Therapeutic Implications and Regulations of Protein Post-translational Modifications in Parkinsons Disease

Parkinsons disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron loss and alpha-synuclein aggregation. This comprehensive review examines the intricate role of post-translational modifications (PTMs) in PD pathogenesis, focusing on DNA methylation, histone modifications, phosphorylation, SUMOylation, and ubiquitination. Targeted PTM modulation, particularly in key proteins like Parkin, DJ1, and PINK1, emerges as a promising therapeutic strategy for mitigating dopaminergic degeneration in PD. Dysregulated PTMs significantly contribute to the accumulation of toxic protein aggregates and dopaminergic neuronal dysfunction observed in PD. Targeting PTMs, including epigenetic strategies, addressing aberrant phosphorylation events, and modulating SUMOylation processes, provides potential avenues for intervention. The ubiquitin-proteasome system, governed by enzymes like Parkin and Nedd4, offers potential targets for clearing misfolded proteins and developing disease-modifying interventions. Compounds like ginkgolic acid, SUMO E1 enzyme inhibitors, and natural compounds like Indole-3-carbinol illustrate the feasibility of modulating PTMs for therapeutic purposes in PD. This review underscores the therapeutic potential of PTM-targeted interventions in modulating PD-related pathways, emphasizing the need for further research in this promising area of Parkinsons disease therapeutics.

Preclinical validation of a novel brain-penetrant PET ligand for visualization of histone deacetylase 6: a potential imaging target for neurodegenerative diseases

PURPOSE

Histone deacetylase 6 (HDAC6) has emerged as a therapeutic target for neurodegenerative diseases such as Alzheimer's disease. Noninvasive imaging of HDAC6 in the brain by positron emission tomography (PET) would accelerate research into its roles in these diseases. We recently discovered an 18F-labeled derivative of the selective HDAC6 inhibitor SW-100 ([18F]FSW-100) as a potential candidate for brain HDAC6 radioligand. As a mandatory step prior to clinical translation, we performed preclinical validation of [18F]FSW-100.

METHODS

Process validation of [18F]FSW-100 radiosynthesis for clinical use and assessment of preclinical toxicity and radiation dosimetry estimated from mouse distribution data were performed. In vitro selectivity of FSW-100 for 28 common receptors in the brain and HDAC isoforms was characterized. [18F]FSW-100 PET imaging was performed in non-human primates in a conscious state to estimate the feasibility of HDAC6 imaging in humans.

RESULTS

Three consecutive validation runs of the automated radiosynthesis gave [18F]FSW-100 injections with radiochemical yields of 12%, and the injections conformed to specified quality control criteria for batch release. No acute toxicity was observed for non-radiolabeled FSW-100 or radioactivity decayed [18F]FSW-100 injection, and the former was negative in the Ames test. The whole-body effective dose estimated from biodistribution in mice was within the range of that of previously reported 18F-radioligands in humans. In vitro selectivity against common receptors and other HDAC isoforms was confirmed. [18F]FSW-100 demonstrated good penetration in monkey brain, and in vivo blocking studies suggested that the uptake was specific.

CONCLUSION

These results support the clinical utility of [18F]FSW-100 for in vivo imaging of HDAC6 in the brain.

Autophagy mediates the clearance of oligodendroglial SNCA/alpha-synuclein and TPPP/p25A in multiple system atrophy models

Accumulation of the neuronal protein SNCA/alpha-synuclein and of the oligodendroglial phosphoprotein TPPP/p25A within the glial cytoplasmic inclusions (GCIs) represents the key histophathological hallmark of multiple system atrophy (MSA). Even though the levels/distribution of both oligodendroglial SNCA and TPPP/p25A proteins are critical for disease pathogenesis, the proteolytic mechanisms involved in their turnover in health and disease remain poorly understood. Herein, by pharmacological and molecular modulation of the autophagy-lysosome pathway (ALP) and the proteasome we demonstrate that the endogenous oligodendroglial SNCA and TPPP/p25A are degraded mainly by the ALP in murine primary oligodendrocytes and oligodendroglial cell lines under basal conditions. We also identify a KFERQ-like motif in the TPPP/p25A sequence that enables its effective degradation via chaperone-mediated autophagy (CMA) in an in vitro system of rat brain lysosomes. Furthermore, in a MSA-like setting established by addition of human recombinant SNCA pre-formed fibrils (PFFs) as seeds of pathological SNCA, we thoroughly characterize the contribution of CMA and macroautophagy in particular, in the removal of the exogenously added and the seeded oligodendroglial SNCA pathological assemblies. We also show that PFF treatment impairs autophagic flux and that TPPP/p25A exerts an inhibitory effect on macroautophagy, while at the same time CMA is upregulated to remove the pathological SNCA species formed within oligodendrocytes. Finally, augmentation of CMA or macroautophagy accelerates the removal of the engendered pathological SNCA conformations further suggesting that autophagy targeting may represent a successful approach for the clearance of pathological SNCA and/or TPPP/p25A in the context of MSA.Abbreviations: 3MA: 3-methyladenine; ACTB: actin, beta; ALP: autophagy-lysosome pathway; ATG5: autophagy related 5; AR7: atypical retinoid 7; CMA: chaperone-mediated autophagy; CMV: cytomegalovirus; CTSD: cathepsin D; DAPI: 4',6-diamidino-2-phenylindole; DMEM: Dulbecco's modified Eagle's medium; Epox: epoxomicin; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GCIs: glial cytoplasmic inclusions; GFP: green fluorescent protein; HMW: high molecular weight; h: hours; HSPA8/HSC70: heat shock protein 8; LAMP1: lysosomal-associated membrane protein 1; LAMP2A: lysosomal-associated membrane protein 2A; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mcherry: monomeric cherry; MFI: mean fluorescence intensity; mRFP: monomeric red fluorescent protein; MSA: multiple system atrophy; OLN: oligodendrocytes; OPCs: oligodendroglial progenitor cells; PBS: phosphate-buffered saline; PC12: pheochromocytoma cell line; PD: Parkinson disease; PFFs: pre-formed fibrils; PIs: protease inhibitors; PSMB5: proteasome (prosome, macropain) subunit, beta type 5; Rap: rapamycin; RFP: red fluorescent protein; Scr: scrambled; SDS: sodium dodecyl sulfate; SE: standard error; siRNAs: small interfering RNAs; SNCA: synuclein, alpha; SQSTM1: sequestosome 1; TPPP: tubulin polymerization promoting protein; TUBA: tubulin, alpha; UPS: ubiquitin-proteasome system; WT: wild type.

An Update on the Critical Role of α-Synuclein in Parkinson's Disease and Other Synucleinopathies: from Tissue to Cellular and Molecular Levels

The aggregation of alpha-synuclein (α-Syn) plays a critical role in the development of Parkinson's disease (PD) and other synucleinopathies. α-Syn, which is encoded by the SNCA gene, is a lysine-rich soluble amphipathic protein normally expressed in neurons. Located in the cytosolic domain, this protein has the ability to remodel itself in plasma membranes, where it assumes an alpha-helix conformation. However, the protein can also adopt another conformation rich in cross-beta sheets, undergoing mutations and post-translational modifications, then leading the protein to an unusual aggregation in the form of Lewy bodies (LB), which are cytoplasmic inclusions constituted predominantly by α-Syn. Pathogenic mechanisms affecting the structural and functional stability of α-Syn - such as endoplasmic reticulum stress, Golgi complex fragmentation, disfunctional protein degradation systems, aberrant interactions with mitochondrial membranes and nuclear DNA, altered cytoskeleton dynamics, disrupted neuronal plasmatic membrane, dysfunctional vesicular transport, and formation of extracellular toxic aggregates - contribute all to the pathogenic progression of PD and synucleinopathies. In this review, we describe the collective knowledge on this topic and provide an update on the critical role of α-Syn aggregates, both at the cellular and molecular levels, in the deregulation of organelles affecting the cellular homeostasis and leading to neuronal cell death in PD and other synucleinopathies.

