In vivo peripheral nervous system insulin signaling

Transcriptomic profiling of sciatic nerves and dorsal root ganglia reveals site-specific effects of prediabetic neuropathy

Peripheral neuropathy (PN) is a severe and frequent complication of obesity, prediabetes, and type 2 diabetes characterized by progressive distal-to-proximal peripheral nerve degeneration. However, a comprehensive understanding of the mechanisms underlying PN, and whether these mechanisms change during PN progression, is currently lacking. Here, gene expression data were obtained from distal (sciatic nerve; SCN) and proximal (dorsal root ganglia; DRG) injury sites of a high-fat diet (HFD)-induced mouse model of obesity/prediabetes at early and late disease stages. Self-organizing map and differentially expressed gene analyses followed by pathway enrichment analysis identified genes and pathways altered across disease stage and injury site. Pathways related to immune response, inflammation, and glucose and lipid metabolism were consistently dysregulated with HFD-induced PN, irrespective of injury site. However, regulation of oxidative stress was unique to the SCN while dysregulated Hippo and Notch signaling were only observed in the DRG. The role of the immune system and inflammation in disease progression was supported by an increase in the percentage of immune cells in the SCN with PN progression. Finally, when comparing these data to transcriptomic signatures from human patients with PN, we observed conserved pathways related to metabolic dysregulation across species, highlighting the translational relevance of our mouse data. Our findings demonstrate that PN is associated with distinct site-specific molecular re-programming in the peripheral nervous system, identifying novel, clinically relevant therapeutic targets.

The Effects of Insulin on Immortalized Rat Schwann Cells, IFRS1

Schwann cells play an important role in peripheral nerve function, and their dysfunction has been implicated in the pathogenesis of diabetic neuropathy and other demyelinating diseases. The physiological functions of insulin in Schwann cells remain unclear and therefore define the aim of this study. By using immortalized adult Fischer rat Schwann cells (IFRS1), we investigated the mechanism of the stimulating effects of insulin on the cell proliferation and expression of myelin proteins (myelin protein zero (MPZ) and myelin basic protein (MBP). The application of insulin to IFRS1 cells increased the proliferative activity and induced phosphorylation of Akt and ERK, but not P38-MAPK. The proliferative potential of insulin-stimulated IFRS1 was significantly suppressed by the addition of LY294002, a PI3 kinase inhibitor. The insulin-stimulated increase in MPZ expression was significantly suppressed by the addition of PD98059, a MEK inhibitor. Furthermore, insulin-increased MBP expression was significantly suppressed by the addition of LY294002. These findings suggest that both PI3-K/Akt and ERK/MEK pathways are involved in insulin-induced cell growth and upregulation of MPZ and MBP in IFRS1 Schwann cells.

Sex differences in insulin resistance, but not peripheral neuropathy, in a diet-induced prediabetes mouse model

Peripheral neuropathy (PN) is a common complication of prediabetes and diabetes and is an increasing problem worldwide. Existing PN treatments rely solely on glycemic control, which is effective in type 1 but not type 2 diabetes. Sex differences in response to anti-diabetic drugs further complicate the identification of effective PN therapies. Preclinical research has been primarily carried out in males, highlighting the need for increased sex consideration in PN models. We previously reported PN sex dimorphism in obese leptin-deficient ob/ob mice. This genetic model is inherently limited, however, owing to leptin's role in metabolism. Therefore, the current study goal was to examine PN and insulin resistance in male and female C57BL6/J mice fed a high-fat diet (HFD), an established murine model of human prediabetes lacking genetic mutations. HFD mice of both sexes underwent longitudinal phenotyping and exhibited expected metabolic and PN dysfunction compared to standard diet (SD)-fed animals. Hindpaw thermal latencies to heat were shorter in HFD females versus HFD males, as well as SD females versus males. Compared to HFD males, female HFD mice exhibited delayed insulin resistance, yet still developed the same trajectory of nerve conduction deficits and intraepidermal nerve fiber density loss. Subtle differences in adipokine levels were also noted by sex and obesity status. Collectively, our results indicate that although females retain early insulin sensitivity upon HFD challenge, this does not protect them from developing the same degree of PN as their male counterparts. This article has an associated First Person interview with the first author of the paper.

