Disentangling a new paradigm for mutual homeostasis of aggregation-prone proteins

The dosage of proteins must be regulated in cells, especially for those containing low complexity domains (LCD-proteins) due to their propensity for condensation and aberrant aggregation in neurodegeneration. We recently identified a new mechanism, ‘interstasis’, that achieves dosage-responsive co-regulation of LCD-proteins via RNA-protein condensates called nuclear speckles. Speckles can sequester excess LCD-proteins, along with the mRNAs that encode them, thus preventing protein production until the cell lowers their combined dosage. I will apply innovative biophysical tools exploiting microfluidics and atomic force microscopy applied to reconstituted speckles to first ask how they achieve selectivity for mRNAs that encode LCD-proteins. I will then address a new paradigm whereby this selective protein-RNA condensation contributes to mutual homeostasis of LCD-proteins, using cellular and reconstituted systems. I will modulate the threshold of interstasis in neuronal models of amyotrophic lateral sclerosis and measure proteome-wide solubility changes using mass spectrometry. To reveal the interactions underlying these changes, I will simulate interstasis and disease states in vitro and study the biophysical changes of speckles and specific LCD-proteins, by modulating stoichiometries, LCD multivalency, and post-translational modifications. This will provide a crucial understanding of the roles of RNA-protein condensates in counteracting aggregation of LCD-proteins in the early stages of neurodegeneration.