Calciprotein Particle Synthesis Strategy Determines In Vitro Calcification Potential

The formation and function of calciprotein particles

Vertebrate extracellular fluids lie below the threshold for spontaneous calcium phosphate (Ca-Pi) precipitation; yet, they remain supersaturated enough to foster crystal growth if unchecked. Calciprotein particles (CPP) and their smaller precursor calciprotein monomers (CPM) have emerged as fast-acting "mineral buffers" that mitigate abrupt local oversaturation. Although these complexes typically contain only trace amounts of Ca-Pi relative to total plasma levels, they exhibit remarkably high turnover kinetics, with clearance from the circulation within minutes, far outpacing hormonal loops that operate on timescales of hours to days. By forming ephemeral colloidal assemblies, CPM/CPP help maintain fluid-phase stability and avert uncontrolled crystallization "accidents" in microenvironments such as the intestine or bone-remodeling sites. However, under chronic mineral stress, such as in chronic kidney disease, multiple inhibitory factors (e.g., fetuin-A, pyrophosphate) can become deficient, enabling persistent generation of more advanced, crystalline CPP species. These "modified" CPP can adsorb additional ligands (e.g., apolipoproteins, microbial remnants, growth factors) and have been linked to inflammatory and pro-calcific changes in vascular and immune cells. Despite their minor quantitative contribution, these rapidly mobilized colloids may exert outsized influence on vascular and skeletal homeostasis, underscoring the need to clarify their origins, biological roles, and potential therapeutic targeting in disorders of mineral metabolism.

Roxadustat increases markers of calcification in patients with end-stage kidney disease: prospective cohort study

To determine the effects of roxadustat on calcification when treating renal anemia in end-stage kidney disease (ESKD) patients. A prospective cohort study enrolled participants with ESKD that either received roxadustat or no roxadustat treatment. The primary outcome was the change in the degree of hydroxyapatite (HAP) crystals deposition. The secondary outcome was calcification propensity, a measure of calcification, allowing an evaluation of the conversion process from primary to secondary calciprotein particles. A total of 205 patients were enrolled, and 187 (91.2%) completed follow-up for inclusion in the analysis and were divided into the roxadustat group (n = 92) and the control group (n = 95) based on whether or not they were taking roxadustat regularly over a 6-mo period. After roxadustat administration for 6 mo, patients exhibited increases in total RBC counts, hematocrit, hemoglobin, calcium, total ferritin binding, intact fibroblast growth factor 23 (iFGF23), secreted phosphoprotein 24 (SPP24), and calcification propensity relative to pre-treatment levels. No significant differences were observed in patients who were not treated with roxadustat. The deposition degree of HAP crystals increased by 5% and 0.01% following treatment with roxadustat in the roxadustat group and control group, respectively. Linear regression analysis found that the use of roxadustat was an independent risk factor for calcification propensity and changes in the degree of HAP crystals deposition. Roxadustat treatment may increase iFGF23, SPP24, and calcification propensity in patients with ESKD when treating these patients for renal anemia. Therefore, the clinicians should aware of this risk when treating patients with prolyl hydroxylase domain inhibitors.

Simultaneous Measurement of Calciprotein Particles with Different Assays and Clinical Outcomes in CKD

Key Points

Multiple assays are available to measure calciprotein particles (CPP), but a direct comparison of their association with clinical outcomes is lacking. Higher levels of high-density CPP and secondary CPP were independently associated with all-cause mortality in a CKD cohort. Higher levels of low-density CPP, primary CPP, and secondary CPP were each independently associated with progression of CKD.

Background

Mineral metabolism abnormalities are an almost universal feature of CKD and are believed to contribute to the development of many adverse events. Calciprotein particles (CPP) form spontaneously in biological fluids from calcium and phosphate together with the glycoprotein fetuin-A. While CPP formation is a part of normal mineral homeostasis, levels are chronically elevated in CKD. Several assays have been developed to measure serum CPP, each linking high CPP levels to adverse clinical outcomes in CKD. However, direct comparisons among these assays are lacking. The aim of this exploratory study was to compare these assays to assess their association with CKD progression and mortality.

