Trim39 regulates neuronal apoptosis by acting as a SUMO-targeted E3 ubiquitin-ligase for the transcription factor NFATc3

NFATc3 is the predominant member of the NFAT family of transcription factors in neurons, where it plays a pro-apoptotic role. Mechanisms controlling NFAT protein stability are poorly understood. Here we identify Trim39 as an E3 ubiquitin-ligase of NFATc3. Indeed, Trim39 binds and ubiquitinates NFATc3 in vitro and in cells where it reduces NFATc3 protein level and transcriptional activity. In contrast, silencing of endogenous Trim39 decreases NFATc3 ubiquitination and increases its activity, thereby resulting in enhanced neuronal apoptosis. We also show that Trim17 inhibits Trim39-mediated ubiquitination of NFATc3 by reducing both the E3 ubiquitin-ligase activity of Trim39 and the NFATc3/Trim39 interaction. Moreover, we identify Trim39 as a new SUMO-targeted E3 ubiquitin-ligase (STUbL). Indeed, mutation of SUMOylation sites in NFATc3 or SUMO-interacting motifs in Trim39 reduces NFATc3/Trim39 interaction and Trim39-induced ubiquitination of NFATc3. In addition, Trim39 preferentially ubiquitinates SUMOylated forms of NFATc3 in vitro. As a consequence, a SUMOylation-deficient mutant of NFATc3 exhibits increased stability and pro-apoptotic activity in neurons. Taken together, these data indicate that Trim39 modulates neuronal apoptosis by acting as a STUbL for NFATc3.

To Ubiquitinate or Not to Ubiquitinate: TRIM17 in Cell Life and Death

TRIM17 is a member of the TRIM family, a large class of RING-containing E3 ubiquitin-ligases. It is expressed at low levels in adult tissues, except in testis and in some brain regions. However, it can be highly induced in stress conditions which makes it a putative stress sensor required for the triggering of key cellular responses. As most TRIM members, TRIM17 can act as an E3 ubiquitin-ligase and promote the degradation by the proteasome of substrates such as the antiapoptotic protein MCL1. Intriguingly, TRIM17 can also prevent the ubiquitination of other proteins and stabilize them, by binding to other TRIM proteins and inhibiting their E3 ubiquitin-ligase activity. This duality of action confers several pivotal roles to TRIM17 in crucial cellular processes such as apoptosis, autophagy or cell division, but also in pathological conditions as diverse as Parkinson's disease or cancer. Here, in addition to recent data that endorse this duality, we review what is currently known from public databases and the literature about TRIM17 gene regulation and expression, TRIM17 protein structure and interactions, as well as its involvement in cell physiology and human disorders.

The E3 Ubiquitin Ligases TRIM17 and TRIM41 Modulate α-Synuclein Expression by Regulating ZSCAN21

Although accumulating data indicate that increased α-synuclein expression is crucial for Parkinson disease (PD), mechanisms regulating the transcription of its gene, SNCA, are largely unknown. Here, we describe a pathway regulating α-synuclein expression. Our data show that ZSCAN21 stimulates SNCA transcription in neuronal cells and that TRIM41 is an E3 ubiquitin ligase for ZSCAN21. In contrast, TRIM17 decreases the TRIM41-mediated degradation of ZSCAN21. Silencing of ZSCAN21 and TRIM17 consistently reduces SNCA expression, whereas TRIM41 knockdown increases it. The mRNA levels of TRIM17, ZSCAN21, and SNCA are simultaneously increased in the midbrains of mice following MPTP treatment. In addition, rare genetic variants in ZSCAN21, TRIM17, and TRIM41 genes occur in patients with familial forms of PD. Expression of variants in ZSCAN21 and TRIM41 genes results in the stabilization of the ZSCAN21 protein. Our data thus suggest that deregulation of the TRIM17/TRIM41/ZSCAN21 pathway may be involved in the pathogenesis of PD.

GNIP1 E3 ubiquitin ligase is a novel player in regulating glycogen metabolism in skeletal muscle

BACKGROUND

Glycogenin-interacting protein 1 (GNIP1) is a tripartite motif (TRIM) protein with E3 ubiquitin ligase activity that interacts with glycogenin. These data suggest that GNIP1 could play a major role in the control of glycogen metabolism. However, direct evidence based on functional analysis remains to be obtained.

OBJECTIVES

The aim of this study was 1) to define the expression pattern of glycogenin-interacting protein/Tripartite motif containing protein 7 (GNIP/TRIM7) isoforms in humans, 2) to test their ubiquitin E3 ligase activity, and 3) to analyze the functional effects of GNIP1 on muscle glucose/glycogen metabolism both in human cultured cells and in vivo in mice.

RESULTS

We show that GNIP1 was the most abundant GNIP/TRIM7 isoform in human skeletal muscle, whereas in cardiac muscle only TRIM7 was expressed. GNIP1 and TRIM7 had autoubiquitination activity in vitro and were localized in the Golgi apparatus and cytosol respectively in LHCN-M2 myoblasts. GNIP1 overexpression increased glucose uptake in LHCN-M2 myotubes. Overexpression of GNIP1 in mouse muscle in vivo increased glycogen content, glycogen synthase (GS) activity and phospho-GSK-3α/β (Ser21/9) and phospho-Akt (Ser473) content, whereas decreased GS phosphorylation in Ser640. These modifications led to decreased blood glucose levels, lactate levels and body weight, without changing whole-body insulin or glucose tolerance in mouse.

CONCLUSION

GNIP1 is an ubiquitin ligase with a markedly glycogenic effect in skeletal muscle.

