Small heat shock proteins operate as molecular chaperones in the mitochondrial intermembrane space.

Mitochondria are complex organelles with different compartments, each harbouring their own protein quality control factors. While chaperones of the mitochondrial matrix are well characterized, it is poorly understood which chaperones protect the mitochondrial intermembrane space. Here we show that cytosolic small heat shock proteins are imported under basal conditions into the mitochondrial intermembrane space, where they operate as molecular chaperones. Protein misfolding in the mitochondrial intermembrane space leads to increased recruitment of small heat shock proteins. Depletion of small heat shock proteins leads to mitochondrial swelling and reduced respiration, while aggregation of aggregation-prone substrates is countered in their presence. Charcot-Marie-Tooth disease-causing mutations disturb the mitochondrial function of HSPB1, potentially linking previously observed mitochondrial dysfunction in Charcot-Marie-Tooth type 2F to its role in the mitochondrial intermembrane space. Our results reveal that small heat shock proteins form a chaperone system that operates in the mitochondrial intermembrane space.

NCAM1 and GDF15 are biomarkers of Charcot-Marie-Tooth disease in patients and mice.

Molecular markers scalable for clinical use are critical for the development of effective treatments and the design of clinical trials. Here, we identify proteins in sera of patients and mouse models with Charcot-Marie-Tooth disease (CMT) with characteristics that make them suitable as biomarkers in clinical practice and therapeutic trials. We collected serum from mouse models of CMT1A (C61 het), CMT2D (GarsC201R, GarsP278KY), CMT1X (Gjb1-null), CMT2L (Hspb8K141N) and from CMT patients with genotypes including CMT1A (PMP22d), CMT2D (GARS), CMT2N (AARS) and other rare genetic forms of CMT. The severity of neuropathy in the patients was assessed by the CMT Neuropathy Examination Score (CMTES). We performed multitargeted proteomics on both sample sets to identify proteins elevated across multiple mouse models and CMT patients. Selected proteins and additional potential biomarkers, such as growth differentiation factor 15 (GDF15) and cell free mitochondrial DNA, were validated by ELISA and quantitative PCR, respectively. We propose that neural cell adhesion molecule 1 (NCAM1) is a candidate biomarker for CMT, as it was elevated in Gjb1-null, Hspb8K141N, GarsC201R and GarsP278KY mice as well as in patients with both demyelinating (CMT1A) and axonal (CMT2D, CMT2N) forms of CMT. We show that NCAM1 may reflect disease severity, demonstrated by a progressive increase in mouse models with time and a significant positive correlation with CMTES neuropathy severity in patients. The increase in NCAM1 may reflect muscle regeneration triggered by denervation, which could potentially track disease progression or the effect of treatments. We found that member proteins of the complement system were elevated in Gjb1-null and Hspb8K141N mouse models as well as in patients with both demyelinating and axonal CMT, indicating possible complement activation at the impaired nerve terminals. However, complement proteins did not correlate with the severity of neuropathy measured on the CMTES scale. Although the complement system does not seem to be a prognostic biomarker, we do show complement elevation to be a common disease feature of CMT, which may be of interest as a therapeutic target. We also identify serum GDF15 as a highly sensitive diagnostic biomarker, which was elevated in all CMT genotypes as well as in Hspb8K141N, Gjb1-null, GarsC201R and GarsP278KY mouse models. Although we cannot fully explain its origin, it may reflect increased stress response or metabolic disturbances in CMT. Further large and longitudinal patient studies should be performed to establish the value of these proteins as diagnostic and prognostic molecular biomarkers for CMT.

Induced pluripotent stem cell-derived motor neurons of CMT type 2 patients reveal progressive mitochondrial dysfunction.

