A Chinese hamster ovary cell mutant F2A8 utilizes polyprenol rather than dolichol for its lipid-dependent asparagine-linked glycosylation reactions

A pseudoautosomal glycosylation disorder prompts the revision of dolichol biosynthesis

Dolichol is a lipid critical for N-glycosylation as a carrier for activated sugars and nascent oligosaccharides. It is commonly thought to be directly produced from polyprenol by the enzyme SRD5A3. Instead, we found that dolichol synthesis requires a three-step detour involving additional metabolites, where SRD5A3 catalyzes only the second reaction. The first and third steps are performed by DHRSX, whose gene resides on the pseudoautosomal regions of the X and Y chromosomes. Accordingly, we report a pseudoautosomal-recessive disease presenting as a congenital disorder of glycosylation in patients with missense variants in DHRSX (DHRSX-CDG). Of note, DHRSX has a unique dual substrate and cofactor specificity, allowing it to act as a NAD+-dependent dehydrogenase and as a NADPH-dependent reductase in two non-consecutive steps. Thus, our work reveals unexpected complexity in the terminal steps of dolichol biosynthesis. Furthermore, we provide insights into the mechanism by which dolichol metabolism defects contribute to disease.

Metabolomics profiling reveals new aspects of dolichol biosynthesis in Plasmodium falciparum

The cis-polyisoprenoid lipids namely polyprenols, dolichols and their derivatives are linear polymers of several isoprene units. In eukaryotes, polyprenols and dolichols are synthesized as a mixture of four or more homologues of different length with one or two predominant species with sizes varying among organisms. Interestingly, co-occurrence of polyprenols and dolichols, i.e. detection of a dolichol along with significant levels of its precursor polyprenol, are unusual in eukaryotic cells. Our metabolomics studies revealed that cis-polyisoprenoids are more diverse in the malaria parasite Plasmodium falciparum than previously postulated as we uncovered active de novo biosynthesis and substantial levels of accumulation of polyprenols and dolichols of 15 to 19 isoprene units. A distinctive polyprenol and dolichol profile both within the intraerythrocytic asexual cycle and between asexual and gametocyte stages was observed suggesting that cis-polyisoprenoid biosynthesis changes throughout parasite’s development. Moreover, we confirmed the presence of an active cis-prenyltransferase (PfCPT) and that dolichol biosynthesis occurs via reduction of the polyprenol to dolichol by an active polyprenol reductase (PfPPRD) in the malaria parasite.

MPDU1 regulates CEACAM1 and cell adhesion in vitro and in vivo

N-linked glycosylation (NLG) is a co-translational modification that is essential for the folding, stability, and trafficking of transmembrane (TM) and secretory glycoproteins. Efficient NLG requires the stepwise synthesis and en bloc transfer of a 14-sugar carbohydrate known as a lipid-linked oligosaccharide (LLO). The genetics of LLO biosynthesis have been established in yeast and Chinese hamster systems, but human models of LLO biosynthesis are lacking. In this study we report that Kato III human gastric cancer cells represent a model of deficient LLO synthesis, possessing a homozygous deletion of the LLO biosynthesis factor, MPDU1. Kato III cells lacking MPDU1 have all the hallmarks of a glycosylation-deficient cell line, including altered sensitivity to lectins and the formation of truncated LLOs. Analysis of transcription using an expression microarray and protein levels using a proteome antibody array reveal changes in the expression of several membrane proteins, including the metalloprotease ADAM-15 and the cell adhesion molecule CEACAM1. Surprisingly, the restoration of MPDU1 expression in Kato III cells demonstrated a clear phenotype of increased cell-cell adhesion, a finding that was confirmed in vivo through analysis of tumor xenografts. These experiments also confirmed that protein levels of CEACAM-1, which functions in cell adhesion, is dependent on LLO biosynthesis in vivo. Kato III cells and the MPDU1-rescued Kato IIIM cells therefore provide a novel model to examine the consequences of defective LLO biosynthesis both in vitro and in vivo.

