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Synucleinopathies are neurodegenerative disorders characterized by the accumulation of α -synuclein containing Lewy bodies. Ubiquitination, a key post-translational modification, has been recognized as a pivotal regulator of α -synuclein's cellular dynamics, influencing its degradation, aggregation, and associated neurotoxicity. This review examines comprehensively the current understanding of α -synuclein ubiquitination and its role in the pathogenesis of synucleinopathies, particularly in the context of Parkinson's disease. We explore the molecular mechanisms responsible for α -synuclein ubiquitination, with a focus on the roles of E3 ligases and deubiquitinases implicated in the degradation process which occurs primarily through the endosomal lysosomal pathway. The review further discusses how the dysregulation of these mechanisms contributes to α -synuclein aggregation and LB formation and offers suggestions for future investigations into the role of α -synuclein ubiquitination. Understanding these processes may shed light on potential therapeutic avenues that can modulate α -synuclein ubiquitination to alleviate its pathological impact in synucleinopathies., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Ho and Wing.)

Aims: The objective of this umbrella review and meta-analysis was to evaluate the effect of diabetes on risk of dementia, as well as the mitigating effect of antidiabetic treatments., Materials and Methods: We conducted a systematic umbrella review on diabetes and its treatment, and a meta-analysis focusing on treatment. We searched MEDLINE/PubMed, Embase, PsycINFO, CINAHL and the Cochrane Library for systematic reviews and meta-analyses assessing the risk of cognitive decline/dementia in individuals with diabetes until 2 July 2023. We conducted random-effects meta-analyses to obtain risk ratios and 95% confidence intervals estimating the association of metformin, thiazolidinediones, pioglitazone, dipeptidyl peptidase-4 inhibitors, α-glucosidase inhibitors, meglitinides, insulin, sulphonylureas, glucagon-like peptide-1 receptor agonists (GLP1RAs) and sodium-glucose cotransporter-2 inhibitors (SGLT2is) with risk of dementia from cohort/case-control studies. The subgroups analysed included country and world region. Risk of bias was assessed with the AMSTAR tool and Newcastle-Ottawa Scale., Results: We included 100 reviews and 27 cohort/case-control studies (N = 3 046 661). Metformin, thiazolidinediones, pioglitazone, GLP1RAs and SGLT2is were associated with significant reduction in risk of dementia. When studies examining metformin were divided by country, the only significant effect was for the United States. Moreover, the effect of metformin was significant in Western but not Eastern populations. No significant effect was observed for dipeptidyl peptidase-4 inhibitors, α-glucosidase inhibitors, or insulin, while meglitinides and sulphonylureas were associated with increased risk., Conclusions: Metformin, thiazolidinediones, pioglitazone, GLP1RAs and SGLT2is were associated with reduced risk of dementia. More longitudinal studies aimed at determining their relative benefit in different populations should be conducted., (© 2023 The Authors. Diabetes, Obesity and Metabolism published by John Wiley & Sons Ltd.)

The USP19 deubiquitinase is found in a locus associated with Parkinson's Disease (PD), interacts with chaperonins, and promotes secretion of α-synuclein (α-syn) through the misfolding-associated protein secretion (MAPS) pathway. Since these processes might modulate the processing of α-syn aggregates in PD, we inactivated USP19 (KO) in mice expressing the A53T mutation of α-syn and in whom α-syn preformed fibrils (PFF) had been injected in the striatum. Compared to WT, KO brains showed decreased accumulation of phospho-synuclein (pSyn) positive aggregates. This improvement was associated with less activation of microglia and improved performance in a tail-suspension test. Exposure of primary neurons from WT and KO mice to PFF in vitro also led to decreased accumulation of pSyn aggregates. KO did not affect uptake of PFF nor propagation of aggregates in the cultured neurons. We conclude that USP19 instead modulates intracellular dynamics of aggregates. At an early time following PFF injection when the number of pSyn-positive neurons were similar in WT and KO brains, the KO neurons contained less aggregates. KO brain aggregates stained more intensely with anti-ubiquitin antibodies. Immunoprecipitation of soluble proteins from WT and KO brains with antibodies to pSyn showed higher levels of ubiquitinated oligomeric species in the KO samples. We propose that the improved pathology in USP19 KO brains may arise from decreased formation or enhanced clearance of the more ubiquitinated aggregates and/or enhanced disassembly towards more soluble oligomeric species. USP19 inhibition may represent a novel therapeutic approach that targets the intracellular dynamics of α-syn complexes., (© 2023. The Author(s).)

