The gene encoding gigaxonin, a new member of the cytoskeletal BTB/kelch repeat family, is mutated in giant axonal neuropathy

Ubiquitin ligase activity of Cul3-KLHL7 protein is attenuated by autosomal dominant retinitis pigmentosa causative mutation

Substrate-specific protein degradation mediated by the ubiquitin proteasome system (UPS) is crucial for the proper function of the cell. Proteins are specifically recognized and ubiquitinated by the ubiquitin ligases (E3s) and are then degraded by the proteasome. BTB proteins act as the substrate recognition subunit that recruits their cognate substrates to the Cullin 3-based multisubunit E3s. Recently, it was reported that missense mutations in KLHL7, a BTB-Kelch protein, are related to autosomal dominant retinitis pigmentosa (adRP). However, the involvement of KLHL7 in the UPS and the outcome of the adRP causative mutations were unknown. In this study, we show that KLHL7 forms a dimer, assembles with Cul3 through its BTB and BACK domains, and exerts E3 activity. Lys-48-linked but not Lys-63-linked polyubiquitin chain co-localized with KLHL7, which increased upon proteasome inhibition suggesting that KLHL7 mediates protein degradation via UPS. An adRP-causative missense mutation in the BACK domain of KLHL7 attenuated only the Cul3 interaction but not dimerization. Nevertheless, the incorporation of the mutant as a heterodimer in the Cul3-KLHL7 complex diminished the E3 ligase activity. Together, our results suggest that KLHL7 constitutes a Cul3-based E3 and that the disease-causing mutation inhibits ligase activity in a dominant negative manner, which may lead to the inappropriate accumulation of the substrates targeted for proteasomal degradation.

A BTB domain-containing gene is upregulated by immune challenge

20-Hydroxyecdysone (20E) is an important hormone that regulates the development of insects. Although previous evidence revealed that 20E promotes innate immunity in insects, the mechanism involved is still unclear. In this study, the HaBBP gene from Helicoverpa armigera is cloned, which contains BTB (broad-complex, tramtrack, and bric-a-brac), a BACK (BTB and carboxyl-terminus kelch repeats), and PHR (PAM, highwire, and RPM) domains. RT-PCR analysis of HaBBP and western blot analysis of HaBBP show that the mRNA and protein level are higher in the fat body and hemocytes during the molting and metamorphic stages compared with the feeding stage. HaBBP was upregulated by 20E in hemocytes. Knockdown of the 20E receptor EcR-B1 and the heterodimeric partner ultraspiracle protein USP1 in an epidermal cell line (HaEpi) blocked the transcription of HaBBP. HaBBP is distributed in granulocytes and plasmatocytes. Immune stimulation by Escherichia coli caused the upregulation of HaBBP in both hemocytes and fat body. Thus, HaBBP is regulated by the 20E signaling pathway, and is likely involved in the insect innate immunity.

Sensory-motor deficits and neurofilament disorganization in gigaxonin-null mice

Background

Giant Axonal Neuropathy (GAN) is a fatal neurodegenerative disorder with early onset characterized by a severe deterioration of the peripheral and central nervous system, involving both the motor and the sensory tracts and leading to ataxia, speech defect and intellectual disabilities. The broad deterioration of the nervous system is accompanied by a generalized disorganization of the intermediate filaments, including neurofilaments in neurons, but the implication of this defect in disease onset or progression remains unknown. The identification of gigaxonin, the substrate adaptor of an E3 ubiquitin ligase, as the defective protein in GAN allows us to now investigate the crucial role of the gigaxonin-E3 ligase in sustaining neuronal and intermediate filament integrity. To study the mechanisms controlled by gigaxonin in these processes and to provide a relevant model to test the therapeutic approaches under development for GAN, we generated a Gigaxonin-null mouse by gene targeting.

Results

We investigated for the first time in Gigaxonin-null mice the deterioration of the motor and sensory functions over time as well as the spatial disorganization of neurofilaments. We showed that gigaxonin depletion in mice induces mild but persistent motor deficits starting at 60 weeks of age in the 129/SvJ-genetic background, while sensory deficits were demonstrated in C57BL/6 animals. In our hands, another gigaxonin-null mouse did not display the early and severe motor deficits reported previously. No apparent neurodegeneration was observed in our knock-out mice, but dysregulation of neurofilaments in proximal and distal axons was massive. Indeed, neurofilaments were not only more abundant but they also showed the abnormal increase in diameter and misorientation that are characteristics of the human pathology.

Conclusions

Together, our results show that gigaxonin depletion in mice induces mild motor and sensory deficits but recapitulates the severe neurofilament dysregulation seen in patients. Our model will allow investigation of the role of the gigaxonin-E3 ligase in organizing neurofilaments and may prove useful in understanding the pathological processes engaged in other neurodegenerative disorders characterized by accumulation of neurofilaments and dysfunction of the Ubiquitin Proteasome System, such as Amyotrophic Lateral Sclerosis, Huntington's, Alzheimer's and Parkinson's diseases.

