BDNF modulate mechanical allodynia in mouse models of diabetic neuropathy

Diabetic neuropathy is a debilitating disorder characterized by spontaneous and mechanical pain. The role of skin mechanoreceptors in the development of mechanical pain (allodynia) is unclear. We discovered that mice with diabetic neuropathy had decreased sirtuin 1 (SIRT1) deacetylase activity in foot skin, leading to reduced expression of brain-derived neurotrophic factor (BDNF) and subsequent loss of innervation in Meissner corpuscles, a mechanoreceptor expressing the BDNF receptor TrkB. When SIRT1 was depleted from skin, the mechanical allodynia worsened in diabetic neuropathy mice, likely due to retrograde degeneration of the Meissner-corpuscle innervating Aβ axons and aberrant formation of Meissner corpuscles which may have increased the mechanosensitivity. The same phenomenon was also noted in skin BDNF knockout mice. Furthermore, overexpression of SIRT1 in skin induced Meissner corpuscle reinnervation and regeneration, resulting in significant improvement of diabetic mechanical allodynia. Overall, the findings suggested that skin-derived SIRT1 and BDNF function in the same pathway in skin sensory apparatus regeneration and highlighted the potential of developing topical SIRT1-activating compounds as a novel treatment for diabetic mechanical allodynia.

The Role of Insulin-like Growth Factor (IGF) System in the Corneal Epithelium Homeostasis-From Limbal Epithelial Stem Cells to Therapeutic Applications

The corneal epithelium, comprising three layers of cells, represents the outermost portion of the eye and functions as a vital protective barrier while concurrently serving as a critical refractive structure. Maintaining its homeostasis involves a complex regenerative process facilitated by the functions of the lacrimal gland, tear film, and corneal nerves. Crucially, limbal epithelial stem cells located in the limbus (transitional zone between the cornea and the conjunctiva) are instrumental for the corneal epithelium integrity by replenishing and renewing cells. Re-epithelialization failure results in persistent defects, often associated with various ocular conditions including diabetic keratopathy. The insulin-like growth factor (IGF) system is a sophisticated network of insulin and other proteins essential for numerous physiological processes. This review examines its role in maintaining the corneal epithelium homeostasis, with a special focus on the interplay with corneal limbal stem cells and the potential therapeutic applications of the system components.

Rat Model of Type 2 Diabetes Mellitus Recapitulates Human Disease in the Anterior Segment of the Eye

Changes in the anterior segment of the eye due to type 2 diabetes mellitus (T2DM) are not well-characterized, in part due to the lack of a reliable animal model. This study evaluates changes in the anterior segment, including crystalline lens health, corneal endothelial cell density, aqueous humor metabolites, and ciliary body vasculature, in a rat model of T2DM compared with human eyes. Male Sprague-Dawley rats were fed a high-fat diet (45% fat) or normal diet, and rats fed the high-fat diet were injected with streptozotocin i.p. to generate a model of T2DM. Cataract formation and corneal endothelial cell density were assessed using microscopic analysis. Diabetes-related rat aqueous humor alterations were assessed using metabolomics screening. Transmission electron microscopy was used to assess qualitative ultrastructural changes ciliary process microvessels at the site of aqueous formation in the eyes of diabetic rats and humans. Eyes from the diabetic rats demonstrated cataracts, lower corneal endothelial cell densities, altered aqueous metabolites, and ciliary body ultrastructural changes, including vascular endothelial cell activation, pericyte degeneration, perivascular edema, and basement membrane reduplication. These findings recapitulated diabetic changes in human eyes. These results support the use of this model for studying ocular manifestations of T2DM and support a hypothesis postulating blood-aqueous barrier breakdown and vascular leakage at the ciliary body as a mechanism for diabetic anterior segment pathology.

Comparison of western diet-induced obesity and streptozotocin mouse models: insights into energy balance, somatosensory dysfunction, and cardiac autonomic neuropathy

Metabolic disorders such as obesity and type 2 diabetes (T2D) are increasingly prevalent worldwide, necessitating a deeper comprehension of their underlying mechanisms. However, translating findings from animal research to human patients remains challenging. This study aimed to investigate the long-term effects of Streptozotocin (STZ) on metabolic, cardiac, and somatosensory function in mice fed a Western diet (WD) of high fat, sucrose, and cholesterol with low doses of STZ administration compared to mice fed WD alone. In our research, we thoroughly characterized energy balance and glucose homeostasis, as well as allodynia and cardiac function, all of which have been previously shown to be altered by WD feeding. Notably, our findings revealed that the treatment of WD-fed mice with STZ exacerbated dysfunction in glucose homeostasis via reduced insulin secretion in addition to impaired peripheral insulin signaling. Furthermore, both WD and WD + STZ mice exhibited the same degree of cardiac autonomic neuropathy, such as reduced heart rate variability and decreased protein levels of cardiac autonomic markers. Furthermore, both groups developed the same symptoms of neuropathic pain, accompanied by elevated levels of activating transcription factor 3 (Atf3) in the dorsal root ganglia. These discoveries enhance our understanding of metabolic activity, insulin resistance, neuropathy, and cardiac dysfunction of diet-induced models of obesity and diabetes. The exacerbation of impaired insulin signaling pathways by STZ did not lead to or worsen cardiac and somatosensory dysfunction. Additionally, they offer valuable insights into suitable diet induced translational mouse models, thereby advancing the development of potential interventions for associated conditions.

In vivo corneal confocal microscopy as a non-invasive test to assess obesity induced small fibre nerve damage and inflammation

PURPOSE

To investigate small nerve fibre damage and inflammation at the level of the sub-basal nerve plexus (SNP) of severe obese patients and compare the results with those of healthy subjects.

METHODS

This cross-sectional, observational study investigated the data of 28 patients (14 out of 28 prediabetic or diabetic) with severe obesity (Body Mass Index; BMI ≥ 40) and 20 healthy subjects. Corneal nerve fibre density (CNFD), branch density (CNBD), fibre length (CNFL), nerve fibre area (CNFA), nerve fibre width (CNFW), and nerve fractal dimension (CNFrD) and dendritic cell (DC) density were evaluated using in vivo confocal microscopy (IVCM, Heidelberg Retinal Tomograph III Rostock Cornea Module). Automatic CCMetrics software (University of Manchester, UK) was used for quantitative analysis of SNP.

RESULTS

Mean age was 48.4±7.4 and 45.1 ± 5.8 in the control and obese group, respectively (p = 0.09). Mean BMI were 49.1 ± 7.8 vs. 23.3 ± 1.4 in obese vs. control group, respectively (p < 0.001). Mean CNFD, CNBD, CNFL, CNFA, CNFW were significantly reduced in obese group compared with those in the control group (always p < 0.05, respectively). There were no significant differences in any ACCMetrics parameters between prediabetic/diabetic and non-diabetic obese patients. Increased DC densities were detected in obese group compared with those in control group (p < 0.0001). There were significant correlations between BMI scores and SNP parameters.

CONCLUSION

Imaging with IVCM is a feasible, non-invasive method to detect and quantify occult corneal nerve damage and increased inflammation in patients with obesity. This study suggests that obesity may be a separate risk factor for peripheral neuropathy regardless of DM.

