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From the Department of Medicine and Department of Physiology and Biophysics, Division of Nephrology, Nephrology Research and Training Center, Comprehensive Cancer Center, and Cell Adhesion and Matrix Research Center, University of Alabama at Birmingham, Birmingham, Alabama; and the Department of Veterans Affairs Medical Center, Birmingham, Alabama
| Abstract |
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and
light chains interacted with THP. These findings were
confirmed in a series of competition studies using a synthetic peptide
that corresponded to the CDR3 region and purified THP and light chains.
Variations in the CDR3 sequence of the light chain affected binding.
Thus, the current studies increase our understanding of the
process of cast formation and provide an opportunity to develop
strategies that may inhibit this interaction and prevent the clinical
manifestations of myeloma kidney.
| Introduction |
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Cast nephropathy represents the most common cause of renal failure in multiple myeloma.7 To initiate cast formation, light chains bind to a specific peptide domain on Tamm-Horsfall protein (THP),8-12 which is synthesized exclusively by cells of the thick ascending limb of the loop of Henle.13,14 Co-aggregation of light chains with THP produces the intraluminal casts that are the prominent feature of myeloma kidney.8 The electrolyte composition of the tubule fluid as well as tubule fluid flow rates and amount of THP8-12 modulate binding. The structure of the light chain plays an important role in association with THP10 and may also promote homotypic aggregation.15 Although myeloma kidney is potentially reversible, prevention of cast formation is the key to controlling the problem. Understanding the protein interactions involved in cast formation represents the initial advance in development of potential treatment strategies designed to prevent myeloma kidney. The current study determined the domain on the light chain involved in binding THP.
| Materials and Methods |
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The yeast two-hybrid system (Matchmaker LexA Two-Hybrid System; Clontech Lab. Inc., Palo Alto, CA) was used initially to detect binding interactions between THP and immunoglobulin light chains. This approach was similar to the original description of the yeast two-hybrid assay,16 but was a LexA-based interaction trap system.17 The host strain in these experiments was Saccharomyces cerevisiae EGY48[p8op-lacZ]. The bait consisted of two fragments of human THP that were obtained by polymerase chain reaction using cDNA that was provided by Genentech, Inc. (South San Francisco, CA). Description of the cloning and characterization of THP has been published.18 All primers used in this study were obtained commercially (Operon Tech. Inc., Alameda, CA). Because there is a single binding domain (amino acid residues 225 to 233) for light chains on THP,10,12 the present study used two fragments of THP that contained this domain. A 787-bp fragment (encoding amino acid residues 148 to 410, termed THP787) was polymerase chain reaction-amplified using 5'-CCGGAATTCCAATGTGGTGGGCAGCTACTT-G-3' as the forward primer and 5'-ACGCTCGAGCTCCACGGAGCTGGGGTCTGTGC-3' as the reverse primer. Underlined sequences in the upstream and downstream primers contained an EcoRI and XhoI site, respectively. A 248-bp fragment (encoding amino acid residues 188 to 270, termed THP248) was created using 5'-CCGGAATTCCGCATGGCCGAGACCTGCGTGC as the forward primer and 5'-ACGCTCGAGCTCCACGGAGCTGGGGTCTGTGC3' as the reverse primer. THP787 and THP248 were each ligated into pBluescript II SK(-) vector (Stratagene, La Jolla, CA) using EcoRI and XhoI and then cloned. Sequences of both products were confirmed using a kit (T7 Sequenase DNA sequencing kit; Amersham Life Science, Inc., Cleveland, OH). THP787 and THP248 were then cloned into the pLexA expression vector using EcoRI and XhoI.
Representative cDNAs encoding immunoglobulin
and
light chains
were generous gifts from Dr. S. Louis Bridges, Jr., at the University
of Alabama at Birmingham, Birmingham, AL. A total of four unique
and seven
light chains were used. Descriptions of these molecules
have been published.19-21
Each cDNA was used as a
template in polymerase chain reactions to generate fragments that were
initially cloned into pBluescript II SK(-) for sequencing and then
inserted in-frame into pB42AD expression vector, using EcoRI
and XhoI. The expressed fusion proteins consisted of the
light chain of interest and the LexA transcriptional activation domain.
