Peripheral nervous system insulin resistance in ob/ob mice
© Grote et al.; licensee BioMed Central Ltd. 2013
Received: 20 March 2013
Accepted: 19 April 2013
Published: 10 May 2013
A reduction in peripheral nervous system (PNS) insulin signaling is a proposed mechanism that may contribute to sensory neuron dysfunction and diabetic neuropathy. Neuronal insulin resistance is associated with several neurological disorders and recent evidence has indicated that dorsal root ganglion (DRG) neurons in primary culture display altered insulin signaling, yet in vivo results are lacking. Here, experiments were performed to test the hypothesis that the PNS of insulin-resistant mice displays altered insulin signal transduction in vivo. For these studies, nondiabetic control and type 2 diabetic ob/ob mice were challenged with an intrathecal injection of insulin or insulin-like growth factor 1 (IGF-1) and downstream signaling was evaluated in the DRG and sciatic nerve using Western blot analysis.
The results indicate that insulin signaling abnormalities documented in other “insulin sensitive” tissues (i.e. muscle, fat, liver) of ob/ob mice are also present in the PNS. A robust increase in Akt activation was observed with insulin and IGF-1 stimulation in nondiabetic mice in both the sciatic nerve and DRG; however this response was blunted in both tissues from ob/ob mice. The results also suggest that upregulated JNK activation and reduced insulin receptor expression could be contributory mechanisms of PNS insulin resistance within sensory neurons.
These findings contribute to the growing body of evidence that alterations in insulin signaling occur in the PNS and may be a key factor in the pathogenesis of diabetic neuropathy.
KeywordsDiabetic neuropathy Neuronal insulin resistance Neurotrophic support
Diabetes and metabolic syndrome are risk factors for several neurological diseases, and emerging evidence has indicated that neuronal insulin resistance may be involved in disease pathogenesis . While altered insulin signaling is known to be the key factor in the development of diabetes, the role that it plays in diabetic neuropathy (DN) is not well understood. However, it has been demonstrated that neuronally-targeted insulin treatment can improve signs of neuropathy without altering blood glucose levels [2–4]. Recent evidence suggests that cultured sensory neurons from insulin-resistant mice display classic signs of insulin resistance and that insulin resistance may be contributing to mitochondrial dysfunction and increased ROS in DN [5–7]. Furthermore, clinical evidence has reported that insulin resistance appears to be an independent risk factor for both autonomic and peripheral neuropathy .
Although neurons do not appear to rely on insulin for glucose uptake , insulin does have an important role in both the CNS and PNS. Insulin promotes in vivo nerve regeneration [4, 10, 11], induces neurite outgrowth [12, 13], maintains neuronal mitochondrial function [14, 15], supports memory formation [16, 17], and regulates hypothalamic metabolic control [18, 19]. While the exact mechanisms through which insulin promotes these functions remain unclear, insulin is considered a potent neurotrophic factor key to maintaining proper neuronal function.
Insulin and insulin-like growth factor 1 (IGF-1) signaling is propagated by phosphorylation events that begin with the intrinsic tyrosine kinase activity of the insulin or IGF receptor (reviewed in [20, 21]) and continue with subsequent activation of both the PI3K-Akt and MAPK cascades. While these pathways are well defined in muscle, adipose, and liver, insulin signaling and its actions in the PNS are poorly understood.
In an insulin-resistant state, the cellular effects of insulin are blunted due to improper signal propagation resulting from several different mechanisms, including 1) degradation of the insulin receptor [22–25], 2) removal of key tyrosine phosphorylation sites by overactivation of protein tyrosine phosphatases [26–29], and 3) increased phosphorylation at inhibitory IRS serine residues due to elevated stress kinases, such as JNK [30–35]. However, the extent to which these mechanisms affect insulin signal transduction in the PNS is not clear.
Growing evidence suggests that neurons may become insulin resistant similar to other tissues. However, no in vivo evidence of PNS insulin resistance has been presented, and the cellular mechanisms associated with PNS insulin resistance have not been thoroughly investigated. Here, we demonstrate that the DRG and sciatic nerve of ob/ob mice display reduced insulin signaling in response to an intrathecal injection of insulin. Furthermore, the PNS of ob/ob mice has alterations in cellular mechanisms of insulin resistance, including decreased DRG insulin receptor expression and upregulation of JNK activity in the sciatic nerve.
