Peripheral myelin protein 22 kDa and protein zero: domain specific trans-interactions

Disrupting the transmembrane domain interface between PMP22 and MPZ causes peripheral neuropathy

Peripheral Myelin Protein 22 (PMP22) and MPZ are abundant myelin membrane proteins in Schwann cells. The MPZ adhesion protein holds myelin wraps together across the intraperiod line. PMP22 is a tetraspan protein belonging to the Claudin superfamily. Loss of either MPZ or PMP22 causes severe demyelinating Charcot-Marie-Tooth (CMT) peripheral neuropathy, and duplication of PMP22 causes the most common form of CMT, CMT1A. Yet, the molecular functions provided by PMP22 and how its alteration causes CMT are unknown. Here, we find MPZ and PMP22 form a specific complex through interfaces within their transmembrane domains. We also find that the PMP22 A67T patient variant that causes a loss-of-function (hereditary neuropathy with pressure palsies) phenotype maps to this interface, and blocks MPZ association without affecting localization to the plasma membrane or interactions with other proteins. These data define the molecular basis for the MPZ ∼ PMP22 interaction and indicate this complex fulfills an important function in myelinating cells.

Disrupting the transmembrane domain interface between PMP22 and MPZ causes peripheral neuropathy

PMP22 and MPZ are abundant myelin membrane proteins in Schwann cells. The MPZ adhesion protein holds myelin wraps together across the intraperiod line. PMP22 is a tetraspan protein belonging to the Claudin superfamily. Loss of either MPZ or PMP22 causes severe demyelinating Charcot-Marie-Tooth (CMT) peripheral neuropathy, and duplication of PMP22 causes the most common form of CMT, CMT1A. Yet, the molecular functions provided by PMP22 and how its alteration causes CMT are unknown. Here we find MPZ and PMP22 form a specific complex through interfaces within their transmembrane domains. We also find that the PMP22 A67T patient variant that causes a loss-of-function (Hereditary Neuropathy with Pressure Palsies) phenotype maps to this interface, and blocks MPZ association without affecting localization to the plasma membrane or interactions with other proteins. These data define the molecular basis for the MPZ~PMP22 interaction and indicate this complex fulfills an important function in myelinating cells.

Complete Loss of Myelin protein zero (MPZ) in a patient with a late onset Charcot-Marie-Tooth (CMT)

Charcot-Marie-Tooth (CMT) comprises a group of hereditary neuropathies with clinical, epidemiological, and molecular heterogeneity in which variants in more than 80 different genes have been reported. One of the important genes which cause 5% of all CMT cases is Myelin protein zero (P0, MPZ). Variants in this gene have been reported in association with different forms of CMT including classical CMT1, severe DSS (CMT3B), DI-CMT, CMT2I and CMT2J with autosomal dominant (AD) inheritance. To our knowledge, MPZ variants have not been described in autosomal recessive (AR) form of CMT in previous studies. Moreover, its complete deletion has not been reported in human. Here, we described clinical characteristics of a patient with CMT symptoms who demonstrated manifestations of the disease late in his life. We performed exome sequencing for identifying CMT subtype and its associated gene, and follow that co-segregation analysis has been done to characterize inheritance pattern of the disorder. Through using exome sequencing, we identified a novel 4074 bp homozygote deletion which encompasses all 6 exons of the MPZ gene in this patient. After identifying the alteration, variant confirmation and co-segregation analysis have been performed by using specific primers. Our result revealed that the patient's parents were heterozygous for the alteration and they did not show any symptoms of CMT. Although most MPZ variants have been described with early onset CMT with AD pattern of inheritance, the reported patient in our study had late onset form and his parents did not show any symptoms. Considering substantial role of MPZ protein in the biogenesis of peripheral nervous system (PNS) myelin, we proposed that there should be another protein in PNS that compensates for lack of MPZ protein. Taken together, our finding is the first report of MPZ association with AR form of CMT with late onset features. Moreover, our results propose the presence of another protein in PNS myelin biogenesis and its assembly. However, functional studies alongside with other molecular studies are needed to confirm our results and identify the proposed protein.