Type 2 diabetes influences intraepithelial corneal nerve parameters and corneal stromal-epithelial nerve penetration sites

INTRODUCTION

The quantification of intraepithelial corneal basal nerve parameters by in vivo confocal microscopy represents a promising modality to identify the earliest manifestations of diabetic peripheral neuropathy. However, its diagnostic accuracy is hampered by its dependence on neuron length, with minimal consideration for other parameters, including the origin of these nerves, the corneal stromal-epithelial nerve penetration sites. This study sought to utilize high-resolution images of murine corneal nerves to analyze comprehensively the morphological changes associated with type 2 diabetes progression.

MATERIALS AND METHODS

βIII-Tubulin immunostained corneas from prediabetic and type 2 diabetic mice and their respective controls were imaged by scanning confocal microscopy and analyzed automatically for nerve parameters. Additionally, the number and distribution of penetration sites was manually ascertained and the average length of the axons exiting them was computed.

RESULTS

The earliest detectable changes included a significant increase in nerve density (6.06 ± 0.41% vs 8.98 ± 1.99%, P = 0.03) and branching (2867.8 ± 271.3/mm² vs 4912.1 ± 1475.3/mm² , P = 0.03), and in the number of penetration sites (258.80 ± 20.87 vs 422.60 ± 63.76, P = 0.0002) at 8 weeks of age. At 16 weeks, corneal innervation decreased, most notably in the periphery. The number of penetration sites remained significantly elevated relative to controls throughout the monitoring period. Similarly, prediabetic mice exhibited an increased number of penetration sites (242.2 ± 13.55 vs 305.6 ± 30.96, P = 0.003) without significant changes to the nerves.

CONCLUSIONS

Our data suggest that diabetic peripheral neuropathy may be preceded by a phase of neuron growth rather than regression, and that the peripheral cornea is more sensitive than the center for detecting changes in innervation.

A Low-Fat/Sucrose Diet Rich in Complex Carbohydrates Reverses High-Fat/Sucrose Diet-Induced Corneal Dysregulation

High-fat/sucrose diet feeding in mice causes loss of corneal nerve function and impairs corneal wound healing. While changing to a diet with a low fat/sugar composition and enrichments in complex carbohydrates mitigates the reduction in nerve function, it remains to be determined if it has an effect on corneal wound healing. In this study, 6-week-old C57BL/6 male mice were fed either a normal diet or a high-fat/sucrose diet for 20 weeks. A third group (diet reversal) was placed on a high-fat/sucrose diet for 10 weeks followed by a normal diet for an additional 10 weeks. A central corneal epithelial abrasion wound was created, and wound closure was monitored. Neutrophil and platelet recruitment was assessed by immunofluorescence microscopy. Mice fed the high-fat/sucrose diet-only had greater adiposity (p < 0.005) than normal diet-only fed mice; diet reversal markedly reduced adiposity. Following corneal abrasion, wound closure was delayed by ~6 h (p ≤ 0.01) and, at 30 h post-wounding, fewer neutrophils reached the wound center and fewer extravascular platelets were present at the limbus (p < 0.05). Diet restored normal wound closure and neutrophil and platelet influx in the injured cornea. These data suggest compositional changes to the diet may be an effective diet-based therapeutic strategy for maintaining or restoring corneal health.

Corneal Confocal Microscopy as a Quantitative Imaging Biomarker of Diabetic Peripheral Neuropathy: A Review

Distal symmetric polyneuropathy (DPN), particularly chronic sensorimotor DPN, represents one of the most frequent complications of diabetes, affecting 50% of diabetic patients and causing an enormous financial burden. Whilst diagnostic methods exist to detect and monitor this condition, they have significant limitations, mainly due to their high subjectivity, invasiveness, and non-repeatability. Corneal confocal microscopy (CCM) is an in vivo, non-invasive, and reproducible diagnostic technique for the study of all corneal layers including the sub-basal nerve plexus, which represents part of the peripheral nervous system. We reviewed the current literature on the use of CCM as an instrument in the assessment of diabetic patients, particularly focusing on its role in the study of sub-basal nerve plexus alterations as a marker of DPN. CCM has been demonstrated to be a valid in vivo tool to detect early sub-basal nerve plexus damage in adult and pediatric diabetic patients, correlating with the severity of DPN. Despite its great potential, CCM has still limited application in daily clinical practice, and more efforts still need to be made to allow the dissemination of this technique among doctors taking care of diabetic patients.

The recovery of ocular surface after bariatric surgery in morbid obese patients

Aim: To evaluate the alterations of the ocular surface in morbid obese patients after bariatric surgery. Material and Method: The morbid obese patients who underwent sleeve gastrectomy surgery between February 2019 and September 2020 at Department of General Surgery in Balıkesir University Medicine Faculty were evaluated were evaluated in preoperative/postoperative 6th month period. The body-mass index (BMI), abdominal circumference (AC), Ocular Surface Disease Index (OSDI) questionnaire, tear osmolarity, Oxford ocular surface staining score, and Schirmer’s test were performed. Preoperative and postoperative values were compared. Results: The study included 68 eyes of 68 patients (33.76±9.85 years). The BCVA was improved from 0.98±0.11 to 1.00, the BMI was changed from 45.11±2.23 to 30.70±4.92 kg/m2 (p

The impact of sensory neuropathy and inflammation on epithelial wound healing in diabetic corneas

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes, with several underlying pathophysiological mechanisms, some of which are still uncertain. The cornea is an avascular tissue and sensitive to hyperglycemia, resulting in several diabetic corneal complications including delayed epithelial wound healing, recurrent erosions, neuropathy, loss of sensitivity, and tear film changes. The manifestation of DPN in the cornea is referred to as diabetic neurotrophic keratopathy (DNK). Recent studies have revealed that disturbed epithelial-neural-immune cell interactions are a major cause of DNK. The epithelium is supplied by a dense network of sensory nerve endings and dendritic cell processes, and it secretes growth/neurotrophic factors and cytokines to nourish these neighboring cells. In turn, sensory nerve endings release neuropeptides to suppress inflammation and promote epithelial wound healing, while resident immune cells provide neurotrophic and growth factors to support neuronal and epithelial cells, respectively. Diabetes greatly perturbs these interdependencies, resulting in suppressed epithelial proliferation, sensory neuropathy, and a decreased density of dendritic cells. Clinically, this results in a markedly delayed wound healing and impaired sensory nerve regeneration in response to insult and injury. Current treatments for DPN and DNK largely focus on managing the severe complications of the disease. Cell-based therapies hold promise for providing more effective treatment for diabetic keratopathy and corneal ulcers.

An improved model of type 2 diabetes with effects on glucose tolerance, neuropathy and retinopathy with and without obesity

RATIONALE

Type 2 diabetes (T2D) costs billions of dollars annually, is also associated with pain (diabetic neuropathy), as well as retinopathy, lower urinary tract/urinary bladder dysfunction, depression, and systemic inflammation, affecting quality of life for patients. To that end, animal models are utilized to explore potential treatments, but may not reflect the complexity of the condition.

OBJECTIVE

We aimed to test an improved model of T2D that more closely mimics the clinical mechanisms and symptoms in an outbred strain of mouse.

FINDINGS

Male and female CD-1 mice (n = 72) were fed one of four diets: regular chow (REG), our Standard American Diet (SAD), a revised SAD (SAD2), or the commonly-used high-fat diet (HFD). Overall, HFD- and SAD-fed mice had significant weight gain and increased fat mass. Following injury, the SAD- and SAD2-fed mice showed protracted recovery, but the HFD-fed mice did not. Similarly, SAD- and SAD2-fed mice showed impaired retinal function compared to REG-fed mice, but the HFD-fed mice did not.

