Lysosomal proteases are involved in generation of N-terminal huntingtin fragments

Huntington's Disease: Complex Pathogenesis and Therapeutic Strategies

Huntington's disease (HD) arises from the abnormal expansion of CAG repeats in the huntingtin gene (HTT), resulting in the production of the mutant huntingtin protein (mHTT) with a polyglutamine stretch in its N-terminus. The pathogenic mechanisms underlying HD are complex and not yet fully elucidated. However, mHTT forms aggregates and accumulates abnormally in neuronal nuclei and processes, leading to disruptions in multiple cellular functions. Although there is currently no effective curative treatment for HD, significant progress has been made in developing various therapeutic strategies to treat HD. In addition to drugs targeting the neuronal toxicity of mHTT, gene therapy approaches that aim to reduce the expression of the mutant HTT gene hold great promise for effective HD therapy. This review provides an overview of current HD treatments, discusses different therapeutic strategies, and aims to facilitate future therapeutic advancements in the field.

Interaction between huntingtin exon 1 and HEAT repeat structure probed by chimeric model proteins

Huntington disease (HD) is associated with aggregation of huntingtin (HTT) protein containing over 35 continuous Q residues within the N-terminal exon 1 encoded region. The C-terminal of the HTT protein consists mainly of HEAT repeat structure which serves as a scaffold for multiple cellular activities. Structural and biochemical analysis of the intact HTT protein has been hampered by its huge size (~300 kDa) and most in vitro studies to date have focused on the properties of the exon 1 region. To explore the interaction between HTT exon 1 and the HEAT repeat structure, we constructed chimeric proteins containing the N-terminal HTT exon 1 region and the HEAT repeat protein PR65/A. The results indicate that HTT exon 1 slightly destabilizes the downstream HEAT repeat structure and endows the HEAT repeat structure with more conformational flexibility. Wild-type and pathological lengths of polyQ did not show differences in the interaction between HTT exon 1 and the HEAT repeats. With the C-terminal fusion of PR65/A, HTT exon 1 containing pathological lengths of polyQ could still form amyloid fibrils, but the higher-order architecture of fibrils and kinetics of fibril formation were affected by the C-terminal fusion of HEAT repeats. This indicates that interaction between HTT exon 1 and HEAT repeat structure is compatible with both normal function of HTT protein and the pathogenesis of HD, and this study provides a potential model for further exploration.

The Role of Cysteine Protease Cathepsins B, H, C, and X/Z in Neurodegenerative Diseases and Cancer

Papain-like cysteine proteases are composed of 11 human cysteine cathepsins, originally located in the lysosomes. They exhibit broad specificity and act as endopeptidases and/or exopeptidases. Among them, only cathepsins B, H, C, and X/Z exhibit exopeptidase activity. Recently, cysteine cathepsins have been found to be present outside the lysosomes and often participate in various pathological processes. Hence, they have been considered key signalling molecules. Their potentially hazardous proteolytic activities are tightly regulated. This review aims to discuss recent advances in understanding the structural aspects of these four cathepsins, mechanisms of their zymogen activation, regulation of their activities, and functional aspects of these enzymes in neurodegeneration and cancer. Neurodegenerative effects have been evaluated, particularly in Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, and neuropsychiatric disorders. Cysteine cathepsins also participate in tumour progression and metastasis through the overexpression and secretion of proteases, which trigger extracellular matrix degradation. To our knowledge, this is the first review to provide an in-depth analysis regarding the roles of cysteine cathepsins B, H, C, and X in neurodegenerative diseases and cancer. Further advances in understanding the functions of cysteine cathepsins in these conditions will result in the development of novel, targeted therapeutic strategies.

Reciprocal effects of alpha-synuclein aggregation and lysosomal homeostasis in synucleinopathy models

BACKGROUND

Lysosomal dysfunction has been implicated in a number of neurodegenerative diseases such as Parkinson's disease (PD). Various molecular, clinical and genetic studies have highlighted a central role of lysosomal pathways and proteins in the pathogenesis of PD. Within PD pathology the synaptic protein alpha-synuclein (αSyn) converts from a soluble monomer to oligomeric structures and insoluble amyloid fibrils. The aim of this study was to unravel the effect of αSyn aggregates on lysosomal turnover, particularly focusing on lysosomal homeostasis and cathepsins. Since these enzymes have been shown to be directly involved in the lysosomal degradation of αSyn, impairment of their enzymatic capacity has extensive consequences.

METHODS

We used patient-derived induced pluripotent stem cells and a transgenic mouse model of PD to examine the effect of intracellular αSyn conformers on cell homeostasis and lysosomal function in dopaminergic (DA) neurons by biochemical analyses.

RESULTS

We found impaired lysosomal trafficking of cathepsins in patient-derived DA neurons and mouse models with αSyn aggregation, resulting in reduced proteolytic activity of cathepsins in the lysosome. Using a farnesyltransferase inhibitor, which boosts hydrolase transport via activation of the SNARE protein ykt6, we enhanced the maturation and proteolytic activity of cathepsins and thereby decreased αSyn protein levels.

CONCLUSIONS

Our findings demonstrate a strong interplay between αSyn aggregation pathways and function of lysosomal cathepsins. It appears that αSyn directly interferes with the enzymatic function of cathepsins, which might lead to a vicious cycle of impaired αSyn degradation. Lysosomal trafficking of cathepsin D (CTSD), CTSL and CTSB is disrupted when alpha-synuclein (αSyn) is aggregated. This results in a decreased proteolytic activity of cathepsins, which directly mediate αSyn clearance. Boosting the transport of the cathepsins to the lysosome increases their activity and thus contributes to efficient αSyn degradation.

The role of lysosomal cathepsins in neurodegeneration: Mechanistic insights, diagnostic potential and therapeutic approaches

Lysosomes are ubiquitous organelles with a fundamental role in maintaining cellular homeostasis by mediating degradation and recycling processes. Cathepsins are the most abundant lysosomal hydrolyses and are responsible for the bulk degradation of various substrates. A correct autophagic function is essential for neuronal survival, as most neurons are post-mitotic and thus susceptible to accumulate cellular components. Increasing evidence suggests a crucial role of the lysosome in neurodegeneration as a key regulator of aggregation-prone and disease-associated proteins, such as α-synuclein, β-amyloid and huntingtin. Particularly, alterations in lysosomal cathepsins CTSD, CTSB and CTSL can contribute to the pathogenesis of neurodegenerative diseases as seen for neuronal ceroid lipofuscinosis, synucleinopathies (Parkinson's disease, Dementia with Lewy Body and Multiple System Atrophy) as well as Alzheimer's and Huntington's disease. In this review, we provide an overview of recent evidence implicating CTSD, CTSB and CTSL in neurodegeneration, with a special focus on the role of these enzymes in α-synuclein metabolism. In addition, we summarize the potential role of lysosomal cathepsins as clinical biomarkers in neurodegenerative diseases and discuss potential therapeutic approaches by targeting lysosomal function.

