Recent advances in the pathogenesis and therapeutics of amyotrophic lateral sclerosis.

Abstract

This article is part of a themed issue on Recent advances in ALS pathogenesis and therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.6/issuetoc Amyotrophic lateral sclerosis (ALS) is a progressive and highly disabling neurodegenerative disorder, with an incidence of 1–5 cases/100,000 subjects and characterized by muscle denervation, weakness, atrophy, and paralysis, frequently leading to patient death (by respiratory failure) in 3–5 years after diagnosis. The disease starts with a progressive loss of upper and lower motor neurons triggered by a complex aetiology (genes and environmental factors). Sporadic forms of the disease are the most abundant (up to 90% of cases), but they are clinically and histopathologically indistinguishable from the different familial forms described so far, which involve mutations in more than 25 ALS-related genes, among which the most relevant, for different reasons, are (i) SOD1 encoding the key antioxidant enzyme superoxide dismutase-1; (ii) TARDBP and FUS encoding the proteins TAR-DNA binding protein-43 (TDP-43) or fused in sarcoma (FUS), respectively, involved in pre-mRNA splicing, transport, and stability; and (iii) C9orf72 encoding a protein involved in intracellular trafficking in neurons and other cell functions not completely understood yet. Changes in SOD-1, TARDBP, FUS, and C9orf72 are present in most of the cases (approximately 70%) of familial ALS (Kim, Gautier, Tassoni-Tsuchida, Ma, & Gitler, 2020). SOD1 was the first ALS-related gene to be identified in 1993 (Rosen et al., 1993), and its discovery represented an important drive in the research in ALS, in part derived from the rapid development of transgenic animal models (most of them in mice) overexpressing the different human SOD1 mutations (the most abundant being G93A; Ripps, Huntley, Hof, Morrison, & Gordon, 1995), which served for years as the unique tool for the study of ALS pathogenesis and therapeutics. Such drive is evident looking at the data presented in Figure 1, which shows the progression in the number of articles published with studies conducted in ALS and collected in PubMed during the last 50 years. As seen in Figure 1, the subsequent discoveries of new ALS-linked genes (TARDBP, FUS, and C9orf72) further increased the growth of research into ALS, again by providing new transgenic models overexpressing the most frequent mutations found in these genes (Van Damme, Robberecht, & Van Den Bosch, 2017). As mentioned above, sporadic and familial cases of ALS are clinically indistinguishable, and the same is true for the pathogenic mechanisms underlying the death of the motor neurons. The progressive discovery of the different ALS-related genes, as well as the development of experimental models based on these genes, has contributed to a better understanding of these pathogenic mechanisms, as indicated in Figure 2. ALS is a disease with a clear unmet therapeutic need for effective drugs as all those presently available have modest effects, extending the lifespan only for a few months. These therapeutic agents include the anti-excitotoxic agent riluzole (Rilutek®), firstly approved in 1995 and for long time the only available medicine for the treatment of ALS patients. In 2015, the anti-oxidant agent edaravone (Radicava®) was approved in Japan and subsequently approved in the United States (2017) and Canada (2018). Also, in 2015, the anti-inflammatory tyrosine kinase inhibitor masitinib (Kinavet-CA1®) was designed as an orphan drug for the treatment of ALS. These are the only available disease-modifying medicines for patients. However, during years, many other potential therapeutic agents, with different mechanisms of action (anti-excitotoxic, anti-oxidant, anti-inflammatory, mitochondria-targeted, neurotrophic factors and other agents) have been clinically tested. Unfortunately, although these agents exhibited a good profile of safety and efficacy in animal models, they were unable to reproduce these benefits in the clinical trials (see Figure 3 as a summary of some of the most relevant therapeutic agents evaluated at the clinical level in the last years). A wide range of factors including the predictability of the experimental models, the late stage at which the clinical trials were initiated, the heterogeneity of pathogenic mechanisms occurring in ALS or the need for multi-target strategies, have been considered as the possible reasons for these failures. The identification during the last 15 years of new ALS-related genes has provided key information on the molecular and cellular mechanisms initiating the pathology or contributing to the disease progression. These discoveries have contributed to the identification of new targets for the development of neuroprotective therapies. In addition, these new genes have related ALS to other neurodegenerative pathologies such as frontotemporal dementia (FTD), so that they are currently considered as two sides of the same pathology. The aim of this Themed Issue is to put together all these recent advances on ALS and related pathologies in order to provide the scientific community with an updated view on the aetiology of ALS, the pathogenic mechanisms involved in the degeneration of motor neurons, and the current development of novel and efficacious therapies. In this Themed Issue, which stemmed from the “International Workshop on ALS: new genes, new treatments, new hopes” organized