Ethanol-induced impairment in the biosynthesis of N-linked glycosylation

Deficiency in N-linked protein glycosylation is a long-known characteristic of alcoholic liver disease and congenital disorders of glycosylation. Previous investigations of ethanol-induced glycosylation deficiency demonstrated perturbations in the early steps of substrate synthesis and in the final steps of capping N-linked glycans in the Golgi. The significance of the biosynthesis of N-glycan precursors in the endoplasmic reticulum, however, has not yet been addressed in alcoholic liver disease. Ethanol-metabolizing hepatoma cells were treated with increasing concentrations of ethanol. Transcript analysis of genes involved in the biosynthesis of N-glycans, activity assays of related enzymes, dolichol-phosphate quantification, and analysis of dolichol-linked oligosaccharides were performed. Upon treatment of cells with ethanol, we found a decrease in the final N-glycan precursor Dol-PP-GlcNAc(2) Man(9) Glc(3) and in C95- and C100-dolichol-phosphate levels. Transcript analysis of genes involved in N-glycosylation showed a 17% decrease in expression levels of DPM1, a subunit of the dolichol-phosphate-mannose synthase, and an 8% increase in RPN2, a subunit of the oligosaccharyl transferase. Ethanol treatment decreases the biosynthesis of dolichol-phosphate. Consequently, the formation of N-glycan precursors is affected, resulting in an aberrant precursor assembly. Messenger RNA levels of genes involved in N-glycan biosynthesis are slightly affected by ethanol treatment, indicating that the assembly of N-glycan precursors is not regulated at the transcriptional level. This study confirms that ethanol impairs N-linked glycosylation by affecting dolichol biosynthesis leading to impaired dolichol-linked oligosaccharide assembly. Together our data help to explain the underglycosylation phenotype observed in alcoholic liver disease and congenital disorders of glycosylation.

At the membrane frontier: a prospectus on the remarkable evolutionary conservation of polyprenols and polyprenyl-phosphates

Long-chain polyprenols and polyprenyl-phosphates are ubiquitous and essential components of cellular membranes throughout all domains of life. Polyprenyl-phosphates, which include undecaprenyl-phosphate in bacteria and the dolichyl-phosphates in archaea and eukaryotes, serve as specific membrane-bound carriers in glycan biosynthetic pathways responsible for the production of cellular structures such as N-linked protein glycans and bacterial peptidoglycan. Polyprenyl-phosphates are the only form of polyprenols with a biochemically-defined role; however, unmodified or esterified polyprenols often comprise significant percentages of the cellular polyprenol pool. The strong evolutionary conservation of unmodified polyprenols as membrane constituents and polyprenyl-phosphates as preferred glycan carriers in biosynthetic pathways is poorly understood. This review surveys the available research to explore why unmodified polyprenols have been conserved in evolution and why polyprenyl-phosphates are universally and specifically utilized for membrane-bound glycan assembly.

Structure and synthesis of polyisoprenoids used in N-glycosylation across the three domains of life

N-linked protein glycosylation was originally thought to be specific to eukaryotes, but evidence of this post-translational modification has now been discovered across all domains of life: Eucarya, Bacteria, and Archaea. In all cases, the glycans are first assembled in a step-wise manner on a polyisoprenoid carrier lipid. At some stage of lipid-linked oligosaccharide synthesis, the glycan is flipped across a membrane. Subsequently, the completed glycan is transferred to specific asparagine residues on the protein of interest. Interestingly, though the N-glycosylation pathway seems to be conserved, the biosynthetic pathways of the polyisoprenoid carriers, the specific structures of the carriers, and the glycan residues added to the carriers vary widely. In this review we will elucidate how organisms in each basic domain of life synthesize the polyisoprenoids that they utilize for N-linked glycosylation and briefly discuss the subsequent modifications of the lipid to generate a lipid-linked oligosaccharide.

The ins(ide) and out(side) of dolichyl phosphate biosynthesis and recycling in the endoplasmic reticulum

The precursor oligosaccharide donor for protein N-glycosylation in eukaryotes, Glc3Man9GlcNAc(2)-P-P-dolichol, is synthesized in two stages on both leaflets of the rough endoplasmic reticulum (ER). There is good evidence that the level of dolichyl monophosphate (Dol-P) is one rate-controlling factor in the first stage of the assembly process. In the current topological model it is proposed that ER proteins (flippases) then mediate the transbilayer movement of Man-P-Dol, Glc-P-Dol, and Man5GlcNAc(2)-P-P-Dol from the cytoplasmic leaflet to the lumenal leaflet. The rate of flipping of the three intermediates could plausibly influence the conversion of Man5GlcNAc(2)-P-P-Dol to Glc3Man(9)GlcNAc(2)-P-P-Dol in the second stage on the lumenal side of the rough ER. This article reviews the current understanding of the enzymes involved in the de novo biosynthesis of Dol-P and other polyisoprenoid glycosyl carrier lipids and speculates about the role of membrane proteins and enzymes that could be involved in the transbilayer movement of the lipid intermediates and the recycling of Dol-P and Dol-P-P discharged during glycosylphosphatidylinositol anchor biosynthesis, N-glycosylation, and O- and C-mannosylation reactions on the lumenal surface of the rough ER.