Insulin resistance in ob/ob mice
Mechanical allodynia in ob/ob mice
Blunted insulin and IGF-1 Akt activation in ob/ob DRG and sciatic nerve
Insulin stimulation causes a robust activation of Akt in insulin-sensitive tissues like muscle and adipose, as well as in neurons of both the peripheral and central nervous systems. Moreover, reduced insulin-induced activation of Akt is a hallmark of insulin resistance [5, 6, 37, 38]. Here, nondiabetic and diabetic ob/ob mice were administered either intrathecal PBS or insulin and the DRG and sciatic nerve were harvested 30 minutes later for Western blot analysis to assess Akt activation. Both nondiabetic control and ob/ob mice display significantly elevated blood glucose levels following intrathecal injection of PBS. Nondiabetic mice glucose levels increased from 6.6 ± 0.4 mmol/L to 8.6 ± 0.5 mmol/L, whereas ob/ob levels increased from 12.1 ± 2.4 mmol/L to 22.3 ± 2.4 mmol/L. Glucose levels in nondiabetic mice significantly decreased from 7.0 ± 0.3 mmol/L to 3.3 ± 0.3 mmol/L after intrathecal insulin injection. Ob/ob mice glucose levels 30 minutes after insulin injection were not significantly different from baseline, starting at 12.7 ± 0.9 mmol/L and ending at 12.3 ± 1.5 mmol/L after 30 minutes.
Insulin signaling downstream of Akt in the DRG and sciatic nerve
Downstream Akt pathway activation in the DRG and sciatic nerve after intrathecal insulin stimulation
Protein of interest
Insulin-induced fold change
Insulin-induced fold change
Insulin-induced fold change
Insulin-induced fold change
The PNS of ob/ob mice display reduced insulin receptor expression and increased JNK activation
Our previous studies in primary DRG cultures reported an upregulation of IRS2 serine phosphorylation , a recognized mechanism of insulin resistance in muscle and adipose. In the current study, we investigated both IRS1 (muscle and adipose isoform)  and IRS2 (neural isoform) [6, 42] serine phosphorylation. In contrast to neurons in vitro, IRS serine phosphorylation does not appear to be significantly affected in the PNS in vivo within this model, (data not shown). Interestingly, there was significant activation of the stress kinase JNK (p(Thr183/Tyr185)JNK/total JNK) in the sciatic nerve of ob/ob mice compared to nondiabetic mice (Figure 6D) and a similar pattern of activated JNK was observed in the DRG of ob/ob mice, however significance was not reached (nondiabetic vs. diabetic p = 0.122) (Figure 6C).
In addition to stress kinase activation and reduced insulin receptor expression, insulin resistance can also be induced by over activation of tyrosine phosphatases . However, PTP1B expression was not elevated in the DRG or sciatic nerve of ob/ob mice, nor did insulin stimulation appear to alter its expression levels (Figure 6E,F, respectively).
Diabetic neuropathy is associated with profound loss of distal limb sensation and/or pain, causing significant decline in the quality of life and potential morbidity and mortality for patients. Currently, there are no clinical treatments that successfully improve neuropathic damage to peripheral sensory nerve fibers, likely due to the multifactorial etiology of neuropathy development and progression. Here, we have demonstrated in vivo PNS insulin resistance in ob/ob mice. These results are consistent with recent in vitro studies and support the view that altered insulin signaling may contribute to DN . A robust activation of insulin-sensitive pathways was observed in the DRG and sciatic nerve of nondiabetic mice, with a blunted response in both tissues from insulin-resistant ob/ob mice. While no one mechanism of insulin resistance was clearly prevalent, significant changes were seen in two known pathways of insulin resistance, including increased JNK activity and reduced insulin receptor expression. Although more research is needed to fully elucidate the pathways leading to PNS insulin resistance, these results suggest that cellular mechanisms of insulin resistance that have been defined in muscle may also play an important role in the PNS.
