Merete Albrechtsen

IT University of Copenhagen, København, Capital Region, Denmark

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Publications (34)150.56 Total impact

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    ABSTRACT: Arbuscular mycorrhizal fungi (AMF) have a key role in plant phosphate (Pi) uptake by their efficient capture of soil phosphorus (P) that is transferred to the plant via Pi transporters in the root cortical cells. The activity of this mycorrhizal Pi uptake pathway is often associated with down-regulation of Pi transporter genes in the direct Pi uptake pathway. As the total Pi taken up by the plant is determined by the combined activity of mycorrhizal and direct pathways, it is important to understand the interplay between these, in particular the actual activity of the pathways. To study this interplay we modulated the delivery of Pi via the mycorrhizal pathway in Pisum sativum by two means: 1) Partial down-regulation by virus induced gene silencing of PsPT4, a putative Pi transporter gene in the mycorrhizal pathway. This resulted in decreased fungal development in roots and soil and led to reduced plant Pi uptake. 2) Changing the percentage of AMF-colonized root length by using non-, half-mycorrhizal or full-mycorrhizal split root systems. The combination of split roots, use of (32) P and (33) P isotopes and partial silencing of PsPT4 enabled us to show that the expression of PsPT1, a putative Pi transporter gene in the direct pathway, was negatively correlated with increasing mycorrhizal uptake capacity of the plant, both locally and systemically. However, transcript changes in PsPT1 were not translated into corresponding, systemic changes in actual direct Pi uptake. Our results suggest that AMF have a limited long-distance impact on the direct pathway.
    Physiologia Plantarum 02/2013; 149(2). DOI:10.1111/ppl.12030 · 3.26 Impact Factor
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    ABSTRACT: Inorganic phosphate (Pi) is an easily accessible form of phosphorus for plants. Plant Pi uptake is usually limited however by slow Pi diffusion through the soil which strongly adsorps phosphate species. Plants have developed mechanisms to increase Pi availability. There are also abiotic (phosphate level) and biotic (e.g., mycorrhizal) factors regulating the expression of Pi-responsive genes. Transcription factors binding to the promoters of Pi-responsive genes activate different pathways of Pi transport, distribution, and homeostasis maintenance. Pi metabolism involves not only functional proteins but also microRNAs and other non-coding RNAs.
    Frontiers in Plant Science 03/2012; 3:58. DOI:10.3389/fpls.2012.00058 · 3.95 Impact Factor
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    ABSTRACT: Gene silencing through RNA interference (RNAi) has revolutionized the study of gene function, particularly in non-model insects. However, in Lepidoptera (moths and butterflies) RNAi has many times proven to be difficult to achieve. Most of the negative results have been anecdotal and the positive experiments have not been collected in such a way that they are possible to analyze. In this review, we have collected detailed data from more than 150 experiments including all to date published and many unpublished experiments. Despite a large variation in the data, trends that are found are that RNAi is particularly successful in the family Saturniidae and in genes involved in immunity. On the contrary, gene expression in epidermal tissues seems to be most difficult to silence. In addition, gene silencing by feeding dsRNA requires high concentrations for success. Possible causes for the variability of success in RNAi experiments in Lepidoptera are discussed. The review also points to a need to further investigate the mechanism of RNAi in lepidopteran insects and its possible connection to the innate immune response. Our general understanding of RNAi in Lepidoptera will be further aided in the future as our public database at http://insectacentral.org/RNAi will continue to gather information on RNAi experiments
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    ABSTRACT: Gene silencing vectors based on Barley stripe mosaic virus (BSMV) are used extensively in cereals to study gene function, but nearly all studies have been limited to genes expressed in leaves of barley and wheat. However since many important aspects of plant biology are based on root-expressed genes we wanted to explore the potential of BSMV for silencing genes in root tissues. Furthermore, the newly completed genome sequence of the emerging cereal model species Brachypodium distachyon as well as the increasing amount of EST sequence information available for oat (Avena species) have created a need for tools to study gene function in these species. Here we demonstrate the successful BSMV-mediated virus induced gene silencing (VIGS) of three different genes in barley roots, i.e. the barley homologues of the IPS1, PHR1, and PHO2 genes known to participate in Pi uptake and reallocation in Arabidopsis. Attempts to silence two other genes, the Pi transporter gene HvPht1;1 and the endo-β-1,4-glucanase gene HvCel1, in barley roots were unsuccessful, probably due to instability of the plant gene inserts in the viral vector. In B. distachyon leaves, significant silencing of the PHYTOENE DESATURASE (BdPDS) gene was obtained as shown by photobleaching as well as quantitative RT-PCR analysis. On the other hand, only very limited silencing of the oat AsPDS gene was observed in both hexaploid (A. sativa) and diploid (A. strigosa) oat. Finally, two modifications of the BSMV vector are presented, allowing ligation-free cloning of DNA fragments into the BSMV-γ component. Our results show that BSMV can be used as a vector for gene silencing in barley roots and in B. distachyon leaves and possibly roots, opening up possibilities for using VIGS to study cereal root biology and to exploit the wealth of genome information in the new cereal model plant B. distachyon. On the other hand, the silencing induced by BSMV in oat seemed too weak to be of practical use. The new BSMV vectors modified for ligation-free cloning will allow rapid insertion of plant gene fragments for future experiments.
