A catalytic independent function of the deubiquitinating enzyme USP14 regulates hippocampal synaptic short-term plasticity and vesicle number

Modulation of temporoammonic-CA1 synapses by neuropeptide Y is through Y1 receptors in mice

Reduced levels of neuropeptide Y (NPY), an abundant neuromodulator in the brain, are linked to multiple neuropsychiatric disorders, including post-traumatic stress disorder (PTSD). The CA1 region of hippocampus is important for anxiety regulation and highly expresses NPY. Injecting NPY into CA1 is anxiolytic and alleviates behavioral symptoms in a model of traumatic stress; these anxiolytic effects are blocked by a Y1 receptor antagonist. However the location of Y1Rs that mediate NPY's anxiolytic effects in CA1 remains unclear. CA1 receives inputs from entorhinal cortex through the temporammonic pathway (TA), which is important for fear learning and sensitive to stress. Our lab previously showed that NPY reduces TA-evoked synaptic responses, however, the subtype of NPY receptor mediating this reduction is unknown. Here we demonstrate that in mice both exogenous (bath-applied) and endogenously-released NPY act through Y1 receptors in the TA pathway. This is the first demonstration of Y1 receptor-mediated effect on synaptic function in CA1. Interestingly, chronic overexpression of NPY (in NPY-expressing interneurons) impairs the sensitivity of the TA-evoked synaptic response to a Y1 receptor agonist. However, the long-known NPY Y2 receptor-mediated effect on the Schaffer collateral (SC) pathway is unaffected by NPY overexpression. Therefore, NPY can have a pathway-specific impact on synaptic transmission in CA1 based on the differential expression of NPY receptors and their response to overexpression of NPY. Our results demonstrating that NPY acts at Y1 receptors in the TA pathway are consistent with the idea that the TA pathway underlies the anxiolytic effects of NPY in CA1.

HERC1 E3 Ubiquitin Ligase Is Necessary for Autophagy Processes and for the Maintenance and Homeostasis of Vesicles in Motor Nerve Terminals, but Not for Proteasomal Activity

The ubiquitin proteasome system (UPS) is implicated in protein homeostasis. One of the proteins involved in this system is HERC1 E3 ubiquitin ligase, which was associated with several processes including the normal development and neurotransmission at the neuromuscular junction (NMJ), autophagy in projection neurons, myelination of the peripheral nervous system, among others. The tambaleante (tbl) mouse model carries the spontaneous mutation Gly483Glu substitution in the HERC1 E3 protein. Using this model, we analyzed the implication of HERC1 E3 ubiquitin ligase in the activity of UPS, autophagy, and synaptic homeostasis in brain and muscle tissues. Regarding UPS, no differences were found in its activity nor in the specific gene expression in both brain and muscle tissues from tbl compared with the control littermates. Furthermore, the use of the specific UPS inhibitor (MG-132), did not alter the evoked neurotransmitter release in the levator auris longus (LAL) muscle. Interestingly, the expression of the autophagy-related gene p62 was significantly increased in the muscle of tbl compared to the control littermates. Indeed, impaired evoked neurotransmitter release was observed with the autophagy inhibitor Wortmannin. Finally, altered levels of Clathrin and Synaptophysin were detected in muscle tissues. Altogether, our findings show that HERC1 E3 ubiquitin ligase mutation found in tbl mice alters autophagy and vesicular recycling without affecting proteasomal function.

Proteotoxic stress and the ubiquitin proteasome system

The ubiquitin proteasome system maintains protein homeostasis by regulating the breakdown of misfolded proteins, thereby preventing misfolded protein aggregates. The efficient elimination is vital for preventing damage to the cell by misfolded proteins, known as proteotoxic stress. Proteotoxic stress can lead to the collapse of protein homeostasis and can alter the function of the ubiquitin proteasome system. Conversely, impairment of the ubiquitin proteasome system can also cause proteotoxic stress and disrupt protein homeostasis. This review examines two impacts of proteotoxic stress, 1) disruptions to ubiquitin homeostasis (ubiquitin stress) and 2) disruptions to proteasome homeostasis (proteasome stress). Here, we provide a mechanistic description of the relationship between proteotoxic stress and the ubiquitin proteasome system. This relationship is illustrated by findings from several protein misfolding diseases, mainly neurodegenerative diseases, as well as from basic biology discoveries from yeast to mammals. In addition, we explore the importance of the ubiquitin proteasome system in endoplasmic reticulum quality control, and how proteotoxic stress at this organelle is alleviated. Finally, we highlight how cells utilize the ubiquitin proteasome system to adapt to proteotoxic stress and how the ubiquitin proteasome system can be genetically and pharmacologically manipulated to maintain protein homeostasis.

In the moonlight: non-catalytic functions of ubiquitin and ubiquitin-like proteases

Proteases that cleave ubiquitin or ubiquitin-like proteins (UBLs) are critical players in maintaining the homeostasis of the organism. Concordantly, their dysregulation has been directly linked to various diseases, including cancer, neurodegeneration, developmental aberrations, cardiac disorders and inflammation. Given their potential as novel therapeutic targets, it is essential to fully understand their mechanisms of action. Traditionally, observed effects resulting from deficiencies in deubiquitinases (DUBs) and UBL proteases have often been attributed to the misregulation of substrate modification by ubiquitin or UBLs. Therefore, much research has focused on understanding the catalytic activities of these proteins. However, this view has overlooked the possibility that DUBs and UBL proteases might also have significant non-catalytic functions, which are more prevalent than previously believed and urgently require further investigation. Moreover, multiple examples have shown that either selective loss of only the protease activity or complete absence of these proteins can have different functional and physiological consequences. Furthermore, DUBs and UBL proteases have been shown to often contain domains or binding motifs that not only modulate their catalytic activity but can also mediate entirely different functions. This review aims to shed light on the non-catalytic, moonlighting functions of DUBs and UBL proteases, which extend beyond the hydrolysis of ubiquitin and UBL chains and are just beginning to emerge.