Axonal Charcot-Marie-Tooth neuropathies (CMT type 2) are caused by inherited mutations in various genes functioning in different pathways. The types of genes and multiplicity of mutations reflect the clinical and genetic heterogeneity in CMT2 disease, which complicates its diagnosis and has inhibited the development of therapies. Here, we used CMT2 patient-derived pluripotent stem cells (iPSCs) to identify common hallmarks of axonal degeneration shared by different CMT2 subtypes. We compared the cellular phenotypes of neurons differentiated from CMT2 patient iPSCs with those from healthy controls and a CRISPR/Cas9-corrected isogenic line. Our results demonstrated neurite network alterations along with extracellular electrophysiological abnormalities in the differentiated motor neurons. Progressive deficits in mitochondrial and lysosomal trafficking, as well as in mitochondrial morphology, were observed in all CMT2 patient lines. Differentiation of the same CMT2 iPSC lines into peripheral sensory neurons only gave rise to cellular phenotypes in subtypes with sensory involvement, supporting the notion that some gene mutations predominantly affect motor neurons. We revealed a common mitochondrial dysfunction in CMT2-derived motor neurons, supported by alterations in the expression pattern and oxidative phosphorylation, which could be recapitulated in the sciatic nerve tissue of a symptomatic mouse model. Inhibition of a dual leucine zipper kinase could partially ameliorate the mitochondrial disease phenotypes in CMT2 subtypes. Altogether, our data reveal shared cellular phenotypes across different CMT2 subtypes and suggests that targeting such common pathomechanisms could allow the development of a uniform treatment for CMT2.

Neuropathy-causing mutations in HSPB1 impair autophagy by disturbing the formation of SQSTM1/p62 bodies.

HSPB1 (heat shock protein family B [small] member 1) is a ubiquitously expressed molecular chaperone. Most mutations in HSPB1 cause axonal Charcot-Marie-Tooth neuropathy and/or distal hereditary motor neuropathy. In this study we show that mutations in HSPB1 lead to impairment of macroautophagic/autophagic flux. In HSPB1 knockout cells, we demonstrate that HSPB1 is necessary for autophagosome formation, which was rescued upon re-expression of HSPB1. Employing a label-free LC-MS/MS analysis on the various HSPB1 variants (wild type and mutants), we identified autophagy-specific interactors. We reveal that the wild-type HSPB1 protein binds to the autophagy receptor SQSTM1/p62 and that the PB1 domain of SQSTM1 is essential for this interaction. Mutations in HSPB1 lead to a decrease in the formation of SQSTM1/p62 bodies, and subsequent impairment of phagophore formation, suggesting a regulatory role for HSPB1 in autophagy via interaction with SQSTM1. Remarkably, autophagy deficits could also be confirmed in patient-derived motor neurons thereby indicating that the impairment of autophagy might be one of the pathomechanisms by which mutations in HSPB1 lead to peripheral neuropathy. Abbreviations: ACD: alpha-crystallin domain; ALS: amyotrophic lateral sclerosis; ATG14: autophagy related 14; BAG1/3: BCL2 associated athanogene 1/3; CMT: Charcot-Marie-Tooth; dHMN: distal hereditary motor neuropathy; GFP: green fluorescent protein; HSPA8: heat shock protein family A (Hsp70) member 8; HSPB1/6/8: heat shock protein family B (small) member 1/6/8; LIR: LC3-interacting region; LC3B: microtubule associated protein 1 light chain 3 beta; PB1: Phox and Bem1; SQSTM1: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; UBA: ubiquitin-associated; WIPI1: WD repeat domain, phosphoinositide interacting 1; WT: wild-type.

Sensory neuropathy-causing mutations in ATL3 affect ER-mitochondria contact sites and impair axonal mitochondrial distribution.

Axonopathies are neurodegenerative disorders caused by axonal degeneration, affecting predominantly the longest neurons. Several of these axonopathies are caused by genetic defects in proteins involved in the shaping and dynamics of the endoplasmic reticulum (ER); however, it is unclear how these defects impinge on neuronal survival. Given its central and widespread position within a cell, the ER is a pivotal player in inter-organelle communication. Here, we demonstrate that defects in the ER fusion protein ATL3, which were identified in patients suffering from hereditary sensory and autonomic neuropathy, result in an increased number of ER-mitochondria contact sites both in HeLa cells and in patient-derived fibroblasts. This increased contact is reflected in higher phospholipid metabolism, upregulated autophagy and augmented Ca2+ crosstalk between both organelles. Moreover, the mitochondria in these cells display lowered motility, and the number of axonal mitochondria in neurons expressing disease-causing mutations in ATL3 is strongly decreased. These results underscore the functional interdependence of subcellular organelles in health and disease and show that disorders caused by ER-shaping defects are more complex than previously assumed.

Sensory-Neuropathy-Causing Mutations in ATL3 Cause Aberrant ER Membrane Tethering.

