Abstract
Astroglial pathology is seen in various neurodegenerative diseases including frontotemporal dementia (FTD), which can be caused by mutations in the gene encoding the microtubule-associated protein TAU (MAPT). Here, we applied a stem cell model of FTD to examine if FTD astrocytes carry an intrinsic propensity to degeneration and to determine if they can induce non-cell-autonomous effects in neighboring neurons. We utilized CRISPR/Cas9 genome editing in human induced pluripotent stem (iPS) cell-derived neural progenitor cells (NPCs) to repair the FTD-associated N279K MAPT mutation. While astrocytic differentiation was not impaired in FTD NPCs derived from one patient carrying the N279K MAPT mutation, FTD astrocytes appeared larger, expressed increased levels of 4R-TAU isoforms, demonstrated increased vulnerability to oxidative stress and elevated protein ubiquitination and exhibited disease-associated changes in transcriptome profiles when compared to astrocytes derived from one control individual and to the isogenic control. Interestingly, co-culture experiments with FTD astrocytes revealed increased oxidative stress and robust changes in whole genome expression in previously healthy neurons. Our study highlights the utility of iPS cell-derived NPCs to elucidate the role of astrocytes in the pathogenesis of FTD.
Highlights
Despite well-documented histomorphologic changes of astrocytes in Frontotemporal dementia (FTD), it is currently not known if astrocytes play an active role in the pathogenesis of FTD
While we have previously demonstrated unfolded protein response (UPR) activation in induced pluripotent stem (iPS) cell-derived neurons carrying N279K MAPT10, we did not observe an increase in p-PERK and BiP in N279K MAPT FTD astrocytes indicating neural cell type-specific differences in UPR in FTD (Fig. S2h,i)
Neurite density and synaptic coverage of CtrlGFP neuronal profiles were not altered by co-culture with either astrocyte type (Fig. 4b–d; Fig. S4a,b and S6j,k), we observed an increased susceptibility of CtrlGFP neurons to rotenone-induced oxidative stress after co-culture with N279K MAPT astrocytes (Fig. 4e,f; Fig. S6l). 48 hours after application of rotenone, we found a significantly lower number of CtrlGFP neurons when co-cultured on FTD astrocytes compared to CtrlGFP neurons co-cultured on Ctrl astrocytes, indicating an increased vulnerability of neurons in the presence of FTD astrocytes (Fig. 4e,f; Fig. S6l)
Summary
Derivation and characterization of FTD and control NPCs. NPCs were generated from human iPS cells carrying the N279K MAPT mutation (FTD NPCs) and were differentiated in parallel with human iPS cells from a healthy control (Ctrl) individual carrying wildtype MAPT (C1 NPCs). This analysis revealed only one gene, ANXA2, to be significantly upregulated in neurons after co-culture with FTD astrocytes while expression levels of the other previously identified genes (TCEAL7, GAS7, DYSF, CXCL12, NPY, MMP14, NELL2, MAOB) were comparable (Fig. 4g; Figs S4i and S6m) These observations indicated that ANXA2 may play an important role in astrocyte-neuron-crosstalk in the context of FTD. In line with our previous observations, upregulated genes in CtrlGFP neurons co-cultured with N279K MAPT astrocytes were associated with apoptosis (CXCR4, ESPL1, MX1), response to stress (CXCR4, KLRG1, TDG), system development (ANXA2, CSDE1, NHLH1, NTF3) and signal transduction (CXCR4, EFS, KLRG1, MX1, NTF3, RBPJL) (Fig. 4i; Fig. S4l,m) These findings indicated that FTD astrocytes have the ability to significantly influence neural programs in previously healthy neurons and demonstrate astrocyte-mediated non-cell-autonomous changes in FTD, which have been described as an important pathogenic component in other neurodegenerative diseases such as ALS5,6. Our human neural stem cell model of FTD does provide a suitable platform to identify disease phenotypes in patient-derived astrocytes, but it serves as an invaluable tool for future, therapy-oriented drug screens on FTD astrocytes, which may significantly influence neuronal degeneration in FTD
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