Clinical and molecular characterization of ataxia with oculomotor apraxia patients in Saudi Arabia

Background

Autosomal recessive ataxias represent a group of clinically overlapping disorders. These include ataxia with oculomotor apraxia type1 (AOA1), ataxia with oculomotor apraxia type 2 (AOA2) and ataxia-telangiectasia-like disease (ATLD). Patients are mainly characterized by cerebellar ataxia and oculomotor apraxia. Although these forms are not quite distinctive phenotypically, different genes have been linked to these disorders. Mutations in the APTX gene were reported in AOA1 patients, mutations in SETX gene were reported in patients with AOA2 and mutations in MRE11 were identified in ATLD patients. In the present study we describe in detail the clinical features and results of genetic analysis of 9 patients from 4 Saudi families with ataxia and oculomotor apraxia.

Methods

This study was conducted in the period between 2005-2010 to clinically and molecularly characterize patients with AOA phenotype. Comprehensive sequencing of all coding exons of previously reported genes related to this disorder (APTX, SETX and MRE11).

Results

A novel nonsense truncating mutation c.6859 C > T, R2287X in SETX gene was identified in patients from one family with AOA2. The previously reported missense mutation W210C in MRE11 gene was identified in two families with autosomal recessive ataxia and oculomotor apraxia.

Conclusion

Mutations in APTX , SETX and MRE11 are common in patients with autosomal recessive ataxia and oculomotor apraxia. The results of the comprehensive screening of these genes in 4 Saudi families identified mutations in SETX and MRE11 genes but failed to identify mutations in APTX gene.

Foot pad skin biopsy in mouse models of hereditary neuropathy

Numerous transgenic and knockout mouse models of human hereditary neuropathies have become available over the past decade. We describe a simple, reproducible, and safe biopsy of mouse skin for histopathological evaluation of the peripheral nervous system (PNS) in models of hereditary neuropathies. We compared the diagnostic outcome between sciatic nerve and dermal nerves found in skin biopsy (SB) from the hind foot. A total of five animal models of different Charcot-Marie-Tooth neuropathies, and one model of congenital muscular dystrophy associated neuropathy were examined. In wild type mice, dermal nerve fibers were readily identified by immunohistochemistry, light, and electron microscopy and they appeared similar to myelinated fibers in sciatic nerve. In mutant mice, SB manifested myelin abnormalities similar to those observed in sciatic nerves, including hypomyelination, onion bulbs, myelin outfolding, redundant loops, and tomacula. In many strains, however, SB showed additional abnormalities—fiber loss, dense neurofilament packing with lower phosphorylation status, and axonal degeneration—undetected in sciatic nerve, possibly because SB samples distal nerves. SB, a reliable technique to investigate peripheral neuropathies in human beings, is also useful to investigate animal models of hereditary neuropathies. Our data indicate that SB may reveal distal axonal pathology in mouse models and permits sequential follow-up of the neuropathy in an individual mouse, thereby reducing the number of mice necessary to document pathology of the PNS. © 2010 Wiley-Liss, Inc.

Molecular signaling how do axons die?

Axons depend critically on axonal transport both for supplying materials and for communicating with cell bodies. This chapter looks at each activity, asking what aspects are essential for axon survival. Axonal transport declines in neurodegenerative disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and in normal ageing, but whether all cargoes are equally affected and what limits axon survival remains unclear. Cargoes can be differentially blocked in some disorders, either individually or in groups. Each missing protein cargo results in localized loss-of-function that can be partially modeled by disrupting the corresponding gene, sometimes with surprising results. The axonal response to losing specific proteins also depends on the rates of protein turnover and on whether the protein can be locally synthesized. Among cargoes with important axonal roles are components of the PI3 kinase, Mek/Erk, and Jnk signaling pathways, which help to communicate with cell bodies and to regulate axonal transport itself. Bidirectional trafficking of Bdnf, NT-3, and other neurotrophic factors contribute to intra- and intercellular signaling, affecting the axon's cellular environment and survival. Finally, several adhesion molecules and gangliosides are key determinants of axon survival, probably by mediating axon-glia interactions. Thus, failure of long-distance intracellular transport can deprive axons of one, few, or many cargoes. This can lead to axon degeneration either directly, through the absence of essential axonal proteins, or indirectly, through failures in communication with cell bodies and nonneuronal cells.

BTB-Kelch proteins and ubiquitination of kainate receptors

Kainate receptors (KAR) form a class of glutamate receptors that have been implicated in epilepsy, stroke, Alzheimer's and neuropathic pain.1 KAR subtypes are known to be segregated to specific locations within neurons and play significant roles in synaptic transmission and plasticity.2 Increasing evidence suggests a the role for ubiqutination in regulating the number of synaptic neurotransmitter receptors.3-5 The ubiquitin pathway consists of activation (E1), conjugation (E2) and ligation (E3). Cullins form the largest family of E3 ligase complexes. We have recently shown that the BTB/Kelch domain proteins, actinfilin and mayven, bind both Cul3 and specific KAR subtypes (GluR6 and GluR5-2b) to target these KARs for ubiquitination and degradation.5 In this chapter we will review how these interactions occur, what they mean for the stability of KARs and their associated proteins and how, in turn, they may affect synaptic functions in the central nervous system.