Recombinant pro-CTSD (cathepsin D) enhances SNCA/α-Synuclein degradation in α-Synucleinopathy models

Parkinson disease (PD) is a neurodegenerative disorder characterized by the abnormal intracellular accumulation of SNCA/α-synuclein. While the exact mechanisms underlying SNCA pathology are not fully understood, increasing evidence suggests the involvement of autophagy as well as lysosomal deficiencies. Because CTSD (cathepsin D) has been proposed to be the major lysosomal protease involved in SNCA degradation, its deficiency has been linked to the presence of insoluble SNCA conformers in the brain of mice and humans as well as to the transcellular transmission of SNCA aggregates. We here postulate that SNCA degradation can be enhanced by the application of the recombinant human proform of CTSD (rHsCTSD). Our results reveal that rHsCTSD is efficiently endocytosed by neuronal cells, correctly targeted to lysosomes and matured to an enzymatically active protease. In dopaminergic neurons derived from induced pluripotent stem cells (iPSC) of PD patients harboring the A53T mutation within the SNCA gene, we confirm the reduction of insoluble SNCA after treatment with rHsCTSD. Moreover, we demonstrate a decrease of pathological SNCA conformers in the brain and within primary neurons of a ctsd-deficient mouse model after dosing with rHsCTSD. Boosting lysosomal CTSD activity not only enhanced SNCA clearance in human and murine neurons as well as tissue, but also restored endo-lysosome and autophagy function. Our findings indicate that CTSD is critical for SNCA clearance and function. Thus, enzyme replacement strategies utilizing CTSD may also be of therapeutic interest for the treatment of PD and other synucleinopathies aiming to decrease the SNCA burden.Abbreviations: aa: amino acid; SNCA/α-synuclein: synuclein alpha; APP: amyloid beta precursor protein; BBB: blood brain barrier; BF: basal forebrain; CBB: Coomassie Brilliant Blue; CLN: neuronal ceroid lipofuscinosis; CNL10: neuronal ceroid lipofuscinosis type 10; Corr.: corrected; CTSD: cathepsin D; CTSB: cathepsin B; DA: dopaminergic; DA-iPSn: induced pluripotent stem cell-derived dopaminergic neurons; dox: doxycycline; ERT: enzyme replacement therapy; Fx: fornix, GBA/β-glucocerebrosidase: glucosylceramidase beta; h: hour; HC: hippocampus; HT: hypothalamus; i.c.: intracranially; IF: immunofluorescence; iPSC: induced pluripotent stem cell; KO: knockout; LAMP1: lysosomal associated membrane protein 1; LSDs: lysosomal storage disorders; MAPT: microtubule associated protein tau; M6P: mannose-6-phosphate; M6PR: mannose-6-phosphate receptor; MB: midbrain; mCTSD: mature form of CTSD; neurofil.: neurofilament; PD: Parkinson disease; proCTSD: proform of CTSD; PRNP: prion protein; RFU: relative fluorescence units; rHsCTSD: recombinant human proCTSD; SAPC: Saposin C; SIM: structured illumination microscopy; T-insol: Triton-insoluble; T-sol: Triton-soluble; TEM: transmission electron microscopy, TH: tyrosine hydroxylase; Thal: thalamus.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

Effects of flanking sequences and cellular context on subcellular behavior and pathology of mutant HTT

Huntington's disease (HD) is caused by an expansion of a poly glutamine (polyQ) stretch in the huntingtin protein (HTT) that is necessary to cause pathology and formation of HTT aggregates. Here we ask whether expanded polyQ is sufficient to cause pathology and aggregate formation. By addressing the sufficiency question, one can identify cellular processes and structural parameters that influence HD pathology and HTT subcellular behavior (i.e. aggregation state and subcellular location). Using Drosophila, we compare the effects of expressing mutant full-length human HTT (fl-mHTT) to the effects of mutant human HTTexon1 and to two commonly used synthetic fragments, HTT171 and shortstop (HTT118). Expanded polyQ alone is not sufficient to cause inclusion formation since full-length HTT and HTTex1 with expanded polyQ are both toxic although full-length HTT remains diffuse while HTTex1 forms inclusions. Further, inclusions are not sufficient to cause pathology since HTT171-120Q forms inclusions but is benign and co-expression of HTT171-120Q with non-aggregating pathogenic fl-mHTT recruits fl-mHTT to aggregates and rescues its pathogenicity. Additionally, the influence of sequences outside the expanded polyQ domain is revealed by finding that small modifications to the HTT118 or HTT171 fragments can dramatically alter their subcellular behavior and pathogenicity. Finally, mutant HTT subcellular behavior is strongly modified by different cell and tissue environments (e.g. fl-mHTT appears as diffuse nuclear in one tissue and diffuse cytoplasmic in another but toxic in both). These observations underscore the importance of cellular and structural context for the interpretation and comparison of experiments using different fragments and tissues to report the effects of expanded polyQ.

Adenosine A1 Receptor Agonist (R-PIA) before Pilocarpine Modulates Pro- and Anti-Apoptotic Factors in an Animal Model of Epilepsy

We aimed to characterize the mechanisms involved in neuroprotection by R-PIA administered before pilocarpine-induced seizures. Caspase-1 and caspase-3 activities were assayed using fluorimetry, and cathepsin D, HSP-70, and AKT expression levels were assayed using Western Blot of hippocampal samples. R-PIA was injected before pilocarpine (PILO), and four groups were studied at 1 h 30 min and 7 days following initiation of status epilepticus (SE): PILO, R-PIA+PILO, SALINE, and R-PIA+SALINE. At 1 h 30 min, significantly higher activities of caspase-1 and -3 were observed in the PILO group than in the SALINE group. Caspase-1 and -3 activities were higher in the R-PIA+PILO group than in the PILO group. At 7 days following SE, caspase-1 and -3 activities were higher than in the initial post-seizure phase compared to the SALINE group. The pretreatment of rats receiving PILO significantly reduced caspase activities compared to the PILO group. Expression of HSP-70, AKT, and cathepsin D was significantly higher in the PILO group than in the SALINE. In the R-PIA+PILO group, the expression of AKT and HSP-70 was greater than in rats receiving only PILO, while cathepsin D presented decreased expression. Pretreatment with R-PIA in PILO-injected rats strongly inhibited caspase-1 and caspase-3 activities and cathepsin D expression. It also increased expression levels of the neuroprotective proteins HSP-70 and AKT, suggesting an important role in modulating the cellular survival cascade.

An altered secretome is an early marker of the pathogenesis of CLN6 Batten disease

Neuronal ceroid lipofuscinoses (NCLs) are a group of inherited childhood neurodegenerative disorders. In addition to the accumulation of auto-fluorescent storage material in lysosomes, NCLs are largely characterised by region-specific neuroinflammation that can predict neuron loss. These phenotypes suggest alterations in the extracellular environment-making the secretome an area of significant interest. This study investigated the secretome in the CLN6 (ceroid-lipofuscinosis neuronal protein 6) variant of NCL. To investigate the CLN6 secretome, we co-cultured neurons and glia isolated from Cln6^nclf or Cln6^± mice, and utilised mass spectrometry to compare protein constituents of conditioned media. The significant changes noted in cathepsin enzymes, were investigated further via western blotting and enzyme activity assays. Viral-mediated gene therapy was used to try and rescue the wild-type phenotype and restore the secretome-both in vitro in co-cultures and in vivo in mouse plasma. In Cln6^nclf cells, proteomics revealed a marked increase in catabolic and cytoskeletal-associated proteins-revealing new similarities between the pathogenic signatures of NCLs with other neurodegenerative disorders. These changes were, in part, corrected by gene therapy intervention, suggesting these proteins as candidate in vitro biomarkers. Importantly, these in vitro changes show promise for in vivo translation, with Cathepsin L (CTSL) activity reduced in both co-cultures and Cln6^nclf plasma samples post gene-therapy. This work suggests the secretome plays a role in CLN6 pathogenesis and highlights its potential use as an in vitro model. Proteomic changes present a list of candidate biomarkers for monitoring disease and assessing potential therapeutics in future studies.

