Modelling motor neuron disease in fruit flies: Lessons from spinal muscular atrophy

Functional characterisation of the ACE2 orthologues in Drosophila provides insights into the neuromuscular complications of COVID-19

SARS-CoV-2, the virus responsible for the coronavirus disease of 2019 (COVID-19), gains cellular entry via interaction with the angiotensin-converting enzyme 2 (ACE2) receptor of host cells. Although SARS-CoV-2 mainly targets the respiratory system, the neuromuscular system also appears to be affected in a large percentage of patients with acute or chronic COVID-19. The cause of the well-described neuromuscular manifestations resulting from SARS-CoV-2 infection remains unresolved. These may result from the neuromuscular-invasive capacity of the virus leading to direct injury. Alternatively, they may be the consequence of ACE2 inactivation either due to viral infection, ACE2 autoantibodies or both. Here, we made use of the Drosophila model to investigate whether ACE2 downregulation is sufficient to induce neuromuscular phenotypes. We show that moderate gene silencing of ACE2 orthologues Ance or Ance3 diminished survival on exposure to thermal stress only upon induction of neuromuscular fatigue driven by increased physical activity. A strong knockdown of Ance or Ance3 directed to muscle reduced or abolished adult viability and caused obvious motoric deficits including reduced locomotion and impaired flight capacity. Selective knockdown of Ance and Ance3 in neurons caused wing defects and an age-dependent decline in motor behaviour, respectively, in adult flies. Interestingly, RNA sequencing allowed us to discover several differentially spliced genes that are required for synaptic function downstream of Ance or Ance3 depletion. Our findings are therefore supportive of the notion that loss of a RAS-independent function for ACE2 contributes to the neuromuscular manifestations associated with SARS-CoV-2 infection.

Genetic landscape of ALS in Malta based on a quinquennial analysis

Genetic risk for amyotrophic lateral sclerosis (ALS) is highly elevated in genetic isolates, like the island population of Malta in the south of Europe, providing a unique opportunity to investigate the genetics of this disease. Here we characterize the clinical phenotype and genetic profile of the largest series of Maltese ALS patients to date identified throughout a 5-year window. Cases and controls underwent neuromuscular assessment and analysis of rare variants in ALS causative or risk genes following whole-genome sequencing. Potentially damaging variants or repeat expansions were identified in more than 45% of all patients. The most commonly affected genes were ALS2, DAO, SETX and SPG11, an infrequent cause of ALS in Europeans. We also confirmed a significant association between ATXN1 intermediate repeats and increased disease risk. Damaging variants in major ALS genes C9orf72, SOD1, TARDBP and FUS were however either absent or rare in Maltese ALS patients. Overall, our study underscores a population that is an outlier within Europe and one that represents a high percentage of genetically explained cases.

Rbfox1 is required for myofibril development and maintaining fiber type-specific isoform expression in Drosophila muscles

Protein isoform transitions confer muscle fibers with distinct properties and are regulated by differential transcription and alternative splicing. RNA-binding Fox protein 1 (Rbfox1) can affect both transcript levels and splicing, and is known to contribute to normal muscle development and physiology in vertebrates, although the detailed mechanisms remain obscure. In this study, we report that Rbfox1 contributes to the generation of adult muscle diversity in Drosophila Rbfox1 is differentially expressed among muscle fiber types, and RNAi knockdown causes a hypercontraction phenotype that leads to behavioral and eclosion defects. Misregulation of fiber type-specific gene and splice isoform expression, notably loss of an indirect flight muscle-specific isoform of Troponin-I that is critical for regulating myosin activity, leads to structural defects. We further show that Rbfox1 directly binds the 3'-UTR of target transcripts, regulates the expression level of myogenic transcription factors myocyte enhancer factor 2 and Salm, and both modulates expression of and genetically interacts with the CELF family RNA-binding protein Bruno1 (Bru1). Rbfox1 and Bru1 co-regulate fiber type-specific alternative splicing of structural genes, indicating that regulatory interactions between FOX and CELF family RNA-binding proteins are conserved in fly muscle. Rbfox1 thus affects muscle development by regulating fiber type-specific splicing and expression dynamics of identity genes and structural proteins.

