The Interactive Fly

Zygotically transcribed genes

Ubiquitination and protein degradation

Ubiquitin (Ub) is a member of a family of conserved polypeptides that are covalently attached to protein substrates. Multiple rounds of modification create a poly(Ub) chain on the substrate that targets the substrate for degradation by the proteasome. The transfer of free Ub onto a protein substrate is a multistep process. E1 activates free Ub at the expense of ATP. Ub is then transferred to an E2 (or ubiquitin protein-conjugating enzyme). It is believed that each E2 is responsible for ubiquitinating distinct substrates. Although a free E2 enzyme may directly transfer Ub onto a substrate in a purified system, this reaction is promoted by additional proteins referred to as E3s or ubiquitin protein ligases. Some E3s act as intermediary Ub carriers in the transfer of Ub from E2 to substrate. Other E3s act as adapters, tethering E2 to E2's substrates. It turns out that a variety of structurally distinct E3 proteins each serve to regulate the interaction between E2 proteins and various distinct substrates.

  • Proteasome, but not autophagy, disruption results in severe eye and wing dysmorphia: a subunit- and regulator-dependent process in Drosophila
  • The deubiquitinase Leon/USP5 regulates ubiquitin homeostasis during Drosophila development
  • USP5/Leon deubiquitinase confines postsynaptic growth by maintaining ubiquitin homeostasis through Ubiquilin
  • Conserved properties of Drosophila Insomniac link sleep regulation and synaptic function
  • Cul3 and insomniac are required for rapid ubiquitination of postsynaptic targets and retrograde homeostatic signaling
  • Pri sORF peptides induce selective proteasome-mediated protein processing
  • Variation in Dube3a expression affects neurotransmission at the Drosophila neuromuscular junction
  • Quantitative proteomics reveals extensive changes in the ubiquitinome after perturbation of the proteasome by targeted dsRNA mediated subunit knockdown in Drosophila
  • A proteomics approach to identify targets of the ubiquitin-like molecule Urm1 in Drosophila melanogaster
  • A new Drosophila model of Ubiquilin knockdown shows the effect of impaired proteostasis on locomotive and learning abilities
  • Mask mitigates MAPT- and FUS-induced degeneration by enhancing autophagy through lysosomal acidification
  • Expression and regulation of deubiquitinase-resistant, unanchored ubiquitin chains in Drosophila
  • Proteasome activity determines pupation timing through the degradation speed of timer molecule Blimp-1
  • Developmental and tissue specific changes of ubiquitin forms in Drosophila melanogaster
  • Unanchored ubiquitin chains do not lead to marked alterations in gene expression in Drosophila melanogaster
  • Proteasome dysfunction induces excessive proteome instability and loss of mitostasis that can be mitigated by enhancing mitochondrial fusion or autophagy
  • Usp14 is required for spermatogenesis and ubiquitin stress responses in Drosophila melanogaster
  • Proteotoxic stress is a driver of the loser status and cell competition
  • Isoleucine 44 Hydrophobic Patch Controls Toxicity of Unanchored, Linear Ubiquitin Chains through NF-kappaB Signaling
  • Deubiquitinase USP7 regulates Drosophila aging through ubiquitination and autophagy
  • Slowed protein turnover in aging Drosophila reflects a shift in cellular priorities
  • The linear ubiquitin E3 ligase-Relish pathway is involved in the regulation of proteostasis in Drosophila muscle during aging
  • The ubiquitin ligase Ariadne-1 regulates neurotransmitter release via ubiquitination of NSF
  • Proteasome stress in skeletal muscle mounts a long-range protective response that delays retinal and brain aging
  • Stuxnet facilitates the degradation of polycomb protein during development

  • Ubiquitin ligases
    Anaphase promoting complex and its regulators
    Proteins that regulate Tramtrack degradation
    Other proteins that regulate protein degradation
    Ubiquitin activating enzymes

    Ubiquitin conjugating enzymes

    Proteasome, but not autophagy, disruption results in severe eye and wing dysmorphia: a subunit- and regulator-dependent process in Drosophila

    Proteasome-dependent and autophagy-mediated degradation of eukaryotic cellular proteins represent the two major proteostatic mechanisms that are critically implicated in a number of signaling pathways and cellular processes. Deregulation of functions engaged in protein elimination frequently leads to development of morbid states and diseases. In this context, and through the utilization of GAL4/UAS genetic tool, this study examined the in vivo contribution of proteasome and autophagy systems in Drosophila eye and wing morphogenesis. By exploiting the ability of GAL4-ninaE. GMR and P{GawB}Bx(MS1096) genetic drivers to be strongly and preferentially expressed in the eye and wing discs, respectively, this study proved that proteasomal integrity and ubiquitination proficiency essentially control fly's eye and wing development. Indeed, subunit- and regulator-specific patterns of severe organ dysmorphia were obtained after the RNAi-induced downregulation of critical proteasome components (Rpn1, Rpn2, alpha5, beta5 and beta6) or distinct protein-ubiquitin conjugators (UbcD6, but not UbcD1 and UbcD4). Proteasome deficient eyes presented with either rough phenotypes or strongly dysmorphic shapes, while transgenic mutant wings were severely folded and carried blistered structures together with loss of vein differentiation. Moreover, transgenic fly eyes overexpressing the UBP2-yeast deubiquitinase enzyme were characterized by an eyeless-like phenotype. Therefore, the proteasome/ubiquitin proteolytic activities are undoubtedly required for the normal course of eye and wing development. In contrast, the RNAi-mediated downregulation of critical Atg (1, 4, 7, 9 and 18) autophagic proteins revealed their non-essential, or redundant, functional roles in Drosophila eye and wing formation under physiological growth conditions, since their reduced expression levels could only marginally disturb wing's, but not eye's, morphogenetic organization and architecture. However, Atg9 proved indispensable for the maintenance of structural integrity of adult wings in aged flies. In all, these findings clearly demonstrate the gene-specific fundamental contribution of proteasome, but not autophagy, in invertebrate eye and wing organ development (Velentzas, 2013).

    The deubiquitinase Leon/USP5 regulates ubiquitin homeostasis during Drosophila development

    Ubiquitination and the reverse process deubiquitination regulate protein stability and function during animal development. The Drosophila USP5 homolog Leon functions as other family members of unconventional deubiquitinases, disassembling free, substrate-unconjugated polyubiquitin chains to replenish the pool of mono-ubiquitin, and maintaining cellular ubiquitin homeostasis. However, the significance of Leon/USP5 in animal development is still unexplored. This study generated leon mutants to show that Leon is essential for animal viability and tissue integrity during development. Both free and substrate-conjugated polyubiquitin chains accumulate in leon mutants, suggesting that abnormal ubiquitin homeostasis caused tissue disorder and lethality in leon mutants. Further analysis of protein expression profiles in leon mutants shows that the levels of all proteasomal subunits were elevated. Also, proteasomal enzymatic activities were elevated in leon mutants. However, proteasomal degradation of ubiquitinated substrates was impaired. Thus, aberrant ubiquitin homeostasis in leon mutants disrupts normal proteasomal degradation, which is compensated by elevating the levels of proteasomal subunits and activities. Ultimately, the failure to fully compensate the dysfunctional proteasome in leon mutants leads to animal lethality and tissue disorder (Wang, 2014).

    USP5/Leon deubiquitinase confines postsynaptic growth by maintaining ubiquitin homeostasis through Ubiquilin

    Synapse formation and growth are tightly controlled processes. How synaptic growth is terminated after reaching proper size remains unclear. This study shows that Leon, the Drosophila USP5 deubiquitinase, controls postsynaptic growth. In leon mutants, postsynaptic specializations of neuromuscular junctions are dramatically expanded, including the subsynaptic reticulum, the postsynaptic density, and the glutamate receptor cluster. Expansion of these postsynaptic features is caused by a disruption of ubiquitin homeostasis with accumulation of free ubiquitin chains and ubiquitinated substrates in the leon mutant. Accumulation of Ubiquilin (Ubqn), the ubiquitin receptor whose human homolog ubiquilin 2 is associated with familial amyotrophic lateral sclerosis, also contributes to defects in postsynaptic growth and ubiquitin homeostasis. Importantly, accumulations of postsynaptic proteins cause different aspects of postsynaptic overgrowth in leon mutants. Thus, the deubiquitinase Leon maintains ubiquitin homeostasis and proper Ubqn levels, preventing postsynaptic proteins from accumulation to confine postsynaptic growth (Wang, 2017).

    A synapse is a specialized structure where signals are transmitted from a neuron to another neuron or other target cells such as muscles. Proper synapse formation is prerequisite to building functional synapses and constructing neuronal circuits. Synapse abnormalities are suggested to induce neurological and psychological disorders such as autism spectrum disorders and fragile X syndrome. Formation of postsynapses requires coordinated formation of several specialized structures. One prominent postsynaptic feature at neuromuscular junctions (NMJs) is the extensively folded muscular membranes. Specialized folding of postjunctional membranes is thought to increase the area exposed to the synaptic cleft and ensure the effectiveness of neuromuscular transmission. In addition to membrane specializations, the postsynaptic density (PSD) is also a common element whose size requires proper control. The PSD contains scaffolding proteins that recruit signaling protein complexes and neurotransmitter receptors, matching precisely the presynaptic active zones. Formations of postsynaptic membrane and PSD are tightly controlled and coordinated yet these processes remain elusive (Wang, 2017).

    The Drosophila NMJ is a model to study synapse formation and activity-dependent synapse remodeling. Synaptic boutons are swollen structures of axonal terminals embedded in highly folded muscular membranes called the subsynaptic reticulum (SSR) and each bouton contains tens of neurotransmitter release sites paired with PSDs. During larval development, the SSR and the PSD concomitantly form and gradually increase their sizes. Two crucial factors, postsynaptic density protein-95/Discs large (Dlg) localized at the SSR and Drosophila p21-activating kinase (dPak) localized at the PSD, regulate SSR formation. At the PSD, two types of localized glutamate receptors (GluRs), IIA and IIB, appear in distinct GluR clusters. The abundance of GluRIIA at the PSD is regulated by PSD-localized dPak and the SSR-localized NF-κB complex, NF-κB/Dorsal (Dl), IκB/Cactus (Cact) and IRAK/Pelle (Pll) (Zhou, 2015 and references therein). Thus, the postsynaptic protein could localize at either SSR or PSD, and confer growth regulation on SSR, PSD or both (Wang, 2017).

    Ubiquitination plays essential roles in various cellular processes including synaptic growth. Ubiquitin species are dynamically balanced among free and substrate-conjugated forms of mono-ubiquitin and ubiquitin chains. Ubiquitin homeostasis, i.e. the maintenance of diverse ubiquitin species in proper proportions and levels, is regulated in cellular growth and differentiation, a large superfamily of ubiquitin regulators, participate in the dynamic equilibrium of ubiquitin species. While some DUBs process newly synthesized ubiquitin precursors for ubiquitin supply, others recycle ubiquitin by cleaving ubiquitin chains from protein substrates prior to proteasomal degradation. USP5, the focus of this study, is dedicated to disassembly of free ubiquitin chains for recycling. Physiologically, heat shock stress in yeast causes a reduction of the mono-ubiquitin level. To compensate for ubiquitin depletion, the level of the DUB Doa4 is elevated, leading to an increase in the mono-ubiquitin level by cleaving free ubiquitin chains. The ataxia mice axJ, carrying mutations in the DUB USP14, displayed nerve swelling and abnormal neurotransmission at NMJs. The defects are caused by a reduction in the ubiquitin level as lower ubiquitin levels were detected in the mutant mice and introducing an ubiquitin transgene suppressed the axJ phenotypes. Thus, regulation of the ubiquitin level is a critical step in synapse development and for preventing neurological disorders (Wang, 2017).

