PMP22 accumulation in aggresomes: implications for CMT1A pathology

Molecular mechanisms and therapeutic strategies for neuromuscular diseases

Neuromuscular diseases encompass a heterogeneous array of disorders characterized by varying onset ages, clinical presentations, severity, and progression. While these conditions can stem from acquired or inherited causes, this review specifically focuses on disorders arising from genetic abnormalities, excluding metabolic conditions. The pathogenic defect may primarily affect the anterior horn cells, the axonal or myelin component of peripheral nerves, the neuromuscular junction, or skeletal and/or cardiac muscles. While inherited neuromuscular disorders have been historically deemed not treatable, the advent of gene-based and molecular therapies is reshaping the treatment landscape for this group of condition. With the caveat that many products still fail to translate the positive results obtained in pre-clinical models to humans, both the technological development (e.g., implementation of tissue-specific vectors) as well as advances on the knowledge of pathogenetic mechanisms form a collective foundation for potentially curative approaches to these debilitating conditions. This review delineates the current panorama of therapies targeting the most prevalent forms of inherited neuromuscular diseases, emphasizing approved treatments and those already undergoing human testing, offering insights into the state-of-the-art interventions.

Clinical practice and outcomes of preimplantation genetic testing for CMT1A using a novel direct detection method

Background

Charcot-Marie-Tooth type 1A (CMT1A), the most frequent type of Charcot-Marie-Tooth disease, is mainly caused by a 1.4-Mb duplication containing the PMP22 gene. There is no effective treatment other than general supportive care and symptomatic treatment. Preimplantation genetic testing for monogenic defects (PGT-M) is an alternative approach for obtaining healthy babies.

Methods

A new technology and analysis method based on next-generation sequencing (NGS) was developed to detect duplication mutations directly. Simultaneously, aneuploidy and linkage analyses were performed to achieve a comprehensive and accurate embryo diagnosis.

Results

Eight couples were recruited in this study; PMP22 duplication was validated in seven couples, and PMP22 splicing mutation was found in one. Forty-five embryos from 12 PGT cycles were successfully detected using this novel method. The direct detection results for all embryos were consistent with the linkage analyses, suggesting a 100 % accuracy rate, and the aneuploidy rate of the biopsied blastocysts was 33.3 %. Eventually, 18 of the 45 diagnosed embryos were deemed suitable for transfer. Four healthy babies from three families were delivered and their genetic status confirmed by amniocentesis. Additionally, there were no adverse effects of anesthesia or increased pregnancy complications during PGT-M in female patients with CMT1A.

Conclusions

This study provided a simple, reliable, and efficient method that can directly detect PMP22 mutations based on NGS data and does not require positive family members. A clinical workflow for CMT1A interruption in the offspring before embryo implantation is also summarized.

Proteostasis plays an important role in demyelinating Charcot Marie Tooth disease

Type 1 Charcot-Marie-Tooth disease (CMT1) is the most common demyelinating peripheral neuropathy. Patients suffer from progressive muscle weakness and sensory problems. The underlying disease mechanisms of CMT1 are still unclear and no therapy is currently available, hence patients completely rely on supportive care. Balancing protein levels is a complex multistep process fundamental to maintain cells in their healthy state and a disrupted proteostasis is a hallmark of several neurodegenerative diseases. When protein misfolding occurs, protein quality control systems are activated such as chaperones, the lysosomal-autophagy system and proteasomal degradation to ensure proper degradation. However, in pathological circumstances, these mechanisms are overloaded and thereby become inefficient to clear the load of misfolded proteins. Recent evidence strongly indicates that a disbalance in proteostasis plays an important role in several forms of CMT1. In this review, we present an overview of the protein quality control systems, their role in CMT1, and potential treatment strategies to restore proteostasis.

TGFβ4 alleviates the phenotype of Charcot-Marie-Tooth disease type 1A

The duplication of the peripheral myelin protein 22 (PMP22) gene causes a demyelinating type of neuropathy, commonly known as Charcot-Marie-Tooth disease type 1A (CMT1A). Development of effective drugs for CMT1A still remains as an unmet medical need. In the present study, we assessed the role of the transforming growth factor beta 4 (TGFβ4)/Nodal axis in the pathogenesis of CMT1A. First, we identified PMP22 overexpression-induced Nodal expression in Schwann cells, which might be one of the downstream effectors in CMT1A. Administration of Nodal protein at the developmental stage of peripheral nerves induced the demyelinating phenotype in vivo. Second, we further isolated TGFβ4 as an antagonist that could abolish Nodal-induced demyelination. Finally, we developed a recombinant TGFβ4-fragment crystallizable (Fc) fusion protein, CX201, and demonstrated that its application had promyelinating efficacy in Schwann cells. CX201 administration improved the demyelinating phenotypes of CMT1A mouse models at both pre-symptomatic and post-symptomatic stages. These results suggest that the TGFβ4/Nodal axis plays a crucial role in the pathogenesis of CMT1A and might be a potential therapeutic target for CMT1A.

CMT1A current gene therapy approaches and promising biomarkers

Charcot-Marie-Tooth neuropathies (CMT) constitute a group of common but highly heterogeneous, non-syndromic genetic disorders affecting predominantly the peripheral nervous system. CMT type 1A (CMT1A) is the most frequent type and accounts for almost ~50% of all diagnosed CMT cases. CMT1A results from the duplication of the peripheral myelin protein 22 (PMP22) gene. Overexpression of PMP22 protein overloads the protein folding apparatus in Schwann cells and activates the unfolded protein response. This leads to Schwann cell apoptosis, dys- and de- myelination and secondary axonal degeneration, ultimately causing neurological disabilities. During the last decades, several different gene therapies have been developed to treat CMT1A. Almost all of them remain at the pre-clinical stage using CMT1A animal models overexpressing PMP22. The therapeutic goal is to achieve gene silencing, directly or indirectly, thereby reversing the CMT1A genetic mechanism allowing the recovery of myelination and prevention of axonal loss. As promising treatments are rapidly emerging, treatment-responsive and clinically relevant biomarkers are becoming necessary. These biomarkers and sensitive clinical evaluation tools will facilitate the design and successful completion of future clinical trials for CMT1A.

Electroceutical approach ameliorates intracellular PMP22 aggregation and promotes pro-myelinating pathways in a CMT1A in vitro model

Charcot-Marie-Tooth disease subtype 1A (CMT1A) is one of the most prevalent demyelinating peripheral neuropathies worldwide, caused by duplication of the peripheral myelin protein 22 (PMP22) gene, which is expressed primarily in Schwann cells (SCs). PMP22 overexpression in SCs leads to intracellular aggregation of the protein, which eventually results in demyelination. Unfortunately, previous biochemical approaches have not resulted in an approved treatment for CMT1A disease, compelling the pursuit for a biophysical approach such as electrical stimulation (ES). However, the effects of ES on CMT1A SCs have remained unexplored. In this study, we established PMP22-overexpressed Schwannoma cells as a CMT1A in vitro model, and investigated the biomolecular changes upon applying ES via a custom-made high-throughput ES platform, screening for the condition that delivers optimal therapeutic effects. While PMP22-overexpressed Schwannoma exhibited intracellular PMP22 aggregation, ES at 20 Hz for 1 h improved this phenomenon, bringing PMP22 distribution closer to healthy condition. ES at this condition also enhanced the expression of the genes encoding myelin basic protein (MBP) and myelin-associated glycoprotein (MAG), which are essential for assembling myelin sheath. Furthermore, ES altered the gene expression for myelination-regulating transcription factors Krox-20, Oct-6, c-Jun and Sox10, inducing pro-myelinating effects in PMP22-overexpressed Schwannoma. While electroceuticals has previously been applied in the peripheral nervous system towards acquired peripheral neuropathies such as pain and nerve injury, this study demonstrates its effectiveness towards ameliorating biomolecular abnormalities in an in vitro model of CMT1A, an inherited peripheral neuropathy. These findings will facilitate the clinical translation of an electroceutical treatment for CMT1A.