Neurons and Glia Interplay in α-Synucleinopathies

Accumulation of the neuronal presynaptic protein alpha-synuclein within proteinaceous inclusions represents the key histophathological hallmark of a spectrum of neurodegenerative disorders, referred to by the umbrella term a-synucleinopathies. Even though alpha-synuclein is expressed predominantly in neurons, pathological aggregates of the protein are also found in the glial cells of the brain. In Parkinson's disease and dementia with Lewy bodies, alpha-synuclein accumulates mainly in neurons forming the Lewy bodies and Lewy neurites, whereas in multiple system atrophy, the protein aggregates mostly in the glial cytoplasmic inclusions within oligodendrocytes. In addition, astrogliosis and microgliosis are found in the synucleinopathy brains, whereas both astrocytes and microglia internalize alpha-synuclein and contribute to the spread of pathology. The mechanisms underlying the pathological accumulation of alpha-synuclein in glial cells that under physiological conditions express low to non-detectable levels of the protein are an area of intense research. Undoubtedly, the presence of aggregated alpha-synuclein can disrupt glial function in general and can contribute to neurodegeneration through numerous pathways. Herein, we summarize the current knowledge on the role of alpha-synuclein in both neurons and glia, highlighting the contribution of the neuron-glia connectome in the disease initiation and progression, which may represent potential therapeutic target for a-synucleinopathies.

Preclinical Evaluation of an 18F-Labeled SW-100 Derivative for PET Imaging of Histone Deacetylase 6 in the Brain

Histone deacetylase 6 (HDAC6), an enzyme involved in protein degradation, exhibits several unique properties, such as cytoplasmic localization and ubiquitin binding. HDAC6 has emerged as an interesting therapeutic target in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Techniques enabling noninvasive HDAC6 imaging in the brain could enhance understanding of its pathologic role, but development of brain-penetrating radioligands for HDACs imaging by positron emission tomography (PET) remains challenging. Here, we report the synthesis and evaluation of an ¹⁸F-labeled tetrahydroquinoline derivative, [¹⁸F]2, based on the HDAC6 selective inhibitor SW-100 as a brain HDAC6 imaging radioligand. [¹⁸F]2 was synthesized via copper-mediated radiofluorination from an arylboronic precursor, followed by removal of the catalyst by solid-phase extraction and then hydroxamic acid formation. [¹⁸F]2 demonstrated good penetration and moderate stability in the mouse brain. In mouse plasma, however, [¹⁸F]2 was rapidly metabolized to a corresponding carboxylic acid form. Blocking studies in mice with unlabeled compound 2 and HDAC6 selective inhibitors, including tubastatin A and ACY-775, demonstrated that the HDAC6 inhibitors displaced over 80% of [¹⁸F]2 taken up in the brain, indicating selective binding of [¹⁸F]2. These results suggest that [¹⁸F]2 is a potentially useful PET radioligand for brain HDAC6 imaging.

HDAC6 ZnF UBP as the Modifier of Tau Structure and Function

Histone deacetylase 6 is a class II histone deacetylase primarily present in the cytoplasm and involved in the regulation of various cellular functions. It consists of two catalytic deacetylase domains and a unique zinc finger ubiquitin binding protein domain, which sets it apart from other HDACs. HDAC6 is known to regulate cellular activities by modifying the function of microtubules, HSP90, and cortactin through deacetylation. Apart from the catalytic activity of HDAC6, it interacts with other proteins through either the SE14 domain or the ZnF UBP domain to modulate their functions. Here, we have studied the role of the HDAC6 ZnF UBP domain as a modifier of Tau aggregation by its direct interaction with the polyproline region/repeat region of Tau. Interaction of HDAC6 ZnF UBP with Tau was found to reduce the propensity of Tau to self-aggregate and to disaggregate preformed aggregates in a concentration-dependent manner and also bring about the conformational changes in Tau protein. The interaction of HDAC6 ZnF UBP with Tau results in its degradation, suggesting either proteolytic activity of HDAC6 ZnF UBP or its role in enhancing autoproteolysis of Tau.

Histone Deacetylase 6 and the Disease Mechanisms of α-Synucleinopathies

The abnormal accumulation of α-Synuclein (α-Syn) is a prominent pathological feature in a group of diseases called α-Synucleinopathies, such as Parkinson’s disease, dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). The formation of Lewy bodies (LBs) and glial cytoplasmic inclusions (GCIs) in neurons and oligodendrocytes, respectively, is highly investigated. However, the molecular mechanisms behind α-Syn improper folding and aggregation remain unclear. Histone deacetylase 6 (HDAC6) is a Class II deacetylase, containing two active catalytic domains and a ubiquitin-binding domain. The properties of HDAC6 and its exclusive cytoplasmic localization allow HDAC6 to modulate the microtubule dynamics, acting as a specific α-tubulin deacetylase. Also, HDAC6 can bind ubiquitinated proteins, facilitating the formation of the aggresome, a cellular defense mechanism to cope with higher levels of misfolded proteins. Several studies report that the aggresome shares similarities in size and composition with LBs and GCIs. HDAC6 is found to co-localize with α-Syn in neurons and in oligodendrocytes, together with other aggresome-related proteins. The involvement of HDAC6 in several neurodegenerative diseases is already under discussion, however, the results obtained by modulating HDAC6 activity are not entirely conclusive. The main goal of this review is to summarize and critically discuss previous in vitro and in vivo data regarding the specific role of HDAC6 in the context of α-Syn accumulation and protein aggregation in α-Synucleinopathies.

Phospho-HDAC6 Gathers Into Protein Aggregates in Parkinson's Disease and Atypical Parkinsonisms

HDAC6 is a unique histone deacetylase that targets cytoplasmic non-histone proteins and has a specific ubiquitin-binding activity. Both of these activities are required for HDAC6-mediated formation of aggresomes, which contain misfolded proteins that will ultimately be degraded via autophagy. HDAC6 deacetylase activity is increased following phosphorylation on serine 22 (phospho-HDAC6). In human, HDAC6 localizes in neuronal Lewy bodies in Parkinson’s disease (PD) and in oligodendrocytic Papp–Lantos bodies in multiple system atrophy (MSA). However, the expression of phospho-HDAC6 in post-mortem human brains is currently unexplored. Here, we evaluate and compare the distribution of HDAC6 and its phosphorylated form in human brains obtained from patients affected by three forms of parkinsonism: two synucleinopathies (PD and MSA) and a tauopathy (progressive supranuclear palsy, PSP). We find that both HDAC6 and its phosphorylated form localize with pathological protein aggregates, including α-synuclein-positive Lewy bodies in PD and Papp–Lantos bodies in MSA, and phospho-tau-positive neurofibrillary tangles in PSP. We further find a direct interaction of HDAC6 with α-synuclein with proximity ligation assay (PLA) in neuronal cell of PD patients. Taken together, our findings suggest that both HDAC6 and phospho-HDAC6 regulate the homeostasis of intra-neuronal proteins in parkinsonism.

Chaperones and Proteostasis: Role in Parkinson's Disease

Proper folding to attain a defined three-dimensional structure is a prerequisite for the functionality of a protein. Improper folding that eventually leads to formation of protein aggregates is a hallmark of several neurodegenerative disorders. Loss of protein homeostasis triggered by cellular stress conditions is a major contributing factor for the formation of these toxic aggregates. A conserved class of proteins called chaperones and co-chaperones is implicated in maintaining the cellular protein homeostasis. Expanding the body of evidence highlights the role of chaperones as central mediators in the formation, de-aggregation and degradation of the aggregates. Altered expression and function of chaperones is associated with many neurodegenerative diseases including Parkinson’s disease. Several studies indicate that chaperones are at the center of the cause and effect cycle of this disease. An overview of the various chaperones that are associated with homeostasis of Parkinson’s disease-related proteins and their role in pathogenicity will be discussed in this review.

