Sumoylation in neurons: nuclear and synaptic roles?

Guanosine increases global SUMO1-ylation in the hippocampus of young and aged mice and improves the short-term memory of young mice

The nucleoside guanosine is an endogenous neuromodulator associated with neuroprotection. The roles of guanosine during aging are still not fully elucidated. Guanosine modulates SUMOylation in neurons and astrocytes in vitro, but it is not known whether guanosine can modulate SUMOylation in vivo and improve cognitive functions during aging. SUMOylation is a post-translational protein modification with potential neuroprotective roles. In this follow-up study, we investigated whether guanosine could modulate SUMOylation in vivo and behavior in young and aged mice. Young (3-month-old) and aged (24-month-old) C57BL/6 mice were treated with guanosine (8 mg/kg intraperitoneal) daily for 14 days. Starting on day 8 of treatment, the following behavioral tests were performed: open field, novel object location, Y-maze, sucrose splash test, and tail suspension test. Treatment with guanosine did not change the locomotor activity of young or aged mice in the open-field test. Treatment with guanosine improved short-term memory only for young mice but did not change the working memory of either young or aged mice, as evaluated using object recognition and the Y-maze tests, respectively. Depressive-like behaviors, such as impaired grooming evaluated through the splash test, did not change in either young or aged mice. However, young mice treated with guanosine increased their immobility time in the tail suspension test, suggesting an effect on behavioral coping strategies. Global SUMO1-ylation was significantly increased in the hippocampus of young and aged mice after 14 days of treatment with guanosine, whereas no changes were detected in the cerebral cortex of either young or aged mice. Our findings demonstrate that guanosine also targets hippocampal SUMOylation in vivo, thereby contributing to a deeper understanding of its mechanisms of action. This highlights the involvement of SUMOylation in guanosine's modulatory and neuroprotective effects.

SENP5 deteriorates traumatic brain injury via SUMO2-dependent suppression of E2F1 SUMOylation

Traumatic brain injury (TBI) represents a main public health concern during the past decade, attracting considerable interest because of its rising prevalence, wide-ranging risk factors and lifelong familial and societal influence. SUMO2 can conjugate to substrates upon various cellular stresses. Nevertheless, whether and how SUMO2-specific proteases partake in TBI is less understood. The aim of this study is to dissect the effects of SUMO-specific peptidase 5 (SENP5) on accentuating TBI in rats in an effort to unveil its underlying mechanism. SENP5 is overexpressed in hippocampal tissues of TBI rats, and inhibition of SENP5 reduces neurological function scores, decreases brain water content, inhibits apoptosis in hippocampal tissues, and attenuates brain injury caused in rats. Moreover, SENP5 inhibits the SUMOylation level of E2F transcription factor 1 (E2F1) and increases the protein expression of E2F1. Silencing of E2F1 blocks the p53 signaling pathway. Overexpression of E2F1 partially reverses the protective effect of sh-SENP5 on TBI in rats. These findings reveal an essential role of SENP5 and the SUMOylation status of E2F1 in the TBI development.

SUMO1 hinders α-Synuclein fibrillation by inducing structural compaction

Small Ubiquitin-like Modifier 1 (SUMO1) is an essential protein for many cellular functions, including regulation, signaling, etc., achieved by a process known as SUMOylation, which involves covalent attachment of SUMO1 to target proteins. SUMO1 also regulates the function of several proteins via non-covalent interactions involving the hydrophobic patch in the target protein identified as SUMO Binding or Interacting Motif (SBM/SIM). Here, we demonstrate a crucial functional potential of SUMO1 mediated by its non-covalent interactions with α-Synuclein, a protein responsible for many neurodegenerative diseases called α-Synucleinopathies. SUMO1 hinders the fibrillation of α-Synuclein, an intrinsically disordered protein (IDP) that undergoes a transition to β-structures during the fibrillation process. Using a plethora of biophysical techniques, we show that SUMO1 transiently binds to the N-terminus region of α-Synuclein non-covalently and causes structural compaction, which hinders the self-association process and thereby delays the fibrillation process. On the one hand, this study demonstrates an essential functional role of SUMO1 protein concerning neurodegeneration; it also illustrates the commonly stated mechanism that IDPs carry out multiple functions by structural adaptation to suit specific target proteins, on the other. Residue-level details about the SUMO1-α-Synuclein interaction obtained here also serve as a reliable approach for investigating the detailed mechanisms of IDP functions.

Contributive Role of Hyperglycemia and Hypoglycemia Towards the Development of Alzheimer's Disease

Alzheimer's disease (AD) is one of the causes of dementia that results from several infections/biological conditions leading to either cell disruption or loss of neuronal communication. Studies have documented the accumulation of two proteins, beta-amyloid (Aβ), which accumulates on the exteriors of neurons, and tau (Tau), which assembles at the interiors of brain cells and is chiefly liable for the progression of the disease. Several molecular and cellular pathways account for the accumulation of amyloid-β and the formation of neurofibrillary tangles, which are phosphorylated variants of Tau protein. Moreover, research has revealed a potential connection between AD and diabetes. It has also been demonstrated that both hypoglycemia and hyperglycemia have a significant role in the development of AD. In addition, SUMO (small ubiquitin-like modifier protein) plays a crucial role in the pathogenesis of AD. SUMOylation is the process by which modification of amyloid precursor protein (APP) and Tau takes place. Furthermore, Drosophila melanogaster has proven to be an efficient model organism in studies to establish the relationship between AD and variations in blood glucose levels. In addition, the review successfully identifies the common pathway that links the effects of fluctuations in glucose levels on AD pathogenesis and advancements.