CONCLUSIONS

The SAD and SAD2 more closely model the problematic dietary intake and subsequent clinical symptoms associated with T2D.

POTENTIAL IMPACT OF STUDY

The adjusted SAD2 may be a better representation of a human-translatable diet than the SAD and HFD, and may allow for increased advances in the investigation of T2D-related symptoms.

Using Corneal Confocal Microscopy to Identify Therapeutic Agents for Diabetic Neuropathy

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

Dissociation between Corneal and Cardiometabolic Changes in Response to a Time-Restricted Feeding of a High Fat Diet

Mice fed a high fat diet (HFD) ab libitum show corneal dysregulation, as evidenced by decreased sensitivity and impaired wound healing. Time-restricted (TR) feeding can effectively mitigate the cardiometabolic effects of an HFD. To determine if TR feeding attenuates HFD-induced corneal dysregulation, this study evaluated 6-week-old C57BL/6 mice fed an ad libitum normal diet (ND), an ad libitum HFD, or a time-restricted (TR) HFD for 10 days. Corneal sensitivity was measured using a Cochet-Bonnet aesthesiometer. A corneal epithelial abrasion wound was created, and wound closure was monitored for 30 h. Neutrophil and platelet recruitment were assessed by immunofluorescence microscopy. TR HFD fed mice gained less weight (p < 0.0001), had less visceral fat (p = 0.015), and had reduced numbers of adipose tissue macrophages and T cells (p < 0.05) compared to ad libitum HFD fed mice. Corneal sensitivity was reduced in ad libitum HFD and TR HFD fed mice compared to ad libitum ND fed mice (p < 0.0001). Following epithelial abrasion, corneal wound closure was delayed (~6 h), and neutrophil and platelet recruitment was dysregulated similarly in ad libitum and TR HFD fed mice. TR HFD feeding appears to mitigate adipose tissue inflammation and adiposity, while the cornea remains sensitive to the pathologic effects of HFD feeding.

Pathogenesis, diagnosis and clinical management of diabetic sensorimotor peripheral neuropathy

Diabetic sensorimotor peripheral neuropathy (DSPN) is a serious complication of diabetes mellitus and is associated with increased mortality, lower-limb amputations and distressing painful neuropathic symptoms (painful DSPN). Our understanding of the pathophysiology of the disease has largely been derived from animal models, which have identified key potential mechanisms. However, effective therapies in preclinical models have not translated into clinical trials and we have no universally accepted disease-modifying treatments. Moreover, the condition is generally diagnosed late when irreversible nerve damage has already taken place. Innovative point-of-care devices have great potential to enable the early diagnosis of DSPN when the condition might be more amenable to treatment. The management of painful DSPN remains less than optimal; however, studies suggest that a mechanism-based approach might offer an enhanced benefit in certain pain phenotypes. The management of patients with DSPN involves the control of individualized cardiometabolic targets, a multidisciplinary approach aimed at the prevention and management of foot complications, and the timely diagnosis and management of neuropathic pain. Here, we discuss the latest advances in the mechanisms of DSPN and painful DSPN, originating both from the periphery and the central nervous system, as well as the emerging diagnostics and treatments.

Corneal Keratocyte Density and Corneal Nerves Are Reduced in Patients With Severe Obesity and Improve After Bariatric Surgery

Purpose

Obesity is associated with peripheral neuropathy, which bariatric surgery may ameliorate. The aim of this study was to assess whether corneal confocal microscopy can show a change in corneal nerve morphology and keratocyte density in subjects with severe obesity after bariatric surgery.

Methods

Twenty obese patients with diabetes (n = 13) and without diabetes (n = 7) underwent assessment of hemoglobin A1c (HbA1c), lipids, IL-6, highly sensitive C-reactive protein (hsCRP), and corneal confocal microscopy before and 12 months after bariatric surgery. Corneal nerve fiber density (CNFD), corneal nerve branch density (CNBD), corneal nerve fiber length (CNFL), and keratocyte density (KD) from the anterior, middle, and posterior stroma were quantified. Twenty-two controls underwent assessment at baseline only.

Results

CNFL (P < 0.001), CNBD (P < 0.05), and anterior (P < 0.001), middle (P < 0.001), and posterior (P < 0.001) keratocyte densities were significantly lower in obese patients compared to controls, and anterior keratocyte density (AKD) correlated with CNFL. Twelve months after bariatric surgery, there were significant improvements in body mass index (BMI; P < 0.001), HDL cholesterol (P < 0.05), hsCRP (P < 0.001), and IL-6 (P < 0.01). There were significant increases in CNFD (P < 0.05), CNBD (P < 0.05), CNFL (P < 0.05), and anterior (P < 0.05) and middle (P < 0.001) keratocyte densities. The increase in AKD correlated with a decrease in BMI (r = -0.55, P < 0.05) and triglycerides (r = -0.85, P < 0.001). There were no significant correlations between the change in keratocyte densities and corneal nerve fiber or other neuropathy measures.

Conclusions

Corneal confocal microscopy demonstrates early small fiber damage and reduced keratocyte density in obese patients. Bariatric surgery leads to weight reduction and improvement in lipids and inflammation and an improvement in keratocyte density and corneal nerve regeneration.

Corneal Confocal Microscopy: A Biomarker for Diabetic Peripheral Neuropathy

PURPOSE

Diagnosing early diabetic peripheral neuropathy remains a challenge due to deficiencies in currently advocated end points. The cornea is densely innervated with small sensory fibers, which are structurally and functionally comparable to intraepidermal nerve fibers. Corneal confocal microscopy is a method for rapid, noninvasive scanning of the living cornea with high resolution and magnification.

METHODS

This narrative review presents the framework for the development of biomarkers and the literature on the use and adoption of corneal confocal microscopy as an objective, diagnostic biomarker in experimental and clinical studies of diabetic peripheral neuropathy. A search was performed on PubMed and Google Scholar based on the terms "corneal confocal microscopy," "diabetic neuropathy," "corneal sensitivity," and "clinical trials."

FINDINGS

A substantial body of evidence underpins the thesis that corneal nerve loss predicts incident neuropathy and progresses with the severity of diabetic peripheral neuropathy. Corneal confocal microscopy also identifies early corneal nerve regeneration, strongly arguing for its inclusion as a surrogate end point in clinical trials of disease-modifying therapies.

IMPLICATIONS

There are sufficient diagnostic and prospective validation studies to fulfill the US Food and Drug Administration criteria for a biomarker to support the inclusion of corneal confocal microscopy as a primary end point in clinical trials of disease-modifying therapies in diabetic neuropathy.

Neuronal-epithelial cell alignment: A determinant of health and disease status of the cornea

PURPOSE

How sensory neurons and epithelial cells interact with one another, and whether this association can be considered an indicator of health or disease is yet to be elucidated.

METHODS

Herein, we used the cornea, Confetti mice, a novel image segmentation algorithm for intraepithelial corneal nerves which was compared to and validated against several other analytical platforms, and three mouse models to delineate this paradigm. For aging, eyes were collected from 2 to 52 week-old normal C57BL/6 mice (n ≥ 4/time-point). For wound-healing and limbal stem cell deficiency, 7 week-old mice received a limbal-sparing or limbal-to-limbal epithelial debridement to their right cornea, respectively. Eyes were collected 2-16 weeks post-injury (n=4/group/time-point), corneas procured, immunolabelled with βIII-tubulin, flat-mounted, imaged by scanning confocal microscopy and analyzed for nerve and epithelial-specific parameters.

