A new class mutation of low density lipoprotein receptor with altered carbohydrate chains

Maintaining order: COG complex controls Golgi trafficking, processing, and sorting

The conserved oligomeric Golgi (COG) complex, a multisubunit tethering complex of the CATCHR (complexes associated with tethering containing helical rods) family, controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle targeting within the Golgi. In humans, COG defects lead to severe multisystemic diseases known as COG-congenital disorders of glycosylation (COG-CDG). The COG complex both physically and functionally interacts with all classes of molecules maintaining intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, and vesicular coats. Here, we review our current knowledge of COG-related trafficking and glycosylation defects in humans and model organisms, and analyze possible scenarios for the molecular mechanism of the COG orchestrated vesicle targeting.

Site-specific O-glycosylation of members of the low-density lipoprotein receptor superfamily enhances ligand interactions

The low-density lipoprotein receptor (LDLR) and related receptors are important for the transport of diverse biomolecules across cell membranes and barriers. Their functions are especially relevant for cholesterol homeostasis and diseases, including neurodegenerative and kidney disorders. Members of the LDLR-related protein family share LDLR class A (LA) repeats providing binding properties for lipoproteins and other biomolecules. We previously demonstrated that short linker regions between these LA repeats contain conserved O-glycan sites. Moreover, we found that O-glycan modifications at these sites are selectively controlled by the GalNAc-transferase isoform, GalNAc-T11. However, the effects of GalNAc-T11-mediated O-glycosylation on LDLR and related receptor localization and function are unknown. Here, we characterized O-glycosylation of LDLR-related proteins and identified conserved O-glycosylation sites in the LA linker regions of VLDLR, LRP1, and LRP2 (Megalin) from both cell lines and rat organs. Using a panel of gene-edited isogenic cell line models, we demonstrate that GalNAc-T11-mediated LDLR and VLDLR O-glycosylation is not required for transport and cell-surface expression and stability of these receptors but markedly enhances LDL and VLDL binding and uptake. Direct ELISA-based binding assays with truncated LDLR constructs revealed that O-glycosylation increased affinity for LDL by ∼5-fold. The molecular basis for this observation is currently unknown, but these findings open up new avenues for exploring the roles of LDLR-related proteins in disease.

COG6 interacts with a subset of the Golgi SNAREs and is important for the Golgi complex integrity

Vesicular tethers and SNAREs are two key protein components that govern docking and fusion of intracellular membrane carriers in eukaryotic cells. The conserved oligomeric Golgi (COG) complex has been specifically implicated in the tethering of retrograde intra-Golgi vesicles. Using yeast two-hybrid and co-immunoprecipitation approaches, we show that the COG6 subunit of the COG complex is capable of interacting with a subset of Golgi SNAREs, namely STX5, STX6, GS27 and SNAP29. Interaction with SNAREs is accomplished via the universal SNARE-binding motif of COG6. Overexpression of COG6, or its depletion from cells, disrupts the integrity of the Golgi complex. Importantly, COG6 protein lacking the SNARE-binding domain is deficient in Golgi binding, and is not capable of inducing Golgi complex fragmentation when overexpressed. These results indicate that COG6-SNARE interactions are important for both COG6 localization and Golgi integrity.

Role of the conserved oligomeric Golgi (COG) complex in protein glycosylation

The Golgi apparatus is a central hub for both protein and lipid trafficking/sorting and is also a major site for glycosylation in the cell. This organelle employs a cohort of peripheral membrane proteins and protein complexes to keep its structural and functional organization. The conserved oligomeric Golgi (COG) complex is an evolutionary conserved peripheral membrane protein complex that is proposed to act as a retrograde vesicle tethering factor in intra-Golgi trafficking. The COG protein complex consists of eight subunits, distributed in two lobes, Lobe A (Cog1-4) and Lobe B (Cog5-8). Malfunctions in the COG complex have a significant impact on processes such as protein sorting, glycosylation, and Golgi integrity. A deletion of Lobe A COG subunits in yeasts causes severe growth defects while mutations in COG1, COG7, and COG8 in humans cause novel types of congenital disorders of glycosylation. These pathologies involve a change in structural Golgi phenotype and function. Recent results indicate that down-regulation of COG function results in the resident Golgi glycosyltransferases/glycosidases to be mislocalized or degraded.