Bicuculline restores frequency-dependent hippocampal I/E ratio and circuit function in PGC-1ɑ null mice

Altered inhibition/excitation (I/E) balance contributes to various brain disorders. Dysfunctional GABAergic interneurons enhance or reduce inhibition, resulting in I/E imbalances. Differences in short-term plasticity between excitation and inhibition cause frequency-dependence of the I/E ratio, which can be altered by GABAergic dysfunction. However, it is unknown whether I/E imbalances can be rescued pharmacologically using a single dose when the imbalance magnitude is frequency-dependent. Loss of PGC-1α (peroxisome proliferator activated receptor γ coactivator 1α) causes transcriptional dysregulation in hippocampal GABAergic interneurons. PGC-1α-/- slices have enhanced baseline inhibition onto CA1 pyramidal cells, causing increased I/E ratio and impaired circuit function. High frequency stimulation reduces the I/E ratio and recovers circuit function in PGC-1α-/- slices. Here we tested if using a low dose of bicuculline that can restore baseline I/E ratio can also rescue the frequency-dependent I/E imbalances in these mice. Remarkably, bicuculline did not reduce the I/E ratio below that of wild type during high frequency stimulation. Interestingly, bicuculline enhanced the paired-pulse ratio (PPR) of disynaptic inhibition without changing the monosynaptic inhibition PPR, suggesting that bicuculline modifies interneuron recruitment and not GABA release. Bicuculline improved CA1 output in PGC-1α-/- slices, enhancing EPSP-spike coupling to wild type levels at high and low frequencies. Our results show that it is possible to rescue frequency-dependent I/E imbalances in an animal model of transcriptional dysregulation with a single treatment.

USP14: Structure, Function, and Target Inhibition

Ubiquitin-specific protease 14 (USP14), a deubiquitinating enzyme (DUB), is associated with proteasomes and exerts a dual function in regulating protein degradation. USP14 protects protein substrates from degradation by removing ubiquitin chains from proteasome-bound substrates, whereas promotes protein degradation by activating the proteasome. Increasing evidence have shown that USP14 is involved in several canonical signaling pathways, correlating with cancer, neurodegenerative diseases, autophagy, immune responses, and viral infections. The activity of USP14 is tightly regulated to ensure its function in various cellular processes. Structural studies have demonstrated that free USP14 exists in an autoinhibited state with two surface loops, BL1 and BL2, partially hovering above and blocking the active site cleft binding to the C-terminus of ubiquitin. Hence, both proteasome-bound and phosphorylated forms of USP14 require the induction of conformational changes in the BL2 loop to activate its deubiquitinating function. Due to its intriguing roles in the stabilization of disease-causing proteins and oncology targets, USP14 has garnered widespread interest as a therapeutic target. In recent years, significant progress has been made on identifying inhibitors targeting USP14, despite the complexity and challenges in improving their selectivity and affinity for USP14. In particular, the crystal structures of USP14 complexed with IU1-series inhibitors revealed the underlying allosteric regulatory mechanism and enabled the further design of potent inhibitors. In this review, we summarize the current knowledge regarding the structure, regulation, pathophysiological function, and selective inhibition of USP14, including disease associations and inhibitor development.

Small-Molecule Inhibitors Targeting Proteasome-Associated Deubiquitinases

The 26S proteasome is the principal protease for regulated intracellular proteolysis. This multi-subunit complex is also pivotal for clearance of harmful proteins that are produced throughout the lifetime of eukaryotes. Recent structural and kinetic studies have revealed a multitude of conformational states of the proteasome in substrate-free and substrate-engaged forms. These conformational transitions demonstrate that proteasome is a highly dynamic machinery during substrate processing that can be also controlled by a number of proteasome-associated factors. Essentially, three distinct family of deubiquitinases–USP14, RPN11, and UCH37–are associated with the 19S regulatory particle of human proteasome. USP14 and UCH37 are capable of editing ubiquitin conjugates during the process of their dynamic engagement into the proteasome prior to the catalytic commitment. In contrast, RPN11-mediated deubiquitination is directly coupled to substrate degradation by sensing the proteasome’s conformational switch into the commitment steps. Therefore, proteasome-bound deubiquitinases are likely to tailor the degradation events in accordance with substrate processing steps and for dynamic proteolysis outcomes. Recent chemical screening efforts have yielded highly selective small-molecule inhibitors for targeting proteasomal deubiquitinases, such as USP14 and RPN11. USP14 inhibitors, IU1 and its progeny, were found to promote the degradation of a subset of substrates probably by overriding USP14-imposed checkpoint on the proteasome. On the other hand, capzimin, a RPN11 inhibitor, stabilized the proteasome substrates and showed the anti-proliferative effects on cancer cells. It is highly conceivable that these specific inhibitors will aid to dissect the role of each deubiquitinase on the proteasome. Moreover, customized targeting of proteasome-associated deubiquitinases may also provide versatile therapeutic strategies for induced or repressed protein degradation depending on proteolytic demand and cellular context.

Discovery of new promising USP14 inhibitors: computational evaluation of the thumb-palm pocket

Ubiquitin-specific protease 14 (USP14) is a member of the deubiquitinating enzymes (DUBs) involved in disrupting the ubiquitin-proteasome regulation system, responsible for the degradation of impaired and misfolded proteins, which is an essential mechanism in eukaryotic cells. The involvement of USP14 in cancer progression and neurodegenerative disorders has been reported. Thereof USP14 is a prime therapeutic target; hence, designing efficacious inhibitors against USP14 is central in curbing these conditions. Herein, we relied on structural bioinformatics methods incorporating molecular docking, molecular mechanics generalized born surface area (MM-GBSA), molecular dynamics simulation (MD simulation), and ADME to identify potential allosteric USP14 inhibitors. A library of over 733 compounds from the PubChem repository with >90% match to the IU1 chemical structure was screened in a multi-step framework to attain prospective drug-like inhibitors. Two potential lead compounds (CID 43013232 and CID 112370349) were shown to record better binding affinity compared to IU1, but with subtle difference to IU1-47, a 10-fold potent compound when compared to IU1. The stability of the lead molecules complexed with USP14 was studied via MD simulation. The molecules were found to be stable within the binding site throughout the 50 ns simulation time. Moreover, the protein-ligand interactions across the simulation run time suggest Phe331, Tyr476, and Gln197 as crucial residues for USP14 inhibition. Furthermore, in-silico pharmacological evaluation revealed the lead compounds as pharmacological sound molecules. Overall, the methods deployed in this study revealed two novel candidates that may show selective inhibitory activity against USP14, which could be exploited to produce potent and harmless USP14 inhibitors.Communicated by Ramaswamy H. Sarma.