Implantable Electroceutical Approach Improves Myelination by Restoring Membrane Integrity in a Mouse Model of Peripheral Demyelinating Neuropathy

Although many efforts are undertaken to treat peripheral demyelinating neuropathies based on biochemical interventions, unfortunately, there is no approved treatment yet. Furthermore, previous studies have not shown improvement of the myelin membrane at the biomolecular level. Here, an electroceutical treatment is introduced as a biophysical intervention to treat Charcot-Marie-Tooth (CMT) disease-the most prevalent peripheral demyelinating neuropathy worldwide-using a mouse model. The specific electrical stimulation (ES) condition (50 mV mm^-1 , 20 Hz, 1 h) for optimal myelination is found via an in vitro ES screening system, and its promyelinating effect is validated with ex vivo dorsal root ganglion model. Biomolecular investigation via time-of-flight secondary ion mass spectrometry shows that ES ameliorates distribution abnormalities of peripheral myelin protein 22 and cholesterol in the myelin membrane, revealing the restoration of myelin membrane integrity. ES intervention in vivo via flexible implantable electrodes shows not only gradual rehabilitation of mouse behavioral phenotypes (balance and endurance), but also restored myelin thickness, compactness, and membrane integrity. This study demonstrates, for the first time, that an electroceutical approach with the optimal ES condition has the potential to treat CMT disease and restore impaired myelin membrane integrity, shifting the paradigm toward practical interventions for peripheral demyelinating neuropathies.

A translatable RNAi-driven gene therapy silences PMP22/Pmp22 genes and improves neuropathy in CMT1A mice

Charcot-Marie-Tooth disease type 1A (CMT1A), the most common inherited demyelinating peripheral neuropathy, is caused by PMP22 gene duplication. Overexpression of WT PMP22 in Schwann cells destabilizes the myelin sheath, leading to demyelination and ultimately to secondary axonal loss and disability. No treatments currently exist that modify the disease course. The most direct route to CMT1A therapy will involve reducing PMP22 to normal levels. To accomplish this, we developed a gene therapy strategy to reduce PMP22 using artificial miRNAs targeting human PMP22 and mouse Pmp22 mRNAs. Our lead therapeutic miRNA, miR871, was packaged into an adeno-associated virus 9 (AAV9) vector and delivered by lumbar intrathecal injection into C61-het mice, a model of CMT1A. AAV9-miR871 efficiently transduced Schwann cells in C61-het peripheral nerves and reduced human and mouse PMP22 mRNA and protein levels. Treatment at early and late stages of the disease significantly improved multiple functional outcome measures and nerve conduction velocities. Furthermore, myelin pathology in lumbar roots and femoral motor nerves was ameliorated. The treated mice also showed reductions in circulating biomarkers of CMT1A. Taken together, our data demonstrate that AAV9-miR871–driven silencing of PMP22 rescues a CMT1A model and provides proof of principle for treating CMT1A using a translatable gene therapy approach.

Treatment with IFB-088 Improves Neuropathy in CMT1A and CMT1B Mice

Charcot-Marie-Tooth disease type 1A (CMT1A), caused by duplication of the peripheral myelin protein 22 (PMP22) gene, and CMT1B, caused by mutations in myelin protein zero (MPZ) gene, are the two most common forms of demyelinating CMT (CMT1), and no treatments are available for either. Prior studies of the MpzSer63del mouse model of CMT1B have demonstrated that protein misfolding, endoplasmic reticulum (ER) retention and activation of the unfolded protein response (UPR) contributed to the neuropathy. Heterozygous patients with an arginine to cysteine mutation in MPZ (MPZR98C) develop a severe infantile form of CMT1B which is modelled by MpzR98C/ + mice that also show ER stress and an activated UPR. C3-PMP22 mice are considered to effectively model CMT1A. Altered proteostasis, ER stress and activation of the UPR have been demonstrated in mice carrying Pmp22 mutations. To determine whether enabling the ER stress/UPR and readjusting protein homeostasis would effectively treat these models of CMT1B and CMT1A, we administered Sephin1/IFB-088/icerguestat, a UPR modulator which showed efficacy in the MpzS63del model of CMT1B, to heterozygous MpzR98C and C3-PMP22 mice. Mice were analysed by behavioural, neurophysiological, morphological and biochemical measures. Both MpzR98C/ + and C3-PMP22 mice improved in motor function and neurophysiology. Myelination, as demonstrated by g-ratios and myelin thickness, improved in CMT1B and CMT1A mice and markers of UPR activation returned towards wild-type values. Taken together, our results demonstrate the capability of IFB-088 to treat a second mouse model of CMT1B and a mouse model of CMT1A, the most common form of CMT. Given the recent benefits of IFB-088 treatment in amyotrophic lateral sclerosis and multiple sclerosis animal models, these data demonstrate its potential in managing UPR and ER stress for multiple mutations in CMT1 as well as in other neurodegenerative diseases.

Curcumin and Ethanol Effects in Trembler-J Schwann Cell Culture

Charcot-Marie-Tooth (CMT) syndrome is the most common progressive human motor and sensory peripheral neuropathy. CMT type 1E is a demyelinating neuropathy affecting Schwann cells due to peripheral-myelin-protein-22 (PMP22) mutations, modelized by Trembler-J mice. Curcumin, a natural polyphenol compound obtained from turmeric (Curcuma longa), exhibits dose- and time-varying antitumor, antioxidant and neuroprotective properties, however, the neurotherapeutic actions of curcumin remain elusive. Here, we propose curcumin as a possible natural treatment capable of enhancing cellular detoxification mechanisms, resulting in an improvement of the neurodegenerative Trembler-J phenotype. Using a refined method for obtaining enriched Schwann cell cultures, we evaluated the neurotherapeutic action of low dose curcumin treatment on the PMP22 expression, and on the chaperones and autophagy/mammalian target of rapamycin (mTOR) pathways in Trembler-J and wild-type genotypes. In wild-type Schwann cells, the action of curcumin resulted in strong stimulation of the chaperone and macroautophagy pathway, whereas the modulation of ribophagy showed a mild effect. However, despite the promising neuroprotective effects for the treatment of neurological diseases, we demonstrate that the action of curcumin in Trembler-J Schwann cells could be impaired due to the irreversible impact of ethanol used as a common curcumin vehicle necessary for administration. These results contribute to expanding our still limited understanding of PMP22 biology in neurobiology and expose the intrinsic lability of the neurodegenerative Trembler-J genotype. Furthermore, they unravel interesting physiological mechanisms of cellular resilience relevant to the pharmacological treatment of the neurodegenerative Tremble J phenotype with curcumin and ethanol. We conclude that the analysis of the effects of the vehicle itself is an essential and inescapable step to comprehensibly assess the effects and full potential of curcumin treatment for therapeutic purposes.

The molecular and cellular insight into the toxicology of bortezomib-induced peripheral neuropathy

The proteasome inhibitor bortezomib (BTZ) is a first-line antitumor drug, mainly used for multiple myeloma treatment. However, BTZ shows prominent toxicity in the peripheral nervous system, termed BTZ-induced peripheral neuropathy (BIPN). BIPN is characterized by neuropathic pain, resulting in a dose reduction or even treatment withdrawal. To date, the pathological mechanism of BIPN has not been elucidated. There is still no effective strategy to prevent or treat BIPN. This review summarizes the pathological mechanisms of BIPN, which involves the pathological changes of Schwann cells, neurons, astrocytes and macrophages. A better knowledge of the pathological mechanisms of BIPN would provide new ideas for therapeutic interventions of BIPN patients.