Dominant mutations in KBTBD13, a member of the BTB/Kelch family, cause nemaline myopathy with cores

We identified a member of the BTB/Kelch protein family that is mutated in nemaline myopathy type 6 (NEM6), an autosomal-dominant neuromuscular disorder characterized by the presence of nemaline rods and core lesions in the skeletal myofibers. Analysis of affected families allowed narrowing of the candidate region on chromosome 15q22.31, and mutation screening led to the identification of a previously uncharacterized gene, KBTBD13, coding for a hypothetical protein and containing missense mutations that perfectly cosegregate with nemaline myopathy in the studied families. KBTBD13 contains a BTB/POZ domain and five Kelch repeats and is expressed primarily in skeletal and cardiac muscle. The identified disease-associated mutations, C.742C>A (p.Arg248Ser), c.1170G>C (p.Lys390Asn), and c.1222C>T (p.Arg408Cys), located in conserved domains of Kelch repeats, are predicted to disrupt the molecule's beta-propeller blades. Previously identified BTB/POZ/Kelch-domain-containing proteins have been implicated in a broad variety of biological processes, including cytoskeleton modulation, regulation of gene transcription, ubiquitination, and myofibril assembly. The functional role of KBTBD13 in skeletal muscle and the pathogenesis of NEM6 are subjects for further studies.

Canine inherited motor and sensory neuropathies: an updated classification in 22 breeds and comparison to Charcot-Marie-Tooth disease

Canine inherited neuropathies form a group of degenerative diseases affecting motor and/or sensory and autonomic peripheral nerves. There is now a large number of inherited motor and sensory neuropathies (IMSN) reported in the veterinary literature, for which clinical, electrophysiological, histopathological and mode of inheritance data are available. Their resemblance with Charcot-Marie-Tooth disease in humans is suggested, although direct comparison is difficult due to the small number of cases described in each breed and the lack of genetic knowledge in dogs. Charcot-Marie-Tooth disease forms a wide group of hereditary neuropathies for which a genetic mutation is recognised in more than 70% of patients. In dogs, no genetic mutation has so far been identified and the knowledge available for human hereditary neuropathies may be useful to identify genetic mutations in dogs. This review provides an update on data available on inherited neuropathy in Leonberger dogs and three new degenerative neuropathies are briefly described in two Russian Black terriers, two Cocker Spaniels and a Podhale Shepherd dog.

Mechanisms involved in the blood-testis barrier increased permeability induced by EMP

The blood-testis barrier (BTB) plays an important role in male reproductive system. Lots of environmental stimulations can increase the permeability of BTB and then result in antisperm antibody (AsAb) generation, which is a key step in male immune infertility. Here we reported the results of male mice exposed to electromagnetic pulse (EMP) by measuring the expression of tight-junction-associated proteins (ZO-1 and Occludin), vimentin microfilaments, and transforming growth factor-beta (TGF-beta3) as well as AsAb level in serum. Male BALB/c mice were sham exposed or exposed to EMP at two different intensities (200kV/m and 400kV/m) for 200 pulses. The testes were collected at different time points after EMP exposure. Immunofluorescence histocytochemistry, western blotting, laser confocal microscopy and RT-PCR were used in this study. Compared with sham group, the expression of ZO-1 and TGF-beta3 significantly decreased accompanied with unevenly stained vimentin microfilaments and increased serum AsAb levels in EMP-exposed mice. These results suggest a potential BTB injury and immune infertility in male mice exposed to a certain intensity of EMP.

Kelch-like homologue 9 mutation is associated with an early onset autosomal dominant distal myopathy

Distal myopathies are a heterogeneous group of disorders characterized by progressive weakness and muscular atrophy, beginning in distal limb muscles and affecting proximal limb muscles at a later stage. We studied a large German kindred with 10 affected members. Weakness and atrophy of the anterior tibial muscles started between the ages of 8 and 16 years, followed by atrophy of intrinsic hand muscles. Progression was slow, and patients retained the ability to walk until the seventh decade. Serum creatinine kinase levels were increased in the range of 150–1400 U/l. Muscle biopsies showed myopathic changes, whereas immunohistochemistry showed normal expression of marker proteins for muscular dystrophies. Patients had reduced sensation with stocking-glove distribution in the distal limbs in later life. Nerve conduction studies revealed no evidence of neuropathy. Genome-wide linkage analysis in this family revealed a new locus for distal myopathy at 9p21.2-p22.3 (multipoint logarithm of the odds ratio = 4.21). By positional cloning we found a heterozygous mutation L95F in the Kelch-like homologue 9 gene, encoding a bric-a-brac Kelch protein. Molecular modelling indicated that the mutation may interfere with the interaction of the bric-a-brac domain with Cullin 3. Coimmunoprecipitation experiments confirmed that the mutation reduces association with Cullin 3 in the Kelch-like homologue 9-Cullin 3–E3 ubiquitin ligase complex, which is involved in ubiquitin-dependent protein degradation. We identified a unique form of early onset autosomal dominant distal myopathy which is associated with a Kelch-like homologue 9 mutation and interferes with normal skeletal muscle through a novel pathogenetic mechanism.

