Dysregulation of Exosome Cargo by Mutant Tau Expressed in Human-induced Pluripotent Stem Cell (iPSC) Neurons Revealed by Proteomics Analyses*

Accumulation and propagation of Tau is a neuropathological hallmark of Alzheimer's disease, frontotemporal dementia, and related tauopathies. Extracellular vesicles, especially exosomes, participate in Tau propagation in brain, including those produced by human neurons expressing mutant Tau (mTau). The goal of this study was to define the cargo of these mTau exosomes by proteomics analyses. Findings showed that mTau dysregulates the exosome by acquisition, loss, and up- or downregulation of proteins, resulting in pathogenic exosomes capable of propagating Tau neuropathology. Graphical Abstract Highlights Mutant Tau dysregulates exosome cargo generated by human iPSC neurons. Expression of P301L and V337M mutations of Tau in human iPSC neurons result in recruitment of distinct proteins to exosomes, and absence of control proteins to exosomes. Tau mutations of FTDP-17 result in up- and downregulation of exosome proteins. Altered proteome cargo of mTau exosomes may be involved in Tau pathogenesis in vivo in brain. Accumulation and propagation of hyperphosphorylated Tau (p-Tau) is a common neuropathological hallmark associated with neurodegeneration of Alzheimer's disease (AD), frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), and related tauopathies. Extracellular vesicles, specifically exosomes, have recently been demonstrated to participate in mediating Tau propagation in brain. Exosomes produced by human induced pluripotent stem cell (iPSC)-derived neurons expressing mutant Tau (mTau), containing the P301L and V337M Tau mutations of FTDP-17, possess the ability to propagate p-Tau pathology after injection into mouse brain. To gain an understanding of the mTau exosome cargo involved in Tau pathogenesis, these pathogenic exosomes were analyzed by proteomics and bioinformatics. The data showed that mTau expression dysregulates the exosome proteome to result in 1) proteins uniquely present only in mTau, and not control exosomes, 2) the absence of proteins in mTau exosomes, uniquely present in control exosomes, and 3) shared proteins which were significantly upregulated or downregulated in mTau compared with control exosomes. Notably, mTau exosomes (not control exosomes) contain ANP32A (also known as I1PP2A), an endogenous inhibitor of the PP2A phosphatase which regulates the phosphorylation state of p-Tau. Several of the mTau exosome-specific proteins have been shown to participate in AD mechanisms involving lysosomes, inflammation, secretases, and related processes. Furthermore, the mTau exosomes lacked a substantial portion of proteins present in control exosomes involved in pathways of localization, vesicle transport, and protein binding functions. The shared proteins present in both mTau and control exosomes represented exosome functions of vesicle-mediated transport, exocytosis, and secretion processes. These data illustrate mTau as a dynamic regulator of the biogenesis of exosomes to result in acquisition, deletion, and up- or downregulation of protein cargo to result in pathogenic mTau exosomes capable of in vivo propagation of p-Tau neuropathology in mouse brain.