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Infect Immun, April 1998, p. 1783-1786, Vol. 66, No. 4
St. John's Cardiovascular Research Center,
Division of Infectious Diseases, Department of Medicine,
Harbor-UCLA Research and Education Institute, Torrance, California
905021;
Department of Microbiology
and Immunology, Georgetown University, Washington, D.C.
200072; and
UCLA School of Medicine,
Los Angeles, California 900243
Received 13 November 1997/Returned for modification 8 December
1997/Accepted 15 January 1998
To identify genes encoding adhesins that mediate the binding of
Candida albicans to endothelial cells, a genomic library
from this organism was constructed and used to transform
Saccharomyces cerevisiae. These transformed organisms were
screened for adherence to endothelial cells, and a highly adherent
clone was identified. The adherence of this clone to endothelial cells
was over 100-fold greater than that of control S. cerevisiae transformed with the empty plasmid. This clone also
exhibited enhanced adherence to epithelial cells. The C. albicans gene contained within this clone was found to be
ALS1. These results indicate that ALS1 may
encode a candidal adhesin.
The opportunistic pathogen
Candida albicans disseminates hematogenously in susceptible
hosts. During the process of hematogenous dissemination, it is likely
that blood-borne organisms must first adhere to and then penetrate
through the endothelial cell lining of the vasculature to invade the
target organs. For this reason, the adherence of C. albicans to the vascular endothelium is likely a pivotal step in
the initiation of a hematogenously disseminated infection.
Characterizing the adhesins that mediate the binding of C. albicans to endothelial cells is important for understanding the
mechanism by which this process occurs and developing therapeutic strategies to block it.
Although considerable investigative effort has been devoted to
identifying candidal adhesins, it has been difficult to characterize these adhesins at the molecular level. Previously, we used
complementation cloning to identify CAD1/AAF1, a gene from
C. albicans that, when expressed in Saccharomyces
cerevisiae, induces enhanced adherence to endothelial cells and
flocculation in vitro (3). However, the sequence of the
predicted protein encoded by CAD1/AAF1 and the finding that
the CAD1/AAF1 protein does not localize to the plasma membrane or cell
wall indicate that the gene does not encode a cell surface adhesin.
Furthermore, the endothelial cell adherence of homozygous
cad1/AAF1 null mutants of C. albicans is
similar to that of the wild-type parent strain (12). These
results indicate that the protein encoded by CAD1/AAF1 does
not contribute significantly to the adherence of C. albicans to endothelial cells in vitro.
In the present series of investigations, we constructed a genomic
library of C. albicans DNA in an S. cerevisiae expression vector. By screening for adherence to human
endothelial cells, we identified a clone that was highly adherent to
both endothelial and epithelial cells. This clone was found to express
ALS1, a gene that has homology to S. cerevisiae
AG Genomic library construction.
To construct the genomic
library, DNA from C. albicans SC5314 was
mechanically sheared and the ends were made blunt with T4 DNA
polymerase (New England Biolabs, Beverly, Mass.). The DNA fragments
were fractionated by agarose gel electrophoresis, after which fragments
of 3 to 7 kb were pooled and ligated to XhoI adaptors. The
resulting pool of DNA was ligated to XhoI-digested Selection of adherent clones.
To select clones with enhanced
adherence to endothelial cells, S. cerevisiae S150-2B
(leu2 his3 trp1 ura3) was transformed with four pYesR
sublibraries by the method of Gietz et al. (6). The four
pools of yeast, each containing a sublibrary, were grown in minimal
medium (yeast nitrogen base broth without amino acids [Difco, Detroit,
Mich.] supplemented with 80 µg of L-leucine per ml, 60 µg of L-histidine per ml, and 60 µg of
L-tryptophan per ml and containing either galactose,
raffinose, or glucose [each 2%, wt/vol]). Expression of the candidal
genes was induced by incubating the transformants in minimal medium
plus galactose on a rotary shaker overnight at 30°C. The
organisms were harvested by centrifugation, washed twice in Dulbecco's
phosphate-buffered saline (PBS), and then suspended in PBS
containing Mg2+ and Ca2+
(PBS++). Next, a total of 3 × 108 induced
cells was added to confluent monolayers of human umbilical vein
endothelial cells in 100-mm-diameter tissue culture dishes. These
endothelial cells had been isolated and grown by our modification of
the method of Jaffe et al. (2, 9). The S. cerevisiae cells were incubated with the endothelial cells for 30 min at 37°C in 5% CO2, after which the nonadherent
clones were removed by rinsing with warm PBS++ in a
standardized manner. The endothelial cells and adherent clones were
removed from the tissue culture dishes with a cell scraper and then
sonicated briefly to lyse the endothelial cells. The S. cerevisiae cells were then transferred to minimal medium plus
galactose and incubated overnight at 30°C.
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Expression of the Candida albicans Gene
ALS1 in Saccharomyces cerevisiae Induces
Adherence to Endothelial and Epithelial Cells
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ABSTRACT
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1 and is a member of the immunoglobulin gene
superfamily. These results suggest that ALS1 may encode
a candidal adhesin.
-Yes-R (kindly provided by Ronald W. Davis of Stanford University) (1, 11). The ligation mixture was packaged into
phage with the
phage packaging system (Stratagene, La Jolla, Calif.). A total of
1 × 107 independent clones were selected and divided
into 20 sublibraries of 5 × 105 clones each. To
convert the phage library into a pYesR plasmid library, the phages
were transfected into a cre-producing strain of
Escherichia coli, BNN132 (Ronald W. Davis). Plasmid
pYesR is a single-copy vector that contains the S. cerevisiae GAL1 promoter so that expression of the candidal genes
within the library can be induced. Analysis of the plasmids contained
within representative transformants revealed that 95% contained
inserts of candidal DNA with an average size of 4.3 kb.