Nuclear matrix protein (NRP/B) modulates the nuclear factor (Erythroid-derived 2)-related 2 (NRF2)-dependent oxidative stress response

Reactive molecules have diverse effects on cells and contribute to several pathological conditions. Cells have evolved complex protective systems to neutralize these molecules and restore redox homeostasis. Previously, we showed that association of nuclear factor (NF)-erythroid-derived 2 (E2)-related factor 2 (NRF2) with the nuclear matrix protein NRP/B was essential for the transcriptional activity of NRF2 target genes in tumor cells. The present study demonstrates the molecular mechanism by which NRP/B, via NRF2, modulates the transcriptional activity of antioxidant response element (ARE)-driven genes. NRP/B is localized in the nucleus of primary brain tissue and human neuroblastoma (SH-SY5Y) cells. Treatment with hydrogen peroxide (H(2)O(2)) enhances the nuclear colocalization of NRF2 and NRP/B and induces heme oxygenase 1 (HO1). Treatment of NRP/B or NRF2 knockdowns with H(2)O(2) induced apoptosis. Co-expression of NRF2 with members of the Kelch protein family, NRP/B, MAYVEN, or MAYVEN-related protein 2 (MRP2), revealed that the NRF2-NRP/B complex is important for the transcriptional activity of ARE-driven genes HO1 and NAD(P)H:quinine oxidoreductase 1 (NQO1). NRP/B interaction with Nrf2 was mapped to NRF2 ECH homology 4 (Neh4)/Neh5 regions of NRF2. NRP/B mutations that resulted in low binding affinity to NRF2 were unable to activate NRF2-modulated transcriptional activity of the ARE-driven genes, HO1 and NQO1. Thus, the interaction of NRP/B with the Neh4/Neh5 domains of NRF2 is indispensable for activation of NRF2-mediated ARE-driven antioxidant and detoxifying genes that confer cellular defense against oxidative stress-induced damage.

KBTBD7, a novel human BTB-kelch protein, activates transcriptional activities of SRE and AP-1

In this study, a novel member of BTB-kelch proteins, named KBTBD7, was cloned from a human embryonic heart cDNA library. The cDNA of KBTBD7 is 3,008 bp long and encodes a protein product of 684 amino acids (77.2 kD). This protein is highly conserved in evolution across different species. Western blot analysis indicates that a 77 kD protein specific for KBTBD7 is wildly expressed in all embryonic tissues examined. In COS-7 cells, KBTBD7 proteins are localized to the cytoplasm. KBTBD7 is a transcription activator when fused to GAL4 DNA-binding domain. Deletion analysis indicates that the BTB domain and kelch repeat motif are main regions for transcriptional activation. Overexpression of KBTBD7 in MCF-7 cells activates the transcriptional activities of activator protein-1 (AP-1) and serum response element (SRE), which can be relieved by siRNA. These results suggest that KBTBD7 proteins may act as a new transcriptional activator in mitogen-activated protein kinase (MAPK) signaling. [BMB reports 2010; 43(1): 17-22].

Inhibition of angiogenesis by the BTB domain of promyelocytic leukemia zinc finger protein

Promyelocytic leukemia zinc finger is a negative regulator of cell cycle progression. In this study, we showed that PLZF inhibits endothelial cell angiogenesis using a human umbilical vein endothelial cell system. We also focused on characterizing the specific function of the BTB domain of PLZF as a novel apoptotic and anti-angiogenic protein via deletion mapping analysis. The BTB domain directly inhibited tube formation, as well as the biological functions of angiostatic activity in vivo, and reduced the expression of p-Akt and p-eNOS, which play a significant role in angiogenesis when stimulated by VEGF. These results strongly suggest that the BTB domain could potentially modulate the apoptotic and anti-angiogenic effects of PLZF.

Estradiol therapy in adulthood reverses glial and neuronal alterations caused by perinatal asphyxia

The capacity of the ovarian hormone 17beta-estradiol to prevent neurodegeneration has been characterized in several animal models of brain and spinal cord pathology. However, the potential reparative activity of the hormone under chronic neurodegenerative conditions has received less attention. In this study we have assessed the effect of estradiol therapy in adulthood on chronic glial and neuronal alterations caused by perinatal asphyxia (PA) in rats. Four-month-old male Sprague-Dawley rats submitted to PA just after delivery, and their control littermates, were injected for 3 consecutive days with 17beta estradiol or vehicle. Animals subjected to PA and treated with vehicle showed an increased astrogliosis, focal swelling and fragmented appearance of MAP-2 immunoreactive dendrites, decreased MAP-2 immunoreactivity and decreased phosphorylation of high and medium molecular weight neurofilaments in the hippocampus, compared to control animals. Estradiol therapy reversed these alterations. These findings indicate that estradiol is able to reduce, in adult animals, chronic reactive astrogliosis and neuronal alterations caused by an early developmental neurodegenerative event, suggesting that the hormone might induce reparative actions in the Central Nervous System (CNS).

Giant Axonal Neuropathy: A Pictorial Essay and Review of Literature

Giant axonal neuropathy (GAN) is a neurodegenerative disorder classified within the hereditary motor and sensory neuropathies, affecting both the peripheral and central nervous systems. GAN typically presents in early childhood before the age of five years and progresses to death usually by early adulthood. Various imaging findings in giant axonal neuropathy have been described and documented in literature in the form multiple case reports. We here present a pictorial essay of all the major imaging GAN findings described in the literature. In addition, involvement of the dentate nucleus hitherto not described in the literature was noted in the present case.

