A tripartite nuclear localization signal in the PDZ-domain protein L-periaxin

Dynamic early clusters of nodal proteins contribute to node of Ranvier assembly during myelination of peripheral neurons

Voltage-gated sodium channels cluster in macromolecular complexes at nodes of Ranvier to promote rapid nerve impulse conduction in vertebrate nerves. Node assembly in peripheral nerves is thought to be initiated at heminodes at the extremities of myelinating Schwann cells, and fusion of heminodes results in the establishment of nodes. Here we show that assembly of 'early clusters' of nodal proteins in the murine axonal membrane precedes heminode formation. The neurofascin (Nfasc) proteins are essential for node assembly, and the formation of early clusters also requires neuronal Nfasc. Early clusters are mobile and their proteins are dynamically recruited by lateral diffusion. They can undergo fusion not only with each other but also with heminodes, thus contributing to the development of nodes in peripheral axons. The formation of early clusters constitutes the earliest stage in peripheral node assembly and expands the repertoire of strategies that have evolved to establish these essential structures.

Completion of neuronal remodeling prompts myelination along developing motor axon branches

Neuronal remodeling and myelination are two fundamental processes during neurodevelopment. How they influence each other remains largely unknown, even though their coordinated execution is critical for circuit function and often disrupted in neuropsychiatric disorders. It is unclear whether myelination stabilizes axon branches during remodeling or whether ongoing remodeling delays myelination. By modulating synaptic transmission, cytoskeletal dynamics, and axonal transport in mouse motor axons, we show that local axon remodeling delays myelination onset and node formation. Conversely, glial differentiation does not determine the outcome of axon remodeling. Delayed myelination is not due to a limited supply of structural components of the axon-glial unit but rather is triggered by increased transport of signaling factors that initiate myelination, such as neuregulin. Further, transport of promyelinating signals is regulated via local cytoskeletal maturation related to activity-dependent competition. Our study reveals an axon branch-specific fine-tuning mechanism that locally coordinates axon remodeling and myelination.

Neurofascin and Kv7.3 are delivered to somatic and axon terminal surface membranes en route to the axon initial segment

Ion channel complexes promote action potential initiation at the mammalian axon initial segment (AIS), and modulation of AIS size by recruitment or loss of proteins can influence neuron excitability. Although endocytosis contributes to AIS turnover, how membrane proteins traffic to this proximal axonal domain is incompletely understood. Neurofascin186 (Nfasc186) has an essential role in stabilising the AIS complex to the proximal axon, and the AIS channel protein Kv7.3 regulates neuron excitability. Therefore, we have studied how these proteins reach the AIS. Vesicles transport Nfasc186 to the soma and axon terminal where they fuse with the neuronal plasma membrane. Nfasc186 is highly mobile after insertion in the axonal membrane and diffuses bidirectionally until immobilised at the AIS through its interaction with AnkyrinG. Kv7.3 is similarly recruited to the AIS. This study reveals how key proteins are delivered to the AIS and thereby how they may contribute to its functional plasticity.

Input-Output Relationship of CA1 Pyramidal Neurons Reveals Intact Homeostatic Mechanisms in a Mouse Model of Fragile X Syndrome

Cellular hyperexcitability is a salient feature of fragile X syndrome animal models. The cellular basis of hyperexcitability and how it responds to changing activity states is not fully understood. Here, we show increased axon initial segment length in CA1 of the Fmr1^-/y mouse hippocampus, with increased cellular excitability. This change in length does not result from reduced AIS plasticity, as prolonged depolarization induces changes in AIS length independent of genotype. However, depolarization does reduce cellular excitability, the magnitude of which is greater in Fmr1^-/y neurons. Finally, we observe reduced functional inputs from the entorhinal cortex, with no genotypic difference in the firing rates of CA1 pyramidal neurons. This suggests that AIS-dependent hyperexcitability in Fmr1^-/y mice may result from adaptive or homeostatic regulation induced by reduced functional synaptic connectivity. Thus, while AIS length and intrinsic excitability contribute to cellular hyperexcitability, they may reflect a homeostatic mechanism for reduced synaptic input onto CA1 neurons.

Flexible Players within the Sheaths: The Intrinsically Disordered Proteins of Myelin in Health and Disease

Myelin ensheathes selected axonal segments within the nervous system, resulting primarily in nerve impulse acceleration, as well as mechanical and trophic support for neurons. In the central and peripheral nervous systems, various proteins that contribute to the formation and stability of myelin are present, which also harbor pathophysiological roles in myelin disease. Many myelin proteins have common attributes, including small size, hydrophobic segments, multifunctionality, longevity, and regions of intrinsic disorder. With recent advances in protein biophysical characterization and bioinformatics, it has become evident that intrinsically disordered proteins (IDPs) are abundant in myelin, and their flexible nature enables multifunctionality. Here, we review known myelin IDPs, their conservation, molecular characteristics and functions, and their disease relevance, along with open questions and speculations. We place emphasis on classifying the molecular details of IDPs in myelin, and we correlate these with their various functions, including susceptibility to post-translational modifications, function in protein–protein and protein–membrane interactions, as well as their role as extended entropic chains. We discuss how myelin pathology can relate to IDPs and which molecular factors are potentially involved.

Novel mutation in the periaxin gene causal to Charcot-Marie-Tooth disease type 4F

Charcot–Marie–Tooth (CMT) disease is the most common hereditary neuropathy. Mutations in the periaxin gene (PRX) can cause CMT type 4F, an autosomal recessive neuropathy, which is clinically characterized by slowly progressive distal muscle atrophy and weakness, with pes cavus deformity of the foot, and the absence of deep tendon reflexes. To date, dozens of reports of PRX mutations have been published worldwide, but none have been reported in Chinese patients. Here, we describe a 14-year-old Chinese boy with neuropathy characterized by slowly progressive limb weakness and atrophy, as well as sensory ataxia, whose cerebrospinal protein levels were 1627 mg/L. Genetic analysis identified a novel homozygous mutation, c.1174C>T (p.R392X), in exon 6 of PRX, which is the first case of its kind recorded in China.

