ApoB SURFs a Ride from the ER to the Golgi

Chylomicrons and very-low-density lipoproteins (VLDLs) are large, complex cargos that may require specific chaperones for efficient transport from the ER to Golgi. In this issue of Cell Metabolism, Wang et al., 2020Wang X. Wang H. Xu B. Huang D. Nie C. Pu L. Zajac G.J.M. Yan H. Zhao J. Shi F. et al.Receptor-mediated ER export of lipoproteins controls lipid homeostasis in mice and humans.Cell Metab. 2020; 33 (this issue): 350-366Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar identify SURF4, in coordination with SAR1B, as an essential player in COPII transport of VLDLs from ER to Golgi, suggesting that SURF4 may be a target for approaches aimed at reducing secretion of triglyceride-rich, atherogenic lipoproteins from the liver. Chylomicrons and very-low-density lipoproteins (VLDLs) are large, complex cargos that may require specific chaperones for efficient transport from the ER to Golgi. In this issue of Cell Metabolism, Wang et al., 2020Wang X. Wang H. Xu B. Huang D. Nie C. Pu L. Zajac G.J.M. Yan H. Zhao J. Shi F. et al.Receptor-mediated ER export of lipoproteins controls lipid homeostasis in mice and humans.Cell Metab. 2020; 33 (this issue): 350-366Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar identify SURF4, in coordination with SAR1B, as an essential player in COPII transport of VLDLs from ER to Golgi, suggesting that SURF4 may be a target for approaches aimed at reducing secretion of triglyceride-rich, atherogenic lipoproteins from the liver. Very-low-density lipoproteins (VLDLs) are spherical aggregates comprised of a core of thousands of triglyceride (TG) and cholesterol ester molecules with a monolayer cover of amphipathic phospholipids, and a small quantity of free cholesterol. VLDLs are also comprised of several proteins, including apolipoprotein B100 (hereafter referred to as apoB), which is required for the proper assembly and secretion of VLDLs. ApoB is comprised of 4,536 amino acids, including a globular amino-terminal domain, two very large highly lipophilic domains separated by a short amphipathic region, and an amphipathic carboxy-terminal domain. Studies of the complicated intracellular itinerary of this very complex polypeptide emerged in the second half of the 1980s and, for the next decade, focused on co-translational lipidation of apoB during its translocation across the ER membrane and co-translocational proteasomal degradation of apoB when lipidation was inadequate. Other types of ER-associated degradation (ERAD) were also identified during this time (Fisher and Ginsberg, 2002Fisher E.A. Ginsberg H.N. Complexity in the secretory pathway: the assembly and secretion of apolipoprotein B-containing lipoproteins.J. Biol. Chem. 2002; 277: 17377-17380Abstract Full Text Full Text PDF PubMed Scopus (362) Google Scholar). In the first decades of this century, characterization of additional components of apoB's itinerary, including post-ER presecretory proteolysis (PERPP), stepwise lipidation converting nascent lipid-poor VLDL that initially enters the ER lumen into the mature lipid-rich VLDL that is secreted, and the sites of that lipidation, have expanded our understanding of the early trafficking of VLDL (Olofsson and Borén, 2012Olofsson S.O. Borén J. Apolipoprotein B secretory regulation by degradation.Arterioscler. Thromb. Vasc. Biol. 2012; 32: 1334-1338Crossref PubMed Scopus (49) Google Scholar). A less well-defined component of VLDL's cellular itinerary, despite more than a decade of investigation, is its transport from the ER to the Golgi. Although the COPII vesicular transport system has been extensively characterized, there has been uncertainty whether a cargo as large as VLDL (diameters range from 30 to 100 nm) can "fit" into canonical COPII vesicles (diameter: 55–80 nm), or require additional proteins that would facilitate generation of larger COPII vesicles (Brodsky et al., 2004Brodsky J.L. Gusarova V. Fisher E.A. Vesicular trafficking of hepatic apolipoprotein B100 and its maturation to very low-density lipoprotein particles; studies from cells and cell-free systems.Trends Cardiovasc. Med. 2004; 14: 127-132Crossref PubMed Scopus (22) Google Scholar). In a new study in this issue of Cell Metabolism, Wang et al., 2020Wang X. Wang H. Xu B. Huang D. Nie C. Pu L. Zajac G.J.M. Yan H. Zhao J. Shi F. et al.Receptor-mediated ER export of lipoproteins controls lipid homeostasis in mice and humans.Cell Metab. 