Clinical and genetic studies in a Chinese family with giant axonal neuropathy

The objective of the study was to investigate a girl with giant axonal neuropathy and detect the mutation of GAN gene in her family. The encoding exons of GAN gene were amplified from genomic DNA of the proband and her parents by polymerase chain reaction and directly sequenced after purification. The proband manifested typical neurological symptoms and pathological abnormalities. The case had 2 heterozygous missense mutations in GAN gene: 1. c. 224 T>A in exon 2, her mother was a heterozygote of this mutation and had normal phenotype; 2. c.1634G>A in exon 10, and her father was a heterozygote of this mutation and had normal phenotype. Both of the mutations caused amino acid changes in the gigaxonin protein. In this family, missense mutation of c.224 T>A and missense mutation of c.1634G>A in GAN gene caused the phenotype of giant axonal neuropathy in the proband. Her parents are heterozygotes of the disease without symptoms.

RhoB induces apoptosis via direct interaction with TNFAIP1 in HeLa cells

RhoB, a tumor suppressor, has emerged as an interesting cancer target, and extensive studies aimed at understanding its role in apoptosis have been performed. In our study, we investigated the involvement of RhoB-interacting molecules in apoptosis. To identify RhoB-interacting proteins, we performed yeast-two hybrid screening assays using RhoB as a bait and isolated TNFAIP1, a TNFalpha-induced protein containing the BTB/POZ domain. The interaction between RhoB and TNFAIP1 was demonstrated in vivo through coimmunoprecipitation studies and in vitro binding assays. RFP-TNFAIP1 was found to be partially colocalized with EGFP-RhoB. The partial colocalization of RhoB and TNFAIP1 in endosomes suggests that RhoB-TNFAIP1 interactions may have a functional role in apoptosis. TNFAIP1 elicited proapoptotic activity, while simultaneous expression of RhoB and TNFAIP1 resulted in a dramatic increase in apoptosis in HeLa cells. Furthermore, knockdown of RhoB using siRNA clearly rescued cells from apoptosis induced by TNFAIP1. This finding suggests that interactions between RhoB and TNFAIP1 are crucial for induction of apoptosis in HeLa cells. The observation of increased SAPK/JNK phosphorylation in apoptotic cells and the finding that a JNK inhibitor suppressed apoptosis indicates that SAPK/JNK signaling may be involved in apoptosis induced by RhoB-TNFAIP1 interactions. In conclusion, we found that RhoB interacts with TNFAIP1 to regulate apoptosis via a SAPK/JNK-mediated signal transduction mechanism.

COP9 limits dendritic branching via Cullin3-dependent degradation of the actin-crosslinking BTB-domain protein Kelch

Components of the COP9 signalosome (CSN), a key member of the conserved 26S proteasome degradation pathway, have been detected to be altered in patients of several debilitating syndromes. These findings suggest that CSN acts in neural circuits, but the exact function of CSN in brain remains unidentified. Previously, using Drosophila peripheral nervous system (PNS) as a model system, we determined that CSN is a critical regulator of dendritic morphogenesis. We found that defects in CSN led to the strikingly contrast phenotype of either reducing or stimulating dendritic branching. In particular, we have reported that CSN stimulates dendritic branching via Cullin1-mediated proteolysis. Here we describe that CSN inhibits dendritic arborization in PNS neurons acting via control of Cullin3 function: loss of Cullin3 causes excessive dendritic branching. We also identified a downstream target for Cullin3-dependent degradation in neurons – the actin-crosslinking BTB-domain protein Kelch. Inappropriate accumulation of Kelch, either due to the impaired Cullin3-dependent turnover, or ectopic expression of Kelch, leads to uncontrolled dendritic branching. These findings indicate that the CSN pathway modulates neuronal network in a multilayer manner, providing the foundation for new insight into the CSN role in human mental retardation disorders and neurodegenerative disease.

Gigaxonin mutation analysis in patients with NIFID

Neuronal intermediate filament inclusion disease (NIFID) is a frontotemporal lobar degeneration (FTLD) characterized by frontotemporal dementia (FTD), pyramidal and extrapyramidal signs. The disease is histologically characterized by the presence of abnormal neuronal cytoplasmic inclusions (NCIs) which contain α-internexin and other neuronal intermediate filament (IF) proteins. Gigaxonin (GAN) is a cytoskeletal regulating protein and the genetic cause of giant axonal neuropathy. Since the immunoreactive profile of NCIs in NIFID is similar to that observed in brain sections from Gan(Δex1/Δex1) mice, we speculated that GAN could be a candidate gene causing NIFID. Therefore, we performed a mutation analysis of GAN in NIFID patients. Although the NCIs of NIFID and Gan(Δex1/Δex1) mice were immunohistochemically similar, no GAN variant was identified in DNA obtained from well-characterized cases of NIFID.