Cerebrospinal fluid endo-lysosomal proteins as potential biomarkers for Huntington's disease

Molecular markers derived from cerebrospinal fluid (CSF) represent an accessible means of exploring the pathobiology of Huntington's disease (HD) in vivo. The endo-lysosomal/autophagy system is dysfunctional in HD, potentially contributing to disease pathogenesis and representing a potential target for therapeutic intervention. Several endo-lysosomal proteins have shown promise as biomarkers in other neurodegenerative diseases; however, they have yet to be fully explored in HD. We performed parallel reaction monitoring mass spectrometry analysis (PRM-MS) of multiple endo-lysosomal proteins in the CSF of 60 HD mutation carriers and 20 healthy controls. Using generalised linear models controlling for age and CAG, none of the 18 proteins measured displayed significant differences in concentration between HD patients and controls. This was affirmed by principal component analysis, in which no significant difference across disease stage was found in any of the three components representing lysosomal hydrolases, binding/transfer proteins and innate immune system/peripheral proteins. However, several proteins were associated with measures of disease severity and cognition: most notably amyloid precursor protein, which displayed strong correlations with composite Unified Huntington's Disease Rating Scale, UHDRS Total Functional Capacity, UHDRS Total Motor Score, Symbol Digit Modalities Test and Stroop Word Reading. We conclude that although endo-lysosomal proteins are unlikely to have value as disease state CSF biomarkers for Huntington's disease, several proteins demonstrate associations with clinical severity, thus warranting further, targeted exploration and validation in larger, longitudinal samples.

Could metformin be therapeutically useful in Huntington's disease?

Emerging evidence suggest that dimethylbiguanide (metformin), a first-line drug for type 2 diabetes mellitus, could be neuroprotective in a range of brain pathologies, which include neurodegenerative diseases and brain injury. However, there are also contraindications that associate metformin treatment with cognitive impairment as well as adverse outcomes in Alzheimer's disease and Parkinson's disease animal models. Recently, a beneficial effect of metformin in animal models of Huntington's disease (HD) has been strengthened by multiple reports. In this brief review, the findings associated with the effects of metformin in attenuating neurodegenerative diseases are discussed, focusing on HD-associated pathology and the potential underlying mechanisms highlighted by these studies. The mechanism of action of metformin is complex, and its therapeutic efficacy is therefore expected to be dependent on the disease context. The key metabolic pathways that are effectively affected by metformin, such as AMP-activated protein kinase activation, may be altered in the later decades of the human lifespan. In this regard, metformin may nonetheless be therapeutically useful for neurological diseases with early pathological onsets, such as HD.

Lysosomes as a therapeutic target

Lysosomes are membrane-bound organelles with roles in processes involved in degrading and recycling cellular waste, cellular signalling and energy metabolism. Defects in genes encoding lysosomal proteins cause lysosomal storage disorders, in which enzyme replacement therapy has proved successful. Growing evidence also implicates roles for lysosomal dysfunction in more common diseases including inflammatory and autoimmune disorders, neurodegenerative diseases, cancer and metabolic disorders. With a focus on lysosomal dysfunction in autoimmune disorders and neurodegenerative diseases - including lupus, rheumatoid arthritis, multiple sclerosis, Alzheimer disease and Parkinson disease - this Review critically analyses progress and opportunities for therapeutically targeting lysosomal proteins and processes, particularly with small molecules and peptide drugs.

Proteomics of the corpus callosum to identify novel factors involved in hypomyelinated Niemann-Pick Type C disease mice

Hypomyelination in the central nerves system (CNS) is one of the most obviously pathological features in Niemann-Pick Type C disease (NPC), which is a rare neurodegenerative disorder caused by mutations in the NPC intracellular cholesterol transporter 1 or 2 (Npc1 or Npc2). Npc1 plays key roles in both neurons and oligodendrocytes during myelination, however, the linkage between the disturbed cholesterol transport and inhibited myelination is unrevealed. In this study, mass spectrometry (MS)-based differential quantitative proteomics was applied to compare protein composition in the corpus callosum between wild type (WT) and NPC mice. In total, 3009 proteins from both samples were identified, including myelin structural proteins, neuronal proteins, and astrocyte-specific proteins. In line to hypomyelination, our data revealed downregulation of myelin structural and indispensable proteins in Npc1 mutant mice. Notably, the reduced ceramide synthase 2 (Cers2), UDP glycosyltransferase 8 (Ugt8), and glycolipid transfer protein (Gltp) indicate the altered sphingolipid metabolism in the disease and the involvement of Gltp in myelination. The identification of most reported myelin structural proteins and proteins from other cell types advocates the use of the corpus callosum to investigate proteins in different cell types that regulate myelination.

Lysosomal Cathepsin Protease Gene Expression Profiles in the Human Brain During Normal Development

Cathepsin protease genes are necessary for protein homeostasis in normal brain development and function. The diversity of the 15 cathepsin protease activities raises the question of what are the human brain expression profiles of the cathepsin genes during development from prenatal and infancy to childhood, adolescence, and young adult stages. This study, therefore, evaluated the cathepsin gene expression profiles in 16 human brain regions during development by quantitative RNA-sequencing data obtained from the Allen Brain Atlas resource. Total expression of all cathepsin genes was the lowest at the early prenatal stage which became increased at the infancy stage. During infancy to young adult phases, total gene expression was similar. Interestingly, the rank ordering of gene expression among the cathepsins was similar throughout the brain at the age periods examined, showing (a) high expression of cathepsins B, D, and F; (b) moderate expression of cathepsins A, L, and Z; (c) low expression of cathepsins C, H, K, O, S, and V; and (d) very low expression of cathepsins E, G, and W. Results show that the human brain utilizes well-defined, balanced patterns of cathepsin gene expression throughout the different stages of human brain development. Knowledge gained by this study of the gene expression profiles of lysosomal cathepsin proteases among human brain regions during normal development is important for advancing future investigations of how these cathepsins are dysregulated in lysosomal-related brain disorders that affect infants, children, adolescents, and young adults.

Unconventional Secretion and Intercellular Transfer of Mutant Huntingtin

The mechanism of intercellular transmission of pathological agents in neurodegenerative diseases has received much recent attention. Huntington’s disease (HD) is caused by a monogenic mutation in the gene encoding Huntingtin (HTT). Mutant HTT (mHTT) harbors a CAG repeat extension which encodes an abnormally long polyglutamine (polyQ) repeat at HTT’s N-terminus. Neuronal pathology in HD is largely due to the toxic gain-of-function by mHTT and its proteolytic products, which forms both nuclear and cytoplasmic aggregates that perturb nuclear gene transcription, RNA splicing and transport as well cellular membrane dynamics. The neuropathological effects of mHTT have been conventionally thought to be cell-autonomous in nature. Recent findings have, however, indicated that mHTT could be secreted by neurons, or transmitted from one neuronal cell to another via different modes of unconventional secretion, as well as via tunneling nanotubes (TNTs). These modes of transmission allow the intercellular spread of mHTT and its aggregates, thus plausibly promoting neuropathology within proximal neuronal populations and between neurons that are connected within neural circuits. Here, the various possible modes for mHTT’s neuronal cell exit and intercellular transmission are discussed.