    Drosophila USP5/Leon is essential to maintain ubiquitin homeostasis during tissue formation and controls activation of apoptosis and the JNK pathway during eye development. This study characterized the role of Leon in postsynaptic growth after synapse formation. In leon mutants, while the presynapse maintains normal morphology, the postsynapse overelaborates, displaying expanded SSR, enlarged PSD and excess PSD-localized GluR clusters. Free ubiquitin chains and ubiquitinated substrates accumulate in leon postsynapses, revealing defects in ubiquitin homeostasis. Genetic analysis shows that accumulations of several postsynaptic proteins accounts for overelaborated postsynaptic structures. The ubiquitin receptor Ubiquilin (Ubqn) recognizes and transfers ubiquitinated substrates to the proteasome for degradation. The Ubqn level is elevated in leon postsynapses and reducing the Ubqn level suppresses leon mutant phenotypes. Importantly, co-overexpression of free ubiquitin chains and Ubqn promotes expansion of these postsynaptic features. Thus, ubiquitin homeostasis such as disassembly of free ubiquitin chains, timely degradation of proteins, and normal function of the ubiquitin receptor Ubqn are compromised in leon mutants, leading to postsynaptic overgrowth (Wang, 2017).

    Conserved properties of Drosophila Insomniac link sleep regulation and synaptic function

    Sleep is an ancient animal behavior that is regulated similarly in species ranging from flies to humans. Various genes that regulate sleep have been identified in invertebrates, but whether the functions of these genes are conserved in mammals remains poorly explored. Drosophila insomniac (inc) mutants exhibit severely shortened and fragmented sleep. Inc protein physically associates with the Cullin-3 (Cul3) ubiquitin ligase, and neuronal depletion of Inc or Cul3 strongly curtails sleep, suggesting that Inc is a Cul3 adaptor that directs the ubiquitination of neuronal substrates that impact sleep. Three proteins similar to Inc exist in vertebrates-KCTD2, KCTD5, and KCTD17-but are uncharacterized within the nervous system and their functional conservation with Inc has not been addressed. This study shows that Inc and its mouse orthologs exhibit striking biochemical and functional interchangeability within Cul3 complexes. Remarkably, KCTD2 and KCTD5 restore sleep to inc mutants, indicating that they can substitute for Inc in vivo and engage its neuronal targets relevant to sleep. Inc and its orthologs localize similarly within fly and mammalian neurons and can traffic to synapses, suggesting that their substrates may include synaptic proteins. Consistent with such a mechanism, inc mutants exhibit defects in synaptic structure and physiology, indicating that Inc is essential for both sleep and synaptic function. These findings reveal that molecular functions of Inc are conserved through ~600 million years of evolution and support the hypothesis that Inc and its orthologs participate in an evolutionarily conserved ubiquitination pathway that links synaptic function and sleep regulation (Li, 2017).

    The presence of sleep states in diverse animals has been suggested to reflect a common purpose for sleep and the conservation of underlying regulatory mechanisms. This study has shown that attributes of the Insomniac protein likely to underlie its impact on sleep in Drosophila-its ability to function as a multimeric Cul3 adaptor and engage neuronal targets that impact sleep-are functionally conserved in its mammalian orthologs. This comparative analysis of Inc family members in vertebrate and invertebrate neurons furthermore reveals that these proteins can traffic to synapses and that Inc itself is essential for normal synaptic structure and excitability. These findings support the hypothesis that Inc family proteins serve as Cul3 adaptors and direct the ubiquitination of conserved neuronal substrates that impact sleep and synaptic function (Li, 2017).

    The ability of KCTD2 and KCTD5 to substitute for Inc in the context of sleep is both surprising and notable given the complexity of sleep-wake behavior and the likely functions of these proteins as Cul3 adaptors. Adaptors are multivalent proteins that self-associate, bind Cul3, and recruit substrates, and these interactions are further regulated by additional post-translational mechanisms. The findings indicate that KCTD2 and KCTD5 readily substitute for Inc within oligomeric Inc-Cul3 complexes, and strongly suggest that these proteins recapitulate other aspects of Inc function in vivo including the ability to engage neuronal targets that impact sleep. The simplest explanation for why KCTD2 and KCTD5 have retained the apparent ability to engage Inc targets despite the evolutionary divergence of Drosophila and mammals is that orthologs of Inc targets are themselves conserved in mammals. This inference draws support from manipulations of Drosophila Roadkill/HIB and its mammalian ortholog SPOP, Cul3 adaptors of the MATH-BTB family that regulate the conserved Hedgehog signaling pathway. While the ability of SPOP to substitute for HIB has not been assessed by rescue at an organismal level, clonal analysis in Drosophila indicates that ectopically expressed mouse SPOP can degrade the endogenous HIB substrate Cubitus Interruptus (Ci), and conversely, that HIB can degrade mammalian Gli proteins that are the conserved orthologs of Ci and substrates of SPOP. By analogy, Inc targets that impact sleep are likely to have orthologs in vertebrates that are recruited by KCTD2 and KCTD5 to Cul3 complexes. While the manipulations do not resolve whether KCTD17 can substitute for Inc in vivo, the ability of KCTD17 to assemble with fly Inc and Cul3 suggests that functional divergence among mouse Inc orthologs may arise outside of the BTB domain, and in particular may reflect properties of their C-termini including the ability to recruit substrates (Li, 2017).

    The finding that Inc can transit to synapses and is required for normal synaptic function is intriguing in light of hypotheses that invoke synaptic homeostasis as a key function of sleep. While ubiquitin-dependent mechanisms contribute to synaptic function and plasticity and sleep is known to influence synaptic remodeling in both vertebrates and invertebrates, molecular links between ubiquitination, synapses, and sleep remain poorly explored. Other studies in flies have indicated that regulation of RNA metabolism may similarly couple synaptic function and the control of sleep. Alterations in the activity of the Fragile X mental retardation protein (FMR), a regulator of mRNA translation, cause defects in the elaboration of neuronal projections and the formation of synapses as well as changes in sleep duration and consolidation. Loss of Adar, a deaminase that edits RNA, leads to increased sleep through altered glutamatergic synaptic function. Like Inc, these proteins are conserved in mammals, suggesting that further studies in flies may provide insights into diverse mechanisms by which sleep influences synaptic function and conversely, how changes in synapses may impact the regulation of sleep (Li, 2017).

    These findings at a model synapse suggest that the impact of Inc on synaptic function may be intimately linked to its influence on sleep but do not yet resolve important aspects of such a mechanism. The synaptic phenotypes of inc mutants-increased synaptic growth, decreased evoked neurotransmitter release, and modest effects on spontaneous neurotransmission-are qualitatively distinct from those of other short sleeping mutants. Shaker (Sh) and Hyperkinetic (Hk) mutations decrease sleep in adults but increase both excitability and synaptic growth at the NMJ, suggesting that synaptic functions of Inc may affect sleep by a mechanism different than broad neuronal hyperexcitability. While a parsimonious model is that Inc directs the ubiquitination of a target critical for synaptic transmission both at the larval NMJ and in neuronal populations that promote sleep, this hypothesis awaits the elucidation of Inc targets, definition of the temporal requirements of Inc activity, and further mapping of the neuronal populations through which Inc impacts sleep. Finally, determining the localization of endogenous Inc within neurons is essential to distinguish possible presynaptic and postsynaptic functions of Inc and whether Inc engages local synaptic proteins or extrasynaptic targets that ultimately influence synaptic function (Li, 2017).

    A clear implication of these findings is that neuronal targets and synaptic functions of Inc may be conserved in other animals. While the impact of Inc orthologs on sleep in vertebrates is as yet unknown, findings from C. elegans support the notion that conserved molecular functions of Inc and Cul3 may underlie similar behavioral outputs in diverse organisms. INSO-1/C52B11.2, the only C. elegans ortholog of Inc, interacts with Cul3, and RNAi against Cul3 and INSO-1 reduces the duration of lethargus, a quiescent sleep-like state, suggesting that effects of Cul3- and Inc-dependent ubiquitination on sleep may be evolutionarily conserved. The functions of Inc orthologs and Cul3 in the mammalian nervous system await additional characterization, but emerging data suggest functions relevant to neuronal physiology and disease. Human mutations at the KCTD2/ATP5H locus are associated with Alzheimer's disease, and mutations of KCTD17 with myoclonic dystonia. Cul3 lesions have been associated in several studies with autism spectrum disorders and comorbid sleep disturbances. More generally, autism spectrum disorders are commonly associated with sleep deficits and are thought to arise in many cases from altered synaptic function, but molecular links to sleep remain fragmentary. Studies of Inc family members and their conserved functions in neurons are likely to broaden understanding of how ubiquitination pathways may link synaptic function to the regulation of sleep and other behaviors (Li, 2017).

    Cul3 and insomniac are required for rapid ubiquitination of postsynaptic targets and retrograde homeostatic signaling

    At the Drosophila neuromuscular junction, inhibition of postsynaptic glutamate receptors activates retrograde signaling that precisely increases presynaptic neurotransmitter release to restore baseline synaptic strength. However, the nature of the underlying postsynaptic induction process remains enigmatic. In this study a forward genetic screen is described to discover factors in the postsynaptic compartment necessary to generate retrograde homeostatic signaling. This approach identified insomniac (inc), a putative adaptor for the Cullin-3 (Cul3) ubiquitin ligase complex, which together with Cul3 is essential for normal sleep regulation. Interestingly, it was found that Inc and Cul3 rapidly accumulate at postsynaptic compartments following acute receptor inhibition and are required for a local increase in mono-ubiquitination. Finally, it was shown that Peflin, a Ca(2+)-regulated Cul3 co-adaptor, is necessary for homeostatic communication, suggesting a relationship between Ca(2+) signaling and control of Cul3/Inc activity in the postsynaptic compartment. This study suggests that Cul3/Inc-dependent mono-ubiquitination, compartmentalized at postsynaptic densities, gates retrograde signaling and provides an intriguing molecular link between the control of sleep and homeostatic plasticity at synapses (Kikuma, 2019).

    By screening >300 genes with putative functions at synapses, this study has identified inc as a key postsynaptic regulator of retrograde homeostatic signaling at the Drosophila NMJ. The data suggest that Inc and Cul3 are recruited to the postsynaptic compartment within minutes of glutamate receptor perturbation, where they promote local mono-ubiquitination. Inc/Cul3 appear to function downstream of or in parallel to CaMKII and upstream of retrograde signaling during PHP. Pef was identified as a putative co-adaptor that may work with Inc/Cul3 to link Ca2+ signaling in the postsynaptic compartment with membrane trafficking and retrograde communication. Altogether, these findings implicate a post translational signaling system involving mono-ubiquitination in the induction of retrograde homeostatic signaling at postsynaptic compartments (Kikuma, 2019).

