Oral curcumin mitigates the clinical and neuropathologic phenotype of the Trembler-J mouse: a potential therapy for inherited neuropathy

Mutations in myelin genes cause inherited peripheral neuropathies that range in severity from adult-onset Charcot-Marie-Tooth disease type 1 to childhood-onset Dejerine-Sottas neuropathy and congenital hypomyelinating neuropathy. Many myelin gene mutants that cause severe disease, such as those in the myelin protein zero gene (MPZ) and the peripheral myelin protein 22 gene (PMP22), appear to make aberrant proteins that accumulate primarily within the endoplasmic reticulum (ER), resulting in Schwann cell death by apoptosis and, subsequently, peripheral neuropathy. We previously showed that curcumin supplementation could abrogate ER retention and aggregation-induced apoptosis associated with neuropathy-causing MPZ mutants. We now show reduced apoptosis after curcumin treatment of cells in tissue culture that express PMP22 mutants. Furthermore, we demonstrate that oral administration of curcumin partially mitigates the severe neuropathy phenotype of the Trembler-J mouse model in a dose-dependent manner. Administration of curcumin significantly decreases the percentage of apoptotic Schwann cells and results in increased number and size of myelinated axons in sciatic nerves, leading to improved motor performance. Our findings indicate that curcumin treatment is sufficient to relieve the toxic effect of mutant aggregation-induced apoptosis and improves the neuropathologic phenotype in an animal model of human neuropathy, suggesting a potential therapeutic role in selected forms of inherited peripheral neuropathies.     

Mutations in the cytoplasmic domain of P0 reveal a role for PKC-mediated phosphorylation in adhesion and myelination.

Mutations in P0 (MPZ), the major myelin protein of the peripheral nervous system, cause the inherited demyelinating neuropathy Charcot-Marie-Tooth disease type 1B. P0 is a member of the immunoglobulin superfamily and functions as a homophilic adhesion molecule. We now show that point mutations in the cytoplasmic domain that modify a PKC target motif (RSTK) or an adjacent serine residue abolish P0 adhesion function and can cause peripheral neuropathy in humans. Consistent with these data, PKCalpha along with the PKC binding protein RACK1 are immunoprecipitated with wild-type P0, and inhibition of PKC activity abolishes P0-mediated adhesion. Point mutations in the RSTK target site that abolish adhesion do not alter the association of PKC with P0; however, deletion of a 14 amino acid region, which includes the RSTK motif, does abolish the association. Thus, the interaction of PKCalpha with the cytoplasmic domain of P0 is independent of specific target residues but is dependent on a nearby sequence. We conclude that PKC-mediated phosphorylation of specific residues within the cytoplasmic domain of P0 is necessary for P0-mediated adhesion, and alteration of this process can cause demyelinating neuropathy in humans.     

Mutational analysis of the MPZ, PMP22 and Cx32 genes in patients of Spanish ancestry with Charcot-Marie-Tooth disease and hereditary neuropathy with liability to pressure palsies

Charcot-Marie-Tooth disease (CMT) and hereditary neuropathy with liability to pressure palsies (HNPP) are two inherited peripheral neuropathies. The most prevalent mutations are a reciprocal 1.5-Mb duplication and 1.5-Mb deletion, respectively, at the CMT1A/HNPP locus on chromosome 17p11.2. Point mutations in the coding region of the myelin genes, peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) or connexin 32 (Cx32) have been reported in CMT patients, including CMT type 1 (CMT1), CMT type 2 (CMT2) and Déjérine-Sottas neuropathy (DS) patients, and only in the coding region of PMP22 in HNPP families lacking a deletion. We have investigated point and small mutations in the MPZ, PMP22 and Cx32 genes in a series of patients of Spanish ancestry: 47 CMT patients without duplications, and 5 HNPP patients without deletions. We found 15 different mutations in 16 CMT patients (34%). Nine different mutations in ten patients were detected in the Cx32 gene, this being the most frequently involved gene in this series, whereas five mutations involved the MPZ gene and only one the PMP22 gene. Six out of nine nucleotide substitutions in the Cx32 gene involved two codons encoding arginine at positions 164 and 183, suggesting that these two codons may constitute two Cx32 regions prone to mutate in the Spanish population. Analysis of HNPP patients revealed a 5′ splicing mutation in intron 1 of the PMP22 gene in a family with autosomal dominance, which confirms allelic heterogeneity in HNPP. Ectopic mRNA analysis on leukocytes suggests that this mutation might behave as a null allele. Received: 25 July 1996 / Revised: 15 November 1996     