Examination of genetic and pharmacological tools to study the proteasomal deubiquitinating enzyme ubiquitin-specific protease 14 in the nervous system

Strategies for enhancing protein degradation have been proposed for treating neurological diseases associated with a decline in proteasome activity. A proteasomal deubiquitinating enzyme that controls substrate entry into proteasomes, ubiquitin-specific protease 14 (USP14), is an attractive candidate for therapies that modulate proteasome activity. This report tests the validity of genetic and pharmacological tools to study USP14's role in regulating protein abundance. Although previous studies implicated USP14 in the degradation of microtubule associate protein tau, tar DNA binding protein, and prion protein, the levels of these proteins were similar in our neurons cultured from wild type and USP14-deficient mice. Neither loss nor over-expression of USP14 affected the levels of these proteins in mice, implying that modifying the amount of USP14 is not sufficient to alter their steady-state levels. However, neuronal over-expression of a catalytic mutant of USP14 showed that manipulating USP14's ubiquitin-hydrolase activity altered the levels of specific proteins in vivo. Although pharmacological inhibitors of USP14's ubiquitin-hydrolase activity reduced microtubule associate protein tau, tar DNA binding protein, and prion protein in culture, the effect was similar in wild type and USP14-deficient neurons, thus impacting their use for specifically evaluating USP14 in a therapeutic manner. While examining how targeting USP14 may affect other proteins in vivo, this report showed that fatty acid synthase, v-rel reticuloendotheliosis viral oncogene homolog, CTNNB1, and synaptosome associated protein 23 are reduced in USP14-deficient mice; however, loss of USP14 differentially altered the levels of these proteins in the liver and brain. As such, it is critical to more thoroughly examine how inhibiting USP14 alters protein abundance to determine if targeting USP14 will be a beneficial strategy for treating neurodegenerative diseases.

Ubiquitin and Ubiquitin-Like Proteins in the Critical Equilibrium between Synapse Physiology and Intellectual Disability

Posttranslational modifications (PTMs) represent a dynamic regulatory system that precisely modulates the functional organization of synapses. PTMs consist in target modifications by small chemical moieties or conjugation of lipids, sugars or polypeptides. Among them, ubiquitin and a large family of ubiquitin-like proteins (UBLs) share several features such as the structure of the small protein modifiers, the enzymatic cascades mediating the conjugation process, and the targeted aminoacidic residue. In the brain, ubiquitination and two UBLs, namely sumoylation and the recently discovered neddylation orchestrate fundamental processes including synapse formation, maturation and plasticity, and their alteration is thought to contribute to the development of neurological disorders. Remarkably, emerging evidence suggests that these pathways tightly interplay to modulate the function of several proteins that possess pivotal roles for brain homeostasis as well as failure of this crosstalk seems to be implicated in the development of brain pathologies. In this review, we outline the role of ubiquitination, sumoylation, neddylation, and their functional interplay in synapse physiology and discuss their implication in the molecular pathogenesis of intellectual disability (ID), a neurodevelopmental disorder that is frequently comorbid with a wide spectrum of brain pathologies. Finally, we propose a few outlooks that might contribute to better understand the complexity of these regulatory systems in regard to neuronal circuit pathophysiology.

Ubiquitin biology in neurodegenerative disorders: From impairment to therapeutic strategies

The abnormal accumulation of neurotoxic proteins is the typical hallmark of various age-related neurodegenerative disorders (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis and Multiple sclerosis. The anomalous proteins, such as Aβ, Tau in Alzheimer's disease and α-synuclein in Parkinson's disease, perturb the neuronal physiology and cellular homeostasis in the brain thereby affecting the millions of human lives across the globe. Here, ubiquitin proteasome system (UPS) plays a decisive role in clearing the toxic metabolites in cells, where any aberrancy is widely reported to exaggerate the neurodegenerative pathologies. In spite of well-advancement in the ubiquitination research, their molecular markers and mechanisms for target-specific protein ubiquitination and clearance remained elusive. Therefore, this review substantiates the role of UPS in the brain signaling and neuronal physiology with their mechanistic role in the NDD's specific pathogenic protein clearance. Moreover, current and future promising therapies are discussed to target UPS-mediated neurodegeneration for better public health.

Deubiquitination Reactions on the Proteasome for Proteasome Versatility

The 26S proteasome, a master player in proteolysis, is the most complex and meticulously contextured protease in eukaryotic cells. While capable of hosting thousands of discrete substrates due to the selective recognition of ubiquitin tags, this protease complex is also dynamically checked through diverse regulatory mechanisms. The proteasome's versatility ensures precise control over active proteolysis, yet prevents runaway or futile degradation of many essential cellular proteins. Among the multi-layered processes regulating the proteasome's proteolysis, deubiquitination reactions are prominent because they not only recycle ubiquitins, but also impose a critical checkpoint for substrate degradation on the proteasome. Of note, three distinct classes of deubiquitinating enzymes-USP14, RPN11, and UCH37-are associated with the 19S subunits of the human proteasome. Recent biochemical and structural studies suggest that these enzymes exert dynamic influence over proteasome output with limited redundancy, and at times act in opposition. Such distinct activities occur spatially on the proteasome, temporally through substrate processing, and differentially for ubiquitin topology. Therefore, deubiquitinating enzymes on the proteasome may fine-tune the degradation depending on various cellular contexts and for dynamic proteolysis outcomes. Given that the proteasome is among the most important drug targets, the biology of proteasome-associated deubiquitination should be further elucidated for its potential targeting in human diseases.

