Ubiquilin-1 overexpression increases the lifespan and delays accumulation of Huntingtin aggregates in the R6/2 mouse model of Huntington's disease

Neurodegenerative Diseases: New Hopes and Perspectives

Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, Huntington's disease, and Friedrich ataxia are all incurable neurodegenerative diseases defined by the continuous progressive loss of distinct neuronal subtypes. Despite their rising prevalence among the world's ageing population, fewer advances have been made in the concurrent massive efforts to develop newer drugs. Recently, there has been a shift in research focus towards the discovery of new therapeutic agents for neurodegenerative diseases. In this review, we have summarized the recently developed therapies and their status in the management of neurodegenerative diseases.

UBQLN proteins in health and disease with a focus on UBQLN2 in ALS/FTD

Ubiquilin (UBQLN) proteins are a dynamic and versatile family of proteins found in all eukaryotes that function in the regulation of proteostasis. Besides their canonical function as shuttle factors in delivering misfolded proteins to the proteasome and autophagy systems for degradation, there is emerging evidence that UBQLN proteins play broader roles in proteostasis. New information suggests the proteins function as chaperones in protein folding, protecting proteins prior to membrane insertion, and as guardians for mitochondrial protein import. In this review, we describe the evidence for these different roles, highlighting how different domains of the proteins impart these functions. We also describe how changes in the structure and phase separation properties of UBQLNs may regulate their activity and function. Finally, we discuss the pathogenic mechanisms by which mutations in UBQLN2 cause amyotrophic lateral sclerosis and frontotemporal dementia. We describe the animal model systems made for different UBQLN2 mutations and how lessons learnt from these systems provide fundamental insight into the molecular mechanisms by which UBQLN2 mutations drive disease pathogenesis through disturbances in proteostasis.

Nuclear and cytoplasmic huntingtin inclusions exhibit distinct biochemical composition, interactome and ultrastructural properties

Despite the strong evidence linking the aggregation of the Huntingtin protein (Htt) to the pathogenesis of Huntington's disease (HD), the mechanisms underlying Htt aggregation and neurodegeneration remain poorly understood. Herein, we investigated the ultrastructural properties and protein composition of Htt cytoplasmic and nuclear inclusions in mammalian cells and primary neurons overexpressing mutant exon1 of the Htt protein. Our findings provide unique insight into the ultrastructural properties of cytoplasmic and nuclear Htt inclusions and their mechanisms of formation. We show that Htt inclusion formation and maturation are complex processes that, although initially driven by polyQ-dependent Htt aggregation, also involve the polyQ and PRD domain-dependent sequestration of lipids and cytoplasmic and cytoskeletal proteins related to HD dysregulated pathways; the recruitment and accumulation of remodeled or dysfunctional membranous organelles, and the impairment of the protein quality control and degradation machinery. We also show that nuclear and cytoplasmic Htt inclusions exhibit distinct biochemical compositions and ultrastructural properties, suggesting different mechanisms of aggregation and toxicity.

Abnormally elevated ubiquilin‑1 expression in breast cancer regulates metastasis and stemness via AKT signaling

Ubiquilin-1 (UBQLN1) is an essential factor for the maintenance of proteostasis in cells. It is important for the regulation of different protein degradation mechanisms, including the ubiquitin-proteasome system, autophagy and endoplasmic reticulum-associated protein degradation pathways. However, the role of UBQLN1 in cancer progression remains largely unknown. In the present study, the expression, functions and molecular mechanisms of UBQLN1 in breast cancer tissue samples and cell lines were explored. Immunohistochemical and bioinformatics analyses revealed that UBQLN1 expression was significantly upregulated in breast cancer tissues and cell lines. UBQLN1 expression in breast cancer was significantly associated with lymph node metastasis and TNM stage. Moreover, a high UBQLN1 expression was a predictor of an unfavorable survival in patients with breast cancer. In vitro, UBQLN1 silencing markedly inhibited cell migration and invasion, epithelial-to-mesenchymal transition (EMT) and MMP expression. UBQLN1 silencing attenuated the stem cell-like properties of breast cancer cells, including their mammosphere-forming abilities. UBQLN1 knockdown also enhanced breast cancer cell chemosensitivity to paclitaxel. The expression levels of the stem cell markers. Aldehyde dehydrogenase 1 (ALDH1), Oct-4 and Sox2 were significantly decreased in the cells in which UBQLN1 was silenced, whereas breast cancer stem cells exhibited an increased expression of UBQLN1. Mechanistically, UBQLN1 knockdown inhibited the activation of AKT signaling, as revealed by the increased PTEN expression and the decreased expression of phosphorylated AKT in cells in which UBQLN1 was silenced. On the whole, the present study demonstrates that UBQLN1 is aberrantly upregulated in breast cancer and predicts a poor prognosis. The silencing of UBQLN1 inhibited the invasion, EMT and stemness of breast cancer cells, possibly via AKT signaling.

