Christopher G. Burd

In organelle biogenesis pathways, the Golgi apparatus plays a central role in synthesizing and distributing complex sphingolipids, glycerolipids, and cholesterol, which form the membranes of organelles. The concept of 'lipid flow' describes the circulation of lipids throughout the endomembrane system, mediated by vesicle-based carriers and lipid transfer proteins that directly transfer lipids at sites of organelle-organelle contact. The synthesis and trafficking of lipids are closely interconnected, as they undergo successive enzymatic transformations that begin in the endoplasmic reticulum and are completed in the Golgi apparatus before being distributed to other organelles. A phosphatidylinositol 4-phosphate signaling network that converges on the trans cisterna and trans Golgi network compartments regulates lipid flow to, though, and out of the Golgi apparatus, acting as a rheostat that tunes the rates and specificities of lipid trafficking by controlling phosphatidylinositol 4-phosphate levels in Golgi membranes. The integration of glycolipid and glycoprotein processing reactions within the Golgi with lipid sorting and trafficking pathways suggests that the Golgi apparatus serves as a lipid-based sorting station at the heart of organelle biogenesis pathways.

Homeostatic pathways maintain the lipid composition of organelle membranes, and mechanistic links between lipid sensing, synthesis, and trafficking are lacking. Acute depletion of cell cholesterol elicits an increase in the rate of very-long-chain (VLC) sphingomyelin synthesis in the Golgi apparatus, thereby promoting cholesterol retention in the plasma membrane. Stable isotope metabolic analyses and lipid trafficking assays showed that the increase in VLC-sphingomyelin results from an increase in the rate of coatomer II–dependent trafficking of VLC-ceramide from the endoplasmic reticulum to the Golgi apparatus. An integral membrane protein of the coatomer II network, cTAGE5, is required for endoplasmic reticulum-to-Golgi trafficking of ceramide and cTAGE5 overexpression caused herniations of the endoplasmic reticulum network that entrapped a synthetic ceramide analog to which cTAGE5 could be photochemically cross-linked. We propose that cTAGE5 is a ceramide sensor for export of VLC-ceramide from the endoplasmic reticulum exit site.

Within cellular membranes, sphingomyelin is associated with cholesterol and this complex facilitates homeostatic regulation of membrane viscosity. Acute cholesterol depletion increases the synthesis of very-long-chain (VLC) sphingomyelin, but a link between lipid sensing and sphingolipid synthesis is lacking. Using sphingolipid metabolic flux analysis, we observed that VLC-ceramide, the precursor to VLC complex sphingolipids that are produced in the Golgi apparatus, was rapidly consumed after cholesterol depletion, while synthesis of long-chain sphingolipids was unaffected. Sphingolipid trafficking assays showed that cholesterol depletion enhances VLC-Ceramide trafficking from the endoplasmic reticulum to the Golgi apparatus. Changes in the sizes of coatomer II ER exit sites were correlated with increased VLC-Ceramide trafficking and concomitant increase in sphingomyelin. Depletion of Sec16A, a component of the COPII network, abolished VLC-SM synthesis. This study reveals ER-to-Golgi trafficking of VLC-Ceramide as a key regulatory node in organelle membrane homeostasis pathways. Summary In cellular membranes, sphingomyelin is associated with cholesterol. Metabolic flux analysis of sphingolipid metabolism showed that synthesis rate of sphingomyelin, but not ceramide, was increased after depletion of cholesterol due increased rate of COPII-dependent ER-to-Golgi transport of ceramide.

Bridge-like lipid transfer protein family member 2 (BLTP2) is an evolutionary conserved protein with unknown function(s). The absence of BLTP2 in Drosophila melanogaster results in impaired cellular secretion and larval death, while in mice ( Mus musculus), it causes preweaning lethality. Structural predictions propose that BLTP2 belongs to the repeating β-groove domain-containing (also called the VPS13) protein family, forming a long tube with a hydrophobic core, suggesting that it operates as a lipid transfer protein (LTP). We establish BLTP2 as a negative regulator of ciliogenesis in RPE-1 cells based on a strong genetic interaction with WDR44, a gene that also suppresses ciliogenesis. Like WDR44, BLTP2 localizes to membrane contact sites involving the endoplasmic reticulum and the tubular endosome network in HeLa cells and that BLTP2 depletion enhanced ciliogenesis in RPE-1 cells grown in serum-containing medium, a condition where ciliogenesis is normally suppressed. This study establishes human BLTP2 as a putative LTP acting between tubular endosomes and ER that regulates primary cilium biogenesis.