Insights into the pathogenesis of multiple system atrophy: focus on glial cytoplasmic inclusions

Multiple system atrophy (MSA) is a debilitating and fatal neurodegenerative disorder. The disease severity warrants urgent development of disease-modifying therapy, but the disease pathogenesis is still enigmatic. Neurodegeneration in MSA brains is preceded by the emergence of glial cytoplasmic inclusions (GCIs), which are insoluble α-synuclein accumulations within oligodendrocytes (OLGs). Thus, preventive strategies against GCI formation may suppress disease progression. However, although numerous studies have tried to elucidate the molecular pathogenesis of GCI formation, difficulty remains in understanding the pathological interaction between the two pivotal aspects of GCIs; α-synuclein and OLGs. The difficulty originates from several enigmas: 1) what triggers the initial generation and possible propagation of pathogenic α-synuclein species? 2) what contributes to OLG-specific accumulation of α-synuclein, which is abundantly expressed in neurons but not in OLGs? and 3) how are OLGs and other glial cells affected and contribute to neurodegeneration? The primary pathogenesis of GCIs may involve myelin dysfunction and dyshomeostasis of the oligodendroglial cellular environment such as autophagy and iron metabolism. We have previously reported that oligodendrocyte precursor cells are more prone to develop intracellular inclusions in the presence of extracellular fibrillary α-synuclein. This finding implies a possibility that the propagation of GCI pathology in MSA brains is mediated through the internalization of pathological α-synuclein into oligodendrocyte precursor cells. In this review, in order to discuss the pathogenesis of GCIs, we will focus on the composition of neuronal and oligodendroglial inclusions in synucleinopathies. Furthermore, we will introduce some hypotheses on how α-synuclein pathology spreads among OLGs in MSA brains, in the light of our data from the experiments with primary oligodendrocyte lineage cell culture. While various reports have focused on the mysterious source of α-synuclein in GCIs, insights into the mechanism which regulates the uptake of pathological α-synuclein into oligodendroglial cells may yield the development of the disease-modifying therapy for MSA. The interaction between glial cells and α-synuclein is also highlighted with previous studies of post-mortem human brains, cultured cells, and animal models, which provide comprehensive insight into GCIs and the MSA pathomechanisms.

A New Histone Deacetylase Inhibitor Enhances Radiation Sensitivity through the Induction of Misfolded Protein Aggregation and Autophagy in Triple-Negative Breast Cancer

Radiation therapy (RT) is one of the main treatments for triple-negative breast cancer (TNBC). However, many patients experience RT failure due to the metastatic potential of RT and the radiation resistance of several cancers. Histone deacetylase inhibitors (HDACis) can serve as radiosensitizers. In this study, we investigated whether a novel HDACi, TMU-35435, could reinforce radiosensitivity through the induction of misfolded protein aggregation and autophagy in TNBC. Significantly enhanced toxicity was found for the combination treatment compared with TMU-35435 or irradiation (IR) treatment alone in TNBC cells. The combination treatment induced misfolded protein aggregation and TMU-35435 inhibited the interaction of HDAC6 with dynein. Furthermore, the combined treatment induced endoplasmic reticulum (ER) stress but did not trigger apoptosis. In addition, the combination treatment caused autophagic cell death. Tumor growth in the mouse of model orthotopic breast cancer was suppressed by the combination treatment through the induction of ER stress and autophagy. These findings support the future evaluation of the novel HDACi TMU-35435, as a potent radiosensitizer in TNBC.

The role of lysosomes and autophagosomes in frontotemporal lobar degeneration

INTRODUCTION

Cell biological and genetic evidence implicate failures in degrading aggregating proteins, such as tau and TDP-43, through the autophagy or lysosomal pathways in the pathogenesis of frontotemporal lobar degeneration (FTLD).

METHODS

We investigated changes in the degradative pathways in 60 patients with different pathological or genetic forms of FTLD employing immunohistochemistry for marker proteins such as lysosomal-associated membrane proteins 1 (LAMP-1) and 2 (LAMP-2), cathepsin D (CTSD) and microtubule-associated protein 1 light chain 3 alpha (LC3A). Immunostained sections were qualitatively and semi-quantitatively assessed for the appearance, distribution and intensity of staining in neurones of the dentate gyrus (DG) and CA4 region of the hippocampus, and the temporal cortex (Tcx).

RESULTS

Lower levels of neuronal LAMP-1 immunostaining were present in the DG and Tcx in FTLD-tau compared to FTLD-TDP. There was less LAMP-1 immunostaining in FTLD-tau with MAPT mutations, and FTLD-tau with Pick bodies, compared to FTLD-TDP types A and B, and less LAMP-1 immunostaining in FTLD-TDP type C than in FTLD-TDP types A and B. There was greater LAMP-1 immunostaining in GRN mutation which may reflect the underlying type A histology rather than mutation. There were no differences in neuronal LAMP-2, CTSD, EEA-1 or LC3A immunostaining between any of the five FTLD histological or four genetic groups, nor between FTLD-TDP and FTLD-tau.

CONCLUSIONS

The underlying pathological mechanism in FTLD-tau may lie with a relative deficiency of lysosomes, or defective vesicular transport, whereas the failure to clear TDP-43 aggregates may lie with lysosomal dysfunction rather than a lack of available lysosomes or degradative enzymes.

TIGAR inclusion pathology is specific for Lewy body diseases

BACKGROUND

We previously reported up-regulation of tigarb (the zebrafish orthologue of human TIGAR, TP53 - Induced Glycolysis and Apoptosis Regulator) in a zebrafish pink1^-/- model of Parkinson's disease (PD). Genetic inactivation of tigarb led to the rescue of dopaminergic neurons and mitochondrial function in pink^-/- zebrafish. The aim of this study was to determine the relevance of TIGAR for human PD, investigate its disease specificity and identify relevant upstream and downstream mechanisms.

MATERIALS AND METHODS

TIGAR Immunohistochemistry, using a range of antibodies, was undertaken for detailed assessment of TIGAR in formalin-fixed, paraffin-embedded tissue from post mortem brains of PD patients and other neurodegenerative disorders (n = 10 controls, 10 PD cases, 10 dementia with Lewy bodies, 5 motor neurone disease (MND), 3 multiple system atrophy (MSA)) and complemented by immunohistochemistry for p53, hexokinase I (HK-I) and hexokinase II (HK-II; n = 4 control, 4 PD, and 4 dementia with Lewy bodies).

RESULTS

TIGAR was detected in Lewy bodies and Lewy neurites in the substantia nigra of sporadic PD and Dementia with Lewy bodies (DLB) patients. Staining of adjacent sections and double staining confirmed the presence of TIGAR alongside alpha-synuclein in these LB and neurites. In contrast, TIGAR-positive aggregates were not seen in cortical Lewy bodies. TIGAR protein was also absent in both TDP-43-positive inclusions in MND and glial cytoplasmic inclusions in MSA. Subsequent investigation of the TIGAR-upstream regulator p53 and the downstream targets HK-I and HK-II in PD brains suggested a possible mild increase in HK-I.

CONCLUSIONS

TIGAR protein, is present in SN Lewy bodies of both sporadic PD and DLB. The absence of TIGAR protein in the pathological inclusions of MND or MSA suggests disease specificity and further raises the possibility that TIGAR may be involved in PD pathogenesis.

Dynactin is involved in Lewy body pathology

Dynactin forms a protein complex with dynein that retrogradely transports cargo along microtubules. Dysfunction of this dynein-dynactin complex causes several neurodegenerative diseases such as Perry syndrome, motor neuron diseases and progressive supranuclear palsy. Recently, we reported colocalization of phosphorylated α-synuclein (p-SNCA) and the largest subunit of dynactin (DCTN1) in Lewy body (LB)-like structures in Perry syndrome. Previous reports have not focused on the relationship between dynactin and synucleinopathies. Thus, we examined autopsied human brains from patients with Parkinson's disease, dementia with LBs, and multiple system atrophy using immunohistochemistry for p-SNCA, DCTN1, dynactin 2 (DCTN2, dynamitin) and dynein cytoplasmic 1 intermediate chain 1 (DYNC1I1). We also examined microtubule affinity-regulating kinases (MARKs), which phosphorylate microtubule-associated proteins and trigger microtubule disruption. Both brainstem-type and cortical LBs were immunopositive for DCTN1, DCTN2, DYNC1I1 and p-MARK and their staining often overlapped with p-SNCA. Lewy neurites were also immunopositive for DCTN1, DCTN2 and DYNC1I1. However, p-SNCA-positive inclusions of multiple system atrophy, which included both glial and neuronal cytoplasmic inclusions, were immunonegative for DCTN1, DCTN2, DYNC1I1 and p-MARK. Thus, immunohistochemistry for dynein-dynactin complex molecules, especially DCTN1, can clearly distinguish LBs from neuronal cytoplasmic inclusions. Our results suggest that dynactin is closely associated with LB pathology.

