Diabetic neuropathy in type 1 and type 2 diabetes and the effects of C-peptide

Fasted C-Peptide Distribution and Associated Clinical Factors in Adults With Longstanding Type 1 Diabetes: Analysis of the Canadian Study of Longevity in Type 1 Diabetes

OBJECTIVE

Although insulin production is reportedly retained in many people with longstanding type 1 diabetes (T1D), the magnitude and relevance of connecting peptide (C-peptide) production are uncertain. In this study, we aimed to define fasted C-peptide distributions and associated clinical factors.

METHODS

In a cross-sectional analysis of the Canadian Study of Longevity, fasted serum and urinary C-peptide was measured in 74 patients with longstanding T1D (duration ≥50 years) and 75 age- and sex-matched controls. Extensive phenotyping for complications was performed and patient-reported variables were included. C-peptide distributions were analyzed, and multivariable logistic regression was used to assess the variable association in participants with T1D.

RESULTS

The 74 participants with T1D had a mean age of 66±8 years, a disease duration of 54 (interquartile range 52 to 58) years, and a glycated hemoglobin (A1C) of 7.4%±0.8% (56.8±9.15 mmol/mol). The 75 controls had a mean age of 65±8 years and an A1C of 5.7%±0.4% (38.4±4.05 mmol/mol). Participants with T1D had lower fasted serum C-peptide than controls (0.013±0.022 vs 1.595±1.099 nmol/L, p<0.001). Of the participants with T1D, C-peptide was detectable in 30 of 73 (41%) serum samples, 32 of 74 (43%) urine samples, and 48 of 74 (65%) for either serum or urine. The variables independently associated with detectable serum or urinary C-peptide were lower total daily insulin requirement (odds ratio 2.351 [for 1 lower unit/kg], p=0.013) and lower hypoglycemia worry score (odds ratio 1.059 [for 1 point lower on the worry subscore of the Hypoglycemia Fear Survey], p=0.030).

CONCLUSIONS

Although detectable C-peptide in longstanding diabetes was common, the magnitude of concentration was extremely low when compared with age- and sex-matched controls. Despite minimal detectability, its presence is validated by lower insulin requirements and strongly associated with lower hypoglycemia worry.

Peripheral Neuropathy Phenotyping in Rat Models of Type 2 Diabetes Mellitus: Evaluating Uptake of the Neurodiab Guidelines and Identifying Future Directions

Diabetic peripheral neuropathy (DPN) affects over half of type 2 diabetes mellitus (T2DM) patients, with an urgent need for effective pharmacotherapies. While many rat and mouse models of T2DM exist, the phenotyping of DPN has been challenging with inconsistencies across laboratories. To better characterize DPN in rodents, a consensus guideline was published in 2014 to accelerate the translation of preclinical findings. Here we review DPN phenotyping in rat models of T2DM against the 'Neurodiab' criteria to identify uptake of the guidelines and discuss how DPN phenotypes differ between models and according to diabetes duration and sex. A search of PubMed, Scopus and Web of Science databases identified 125 studies, categorised as either diet and/or chemically induced models or transgenic/spontaneous models of T2DM. The use of diet and chemically induced T2DM models has exceeded that of transgenic models in recent years, and the introduction of the Neurodiab guidelines has not appreciably increased the number of studies assessing all key DPN endpoints. Combined high-fat diet and low dose streptozotocin rat models are the most frequently used and well characterised. Overall, we recommend adherence to Neurodiab guidelines for creating better animal models of DPN to accelerate translation and drug development.

Minocycline alleviates nociceptive response through modulating the expression of NR2B subunit of NMDA receptor in spinal cord of rat model of painful diabetic neuropathy

Background

It has been reported that neuropathic pain can be overcome by targeting the NR2B subunit of N-methyl-D-aspartate receptors (NR2B). This study aimed to investigate the effects of minocycline on phosphorylated and total expression of NR2B in the spinal cord of rats with diabetic neuropathic pain.

Methods

A total of 32 Sprague-Dawley male rats were randomly assigned into four groups (n = 8); control healthy, control diabetic (PDN), and PDN rats that received 80 µg or 160 µg intrathecal minocycline respectively. The rats were induced to develop diabetes and allowed to develop into the early phase of PDN for two weeks. Hot-plate and formalin tests were conducted. Intrathecal treatment of minocycline or normal saline was conducted for 7 days. The rats were sacrificed to obtain the lumbar enlargement region of the spinal cord (L4-L5) for immunohistochemistry and western blot analyses to determine the expression of phosphorylated (pNR2B) and total NR2B (NR2B).

