Dystrophic neurites express C9orf72 in Alzheimer's disease brains

Young Onset Alzheimer's Disease Associated with C9ORF72 Hexanucleotide Expansion: Further Evidence for a Still Unsolved Association

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are recognized as part of a disease continuum (FTD-ALS spectrum), in which the most common genetic cause is chromosome 9 open reading frame 72 (C9ORF72) gene hexanucleotide repeat expansion. The clinical phenotype of patients carrying this expansion varies widely and includes diseases beyond the FTD-ALS spectrum. Although a few cases of patients with C9ORF72 expansion and a clinical or biomarker-supported diagnosis of Alzheimer's disease (AD) have been described, they have been considered too sparse to establish a definite association between the C9ORF72 expansion and AD pathology. Here, we describe a C9ORF72 family with pleomorphic phenotypical expressions: a 54-year-old woman showing cognitive impairment and behavioral disturbances with both neuroimaging and cerebrospinal fluid (CSF) biomarkers consistent with AD pathology, her 49-year-old brother with typical FTD-ALS, and their 63-year-old mother with the behavioral variant of FTD and CSF biomarkers suggestive of AD pathology. The young onset of disease in all three family members and their different phenotypes and biomarker profiles make the simple co-occurrence of different diseases an extremely unlikely explanation. Our report adds to previous findings and may contribute to further expanding the spectrum of diseases associated with C9ORF72 expansion.

rs56405341 Variant Associates with Expression of C4orf33 and C4orf33 Was Downregulated in Alzheimer's Disease and Progressive Supranuclear Palsy

The first primary age-related tauopathy (PART) genome-wide association study confirmed significant associations of Alzheimer's disease (AD) and progressive supranuclear palsy (PSP) genetic variants with PART, and highlighted a novel genetic variant rs56405341. Here, we perform a comprehensive analysis of rs56405341. We found that rs56405341 was significantly associated with C4orf33 mRNA expression, but not JADE1 mRNA expression in multiple brain tissues. C4orf33 was mainly expressed in cerebellar hemisphere and cerebellum, and JADE1 was mainly expressed in thyroid, and coronary artery. Meanwhile, we found significantly downregulated C4orf33 expression both AD and PSP compared with normal controls, respectively.

Therapeutic Targeting of Rab GTPases: Relevance for Alzheimer's Disease

Rab GTPases (Rabs) are small proteins that play crucial roles in vesicle transport and membrane trafficking. Owing to their widespread functions in several steps of vesicle trafficking, Rabs have been implicated in the pathogenesis of several disorders, including cancer, diabetes, and multiple neurodegenerative diseases. As treatments for neurodegenerative conditions are currently rather limited, the identification and validation of novel therapeutic targets, such as Rabs, is of great importance. This review summarises proof-of-concept studies, demonstrating that modulation of Rab GTPases in the context of Alzheimer's disease (AD) can ameliorate disease-related phenotypes, and provides an overview of the current state of the art for the pharmacological targeting of Rabs. Finally, we also discuss the barriers and challenges of therapeutically targeting these small proteins in humans, especially in the context of AD.

Correlation between C9ORF72 mutation and neurodegenerative diseases: A comprehensive review of the literature

Chromosome 9 open reading frame 72 (C9ORF72) encodes a 54-kDa protein with unknown function that is expressed at high levels in the central nervous system. The C9ORF72 hexanucleotide amplification is one of the most recently discovered repetitive amplification diseases related to neurodegeneration. Its association with amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) spectrum diseases has been fully established, although a causative role for C9ORF72 in Alzheimer's disease (AD) and Parkinson's disease (PD) remains to be established. Therefore, in this article, we will review the evidence for C9ORF72 as a causative factor in neurodegenerative diseases, the underlying mechanisms, and the potential for targeting C9ORF72 as a strategy to alleviate neurodegenerative disease progression.