Polyoma middle T antigen or v-src desensitizes human epidermal growth factor receptor function and interference by a monensin-resistant mutation in mouse Balb/3T3 cells

Epidermal growth factor (EGF)-induced down-regulation of its receptor is an obligatory pathway for cellular regulation of EGF-specific receptor (EGF-R) in normal and malignant cells. BNER4 cells are mouse Balb/3T3 cells transfected with the human EGF-R complementary DNA (cDNA). Polyoma middle T antigen-transfectants of BNER4, B4/MT-2, B4/MT-13, B4/MT-23, and B4/MT-24, showed diminished down-regulation of cell surface human EGF-R in response to EGF relative to the parental BNER4 cells. Also, the v-src-transfectants B4/SRC-13 and B4/SRC-24 showed much less down-regulation than BNER4 cells, whereas H-ras-transfectants of BNER4, B4/RAS-24 and B4/RAS-25, showed EGF-induced down-regulation of the cell surface EGF-R similar to that of BNER4. EGF induced DNA synthesis more than 20-fold in BNER4, but induced only about a 1.5- to 6-fold increase in the middle T antigen- and v-src-transfectants. EGF-Rs of the middle T antigen-transfectants were metabolically stable in the presence of EGF in comparison with their parental BNER4 cells. EGF-Rs of BNER4 cells degraded with half-lives of about 2 h in the presence of EGF, but those of the middle T antigen transformants were found to be highly stabilized in the presence of EGF. On the other hand, transfection with polyoma middle T antigen (MTAg) cDNA causes malignant transformation of Balb/3T3 cells, but not its monensin (an ionophoric antibiotic)-resistant mutant MO-5 cells, which have no significant EGF binding activity. Transfection of human EGF-R cDNA into MO-5 leads to the expression of high levels of human EGF-R in MNER31. Unlike the polyoma MTAg transfectants of BNER4, EGF-R in polyoma MTAg cDNA-transfectants into MNER31, M31/MT-13 and M31/MT-14, were down-regulated to levels similar to those of their parental MNER31. Exposure to EGF induced a more than 10-fold increase in DNA synthesis of quiescent BNER4, MNER31, M31/MT-13, and M31/MT-14 cells. Polyoma middle T antigen or v-src appears to modulate EGF-induced down-regulation of EGF-R, possibly through interaction of the receptor with the viral oncogenes, and this interaction may be altered in the mutant.

Rapid turnover of low density lipoprotein receptor in human monocytic THP-1 cells

We examined whether human monocyte-derived macrophages had low density lipoprotein (LDL) receptors with a short life span. The human monocytic leukemia cell line, THP-1, was highly differentiated when treated with phorbol ester. LDL receptors degraded rapidly with half-lives of 3-4 h in THP-1 cells before phorbol ester treatment. During the transition into monocytic cells, expression of the LDL receptor gene was not affected. However, relative degradation rates of LDL receptors normalized by those of cellular total proteins were about twice as fast in phorbol ester-treated THP-1 cells compared to untreated cells.