Primers and light chains used in these reactions were shown in Tables 1 and 2
. A
series of truncation and deletion mutants of the
light chain,
SSH23, and the
light chain, ITPBLL1, were also created (Figure 1)
. Restricting the 506-bp fragment with
BsiZI that cut the insert in two unique sites, and
subsequently purifying and intramolecularly re-ligating the product
created a truncation mutant of SSH23. To generate the SSH23pep
construct, two 5'-phosphorylated synthetic oligonucleotides,
5'-CCGGAATTCATGCAAGGTACACACTGGCCTCCGCTCACTCTCGAGCGT-3
'and
5'-ACGCTCGAGAGTGAGCGGAGGCCAGTGTGTACCTTGCATGAATTCCGG-3',
were annealed and ligated into pBluescript II SK(-) for sequencing and
then into pLexA, using EcoRI and XhoI. This
sequence encoded a 10-amino acid peptide (MQGTHWPPLT) that corresponded
to the amino acid residues from 94 to 103 on SSH23. ITPBLL1pep, which
encoded a 10 amino acid peptide (QVWDSTSDHY) that corresponded to amino
acid residues 88 to 97 of ITPBLL1, was prepared in similar manner using
5'-phosphorylated
5'-CCGGAATTCCAGGTATGGGATAGTACTAGTGATCATTATCTCGA-GCGT-3'
and
5'-ACGCTCGAGATAATGATCACTAGTACTATCCCATACCTGGAATTCCGG-3'.
Both SSH23pep and ITPBLL1pep corresponded to the
complementarity-determining region (CDR)3 of light chains. Automated
DNA sequencing confirmed proper construction of all plasmids and
authenticity of the sequences.
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To verify expression of the fusion proteins, selected samples were examined using Western blotting. Yeast cells were pelleted, then resuspended in cracking buffer, 8 mol/L urea, 5% sodium dodecyl sulfate, 40 mmol/L Tris-HCl, 0.1 mmol/L ethylenediaminetetraacetic acid, 0.08% ß-mercaptoethanol and a combination of protease inhibitors (Complete; Boehringer Mannheim, Mannheim, Germany) was included. The yeast were disrupted by vigorous shaking for 3 minutes at 5,000 rpm (MiniBeadbeater; Biospec Products Co., Bartlesville, OK) after addition of 425 to 600 µmol/L glass beads (Sigma Chemical Co., St. Louis, MO). Cell debris was removed by centrifugation at 14,000 rpm for 5 minutes at 4°C. Western blot detection of THP and light chains then proceeded in standard manner.23,24 Briefly, the supernatant fractions were boiled briefly, then separated using 8 or 12% polyacrylamide gels and transferred onto nitrocellulose membranes. Membranes were then probed using rabbit anti-human THP antiserum (Biomedical Technologies, Inc., Stoughton, MA) or rabbit anti-human Ig/L-chain type lambda and type kappa (Behringwerke AG, Marburg, Germany).
Purification of Human THP and Immunoglobulin Light Chains
Human THP and light chains were obtained from urine in standard
manner.8-12
THP was purified from urine of a healthy
adult male by precipitation in 0.64 mol/L NaCl, followed by dialysis
and lyophilization. Purified human THP was biotinylated as described
previously,25
using sulfo-N-hydroxysuccinimide
biotin (ImmunoPure Sulfo-NHS-Biotin; Pierce Chemical Co., Rockford,
IL), followed by dialysis against phosphate-buffered saline (PBS) at
4°C for 24 hours to remove free biotin. Sodium dodecyl
sulfate-polyacrylamide gel electrophoresis identified a single band at
100 kd (data not shown). Six different light chains (three
and
three
) were also used in this study and were obtained from patients
who had light chain proteinuria and clinical renal failure. The
patients who donated
2 and
5 had biopsy-proven cast nephropathy.
Patients who donated
5,
3, and
7 had clinical presentations
that were compatible with cast nephropathy, but did not undergo kidney
biopsy. The patient who donated
6 had Waldenströms
macroglobulinemia and progressive renal failure and was also not
biopsied. These light chains were purified from urine by precipitation
using 70% ammonium sulfate, followed by ion-exchange
chromatography.4,5,8,9
Purified light chains were dialyzed
and lyophilized. A single band at
22 kd was identified by sodium
dodecyl sulfate-polyacrylamide gel electrophoresis (data not shown).
Proteins were kept at -20°C until use.
Peptide Competition Experiments
A synthetic peptide was obtained commercially (Research Genetics Inc., Huntsville, AL). MQGTHWPPLT corresponded to the CDR3 region of SSH23. Peptides were purified by high-pressure liquid chromatography; the molecular masses were confirmed by mass spectrometry.
