Microtubule-associated protein 1B: a neuronal binding partner for gigaxonin

Expanding the genetic spectrum of giant axonal neuropathy: Two novel variants in Iranian families

BACKGROUND

Giant axonal neuropathy (GAN) is a progressive childhood hereditary polyneuropathy that affects both the peripheral and central nervous systems. Disease-causing variants in the gigaxonin gene (GAN) cause autosomal recessive giant axonal neuropathy. Facial weakness, nystagmus, scoliosis, kinky or curly hair, pyramidal and cerebellar signs, and sensory and motor axonal neuropathy are the main symptoms of this disorder. Here, we report two novel variants in the GAN gene from two unrelated Iranian families.

METHODS

Clinical and imaging data of patients were recorded and evaluated, retrospectively. Whole-exome sequencing (WES) was undertaken in order to detect disease-causing variants in participants. Confirmation of a causative variant in all three patients and their parents was carried out using Sanger sequencing and segregation analysis. In addition, for comparing to our cases, we reviewed all relevant clinical data of previously published cases of GAN between the years 2013-2020.

RESULTS

Three patients from two unrelated families were included. Using WES, we identified a novel nonsense variant [NM_022041.3:c.1162del (p.Leu388Ter)], in a 7-year-old boy of family 1, and a likely pathogenic missense variant [NM_022041.3:c.370T>A (p.Phe124Ile)], in two affected siblings of the family 2. Clinical examination revealed typical features of GAN-1 in all three patients, including walking difficulties, ataxic gait, kinky hair, sensory-motor polyneuropathy, and nonspecific neuroimaging abnormalities. Review of 63 previously reported cases of GAN indicated unique kinky hair, gait problem, hyporeflexia/areflexia, and sensory impairment were the most commonly reported clinical features.

CONCLUSIONS

One homozygous nonsense variant and one homozygous missense variant in the GAN gene were discovered for the first time in two unrelated Iranian families that expand the mutation spectrum of GAN. Imaging findings are nonspecific, but the electrophysiological study in addition to history is helpful to achieve the diagnosis. The molecular test confirms the diagnosis.

Genetic Approaches for the Treatment of Giant Axonal Neuropathy

Giant axonal neuropathy (GAN) is a pediatric, hereditary, neurodegenerative disorder that affects both the central and peripheral nervous systems. It is caused by mutations in the GAN gene, which codes for the gigaxonin protein. Gigaxonin plays a role in intermediate filament (IF) turnover hence loss of function of this protein leads to IF aggregates in various types of cells. These aggregates can lead to abnormal cellular function that manifests as a diverse set of symptoms in persons with GAN including nerve degeneration, cognitive issues, skin diseases, vision loss, and muscle weakness. GAN has no cure at this time. Currently, an adeno-associated virus (AAV) 9-mediated gene replacement therapy is being tested in a phase I clinical trial for the treatment of GAN. This review paper aims to provide an overview of giant axonal neuropathy and the current efforts at developing a treatment for this devastating disease.

E3 Ubiquitin Ligases in Neurological Diseases: Focus on Gigaxonin and Autophagy

Ubiquitination is a dynamic post-translational modification that regulates the fate of proteins and therefore modulates a myriad of cellular functions. At the last step of this sophisticated enzymatic cascade, E3 ubiquitin ligases selectively direct ubiquitin attachment to specific substrates. Altogether, the ∼800 distinct E3 ligases, combined to the exquisite variety of ubiquitin chains and types that can be formed at multiple sites on thousands of different substrates confer to ubiquitination versatility and infinite possibilities to control biological functions. E3 ubiquitin ligases have been shown to regulate behaviors of proteins, from their activation, trafficking, subcellular distribution, interaction with other proteins, to their final degradation. Largely known for tagging proteins for their degradation by the proteasome, E3 ligases also direct ubiquitinated proteins and more largely cellular content (organelles, ribosomes, etc.) to destruction by autophagy. This multi-step machinery involves the creation of double membrane autophagosomes in which engulfed material is degraded after fusion with lysosomes. Cooperating in sustaining homeostasis, actors of ubiquitination, proteasome and autophagy pathways are impaired or mutated in wide range of human diseases. From initial discovery of pathogenic mutations in the E3 ligase encoding for E6-AP in Angelman syndrome and Parkin in juvenile forms of Parkinson disease, the number of E3 ligases identified as causal gene for neurological diseases has considerably increased within the last years. In this review, we provide an overview of these diseases, by classifying the E3 ubiquitin ligase types and categorizing the neurological signs. We focus on the Gigaxonin-E3 ligase, mutated in giant axonal neuropathy and present a comprehensive analysis of the spectrum of mutations and the recent biological models that permitted to uncover novel mechanisms of action. Then, we discuss the common functions shared by Gigaxonin and the other E3 ligases in cytoskeleton architecture, cell signaling and autophagy. In particular, we emphasize their pivotal roles in controlling multiple steps of the autophagy pathway. In light of the various targets and extending functions sustained by a single E3 ligase, we finally discuss the challenge in understanding the complex pathological cascade underlying disease and in designing therapeutic approaches that can apprehend this complexity.

Tubulin Folding Cofactor TBCB is a Target of the Salmonella Effector Protein SseK1

Salmonella enterica serovar Typhimurium is a human and animal pathogen that uses type III secretion system effectors to manipulate the host cell and fulfill infection. SseK1 is a Salmonella effector with glycosyltransferase activity. We carried out a yeast two-hybrid screen and have identified tubulin-binding cofactor B (TBCB) as a new binding partner for this effector. SseK1 catalyzed the addition of N-acetylglucosamine to arginine on TBCB, and its expression promoted the stabilization of the microtubule cytoskeleton of HEK293T cells. The conserved Asp-x-Asp (DxD) motif that is essential for the activity of SseK1 was required for the binding and modification of TBCB and for the effect on the cytoskeleton. Our study has identified a novel target for SseK1 and suggests that this effector may have a role in the manipulation of the host cell microtubule network to provide a safe niche for this pathogen.