Protein conformation and disease: pathological consequences of analogous mutations in homologous proteins

The antibody light chain variable domain (V(L))(1) and myelin protein zero (MPZ) are representatives of the functionally diverse immunoglobulin superfamily. The V(L) is a subunit of the antigen-binding component of antibodies, while MPZ is the major membrane-linked constituent of the myelin sheaths that coat peripheral nerves. Despite limited amino acid sequence homology, the conformations of the core structures of the two proteins are largely superimposable. Amino acid variations in V(L) account for various conformational disease outcomes, including amyloidosis. However, the specific amino acid changes in V(L) that are responsible for disease have been obscured by multiple concurrent primary structure alterations. Recently, certain demyelination disorders have been linked to point mutations and single amino acid polymorphisms in MPZ. We demonstrate here that some pathogenic variations in MPZ correspond to changes suspected of determining amyloidosis in V(L). This unanticipated observation suggests that studies of the biophysical origin of conformational disease in one member of a superfamily of homologous proteins may have implications throughout the superfamily. In some cases, findings may account for overt disease; in other cases, due to the natural repertoire of inherited polymorphisms, variations in a representative protein may predict subclinical impairment of homologous proteins.     

Screening of the myelin protein zero gene in patients with Charcot-Marie-Tooth disease

The myelin protein zero gene (MPZ) coding for the most abundant protein of the peripheral myelin was shown to be mutated in Charcot-Marie-Tooth type 1B disease (CMT1B). Later on MPZ mutations have been shown in axonal type of CMT (CMT2). Recently three novel MPZ gene mutations were reported in congenital hypomyelinating neuropathy (CHN). In contrast to the previously reported studies, focused on CMT1B disease, we aimed to analyze the coding and promoter sequences of the MPZ gene in a group of patients with three CMT phenotypes i.e.: CMT1, CMT2 and CHN. Over 500 PCR products were screened by single strand conformation polymorphism analysis (SSCP) and heteroduplex analysis (HA). In one CMT2 family we founded the E56K mutation in the MPZ gene and in one CHN patient the T124K substitution was detected. In agreement with previously reported studies we conclude that MPZ gene screening should be performed for wide phenotype spectrum of CMT.     

In silico and in vitro effects of the I30T mutation on myelin protein zero instability in the cell membrane

Charcot‐Marie‐Tooth (CMT) diseases are a heterogeneous group of genetic peripheral neuropathies caused by mutations in a variety of genes, which are involved in the development and maintenance of peripheral nerves. Myelin protein zero (MPZ) is expressed by Schwann cells, and MPZ mutations can lead to primarily demyelinating polyneuropathies including CMT type 1B. Different mutations demonstrate various forms of disease pathomechanisms, which may be beneficial in understanding the disease cellular pathology. Our molecular dynamics simulation study on the possible impacts of I30T mutation on the MPZ protein structure suggested a higher hydrophobicity and thus lower stability in the membranous structures. A study was also conducted to predict native/mutant MPZ interactions. To validate the results of the simulation study, the native and mutant forms of the MPZ protein were separately expressed in a cellular model, and the protein trafficking was chased down in a time course pattern. In vitro studies provided more evidence on the instability of the MPZ protein due to the mutation. In this study, qualitative and quantitative approaches were adopted to confirm the instability of mutant MPZ in cellular membranes.     

Multiple disease-linked myotubularin mutations cause NFL assembly defects in cultured cells and disrupt myotubularin dimerization

Charcot-Marie-Tooth disease (CMT) is an inherited peripheral neuropathy that has been linked to mutations in multiple genes. Mutations in the neurofilament light ( NFL ) chain gene lead to the CMT2E form whereas mutations in the myotubularin-related protein 2 and 13 ( MTMR2 and MTMR13 ) genes lead to the CMT4B form. These two forms share characteristic pathological hallmarks on nerve biopsies including concentric sheaths ('onion bulbs') and, in at least one case, myelin loops. In addition, MTMR2 protein has been shown to interact physically with both NFL and MTMR13. Here, we present evidence that CMT-linked mutations of MTMR2 can cause NFL aggregation in a cell line devoid of endogenous intermediate filaments, SW13vim⁻. Mutations in the protein responsible for X-linked myotubular myopathy (myotubularin, MTM1) also induced NFL abnormalities in these cells. We also show that two MTMR2 mutant proteins, G103E and R283W, are unable to form dimers and undergo phosphorylation in vivo , implicating impaired complex formation in myotubularin-related pathology.     