These experiments used an in vivo approach to support the mounting in vitro evidence pointing to PNS insulin resistance in diabetes. Interestingly, Akt activation was very prominent in the DRG and sciatic nerve of nondiabetic mice, yet very few significant changes were seen in downstream signaling molecules. This may be due to a temporal effect, as downstream mediators of the Akt pathway may have not yet been activated during the 30-minute stimulation period used for this study. However, it is also plausible that the downstream Akt signaling proteins explored in this study do not play a prominent role in insulin pathways within the DRG. Instead of driving protein synthesis through mTor and p70S6K or regulation of GSK3β actions, insulin may be playing a more important role in lipid and glucose metabolism, gene regulation, or mitochondrial maintenance in peripheral neurons. Further studies are underway to explore these other downstream components of insulin signaling and to define the temporal components of this signaling pathway.
An additional caveat to this study is the use of leptin-deficient ob/ob mice. Leptin’s role in the nervous system is receiving increasing attention, and it may have a neuroprotective role . It is not known how reduced neuronal leptin may have contributed to our results. Thus, confirming these results in a high-fat diet model of obesity will be an important step to further investigating PNS insulin resistance.
In experiments presented here, it appeared that insulin produced a stronger Akt activation in the sciatic nerve compared to the DRG (Figure 3), whereas IGF-1 produced a stronger Akt activation in the DRG compared to the sciatic nerve (Figure 5). These results point to an apparent separation in insulin/IGF-1 signaling support within the PNS. One plausible explanation may be that insulin and IGF-1 have different actions on the DRG soma and satellite cells compared to sensory axons, motor axons, and Schwann cells in the peripheral nerve, leading to alternative signaling profiles. How this potential divergence in signaling may affect sensory neuron function is yet to be determined and ongoing research is further delineating the differential roles that insulin and IGF-1 may play in sensory nerve biology.
In ob/ob mice, both the DRG and sciatic nerve displayed reduced insulin-induced Akt activation, a classic indication of insulin resistance. Several mechanisms of insulin resistance outlined in muscle also appear to be altered in the PNS, and may be contributing to the observed reduction in insulin signal transduction. However, these results must be interpreted with caution as significant changes were not seen consistently across PNS tissues, and further research will need to be completed to fully establish a clear mechanism. Interestingly, no change in baseline Akt activation levels was observed between nondiabetic and ob/ob mice as may be expected in states of insulin resistance. These results are intriguing and suggest that future research focusing on pathways driving Akt signaling is warranted.
Hyperinsulinemia can promote insulin resistance through downregulation of the insulin receptor . This effect was demonstrated in our data. The ob/ob mice in this cohort had serum insulin levels 34.3 fold higher than nondiabetic mice and the DRG of ob/ob mice displayed significantly lower insulin receptor expression. Thus, the extreme hyperinsulinemia in the ob/ob mice may be promoting insulin receptor downregulation and contributing to PNS insulin resistance. This idea is supported by a recent study that reported a significant decrease in insulin receptor mRNA in cultured DRG neurons that displayed insulin resistance when treated with high levels of insulin .
An alternative mediator of insulin resistance is the stress kinase JNK, which is activated in response to various cellular stressors, including low grade chronic inflammation induced by obesity [33, 45]. In fact, ob/ob mice with a JNK null mutation have improved whole body glucose tolerance and insulin sensitivity . Additionally, JNK activation has been implicated in altered neurofilament phosphorylation in the PNS of diabetic rats . JNK activation is proposed to promote insulin resistance through upregulation of IRS serine phosphorylation, and IRS is a key common signaling component of both the insulin and IGF-1 pathways. In the current study we observed increased JNK activation without a significant elevation in either IRS1 or IRS2 serine phosphorylation. Some controversy does exist as to which serine sites are most important in insulin resistance, thus the serine sites that we probed (p(ser731)IRS2 and (p(ser307)IRS1) may not be heavily involved in inhibiting insulin signaling in the PNS. More powerful approaches, such as mass spectrometry, may be needed to establish a global change in the IRS phosphorylation profile within the PNS .
Another possible component of the insulin receptor signaling pathway that could be affected in insulin resistance is PTP1B. PTP1B is the canonical member of protein tyrosine phosphatases and serves an important role in insulin signaling regulation . Overexpression of PTP1B has been linked to insulin resistance in peripheral tissues of ob/ob mice  and PTP1B knockout mice display increased insulin sensitivity . In the current study, we did not detect significant upregulation of PTP1B in the DRG or sciatic nerve of insulin resistant mice. While there was no change in PTP1B expression, there still could be alterations in phosphatase activity and further studies are underway to explore this possibility.
