Optimizing hepcidin measurement with a proficiency test framework and standardization improvement

Objectives

Hepcidin measurement advances insights in pathophysiology, diagnosis, and treatment of iron disorders, but requires analytically sound and standardized measurement procedures (MPs). Recent development of a two-level secondary reference material (sRM) for hepcidin assays allows worldwide standardization. However, no proficiency testing (PT) schemes to ensure external quality assurance (EQA) exist and the absence of a high calibrator in the sRM set precludes optimal standardization.

Methods

We developed a pilot PT together with the Dutch EQA organization Stichting Kwaliteitsbewaking Medische Laboratoriumdiagnostiek (SKML) that included 16 international hepcidin MPs. The design included 12 human serum samples that allowed us to evaluate accuracy, linearity, precision and standardization potential. We manufactured, value-assigned, and validated a high-level calibrator in a similar manner to the existing low- and middle-level sRM.

Results

The pilot PT confirmed logistical feasibility of an annual scheme. Most MPs demonstrated linearity (R2>0.99) and precision (duplicate CV>12.2%), although the need for EQA was shown by large variability in accuracy. The high-level calibrator proved effective, reducing the inter-assay CV from 42.0% (unstandardized) to 14.0%, compared to 17.6% with the two-leveled set. The calibrator passed international homogeneity criteria and was assigned a value of 9.07±0.24 nmol/L.

Conclusions

We established a framework for future PT to enable laboratory accreditation, which is essential to ensure quality of hepcidin measurement and its use in patient care. Additionally, we showed optimized standardization is possible by extending the current sRM with a third high calibrator, although international implementation of the sRM is a prerequisite for its success.

Toward Worldwide Hepcidin Assay Harmonization: Identification of a Commutable Secondary Reference Material

BACKGROUND

Absolute plasma hepcidin concentrations measured by various procedures differ substantially, complicating interpretation of results and rendering reference intervals method dependent. We investigated the degree of equivalence achievable by harmonization and the identification of a commutable secondary reference material to accomplish this goal.

METHODS

We applied technical procedures to achieve harmonization developed by the Consortium for Harmonization of Clinical Laboratory Results. Eleven plasma hepcidin measurement procedures (5 mass spectrometry based and 6 immunochemical based) quantified native individual plasma samples (n = 32) and native plasma pools (n = 8) to assess analytical performance and current and achievable equivalence. In addition, 8 types of candidate reference materials (3 concentrations each, n = 24) were assessed for their suitability, most notably in terms of commutability, to serve as secondary reference material.

RESULTS

Absolute hepcidin values and reproducibility (intrameasurement procedure CVs 2.9%–8.7%) differed substantially between measurement procedures, but all were linear and correlated well. The current equivalence (intermeasurement procedure CV 28.6%) between the methods was mainly attributable to differences in calibration and could thus be improved by harmonization with a common calibrator. Linear regression analysis and standardized residuals showed that a candidate reference material consisting of native lyophilized plasma with cryolyoprotectant was commutable for all measurement procedures. Mathematically simulated harmonization with this calibrator resulted in a maximum achievable equivalence of 7.7%.

CONCLUSIONS

The secondary reference material identified in this study has the potential to substantially improve equivalence between hepcidin measurement procedures and contributes to the establishment of a traceability chain that will ultimately allow standardization of hepcidin measurement results.

Evaluation of a new p16INK4A ELISA test and a high-risk HPV DNA test for cervical cancer screening: Results from proof-of-concept study