RESULTS

Our data indicate that nerve features are dynamic during aging and their curvilinear arrangement align with corneal epithelial migratory tracks. Moderate corneal injury prompted axonal regeneration and recovery of nerve fiber features. Limbal stem cell deficient corneas displayed abnormal nerve morphology, and fibers no longer aligned with corneal epithelial migratory tracks. Mechanistically, we discovered that nerve pattern restoration relies on the number and distribution of stromal-epithelial nerve penetration sites.

CONCLUSIONS

Microstructural changes to innervation may explain corneal complications related to aging and/or disease and facilitate development of new assays for diagnosis and/or classification of ocular and systemic diseases.

Impacts of high fat diet on ocular outcomes in rodent models of visual disease

High fat diets (HFD) have been utilized in rodent models of visual disease for over 50 years to model the effects of lipids, metabolic dysfunction, and diet-induced obesity on vision and ocular health. HFD treatment can recapitulate the pathologies of some of the leading causes of blindness, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR) in rodent models of visual disease. However, there are many important factors to consider when using and interpreting these models. To synthesize our current understanding of the importance of lipid signaling, metabolism, and inflammation in HFD-driven visual disease processes, we systematically review the use of HFD in mouse and rat models of visual disease. The resulting literature is grouped into three clusters: models that solely focus on HFD treatment, models of diabetes that utilize both HFD and streptozotocin (STZ), and models of AMD that utilize both HFD and genetic models and/or other exposures. Our findings show that HFD profoundly affects vision, retinal function, many different ocular tissues, and multiple cell types through a variety of mechanisms. We delineate how HFD affects the cornea, lens, uvea, vitreous humor, retina, retinal pigmented epithelium (RPE), and Bruch's membrane (BM). Furthermore, we highlight how HFD impairs several retinal cell types, including glia (microglia), retinal ganglion cells, bipolar cells, photoreceptors, and vascular support cells (endothelial cells and pericytes). However, there are a number of gaps, limitations, and biases in the current literature. We highlight these gaps and discuss experimental design to help guide future studies. Very little is known about how HFD impacts the lens, ciliary bodies, and specific neuronal populations, such as rods, cones, bipolar cells, amacrine cells, and retinal ganglion cells. Additionally, sex bias is an important limitation in the current literature, with few HFD studies utilizing female rodents. Future studies should use ingredient-matched control diets (IMCD), include both sexes in experiments to evaluate sex-specific outcomes, conduct longitudinal metabolic and visual measurements, and capture acute outcomes. In conclusion, HFD is a systemic exposure with profound systemic effects, and rodent models are invaluable in understanding the impacts on visual and ocular disease.

Exendin-4 promotes bone formation in diabetic states via HDAC1-Wnt/β-catenin axis

Exendin-4 has been found to have hypoglycemic effect and prevent bone loss in diabetic patients, but its mechanism of preventing bone loss is still unclear. In this study, high-fat diet combined with streptozotocin was used to establish type 2 diabetes mellitus (T2DM) mice, and bone marrow mesenchyme stem cells (BMSCs) were isolated for osteogenic induction in vitro. Alizarin red staining and ALP activity detection were used to observe the effect of exendin-4 on osteogenic differentiation of BMSCs. Western blot was used to detect the proteins expression in BMSCs. In vivo, the effects of exendin-4 treatment on body weight, blood glucose, bone density and bone quality of T2DM mice were observed by treatment with exendin-4. The results showed that exendin-4 promoted osteogenic differentiation of T2DM derived BMSCs, down-regulated histone deacetylase 1 (HDAC1) and p-β-Catenin proteins expression, and up-regulated Wnt3, β-Catenin and runt-related transcription factor 2 (Runx 2) proteins expression. In vivo, exendin-4 effectively suppressed the blood glucose and increased body weight of T2DM mice, and significantly improved bone density and bone quality of the right tibia. Interestingly, by over-expression of HDAC1 in BMSCs, the effect of exendin-4 on promoting osteogenic differentiation of BMSCs was attenuated, and the regulation of Wnt3a, β-Catenin, p-β-Catenin or Runx2 proteins were reversed. By injecting adenovirus containing HDAC1 into the right tibia of mice, the effect of exendin-4 on bone density and bone quality of T2DM mice was significantly attenuated. All above results suggest that the HDAC1-Wnt/β-Catenin signal axis is involved in the anti-diabetic bone loss effect of exendin-4, and HDAC1 may be the target of exendin-4.

The effect of high-fat diet-induced metabolic disturbance on corneal neuroimmune features

PURPOSE

The highly innervated cornea is susceptible to nerve loss secondary to systemic diseases such as diabetes and metabolic disturbances caused by high-fat diet. In this study, we characterize the effect of high-fat diet on the mouse corneal neuroimmune phenotype, including changes to corneal nerve density and resident immune cells, alongside the clinical assessment of corneal thickness and endothelial cell density.

METHODS

Male C57Bl6/J mice, aged 10 weeks, were fed a high-fat diet (60 kcal% fat, 5.2 kcal/g) or control diet (10 kcal%, 3.8 kcal/g) for 16 weeks. At the study endpoint, metabolic parameters (HbA1c, weight, fasting glucose, body fat) were measured to confirm metabolic disturbance. Clinical imaging of the anterior segment was performed using optical coherence tomography to measure the corneal epithelial and stromal thickness. Corneal sensory nerves were visualized using flatmount immunostaining and confocal microscopy. The topographical distribution and density of sensory nerves (BIII-tubulin⁺), intraepithelial CD45⁺ and MHC- II⁺ cells, stromal macrophages (IBA1⁺CD206⁺) and endothelial cells (ZO-1⁺) were analysed using FIJI.

RESULTS

High-fat diet mice had significantly higher blood HbA1c, higher body weight, a higher percentage of body fat and elevated fasting glucose compared to the control diet mice. Corneal epithelial and stromal thickness was similar in both groups. The sum length of the basal nerve plexus was lower in the central and peripheral cornea of mice fed a high-fat diet. In contrast, the sum length of superficial nerve terminals was similar between groups. Epithelial immune cell density was two-fold higher in the central corneas of high-fat diet mice compared to control diet mice. IBA1⁺CD206⁺ macrophage density was similar in the anterior stroma of both groups but was significantly higher in the posterior stroma of the peripheral cornea in the high-fat diet mice compared to controls. The percentage of nerve-associated MHC-II⁺ cells in the epithelium and stroma was higher in HFD mice compared to controls. Endothelial cell density was similar in the corneas of high-fat diet mice compared to controls.

CONCLUSION

Together with corneal neuropathy, corneal immune cells in mice fed a high-fat diet were differentially affected depending on their topographical distribution and location within cornea, and appeared in closer proximity to epithelial and stromal nerves, suggesting a local neuroimmune disruption induced by systemic metabolic disturbance.

