Fingolimod for the treatment of neurological diseases—state of play and future perspectives

Sphingolipids are a fascinating class of signaling molecules derived from the membrane lipid sphingomyelin. They show abundant expression in the brain. Complex sphingolipids such as glycosphingolipids (gangliosides and cerebrosides) regulate vesicular transport and lysosomal degradation and their dysregulation can lead to storage diseases with a neurological phenotype. More recently, simple sphingolipids such ceramide, sphingosine and sphingosine 1-phosphate (S1P) were discovered to signal in response to many extracellular stimuli. Forming an intricate signaling network, the balance of these readily interchangeable mediators is decisive for cell fate under stressful conditions.The immunomodulator fingolimod is the prodrug of an S1P receptor agonist. Following receptor activation, the drug leads to downregulation of the S1P1 receptor with the consequence of functional antagonism. Being the first drug to modulate the sphingolipid signaling pathway, it was marketed in 2010 for the treatment of multiple sclerosis (MS). At that time, immunomodulation was widely accepted as the key mechanism of fingolimod's efficacy in MS.But given the excellent passage of this lipophilic compound into the brain and its massive brain accumulation as well as the abundant expression of S1P receptors on brain cells, it is conceivable that fingolimod also affects brain cells directly. Indeed, a seminal study showed that the protective effect of fingolimod in experimental autoimmune encephalitis (EAE), a murine MS model, is lost in mice lacking the S1P1 receptor on astrocytes, arguing for a specific role of astrocytic S1P signaling in multiple sclerosis. In this review, we discuss the role of sphingolipid mediators and their metabolizing enzymes in neurologic diseases and putative therapeutic strategies arising thereof

Innate immunity in progressive multiple sclerosis

Multiple sclerosis is (MS) is a chronic inflammatory autoimmune disease of central nervous system (CNS) leading to demyelination, axonal damage and neurological handicap, often affecting young adults. A majority of patients with MS initiate their disease with clinical bouts and relapses, but with time convert to a progressive course with dampened signs of CNS inflammation but increasing neurological deficits. This thesis is focused on highlighting the differences in levels of key immune mediators, neurofilament-light (NFL), and kynurenine pathway in different phases of MS and in an animal model of neurodegeneration. In Study I , we determined levels of NFL, complement C3 and activity of the two main acetylcholine hydrolyzing enzymes, AChE and BChE, in cerebrospinal fluid (CSF) from patients with MS and controls. Levels of C3 were higher in MS patients compared to controls and correlated with MS disease disability and NFL. The BChE activity was correlated with C3 and NFL in individual samples suggesting a potential link between intrathecal cholinergic activity and complement activation. The results motivate further studies on the regulation and effector functions of the complement system in MS, and its relation to cholinergic tone. In Study II , we identified a strong naturally occurring cis -regulatory influence on the local expression of complement receptor 2 (Cr2) in the rat spinal cord and increased soluble CR2 (sCR2) in the CSF of nerve injured rates. In transgenic mice loss of Cr2 resulted in increased loss of synapses in the axotomized motor neuron pool. In humans increased sCR2 levels were detected in the CSF of patients with MS as compared to controls, identifying CR2 as a potential novel biomarker of CNS inflammation. These results propose a new role for CR2/sCR2 as a modulator of innate immune reactions and synaptic plasticity in the CNS. In Study III , we determined levels of tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA) and quinolinic acid (QUIN) in CSF. The absolute QUIN levels and the QUIN/KYN ratio were increased in MS during relapse (RRMS). Interestingly, secondary progressive MS (SPMS) displayed lower TRP and KYNA, while primary progressive (PPMS) patients displayed increased levels of all metabolites, similar to a group of inflammatory neurological disease controls. In addition, MS patients with active disease and short disease duration were prospectively evaluated for neuropsychiatric symptoms. Depressed patients displayed higher KYNA/TRP and KYN/TRP ratios, mainly due to low TRP levels. These results demonstrate that clinical disease activity and differences in disease courses are reflected by changes in KP metabolites. Increased QUIN levels of RRMS patients in relapse and generally decreased levels of TRP in SPMS may relate to neurotoxicity and failure of remyelination, respectively. In Study IV , we analyzed the main monocytes subsets and/or expression of the chemokine receptors CCR2 or CX3CR1 in relation to different MS disease courses, and after treatment with dimethyl fumarate (DMF). In contrast to the prior studies we could not detect significant quantitative or qualitative differences in the monocyte population between different MS disease stages. DMF treatment resulted in a heterogeneous response, with both expansion and reduction of non-classical monocyte subsets in a proportion of patients. In summary and in context of current knowledge, my findings suggest that later stages of MS is characterized less of adaptive and innate cellular alterations in the periphery, also supported by the relative lack of efficacy of current therapies in MS directed mainly at modulating the adaptive immune defense. However, findings of altered complement expression and metabolic changes involving the KP may reflect low grade widespread tissue responses that can exert effects on synaptic remodeling and neuronal transmission. These pathways deserve attention as potential therapeutic targets in later stages of MS