Activation of 20-HETE Synthase Triggers Oxidative Injury and Peripheral Nerve Damage in Type 2 Diabetic Mice

Diabetic Peripheral Neuropathy (DPN), highly prevalent among patients with diabetes, is characterized by peripheral nerve dysfunction. Reactive Oxygen Species (ROS) overproduction has been suggested to orchestrate diabetic complications including DPN. Untargeted antioxidant therapy has exhibited limited efficacy, highlighting a critical need to explore ROS sources altered in a cell-specific manner in DPN. Cytochromes P450 (CYP) enzymes are prominent sources of ROS. Particularly, the 20-HETE synthase, CYP4A, is reported to mediate diabetes-induced renal, retinal, and cardiovascular injuries. This work investigates the role of CYP4A/20-HETE in DPN and their mechanisms of action. Non-obese type 2 Diabetic mice (MKR) were used and treated with a CYP4A-inhibitor (HET0016) or AMPK-activator (Metformin). Peripheral nerves of MKR mice reflect increased CYP4A and 20-HETE levels, concurrent with altered myelin proteins and sensorimotor deficits. This was associated with increased ROS production and altered Beclin-1 and LC3 protein levels, indicative of disrupted autophagic responses in tandem with AMPK inactivation. AMPK activation via Metformin restored nerve integrity, reduced ROS production, and regulated autophagy. Interestingly, similar outcomes were revealed upon HET0016 treatment whereby ROS production, autophagic responses, and AMPK signaling were normalized in diabetic mice. Altogether, the results highlight hyperglycemia-mediated oxidative injury in DPN through a novel CYP4A/20-HETE/AMPK pathological axis. PERSPECTIVE: To our knowledge, this is the first study to highlight the role of CYPs/20-HETE-induced oxidative injury in the pathogenesis of diabetic peripheral neuropathy. Targeting the identified pathological axis CYP4A/20-HETE/AMPK may be of clinical potential in predicting and alleviating peripheral nerve injury in patients with Type 2 Diabetes Mellitus.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

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.

How Does Protein Zero Assemble Compact Myelin?

Myelin protein zero (P0), a type I transmembrane protein, is the most abundant protein in peripheral nervous system (PNS) myelin-the lipid-rich, periodic structure of membrane pairs that concentrically encloses long axonal segments. Schwann cells, the myelinating glia of the PNS, express P0 throughout their development until the formation of mature myelin. In the intramyelinic compartment, the immunoglobulin-like domain of P0 bridges apposing membranes via homophilic adhesion, forming, as revealed by electron microscopy, the electron-dense, double "intraperiod line" that is split by a narrow, electron-lucent space corresponding to the extracellular space between membrane pairs. The C-terminal tail of P0 adheres apposing membranes together in the narrow cytoplasmic compartment of compact myelin, much like myelin basic protein (MBP). In mouse models, the absence of P0, unlike that of MBP or P2, severely disturbs myelination. Therefore, P0 is the executive molecule of PNS myelin maturation. How and when P0 is trafficked and modified to enable myelin compaction, and how mutations that give rise to incurable peripheral neuropathies alter the function of P0, are currently open questions. The potential mechanisms of P0 function in myelination are discussed, providing a foundation for the understanding of mature myelin development and how it derails in peripheral neuropathies.

Molecular structure and function of myelin protein P0 in membrane stacking

Compact myelin forms the basis of nerve insulation essential for higher vertebrates. Dozens of myelin membrane bilayers undergo tight stacking, and in the peripheral nervous system, this is partially enabled by myelin protein zero (P0). Consisting of an immunoglobulin (Ig)-like extracellular domain, a single transmembrane helix, and a cytoplasmic extension (P0ct), P0 harbours an important task in ensuring the integrity of compact myelin in the extracellular compartment, referred to as the intraperiod line. Several disease mutations resulting in peripheral neuropathies have been identified for P0, reflecting its physiological importance, but the arrangement of P0 within the myelin ultrastructure remains obscure. We performed a biophysical characterization of recombinant P0ct. P0ct contributes to the binding affinity between apposed cytoplasmic myelin membrane leaflets, which not only results in changes of the bilayer properties, but also potentially involves the arrangement of the Ig-like domains in a manner that stabilizes the intraperiod line. Transmission electron cryomicroscopy of native full-length P0 showed that P0 stacks lipid membranes by forming antiparallel dimers between the extracellular Ig-like domains. The zipper-like arrangement of the P0 extracellular domains between two membranes explains the double structure of the myelin intraperiod line. Our results contribute to the understanding of PNS myelin, the role of P0 therein, and the underlying molecular foundation of compact myelin stability in health and disease.