Ubiquilin-2 differentially regulates polyglutamine disease proteins

Divergent protein context helps explain why polyglutamine expansion diseases differ clinically and pathologically. This heterogeneity may also extend to how polyglutamine disease proteins are handled by cellular pathways of proteostasis. Studies suggest, for example, that the ubiquitin-proteasome shuttle protein Ubiquilin-2 (UBQLN2) selectively interacts with specific polyglutamine disease proteins. Here we employ cellular models, primary neurons and mouse models to investigate the potential differential regulation by UBQLN2 of two polyglutamine disease proteins, huntingtin (HTT) and ataxin-3 (ATXN3). In cells, overexpressed UBQLN2 selectively lowered levels of full-length pathogenic HTT but not of HTT exon 1 fragment or full-length ATXN3. Consistent with these results, UBQLN2 specifically reduced accumulation of aggregated mutant HTT but not mutant ATXN3 in mouse models of Huntington's disease (HD) and spinocerebellar ataxia type 3 (SCA3), respectively. Normally a cytoplasmic protein, UBQLN2 translocated to the nuclei of neurons in HD mice but not in SCA3 mice. Remarkably, instead of reducing the accumulation of nuclear mutant ATXN3, UBQLN2 induced an accumulation of cytoplasmic ATXN3 aggregates in neurons of SCA3 mice. Together these results reveal a selective action of UBQLN2 toward polyglutamine disease proteins, indicating that polyglutamine expansion alone is insufficient to promote UBQLN2-mediated clearance of this class of disease proteins. Additional factors, including nuclear translocation of UBQLN2, may facilitate its action to clear intranuclear, aggregated disease proteins like HTT.

New Avenues for the Treatment of Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by a CAG expansion in the HD gene. The disease is characterized by neurodegeneration, particularly in the striatum and cortex. The first symptoms usually appear in mid-life and include cognitive deficits and motor disturbances that progress over time. Despite being a genetic disorder with a known cause, several mechanisms are thought to contribute to neurodegeneration in HD, and numerous pre-clinical and clinical studies have been conducted and are currently underway to test the efficacy of therapeutic approaches targeting some of these mechanisms with varying degrees of success. Although current clinical trials may lead to the identification or refinement of treatments that are likely to improve the quality of life of those living with HD, major efforts continue to be invested at the pre-clinical level, with numerous studies testing novel approaches that show promise as disease-modifying strategies. This review offers a detailed overview of the currently approved treatment options for HD and the clinical trials for this neurodegenerative disorder that are underway and concludes by discussing potential disease-modifying treatments that have shown promise in pre-clinical studies, including increasing neurotropic support, modulating autophagy, epigenetic and genetic manipulations, and the use of nanocarriers and stem cells.

Intracellular Sources of ROS/H2O2 in Health and Neurodegeneration: Spotlight on Endoplasmic Reticulum

Reactive oxygen species (ROS) are produced continuously throughout the cell as products of various redox reactions. Yet these products function as important signal messengers, acting through oxidation of specific target factors. Whilst excess ROS production has the potential to induce oxidative stress, physiological roles of ROS are supported by a spatiotemporal equilibrium between ROS producers and scavengers such as antioxidative enzymes. In the endoplasmic reticulum (ER), hydrogen peroxide (H2O2), a non-radical ROS, is produced through the process of oxidative folding. Utilisation and dysregulation of H2O2, in particular that generated in the ER, affects not only cellular homeostasis but also the longevity of organisms. ROS dysregulation has been implicated in various pathologies including dementia and other neurodegenerative diseases, sanctioning a field of research that strives to better understand cell-intrinsic ROS production. Here we review the organelle-specific ROS-generating and consuming pathways, providing evidence that the ER is a major contributing source of potentially pathologic ROS.

Spatiotemporal analysis of soluble aggregates and autophagy markers in the R6/2 mouse model

Maintenance of cellular proteostasis is vital for post-mitotic cells like neurons to sustain normal physiological function and homeostasis, defects in which are established hallmarks of several age-related conditions like AD, PD, HD, and ALS. The Spatio-temporal accumulation of aggregated proteins in the form of inclusion bodies/plaques is one of the major characteristics of many neurodegenerative diseases, including Huntington's disease (HD). Toxic accumulation of HUNTINGTIN (HTT) aggregates in neurons bring about the aberrant phenotypes of HD, including severe motor dysfunction, dementia, and cognitive impairment at the organismal level, in an age-dependent manner. In several cellular and animal models, aggrephagy induction has been shown to clear aggregate-prone proteins like HTT and ameliorate disease pathology by conferring neuroprotection. In this study, we used the mouse model of HD, R6/2, to understand the pathogenicity of mHTT aggregates, primarily focusing on autophagy dysfunction. We report that basal autophagy is not altered in R6/2 mice, whilst being functional at a steady-state level in neurons. Moreover, we tested the efficacy of a known autophagy modulator, Nilotinib (Tasigna™), presently in clinical trials for PD, and HD, in curbing mHTT aggregate growth and their potential clearance, which was ineffective in both inducing autophagy and rescuing the pathological phenotypes in R6/2 mice.

