Cargo receptor-assisted endoplasmic reticulum export of pathogenic α1-antitrypsin polymers

RNAi targeting LMAN1-MCFD2 complex promotes anticoagulation in mice

Combined deficiency of coagulation factor V (FV) and factor VIII (FVIII) is a rare bleeding disease caused by variants in either lectin mannose binding 1 (LMAN1) or multiple coagulation factor deficiency 2 (MCFD2) gene. Reducing the level of FVIII by inhibiting the LMAN1-MCFD2 complex may become a new anticoagulant approach. We aimed to find a new therapeutic option for anticoagulation by RNA interference (RNAi) targeting LMAN1 and MCFD2. siRNA sequences with cross-homology between mice and humans were designed based on LMAN1 or MCFD2 transcripts in NCBI and were screened with the Dual-Luciferase reporter assay. The optimal siRNAs were chemically modified and conjugated with three N-acetylgalactosamine molecules (GalNAc-siRNA), promoting their targeted delivery to the liver. The expression of LMAN1 and MCFD2 in cell lines or mice was examined by RT-qPCR and western blotting. For the mice administered with siRNA, we assessed their coagulation function by measuring APTT and the activity of FVIII factor. After administration, siRNAs GalNAc-LMAN1 and GalNAc-MCFD2 demonstrated effective and persistent LMAN1 and MCFD2 inhibition. 7 days after injection of 3mg/kg GalNAc-LMAN1, the LMAN1 mRNA levels reduced to 19.97% ± 3.78%. MCFD2 mRNA levels reduced to 32.22% ± 13.14% with injection of 3mg/kg GalNAc-MCFD2. After repeated administration, APTT was prolonged and the FVIII activity was remarkably decreased. The tail bleeding test of mice showed that the amount of bleeding in the treated group did not significantly increase compared with the control group. Our study confirms that therapy with RNAi targeting LMAN1-MCFD2 complex is effective and can be considered a viable option for anticoagulation drugs. However, the benefits and potential risk of bleeding in thrombophilic mice model needs to be evaluated.

A genome-wide CRISPR/Cas9 screen identifies calreticulin as a selective repressor of ATF6α

Activating transcription factor 6 (ATF6) is one of three endoplasmic reticulum (ER) transmembrane stress sensors that mediate the unfolded protein response (UPR). Despite its crucial role in long-term ER stress adaptation, regulation of ATF6 alpha (α) signalling remains poorly understood, possibly because its activation involves ER-to-Golgi and nuclear trafficking. Here, we generated an ATF6α/Inositol-requiring kinase 1 (IRE1) dual UPR reporter CHO-K1 cell line and performed an unbiased genome-wide CRISPR/Cas9 mutagenesis screen to systematically profile genetic factors that specifically contribute to ATF6α signalling in the presence and absence of ER stress. The screen identified both anticipated and new candidate genes that regulate ATF6α activation. Among these, calreticulin (CRT), a key ER luminal chaperone, selectively repressed ATF6α signalling: Cells lacking CRT constitutively activated a BiP::sfGFP ATF6α-dependent reporter, had higher BiP levels and an increased rate of trafficking and processing of ATF6α. Purified CRT interacted with the luminal domain of ATF6α in vitro and the two proteins co-immunoprecipitated from cell lysates. CRT depletion exposed a negative feedback loop implicating ATF6α in repressing IRE1 activity basally and overexpression of CRT reversed this repression. Our findings indicate that CRT, beyond its known role as a chaperone, also serves as an ER repressor of ATF6α to selectively regulate one arm of the UPR.

COPII cage assembly factor Sec13 integrates information flow regulating endomembrane function in response to human variation

How information flow is coordinated for managing transit of 1/3 of the genome through endomembrane pathways by the coat complex II (COPII) system in response to human variation remains an enigma. By examining the interactome of the COPII cage-assembly component Sec13, we show that it is simultaneously associated with multiple protein complexes that facilitate different features of a continuous program of chromatin organization, transcription, translation, trafficking, and degradation steps that are differentially sensitive to Sec13 levels. For the trafficking step, and unlike other COPII components, reduction of Sec13 expression decreased the ubiquitination and degradation of wild-type (WT) and F508del variant cargo protein cystic fibrosis transmembrane conductance regulator (CFTR) leading to a striking increase in fold stability suggesting that the events differentiating export from degradation are critically dependent on COPII cage assembly at the ER Golgi intermediate compartment (ERGIC) associated recycling and degradation step linked to COPI exchange. Given Sec13's multiple roles in protein complex assemblies that change in response to its expression, we suggest that Sec13 serves as an unanticipated master regulator coordinating information flow from the genome to the proteome to facilitate spatial covariant features initiating and maintaining design and function of membrane architecture in response to human variation.

