TAR DNA-binding protein 43 (TDP-43) is an evolutionarily conserved RNA/DNA-binding protein that is nuclear-enriched in healthy cells, but deposited in the cytoplasm as aggregates in affected neurons in certain neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). We have previously developed an optogenetic TDP-43 variant (opTDP-43h) whose oligomerization status can be modulated via the CRY2olig tag, which self-assembles upon absorption of blue light. Illumination of zebrafish spinal motor neurons expressing opTDP-43h with a blue light triggers its cytoplasmic mislocalization, eventually leading to cytoplasmic deposition of opTDP-43h aggregates. Intriguingly, a light illumination-dependent transient opTDP-43 mislocalization can halt motor axon outgrowth, even in the absence of cytoplasmic deposition of opTDP-43 aggregates. These observations point toward an oligomerization-dependent, but aggregation-independent, cytotoxic effect of TDP-43 that might contribute to pathogenesis of ALS. In the present review, we would like to overview the zebrafish ALS model based on the optogenetic TDP-43, and then discuss about the potential mechanisms of TDP-43 cytotoxicity that trigger and/or promote motor neuron degeneration in ALS.