    Although forward genetic screens have been very successful in identifying genes required in the presynaptic neuron for the expression of PHP, these screens have provided less insight into the postsynaptic mechanisms that induce retrograde homeostatic signaling. It seems clear that many genes acting presynaptically are individually required for PHP, with loss of any one completely blocking PHP expression. Indeed, ~25 genes that function in neurons have thus far been implicated in PHP expression. In contrast, forward genetic screens have largely failed to uncover new genes functioning in the postsynaptic muscle during PHP, implying some level of redundancy. The specific postsynaptic induction mechanisms driving retrograde PHP signaling have therefore remained unclear, and are further complicated by cap-dependent translation and metabolic pathways that contribute to sustaining PHP expression over chronic, but not acute, time scales. Therefore, it is perhaps not surprising that despite screening hundreds of mutants, this study found only a single gene, insomniac, to be required for PHP induction. Inc is expressed in the nervous system and can traffic to the presynaptic terminals of motor neuron. In the context of PHP signaling, however, inc was found to be required in the postsynaptic compartment, where it functions downstream of or in parallel to CaMKII. One attractive possibility is that a reduction in CaMKII-dependent phosphorylation of postsynaptic targets enables subsequent ubiquitination by Cul3-Inc complexes, and that this modification ultimately drives retrograde signaling during PHP. Indeed, reciprocal influences of phosphorylation and ubiquitination on shared targets are a common regulatory feature in a variety of signaling systems. The dynamic interplay of phosphorylation and ubiquitination in the postsynaptic compartment may enable a sensitive and tunable mechanism for controlling the timing and calibrating the amplitude of retrograde signaling at the NMJ (Kikuma, 2019).

    The substrates targeted by Inc and Cul3 during PHP induction are not known, but the identification of mono-ubiquitination in the postsynaptic compartment during PHP signaling and the putative Cul3 co-adaptor Peflin provides a foundation from which to assess possible candidates and pathways. In mammals, Pef forms a complex with another Ca2+ binding protein, ALG2, to confer Ca2+ regulation to membrane trafficking pathways. Moreover, Pef/ALG2 were recently found to serve as target-specific co-adaptors for Cul3-KLHL12. In particular, SEC31 and other components involved in ER-mediated membrane trafficking pathways were shown to be targeted for mono-ubiquitination, which in turn modulate Collagen secretion. One attractive possibility, therefore, is that Cul3/Inc could respond to changes in Ca2+ in the postsynaptic compartment through regulation by Pef during PHP signaling to control membrane trafficking pathways. Importantly, the subsynaptic reticulum (SSR) is a complex and membraneous network at the Drosophila NMJ, where electrical, Ca2+-dependent, and membrane trafficking pathways in the postsynaptic compartment are integrated (Teodoro, 2013; Nguyen, 2016). Indeed, Multiplexin, a fly homolog of Collagen XV/XVIII and a proposed retrograde signal, is secreted into the synaptic cleft and is required for trans-synaptic retrograde signaling during PHP (Wang, 2014). In addition, another proposed retrograde signal and secreted protein, Semaphorin 2B, was recently shown to function postsynaptically in retrograde PHP signaling (Orr, 2017). However, inc does not appear to be the closest Drosophila ortholog to KLHL12, and it is therefore possible that Pef and Cul3/Inc regulate postsynaptic PHP signaling through a more indirect mechanism (Kikuma, 2019).

    While the precise relationships between CaMKII, Inc, Cul3, and Pef are currently unclear, the activity of membrane trafficking pathways could ultimately be targeted for modulation by Ca2+- and Cul3/Inc-dependent signaling during PHP induction. First, a role for postsynaptic membrane trafficking and elaboration during PHP signaling has already been suggested. In addition, extracellular Ca2+ does not appear to be involved in rapid PhTx-dependent PHP induction. It is therefore tempting to speculate that Ca2+ release from the postsynaptic SSR during rapid PHP signaling may influence Cul3/Inc activity through Pef-dependent regulation, as transient changes in ER-derived Ca2+-signaling controls Pef-dependent recruitment of Cul3 (McGourty, 2016). Alternatively, postsynaptic scaffolds and/or glutamate receptors themselves may be targeted by Cul3/Inc at the Drosophila NMJ, given that these proteins are involved in ubiquitin-mediated signaling and remodeling at dendritic spines. Consistent with this idea, there is evidence that signaling complexes composed of neurotransmitter receptors, CaMKII, and membrane-associated guanylate kinases are intimately associated at postsynaptic densities in Drosophila, as they are in the mammalian central nervous system. There has been speculation that these complexes are targets for modulation during PHP signaling. Although these models are not mutually exclusive, further studies will be required to determine the specific substrates and signal transduction mechanisms through which Cul3/Inc and Pef initiate and sustain retrograde homeostatic communication in postsynaptic compartments (Kikuma, 2019).

    While it is well established that the ubiquitin proteasome system can sculpt and remodel synaptic architecture, the importance of mono-ubiquitination at synapses is less studied. Ubiquitin-dependent pathways play key roles in synaptic structure, function, and degeneration, and also contribute to activity-dependent dendritic growth. However, the fact that some proteins persist for long periods at synapses suggests that modification of these proteins by ubiquitin likely include non-degredative and reversible mechanisms. Indeed, a recent study revealed a remarkable heterogeneity in the stability of synaptic proteins, with some short lived and rapidly turned over, while others persisting for long time scales, with half lives of months or longer. At the Drosophila NMJ, rapid ubiquitin-dependent proteasomal degradation at presynaptic terminals is necessary for the expression of PHP through modulation of the synaptic vesicle pool (Wentzel, 2018). In contrast, postsynaptic proteasomal degradation does not appear to be involved in rapid PHP signaling, suggesting that ubiquitin-dependent pathways in the postsynaptic compartment contribute to PHP signaling by non-degradative mechanisms. The current data demonstrate that Cul3, Inc, and Pef function in muscle to enable retrograde PHP signaling, and suggest that Cul3/Inc rapidly trigger mono-ubiquitination at postsynaptic densities following glutamate receptor perturbation. Interestingly, synaptic proteins can be ubiquitinated in <15 s following depolarization-induced Ca2+ influx at synapses (Chen, 2003) and changes in intracellular Ca2+ can activate Pef and Cul3 signaling with similar rapidity. Therefore, both poly- and mono-ubiquination may function in combination with other rapid and reversible processes, including phosphorylation at postsynaptic compartments to enable robust and diverse signaling outcomes during the induction of homeostatic plasticity (Kikuma, 2019).

    A prominent hypothesis postulates that a major function of sleep is to homeostatically regulate synaptic strength following experience-dependent changes that accrue during wakefulness. Several studies have revealed changes in neuronal firing rates and synapses during sleep/wake behavior, yet few molecular mechanisms that directly associate the electrophysiological process of homeostatic synaptic plasticity and sleep have been identified. The finding that inc is required for the homeostatic control of synaptic strength provides an intriguing link to earlier studies, which implicate inc in the regulation of sleep. It remains to be determined to what extent the role of inc in controlling PHP signaling at the NMJ is related to the impact of inc on sleep and, if so, whether Inc targets the same substrates to regulate these processes. Interestingly, virtually all neuropsychiatric disorders are associated with sleep dysfunction, including those associated with homeostatic plasticity and Fragile X Syndrome, and sleep behavior is also disrupted by mutations in the Drosophila homolog of FMRP, dfmr1. Further investigation of this intriguing network of genes involved in the homeostatic control of sleep and synaptic plasticity may help solve the biological mystery that is sleep and also shed light on the etiology of neuropsychiatric diseases (Kikuma, 2019).

    Pri sORF peptides induce selective proteasome-mediated protein processing

    A wide variety of RNAs encode small open-reading-frame (smORF/sORF) peptides, but their functions are largely unknown. This study shows that Drosophila polished-rice (pri) sORF peptides trigger proteasome-mediated protein processing, converting the Shavenbaby (Svb) transcription repressor into a shorter activator. A genome-wide RNA interference screen identifies an E2-E3 ubiquitin-conjugating complex, UbcD6-Ubr3, which targets Svb to the proteasome in a pri-dependent manner. Upon interaction with Ubr3, Pri peptides promote the binding of Ubr3 to Svb. Ubr3 can then ubiquitinate the Svb N terminus, which is degraded by the proteasome. The C-terminal domains protect Svb from complete degradation and ensure appropriate processing. These data show that Pri peptides control selectivity of Ubr3 binding, which suggests that the family of sORF peptides may contain an extended repertoire of protein regulators (Zanet, 2015).

    Eukaryotic genomes encode many noncoding RNAs (ncRNAs) that lack the classical hallmarks of protein-coding genes. However, both ncRNAs and mRNAs often contain small open reading frames (sORFs), and there is growing evidence that they can produce peptides, from yeast to plants or humans. The polished rice or tarsal-less (pri) RNA contains four sORFs that encode highly related 11- to 32-amino acid peptides, required for embryonic development across insect species. In flies, pri is essential for the differentiation of epidermal outgrowths called trichomes. Trichome development is governed by the Shavenbaby (Svb) transcription factor; however, only in the presence of pri can Svb turn on the program of trichome development, i.e., activate expression of cellular effectors. Indeed, the Svb protein is translated as a large repressor, pri then induces truncation of its N-terminal region, which leads to a shorter activator (Kondo, 2010). Thereby, pri defines the developmental timing of epidermal differentiation, in a direct response to systemic ecdysone hormonal signaling (Chanut-Delalande, 2014). Although there is currently a clear framework for the developmental functions of pri, how these small peptides can trigger Svb processing is unknown (Zanet, 2015).

    To identify factors required for Svb processing in response to pri, a genome-wide RNA interference (RNAi) screen was performed in a cell line coexpressing green fluorescent protein (GFP)-tagged Svb and pri. An automated assay was set up quantifying Svb processing for each of the Drosophila genes, with an inhibitory score reflecting the proportion of cells unable to cleave off the Svb N terminus. pri RNAi displayed the highest score, which validated this approach to identifying molecular players in Svb processing. Methods used to evaluate results from genome-wide screening all converged on a key role for the proteasome. For instance, COMPLEAT, a bioinformatic framework based on protein complex analysis, identified the proteasome in 66 out of the 71 top predictions. A survey of individual proteasome subunits indicated that both the 20S catalytic core and the 19S regulatory particles are required for Svb processing. Chemical proteasome inhibitors independently confirmed this conclusion, because they also prevented pri-induced Svb processing. These data thus provide compelling evidence that Svb processing results from a pri-dependent proteolysis by the proteasome (Zanet, 2015).

    To investigate how pri regulates proteolysis of Svb, the protein region(s) in Svb were identified that are involved in pri-dependent processing. Systematic deletions demonstrated the importance of the Svb N terminus for pri response and restricted the minimal motif to the N-terminal 31 amino acids. Deletion of this motif within an otherwise full-length protein (Δ31) made Svb refractory to pri. Conversely, the Svb N terminus when fused to GFP (1s::GFP) was sufficient to transform this protein into a pri target and to make GFP sensitive to pri. Unlike Svb, however, 1s::GFP was completely degraded by the proteasome upon pri expression (Zanet, 2015).

