Huntingtin lowering impairs the maturation and synchronized synaptic activity of human cortical neuronal networks derived from induced pluripotent stem cells.

Despite growing descriptions of wild-type Huntingtin (wt-HTT) roles in both adult brain function and, more recently, development, several clinical trials are exploring HTT-lowering approaches that target both wt-HTT and the mutant isoform (mut-HTT) responsible for Huntington's disease (HD). This non-selective targeting is based on the autosomal dominant inheritance of HD, supporting the idea that mut-HTT exerts its harmful effects through a toxic gain-of-function or a dominant-negative mechanism. However, the precise amount of wt-HTT needed for healthy neurons in adults and during development remains unclear. In this study, we address this question by examining how wt-HTT loss affects human neuronal network formation, synaptic maturation, and homeostasis in vitro. Our findings establish a role of wt-HTT in the maturation of dendritic arborization and the acquisition of network-wide synchronized activity by human cortical neuronal networks modeled in vitro. Interestingly, the network synchronization defects only became apparent when more than two-thirds of the wt-HTT protein was depleted. Our study underscores the critical need to precisely understand wt-HTT role in neuronal health. It also emphasizes the potential risks of excessive wt-HTT loss associated with non-selective therapeutic approaches targeting both wt- and mut-HTT isoforms in HD patients.

Sulfur starvation-induced autophagy in Saccharomyces cerevisiae involves SAM-dependent signaling and transcription activator Met4.

Autophagy is a key lysosomal degradative mechanism allowing a prosurvival response to stresses, especially nutrient starvation. Here we investigate the mechanism of autophagy induction in response to sulfur starvation in Saccharomyces cerevisiae. We found that sulfur deprivation leads to rapid and widespread transcriptional induction of autophagy-related (ATG) genes in ways not seen under nitrogen starvation. This distinctive response depends mainly on the transcription activator of sulfur metabolism Met4. Consistently, Met4 is essential for autophagy under sulfur starvation. Depletion of either cysteine, methionine or SAM induces autophagy flux. However, only SAM depletion can trigger strong transcriptional induction of ATG genes and a fully functional autophagic response. Furthermore, combined inactivation of Met4 and Atg1 causes a dramatic decrease in cell survival under sulfur starvation, highlighting the interplay between sulfur metabolism and autophagy to maintain cell viability. Thus, we describe a pathway of sulfur starvation-induced autophagy depending on Met4 and involving SAM as signaling sulfur metabolite.

Mediator, known as a coactivator, can act in transcription initiation in an activator-independent manner in vivo.

Mediator is an evolutionarily conserved complex best known for its role as a coactivator responsible for transducing regulatory signals from DNA-bound activators to the basal RNA polymerase II (Pol II) machinery that initiates transcription from promoters of protein-encoding genes. By exploiting our in vivo activator-independent transcription assay in Saccharomyces cerevisiae, in combination with new temperature sensitive (ts) mutants of Med14 N-terminal half exhibiting widespread transcriptional defects, and existing ts mutants of Kin28 and Med17, we show that, in the absence of activator: (i) Mediator can associate with a promoter as a form devoid of the Cyclin-dependent kinase 8 (CDK8) module, and this association remains regulated by Kin28; (ii) Mediator can stimulate the assembly of the entire Pol II initiation machinery. Although the literature emphasizes the role of the interaction between activators and Mediator, together our results support the view that Mediator is able to act through a dual mechanism in vivo, activator-dependent but also activator-independent, therefore not always as a coactivator.

Selenium-regulated hierarchy of human selenoproteome in cancerous and immortalized cells lines.

BACKGROUND: Selenoproteins (25 genes in human) co-translationally incorporate selenocysteine using a UGA codon, normally used as a stop signal. The human selenoproteome is primarily regulated by selenium bioavailability with a tissue-specific hierarchy. METHODS: We investigated the hierarchy of selenoprotein expression in response to selenium concentration variation in four cell lines originating from kidney (HEK293, immortalized), prostate (LNCaP, cancer), skin (HaCaT, immortalized) and liver (HepG2, cancer), using complementary analytical methods. We performed (i) enzymatic activity, (ii) RT-qPCR, (iii) immuno-detection, (iv) selenium-specific mass spectrometric detection after non-radioactive 76Se labeling of selenoproteins, and (v) luciferase-based reporter constructs in various cell extracts. RESULTS: We characterized cell-line specific alterations of the selenoproteome in response to selenium variation that, in most of the cases, resulted from a translational control of gene expression. We established that UGA-selenocysteine recoding efficiency, which depends on the nature of the SECIS element, dictates the response to selenium variation. CONCLUSIONS: We characterized that selenoprotein hierarchy is cell-line specific with conserved features. This analysis should be done prior to any experiments in a novel cell line. GENERAL SIGNIFICANCE: We reported a strategy based on complementary methods to evaluate selenoproteome regulation in human cells in culture.