The leptin receptor mutation of the obese Zucker rat causes sciatic nerve demyelination with a centripetal pattern defect

Young male Zucker rats with a leptin receptor mutation are obese, have a non-insulin-dependent diabetes mellitus (NIDDM), and other endocrinopathies. Tibial branches of the sciatic nerve reveal a progressive demyelination that progresses out of the Schwann cells (SCs) where electron-contrast deposits are accumulated while the minor lines or intermembranous SC contacts display exaggerated spacings. Cajal bands contain diversely contrasted vesicles adjacent to the abaxonal myelin layer with blemishes; they appear dispatched centripetally out of many narrow electron densities, regularly spaced around the myelin annulus. These anomalies widen and yield into sectors across the stacked myelin layers. Throughout the worse degradations, the adaxonal membrane remains along the axonal neuroplasm. This peripheral neuropathy with irresponsive leptin cannot modulate hypothalamic-pituitary-adrenal axis and SC neurosteroids, thus exacerbates NIDDM condition. Additionally, the ultrastructure of the progressive myelin alterations may have unraveled a peculiar, centripetal mode of trafficking maintenance of the peripheral nervous system myelin, while some adhesive glycoproteins remain between myelin layers, somewhat hindering the axon mutilation. Heading title: Peripheral neuropathy and myelin.

A dual role for Integrin α6β4 in modulating hereditary neuropathy with liability to pressure palsies

Peripheral myelin protein 22 (PMP22) is a component of compact myelin in the peripheral nervous system. The amount of PMP22 in myelin is tightly regulated, and PMP22 over or under-expression cause Charcot-Marie-Tooth 1A (CMT1A) and Hereditary Neuropathy with Pressure Palsies (HNPP). Despite the importance of PMP22, its function remains largely unknown. It was reported that PMP22 interacts with the β4 subunit of the laminin receptor α6β4 integrin, suggesting that α6β4 integrin and laminins may contribute to the pathogenesis of CMT1A or HNPP. Here we asked if the lack of α6β4 integrin in Schwann cells influences myelin stability in the HNPP mouse model. Our data indicate that PMP22 and β4 integrin may not interact directly in myelinating Schwann cells, however, ablating β4 integrin delays the formation of tomacula, a characteristic feature of HNPP. In contrast, ablation of integrin β4 worsens nerve conduction velocities and non-compact myelin organization in HNPP animals. This study demonstrates that indirect interactions between an extracellular matrix receptor and a myelin protein influence the stability and function of myelinated fibers.

Clinical and Molecular Characterization of PMP22 point mutations in Taiwanese patients with Inherited Neuropathy

Point mutations in the peripheral myelin protein 22 (PMP22) gene have been identified to cause demyelinating Charcot-Marie-Tooth disease (CMT) and hereditary neuropathy with liability to pressure palsy (HNPP). To investigate the mutation spectrum of PMP22 in Han-Chinese population residing in Taiwan, 53 patients with molecularly unassigned demyelinating CMT and 52 patients with HNPP-like neuropathy of unknown genetic causes were screened for PMP22 mutations by Sanger sequencing. Three point mutations were identified in four patients with demyelinating CMT, including c.256 C > T (p.Q86X) in two, and c.310delA (p.I104FfsX7) and c.319 + 1G > A in one each. One PMP22 missense mutation, c.124 T > C (p.C42R), was identified in a patient with HNPP-like neuropathy. The clinical presentations of these mutations vary from mild HNPP-like syndrome to severe infantile-onset demyelinating CMT. In vitro analyses revealed that both PMP22 p.Q86X and p.I104FfsX7 mutations result in truncated PMP22 proteins that are almost totally retained within cytosol, whereas the p.C42R mutation partially impairs cell membrane localization of PMP22 protein. In conclusion, PMP22 point mutations account for 7.5% and 1.9% of demyelinating CMT and HNPP patients with unknown genetic causes, respectively. This study delineates the clinical and molecular features of PMP22 point mutations in Taiwan, and emphasizes their roles in demyelinating CMT or HNPP-like neuropathy.

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.

Topologically Diverse Human Membrane Proteins Partition to Liquid-Disordered Domains in Phase-Separated Lipid Vesicles

The integration of membrane proteins into “lipid raft” membrane domains influences many biochemical processes. The intrinsic structural properties of membrane proteins are thought to mediate their partitioning between membrane domains. However, whether membrane topology influences the targeting of proteins to rafts remains unclear. To address this question, we examined the domain preference of three putative raft-associated membrane proteins with widely different topologies: human caveolin-3, C99 (the 99 residue C-terminal domain of the amyloid precursor protein), and peripheral myelin protein 22. We find that each of these proteins are excluded from the ordered domains of giant unilamellar vesicles containing coexisting liquid-ordered and liquid-disordered phases. Thus, the intrinsic structural properties of these three topologically distinct disease-linked proteins are insufficient to confer affinity for synthetic raft-like domains.