Development of Ubiquitin Tools for Studies of Complex Ubiquitin Processing Protein Machines

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Ubiquitination is one of the most extensive post-translational modifications in eukaryotes and is involved in various physiological processes such as protein degradation, autophagy, protein interaction, and protein localization. The ubiquitin (Ub)-related protein machines include Ub-activating enzymes (E1s), Ub-conjugating enzymes (E2s), Ub ligases (E3s), deubiquitinating enzymes (DUBs), p97, and the proteasomes. In recent years, the role of DUBs has been extensively studied and relatively well understood. On the other hand, the functional mechanisms of the other more complex ubiquitin-processing protein machines (e.g., E3, p97, and proteasomes) are still to be sufficiently well explored due to their intricate nature. One of the hurdles facing the studies of these complex protein machines is the challenge of developing tailor-designed structurally defined model substrates, which unfortunately cannot be directly obtained using recombinant technology. Consequently, the acquisition and synthesis of the ubiquitin tool molecules are essential for the elucidation of the functions and structures of the complex ubiquitin-processing protein machines. This paper aims to highlight recent studies on these protein machines based on the synthetic ubiquitin tool molecules.

Sustained ER stress promotes hyperglycemia by increasing glucagon action through the deubiquitinating enzyme USP14

Endoplasmic reticulum (ER) stress plays an important role in metabolic diseases like obesity and type 2 diabetes mellitus (T2DM), although the underlying mechanisms and regulatory pathways remain to be elucidated. Here, we induced chronic low-grade ER stress in lean mice to levels similar to those in high-fat diet (HFD)-fed obese mice and found that it promoted hyperglycemia due to enhanced hepatic gluconeogenesis. Mechanistically, sustained ER stress up-regulated the deubiquitinating enzyme ubiquitin-specific peptidase 14 (USP14), which increased the stability and levels of 3',5'-cyclic monophosphate-responsive element binding (CREB) protein (CBP) to enhance glucagon action and hepatic gluconeogenesis. Exogenous overexpression of USP14 in the liver significantly increased hepatic glucose output. Consistent with this, liver-specific knockdown of USP14 abrogated the effects of ER stress on glucose metabolism, and also improved hyperglycemia and glucose intolerance in obese mice. In conclusion, our findings show a mechanism underlying ER stress-induced disruption of glucose homeostasis, and present USP14 as a potential therapeutic target against T2DM.

Structurally-defined deubiquitinase inhibitors provide opportunities to investigate disease mechanisms

The Ubiquitin/Proteasome System comprises an essential cellular mechanism for regulated protein degradation. Ubiquitination may also promote the assembly of protein complexes that initiate intracellular signaling cascades. Thus, proper regulation of substrate protein ubiquitination is essential for maintaining normal cellular physiology. Deubiquitinases are the class of enzymes responsible for removing ubiquitin modifications from target proteins and have been implicated in regulating human disease. As such, deubiquitinases are now recognized as emerging drug targets. Small molecule deubiquitinase inhibitors have been developed; among those, inhibitors for the deubiquitinases USP7 and USP14 are the best-characterized given that they are structurally validated. In this review we discuss the normal physiological roles of the USP7 and USP14 deubiquitinases as well as the pathological conditions associated with their dysfunction, with a focus on oncology and neurodegenerative diseases. We also review structural biology of USP7 and USP14 enzymes and the characterization of their respective inhibitors, highlighting the various molecular mechanisms by which these deubiquitinases may be functionally inhibited. Finally, we summarize the cellular and in vivo studies performed using the structurally-validated USP7 and USP14 inhibitors.

USP14 promotes K63-linked RIG-I deubiquitination and suppresses antiviral immune responses

Retinoic acid-inducible gene I (RIG-I) is a critical RNA virus sensor that initiates antiviral immune response through K63-linked ubiquitination. In this study, we demonstrated USP14, a deubiquitinating enzyme, as a negative regulator in antiviral responses by directly deubiquitinating K63-linked RIG-I. USP14 knockdown significantly enhanced RIG-I-triggered type I IFN signaling and inhibited vesicular stomatitis virus (VSV) replication both in mouse peritoneal macrophages and THP1 cells. USP14 overexpression in HeLa cells attenuated RIG-I-triggered IFN-β expression and promoted VSV replication. Besides, USP14-specific inhibitor, IU1, increased RIG-I-mediated type I IFN production and antiviral responses in vitro and in vivo. In addition, USP14 could interact with RIG-I and remove RIG-I K63-linked polyubiquitination chains. This article is the first to report that USP14 acts as a negative regulator in antiviral response through deubiquitinating K63-linked RIG-I. These findings provide insights into a potential new therapy targeting USP14 for RNA virus-related diseases.

A 13-year-old girl with 18p deletion syndrome presenting Turner syndrome-like clinical features of short stature, short webbed neck, low posterior hair line, puffy eyelids and increased carrying angle of the elbows

OBJECTIVE

We report a 13-year-old girl with 18p deletion syndrome presenting Turner syndrome-like clinical features.

CASE REPORT

A 13-year-old girl was referred for genetic counseling of Turner syndrome-like clinical features of short stature, short webbed neck, low posterior hair line, puffy eyelids and increased carrying angle of the elbows. The girl also had mild intellectual disability, psychomotor developmental delay, speech disorder, high-arched palate, hypertelorism and mid-face hypoplasia. Cytogenetic analysis of the girl revealed a karyotype of 46,XX,del(18) (p11.2). The parental karyotypes were normal. Array comparative genomic hybridization analysis on the DNA extracted from the peripheral blood revealed a 13.93-Mb deletion of 18p11.32-p11.21 or arr 18p11.32p11.21 (148,993-14,081,858) × 1.0 [GRCh37 (hg19)] encompassing 52 Online Mendelian Inheritance in Man (OMIM) genes including USP14, TYMS, SMCHD1, TGIF1, LAMA1, TWSG1, GNAL and PTPN2. Polymorphic DNA marker analysis revealed a maternal origin of the deletion.

CONCLUSION

Females with Turner syndrome-like clinical features in association with intellectual disability, facial dysmorphism and psychomotor developmental delay should be suspected of having chromosome deletion syndromes.