The relationship between HDAC6, CXCR3, and SIRT1 genes expression levels with progression of primary open-angle glaucoma

BACKGROUND

Primary open-angle glaucoma (POAG) belongs to neurodegenerative diseases. Its etiology is not fully understood. However, a lot of reports have indicated that many biochemical molecules are involved in the retinal ganglion cell damage. Therefore, the purpose of this study was to evaluate a relationship between HDAC6, CXCR3, and SIRT1 genes expression levels with the occurrence risk of POAG and its progression.

MATERIALS AND METHODS

The study included 34 glaucoma patients and 32 subjects without glaucoma symptoms. RNA was isolated from peripheral blood lymphocytes. Level of mRNA expression was determined by real-time PCR method.

RESULTS

Our results have shown significant association of the HDAC6 and SIRT1 expression levels with progression of POAG according to rim area (RA) value, p = 0.041; p = 0.012. Moreover, the analysis of the CXCR3 expression level showed a correlation with progression of POAG based on RA and cup disc ratio (c/d) value, p = 0.006 and p = 0.012, respectively.

CONCLUSIONS

The expression level of HDAC6, CXCR3, and SIRT1 genes may be involved in the progression of POAG.

microRNAs in Parkinson's Disease: From Pathogenesis to Novel Diagnostic and Therapeutic Approaches

Parkinson’s disease (PD) is the most prevalent central nervous system (CNS) movement disorder and the second most common neurodegenerative disease overall. PD is characterized by the progressive loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNpc) within the midbrain, accumulation of alpha-synuclein (α-SYN) in Lewy bodies and neurites and excessive neuroinflammation. The neurodegenerative processes typically begin decades before the appearance of clinical symptoms. Therefore, the diagnosis is achievable only when the majority of the relevant DAergic neurons have already died and for that reason available treatments are only palliative at best. The causes and mechanism(s) of this devastating disease are ill-defined but complex interactions between genetic susceptibility and environmental factors are considered major contributors to the etiology of PD. In addition to the role of classical gene mutations in PD, the importance of regulatory elements modulating gene expression has been increasingly recognized. One example is the critical role played by microRNAs (miRNAs) in the development and homeostasis of distinct populations of neurons within the CNS and, in particular, in the context of PD. Recent reports demonstrate how distinct miRNAs are involved in the regulation of PD genes, whereas profiling approaches are unveiling variations in the abundance of certain miRNAs possibly relevant either to the onset or to the progression of the disease. In this review, we provide an overview of the miRNAs recently found to be implicated in PD etiology, with particular focus on their potential relevance as PD biomarkers, as well as their possible use in PD targeted therapy.

Alteration of autophagy-related proteins in peripheral blood mononuclear cells of patients with Parkinson's disease

Previous postmortem studies demonstrated dysregulation of autophagy in patients with Parkinson's disease (PD). To clarify whether this alteration reflects a fundamental aspect of PD or represents the final stage of autophagy dysregulation resulting from a long neurodegenerative process, we focused on basal autophagy in peripheral blood mononuclear cells (PBMCs) of PD patients (n = 35) and controls (n = 23). The whole-transcriptome assay revealed downregulation of mRNAs for 6 core regulators of autophagy (UNC-51-like kinase [ULK] 3, autophagy-related [Atg] 2A, Atg4B, Atg5, Atg16L1, and histone deacetylase 6). Reverse transcription-polymerase chain reaction and Western blot analysis confirmed significantly increased protein levels of upstream autophagy (ULK1, Beclin1, and autophagy/beclin1 regulator 1) with negative feedback of mRNA expression for these proteins in PD. These protein levels were correlated with increased levels of α-synuclein in PBMCs. The expression level of the oligomeric form of α-synuclein in PBMCs paralleled the clinical severity of PD and the degeneration of cardiac sympathetic nerves. Basal activity of autophagy can be lower in patients with PD. Alteration of basal autophagy may be a fundamental aspect of PD.

The Neuroprotective Role of Protein Quality Control in Halting the Development of Alpha-Synuclein Pathology

Synucleinopathies are a family of neurodegenerative disorders that comprises Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Each of these disorders is characterized by devastating motor, cognitive, and autonomic consequences. Current treatments for synucleinopathies are not curative and are limited to improvement of quality of life for affected individuals. Although the underlying causes of these diseases are unknown, a shared pathological hallmark is the presence of proteinaceous inclusions containing the α-synuclein (α-syn) protein in brain tissue. In the past few years, it has been proposed that these inclusions arise from the self-templated, prion-like spreading of misfolded and aggregated forms of α-syn throughout the brain, leading to neuronal dysfunction and death. In this review, we describe how impaired protein homeostasis is a prominent factor in the α-syn aggregation cascade, with alterations in protein quality control (PQC) pathways observed in the brains of patients. We discuss how PQC modulates α-syn accumulation, misfolding and aggregation primarily through chaperoning activity, proteasomal degradation, and lysosome-mediated degradation. Finally, we provide an overview of experimental data indicating that targeting PQC pathways is a promising avenue to explore in the design of novel neuroprotective approaches that could impede the spreading of α-syn pathology and thus provide a curative treatment for synucleinopathies.

Novel Proteasome Inhibitors and Histone Deacetylase Inhibitors: Progress in Myeloma Therapeutics

The unfolded protein response is responsible for the detection of misfolded proteins and the coordination of their disposal and is necessary to maintain the cellular homoeostasis. Multiple myeloma cells secrete large amounts of immunoglobulins, proteins that need to be correctly folded by the chaperone system. If this process fails, the misfolded proteins have to be eliminated by the two main garbage-disposal systems of the cell: proteasome and aggresome. The blockade of either of these systems will result in accumulation of immunoglobulins and other toxic proteins in the cytoplasm and cell death. The simultaneous inhibition of the proteasome, by proteasome inhibitors (PIs) and the aggresome, by histone deacetylase inhibitors (HDACi) results in a synergistic increase in cytotoxicity in myeloma cell lines. This review provides an overview of mechanisms of action of second-generation PIs and HDACi in multiple myeloma (MM), the clinical results currently observed with these agents and assesses the potential therapeutic impact of the different agents in the two classes. The second-generation PIs offer benefits in terms of increased efficacy, reduced neurotoxicity as off-target effect and may overcome resistance to bortezomib because of their different chemical structure, mechanism of action and biological properties. HDACi with anti-myeloma activity in clinical development discussed in this review include vorinostat, panobinostat and selective HDAC6 inhibitor, ricolinostat.

