Alterations in gene expression associated with primary demyelination and remyelination in the peripheral nervous system

Baseline effects of lysophosphatidylcholine and nerve growth factor in a rat model of sciatic nerve regeneration after crush injury

Schwann cells play a major role in helping heal injured nerves. They help clear debris, produce neurotrophins, upregulate neurotrophin receptors, and form bands of Büngner to guide the healing nerve. But nerves do not always produce enough neurotrophins and neurotrophin receptors to repair themselves. Nerve growth factor (NGF) is an important neurotrophin for promoting nerve healing and lysophosphatidylcholine (LPC) has been shown to stimulate NGF receptors (NGFR). This study tested the administration of a single intraneural injection of LPC (1 mg/mL for single LPC injection and 10 mg/mL for multiple LPC injections) at day 0 and one (day 7), two (days 5 and 7), or three (days 5, 7, and 9) injections of NGF (160 ng/mL for single injections and 80 ng/mL for multiple injections) to determine baseline effects on crushed sciatic nerves in rats. The rats were randomly divided into four groups: control, crush, crush-NGF, and crush-LPC-NGF. The healing of the nerves was measured weekly by monitoring gait; electrophysiological parameters: compound muscle action potential (CMAP) amplitudes; and morphological parameters: total fascicle areas, myelinated fiber counts, fiber densities, fiber packing, and mean g-ratio values at weeks 3 and 6. The crush, crush-NGF, and crush-LPC-NGF groups statistically differed from the control group for all six weeks for the electrophysiological parameters but only differed from the control group at week 3 for the morphological parameters. The crush, crush-NGF, and crush-LPC-NGF groups did not differ from each other over the course of the study. Single injections of LPC and NGF one week apart or multiple treatments of NGF at 5, 7 and 9 days post-injury did not alter the healing rate of the sciatic nerves during weeks 1-6 of the study. These findings are important to define the baseline effects of NGF and LPC injections, as part of a larger effort to determine the minimal dose regimen of NGF to regenerate peripheral nerves.

The role of oxidative stress in nervous system aging

While oxidative stress is implicated in aging, the impact of oxidative stress on aging in the peripheral nervous system is not well understood. To determine a potential mechanism for age-related deficits in the peripheral nervous system, we examined both functional and morphological changes and utilized microarray technology to compare normal aging in wild-type mice to effects in copper/zinc superoxide dismutase-deficient (Sod1(-/-)) mice, a mouse model of increased oxidative stress. Sod1(-/-) mice exhibit a peripheral neuropathy phenotype with normal sensory nerve function and deficits in motor nerve function. Our data indicate that a decrease in the synthesis of cholesterol, which is vital to myelin formation, correlates with the structural deficits in axons, myelin, and the cell body of motor neurons in the Sod1(+/+) mice at 30 months and the Sod1(-/-) mice at 20 months compared with mice at 2 months. Collectively, we have demonstrated that the functional and morphological changes within the peripheral nervous system in our model of increased oxidative stress are manifested earlier and resemble the deficits observed during normal aging.

Biophysical stimulation induces demyelination via an integrin-dependent mechanism

OBJECTIVE

Chronic nerve compression (CNC) injuries occur when peripheral nerves are subjected to sustained mechanical forces, with increasing evidence implicating Schwann cells as key mediators. Integrins, a family of transmembrane adhesion molecules that are capable of intracellular signaling, have been implicated in a variety of biological processes such as myelination and nerve regeneration. In this study, we seek to define the physical stimuli mediating demyelination and to determine whether integrin plays a role in the demyelinating response.

METHODS

We used a previously described in vitro model of CNC injury where myelinating neuron-Schwann cell cocultures were subjected to independent manipulations of hydrostatic pressure, hypoxia, and glucose deprivation in a custom bioreactor. We assessed whether demyelination increased in response to applied manipulation and determined whether integrin-associated signaling cascades are upregulated.

RESULTS

Biophysical stimulation of neural tissue induced demyelination and Schwann cell proliferation without neuronal or glial cytotoxicity or apoptosis. Although glucose deprivation and hypoxia independently had minor effects on myelin stability, together they potentiated the demyelinating effects of hydrostatic compression, and in combination, significantly destabilized myelin. Biophysical stimuli transiently increased phosphorylation of the integrin-associated tyrosine kinase Src within Schwann cells. Silencing this integrin signaling cascade blocked Src activation and prevented pressure-induced demyelination. Colocalization analysis indicated that Src is localized within Schwann cells.

INTERPRETATION

These results indicate that myelin is sensitive to CNC injury and support the novel concept that myelinating cocultures respond directly to mechanical loading via activating an integrin signaling cascade.