Ezrin interacts with L-periaxin by the "head to head and tail to tail" mode and influences the location of L-periaxin in Schwann cell RSC96

In the peripheral nervous system (PNS), Schwann cells (SCs) are required for the myelination of axons. Periaxin (PRX), one of the myelination proteins expressed in SCs, is critical for the normal development and maintenance of PNS. As a member of the ERM (ezrin-radxin-moesin) protein family, ezrin holds our attention since their link to the formation of the nodes of Ranvier. Furthermore, PRX and ezrin are co-expressed in cytoskeletal complexes with periplakin and desmoyokin in lens fiber cells. In the present study, we observed that L-periaxin and ezrin interacted in a "head to head and tail to tail" mode in SC RSC96 through NLS3 region of L-periaxin with F3 subdomain of ezrin interaction, and the region of L-periaxin (residues 1368-1461) with ezrin (residues 475-557) interaction. A phosphorylation-mimicking mutation of ezrin resulted in L-periaxin accumulation on SC RSC96 membrane. Ezrin could inhibit the self-association of L-periaxin, and ezrin overexpression in sciatic nerve injury rats could facilitate the repair of impaired myelin sheath. Therefore, the interaction between L-periaxin and ezrin may adopt a close form to complete protein accumulation and to participate in myelin sheath maintenance.

MotifAnalyzer-PDZ: A computational program to investigate the evolution of PDZ-binding target specificity

Recognition of short linear motifs (SLiMs) or peptides by proteins is an important component of many cellular processes. However, due to limited and degenerate binding motifs, prediction of cellular targets is challenging. In addition, many of these interactions are transient and of relatively low affinity. Here, we focus on one of the largest families of SLiM-binding domains in the human proteome, the PDZ domain. These domains bind the extreme C-terminus of target proteins, and are involved in many signaling and trafficking pathways. To predict endogenous targets of PDZ domains, we developed MotifAnalyzer-PDZ, a program that filters and compares all motif-satisfying sequences in any publicly available proteome. This approach enables us to determine possible PDZ binding targets in humans and other organisms. Using this program, we predicted and biochemically tested novel human PDZ targets by looking for strong sequence conservation in evolution. We also identified three C-terminal sequences in choanoflagellates that bind a choanoflagellate PDZ domain, the Monsiga brevicollis SHANK1 PDZ domain (mbSHANK1), with endogenously-relevant affinities, despite a lack of conservation with the targets of a homologous human PDZ domain, SHANK1. All three are predicted to be signaling proteins, with strong sequence homology to cytosolic and receptor tyrosine kinases. Finally, we analyzed and compared the positional amino acid enrichments in PDZ motif-satisfying sequences from over a dozen organisms. Overall, MotifAnalyzer-PDZ is a versatile program to investigate potential PDZ interactions. This proof-of-concept work is poised to enable similar types of analyses for other SLiM-binding domains (e.g., MotifAnalyzer-Kinase). MotifAnalyzer-PDZ is available at http://motifAnalyzerPDZ.cs.wwu.edu.

Direct Binding of the Flexible C-Terminal Segment of Periaxin to β4 Integrin Suggests a Molecular Basis for CMT4F

The process of myelination in the nervous system requires a coordinated formation of both transient and stable supramolecular complexes. Myelin-specific proteins play key roles in these assemblies, which may link membranes to each other or connect the myelinating cell cytoskeleton to the extracellular matrix. The myelin protein periaxin is known to play an important role in linking the Schwann cell cytoskeleton to the basal lamina through membrane receptors, such as the dystroglycan complex. Mutations that truncate periaxin from the C terminus cause demyelinating peripheral neuropathy, Charcot-Marie-Tooth (CMT) disease type 4F, indicating a function for the periaxin C-terminal region in myelination. We identified the cytoplasmic domain of β4 integrin as a specific high-affinity binding partner for periaxin. The C-terminal region of periaxin remains unfolded and flexible when bound to the third fibronectin type III domain of β4 integrin. Our data suggest that periaxin is able to link the Schwann cell cytoplasm to the basal lamina through a two-pronged interaction via different membrane protein complexes, which bind close to the N and C terminus of this elongated, flexible molecule.

Expression of periaxin (PRX) specifically in the human cerebrovascular system: PDZ domain-mediated strengthening of endothelial barrier function

Regulation of cerebral endothelial cell function plays an essential role in changes in blood-brain barrier permeability. Proteins that are important for establishment of endothelial tight junctions have emerged as critical molecules, and PDZ domain containing-molecules are among the most important. We have discovered that the PDZ-domain containing protein periaxin (PRX) is expressed in human cerebral endothelial cells. Surprisingly, PRX protein is not detected in brain endothelium in other mammalian species, suggesting that it could confer human-specific vascular properties. In endothelial cells, PRX is predominantly localized to the nucleus and not tight junctions. Transcriptome analysis shows that PRX expression suppresses, by at least 50%, a panel of inflammatory markers, of which 70% are Type I interferon response genes; only four genes were significantly activated by PRX expression. When expressed in mouse endothelial cells, PRX strengthens barrier function, significantly increases transendothelial electrical resistance (~35%; p < 0.05), and reduces the permeability of a wide range of molecules. The PDZ domain of PRX is necessary and sufficient for its barrier enhancing properties, since a splice variant (S-PRX) that contains only the PDZ domain, also increases barrier function. PRX also attenuates the permeability enhancing effects of lipopolysaccharide. Collectively, these studies suggest that PRX could potentially regulate endothelial homeostasis in human cerebral endothelial cells by modulating inflammatory gene programs.

The role of IL‑16 gene polymorphisms in endometriosis

Endometriosis is one of the most common gynecological diseases affecting up to 10% of the female population of childbearing age and a major cause of pain and infertility. It is influenced by multiple genetic, epigenetic and environmental factors. Interleukin-16 (IL-16) is a proinflammatory cytokine playing a pivotal role in many inflammatory and autoimmune diseases as well as in the pathogenesis of endometriosis. The aim of the present study was to evaluate the association of two IL-16 gene single nucleotide polymorphisms (SNPs), rs4072111 and rs11556218, with the risk of endometriosis in women from Greece as well as to gain insight about the structural consequences of these two exonic SNPs regarding development of the disease. A total of 159 women with endometriosis (stages I–IV) hospitalized for endometriosis, diagnosed by laparoscopic intervention and histologically confirmed, and 146 normal controls were recruited and genotyped. Subjects were genotyped using a polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) strategy. A significant association was detected regarding the GG and GT genotype as well as 'G' allele of rs11556218 in patients with endometriosis. The rs4072111 SNP of the IL-16 gene was not found to be associated with an increased susceptibility to endometriosis either for all patients (stages I–IV) or for stage III and IV of the disease only. Our results demonstrated that rs11556218 is associated with endometriosis in Greek women, probably by resulting in the aberrant expression of IL-16, as suggested by the bioinformatics analysis conducted on the SNP-derived protein sequences, which indicated a possible association between mutation and functional modification of Pro-IL-16.

