Rescue and regeneration of injured peripheral nerve axons by intrathecal insulin

Hyperinsulinemia induces insulin resistance in dorsal root ganglion neurons

Insulin resistance (IR) is the major feature of metabolic syndrome, including type 2 diabetes. IR studies are mainly focused on peripheral tissues, such as muscle and liver. There is, however, little knowledge about IR in neurons. In this study, we examined whether neurons develop IR in response to hyperinsulinemia. We first examined insulin signaling using adult dorsal root ganglion neurons as a model system. Acute insulin treatment resulted in time- and concentration-dependent activation of the signaling cascade, including phosphorylation of the insulin receptor, Akt, p70S6K, and glycogen synthase kinase-3β. To mimic hyperinsulinemia, cells were pretreated with 20 nM insulin for 24 h and then stimulated with 20 nM insulin for 15 min. Chronic insulin treatment resulted in increased basal Akt phosphorylation. More importantly, acute insulin stimulation after chronic insulin treatment resulted in blunted phosphorylation of Akt, p70S6K, and glycogen synthase kinase-3β. Interestingly, when the cells were treated with phosphatidylinositol 3-kinase pathway inhibitor, but not MAPK pathway inhibitor, chronic insulin treatment did not block acute insulin treatment-induced Akt phosphorylation. Insulin-induced Akt phosphorylation was lower in dorsal root ganglion neurons from BKS-db/db compared with control BKS-db+ mice. This effect was age dependent. Our results suggest that hyperinsulinemia cause IR by disrupting the Akt-mediated pathway. We also demonstrate that hyperinsulinemia increases the mitochondrial fission protein dynamin-related protein 1. Our results suggest a new theory for the etiology of diabetic neuropathy, i.e. that, similar to insulin dependent tissues, neurons develop IR and, in turn, cannot respond to the neurotrophic properties of insulin, resulting in neuronal injury and the development of neuropathy.

Insulin receptor substrate 2 expression and involvement in neuronal insulin resistance in diabetic neuropathy

Insulin signaling depends on tyrosine phosphorylation of insulin receptor substrates (IRSs) to mediate downstream effects; however, elevated serine phosphorylation of IRS impairs insulin signaling. Here, we investigated IRS protein expression patterns in dorsal root ganglia (DRG) of mice and whether their signaling was affected by diabetes. Both IRS1 and IRS2 are expressed in DRG; however, IRS2 appears to be the prevalent isoform and is expressed by many DRG neuronal subtypes. Phosphorylation of Ser(731)IRS2 was significantly elevated in DRG neurons from type 1 and type 2 diabetic mice. Additionally, Akt activation and neurite outgrowth in response to insulin were significantly decreased in DRG cultures from diabetic ob/ob mice. These results suggest that DRG neurons express IRS proteins that are altered by diabetes similar to other peripheral tissues, and insulin signaling downstream of the insulin receptor may be impaired in sensory neurons and contribute to the pathogenesis of diabetic neuropathy.

Forced-exercise delays neuropathic pain in experimental diabetes: effects on voltage-activated calcium channels

Physical exercise produces a variety of psychophysical effects, including altered pain perception. Elevated levels of centrally produced endorphins or endocannabinoids are implicated as mediators of exercise-induced analgesia. The effect of exercise on the development and persistence of disease-associated acute/chronic pain remains unclear. In this study, we quantified the physiological consequence of forced-exercise on the development of diabetes-associated neuropathic pain. Euglycemic control or streptozotocin (STZ)-induced diabetic adult male rats were subdivided into sedentary or forced-exercised (2-10 weeks, treadmill) subgroups and assessed for changes in tactile responsiveness. Two weeks following STZ-treatment, sedentary rats developed a marked and sustained hypersensitivity to von Frey tactile stimulation. By comparison, STZ-treated diabetic rats undergoing forced-exercise exhibited a 4-week delay in the onset of tactile hypersensitivity that was independent of glucose control. Exercise-facilitated analgesia in diabetic rats was reversed, in a dose-dependent manner, by naloxone. Small-diameter (< 30 μm) DRG neurons harvested from STZ-treated tactile hypersensitive diabetic rats exhibited an enhanced (2.5-fold) rightward (depolarizing) shift in peak high-voltage activated (HVA) Ca(2+) current density with a concomitant appearance of a low-voltage activated (LVA) Ca(2+) current component. LVA Ca(2+) currents present in DRG neurons from hypersensitive diabetic rats exhibited a marked depolarizing shift in steady-state inactivation. Forced-exercise attenuated diabetes-associated changes in HVA Ca(2+) current density while preventing the depolarizing shift in steady-state inactivation of LVA Ca(2+) currents. Forced-exercise markedly delays the onset of diabetes-associated neuropathic pain, in part, by attenuating associated changes in HVA and LVA Ca(2+) channel function within small-diameter DRG neurons possibly by altering opioidergic tone.