Occupation and amyotrophic lateral sclerosis risk: a case-control study in the isolated island population of Malta

Objective: Amyotrophic lateral sclerosis (ALS) is a mostly sporadic neurodegenerative disease. The role of environmental factors has been extensively investigated but associations remain controversial. Considering that a substantial proportion of adult life is spent at work, identifying occupations and work-related exposures is considered an effective way to detect factors that increase ALS risk. This process may be further facilitated in population isolates due to environmental and genetic homogeneity. Our study investigated occupations and occupational exposures potentially associated with ALS risk in the isolated island population of Malta, using a case-control study design. Methods: Patients with ALS and randomly identified matched controls (1:1) were recruited throughout a four-year window, from 2017 through 2020. Data on educational level, residence, main occupation, smoking, and alcohol history were collected. Results: We found that compared to controls (44.4%), a higher percentage (73.7%) of ALS patients reported a blue-collar job as their main occupation (OR 2.04, 95% CI 1.2-3.72; p = 0.0072). Through regression analysis, craft and related trades occupations such as carpentry and construction (ISCO-08 major group 7), were found to be positively associated with ALS, with patients in this occupational category found to be more prone to develop bulbar-onset ALS (p = 0.0297). Overall, patients with ALS reported a significantly higher exposure to work-related strenuous physical activity (OR 2.35, 95% CI 1.53-3.59; p = 0.0002). Conclusion: Our findings suggest that manual workers particularly those working in the carpentry and construction industries have an increased ALS risk, possibly due to a history of intense or sustained physical activity.

Genetic analysis of ALS cases in the isolated island population of Malta

Genetic isolates are compelling tools for mapping genes of inherited disorders. The archipelago of Malta, a sovereign microstate in the south of Europe is home to a geographically and culturally isolated population. Here, we investigate the epidemiology and genetic profile of Maltese patients with amyotrophic lateral sclerosis (ALS), identified throughout a 2-year window. Cases were largely male (66.7%) with a predominant spinal onset of symptoms (70.8%). Disease onset occurred around mid-age (median age: 64 years, men; 59.5 years, female); 12.5% had familial ALS (fALS). Annual incidence rate was 2.48 (95% CI 1.59–3.68) per 100,000 person-years. Male-to-female incidence ratio was 1.93:1. Prevalence was 3.44 (95% CI 2.01–5.52) cases per 100,000 inhabitants on 31^st December 2018. Whole-genome sequencing allowed us to determine rare DNA variants that change the protein-coding sequence of ALS-associated genes. Interestingly, the Maltese ALS patient cohort was found to be negative for deleterious variants in C9orf72, SOD1, TARDBP or FUS genes, which are the most commonly mutated ALS genes globally. Nonetheless, ALS-associated repeat expansions were identified in ATXN2 and NIPA1. Variants predicted to be damaging were also detected in ALS2, DAO, DCTN1, ERBB4, SETX, SCFD1 and SPG11. A total of 40% of patients with sporadic ALS had a rare and deleterious variant or repeat expansion in an ALS-associated gene, whilst the genetic cause of two thirds of fALS cases could not be pinpointed to known ALS genes or risk loci. This warrants further studies to elucidate novel genes that cause ALS in this unique population isolate.

Drug Screening and Drug Repositioning as Promising Therapeutic Approaches for Spinal Muscular Atrophy Treatment

Spinal muscular atrophy (SMA) is the most common genetic disease affecting infants and young adults. Due to mutation/deletion of the survival motor neuron (SMN) gene, SMA is characterized by the SMN protein lack, resulting in motor neuron impairment, skeletal muscle atrophy and premature death. Even if the genetic causes of SMA are well known, many aspects of its pathogenesis remain unclear and only three drugs have been recently approved by the Food and Drug Administration (Nusinersen-Spinraza; Onasemnogene abeparvovec or AVXS-101-Zolgensma; Risdiplam-Evrysdi): although assuring remarkable results, the therapies show some important limits including high costs, still unknown long-term effects, side effects and disregarding of SMN-independent targets. Therefore, the research of new therapeutic strategies is still a hot topic in the SMA field and many efforts are spent in drug discovery. In this review, we describe two promising strategies to select effective molecules: drug screening (DS) and drug repositioning (DR). By using compounds libraries of chemical/natural compounds and/or Food and Drug Administration-approved substances, DS aims at identifying new potentially effective compounds, whereas DR at testing drugs originally designed for the treatment of other pathologies. The drastic reduction in risks, costs and time expenditure assured by these strategies make them particularly interesting, especially for those diseases for which the canonical drug discovery process would be long and expensive. Interestingly, among the identified molecules by DS/DR in the context of SMA, besides the modulators of SMN2 transcription, we highlighted a convergence of some targeted molecular cascades contributing to SMA pathology, including cell death related-pathways, mitochondria and cytoskeleton dynamics, neurotransmitter and hormone modulation.

