SUMOylation at K340 inhibits tau degradation through deregulating its phosphorylation and ubiquitination

Intracellular accumulation of the abnormally modified tau is hallmark pathology of Alzheimer’s disease (AD), but the mechanism leading to tau aggregation is not fully characterized. Here, we studied the effects of tau SUMOylation on its phosphorylation, ubiquitination, and degradation. We show that tau SUMOylation induces tau hyperphosphorylation at multiple AD-associated sites, whereas site-specific mutagenesis of tau at K340R (the SUMOylation site) or simultaneous inhibition of tau SUMOylation by ginkgolic acid abolishes the effect of small ubiquitin-like modifier protein 1 (SUMO-1). Conversely, tau hyperphosphorylation promotes its SUMOylation; the latter in turn inhibits tau degradation with reduction of solubility and ubiquitination of tau proteins. Furthermore, the enhanced SUMO-immunoreactivity, costained with the hyperphosphorylated tau, is detected in cerebral cortex of the AD brains, and β-amyloid exposure of rat primary hippocampal neurons induces a dose-dependent SUMOylation of the hyperphosphorylated tau. Our findings suggest that tau SUMOylation reciprocally stimulates its phosphorylation and inhibits the ubiquitination-mediated tau degradation, which provides a new insight into the AD-like tau accumulation.Alzheimer’s disease (AD) is the most common neurodegenerative disorder in the elderly. Intracellular accumulation of neurofibrillary tangles (NFTs) and extracellular precipitation of senile plaques are the most prominent pathological hallmarks of AD (1–3). The clinical-to-pathological correlation studies have demonstrated that the number of NFTs consisting of hyperphosphorylated tau correlates with the degree of dementia in AD (4–6). Tau is the major microtubule-associated protein that normally contains 2–3 mol of phosphate per mole of tau protein. In AD brains, tau is abnormally hyperphosphorylated (namely AD-P-tau) and the phosphate level increases to 5–9 mol phosphate per mole tau (4). AD-P-tau does not bind to tubulin and become incompetent in promoting microtubule assembly and maintaining the stability of the microtubules. The AD-P-tau also sequesters normal tau from microtubules (7), and serves as a template for the conversion of normal tau into misfolded protein in a prion-like manner (8). In addition to hyperphosphorylation, tau is also contains other posttranslational modifications, such as ubiquitination and SUMOylation (5, 9–11). The abnormal modification of tau also decreases its solubility, and ∼40% of the hyperphosphorylated tau in AD brains has been isolated as sedimentable nonfibril cytosolic protein (1, 12). Although the mechanisms underlying the formation of the NFTs remain unclear, the altered tau modifications and impaired degradation are believed to play a role. Therefore, clarifying the mechanism that may cause tau accumulation is of great significance for understanding the pathogenesis of AD and for developing new therapeutics.Like other proteins, tau can be degraded by autophagy-lysosomal and ubiqutin-proteasomal systems under physiological conditions. In mouse cortical neurons, a C-terminal–truncated form of tau that mimics tau cleaved at Asp421 (tauΔC) is removed by macroautophagic and lysosomal mechanisms (13). Lysosomal perturbation inhibits the clearance of tau with accumulation and aggregation of tau in M1C cells (14). Cathepsin D released from lysosome can degrade tau in cultured hippocampal slices (15). Inhibition of the autophagic vacuole formation leads to a noticeable accumulation of tau (14). Studies also suggest that tau protein is degraded in an ubiquitin-, ATP-, and 26S proteasome-, but not a 20S proteasome-dependent manner under normal conditions (16). When the cells are exposed to the stresses, CHIP, a ubiquitin ligase that interacts directly with Hsp70/90, can induce tau ubiquitination and thus selectively reduce the level of detergent insoluble tau (17). The compensatory activation of autophagy-lysosomal or ubiqutin-proteasomal system can antagonize tau aggregation; therefore, tau accumulation does not show in the early stage of AD. During the evolution of AD, a gradual impairment of autophagy-lysosomal system and ubiqutin-proteasomal system has been detected at later stage of the disease (18–20). Studies suggest that the ubiquitin-mediated degradation pathway seems ineffective in removing the tau-positive fibrillar structures in the AD brains (21–23); however, the mechanisms underlying the impairment of the ubiqutin-proteasomal system are elusive.Ubiquitin is an important component of the cellular defense system that tags abnormal proteins for their degradation by ATP-dependent nonlysosomal proteases (24). Monoclonal antibodies 3-39 and 5-25 raised against paired helical filaments of NFTs have been shown to recognize ubiquitin (25). Meanwhile, tau can be sumoylated at K340 in vitro by SUMO-1 (small ubiquitin-like modifier protein-1) and to a lesser extent by SUMO2 and SUMO3 (9–11). Moreover, SUMO-1 immunoreactivity was colocalized with tau aggregates in neuritic plaques of APP transgenic mice (11). It is well known that SUMO share similarities with ubiquitin in both the structure and the biochemistry of their conjugation (26). Therefore, tau SUMOylation may compete against its ubiquitination and thus suppress tau degradation. In the present study, we found that tau SUMOylation reciprocally stimulates its phosphorylation and thus inhibits the ubiquitination and degradation of tau proteins.     