Endoplasmic reticulum quality control and dysmyelination

Dysmyelination contributes to several human diseases including multiple sclerosis, Charcot-Marie-Tooth, leukodystrophies, and schizophrenia and can result in serious neurological disability. Properly formed, compacted myelin sheaths are required for appropriate nerve conduction velocities and the health and survival of neurons. Many different molecular mechanisms contribute to dysmyelination and many of these mechanisms originate at the level of the endoplasmic reticulum. The endoplasmic reticulum is a critical organelle for myelin biosynthesis and maintenance as the site of myelin protein folding quality control, Ca2+ homeostasis, cholesterol biosynthesis, and modulation of cellular stress. This review paper highlights the role of the endoplasmic reticulum and its resident molecules as an upstream and dynamic contributor to myelin and myelin pathologies.

Unravelling crucial biomechanical resilience of myelinated peripheral nerve fibres provided by the Schwann cell basal lamina and PMP22

There is an urgent need for the research of the close and enigmatic relationship between nerve biomechanics and the development of neuropathies. Here we present a research strategy based on the application atomic force and confocal microscopy for simultaneous nerve biomechanics and integrity investigations. Using wild-type and hereditary neuropathy mouse models, we reveal surprising mechanical protection of peripheral nerves. Myelinated peripheral wild-type fibres promptly and fully recover from acute enormous local mechanical compression while maintaining functional and structural integrity. The basal lamina which enwraps each myelinated fibre separately is identified as the major contributor to the striking fibre's resilience and integrity. In contrast, neuropathic fibres lacking the peripheral myelin protein 22 (PMP22), which is closely connected with several hereditary human neuropathies, fail to recover from light compression. Interestingly, the structural arrangement of the basal lamina of Pmp22^−/− fibres is significantly altered compared to wild-type fibres. In conclusion, the basal lamina and PMP22 act in concert to contribute to a resilience and integrity of peripheral nerves at the single fibre level. Our findings and the presented technology set the stage for a comprehensive research of the links between nerve biomechanics and neuropathies.

Glycans of myelin proteins

Human P0 is the main myelin glycoprotein of the peripheral nervous system. It can bind six different glycans, all linked to Asn(93) , the unique glycosylation site. Other myelin glycoproteins, also with a single glycosylation site (PMP22 at Asn(36) , MOG at Asn(31) ), bind only one glycan. The MAG has 10 glycosylation sites; the glycoprotein OMgp has 11 glycosylation sites. Aside from P0, no comprehensive data are available on other myelin glycoproteins. Here we review and analyze all published data on the physicochemical structure of the glycans linked to P0, PMP22, MOG, and MAG. Most data concern bovine P0, whose glycan moieties have an MW ranging from 1,294.56 Da (GP3) to 2,279.94 Da (GP5). The pI of glycosylated P0 protein varies from pH 9.32 to 9.46. The most charged glycan is MS2 containing three sulfate groups and one glucuronic acid; whereas the least charged one is the BA2 residue. All glycans contain one fucose and one galactose. The most mannose rich are the glycans MS2 and GP4, each of them has four mannoses; OPPE1 contains five N-acetylglucosamines and one sulfated glucuronic acid; GP4 contains one sialic acid. Furthermore, human P0 variants causing both gain and loss of glycosylation have been described and cause peripheral neuropathies with variable clinical severity. In particular, the substitution T(95) →M is a very common in Europe and is associated with a late-onset axonal neuropathy. Although peripheral myelin is made up largely of glycoproteins, mutations altering glycosylation have been described only in P0. This attractive avenue of research requires further study.

Myelin-specific proteins: a structurally diverse group of membrane-interacting molecules

The myelin sheath is a multilayered membrane in the nervous system, which has unique biochemical properties. Myelin carries a set of specific high-abundance proteins, the structure and function of which are still poorly understood. The proteins of the myelin sheath are involved in a number of neurological diseases, including autoimmune diseases and inherited neuropathies. In this review, we briefly discuss the structural properties and functions of selected myelin-specific proteins (P0, myelin oligodendrocyte glycoprotein, myelin-associated glycoprotein, myelin basic protein, myelin-associated oligodendrocytic basic protein, P2, proteolipid protein, peripheral myelin protein of 22 kDa, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and periaxin); such properties include, for example, interactions with lipid bilayers and the presence of large intrinsically disordered regions in some myelin proteins. A detailed understanding of myelin protein structure and function at the molecular level will be required to fully grasp their physiological roles in the myelin sheath.

