Variant APOL1 protein in plasma associates with larger particles in humans and mouse models of kidney injury

BACKGROUND

Genetic variants in apolipoprotein L1 (APOL1), a protein that protects humans from infection with African trypanosomes, explain a substantial proportion of the excess risk of chronic kidney disease affecting individuals with sub-Saharan ancestry. The mechanisms by which risk variants damage kidney cells remain incompletely understood. In preclinical models, APOL1 expressed in podocytes can lead to significant kidney injury. In humans, studies in kidney transplant suggest that the effects of APOL1 variants are predominantly driven by donor genotype. Less attention has been paid to a possible role for circulating APOL1 in kidney injury.

METHODS

Using liquid chromatography-tandem mass spectrometry, the concentrations of APOL1 were measured in plasma and urine from participants in the Seattle Kidney Study. Asymmetric flow field-flow fractionation was used to evaluate the size of APOL1-containing lipoprotein particles in plasma. Transgenic mice that express wild-type or risk variant APOL1 from an albumin promoter were treated to cause kidney injury and evaluated for renal disease and pathology.

RESULTS

In human participants, urine concentrations of APOL1 were correlated with plasma concentrations and reduced kidney function. Risk variant APOL1 was enriched in larger particles. In mice, circulating risk variant APOL1-G1 promoted kidney damage and reduced podocyte density without renal expression of APOL1.

CONCLUSIONS

These results suggest that plasma APOL1 is dynamic and contributes to the progression of kidney disease in humans, which may have implications for treatment of APOL1-associated kidney disease and for kidney transplantation.

Tandem Mass Spectrometry-Based Amyloid Typing Using Manual Microdissection and Open-Source Data Processing

OBJECTIVES

Standard implementations of amyloid typing by liquid chromatography-tandem mass spectrometry use capabilities unavailable to most clinical laboratories. To improve accessibility of this testing, we explored easier approaches to tissue sampling and data processing.

METHODS

We validated a typing method using manual sampling in place of laser microdissection, pairing the technique with a semiquantitative measure of sampling adequacy. In addition, we created an open-source data processing workflow (Crux Pipeline) for clinical users.

RESULTS

Cases of amyloidosis spanning the major types were distinguishable with 100% specificity using measurements of individual amyloidogenic proteins or in combination with the ratio of λ and κ constant regions. Crux Pipeline allowed for rapid, batched data processing, integrating the steps of peptide identification, statistical confidence estimation, and label-free protein quantification.

CONCLUSIONS

Accurate mass spectrometry-based amyloid typing is possible without laser microdissection. To facilitate entry into solid tissue proteomics, newcomers can leverage manual sampling approaches in combination with Crux Pipeline and related tools.

Small volume retinol binding protein measurement by liquid chromatography-tandem mass spectrometry

BACKGROUND

The measurement of plasma concentrations of retinol binding protein is a component of nutritional assessment in neonatal intensive care. However, serial testing in newborns is hampered by the limited amount of blood that can be sampled. Limitations are most severe with preterm infants, for whom close monitoring may be most important.

METHODS

We developed an assay to quantify retinol binding protein using trypsin digestion and liquid chromatography-tandem mass spectrometry, which requires a serum or plasma volume of 5 µl. Additionally, we validated the method according to current recommendations and performed comparison with a standard nephelometry platform in clinical use.

RESULTS

The assay demonstrated linearity from below 1 mg/dL (0.48 µM) to more than 20 mg/dL (9.7 µM), and an imprecision of 11.8% at 0.43 mg/dL (0.21 µM). The distribution of results observed with the new method was different when compared with nephelometry.

CONCLUSION

Liquid chromatography-tandem mass spectrometry facilitated testing a smaller sample volume, thereby increasing the ability to monitor key nutritional markers in premature infants. The differences in results compared with a commercially-available nephelometric assay revealed questionable results for lower concentrations by immunoassay.

Development and Validation of a Novel LC-MS/MS Assay for C-Peptide in Human Serum

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Quantification of C-peptide without an antibody or multidimensional chromatography.