Autophagy Activation by Transcription Factor EB (TFEB) in Striatum of HDQ175/Q7 Mice

Background: Mutant huntingtin (mHTT) is encoded by the Huntington’s disease (HD) gene and its accumulation in the brain contributes to HD pathogenesis. Reducing mHTT levels through activation of the autophagosome-lysosomal pathway may have therapeutic benefit. Transcription factor EB (TFEB) regulates lysosome biogenesis and autophagy.

Objective: To examine if increasing TFEB protein levels in HD mouse striatum induces autophagy and influences mHTT levels.

Methods: We introduced cDNA encoding TFEB with an HA tag (TFEB-HA) under the control of neuron specific synapsin 1 promoter into the striatum of 3 month old HD^Q175/Q7 mice using adeno-associated virus AAV2/9. The levels of exogenous TFEB were analyzed using qPCR and Western blot. Proteins involved in autophagy, levels of huntingtin, and striatal-enriched proteins were examined using biochemical and/or immunohistochemical methods.

Results: In HD mice expressing TFEB-HA, HA immunoreactivity distributed throughout the striatum in neuronal cell bodies and processes and preferentially in neuronal nuclei and overlapped with a loss of DARPP32 immunoreactivity. TFEB-HA mRNA and protein were detected in striatal lysates. There were increased levels of proteins involved with autophagosome/lysosome activity including LAMP-2A, LC3II, and cathepsin D and reduced levels of mutant HTT and the striatal enriched proteins DARPP32 and PDE10A. Compared to WT mice, HD^Q175/Q7 mice had elevated levels of the ER stress protein GRP78/BiP and with TFEB-HA expression, increased levels of the astrocyte marker GFAP and pro-caspase 3.

Conclusion: These results suggest that TFEB expression in the striatum of HD^Q175/Q7 mice stimulates autophagy and lysosome activity, and lowers mHTT, but may also increase a neuronal stress response.

Effects of Exogenous NUB1 Expression in the Striatum of HDQ175/Q7 Mice

BACKGROUND

Reducing mutant huntingtin (mHTT) in neurons may be a therapy for Huntington's disease (HD). Elevating NUB1 protein reduced mHTT levels in cell and fly models of HD through a proteasome dependent mechanism.

OBJECTIVE

To examine the effects of augmenting NUB1 in HD mouse striatum on mHTT levels.

METHODS

Striata of HDQ175/Q7 mice were injected at 3 months of age with recombinant AAV2/9 coding for NUB1 or GFP under the control of the neuron specific human synapsin 1 promoter and examined 6 months post-injection for levels of huntingtin, the striatal markers DARPP32 and PDE10A, the astrocyte marker GFAP, and the autophagy and mHTT aggregate marker P62 using immunolabeling of brain sections and Western blot assay of striatal subcellular fractions.

RESULTS

By Western blot human HD brain had only one of the two variants of NUB1 present in human control brain. In striatum of WT and HD mice NUB1 was localized in medium size neurons and enriched in the nucleus of large neurons. In the striatum of NUB1 injected HD mice, there was widespread neuronal distribution of exogenous NUB1 labeling and protein levels were ∼2.5-fold endogenous levels. DARPP32 and GFAP distribution and levels were unchanged but PDE10A levels were lower in crude homogenates and P62 was increased in nuclear enriched P1 fractions. Elevating NUB1 did not change levels of full-length mHTT or the number and size of mHTT (S830) positive nuclear inclusions.

CONCLUSION

Findings suggest that increasing NUB1 protein in striatal neurons of HDQ175/Q7 mice in vivo may be relatively safe but is ineffective in reducing mHTT. Increased NUB1 expression in HD striatum alters PDE10A and P62 which are known to be influenced by mHTT.

Lysosomal cathepsins and their regulation in aging and neurodegeneration

Lysosomes and lysosomal hydrolases, including the cathepsins, have been shown to change their properties with aging brain a long time ago, although their function was not really understood. The first biochemical and clinical studies were followed by a major expansion in the last 20 years with the development of animal disease models and new approaches leading to a major advancement of understanding of the role of physiological and degenerative processes in the brain at the molecular level. This includes the understanding of the major role of autophagy and the cathepsins in a number of diseases, including its critical role in the neuronal ceroid lipofuscinosis. Similarly, cathepsins and some other lysosomal proteases were shown to have important roles in processing and/or degradation of several important neuronal proteins, thereby having either neuroprotective or harmful roles. In this review, we discuss lysosomal cathepsins and their regulation with the focus on cysteine cathepsins and their endogenous inhibitors, as well as their role in several neurodegenerative diseases.

Huntingtin Subcellular Localisation Is Regulated by Kinase Signalling Activity in the StHdhQ111 Model of HD

Huntington's disease is a neurodegenerative disorder characterised primarily by motor abnormalities, and is caused by an expanded polyglutamine repeat in the huntingtin protein. Huntingtin dynamically shuttles between subcellular compartments, and the mutant huntingtin protein is mislocalised to cell nuclei, where it may interfere with nuclear functions, such as transcription. However, the mechanism by which mislocalisation of mutant huntingtin occurs is currently unknown. An immortalised embryonic striatal cell model of HD (StHdhQ111) was stimulated with epidermal growth factor in order to determine whether the subcellular localisation of huntingtin is dependent on kinase signalling pathway activation. Aberrant phosphorylation of AKT and MEK signalling pathways was identified in cells carrying mutant huntingtin. Activity within these pathways was found to contribute to the regulation of huntingtin and mutant huntingtin localisation, as well as to the expression of immediate-early genes. We propose that altered kinase signalling is a phenotype of Huntington's disease that occurs prior to cell death; specifically, that altered kinase signalling may influence huntingtin localisation, which in turn may impact upon nuclear processes such as transcriptional regulation. Aiming to restore the balance of activity between kinase signalling networks may therefore prove to be an effective approach to delaying Huntington's disease symptom development and progression.

