HDAC6 regulates mutant SOD1 aggregation through two SMIR motifs and tubulin acetylation

Epigenetics in the formation of pathological aggregates in amyotrophic lateral sclerosis

The progressive degeneration of motor neurons in amyotrophic lateral sclerosis (ALS) is accompanied by the formation of a broad array of cytoplasmic and nuclear neuronal inclusions (protein aggregates) largely containing RNA-binding proteins such as TAR DNA-binding protein 43 (TDP-43) or fused in sarcoma/translocated in liposarcoma (FUS/TLS). This process is driven by a liquid-to-solid phase separation generally from proteins in membrane-less organelles giving rise to pathological biomolecular condensates. The formation of these protein aggregates suggests a fundamental alteration in the mRNA expression or the levels of the proteins involved. Considering the role of the epigenome in gene expression, alterations in DNA methylation, histone modifications, chromatin remodeling, non-coding RNAs, and RNA modifications become highly relevant to understanding how this pathological process takes effect. In this review, we explore the evidence that links epigenetic mechanisms with the formation of protein aggregates in ALS. We propose that a greater understanding of the role of the epigenome and how this inter-relates with the formation of pathological LLPS in ALS will provide an attractive therapeutic target.

Screens in aging-relevant human ALS-motor neurons identify MAP4Ks as therapeutic targets for the disease

Effective therapeutics is much needed for amyotrophic lateral sclerosis (ALS), an adult-onset neurodegenerative disease mainly affecting motor neurons. By screening chemical compounds in human patient-derived and aging-relevant motor neurons, we identify a neuroprotective compound and show that MAP4Ks may serve as therapeutic targets for treating ALS. The lead compound broadly improves survival and function of motor neurons directly converted from human ALS patients. Mechanistically, it works as an inhibitor of MAP4Ks, regulates the MAP4Ks-HDAC6-TUBA4A-RANGAP1 pathway, and normalizes subcellular distribution of RANGAP1 and TDP-43. Finally, in an ALS mouse model we show that inhibiting MAP4Ks preserves motor neurons and significantly extends animal lifespan.

Linking acetylated α-Tubulin redistribution to α-Synuclein pathology in brain of Parkinson's disease patients

Highly specialized microtubules in neurons are crucial to both health and disease of the nervous system, and their properties are strictly regulated by different post-translational modifications, including α-Tubulin acetylation. An imbalance in the levels of acetylated α-Tubulin has been reported in experimental models of Parkinson's disease (PD) whereas pharmacological or genetic modulation that leads to increased acetylated α-Tubulin successfully rescues axonal transport defects and inhibits α-Synuclein aggregation. However, the role of acetylation of α-Tubulin in the human nervous system is largely unknown as most studies are based on in vitro evidence. To capture the complexity of the pathological processes in vivo, we analysed post-mortem human brain of PD patients and control subjects. In the brain of PD patients at Braak stage 6, we found a redistribution of acetylated α-Tubulin, which accumulates in the neuronal cell bodies in subcortical structures but not in the cerebral cortex, and decreases in the axonal compartment, both in putamen bundles of fibres and in sudomotor fibres. High-resolution and 3D reconstruction analysis linked acetylated α-Tubulin redistribution to α-Synuclein oligomerization and to phosphorylated Ser 129 α-Synuclein, leading us to propose a model for Lewy body (LB) formation. Finally, in post-mortem human brain, we observed threadlike structures, resembling tunnelling nanotubes that contain α-Synuclein oligomers and are associated with acetylated α-Tubulin enriched neurons. In conclusion, we support the role of acetylated α-Tubulin in PD pathogenesis and LB formation.

Genomic and transcriptomic advances in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder and the most common motor neuron disease. ALS shows substantial clinical and molecular heterogeneity. In vitro and in vivo models coupled with multiomic techniques have provided important contributions to unraveling the pathomechanisms underlying ALS. To date, despite promising results and accumulating knowledge, an effective treatment is still lacking. Here, we provide an overview of the literature on the use of genomics, epigenomics, transcriptomics and microRNAs to deeply investigate the molecular mechanisms developing and sustaining ALS. We report the most relevant genes implicated in ALS pathogenesis, discussing the use of different high-throughput sequencing techniques and the role of epigenomic modifications. Furthermore, we present transcriptomic studies discussing the most recent advances, from microarrays to bulk and single-cell RNA sequencing. Finally, we discuss the use of microRNAs as potential biomarkers and promising tools for molecular intervention. The integration of data from multiple omic approaches may provide new insights into pathogenic pathways in ALS by shedding light on diagnostic and prognostic biomarkers, helping to stratify patients into clinically relevant subgroups, revealing novel therapeutic targets and supporting the development of new effective therapies.

Chemical screens in aging-relevant human motor neurons identify MAP4Ks as therapeutic targets for amyotrophic lateral sclerosis

Effective therapeutics is much needed for amyotrophic lateral sclerosis (ALS), an adult-onset neurodegenerative disease mainly affecting motor neurons. By screening chemical compounds in human patient-derived and aging-relevant motor neurons, we identify a neuroprotective compound and show that MAP4Ks may serve as therapeutic targets for treating ALS. The lead compound broadly improves survival and function of motor neurons directly converted from human ALS patients. Mechanistically, it works as an inhibitor of MAP4Ks, regulates the MAP4Ks-HDAC6-TUBA4A-RANGAP1 pathway, and normalizes subcellular distribution of RANGAP1 and TDP-43. Finally, in an ALS mouse model we show that inhibiting MAP4Ks preserves motor neurons and significantly extends animal lifespan.

Disruption of axonal transport in neurodegeneration

Neurons are markedly compartmentalized, which makes them reliant on axonal transport to maintain their health. Axonal transport is important for anterograde delivery of newly synthesized macromolecules and organelles from the cell body to the synapse and for the retrograde delivery of signaling endosomes and autophagosomes for degradation. Dysregulation of axonal transport occurs early in neurodegenerative diseases and plays a key role in axonal degeneration. Here, we provide an overview of mechanisms for regulation of axonal transport; discuss how these mechanisms are disrupted in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, hereditary spastic paraplegia, amyotrophic lateral sclerosis, and Charcot-Marie-Tooth disease; and discuss therapeutic approaches targeting axonal transport.

