• Different types of proteopathic seeds are secreted in association with extracellular vesicles • Receptor-ligand interactions are important drivers of direct cell-to-cell and extracellular vesicle-mediated spreading of protein misfolding
• Viral glycoproteins mediating attachment and membrane fusion strongly enhance aggregate inducing capacity in recipient cells
Aberrant folding and aggregation of host-encoded proteins into ordered protein assemblies is a pathological hallmark of neurodegenerative diseases such as prion diseases, Alzheimer's disease (AD) and Parkinson's disease (PD). Disease-associated protein deposition usually starts locally, subsequently spreading stereotypically to other brain regions 1 . AD, the most prevalent neurodegenerative disease, is associated with the extracellular accumulation of Aβ amyloid and intracellular inclusion of misfolded microtubule-binding protein Tau as in neurofibrillary tangles 2, 3 . The formation of Tau aggregates is also a hallmark of other neurodegenerative diseases, collectively known as tauopathies 4 . In PD, normally soluble α-synuclein is deposited into aberrant neuronal inclusions 5 . Transmissible spongiform encephalopathies or prion diseases constitute a special group of neurodegenerative diseases affecting humans and other mammals 6 .
Prion diseases can occur sporadically, be caused by mutations in the prion protein gene PRNP or can be acquired by infection or iatrogenic transmission 7 .
Pathogenic protein aggregates form in a time-limiting nucleation dependent process in which soluble aggregation-prone proteins form oligomers that subsequently grow into highly ordered, beta-sheet rich fibrils 8 . In vitro assays demonstrate that the lag phase required for seed formation drastically shortens in the presence of pre-formed polymers that act as seeds 9 . Interestingly, such proteopathic seeds not only recruit monomeric protein in the affected cell, but also in unaffected cells upon intercellular transmission, a process that likely underlies the often observed stereotypical spreading of protein misfolding in neurodegenerative diseases 1 . In fact, intercellular transmission of proteopathic seeds is regarded as a common mechanism of neurodegenerative diseases 10 .
The precise mechanism of intercellular aggregate transfer and induction of new aggregates is unclear, but appears to involve release of free protein aggregates, direct cell-to-cell contact by cytonemes such as tunneling nanotubes (TNTs) [11] [12] [13] , or release of extracellular vesicles (EV) containing small proteopathic seeds 14 . The extent to which these three routes contribute to the spreading of protein misfolding remains unclear.
EV are nanosized communication vesicles secreted by all cells under physiological and pathological conditions 15 . EV differ in their cellular origin, with some budding directly from the cell surface (so-called microvesicles), and others being secreted when multivesicular bodies of endosomal origin fuse with the plasma membrane and release exosomes into the extracellular space. Secreted vesicles are now generally referred to as EV due to substantial overlap of microvesicles and exosomes in terms of size, surface markers and function 16 . Soluble and aggregated isoforms of a diverse number of neurodegenerative diseases have been found secreted by neurons and other cells either as free proteins or in association with EV [17] [18] [19] [20] [21] . While EV containing proteopathic seeds have been shown to seed protein aggregation in vitro and in vivo, the efficiency of protein aggregate transfer and subsequent seeding through this route is unclear. Only a small fraction of soluble or aggregated proteins are released associated with EV, while the vast majority is freely secreted. For example, less than 1% of total secreted Aβ associated with EV 17 , and only 3% of total secreted α-synuclein was found in the EV fraction 22 . Rat cortical neurons have been shown to secrete Tau, but again very little (3%) was associated with EV 19 . Importantly, EV fractions purified from conditioned medium of N2a cells overexpressing an aggregated Tau mutant induced Tau aggregation in less than 0,1% of recipient cells, arguing that EV-mediated aggregate induction can be rather inefficient 19 .
EV isolated from different donor cells exhibit a marked cell tropism 23, 24 .