AMBRA1, a novel α-synuclein-binding protein, is implicated in the pathogenesis of multiple system atrophy

The accumulation of abnormal α-synuclein is the major histopathological feature of Lewy body disease and multiple system atrophy (MSA), which are referred to as synucleinopathies. Cytoplasmic degradation systems, such as the autophagy-lysosome and proteasome pathways, are involved in their pathogenesis. Autophagy is tightly regulated by several upstream proteins including UNC-51-like kinase 1/2, beclin1, vacuolar protein sorting-associated protein 34 and autophagy/beclin1 regulator 1 (AMBRA1). Recently, we revealed that both cortical and brainstem-type Lewy bodies were immunopositive for several upstream proteins of autophagy. Therefore, we conducted the present study to elucidate the role of upstream proteins of autophagy in the pathogenesis of MSA. Pathological and biochemical analyses using human brain samples revealed that AMBRA1 is a component of the pathological hallmarks of MSA and upstream proteins of autophagy are impaired in the MSA brain. In vitro and in vivo analyses revealed a ninefold stronger affinity of AMBRA1 with α-synuclein phosphorylated at serine 129 compared with non-phosphorylated α-synuclein. Furthermore, a weak but significant correlation between AMBRA1 overexpression and reduction of abnormal α-synuclein was observed. Silencing AMBRA1 function caused aggregates of α-synuclein in the cytoplasm of mouse primary cultured neurons, which was simulated by the treatment of Bafilomycin, an autophagy inhibitor. Our results demonstrated for the first time that AMBRA1 is a novel hub binding protein of α-synuclein and plays a central role in the pathogenesis of MSA through the degradative dynamics of α-synuclein. These results raise the possibility that molecular modulation targeting AMBRA1 can be a promising candidate for the treatment of synucleinopathies.

Protein aggregate formation in oligodendrocytes: tau and the cytoskeleton at the intersection of neuroprotection and neurodegeneration

Oligodendrocytes are dependent on an intact, dynamic microtubule (MT) network, which participates in the elaboration and stabilization of myelin forming extensions, and is essential for cellular sorting processes. The microtubule-associated protein tau is constituent of oligodendrocytes. During culture maturation it is developmentally regulated and important for MT stability, MT formation and intracellular trafficking. Downregulation of tau impairs process outgrowth and the transport of myelin basic protein (MBP) mRNA to the cell periphery. Cells fail to differentiate into MBP-expressing, sheet-forming oligodendrocytes. Tau-positive inclusions originating in oligodendrocytes and white matter pathology are prominent in frontotemporal dementias, such as Pick's disease, progressive supranuclear palsy and corticobasal degeneration. An impairment or overload of the proteolytic degradation systems, i.e. the ubiquitin proteasomal system and the lysosomal degradation pathway, has been connected to the formation of protein aggregates. Large protein aggregates are excluded from the proteasome and degraded by autophagy, which is a highly selective process and requires receptor proteins for ubiquitinated proteins, including histone deacetylase 6 (HDAC6). HDAC6 is present in oligodendrocytes, and α-tubulin and tau are substrates of HDAC6. In this review our current knowledge of the role of tau and protein aggregate formation in oligodendrocyte cell culture systems is summarized.

Changes in the miRNA-mRNA Regulatory Network Precede Motor Symptoms in a Mouse Model of Multiple System Atrophy: Clinical Implications

Multiple system atrophy (MSA) is a fatal rapidly progressive α-synucleinopathy, characterized by α-synuclein accumulation in oligodendrocytes. It is accepted that the pathological α-synuclein accumulation in the brain of MSA patients plays a leading role in the disease process, but little is known about the events in the early stages of the disease. In this study we aimed to define potential roles of the miRNA-mRNA regulatory network in the early pre-motor stages of the disease, i.e., downstream of α-synuclein accumulation in oligodendroglia, as assessed in a transgenic mouse model of MSA. We investigated the expression patterns of miRNAs and their mRNA targets in substantia nigra (SN) and striatum, two brain regions that undergo neurodegeneration at a later stage in the MSA model, by microarray and RNA-seq analysis, respectively. Analysis was performed at a time point when α-synuclein accumulation was already present in oligodendrocytes at neuropathological examination, but no neuronal loss nor deficits of motor function had yet occurred. Our data provide a first evidence for the leading role of gene dysregulation associated with deficits in immune and inflammatory responses in the very early, non-symptomatic disease stages of MSA. While dysfunctional homeostasis and oxidative stress were prominent in SN in the early stages of MSA, in striatum differential gene expression in the non-symptomatic phase was linked to oligodendroglial dysfunction, disturbed protein handling, lipid metabolism, transmembrane transport and altered cell death control, respectively. A large number of putative miRNA-mRNAs interaction partners were identified in relation to the control of these processes in the MSA model. Our results support the role of early changes in the miRNA-mRNA regulatory network in the pathogenesis of MSA preceding the clinical onset of the disease. The findings thus contribute to understanding the disease process and are likely to pave the way towards identifying disease biomarkers for early diagnosis of MSA.

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.

Alteration of Upstream Autophagy-Related Proteins (ULK1, ULK2, Beclin1, VPS34 and AMBRA1) in Lewy Body Disease

Autophagy is associated with the pathogenesis of Lewy body disease, including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). It is known that several downstream autophagosomal proteins are incorporated into Lewy bodies (LBs). We performed immunostaining and Western blot analysis using a cellular model of PD and human brain samples to investigate the involvement of upstream autophagosomal proteins (ULK1, ULK2, Beclin1, VPS34 and AMBRA1), which initiate autophagy and form autophagosomes. Time course analysis of cultured cells transfected with flag-α-synuclein and synphilin-1 revealed upregulation of these upstream proteins with accumulation of LB-like inclusions. In human specimens, only mature LBs were positive for upstream autophagosomal proteins. Western blotting of fractionated brain lysates showed that upstream autophagosomal proteins were detected in the soluble and insoluble fraction in DLB, corresponding to the bands of phosphorylated α-synuclein. However, Western blot analysis of total brain lysates in PD and DLB showed that the increase of upstream autophagosomal proteins was only partial. The quantitative, qualitative and locational alteration of upstream autophagosomal proteins in the present study indicates their involvement in the pathogenesis of LB disease. Our data also suggest that misinduction or impairment of upstream autophagy might occur in the disease process of LB disease.

Direct and/or Indirect Roles for SUMO in Modulating Alpha-Synuclein Toxicity

α-Synuclein inclusion bodies are a pathological hallmark of several neurodegenerative diseases, including Parkinson’s disease, and contain aggregated α-synuclein and a variety of recruited factors, including protein chaperones, proteasome components, ubiquitin and the small ubiquitin-like modifier, SUMO-1. Cell culture and animal model studies suggest that misfolded, aggregated α-synuclein is actively translocated via the cytoskeletal system to a region of the cell where other factors that help to lessen the toxic effects can also be recruited. SUMO-1 covalently conjugates to various intracellular target proteins in a way analogous to ubiquitination to alter cellular distribution, function and metabolism and also plays an important role in a growing list of cellular pathways, including exosome secretion and apoptosis. Furthermore, SUMO-1 modified proteins have recently been linked to cell stress responses, such as oxidative stress response and heat shock response, with increased SUMOylation being neuroprotective in some cases. Several recent studies have linked SUMOylation to the ubiquitin-proteasome system, while other evidence implicates the lysosomal pathway. Other reports depict a direct mechanism whereby sumoylation reduced the aggregation tendency of α-synuclein, and reduced the toxicity. However, the precise role of SUMO-1 in neurodegeneration remains unclear. In this review, we explore the potential direct or indirect role(s) of SUMO-1 in the cellular response to misfolded α-synuclein in neurodegenerative disorders.

Glia and alpha-synuclein in neurodegeneration: A complex interaction

α-Synucleinopathies (ASP) comprise adult-onset, progressive neurodegenerative disorders such as Parkinson's disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA) that are characterized by α-synuclein (AS) aggregates in neurons or glia. PD and DLB feature neuronal AS-positive inclusions termed Lewy bodies (LB) whereas glial cytoplasmic inclusions (GCIs, Papp-Lantos bodies) are recognized as the defining hallmark of MSA. Furthermore, AS-positive cytoplasmic aggregates may also be seen in astroglial cells of PD/DLB and MSA brains. The glial AS-inclusions appear to trigger reduced trophic support resulting in neuronal loss. Moreover, microgliosis and astrogliosis can be found throughout the neurodegenerative brain and both are key players in the initiation and progression of ASP. In this review, we will highlight AS-dependent alterations of glial function and their impact on neuronal vulnerability thereby providing a detailed summary on the multifaceted role of glia in ASP.