The role of altered protein acetylation in neurodegenerative disease

Acetylation is a key post-translational modification (PTM) involved in the regulation of both histone and non-histone proteins. It controls cellular processes such as DNA transcription, RNA modifications, proteostasis, aging, autophagy, regulation of cytoskeletal structures, and metabolism. Acetylation is essential to maintain neuronal plasticity and therefore essential for memory and learning. Homeostasis of acetylation is maintained through the activities of histone acetyltransferases (HAT) and histone deacetylase (HDAC) enzymes, with alterations to these tightly regulated processes reported in several neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). Both hyperacetylation and hypoacetylation can impair neuronal physiological homeostasis and increase the accumulation of pathophysiological proteins such as tau, α-synuclein, and Huntingtin protein implicated in AD, PD, and HD, respectively. Additionally, dysregulation of acetylation is linked to impaired axonal transport, a key pathological mechanism in ALS. This review article will discuss the physiological roles of protein acetylation and examine the current literature that describes altered protein acetylation in neurodegenerative disorders.

Stress granules in the spinal muscular atrophy and amyotrophic lateral sclerosis: The correlation and promising therapy

Increasing genetic and biochemical evidence has broadened our view of the pathomechanisms that lead to Spinal muscular atrophy (SMA) and Amyotrophic lateral sclerosis (ALS), two fatal neurodegenerative diseases with similar symptoms and causes. Stress granules are dynamic cytosolic storage hubs for mRNAs in response to stress exposures, that are evolutionarily conserved cytoplasmic RNA granules in somatic cells. A lot of previous studies have shown that the impaired stress granules are crucial events in SMA/ALS pathogenesis. In this review, we described the key stress granules related RNA binding proteins (SMN, TDP-43, and FUS) involved in SMA/ALS, summarized the reported mutations in these RNA binding proteins involved in SMA/ALS pathogenesis, and discussed the mechanisms through which stress granules dynamics participate in the diseases. Meanwhile, we described the applications and limitation of current therapies targeting SMA/ALS. We futher proposed the promising targets on stress granules in the future therapeutic interventions of SMA/ALS.

The Role of Epigenetic Mechanisms in Autoimmune, Neurodegenerative, Cardiovascular, and Imprinting Disorders

Epigenetic mutations like aberrant DNA methylation, histone modifications, or RNA silencing are found in a number of human diseases. This review article discusses the epigenetic mechanisms involved in neurodegenerative disorders, cardiovascular disorders, auto-immune disorder, and genomic imprinting disorders. In addition, emerging epigenetic therapeutic strategies for the treatment of such disorders are presented. Medicinal chemistry campaigns highlighting the efforts of the chemists invested towards the rational design of small molecule inhibitors have also been included. Pleasingly, several classes of epigenetic inhibitors, DNMT, HDAC, BET, HAT, and HMT inhibitors along with RNA based therapies have exhibited the potential to emerge as therapeutics in the longer run. It is quite hopeful that epigenetic modulator-based therapies will advance to clinical stage investigations by leaps and bounds.

Modeling gain-of-function and loss-of-function components of SPAST-based hereditary spastic paraplegia using transgenic mice

Hereditary spastic paraplegia (HSP) is a disease in which dieback degeneration of corticospinal tracts, accompanied by axonal swellings, leads to gait deficiencies. SPG4-HSP, the most common form of the disease, results from mutations of human spastin gene (SPAST), which is the gene that encodes spastin, a microtubule-severing protein. The lack of a vertebrate model that recapitulates both the etiology and symptoms of SPG4-HSP has stymied the development of effective therapies for the disease. hSPAST-C448Y mice, which express human mutant spastin at the ROSA26 locus, display corticospinal dieback and gait deficiencies but not axonal swellings. On the other hand, mouse spastin gene (Spast)-knockout (KO) mice display axonal swellings but not corticospinal dieback or gait deficiencies. One possibility is that reduced spastin function, resulting in axonal swellings, is not the cause of the disease but exacerbates the toxic effects of the mutant protein. To explore this idea, Spast-KO and hSPAST-C448Y mice were crossbred, and the offspring were compared with the parental lines via histological and behavioral analyses. The crossbred animals displayed axonal swellings as well as earlier onset, worsened gait deficiencies and corticospinal dieback compared with the hSPAST-C448Y mouse. These results, together with observations on changes in histone deacetylases 6 and tubulin modifications in the axon, indicate that each of these three transgenic mouse lines is valuable for investigating a different component of the disease pathology. Moreover, the crossbred mice are the best vertebrate model to date for testing potential therapies for SPG4-HSP.

The PRMT5/HURP axis retards Golgi repositioning by stabilizing acetyl-tubulin and Golgi apparatus during cell migration

The Golgi apparatus (GA) translocates to the cell leading end during directional migration, thereby determining cell polarity and transporting essential factors to the migration apparatus. The study provides mechanistic insights into how GA repositioning (GR) is regulated. We show that the methyltransferase PRMT5 methylates the microtubule regulator HURP at R122. The HURP methylation mimicking mutant 122F impairs GR and cell migration. Mechanistic studies revealed that HURP 122F or endogenous methylated HURP, that is, HURP m122, interacts with acetyl-tubulin. Overexpression of HURP 122F stabilizes the bundling pattern of acetyl-tubulin by decreasing the sensitivity of the latter to a microtubule disrupting agent nocodazole. HURP 122F also rigidifies GA via desensitizing the organelle to several GA disrupting chemicals. Similarly, the acetyl-tubulin mimicking mutant 40Q or tubulin acetyltransferase αTAT1 can rigidify GA, impair GR, and retard cell migration. Reversal of HURP 122F-induced GA rigidification, by knocking down GA assembly factors such as GRASP65 or GM130, attenuates 122F-triggered GR and cell migration. Remarkably, PRMT5 is found downregulated and the level of HURP m122 is decreased during the early hours of wound healing-based cell migration, collectively implying that the PRMT5-HURP-acetyl-tubulin axis plays the role of brake, preventing GR and cell migration before cells reach empty space.

Potential Therapeutic Role of HDAC Inhibitors in FUS-ALS

Mutations in the FUS gene cause amyotrophic lateral sclerosis (ALS-FUS). However, the exact pathogenic mechanism of mutant fused in sarcoma (FUS) protein is not completely understood. FUS is an RNA binding protein (RBP) localized predominantly in the nucleus, but ALS-linked FUS mutations can affect its nuclear localization signal impairing its import into the nucleus. This mislocalization to the cytoplasm facilitates FUS aggregation in cytoplasmic inclusions. Therapies targeting post translational modifications are rising as new treatments for ALS, in particular acetylation which could have a role in the dynamics of RBPs. Research using histone deacetylase (HDAC) inhibitors in FUS-ALS models showed that HDACs can influence cytoplasmic FUS localization. Inhibition of HDACs could promote acetylation of the FUS RNA binding domain (RRM) and altering its RNA interactions resulting in FUS maintenance in the nucleus. In addition, acetylation of FUS RRMs might also favor or disfavor its incorporation into pathological inclusions. In this review, we summarize and discuss the evidence for the potential role of HDACs in the context of FUS-ALS and we propose a new hypothesis based on this overview.