The PMP22 gene and its related diseases

Peripheral myelin protein-22 (PMP22) is primarily expressed in the compact myelin of the peripheral nervous system. Levels of PMP22 have to be tightly regulated since alterations of PMP22 levels by mutations of the PMP22 gene are responsible for >50 % of all patients with inherited peripheral neuropathies, including Charcot-Marie-Tooth type-1A (CMT1A) with trisomy of PMP22, hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22, and CMT1E with point mutations of PMP22. While overexpression and point-mutations of the PMP22 gene may produce gain-of-function phenotypes, deletion of PMP22 results in a loss-of-function phenotype that reveals the normal physiological functions of the PMP22 protein. In this article, we will review the basic genetics, biochemistry and molecular structure of PMP22, followed by discussion of the current understanding of pathogenic mechanisms involving in the inherited neuropathies with mutations in PMP22 gene.

Rapamycin activates autophagy and improves myelination in explant cultures from neuropathic mice

Misexpression and cytosolic retention of peripheral myelin protein 22 (PMP22) within Schwann cells (SCs) is associated with a genetically heterogeneous group of demyelinating peripheral neuropathies. PMP22 overproducer C22 and spontaneous mutant Trembler J (TrJ) mice display neuropathic phenotypes and affected nerves contain abnormally localized PMP22. Nutrient deprivation-induced autophagy is able to suppress the formation of PMP22 aggregates in a toxin-induced cellular model, and improve locomotor performance and myelination in TrJ mice. As a step toward therapies, we assessed whether pharmacological activation of autophagy by rapamycin (RM) could facilitate the processing of PMP22 within neuropathic SCs and enhance their capacity to myelinate peripheral axons. Exposure of mouse SCs to RM induced autophagy in a dose- and time-dependent manner and decreased the accumulation of poly-ubiquitinated substrates. The treatment of myelinating dorsal root ganglion (DRG) explant cultures from neuropathic mice with RM (25 nm) improved the processing of PMP22 and increased the abundance and length of myelin internodes, as well as the expression of myelin proteins. Notably, RM is similarly effective in both the C22 and TrJ model, signifying that the benefit overlaps among distinct genetic models of PMP22 neuropathies. Furthermore, lentivirus-mediated shRNA knockdown of the autophagy-related gene 12 (Atg12) abolished the activation of autophagy and the increase in myelin proteins, demonstrating that autophagy is critical for the observed improvement. Together, these results support the potential use of RM and other autophagy-enhancing compounds as therapeutic agents for PMP22-associated demyelinating neuropathies.

Compound heterozygous PMP22 deletion mutations causing severe Charcot-Marie-Tooth disease type 1

We present a 3⅓-year-old girl with severe Charcot-Marie-Tooth disease type 1 (Dejerine-Sottas disease), who was a compound heterozygote carrying a deletion of the whole peripheral myelin protein 22 (PMP22) and a deletion of exon 5 in the other PMP22 allele. Haplotype analyses and sequence determination revealed a 11.2 kb deletion spanning from intron 4 to 3'-region of PMP22, which was likely generated by nonhomologous end joining. Severely affected patients carrying a PMP22 deletion must be analyzed for the mutations of the other copy of PMP22.

