The role of gigaxonin in the degradation of the glial-specific intermediate filament protein GFAP

Versatility of vimentin assemblies: From filaments to biomolecular condensates and back

Cytoskeletal structures shape and confer resistance to cells. The intermediate filament protein vimentin forms versatile structures that play key roles in cytoskeletal crosstalk, in the integration of cellular responses to a variety of external and internal cues, and in the defense against stress. Such multifaceted roles can be fulfilled thanks to the vast variety of vimentin proteoforms, which in turn arise from the combinations of a myriad of tightly regulated posttranslational modifications. Diverse vimentin proteoforms will differentially shape its polymeric assemblies, underlying vimentin ability to organize in filaments, bundles, squiggles, droplets, cell surface-bound and/or various secreted forms. Interestingly, certain vimentin dots or droplets have been lately categorized as biomolecular condensates. Biomolecular condensates are phase-separated membraneless structures that are critical for the organization of cellular components and play important roles in pathophysiology. Recent findings have unveiled the importance of low complexity sequence domains in vimentin filament assembly. Moreover, several oxidants trigger the transition of vimentin filaments into phase-separated biomolecular condensates, a reversible process that may provide clues on the role of condensates as seeds for filament formation. Revisiting previous results in the light of recent knowledge prompts the hypothesis that vimentin condensates could play a role in traffic of filament precursors, cytoskeletal crosstalk and cellular responses to stress. Deciphering the "vimentin posttranslational modification code", that is, the structure-function relationships of vimentin proteoforms, constitutes a major challenge to understand the regulation of vimentin behavior and its multiple personalities. This will contribute to unveil essential cellular mechanisms and foster novel opportunities for drug discovery.

Deletions in Glial Fibrillary Acidic Protein Leading to Alterations in Intermediate Filament Assembly and Network Formation

Glial fibrillary acidic protein (GFAP) is classified as a type III intermediate filament protein predominantly expressed in mature astrocytes. It has the ability to self-assemble into 10 nm filaments in vitro, making it particularly valuable for elucidating the sequences essential for filament assembly. In this study, we created a series of deletion mutants targeting sequences in the N-terminal, C-terminal, and central rod domains to explore the sequences critical for the assembly of GFAP into 10 nm filaments. The impact of these deletions on filament formation was evaluated through in vitro assembly studies and transduction assays conducted with primary astrocytes. Our data revealed that deletions at the carboxy end resulted in abnormalities in either filament diameter calibration or lateral association, whereas deletions at the amino-terminal end significantly disrupted the filament assembly process, particularly restricting filament elongation. Furthermore, we discovered that the filament-forming sequences within the rod domain varied in their contributions to filament assembly and network formation. These findings enhance our understanding of the GFAP assembly process in vitro and provide a detailed mapping of the essential regions required for GFAP assembly. These insights hold significant implications for Alexander disease arising from deletion mutations in GFAP.

Gigaxonin, mutated in Giant Axonal Neuropathy, interacts with TDP-43 and other RNA binding proteins

Giant Axonal Neuropathy (GAN) is a neurodegenerative disease caused by loss-of-function mutations in the KLHL16 gene, encoding the cytoskeleton regulator gigaxonin. In the absence of functional gigaxonin, intermediate filament (IF) proteins accumulate in neurons and other cell types due to impaired turnover and transport. GAN neurons exhibit distended, swollen axons and distal axonal degeneration, but the mechanisms behind this selective neuronal vulnerability are unknown. Our objective was to identify novel gigaxonin interactors pertinent to GAN neurons. Unbiased proteomics revealed a statistically significant predominance of RNA-binding proteins (RBPs) within the soluble gigaxonin interactome and among differentially-expressed proteins in iPSC-neuron progenitors from a patient with classic GAN. Among the identified RBPs was TAR DNA-binding protein 43 (TDP-43), which associated with the gigaxonin protein and its mRNA transcript. TDP-43 co-localized within large axonal neurofilament IFs aggregates in iPSC-motor neurons derived from a GAN patient with the 'axonal CMT-plus' disease phenotype. Our results implicate RBP dysfunction as a potential underappreciated contributor to GAN-related neurodegeneration.

