Sequestration of TDP-43^216-414 Aggregates by Cytoplasmic Expression of the proSAAS Chaperone

As neurons age, protein homeostasis becomes less efficient, resulting in misfolding and aggregation. Chaperone proteins perform vital functions in the maintenance of cellular proteostasis, and chaperone-based therapies that promote sequestration of toxic aggregates may prove useful in blocking the development of neurodegenerative disease. We previously demonstrated that proSAAS, a small secreted neuronal protein, exhibits potent chaperone activity against protein aggregation in vitro and blocks the cytotoxic effects of amyloid and synuclein oligomers in cell culture systems. We now examine whether cytoplasmic expression of proSAAS results in interactions with protein aggregates in this cellular compartment. We report that expression of proSAAS within the cytoplasm generates dense, membraneless 2 μm proSAAS spheres which progressively fuse to form larger spheres, suggesting liquid droplet-like properties. ProSAAS spheres selectively accumulate a C-terminally truncated fluorescently tagged form of TDP-43, initiating its cellular redistribution; these TDP-43-containing spheres also exhibit dynamic fusion. Efficient encapsulation of TDP-43 into proSAAS spheres is driven by its C-terminal prion-like domain; spheres must be formed for sequestration to occur. Three proSAAS sequences, a predicted coiled-coil, a conserved region (residues 158-169), and the positively charged sequence 181-185, are all required for proSAAS to form spheres able to encapsulate TDP-43 aggregates. Substitution of lysines for arginines in the 181-185 sequence results in nuclear translocation of proSAAS and encapsulation of nuclear-localized TDP-43^216-414. As a functional output, we demonstrate that proSAAS expression results in cytoprotection against full-length TDP-43 toxicity in yeast. We conclude that proSAAS can act as a functional holdase for TDP-43 via this phase-separation property, representing a cytoprotectant whose unusual biochemical properties can potentially be exploited in the design of therapeutic molecules.