A dimer-monomer switch controls CHIP-dependent substrate ubiquitylation and processing.

The high substrate selectivity of the ubiquitin/proteasome system is mediated by a large group of E3 ubiquitin ligases. The ubiquitin ligase CHIP regulates the degradation of chaperone-controlled and chaperone-independent proteins. To understand how CHIP mediates substrate selection and processing, we performed a structure-function analysis of CHIP and addressed its physiological role in Caenorhabditis elegans and human cells. The conserved function of CHIP in chaperone-assisted degradation requires dimer formation to mediate proteotoxic stress resistance and to prevent protein aggregation. The CHIP monomer, however, promotes the turnover of the membrane-bound insulin receptor and longevity. The dimer-monomer transition is regulated by CHIP autoubiquitylation and chaperone binding, which provides a feedback loop that controls CHIP activity in response to cellular stress. Because CHIP also binds other E3 ligases, such as Parkin, the molecular switch mechanism described here could be a general concept for the regulation of substrate selectivity and ubiquitylation by combining different E3s.

In Vitro Analysis of E3 Ubiquitin Ligase Function.

The covalent attachment of ubiquitin (Ub) to internal lysine residue(s) of a substrate protein, a process termed ubiquitylation, represents one of the most important post-translational modifications in eukaryotic organisms. Ubiquitylation is mediated by a sequential cascade of three enzyme classes including ubiquitin-activating enzymes (E1 enzymes), ubiquitin-conjugating enzymes (E2 enzymes), and ubiquitin ligases (E3 enzymes), and sometimes, ubiquitin-chain elongation factors (E4 enzymes). Here, in vitro protocols for ubiquitylation assays are provided, which allow the assessment of E3 ubiquitin ligase activity, the cooperation between E2-E3 pairs, and substrate selection. Cooperating E2-E3 pairs can be screened by monitoring the generation of free poly-ubiquitin chains and/or auto-ubiquitylation of the E3 ligase. Substrate ubiquitylation is defined by selective binding of the E3 ligase and can be detected by western blotting of the in vitro reaction. Furthermore, an E2~Ub discharge assay is described, which is a useful tool for the direct assessment of functional E2-E3 cooperation. Here, the E3-dependent transfer of ubiquitin is followed from the corresponding E2 enzyme onto free lysine amino acids (mimicking substrate ubiquitylation) or internal lysines of the E3 ligase itself (auto-ubiquitylation). In conclusion, three different in vitro protocols are provided that are fast and easy to perform to address E3 ligase catalytic functionality.

Ubiquitin-dependent regulation of a conserved DMRT protein controls sexually dimorphic synaptic connectivity and behavior.

Sex-specific synaptic connectivity is beginning to emerge as a remarkable, but little explored feature of animal brains. We describe here a novel mechanism that promotes sexually dimorphic neuronal function and synaptic connectivity in the nervous system of the nematode Caenorhabditis elegans. We demonstrate that a phylogenetically conserved, but previously uncharacterized Doublesex/Mab-3 related transcription factor (DMRT), dmd-4, is expressed in two classes of sex-shared phasmid neurons specifically in hermaphrodites but not in males. We find dmd-4 to promote hermaphrodite-specific synaptic connectivity and neuronal function of phasmid sensory neurons. Sex-specificity of DMD-4 function is conferred by a novel mode of posttranslational regulation that involves sex-specific protein stabilization through ubiquitin binding to a phylogenetically conserved but previously unstudied protein domain, the DMA domain. A human DMRT homolog of DMD-4 is controlled in a similar manner, indicating that our findings may have implications for the control of sexual differentiation in other animals as well.

Regulation of E3 ubiquitin ligases by homotypic and heterotypic assembly.

Protein ubiquitylation is essential for the maintenance of cellular homeostasis. E3 ubiquitin ligases are key components of the enzymatic machinery catalyzing the attachment of ubiquitin to substrate proteins. Consequently, enzymatic dysfunction has been associated with medical conditions including cancer, diabetes, and cardiovascular and neurodegenerative disorders. To safeguard substrate selection and ubiquitylation, the activity of E3 ligases is tightly regulated by post-translational modifications including phosphorylation, sumoylation, and ubiquitylation, as well as binding of alternative adaptor molecules and cofactors. Recent structural studies identified homotypic and heterotypic interactions between E3 ligases, adding another layer of control for rapid adaptation to changing environmental and physiological conditions. Here, we discuss the regulation of E3 ligase activity by combinatorial oligomerization and summarize examples of associated ubiquitylation pathways and mechanisms.