Local insulin and the rapid regrowth of diabetic epidermal axons

Insulin deficiency may contribute toward the neurological deficits of diabetic polyneuropathy (DPN). In particular, the unique trophic properties of insulin, acting on sensory neuron and axon receptors offer an approach toward reversing loss of skin axons that develops during diabetes. Here we examined how local cutaneous insulin, acting on axon receptors, influences innervation of the epidermis. That cutaneous axons might be amenable to regrowth was suggested by confirming that a high proportion of epidermal axons expressed GAP43/B50, a growth associated protein. Also, IRβ (insulin receptor subunit β) mRNA was expressed and upregulated in the footpads of diabetic mice and protein expression was upregulated in their sensory dorsal root ganglia. Moreover, footpads expressed mRNAs of the downstream insulin transduction molecules, IRS-1 and IRS-2. IRβ protein was identified in dermal axons, some epidermal sensory axons, and in keratinocytes. In separate models of experimental diabetes, we identified a surprising and rapid local response of this axon population to insulin. C57BL/6J streptozotocin (STZ) injected mice, as a model of type 1 diabetes and dbdb mice, as a model of type 2 diabetes were both evaluated after 3 months of diabetes duration. Local hindpaw plantar injections of low dose subhypoglycemic insulin (that did not alter diabetic hyperglycemia) and carrier (into the opposite paw) were given over two days and innervation studied at 5 days. Insulin injections in both models were associated with an ipsilateral rise in the density of PGP 9.5 labeled diabetic epidermal axons at 5 days, compared to that of their contralateral carrier injected hindpaw. Nondiabetic controls did not have changes in innervation following insulin. In a separate cohort of STZ diabetic mice and controls evaluated for paw sensation, there was mild improvement in mechanical, but not thermal sensation at 2 weeks after insulin injection in diabetics but not controls. Fine unmyelinated epidermal axons have considerable plasticity. Here we identify a rapid improvement of skin innervation by doses of insulin insufficient to alter glycemia or innervation of the opposite paw. Local direct insulin signaling of receptors expressed on diabetic cutaneous axons may reverse retraction of their branches during experimental DPN.

Mitochondrial respiratory chain dysfunction in dorsal root ganglia of streptozotocin-induced diabetic rats and its correction by insulin treatment

OBJECTIVE

Impairments in mitochondrial physiology may play a role in diabetic sensory neuropathy. We tested the hypothesis that mitochondrial dysfunction in sensory neurons is due to abnormal mitochondrial respiratory function.

RESEARCH DESIGN AND METHODS

Rates of oxygen consumption were measured in mitochondria from dorsal root ganglia (DRG) of 12- to- 22-week streptozotocin (STZ)-induced diabetic rats, diabetic rats treated with insulin, and age-matched controls. Activities and expression of components of mitochondrial complexes and reactive oxygen species (ROS) were analyzed.

RESULTS

Rates of coupled respiration with pyruvate + malate (P + M) and with ascorbate + TMPD (Asc + TMPD) in DRG were unchanged after 12 weeks of diabetes. By 22 weeks of diabetes, respiration with P + M was significantly decreased by 31-44% and with Asc + TMPD by 29-39% compared with control. Attenuated mitochondrial respiratory activity of STZ-diabetic rats was significantly improved by insulin that did not correct other indices of diabetes. Activities of mitochondrial complexes I and IV and the Krebs cycle enzyme, citrate synthase, were decreased in mitochondria from DRG of 22-week STZ-diabetic rats compared with control. ROS levels in perikarya of DRG neurons were not altered by diabetes, but ROS generation from mitochondria treated with antimycin A was diminished compared with control. Reduced mitochondrial respiratory function was associated with downregulation of expression of mitochondrial proteins.

