Axonal transport of choline acetyltransferase and 6-phosphofructokinase activities in genetically diabetic mice

Selective antagonism of muscarinic receptors is neuroprotective in peripheral neuropathy

Sensory neurons have the capacity to produce, release, and respond to acetylcholine (ACh), but the functional role of cholinergic systems in adult mammalian peripheral sensory nerves has not been established. Here, we have reported that neurite outgrowth from adult sensory neurons that were maintained under subsaturating neurotrophic factor conditions operates under cholinergic constraint that is mediated by muscarinic receptor-dependent regulation of mitochondrial function via AMPK. Sensory neurons from mice lacking the muscarinic ACh type 1 receptor (M1R) exhibited enhanced neurite outgrowth, confirming the role of M1R in tonic suppression of axonal plasticity. M1R-deficient mice made diabetic with streptozotocin were protected from physiological and structural indices of sensory neuropathy. Pharmacological blockade of M1R using specific or selective antagonists, pirenzepine, VU0255035, or muscarinic toxin 7 (MT7) activated AMPK and overcame diabetes-induced mitochondrial dysfunction in vitro and in vivo. These antimuscarinic drugs prevented or reversed indices of peripheral neuropathy, such as depletion of sensory nerve terminals, thermal hypoalgesia, and nerve conduction slowing in diverse rodent models of diabetes. Pirenzepine and MT7 also prevented peripheral neuropathy induced by the chemotherapeutic agents dichloroacetate and paclitaxel or HIV envelope protein gp120. As a variety of antimuscarinic drugs are approved for clinical use against other conditions, prompt translation of this therapeutic approach to clinical trials is feasible.

Age-dependent slowing of enteric axonal transport in insulin-resistant mice

AIM: To investigate retrograde tracer transport by gastric enteric neurons in insulin resistant mice with low or high glycosylated hemoglobin (Hb).

METHODS: Under anesthesia, the retrograde tracer fluorogold was superficially injected into the fundus or antrum using a microsyringe in KK Cg-Ay/J mice prior to onset of type 2 diabetes mellitus (T2DM; 4 wk of age), at onset of T2DM (8 wk of age), and after 8, 16, or 24 wk of untreated T2DM and in age-matched KK/HIJ mice. Six days later, mice were sacrificed by CO₂ narcosis followed by pneumothorax. Stomachs were removed and fixed. Sections from fundus, corpus and antrum were excised and mounted on a glass slide. Tracer-labeled neurons were viewed using a microscope and manually counted. Data were expressed as the number of neurons in short and long descending and ascending pathways and in local fundus and antrum pathways, and the number of neurons in all regions labeled after injection of tracer into either the fundus or the antrum.

RESULTS: By 8 wk of age, body weights of KKAy mice (n = 12, 34 ± 1 g) were heavier than KK mice (n = 17, 29 ± 1 g; F (4, 120) = 4.414, P = 0.002] and glycosylated Hb was higher [KK: (n = 7), 4.97% ± 0.04%; KKAy: (n = 6), 6.57% ± 0.47%; F (1, 26) = 24.748, P < 0.001]. The number of tracer labeled enteric neurons was similar in KK and KKAy mice of all ages in the short descending pathway [F (1, 57) = 2.374, P = 0.129], long descending pathway [F (1, 57) = 0.922, P = 0.341], local fundus pathway [F (1, 53) = 2.464, P = 0.122], local antrum pathway [F (1, 57) = 0.728, P = 0.397], and short ascending pathway [F (1, 53) = 2.940, P = 0.092]. In the long ascending pathway, fewer tracer-labeled neurons were present in KKAy as compared to KK mice [KK: (n = 34), 302 ± 17; KKAy: (n = 29), 230 ± 15; F (1, 53) = 8.136, P = 0.006]. The number of tracer-labeled neurons was decreased in all mice by 16 wk as compared to 8 wk of age in the short descending pathway [8 wk: (n = 15), 305 ± 26; 16 wk: (n = 13), 210 ± 30; F (4, 57) = 9.336, P < 0.001], local antrum pathway [8 wk: (n = 15), 349 ± 20; 16 wk: (n = 13), 220 ± 33; F (4, 57) = 8.920, P < 0.001], short ascending pathway [8 wk: (n = 14), 392 ± 15; 16 wk: (n = 14), 257 ± 33; F (4, 53) = 17.188, P < 0.001], and long ascending pathway [8 wk: (n = 14), 379 ± 39; 16 wk: (n = 14), 235 ± 26; F (4, 53) = 24.936, P < 0.001]. The number of tracer-labeled neurons decreased at 24 wk of age in the local fundus pathway [8 wk: (n = 14), 33 ± 11; 24 wk: (n = 12), 3 ± 2; F (4, 53) = 5.195, P = 0.001] and 32 wk of age in the long descending pathway [8 wk: (n = 15), 16 ± 3; 32 wk: (n = 12), 3 ± 2; F (4, 57) = 2.944, P = 0.028]. The number of tracer-labeled enteric neurons was correlated to final body weight for local fundus and ascending pathways [KK: (n = 34), r = -0.746, P < 0.001; KKAy: (n = 29), r = -0.842, P < 0.001] as well as local antrum and descending pathways [KK (n = 36), r = -0.660, P < 0.001; KKAy (n = 31), r = -0.622, P < 0.001]. In contrast, glycosylated Hb was not significantly correlated to number of tracer-labeled neurons [KK (n = 17), r = -0.164, P = 0.528; KKAy (n = 16), r = -0.078, P = 0.774].