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

Modulation of Sensory Nerve Function by Insulin: Possible Relevance to Pain, Inflammation and Axon Growth

Insulin, besides its pivotal role in energy metabolism, may also modulate neuronal processes through acting on insulin receptors (InsRs) expressed by neurons of both the central and the peripheral nervous system. Recently, the distribution and functional significance of InsRs localized on a subset of multifunctional primary sensory neurons (PSNs) have been revealed. Systematic investigations into the cellular electrophysiology, neurochemistry and morphological traits of InsR-expressing PSNs indicated complex functional interactions among specific ion channels, proteins and neuropeptides localized in these neurons. Quantitative immunohistochemical studies have revealed disparate localization of the InsRs in somatic and visceral PSNs with a dominance of InsR-positive neurons innervating visceral organs. These findings suggested that visceral spinal PSNs involved in nociceptive and inflammatory processes are more prone to the modulatory effects of insulin than somatic PSNs. Co-localization of the InsR and transient receptor potential vanilloid 1 (TRPV1) receptor with vasoactive neuropeptides calcitonin gene-related peptide and substance P bears of crucial importance in the pathogenesis of inflammatory pathologies affecting visceral organs, such as the pancreas and the urinary bladder. Recent studies have also revealed significant novel aspects of the neurotrophic propensities of insulin with respect to axonal growth, development and regeneration.

Disrupting insulin signaling in Schwann cells impairs myelination and induces a sensory neuropathy

Although diabetic mice have been studied for decades, little is known about the cell type specific contributions to diabetic neuropathy (DN). Schwann cells (SCs) myelinate and provide trophic support to peripheral nervous system axons. Altered SC metabolism leads to myelin defects, which can be seen both in inherited and DNs. How SC metabolism is altered in DN is not fully understood, but it is clear that insulin resistance underlies impaired lipid metabolism in many cell types throughout the body via the phosphoinositide 3-kinase/protein kinase b (PKB)/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway. Here, we created an insulin resistant SC by deleting both insulin receptor (INSR) and insulin-like growth factor receptor 1 (IGF1R), to determine the role of this signaling pathway in development and response to injury in order to understand SC defects in DN. We found that myelin is thinner throughout development and adulthood in INSR/IGF1R Schwann cell specific knock out mice. The nerves of these mutant mice had reduced expression of key genes that mediate fatty acid and cholesterol synthesis due to reduced mTOR-sterol regulatory element-binding protein signaling. In adulthood, these mice show sensory neuropathy phenotypes reminiscent of diabetic mice. Altogether, these data suggest that SCs may play an important role in DN and targeting their metabolism could lead to new therapies for DN.

Insulin Confers Differing Effects on Neurite Outgrowth in Separate Populations of Cultured Dorsal Root Ganglion Neurons: The Role of the Insulin Receptor

Apart from its pivotal role in the regulation of carbohydrate metabolism, insulin exerts important neurotrophic and neuromodulator effects on dorsal root ganglion (DRG) neurons. The neurite outgrowth-promoting effect is one of the salient features of insulin's action on cultured DRG neurons. Although it has been established that a significant population of DRG neurons express the insulin receptor (InsR), the significance of InsR expression and the chemical phenotype of DRG neurons in relation to the neurite outgrowth-promoting effect of insulin has not been studied. Therefore, in this study by using immunohistochemical and quantitative stereological methods we evaluated the effect of insulin on neurite outgrowth of DRG neurons of different chemical phenotypes which express or lack the InsR. Insulin, at a concentration of 10 nM, significantly increased total neurite length, the length of the longest neurite and the number of branch points of cultured DRG neurons as compared to neurons cultured in control medium or in the presence of 1 μM insulin. In both the control and the insulin exposed cultures, ∼43% of neurons displayed InsR-immunoreactivity. The proportions of transient receptor potential vanilloid type 1 receptor (TRPV1)-immunoreactive (IR), calcitonin gene-related peptide (CGRP)-IR and Bandeiraea simplicifolia isolectin B4 (IB4)-binding neurons amounted to ∼61%, ∼57%, and ∼31% of DRG neurons IR for the InsR. Of the IB4-positive population only neurons expressing the InsR were responsive to insulin. In contrast, TRPV1-IR nociceptive and CGRP-IR peptidergic neurons showed increased tendency for neurite outgrowth which was further enhanced by insulin. However, the responsiveness of DRG neurons expressing the InsR was superior to populations of DRG neurons which lack this receptor. The findings also revealed that besides the expression of the InsR, inherent properties of peptidergic, but not non-peptidergic nociceptive neurons may also significantly contribute to the mechanisms of neurite outgrowth of DRG neurons. These observations suggest distinct regenerative propensity for differing populations of DRG neurons which is significantly affected through insulin receptor signaling.