Glial fibrillary acidic protein is pathologically modified in Alexander disease

Here, we describe pathological events potentially involved in the disease pathogenesis of Alexander disease (AxD). This is a primary genetic disorder of astrocyte caused by dominant gain-of-function mutations in the gene coding for an intermediate filament protein glial fibrillary acidic protein (GFAP). Pathologically, this disease is characterized by the upregulation of GFAP and its accumulation as Rosenthal fibers. Although the genetic basis linking GFAP mutations with Alexander disease has been firmly established, the initiating events that promote GFAP accumulation and the role of Rosenthal fibers (RFs) in the disease process remain unknown. Here, we investigate the hypothesis that disease-associated mutations promote GFAP aggregation through aberrant posttranslational modifications. We found high molecular weight GFAP species in the RFs of AxD brains, indicating abnormal GFAP crosslinking as a prominent pathological feature of this disease. In vitro and cell-based studies demonstrate that cystine-generating mutations promote GFAP crosslinking by cysteine-dependent oxidation, resulting in defective GFAP assembly and decreased filament solubility. Moreover, we found GFAP was ubiquitinated in RFs of AxD patients and rodent models, supporting this modification as a critical factor linked to GFAP aggregation. Finally, we found that arginine could increase the solubility of aggregation-prone mutant GFAP by decreasing its ubiquitination and aggregation. Our study suggests a series of pathogenic events leading to AxD, involving interplay between GFAP aggregation and abnormal modifications by GFAP ubiquitination and oxidation. More important, our findings provide a basis for investigating new strategies to treat AxD by targeting abnormal GFAP modifications.

Targeting kelch-like (KLHL) proteins: achievements, challenges and perspectives

Kelch-like proteins (KLHLs) are a large family of BTB-containing proteins. KLHLs function as the substrate adaptor of Cullin 3-RING ligases (CRL3) to recognize substrates. KLHLs play pivotal roles in regulating various physiological and pathological processes by modulating the ubiquitination of their respective substrates. Mounting evidence indicates that mutations or abnormal expression of KLHLs are associated with various human diseases. Targeting KLHLs is a viable strategy for deciphering the KLHLs-related pathways and devising therapies for associated diseases. Here, we comprehensively review the known KLHLs inhibitors to date and the brilliant ideas underlying their development.

Stability dynamics of neurofilament and GFAP networks and protein fragments

Neurofilaments (NFs) and GFAP are cytoskeletal intermediate filaments (IFs) that support cellular processes unfolding within the uniquely complex environments of neurons and astrocytes, respectively. This review highlights emerging concepts on the transitions between stable and destabilized IF networks in the nervous system. While self-association between transiently structured low-complexity IF domains promotes filament assembly, the opposing destabilizing actions of phosphorylation-mediated filament severing facilitate faster intracellular transport. Cellular proteases, including caspases and calpains, produce a variety of IF fragments, which may interact with N-degron and C-degron pathways of the protein degradation machinery. The rapid adoption of NF and GFAP-based clinical biomarker tests is contrasted with the lagging understanding of the dynamics between the native IF proteins and their fragments.