GLP-1 promotes osteogenic differentiation of human ADSCs via the Wnt/GSK-3β/β-catenin pathway

Glucagon-like peptide-1 (GLP-1) analogues are promising anti-diabetic drugs which had been shown to have beneficial effects on bone metabolism in clinical practice, but the molecular mechanism remains unclear. In this study, we evaluated whether GLP-1 can affect the "intestine-fat-bone axis" via the Wnt/GSK-3β/β-catenin pathway. We established a diabetic mouse model and then treated mice with GLP-1 analogue liraglutide. The results showed that after liraglutide treatment, glucose tolerance and insulin tolerance were significantly improved in diabetic mice as expected. Moreover, osteogenic markers such as collagenⅠ, Runx2 and OCN were upregulated; and the adipogenic differentiation markers C/EBP-α and PPAR-γ were downregulated, these results indicated that liraglutide could ameliorate the osteogenic metabolism in diabetic mice. In the cell model, human ADSCs (hADSCs) were cultured and induced to undergo osteogenic and adipogenic differentiation under high glucose conditions in vitro and then treated with GLP-1. The results showed that GLP-1 repressed the induction of adipocyte differentiation biomarkers and the secretion of GSK-3β in a dose-dependent manner. In addition, GLP-1 enhanced the expression of osteoblastogenic biomarkers, such as OCN, Runx2 and collagenⅠ, and promoted osteoblastic mineralization. These effects were substantially suppressed by the Wnt signal recombinant human DKK-1 or activated by Wnt pathway agonist LiCl. Silencing of GSK-3β showed that the levels of β-catenin, GSK-3β and Runx2 were significantly increased by 2.46-, 2.05-, 4.44-fold after GLP-1 treatment compared to that observed in the GSK-3β lentiviral group, respectively. We conclude that GLP-1 promotes the osteogenic differentiation of hADSCs via the Wnt/GSK-3β/β-catenin pathway.

Corneal dysfunction precedes the onset of hyperglycemia in a mouse model of diet-induced obesity

PURPOSE

The purpose of this study was to use a mouse model of diet-induced obesity to determine if corneal dysfunction begins prior to the onset of sustained hyperglycemia and if the dysfunction is ameliorated by diet reversal.

METHODS

Six-week-old male C57BL/6 mice were fed a high fat diet (HFD) or a normal diet (ND) for 5-15 weeks. Diet reversal (DiR) mice were fed a HFD for 5 weeks, followed by a ND for 5 or 10 weeks. Corneal sensitivity was determined using aesthesiometry. Corneal cytokine expression was analyzed using a 32-plex Luminex assay. Excised corneas were prepared for immunofluorescence microscopy to evaluate diet-induced changes and wound healing. For wounding studies, mice were fed a HFD or a ND for 10 days prior to receiving a central 2mm corneal abrasion.

RESULTS

After 10 days of HFD consumption, corneal sensitivity declined. By 10 weeks, expression of corneal inflammatory mediators increased and nerve density declined. While diet reversal restored nerve density and sensitivity, the corneas remained in a heightened inflammatory state. After 10 days on the HFD, corneal circadian rhythms (limbal neutrophil accumulation, epithelial cell division and Rev-erbα expression) were blunted. Similarly, leukocyte recruitment after wounding was dysregulated and accompanied by delays in wound closure and nerve recovery.

CONCLUSION

In the mouse, obesogenic diet consumption results in corneal dysfunction that precedes the onset of sustained hyperglycemia. Diet reversal only partially ameliorated this dysfunction, suggesting a HFD diet may have a lasting negative impact on corneal health that is resistant to dietary therapeutic intervention.

Characterization of experimental diabetic neuropathy using multicontrast magnetic resonance neurography at ultra high field strength

In light of the limited treatment options of diabetic polyneuropathy (DPN) available, suitable animal models are essential to investigate pathophysiological mechanisms and to identify potential therapeutic targets. In vivo evaluation with current techniques, however, often provides only restricted information about disease evolution. In the study of patients with DPN, magnetic resonance neurography (MRN) has been introduced as an innovative diagnostic tool detecting characteristic lesions within peripheral nerves. We developed a novel multicontrast ultra high field MRN strategy to examine major peripheral nerve segments in diabetic mice non-invasively. It was first validated in a cross-platform approach on human nerve tissue and then applied to the popular streptozotocin(STZ)-induced mouse model of DPN. In the absence of gross morphologic alterations, a distinct MR-signature within the sciatic nerve was observed mirroring subtle changes of the nerves' fibre composition and ultrastructure, potentially indicating early re-arrangements of DPN. Interestingly, these signal alterations differed from previously reported typical nerve lesions of patients with DPN. The capacity of our approach to non-invasively assess sciatic nerve tissue structure and function within a given mouse model provides a powerful tool for direct translational comparison to human disease hallmarks not only in diabetes but also in other peripheral neuropathic conditions.

Hyperglycemia Suppresses Age-Related Increases in Corneal Peripheral Sensory Nerves in Wistar Bon Kobori (WBN/Kob) Rats

Purpose

Nerve fiber density in the cornea is an alternative marker for diabetic peripheral neuropathy combined with intraepidermal nerve fiber density (IENFD). Recent studies investigated corneal nerves using rodent models of diabetes. Male Wistar Bon Kobori (WBN/Kob) rats spontaneously develop long-lasting diabetes and human-like diabetic peripheral neuropathy with vascular lesions. This study investigated corneal nerve fiber density and IENFD in diabetic male WBN/Kob rats as morphological markers of diabetic peripheral neuropathy.

Methods

Male WBN/Kob rats exhibit abnormal glucose tolerance and diabetes at approximately 30 weeks of age, which progresses until approximately 90 weeks of age. Male WBN/Kob rats aged 36 and 90 weeks were therefore used for histological investigations and compared with age-matched nondiabetic female rats.

Results

Terminal epithelial nerve density and subbasal nerve plexus density in the central cornea were significantly greater in nondiabetic female rats aged 90 weeks when compared with nondiabetic female rats aged 36 weeks. However, terminal epithelial nerve density and subbasal nerve plexus density did not increase with age in diabetic male WBN/Kob rats, instead lowering by up to 40%, relative to measurements in nondiabetic female rats aged 90 weeks. However, this difference was not statistically significant. IENFD was significantly lower in diabetic male rats aged 90 weeks than in male rats aged 36 weeks, but did not differ between diabetic male rats and nondiabetic female rats aged 90 weeks.

Conclusions

In WBN/Kob rats, hyperglycemia suppresses an age-related increase in peripheral sensory corneal nerve density; therefore, corneal sensory nerves may be important morphological markers of diabetic peripheral sensory neuropathy.

The Effects of Diabetes and High-Fat Diet on Polymodal Nociceptor and Cold Thermoreceptor Nerve Terminal Endings in the Corneal Epithelium

Purpose

There is a substantial body of evidence indicating that corneal sensory innervation is affected by pathology in a range of diseases. However, there are no published studies that have directly assessed whether the nerve fiber density of the different subpopulations of corneal sensory neurons are differentially affected. The present study explored the possibility that the intraepithelial nerve fiber density of corneal polymodal nociceptors and cold thermoreceptors are differentially affected in mice fed with a high-fat high cholesterol (HFHC; 21% fat, 2% cholesterol) diet and in those that also have diabetes.

Methods

The mice were fed the HFHC diet for the duration of the experiment (up to 40 weeks). Mice in the diabetes group had hyperglycaemia induced with streptozotocin after 15 weeks on the HFHC diet. Age-matched control animals were fed a standard diet. All corneal nerve fibers were labeled with a pan neuronal antibody (antiprotein gene product 9.5), and polymodal nociceptors and cold thermoreceptors were labeled with antibodies directed against transient receptor potential cation channel, subfamily V, member 1 and transient receptor potential cation channel subfamily M member 8, respectively.

Results

The mice fed a HFHC diet and those that in addition have hyperglycemia have similar reductions in corneal nerve fiber density consistent with small fiber neuropathy. Importantly, both treatments more markedly affected the intraepithelial axons of cold thermoreceptors than those of polymodal nociceptors.