Schwann Cell O-GlcNAc Glycosylation Is Required for Myelin Maintenance and Axon Integrity

Schwann cells (SCs), ensheathing glia of the peripheral nervous system, support axonal survival and function. Abnormalities in SC metabolism affect their ability to provide this support and maintain axon integrity. To further interrogate this metabolic influence on axon-glial interactions, we generated OGT-SCKO mice with SC-specific deletion of the metabolic/nutrient sensing protein O-GlcNAc transferase that mediates the O-linked addition of N-acetylglucosamine (GlcNAc) moieties to Ser and Thr residues. The OGT-SCKO mice develop tomaculous demyelinating neuropathy characterized by focal thickenings of the myelin sheath (tomacula), progressive demyelination, axonal loss, and motor and sensory nerve dysfunction. Proteomic analysis identified more than 100 O-GlcNAcylated proteins in rat sciatic nerve, including Periaxin (PRX), a myelin protein whose mutation causes inherited neuropathy in humans. PRX lacking O-GlcNAcylation is mislocalized within the myelin sheath of these mutant animals. Furthermore, phenotypes of OGT-SCKO and Prx-deficient mice are very similar, suggesting that metabolic control of PRX O-GlcNAcylation is crucial for myelin maintenance and axonal integrity.

SIGNIFICANCE STATEMENT

The nutrient sensing protein O-GlcNAc transferase (OGT) mediates post-translational O-linked N-acetylglucosamine (GlcNAc) modification. Here we find that OGT functions in Schwann cells (SCs) to maintain normal myelin and prevent axonal loss. SC-specific deletion of OGT (OGT-SCKO mice) causes a tomaculous demyelinating neuropathy accompanied with progressive axon degeneration and motor and sensory nerve dysfunction. We also found Periaxin (PRX), a myelin protein whose mutation causes inherited neuropathy in humans, is O-GlcNAcylated. Importantly, phenotypes of OGT-SCKO and Prx mutant mice are very similar, implying that compromised PRX function contributes to the neuropathy of OGT-SCKO mice. This study will be useful in understanding how SC metabolism contributes to PNS function and in developing new strategies for treating peripheral neuropathy by targeting SC function.

Retracted Article: Self-association of L-periaxin occurs via its acidic domain and NLS2/NLS3, and affects its trafficking in RSC96 cells

Periaxin (PRX) protein was first identified in myelinating Schwann cells through the screening of cytoskeleton-associated proteins in peripheral nerve myelination.

Restoration of SMN in Schwann cells reverses myelination defects and improves neuromuscular function in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of SMN protein, primarily affecting lower motor neurons. Recent evidence from SMA and related conditions suggests that glial cells can influence disease severity. Here, we investigated the role of glial cells in the peripheral nervous system by creating SMA mice selectively overexpressing SMN in myelinating Schwann cells (Smn^−/−;SMN2^tg/0;SMN1^SC). Restoration of SMN protein levels restricted solely to Schwann cells reversed myelination defects, significantly improved neuromuscular function and ameliorated neuromuscular junction pathology in SMA mice. However, restoration of SMN in Schwann cells had no impact on motor neuron soma loss from the spinal cord or ongoing systemic and peripheral pathology. This study provides evidence for a defined, intrinsic contribution of glial cells to SMA disease pathogenesis and suggests that therapies designed to include Schwann cells in their target tissues are likely to be required in order to rescue myelination defects and associated disease symptoms.

Intermolecular disulfide bond in the dimerization of S-periaxin mediated by Cys88 and Cys139

Periaxin is expressed in mammalian Schwann cells and lens fiber cells, and has been identified in a screen for cytoskeleton-associated proteins. Charcot-Marie-Tooth 4F is caused by losses or mutations of theperiaxingene. Theperiaxingene encodes two protein isoforms, namely, L-periaxin and S-periaxin.S-periaxin contains 147 amino acid residues and has an N-terminal PDZ domain. In this paper, S-periaxin was reported to be homodimerized through the formation of intermolecular disulfide bonds with its Cys88 and Cys139 residues under mild oxidation conditions. The covalent dimer of S-periaxin was also observed by western blot analysis and bimolecular fluorescence complementation analyses. S-periaxin dimerization formation could be regulated by cellular redox fluctuations. These results offer a possible mechanism to the formation of periaxin complexes, improvement of complex stability, and establishment of a link between the extracellular matrix and the cytoskeleton.

L-periaxin interacts with S-periaxin through its PDZ domain

Periaxin was first identified as a protein in myelinating Schwann cells through a screen of novel cytoskeleton-associated proteins in peripheral nerve myelination. The periaxin gene encodes two isoforms, namely, L- and S-periaxin, which are 1461 and 147 residues in size, respectively. Several loss-of-function mutations linked to autosomal recessive Dejerine-Sottas neuropathy and demyelinating Charcot-Marie-Tooth disease in periaxin have been described. In this study, the colocolization of L- and S-periaxin in the cytoplasm of RSC96 cells was found by immunofluorescence assays. The interaction between these two isoforms was confirmed by co-immunoprecipitation, fluorescence complementation experiment, and GST pull-down assay. Results also showed that the two periaxin isoforms interacted in the cytoplasm through the PDZ domain, and their interaction prevented the homodimerization of L-periaxin. S-periaxin may regulate the function of L-periaxin in Schwann cells.

Retracted Article: Spectrin-like domain 2 of DRP2 serves as a novel binding region for the NLS2 and 3 sub-domains of L-periaxin

The NLS1 domain of L-periaxin involved in nucleo-cytoplasmic shuttling, NLS2 and 3 participated in interaction with spectrin-like domain 2 of DRP2. The binding model of DRP2 and L-periaxin is crucial for understanding the role of L-periaxin in PNS.

Nuclear export of L-periaxin, mediated by its nuclear export signal in the PDZ domain

Myelinating Schwann cells specifically express L-periaxin (L-PRX) in the mammalian peripheral nervous system. Several loss-of-function mutations in periaxin have been described and linked to autosomal recessive Dejerine Sottas neuropathy and to demyelinating Charcot-Marie-Tooth disease. The localization of L-periaxin is developmentally regulated in the nucleus and the plasma membrane of Schwann cells. In this study, L-periaxin, which contains a PDZ domain, a nuclear localization signal (NLS) domain, a repeat domain, and an acidic domain, was localized in the cytoplasm of RSC96 cells. By contrast, a mutant L-periaxin with a deleted PDZ domain was localized mainly in the nucleus of RSC96 cells. After a nuclear cyclin A1, which is localized exclusively in the nucleus, was fused with the PDZ domain, cyclinA1was found in the cytoplasm of RSC96 cells. Treatment with leptomycin B (LMB), a specific inhibitor of nuclear export mediated by leucine-rich nuclear export signal (NES), also causes nuclear accumulation of wild-type L-periaxin. Double leucine mutation (L83, 85Q) in the putative NES in the PDZ domain prevented L-periaxin nuclear export and induced nuclear accumulation. These results suggested that the localization of L-periaxin in the cytoplasm is supported by NES in the PDZ domain.