Regulation of Serotonin 1A Receptor SUMOylation by SENP2 and PIASxα

Serotonin 1A receptors (5-HT1ARs) are implicated in the control of mood, cognition, and memory and in various neuropsychiatric disorders such as depression and anxiety. As such, understanding the regulation of 5-HT1ARs will inform the development of better treatment approaches. We previously demonstrated 5-HT1ARs are SUMOylated by SUMO1 in the rat brain. Agonist stimulation increased SUMOylation and was further enhanced when combined with 17β-estradiol-3-benzoate (EB), which are treatments that cause the transient and prolonged desensitization of 5-HT1AR signaling, respectively. In the current study, we identified the protein inhibitor of activated STAT (PIAS)xα as the enzyme that facilitates SUMOylation, and SENP2 as the protein that catalyzes the deSUMOylation of 5-HT1ARs. We demonstrated that PIASxα significantly increased in the membrane fraction of rats co-treated with EB and an agonist, compared to either the EB-treated or vehicle-treated groups. The acute treatment with an agonist alone shifted the location of SENP2 from the membrane to the cytoplasmic fraction, but it has little effect on PIASxα. Hence, two separate mechanisms regulate SUMOylation and the activity of 5-HT1ARs by an agonist and EB. The effects of EB on 5-HT1AR SUMOylation and signaling may be related to the higher incidence of mood disorders in women during times with large fluctuations in estrogens. Targeting the SUMOylation of 5-HT1ARs could have important clinical relevance for the therapy for several neuropsychiatric disorders in which 5-HT1ARs are implicated.

Cell type-specific transcriptional programs in mouse prefrontal cortex during adolescence and addiction

Coordinated activity-induced transcriptional changes across multiple neuron subtypes of the prefrontal cortex (PFC) play a pivotal role in encoding and regulating major cognitive behaviors. Yet, the specific transcriptional programs in each neuron subtype remain unknown. Using single-cell RNA sequencing (scRNA-seq), here we comprehensively classify all unique cell subtypes in the PFC. We analyze transcriptional dynamics of each cell subtype under a naturally adaptive and an induced condition. Adaptive changes during adolescence (between P21 and P60), a highly dynamic phase of postnatal neuroplasticity, profoundly impacted transcription in each neuron subtype, including cell type-specific regulation of genes implicated in major neuropsychiatric disorders. On the other hand, an induced plasticity evoked by chronic cocaine addiction resulted in progressive transcriptional changes in multiple neuron subtypes and became most pronounced upon prolonged drug withdrawal. Our findings lay a foundation for understanding cell type-specific postnatal transcriptional dynamics under normal PFC function and in neuropsychiatric disease states.

Variation in expression of small ubiquitin-like modifiers in injured sciatic nerve of mice

Small ubiquitin-like modifiers (SUMOs) have been shown to regulate axonal regeneration, signal transduction, neuronal migration, and myelination, by covalently and reversibly attaching to the protein substrates during neuronal cell growth, development, and differentiation. It has not been reported whether SUMOs play a role in peripheral nerve injury and regeneration. To investigate any association between SUMOylation and potential neuroprotective effects during peripheral nerve injury and regeneration, C57/BL mice were randomly divided into sham and experimental groups. The sciatic nerve was exposed only in the sham group. The experimental group underwent neurotomy and epineurial neurorrhaphy. Real-time quantitative polymerase chain reaction and western blot assay results revealed different mRNA and protein expression levels of SUMO1, SUMO2, SUMO3 and UBC9 in sciatic nerve tissue (containing both 5 mm of proximal and distal stumps at the injury site) at various time points after injury. Compared with the sham group, protein levels of SUMO1 and SUMO2/3 increased in both their covalent and free states after sciatic nerve injury in the experimental group, especially in the covalent state. UBC9 protein levels showed similar changes to those of SUMO1 and SUMO2/3 in the covalent states. Immunohistochemical staining demonstrated that SUMO1 and SUMO2/3 immunopositivities were higher in the experimental group than in the sham group. Our results verified that during the repair of sciatic nerve injury, the mRNA and protein expression of SUMO1, SUMO2, SUMO3 and UBC9 in injured nerve tissues changed in varying patterns and there were clear changes in the expression of SUMO-related proteins. These findings reveal that SUMOs possibly play an important role in the repair of peripheral nerve injury. All animal protocols were approved by the Institutional Animal Care and Use Committee of Tianjin Fifth Central Hospital, China (approval No. TJWZXLL2018041) on November 8, 2018.

Enzyme Complexes Important for the Glutamate-Glutamine Cycle

Transient multienzyme and/or multiprotein complexes (metabolons) direct substrates toward specific pathways and can significantly influence the metabolism of glutamate and glutamine in the brain. Glutamate is the primary excitatory neurotransmitter in brain. This neurotransmitter has essential roles in normal brain function including learning and memory. Metabolism of glutamate involves the coordinated activity of astrocytes and neurons and high affinity transporter proteins that are selectively distributed on these cells. This chapter describes known and possible metabolons that affect the metabolism of glutamate and related compounds in the brain, as well as some factors that can modulate the association and dissociation of such complexes, including protein modifications by acylation reactions (e.g., acetylation, palmitoylation, succinylation, SUMOylation, etc.) of specific residues. Development of strategies to modulate transient multienzyme and/or enzyme-protein interactions may represent a novel and promising therapeutic approach for treatment of diseases involving dysregulation of glutamate metabolism.

Regulation of mGluR7 trafficking by SUMOylation in neurons

SUMOylation is a post-translational modification by which Small Ubiquitin-like MOdifier (SUMO) proteins are covalently linked to the lysine residues of target proteins via an enzymatic cascade. SUMOylation at the synapse plays an important regulatory role in a wide variety of neuronal function such as synapse formation and receptor endocytosis. The metabotropic glutamate receptor type 7 (mGluR7), a presynaptic G protein-coupled receptor, modulates excitatory neurotransmission and synaptic plasticity by inhibiting neurotransmitter release. The SUMO conjugation of mGluR7 has been demonstrated from several in vitro studies, however, it has not been successful in identifying SUMOylation of full-length mGluR7 in vivo. In the present study, we find that mGluR7 at Lys889 is a target of SUMO conjugation, which is impeded by SUMO-specific isopeptidase SENP1 in HEK 293T cells. In addition, we identify SUMOylated mGluR7 both in brain and primary cortical neurons, that is reduced by the treatment of L-AP4, mGluR7 agonist. We find that deSUMOylated mutation in mGluR7 or overexpression of SENP-1 markedly increases mGluR7 internalization in hippocampal neurons, indicating that endocytosis of mGluR7 is enhanced by the reduced SUMO conjugation of mGluR7. Furthermore, Ser862 phosphorylation facilitates SUMO conjugation of mGluR7. Together, these results reveal that SUMOylation of mGluR7 at Lys889 is required for stable surface expression of mGluR7 in neurons.