It will be important to put the current results in context with other contributory mechanisms of DN, including glucose and/or lipid mediated toxicity as well as oxidative stress . We postulate that the metabolic dysfunction associated with hyperglycemia and dyslipidemia in concert with reduced neurotrophic support promotes deterioration and reduced regeneration of the distal axon. Furthermore, the loss of appropriate insulin signaling could make neurons even more susceptible to these pathogenic cascades. It should be noted that both intrathecal and intraperitoneal insulin injections altered blood glucose levels. Thus, these results should be viewed in the context that glucose levels were also transiently altered by insulin administration. Further research into disrupted PNS insulin signaling relative to other pathogenic mechanisms is needed, as this will be a key step in translating these basic science results into clinical applications.
Insulin resistance is emerging as a potential mediator of several neurological syndromes (reviewed in ). This study, along with recent data of in vitro DRG insulin resistance, strongly supports altered insulin signaling as a pathogenic mechanism in DN. While deficient insulin signaling has been a proposed contributor to DN in type 1 models for some time [3, 4, 14, 43, 50], little has been known about insulin signaling effectiveness in type 2 (hyperinsulinemic) models of DN. We observed reduced insulin signaling in vivo in the PNS of type 2 diabetic ob/ob mice and suggest possible mechanisms that may be contributing to these changes. It is now becoming evident that decreased insulin neurotrophic support in the PNS is an integral part of DN and may be a congruent mechanism between type 1 and type 2 diabetic models of DN, as both have reduced insulin signaling either due to insulinopenia or neuronal insulin resistance.
Future studies will focus on mechanisms through which insulin supports proper PNS function, as revealing these pathways may provide insight into how decreased insulin support contributes to the pathogenesis of DN. Furthermore, delineating the details of PNS insulin signaling may open new avenues for therapeutic intervention in patients with DN.
All experiments were approved by the University of Kansas Medical Center Institutional Animal Care and Use Committee. Male ob/ob leptin null mutant and age-matched control mice (ob/+) were purchased from Jackson Laboratories (Bar Harbor, Maine) at 8 weeks of age. Mice were given access to food and water ad libitum and housed on a 12-hour light/dark cycle. Weekly blood glucose (Glucose Diagnostic Assay Sigma-Aldrich, St. Louis, MO), serum insulin (Insulin ELISA Alpco, Salem, NH) and weights were monitored and mice were sacrificed at 11 weeks of age.
Glucose tolerance test
At 9 weeks of age, an intraperitoneal glucose tolerance test (IPGTT) was used to assess the response of mice to a glucose challenge. After a 6-hour fast, mice were given an intraperitoneal injection of glucose at 1g of glucose per kg body weight. Blood glucose levels were measured via tail clip immediately prior to the glucose bolus and then at 15, 30, 60, and 120 minutes after injection.
Insulin tolerance test
At 10 weeks of age, mice underwent an insulin tolerance test (ITT). Mice were fasted for 6 hours and then administered IP insulin (Humulin R, Lilly, Indianapolis, Indiana) at a dosage of 1.5 U per kg body weight. Blood glucose levels were monitored immediately prior to insulin injection and then at 15, 30, 60, and 120 minutes thereafter.
Fasting insulin and fasting glucose levels were used to calculate the homeostatic model assessment of insulin resistance (HOMA-IR). Scores were calculated with the following equation: (blood glucose (mg/dl) X (serum insulin (uU/mL))/405) .
Mechanical behavioral responses to Semmes Weinstein-von Frey monofilaments (0.07 to 5.0 g) were assessed at 8, 9, 10, and 11 weeks of age. Mice underwent acclimation 2 days prior to the first day of behavioral testing. Mice were placed in individual clear plastic cages (11×5×3.5 cm) on a wire mesh grid 55 cm above the table and were acclimated for 30 minutes prior to behavioral analysis. The filaments were applied perpendicularly to the plantar surface of the hindpaw until the filament bent. Testing began with the 0.7 g filament, and in the presence of a response, the next smaller filament was applied. If no response was observed, the next larger filament was used. Filaments were applied until there was a change in response, followed by an additional 4 more applications. The withdrawal threshold was calculated using the formula from the up-down method previously described .