Competition experiments were performed using solid-phase enzyme-linked immunosorbent assay and Western and dot blotting. To perform enzyme-linked immunosorbent assay, wells of microtiter plates were coated with each of the six purified light chains, 0.2 mmol/L in PBS, and incubated overnight at room temperature. Biotinylated human THP, 0.2 µmol/L, was incubated overnight at 4°C alone or with the CDR3 peptide. The concentration of the peptides ranged from 0 to 4 mmol/L. After washing the wells with 0.05% Tween 20 in PBS and blocking with 1% bovine serum albumin in PBS, the pre-incubated, THP/peptide mixtures were added to the wells and incubated at 37°C for 2 hours. After washing, samples were incubated with avidin conjugated with horseradish peroxidase (ImmunoPure avidin, horseradish peroxidase conjugated; Pierce Chemical Co., Rockford, IL), 1:2,000 dilution in PBS. After additional washes, wells were developed using Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) as substrate in citrate-phosphate buffer, pH 4.2. Optical density was determined at 405 nm using a microplate reader (VERSAmax; Molecular Devices Corp., Menlo Pack, CA).
In other competition experiments, 10 µg samples of each of the six light chains were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis using 12% polyacrylamide gels and then transferred onto a nitrocellulose membrane. Twenty µg of yeast extract containing the fusion proteins were dot-blotted directly onto a nitrocellulose membrane. After blocking and washing, the blots were probed with 0.2 µmol/L biotinylated THP or 0.2 µmol/L biotinylated THP that had been pre-incubated with CDR3 peptide, 4 mmol/L, overnight at 4°C. After additional washes, the membranes were incubated with streptavidin-conjugated HRP, 1:10,000 dilution in Tris-buffered saline. The membranes were developed using ECL Western blotting system and Hyperfilm (Amersham Pharmacia Biotech, Piscataway, NJ).
Statistical Analysis
All data were presented as mean ± SE. Significant differences were determined using one-way analysis of variance with standard post hoc testing (Statview, version 5.0; SAS Institute, Inc., Cary, NC). A P value of < 0.05 assigned statistical significance.
| Results |
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Initial experiments ensured that the hybrid proteins that
consisted of the pLexA DNA-binding domain and the two fragments of
human THP did not, by themselves, trans-activate the
reporter genes. Both peptide segments of THP contained the known single
binding site for light chains.10,12
The light chain
constructs that were tested are shown in Tables 1 and 2
. Western
analysis of extracts of representative co-transformed yeast confirmed
expression of fusion proteins that contained THP and light chain
(Figure 1)
. When co-expressed with pLexA-THP248 and pLexA-THP787,
fusion proteins that consisted of the LexA activation domain and both
and
light chains activated both LEU2 and
lacZ reporter genes. Reporter gene activity was strictly
galactose-dependent. There were no interactions among any of the light
chain constructs and pLexA-Lam, which encoded human lamin C protein.
Also, transformation of yeast with pB42AD-T, which encoded the SSV40
large T-antigen, and either pLexA-THP248 or pLexA-THP787 did not
activate either reporter gene. Interactions were further quantified
using a liquid culture assay of ß-galactosidase activity (Table 3)
. Although all of the light chains
interacted with THP, the relative strength of the interactions differed
among the 11 light chains examined. The
VI light chain, ITPBLL75,
showed a low-affinity interaction: yeast transformed with this
construct grew slowly in leucine-deficient medium and possessed low
ß-galactosidase activity. The
IIIa light chain, ITPBLL1,
demonstrated the highest binding affinity among the light chains
tested.
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To identify the domain on the light chain that interacted with
THP, a series of truncated constructs of SSH23 and ITPBLL1 (Figure 2)
were tested for binding to the THP
fragments in vivo. Deletion of the N-terminal portions of
the variable regions of both SSH23 and ITPBLL1 did not abolish
interaction with THP. Removal of the constant homology domains of the
light chains also did not alter binding. In contrast, SSH23
, which
lacked a portion of the third and fourth framework regions and the CDR3
region, did not interact with THP. The two 10-amino acid peptide
sequences that represented the CDR3 portions of both molecules
interacted with THP in vivo.
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Biotinylated human THP bound to six different light chains, three
and three
, which had been purified from patients who had renal
failure and light chain proteinuria. Pre-incubation of the THP with a
synthetic 10-amino acid peptide that represented the CDR3 region of
SSH23 effectively inhibited interaction of THP with the bound light
chains (Figure 3)
. Using enzyme-linked
immunosorbent assay, this peptide inhibited, in a dose-response manner,
binding of THP to these light chains (Figure 3)
. Extracts from yeast
that had been transformed with the various light chain constructs were
dot blotted and then incubated with biotinylated THP to show binding
(Figure 4)
. As anticipated from the
in vivo studies, THP bound to samples that contained SSH23,
SSH23b, SSH23c, and SSH23pep, but did not interact with samples
containing SSH23a and SSH23
. THP also bound to samples that
contained ITPBLL1, ITPBLL1b, and ITPBLL1c, but not extracts from yeast
that expressed ITPBLL1a and ITPBLL1d. These findings confirmed the
in vivo studies described above. Pre-incubation of THP with
the CDR3 peptide resulted in inhibition of binding to these samples.