    Recent studies have shown that structural features of proteins influence their degradation by the proteasome: Whereas unstructured substrates, such as intrinsically disordered regions, favor degradation, tightly folded domains can resist proteasome progression. Analysis of Svb sequences predicted intrinsically disordered features throughout its N-terminal moiety, which is degraded. By contrast, the proteasome-resistant C-terminal moiety comprises two folded regions: the transcriptional activation and zinc finger domains. Within the transcriptional activation region, amino acids 532 to 701 protected Svb from complete degradation. Indeed, the C-terminally truncated mutants of 1 to 701 amino acids (and longer) were still processed, whereas mutants shortened by 1 to 532 amino acids (and shorter) were fully degraded. Whether other folded domains would also protect Svb from complete degradation was tested and it was found that attaching zinc fingers to short Svb mutants-otherwise degraded upon pri expression-was sufficient to restore processing. Likewise, the DNA binding domain of Gal4 protected against degradation, which indicated that even a heterologous protein domain with strong structure can protect Svb from full degradation in response to pri. Hence, distinct regions of Svb mediate its processing by the proteasome: the 31 N-terminal residues act as a pri-dependent degradation signal, or degron, and C-terminal domains act as stabilizing features that prevent complete degradation (Zanet, 2015).

    Proteins are targeted to the proteasome by the covalent attachment of ubiquitin to Lys residues. The Svb N terminus is highly conserved from insects to human; it comprises two invariant Lys residues (K3 and K8) and a third one at a less constrained position (K28 in Drosophila). Individual Lys substitutions had only a weak effect or no effect, whereas simultaneous mutation of all three Lys (3Kmut) abolished Svb processing. Furthermore, strong pri-dependent ubiquitination of Svb was detected when the proteasome was inhibited. By contrast, this was no longer seen in the 3Kmut variant, which demonstrated the key role of these three Lys in ubiquitin-dependent Svb processing (Zanet, 2015).

    Ubiquitin conjugation requires three enzymes (E1, E2, and E3); specificity is generally conferred by the E3 ubiquitin ligases that recognize and bind to substrates. A prominent hit from the RNAi screen was Ubr3 (7 hits out of the top 15), which encodes an E3. Ranking all Drosophila ubiquitin enzymes by their inhibitory score confirmed that Ubr3 was the major E3 required for Svb processing and identified UbcD6 (Rad6) as its associated E2, consistent with evidence that human Ubr3 also forms a complex with UbcD6. Like many proteasome factors, Ubr3 has a broad subcellular distribution in cytoplasm and nuclei, whereas Svb and UbcD6 are nuclear proteins. Svb processing still occurred normally when nuclear export was impaired, which indicated that the proteolytic activation of Svb takes place within the nucleus (Zanet, 2015).

    Several additional lines of evidence support the conclusion that Ubr3 mediates the function of pri for Svb ubiquitination. First, Ubr3 coimmunoprecipitated with Svb in a pri-dependent manner and ubiquitinated Svb was found in a complex with Ubr3 upon proteasome inhibition. Second, the N terminus of Svb was sufficient for Ubr3 binding in response to pri. Note that a functional N-terminal degron in Svb was required for its interaction with Ubr3, because the ubiquitin-resistant 3Kmut variant no longer bound Ubr3. Third, in protein extracts from cells that do not express pri, addition of synthetic Pri peptide was sufficient to promote Ubr3-Svb interaction in vitro, in a dose-dependent manner. By contrast, a peptide of the same composition but in a 'scrambled' sequence lacked activity (Zanet, 2015).

    Although critical for the binding of Ubr3 to the Svb N terminus, Pri peptides are, however, not indispensable for Ubr3 activity. pri did not influence the binding of Ubr3 to Ape1 (Rrp1), a factor involved in DNA repair and regulated by Ubr3-dependent proteasome degradation. Also, the interaction of Ubr3 with DIAP1, which inhibits apoptosis, occurred with or without pri. Moreover, Pri peptides interacted with Ubr3, even in the absence of Svb. Finally, the isolated UBR-box of Ubr3 no longer required Pri peptides to bind Svb, which suggested that other Ubr3 motifs prevent Svb interaction in the absence of pri. It is therefore concluded that Pri peptides directly regulate the selectivity of Ubr3 for binding to the Svb N terminus and, thereby, trigger Svb ubiquitination and processing by the proteasome (Zanet, 2015).

    Recently a Ubr3 loss-of-function allele was isolated, and its phenotype in the differentiation of epidermal cells was assayed. As observed for pri mutants, embryos lacking Ubr3 were unable to differentiate trichomes and to process Svb. Moreover, inactivation of either UbcD6 or Ubr3 prevented formation of adult trichomes in mosaic animals. When compared with their wild-type neighbors, Ubr3-null cells accumulated the repressor form of Svb, which demonstrated Ubr3's essential role for Svb processing in vivo (Zanet, 2015).

    Taken together, thes data show that Pri peptides control the binding of the Ubr3 ubiquitin ligase to Svb and activate its processing by the proteasome. In the absence of Pri, Ubr3 nonetheless recognizes other substrates, which shows that a main role for Pri peptides is to modify the binding selectivity of Ubr3. This could potentially be achieved through a conformational change in Ubr3 protein, as proposed for Ubr1, that unmasked the recognition site for Svb upon Pri peptide binding to Ubr3 (Zanet, 2015).

    Although recent work has uncovered thousands of novel sORF peptides, only a handful of their molecular targets have yet been identified. sORF peptides have recently been found to bind and regulate the Ca2+ uptake SERCA protein, the heterotrimeric guanine nucleotide-binding protein coupled signaling APJ (Apelin), and the DNA repair protein Ku. Protein-protein interactions often involve small protein regions, and artificial peptides that mimic these binding surfaces have been proven to be potent modulators of protein complexes. It is proposed that sORF-encoded peptides provide an unexplored reservoir of protein-binding interfaces, well suited to regulate the activity of a wide range of cellular factors (Zanet, 2015).

    Variation in Dube3a expression affects neurotransmission at the Drosophila neuromuscular junction

    Changes in UBE3A expression levels in neurons can cause neurogenetic disorders ranging from Angelman syndrome (AS) (decreased levels) to autism (increased levels). This study investigated the effects on neuronal function of varying UBE3A levels using the Drosophila neuromuscular junction as a model for both of these neurogenetic disorders. Stimulations that evoked excitatory junction potentials (EJPs) at 1 Hz intermittently failed to evoke EJPs at 15 Hz in a significantly higher proportion of Dube3a over-expressors using the pan neuronal GAL4 driver C155-GAL4 (C155-GAL4>UAS-Dube3a) relative to controls (C155>+ alone). However, in the Dube3a over-expressing larval neurons with no failures, there was no difference in EJP amplitude at the beginning of the train, or the rate of decrease in EJP amplitude over the course of the train compared to controls. In the absence of tetrodotoxin (TTX), spontaneous EJPs were observed in significantly more C155-GAL4>UAS-Dube3a larva compared to controls. In the presence of TTX, spontaneous and evoked EJPs were completely blocked and mEJP amplitude and frequency did not differ among genotypes. These data suggest that over-expression of wild type Dube3a, but not a ubiquitination defective Dube3a-C/A protein, compromises the ability of motor neuron axons to support closely spaced trains of action potentials, while at the same time increasing excitability. EJPs evoked at 15 Hz in the absence of Dube3a (Dube3a15b homozygous mutant larvae) decay more rapidly over the course of 30 stimulations compared to w1118 controls, and Dube3a15b larval muscles have significantly more negative resting membrane potentials (RMP). However, these results could not be recapitulated using RNAi knockdown of Dube3a in muscle or neurons alone, suggesting more global developmental defects contribute to this phenotype. These data suggest that reduced UBE3A expression levels may cause global changes that affect RMP and neurotransmitter release from motorneurons at the neuromuscular junction. Similar affects of under- and over-expression of UBE3A on membrane potential and synaptic transmission may underlie the synaptic plasticity defects observed in both AS and autism (Valdez, 2015).

    Angelman syndrome (AS) is a devastating human neurological disorder characterized by cognitive and behavioral defects, muscle hypotonia as well as jerky limb movements and a debilitating ataxic gait. Mouse models of UBE3A maternal loss of function exhibit deficits in learning, hippocampal long term potentiation, and experience-dependent maturation of the neocortex, which may represent alterations in calcium/calmodulin-dependent protein kinase II, properties of axonal initial segment, postsynaptic regulation of glutamatergic signaling, and dendrite morphogenesis. The ataxic gait phenotype of AS is clearly recapitulated in mice deficient for Ube3a as demonstrated by rotarod performance, gait analysis, and cerebellar controlled licking behavior. Although these gait phenotypes appear to be primarily due to a decrease in inhibitory signals in the cerebellum, a comprehensive analysis of motor neuron function in the absence of UBE3A has not yet been performed and rescue of Ube3a levels in the cerebellum of Ube3a deficient mice does not always rescue the ataxic gait phenotype (Valdez, 2015).

    Duplications of the same region deleted in the majority of individuals with AS are the second most common genetic lesion (3-5% of cases) found in autism. Just as maternal deletion is required for an AS phenotype, maternal duplications of 15q are specifically associated with increased autism risk. A mouse model with a duplication syntenic to human interstitial duplications of 15q11.2-q13, displayed behavioral deficits characteristic of autism, possibly caused by a deficit in 5-HT2c receptor signaling. These data support the hypothesis that the level of UBE3A expressed from the maternal allele in neurons is critical to neuronal development and function; deficiency for maternal UBE3A resulting in Angelman syndrome and duplication of maternal UBE3A driving increased autism risk (Valdez, 2015).

    Drosophila models of Dube3a deficiency [the orthologue to UBE3A in flies have revealed that the loss of Dube3a in neurons results in decreased dendritic arborization in larval peripheral neurons, decreased dopamine levels in adult fly brain, and a clearly measurable defect in climbing ability in adult flies. Adult flies deficient for Dube3a or expressing wild type Dube3a in neurons showed significant defects in climbing ability that were ubiquitin ligase dependent, implying an underlying defect at the neuromuscular junction that may also depend on Dube3a ubiquitination. Previous work has shown that Dube3a loss of function causes changes in the expression of various protein components of the actin cytoskeleton eventually leading to a measurable loss of filamentous actin in the larval muscle wall, so this effect may also be due to muscle developmental defects (Valdez, 2015).

    The fly neuromuscular junction (NMJ) is an excellent model for examination of genes involved in synapse formation, function and regulation, but can also be used to examine the effects post-synaptic defects in larval muscle on neurophysiology. Studies of mammalian synapses in the brain have pointed to a pivotal role for the ubiquitin proteasome system in both pre and post-synaptic regulation and this is also true for the development and function of the fly NMJ. To find out how changes in Dube3a levels affected neuronal function (both axonal and synaptic) at the NMJ this study examined synaptic transmission at 3rd instar larval NMJ under conditions of both loss and over-expression of Dube3a. Defects were identifed in axonal propagation of action potentials and synaptic transmission associated with changes in Dube3a in motor neurons. This study provides evidence that the phenotypes observed in humans and mice with decreased or elevated Ube3a may be at least in part related to defects in axonal and synaptic function (Valdez, 2015).