Genome-wide screen identifies drug-induced regulation of the gene giant axonal neuropathy (Gan) in a mouse model of antiretroviral-induced painful peripheral neuropathy

Painful peripheral neuropathy is a debilitating complication of the treatment of HIV with nucleoside reverse transcriptase inhibitors (NRTIs). Patients are living longer with these drugs; however many develop excruciating, unremitting, and often treatment-limiting neuropathy that is resistant to conventional pain management therapies. Improving patient comfort and quality of life is paramount and depends on a clearer understanding of this devastating side effect. The mechanisms underlying the development of NRTI-induced neuropathy, however, remain unclear. Using a mouse model of NRTI-induced neuropathy, the authors conducted an unbiased whole-genome microarray screen to identify molecular targets in the spinal dorsal horn, which is the location where integration of ascending sensory transmission and descending modulatory effects occur. Analysis of the microarray data identified a change in the gene giant axonal neuropathy 1 (Gan1). Mutation of this gene has been linked to the development of giant axonal neuropathy (GAN), a rare autosomal recessive condition characterized by a progressive sensorimotor neuropathy. Gan1 has not been previously linked to nerve pathologies in other populations. In this study, downregulation of the Gan1 gene and the gene protein product, gigaxonin, was validated via quantitative polymerase chain reaction ([qPCR] gene expression) and Western blot analyses (protein level). Our report is the first to suggest that Gan1 might be a novel molecular target in the development of NRTI-induced peripheral neuropathy with implications for new therapeutic approaches to preventing or reducing a significant side effect of HIV treatment.

Dysfunctions of neuronal and glial intermediate filaments in disease

Intermediate filaments (IFs) are abundant structures found in most eukaryotic cells, including those in the nervous system. In the CNS, the primary components of neuronal IFs are alpha-internexin and the neurofilament triplet proteins. In the peripheral nervous system, a fifth neuronal IF protein known as peripherin is also present. IFs in astrocytes are primarily composed of glial fibrillary acidic protein (GFAP), although vimentin is also expressed in immature astrocytes and some mature astrocytes. In this Review, we focus on the IFs of glial cells (primarily GFAP) and neurons as well as their relationship to different neurodegenerative diseases.

Printor, a novel torsinA-interacting protein implicated in dystonia pathogenesis

Early onset generalized dystonia (DYT1) is an autosomal dominant neurological disorder caused by deletion of a single glutamate residue (torsinA DeltaE) in the C-terminal region of the AAA(+) (ATPases associated with a variety of cellular activities) protein torsinA. The pathogenic mechanism by which torsinA DeltaE mutation leads to dystonia remains unknown. Here we report the identification and characterization of a 628-amino acid novel protein, printor, that interacts with torsinA. Printor co-distributes with torsinA in multiple brain regions and co-localizes with torsinA in the endoplasmic reticulum. Interestingly, printor selectively binds to the ATP-free form but not to the ATP-bound form of torsinA, supporting a role for printor as a cofactor rather than a substrate of torsinA. The interaction of printor with torsinA is completely abolished by the dystonia-associated torsinA DeltaE mutation. Our findings suggest that printor is a new component of the DYT1 pathogenic pathway and provide a potential molecular target for therapeutic intervention in dystonia.

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.

Mutations in a BTB-Kelch protein, KLHL7, cause autosomal-dominant retinitis pigmentosa

Retinitis pigmentosa (RP) refers to a genetically heterogeneous group of progressive neurodegenerative diseases that result in dysfunction and/or death of rod and cone photoreceptors in the retina. So far, 18 genes have been identified for autosomal-dominant (ad) RP. Here, we describe an adRP locus (RP42) at chromosome 7p15 through linkage analysis in a six-generation Scandinavian family and identify a disease-causing mutation, c.449G-->A (p.S150N), in exon 6 of the KLHL7 gene. Mutation screening of KLHL7 in 502 retinopathy probands has revealed three different missense mutations in six independent families. KLHL7 is widely expressed, including expression in rod photoreceptors, and encodes a 75 kDa protein of the BTB-Kelch subfamily within the BTB superfamily. BTB-Kelch proteins have been implicated in ubiquitination through Cullin E3 ligases. Notably, all three putative disease-causing KLHL7 mutations are within a conserved BACK domain; homology modeling suggests that mutant amino acid side chains can potentially fill the cleft between two helices, thereby affecting the ubiquitination complexes. Mutations in an identical region of another BTB-Kelch protein, gigaxonin, have previously been associated with giant axonal neuropathy. Our studies suggest an additional role of the ubiquitin-proteasome protein-degradation pathway in maintaining neuronal health and in disease.

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.

NAC1, a POZ/BTB protein that functions as a corepressor

We now demonstrate that NAC1 acts as a corepressor for other POZ/BTB proteins. NAC1 is a POZ/BTB motif containing transcriptional repressor protein. In a mammalian two hybrid assay in neuronal (N2A) cells and non-neuronal (HEK 293T) cells, VP16 activation domain tagged NAC1 resulted in significant reversal of transcriptional inhibition with the Gal4-ZID, Gal4-BCL6, Gal4-ZF5, and kelch proteins Gal4-MAYVEN and Gal4-NRP/B fusion proteins. We also observed similar results with another corepressor, BCoR Gal4 fusion protein. NAC1 potentiated ZF5 mediated repression in Gal4-DBD fusion transient assays. GST pulldown assays further confirmed protein-protein interactions between these proteins and NAC1. Both the NAC1 isoforms demonstrated selective interaction through the POZ/BTB domain but not with the non-POZ/BTB region. Endogenous NAC1 and BCL6 physically associated in CNS regions. Strikingly, NAC1 did not interact with the pro-myelocytic leukemia zinc finger protein (PLZF), another POZ/BTB protein that is not found in the adult brain. Therefore, we conclude that NAC1 functions as a corepressor for POZ/BTB proteins expressed in the mature CNS.

