Sequestration of TDP-43^216-414aggregates by cytoplasmic expression of the proSAAS chaperone

Abstract

Chaperone proteins perform vital functions in the maintenance of cellular proteostasis and play important roles during the development of neurodegenerative diseases involving protein aggregation. We have previously reported that a secreted neuronal chaperone known as proSAAS exhibits potent chaperone activity in vitro against protein aggregation and blocks the cytotoxic effects of amyloid and α-synuclein oligomers. Here we report that overexpression of proSAAS generates dense, membraneless 2 µm spheres which can increase by fusion up to 4 µM during expression within the cytoplasm. The presence of dense proSAAS spheres was confirmed using electron microscopy. ProSAAS spheres selectively sequestered GFP-TDP-43 216-414 within their cores, resulting in cellular redistribution and retardation of degradation. ProSAAS expression was protective against TDP-43 cytotoxicity in a yeast model system. Aggregate sequestration via proSAAS encapsulation may provide protection from cell-to-cell transmission of aggregates and explain the as-yet unclear mechanism underlying the cytoprotective chaperone action of proSAAS.

Secreted Chaperones in Neurodegeneration

“…These chaperones, like their client counterparts, contain IDRs of varying degrees that essentially enable the holdase and sequestrase activities, and allows them to form functional multimers that can bind to toxic protein species, preventing cellular damage. Support for this mechanism has been presented for clusterin, BRICHOS proteins, and cytoplasmic sHsps ( Rohne et al, 2016 ; Chen et al, 2017 ; Mogk et al, 2019 ) and recent work shows similar properties for GRN-C ( Bhopatkar et al, 2020 ) and for proSAAS ( Peinado et al, 2020 ). In addition, the finding of reduced plaque number in AD model mice lacking 7B2 expression ( Jarvela et al, 2018 ) or clusterin ( DeMattos et al, 2002 ; Wojtas et al, 2017 ) suggests possible roles for 7B2 and clusterin in sequestration events ( Figure 1 , right panel).…”

“…To date known proSAAS clients include Abeta ( Hoshino et al, 2014 ); α-synuclein ( Jarvela et al, 2016 ); and islet amyloid polypeptide ( Peinado et al, 2013 ). Exciting new results indicate that cytoplasmic expression of proSAAS results in the formation of phase-separated proSAAS spheres which are able to trap the aggregating protein TDP-43 214–414 within their cores ( Peinado et al, 2020 ). Further structure-function analysis should permit us to determine the self-associating domains of proSAAS as well as the residues lining the sphere interior, which clearly favor aggregate binding.…”

Release of frustration drives corneal amyloid disaggregation by brain chaperone

“…However, the protecting effect might not be sufficient to clear amyloid deposits in an adverse genetic or environmental background 9 . Conversely, the cerebrospinal fluid (CSF) has evolved to have a unique set of chaperones to deal with polymorphic amyloid deposits such as Aβ amyloids 21 . Since most amyloid deposits are found in the extracellular region where the amount of ATP is low, the majority of the amyloid chaperones in the CSF are ATP independent 22,23 .…”