SDHA gain-of-function engages inflammatory mitochondrial retrograde signaling via KEAP1-Nrf2.

Whether screening the metabolic activity of immune cells facilitates discovery of molecular pathology remains unknown. Here we prospectively screened the extracellular acidification rate as a measure of glycolysis and the oxygen consumption rate as a measure of mitochondrial respiration in B cells from patients with primary antibody deficiency. The highest oxygen consumption rate values were detected in three study participants with persistent polyclonal B cell lymphocytosis (PPBL). Exome sequencing identified germline mutations in SDHA, which encodes succinate dehydrogenase subunit A, in all three patients with PPBL. SDHA gain-of-function led to an accumulation of fumarate in PPBL B cells, which engaged the KEAP1-Nrf2 system to drive the transcription of genes encoding inflammatory cytokines. In a single patient trial, blocking the activity of the cytokine interleukin-6 in vivo prevented systemic inflammation and ameliorated clinical disease. Overall, our study has identified pathological mitochondrial retrograde signaling as a disease modifier in primary antibody deficiency.

Transcription factors TEAD2 and E2A globally repress acetyl-CoA synthesis to promote tumorigenesis.

Acetyl-coenzyme A (acetyl-CoA) plays an important role in metabolism, gene expression, signaling, and other cellular processes via transfer of its acetyl group to proteins and metabolites. However, the synthesis and usage of acetyl-CoA in disease states such as cancer are poorly characterized. Here, we investigated global acetyl-CoA synthesis and protein acetylation in a mouse model and patient samples of hepatocellular carcinoma (HCC). Unexpectedly, we found that acetyl-CoA levels are decreased in HCC due to transcriptional downregulation of all six acetyl-CoA biosynthesis pathways. This led to hypo-acetylation specifically of non-histone proteins, including many enzymes in metabolic pathways. Importantly, repression of acetyl-CoA synthesis promoted oncogenic dedifferentiation and proliferation. Mechanistically, acetyl-CoA synthesis was repressed by the transcription factors TEAD2 and E2A, previously unknown to control acetyl-CoA synthesis. Knockdown of TEAD2 and E2A restored acetyl-CoA levels and inhibited tumor growth. Our findings causally link transcriptional reprogramming of acetyl-CoA metabolism, dedifferentiation, and cancer.

Phenotypic variation of Salmonella in host tissues delays eradication by antimicrobial chemotherapy.

Antibiotic therapy often fails to eliminate a fraction of transiently refractory bacteria, causing relapses and chronic infections. Multiple mechanisms can induce such persisters with high antimicrobial tolerance in vitro, but their in vivo relevance remains unclear. Using a fluorescent growth rate reporter, we detected extensive phenotypic variation of Salmonella in host tissues. This included slow-growing subsets as well as well-nourished fast-growing subsets driving disease progression. Monitoring of Salmonella growth and survival during chemotherapy revealed that antibiotic killing correlated with single-cell division rates. Nondividing Salmonella survived best but were rare, limiting their impact. Instead, most survivors originated from abundant moderately growing, partially tolerant Salmonella. These data demonstrate that host tissues diversify pathogen physiology, with major consequences for disease progression and control.

Nicotinamide N-methyltransferase sustains a core epigenetic program that promotes metastatic colonization in breast cancer.

Metastatic colonization of distant organs accounts for over 90% of deaths related to solid cancers, yet the molecular determinants of metastasis remain poorly understood. Here, we unveil a mechanism of colonization in the aggressive basal-like subtype of breast cancer that is driven by the NAD+ metabolic enzyme nicotinamide N-methyltransferase (NNMT). We demonstrate that NNMT imprints a basal genetic program into cancer cells, enhancing their plasticity. In line, NNMT expression is associated with poor clinical outcomes in patients with breast cancer. Accordingly, ablation of NNMT dramatically suppresses metastasis formation in pre-clinical mouse models. Mechanistically, NNMT depletion results in a methyl overflow that increases histone H3K9 trimethylation (H3K9me3) and DNA methylation at the promoters of PR/SET Domain-5 (PRDM5) and extracellular matrix-related genes. PRDM5 emerged in this study as a pro-metastatic gene acting via induction of cancer-cell intrinsic transcription of collagens. Depletion of PRDM5 in tumor cells decreases COL1A1 deposition and impairs metastatic colonization of the lungs. These findings reveal a critical activity of the NNMT-PRDM5-COL1A1 axis for cancer cell plasticity and metastasis in basal-like breast cancer.