Identification of Novel Compound Heterozygous Mutations in the GAN Gene of a Chinese Patient Diagnosed With Giant Axonal Neuropathy

Giant axonal neuropathy (GAN) is a very rare autosomal recessive disorder characterized by abnormally large and dysfunctional neuronal axons. Mutations in the GAN gene have been identified as the cause of this disorder. In this report, we performed a detailed phenotypic assessment of a Chinese patient with GAN. An array-based exon capture test and targeted next-generation sequencing were used to detect the suspected mutation sites. Compound heterozygous mutations of p.S79L (c.236C > T) in the BTB domain and p.T489S (c.1466C > G) in the kelch domain were identified in the proband’s genome. S79L was a known mutation, and T489S was reported for the first time. The p.S79L and p.T489S were confirmed in the proband’s mother and father, respectively. Both mutations were located in highly conserved regions and affected the predicted protein crystal structures. The proband’s sural biopsy revealed the classical GAN phenotype of swollen axons filled with closely packed neurofilaments. The combined application of the next-generation sequencing platform and bioinformatics analyses was an effective method for diagnosing GAN. The novel compound mutations of S79L and T489S in the GAN gene were likely the cause of the patient’s GAN symptoms. Our findings enrich the spectrum of mutations associated with this rare type of axonopathy.

A Pathogenic Missense Variant (c.1617G>A, p.Met539Ile) in UBA1 Causing Infantile X-Linked Spinal Muscular Atrophy (SMAX2)

Background: Infantile X-linked spinal muscular atrophy (SMAX2) is a rare type of spinal muscular atrophy associated with UBA1 variants. Methods: Clinical imaging and neurophysiological tests were performed on a Chinese patient with SMAX2. Further, focused panel sequencing of UBA1 was carried out on samples of both the proband and his maternal relatives. Results: The proband, a 4-year-old boy with the SMAX2 phenotype, suffered from reduced exercise capacity since infancy. His other symptoms included speech difficulties, severe nasal tone, reduced distal muscle strength, areflexia, and inadequate sucking ability. The brain MRI of the proband's showed normal results but the electromyography results showed multiple peripheral neurogenic lesions. Five male members of the proband's family were affected with the SMAX2 phenotype. They presented similar symptoms and had experienced a long and autonomous life. Molecular analysis revealed a novel missense variant (c.1617G>A, p.Met539Ile) in the exon 15 of UBA1. The proband's mother, as well as grandmother, carried the heterozygous missense UBA1 variant; whereas, the male patients from the family carried the hemizygotic variant. Conclusions: The affected members in this Chinese family showed unique features such as extended life span, no fractures, and cramps as compared with previously reported SMAX2 cases. The novel missense variant (c.1617G>A (p.Met539Ile) in UBA1 highlights the critical role of this gene in causing SMAX2 phenotype.

Two novel mutations in the GAN gene causing giant axonal neuropathy

BACKGROUND

Giant axonal neuropathy (GAN) is a rare neurodegenerative disease transmitted in an autosomal recessive mode. This disorder presents motor and sensitive symptoms with an onset in early childhood. Progressive neurodegeneration makes the patients wheelchair dependent by the end of the second decade of life. Affected individuals do not survive beyond the third decade of life. Molecular analysis has identified mutations in the gene GAN in patients with this disorder. This gene produces a protein called gigaxonin which is presumably involved in protein degradation via the ubiquitin-proteasome system. However, the underlying molecular mechanism is not clearly understood yet.

METHODS

Here we present the first patient from Mexico with clinical data suggesting GAN. Sequencing of the GAN gene was carried out. Changes in the nucleotide sequence were investigated for their possible impact on protein function and structure using the publicly available prediction tools PolyPhen-2 and PANTHER.

RESULTS

The patient is a compound heterozygous carrying two novel mutations in the GAN gene. The sequence analysis revealed two missense mutations in the Kelch repeats domain. In one allele, a C>T transition was found in exon 9 at the nucleotide position 55393 (g.55393C>T). In the other allele, a transversion G>T in exon 11 at the nucleotide position 67471 (g.67471G>T) was observed. Both of the bioinformatic tools predicted that these amino acid substitutions would have a negative impact on gigaxonin's function.

CONCLUSION

This work provides useful information for health professionals and expands the spectrum of disease-causing mutations in the GAN gene and it is the first documented case in Mexican population.

MAP1B Light Chain Modulates Synaptic Transmission via AMPA Receptor Intracellular Trapping

The regulated transport of AMPA-type glutamate receptors (AMPARs) to the synaptic membrane is a key mechanism to determine the strength of excitatory synaptic transmission in the brain. In this work, we uncovered a new role for the microtubule-associated protein MAP1B in modulating access of AMPARs to the postsynaptic membrane. Using mice and rats of either sex, we show that MAP1B light chain (LC) accumulates in the somatodendritic compartment of hippocampal neurons, where it forms immobile complexes on microtubules that limit vesicular transport. These complexes restrict AMPAR dendritic mobility, leading to the intracellular trapping of receptors and impairing their access to the dendritic surface and spines. Accordingly, increasing MAP1B-LC expression depresses AMPAR-mediated synaptic transmission. This effect is specific for the GluA2 subunit of the AMPAR and requires glutamate receptor interacting protein 1 (GRIP1) interaction with MAP1B-LC. Therefore, MAP1B-LC represents an alternative link between GRIP1-AMPARs and microtubules that does not result in productive transport, but rather limits AMPAR availability for synaptic insertion, with a direct impact on synaptic transmission.SIGNIFICANCE STATEMENT The ability of neurons to modify their synaptic connections, known as synaptic plasticity, is accepted as the cellular basis for learning and memory. One mechanism for synaptic plasticity is the regulated addition and removal of AMPA-type glutamate receptors (AMPARs) at excitatory synapses. In this study, we found that a microtubule-associated protein, MAP1B light chain (MAP1B-LC), participates in this process. MAP1B-LC forms immobile complexes along dendrites. These complexes limit intracellular vesicular trafficking and trap AMPARs inside the dendritic shaft. In this manner, MAP1B restricts the access of AMPARs to dendritic spines and the postsynaptic membrane, contributing to downregulating synaptic transmission.

