Autophagy and ALS: mechanistic insights and therapeutic implications

Mechanisms of protein homeostasis are crucial for overseeing the clearance of misfolded and toxic proteins over the lifetime of an organism, thereby ensuring the health of neurons and other cells of the central nervous system. The highly conserved pathway of autophagy is particularly necessary for preventing and counteracting pathogenic insults that may lead to neurodegeneration. In line with this, mutations in genes that encode essential autophagy factors result in impaired autophagy and lead to neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS). However, the mechanistic details underlying the neuroprotective role of autophagy, neuronal resistance to autophagy induction, and the neuron-specific effects of autophagy-impairing mutations remain incompletely defined. Further, the manner and extent to which non-cell autonomous effects of autophagy dysfunction contribute to ALS pathogenesis are not fully understood. Here, we review the current understanding of the interplay between autophagy and ALS pathogenesis by providing an overview of critical steps in the autophagy pathway, with special focus on pivotal factors impaired by ALS-causing mutations, their physiologic effects on autophagy in disease models, and the cell type-specific mechanisms regulating autophagy in non-neuronal cells which, when impaired, can contribute to neurodegeneration. This review thereby provides a framework not only to guide further investigations of neuronal autophagy but also to refine therapeutic strategies for ALS and related neurodegenerative diseases.Abbreviations: ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CHMP2B: charged multivesicular body protein 2B; DPR: dipeptide repeat; FTD: frontotemporal dementia; iPSC: induced pluripotent stem cell; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; PINK1: PTEN induced kinase 1; RNP: ribonuclear protein; sALS: sporadic ALS; SPHK1: sphingosine kinase 1; TARDBP/TDP-43: TAR DNA binding protein; TBK1: TANK-binding kinase 1; TFEB: transcription factor EB; ULK: unc-51 like autophagy activating kinase; UPR: unfolded protein response; UPS: ubiquitin-proteasome system; VCP: valosin containing protein.

Cause or Effect: Misregulation of microRNA Pathways in Neurodegeneration

During normal aging or neurodegenerative diseases, neuronal survival and function depend on protein homeostasis, which is regulated by multiple mechanisms, including the microRNA (miRNA) pathway. In different cells types, the absence of Dicer, a key miRNA processing enzyme, leads to neurodegeneration through cell-autonomous and non-cell-autonomous mechanisms. Loss of certain miRNAs also causes neurodegeneration in some model organisms. On the other hand, miRNA expression is misregulated in patients with different neurodegenerative diseases. Thus, the miRNA pathway appears to be essential in the pathogenesis of several age-dependent neurodegenerative conditions; however, our understanding of the underlying mechanism remains rudimentary. The precise causal relationships between specific miRNAs and neurodegeneration in humans need to be further investigated.

Autophagy as a common pathway in amyotrophic lateral sclerosis

Age-dependent neurodegenerative diseases are associated with a decline in protein quality control systems including autophagy. Amyotrophic lateral sclerosis (ALS) is a motor neuron degenerative disease of complex etiology with increasing connections to other neurodegenerative conditions such as frontotemporal dementia. Among the diverse genetic causes for ALS, a striking feature is the common connection to autophagy and its associated pathways. There is a recurring theme of protein misfolding as in other neurodegenerative diseases, but importantly there is a distinct common thread among ALS genes that connects them to the cascade of autophagy. However, the roles of autophagy in ALS remain enigmatic and it is still unclear whether activation or inhibition of autophagy would be a reliable avenue to ameliorate the disease. The main evidence that links autophagy to different genetic forms of ALS is discussed.

Frontotemporal dementia caused by CHMP2B mutation is characterised by neuronal lysosomal storage pathology

Mutations in the charged multivesicular body protein 2B (CHMP2B) cause frontotemporal dementia (FTD). We report that mice which express FTD-causative mutant CHMP2B at physiological levels develop a novel lysosomal storage pathology characterised by large neuronal autofluorescent aggregates. The aggregates are an early and progressive pathology that occur at 3 months of age and increase in both size and number over time. These autofluorescent aggregates are not observed in mice expressing wild-type CHMP2B, or in non-transgenic controls, indicating that they are a specific pathology caused by mutant CHMP2B. Ultrastructural analysis and immuno- gold labelling confirmed that they are derived from the endolysosomal system. Consistent with these findings, CHMP2B mutation patient brains contain morphologically similar autofluorescent aggregates. These aggregates occur significantly more frequently in human CHMP2B mutation brain than in neurodegenerative disease or age-matched control brains. These data suggest that lysosomal storage pathology is the major neuronal pathology in FTD caused by CHMP2B mutation. Recent evidence suggests that two other genes associated with FTD, GRN and TMEM106B are important for lysosomal function. Our identification of lysosomal storage pathology in FTD caused by CHMP2B mutation now provides evidence that endolysosomal dysfunction is a major degenerative pathway in FTD.