    Plant Methods 11/2010; 6(1):26. DOI:10.1186/1746-4811-6-26 · 2.59 Impact Factor
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    ABSTRACT: Gene silencing through RNA interference (RNAi) has revolutionized the study of gene function, particularly in non-model insects. However, in Lepidoptera (moths and butterflies) RNAi has many times proven to be difficult to achieve. Most of the negative results have been anecdotal and the positive experiments have not been collected in such a way that they are possible to analyze. In this review, we have collected detailed data from more than 150 experiments including all to date published and many unpublished experiments. Despite a large variation in the data, trends that are found are that RNAi is particularly successful in the family Saturniidae and in genes involved in immunity. On the contrary, gene expression in epidermal tissues seems to be most difficult to silence. In addition, gene silencing by feeding dsRNA requires high concentrations for success. Possible causes for the variability of success in RNAi experiments in Lepidoptera are discussed. The review also points to a need to further investigate the mechanism of RNAi in lepidopteran insects and its possible connection to the innate immune response. Our general understanding of RNAi in Lepidoptera will be further aided in the future as our public database at http://insectacentral.org/RNAi will continue to gather information on RNAi experiments.
    Journal of insect physiology 11/2010; 57(2):231-45. DOI:10.1016/j.jinsphys.2010.11.006 · 2.50 Impact Factor
  • M Dinesen · M Lundmark · M Albrechtsen
    Archives of Virology 07/2009; 154(7):1173-5. DOI:10.1007/s00705-009-0405-x · 2.28 Impact Factor
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    ABSTRACT: RNA-directed RNA polymerases (RDRs) play crucial roles in the RNA silencing response of plants by enhancing and maintaining silencing signals. At least two members of the RDR group, namely RDR1 and RDR6, are implicated in defence against plant viruses. RDRs have so far only been characterized in dicot species. In this report, we identified and characterized HvRDR1, HvRDR2 and HvRDR6 genes in the monocot plant barley (Hordeum vulgare). We analysed their expression under various biotic and abiotic stresses including fungal and viral infections, salicylic acid treatment as well as during plant development. The different classes and subclasses of barley RDRs displayed contrasting expression patterns during pathogen challenge and development suggesting their involvement in specific regulatory pathways. Their response to heat and salicylic acid treatment suggests a conserved pattern of expression of these genes between monocot and dicot plant species. The existence of two HvRDR1 and two HvRDR6 genes suggests an evolutionary selection for specialization in response to biotic and abiotic stresses after gene duplication.