Coordinate nuclear targeting of the FANCD2 and FANCI proteins via a FANCD2 nuclear localization signal

Fanconi anemia (FA) is a rare recessive disease, characterized by congenital defects, bone marrow failure, and increased cancer susceptibility. FA is caused by biallelic mutation of any one of sixteen genes. The protein products of these genes function cooperatively in the FA-BRCA pathway to repair DNA interstrand crosslinks (ICLs). A central step in the activation of this pathway is the monoubiquitination of the FANCD2 and FANCI proteins. Monoubiquitinated FANCD2 and FANCI localize to discrete chromatin regions where they function in ICL repair. Despite their critical role in ICL repair, very little is known about the structure, function, and regulation of the FANCD2 and FANCI proteins, or how they are targeted to the nucleus and chromatin. In this study, we describe the functional characterization of an amino-terminal FANCD2 nuclear localization signal (NLS). We demonstrate that the amino terminal 58 amino acids of FANCD2 can promote the nuclear expression of GFP and is necessary for the nuclear localization of FANCD2. Importantly, mutation of this FANCD2 NLS reveals that intact FANCD2 is required for the nuclear localization of a subset of FANCI. In addition, the NLS is necessary for the efficient monoubiquitination of FANCD2 and FANCI and, consequently, for their localization to chromatin. As a result, FANCD2 NLS mutants fail to rescue the ICL sensitivity of FA-D2 patient cells. Our studies yield important insight into the domain structure of the poorly characterized FANCD2 protein, and reveal a previously unknown mechanism for the coordinate nuclear import of a subset of FANCD2 and FANCI, a key early step in the cellular ICL response.

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.

Exome sequencing of senescence-accelerated mice (SAM) reveals deleterious mutations in degenerative disease-causing genes

BACKGROUND

Senescence-accelerated mice (SAM) are a series of mouse strains originally derived from unexpected crosses between AKR/J and unknown mice, from which phenotypically distinct senescence-prone (SAMP) and -resistant (SAMR) inbred strains were subsequently established. Although SAMP strains have been widely used for aging research focusing on their short life spans and various age-related phenotypes, such as immune dysfunction, osteoporosis, and brain atrophy, the responsible gene mutations have not yet been fully elucidated.

RESULTS

To identify mutations specific to SAMP strains, we performed whole exome sequencing of 6 SAMP and 3 SAMR strains. This analysis revealed 32,019 to 38,925 single-nucleotide variants in the coding region of each SAM strain. We detected Ogg1 p.R304W and Mbd4 p.D129N deleterious mutations in all 6 of the SAMP strains but not in the SAMR or AKR/J strains. Moreover, we extracted 31 SAMP-specific novel deleterious mutations. In all SAMP strains except SAMP8, we detected a p.R473W missense mutation in the Ldb3 gene, which has been associated with myofibrillar myopathy. In 3 SAMP strains (SAMP3, SAMP10, and SAMP11), we identified a p.R167C missense mutation in the Prx gene, in which mutations causing hereditary motor and sensory neuropathy (Dejerine-Sottas syndrome) have been identified. In SAMP6 we detected a p.S540fs frame-shift mutation in the Il4ra gene, a mutation potentially causative of ulcerative colitis and osteoporosis.

CONCLUSIONS

Our data indicate that different combinations of mutations in disease-causing genes may be responsible for the various phenotypes of SAMP strains.

Sulfatide decrease in myelin influences formation of the paranodal axo-glial junction and conduction velocity in the sciatic nerve

Cerebroside sulfotransferase (CST) catalyzes the production of sulfatide, which is one of the major glycolipids in myelin. Homozygous CST knockout mice were shown to be completely deficient in sulfatide. They were born healthy but began to display progressive neurological deficits from 6 weeks of age. Severe abnormalities of paranodal regions and changes in axonal ion channel distribution were prominent in both the central and peripheral nervous systems. But whether partial decreases in myelin sulfatide levels influence paranodal formation, as well as nerve conduction velocity (NCV), is largely unknown. To determine the functional significance of sulfatide content in myelin, we performed electrophysiological, morphological, and biochemical analyses using heterozygote, homozygote, and wild-type mouse peripheral nerves and compared the results with individual sulfatide content. NCVs were significantly reduced in homozygote animals compared with wild-type mice. In contrast, these values were markedly varied in individual heterozygote mice. On the basis of NCV values, we divided heterozygous mice into two groups: mice with mild but significant reduction of NCV and those with normal NCV. Teased nerve fibers obtained from individual mouse sciatic nerves were immunostained, and Na(+) channel and Caspr cluster lengths were measured to determine abnormal levels of junctional formation at the paranode. Furthermore, sulfatide content in each sciatic nerve was examined by thin layer chromatography. The results demonstrated significant correlations among sulfatide level, severity of paranodal abnormality, and reduction of NCV. Thus, the fine regulation of myelin sulfatide content by CST is important for normal function of myelinated axons.

Drp2 and periaxin form Cajal bands with dystroglycan but have distinct roles in Schwann cell growth

Cajal bands are cytoplasmic channels flanked by appositions where the abaxonal surface of Schwann cell myelin apposes and adheres to the overlying plasma membrane. These appositions contain a dystroglycan complex that includes periaxin and dystrophin-related protein 2 (Drp2). Loss of periaxin disrupts appositions and Cajal bands in Schwann cells and causes a severe demyelinating neuropathy in mouse and human. Here, we investigated the role of mouse Drp2 in apposition assembly and Cajal band function and compared it with periaxin. We show that periaxin and Drp2 are not only both required to form appositions, but they must also interact. Periaxin-Drp2 interaction is also required for Drp2 phosphorylation, but phosphorylation is not required for the assembly of appositions. Drp2 loss causes corresponding increases in Dystrophin family members, utrophin and dystrophin Dp116, although dystroglycan remains unchanged. We also show that all dystroglycan complexes in Schwann cells use the uncleaved form of β-dystroglycan. Drp2-null Schwann cells have disrupted appositions and Cajal bands, and they undergo focal hypermyelination and concomitant demyelination. Nevertheless, they do not have the short internodal lengths and associated reduced nerve conduction velocity seen in the absence of periaxin, showing that periaxin regulates Schwann cell elongation independent of its role in the dystroglycan complex. We conclude that the primary role of the dystroglycan complex in appositions is to stabilize and limit the radial growth of myelin.