Tracing genetic diversity captures the molecular basis of misfolding disease

Genetic variation in human populations can result in the misfolding and aggregation of proteins, giving rise to systemic and neurodegenerative diseases that require management by proteostasis. Here, we define the role of GRP94, the endoplasmic reticulum Hsp90 chaperone paralog, in managing alpha-1-antitrypsin deficiency on a residue-by-residue basis using Gaussian process regression-based machine learning to profile the spatial covariance relationships that dictate protein folding arising from sequence variants in the population. Covariance analysis suggests a role for the ATPase activity of GRP94 in controlling the N- to C-terminal cooperative folding of alpha-1-antitrypsin responsible for the correction of liver aggregation and lung-disease phenotypes of alpha-1-antitrypsin deficiency. Gaussian process-based spatial covariance profiling provides a standard model built on covariant principles to evaluate the role of proteostasis components in guiding information flow from genome to proteome in response to genetic variation, potentially allowing us to intervene in the onset and progression of complex multi-system human diseases.

Multiple Genes Core to ERAD, Macroautophagy and Lysosomal Degradation Pathways Participate in the Proteostasis Response in α1-Antitrypsin Deficiency

BACKGROUND & AIMS

In the classic form of α1-antitrypsin deficiency (ATD), the misfolded α1-antitrypsin Z (ATZ) variant accumulates in the endoplasmic reticulum (ER) of liver cells. A gain-of-function proteotoxic mechanism is responsible for chronic liver disease in a subgroup of homozygotes. Proteostatic response pathways, including conventional endoplasmic reticulum-associated degradation and autophagy, have been proposed as the mechanisms that allow cellular adaptation and presumably protection from the liver disease phenotype. Recent studies have concluded that a distinct lysosomal pathway called endoplasmic reticulum-to-lysosome completely supplants the role of the conventional macroautophagy pathway in degradation of ATZ. Here, we used several state-of-the-art approaches to characterize the proteostatic responses more fully in cellular systems that model ATD.

METHODS

We used clustered regularly interspaced short palindromic repeats (CRISPR)-mediated genome editing coupled to a cell selection step by fluorescence-activated cell sorter to perform screening for proteostasis genes that regulate ATZ accumulation and combined that with selective genome editing in 2 other model systems.

RESULTS

Endoplasmic reticulum-associated degradation genes are key early regulators and multiple autophagy genes, from classic as well as from ER-to-lysosome and other newly described ER-phagy pathways, participate in degradation of ATZ in a manner that is temporally regulated and evolves as ATZ accumulation persists. Time-dependent changes in gene expression are accompanied by specific ultrastructural changes including dilation of the ER, formation of globular inclusions, budding of autophagic vesicles, and alterations in the overall shape and component parts of mitochondria.

CONCLUSIONS

Macroautophagy is a critical component of the proteostasis response to cellular ATZ accumulation and it becomes more important over time as ATZ synthesis continues unabated. Multiple subtypes of macroautophagy and nonautophagic lysosomal degradative pathways are needed to respond to the high concentrations of misfolded protein that characterizes ATD and these pathways are attractive candidates for genetic variants that predispose to the hepatic phenotype.

Intestinal SURF4 is essential for apolipoprotein transport and lipoprotein secretion

OBJECTIVE

Lipoprotein assembly and secretion in the small intestine are critical for dietary fat absorption. Surfeit locus protein 4 (SURF4) serves as a cargo receptor, facilitating the cellular transport of multiple proteins and mediating hepatic lipid secretion in vivo. However, its involvement in intestinal lipid secretion is not fully understood. In this study, we investigated the role of SURF4 in intestinal lipid absorption.