Identification of the regulatory region of the peripheral myelin protein 22 (PMP22) gene that directs temporal and spatial expression in development and regeneration of peripheral nerves

Minor changes in PMP22 gene dosage have profound effects on the development and maintenance of peripheral nerves. This is evident from the genetic disease mechanisms in Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) as well as transgenic animals with altered PMP22 gene dosage. Thus, regulation of PMP22 is a crucial aspect in understanding the function of this protein in health and disease. In this study, we have generated transgenic mice containing 10 kb of the 5'-flanking region of the PMP22 gene, including the two previously identified alternative promoters, fused to a lacZ reporter gene. We show that this part of the PMP22 gene contains the necessary information to mirror the endogenous expression pattern in peripheral nerves during development and regeneration and in mouse models of demyelination due to genetic lesions. Transgene expression is strongly regulated during myelination, demyelination, and remyelination in Schwann cells, demonstrating the crucial influence of neuron-Schwann cell interactions in the regulation of PMP22. In addition, the region of the PMP22 gene present on this transgene confers also neuronal expression in sensory and motor neurons. These results provide the crucial basis for further dissection of the elements that direct the temporal and spatial regulation of the PMP22 gene and to elucidate the molecular basis of the master program regulating peripheral nerve myelination.

Vascular endothelial growth factor expression in peripheral nerves and dorsal root ganglia in diabetic neuropathy in rats

Vascular alterations of peripheral nerves occuring after mechanical injury or in metabolic disorders are well described. It is thought that vascular endothelial growth factor (VEGF), a potent growth factor for angiogenesis, also plays an important role for regeneration of nervous tissue. We used a rat model of type I diabetes (streptozotozin-induced) with sensory neuropathy and with chronic hyperglycemia over 12 weeks. A monoclonal antibody to VEGF was used for immunohistochemistry of sciatic nerves and dorsal root ganglia (DRG). Intense VEGF staining was detected in cell bodies and nerve fibers of animals with chronic diabetes. Healthy control groups expressed no or very little VEGF and animals treated with insulin to prevent neuropathy and severe hyperglycemia showed significantly lower immunostaining for VEGF. After application of nerve growth factor (NGF), which is known to improve axonal and Schwann cell regeneration, a markedly decreased expression of VEGF was seen in diabetic animals. In contrast, enhanced VEGF staining was noted in NGF-treated healthy controls of the same age and body weight as the diabetic rats. Similar findings were made in diabetic animals treated with both, insulin and NGF. We conclude that functional alteration of peripheral nerves causes up-regulation of VEGF in Schwann cells and neurons. With functional restitution of nervous tissue, i.e. under insulin and/or NGF treatment VEGF expression decreases significantly. Additionally, NGF may stimulate VEGF in normal controls. The production of VEGF may play a role in complete nerve regeneration and its regulation may reflect the functional state of peripheral nerves.

Tellurium causes dose-dependent coordinate down-regulation of myelin gene expression

Exposure of developing rats to a diet containing elemental tellurium systemically inhibits cholesterol synthesis at the level of squalene epoxidase. At high tellurium exposure levels (> 0.1% in the diet), there is an associated segmental demyelination of the PNS. Low levels of dietary tellurium (0.0001%) led to in vivo inhibition of squalene epoxidase activity in sciatic nerve, and inhibition increased with increasing exposure levels. With increasing dose and increasing exposure times, there was an increasing degree of demyelination and increasing down-regulation of mRNA levels for myelin P0 protein, ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis), and HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis). Because these were all down-regulated in parallel, we conclude there is coordinate regulation of the entire program for myelin synthesis in Schwann cells. An anomaly was that at early time points and low tellurium levels, mRNA levels for HMG-CoA reductase were slightly elevated, presumably in response to tellurium-induced sterol deficits. We suggest the eventual down-regulation relates to a separate mechanism by which Schwann cells regulate cholesterol synthesis, related to the need for coordinate synthesis of myelin components. Levels of mRNA for the low-affinity nerve growth factor receptor (indicator of alterations in axon-Schwann cell interactions) and for lysozyme (marker for phagocytic macrophages) were both up-regulated in a dose- and time-dependent manner which correlated with the presence of segmental demyelination. Levels of mRNA coding for myelin-related proteins were down-regulated at low tellurium exposure levels, without demyelination or up-regulation of nerve growth factor receptor. This suggests the down-regulation is related to the tellurium-induced cholesterol deficit, and not to the loss of axonal contact associated with early stages of demyelination or to the entry of activated macrophages.