Methods

We measured baseline CPP levels using various methods on stored serum samples from 189 participants in a prospective observational cohort study of patients with CKD stage 3/4. Three assays were used to measure CPP: (1 ) ELISA (fetuin-A CPP), (2 ) low-density CPP (L-CPP) and high-density CPP by gel filtration, and (3 ) primary CPP and secondary CPP (CPP-II) by flow cytometry. We examined the association of each CPP measure with all-cause mortality and CKD progression using unadjusted and covariate-adjusted Cox proportional hazard regression models.

Results

In fully adjusted models, high-density CPP (hazard ratio [HR], 1.53; 95% confidence interval [CI], 1.05 to 2.23; P = 0.028) and CPP-II (HR, 3.11; 95% CI, 1.97 to 4.98; P < 0.001) were significantly associated with an increased risk of all-cause mortality. In addition, L-CPP (HR, 1.91; 95% CI, 1.43 to 2.62; P < 0.001), primary CPP (HR, 1.31; 95% CI, 1.04 to 1.65; P = 0.023), and CPP-II (HR, 1.66; 95% CI, 1.20 to 2.29; P = 0.002) were significantly associated with CKD progression.

Conclusions

In a cohort of patients with CKD stages 3 and 4, serum CPP-II levels demonstrated the strongest association with all-cause mortality, while L-CPP levels showed the strongest association with CKD progression.

Size-dependent interactions between calciprotein particles and vascular endothelium

The underlying mechanisms governing the interactions between nanoparticles and vascular endothelial barrier remain largely unexplored, which is crucial for the optimal design of nanoparticles for clinical applications. In this study, the size-dependent interactions between calciprotein particles (CPPs) and endothelial cells (ECs) were investigated using a rat model of chronic kidney disease (CKD) induced by 5/6 nephrectomy. Two primary types of CPP1 were studied: small-sized CPP1 (S-CPP1, <50 nm) and larger CPP1 (L-CPP1, <100 nm), detected three and five weeks post-surgery, respectively. By adjusting the amounts of Ca2+, HPO4 2- and H2PO4 - ions in Dulbecco's Modified Eagle Medium supplemented with 10 % (V/V) fetal bovine serum and 1 % (V/V) Pen-Strep solution, S-CPP1 (<50 nm) with an elliptical shape, L-CPP1 (50-100 nm), and secondary CPPs (CPP2, >100 nm) with a needle-like crystalline structure, resembling endogenous CPPs, were synthesized. The results showed that S-CPP1 significantly increased endothelial permeability at concentrations of 445 μg/mL and 890 μg/mL, thereby disrupting the integrity of the endothelial barrier formed by a confluent monolayer of ECs. Immunofluorescence analysis revealed that L-CPP1 was internalized by ECs via endocytosis, while S-CPP1 disrupted VE-cadherin junctions, leading to irregular cell morphology and widened intercellular gaps. These structural changes likely contribute to medial calcification as CPPs accumulate within the circulatory system. In conclusion, the findings underscore that the interaction between CPPs and the vascular endothelium is strongly size-dependent, with significant implications for vascular system health and the design of nanoparticle-based therapies.

Effects of calciprotein particles on EMT induction in an in vitro 3D-cultured proximal tubule epithelial cell model of CKD

Calciprotein particles (CPPs) are blood-borne circulating nanoparticles composed of calcium phosphate and proteins that are known to exacerbate pathological processes such as chronic kidney disease-mineral bone disorder (CKD-MBD). Despite the significant interest in CKD-MBD pathogenesis, research directly addressing CPP-induced fibrosis in renal proximal tubules is rare, largely owing to the lack of suitable in vitro tissue models. Our study confirmed that 3D-cultured renal proximal tubule epithelial cells (PTECs) exhibited enhanced pathological characteristics compared to 2D-cultured PTECs when treated with CPPs, a key factor in CKD-MBD, and the uremic toxin. 3D-cultured PTECs under CKD-inducing conditions by CPPs were associated with epithelial-mesenchymal transition (EMT), mediated by transforming growth factor-β1 (TGF-β1), with notable changes in early EMT marker expression. Furthermore, this was attributed to increased expression of the calcium-sensing receptor (CASR), a receptor for CPPs, and activation of the downstream cell division control protein 42 (CDC42), leading to EMT progression. This study underscores the potential of PTEC-on-a-chip systems to serve as drug testing models, given the heightened sensitivity of these cells to external environments. This approach provides a better understanding of the pathological features of CKD and could contribute to the development of more effective in vitro models and therapeutics.