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.

A deletion mutation in Slc12a6 is associated with neuromuscular disease in gaxp mice

Giant axonopathy (gaxp), an autosomal recessive mouse mutation, exhibits ataxia of the hind legs with a slight side-to-side wobble while walking. Within the genomic region of the gaxp locus, a total of 94 transcripts were identified; the annotation of these genes using OMIM and PubMed yielded three potential candidate genes. By cDNA microarray analysis, 54 genes located on or near the gaxp locus were found to exhibit differential expression between gaxp and littermate controls. Based on microarray data and the known function of genes identified, Slc12a6 was selected as the primary candidate gene and analyzed using the Reveal technology of SpectruMedix. A 17-base deletion was detected from within exon 4 of Slc12a6. Reverse transcriptase polymerase chain reaction validated the difference in Slc12a6 expression in different types of mice at the mRNA level, revealing a marked reduction in gaxp mice. Western blot analysis indicated that the protein product of Slc12a6, the K(+)-Cl(-) cotransporter Kcc3, was not detectable in gaxp mice. The causative role of the exon 4 mutation within Slc12a6 in the gaxp phenotype was further confirmed by screening multiple inbred strains and by excluding the mutation of nearby genes within the gaxp locus.

Ectromelia virus BTB/kelch proteins, EVM150 and EVM167, interact with cullin-3-based ubiquitin ligases

Cellular proteins containing BTB and kelch domains have been shown to function as adapters for the recruitment of substrates to cullin-3-based ubiquitin ligases. Poxviruses are the only family of viruses known to encode multiple BTB/kelch proteins, suggesting that poxviruses may modulate the ubiquitin pathway through interaction with cullin-3. Ectromelia virus encodes four BTB/kelch proteins and one BTB-only protein. Here we demonstrate that two of the ectromelia virus-encoded BTB/kelch proteins, EVM150 and EVM167, interacted with cullin-3. Similar to cellular BTB proteins, the BTB domain of EVM150 and EVM167 was necessary and sufficient for cullin-3 interaction. During infection, EVM150 and EVM167 localized to discrete cytoplasmic regions, which co-localized with cullin-3. Furthermore, EVM150 and EVM167 co-localized and interacted with conjugated ubiquitin, as demonstrated by confocal microscopy and co-immunoprecipitation. Our findings suggest that the ectromelia virus-encoded BTB/kelch proteins, EVM150 and EVM167, interact with cullin-3 potentially functioning to recruit unidentified substrates for ubiquitination.

A pachygyria-causing alpha-tubulin mutation results in inefficient cycling with CCT and a deficient interaction with TBCB

The agyria (lissencephaly)/pachygyria phenotypes are catastrophic developmental diseases characterized by abnormal folds on the surface of the brain and disorganized cortical layering. In addition to mutations in at least four genes--LIS1, DCX, ARX and RELN--mutations in a human alpha-tubulin gene, TUBA1A, have recently been identified that cause these diseases. Here, we show that one such mutation, R264C, leads to a diminished capacity of de novo tubulin heterodimer formation. We identify the mechanisms that contribute to this defect. First, there is a reduced efficiency whereby quasinative alpha-tubulin folding intermediates are generated via ATP-dependent interaction with the cytosolic chaperonin CCT. Second, there is a failure of CCT-generated folding intermediates to stably interact with TBCB, one of the five tubulin chaperones (TBCA-E) that participate in the pathway leading to the de novo assembly of the tubulin heterodimer. We describe the behavior of the R264C mutation in terms of its effect on the structural integrity of alpha-tubulin and its interaction with TBCB. In spite of its compromised folding efficiency, R264C molecules that do productively assemble into heterodimers are capable of copolymerizing into dynamic microtubules in vivo. The diminished production of TUBA1A tubulin in R264C individuals is consistent with haploinsufficiency as a cause of the disease phenotype.

Crystal structure of the Bach1 BTB domain and its regulation of homodimerization

The BTB/POZ domain is known as a protein-protein interaction motif that mediates homodimer and higher order self-associations. Proteins containing the BTB domain exist throughout eukaryotes; however, there is little information about the mechanism that determines the oligomeric state of the BTB domain. To address this question, we have determined the X-ray structure of the mouse Bach1 BTB domain. The present structure is similar to the previously determined BTB domain folds, including the human Bach1 BTB domain; however, distinct structural features are present, such as a novel homodimer interaction surface. The homodimer formation was found to involve a novel hydrogen bond network and interactions between hydrophobic surfaces of the kinked N-terminus (N-hook) and the partner's C-terminal residues. The deletion of the N-hook resulted in the conversion of the homodimer into a monomer in solution, indicating that the N-hook promotes the homodimerization of the mBach1 BTB domain. We have also found that the BTB domain of Bach2, a protein highly related to Bach1, is present as a monomer due to a short peptide insertion at the N-hook. These results represent the first example of the key modulatory element of BTB domain homodimerization.

