Growth factors and diabetic neuropathy

Early Biomarkers of Neurodegenerative and Neurovascular Disorders in Diabetes

Diabetes mellitus (DM) is a common disease worldwide. There is a strong association between DM and neurovascular and neurodegenerative disorders. The first group mainly consists of diabetic retinopathy, diabetic neuropathy and stroke, whereas, the second group includes Alzheimer's disease, Parkinson's disease, mild cognitive impairment and dementia. The aforementioned diseases have a common pathophysiological background including insulin resistance, oxidative stress, atherosclerosis and vascular injury. The increasing prevalence of neurovascular and neurodegenerative disorders among diabetic patients has resulted in an urgent need to develop biomarkers for their prediction and/or early detection. The aim of this review is to present the potential application of the most promising biomarkers of diabetes-related neurodegenerative and neurovascular disorders, including amylin, β-amyloid, C-reactive protein (CRP), dopamine, gamma-glutamyl transferase (GGT), glycogen synthase kinase 3β, homocysteine, microRNAs (mi-RNAs), paraoxonase 1, phosphoinositide 3-kinases, tau protein and various growth factors. The most clinically promising biomarkers of neurovascular and neurodegenerative complications in DM are hsCRP, GGT, homocysteine and miRNAs. However, all biomarkers discussed in this review could become a part of the potential multi-biomarker screening panel for diabetic patients at risk of neurovascular and neurodegenerative complications.

Acetyl-L-carnitine (levacecarnine) in the treatment of diabetic neuropathy. A long-term, randomised, double-blind, placebo-controlled study

OBJECTIVE

To assess the efficacy and tolerability of acetyl-L-carnitine (levacecarnine; LAC) versus placebo in the treatment of diabetic neuropathy, mainly by evaluating the effects of treatment on electrophysiological parameters and pain symptoms.

DESIGN

This was a multicentre (n = 20), randomised, double-blind, placebo-controlled, parallel-group study.

PATIENTS

333 patients meeting clinical and/or neurophysiological criteria for diabetic neuropathy were enrolled.

INTERVENTIONS

Patients were randomised to treatment with LAC or placebo. LAC (or placebo) was started intramuscularly at a dosage of 1000 mg/day for 10 days and continued orally at a dosage of 2000 mg/day for the remainder of the study (355 days). MAIN OUTCOME PARAMETERS AND RESULTS: The main efficacy parameter was the effect of treatment on 6- and 12-month changes from baseline in nerve conduction velocity (NCV) and amplitude in the sensory (ulnar, sural and median) and motor (median, ulnar and peroneal) nerves. The effect of treatment on pain was also evaluated by means of a visual analogue scale (VAS). Among the 294 patients with impaired electrophysiological parameters at baseline, those treated with LAC showed a statistically significant improvement in mean NCV and amplitude compared with placebo (p < 0.01). The greatest changes in NCV (at 12 months) were observed in the sensory sural nerve (7 m/sec in the LAC group vs +1.0 m/sec in the placebo group), sensory ulnar nerve (+2.9 vs +0.1 m/sec, respectively) and motor peroneal nerve (+2.7 vs -0.2 m/sec), whereas the greatest changes in amplitude were recorded in the motor peroneal nerve (+2.2 vs +0.1 mV). After 12 months of treatment, mean VAS scores for pain were significantly reduced from baseline by 39% in LAC-treated patients (p < 0.0 vs baseline) compared with 8% in placebo recipients. LAC was well tolerated over the study period.

CONCLUSIONS

LAC was effective and well tolerated in improving neurophysiological parameters and in reducing pain over a 1-year period. LAC is, therefore, a promising treatment option in patients with diabetic neuropathy.

Acquired canine peripheral neuropathies

Accurate diagnosis of the many causes of acute and chronic peripheral neuropathy in the dog presents a challenging prospect for any clinician. Being able to accurately localize the observed neurologic signs to the peripheral nervous system is the first challenge. Once this is accomplished, a logical series of diagnostic steps should be pursued so as to have the best chance of reaching a final etiologic diagnosis. Specific therapy can then be instituted to attempt to halt or, in some cases, reverse the peripheral nerve dysfunction.