A perspective on therapies for amyotrophic lateral sclerosis: can disease progression be curbed?

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease involving both upper and lower motor neurons, leading to paralysis and eventually death. Symptomatic treatments such as inhibition of salivation, alleviation of muscle cramps, and relief of spasticity and pain still play an important role in enhancing the quality of life. To date, riluzole and edaravone are the only two drugs approved by the Food and Drug Administration for the treatment of ALS in a few countries. While there is adequate consensus on the modest efficacy of riluzole, there are still open questions concerning the efficacy of edaravone in slowing the disease progression. Therefore, identification of novel therapeutic strategies is urgently needed. Impaired autophagic process plays a critical role in ALS pathogenesis. In this review, we focus on therapies modulating autophagy in the context of ALS. Furthermore, stem cell therapies, gene therapies, and newly-developed biomaterials have great potentials in alleviating neurodegeneration, which might halt the disease progression. In this review, we will summarize the current and prospective therapies for ALS.

Opportunities for histone deacetylase inhibition in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease. ALS patients suffer from a progressive loss of motor neurons, leading to respiratory failure within 3 to 5 years after diagnosis. Available therapies only slow down the disease progression moderately or extend the lifespan by a few months. Epigenetic hallmarks have been linked to the disease, creating an avenue for potential therapeutic approaches. Interference with one class of epigenetic enzymes, histone deacetylases, has been shown to affect neurodegeneration in many preclinical models. Consequently, it is crucial to improve our understanding about histone deacetylases and their inhibitors in (pre)clinical models of ALS. We conclude that selective inhibitors with high tolerability and safety and sufficient blood-brain barrier permeability will be needed to interfere with both epigenetic and non-epigenetic targets of these enzymes. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.6/issuetoc.

Epigenetics and Communication Mechanisms in Microglia Activation with a View on Technological Approaches

Microglial cells, the immune cells of the central nervous system (CNS), play a crucial role for the proper brain development and function and in CNS homeostasis. While in physiological conditions, microglia continuously check the state of brain parenchyma, in pathological conditions, microglia can show different activated phenotypes: In the early phases, microglia acquire the M2 phenotype, increasing phagocytosis and releasing neurotrophic and neuroprotective factors. In advanced phases, they acquire the M1 phenotype, becoming neurotoxic and contributing to neurodegeneration. Underlying this phenotypic change, there is a switch in the expression of specific microglial genes, in turn modulated by epigenetic changes, such as DNA methylation, histones post-translational modifications and activity of miRNAs. New roles are attributed to microglial cells, including specific communication with neurons, both through direct cell–cell contact and by release of many different molecules, either directly or indirectly, through extracellular vesicles. In this review, recent findings on the bidirectional interaction between neurons and microglia, in both physiological and pathological conditions, are highlighted, with a focus on the complex field of microglia immunomodulation through epigenetic mechanisms and/or released factors. In addition, advanced technologies used to study these mechanisms, such as microfluidic, 3D culture and in vivo imaging, are presented.

Autophagy regulation by acetylation-implications for neurodegenerative diseases

Posttranslational modifications of proteins, such as acetylation, are essential for the regulation of diverse physiological processes, including metabolism, development and aging. Autophagy is an evolutionarily conserved catabolic process that involves the highly regulated sequestration of intracytoplasmic contents in double-membrane vesicles called autophagosomes, which are subsequently degraded after fusing with lysosomes. The roles and mechanisms of acetylation in autophagy control have emerged only in the last few years. In this review, we describe key molecular mechanisms by which previously identified acetyltransferases and deacetylases regulate autophagy. We highlight how p300 acetyltransferase controls mTORC1 activity to regulate autophagy under starvation and refeeding conditions in many cell types. Finally, we discuss how altered acetylation may impact various neurodegenerative diseases in which many of the causative proteins are autophagy substrates. These studies highlight some of the complexities that may need to be considered by anyone aiming to perturb acetylation under these conditions.

Epigenetic Regulation of ALS and CMT: A Lesson from Drosophila Models

Amyotrophic lateral sclerosis (ALS) is the third most common neurodegenerative disorder and is sometimes associated with frontotemporal dementia. Charcot–Marie–Tooth disease (CMT) is one of the most commonly inherited peripheral neuropathies causing the slow progression of sensory and distal muscle defects. Of note, the severity and progression of CMT symptoms markedly vary. The phenotypic heterogeneity of ALS and CMT suggests the existence of modifiers that determine disease characteristics. Epigenetic regulation of biological functions via gene expression without alterations in the DNA sequence may be an important factor. The methylation of DNA, noncoding RNA, and post-translational modification of histones are the major epigenetic mechanisms. Currently, Drosophila is emerging as a useful ALS and CMT model. In this review, we summarize recent studies linking ALS and CMT to epigenetic regulation with a strong emphasis on approaches using Drosophila models.

Regulation of Functional Protein Aggregation by Multiple Factors: Implications for the Amyloidogenic Behavior of the CAP Superfamily Proteins

The idea that amyloid fibrils and other types of protein aggregates are toxic for cells has been challenged by the discovery of a variety of functional aggregates. However, an identification of crucial differences between pathological and functional aggregation remains to be explored. Functional protein aggregation is often reversible by nature in order to respond properly to changing physiological conditions of the cell. In addition, increasing evidence indicates that fast fibril growth is a feature of functional amyloids, providing protection against the long-term existence of potentially toxic oligomeric intermediates. It is becoming clear that functional protein aggregation is a complexly organized process that can be mediated by a multitude of biomolecular factors. In this overview, we discuss the roles of diverse biomolecules, such as lipids/membranes, glycosaminoglycans, nucleic acids and metal ions, in regulating functional protein aggregation. Our studies on the protein GAPR-1 revealed that several of these factors influence the amyloidogenic properties of this protein. These observations suggest that GAPR-1, as well as the cysteine-rich secretory proteins, antigen 5 and pathogenesis-related proteins group 1 (CAP) superfamily of proteins that it belongs to, require the assembly into an amyloid state to exert several of their functions. A better understanding of functional aggregate formation may also help in the prevention and treatment of amyloid-related diseases.

Mitochondrial Dysfunction, Neurogenesis, and Epigenetics: Putative Implications for Amyotrophic Lateral Sclerosis Neurodegeneration and Treatment

Amyotrophic lateral sclerosis (ALS) is a progressive and devastating multifactorial neurodegenerative disorder. Although the pathogenesis of ALS is still not completely understood, numerous studies suggest that mitochondrial deregulation may be implicated in its onset and progression. Interestingly, mitochondrial deregulation has also been associated with changes in neural stem cells (NSC) proliferation, differentiation, and migration. In this review, we highlight the importance of mitochondrial function for neurogenesis, and how both processes are correlated and may contribute to the pathogenesis of ALS; we have focused primarily on preclinical data from animal models of ALS, since to date no studies have evaluated this link using human samples. As there is currently no cure and no effective therapy to counteract ALS, we have also discussed how improving neurogenic function by epigenetic modulation could benefit ALS. In support of this hypothesis, changes in histone deacetylation can alter mitochondrial function, which in turn might ameliorate cellular proliferation as well as neuronal differentiation and migration. We propose that modulation of epigenetics, mitochondrial function, and neurogenesis might provide new hope for ALS patients, and studies exploring these new territories are warranted in the near future.