Patient iPSC-Derived Neurons for Disease Modeling of Frontotemporal Dementia with Mutation in CHMP2B

The truncated mutant form of the charged multivesicular body protein 2B (CHMP2B) is causative for frontotemporal dementia linked to chromosome 3 (FTD3). CHMP2B is a constituent of the endosomal sorting complex required for transport (ESCRT) and, when mutated, disrupts endosome-to-lysosome trafficking and substrate degradation. To understand the underlying molecular pathology, FTD3 patient induced pluripotent stem cells (iPSCs) were differentiated into forebrain-type cortical neurons. FTD3 neurons exhibited abnormal endosomes, as previously shown in patients. Moreover, mitochondria of FTD3 neurons displayed defective cristae formation, accompanied by deficiencies in mitochondrial respiration and increased levels of reactive oxygen. In addition, we provide evidence for perturbed iron homeostasis, presenting an in vitro patient-specific model to study the effects of iron accumulation in neurodegenerative diseases. All phenotypes observed in FTD3 neurons were rescued in CRISPR/Cas9-edited isogenic controls. These findings illustrate the relevance of our patient-specific in vitro models and open up possibilities for drug target development.

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FTD3 neurons show abnormalities in endosomes and mitochondria

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Parkinson's and Alzheimer's disease core genes are altered in FTD3 neurons

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Iron homeostasis is perturbed in FTD3 neurons

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Impairments in FTD3 neurons are rescued in CRISPR/Cas9-edited isogenic controls

Glutamate-glutamine homeostasis is perturbed in neurons and astrocytes derived from patient iPSC models of frontotemporal dementia

Frontotemporal dementia (FTD) is amongst the most prevalent early onset dementias and even though it is clinically, pathologically and genetically heterogeneous, a crucial involvement of metabolic perturbations in FTD pathology is being recognized. However, changes in metabolism at the cellular level, implicated in FTD and in neurodegeneration in general, are still poorly understood. Here we generate induced human pluripotent stem cells (hiPSCs) from patients carrying mutations in CHMP2B (FTD3) and isogenic controls generated via CRISPR/Cas9 gene editing with subsequent neuronal and glial differentiation and characterization. FTD3 neurons show a dysregulation of glutamate-glutamine related metabolic pathways mapped by ¹³C-labelling coupled to mass spectrometry. FTD3 astrocytes show increased uptake of glutamate whilst glutamate metabolism is largely maintained. Using quantitative proteomics and live-cell metabolic analyses, we elucidate molecular determinants and functional alterations of neuronal and glial energy metabolism in FTD3. Importantly, correction of the mutations rescues such pathological phenotypes. Notably, these findings implicate dysregulation of key enzymes crucial for glutamate-glutamine homeostasis in FTD3 pathogenesis which may underlie vulnerability to neurodegeneration. Neurons derived from human induced pluripotent stem cells (hiPSCs) of patients carrying mutations in CHMP2B (FTD3) display major metabolic alterations compared to CRISPR/Cas9 generated isogenic controls. Using quantitative proteomics, ¹³C-labelling coupled to mass spectrometry metabolic mapping and seahorse analyses, molecular determinants and functional alterations of neuronal and astrocytic energy metabolism in FTD3 were characterized. Our findings implicate dysregulation of glutamate-glutamine homeostasis in FTD3 pathogenesis. In addition, FTD3 neurons recapitulate glucose hypometabolism observed in FTD patient brains. The impaired mitochondria function found here is concordant with disturbed TCA cycle activity and decreased glycolysis in FTD3 neurons. FTD3 neuronal glutamine hypermetabolism is associated with up-regulation of PAG expression and, possibly, ROS production. Distinct compartments of glutamate metabolism can be suggested for the FTD3 neurons. Endogenous glutamate generated from glutamine via PAG may enter the TCA cycle via AAT (left side of neuron) while exogenous glutamate taken up from the extracellular space may be incorporated into the TCA cycle via GDH (right side of the neuron) FTD3 astrocytic glutamate uptake is upregulated whilst glutamate metabolism is largely maintained. Finally, pharmacological reversal of glutamate hypometabolism manifesting from decreased GDH expression should be explored as a novel therapeutic intervention for treating FTD3.

