Mutant HSPB1 Research Articles

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.

Pilot phenotype and natural history study of hereditary neuropathies caused by mutations in the HSPB1 gene

Mutations in HSPB1 are one of the commonest causes of distal Hereditary Motor Neuropathy (dHMN). Transgenic mouse models of the disease have identified HDAC6 inhibitors as promising treatments for the condition paving the way for human trials. A detailed phenotype and natural history study of HSPB1 neuropathy is therefore required in order to inform the duration and outcome measures of any future trials. Clinical and neurophysiological data and lower limb muscle MRI were collected both prospectively and retrospectively from patients with mutations in HSPB1. The natural history was assessed by recording the weighted Charcot–Marie–Tooth Examination Score (CMTES) at annual intervals in a subset of patients. 20 patients from 14 families were recruited into the study. The average age of onset was in the 4th decade. Patients presented with a length dependent neuropathy but with early ankle plantar flexion weakness. Neurophysiology confirmed a motor neuropathy but also showed sensory nerve involvement in most patients. Cross sectional muscle MRI revealed soleus and medial gastrocnemius fat infiltration as an early signature of mutant HSPB1 disease. In this study neither semi quantitative muscle MRI, the CMTES nor neurophysiology were able to detect disease progression in HSPB1 neuropathy over 1 or 2 years. Further studies are therefore required to identify a suitable biomarker before clinical trials in HSPB1 neuropathy can be undertaken.

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

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. 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. 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. 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. 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.

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-013-1133-6) contains supplementary material, which is available to authorized users.

Phenotype of Cardiomyopathy in Cardiac-specific Heat Shock Protein B8 K141N Transgenic Mouse

A K141N missense mutation in heat shock protein (HSP) B8, which belongs to the small HSP family, causes distal hereditary motor neuropathy, which is characterized by the formation of inclusion bodies in cells. Although the HSPB8 gene causes hereditary motor neuropathy, obvious expression of HSPB8 is also observed in other tissues, such as the heart. The effects of a single mutation in HSPB8 upon the heart were analyzed using rat neonatal cardiomyocytes. Expression of HSPB8 K141N by adenoviral infection resulted in increased HSPB8-positive aggregates around nuclei, whereas no aggregates were observed in myocytes expressing wild-type HSPB8. HSPB8-positive aggresomes contained amyloid oligomer intermediates that were detected by a specific anti-oligomer antibody (A11). Expression of HSPB8 K141N induced slight cellular toxicity. Recombinant HSPB8 K141N protein showed reactivity against the anti-oligomer antibody, and reactivity of the mutant HSPB8 protein was much higher than that of wild-type HSPB8 protein. To extend our in vitro study, cardiac-specific HSPB8 K141N transgenic (TG) mice were generated. Echocardiography revealed that the HSPB8 K141N TG mice exhibited mild hypertrophy and apical fibrosis as well as slightly reduced cardiac function, although no phenotype was detected in wild-type HSPB8 TG mice. A single point mutation of HSPB8, such as K141N, can cause cardiac disease.

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.

Mutant HSPB1 overexpression in neurons is sufficient to cause age-related motor neuronopathy in mice

The small heat shock protein HSPB1 is a multifunctional, α-crystallin-based protein that has been shown to be neuroprotective in animal models of motor neuron disease and peripheral nerve injury. Missense mutations in HSPB1 result in axonal Charcot–Marie–Tooth disease with minimal sensory involvement (CMT2F) and distal hereditary motor neuropathy type 2 (dHMN-II). These disorders are characterized by a selective loss of motor axons in peripheral nerve resulting in distal muscle weakness and often severe disability. To investigate the pathogenic mechanisms of HSPB1 mutations in motor neurons in vivo, we have developed and characterized transgenic PrP-HSPB1 and PrP-HSPB1(R136W) mice. These mice express the human HSPB1 protein throughout the nervous system including in axons of peripheral nerve. Although both mouse strains lacked obvious motor deficits, the PrP-HSPB1(R136W) mice developed an age-dependent motor axonopathy. Mutant mice showed axonal pathology in spinal cord and peripheral nerve with evidence of impaired neurofilament cytoskeleton, associated with organelle accumulation. Accompanying these findings, increases in the number of Schmidt–Lanterman incisures, as evidence of impaired axon–Schwann cell interactions, were present. These observations suggest that overexpression of HSPB1(R136W) in neurons is sufficient to cause pathological and electrophysiological changes in mice that are seen in patients with hereditary motor neuropathy.

