James W. Russell

High glucose-induced oxidative stress and mitochondrial dysfunction in neurons

The current study examines the association between glucose induction of reactive oxygen species (ROS), mitochondrial (Mt) depolarization, and programmed cell death in primary neurons. In primary dorsal root ganglion (DRG) neurons, 45 mM glucose rapidly induces a peak rise in ROS Corresponding to a 50% increase in mean Mt size at 6 h (P<0.001). This is coupled with loss of regulation of the Mt membrane potential (Mt membrane hyperpolarization, followed by depolarization, MMD), partial depletion of ATP, and activation of caspase‐3 and ‐9. Glucose‐induced activation of ROS, MMD, and caspase‐3 and ‐9 activation is inhibited by myxothiazole and thenoyltrifluoroacetone (P<0.001), which inhibit specific components of the Mt electron transfer chain. Similarly, MMD and caspase‐3 activation are inhibited by 100 fM bongkrekic acid (an inhibitor of the adenosine nucleotide translocase ANT). These results indicate that mild increases in glucose induce ROS and Mt swelling that precedes neuronal apoptosis. Glucotoxicity is blocked by inhibiting ROS induction, MMD, or caspase cleavage by specific inhibitors of electron transfer, or by stabilizing the ANT.—Russell, J. W., Golovoy, D., Vincent, A. M., Mahendru, P., Olzmann, J. A., Mentzer, A., Feldman, E. L. High glucose‐induced oxidative stress and mitochondrial dysfunction in neurons. FASEB J. 16, 1738–1748 (2002)

Oxidative stress and programmed cell death in diabetic neuropathy.

Recent evidence in both animal models and human sural nerve biopsies indicates an association with oxidative stress, mitochondrial (Mt) membrane depolarization (MMD), and induction of programmed cell death (PCD). In streptozotocin (STZ)‐treated diabetic rats, hyperglycemia induces typical apoptotic changes as well as swelling and disruption of the Mt cristae in diabetic dorsal root ganglion neurons (DRG) and Schwann cells (SC), but these changes are only rarely observed in control neurons. In human sural nerve biopsies, from patients with diabetic sensory neuropathy, there is transmission electromicrograph evidence of swelling and disruption of the Mt and cristae compared to patients without peripheral neuropathy. In human SH‐SY5Y neurons, rat sensory neurons, and SC, in vivo, there is an increase in reactive oxygen species (ROS) after exposure to 20 mM added glucose. In parallel, there is an initial Mt membrane hyperpolarization followed by depolarization (MMD). In turn, MMD is coupled with cleavage of caspases. Various strategies aimed at inhibiting the oxidative burst, or stabilizing the ΔΨM, block induction of PCD. First, growth factors such as NGF can block induction of ROS and/or stabilize the ΔΨM. This, in turn, is associated with inhibition of PCD. Second, reduction of ROS generation in neuronal Mt prevents neuronal PCD. Third, up‐regulation of uncoupling proteins (UCPs), which stabilize the ΔΨM, blocks induction of caspase cleavage. Collectively, these findings indicate that hyperglycemic conditions observed in diabetes mellitus are associated with oxidative stress‐induced neuronal and SC death, and targeted therapies aimed at regulating ROS may prove effective in therapy of diabetic neuropathy.

Impaired glucose tolerance—does it cause neuropathy?

The publication of the Diabetes Control and Complications Trial (DCCT) laid to rest much of the controversy surrounding the role of hyperglycemia in diabetic neuropathy. This study showed that intensive insulin therapy, coupled with improved glycemic control, reduces the severity of diabetic complications and, more importantly, decreases the risk of developing these complications. This was the first large prospective study to show that careful regulation of blood glucose can prevent development of neuropathy in diabetic patients. Despite the evidence that hyperglycemia is coupled with neuropathy, it has been assumed that neuropathy results only from significant hyperglycemia and is not related to impaired glucose tolerance (IGT). In the presence of mild and episodic hyperglycemia, alternative causes for neuropathy are sought.