The impact of proteostasis dysfunction secondary to environmental and genetic causes on neurodegenerative diseases progression and potential therapeutic intervention

Aggregation of particular proteins in the form of inclusion bodies or plaques followed by neuronal death is a hallmark of neurodegenerative proteopathies such as primary Parkinsonism, Alzheimer's disease, Lou Gehrig's disease, and Huntington's chorea. Complex polygenic and environmental factors implicated in these proteopathies. Accumulation of proteins in these disorders indicates a substantial disruption in protein homeostasis (proteostasis). Proteostasis or cellular proteome homeostasis is attained by the synchronization of a group of cellular mechanisms called the proteostasis network (PN), which is responsible for the stability of the proteome and achieves the equilibrium between synthesis, folding, and degradation of proteins. In this review, we will discuss the different types of PN and the impact of PN component dysfunction on the four major neurodegenerative diseases mentioned earlier. Graphical abstract.

TNF receptor-associated factor 6 interacts with ALS-linked misfolded superoxide dismutase 1 and promotes aggregation

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the selective loss of motor neurons leading to paralysis. Mutations in the gene encoding superoxide dismutase 1 (SOD1) are the second most common cause of familial ALS, and considerable evidence suggests that these mutations result in an increase in toxicity due to protein misfolding. We previously demonstrated in the SOD1^G93A rat model that misfolded SOD1 exists as distinct conformers and forms deposits on mitochondrial subpopulations. Here, using SOD1^G93A rats and conformation-restricted antibodies specific for misfolded SOD1 (B8H10 and AMF7-63), we identified the interactomes of the mitochondrial pools of misfolded SOD1. This strategy identified binding proteins that uniquely interacted with either AMF7-63 or B8H10-reactive SOD1 conformers as well as a high proportion of interactors common to both conformers. Of this latter set, we identified the E3 ubiquitin ligase TNF receptor-associated factor 6 (TRAF6) as a SOD1 interactor, and we determined that exposure of the SOD1 functional loops facilitates this interaction. Of note, this conformational change was not universally fulfilled by all SOD1 variants and differentiated TRAF6 interacting from TRAF6 noninteracting SOD1 variants. Functionally, TRAF6 stimulated polyubiquitination and aggregation of the interacting SOD1 variants. TRAF6 E3 ubiquitin ligase activity was required for the former but was dispensable for the latter, indicating that TRAF6-mediated polyubiquitination and aggregation of the SOD1 variants are independent events. We propose that the interaction between misfolded SOD1 and TRAF6 may be relevant to the etiology of ALS.

Pharmacological intervention of histone deacetylase enzymes in the neurodegenerative disorders

Reversal of aging symptoms and related disorders are the challenging task where epigenetic is a crucial player that includes DNA methylation, histone modification; chromatin remodeling and regulation that are linked to the progression of various neurodegenerative disorders (NDDs). Overexpression of various histone deacetylase (HDACs) can activate Glycogen synthase kinase 3 which promotes the hyperphosphorylation of tau and inhibits its degradation. While HDAC is important for maintaining the neuronal morphology and brain homeostasis, at the same time, these enzymes are promoting neurodegeneration, if it is deregulated. Different experimental models have also confirmed the neuroprotective effects caused by HDAC enzymes through the regulation of neuronal apoptosis, inflammatory response, DNA damage, cell cycle regulation, and metabolic dysfunction. Apart from transcriptional regulation, protein-protein interaction, histone post-translational modifications, deacetylation mechanism of non-histone protein and direct association with disease proteins have been linked to neuronal imbalance. Histone deacetylases inhibitors (HDACi) can be able to alter gene expression and shown its efficacy on experimental models, and in clinical trials for NDD's and found to be a very promising therapeutic agent with certain limitation, for instance, non-specific target effect, isoform-selectivity, specificity, and limited number of predicted biomarkers. Herein, we discussed (i) the catalytic mechanism of the deacetylation process of various HDAC's in in vivo and in vitro experimental models, (ii) how HDACs are participating in neuroprotection as well as in neurodegeneration, (iii) a comprehensive role of HDACi in maintaining neuronal homeostasis and (iv) therapeutic role of biomolecules to modulate HDACs.

Amyloid cross-seeding raises new dimensions to understanding of amyloidogenesis mechanism

Hallmarks of most of the amyloid pathologies are surprisingly found to be heterocomponent entities such as inclusions and plaques which contain diverse essential proteins and metabolites. Experimental studies have already revealed the occurrence of coaggregation and cross-seeding during amyloid formation of several proteins and peptides, yielding multicomponent assemblies of amyloid nature. Further, research reports on the co-occurrence of more than one type of amyloid-linked pathologies in the same individual suggest the possible cross-talk among the disease related amyloidogenic protein species during their amyloid growth. In this review paper, we have tried to gain more insight into the process of coaggregation and cross-seeding during amyloid aggregation of proteins, particularly focusing on their relevance to the pathogenesis of the protein misfolding diseases. Revelation of amyloid cross-seeding and coaggregation seems to open new dimensions in our mechanistic understanding of amyloidogenesis and such knowledge may possibly inspire better designing of anti-amyloid therapeutics.

The Acetylation of Lysine-376 of G3BP1 Regulates RNA Binding and Stress Granule Dynamics

Stress granules (SGs) are ribonucleoprotein aggregates that form in response to stress conditions. The regulation of SG dynamics is not fully understood. Permanent pathological SG-like structures were reported in neurodegenerative diseases such as amyotrophic lateral sclerosis. The Ras GTPase-activating protein-binding protein G3BP1 is a central regulator of SG dynamics. We found that the lysine 376 residue (K376) of G3BP1, which is in the RRM RNA binding domain, was acetylated. Consequently, G3BP1 RNA binding was impaired by K376 acetylation. In addition, the acetylation-mimicking mutation K376Q impaired the RNA-dependent interaction of G3BP1 with poly(A)-binding protein 1 (PABP1), but its RNA-independent interactions with caprin-1 and USP10 were little affected. The formation of G3BP1 SGs depended on G3BP1 RNA binding; thus, replacement of endogenous G3BP1 with the K376Q mutant or the RNA binding-deficient F380L/F382L mutant interfered with SG formation. Significant G3BP1 K376 acetylation was detected during SG resolution, and K376-acetylated G3BP1 was seen outside SGs. G3BP1 acetylation is regulated by histone deacetylase 6 (HDAC6) and CBP/p300. Our data suggest that the acetylation of G3BP1 facilitates the disassembly of SGs, offering a potential avenue to mitigate hyperactive stress responses under pathological conditions.