Background: Sex differences in the pathogenesis of hypertension exist. While gut microbiota (GM) has been associated with hypertension, it is unclear whether there are sex-linked differences in the association between GM and hypertension., Methods: We conducted a cross-sectional study to investigate the sex differences in associations between GM characterized by shotgun sequencing, GM-derived short-chain fatty acids, and 24-hour ambulatory blood pressure in 241 Hong Kong Chinese (113 men and 128 women; mean age, 54±6 years)., Results: The hypertensive group was associated with GM alterations; however, significant differences in β-diversity and GM composition in hypertensive versus normotensive groups were only observed in women and not in men under various statistical models adjusting for the following covariates: age, sex, body mass index, sodium intake estimated by spot urine analysis, blood glucose, triglycerides, low- and high-density lipoprotein cholesterol, smoking, menopause, and fatty liver status. Specifically, Ruminococcus gnavus , Clostridium bolteae , and Bacteroides ovatus were significantly more abundant in the hypertensive women, whereas Dorea formicigenerans was more abundant in the normotensive women. No bacterial species were found to be significantly associated with hypertension in men. Furthermore, total plasma short-chain fatty acids and propionic acid were independent predictors of systolic and diastolic blood pressure in women but not men., Conclusions: GM dysregulation was strongly associated with 24-hour ambulatory blood pressure in women but not men, which may be mediated through propionic acid. Our work suggests that sex differences may be an important consideration while assessing the role of GM in the development and treatment of hypertension., Competing Interests: Disclosures K.K. Lau received grants from Research Fund Secretariat of the Food and Health Bureau, Innovation and Technology Bureau, Research Grants Council, Amgen, Boehringer Ingelheim, Eisai, and Pfizer and consultation fees from Amgen, Boehringer Ingelheim, Daiichi Sankyo, and Sanofi, all outside the submitted work. S.Y. Leung has received research sponsorship from Pfizer, Merck, Servier, and Curegenix. S.K. Chung received a faculty research grant (FRG-22-024-FMD) and Dr Neher’s Biophysics Laboratory for Innovative Drug Discovery from the Macao Science and Technology Development Fund (FDCT project code: 001/2020/ALC), all outside of the submitted work. The other authors report no conflicts.

Importance: Cross-sectional studies have shown that combined low muscle mass and strength are associated with cognitive impairment. Whether low muscle mass, reflective of physiologic reserve, is independently associated with faster cognitive decline remains unknown., Objective: To investigate the associations between low muscle mass and cognitive decline in 3 distinct domains among adults aged at least 65 years., Design, Setting, and Participants: The Canadian Longitudinal Study on Aging is a prospective population-based cohort study of community-dwelling adults. Enrollment occurred from 2011 to 2015 with a 3-year follow-up. Analyses for this study were conducted on those aged at least 65 years from April 24 to August 12, 2020., Exposure: Appendicular lean soft tissue mass (ALM) was assessed by dual energy x-ray absorptiometry. Low ALM was identified using the sex-specific Canadian cut points., Main Outcomes and Measures: Memory was assessed using the Rey auditory verbal learning test. Executive function was assessed using the mental alternation test, Stroop high interference (words/dot) test, the animal fluency test, and the controlled oral word association test. Psychomotor speed was assessed using computer-administered choice reaction time. Composite scores by domain were created., Results: Of 8279 participants, 4003 (48%) were female, 8005 (97%) were White, and the mean (SD) age was 72.9 (5.6) years. A total of 1605 participants (19.4%) had low ALM at baseline. Participants with low ALM were older, had lower body mass index and physical activity level. The presence of low ALM at baseline was associated with faster 3-year cognitive decline in executive functions and psychomotor speed from multiple linear regressions. After adjusting for covariates including age, level of education, percentage body fat, and handgrip strength, low ALM remained independently associated with executive function decline (standardized β: -0.032; P = .03) only. Low ALM was not associated with memory., Conclusions and Relevance: This cohort study found longitudinal associations between low ALM and cognition in aging. Identification of older adults with low muscle mass, a targetable modifiable factor, may help estimate those at risk for accelerated executive function decline. Further longer-term investigation of associations is warranted.