Sigma-1 receptor is involved in degradation of intranuclear inclusions in a cellular model of Huntington's disease

The sigma-1 receptor (SIGMAR1) is one of the endoplasmic reticulum (ER) chaperones, which participate in the degradation of misfolded proteins via the ER-related degradation machinery linked to the ubiquitin-proteasome pathway. ER dysfunction in the formation of inclusion bodies in various neurodegenerative diseases has also become evident. Recently, we demonstrated that accumulation of SIGMAR1 was common to neuronal nuclear inclusions in polyglutamine diseases including Huntington's disease. Our study also indicated that SIGMAR1 might shuttle between the cytoplasm and the nucleus. In the present study, we investigated the role of SIGMAR1 in nuclear inclusion (NI) formation, using HeLa cells transfected with N-terminal mutant huntingtin. Cell harboring the mutant huntingtin produced SIGMAR1-positive NIs. SIGMAR1 siRNA and a specific inhibitor of the proteasome (epoxomicin) caused significant accumulation of aggregates in the cytoplasm and nucleus. A specific inhibitor of exportin 1 (leptomycin B) also caused NIs. Huntingtin became insolubilized in Western blot analysis after treatments with SIGMAR1 siRNA and epoxomicin. Furthermore, proteasome activity increased chronologically along with the accumulation of mutant huntingtin, but was significantly reduced in cells transfected with SIGMAR1 siRNA. By contrast, overexpression of SIGMAR1 reduced the accumulation of NIs containing mutant huntingtin. Although the LC3-I level was decreased in cells treated with both SIGMAR1 siRNA and control siRNA, the levels of LC3-II and p62 were unchanged. SIGMAR1 agonist and antagonist had no effect on cellular viability and proteasome activity. These findings suggest that the ubiquitin-proteasome pathway is implicated in NI formation, and that SIGMAR1 degrades aberrant proteins in the nucleus via the ER-related degradation machinery. SIGMAR1 might be a promising candidate for therapy of Huntington's disease.

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.

An acetylation switch controls TDP-43 function and aggregation propensity

TDP-43 pathology is a disease hallmark that characterizes amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP). Although a critical role for TDP-43 as an RNA-binding protein has emerged, the regulation of TDP-43 function is poorly understood. Here, we identify lysine acetylation as a novel post-translational modification controlling TDP-43 function and aggregation. We provide evidence that TDP-43 acetylation impairs RNA binding and promotes accumulation of insoluble, hyper-phosphorylated TDP-43 species that largely resemble pathological inclusions in ALS and FTLD-TDP. Moreover, biochemical and cell-based assays identify oxidative stress as a signalling cue that promotes acetylated TDP-43 aggregates that are readily engaged by the cellular defense machinery. Importantly, acetylated TDP-43 lesions are found in ALS patient spinal cord, indicating that aberrant TDP-43 acetylation and loss of RNA binding are linked to TDP-43 proteinopathy. Thus, modulating TDP-43 acetylation represents a plausible strategy to fine-tune TDP-43 activity, which could provide new therapeutic avenues for TDP-43 proteinopathies.

Models of α-synuclein aggregation in Parkinson's disease

Parkinson's disease (PD) is not only characterized by motor disturbances but also, by cognitive, sensory, psychiatric and autonomic dysfunction. It has been proposed that some of these symptoms might be related to the widespread pathology of α-synuclein (α-syn) aggregation in different nuclei of the central and peripheral nervous system. However, the pathogenic formation of α-syn aggregates in different brain areas of PD patients is poorly understood. Most experimental models of PD are valuable to assess specific aspects of its pathogenesis, such as toxin-induced dopaminergic neurodegeneration. However, new models are required that reflect the widespread and progressive formation of α-syn aggregates in different brain areas. Such α-syn aggregation is induced in only a few animal models, for example perikaryon inclusions are found in rats administered rotenone, aggregates with a neuritic morphology develop in mice overexpressing either mutated or wild-type α-syn, and in Smad3 deficient mice, aggregates form extensively in the perikaryon and neurites of specific brain nuclei. In this review we focus on α-syn aggregation in the human disorder, its genetics and the availability of experimental models. Indeed, evidences show that dopamine (DA) metabolism may be related to α-syn and its conformational plasticity, suggesting an interesting link between the two pathological hallmarks of PD: dopaminergic neurodegeneration and Lewy body (LB) formation.

ATP13A2 and Alpha-synuclein: a Metal Taste in Autophagy

Parkinson's Disease (PD) is a complex and multifactorial disorder of both idiopathic and genetic origin. Thus far, more than 20 genes have been linked to familial forms of PD. Two of these genes encode for ATP13A2 and alpha-synuclein (asyn), proteins that seem to be members of a common network in both physiological and disease conditions. Thus, two different hypotheses have emerged supporting a role of ATP13A2 and asyn in metal homeostasis or in autophagy. Interestingly, an appealing theory might combine these two cellular pathways. Here we review the novel findings in the interaction between these two proteins and debate the exciting roads still ahead.

Multiple system atrophy: genetic or epigenetic?

Multiple system atrophy (MSA) is a rare, late-onset and fatal neurodegenerative disease including multisystem neurodegeneration and the formation of α-synuclein containing oligodendroglial cytoplasmic inclusions (GCIs), which present the hallmark of the disease. MSA is considered to be a sporadic disease; however certain genetic aspects have been studied during the last years in order to shed light on the largely unknown etiology and pathogenesis of the disease. Epidemiological studies focused on the possible impact of environmental factors on MSA disease development. This article gives an overview on the findings from genetic and epigenetic studies on MSA and discusses the role of genetic or epigenetic factors in disease pathogenesis.

Inhibition of HDAC6 modifies tau inclusion body formation and impairs autophagic clearance

Proteinaceous inclusions in nerve cells and glia are a defining neuropathological hallmark in a variety of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). Their occurrence may be related to malfunctions of the proteolytic degradation systems. In cultured oligodendrocytes, proteasomal inhibition leads to protein aggregate formation resembling coiled bodies, which are characteristic for PSP and CBD. Large protein aggregates are excluded from the proteasome and can only be degraded by autophagy, a lysosomal pathway. Autophagy is a highly selective process, which requires a variety of receptor proteins for ubiquitinated proteins, such as p62 and histone deacetylase 6 (HDAC6). HDAC6 is mainly localized in the cytoplasm, and alpha-tubulin is its major substrate. HDAC6 is considered as a sensor of proteasomal stress; it is involved in the autophagosomal pathway and can mediate the retrograde transport of ubiquitinated proteins along the microtubules. As we have shown recently, HDAC6 is present in oligodendrocytes and its inhibition leads to morphological alterations, microtubule bundling, modulation of acetylation, and phosphorylation of the microtubule-associated protein tau. The present study was undertaken to investigate whether HDAC6 is involved in protein aggregate formation in oligodendrocytes and whether its inhibition modifies the consequences of MG-132-induced inhibition of the ubiquitin proteasome system (UPS). The data show that HDAC6 and acetylated tau are recruited to protein aggregates after proteasomal inhibition. Pharmacological inhibition of HDAC6 by the selective inhibitor tubastatin A (TST) and its small hairpin RNA (shRNA)-mediated downregulation alters the assembly of MG-132-induced compact protein aggregates. After TST treatment, they appear more diffusely dispersed throughout the cytoplasm. This is not a protective means but promotes the onset of apoptotic cell death. Furthermore, the heat shock response is altered, and TST suppresses the MG-132-stimulated induction of HSP70. To test whether the alteration of protein aggregate formation is related to the influence of HDAC6 on the autophagic degradation system, an oligodendroglial cell line, i.e., OLN-93 cells stably expressing green fluorescent protein (GFP)-microtubule associated protein light chain 3 (LC3) and tau, was used. During autophagosome formation, endogenous LC3 is processed to LC3-I, which is then converted to LC3-II. An increase of LC3-II is used as a reliable marker for autophagosome formation and abundance. It is demonstrated that inhibition of HDAC6 leads to the accumulation of LC3-positive autophagosomal vacuoles and an increase in LC3-II immunoreactivity, but the autophagic flux is rather impaired. Hence, the inhibition or dysregulation of HDAC6 contributes to stress responses and pathological processes in oligodendrocytes.