Class I and II histone deacetylase expression is not altered in human amyotrophic lateral sclerosis: Neuropathological and positron emission tomography molecular neuroimaging evidence

INTRODUCTION

There is an unmet need for mechanism-based biomarkers and effective disease modifying treatments in amyotrophic lateral sclerosis (ALS). Previous findings have provided evidence that histone deacetylases (HDAC) are altered in ALS, providing a rationale for testing HDAC inhibitors as a therapeutic option.

METHODS

We measured class I and II HDAC protein and transcript levels together with acetylation levels of downstream substrates by using Western blotting in postmortem tissue of ALS and controls. [¹¹ C]Martinostat, a novel HDAC positron emission tomography ligand, was also used to assess in vivo brain HDAC alterations in patients with ALS and healthy controls (HC).

RESULTS

There was no significant difference in HDAC levels between patients with ALS and controls as measured by Western blotting and reverse-transcription quantitative polymerase chain reaction. Similarly, no differences were detected in [¹¹ C]Martinostat-positron emission tomography uptake in ALS participants compared with HCs.

DISCUSSION

These findings provide evidence that alterations in HDAC isoforms are not a dominant pathological feature at the bulk tissue level in ALS.

Structural Properties and Interaction Partners of Familial ALS-Associated SOD1 Mutants

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron degenerative disease in adults and has also been proven to be a type of conformational disease associated with protein misfolding and dysfunction. To date, more than 150 distinct genes have been found to be associated with ALS, among which Superoxide Dismutase 1 (SOD1) is the first and the most extensively studied gene. It has been well-established that SOD1 mutants-mediated toxicity is caused by a gain-of-function rather than the loss of the detoxifying activity of SOD1. Compared with the clear autosomal dominant inheritance of SOD1 mutants in ALS, the potential toxic mechanisms of SOD1 mutants in motor neurons remain incompletely understood. A large body of evidence has shown that SOD1 mutants may adopt a complex profile of conformations and interact with a wide range of client proteins. Here, in this review, we summarize the fundamental conformational properties and the gained interaction partners of the soluble forms of the SOD1 mutants which have been published in the past decades. Our goal is to find clues to the possible internal links between structural and functional anomalies of SOD1 mutants, as well as the relationships between their exposed epitopes and interaction partners, in order to help reveal and determine potential diagnostic and therapeutic targets.

HDAC6 Modulates Signaling Pathways Relevant to Synaptic Biology and Neuronal Differentiation in Human Stem-Cell-Derived Neurons

Recent studies show that histone deacetylase 6 (HDAC6) has important roles in the human brain, especially in the context of a number of nervous system disorders. Animal models of neurodevelopmental, neurodegenerative, and neuropsychiatric disorders show that HDAC6 modulates important biological processes relevant to disease biology. Pan-selective histone deacetylase (HDAC) inhibitors had been studied in animal behavioral assays and shown to induce synaptogenesis in rodent neuronal cultures. While most studies of HDACs in the nervous system have focused on class I HDACs located in the nucleus (e.g., HDACs 1,2,3), recent findings in rodent models suggest that the cytoplasmic class IIb HDAC, HDAC6, plays an important role in regulating mood-related behaviors. Human studies suggest a significant role for synaptic dysfunction in the prefrontal cortex (PFC) and hippocampus in depression. Studies of HDAC inhibitors (HDACi) in human neuronal cells show that HDAC6 inhibitors (HDAC6i) increase the acetylation of specific lysine residues in proteins involved in synaptogenesis. This has led to the hypothesis that HDAC6i may modulate synaptic biology not through effects on the acetylation of histones, but by regulating acetylation of non-histone proteins.

Aggregation of the nucleic acid-binding protein TDP-43 occurs via distinct routes that are coordinated with stress granule formation

TAR DNA-binding protein 43 (TDP-43) is a nucleic acid-binding protein, and its aggregation represents the defining pathology in amyotrophic lateral sclerosis (ALS) and related proteinopathies. Recent studies implicate cytoplasmic stress granules (SGs) as hubs that may facilitate TDP-43 aggregation. Here, using cellular fractionation, biochemical analyses, and histological assays, we show that TDP-43 targeted to the cytoplasm has multiple fates. Whereas a TDP-43 subpopulation is indeed recruited to SGs, mature aggregated TDP-43, produced with aggregate-prone TDP-43 variants or exposure to oxidative stress, generates distinct TDP-43 inclusions that are surprisingly devoid of SGs. Consistent with this observation, we found that SG components are predominantly excluded from TDP-43 pathology in motor neurons from individuals with ALS. We generated de novo SGs by expressing the fragile X protein (FMRP) and found that rather than directly engaging TDP-43 aggregates, SGs can sequester the proteostasis factor histone deacetylase 6 (HDAC6) and thereby impede TDP-43 clearance from cells. These findings indicate that SGs form distinct cytoplasmic structures that can indirectly enhance TDP-43 aggregation. Therapeutic approaches that inhibit SG formation may therefore be effective at suppressing TDP-43-mediated toxicity in patients with ALS and related TDP-43 proteinopathies.

Epigenetics in amyotrophic lateral sclerosis: a role for histone post-translational modifications in neurodegenerative disease

Amyotrophic lateral sclerosis (ALS) is the third most common adult onset neurodegenerative disorder worldwide. It is generally characterized by progressive paralysis starting at the limbs ultimately leading to death caused by respiratory failure. There is no cure and current treatments fail to slow the progression of the disease. As such, new treatment options are desperately needed. Epigenetic targets are an attractive possibility because they are reversible. Epigenetics refers to heritable changes in gene expression unrelated to changes in DNA sequence. Three main epigenetic mechanisms include the methylation of DNA, microRNAs and the post-translational modification of histone proteins. Histone modifications occur in many amino acid residues and include phosphorylation, acetylation, methylation as well as other chemical moieties. Recent evidence points to a possible role for epigenetic mechanisms in the etiology of ALS. Here, we review recent advances linking ALS and epigenetics, with a strong focus on histone modifications. Both local and global changes in histone modification profiles are associated with ALS drawing attention to potential targets for future diagnostic and treatment approaches.