PMP22 expression in dermal nerve myelin from patients with CMT1A

Charcot-Marie-Tooth disease type 1A (CMT1A) is caused by a 1.4 Mb duplication on chromosome 17p11.2, which contains the peripheral myelin protein-22 (PMP22) gene. Increased levels of PMP22 in compact myelin of peripheral nerves have been demonstrated and presumed to cause the phenotype of CMT1A. The objective of the present study was to determine whether an extra copy of the PMP22 gene in CMT1A disrupts the normally coordinated expression of PMP22 protein in peripheral nerve myelin and to evaluate PMP22 over-expression in patients with CMT1A and determine whether levels of PMP22 are molecular markers of disease severity. PMP22 expression was measured by taking skin biopsies from patients with CMT1A (n = 20) and both healthy controls (n = 7) and patients with Hereditary Neuropathy with liability to Pressure Palsies (HNPP) (n = 6), in which patients have only a single copy of PMP22. Immunological electron microscopy was performed on the skin biopsies to quantify PMP22 expression in compact myelin. Similar biopsies were analysed by real time PCR to measure PMP22 mRNA levels. Results were also correlated with impairment in CMT1A, as measured by the validated CMT Neuropathy Score. Most, but not all patients with CMT1A, had elevated PMP22 levels in myelin compared with the controls. The levels of PMP22 in CMT1A were highly variable, but not in HNPP or the controls. However, there was no correlation between neurological disabilities and the level of over-expression of PMP22 protein or mRNA in patients with CMT1A. The extra copy of PMP22 in CMT1A results in disruption of the tightly regulated expression of PMP22. Thus, variability of PMP22 levels, rather than absolute level of PMP22, may play an important role in the pathogenesis of CMT1A.

Diagnostic value of ultrastructural nerve examination in Charcot-Marie-Tooth disease: two CMT 1B cases with pseudo-recessive inheritance

We report two sporadic patients of CMT disease in different consanguineous families. The electrophysiological examination led to the diagnosis of a severe demyelinating neuropathy. The nerve biopsies exhibited numerous outfoldings of the myelin sheaths and onion-bulb proliferations. The consanguinity and the histological findings pointed to a diagnosis of CMT 4B. However, the detection of abnormal and regular widenings between the major dense lines of the myelin lamellae by electron microscopy led us to search for a P0 gene mutation. Two heterozygous mutations of this gene were identified: S63F and N131Y. Different aspects of uncompacted myelin lamellae have been described in some cases of P0 mutations and a few now appear to be quite specific to it. More than 30 genes are implicated in CMT and as mutation search is time- and money-consuming, we believe that in some selected patients ultrastructural examination of nerves, among other criteria, helps orientate the molecular diagnosis of CMT.

Developmental abnormalities in the nerves of peripheral myelin protein 22-deficient mice

Peripheral myelin protein 22 (PMP22) is a tetraspan glycoprotein whose misexpression is associated with a family of hereditary peripheral neuropathies. In a recent report, we have characterized a novel PMP22-deficient mouse model in which the first two coding exons were replaced by the lacZ reporter. To investigate further the myelin abnormalities in the absence of PMP22, sciatic nerves and dorsal root ganglion (DRG) neuron explant cultures from PMP22-deficient mice were studied at various stages of myelination. Throughout the first 3 months of postnatal development, myelin protein and beta4 integrin levels are dramatically reduced, whereas p75 and beta1 integrin remain elevated. By immunostaining, the distributions of several glial proteins, including beta4 integrin, the voltage-gated potassium channel Kv1.1, and E-cadherin, are altered. Schwann cells from PMP22-deficient mice are able to produce limited amounts of myelin in DRG explant cultures, yet the internodal segments are dramatically fewer and shorter. The comparison of PMP22-deficient mice with other PMP22 mutant models reveals that the decrease in beta4 integrin is specific to an absence of PMP22. Furthermore, whereas lysosome-associated membrane protein 1 and ubiquitin are notably up-regulated in nerves of PMP22-deficient mice, heat shock protein 70 levels remain constant or decrease compared with wild-type or PMP22 mutant samples. Together these results support a role for PMP22 in the early events of peripheral nerve myelination. Additionally, although myelin abnormalities are a commonality among PMP22 neuropathic models, the underlying subcellular mechanisms are distinct and depend on the specific genetic abnormality.

Animal models of Charcot-Marie-Tooth disease type 1A

The most frequent genetic subtype of Charcot-Marie-Tooth disease is CMT1A, linked to chromosome 17p11.2. In the majority of cases, CMT1A is a gene dosage disease associated with a 1.5 Mb large genomic duplication. Transgenic models with extra copies of the Pmp22 gene have provided formal proof that overexpression of only this candidate gene is sufficent to cause peripheral demyelination, onion bulb formation, secondary axonal loss, and progressive muscle atrophy, the pathological hallmarks of CMT1A. The transgenic CMT rat with about 1.6-fold PMP22 overexpression exhibits clinical abnormalities, such as reduced nerve conduction velocity and lower grip strength that mimick findings in CMT1A patients. Also transgenic mice, carrying yeast artifical chromosomes as Pmp22 transgenes, demonstrate the variability of disease expression as a function of increased gene dosage. Recently, the first rational experimental therapies of CMT1A were tested, using transgenic animal models. In one proof-of-principle study with the CMT rat, a synthetic antagonist of the nuclear progesterone receptor was shown to reduce PMP22 overexpression and to ameliorate the clinical severity. In another study, administration of ascorbic acid, an essential factor of in vitro myelination, prolonged the survival and restored myelination of a dysmyelinated mouse model. Application of gene expression analysis to nerve biopsies that are readily available from such CMT1A animal models might identify additional pharmacological targets.