CTNNB1-related disorder is an autosomal dominant neurodevelopmental disorder characterized by a variable degree of cognitive impairment, microcephaly, truncal hypotonia, peripheral spasticity, visual defects, and dysmorphic features. In this case series, we report the clinical and molecular findings of nine Chinese patients affected by CTNNB1-related disorders. The facial features of these affected individuals appear to resemble what had been previously described, with thin upper lip (77.8%) and hypoplastic alae nasi (77.8%) being the most common. Frequently reported clinical characteristics in our cohort include developmental delay (100%), peripheral spasticity (88.9%), truncal hypotonia (66.7%), microcephaly (66.7%), and dystonia (44.4%). While various eye manifestations were reported, two affected individuals (22.2%) in our cohort had familial exudative vitreoretinopathy. One of the affected individuals had craniosynostosis, a feature not reported in the literature before. To our knowledge, this is the first reported Chinese case series of CTNNB1-related neurodevelopmental disorders. Further studies are required to look into whether ethnic differences play a role in phenotypic variations., (© 2021 Wiley Periodicals LLC.)

To examine reliability and validity of the new Social Motor Function Classification System for Children with Autism Spectrum Disorders (SMFCS-ASD). The SMFCS-ASD reliability was examined on 25 children (62.4 months SD 7.8) with ASD among six physical therapists. The validity study involved 1001 children (57.0 months, SD 9.9) with ASD using the gross motor scale (GMS) of the Peabody Developmental Motor Scales (PDMS-2). The indices of agreement and reliability across six examiners were moderate to substantial (Cohen's κ ≤ 0.65 and ICC > 0.90, all p < 0.001). The SMFCS-ASD was significantly correlated with the GMS of PDMS-2 (all rho from 0.61 to 0.76, p < 0.001). The SMFCS-ASD was reliable and significantly correlated with the GMS of the PDMS-2.

Homeostatic regulation of energy and metabolic status requires that anabolic and catabolic signaling pathways be precisely regulated and coordinated. Mammalian/mechanistic target of rapamycin complex 1 (mTORC1) is a mega protein complex that promotes energy-consuming anabolic processes of protein and nucleic acid synthesis as well lipogenesis in times of energy and nutrient abundance. However, it is best characterized as the regulator of steps leading to protein synthesis. The ubiquitin-proteasome proteolytic system (UPS) is a major intracellular proteolytic system whose activity is increased during periods of nutrient scarcity and in muscle wasting conditions such as cachexia. Recent studies have examined the impact of mTORC1 on levels and functions of the 26S proteasome, the mega protease complex of the UPS. Here we first briefly review current understanding of the regulation of mTORC1, the UPS, and the 26S proteasome complex. We then review evidence of the effect of each complex on the abundance and functions of the other. Given the fact that drugs that inhibit either complex are either in clinical trials or are approved for treatment of cancer, a muscle wasting condition, we identify studying the effect of combinatory mTORC1-proteasome inhibition on skeletal muscle mass and health as a critical area requiring investigation., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Background: Aging is associated with sarcopenia (low muscle mass) and dynapenia (low muscle strength) leading to disability and mortality. Widely used previous cut-points for sarcopenia were established from dated, small, or pooled cohorts. We aimed to identify cut-points of low strength as a determinant of impaired physical performance and cut-points of low appendicular lean mass (ALM) as a predictor of low strength in a single, large, and contemporary cohort of community-dwelling older adults and compare these criteria with others., Methods: Cross-sectional analyses were conducted on baseline data from 4725 and 4363 community-dwelling men and women (65-86 years, 96.8% Caucasian) of the Canadian longitudinal study on aging comprehensive cohort. Physical performance was evaluated from gait speed, timed up-and-go, chair rise, and balance tests; a weighted-sum score was computed using factor analysis. Strength was measured by handgrip dynamometry; ALM, by dual-energy X-ray absorptiometry and ALM index (ALMI; kg/m 2 ), was calculated. Classification and regression tree analyses determined optimal sex-specific cut-points of ALMI predicting low strength and of strength predicting impaired physical performance (score < 1.5 SD below the sex-specific mean)., Results: Modest associations were found between ALMI and strength and between strength and physical performance score in both sexes. ALMI was not an independent predictor of physical performance score. Cut-points of <33.1 and <20.4 kg were found to define dynapenia in men and in women, respectively, corresponding to 21.5% and 24.0% prevalence rates. Sarcopenia cut-points were <7.76 kg/m 2 in men and <5.72 kg/m 2 in women; prevalence rates of 21.7% and 13.7%. Overall, 8.3% of men and 5.5% of women had sarco-dynapenia. Sarcopenic were older and had lower fat mass and body mass index (BMI) than non-sarcopenic participants. While the agreement between current criteria and the updated European Working Group for Sarcopenia in Older Persons recommendations was fair, we found only slight agreement with the Foundation for the National Institute of Health sarcopenia project. Older persons identified with sarcopenia as per the Foundation for the National Institute of Health criteria (using ALM/BMI as the index) have higher BMI and fat mass compared with non-sarcopenic and have normal ALMI as per our criteria., Conclusions: The proposed function-derived cut-points established from this single, large, and contemporary Canadian cohort should be used for the identification of sarcopenia and dynapenia in Caucasian older adults. We advise on using criteria based on ALMI in the diagnosis of sarcopenia. The modest agreement between sarcopenia and dynapenia denotes potential distinct health implications justifying to study both components separately., (© 2019 The Authors Journal of Cachexia, Sarcopenia and Muscle published by John Wiley & Sons Ltd on behalf of Society on Sarcopenia, Cachexia and Wasting Disorders.)

