Quantitative Imaging of Specific Proteins in the Human Retina by Laser Ablation ICPMS using Bioconjugated Metal Nanoclusters as Labels

Unveiling compositional images of specific proteins in individual cells by LA-ICP-MS: Labelling with ruthenium red and metal nanoclusters

BACKGROUND

Recent biological studies have demonstrated that changes can occur in the cellular genome and proteome due to variations in cell volume. Therefore, it is imperative to take cell volume into account when analyzing a target protein. This consideration becomes especially critical in experimental models involving cells subjected to different treatments. Failure to consider cell volume could obscure the studied biological phenomena or lead to erroneous conclusions. However, quantitative imaging of proteins within cells by LA-ICP-MS is limited by the lack of methods that provide the protein concentration (protein mass over cell volume) rather than just protein mass within individual cells.

RESULTS

The combination of a metal tagged immunoprobe with ruthenium red (RR) labelling enables the simultaneous analysis of a specific protein and the cell volume in each cell analyzed by LA-ICP-(Q)MS. The results indicate that the CYP1B1 concentration exhibits a quasi-normally distribution in control ARPE-19 cells, whereas AAPH-treated cells reveal the presence of two distinct cell groups, responding and non-responding cells to an in vitro induced oxidative stress. The labelling of the membrane with RR and the measurement of Ru mass in each cell by LA-ICP-MS offers higher precision compared to manually delimitation of the cell perimeter and eliminates the risk of biased information, which can be prone to inter-observer variability. The proposed procedure is fast and minimizes errors in cell area assignment and offers the possibility to carry out a faster data treatment approach if just relative volumes are compared, which can be advantageous for specific applications.

SIGNIFICANCE AND NOVELTY

This work presents an innovative strategy to directly study the distribution and concentration of proteins within individual cells by LA-ICP-MS. This method employs ruthenium red as a cell volume marker and Au nanoclusters (AuNCs) tagged immunoprobes to label the protein of interest. Furthermore, the proposed labelling strategy enables rapid data processing, allowing for the calculation of relative concentrations and thus facilitating the comparison across large datasets. As a proof-of-concept, the concentration of the CYP1B1 protein was quantified in ARPE-19 cells under both control and oxidative stress conditions.

Single Cell-ICP-ToF-MS for the Multiplexed Determination of Proteins: Evaluation of the Cellular Stress Response

An automated and straightforward detection and data treatment strategy for the determination of the protein relative concentration in individual human cells by single cell-inductively coupled plasma-time-of-flight mass spectrometry (sc-ICP-ToF-MS) is proposed. Metal nanocluster (NC)-labeled specific antibodies for the target proteins were employed, and ruthenium red (RR) staining, which binds to the cells surface, was used to determine the number of cell events as well as to evaluate the relative volume of the cells. As a proof of concept, the expression of hepcidin, metallothionein-2, and ferroportin employing specific antibodies labeled with IrNCs, PtNCs, and AuNCs, respectively, was investigated by sc-ICP-ToF-MS in human ARPE-19 cells. Taking into account that ARPE-19 cells are spherical in suspension and RR binds to the surface of the cells, the Ru intensity was related to the cell volume (i.e., the cell volume is directly proportional to (Ru intensity)3/2), making it possible to determine not only the mass of the target proteins in each individual cell but also the relative concentration. The proposed approach is of particular interest in comparing cell cultures subjected to different supplementations. ARPE-19 cell cultures under two stress conditions were compared: a hyperglycemic model and an oxidative stress model. The comparison of the control with treated cells shows not only the mass of analyzed species but also the relative changes in the cell volume and concentration of target proteins, clearly allowing the identification of subpopulations under the respective treatment.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)-based strategies applied for the analysis of metal-binding protein in biological samples: an update on recent advances

New analytical strategies for metal-binding protein facilitate researchers learning about how metals play a significant role in life. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) offers many advantages for the metal analysis of biological samples and shows a promising future in protein analysis, but recent advances in LA-ICP-MS-based strategies for identifying metal-binding proteins via endogenous metals remain less updated yet. To present the current status in this field, the main analytical strategies for metal-binding proteins with LA-ICP-MS are reviewed here, including in situ analysis of biospecimens and ex situ analysis with gel electrophoresis. A critical discussion of challenges and future perspectives is also given. Multifarious laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS)-based strategies have been developed and applied to investigate the metal-binding proteins in biospecimens in situ or through gel electrophoresis ex situ over the past decades, facilitating researchers disclosing how essential metals are implicated in life or what proteins toxic metals will target.