Mapping of SUMO sites and analysis of SUMOylation changes induced by external stimuli

SUMOylation is an essential ubiquitin-like modification involved in important biological processes in eukaryotic cells. Identification of small ubiquitin-related modifier (SUMO)-conjugated residues in proteins is critical for understanding the role of SUMOylation but remains experimentally challenging. We have set up a powerful and high-throughput method combining quantitative proteomics and peptide immunocapture to map SUMOylation sites and have analyzed changes in SUMOylation in response to stimuli. With this technique we identified 295 SUMO1 and 167 SUMO2 sites on endogenous substrates of human cells. We further used this strategy to characterize changes in SUMOylation induced by listeriolysin O, a bacterial toxin that impairs the host cell SUMOylation machinery, and identified several classes of host proteins specifically deSUMOylated in response to this toxin. Our approach constitutes an unprecedented tool, broadly applicable to various SUMO-regulated cellular processes in health and disease.Posttranslational modifications (PTMs) are key mechanisms used by both prokaryotes and eukaryotes to regulate protein activity specifically, locally, and temporally. Ubiquitin and ubiquitin-like proteins (UBLs) constitute a specific class of small protein modifiers that can be covalently attached to a target protein via the formation of an isopeptide bond in a reversible manner. Small ubiquitin-related modifier (SUMO), one of these UBLs, is an essential PTM in eukaryotic cells that is involved in various cellular functions including gene expression regulation, DNA repair, intracellular transport, and response to viral and bacterial infections (1–5). The human genome encodes three different functional SUMO isoforms (SUMO1, SUMO2, and SUMO3) that are conjugated to distinct but overlapping sets of target proteins (1, 2, 6). Conjugation of SUMO to its targets in humans requires an E1-activating enzyme (the SAE1/SAE2 heterodimer), an E2-conjugating enzyme (Ubc9), and several E3 SUMO enzymes. Once conjugated to its target, SUMO can be deconjugated by several different SUMO isopeptidases that tightly regulate the SUMOylation levels of proteins (7).Since the discovery of SUMO two decades ago, much effort has been dedicated to the identification of SUMO-conjugated proteins in different organisms including yeast, plants, and mammals (8). However, isolation of SUMOylated proteins has proven to be challenging. Indeed, for most SUMO substrates, only a small proportion of the total amount of protein is SUMO-modified. In addition, the high activity of SUMO isopeptidases in cell lysates results in the rapid loss of SUMO conjugation in the absence of appropriate inhibitors. Thus, the most common approach used to isolate SUMOylated proteins is based on the expression of His-tagged versions of SUMO allowing the purification of SUMO-conjugated proteins by nickel chromatography under denaturing conditions (8, 9). Denaturing conditions inactivate SUMO isopeptidases and also prevent contamination by proteins interacting noncovalently with SUMO via specific domains such as SUMO-interacting motifs (SIMs) (2). Once SUMOylated proteins have been isolated, their analysis by mass spectrometry (MS) has been widely used to identify SUMO-modified proteins and, albeit less successfully, SUMO-conjugation sites.Mapping the exact lysine residue to which SUMO is attached in modified proteins is a critical step to get further insight into the function of SUMOylation. Indeed, the identification of SUMO sites allows the generation of non-SUMOylatable mutants and the study of associated phenotypes. Identification of SUMO sites by MS is not straightforward (8). Unlike ubiquitin, which leaves a small diglycine (GG) signature tag on the modified lysine residue after trypsin digestion, SUMO leaves a larger signature that severely hampers the identification of modified peptides.In addition to the identification of the SUMO site per se, a comparison of the SUMOylation status of sites in different cell-growth conditions is critical for better characterizing the biological implications of SUMOylation. For example, analysis of SUMOylation changes induced after heat shock, arsenic treatment, inhibition of the proteasome, or during the cell cycle has led to numerous insights into the role of SUMOylation in cell physiology (refs. 10–14 and reviewed in ref. 2). Here, we devised a performant approach which combines the use of SUMO variants, peptide immunocapture, and quantitative proteomics for high-throughput identification of SUMO sites. We then show that our approach is able to characterize global changes in the cell SUMOylome in response to a given stimulus, such as exposure to a bacterial toxin, listeriolysin O (LLO).     