Aims/hypothesis: Elucidating the molecular mechanisms of fat accumulation and its metabolic consequences is crucial to understanding and treating obesity, an epidemic disease. We have previously observed that Usp19 deubiquitinating enzyme-null mice (Usp19 -/- ) have significantly lower fat mass than wild-type (WT) mice. Thus, this study aimed to provide further understanding of the role of ubiquitin-specific peptidase 19 (USP19) in fat development, obesity and diabetes., Methods: In this study, the metabolic phenotypes of WT and Usp19 -/- mice were compared. The stromal vascular fractions (SVFs) of inguinal fat pads from WT and Usp19 -/- mice were isolated and cells were differentiated into adipocytes in culture to assess their adipogenic capacity. Mice were fed a high-fat diet (HFD) for 18 weeks. Body composition, glucose metabolism and metabolic variables were assessed. In addition, following insulin injection, signalling activity was analysed in the muscle, liver and adipose tissue. Finally, the correlation between the expression of Usp19 mRNA and adipocyte function genes in human adipose tissue was analysed., Result: Upon adipogenic differentiation, SVF cells from Usp19 -/- failed to accumulate lipid and upregulate adipogenic genes, unlike cells from WT mice. Usp19 -/- mice were also found to have smaller fat pads throughout the lifespan and a higher percentage of lean mass, compared with WT mice. When fed an HFD, Usp19 -/- mice were more glucose tolerant, pyruvate tolerant and insulin sensitive than WT mice. Moreover, HFD-fed Usp19 -/- mice had enhanced insulin signalling in the muscle and the liver, but not in adipose tissue. Finally, USP19 mRNA expression in human adipose tissue was positively correlated with the expression of important adipocyte genes in abdominal fat depots, but not subcutaneous fat depots., Conclusions/interpretation: USP19 is an important regulator of fat development. Its inactivation in mice exerts effects on multiple tissues, which may protect against the negative metabolic effects of high-fat feeding. These findings suggest that inhibition of USP19 could have therapeutic potential to protect from the deleterious consequences of obesity and diabetes.

Muscle atrophy arises because of many chronic illnesses, as well as from prolonged glucocorticoid treatment and nutrient deprivation. We previously demonstrated that the USP19 deubiquitinating enzyme plays an important role in chronic glucocorticoid- and denervation-induced muscle wasting. However, the mechanisms by which USP19 exerts its effects remain unknown. To explore this further, we fasted mice for 48 hours to try to identify early differences in the response of wild-type and USP19 knockout (KO) mice that could yield insights into the mechanisms of USP19 action. USP19 KO mice manifested less myofiber atrophy in response to fasting due to increased rates of protein synthesis. Insulin signaling was enhanced in the KO mice, as revealed by lower circulating insulin levels, increased insulin-stimulated glucose disposal and phosphorylation of Akt and S6K in muscle, and improved overall glucose tolerance. Glucocorticoid signaling, which is essential in many conditions of atrophy, was decreased in KO muscle, as revealed by decreased expression of glucocorticoid receptor (GR) target genes upon both fasting and glucocorticoid treatment. This decreased GR signaling was associated with lower GR protein levels in the USP19 KO muscle. Restoring the GR levels in USP19-deficient muscle was sufficient to abolish the protection from myofiber atrophy. Expression of GR target genes also correlated with that of USP19 in human muscle samples. Thus, USP19 modulates GR levels and in so doing may modulate both insulin and glucocorticoid signaling, two critical pathways that control protein turnover in muscle and overall glucose homeostasis.