p16(INK4a), a cell cycle regulation protein, accumulates in abnormal epithelial cells infected with high-risk human papilloma virus (HPV). In immunostaining studies, p16(INK4a) has shown potential as a marker of high grade cervical intraepithelial neoplasia (CIN) and invasive cervical cancer. To evaluate its potential use in cervical cancer screening, we conducted a feasibility study to compare the performance of a new enzyme linked immunosorbant assay (ELISA) for p16(INK4a) (mtm laboratories, Heidelberg, Germany) to that of the Hybrid Capture 2 (hc2) test for high-risk HPV DNA for the detection of CIN3. Three hundred and nineteen women were referred from Western Washington Planned Parenthood clinics for colposcopy examination and cervical biopsy because of abnormal Pap test results. Cervical samples were obtained from study participants for p16(INK4a) ELISA, liquid-based cytology and hc2. The order (first and second) for obtaining samples for cervical cytology and p16(INK4a) ELISA changed with every other subject. Concentrations of p16(INK4a) protein were higher when the sample was taken before the cytology. The sensitivity of p16(INK4a) ELISA (concentration > or = 8 units/ml) taken as first sample was 90.0% for CIN3, and the sensitivity of HC2 taken as a second sample was 85%. In the same group, the specificity of p16(INK4a) ELISA (46.9%) was slightly better than hc2 (35.4%) Results from this proof-of-concept study suggest that p16(INK4a) ELISA has a similar sensitivity and slightly better specificity for CIN3 compared to hc2. These findings support proceeding with a larger study with samples from a population of women presenting for routine cytology screening.

Identification of high-grade cervical dysplasia by the detection of p16INK4a in cell lysates obtained from cervical samples

BACKGROUND

Current cervical cancer screening approaches are based on cytology supplemented by human papillomavirus (HPV) testing in some settings. Whereas cytology is laborious and depends on the cytologists' experience, HPV testing has limited specificity when it is used to detect high-grade lesions. A dichotomous test to identify high-grade lesions with greater specificity may be a useful tool for cervical cancer screening. p16(INK4a) is a cell-cycle regulator that has demonstrated strong overexpression in cervical precancer cells and cervical cancer induced by the deregulated expression of HPV oncogenes.

METHODS

The authors used a sandwich enzyme-linked immunosorbent assay (ELISA) to quantify the amount of solubilized p16(INK4a) protein in lysates that were prepared from cervical samples to detect high-grade cervical lesions. In total, 187 specimens that were obtained after sampling for conventional cytology in women who attended a cervical colposcopy clinic were analyzed. Seventy-six women underwent a biopsy, and 45 of those women showed histologically confirmed, high-grade cervical intraepithelial neoplasia.

RESULTS

For 76 women with biopsy-proven diagnoses, receiver operating characteristic (ROC) analysis of different cutoff values showed an area under the ROC curve of 0.89 for the detection of high-grade cervical dysplasia. At a cutoff value of 8 U/mL, the sensitivity of the p16(INK4a) ELISA for detecting high-grade dysplastic cervical lesions was 96%.

CONCLUSIONS

The data obtained in this study suggested that ELISA-based quantification of solubilized p16(INK4a) protein may have high sensitivity for detecting cervical precancer. Further population-based studies will be necessary to analyze the specificity and predictive values of p16(INK4a) protein quantification in cervical samples.

Formation of molecular complexes by N-methyl-D-aspartate receptor subunit NR2B and ryanodine receptor 2 in neonatal rat myocard

The N-methyl-d-aspartate (NMDA) receptor is a glutamate gated cation channel prevalent in the postsynaptic membranes of central nervous system neurons. The neurotransmitter receptor complex is thought to represent a tetramer where variable NR2 or NR3 polypeptides form heteromeric assemblies with an obligatory NR1 subunit. Recently, we showed that cardiac myocytes from perinatal rats transiently express the NMDA receptor subunit NR2B, the function of which in heart is unknown. To characterize the cardiac NR2B protein, we determined its subcellular distribution and specific molecular interaction partners. By immunostaining of rat heart tissue slices and acutely dissociated cardiac myocytes, the NR2B antigen was localized at the sarcomeric Z-bands. Using immunoprecipitation of detergent-solubilized NR2B protein and subsequent analysis employing matrix-assisted laser desorption/ionization time of flight mass spectrometry, ryanodine receptor 2 was identified as a molecular interaction partner of the cardiac NR2B polypeptide. Differences in antibody recognition indicate that the cardiac NR2B polypeptide carries a structurally altered C terminus as compared with the NR2B variant prevalent in central nervous system. Based on its localization and protein interaction, the function of cardiac NR2B protein may relate to mechanosensitivity or play a role in the regulation of the contractile apparatus of neonatal heart.

