Altered proteasomal function due to the expression of polyglutamine-expanded truncated N-terminal huntingtin induces apoptosis by caspase activation through mitochondrial cytochrome c release

Abstract

Expansion of CAG repeats within the coding region of target genes is the cause of several autosomal dominant neurodegenerative diseases including Huntington's disease (HD). A hallmark of HD is the proteolytic production of N-terminal fragments of huntingtin containing polyglutamine repeats that form ubiquitinated aggregates in the nucleus and cytoplasm of the affected neurons. In this study, we used an ecdysone-inducible stable mouse neuro2a cell line that expresses truncated N-terminal huntingtin (tNhtt) with different polyglutamine length, along with mice transgenic for HD exon 1, to demonstrate that the ubiquitin-proteasome pathway is involved in the pathogenesis of HD. Proteasomal 20S core catalytic component was redistributed to the polyglutamine aggregates in both the cellular and transgenic mouse models. Proteasome inhibitor dramatically increased the rate of aggregate formation caused by tNhtt protein with 60 glutamine (60Q) repeats, but had very little influence on aggregate formation by tNhtt protein with 150Q repeats. Both normal and polyglutamine-expanded tNhtt proteins were degraded by proteasome, but the rate of degradation was inversely proportional to the repeat length. The shift of the proteasomal components from the total cellular environment to the aggregates, as well as the comparatively slower degradation of tNhtt with longer polyglutamine, decreased the proteasome's availability for degrading other key target proteins, such as p53. This altered proteasomal function was associated with disrupted mitochondrial membrane potential, released cytochrome c from mitochondria into the cytosol and activated caspase-9- and caspase-3-like proteases. These results suggest that the impaired proteasomal function plays an important role in polyglutamine protein-induced cell death.

Proteasome inhibitor‐induced apoptosis in human monocyte‐derived dendritic cells

“…We as well as other groups have recently confirmed the involvement of the mitochondrial pathway and of caspases in apoptosis due to proteasome inhibition [13][14][15][16][17]. The observation of cytochrome c cytosolic relocalization and DW m dissipation in DC treated with bortezomib demonstrates the eventual loss of mitochondria integrity in response to this drug.…”

“…The mitochondrial apoptotic pathway has been shown to play a role in proteasome inhibitor-induced apoptosis in several tumor cell models [13][14][15][16][17][18][19]. Therefore, we evaluated mitochondria integrity in DC exposed to bortezomib.…”

“…How proteasome inhibition translates into a potent cytotoxic effect in tumor cells is not fully understood, even though inhibition of NF-jB and deregulated expression of p53, caspases, Bcl-2 family members, CDC25 family proteins and cyclins were all proposed to be relevant in this context [1,2]. Ultimately, deregulated protein turnover by proteasome inhibition leads to activation of the intrinsic apoptotic pathway as shown by mitochondria dysfunction, production of reactive oxygen species, and endoplasmic reticulum stress, which have all been observed in response to these drugs [13][14][15][16][17][18][19].…”

The roles of the proteasome pathway in signal transduction and neurodegenerative diseases

“…The mechanisms by which mutant htt causes disease remain elusive but it has been hypothesized that the UPS is involved in many aspects of HD pathogenesis [81,82] . Htt is a substrate of the UPS and ubiquitylated mutant htt accumulates in cells [82,83] .…”

“…Htt is a substrate of the UPS and ubiquitylated mutant htt accumulates in cells [82,83] . Aggregates are a pathologic hallmark of HD and many other neurodegenerative diseases.…”