Overexpression of UBQLN1 reduces neuropathology in the P497S UBQLN2 mouse model of ALS/FTD

Missense mutations in UBQLN2 cause X-linked dominant inheritance of amyotrophic lateral sclerosis with frontotemporal dementia (ALS/FTD). UBQLN2 belongs to a family of four highly homologous proteins expressed in humans that play diverse roles in maintaining proteostasis, but whether one isoform can substitute for another is not known. Here, we tested whether overexpression of UBQLN1 can alleviate disease in the P497S UBQLN2 mouse model of ALS/FTD by crossing transgenic (Tg) mouse lines expressing the two proteins and characterizing the resulting genotypes using a battery of pathologic and behavioral tests. The pathologic findings revealed UBQLN1 overexpression dramatically reduced the burden of UBQLN2 inclusions, neuronal loss and disturbances in proteostasis in double Tg mice compared to single P497S Tg mice. The beneficial effects of UBQLN1 overexpression were primarily confirmed by behavioral improvements seen in rotarod performance and grip strength in male, but not female mice. Paradoxically, although UBQLN1 overexpression reduced pathologic signatures of disease in P497S Tg mice, female mice had larger percentage of body weight loss than males, and this correlated with a corresponding lack of behavioral improvements in the females. These findings lead us to speculate that methods to upregulate UBQLN1 expression may reduce pathogenicity caused by UBQLN2 mutations, but may also lead to gender-specific outcomes that will have to be carefully weighed with the therapeutic benefits of UBQLN1 upregulation.

The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges

Ubiquitin Proteasome System (UPS) is an adaptable and finely tuned system that sustains proteostasis network under a large variety of physiopathological conditions. Its dysregulation is often associated with the onset and progression of human diseases; hence, UPS modulation has emerged as a promising new avenue for the development of treatments of several relevant pathologies, such as cancer and neurodegeneration. The clinical interest in proteasome inhibition has considerably increased after the FDA approval in 2003 of bortezomib for relapsed/refractory multiple myeloma, which is now used in the front-line setting. Thereafter, two other proteasome inhibitors (carfilzomib and ixazomib), designed to overcome resistance to bortezomib, have been approved for treatment-experienced patients, and a variety of novel inhibitors are currently under preclinical and clinical investigation not only for haematological malignancies but also for solid tumours. However, since UPS collapse leads to toxic misfolded proteins accumulation, proteasome is attracting even more interest as a target for the care of neurodegenerative diseases, which are sustained by UPS impairment. Thus, conceptually, proteasome activation represents an innovative and largely unexplored target for drug development. According to a multidisciplinary approach, spanning from chemistry, biochemistry, molecular biology to pharmacology, this review will summarize the most recent available literature regarding different aspects of proteasome biology, focusing on structure, function and regulation of proteasome in physiological and pathological processes, mostly cancer and neurodegenerative diseases, connecting biochemical features and clinical studies of proteasome targeting drugs.

Therapeutic Advances for Huntington's Disease

Huntington’s disease (HD) is a progressive neurological disease that is inherited in an autosomal fashion. The cause of disease pathology is an expansion of cytosine-adenine-guanine (CAG) repeats within the huntingtin gene (HTT) on chromosome 4 (4p16.3), which codes the huntingtin protein (mHTT). The common symptoms of HD include motor and cognitive impairment of psychiatric functions. Patients exhibit a representative phenotype of involuntary movement (chorea) of limbs, impaired cognition, and severe psychiatric disturbances (mood swings, depression, and personality changes). A variety of symptomatic treatments (which target glutamate and dopamine pathways, caspases, inhibition of aggregation, mitochondrial dysfunction, transcriptional dysregulation, and fetal neural transplants, etc.) are available and some are in the pipeline. Advancement in novel therapeutic approaches include targeting the mutant huntingtin (mHTT) protein and the HTT gene. New gene editing techniques will reduce the CAG repeats. More appropriate and readily tractable treatment goals, coupled with advances in analytical tools will help to assess the clinical outcomes of HD treatments. This will not only improve the quality of life and life span of HD patients, but it will also provide a beneficial role in other inherited and neurological disorders. In this review, we aim to discuss current therapeutic research approaches and their possible uses for HD.

Recognition and Degradation of Mislocalized Proteins in Health and Disease

A defining feature of eukaryotic cells is the segregation of complex biochemical processes among different intracellular compartments. The protein targeting, translocation, and trafficking pathways that sustain compartmentalization must recognize a diverse range of clients via degenerate signals. This recognition is imperfect, resulting in polypeptides at incorrect cellular locations. Cells have evolved mechanisms to selectively recognize mislocalized proteins and triage them for degradation or rescue. These spatial quality control pathways maintain cellular protein homeostasis, become especially important during organelle stress, and might contribute to disease when they are impaired or overwhelmed.