Exchange protein directly activated by cAMP plays a critical role in bacterial invasion during fatal rickettsioses

Rickettsiae are responsible for some of the most devastating human infections. A high infectivity and severe illness after inhalation make some rickettsiae bioterrorism threats. We report that deletion of the exchange protein directly activated by cAMP (Epac) gene, Epac1, in mice protects them from an ordinarily lethal dose of rickettsiae. Inhibition of Epac1 suppresses bacterial adhesion and invasion. Most importantly, pharmacological inhibition of Epac1 in vivo using an Epac-specific small-molecule inhibitor, ESI-09, completely recapitulates the Epac1 knockout phenotype. ESI-09 treatment dramatically decreases the morbidity and mortality associated with fatal spotted fever rickettsiosis. Our results demonstrate that Epac1-mediated signaling represents a mechanism for host–pathogen interactions and that Epac1 is a potential target for the prevention and treatment of fatal rickettsioses.Rickettsiae are responsible for some of the most devastating human infections (1–4). It has been forecasted that temperature increases attributable to global climate change will lead to more widespread distribution of rickettsioses (5). These tick-borne diseases are caused by obligately intracellular bacteria of the genus Rickettsia, including Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever (RMSF) in the United States and Latin America (2, 3), and Rickettsia conorii, the causative agent of Mediterranean spotted fever endemic to southern Europe, North Africa, and India (6). A high infectivity and severe illness after inhalation make some rickettsiae (including Rickettsia prowazekii, R. rickettsii, Rickettsia typhi, and R. conorii) bioterrorism threats (7). Although the majority of rickettsial infections can be controlled by appropriate broad-spectrum antibiotic therapy if diagnosed early, up to 20% of misdiagnosed or untreated (1, 3) and 5% of treated RMSF cases (8) result in a fatal outcome caused by acute disseminated vascular endothelial infection and damage (9). Fatality rates as high as 32% have been reported in hospitalized patients diagnosed with Mediterranean spotted fever (10). In addition, strains of R. prowazekii resistant to tetracycline and chloramphenicol have been developed in laboratories (11). Disseminated endothelial infection and endothelial barrier disruption with increased microvascular permeability are the central features of SFG rickettsioses (1, 2, 9). The molecular mechanisms involved in rickettsial infection remain incompletely elucidated (9, 12). A comprehensive understanding of rickettsial pathogenesis and the development of novel mechanism-based treatment are urgently needed.Living organisms use intricate signaling networks for sensing and responding to changes in the external environment. cAMP, a ubiquitous second messenger, is an important molecular switch that translates environmental signals into regulatory effects in cells (13). As such, a number of microbial pathogens have evolved a set of diverse virulence-enhancing strategies that exploit the cAMP-signaling pathways of their hosts (14). The intracellular functions of cAMP are predominantly mediated by the classic cAMP receptor, protein kinase A (PKA), and the more recently discovered exchange protein directly activated by cAMP (Epac) (15). Thus, far, two isoforms, Epac1 and Epac2, have been identified in humans (16, 17). Epac proteins function by responding to increased intracellular cAMP levels and activating the Ras superfamily small GTPases Ras-proximate 1 and 2 (Rap1 and Rap2). Accumulating evidence demonstrates that the cAMP/Epac1 signaling axis plays key regulatory roles in controlling various cellular functions in endothelial cells in vitro, including cell adhesion (18–21), exocytosis (22), tissue plasminogen activator expression (23), suppressor of cytokine signaling 3 (SOCS-3) induction (24–27), microtubule dynamics (28, 29), cell–cell junctions, and permeability and barrier functions (30–37). Considering the critical importance of endothelial cells in rickettsioses, we examined the functional roles of Epac1 in rickettsial pathogenesis in vivo, taking advantage of the recently generated Epac1 knockout mouse (38) and Epac-specific inhibitors (39, 40) generated from our laboratory. Our studies demonstrate that Epac1 plays a key role in rickettsial infection and represents a therapeutic target for fatal rickettsioses.     