Quantitative analysis of fission yeast transcriptomes and proteomes in proliferating and quiescent cells.

Data on absolute molecule numbers will empower the modeling, understanding, and comparison of cellular functions and biological systems. We quantified transcriptomes and proteomes in fission yeast during cellular proliferation and quiescence. This rich resource provides the first comprehensive reference for all RNA and most protein concentrations in a eukaryote under two key physiological conditions. The integrated data set supports quantitative biology and affords unique insights into cell regulation. Although mRNAs are typically expressed in a narrow range above 1 copy/cell, most long, noncoding RNAs, except for a distinct subset, are tightly repressed below 1 copy/cell. Cell-cycle-regulated transcription tunes mRNA numbers to phase-specific requirements but can also bring about more switch-like expression. Proteins greatly exceed mRNAs in abundance and dynamic range, and concentrations are regulated to functional demands. Upon transition to quiescence, the proteome changes substantially, but, in stark contrast to mRNAs, proteins do not uniformly decrease but scale with cell volume.

Hepatic stellate cells suppress NK cell-sustained breast cancer dormancy.

The persistence of undetectable disseminated tumour cells (DTCs) after primary tumour resection poses a major challenge to effective cancer treatment1-3. These enduring dormant DTCs are seeds of future metastases, and the mechanisms that switch them from dormancy to outgrowth require definition. Because cancer dormancy provides a unique therapeutic window for preventing metastatic disease, a comprehensive understanding of the distribution, composition and dynamics of reservoirs of dormant DTCs is imperative. Here we show that different tissue-specific microenvironments restrain or allow the progression of breast cancer in the liver-a frequent site of metastasis4 that is often associated with a poor prognosis5. Using mouse models, we show that there is a selective increase in natural killer (NK) cells in the dormant milieu. Adjuvant interleukin-15-based immunotherapy ensures an abundant pool of NK cells that sustains dormancy through interferon-γ signalling, thereby preventing hepatic metastases and prolonging survival. Exit from dormancy follows a marked contraction of the NK cell compartment and the concurrent accumulation of activated hepatic stellate cells (aHSCs). Our proteomics studies on liver co-cultures implicate the aHSC-secreted chemokine CXCL12 in the induction of NK cell quiescence through its cognate receptor CXCR4. CXCL12 expression and aHSC abundance are closely correlated in patients with liver metastases. Our data identify the interplay between NK cells and aHSCs as a master switch of cancer dormancy, and suggest that therapies aimed at normalizing the NK cell pool might succeed in preventing metastatic outgrowth.

RIPK1 and Caspase-8 Ensure Chromosome Stability Independently of Their Role in Cell Death and Inflammation.

Receptor-interacting protein kinase (RIPK) 1 functions as a key mediator of tissue homeostasis via formation of Caspase-8 activating ripoptosome complexes, positively and negatively regulating apoptosis, necroptosis, and inflammation. Here, we report an unanticipated cell-death- and inflammation-independent function of RIPK1 and Caspase-8, promoting faithful chromosome alignment in mitosis and thereby ensuring genome stability. We find that ripoptosome complexes progressively form as cells enter mitosis, peaking at metaphase and disassembling as cells exit mitosis. Genetic deletion and mitosis-specific inhibition of Ripk1 or Caspase-8 results in chromosome alignment defects independently of MLKL. We found that Polo-like kinase 1 (PLK1) is recruited into mitotic ripoptosomes, where PLK1's activity is controlled via RIPK1-dependent recruitment and Caspase-8-mediated cleavage. A fine balance of ripoptosome assembly is required as deregulated ripoptosome activity modulates PLK1-dependent phosphorylation of downstream effectors, such as BUBR1. Our data suggest that ripoptosome-mediated regulation of PLK1 contributes to faithful chromosome segregation during mitosis.