Identification of Schizosaccharomyces pombe prenol as dolichol-16,17

The identity of the prenol involved in N-linked glycosylation in the fission yeast Schizosaccharomyces pombe was unknown. In order to determine the identity of the prenol, S. pombe cells were incubated with a metabolic precursor of prenol, tritiated mevalonolactone. The cells incorporated only a modest amount of label, about 1000 dpm per million cells, into base-stable lipid and only 1% of that radioactivity was incorporated into prenol. We found by normal phase silica HPLC and more directly by the lack of reactivity with MnO2 that the labeled lipid was predominantly dolichol, not polyprenol. Reverse phase HPLC demonstrated that in S. pombe dolichol ranged between 14 and 18 isoprene units with dolichol-16,17 being the most abundant prenol. This dolichol is of an intermediate length, between the dolichol of S. cerevisiae and that of mammalian cells.

Synthesis of dolichol in a polyprenol reductase mutant is restored by elevation of cis-prenyl transferase activity

CHB11-1-3 is a glycosylation mutant of Chinese hamster ovary (CHO) cells, isolated by screening mutagenized cells for those with decreased intracellular lysosomal enzyme activity [C. W. Hall et al. (1986) Mol. Cell. Biochem. 72, 35-45]. CHB11-1-3 synthesizes the lipid polyprenol, the metabolic precursor of dolichol, rather than dolichol, indicating a defect in polyprenol reductase. This defect was demonstrated previously in Lec9 CHO mutants, and cell fusion experiments confirmed that CHB11-1-3 is a member of this complementation group. A revertant of CHB11-1-3, CHBREV, isolated for its ability to grow at 39 degrees C, synthesizes dolichol at near-normal levels. CHBREV is probably a second-site revertant, because it synthesizes three to four times as much polyprenol as CHB11-1-3 and exhibits a similar elevation in the specific activity of cis-prenyl transferase. This higher activity appears to reflect an increase in enzyme molecules rather than the presence of an activator or absence of an inhibitor. These results suggest that CHB11-1-3 is a "K(m)" mutant, because synthesis of higher amounts of the substrate of polyprenol reductase obviates the defect.

A partial deficiency of dehydrodolichol reduction is a cause of carbohydrate-deficient glycoprotein syndrome type I

Carbohydrate-deficient glycoprotein (CDG) syndrome type I is a congenital disorder that involves the underglycosylation of N-glycosylated glycoproteins (Yamashita, K., Ideo, H., Ohkura, T., Fukushima, K., Yuasa, I., Ohno, K., and Takeshita, K. (1993) J. Biol. Chem. 268, 5783-5789). In an effort to further elucidate the biochemical basis of CDG syndrome type I in our patients, we investigated the defect in the multi-step pathway for biosynthesis of lipid-linked oligosaccharides (LLO) by the metabolic labeling method using [3H]glucosamine, [3H]mannose, and [3H]mevalonate. The LLO levels in synchronized cultures of fibroblasts from these patients were severalfold lower than those in control fibroblasts in the S phase, and the oligosaccharides released from LLO showed the same structural composition, Glc1 approximately 3.Man9.GlcNAc.GlcNAc, in the case of both the patients and controls. The amount of [3H]mannose incorporated into mannose 6-phosphate, mannose 1-phosphate, and GDP-mannose was greater in fibroblasts from these patients than in the control fibroblasts in the G1 period, although the ratios of these acidic mannose derivatives as indicated by the relative levels of radioactivity were the same for the two types of fibroblasts. Furthermore, upon metabolic labeling with [3H]mevalonate, the level of [3H]dehydrodolichol in fibroblasts from these patients increased in the S phase, and the levels of [3H]dolichol and [3H]dolichol-PP oligosaccharides concomitantly decreased, although the chain length distribution of the respective dolichols and dehydrodolichols was the same in the two types of fibroblasts. These results indicate that the conversion of dehydrodolichol to dolichol is partially defective in our patients and that the resulting loss of dolichol leads directly to underglycosylation.