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High-throughput method with good comparability to reference measurement procedure.

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Proteolysis improves limit of detection over intact C-peptide.

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Glu-C is an important proteolytic enzyme for targeted proteomic workflows.

Introduction

 C-peptide is used as a marker of endogenous insulin secretion in the assessment of residual β-cell function in diabetes and in the diagnostic workup of hypoglycemia. Previously developed LC-MS/MS methods to quantify serum concentrations of C-peptide have monitored intact peptide, which ionizes poorly. As a result, methods have leveraged immunoaffinity enrichment or two-dimensional chromatography. In this study, we aimed to use proteolysis during sample preparation to enhance the sensitivity of traditional LC-MS/MS.

Methods

 Due to the absence of arginine and lysine residues in C-peptide, we utilized Glu-C as the proteolytic enzyme in the method. After protein precipitation using acetonitrile and solid phase extraction with mixed anion exchange, lower molecular weight polypeptides were reduced, alkylated, and proteolyzed. The two amino-terminal peptide fragments, EAEDLQVGQVE and LGGGPGAGSLQPLALE, were monitored using multiple reaction monitoring in positive ion mode (Acquity ULPC-Xevo TQ-S, Waters). The former peptide was used for quantification and the latter for quality assurance.

Results

 Glu-C was determined to be a reliable proteolytic enzyme with monotonic digestion kinetics. The assay was linear between 0.1 and 15 ng/mL and had a lower limit of quantification of 0.06 ng/mL. Total imprecision was 7.7 %CV and long-term imprecision at 0.16 ng/mL was 10.0%. Spike-recovery experiments demonstrated a mean recovery of 98.2 % (± 9.1 %) and the method compared favorably with a commercially available immunoassay and a reference measurement procedure.

Conclusion

 Protein precipitation with solid phase extraction and proteolysis with Glu-C is a robust sample preparation method for quantification of C-peptide in human serum by LC-MS/MS.

Vitamin D in human serum and adipose tissue after supplementation

BACKGROUND

Serum 25-hydroxyvitamin D [25(OH)D] concentration is an indicator of vitamin D exposure, but it is also influenced by clinical characteristics that affect 25(OH)D production and clearance. Vitamin D is the precursor to 25(OH)D but is analytically challenging to measure in biological specimens.

OBJECTIVES

We aimed to develop and validate a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantification of vitamins D3 and D2 in serum and to explore the potential of circulating vitamin D as a biomarker of exposure in supplementation trials.

METHODS

The method was validated using guideline C62-A from the Clinical and Laboratory Standards Institute and was applied in 2 pilot clinical trials of oral vitamin D3 supplementation. Pilot study 1 included 22 adults randomly assigned to placebo or 2000 IU/d. Blood was collected at baseline, 1, 3, 6, and 12 mo. Pilot study 2 included 15 adults randomly assigned to 2000 or 4000 IU/d. Blood and subcutaneous (SUBQ) adipose tissue were collected at baseline and 3 mo.

RESULTS

In study 1, mean change (baseline to 3 mo) in serum vitamin D3 was -0.1 ng/mL in the placebo group and 6.8 ng/mL in the 2000 IU/d group (absolute difference: 6.9; 95% CI: 4.5, 9.3 ng/mL). In study 2, mean change (baseline to 3 mo) in serum vitamin D3 was 10.4 ng/mL in the 2000 IU/d group and 22.2 ng/mL in the 4000 IU/d group (fold difference: 2.15; 95% CI: 1.40, 3.37). Serum and adipose tissue vitamin D3 concentrations were correlated, and the dose-response of vitamin D3 in adipose mirrored that in serum.

CONCLUSIONS

We validated a sensitive, robust, and high-throughput LC-MS/MS method to quantify vitamins D3 and D2 in serum. Serum and SUBQ adipose tissue vitamin D3 concentrations increased proportionally to dose with 3 mo of daily supplementation.These trials were registered at clinicaltrials.gov as NCT00552409 (pilot study 1) and NCT01477034 (pilot study 2).