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.

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With the rapid development of China's BeiDou Navigation Satellite System (BDS), the application of real-time precise point positioning (RTPPP) based on BDS has become an active research area in the field of Global Navigation Satellite Systems (GNSS). BDS has provided the service of broadcasting RTPPP information. It indicates that BDS has become the second satellite system that provides RTPPP services, following Galileo among the GNSS, but work based on this direction has yet to be explored. Therefore, this paper evaluates the performance of precise point positioning (PPP) service using a software-defined receiver (SDR). An experiment was carried out to verify the feasibility of the SDR. The PPP-B2b signal was processed to obtain PPP service information, including orbit corrections, clock corrections, and differential code bias corrections. The timevarying attributes of these corrections of BDS and GPS are evaluated, and the integrity and stability of the PPP service were analyzed. The results show the PPP-B2b signal can stably provide PPP services for satellites in the Asia-Pacific region, including centimeter to decimeter-level orbit corrections and meter-level clock corrections for BDS satellites. At the same time, PPP services provide decimeter to meter-level orbit correction and meter-level clock correction for GPS satellites. Finally, detection tip for bitstream availability in SDR is proposed. Some content which is not defined in the official document, such as the PPP-B2b frame arrangement, various correction update cycles and the progress of PPP service are discussed. INDEX TERMS Real-time precise point positioning (RTPPP); BeiDou Navigation Satellite System (BDS); Signal processing; Software-defined receiver (SDR). I. INTRODUCTION The development of Chinese BeiDou navigation satellite system (BDS) can be divided into three steps. The first step was to construct the BeiDou Satellite Navigation Demonstration System (BDS-1) [1]. Using an active positioning scheme, the system provided users in China with positioning, timing, wide-area differential and short message communication services. Since 2003, the third BeiDou navigation experiment satellite was launched, further enhancing the performance of the BeiDou Navigation Satellite Demonstration System [2]. The second step was the construction of the BeiDou Satellite Navigation Regional System (BDS-2). In addition to a technical scheme compatible with that of BDS-1, BDS-2 further included a passive positioning scheme, and provided users in the Asia-Pacific region with positioning, velocity measurement, timing, and short message communication

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