Reallocation of ribosomes as a disease mechanism of Amyotrophic Lateral Sclerosis
The molecular mechanism underlying Amyotrophic Lateral Sclerosis (ALS) is still largely unclear. The RNA binding protein TDP-43 is a major contributor and hallmark of the disease. Here, we show that in multiple TDP-43 models of ALS the alterations of the physiological crosstalk between free RNA/granules and polysomes lead to the global reorganisation of translational resources. We exploited in cellulo and in vivo models of TDP-43 proteinopathy and a unique tag-free miniaturised polysomal profiling coupled with RNA-Seq and biochemical assays with subcellular (i.e. cell body and axon) and compartment (i.e. RNA granules and polysomes) resolution. Using these models, we identified: i) a widespread axonal- and polysome-specific downregulation of TDP-43 target mRNAs, ii) the rewiring of mRNAs in granule and polysome compartments, iii) decrease in ribosome kinetics, iv) a robust ribosome engagement with residual mRNAs and v) downstream alterations in protein synthesis of TDP-43 non-target mRNAs. Irrespective of the ALS model used, we observed a snowball effect in RNA metabolism with redistribution of translational resources. Based on our results we propose a systems-oriented mechanism of disease pathogenesis, leveraging on the conceptual framework of reallocation of cellular resources. Overall design: In this study, we performed RNA-Seq of total, polysomal and free/granule-associated RNA from mouse primary cortical neurons control and expressing tRFP, exogenous TDP43-WT or TDP43-A315T to investigate the repertoires of defects caused by TDP-43 proteinopathy at subcellular (cell body vs axon) and compartment (RNA granules and polysomes) levels.