CONCLUSIONS

Mitochondrial dysfunction in sensory neurons from type 1 diabetic rats is associated with impaired rates of respiratory activity and occurs without a significant rise in perikaryal ROS.

Recovery of whisking function promoted by manual stimulation of the vibrissal muscles after facial nerve injury requires insulin-like growth factor 1 (IGF-1)

Recently, we showed that manual stimulation (MS) of denervated vibrissal muscles enhanced functional recovery following facial nerve cut and suture (FFA) by reducing poly-innervation at the neuro-muscular junctions (NMJ). Although the cellular correlates of poly-innervation are established, with terminal Schwann cells (TSC) processes attracting axon sprouts to "bridge" adjacent NMJ, molecular correlates are poorly understood. Since quantitative RT-PCR revealed a rapid increase of IGF-1 mRNA in denervated muscles, we examined the effect of daily MS for 2 months after FFA in IGF-1(+/-) heterozygous mice; controls were wild-type (WT) littermates including intact animals. We quantified vibrissal motor performance and the percentage of NMJ bridged by S100-positive TSC. There were no differences between intact WT and IGF-1(+/-) mice for vibrissal whisking amplitude (48 degrees and 49 degrees ) or the percentage of bridged NMJ (0%). After FFA and handling alone (i.e. no MS) in WT animals, vibrissal whisking amplitude was reduced (60% lower than intact) and the percentage of bridged NMJ increased (42% more than intact). MS improved both the amplitude of vibrissal whisking (not significantly different from intact) and the percentage of bridged NMJ (12% more than intact). After FFA and handling in IGF-1(+/-) mice, the pattern was similar (whisking amplitude 57% lower than intact; proportion of bridged NMJ 42% more than intact). However, MS did not improve outcome (whisking amplitude 47% lower than intact; proportion of bridged NMJ 40% more than intact). We conclude that IGF-I is required to mediate the effects of MS on target muscle reinnervation and recovery of whisking function.

Diabetes and failure of axon regeneration in peripheral neurons

Peripheral neurons are targeted by a 'double hit' during diabetes mellitus. First, they are damaged directly; diabetic polyneuropathy is a progressive neurodegenerative disorder that involves sensory, autonomic and eventually motor neurons. The second 'hit' involves a profound impairment in the ability of peripheral axons to regenerate. This is important because the impairment impacts on how patients may recover from polyneuropathy. Moreover, diabetic patients also develop direct focal injures of peripheral nerves, such as carpal tunnel syndrome and ulnar neuropathy at the elbow. Their response to the treatment of these selective injuries is also impaired. Regeneration of peripheral neurons is normally a complex process that involves axon sprouting, upregulation of molecular regeneration programs, clearance of pathways for axon regrowth, maturation of new axons and reconnection to their targets. Schwann cells and perineuronal glial cells provide support during many of these processes. However, in diabetes mellitus a number of these steps may be independently impaired. In this brief article, we discuss evidence for several of these mechanisms of regenerative failure in diabetes.

Igf1r+/CD34+ immature ICC are putative adult progenitor cells, identified ultrastructurally as fibroblast-like ICC in Ws/Ws rat colon