Drosophila seminal protein ovulin mediates ovulation through female octopamine neuronal signaling

Across animal taxa, seminal proteins are important regulators of female reproductive physiology and behavior. However, little is understood about the physiological or molecular mechanisms by which seminal proteins effect these changes. To investigate this topic, we studied the increase in Drosophila melanogaster ovulation behavior induced by mating. Ovulation requires octopamine (OA) signaling from the central nervous system to coordinate an egg’s release from the ovary and its passage into the oviduct. The seminal protein ovulin increases ovulation rates after mating. We tested whether ovulin acts through OA to increase ovulation behavior. Increasing OA neuronal excitability compensated for a lack of ovulin received during mating. Moreover, we identified a mating-dependent relaxation of oviduct musculature, for which ovulin is a necessary and sufficient male contribution. We report further that oviduct muscle relaxation can be induced by activating OA neurons, requires normal metabolic production of OA, and reflects ovulin’s increasing of OA neuronal signaling. Finally, we showed that as a result of ovulin exposure, there is subsequent growth of OA synaptic sites at the oviduct, demonstrating that seminal proteins can contribute to synaptic plasticity. Together, these results demonstrate that ovulin increases ovulation through OA neuronal signaling and, by extension, that seminal proteins can alter reproductive physiology by modulating known female pathways regulating reproduction.Throughout internally fertilizing animals, seminal proteins play important roles in regulating female fertility by altering female physiology and, in some cases, behavior after mating (reviewed in refs. 1–3). Despite this, little is understood about the physiological mechanisms by which seminal proteins induce postmating changes and how their actions are linked with known networks regulating female reproductive physiology.In Drosophila melanogaster, the suite of seminal proteins has been identified, as have many seminal protein-dependent postmating responses, including changes in egg production and laying, remating behavior, locomotion, feeding, and in ovulation rate (reviewed in refs. 2 and 3). For example, the Drosophila seminal protein ovulin elevates ovulation rate to maximal levels during the 24 h following mating (4, 5), and the seminal protein sex peptide (SP) suppresses female mating receptivity and increases egg-laying behavior for several days after mating (6–10). However, although a receptor for SP has been identified (11), along with elements of the neural circuit in which it is required (12–14), SP’s mechanism of action has not yet been linked to regulatory networks known to control postmating behaviors. Thus, a crucial question remains: how do male-derived seminal proteins interact with regulatory networks in females to trigger postmating responses?We addressed this question by examining the stimulation of Drosophila ovulation by the seminal protein ovulin. In insects, ovulation, defined here as the release of an egg from the ovary to the uterus, is among the best understood reproductive processes in terms of its physiology and neurogenetics (15–27). In D. melanogaster, ovulation requires input from neurons in the abdominal ganglia that release the catecholaminergic neuromodulators octopamine (OA) and tyramine (17, 18, 28). Drosophila ovulation also requires an OA receptor, OA receptor in mushroom bodies (OAMB) (19, 20). Moreover, it has been proposed that OA may integrate extrinsic factors to regulate ovulation rates (17). Noradrenaline, the vertebrate structural and functional equivalent to OA (29, 30), is important for mammalian ovulation, and its dysregulation has been associated with ovulation disorders (31–38). In this paper we investigate the role of neurons that release OA and tyramine in ovulin’s action. For simplicity, we refer to these neurons as “OA neurons” to reflect the well-established role of OA in ovulation behavior (16–20, 22).We investigated how action of the seminal protein ovulin relates to the conserved canonical neuromodulatory pathway that regulates ovulation physiology (39–41). We found that ovulin increases ovulation and egg laying through OA neuronal signaling. We also found that ovulin relaxes oviduct muscle tonus, a postmating process that is also mediated by OA neuronal signaling. Finally, subsequent to these effects we detected an ovulin-dependent increase in synaptic sites between OA motor neurons and oviduct muscle, suggesting that ovulin’s stimulation of OA neurons could have increased their synaptic activity. These results suggest that ovulin affects ovulation by manipulating the gain of a neuromodulatory pathway regulating ovulation physiology.     