The correlation between insulin and OCT-6 transcription factor in Schwann cells and sciatic nerve of diabetic rats

Insulin signal is one of the vital signaling cascade required for Schwann cells to myelinate the axons of peripheral nervous system (PNS). Myelin formation of peripheral nerve is a complex molecular event controlled by different neurotrophic and transcription factors. The altered or failure in this signaling progression is one of the reasons behind the demyelination of peripheral neurons in diabetic peripheral neuropathy (DPN). The Schwann cell in PNS includes POU domain transcription factor OCT-6 expression. This factor is considered as crucial for the initiation and enhancement of myelination during nerve regeneration. To know the importance of OCT-6 gene, here we studied the long term expression of OCT-6 nuclear protein in sciatic nerve of normal and diabetic neuropathic rats. Also for the first time we elucidated the role of insulin in controlling the expression of OCT-6 in hyperglycemic Schwann cells and sciatic nerve of diabetic neuropathic rats. The results shows that, there will be long term OCT-6 expression in sciatic nerve of adult rats and also their significant decrease is observed in the diabetic condition. But, addition of Insulin for primary Schwann cells and diabetic rats shows the increased OCT-6 expression in both invivo and invitro. Together these results indicate the failure of OCT-6 support in neuropathy and also the importance of insulin signaling cascade in the expression of OCT-6 transcription factor.

Deletion of the insulin receptor in sensory neurons increases pancreatic insulin levels

Insulin is known to have neurotrophic properties and loss of insulin support to sensory neurons may contribute to peripheral diabetic neuropathy (PDN). Here, genetically-modified mice were generated in which peripheral sensory neurons lacked the insulin receptor (SNIRKO mice) to determine whether disrupted sensory neuron insulin signaling plays a crucial role in the development of PDN and whether SNIRKO mice develop symptoms of PDN due to reduced insulin neurotrophic support. Our results revealed that SNIRKO mice were euglycemic and never displayed significant changes in a wide range of sensorimotor behaviors, nerve conduction velocity or intraepidermal nerve fiber density. However, SNIRKO mice displayed elevated serum insulin levels, glucose intolerance, and increased insulin content in the islets of Langerhans of the pancreas. These results contribute to the growing idea that sensory innervation of pancreatic islets is key to regulating islet function and that a negative feedback loop of sensory neuron insulin signaling keeps this regulation in balance. Our results suggest that a loss of insulin receptors in sensory neurons does not lead to peripheral nerve dysfunction. The SNIRKO mice will be a powerful tool to investigate sensory neuron insulin signaling and may give a unique insight into the role that sensory neurons play in modifying islet physiology.

Modulation of diet-induced mechanical allodynia by metabolic parameters and inflammation

Dietary-associated diseases have increased tremendously in our current population, yet key molecular changes associated with high-fat diets that cause clinical pre-diabetes, obesity, hyperglycemia, and peripheral neuropathy remain unclear. This study examines molecular and metabolic aspects altered by voluntary exercise and a high-fat diet in the mouse dorsal root ganglion. Mice were examined for changes in mRNA and proteins encoding anti-inflammatory mediators, metabolic-associated molecules, and pain-associated ion channels. Proteins involved in the synaptosomal complex and pain-associated TRP ion channels decrease in the dorsal root ganglion of high-fat exercise animals relative to their sedentary controls. Exercise reversed high-fat diet induced mechanical allodynia without affecting weight gain, elevated blood glucose, and utilization of fat as a fuel source. Independent of weight or fat mass changes, high-fat exercised mice display reduced inflammation-associated mRNAs. The benefits of exercise on abnormal peripheral nerve function appear to occur independent of systemic metabolic changes, suggesting that the utilization of fats and inflammation in the peripheral nervous system may be key for diet-induced peripheral nerve dysfunction and the response to exercise.

Association of Insulin and Cholesterol Levels With Peripheral Nervous System Function in Overweight Adults: A 3-Year Follow-up

PURPOSE

The purpose of this prospective 3-year follow-up was to investigate the association of glucose, insulin, and cholesterol levels with peripheral nervous system function in overweight and obese subjects.

METHODS

Forty nondiabetic overweight and obese adults were enrolled, of whom 29 completed the follow-up. Peripheral nervous system function was measured and defined by conduction studies of the peroneal motor nerve and the radial, sural, and medial plantar sensory nerves. Serum insulin and glucose levels were determined with an oral glucose tolerance test, and cholesterol levels were measured. The measurements were performed at baseline and after 3 years.

RESULTS

The change in serum insulin level at 120 minutes after an oral glucose tolerance test was positively associated with changes in peroneal nerve conduction velocities and F-wave mean, sural nerve conduction and medial plantar nerve conduction velocities. Action potential amplitudes decreased consistently and significantly in all sensory nerves.