Glycosylation limits forward trafficking of the tetraspan membrane protein PMP22

Peripheral myelin protein 22 (PMP22) folds and trafficks inefficiently, with only 20% of newly expressed protein trafficking to the cell surface. This behavior is exacerbated in many of the mutants associated with Charcot–Marie–Tooth disease, motivating further study. Here we characterized the role of N-glycosylation in limiting PMP22 trafficking. We first eliminated N-glycosylation using an N41Q mutation, which resulted in an almost 3-fold increase in trafficking efficiency of wildtype (WT) PMP22 and a 10-fold increase for the severely unstable L16P disease mutant in HEK293 cells, with similar results in Schwann cells. Total cellular levels were also much higher for the WT/N41Q mutant, although not for the L16P/N41Q form. Depletion of oligosaccharyltransferase OST-A and OST-B subunits revealed that WT PMP22 is N-glycosylated posttranslationally by OST-B, whereas L16P is cotranslationally glycosylated by OST-A. Quantitative proteomic screens revealed similarities and differences in the interactome for WT, glycosylation-deficient, and unstable mutant forms of PMP22 and also suggested that L16P is sequestered at earlier stages of endoplasmic reticulum quality control. CRISPR knockout studies revealed a role for retention in endoplasmic reticulum sorting receptor 1 (RER1) in limiting the trafficking of all three forms, for UDP-glucose glycoprotein glucosyltransferase 1 (UGGT1) in limiting the trafficking of WT and L16P but not N41Q, and calnexin (CNX) in limiting the trafficking of WT and N41Q but not L16P. This work shows that N-glycosylation is a limiting factor to forward trafficking PMP22 and sheds light on the proteins involved in its quality control.

Ion mobility-mass spectrometry reveals the role of peripheral myelin protein dimers in peripheral neuropathy

Peripheral myelin protein (PMP22) is an integral membrane protein that traffics inefficiently even in wild-type (WT) form, with only 20% of the WT protein reaching its final plasma membrane destination in myelinating Schwann cells. Misfolding of PMP22 has been identified as a key factor in multiple peripheral neuropathies, including Charcot-Marie-Tooth disease and Dejerine-Sottas syndrome. While biophysical analyses of disease-associated PMP22 mutants show altered protein stabilities, leading to reduced surface trafficking and loss of PMP22 function, it remains unclear how destabilization of PMP22 mutations causes mistrafficking. Here, native ion mobility-mass spectrometry (IM-MS) is used to compare the gas phase stabilities and abundances for an array of mutant PM22 complexes. We find key differences in the PMP22 mutant stabilities and propensities to form homodimeric complexes. Of particular note, we observe that severely destabilized forms of PMP22 exhibit a higher propensity to dimerize than WT PMP22. Furthermore, we employ lipid raft-mimicking SCOR bicelles to study PMP22 mutants, and find that the differences in dimer abundances are amplified in this medium when compared to micelle-based data, with disease mutants exhibiting up to 4 times more dimer than WT when liberated from SCOR bicelles. We combine our findings with previous cellular data to propose that the formation of PMP22 dimers from destabilized monomers is a key element of PMP22 mistrafficking.

Influence of spatial structure on protein damage susceptibility: a bioinformatics approach

Aging research is a very popular field of research in which the deterioration or decline of various physiological features is studied. Here we consider the molecular level, which can also have effects on the macroscopic level. The proteinogenic amino acids differ in their susceptibilities to non-enzymatic modification. Some of these modifications can lead to protein damage and thus can affect the form and function of proteins. For this, it is important to know the distribution of amino acids between the protein shell/surface and the core. This was investigated in this study for all known structures of peptides and proteins available in the PDB. As a result, it is shown that the shell contains less susceptible amino acids than the core with the exception of thermophilic organisms. Furthermore, proteins could be classified according to their susceptibility. This can then be used in applications such as phylogeny, aging research, molecular medicine, and synthetic biology.

Genetic mechanisms of peripheral nerve disease

Peripheral neuropathies of genetic etiology are a very diverse group of disorders manifesting either as non-syndromic inherited neuropathies without significant manifestations outside the peripheral nervous system, or as part of a systemic or syndromic genetic disorder. The former and most frequent group is collectively known as Charcot-Marie-Tooth disease (CMT), with prevalence as high as 1:2,500 world-wide, and has proven to be genetically highly heterogeneous. More than 100 different genes have been identified so far to cause various CMT forms, following all possible inheritance patterns. CMT causative genes belong to several common functional pathways that are essential for the integrity of the peripheral nerve. Their discovery has provided insights into the normal biology of axons and myelinating cells, and has highlighted the molecular mechanisms including both loss of function and gain of function effects, leading to peripheral nerve degeneration. Demyelinating neuropathies result from dysfunction of genes primarily affecting myelinating Schwann cells, while axonal neuropathies are caused by genes affecting mostly neurons and their long axons. Furthermore, mutation in genes expressed outside the nervous system, as in the case of inherited amyloid neuropathies, may cause peripheral neuropathy resulting from accumulation of β-structured amyloid fibrils in peripheral nerves in addition to various organs. Increasing insights into the molecular-genetic mechanisms have revealed potential therapeutic targets. These will enable the development of novel therapeutics for genetic neuropathies that remain, in their majority, without effective treatment.

A novel histone deacetylase 6 inhibitor improves myelination of Schwann cells in a model of Charcot-Marie-Tooth disease type 1A

BACKGROUND AND PURPOSE

Charcot-Marie-Tooth (CMT) disease is the most common hereditary peripheral neuropathy. CMT type 1A (CMT1A) accounts for approximately 50% of CMT patients and is linked to PMP22 gene duplication. Histone deacetylase-6 (HDAC6) has pleiotropic effects, such as regulating lipid homeostasis and cellular stress. Although HDAC6 has been regarded as a promising drug target for neurodegenerative diseases, its inhibition has not yet been tested in CMT1A. Here we have tested the therapeutic potential of CKD-504, a clinical stage HDAC6 inhibitor, in a mouse model of CMT1A EXPERIMENTAL APPROACH: The potency and selectivity of CKD-504 was evaluated, using a HDAC enzyme panel assay and western blots. The therapeutic potential of CKD-504 was evaluated using behavioural testing and electrophysiological assessments in the C22 mouse model of CMT1A. PMP22 protein expression and aggregation were analysed in mesenchymal stem cell-derived Schwann cells from CMT1A patients and sciatic nerves from C22 mice.

KEY RESULTS

The HDAC6 inhibitor, CKD-504, modulated molecular chaperon proteins such as HSP90 and HSP70, which are involved in the folding/refolding of proteins such as PMP22. CKD-504 treatment restored myelination in both mesenchymal stem cell-derived Schwann cells from CMT1A patients and sciatic nerves of C22 mice and improved the axonal integrity of the sciatic nerve, leading to behavioural, electrophysiological, and histological improvements in C22 mice.

CONCLUSION AND IMPLICATIONS

A novel HDAC6 inhibitor, CKD-504, has potent therapeutic efficacy for CMT1A.

Direct relationship between increased expression and mistrafficking of the Charcot-Marie-Tooth-associated protein PMP22

Charcot-Marie-Tooth disease (CMT) is a neuropathy of the peripheral nervous system that afflicts ∼1:2500 people. The most common form of this disease (CMT1A, 1:4000) is associated with duplication of chromosome fragment 17p11.2-12, which results in a third WT PMP22 allele. In rodent models overexpressing the PMP22 (peripheral myelin protein 22) protein and in dermal fibroblasts from patients with CMT1A, PMP22 aggregates have been observed. This suggests that overexpression of PMP22 under CMT1A conditions overwhelms the endoplasmic reticulum quality control system, leading to formation of cytotoxic aggregates. In this work, we used a single-cell flow-cytometry trafficking assay to quantitatively examine the relationship between PMP22 expression and trafficking efficiency in individual cells. We observed that as expression of WT or disease variants of PMP22 is increased, the amount of intracellular PMP22 increases to a greater extent than the amount of surface-trafficked protein. This was true for both transiently transfected cells and PMP22 stable expressing cells. Our results support the notion that overexpression of PMP22 in CMT1A leads to a disproportionate increase in misfolding and mistrafficking of PMP22, which is likely a contributor to disease pathology and progression.