    This study demonstrates that both over-expression and deficiency for Dube3a, the fly orthologue of human UBE3A, alters neurotransmission at the neuromuscular junction in Drosophila melanogaster 3rd instar larvae. In a significant proportion of larvae expressing elevated levels of Dube3a in neurons, rapid stimulation of motor nerves intermittently fails to evoke an EJP, and spontaneous depolarizations resembling evoked EJPs are frequently observed in the absence of TTX. However, the amplitude of the first EJP in the train of evoked EJPs and the amplitude and frequency of mEJPs does not vary between any of the genotypes, indicating that this is an axonal rather than vesicle recycling issue. Also, in over-expressors that do not exhibit evoked EJP failure, EJP amplitude does not change more than controls during rapid stimulation. Finally, the spontaneous depolarizations are not observed in larvae over-expressing a ubiquitination defective form of Dube3a (Dube3a-C/A) indicating that the phenomena is dependent on the ubiquitin ligase function of the Dube3a protein. These data could be explained by assuming that evoked EJP failure and spontaneous depolarizations result from regulation of Dube3a ubiquitin target(s) in motor neuron axons rather than directly on the release of neurotransmitter at the synapse. One possible explanation for the spontaneous depolarizations and failures is that Dube3a over-expression results in a depolarization of the RMP of the motor neurons. It is possible that a depolarized membrane potential could result in inactivation of Na+ channels, which could lead to inability of the axons to conduct closely spaced action potentials. At the same time, depolarization of the membrane potential could increase excitability of the axon by bringing it closer to the potential where large numbers of Na+ channels begin to activate. Under this condition any minor perturbation of the axon membrane potential could trigger an action potential in the motor neuron and subsequent EJP in the targeted muscle. The spontaneous depolarizations often appear as bursts, the termination of which might be also be explained by Na+ channel inactivation, similar to the intermittent failures observed at rapid stimulation rates. There was no significant difference in muscle RMP in Dube3a over-expressors versus controls, which is expected since the C155-GAL4 driver employed selectively targets neurons and not muscle (Valdez, 2015).

    Complete loss of Dube3a expression in the mutant resulta in a different pattern of effects from over-expression. In w1118; Dube3a15b/Dube3a15b larvae, which make no functional Dube3a protein, the EJP decreases more rapidly in response to rapid stimulation compared to their w1118 controls. This is typically referred to as short term depression (STD). The observation of apparent STD in Dube3a15b larvae could be related to the observation that short term facilitation (STF) is less frequently observed in Dube3a15b versus their w1118 controls. STD is thought to be due to a depletion of the readily releasable pool of synaptic vesicles, while STF is thought to be the result of Ca2+ build up in the terminal due to rapid successive depolarizations. At the stimulation rate of 15 Hz, the overall change in EJP amplitude could be a balance between STF and STD. Possibly, a deficit in STF in Dube3a15b larvae could have led to an overall faster decrease in EJP amplitude relative to w1118 controls (Valdez, 2015).

    Also, the RMP in the muscles of Dube3a15b mutants is significantly more negative than their w1118 controls. These data may reflect a deficit in one or more of the processes or elements involved in maintenance of the RMP. A recent study suggests that Na+/K+ ATPase is ubiquitinated in a Dube3a dependent manner. One might expect that if the loss of Dube3a is causing the more negative RMP in muscle via an effect on Na+/K+ ATPase levels or activity, then the motor neurons may also be affected because regulation of muscle and nerve cell membrane potential both depend on the Na+/K+ ATPase. Nevertheless, changes in resting K+ levels due to leakage across the membrane could also explain these findings. However, it may be more than a coincidence that the effects of over-expression of Dube3a results in increased evoked EJP failures and increased spontaneous, both of which may be indications of a depolarized RMP in motor neurons in the corresponding larvae. Over-expression of Dube3a may have the opposite effects on RMP as loss of Dube3a via opposing actions on this ubiquitin target (Valdez, 2015).

    The data on the structure of the synaptic active zones suggests that C155>Dube3a-27 and C155>Dube3a-51 larvae have fewer active zones and that C155>Dube3a-51 also have smaller synaptic vesicles relative to the other genotypes. It was also shown that there is a slight increase in synaptic zone density by NC82 staining, however these results did not reach significance despite the large dataset analyzed. These effects of altered Dube3a expression do not seem to explain the effects of over-expression or deficiency on the electrophysiological paradigms employed. However, they may later prove to be important observations that explain deficits in synaptic transmission not tested in the present study. In mouse models of both Angelman syndrome (decreased Ube3a) and Duplication 15q autism (elevated Ube3a) there are defects in glutamatergic synaptic transmission. This study shows that these defects in glutamatergic signaling can be recapitulated in the fly models for both syndromes as well, validating the fly model system for both syndromes. Thus, in a simple and easy to manipulate model system, the Drosophila NMJ, one can now investigate the downstream effects of changes in Dube3a levels on potential ubiquitin targets in the context of neuronal function. Some putative Ube3a protein targets such as Arc and CamKII have been known for some time, while an entirely new set of potential Dube3a targets has been recently identified through a proteomic screen in flies. It can be anticipated that by manipulating the putative targets of Dube3a in the fly NMJ system through shRNAi knock down or mutations in these genes one can begin to unravel the molecular mechanism behind the neurological defects observed in humans with both AS and Duplication 15q autism (Valdez, 2015).

    Quantitative proteomics reveals extensive changes in the ubiquitinome after perturbation of the proteasome by targeted dsRNA mediated subunit knockdown in Drosophila

    The ubiquitin-proteasome system (UPS), a highly regulated mechanism including the active marking of proteins by ubiquitin in order to be degraded, is critical in regulating proteostasis. Dysfunctioning of the UPS has been implicated in diseases such as cancer and neurodegenerative disorders. This study investigated the effects of proteasome malfunctioning on global proteome and ubiquitinome dynamics using SILAC proteomics in Drosophila S2 cells. dsRNA mediated knockdown of specific proteasome target subunits is used to inactivate the proteasome. Upon this perturbation, both the global proteome and the ubiquitinome become modified to a great extent, the overall impact on the ubiquitinome being most dramatic. The abundances of approx. 10% of all proteins are increased, while the abundances of the far majority of over 14 thousand detected diGly peptides are increased, suggesting that the pool of ubiquitinated proteins is highly dynamic. Remarkably, several proteins show heterogeneous ubiquitination dynamics, with different lysine residues on the same protein showing either increased or decreased ubiquitination. This suggests the occurrence of simultaneous and functionally different ubiquitination events. This strategy offers a powerful tool to study the response of the ubiquitinome upon interruption of normal UPS activity by targeted interference and opens up new avenues for the dissection of the mode of action of individual components of the proteasome. Since this is the first comprehensive ubiquitinome screen upon proteasome malfunctioning in a fruit fly cell system, this data set will serve as a valuable repository for the Drosophila community (Sap, 2017).

    A proteomics approach to identify targets of the ubiquitin-like molecule Urm1 in Drosophila melanogaster

    By covalently conjugating to target proteins, ubiquitin-like modifiers (UBLs) act as important regulators of target protein localization and activity. The most ancient and one of the least studied UBLs is Urm1, a dual-function protein that in parallel to performing similar functions as its prokaryotic ancestors in tRNA modification. Affinity purification followed by mass spectrometry were used to identify putative targets of Urm1 conjugation (urmylation) at three developmental stages of the Drosophila melanogaster lifecycle. Altogether 79 Urm1-interacting proteins were recovered in Drosophila, which include the already established Urm1 binding partners Prx5 and Uba4, together with 77 candidate urmylation targets that are completely novel in the fly. Among these, the majority was exclusively identified during either embryogenesis, larval stages or in adult flies. Biochemical evidence is presented that four of these proteins are covalently conjugated by Urm1, whereas the fifth verified Urm1-binding protein appears to interact with Urm1 via non-covalent means. Besides recapitulating the previously established roles of Urm1 in tRNA modification and during oxidative stress, functional clustering of the newly identified Urm1-associated proteins further positions Urm1 in protein networks that control other types of cellular stress, such as immunological threats and DNA damage. In addition, the functional characteristics of several of the candidate targets strongly match the phenotypes displayed by Urm1n123 null animals, including embryonic lethality, reduced fertility and shortened lifespan. In conclusion, this identification of candidate targets of urmylation significantly increases the knowledge of Urm1 and presents an excellent starting point for unravelling the role of Urm1 in the context of a complex living organism (Khoshnood, 2017).

    A new Drosophila model of Ubiquilin knockdown shows the effect of impaired proteostasis on locomotive and learning abilities

    Ubiquilin(UBQLN) plays a crucial role in cellular proteostasis through its involvement in the ubiquitin proteasome system and autophagy. Mutations in the UBQLN2 gene have been implicated in amyotrophic lateral sclerosis (ALS) and ALS with frontotemporal lobar dementia (ALS/FTLD). Previous studies reported a key role for UBQLN in Alzheimer's disease (AD); however, the mechanistic involvement of UBQLN in other neurodegenerative diseases remains unclear. The genome of Drosophila contains a single UBQLN homolog (dUbqn) that shows high similarity to UBQLN1 and UBQLN2; therefore, the fly is a useful model for characterizing the role of UBQLN in vivo in neurological disorders affecting locomotion and learning abilities. This study performed a phenotypic and molecular characterization of diverse dUbqn RNAi lines. The depletion of dUbqn induced the accumulation of polyubiquitinated proteins and caused morphological defects in various tissues. The results showed that structural defects in larval neuromuscular junctions, abdominal neuromeres, and mushroom bodies correlated with limited abilities in locomotion, learning, and memory. These results contribute to understanding of the impact of impaired proteostasis in neurodegenerative diseases and provide a useful Drosophila model for the development of promising therapies for ALS and FTLD (Jantrapirom, 2018).

    Mask mitigates MAPT- and FUS-induced degeneration by enhancing autophagy through lysosomal acidification

    This study shows that Mask, an Ankyrin-repeat and KH-domain containing protein, plays a key role in promoting autophagy flux and mitigating degeneration caused by protein aggregation or impaired ubiquitin-proteasome system (UPS) function. In Drosophila eye models of human tauopathy or amyotrophic lateral sclerosis diseases, loss of Mask function enhanced, while gain of Mask function mitigated, eye degenerations induced by eye-specific expression of human pathogenic MAPT/TAU or FUS proteins. The fly larval muscle, a more accessible tissue, was then used to study the underlying molecular mechanisms in vivo. Mask was found to modulate the global abundance of K48- and K63-ubiquitinated proteins by regulating macroautophagy/autophagy-lysosomal-mediated degradation, but not UPS function. Indeed, upregulation of Mask compensated the partial loss of UPS function. It was further demonstrated that Mask promotes autophagic flux by enhancing lysosomal function, and that Mask is necessary and sufficient for promoting the expression levels of the proton-pumping vacuolar (V)-type ATPases in a TFEB-independent manner. Moreover, the beneficial effects conferred by Mask expression on the UPS dysfunction and neurodegenerative models depend on intact autophagy-lysosomal pathway. These findings highlight the importance of lysosome acidification in cellular surveillance mechanisms and establish a model for exploring strategies to mitigate neurodegeneration by boosting lysosomal function (Zhu, 2017).

    Misfolded protein aggregates in and outside of cells in the central nervous system are pathological hallmarks of many neurodegenerative disorders including Alzheimer (AD), Parkinson (PD), Huntington (HD) diseases and amyotrophic lateral sclerosis (ALS). Interestingly, many of the aggregated proteins (such as MAPT (TAU) and APP for Alzheimer disease, SNCA/α-synuclein for Parkinson disease, HTT (Huntingtin) for Huntington disease, FUS, SOD1 and TARDBP/TDP-43 for ALS) can serve as seeds for 'prion-like' spreading of the aggregation within and among cells. It is not entirely clear whether these aggregates are the causes or the results of progressive and cell-type-specific neurodegeneration. However, mounting evidence suggests that clearance and prevention of these toxic protein aggregates are beneficial for meliorating degeneration (Zhu, 2017).