Peripheral neuropathy does not invariably coexist with autonomic neuropathy in diabetes mellitus

Background: Peripheral somatic and autonomic neuropathies are the most common types of diabetic polyneuropathy. Although duration and degree of hyperglycemia are considered to be risk factors for both autonomic and peripheral neuropathy, recent studies have raised the question of a different development and natural history of these neuropathies in diabetes. In addition, a few studies have investigated the relationship between chronic painful and autonomic neuropathy. The aim of this study was to investigate to what extent autonomic and peripheral neuropathy coexist, as well as whether painful neuropathy is more common in diabetic patients with autonomic neuropathy. Methods: Subjects with type 1 (n=52; mean age 31.7 years) and type 2 diabetes (n=53; mean age 54.5 years) were studied. Evaluation of peripheral neuropathy was based on clinical symptoms (neuropathic symptom score), signs (neuropathy disability score), and quantitative sensory testing (vibration perception threshold). Assessment of autonomic neuropathy was based on the battery of standardized cardiovascular autonomic function tests. Results: Prevalence rates of pure autonomic and of pure peripheral neuropathy in patients with type 1diabetes were 28.8 and 13.5%, respectively. The respective rates in patients with type 2 diabetes were 20.7% (P=0.33 vs. type 1 diabetes) and 20.7% (P=0.32). Peripheral and autonomic neuropathy coexisted in 28.8% of type 1 and in 45.3% of type 2 diabetic subjects (P=0.08). Prevalence rates of chronic painful neuropathy in subjects with type 1 diabetes, with and without autonomic neuropathy, were 16.6 and 22.7%, respectively (P=0.85) and in type 2 diabetic subjects 20 and 22.2%, respectively (P=0.58). Multivariate analysis after adjustment for age, sex, blood pressure, duration of diabetes, HBA(1c), and presence of retinopathy or microalbuminuria showed that neither the indices of peripheral nerve function (neuropathic symptom score, neuropathy disability score, vibration perception threshold) nor the presence of peripheral neuropathy or chronic painful neuropathy are associated with the presence of autonomic neuropathy in individuals with either type 1 or type 2 diabetes. Conclusions: Peripheral and autonomic neuropathies do not invariably coexist in diabetes. In addition, chronic painful neuropathy may be present irrespective of the presence of autonomic neuropathy.

Contraction-induced muscle fiber damage is increased in soleus muscle of streptozotocin-diabetic rats and is associated with elevated expression of brain-derived neurotrophic factor mRNA in muscle fibers and activated satellite cells

The expression of brain-derived neurotrophic factor (BDNF) is elevated in the soleus muscle of streptozotocin-diabetic rats. To determine whether this diabetes-induced elevation was associated with or enhanced by muscle activity we have induced high-intensity muscle contraction by electrically stimulating the sciatic nerve. In 6-week diabetic rats, intense contraction of the soleus muscle resulted in a two- to four-fold elevation of BDNF mRNA and increased plasma levels of creatine kinase that were associated with severe focal muscle fiber damage and concomitant satellite cell activation. Focal muscle fiber damage and concomitant satellite cell activation were also observed in the soleus muscle of nonstimulated diabetic rats, but to a much lesser extent. No effects of muscle contraction, i.e., experimentally induced or during normal daily activity, on muscle fiber structure or BDNF mRNA expression were seen in diabetic extensor digitorum longus (EDL) muscle. Using a nonradioactive in situ hybridization technique for electron microscopy, the elevated expression of BDNF mRNA in the diabetic soleus muscle was localized within muscle fibers as well as activated satellite cells. This study shows that diabetic soleus muscle, in contrast to diabetic EDL and to soleus and EDL muscle of normal animals, is highly susceptible to contraction-induced damage. Intense contraction and the associated muscle fiber damage in the diabetic soleus muscle result in an upregulation of BDNF mRNA in muscle fibers and activated satellite cells, which may be involved in the restoration and/or maintenance of nerve/muscle integrity.