HspB8 mutation causing hereditary distal motor neuropathy impairs lysosomal delivery of autophagosomes

HspB8, a small heat-shock protein implicated in autophagy, is mutated in patients with distal hereditary motor neuropathy type II (dHMNII). Autophagy is essential for maintaining protein homeostasis in the central nervous system, but its role has not been investigated in peripheral motor neurons. We used a novel, multispectral-imaging flow cytometry assay to measure autophagy in cells. This assay revealed that over-expression of wild-type HspB8 in motor neuron-like NSC34 cells led to an increased co-localisation of autophagosomes with the lysosomes. By contrast, over-expression of mutant HspB8 resulted in autophagosomes that co-localised with protein aggregates but failed to co-localise with the lysosomes. A similar impairment of autophagy could also be demonstrated in peripheral blood mononuclear cells from two dHMNII patients with the HspB8(K141E) mutation. We conclude that defects in HspB8-mediated autophagy are likely to contribute to dHMNII pathology and their detection in peripheral blood mononuclear cells could be a useful, accessible biomarker for the disease.

Microtubule dynamics in the peripheral nervous system

The special architecture of neurons in the peripheral nervous system, with axons extending for long distances, represents a major challenge for the intracellular transport system. Two recent studies show that mutations in the small heat shock protein HSPB1, which cause an axonal type of Charcot-Marie-Tooth (CMT) neuropathy, affect microtubule dynamics and impede axonal transport. Intriguingly, while at presymptomatic age the neurons in the mutant HSPB1 mouse show a hyperstable microtubule network, at postsymptomatic age, the microtubule network completely lost its stability as reflected by a marked decrease in tubulin acetylation levels. We here propose a model explaining the role of microtubule stabilization and tubulin acetylation in the pathogenesis of HSPB1 mutations.

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.

HDAC6 inhibitors reverse axonal loss in a mouse model of mutant HSPB1–induced Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is the most common inherited disorder of the peripheral nervous system. Mutations in the 27-kDa small heat-shock protein gene (HSPB1) cause axonal CMT or distal hereditary motor neuropathy (distal HMN). We developed and characterized transgenic mice expressing two different HSPB1 mutations (S135F and P182L) in neurons only. These mice showed all features of CMT or distal HMN dependent on the mutation. Expression of mutant HSPB1 decreased acetylated α-tubulin abundance and induced severe axonal transport deficits. An increase of α-tubulin acetylation induced by pharmacological inhibition of histone deacetylase 6 (HDAC6) corrected the axonal transport defects caused by HSPB1 mutations and rescued the CMT phenotype of symptomatic mutant HSPB1 mice. Our findings demonstrate the pathogenic role of α-tubulin deacetylation in mutant HSPB1-induced neuropathies and offer perspectives for using HDAC6 inhibitors as a therapeutic strategy for hereditary axonopathies.

Mutant HSPB8 causes motor neuron-specific neurite degeneration

Missense mutations (K141N and K141E) in the α-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.

Abnormal interaction of motor neuropathy-associated mutant HspB8 (Hsp22) forms with the RNA helicase Ddx20 (gemin3)