Novel antibodies reveal presynaptic localization of C9orf72 protein and reduced protein levels in C9orf72 mutation carriers

Hexanucleotide repeat expansion in C9orf72 is the most common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis, but the pathogenic mechanism of this mutation remains unresolved. Haploinsufficiency has been proposed as one potential mechanism. However, insights if and how reduced C9orf72 proteins levels might contribute to disease pathogenesis are still limited because C9orf72 expression, localization and functions in the central nervous system (CNS) are uncertain, in part due to the poor specificity of currently available C9orf72 antibodies.

Here, we generated and characterized novel knock-out validated monoclonal rat and mouse antibodies against C9orf72. We found that C9orf72 is a low abundant, cytoplasmic, highly soluble protein with the long 481 amino acid isoform being the predominant, if not exclusively, expressed protein isoform in mouse tissues and human brain. As consequence of the C9orf72 repeat expansion, C9orf72 protein levels in the cerebellum were reduced to 80% in our series of C9orf72 mutation carriers (n = 17) compared to controls (n = 26). However, no associations between cerebellar protein levels and clinical phenotypes were seen. Finally, by utilizing complementary immunohistochemical and biochemical approaches including analysis of human iPSC derived motor neurons, we identified C9orf72, in addition to its association to lysosomes, to be localized to the presynapses and able to interact with all members of the RAB3 protein family, suggestive of a role for C9orf72 in regulating synaptic vesicle functions by potentially acting as guanine nucleotide exchange factor for RAB3 proteins.

In conclusion, our findings provide further evidence for haploinsufficiency as potential mechanism in C9orf72 pathogenesis by demonstrating reduced protein levels in C9orf72 mutation carriers and important novel insights into the physiological role of C9orf72 in the CNS. Moreover, the described novel monoclonal C9orf72 antibodies will be useful tools to further dissect the cellular and molecular functions of C9orf72.

Immunohistochemical detection of C9orf72 protein in frontotemporal lobar degeneration and motor neurone disease: patterns of immunostaining and an evaluation of commercial antibodies

We have employed as ‘gold standards’ two in-house, well-characterised and validated polyclonal antibodies, C9-L and C9-S, which detect the longer and shorter forms of C9orf72, and have compared seven other commercially available antibodies with these in order to evaluate the utility of the latter as credible tools for the demonstration of C9orf72. C9-L and C9-S antibodies immunostained cytoplasmic ‘speckles’, and the nuclear membrane, respectively, in cerebellar Purkinje cells of the cerebellum in patients with behavioural variant frontotemporal dementia (bvFTD) with amyotrophic lateral sclerosis (ALS), and in patients with ALS alone. Similar staining was seen in Purkinje cells in healthy control tissues and in other neurodegenerative disorders, and in pyramidal cells of CA4 and dentate gyrus of hippocampus. However, in the spinal cord there was little cytoplasmic staining with C9-L antibody. C9-S antibody immunostained the nuclear membrane of anterior horn cells in healthy neurons. In patients with bvFTD + ALS, or ALS alone, this C9-S nuclear staining was redistributed to the plasma membrane. In those patients with bvFTD + ALS or ALS bearing an expansion in C9orf72, none of the commercially available antibodies detected TDP-43 inclusions in anterior horn cells, nor were dipeptide repeat proteins demonstrated. Five of the commercial antibodies provided immunohistochemical staining patterns similar in morphological appearance to the in-house C9-L antibody, but distinct from C9-S antibody. However, only three showed sufficient specificity and intensity of staining for C9orf72 at acceptably low concentrations, to make them of practical value and sufficiently reliable for the detection of at least the longer form of C9orf72 protein.