The emerging roles of microRNAs in the pathogenesis of frontotemporal dementia-amyotrophic lateral sclerosis (FTD-ALS) spectrum disorders

Increasing evidence suggests that frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) share some clinical, pathological, and molecular features as part of a common neurodegenerative spectrum disorder. In recent years, enormous progress has been made in identifying both pathological proteins and genetic mutations associated with FTD-ALS. However, the molecular pathogenic mechanisms of disease onset and progression remain largely unknown. Recent studies have uncovered unexpected links between FTD-ALS and multiple aspects of RNA metabolism, setting the stage for further understanding of the disorder. Here, the authors will focus on microRNAs and review the emerging roles of these small RNAs in several aspects of FTD-ALS pathogenesis.

Lessons learned from CHMP2B, implications for frontotemporal dementia and amyotrophic lateral sclerosis

Frontotemporal dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS) are two neurodegenerative diseases with clinical, genetic and pathological overlap. As such, they are commonly regarded as a single spectrum disorder, with pure FTD and pure ALS representing distinct ends of a continuum. Dysfunctional endo-lysosomal and autophagic trafficking, leading to impaired proteostasis is common across the FTD-ALS spectrum. These pathways are, in part, mediated by CHMP2B, a protein that coordinates membrane scission events as a core component of the ESCRT machinery. Here we review how ALS and FTD disease causing mutations in CHMP2B have greatly contributed to our understanding of how endosomal-lysosomal and autophagic dysfunction contribute to neurodegeneration, and how in vitro and in vivo models have helped elucidate novel candidates for potential therapeutic intervention with implications across the FTD-ALS spectrum.

The ESCRT protein CHMP2B acts as a diffusion barrier on reconstituted membrane necks

Endosomal sorting complexes required for transport (ESCRT)-III family proteins catalyze membrane remodeling processes that stabilize and constrict membrane structures. It has been proposed that stable ESCRT-III complexes containing CHMP2B could establish diffusion barriers at the post-synaptic spine neck. In order to better understand this process, we developed a novel method based on fusion of giant unilamellar vesicles to reconstitute ESCRT-III proteins inside GUVs, from which membrane nanotubes are pulled. The new assay ensures that ESCRT-III proteins polymerize only when they become exposed to physiologically relevant membrane topology mimicking the complex geometry of post-synaptic spines. We establish that CHMP2B, both full-length and with a C-terminal deletion (ΔC), preferentially binds to membranes containing phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Moreover, we show that CHMP2B preferentially accumulates at the neck of membrane nanotubes, and provide evidence that CHMP2B-ΔC prevents the diffusion of PI(4,5)P2 lipids and membrane-bound proteins across the tube neck. This indicates that CHMP2B polymers formed at a membrane neck may function as a diffusion barrier, highlighting a potential important function of CHMP2B in maintaining synaptic spine structures.

Astrocytic reactivity triggered by defective autophagy and metabolic failure causes neurotoxicity in frontotemporal dementia type 3

Frontotemporal dementia type 3 (FTD3), caused by a point mutation in the charged multivesicular body protein 2B (CHMP2B), affects mitochondrial ultrastructure and the endolysosomal pathway in neurons. To dissect the astrocyte-specific impact of mutant CHMP2B expression, we generated astrocytes from human induced pluripotent stem cells (hiPSCs) and confirmed our findings in CHMP2B mutant mice. Our data provide mechanistic insights into how defective autophagy causes perturbed mitochondrial dynamics with impaired glycolysis, increased reactive oxygen species, and elongated mitochondrial morphology, indicating increased mitochondrial fusion in FTD3 astrocytes. This shift in astrocyte homeostasis triggers a reactive astrocyte phenotype and increased release of toxic cytokines, which accumulate in nuclear factor kappa b (NF-κB) pathway activation with increased production of CHF, LCN2, and C3 causing neurodegeneration.

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FTD3 iPSC-derived astrocytes display impaired autophagy

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Impaired autophagy affects mitochondria turnover, glucose hypometabolism and TCA cycle

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FTD3 astrocytes contribute to reactive gliosis by increased C3, LCN2, IL6, and IL8

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Reactive astrocyte phenotypes are present in both in vitro and in vivo models

SIRT6 Protects Against Myocardial Ischemia-Reperfusion Injury by Attenuating Aging-Related CHMP2B Accumulation