Treatment of diabetic polyneuropathy with the antioxidant thioctic acid (alpha-lipoic acid): a two year multicenter randomized double-blind placebo-controlled trial (ALADIN II). Alpha Lipoic Acid in Diabetic Neuropathy

Short-term trials with the antioxidant thioctic acid (TA) appear to improve neuropathic symptoms in diabetic patients, but the long-term response remains to be established. Therefore, Type 1 and Type 2 diabetic patients with symptomatic polyneuropathy were randomly assigned to three treatment regimens: (1) 2 x 600(mg of TA (TA 1200), (2) 600)mg of TA plus placebo (PLA) (TA 600) or (3) placebo and placebo (PLA). A trometamol salt solution of TA of 1200 or 600 mg or PLA was intravenously administered once daily for five consecutive days before enrolling the patients in the oral treatment phase. The study was prospective, PLA-controlled, randomized, double-blind and conducted for two years. Severity of diabetic neuropathy was assessed by the Neuropathy Disability Score (NDS) and electrophysiological attributes of the sural (sensory nerve conduction velocity (SNCV), sensory nerve action potential (SNAP)) and the tibial (motor nerve conduction velocity (MNCV), motor nerve distal latency (MNDL)) nerve. Statistical analysis was performed after independent reviewers excluded all patients with highly variable data allowing a final analysis of 65 patients (TA 1200: n = 18, TA 600: n = 27; PLA: n = 20). At baseline no significant differences were noted between the groups regarding the demographic variables and peripheral nerve function parameters for these 65 patients. Statistically significant changes after 24 months between TA and PLA were observed (mean +/- SD) for sural SNCV: +3.8 +/- 4.2 m/s in TA 1200, +3.0+/-3.0m/s in TA 600, -0.1+/-4.8m/s in PLA (p < 0.05 for TA 1200 and TA 600 vs. PLA); sural SNAP: +0.6+/-2.5 microV in TA 1200, +0.3+/-1.4 microV in TA 600, -0.7 +/- 1.5 microV in PLA (p = 0.076 for TA 1200 vs. PLA and p < 0.05 for TA 600 vs. PLA), and in tibial MNCV: +/- 1.2 +/- 3.8 m/s in TA 1200, -0.3 +/- 5.2 m/s in TA 600, 1.5 +/- 2.9 m/s in PLA (p < 0.05 for TA 1200 vs. PLA). No significant differences between the groups after 24 months were noted regarding the tibial MNDL and the NDS. We conclude that in a subgroup of patients after exclusion of patients with excessive test variability throughout the trial, TA appeared to have a beneficial effect on several attributes of nerve conduction.

Tissue nerve growth factor concentrations in neuromuscular diseases

In order to investigate the possible influences of pathological processes on muscle NGF levels in human subjects, we measured the NGF concentrations in muscle biopsies from 35 male and 16 female subjects (controls, n = 14; amyotrophic lateral sclerosis, n = 20; inflammatory myopathy, n = 6; muscular dystrophy, n = 11). The NGF concentration in each group was as follows: controls, 1.73 +/- 0.3 pg/mg protein (mean +/- S.E.M.); muscular dystrophies, 1.73 +/- 0.48 pg/mg protein; inflammatory myopathies, 2.28 +/- 1.45 pg/mg protein; amyotrophic lateral sclerosis, 4.15 +/- 0.79 pg/mg protein. The tissue NGF concentrations were significantly (140%) higher in patients with ALS than in the control subjects (P < 0.05, Wilcoxon signed rank test). Age and gender had no influence on tissue NGF concentrations. We conclude that the NGF increases observed here in affected muscle in amyotrophic lateral sclerosis can best be explained in terms of rapidly progressing denervation processes. Copyright 1998 Lippincott Williams & Wilkins

Diabetic neuropathy

Better clinical characteristics and a standardized approach to the definition of neuropathy has enabled us to define more precisely the natural history of diabetic neuropathy. Detailed studies on the pathology and pathogenesis have allowed dissection of important pathogenetic pathways. Effective treatment is currently limited, although a number of new and potentially important therapeutic interventions, including modification of the vascular supply and antioxidant status and growth factors, may prove to be of benefit in preventing damage and also promoting repair of peripheral nerves in human diabetic neuropathy.