Olesoxime suppresses calpain activation and mutant huntingtin fragmentation in the BACHD rat

Huntington's disease is a fatal human neurodegenerative disorder caused by a CAG repeat expansion in the HTT gene, which translates into a mutant huntingtin protein. A key event in the molecular pathogenesis of Huntington's disease is the proteolytic cleavage of mutant huntingtin, leading to the accumulation of toxic protein fragments. Mutant huntingtin cleavage has been linked to the overactivation of proteases due to mitochondrial dysfunction and calcium derangements. Here, we investigated the therapeutic potential of olesoxime, a mitochondria-targeting, neuroprotective compound, in the BACHD rat model of Huntington's disease. BACHD rats were treated with olesoxime via the food for 12 months. In vivo analysis covered motor impairments, cognitive deficits, mood disturbances and brain atrophy. Ex vivo analyses addressed olesoxime's effect on mutant huntingtin aggregation and cleavage, as well as brain mitochondria function. Olesoxime improved cognitive and psychiatric phenotypes, and ameliorated cortical thinning in the BACHD rat. The treatment reduced cerebral mutant huntingtin aggregates and nuclear accumulation. Further analysis revealed a cortex-specific overactivation of calpain in untreated BACHD rats. Treated BACHD rats instead showed significantly reduced levels of mutant huntingtin fragments due to the suppression of calpain-mediated cleavage. In addition, olesoxime reduced the amount of mutant huntingtin fragments associated with mitochondria, restored a respiration deficit, and enhanced the expression of fusion and outer-membrane transport proteins. In conclusion, we discovered the calpain proteolytic system, a key player in Huntington's disease and other neurodegenerative disorders, as a target of olesoxime. Our findings suggest that olesoxime exerts its beneficial effects by improving mitochondrial function, which results in reduced calpain activation. The observed alleviation of behavioural and neuropathological phenotypes encourages further investigations on the use of olesoxime as a therapeutic for Huntington's disease.

Huntingtin proteolysis releases non-polyQ fragments that cause toxicity through dynamin 1 dysregulation

Cleavage of mutant huntingtin (HTT) is an essential process in Huntington's disease (HD), an inherited neurodegenerative disorder. Cleavage generates N-ter fragments that contain the polyQ stretch and whose nuclear toxicity is well established. However, the functional defects induced by cleavage of full-length HTT remain elusive. Moreover, the contribution of non-polyQ C-terminal fragments is unknown. Using time- and site-specific control of full-length HTT proteolysis, we show that specific cleavages are required to disrupt intramolecular interactions within HTT and to cause toxicity in cells and flies. Surprisingly, in addition to the canonical pathogenic N-ter fragments, the C-ter fragments generated, that do not contain the polyQ stretch, induced toxicity via dilation of the endoplasmic reticulum (ER) and increased ER stress. C-ter HTT bound to dynamin 1 and subsequently impaired its activity at ER membranes. Our findings support a role for HTT on dynamin 1 function and ER homoeostasis. Proteolysis-induced alteration of this function may be relevant to disease.

From pathways to targets: understanding the mechanisms behind polyglutamine disease

The history of polyglutamine diseases dates back approximately 20 years to the discovery of a polyglutamine repeat in the androgen receptor of SBMA followed by the identification of similar expansion mutations in Huntington's disease, SCA1, DRPLA, and the other spinocerebellar ataxias. This common molecular feature of polyglutamine diseases suggests shared mechanisms in disease pathology and neurodegeneration of disease specific brain regions. In this review, we discuss the main pathogenic pathways including proteolytic processing, nuclear shuttling and aggregation, mitochondrial dysfunction, and clearance of misfolded polyglutamine proteins and point out possible targets for treatment.

Identification of a small molecule that induces ATG5-and-cathepsin-l-dependent cell death and modulates polyglutamine toxicity

Non-apoptotic cell death mechanisms are largely uncharacterized despite their importance in physiology and disease [1]. Here we sought to systematically identify non-apoptotic cell death pathways in mammalian cells. We screened 69,612 compounds for those that induce non-canonical cell death by counter screening in the presence of inhibitors of apoptosis and necrosis. We further selected compounds that require active protein synthesis for inducing cell death. Using this tiered approach, we identified NID-1 (Novel Inducer of Death-1), a small molecule that induces an active, energy-dependent cell death in diverse mammalian cell lines. NID-1-induced death required components of the autophagic machinery, including ATG5, and the lysosomal hydrolase cathepsin L, but was distinct from classical macroautophagy. Since macroautophagy can prevent cell death in several contexts, we tested and found that NID-1 suppressed cell death in a cell-based model of Huntington's disease, suggesting that NID-1 activates a specific pathway. Thus the discovery of NID-1 identifies a previously unexplored cell death pathway, and modulating this pathway may have therapeutic applications. Furthermore, these findings provide a proof-of-principle for using chemical screening to identify novel cell death paradigms.

Analysis of proteolytic processes and enzymatic activities in the generation of huntingtin n-terminal fragments in an HEK293 cell model

BACKGROUND

N-terminal fragments of mutant huntingtin (htt) that terminate between residues 90-115, termed cleavage product A or 1 (cp-A/1), form intracellular and intranuclear inclusion bodies in the brains of patients with Huntington's disease (HD). These fragments appear to be proteolytic products of the full-length protein. Here, we use an HEK293 cell culture model to investigate huntingtin proteolytic processing; previous studies of these cells have demonstrated cleavage of htt to cp-A/1 like htt fragments.

RESULTS

Recombinant N-terminal htt fragments, terminating at residue 171 (also referred to as cp-B/2 like), were efficiently cleaved to produce cp-A/1 whereas fragments representing endogenous caspase, calpain, and metalloproteinase cleavage products, terminating between residues 400-600, were inefficiently cleaved. Using cysteine-labeling techniques and antibody binding mapping, we localized the C-terminus of the cp-A/1 fragments produced by HEK293 cells to sequences minimally limited by cysteine 105 and an antibody epitope composed of residues 115-124. A combination of genetic and pharmacologic approaches to inhibit potential proteases, including γ-secretase and calpain, proved ineffective in preventing production of cp-A/1.

CONCLUSIONS

Our findings indicate that HEK293 cells express a protease that is capable of efficiently cleaving cp-B/2 like fragments of htt with normal or expanded glutamine repeats. For reasons that remain unclear, this protease cleaves longer htt fragments, with normal or expanded glutamine expansions, much less efficiently. The protease in HEK293 cells that is capable of generating a cp-A/1 like htt fragment may be a novel protease with a high preference for a cp-B/2-like htt fragment as substrate.

Native mutant huntingtin in human brain: evidence for prevalence of full-length monomer

Huntington disease (HD) is caused by polyglutamine expansion in the N terminus of huntingtin (htt). Analysis of human postmortem brain lysates by SDS-PAGE and Western blot reveals htt as full-length and fragmented. Here we used Blue Native PAGE (BNP) and Western blots to study native htt in human postmortem brain. Antisera against htt detected a single band broadly migrating at 575-850 kDa in control brain and at 650-885 kDa in heterozygous and Venezuelan homozygous HD brains. Anti-polyglutamine antisera detected full-length mutant htt in HD brain. There was little htt cleavage even if lysates were pretreated with trypsin, indicating a property of native htt to resist protease cleavage. A soluble mutant htt fragment of about 180 kDa was detected with anti-htt antibody Ab1 (htt-(1-17)) and increased when lysates were treated with denaturants (SDS, 8 M urea, DTT, or trypsin) before BNP. Wild-type htt was more resistant to denaturants. Based on migration of in vitro translated htt fragments, the 180-kDa segment terminated ≈htt 670-880 amino acids. If second dimension SDS-PAGE followed BNP, the 180-kDa mutant htt was absent, and 43-50 kDa htt fragments appeared. Brain lysates from two HD mouse models expressed native full-length htt; a mutant fragment formed if lysates were pretreated with 8 M urea + DTT. Native full-length mutant htt in embryonic HD(140Q/140Q) mouse primary neurons was intact during cell death and when cell lysates were exposed to denaturants before BNP. Thus, native mutant htt occurs in brain and primary neurons as a soluble full-length monomer.

