Combined PTEN Knockdown and Local Insulin in Chronic Experimental Diabetic Neuropathy.

Diabetic polyneuropathy (DPN) renders progressive sensory neurodegeneration linked to hyperglycemia and its associated metabolopathy. We hypothesized that there may be additive impacts of direct insulin signaling, independent of glycemia and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) knockdown on neuropathy. Our targets for combined interventions were neurons and Schwann cells (SCs) in vitro and chronic type 1 DPN in mice. Insulin receptor expression was not altered by high-glucose conditions in neurons or SCs, and insulin promoted survival of neurons and proliferation of SCs in vitro. There were additive impacts between insulin signaling and PTEN knockdown in sensory neuron outgrowth and in axon myelination by SCs. In a chronic mouse model of experimental DPN, unilateral intra-hind paw injections of a PTEN siRNA and local insulin had additive impacts on correcting key features of chronic experimental DPN independent of glycemia, including motor axon conduction and thermal and mechanical sensory loss. Moreover, combined interventions improved sural and tibial nerve myelin thickness, hind paw epidermal innervation, and pAkt expression in dorsal root ganglion sensory neurons. We conclude that local PTEN inhibition or knockdown and insulin provide additive trophic support for sensory neurons and SCs while reversing key abnormalities of experimental DPN but without requiring metabolic correction. ARTICLE HIGHLIGHTS: Impaired growth and plasticity of neurons may contribute to chronic diabetic polyneuropathy. Both direct insulin signaling of neurons and neuron knockdown of the protein phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a roadblock to neuronal regenerative growth, offer direct support of neurons. Direct insulin and PTEN knockdown using siRNA had additive impacts on neuron survival, Schwann cell proliferation, neuron outgrowth, and myelination in vitro. Combined local insulin and PTEN siRNA hind paw injections improved abnormalities in chronic experimental diabetic polyneuropathy, including sensory axon loss, independently of glycemia.

Insulin enhances neurite extension and myelination of diabetic neuropathy neurons.

Background: The authors established an in vitro model of diabetic neuropathy based on the culture system of primary neurons and Schwann cells (SCs) to mimic similar symptoms observed in in vivo models of this complication, such as impaired neurite extension and impaired myelination. The model was then utilized to investigate the effects of insulin on enhancing neurite extension and myelination of diabetic neurons. Methods: SCs and primary neurons were cultured under conditions mimicking hyperglycemia prepared by adding glucose to the basal culture medium. In a single culture, the proliferation and maturation of SCs and the neurite extension of neurons were evaluated. In a co-culture, the percentage of myelination of diabetic neurons was investigated. Insulin at different concentrations was supplemented to culture media to examine its effects on neurite extension and myelination. Results: The cells showed similar symptoms observed in in vivo models of this complication. In a single culture, hyperglycemia attenuated the proliferation and maturation of SCs, induced apoptosis, and impaired neurite extension of both sensory and motor neurons. In a co-culture of SCs and neurons, the percentage of myelinated neurites in the hyperglycemia-treated group was significantly lower than that in the control group. This impaired neurite extension and myelination was reversed by the introduction of insulin to the hyperglycemic culture media. Conclusions: Insulin may be a potential candidate for improving diabetic neuropathy. Insulin can function as a neurotrophic factor to support both neurons and SCs. Further research is needed to discover the potential of insulin in improving diabetic neuropathy.

Diabetic neuropathy research: from mouse models to targets for treatment.

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

Impaired peripheral nerve regeneration in type-2 diabetic mouse model.

Peripheral neuropathy is one of the most common and serious complications of type-2 diabetes. Diabetic neuropathy is characterized by a distal symmetrical sensorimotor polyneuropathy, and its incidence increases in patients 40 years of age or older. In spite of extensive research over decades, there are few effective treatments for diabetic neuropathy besides glucose control and improved lifestyle. The earliest changes in diabetic neuropathy occur in sensory nerve fibers, with initial degeneration and regeneration resulting in pain. To seek its effective treatment, here we prepared a type-2 diabetic mouse model by giving mice 2 injections of streptozotocin and nicotinamide and examining the ability for nerve regeneration by using a sciatic nerve transection-regeneration model previously established by us. Seventeen weeks after the last injection, the mice exhibited symptoms of type-2 diabetes, that is, impaired glucose tolerance, decreased insulin level, mechanical hyperalgesia, and impaired sensory nerve fibers in the plantar skin. These mice showed delayed functional recovery and nerve regeneration by 2 weeks compared with young healthy mice and by 1 week compared with age-matched non-diabetic mice after axotomy. Furthermore, type-2 diabetic mice displayed increased expression of PTEN in their DRG neurons. Administration of a PTEN inhibitor at the cutting site of the nerve for 4 weeks promoted the axonal transport and functional recovery remarkably. This study demonstrates that peripheral nerve regeneration was impaired in type-2 diabetic model and that its combination with sciatic nerve transection is suitable for the study of the pathogenesis and treatment of early diabetic neuropathy.