Immortalized cortical neurons expressing caspase-cleaved tau are sensitized to endoplasmic reticulum stress induced cell death

Abstract

It has been previously reported that an Asp421 cleaved form of tau is toxic when expressed in cells. The purpose of this study was to understand if, and in what manner, the presence of Asp421 cleaved tau in neurons, which is generated by caspase cleavage, might facilitate neuronal death in Alzheimer's disease (AD). For these studies we used immortalized cortical neurons that inducibly express either a full-length tau isoform (T4) or an isoform that has been pseudo-truncated at Asp421 (T4C3), to mimic caspase-3 cleavage. Neurons expressing either T4 or T4C3 were treated with thapsigargin, a drug, which has been shown to induce endoplasmic reticulum (ER) stress. Following long-term treatment with thapsigargin, cells expressing T4C3 presented with a marked increase in cell toxicity, underscored by differential activation of caspase-3 in comparison with cells expressing T4. Furthermore, we found that an inhibitor of the ERK1/2 signaling pathway, which is upregulated to different extents in each cell type, significantly reduced toxicity in both T4 and T4C3 cells. Our results suggest that the presence of Asp421 cleaved tau may sensitize neurons to ER stressors and possibly potentiate cell death processes during AD progression.

Caspase-cleaved Tau Expression Induces Mitochondrial Dysfunction in Immortalized Cortical Neurons

“…Chronic Treatment with Thapsigargin Induces Mitochondrial Fragmentation in T4-expressing Cells-Prolonged treatment of T4C3 cells with 1 M thapsigargin results in significantly more cell death in comparison with T4 cells (12). In the same studies we observed that 6 h of thapsigargin treatment results in a significant increase in caspase-cleaved tau in T4 cells as a result of caspase activation (12).…”

“…To study the role that Asp-421-cleaved tau may play in compromising mitochondrial function, we used immortalized cortical neurons that inducibly express either a full-length form of tau (T4) or a tau isoform that has been truncated at Asp-421 (T4C3). The cells expressing T4C3 showed an increase in cell toxicity, as measured by lactate dehydrogenase release, compared with T4-expressing cells (12). In this study we found that mitochondria in the T4C3-expressing cells presented with an abnormal morphology, characterized by a decrease in mitochondrial length, suggestive of fragmentation.…”

“…Immortalized Cortical Cells-Immortalized cortical neurons were induced to express tau by treatment with, doxycycline (2 g/ml) for 48 h (12,29). In the absence of doxycycline, inducible cells express an almost undetectable amount of tau, as measured by Western blotting (29).…”

“…In the absence of doxycycline, inducible cells express an almost undetectable amount of tau, as measured by Western blotting (29). However, treatment with doxycycline resulted in a robust increase in tau expression to levels approximately equivalent to concentrations seen in rat primary neuronal cortical cultures from E18 rat cortices (12). Considering that mitochondrial dysfunction occurs in the AD brain (30,31), we investigated the possibility that T4C3 may sensitize cells to a loss of viability by compromising mitochondrial function.…”

“…Given the fact that both caspase cleavage of tau and mitochondrial dysfunction are early events in the pathogenesis of AD (7,14) and the finding that Asp-421-cleaved tau is toxic in our cell culture model (12), the focus of this study was to determine whether and how Asp-421-cleaved tau affected mitochondrial function. To study the role that Asp-421-cleaved tau may play in compromising mitochondrial function, we used immortalized cortical neurons that inducibly express either a full-length form of tau (T4) or a tau isoform that has been truncated at Asp-421 (T4C3).…”