Testing the adherence induced by pYF-5. After five rounds of selection, 12 clones were chosen and their adherence to endothelial cells was tested and compared to the adherence of control S. cerevisiae transformed with the empty plasmid. One clone that exhibited significantly greater adherence than did the control organism was identified. The plasmid contained within this clone was designated pYF-5. The adherence of this clone to endothelial cells was then determined. It was grown in minimal medium plus galactose to induce expression of the candidal gene in pYF-5. This clone was also grown in minimal medium plus glucose to suppress the expression of this gene. The adherence of this organism grown under these two conditions was compared to that of control organisms that contained the empty plasmid and were grown under identical conditions. In these experiments, C. albicans SC5314 was included as a positive control.
When grown in the presence of galactose, the clone containing pYF-5 exhibited 68-fold greater adherence to endothelial cells than it did when grown in the presence of glucose (P < 0.001 by the Kruskall-Wallace test) (Fig. 1A). Its adherence was also at least 100-fold greater than that of S. cerevisiae that had been transformed with vector alone and grown in either galactose or glucose (P < 0.001 for each comparison). The adherence of this clone was almost twofold greater than that of C. albicans SC5314 (P < 0.001). These findings provide strong evidence that the increased adherence of the clone containing pYF-5 was due to the expression of the candidal gene contained within this clone.
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pYF-5 contained ALS1. We next analyzed the candidal DNA insert contained within pYF-5. Digestion of pYF-5 with XhoI released a single 6-kb insert. This insert was used as a probe for Southern blotting of DNA from C. albicans SC5314 to confirm that the insert was part of the C. albicans genome (data not shown). Next, the 1.1 kb of DNA adjacent to the GAL1 promoter within pYF-5 was sequenced. The beginning of an open reading frame was identified 82 bp downstream from this promoter. Its sequence was identical to that of ALS1, which has previously been isolated by Hoyer et al. (8).
ALS1 is part of the ALS gene family, the members of which are characterized by the presence of conserved tandem repeats (8). Certain members of this gene family have been sequenced, and the 5' ends of some of these genes are reported to have significant homology with ALS1 (7). Therefore, we performed further analyses to confirm that the insert contained within pYF-5 was ALS1 and not another member of the ALS gene family. We first constructed primers based on nucleotides 1 to 22 (primer 1; sense) and 1296 to 1280 (primer 2; antisense) of the 5' end of the published ALS1 sequence (Fig. 2). When used in PCR with pYF-5 as the template, the expected 1.3-kb fragment was amplified. Similarly, PCR amplification using primers from positions 2399 to 2420 (primer 3; sense) and 3786 to 3763 (primer 4; antisense) of the 3' end of the published ALS1 sequence yielded a product that was 1.4 kb, as expected. However, when primers 1 and 4 were used to PCR amplify a fragment from pYF-5, the resulting 4.9-kb product was 1.1 kb longer than was predicted by the published sequence of ALS1 (8). Although the published sequence of ALS1 is 3.8 kb, it has been reported that the size of this gene exhibits strain-to-strain differences in the number of repeats (8). Our PCR results suggest that the additional 1.1 kb within ALS1 obtained from C. albicans SC5314 is due to the presence of additional tandem repeats. Each tandem repeat unit in ALS1 is 108 bp in length; therefore, we estimate that the allele of ALS1 in pYF-5 contains 10 additional tandem repeats (Fig. 2).
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The ALS1 gene product has motifs characteristic of a
cell surface protein.
The predicted ALS1 protein has several
motifs which are characteristic of a protein expressed on the cell
surface (8). First, there appears to be a
glycosylphosphatidylinositol attachment site in its C terminus.
Second, the N terminus contains a region resembling a signal peptide.
Third, the ALS1 protein is predicted to be highly glycosylated. Fourth,
the N and C termini of ALS1 have extensive homology with the
S. cerevisiae AG
1 gene product, a glycoprotein that
mediates cell-to-cell adhesion during mating (10).
Interestingly, the N termini of both the ALS1 and
AG
1 gene products have homology with the immunoglobulin
superfamily (13). This superfamily contains adhesins such as
intercellular adhesion molecules 1, 2, and 3 which mediate the
cell-to-cell attachment of mammalian cells. This similarity to
S. cerevisiae and mammalian adhesins is consistent with
the hypothesis that ALS1 encodes a candidal adhesin. Our
finding that expressing this gene in S. cerevisiae
causes a very large increase in adherence to endothelial and epithelial
cells provides additional support for this hypothesis.
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ACKNOWLEDGMENTS |
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We thank the Perinatal nurses at Harbor-UCLA Medical Center for collecting umbilical cords; Alison Orozco, Toshiko Lamkin, and Michael Mador for helping with tissue culture; and Toyota USA for donating the Olympus phase-contrast microscope used in these studies.
This work was supported in part by Public Health Service grants R01 AI-19990, P01 AI-37194, R29 AI040636, and MO1 RR00425 from the National Institutes of Health.
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FOOTNOTES |
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* Corresponding author. Mailing address: Division of Infectious Diseases, Harbor-UCLA Research and Education Institute, Bldg. RB-2, 1124 West Carson St., Torrance, CA 90502. Phone: (310) 222-6426. Fax: (310) 782-2016. E-mail: Filler{at}AFP76.HUMC.EDU.
Editor: T. R. Kozel
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