Defining the earliest pathological changes of Alzheimer's disease

The prospects for effectively treating well-established dementia, such as Alzheimer's disease (AD), are slim, due to the destruction of key brain pathways that underlie higher cognitive function. There has been a substantial shift in the field towards detecting conditions such as AD in their earliest stages, which would allow preventative or therapeutic approaches to substantially reduce risk and/or slow the progression of disease. AD is characterized by hallmark pathological changes such as extracellular Aβ plaques and intracellular neurofibrillary pathology, which selectively affect specific subclasses of neurons and brain circuits. Current evidence indicates that Aβ plaques begin to form many years before overt dementia, a gradual and progressive pathology which offers a potential target for early intervention. Early Aβ changes in the brain result in localized damage to dendrites, axonal processes and synapses, to which excitatory synapses and the processes of projection neurons are highly vulnerable. Aβ pathology is replicated in a range of transgenic models overexpressing mutant human familial AD genes (e.g. APP and presenilin 1). Studying the development of aberrant regenerative and degenerative changes in neuritic processes associated with Aβ plaques may represent the best opportunity to understand the relationship between the pathological hallmarks of AD and neuronal damage, and to develop early interventions to prevent, slow down or mitigate against Aβ pathology and/or the neuronal alterations that leads to cognitive impairment.

C9orf72 binds SMCR8, localizes to lysosomes, and regulates mTORC1 signaling

C9orf72 interacts strongly with SMCR8 and depends on this interaction for its stability. Lysosomes are major sites of C9orf72 subcellular localization, and abnormal lysosome morphology is seen in its absence. Defects are found in the regulation of the lysosome-localized mTORC1 signaling pathway in C9orf72 KO cells.

Hexanucleotide expansion in an intron of the C9orf72 gene causes amyotrophic lateral sclerosis and frontotemporal dementia. However, beyond bioinformatics predictions that suggested structural similarity to folliculin, the Birt-Hogg-Dubé syndrome tumor suppressor, little is known about the normal functions of the C9orf72 protein. To address this problem, we used genome-editing strategies to investigate C9orf72 interactions, subcellular localization, and knockout (KO) phenotypes. We found that C9orf72 robustly interacts with SMCR8 (a protein of previously unknown function). We also observed that C9orf72 localizes to lysosomes and that such localization is negatively regulated by amino acid availability. Analysis of C9orf72 KO, SMCR8 KO, and double-KO cell lines revealed phenotypes that are consistent with a function for C9orf72 at lysosomes. These include abnormally swollen lysosomes in the absence of C9orf72 and impaired responses of mTORC1 signaling to changes in amino acid availability (a lysosome-dependent process) after depletion of either C9orf72 or SMCR8. Collectively these results identify strong physical and functional interactions between C9orf72 and SMCR8 and support a lysosomal site of action for this protein complex.

C9ORF72 expression and cellular localization over mouse development

INTRODUCTION

A majority of familial frontotemporal lobar dementia and amyotrophic lateral sclerosis cases are associated with a large repeat expansion in a non-coding region of the C9ORF72 gene. Currently, little is known about the normal function and the expression pattern of the C9ORF72 protein. The aims of this study were to characterize the expression pattern and cellular localization of the three reported mouse isoforms of C9orf72, over a developmental time-course in primary cultured cortical neurons and brain tissue from C57BL/6 mice.

RESULTS

We demonstrated that the different isoforms of C9ORF72 at the mRNA and protein level undergo alterations in expression during development and into adulthood. Cellular fractionation and immunofluorescence demonstrated that levels of nuclear and cytoplasmic expression of isoforms changed significantly over the time course. Additionally, immunofluorescence studies showed C9ORF72 labeling as puncta throughout neurons, extending beyond the microtubule cytoskeleton into actin-rich structures such as filopodia and growth cones. Finally, synaptosome preparations demonstrated the presence of C9ORF72 isoform 1 in synaptic-rich fractions from adult mouse brain.

CONCLUSION

In summary, the presence of C9ORF72 as puncta and within synaptic-rich fractions may indicate involvement at the synapse and differential expression of isoforms in nuclei and cytoplasm may suggest distinct roles for the isoforms. Determining the physiological role of C9ORF72 protein may help to determine the role it plays in disease.

