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Supplementary key words mevalonate • lanosterol • sterol intermediates • 3-hydroxy-3-methylglutaryl-coenzyme A reductase degradation • sterol regulatory element-binding protein-2 cleavage • clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) • sterol regulatory element-binding protein-2 Cholesterol is an essential lipid for mammals. It regulates membrane fluidity and functions, serves as the precursor for steroid hormones and bile acids, and covalently modifies the Hedgehog and Smoothened proteins (1-3). A high level of blood cholesterol is a major risk factor for cardiovascular disease, and cholesterol lowering is an effective way to treat the cardiovascular disease (4-7). Cholesterol is synthesized from acetyl-CoA through over 30 steps of reactions (Fig. 1). The cholesterol biosynthetic pathway is also known as the mevalonate pathway. Mevalonate is a key intermediate synthesized from HMG-CoA by HMG-CoA reductase (HMGCR), an ER-localized ratelimiting enzyme of the mevalonate pathway (8). Cholesterol biosynthesis is governed by two feedback regulatory mechanisms: the sterol-induced degradation of HMGCR (9) and inactivation of SREBP-2, the latter of which controls transcription of the genes involved in cholesterol biosynthesis and uptake (10). When the cellular sterol level is Abstract Sterol-regulated HMG-CoA reductase (HMGCR) degradation and SREBP-2 cleavage are two major feedback regulatory mechanisms governing cholesterol biosynthesis. Reportedly, lanosterol selectively stimulates HMGCR degradation, and cholesterol is a specific regulator of SREBP-2 cleavage. However, it is unclear whether other endogenously generated sterols regulate these events. Here, we investigated the sterol intermediates from the mevalonate pathway of cholesterol biosynthesis using a CRISPR/Cas9mediated genetic engineering approach. With a constructed HeLa cell line expressing the mevalonate transporter, we individually deleted genes encoding major enzymes in the mevalonate pathway, used lipidomics to measure sterol intermediates, and examined HMGCR and SREBP-2 statuses. We found that the C4-dimethylated sterol intermediates, including lanosterol, 24,25-dihydrolanosterol, follicular fluid meiosis activating sterol, testis meiosis activating sterol, and dihydro-testis meiosis activating sterol, were significantly upregulated upon mevalonate loading. These intermediates augmented both degradation of HMGCR and inhibition of SREBP-2 cleavage. The accumulated lanosterol induced rapid degradation of HMGCR, but did not inhibit SREBP-2 cleavage. The newly synthesized cholesterol from the mevalonate pathway is dispensable for inhibiting SREBP-2 cleavage. Together, these results suggest that lanosterol is a bona fide endogenous regulator that specifically promotes HMGCR degradation, and that other C4-dimethylated sterol intermediates may regulate both HMGCR degradation and SREBP-2 cleavage.

Toll-like receptors (TLRs) are pattern recognition receptors that sense a variety of pathogens, initiate innate immune responses, and direct adaptive immunity. All TLRs except TLR3 recruit the adaptor MyD88 to ultimately elicit inflammatory gene expression, whereas TLR3 and internalized TLR4 use TIR-domain-containing adaptor TRIF for the induction of type I interferon and inflammatory cytokines. Here, we identify the WD repeat and FYVE-domaincontaining protein WDFY1 as a crucial adaptor protein in the TLR3/4 signaling pathway. Overexpression of WDFY1 potentiates TLR3-and TLR4-mediated activation of NF-jB, interferon regulatory factor 3 (IRF3), and production of type I interferons and inflammatory cytokines. WDFY1 depletion has the opposite effect. WDFY1 interacts with TLR3 and TLR4 and mediates the recruitment of TRIF to these receptors. Our findings suggest a crucial role for WDFY1 in bridging the TLR-TRIF interaction, which is necessary for TLR signaling.

Innate immunity mediated by Toll signalling has been extensively studied, but how Toll signalling is precisely controlled in balancing innate immune responses remains poorly understood. It was reported that the plasma membrane localization of Drosophila MyD88 is necessary for the recruitment of cytosolic adaptor Tube to the cell surface, thus contributing to Toll signalling transduction. Here we demonstrate that Drosophila Pellino functions as a negative regulator in Toll-mediated signalling. We show that Pellino accumulates at the plasma membrane upon the activation of Toll signalling in a MyD88-dependent manner. Moreover, we find that Pellino is associated with MyD88 via its CTE domain, which is necessary and sufficient to promote Pellino accumulation at the plasma membrane where it targets MyD88 for ubiquitination and degradation. Collectively, our study uncovers a mechanism by which a feedback regulatory loop involving MyD88 and Pellino controls Toll-mediated signalling, thereby maintaining homeostasis of host innate immunity.