ALK signaling primes the DNA damage response sensitizing ALK-driven neuroblastoma to therapeutic ATR inhibition.

High-risk neuroblastoma (NB) is a significant clinical challenge. MYCN and Anaplastic Lymphoma Kinase (ALK), which are often involved in high-risk NB, lead to increased replication stress in cancer cells, suggesting therapeutic strategies. We previously identified an ATR (ataxia telangiectasia and Rad3-related)/ALK inhibitor (ATRi/ALKi) combination as such a strategy in two independent genetically modified mouse NB models. Here, we identify an underlying molecular mechanism, in which ALK signaling leads to phosphorylation of ATR and CHK1, supporting an effective DNA damage response. The importance of ALK inhibition is supported by mouse data, in which ATRi monotreatment resulted in a robust initial response, but subsequent relapse, in contrast to a 14-d ALKi/ATRi combination treatment that resulted in a robust and sustained response. Finally, we show that the remarkable response to the 14-d combined ATR/ALK inhibition protocol reflects a robust differentiation response, reprogramming tumor cells to a neuronal/Schwann cell lineage identity. Our results identify an ability of ATR inhibition to promote NB differentiation and underscore the importance of further exploring combined ALK/ATR inhibition in NB, particularly in high-risk patient groups with oncogene-induced replication stress.

Regulation of α-synuclein by chaperones in mammalian cells.

Neurodegeneration in patients with Parkinson's disease is correlated with the occurrence of Lewy bodies-intracellular inclusions that contain aggregates of the intrinsically disordered protein α-synuclein1. The aggregation propensity of α-synuclein in cells is modulated by specific factors that include post-translational modifications2,3, Abelson-kinase-mediated phosphorylation4,5 and interactions with intracellular machineries such as molecular chaperones, although the underlying mechanisms are unclear6-8. Here we systematically characterize the interaction of molecular chaperones with α-synuclein in vitro as well as in cells at the atomic level. We find that six highly divergent molecular chaperones commonly recognize a canonical motif in α-synuclein, consisting of the N terminus and a segment around Tyr39, and hinder the aggregation of α-synuclein. NMR experiments9 in cells show that the same transient interaction pattern is preserved inside living mammalian cells. Specific inhibition of the interactions between α-synuclein and the chaperone HSC70 and members of the HSP90 family, including HSP90ß, results in transient membrane binding and triggers a remarkable re-localization of α-synuclein to the mitochondria and concomitant formation of aggregates. Phosphorylation of α-synuclein at Tyr39 directly impairs the interaction of α-synuclein with chaperones, thus providing a functional explanation for the role of Abelson kinase in Parkinson's disease. Our results establish a master regulatory mechanism of α-synuclein function and aggregation in mammalian cells, extending the functional repertoire of molecular chaperones and highlighting new perspectives for therapeutic interventions for Parkinson's disease.

Data-independent acquisition: A milestone and prospect in clinical mass spectrometry-based proteomics.

