Nicotinamide Riboside Opposes Type 2 Diabetes and Neuropathy in Mice.

Male C57BL/6J mice raised on high fat diet (HFD) become prediabetic and develop insulin resistance and sensory neuropathy. The same mice given low doses of streptozotocin are a model of type 2 diabetes (T2D), developing hyperglycemia, severe insulin resistance and diabetic peripheral neuropathy involving sensory and motor neurons. Because of suggestions that increased NAD(+) metabolism might address glycemic control and be neuroprotective, we treated prediabetic and T2D mice with nicotinamide riboside (NR) added to HFD. NR improved glucose tolerance, reduced weight gain, liver damage and the development of hepatic steatosis in prediabetic mice while protecting against sensory neuropathy. In T2D mice, NR greatly reduced non-fasting and fasting blood glucose, weight gain and hepatic steatosis while protecting against diabetic neuropathy. The neuroprotective effect of NR could not be explained by glycemic control alone. Corneal confocal microscopy was the most sensitive measure of neurodegeneration. This assay allowed detection of the protective effect of NR on small nerve structures in living mice. Quantitative metabolomics established that hepatic NADP(+) and NADPH levels were significantly degraded in prediabetes and T2D but were largely protected when mice were supplemented with NR. The data justify testing of NR in human models of obesity, T2D and associated neuropathies.

Rat Models of Diet-Induced Obesity and High Fat/Low Dose Streptozotocin Type 2 Diabetes: Effect of Reversal of High Fat Diet Compared to Treatment with Enalapril or Menhaden Oil on Glucose Utilization and Neuropathic Endpoints.

We examined whether reversal of high fat diet, stimulating weight loss, compared to two treatments previously shown to have beneficial effects, could improve glucose utilization and peripheral neuropathy in animal models of obesity and type 2 diabetes. Rats were fed a high fat diet and treated with a low dose of streptozotocin to create models of diet induced obesity or type 2 diabetes, respectively. Afterwards, rats were transferred to a normal diet or treated with enalapril or dietary enrichment with menhaden oil for 12 weeks. Obesity and to a greater extent type 2 diabetes were associated with impaired glucose utilization and peripheral neuropathy. Placing obese rats on a normal diet improved glucose utilization. Steatosis but not peripheral neuropathy was improved after placing obese or diabetic rats on a normal diet. Treating obese and diabetic rats with enalapril or a menhaden oil enriched diet generally improved peripheral neuropathy endpoints. In summary, dietary improvement with weight loss in obese or type 2 diabetic rats was not sufficient to correct peripheral neuropathy. These results further stress the need for discovery of a comprehensive treatment for peripheral neuropathy.

Effect of combination therapy consisting of enalapril, α-lipoic acid, and menhaden oil on diabetic neuropathy in a high fat/low dose streptozotocin treated rat.

We have previously demonstrated that treating diabetic rats with enalapril, an angiotensin converting enzyme (ACE) inhibitor, α-lipoic acid, an antioxidant, or menhaden oil, a natural source of omega-3 fatty acids can partially improve diabetic peripheral neuropathy. In this study we sought to determine the efficacy of combining these three treatments on vascular and neural complications in a high fat fed low dose streptozotocin treated rat, a model of type 2 diabetes. Rats were fed a high fat diet for 8 weeks followed by a 30 mg/kg dose of streptozotocin. Eight weeks after the onset of hyperglycemia diabetic rats were treated with a combination of enalapril, α-lipoic acid and menhaden oil. Diabetic rats not receiving treatment were continued on the high fat diet. Glucose clearance was impaired in diabetic rats and significantly improved with treatment. Diabetes caused steatosis, elevated serum lipid levels, slowing of motor and sensory nerve conduction, thermal hypoalgesia, reduction in intraepidermal nerve fiber profiles, decrease in cornea sub-basal nerve fiber length and corneal sensitivity and impairment in vascular relaxation to acetylcholine and calcitonin gene-related peptide in epineurial arterioles of the sciatic nerve. Treating diabetic rats with the combination of enalapril, α-lipoic acid and menhaden oil reversed all these deficits to near control levels except for motor nerve conduction velocity which was also significantly improved compared to diabetic rats but remained significantly decreased compared to control rats. These studies suggest that a combination therapeutic approach may be most effective for treating vascular and neural complications of type 2 diabetes.

Effect of enriching the diet with menhaden oil or daily treatment with resolvin D1 on neuropathy in a mouse model of type 2 diabetes.