The endoplasmic reticulum (ER) is a complex network of sheets and tubules that is continuously remodeled. The relevance of this membrane dynamics is underscored by the fact that mutations in atlastins (ATLs), the ER fusion proteins in mammals, cause neurodegeneration. How defects in this process disrupt neuronal homeostasis is unclear. Using electron microscopy (EM) volume reconstruction of transfected cells, neurons, and patient fibroblasts, we show that hereditary sensory and autonomic neuropathy (HSAN)-causing ATL3 mutants promote aberrant ER tethering hallmarked by bundles of laterally attached ER tubules. In vitro, these mutants cause excessive liposome tethering, recapitulating the results in cells. Moreover, ATL3 variants retain their dimerization-dependent GTPase activity but are unable to promote membrane fusion, suggesting a defect in an intermediate step of the ATL3 functional cycle. Our data show that the effects of ATL3 mutations on ER network organization go beyond a loss of fusion and shed light on neuropathies caused by atlastin defects.

A knock-in/knock-out mouse model of HSPB8-associated distal hereditary motor neuropathy and myopathy reveals toxic gain-of-function of mutant Hspb8.

Mutations in the small heat shock protein B8 gene (HSPB8/HSP22) have been associated with distal hereditary motor neuropathy, Charcot-Marie-Tooth disease, and recently distal myopathy. It is so far not clear how mutant HSPB8 induces the neuronal and muscular phenotypes and if a common pathogenesis lies behind these diseases. Growing evidence points towards a role of HSPB8 in chaperone-associated autophagy, which has been shown to be a determinant for the clearance of poly-glutamine aggregates in neurodegenerative diseases but also for the maintenance of skeletal muscle myofibrils. To test this hypothesis and better dissect the pathomechanism of mutant HSPB8, we generated a new transgenic mouse model leading to the expression of the mutant protein (knock-in lines) or the loss-of-function (functional knock-out lines) of the endogenous protein Hspb8. While the homozygous knock-in mice developed motor deficits associated with degeneration of peripheral nerves and severe muscle atrophy corroborating patient data, homozygous knock-out mice had locomotor performances equivalent to those of wild-type animals. The distal skeletal muscles of the post-symptomatic homozygous knock-in displayed Z-disk disorganisation, granulofilamentous material accumulation along with Hspb8, αB-crystallin (HSPB5/CRYAB), and desmin aggregates. The presence of the aggregates correlated with reduced markers of effective autophagy. The sciatic nerve of the homozygous knock-in mice was characterized by low autophagy potential in pre-symptomatic and Hspb8 aggregates in post-symptomatic animals. On the other hand, the sciatic nerve of the homozygous knock-out mice presented a normal morphology and their distal muscle displayed accumulation of abnormal mitochondria but intact myofiber and Z-line organisation. Our data, therefore, suggest that toxic gain-of-function of mutant Hspb8 aggregates is a major contributor to the peripheral neuropathy and the myopathy. In addition, mutant Hspb8 induces impairments in autophagy that may aggravate the phenotype.

Mutant HSPB1 causes loss of translational repression by binding to PCBP1, an RNA binding protein with a possible role in neurodegenerative disease.

The small heat shock protein HSPB1 (Hsp27) is an ubiquitously expressed molecular chaperone able to regulate various cellular functions like actin dynamics, oxidative stress regulation and anti-apoptosis. So far disease causing mutations in HSPB1 have been associated with neurodegenerative diseases such as distal hereditary motor neuropathy, Charcot-Marie-Tooth disease and amyotrophic lateral sclerosis. Most mutations in HSPB1 target its highly conserved α-crystallin domain, while other mutations affect the C- or N-terminal regions or its promotor. Mutations inside the α-crystallin domain have been shown to enhance the chaperone activity of HSPB1 and increase the binding to client proteins. However, the HSPB1-P182L mutation, located outside and downstream of the α-crystallin domain, behaves differently. This specific HSPB1 mutation results in a severe neuropathy phenotype affecting exclusively the motor neurons of the peripheral nervous system. We identified that the HSPB1-P182L mutant protein has a specifically increased interaction with the RNA binding protein poly(C)binding protein 1 (PCBP1) and results in a reduction of its translational repressive activity. RNA immunoprecipitation followed by RNA sequencing on mouse brain lead to the identification of PCBP1 mRNA targets. These targets contain larger 3'- and 5'-UTRs than average and are enriched in an RNA motif consisting of the CTCCTCCTCCTCC consensus sequence. Interestingly, next to the clear presence of neuronal transcripts among the identified PCBP1 targets we identified known genes associated with hereditary peripheral neuropathies and hereditary spastic paraplegias. We therefore conclude that HSPB1 can mediate translational repression through interaction with an RNA binding protein further supporting its role in neurodegenerative disease.

