Defective recognition of LC3B by mutant SQSTM1/p62 implicates impairment of autophagy as a pathogenic mechanism in ALS-FTLD
Oldham, Neil J.
Searle, Mark S.
- Publisher: Taylor & Francis
(issn: 1554-8627, vol:
VDP::Medisinske Fag: 700::Basale medisinske, odontologiske og veterinærmedisinske fag: 710::Medisinsk mikrobiologi: 715 | autophagy | LIR | Atg8/LC3 | FTLD | SQSTM1/p62 | ALS | VDP::Medical disciplines: 700::Basic medical, dental and veterinary science disciplines: 710::Medical microbiology: 715 | Basic Research Paper
Published version. Source at <a href=http://dx.doi.org/10.1080/15548627.2016.1170257> http://dx.doi.org/10.1080/15548627.2016.1170257 </a>
Growing evidence implicates impairment of autophagy as a candidate pathogenic mechanism in the
spectrum of neurodegenerative disorders which includes amyotrophic lateral sclerosis and frontotemporal
lobar degeneration (ALS-FTLD). SQSTM1, which encodes the autophagy receptor SQSTM1/p62, is genetically
associated with ALS-FTLD, although to date autophagy-relevant functional defects in disease-associated
variants have not been described. A key protein-protein interaction in autophagy is the recognition of a lipidanchored
form of LC3 (LC3-II) within the phagophore membrane by SQSTM1, mediated through its LC3-
interacting region (LIR), and notably some ALS-FTLD mutations map to this region. Here we show that
although representing a conservative substitution and predicted to be benign, the ALS-associated L341V
mutation of SQSTM1 is defective in recognition of LC3B. We place our observations on a firm quantitative
footing by showing the L341V-mutant LIR is associated with a »3-fold reduction in LC3B binding affinity and
using protein NMR we rationalize the structural basis for the effect. This functional deficit is realized in motor
neuron-like cells, with the L341V mutant EGFP-mCherry-SQSTM1 less readily incorporated into acidic
autophagic vesicles than the wild type. Our data supports a model in which the L341V mutation limits the
critical step of SQSTM1 recruitment to the phagophore. The oligomeric nature of SQSTM1, which presents
multiple LIRs to template growth of the phagophore, potentially gives rise to avidity effects which amplify
the relatively modest impact of any single mutation on LC3B binding. Over the lifetime of a neuron, impaired
autophagy could expose a vulnerability, which ultimately tips the balance from cell survival toward cell death.