Impaired autophagic flux induces aging-related ischemia vulnerability, which is the hallmark pathology in cardiac aging. Our previous work has confirmed that the accumulation of charged multivesicular body protein 2B (CHMP2B), a subunit of the ESCRT-III complex, in the heart can impair autophagy flux. However, whether CHMP2B accumulation contributes to aging-related intolerance to ischemia/reperfusion (I/R) injury and the regulatory mechanism for CHMP2B in aged heart remain elusive. The cardiac CHMP2B level was significantly higher in aged human myocardium than that in young myocardium. Increased CHMP2B were shown to inhibit autophagic flux leading to the deterioration of MI/R injury in aged mice hearts. Interestingly, a negative correlation was observed between SIRT6 and CHMP2B expression in human heart samples. Specific activation of SIRT6 suppressed CHMP2B accumulation and ameliorated autophagy flux in aged hearts. Using myocardial-specific SIRT6 heterozygous knockout mice and recovery experiments confirmed that SIRT6 regulated myocardial CHMP2B levels. Finally, activation of SIRT6 decreased acetylation of FoxO1 to promote its transcriptional function on Atrogin-1, a muscle-specific ubiquitin ligase, which subsequently enhanced the degradation of CHMP2B by Atrogin-1. This is a novel mechanism for SIRT6 against aging-related myocardial ischemia vulnerability, particularly by preventing excessive accumulation of autophagy key factors.

The ESCRT-III protein VPS4, but not CHMP4B or CHMP2B, is pathologically increased in familial and sporadic ALS neuronal nuclei

Nuclear pore complex injury has recently emerged as an early and significant contributor to familial and sporadic ALS disease pathogenesis. However, the molecular events leading to this pathological phenomenon characterized by the reduction of specific nucleoporins from neuronal nuclear pore complexes remain largely unknown. This is due in part to a lack of knowledge regarding the biological pathways and proteins underlying nuclear pore complex homeostasis specifically in human neurons. We have recently uncovered that aberrant nuclear accumulation of the ESCRT-III protein CHMP7 initiates nuclear pore complex in familial and sporadic ALS neurons. In yeast and non-neuronal mammalian cells, nuclear relocalization of CHMP7 has been shown to recruit the ESCRT-III proteins CHMP4B, CHMP2B, and VPS4 to facilitate nuclear pore complex and nuclear envelope repair and homeostasis. Here, using super resolution structured illumination microscopy, we find that neither CHMP4B nor CHMP2B are increased in ALS neuronal nuclei. In contrast, VPS4 expression is significantly increased in ALS neuronal nuclei prior to the emergence of nuclear pore injury in a CHMP7 dependent manner. However, unlike our prior CHMP7 knockdown studies, impaired VPS4 function does not mitigate alterations to the NPC and the integral transmembrane nucleoporin POM121. Collectively our data suggest that while alterations in VPS4 subcellular localization appear to be coincident with nuclear pore complex injury, therapeutic efforts to mitigate this pathogenic cascade should be targeted towards upstream events such as the nuclear accumulation of CHMP7 as we have previously described.

Neuroprotective activity of ursodeoxycholic acid in CHMP2BIntron5 models of frontotemporal dementia

Frontotemporal dementia (FTD) is one of the most prevalent forms of early-onset dementia. It represents part of the FTD-Amyotrophic Lateral Sclerosis (ALS) spectrum, a continuum of genetically and pathologically overlapping disorders. FTD-causing mutations in CHMP2B, a gene encoding a core component of the heteromeric ESCRT-III Complex, lead to perturbed endosomal-lysosomal and autophagic trafficking with impaired proteostasis. While CHMP2B mutations are rare, dysfunctional endosomal-lysosomal signalling is common across the FTD-ALS spectrum. Using our established Drosophila and mammalian models of CHMP2BIntron5 induced FTD we demonstrate that the FDA-approved compound Ursodeoxycholic Acid (UDCA) conveys neuroprotection, downstream of endosomal-lysosomal dysfunction in both Drosophila and primary mammalian neurons. UDCA exhibited a dose dependent rescue of neuronal structure and function in Drosophila pan-neuronally expressing CHMP2BIntron5. Rescue of CHMP2BIntron5 dependent dendritic collapse and apoptosis with UDCA in rat primary neurons was also observed. UDCA failed to ameliorate aberrant accumulation of endosomal and autophagic organelles or ubiquitinated neuronal inclusions in both models. We demonstrate the neuroprotective activity of UDCA downstream of endosomal-lysosomal and autophagic dysfunction, delineating the molecular mode of action of UDCA and highlighting its potential as a therapeutic for the treatment of FTD-ALS spectrum disorders.