MMP2-9 cleavage of dystroglycan alters the size and molecular composition of Schwann cell domains

Myelinating glial cells exhibit a spectacular cytoarchitecture, because they polarize on multiple axes and domains. How this occurs is essentially unknown. The dystroglycan-dystrophin complex is required for the function of myelin-forming Schwann cells. Similar to other tissues, the dystroglycan complex in Schwann cells localizes with different dystrophin family members in specific domains, thus promoting polarization. We show here that cleavage of dystroglycan by matrix metalloproteinases 2 and 9, an event that is considered pathological in most tissues, is finely and dynamically regulated in normal nerves and modulates dystroglycan complex composition and the size of Schwann cell compartments. In contrast, in nerves of Dy(2j/2j) mice, a model of laminin 211 deficiency, metalloproteinases 2 and 9 are increased, causing excessive dystroglycan cleavage and abnormal compartments. Pharmacological inhibition of cleavage rescues the cytoplasmic defects of Dy(2j/2j) Schwann cells. Thus, regulated cleavage may be a general mechanism to regulate protein complex composition in physiological conditions, whereas unregulated processing is pathogenic and a target for treatment in disease.

Self-regulated alternative splicing at the AHNAK locus

AHNAK is a 700-kDa protein involved in cytoarchitecture and calcium signaling. It is secondarily reduced in muscle of dysferlinopathy patients and accumulates in muscle of calpainopathy patients, both affected by a muscular dystrophy. AHNAK directly interacts with dysferlin. This interaction is lost on cleavage of AHNAK by the protease calpain 3, explaining the molecular observations in patients. Currently, little is known of AHNAK regulation. We describe the self-regulation of multiple mRNA transcripts emanating from the AHNAK locus in muscle cells. We show that the AHNAK gene consists of a 17-kb exon flanked by multiple small exons. This genetic structure is shared by AHNAK2 and Periaxin, which share a common ancestor. Two major AHNAK transcripts are differentially expressed during muscle differentiation that encode for a small (17-kDa) and a large (700-kDa) protein isoform. These proteins interact in the cytoplasm, but the small AHNAK is also present in the nucleus. During muscle differentiation the small AHNAK is strongly increased, thereby establishing a positive feedback loop to regulate mRNA splicing of its own locus. A small 17-kDa isoform of Periaxin similarly traffics between the cytoplasm and the nucleus to regulate mRNA splicing. Thus, AHNAK constitutes a novel mechanism in post-transcriptional control of gene expression.

A critical role for Neurofascin in regulating action potential initiation through maintenance of the axon initial segment

The axon initial segment (AIS) is critical for the initiation and propagation of action potentials. Assembly of the AIS requires interactions between scaffolding molecules and voltage-gated sodium channels, but the molecular mechanisms that stabilize the AIS are poorly understood. The neuronal isoform of Neurofascin, Nfasc186, clusters voltage-gated sodium channels at nodes of Ranvier in myelinated nerves: here, we investigate its role in AIS assembly and stabilization. Inactivation of the Nfasc gene in cerebellar Purkinje cells of adult mice causes rapid loss of Nfasc186 from the AIS but not from nodes of Ranvier. This causes AIS disintegration, impairment of motor learning and the abolition of the spontaneous tonic discharge typical of Purkinje cells. Nevertheless, action potentials with a modified waveform can still be evoked and basic motor abilities remain intact. We propose that Nfasc186 optimizes communication between mature neurons by anchoring the key elements of the adult AIS complex.

Effect of pressure overload-induced hypertrophy on the expression and localization of p38 MAP kinase isoforms in the mouse heart

p38 mitogen-activated protein kinases (MAPKs) are serine/threonine specific protein kinases that respond to cellular stress and regulate a broad range of cellular activities. There are four major isoforms of p38 MAPK: alpha, beta, gamma, and delta. To date, the prominent isoform in heart has been thought to be p38alpha. We examined the expression of each p38 isoform at both the mRNA and protein level in murine heart. mRNA for all four p38 isoforms was detected. p38gamma and p38delta were expressed at protein levels comparable to p38alpha and 38beta, respectively. In the early phase of pressure-overload hypertrophy (1-7 days after constriction of the transverse aorta), the abundance of p38beta, p38gamma and p38delta mRNA increased; however, no corresponding changes were detected at the protein level. Confocal immunofluorescence studies revealed p38alpha and p38gamma in both the cytoplasm and nucleus. In the established phase of hypertrophy induced by chronic pressure overload (7-28 days after constriction of the transverse aorta), p38gamma immunoreactivity accumulated in the nucleus whereas the distribution of p38alpha remained unaffected. Hence, both p38alpha and p38gamma are prominent p38 isoforms in heart and p38gamma may play a role in mediating the changes in gene expression associated with cardiac remodeling during pressure-overload hypertrophy.

The mitogaligin protein is addressed to the nucleus via a non-classical localization signal

Mitogaligin, a protein encoded by galig, an internal cytotoxic gene of the galectin-3 locus, is mostly a mitochondrial protein. Mitochondrial targeting is due to an already identified mitochondrial localization signal. Interaction of mitogaligin with mitochondria leads to cytochrome c cytosolic leakage and ultimately to cell death. We have previously pointed out that mitogaligin can also be directed to the nucleus when the mitochondrial addressing signal is inactivated, indicating a possible dual intracellular localization of the protein. When expressed in the nucleus, mitogaligin exhibits also apoptotic properties leading to cell death. In this report, we show that nuclear addressing of mitogaligin depends on a sequence differing from classical signals containing basic, lysine or proline-tyrosine rich residues. The signal consists of a long sequence of amino acids residues based on a series of a short repetitive degenerated sequence.