METHODS

We generated intestine-specific Surf4 knockout mice and characterized the phenotypes. Additionally, we investigated the underlying mechanisms of SURF4 in intestinal lipid secretion using proteomics and cellular models.

RESULTS

We unveiled that SURF4 is indispensable for apolipoprotein transport and lipoprotein secretion. Intestine-specific Surf4 knockout mice exhibited ectopic lipid deposition in the small intestine and hypolipidemia. Deletion of SURF4 impeded the transport of apolipoprotein A1 (ApoA1), proline-rich acidic protein 1 (PRAP1), and apolipoprotein B48 (ApoB48) and hindered the assembly and secretion of chylomicrons and high-density lipoproteins.

CONCLUSIONS

SURF4 emerges as a pivotal regulator of intestinal lipid absorption via mediating the secretion of ApoA1, PRAP1 and ApoB48.

Identification of LMAN1- and SURF4-Dependent Secretory Cargoes

Most proteins secreted into the extracellular space are first recruited from the endoplasmic reticulum into coat protein complex II (COPII)-coated vesicles or tubules that facilitate their transport to the Golgi apparatus. Although several secreted proteins have been shown to be actively recruited into COPII vesicles and tubules by the cargo receptors LMAN1 and SURF4, the full cargo repertoire of these receptors is unknown. We now report mass spectrometry analysis of conditioned media and cell lysates from HuH7 cells CRISPR targeted to inactivate the LMAN1 or SURF4 gene. We found that LMAN1 has limited clients in HuH7 cells, whereas SURF4 traffics a broad range of cargoes. Analysis of putative SURF4 cargoes suggests that cargo recognition is governed by complex mechanisms rather than interaction with a universal binding motif..

Alpha-1 antitrypsin protects against phosgene-induced acute lung injury by activating the ID1-dependent anti-inflammatory response

Phosgene is widely used as an industrial chemical, and phosgene inhalation causes acute lung injury (ALI), which may further progress into pulmonary edema. Currently, an antidote for phosgene poisoning is not known. Alpha-1 antitrypsin (α1-AT) is a protease inhibitor used to treat patients with emphysema who are deficient in α1-AT. Recent studies have revealed that α1-AT has both anti-inflammatory and anti-SARS-CoV-2 effects. Herein, we aimed to investigate the role of α1-AT in phosgene-induced ALI. We observed a time-dependent increase in α1-AT expression and secretion in the lungs of rats exposed to phosgene. Notably, α1-AT was derived from neutrophils but not from macrophages or alveolar type II cells. Moreover, α1-AT knockdown aggravated phosgene- and lipopolysaccharide (LPS)-induced inflammation and cell death in human bronchial epithelial cells (BEAS-2B). Conversely, α1-AT administration suppressed the inflammatory response and prevented death in LPS- and phosgene-exposed BEAS-2B cells. Furthermore, α1-AT treatment increased the inhibitor of DNA binding 1 (ID1) gene expression, which suppressed NF-κB pathway activation, reduced inflammation, and inhibited cell death. These data demonstrate that neutrophil-derived α1-AT acts as a self-protective mechanism, which protects against phosgene-induced ALI by activating the ID1-dependent anti-inflammatory response. This study may provide novel strategies for the treatment of patients with phosgene-induced ALI.

Mammalian cargo receptors for endoplasmic reticulum-to-Golgi transport: mechanisms and interactions

Proteins that are destined to enter the secretory pathway are synthesized on the rough endoplasmic reticulum (ER) and then translocated into the ER lumen, where they undergo posttranslational modifications, folding, and assembly. After passing a quality control system, the cargo proteins are packaged into coat protein complex II (COPII) vesicles to exit the ER. In metazoans, most COPII subunits have multiple paralogs, enabling COPII vesicles the flexibility to transport a diverse range of cargo. The cytoplasmic domains of transmembrane proteins can interact with SEC24 subunits of COPII to enter the ER exit sites. Some transmembrane proteins may also act as cargo receptors that bind soluble secretory proteins within the ER lumen, enabling them to enter COPII vesicles. The cytoplasmic domains of cargo receptors also contain coat protein complex I binding motifs that allow for their cycling back to the ER after unloading their cargo in the ER-Golgi intermediate compartment and cis-Golgi. Once unloaded, the soluble cargo proteins continue maturation through the Golgi before reaching their final destinations. This review provides an overview of receptor-mediated transport of secretory proteins from the ER to the Golgi, with a focus on the current understanding of two mammalian cargo receptors: the LMAN1-MCFD2 complex and SURF4, and their roles in human health and disease.