Metformin-Enhanced Cardiac AMP-Activated Protein Kinase/Atrogin-1 Pathways Inhibit Charged Multivesicular Body Protein 2B Accumulation in Ischemia-Reperfusion Injury

Background: Cardiac autophagic flux is impaired during myocardial ischemia/reperfusion (MI/R). Impaired autophagic flux may exacerbate MI/R injury. Charged multivesicular body protein 2B (CHMP2B) is a subunit of the endosomal sorting complex required for transport (ESCRT-III) complex that is required for autophagy. However, the reverse role of CHMP2B accumulation in autophagy and MI/R injury has not been established. The objective of this article is to elucidate the roles of AMP-activated protein kinase (AMPK)/atrogin-1 pathways in inhibiting CHMP2B accumulation in ischemia-reperfusion injury. Methods: Male C57BL/6 mice (3-4 months) and H9c2 cardiomyocytes were used to evaluate MI/R and hypoxia/reoxygenation (H/R) injury in vivo and in vitro, respectively. MI/R was built by a left lateral thoracotomy and occluded the left anterior descending artery. H9c2 cells were firstly treated in 95% N2 and 5% CO2 for 15 h and reoxygenation for 1 h. Metformin (100 mg/kg/d) and CHMP2B (Ad-CHMP2B) transfected adenoviruses were administered to the mice. The H9c2 cells were treated with metformin (2.5 mM), MG-132 (10 μM), bafilomycin A1 (10 nM), and compound C (20 μM). Results: Autophagic flux was found to be inhibited in H/R-treated cardiomyocytes and MI/R mice, with elevated cardiac CHMP2B accumulation. Upregulated CHMP2B levels in the in vivo and in vitro experiments were shown to inhibit autophagic flux leading to the deterioration of H/R-cardiomyocytes and MI/R injury. This finding implies that CHMP2B accumulation increases the risk of myocardial ischemia. Metformin suppressed CHMP2B accumulation and ameliorated H/R-induced autophagic dysfunction by activating AMPK. Activated AMPK upregulated the messenger RNA expression and protein levels of atrogin-1, a muscle-specific ubiquitin ligase, in the myocardium. Atrogin-1 significantly enhanced the interaction between atrogin-1 and CHMP2B, therefore, promoting CHMP2B degradation in the MI/R myocardium. Finally, this study revealed that metformin-inhibited CHMP2B accumulation induced autophagic impairment and ischemic susceptibility in vivo through the AMPK-regulated CHMP2B degradation by atrogin-1. Conclusion: Impaired CHMP2B clearance in vitro and in vivo inhibits autophagic flux and weakens the myocardial ischemic tolerance. Metformin treatment degrades CHMP2B through the AMPK-atrogin-1-dependent pathway to maintain the homeostasis of autophagic flux. This is a novel mechanism that enriches the understanding of cardioprotection.

Inflammatory markers of CHMP2B-mediated frontotemporal dementia

Histopathological studies and animal models have suggested an inflammatory component in the pathomechanism of the CHMP2B associated frontotemporal dementia (FTD-3). In this cross-sectional study, serum and cerebrospinal fluid were analyzed for inflammatory markers in CHMP2B mutation carriers. Serum levels of CCL4 were increased throughout life. Serum levels of IL-15, CXCL10, CCL22 and TNF-α were significantly associated with cognitive decline, suggesting a peripheral inflammatory response to neurodegeneration. CSF levels of sTREM2 appeared to increase more rapidly with age in CHMP2B mutation carriers. The identification of a peripheral inflammatory response to disease progression supports the involvement of an inflammatory component in FTD-3.

TMEM106B and ApoE polymorphisms in CHMP2B-mediated frontotemporal dementia (FTD-3)

Single-nucleotide polymorphisms in the TMEM106B gene have been identified as a risk factor in frontotemporal dementia (FTD). The major allele of SNP rs3173615 is a risk factor in sporadic FTD, whereas the minor allele seems protective in GRN- and C9orf72-mediated FTD. The role of apolipoprotein E (ApoE) in FTD is uncertain, though an established risk factor in Alzheimer's disease. In a unique Danish family, inherited FTD is caused by a mutation in the CHMP2B gene located on chromosome 3 (FTD-3). In this family, both risk factors TMEM106B and ApoE were analyzed and correlated to age at onset (AAO) and progression in terms of age at institutionalization (AAI) and age at death (AAD). Although TMEM106B and CHMP2B share cellular function in that both localize to endolysosomes, TMEM106B genotypes appeared to have no influence on the clinical disease course. ApoE ε4 was found to be a protective factor with later AAO and AAI, whereas ε2 seemed to aggravate the disease with earlier AAO and AAD. These results indicate ApoE ε2 as a risk factor in FTD-3 and suggest a protective role of ε4.

Serum Neurofilament Light in Patients with Frontotemporal Dementia Caused by CHMP2B Mutation

INTRODUCTION

The potential of neurofilament light (NfL) as a blood-based biomarker is currently being investigated in autosomal dominant neurodegenerative disease. This study explores the clinical utility of serum-NfL in frontotemporal dementia due to CHMP2B mutation (FTD-3).