2020; 33 (this issue): 350-366Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar used an impressive array of approaches to identify a role for SURF4 as a unique component of COPII vesicles, both for carrying VLDL out of the ER and targeting the vesicle for retrieval from the Golgi back to the ER. They began their studies examining the effect of liver-specific knockout of the cytosolic protein SAR1B, a well-characterized member of the Sar1-ADP-ribosylation factor family of small GTPases that play early, important roles in the formation of COPII vesicles. SAR1B is critical for the formation of very large COPII vesicles in the small intestine that transport chylomicrons (diameters ranging from 70 to 600 nm) from the ER to the Golgi in enterocytes. Loss-of-function mutations of SAR1B lead to chylomicron retention disease, also known as Anderson's disease, which is characterized by the absence of circulating chylomicrons, severe fat malabsorption, and failure to thrive (Peretti et al., 2009Peretti N. Roy C.C. Sassolas A. Deslandres C. Drouin E. Rasquin A. Seidman E. Brochu P. Vohl M.C. Labarge S. et al.Chylomicron retention disease: a long term study of two cohorts.Mol. Genet. Metab. 2009; 97: 136-142Crossref PubMed Scopus (32) Google Scholar). The authors established an SAR1B liver-specific knockout mouse and demonstrated significant defects in TG and apoB secretion, hepatic steatosis, and marked reductions in plasma levels of cholesterol, TG, apoB, and apoA-I. The effects of SAR1B loss of function appeared to be specific for VLDL, as plasma levels of albumin and alpha1-antitrypsin were unaffected. The SAR1B knockout phenotype confirmed reductions in plasma lipids and hepatic lipoproteins present in patients with Anderson's disease, and was consistent with stable isotope kinetics studies in such patients that demonstrated a 60% reduction in VLDL apoB secretion and a 30% reduction in HDL apoA-I secretion (Ouguerram et al., 2012Ouguerram K. Zaïr Y. Kasbi-Chadli F. Nazih H. Bligny D. Schmitz J. Aparicio T. Chétiveaux M. Magot T. Aggerbeck L.P. et al.Low rate of production of apolipoproteins B100 and AI in 2 patients with Anderson disease (chylomicron retention disease).Arterioscler. Thromb. Vasc. Biol. 2012; 32: 1520-1525Crossref PubMed Scopus (8) Google Scholar). Wang and colleagues next identified SURF4 as an SAR1B partner using a cell-based proximity-proteomic assay. SURF4 has 260 amino acids comprising 6 predicted transmembrane domains, including one large ER luminal loop, and a cytosolic sorting motif for ER retrieval. It is the human homolog of SFT-4 in C. elegans, which is necessary for the ER export of yolk proteins, such as VIT-2, and cargo receptors of the Erv29p family that bind soluble cargo and COPII components in yeast. Knockout of SFT-4 resulted in reduced ER exit of VIT-2 in C. elegans, and knockdown of SURF4 in HepG2 cells led to reduced numbers of ER exit sites and decreased secretion of apoB (Saegusa et al., 2018Saegusa K. Sato M. Morooka N. Hara T. Sato K. SFT-4/Surf4 control ER export of soluble cargo proteins and participate in ER exit site organization.J. Cell Biol. 2018; 217: 2073-2085Crossref PubMed Scopus (20) Google Scholar). Wang et al. generated a liver-specific knockout of SURF4, which resulted in near total knockdown of plasma TG and cholesterol in all lipoprotein fractions, concomitant with inhibition of TG secretion and significant hepatic steatosis. Of note, these mice showed no evidence of nonalcoholic steatohepatitis (NASH), including no fibrosis, at 13 months of age. Transmission EM studies from livers of the knockout mice demonstrated accumulation of lipoproteins in the ER and their absence in the Golgi. The authors presented immuno-EM using an anti-apoB antibody, but these difficult studies were not convincing. Thus, it is uncertain if the lipoproteins seen with TEM were apoB-lipoproteins or just ER-lumenal lipid droplets. The authors also