Reduction of mutant huntingtin accumulation and toxicity by lysosomal cathepsins D and B in neurons

Background

Huntington's disease is caused by aggregation of mutant huntingtin (mHtt) protein containing more than a 36 polyQ repeat. Upregulation of macroautophagy was suggested as a neuroprotective strategy to degrade mutant huntingtin. However, macroautophagy initiation has been shown to be highly efficient in neurons whereas lysosomal activities are rate limiting. The role of the lysosomal and other proteases in Huntington is not clear. Some studies suggest that certain protease activities may contribute to toxicity whereas others are consistent with protection. These discrepancies may be due to a number of mechanisms including distinct effects of the specific intermediate digestion products of mutant huntingtin generated by different proteases. These observations suggested a critical need to investigate the consequence of upregulation of individual lysosomal enzyme in mutant huntingtin accumulation and toxicity.

Results

In this study, we used molecular approaches to enhance lysosomal protease activities and examined their effects on mutant huntingtin level and toxicity. We found that enhanced expression of lysosomal cathepsins D and B resulted in their increased enzymatic activities and reduced both full-length and fragmented huntingtin in transfected HEK cells. Furthermore, enhanced expression of cathepsin D or B protected against mutant huntingtin toxicity in primary neurons, and their neuroprotection is dependent on macroautophagy.

Conclusions

These observations demonstrate a neuroprotective effect of enhancing lysosomal cathepsins in reducing mutant huntingtin level and toxicity in transfected cells. They highlight the potential importance of neuroprotection mediated by cathepsin D or B through macroautophagy.

Cysteine proteases bleomycin hydrolase and cathepsin Z mediate N-terminal proteolysis and toxicity of mutant huntingtin

N-terminal proteolysis of huntingtin is thought to be an important mediator of HD pathogenesis. The formation of short N-terminal fragments of huntingtin (cp-1/cp-2, cp-A/cp-B) has been demonstrated in cells and in vivo. We previously mapped the cp-2 cleavage site by mass spectrometry to position Arg167 of huntingtin. The proteolytic enzymes generating short N-terminal fragments of huntingtin remain unknown. To search for such proteases, we conducted a genome-wide screen using an RNA-silencing approach and an assay for huntingtin proteolysis based on the detection of cp-1 and cp-2 fragments by Western blotting. The primary screen was carried out in HEK293 cells, and the secondary screen was carried out in neuronal HT22 cells, transfected in both cases with a construct encoding the N-terminal 511 amino acids of mutant huntingtin. For additional validation of the hits, we employed a complementary assay for proteolysis of huntingtin involving overexpression of individual proteases with huntingtin in two cell lines. The screen identified 11 enzymes, with two major candidates to carry out the cp-2 cleavage, bleomycin hydrolase (BLMH) and cathepsin Z, which are both cysteine proteases of a papain-like structure. Knockdown of either protease reduced cp-2 cleavage, and ameliorated mutant huntingtin induced toxicity, whereas their overexpression increased the cp-2 cleavage. Both proteases partially co-localized with Htt in the cytoplasm and within or in association with early and late endosomes, with some nuclear co-localization observed for cathepsin Z. BLMH and cathepsin Z are expressed in the brain and have been associated previously with neurodegeneration. Our findings further validate the cysteine protease family, and BLMH and cathepsin Z in particular, as potential novel targets for HD therapeutics.

Small changes, big impact: posttranslational modifications and function of huntingtin in Huntington disease

Huntington disease (HD) is a neurodegenerative disorder caused by an elongated polyglutamine tract in huntingtin (htt). htt normally undergoes different posttranslational modifications (PTMs), including phosphorylation, SUMOylation, ubiquitination, acetylation, proteolytic cleavage, and palmitoylation. In the presence of the HD mutation, some PTMs are significantly altered and can result in changes in the clinical phenotype. A rate-limiting PTM is defined as one that can result in significant effects on the phenotype in animal models. For example, the prevention of proteolysis at D586 as well as constitutive phosphorylation at S13 and S16 can obviate the expression of phenotypic features of HD. The enzymes involved in these modifications such as caspase-6, the IκB kinase (IKK) complex, and still to be characterized phosphatases therefore represent promising therapeutic targets for HD. Identifying and testing specific modulators of PTMs now constitute the next big challenges in order to further validate these targets and proceed towards the goal of a mechanism-based treatment for HD.

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

Transgenic Animal Models of Huntington's Disease

Huntington's disease (HD) is a devastating neurodegenerative disorder that currently has no cure. In order to develop effective treatment, an understanding of HD pathogenesis and the evaluation of therapeutic efficacy of novel medications with the aid of animal models are critical steps. Transgenic animals sharing similar genetic defects that lead to HD have provided important discoveries in HD mechanisms that cell models are not able to replicate, which include psychiatric impairment, cognitive behavioral impact, and motor functions. Although transgenic HD rodent models have been widely used in HD research, it is clear that an animal model with comparable physiology to man, similar genetic defects that lead to HD, and the ability to develop similar cognitive and behavioral impairments is critical for explaining HD pathogenesis and the development of cures. Compared to HD rodents, HD transgenic nonhuman primates have not only developed comparable neuropathology but also present HD clinical features such as rigidity, seizure, dystonia, bradykinesia, and chorea that no other animal model has been able to replicate. Distinctive degenerating neurons and the accumulation of neuropil aggregates observed in HD monkey brain strongly support the hypothesis that the unique neuropathogenic events seen in HD monkey brain recapitulate HD in man. The latest development of transgenic HD primates has opened a new era of animal modeling that better represents human genetic disorders such as HD, which will accelerate the development of diagnostic tools and identifying novel biomarkers through longitudinal studies including gene expression and metabolite profiling, and noninvasive imaging. Furthermore, novel treatments with predictable efficacy in human patients can be developed using HD monkeys because of comparable neuropathology and clinical features.

Huntingtin cleavage product A forms in neurons and is reduced by gamma-secretase inhibitors

Background

The mutation in Huntington's disease is a polyglutamine expansion near the N-terminus of huntingtin. Huntingtin expressed in immortalized neurons is cleaved near the N-terminus to form N-terminal polypeptides known as cleavage products A and B (cpA and cpB). CpA and cpB with polyglutamine expansion form inclusions in the nucleus and cytoplasm, respectively. The formation of cpA and cpB in primary neurons has not been established and the proteases involved in the formation of these fragments are unknown.

Results

Delivery of htt cDNA into the mouse striatum using adeno-associated virus or into primary cortical neurons using lentivirus generated cpA and cpB, indicating that neurons in brain and in vitro can form these fragments. A screen of small molecule protease inhibitors introduced to clonal striatal X57 cells and HeLa cells identified compounds that reduced levels of cpA and are inhibitors of the aspartyl proteases cathepsin D and cathepsin E. The most effective compound, P1-N031, is a transition state mimetic for aspartyl proteases. By western blot analysis, cathepsin D was easily detected in clonal striatal X57 cells, mouse brain and primary neurons, whereas cathepsin E was only detectible in clonal striatal X57 cells. In primary neurons, levels of cleavage product A were not changed by the same compounds that were effective in clonal striatal cells or by mRNA silencing to partially reduce levels of cathepsin D. Instead, treating primary neurons with compounds that are known to inhibit gamma secretase activity either indirectly (Imatinib mesylate, Gleevec) or selectively (LY-411,575 or DAPT) reduced levels of cpA. LY-411,575 or DAPT also increased survival of primary neurons expressing endogenous full-length mutant huntingtin.