Cryogenic Laser Ablation in a Rapid Cooling Chamber Ensures Excellent Elemental Imaging in Fresh Biological Tissues

Laser ablation inductively coupled plasma mass spectrometry imaging of biologically significant targets largely relies on maintaining the original structures of samples. The temperature regulation capability of the ablation cell is crucial. Herein, a rapid cooling cryogenic sample cell (RCCSC) was developed. In the RCCSC chamber, the temperature reduces to -20 °C in 4 min with a minimum 10 h variation of ±0.1 °C at -26 °C. Improvements on the precision were achieved for the elements of interest in NIST 612 and spiked agarose gel under cryogenic conditions. The limits of detection improved by up to 1.57, 1.70, 3.26, and 1.33 fold for ⁶³Cu, ⁶⁶Zn, ⁵⁷Fe, and ¹⁴⁰Ce in agarose gel, respectively, were obtained under cryogenic conditions compared with those at room temperature. In a time period of testing (10 h), the cryogenic ablation maintains the native state of biological tissues with a high water content to ensure better elemental imaging by reducing thermal effects in ablation and suppressing evaporation of water. The rapid cooling cryogenic ablation significantly improves elemental imaging, as demonstrated by the imaging of various elements in coriander leaves. The present study may provide further insights into elemental distributions in fresh biological samples.

Review about Powerful Combinations of Advanced and Hyphenated Sample Introduction Techniques with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) for Elucidating Trace Element Species in Pathologic Conditions on a Molecular Level

Element analysis in clinical or biological samples is important due to the essential role in clinical diagnostics, drug development, and drug-effect monitoring. Particularly, the specific forms of element binding, actual redox state, or their spatial distribution in tissue or in single cells are of interest in medical research. This review summarized exciting combinations of sophisticated sample delivery systems hyphenated to inductively coupled plasma-mass spectrometry (ICP-MS), enabling a broadening of information beyond the well-established outstanding detection capability. Deeper insights into pathological disease processes or intracellular distribution of active substances were provided, enabling a better understanding of biological processes and their dynamics. Examples were presented from spatial elemental mapping in tissue, cells, or spheroids, also considering elemental tagging. The use of natural or artificial tags for drug monitoring was shown. In the context of oxidative stress and ferroptosis iron, redox speciation gained importance. Quantification methods for Fe2+, Fe3+, and ferritin-bound iron were introduced. In Wilson’s disease, free and exchangeable copper play decisive roles; the respective paragraph provided information about hyphenated Cu speciation techniques, which provide their fast and reliable quantification. Finally, single cell ICP-MS provides highly valuable information on cell-to-cell variance, insights into uptake of metal-containing drugs, and their accumulation and release on the single-cell level.

Performing 2D-1D-2D Mass Spectrometry Imaging Using Strings

The mass spectrometry imaging (MSI) technique is widely used in several fields due to its ability to provide spatial information of samples. However, for existing MSI methods, the sample is typically placed on a two-dimensional (2D) platform and is scanned back and forth. As a result, the platform size limits the imaging size. This paper proposes a new MSI method that involves the initial imprinting of chemicals on a two-dimensional string plane area. The string plane was then unraveled to a one-dimensional (1D) string, and the chemicals imprinted on it were ionized using a lab-made ion source. Finally, a 2D MSI image was reconstructed through data processing (2D-1D-2D mass imaging). Compared with traditional MSI methods, the imaging size is no longer limited by the platform size, making it possible to perform the MSI of large samples. As proof of concept, this method was used to image an intact seedling of Broussonetia papyrifera. As a result, clear and overall MS images were obtained, demonstrating the ability of this method to analyze large samples.

Quantitative Mass Spectrometry Imaging of Biological Systems

Mass spectrometry imaging (MSI) is a powerful, label-free technique that provides detailed maps of hundreds of molecules in complex samples with high sensitivity and subcellular spatial resolution. Accurate quantification in MSI relies on a detailed understanding of matrix effects associated with the ionization process along with evaluation of the extraction efficiency and mass-dependent ion losses occurring in the analysis step. We present a critical summary of approaches developed for quantitative MSI of metabolites, lipids, and proteins in biological tissues and discuss their current and future applications.