Subcellular diversion of cholesterol by gain- and loss-of-function mutations in PMP22

Abnormalities of the peripheral myelin protein 22 (PMP22) gene, including duplication, deletion and point mutations are a major culprit in Type 1 Charcot-Marie-Tooth (CMT) diseases. The complete absence of PMP22 alters cholesterol metabolism in Schwann cells, which likely contributes to myelination deficits. Here, we examined the subcellular trafficking of cholesterol in distinct models of PMP22-linked neuropathies. In Schwann cells from homozygous Trembler J (TrJ) mice carrying a Leu16Pro mutation, cholesterol was retained with TrJ-PMP22 in the Golgi, alongside a corresponding reduction in its plasma membrane level. PMP22 overexpression, which models CMT1A caused by gene duplication, triggered cholesterol sequestration to lysosomes, and reduced ATP-binding cassette transporter-dependent cholesterol efflux. Conversely, lysosomal targeting of cholesterol by U18666A treatment increased wild type (WT)-PMP22 levels in lysosomes. Mutagenesis of a cholesterol recognition motif, or CRAC domain, in human PMP22 lead to increased levels of PMP22 in the ER and Golgi compartments, along with higher cytosolic, and lower membrane-associated cholesterol. Importantly, cholesterol trafficking defects observed in PMP22-deficient Schwann cells were rescued by WT but not CRAC-mutant-PMP22. We also observed that myelination deficits in dorsal root ganglia explants from heterozygous PMP22-deficient mice were improved by cholesterol supplementation. Collectively, these findings indicate that PMP22 is critical in cholesterol metabolism, and this mechanism is likely a contributing factor in PMP22-linked hereditary neuropathies. Our results provide a basis for understanding how altered expression of PMP22 impacts cholesterol metabolism.

HSP90 Inhibitor, NVP-AUY922, Improves Myelination in Vitro and Supports the Maintenance of Myelinated Axons in Neuropathic Mice

Hereditary demyelinating neuropathies linked to peripheral myelin protein 22 (PMP22) involve the disruption of normal protein trafficking and are therefore relevant targets for chaperone therapy. Using a small molecule HSP90 inhibitor, EC137, in cell culture models, we previously validated the chaperone pathway as a viable target for therapy development. Here, we tested five commercially available inhibitors of HSP90 and identified BIIB021 and AUY922 to support Schwann cell viability and enhance chaperone expression. AUY922 showed higher efficacy, compared to BIIB021, in enhancing myelin synthesis in dorsal root ganglion explant cultures from neuropathic mice. For in vivo testing, we randomly assigned 2–3 month old C22 and 6 week old Trembler J (TrJ) mice to receive two weekly injections of either vehicle or AUY922 (2 mg/kg). By the intraperitoneal (i.p.) route, the drug was well-tolerated by all mice over the 5 month long study, without influence on body weight or general grooming behavior. AUY922 improved the maintenance of myelinated nerves of both neuropathic models and attenuated the decline in rotarod performance and peak muscle force production in C22 mice. These studies highlight the significance of proteostasis in neuromuscular function and further validate the HSP90 pathway as a therapeutic target for hereditary neuropathies.

Peripheral myelin protein 22 modulates store-operated calcium channel activity, providing insights into Charcot-Marie-Tooth disease etiology

Charcot-Marie-Tooth (CMT) disease is a peripheral neuropathy associated with gene duplication and point mutations in the peripheral myelin protein 22 (PMP22) gene. However, the role of PMP22 in Schwann cell physiology and the mechanisms by which PMP22 mutations cause CMT are not well-understood. On the basis of homology between PMP22 and proteins associated with modulation of ion channels, we hypothesized that PMP22 alters ion channel activity. Using whole-cell electrophysiology, we show here that heterologous PMP22 expression increases the amplitude of currents similar to those ascribed to store-operated calcium (SOC) channels, particularly those involving transient receptor canonical channel 1 (TrpC1). These channels help replenish Ca²⁺ in the endoplasmic reticulum (ER) following stimulus-induced depletion. Currents with similar properties were recorded in WT but not pmp22 ^-/- mouse Schwann cells. Heterologous expression of the CMT-associated PMP22_L16P variant, which fails to reach the plasma membrane and localizes to the ER, led to larger currents than WT PMP22. Similarly, Schwann cells isolated from Trembler J (TrJ; PMP22_L16P) mice had larger currents than WT littermates. Calcium imaging in live nerves and cultured Schwann cells revealed elevated intracellular Ca²⁺ in TrJ mice compared with WT. Moreover, we found that PMP22 co-immunoprecipitated with stromal interaction molecule 1 (STIM1), the Ca²⁺ sensor SOC channel subunit in the ER. These results suggest that in the ER, PMP22 interacts with STIM1 and increases Ca²⁺ influx through SOC channels. Excess or mutant PMP22 in the ER may elevate intracellular Ca²⁺ levels, which could contribute to CMT pathology.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux

The absence of functional peripheral myelin protein 22 (PMP22) is associated with shortened lifespan in rodents and severe peripheral nerve myelin abnormalities in several species including humans. Schwann cells and nerves from PMP22 knock-out (KO) mice show deranged cholesterol distribution and aberrant lipid raft morphology, supporting an unrecognized role for PMP22 in cellular lipid metabolism. To examine the mechanisms underlying these abnormalities, we studied Schwann cells and nerves from male and female PMP22 KO mice. Whole-cell current-clamp recordings in cultured Schwann cells revealed increased membrane capacitance and decreased membrane resistance in the absence of PMP22, which was consistent with a reduction in membrane cholesterol. Nerves from PMP22-deficient mice contained abnormal lipid droplets, with both mRNA and protein levels of apolipoprotein E (apoE) and ATP-binding cassette transporter A1 (ABCA1) being highly upregulated. Despite the upregulation of ABCA1 and apoE, the absence of PMP22 resulted in reduced localization of the transporter to the cell membrane and diminished secretion of apoE. The absence of PMP22 also impaired ABCA1-mediated cholesterol efflux capacity. In nerves from ABCA1 KO mice, the expression of PMP22 was significantly elevated and the subcellular processing of the overproduced protein was aberrant. In wild-type samples, double immunolabeling identified overlapping distribution of PMP22 and ABCA1 at the Schwann cell plasma membrane and the two proteins were coimmunoprecipitated from Schwann cell and nerve lysates. Together, these results reveal a novel role for PMP22 in regulating lipid metabolism and cholesterol trafficking through functional interaction with the cholesterol efflux regulatory protein ABCA1.SIGNIFICANCE STATEMENT Understanding the subcellular events that underlie abnormal myelin formation in hereditary neuropathies is critical for advancing therapy development. Peripheral myelin protein 22 (PMP22) is an essential peripheral myelin protein because its genetic abnormalities account for ∼80% of hereditary neuropathies. Here, we demonstrate that in the absence of PMP22, the cellular and electrophysiological properties of the Schwann cells' plasma membrane are altered and cholesterol trafficking and lipid homeostasis are perturbed. The molecular mechanisms for these abnormalities involve a functional interplay among PMP22, cholesterol, apolipoprotein E, and the major cholesterol-efflux transporter protein ATP-binding cassette transporter A1 (ABCA1). These findings establish a critical role for PMP22 in the maintenance of cholesterol homeostasis in Schwann cells.

Elevated Peripheral Myelin Protein 22, Reduced Mitotic Potential, and Proteasome Impairment in Dermal Fibroblasts from Charcot-Marie-Tooth Disease Type 1A Patients

A common form of hereditary autosomal dominant demyelinating neuropathy known as Charcot-Marie-Tooth disease type 1A (CMT1A) is linked with duplication of the peripheral myelin protein 22 (PMP22) gene. Although studies from animal models have led to better understanding of the pathobiology of these neuropathies, there continues to be a gap in the translation of findings from rodents to humans. Because PMP22 was originally identified in fibroblasts as growth arrest specific gene 3 (gas3) and is expressed broadly in the body, it was tested whether skin cells from neuropathic patients would display the cellular pathology observed in Schwann cells from rodent models. Dermal fibroblasts from two CMT1A pedigrees with confirmed PMP22 gene duplication were studied. Samples from age-matched non-neuropathic individuals were used as controls. CMT1A patient–derived cultures contain approximately 1.5-fold elevated levels of PMP22 mRNA, exhibit reduced mitotic potential, and display intracellular protein aggregates as compared to cells from unaffected individuals. The presence of cytosolic PMP22 coincides with a decrease in proteasome activity and an increase in autophagy-lysosomal proteins, including LC3-II and LAMP1. These results indicate that the abnormalities in the subcellular processing of excess PMP22 elicit a detectable response in human CMT1A fibroblasts, a phenotype that resembles Schwann cells from neuropathic mice. These findings support the use of human CMT1A fibroblasts as a platform for therapy testing.