ALS mutations of FUS suppress protein translation and disrupt the regulation of nonsense-mediated decay

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by preferential motor neuron death. Approximately 15% of ALS cases are familial, and mutations in the fused in sarcoma (FUS) gene contribute to a subset of familial ALS cases. FUS is a multifunctional protein participating in many RNA metabolism pathways. ALS-linked mutations cause a liquid-liquid phase separation of FUS protein in vitro, inducing the formation of cytoplasmic granules and inclusions. However, it remains elusive what other proteins are sequestered into the inclusions and how such a process leads to neuronal dysfunction and degeneration. In this study, we developed a protocol to isolate the dynamic mutant FUS-positive cytoplasmic granules. Proteomic identification of the protein composition and subsequent pathway analysis led us to hypothesize that mutant FUS can interfere with protein translation. We demonstrated that the ALS mutations in FUS indeed suppressed protein translation in N2a cells expressing mutant FUS and fibroblast cells derived from FUS ALS cases. In addition, the nonsense-mediated decay (NMD) pathway, which is closely related to protein translation, was altered by mutant FUS. Specifically, NMD-promoting factors UPF1 and UPF3b increased, whereas a negative NMD regulator, UPF3a, decreased, leading to the disruption of NMD autoregulation and the hyperactivation of NMD. Alterations in NMD factors and elevated activity were also observed in the fibroblast cells of FUS ALS cases. We conclude that mutant FUS suppresses protein biosynthesis and disrupts NMD regulation, both of which likely contribute to motor neuron death.

Epothilone D accelerates disease progression in the SOD1G93A mouse model of amyotrophic lateral sclerosis

AIMS

Degeneration of the distal neuromuscular circuitry is a hallmark pathology of Amyotrophic Lateral Sclerosis (ALS). The potential for microtubule dysfunction to be a critical pathophysiological mechanism in the destruction of this circuitry is increasingly being appreciated. Stabilization of microtubules to improve neuronal integrity and pathology has been shown to be a particularly favourable approach in other neurodegenerative diseases. We present evidence here that treatment with the microtubule-targeting compound Epothilone D (EpoD) both positively and negatively affects the spinal neuromuscular circuitry in the SOD1^G93A mouse model of ALS.

METHODS

SOD1^G93A mice were treated every 5 days with 2 mg/kg EpoD. Evaluation of motor behaviour, neurological phenotype and survival was completed, with age-dependent histological characterization also conducted, using the thy1-YFP mouse. Motor neuron degeneration, axonal integrity, neuromuscular junction (NMJ) health and gliosis were also assessed.

RESULTS

EpoD treatment prevented loss of the spinal motor neuron soma, and distal axon degeneration, early in the disease course. This, however, was not associated with protection of the NMJ synapse and did not improve motor phenotype or clinical progression. EpoD administration was also found to be neurotoxic at later disease stages. This was evidenced by accelerated motor neuron cell body loss, increasing gliosis, and was associated with detrimental outcomes to motor behaviour, clinical assessment and survival.

CONCLUSIONS

The results suggest that EpoD accelerates disease progression in the SOD1^G93A mouse model of ALS, and highlights that the pathophysiological involvement of microtubules in ALS is an evolving and underappreciated phenomenon.

HDAC3 promotes meiotic apparatus assembly in mouse oocytes by modulating tubulin acetylation

Histone deacetylases (HDACs) have been shown to deacetylate numerous cellular substrates that govern a wide array of biological processes. HDAC3, a member of the Class I HDACs, is a highly conserved and ubiquitously expressed protein. However, its roles in meiotic oocytes are not known. In the present study, we find that mouse oocytes depleted of HDAC3 are unable to completely progress through meiosis, and are blocked at metaphase I. These HDAC3 knockdown oocytes show spindle/chromosome organization failure, with severely impaired kinetochore-microtubule attachments. Consistent with this, the level of BubR1, a central component of the spindle assembly checkpoint, at kinetochores is dramatically increased in metaphase oocytes following HDAC3 depletion. Knockdown and overexpression experiments reveal that HDAC3 modulates the acetylation status of α-tubulin in mouse oocytes. Importantly, the deacetylation mimetic mutant tubulin-K40R can partly rescue the defective phenotypes of HDAC3 knockdown oocytes. Our data support a model whereby HDAC3, through deacetylating tubulin, promotes microtubule stability and the establishment of kinetochore-microtubule interaction, consequently ensuring proper spindle morphology, accurate chromosome movement and orderly meiotic progression during oocyte maturation.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Amyotrophic lateral sclerosis (ALS) is an aggressive multifactorial disease converging on a common pathology: the degeneration of motor neurons (MNs), their axons and neuromuscular synapses. This vulnerability and dysfunction of MNs highlights the dependency of these large cells on their intracellular machinery. Neuronal microtubules (MTs) are intracellular structures that facilitate a myriad of vital neuronal functions, including activity dependent axonal transport. In ALS, it is becoming increasingly apparent that MTs are likely to be a critical component of this disease. Not only are disruptions in this intracellular machinery present in the vast majority of seemingly sporadic cases, recent research has revealed that mutation to a microtubule protein, the tubulin isoform TUBA4A, is sufficient to cause a familial, albeit rare, form of disease. In both sporadic and familial disease, studies have provided evidence that microtubule mediated deficits in axonal transport are the tipping point for MN survivability. Axonal transport deficits would lead to abnormal mitochondrial recycling, decreased vesicle and mRNA transport and limited signaling of key survival factors from the neurons peripheral synapses, causing the characteristic peripheral "die back". This disruption to microtubule dependant transport in ALS has been shown to result from alterations in the phenomenon of microtubule dynamic instability: the rapid growth and shrinkage of microtubule polymers. This is accomplished primarily due to aberrant alterations to microtubule associated proteins (MAPs) that regulate microtubule stability. Indeed, the current literature would argue that microtubule stability, particularly alterations in their dynamics, may be the initial driving force behind many familial and sporadic insults in ALS. Pharmacological stabilization of the microtubule network offers an attractive therapeutic strategy in ALS; indeed it has shown promise in many neurological disorders, ALS included. However, the pathophysiological involvement of MTs and their functions is still poorly understood in ALS. Future investigations will hopefully uncover further therapeutic targets that may aid in combating this awful disease.

Stress granules at the intersection of autophagy and ALS

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal disease caused by loss of upper and lower motor neurons. The majority of ALS cases are classified as sporadic (80-90%), with the remaining considered familial based on patient history. The last decade has seen a surge in the identification of ALS-causing genes - including TARDBP (TDP-43), FUS, MATR3 (Matrin-3), C9ORF72 and several others - providing important insights into the molecular pathways involved in pathogenesis. Most of the protein products of ALS-linked genes fall into two functional categories: RNA-binding/homeostasis and protein-quality control (i.e. autophagy and proteasome). The RNA-binding proteins tend to be aggregation-prone with low-complexity domains similar to the prion-forming domains of yeast. Many also incorporate into stress granules (SGs), which are cytoplasmic ribonucleoprotein complexes that form in response to cellular stress. Mutant forms of TDP-43 and FUS perturb SG dynamics, lengthening their cytoplasmic persistence. Recent evidence suggests that SGs are regulated by the autophagy pathway, suggesting a unifying connection between many of the ALS-linked genes. Persistent SGs may give rise to intractable aggregates that disrupt neuronal homeostasis, thus failure to clear SGs by autophagic processes may promote ALS pathogenesis. This article is part of a Special Issue entitled SI:Autophagy.