Results

PDN rats showed enhanced flinching (phase 1: p < 0.001, early phase 2: p < 0.001, and late phase 2: p < 0.05) and licking responses (phase 1: p < 0.001 and early phase 2: p < 0.05). PDN rats were also associated with higher spinal expressions of pNR2B and NR2B (p < 0.001) but no significant effect on thermal hyperalgesia. Minocycline inhibited formalin-induced flinching and licking responses (phase 1: p < 0.001, early phase 2: p < 0.001, and late phase 2: p < 0.05) in PDN rats with lowered spinal expressions of pNR2B (p < 0.01) and NR2B (p < 0.001) in a dose-dependent manner.

Conclusion

Minocycline alleviates nociceptive responses in PDN rats, possibly via suppression of NR2B activation. Therefore, minocycline could be one of the potential therapeutic antinociceptive drugs for the management of neuropathic pain.

Non-glucose risk factors in the pathogenesis of diabetic peripheral neuropathy

In this review, we consider the diverse risk factors in diabetes patients beyond hyperglycemia that are being recognized as contributors to diabetic peripheral neuropathy (DPN). Interest in such alternative mechanisms has been encouraged by the recognition that neuropathy occurs in subjects with metabolic syndrome and pre-diabetes and by the reporting of several large clinical studies that failed to show reduced prevalence of neuropathy after intensive glucose control in patients with type 2 diabetes. Animal models of obesity, dyslipidemia, hypertension, and other disorders common to both pre-diabetes and diabetes have been used to highlight a number of plausible pathogenic mechanisms that may either damage the nerve independent of hyperglycemia or augment the toxic potential of hyperglycemia. While pathogenic mechanisms stemming from hyperglycemia are likely to be significant contributors to DPN, future therapeutic strategies will require a more nuanced approach that considers a range of concurrent insults derived from the complex pathophysiology of diabetes beyond direct hyperglycemia.

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

Axonal dysfunction prior to neuropathy onset in type 1 diabetes

BACKGROUND

The present study was undertaken to determine whether there were changes evident in axonal membrane function prior to the onset of neuropathy in patients with type 1 and type 2 diabetes.

METHODS

From a cohort of 110 consecutive referrals, nerve excitability was investigated in 40 diabetic patients without clinical evidence of neuropathy (20 type 1 diabetic patients and 20 type 2 diabetic patients). Groups were matched for gender, disease duration and HbA(1c). Studies were also undertaken in two control groups, younger controls and older controls, matched for age and gender with the diabetic cohorts.

RESULTS

Subjects with type 1 diabetes demonstrated significant nerve excitability abnormalities when compared with younger normal controls. Specifically, type 1 subjects showed a significant reduction at multiple time points in both depolarising and hyperpolarising threshold electrotonus. Additionally, the relative refractory period was prolonged (type 1, 3.19 ms; younger normal controls, 3.0 ms; p < 0.05) and superexcitability was reduced (type 1, -23.12%; younger normal controls, -26.37%; p < 0.05), consistent with axonal membrane depolarisation. Correlations were identified in type 1 patients between disease duration and nerve excitability parameters, including the relative refractory period (r = -0.533, p < 0.05). In contrast, only minor non-specific changes were noted in the type 2 group.

DISCUSSION

This study provides clear evidence of altered axonal function in patients with type 1 diabetes in the absence of clinical neuropathy. These findings suggest that altered axonal membrane potential may precede neuropathy onset in type 1 diabetes and as such may indicate a window of opportunity to intervene and potentially reverse axonal membrane dysfunction before the development of irreversible neuropathy.

Contribution of mitochondria to pain in diabetic neuropathy

Diabetes is a metabolic disease affecting approximately 300 million people worldwide. Neuropathy is one of its frequent complications, and may affect sensory, motor, and autonomic nerves. Its pathophysiology has not fully been elucidated. Several hypotheses have been proposed, and mitochondria have been suggested to play a significant role. This article reviews the mechanisms involved in mitochondrial dysfunction and development of diabetic neuropathy, consisting mainly of oxidative and inflammatory stress, changes in intracellular calcium regulation, apoptotic processes, and changes in mitochondrial structure and function that may lead to development of diabetic neuropathy.