Trim17-mediated ubiquitination and degradation of Mcl-1 initiate apoptosis in neurons

Short-term proteasome inhibition has been shown to prevent neuronal apoptosis. However, the key pro-survival proteins that must be degraded for triggering neuronal death are mostly unknown. Here, we show that Mcl-1, an anti-apoptotic Bcl-2 family member, is degraded by the proteasome during neuronal apoptosis. Using primary cultures of cerebellar granule neurons deprived of serum and KCl, we found that ubiquitination and proteasomal degradation of Mcl-1 depended on its prior phosphorylation by GSK3, providing the first insight into post-translational regulation of Mcl-1 in neurons. In a previous study, we have reported that the E3 ubiquitin-ligase Trim17 is both necessary and sufficient for neuronal apoptosis. Here, we identified Trim17 as a novel E3 ubiquitin-ligase for Mcl-1. Indeed, Trim17 co-immunoprecipitated with Mcl-1. Trim17 ubiquitinated Mcl-1 in vitro. Overexpression of Trim17 decreased the protein level of Mcl-1 in a phosphorylation- and proteasome-dependent manner. Finally, knock down of Trim17 expression reduced both ubiquitination and degradation of Mcl-1 in neurons. Moreover, impairment of Mcl-1 phosphorylation, by kinase inhibition or point mutations, not only decreased ubiquitination and degradation of Mcl-1, but also blocked the physical interaction between Trim17 and Mcl-1. As this stabilization of Mcl-1 increased its neuroprotective effect, our data strongly suggest that Trim17-mediated ubiquitination and degradation of Mcl-1 is necessary for initiating neuronal death.

NF-Y is essential for expression of the proapoptotic bim gene in sympathetic neurons

Neuronal apoptosis has a major role during development and aberrant apoptosis contributes to the pathology of certain neurological conditions. Studies with nerve growth factor (NGF)-dependent sympathetic neurons have provided important insights into the molecular mechanisms of neuronal apoptosis and the signalling pathways that regulate the cell death programme in neurons. The BH3-only protein Bim is a critical mediator of apoptosis in many cell types and in sympathetic neurons is required for NGF withdrawal-induced death. However, regulation of bim expression is complex and remains incompletely understood. We report that a conserved inverted CCAAT box (ICB) in the rat bim promoter is bound by the heterotrimeric transcription factor NF-Y. Interestingly, NF-Y is required for bim promoter activity and its induction following NGF withdrawal. We demonstrate that NF-Y activity is essential for endogenous Bim expression and contributes to NGF withdrawal-induced death. Furthermore, we find that the transcriptional coactivators CBP and p300 interact with NF-Y and FOXO3a and bind to this region of the bim promoter. The amount of CBP/p300 bound to bim increases after NGF deprivation and inhibition of CBP/p300 activity reduces bim induction. Our results indicate that NF-Y cooperates with FOXO3a to recruit CBP/p300 to the bim promoter to form a stable multi-protein/DNA complex that activates bim transcription after survival factor withdrawal.

ATF4 degradation relies on a phosphorylation-dependent interaction with the SCF(betaTrCP) ubiquitin ligase

The ubiquitin-proteasome pathway regulates gene expression through protein degradation. Here we show that the F-box protein betaTrCP, the receptor component of the SCF E3 ubiquitin ligase responsible for IkappaBalpha and beta-catenin degradation, is colocalized in the nucleus with ATF4, a member of the ATF-CREB bZIP family of transcription factors, and controls its stability. Association between the two proteins depends on ATF4 phosphorylation and on ATF4 serine residue 219 present in the context of DSGXXXS, which is similar but not identical to the motif found in other substrates of betaTrCP. ATF4 ubiquitination in HeLa cells is enhanced in the presence of betaTrCP. The F-box-deleted betaTrCP protein behaves as a negative transdominant mutant that inhibits ATF4 ubiquitination and degradation and, subsequently, enhances its activity in cyclic AMP-mediated transcription. ATF4 represents a novel substrate for the SCF(betaTrCP) complex, which is the first mammalian E3 ubiquitin ligase identified so far for the control of the degradation of a bZIP transcription factor.

The F-box protein beta-TrCP associates with phosphorylated beta-catenin and regulates its activity in the cell

Defects in beta-catenin regulation contribute to the neoplastic transformation of mammalian cells. Dysregulation of beta-catenin can result from missense mutations that affect critical sites of phosphorylation by glycogen synthase kinase 3beta (GSK3beta). Given that phosphorylation can regulate targeted degradation of beta-catenin by the proteasome, beta-catenin might interact with an E3 ubiquitin ligase complex containing an F-box protein, as is the case for certain cell cycle regulators. Accordingly, disruption of the Drosophila F-box protein Slimb upregulates the beta-catenin homolog Armadillo. We reasoned that the human homologs of Slimb - beta-TrCP and its isoform beta-TrCP2 (KIAA0696) - might interact with beta-catenin. We found that the binding of beta-TrCP to beta-catenin was direct and dependent upon the WD40 repeat sequences in beta-TrCP and on phosphorylation of the GSK3beta sites in beta-catenin. Endogenous beta-catenin and beta-TrCP could be coimmunoprecipitated from mammalian cells. Overexpression of wild-type beta-TrCP in mammalian cells promoted the downregulation of beta-catenin, whereas overexpression of a dominant-negative deletion mutant upregulated beta-catenin protein levels and activated signaling dependent on the transcription factor Tcf. In contrast, beta-TrCP2 did not associate with beta-catenin. We conclude that beta-TrCP is a component of an E3 ubiquitin ligase that is responsible for the targeted degradation of phosphorylated beta-catenin.