Genomic disorders 20 years on-mechanisms for clinical manifestations

Genomic disorders result from copy-number variants (CNVs) or submicroscopic rearrangements of the genome rather than from single nucleotide variants (SNVs). Diverse technologies, including array comparative genomic hybridization (aCGH) and single nucleotide polymorphism (SNP) microarrays, and more recently, whole genome sequencing and whole-exome sequencing, have enabled robust genome-wide unbiased detection of CNVs in affected individuals and in reportedly healthy controls. Sequencing of breakpoint junctions has allowed for elucidation of upstream mechanisms leading to genomic instability and resultant structural variation, whereas studies of the association between CNVs and specific diseases or susceptibility to morbid traits have enhanced our understanding of the downstream effects. In this review, we discuss the hallmarks of genomic disorders as they were defined during the first decade of the field, including genomic instability and the mechanism for rearrangement defined as nonallelic homologous recombination (NAHR); recurrent vs nonrecurrent rearrangements; and gene dosage sensitivity. Moreover, we highlight the exciting advances of the second decade of this field, including a deeper understanding of genomic instability and the mechanisms underlying complex rearrangements, mechanisms for constitutional and somatic chromosomal rearrangements, structural intra-species polymorphisms and susceptibility to NAHR, the role of CNVs in the context of genome-wide copy number and single nucleotide variation, and the contribution of noncoding CNVs to human disease.

Peripheral myelin protein 22 alters membrane architecture

Peripheral myelin protein 22 (PMP22) is highly expressed in myelinating Schwann cells of the peripheral nervous system. PMP22 genetic alterations cause the most common forms of Charcot-Marie-Tooth disease (CMTD), which is characterized by severe dysmyelination in the peripheral nerves. However, the functions of PMP22 in Schwann cell membranes remain unclear. We demonstrate that reconstitution of purified PMP22 into lipid vesicles results in the formation of compressed and cylindrically wrapped protein-lipid vesicles that share common organizational traits with compact myelin of peripheral nerves in vivo. The formation of these myelin-like assemblies depends on the lipid-to-PMP22 ratio, as well as on the PMP22 extracellular loops. Formation of the myelin-like assemblies is disrupted by a CMTD-causing mutation. This study provides both a biochemical assay for PMP22 function and evidence that PMP22 directly contributes to membrane organization in compact myelin.

Endoplasmic Reticulum Protein Quality Control Failure in Myelin Disorders

Reaching the correct three-dimensional structure is crucial for the proper function of a protein. The endoplasmic reticulum (ER) is the organelle where secreted and transmembrane proteins are synthesized and folded. To guarantee high fidelity of protein synthesis and maturation in the ER, cells have evolved ER-protein quality control (ERQC) systems, which assist protein folding and promptly degrade aberrant gene products. Only correctly folded proteins that pass ERQC checkpoints are allowed to exit the ER and reach their final destination. Misfolded glycoproteins are detected and targeted for degradation by the proteasome in a process known as endoplasmic reticulum-associated degradation (ERAD). The excess of unstructured proteins in the ER triggers an adaptive signal transduction pathway, called unfolded protein response (UPR), which in turn potentiates ERQC activities in order to reduce the levels of aberrant molecules. When the situation cannot be restored, the UPR drives cells to apoptosis. Myelin-forming cells of the central and peripheral nervous system (oligodendrocytes and Schwann cells) synthesize a large amount of myelin proteins and lipids and therefore are particularly susceptible to ERQC failure. Indeed, deficits in ERQC and activation of ER stress/UPR have been implicated in several myelin disorders, such as Pelizaeus-Merzbacher and Krabbe leucodystrophies, vanishing white matter disease and Charcot-Marie-Tooth neuropathies. Here we discuss recent evidence underlying the importance of proper ERQC functions in genetic disorders of myelinating glia.

Tuning PAK Activity to Rescue Abnormal Myelin Permeability in HNPP

Schwann cells in the peripheral nervous systems extend their membranes to wrap axons concentrically and form the insulating sheath, called myelin. The spaces between layers of myelin are sealed by myelin junctions. This tight insulation enables rapid conduction of electric impulses (action potentials) through axons. Demyelination (stripping off the insulating sheath) has been widely regarded as one of the most important mechanisms altering the action potential propagation in many neurological diseases. However, the effective nerve conduction is also thought to require a proper myelin seal through myelin junctions such as tight junctions and adherens junctions. In the present study, we have demonstrated the disruption of myelin junctions in a mouse model (Pmp22+/-) of hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of Pmp22 gene. We observed a robust increase of F-actin in Pmp22+/- nerve regions where myelin junctions were disrupted, leading to increased myelin permeability. These abnormalities were present long before segmental demyelination at the late phase of Pmp22+/- mice. Moreover, the increase of F-actin levels correlated with an enhanced activity of p21-activated kinase (PAK1), a molecule known to regulate actin polymerization. Pharmacological inhibition of PAK normalized levels of F-actin, and completely prevented the progression of the myelin junction disruption and nerve conduction failure in Pmp22+/- mice. Our findings explain how abnormal myelin permeability is caused in HNPP, leading to impaired action potential propagation in the absence of demyelination. We call it "functional demyelination", a novel mechanism upstream to the actual stripping of myelin that is relevant to many demyelinating diseases. This observation also provides a potential therapeutic approach for HNPP.

Conformational Stability and Pathogenic Misfolding of the Integral Membrane Protein PMP22

Despite broad biochemical relevance, our understanding of the physiochemical reactions that limit the assembly and cellular trafficking of integral membrane proteins remains superficial. In this work, we report the first experimental assessment of the relationship between the conformational stability of a eukaryotic membrane protein and the degree to which it is retained by cellular quality control in the secretory pathway. We quantitatively assessed both the conformational equilibrium and cellular trafficking of 12 variants of the α-helical membrane protein peripheral myelin protein 22 (PMP22), the intracellular misfolding of which is known to cause peripheral neuropathies associated with Charcot–Marie–Tooth disease (CMT). We show that the extent to which these mutations influence the energetics of Zn(II)-mediated PMP22 folding is proportional to the observed reduction in cellular trafficking efficiency. Strikingly, quantitative analyses also reveal that the reduction of motor nerve conduction velocities in affected patients is proportional to the extent of the mutagenic destabilization. This finding provides compelling evidence that the effects of these mutations on the energetics of PMP22 folding lie at the heart of the molecular basis of CMT. These findings highlight conformational stability as a key factor governing membrane protein biogenesis and suggest novel therapeutic strategies for CMT.

Molecular and clinical features of inherited neuropathies due to PMP22 duplication

PMP22 is a transmembrane glycoprotein component of myelin, important for myelin functioning. Mutation of PMP22 gene which encodes for the production of PMP22 glycoprotein is associated with a variety of inherited neuropathies. This literature review sought to review the molecular mechanism and clinical features of inherited neuropathies caused by PMP22 duplication. PMP22 duplication causes CMT1A which accounts for more than half of all CMT cases and about 70% of CMT1 cases. It manifests with muscle weakness, depressed reflexes, impaired distal sensation, hand and foot deformities, slowing of NCV and onion bulbs. With no specific treatment available, it is managed conservatively. Future treatment may be based on the molecular genetics of the disease.