Ectodermal-Neural Cortex 1 Isoforms Have Contrasting Effects on MC3T3-E1 Osteoblast Mineralization and Gene Expression

The importance of Wnt pathway signaling in development of bone has been well established. Here we investigated the role of a known Wnt target, ENC1 (ectodermal-neural cortex 1; NRP/B), in osteoblast differentiation. Enc1 expression was detected in mouse osteoblasts, chondrocytes, and osteocytes by in situ hybridization, and osteoblastic expression was verified in differentiating primary cultures and MC3T3-E1 pre-osteoblast cells, with 57 kDa and 67 kDa ENC1 protein isoforms detected throughout differentiation. Induced knockdown of both ENC1 isoforms reduced alkaline phosphatase staining and virtually abolished MC3T3-E1 mineralization. At culture confluence, Alpl (alkaline phosphatase liver/bone/kidney) expression was markedly reduced compared with control cells, and there was significant and coordinated alteration of other genes involved in cellular phosphate biochemistry. In contrast, with 67 kDa-selective knockdown mineralized nodule formation was enhanced and there was a two-fold increase in Alpl expression at confluence. There was enhanced expression of Wnt/β-catenin target genes with knockdown of both isoforms at this time-point and a five-fold increase in Frzb (Frizzled related protein) with 67 kDa-selective knockdown at mineralization, indicating possible ENC1 interactions with Wnt signaling in osteoblasts. These results are the first to demonstrate a role for ENC1 in the control of osteoblast differentiation. Additionally, the contrasting mineralization phenotypes and transcriptional patterns seen with coordinate knockdown of both ENC1 isoforms vs selective knockdown of 67 kDa ENC1 suggest opposing roles for the isoforms in regulation of osteoblastic differentiation, through effects on Alpl expression and phosphate cellular biochemistry. This study is the first to report differential roles for the ENC1 isoforms in any cell lineage. J. Cell. Biochem. 118: 2141-2150, 2017. © 2016 Wiley Periodicals, Inc.

Neurofilaments and Neurofilament Proteins in Health and Disease

SUMMARYNeurofilaments (NFs) are unique among tissue-specific classes of intermediate filaments (IFs) in being heteropolymers composed of four subunits (NF-L [neurofilament light]; NF-M [neurofilament middle]; NF-H [neurofilament heavy]; and α-internexin or peripherin), each having different domain structures and functions. Here, we review how NFs provide structural support for the highly asymmetric geometries of neurons and, especially, for the marked radial expansion of myelinated axons crucial for effective nerve conduction velocity. NFs in axons extensively cross-bridge and interconnect with other non-IF components of the cytoskeleton, including microtubules, actin filaments, and other fibrous cytoskeletal elements, to establish a regionally specialized network that undergoes exceptionally slow local turnover and serves as a docking platform to organize other organelles and proteins. We also discuss how a small pool of oligomeric and short filamentous precursors in the slow phase of axonal transport maintains this network. A complex pattern of phosphorylation and dephosphorylation events on each subunit modulates filament assembly, turnover, and organization within the axonal cytoskeleton. Multiple factors, and especially turnover rate, determine the size of the network, which can vary substantially along the axon. NF gene mutations cause several neuroaxonal disorders characterized by disrupted subunit assembly and NF aggregation. Additional NF alterations are associated with varied neuropsychiatric disorders. New evidence that subunits of NFs exist within postsynaptic terminal boutons and influence neurotransmission suggests how NF proteins might contribute to normal synaptic function and neuropsychiatric disease states.

A review of gigaxonin mutations in giant axonal neuropathy (GAN) and cancer

Gigaxonin, the product of GAN gene localized to chromosome 16, is associated with the early onset neuronal degeneration disease giant axonal neuropathy (GAN). Gigaxonin is an E3 ubiquitin ligase adaptor protein involved in intermediate filament processing in neural cells, and vimentin filaments in fibroblasts. Mutations of the gene cause pre-neural filaments to accumulate and form giant axons resulting in the inhibition of neural cell signaling. Analysis of the catalog of somatic mutations in cancer, driver DB and IDGC data portal databases containing 21,000 tumor genomic sequences has identified GAN patient mutations in cancer cell lines and primary tumors. The database search has also shown the presence of identical missense and nonsense gigaxonin mutations in GAN and colon cancer. These mutations frequently occur in the domains associated with protein homodimerization and substrate interaction such as Broad-Complex, Tramtrack and Bric a brac (BTB), BTB associated C-terminal KELCH (BACK), and KELCH repeats. Analysis of the International HapMap Project database containing 1200 normal genomic sequences has identified a single nucleotide polymorphism (SNP), rs2608555, in exon 8 of the gigaxonin sequence. While this SNP is present in >40 % of Caucasian population, it is present in less than 10 % of Japanese and Chinese populations. Although the role of gigaxonin polymorphism is not yet known, CFTR and MDR1 gene studies have shown that silent mutations play a role in the instability and aberrant splicing and folding of mRNAs. We believe that molecular and functional investigation of gigaxonin mutations including the exon 8 polymorphism could lead to an improved understanding of the relationship between GAN and cancer.

Kelch Domain of Gigaxonin Interacts with Intermediate Filament Proteins Affected in Giant Axonal Neuropathy

Patients with giant axonal neuropathy (GAN) show progressive loss of motor and sensory function starting in childhood and typically live for less than 30 years. GAN is caused by autosomal recessive mutations leading to low levels of gigaxonin (GIG), a ubiquitously-expressed BTB/Kelch cytoplasmic protein believed to be an E3 ligase substrate adaptor. GAN pathology is characterized by aggregates of intermediate filaments (IFs) in multiple tissues. To delineate the molecular pathway between GIG deficiency and IF pathology, we undertook a proteomic screen to identify the normal binding partners of GIG. Prominent among them were several classes of IFs, including the neurofilament subunits whose accumulation leads to the axonal swellings for which GAN is named. We showed these interactions were dependent on the Kelch domain of GIG. Furthermore, we identified the E3 ligase MYCBP2 and the heat shock proteins HSP90AA1/AB1 as interactors with the BTB domain that may result in the ubiquitination and subsequent degradation of intermediate filaments. Our open-ended proteomic screen provides support to GIG’s role as an adaptor protein, linking IF proteins through its Kelch domain to the ubiquitin pathway proteins via its BTB domain, and points to future approaches for reversing the phenotype in human patients.