Spatiotemporal distribution of SUMOylation components during mouse brain development

Posttranslational modification of proteins might play an important role in brain cellular dynamics via the rapid turnover or functional change of critical proteins controlling neuronal differentiation or synaptic transmission. Small ubiquitin-like modifier protein (SUMO) is a family of ubiquitin-like small proteins that are covalently attached to target proteins to modify their function posttranslationally. Many cellular processes, such as transcription and protein trafficking, are regulated by SUMOylation, but its functional significance in the brain remains unclear. Although developmental regulation of SUMOylation levels in rat brain was recently demonstrated, no comparative immunohistochemical analysis of the cellular distribution profiles of SUMOylation components, including SUMO1, SUMO2/3, and Ubc9, has been undertaken so far. The present study used immunohistochemical and immunoblot analysis with the different developmental stages of mice and demonstrated the developmentally regulated distribution of SUMO1, SUMO2/3, and Ubc9 in the brain. During embryonic development, SUMOylation by SUMO1 and SUMO2/3 occurred in the nucleoplasm of nestin-positive neural stem cells. Although the total amount of SUMO-modified proteins decreased during postnatal brain development, intense and persistent accumulation of SUMO2/3 was detected throughout life in neural progenitor populations in neurogenic regions, including the subventricular zone and the hippocampal subgranular zone. In contrast, many neurons in the adult brain accumulated SUMO1 rather than SUMO2/3. Heavy immunoreactivity of SUMO1 was found in large projection neurons in the brainstem, whereas SUMO2/3 was almost absent from these areas. This heterogeneous distribution implies that both proteins play a specific and unique role in the brain.

Non-nuclear function of sumoylated proteins

Post-translational modification by the SUMO moiety is now regarded as one of the key regulatory modifications in eukaryotic cells. Up to now, plenty of sumoylated proteins have been found to be involved in nuclear processes such as chromatin organization, transcription and DNA repair as well as in other cellular functions. Since the number of data concerning sumoylated proteins and their function outside the nucleus has grown rapidly, in this review we summarized the results describing the non-nuclear role of SUMO substrates. In particular, we focused on the role of sumoylation in the regulation of channel activity, receptor function, G-protein signaling, activity of enzymes, cytoskeletal organization, exocytosis, autophagy and mitochondrial dynamics.

CREB SUMOylation by the E3 ligase PIAS1 enhances spatial memory

cAMP-responsive element binding protein (CREB) phosphorylation and signaling plays an important role in long-term memory formation, but other posttranslational modifications of CREB are less known. Here, we found that CREB1Δ, the short isoform of CREB, could be sumoylated by the small ubiquitin-like modifier (SUMO) E3 ligase protein inhibitor of activated STAT1 (PIAS1) at Lys271 and Lys290 and PIAS1 SUMOylation of CREB1Δ increased the expression level of CREB1Δ. CREB1Δ could also be sumoylated by other PIAS family proteins, but not by the E3 ligases RanBP2 and Pc2 or by the E2 ligase Ubc9. Furthermore, water maze training increased the level of endogenous CREB SUMOylation in rat CA1 neurons determined by in vitro SUMOylation assay, but this effect was not observed in other brain areas. Moreover, transduction of Lenti-CREBWT to rat CA1 area facilitated, whereas transduction of Lenti-CREB double sumo-mutant (CREBK271RK290R) impaired, spatial learning and memory performance. Transduction of Lenti-CREBWT-SUMO1 fusion vector to rat CA1 area showed a more significant effect in enhancing spatial learning and memory and CREB SUMOylation. Lenti-CREBWT transduction increased, whereas Lenti-CREBK271RK290R transduction decreased, CREB DNA binding to the brain-derived neurotrophic factor (bdnf) promoter and decreased bdnf mRNA expression. Knock-down of PIAS1 expression in CA1 area by PIAS1 siRNA transfection impaired spatial learning and memory and decreased endogenous CREB SUMOylation. In addition, CREB SUMOylation was CREB phosphorylation dependent and lasted longer. Therefore, CREB phosphorylation may be responsible for signal transduction during the early phase of long-term memory formation, whereas CREB SUMOylation sustains long-term memory.

Gephyrin: a master regulator of neuronal function?

The neurotransmitters GABA and glycine mediate fast synaptic inhibition by activating ligand-gated chloride channels--namely, type A GABA (GABA(A)) and glycine receptors. Both types of receptors are anchored postsynaptically by gephyrin, which self-assembles into a scaffold and interacts with the cytoskeleton. Current research indicates that postsynaptic gephyrin clusters are dynamic assemblies that are held together and regulated by multiple protein-protein interactions. Moreover, post-translational modifications of gephyrin regulate the formation and plasticity of GABAergic synapses by altering the clustering properties of postsynaptic scaffolds and thereby the availability and function of receptors and other signalling molecules. Here, we discuss the formation and regulation of the gephyrin scaffold, its role in GABAergic and glycinergic synaptic function and the implications for the pathophysiology of brain disorders caused by abnormal inhibitory neurotransmission.