Independent of their uptake mechanisms, EV must merge with cellular membranes to release their cargo into the cytosol. EV docking onto target cells and subsequent uptake are selective processes that require specific membrane interactions 24 . Consequently, receptor-ligand interactions between EV and recipient cells will likely modulate the spreading behavior of proteopathic seed cargo. While integrins and proteoglycans have been identified that adhere EV to target cells, most receptor-ligand pairs that underlie these targeted interactions are so far unknown 24, 25 .
We reasoned that the poor aggregate inducing activity of some seed-containing EV could be due to the lack of specific ligands required for host receptor interactions and/or subsequent fusion processes. Membrane contact and fusion can be facilitated by viral glycoproteins that allow viruses to adhere to and penetrate into their target cells. The contact with specific receptors on apposed membranes leads to conformational transitions in the viral glycoproteins, thereby bringing the two membranes in close proximity and enforcing bilayer merger. Viral glycoproteins are routinely used to pseudotype genetically engineered viral vectors for efficient cargo delivery.
The vesicular stomatitis virus glycoprotein VSV-G is the sole surface protein of this virus belonging to the rhabdovirus genus that mediates the binding to the LDL receptors present on most cell types 26 . A conformational change in VSV-G subsequently results in fusion of host and viral lipid bilayers 27 . Ectopically expressed VSV-G is not only found on the cell surface but also decorates EV in the absence of other viral constituents 28 . It has been recently demonstrated that VSV-G enhances EV-mediated cargo delivery to recipient cells 28, 29 . We here expressed VSV-G in three different cellular models that propagate different protein aggregates. Coculture of VSV-Gexpressing donor cells with recipient cells strongly increased protein aggregate induction in the latter. Further, expression of VSV-G also promoted the secretion of VSV-G-coated EV with drastically enhanced aggregate-inducing capacity in recipient cells. Intriguingly, interactions between SARS-CoV-2 spike S protein and its receptor ACE2 similarly contributed to the spreading of cytosolic prions and Tau aggregates. Thus, efficient intercellular proteopathic seed transfer is strongly controlled by receptor-ligand interactions. Further, our data raise the intriguing possibility that viral glycoproteins, expressed during acute or chronic infection, could facilitate the spreading of protein misfolding in vivo.
To study the intercellular dissemination and propagation of proteinaceous seeds, we have implemented cellular models that rely on the ectopic expression of a yeast prion domain in the cytosol of mammalian cells. The Saccharomyces cerevisiae Sup35 translation termination factor is the prototype prion protein of lower eukaryotes. Its classification as a prion protein is based on the fact that it can exist in a functional soluble isoform and as an inactive, cross-beta sheet polymer that self-replicates by imprinting its conformation onto soluble protein of the same kind 30 . When expressed in the mammalian cytosol, the NM prion domain of Sup35 exists in a soluble state and is non-functional 31 . Exposure of cells to highly ordered protein fibrils composed of recombinant NM leads to the aggregation of the ectopically expressed NM, thereby inducing self-replicating NM prions that are heritable by progeny 31 . NM prions are also transmissible to naïve bystander cells by direct cell-to-cell contact 32 and EV 33 .
Interestingly, we found that donor cell populations drastically differ in their NM aggregate-inducing activity, an effect that also depended on the recipient cell line 33, 34 .
For example, coculture of HEK cells propagating HA-epitope tagged NM prions (NM-HA agg ) only poorly induced NM-GFP prions (NM-GFP agg ) in recipients (suppl. figure 1) .
Likewise, EV secreted by HEK NM-HA agg cells contained substantial amounts of aggregated NM 34 , yet were ineffective at inducing NM-GFP aggregation in recipient cells (suppl. figure 1) . We hypothesized that one reason for the poor NM aggregate induction in some recipient cell populations could be inefficient membrane contact and fusion of either EV with target cells or donor and recipient cells. Since vesicular stomatitis virus glycoprotein VSV-G binds to the broadly expressed LDL receptor and therefore increases cell-to-cell membrane contact 26 , we assessed whether the expression of VSV-G facilitates protein aggregate transmission. We first tested if ectopic VSV-G expression increased cell-to-cell spreading of NM prions from donor to recipient N2a or HEK cells (Fig. 1a) . As donors, we chose HEK NM-HA agg cells 34 (Fig. 1g, h) .