Intermediate filament dysregulation in astrocytes in the human disease model of KLHL16 mutation in giant axonal neuropathy (GAN)

Giant Axonal Neuropathy (GAN) is a pediatric neurodegenerative disease caused by KLHL16 mutations. KLHL16 encodes gigaxonin, which regulates intermediate filament (IF) turnover. Previous neuropathological studies and examination of postmortem brain tissue in the current study revealed involvement of astrocytes in GAN. To develop a clinically-relevant model, we reprogrammed skin fibroblasts from seven GAN patients to pluripotent stem cells (iPSCs), which were used to generate neural progenitor cells (NPCs), astrocytes, and brain organoids. Multiple isogenic control clones were derived via CRISPR/Cas9 gene editing of one patient line carrying the G332R gigaxonin mutation. All GAN iPSCs were deficient for gigaxonin and displayed patient-specific increased vimentin expression. GAN NPCs had lower nestin expression and fewer nestin-positive cells compared to isogenic controls, but nestin morphology was unaffected. GAN brain organoids were marked by the presence of neurofilament and GFAP aggregates. GAN iPSC-astrocytes displayed striking dense perinuclear vimentin and GFAP accumulations and abnormal nuclear morphology. In over-expression systems, GFAP oligomerization and perinuclear aggregation were augmented in the presence of vimentin. GAN patient cells with large perinuclear vimentin aggregates accumulated significantly more nuclear KLHL16 mRNA compared to cells without vimentin aggregates. As an early effector of KLHL16 mutations, vimentin may be a potential target in GAN.

Gigaxonin is required for intermediate filament transport

Gigaxonin is an adaptor protein for E3 ubiquitin ligase substrates. It is necessary for ubiquitination and degradation of intermediate filament (IF) proteins. Giant axonal neuropathy is a pathological condition caused by mutations in the GAN gene that encodes gigaxonin. This condition is characterized by abnormal accumulation of IFs in both neuronal and non-neuronal cells; however, it is unclear what causes IF aggregation. In this work, we studied the dynamics of IFs using their subunits tagged with a photoconvertible protein mEOS 3.2. We have demonstrated that the loss of gigaxonin dramatically inhibited transport of IFs along microtubules by the microtubule motor kinesin-1. This inhibition was specific for IFs, as other kinesin-1 cargoes, with the exception of mitochondria, were transported normally. Abnormal distribution of IFs in the cytoplasm can be rescued by direct binding of kinesin-1 to IFs, demonstrating that transport inhibition is the primary cause for the abnormal IF distribution. Another effect of gigaxonin loss was a more than 20-fold increase in the amount of soluble vimentin oligomers in the cytosol of gigaxonin knock-out cells. We speculate that these oligomers saturate a yet unidentified adapter that is required for kinesin-1 binding to IFs, which might inhibit IF transport along microtubules causing their abnormal accumulation.

Intermediate filament dysregulation and astrocytopathy in the human disease model of KLHL16 mutation in giant axonal neuropathy (GAN)

Giant Axonal Neuropathy (GAN) is a pediatric neurodegenerative disease caused by KLHL16 mutations. KLHL16 encodes gigaxonin, a regulator of intermediate filament (IF) protein turnover. Previous neuropathological studies and our own examination of postmortem GAN brain tissue in the current study revealed astrocyte involvement in GAN. To study the underlying mechanisms, we reprogrammed skin fibroblasts from seven GAN patients carrying different KLHL16 mutations to iPSCs. Isogenic controls with restored IF phenotypes were derived via CRISPR/Cas9 editing of one patient carrying a homozygous missense mutation (G332R). Neural progenitor cells (NPCs), astrocytes, and brain organoids were generated through directed differentiation. All GAN iPSC lines were deficient for gigaxonin, which was restored in the isogenic control. GAN iPSCs displayed patient-specific increased vimentin expression, while GAN NPCs had decreased nestin expression compared to isogenic control. The most striking phenotypes were observed in GAN iPSC-astrocytes and brain organoids, which exhibited dense perinuclear IF accumulations and abnormal nuclear morphology. GAN patient cells with large perinuclear vimentin aggregates accumulated nuclear KLHL16 mRNA. In over-expression studies, GFAP oligomerization and perinuclear aggregation were potentiated in the presence of vimentin. As an early effector of KLHL16 mutations, vimentin may serve as a potential therapeutic target in GAN.