IGFs and the Nervous System

Insulin-like growth factors-I and -II (IGF-I and IGF-II) are peptide growth factors closely related in sequence to insulin (1,2). In the nervous system, the IGFs, IGF receptors, and IGF binding proteins (IGFBPS) are widely expressed and promote proliferation, survival, and differentiation of neuronal and nonneuronal cells. In addition, high levels of IGFs and IGFBPs are found in a variety of nervous system tissues during development. Because of their potent survival and differentiation activities, the IGFs are currently being tested as therapeutics for a variety of neuronal pathologies, including various neuropathies, motor neuron disease, and physical or hypoxic—ischemic insults. The physiological effects of the IGFs in the nervous system are thought to be mediated by the IGF-I receptor, while the IGF-II receptor, in conjunction with the IGFBPs, regulates IGF bioavailability.

Clinical and pathologic features of focal myositis

To clarify the nosology of focal myositis (FM), we report the clinical and pathologic features of eight patients presenting with focal enlargement of one muscle. Most patients improved without immunosuppressive therapy, and none developed polymyositis. Pathologic features were those of an inflammatory myopathy, with muscle fiber hypertrophy and moderate to severe inflammation. In most cases, a clustering of tightly packed muscle fibers, enveloped by a thick bundle of fibrosis, was associated with the diagnosis of FM. Immunohistochemistry showed T cell predominance within the interstitial infiltrates in all cases. No evidence of vasculitis was present. Our findings suggest that FM is a benign condition that has certain clinical features separating it from other inflammatory myopathies. Pathologic changes, such as large clusters of nesting muscle fibers surrounded by thick fibrosis, are more characteristic of FM than polymyositis.

IGF‐I Promotes Peripheral Nervous System Myelination

ABSTRACT: Insulin‐like growth factor‐I (IGF‐I) promotes the proliferation and differentiation of Schwann cells (SC). We use SC/dorsal root ganglion neuron (DRG) cocultures to examine the effects of IGF‐I on the interaction between axons and SC. As SC extend processes toward the axon in the presence of IGF‐I, these processes attach to and ensheath axons. Continued IGF‐I exposure leads to enhanced P0 expression and long‐term myelination. No myelination occurs in the absence of IGF‐I. These data imply that IGF‐I is critical not only for SC attachment and ensheathment of axons but also for long‐term myelination.

Protection against glucose-induced neuronal death by NAAG and GCP II inhibition is regulated by mGluR3

Glutamate carboxypeptidase II (GCP II) inhibition has previously been shown to be protective against long‐term neuropathy in diabetic animals. In the current study, we have determined that the GCP II inhibitor 2‐(phosphonomethyl) pentanedioic acid (2‐PMPA) is protective against glucose‐induced programmed cell death (PCD) and neurite degeneration in dorsal root ganglion (DRG) neurons in a cell culture model of diabetic neuropathy. In this model, inhibition of caspase activation is mediated through the group II metabotropic glutamate receptor, mGluR3. 2‐PMPA neuroprotection is completely reversed by the mGluR3 antagonist (S)‐α‐ethylglutamic acid (EGLU). In contrast, group I and III mGluR inhibitors have no effect on 2‐PMPA neuroprotection. Furthermore, we show that two mGluR3 agonists, the direct agonist (2R,4R)‐4‐aminopyrrolidine‐2, 4‐dicarboxylate (APDC) and N‐acetyl‐aspartyl‐glutamate (NAAG) provide protection to neurons exposed to high glucose conditions, consistent with the concept that 2‐PMPA neuroprotection is mediated by increased NAAG activity. Inhibition of GCP II or mGluR3 may represent a novel mechanism to treat neuronal degeneration under high‐glucose conditions.