Self-assembly of alkynylplatinum(II) terpyridine amphiphiles into nanostructures via steric control and metal–metal interactions

A series of mono- and dinuclear alkynylplatinum(II) terpyridine complexes containing the hydrophilic oligo(para-phenylene ethynylene) with two 3,6,9-trioxadec-1-yloxy chains was designed and synthesized. The mononuclear alkynylplatinum(II) terpyridine complex was found to display a very strong tendency toward the formation of supramolecular structures. Interestingly, additional end-capping with another platinum(II) terpyridine moiety of various steric bulk at the terminal alkyne would lead to the formation of nanotubes or helical ribbons. These desirable nanostructures were found to be governed by the steric bulk on the platinum(II) terpyridine moieties, which modulates the directional metal−metal interactions and controls the formation of nanotubes or helical ribbons. Detailed analysis of temperature-dependent UV-visible absorption spectra of the nanostructured tubular aggregates also provided insights into the assembly mechanism and showed the role of metal−metal interactions in the cooperative supramolecular polymerization of the amphiphilic platinum(II) complexes.Square-planar d⁸ platinum(II) polypyridine complexes have long been known to exhibit intriguing spectroscopic and luminescence properties (1–54) as well as interesting solid-state polymorphism associated with metal−metal and π−π stacking interactions (1–14, 25). Earlier work by our group showed the first example, to our knowledge, of an alkynylplatinum(II) terpyridine system [Pt(tpy)(C ≡ CR)]⁺ that incorporates σ-donating and solubilizing alkynyl ligands together with the formation of Pt···Pt interactions to exhibit notable color changes and luminescence enhancements on solvent composition change (25) and polyelectrolyte addition (26). This approach has provided access to the alkynylplatinum(II) terpyridine and other related cyclometalated platinum(II) complexes, with functionalities that can self-assemble into metallogels (27–31), liquid crystals (32, 33), and other different molecular architectures, such as hairpin conformation (34), helices (35–38), nanostructures (39–45), and molecular tweezers (46, 47), as well as having a wide range of applications in molecular recognition (48–52), biomolecular labeling (48–52), and materials science (53, 54). Recently, metal-containing amphiphiles have also emerged as a building block for supramolecular architectures (42–44, 55–59). Their self-assembly has always been found to yield different molecular architectures with unprecedented complexity through the multiple noncovalent interactions on the introduction of external stimuli (42–44, 55–59).Helical architecture is one of the most exciting self-assembled morphologies because of the uniqueness for the functional and topological properties (60–69). Helical ribbons composed of amphiphiles, such as diacetylenic lipids, glutamates, and peptide-based amphiphiles, are often precursors for the growth of tubular structures on an increase in the width or the merging of the edges of ribbons (64, 65). Recently, the optimization of nanotube formation vs. helical nanostructures has aroused considerable interests and can be achieved through a fine interplay of the influence on the amphiphilic property of molecules (66), choice of counteranions (67, 68), or pH values of the media (69), which would govern the self-assembly of molecules into desirable aggregates of helical ribbons or nanotube scaffolds. However, a precise control of supramolecular morphology between helical ribbons and nanotubes remains challenging, particularly for the polycyclic aromatics in the field of molecular assembly (64–69). Oligo(para-phenylene ethynylene)s (OPEs) with solely π−π stacking interactions are well-recognized to self-assemble into supramolecular system of various nanostructures but rarely result in the formation of tubular scaffolds (70–73). In view of the rich photophysical properties of square-planar d⁸ platinum(II) systems and their propensity toward formation of directional Pt···Pt interactions in distinctive morphologies (27–31, 39–45), it is anticipated that such directional and noncovalent metal−metal interactions might be capable of directing or dictating molecular ordering and alignment to give desirable nanostructures of helical ribbons or nanotubes in a precise and controllable manner.Herein, we report the design and synthesis of mono- and dinuclear alkynylplatinum(II) terpyridine complexes containing hydrophilic OPEs with two 3,6,9-trioxadec-1-yloxy chains. The mononuclear alkynylplatinum(II) terpyridine complex with amphiphilic property is found to show a strong tendency toward the formation of supramolecular structures on diffusion of diethyl ether in dichloromethane or dimethyl sulfoxide (DMSO) solution. Interestingly, additional end-capping with another platinum(II) terpyridine moiety of various steric bulk at the terminal alkyne would result in nanotubes or helical ribbons in the self-assembly process. To the best of our knowledge, this finding represents the first example of the utilization of the steric bulk of the moieties, which modulates the formation of directional metal−metal interactions to precisely control the formation of nanotubes or helical ribbons in the self-assembly process. Application of the nucleation–elongation model into this assembly process by UV-visible (UV-vis) absorption spectroscopic studies has elucidated the nature of the molecular self-assembly, and more importantly, it has revealed the role of metal−metal interactions in the formation of these two types of nanostructures.