    Biochimica et Biophysica Acta 05/2009; 1789(5):375-85. DOI:10.1016/j.bbagrm.2009.03.003 · 4.66 Impact Factor
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    M Bruun-Rasmussen · C T Madsen · E Johansen · M Albrechtsen
    Archives of Virology 02/2008; 153(1):223-6. DOI:10.1007/s00705-007-1057-3 · 2.28 Impact Factor
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    ABSTRACT: Virus-induced gene silencing (VIGS) can be used as a powerful tool for functional genomics studies in plants. With this approach, it is possible to target most genes and downregulate the messenger (m)RNA in a sequence-specific manner. Barley stripe mosaic virus (BSMV) is an established VIGS vector for barley and wheat; however, silencing using this vector is generally transient, with efficient silencing often being confined to the first two or three systemically infected leaves. To investigate this further, part of the barley Phytoene desaturase (PDS) gene was inserted into BSMV and the resulting photobleaching in infected barley plants was used as a reporter for silencing. In addition, downregulation of PDS mRNA was measured by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR). Using fragments of PDS ranging from 128 to 584 nucleotides in BSMV, we observed that insert length influenced stability but not efficiency of VIGS. Silencing was transient in most cases; however, the decrease in PDS mRNA levels measured by qRT-PCR began earlier and lasted longer than the photobleaching. Occasionally, silencing persisted and could be transmitted through seed as well as via mechanical inoculation, although large parts of the insert had been lost from the virus vector. The instability of the insert, observed consistently throughout our experiments, offers an explanation for the transient nature of silencing when using BSMV as a VIGS vector.
    Molecular Plant-Microbe Interactions 12/2007; 20(11):1323-31. DOI:10.1094/MPMI-20-11-1323 · 4.46 Impact Factor
  • Bodil Jørgensen · Merete Albrechtsen
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    ABSTRACT: Induction of post-transcriptional gene silencing (PTGS) by transgenes can be exploited in the genetic engineering of plants for virus resistance, altered lipid or polysaccharide composition, delayed flowering, reduced toxin or allergen content, and many other desired traits. Transformation with constructs based on virus coat protein (CP) genes or other viral genes has been successfully used to engineer PTGS-mediated virus resistance into a large number of crop plants and some transgenic lines have been commercially exploited. However the discovery that plant viruses encode suppressors of gene silencing has raised concerns that virus infection of crop plants might reverse the new silencing-based traits. Most studies of virus suppression of silencing have used model systems based on silencing of reporter genes. A few studies have analysed the effects of virus infections on plants with genetically engineered virus resistance based on either a simple sense or an inverted repeat construct. We decided to use genetically engineered virus resistance in potato as a model system for further studies of the effect of virus infection on genetically engineered traits. We present for the first time a comparison of simple and inverted repeat constructs with respect to resistance stability upon challenge with another virus. We found that silencing induced by an inverted repeat construct persisted after virus infection where silencing induced by a simple sense construct failed. Furthermore, even with a simple sense construct the virus-induced suppression of silencing was partial and transient, suggesting that silencing based traits can be agronomically highly useful.
    Molecular Breeding 04/2007; 19(4):371-376. DOI:10.1007/s11032-006-9068-4 · 2.28 Impact Factor
  • Birgit Otzen Petersen · Merete Albrechtsen
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    ABSTRACT: RNA silencing is a sequence-specific RNA degradation mechanism found in most eukaryotes, where small cleavage products (siRNAs) of double stranded RNA (dsRNA) mediate silencing of genes with sequence identity to the dsRNA inducer. In several systems, silencing has been found to spread from the dsRNA inducer sequence into upstream or downstream regions of the target RNA, a phenomenon termed transitive silencing. In nematodes, silencing spreads only in the 3'-5' direction along the target mRNA by siRNAs serving as primers for cRNA synthesis by RNA-dependent RNA polymerase. In plants, transitive silencing is seen in both directions suggesting that at least some cRNA synthesis occurs by un-primed initiation at the 3' end of mRNAs. Replicating plant viruses trigger an RNA silencing defence response that degrades the viral RNA, thus tempering the virus infection. Likewise, fragments of plant genes inserted into a virus will become targets for degradation, leading to virus-induced gene silencing (VIGS) of the homologous plant mRNAs. We have analyzed the spreading of gene silencing in VIGS experiments using a transgene and two endogenous genes as targets. In Nicotiana benthamiana plants expressing a beta-glucuronidase (GUS) transgene, a Potato virus X vector carrying a 5' fragment of the GUS gene induced silencing which spread to downstream regions of the transgene mRNA including the 3'-untranslated region. Conversely, silencing induced by a 3' fragment spread only for a limited distance in the 3'-5' direction. Silencing induced by a central GUS gene fragment spread only into downstream regions. Similar analyses using the endogenous plant genes, magnesium chelatase subunit I (ChlI) and an RNase L inhibitor homologue (RLIh), revealed no spreading along target sequences. This implies that transitive silencing in plants occurs by un-primed cRNA synthesis from the 3' end of targeted (transgene) transcripts, and not by siRNA-primed cRNA synthesis.