Induction by beta-bungarotoxin of apoptosis in cultured hippocampal neurons is mediated by Ca(2+)-dependent formation of reactive oxygen species

The component of the venom of the Taiwanese banded krait Bungarus multicinctus, beta-bungarotoxin (beta-BuTx), acts as an extremely potent inducer of neuronal apoptosis when applied to rat hippocampal cultures. While induction of cell death is dependent on toxin binding to voltage-activated K+ channels and subsequent internalization, the pro-apoptotic signals triggered by picomolar concentrations of beta-BuTx are not understood. Following toxin binding, a dramatic increase in intracellular Ca2+ became detectable after 30 min, and in reactive oxygen species (ROS) after 3-4 h. Conversely, Ca2+ chelators, radical quenchers and antioxidants efficiently antagonized beta-BuTx induced apoptosis. As shown for the antioxidant 2,3-dihydroxybenzoic acid, analysis by matrix assisted laser desorbtion-time of flight (MALDI-TOF) mass spectrometry excluded the protective effects to be due to reductive cleavage of the toxic beta-BuTx dimer. Inhibitors of the intracellular antioxidant defence system enhanced neuronal susceptibility to beta-BuTx, supporting the essential role of ROS in beta-BuTx-initiated apoptosis. Cell damage was accompanied by an accumulation of markers of oxidative cell stress, phospholipid hydroxyperoxides and the lipid peroxidation product, malonyl dialdehyde. These observations indicate that beta-BuTx-induced cell death resulted from an intracellular signalling cascade involving subsequent stages of a dramatic rise in free Ca2+, the accumulation of ROS, membrane lipid peroxidation and, finally, apoptosis.

Beta-bungarotoxin is a potent inducer of apoptosis in cultured rat neurons by receptor-mediated internalization

The neurotoxic phospholipase A(2), beta-bungarotoxin (beta-BuTx), is a component of the snake venom from the Taiwanese banded krait Bungarus multicinctus. beta-BuTx affects presynaptic nerve terminal function of the neuromuscular junction and induces widespread neuronal cell death throughout the mammalian and avian CNS. To analyse the initial events of beta-BuTx-mediated cell death, the toxin was applied to cultured rat hippocampal neurons where it induced neuronal cell death in a concentration-dependent manner (EC(50) approximately equal to 5 x 10(-13) M) within 24 h. Fluorescence labelled beta-BuTx was completely incorporated by neurons within < 10 min. Binding and uptake of beta-BuTx, as well as induction of cell death, were efficiently antagonized by preincubation with dendrotoxin I, a blocker of voltage-gated potassium channels devoid of phospholipase activity. Binding of beta-BuTx was selective for neurofilament-positive cells. As evident from intense annexin-V and TUNEL stainings, application of beta-BuTx induced apoptotic cell death exclusively in neurons, leaving astrocytes unaffected. No evidence was obtained for any contribution of either caspases or calpains to beta-BuTx-induced apoptosis, consistent with the inability of the inhibitors Z-Asp-DCB and calpeptin, respectively, to protect neurons from beta-BuTx-induced cell death. These observations indicate that induction of cell death by beta-BuTx comprises several successive phases: (i) binding to neuronal potassium channels is the initial event, followed by (ii) internalization and (iii) induction of apoptotic cell death via a caspase-independent pathway.

Critical illness polyneuropathy: clinical findings and cell culture assay of neurotoxicity assessed by a prospective study

OBJECTIVE

First, to evaluate the role of typical intensive care-related conditions like sepsis, prolonged ventilation, drug effects and metabolic disorders in the pathogenesis of critical illness polyneuropathy (CIP); second, to investigate the possible significance of patient serum neurotoxicity assessed by an in vitro cytotoxicity assay with respect to CIP development.

DESIGN

Prospective study.

SETTING

Neurological intensive care unit.

PATIENTS AND PARTICIPANTS

Twenty-eight patients who were on mechanical respiratory support for at least 4 days during a 21-month study period.

RESULTS

Diagnosis of CIP was established by clinical and electrophysiological examination in 16 (57%) of 28 patients. Patients were investigated on days 4, 8 and 14 of mechanical ventilation. Two of 16 CIP patients had clinical signs of polyneuropathy at initial examination. Factors that correlated significantly with the development of CIP were: the multiple organ failure score on day 8 of ventilation, the total duration of respiratory support, the presence of weaning problems and the manifestation of complicating sepsis and/or lung failure. The in vitro toxicity assay showed serum neurotoxicity in 12 of 16 CIP patients. Electrophysiological investigations yielded false positive results of the toxicity assay in six patients (not developing CIP) and false negative results in four patients (developing clinical and electrophysiological signs of CIP). Statistical analysis did not reveal a significant correlation between the diagnosis of CIP and the finding serum neurotoxicity.

