Emerging evidence for dysregulated proteome cargoes of tau-propagating extracellular vesicles driven by familial mutations of tau and presenilin

Abstract

Tau propagation, pathogenesis, and neurotoxicity are hallmarks of neurodegenerative diseases that result in cognitive impairment. Tau accumulates in Alzheimer's disease (AD), frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), chronic traumatic encephalopathy (CTE), progressive supranuclear palsy, and related tauopathies. Knowledge of the mechanisms for tau propagation in neurodegeneration is necessary for understanding the development of dementia. Exosomes, known as extracellular vesicles (EVs), have emerged as participants in promoting tau propagation. Recent findings show that EVs generated by neurons expressing familial mutations of tauopathies of FTDP-17 (P301L and V337M) (mTau) and presenilin (A246E) (mPS1) in AD induce tau propagation and accumulation after injection into rodent brain. To gain knowledge of the proteome cargoes of the mTau and mPS1 EVs that promote tau pathogenesis, this review compares the proteomes of these EVs, which results in important new questions concerning EV mechanisms of tau pathogenesis. Proteomics data show that EVs produced by mTau- and mPS1-expressing iPSC neurons share proteins involved in exocytosis and vesicle secretion and, notably, these EVs also possess differences in protein components of vesicle-mediated transport, extracellular functions, and cell adhesion. It will be important for future studies to gain an understanding of the breadth of familial genetic mutations of tau, presenilin, and other genes in promoting EV initiation of tau propagation and pathogenesis. Furthermore, elucidation of EV cargo components that mediate tau propagation will have potential as biomarkers and therapeutic strategies to ameliorate dementia of tauopathies.

Keywords: Alzheimer’s disease; Tauopathies; exosome; extracellular vesicle; frontotemporal dementia; mutant presenilin; mutant tau; proteomics.

Figure 1

Analysis of proteome cargoes of tau-propagating EVs generated by neurons expressing familial mutant forms of tau and presenilin. EVs: extracellular vesicles. Tau-propagating EVs produced by human iPSC neurons expressing familial mutant tau P301L and V337 mutations of FTDP-17^[24], and mutant presenilin of A246E^[25] of Alzheimer’s disease were analyzed for (A) initiation of tau propagation in the rodent brain; (B) comparison of mutant EV proteome cargoes compared to wild-type control EVs; (C) comparison of mTau and mPS1 EVs proteomes that promote tau pathogenesis. The mTau iPSC neuronal cell line was obtained by lentivirus expression in a normal iPSC cell line, with a control consisting of expressing control lentivirus without the mutant Tau construct^[24]. The mPS1 iPSC neuronal cell line was generated by reprogramming from a biopsy from a patient harboring the mPS1, and the control wild-type PS1 iPSC cell line was generated by reprogramming from a biopsy from a normal healthy patient having wild-type PS1^[25]. It is noted that the mTau and mPS1 iPSC neurons are generated from different human patient biopsies and, therefore, possess different genetic backgrounds. The study of human iPSCs from different genetic backgrounds is logical to gain an understanding of tauopathies that afflict various human populations (The BioRender resource was used for the preparation of Figure 1).

Figure 2

Protein network analysis of proteins shared by mTau and mPS1 EVs. (A) Shared biological pathways of mTau and mPS1 EV proteomes. Shared proteins of mTau and mTPS1 EVs^[24,25] were assessed for functional biological pathways by GO analysis using FDR significance levels of < 0.05. In fact, highly significant FDRs are indicated of 10^-24 to 10^-28; (B) Hub proteins of interaction networks. Protein components of hubs of protein interaction networks of groups 1-3 are listed. Functions of these proteins are provided in Supplementary Table 2; (C) STRING-db protein interaction networks of proteins shared by mTau and mPS1 EVs. Interaction utilized scores set to high confidence (0.7 on a scale of 0-1) that predicted interactions exist among the proteins illustrated. EVs: extracellular vesicles.

Figure 3

Proteins that are downregulated, upregulated, or of similar levels in mTau compared to mPS1 EVs. (A) Heatmap with hierarchical clustering illustrates downregulated and upregulated proteins in mTau compared to mPS1 EVs. The heatmap shows log₂(mTau/mPS1) ratios that were significant (P < 0.05); (B) Downregulated and upregulated proteins in mTau vs. mPS1 EVs. Protein components found to be significantly downregulated or upregulated in mTau compared to mPS1 proteomes are listed by gene and protein names. Significance is defined as P < 0.05 for log₂ ratios of (mTau/mPS1) quantitation of proteins. Functions of these proteins are provided in Supplementary Table 4; (C) Proteins at similar levels in mTau and mPS1 EVs. Proteins present in both mTau and mPS1 EVs of similar levels are listed by gene names. EVs: extracellular vesicles.

Figure 4

Proteins present in only mTau EVs (not mPS1 EVs). (A) Proteins present in only mTau EVs (not mPS1 EVs). Such proteins are listed by their gene symbol names; (B) GO analysis of proteins in only mTau EVs. GO analysis of proteins present in only the mTau EVs (and not in the mPS1 EVs) indicated significant cell component pathways (FDR < 0.05). EVs: extracellular vesicles.

Figure 5

Protein network of proteins present in only mPS1 EVs (not mTau EVs). (A) STRING-db protein interaction networks of proteins present in only mPS1 EVs. Interactions utilized scores set to high confidence that predicted interactions exist among the proteins illustrated; (B) Hub proteins of interaction networks. Protein components of three hubs of protein interaction networks are illustrated. Functions of proteins are provided in Supplementary Table 5.

Figure 6

Tau mutations and tau protein isoforms. (A) Tau mutations. Tau missense and deletion mutations located in six tau isoforms (listed in legend for tau isoforms) are illustrated. The mutations are mapped for the representative 2N4R isoform; (B) Tau protein isoforms. Six CNS tau isoforms are illustrated with respect to the N1 and N2 domains at the N-terminal regions with the four R1, R2, R3, and R4 domains of the MTBR. The six tau isoforms are shown as 2N4R, 2N3R, 1N4R, 1N3R, 0N4R, and 0N3R. MTBR: microtubule-binding domain region.

Figure 7

PS1 mutations (PS1: presenilin 1). Presenilin 1 is one of the four catalytic subunits of the γ-secretase protein complex^[50]. Over 300 PS1 mutations have been identified, covering ~ 25% of the PS1 residues, which account for the majority of FAD mutations^[40]. Most are missense mutations that localize in the TMDs and in the HLs. Upon assembly and maturation of the complex, presenilin 1 is cleaved within the large cytoplasmic loop into two fragments, the NFT comprising of TMDs 1-6 (blue) and the CTF comprising TMDs 7-9^[51]. Cleavage occurs between the two aspartate active site residues in TMDs 6 and 7 (labeled D)^[52]. The FAD mutation A2456 generated for the iPSC neurons in the mPS1 study^[24,25] is shown in red. FAD: familial AD; TMDs: transmembrane domains; HLs: hydrophilic loops; NFT: N-terminal fragment; CTF: C-terminal fragment.