FAK is required for axonal sorting by Schwann cells

Signaling by laminins and axonal neuregulin has been implicated in regulating axon sorting by myelin-forming Schwann cells. However, the signal transduction mechanisms are unknown. Focal adhesion kinase (FAK) has been linked to α6β1 integrin and ErbB receptor signaling, and we show that myelination by Schwann cells lacking FAK is severely impaired. Mutant Schwann cells could interdigitate between axon bundles, indicating that FAK signaling was not required for process extension. However, Schwann cell FAK was required to stimulate cell proliferation, suggesting that amyelination was caused by insufficient Schwann cells. ErbB2 receptor and AKT were robustly phosphorylated in mutant Schwann cells, indicating that neuregulin signaling from axons was unimpaired. These findings demonstrate the vital relationship between axon defasciculation and Schwann cell number and show the importance of FAK in regulating cell proliferation in the developing nervous system.

Molecular basis of cellular localization of poly C binding protein 1 in neuronal cells

Poly C binding protein 1 (PCBP) is involved in the transcriptional regulation of neuronal mu-opioid receptor gene. In this study, we examined the molecular basis of PCBP cellular/nuclear localization in neuronal cells using EGFP fusion protein. PCBP, containing three KH domains and a variable domain, distributed in cytoplasm and nucleus with a preferential nuclear expression. Domain-deletional analyses suggested the requirement of variable and KH3 domains for strong PCBP nuclear expression. Within the nucleus, a low nucleolar PCBP expression was observed, and PCBP variable domain contributed to this restricted nucleolar expression. Furthermore, the punctate nuclear pattern of PCBP was correlated to its single-stranded (ss) DNA binding ability, with both requiring cooperativity of at least three sequential domains. Collectively, certain PCBP domains thus govern its nuclear distribution and transcriptional regulatory activity in the nucleus of neurons, whereas the low nucleolar expression implicates the disengagement of PCBP in the ribosomal RNA synthesis.

Charcot-marie-tooth disease: seventeen causative genes

Charcot-Marie-Tooth disease (CMT) is the most common form of inherited motor and sensory neuropathy. Moreover, CMT is a genetically heterogeneous disorder of the peripheral nervous system, with many genes identified as CMT-causative. CMT has two usual classifications: type 1, the demyelinating form (CMT1); and type 2, the axonal form (CMT2). In addition, patients are classified as CMTX if they have an X-linked inheritance pattern and CMT4 if the inheritance pattern is autosomal recessive. A large amount of new information on the genetic causes of CMT has become available, and mutations causing it have been associated with more than 17 different genes and 25 chromosomal loci. Advances in our understanding of the molecular basis of CMT have revealed an enormous diversity in genetic mechanisms, despite a clinical entity that is relatively uniform in presentation. In addition, recent encouraging studies - shown in CMT1A animal models - concerning the therapeutic effects of certain chemicals have been published; these suggest potential therapies for the most common form of CMT, CMT1A. This review focuses on the inherited motor and sensory neuropathy subgroup for which there has been an explosion of new molecular genetic information over the past decade.

Periaxin mutation in Japanese patients with Charcot-Marie-Tooth disease

Periaxin (PRX) plays an important role in the myelination of the peripheral nerve and consequently in the pathogenesis of Charcot-Marie-Tooth disease (CMT). To date, nine nonsense or frameshift PRX mutations have been reported in eight families with CMT. The patients with PRX mutations appeared to show characteristic clinical features with early onset but slow or no progression, a common result of mutations that lead to missing a C-terminal acidic domain. Here, we report a Japanese CMT patient with these characteristic clinical features, who was a compound heterozygote for PRX R1070X and L132FsX153 mutations. We previously reported that three Japanese isolated families also had the homozygous R1070X mutation. To examine the potential founder effect of the R1070X mutation in the Japanese population, we performed haplotype analysis and found that each R1070X allele lay on a different haplotype background in these four families. Therefore, the high frequency of the R1070X mutation among the Japanese population is not likely the consequence of a founder effect, but probably a result of a mutation hot spot.

Neurofascins Are Required to Establish Axonal Domains for Saltatory Conduction

Voltage-gated sodium channels are concentrated in myelinated nerves at the nodes of Ranvier flanked by paranodal axoglial junctions. Establishment of these essential nodal and paranodal domains is determined by myelin-forming glia, but the mechanisms are not clear. Here, we show that two isoforms of Neurofascin, Nfasc155 in glia and Nfasc186 in neurons, are required for the assembly of these specialized domains. In Neurofascin-null mice, neither paranodal adhesion junctions nor nodal complexes are formed. Transgenic expression of Nfasc155 in the myelinating glia of Nfasc-/- nerves rescues the axoglial adhesion complex by recruiting the axonal proteins Caspr and Contactin to the paranodes. However, in the absence of Nfasc186, sodium channels remain diffusely distributed along the axon. Our study shows that the two major Neurofascins play essential roles in assembling the nodal and paranodal domains of myelinated axons; therefore, they are essential for the transition to saltatory conduction in developing vertebrate nerves.

Cell autonomy of the mouse claw paw mutation

Mice homozygous for the autosomal recessive mutation claw paw (clp) are characterized by limb posture abnormalities and congenital hypomyelination, with delayed onset of myelination of the peripheral nervous system but not the central nervous system. Although this combination of limb and peripheral nerve abnormalities in clp/clp mice might suggest a common neurogenic origin of the syndrome, it is not clear whether the clp gene acts primarily in the neurone, the Schwann cell or both. In the work described here, we address this question of cell autonomy of the clp mutation through reciprocal nerve grafting experiments between wild-type and clp/clp animals. Our results demonstrate that the clp mutation affects the Schwann cell compartment and possibly also the neuronal compartment. These data suggest that the clp gene product is expressed in Schwann cells as well as neurones and is likely to be involved in direct axon--Schwann cell interactions. Within the Schwann cell, clp affects a myelin-related signaling pathway that regulates periaxin and Krox-20 expression, but not Oct-6.

Periaxin mutation causes early-onset but slow-progressive Charcot-Marie-Tooth disease

Periaxin (PRX) plays a significant role in the myelination of the peripheral nerve. To date, seven non-sense or frameshift PRX mutations have been reported in six pedigrees with Dejerine-Sottas neuropathy or severe Charcot-Marie-Tooth neuropathy (CMT). We detected a PRX mutation in three patients in the screening of 66 Japanese demyelinating CMT patients who were negative for the gene mutation causing dominant or X-linked demyelinating CMT. Three unrelated patients were homozygous for a novel R1070X mutation and presented early-onset but slowly progressive distal motor and sensory neuropathies. Mutations lacking the carboxyl-terminal acidic domain may show loss-of-function effects and cause severe demyelinating CMT.