Patterns of ubiquitylation and SUMOylation associated with exposure to anoxia in embryos of the annual killifish Austrofundulus limnaeus

Embryos of the annual killifish Austrofundulus limnaeus acquire extreme tolerance to anoxia during embryonic development. These embryos can survive environmental and cellular conditions that would likely result in death in the majority of vertebrate cells, despite experiencing a massive loss of ATP. It is highly likely that the initial response to anoxia must quickly alter cellular physiology to reprogram cell signaling and metabolic pathways to support anaerobiosis. Covalent protein modifications are a mechanism that can quickly act to effect large-scale changes in protein structure and function and have been suggested by others to play a key role in mammalian ischemia tolerance. Using Western blot analysis, we explored patterns of protein ubiquitylation and SUMOylation in embryos of A. limnaeus exposed to anoxia and anoxic preconditioning. Surprisingly, we report stage-specific protein ubiquitylation patterns that suggest different mechanisms for altering protein turnover in dormant and actively developing embryos that both survive long-term anoxia. Anoxic preconditioning does not appear to alter levels of ubiquitin conjugates in a unique manner. Global SUMOylation of proteins does not change in response to anoxia, but there are stage-specific changes in SUMOylation of specific protein bands. Contrary to other systems, global changes in protein SUMOylation may not be required to support long-term tolerance to anoxia in embryos of A. limnaeus. These data lead us to conclude that embryos of A. limnaeus respond to anoxia in a unique manner compared to other vertebrate models of anoxia tolerance and may provide novel mechanisms for engineering vertebrate tissues to survive long-term anoxia.

Estradiol potentiates 8-OH-DPAT-induced sumoylation of 5-HT₁A receptor: characterization and subcellular distribution of sumoylated 5-HT₁A receptors

Sumoylation is a recently described post-translational modification and only a few sumoylated neurotransmitter receptors are known. Through the present studies, we discovered that serotonin1A receptors (5-HT1A-Rs) can be sumoylated by SUMO1 (small-ubiquitin-related modifier 1) protein. The SUMO1-5-HT1A-R is ∼55kDa, is located in the membrane fraction, but not the cytosol, and is distributed in all of the brain regions expressing 5-HT1A-Rs examined. Acute stimulation of 5-HT1A-Rs significantly increased SUMO1-5-HT1A-R in rat hypothalamus. Pre-treatment with estradiol for 2 days, which causes a partial desensitization of 5-HT1A-R signaling, potentiated agonist-induced increases in SUMO1-5-HT1A-Rs in the hypothalamus of ovariectomized rats. Using discontinuous gradient centrifugation followed by digitonin treatment, we found that the majority of SUMO1-5-HT1A-Rs is co-localized with endoplasmic-reticulum and trans-Golgi-network markers. Although a small proportion of SUMO1-5-HT1A-Rs are located in the detergent resistant microdomain (DRM) that contain active G-protein coupled receptors, their distribution was different from that of the Gαz protein that couples to the receptors. These data suggest that the SUMO1-5-HT1A-Rs are an inactive form of 5-HT1A-Rs, a finding further supported by results showing minimal 5-HT1A-R agonist binding to SUMO1-5-HT1A-Rs. Furthermore, SUMO1-5-HT1A-Rs in the DRM were increased by treatment with a 5-HT1A-R agonist, 8-OH-DPAT ((+)8-hydroxy-2-dipropylaminotetralin). Together, these data suggest that sumoylation of 5-HT1A-Rs may be related to 5-HT1A-R trafficking and internalization, which may contribute to 5-HT1A-R desensitization. Since 5-HT1A-Rs play an important role in mood regulation, the present results significantly impact on the understanding of the pathogenesis of affective disorders and development of better therapeutic approaches for these diseases.

The SUMO protease Verloren regulates dendrite and axon targeting in olfactory projection neurons

Sumoylation is a post-translational modification regulating numerous biological processes. Small ubiquitin-like modifier (SUMO) proteases are required for the maturation and deconjugation of SUMO proteins, thereby either promoting or reverting sumoylation to modify protein function. Here, we show a novel role for a predicted SUMO protease, Verloren (Velo), during projection neuron (PN) target selection in the Drosophila olfactory system. PNs target their dendrites to specific glomeruli within the antennal lobe (AL) and their axons stereotypically into higher brain centers. We uncovered mutations in velo that disrupt PN targeting specificity. PN dendrites that normally target to a particular dorsolateral glomerulus instead mistarget to incorrect glomeruli within the AL or to brain regions outside the AL. velo mutant axons also display defects in arborization. These phenotypes are rescued by postmitotic expression of Velo in PNs but not by a catalytic domain mutant of Velo. Two other SUMO proteases, DmUlp1 and CG12717, can partially compensate for the function of Velo in PN dendrite targeting. Additionally, mutations in SUMO and lesswright (which encodes a SUMO conjugating enzyme) similarly disrupt PN targeting, confirming that sumoylation is required for neuronal target selection. Finally, genetic interaction studies suggest that Velo acts in SUMO deconjugation rather than in maturation. Our study provides the first in vivo evidence for a specific role of a SUMO protease during neuronal target selection that can be dissociated from its functions in neuronal proliferation and survival.

NMDA receptor signaling mediates the expression of protein inhibitor of activated STAT1 (PIAS1) in rat hippocampus

Protein inhibitor of activated STAT1 (PIAS1) was shown to play an important role in inflammation and innate immune response, but how PIAS1 is regulated is not known. We have recently demonstrated that PIAS1 enhances spatial learning and memory performance in rats. In this study, we examined the signaling pathway and neural mechanism that regulate PIAS1 expression in the brain by using pharmacological and molecular approaches. Our results revealed that pias1 gene expression is rapidly induced upon NMDA receptor activation in rat hippocampus, but this effect is blocked by transfection of sub-threshold concentrations of ERK1 siRNA/ERK2 siRNA or CREB siRNA. Pias1 gene expression is similarly induced by overexpression of the ERK1/ERK2 plasmids in rat hippocampus, and this effect is also blocked by sub-threshold concentration of CREB siRNA transfection. On the other hand, transfection of ERK1 siRNA/ERK2 siRNA or CREB siRNA at a higher concentration is sufficient to down-regulate PIAS1 expression. Inhibition of PI-3 kinase signaling and CaMKII signaling, which both result in CREB inactivation, similarly decreases PIAS1 expression. But NMDA and MK-801 do not affect the expression of IL-6 and TNFα. NMDA also did not affect the expression of PIAS2, PIAS3 and PIAS4. Further, pias1 mRNA has a similar degradation rate to that of the zif268 gene. These results together suggest that pias1 may function as an immediate early gene in an activity-dependent manner and PIAS1 expression is regulated by the NMDA-MAPK/ERK-CREB signaling pathway implicated in neuronal plasticity.