Therapeutic Efficacy of E-64-d, a Selective Calpain Inhibitor, in Experimental Acute Spinal Cord Injury

This study aims to investigate the therapeutic effect of calpain inhibitor E-64-d on SCI and to find a new approach to treat SCI. When an SCI rat model was established, it was immediately administered with E-64-d. RT-PCR and Western blotting were used to determine the protein and mRNA levels of calpain 1 and 68-kD NFP. TUNEL staining and NeuN labeling were performed to analyze neuronal apoptosis in the lesion. Immunohistochemistry assay was carried out to observe the expressions of calpain 1 and GFAP. Cyclooxygenase-2 activity was measured to show the immune response status. Locomotor function was evaluated by inclined plane test and Basso, Beattie, and Bresnahan locomotor rating scale. The results showed that calpain 1 was activated after SCI occurred. Treatment with E-64-d decreased expressions of calpain 1 and GFAP, alleviated neuronal apoptosis, inhibited cyclooxygenase-2 activity, and resulted in the promoted locomotor function. Furthermore, combination of E-64-d and MP had better efficacy than did E-64-d or MP alone. E-64-d is expected to be applied to treat SCI, and its alliance with MP may provide a valid strategy for SCI therapy.

Autism and Intellectual Disability-Associated KIRREL3 Interacts with Neuronal Proteins MAP1B and MYO16 with Potential Roles in Neurodevelopment

Cell-adhesion molecules of the immunoglobulin superfamily play critical roles in brain development, as well as in maintaining synaptic plasticity, the dysfunction of which is known to cause cognitive impairment. Recently dysfunction of KIRREL3, a synaptic molecule of the immunoglobulin superfamily, has been implicated in several neurodevelopmental conditions including intellectual disability, autism spectrum disorder, and in the neurocognitive delay associated with Jacobsen syndrome. However, the molecular mechanisms of its physiological actions remain largely unknown. Using a yeast two-hybrid screen, we found that the KIRREL3 extracellular domain interacts with brain expressed proteins MAP1B and MYO16 and its intracellular domain can potentially interact with ATP1B1, UFC1, and SHMT2. The interactions were confirmed by co-immunoprecipitation and colocalization analyses of proteins expressed in human embryonic kidney cells, mouse neuronal cells, and rat primary neuronal cells. Furthermore, we show KIRREL3 colocalization with the marker for the Golgi apparatus and synaptic vesicles. Previously, we have shown that KIRREL3 interacts with the X-linked intellectual disability associated synaptic scaffolding protein CASK through its cytoplasmic domain. In addition, we found a genomic deletion encompassing MAP1B in one patient with intellectual disability, microcephaly and seizures and deletions encompassing MYO16 in two unrelated patients with intellectual disability, autism and microcephaly. MAP1B has been previously implicated in synaptogenesis and is involved in the development of the actin-based membrane skeleton. MYO16 is expressed in hippocampal neurons and also indirectly affects actin cytoskeleton through its interaction with WAVE1 complex. We speculate KIRREL3 interacting proteins are potential candidates for intellectual disability and autism spectrum disorder. Moreover, our findings provide further insight into understanding the molecular mechanisms underlying the physiological action of KIRREL3 and its role in neurodevelopment.

The MAP1B case: an old MAP that is new again

The functions of microtubule-associated protein 1B (MAP1B) have historically been linked to the development of the nervous system, based on its very early expression in neurons and glial cells. Moreover, mice in which MAP1B is genetically inactivated have been used extensively to show its role in axonal elongation, neuronal migration, and axonal guidance. In the last few years, it has become apparent that MAP1B has other cellular and molecular functions that are not related to its microtubule-stabilizing properties in the embryonic and adult brain. In this review, we present a systematic review of the canonical and novel functions of MAP1B and propose that, in addition to regulating the polymerization of microtubule and actin microfilaments, MAP1B also acts as a signaling protein involved in normal physiology and pathological conditions in the nervous system.

Restoration of cytoskeleton homeostasis after gigaxonin gene transfer for giant axonal neuropathy

Giant axonal neuropathy (GAN) is caused by loss of function of the gigaxonin protein. On a cellular level GAN is characterized by intermediate filament (IF) aggregation, leading to a progressive and fatal peripheral neuropathy in humans. This study sought to determine if re-introduction of the GAN gene into GAN-deficient cells and mice would restore proper cytoskeleton IF homeostasis. Treatment of primary skin fibroblast cultures from three different GAN patients with an adeno-associated virus type 2 (AAV2) vector containing a normal human GAN transgene significantly reduced the number of cells displaying vimentin IF aggregates. A proteomic analysis of these treated cells was also performed, wherein the abundance of 32 of 780 identified proteins significantly changed in response to gigaxonin gene transfer. While 29 of these responding proteins have not been directly described in association with gigaxonin, three were previously identified as being disregulated in GAN and were now shifted toward normal levels. To assess the potential application of this approach in vivo and eventually in humans, GAN mice received an intracisternal injection of an AAV9/GAN vector to globally deliver the GAN gene to the brainstem and spinal cord. The treated mice showed a nearly complete clearance of peripherin IF accumulations at 3 weeks post-injection. These studies demonstrate that gigaxonin gene transfer can reverse the cellular IF aggregate pathology associated with GAN.

The emerging family of CULLIN3-RING ubiquitin ligases (CRL3s): cellular functions and disease implications

Protein ubiquitylation is a post-translational modification that controls all aspects of eukaryotic cell functionality, and its defective regulation is manifested in various human diseases. The ubiquitylation process requires a set of enzymes, of which the ubiquitin ligases (E3s) are the substrate recognition components. Modular CULLIN-RING ubiquitin ligases (CRLs) are the most prevalent class of E3s, comprising hundreds of distinct CRL complexes with the potential to recruit as many and even more protein substrates. Best understood at both structural and functional levels are CRL1 or SCF (SKP1/CUL1/F-box protein) complexes, representing the founding member of this class of multimeric E3s. Another CRL subfamily, called CRL3, is composed of the molecular scaffold CULLIN3 and the RING protein RBX1, in combination with one of numerous BTB domain proteins acting as substrate adaptors. Recent work has firmly established CRL3s as major regulators of different cellular and developmental processes as well as stress responses in both metazoans and higher plants. In humans, functional alterations of CRL3s have been associated with various pathologies, including metabolic disorders, muscle, and nerve degeneration, as well as cancer. In this review, we summarize recent discoveries on the function of CRL3s in both metazoans and plants, and discuss their mode of regulation and specificities.