Identification of LMAN1 and SURF4 dependent secretory cargoes

Most proteins secreted into the extracellular space are first recruited from the endoplasmic reticulum into coat protein complex II (COPII)-coated vesicles or tubules that facilitate their transport to the Golgi apparatus. Although several secreted proteins have been shown to be actively recruited into COPII vesicles/tubules by the cargo receptors LMAN1 and SURF4, the full cargo repertoire of these receptors is unknown. We now report mass spectrometry analysis of conditioned media and cell lysates from HuH7 cells CRISPR targeted to inactivate the LMAN1 or SURF4 gene. We found that LMAN1 has limited clients in HuH7 cells whereas SURF4 traffics a broad range of cargoes. Analysis of putative SURF4 cargoes suggests that cargo recognition is governed by complex mechanisms rather than interaction with a universal binding motif.

Shaping the future from the small scale: dry powder inhalation of CRISPR-Cas9 lipid nanoparticles for the treatment of lung diseases

INTRODUCTION

Most lung diseases are serious conditions resulting from genetic and environmental causes associated with high mortality and severe symptoms. Currently, treatments available have a palliative effect and many targets are still considered undruggable. Gene therapy stands as an attractive approach to offering innovative therapeutic solutions. CRISPRCas9 has established a remarkable potential for genome editing with high selectivity to targeted mutations. To ensure high efficacy with minimum systemic exposure, the delivery and administration route are key components that must be investigated.

AREAS COVERED

This review is focused on the delivery of CRISPRCas9 to the lungs, taking advantage of lipid nanoparticles (LNPs), the most clinically advanced nucleic acid carriers. We also aim to highlight the benefits of pulmonary administration as a local delivery route and the use of spray drying to prepare stable nucleic-acid-based dry powder formulations that can overcome multiple lung barriers.

EXPERT OPINION

Exploring the pulmonary administration to deliver CRISPRCas9 loaded in LNPs as a dry powder increases the chances to achieve high efficacy and reduced adverse effects. CRISPRCas9 loaded in LNP-embedded microparticles has not yet been reported in the literature but has the potential to reach and accumulate in target cells in the lung, thus, enhancing overall efficacy and safety.

Surf4 (Surfeit Locus Protein 4) Deficiency Reduces Intestinal Lipid Absorption and Secretion and Decreases Metabolism in Mice

BACKGROUND

Postprandial dyslipidemia is a causative risk factor for cardiovascular disease. The majority of absorbed dietary lipids are packaged into chylomicron and then delivered to circulation. Previous studies showed that Surf4 (surfeit locus protein 4) mediates very low-density lipoprotein secretion from hepatocytes. Silencing hepatic Surf4 markedly reduces the development of atherosclerosis in different mouse models of atherosclerosis without causing hepatic steatosis. However, the role of Surf4 in chylomicron secretion is unknown.

METHODS

We developed inducible intestinal-specific Surf4 knockdown mice (Surf4^IKO) using Vil1Cre-ER^T2 and Surf4^flox mice. Metabolic cages were used to monitor mouse metabolism. Enzymatic kits were employed to measure serum and tissue lipid levels. The expression of target genes was detected by qRT-PCR and Western Blot. Transmission electron microscopy and radiolabeled oleic acid were used to assess the structure of enterocytes and intestinal lipid absorption and secretion, respectively. Proteomics was performed to determine changes in protein expression in serum and jejunum.