Upregulation of PSMB8 and cathepsins in the human brains of dementia with Lewy bodies

Proteasome and lysosome are responsible for the homeostasis of proteins, lipids and carbohydrates in cells. Numerous reports indicate the proteolytic pathways have altered functions during neurodegeneration and aging. Dementia with Lewy bodies (DLB) is one of the leading forms of dementia, and the proteolytic alteration in DLB has not yet been fully investigated. This study shows that the components of proteasome and lysosome had selectively altered gene expression and enzymatic functions. Specifically, PSMB8, an inducible proteasomal β subunit, had elevated mRNA level and protein level in DLB brain compared with age-matched controls. The proteasomal caspase-like peptidase showed significant decreased activity in DLB brains and the trypsin-like/chemotrypsin-like activities did not reach statistical significance. Lysosomal cathepsin B and D had elevated mRNA levels while only cathepsin B showed elevated enzymatic activity in DLB brains. This data indicate that the alteration of proteolytic pathways is highly selective and comprehensive. Further study to elucidate the correlation between neurodegenerative development and the alteration of proteolytic pathways would be important for therapeutic development.

Small molecule modulator of protein disulfide isomerase attenuates mutant huntingtin toxicity and inhibits endoplasmic reticulum stress in a mouse model of Huntington's disease

Huntington's disease (HD) is caused by a cytosine-adenine-guanine (CAG) trinucleotide repeat expansion in the huntingtin (HTT) gene encoding an elongated polyglutamine tract within the N-terminal of the huntingtin protein (Htt) and leads to Htt misfolding, aberrant protein aggregation, and progressive appearance of disease symptoms. Chronic activation of endoplasmic reticulum (ER) stress by mutant Htt (mHtt) results in cellular dysfunction and ultimately cell death. Protein disulfide isomerase (PDI) is a chaperone protein located in the ER. Our previous studies demonstrated that mHtt caused PDI to accumulate at mitochondria-associated ER membranes and triggered cell death, and that modulating PDI activity using small molecules protected cells again mHtt toxicity in cell and brain slice models of HD. In this study, we demonstrated that PDI is upregulated in the HD human brain, in cell and mouse models. Chronic administration of a reversible, brain penetrable small molecule PDI modulator, LOC14 (20 mg/kg/day), significantly improved motor function, attenuated brain atrophy and extended survival in the N171-82Q HD mice. Moreover, LOC14 preserved medium spiny neuronal marker dopamine- and cyclic-AMP-regulated phosphoprotein of molecular weight 32 000 (DARPP32) levels in the striatum of HD mice. Mechanistic study revealed that LOC14 suppressed mHtt-induced ER stress, indicated by repressing the abnormally upregulated ER stress proteins in HD models. These findings suggest that LOC14 is promising to be further optimized for clinical trials of HD, and modulation of signaling pathways coping with ER stress may constitute an attractive approach to reduce mHtt toxicity and identify new therapeutic targets for treatment of HD.

Distinct proteostasis circuits cooperate in nuclear and cytoplasmic protein quality control

Protein misfolding is linked to a wide array of human disorders, including Alzheimer's disease, Parkinson's disease and type II diabetes^1,2. Protective cellular protein quality control (PQC) mechanisms have evolved to selectively recognize misfolded proteins and limit their toxic effects^3-9, thus contributing to the maintenance of the proteome (proteostasis). Here we examine how molecular chaperones and the ubiquitin-proteasome system cooperate to recognize and promote the clearance of soluble misfolded proteins. Using a panel of PQC substrates with distinct characteristics and localizations, we define distinct chaperone and ubiquitination circuitries that execute quality control in the cytoplasm and nucleus. In the cytoplasm, proteasomal degradation of misfolded proteins requires tagging with mixed lysine 48 (K48)- and lysine 11 (K11)-linked ubiquitin chains. A distinct combination of E3 ubiquitin ligases and specific chaperones is required to achieve each type of linkage-specific ubiquitination. In the nucleus, however, proteasomal degradation of misfolded proteins requires only K48-linked ubiquitin chains, and is thus independent of K11-specific ligases and chaperones. The distinct ubiquitin codes for nuclear and cytoplasmic PQC appear to be linked to the function of the ubiquilin protein Dsk2, which is specifically required to clear nuclear misfolded proteins. Our work defines the principles of cytoplasmic and nuclear PQC as distinct, involving combinatorial recognition by defined sets of cooperating chaperones and E3 ligases. A better understanding of how these organelle-specific PQC requirements implement proteome integrity has implications for our understanding of diseases linked to impaired protein clearance and proteostasis dysfunction.