Activation of AMPK has been associated with pro-atrophic signaling in muscle. However, AMPK also has anti-inflammatory effects, suggesting that in cachexia, a syndrome of inflammatory-driven muscle wasting, AMPK activation could be beneficial. Here we show that the AMPK agonist AICAR suppresses IFNγ/TNFα-induced atrophy, while the mitochondrial inhibitor metformin does not. IFNγ/TNFα impair mitochondrial oxidative respiration in myotubes and promote a metabolic shift to aerobic glycolysis, similarly to metformin. In contrast, AICAR partially restored metabolic function. The effects of AICAR were prevented by the AMPK inhibitor Compound C and were reproduced with A-769662, a specific AMPK activator. AICAR and A-769662 co-treatment was found to be synergistic, suggesting that the anti-cachectic effects of these drugs are mediated through AMPK activation. AICAR spared muscle mass in mouse models of cancer and LPS induced atrophy. Together, our findings suggest a dual function for AMPK during inflammation-driven atrophy, wherein it can play a protective role when activated exogenously early in disease progression, but may contribute to anabolic suppression and atrophy when activated later through mitochondrial dysfunction and subsequent metabolic stress., (© 2018 The Authors. Published under the terms of the CC BY 4.0 license.)

Osteoporosis and sarcopenia are common comorbid diseases, yet their shared mechanisms are largely unknown. We found that genetic variation near FAM210A was associated, through large genome-wide association studies, with fracture, bone mineral density (BMD), and appendicular and whole body lean mass, in humans. In mice, Fam210a was expressed in muscle mitochondria and cytoplasm, as well as in heart and brain, but not in bone. Grip strength and limb lean mass were reduced in tamoxifen-inducible Fam210a homozygous global knockout mice ( TFam210a -/- ), and in tamoxifen-inducible Fam210 skeletal muscle cell-specific knockout mice ( TFam210aMus -/- ). Decreased BMD, bone biomechanical strength, and bone formation, and elevated osteoclast activity with microarchitectural deterioration of trabecular and cortical bones, were observed in TFam210a -/- mice. BMD of male TFam210aMus -/- mice was also reduced, and osteoclast numbers and surface in TFam210aMus -/- mice increased. Microarray analysis of muscle cells from TFam210aMus -/- mice identified candidate musculoskeletal modulators. FAM210A , a novel gene, therefore has a crucial role in regulating bone structure and function, and may impact osteoporosis through a biological pathway involving muscle as well as through other mechanisms., Competing Interests: The authors declare no conflict of interest.

Spermatogenesis consists of a series of highly regulated processes that include mitotic proliferation, meiosis and cellular remodeling. Although alterations in gene expression are well known to modulate spermatogenesis, posttranscriptional mechanisms are less well defined. The ubiquitin proteasome system plays a significant role in protein turnover and may be involved in these posttranscriptional mechanisms. We previously identified ubiquitin ligase Huwe1 in the testis and showed that it can ubiquitinate histones. Since modulation of histones is important at many steps in spermatogenesis, we performed a complete characterization of the functions of Huwe1 in this process by examining the effects of its inactivation in the differentiating spermatogonia, spermatocytes and spermatids. Inactivation of Huwe1 in differentiating spermatogonia led to their depletion and formation of fewer pre-leptotene spermatocytes. The cell degeneration was associated with an accumulation of DNA damage response protein γH2AX, impaired downstream signalling and apoptosis. Inactivation of Huwe1 in spermatocytes indicated that Huwe1 is not essential for meiosis and spermiogenesis, but can result in accumulation of γH2AX. Collectively, these results provide a comprehensive survey of the functions of Huwe1 in spermatogenesis and reveal Huwe1's critical role as a modulator of the DNA damage response pathway in the earliest steps of spermatogonial differentiation.

