Co-occurrence of MS and ALS: a clue in favor of common pathophysiological findings?

Neuroinflammation is involved in several neurodegenerative disorders and emerging evidence indicates that it constitutes a critical process that is required for the progression of neurodegeneration. Microglial activation constitutes a central event in neuroinflammation. Furthermore, microglia can not only be activated with an inflammatory and neurotoxic phenotype (M1-like phenotype), but they also can acquire a neurosupportive functional phenotype (M2-like phenotype) characterised by the production of anti-inflammatory mediators and neurotrophic factors. Importantly, during the past decade, several studies have shown that CD4+ T-cells infiltrate the central nervous system (CNS) in many neurodegenerative disorders, in which their participation has a critical influence on the outcome of microglial activation and consequent neurodegeneration. In this review, we focus on the analysis of the interplay of the different sub-populations of CD4+ T-cells infiltrating the CNS and how they participate in regulating the outcome of neuroinflammation and neurodegeneration in the context of Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis and multiple sclerosis. In this regard, encephalitogenic inflammatory CD4+ T-cells, such as Th1, Th17, GM-CSF-producer CD4+ T-cells and γδT-cells, strongly contribute to chronic neuroinflammation, thus perpetuating neurodegenerative processes. In contrast, encephalitogenic or meningeal Tregs and Th2 cells decrease inflammatory functions in microglial cells and promote a neurosupportive microenvironment. Moreover, whereas some neurodegenerative disorders such as multiple sclerosis, Parkinson’s disease and Alzheimer’s disease involve the participation of inflammatory CD4+ T-cells 'naturally', the physiopathology of other neurodegenerative diseases, such as amyotrophic lateral sclerosis, is associated with the participation of anti-inflammatory CD4+ T-cells that delay the neurodegenerative process. Thus, current evidence supports the hypothesis that the involvement of CD4+ T-cells against CNS antigens constitutes a key component in regulating the progression of the neurodegenerative process.

Crossover in the pathogenic mechanisms of amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) has been described but is poorly understood. A GGGGCC hexanucleotide repeat expansion of C9ORF72 has recently been identified in a significant proportion of patients with ALS. In approximately 650 patients diagnosed with ALS from the North of England we identified seven patients who initially presented with MS. DNA obtained from five patients with MS-ALS and 215 patients with MS alone was screened for the C9ORF72 expansion. Post-mortem material was examined from two patients with MS-ALS. Gene expression profiling was performed on lymphoblastoid cells and levels of CXCL10 were measured in cerebrospinal fluid (CSF) from patients with ALS with and without the C9ORF72 expansion and controls. Concurrence of MS and ALS is higher than expected in our population. The C9ORF72 expansion was identified in 80% of patients with MS-ALS but not in those with MS alone. In the presence of preceding MS, C9ORF72-ALS was more rapidly progressive. MetaCore analysis identified alteration of the NF-кB pathway in C9ORF72-ALS and non-C9ORF72-ALS. NF-кB activation is associated with increased expression of the neuroprotective cytokine CXCL10 but, in C9ORF72-ALS, CXCL10 is downregulated and CSF levels are reduced. We propose that MS-associated neuroinflammation may affect penetrance and progression of the C9ORF72 expansion. In particular, the NF-кB pathway is activated in MS and appears to be dysfunctional in C9ORF72-ALS. Aberrant downregulation of CXCL10 may explain the predisposition of C9ORF72 expansion carriers to develop ALS in the context of MS and NF-кB activation, and offers a potential therapeutic target.

There is a high incidence on Guam of a severe tauopathy known as the Parkinson- dementia complex (PDC). It is linked with an even more malignant amyotrophic lateral sclerosis (ALS) syndrome. There is great interest in determining the cause, or causes, of the Guam ALS/PDC syndrome because insight might be gained regarding ALS and the more common tauopathies found throughout the world. Research into the disorder is stimulated by hypotheses as to cause. Such hypotheses should be compatible with the known epidemiology and pathology of the syndrome. These include a high, if not exclusive, restriction to the Chamorro population, familial occurrence, a regional variation on Guam itself, a definite persistence but with declining incidence, and a possible duplication in isolated villages on the Kii peninsula of Japan. Proposed causation factors should also be able to reproduce the syndrome in experimental systems. This includes induction of neurofibrillary tangles with a tau isoform distribution similar to that of Alzheimer disease and association of the lesions with TDP-43 and Lrrk2. A recurring hypothesis as to causation is exposure to Cycas micronesica, the false Sago palm known locally as fadang. We review the reasons why this hypothesis falls short of the minimal criteria needed for further serious consideration and discuss some other possibilities that should not be excluded.