ALS Model Worms Sicken Via Innate Immune Pathway

In the motor neurons of the humble nematode, researchers have found a link between motor neuron degeneration and an ALS gene. Scientists from the Université de Montréal in Canada discovered that the homolog of UNC13A was required for ALS model worms to enact an inappropriate innate immune response that leads to neurodegeneration. Moreover, the kinase cascade leading to the immune response kicked off with the worm version of SARM1. SARM1 was fingered as a possible ALS gene in a meta-analysis of genome-wide association data, though it is better known for its role in axon degeneration (Fogh et al., 2014; Jun 2012 news; Jan 2015 news; Apr 2015 news story). Any of the kinases downstream of SARM1 might make an easy target for medications, suggested senior author Alex Parker.

Pathway to Neurodegeneration.

In ALS model worms, motor neurons secrete an unknown factor (blue ovals) that activates innate immunity in the intestine and promotes neurodegeneration in a paracrine manner. [Courtesy of Vériepè et al., Nature Communications]

Several researchers have observed the importance of the innate immune system in mouse ALS models and in human tissues (see Oct 2008 news; Nov 2009 news; Mar 2010 news). First author Julie Vériepè decided to investigate innate immunity in the lab’s Caenorhabditis elegans models of ALS due to toxic TDP-43 or FUS (Vaccaro et al., 2012).

Expressing mutant human TDP-43 or FUS in the nematode’s motor neurons leads to neurodegeneration and paralysis, with the majority of worms unable to move by the time they reach 12 days old. To find out if those worms activated the innate immune response, Vériepè crossed them with a reporter strain toting green fluorescent protein attached to the antimicrobial peptide NLP-29. Though they suffered no infection, the worms fluoresced green. Next Vériepè crossed the TDP-43 and FUS strains with mutants for TIR-1, the worm homolog of SARM1, which activates production of antimicrobials including NLP-29. Their glow diminished, indicating the TIR-1 pathway normally activated the immune response. In addition, most of the worms were still wriggling after 12 days, and they were less likely to exhibit neurodegeneration.

The green fluorescence was a bit curious, given that the TDP-43 and FUS were in neurons, but NLP-29 is expressed in the worm’s intestine. “How does the nervous system communicate with the intestine to do this?” wondered Parker. He and Vériepè reasoned that the sick motor neurons must secrete some factor that signaled the intestine to turn on innate immunity. To prove this, they deleted two genes crucial for neurosecretion, unc-13 and unc-31, in their TDP-43 model with the NLP-29 reporter. Worms missing one or the other secretory factor glowed only dimly, indicating neurosecretion was necessary to activate the innate immune response. Their neurons were also less likely to degenerate. While about half of TDP-43 worms exhibit degenerating axons, only 20 to 30 percent of the secretion mutants did. “Neurons are transmitting some information to the immune system,” commented Janice Robertson of the University of Toronto. “This is the first time that has been demonstrated so clearly,” she said. Robertson was not involved in the paper.

Was it immune activity in the intestine, then, that was attacking the motor neurons? Not so, Vériepè found. She used RNA interference to knock out TIR-1 in worm strains that were sensitive to RNAi in only in their neurons, or in only the intestine. While eliminating TIR-1 in the intestine did not offset paralysis in her model, eliminating it from neurons did. Thus, the authors concluded that the factor secreted from neurons must act in a paracrine fashion (see image above).

Next, Vériepè investigated the kinases downstream from TIR-1. Crossing the TDP-43 worms with null mutants for NSY-1, SEK-1, PMK-1, or the transcription factor ATF-7, resulted in progeny that were more motile, and less prone to neurodegeneration, than the parent strain. A p38 inhibitor, SB203580, had the same effect, diminishing neurodegeneration, paralysis, and fluorescence of the NLP-29 reporter. “Inappropriate activation of the innate immune response in motor neurons leads to neurodegeneration,” Parker concluded.