The colon of Ws/Ws mutant rats shows impairment of pacemaker activity and altered inhibitory neurotransmission. The present study set out to find structural correlates to these findings to resolve mechanisms. In the colon of Ws/Ws rats, interstitial cells of Cajal associated with Auerbach's plexus (ICC-AP) were significantly decreased and ICC located at the submuscular plexus and intramuscular ICC were rarely observed based on immunohistochemistry and electron microscopy. Ultrastructural investigations revealed that there was no overall loss of all types of interstitial cells combined. Where loss of ICC was observed, a marked increase in fibroblast-like ICC (FL-ICC) was found at the level of AP. Immunoelectron microscopy proved FL-ICC to be c-Kit(-) but gap junction coupled to each other and to c-Kit(+) ICC; they were associated with enteric nerves and occupied space normally occupied by ICC in the wild-type rat colon, suggesting them to be immature ICC. In addition, a marked increase in immunoreactivity for insulin-like growth factor 1 receptor (Igf1r) occurred, co-localized with CD34 but not with c-Kit. A significantly higher number of Igf1r(+)/CD34(+) cells were found in Ws/Ws compared to wild-type rat colons. These CD34(+)/Igf1r(+) cells in the Ws/Ws colon occupied the same space as FL-ICC. Hence we propose that a subset of immature ICC (FL-ICC) consists of adult progenitor cells. Immunohistochemistry revealed a reduction of neurons positive for neuronal nitric oxide synthase. The functional capabilities of the immature ICC and the regenerative capabilities of the adult progenitor cells need further study. The morphological features described here show that the loss of pacemaker activity is not associated with failure to develop a network of interstitial cells around AP but a failure to develop this network into fully functional pacemaker cells. The reduction in nitrergic innervation associated with the Ws mutation may be the result of a reduction in nitrergic neurons.

Differentiation and migration of neural crest stem cells are stimulated by pancreatic islets

Neural crest stem cells (NCSCs) migrate during embryonic development towards the endoderm-derived pancreas and the interaction between NCSCs and beta-cell progenitors is crucial for their mutual differentiation. In diabetes, loss of beta-cells or impaired beta-cell function is accompanied by nerve degeneration, which contributes to the progression of the disease. Here we show that adult pancreatic islets markedly promote differentiation of NCSCs towards neuronal phenotype in vitro and in vivo after transplantation and increase their migration towards islets. These findings indicate that pancreatic islets can be used to promote differentiation of NCSCs towards neuronal phenotype and that this in-vitro system may help elucidate interactions between NCSCs and healthy or diseased beta-cells.

Elevated triglycerides correlate with progression of diabetic neuropathy

OBJECTIVE

To evaluate mechanisms underlying diabetic neuropathy progression using indexes of sural nerve morphometry obtained from two identical randomized, placebo-controlled clinical trials.

RESEARCH DESIGN AND METHODS

Sural nerve myelinated fiber density (MFD), nerve conduction velocities (NCVs), vibration perception thresholds, clinical symptom scores, and a visual analog scale for pain were analyzed in participants with diabetic neuropathy. A loss of > or =500 fibers/mm(2) in sural nerve MFD over 52 weeks was defined as progressing diabetic neuropathy, and a MFD loss of < or =100 fibers/mm(2) during the same time interval as nonprogressing diabetic neuropathy. The progressing and nonprogressing cohorts were matched for baseline characteristics using an O'Brien rank-sum and baseline MFD.

RESULTS

At 52 weeks, the progressing cohort demonstrated a 25% decrease (P < 0.0001) from baseline in MFD, while the nonprogressing cohort remained unchanged. MFD was not affected by active drug treatment (P = 0.87), diabetes duration (P = 0.48), age (P = 0.11), or BMI (P = 0.30). Among all variables tested, elevated triglycerides and decreased peroneal motor NCV at baseline significantly correlated with loss of MFD at 52 weeks (P = 0.04).

CONCLUSIONS

In this cohort of participants with mild to moderate diabetic neuropathy, elevated triglycerides correlated with MFD loss independent of disease duration, age, diabetes control, or other variables. These data support the evolving concept that hyperlipidemia is instrumental in the progression of diabetic neuropathy.

