Abstract
Human iPSC lines represent a powerful translational model of tauopathies. We have recently described a pathophysiological phenotype of neuronal excitability of human cells derived from the patients with familial frontotemporal dementia and parkinsonism (FTDP-17) caused by the MAPT 10+16 splice-site mutation. This mutation leads to the increased splicing of 4R tau isoforms. However, the role of different isoforms of tau protein in initiating neuronal dementia-related dysfunction, and the causality between the MAPT 10+16 mutation and altered neuronal activity have remained unclear. Here, we employed genetically engineered cells, in which the IVS10+16 mutation was introduced into healthy donor iPSCs to increase the expression of 4R tau isoform in exon 10, aiming to explore key physiological traits of iPSC-derived MAPT IVS10+16 neurons using patch-clamp electrophysiology and multiphoton fluorescent imaging techniques. We found that during late in vitro neurogenesis (from ~180 to 230 days) iPSC-derived cortical neurons of the control group (parental wild-type tau) exhibited membrane properties compatible with “mature” neurons. In contrast, MAPT IVS10+16 neurons displayed impaired excitability, as reflected by a depolarized resting membrane potential, an increased input resistance, and reduced voltage-gated Na+- and K+-channel-mediated currents. The mutation changed the channel properties of fast-inactivating Nav and decreased the Nav1.6 protein level. MAPT IVS10+16 neurons exhibited reduced firing accompanied by a changed action potential waveform and severely disturbed intracellular Ca2+ dynamics, both in the soma and dendrites, upon neuronal depolarization. These results unveil a causal link between the MAPT 10+16 mutation, hence overproduction of 4R tau, and a dysfunction of human cells, identifying a biophysical basis of changed neuronal activity in 4R tau-triggered dementia. Our study lends further support to using iPSC lines as a suitable platform for modelling tau-induced human neuropathology in vitro.
Highlights
The deposition of abnormal tau protein is a hallmark for a large group of human cognitive disorders, which includes Alzheimer’s disease [1, 2], several forms of parkinsonism [3] or frontotemporal lobar degeneration (FTLD)—such as corticobasal degeneration, progressive supranuclear palsy, inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17)—among others [4,5,6]
This study provides an electrophysiological characterisation of human cells with the genetically engineered pathogenic IVS10 +16 monoallelic mutation, which was introduced into the healthy donor cells to increase splicing of the 4R tau protein isoform
The findings are consistent with the earlier described phenotype of pathophysiological excitability of the cells derived from FTDP-17 patient samples, thereby confirming that overproduction of 4R tau by introducing the MAPT 10+16 mutation into healthy cells effectively reproduces the pathogenesis of neurons derived from patient samples
Summary
The deposition of abnormal tau protein is a hallmark for a large group of human cognitive disorders (tauopathies), which includes Alzheimer’s disease [1, 2], several forms of parkinsonism [3] or frontotemporal lobar degeneration (FTLD)—such as corticobasal degeneration, progressive supranuclear palsy, inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17)—among others [4,5,6]. The primary underlying molecular mechanism, as established to date, includes genetically triggered self-aggregation of tau protein [7] followed by conformational changes in the microtubule dynamics [8, 9]. The latter can provoke neurodegeneration due to induced neuronal cell death—directly or via calcium-induced excitotoxicity [10,11,12] —and a cognitive decline in patients with tauopathy [13]. Cumulative evidence indicates that correct splicing (balanced 3R/4R ratio) is required for normal neuronal function: several MAPT mutations causing overproduction of 4R tau (inclusion of exon 10) have effectively triggered neurodegeneration linked to dementia [4,5,6]. Immense progress has been made over the past decade in our understanding of the molecular biology of tau protein, the exact mechanism(s) by which various tau isoforms affect neuronal activity and initiate neuronal dysfunction remain largely unclear
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