Alterations in stress granule dynamics driven by TDP-43 and FUS: a link to pathological inclusions in ALS?

Stress granules (SGs) are RNA-containing cytoplasmic foci formed in response to stress exposure. Since their discovery in 1999, over 120 proteins have been described to be localized to these structures (in 154 publications). Most of these components are RNA binding proteins (RBPs) or are involved in RNA metabolism and translation. SGs have been linked to several pathologies including inflammatory diseases, cancer, viral infection, and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). In ALS and FTD, the majority of cases have no known etiology and exposure to external stress is frequently proposed as a contributor to either disease initiation or the rate of disease progression. Of note, both ALS and FTD are characterized by pathological inclusions, where some well-known SG markers localize with the ALS related proteins TDP-43 and FUS. We propose that TDP-43 and FUS serve as an interface between genetic susceptibility and environmental stress exposure in disease pathogenesis. Here, we will discuss the role of TDP-43 and FUS in SG dynamics and how disease-linked mutations affect this process.

Cellular Signature of SIL1 Depletion: Disease Pathogenesis due to Alterations in Protein Composition Beyond the ER Machinery

SIL1 acts as nucleotide exchange factor for the endoplasmic reticulum chaperone BiP. Mutations of SIL1 cause Marinesco-Sjögren syndrome (MSS), a neurodegenerative disorder. Moreover, a particular function of SIL1 for etiopathology of amyotrophic lateral sclerosis (ALS) was highlighted, thus declaring the functional SIL1-BiP complex as a modifier for neurodegenerative disorders. Thereby, depletion of SIL1 was associated with an earlier manifestation and in strengthened disease progression in ALS. Owing to the absence of appropriate in vitro models, the precise cellular pathophysiological mechanisms leading to neurodegeneration in MSS and triggering the same in further disorders like ALS are still elusive. We found that SIL1 depletion in human embryonic kidney 293 (HEK293) cells led to structural changes of the endoplasmic reticulum (ER) including the nuclear envelope and mitochondrial degeneration that closely mimic pathological alterations in MSS and ALS. Functional studies revealed disturbed protein transport, cytotoxicity with reduced proliferation and viability, accompanied by activation of cellular defense mechanisms including the unfolded protein response, ER-associated degradation pathway, proteolysis, and expression of apoptotic and survival factors. Our data moreover indicated that proteins involved in cytoskeletal organization, vesicular transport, mitochondrial function, and neurological processes contribute to SIL1 pathophysiology. Altered protein expression upon SIL1 depletion in vitro could be confirmed in Sil1-deficient motoneurones for paradigmatic proteins belonging to different functional classes. Our results demonstrate that SIL1-depleted HEK293 cells are an appropriate model to identify proteins modulated by SIL1 expression level and contributing to neurodegeneration in MSS and further disorders like ALS. Thereby, our combined results point out that proteins beyond such involved ER-related protein processing are affected by SIL1 depletion.

The altered autophagy mediated by TFEB in animal and cell models of amyotrophic lateral sclerosis

Autophagy is an intracellular degradation process that clears away aggregated proteins or aged and damaged organelles. Abnormalities in autophagy result in defects in clearance of these misfolded and aggregate proteins, which have been associated with neurodegenerative disorders. A key neuropathological hallmark of amyotrophic lateral sclerosis (ALS) that contributes to the progressive loss of motor neurons is abnormal protein aggregation of mutant Cu/Zn superoxide dismutase1 (SOD1). TFEB is a recently described gene that regulates autophagy. Several studies have reported that autophagy is altered in ALS, but little is known about the role and mechanisms of TFEB-mediated autophagy during the progression of ALS. In this study, altered expression of TFEB and Beclin-1 were detected in the spinal cords of ALS transgenic mice at different stages and in an NSC-34 cell model with the SOD1-G93A mutation using RT-PCR, western blot, and immunohistochemistry. The majority of cells positive for TFEB and Beclin-1 are β-tubulin III-labeled neurons, especially in the anterior horn of the gray matter. Overexpression of TFEB in NSC-34 cells with the SOD1-G93A mutation increased the mRNA and protein levels of Beclin-1, accompanied by increased levels of LC3-II protein. MTS assay revealed that TFEB overexpression increased proliferation and survival of NSC-34 cells with the SOD1-G93A mutation. Our findings suggest that TFEB promotes autophagy by enhancing the expression of Beclin-1. The altered autophagy mediated by TFEB is a key element in the pathogenesis of ALS, making TFEB a very promising target for the development of novel drugs and new gene therapeutics for ALS.

Uncoupling of Protein Aggregation and Neurodegeneration in a Mouse Amyotrophic Lateral Sclerosis Model

Aberrant accumulation of protein aggregates is a pathological hallmark of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Although a buildup of protein aggregates frequently leads to cell death, whether it is the key pathogenic factor in driving neurodegenerative disease remains controversial. HDAC6, a cytosolic ubiquitin-binding deacetylase, has emerged as an important regulator of ubiquitin-dependent quality control autophagy, a lysosome-dependent degradative system responsible for the disposal of misfolded protein aggregates and damaged organelles. Here, we show that in cell models HDAC6 plays a protective role against multiple disease-associated and aggregation-prone cytosolic proteins by facilitating their degradation. We further show that HDAC6 is required for efficient localization of lysosomes to protein aggregates, indicating that lysosome targeting to autophagic substrates is regulated. Supporting a critical role of HDAC6 in protein aggregate disposal in vivo, genetic ablation of HDAC6 in a transgenic SOD1G93A mouse, a model of ALS, leads to dramatic accumulation of ubiquitinated SOD1G93A protein aggregates. Surprisingly, despite a robust buildup of SOD1G93A aggregates, deletion of HDAC6 only moderately modified the motor phenotypes. These findings indicate that SOD1G93A aggregation is not the only determining factor to drive neurodegeneration in ALS, and that HDAC6 likely modulates neurodegeneration through additional mechanisms beyond protein aggregate clearance.

