Myelin protein zero: mutations in the cytoplasmic domain interfere with its cellular trafficking

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

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.

Molecular basis of axonal dysfunction and traffic impairments in CMT

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders. It comprises a group of diseases caused by mutations in genes involved in Schwann cells homeostasis and neuronal function that affect the peripheral nerves. So far mutations in more than 33 genes have been identified causing either the demyelinating form (CMT1) or the axonal form (CMT2). Genes involving a large variety of unrelated functions may lead to the same phenotype when mutated. Our review will focus on the common link between genes causing axonal phenotypes like MFN2, KIF1B, DYNC1H1, Rab7, TRPV4, ARSs, NEFL, HSPB1, MPZ, and HSPB8. While KIF1B and DYNC1H1, two genes coding for molecular motors, are directly linked to axonal transport, the involvement of the other CMT2-causing genes in this function is less obvious. However, the last years have seen a growing list of evidence demonstrating that intracellular trafficking and mitochondrial dynamics might be dysfunctional in CMT2, and these mechanisms might present a common link between dissimilar CMT2-causing genes. The involvement of impaired transport in the pathogenesis of other rare neurological diseases or recessive CMT2 is also discussed.

Mpz gene suppression by shRNA increases Schwann cell apoptosis in vitro

We investigated the effects of short hairpin RNA (shRNA) on myelin protein zero (MPZ) gene expression in Schwann cells (SCs) in vitro and determined the effects of the MPZ gene suppression on the survival of SCs. The MPZ-specific shRNA was introduced into a lentiviral vector for expression under the U6 promoter, and the viral vector-based shRNAs were used to infect cultured SCs. The efficiency of MPZ knockdown was analyzed by real time-PCR (RT-PCR) and western blotting. Flow cytometric analysis and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) were used to determine the cell cycle and the amount of apoptosis of SCs. We found that MPZ shRNAs significantly inhibited the expression of the MPZ gene and induced SC apoptosis in vitro. These results provided interesting experimental evidence for understanding the mechanism of demyelinating neuropathies caused by MPZ gene malfunction.

Subcellular localization of Mayven following expression of wild type and mutant EGFP tagged cDNAs

BACKGROUND

Process formation by glial cells is crucial to their function. Mayven, an actin binding, multi-domain polypeptide, and member of the BTB-BACK-Kelch family have been shown to be important in oligodendrocyte process extension. To assess the role of Mayven in neural cell process extension we have tracked the subcellular distribution of exogenous Mayven following expression of a rat Mayven -EGFP cDNA in a variety of neural cell backgrounds and specifically in OEC tranfectants following drug treatment to disrupt the integrity of the cytoskeleton. A comparison was made between the subcellular localization following transient transfection of OECs with full-length Mayven cDNA and a series of mutant domain constructs.

RESULTS

The subcellular location of Mayven in OEC transfectants showed a characteristic distribution with intense foci of staining towards the process tips corresponding to regions of accumulated Mayven overlapping in part with lammelipodial actin and was absent from the filipodia and the outer membrane. This signature pattern was also observed in Schwann cells, Oli-Neu cells, astrocytes and the neuroblastoma cell line B104 transfectants and resembled the exogenous and endogenous Mayven distribution in oligodendrocytes. This contrasted with the localization pattern in non-neural cells. There was a re-localization of Mayven in OEC transfectants following drug treatment to challenge the integrity of the actin cytoskeleton while breakdown of the microtubular component had no discernible impact on the accumulation of Mayven in the process tips. Deletion of the first three amino acids of the SH3 motif of the putative Fyn Kinase binding domain at the amino terminus significantly compromised this signature pattern as did the removal of the last Kelch repeat unit of six unit Kelch domain comprising the carboxyl terminus. In addition, there was a reduction in process length in mutant transfectants. Co-expression studies with a haemagglutinin (HA) tagged wild type Mayven cDNA and EGFP tagged mutant cDNAs suggested a homomeric interaction mediated by the BTB/POZ domain.

CONCLUSIONS

Exogenous Mayven is transported to the lamellipodia in neural transfectants associating with the actin cytoskeletal network. In addition to the importance of the internal BTB/POZ domain, this subcellular distribution pattern is dependent on the presence of an intact amino and carboxyl terminus.

Cellular characterization of MPZ mutations presenting with diverse clinical phenotypes

Mutations in MPZ, which encodes myelin protein zero (P(0)), may lead to different subtypes of Charcot-Marie-Tooth disease (CMT). The aim of this study was to characterize the cellular manifestations of various MPZ mutations associated with CMT1, Dejerine-Sottas syndrome (DSS) and CMT2, and to correlate their cellular and clinical phenotypes. Nine P(0) mutants associated with CMT1 (P(0)S63F, R98H, R277S, and S233fs), DSS (P(0) I30T and R98C), and CMT2 (P(0)S44F, D75V, and T124M), were investigated. Wild-type and mutant P(0) fused with fluorescent proteins were expressed in vitro to monitor their intracellular localization. An adhesiveness assay was used to evaluate the adhesiveness of the transfected cells. Protein localization and cell adhesiveness of each mutant protein were compared and correlated with their clinical phenotypes. Three different intracellular localization patterns of the mutant P(0) were observed. Wild-type P(0), P(0)I30T, S44F, S63F, D75V, T124M, and R227S were mostly localized on the cell membrane, P(0)R98H, and R98C were found in the endoplasmic reticulum (ER) or Golgi apparatus, and P(0)S233fs formed aggregates within the ER. Cells expressing mutant P(0), as compared with those expressing wild-type P(0), demonstrated variable degrees of reduction in the cell adhesiveness. The molecular patho-mechanisms of MPZ mutations are likely very complex and the clinical phenotype must be influenced by many genetic or environmental factors. This complexity may contribute to the highly variable clinical manifestations resulting from different MPZ mutations.

Myelin sheaths are formed with proteins that originated in vertebrate lineages

All vertebrate nervous systems, except those of agnathans, make extensive use of the myelinated fiber, a structure formed by coordinated interplay between neuronal axons and glial cells. Myelinated fibers, by enhancing the speed and efficiency of nerve cell communication allowed gnathostomes to evolve extensively, forming a broad range of diverse lifestyles in most habitable environments. The axon-covering myelin sheaths are structurally and biochemically novel as they contain high portions of lipid and a few prominent low molecular weight proteins often considered unique to myelin. Here we searched genome and EST databases to identify orthologs and paralogs of the following myelin-related proteins: (1) myelin basic protein (MBP), (2) myelin protein zero (MPZ, formerly P0), (3) proteolipid protein (PLP1, formerly PLP), (4) peripheral myelin protein-2 (PMP2, formerly P2), (5) peripheral myelin protein-22 (PMP22) and (6) stathmin-1 (STMN1). Although widely distributed in gnathostome/vertebrate genomes, neither MBP nor MPZ are present in any of nine invertebrate genomes examined. PLP1, which replaced MPZ in tetrapod CNS myelin sheaths, includes a novel 'tetrapod-specific' exon (see also Möbius et al., 2009). Like PLP1, PMP2 first appears in tetrapods and like PLP1 its origins can be traced to invertebrate paralogs. PMP22, with origins in agnathans, and STMN1 with origins in protostomes, existed well before the evolution of gnathostomes. The coordinated appearance of MBP and MPZ with myelin sheaths and of PLP1 with tetrapod CNS myelin suggests interdependence - new proteins giving rise to novel vertebrate structures.

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.