An autopsied case of unclassifiable sporadic four-repeat tauopathy presenting with parkinsonism and speech disturbances

Genetically engineered MAPT 10+16 mutation causes pathophysiological excitability of human iPSC-derived neurons related to 4R tau-induced dementia

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.

Unclassified four-repeat tauopathy associated with familial parkinsonism and progressive respiratory failure

We describe an autopsied patient with familial parkinsonism and unclassified four repeat-tau (4R-tau) aggregation. She presented with bradykinesia, truncal dystonia, and mild amnesia at the age of 61 and then exhibited body weight loss (15 kg over 8 months), sleep disturbances, and progressive respiratory failure with CO₂ narcosis. She died of respiratory failure at the age of 62, 14 months after disease onset. Her brother also showed parkinsonism at the age of 58 and suddenly died 6 months later. Postmortem examination revealed 4R-tau aggregation, which was characterized by neuronal globose-type tangles or pretangles, bush-like or miscellaneous astrocytic inclusions, and coiled bodies. The temporal tip, the striatum, the substantia nigra, the tegmentum of the midbrain, the medullary reticular formation, and the spinal cord were severely involved with tau aggregation. Argyrophilic grains and ballooned neurons were also found in the medial temporal structures, however, extensions of the 4R-aggregations in the case were clearly broader than those of the argyrophilic grains. Western blot analysis of sarkosyl-insoluble fractions from brain lysates revealed prominent bands of tau at both 33 kDa and 37 kDa. Genetic examinations did not reveal any known pathogenic mutations in MAPT, DCTN-1, PSEN-1, or familial or young-onset parkinsonism-related genes. The clinical manifestations, pathologic findings, and biochemical properties of aggregated tau in our patient cannot be explained by argyrophilic grain disease or other known 4R-tauopathies alone. Our results further extend the clinical and neuropathologic spectra of 4R-tauopathy.

An autopsied case of corticobasal degeneration presenting with frontotemporal dementia followed by myoclonus

A Japanese woman developed frontotemporal dementia (FTD)-like symptoms of abnormal behavior, such as stereotyped behavior and disinhibition. The patient developed these symptoms at the age of 59 years, although aphasia symptoms were not apparent at early disease stages. Progressive parkinsonism was dominant on the left side, and conspicuous myoclonus was recognized in the late disease stage. MRI indicated severe, right side-dominant frontotemporal lobe atrophy with white matter degeneration. Brainstem and cerebellar atrophy were also observed. The patient underwent gastrostomy 7 years after the onset of her symptoms and died at the age of 70 years. Neuropathological examination showed diffuse neuron loss with gliosis and tissue rarefaction in the frontotemporal lobe, particularly in the right anterior portion of the frontal lobe. Spongiform changes and ballooned neurons were also observed in the frontotemporal cortex, particularly in the superficial layer and deeper layers, respectively. Gallyas-Braak silver staining and anti-phosphorylated tau immunostaining indicated numerous argyrophilic and tau-positive structures, including pretangles, astrocytic plaques, coiled bodies and neuropil threads. We speculate that the myoclonus observed in the present case was at least partly caused by or related to the spongiform change and ballooned neurons in the cerebral cortex. The clinical phenotypes of corticobasal degeneration (CBD) vary considerably, and the clinical presentation of the present patient was compatible with frontal behavioral-spatial syndrome in the early disease stage. However, in the later disease stages, her symptoms were reflective of corticobasal syndrome. Because it is not rare for the clinical phenotype to change along with disease progression in cases of CBD, we should consider CBD in cases presenting with FTD at symptom onset.