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.
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
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