Periaxin mutations cause recessive Dejerine-Sottas neuropathy

The periaxin gene (PRX) encodes two PDZ-domain proteins, L- and S-periaxin, that are required for maintenance of peripheral nerve myelin. Prx(-/-) mice develop a severe demyelinating peripheral neuropathy, despite apparently normal initial formation of myelin sheaths. We hypothesized that mutations in PRX could cause human peripheral myelinopathies. In accordance with this, we identified three unrelated Dejerine-Sottas neuropathy patients with recessive PRX mutations-two with compound heterozygous nonsense and frameshift mutations, and one with a homozygous frameshift mutation. We mapped PRX to 19q13.13-13.2, a region recently associated with a severe autosomal recessive demyelinating neuropathy in a Lebanese family (Delague et al. 2000) and syntenic to the location of Prx on murine chromosome 7 (Gillespie et al. 1997).     

A novel Gly137Asp MPZ mutation in a Charcot-Marie-Tooth disease type 1B family

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous peripheral neuropathy. Myelin protein zero (MPZ) is a major myelin protein of the peripheral nerve and mutations in its gene cause a wide phenotypic spectrum including CMT1B, CMT2I, CMT2J and Dejerine-Sottas syndrome. The objective of this study was to find the causative mutation in a Korean large autosomal dominant demyelinating CMT family (FC240). Clinical disabilities were measured by a functional disability scale (FDS), a CMT neuropathy score (CMTNS), and a 9-hole peg test (9-HPT). Mutational analysis of the FC240 family revealed a novel c.410G>A (Gly137Asp) mutation in the MPZ gene. The mutation was completely co-segregated with affected members in the family, and was not found in controls. The clinical features and electrophysiological findings were compatible with CMT1B. Clinical symptoms revealed phenotypic diversities among family members and generations within the same family. In addition, the present patients with Gly137Asp mutation showed earlier age at onset and slow progression than the reported patient with Gly137Ser. Therefore, it seems that there were wide phenotypic variations within the same family harboring Gly137Asp mutation, and between Gly137Asp and Gly137Ser mutations.     

Mutations in MTMR13, a new pseudophosphatase homologue of MTMR2 and Sbf1, in two families with an autosomal recessive demyelinating form of Charcot-Marie-Tooth disease associated with early-onset glaucoma

Charcot-Marie-Tooth disease (CMT) with autosomal recessive (AR) inheritance is a heterogeneous group of inherited motor and sensory neuropathies. In some families from Japan and Brazil, a demyelinating CMT, mainly characterized by the presence of myelin outfoldings on nerve biopsies, cosegregated as an autosomal recessive trait with early-onset glaucoma. We identified two such large consanguineous families from Tunisia and Morocco with ages at onset ranging from 2 to 15 years. We mapped this syndrome to chromosome 11p15, in a 4.6-cM region overlapping the locus for an isolated demyelinating ARCMT (CMT4B2). In these two families, we identified two different nonsense mutations in the myotubularin-related 13 gene, MTMR13. The MTMR protein family includes proteins with a phosphoinositide phosphatase activity, as well as proteins in which key catalytic residues are missing and that are thus called "pseudophosphatases." MTM1, the first identified member of this family, and MTMR2 are responsible for X-linked myotubular myopathy and Charcot-Marie-Tooth disease type 4B1, an isolated peripheral neuropathy with myelin outfoldings, respectively. Both encode active phosphatases. It is striking to note that mutations in MTMR13 also cause peripheral neuropathy with myelin outfoldings, although it belongs to a pseudophosphatase subgroup, since its closest homologue is MTMR5/Sbf1. This is the first human disease caused by mutation in a pseudophosphatase, emphasizing the important function of these putatively inactive enzymes. MTMR13 may be important for the development of both the peripheral nerves and the trabeculum meshwork, which permits the outflow of the aqueous humor. Both of these tissues have the same embryonic origin.     