Case report: Alexander's disease with "head drop" as the main symptom and literature review

Alexander's disease (AxD) is a rare autosomal dominant hereditary disorder that is caused by the mutations in the GFAP gene, which encodes the glial fibrillary acidic protein (GFAP). This neurogenerative disease has many clinical manifestations, and the onset of disease spans a wide range of ages, from newborns to children, adults, and even the elderly. An overaccumulation of the expression of GFAP has a close causal relationship with the pathogenesis of Alexander's disease. Usually, the disease has severe morbidity and high mortality, and can be divided into three distinct subgroups that are based on the age of clinical presentation: infantile (0-2 years), juvenile (2-13 years), and adult (>13 years). Children often present with epilepsy, macrocephaly, and psychomotor retardation, while adolescents and adults mainly present with muscle weakness, spasticity, and bulbar symptoms. Atonic seizures are a type of epilepsy that often appears in the Lennox-Gastaut syndrome and myoclonic-astatic epilepsy in early childhood; however, the prognosis is often poor. Atonic episodes are characterized by a sudden or frequent reduction in muscle tone that can be local (such as head, neck, or limb) or generalized. Here, we report a 4-year-old girl whose main symptoms were intermittent head drop movements, which could break the frontal frame and even bleed in severe conditions. A video-encephalography (VEEG) showed that the nodding movements were atonic seizures. A head magnetic resonance imaging (MRI) revealed abnormal signals in the bilateral paraventricular and bilateral subfrontal cortex. The gene detection analyses indicated that the GFAP gene exon 1 c.262 C>T was caused by a heterozygous mutation, as both her parents were of the wild-type. The girl had no other abnormal manifestations except atonic seizures. She could communicate normally and go to kindergarten. After an oral administration of sodium valproate, there were no atonic attacks. Although epilepsy is a common symptom of Alexander's disease, atonic seizures have not been reported to date. Therefore, we report a case of Alexander's disease with atonic seizures as the main symptom and provide a review of the literature.

The roles of KLHL family members in human cancers

The Kelch-like (KLHL) family members consist of three domains: bric-a-brac, tramtrack, broad complex/poxvirus and zinc finger domain, BACK domain and Kelch domain, which combine and interact with Cullin3 to form an E3 ubiquitin ligase. Research has indicated that KLHL family members ubiquitinate target substrates to regulate physiological and pathological processes, including tumorigenesis and progression. KLHL19, a member of the KLHL family, is associated with tumorigenesis and drug resistance. However, the regulation and cross talks of other KLHL family members, which also play roles in cancer, are still unclear. Our review mainly explores studies concerning the roles of other KLHL family members in tumor-related regulation to provide novel insights into KLHL family members.

Symptomatic care of late-onset Alexander disease presenting with area postrema-like syndrome with prednisolone; a case report

Background

Alexander disease (AxD) is classified into AxD type I (infantile) and AxD type II (juvenile and adult form). We aimed to determine the potential genetic cause(s) contributing to the AxD type II manifestations in a 9-year-old male who presented area postrema-like syndrome and his vomiting and weight loss improved after taking prednisolone.

Case presentation

A normal cognitive 9-year-old boy with persistent nausea, vomiting, and a significant weight loss at the age of 6 years was noticed. He also experienced an episode of status epilepticus with generalized atonic seizures. He showed non-febrile infrequent multifocal motor seizures at the age of 40 days which were treated with phenobarbital. He exhibited normal physical growth and neurologic developmental milestones by the age of six. Occasionally vomiting unrelated to feeding was reported. Upon examination at 9 years, a weak gag reflex, prominent drooling, exaggerated knee-deep tendon reflexes (3+), and nasal tone speech was detected. All gastroenterological, biochemical, and metabolic assessments were normal. Brain magnetic resonance imaging (MRI) revealed bifrontal confluent deep and periventricular white matter signal changes, fine symmetric frontal white matter and bilateral caudate nucleus involvements with garland changes, and a hyperintense tumefactive-like lesion in the brain stem around the floor of the fourth ventricle and area postrema with contrast uptake in post-contrast T1-W images. Latter MRI at the age of 8 years showed enlarged area postrema lesion and bilateral middle cerebellar peduncles and dentate nuclei involvements. Due to clinical and genetic heterogeneities, whole-exome sequencing was performed and the candidate variant was confirmed by Sanger sequencing. A de novo heterozygous mutation, NM_001242376.1:c.262 C > T;R88C in exon 1 of the GFAP (OMIM: 137,780) was verified. Because of persistent vomiting and weight loss of 6.0 kg, prednisolone was prescribed which brought about ceasing vomiting and led to weight gaining of 3.0 kg over the next 3 months after treatment. Occasional attempts to discontinue prednisolone had been resulting in the reappearance of vomiting.