Amyotrophic lateral sclerosis as a spatiotemporal mislocalization disease: location, location, location

Spatiotemporal localization of signals is a fundamental feature impacting cell survival and proper function. The cell needs to respond in an accurate manner in both space and time to both intra- and intercellular environment cues. The regulation of this comprehensive process involves the cytoskeleton and the trafficking machinery, as well as local protein synthesis and ligand-receptor mechanisms. Alterations in such mechanisms can lead to cell dysfunction and disease. Motor neurons that can extend over tens of centimeters are a classic example for the importance of such events. Changes in spatiotemporal localization mechanisms are thought to play a role in motor neuron degeneration that occurs in amyotrophic lateral sclerosis (ALS). In this review we will discuss these mechanisms and argue that possible misregulated factors can lead to motor neuron degeneration in ALS.

Class II HDACs and neuronal regeneration

The vastly more superior regenerative capacity of the axons of peripheral nerves over central nervous system (CNS) neurons has been partly attributed to the former's intrinsic capacity to initiate and sustain the functionality of a new growth cone. Growth cone generation involves a myriad of processes that centers around the organization of microtubule bundles. Histone deacetylases (HDACs) modulate a wide range of key neuronal processes such as neural progenitor differentiation, learning and memory, neuronal death, and degeneration. HDAC inhibitors have been shown to be beneficial in attenuating neuronal death and promoting neurite outgrowth and axonal regeneration. Recent advances have provided insights on how manipulating HDAC activities, particularly the type II HDACs 5 and 6, which deacetylate tubulin, may benefit axonal regeneration. These advances are discussed herein.

Histone deacetylase 6 is a FoxO transcription factor-dependent effector in skeletal muscle atrophy

Skeletal muscle atrophy is a severe condition of muscle mass loss. Muscle atrophy is caused by a down-regulation of protein synthesis and by an increase of protein breakdown due to the ubiquitin-proteasome system and autophagy activation. Up-regulation of specific genes, such as the muscle-specific E3 ubiquitin ligase MAFbx, by FoxO transcription factors is essential to initiate muscle protein ubiquitination and degradation during atrophy. HDAC6 is a particular HDAC, which is functionally related to the ubiquitin proteasome system via its ubiquitin binding domain. We show that HDAC6 is up-regulated during muscle atrophy. HDAC6 activation is dependent on the transcription factor FoxO3a, and the inactivation of HDAC6 in mice protects against muscle wasting. HDAC6 is able to interact with MAFbx, a key ubiquitin ligase involved in muscle atrophy. Our findings demonstrate the implication of HDAC6 in skeletal muscle wasting and identify HDAC6 as a new downstream target of FoxO3a in stress response. This work provides new insights in skeletal muscle atrophy development and opens interesting perspectives on HDAC6 as a valuable marker of muscle atrophy and a potential target for pharmacological treatments.

Exploring new pathways of neurodegeneration in ALS: the role of mitochondria quality control

Neuronal cells are highly dependent on mitochondria, and mitochondrial dysfunction is associated with neurodegenerative diseases. As perturbed mitochondrial function renders neurons extremely sensitive to a wide variety of insults, such as oxidative stress and bioenergetic defects, mitochondrial defects can profoundly affect neuronal fate. Several studies have linked ALS with mitochondrial dysfunction, stemming from observations of mitochondrial abnormalities, both in patients and in cellular and mouse models of familial forms of ALS. Mitochondrial changes have been thoroughly investigated in mutants of superoxide dismutase 1 (SOD1), one of the most common causes of familial ALS, for which excellent cellular and animal models are available, but recently evidence is emerging also in other forms of ALS, both familial and sporadic. Mitochondrial defects in ALS involve many critical physiopathological processes, from defective bioenergetics to abnormal calcium homeostasis, altered morphology and impaired trafficking. In this review, we summarize established evidence of mitochondrial dysfunction in ALS, especially in SOD1 mutant models of familial ALS. The main focus of the review is on defective mitochondrial quality control (MQC) in ALS. MQC operates at multiple levels to clear damaged proteins through proteostasis and to eliminate irreparably damaged organelles through mitophagy. However, since ALS motor neurons progressively accumulate damaged mitochondria, it is plausible that the MQC is ineffective or overwhelmed by excessive workload imposed by the chronic and extensive mitochondrial damage. This article is part of a Special Issue entitled ALS complex pathogenesis.

Tissue-specific deregulation of selected HDACs characterizes ALS progression in mouse models: pharmacological characterization of SIRT1 and SIRT2 pathways

Acetylation homeostasis is thought to play a role in amyotrophic lateral sclerosis, and treatment with inhibitors of histone deacetylases has been considered a potential and attractive therapeutic approach, despite the lack of a thorough study of this class of proteins. In this study, we have considerably extended previous knowledge on the expression of 13 histone deacetylases in tissues (spinal cord and muscle) from mice carrying two different ALS-linked SOD1 mutations (G93A-SOD1 and G86R-SOD1). We have then focused on class III histone deacetylases SIRT1 and SIRT2 that are considered relevant in neurodegenerative diseases. SIRT1 decreases in the spinal cord, but increases in muscle during the progression of the disease, and a similar expression pattern is observed in the corresponding cell models (neuroblastoma and myoblasts). SIRT2 mRNA expression increases in the spinal cord in both G93A-SOD1 and G86R-SOD1 mice but protein expression is substantially unchanged in all the models examined. At variance with other sirtuin modulators (sirtinol, AGK2 and SRT1720), the well-known SIRT1 inhibitor Ex527 has positive effects on survival of neuronal cells expressing mutant SOD1, but this effect is neither mediated by SIRT1 inhibition nor by SIRT2 inhibition. These data call for caution in proposing sirtuin modulation as a target for treatment.

Proteomic analysis reveals differentially regulated protein acetylation in human amyotrophic lateral sclerosis spinal cord

Amyotrophic lateral sclerosis (ALS) is a progressive fatal neurodegenerative disease that primarily affects motor neurons in the brain and spinal cord. Histone deacetylase (HDAC) inhibitors have neuroprotective effects potentially useful for the treatment of neurodegenerative diseases including ALS; however, the molecular mechanisms underlying their potential efficacy is not well understood. Here we report that protein acetylation in urea-soluble proteins is differently regulated in post-mortem ALS spinal cord. Two-dimensional electrophoresis (2-DE) analysis reveals several protein clusters with similar molecular weight but different charge status. Liquid chromatography and tandem mass spectrometry (LC-MS/MS) identifies glial fibrillary acidic protein (GFAP) as the dominant component in the protein clusters. Further analysis indicates six heavily acetylated lysine residues at positions 89, 153, 189, 218, 259 and 331 of GFAP. Immunoprecipitation followed by Western blotting confirms that the larger form of GFAP fragments are acetylated and upregulated in ALS spinal cord. Further studies demonstrate that acetylation of the proteins additional to GFAP is differently regulated, suggesting that acetylation and/or deacetylation play an important role in pathogenesis of ALS.