Activation of GlcNAc-P-P-dolichol synthesis by mannosylphosphoryldolichol is stereospecific and requires a saturated alpha-isoprene unit

Exogenous mannosylphosphoryldolichol (Man-P-Dol) has previously been shown to stimulate UDP-GlcNAc:dolichyl phosphate N-acetylglucosamine 1-phosphate transferase (GPT1), the enzyme catalyzing the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-Dol). To define the structural specificity of the mannolipid-mediated activation of GPT1, the ability of a variety of mannosylphosphorylisoprenols to stimulate GlcNAc-lipid biosynthesis in microsomal preparations from retinas of the embryonic chick has been tested. For these comparisons several Man-P-isoprenols were synthesized enzymatically and chemically. The catalytic efficiency of activation expressed as the Vmax/Ka ratio was substantially higher for Man-P-Dol95 than for mannosylphosphorylpolyprenol95 (Man-P-Poly95), demonstrating that the saturated alpha-isoprene unit of the dolichyl moiety influences the mannolipid-enzyme interaction. The degree of activation increased with chain length and hydrophobicity of the dolichyl moiety when Man-P-dolichols containing 2, 11, and 19 isoprene units were evaluated. A strict stereospecificity was exhibited as beta-Man-P-Dol95 provided a 100-fold greater stimulation than the corresponding alpha-stereoisomer. The recognition of the saturated alpha-isoprene unit, the influence of chain length, and the strict stereospecificity of the interaction between beta-Man-P-Dol and GPT1 extend the description of the mannolipid-enzyme interaction and provide strong new evidence that Man-P-Dol levels can influence the rate of GlcNAc-P-P-Dol synthesis.

Lec15 cells transfer glucosylated oligosaccharides to protein

B4-2-1 cells (Lec15 cells) are Chinese hamster ovary cells deficient in mannosylphosphoryldolichol synthase activity. They synthesize the truncated lipid intermediate Man5GlcNAc2-P-P-dolichol rather than the Glc3Man9GlcNAc2-P-P-dolichol synthesized by wild-type cells. In this report we present evidence that these cells did synthesize glucosylated Man5GlcNAc2-P-P-dolichol, but this species represented only a minor fraction of the labeled oligosaccharide-lipid. On the other hand, glucosylated oligosaccharides were a major species transferred to protein in these cells, showing that in vivo, glucosylated oligosaccharides are preferentially transferred to protein. The truncated oligosaccharides found in B4-2-1 cells were removed from the protein by N-glycanase treatment, since they were resistant to both endo-beta-N-acetylglucosaminidase H and F activity. B4-2-1 cells processed the glucosylated, truncated oligosaccharides transferred to G protein of vesicular stomatitis virus, leading to infectious virus.

An FM3A mutant, G258, with a mutation that affects both cell growth and oligosaccharide-lipid synthesis

I have previously isolated a temperature-sensitive FM3A mutant (G258) defective in asparagine-linked glycosylation. G258 shows not only the temperature sensitivity for cell growth, but also temperature sensitivity for synthesis of oligosaccharide-lipid (Nishikawa, Y. (1984) J. Cell. Physiol. 119, 260-266). In the present study, I isolated revertants for cell growth from G258 cells. All three growth revertants also showed reversion on the synthesis of oligosaccharide-lipid. These results imply that the temperature sensitivity for oligosaccharide-lipid synthesis of G258 couples with the temperature sensitivity for cell growth of the mutant. A possible mechanism of the coupling between impaired oligosaccharide-lipid synthesis and growth arrest of G258 cells at 39 degrees C is discussed.

Complementing mutant alleles define three loci involved in mannosylation of Man5-GlcNAc2-P-P-dolichol in Chinese hamster ovary cells

Dolichol-linked oligosaccharides consisting of two N-acetylglucosamine, nine mannose, and three glucose residues (Glc3Man9GlcNAc2) are transferred to proteins that contain the consensus sequence Asn-X-Ser/Thr. This transfer occurs upon protein import into the lumen of the endoplasmic reticulum. An intermediate in the biosynthesis of the Glc3Man9GlcNAc2 lipid-linked oligosaccharide contains two GlcNAc and five mannose residues. This intermediate serves as a substrate for further mannosylation and glucosylation before transfer to protein. The addition of the sixth mannose residue to this intermediate requires the enzyme mannosyltransferase VI and the mannose donor, mannose-P-dolichol. Several different CHO cell line mutants that fail to efficiently catalyze this transfer have been described. In this report, we examine seven independent mutant cell lines with various biochemical phenotypes and demonstrate that all can be assigned to one of three genetic complementation groups. One mutation affects mannose-P-dolichol biosynthesis (Lec15), three affect dolichol phosphate biosynthesis (Lec9), and three appear to affect the functional orientation of enzyme substrates (PIR).

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.