Data-independent acquisition (DIA) has revolutionized the field of mass spectrometry (MS)-based proteomics over the past few years. DIA stands out for its ability to systematically sample all peptides in a given mass-to-charge range, allowing an unbiased acquisition of proteomics data. This greatly mitigates the issue of missing values and significantly enhances quantitative accuracy, precision, and reproducibility compared to many traditional methods. This review focuses on the critical role of DIA analysis software tools, primarily focusing on their capabilities and the challenges they address in proteomic research. Advances in MS technology, such as trapped ion mobility spectrometry, or high field asymmetric waveform ion mobility spectrometry require sophisticated analysis software capable of handling the increased data complexity and exploiting the full potential of DIA. We identify and critically evaluate leading software tools in the DIA landscape, discussing their unique features, and the reliability of their quantitative and qualitative outputs. We present the biological and clinical relevance of DIA-MS and discuss crucial publications that paved the way for in-depth proteomic characterization in patient-derived specimens. Furthermore, we provide a perspective on emerging trends in clinical applications and present upcoming challenges including standardization and certification of MS-based acquisition strategies in molecular diagnostics. While we emphasize the need for continuous development of software tools to keep pace with evolving technologies, we advise researchers against uncritically accepting the results from DIA software tools. Each tool may have its own biases, and some may not be as sensitive or reliable as others. Our overarching recommendation for both researchers and clinicians is to employ multiple DIA analysis tools, utilizing orthogonal analysis approaches to enhance the robustness and reliability of their findings.

Centriolar distal appendages activate the centrosome-PIDDosome-p53 signalling axis via ANKRD26.

Centrosome amplification results into genetic instability and predisposes cells to neoplastic transformation. Supernumerary centrosomes trigger p53 stabilization dependent on the PIDDosome (a multiprotein complex composed by PIDD1, RAIDD and Caspase-2), whose activation results in cleavage of p53's key inhibitor, MDM2. Here, we demonstrate that PIDD1 is recruited to mature centrosomes by the centriolar distal appendage protein ANKRD26. PIDDosome-dependent Caspase-2 activation requires not only PIDD1 centrosomal localization, but also its autoproteolysis. Following cytokinesis failure, supernumerary centrosomes form clusters, which appear to be necessary for PIDDosome activation. In addition, in the context of DNA damage, activation of the complex results from a p53-dependent elevation of PIDD1 levels independently of centrosome amplification. We propose that PIDDosome activation can in both cases be promoted by an ANKRD26-dependent local increase in PIDD1 concentration close to the centrosome. Collectively, these findings provide a paradigm for how centrosomes can contribute to cell fate determination by igniting a signalling cascade.

Glucocorticoids promote breast cancer metastasis.

Diversity within or between tumours and metastases (known as intra-patient tumour heterogeneity) that develops during disease progression is a serious hurdle for therapy1-3. Metastasis is the fatal hallmark of cancer and the mechanisms of colonization, the most complex step in the metastatic cascade4, remain poorly defined. A clearer understanding of the cellular and molecular processes that underlie both intra-patient tumour heterogeneity and metastasis is crucial for the success of personalized cancer therapy. Here, using transcriptional profiling of tumours and matched metastases in patient-derived xenograft models in mice, we show cancer-site-specific phenotypes and increased glucocorticoid receptor activity in distant metastases. The glucocorticoid receptor mediates the effects of stress hormones, and of synthetic derivatives of these hormones that are used widely in the clinic as anti-inflammatory and immunosuppressive agents. We show that the increase in stress hormones during breast cancer progression results in the activation of the glucocorticoid receptor at distant metastatic sites, increased colonization and reduced survival. Our transcriptomics, proteomics and phospho-proteomics studies implicate the glucocorticoid receptor in the activation of multiple processes in metastasis and in the increased expression of kinase ROR1, both of which correlate with reduced survival. The ablation of ROR1 reduced metastatic outgrowth and prolonged survival in preclinical models. Our results indicate that the activation of the glucocorticoid receptor increases heterogeneity and metastasis, which suggests that caution is needed when using glucocorticoids to treat patients with breast cancer who have developed cancer-related complications.

Robust, Precise, and Deep Proteome Profiling Using a Small Mass Range and Narrow Window Data-Independent-Acquisition Scheme.