CONCLUSIONS

The change in serum insulin level at 120 minutes appears to be positively associated with changes in nerve conduction velocities more than 3 years but not with nerve action potential amplitudes. Significant decreases in the action potential amplitudes of all sensory nerves suggest that such changes might be the earliest detectable sign of damage to the peripheral nervous system in overweight and obese people without type 2 diabetes.

Insulin prevents aberrant mitochondrial phenotype in sensory neurons of type 1 diabetic rats

Diabetic neuropathy affects approximately 50% of diabetic patients. Down-regulation of mitochondrial gene expression and function has been reported in both human tissues and in dorsal root ganglia (DRG) from animal models of type 1 and type 2 diabetes. We hypothesized that loss of direct insulin signaling in diabetes contributes to loss of mitochondrial function in DRG neurons and to development of neuropathy. Sensory neurons obtained from age-matched adult control or streptozotocin (STZ)-induced type 1 diabetic rats were cultured with or without insulin before determining mitochondrial respiration and expression of mitochondrial respiratory chain and insulin signaling-linked proteins. For in vivo studies age-matched control rats and diabetic rats with or without trace insulin supplementation were maintained for 5months before DRG were analyzed for respiratory chain gene expression and cytochrome c oxidase activity. Insulin (10nM) significantly (P<0.05) increased phosphorylation of Akt and P70S6K by 4-fold and neurite outgrowth by 2-fold in DRG cultures derived from adult control rats. Insulin also augmented the levels of selective mitochondrial respiratory chain proteins and mitochondrial bioenergetics parameters in DRG cultures from control and diabetic rats, with spare respiratory capacity increased by up to 3-fold (P<0.05). Insulin-treated diabetic animals exhibited improved thermal sensitivity in the hind paw and had increased dermal nerve density compared to untreated diabetic rats, despite no effect on blood glucose levels. In DRG of diabetic rats there was suppressed expression of mitochondrial respiratory chain proteins and cytochrome c oxidase activity that was corrected by insulin therapy. Insulin elevates mitochondrial respiratory chain protein expression and function in sensory neurons and this is associated with enhanced neurite outgrowth and protection against indices of neuropathy.

A Role for Insulin in Diabetic Neuropathy

The peripheral nervous system is one of several organ systems that are profoundly affected in diabetes. The longstanding view is that insulin does not have a major role in modulating neuronal function in both central and peripheral nervous systems is now being challenged. In the setting of insulin deficiency or excess insulin, it is logical to propose that insulin dysregulation can contribute to neuropathic changes in sensory neurons. This is particularly important as sensory nerve damage associated with prediabetes, type 1 and type 2 diabetes is so prevalent. Here, we discuss the current experimental literature related to insulin's role as a potential neurotrophic factor in peripheral nerve function, as well as the possibility that insulin deficiency plays a role in diabetic neuropathy. In addition, we discuss how sensory neurons in the peripheral nervous system respond to insulin similar to other insulin-sensitive tissues. Moreover, studies now suggest that sensory neurons can also become insulin resistant like other tissues. Collectively, emerging studies are revealing that insulin signaling pathways are active contributors to sensory nerve modulation, and this review highlights this novel activity and should provide new insight into insulin's role in both peripheral and central nervous system diseases.

Effects of insulin on peripheral nerves

AIMS

To assess the effects of insulin on peripheral nerve under normoglycemic and hyperglycemic conditions in the presence and absence of anoxia.

METHODS

This study uses the in-vitro sciatic nerve model to assess the effect of insulin on peripheral nerve with the nerve action potential (NAP) as an index of nerve function.

RESULTS

Under normoglycemic conditions, low concentrations of regular insulin (0.01nM) reduced the conduction velocity of oxygenated nerves. Hyperglycemia increased the duration of the NAP and this increase was nearly completely eliminated by insulin in the 0.1nM-100nM concentration range. Insulin (1nM) also had effects on normoglycemic nerves exposed to intermittent anoxia, producing a decrease in the paired-pulse response and NAP amplitude and an increase in peak duration. This was associated with a reduced time to anoxia-induced conduction block. Similar effects were seen when regular insulin was replaced by insulin detemir, but the latter required much higher concentrations.

CONCLUSIONS

Insulin has concentration dependent effects on the peripheral nerve that are dependent on glucose and anoxia. These effects may be important in modulating neuropathic consequences of diabetes.