Nutrient excess and altered mitochondrial proteome and function contribute to neurodegeneration in diabetes

Diabetic neuropathy is a major complication of diabetes that results in the progressive deterioration of the sensory nervous system. Mitochondrial dysfunction has been proposed to play an important role in the pathogenesis of the neurodegeneration observed in diabetic neuropathy. Our recent work has shown that mitochondrial dysfunction occurs in dorsal root ganglia (DRG) sensory neurons in streptozotocin (STZ) induced diabetic rodents. In neurons, the nutrient excess associated with prolonged diabetes may trigger a switching off of AMP kinase (AMPK) and/or silent information regulator T1 (SIRT1) signaling leading to impaired peroxisome proliferator-activated receptor γ coactivator-1 (PGC-1α) expression/activity and diminished mitochondrial activity. This review briefly summarizes the alterations of mitochondrial function and proteome in sensory neurons of STZ-diabetic rodents. We also discuss the possible involvement of AMPK/SIRT/PGC-1α pathway in other diabetic models and different tissues affected by diabetes.

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 stress and the pathogenesis of diabetic neuropathy

Diabetic neuropathy is a major complication of diabetes that affects the sensory and autonomic nervous systems and leads to significant morbidity and impact on quality of life of patients. Mitochondrial stress has been proposed as a major mediator of neurodegeneration in diabetes. This review briefly summarizes the nature of sensory and autonomic nerve dysfunction and presents these findings in the context of diabetes-induced nerve degeneration mediated by alterations in mitochondrial ultrastructure, physiology and trafficking. Diabetes-induced dysfunction in calcium homeostasis is discussed at length and causative associations with sub-optimal mitochondrial physiology are developed. It is clear that across a range of complications of diabetes that mitochondrial physiology is impaired, in general a reduction in electron transport chain capability is apparent. This abnormal activity may predispose mitochondria to generate elevated reactive oxygen species (ROS), although experimental proof remains lacking, but more importantly will deleteriously alter the bioenergetic status of neurons. It is proposed that the next five years of research should focus on identifying changes in mitochondrial phenotype and associated cellular impact, identifying sources of ROS in neurons and analyzing mitochondrial trafficking under diabetic conditions.

Prevalence of other diabetes-associated complications and comorbidities and its impact on health care charges among patients with diabetic neuropathy

INTRODUCTION

Diabetic neuropathy (DN) is a common complication associated with diabetes. This study assesses the prevalence of other diabetes-related complications or comorbidities among DN patients and its marginal contribution to health care charges.

METHODS

Using administrative claims database, we studied commercially insured patients below 65 years old with at least one claim of DN anytime from July 2004 through June 2005 (Year 1). Using propensity scoring, a 10:1 ratio of demographically matched controls with diabetes but no DN was constructed. Both DN patients and controls had 12 months of continuous enrollment in Year 1 and Year 2 (July 2005-June 2006). We compared the Year 1 prevalence of other diabetes-associated complications or comorbidities between DN patients and diabetic controls. Controlling for comorbidities, we used multivariate regressions to examine the incremental impact of DN or any other diabetes-related complication or comorbidity on Year 2 health care charges.

RESULTS

A higher percentage of DN patients had at least one other diabetes-related complication or comorbidity than diabetic controls. Individuals with DN had a higher prevalence of each individual other diabetes-related complication or comorbidity. Controlling for comorbidities, the presence of any other diabetes-related complication or comorbidity was statistically associated with higher outpatient pharmacy and total charges for both DN patients and controls. Total and outpatient pharmacy charges were also significantly higher for DN patients than for controls, among those with or without any other diabetes-related complications or comorbidities.

CONCLUSIONS

DN can occur in the absence of other diabetes-related complications or comorbidities. The presence of DN and any other diabetes-related complications or comorbidities significantly increases health care charges.

Abnormal calcium homeostasis in peripheral neuropathies

Abnormal neuronal calcium (Ca2+) homeostasis has been implicated in numerous diseases of the nervous system. The pathogenesis of two increasingly common disorders of the peripheral nervous system, namely neuropathic pain and diabetic polyneuropathy, has been associated with aberrant Ca2+ channel expression and function. Here we review the current state of knowledge regarding the role of Ca2+ dyshomeostasis and associated mitochondrial dysfunction in painful and diabetic neuropathies. The central impact of both alterations of Ca2+ signalling at the plasma membrane and also intracellular Ca2+ handling on sensory neurone function is discussed and related to abnormal endoplasmic reticulum performance. We also present new data highlighting sub-optimal axonal Ca2+ signalling in diabetic neuropathy and discuss the putative role for this abnormality in the induction of axonal degeneration in peripheral neuropathies. The accumulating evidence implicating Ca2+ dysregulation in both painful and degenerative neuropathies, along with recent advances in understanding of regional variations in Ca2+ channel and pump structures, makes modulation of neuronal Ca2+ handling an increasingly viable approach for therapeutic interventions against the painful and degenerative aspects of many peripheral neuropathies.