Pathomechanisms of mutant proteins in Charcot-Marie-Tooth disease

We review the putative functions and malfunctions of proteins encoded by genes mutated in Charcot-Marie-Tooth disease (CMT; inherited motor and sensory neuropathies) in normal and affected peripheral nerves. Some proteins implicated in demyelinating CMT, peripheral myelin protein 22, protein zero (P0), and connexin32 (Cx32/GJB1) are crucial components of myelin. Periaxin is involved in connecting myelin to the surrounding basal lamina. Early growth response 2 (EGR2) and Sox10 are transcriptional regulators of myelin genes. Mutations in the small integral membrane protein of lysosome/late endosome, the myotubularin-related protein 2 (MTMR2), and MTMR13/set-binding factor 2 are involved in vesicle and membrane transport and the regulation of protein degradation. Pathomechanisms related to alterations of these processes are a widespread phenomenon in demyelinating neuropathies because mutations of myelin components may also affect protein biosynthesis, transport, and/or degradation. Related disease mechanisms are also involved in axonal neuropathies although there is considerably more functional heterogeneity. Some mutations, most notably in P0, GJB1, ganglioside-induced differentiation-associated protein 1 (GDAP1), neurofilament light chain (NF-L), and dynamin 2 (DNM2), can result in demyelinating or axonal neuropathies introducing additional complexity in the pathogenesis. Often, this relates to the intimate connection between Schwann cells and neurons/axons leading to axonal damage even if the mutation-caused defect is Schwann-cell-autonomous. This mechanism is likely for P0 and Cx32 mutations and provides the basis for the unifying hypothesis that also demyelinating neuropathies develop into functional axonopathies. In GDAP1 and DNM2 mutants, both Schwann cells and axons/neurons might be directly affected. NF-L mutants have a primary neuronal defect but also cause demyelination. The major challenge ahead lies in determining the individual contributions by neurons and Schwann cells to the pathology over time and to delineate the detailed molecular functions of the proteins associated with CMT in health and disease.

Myelin disorders: Causes and perspectives of Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth (CMT) disease is a common hereditary neuropathy that causes progressive distally pronounced muscle weakness and can lead to life-long disability in patients. In most cases, the disorder has been associated with a partial duplication of human chromosome 17 (CMT1A), causing 1.5-fold overexpression of the peripheral myelin protein 22 kDa (PMP22). Increased PMP22 gene dosage results in demyelination, secondary axonal loss, and neurogenic muscle atrophy. Experimental therapeutic approaches based on the role of progesterone and ascorbic acid in myelin formation recently have reached preclinical proof-of-principle trials in rodents. It was shown that progesterone receptor antagonists can reduce PMP22 overexpression and clinical severity in a CMT1A rat model. Furthermore, ascorbic acid treatment reduced premature death and demyelination in a CMT1A mouse model. Thus, basic research has opened up new vistas for the understanding and treatment of hereditary neuropathies.

Peripheral myelin protein 22 is in complex with alpha6beta4 integrin, and its absence alters the Schwann cell basal lamina

Peripheral myelin protein 22 (PMP22) is a tetraspan membrane glycoprotein, the misexpression of which is associated with hereditary demyelinating neuropathies. Myelinating Schwann cells (SCs) produce the highest levels of PMP22, yet the function of the protein in peripheral nerve biology is unresolved. To investigate the potential roles of PMP22, we engineered a novel knock-out (-/-) mouse line by replacing the first two coding exons of pmp22 with the lacZ reporter. PMP22-deficient mice show strong beta-galactosidase reactivity in peripheral nerves, cartilage, intestines, and lungs, whereas phenotypically they display the characteristics of tomaculous neuropathy. In the absence of PMP22, myelination of peripheral nerves is delayed, and numerous axon-SC profiles show loose basal lamina, suggesting altered interactions of the glial cells with the extracellular matrix. The levels of beta4 integrin, a molecule involved in the linkage between SCs and the basal lamina, are severely reduced in nerves of PMP22-deficient mice. During early stages of myelination, PMP22 and beta4 integrin are coexpressed at the cell surface and can be coimmunoprecipitated together with laminin and alpha6 integrin. In agreement, in clone A colonic carcinoma cells, epitope-tagged PMP22 forms a complex with beta4 integrin. Together, these data indicate that PMP22 is a binding partner in the integrin/laminin complex and is involved in mediating the interaction of SCs with the extracellular environment.