Charcot-Marie-Tooth-linked mutant GARS is toxic to peripheral neurons independent of wild-type GARS levels

Charcot-Marie-Tooth disease type 2D (CMT2D) is a dominantly inherited peripheral neuropathy caused by missense mutations in the glycyl-tRNA synthetase gene (GARS). In addition to GARS, mutations in three other tRNA synthetase genes cause similar neuropathies, although the underlying mechanisms are not fully understood. To address this, we generated transgenic mice that ubiquitously over-express wild-type GARS and crossed them to two dominant mouse models of CMT2D to distinguish loss-of-function and gain-of-function mechanisms. Over-expression of wild-type GARS does not improve the neuropathy phenotype in heterozygous Gars mutant mice, as determined by histological, functional, and behavioral tests. Transgenic GARS is able to rescue a pathological point mutation as a homozygote or in complementation tests with a Gars null allele, demonstrating the functionality of the transgene and revealing a recessive loss-of-function component of the point mutation. Missense mutations as transgene-rescued homozygotes or compound heterozygotes have a more severe neuropathy than heterozygotes, indicating that increased dosage of the disease-causing alleles results in a more severe neurological phenotype, even in the presence of a wild-type transgene. We conclude that, although missense mutations of Gars may cause some loss of function, the dominant neuropathy phenotype observed in mice is caused by a dose-dependent gain of function that is not mitigated by over-expression of functional wild-type protein.     

Neurotrophin-4 induces myelin protein zero expression in cultured Schwann cells via the TrkB/PI3K/Akt/mTORC1 pathway

Myelin formation during peripheral nervous system development, as well as myelin repair after injury and in disease, requires multiple intrinsic and extrinsic signals. Neurotrophin-4 (NT-4) is a member of the neurotrophin family, which regulates the development of neuronal networks by participating in the growth of neuronal processes, synaptic development and plasticity, neuronal survival, and differentiation. However, the intracellular signaling pathways by which NT-4 participates in myelination by Schwann cells remain elusive. In this study, we examined the effects of NT-4 on the expression of compact myelin proteins in cultured Schwann cells. Using real-time quantitative RT-PCR and western blotting, we found that NT-4 could significantly enhance the expression of myelin protein zero (MPZ) but not the expression of myelin basic protein or peripheral myelin protein 22. Further, knockdown of truncated TrkB with small interfering RNA could eliminate the effect of NT-4 on MPZ expression. Moreover, we demonstrated that the NT-4-enhanced MPZ expression depended on Akt and mTORC1 signaling. Taken together, these results suggest that NT-4 binds TrkB to enhance the expression of MPZ in Schwann cells, probably through the PI3K/Akt/mTORC1 signaling pathway, thus contributing to myelination.     

Loss-of-Function Alanyl-tRNA Synthetase Mutations Cause an Autosomal-Recessive Early-Onset Epileptic Encephalopathy with Persistent Myelination Defect

Mutations in genes encoding aminoacyl-tRNA synthetases are known to cause leukodystrophies and genetic leukoencephalopathies—heritable disorders that result in white matter abnormalities in the central nervous system. Here we report three individuals (two siblings and an unrelated individual) with severe infantile epileptic encephalopathy, clubfoot, absent deep tendon reflexes, extrapyramidal symptoms, and persistently deficient myelination on MRI. Analysis by whole exome sequencing identified mutations in the nuclear-encoded alanyl-tRNA synthetase (AARS) in these two unrelated families: the two affected siblings are compound heterozygous for p.Lys81Thr and p.Arg751Gly AARS, and the single affected child is homozygous for p.Arg751Gly AARS. The two identified mutations were found to result in a significant reduction in function. Mutations in AARS were previously associated with an autosomal-dominant inherited form of axonal neuropathy, Charcot-Marie-Tooth disease type 2N (CMT2N). The autosomal-recessive AARS mutations identified in the individuals described here, however, cause a severe infantile epileptic encephalopathy with a central myelin defect and peripheral neuropathy, demonstrating that defects of alanyl-tRNA charging can result in a wide spectrum of disease manifestations.     

Common themes in peripheral neuropathy disease genes

After a century of study, mutations in connexin32, peripheral myelin protein22, and protein zero are now known to culminate in the prototypical phenotype of Charcot-Marie-Tooth disease. Many of these mutations have been modeled in rodents and in tissue culture. Consequently, structure-function predictions for these mutations are now possible and detailed analyses of many of them are ongoing. Despite the marked differences in the functions of these three proteins, it is profitable to consider the many similarities between them, including the types of mutational mechanisms and their effects on myelin structure and function. Accordingly, the biology and genetics of Charcot-Marie-Tooth disease and other inherited peripheral neuropathies due to mutations in these proteins are reviewed.     