The purpose of this study was to determine the effect of supplementing the diet of a mouse model of type 2 diabetes with menhaden (fish) oil or daily treatment with resolvin D1 on diabetic neuropathy. The end points evaluated included motor and sensory nerve conduction velocity, thermal sensitivity, innervation of sensory nerves in the cornea and skin, and the retinal ganglion cell complex thickness. Menhaden oil is a natural source for n-3 polyunsaturated fatty acids, which have been shown to have beneficial effects in other diseases. Resolvin D1 is a metabolite of docosahexaenoic acid and is known to have anti-inflammatory and neuroprotective properties. To model type 2 diabetes, mice were fed a high-fat diet for 8 wk followed by a low dosage of streptozotocin. After 8 wk of hyperglycemia, mice in experimental groups were treated for 6 wk with menhaden oil in the diet or daily injections of 1 ng/g body wt resolvin D1. Our findings show that menhaden oil or resolvin D1 did not improve elevated blood glucose, HbA1C, or glucose utilization. Untreated diabetic mice were thermal hypoalgesic, had reduced motor and sensory nerve conduction velocities, had decreased innervation of the cornea and skin, and had thinner retinal ganglion cell complex. These end points were significantly improved with menhaden oil or resolvin D1 treatment. Exogenously, resolvin D1 stimulated neurite outgrowth from primary cultures of dorsal root ganglion neurons from normal mice. These studies suggest that n-3 polyunsaturated fatty acids derived from fish oil could be an effective treatment for diabetic neuropathy.

Vitamin A deficiency alters the pulmonary parenchymal elastic modulus and elastic fiber concentration in rats.

BACKGROUND: Bronchial hyperreactivity is influenced by properties of the conducting airways and the surrounding pulmonary parenchyma, which is tethered to the conducting airways. Vitamin A deficiency (VAD) is associated with an increase in airway hyperreactivity in rats and a decrease in the volume density of alveoli and alveolar ducts. To better define the effects of VAD on the mechanical properties of the pulmonary parenchyma, we have studied the elastic modulus, elastic fibers and elastin gene-expression in rats with VAD, which were supplemented with retinoic acid (RA) or remained unsupplemented. METHODS: Parenchymal mechanics were assessed before and after the administration of carbamylcholine (CCh) by determining the bulk and shear moduli of lungs that that had been removed from rats which were vitamin A deficient or received a control diet. Elastin mRNA and insoluble elastin were quantified and elastic fibers were enumerated using morphometric methods. Additional morphometric studies were performed to assess airway contraction and alveolar distortion. RESULTS: VAD produced an approximately 2-fold augmentation in the CCh-mediated increase of the bulk modulus and a significant dampening of the increase in shear modulus after CCh, compared to vitamin A sufficient (VAS) rats. RA-supplementation for up to 21 days did not reverse the effects of VAD on the elastic modulus. VAD was also associated with a decrease in the concentration of parenchymal elastic fibers, which was restored and was accompanied by an increase in tropoelastin mRNA after 12 days of RA-treatment. Lung elastin, which was resistant to 0.1 N NaOH at 98 degrees, decreased in VAD and was not restored after 21 days of RA-treatment. CONCLUSION: Alterations in parenchymal mechanics and structure contribute to bronchial hyperreactivity in VAD but they are not reversed by RA-treatment, in contrast to the VAD-related alterations in the airways.

Retinoic acid reverses the airway hyperresponsiveness but not the parenchymal defect that is associated with vitamin A deficiency.

Airway hyperresponsiveness (AHR) is influenced by structural components of the bronchial wall, including the smooth muscle and connective tissue elements and the neuromuscular function. AHR is also influenced by parenchymally derived tethering forces on the bronchial wall, which maintain airway caliber by producing outward radial traction. Our previous work has shown that vitamin A-deficient (VAD) rats exhibit cholinergic hyperresponsiveness and a decrease in the expression and function of the muscarinic-2 receptors (M2R). We hypothesized that if decreases in radial traction from airway or parenchymal structures contributed to the VAD-related increase in AHR, then the radial traction would normalize more slowly than VAD-related alterations in neurotransmitter signaling. Rats remained vitamin A sufficient (VAS) or were rendered VAD and then maintained on the VAD diet in the presence or absence of supplementation with all-trans retinoic acid (RA). VAD was associated with an approximately twofold increase in respiratory resistance and elastance compared with VAS rats. Exposure to RA for 12 days but not 4 days restored resistance and elastance to control (VAS) levels. In VAD rats, AHR was accompanied by decreases in bronchial M2R gene expression and function, which were restored after 12 days of RA supplementation. Subepithelial bronchial elastic fibers were decreased by approximately 50% in VAD rats and were significantly restored by RA. The increase in AHR that is associated with VAD is accompanied by decreases in M2R expression and function that can be restored by RA and a reduction in airway elastic fibers that can be partially restored by RA.