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| Discussion |
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and
light
chains was the CDR3 region in the variable domain of the molecules. The
yeast two-hybrid findings were confirmed by showing a synthetic peptide
that corresponded to the CDR3 region of SSH23 inhibited binding of THP
to six different light chains from patients who had light chain
proteinuria and clinical manifestations of renal failure. This peptide
also inhibited binding of purified THP to the fusion proteins that were
present in the yeast extracts (Figure 4)
Every light chain contains two globular subunits that are termed
variable and constant homology domains.38
The variable
domain consists of a series of four framework regions that form
irregular ß-pleated sheets that surround a tightly packed hydrophobic
interior.39,40
Three hypervariable segments, termed CDRs,
configure part of the antigen-binding site on the immunoglobulin
molecule.38,41
The CDRs form loop structures and represent
the regions of sequence variability among light
chains.42-44
Thus, although possessing similar
structures, no two light chains are identical. The CDR3 region is
perhaps the most variable portion of the molecule, in part because of
V-J recombination and because of the capability of terminal
deoxynucleotidyl transferase to insert nongermline-encoded nucleotides
in this region.19,20,45
It is therefore interesting that
CDR3 binds THP, although there are similarities in this region among
many
and
light chains. Kabat and colleagues,46
first identified the appearance of certain amino acid residues in CDRs
of heavy and light chains. Mian and associates47
suggested
that amphipathic amino acids, especially tryptophan and tyrosine, are
frequently present in CDR regions, because they permit flexibility to
interact with a wide range of antigens. Thus, although required to
promote antigen binding, these residues also allow cross-reactivity
with other proteins. However, the relative affinities of the light
chains for THP varied, with one of the
light chains, ITPBLL75, in
the present study showing very low affinity. The Kyte-Doolittle
hydropathy plot of the CDR3 region of ITPBLL75 differed from the plots
of SSH23 and ITPBLL1, which were light chains that showed higher
binding affinities for THP. Thus, the entire domain appeared to
modulate the interaction.
In summary, the identification of the CDR3 region as the single binding site of light chains for THP provides new insights into the pathogenesis of cast nephropathy related to multiple myeloma. Differences in the CDR3 region accounts for the variable affinity of light chains for THP, when this process is examined under controlled conditions. Our previous studies, using a rodent model of in vivo cast formation, showed that binding of light chain to THP was required for cast formation.8,9,48 However, other factors modulate this interaction and determine the clinical expression of the disease.9-11 For example, a light chain from a patient who had no clinically evident renal dysfunction bound THP in vitro, albeit at lower affinity. This same light chain did not obstruct the lumen of perfused nephrons of euvolemic rats, but did obstruct nephrons of hydropenic rats.9 Thus, although there is only a general correlation between binding affinity and clinical cast nephropathy, all tested light chains that potentially form casts in vivo also bind THP.8-12,48 Certainly, not all light chains are nephrotoxic. Some patients excrete grams of light chains in the urine and yet do not manifest renal injury clinically.49 The present study demonstrated that the amino acid sequence of the CDR3 region, along with other factors previously reported,8-12 modulates the binding of THP to light chains and subsequent development of clinical manifestations of renal failure. Finally, because the basic mechanism of this process was elucidated, these studies have provided an opportunity to pursue strategies that inhibit this interaction and potentially prevent the severe renal failure that occurs in this setting.
| Acknowledgements |
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| Footnotes |
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Supported by The Office of Research and Development, Medical Research Service, Department of Veterans Affairs, and National Institutes of Health grant (R01 DK46199).
Accepted for publication January 23, 2001.
| References |
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-globulins. J Exp Med 1962, 116:207-227[Abstract]
gene segment utilization on CDR3 length. Mol Med 1998, 4:525-553[Medline]
light chains expressed in patients with rheumatoid arthritis and in normal individuals. J Clin Invest 1995, 96:831-841
II gene segments. Arth Rheum 1992, 35:900-904[Medline]
genes with no or little somatic mutation throughout life. Eur J Immunol 1993, 23:3272-3277[Medline]
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