In recent years, a plethora of different data-independent acquisition methods have been developed for proteomics to cover a wide range of requirements. Current deep proteome profiling methods rely on fractionations, elaborate chromatography, and mass spectrometry setups or display suboptimal quantitative precision. We set out to develop an easy-to-use one shot DIA method that achieves high quantitative precision and high proteome coverage. We achieve this by focusing on a small mass range of 430-670 m/z using small isolation windows without overlap. With this new method, we were able to quantify >9200 protein groups in HEK lysates with an average coefficient of variance of 3.2%. To demonstrate the power of our newly developed narrow mass range method, we applied it to investigate the effect of PGC-1α knockout on the skeletal muscle proteome in mice. Compared to a standard data-dependent acquisition method, we could double proteome coverage and, most importantly, achieve a significantly higher quantitative precision, as compared to a previously proposed DIA method. We believe that our method will be especially helpful in quantifying low abundant proteins in samples with a high dynamic range. All raw and result files are available at massive.ucsd.edu (MSV000092186).

Prevention of dsRNA-induced interferon signaling by AGO1x is linked to breast cancer cell proliferation.

Translational readthrough, i.e., elongation of polypeptide chains beyond the stop codon, was initially reported for viral RNA, but later found also on eukaryotic transcripts, resulting in proteome diversification and protein-level modulation. Here, we report that AGO1x, an evolutionarily conserved translational readthrough isoform of Argonaute 1, is generated in highly proliferative breast cancer cells, where it curbs accumulation of double-stranded RNAs (dsRNAs) and consequent induction of interferon responses and apoptosis. In contrast to other mammalian Argonaute protein family members with primarily cytoplasmic functions, AGO1x exhibits nuclear localization in the vicinity of nucleoli. We identify AGO1x interaction with the polyribonucleotide nucleotidyltransferase 1 (PNPT1) and show that the depletion of this protein further augments dsRNA accumulation. Our study thus uncovers a novel function of an Argonaute protein in buffering the endogenous dsRNA-induced interferon responses, different than the canonical function of AGO proteins in the miRNA effector pathway. As AGO1x expression is tightly linked to breast cancer cell proliferation, our study thus suggests a new direction for limiting tumor growth.

HLA-DP on Epithelial Cells Enables Tissue Damage by NKp44+ Natural Killer Cells in Ulcerative Colitis.

BACKGROUND & AIMS: Ulcerative colitis (UC) is characterized by severe inflammation and destruction of the intestinal epithelium, and is associated with specific risk single nucleotide polymorphisms in HLA class II. Given the recently discovered interactions between subsets of HLA-DP molecules and the activating natural killer (NK) cell receptor NKp44, genetic associations of UC and HLA-DP haplotypes and their functional implications were investigated. METHODS: HLA-DP haplotype and UC risk association analyses were performed (UC: n = 13,927; control: n = 26,764). Expression levels of HLA-DP on intestinal epithelial cells (IECs) in individuals with and without UC were quantified. Human intestinal 3-dimensional (3D) organoid cocultures with human NK cells were used to determine functional consequences of interactions between HLA-DP and NKp44. RESULTS: These studies identified HLA-DPA1∗01:03-DPB1∗04:01 (HLA-DP401) as a risk haplotype and HLA-DPA1∗01:03-DPB1∗03:01 (HLA-DP301) as a protective haplotype for UC in European populations. HLA-DP expression was significantly higher on IECs of individuals with UC compared with controls. IECs in human intestinal 3D organoids derived from HLA-DP401pos individuals showed significantly stronger binding of NKp44 compared with HLA-DP301pos IECs. HLA-DP401pos IECs in organoids triggered increased degranulation and tumor necrosis factor production by NKp44+ NK cells in cocultures, resulting in enhanced epithelial cell death compared with HLA-DP301pos organoids. Blocking of HLA-DP401-NKp44 interactions (anti-NKp44) abrogated NK cell activity in cocultures. CONCLUSIONS: We identified an UC risk HLA-DP haplotype that engages NKp44 and activates NKp44+ NK cells, mediating damage to intestinal epithelial cells in an HLA-DP haplotype-dependent manner. The molecular interaction between NKp44 and HLA-DP401 in UC can be targeted by therapeutic interventions to reduce NKp44+ NK cell-mediated destruction of the intestinal epithelium in UC.

