OptoGranules reveal the evolution of stress granules to ALS-FTD pathology

Stress granules are non-membranous assemblies of mRNA and protein that form in response to a variety of stressors. Genetic, pathologic, biophysical and cell biological studies have implicated disturbances in the dynamics of membrane-less organelles, such as stress granules, as a pathobiological component of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)1-12. This confluence of evidence has inspired the hypothesis that these diseases reflect an underlying disturbance in the dynamics and material properties of stress granules; however, this concept has remained largely untestable in available models of stress granule assembly, which require the confounding variable of exogenous stressors. Here we demonstrate the development and use of a light-inducible stress granule system, termed OptoGranules, which permits discrete, experimental control of the dynamics and material properties of stress granules in living cells in the absence of exogenous stressors. The nucleator in this system is Opto-G3BP1, a light-sensitive chimeric protein assembled from the intrinsically disordered region (IDR) and RNA-binding domain of G3BP1 combined with the light-sensitive oligomerization domain of Arabidopsis thaliana cryptochrome 2 (CRY2) photolyase homology region (PHR). Upon stimulation with blue light, Opto-G3BP1 initiates the rapid assembly of dynamic, cytoplasmic, liquid granules that are composed of canonical stress granule components, including G3BP1, PABP, TIA1, TIAR, eIF4G, eIF3{eta}, ataxin 2, GLE1, TDP-43 and polyadenylated RNA. With this system, we demonstrate that persistent or repetitive assembly of stress granules is cytotoxic and is accompanied by the evolution of stress granules to neuronal cytoplasmic inclusions that recapitulate the pathology of ALS-FTD.