Ubiquitylation Pathways In Insulin Signaling and Organismal Homeostasis.

The insulin/insulin-like growth factor-1 (IGF-1) signaling (IIS) pathway is a pivotal genetic program regulating cell growth, tissue development, metabolic physiology, and longevity of multicellular organisms. IIS integrates a fine-tuned cascade of signaling events induced by insulin/IGF-1, which is precisely controlled by post-translational modifications. The ubiquitin/proteasome-system (UPS) influences the functionality of IIS through inducible ubiquitylation pathways that regulate internalization of the insulin/IGF-1 receptor, the stability of downstream insulin/IGF-1 signaling targets, and activity of nuclear receptors for control of gene expression. An age-related decline in UPS activity is often associated with an impairment of IIS, contributing to pathologies such as cancer, diabetes, cardiovascular, and neurodegenerative disorders. Recent findings identified a key role of diverse ubiquitin modifications in insulin signaling decisions, which governs dynamic adaption upon environmental and physiological changes. In this review, we discuss the mutual crosstalk between ubiquitin and insulin signaling pathways in the context of cellular and organismal homeostasis.

The Ubiquitin Ligase CHIP Integrates Proteostasis and Aging by Regulation of Insulin Receptor Turnover.

Aging is attended by a progressive decline in protein homeostasis (proteostasis), aggravating the risk for protein aggregation diseases. To understand the coordination between proteome imbalance and longevity, we addressed the mechanistic role of the quality-control ubiquitin ligase CHIP, which is a key regulator of proteostasis. We observed that CHIP deficiency leads to increased levels of the insulin receptor (INSR) and reduced lifespan of worms and flies. The membrane-bound INSR regulates the insulin and IGF1 signaling (IIS) pathway and thereby defines metabolism and aging. INSR is a direct target of CHIP, which triggers receptor monoubiquitylation and endocytic-lysosomal turnover to promote longevity. However, upon proteotoxic stress conditions and during aging, CHIP is recruited toward disposal of misfolded proteins, reducing its capacity to degrade the INSR. Our study indicates a competitive relationship between proteostasis and longevity regulation through CHIP-assisted proteolysis, providing a mechanistic concept for understanding the impact of proteome imbalance on aging.

Endotoxin-induced silencing of mesoderm induction and functional differentiation: role of HMGB1 in pluripotency and infection.

OBJECTIVES: Mechanisms underpinning Gram-negative bacterial vaginosis-induced birth anomalies are obscure. Ethical issues limit such studies on peri-implantation-stage human embryos. Here we have used embryoid bodies (EBs) as an in vitro model to examine the effect of Gram-negative bacterial endotoxins/lipopolysaccharides (LPS) on the faithful induction of germ lineages during embryogenesis. The role of LPS-inducible cytokine and pluripotency-related DNA-binding protein HMGB1 was also studied in these EBs. METHODS: EBs derived from the human embryonic stem cell line HUES9 were exposed to 12.5 pg/ml of LPS for 48 h. The expression profile of the ectoderm, endoderm, mesoderm and trophectoderm lineage markers, such as beta III-tubulin, GATA4, BMP2, Brachury and beta-hCG, were studied, by RT-PCR and immunofluorescence. Inhibition of mesoderm induction was confirmed by RT-PCR analysis for hANP, cTnT, ABCG2, GATA2, BMP4 and HAND1. Osteoblast differentiation was induced in the EBs, and confirmed by von Kosa and Alizarin red staining. A comet assay was also carried out to assess the degree of apoptosis in these EBs. RESULTS AND CONCLUSIONS: We found that the LPS-treated EBs were selectively silenced for mesoderm markers and failed to differentiate into functional osteoblasts. HMGB1 expression was absent in the normal EBs and was found to be localized in the cytoplasm of the LPS-treated EBs. Overall, our data indicate that endotoxin-induced HMGB1 expression in the peri-implantation-stage embryos can bring about severe birth defects of, for example, the bone and heart. This study also indicates that HMGB1 could be involved in maintenance of pluripotency in the human embryonic stem cells by impeding their differentiation.