Generation and nuclear translocation of sumoylated transmembrane fragment of cell adhesion molecule L1

The functions of the cell adhesion molecule L1 in the developing and adult nervous system are triggered by homophilic and heterophilic interactions that stimulate signal transductions that activate cellular responses. Here, we show that stimulation of signaling by function-triggering L1 antibodies or L1-Fc leads to serine protease-dependent cleavage of full-length L1 at the plasma membrane and generation of a sumoylated transmembrane 70-kDa fragment comprising the intracellular and transmembrane domains and part of the extracellular domain. The 70-kDa transmembrane fragment is transported from the plasma membrane to a late endosomal compartment, released from endosomal membranes into the cytoplasm, and transferred from there into the nucleus by a pathway that depends on importin and chromatin-modifying protein 1. Mutation of the sumoylation site at Lys(1172) or of the nuclear localization signal at Lys(1147) abolished L1-stimulated generation or nuclear import of the 70-kDa fragment, respectively. Nuclear import of the 70-kDa fragment may activate cellular responses in parallel or in association with phosphorylation-dependent signaling pathways. Alterations in the levels of the 70-kDa fragment during development and in the adult after spinal cord injury or in a mouse model of Alzheimer disease suggest that this fragment is functionally implicated in development, regeneration, neurodegeneration, tumorigenesis, and possibly synaptic plasticity in the mature nervous system.

Changes in the striatal proteome of YAC128Q mice exhibit gene-environment interactions between mutant huntingtin and manganese

Huntington's disease (HD) is a neurodegenerative disorder caused by expansion of a CAG repeat within the Huntingtin (HTT) gene, though the clinical presentation of disease and age-of-onset are strongly influenced by ill-defined environmental factors. We recently reported a gene-environment interaction wherein expression of mutant HTT is associated with neuroprotection against manganese (Mn) toxicity. Here, we are testing the hypothesis that this interaction may be manifested by altered protein expression patterns in striatum, a primary target of both neurodegeneration in HD and neurotoxicity of Mn. To this end, we compared striatal proteomes of wild-type and HD (YAC128Q) mice exposed to vehicle or Mn. Principal component analysis of proteomic data revealed that Mn exposure disrupted a segregation of WT versus mutant proteomes by the major principal component observed in vehicle-exposed mice. Identification of altered proteins revealed novel markers of Mn toxicity, particularly proteins involved in glycolysis, excitotoxicity, and cytoskeletal dynamics. In addition, YAC128Q-dependent changes suggest that axonal pathology may be an early feature in HD pathogenesis. Finally, for several proteins, genotype-specific responses to Mn were observed. These differences include increased sensitivity to exposure in YAC128Q mice (UBQLN1) and amelioration of some mutant HTT-induced alterations (SAE1, ENO1). We conclude that the interaction of Mn and mutant HTT may suppress proteomic phenotypes of YAC128Q mice, which could reveal potential targets in novel treatment strategies for HD.

SUMO-SIM interactions regulate the activity of RGSZ2 proteins

The RGSZ2 gene, a regulator of G protein signaling, has been implicated in cognition, Alzheimer's disease, panic disorder, schizophrenia and several human cancers. This 210 amino acid protein is a GTPase accelerating protein (GAP) on Gαi/o/z subunits, binds to the N terminal of neural nitric oxide synthase (nNOS) negatively regulating the production of nitric oxide, and binds to the histidine triad nucleotide-binding protein 1 at the C terminus of different G protein-coupled receptors (GPCRs). We now describe a novel regulatory mechanism of RGS GAP function through the covalent incorporation of Small Ubiquitin-like MOdifiers (SUMO) into RGSZ2 RGS box (RH) and the SUMO non covalent binding with SUMO-interacting motifs (SIM): one upstream of the RH and a second within this region. The covalent attachment of SUMO does not affect RGSZ2 binding to GPCR-activated GαGTP subunits but abolishes its GAP activity. By contrast, non-covalent binding of SUMO with RH SIM impedes RGSZ2 from interacting with GαGTP subunits. Binding of SUMO to the RGSZ2 SIM that lies outside the RH does not affect GαGTP binding or GAP activity, but it could lead to regulatory interactions with sumoylated proteins. Thus, sumoylation and SUMO-SIM interactions constitute a new regulatory mechanism of RGS GAP function and therefore of GPCR cell signaling as well.

Emerging roles of the SUMO pathway in development

Sumoylation is a reversible post-translational modification that targets a variety of proteins mainly within the nucleus, but also in the plasma membrane and cytoplasm of the cell. It controls diverse cellular mechanisms such as subcellular localization, protein-protein interactions, or transcription factor activity. In recent years, the use of several developmental model systems has unraveled many critical functions for the sumoylation system in the early life of diverse species. In particular, detailed analyses of mutant organisms in both the components of the SUMO pathway and their targets have established the importance of the SUMO system in early developmental processes, such as cell division, cell lineage commitment, specification, and/or differentiation. In addition, an increasing number of developmental proteins, including transcription factors and epigenetic regulators, have been identified as sumoylation substrates. Sumoylation acts on these targets through various mechanisms. For example, this modification has been involved in converting a transcription factor from an activator to a repressor or in regulating the localization and/or stability of numerous transcription factors. This review will summarize current information on the function of sumoylation in embryonic development in different species from yeast to mammals.