    Two major pathways collaborate in regulating intracellular protein degradation: the ubiquitin-proteasome system (UPS) and the autophagy-lysosomal system. Under the normal conditions, UPS serves as the primary route for rapid protein turnover while autophagy mainly degrades long-lived proteins and large cellular organelles under basal conditions and can be robustly induced in face of stresses such as starvation, organelle damage or accumulation of misfolded proteins. However when it comes to degradation of damaged proteins in diseased states, autophagy has been shown to play at least an equally important role as UPS.5. Many of the neurodegenerative disease-related proteins are delivered to autophagic vacuoles and degraded by the autophagy pathway. Meanwhile, impairment of autophagy in the mouse brain causes neurodegeneration associated with ubiquitin-positive protein aggregation. These data suggest that UPS and autophagy are both indispensable in maintaining cellular protein homeostasis. Furthermore, recent studies indicate that UPS and autophagy pathways coordinate with each other to prevent accumulation of toxic protein aggregates, so that enhanced activity of one pathway can compensate if the other is compromised (Zhu, 2017).

    Both UPS and autophagy degradation systems are complex processes consisting of chains of sequential events orchestrated by a large group of proteins. To understand their coordinated action, it is necessary to identify novel players that are necessary and sufficient to mediate the compensatory function between the two systems. This study shows Mask, a conserved protein with Ankyrin repeats and a KH domain, as a novel and critical player in such a context. Initially identified as a modulator of receptor tyrosine signaling during Drosophila development (Smith, 2002), Mask has recently been shown to function as a cofactor of the Hippo pathway effector Yorkie and together they regulate target gene transcription with another transcription cofactor (Scalloped) during cell proliferation (Sansores-Garcia, 2013; Sidor, 2013). The human ortholog of Mask, ANKHD1, is highly expressed in several cancer cell lines. Loss of mask function rescues the mitochondrial defects and muscle degeneration observed with pink1 and park mutants (Zhu, 2015). This study shows that in MAPT- and FUS-induced eye degeneration fly models, loss of Mask function enhances degeneration, while gain of Mask function suppresses degeneration. By enhancing V-type ATPase expression, Mask promotes lysosome acidification and autophagic flux; Mask is necessary and sufficient to mediate a compensatory effect for partial loss of UPS function, to increase clearance of ubiquitinated proteins, and to protect against degeneration induced by aggregation-prone mutations (Zhu, 2017).

    Autophagy, an evolutionarily conserved cellular mechanism that preserves metabolic homeostasis during nutrient unavailability, is traditionally regarded as a self-eating degradative process with limited selectivity. However, mounting evidence suggests that both micro- and macro-autophagy can play cytoprotective roles to specifically target damaged and toxic organelles and proteins for clearance under pathological conditions. The mechanism of selective autophagy is unclear. There is some evidence that autophagy receptors can recognize ubiquitin-dependent and ubiquitin-independent signals for selective degradation. Autophagy is a multistep process including nucleation, autophagosome formation and fusion with lysosomes and each step can be regulated to enhance degradation of damaged cellular components. Research has emerged showing TFEB is a potent regulator of the autophagy-lysosomal pathway whose activation can promote lysosomal function and mitigate disease in a range of neurodegenerative disorders. This study shows that Mask acts in a TFEB-independent manner to boost the expression of V-ATPase subunits. This study provides novel evidence that lysosome function is not only required for the normal clearance of ubiquitinated and misfolded proteins, but its activity can also be boosted potential through enhanced lysosomal acidification, to mitigate cellular degeneration caused by toxic protein aggregation (Zhu, 2017).

    Mask is well positioned to regulate lysosome-mediated clearance of ubiquitinated and misfolded proteins. As a positive regulator of several V-type ATPase V1 subunits expression, Mask function is necessary and sufficient to promote lysosomal acidification and autophagosome degradation in a cell-autonomous manner. When the UPS function is impaired, increased Mask expression is sufficient to increase autophagic flux, which in turn compensates the partial loss of the proteasome-mediated degradation. Interestingly, even when UPS function is intact, levels of Mask activity impact the abundance of UPS-dependent (K48) and -independent (such as K63) ubiquitin-conjugated proteins, suggesting that autophagy and lysosome-mediated degradation plays an important role for basal protein homeostasis. Under pathological conditions such as UPS inactivation or excessive accumulation of disease proteins, upregulation of Mask activity substantially suppressed the cellular degeneration phenotypes in both muscles and photoreceptors, potentially through Mask-mediated increase of autophagy and lysosome activities and subsequent degradation of harmful protein aggregates, as suggested by the current biochemical and genetic analyses. In support of this notion, upregulation of Mask promotes autophagic flux in larval muscles, adult eyes and adult brains (Zhu, 2017).

    This work in the Drosophila model organism yielded new insight into Mask-mediated cellular protective mechanisms that regulate lysosomal function in normal and stressed conditions caused by misfolding-prone disease proteins or impaired UPS. Such mechanisms may provide a therapeutic approach for the treatment of a group of neurodegenerative disorders caused by intracellular inclusions (Zhu, 2017).

    Expression and regulation of deubiquitinase-resistant, unanchored ubiquitin chains in Drosophila

    The modifier protein, ubiquitin (Ub) regulates various cellular pathways by controlling the fate of substrates to which it is conjugated. Ub moieties are also conjugated to each other, forming chains of various topologies. In cells, poly-Ub is attached to proteins and also exists in unanchored form. Accumulation of unanchored poly-Ub is thought to be harmful and quickly dispersed through dismantling by deubiquitinases (DUBs). This study asked whether disassembly by DUBs is necessary to control unanchored Ub chains in vivo. Drosophila melanogaster lines were generated that express Ub chains non-cleavable into mono-Ub by DUBs. These chains are rapidly modified with different linkages and represent various types of unanchored species. Unanchored poly-Ub is not devastating in Drosophila, under normal conditions or during stress. The DUB-resistant, free Ub chains are degraded by the proteasome, at least in part through the assistance of VCP and its cofactor, p47. Also, unanchored poly-Ub that cannot be cleaved by DUBs can be conjugated en bloc, in vivo. These results indicate that unanchored poly-Ub species need not be intrinsically toxic; they can be controlled independently of DUB-based disassembly by being degraded, or through conjugation onto other proteins (Blount, 2018).

    Proteasome dysfunction induces excessive proteome instability and loss of mitostasis that can be mitigated by enhancing mitochondrial fusion or autophagy

    The ubiquitin-proteasome pathway (UPP) is central to proteostasis network (PN) functionality and proteome quality control. Yet, the functional implication of the UPP in tissue homeodynamics at the whole organism level and its potential cross-talk with other proteostatic or mitostatic modules are not well understood. This study shows that knock down (KD) of proteasome subunits in Drosophila flies, induced, for most subunits, developmental lethality. Ubiquitous or tissue specific proteasome dysfunction triggered systemic proteome instability and activation of PN modules, including macroautophagy/autophagy, molecular chaperones and the antioxidant cncC (the fly ortholog of NFE2L2/Nrf2) pathway. Also, proteasome KD increased genomic instability, altered metabolic pathways and severely disrupted mitochondrial functionality, triggering a cncC-dependent upregulation of mitostatic genes and enhanced rates of mitophagy. Whereas, overexpression of key regulators of antioxidant responses (e.g., cncC or foxo) could not suppress the deleterious effects of proteasome dysfunction; these were alleviated in both larvae and adult flies by modulating mitochondrial dynamics towards increased fusion or by enhancing autophagy. These findings reveal the extensive functional wiring of genomic, proteostatic and mitostatic modules in higher metazoans. Also, they support the notion that age-related increase of proteotoxic stress due to decreased UPP activity deregulates all aspects of cellular functionality being thus a driving force for most age-related diseases (Tsakiri, 2019).

    Usp14 is required for spermatogenesis and ubiquitin stress responses in Drosophila melanogaster
    Deubiquitylating (DUB) enzymes free covalently linked ubiquitin moieties from ubiquitin-ubiquitin and ubiquitin-protein conjugates, and thereby maintain the equilibrium between free and conjugated ubiquitin moieties and regulate ubiquitin-mediated cellular processes. This study performed genetic analyses of mutant phenotypes in Drosophila melanogaster and demonstrates that loss of Usp14 function results in male sterility, with defects in spermatid individualization and reduced testicular free monoubiquitin levels. These phenotypes were rescued by germline-specific overexpression of wild-type Usp14. Synergistic genetic interactions with Ubi-p63E and cycloheximide sensitivity suggest that ubiquitin shortage is a primary cause of male sterility. In addition, Usp14 is predominantly expressed in testes in Drosophila, indicating a higher demand for this DUB in testes that is also reflected by testis-specific loss-of-function Usp14 phenotypes. Collectively, these results suggest a major role of Usp14 in maintaining normal steady state free monoubiquitin levels during the later stages of Drosophila spermatogenesis (Kovacs, 2020).

    Proteotoxic stress is a driver of the loser status and cell competition

    Cell competition allows winner cells to eliminate less fit loser cells in tissues. In Minute cell competition, cells with a heterozygous mutation in ribosome genes, such as RpS3(+/-) cells, are eliminated by wild-type cells. How cells are primed as losers is partially understood and it has been proposed that reduced translation underpins the loser status of ribosome mutant, or Minute, cells. Using Drosophila this study shows that reduced translation does not cause cell competition. Instead, proteotoxic stress was identified as the underlying cause of the loser status for Minute competition and competition induced by mahjong, an unrelated loser gene. RpS3(+/-) cells exhibit reduced autophagic and proteasomal flux, accumulate protein aggregates and can be rescued from competition by improving their proteostasis. Conversely, inducing proteotoxic stress is sufficient to turn otherwise wild-type cells into losers. Thus, it is proposed that tissues may preserve their health through a proteostasis-based mechanism of cell competition and cell selection (Baumgartner, 2021).

    Isoleucine 44 Hydrophobic Patch Controls Toxicity of Unanchored, Linear Ubiquitin Chains through NF-kappaB Signaling

    Ubiquitination is a post-translational modification that regulates cellular processes by altering the interactions of proteins to which ubiquitin, a small protein adduct, is conjugated. Ubiquitination yields various products, including mono- and poly-ubiquitinated substrates, as well as unanchored poly-ubiquitin chains whose accumulation is considered toxic. Previous work has shown that transgenic, unanchored poly-ubiquitin is not problematic in Drosophila melanogaster. In the fruit fly, free chains exist in various lengths and topologies and are degraded by the proteasome; they are also conjugated onto other proteins as one unit, eliminating them from the free ubiquitin chain pool. To further explore the notion of unanchored chain toxicity, this study examined when free poly-ubiquitin might become problematic. It was found that unanchored chains can be highly toxic if they resemble linear poly-ubiquitin that cannot be modified into other topologies. These species upregulate NF-κB signaling, and modulation of the levels of NF-κB components reduces toxicity. In additional studies,toxicity from untethered, linear chains was shown to be regulated by isoleucine 44, which anchors a key interaction site for ubiquitin. It is concluded that free ubiquitin chains can be toxic, but only in uncommon circumstances, such as when the ability of cells to modify and regulate them is markedly restricted (Blount, 2020).