Tissue concentrations of nerve growth factor in aging rat heart and skeletal muscle

In order to examine the association between adult nerve growth factor (NGF) levels and age-related changes in skeletal and heart muscle mass, we determined NGF concentrations in both tissues. NGF concentrations in rat heart muscle were significantly higher than those in skeletal muscle. NGF concentrations in heart muscle had a significant positive correlation with heart muscle wet weight. A causal association may exist between age-related changes in adult heart muscle mass and tissue NGF levels (in contrast to skeletal muscle). Among the potential clinical implications for skeletal muscle, it appears that age-related delay or deterioration in regeneration processes in neuromuscular diseases, or age-related decline in skeletal muscle mass, are not caused by reduced tissue NGF concentrations.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Colocalisation of insulin and IGF-1 receptors in cultured rat sensory and sympathetic ganglion cells

Peripheral sensory and autonomic neurons are known to possess insulin receptors. These have been considered to be of the peripheral type, i.e. similar to those of hepatic and fat cells rather than of the brain type which show dual specificity for both insulin and insulin-like growth factor (IGF-1). We have examined the localisation of insulin and IGF-1 receptors in cultured sensory and sympathetic ganglion cells using confocal microscopy and indirect labelling with FITC (fluorescein isothiocyanate) and TRITC (tetramethyl rhodamine isothiocyanate) respectively. We have shown that in cultured U266B1 multiple myeloma cells these receptors display separate localisation, whereas they are colocalised in IM-9 lymphocytes which are known to possess hybrid receptors. We have confirmed the sequestration of insulin and IGF-1 receptors in the cytoplasm of sensory and sympathetic neurons, consistent with a brain-type receptor. The colocalisation of insulin and IGF-1 receptors in sensory and sympathetic ganglion cells is consistent with the view that they are hybrid receptors, similar to those present in the CNS. The function of these receptors, as suggested for the CNS, may be related to trophic support for neurons.

Diabetes and the nervous system

Hyperglycemia and its vascular complications affect the entire nervous system, contributing to increased morbidity and mortality. Chronic hyperglycemia is not only a known and major risk factor for cerebral vascular diseases but also the presence of hyperglycemia at the time of a cerebrovascular event may adversely influence the outcome. It also affects the treatment of some neurodegenerative disorders, and there are suggestions that diabetes may in fact suffer from a "chronic diabetic encephalopathy." Its varied effects on the peripheral nervous system result in several forms of diabetic neuropathies, the exact pathogenesis of which is still obscure. There is, however, some new information that may link metabolic and vascular hypotheses.

Treatment of symptomatic diabetic peripheral neuropathy with the anti-oxidant alpha-lipoic acid. A 3-week multicentre randomized controlled trial (ALADIN Study)