Imaging vicinal dithiol of arsenic-binding proteins in the mouse brain with amplification by gold nanocluster Au22(GSH)18

A quantitative imaging strategy for the vicinal dithiol (VD) of arsenic-binding proteins in the mouse brain is reported. 2-p-Aminophenyl-1,3,2-dithiarsenolane (PAO-EDT) couples to gold nanoclusters Au₂₂(GSH)₁₈ to form conjugate Au₂₂-PAO-EDT (APE). PAO-EDT in APE selectively binds VD with 1 : 1 stoichiometry. After tagging the mouse brain with APE, VD imaging is realized by laser ablation ICP-MS. VD correlates linearly with ¹⁹⁷Au in APE offering a 22-fold amplification and a LOD of 5.43 nM. It is found that the cerebral cortex and hippocampus are most affected in an arsenic poisoned mouse brain. This study provides useful information for further understanding the mechanisms underlying the biological effects of arsenic on the living body.

Quantitative imaging of amyloid beta peptide (Aβ) in Alzheimer's brain tissue by laser ablation ICP-MS using gold nanoparticles as labels

Quantitative imaging of amyloid beta (Aβ) in brain is of great significance for pathological study and follow-up drug development of Alzheimer's disease (AD). In this work, a method using antibody-conjugated gold nanoparticles (AuNPs) was established for quantitative imaging of Aβ peptide in the brain of AD mouse by Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Aβ antibody (Anti-Aβ) was labeled with AuNPs to form the conjugate AuNPs-Anti-Aβ which was immunoreactive with Aβ in the brain slice of mouse. Quantitative imaging of Au was acquired with homogenized brain slice matrix-matched standards as external calibrants which were made by immersing in gold standard solution with different concentrations. Furthermore, the stoichiometric ratios between metal conjugates and Aβ were optimized, and the immunoreaction efficiency after labeling was also investigated. According to the molar relationship between AuNPs and Anti-Aβ (1:4.3) and the ratio of Anti-Aβ to Aβ (1:1), quantitative imaging of Aβ in brain was accomplished. The method intuitively displayed the location and concentration of Aβ aggregation, which was consistent with traditional immunohistochemical staining. Since the numerous gold atoms contained in AuNPs can enhance the signal of Aβ, the method is more intuitive and sensitive. The proposed methodology is potential in investigating the quantitative imaging of biomarker heterogeneity, and is useful to understand such complex brain mechanisms in the future.

Multiplex bioimaging of proteins-related to neurodegenerative diseases in eye sections by laser ablation - Inductively coupled plasma - Mass spectrometry using metal nanoclusters as labels

Simultaneous determination of proteins with micrometric resolution is a significant challenge. In this study, laser ablation (LA) inductively coupled plasma - mass spectrometry (ICP-MS) was employed to quantify the distribution of proteins associated to the eye disease age-related macular degeneration (AMD) using antibodies labelled with three different metal nanoclusters (MNCs). PtNCs, AuNCs and AgNCs contain hundreds of metal atoms and were used to detect metallothionein 1/2 (MT1/2), complement factor H (CFH) and amyloid precursor protein (APP) in retina, ciliary body, retinal pigment epithelium (RPE), choroid and sclera from human cadaveric eye sections. First, the labelling of MNCs bioconjugated primary antibodies (Ab) was optimised following an immunolabelling protocol to avoid the non-specific interaction of MNCs with the tissue. Then, the LA and ICP-MS conditions were studied to obtain high-resolution images for the simultaneous detection of the three labels at the same tissue section. A significant signal amplification was found when using AuNCs, AgNCs and PtNCs labelled Ab of 310, 723 and 1194 respectively. After the characterisation of MNCs labelled immunoprobes, the Ab labelling was used for determination of MT1/2, CFH and APP in the RPE-choroid-sclera, where accumulation of extracellular deposits related to AMD was observed. Experimental results suggest that this method is fully suitable for the simultaneous detection of at least three different proteins.

Peptide and protein modified metal clusters for cancer diagnostics

The biomedical features of metal clusters have been explored in tumor diagnostic applications in recent years. Peptide or protein protected metal clusters with low toxicity, ultra-small size and good biocompatibility are ideal bioanalytical tools, and exhibit better cancer diagnostic properties that have been attractive to oncologists. This perspective provides a rigorous but succinct overview of cancer diagnosis as a working concept for metal clusters by reporting the latest significant advances in the applications of metal clusters in tumor-related bioanalysis and diagnosis. The materials design principles, bioanalytical mechanisms and biomedical applications of metal clusters are described, and then the potential challenges and prospects of metal clusters in cancer diagnosis are discussed. A perspective addressing the role of metal clusters in this field is required to understand their effects and functions, as well as for the scientific community to further advance the development of metal clusters for broader diagnostic applications.