Characterization of New Transgenic Mouse Models for Two Charcot-Marie-Tooth-Causing HspB1 Mutations using the Rosa26 Locus.

BACKGROUND: Charcot-Marie-Tooth (CMT) and associated neuropathies, the most common inherited diseases of the peripheral nervous system, remain so far incurable. Three existing murine models of Charcot-Marie-Tooth type 2F (CMT2F) and/or distal hereditary motor neuropathy type IIb (dHMNIIb), caused by mutations in the small heat shock protein B1 gene (HSPB1/HSP27), partially recapitulate the hallmarks of peripheral neuropathy. Because these models overexpress the HSPB1 mutant proteins they differ from the patients' situation. OBJECTIVE: To overcome the possible bias induced by overexpression, we generated and characterized a transgenic model in which the wild type or mutant HSPB1 protein was expressed at a moderate, more physiologically relevant level. METHODS: We generated a new transgenic mouse model in which a human wild type (hHSPB1WT) or mutant (hHSPB1R127W; hHSPB1P182L) HSPB1 transgene was integrated in the mouse ROSA26 locus. The motor and sensory functions of the mice was assessed at 3, 6, 9, 12 and 18 month. RESULTS: However, the mice expressing the mutant hHSPB1 do not develop motor or sensory deficits and do not show any sign of axonal degeneration, even at late age. Quantitative PCR analyses reveal contrasting tissue-specific expression pattern for the endogenous mouse and exogenous human HSPB1 and show that the ratio of human HSPB1 to the endogenous mouse HspB1 is lower in the sciatic nerve and spinal cord compared to the brain. CONCLUSION: These results suggest that expressing the transgene at a physiological level using the ROSA26 locus may not be sufficient to model inherited peripheral neuropathies caused by mutation in HSPB1.

Molecular Chaperones in the Pathogenesis of Amyotrophic Lateral Sclerosis: The Role of HSPB1.

Genetic discoveries in amyotrophic lateral sclerosis (ALS) have a significant impact on deciphering molecular mechanisms of motor neuron degeneration but, despite recent advances, the etiology of most sporadic cases remains elusive. Several cellular mechanisms contribute to the motor neuron degeneration in ALS, including RNA metabolism, cellular interactions between neurons and nonneuronal cells, and seeding of misfolded protein with prion-like propagation. In this scenario, the importance of protein turnover and degradation in motor neuron homeostasis gained increased recognition. In this study, we evaluated the role of the candidate gene HSPB1, a molecular chaperone involved in several proteome-maintenance functions. In a cohort of 247 unrelated Italian ALS patients, we identified two variants (c.570G>C, p.Gln190His and c.610dupG, p.Ala204Glyfs* 6). Functional characterization of the p.Ala204Glyfs* 6 demonstrated that the mutant protein alters HSPB1 dynamic equilibrium, sequestering the wild-type protein in a stable dimer and resulting in a loss of chaperone-like activity. Our results underline the relevance of identifying rare but pathogenic variations in sporadic neurodegenerative diseases, suggesting a possible correlation between specific pathomechanisms linked to HSPB1 mutations and the associated neurological phenotype. Our study provides additional lines of evidence to support the involvement of HSPB1 in the pathogenesis of sporadic ALS.

Nlrp6 promotes recovery after peripheral nerve injury independently of inflammasomes.

