Introduction

Amyotrophic lateral sclerosis (ALS), one of the most severe clinical conditions, is characterized by upper and lower motor neuron degeneration leading to severe disability, paralysis and death. After more than 150 years since Charcot described it, the disease is still orphan of therapy. The enormous advancements in scientific research over the past two decades have highlighted the complexity of ALS in terms of clinical phenotype, genetics and pathophysiological processes, markedly changing the concept of ALS as a single entity that affects a unique cell population, and strengthening the hypothesis that it is a multisystem, multicellular and multifactorial disease. This minisymposium aims to bring some clarity to the clinical and biological complexity of the disease with the perspective, eventually, to unravel the right pathway to reach maximal therapeutic benefit.

Clinical heterogeneity is a well‐recognized feature of the disease, as the variability in the clinical onset and the disease duration is large and, although most patients with ALS die within 3–5 years after symptom onset, some survive even more than two decades 16. In addition, recent breakthrough discoveries have demonstrated that clinical manifestations associated with ALS‐related genes are not circumscribed to the motor neuron damage but involve a spectrum of diseases with overlappimg phenotypes. These findings are reviewed by Sabatelli et al 18. After the discovery, in 1993, of the superoxide dismutase 1 (SOD1) gene mutation still linked to a pure motor neuron disorder, the majority of ALS‐related genes identified over the last 5 years have been associated to a broad spectrum of different clinical manifestations ranging from the most common fronto‐temporal dementia (FTD) to myopathies and mitochondrial disorders. This underlies the pleiotropy of the ALS related genes and confirm the view that ALS may be part of a multisystem neurological disease. In addition, the recent genetic breakthrough has provided new insights in the mechanisms underlying the motor neuron degeneration with the unquestionable evidence that alteration in RNA metabolism is a new key player in the ALS pathogenesis 11. This aspect is reviewed in the article by Rossi et al 17. Emphasis is given to the role of TDP‐43 and FUS, two members of a family of heterogenous ribonucleoproteins that by shuttling between the nucleus and the cytoplasm may control various crucial functions in RNA‐related pathways, including transcription, pre‐mRNA splicing, mRNA trafficking and local translation 4. The recent discovery of a ALS/FTD‐gene related mutation consisting of an expansion of G4C2 hexanucleotide repeats between the noncoding exons 1a and 1b of the C9orf72 gene, has reinforced the hypothesis of RNA dys‐metabolism in ALS pathogenesis 3. More recently, it has been hypothesized that the G4C2 repeat expansion toxicity is due to the alteration of proteins trafficking between the nucleus and cytoplasm 20 and to the formation of dipetide repeat proteins aggregates 6. Thus, protein aggregation appears a converging mechanism in the toxicity of SOD1, TDP‐43, FUS and C9orf72 mutations strengthening the old hypothesis of ALS as a proteinopathy 19. An emerging and interesting facet of the toxicity of protein aggregates in ALS is the possibility that these aggregates do propagate to neighboring cells via a prion‐like mechanism which might contribute to the spreading of the ALS symptoms from a focal onset and to the non‐cell autonomous nature of the disease 14. This process may involve transfer of extracellular aggregates released from dying neurons or mediated by exosomes.

Motor neurons do not die alone in ALS. Puentes et al 15 here review the key role of non‐neuronal cells in the progressive dysfunction and loss of motor neurons in ALS patients and animal models. In particular, they focus on the contribution of innate and adaptive immune responses and on blood‐CNS barriers in governing the balance between neuronal repair and neuronal damage in ALS. The innate immunity which involves mostly reactive microglia/macrophages and astrocytes resident in the CNS when activated by aggregated proteins or danger signals may provide the first line of defence for a protective action on degenerating neurons. This may function properly only when sustained by the action of the adaptive immunity mediated by CD4+ regulatory T cells (Treg) 1. Conversely, when both the immune responses switch to Th1/M1 immunophenotype the neurotoxic effect prevails, defining the progression and the final outcome of the disease. This dual role of immunity either pathogenic or neuroprotective may explain why immuno‐modulating treatments have been overall disappointing in ALS patients and only partially successful in animal models.

Genetic findings along with clinical and pathological observations indicate that ALS may be linked to different forms of muscular disorders. This suggests that the atrophy of the skeletal muscles in ALS is not only secondary to degeneration of motor neurons but may play an active role in the pathogenesis and progression of the disease. The review by Loeffler et al 5 describes the characteristics of muscle pathology in ALS patients and animal models and discusses the mechanisms through which it may contribute to the disease. In addition, it provides an overview of the therapeutic strategies proposed to alleviate muscle pathology. As there is growing evidence that treatments that protect spinal motor neurons are not sufficient to increase the survival in ALS mouse models 8, an approach targeting both the motor neuron and the skeletal muscle could be a winning strategy for a real effective therapy.

Simultaneously with the discovery of new ALS‐associated genes, there has been an impressive development of in vitro technologies that have enabled the modeling of this disease from ALS patients‐derived cells. Myszczynska and Ferraiuolo 9 present critical overview of the recent advances in the technology based on cell reprogramming protocols to induce pluripotent stem cells (iPSCs) from familial and sporadic ALS patients. In particular, they stress the great potential and promises of this model in ALS research, not neglecting the limitations that still exist in the use of these cells. Particular emphasis is given to the development of a novel protocol for fibroblasts reprogramming that manages to bypass the iPSC stage by converting fibroblast from skin biopsies directly into induced neural progenitor cells. Ideally, this technology could lead to the development of models for each ALS patient while still alive, allowing eventually to develop a personalized therapy.

The discovery of new ALS‐linked genes also gave a big boost in the development of new genetically modified animal models. Soon after the identification of TDP43, FUS and VCP mutations in familial ALS, different groups have generated different animal models, particularly rodents. Most of these models, showing variable phenotypes, have been widely described in recent reviews 8, 13, however, in large part these models do not recapitulate ALS‐like phenotype to the extent of SOD1 transgenic mouse model. The study of transgenic mouse models overexpressing mutant SOD1 over the past 20 years has been instrumental to our knowledge of the underlying pathophysiology of ALS 2. The discovery of phenotype variability in terms of disease onset and progression, due to their genetic background 7, 12 has provided insight in the mechanisms that could impact on the disease course and for the discovery of promising prognostic biomarkers of phenotypic variability in ALS patients. Nardo et al 10 here summarize the results obtained from the analysis of two SOD1G93A mouse strains exhibiting fast or slow disease progression and provide key elements to understand the causes of this variability. Unravelling these mechanisms allow to design new treatments that may successfully impact on the disease course and to possibly stratify patients for targeted clinical trials.