Spermatogenesis is sustained by a heterogeneous population of spermatogonia that includes the spermatogonial stem cells. However, the mechanisms underlying their establishment from gonocyte embryonic precursors and their maintenance thereafter remain largely unknown. In this study, we report that inactivation of the ubiquitin ligase Huwe1 in male germ cells in mice led to the degeneration of spermatogonia in neonates and resulted in a Sertoli cell-only phenotype in the adult. Huwe1 knockout gonocytes showed a decrease in mitotic re-entry, which inhibited their transition to spermatogonia. Inactivation of Huwe1 in primary spermatogonial culture or the C18-4 cell line resulted in cell degeneration. Degeneration of Huwe1 knockout spermatogonia was associated with an increased level of histone H2AX and an elevated DNA damage response that led to apparent mitotic catastrophe but not apoptosis or senescence. Blocking this increase in H2AX prevented the degeneration of Huwe1-depleted cells. Taken together, these results reveal a previously undefined role of Huwe1 in orchestrating the physiological DNA damage response in the male germline that contributes to the establishment and maintenance of spermatogonia., (Copyright © 2017 Endocrine Society.)

Kidney cell injury may be associated with protein misfolding and induction of endoplasmic reticulum (ER) stress. Examples include complement-induced glomerular epithelial cell (GEC)/podocyte injury in membranous nephropathy and ischemia-reperfusion injury. Renal cell injury can also result from mutations in integral proteins, which lead to their misfolding and accumulation. Certain nephrin missense mutants misfold, accumulate in the ER, and induce ER stress. We examined if enhancement of ubiquitin-proteasome system function may facilitate proteostasis and confer protection against injury. Ubiquitin-specific protease 14 (Usp14) is reported to retard proteasomal protein degradation. Thus inhibition of Usp14 may enhance degradation of misfolded proteins and attenuate cell injury. In GEC, the reporter proteins GFP u (a "misfolded" protein) and CD3δ (an ER-associated degradation substrate) undergo time-dependent proteasomal degradation. Complement did not affect degradation of CD3δ-yellow fluorescent protein (YFP), but accelerated degradation of GFP u , and the Usp14-directed inhibitor IU1 further accelerated this degradation. Conversely, overexpression of Usp14 reduced degradation of GFP u and CD3δ-YFP. In 293T cells, IU1 did not enhance degradation of disease-associated nephrin missense mutants I171N and S724C, whereas overexpression of Usp14 reduced degradation. IU1 was cytoprotective after injury induced by the ER stressor tunicamycin and in vitro ischemia-reperfusion, but did not affect complement-induced cytotoxicity. In conclusion, Usp14 controls proteasomal degradation of some misfolded proteins. In addition, a Usp14-directed inhibitor reduces cytotoxicity in the context of global protein misfolding during certain types of renal cell injury., (Copyright © 2016 the American Physiological Society.)

The ubiquitin proteasome system is well recognized to be involved in mediating muscle atrophy in response to diverse catabolic conditions. To date, almost all of the genes that have been implicated are ubiquitin ligases. Although ubiquitination is modulated also by deubiquitinating enzymes, the roles of these enzymes in muscle wasting remains largely unexplored. In this article, the potential roles of deubiquitinating enzymes in regulating muscle size are discussed. This is followed by a review of the roles described for USP19, the deubiquitinating enzyme that has been most studied in muscle wasting. This enzyme is upregulated in muscle in many catabolic conditions and its inactivation leads to protection from muscle loss induced by stimuli that are common in many illnesses causing cachexia. It can regulate both protein synthesis and protein degradation as well as myogenesis, thereby modulating the key processes that control muscle mass. Roles for other deubiquitinating enzymes remain possible and to be explored., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Muscle atrophy complicates many diseases as well as aging, and its presence predicts both decreased quality of life and survival. Much work has been conducted to define the molecular mechanisms involved in maintaining protein homeostasis in muscle. To date, the ubiquitin proteasome system (UPS) has been shown to play an important role in mediating muscle wasting. In this review, we have collated the enzymes in the UPS whose roles in muscle wasting have been confirmed through loss-of-function studies. We have integrated information on their mechanisms of action to create a model of how they work together to produce muscle atrophy. These enzymes are involved in promoting myofibrillar disassembly and degradation, activation of autophagy, inhibition of myogenesis as well as in modulating the signaling pathways that control these processes. Many anabolic and catabolic signaling pathways are involved in regulating these UPS genes, but none appear to coordinately regulate a large number of these genes. A number of catabolic signaling pathways appear to instead function by inhibition of the insulin/IGF-I/protein kinase B anabolic pathway. This pathway is a critical determinant of muscle mass, since it can suppress key ubiquitin ligases and autophagy, activate protein synthesis, and promote myogenesis through its downstream mediators such as forkhead box O, mammalian target of rapamycin, and GSK3β, respectively. Although much progress has been made, a more complete inventory of the UPS genes involved in mediating muscle atrophy, their mechanisms of action, and their regulation will be useful for identifying novel therapeutic approaches to this important clinical problem., (Copyright © 2016 the American Physiological Society.)