Multiplex LA-ICP-MS bio-imaging of brain tissue of a parkinsonian mouse model stained with metal-coded affinity-tagged antibodies and coated with indium-spiked commercial inks as internal standards

BACKGROUND

Immunohistochemistry techniques represent a powerful tool to detect and quantify disease related proteins. Improvements were accomplished by tagged antibodies using laser ablation and inductively coupled plasma mass spectrometry (LA-ICP-MS). However, these approaches are effected by day-to-variations due to instrumental drift.

NEW METHOD

Brain tissue from line 62, a Parkinson's disease model, and control mice were incubated with four antibodies relevant to the disease and standardized to three house-keeping proteins. In addition, a new standardization approach was developed and the results compared. This new approach consisted of coating specimens with gelatin and printing an indium-doped ink with a commercial ink jet printer. Furthermore, the method was evaluated for different ablation spot sizes with respect to resolution and signal-to-noise ratio.

RESULTS

Normalization using house-keeping proteins led to high background signals even at high resolution. Normalization using indium-doped ink improved the signal-to-noise ratio even when small laser spot sizes were used and further improved by overlaying tissue specimen with gelatin.

COMPARISON WITH EXISTING METHODS

Line 62 mice had more α-Synuclein and gliosis but decreased numbers of neurons, as found by conventional immunohistochemistry. These data are in line with the results obtained by LA-ICP-MS with indium standardization. However, differences between L62 and controls for tyrosine hydroxylase were only detected by LA-ICP-MS.

CONCLUSIONS

Internal standardisation using indium-doped inks is an effective method to overcome day-to-day variations and instrumental drifts. The new approach results in an increased signal-to-noise ratio and only under these conditions small but significant changes were detected, as seen for tyrosine hydroxylase.

Laser ablation ICP-MS for simultaneous quantitative imaging of iron and ferroportin in hippocampus of human brain tissues with Alzheimer's disease

Laser ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS) is proposed for a better understanding of metals and proteins distribution in micrometre structures of human brain tissues. Simultaneous absolute quantitative imaging of Fe and ferroportin (FPN), in 5 µm thick tissue sections of the stratum pyramidale of hippocampus CA1 region, was carried out for Alzheimer disease (AD) patients and healthy controls (HC). For the imaging of FPN by LA-ICP-MS, antibodies were labelled via carbodiimide crosslinking with fluorescent gold nanoclusters (AuNCs) of 2.2 nm diameter, enabling a high amplification (314 gold atoms per NC). Laboratory made gelatin standards containing Fe and Au were used for LA-ICP-MS calibration. Results showed that iron presents an increased concentration in AD donors compared with HC donors, whereas similar concentrations of FPN in AD donors with respect to HC donors were obtained. The average absolute FPN concentrations in selected areas obtained with the proposed AuNCs method were compared with the levels obtained by densitometric analysis with a traditional IHC approach, observing a similar trend in all cases.

A reflection on the role of ICP-MS in proteomics: Update and future perspective

Mass spectrometry is the technique of reference for the identification and quantification of proteins. Whereas ESI and MALDI ionization sources are inherently not quantitative being highly influenced by the chemical nature of the analyte and the matrix, ICP-MS uses a hard ionization source that destroys proteins into its atoms and measures the elemental signal, which is independent of its chemical environment. As a consequence, ICP-MS turns up as an excellent technique for the screening, mapping and quantification of peptides and proteins in a sample through elemental detection (any element but C, H, N, or O) once they have been previously separated by chromatography. In this time, great efforts have been put in developing instrumentation and new methodologies that enable a better, more efficient, and more useful analysis of proteins with ICP-MS. Moreover, quantitative capabilities but lack of molecular information of ICP has led to a synergic relationship both with identifying capabilities of ESI-MS, or the use of protein-specific antibodies carrying an elemental label. JOURNAL SIGNIFICANCE: We are delighted to participate in this special issue and have the chance to congratulate Journal of Proteomics for its 10th Anniversary, and wish for many further successful anniversaries. During this last decade, Journal of Proteomics has been a clear promotor of works integrating ICP-MS for proteomics analysis. In fact, already in 2009, a review was published by invitation of the editor in chief focused on the established and potential role of ICP-MS in different areas of the proteomics analysis at the time: "The emerging role of ICP-MS in proteomics" [1]. Even though ICP-MS is not fully known or acknowledged in the proteomics world yet, its impact was significant as demonstrated by the really high interest in such publication (over 150 citations). Since then, several excellent papers relating to ICP-MS applications in proteomics have been published in this journal. Following the trend, we expect through this personal view of the current standing of ICP-MS in proteomics to enlighten the readers of Journal of Proteomics with a vision of the full present and future potential of ICP-MS in proteomics.