Controversies and priorities in amyotrophic lateral sclerosis

Two decades after the discovery that 20% of familial amyotrophic lateral sclerosis (ALS) cases were linked to mutations in the superoxide dismutase-1 (SOD1) gene, a substantial proportion of the remainder of cases of familial ALS have now been traced to an expansion of the intronic hexanucleotide repeat sequence in C9orf72. This breakthrough provides an opportunity to re-evaluate longstanding concepts regarding the cause and natural history of ALS, coming soon after the pathological unification of ALS with frontotemporal dementia through a shared pathological signature of cytoplasmic inclusions of the ubiquitinated protein TDP-43. However, with profound clinical, prognostic, neuropathological, and now genetic heterogeneity, the concept of ALS as one disease appears increasingly untenable. This background calls for the development of a more sophisticated taxonomy, and an appreciation of ALS as the breakdown of a wider network rather than a discrete vulnerable population of specialised motor neurons. Identification of C9orf72 repeat expansions in patients without a family history of ALS challenges the traditional division between familial and sporadic disease. By contrast, the 90% of apparently sporadic cases and incomplete penetrance of several genes linked to familial cases suggest that at least some forms of ALS arise from the interplay of multiple genes, poorly understood developmental, environmental, and age-related factors, as well as stochastic events

Absence of system xc⁻ on immune cells invading the central nervous system alleviates experimental autoimmune encephalitis

Background: Multiple sclerosis (MS) is an autoimmune demyelinating disease that affects the central nervous system (CNS), leading to neurodegeneration and chronic disability. Accumulating evidence points to a key role for neuroinflammation, oxidative stress, and excitotoxicity in this degenerative process. System x(c)- or the cystine/glutamate antiporter could tie these pathological mechanisms together: its activity is enhanced by reactive oxygen species and inflammatory stimuli, and its enhancement might lead to the release of toxic amounts of glutamate, thereby triggering excitotoxicity and neurodegeneration. Methods: Semi-quantitative Western blotting served to study protein expression of xCT, the specific subunit of system x(c)-, as well as of regulators of xCT transcription, in the normal appearing white matter (NAWM) of MS patients and in the CNS and spleen of mice exposed to experimental autoimmune encephalomyelitis (EAE), an accepted mouse model of MS. We next compared the clinical course of the EAE disease, the extent of demyelination, the infiltration of immune cells and microglial activation in xCT-knockout (xCT(-/-)) mice and irradiated mice reconstituted in xCT(-/-) bone marrow (BM), to their proper wild type (xCT(+/+)) controls. Results: xCT protein expression levels were upregulated in the NAWM of MS patients and in the brain, spinal cord, and spleen of EAE mice. The pathways involved in this upregulation in NAWM of MS patients remain unresolved. Compared to xCT(+/+) mice, xCT(-/-) mice were equally susceptible to EAE, whereas mice transplanted with xCT(-/-) BM, and as such only exhibiting loss of xCT in their immune cells, were less susceptible to EAE. In none of the above-described conditions, demyelination, microglial activation, or infiltration of immune cells were affected. Conclusions: Our findings demonstrate enhancement of xCT protein expression in MS pathology and suggest that system x(c)- on immune cells invading the CNS participates to EAE. Since a total loss of system x(c)- had no net beneficial effects, these results have important implications for targeting system x(c)- for treatment of MS