A number of missense mutations in the two related small heat shock proteins HspB8 (Hsp22) and HspB1 (Hsp27) have been associated with the inherited motor neuron diseases (MND) distal hereditary motor neuropathy and Charcot-Marie-Tooth disease. HspB8 and HspB1 interact with each other, suggesting that these two etiologic factors may act through a common biochemical mechanism. However, their role in neuron biology and in MND is not understood. In a yeast two-hybrid screen, we identified the DEAD box protein Ddx20 (gemin3, DP103) as interacting partner of HspB8. Using co-immunoprecipitation, chemical cross-linking, and in vivo quantitative fluorescence resonance energy transfer, we confirmed this interaction. We also show that the two disease-associated mutant HspB8 forms have abnormally increased binding to Ddx20. Ddx20 itself binds to the survival-of-motor-neurons protein (SMN protein), and mutations in the SMN1 gene cause spinal muscular atrophy, another MND and one of the most prevalent genetic causes of infant mortality. Thus, these protein interaction data have linked the three etiologic factors HspB8, HspB1, and SMN protein, and mutations in any of their genes cause the various forms of MND. Ddx20 and SMN protein are involved in spliceosome assembly and pre-mRNA processing. RNase treatment affected the interaction of the mutant HspB8 with Ddx20 suggesting RNA involvement in this interaction and a potential role of HspB8 in ribonucleoprotein processing.

Asymmetrical late onset motor neuropathy associated with a novel mutation in the small heat shock protein HSPB1 (HSP27)

Distal hereditary motor neuropathy, also known as distal spinal muscular atrophy, is characterised by slowly progressive weakness and wasting of the hands and feet and has a heterogeneous genetic basis....

A mutation in the small heat-shock protein HSPB1 leading to distal hereditary motor neuronopathy disrupts neurofilament assembly and the axonal transport of specific cellular cargoes

Distal hereditary motor neuronopathies (dHMNs) are a clinically and genetically heterogeneous group of disorders in which motor neurons selectively undergo age-dependant degeneration. Mutations in the small heat-shock protein HSPB1 (HSP27) are responsible for one form of dHMN. In this study, we have analysed the effect of expressing a form of mutant HSPB1 in primary neuronal cells in culture. Mutant (P182L) but not wild-type HSPB1 led to the formation of insoluble intracellular aggregates and to the sequestration in the cytoplasm of selective cellular components, including neurofilament middle chain subunit (NF-M) and p150 dynactin. These findings suggest a possible pathogenic mechanism for HSPB1 whereby the mutation may lead to preferential motor neuron loss by disrupting selective components essential for axonal structure and transport.

Hot-spot residue in small heat-shock protein 22 causes distal motor neuropathy.

Distal hereditary motor neuropathies are pure motor disorders of the peripheral nervous system resulting in severe atrophy and wasting of distal limb muscles. In two pedigrees with distal hereditary motor neuropathy type II linked to chromosome 12q24.3, we identified the same mutation (K141N) in small heat-shock 22-kDa protein 8 (encoded by HSPB8; also called HSP22). We found a second mutation (K141E) in two smaller families. Both mutations target the same amino acid, which is essential to the structural and functional integrity of the small heat-shock protein alphaA-crystallin. This positively charged residue, when mutated in other small heat-shock proteins, results in various human disorders. Coimmunoprecipitation experiments showed greater binding of both HSPB8 mutants to the interacting partner HSPB1. Expression of mutant HSPB8 in cultured cells promoted formation of intracellular aggregates. Our findings provide further evidence that mutations in heat-shock proteins have an important role in neurodegenerative disorders.

Mutant small heat-shock protein 27 causes axonal Charcot-Marie-Tooth disease and distal hereditary motor neuropathy.

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuromuscular disease and is characterized by considerable clinical and genetic heterogeneity. We previously reported a Russian family with autosomal dominant axonal CMT and assigned the locus underlying the disease (CMT2F; OMIM 606595) to chromosome 7q11-q21 (ref. 2). Here we report a missense mutation in the gene encoding 27-kDa small heat-shock protein B1 (HSPB1, also called HSP27) that segregates in the family with CMT2F. Screening for mutations in HSPB1 in 301 individuals with CMT and 115 individuals with distal hereditary motor neuropathies (distal HMNs) confirmed the previously observed mutation and identified four additional missense mutations. We observed the additional HSPB1 mutations in four families with distal HMN and in one individual with CMT neuropathy. Four mutations are located in the Hsp20-alpha-crystallin domain, and one mutation is in the C-terminal part of the HSP27 protein. Neuronal cells transfected with mutated HSPB1 were less viable than cells expressing the wild-type protein. Cotransfection of neurofilament light chain (NEFL) and mutant HSPB1 resulted in altered neurofilament assembly in cells devoid of cytoplasmic intermediate filaments.