Post-translational modification biology of glutamate receptors and drug addiction

Post-translational covalent modifications of glutamate receptors remain a hot topic. Early studies have established that this family of receptors, including almost all ionotropic and metabotropic glutamate receptor subtypes, undergoes active phosphorylation at serine, threonine, or tyrosine residues in their intracellular domains. Recent evidence identifies several glutamate receptor subtypes to be direct substrates for palmitoylation at cysteine residues. Other modifications such as ubiquitination and sumoylation at lysine residues also occur to certain glutamate receptors. These modifications are dynamic and reversible in nature and are regulatable by changing synaptic inputs. The regulated modifications significantly impact the receptor in many ways, including interrelated changes in biochemistry (synthesis, subunit assembling, and protein–protein interactions), subcellular redistribution (trafficking, endocytosis, synaptic delivery, and clustering), and physiology, usually associated with changes in synaptic plasticity. Glutamate receptors are enriched in the striatum and cooperate closely with dopamine to regulate striatal signaling. Emerging evidence shows that modification processes of striatal glutamate receptors are sensitive to addictive drugs, such as psychostimulants (cocaine and amphetamine). Altered modifications are believed to be directly linked to enduring receptor/synaptic plasticity and drug-seeking. This review summarizes several major types of modifications of glutamate receptors and analyzes the role of these modifications in striatal signaling and in the pathogenesis of psychostimulant addiction.

Caveolin-3 undergoes SUMOylation by the SUMO E3 ligase PIASy: sumoylation affects G-protein-coupled receptor desensitization

Caveolin (Cav) proteins in the plasma membrane have numerous binding partners, but the determinants of these interactions are poorly understood. We show here that Cav-3 has a small ubiquitin-like modifier (SUMO) consensus motif (ΨKX(D/E, where Ψ is a hydrophobic residue)) near the scaffolding domain and that Cav-3 is SUMOylated in a manner that is enhanced by the SUMO E3 ligase PIASy (protein inhibitor of activated STAT-y). Site-directed mutagenesis revealed that the consensus site lysine is the preferred SUMOylation site but that mutation of all lysines is required to abolish SUMOylation. Co-expression of a SUMOylation-deficient mutant of Cav-3 with β-adrenergic receptors (βARs) alters the expression level of β(2)ARs but not β(1)ARs following agonist stimulation, thus implicating Cav-3 SUMOylation in the mechanisms for β(2)AR but not β(1)AR desensitization. Expression of endothelial nitric-oxide synthase (NOS3) was not altered by the SUMOylation-deficient mutant. Thus, SUMOylation is a covalent modification of caveolins that influence the regulation of certain signaling partners.

Analysis of metabotropic glutamate receptor 7 as a potential substrate for SUMOylation

Group III metabotropic glutamate receptors (mGluRs) undergo post-translational modification by SUMO in in vitro assays but the SUMOylation of full-length mGluRs in mammalian cells has not been reported. Here we investigated SUMOylation of mGluR7 in HEK293 cells and primary cortical neurons in an attempt to confirm SUMOylation and define physiological effects on mGluR7 function. Using a recombinant bacterial expression assay we validated in vitro SUMOylation of the C-terminal domain of mGluR7 by both SUMO-1 and SUMO-2 and show that a single lysine residue (K889) in mGluR7 is required for SUMOylation. However, using a range of approaches, we were unable to detect SUMOylation of full-length mGluR7 in either heterologous cells or neurons. Further, we observed no differences in receptor stability or surface expression between wild-type and a non-SUMOylatable point mutant mGluR7. Thus, our results question whether mGluR7, and by implication other group III mGluRs, are physiologically relevant neuronal SUMO substrates.

Novel role and mechanism of protein inhibitor of activated STAT1 in spatial learning

By using differential display PCR, we have previously identified 98 cDNA fragments from rat dorsal hippocampus, which are expressed differentially between the fast learners and slow learners from water-maze learning task. One cDNA fragment, which showed a higher expression level in fast learners, encodes the rat protein inhibitor of activated STAT1 (pias1) gene. Spatial training induced a significant increase in PIAS1 expression in rat hippocampus. Transient transfection of the wild-type (WT) PIAS1 plasmid to CA1 neurons facilitated, whereas transfection of PIAS1 siRNA impaired spatial learning in rats. Meanwhile, PIAS1WT increased STAT1 sumoylation, decreased STAT1 DNA binding and decreased STAT1 phosphorylation at Tyr-701 associated with spatial learning facilitation. But PIAS1 siRNA transfection produced an opposite effect on these measures associated with spatial learning impairment. Further, transfection of STAT1 sumoylation mutant impaired spatial acquisition, whereas transfection of STAT1 phosphorylation mutant blocked the impairing effect of PIAS1 siRNA on spatial learning. In this study, we first demonstrate the role of PIAS1 in spatial learning. Both posttranslational modifications (increased sumoylation and decreased phosphorylation) mediate the effect of PIAS1 on spatial learning facilitation.

Hippocampal gene profiling: toward a systems biology of the hippocampus

Transcriptomics and proteomics approaches give a unique perspective for understanding brain and hippocampal functions but also pose unique challenges because of the singular complexity of the nervous system. The proliferation of genome-wide expression studies during the last decade has provided important insight into the molecular underpinnings of brain anatomy, neural plasticity, and neurological diseases. Microarray technology has dominated transcriptomics research, but this situation is rapidly changing with the recent technological advances in high-throughput sequencing. The full potential of transcriptomics in the neurosciences will be achieved as a result of its integration with other "-omics" disciplines as well as the development of novel analytical bioinformatics and systems biology tools for meta-analysis. Here, we review some of the most relevant advances in the gene profiling of the hippocampus, its relationship with proteomics approaches, and the promising perspectives for the future.

Delta(9)-tetrahydrocannabinol regulates the p53 post-translational modifiers Murine double minute 2 and the Small Ubiquitin MOdifier protein in the rat brain

The phytocannabinoid Delta(9)-Tetrahydrocannabinol (Delta(9)-THC), the main psychoactive cannabinoid in cannabis, activates a number of signalling cascades including p53. This study examines the role of Delta(9)-THC in regulating the p53 post-translational modifier proteins, Murine double minute (Mdm2) and Small Ubquitin-like MOdifier protein 1 (SUMO-1) in cortical neurons. Delta(9)-THC increased both Mdm2 and SUMO-1 protein expression and induced the deSUMOylation of p53 in a cannabinoid receptor type 1 (CB(1))-receptor dependent manner. We demonstrate that Delta(9)-THC decreased the SUMOylation of the CB(1) receptor. The data reveal a novel role for cannabinoid receptor activation in modulating the SUMO regulatory system.