    Plant Molecular Biology 08/2005; 58(4):575-83. DOI:10.1007/s11103-005-7307-4 · 4.07 Impact Factor
  • Birgit Otzen Petersen · Bodil Jørgensen · Merete Albrechtsen
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    ABSTRACT: Based on the sequences of mammalian RNase L inhibitors (RLI), homologues were identified in Arabidopsis thaliana and Nicotiana plant species. RLI homologue (RLIh) cDNAs were cloned from Nicotiana benthamiana and Nicotiana tabacum and used to induce post-transcriptional gene silencing (PTGS) in Nicotiana species. Downregulation of RLIh in N. benthamiana plants by virus-induced gene silencing had severe effects on plant morphology and plant growth but did not affect the susceptibility of the plant to infection by Potato virus Y (PVY). Upon transformation of N. benthamiana and N. tabacum with an inverted repeat of RLIh only one transformant, X#2.1, displayed an abnormal phenotype characterized by reduced growth and distorted leaves with white spots. Northern blotting revealed the presence of RLIh-specific small interfering RNAs in X#2.1 but not in 14 other transformants with a normal phenotype. Based on our observations we propose that RLIh has an essential role in plant growth and development.
    Plant Science 12/2004; 167(6):1283-1289. DOI:10.1016/j.plantsci.2004.06.030 · 4.11 Impact Factor
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    ABSTRACT: Target regions for posttranscriptional silencing of transgenes often reside in the 3' region of the coding sequence, although there are exceptions. To resolve if the target region is determined by the gene undergoing silencing rather than by the structure of the transgene loci or the plant genetic background, we have performed detailed analyses of target regions in three spontaneously beta-glucuronidase (GUS) silencing tobacco lines of different origin. From quantitative cosuppression experiments, we show that the main target region in all three tobacco lines is found within the 3' half of the GUS coding region but upstream of the last 200 nt. The quantities of small (21-25 nt) RNAs homologous to 5' or 3' regions of the GUS coding sequence were found to correlate approximately with the target strength of the corresponding regions. These results suggest that transgene locus structure and plant genetic background are not major determinants of silencing target regions. We also show that virus-induced gene silencing (VIGS) of GUS in Nicotiana benthamiana is induced equally effectively with Potato virus X carrying either the 5' or 3' third of the GUS coding region. This indicates that both regions can act as efficient inducers as well as targets of posttranscriptional silencing, although the 3' region is the predominant target region in the spontaneously silencing transgenic plant lines examined. Finally, we investigated spreading of the target region in the N. benthamiana plants undergoing VIGS. Surprisingly, only evidence for spreading of the target region in the 5'-3' direction was obtained. This finding may help explain why the majority of target regions examined to date lie within the 3' region of transgenes.
    RNA 09/2002; 8(8):1034-44. DOI:10.1017/S1355838202026080 · 4.62 Impact Factor
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    Lilli Sander · Robin Child · Peter Ulvskov · Merete Albrechtsen · Bernhard Borkhardt
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    ABSTRACT: The oilseed rape (Brassica napus) endo-polygalacturonase (endo-PG) RDPG1 is involved in middle lamella breakdown during silique opening. We investigated tissue-specific expression of RDPG1 in transgenic Arabidopsis thaliana. Cellular localization of endo-PG protein in Arabidopsis siliques was determined by immuno-electron microscopy. An Arabidopsis orthologue, ADPG1, was isolated and aligned with the sequence of RDPG1. The proximal 5' sequences as well as introns are largely conserved. Analysis of the histological GUS-staining pattern of two RDPG1 promoter-GUS (beta-glucuronidase) constructs in transgenic Arabidopsis revealed that the conserved proximal part of the 5'-flanking region directs expression in dehiscence zones of siliques and anthers, floral abscission zones and stylar tissues during pollen tube growth, branch points between stems and pedicel and expression associated with the apical meristem of seedlings, while the distal part of the RDPG1 5'-flanking region contains elements involved in vascular-associated expression in petals, cotyledons and roots. Subsequent RT-PCR analysis, on RNA from the corresponding rape tissues, confirms the staining pattern revealed in transgenic Arabidopsis, thereby justifying the use of Arabidopsis as a reliable model system for analysis of oilseed rape regulatory sequences.