Locally synthesized calcitonin gene-related Peptide has a critical role in peripheral nerve regeneration

Regeneration of peripheral nerves involves complex and intimate interactions between axons and Schwann cells. Here, we show that local axon synthesis and action of the neuropeptide calcitonin gene-related peptide (CGRP) is critical for this collaboration. After peripheral sural sensory axon injury in rats, we observed an unexpectedly large proportion of axons that newly expressed CGRP during regeneration. Intense peptide expression accompanied local rises in alphaCGRP mRNA in the nerve trunk, and there was evidence of transport of alphaCGRP mRNA into regenerating axons, indicating intra-axonal peptide synthesis. Calcitonin gene-related peptide receptor and its receptor activity modifying protein were expressed onadjacent Schwann cells, where they were available for signaling. Moreover, exogenous CGRP induced proliferation in isolated adult Schwann cells. New axon outgrowth and CGRP expression depended on local peptide synthesis and were inhibited by exposure tolocal translation inhibitors. Local delivery of siRNAs to either alphaCGRP or receptor activity modifying protein 1 to sites of nerve transection was associated with severe disruption of axon outgrowth.These findings indicate that robust localized intra-axonal translation of the CGRP neuropeptide during regeneration signals Schwann cell proliferation, behavior that is critical for partnering during adult peripheral nerve regrowth.

Intranasal insulin ameliorates experimental diabetic neuropathy

OBJECTIVE

We hypothesized that intranasal insulin (I-I) delivery targets the nervous system while avoiding potential adverse systemic effects when compared with subcutaneous insulin (S-I) for experimental streptozotocin-induced diabetic peripheral neuropathy (DPN).

RESEARCH DESIGN AND METHODS

I-I or S-I at 0.87 IU daily or placebo were delivered in separate cohorts of diabetic and nondiabetic CD1 mice during 8 months of diabetes. Radiolabeled insulin detection was used to compare delivery and biodistribution for I-I and S-I. Biweekly behavioral testing and monthly electrophysiological and quantitative studies assessed progression of DPN. At and before end point, morphometric analysis of DRG, peripheral nerve, distal epidermal innervation, and specific molecular markers were evaluated.

RESULTS

Radiolabeled I-I resulted in more rapid and concentrated delivery to the spinal cord and DRG with less systemic insulin exposure. When compared with S-I or intranasal placebo, I-I reduced overall mouse mortality and sensory loss while improving neuropathic pain and electrophysiological/morphological abnormalities in diabetic mice. I-I restored mRNA and protein levels of phosphoinositide 3-kinase/Akt, cyclic AMP response element-binding protein, and glycogen synthase kinase 3beta to near normal levels within diabetic DRGs.

CONCLUSIONS

I-I slows the progression of experimental DPN in streptozotocin mice, avoids adverse effects associated with S-I treatment, and prolongs lifespan when compared with S-I. I-I may be a promising approach for the treatment of DPN.

Diabetic polyneuropathy: an update

PURPOSE OF REVIEW

To examine current issues in the diagnosis and treatment of diabetic polyneuropathy.

RECENT FINDINGS

Diabetic neuropathies are common and rising in prevalence with the global burden of type 2 diabetes. Polyneuropathy is also emerging as a complication of impaired glucose tolerance, without frank diabetes. Ideas about the pathogenesis of diabetic polyneuropathy have evolved, and some new pathogenic mechanisms are being considered. These include roles for direct insulin signaling on neurons and axons that transduce growth signals, actions of the cleaved C-peptide product of the insulin prohormone, abnormal signaling by advanced glycation endproducts on neuronal and glial receptors and activation of poly (ADP-ribose) polymerase in microvessels and neurons. Manipulation of these pathways may offer new therapeutic approaches. Ideas about neuronal targets in the treatment of neuropathic pain have also advanced emphasizing abnormal sodium and calcium channel signaling.

SUMMARY

Consideration of diabetic polyneuropathy as a unique neurodegenerative condition has generated interest in new pathways involved in its development. A new round of clinical trials that address its pathogenesis may be welcome, as recent attempts have been largely disappointing. In the interim, several forms of therapy for neuropathic pain are available.