Structural alterations and pain perception at the ankle joint in patients with haemophilia.

BACKGROUND: Patients with haemophilia (PwH) suffer from chronic pain due to joint alterations induced by recurring haemorrhage. OBJECTIVES: This study aimed to investigate the relationship between structural alterations and pain perception at the ankle joint in PwH. PATIENTS/METHODS: Ankle joints of 79 PwH and 57 healthy controls (Con) underwent ultrasound examination (US) and assessment of pain sensitivity via pressure pain thresholds (PPT). US discriminated between joint activity (synovitis) and joint damage (cartilage and/or bone degeneration) applying the HEAD-US protocol. Based on US-findings, five subgroups were built: PwH with activity/damage, PwH with activity/no damage, PwH with no activity/no damage, controls with activity/no damage and controls with no activity/no damage. RESULTS: Joint activity and joint damage were significantly increased in ankles of PwH compared to Con (p ≤.001). Subgroup analysis revealed that structural alterations negatively impact pain perception. This is particularly evident when comparing PwH with both activity/damage to PwH with no activity/no damage at the tibiotalar joint (p = .001). At the fibulotalar joint, no significant differences were observed between PwH subgroups. Further analysis showed that both joint activity and joint damage result in an increase in pain sensitivity (p ≤.001). CONCLUSION: The data suggest a relation between joint activity, joint damage and pain perception in PwH. Even minor changes due to synovitis appear to affect pain perception, with the effect not intensifying at higher levels of inflammation. In terms of joint damage, severe degeneration leads to a sensitised pain state most robustly, whereas initial changes do not seem to significantly affect pain perception.

Functional clinical motor performance tests to assess potential fall risks in patients with haemophilia: A case-control study.

INTRODUCTION: Patients with haemophilia (PwH) are at increased risk of falls due to haemophilic arthropathy. Yet, studies on clinical tests associated with the risk of falling are scarce in PwH. AIMS: (1) To evaluate the feasibility of different clinical motor performance tests associated with the risk of falling in PwH; (2) to evaluate PwH's performance of these tests compared to a control group; (3) to identify possible influencing factors that affect performance. METHODS: Twenty-nine severe and moderate PwH (57.0 years, IQR: 48.0-61.5) and 29 healthy age- and BMI-matched control participants (CG) performed 13 different clinical tests (SPPB, timed up and go, push and release, functional reach, single-leg stance, knee and grip strength). Haemophilia joint health score (HJHS), kinesiophobia (TSK-11), subjective physical performance (HEP-Test-Q), falls efficiency (FES-I) and falls were assessed. RESULTS: No adverse events occurred. PwH showed impaired performance in all clinical tests, a lower falls efficiency and a higher HJHS than CG. PwH with higher HJHS, lower HEP-Test-Q and higher TSK-11 scores showed higher deficits. Largest discrepancies were observed in the single-leg stance with eyes open and knee extensor strength, where orthopaedically majorly affected PwH showed worse performance compared to minorly affected PwH and the CG, respectively. The prevalence of ≥1 fall in the last year was 27.6% (PwH) and 10.3% (CG). CONCLUSION: These clinical tests are feasible in PwH. Impaired joint status, a high kinesiophobia and low physical performance impair performance. These tests can be used by clinicians for gaining specific information on functional motor abilities of patients.

Physical activity and factor VIII levels in patients with haemophilia: A real-world prospective observational study.