Proteome-wide analysis of USP14 substrates revealed its role in hepatosteatosis via stabilization of FASN

Ubiquitin-specific protease 14 (USP14) is one of the major proteasome-associated deubiquitinating enzymes critical for proteome homeostasis. However, substrates of USP14 remain largely unknown, hindering the understanding of its functional roles. Here we conduct a comprehensive proteome, ubiquitinome and interactome analysis for USP14 substrate screening. Bioinformatics analysis reveals broad new potential roles of USP14, especially in lipid and carbohydrate metabolism. Among the potential substrates identified, we show that fatty acid synthase (FASN), a key enzyme involved in hepatic lipogenesis, is a bona fide substrate of USP14. USP14 directly interacts with and increases FASN stability. As a result, overexpression of USP14 promotes liver triglyceride accumulation in C57BL/6 mice, whereas genetic ablation or pharmacological inhibition of USP14 ameliorates hepatosteatosis, hyperglycemia and insulin resistance in obese mice. In conclusion, our findings reveal for the first time an indispensable role of USP14 in hepatosteatosis through FASN stabilization.

Ubiquitin-specific protease 14 (USP14) is a proteasome-associated deubiquitinating enzyme with known roles in physiology and disease. Here the authors show that fatty acid synthase (FASN) is a substrate of USP14, and that by stabilizing FASN, it plays a role in hepatosteatosis.

Stretchable organic optoelectronic sensorimotor synapse

Emulation of human sensory and motor functions becomes a core technology in bioinspired electronics for next-generation electronic prosthetics and neurologically inspired robotics. An electronic synapse functionalized with an artificial sensory receptor and an artificial motor unit can be a fundamental element of bioinspired soft electronics. Here, we report an organic optoelectronic sensorimotor synapse that uses an organic optoelectronic synapse and a neuromuscular system based on a stretchable organic nanowire synaptic transistor (s-ONWST). The voltage pulses of a self-powered photodetector triggered by optical signals drive the s-ONWST, and resultant informative synaptic outputs are used not only for optical wireless communication of human-machine interfaces but also for light-interactive actuation of an artificial muscle actuator in the same way that a biological muscle fiber contracts. Our organic optoelectronic sensorimotor synapse suggests a promising strategy toward developing bioinspired soft electronics, neurologically inspired robotics, and electronic prostheses.

Target-cell-specific Short-term Plasticity Reduces the Excitatory Drive onto CA1 Interneurons Relative to Pyramidal Cells During Physiologically-derived Spike Trains

Short-term plasticity enables synaptic strength to be dynamically regulated by input timing. Excitatory synapses arising from the same axon can have profoundly different presynaptic forms of short-term plasticity onto inhibitory and excitatory neurons. We previously showed that Schaffer collateral synapses onto most hippocampal CA1 stratum radiatum interneurons have less paired-pulse facilitation than synapses onto CA1 pyramidal cells, but little difference in steady-state short-term depression. However, less is known about how synapses onto interneurons respond to temporally complex patterns that occur in vivo. Here we compared Schaffer collateral synapses onto stratum radiatum interneurons and pyramidal cells in acute hippocampal slices in response to physiologically-derived spike trains. We find that synapses onto interneurons have less short-term facilitation than synapses onto pyramidal cells, and a subset expresses only short-term depression. Mathematical modeling predicts this target cell-specific short-term plasticity occurs through differences in initial release probability. All three groups have more short-term facilitation during physiologically-derived train stimulation than during constant-frequency stimulation at the same frequency, indicating that variability in stimulus timing is important. These target-cell specific differences in short-term plasticity reduce the strength of excitatory input onto interneurons relative to pyramidal cells, and of depression interneurons relative to facilitation interneurons, during high frequency portions of the train. This occurs to a similar extent at 25 °C and at 33 °C, and is even greater at physiological extracellular calcium. Target-cell specific differences in short-term plasticity enable synapses to have different temporal filtering characteristics, which may help to dynamically regulate the balance of inhibition and excitation in CA1.

USP14 de-ubiquitinates vimentin and miR-320a modulates USP14 and vimentin to contribute to malignancy in gastric cancer cells

Vimentin plays important roles in the epithelial-to-mesenchymal transition (EMT). In this study, we found that vimentin was highly expressed in human gastric cancer (GC) tissues and cell lines and significantly promoted cell growth, migration and invasion. Ubiquitin-specific protease 14 (USP14) interacted with the vimentin protein, which led to its de-ubiquitination. miR-320a was found to bind to the 3′UTR of both vimentin and USP14 transcripts and downregulate the expression of both proteins. The downregulation of miR-320a upregulates vimentin expression by directly binding to the 3′UTR of vimentin to derepress expression and indirectly by augmenting USP14 to increase vimentin stability in GC cells. Taken together, these results provide new insight into malignancy in gastric cancers.

Meddling with Fate: The Proteasomal Deubiquitinating Enzymes

Three deubiquitinating enzymes-Rpn11, Usp14, and Uch37-are associated with the proteasome regulatory particle. These enzymes allow proteasomes to remove ubiquitin from substrates before they are translocated into the core particle to be degraded. Although the translocation channel is too narrow for folded proteins, the force of translocation unfolds them mechanically. As translocation proceeds, ubiquitin chains bound to substrate are drawn to the channel's entry port, where they can impede further translocation. Rpn11, situated over the port, can remove these chains without compromising degradation because substrates must be irreversibly committed to degradation before Rpn11 acts. This coupling between deubiquitination and substrate degradation is ensured by the Ins-1 loop of Rpn11, which controls ubiquitin access to its catalytic site. In contrast to Rpn11, Usp14 and Uch37 can rescue substrates from degradation by promoting substrate dissociation from the proteasome prior to the commitment step. Uch37 is unique in being a component of both the proteasome and a second multisubunit assembly, the INO80 complex. However, only recruitment into the proteasome activates Uch37. Recruitment to the proteasome likewise activates Usp14. However, the influence of Usp14 on the proteasome depends on the substrate, due to its marked preference for proteins that carry multiple ubiquitin chains. Usp14 exerts complex control over the proteasome, suppressing proteasome activity even when inactive in deubiquitination. A major challenge for the field will be to elucidate the specificities of Rpn11, Usp14, and Uch37 in greater depth, employing not only model in vitro substrates but also their endogenous targets.