Inducible HSP70 is critical in preventing the aggregation and enhancing the processing of PMP22

Chaperones, also called heat shock proteins (HSPs), transiently interact with proteins to aid their folding, trafficking, and degradation, thereby directly influencing the transport of newly synthesized molecules. Induction of chaperones provides a potential therapeutic approach for protein misfolding disorders, such as peripheral myelin protein 22 (PMP22)-associated peripheral neuropathies. Cytosolic aggregates of PMP22, linked with a demyelinating Schwann cell phenotype, result in suppression of proteasome activity and activation of proteostatic mechanisms, including the heat shock pathway. Although the beneficial effects of chaperones in preventing the aggregation and improving the trafficking of PMP22 have been repeatedly observed, the requirement for HSP70 in events remains elusive. In this study, we show that activation of the chaperone pathway in fibroblasts from PMP22 duplication-associated Charcot–Marie–Tooth disease type 1A patient with an FDA-approved small molecule increases HSP70 expression and attenuates proteasome dysfunction. Using cells from an HSP70.1/3^−/− (inducible HSP70) mouse model, we demonstrate that under proteotoxic stress, this chaperone is critical in preventing the aggregation of PMP22, and this effect is aided by macroautophagy. When examined at steady-state, HSP70 appears to play a minor role in the trafficking of wild-type-PMP22, while it is crucial for preventing the buildup of the aggregation-prone Trembler-J-PMP22. HSP70 aids the processing of Trembler-J-PMP22 through the Golgi and its delivery to lysosomes via Rab7-positive vesicles. Together, these results demonstrate a key role for inducible HSP70 in aiding the processing and hindering the accumulation of misfolded PMP22, which in turn alleviates proteotoxicity within the cells.

Polytherapy with a combination of three repurposed drugs (PXT3003) down-regulates Pmp22 over-expression and improves myelination, axonal and functional parameters in models of CMT1A neuropathy

Charcot-Marie-Tooth disease type 1A (CMT1A) is the most common inherited sensory and motor peripheral neuropathy. It is caused by PMP22 overexpression which leads to defects of peripheral myelination, loss of long axons, and progressive impairment then disability. There is no treatment available despite observations that monotherapeutic interventions slow progression in rodent models. We thus hypothesized that a polytherapeutic approach using several drugs, previously approved for other diseases, could be beneficial by simultaneously targeting PMP22 and pathways important for myelination and axonal integrity. A combination of drugs for CMT1A polytherapy was chosen from a group of authorised drugs for unrelated diseases using a systems biology approach, followed by pharmacological safety considerations. Testing and proof of synergism of these drugs were performed in a co-culture model of DRG neurons and Schwann cells derived from a Pmp22 transgenic rat model of CMT1A. Their ability to lower Pmp22 mRNA in Schwann cells relative to house-keeping genes or to a second myelin transcript (Mpz) was assessed in a clonal cell line expressing these genes. Finally in vivo efficacy of the combination was tested in two models: CMT1A transgenic rats, and mice that recover from a nerve crush injury, a model to assess neuroprotection and regeneration. Combination of (RS)-baclofen, naltrexone hydrochloride and D-sorbitol, termed PXT3003, improved myelination in the Pmp22 transgenic co-culture cellular model, and moderately down-regulated Pmp22 mRNA expression in Schwannoma cells. In both in vitro systems, the combination of drugs was revealed to possess synergistic effects, which provided the rationale for in vivo clinical testing of rodent models. In Pmp22 transgenic CMT1A rats, PXT3003 down-regulated the Pmp22 to Mpz mRNA ratio, improved myelination of small fibres, increased nerve conduction and ameliorated the clinical phenotype. PXT3003 also improved axonal regeneration and remyelination in the murine nerve crush model. Based on these observations in preclinical models, a clinical trial of PTX3003 in CMT1A, a neglected orphan disease, is warranted. If the efficacy of PTX3003 is confirmed, rational polytherapy based on novel combinations of existing non-toxic drugs with pleiotropic effects may represent a promising approach for rapid drug development.

The safety dance: biophysics of membrane protein folding and misfolding in a cellular context

Most biological processes require the production and degradation of proteins, a task that weighs heavily on the cell. Mutations that compromise the conformational stability of proteins place both specific and general burdens on cellular protein homeostasis (proteostasis) in ways that contribute to numerous diseases. Efforts to elucidate the chain of molecular events responsible for diseases of protein folding address one of the foremost challenges in biomedical science. However, relatively little is known about the processes by which mutations prompt the misfolding of α-helical membrane proteins, which rely on an intricate network of cellular machinery to acquire and maintain their functional structures within cellular membranes. In this review, we summarize the current understanding of the physical principles that guide membrane protein biogenesis and folding in the context of mammalian cells. Additionally, we explore how pathogenic mutations that influence biogenesis may differ from those that disrupt folding and assembly, as well as how this may relate to disease mechanisms and therapeutic intervention. These perspectives indicate an imperative for the use of information from structural, cellular, and biochemical studies of membrane proteins in the design of novel therapeutics and in personalized medicine.

LITAF mutations associated with Charcot-Marie-Tooth disease 1C show mislocalization from the late endosome/lysosome to the mitochondria

Charcot-Marie-Tooth (CMT) disease is one of the most common heritable neuromuscular disorders, affecting 1 in every 2500 people. Mutations in LITAF have been shown to be causative for CMT type 1C disease. In this paper we explore the subcellular localization of wild type LITAF and mutant forms of LITAF known to cause CMT1C (T49M, A111G, G112S, T115N, W116G, L122V and P135T). The results show that LITAF mutants A111G, G112S, W116G, and T115N mislocalize from the late endosome/lysosome to the mitochondria while the mutants T49M, L122V, and P135T show partial mislocalization with a portion of the total protein present in the late endosome/lysosome and the remainder of the protein localized to the mitochondria. This suggests that different mutants of LITAF will produce differing severity of disease. We also explored the effect of the presence of mutant LITAF on wild-type LITAF localization. We showed that in cells heterozygous for LITAF, CMT1C mutants T49M and G112S are dominant since wild-type LITAF localized to the mitochondria when co-transfected with a LITAF mutant. Finally, we demonstrated how LITAF transits to the endosome and mitochondria compartments of the cell. Using Brefeldin A to block ER to Golgi transport we demonstrated that wild type LITAF traffics through the secretory pathway to the late endosome/lysosome while the LITAF mutants transit to the mitochondria independent of the secretory pathway. In addition, we demonstrated that the C-terminus of LITAF is necessary and sufficient for targeting of wild-type LITAF to the late endosome/lysosome and the mutants to the mitochondria. Together these data provide insight into how mutations in LITAF cause CMT1C disease.

Contribution of small heat shock proteins to muscle development and function

Investigations undertaken over the past years have led scientists to introduce the concept of protein quality control (PQC) systems, which are responsible for polypeptide processing. The PQC system monitors proteostasis and involves activity of different chaperones such as small heat shock proteins (sHSPs). These proteins act during normal conditions as housekeeping proteins regulating cellular processes, and during stress conditions. They also mediate the removal of toxic misfolded polypeptides and thereby prevent development of pathogenic states. It is postulated that sHSPs are involved in muscle development. They could act via modulation of myogenesis or by maintenance of the structural integrity of signaling complexes. Moreover, mutations in genes coding for sHSPs lead to pathological states affecting muscular tissue functioning. This review focuses on the question how sHSPs, still relatively poorly understood proteins, contribute to the development and function of three types of muscle tissue: skeletal, cardiac and smooth.

Biochemical characterization of protein quality control mechanisms during disease progression in the C22 mouse model of CMT1A

Charcot–Marie–Tooth disease type 1A (CMT1A) is a hereditary demyelinating neuropathy linked with duplication of the peripheral myelin protein 22 (PMP22) gene. Transgenic C22 mice, a model of CMT1A, display many features of the human disease, including slowed nerve conduction velocity and demyelination of peripheral nerves. How overproduction of PMP22 leads to compromised myelin and axonal pathology is not fully understood, but likely involves subcellular alterations in protein homoeostatic mechanisms within affected Schwann cells. The subcellular response to abnormally localized PMP22 includes the recruitment of the ubiquitin–proteasome system (UPS), autophagosomes and heat-shock proteins (HSPs). Here we assessed biochemical markers of these protein homoeostatic pathways in nerves from PMP22-overexpressing neuropathic mice between the ages of 2 and 12 months to ascertain their potential contribution to disease progression. In nerves of 3-week-old mice, using endoglycosidases and Western blotting, we found altered processing of the exogenous human PMP22, an abnormality that becomes more prevalent with age. Along with the ongoing accrual of misfolded PMP22, the activity of the proteasome becomes compromised and proteins required for autophagy induction and lysosome biogenesis are up-regulated. Moreover, cytosolic chaperones are consistently elevated in nerves from neuropathic mice, with the most prominent change in HSP70. The gradual alterations in protein homoeostatic response are accompanied by Schwann cell de-differentiation and macrophage infiltration. Together, these results show that while subcellular protein quality control mechanisms respond appropriately to the presence of the overproduced PMP22, with aging they are unable to prevent the accrual of misfolded proteins.