SMN complex member Gemin3 self-interacts and has a functional relationship with ALS-linked proteins TDP-43, FUS and Sod1

The predominant motor neuron disease in infants and adults is spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), respectively. SMA is caused by insufficient levels of the Survival Motor Neuron (SMN) protein, which operates as part of the multiprotein SMN complex that includes the DEAD-box RNA helicase Gemin3/DDX20/DP103. C9orf72, SOD1, TDP-43 and FUS are ranked as the four major genes causing familial ALS. Accumulating evidence has revealed a surprising molecular overlap between SMA and ALS. Here, we ask the question of whether Drosophila can also be exploited to study shared pathogenic pathways. Focusing on motor behaviour, muscle mass and survival, we show that disruption of either TBPH/TDP-43 or Caz/FUS enhance defects associated with Gemin3 loss-of-function. Gemin3-associated neuromuscular junction overgrowth was however suppressed. Sod1 depletion had a modifying effect in late adulthood. We also show that Gemin3 self-interacts and Gem3^ΔN, a helicase domain deletion mutant, retains the ability to interact with its wild-type counterpart. Importantly, mutant:wild-type dimers are favoured more than wild-type:wild-type dimers. In addition to reinforcing the link between SMA and ALS, further exploration of mechanistic overlaps is now possible in a genetically tractable model organism. Notably, Gemin3 can be elevated to a candidate for modifying motor neuron degeneration.

Comprehensive Modeling of Spinal Muscular Atrophy in Drosophila melanogaster

Spinal muscular atrophy (SMA) is a neurodegenerative disorder that affects motor neurons, primarily in young children. SMA is caused by mutations in the Survival Motor Neuron 1 (SMN1) gene. SMN functions in the assembly of spliceosomal RNPs and is well conserved in many model systems including mouse, zebrafish, fruit fly, nematode, and fission yeast. Work in Drosophila has focused on the loss of SMN function during larval stages, primarily using null alleles or strong hypomorphs. A systematic analysis of SMA-related phenotypes in the context of moderate alleles that more closely mimic the genetics of SMA has not been performed in the fly, leading to debate over the validity and translational value of this model. We, therefore, examined 14 Drosophila lines expressing SMA patient-derived missense mutations in Smn, with a focus on neuromuscular phenotypes in the adult stage. Animals were evaluated on the basis of organismal viability and longevity, locomotor function, neuromuscular junction structure, and muscle health. In all cases, we observed phenotypes similar to those of SMA patients, including progressive loss of adult motor function. The severity of these defects is variable and forms a broad spectrum across the 14 lines examined, recapitulating the full range of phenotypic severity observed in human SMA. This includes late-onset models of SMA, which have been difficult to produce in other model systems. The results provide direct evidence that SMA-related locomotor decline can be reproduced in the fly and support the use of patient-derived SMN missense mutations as a comprehensive system for modeling SMA.

Genetic screen identifies a requirement for SMN in mRNA localisation within the Drosophila oocyte

Objective

Spinal muscular atrophy (SMA) results from insufficient levels of the survival motor neuron (SMN) protein. Drosophila is conducive to large-scale genetic-modifier screens which can reveal novel pathways underpinning the disease mechanism. We tested the ability of a large collection of genomic deletions to enhance SMN-dependent lethality. To test our design, we asked whether our study can identify loci containing genes identified in previous genetic screens. Our objective was to find a common link between genes flagged in independent screens, which would allow us to expose novel functions for SMN in vivo.

Results

Out of 128 chromosome deficiency lines, 12 (9.4%) were found to consistently depress adult viability when crossed to SMN loss-of-function heterozygotes. In their majority, the enhancing deletions harboured genes that were previously identified as genetic modifiers, hence, validating the design of the screen. Importantly, gene overlap allowed us to flag genes with a role in post-transcriptional regulation of mRNAs that are crucial for determining the axes of the oocyte and future embryo. We find that SMN is also required for the correct localisation of gurken and oskar mRNAs in oocytes. These findings extend the role of SMN in oogenesis by identifying a key requirement for mRNA trafficking.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3496-1) contains supplementary material, which is available to authorized users.