BACKGROUND: NOD-like receptors (Nlrs) are key regulators of immune responses during infection and autoimmunity. A subset of Nlrs assembles inflammasomes, molecular platforms that are activated in response to endogenous danger and microbial ligands and that control release of interleukin (IL)-1ß and IL-18. However, their role in response to injury in the nervous system is less understood. METHODS: In this study, we investigated the expression profile of major inflammasome components in the peripheral nervous system (PNS) and explored the physiological role of different Nlrs upon acute nerve injury in mice. RESULTS: While in basal conditions, predominantly members of NOD-like receptor B (Nlrb) subfamily (NLR family, apoptosis inhibitory proteins (NAIPs)) and Nlrc subfamily (ICE-protease activating factor (IPAF)/NOD) are detected in the sciatic nerve, injury causes a shift towards expression of the Nlrp family. Sterile nerve injury also leads to an increase in expression of the Nlrb subfamily, while bacteria trigger expression of the Nlrc subfamily. Interestingly, loss of Nlrp6 led to strongly impaired nerve function upon nerve crush. Loss of the inflammasome adaptor apoptosis-associated speck-like protein containing a CARD (ASC) and effector caspase-1 and caspase-11 did not affect sciatic nerve function, suggesting that Nlrp6 contributed to recovery after peripheral nerve injury independently of inflammasomes. In line with this, we did not detect release of mature IL-1ß upon acute nerve injury despite potent induction of pro-IL-1ß and inflammasome components Nlrp3 and Nlrp1. However, Nlrp6 deficiency was associated with increased pro-inflammatory extracellular regulated MAP kinase (ERK) signaling, suggesting that hyperinflammation in the absence of Nlrp6 exacerbated peripheral nerve injury. CONCLUSIONS: Together, our observations suggest that Nlrp6 contributes to recovery from peripheral nerve injury by dampening inflammatory responses independently of IL-1ß and inflammasomes.

HSPB1 facilitates the formation of non-centrosomal microtubules.

The remodeling capacity of microtubules (MT) is essential for their proper function. In mammals, MTs are predominantly formed at the centrosome, but can also originate from non-centrosomal sites, a process that is still poorly understood. We here show that the small heat shock protein HSPB1 plays a role in the control of non-centrosomal MT formation. The HSPB1 expression level regulates the balance between centrosomal and non-centrosomal MTs. The HSPB1 protein can be detected specifically at sites of de novo forming non-centrosomal MTs, while it is absent from the centrosomes. In addition, we show that HSPB1 binds preferentially to the lattice of newly formed MTs in vitro, suggesting that its function occurs by stabilizing MT seeds. Our findings open new avenues for the understanding of the role of HSPB1 in the development, maintenance and protection of cells with specialized non-centrosomal MT arrays.

Charcot-Marie-Tooth causing HSPB1 mutations increase Cdk5-mediated phosphorylation of neurofilaments.

Mutations in the small heat shock protein HSPB1 (HSP27) are a cause of axonal Charcot-Marie-Tooth neuropathy (CMT2F) and distal hereditary motor neuropathy. To better understand the effect of mutations in HSPB1 on the neuronal cytoskeleton, we stably transduced neuronal cells with wild-type and mutant HSPB1 and investigated axonal transport of neurofilaments (NFs). We observed that mutant HSPB1 affected the binding of NFs to the anterograde motor protein kinesin, reducing anterograde transport of NFs. These deficits were associated with an increased phosphorylation of NFs and cyclin-dependent kinase Cdk5. As Cdk5 mediates NF phosphorylation, inhibition of Cdk5/p35 restored NF phosphorylation level, as well as NF binding to kinesin in mutant HSPB1 neuronal cells. Altogether, we demonstrate that HSPB1 mutations induce hyperphosphorylation of NFs through Cdk5 and reduce anterograde transport of NFs.

Molecular defects in the motor adaptor BICD2 cause proximal spinal muscular atrophy with autosomal-dominant inheritance.

The most common form of spinal muscular atrophy (SMA) is a recessive disorder caused by deleterious SMN1 mutations in 5q13, whereas the genetic etiologies of non-5q SMA are very heterogeneous and largely remain to be elucidated. In a Bulgarian family affected by autosomal-dominant proximal SMA, we performed genome-wide linkage analysis and whole-exome sequencing and found a heterozygous de novo c.320C>T (p.Ser107Leu) mutation in bicaudal D homolog 2 (Drosophila) (BICD2). Further analysis of BICD2 in a cohort of 119 individuals with non-5q SMA identified a second de novo BICD2 mutation, c.2321A>G (p.Glu774Gly), in a simplex case. Detailed clinical and electrophysiological investigations revealed that both families are affected by a very similar disease course, characterized by early childhood onset, predominant involvement of lower extremities, and very slow disease progression. The amino acid substitutions are located in two interaction domains of BICD2, an adaptor protein linking the dynein molecular motor with its cargo. Our immunoprecipitation and localization experiments in HeLa and SH-SY5Y cells and affected individuals' lymphoblasts demonstrated that p.Ser107Leu causes increased dynein binding and thus leads to accumulation of BICD2 at the microtubule-organizing complex and Golgi fragmentation. In addition, the altered protein had a reduced colocalization with RAB6A, a regulator of vesicle trafficking between the Golgi and the endoplasmic reticulum. The interaction between p.Glu744Gly altered BICD2 and RAB6A was impaired, which also led to their reduced colocalization. Our study identifies BICD2 mutations as a cause of non-5q linked SMA and highlights the importance of dynein-mediated motility in motor neuron function in humans.