Neuronal Targeting in Diabetes Mellitus: A Story of Sensory Neurons and Motor Neurons

Diabetes mellitus targets the peripheral nervous system in unique but disabling ways. Although several mechanisms may target peripheral neurons, they render a degenerative pattern of damage that begins in distal terminals. Moreover, sensory neurons are involved early, motor neurons later. By studying a variety of diabetic neuropathy models in rats, mice, and other species, an overall appreciation of its neurodegeneration emerges. Understanding how mechanisms of diabetes complications target peripheral neurons selectively may offer opportunities to intervene before irretrievable neuron loss develops.NEUROSCIENTIST 14(4):311–318, 2008. DOI: 10.1177/1073858408316175

Blockade of hexokinase activity and binding to mitochondria inhibits neurite outgrowth in cultured adult rat sensory neurons

Hexokinase is known as the first enzyme and rate-limiting step in glycolysis. The role of hexokinase activity and localization in regulating the rate of axonal regeneration was studied in cultured adult sensory neurons of dorsal root ganglia (DRG). Immunofluorescent staining of DRG demonstrated that small-medium neurons and satellite cells exhibited high levels of expression of hexokinase I. Large neurons had negative staining for hexokinase I. Intracellular localization and biochemical studies in cultured adult rat sensory neurons revealed that hexokinase I was almost exclusively found in the mitochondrial compartment. The hypothesis that neurotrophic factor dependent activation of Akt would regulate hexokinase association with the mitochondria was tested and quantitative Western blotting showed no effect of blockade of the phosphoinositide 3-kinase (PI 3-kinase)/Akt pathway using the inhibitor LY294002, indicating this interaction of hexokinase with mitochondria was not neurotrophic factor or Akt-dependent. Finally, pharmacological blockade of hexokinase activity and inhibition of localization to the mitochondrial compartment with hexokinase II VDAC binding domain (Hxk2VBD) peptide caused a significant inhibition of neurotrophic factor-directed axon outgrowth. The results support a key role for hexokinase activity and/or localization to the mitochondria in the regulation of neurite outgrowth in cultured adult sensory neurons.

Early changes in insulin receptor signaling and pain sensation in streptozotocin-induced diabetic neuropathy in rats

The objective of the present study was to evaluate the time course of changes in peripheral nerve insulin receptor (IR) signaling and compare observed findings with behavioral responses to noxious mechanical and thermal stimuli in streptozotocin (STZ)-diabetic rats over 12 weeks of diabetes. Diabetic rats developed mechanical hyperalgesia, as indicated by decreased paw withdrawal thresholds to mechanical stimuli that were detectable after 2 weeks of diabetes; they also developed thermal hypoalgesia, as indicated by increased tail flick latencies to thermal stimuli that were detectable at 1 week of diabetes. Western blot analysis revealed decreased phosphorylated: total IR protein ratio that was detectable as early as 2 weeks of diabetes, whereas phosphorylated:total Akt protein ratio was decreased at 2 weeks and increased at 12 weeks of diabetes with unchanged PI-3K protein levels. To our knowledge, the present study is the first to demonstrate that impaired peripheral nerve IR signaling, as indicated by decreased phosphorylated:total IR protein ratio, coincides with early mechanical hyperalgesia and thermal hypoalgesia in STZ-diabetic rats. This finding may improve understanding of how altered pain sensation develops rapidly in this model.

PERSPECTIVE

This study examined peripheral nerve IR signaling during the early course of altered nociception in STZ-diabetic rats. In diabetic rats, impaired peripheral nerve IR signaling is observed shortly after STZ injection, as is altered nociception. This finding suggests a possible role of impaired IR signaling in diabetic sensory neuropathy.

Diabetes mellitus and the peripheral nervous system: manifestations and mechanisms

Diabetes targets the peripheral nervous system with several different patterns of damage and several mechanisms of disease. Diabetic polyneuropathy (DPN) is a common disorder involving a large proportion of diabetic patients, yet its pathophysiology is controversial. Mechanisms considered have included polyol flux, microangiopathy, oxidative stress, abnormal signaling from advanced glycation endproducts and growth factor deficiency. Although some clinical trials have demonstrated modest benefits in disease stabilization or pain therapy in DPN, robust therapy capable of reversing the disease is unavailable. In this review, general aspects of DPN and other diabetic neuropathies are examined, including a summary of recent therapeutic trials. A particular emphasis is placed on the evidence that the neurobiology of DPN reflects a unique yet common and disabling neurodegenerative disorder.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.