CONCLUSION

The results support the hypothesis of a multi-factorial aetiopathogenesis of CIP. We observed serum neurotoxicity in the majority of CIP patients, indicating the possible involvement of a so far unknown, low-molecular-weight neurotoxic agent in CIP pathogenesis.

Transient expression of NMDA receptor subunit NR2B in the developing rat heart

NMDA receptors represent a subtype of the ionotropic glutamate receptor family, comprising three classes of subunits (NR1, NR2A-D, NR3), which exhibit distinct patterns of regional and developmental expression in the CNS. Recently, some NMDA receptor subunits have also been described in adult extraneuronal tissues and keratinocytes. However, their developmental expression patterns are currently unknown. With use of RT-PCR and western blot analysis, the expression of NMDA receptor subunit NR2B was investigated in the developing rat heart. NR2B mRNA and protein were detected in heart tissue of rats from embryonic day 14 until postnatal day 21 but disappeared 10 weeks after birth. In contrast, no NMDA receptor subunit NR1, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor subunit GluR2, or anchoring postsynaptic density protein-95 could be detected in rat heart at any developmental stage. Confocal microscopy of cultured cardiac myocytes (CMs) from neonatal rats revealed distinct NR2B staining mainly of intracellular structures. However, no functional NMDA receptor could be detected on CMs by whole-cell recordings. In conclusion, high concentrations of NR2B protein can be detected in early rat heart development, but its function still remains elusive.

Cellular and subcellular localization of the 2B-subunit of the NMDA receptor in the adult rat telencephalon

NMDA receptors (NR) are encoded by a family of genes including those of the NR1 and NR2A-D subunits. In situ hybridization has revealed that NR1, comprising eight splice variants, is ubiquitously expressed in the central nervous system (CNS) while the expression of NR2 isoforms is restricted to particular CNS regions. We report on the cellular and ultrastructural distribution of the NR2B polypeptide in rat telencephalon. In the telencephalon, the hippocampus represented the most intensively immunolabeled region. Here, predominantly the CA pyramidal neurons were heavily stained. Intense immunoreactivity (IR) was also detected in cortical neurons, in particular in pyramidal-like ones of layers II/III and V. On the ultrastructural level, the NR2B subunit was present not only in synaptic complexes where it usually was present in postsynaptic sites but in addition could be located at extrasynaptic sites. Furthermore, preliminary evidence indicates a presynaptic location of NR2B in some rare cases. NR2B antigen distribution is consistent with that of corresponding transcripts. Indeed, NR2B immunoreactivity coincides largely with that for NR1, indicating that both subunits are coexpressed in numerous cortical and hippocampal neurons.

The NMDA receptor subunit NR2B of neonatal rat brain: complex formation and enrichment in axonal growth cones

The N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors comprises a family of highly homologous subunits which assemble into oligomeric protein complexes. Alterations in subunit composition are developmentally regulated, leading to functionally distinct receptor populations. Here, the contribution of the subunit NR2B to NMDA receptor complex formation was analysed in neonatal rat brain, employing polyclonal antibodies raised against NR2B-specific synthetic peptides. By hydrodynamic size fractionation of the solubilized receptor protein and chemical cross-linking, NR2B antigen was found to be associated with several protein species of up to 690 kDa molecular weight. These observations show NR2B to be part of a multimeric receptor complex. Fractionation of cortex homogenates from E18 rat embryos on sucrose density gradients revealed NR2B polypeptide to be highly enriched in axonal growth cones. A similar distribution was found by fluorescence microscopy of immature hippocampal neurons, showing a preferential accumulation of NR2B antigen in axonal growth cones and varicosities. In mature cells, NR2B antigen displayed a punctated distribution pattern with redistribution to somato-dendritic spheres. The association of NR2B with axonal growth cones and processes of immature neurons suggests a role of NMDA receptors in the regulation of neurite outgrowth and migration.