“The results look clean,” commented Chris Link of the University of Colorado in Boulder. However, he wondered how mutant TDP-43 and FUS initiate an immune cascade. The neurons might release the secreted factor in response to damage done by the mutant proteins, Robertson said. Alternatively, Parker and Vériepè suggested that the neuron might interpret aggregated TDP-43 or FUS as an infection. Link offered a similar hypothesis, suggesting that the mutant proteins might alter RNA processing, leading to double-stranded RNAs that mimic viruses.

The next challenge will be to investigate this pathway in people and other mammals, commented Isaac Chiu of Harvard Medical School. He noted that nematodes lack the “professional” immune cells mammals have, like microglia. However, neurons mount their own innate immune response in humans as well as worms, and several genes point to an immune angle to ALS. This study was the first to describe an UNC13A function relevant to motor neuron death, via innate immunity, noted Michael van Es of the University Medical Center Utrecht in the Netherlands, who first linked UNC13A and ALS (see Sep 2009 news). Recently, scientists have identified mutations in the immunomodulators TBK1 and TREM2 as an ALS gene and risk factor, respectively. Moreover, inhibiting p38 protected mice from neurodegeneration induced by another ALS gene, SOD1 (Dewil et al., 2007).

Some aspects of the TIR-1 pathway are similar to the SARM1 axon degeneration cascade in people, Parker said. Therefore, researchers found it plausible that some aspects of the TIR-1/SARM1 pathway might be at work in human ALS, and Parker is already investigating small molecules that might inhibit the cascade.—Amber Dance

Comments

  1. I think this is a spectacular paper. The researchers are the first to provide any functional evidence on how UNC13a might be involved in motor neuron death. This is important considering UNC13a confers risk for developing ALS, but there two studies suggesting that the gene negatively influences survival in ALS in a profound way (6 to 12 months shorter lifespan).

    Understanding the mechanisms underpinning the role of UNC13a in ALS, therefore, seems a logical step for identifying potential therapeutic targets. Indeed, it seems Vérièpe and colleagues have not only found that the innate immune system is involved, but have unraveled a considerable part of the pathways involved. Undoubtedly, this paper will help identify further treatment strategies for ALS.  

    View all comments by Michael van Es

  2. The article by Vérièpe et al. provides elegant evidence supporting the involvement of innate immunity and cell-non-autonomous pathways in ALS. Expression of TDP-43 and FUS in C. elegans motor neurons led to motility defects and motor neuron degeneration by activating an innate immune response pathway. Although innate immune microglia and an adaptive immune system are lacking in C. elegans, several specific steps leading to neurodegeneration parallel pathways of disease activation in mammalian ALS. The most cogent finding is that UNC-13-mediated secretion of signals from motor neurons was required for induction of the immune response and degeneration of motor neurons.

    The UNC-13-mediated secretions activated the TIR-1 pathway and innate response genes in the distal intestinal and hypodermal cells, just as secreted molecules activate glial TLR-4 and NF-κB innate response genes in microglia in mammalian ALS. UNC-13-mediated secretions from motor neurons also led to neuronal TIR-1 activation initiating the neurodegenerative cascade. The molecular composition of the secreted factors as well as their specific receptors remains to be identified. Nevertheless, the key involvement of UNC-13 supports the importance of cell-non-autonomous pathways in TDP-43 and FUS-mediated neurodegeneration in C. elegans, and by analogy in human ALS, especially since the human orthologue, UNC13A, has been reported to modify disease progression and duration in ALS. Such studies also support the importance of the immune system in motor neuron degeneration.

    Identifying and neutralizing these secretions offers a potential therapeutic approach to preventing pro-inflammatory immune activation and the resulting neurodegenerative cascade in ALS.

