In 1993, amyotrophic lateral sclerosis (ALS) was linked to a genetic mutation in the SOD1 gene. It was a landmark discovery, as were others like it at the time linking neurological diseases to heritable, genetic sources. For example, APO-E, the gene linked to Alzheimer’s disease, was discovered in the same year and the gene for Parkinson’s disease just a few years later.

Now, 25 years later, a new wave of causal relationships to devastating neurological diseases has arrived. Because, for each of these neurological disorders, the genetic component accounts for a small number of cases, making it only part of the story. The rest of the story, it seems, is being told by the microbiome. And, a new study published today in Nature adds ALS to the list, with the first report that gut dysbiosis plays an important role in ALS manifestations in transgenic mice, providing the first evidence that the microbiome could potentially modulate the disease course.

The work, led by Israeli labs from the Weizmann Institute and the Hadassah-Hebrew University Medical Center, is published today in a paper titled “Potential roles of gut microbiome and metabolites in modulating ALS in mice.”

The team used, according to Eran Elinav, MD, PhD, professor at the Weizmann Institute of Science and co-senior author on the paper, the “most widely used ALS mouse model” that harbors the same human mutation in familial cases of ALS—SOD1.

Elinav tells GEN that they were “surprised” that depletion of the microbiome in the SOD1 mice by antibiotic administration led to a “much more severe” disease in the mice. This particular result was also surprising to Sam Sisodia, PhD, professor in the department of neurobiology at the University of Chicago, noting that this is “quite the opposite of what is observed in mouse models of Aβ amyloidosis.”

To validate these findings, the group moved the mice to a germ-free facility which, according to Elinav, was not an easy task. He explains that it took almost three years because the mice kept dying, an observation that in and of itself “told us something very interesting about the potential effect of the lack of a microbiome in these mice.”

They did, with persistence, generate a small colony of the SOD1 mice in the sterile facility which allowed them to characterize their gut microbes. They showed that the microbiome of the SOD1 mice differed from their wild type littermates. They also found that the microbiome was different months before clinical symptoms and signs of ALS in these mice. Although this was interesting, Elinav notes that this “was not proof” that the microbiome was causing changes in ALS progression.

They dug deeper, identifying roughly a dozen strains of bacteria that they suspected of modulating the disease in the mice, and in a series of “painstaking” experiments, according to Elinav, they gave the SOD1 mice each of the bacteria—one at a time. They found that one species of bacteria, in particular, was making the ALS worse in the mice. But, even more interestingly, one species of bacteria, Akkermansia muciniphila, led to the amelioration of motor phenotypes and pathology in these animals, in addition to improving their survival rate.

How does the Akkermansia alter the progression of ALS in the mouse? Like other gut-brain findings, the team suggest a link to the production of small molecules by the bacteria. They honed in on one—nicotinamide (NAM) which was able, when given alone, to enter the bloodstream of the mice from the gut and alter gene expression in the brain. The genes that were altered, as measured by RNA-seq, were involved in mediating oxidative damage and mitochondrial functionality. Because this was the first time that bacteria were shown to elevate NAD, the group did a control experiment using a strain of E.coli that could generate NAD (although the levels are not as high as Akkermansia) and compared it with a strain of E.coli that could not generate NAD. Only the strain that could make NAD had an effect on the mice.

The group wanted to move some of their conceptual findings from mice into humans, although Elinav notes that they only “scratched the surface.” Using ALS patients and their family members, namely household members who generally share much of their gut microbiome, they sequenced the microbiomes and found distinct differences in their bacterial composition. Further, metabolic profiling showed alterations in patients versus controls. Namely, the ALS patients had lower NAD in the blood and the CSF when compared to their controls.

The most exciting part of this work for Elinav is that “the microbiome could provide an important and prominent modulatory effect mediated by specific bacteria and specific small molecules” giving “a more complex view” of this disease as being a combination of genetic risks of susceptibility and the environmental signals which modify it.

Compelling, commonplace, and course changing

“This story was not entirely surprising,” notes Sam Sisodia, PhD, professor in the department of neurobiology at the University of Chicago. “I expected that something along these lines would be published soon given the interest in the gut-brain connection in disease models of Alzheimer’s disease and Parkinson’s disease.”

“There is a lot we don’t know,” notes Vicki Hertzberg, PhD, professor and director of the Center for Data Science at the Neil Hodgson Woodruff School of Nursing at Emory University, adding that “this is an excellent paper, yet it leaves so many questions unanswered.”

Sisodia would like to know what would happen if they colonized the SOD1 mice with human fecal samples. And, Hertzberg was curious about the patients, and if any of them have familial ALS and the differences that could have occurred between those with a SOD1 mutation versus none. The authors answered this point, noting that they did not assess SOD1 genotypes for the patients, so, expect the number to be low as that mutation accounts for only 10% of all cases.

Teepu Siddique, MD, professor of neurology and cell and developmental biology at Northwestern University, who was a member of the 1993 team that first discovered the SOD1 gene and went on to develop the SOD1 transgenic mouse used in this study, calls the work, “very intriguing.” He would have liked for the team to have experimented with more than one ALS mouse model but his interest is piqued by the patient data presented in the paper. He notes the less than ideal ratio of patient to control samples and notes that reproduction of these results will be critical in the long term adoption of the microbiome as a mediator of ALS progression.

Elinav is not worried about this work receiving the same type of criticism that similar papers have received in the past. He says that some experiments were repeated six or seven times. And, even though “people can say whatever they want,” the “state of the art of microbiome research is to try to elucidate causality and mechanistic insight instead of correlation.” He emphasizes that the data and the discovery have to be presented in a “fair and comprehensive manner.”

“We need to be very careful here, as this is a very preliminary study,” notes Elinav. This is “not aimed in any way or manner to suggest a treatment or intervention” and needs to be validated by larger studies.

That said, Marc Gotkine, MD, senior physician in the department of neurology at the Hadassah Medical Center and co-senior author on the paper, is excited about the translational aspect of this work. “For me,” notes Gotkine, “the most exciting aspect of this research is the potentially rapid translation to human therapeutic trials” noting that “manipulating the microbiome in people with ALS is a viable therapeutic option.”

Eran Segal, PhD, professor, department of computer science and applied math at the Weizmann Institute of Science and co-senior author on the paper, notes simply that this work “opens up a research avenue” of a “previously unexplored avenue of research in the understanding and treatment of ALS.” And, just like the genetic discoveries did 25 years ago, if these recent microbiome studies can be reproduced to show that the gut microbiome has a causal role in the mediating disease onset, progression, and severity of these devastating diseases, this field may change the course of these diseases as we know them.