New mutations, genotype phenotype studies and manifesting carriers in giant axonal neuropathy

Giant axonal neuropathy (GAN; MIM 256850) is a severe childhood onset autosomal recessive sensorimotor neuropathy affecting both the peripheral nerves and the central nervous system. Bomont and colleagues identified a novel ubiquitously expressed gene they named Gigaxonin on chromosome 16q24 as the cause of GAN in a number of families. We analysed five families with GAN for mutations in the Gigaxonin gene and mutations were found in four families; three families had homozygous mutations, one had two compound heterozygous mutations and one family had no mutation identified. All families had the typical clinical features, kinky hair and nerve biopsy. We report some unusual clinical features associated with GAN and Gigaxonin mutations as well as confirm the heterogeneity in GAN and the identification of two families with manifesting carriers.

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.

Conditional NF-L transgene expression in mice for in vivo analysis of turnover and transport rate of neurofilaments

We generated mice with doxycycline control of a human neurofilament light (NF-L) transgene in the context of the absence (tTA;hNF-L;NF-L(-/-)) or presence (tTA;hNF-L;NF-L(+/-)) of endogenous mouse NF-L proteins. Doxycycline treatment caused the rapid disappearance of human NF-L (hNF-L) mRNA in tTA;hNF-L mice, but the hNF-L proteins remained with a half-life of 3 weeks in the brain. In the sciatic nerve, the disappearance of hNF-L proteins after doxycycline treatment occurred in synchrony along the sciatic nerve, suggesting a proteolysis of NF proteins along the entire axon. The presence of permanent NF network in tTA;hNF-L;NF-L(+/-) mice further stabilized and extended longevity of hNF-L proteins by several months. Surprisingly, after cessation of doxycycline treatment, there was no evidence of leading front of newly synthesized hNF-L proteins migrating into sciatic nerve axons devoid of NF structures. The hNF-L proteins detected at weekly intervals reappeared and accumulated in synchrony at similar rate along nerve segments, a phenomenon consistent with a fast hNF-L transport into axons. We estimated the hNF-L transport rate to be of approximately 10 mm/d in axons devoid of NF structures based on the use of an adenovirus encoding tet-responsive transcriptional activator to transactivate the hNF-L transgene in hypoglossal motor neurons. These results provide in vivo evidence that the stationary NF network in axons is a key determinant of half-life and transport rate of NF proteins.

Using FlyAtlas to identify better Drosophila melanogaster models of human disease

FlyAtlas, a new online resource, provides the most comprehensive view yet of expression in multiple tissues of Drosophila melanogaster. Meta-analysis of the data shows that a significant fraction of the genome is expressed with great tissue specificity in the adult, demonstrating the need for the functional genomic community to embrace a wide range of functional phenotypes. Well-known developmental genes are often reused in surprising tissues in the adult, suggesting new functions. The homologs of many human genetic disease loci show selective expression in the Drosophila tissues analogous to the affected human tissues, providing a useful filter for potential candidate genes. Additionally, the contributions of each tissue to the whole-fly array signal can be calculated, demonstrating the limitations of whole-organism approaches to functional genomics and allowing modeling of a simple tissue fractionation procedure that should improve detection of weak or tissue-specific signals.

New movements in neurofilament transport, turnover and disease

Revealing the mechanisms by which neurofilament transport and turnover are regulated has proven difficult over the years but recent studies have given new insight into these processes. Mature neurofilament fibers may incorporate a fourth functional subunit, alpha-internexin, as new evidence suggests. Recent findings have made the role of phosphorylation in regulating neurofilament transport velocity controversial. Kinesin and dynein may transport neurofilaments in slow axonal transport as they have been found to associate with neurofilaments. Neurofilament transport and turnover rates may be reduced depending on the existing stationary neurofilament network. Finally, mutations in neurofilament light that have been linked to Charcot-Marie-Tooth disease as well as other neurofilament abnormalities in human disease are discussed.

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.

Process elongation of oligodendrocytes is promoted by the Kelch-related protein MRP2/KLHL1

Oligodendrocytes (OLGs) are generated by progenitor cells that are committed to differentiating into myelin-forming cells of the central nervous system. Rearrangement of the cytoskeleton leading to the extension of cellular processes is essential for the myelination of axons by OLGs. Here, we have characterized a new member of the Kelch-related protein family termed MRP2 (for Mayven-related protein 2) that is specifically expressed in brain. MRP2/KLHL1 is expressed in oligodendrocyte precursors and mature OLGs, and its expression is up-regulated during OLG differentiation. MRP2/KLHL1 expression was abundant during the specific stages of oligodendrocyte development, as identified by A2B5-, O4-, and O1-specific oligodendrocyte markers. MRP2/KLHL1 was localized in the cytoplasm and along the cell processes. Moreover, a direct endogenous association of MRP2/KLHL1 with actin was observed, which was significantly increased in differentiated OLGs compared with undifferentiated OLGs. Overexpression of MRP2/KLHL1 resulted in a significant increase in the process extension of rat OLGs, whereas MRP2/KLHL1 antisense reduced the process length of primary rat OLGs. Furthermore, murine OLGs isolated from MRP2/KLHL1 transgenic mice showed a significant increase in the process extension of OLGs compared with control wild-type murine OLGs. These studies provide insights into the role of MRP2/KLHL1, through its interaction with actin, in the process elongation of OLGs.