Some molecular effectors of antidepressant action of quetiapine revealed by DNA microarray in the frontal cortex of anhedonic rats

OBJECTIVES AND METHODS

We have previously demonstrated that quetiapine (QTP) had antidepressant-like action by using the chronic mild stress (CMS) paradigm, an animal model of human depression. The aim of this study was to investigate the molecular mechanism(s) of QTP antidepressant effect by coupling the CMS protocol with Affymetrix microarray technology to screen the entire rat genome for gene changes in the frontal cortex.

RESULTS

The genes regulated by the administration of CMS whose transcription was reversed by chronic QTP treatment (2 mg/kg/day) were 42 (23 upregulated and 19 downregulated). The transcripts that showed no significant altered expression levels in anhedonic rats but were regulated by the administration of QTP were 19 (nine upregulated and 10 downregulated). On the whole, the action of QTP prevented the stress-induced impairment of some processes involved in central nervous system development or having a crucial role for viability of neural cells and cell-cell communications, like regulation of signal transduction, inorganic cation transport, membrane organization, and neurite morphogenesis. For 11 genes (Ptgs2, Gad1, Plcb1, Camk2a, Homer1, Senp2, Junb, Nfib, Hes5, Capon, and Marcks), significant differential expressions were confirmed by real-time reverse-transcriptase polymerase chain reaction.

CONCLUSION

We have shown that chronic QTP treatment prevented anhedonia and reversed, at least in part, the changes of gene expression induced by CMS in the rat frontal cortex. We have also identified and confirmed by two different methods that 11 genes, representing molecular targets of QTP, are presumably the effectors of its clinical efficacy.

SUMOylation participates in induction of ischemic tolerance

Ground squirrels in hibernation torpor have been shown to have striking increases in global SUMOylation on tissue immunoblots. Here, we find evidence that global SUMOylation is also involved in ischemic tolerance in primary cortical neuronal cultures (from rats and mice) and SHSY5Y human neuroblastoma cells. Cultured cortical neurons preconditioned by sublethal oxygen/glucose deprivation (OGD) were less vulnerable to severe OGD than non-preconditioned neurons. Preconditioned neurons maintained elevated SUMO-1 conjugation levels (and, to a lesser extent those of SUMO-2/3) on western blots in contrast to non-preconditioned cells. Further, cortical neurons and SHSY5Y cells in which transfected SUMO-1 or SUMO-2 were over-expressed showed increased survival after severe OGD. In contrast, cell cultures subjected to depletion of endogenous SUMO-1 protein by RNAi had reduced survival after exposure to this form of in vitro ischemia and an attenuated protective response to preconditioning. These findings suggest that maintenance of a globally elevated SUMO-1 (and maybe SUMO-2/3) conjugation level as revealed by immunoblot assays is a component of ischemic tolerance.

Investigating the mechanisms underlying neuronal death in ischemia using in vitro oxygen-glucose deprivation: potential involvement of protein SUMOylation

It is well established that brain ischemia can cause neuronal death via different signaling cascades. The relative importance and interrelationships between these pathways, however, remain poorly understood. Here is presented an overview of studies using oxygen-glucose deprivation of organotypic hippocampal slice cultures to investigate the molecular mechanisms involved in ischemia. The culturing techniques, setup of the oxygen-glucose deprivation model, and analytical tools are reviewed. The authors focus on SUMOylation, a posttranslational protein modification that has recently been implicated in ischemia from whole animal studies as an example of how these powerful tools can be applied and could be of interest to investigate the molecular pathways underlying ischemic cell death.

Knock-in mice lacking the PDZ-ligand motif of mGluR7a show impaired PKC-dependent autoinhibition of glutamate release, spatial working memory deficits, and increased susceptibility to pentylenetetrazol

The metabotropic glutamate receptor 7 (mGluR7) is widely expressed throughout the brain and primarily localized at presynaptic active zones, where it is thought to regulate neurotransmitter release. Protein interacting with C kinase 1 (PICK1), a postsynaptic density protein-95/disc-large tumor suppressor protein/zonula occludens-1 (PDZ)-domain protein, binds to the three C-terminal amino acids (-LVI) of the predominant mGluR7 splice variant, mGluR7a, and has been implicated in the synaptic clustering of this receptor. Here, we generated knock-in mice in which the C-terminal LVI coding sequence of exon 10 of the mGluR7 gene was replaced by three alanine codons (-AAA). Immunoprecipitation showed that the PICK1-mGluR7a interaction is disrupted in mGluR7a(AAA/AAA) mice. However, the synaptic localization of mGluR7a was not altered in cultured hippocampal neurons and brain sections prepared from the knock-in animals. In cerebellar granule cell cultures, the group III mGluR agonist l-AP-4 decreased the frequency of spontaneous excitatory currents in neurons derived from wild-type but not mGluR7a(AAA/AAA) mice, consistent with the interaction between mGluR7a and PICK1 being required for protein kinase C-mediated inhibition of glutamate release. At the behavioral level, the mGluR7a(AAA/AAA) mice showed no deficits in motor coordination, pain sensitivity, and anxiety but exhibited significant defects in hippocampus-dependent spatial working memory. In addition, they displayed a high susceptibility to the convulsant drug pentylenetetrazole. Together, these results indicate that PICK1 binding to the C-terminal region of mGluR7a is crucial for agonist-triggered presynaptic signaling in vivo.

