The interneuron hypothesis of amyotrophic lateral sclerosis

This month’s issue of Brain features a pivotal article by Vucic and colleagues¹ outlining their neurophysiological findings in patients with amyotrophic lateral sclerosis (ALS). As I read this article for the first time, I was reminded of that wonderfully dark scene in Raiders of the Lost Ark where Indiana Jones and his friend, Sallah, realize the Germans have been excavating for the Ark in the wrong spot. When the realization strikes them, they shout in unison, ‘They’re digging in the wrong place!’—which perfectly captures my reaction when I first read this article. It crystallized around the idea that the ALS field has been overly focused on motor neurons for the past century, but we also need to consider other areas, particularly the interneurons.

How does the Vucic paper lead us to such a conclusion? To begin, let us summarize their key findings: the researchers merged transcranial magnetic stimulation with advanced EEG recordings of the primary motor cortex in 21 patients diagnosed with ALS and compared these results to those of healthy individuals.¹ This innovative methodology allows for the direct evaluation of cortical function in isolation, removing information from the brainstem and spinal cord. The resulting dataset provides numerous valuable insights into this complex neurodegenerative disease. For instance, the authors suggest that these electrical brain wave patterns may serve as clinically relevant biomarkers for disease progression and even for potential therapeutic effects in ALS patients. More fundamentally, their research shows that ALS patients consistently exhibit patterns of cortical hyperexcitability due to impaired functioning of inhibitory cortical interneurons.

Like all clinical studies, this one has limitations and opportunities for improvement. Future research is needed to replicate these findings at different centres, refine the specific neurophysiological settings suitable for clinical application, and examine changes in cortical hyperexcitability among patients over time. It would be interesting to compare the findings in ALS patients with those of other neurodegenerative diseases, such as frontotemporal dementia. Additionally, it would be valuable to determine whether riluzole or edaravone influenced the recordings after a dose or over an extended period. Nevertheless, the key point that ALS patients exhibit cortical hyperexcitability due to interneuron dysfunction is compelling and aligns with other observations in ALS patients, as we will now discuss.

Growing evidence indicates that interneurons play a vital role in the development of ALS. In 2004, Maekawa and her colleagues² performed autopsies to determine which brain cells were most impacted in ALS patients. Beyond the anticipated decrease in pyramidal motor neurons, a striking discovery was that the damage to interneurons—particularly parvalbumin and calretinin interneurons in the primary motor cortex—was more significant than previously recognized in ALS.

Even with its captivating story, the Maekawa paper was largely overlooked for two decades until a talented research fellow, Sara Saez-Atienzar, rediscovered it during her time in my laboratory. She analysed a comprehensive genomic dataset that included 20 000 patients with ALS and 60 000 control subjects. Although her initial goal was to uncover pathways associated with the disease, she shifted her focus to specifically identify the cell types linked to this deadly neurodegenerative disorder. Consequently, she identified GABAergic cortical interneurons as a significant contributor to ALS pathogenesis.³

In summary, we now have three distinct types of evidence showing that interneurons play a crucial role in the pathogenesis of ALS: neuropathological findings from autopsies,² extensive genomic data,³ and, most recently from the Vucic group, neurophysiological data from living patients.¹ Other work from mouse models of ALS also supports the role of interneurons,⁴ and PET imaging studies in patients have pointed towards widespread loss of GABA receptors.^5,6

What is the role of interneurons? Essentially, they function as suppressors of electrical activity by limiting the firing rates of excitatory neurons. Consequently, it’s not unexpected that interneurons are strategically positioned in the areas of the CNS that contain excitatory neurons, such as the frontal and motor cortices. One straightforward theory suggests that when inhibitory neurons are turned off, excitatory neurons can fire more frequently, leading to a state of hyperexcitability that may result in excitotoxicity.

Conversely, a more complex perspective proposes that the inherent hyperexcitability of excitatory neurons in ALS patients strains interneurons, ultimately leading to their demise. In this view, the primary defect originates within the excitatory motor neurons, and the death of the interneurons is a secondary effect, though one that may contribute to a harmful positive feedback loop. However, our own data is based on genetic variants indicating that the death of interneurons is embedded in their DNA rather than merely a bystander effect.³

Recognizing interneurons as vital contributors in ALS carries significant implications for developing therapies. Drugs enhancing GABAergic interneuronal function could be beneficial. Already, we see the emergence of GABAergic medications such as acamprosate, but also benzodiazepines, gabapentin and other antiepileptic drugs, in our drug repurposing initiatives.⁷ In conclusion, Vucic and colleagues¹ present further evidence, based in living patients, that interneurons are critical players in ALS pathogenesis. This insight almost compels me to dance around the room like Sallah, singing that old Gilbert and Sullivan favourite, ‘I am the monarch of the sea, I am the ruler of the Queen’s Navee’.

Funding

This work was supported by the Intramural Research Program of the National Institutes of Health, the National Institute on Aging (1ZIAAG000933). B.J.T. receives research support from Cerevel Therapeutics.

Competing interests

B.J.T. has a patent pending (U.S. Patent Application No. 63/717,807) on the diagnostic testing for ALS based on the proteomic panel. B.J.T. holds patents on the clinical testing and therapeutic intervention for the hexanucleotide repeat expansion of C9orf72. B.J.T is an Associate Editor of Brain. The author reports no competing interests related to this work.