Conclusion

We show that cpA and cpB are produced from a larger huntingtin fragment in vivo in mouse brain and in primary neuron cultures. The aspartyl protease involved in forming cpA has cathepsin-D like properties in immortalized neurons and gamma secretase-like properties in primary neurons, suggesting that cell type may be a critical factor that specifies the aspartyl protease responsible for cpA. Since gamma secretase inhibitors were also protective in primary neurons, further study of the role of gamma-secretase activity in HD neurons is justified.

Modulation of mutant huntingtin N-terminal cleavage and its effect on aggregation and cell death

Huntington's disease (HD) is a neurodegenerative disorder caused by a polyglutamine expansion near the N-terminus of huntingtin. A neuropathological hallmark of Huntington's disease is the presence of intracellular aggregates composed of mutant huntingtin N-terminal fragments in human postmortem brain, animal models, and cell culture models. It has been found that N-terminal fragments of the mutant huntingtin protein are more toxic than the full-length protein. Therefore, proteolytic processing of mutant huntingtin may play a key event in the pathogenesis of HD. Here, we present evidence that the region in huntingtin covering amino acids 116 to 125 is critical for N-terminal proteolytic processing. Within this region, we have identified mutations that either strongly reduce or enhance N-terminal cleavage. We took advantage of this effect and demonstrate that the mutation Δ121-122 within the putative cleavage region enhances N-terminal cleavage of huntingtin and the aggregation of N-terminal fragments. Furthermore, this particular deletion increased the activation of apoptotic processes and decreased neuronal cell viability. Our data indicate that the N-terminal proteolytic processing of mutant huntingtin can be modulated with an effect on aggregation and cell death rate.

Matrix metalloproteinases are modifiers of huntingtin proteolysis and toxicity in Huntington's disease

Proteolytic cleavage of huntingtin (Htt) is known to be a key event in the pathogenesis of Huntington's disease (HD). Our understanding of proteolytic processing of Htt has thus far focused on the protease families-caspases and calpains. Identifying critical proteases involved in Htt proteolysis and toxicity using an unbiased approach has not been reported. To accomplish this, we designed a high-throughput western blot-based screen to examine the generation of the smallest N-terminal polyglutamine-containing Htt fragment. We screened 514 siRNAs targeting the repertoire of human protease genes. This screen identified 11 proteases that, when inhibited, reduced Htt fragment accumulation. Three of these belonged to the matrix metalloproteinase (MMP) family. One family member, MMP-10, directly cleaves Htt and prevents cell death when knocked down in striatal Hdh(111Q/111Q) cells. Correspondingly, MMPs are activated in HD mouse models, and loss of function of Drosophila homologs of MMPs suppresses Htt-induced neuronal dysfunction in vivo.

Proteolysis of mutant huntingtin produces an exon 1 fragment that accumulates as an aggregated protein in neuronal nuclei in Huntington disease

Huntingtin proteolysis has been implicated in the molecular pathogenesis of Huntington disease (HD). Despite an intense effort, the identity of the pathogenic smallest N-terminal fragment has not been determined. Using a panel of anti-huntingtin antibodies, we employed an unbiased approach to generate proteolytic cleavage maps of mutant and wild-type huntingtin in the HdhQ150 knock-in mouse model of HD. We identified 14 prominent N-terminal fragments, which, in addition to the full-length protein, can be readily detected in cytoplasmic but not nuclear fractions. These fragments were detected at all ages and are not a consequence of the pathogenic process. We demonstrated that the smallest fragment is an exon 1 huntingtin protein, known to contain a potent nuclear export signal. Prior to the onset of behavioral phenotypes, the exon 1 protein, and possibly other small fragments, accumulate in neuronal nuclei in the form of a detergent insoluble complex, visualized as diffuse granular nuclear staining in tissue sections. This methodology can be used to validate the inhibition of specific proteases as therapeutic targets for HD by pharmacological or genetic approaches.

The crystal structure of caspase-6, a selective effector of axonal degeneration

Neurodegenerative diseases pose one of the most pressing unmet medical needs today. It has long been recognized that caspase-6 may play a role in several neurodegenerative diseases for which there are currently no disease-modifying therapies. Thus it is a potential target for neurodegenerative drug development. In the present study we report on the biochemistry and structure of caspase-6. As an effector caspase, caspase-6 is a constitutive dimer independent of the maturation state of the enzyme. The ligand-free structure shows caspase-6 in a partially mature but latent conformation. The cleaved inter-domain linker remains partially inserted in the central groove of the dimer, as observed in other caspases. However, in contrast with the structures of other caspases, not only is the catalytic machinery misaligned, but several structural elements required for substrate recognition are missing. Most importantly, residues forming a short anti-parallel β-sheet abutting the substrate in other caspase structures are part of an elongation of the central α-helix. Despite the dramatic structural changes that are required to adopt a canonical catalytically competent conformation, the pre-steady-state kinetics exhibit no lag phase in substrate turnover. This suggests that the observed conformation does not play a regulatory role in caspase-6 activity. However, targeting the latent conformation in search for specific and bio-available caspase-6 inhibitors might offer an alternative to active-site-directed approaches.

Tau fragmentation, aggregation and clearance: the dual role of lysosomal processing

Aggregation and cleavage are two hallmarks of Tau pathology in Alzheimer disease (AD), and abnormal fragmentation of Tau is thought to contribute to the nucleation of Tau paired helical filaments. Clearance of the abnormally modified protein could occur by the ubiquitin-proteasome and autophagy-lysosomal pathways, the two major routes for protein degradation in cells. There is a debate on which of these pathways contributes to clearance of Tau protein and of the abnormal Tau aggregates formed in AD. Here, we demonstrate in an inducible neuronal cell model of tauopathy that the autophagy-lysosomal system contributes to both Tau fragmentation into pro-aggregating forms and to clearance of Tau aggregates. Inhibition of macroautophagy enhances Tau aggregation and cytotoxicity. The Tau repeat domain can be cleaved near the N terminus by a cytosolic protease to generate the fragment F1. Additional cleavage near the C terminus by the lysosomal protease cathepsin L is required to generate Tau fragments F2 and F3 that are highly amyloidogenic and capable of seeding the aggregation of Tau. We identify in this work that components of a selective form of autophagy, chaperone-mediated autophagy, are involved in the delivery of cytosolic Tau to lysosomes for this limited cleavage. However, F1 does not fully enter the lysosome but remains associated with the lysosomal membrane. Inefficient translocation of the Tau fragments across the lysosomal membrane seems to promote formation of Tau oligomers at the surface of these organelles which may act as precursors of aggregation and interfere with lysosomal functioning.