Inadequate ubiquitination-proteasome coupling contributes to myocardial ischemia-reperfusion injury

The ubiquitin-proteasome system (UPS) degrades a protein molecule via 2 main steps: ubiquitination and proteasomal degradation. Extraproteasomal ubiquitin receptors are thought to couple the 2 steps, but this proposition has not been tested in vivo with vertebrates. More importantly, impaired UPS performance plays a major role in cardiac pathogenesis, including myocardial ischemia-reperfusion injury (IRI), but the molecular basis of UPS impairment remains poorly understood. Ubiquilin1 is a bona fide extraproteasomal ubiquitin receptor. Here, we report that mice with a cardiomyocyte-restricted knockout of Ubiquilin1 (Ubqln1-CKO mice) accumulated a surrogate UPS substrate (GFPdgn) and increased myocardial ubiquitinated proteins without altering proteasome activities, resulting in late-onset cardiomyopathy and a markedly shortened life span. When subject to regional myocardial ischemia-reperfusion, young Ubqln1-CKO mice showed substantially exacerbated cardiac malfunction and enlarged infarct size, and conversely, mice with transgenic Ubqln1 overexpression displayed attenuated IRI. Furthermore, Ubqln1 overexpression facilitated proteasomal degradation of oxidized proteins and the degradation of a UPS surrogate substrate in cultured cardiomyocytes without increasing autophagic flux. These findings demonstrate that Ubiquilin1 is essential to cardiac ubiquitination-proteasome coupling and that an inadequacy in the coupling represents a major pathogenic factor for myocardial IRI; therefore, strategies to strengthen coupling have the potential to reduce IRI.

New directions in therapeutics for Huntington disease

Huntington disease (HD) is an autosomal dominantly inherited neurodegenerative disease that affects motor, cognitive and psychiatric functions, and ultimately leads to death. The pathology of the disease is based on an expansion of CAG repeats in exon 1 of the huntingtin gene on chromosome 4, which produces a mutant huntingtin protein (mHtt). This protein is involved in neurotoxicity and brain atrophy, and can form β-sheets and abnormal mHtt aggregates. Currently, there are no approved effective treatments for HD, although tetrabenazine (Xenazine™) and deutetrabenazine (AUSTEDO™) have been approved for treatment of the motor symptom chorea in HD. This literature review aims to address the latest research on promising therapeutics based on influencing the hypothesized pathological mechanisms.

Proteostasis of Huntingtin in Health and Disease

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by motor dysfunction, cognitive deficits and psychosis. HD is caused by mutations in the Huntingtin (HTT) gene, resulting in the expansion of polyglutamine (polyQ) repeats in the HTT protein. Mutant HTT is prone to aggregation, and the accumulation of polyQ-expanded fibrils as well as intermediate oligomers formed during the aggregation process contribute to neurodegeneration. Distinct protein homeostasis (proteostasis) nodes such as chaperone-mediated folding and proteolytic systems regulate the aggregation and degradation of HTT. Moreover, polyQ-expanded HTT fibrils and oligomers can lead to a global collapse in neuronal proteostasis, a process that contributes to neurodegeneration. The ability to maintain proteostasis of HTT declines during the aging process. Conversely, mechanisms that preserve proteostasis delay the onset of HD. Here we will review the link between proteostasis, aging and HD-related changes.

Reduced body weight gain in ubiquilin-1 transgenic mice is associated with increased expression of energy-sensing proteins

Ubiquilin-1 (Ubqln1), a ubiquitin-like protein, is implicated in a variety of pathophysiological processes, but its role in mediating body weight gain or metabolism has not been determined. Here, we demonstrate that global overexpression of Ubqln1 in a transgenic (Tg) mouse reduces the animal's body weight gain. The decreased body weight gain in Tg mice is associated with lower visceral fat content and higher metabolic rate. The Ubqln1 Tg mice exhibited reduced leptin and insulin levels as well as increased insulin sensitivity manifested by homeostatic model assessment of insulin resistance. Additionally, the reduced body weight in Tg mice was associated with the upregulation of two energy-sensing proteins, sirtuin1 (SIRT1) in the hypothalamus and AMP-activated protein kinase (AMPK) in the skeletal muscle. Consistent with the in vivo results, overexpression of Ubqln1 significantly increased SIRT1 and AMPK levels in the mouse embryonic fibroblast cell culture. Thus, our results not only establish the link between Ubqln1 and body weight regulation but also indicate that the metabolic function of Ubqln1 on body weight may be through regulating energy-sensing proteins.

The Ubiquitin Receptor ADRM1 Modulates HAP40-Induced Proteasome Activity

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by an N-terminal expansion of polyglutamine stretch (polyQ) of huntingtin (Htt) protein. HAP40 is a huntingtin-associated protein with unknown cellular functions. Increased HAP40 expression has been reported in the brain of HD patients and HD mouse model. However, the relationship between the elevation of HAP40 and HD etiology remains elusive. In this study, we demonstrated that overexpression of HAP40 enhanced accumulation of mutant Htt aggregates and caused defects in proteasome function. Specifically, excess HAP40 interfered with adhesion-regulating molecule 1 (ADRM1), a proteasome ubiquitin receptor, to regulate the proteasome-dependent pathway. Increasing ADRM1 in the presence of excess HAP40 alleviated mutant Htt aggregates and at the same time, restored the cell viability. Reducing ADRM1 in the absence of excess HAP40; on the other hand, increased mutant Htt aggregates and decreased the cell viability. Our data provide compelling evidence to support that ADRM1 plays an important role in mediating removal of mutant Htt aggregates when excess HAP40 is present. ADRM1-dependent ubiquitin proteasome system (UPS) may be a general mechanism to guard cells from mutant Htt toxicity.