A dual role of erbB2 in myelination and in expansion of the schwann cell precursor pool

Neuregulin-1 provides an important axonally derived signal for the survival and growth of developing Schwann cells, which is transmitted by the ErbB2/ErbB3 receptor tyrosine kinases. Null mutations of the neuregulin-1, erbB2, or erbB3 mouse genes cause severe deficits in early Schwann cell development. Here, we employ Cre-loxP technology to introduce erbB2 mutations late in Schwann cell development, using a Krox20-cre allele. Cre-mediated erbB2 ablation occurs perinatally in peripheral nerves, but already at E11 within spinal roots. The mutant mice exhibit a widespread peripheral neuropathy characterized by abnormally thin myelin sheaths, containing fewer myelin wraps. In addition, in spinal roots the Schwann cell precursor pool is not correctly established. Thus, the Neuregulin signaling system functions during multiple stages of Schwann cell development and is essential for correct myelination. The thickness of the myelin sheath is determined by the axon diameter, and we suggest that trophic signals provided by the nerve determine the number of times a Schwann cell wraps an axon.     

Binary specification of nonsense codons by splicing and cytoplasmic translation.

Premature translation termination codons resulting from nonsense or frameshift mutations are common causes of genetic disorders. Complications arising from the synthesis of C-terminally truncated polypeptides can be avoided by 'nonsense-mediated decay' of the mutant mRNAs. Premature termination codons in the beta-globin mRNA cause the common recessive form of beta-thalassemia when the affected mRNA is degraded, but the more severe dominant form when the mRNA escapes nonsense-mediated decay. We demonstrate that cells distinguish a premature termination codon within the beta-globin mRNA from the physiological translation termination codon by a two-step specification mechanism. According to the binary specification model proposed here, the positions of splice junctions are first tagged during splicing in the nucleus, defining a stop codon operationally as a premature termination codon by the presence of a 3' splicing tag. In the second step, cytoplasmic translation is required to validate the 3' splicing tag for decay of the mRNA. This model explains nonsense-mediated decay on the basis of conventional molecular mechanisms and allows us to propose a common principle for nonsense-mediated decay from yeast to man.     

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.     

Mutations in the neurofilament light gene linked to Charcot-Marie-Tooth disease cause defects in transport

Neurofilament light gene mutations have been linked to a subset of patients with Charcot-Marie-Tooth disease, the most common inherited motor and sensory neuropathy. We have previously shown that Charcot-Marie-Tooth-linked mutant neurofilament light assembles abnormally in non-neuronal cells. In this study, we have characterized the effects of expression of mutant neurofilament light proteins on axonal transport in a neuronal cell culture model. We demonstrated that the Charcot-Marie-Tooth-linked neurofilament light mutations: (i) affect the axonal transport of mutant neurofilaments; (ii) have a dominant-negative effect on the transport of wild-type neurofilaments; (iii) affect the transport of mitochondria and the anterograde axonal transport marker human amyloid precursor protein; (iv) result in alterations of retrograde axonal transport and (v) cause fragmentation of the Golgi apparatus. Increased neuritic degeneration was observed in neuronal cells overexpressing neurofilament light mutants. Our results suggest that these generalized axonal transport defects could be responsible for the neuropathy in Charcot-Marie-Tooth disease.     

Transmembrane domain of myelin protein zero can form dimers: possible implications for myelin construction

Myelin protein zero (MPZ) is the major integral membrane protein of peripheral nerve myelin in higher vertebrates, mediating homoadhesion of the multiple, spiraling wraps of the myelin sheath. Previous studies have shown that full-length MPZ can form dimers and tetramers, and biochemical studies on the extracellular domain (ECD) indicate that it can form a tetramer, albeit very weakly. On the basis of cross-linking studies and equilibrium sedimentation of a transmembrane (TM) domain peptide (MPZ-TM), we find that the MPZ-TM can form homodimers. We further characterized the dimer by measuring the effects of alanine and leucine substitutions on the ability of the TM to dimerize in Escherichia coli membranes. Our results indicate that the primary packing interface for the MPZ TM homodimer is a glycine zipper (GxxxGxxxG) motif. We also find that the G134R mutation, which lies within the glycine zipper packing interface and causes Charcot-Marie-Tooth disease type 1B, severely inhibits dimerization, suggesting that dimerization of the TM domain may be important for the normal functioning of MPZ. By combining our new results with prior work, we suggest a new model for an MPZ lattice that may form during the construction of myelin.