Mutant huntingtin N-terminal fragments of specific size mediate aggregation and toxicity in neuronal cells

Huntingtin proteolysis is implicated in Huntington disease pathogenesis, yet, the nature of huntingtin toxic fragments remains unclear. Huntingtin undergoes proteolysis by calpains and caspases within an N-terminal region between amino acids 460 and 600. We have focused on proteolytic steps producing shorter N-terminal fragments, which we term cp-1 and cp-2 (distinct from previously described cp-A/cp-B). We used HEK293 cells to express the first 511 residues of huntingtin and further define the cp-1 and cp-2 cleavage sites. Based on epitope mapping with huntingtin-specific antibodies, we found that cp-1 cleavage occurs between residues 81 and 129 of huntingtin. Affinity and size exclusion chromatography were used to further purify huntingtin cleavage products and enrich for the cp-1/cp-2 fragments. Using mass spectrometry, we found that the cp-2 fragment is generated by cleavage of huntingtin at position Arg(167). This site was confirmed by deletion analysis and specific detection with a custom-generated cp-2 site neo-epitope antibody. Furthermore, alterations of this cleavage site resulted in a decrease in toxicity and an increase in aggregation of huntingtin in neuronal cells. These data suggest that cleavage of huntingtin at residue Arg(167) may mediate mutant huntingtin toxicity in Huntington disease.

Cathepsin D--many functions of one aspartic protease

For years, it has been held that cathepsin D (CD) is involved in rather non-specific protein degradation in a strongly acidic milieu of lysosomes. Studies with CD knock-out mice revealed that CD is not necessary for embryonal development, but it is indispensable for postnatal tissue homeostasis. Mutation that abolishes CD enzymatic activity causes neuronal ceroid lipofuscinosis (NCL) characterized by severe neurodegeneration, developmental regression, visual loss and epilepsy in both animals and humans. In the last decade, however, an increasing number of studies demonstrated that enzymatic function of CD is not restricted solely to acidic milieu of lysosomes with important consequences in regulation of apoptosis. In addition to CD enzymatic activity, it has been shown that apoptosis is also regulated by catalytically inactive mutants of CD which suggests that CD interacts with other important molecules and influences cell signaling. Moreover, procathepsin D (pCD), secreted from cancer cells, acts as a mitogen on both cancer and stromal cells and stimulates their pro-invasive and pro-metastatic properties. Numerous studies found that pCD/CD level represents an independent prognostic factor in a variety of cancers and is therefore considered to be a potential target of anti-cancer therapy. Studies dealing with functions of cathepsin D are complicated by the fact that there are several simultaneous forms of CD in a cell-pCD, intermediate enzymatically active CD and mature heavy and light chain CD. It became evident that these forms may differently regulate the above-mentioned processes. In this article, we review the possible functions of CD and its various forms in cells and organisms during physiological and pathological conditions.

Autophagy: basic principles and relevance to disease

Autophagy is a process by which cytoplasmic components are sequestered in double membrane vesicles and degraded upon fusion with lysosomal compartments. In yeast, autophagy is activated in response to changes in the extracellular milieu. Depending upon the stimulus, autophagy can degrade cytoplasmic contents nonspecifically or can target the degradation of specific cellular components. Both of these have been adopted in higher eukaryotes and account for the expanding role of autophagy in various cellular processes, as well as contribute to the variation in cellular outcomes after induction of autophagy. In some cases, autophagy appears to be an adaptive response, whereas under other circumstances it is involved in cell death. In mammals, autophagy has been implicated in either the pathogenesis or response to a wide variety of diseases, including neurodegenerative disease, chronic bacterial and viral infections, atherosclerosis, and cancer. As the basic molecular pathways that regulate autophagy are elucidated, the relationship of autophagy to the pathogenesis of various disease states emerges.

A mutant ataxin-3 fragment results from processing at a site N-terminal to amino acid 190 in brain of Machado-Joseph disease-like transgenic mice

Machado-Joseph disease also called spinocerebellar ataxia type 3 (MJD/SCA3) is a hereditary and neurodegenerative movement disorder caused by ataxin-3 with a polyglutamine expansion (mutant ataxin-3). Neuronal loss in MJD/SCA3 is associated with a mutant ataxin-3 toxic fragment. Defining mutant ataxin-3 proteolytic site(s) could facilitate the identification of the corresponding enzyme(s). Previously, we reported a mutant ataxin-3 mjd1a fragment in the brain of transgenic mice (Q71) that contained epitopes C-terminal to amino acid 220. In this study, we generated and characterized neuroblastoma cells and transgenic mice expressing mutant ataxin-3 mjd1a lacking amino acids 190-220 (deltaQ71). Less deltaQ71 than Q71 fragments were detected in the cell but not mouse model. The transgenic mice developed an MJD/SCA3-like phenotype and their brain homogenates had a fragment containing epitopes C-terminal to amino acid 220. Our results support the toxic fragment hypothesis and narrow the mutant ataxin-3 cleavage site to the N-terminus of amino acid 190.

Characterization of Huntingtin Pathologic Fragments in Human Huntington Disease, Transgenic Mice, and Cell Models

Huntington disease (HD) is caused by the expansion of a glutamine (Q) repeat near the N terminus of huntingtin (htt), resulting in altered conformation of the mutant protein to produce, most prominently in brain neurons, nuclear and cytoplasmic inclusion pathology. The inclusions and associated diffuse accumulation of mutant htt in nuclei are composed of N-terminal fragments of mutant protein. Here, we used a panel of peptide antibodies to characterize the htt protein pathologies in brain tissues from human HD, and a transgenic mouse model created by expressing the first 171 amino acids of human htt with 82Q (htt-N171-82Q). In tissues from both sources, htt pathologic features in nuclei were detected by antibodies to htt peptides 1-17 and 81-90 but not 115-129 (wild-type huntingtin numbering with 23 repeats). Human HEK 293 cells transfected with expression vectors that encode either the N-terminal 233 amino acids of human htt (htt-N233-82Q) or htt-N171-18Q accumulated smaller N-terminal fragments with antibody-binding characteristics identical to those of pathologic peptides. We conclude that the mutant htt peptides that accumulate in pathologic structures of human HD and httN171-82Q in mice are produced by similar, yet to be defined, proteolytic events in a region of the protein near or within amino acids 90-115.

Extended polyglutamine repeats trigger a feedback loop involving the mitochondrial complex III, the proteasome and huntingtin aggregates

Mitochondrial abnormalities represent a major cytopathology in Huntington's disease (HD), a fatal neurodegenerative disease caused by CAG repeat expansions in the gene encoding huntingtin (Htt). In the present study, we investigated whether defects in the mitochondrial respiratory function are consequences of the expression of mutant Htt or they promote the formation of Htt aggregates. To take advantage of existing mitochondrial DNA mutants, we developed human osteosarcoma 143B cells expressing mutant Htt in an inducible manner and found that cells expressing mutant Htt but not wild-type Htt exhibited a reduced activity of complex III and an increased activity of complex IV. Conversely, pharmacological treatments that inhibited complex III activity significantly promoted the formation of Htt aggregates. This complex III-mediated modulation of Htt aggregates was also observed in a neuronal progenitor RN33B cell line transduced by lentivirus carrying mutant Htt. This effect of complex III inhibition on the Htt aggregates appeared to be mediated by the inhibition of proteasome activity, but not by ATP depletion or production of reactive oxygen species. Accordingly, complex III mutant cells also showed decreased proteasome activity. These results suggest the presence of a feedback system connecting the mitochondrial respiratory complex III and the production of Htt aggregates. Our results suggest that therapeutic interventions targeting complex III and/or proteasome could ameliorate the progress of HD.