METHODS

This cross-sectional study included serum and CSF data from 38 members of the Danish FTD-3 family: 12 affected CHMP2B mutation carriers, 10 presymptomatic carriers, and 16 noncarriers. Serum-NfL levels measured by single-molecule array (Simoa) technology were tested for associations with the clinical groups and clinical parameters. Serum and CSF data were compared, and CSF/serum-albumin ratio was included as a measure of blood-brain barrier (BBB) function.

RESULTS

Serum-NfL concentrations were significantly increased in symptomatic CHMP2B mutation carriers compared to presymptomatic carriers and in both groups compared to healthy family controls. Serum-NfL levels appear to increase progressively with age in presymptomatic carriers, and this is perhaps followed by a change in trajectory when patients become symptomatic. Measurements of NfL in serum and CSF were highly correlated and fold-changes in serum and CSF between clinical groups were similar. Increase in serum-NFL levels was correlated with reduced ACE-score. Higher CSF/serum-albumin ratios were demonstrated in FTD-3 patients, but this did not affect the significant associations between serum-NfL and clinical groups.

CONCLUSION

Serum-NfL could be utilized as an accurate surrogate marker of CSF levels to segregate symptomatic CHMP2B carriers, presymptomatic carriers, and non-carriers. The observed indication of BBB dysfunction in FTD-3 patients did not confound this use of serum-NfL. The results support the occurrence of mutation-related differences in NfL dynamics in familial FTD.

A novel synaptopathy-defective synaptic vesicle protein trafficking in the mutant CHMP2B mouse model of frontotemporal dementia

Mutations in the ESCRT-III subunit CHMP2B cause frontotemporal dementia (FTD) and lead to impaired endolysosomal trafficking and lysosomal storage pathology in neurons. We investigated the effect of mutant CHMP2B on synaptic pathology, as ESCRT function was recently implicated in the degradation of synaptic vesicle (SV) proteins. We report here that expression of C-terminally truncated mutant CHMP2B results in a novel synaptopathy. This unique synaptic pathology is characterised by selective retention of presynaptic SV trafficking proteins in aged mutant CHMP2B transgenic mice, despite significant loss of postsynaptic proteins. Furthermore, ultrastructural analysis of primary cortical cultures from transgenic CHMP2B mice revealed a significant increase in the number of presynaptic endosomes, while neurons expressing mutant CHMP2B display defective SV recycling and alterations to functional SV pools. Therefore, we reveal how mutations in CHMP2B affect specific presynaptic proteins and SV recycling, identifying CHMP2B FTD as a novel synaptopathy. This novel synaptopathic mechanism of impaired SV physiology may be a key early event in multiple forms of FTD, since proteins that mediate the most common genetic forms of FTD all localise at the presynapse.

Expression of a human variant of CHMP2B linked to neurodegeneration in Drosophila external sensory organs leads to cell fate transformations associated with increased Notch activity

Proper function of cell signaling pathways is dependent upon regulated membrane trafficking events that lead to the endocytosis, recycling, and degradation of cell surface receptors. The endosomal complexes required for transport (ESCRT) genes play a critical role in the sorting of ubiquitinated cell surface proteins. CHMP2B^Intron5 , a truncated form of a human ESCRT-III protein, was discovered in a Danish family afflicted by a hereditary form of frontotemporal dementia (FTD). Although the mechanism by which the CHMP2B mutation in this family causes FTD is unknown, the resulting protein has been shown to disrupt normal endosomal-lysosomal pathway function and leads to aberrant regulation of signaling pathways. Here we have misexpressed CHMP2B^Intron5 in the developing Drosophila external sensory (ES) organ lineage and demonstrate that it is capable of altering cell fates. Each of the cell fate transformations seen is compatible with an increase in Notch signaling. Furthermore, this interpretation is supported by evidence that expression of CHMP2B^Intron5 in the notum environment is capable of raising the levels of Notch signaling. As such, these results add to a growing body of evidence that CHMP2B^Intron5 can act rapidly to disrupt normal cellular function via the misregulation of critical cell surface receptor function.

Cytoplasmic YAP1-mediated ESCRT-III assembly promotes autophagic cell death and is ubiquitinated by NEDD4L in breast cancer

BACKGROUND

Nuclear Yes1-associated transcriptional regulator (YAP1) promotes tumor progression. However, the function of cytoplasmic YAP1 in breast cancer cells and its impact on the survival of breast cancer patients remain unclear. Our research aimed to explore the biological function of cytoplasmic YAP1 in breast cancer cells and the possibility of cytoplasmic YAP1 as a predictive marker of breast cancer survival.