Bridge-like lipid transfer protein family member 2 (BLTP2) is an evolutionary conserved protein with unknown function(s). The absence of BLTP2 in Drosophila melanogaster results in impaired cellular secretion and larval death, while in mice ( Mus musculus ), it causes preweaning lethality. Structural predictions propose that BLTP2 belongs to the repeating β-groove domain-containing (also called the VPS13) protein family, forming a long tube with a hydrophobic core, suggesting that it operates as a lipid transfer protein (LTP). We establish BLTP2 as a negative regulator of ciliogenesis in RPE-1 cells based on a strong genetic interaction with WDR44 , a gene that also suppresses ciliogenesis. Like WDR44, BLTP2 localizes to membrane contact sites involving the endoplasmic reticulum and the tubular endosome network in HeLa cells and that BLTP2 depletion enhanced ciliogenesis by serum-fed RPE-1 cells, a condition where ciliogenesis is normally suppressed. This study establishes human BLTP2 as a putative lipid transfer protein acting between tubular endosomes and ER that regulates primary cilium biogenesis. Significance statement: We show the involvement of an ER-localized bridge-like lipid transfer protein, BLTP2, in ciliogenesis and establish that BLTP2 is enriched at organelle-organelle membrane contact sites involving the endoplasmic reticulum (ER) and the tubular endosome network (TEN). These results implicate, for the first time, the involvement of bulk lipid transfer between the ER and TEN in regulating ciliogenesis.

Sphingomyelin plays a key role in cellular cholesterol homeostasis by binding to and sequestering cholesterol in the plasma membrane. We discovered that synthesis of very long chain (VLC) sphingomyelins is inversely regulated by cellular cholesterol levels; acute cholesterol depletion elicited a rapid induction of VLC-sphingolipid synthesis, increased trafficking to the Golgi apparatus and plasma membrane, while cholesterol loading reduced VLC-sphingolipid synthesis. This sphingolipid–cholesterol metabolic axis is distinct from the sterol responsive element binding protein pathway as it requires ceramide synthase 2 (CerS2) activity, epidermal growth factor receptor signaling, and was unaffected by inhibition of protein translation. Depletion of VLC-ceramides reduced plasma membrane cholesterol content, reduced plasma membrane lipid packing, and unexpectedly resulted in the accumulation of cholesterol in the cytoplasmic leaflet of the lysosome membrane. This study establishes the existence of a cholesterol–sphingolipid regulatory axis that maintains plasma membrane lipid homeostasis via regulation of sphingomyelin synthesis and trafficking.

The sorting and trafficking of lipids between organelles gives rise to a dichotomy of bulk membrane properties between organelles of the secretory and endolysosome networks, giving rise to two “membrane territories” based on differences in lipid-packing density, net membrane charge, and bilayer leaflet asymmetries. The cellular organelle membrane dichotomy emerges from ER-to-PM anterograde membrane trafficking and the synthesis of sphingolipids and cholesterol flux at the trans-Golgi network, which constitutes the interface between the two membrane territories. Organelle homeostasis is maintained by vesicle-mediated retrieval of bulk membrane from the distal organelles of each territory to the endoplasmic reticulum or plasma membrane and by soluble lipid transfer proteins that traffic particular lipids. The concept of cellular membrane territories emphasizes the contrasting features of organelle membranes of the secretory and endolysosome networks and the essential roles of lipid-sorting pathways that maintain organelle function.

Sphingomyelin is a dominant sphingolipid in mammalian cells. Its production in the trans-Golgi traps cholesterol synthesized in the ER to promote formation of a sphingomyelin/sterol gradient along the secretory pathway. This gradient marks a fundamental transition in physical membrane properties that help specify organelle identify and function. We previously identified mutations in sphingomyelin synthase SMS2 that cause osteoporosis and skeletal dysplasia. Here, we show that SMS2 variants linked to the most severe bone phenotypes retain full enzymatic activity but fail to leave the ER owing to a defective autonomous ER export signal. Cells harboring pathogenic SMS2 variants accumulate sphingomyelin in the ER and display a disrupted transbilayer sphingomyelin asymmetry. These aberrant sphingomyelin distributions also occur in patient-derived fibroblasts and are accompanied by imbalances in cholesterol organization, glycerophospholipid profiles, and lipid order in the secretory pathway. We postulate that pathogenic SMS2 variants undermine the capacity of osteogenic cells to uphold nonrandom lipid distributions that are critical for their bone forming activity.

The trans-Golgi network coordinates sorting and secretion of proteins and lipids to cellular organelles and the plasma membrane. It is revealed that Sdc1, a heparin sulfated proteoglycan present on basolateral membranes of polarized epithelia, requires the golgin GOPC for basolateral targeting, in contrast to its sorting requirements in nonpolarized cells.

The Golgi functions principally in the biogenesis and trafficking of glycoproteins and lipids. It is compartmentalized into multiple flattened adherent membrane sacs termed cisternae, which each contain a distinct repertoire of resident proteins, principally enzymes that modify newly synthesized proteins and lipids sequentially as they traffic through the stack of Golgi cisternae. Upon reaching the final compartments of the Golgi, the trans cisterna and trans-Golgi network (TGN), processed glycoproteins and lipids are packaged into coated and non-coated transport carriers derived from the trans Golgi and TGN. The cargoes of clathrin-coated vesicles are chiefly residents of endo-lysosomal organelles, while uncoated carriers ferry cargo to the cell surface. There are outstanding questions regarding the mechanisms of protein and lipid sorting within the Golgi for export to different organelles. Nonetheless, conceptual advances have begun to define the key molecular features of cargo clients and the mechanisms underlying their sorting into distinct export pathways, which we have collated in this Cell Science at a Glance article and the accompanying poster.