Fingolimod impedes Schwann cell-mediated myelination: implications for the treatment of immune neuropathies?

BACKGROUND: Fingolimod (FTY720), a first-in-class sphingosine-1-phosphate (S1P) receptor agonist, is a recently approved drug for treating relapsing multiple sclerosis. Experimental evidence suggests that FTY720 not only exhibits anti-inflammatory properties but also promotes myelination in the central nervous system by direct interaction with oligodendrocytes. OBJECTIVE: To assess the effects of FTY720 on Schwann cells (SCs) and peripheral nerve myelination. DESIGN: Receptor expression studies and myelination were investigated in primary rat SCs and rat neuronal/SC cocultures. Cells were treated with physiologically relevant concentrations of the active phosphorylated form of FTY720 (FTY720P). In addition, S1P receptor expression was corroborated in human and rat peripheral nerve tissue sections. RESULTS: Schwann cells express all known S1P receptors on the RNA level, not altered by FTY720P. In the myelination model, treatment with FTY720P resulted in a significant reduction of quantitative myelin formation. FTY720P induced reactive oxygen species in SCs associated with apoptosis of these cells, as demonstrated by the detection of cysteine aspartic acid­specific protease 3 and 7, as well as terminal deoxynucleotidyl transferase dUTP nick-end labeling. This effect was dependent of S1P signaling because the blocking of S1P receptors ameliorated reactive oxygen species production, SC apoptosis, and myelin loss. CONCLUSIONS: FTY720P at greater concentrations induces apoptosis in SCs and may interfere with peripheral nerve myelination.

Levetiracetam exhibits protective properties on rat Schwann cells in vitro.

Oxidative stress and inflammation represent pathways causing substantial damage to the peripheral nervous system. Levetiracetam (LEV) is a commonly used antiepileptic drug targeting high-voltage activated N-type calcium channels. Recent evidence suggests that LEV may also act as a histone deacetylase inhibitor, suggesting that this drug exhibits both anti-inflammatory and anti-oxidative effects, and as such may represent an interesting candidate for treating inflammatory diseases affecting the peripheral nerve. Therefore, we analysed the influence of LEV ex vivo on purified Schwann cells from neonatal P3 rats as well as on dorsal root ganglia prepared from E15 rat embryos. LEV diminished a lipopolysaccharide (LPS)-induced increase of the pro-inflammatory signature molecules tumour necrosis factor alpha, matrix metalloproteinase 9 (MMP-9), and caspase 6. Furthermore, LEV decreased LPS-induced cell death and protected cells against oxidative stress in a glutamate-based oxidative stress model. MMP-2 activity, usually elevated during myelination and repair, was also found to be up-regulated following LEV, while LEV exhibited no negative effects on myelination. Intracellular sodium or calcium concentrations were unaltered by LEV. Thus, LEV may be a promising, well-tolerated drug that - besides its antiepileptic potential - mediates anti-inflammatory, anti-oxidative, and anti-apoptotic properties that may potentially be useful in treating diseases of the peripheral nerve.

Matrix metalloproteinase-2 is involved in myelination of dorsal root ganglia neurons.

Matrix metalloproteinases (MMPs) comprise a large family of endopeptidases that are capable of degrading all extracellular matrix components. There is increasing evidence that MMPs are not only involved in tissue destruction but may also exert beneficial effects during axonal regeneration and nerve remyelination. Here, we provide evidence that MMP-2 (gelatinase A) is associated with the physiological process of myelination in the peripheral nervous system (PNS). In a myelinating co-culture model of Schwann cells and dorsal root ganglia neurons, MMP-2 expression correlated with the degree of myelination as determined by immunocytochemistry, zymography, and immunosorbent assay. Modulation of MMP-2 activity by chemical inhibitors led to incomplete and aberrant myelin formation. In vivo MMP-2 expression was detected in the cerebrospinal fluid (CSF) of patients with Guillain-Barré syndrome as well as in CSF and sural nerve biopsies of patients with chronic inflammatory demyelinating polyneuropathy. Our findings suggest an important, previously unrecognized role for MMP-2 during myelination in the PNS. Endogenous or exogenous modulation of MMP-2 activity may be a relevant target to enhance regeneration in demyelinating diseases of the PNS.

Role of nitric oxide as mediator of nerve injury in inflammatory neuropathies.

Different lines of evidence suggest that nitric oxide (NO) plays a key role in the pathogenesis of inflammatory neuropathies; however, it is still unclear which structures in the peripheral nerve are the primary targets of NO-mediated nerve injury. To address this issue, we determined the expression of NO metabolites in sural nerve biopsies and in cerebrospinal fluid from patients with inflammatory neuropathies and studied the pathologic effects of NO in an in vitro model of myelinated Schwann cell-neuron cocultures. In cerebrospinal fluid samples, nitrite levels remained unaltered; however, nitrotyrosine, a marker for peroxynitrite formation, could be identified in nerve biopsies from patients with inflammatory neuropathies. In an in vitro model of Schwann cell neuron cocultures, high concentrations of NO induced robust demyelination, which was the result of NO-mediated axonal injury, whereas Schwann cell viability remained unaffected. These findings suggest that in contrast to Schwann cells, sensory neurons are the primary target of NO-mediated cytotoxicity and the loss of myelin is the result of selective damage to axons rather than a direct harmful effect to Schwann cells. Our findings imply that NO contributes to the pathologic changes seen in the inflamed peripheral nervous system, which is characterized by the features of axonal injury and subsequent myelin degradation, previously described as Wallerian-like degeneration.