FUS/TLS acts as an aggregation-dependent modifier of polyglutamine disease model mice

FUS/TLS is an RNA/DNA-binding protein associated with neurodegenerative diseases including amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Previously, we found that a prion-like domain in the N-terminus of FUS/TLS mediates co-aggregation between FUS/TLS and mutant huntingtin, the gene product of Huntington's disease (HD). Here, we show that heterozygous knockout of FUS/TLS worsened the phenotypes of model mice of (HD, but not spinal and bulbar muscular atrophy (SBMA). This difference was correlated with the degree of pathological association between disease proteins and FUS/TLS. Co-aggregation between FUS/TLS and mutant huntingtin resulted in the depletion of free FUS/TLS protein in HD mice that was detected as a monomer in SDS-PAGE analysis. Recently, we found that FUS/TLS paralogs, TAF15 and EWS, were up-regulated in homozygous FUS/TLS knockout mice. These two proteins were up-regulated in both HD and FUS/TLS heterozygote mice, and were further elevated in HD-TLS^+/- double mutant mice, consistent with the functional impairment of FUS/TLS. These results suggest that FUS/TLS sequestration by co-aggregation is a rate-limiting factor of disease phenotypes of HD and that inclusions may have an adverse aspect, rather than being simply benign or protective. In addition, our results highlight inclusions as repositories of potential modifiers of neurodegeneration.

Ubiquilins Chaperone and Triage Mitochondrial Membrane Proteins for Degradation

We investigated how mitochondrial membrane proteins remain soluble in the cytosol until their delivery to mitochondria or degradation at the proteasome. We show that Ubiquilin family proteins bind transmembrane domains in the cytosol to prevent aggregation and temporarily allow opportunities for membrane targeting. Over time, Ubiquilins recruit an E3 ligase to ubiquitinate bound clients. The attached ubiquitin engages Ubiquilin's UBA domain, normally bound to an intramolecular UBL domain, and stabilizes the Ubiquilin-client complex. This conformational change precludes additional chances at membrane targeting for the client, while simultaneously freeing Ubiquilin's UBL domain for targeting to the proteasome. Loss of Ubiquilins by genetic ablation or sequestration in polyglutamine aggregates leads to accumulation of non-inserted mitochondrial membrane protein precursors. These findings define Ubiquilins as a family of chaperones for cytosolically exposed transmembrane domains and explain how they use ubiquitin to triage clients for degradation via coordinated intra- and intermolecular interactions.

The Unfolded Protein Response and the Role of Protein Disulfide Isomerase in Neurodegeneration

The maintenance and regulation of proteostasis is a critical function for post-mitotic neurons and its dysregulation is increasingly implicated in neurodegenerative diseases. Despite having different clinical manifestations, these disorders share similar pathology; an accumulation of misfolded proteins in neurons and subsequent disruption to cellular proteostasis. The endoplasmic reticulum (ER) is an important component of proteostasis, and when the accumulation of misfolded proteins occurs within the ER, this disturbs ER homeostasis, giving rise to ER stress. This triggers the unfolded protein response (UPR), distinct signaling pathways that whilst initially protective, are pro-apoptotic if ER stress is prolonged. ER stress is increasingly implicated in neurodegenerative diseases, and emerging evidence highlights the complexity of the UPR in these disorders, with both protective and detrimental components being described. Protein Disulfide Isomerase (PDI) is an ER chaperone induced during ER stress that is responsible for the formation of disulfide bonds in proteins. Whilst initially considered to be protective, recent studies have revealed unconventional roles for PDI in neurodegenerative diseases, distinct from its normal function in the UPR and the ER, although these mechanisms remain poorly defined. However, specific aspects of PDI function may offer the potential to be exploited therapeutically in the future. This review will focus on the evidence linking ER stress and the UPR to neurodegenerative diseases, with particular emphasis on the emerging functions ascribed to PDI in these conditions.