An inhibitor of the proteasomal deubiquitinating enzyme USP14 induces tau elimination in cultured neurons

The ubiquitin-proteasome system (UPS) is responsible for most selective protein degradation in eukaryotes and regulates numerous cellular processes, including cell cycle control and protein quality control. A component of this system, the deubiquitinating enzyme USP14, associates with the proteasome where it can rescue substrates from degradation by removal of the ubiquitin tag. We previously found that a small-molecule inhibitor of USP14, known as IU1, can increase the rate of degradation of a subset of proteasome substrates. We report here the synthesis and characterization of 87 variants of IU1, which resulted in the identification of a 10-fold more potent USP14 inhibitor that retains specificity for USP14. The capacity of this compound, IU1-47, to enhance protein degradation in cells was tested using as a reporter the microtubule-associated protein tau, which has been implicated in many neurodegenerative diseases. Using primary neuronal cultures, IU1-47 was found to accelerate the rate of degradation of wild-type tau, the pathological tau mutants P301L and P301S, and the A152T tau variant. We also report that a specific residue in tau, lysine 174, is critical for the IU1-47-mediated tau degradation by the proteasome. Finally, we show that IU1-47 stimulates autophagic flux in primary neurons. In summary, these findings provide a powerful research tool for investigating the complex biology of USP14.

Klotho regulates CA1 hippocampal synaptic plasticity

Global klotho overexpression extends lifespan while global klotho-deficiency shortens it. As well, klotho protein manipulations inversely regulate cognitive function. Mice without klotho develop rapid onset cognitive impairment before they are 2months old. Meanwhile, adult mice overexpressing klotho show enhanced cognitive function, particularly in hippocampal-dependent tasks. The cognitive enhancing effects of klotho extend to humans with a klotho polymorphism that increases circulating klotho and executive function. To affect cognitive function, klotho could act in or on the synapse to modulate synaptic transmission or plasticity. However, it is not yet known if klotho is located at synapses, and little is known about its effects on synaptic function. To test this, we fractionated hippocampi and detected klotho expression in both pre and post-synaptic compartments. We find that loss of klotho enhances both pre and post-synaptic measures of CA1 hippocampal synaptic plasticity at 5weeks of age. However, a rapid loss of synaptic enhancement occurs such that by 7weeks, when mice are cognitively impaired, there is no difference from wild-type controls. Klotho overexpressing mice show no early life effects on synaptic plasticity, but decreased CA1 hippocampal long-term potentiation was measured at 6months of age. Together these data suggest that klotho affects cognition, at least in part, by regulating hippocampal synaptic plasticity.

Sphingosine 1-phosphate lyase ablation disrupts presynaptic architecture and function via an ubiquitin- proteasome mediated mechanism

The bioactive lipid sphingosine 1-phosphate (S1P) is a degradation product of sphingolipids that are particularly abundant in neurons. We have shown previously that neuronal S1P accumulation is toxic leading to ER-stress and an increase in intracellular calcium. To clarify the neuronal function of S1P, we generated brain-specific knockout mouse models in which S1P-lyase (SPL), the enzyme responsible for irreversible S1P cleavage was inactivated. Constitutive ablation of SPL in the brain (SPL^fl/fl/Nes) but not postnatal neuronal forebrain-restricted SPL deletion (SPL^fl/fl/CaMK) caused marked accumulation of S1P. Hence, altered presynaptic architecture including a significant decrease in number and density of synaptic vesicles, decreased expression of several presynaptic proteins, and impaired synaptic short term plasticity were observed in hippocampal neurons from SPL^fl/fl/Nes mice. Accordingly, these mice displayed cognitive deficits. At the molecular level, an activation of the ubiquitin-proteasome system (UPS) was detected which resulted in a decreased expression of the deubiquitinating enzyme USP14 and several presynaptic proteins. Upon inhibition of proteasomal activity, USP14 levels, expression of presynaptic proteins and synaptic function were restored. These findings identify S1P metabolism as a novel player in modulating synaptic architecture and plasticity.

Bassoon and piccolo regulate ubiquitination and link presynaptic molecular dynamics with activity-regulated gene expression

Release of neurotransmitter is executed by complex multiprotein machinery, which is assembled around the presynaptic cytomatrix at the active zone. One well-established function of this proteinaceous scaffold is the spatial organization of synaptic vesicle cluster, the protein complexes that execute membrane fusion and compensatory endocytosis, and the transmembrane molecules important for alignment of pre- and postsynaptic structures. The presynaptic cytomatrix proteins function also in processes other than the formation of a static frame for assembly of the release apparatus and synaptic vesicle cycling. They actively contribute to the regulation of multiple steps in this process and are themselves an important subject of regulation during neuronal plasticity. We are only beginning to understand the mechanisms and signalling pathways controlling these regulations. They are mainly dependent on posttranslational modifications, including phosphorylation and small-molecules conjugation, such as ubiquitination. Ubiquitination of presynaptic proteins might lead to their degradation by proteasomes, but evidence is growing that this modification also affects their function independently of their degradation. Signalling from presynapse to nucleus, which works on a much slower time scale and more globally, emerged as an important mechanism for persistent usage-dependent and homeostatic neuronal plasticity. Recently, two new functions for the largest presynaptic scaffolding proteins bassoon and piccolo emerged. They were implied (1) in the regulation of specific protein ubiquitination and proteasome-mediated proteolysis that potentially contributes to short-term plasticity at the presynapse and (2) in the coupling of activity-induced molecular rearrangements at the presynapse with reprogramming of expression of neuronal activity-regulated genes.