RESULTS

Surf4^IKO mice, especially male Surf4^IKO mice, displayed significant body weight loss, increased mortality, and reduced metabolism. Surf4^IKO mice exhibited lipid accumulation in enterocytes and impaired fat absorption and secretion. Lipid droplets and small lipid vacuoles were accumulated in the cytosol and the endoplasmic reticulum lumen of the enterocytes of Surf4^IKO mice, respectively. Surf4 colocalized with apoB and co-immunoprecipitated with apoB48 in differentiated Caco-2 cells. Intestinal Surf4 deficiency also significantly reduced serum triglyceride, cholesterol, and free fatty acid levels in mice. Proteomics data revealed that diverse pathways were altered in Surf4^IKO mice. In addition, Surf4^IKO mice had mild liver damage, decreased liver size and weight, and reduced hepatic triglyceride levels.

CONCLUSIONS

Our findings demonstrate that intestinal Surf4 plays an essential role in lipid absorption and chylomicron secretion and suggest that the therapeutic use of Surf4 inhibition requires highly cell/tissue-specific targeting.

Surf4, cargo trafficking, lipid metabolism, and therapeutic implications

Surfeit 4 is a polytopic transmembrane protein that primarily resides in the endoplasmic reticulum (ER) membrane. It is ubiquitously expressed and functions as a cargo receptor, mediating cargo transport from the ER to the Golgi apparatus via the canonical coat protein complex II (COPII)-coated vesicles or specific vesicles. It also participates in ER-Golgi protein trafficking through a tubular network. Meanwhile, it facilitates retrograde transportation of cargos from the Golgi apparatus to the ER through COPI-coated vesicles. Surf4 can selectively mediate export of diverse cargos, such as PCSK9 very low-density lipoprotein (VLDL), progranulin, α1-antitrypsin, STING, proinsulin, and erythropoietin. It has been implicated in facilitating VLDL secretion, promoting cell proliferation and migration, and increasing replication of positive-strand RNA viruses. Therefore, Surf4 plays a crucial role in various physiological and pathophysiological processes and emerges as a promising therapeutic target. However, the molecular mechanisms by which Surf4 selectively sorts diverse cargos for ER-Golgi protein trafficking remain elusive. Here, we summarize the most recent advances in Surf4, focusing on its role in lipid metabolism.

Cargo selection in endoplasmic reticulum-to-Golgi transport and relevant diseases

Most proteins destined for the extracellular space or various intracellular compartments must traverse the intracellular secretory pathway. The first step is the recruitment and transport of cargoes from the endoplasmic reticulum (ER) lumen to the Golgi apparatus by coat protein complex II (COPII), consisting of five core proteins. Additional ER transmembrane proteins that aid cargo recruitment are referred to as cargo receptors. Gene duplication events have resulted in multiple COPII paralogs present in the mammalian genome. Here, we review the functions of each COPII protein, human disorders associated with each paralog, and evidence for functional conservation between paralogs. We also provide a summary of current knowledge regarding two prototypical cargo receptors in mammals, LMAN1 and SURF4, and their roles in human health and disease.

Export of polybasic motif-containing secretory proteins BMP8A and SFRP1 from the endoplasmic reticulum is regulated by surfeit locus protein 4

In the conventional secretory pathway, cargo receptors play important roles in exporting newly synthesized secretory proteins from the endoplasmic reticulum (ER). We previously showed that a cargo receptor, surfeit locus protein 4 (SURF4), promotes ER export of a soluble signaling molecule, sonic hedgehog, via recognizing the polybasic residues within its Cardin-Weintraub motif. In addition to sonic hedgehog, we found 30 more secretory proteins containing the polybasic motif (K/R)(K/R)(K/R)XX(K/R)(K/R), but whether SURF4 plays a general role in mediating ER export of these secretory proteins is unclear. Here, we analyzed the trafficking of four of these secretory proteins: desert hedgehog, Indian hedgehog, bone morphogenetic protein 8A (BMP8A), and secreted frizzled-related protein 1 (SFRP1). We found that the polybasic motifs contained in these cargo proteins are important for their ER export. Further analyses indicated that the polybasic motifs of BMP8A and SFRP1 interact with the triacidic motif on the predicted first luminal domain of SURF4. These interactions with SURF4 are essential and sufficient for the ER-to-Golgi trafficking of BMP8A and SFRP1. Moreover, we demonstrated that SURF4 localizes at a subpopulation of ER exit sites to regulate the ER export of its clients. Taken together, these results suggest that SURF4 is recruited to specific ER exit sites and plays a general role in capturing polybasic motif-containing secretory cargo proteins through electrostatic interactions.

