Small Heat-Shock Protein HSPB1 Mutants Stabilize Microtubules in Charcot-Marie-Tooth Neuropathy

Abstract

Mutations in the small heat shock protein HSPB1 (HSP27) are causative for Charcot-Marie-Tooth (CMT) neuropathy. We previously showed that a subset of these mutations displays higher chaperone activity and enhanced affinity to client proteins. We hypothesized that this excessive binding property might cause the HSPB1 mutant proteins to disturb the function of proteins essential for the maintenance or survival of peripheral neurons. In the present work, we explored this hypothesis further and compared the protein complexes formed by wild-type and mutant HSPB1. Tubulin came out as the most striking differential interacting protein, with hyperactive mutants binding more strongly to both tubulin and microtubules. This anomalous binding leads to a stabilization of the microtubule network in a microtubule-associated protein-like manner as reflected by resistance to cold depolymerization, faster network recovery after nocodazole treatment, and decreased rescue and catastrophe rates of individual microtubules. In a transgenic mouse model for mutant HSPB1 that recapitulates all features of CMT, we could confirm the enhanced interaction of mutant HSPB1 with tubulin. Increased stability of the microtubule network was also clear in neurons isolated from these mice. Since neuronal cells are particularly vulnerable to disturbances in microtubule dynamics, this mechanism might explain the neuron-specific CMT phenotype caused by HSPB1 mutations.

Small heat shock proteins in ageing and age-related diseases

“…In addition, overexpression of the HSPB1 R136W mutant isoform results in severe axonopathy, defective neurofilament cytoskeleton and accumulation of mitochondria along axons (Srivastava et al 2012). Moreover, the hyperactive HSPB1 mutants (R127W, S135F and R136W) have been reported to display enhanced affinity to tubulin and microtubule stabilization (Almeida-Souza et al 2011). Thus, the recapitulation of the CMT condition in mice supports the requirement for intact HSPB1 function for preserving cytoskeleton integrity in neurons.…”

“…The recent generation of mouse models has provided novel insights concerning the relation between HSP mutations and CMT. Overexpression of either mutant HSPB1 S135F or HSPB1 P182L isoforms in the neuronal system leads to pronounced motor defects and reduced muscle strength, as monitored through behavioural tests (d'Ydewalle et al 2011;Lee et al 2015). While there is support for aberrant myelination in the latter study using the HSPB1 S135F model, a decrease in the levels of a-tubulin acetylation seems to be a common denominator of the observed pathology.…”

“…While there is support for aberrant myelination in the latter study using the HSPB1 S135F model, a decrease in the levels of a-tubulin acetylation seems to be a common denominator of the observed pathology. Tubulin hypoacetylation is associated with impaired transport of mitochondria across axons and in neurites in HSPB1 S135F overexpressing animals, and this defect can be reversed upon administration of histone deacetylase inhibitors (d'Ydewalle et al 2011). In addition, overexpression of the HSPB1 R136W mutant isoform results in severe axonopathy, defective neurofilament cytoskeleton and accumulation of mitochondria along axons (Srivastava et al 2012).…”

Phosphorylation-dependent subcellular localization of the small heat shock proteins HspB1/Hsp25 and HspB5/αB-crystallin in cultured hippocampal neurons

“…It remains to be determined to which types of filaments they bind. pHspB1 likely binds to neurofilaments or microtubules since overexpression of various mutants of HspB1 have been shown to disrupt neurofilament assembly or to disturb microtubule dynamics (Ackerley et al 2006;Almeida-Souza et al 2011;Evgrafov et al 2004). …”

The functional roles of the unstructured N- and C-terminal regions in αB-crystallin and other mammalian small heat-shock proteins

“…A common theme emerges from these studies: the assembly and dynamics of all three major cytoskeletal elements in the cell are modulated by sHsps, including Hsp27, αBc, HspB7 and Hsp22. Whilst studies sometimes focus on one specific element of the cytoskeleton (Almeida-Souza et al 2011;Shimizu et al 2016), it is obvious that both the competence and the integration of the different elements of the cytoskeleton rely on sHsps.…”