Juan Xiong

Sigma Non-Opioid Intracellular Receptor 1 (SigmaR1) is a member of the sigma family of receptors that interacts with a variety of psychotomimetic drugs and is involved in a wide range of cellular and physiological functions. Despite its increasing importance in human physiology and disease, the subcellular localization of SigmaR1 and its molecular function remain poorly defined. Using endogenous tagging and cell fractionation, we show that SigmaR1 is a type II integral ER membrane protein that is specifically enriched at ER sheets. A short region at the N-terminus of SigmaR1 promotes its ER-sheet localization. Importantly, our biochemical studies demonstrate that SigmaR1 directly interacts with components of the translocon complex including TRAPα and Nicalin. In addition, we found that a β-barrel at the C-terminal of SigmaR1 binds phosphatidylcholine (PC), and the binding of PC strengthens the association of SigmaR1 with the translocon complex. SigmaR1 knockout systematically impaired cellular protein and lipid homeostasis, resulting in accumulation of lipid droplets in hepatocytes. Collectively, we propose that SigmaR1 is an auxiliary translocon factor that binds lipids to regulate protein and lipid droplet homeostasis, which may underlie the broad and vital roles of SigmaR1 in physiology and disease.

Lipid droplets (LDs) are evolutionarily conserved organelles essential for cellular metabolism. They form and grow at the endoplasmic reticulum (ER), requiring lipid transfer between these compartments, yet the underlying molecular mechanisms remain elusive. We identify Sec14L6, a unique Sec14 family member, as a lipid transporter regulating phosphoinositide (PIP) homeostasis and LD biogenesis, promoting adipogenic differentiation of mesenchymal stem cells. Sec14L6 directly binds the LD biogenesis factor ACSL3, which facilitates the association of Sec14L6 with LD surface. Furthermore, the ER membrane protein PGRMC1 recruits Sec14L6 to the ER. Targeted lipidomics revealed profound PIP dysregulation in Sec14L6-KO cells: LDs accumulated phosphoinositide-4-phosphate (PI4P) and PI(4,5)P₂, while these PIPs were reduced within the ER. In vitro assays demonstrated that Sec14L6 transports PI4P and PI(4,5)P₂. Sec14L6 knockout significantly impaired LD formation; this defect was rescued by wild-type Sec14L6, but not by lipid-transfer-deficient mutants. Our study reveals an essential role for Sec14L6 in PIP homeostasis and promotes LD biogenesis through lipid transfer between the ER and LDs.

Lipid droplets (LDs) are evolutionarily conserved organelles crucial for cellular metabolism. Their biogenesis and growth occur in the endoplasmic reticulum (ER) and rely on lipid transfer between the ER and LDs. However, the molecular mechanisms remain poorly understood. In this study, we identified Sec14L6, a unique Sec14 protein family member, as a lipid transporter that regulates phosphoinositide homeostasis and biogenesis of LDs, and is required for the differentiation of adipose-derived mesenchymal stem cells. Sec14L6 directly binds to ACSL3, a known LD biogenesis factor, which facilitates the association of Sec14L6 with LD surface. Furthermore, we identify PGRMC1, an ER membrane protein, as an adaptor that recruits Sec14L6 to the ER, specifying a role for Sec14L6 at ER-LD interface. Targeted lipidomics revealed profound dysregulation of PIP homeostasis: residual LDs from Sec14L6-KO cells exhibited aberrant accumulation of PI4P and PI(4,5)P2, concomitant with a reduction of these PIPs within the ER compartment. In vitro assays demonstrate that Sec14L6 preferentially transports phosphoinositide-4-phosphate (PI4P) and PI(4,5)P2. Sec14L6 knockout (KO) severely impaired nascent LD formation, a defect rescued by wild-type Sec14L6 but not by lipid-transfer-deficient mutants. Our study identified Sec14L6 as a new factor that regulates PIP homeostasis and biogenesis of LDs via lipid transfer at ER-LD interface.

Sigma Non-Opioid Intracellular Receptor 1 (SigmaR1) is a member of the sigma family of receptors that interacts with a variety of psychotomimetic drugs and has been involved in a wide range of cellular and physiological functions. Despite its increasing importance in human physiology and disease, the subcellular localization of SigmaR1 and its molecular function remain poorly defined. Using endogenous tagging and cell fractionation, we show that SigmaR1 is a type II integral ER membrane protein that is specifically enriched at ER sheet. A short region at the N-terminus of SigmaR1 is sufficient and required for its ER-sheet localization. Importantly, our biochemical studies demonstrate that SigmaR1 directly interacts with several key components of the transposon complex including TRAPα and Nicalin via the N-terminal region. Interestingly, we found that the β-barrel structure at the C-terminal of SigmaR1 binds to phosphatidylcholine. SigmaR1 knockout systematically impaired cellular protein and lipid homeostasis, resulting in an accumulation of lipid droplets in hepatocytes. Collectively, we propose that SigmaR1 is a lipid-binding component of the translocon complex, which may contribute to the broad and vital roles of SigmaR1 in physiology and disease.

