Applying Hsp104 to protein-misfolding disorders

Abstract

Hsp104, a hexameric AAA+ ATPase found in yeast, transduces energy from cycles of ATP binding and hydrolysis to resolve disordered protein aggregates and cross-beta amyloid conformers. These disaggregation activities are often co-ordinated by the Hsp70 chaperone system and confer considerable selective advantages. First, renaturation of aggregated conformers by Hsp104 is critical for yeast survival after various environmental stresses. Second, amyloid remodeling by Hsp104 enables yeast to exploit multifarious prions as a reservoir of beneficial and heritable phenotypic variation. Curiously, although highly conserved in plants, fungi and bacteria, Hsp104 orthologues are absent from metazoa. Indeed, metazoan proteostasis seems devoid of a system that couples protein disaggregation to renaturation. Here, we review recent endeavors to enhance metazoan proteostasis by applying Hsp104 to the specific protein-misfolding events that underpin two deadly neurodegenerative amyloidoses. Hsp104 potently inhibits Abeta42 amyloidogenesis, which is connected with Alzheimer's disease, but appears unable to disaggregate preformed Abeta42 fibers. By contrast, Hsp104 inhibits and reverses the formation of alpha-synuclein oligomers and fibers, which are connected to Parkinson's disease. Importantly, Hsp104 antagonizes the degeneration of dopaminergic neurons induced by alpha-synuclein misfolding in the rat substantia nigra. These studies raise hopes for developing Hsp104 as a therapeutic agent.

Fig. 1

Protein disaggregation by Hsp104. Hsp104 couples energy from ATP binding and hydrolysis to dissolve denatured protein aggregates and amyloid fibers. Hsp70 and Hsp40 co-ordinate the disaggregation of denatured protein aggregates, whereas they are less essential for the dissolution of amyloid structure. Note the 3-tiered structure of Hsp104, and the substrate-binding Tyr motifs that project into the central channel (denoted by small Y-shapes).

Fig. 2

Co-ordination of Hsp104 ATPase activity is required for protein disaggregation. (a) In the presence of ATP, Hsp104 engages denatured aggregates too transiently to initiate disaggregation. (b) In the presence of specific mixtures of ATP and ATPγS (e.g., 3 ATP : 1 ATPγS), Hsp104 is able to productively couple substrate binding to translocation and disaggregation. Only specific ratios of ATP and ATPγS are effective, and pure ATPγS inhibits activity (Doyle et al. 2007b). (c) In the presence of ATP, specific Hsp104 mutants with reduced ATPase activity at NBD2, such as N728A or K620T, can productively couple substrate binding to translocation and disaggregation (Doyle et al. 2007b). Despite this innate ability to disaggregate some disordered aggregates, these mutants fail to disaggregate amyloid and are unable to synergize with the Hsp70 chaperone system and are defective in vivo (Doyle et al. 2007b; Hattendorf and Lindquist 2002; Shorter and Lindquist 2004). (d) In the presence of ATP, Hsp70 and Hsp40 ensure productive substrate binding by Hsp104, which leads to translocation and disaggregation. (e) In the presence of ATP, Hsp104 is able to bind and disaggregate amyloid substrates.