    Deubiquitinase USP7 regulates Drosophila aging through ubiquitination and autophagy

    Ubiquitination-mediated protein degradation is the selective degradation of diverse forms of damaged proteins that are tagged with ubiquitin, while deubiquitinating enzymes reverse ubiquitination-mediated protein degradation by removing the ubiquitin chain from the target protein. The interactions of ubiquitinating and deubiquitinating enzymes are required to maintain protein homeostasis. The ubiquitin-specific protease USP7 is a deubiquitinating enzyme that indirectly plays a role in repairing DNA damage and development. However, the mechanism of its participation in aging has not been fully explored. Regarding this issue, this study found that USP7 was necessary to maintain the normal lifespan of Drosophila melanogaster, and knockdown of dusp7 shortened the lifespan and reduced the ability of Drosophila to cope with starvation, oxidative stress and heat stress. Furthermore, this study showed that the ability of USP7 to regulate aging depends on the autophagy and ubiquitin signaling pathways. Furthermore, 2,5-dimethyl-celecoxib (DMC), a derivative of celecoxib, can partially restore the shortened lifespan and aberrant phenotypes caused by dusp7 knockdown. These results suggest that USP7 is an important factor involved in the regulation of aging, and related components in this regulatory pathway may become new targets for anti-aging treatments (Cui, 2020).

    Slowed protein turnover in aging Drosophila reflects a shift in cellular priorities

    The accumulation of protein aggregates and dysfunctional organelles as organisms age has led to the hypothesis that aging involves general breakdown of protein quality control. This hypothesis was tested using a proteomic and informatic approach in the fruit fly Drosophila melanogaster. Turnover of most proteins was markedly slower in old flies. However, ribosomal and proteasomal proteins maintained high turnover rates, suggesting that the observed slowdowns in protein turnover might not be due to a global failure of quality control. As protein turnover reflects the balance of protein synthesis and degradation, whether decreases in synthesis or decreases in degradation would best explain the observed slowdowns in protein turnover was investigated. It was found that while many individual proteins in old flies showed slower turnover due to decreased degradation, an approximately equal number showed slower turnover due to decreased synthesis, and enrichment analyses revealed that translation machinery itself was less abundant. Mitochondrial complex I subunits and glycolytic enzymes were decreased in abundance as well, and proteins involved in glutamine-dependent anaplerosis were increased, suggesting that old flies modify energy production to limit oxidative damage. Together, these findings suggest that age-related proteostasis changes in Drosophila represent a coordinated adaptation rather than a systems collapse (Vincow, 2021).

    The linear ubiquitin E3 ligase-Relish pathway is involved in the regulation of proteostasis in Drosophila muscle during aging

    Linear ubiquitination is an atypic ubiquitination process that directly connects the N- and C-termini of ubiquitin and is catalyzed by HOIL-1-interacting protein (HOIP). It is involved in the immune response or apoptosis by activating the nuclear factor-κB pathway and is associated with polyglucosan body myopathy 1, an autosomal recessive disorder with progressive muscle weakness and cardiomyopathy. However, little is currently known regarding the function of linear ubiquitination in muscles. This study investigated the role of linear ubiquitin E3 ligase (LUBEL), a Drosophila HOIP ortholog, in the development and aging of muscles. The muscles of the flies with down-regulation of LUBEL or its downstream factors, kenny and Relish, developed normally, and there were no obvious abnormalities in function in young flies. However, the locomotor activity of the LUBEL RNAi flies was reduced compared to age-matched control, while LUBEL RNAi did not affect the increased mitochondrial fusion or myofiber disorganization during aging. Interestingly, the accumulation of polyubiquitinated protein aggregation during aging decreased in muscles by silencing LUBEL, kenny, or Relish. Meanwhile, the levels of autophagy and global translation, which are implicated in the maintenance of proteostasis, did not change due to LUBEL down-regulation. In conclusion, a new role of linear ubiquitination is proposed in proteostasis in the muscle aging (Lee, 2021).

    The ubiquitin ligase Ariadne-1 regulates neurotransmitter release via ubiquitination of NSF

    Ariadne-1 (Ari-1) is an E3 ubiquitin-ligase essential for neuronal development, but whose neuronal substrates are yet to be identified. To search for putative Ari-1 substrates, this study used an in vivo ubiquitin biotinylation strategy coupled to quantitative proteomics of Drosophila heads. Sixteen candidates were identified that met the established criteria: a significant change of at least two-fold increase on ubiquitination, with at least two unique peptides identified. Amongst those candidates, Comatose (Comt), the homologue of the N-ethylmaleimide sensitive factor (NSF), which is involved in neurotransmitter release, was identified. Using a pulldown approach that relies on the overexpression and stringent isolation of a GFP-fused construct, Comt/NSF was validated to be an ubiquitination substrate of Ari-1 in fly neurons, resulting in the preferential monoubiquitination of Comt/NSF. The possible functional relevance of this modification was tested using Ari-1 loss of function mutants, which displayed a lower rate of spontaneous neurotransmitter release due to failures at the pre-synaptic side. By contrast, evoked release in Ari-1 mutants was enhanced compared to controls in a Ca(2+) dependent manner without modifications in the number of active zones, indicating that the probability of release per synapse is increased in these mutants. This phenotype distinction between spontaneous versus evoked release suggests that NSF activity may discriminate between these two types of vesicle fusion. These results thus provide a mechanism to regulate NSF activity in the synapse through Ari-1-dependent ubiquitination (Ramirez, 2021).

    Proteasome stress in skeletal muscle mounts a long-range protective response that delays retinal and brain aging

    Neurodegeneration in the central nervous system (CNS) is a defining feature of organismal aging that is influenced by peripheral tissues. Clinical observations indicate that skeletal muscle influences CNS aging, but the underlying muscle-to-brain signaling remains unexplored. In Drosophila, this study found that moderate perturbation of the proteasome in skeletal muscle induces compensatory preservation of CNS proteostasis during aging. Such long-range stress signaling depends on muscle-secreted Amyrel amylase. Mimicking stress-induced Amyrel upregulation in muscle reduces age-related accumulation of poly-ubiquitinated proteins in the brain and retina via chaperones. Preservation of proteostasis stems from the disaccharide maltose, which is produced via Amyrel amylase activity. Correspondingly, RNAi for SLC45 maltose transporters reduces expression of Amyrel-induced chaperones and worsens brain proteostasis during aging. Moreover, maltose preserves proteostasis and neuronal activity in human brain organoids challenged by thermal stress. Thus, proteasome stress in skeletal muscle hinders retinal and brain aging by mounting an adaptive response via amylase/maltose (Rai, 2021).

    Stuxnet facilitates the degradation of polycomb protein during development
    Polycomb-group (PcG) proteins function to ensure correct deployment of developmental programs by epigenetically repressing target gene expression. Despite the importance, few studies have been focused on the regulation of PcG activity itself. This study reports a Drosophila gene, stuxnet (stx), that controls Pc protein stability. Heightened stx activity leads to homeotic transformation, reduced Pc activity, and de-repression of PcG targets. Conversely, stx mutants, which can be rescued by decreased Pc expression, display developmental defects resembling hyperactivation of Pc. Biochemical analyses provide a mechanistic basis for the interaction between stx and Pc; stx facilitates Pc degradation in the proteasome, independent of ubiquitin modification. Furthermore, this mode of regulation is conserved in vertebrates. Mouse stx promotes degradation of Cbx4, an orthologous Pc protein, in vertebrate cells and induces homeotic transformation in Drosophila. These results highlight an evolutionarily conserved mechanism of regulated protein degradation on PcG homeostasis and epigenetic activity (Du, 2016).

    Polycomb-group (PcG) genes were first identified in Drosophila for their roles in maintaining correct expression patterns of homeotic genes. PcG-mediated transcription silencing was later proved to be a well-conserved regulatory mechanism throughout metazoans. Classical PcG targets, such as Hox genes, play important roles in biological processes ranging from stem cell maintenance to genomic imprinting. Recent genome-wide studies unveiled additional PcG targets, many of which encode transcription factors and cell-signaling proteins that regulate a diverse array of downstream effectors. Thus, PcG may act in a much broader spectrum of cellular processes than previously anticipated (Du, 2016).

    PcG silencing depends primarily on the activities of two Polycomb repressive complexes (PRC). In Drosophila, PRC1 is composed of Pc (Polycomb), Ph (Polyhomeotic), Psc (Posterior sex combs), and Sce (Sex combs extra). The main subunits of the PRC2 include Esc (Extra sex combs), E(z) (Enhancer of zeste), Su(z)12 (Suppressor of zeste 12) and Caf1 (Chromatin assembly factor 1). Relying on the presence of a conserved enzymatic SET domain in E(z), PRC2 catalyzes tri-methylation of histone H3 at Lys 27 (H3K27me3). Pc then employs its chromo domain to recognize H3K27me3 mark, resulting in recruitment of PRC1 to PcG targets. Mechanisms utilized by PRC1 to silence target genes include histone H2A mono-ubiquitination, chromatin compaction, and direct interaction with the general transcription machinery (Du, 2016).

    While intensive studies have been focused on uncovering mechanisms by which PcG proteins epigenetically repress target gene expression, few are devoted to define how the PcG activities are regulated. Nevertheless, several transcription factors and microRNAs are known to directly modulate PcG expression. Feedback regulatory loops may also be important to maintain proper expression of PcG, which themselves are subject to epigenetic repression. Furthermore, post-translational modifications on several PcG proteins have been reported, and the importance of such modifications has only been revealed recently. For example, SUMOylation is shown to modulate PcG activity by affecting chromatin targeting of the Pc protein, and O-GlcNAcylation has been demonstrated to prevent aggregation of PRC1 subunit Ph in Drosophila (Du, 2016).

    This report describe that a Drosophila gene CG32676, which was named stuxnet (stx), functions through ubiquitin-independent degradation (UID) to control Pc protein stability and thereby PcG-mediated epigenetic repression. This study shows further that vertebrate Stx regulates orthologous Pc protein in the same fashion. Together, these results highlight a conserved regulatory mechanism for Pc, the founding member of the PcG family of proteins (Du, 2016).

    Taking advantage of genetic tools available in Drosophila, the function of a UBL-domain-containing protein, Stx, was examined, and its unexpected role of regulated Pc protein degradation in epigenetic repression. These analyses on classical PcG targets demonstrate that Stx functions as a Pc-specific regulator that negatively modulates the PcG activity. Importantly, this mode of regulation was found to be conserved from flies to vertebrates (Du, 2016).

    stx activity is essential for Drosophila development. The fact that pupal lethal phenotype associated with loss-of-function stx mutations can be rescued by removing 50% of Pc activity strongly supports that modulating the Pc expression is the major developmental process regulated by stx. Stx might not be a constitutive component of the canonical PRC1. However, the ability of Stx to reduce Pc recruitment to target gene loci argues that Stx may act as a gatekeeper for control of Pc availability to form highly dynamic PRC complexes on target chromatin. As stx activity is necessary for PcG target expression, Stx could function in an intrinsic machinery to regulate Pc protein homeostasis. Stx directly binds Pc through a serine-rich PcB domain and interacts with the proteasome through the UBL domain. As Pc protein degradation does not rely on ubiquitination, the UBL domain in Stx, upon interaction with Pc, could serve as a recognition signal that marks Pc protein for degradation in the proteasome. Thus, a model is proposed in which Stx acts first as an adapter and then a chaperone-like protein to facilitate proteasomal degradation of Pc, resulting in altered PcG activity in animal development. Intriguingly, upon inspection of modENCODE database, multiple binding sites were found for PcG components, including Pc, Psc, Sce, and Pho, and Ubx, which is itself a PcG target, thus pointing to the existence of a potential feedback loop between Stx and PcG activity (Du, 2016).