The beneficial effects of C-Peptide on diabetic polyneuropathy

Diabetic polyneuropathy (DPN) is a common complication in diabetes. At present, there is no adequate treatment, and DPN is often debilitating for patients. It is a heterogeneous disorder and differs in type 1 and type 2 diabetes. An important underlying factor in type 1 DPN is insulin deficiency. Proinsulin C-peptide is a critical element in the cascade of events. In this review, we describe the physiological role of C-peptide and how it provides an insulin-like signaling function. Such effects translate into beneficial outcomes in early metabolic perturbations of neural Na+/K+-ATPase and nitric oxide (NO) with subsequent preventive effects on early nerve dysfunction. Further corrective consequences resulting from this signaling cascade have beneficial effects on gene regulation of early gene responses, neurotrophic factors, their receptors, and the insulin receptor itself. This may lead to preventive and corrective results to nerve fiber degeneration and loss, as well as, promotion of nerve fiber regeneration with respect to sensory somatic fibers and small nociceptive nerve fibers. A characteristic abnormality of type 1 DPN is nodal and paranodal degeneration with severe consequences for myelinated fiber function. This review deals in detail with the underlying insulin-deficiency-related molecular changes and their correction by C-peptide. Based on these observations, it is evident that continuous maintenance of insulin-like actions by C-peptide is needed in peripheral nerve to minimize the sequences of metabolic and molecular abnormalities, thereby ameliorating neuropathic complications. There is now ample evidence demonstrating that C-peptide replacement in type 1 diabetes promotes insulin action and signaling activities in a more enhanced, prolonged, and continuous fashion than does insulin alone. It is therefore necessary to replace C-peptide to physiological levels in diabetic patients. This will have substantial beneficial effects on type 1 DPN.

The effects of C-peptide on type 1 diabetic polyneuropathies and encephalopathy in the BB/Wor-rat

Diabetic polyneuropathy (DPN) occurs more frequently in type 1 diabetes resulting in a more severe DPN. The differences in DPN between the two types of diabetes are due to differences in the availability of insulin and C-peptide. Insulin and C-peptide provide gene regulatory effects on neurotrophic factors with effects on axonal cytoskeletal proteins and nerve fiber integrity. A significant abnormality in type 1 DPN is nodal degeneration. In the type 1 BB/Wor-rat, C-peptide replacement corrects metabolic abnormalities ameliorating the acute nerve conduction defect. It corrects abnormalities of neurotrophic factors and the expression of neuroskeletal proteins with improvements of axonal size and function. C-peptide corrects the expression of nodal adhesive molecules with prevention and repair of the functionally significant nodal degeneration. Cognitive dysfunction is a recognized complication of type 1 diabetes, and is associated with impaired neurotrophic support and apoptotic neuronal loss. C-peptide prevents hippocampal apoptosis and cognitive deficits. It is therefore clear that substitution of C-peptide in type 1 diabetes has a multitude of effects on DPN and cognitive dysfunction. Here the effects of C-peptide replenishment will be extensively described as they pertain to DPN and diabetic encephalopathy, underpinning its beneficial effects on neurological complications in type 1 diabetes.

Diabetic neuropathy differs in type 1 and type 2 diabetes

In this article we describe differences in early metabolic abnormalities between type 1 and type 2 diabetic polyneuropathy (DPN), and how these differences lead to milder initial functional defects in type 2 diabetes, despite the same hyperglycemic exposures. This early reversible metabolic phase is progressively overshadowed by structural degenerative changes eventually resulting in nerve fiber loss. In comparison, the late structural phase of DPN affects type 1 diabetes more severely. Progressive axonal atrophy and loss is hence expressed to a larger extent in type 1 diabetes. In addition, type 1 DPN is characterized by paranodal degenerative changes not seen in type 2 DPN. These differences can be related to the differences in insulin action and signal transduction affecting the expression of neurotrophic factors and their receptors in type 1 diabetes. Downstream effects on neuroskeletal and adhesive proteins, their posttranslational modifications, and nociceptive peptides underlie the more severe resultant pathology in type 1 DPN. These differences in underlying mechanisms should be seriously considered in the future design of interventional paradigms to combat these common conditions.