Insulin influenced expression of myelin proteins in diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is one of the downstream complications of diabetes. This complication is caused by the deficiency of insulin action and subsequent hyperglycemia, but the details of their pathogenesis remain unclear. Hence, it is of critical importance to understand how such hormonal variation affects the expression of myelin proteins such as myelin basic protein (MBP) and myelin associated glycoprotein (MAG) in the peripheral nerve. An earlier report from our lab has demonstrated the expression of insulin receptors (IR) in Schwann cells (SCs) of sciatic nerve. To assess the neurotrophic role of insulin in diabetic neuropathy, we studied the expression of these myelin proteins under control, DPN and insulin treated DPN subjects at developmental stages. Further, the expression of these myelin proteins was correlated with the expression of insulin receptor. Expression of myelin proteins was significantly reduced in the diabetic model compared to normal, and upregulated in insulin treated diabetic rats. Similarly, an in vitro study was also carried out in SCs grown at high glucose and insulin treated conditions. The expression pattern of myelin proteins in SCs was comparable to that of in vivo samples. In addition, quantitative study of myelin genes by real time PCR has also showed the significant expression pattern change in the insulin treated and non-treated DPN subjects. Taken together, these results corroborate the critical importance of insulin as a neurotrophic factor in demyelinized neurons in diabetic neuropathy.

The dipeptidyl peptidase IV inhibitor vildagliptin suppresses development of neuropathy in diabetic rodents: effects on peripheral sensory nerve function, structure and molecular changes

Incretin-related therapy was found to be beneficial for experimental diabetic neuropathy, but its mechanism is obscure. The purpose of this study is to explore the mechanism through which dipeptidyl peptidase IV inhibitor, vildagliptin (VG), influences neuropathy in diabetic rodents. To this end, non-obese type 2 diabetic Goto-Kakizaki rats (GK) and streptozotocin (STZ)-induced diabetic mice were treated with VG orally. Neuropathy was evaluated by nerve conduction velocity (NCV) in both GK and STZ-diabetic mice, whereas calcitonin-gene-related peptide expressions, neuronal cell size of dorsal root ganglion (DRG) and intraepidermal nerve fiber density were examined in GK. DRG from GK and STZ-diabetic mice served for the analyses of GLP-1 and insulin signaling. As results, VG treatment improved glucose intolerance and increased serum insulin and GLP-1 in GK accompanied by the amelioration of delayed NCV and neuronal atrophy, reduced calcitonin-gene-related peptide expressions and intraepidermal nerve fiber density. Diet restriction alone did not significantly influence these measures. Impaired GLP-1 signals such as cAMP response element binding protein, protein kinase B/Akt (PKB/Akt) and S6RP in DRG of GK were restored in VG-treated group, but the effect was equivocal in diet-treated GK. Concurrently, decreased phosphorylation of insulin receptor substrate 2 in GK was corrected by VG treatment. Consistent with the effect on GK, VG treatment improved NCV in diabetic mice without influence on hyperglycemia. DRG of VG-treated diabetic mice were characterized by correction of GLP-1 signals and insulin receptor substrate 2 phosphorylation without effects on insulin receptor β expression. The results suggest close association of neuropathy development with impaired signaling of insulin and GLP-1 in diabetic rodents. Diabetic neurons are resistant to insulin and such insulin resistance may contribute to development of neuropathy. DPP-IV inhibitor, vildagliptin, corrected insulin resistance and improved neuropathy irrespective of blood glucose via augmented action of GLP-1.

Activation of the insulin-signaling pathway in sciatic nerve and hippocampus of type 1 diabetic rats

Peripheral neuropathy is a major complication associated with diabetes and central neuropathy characterized by Alzheimer's disease-like features in the brain is associated with increased dementia risk for patients with diabetes. Although glucose uptake into the cells of the nervous system is insulin-independent, contribution of impaired insulin support is clearly recognized to play a role, however not yet fully understood, in the development of neuropathy. In this study, we assessed the direct role of insulin on the peripheral nervous system (PNS) and central nervous system (CNS) of insulin-dependent type 1 diabetic rats. Fresh sciatic nerve and hippocampus from control and diabetic rats were incubated with varied ex vivo concentrations of insulin and phosphorylation levels of insulin receptor and glycogen synthase kinase-3 (GSK3β) were assessed by Western blot analysis. Both the sciatic nerve and hippocampus from type 1 diabetic rats were highly responsive to exogenous insulin with a significantly increased phosphorylation of insulin receptor and GSK3 compared to tissues from control rats. Further, sustained in vivo insulin delivery, not sufficient to restore normal blood glucose, normalized the activation of both insulin receptor and GSK3 in both PNS and CNS tissues. These results suggest that the insulin-signaling pathway is responsive to exogenous insulin in the nervous system of insulin-deficient type 1 diabetic rats and that constant insulin delivery restore normal nerve function and may protect PNS and CNS from damage.