Aggregate-prone proteins with polyglutamine and polyalanine expansions are degraded by autophagy

Abstract

Protein conformational disorders (PCDs), such as Alzheimer's disease, Huntington's disease (HD), Parkinson's disease and oculopharyngeal muscular dystrophy, are associated with proteins that misfold and aggregate. Here we have used exon 1 of the HD gene with expanded polyglutamine [poly(Q)] repeats and enhanced green fluorescent protein tagged to 19 alanines as models for aggregate-prone proteins, to investigate the pathways mediating their degradation. Autophagy is involved in the degradation of these model proteins, since they accumulated when cells were treated with different inhibitors acting at distinct stages of the autophagy-lysosome pathway, in two different cell lines. Furthermore, rapamycin, which stimulates autophagy, enhanced the clearance of our aggregate-prone proteins. Rapamycin also reduced the appearance of aggregates and the cell death associated with the poly(Q) and polyalanine [poly(A)] expansions. Since rapamycin is used clinically, this drug or related analogues may be suitable candidates for therapeutic investigation in HD and related diseases. We have also re-examined the role of the proteasome, since previous studies in poly(Q) diseases have used lactacystin as an inhibitor--recent studies have shown that lactacystin may also affect lysosomal function. Both lactacystin and the specific proteasomal inhibitor epoxomicin increased soluble protein levels of the poly(Q) constructs, suggesting that these are also cleared by the proteasome. However, while poly(Q) aggregation was enhanced by lactacystin in our inducible PC12 cell model, aggregation was reduced by epoxomicin, suggesting that some other protein(s) induced by epoxomicin may regulate poly(Q) aggregation.

Impairing the bioenergetic status and the biogenesis of mitochondria triggers mitophagy in yeast

“…Autophagic response was already reported to be induced by misfolded proteins. 29,30 However, if the Dfmc1 model used here effectively accumulates a-F1 and b-F1 as large matricial aggregates under autophagy-promoting conditions 20,21 (heat stress and anaerobiosis), such aggregates also accumulate in cells grown under aerobiosis (Figure 2d), without induction of autophagy. Hence, protein misfolding is not the 'stressinducing' event of autophagy in our study.…”

“…29,30 Furthermore, in yeast, impairment of the Yme1p-mediated degradation of Cox2p subunits that are not correctly assembled in cytochrome c oxidase complex of the respiratory chain, leads to mitochondrial degradation by a poorly characterised process. 31,32 As already stated, Dfmc1 cells grown at nonpermissive temperature are known to accumulate a-F1 and b-F1 aggregates in the mitochondrial matrix.…”

The latent membrane protein 1 oncogene modifies B-cell physiology by regulating autophagy

“…The mechanism to downregulate LMP1 may be a substitute for withdrawal of ligand for liganddependent receptors such as CD40. It has been shown previously that aggregate-prone proteins with polyglutamine and polyalanine expansions are degraded by autophagy (Ravikumar et al, 2002). LMP1's induction of autophagy may facilitate the clearance of its own aggregates and allow cells to escape from direct immune recognition by CD4 þ T cells.…”

The pleiotropic role of autophagy: from protein metabolism to bactericide

“…16,17 Inhibition of autophagy induces aggregate formation, whereas rapamycin-induced upregulation of autophagy reduces it. [18][19][20][21] Furthermore, amelioration of neurodegeneration and improvement of behavior were reported in a D. melanogaster Huntington model following treatment with a rapamycin analog, CCI-779. 21 It has been speculated that aggregates can be degraded by autophagy Figure 2 Molecular mechanism of autophagy regulation and autophagosome formation.…”