Mutant HSPB8 causes protein aggregates and a reduced mitochondrial membrane potential in dermal fibroblasts from distal hereditary motor neuropathy patients.

Missense mutations in the small heat shock protein HSPB8 cause distal hereditary motor neuropathy (dHMN) and axonal Charcot-Marie-Tooth disease (CMT2L). We previously demonstrated that, despite the ubiquitous expression of HSPB8, motor neurons appear to be predominantly affected by HSPB8 mutations. Here, we studied the effect of mutant HSPB8 in primary fibroblast cultures derived from dHMN patients' skin biopsy. In early passage cultures, we observed in all patients' fibroblasts HSPB8 protein aggregates that were not detected in control cells. After applying heat shock stress on the patients' early passage cultured cells, the protein aggregates coalesced into larger formations, while in control cells a homogenous upregulation of HSPB8 protein expression was seen. We also found a reduction in the mitochondrial membrane potential in the early passage cultures. After three months in culture, the number of cells with aggregates had become indistinguishable from that in controls and the mitochondrial membrane potential had returned to normal. These results emphasize the possible drawbacks of using patients' non-neuronal cells to study neuropathological disease mechanisms.

Small heat-shock protein HSPB1 mutants stabilize microtubules in Charcot-Marie-Tooth neuropathy.

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.

Mutant HSPB8 causes motor neuron-specific neurite degeneration.

Missense mutations (K141N and K141E) in the alpha-crystallin domain of the small heat shock protein HSPB8 (HSP22) cause distal hereditary motor neuropathy (distal HMN) or Charcot-Marie-Tooth neuropathy type 2L (CMT2L). The mechanism through which mutant HSPB8 leads to a specific motor neuron disease phenotype is currently unknown. To address this question, we compared the effect of mutant HSPB8 in primary neuronal and glial cell cultures. In motor neurons, expression of both HSPB8 K141N and K141E mutations clearly resulted in neurite degeneration, as manifested by a reduction in number of neurites per cell, as well as in a reduction in average length of the neurites. Furthermore, expression of the K141E (and to a lesser extent, K141N) mutation also induced spheroids in the neurites. We did not detect any signs of apoptosis in motor neurons, showing that mutant HSPB8 resulted in neurite degeneration without inducing neuronal death. While overt in motor neurons, these phenotypes were only very mildly present in sensory neurons and completely absent in cortical neurons. Also glial cells did not show an altered phenotype upon expression of mutant HSPB8. These findings show that despite the ubiquitous presence of HSPB8, only motor neurons appear to be affected by the K141N and K141E mutations which explain the predominant motor neuron phenotype in distal HMN and CMT2L.

Increased monomerization of mutant HSPB1 leads to protein hyperactivity in Charcot-Marie-Tooth neuropathy.

Small heat shock proteins are molecular chaperones capable of maintaining denatured proteins in a folding-competent state. We have previously shown that missense mutations in the small heat shock protein HSPB1 (HSP27) cause distal hereditary motor neuropathy and axonal Charcot-Marie-Tooth disease. Here we investigated the biochemical consequences of HSPB1 mutations that are known to cause peripheral neuropathy. In contrast to other chaperonopathies, our results revealed that particular HSPB1 mutations presented higher chaperone activity compared with wild type. Hyperactivation of HSPB1 was accompanied by a change from its wild-type dimeric state to a monomer without dissociation of the 24-meric state. Purification of protein complexes from wild-type and HSPB1 mutants showed that the hyperactive isoforms also presented enhanced binding to client proteins. Furthermore, we show that the wild-type HSPB1 protein undergoes monomerization during heat-shock activation, strongly suggesting that the monomer is the active form of the HSPB1 protein.