A joint research group in Japan reproduced amyotrophic lateral sclerosis (ALS) symptoms in zebrafish by remotely controlling a disease-associated protein molecule with light. In ALS, motor neurons progressively degenerate and accumulate inclusions containing an aggregated form of the TDP-43 protein. Researchers at Tokyo Medical University and the National Institute of Genetics in Mishima devised a new TDP-43 variant by attaching human TDP-43 to a plant protein that forms protein aggregates when it absorbs blue light. This light-controlled optogenetic (op)TDP-43 functions normally as TDP-43 in the dark, but gradually forms aggregates when illuminated by blue light. The researchers focused on the motor neurons of zebrafish because they share several characteristics with human motor neurons. Whole cells can be visualized because of the transparent zebrafish body. The researchers expressed the opTDP-43 in the zebrafish motor neurons and found that key ALS pathologies appeared when the fish were illuminated by blue light. The whole cells of spinal motor neurons are visualized in an intact zebrafish larva (white/red). Skeletal muscles are shown in blue. Courtesy of Kazuhide Asakawa. Using the translucent zebrafish neuromuscular system, the researchers demonstrated that short-term light stimulation reversibly induced cytoplasmic opTDP-43 mislocalization, but not aggregation, in the spinal motor neuron, leading to an axon outgrowth defect associated with myofiber denervation. In contrast, opTDP-43 formed pathological aggregates in the cytoplasm after longer-term illumination and seeded nonoptogenetic TDP-43 aggregation. The scientists observed that the connection between the motor neurons and the muscles weakened even when light illumination ceased and before the opTDP-43 became aggregated. This result suggests that motor neurons are already damaged before TDP-43 develops into the large aggregates that are characteristic in the terminal phase of the disease. “We think that the small TDP-43 assemblies, which are called TDP-43 oligomers, might be more toxic to motor neurons than the larger aggregates,” professor Kazuhide Asakawa said. “We can now produce an ALS-like state in a temporary and spatially tuned manner by controlling light intensity and the position of illumination,” Asakawa said. “Our ultimate goal over the next few years is to identify chemicals that prevent optogenetic TDP-43 from forming oligomers and aggregates, and we hope such chemicals will be used for ALS treatment.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-020-14815-x). An optogenetic ALS zebrafish showing motor decline after blue light illumination (right). Courtesy of Kazuhide Asakawa.