Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disorder marked by loss of motor neurons of the spinal cord and cortex. ALS is characterized by significant heterogeneity in disease onset and patient presentation. Over the last twenty years, mutations in over 35 different genes have been uncovered as causal in the development of familial forms of ALS (fALS); however, fALS only accounts for roughly 10% of all ALS cases. The remaining 90% of patients suffer from sporadic ALS (sALS), with no family history of disease and unknown causes of pathogenesis. Regardless of all this genetic and pathogenic complexity, remarkably nearly every single ALS patient (both fALS and sALS) share a common neuropathology in the form of aberrant cytoplasmic inclusions of a protein called TAR DNA-binding protein of 43 kDa (TDP-43) found in degenerating regions of the nervous system. Insight drawn from studies of fALS-linked mutations have implicated stress granule dysfunction in disease development, but the exact mechanisms by which the stress granules may directly regulate TDP-43 aggregation remains unclear. By utilizing a novel optogenetic approach to induce either TDP-43 inclusions or functional stress granules, we can manipulate these processes at a previously unattainable level of control. These experiments will explore the contribution of stress granule dysfunction to TDP-43 inclusion neurotoxicity. We hypothesize that perturbations in stress granule dynamics contribute to neurotoxic TDP-43 aggregation in ALS. We will first investigate the contribution of stress granules to light-induced TDP-43 inclusions. We will then determine if chronic or persistent stress granule formation initiates TDP-43 proteinopathy. Finally, we will perform a genome-wide RNAi screen to identify novel pathways that protect against TDP-43 inclusion toxicity and assess whether this protection is dependent on stress granule regulation.