Selective inhibition of protein secretion by abrogating receptor-coat interactions during ER export

Protein secretion is an essential process that drives cell growth, movement, and communication. Protein traffic within the secretory pathway occurs via transport intermediates that bud from one compartment and fuse with a downstream compartment to deliver their contents. Here, we explore the possibility that protein secretion can be selectively inhibited by perturbing protein-protein interactions that drive capture into transport vesicles. Human proprotein convertase subtilisin/kexin type 9 (PCSK9) is a determinant of cholesterol metabolism whose secretion is mediated by a specific cargo adaptor protein, SEC24A. We map a series of protein-protein interactions between PCSK9, its endoplasmic reticulum (ER) export receptor SURF4, and SEC24A that mediate secretion of PCSK9. We show that the interaction between SURF4 and SEC24A can be inhibited by 4-phenylbutyrate (4-PBA), a small molecule that occludes a cargo-binding domain of SEC24. This inhibition reduces secretion of PCSK9 and additional SURF4 clients that we identify by mass spectrometry, leaving other secreted cargoes unaffected. We propose that selective small-molecule inhibition of cargo recognition by SEC24 is a potential therapeutic intervention for atherosclerosis and other diseases that are modulated by secreted proteins.

Progress of delivery methods for CRISPR-Cas9

INTRODUCTION

Gene therapy is becoming increasingly common in clinical practice, giving hope for the correction of a wide range of human diseases and defects. The CRISPR/Cas9 system, consisting of the Cas9 nuclease and single-guide RNA (sgRNA), has revolutionized the field of gene editing. However, efficiently delivering the CRISPR-Cas9 to the target organ or cell remains a significant challenge. In recent years, with rapid advances in nanoscience, materials science, and medicine, researchers have developed various technologies that can deliver CRISPR-Cas9 in different forms for in vitro and in vivo gene editing. Here, we review the development of the CRISPR-Cas9 and describe the delivery forms and the vectors that have emerged in CRISPR-Cas9 delivery, summarizing the key barriers and the promising strategies that vectors currently face in delivering the CRISPR-Cas9.

AREAS COVERED

With the rapid development of CRISPR-Cas9, delivery methods are becoming increasingly important in the in vivo delivery of CRISPR-Cas9.

EXPERT OPINION

CRISPR-Cas9 is becoming increasingly common in clinical trials. However, the complex nuclease and protease environment is a tremendous challenge for in vivo clinical applications. Therefore, the development of delivery methods is highly likely to take the application of CRISPR-Cas9 technology to another level.

LMAN1-MCFD2 complex is a cargo receptor for the ER-Golgi transport of α1-antitrypsin

α1-antitrypsin (AAT) is a serine protease inhibitor synthesized in hepatocytes and protects the lung from damage by neutrophil elastase. AAT gene mutations result in AAT deficiency (AATD), which leads to lung and liver diseases. The AAT Z variant forms polymer within the endoplasmic reticulum (ER) of hepatocytes and results in reduction of AAT secretion and severe disease. Previous studies demonstrated a secretion defect of AAT in LMAN1 deficient cells, and mild decreases in AAT levels in male LMAN1 and MCFD2 deficient mice. LMAN1 is a transmembrane lectin that forms a complex with a small soluble protein MCFD2. The LMAN1-MCFD2 protein complex cycles between the ER and the Golgi. Here we report that LMAN1 and MCFD2 knockout (KO) HepG2 and HEK293T cells display reduced AAT secretion and elevated intracellular AAT levels due to a delayed ER-to-Golgi transport of AAT. Secretion defects in KO cells were rescued by wild-type LMAN1 or MCFD2, but not by mutant proteins. Elimination of the second glycosylation site of AAT abolished LMAN1 dependent secretion. Co-immunoprecipitation experiment in MCFD2 KO cells suggested that AAT interaction with LMAN1 is independent of MCFD2. Furthermore, our results suggest that secretion of the Z variant, both monomers and polymers, is also LMAN1-dependent. Results provide direct evidence supporting that the LMAN1-MCFD2 complex is a cargo receptor for the ER-to-Golgi transport of AAT and that interactions of LMAN1 with an N-glycan of AAT is critical for this process. These results have implications in production of recombinant AAT and in developing treatments for AATD patients.

