Frontotemporal dementia-associated N279K tau mutant disrupts subcellular vesicle trafficking and induces cellular stress in iPSC-derived neural stem cells

Wren et al. Molecular Neurodegeneration (2015) 10:46 DOI 10.1186/s13024-015-0042-7 RESEARCH ARTICLE Open Access Frontotemporal dementia-associated N279K tau mutant disrupts subcellular vesicle trafficking and induces cellular stress in iPSC-derived neural stem cells Melissa C. Wren1, Jing Zhao1, Chia-Chen Liu1, Melissa E. Murray1, Yuka Atagi1, Mary D. Davis1, Yuan Fu1, Hirotaka J. Okano2, Kotaro Ogaki1, Audrey J. Strongosky3, Pawel Tacik3, Rosa Rademakers1, Owen A. Ross1, Dennis W. Dickson1, Zbigniew K. Wszolek3, Takahisa Kanekiyo1* and Guojun Bu1* Abstract Background: Pallido-ponto-nigral degeneration (PPND), a major subtype of frontotemporal dementia with parkinsonism related to chromosome 17 (FTDP-17), is a progressive and terminal neurodegenerative disease caused by c.837 T > G mutation in the MAPT gene encoding microtubule-associated protein tau (rs63750756; N279K). This MAPT mutation induces alternative splicing of exon 10, resulting in a modification of microtubule-binding region of tau. Although mutations in the MAPT gene have been linked to multiple tauopathies including Alzheimer’s disease, frontotemporal dementia and progressive supranuclear palsy, knowledge regarding how tau N279K mutation causes PPND/FTDP-17 is limited. Results: We investigated the underlying disease mechanism associated with the N279K tau mutation using PPND/ FTDP-17 patient-derived induced pluripotent stem cells (iPSCs) and autopsy brains. In iPSC-derived neural stem cells (NSCs), the N279K tau mutation induced an increased ratio of 4-repeat to 3-repeat tau and accumulation of stress granules indicating elevated cellular stress. More significant, NSCs derived from patients with the N279K tau mutation displayed impaired endocytic trafficking as evidenced by accumulation of endosomes and exosomes, and a reduction of lysosomes. Since there were no significant differences in cellular stress and distribution of subcellular organelles between control and N279K skin fibroblasts, N279K-related vesicle trafficking defects are likely specific to the neuronal lineage. Consistently, the levels of intracellular/luminal vesicle and exosome marker flotillin-1 were significantly increased in frontal and temporal cortices of PPND/FTDP-17 patients with the N279K tau mutation, events that were not seen in the occipital cortex which is the most spared cortical region in the patients. Conclusion: Together, our results demonstrate that alterations of intracellular vesicle trafficking in NSCs/neurons likely contribute to neurodegeneration as an important disease mechanism underlying the N279K tau mutation in PPND/FTDP-17. Keywords: FTDP-17, iPSCs, N279K, Neural stem cells, PPND, Tau * Correspondence: ; Melissa C. Wren and Jing Zhao are co-first authors 1 Department of Neuroscience, Mayo Clinic Jacksonville, 4500 San Pablo Road, Jacksonville, FL 32224, USA Full list of author information is available at the end of the article © 2015 Wren et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Wren et al. Molecular Neurodegeneration (2015) 10:46 Background Frontotemporal dementia with parkinsonism related to chromosome 17 (FTDP-17) prevails as one of the most common form of early-onset dementia [1]. FTDP-17 presents as a fulminant progressive neurodegenerative dementia, with no curative treatment or effective palliative relief. Clinically, FTDP-17 displays a triad of symptoms, with parkinsonism, behavioral changes and personality dysfunctions, and later cognitive impairments [1, 2]. Two genetic causes have been identified that lead to distinct forms of FTDP-17; mutations in progranulin (GRN) cause FTDP-17U (ubiquitin) with TDP-43 pathology and mutations in the microtubule-associated protein tau (MAPT) gene cause FTDP-17T (tau) [3]. Approximately one-half of all FTDP-17 cases are caused by autosomal dominant mutations in MAPT. The pallido-ponto-nigral degeneration (PPND) family is the largest and most comprehensively studied kindred of tau N279K mutant carriers and of all FTDP-17 families, containing to date, 59 affected individuals [4]. The N279K substitution is encoded within exon 10 of the MAPT gene locus and is one of the most frequent mutations in FTDP-17T patients [2]; together with mutations P301L and intron 10 + 16. Together these three mutations account for up to 60 % of FTDP-17T cases [5]. Multiple MAPT transcripts exist due to the alternative splicing of the MAPT gene, and transcripts of MAPT can be classified based on the inclusion/exclusion of exon 10 [6]. Most MAPT mutations including the N279K mutation increase the transcript containing exon 10 resulting in the overproduction of tau protein isoforms containing four tandem microtubule-binding domain repeats (4Rtau); whereas levels of the other major isoform in human brain containing only three repeats (3R-tau) is believed to be unaffected [7]. Although it is not fully understood how the increase in 4R-tau contributes to disease pathogenesis, MAPT mutations have been shown to cause multiple tauopathies [8–10]. It is believed that tauopathies are caused by aberrant hyperphosphorylation of tau, leading to the assembly of variable neurotoxic tau aggregates and deposition of insoluble tau fibers in both neurons and glia [11]. Progressive tau deposition in FTDP-17T patients is associated with severe neocortical atrophy of the frontal and temporal lobes, in association with the degeneration of medial temporal structures [12]. Destruction of the basal ganglia and depigmentation of the substantia nigra may also be present to some degree in MAPT mutant carriers, but are documented to be a consistent pathological hallmark in N279K kindreds [13, 14]. The exact mechanism owing to the cell death of specific neuronal populations in FTDP-17T remains to be identified; however, the characteristic dominant penetrance of the N279K tau mutant suggests a gain-of-toxic-function that results in specific subsets of degenerating cells. The Page 2 of 13 induced pluripotent stem cell (iPSC) technology provides a novel and unparalleled approach to aid the study of molecular and cellular dynamics of disease pathogenesis by utilizing patient-specific cells with pathologically linked genetic mutations on the inherent genetic background [15], which has facilitated its exponential use as a novel tool to understand the pathogenesis of multiple neurodegenerative diseases [16]. Herein, we investigated the pathogenic mechanism underlying FTDP-17T (...truncated)