VSV-G is a type III fusogen present in an inactive pre-fusion state at neutral pH that becomes activated by low pH in the endolysosomal system 35 . The occasional syncytia formation argued that at least some of the aggregate induction events could be due to extracellular VSV-G activation and subsequent cell fusion rather than contact and fusion at local cell contacts, such as cytonemes. We thus tested the effect of VSV-G expression on cell-free NM-GFP aggregate induction. EV were isolated from N2a or HEK donor cells transfected with empty vector or VSV-G coding vector. EV were subsequently added to recipient N2a or HEK cells NM-GFP sol cells (Fig. 2a) . EV fractions isolated from donor populations contained VSV-G, NM-HA and EV marker proteins Flotillin1, Hsp70/72, ALIX and GAPDH (Fig. 2b) . The presence of VSV-G on EV isolated from donor N2a or HEK NM-HA agg cells strongly increased intercellular aggregate induction when EV were added to HEK cells expressing NM-GFP sol (Fig. 2c) . Sonication of EV fractions prior to their addition to cells strongly reduced aggregate induction, demonstrating that intact EV were required for efficient prion spreading (Fig. 2c) .
Furthermore, expression of VSV-G proved non-toxic to donor cells in our experimental setup (Fig. 2d) . Thus, the increased aggregate induction efficiency was unlikely due to free naked NM-HA aggregates released by dying cells.
The effect of VSV-G expression on prion induction could either be due to enhanced production of EV, enhanced membrane docking and fusion of EV and recipient cells, or both. Consistent with previous findings 29 , we observed an approximately twofold increase in EV release upon transient expression of VSV-G in donor cell clones (Fig.   2e ). VSV-G pseudotyping did not change the size distribution of released EV (Fig. 2f ).
When adjusted for particle numbers, still an increase in intercellular aggregate induction was observed when EV were pseudotyped with VSV-G (Fig. 2g) . Pseudotyped EV secreted by donor cell lines also induced NM-GFP aggregation in primary human astrocytes ectopically expressing soluble NM-GFP, demonstrating that VSV-G mediated aggregate induction was not restricted to permanent cell cultures (Fig. 2h, i) . We conclude that VSV-G expressed by donor cells can strongly increase intercellular NM prion induction by mediating efficient contact and fusion of donor and recipient cell membranes as well as of EV and target cells.
Accumulating evidence suggests that seeding-competent Tau species can spread from cell-to-cell by EV 19 . To assess the effect of receptor-ligand interactions on intercellular Tau aggregate induction, we employed a previously published Tau cell model 36 . We engineered HEK cells to stably express the aggregation competent Tau repeat-domainspanning amino acid residues 244-372 (carrying the two-point mutations P301L/V337M and fused to GFP -hereafter termed Tau-GFP) and exposed them to homogenates extracted from affected brain regions from patients with Alzheimer's disease (AD), cortical basal degeneration (CBD), progressive supranuclear palsy (PSP) or frontotemporal lobar degeneration with Tau pathology (FTLD-Tau). All patient brain homogenates contained aggregated Tau, as revealed by pronase digestion (suppl. figure   3a ). Upon limiting dilution cloning, we established HEK cell clones Tau-GFP AD , Tau-GFP FTLD , Tau-GFP PSP and Tau-GFP CBD stably producing Tau aggregates (Fig. 3a) .
Tau-GFP in all cell clones exposed to patient brain homogenate, but not in control cells exposed to control brain homogenate (Fig. 3b, suppl. figure 3b ).