VPS13B/COH1 is the only known causative factor for Cohen syndrome, an early-onset autosomal recessive developmental disorder with intellectual inability, developmental delay, joint hypermobility, myopia, and facial dysmorphism as common features, but the molecular basis of VPS13B/COH1 in pathogenesis remains largely unclear. Here, we identify Sec23 interacting protein (Sec23IP) at the ER exit site (ERES) as a VPS13B adaptor that recruits VPS13B to ERES–Golgi interfaces. VPS13B interacts directly with Sec23IP via the VPS13 adaptor binding domain (VAB), and the interaction promotes the association between ERES and the Golgi. Disease-associated missense mutations of VPS13B–VAB impair the interaction with Sec23IP. Knockout of VPS13B or Sec23IP blocks the formation of tubular ERGIC, an unconventional cargo carrier that expedites ER-to-Golgi transport. In addition, depletion of VPS13B or Sec23IP delays ER export of procollagen, suggesting a link between procollagen secretion and joint laxity in patients with Cohen disease. Together, our study reveals a crucial role of VPS13B–Sec23IP interaction at the ERES–Golgi interface in the pathogenesis of Cohen syndrome.

In metazoans the endoplasmic reticulum (ER) undergoes extensive remodeling during the cell cycle. The endosomal sorting complexes required for transport (ESCRT) protein CHMP7 coordinates ESCRT-III dependent nuclear envelope reformation during mitotic exit. However, potential roles of ER-associated CHMP7 at non-mitotic stages remain unclear. Here we discovered a new role of CHMP7 in mediating three-way ER and ER-mitochondrial membrane contact sites (MCSs). We showed that CHMP7 localizes to multiple cellular membranes including the ER, mitochondrial-associated membranes (MAMs) and the outer mitochondrial membrane (OMM) via its N-terminal membrane-binding domain. CHMP7 undergoes dynamic assembly at three-way ER junctions and ER-mitochondrial MCSs through hydrophobic interactions among α helix-1 and α helix-2 of the C-terminal CHMP-like domain, which was required for tethering different organelles in vivo. Furthermore, CHMP7 mediates the formation of three-way ER junctions in parallel with Atlastins (ATLs). Importantly, CHMP7 also regulates ER-mitochondrial interactions and its depletion affects mitochondrial division independently of ESCRT complex. Taken together, our results suggest a direct role of CHMP7 in the formation of the ER contacts in interphase.

Double lumen endobronchial ventilation in McKeown esophagectomy is common for esophageal cancer. In spite of most patients could be extubated immediately after surgery under adequate multimodal analgesia, still some patients require extended mechanical ventilation or airway support post-surgery because of pain or difficult respiration. The present study reported a novel challenge for McKeown esophagectomy with discontinuous spontaneous ventilating anesthesia by the laryngeal mask. Three esophageal cancer patients underwent McKeown esophagectomy under discontinuous spontaneous ventilating anesthesia with local and regional analgesia and appropriate sedation. Two of them were accomplished under non-intubated video-assisted thoracoscopic surgery (NIVATS), and then, the abdominal and neck surgery was managed under laryngeal mask airway with appropriate muscle relaxation. One patient was endured high PetCO₂ level, and converted to regular double lumen endobronchial intubation for safety. However, from the two successful cases, we still proved that the discontinuous spontaneous ventilating anesthesia achieved the same anesthetic effect as bronchial intubation under general anesthesia for McKeown esophagectomy, which reduced the postoperative pharyngeal discomfort, might be beneficial to the patients for enhanced recovery after surgery (ERAS).

Membrane contact sites (MCSs) between the endoplasmic reticulum (ER) and late endosomes/lysosomes (LE/lys) are emerging as critical hubs for diverse cellular events, and changes in their extents are linked to severe neurological diseases. While recent studies show that the synaptotagmin-like mitochondrial-lipid-binding (SMP) domain-containing protein PDZD8 may mediate the formation of ER–LE/lys MCSs, the cellular functions of PDZD8 remain largely elusive. Here, we attempt to investigate the lipid transfer activities of PDZD8 and the extent to which its cellular functions depend on its lipid transfer activities. In accordance with recent studies, we demonstrate that PDZD8 is a protrudin (ZFYVE27)-interacting protein and that PDZD8 acts as a tether at ER–LE/lys MCSs. Furthermore, we discover that the SMP domain of PDZD8 binds glycerophospholipids and ceramides both in vivo and in vitro, and that the SMP domain can transport lipids between membranes in vitro. Functionally, PDZD8 is required for LE/lys positioning and neurite outgrowth, which is dependent on the lipid transfer activity of the SMP domain. This article has an associated First Person interview with the first author of the paper.