    Altered Pc protein abundance has been noted in several biological processes. In the Sce mutant fly embryos, the bulk level of Pc protein is significantly reduced, but Ph and Psc are not affected. Similar results have been reported in mouse ES cells for RING1B and Cbx4, mammalian orthologs of Sce and Pc. However, the significance of such regulation was not understood. It is suspected that binding with Sce might stabilize Pc, which is crucial for PRC1 assembly. It is interesting to note that the level of Pc changes rapidly in the cell cycle. The oscillation of Pc protein during the cell cycle is thought to be important for establishment and maintenance of cellular epigenetic memory. The observation of the reciprocal expression pattern of Pc and Stx as well as the ability of Stx to control Pc abundance in cell cycle are in favor of a notion that regulated Pc protein stability may be one way to dynamically control Pc activity in physiological contexts. How Stx participates in such regulation is an interesting question that awaits further exploration (Du, 2016).

    The PRC1 is composed of four core subunits, each of which has unique molecular activities non-exchangeable among each other. However, the loss-of-function phenotypes of individual PRC1 subunits in Drosophila only partially overlap, revealing the complexity of PRC1 regulation in various cellular processes. The differential requirement of PRC1 subunits in development might be due to the presence of distinct PRC complexes in a temporal and tissue-specific manner. This view is further complicated in vertebrates by partially redundant orthologous PRC1 proteins and the formation of multiple non-canonical complexes. Thus, it will be necessary to explore the regulatory machineries utilized by individual PRC1 components to better understand how PRC complexes exert versatile functions in vivo. This study has shown that Stx targets Pc for proteasomal degradation, but whether parallel regulators exist for other PRC1 components is still unknown (Du, 2016).

    This study of Stx regulation on Pc stability reveals that the activity of Pc protein, the founding member of the PRC complexes, can be controlled through regulated protein degradation. Surprisingly, it was found that fly Pc protein is largely regulated by UID. The list of substrates that undergo UID has expanded rapidly in recent years. Intriguingly, many UID substrates are localized to the nucleus, including transcription factors and chromatin remodeling factors. The addition of Pc, a key epigenetic regulator, to this list leads to the belief that UID in the nucleus may participate in the control of gene expression (Du, 2016).

    Consistent with a role of Stx on Pc stability in Drosophila development, proteasomal degradation has been reported to affect the stability of several PcG components in cultured vertebrate cells, including three PRC1 proteins BMI1, RING1B, and PHC, and one PRC2 protein EZH2. It is thus highly likely that protein degradation may play a general role in regulating PcG activity (Du, 2016).

    Appropriate PcG activity is essential for stem cell maintenance and lineage specification in vertebrates. Altered PcG activity is associated with malignant human diseases, including cancer. Furthermore, dysregulated stx expression and Stx mutations are reported in several forms of cancer in the COSMIC database. Consistently, genes co-expressed with stx shown in COXPRESdb are clustered into pathways in cancer as well as Notch and MAPK signaling pathways. Very recently, Stx mutations were found in patients with autism spectrum disorders (ASD) by whole-exome sequencing. Given the strong connection between PcG and ASD, Stx may play a role in ASD through its regulation of PcG activity. Thus, the identification of regulators of PcG activity, such as Stx, may provide additional therapeutic targets for relevant diseases (Du, 2016).

    Proteasome activity determines pupation timing through the degradation speed of timer molecule Blimp-1

    The transcriptional repressor Blimp-1 is a labile protein. This characteristic is key for determining pupation timing because the timing of the disappearance of Blimp-1 affects pupation timing by regulating the expression of its target betaftz-f1. However, the molecular mechanisms that regulate the protein turnover of Blimp-1 are still unclear. This study demonstrates that Blimp-1 is regulated by the ubiquitin proteasome system. Blimp-1 degradation is inhibited by proteasome inhibitor MG132. Pupation timing was delayed in mutants of 26S proteasome subunits as well as FBXO11, which recruits target proteins to the 26S proteasome as a component of the SCF ubiquitin ligase complex by slowing down the degradation speed of Blimp-1. Delay in pupation timing in the FBXO11 mutant was suppressed by the induction of betaFTZ-F1. Furthermore, fat-body-specific knockdown of proteasomal activity was sufficient to induce a delay in pupation timing. These results suggest that Blimp-1 is degraded by the 26S proteasome and is recruited by FBXO11 in the fat body, which is important for determining pupation timing (Aly, 2018).

    This study showed that Drosophila Blimp-1 is degraded by the 26S proteasome system and is recruited by FBXO11 as the substrate-recognition component of the SCF complex. Furthermore, this study showed the importance of proteasome activity in the fat body to determine pupation timing. The results are correlated with previously described results that the biological timer system for pupation is located in the fat body (Akagi, 2016; Aly, 2018 and references therein).

    A delay was observed in pupation timing in all of the examined heterozygous mutants of 26S proteasome components. These results suggest gene dosage effects due to loss-of-function mutations of these 26S proteasome components. In addition, a heterozygous mutant of recruiter FBXO11 also exhibited the same level of delay in pupation timing. These results indicate that the expression level of these components is an important factor to determine pupation timing; therefore, pupation timing can be controlled by the expression level of these components. Thus, it is assumed that the UPS contributes to determine pupation timing as one of the components in the biological timer during the early prepupal period. Of note, a sudden increase in the concentration of the 26S proteasome at 0 to 4 hr APF has been reported, suggesting the importance of protein degradation in developmental control. Furthermore, RNA-Seq data in the modENCODE developmental transcriptome of D. melanogaster showed that the expression of the FBXO11 increases gradually from the 3rd instar larval stage (L3) to a moderately high level at pupation and then starts to decrease again 24 hr later. These developmental changes may allow control of the degradation speed of specific targets, including Blimp-1, among many UPS target proteins that must be degraded at appropriate time points (Aly, 2018).

    This study has shown that both the Blimp-1 and βftz-f1 are induced by 20E and are temporally expressed in almost all organs), but the identified target genes are still limited in number. βFTZ-F1 has multiple functions in each organ during the mid to late prepupal period. For instance, βFTZ-F1 regulates two pupal cuticle genes that are expressed in slightly different parts of the epidermis, and it also regulates a protease that is expressed in the fat body and contributes to its morphological change. Furthermore, the expression of βFTZ-F1 in the inka cells is essential for releasing the ecdysis-triggering hormone ETH, which induces pupation in the late prepupal period, and also βFTZ-F1 expression in muscles is necessary to determine the timing of muscle apoptosis during metamorphosis. Moreover, βFTZ-F1 is a master regulator of late prepupal gene expression, which is essential for histolysis of the salivary gland cells during the early pupal period. In addition, the expression timing of βFTZ-F1 is not completely the same among different organs. In a large transcriptional profiling platform, involving 29 dissected tissues from larval, pupal, and adult stages of Drosophila, FBXO11 appeared to be expressed in many tissues and/or during development with specific upregulation in the fat body from L3 up to pupation. It is deduced that the expression levels of the 26S proteasome and FBXO11 may differ depending on tissue and contribute to the determination of timing of tissue-specific developmental events through control of the degradation speed of Blimp-1 (Aly, 2018).

    In C. elegans, Blmp-1 was previously identified using RNAi-based suppressor screening to suppress dre-1 heterochronic phenotypes. A dre-1 mutant showed retarded migration of the gonad, whereas a Blmp-1 mutant showed precocious gonadal migration during L2 to L3 larva and was able to suppress the retarded phenotype of dre-1. In addition, precocious fusion and differentiation of epidermal stem cells, called seam cells, was partially suppressed by the Blmp-1 mutant in C. elegans. Moreover, similar genetic interactions were observed between DRE-1 and Blmp-1 for dauer formation. These observations suggest a conserved role of Blimp-1 degradation for the determination of developmental timing across taxa (Aly, 2018).

    Developmental and tissue specific changes of ubiquitin forms in Drosophila melanogaster

    In most Eukaryotes, ubiquitin either exists as free monoubiquitin or as a molecule that is covalently linked to other proteins. These two forms cycle between each other and due to the concerted antagonistic activity of ubiquitylating and deubiquitylating enzymes, an intracellular ubiquitin equilibrium is maintained that is essential for normal biological function. However, measuring the level and ratio of these forms of ubiquitin has been difficult and time consuming. This paper has adapted a simple immunoblotting technique to monitor ubiquitin content and equilibrium dynamics in different developmental stages and tissues of Drosophila. The data show that the level of total ubiquitin is distinct in different developmental stages, lowest at the larval-pupal transition and in three days old adult males, and highest in first instar larvae. Interestingly, the ratio of free mono-ubiquitin remains within 30-50% range of the total throughout larval development, but peaks to 70-80% at the larval-pupal and the pupal-adult transitions. It stays within the 70-80% range in adults. In developmentally and physiologically active tissues, the ratio of free ubiquitin is similarly high, most likely reflecting a high demand for ubiquitin availability. This method was used to demonstrate the disruption of the finely tuned ubiquitin equilibrium by the abolition of proteasome function or the housekeeping deubiquitylase, Usp5. These data support the notion that the ubiquitin equilibrium is regulated by tissue- and developmental stage-specific mechanisms (Nagy, 2018).

    Unanchored ubiquitin chains do not lead to marked alterations in gene expression in Drosophila melanogaster

    The small protein modifier, ubiquitin regulates various aspects of cellular biology through its chemical conjugation onto proteins. Ubiquitination of proteins presents itself in numerous iterations, from a single mono-ubiquitination event to chains of poly-ubiquitin. Ubiquitin chains can be attached onto other proteins or can exist as unanchored species - i.e. free from another protein. Unanchored ubiquitin chains are thought to be deleterious to the cell and rapidly disassembled into mono-ubiquitin. A recent study examined the toxicity and utilization of unanchored poly-ubiquitin in Drosophila melanogaster. Free poly-ubiquitin species were found to be largely innocuous to flies, and free poly-ubiquitin can be controlled by being degraded by the proteasome or by being conjugated onto another protein as a single unit. To explore whether an organismal defense is mounted against unanchored chains, RNA-Seq analyses was conducted to examine the transcriptomic impact of free poly-ubiquitin in the fly. Approximately 90 transcripts were found whose expression is altered in the presence of different types of unanchored poly-ubiquitin. The set of genes identified was essentially devoid of ubiquitin-, proteasome- or autophagy-related components. The seeming absence of a large and multipronged response to unanchored poly-ubiquitin supports the conclusion that these species need not be toxic in vivo and underscores the need to reexamine the role of free ubiquitin chains in the cell (Blount, 2019).


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    date revised: 15 February 2021

    Zygotically transcribed genes

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