Anti-oxidant treatment has been shown to prevent nerve dysfunction in experimental diabetes mellitus, thus providing a rationale of potential therapeutic value for diabetic patients. The effects of the anti-oxidant alpha-lipoic acid (thioctic acid) were studied in a 3-week multicentre, randomized, double-blind placebo-controlled trial (Alpha-Lipoic Acid in Diabetic Neuropathy; ALADIN) in 328 non-insulin-dependent diabetic patients with symptomatic peripheral neuropathy who were randomly assigned to treatment with intravenous infusion of alpha-lipoic acid using three doses (1200, 600, or 100 mg ALA) or placebo (PLAC). Neuropathic symptoms (pain, burning, paraesthesiae, and numbness) were scored at baseline and at each visit (days 2-5, 8-12, and 15-19) prior to infusion. In addition, the Hamburg Pain Adjective List, a multidimensional specific pain questionnaire, and the Neuropathy Symptom and Disability Scores were assessed at baseline and day 19. According to the protocol 260 (65/63/66/66) patients completed the study. The total symptom score in the feet decreased from baseline to day 19 by -4.5 +/- 3.7 (-58.6%) points (mean +/- SD) in ALA 1200, -5.0 +/- 4.1 (-63.5%) points in ALA 600, -3.3 +/- 2.8 (-43.2%) points in ALA 100, and -2.6 +/- 3.2 (-38.4%) points in PLAC (ALA 1200 vs PLAC: p = 0.003; ALA 600 vs PLAC: p < 0.001). The response rates after 19 days, defined as an improvement in the total symptom score of at least 30%, were 70.8% in ALA 1200, 82.5% in ALA 600, 65.2% in ALA 100, and 57.6% in PLAC (ALA 600 vs PLAC; p = 0.002). The total scale of the Pain Adjective List was significantly reduced in ALA 1200 and ALA 600 as compared with PLAC after 19 days (both p < 0.01). The rates of adverse events were 32.6% in ALA 1200, 18.2% in ALA 600, 13.6% in ALA 100, and 20.7% in PLAC. These findings substantiate that intravenous treatment with alpha-lipoic acid using a dose of 600 mg/day over 3 weeks is superior to placebo in reducing symptoms of diabetic peripheral neuropathy, without causing significant adverse reactions.

Peripheral neuropathies and neurotrophic factors: animal models and clinical perspectives

A large body of data exists showing that a wide variety of neurotrophic factors can promote the survival or growth of different neuronal populations in vitro. More recently, several studies have been published on the survival-promoting effects of particular factors in animal models of peripheral neuropathies. Thus, the effect of axotomy on neuropeptide expression in dorsal root ganglion cells is partially reversed by nerve growth factor treatment, and the effect on choline acetyltransferase expression in motoneurones is partially reversed by glial-derived neurotrophic factor, neurotrophin-4/5 and brain-derived neurotrophic factor. Nerve growth factor also ameliorates some of the changes seen in sensory neurones in animal models of diabetic neuropathy and small fibre cytostatic drug neuropathy, whereas neurotrophin-3 has been found to reverse some changes in large sensory neurones associated with cisplatin neurotoxicity. The results of these studies provide grounds for optimism in the clinical uses of such factors, and, indeed, several clinical studies are now under way.

Insulin-like growth factor-I and IGF-I receptors in diabetic patients with neuropathy

Since a number of animal studies have shown that insulin-like growth I (IGF-I) stimulates nerve regeneration, the aim of our study was to evaluate the possible relationship between IGF-I and IGF-I receptors in diabetic patients with peripheral neuropathy. One hundred and four patients with Type 2 diabetes (57 with peripheral neuropathy and 47 non-neuropathic) were studied. Controls were 17 non-diabetic persons. After an overnight fast, blood was taken for IGF-I, IGF-I receptors, glucose, HbA1, C-peptide, and insulin. The neuropathy study group had significantly lower levels of IGF-I:144.5 ng ml-1 (57.5-363.0, 95% confidence limits) compared to controls: 186.2 ng ml-1 (93.3-371.5), p < 0.01, and to diabetic patients without neuropathy: 173.7 ng ml-1 (83.1-363.0), p < 0.01. The study group also had a lower number of IGF-I receptors per red cell: 22.9 x 10(3) (13.08-38.01) vs control subjects: 28.1 x 10(3) (18.62-42.65), p < 0.01, and non-neuropathic diabetic patients: 26.3 x 10(3) (16.59-41.68), p < 0.01. In diabetic subjects there was a positive correlation (r = 0.20, p < 0.05) between IGF-I and HbA1, while in the neuropathy group there was a negative correlation between the score for nerve dysfunction with the IGF-I (r = -0.39, p < 0.01) and with IGF-I receptors (r = -0.34, p < 0.01). We conclude that in diabetic patients with peripheral neuropathy there are abnormalities of IGF-I and IGF-I receptors which may contribute to impaired neuronal regeneration.