Transcriptome analysis of newt lens regeneration reveals distinct gradients in gene expression patterns

Regeneration of the lens in newts is quite a unique process. The lens is removed in its entirety and regeneration ensues from the pigment epithelial cells of the dorsal iris via transdifferentiation. The same type of cells from the ventral iris are not capable of regenerating a lens. It is, thus, expected that differences between dorsal and ventral iris during the process of regeneration might provide important clues pertaining to the mechanism of regeneration. In this paper, we employed next generation RNA-seq to determine gene expression patterns during lens regeneration in Notophthalmus viridescens. The expression of more than 38,000 transcripts was compared between dorsal and ventral iris. Although very few genes were found to be dorsal- or ventral-specific, certain groups of genes were up-regulated specifically in the dorsal iris. These genes are involved in cell cycle, gene regulation, cytoskeleton and immune response. In addition, the expression of six highly regulated genes, TBX5, FGF10, UNC5B, VAX2, NR2F5, and NTN1, was verified using qRT-PCR. These graded gene expression patterns provide insight into the mechanism of lens regeneration, the markers that are specific to dorsal or ventral iris, and layout a map for future studies in the field.

The light chains of microtubule-associated proteins MAP1A and MAP1B interact with α1-syntrophin in the central and peripheral nervous system

Microtubule-associated proteins of the MAP1 family (MAP1A, MAP1B, and MAP1S) share, among other features, a highly conserved COOH-terminal domain approximately 125 amino acids in length. We conducted a yeast 2-hybrid screen to search for proteins interacting with this domain and identified α1-syntrophin, a member of a multigene family of adapter proteins involved in signal transduction. We further demonstrate that the interaction between the conserved COOH-terminal 125-amino acid domain (which is located in the light chains of MAP1A, MAP1B, and MAP1S) and α1-syntrophin is direct and occurs through the pleckstrin homology domain 2 (PH2) and the postsynaptic density protein 95/disk large/zonula occludens-1 protein homology domain (PDZ) of α1-syntrophin. We confirmed the interaction of MAP1B and α1-syntrophin by co-localization of the two proteins in transfected cells and by co-immunoprecipitation experiments from mouse brain. In addition, we show that MAP1B and α1-syntrophin partially co-localize in Schwann cells of the murine sciatic nerve during postnatal development and in the adult. However, intracellular localization of α1-syntrophin and other Schwann cell proteins such as ezrin and dystrophin-related protein 2 (DRP2) and the localization of the axonal node of Ranvier-associated protein Caspr1/paranodin were not affected in MAP1B null mice. Our findings add to a growing body of evidence that classical MAPs are likely to be involved in signal transduction not only by directly modulating microtubule function, but also through their interaction with signal transduction proteins.

Proteomic analysis in giant axonal neuropathy: new insights into disease mechanisms

INTRODUCTION

Giant axonal neuropathy (GAN) is a progressive hereditary disease that affects the peripheral and central nervous systems. It is characterized morphologically by aggregates of intermediate filaments in different tissues. Mutations have been reported in the gene that codes for gigaxonin. Nevertheless, the underlying molecular mechanism remains obscure.

METHODS

Cell lines from 4 GAN patients and 4 controls were analyzed by iTRAQ.

RESULTS

Among the dysregulated proteins were ribosomal protein L29, ribosomal protein L37, galectin-1, glia-derived nexin, and aminopeptidase N. Also, nuclear proteins linked to formin-binding proteins were found to be dysregulated. Although the major role of gigaxonin is reported to be degradation of cytoskeleton-associated proteins, the amount of 76 structural cytoskeletal proteins was unaltered.

CONCLUSIONS

Several of the dysregulated proteins play a role in cytoskeletal reorganization. Based on these findings, we speculate that disturbed cytoskeletal regulation is responsible for formation of aggregates of intermediate filaments.

Cytoskeletal transition at the paranodes: the Achilles' heel of myelinated axons

Myelination organizes axons into distinct domains that allow nerve impulses to propagate in a saltatory manner. The edges of the myelin sheath are sealed at the paranodes by axon-glial junctions that have a crucial role in organizing the axonal cytoskeleton. Here we propose a model in which the myelinated axons depend on the axon-glial junctions to stabilize the cytoskeletal transition at the paranodes. Thus paranodal regions are likely to be particularly susceptible to damage induced by demyelinating diseases such as multiple sclerosis.

Nemitin, a novel Map8/Map1s interacting protein with Wd40 repeats

In neurons, a highly regulated microtubule cytoskeleton is essential for many cellular functions. These include axonal transport, regional specialization and synaptic function. Given the critical roles of microtubule-associated proteins (MAPs) in maintaining and regulating microtubule stability and dynamics, we sought to understand how this regulation is achieved. Here, we identify a novel LisH/WD40 repeat protein, tentatively named nemitin (neuronal enriched MAP interacting protein), as a potential regulator of MAP8-associated microtubule function. Based on expression at both the mRNA and protein levels, nemitin is enriched in the nervous system. Its protein expression is detected as early as embryonic day 11 and continues through adulthood. Interestingly, when expressed in non-neuronal cells, nemitin displays a diffuse pattern with puncta, although at the ultrastructural level it localizes along the microtubule network in vivo in sciatic nerves. These results suggest that the association of nemitin to microtubules may require an intermediary protein. Indeed, co-expression of nemitin with microtubule-associated protein 8 (MAP8) results in nemitin losing its diffuse pattern, instead decorating microtubules uniformly along with MAP8. Together, these results imply that nemitin may play an important role in regulating the neuronal cytoskeleton through an interaction with MAP8.

MAP1B and clathrin are novel interacting partners of the giant cyto-linker dystonin

Dystonin is a large multidomain cytoskeletal-associated protein that plays an essential role in the nervous system. Loss of dystonin results in neuromuscular dysfunction and early death in a mouse mutant called dystonia musculorum. Conserved among related proteins, the plakin domain is a defining feature of all major dystonin isoforms, yet its interactions have not been explored in detail. The purpose of the present study was to identify novel interacting partners of the plakin domain of the neuronal isoform of dystonin (dystonin-a). Newly identified interacting proteins discovered through a pull-down assay were validated using coimmunoprecipitation, coimmunofluorescence, and proximity ligation assays. Microtubule associated protein 1B (MAP1B), a microtubule stabilizing protein, and clathrin heavy chain, the major component of the clathrin triskelion, were identified as interaction partners for dystonin-a. Increased levels of phosphorylated MAP1B suggest a misregulation of MAP1B and a potentially novel component of the dt pathology. This work will further facilitate our understanding of how cytoskeletal proteins can affect and regulate neurodegenerative disorders.