Isoform-specific antibodies reveal distinct subcellular localizations of C9orf72 in amyotrophic lateral sclerosis

OBJECTIVE

A noncoding hexanucleotide repeat expansion in C9orf72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). It has been reported that the repeat expansion causes a downregulation of C9orf72 transcripts, suggesting that haploinsufficiency may contribute to disease pathogenesis. Two protein isoforms are generated from three alternatively spliced transcripts of C9orf72; a long form (C9-L) and a short form (C9-S), and their function(s) are largely unknown owing to lack of specific antibodies.

METHODS

To investigate C9orf72 protein properties, we developed novel antibodies that recognize either C9-L or C9-S. Multiple techniques, including Western blot, immunohistochemistry, and coimmunoprecipitation, were used to determine the expression levels and subcellular localizations of C9-L and C9-S.

RESULTS

Investigation of expression of C9-L and C9-S demonstrated distinct biochemical profiles, region-specific changes, and distinct subcellular localizations in ALS tissues. In particular, C9-L antibody exhibited a diffuse cytoplasmic staining in neurons and labeled large speckles in cerebellar Purkinje cells. In contrast, C9-S antibody gave very specific labeling of the nuclear membrane in healthy neurons, with apparent relocalization to the plasma membrane of diseased motor neurons in ALS. Coimmunoprecipitation experiments revealed an interaction of the C9-isoforms with both Importin β1 and Ran-GTPase, components of the nuclear pore complex.

INTERPRETATION

Using these antibodies, we have shown that C9orf72 may be involved in nucleocytoplasmic shuttling and this may have relevance to pathophysiology of ALS/FTLD. Our antibodies have provided improved detection of C9orf72 protein isoforms, which will help elucidate its physiological function and role in ALS/FTLD.

Bioinformatics Data Mining Approach Suggests Coexpression of AGTPBP1 with an ALS-linked Gene C9orf72

BACKGROUND

Expanded GGGGCC hexanucleotide repeats located in the noncoding region of the chromosome 9 open reading frame 72 (C9orf72) gene represent the most common genetic abnormality for familial and sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Formation of nuclear RNA foci, accumulation of repeat-associated non-ATG-translated dipeptide-repeat proteins, and haploinsufficiency of C9orf72 are proposed for pathological mechanisms of C9ALS/FTD. However, at present, the physiological function of C9orf72 remains largely unknown.

METHODS

By searching on a bioinformatics database named COXPRESdb composed of the comprehensive gene coexpression data, we studied potential C9orf72 interactors.

RESULTS

We identified the ATP/GTP binding protein 1 (AGTPBP1) gene alternatively named NNA1 encoding a cytosolic carboxypeptidase whose mutation is causative of the degeneration of Purkinje cells and motor neurons as the most significant gene coexpressed with C9orf72. We verified coexpression and interaction of AGTPBP1 and C9orf72 in transfected cells by immunoprecipitation and in neurons of the human brain by double-labeling immunohistochemistry. Furthermore, we found a positive correlation between AGTPBP1 and C9orf72 mRNA expression levels in the set of 21 human brains examined.

CONCLUSIONS

These results suggest that AGTPBP1 serves as a C9orf72 interacting partner that plays a role in the regulation of neuronal function in a coordinated manner within the central nervous system.

TMEM106B expression is reduced in Alzheimer's disease brains

Introduction

TMEM106B is a transmembrane glycoprotein of unknown function located within endosome/lysosome compartments expressed ubiquitously in various cell types. Previously, the genome-wide association study (GWAS) identified a significant association of TMEM106B single nucleotide polymorphisms (SNPs) with development of frontotemporal lobar degeneration with ubiquitinated TAR DNA-binding protein-43 (TDP-43)-positive inclusions (FTLD-TDP), particularly in the patients exhibiting the progranulin (PGRN) gene (GRN) mutations. Recent studies indicate that TMEM106B plays a pathological role in various neurodegenerative diseases, including Alzheimer’s disease (AD). However, at present, the precise levels of TMEM106B expression in AD brains remain unknown.