METHODS

We constructed cell mutant models, including NLS-YAP1^5SA (nuclear localized), YAP1^S94A (incapable of binding to the TEA domain transcription factor family) and YAP1^S127D (cytoplasmic localized), and used Cell Counting Kit-8 (CCK-8) assays, 5-ethynyl-2'-deoxyuridine (EdU) incorporation assays, and Western blotting (WB) analysis to detect cell proliferation and apoptosis. The specific mechanism of cytoplasmic YAP1-mediated endosomal sorting complexes required for transport III (ESCRT-III) assembly was studied by co-immunoprecipitation, immunofluorescence staining, and WB analysis. Epigallocatechin gallate (EGCG) was used to simulate YAP1 retention in the cytoplasm in in vitro and in vivo experiments to study the function of cytoplasmic YAP1. YAP1 binding to NEDD4-like E3 ubiquitin protein ligase (NEDD4L) was identified using mass spectrometry and was verified in vitro. Breast tissue microarrays were used to analyze the relationship between cytoplasmic YAP1 expression and the survival of breast cancer patients.

RESULTS

YAP1 was mainly expressed in the cytoplasm in breast cancer cells. Cytoplasmic YAP1 promoted autophagic death of breast cancer cells. Cytoplasmic YAP1 bound to the ESCRT-III complex subunits charged multivesicular body protein 2B (CHMP2B) and vacuolar protein sorting 4 homolog B (VPS4B), promoting assembly of CHMP2B-VPS4B and activating autophagosome formation. EGCG retained YAP1 in the cytoplasm, promoting the assembly of CHMP2B-VPS4B to promote autophagic death of breast cancer cells. YAP1 bound to NEDD4L, and NEDD4L mediated ubiquitination and degradation of YAP1. Breast tissue microarrays revealed that high levels of cytoplasmic YAP1 were beneficial to the survival of breast cancer patients.

CONCLUSIONS

Cytoplasmic YAP1 mediated autophagic death of breast cancer cells by promoting assembly of the ESCRT-III complex; furthermore, we established a new breast cancer survival prediction model based on cytoplasmic YAP1 expression.

Molecular Genetics of Frontotemporal Dementia Elucidated by Drosophila Models-Defects in Endosomal⁻Lysosomal Pathway

Frontotemporal dementia (FTD) is the second most common senile neurodegenerative disease. FTD is a heterogeneous disease that can be classified into several subtypes. A mutation in CHMP2B locus (CHMP2B^intron5), which encodes a component of endosomal sorting complex required for transport-III (ESCRT-III), is associated with a rare hereditary subtype of FTD linked to chromosome 3 (FTD-3). ESCRT is involved in critical cellular processes such as multivesicular body (MVB) formation during endosomal–lysosomal pathway and autophagy. ESCRT mutants causes diverse physiological defects primarily due to accumulation of endosomes and defective MVBs resulting in misregulation of signaling pathways. Charged multivesicular body protein 2B (CHMP2B) is important for neuronal physiology which especially rely on precise regulation of protein homeostasis due to their post-mitotic status. Drosophila has proven to be an excellent model for charaterization of mechanistic underpinning of neurodegenerative disorders including FTD. In this review, current understanding of various FTD-related mutations is discussed with a focus on Drosophila models of CHMP2B^intron5-associated FTD.

Expression of mutant CHMP2B linked to neurodegeneration in humans disrupts circadian rhythms in Drosophila

Mutations in CHMP2B, an ESCRT-III (endosomal sorting complexes required for transport) component, are associated with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Neurodegenerative disorders including FTD are also associated with a disruption in circadian rhythms, but the mechanism underlying this defect is not well understood. Here, we ectopically expressed the human CHMP2B variant associated with FTD (CHMP2BIntron5) in flies using the GMR-GAL4 driver (GMR>CHMP2BIntron5) and analyzed their circadian rhythms at behavioral, cellular, and biochemical level. In GMR>CHMP2BIntron5 flies, we observed disrupted eclosion rhythms, shortened free-running circadian locomotor period, and reduced levels of timeless (tim) mRNA-a circadian pacemaker gene. We also observed that the GMR-GAL4 driver, primarily known for its expression in the retina, drives expression in a subset of tim expressing neurons in the optic lobe of the brain. The patterning of these GMR- and tim-positive neurons in the optic lobe, which appears distinct from the putative clusters of circadian pacemaker neurons in the fly brain, was disrupted in GMR>CHMP2BIntron5 flies. These results demonstrate that CHMP2BIntron5 can disrupt the normal function of the circadian clock in Drosophila.