Early diabetic neuropathy: Triggers and mechanisms

Peripheral neuropathy, and specifically distal peripheral neuropathy (DPN), is one of the most frequent and troublesome complications of diabetes mellitus. It is the major reason for morbidity and mortality among diabetic patients. It is also frequently associated with debilitating pain. Unfortunately, our knowledge of the natural history and pathogenesis of this disease remains limited. For a long time hyperglycemia was viewed as a major, if not the sole factor, responsible for all symptomatic presentations of DPN. Multiple clinical observations and animal studies supported this view. The control of blood glucose as an obligatory step of therapy to delay or reverse DPN is no longer an arguable issue. However, while supporting evidence for the glycemic hypothesis has accumulated, multiple controversies accumulated as well. It is obvious now that DPN cannot be fully understood without considering factors besides hyperglycemia. Some symptoms of DPN may develop with little, if any, correlation with the glycemic status of a patient. It is also clear that identification of these putative non-glycemic mechanisms of DPN is of utmost importance for our understanding of failures with existing treatments and for the development of new approaches for diagnosis and therapy of DPN. In this work we will review the strengths and weaknesses of the glycemic hypothesis, focusing on clinical and animal data and on the pathogenesis of early stages and triggers of DPN other than hyperglycemia.

C-Peptide replacement therapy and sensory nerve function in type 1 diabetic neuropathy

OBJECTIVE

C-peptide replacement in animals results in amelioration of diabetes-induced functional and structural abnormalities in peripheral nerves. The present study was undertaken to examine whether C-peptide administration to patients with type 1 diabetes and peripheral neuropathy improves sensory nerve function.

RESEARCH DESIGN AND METHODS

This was an exploratory, double-blinded, randomized, and placebo-controlled study with three study groups that was carried out at five centers in Sweden. C-peptide was given as a replacement dose (1.5 mg/day, divided into four subcutaneous doses) or a dose three times higher (4.5 mg/day) during 6 months. Neurological examination and neurophysiological measurements were performed before and after 6 months of treatment with C-peptide or placebo.

RESULTS

The age of the 139 patients who completed the protocol was 44.2 +/- 0.6 (mean +/- SE) years and their duration of diabetes was 30.6 +/- 0.8 years. Clinical neurological impairment (NIA) (score >7 points) of the lower extremities was present in 86% of the patients at baseline. Sensory nerve conduction velocity (SCV) was 2.6 +/- 0.08 SD below body height-corrected normal values at baseline and improved similarly within the two C-peptide groups (P < 0.007). The number of patients responding with a SCV peak potential improvement >1.0 m/s was greater in C-peptide-treated patients than in those receiving placebo (P < 0.03). In the least severely affected patients (SCV < 2.5 SD below normal at baseline, n = 70) SCV improved by 1.0 m/s (P < 0.014 vs. placebo). NIA score and vibration perception both improved within the C-peptide-treated groups (P < 0.011 and P < 0.002). A1C levels (7.6 +/- 0.1% at baseline) decreased slightly but similarly in C-peptide-and placebo-treated patients during the study.

CONCLUSIONS

C-peptide treatment for 6 months improves sensory nerve function in early-stage type 1 diabetic neuropathy.

The effect of insulin deficiency on tau and neurofilament in the insulin knockout mouse

Complications of diabetes mellitus within the nervous system are peripheral and central neuropathy. In peripheral neuropathy, defects in neurofilament and microtubules have been demonstrated. In this study, we examined the effects of insulin deficiency within the brain in insulin knockout mice (I-/-). The I-/- exhibited hyperphosphorylation of tau, at threonine 231, and neurofilament. In addition, we showed hyperphosphorylation of c-Jun N-terminal kinase (JNK) and glycogen synthase kinase 3 beta (GSK-3 beta) at serine 9. Extracellular signal-regulated kinase 1 (ERK 1) showed decrease in phosphorylation, whereas ERK 2 showed no changes. Ultrastructural examination demonstrated swollen mitochondria, endoplasmic reticulum, and Golgi apparatus, and dispersion of the nuclear chromatin. Microtubules showed decrease in the number of intermicrotubule bridges and neurofilament presented as bunches. Thus, lack of insulin brain stimulation induces JNK hyperphosphorylation followed by hyperphosphorylation of tau and neurofilament, and ultrastructural cellular damage, that over time may induce decrease in cognition and learning disabilities.