Exosomes induce endolysosomal permeabilization as a gateway by which exosomal tau seeds escape into the cytosol

Abstract

The microtubule-associated protein tau has a critical role in Alzheimer’s disease and other tauopathies. A proposed pathomechanism in the progression of tauopathies is the trans-synaptic spreading of tau seeds, with a role for exosomes which are secretory nanovesicles generated by late endosomes. Our previous work demonstrated that brain-derived exosomes isolated from tau transgenic rTg4510 mice encapsulate tau seeds with the ability to induce tau aggregation in recipient cells. We had also shown that exosomes can hijack the endosomal pathway to spread through interconnected neurons. Here, we reveal how tau seeds contained within internalized exosomes exploit mechanisms of lysosomal degradation to escape the endosome and induce tau aggregation in the cytosol of HEK293T-derived ‘tau biosensor cells’. We found that the majority of the exosome-containing endosomes fused with lysosomes to form endolysosomes. Exosomes induced their permeabilization, irrespective of the presence of tau seeds, or whether the exosomal preparations originated from mouse brains or HEK293T cells. We also found that permeabilization is a conserved mechanism, operating in both non-neuronal tau biosensor cells and primary neurons. However, permeabilization of endolysosomes only occurred in a small fraction of cells, which supports the notion that permeabilization occurs by a thresholded mechanism. Interestingly, tau aggregation was only induced in cells that exhibited permeabilization, presenting this as an escape route of exosomal tau seeds into the cytosol. Overexpression of RAB7, which is required for the formation of endolysosomes, strongly increased tau aggregation. Conversely, inhibition of lysosomal function with alkalinizing agents, or by knocking-down RAB7, decreased tau aggregation. Together, we conclude that the enzymatic activities of lysosomes permeabilize exosomal and endosomal membranes, thereby facilitating access of exosomal tau seeds to cytosolic tau to induce its aggregation. Our data underscore the importance of endosomal membrane integrity in mechanisms of cellular invasion by misfolded proteins that are resistant to lysosomal degradation.

Exosomal and vesicle‐free tau seeds—propagation and convergence in endolysosomal permeabilization

“…Such a redundancy might explain why e.g. neuronal exosomes are internalized by HEK293T cells and vice versa [10,11,16]. Interestingly and although not completely understood, similar to vesicle-free tau seeds, HSPGs also regulate the internalization of exosomes [68,69] (Fig 3B).…”

“…Of note, tau can form proteopathic seeds that propagate transsynaptically and actively corrupt the proper folding of soluble tau in recipient neurons [7][8][9][10][11][12] (Fig 1B). Neuron-to-neuron propagation of tau seeds involves secretion by donor cells and the subsequent internalization by recipient cells, mostly via endocytosis [11,[13][14][15][16]. However, because seeded tau aggregation occurs in the cytosol, endocytosed tau seeds need to escape from the endosomal entrapment in order to access the cytosol and corrupt the proper folding of physiological endogenous tau.…”

“…However, because seeded tau aggregation occurs in the cytosol, endocytosed tau seeds need to escape from the endosomal entrapment in order to access the cytosol and corrupt the proper folding of physiological endogenous tau. This review focuses on the propagation of tau seeds and on different approaches that have been used to halt their spreading, focusing particularly on the emerging importance of endosomal membrane integrity in the cellular invasion of tau seeds [13,[16][17][18]. We discuss here that endolysosomes appear to be organelles with weak and inherently unstable limiting membranes Accepted Article requiring frequent repair, characteristics that invading proteopathic seeds and exosomes take advantage of to induce rupture, and thereby gain access to the cytosol [13,[16][17][18][19][20][21].…”

PIKfyve activity is required for lysosomal trafficking of tau aggregates and tau seeding

“…Based on our data we hypothesize that by reducing the transport of tau seeds into lysosomes via PIKfyve inhibition, tau seeding can be halted. In line with this idea it has been recently shown that exosomes containing Tau seeds require transport into the lysosome as well as lysosomal rupture to promote Tau aggregation ( 64 ). Whereas tau-seeds-induced rupture of lysosomes has not been shown yet, other protein oligomers such as α-synuclein fibrils are known to induce rupture of the lysosomal compartment and induce seeding ( 65 ).…”

FRET-based screening in HEK293T identifies p38 MAPK and PKC inhibition as therapeutic targets for α-synuclein aggregation

“…However, tau seeding in similar models as ours identifies acidified compartments as a necessary component for tau seeds to enter cells and induce pathology. Disruption of these acidified compartments protected against induced tau aggregation (Polanco et al, 2021).…”