Methods

By quantitative reverse transcription (RT)-PCR (qPCR), western blot and immunohistochemistry, we studied TMEM106B and PGRN expression levels in a series of AD and control brains, including amyotrophic lateral sclerosis, Parkinson’s disease, multiple system atrophy and non-neurological cases.

Results

In AD brains, TMEM106B mRNA and protein levels were significantly reduced, whereas PGRN mRNA levels were elevated, compared with the levels in non-AD brains. In all brains, TMEM106B was expressed in the majority of cortical neurons, hippocampal neurons, and some populations of oligodendrocytes, reactive astrocytes and microglia with the location in the cytoplasm. In AD brains, surviving neurons expressed intense TMEM106B immunoreactivity, while senile plaques, neurofibrillary tangles and the perivascular neuropil, almost devoid of TMEM106B, intensely expressed PGRN.

Conclusions

We found an inverse relationship between TMEM106B (downregulation) and PGRN (upregulation) expression levels in AD brains, suggesting a key role of TMEM106B in the pathological processes of AD.

Mechanisms of toxicity in C9FTLD/ALS

A hexanucleotide repeat expansion within a non-coding region of the C9ORF72 gene is the most common mutation causative of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Elucidating how this bidirectionally transcribed G4C2·C4G2 expanded repeat causes "C9FTLD/ALS" has since become an important goal of the field. Likely pathogenic mechanisms include toxicity induced by repeat-containing RNAs, and loss of C9orf72 function due to epigenetic changes resulting in decreased C9ORF72 mRNA expression. With regards to the former, sense and antisense transcripts of the expanded repeat aberrantly interact with various RNA-binding proteins and form discrete nuclear structures, termed RNA foci. These foci have the capacity to sequester select RNA-binding proteins, thereby impairing their function. (G4C2)exp and (C4G2)exp transcripts also succumb to an alternative fate: repeat-associated non-ATG (RAN) translation. This unconventional mode of translation, which occurs in the absence of an initiating codon, results in the abnormal production of poly(GA), poly(GP), poly(GR), poly(PR) and poly(PA) peptides, collectively referred to as C9RAN proteins. C9RAN proteins form neuronal inclusions throughout the central nervous system of C9FTLD/ALS patients and may contribute to disease pathogenesis. This review aims to summarize the important findings from studies examining mechanisms of disease in C9FTLD/ALS, and will also highlight some of the many questions in need of further investigation.

Reduced C9orf72 protein levels in frontal cortex of amyotrophic lateral sclerosis and frontotemporal degeneration brain with the C9ORF72 hexanucleotide repeat expansion

An intronic G₄C₂ hexanucleotide repeat expansion in C9ORF72 is a major cause of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Several mechanisms including RNA toxicity, repeat-associated non-AUG translation mediated dipeptide protein aggregates, and haploinsufficiency of C9orf72 have been implicated in the molecular pathogenesis of this disorder. The aims of this study were to compare the use of two different Southern blot probes for detection of repeat expansions in an amyotrophic lateral sclerosis and frontotemporal lobar degeneration pathological cohort and to determine the levels of C9orf72 transcript variants and protein isoforms in patients versus control subjects. Our Southern blot studies identified smaller repeat expansions (250–1800 bp) that were only detectable with the flanking probe highlighting the potential for divergent results using different Southern blotting protocols that could complicate genotype–phenotype correlation studies. Further, we characterize a new C9orf72 antibody and show for the first time decreased C9orf72 protein levels in the frontal cortex from patients with a pathological hexanucleotide repeat expansion. These data suggest that a reduction in C9orf72 protein may be a consequence of the disease.