Generated donor cell clones propagating Tau aggregates were either transfected with empty vector or vector coding for VSV-G. Donor clones were subsequently cocultured with recipient cells expressing soluble Tau-FusionRed (Tau-FR sol ) (Fig. 3c) .
Coculture with Mock transfected donor cells resulted in poor induction of Tau-FR foci in recipient cells (Fig. 3d) . No Tau-FR foci were detected when donor cells expressed only soluble Tau-GFP (suppl. figure 3c, 
Tau-FR foci were observed when cells were cocultured with VSV-G-expressing HEK Tau-GFP AD donors (Fig. 3d) . We also observed a significant increase in recipient cells with Tau-FR aggregates when HEK Tau-GFP FTLD , HEK Tau-GFP PSP or HEK Tau-GFP CBD were used as donors (Fig. 3e) . VSV-G-expressing HEK Tau-GFP agg cells also efficiently induced Tau We next tested the effect of VSV-G expression on EV-mediated Tau aggregation.
EV fractions from HEK Tau-GFP AD and Tau-GFP CBD contained VSV-G, aggregated Tau-GFP and EV markers Flotillin1 and Hsp70/72 (Fig. 3f, g) . Addition of VSV-G pseudotyped EV to recipient HEK Tau-FR sol cells drastically increased Tau aggregate induction ( Fig. 3h-j) . Again, destruction of EV by sonication basically abolished aggregate induction, demonstrating that functional EV were required for transmission of seeding-competent Tau (Fig. 3j) . VSV-G expression also increased the number of particles released by donors, but did not change the size distribution of EV (Fig. 3k, l) .
VSV-G pseudotyped EV adjusted to comparable particle numbers also resulted in increased Tau aggregate induction (Fig. 3m) . We conclude that decoration of EV with viral glycoprotein VSV-G mediates attachment and fusion of EV with the target cell, thereby efficiently transmitting seeding-competent Tau seeds.
Transmissible spongiform encephalopathy (TSE) agents, the so far only bona fide mammalian prions, are composed of misfolded prion protein PrP 6 . The conversion of cellular prion protein (PrP C ), a protein tethered to the cell membrane by a glycosylphosphatidyl-anchor, into its infectious aggregated isoform PrP Sc , occurs on the cell surface or along the endocytic pathway 37 . We tested the effect of VSV-G on the intercellular spreading of mouse-adapted scrapie in susceptible cell lines. It has been shown that N2a cells release prion infectivity associated with EV 20, 38 . N2a cells persistently infected with TSE strain 22L (N2a 22L ) were transiently transfected with control plasmid or a plasmid coding for VSV-G (Fig. 4a) . Murine fibroblast cell line L929 39 and CAD5 cells 40 , two cell lines highly permissive to TSE strain 22L, were exposed to VSV-G positive EV (Fig. 4b) and subsequently cultured in the absence of EV for more than 8 passages before testing for the formation of PrP Sc (Fig. 4a) . VSV-G pseudotyping strongly affected PrP Sc accumulation in recipient cells, as determined by proteinase K treatment of cell lysates followed by Western blot analysis (Fig. 4c) . VSV-G decorated EV also increased the number of cells containing PrP Sc aggregates (Fig. 4d,   e) . We conclude that expression of viral ligand VSV-G drastically increases the capacity of donor cells to transmit both cytosolic and membrane-anchored proteopathic seeds to recipient cells.
