Another factor that is involved
Another factor that is involved in the severity of EAE is apoptosis signal-regulating kinase 1 (ASK1), which is one of a growing number of mitogen-activated protein kinase (MAPK) kinase kinases identified in the upstream of the c-Jun N-terminal kinase and p38 MAPK pathways . We previously reported that Toll-like receptor (TLR)-ASK1 signaling is required for chemokine productions in astrocytes and for recruitment of activated microglia into the lesion site during EAE . In addition, the same signaling pathways in microglia seem to modulate progression of demyelination by altering the release of proinflammatory components. These results suggest that VPA and ASK1 may regulate the activity of different cell types that play important roles during EAE. To determine this possibility, in this study, we investigated the therapeutic potential of VPA against optic neuritis in EAE mice and examined whether ASK1 deficiency has synergistic effects with VPA.
Materials and methods
Discussion Previous studies have shown that VPA induces apoptosis in activated T 3ct and maintains the immune homeostasis . In addition, VPA treatment in EAE suppressed the polarization of Th1 and Th17 cells but induced the Th2 and Treg cells, and inhibited lymphocyte proliferation and macrophage activation in vitro. These findings, together with decreased T cell infiltration into optic nerves in our study (Fig. 3D), indicate that VPA targets T cells during EAE to reduce neuroinflammation. Interestingly, VPA also suppressed the activation of glial cells in both spinal cords and optic nerves, which may be a secondary effect after T cell suppression. In contrast to VPA, ASK1 has no effect on T cell proliferation capability in vitro. ASK1 signaling in astrocytes regulates chemokine production and recruitment of activated microglia after the infiltration of T cells into the lesion site. In addition, the same signaling pathway in microglial cells seems to modulate the progress of demyelination by altering the release of proinflammatory components . Thus, the beneficial effects of ASK1 inhibition during EAE may mainly come from suppression of glial cell activation in the late phase of EAE, although T cell activation was suppressed in the spinal cord of ASK1 KO EAE mice (Fig. 2D). In fact, VPA and ASK1 deficiency decreased the clinical score in the early and late phase, respectively (Fig. 1). These findings suggest a possibility that VPA and ASK1 inhibition are effective in different cell types and at different timepoints during EAE. Further studies are required to examine the pathology of spinal cords and optic nerves at various time points during EAE. In this study, we demonstrate that VPA and ASK1 deficiency show clear synergistic effects on retinal neuroprotection during EAE (Fig. 4). There is increasing evidence that MS is not only an inflammatory disease, but also a neurodegenerative disease, and that neuroprotective agents may be effective for MS therapy . VPA has been reported to exert neuroprotective effects by stimulating the ERK pathway in cortical neurons  and in the retina following optic nerve injury , . Also, VPA is an effective HDAC inhibitor ,  and increased histone acetylation is associated with VPA-mediated neuroprotection , , . In addition, VPA reduces glutamatergic excitatory neurotransmission , suggesting that excitotoxic damage in EAE mice may be decreased by VPA treatment. Furthermore, VPA may accelerate the recovery phase of EAE and increase the number of remyelinated axons in the lesion area by recruiting neural stem cells and oligodendrocyte progenitors . Therefore, VPA could effectively ameliorate EAE by exerting neuroprotective effects through multiple mechanisms, in addition to its suppressive effects on T cells. On the other hand, ASK1 deficiency stimulates neuroprotection through different pathways, probably suppression of p38 MAPK etc. , , during EAE as well as after optic nerve injury . Therefore, robust synergistic neuroprotective effects may be achieved by combination of the two.