Fingolimod is the first oral disease-modifying therapy approved for relapsing forms of multiple sclerosis (MS). Following phosphorylation in vivo, the active agent, fingolimod phosphate (fingolimod-P), acts as a sphingosine 1-phosphate (S1P) receptor modulator, binding with high affinity to four of the five known S1P receptors (S1P1, S1P3, S1P4 and S1P5). The mechanism of action of fingolimod in MS has primarily been considered as immunomodulatory, whereby fingolimod-P modulates S1P1 on lymphocytes, selectively retaining autoreactive lymphocytes in lymph nodes to reduce damaging infiltration into the central nervous system (CNS). However, emerging evidence indicates that fingolimod has direct effects in the CNS in MS. For example, in the MS animal model of experimental autoimmune encephalomyelitis (EAE), fingolimod is highly efficacious in both a prophylactic and therapeutic setting, yet becomes ineffective in animals selectively deficient for S1P1 on astrocytes, despite maintained normal immunologic receptor expression and functions, and S1P-mediated immune activities. Here, we review S1P signalling effects relevant to MS in neural cell types expressing S1P receptors, including astrocytes, oligodendrocytes, neurons, microglia and dendritic cells. The direct effects of fingolimod on these CNS cells observed in preclinical studies are discussed in view of the functional consequences of reducing neurodegenerative processes and promoting myelin preservation and repair. The therapeutic implications of S1P modulation in the CNS are considered in terms of the clinical outcomes of MS, such as reducing MS-related brain atrophy, and other CNS disorders. Additionally, we briefly outline other existing and investigational MS therapies that may also have effects in the CNS.

Astrocytes have prominent roles in central nervous system (CNS) function and disease, with subpopulations defined primarily by morphologies and molecular markers often determined in cell culture. Here, we identify an astrocyte subpopulation termed immediate-early astrocytes () that is defined by functional c-Fos activation during CNS disease development. An unbiased screen for CNS cells showing c-Fos activation during experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis (MS), was developed by using inducible, TetTag c-Fos reporter mice that label activated cells with a temporally stable, nuclear green fluorescent protein (GFP). Four-dimensional (3D over time) c-Fos activation maps in the spinal cord were produced by combining tissue clearing (iDISCO) and confocal microscopy that identified onset and expansion of GFP cell populations during EAE. More than 95% of the GFP cells showed glial fibrillary acidic protein (GFAP) immunoreactivity-in contrast to absent or rare labeling of neurons, microglia, and infiltrating immune cells-which constituted that linearly increased in number with progression of EAE. formation was reduced by either astrocyte-specific genetic removal of sphingosine 1-phosphate receptor 1 (S1P) or pharmacological inhibition by fingolimod (FTY720), an FDA-approved MS medicine that can functionally antagonize S1P. s thus represent a functionally defined subset of disease-linked astrocytes that are the first and predominant CNS cell population activated during EAE, and that track with disease severity. Their reduction by a disease-modifying agent supports their therapeutic relevance to MS and potentially other neuroinflammatory and neurodegenerative diseases.

Dimethyl fumarate is an orally bioavailable compound for the treatment of multiple sclerosis and psoriasis. A mechanism involving nuclear factor erythroid 2-like 2 activation has been proposed to account for its efficacy in multiple sclerosis. Here, we report that dimethyl fumarate inhibits expression of integrin α4 on circulating lymphocytes in experimental autoimmune encephalomyelitis mice and also on activated human Jurkat T cells in a manner distinct from nuclear factor erythroid 2-like 2 activation. Our results offer an alternative mechanism for the efficacy of dimethyl fumarate in multiple sclerosis.

FTY720 (fingolimod) is a sphingosine 1-phosphate (S1P) receptor modulator and sphingosine analogue approved for multiple sclerosis (MS) therapy, which can functionally antagonize the S1P receptor, S1P1. Vitamin B12 (B12) deficiency produces neurological manifestations resembling MS. Here, we report a new mechanism where FTY720 suppresses neuroinflammation by regulating B12 metabolic pathways. Nuclear RNA-seq of c-Fos-activated astrocytes (called ieAstrocytes) from experimental autoimmune encephalomyelitis (EAE) spinal cords identified up-regulation of CD320, a transcobalamin 2 (TCN2)-B12 receptor, by S1P1 inhibition. CD320 was reduced in MS plaques. Deficiency of CD320 or dietary B12 worsened EAE and eliminated FTY720 efficacy, while concomitantly down-regulating type I interferon signaling. TCN2 functioned as a chaperone for FTY720 and sphingosine, which induced astrocytic CD320 internalization. An accompanying paper identified a requirement for astrocyte sphingosine kinases in FTY720 efficacy and its altered expression in MS brains, molecularly linking MS and B12 deficiency that can be accessed by sphingolipid/fingolimod metabolic pathways.