In peripheral nerves of neuropathic C22 mice the frequency of cytosolic PMP22 aggregates increases with age, which elicits a response from protein quality control mechanisms. The combined effects of aging and neuropathic genotype exacerbate disease progression leading to nerve defects.

Liposomes to target peripheral neurons and Schwann cells

While a wealth of literature for tissue-specific liposomes is emerging, optimal formulations to target the cells of the peripheral nervous system (PNS) are lacking. In this study, we asked whether a novel formulation of phospholipid-based liposomes could be optimized for preferential uptake by microvascular endothelia, peripheral neurons and Schwann cells. Here, we report a unique formulation consisting of a phospholipid, a polymer surfactant and cholesterol that result in enhanced uptake by targeted cells. Using fluorescently labeled liposomes, we followed particle internalization and trafficking through a distinct route from dextran and escape from degradative compartments, such as lysosomes. In cultures of non-myelinating Schwann cells, liposomes associate with the lipid raft marker Cholera toxin, and their internalization is inhibited by disruption of lipid rafts or actin polymerization. In contrast, pharmacological inhibition of clathrin-mediated endocytosis does not significantly impact liposome entry. To evaluate the efficacy of liposome targeting in tissues, we utilized myelinating explant cultures of dorsal root ganglia and isolated diaphragm preparations, both of which contain peripheral neurons and myelinating Schwann cells. In these models, we detected preferential liposome uptake into neurons and glial cells in comparison to surrounding muscle tissue. Furthermore, in vivo liposome administration by intramuscular or intravenous injection confirmed that the particles were delivered to myelinated peripheral nerves. Within the CNS, we detected the liposomes in choroid epithelium, but not in myelinated white matter regions or in brain parenchyma. The described nanoparticles represent a novel neurophilic delivery vehicle for targeting small therapeutic compounds, biological molecules, or imaging reagents into peripheral neurons and Schwann cells, and provide a major advancement toward developing effective therapies for peripheral neuropathies.

Widespread aggregation and neurodegenerative diseases are associated with supersaturated proteins

The maintenance of protein solubility is a fundamental aspect of cellular homeostasis because protein aggregation is associated with a wide variety of human diseases. Numerous proteins unrelated in sequence and structure, however, can misfold and aggregate, and widespread aggregation can occur in living systems under stress or aging. A crucial question in this context is why only certain proteins appear to aggregate readily in vivo, whereas others do not. We identify here the proteins most vulnerable to aggregation as those whose cellular concentrations are high relative to their solubilities. We find that these supersaturated proteins represent a metastable subproteome involved in pathological aggregation during stress and aging and are overrepresented in biochemical processes associated with neurodegenerative disorders. Consequently, such cellular processes become dysfunctional when the ability to keep intrinsically supersaturated proteins soluble is compromised. Thus, the simultaneous analysis of abundance and solubility can rationalize the diverse cellular pathologies linked to neurodegenerative diseases and aging.

Autophagy promotes oligodendrocyte survival and function following dysmyelination in a long-lived myelin mutant

The Long-Evans shaker (les) rat has a mutation in myelin basic protein that results in severe CNS dysmyelination and subsequent demyelination during development. During this time, les oligodendrocytes accumulate cytoplasmic vesicles, including lysosomes and membrane-bound organelles. However, the mechanism and functional relevance behind these oligodendrocyte abnormalities in les have not been investigated. Using high-magnification electron microscopy, we identified the accumulations in les oligodendrocytes as early and late autophagosomes. Additionally, immunohistochemistry and Western blots showed an increase in autophagy markers in les. However, autophagy did not precede the death of les oligodendrocytes. Instead, upregulating autophagy promoted membrane extensions in les oligodendrocytes in vitro. Furthermore, upregulating autophagy in les rats via intermittent fasting increased the proportion of myelinated axons as well as myelin sheath thickness in les and control rats. Overall, this study provides insight into the abnormalities described in les as well as identifying a novel mechanism that promotes the survival and function of oligodendrocytes.

Reversible folding of human peripheral myelin protein 22, a tetraspan membrane protein

Misfolding of the α-helical membrane protein peripheral myelin protein 22 (PMP22) has been implicated in the pathogenesis of the common neurodegenerative disease known as Charcot-Marie-Tooth disease (CMTD) and also several other related peripheral neuropathies. Emerging evidence suggests that the propensity of PMP22 to misfold in the cell may be due to an intrinsic lack of conformational stability. Therefore, quantitative studies of the conformational equilibrium of PMP22 are needed to gain insight into the molecular basis of CMTD. In this work, we have investigated the folding and unfolding of wild type (WT) human PMP22 in mixed micelles. Both kinetic and thermodynamic measurements demonstrate that the denaturation of PMP22 by n-lauroyl sarcosine (LS) in dodecylphosphocholine (DPC) micelles is reversible. Assessment of the conformational equilibrium indicates that a significant fraction of unfolded PMP22 persists even in the absence of the denaturing detergent. However, we find the stability of PMP22 is increased by glycerol, which facilitates quantitation of thermodynamic parameters. To our knowledge, this work represents the first report of reversible unfolding of a eukaryotic multispan membrane protein. The results indicate that WT PMP22 possesses minimal conformational stability in micelles, which parallels its poor folding efficiency in the endoplasmic reticulum. Folding equilibrium measurements for PMP22 in micelles may provide an approach to assess the effects of cellular metabolites or potential therapeutic agents on its stability. Furthermore, these results pave the way for future investigation of the effects of pathogenic mutations on the conformational equilibrium of PMP22.

Membrane trafficking in neuronal maintenance and degeneration

Defects in membrane trafficking and degradation are hallmarks of most, and maybe all, neurodegenerative disorders. Such defects typically result in the accumulation of undegraded proteins due to aberrant endosomal sorting, lysosomal degradation, or autophagy. The genetic or environmental cause of a specific disease may directly affect these membrane trafficking processes. Alternatively, changes in intracellular sorting and degradation can occur as cellular responses of degenerating neurons to unrelated primary defects such as insoluble protein aggregates or other neurotoxic insults. Importantly, altered membrane trafficking may contribute to the pathogenesis or indeed protect the neuron. The observation of dramatic changes to membrane trafficking thus comes with the challenging need to distinguish pathological from protective alterations. Here, we will review our current knowledge about the protective and destructive roles of membrane trafficking in neuronal maintenance and degeneration. In particular, we will first focus on the question of what type of membrane trafficking keeps healthy neurons alive in the first place. Next, we will discuss what alterations of membrane trafficking are known to occur in Alzheimer's disease and other tauopathies, Parkinson's disease, polyQ diseases, peripheral neuropathies, and lysosomal storage disorders. Combining the maintenance and degeneration viewpoints may yield insight into how to distinguish when membrane trafficking functions protectively or contributes to degeneration.

Myelinating and demyelinating phenotype of Trembler-J mouse (a model of Charcot-Marie-Tooth human disease) analyzed by atomic force microscopy and confocal microscopy

The accumulation of misfolded proteins is associated with various neurodegenerative conditions. Mutations in PMP-22 are associated with the human peripheral neuropathy, Charcot-Marie-Tooth Type 1A (CMT1A). PMP-22 is a short-lived 22 kDa glycoprotein, which plays a key role in the maintenance of myelin structure and compaction, highly expressed by Schwann cells. It forms aggregates when the proteasome is inhibited or the protein is mutated. This study reports the application of atomic force microscopy (AFM) as a detector of profound topographical and mechanical changes in Trembler-J mouse (CMT1A animal model). AFM images showed topographical differences in the extracellular matrix and basal lamina organization of Tr-J/+ nerve fibers. The immunocytochemical analysis indicated that PMP-22 protein is associated with type IV collagen (a basal lamina ubiquitous component) in the Tr-J/+ Schwann cell perinuclear region. Changes in mechanical properties of single myelinating Tr-J/+ nerve fibers were investigated, and alterations in cellular stiffness were found. These results might be associated with F-actin cytoskeleton organization in Tr-J/+ nerve fibers. AFM nanoscale imaging focused on topography and mechanical properties of peripheral nerve fibers might provide new insights into the study of peripheral nervous system diseases.

