Loss of function of XBP1 splicing activity of IRE1α favors B cell tolerance breakdown.

Anti-nuclear antibodies are the hallmark of autoimmune diseases such as systemic lupus erythematosus (SLE) and scleroderma. However, the molecular mechanisms of B cell tolerance breakdown in these pathological contexts are poorly known. The study of rare familial forms of autoimmune diseases could therefore help to better describe common biological mechanisms leading to B cell tolerance breakdown. By Whole-Exome Sequencing, we identified a new heterozygous mutation (p.R594C) in ERN1 gene, encoding IRE1α (Inositol-Requiring Enzyme 1α), in a multiplex family with several members presenting autoantibody-mediated autoimmunity. Using human cell lines and a knock-in (KI) transgenic mouse model, we showed that this mutation led to a profound defect of IRE1α ribonuclease activity on X-Box Binding Protein 1 (XBP1) splicing. The KI mice developed a broad panel of autoantibodies, however in a subclinical manner. These results suggest that a decrease of spliced form of XBP1 (XBP1s) production could contribute to B cell tolerance breakdown and give new insights into the function of IRE1α which are important to consider for the development of IRE1α targeting strategies.

Insights into the structure and function of the RNA ligase RtcB.

To be functional, some RNAs require a processing step involving splicing events. Each splicing event necessitates an RNA ligation step. RNA ligation is a process that can be achieved with various intermediaries such as self-catalysing RNAs, 5'-3' and 3'-5' RNA ligases. While several types of RNA ligation mechanisms occur in human, RtcB is the only 3'-5' RNA ligase identified in human cells to date. RtcB RNA ligation activity is well known to be essential for the splicing of XBP1, an essential transcription factor of the unfolded protein response; as well as for the maturation of specific intron-containing tRNAs. As such, RtcB is a core factor in protein synthesis and homeostasis. Taking advantage of the high homology between RtcB orthologues in archaea, bacteria and eukaryotes, this review will provide an introduction to the structure of RtcB and the mechanism of 3'-5' RNA ligation. This analysis is followed by a description of the mechanisms regulating RtcB activity and localisation, its known partners and its various functions from bacteria to human with a specific focus on human cancer.

Regulation of lipid metabolism by the unfolded protein response.

The endoplasmic reticulum (ER) is the site of protein folding and secretion, Ca2+ storage and lipid synthesis in eukaryotic cells. Disruption to protein folding or Ca2+ homeostasis in the ER leads to the accumulation of unfolded proteins, a condition known as ER stress. This leads to activation of the unfolded protein response (UPR) pathway in order to restore protein homeostasis. Three ER membrane proteins, namely inositol-requiring enzyme 1 (IRE1), protein kinase RNA-like ER kinase (PERK) and activating transcription factor 6 (ATF6), sense the accumulation of unfolded/misfolded proteins and are activated, initiating an integrated transcriptional programme. Recent literature demonstrates that activation of these sensors can alter lipid enzymes, thus implicating the UPR in the regulation of lipid metabolism. Given the presence of ER stress and UPR activation in several diseases including cancer and neurodegenerative diseases, as well as the growing recognition of altered lipid metabolism in disease, it is timely to consider the role of the UPR in the regulation of lipid metabolism. This review provides an overview of the current knowledge on the impact of the three arms of the UPR on the synthesis, function and regulation of fatty acids, triglycerides, phospholipids and cholesterol.

Regulation of hedgehog Ligand Expression by the N-End Rule Ubiquitin-Protein Ligase Hyperplastic Discs and the Drosophila GSK3ß Homologue, Shaggy.

Hedgehog (Hh) morphogen signalling plays an essential role in tissue development and homeostasis. While much is known about the Hh signal transduction pathway, far less is known about the molecules that regulate the expression of the hedgehog (hh) ligand itself. Here we reveal that Shaggy (Sgg), the Drosophila melanogaster orthologue of GSK3ß, and the N-end Rule Ubiquitin-protein ligase Hyperplastic Discs (Hyd) act together to co-ordinate Hedgehog signalling through regulating hh ligand expression and Cubitus interruptus (Ci) expression. Increased hh and Ci expression within hyd mutant clones was effectively suppressed by sgg RNAi, placing sgg downstream of hyd. Functionally, sgg RNAi also rescued the adult hyd mutant head phenotype. Consistent with the genetic interactions, we found Hyd to physically interact with Sgg and Ci. Taken together we propose that Hyd and Sgg function to co-ordinate hh ligand and Ci expression, which in turn influences important developmental signalling pathways during imaginal disc development. These findings are important as tight temporal/spatial regulation of hh ligand expression underlies its important roles in animal development and tissue homeostasis. When deregulated, hh ligand family misexpression underlies numerous human diseases (e.g., colorectal, lung, pancreatic and haematological cancers) and developmental defects (e.g., cyclopia and polydactyly). In summary, our Drosophila-based findings highlight an apical role for Hyd and Sgg in initiating Hedgehog signalling, which could also be evolutionarily conserved in mammals.