Conclusions

This study broadens the spectrum of symptomatic treatment in leukodystrophies and also shows that R88C mutation may lead to a broad range of phenotypes in AxD type II patients.

Effects of Alexander disease-associated mutations on the assembly and organization of GFAP intermediate filaments

Alexander disease is a primary genetic disorder of astrocytes caused by dominant mutations in the gene encoding glial fibrillary acidic protein (GFAP). How single-amino-acid changes can lead to cytoskeletal catastrophe and brain degeneration remains poorly understood. In this study, we have analyzed 14 missense mutations located in the GFAP rod domain to investigate how these mutations affect in vitro filament assembly. Whereas the internal rod mutants assembled into filaments that were shorter than those of wild type, the rod end mutants formed structures with one or more of several atypical characteristics, including short filament length, irregular width, roughness of filament surface, and filament aggregation. When transduced into primary astrocytes, GFAP mutants with in vitro assembly defects usually formed cytoplasmic aggregates, which were more resistant to biochemical extraction. The resistance of GFAP to solubilization was also observed in brain tissues of patients with Alexander disease, in which a significant proportion of insoluble GFAP were accumulated in Rosenthal fiber fractions. These findings provide clinically relevant evidence that link GFAP assembly defects to disease pathology at the tissue level and suggest that altered filament assembly and properties as a result of GFAP mutation are critical initiating factors for the pathogenesis of Alexander disease.

Alexander disease: models, mechanisms, and medicine

Alexander disease is a primary disorder of astrocytes caused by gain-of-function mutations in the gene for glial fibrillary acidic protein (GFAP), which lead to protein aggregation and a reactive astrocyte response, with devastating effects on the central nervous system. Over the past two decades since the discovery of GFAP as the culprit, several cellular and animal models have been generated, and much has been learned about underlying mechanisms contributing to the disease. Despite these efforts, many aspects of Alexander disease have remained enigmatic, particularly the initiating events in GFAP accumulation and astrocyte pathology, the relation between astrocyte dysfunction and myelin deficits, and the variability in age of onset and disease severity. More recent work in both old and new models has begun to address these complex questions and identify new therapeutics that finally offer the promise of effective treatment.

Metabolic Enzyme Alterations and Astrocyte Dysfunction in a Murine Model of Alexander Disease With Severe Reactive Gliosis