Multiple system atrophy: a prototypical synucleinopathy for disease-modifying therapeutic strategies

Despite active fundamental, translational and clinical research, no therapeutic intervention has yet shown convincing effects on disease progression in Parkinson's disease (PD) patients. Indeed, several disease-modification trials failed or proved to be inconclusive due to lack of consistency between clinical rating scales and putative surrogate markers of disease progression, or confounding symptomatic effects of the tested compound. Multiple system atrophy (MSA) is a rapidly progressing orphan disorder leading to severe motor disability within a few years. Together with PD and dementia with Lewy bodies (DLB), MSA belongs to the synucleinopathies, a group of neurodegenerative disorders characterized by the abnormal accumulation of alpha-synuclein. Crucial milestones have been reached for successfully conducting clinical intervention trials in a large number of patients with MSA. In this personal view, we will review evidence, and discuss why MSA could prove the most relevant clinical model for assessing treatments that target mechanisms operating in all synucleinopathies.

Towards translational therapies for multiple system atrophy

Multiple system atrophy (MSA) is a fatal adult-onset neurodegenerative disorder of uncertain etiopathogenesis manifesting with autonomic failure, parkinsonism, and ataxia in any combination. The underlying neuropathology affects central autonomic, striatonigral and olivopontocerebellar pathways and it is associated with distinctive glial cytoplasmic inclusions (GCIs, Papp-Lantos bodies) that contain aggregates of α-synuclein. Current treatment options are very limited and mainly focused on symptomatic relief, whereas disease modifying options are lacking. Despite extensive testing, no neuroprotective drug treatment has been identified up to now; however, a neurorestorative approach utilizing autologous mesenchymal stem cells has shown remarkable beneficial effects in the cerebellar variant of MSA. Here, we review the progress made over the last decade in defining pathogenic targets in MSA and summarize insights gained from candidate disease-modifying interventions that have utilized a variety of well-established preclinical MSA models. We also discuss the current limitations that our field faces and suggest solutions for possible approaches in cause-directed therapies of MSA.

HDAC6 inhibition results in tau acetylation and modulates tau phosphorylation and degradation in oligodendrocytes

Histone deacetylase 6 (HDAC6) is a unique member of the HDAC family. It is localized within the cytoplasm and has unique substrate specificities for nonhistone proteins, such as α-tubulin. Furthermore, it plays a major role in protein aggregate formation and recently was demonstrated to interact with the microtubule associated protein tau and tau was identified as a possible substrate for HDAC6 in neurons. This study was undertaken to investigate whether HDAC6 is present in oligodendrocytes and whether it is involved in tubulin and tau acetylation in these cells. We show for the first time that HDAC6 is expressed in cultured rat brain oligodendrocytes. Its inhibition by the specific HDAC6 inhibitor tubastatin A (TST) leads to morphological alterations, microtubule bundling, and tubulin acetylation, and changes in tau-isoform expression and phosphorylation. Furthermore, the microtubule binding activity of tau was reduced. Using the oligodendroglial cell lines OLN-t40 and OLN-t44, which were genetically engineered to express either the longest human tau isoform with four microtubule binding repeats (4R-tau), or the shortest tau isoform with three repeats (3R-tau), respectively, we demonstrate that tau is acetylated by HDAC6 within the 4R-binding domain. Tau acetylation reduced its turnover rate and acetylated tau was degraded slower in these cells. TST and shRNA-mediated knockdown of HDAC6 in oligodendroglia cells caused an increase in pathological hyperphosphorylated tau detectable with the 12E8 antibody. Hence HDAC6 and dysregulation of the deacetylation and acetylation process in oligodendrocytes may contribute to diseases with oligodendroglial pathology.

An autopsy case of incipient Pick's disease: immunohistochemical profile of early-stage Pick body formation

There is little immunohistochemical information about the early stage of Pick body formation, due to the extremely limited opportunities of studying Pick's disease at the incipient or subclinical stage. We report a 62-year-old man without any clinical manifestations of Pick's disease, who died of B-cell lymphoma of the brainstem. Post mortem examination revealed many Pick bodies without obvious neuronal loss mainly in the left frontal and temporal lobes. Three brains of patients with typical Pick's disease (disease duration: 7, 11 and 16 years) were also examined. Pick bodies were immunopositive for phosphorylated tau and 3-repeat tau, and less consistently for p62 in both incipient and typical cases. In the incipient case, borderline positivity for ubiquitin was evident in only a few Pick bodies, whereas in the typical cases many Pick bodies showed obvious positivity for ubiquitin. These findings suggest that Pick bodies are rarely ubiquitinated in the early stage of Pick body formation.

Inclusion body formation, macroautophagy, and the role of HDAC6 in neurodegeneration

The failure to clear misfolded or aggregated proteins from the cytoplasm of nerve cells and glia is a common pathogenic event in a variety of neurodegenerative disorders. This might be causally related to defects in the major proteolytic systems, i.e., the ubiquitin-proteasomal system and the autophagic pathway. Large protein aggregates and defective organelles are excluded from the proteasome. They can be degraded only by macroautophagy, which is a highly selective process. It requires p62 to act as a bridge connecting ubiquitinated protein aggregates and autophagosomes, and the tubulin deacetylase histone deacetylase 6 (HDAC6). HDAC6 has recently been identified as a constituent in Lewy bodies of Parkinson disease and glial cytoplasmic inclusions of multiple system atrophy. It is considered a sensor of proteasomal inhibition and a cellular stress surveillance factor, and plays a central role in autophagy by controlling the fusion process of autophagosomes with lysosomes. Upon proteasomal inhibition, HDAC6 is relocated and recruited to polyubiquitin-positive aggresomes. Tubulin acetylation is a major consequence of HDAC6 inhibition, and HDAC6 inhibition restores microtubule (MT)-dependent transport mechanisms in neurons. This suggests the involvement of HDAC6 in neurodegenerative diseases. Furthermore, the protein tau seems to be a substrate for HDAC6. Tau acetylation impairs MT assembly and promotes tau fibrillization in vitro. It has been suggested that acetylation and phosphorylation of tau at multiples sites may act synergistically in the pathogenesis of tau fibrillization. In this review, we will survey the process of aggresome formation, macroautophagy and the role of autophagosomal proteins and HDAC6 in inclusion body formation.

Acetyltransferases (HATs) as targets for neurological therapeutics

The acetylation of histone and non-histone proteins controls a great deal of cellular functions, thereby affecting the entire organism, including the brain. Acetylation modifications are mediated through histone acetyltransferases (HAT) and deacetylases (HDAC), and the balance of these enzymes regulates neuronal homeostasis, maintaining the pre-existing acetyl marks responsible for the global chromatin structure, as well as regulating specific dynamic acetyl marks that respond to changes and facilitate neurons to encode and strengthen long-term events in the brain circuitry (e.g., memory formation). Unfortunately, the dysfunction of these finely-tuned regulations might lead to pathological conditions, and the deregulation of the HAT/HDAC balance has been implicated in neurological disorders. During the last decade, research has focused on HDAC inhibitors that induce a histone hyperacetylated state to compensate acetylation deficits. The use of these inhibitors as a therapeutic option was efficient in several animal models of neurological disorders. The elaboration of new cell-permeant HAT activators opens a new era of research on acetylation regulation. Although pathological animal models have not been tested yet, HAT activator molecules have already proven to be beneficial in ameliorating brain functions associated with learning and memory, and adult neurogenesis in wild-type animals. Thus, HAT activator molecules contribute to an exciting area of research.