    View all comments by Stanley H. Appel

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References

News Citations

  1. Staying Alive: Freed of a Single Gene, Severed Axons Defy Death
  2. MAPping Death Pathways in Axons
  3. An Axon Self-Destruct Button Triggers Energy Woes
  4. Microglia in ALS: Helpful, Harmful, or Neutral?
  5. Peripheral Innate Immunity—Not So Peripheral to ALS?
  6. ALS-TDI Scours Transcriptome, Targets CD40L
  7. Research Brief: Latest ALS GWAS Points to Loci on Chromosomes 9, 19

Paper Citations

  1. Fogh I, Ratti A, Gellera C, Lin K, Tiloca C, Moskvina V, Corrado L, Sorarù G, Cereda C, Corti S, Gentilini D, Calini D, Castellotti B, Mazzini L, Querin G, Gagliardi S, Del Bo R, Conforti FL, Siciliano G, Inghilleri M, Saccà F, Bongioanni P, Penco S, Corbo M, Sorbi S, Filosto M, Ferlini A, Di Blasio AM, Signorini S, Shatunov A, Jones A, Shaw PJ, Morrison KE, Farmer AE, Van Damme P, Robberecht W, Chiò A, Traynor BJ, Sendtner M, Melki J, Meininger V, Hardiman O, Andersen PM, Leigh NP, Glass JD, Overste D, Diekstra FP, Veldink JH, van Es MA, Shaw CE, Weale ME, Lewis CM, Williams J, Brown RH, Landers JE, Ticozzi N, Ceroni M, Pegoraro E, Comi GP, D'Alfonso S, van den Berg LH, Taroni F, Al-Chalabi A, Powell J, Silani V, SLAGEN Consortium and Collaborators. A genome-wide association meta-analysis identifies a novel locus at 17q11.2 associated with sporadic amyotrophic lateral sclerosis. Hum Mol Genet. 2014 Apr 15;23(8):2220-31. Epub 2013 Nov 20 PubMed.
  2. Vaccaro A, Tauffenberger A, Aggad D, Rouleau G, Drapeau P, Parker JA. Mutant TDP-43 and FUS cause age-dependent paralysis and neurodegeneration in C. elegans. PLoS One. 2012;7(2):e31321. PubMed.
  3. Dewil M, dela Cruz VF, Van Den Bosch L, Robberecht W. Inhibition of p38 mitogen activated protein kinase activation and mutant SOD1(G93A)-induced motor neuron death. Neurobiol Dis. 2007 May;26(2):332-41. PubMed.

External Citations

  1. UNC13A
  2. SARM1
  3. TDP-43
  4. FUS 
  5. TBK1 
  6. TREM2 
  7. SOD1 

Further Reading

Papers

  1. Ash PE, Zhang YJ, Roberts CM, Saldi T, Hutter H, Buratti E, Petrucelli L, Link CD. Neurotoxic effects of TDP-43 overexpression in C. elegans. Hum Mol Genet. 2010 Aug 15;19(16):3206-18. PubMed.
  2. Saldi TK, Ash PE, Wilson G, Gonzales P, Garrido-Lecca A, Roberts CM, Dostal V, Gendron TF, Stein LD, Blumenthal T, Petrucelli L, Link CD. TDP-1, the Caenorhabditis elegans ortholog of TDP-43, limits the accumulation of double-stranded RNA. EMBO J. 2014 Dec 17;33(24):2947-66. Epub 2014 Nov 12 PubMed.
  3. Zhan L, Xie Q, Tibbetts RS. Opposing roles of p38 and JNK in a Drosophila model of TDP-43 proteinopathy reveal oxidative stress and innate immunity as pathogenic components of neurodegeneration. Hum Mol Genet. 2015 Feb 1;24(3):757-72. Epub 2014 Oct 3 PubMed.

Primary Papers

  1. Vérièpe J, Fossouo L, Parker JA. Neurodegeneration in C. elegans models of ALS requires TIR-1/Sarm1 immune pathway activation in neurons. Nat Commun. 2015 Jun 10;6:7319. PubMed.

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