Next, we tested if glycoproteins associated with human pathogenic viruses could contribute to protein aggregate spreading. SARS-CoV-2 is a novel Betacorona virus that has become a pandemic threat with millions of confirmed cases since its outbreak in December 2019 41 . SARS-CoV-2 binds to its target cells by interaction of its spike protein S with the human angiotensin-converting enzyme 2 receptor (ACE2) [41] [42] [43] . Spike S protein is a large transmembrane protein that is cleaved by host proteases into two subunits responsible for receptor binding and fusion with the host cell membrane. We assessed if ectopic expression of spike S by donor cells modulates proteopathic seed spreading in our cellular models (Fig. 5a) . To this end, donor cell populations propagating NM-HA or Tau-GFP aggregates were transfected with vector coding for SARS-CoV-2 spike S or pcDNA3.1(+) plasmid as Mock control. Both precursor protein and cleaved subunit S1 were identified in lysates of transfected cells, demonstrating that spike S was accurately processed by host proteases (Fig. 5b) . HEK cells overexpressing ACE2 (Fig. 5c) or Vero cells highly susceptible to SARS-CoV-2 41 were used as recipients (Fig. 5a) . Coculture with aggregate-bearing HEK donors overexpressing spike S increased aggregate induction in recipients (Fig. 5d, e) and clearly depended on the expression of the viral ligand ( Fig. 5f-h) . Importantly, spike S also associated with the EV fraction secreted by donor cells (Fig. 5i) . Isolated EV were also tested for their aggregate-inducing capacity in their respective target cells (Fig. 5j) . Spike S expression did not affect EV secretion (Fig. 5k) , but significantly increased numbers of recipient cells with induced aggregates (Fig. 5l, m) . We conclude that glycoproteins from different viral genera facilitate the spreading of proteopathic seeds, suggesting that viral glycoproteins could generally contribute to the intercellular exchange of cellular components.
Dissemination of protein aggregates between cells can occur by secretion of membranefree naked aggregates, secretion as vesicular cargo or via direct cell-to-cell contact. As most disease-associated proteins are secreted in a membrane-free state, it is unclear how much cell-to-cell contact and EV-mediated transfer really contribute to protein aggregate spreading 10 . We reasoned that proteopathic seed transmission involving the latter two pathways is at least partially controlled at the cell entry step. Here we demonstrate that decoration of EV with viral glycoproteins VSV-G or SARS-CoV-2 spike S, which mediate receptor interaction and subsequent merger of opposing membranes, increases protein aggregate induction in recipient cells. Thus, decoration of EV with ligands that mediate effective interaction and fusion with target cells turns them into potent delivery vehicles for proteopathic seeds. The fact that intercellular dissemination of three independent proteopathic seeds could be strongly increased by efficient receptor-ligand interactions clearly shows that mechanisms of intercellular protein aggregate transfer are overlapping. Moreover, the effect of viral glycoproteins on seed transmission suggests that certain viral infections could contribute to the dissemination of proteopathic seeds and ultimately modulate progression of protein misfolding diseases.
Recent findings that disease-associated proteins such as Tau and α-synculein are predominately secreted in a non-membrane bound state 17, 19, 22, 44 have focused research on uptake mechanisms of free oligomers or fibrils, thereby uncovering heparan sulfate proteoglycans 44, 45 and Lrp1 46 as receptors. Still, mechanisms of seed dissemination are not mutually exclusive. The fact that mechanical destruction of EV basically abolished aggregate induction in recipients suggests that, in our experimental setting, vesicle-mediated transfer is much more efficient than uptake of naked seeds from ruptured EV.
Importantly, we previously showed that oligomeric species of NM aggregates packaged in EV were not affected by sonication 33 . Thus, EV equipped with suitable ligands represent highly effective vehicles for transfer of seeding-competent cargo. By contrast, insufficient receptor-ligand interactions constitute barriers to EV-mediated proteopathic seed spreading that can obscure the actual seeding capacity of packaged cargo.
Previous studies have demonstrated that VSV-G-pseudotyped EV share exosome markers and fall in the size range of exosomes that originate from intraluminal budding of vesicles into late endosomes 28 . This is consistent with our findings, demonstrating a consistent EV size of approx. 100 nm unaffected by viral glycoprotein expression. Based on previous evidence, neither size nor marker proteins can specifically discriminate exosomes from plasma membrane-shed vesicles, making such discriminations impossible 16 . Noteworthy, proteopathic seeds have been found secreted both as small exosome-like vesicles and large membrane-shed vesicles, arguing that aggregates composed of the same disease-associated protein can be released from different cellular sites 19, 21, 47, 48 .
