A Charcot-Marie-Tooth-Causing Mutation in HSPB1 Decreases Cell Adaptation to Repeated Stress by Disrupting Autophagic Clearance of Misfolded Proteins

Abstract

Charcot-Marie-Tooth (CMT) disease is the most common inherited neurodegenerative disorder with selective degeneration of peripheral nerves. Despite advances in identifying CMT-causing genes, the underlying molecular mechanism, particularly of selective degeneration of peripheral neurons remains to be elucidated. Since peripheral neurons are sensitive to multiple stresses, we hypothesized that daily repeated stress might be an essential contributor to the selective degeneration of peripheral neurons induced by CMT-causing mutations. Here, we mainly focused on the biological effects of the dominant missense mutation (S135F) in the 27-kDa small heat-shock protein HSPB1 under repeated heat shock. HSPB1S135F presented hyperactive binding to both α-tubulin and acetylated α-tubulin during repeated heat shock when compared with the wild type. The aberrant interactions with tubulin prevented microtubule-based transport of heat shock-induced misfolded proteins for the formation of perinuclear aggresomes. Furthermore, the transport of autophagosomes along microtubules was also blocked. These results indicate that the autophagy pathway was disrupted, leading to an accumulation of ubiquitinated protein aggregates and a significant decrease in cell adaptation to repeated stress. Our findings provide novel insights into the molecular mechanisms of HSPB1S135F-induced selective degeneration of peripheral neurons and perspectives for targeting autophagy as a promising therapeutic strategy for CMT neuropathy.

Neuronal Autophagy: Regulations and Implications in Health and Disease

“…In CMT, the MT-based transport is damaged by the enhanced binding of mutant heat shock protein beta-1 (HSPB1) to tubulin [ 184 ]. The increased binding of mutant HSPB1 to α-tubulin has been shown to inhibit AVs and protein aggregate transport along MTs [ 185 ]. In addition, HSPB1 mutants have been found to interact with p62 [ 186 ], which, in addition to its role in AV formation, has been found to promote AV motility and trafficking along MTs via a direct interaction with dynein [ 152 ].…”

Functional Diversity of Mammalian Small Heat Shock Proteins: A Review

“…In contrast, the S135F missense mutant of HSPB1 has a higher affinity for SQSTM1/p62 and HSPB1 facilitates the formation of SQSTM1/p62 bodies by binding to SQSTM1/p62, which is essential for the nucleation of phagophores that are capable of elongating to become mature autophagosomes [25]. During autophagy, autophagosomes transport from the cytoplasm to the perinuclear region along MTs, ultimately reaching perinuclear lysosomes for degradation [113]. HSPB1 was demonstrated to favor the formation of non-centrosomal MTs, but its binding to MTs appears to be weak and transient [114].…”

“…HSPB1 was demonstrated to favor the formation of non-centrosomal MTs, but its binding to MTs appears to be weak and transient [114]. In contrast, the S135F missense mutant of HSPB1 has a higher affinity for SQSTM1/p62 and α-tubulin, which blocks the MT-dependent transportation of autophagosomes and has a negative effect on its function of promoting autophagy [25,113]. One study reported that sustained HSPB1 expression leads to ER stress, which subsequently activates the AMP-activated protein kinase 1 (AMPK1)/unc-51-like kinase 1 (ULK1) pathway [115].…”

“…On the other hand, mutations generally reduce the chaperone activity of sHSPs, resulting in the aggregation of substrate proteins [197]. Although some mutants, such as HSPB1 S135F, show higher binding affinity for client proteins, they are also impaired in their functions [25,113]. Interestingly, the activity of sHSPs is beneficial in the diseases mentioned above but counterproductive in cancer.…”