Alexander disease (AxD) is a rare and fatal neurodegenerative disorder caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). In this report, a mouse model of AxD (GFAPTg;Gfap+/R236H) was analyzed that contains a heterozygous R236H point mutation in murine Gfap as well as a transgene with a GFAP promoter to overexpress human GFAP. Using label-free quantitative proteomic comparisons of brain tissue from GFAPTg;Gfap+/R236H versus wild-type mice confirmed upregulation of the glutathione metabolism pathway and indicated proteins were elevated in the peroxisome proliferator-activated receptor (PPAR) signaling pathway, which had not been reported previously in AxD. Relative protein-level differences were confirmed by a targeted proteomics assay, including proteins related to astrocytes and oligodendrocytes. Of particular interest was the decreased level of the oligodendrocyte protein, 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (Ugt8), since Ugt8-deficient mice exhibit a phenotype similar to GFAPTg;Gfap+/R236H mice (e.g., tremors, ataxia, hind-limb paralysis). In addition, decreased levels of myelin-associated proteins were found in the GFAPTg;Gfap+/R236H mice, consistent with the role of Ugt8 in myelin synthesis. Fabp7 upregulation in GFAPTg;Gfap+/R236H mice was also selected for further investigation due to its uncharacterized association to AxD, critical function in astrocyte proliferation, and functional ability to inhibit the anti-inflammatory PPAR signaling pathway in models of amyotrophic lateral sclerosis (ALS). Within Gfap+ astrocytes, Fabp7 was markedly increased in the hippocampus, a brain region subjected to extensive pathology and chronic reactive gliosis in GFAPTg;Gfap+/R236H mice. Last, to determine whether the findings in GFAPTg;Gfap+/R236H mice are present in the human condition, AxD patient and control samples were analyzed by Western blot, which indicated that Type I AxD patients have a significant fourfold upregulation of FABP7. However, immunohistochemistry analysis showed that UGT8 accumulates in AxD patient subpial brain regions where abundant amounts of Rosenthal fibers are located, which was not observed in the GFAPTg;Gfap+/R236H mice.

Human iPSC-Derived Astrocytes: A Powerful Tool to Study Primary Astrocyte Dysfunction in the Pathogenesis of Rare Leukodystrophies

Astrocytes are very versatile cells, endowed with multitasking capacities to ensure brain homeostasis maintenance from brain development to adult life. It has become increasingly evident that astrocytes play a central role in many central nervous system pathologies, not only as regulators of defensive responses against brain insults but also as primary culprits of the disease onset and progression. This is particularly evident in some rare leukodystrophies (LDs) where white matter/myelin deterioration is due to primary astrocyte dysfunctions. Understanding the molecular defects causing these LDs may help clarify astrocyte contribution to myelin formation/maintenance and favor the identification of possible therapeutic targets for LDs and other CNS demyelinating diseases. To date, the pathogenic mechanisms of these LDs are poorly known due to the rarity of the pathological tissue and the failure of the animal models to fully recapitulate the human diseases. Thus, the development of human induced pluripotent stem cells (hiPSC) from patient fibroblasts and their differentiation into astrocytes is a promising approach to overcome these issues. In this review, we discuss the primary role of astrocytes in LD pathogenesis, the experimental models currently available and the advantages, future evolutions, perspectives, and limitations of hiPSC to study pathologies implying astrocyte dysfunctions.

Proteasome-mediated degradation of keratins 7, 8, 17 and 18 by mutant KLHL24 in a foetal keratinocyte model: Novel insight in congenital skin defects and fragility of epidermolysis bullosa simplex with cardiomyopathy

Epidermolysis bullosa simplex (EBS) with cardiomyopathy (EBS-KLHL24) is an EBS subtype caused by dominantly inherited, gain-of-function mutations in the gene encoding for the ubiquitin-ligase KLHL24, which addresses specific proteins to proteasomal degradation. EBS-KLHL24 patients are born with extensive denuded skin areas and skin fragility. Whilst skin fragility rapidly ameliorates, atrophy and scarring develop over time, accompanied by life-threatening cardiomyopathy. To date, pathogenetic mechanisms underlying such a unique disease phenotype are not fully characterized. The basal keratin 14 (K14) has been indicated as a KLHL24 substrate in keratinocytes. However, EBS-KLHL24 pathobiology cannot be determined by the mutation-enhanced disruption of K14 alone, as K14 is similarly expressed in foetal and postnatal epidermis and its protein levels are preserved both in vivo and in vitro disease models. In this study, we focused on foetal keratins as additional KLHL24 substrates. We showed that K7, K8, K17 and K18 protein levels are markedly reduced via proteasome degradation in normal foetal keratinocytes transduced with the mutant KLHL24 protein (ΔN28-KLHL24) as compared to control cells expressing the wild-type form. In addition, heat stress led to keratin network defects and decreased resilience in ΔN28-KLHL24 cells. The KLHL24-mediated degradation of foetal keratins could contribute to congenital skin defects in EBS-KLHL24. Furthermore, we observed that primary keratinocytes from EBS-KLHL24 patients undergo accelerated clonal conversion with reduced colony forming efficiency (CFE) and early replicative senescence. Finally, our findings pointed out a reduced CFE in ΔN28-KLHL24-transduced foetal keratinocytes as compared to controls, suggesting that mutant KLHL24 contributes to patients’ keratinocyte clonogenicity impairment.