Molecular determinants of kainate receptor trafficking

Glutamate receptors of the kainate subtype are ionotropic receptors that play a key role in the modulation of neuronal network activity. The role of kainate receptors depends on their precise membrane and subcellular localization in presynaptic, extrasynaptic and postsynaptic domains. These receptors are composed of the combination of five subunits, three of them having several splice variants. The subunits and splice variants show great divergence in their C-terminal cytoplasmic tail domains, which have been implicated in intracellular trafficking of homomeric and heteromeric receptors. Differential trafficking of kainate receptors to specific neuronal compartments likely relies on interactions between the different kainate receptor subunits with distinct subsets of protein partners that interact with C-terminal domains. These C-terminal domains have also been implicated in the degradation of kainate receptors. Finally, the phosphorylation of the C-terminal domain regulates receptor trafficking and function. This review summarizes our knowledge on the regulation of membrane delivery and trafficking of kainate receptors implicating C-terminal domains of the different isoforms and focuses on the identification and characterization of the function of interacting partners.

Transient global cerebral ischemia induces a massive increase in protein sumoylation

A new group of proteins, small ubiquitin-like modifier (SUMO) proteins, has recently been identified and protein sumoylation has been shown to play a major role in various signal transduction pathways. Here, we report that transient global cerebral ischemia induces a marked increase in protein sumoylation. Mice were subjected to 10 mins severe forebrain ischemia followed by 3 or 6 h of reperfusion. Transient cerebral ischemia induced a massive increase in protein sumoylation by SUMO2/3 both in the hippocampus and cerebral cortex. SUMO2/3 conjugation was associated with a decrease in levels of free SUMO2/3. After ischemia, protein levels of the SUMO-conjugating enzyme Ubc9 were transiently decreased in the cortex but not in the hippocampus. We also exposed HT22 cells to arsenite, a respiratory poison that impairs cytoplasmic function and induces oxidative stress. Arsenite exposure induced a marked rise in protein sumoylation, implying that impairment of cytoplasmic function and oxidative stress may be involved in the massive post-ischemic activation of SUMO conjugation described here. Sumoylation of transcription factors has been shown to block their activation, with some exceptions such as the heat-shock factor and the hypoxia-responsive factor, where sumoylation blocks their degradation, and the nuclear factor-kappaB (NF-kappaB) essential modulator where sumoylation leads to an activation of NF-kappaB. Because protein sumoylation is known to be involved in the regulation of various biologic processes, the massive post-ischemic increase in protein sumoylation may play a critical role in defining the final outcome of neurons exposed to transient ischemia.

Concepts in sumoylation: a decade on

A decade has passed since SUMO (small ubiquitin-related modifier) was discovered to be a reversible post-translational protein modifier. During this time many enzymes that participate in regulated SUMO-conjugation and -deconjugation pathways have been identified and characterized. In parallel, the search for SUMO substrates has produced a long list of targets, which appear to be involved in most cellular functions. Sumoylation is a highly dynamic process and its outcomes are extremely diverse, ranging from changes in localization to altered activity and, in some cases, stability of the modified protein. At first glance, these effects have nothing in common; however, it seems that they all result from changes in the molecular interactions of the sumoylated proteins.

RNA editing in regulating gene expression in the brain

Adenosine to inosine RNA editing, catalyzed by Adenosine Deaminases Acting on RNA (ADARs), represents an evolutionary conserved post-transcriptional mechanism which harnesses RNA structures to produce proteins that are not literally encoded in the genome. The species-specific alteration of functionally important residues in a multitude of neuronal ion channels and pre-synaptic proteins through RNA editing has been shown to have profound importance for normal nervous system function in a wide range of invertebrate and vertebrate model organisms. ADARs have also been shown to regulate neuronal gene expression through a remarkable variety of disparate processes, including modulation of the RNAi pathway, the creation of alternative splice sites, and the abolition of stop codons. In addition, ADARs have recently been revealed to have a novel role in the primate lineage: the widespread editing of Alu elements, which comprise approximately 10% of the human genome. Thus, as well as enabling the cell-specific regulation of RNAi and selfish genetic elements, the unshackling of the proteome from the constraints of the genome through RNA editing may have been fundamental to the evolution of complex behavior.

Emerging extranuclear roles of protein SUMOylation in neuronal function and dysfunction

Post-translational protein modifications are integral components of signalling cascades that enable cells to efficiently, rapidly and reversibly respond to extracellular stimuli. These modifications have crucial roles in the CNS, where the communication between neurons is particularly complex. SUMOylation is a post-translational modification in which a member of the small ubiquitin-like modifier (SUMO) family of proteins is conjugated to lysine residues in target proteins. It is well established that SUMOylation controls many aspects of nuclear function, but it is now clear that it is also a key determinant in many extranuclear neuronal processes, and it has also been implicated in a wide range of neuropathological conditions.

Increased protein SUMOylation following focal cerebral ischemia

Stroke is a major cause of death and disability, which involves excessive glutamate receptor activation leading to excitotoxic cell death. We recently reported that SUMOylation can regulate kainate receptor (KAR) function. Here we investigated changes in protein SUMOylation and levels of KAR and AMPA receptor subunits in two different animal stroke models: a rat model of focal ischemia with reperfusion and a mouse model without reperfusion. In rats, transient middle cerebral artery occlusion (MCAO) resulted in a striatal and cortical infarct. A dramatic increase in SUMOylation by both SUMO-1 and SUMO-2/3 was observed at 6h and 24h in the striatal infarct area and by SUMO-2/3 at 24h in the hippocampus, which was not directly subjected to ischemia. In mice, permanent MCAO resulted in a selective cortical infarct. No changes in SUMOylation occurred at 6h but there was increased SUMO-1 conjugation in the cortical infarct and non-ischemic hippocampus at 24h after MCAO. Interestingly, SUMOylation by SUMO-2/3 occurred only outside the infarct area. In both rat and mouse levels of KARs were only decreased in the infarct regions whereas AMPARs were decreased in the infarct and in other brain areas. These results suggest that posttranslational modification by SUMO and down-regulation of AMPARs and KARs may play important roles in the pathophysiological response to ischemia.