    Plant Molecular Biology 08/2001; 46(4):469-79. DOI:10.1023/A:1010619002833 · 4.07 Impact Factor
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    ABSTRACT: Transgenic cv. Folva potato plants expressing the coat protein gene of potato virus Y strain N (PVYN) were produced usingAgrobacterium tumefaciens mediated transformation. Forty independent transformants were selected for resistance screening. Four clones showed complete resistance to mechanical inoculation with all the five PVY isolates tested: the PVYN isolate from which the coat protein gene was derived, two PVYO isolates, and two PVYNTN isolates. Two of the fully resistant clones contained only one copy of the transgene, demonstrating that it is possible by genetic engineering to obtain highly virus resistant potato clones that can also be useful in future breeding programmes.
    Potato Research 05/1996; 39(2):271-282. DOI:10.1007/BF02360919 · 0.84 Impact Factor
  • Helle Hassan · Bernhard Borkhardt · Merete Albrechtsen
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    ABSTRACT: The coat protein from purified particles of pea seedborne mosaic potyvirus (PSbMV) moves in SDS-PAGE with an apparent molecular weight (M(r)) of 36 kDa. However, extracts of PSbMV infected plants prepared with SDS or urea contain PSbMV immunoreactive proteins with apparent M(r) 39 kDa as well as 36 kDa. The low mobility form may be generated from the apparent M(r) 36 kDa form by incubating purified PSbMV particles with healthy plant sap in the presence of denaturing agents. A similar effect is observed with bean yellow mosaic potyvirus, but not with three viruses outside the potyvirus group. Experiments suggest that a soluble plant enzyme is responsible for the conversion, which apparently takes place only in vitro under denaturing conditions. This phenomenon may lead to erroneous conclusions about the M(r) of some viral coat proteins. However, the conversion can be prevented by heat treatment of the plant tissue prior to extraction.
    Journal of Virological Methods 03/1994; 46(2):255-61. DOI:10.1016/0166-0934(94)90108-2 · 1.88 Impact Factor
  • Merete Albrechtsen · Bernhard Borkhardt
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    ABSTRACT: A 45 kD protein (Pro1) derived from the N terminus of the pea seedborne mosaic potyvirus (PSbMV) polyprotein has been detected in extracts of infected pea plants and among in vitro translation products of PSbMV genomic RNA. The genomic region coding for the first 231 amino acids of the PSbMV polyprotein was cloned and expressed in Escherichia coli as a fusion protein with beta-galactosidase. A rabbit antiserum raised against the fusion protein recognized an approximately 45 kD protein in immunoblots of extracts of PSbMV-infected pea leaves that was not present in extracts of healthy leaves. The highest concentration of the 45 kD protein was found in extracts of young leaves, suggesting the protein may be rapidly degraded in vivo. After in vitro translation of PSbMV genomic RNA in a wheat germ extract, the antiserum immunoprecipitated a 45 kD polypeptide as well as some lower molecular weight translation products. On the other hand, an approximately 90 kD polypeptide was immunoprecipitated from in vitro translation products of genomic RNA in a rabbit reticulocyte lysate, corresponding to the combined molecular weights of Pro1 and the helper component predicted from genomic sequence data.