GFAP at 50

Fifty years have passed since the discovery of glial fibrillary acidic protein (GFAP) by Lawrence Eng and colleagues. Now recognized as a member of the intermediate filament family of proteins, it has become a subject for study in fields as diverse as structural biology, cell biology, gene expression, basic neuroscience, clinical genetics and gene therapy. This review covers each of these areas, presenting an overview of current understanding and controversies regarding GFAP with the goal of stimulating continued study of this fascinating protein.

Elevated GFAP isoform expression promotes protein aggregation and compromises astrocyte function

Alexander disease (AxD) caused by mutations in the coding region of GFAP is a neurodegenerative disease characterized by astrocyte dysfunction, GFAP aggregation, and Rosenthal fiber accumulation. Although how GFAP mutations cause disease is not fully understood, Rosenthal fibers could be induced by forced overexpression of human GFAP and this could be lethal in mice implicate that an increase in GFAP levels is central to AxD pathogenesis. Our recent studies demonstrated that intronic GFAP mutations cause disease by altering GFAP splicing, suggesting that an increase in GFAP isoform expression could lead to protein aggregation and astrocyte dysfunction that typify AxD. Here we test this hypothesis by establishing primary astrocyte cultures from transgenic mice overexpressing human GFAP. We found that GFAP-δ and GFAP-κ were disproportionately increased in transgenic astrocytes and both were enriched in Rosenthal fibers of human AxD brains. In vitro assembly studies showed that while the major isoform GFAP-α self-assembled into typical 10-nm filaments, minor isoforms including GFAP-δ, -κ, and -λ were assembly-compromised and aggregation prone. Lentiviral transduction showed that expression of these minor GFAP isoforms decreased filament solubility and increased GFAP stability, leading to the formation of Rosenthal fibers-like aggregates that also disrupted the endogenous intermediate filament networks. The aggregate-bearing astrocytes lost their normal morphology and glutamate buffering capacity, which had a toxic effect on neighboring neurons. In conclusion, our findings provide evidence that links elevated GFAP isoform expression with GFAP aggregation and impaired glutamate transport, and suggest a potential non-cell-autonomous mechanism underlying neurodegeneration through astrocyte dysfunction.

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.

Towards genomic database of Alexander disease to identify variations modifying disease phenotype

Alexander disease (AxD) is an extremely rare neurodegenerative disorder caused by glial fibrillary acidic protein (GFAP) gene mutations. Compared with the cerebral type, which is characterized by infantile onset, the bulbospinal type and intermediate form are associated with a late onset, spanning from juveniles to the elderly, and more diverse clinical spectrum, suggesting the existence of factors contributing to phenotypic diversity. To build a foundation for future genetic studies of this rare disease, we obtained genomic data by whole exome-sequencing (WES) and DNA microarray derived from thirty-one AxD patients with the bulbospinal type and intermediate form. Using this data, we aimed to identify genetic variations determining the age at onset (AAO) of AxD. As a result, WES- or microarray-based association studies between younger (<45 years; n = 13)- and older (≥45 years; n = 18)-onset patients considering the predicted GFAP-mutation pathogenicity identified no genome-wide significant variant. The candidate gene approach identified several variants likely correlated with AAO (p < 0.05): GAN, SLC1A2, CASP3, HDACs, and PI3K. Although we need to replicate the results using an independent population, this is the first step towards constructing a database, which may serve as an important tool to advance our understanding of AxD.