presented immuno-histologic evidence that apoA-I and apoB accumulate co-incidentally in the ER. Of note, despite these marked alterations in lipoprotein transport with ER accumulation of lipids, probably as VLDL, there was no evidence of ER stress or ER autophagy. In a final series of experiments, Wang and colleagues demonstrated that (1) bidirectional flux of SURF4 from ER to Golgi and back to ER was required for the protein's full impact on VLDL secretion; (2) SAR1B and SURF4 acted sequentially to transport VLDL from ER to Golgi; (3) the two proteins were synergistic, at least in terms of the effects of heterozygous loss of function and the hypolipidemic phenotype; and (4) SURF4 loss of function protected mice with reduced LDL receptors from developing atherosclerosis. What do these very interesting studies mean for our understanding of the COPII vesicular transport system from ER to Golgi? While they clearly add another player to the team of proteins involved, they also leave several important questions unanswered. First, SAR1B is an early and critical player in COPII vesicle formation, but the SAR1B loss-of-function phenotype in humans seems to result in relatively isolated defects in transport of large TG-rich lipoproteins. Is this related to the interaction of SURF4 with SAR1B, but not SAR1A, which is thought to be more involved in the generation of smaller COPIII vesicles that carry typical protein cargos (Melville et al., 2020Melville D.B. Studer S. Schekman R. Small sequence variations between two mammalian paralogs of the small GTPase SAR1 underlie functional differences in coat protein complex II assembly.J. Biol. Chem. 2020; 295: 8401-8412Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar)? Second, if SURF4 activity is key to formation of larger COPII vesicles, does it interact with KHLH12 or TANGO1 proteins, also identified as regulating the size of COPII vesicles (McCaughey and Stephens, 2018McCaughey J. Stephens D.J. COPII-dependent ER export in animal cells: adaptation and control for diverse cargo.Histochem. Cell Biol. 2018; 150: 119-131Crossref PubMed Scopus (24) Google Scholar)? Third, is SURF4 the only protein that can target Golgi COPII vesicle back to the ER? Finally, where do proteins such as the ER transmembrane proteins TM6SF2 and ERLIN (Li et al., 2020Li B.-T. Sun M. Li Y.-F. Wang J.-Q. Zhou Z.-M. Song B.-L. Luo J. Disruption of the ERLIN-TM6SF2-APOB complex destabilizes APOB and contributes to non-alcoholic fatty liver disease.PLoS Genet. 2020; 16: e1008955Crossref PubMed Google Scholar), or TorsinA (Shin et al., 2019Shin J.-Y. Hernandez-Ono A. Fedotova T. Östlund C. Lee M.J. Gibeley S.B. Liang C.C. Dauer W.T. Ginsberg H.N. Worman H.J. Nuclear envelope-localized torsinA-LAP1 complex regulates hepatic VLDL secretion and steatosis.J. Clin. Invest. 2019; 129: 4885-4900Crossref PubMed Scopus (21) Google Scholar), a nuclear membrane-associated AAA ATPase, which appear to be critical for the assembly and secretion of mature VLDL, sit along the apoB secretory highway, and do they interact with the SAR1B-SURF4 complex (Figure 1)? The findings of Wang and colleagues also suggest that targeting hepatic SURF4 in humans might reduce plasma lipid levels, thereby reducing cardiovascular disease. Unfortunately, their overall results suggest that knockdown of SURF4 will result in hepatic steatosis that, despite some contrasting data in this paper, may increase the risk for developing NASH and cirrhosis, as has been observed with loss-of-function mutations in apoB or microsomal triglyceride transfer protein that reduce VLDL assembly and secretion. This work was funded by NIH: NHLBI R35 HL135833 to H.N.G. Dr. J.-Y. Shin kindly assisted with the development of the figure. Receptor-Mediated ER Export of Lipoproteins Controls Lipid Homeostasis in Mice and HumansWang et al.Cell MetabolismNovember 12, 2020In BriefXiao-Wei Chen and colleagues report that synergistic pairing between the cargo receptor SURF4 and the GTPase SAR1B defines a selective transport program for circulating lipoproteins from the ER. They also show that genetic depletion of Surf4 in mice strongly protects from atherosclerosis development. Full-Text PDF