Expression of viral envelope protein VSV-G not only increased cell-free 
Frozen postmortem brain tissue samples from neuropathologically confirmed cases of AD, FTLD-Tau, CBD, PSP and controls were provided by the Brain Bank associated with the University Hospital and DZNE Tübingen. All material was collected from donors for or from whom a written informed consent for a brain autopsy and the use of the material and clinical information for research purposes had been obtained. All samples are listed in Supplementary Table 1 .
Ethical approval for usage of human samples for the current study was obtained from 
HEK293T cells were cotransfected with plasmids pRSV-Rev, pMD2.VSV-G, pMDl.g/pRRE, and pRRl.sin.PPT.hCMV.Wpre containing Tau-GFP/FR using lipofectamine2000. Supernatants were harvested and concentrated with PEG method according to published protocols 66 . Cell lines and primary neurons were transduced with lentivirus, and stable cell clones expressing Tau-GFP/FR were selected following limiting dilution cloning 31 .
To prepare exosome-depleted medium, FCS was ultracentrifuged at 100,000 x g, 4 °C for 20 h. Medium supplemented with the exosome-depleted FCS and antibiotics was subsequently filtered through a 0.22 and a 0.1 µM filter-sterilization device (Millipore).
For EV isolation, 2-4x10 6 cells were seeded in a T175 flask in 35 ml exosome-depleted medium to be confluent after 3 days. Cells and cell debris were pelleted by differential centrifugation (300 x g, 10 min; 2,000 x g, 20 min; 16,000 x g, 30 min). The remaining supernatant (conditioned medium) was subjected to ultracentrifugation (UC) (100,000 x g 1 h) using rotors Ti45 or SW32Ti (Beckman Coulter). The pellet was rinsed with PBS and spun again using rotor SW55Ti (100,000 x g 1 h). 
Sedimentation assay was performed as described previously 36 . Briefly, cleared cell lysates with 100 µg total protein was centrifuged at 100,000 x g for 1 h. Pellet was washed with 1.5 ml PBS and centrifuged at 100,000 x g for 30 min. Supernatant fractions were precipitated with 4 x methanol at -20° C overnight, and protein was pelleted at 2,120 x g for 25 min at 4° C (soluble fraction). The pellet (insoluble fraction) and 1/3 of the soluble fraction dissolved in RIPA buffer with 4 % SDS were loaded for western blot analysis.
Pronase digestion experiment was performed as described previously 36 . Briefly, 18 µl cleared cell lysate or brain homogenates (20-100 µg based on Tau aggregate content)
were incubated with 2 µl 1 mg / ml pronase (Roche) at 37° C for one hour. Afterwards, reactions were boiled with 3 x sample buffer, and Tau was detected by Western blot as described below.
Frozen human brain samples were homogenized in lysis buffer (for protein analysis) via Precellys® 24 (Bertin Instruments) with 1.4 mm ceramic beads at 4° C for 4 cycles 5500 rpm 20 s. To prepare 10 % (w/v) clear brain homogenate for aggregate induction, crude homogenates were centrifuged at 872 x g for 5 min at 4° C, and then the supernatants were sonicated with 50 % power for 6min. These homogenates were frozen at -80° C until use. For protein analysis, cleared supernatants were prepared by centrifugation of the crude homogenates at 15,000 x g for 15 min.
To induce Tau aggregation in HEK Tau-GFP sol clone with brain homogenates from different tauopathy patients, cells were plated on 6-well plates at 1x10 6 cells/ well in 2 ml complete medium one day before. Next day, 200 µl 10 % brain homogenates and 4 µl lipofectamine2000 were incubated for 20 min and added to recipient cells to have final 1 % brain homogenates on cells. After 3 days, cells were split and further expanded for limited dilution clone selection as previous described.