Molecular Mechanisms, Diagnosis, and Rational Approaches to Management of and Therapy for Charcot-Marie-Tooth Disease and Related Peripheral Neuropathies

During the last decade, 18 genes and 11 additional loci harboring candidate genes have been associated with Charcot-Marie-Tooth disease (CMT) and related peripheral neuropathies. Ten of these 18 genes have been identified in the last 2 years. This phenomenal pace of CMT gene discovery has fomented an unprecedented explosion of information regarding peripheral nerve biology and its pathologic manifestations in CMT. This review integrates molecular genetics with the clinical phenotypes and provides a flowchart for molecular-based diagnostics. In addition, we discuss rational approaches to molecular therapeutics, including novel biologic molecules (eg, small interfering ribonucleic acid [siRNA], antisense RNA, and ribozymes) that potentially could be used as drugs in the future. These may be applicable in attempts to normalize gene expression in cases of CMT type 1A, wherein a 1.5 Mb genomic duplication causes an increase in gene dosage that is associated with the majority of CMT cases. Aggresome formation by the PMP22 gene product, the disease-associated gene in the duplication cases, could thus be avoided. We also discuss alternative therapeutics, in light of other neurodegenerative disorders, to disrupt such aggresomes. Finally, we review rational therapeutic approaches, including the use of antioxidants such as vitamin E, coenzyme Q10, or lipoic acid to relax potential oxidative stress in peripheral nerves, for CMT management.

Regulation of the myelin gene periaxin provides evidence for Krox-20-independent myelin-related signalling in Schwann cells

We investigated the role of Krox-20 (Egr2), a transcription factor that regulates myelination, in controlling the myelin-associated protein periaxin. In developing Schwann cells, periaxin immunoreactivity appeared at least 2 days before Krox-20-immunopositive nuclei. Consistent with this, in Krox-20 null mice periaxin was upregulated on schedule, albeit to a lower level. In culture Krox-20 and periaxin were upregulated by cAMP as expected for myelin genes. Only those cells with the highest periaxin levels also expressed Krox-20, while other periaxin-positive cells remained Krox-20-negative. Furthermore, cAMP elevated periaxin even in Krox-20 null cells. We also found that in culture enforced Krox-20 expression induced expression of periaxin mRNA and protein in the absence of cAMP elevating agents, and that this induction was inhibited by the co-repressor NAB2. These findings reveal a dual mechanism for periaxin regulation and suggest that the role of Krox-20 is to amplify an earlier Krox-20-independent activation of the periaxin gene. Thus the axonal signals responsible for myelination are only partially transduced in Schwann cells by mechanisms that depend on Krox-20.

Binding of HTLV-1 tax oncoprotein to the precursor of interleukin-16, a T cell PDZ domain-containing protein

HTLV-1 Tax oncoprotein interacts with various cellular factors and modulates transcription and the cell cycle. In that role it is sufficient to create T cell malignancies in the absence of HTLV-1 infection. HTLV-1 Tax protein has been reported to bind to cellular proteins containing PDZ domains in vitro. The precursor of human interleukin 16, pro-IL-16, is an abundant cellular protein present in human peripheral blood T cells. Pro-IL-16 contains three PDZ domains. It has been shown that expression of pro-IL-16 in pro-IL-16 negative cells induces a G(0)/G(1) arrest in the cell cycle. The current studies demonstrate that Tax binds to pro-IL-16 in HTLV-1 infected human T cells. We mapped the Tax binding site to the first PDZ domain of pro-IL-16. Over-expression of Tax in COS cells resulted in fewer cells in G(0)/G(1) consistent with its activity to induce G(1)- to S-phase progression in lymphocytes, while over-expression of pro-IL-16 in COS cells resulted in G(0)/G(1) arrest. Co-expression of wild type Tax with pro-IL-16 in COS cells negated the effects of pro-IL-16, an effect not observed with Tax mutated at its PDZ binding C-terminus. These results suggest that one of the effects of Tax on growth deregulation in HTLV-1 infected T cells might be mediated by its binding to pro-IL-16.

Two separate domains in the golli myelin basic proteins are responsible for nuclear targeting and process extension in transfected cells

The golli products of the myelin basic protein (MBP) gene are expressed in neurons and oligodendrocytes (OLs). In certain neuronal populations, golli proteins undergo translocation between the nucleus and cytoplasm/processes during development. The proteins consist of two domains, a golli domain of 133 amino acids and an MBP domain of variable length. One objective of this study was to identify the sequences responsible for nuclear targeting. Site-directed mutagenesis and deletion analyses were used to generate a series of golli-green fluorescent protein (GFP) DNA constructs that were transfected into OL and neuronal cell lines to follow localization by confocal microscopy. The results indicated that a 36-residue stretch in the MBP domain is essential for nuclear targeting, and the sequence appears to be a nontraditional localization signal motif. The studies also revealed that overexpression of golli proteins could induce dramatic changes in cell morphology. In OL lines, overexpression of intact golli proteins, or golli peptide alone, caused an increase in the length and number of processes, and the elaboration of membrane sheets. In the neuronal lines, there was a dramatic increase in number and length of extensions. The results, consistent with the timing of golli expression in cells during neural development, suggest that golli proteins may be involved in process formation/extension in OLs and neurons during development. These studies have defined two functional domains in the golli protein. Sequences in the MBP domain target the protein into the nucleus and sequences within the golli domain induce process sheet extension in OLs and neurons.

Periaxin mutations cause a broad spectrum of demyelinating neuropathies

Previous studies have demonstrated that apparent loss-of-function mutations in the periaxin gene cause autosomal recessive Dejerine-Sottas neuropathy or severe demyelinating Charcot-Marie-Tooth disease. In this report, we extend the associated phenotypes with the identification of two additional families with novel periaxin gene mutations (C715X and R82fsX96) and provide detailed neuropathology. Each patient had marked sensory involvement; two siblings with a homozygous C715X mutation had much worse sensory impairment than motor impairment. Despite early disease onset, these siblings with the C715X mutation had relatively slow disease progression and adult motor impairment typical of classic demyelinating Charcot-Marie-Tooth neuropathy. In contrast, a patient with the homozygous R82fsX96 mutation had a disease course consistent with Dejerine-Sottas neuropathy. The neuropathology of patients in both families was remarkable for demyelination, onion bulb and occasional tomacula formation with focal myelin thickening, abnormalities of the paranodal myelin loops, and focal absence of paranodal septate-like junctions between the terminal loops and axon. Our study indicates a prominent sensory neuropathy resulting from periaxin gene mutations and suggests a role for the carboxyl terminal domain of the periaxin protein.