The retromer component SNX6 interacts with dynactin p150(Glued) and mediates endosome-to-TGN transport

The retromer is a protein complex that mediates retrograde transport of transmembrane cargoes from endosomes to the trans-Golgi network (TGN). It is comprised of a cargo-selection subcomplex of Vps26, Vps29 and Vps35 and a membrane-binding coat subcomplex of sorting nexins (SNXs). Previous studies identified SNX1/2 as one of the components of the SNX subcomplex, and SNX5/6 as candidates for the second SNX. How the retromer-associated cargoes are recognized and transported by molecular motors are largely unknown. In this study, we found that one of SNX1/2's dimerization partners, SNX6, interacts with the p150(Glued) subunit of the dynein/dynactin motor complex. We present evidence that SNX6 is a component of the retromer, and that recruitment of the motor complex to the membrane-associated retromer requires the SNX6-p150(Glued) interaction. Disruption of the SNX6-p150(Glued) interaction causes failure in formation and detachment of the tubulovesicular sorting structures from endosomes and results in block of CI-MPR retrieval from endosomes to the TGN. These observations indicate that in addition to SNX1/2, SNX6 in association with the dynein/dynactin complex drives the formation and movement of tubular retrograde intermediates.

Neuronal intermediate filaments and neurodegenerative disorders

Intermediate filaments represent the most abundant cytoskeletal element in mature neurons. Mutations and/or accumulations of neuronal intermediate filament proteins are frequently observed in several human neurodegenerative disorders. Although it is now admitted that disorganization of the neurofilament network may be directly involved in neurodegeneration, certain type of perikaryal intermediate filament aggregates confer protection in motor neuron disease. The use of various mouse models provided a better knowledge of the role played by the disorganization of intermediate filaments in the pathogenesis of neurodegenerative disorders, but the mechanisms leading to the formation of these aggregates remain elusive. Here, we will review some neurodegenerative diseases involving intermediate filaments abnormalities and possible mechanisms susceptible to provoke them.

Gigaxonin controls vimentin organization through a tubulin chaperone-independent pathway

Gigaxonin mutations cause the fatal human neurodegenerative disorder giant axonal neuropathy (GAN). Broad deterioration of the nervous system in GAN patients is accompanied by massive disorganization of intermediate filaments (IFs) both in neurons and many non-neuronal cells. With newly developed antibodies, gigaxonin is now shown to be expressed at extremely low levels throughout the nervous system. In lymphoblast cell lines derived from severe and mild forms of GAN, mutations in gigaxonin are shown to yield highly unstable proteins, thereby permitting a rapid diagnostic test for the spectrum of GAN mutations as an alternative to invasive nerve biopsy or systematic sequencing of the GAN gene. Gigaxonin has been proposed as a substrate adaptor for an E3 ubiquitin ligase, which affects proteasome-dependent degradation of microtubule-related proteins including MAP1B, MAP8 and the tubulin folding chaperone TBCB. We demonstrate that, unlike its counterpart TBCE, TBCB only moderately destabilizes microtubules. Neither TBCB abundance nor microtubule organization or densities are altered in GAN mutant fibroblasts, thus demonstrating that altered TBCB levels are not primary determinants of IF disorganization in GAN. Characteristic GAN mutant-induced ovoid aggregates of vimentin are not produced in normal fibroblasts after disrupting microtubule assembly, either by TBCE overexpression or depolymerizing drugs. Thus, IF disorganization in GAN fibroblasts is independent of TBCB and microtubule loss and must be regulated by a yet unidentified mechanism.

The Roc domain of leucine-rich repeat kinase 2 is sufficient for interaction with microtubules

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the leading cause of genetically inherited Parkinson's disease (PD). Although this multidomain protein has been shown to have both GTPase and kinase activities through the Roc and MAPKKK domains, respectively, the protein-protein interactions and pathways involved in LRRK2-mediated signaling remain elusive. Utilizing a combination of protein pull-down assays, mass spectrometry, Western blotting, and immunofluorescence microscopy, this study identifies and describes the interaction between LRRK2 and microtubules. The Roc or GTPase-like domain of LRRK2 is sufficient for interaction with alpha/beta-tubulin heterodimers. This interaction occurs in a guanine nucleotide-independent manner, suggesting that tubulin might not be an effector of the LRRK2 GTPase domain. The R1441C pathogenic mutation, located within the Roc domain, retains interaction with alpha/beta-tubulin heterodimers, suggesting that disruption of this interaction likely is not the mechanism whereby the R1441C mutation leads to disease. At a subcellular level, endogenous LRRK2 protein was found to colocalize with alpha/beta-tubulin in primary hippocampal neurons. These findings are significant in that they link LRRK2 with microtubules, a structural component of the cell that is critically involved in the pathogenesis of several neurodegenerative diseases, including PD.

Modest loss of peripheral axons, muscle atrophy and formation of brain inclusions in mice with targeted deletion of gigaxonin exon 1

Mutations in the gigaxonin gene are responsible for giant axonal neuropathy (GAN), a progressive neurodegenerative disorder associated with abnormal accumulations of Intermediate Filaments (IFs). Gigaxonin is the substrate-specific adaptor for a new Cul3-E3-ubiquitin ligase family that promotes the proteasome dependent degradation of its partners MAP1B, MAP8 and tubulin cofactor B. Here, we report the generation of a mouse model with targeted deletion of Gan exon 1 (Gan(Deltaexon1;Deltaexon1)). Analyses of the Gan(Deltaexon1;Deltaexon1) mice revealed increased levels of various IFs proteins in the nervous system and the presence of IFs inclusion bodies in the brain. Despite deficiency of full length gigaxonin, the Gan(Deltaexon1;Deltaexon1) mice do not develop overt neurological phenotypes and giant axons reminiscent of the human GAN disease. Nonetheless, at 6 months of age the Gan(Deltaexon1;Deltaexon1) mice exhibit a modest hind limb muscle atrophy, a 10% decrease of muscle innervation and a 27% axonal loss in the L5 ventral roots. This new mouse model should provide a useful tool to test potential therapeutic approaches for GAN disease.