FTD/ALS Type 7-Associated Thr104Asn Mutation of CHMP2B Blunts Neuronal Process Elongation, and Is Recovered by Knockdown of Arf4, the Golgi Stress Regulator

Frontotemporal dementia and/or amyotrophic lateral sclerosis type 7 (FTD/ALS7) is an autosomal dominant neurodegenerative disorder characterized by the onset of FTD and/or ALS, mainly in adulthood. Patients with some types of mutations, including the Thr104Asn (T104N) mutation of charged multivesicular body protein 2B (CHMP2B), have predominantly ALS phenotypes, whereas patients with other mutations have predominantly FTD phenotypes. A few mutations result in patients having both phenotypes approximately equally; however, the reason why phenotypes differ depending on the position of the mutation is unknown. CHMP2B comprises one part of the endosomal sorting complexes required for transport (ESCRT), specifically ESCRT-III, in the cytoplasm. We describe here, for the first time, that CHMP2B with the T104N mutation inhibits neuronal process elongation in the N1E-115 cell line, a model line undergoing neuronal differentiation. This inhibitory phenotype was accompanied by changes in marker protein expression. Of note, CHMP2B with the T104N mutation, but not the wild-type form, was preferentially accumulated in the Golgi body. Of the four major Golgi stress signaling pathways currently known, the pathway through Arf4, the small GTPase, was specifically upregulated in cells expressing CHMP2B with the T104N mutation. Conversely, knockdown of Arf4 with the cognate small interfering (si)RNA recovered the neuronal process elongation inhibited by the T104N mutation. These results suggest that the T104N mutation of CHMP2B inhibits morphological differentiation by triggering Golgi stress signaling, revealing a possible therapeutic molecular target for recovering potential molecular and cellular phenotypes underlying FTD/ALS7.

Cortical Frontoparietal Network Dysfunction in CHMP2B-Frontotemporal Dementia

A rare cause of inherited frontotemporal dementia (FTD) is a mutation in the CHMP2B gene on chromosome 3 leading to the autosomal dominantly inherited FTD (CHMP2B-FTD). Since CHMP2B-FTD is clinically well-characterized, and patients show a distinct pattern of executive dysfunction, the condition offers possible insight in the early electroencephalographic (EEG) changes in the cortical networks. Specifically, EEG microstate analysis parses the EEG signals into topographies believed to represent discrete network activations. We investigated the EEG dynamics in patients with symptomatic CHMP2B-FTD (n = 5) as well as pre-symptomatic mutation carriers (n = 5) compared to non-carrier family members (n = 6). The data was parsed into four archetypal microstates and global power was calculated. A trend was found for lower occurrence in microstate D in CHMP2B-FTD (p-value = 0.177, F-value = 2.036). Patients with recent symptom onset (<1 year) showed an increased duration of microstate D, whereas patients who had been symptomatic for longer periods (>2 years) showed decreased duration. Patients with CHMP2B-FTD present with executive dysfunction, and microstate D has previously been shown to be associated with the fronto-parietal network. The biphasic pattern may represent the pathophysiological changes in brain dynamics during neurodegeneration, which may apply to other neurodegenerative diseases.

Six generations of CHMP2B-mediated Frontotemporal Dementia: Clinical features, predictive testing, progression, and survival

OBJECTIVES

Chromosome 3-linked frontotemporal dementia (FTD-3) is caused by a c.532-1G > C mutation in the CHMP2B gene. It is extensively studied in a Danish family comprising one of the largest families with an autosomal dominantly inherited frontotemporal dementia (FTD). This retrospective cohort study utilizes demographics to identify risk factors for onset, progression, life expectancy, and death in CHMP2B-mediated FTD. The pedigree of 528 individuals in six generations is provided, and clinical descriptions are presented. Choices of genetic testing are evaluated.

MATERIALS AND METHODS

Demographic and lifestyle factors were assessed in survival analysis in all identified CHMP2B mutation carriers (44 clinically affected FTD-3 patients and 16 presymptomatic CHMP2B mutation carriers). Predictors of onset and progression included sex, parental disease course, education, and vascular risk factors. Life expectancy was established by matching CHMP2B mutation carriers with average life expectancies in Denmark.

RESULTS

Disease course was not correlated to parental disease course and seemed unmodified by lifestyle factors. Diagnosis was recognized at an earlier age in members with higher levels of education, probably reflecting an early dysexecutive syndrome, unmasked earlier in people with higher work-related requirements. Carriers of the CHMP2B mutation had a significant reduction in life expectancy of 13 years. Predictive genetic testing was chosen by 20% of at-risk family members.

CONCLUSIONS

CHMP2B-mediated FTD is substantiated as an autosomal dominantly inherited disease of complete penetrance. The clinical phenotype is a behavioral variant FTD. The disease course is unpredictable, and life expectancy is reduced. The findings may be applicable to other genetic FTD subtypes.