in Madrid on 30–31 October 2019, with the support of the BJP, readers will find first a review from Javier Riancho, Adolfo López de Munaín, and coworkers dealing with a new conceptualization of the aetiology of ALS. These authors described ALS as a complex disease caused by individual genetic risk, ageing and the influence of environmental factors, which interact to determine the threshold for the onset, progression and prognosis of the disease. They discuss the relevance, for the disease, of those environmental conditions that could be modified and review the evidence for cancer, autoimmunity and metabolic disorders, as important conditions leading to ALS (Riancho et al., 2020). In a second review, Riancho and coworkers addressed ALS from the perspective of a disease beyond the motor system, with emphasis on the sensory system. They combined information coming from patients and from animal models and highlighted the impairment of sensory-motor networks as an important event in this disease that has been frequently ignored or undervalued (Riancho, Paz-Fajardo, & López de Munaín, 2020). In the following review article, Rosario Osta and coworkers collected the experimental evidence generated in their laboratory and in others that support an important contribution of impairments in peripheral tissues, focusing on skeletal muscle. They assess the role played in the disease by impairment in muscle mitochondrial function, energy metabolism, proteostasis and RNA metabolism, as well as a contribution from defects in myogenesis. Most of the information collected by these authors in the review was obtained in the mutant SOD-1 mice, the classic model of ALS based on the first gene identified in relation with ALS. They also addressed the efforts put into the development of novel therapeutic strategies targeting the skeletal muscle to slow down the onset and progression of this disease (Manzano et al., 2020). The ribonucleoprotein TDP-43 discovered in ALS in 2006 is very frequently found in this pathology (and also in FTD), in the form of aggregates in the cytosol, and not only in familial cases derived from mutations in the TARDBP gene but also in many sporadic cases (up to 95% of ALS cases). This is the consequence of a variety of posttranslational modifications (e.g., ubiquitination, phosphorylation, acetylation, sumoylation, and cleavage) occurring in TDP-43, which lead to its conformational alteration, dysregulation, altered cellular location, impaired function, deposition, and aggregation. These phenomena have been extensively reviewed by Emanuele Buratti in his contribution in this Themed Issue in which he emphasized the therapeutic potential of different low MW compounds, able to alter the pathological characteristics of TDP-43, including expression levels, cytoplasmic mislocalization, posttranslational modifications, cleavage, stress granule recruitment, and others (Buratti, 2020). In the same direction as Buratti's review, Ana Martinez and coworkers concentrated their review on the inhibition of several kinases involved in the phosphorylation of TDP-43 and also other important ALS-related proteins, as a potential therapy for ALS. The emphasis was on c-KIT (a tyrosine kinase inhibited by masitinib, currently with an orphan designation for ALS treatment), ROCK (e.g., fasudil), mTOR (e.g., rapamycin), GSK-3 (e.g., lithium), and other inhibitors already in the clinical scenario. The review deals also with many others that have shown efficacy with reasonable safety in preclinical models (e.g., inhibitors for CK-1, CDC7, and MAP3K) and even in early phases of drug discovery, such as inhibitors for the Tau tubulin kinases (TTBKs) and the cyclin-dependent kinases (CDKs) (Palomo, Nozal, Rojas-Prats, Gil, & Martinez, 2020). Sigma-1 receptors are encoded by SIGMAR1 gene, which has been also linked to the development of ALS. Among their different cell substrates in the CNS, sigma-1 receptors are typically found in motor neurons and located in intracellular complexes formed by the endoplasmic reticulum and the mitochondria. They act as chaperone proteins and modulate essential processes for motor neuron survival, so that targeting these receptors may be a useful way to reduce the death of these neurons. This is extensively addressed in the review by Xavier Navarro and coworkers, which seeks to validate the neuroprotective properties of different ligands for these receptors (Herrando-Grabulosa, Gaja-Capdevila, Vela, & Navarro, 2020). ALS is also linked to epigenetic hallmarks so that several lines of research have recently explored the therapeutic potential of this relation. Ludo Van den Bosch and coworkers reviewed in this Themed Issue, the potential of interfering with histone deacetylases, one class of epigenetic enzymes. They explored the potential of known inhibitors such as valproate, lithium, and resveratrol, along with some novel synthetic inhibitors, for the different histone deacetylase subtypes, including the sirtuins, in preclinical models and a few clinical studies, paying particular attention to their ability to cross the blood–brain barrier and their tolerability and safety (Klingl, Pakravan, & Van Den Bosch, 2020). We do hope that these contributions may recruit more resources and efforts that are necessary to continue the identification of novel therapeutic targets and new therapeutic agents to foster the continuous search for drugs clinically effective in slowing ALS progression. Patients urgently need these efforts.