The role of glia in α-synucleinopathies

α-Synuclein (AS)-positive inclusions are the pathological hallmark of Parkinson's disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA), all belonging to the category of α-synucleinopathies. α-Synucleinopathies represent progressive neurodegenerative disorders characterised by increasing incidences in the population over the age of 65. The relevance of glial reactivity and dysfunction in α-synucleinopathies is highlighted by numerous experimental evidences. Glial AS inclusion pathology is prominent in oligodendroglia of MSA (glial cytoplasmic inclusions) and is a common finding in astroglial cells of PD and DLB, resulting in specific dysfunctional responses. Involvement of AS-dependent astroglial and microglial activation in neurodegenerative mechanisms, and therefore in disease initiation and progression, has been suggested. The aim of this review is to summarise and discuss the multifaceted responses of glial cells in α-synucleinopathies. The beneficial, as well as detrimental, effects of glial cells on neuronal viability are taken into consideration to draw an integrated picture of glial roles in α-synucleinopathies. Furthermore, an overview on therapeutic approaches outlines the difficulties of translating promising experimental studies into successful clinical trials targeting candidate glial pathomechanisms.

Brain expression level and activity of HDAC6 protein in neurodegenerative dementia

Histone deacetylase 6 (HDAC6) is a multifunctional cytoplasmic protein that plays an especially critical role in the formation of aggresomes, where aggregates of excess protein are deposited. Previous immunohistochemical studies have shown that HDAC6 accumulates in Lewy bodies in Parkinson's disease and dementia with Lewy bodies (DLB) as well as in glial cytoplasmic inclusions in multiple system atrophy (MSA). However, it is uncertain whether the level and activity of HDAC6 are altered in the brains of patients with neurodegenerative dementia. In the present study, we demonstrated that the level of HDAC6 was not altered in the temporal cortex of patients with Alzheimer's disease and DLB in comparison with controls. In contrast, the level of HDAC6 was significantly increased in the temporal cortex of patients with frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP) and in the cerebellar white matter of patients with MSA. However, the level of acetylated α-tubulin, one of the substrates of HDAC6, was not altered in FTLD-TDP and MSA relative to controls. These findings suggest that the induced level of HDAC6 in the brain is insufficient for manifestation of its activity in FTLD-TDP and MSA.

Autophagic adapter protein NBR1 is localized in Lewy bodies and glial cytoplasmic inclusions and is involved in aggregate formation in α-synucleinopathy

Macroautophagy is a dynamic process whereby cytoplasmic components are initially sequestered within autophagosomes. Recent studies have shown that the autophagosome membrane can selectively recognize ubiquitinated proteins and organelles through interaction with adapter proteins such as p62 and NBR1. Both proteins are structurally similar at the amino acid level, and bind with ubiquitin and ubiquitinated proteins. Although p62 is incorporated into a wide spectrum of pathological inclusions in various neurodegenerative diseases, abnormalities of NBR1 have not been reported in these diseases. Our immunohistochemical examination revealed that the vast majority of Lewy bodies (LBs) in Parkinson's disease and dementia with LBs (DLB) as well as of glial cytoplasmic inclusions in multiple system atrophy (MSA) were positive for NBR1. Neuronal and glial inclusions in tauopathies and TAR DNA-binding protein of 43 kDa proteinopathies were rarely immunolabeled, or were unstained. Using cultured cells bearing LB-like inclusions, formation of α-synuclein aggregates was repressed in cells with NBR1 knockdown. Immunoblot analysis showed that the level of NBR1 was significantly increased by 2.5-fold in MSA, but not in DLB. These findings suggest that NBR1 is involved in the formation of cytoplasmic inclusions in α-synucleinopathy.

Ataxin-3 regulates aggresome formation of copper-zinc superoxide dismutase (SOD1) by editing K63-linked polyubiquitin chains

Polyubiquitination of misfolded proteins, especially K63-linked polyubiquitination, is thought to be associated with the formation of inclusion bodies. However, it is not well explored whether appropriate editing of the different types of ubiquitin linkages by deubiquitinating enzymes (DUBs) affects the dynamics of inclusion bodies. In this study, we report that a specific DUB, ataxin-3, is required for the efficient recruitment of the neurodegenerative disease-associated protein copper-zinc superoxide dismutase (SOD1) to aggresomes. The overexpression of ataxin-3 promotes mutant SOD1 aggresome formation by trimming K63-linked polyubiquitin chains. Moreover, knockdown of ataxin-3 decreases mutant SOD1 aggresome formation and increases cell death induced by mutant SOD1. Thus, our data suggest that the sequestration of misfolded SOD1 into aggresomes, which is driven by ataxin-3, plays an important role in attenuating protein misfolding-induced cell toxicity.

The Lewy body in Parkinson's disease and related neurodegenerative disorders

The histopathological hallmark of Parkinson's disease (PD) is the presence of fibrillar aggregates referred to as Lewy bodies (LBs), in which α-synuclein is a major constituent. Pale bodies, the precursors of LBs, may serve the material for that LBs continue to expand. LBs consist of a heterogeneous mixture of more than 90 molecules, including PD-linked gene products (α-synuclein, DJ-1, LRRK2, parkin, and PINK-1), mitochondria-related proteins, and molecules implicated in the ubiquitin-proteasome system, autophagy, and aggresome formation. LB formation has been considered to be a marker for neuronal degeneration because neuronal loss is found in the predilection sites for LBs. However, recent studies have indicated that nonfibrillar α-synuclein is cytotoxic and that fibrillar aggregates of α-synuclein (LBs and pale bodies) may represent a cytoprotective mechanism in PD.

Genetics and epigenetics of Parkinson's disease

In 1997 a mutation in the a-synuclein (SNCA) gene was associated with familial autosomal dominant Parkinson's disease (PD). Since then, several loci (PARK1-15) and genes have been linked to familial forms of the disease. There is now sufficient evidence that six of the so far identified genes at PARK loci (a-synuclein, leucine-rich repeat kinase 2, parkin, PTEN-induced putative kinase 1, DJ-1, and ATP13A2) cause inherited forms of typical PD or parkinsonian syndromes. Other genes at non-PARK loci (MAPT, SCA1, SCA2, spatacsin, POLG1) cause syndromes with parkinsonism as one of the symptoms. The majority of PD cases are however sporadic "idiopathic" forms, and the recent application of genome-wide screening revealed almost 20 genes that might contribute to disease risk. In addition, increasing evidence suggests that epigenetic mechanisms, such as DNA methylation, histone modifications, and small RNA-mediated mechanisms, could regulate the expression of PD-related genes.

Acetylated tau, a novel pathological signature in Alzheimer's disease and other tauopathies

The microtubule-binding protein, tau, is the major component of neurofibrillary inclusions characteristic of Alzheimer's disease and related neurodegenerative tauopathies. When tau fibrillizes, it undergoes abnormal post-translational modifications resulting in decreased solubility and altered microtubule-stabilizing properties. Recently, we reported that the abnormal acetylation of tau at lysine residue 280 is a novel, pathological post-translational modification. Here, we performed detailed immunohistochemistry to further examine acetylated-tau expression in Alzheimer's disease and other major tauopathies. Immunohistochemistry using a polyclonal antibody specific for acetylated-tau at lysine 280 was conducted on 30 post-mortem central nervous system regions from patients with Alzheimer's disease (10 patients), corticobasal degeneration (5 patients), and progressive supranuclear palsy (5 patients). Acetylated-tau pathology was compared with the sequential emergence of other tau modifications in the Alzheimer's disease hippocampus using monoclonal antibodies to multiple well-characterized tau epitopes. All cases studied showed significant acetylated-tau pathology in a distribution pattern similar to hyperphosphorylated-tau. Acetylated-tau pathology was largely in intracellular, thioflavin-S-positive tau inclusions in Alzheimer's disease, and also thioflavin-S-negative pathology in corticobasal degeneration and progressive supranuclear palsy. Acetylated-tau was present throughout all stages of Alzheimer's disease pathology, but was more prominently associated with pathological tau epitopes in moderate to severe-stage cases. These temporal and morphological immunohistochemical features suggest acetylation of tau at this epitope is preceded by early modifications, including phosphorylation, and followed by later truncation events and cell death in Alzheimer's disease. Acetylation of tau at lysine 280 is a pathological modification that may contribute to tau-mediated neurodegeneration by both augmenting losses of normal tau properties (reduced solubility and microtubule assembly) as well as toxic gains of function (increased tau fibrillization). Thus, inhibiting tau acetylation could be a disease-modifying target for drug discovery target in tauopathies.