Interneuron Transcriptional Dysregulation Causes Frequency-Dependent Alterations in the Balance of Inhibition and Excitation in Hippocampus

Circuit dysfunction in complex brain disorders such as schizophrenia and autism is caused by imbalances between inhibitory and excitatory synaptic transmission (I/E). Short-term plasticity differentially alters responses from excitatory and inhibitory synapses, causing the I/E ratio to change as a function of frequency. However, little is known about I/E ratio dynamics in complex brain disorders. Transcriptional dysregulation in interneurons, particularly parvalbumin interneurons, is a consistent pathophysiological feature of schizophrenia. Peroxisome proliferator activated receptor γ coactivator 1α (PGC-1α) is a transcriptional coactivator that in hippocampus is highly concentrated in inhibitory interneurons and regulates parvalbumin transcription. Here, we used PGC-1α(-/-) mice to investigate effects of interneuron transcriptional dysregulation on the dynamics of the I/E ratio at the synaptic and circuit level in hippocampus. We find that loss of PGC-1α increases the I/E ratio onto CA1 pyramidal cells in response to Schaffer collateral stimulation in slices from young adult mice. The underlying mechanism is enhanced basal inhibition, including increased inhibition from parvalbumin interneurons. This decreases the spread of activation in CA1 and dramatically limits pyramidal cell spiking, reducing hippocampal output. The I/E ratio and CA1 output are partially restored by paired-pulse stimulation at short intervals, indicating frequency-dependent effects. However, circuit dysfunction persists, indicated by alterations in kainate-induced gamma oscillations and impaired nest building. Together, these results show that transcriptional dysregulation in hippocampal interneurons causes frequency-dependent alterations in I/E ratio and circuit function, suggesting that PGC-1α deficiency in psychiatric and neurological disorders contributes to disease by causing functionally relevant alterations in I/E balance.

SIGNIFICANCE STATEMENT

Alteration in the inhibitory and excitatory synaptic transmission (I/E) balance is a fundamental principle underlying the circuit dysfunction observed in many neuropsychiatric and neurodevelopmental disorders. The I/E ratio is dynamic, continuously changing because of synaptic short-term plasticity. We show here that transcriptional dysregulation in interneurons, particularly parvalbumin interneurons, causes frequency-dependent alterations in the I/E ratio and in circuit function in hippocampus. Peroxisome proliferator activated receptor γ coactivator 1α (PGC-1α-deficient) mice have enhanced inhibition in CA1, the opposite of what is seen in cortex. This study fills an important gap in current understanding of how changes in inhibition in complex brain disorders affect I/E dynamics, leading to region-specific circuit dysfunction and behavioral impairment. This study also provides a conceptual framework for analyzing the effects of short-term plasticity on the I/E balance in disease models.

Post-endocytotic Deubiquitination and Degradation of the Metabotropic γ-Aminobutyric Acid Receptor by the Ubiquitin-specific Protease 14

Mechanisms controlling the metabotropic γ-aminobutyric acid receptor (GABAB) cell surface stability are still poorly understood. In contrast with many other G protein-coupled receptors (GPCR), it is not subject to agonist-promoted internalization, but is constitutively internalized and rapidly down-regulated. In search of novel interacting proteins regulating receptor fate, we report that the ubiquitin-specific protease 14 (USP14) interacts with the GABAB(1b)subunit's second intracellular loop. Probing the receptor for ubiquitination using bioluminescence resonance energy transfer (BRET), we detected a constitutive and phorbol 12-myristate 13-acetate (PMA)-induced ubiquitination of the receptor at the cell surface. PMA also increased internalization and accelerated receptor degradation. Overexpression of USP14 decreased ubiquitination while treatment with a small molecule inhibitor of the deubiquitinase (IU1) increased receptor ubiquitination. Treatment with the internalization inhibitor Dynasore blunted both USP14 and IU1 effects on the receptor ubiquitination state, suggesting a post-endocytic site of action. Overexpression of USP14 also led to an accelerated degradation of GABABin a catalytically independent fashion. We thus propose a model whereby cell surface ubiquitination precedes endocytosis, after which USP14 acts as an ubiquitin-binding protein that targets the ubiquitinated receptor to lysosomal degradation and promotes its deubiquitination.

Ubp6 deubiquitinase controls conformational dynamics and substrate degradation of the 26S proteasome

Substrates are targeted for proteasomal degradation through the attachment of ubiquitin chains that need to be removed by proteasomal deubiquitinases prior to substrate processing. In budding yeast, the deubiquitinase Ubp6 trims ubiquitin chains and affects substrate processing by the proteasome, but the underlying mechanisms and its location within the holoenzyme remained elusive. Here we show that Ubp6 activity strongly responds to interactions with the base ATPase and the conformational state of the proteasome. Electron-microscopy analyses reveal that ubiquitin-bound Ubp6 contacts the N-ring and AAA+ ring of the ATPase hexamer, in close proximity to the deubiquitinase Rpn11. Ubiquitin-bound Ubp6 inhibits substrate deubiquitination by Rpn11, stabilizes the substrate-engaged conformation of the proteasome, and allosterically interferes with the engagement of a subsequent substrate. Ubp6 may thus act as an ubiquitin-dependent timer to coordinate individual processing steps at the proteasome and modulate substrate degradation.

Presynaptic size of associational/commissural CA3 synapses is controlled by fibroblast growth factor 22 in adult mice

Associational/commissural CA3-CA3 synapses define the recurrent CA3 network that generates the input to CA1 pyramidal neurons. We quantified the fine structure of excitatory synapses in the stratum radiatum of the CA3d area in adult wild type (WT) and fibroblast growth factor 22 knock-out (FGF22KO) mice by using serial 3D electron microscopy. WT excitatory CA3 synapses are rather small yet range 10 fold in size. Spine size, however, was small and uniform and did not correlate with the size of the synaptic junction. To reveal mechanisms that regulate presynaptic structure, we investigated the role of FGF22, a target-derived signal specific for the distal part of area CA3 (CA3d). In adult FGF22KO mice, postsynaptic properties of associational CA3 synapses were unaltered. Presynaptically, the number of synaptic vesicles (SVs), the bouton volume, and the number of vesicles in axonal regions (the super pool) were reduced. This concurrent decrease suggests concerted control by FGF22 of presynaptic size. This hypothesis is supported by the finding that WT presynapses in the proximal part of area CA3 (CA3p) that do not receive FGF22 signaling in WT mice were smaller than presynapses in CA3d in WT but of comparable size in CA3d of FGF22KO mice. Docked SV density was decreased in CA1, CA3d, and CA3p in FGF22KO mice. Because CA1 and CA3p are not directly affected by the loss of FGF22, the smaller docked SV density may be an adaptation to activity changes in the CA3 network. Thus, docked SV density potentially is a long-term regulator for the synaptic release probability and/or the strength of short-term depression in vivo.