Advances in understanding drug-induced neuropathies

Many commonly used medications have neurotoxic adverse effects; the most common of these is peripheral neuropathy. Neuropathy can be a dose-limiting adverse effect for many medications used in life-threatening conditions, such as malignancy and HIV-related disease. Epidemiological evidence supports previous case reports of HMG-CoA reductase inhibitors (or ‘statins’) causing an axonal sensorimotor neuropathy or a purely small-fibre neuropathy in some patients. The neuropathy improves when the medication is withdrawn. Despite the association between HMG-CoA reductase inhibitors and neuropathy, the risk is low compared with the significant vascular protective benefits. Oxaliplatin, a new platinum chemotherapy agent designed to have fewer adverse effects than other such agents, has been shown to cause a transient initial dysaesthesia in addition to an axonal polyneuropathy. Thalidomide, an old therapy currently being utilised for new therapeutic indications (e.g. treatment of haematological malignancies), is associated with a painful, axonal sensorimotor neuropathy that does not improve on withdrawal of the drug. Nucleoside reverse transcriptase inhibitors are important components of highly active antiretroviral therapy, but are associated with a sensory neuropathy that is likely to be due to a direct effect of these drugs on mitochondrial DNA replication. New research demonstrates that lactate levels may help discriminate between neuropathy caused by nucleoside analogues and HlV-induced neuropathy. Understanding the mechanism of drug-induced neuropathy has led to advances in preventing this disabling condition.

Metabotropic glutamate receptor regulation of neuronal cell death

The metabotropic glutamate receptors (mGluRs) are a family of glutamate-sensitive receptors that regulate neuronal function separately from the ionotropic glutamate receptors. By coupling to guanosine nucleotide-binding proteins (G proteins), mGluRs are able to regulate neuronal injury and survival, likely through a series of downstream protein kinase and cysteine protease signaling pathways that affect mitochondrial regulated programmed cell death (PCD). The physiological relevance of this system is supported by evidence that mGluRs are associated with cell survival in several central nervous system neurodegenerative diseases. Evidence is presented that mGluRs are also able to prevent PCD in the peripheral nervous system, and that this may provide a novel mechanism for treatment of diabetic neuropathy. In dorsal root ganglion (DRG) neurons, a high glucose load increases generation of reactive oxygen species (ROS), destabilizes the inner mitochondrial membrane potential (Deltapsi(M)), induces cytochrome c release from the mitochondrial intermembrane space, and induces downstream activation of caspases. In high-glucose conditions, the group II metabotropic glutamate agonist N-acetylaspartylglutamate (NAAG) blocks caspase activation and is completely reversed by the mGluR3 antagonist (S)-alpha-ethylglutamic acid (EGLU). Furthermore, the direct mGluR3 agonist (2R,4R)-4-aminopyrrolidine-2, 4-dicarboxylate (APDC) prevents induction of ROS. Together these findings are consistent with an emerging concept that mGluRs may protect against cellular injury by regulating oxidative stress in the neuron. More complete understanding of the complex PCD regulatory pathways mediated by mGluRs will provide new therapeutic approaches for the treatment of a wide variety of neurodegenerative diseases.

Metabotropic glutamate receptor 3 protects neurons from glucose‐induced oxidative injury by increasing intracellular glutathione concentration

High glucose concentrations cause oxidative injury and programmed cell death in neurons, and can lead to diabetic neuropathy. Activating the type 3 metabotropic glutamate receptor (mGluR3) prevents glucose‐induced oxidative injury in dorsal root ganglion neurons co‐cultured with Schwann cells. To determine the mechanisms of protection, studies were performed in rat dorsal root ganglion neuron–Schwann cell co‐cultures. The mGluR3 agonist 2R,4R‐4‐aminopyrrolidine‐2,4‐dicarboxylate prevented glucose‐induced inner mitochondrial membrane depolarization, reactive oxygen species accumulation, and programmed cell death, and increased glutathione (GSH) concentration in co‐cultured neurons and Schwann cells, but not in neurons cultured without Schwann cells. Protection was diminished in neurons treated with the GSH synthesis inhibitor l‐buthionine‐sulfoximine, suggesting that mGluR‐mediated protection requires GSH synthesis. GSH precursors and the GSH precursor GSH‐ethyl ester also protected neurons from glucose‐induced injury, indicating that GSH synthesis in Schwann cells, and transport of reaction precursors to neurons, may underlie mGluR‐mediated neuroprotection. These results support the conclusions that activating glial mGluR3 protects neurons from glucose‐induced oxidative injury by increasing free radical scavenging and stabilizing mitochondrial function, through increased GSH antioxidant defense.