INTRODUCTION: Regular physical activity (PA) is recommended for patients with haemophilia (PwH). For PwH it is crucial to ensure a sufficient factor level to prevent PA-induced bleedings. However, there is a gap in the literature dealing with specific factor levels, which are needed when performing specific types of PA. AIM: To provide data on factor VIII (FVIII) levels at the start of PA performed by PwH. METHODS: In this prospective 12-month real-world observational study, 23 PwH recorded every PA they performed and the FVIII levels at the start of the PA using a pharmacokinetic application. PA types were clustered according to the collision and injury risk into three categories (Cat I = low, Cat II = medium, Cat III = high risk). Haemophilia Joint Health Scores (HJHS) were performed at baseline, after 6 and 12 months. RESULTS: 795 PA sessions of Cat I, 193 of Cat II, and 23 of Cat III were documented. FVIII levels at the start of PA were different between categories (Cat I: 29.8 ± 32.1%, Cat II: 38.3 ± 33.4%, Cat III: 86.6 ± 29.2%). Out of all PA sessions, 145 (14%) were performed at a factor level of ≤3%. Three PA-induced bleeding occurred. Baseline HJHS was 14.5 ± 13.6 points and did not change throughout the study. CONCLUSION: This study provides real-life data on FVIII levels at the start of 1011 PA sessions. PwH are mainly active in low-risk sports with higher FVIII levels observed in Cat II and III, respectively. Only three PA-induced bleeding occurred, even though several PA were started with low FVIII levels.

Engagement of TRAIL triggers degranulation and IFNγ production in human natural killer cells.

NK cells utilize a large array of receptors to screen their surroundings for aberrant or virus-infected cells. Given the vast diversity of receptors expressed on NK cells we seek to identify receptors involved in the recognition of HIV-1-infected cells. By combining an unbiased large-scale screening approach with a functional assay, we identify TRAIL to be associated with NK cell degranulation against HIV-1-infected target cells. Further investigating the underlying mechanisms, we demonstrate that TRAIL is able to elicit multiple effector functions in human NK cells independent of receptor-mediated induction of apoptosis. Direct engagement of TRAIL not only results in degranulation but also IFNγ production. Moreover, TRAIL-mediated NK cell activation is not limited to its cognate death receptors but also decoy receptor I, adding a new perspective to the perceived regulatory role of decoy receptors in TRAIL-mediated cytotoxicity. Based on these findings, we propose that TRAIL not only contributes to the anti-HIV-1 activity of NK cells but also possesses a multifunctional role beyond receptor-mediated induction of apoptosis, acting as a regulator for the induction of different effector functions.

Accurate measurement of K/Ca in low-[K] carbonate samples using laser-ablation sector-field inductively coupled plasma mass spectrometry.

RATIONALE: Potassium (K) is a major component of several silicate minerals and seawater, and, therefore, constraining past changes in the potassium cycle is a promising way of tracing large-scale geological processes on Earth. However, [K] measurement using inductively coupled plasma mass spectrometry (ICP-MS) is challenging due to an ArH+ interference, which may be of a similar magnitude to the K+ ion beam in samples with <0.1% m/m [K]. METHODS: In this work, we investigated the effect of the ArH+ interference on K/Ca data quality by comparing results from laser-ablation (LA)-ICP-MS measured in medium and high mass resolution modes and validating our LA results via solution ICP-optical emission spectroscopy (OES) and solution ICP-MS measurements. To do so, we used a wide range of geological reference materials, with a particular focus on marine carbonates, which are potential archives of past changes in the K cycle but are typically characterised by [K] < 200 µg/g. In addition, we examine the degree to which trace-element data quality is driven by downhole fractionation during LA-ICP-MS measurements. RESULTS: Our results show that medium mass resolution (MR) mode is sufficiently capable of minimising the effect of the ArH+ interference on K+ . However, the rate of downhole fractionation for Na and K varies between different samples as a result of their differing bulk composition, resulting in matrix-specific inaccuracy. We show how this can be accounted for via downhole fractionation corrections, resulting in an accuracy of better than 1% and a long-term reproducibility (intermediate precision) of <6% (relative standard deviation) in JCp-1NP using LA-ICP-MS in MR mode. CONCLUSION: Our [K] measurement protocol is demonstrably precise and accurate and applicable to a wide range of materials. The measurement of K/Ca in relatively low-[K] marine carbonates is presented here as a key example of a new application opened up by these advances.