Insulin and IGF-1 injections
Sterile PBS (vehicle), 0.1U (~0.7 nmol) Humulin R insulin, or recombinant IGF-1 equimolar to 0.1U insulin was directly administered to both nondiabetic and ob/ob type 2 diabetic mice via a one-time intrathecal injection. Previously, intrathecal 0.1U insulin and equimolar IGF-1 have been shown to have beneficial effects on the symptoms of DN . All injections were 50 μL and administered with a 1cc 28½ gauge insulin syringe between the L6 and S1 vertebrae. In an additional preliminary study, sterile PBS or insulin was delivered through an intraperitoneal injection at a dose of 3.33 U/kg, such that the total insulin administered was approximately 0.1 U for nondiabetic mice and 0.17U (~1.2 nmol) for ob/ob mice. The doses administered and stimulation time frames used were confirmed to be sufficient for Akt activation in the PNS with dose curve and time course studies (Grote, unpublished observation).
After a 30 minute insulin stimulation period, the right and left lumbar DRG and sciatic nerves were harvested for each sample from 11 week old mice and frozen at −80°C. Tissues were sonicated in Cell Extraction Buffer (Invitrogen, Carlsbad, CA) containing 55.55 μl/ml protease inhibitor cocktail, 200 mM Na3VO4, and 200 mM NaF. Following sonication, protein was extracted on ice for 60 minutes and vortexed every 10 minutes. After centrifugation, protein concentration of the supernatant was measured with a Bradford assay (Bio-Rad, Hercules, CA). Samples were then boiled with Lane Marker Reducing Sample Buffer (Thermo Scientific, Waltham, MA) for 3 minutes. Equal amounts of protein (30 μg) were loaded per lane and samples were separated on a 4-15% gradient tris-glycine gel (Bio-Rad), and then transferred to a nitrocellulose membrane. Membranes were probed with the following primary antibodies and all antibodies were purchased from Cell Signaling (Danvers, MA) unless otherwise noted: total Akt (1:2000), p-(Ser473)Akt (1:500), total p70S6K (1:500), p-(Thr389)p70S6K (1:500), total GSK3β (1:1500), p-(Ser9)GSK3β (1:1000), total JNK (1:1000), p-(Thr183/Tyr185)JNK (1:500), total mTor (1:500), p-(Ser2448)mTor (1:500), Insulin-like growth factor 1 receptor β subunit (1:500), PTP1B (1:500) (Abcam, Cambridge, MA), total AS160 (1:1000) (Millipore, Billerica, MA), p-(Thr642)AS160 (1:500) (Millipore), Insulin Receptor β subunit (1:500) (Santa Cruz, Santa Cruz, CA), and α-tubulin (1:5000) (Abcam). Bands were visualized with either anti-mouse or anti-rabbit HRP-conjugated secondary antibodies (Santa Cruz) and ECL with X-ray film. Densitometry with ImageJ (NIH) was then used to analyze each lane. All samples from each tissue were run simultaneously across multiple gels and each group was equally represented on each gel (approximately 3 nondiabetic PBS, 3 nondiabetic insulin, 3 ob/ob PBS, and 3 ob/ob insulin per gel). Data is presented as the ratio of integrated density of the phosopho-protein normalized to the integrated density of the total protein. The normalized ratio was averaged for each group and the mean ± SEM is represented in the corresponding figures. Representative immunoblots are shown.
All data is expressed as means ± standard error of the mean. IPGTT, ITT, and behavior data were analyzed with a repeated measures analysis of variance (RM-ANOVA). In addition, the area under the curve (AUC) for IPGTT and ITT was analyzed using a Student’s t-test. Blood glucose changes at 30 minutes in response to insulin or IGF-1 were analyzed with a paired Student’s t-test. Western blot results were analyzed with 2-way ANOVA and Bonferroni’s post hoc analysis when appropriate. Outliers greater than or less than 2 standard deviations from the mean were not included in the analysis. All statistical tests were performed using SigmaPlot software and a P value <0.05 was considered significant.
Central nervous system
Peripheral nervous system
Insulin receptor substrate
Mammalian target of rapamycin
Glycogen synthase kinase 3 beta
Mitogen-activated protein kinase
Protein tyrosine phosphatase 1B
c-Jun N-terminal kinase
This work was supported by National Institutes of Health (NIH) grants R01NS43313 (DEW), R01AG031575 (PCG), and P20GM103418 from the Kansas Idea Network of Biomedical Research Excellence (K-INBRE) program of the National Institute of General Medical Sciences, and by the Kansas IDDRC, P30 NICHD HD 002528.
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