Dysbiosis and Neurodegeneration in ALS: Unraveling the Gut-Brain Axis

Abstract

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a neurodegenerative disorder marked by the progressive degeneration of motor neurons in the brain and spinal cord. Despite decades of research, ALS remains incurable, diagnostically elusive, and is accompanied by rapid clinical decline, morbidity, and mortality. Its pathophysiology involves a complex interplay of genetic mutations (SOD1, C9/f72), environmental triggers, oxidative stress, neuroinflammation, and the accumulation of misfolded proteins, such as TDP-43 and SOD1. These factors disrupt cellular homeostasis aggravates excitotoxicity and neuronal death. Existing treatments, such as riluzole (a glutamate release modulator) and edaravone (a free radical scavenger), offer limited benefits, modestly prolonging survival or slowing functional decline without halting progression. Investigational approaches include antisense oligonucleotides targeting mutant SOD1 or C9orf72 genes, stem cell-based motor neuron replacement, and biomarker discovery to enable earlier diagnosis and progression monitoring. ALS patients frequently exhibit gastrointestinal (GI) symptoms, including dysphagia, sialorrhea, constipation, delayed gastric emptying, and pancreatic/parotid deficiencies. These observations underscore a close association between GI dysfunction and ALS pathogenesis. Also, recent studies implicate the gut-brain-microbiota axis in disease evolution, with microbial metabolites influencing neuroimmune interactions, synaptic plasticity, myelination, and skeletal muscle function. These studies indicate that dysbiosis-an imbalance in gut microbiota-may have a crucial role in ALS progression by impairing intestinal barrier integrity, promoting endotoxemia, and driving systemic inflammation. Conversely, ALS progression itself worsens dysbiosis, creating a vicious cycle of neuroinflammation and neurodegeneration. Preclinical and clinical evidence suggests that interventions targeting gut microbiota-such as prebiotics, probiotics, antibiotics, or phage therapy-could alleviate symptoms and slow disease progression and specific probiotic strains have also shown promise in reducing oxidative stress and inflammation in animal models. These findings highlight the urgent need to elucidate the functional role of gut microbiota in ALS to unlock novel diagnostic and therapeutic avenues. This review synthesizes current knowledge on the pathophysiology of ALS, with a focus on the emerging role of the gut-brain-microbiota axis. It highlights how dysbiosis influences diverse disease markers and neurodegenerative mechanisms, offering insights into potential therapeutic strategies and identifying key research gaps and future directions.

Keywords: Amyotrophic lateral sclerosis; Microbiota; Neuroinflammation; Oxidative stress; SCFA.