Ubiquilin-mediated Small Molecule Inhibition of Mammalian Target of Rapamycin Complex 1 (mTORC1) Signaling

Mammalian target of rapamycin complex 1 (mTORC1) is a master regulator of cellular metabolism, growth, and proliferation. mTORC1 has been implicated in many diseases such as cancer, diabetes, and neurodegeneration, and is a target to prolong lifespan. Here we report a small molecule inhibitor (Cbz-B3A) of mTORC1 signaling. Cbz-B3A inhibits the phosphorylation of eIF4E-binding protein 1 (4EBP1) and blocks 68% of translation. In contrast, rapamycin preferentially inhibits the phosphorylation of p70(S6k) and blocks 35% of translation. Cbz-B3A does not appear to bind directly to mTORC1, but instead binds to ubiquilins 1, 2, and 4. Knockdown of ubiquilin 2, but not ubiquilins 1 and 4, decreases the phosphorylation of 4EBP1, suggesting that ubiquilin 2 activates mTORC1. The knockdown of ubiquilins 2 and 4 decreases the effect of Cbz-B3A on 4EBP1 phosphorylation. Cbz-B3A slows cellular growth of some human leukemia cell lines, but is not cytotoxic. Thus Cbz-B3A exemplifies a novel strategy to inhibit mTORC1 signaling that might be exploited for treating many human diseases. We propose that Cbz-B3A reveals a previously unappreciated regulatory pathway coordinating cytosolic protein quality control and mTORC1 signaling.

UPS Activation in the Battle Against Aging and Aggregation-Related Diseases: An Extended Review

Aging is a biological process accompanied by gradual increase of damage in all cellular macromolecules, i.e., nucleic acids, lipids, and proteins. When the proteostasis network (chaperones and proteolytic systems) cannot reverse the damage load due to its excess as compared to cellular repair/regeneration capacity, failure of homeostasis is established. This failure is a major hallmark of aging and/or aggregation-related diseases. Dysfunction of the major cellular proteolytic machineries, namely the proteasome and the lysosome, has been reported during the progression of aging and aggregation-prone diseases. Therefore, activation of these pathways is considered as a possible preventive or therapeutic approach against the progression of these processes. This chapter focuses on UPS activation studies in cellular and organismal models and the effects of such activation on aging, longevity and disease prevention or reversal.

Differential recruitment of UBQLN2 to nuclear inclusions in the polyglutamine diseases HD and SCA3

Accumulation of mutant polyglutamine proteins in intraneuronal inclusions is a hallmark of polyglutamine diseases. Impairment of protein clearance systems and sequestration of clearance-related proteins into inclusions occur in many protein folding diseases, including polyglutamine diseases. The ubiquitin-binding and proteasome adaptor protein UBQLN2 participates in protein homeostasis and localizes to inclusions in various neurodegenerative diseases. Employing mouse models and human brain tissue of Huntington's disease (HD) and spinocerebellar ataxia type 3 (SCA3), we show that UBQLN2 is selectively recruited to inclusions in HD but not SCA3. Consistent with this result, in a cell-based system mutant HTT interacts with UBQLN2 through the UBA domain while the SCA3 disease protein ATXN3, a deubiquitinating enzyme, does not interact with UBQLN2. Differential recruitment of UBQLN2 to aggregates in HD and SCA3 underscores the heterogeneity of inclusions in polyglutamine diseases and suggests that components of neuronal protein quality control may be differentially perturbed in distinct polyQ diseases.

Proteasome activation: An innovative promising approach for delaying aging and retarding age-related diseases

Aging is a natural process accompanied by a progressive accumulation of damage in all constituent macromolecules (nucleic acids, lipids and proteins). Accumulation of damage in proteins leads to failure of proteostasis (or vice versa) due to increased levels of unfolded, misfolded or aggregated proteins and, in turn, to aging and/or age-related diseases. The major cellular proteolytic machineries, namely the proteasome and the lysosome, have been shown to dysfunction during aging and age-related diseases. Regarding the proteasome, it is well established that it can be activated either through genetic manipulation or through treatment with natural or chemical compounds that eventually result to extension of lifespan or deceleration of the progression of age-related diseases. This review article focuses on proteasome activation studies in several species and cellular models and their effects on aging and longevity. Moreover, it summarizes findings regarding proteasome activation in the major age-related diseases as well as in progeroid syndromes.

Ubiquilin 1 Promotes IFN-γ-Induced Xenophagy of Mycobacterium tuberculosis

The success of Mycobacterium tuberculosis (Mtb) as a pathogen rests upon its ability to grow intracellularly in macrophages. Interferon-gamma (IFN-γ) is critical in host defense against Mtb and stimulates macrophage clearance of Mtb through an autophagy pathway. Here we show that the host protein ubiquilin 1 (UBQLN1) promotes IFN-γ-mediated autophagic clearance of Mtb. Ubiquilin family members have previously been shown to recognize proteins that aggregate in neurodegenerative disorders. We find that UBQLN1 can interact with Mtb surface proteins and associates with the bacilli in vitro. In IFN-γ activated macrophages, UBQLN1 co-localizes with Mtb and promotes the anti-mycobacterial activity of IFN-γ. The association of UBQLN1 with Mtb depends upon the secreted bacterial protein, EsxA, which is involved in permeabilizing host phagosomes. In autophagy-deficient macrophages, UBQLN1 accumulates around Mtb, consistent with the idea that it marks bacilli that traffic through the autophagy pathway. Moreover, UBQLN1 promotes ubiquitin, p62, and LC3 accumulation around Mtb, acting independently of the E3 ligase parkin. In summary, we propose a model in which UBQLN1 recognizes Mtb and in turn recruits the autophagy machinery thereby promoting intracellular control of Mtb. Thus, polymorphisms in ubiquilins, which are known to influence susceptibility to neurodegenerative illnesses, might also play a role in host defense against Mtb.