Molecular genetics of autosomal-dominant demyelinating Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous group of disorders and is the most common inherited neuromuscular disorder, with an estimated overall prevalence of 17-40/10,000. Although there has been major advances in the understanding of the genetic basis of CMT in recent years, the most useful classification is still a neurophysiological classification that divides CMT into type 1 (demyelinating; median motor conduction velocity < 38 m/s) and type 2 (axonal; median motor conduction velocity > 38 m/s). An intermediate type is also increasingly being described. Inheritance can be autosomal-dominant (AD), X-linked, or autosomal-recessive (AR). AD CMT1 is the most common type of CMT and was the first form of CMT in which a causative gene was described. This review provides an up-to-date overview of AD CMT1 concentrating on the molecular genetics as the clinical, neurophysiological, and pathological features have been covered elsewhere. Four genes (PMP22, MPZ, LITAF, and EGR2) have been described in the last 15 yr associated with AD CMTI and a further gene (NEFL), originally described as causing AD CMT2 can also cause AD CMT1 (by neurophysiological criteria). Studies have shown many of these genes, when mutated, can cause a wide range of CMT phenotypes from the relatively mild CMT1 to the more severe Dejerine-Sottas disease and congenital hypomyelinating neuropathy, and even in some cases axonal CMT2. This review discusses what is known about these genes and in particular how they cause a peripheral neuropathy, when mutated.

Identification of drug modulators targeting gene-dosage disease CMT1A

The structural integrity of myelin formed by Schwann cells in the peripheral nervous system (PNS) is required for proper nerve conduction and is dependent on adequate expression of myelin genes including peripheral myelin protein 22 (PMP22). Consequently, excess PMP22 resulting from its genetic duplication and overexpression has been directly associated with the peripheral neuropathy called Charcot-Marie-Tooth disease type 1A (CMT1A), the most prevalent type of CMT. Here, in an attempt to identify transcriptional inhibitors with therapeutic value toward CMT1A, we developed a cross-validating pair of orthogonal reporter assays, firefly luciferase (FLuc) and β-lactamase (βLac), capable of recapitulating PMP22 expression, utilizing the intronic regulatory element of the human PMP22 gene. Each compound from a collection of approximately 3,000 approved drugs was tested at multiple titration points to achieve a pharmacological end point in a 1536-well plate quantitative high-throughput screen (qHTS) format. In conjunction with an independent counter-screen for cytotoxicity, the design of our orthogonal screen platform effectively contributed to selection and prioritization of active compounds, among which three drugs (fenretinide, olvanil, and bortezomib) exhibited marked reduction of endogenous Pmp22 mRNA and protein. Overall, the findings of this study provide a strategic approach to assay development for gene-dosage diseases such as CMT1A.

Murine therapeutic models for Charcot-Marie-Tooth (CMT) disease

INTRODUCTION OR BACKGROUND

Charcot-Marie-Tooth (CMT) disease represents a broad group of inherited motor and sensory neuropathies which can originate from various genetic aberrations, e.g. mutations, deletions and duplications.

SOURCES OF DATA

We performed a literature review on murine animal models of CMT disease with regard to experimental therapeutic approaches. Hereby, we focussed on the demyelinating subforms of CMT (CMT1). PubMed items were CMT, animal model, demyelination and therapy.

AREAS OF AGREEMENT

Patients affected by CMT suffer from slowly progressive, distally pronounced muscle atrophy caused by an axonal loss. The disease severity is highly variable and impairments may result in wheelchair boundness. No therapy is available yet.

AREAS OF CONTROVERSY

Numerous rodent models for the various CMT subtypes are available today. The selection of the correct animal model for the specific CMT subtype provides an important prerequisite for the successful translation of experimental findings in patients.

GROWING POINTS

Despite more than 20 years of remarkable progress in CMT research, the disease is still left untreatable. There is a growing number of experimental therapeutic strategies that may be translated into future clinical trials in patients with CMT.

AREAS TIMELY FOR DEVELOPING RESEARCH

The slow disease progression and insensitive outcome measures hamper clinical therapy trials in CMT. Biomarkers may provide powerful tools to monitor therapeutic efficacy. Recently, we have shown that transcriptional profiling can be utilized to assess and predict the disease severity in a transgenic rat model and in affected humans.

F-actin distribution at nodes of Ranvier and Schmidt-Lanterman incisures in mammalian sciatic nerves

Very little is known about the function of the F-actin cytoskeleton in the regeneration and pathology of peripheral nerve fibers. The actin cytoskeleton has been associated with maintenance of tissue structure, transmission of traction and contraction forces, and an involvement in cell motility. Therefore, the state of the actin cytoskeleton strongly influences the mechanical properties of cells and intracellular transport therein. In this work, we analyze the distribution of F-actin at Schmidt-Lanterman Incisures (SLI) and nodes of Ranvier (NR) domains in normal, regenerating and pathologic Trembler J (TrJ/+) sciatic nerve fibers, of rats and mice. F-actin was quantified and it was found increased in TrJ/+, both in SLI and NR. However, SLI and NR of regenerating rat sciatic nerve did not show significant differences in F-actin, as compared with normal nerves. Cytochalasin-D and Latrunculin-A were used to disrupt the F-actin network in normal and regenerating rat sciatic nerve fibers. Both drugs disrupt F-actin, but in different ways. Cytochalasin-D did not disrupt Schwann cell (SC) F-actin at the NR. Latrunculin-A did not disrupt F-actin at the boundary region between SC and axon at the NR domain. We surmise that the rearrangement of F-actin in neurological disorders, as presented here, is an important feature of TrJ/+ pathology as a Charcot-Marie-Tooth (CMT) model.

Accumulation of endogenous LITAF in aggresomes

LITAF is a 161 amino acid cellular protein which includes a proline rich N-terminus and a conserved C-terminal domain known as the simple-like domain. Mutations in LITAF have been identified in Charcot-Marie tooth disease, a disease characterized by protein aggregates. Cells transfected with cellular LITAF reveal that LITAF is localized to late endosomes/lysosomes. Here we investigated the intracellular localization of endogenous LITAF. We demonstrated that endogenous LITAF accumulates at a discrete cytoplasmic site in BGMK cells that we identify as the aggresome. To determine the domain within LITAF that is responsible for the localization of LITAF to aggresomes, we created a construct that contained the C-terminal simple-like domain of LITAF and found that this construct also localizes to aggresomes. These data suggest the simple-like domain is responsible for targeting endogenous LITAF to the aggresome.

Structural basis for the Trembler-J phenotype of Charcot-Marie-Tooth disease

Mutations in peripheral myelin protein 22 (PMP22) can result in the common peripheral neuropathy Charcot-Marie-Tooth disease (CMTD). The Leu16Pro mutation in PMP22 results in misassembly of the protein, which causes the Trembler-J (TrJ) disease phenotype. Here we elucidate the structural defects present in a partially folded state of TrJ PMP22 that are decisive in promoting CMTD-causing misfolding. In this state, transmembrane helices 2-4 (TM2-4) form a molten globular bundle, while transmembrane helix 1 (TM1) is dissociated from this bundle. The TrJ mutation was seen to profoundly disrupt the TM1 helix, resulting in increased backbone dynamics and changes in the tertiary interactions of TM1 with the PMP22 TM2-4 core in the folded state. Consequently, TM1 undergoes enhanced dissociation from the other transmembrane segments in TrJ PMP22, becoming available for recognition and sequestration by protein-folding quality control, which leads to loss of function and toxic accumulation of aggregates that result in CMTD.