Cell pellet collected from one well of 6 well plate was lysed in 1ml lysis buffer. 900 µl of lysates were digested with 20 µg/ ml proteinase K (PK) at 37° C for 30 min for PrP Sc detection. Proteolysis was terminated by addition of 0.5 mM Pefabloc. Rest of the 100 µl lysates for PrP C detection and the digested samples were precipitated with methanol and analyzed by Western blot using anti-PrP antibody 4H11.
For Western blot analysis, protein concentrations were measured by Quick Start TM Bradford Protein assay (Bio-Rad) and proteins were separated on NuPAGE®Novex® 4- 
The image analysis was performed using the Cellvoyager Analysis support software. An 
All analyses were performed using the Prism 6.0 (GraphPad Software v.7.0c). Statistical inter-group comparisons were performed using the one-way ANOVA with a Bonferroni post-test or Student's t test. p values smaller than 0.1 were considered significant. All experiments were performed in triplicates or sextuplicates and repeated at least two times. Error bars represent the standard deviation (SD). f. Size distribution graph of different EV revealed no effect of VSV-G on size of EV. g.
EV preparations adjusted for comparable EV numbers were added to recipient N2a NM-GFP sol cells. Cells fixed 16 h post EV addition were analyzed for the percentage of cells with induced NM-GFP aggregates. Student´s unpaired t-test (n=6, ***, p<0.001). h.
Human astrocytes expressing NM-GFP sol were exposed to VSV-G pseudotyped or nonpseudotyped EV and subsequently assessed for NM-GFP aggregates 1 day later. Nuclei were stained with Hoechst. Scale bar: 20 µm. i. Percentage of astrocytes expressing NM-GFP with induced NM-GFP aggregates upon exposure to EV from either HEK or N2a cells. Statistical analysis was performed using student´s unpaired t-test (n=6, ***, p<0.001). Isolated EV were adjusted to comparable particle numbers and added to recipient HEK-FR sol cells. HEK Tau-FR cells with induced TauFR agg were determined 2 d post EV addition. Statistical analysis was performed using one-way ANOVA (n=6, ***, p<0.001). 3 d later, EV in conditioned medium were harvested and added to prion-permissive CAD5 or L929 cells. Subsequently, recipient cells were passaged 8 times to dilute out inoculum before accumulation of prions was monitored by immunofluorescence staining of Guandidinium hydrochlorid-(GdnHCL) unfolded PrP Sc or proteinase K resistant PrP Sc by Western blot. b. Detection of VSV-G and GAPDH in EV preparations from donor cells. c. Exposure of recipient cells to VSV-G pseudotyped EV from N2a 22L donors leads to efficient infection, as demonstrated by the presence of PK-resistant PrP Sc in cells exposed to pseudotyped EV from scrapie-infected donors. GAPDH and total PrP were detected on a separate blot. d. Detection of PrP Sc in recipient L929 and CAD cells exposed to EV from scrapie-infected donor cells. Eight passages post infection, recipient cells were fixed and protein was denatured using 6 M GdnHCL. Subsequently, PrP Sc was detected using antibody 4H11. Arrowheads indicate PrP Sc signal. e. Quantitative analyses of individual cells with PrP Sc puncta following exposure to EV from transfected donor cells (as of d) (n=6, ***, p<0.001; unpaired student t test). propagating proteopathic seeds were Mock transfected or transfected with plasmid coding for spike S. EV were subsequently added to recipient Vero cells. The percentage of recipients with induced aggregates was assessed 2 d later. k. Expression of spike S does not affect particle numbers. Statistical analysis was performed using one-way ANOVA (n=3). l., m. Percentage of EV-exposed recipient cells with induced NM-GFP agg and Tau-FR AD in the Vero HEK NM-GFP sol or Tau-FR sol recipient cells (n=3, *, p<0.1, ***, p<0.001; unpaired student t test).