Epithelial polarity requires septin coupling of vesicle transport to polyglutamylated microtubules

In epithelial cells, polarized growth and maintenance of apical and basolateral plasma membrane domains depend on protein sorting from the trans-Golgi network (TGN) and vesicle delivery to the plasma membrane. Septins are filamentous GTPases required for polarized membrane growth in budding yeast, but whether they function in epithelial polarity is unknown. Here, we show that in epithelial cells septin 2 (SEPT2) fibers colocalize with a subset of microtubule tracks composed of polyglutamylated (polyGlu) tubulin, and that vesicles containing apical or basolateral proteins exit the TGN along these SEPT2/polyGlu microtubule tracks. Tubulin-associated SEPT2 facilitates vesicle transport by maintaining polyGlu microtubule tracks and impeding tubulin binding of microtubule-associated protein 4 (MAP4). Significantly, this regulatory step is required for polarized, columnar-shaped epithelia biogenesis; upon SEPT2 depletion, cells become short and fibroblast-shaped due to intracellular accumulation of apical and basolateral membrane proteins, and loss of vertically oriented polyGlu microtubules. We suggest that septin coupling of the microtubule cytoskeleton to post-Golgi vesicle transport is required for the morphogenesis of polarized epithelia.

The social network of a cell: recent advances in interactome mapping

Proteins very rarely act in isolation. Biomolecular interactions are central to all biological functions. In human, for example, interference with biomolecular networks often lead to disease. Protein-protein and protein-metabolite interactions have traditionally been studied one by one. Recently, significant progresses have been made in adapting suitable tools for the global analysis of biomolecular interactions. Here we review this suite of powerful technologies that enable an exponentially growing number of large-scale interaction datasets. These new technologies have already contributed to a more comprehensive cartography of several pathways relevant to human pathologies, offering a broader choice for therapeutic targets. Genome-wide scale analyses in model organisms reveal general organizational principles of eukaryotic proteomes. We also review the biochemical approaches that have been used in the past on a smaller scale for the quantification of the binding constant and the thermodynamics parameters governing biomolecular interaction. The adaptation of these technologies to the large-scale measurement of biomolecular interactions in (semi-)quantitative terms represents an important challenge.

Genotype–phenotype analysis in patients with giant axonal neuropathy (GAN)

Giant axonal neuropathy (GAN, MIM: 256850) is a devastating autosomal recessive disorder characterized by an early onset severe peripheral neuropathy, varying central nervous system involvement and strikingly frizzly hair. Giant axonal neuropathy is usually caused by mutations in the gigaxonin gene (GAN) but genetic heterogeneity has been demonstrated for a milder variant of this disease. Here, we report ten patients referred to us for molecular genetic diagnosis. All patients had typical clinical signs suggestive of giant axonal neuropathy. In seven affected individuals, we found disease causing mutations in the gigaxonin gene affecting both alleles: two splice-site and four missense mutations, not reported previously. Gigaxonin binds N-terminally to ubiquitin activating enzyme E1 and C-terminally to various microtubule associated proteins causing their ubiquitin mediated degradation. It was shown for a number of gigaxonin mutations that they impede this process leading to accumulation of microtubule associated proteins and there by impairing cellular functions.

Microtubule-associated protein 1B binds glyceraldehyde-3-phosphate dehydrogenase

Microtubule-associated protein 1B, MAP1B, is a major cytoskeletal protein during brain development and one of the largest brain MAPs associated with microtubules and microfilaments. Here, we identified several proteins that bind to MAP1B via immunoprecipitation with a MAP1B-specific antibody, by one and two-dimensional gel electrophoresis and subsequent mass spectrometry identification of precipitated proteins. In addition to tubulin and actin, a variety of proteins were identified. Among these proteins were glyceraldehyde-3-phosphate dehydrogenase (GAPDH), heat shock protein 8, dihydropyrimidinase related proteins 2 and 3, protein-L-isoaspartate O-methyltransferase, beta-spectrin, and clathrin protein MKIAA0034, linking either directly or indirectly to MAP1B. In particular, GAPDH, a key glycolytic enzyme, was bound in large quantity to the heavy chain of MAP1B in adult brain tissue. In vitro binding studies confirmed a direct binding of GAPDH to MAP1B. In PC12 cells, GAPDH was found in cytoplasm and nuclei and partially co-localized with MAP1B. It disappeared from the cytoplasm under oxidative stress or after a disruption of cytoskeletal elements after colcemid or cytochalasin exposure. GAPDH may be essential in the local energy provision of cytoskeletal structures and MAP1B may help to keep this key enzyme close to the cytoskeleton.

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.

Alterations in lipid metabolism gene expression and abnormal lipid accumulation in fibroblast explants from giant axonal neuropathy patients

Background

Giant axonal neuropathy (GAN) is a hereditary neurological disorder that affects both central and peripheral nerves. The main pathological hallmark of the disease is abnormal accumulations of intermediate filaments (IFs) in giant axons and other cell types. Mutations in the GAN gene, encoding gigaxonin, cause the disease. Gigaxonin is important in controlling protein degradation via the ubiquitin-proteasome system. The goal of this study was to examine global alterations in gene expression in fibroblasts derived from newly identified GAN families compared with normal cells.

Results

We report the characterization of fibroblast explants obtained from two unrelated GAN patients. We identify three novel putative mutant GAN alleles and show aggregation of vimentin IFs in these fibroblasts. By microarray analysis, we also demonstrate that the expression of lipid metabolism genes of the GAN fibroblasts is disrupted, which may account for the abnormal accumulations of lipid droplets in these cells.

Conclusion

Our findings suggest that aberrant lipid metabolism in GAN patients may contribute to the progression of the disease.