Loss of Hepatic Surf4 Depletes Lipid Droplets in the Adrenal Cortex but Does Not Impair Adrenal Hormone Production

The adrenal gland produces steroid hormones to play essential roles in regulating various physiological processes. Our previous studies showed that knockout of hepatic Surf4 (Surf4^LKO) markedly reduced fasting plasma total cholesterol levels in adult mice, including low-density lipoprotein and high-density lipoprotein cholesterol. Here, we found that plasma cholesterol levels were also dramatically reduced in 4-week-old young mice and non-fasted adult mice. Circulating lipoprotein cholesterol is an important source of the substrate for the production of adrenal steroid hormones. Therefore, we investigated whether adrenal steroid hormone production was affected in Surf4^LKO mice. We observed that lacking hepatic Surf4 essentially eliminated lipid droplets and significantly reduced cholesterol levels in the adrenal gland; however, plasma levels of aldosterone and corticosterone were comparable in Surf4^LKO and the control mice under basal and stress conditions. Further analysis revealed that mRNA levels of genes encoding enzymes important for hormone synthesis were not altered, whereas the expression of scavenger receptor class B type I (SR-BI), low-density lipoprotein receptor (LDLR) and 3-hydroxy-3-methyl-glutaryl-CoA reductase was significantly increased in the adrenal gland of Surf4^LKO mice, indicating increased de novo cholesterol biosynthesis and enhanced LDLR and SR-BI-mediated lipoprotein cholesterol uptake. We also observed that the nuclear form of SREBP2 was increased in the adrenal gland of Surf4 ^LKO mice. Taken together, these findings indicate that the very low levels of circulating lipoprotein cholesterol in Surf4^LKO mice cause a significant reduction in adrenal cholesterol levels but do not significantly affect adrenal steroid hormone production. Reduced adrenal cholesterol levels activate SREBP2 and thus increase the expression of genes involved in cholesterol biosynthesis, which increases de novo cholesterol synthesis to compensate for the loss of circulating lipoprotein-derived cholesterol in the adrenal gland of Surf4^LKO mice.

An in vitro vesicle formation assay reveals cargo clients and factors that mediate vesicular trafficking

Protein sorting in the secretory pathway is a fundamentally important cellular process, but the clients of a specific cargo sorting machinery remains largely underinvestigated. Here, utilizing a vesicle formation assay to profile proteins associated with vesicles, we identified cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner or that interact with GTP-bound Sar1A. We found that two of them, FAM84B and PRRC1, regulate anterograde trafficking. Moreover, we revealed specific clients of two export adaptors, SURF4 and ERGIC53. These analyses demonstrate that our approach is powerful to identify factors that regulate vesicular trafficking and to uncover clients of specific cargo receptors, providing a robust method to reveal insights into the secretory pathway.

The fidelity of protein transport in the secretory pathway relies on the accurate sorting of proteins to their correct destinations. To deepen our understanding of the underlying molecular mechanisms, it is important to develop a robust approach to systematically reveal cargo proteins that depend on specific sorting machinery to be enriched into transport vesicles. Here, we used an in vitro assay that reconstitutes packaging of human cargo proteins into vesicles to quantify cargo capture. Quantitative mass spectrometry (MS) analyses of the isolated vesicles revealed cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner. We found that two of them, FAM84B (also known as LRAT domain containing 2 or LRATD2) and PRRC1, contain proline-rich domains and regulate anterograde trafficking. Further analyses revealed that PRRC1 is recruited to endoplasmic reticulum (ER) exit sites, interacts with the inner COPII coat, and its absence increases membrane association of COPII. In addition, we uncovered cargo proteins that depend on GTP hydrolysis to be captured into vesicles. Comparing control cells with cells depleted of the cargo receptors, SURF4 or ERGIC53, we revealed specific clients of each of these two export adaptors. Our results indicate that the vesicle formation assay in combination with quantitative MS analysis is a robust and powerful tool to uncover novel factors that mediate vesicular trafficking and to uncover cargo clients of specific cellular factors.