Tau-induced nuclear envelope invagination causes a toxic accumulation of mRNA in Drosophila

Abstract

The nucleus is a spherical dual-membrane bound organelle that encapsulates genomic DNA. In eukaryotes, messenger RNAs (mRNA) are transcribed in the nucleus and transported through nuclear pores into the cytoplasm for translation into protein. In certain cell types and pathological conditions, nuclei harbor tubular invaginations of the nuclear envelope known as the "nucleoplasmic reticulum." Nucleoplasmic reticulum expansion has recently been established as a mediator of neurodegeneration in tauopathies, including Alzheimer's disease. While the presence of pore-lined, cytoplasm-filled, nuclear envelope invaginations has been proposed to facilitate the rapid export of RNAs from the nucleus to the cytoplasm, the functional significance of nuclear envelope invaginations in regard to RNA export in any disorder is currently unknown. Here, we report that polyadenylated RNAs accumulate within and adjacent to tau-induced nuclear envelope invaginations in a Drosophila model of tauopathy. Genetic or pharmacologic inhibition of RNA export machinery reduces accumulation of polyadenylated RNA within and adjacent to nuclear envelope invaginations and reduces tau-induced neuronal death. These data are the first to point toward a possible role for RNA export through nuclear envelope invaginations in the pathogenesis of a neurodegenerative disorder and suggest that nucleocytoplasmic transport machinery may serve as a possible novel class of therapeutic targets for the treatment of tauopathies.

Keywords: RNA; nuclear export; nucleocytoplasmic transport; nucleus; tau; tauopathy.

Figure 1

Polyadenylated RNA accumulates at nuclear invaginations in tau transgenic Drosophila. (a) Immunostaining of Lamin and fluorescence in situ hybridization of polyadenylated RNA with a poly(dT) nucleotide probe in control and tau transgenic Drosophila brains. Arrowhead indicates nuclear invaginations. (b) Comparison of extranuclear versus intra‐invagination accumulation of polyadenylated RNA in nuclei of tau transgenic Drosophila brains. Arrowhead indicates extranuclear accumulation of polyadenylated RNA, while arrow indicates intra‐invagination accumulation of polyadenylated RNA. (c) Pie chart depicting the percent of invaginated nuclei in tau transgenic Drosophila that are associated with either extranuclear, intra‐invagination, or no polyadenylated RNA accumulation, n = 4, 100 nuclei per fly. All flies are 10 days old. Control is elav‐Gal4/+. Scale bars represent 5 μm

Figure 2

Genetic or pharmacologic inhibition of RNA export machinery reduces tau‐induced toxicity in vivo. (a) Neuronal degeneration assayed by TUNEL staining; n = 6. (b) Western blot showing human tau protein levels in Drosophila heads. Representative images (c) and quantification (d) of the fraction of nuclear invaginations associated with RNA accumulation in Drosophila brains harboring the indicated allele or transgene; n = 4, 100 nuclei per fly. (e) Neuronal degeneration assayed by TUNEL staining in tau transgenic Drosophila treated with either 500 nM leptomycin B (Lept.), 500 nM KPT 350, or the appropriate vehicle. (f) Western blot showing tau protein levels in tau transgenic Drosophila treated with leptomycin B (Lept.), KPT350, or the appropriate vehicle. Arrowheads indicate Lamin invaginations. All flies are 10 days old. Controls are elav‐Gal4/+. Data are presented as the mean ± sem; unpaired t test or ANOVA with Dunnett's post hoc test, *p < 0.05; ***p < 0.001; ****p < 0.0001