More people die from Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), than any other bacterial pathogen. It has long been appreciated that Mtb can survive and divide within macrophages, white blood cells that normally kill bacteria. Macrophages are able to partially control Mtb through a degradative process called autophagy. Autophagy is activated by the cytokine interferon-gamma (IFN-γ), which promotes control of Mtb infection. How the tubercle bacilli are targeted to the autophagy pathway remains unclear. Here we show that the human protein ubiquilin 1 can interact with Mtb surface proteins and associate with Mtb that are present in the host cell cytosol. We propose a model in which activating autophagy with IFN-γ promotes UBQLN1 recruitment to Mtb, which in turn leads to recruitment of the autophagy machinery, autophagy-mediated degradation of the bacteria, and activation of effector T cells. Since IFN-γ is critical in human control of Mtb, our study suggests that polymorphisms in ubiquilins, known to influence susceptibility to neurodegenerative illnesses, might also play a role in host defense against Mtb.

Defective Proteasome Delivery of Polyubiquitinated Proteins by Ubiquilin-2 Proteins Containing ALS Mutations

Ubiquilin proteins facilitate delivery of ubiquitinated proteins to the proteasome for degradation. Interest in the proteins has been heightened by the discovery that gene mutations in UBQLN2 cause dominant inheritance of amyotrophic lateral sclerosis (ALS). However, the mechanisms by which the mutations cause ALS are not known. Here we report on the underlying defect of ubiquilin-2 proteins containing ALS-linked mutations in affecting proteasome-mediated degradation. We found that overexpression of ubiquilin-2 proteins containing any one of five different ALS mutations slow degradation of Myc, a prototypic proteasome substrate. Examination of coprecipitating proteins indicated that the mutant proteins are generally capable of binding polyubiquitinated proteins, but defective in binding the proteasome. GST-pulldown studies revealed that many of the mutants bind weaker to the S5a subunit of the proteasome, compared with wild type (WT) ubiquilin-2 protein. The results suggest the mutant proteins are unable to deliver their captured cargo to the proteasome for degradation, which presumably leads to toxicity. Quantification of cell death is consistent with this idea. Measurement of protein turnover further indicated the mutant proteins have longer half-lives than WT ubiquilin-2. Our studies provide novel insight into the mechanism by which ALS-linked mutations in UBQLN2 interfere with protein degradation.

Signature changes in ubiquilin expression in the R6/2 mouse model of Huntington's disease

Ubiquilin proteins have been implicated in the cause and the pathology of neurodegenerative diseases. In the R6/2 mouse model of Huntington's disease (HD), ubiquilin levels decline during disease progression. Restoration of their levels by transgenic expression of ubiquilin-1 extends survival. Here we provide a comprehensive assessment of the expression and localization of all four ubiquilin proteins in both normal and R6/2-affected mice brains, using antibodies specific for each protein. Ubiquilin-1, 2 and 4 proteins were detected throughout the brain, with increased expression seen in the hippocampus and cerebellum. Ubiquilin-3 expression was not detected. All three ubiquilins expressed in the brain were found in Htt inclusions. Their expression changed during development and disease. Ubiquilin-1 and ubiquilin-2 protein levels decreased from 6 to 18 weeks of mouse development, independent of disease. Ubiquilin-1 and ubiquilin-4 protein levels also changed during HD disease progression. Ubiquilin-4 proteins that are normally expressed in the brain were lost and instead replaced by a novel 115 kDa higher molecular weight immunoreactive band. Taken together, our results demonstrate that all ubiquilin proteins are involved in HD pathology and that distinct changes in the signature of ubiquilin-4 expression could be useful for monitoring end-stage of HD disease.

An update on Huntington's disease: from the gene to the clinic

PURPOSE OF REVIEW

This review highlights the recent advances in Huntington's disease, with a particular focus on development of disease biomarkers for use in therapeutic trials in the premotor phase of the disease, as well as the growing literature regarding pathophysiological mechanisms and their relevance to potential therapeutic targets.

RECENT FINDINGS

There have been continued advances in the development of disease biomarkers, and promising neuroprotection trials are beginning to emerge in the premotor stage of Huntington's disease. Deeper understanding of the pathophysiological mechanisms is being translated into potential therapeutic strategies.

SUMMARY

The premotor stage of Huntington's disease provides an ideal time to trial disease-modifying therapy, but reliable biomarkers are required for monitoring disease progression, and this remains an area of intense research. Our understanding of the underlying pathophysiological mechanisms continues to expand, and a number of promising therapeutic strategies are emerging, including strategies to silence mutant huntingtin expression.