Monensin-resistant LLC-PK1 cell mutants are affected in recycling of the adenylate cyclase-stimulating vasopressin V2-receptor

The ionophore monensin was found to markedly reduce the rate of return of vasopressin V2-receptors to the membrane following down-regulation with [Arg8]vasopressin (AVP), as well as hormone dissociation (unloading) from cells following ligand binding and internalization in LLC-PK1 renal epithelial cells. Monensin-resistant LLC-PK1 mutants were isolated and characterized for V2-receptor recycling. Whilst the MN-41 mutant appeared to be impaired in [3H]AVP internalization, the MN-11 and MN-21 mutants exhibited parental V2-receptor binding and internalization, but markedly impaired receptor recycling subsequent to ligand-dependent receptor down-regulation. Unloading subsequent to ligand binding and internalization at 37 degrees C was also much slower in the mutants either at 37 degrees C or 23 degrees C. In contrast, unloading subsequent to binding at 23 degrees C, or to binding at 37 degrees C in the presence of NH4Cl, was comparable in LLC-PK1 and mutant cells implying the active nature of the recycling process impaired in the mutants. The mutations conferring resistance to monesin thus concomitantly impaired V2-receptor recycling in the mutants. Results argue for a monensin-sensitive endosomal/lysosomal pathway for the renal V2-receptor, representing the first such report for an adenylate cyclase stimulating receptor.

The dysfunctional LDL receptor in a monensin-resistant mutant of Chinese hamster ovary cells lacks selected O-linked oligosaccharides

The Chinese hamster ovary (CHO) cell line Monr31, which is resistant to the cytotoxic ionophore monensin, produces a receptor for the low density lipoprotein (LDL) that has a lowered binding affinity for LDL and is approximately 5 kDa smaller in size than the receptor from parental CHO cells. It has been proposed that the reduced size and affinity for LDL are associated with a reduced level of O-glycosylation of Ser/Thr residues in the receptor. To examine this possibility in more detail, both parental CHO and Monr31 cells were metabolically radiolabeled with [3H]glucosamine, and the labeled LDL receptors were purified by immunoprecipitation and identified by SDS-PAGE-fluorography. The Ser/Thr-linked oligosaccharides in the receptors from both parental CHO and Monr31 cells are mono- and desialylated species having the common core structure Gal beta 1-3GalNAc. The receptor from Monr31 cells, however, contains about one-third fewer Ser/Thr-linked oligosaccharides than the receptor from parental CHO cells. Analysis of the glycopeptides derived from the Monr31 cell LDL receptors indicates that they contain Ser/Thr-linked oligosaccharides only in the clustered domain and are missing Ser/Thr-linked oligosaccharides in the unclustered regions of the protein. Additionally, analysis of a human LDL receptor lacking the domain for attachment of the clustered Ser/Thr-linked oligosaccharides and expressed in both parental CHO and Monr31 cells indicated that the truncated human receptor from Monr31 cells is devoid of Ser/Thr-linked oligosaccharides. In contrast, the truncated human receptor produced by parental CHO cells contains Ser/Thr-linked oligosaccharides contributing approximately 5 kDa to its apparent size. Collectively, these results demonstrate that the LDL receptor produced by the Monr31 cells contains Ser/Thr-linked oligosaccharides in the clustered domain but is missing Ser/Thr-linked oligosaccharides in the unclustered, NH2-terminal domains of the receptor.

Increased cytotoxicity of ricin in a putative Golgi-defective mutant of Chinese hamster ovary cell

We have studied the cytotoxicity of ricin in a monensin-resistant mutant (MonR-31) of Chinese hamster ovary (CHO) cell line which is presumably altered in Golgi functions/structures. The cytotoxicity of ricin was increased in MonR-31 mutant cells compared with that in its parental CHO cells. In wild-type CHO cells, the cytotoxicity of ricin was enhanced by HN4Cl, bafilomycin A1, or nigericin. The enhancement of ricin cytotoxicity by these compounds was greatly reduced in MonR-31 mutant cells. Brefeldin A (BFA), which disrupts the structure of the Golgi apparatus, inhibits the cytotoxicity of ricin in both CHO and MonR-31 cells. We have also examined the effects of glycosylation inhibitors and the removal of high mannose oligosaccharide chains in ricin on the ricin hypersensitivity in MonR-31 cells. The hypersensitivity of MonR-31 cells to ricin is apparently not due to any difference in glycosylation between CHO and MonR-31 cells or in the processing of oligosaccharides on ricin by the target cells. Nigericin at low concentration (10 nM), which has no effect on the cytotoxicity of diphtheria toxin, enhances the ricin cytotoxicity, but inhibits the modeccin cytotoxicity. Our results suggest that important step(s) in the intoxication process of CHO cells by ricin and modeccin take place in the Golgi region.