Conclusions

The results provide evidence that distinct subpopulations of corneal sensory neurons can be differentially affected by pathology.

Extended Duration of Hyperglycemia Result in Human-Like Corneal Nerve Lesions in Mice With Alloxan- and Streptozotocin-Induced Type 1 Diabetes

Purpose

Previous experimental studies assessing corneal nerves as a measure of the severity of diabetic peripheral neuropathy have yielded discordant results; this may have been due to the effect of the short duration of the induced diabetes. We investigated whether increases in the duration of hyperglycemia result in the development of corneal lesions in a mouse model of alloxan (AL)- or streptozotocin (STZ)-induced type 1 diabetes. We further determined whether corneal nerve fiber density, intraepidermal nerve fiber density (IENFD), and sural nerve morphology can be used as morphologic markers of diabetic peripheral neuropathy in rodent models.

Methods

A total of 30 female ICR mice were divided into three groups: those with STZ-induced (STZ group) and AL-induced (AL group) diabetes, and a control group. Hyperglycemia was maintained in diabetic mice for 35 weeks. Animals were euthanized at 41 weeks of age.

Results

Subbasal nerve plexus density (SBNPD) and terminal epithelial nerve density (TEND) in the cornea, as well as IENFD, were significantly lower, and mean sural nerve axon sizes were smaller in mice in the STZ and AL groups than in the control group. There were significant correlations between IENFD and SBNPD, and between IENFD and TEND.

Conclusions

These results indicate that the TEND and SBNTD of the cornea may be useful morphologic markers for diabetic peripheral neuropathy.

Influence of diabetes mellitus on anterior segment of the eye

Diabetes mellitus (DM) has been emerging as one of the most serious health problems worldwide. Ocular complications of DM are currently one of the major causes of blindness in developed countries, among which diabetic retinopathy is relatively well studied and understood. However, although ocular surface complications of DM are common, diabetic complications of anterior segment of the eye, such as, cornea, conjunctiva, and lacrimal glands, are often overlooked. DM is associated with progressive damage to corneal nerves and epithelial cells, which increases the risk of anterior segment disorders including dry eye disease, corneal erosion, persistent epithelial defects, and even sight-threatening corneal ulcer. In this review, the authors will discuss the association of DM with disorders of anterior segment of the eye. Studies indicating the value of corneal nerve assessment as a sensitive, noninvasive, and repeatable biomarker for diabetic neuropathy will also be introduced. In addition, treatment modalities of anterior segment disorders associated with DM is discussed. The studies introduced in this review suggest that early and periodic screening of the anterior segment of the eye, as well as the retina, is important for the optimal treatment of DM.

Juvenile murine models of prediabetes and type 2 diabetes develop neuropathy

Peripheral neuropathy (neuropathy) is a common complication of obesity and type 2 diabetes in children and adolescents. To model this complication in mice, 5-week-old male C57BL/6J mice were fed a high-fat diet to induce diet-induced obesity (DIO), a model of prediabetes, and a cohort of these animals was injected with low-dose streptozotocin (STZ) at 12 weeks of age to induce hyperglycemia and type 2 diabetes. Neuropathy assessments at 16, 24 and 36 weeks demonstrated that DIO and DIO-STZ mice displayed decreased motor and sensory nerve conduction velocities as early as 16 weeks, hypoalgesia by 24 weeks and cutaneous nerve fiber loss by 36 weeks, relative to control mice fed a standard diet. Interestingly, neuropathy severity was similar in DIO and DIO-STZ mice at all time points despite significantly higher fasting glucose levels in the DIO-STZ mice. These mouse models provide critical tools to better understand the underlying pathogenesis of prediabetic and diabetic neuropathy from youth to adulthood, and support the idea that hyperglycemia alone does not drive early neuropathy.

This article has an associated First Person interview with the first author of the paper.

Summary: The mouse models described in this paper provide critical tools to better understand the underlying pathogenesis of prediabetic and diabetic neuropathy from youth to adulthood.

Influence of Sex on Cognition and Peripheral Neurovascular Function in Diabetic Mice

Cognition impairment and peripheral neuropathy (DPN) are two major complications of diabetes. The aim of the present study is to investigate the effect of sex differences on cognition and DPN in diabetic mice. Male and female BKS.Cg-m+/+Leprdb/J (db/db) and db/m mice were used. At ages of 20 and 30 weeks, all animals were subjected to learning, memory and neurological function tests. Regional blood flow in footpad and sciatic nerves were measured using laser Doppler flowmetry. Our data showed that male db/db mice aged 20 weeks and 30 weeks spent significantly more time to locate the hidden platform in the correct quadrant and spent significantly less time exploring the cage with a new stranger mouse compared to aged-matched female db/db mice. Electrophysiological recordings showed that male db mice aged 30 weeks had significantly reduced motor and sensory nerve conduction velocity compared with females. Hot plate and tactile allodynia tests revealed that males exhibited significantly higher thermal and mechanical latency than females. Male db mice aged 30 weeks displayed significantly reduced blood perfusion in sciatic nerve and footpad tissues compared with females. In addition, compared with male and female non-diabetic db/m mice, db/db mice exhibited increased time spent on locating the hidden platform, decreased time spent on exploring the novel odor bead and an unfamiliar mouse, as well as showed significantly lower levels of blood flow, lower velocity of MCV and SCV, higher thermal and mechanical latencies. Blood glucose levels and body weight were not significantly different between male and female diabetic animals (age 30 weeks), but male db mice showed a higher serum total cholesterol content. Together, our data suggest that males develop a greater extent of diabetes-induced cognition deficits and peripheral neurovascular dysfunction than females.

Determination of peripheral neuropathy in high-fat diet fed low-dose streptozotocin-treated female C57Bl/6J mice and Sprague-Dawley rats

AIMS/INTRODUCTION

Peripheral neuropathy is a common complication of diabetes and also occurs in 30% of human obese individuals with impaired glucose tolerance. Even though peripheral neuropathy affects both sexes, most pre-clinical studies have been carried out using male rodents. The aim of the present study was to create diet-induced obesity and type 2 diabetes in female rats and mice in order to examine the development of peripheral neuropathy.

MATERIALS AND METHODS

At 12 weeks-of-age, rats and mice were separated into three groups. Two groups or rats and mice were fed a 60-kcal% high-fat diet for 12 weeks (rats) or 8 weeks (mice). To induce type 2 diabetes, one group of high-fat diet-fed rats and mice were treated with a low dose of streptozotocin. Analyses of multiple neural end-points were carried out 12 weeks later.

RESULTS

Glucose utilization was impaired in diet-induced obese female rats and mice, as was a number of neurological end-points including nerve conduction velocity, intraepidermal and subepithelial corneal nerve fiber densities, and thermal and mechanical sensitivity. When female diet-induced obese rats or mice were made hyperglycemic, glucose utilization and sensory nerve density of the skin and cornea, as well as thermal and mechanical sensitivity, were more significantly impaired compared with diet-induced obese female rodents.

CONCLUSIONS

These studies show that diet-induced obese and type 2 diabetic female rodents develop peripheral neuropathy that is similar to that occurring in male rodents. However, for female rats, more aggressive treatment is required to induce dietary obesity.

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.