Hereditary spastic paraplegia is not associated with C9ORF72 repeat expansions in a Danish cohort

OBJECTIVES

Hereditary spastic paraplegia (HSP) is a heterogeneous group of neurodegenerative disorders characterized by a progressive gait disorder, lower limb spasticity, hyper-reflexia, weakness and extensor plantar responses. Recently, large intronic hexanucleotide repeat expansions (GGGGCC) in C9ORF72 have been found to cause frontotemporal dementia (FTD), amyotrophic lateral sclerosis and FTD with motor neuron disease. Owing to the overlapping phenotypes among HSP, amyotrophic lateral sclerosis and FTD with motor neuron disease along with shared pathological findings, we hypothesized that C9ORF72 expansions might be a genetic risk factor or modifier of HSP.

METHODS

Clinically characterized HSP patients were investigated for elongations in the hexanucleotide repeat of C9ORF72.

RESULTS

Upon analyses of the repeat lengths in the C9ORF72 gene in a Danish cohort of HSP patients, we found no expansions.

CONCLUSION

We conclude that HSP is most likely not associated with repeat expansions in C9ORF72.

C9ORF72 mutations in neurodegenerative diseases

Recent works have demonstrated an expansion of the GGGGCC hexanucleotide repeat in the first intron of chromosome 9 open reading frame 72 (C9ORF72), encoding an unknown C9ORF72 protein, which was responsible for an unprecedented large proportion of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) cases of European ancestry. C9ORF72 is expressed in most tissues including the brain. Emerging evidence has demonstrated that C9ORF72 mutations could reduce the level of C9ORF72 variant 1, which may influence protein expression and the formation of nuclear RNA foci. The spectrum of mutations is broad and provides new insight into neurological diseases. Clinical manifestations of diseases related with C9ORF72 mutations can vary from FTD, ALS, primary lateral sclerosis (PLS), progressive muscular atrophy (PMA), Huntington disease-like syndrome (HDL syndrome), to Alzheimer's disease. In this article, we will review the brief characterizations of the C9ORF72 gene, the expansion mutations, the related disorders, and their features, followed by a discussion of the deficiency knowledge of C9ORF72 mutations. Based on the possible pathological mechanisms of C9ORF72 mutations in ALS and FTD, we can find new targets for the treatment of C9ORF72 mutation-related diseases. Future studies into the mechanisms, taking into consideration the discovery of those disorders, will significantly accelerate new discoveries in this field, including targeting identification of new therapy.

C9orf72 G4C2 repeat expansions in Alzheimer's disease and mild cognitive impairment

C9orf72 G4C2 repeat expansion is a major cause of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Its role in Alzheimer's disease (AD) is less clear. We assessed the prevalence of G4C2 pathogenic repeat expansions in Flanders-Belgian patients with clinical AD or mild cognitive impairment (MCI). In addition, we studied the effect of non-pathogenic G4C2 repeat length variability on susceptibility to AD, and on AD cerebrospinal fluid (CSF) biomarker levels. A pathogenic repeat expansion was identified in 5 of 1217 AD patients (frequency <1%). No pathogenic expansions were observed in patients with MCI (n = 200) or control individuals (n = 1119). Nonpathogenic repeat length variability was not associated with AD, risk of conversion to AD in MCI individuals, or CSF biomarker levels. We conclude that pathogenic C9orf72 G4C2 repeat expansions can be detected in clinical AD patients and could act as a contributor to AD pathogenesis. Non-pathogenic repeat length variability did not affect risk of AD or MCI, nor AD biomarker levels in CSF, indicating that C9orf72 is not a direct AD risk factor.

C9orf72 immunohistochemistry in Alzheimer's disease

Mutation in chromosome 9 open reading frame 72 (C9orf72) is a major genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), referred to as C9FTD/ALS. The function of the protein is currently unknown, and the pathomechanism of C9FTD/ALS remains to be elucidated. The study by Satoh and colleagues in the previous issue of Alzheimer's Research & Therapy presents important new findings on C9orf72 protein expression in neurodegenerative disorders along with characterization of C9orf72 antibodies.