A majority of proteins in the cell can be modified by ubiquitination, thereby altering their function or stability. This ubiquitination is controlled by both ubiquitinating and deubiquitinating enzymes (DUBs). The number of ubiquitin ligases exceeds that of DUBs by about eightfold, indicating that DUBs may have much broader substrate specificity. Despite this, DUBs have been shown to have quite specific physiological functions. This functional specificity is likely due to very precise regulation of activity arising from the sophisticated use of all mechanisms of enzyme regulation. In this commentary, we briefly review key features of DUBs with more emphasis on regulation. In particular, we focus on localization of the enzymes as a critical regulatory mechanism which when integrated with control of expression, substrate activation, allosteric regulation, and post-translational modifications results in precise spatial and temporal deubiquitination of proteins and therefore specific physiological functions. Identification of compounds that target the structural elements in DUBs that dictate localization may be a more promising approach to development of drugs with specificity of action than targeting the enzymatic activity, which for most DUBs is dependent on a thiol group that can react non-specifically with many compounds in large-scale screening.

The ubiquitin system plays a critical role in muscle wasting. Previous work has focused on the roles of ubiquitination. However, a role for deubiquitination in this process has not been established. Because ubiquitin-specific protease (USP)19 deubiquitinating enzyme is induced in skeletal muscle in many catabolic conditions, we generated USP19 knockout (KO) mice. These mice lost less muscle mass than wild-type (WT) animals in response to glucocorticoids, a common systemic cause of muscle atrophy as well as in response to denervation, a model of disuse atrophy. KO mice retained more strength and had less myofiber atrophy with both type I and type IIb fibers being protected. Rates of muscle protein synthesis were similar in WT and KO mice, suggesting that the sparing of atrophy was attributed to suppressed protein degradation. Consistent with this, expression of the ubiquitin ligases MuRF1 and MAFbx/atrogin-1 as well as several autophagy genes was decreased in the muscles of catabolic KO mice. Expression of USP19 correlates with that of MuRF1 and MAFbx/atrogin-1 in skeletal muscles from patients with lung cancer or gastrointestinal cancer, suggesting that USP19 is involved in human muscle wasting. Inhibition of USP19 may be a useful approach to the treatment of many muscle-wasting conditions., (© FASEB.)

The USP19 deubiquitinating enzyme modulates the expression of myogenin and myofibrillar proteins in L6 muscle cells. This raised the possibility that USP19 might regulate muscle cell differentiation. We therefore tested the effects of adenoviral-mediated overexpression or small interfering RNA (siRNA)-mediated silencing of either the cytoplasmic or endoplasmic reticulum (ER)-localized isoforms of USP19. Only the ER-localized isoform of USP19 (USP19-ER) modulated myoblast fusion as well as the expression of myogenin and myofibrillar proteins, and these effects were also dependent on USP19 catalytic activity. USP19-ER inhibited muscle cell differentiation and the induction of CHOP, a transcription factor in the unfolded-protein response (UPR) that is activated during differentiation. Inducing the UPR by creating mild ER stress with thapsigargin was able to reverse the defect in myoblast fusion caused by the overexpression of USP19-ER, suggesting strongly that USP19 exerts its effects on fusion through its effects on UPR signaling. USP19 also functions similarly in vivo, as USP19(-/-) mice display improved muscle regeneration concomitant with enhanced expression of CHOP. Collectively these results implicate a deubiquitinating enzyme as a regulator of the UPR. They also suggest that inhibition of USP19 may be a therapeutic approach for the enhancement of muscle growth following injury., (© 2015 Wiles, Miao, et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)