The HERC1 E3 Ubiquitin Ligase is essential for normal development and for neurotransmission at the mouse neuromuscular junction

The ubiquitin-proteasome system (UPS) plays a fundamental role in protein degradation in neurons, and there is strong evidence that it fulfills a key role in synaptic transmission. The aim of the present work was to study the implication of one component of the UPS, the HERC1 E3 Ubiquitin Ligase, in motor function and neuromuscular transmission. The tambaleante (tbl) mutant mouse carries a spontaneous mutation in HERC1 E3 Ubiquitin Ligase, provoking an ataxic phenotype that develops in the second month of life. Our results show that motor performance in mutant mice is altered at postnatal day 30, before the cerebellar neurodegeneration takes place. This defect is associated with by: (a) a reduction of the motor end-plate area, (b) less efficient neuromuscular activity in vivo, and (c) an impaired evoked neurotransmitter release. Together, these data suggest that the HERC1 E3 Ubiquitin Ligase is fundamental for normal muscle function and that it is essential for neurotransmitter release at the mouse neuromuscular junction.

Building up and knocking down: an emerging role for epigenetics and proteasomal degradation in systems consolidation

Memory formation is a protracted process in which recently acquired events are consolidated to produce stable and specific associations. Initially, newly acquired information undergoes cellular consolidation in the hippocampus, which transiently supports the storage of recently acquired memories. In contrast, remote, or "old" memories are maintained in the cortex and show almost complete independence from the hippocampus. Memories are transferred from the hippocampus to the cortex through a process termed systems consolidation. Emerging evidence suggests that recurrent activation, or "training" of the cortex by the hippocampus is vital to systems consolidation. This process involves prolonged waves of memory-related gene activity in the hippocampus and cortex long after the learning event has terminated. Indeed, molecular events occurring within hours and days of fear conditioning are essential for stabilizing and eventually transitioning the memory to the cortex. It is increasingly evident that molecular mechanisms that exhibit a capacity for prolonged activation may underlie systems consolidation. Processes that have the capacity to control protein abundance over long time scales, such as epigenetic modifications, are prime candidates for the molecular mechanism of systems consolidation. Indeed, recent work has established two types of epigenetic modifications as integral for systems consolidation. First, localized nucleosomal histone variant exchange and histone modifications are integral for early stages of systems consolidation, whereas DNA methylation appears to be utilized to form stable marks that support memory maintenance. Since systems consolidation also requires discrete and time-sensitive changes in protein abundance, additional mechanisms, such as protein degradation, need also be considered, although their role in systems consolidation has yet to be investigated. Here, we discuss the role of molecular mechanisms in systems consolidation and their implications for understanding how memories persist over time.

Distinct effects of ubiquitin overexpression on NMJ structure and motor performance in mice expressing catalytically inactive USP14

Ubiquitin-specific protease 14 (USP14) is a major deubiquitinating enzyme and a key determinant of neuromuscular junction (NMJ) structure and function. We have previously reported dramatic ubiquitin depletion in the nervous systems of the USP14-deficient ataxia (ax^J) mice and demonstrated that transgenic ubiquitin overexpression partially rescues the ax^J neuromuscular phenotype. However, later work has shown that ubiquitin overexpression does not correct the ax^J deficits in hippocampal short term plasticity, and that transgenic expression of a catalytically inactive form of USP14 in the nervous system mimics the neuromuscular phenotype observed in the ax^J mice, but causes a only a modest reduction of free ubiquitin. Instead, increased ubiquitin conjugates and aberrant activation of pJNK are observed in the nervous systems of the USP14 catalytic mutant mice. In this report, we demonstrate that restoring free ubiquitin levels in the USP14 catalytic mutant mice improved NMJ structure and reduced pJNK accumulation in motor neuron terminals, but had a negative impact on measures of NMJ function, such as motor performance and muscle development. Transgenic expression of ubiquitin had a dose-dependent effect on NMJ function in wild type mice: moderate levels of overexpression improved NMJ function while more robust ubiquitin overexpression reduced muscle development and motor coordination. Combined, these results suggest that maintenance of free ubiquitin levels by USP14 contributes to NMJ structure, but that USP14 regulates NMJ function through a separate pathway.

Mice lacking the transcriptional coactivator PGC-1α exhibit alterations in inhibitory synaptic transmission in the motor cortex

Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) is a transcriptional coactivator known to regulate gene programs in a cell-specific manner in energy-demanding tissues, and its dysfunction has been implicated in numerous neurological and psychiatric disorders. Previous work from the Cowell laboratory indicates that PGC-1α is concentrated in inhibitory interneurons and is required for the expression of the calcium buffer parvalbumin (PV) in the cortex; however, the impact of PGC-1α deficiency on inhibitory neurotransmission in the motor cortex is not known. Here, we show that mice lacking PGC-1α exhibit increased amplitudes and decreased frequency of spontaneous inhibitory postsynaptic currents in layer V pyramidal neurons. Upon repetitive train stimulation at the gamma frequency, decreased GABA release is observed. Furthermore, PV-positive interneurons in PGC-1α -/- mice display reductions in intrinsic excitability and excitatory input without changes in gross interneuron morphology. Taken together, these data show that PGC-1α is required for normal inhibitory neurotransmission and cortical PV-positive interneuron function. Given the pronounced motor dysfunction in PGC-1α -/- mice and the essential role of PV-positive interneurons in maintenance of cortical excitatory:inhibitory balance, it is possible that deficiencies in PGC-1α expression could contribute to cortical hyperexcitability and motor abnormalities in multiple neurological disorders.