Folding Landscape of Mutant Huntingtin Exon1: Diffusible Multimers, Oligomers and Fibrils, and No Detectable Monomer.
Expansion of the polyglutamine (polyQ) track of the Huntingtin (HTT) protein above 36 is associated with a sharply enhanced risk of Huntington's disease (HD). Although there is general agreement that HTT toxicity resides primarily in N-terminal fragments such as the HTT exon1 protein, there is no consensus on the nature of the physical states of HTT exon1 that are induced by polyQ expansion, nor on which of these states might be responsible for toxicity. One hypothesis is that polyQ expansion induces an alternative, toxic conformation in the HTT exon1 monomer. Alternative hypotheses posit that the toxic species is one of several possible aggregated states. Defining the nature of the toxic species is particularly challenging because of facile interconversion between physical states as well as challenges to identifying these states, especially in vivo. Here we describe the use of fluorescence correlation spectroscopy (FCS) to characterize the detailed time and repeat length dependent self-association of HTT exon1-like fragments both with chemically synthesized peptides in vitro and with cell-produced proteins in extracts and in living cells. We find that, in vitro, mutant HTT exon1 peptides engage in polyQ repeat length dependent dimer and tetramer formation, followed by time dependent formation of diffusible spherical and fibrillar oligomers and finally by larger, sedimentable amyloid fibrils. For expanded polyQ HTT exon1 expressed in PC12 cells, monomers are absent, with tetramers being the smallest molecular form detected, followed in the incubation time course by small, diffusible aggregates at 6-9 hours and larger, sedimentable aggregates that begin to build up at 12 hrs. In these cell cultures, significant nuclear DNA damage appears by 6 hours, followed at later times by caspase 3 induction, mitochondrial dysfunction, and cell death. Our data thus defines limits on the sizes and concentrations of different physical states of HTT exon1 along the reaction profile in the context of emerging cellular distress. The data provide some new candidates for the toxic species and some new reservations about more well-established candidates. Compared to other known markers of HTT toxicity, nuclear DNA damage appears to be a relatively early pathological event.
Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
Bankanidhi
B
Sahoo
Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
Irene
I
Arduini
Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
Kenneth W
KW
Drombosky
Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
Ravindra
R
Kodali
Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Department of Neurology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
Laurie H
LH
Sanders
Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Department of Neurology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
J Timothy
JT
Greenamyre
Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.|Pittsburgh Institute for Neurodegenerative Diseases, University of Pittsburgh School of Medicine, Pittsburgh, PA, 15260, United States of America.
Ronald
R
Wetzel
D000682:Amyloid / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D000818:Animals; D002470:Cell Survival / Q000235:genetics; D004249:DNA Damage / Q000235:genetics; D000071058:Huntingtin Protein / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D050505:Mutant Proteins / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D009154:Mutation / Q000502:physiology; D016716:PC12 Cells; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010455:Peptides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017510:Protein Folding; D055503:Protein Multimerization; D051381:Rats
D000682:Amyloid; D000071058:Huntingtin Protein; D050505:Mutant Proteins; D010446:Peptide Fragments; D010455:Peptides; C097188:polyglutamine