Vascular and Neural Complications in Type 2 Diabetic Rats: Improvement by Sacubitril/Valsartan Greater Than Valsartan Alone

Previously, we had shown that a vasopeptidase inhibitor drug containing ACE and neprilysin inhibitors was an effective treatment for diabetic vascular and neural complications. However, side effects prevented further development. This led to the development of sacubitril/valsartan, a drug containing angiotensin II receptor blocker and neprilysin inhibitor that we hypothesized would be an effective treatment for diabetic peripheral neuropathy. Using early and late intervention protocols (4 and 12 weeks posthyperglycemia, respectively), type 2 diabetic rats were treated with valsartan or sacubitril/valsartan for 12 weeks followed by an extensive evaluation of vascular and neural end points. The results demonstrated efficacy of sacubitril/valsartan in improving vascular and neural function was superior to valsartan alone. In the early intervention protocol, sacubitril/valsartan treatment was found to slow progression of these deficits and, with late intervention treatment, was found to stimulate restoration of vascular reactivity, motor and sensory nerve conduction velocities, and sensitivity/regeneration of sensory nerves of the skin and cornea in a rat model of type 2 diabetes. These preclinical studies suggest that sacubitril/valsartan may be an effective treatment for diabetic peripheral neuropathy, but additional studies will be needed to investigate these effects further.

High fat diet induces pre-type 2 diabetes with regional changes in corneal sensory nerves and altered P2X7 expression and localization

Type 2 diabetes is one of the leading pathologies that increases the risk of improper wound healing. Obesity has become a major risk factor for this disease that is now considered to be the 4th highest cause of preventable blindness according to the World Health Organization. The cornea is the most densely innervated structure in the human body and senses even the slightest injury. In diabetes, decreased corneal sensitivity secondary to diabetic peripheral neuropathy can lead to increased corneal abrasion, ulceration, and even blindness. In this study, a diet induced obesity (DIO) mouse model of pre-Type 2 diabetes was used to characterize changes in sensory nerves and P2X7, a purinoreceptor, a pain receptor, and an ion channel that is expressed in a number of tissues. Since our previous studies demonstrated that P2X7 mRNA was significantly elevated in diabetic human corneas, we examined P2X7 expression and localization in the DIO murine model at various times after being fed a high fat diet. Fifteen weeks after onset of diet, we found that there was a significant decrease in the density of sub-basal nerves in the DIO mice that was associated with an increase in tortuosity and a decrease in diameter. In addition, P2X7 mRNA expression was significantly greater in the corneal epithelium of DIO mice, and the increase in transcript was enhanced in the central migrating and peripheral regions after injury. Interestingly, confocal microscopy and thresholding analysis revealed that there was a significant increase in P2X7 distal to the injury, which contrasted with a decrease in P2X7-expressing stromal sensory nerves. Therefore, we hypothesize that the P2X7 receptor acts to sense changes at the leading edge following an epithelial abrasion, and this fine-tuned regulation is lost during the onset of diabetes. Further understanding of the corneal changes that occur in diabetes can help us better monitor progression of diabetic complications, as well as develop new therapeutics for the treatment of diabetic corneal dysfunction.

Recovery of the sub-basal nerve plexus and superficial nerve terminals after corneal epithelial injury in mice

Our aim was to compare regeneration of the sub-basal nerve plexus (SBNP) and superficial nerve terminals (SNT) following corneal epithelial injury. We also sought to compare agreement when quantifying nerve parameters using different image analysis techniques. Anesthetized, female C57BL/6 mice received central 1-mm corneal epithelial abrasions. Four-weeks post-injury, eyes were enucleated and processed for PGP9.5 to visualize the corneal nerves using wholemount immunofluorescence staining and confocal microscopy. The percentage area of the SBNP and SNT were quantified using: ImageJ automated thresholds, ImageJ manual thresholds and manual tracings in NeuronJ. Nerve sum length was quantified using NeuronJ and Imaris. Agreement between methods was considered with Bland-Altman analyses. Four-weeks post-injury, the sum length of nerve fibers in the SBNP, but not the SNT, was reduced compared with naïve eyes. In the periphery, but not central cornea, of both naïve and injured eyes, nerve fiber lengths in the SBNP and SNT were strongly correlated. For quantifying SBNP nerve axon area, all image analysis methods were highly correlated. In the SNT, there was poor correlation between manual methods and auto-thresholding, with a trend towards underestimating nerve fiber area using auto-thresholding when higher proportions of nerve fibers were present. In conclusion, four weeks after superficial corneal injury, there is differential recovery of epithelial nerve axons; SBNP sum length is reduced, however the sum length of SNTs is similar to naïve eyes. Care should be taken when selecting image analysis methods to compare nerve parameters in different depths of the corneal epithelium due to differences in background autofluorescence.

Effect of Fish Oil vs. Resolvin D1, E1, Methyl Esters of Resolvins D1 or D2 on Diabetic Peripheral Neuropathy

Objective

Fish oil is enriched in omega-3 polyunsaturated fatty acids primarily eicosapentaenoic and docosahexaenoic fatty acids. Metabolites of these two polyunsaturated fatty acids include the E and D series resolvins. Omega-3 polyunsaturated fatty acids and resolvins have been reported to have anti-inflammatory and neuroprotective properties. The objective of this study was to evaluate the efficacy of menhaden oil, a fish oil derived from the menhaden, resolvins D1 and E1 and the methyl esters of resolvins D1 and D2 on diabetic peripheral neuropathy. Hypothesis being examined was that the methyl esters of resolvins D1 and D2 would be move efficacious than resolvins D1 or E1 due to an extended half-life.

Methods

A model of type 2 diabetes in C57BL/6J mice was created through a combination of a high fat diet followed 8 weeks later with treatment of low dosage of streptozotocin. After 8 weeks of untreated hyperglycemia type 2 diabetic mice were treated for 8 weeks with menhaden oil in the diet or daily injections of 1 ng/g body weight resolvins D1, E1 or methyl esters of resolvins D1 or D2. Afterwards, multiple neurological endpoints were examined.

Results

Menhaden oil or resolvins did not improve hyperglycemia. Untreated diabetic mice were thermal hypoalgesic, had mechanical allodynia, reduced motor and sensory nerve conduction velocities and decreased innervation of the cornea and skin. These endpoints were significantly improved with menhaden oil or resolvin treatment. However, the methyl esters of resolvins D1 or D2, contrary to our hypothesis, were generally less potent than menhaden oil or resolvins D1 or E1.

Conclusion

These studies further support omega-3 polyunsaturated fatty acids derived from fish oil via in part due to their metabolites could be an effective treatment for diabetic neuropathy.

Early ciliary and prominin-1 dysfunctions precede neurogenesis impairment in a mouse model of type 2 diabetes

Diabetes mellitus (DM) is reaching epidemic conditions worldwide and increases the risk for cognition impairment and dementia. Here, we postulated that progenitors in adult neurogenic niches might be particularly vulnerable. Therefore, we evaluated the different components of the mouse subventricular zone (SVZ) during the first week after chemical induction of type 1 and type 2 diabetes-like (T1DM and T2DM) conditions. Surprisingly, only T2DM mice showed SVZ damage. The initial lesions were localized to ependymal cilia, which appeared disorientated and clumped together. In addition, they showed delocalization of the ciliary membrane protein prominin-1. Impairment of neuroprogenitor proliferation, neurogenic marker abnormalities and ectopic migration of neuroblasts were found at a later stage. To our knowledge, our data describe for the first time such an early impact of T2DM on the SVZ. This is consistent with clinical data indicating that brain damage in T2DM patients differs from that in T1DM patients.