Schwann cells and the pathogenesis of inherited motor and sensory neuropathies (Charcot-Marie-Tooth disease)

Over the last 15 years, a number of mutations in a variety of genes have been identified that lead to inherited motor and sensory neuropathies (HMSN), also called Charcot-Marie-Tooth disease (CMT). In this review we will focus on the molecular and cellular mechanisms that cause the Schwann cell pathologies observed in dysmyelinating and demyelinating forms of CMT. In most instances, the underlying gene defects alter primarily myelinating Schwann cells followed by secondary axonal degeneration. The first set of proteins affected by disease-causing mutations includes the myelin components PMP22, P0/MPZ, Cx32/GJB1, and periaxin. A second group contains the regulators of myelin gene transcription EGR2/Krox20 and SOX10. A third group is composed of intracellular Schwann cells proteins that are likely to be involved in the synthesis, transport and degradation of myelin components. These include the myotubularin-related lipid phosphatase MTMR2 and its regulatory binding partner MTMR13/SBF2, SIMPLE, and potentially also dynamin 2. Mutations affecting the mitochondrial fission factor GDAP1 may indicate an important contribution of mitochondria in myelination or myelin maintenance, whereas the functions of other identified genes, including NDRG1, KIAA1985, and the tyrosyl-tRNA synthase YARS, are not yet clear. Mutations in GDAP1, YARS, and the pleckstrin homology domain of dynamin 2 lead to an intermediate form of CMT that is characterized by moderately reduced nerve conduction velocity consistent with minor myelin deficits. Whether these phenotypes originate in Schwann cells or in neurons, or whether both cell types are directly affected, remains a challenging question. However, based on the advances in systematic gene identification in CMT and the analyses of the function and dysfunction of the affected proteins, crucially interconnected pathways in Schwann cells in health and disease have started to emerge. These networks include the control of myelin formation and stability, membrane trafficking, intracellular protein sorting and quality control, and may extend to mitochondrial dynamics and basic protein biosynthesis.

Pathomechanisms of mutant proteins in Charcot-Marie-Tooth disease

We review the putative functions and malfunctions of proteins encoded by genes mutated in Charcot-Marie-Tooth disease (CMT; inherited motor and sensory neuropathies) in normal and affected peripheral nerves. Some proteins implicated in demyelinating CMT, peripheral myelin protein 22, protein zero (P0), and connexin32 (Cx32/GJB1) are crucial components of myelin. Periaxin is involved in connecting myelin to the surrounding basal lamina. Early growth response 2 (EGR2) and Sox10 are transcriptional regulators of myelin genes. Mutations in the small integral membrane protein of lysosome/late endosome, the myotubularin-related protein 2 (MTMR2), and MTMR13/set-binding factor 2 are involved in vesicle and membrane transport and the regulation of protein degradation. Pathomechanisms related to alterations of these processes are a widespread phenomenon in demyelinating neuropathies because mutations of myelin components may also affect protein biosynthesis, transport, and/or degradation. Related disease mechanisms are also involved in axonal neuropathies although there is considerably more functional heterogeneity. Some mutations, most notably in P0, GJB1, ganglioside-induced differentiation-associated protein 1 (GDAP1), neurofilament light chain (NF-L), and dynamin 2 (DNM2), can result in demyelinating or axonal neuropathies introducing additional complexity in the pathogenesis. Often, this relates to the intimate connection between Schwann cells and neurons/axons leading to axonal damage even if the mutation-caused defect is Schwann-cell-autonomous. This mechanism is likely for P0 and Cx32 mutations and provides the basis for the unifying hypothesis that also demyelinating neuropathies develop into functional axonopathies. In GDAP1 and DNM2 mutants, both Schwann cells and axons/neurons might be directly affected. NF-L mutants have a primary neuronal defect but also cause demyelination. The major challenge ahead lies in determining the individual contributions by neurons and Schwann cells to the pathology over time and to delineate the detailed molecular functions of the proteins associated with CMT in health and disease.