Light chain 1 of microtubule-associated protein 1B can negatively regulate the action of Pes1

Pes1 was first identified as the locus affected in the zebrafish mutant pescadillo, which exhibits severe defects in gut and liver development. It has since been demonstrated that loss of Pes1 expression in mammals and yeast affects ribosome biogenesis, resulting in a block in cell proliferation. Pes1 contains a BRCA1 C-terminal domain, a structural motif that has been shown to facilitate protein-protein interactions, suggesting that Pes1 has binding partners. We used a yeast two-hybrid screen to identify putative interacting proteins. We found that light chain 1 of the microtubule-associated protein 1B (Mtap1b-LC1) could partner with Pes1, and deletion analyses revealed a specific interaction of Mtap1b-LC1 with the Pes1 BRCA1 C-terminal domain. We confirmed the integrity of the interaction between Pes1 and Mtap1b-LC1 by co-immunoprecipitation experiments. Protein localization studies in NIH3T3 cells revealed that exogenously expressed Pes1 was typically restricted to nuclei and nucleoli. However, exogenous Pes1 was found predominantly in the cytoplasm in cells that were forced to express Mtap1b-LC1. We also observed that the expression of endogenous Pes1 protein was significantly reduced or undetectable in nuclei when Mtap1b-LC1 was overexpressed, implying that a dynamic interaction exists between the two proteins and that Mtap1b-LC1 has the potential to negatively impact Pes1 function. Finally, we demonstrated that, as is the case when Pes1 expression is depleted by shRNA, overexpression of Mtap1b-LC1 resulted in diminished proliferation of NIH3T3 cells, suggesting that Mtap1b-LC1 has the potential to repress cell proliferation by modulating the nucleolar levels of Pes1.

Retrolinkin, a membrane protein, plays an important role in retrograde axonal transport

Retrograde axonal transport plays an important role in the maintenance of neuronal functions, but the mechanism is poorly defined partly because the constituents of the retrograde transport system and their interactions have yet to be elucidated. Of special interest is how dynein/dynactin motor proteins interact with membrane cargoes. Here, we report that an endosomal vesicle protein, termed retrolinkin, functions as a receptor tethering vesicles to dynein/dynactin through BPAG1n4. Retrolinkin, a membrane protein highly enriched in neuronal endosomes, binds directly to BPAG1n4. Deletion of retrolinkin membrane-association domains disrupts retrograde vesicular transport, recapitulating the BPAG1 null phenotype. We propose that retrolinkin acts with BPAG1n4 to specifically regulate retrograde axonal transport. Our work lays the foundation for understanding fundamental issues of axonal transport and provides insights into the molecular mechanisms underlying human neurodegenerative disorders.

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

A 10-year-old girl with progressive generalized weakness

Case Presentation

Dr. Harvey Sarnat: A.Y. was a 10-year-old Mexican girl who presented with a 7-year history of progressive weakness. She was the full-term product of an uncomplicated pregnancy and delivery, weighing 2850 grams at birth. Early developmental milestones were achieved at the expected rate until age three, when frequent falling was noted. Progressive weakness of her legs ensued, and at age nine years, A.Y. lost the ability to walk beyond a few steps, and shortly thereafter she could not stand without support. She had no seizures, visual disturbance, dysphagia, or incontinence. Previously an excellent student, her academic performance in school had deteriorated over the past year. She was not on any medications. Family history was negative for any known neurological or neuromuscular diseases. A.Y. was an only child. Both parents were alive and well; there was no history of consanguinity.

Impairment of microtubule-dependent trafficking by overexpression of α-synuclein

Abnormal accumulation of alpha-synuclein (alpha-syn) has been linked to several neurological disorders, including Parkinson's disease (PD). However, the underlying mechanism by which alpha-syn accumulation affects neuronal function and survival remains unknown. Here, we provide data suggesting a possible effect of aggregated alpha-syn on the microtubule (MT) network. Consistent with the MT dysfunction, we also observed other degenerative changes, such as neuritic degeneration, trafficking defects, and Golgi fragmentation, which are common pathological features shared by many human neurodegenerative diseases. Neuritic degeneration and Golgi fragmentation were confirmed in primary cultures of dorsal root ganglia (DRG) neurons overexpressing alpha-syn. This effect of alpha-syn seems to have some selectivity to the MT system, as actin microfilaments and MT-independent trafficking remain unaffected. Within the degenerating neurites, we found numerous spherical co-aggregates of alpha-syn and tubulins, from which actin was excluded. These studies suggest that the MT system is a potential target of alpha-syn, and impairment of this system might have impacts on neuronal structure and function.

The MAP1 family of microtubule-associated proteins

MAP1-family proteins are classical microtubule-associated proteins (MAPs) that bind along the microtubule lattice and stabilize microtubules.

MAP1-family proteins are classical microtubule-associated proteins (MAPs) that bind along the microtubule lattice. The founding members, MAP1A and MAP1B, are predominantly expressed in neurons, where they are thought to be important in the formation and development of axons and dendrites. Mammalian genomes usually contain three family members, MAP1A, MAP1B and a shorter, more recently identified gene called MAP1S. By contrast, only one family member, Futsch, is found in Drosophila. After their initial expression, the MAP1A and MAP1B polypeptides are cleaved into light and heavy chains, which are then assembled into mature complexes together with the separately encoded light chain 3 subunit (LC3). Both MAP1A and MAP1B are well known for their microtubule-stabilizing activity, but MAP1 proteins can also interact with other cellular components, including filamentous actin and signaling proteins. Furthermore, the activity of MAP1A and MAP1B is controlled by upstream signaling mechanisms, including the MAP kinase and glycogen synthase kinase-3 β pathways.

Detection of Autoantibodies to the BTB-kelch Protein KLHL7 in Cancer Sera

The aim of the study was to search for novel targets of autoantibodies in patients with paraneoplastic neurological syndromes (PNS). PNS are mediated by immune reactions against autoantigen(s) shared by the cancer cells and the nervous system. By serological screening of a rat cerebellum cDNA expression library using anti-Hu-positive sera from three patients with paraneoplastic encephalomyelitis (PEM), we identified an open reading frame encoding an isoform of the BTB-kelch protein KLHL7. Immunohistochemical studies demonstrated that the KLHL7 protein is expressed in the nuclei of neurones, but not in other tissues including various cancers. However, the KLHL7 protein was detected in the nuclei of cancer cell lines. Antibodies to KLHL7 were detected by an immunoprecipitation assay in sera from 12 of 254 (4.7%) patients with various cancers and 2 of 170 blood donors (1.2%). None of 50 sera from patients with multiple sclerosis were positive for KLHL7 antibodies. Sixteen patients with classical PNS and anti-Hu or anti-Yo antibodies were also negative for KLHL7 antibodies. Seven cancer patients with KLHL7 antibodies had various signs of neurological disease that could be related to cancer, whereas the remaining five seropositive cancer patients had no clinical signs of possible PNS. The present results indicate that KLHL7 antibodies are associated with various cancers, and in some patients also with neurological disease. Whether KLHL7 antibodies can be used as paraneoplastic markers for PNS remains to be determined.