Dietary reversal of neuropathy in a murine model of prediabetes and metabolic syndrome

Patients with metabolic syndrome, which is defined as obesity, dyslipidemia, hypertension and impaired glucose tolerance (IGT), can develop the same macro- and microvascular complications as patients with type 2 diabetes, including peripheral neuropathy. In type 2 diabetes, glycemic control has little effect on the development and progression of peripheral neuropathy, suggesting that other metabolic syndrome components may contribute to the presence of neuropathy. A parallel phenomenon is observed in patients with prediabetes and metabolic syndrome, where improvement in weight and dyslipidemia more closely correlates with restoration of nerve function than improvement in glycemic status. The goal of the current study was to develop a murine model that resembles the human condition. We examined longitudinal parameters of metabolic syndrome and neuropathy development in six mouse strains/genotypes (BKS-wt, BKS-Leprdb/+ , B6-wt, B6-Leprdb/+ , BTBR-wt, and BTBR-Lepob/+ ) fed a 54% high-fat diet (HFD; from lard). All mice fed a HFD developed large-fiber neuropathy and IGT. Changes appeared early and consistently in B6-wt mice, and paralleled the onset of neuropathy. At 36 weeks, B6-wt mice displayed all components of the metabolic syndrome, including obesity, IGT, hyperinsulinemia, dyslipidemia and oxidized low density lipoproteins (oxLDLs). Dietary reversal, whereby B6-wt mice fed a HFD from 4-20 weeks of age were switched to standard chow for 4 weeks, completely normalized neuropathy, promoted weight loss, improved insulin sensitivity, and restored LDL cholesterol and oxLDL by 50% compared with levels in HFD control mice. This dietary reversal model provides the basis for mechanistic studies investigating peripheral nerve damage in the setting of metabolic syndrome, and ultimately the development of mechanism-based therapies for neuropathy.

Effect of tempol on peripheral neuropathy in diet-induced obese and high-fat fed/low-dose streptozotocin-treated C57Bl6/J mice

In this study, we sought to determine the efficacy of tempol on multiple neuropathic endpoints in a diet-induced obese mouse, a model of pre-diabetes, and a high-fat fed low-dose streptozotocin treated mouse, a model of type 2 diabetes. Tempol (4-hydroxy-2,2,6,6-tetramethylpiperdine -1-oxyl) is a low molecular weight, water soluble, membrane permeable, and metal-independent superoxide dismutase mimetic that has been widely used in cellular studies for the removal of intracellular and extracellular superoxide. This in vivo study was designed to be an early intervention. Fourteen weeks post-high-fat diet (6 weeks post-hyperglycemia) control, obese, and diabetic mice were divided into no treatment and treatment groups. The treated mice received tempol by gavage (150 mg/kg in water), while the untreated mice received vehicle. The diet-induced obese and the diabetic mice were maintained on the high-fat diet for the duration of the study, while the control group was maintained on the standard diet. Obesity and diabetes caused slowing of motor and sensory nerve conduction, reduction in intraepidermal nerve fiber density, thermal hypoalgesia, and mechanical allodynia. Treatment with tempol partially or completely protected obese and diabetic mice from these deficits. These studies suggest that tempol or other effective scavengers of reactive oxygen species may be a viable option for treating neural complications associated with obesity or type 2 diabetes.

A systematic review on the impact of diabetes mellitus on the ocular surface

Diabetes mellitus is associated with extensive morbidity and mortality in any human community. It is well understood that the burden of diabetes is attributed to chronic progressive damage in major end-organs, but it is underappreciated that the most superficial and transparent organ affected by diabetes is the cornea. Different corneal components (epithelium, nerves, immune cells and endothelium) underpin specific systemic complications of diabetes. Just as diabetic retinopathy is a marker of more generalized microvascular disease, corneal nerve changes can predict peripheral and autonomic neuropathy, providing a window of opportunity for early treatment. In addition, alterations of immune cells in corneas suggest an inflammatory component in diabetic complications. Furthermore, impaired corneal epithelial wound healing may also imply more widespread disease. The non-invasiveness and improvement in imaging technology facilitates the emergence of new screening tools. Systemic control of diabetes can improve ocular surface health, possibly aided by anti-inflammatory and vasoprotective agents.

Early vs. late intervention of high fat/low dose streptozotocin treated C57Bl/6J mice with enalapril, α-lipoic acid, menhaden oil or their combination: Effect on diabetic neuropathy related endpoints

We have previously demonstrated that enalapril, α-lipoic acid and menhaden (fish) oil has potential as a treatment for diabetic peripheral neuropathy. In this study we sought to determine the efficacy of these treatments individually or in combination on multiple neuropathic endpoints in a high fat fed low dose streptozotocin treated mouse, a model of type 2 diabetes, following early or late intervention. Four or twelve weeks after the onset of hyperglycemia, diabetic mice were treated with enalapril, α-lipoic acid, menhaden oil or their combination for 12 weeks. Afterwards, endpoints including glucose tolerance, motor and sensory nerve conduction velocity, thermal nociception, and intraepidermal and cornea nerve fiber density was determined. Glucose clearance was impaired in diabetic mice and significantly improved only with combination treatment and early intervention. Diabetes caused steatosis, slowing of motor and sensory nerve conduction velocity, thermal hypoalgesia and reduction in intraepidermal and cornea nerve fiber density. Treating diabetic mice with enalapril, α-lipoic acid or menhaden oil partially protected diabetic mice from these deficits, whereas the combination of these three treatments was more efficacious following early or late intervention. These studies suggest that a combination therapy may be more effective for treating neural complications of type 2 diabetes.

Neuronal Changes in the Diabetic Cornea: Perspectives for Neuroprotection

Diabetic neuropathy is associated with neurotrophic ulcerations of the skin and cornea. Decreased corneal sensitivity and impaired innervation lead to weakened epithelial wound healing predisposing patients to ocular complications such as corneal infections, stromal opacification, and surface irregularity. This review presents recent findings on impaired corneal innervation in diabetic individuals, and the findings suggest that corneal neuropathy might be an early indicator of diabetic neuropathy. Additionally, the recent findings for neuroprotective and regenerative therapy for diabetic keratopathy are presented.

Early detection of diabetic neuropathy by investigating CNFL and IENFD in thy1-YFP mice

Small fiber neuropathy is one of the most common and painful long-term complications of diabetes mellitus. Examination of the sub-basal corneal nerve plexus is a promising surrogate marker of diabetic neuropathy. To investigate the efficacy, reliability and reproducibility of in vivo corneal confocal microscopy (IVCCM), we used thy1-YFP mice, which express yellow fluorescence protein (YFP) in nerve fibers. 4 weeks after multiple low-dose injections of streptozotocin, thy1-YFP mice showed manifest diabetes. Subsequent application of insulin-releasing pellets for 8 weeks resulted in a significant reduction of blood glucose concentration and HbA_1c, a significant increase in body weight and no further increase in advanced glycation end products (AGEs). IVCCM, carried out regularly over 12 weeks and analyzed both manually and automatically, revealed a significant loss of corneal nerve fiber length (CNFL) during diabetes manifestation and significant recovery after insulin therapy. Ex vivo analyses of CNFL by YFP-based microscopy confirmed the IVCCM results (with high sensitivity between manual and automated approaches) but demonstrated that the changes were restricted to the central cornea. Peripheral areas, not accessible by IVCCM in mice, remained virtually unaffected. Because parallel assessment of intraepidermal nerve fiber density revealed no changes, we conclude that IVCCM robustly captures early signs of diabetic neuropathy.