Nuclear import impairment causes cytoplasmic trans-activation response DNA-binding protein accumulation and is associated with frontotemporal lobar degeneration

• Published in: Brain (London, England : 1878) England : 1878), 2010-06, Vol.133 (6), p.1763-1771
• Main Authors: Nishimura, Agnes L., Župunski, Vera, Troakes, Claire, Kathe, Claudia, Fratta, Pietro, Howell, Michael, Gallo, Jean–Marc, Hortobágyi, Tibor, Shaw, Christopher E., Rogelj, Boris
• Format: Article
• Language: English
• Subjects: Active Transport, Cell Nucleus - physiology; Aged; alpha Karyopherins - metabolism; Amyotrophic Lateral Sclerosis - metabolism; Animals; beta Karyopherins - metabolism; Biological and medical sciences; Brain - metabolism; Cell Line; Cell Line, Tumor; cellular apoptosis susceptibility; Cellular Apoptosis Susceptibility Protein - metabolism; Cytoplasm - metabolism; Degenerative and inherited degenerative diseases of the nervous system. Leukodystrophies. Prion diseases; DNA-Binding Proteins - metabolism; Female; frontotemporal lobar degeneration; Frontotemporal Lobar Degeneration - metabolism; Glutathione Transferase - metabolism; Humans; karyopherin-β1; Male; Medical sciences; Mice; Middle Aged; Neurology; nuclear transport; Signal Transduction - genetics; Spinal Cord - metabolism; TDP-43
• ISSN: 0006-8950; 1460-2156
• DOI: 10.1093/brain/awq111

Trans-activation response DNA-binding protein (TDP-43) accumulation is the major component of ubiquitinated protein inclusions found in patients with amyotrophic lateral sclerosis, and frontotemporal lobar degeneration with TDP-43 positive ubiquitinated inclusions, recently relabelled the ‘TDP-43 proteinopathies’. TDP-43 is predominantly located in the nucleus, however, in disease it mislocalizes to the cytoplasm where it aggregates to form hallmark pathological inclusions. The identification of TDP-43 mutations in familial and sporadic amyotrophic lateral sclerosis cases confirms its pathogenic role; but it is wild-type TDP-43 that is deposited in the vast majority of TDP-43 proteinopathies, implicating other unknown factors for its mislocalization and aggregation. One such mechanism may be defective nuclear import of TDP-43 protein, as a disruption of its nuclear localization signal leads to mislocalization and aggregation of TDP-43 in the cytoplasm. In order to explore the factors that regulate the nuclear import of TDP-43, we used a small interfering RNA library to silence 82 proteins involved in nuclear transport and found that knockdowns of karyopherin-β1 and cellular apoptosis susceptibility protein resulted in marked cytoplasmic accumulation of TDP-43. In glutathione S-transferase pull-down assays, TDP-43 bound to karyopherin-αs, thereby confirming the classical nuclear import pathway for the import of TDP-43. Analysis of the expression of chosen nuclear import factors in post-mortem brain samples from patients with TDP-43 positive frontotemporal lobar degeneration, and spinal cord samples from patients with amyotrophic lateral sclerosis, revealed a considerable reduction in expression of cellular apoptosis susceptibility protein in frontotemporal lobar degeneration. We propose that cellular apoptosis susceptibility protein associated defective nuclear transport may play a mechanistic role in the pathogenesis of the TDP-43 positive frontotemporal lobar degeneration.