The role of neurofilament aggregation in neurodegeneration: lessons from rare inherited neurological disorders

Many neurodegenerative disorders, including Parkinson's, Alzheimer's, and amyotrophic lateral sclerosis, are well known to involve the accumulation of disease-specific proteins. Less well known are the accumulations of another set of proteins, neuronal intermediate filaments (NFs), which have been observed in these diseases for decades. NFs belong to the family of cytoskeletal intermediate filament proteins (IFs) that give cells their shape; they determine axonal caliber, which controls signal conduction; and they regulate the transport of synaptic vesicles and modulate synaptic plasticity by binding to neurotransmitter receptors. In the last two decades, a number of rare disorders caused by mutations in genes that encode NFs or regulate their metabolism have been discovered. These less prevalent disorders are providing novel insights into the role of NF aggregation in the more common neurological disorders.

Advancing the pathologic phenotype of giant axonal neuropathy: early involvement of the ocular lens

Giant axonal neuropathy (GAN; ORPHA: 643; OMIM# 256850) is a rare, hereditary, pediatric neurodegenerative disorder associated with intracellular accumulations of intermediate filaments (IFs). GAN knockout (KO) mouse models mirror the IF dysregulation and widespread nervous system pathology seen in human GAN. Validation of therapeutic efficacy and viral vector delivery systems with these GAN KO models has provided the springboard for the development of a viral vector being delivered intrathecally in an ongoing Phase I gene therapy clinical trial for the treatment of children with GAN (https://clinicaltrials.gov/ct2/show/NCT02362438). During the course of a comprehensive pathologic characterization of the GAN KO mouse, we discovered the very early and unexpected involvement of the ocular lens. Light microscopy revealed the presence of intracytoplasmic inclusion bodies within lens epithelial cells. The inclusion bodies showed strong immunohistochemical positivity for glial fibrillary acidic protein (GFAP). We confirmed that intracytoplasmic inclusion bodies are also present within lens epithelial cells in human GAN. These IF inclusion bodies in lens epithelial cells are unique to GAN. Similar IF inclusion bodies in lens epithelial cells have not been reported previously in experimental animal models or human diseases. Since current paradigms in drug discovery and drug repurposing for IF-associated disorders are often hindered by lack of validated targets, our findings suggest that lens epithelial cells in the GAN KO mouse may provide a potential target, in vivo and in vitro, for evaluating drug efficacy and alternative therapeutic approaches in promoting the clearance of IF inclusions in GAN and other diseases characterized by intracellular IF accumulations.

Characterization of a panel of monoclonal antibodies recognizing specific epitopes on GFAP

Alexander disease (AxD) is a neurodegenerative disease caused by heterozygous mutations in the GFAP gene, which encodes the major intermediate filament protein of astrocytes. This disease is characterized by the accumulation of cytoplasmic protein aggregates, known as Rosenthal fibers. Antibodies specific to GFAP could provide invaluable tools to facilitate studies of the normal biology of GFAP and to elucidate the pathologic role of this IF protein in disease. While a large number of antibodies to GFAP are available, few if any of them have defined epitopes. Here we described the characterization of a panel of commonly used anti-GFAP antibodies, which recognized epitopes at regions extending across the rod domain of GFAP. We show that all of the antibodies are useful for immunoblotting and immunostaining, and identify a subset that preferentially recognized human GFAP. Using these antibodies, we demonstrate the presence of biochemically modified forms of GFAP in brains of human AxD patients and mouse AxD models. These data suggest that this panel of anti-GFAP antibodies will be useful for studies of animal and cell-based models of AxD and related diseases in which cytoskeletal defects associated with GFAP modifications occur.