Rapid turnover of low-density lipoprotein receptor by a non-lysosomal pathway in mouse macrophage J774 cells and inhibitory effect of brefeldin A

The low-density lipoprotein (LDL) receptor of molecular mass 155 kDa was expressed on the cell surface of cultured mouse macrophage J774 cells. The conversion rate of precursor to mature form of LDL receptor in J774 cells was comparable to that in mouse fibroblast L cells. The half-life of the LDL receptor of J774 cells was about 2 h, that of L cells was about 11 h. The rapid degradation of LDL receptor was not significantly inhibited by the lysosomotropic agents, chloroquine and NH4Cl, nor by the thiol-protease inhibitors leupeptin and E-64. By contrast, incubation at 18 degrees C retarded the degradation of LDL receptor. Treatment of J774 cells with brefeldin A, an inhibitor of membrane transport between the endoplasmic reticulum and the Golgi apparatus, inhibited the rapid turnover of the LDL receptor. Even after a 9-h chase in the presence of brefeldin A, LDL receptor 5-10 kDa smaller than the normal mature form was found to be stable. Rapid turnover of the LDL receptor in the macrophages appeared to occur after exit from the Golgi apparatus, possibly during transport of the LDL receptor to the plasma membrane.

Lectin-resistant CHO cells: selection of seven new mutants resistant to ricin

In attempts to isolate new CHO glycosylation mutants, selection protocols using plant lectins that bind galactose residues of cell surface carbohydrates were applied to mutagenized CHO populations. The lectins were used alone or in combination to obtain seven ricin-resistant phenotypes. Each mutant had distinctive properties compared with previously described ricin-resistant CHO cells. One of the new phenotypes was dominant in somatic cell hybrids, and the others were recessive. Complementation analyses between related lectin-resistant (LecR) phenotypes indicated that each new isolate represented a novel genotype. Five of the mutants had properties typical of new CHO glycosylation mutants. The remaining two mutants were not readily categorized. Although they did not appear to be ricin-internalization or protein-synthesis mutants, they also did not display the marked alterations in sensitivity to several lectins of different sugar specificity expected for glycosylation mutants. The seven new LecR mutants described in these studies brings the total number of different LecR CHO mutants isolated by this and other laboratories to about 40. Criteria for identifying new LecR mutations in CHO cells are discussed.

Insulin receptor and altered glucose transport in a monensin-resistant mutant of Chinese hamster ovary cell

A monensin-resistant mutant Monr-31, derived from Chinese hamster ovary (CHO) cell line, has been shown to have a reduced number of insulin receptors and a reduction in glucose uptake in response to insulin. We have further investigated the possibility that altered glucose uptake in Monr-31 cells is related to an alteration in the activity of the insulin receptor. Uptake of glucosamine, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose in Monr-31 cells was one-half to one-third that of CHO cells. The cellular content of the glucose transporter in Monr-31 was reduced to about one-third that of CHO as assayed by use of an antiglucose transporter antibody. After transfection with the human insulin receptor cDNA, we obtained clones CIR-0 from CHO, and MIR-2 and MIR-15 from Monr-31; CIR-0 expressed a tenfold higher level of the insulin-binding activity than did CHO, and MIR-2 and MIR-15 expressed a 20-fold higher level than did Monr-31. Glucose uptake in both CHO and CIR-0 was significantly enhanced by exogenous insulin, but not in Monr-31, MIR-2, and MIR-15. The beta-subunits of insulin receptor in CHO, CIR-0, Monr-31, and MIR-2 were similarly phosphorylated. The decreased glucose transport activity in Monr-31 cells is discussed in relation to the absence or presence of insulin receptor expression.