Hereditary motor and sensory neuropathies: a biological perspective

Mutations in genes expressed in Schwann cells and the axons they ensheath cause the hereditary motor and sensory neuropathies known as Charcot-Marie-Tooth (CMT) disease. At present, mutations in ten different genes have been identified, chromosomal localisation of many other distinct inherited neuropathies has been mapped, and new genetic causes for inherited neuropathies continue to be discovered. How to keep track of these mutations is a challenge for any neurologist, partly because the mutations are commonly presented as an expanding list to be memorised without a biological context of how the encoded proteins behave in the cell. A further challenge for investigators studying diseases of the peripheral nervous system is the increasing complexity of myelination, axonal function, and interactions between Schwann cells and axons. To address these concerns, we present the mutated genes causing these inherited neuropathies in the context of the cell biology of the Schwann cell and axon, and we begin to develop a model of how the various genes may interact in the pathogenesis of CMT disease.

Understanding Schwann cell-neurone interactions: the key to Charcot-Marie-Tooth disease?

Charcot-Marie-Tooth disease (CMT) comprises a heterogeneous group of disorders. The most frequent subtype is caused by increased PMP22 gene dosage or missense point mutations affecting the PMP22 gene (CMT type 1A; CMT1A). Animal models in rat and mouse with the corresponding PMP22 alterations are available and mimic many aspects of the human diseases. Detailed examinations of the animal mutants, together with complementary data from patients, point towards altered Schwann cell-neurone interactions as a major underlying mechanism of CMT1A and related hereditary neuropathies. This is evident from the finding that mutated proteins affecting either Schwann cells or neurones have a profound influence on their partner cells. Recently, a number of novel genes causing various forms of CMT have been identified which are expressed either mainly by Schwann cells and/or by the accompanying neurones. These genes can be viewed, in analogy to classic experiments routinely performed in lower vertebrates, as the result of a 'functional screen' revealing crucial players in the interactions between Schwann cells and neurones. Studying how Schwann cell and axon-encoded proteins are functionally interconnected will be an exciting task for the future. It will not only yield insights into the molecular and cellular basis of neuropathies but also provide crucial information about the interplay between Schwann cells and neurones in general.

Nesprins: a novel family of spectrin-repeat-containing proteins that localize to the nuclear membrane in multiple tissues

In search of vascular smooth muscle cell differentiation markers, we identified two genes encoding members of a new family of type II integral membrane proteins. Both are ubiquitously expressed, and tissue-specific alternative mRNA initiation and splicing generate at least two major isoforms of each protein, with the smaller isoforms being truncated at the N-terminus. We have named these proteins nesprin-1 and -2 for nuclear envelope spectrin repeat, as they are characterized by the presence of multiple, clustered spectrin repeats, bipartite nuclear localization sequences and a conserved C-terminal, single transmembrane domain. Transient transfection of EGFP-fusion expression constructs demonstrated their localization to the nuclear membrane with a novel C-terminal, TM-domain-containing sequence essential for perinuclear localization. Using antibodies to nesprin-1, we documented its colocalization with LAP1, emerin and lamins at the nuclear envelope, and immunogold labeling confirmed its presence at the nuclear envelope and in the nucleus where it colocalized with heterochromatin. Nesprin-1 is developmentally regulated in both smooth and skeletal muscle and is re-localized from the nuclear envelope to the nucleus and cytoplasm during C2C12 myoblast differentiation. These data and structural analogies with other proteins suggest that nesprins may function as ‘dystrophins of the nucleus’ to maintain nuclear organization and structural integrity.

Protein kinase B phosphorylates AHNAK and regulates its subcellular localization

AHNAK is a ubiquitously expressed giant phosphoprotein that was initially identified as a gene product subject to transcriptional repression in neuroblastoma. AHNAK is predominantly nuclear in cells of nonepithelial origin, but is cytoplasmic or associated with plasma membrane in epithelial cells. In this study we show that the extranuclear localization of AHNAK in epithelial cells depends on the formation of cell-cell contacts. We show that AHNAK is a phosphorylation substrate of protein kinase B (PKB) in vitro and in vivo. Nuclear exclusion of AHNAK is mediated through a nuclear export signal (NES) in a manner that depends on the phosphorylation of serine 5535 of AHNAK by PKB, a process that also plays a major role in determining extranuclear localization of AHNAK. AHNAK is a new PKB substrate whose function, though unknown, is likely to be regulated by its localization, which is in turn regulated by PKB.

Subcellular targeting of the coat protein of African cassava mosaic geminivirus

The coat protein (CP) of geminiviruses is involved in a number of processes during the life cycle of the virus. The predominant function is encapsidation of single-stranded DNA and formation of the virus particle to protect viral DNA during transmission. The CP of monopartite geminiviruses is absolutely essential for virus movement, whereas CP mutants of bipartite geminiviruses are able to infect some host plants systemically, indicating an involvement of the CP in host specificity. During the life cycle of geminiviruses, the viral DNA enters the nucleus of the infected cell where virus replication, transcription, and encapsidation occur. For systemic infection, the virus moves cell-to-cell from the site of inoculation to vascular tissue and via phloem to other plant tissues. To move, viral DNA has to shuttle in and out of the nucleus and through plasmodesmata. Parts of the bipartite African cassava mosaic virus (ACMV) CP were fused with green fluorescent protein (GFP) or beta-glucuronidase (GUS). CP domains were identified that mediate both nuclear import and export, as well as targeting of CP-fusion proteins to the cell periphery. These results indicate that domains of the CP facilitate several aspects of geminivirus movement, including nuclear import and export and transport of the viral genome to the cell periphery.

Specific disruption of a schwann cell dystrophin-related protein complex in a demyelinating neuropathy

Dystroglycan-dystrophin complexes are believed to have structural and signaling functions by linking extracellular matrix proteins to the cytoskeleton and cortical signaling molecules. Here we characterize a dystroglycan-dystrophin-related protein 2 (DRP2) complex at the surface of myelin-forming Schwann cells. The complex is clustered by the interaction of DRP2 with L-periaxin, a homodimeric PDZ domain-containing protein. In the absence of L-periaxin, DRP2 is mislocalized and depleted, although other dystrophin family proteins are unaffected. Disruption of the DRP2-dystroglycan complex is followed by hypermyelination and destabilization of the Schwann cell-axon unit in Prx(-/-) mice. Hence, the DRP2-dystroglycan complex likely has a distinct function in the terminal stages of PNS myelinogenesis, possibly in the regulation of myelin thickness.

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).