CONCLUSION: Since uncontrolled T2DM did not uniformly impair tracer transport in gastric neurons, long ascending neurons may be more susceptible to persistent hyperglycemia and low effective insulin.

Exacerbated brain damage, edema and inflammation in type-2 diabetic mice subjected to focal ischemia

One of the limiting factors in stroke therapeutic development is the use of animal models that do not well represent the underlying medical conditions of patients. In humans, diabetes increases the risk of stroke incidence as well as post-stroke mortality. To understand the mechanisms that render diabetics to increased brain damage, we evaluated the effect of transient middle cerebral artery occlusion in adult db/db mice. The db/db mouse is a model of type-2 diabetes with four times higher blood sugar than its normoglycemic genetic control(db/+ mouse). Following transient middle cerebral artery occlusion, the db/db mice showed significantly higher mortality, bigger infarcts, increased cerebral edema, worsened neurological status compared to db/+ mice. The db/db mice also showed significantly higher post-ischemic inflammatory markers (ICAM1(+) capillaries, extravasated macrophages/neutrophils and exacerbated proinflammatory gene expression) compared to db/+ mice. In addition, the post-ischemic neuroprotective heat-shock chaperone gene expression was curtailed in the db/db compared to db/+ mice.

More severe type 2 diabetes-associated ischemic stroke injury is alleviated in aldose reductase-deficient mice

Aldose reductase (AR), the first enzyme in the polyol pathway, has been implicated in a wide variety of physiological and pathological functions, such as diabetic vascular and neural complications. It is known that diabetes mellitus can exacerbate brain and retina damage after ischemic injuries. However, the underlying mechanisms are not clear. In the present study, we made use of db/db mice with an AR null mutation (AR(-/-)db/db) to understand better the role of AR in the pathogenesis of brain and retina ischemic injuries under diabetic conditions. Cerebral and retinal ischemia was induced by transient middle cerebral artery occlusion in control and diabetic mice either with or without an AR null mutation. Mice were evaluated for neurological deficits after 30 min of ischemia and 23.5 hr of reperfusion. Our results showed that the diabetic db/db mice had significantly more severe neurological deficit and larger brain infarct size than the nondiabetic mice. Compared with wild-type db/db mice, the AR(-/-)db/db mice had significantly lower neurological scores, smaller brain infarct areas, and less hemispheric brain swelling. Retinal swelling was also significantly decreased in the AR(-/-)db/db mice. Less swelling in the brain and retina of the AR(-/-)db/db mice correlated with less expression of the water channel aquaporin 4. Taken together, these data clearly show that deletion of AR leads to less severe brain and retinal ischemic injuries in the diabetic db/db mouse. The present study indicates that inhibition of AR in diabetics may protect against damage in the brain and retina following ischemic reperfusion injury.

Aldose reductase deficiency prevents diabetes-induced blood-retinal barrier breakdown, apoptosis, and glial reactivation in the retina of db/db mice

In 15-month-old db/db mice, signs of diabetic retinopathy, including blood-retinal barrier breakdown, loss of pericytes, neuro-retinal apoptosis, glial reactivation, and proliferation of blood vessels, were evident. These changes in the diabetic retina were associated with increased expression of aldose reductase (AR). To further understand the role of AR in the pathogenesis of diabetic retinopathy, we generated db/db mice with an AR null mutation (AR-/- db/db). AR deficiency led to fewer retinal blood vessels with IgG leakage, suggesting that AR may contribute to blood-retinal barrier breakdown. AR deficiency also prevented diabetes-induced reduction of platelet/endothelial cell adhesion molecule-1 expression and increased expression of vascular endothelial growth factor, which may have contributed to blood-retinal barrier breakdown. In addition, long-term diabetes-induced neuro-retinal stress and apoptosis and proliferation of blood vessels were less prominent in AR-/- db/db mice. These findings indicate that AR is responsible for the early events in the pathogenesis of diabetic retinopathy, leading to a cascade of retinal lesions, including blood-retinal barrier breakdown, loss of pericytes, neuro-retinal apoptosis, glial reactivation, and neovascularization.

Defective activity of Na+,K(+)-ATPase in peripheral nerve of diabetic rats is independent of the axonal transport of the enzyme

This study addressed the question as to whether the reduced activity of Na+,K(+)-ATPase reported to occur in diabetic nerves and to play a crucial role in the pathogenesis of diabetic neuropathy could be due to derangements in the axonal transport of the enzyme. A micromethod was developed to evaluate the ATPase accumulation in individual segments of ligated sciatic nerves from streptozotocin-induced diabetic rats. The results confirmed a approximately 40% decrease in the background activity, but showed that the enzyme was transported at similar rates in both anterograde and retrograde directions, suggesting that the decrease in its activity does not depend on an altered delivery along the axons.