    Virus Genes 02/1994; 8(1):7-13. DOI:10.1007/BF01703597 · 1.84 Impact Factor
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    M G Johnsen · O F Rasmussen · M Albrechtsen · B Borkhardt
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    ABSTRACT: The purpose of this work has been to investigate in vivo transcription and translation products from an open reading frame (ORF) located downstream of the coat protein (CP) cistron on RNA-2 of pea early browning virus (PEBV). This work was initiated as a step towards elucidation of the significance of this putative gene. Sequence data on RNA-2 suggest that a 29,600 Mr (29.6K) protein is translated from the ORF in question. Hybridization with ORF-specific probes on Northern blots with RNA from infected plants showed that PEBV synthesizes a subgenomic RNA encoding CP (RNA-2a) with a size of 3000 nucleotides (nt) and that a putative subgenomic RNA (RNA-2b) encoding the 29.6K protein appears to have a size of 1600 nt. This is 300 nt less than the size predicted from the sequencing data. For antibody production, a cDNA fragment harbouring 85% of the 29.6K ORF was cloned into the pUEX3 expression vector. The resulting plasmid expresses the 29.6K protein as a fusion protein with beta-galactosidase and this protein was used for raising antiserum containing specific anti-29.6K protein antibodies. By using these antibodies on immunoblots it was demonstrated that the 29.6K protein is expressed in infected plants.
    Journal of General Virology 07/1991; 72 ( Pt 6)(6):1223-7. DOI:10.1099/0022-1317-72-6-1223 · 3.53 Impact Factor
  • Merete Albrechtsen · Morten Heide
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    ABSTRACT: High performance liquid chromatography (HPLC) gel filtration has been successfully applied in the purification of elongated and isometric plant viruses. Two different approaches have been tested. In one approach, semi-purified virus particles were dissociated with lithium chloride and the released coat proteins purified by HPLC gel filtration. The purified coat protein was highly immunogenic and gave rise to very specific antisera reacting with intact virus particles as well as with SDS-denatured coat protein monomers. This method is generally applicable only for elongated viruses since many isometric viruses are not dissociated by the lithium chloride treatment. The second approach consisted in gel filtration of native, undissociated virus particles and could be used both with elongated and isometric viruses. Both methods were fast and simple to perform and removed all or most of the contaminating plant proteins as judged by sodium dodecylsulphate gel electrophoresis followed by silver staining or by immunoblotting with antiserum against healthy plant extracts. With both methods the recovery of virus coat protein was about 30% on average.
    Journal of Virological Methods 07/1990; 28(3):245-56. DOI:10.1016/0166-0934(90)90118-Y · 1.88 Impact Factor
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    S C Stocks · M Albrechtsen · M A Kerr
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    ABSTRACT: The expression of the carbohydrate antigen 3-fucosyl-N-acetyl-lactosamine (CD15, LeX) on human neutrophil glycoproteins has been studied by immunoprecipitation and immunoblotting by using monoclonal antibody MC2. The antigen is expressed on membrane glycoproteins of approximate molecular mass 165 and 105 kDa. These glycoproteins include the complement receptor and adhesion molecule, CR3, in which the beta-chain (CD18, 105 kDa) shows much greater expression than the alpha-chain (CD11b, 165 kDa). Most of the 165 kDa CD15 antigen is accounted for by expression on the carcinoembryonic antigen (CEA)-related molecule NCA160. Other members of this family, NCA95, NCA90 and NCA55, which are also found in neutrophils, do not express the CD15 antigen. There is a marked increase in the surface expression of CD15, CR3 and the antigen recognized by anti-CEA antibodies upon activation of neutrophils by the chemotactic peptide N-formylmethionyl-leucylphenylalanine.
    Biochemical Journal 07/1990; 268(2):275-80. DOI:10.1042/bj2680275 · 4.78 Impact Factor

Publication Stats

946 Citations
150.56 Total Impact Points

Institutions

  • 1984–2013
    • IT University of Copenhagen
      København, Capital Region, Denmark
  • 2007–2010
    • Aarhus University
      • Department of Genetics and Biotechnology
      Aarhus, Central Jutland, Denmark
  • 1985–1989
    • University of Dundee
      • School of Medicine
      Dundee, Scotland, United Kingdom
    • Bispebjerg Hospital, Copenhagen University
      København, Capital Region, Denmark