Physiological Concentrations of Calciprotein Particles Trigger Activation and Pro-Inflammatory Response in Endothelial Cells and Monocytes

Supraphysiological concentrations of calciprotein particles (CPPs), which are indispensable scavengers of excessive Ca2+ and PO43- ions in blood, induce pro-inflammatory activation of endothelial cells (ECs) and monocytes. Here, we determined physiological levels of CPPs (10 μg/mL calcium, corresponding to 10% increase in Ca2+ in the serum or medium) and investigated whether the pathological effects of calcium stress depend on the calcium delivery form, such as Ca2+ ions, albumin- or fetuin-centric calciprotein monomers (CPM-A/CPM-F), and albumin- or fetuin-centric CPPs (CPP-A/CPP-F). The treatment with CPP-A or CPP-F upregulated transcription of pro-inflammatory genes (VCAM1, ICAM1, SELE, IL6, CXCL8, CCL2, CXCL1, MIF) and promoted release of pro-inflammatory cytokines (IL-6, IL-8, MCP-1/CCL2, and MIP-3α/CCL20) and pro- and anti-thrombotic molecules (PAI-1 and uPAR) in human arterial ECs and monocytes, although these results depended on the type of cell and calcium-containing particles. Free Ca2+ ions and CPM-A/CPM-F induced less consistent detrimental effects. Intravenous administration of CaCl2, CPM-A, or CPP-A to Wistar rats increased production of chemokines (CX3CL1, MCP-1/CCL2, CXCL7, CCL11, CCL17), hepatokines (hepassocin, fetuin-A, FGF-21, GDF-15), proteases (MMP-2, MMP-3) and protease inhibitors (PAI-1) into the circulation. We concluded that molecular consequences of calcium overload are largely determined by the form of its delivery and CPPs are efficient inducers of mineral stress at physiological levels.

Calciprotein particle counts associate with vascular remodelling in chronic kidney disease

AIMS

Calciprotein particles (CPPs) are circulating calcium and phosphate nanoparticles associated with the development of vascular calcification (VC) in chronic kidney disease (CKD). Although recent studies have been focusing on associations of CPPs with the presence of VC in CKD, insights in the underlying processes and mechanisms by which CPPs might aggravate VC and vascular dysfunction in vivo are currently lacking. Here, we assessed the overall burden of abdominal VC in healthy kidney donors and CKD patients and subsequently performed transcriptome profiling in the vascular tissue obtained from these subjects, linking outcome to CPP counts and calcification propensity.

METHODS AND RESULTS

Calcification scores were quantified in renal arteries, iliac arteries, and abdominal aorta using computed tomography (CT) scans of kidney donors and CKD patients. The vascular tissue was collected from kidney donors (renal artery) and CKD patients (iliac artery), after which bulk RNA sequencing and gene set enrichment analysis (GSEA) were performed on a subset of patients. Calcification propensity (crystallization time, T50) was measured using nephelometry and CPP counts with microparticle flow cytometric analysis. Increased calcification scores (based on CT) were found in CKD patients compared to kidney donors. Transcriptome profiling revealed enrichment for processes related to endothelial activation, inflammation, extracellular matrix (ECM) remodelling, and ossification in CKD vascular biopsies compared to kidney donors. Calcification propensity was increased in CKD, as well as CPP counts, with the latter being significantly associated with markers of vascular remodelling.

CONCLUSION

Our findings reveal that CKD is characterized by systemic VC with increased calcification propensity and CPP counts. Transcriptome profiling showed altered vascular gene expression with enrichment for endothelial activation, inflammation, ECM remodelling, and ossification. Moreover, we demonstrate, for the first time, that vascular remodelling processes are associated with increased circulating CPP counts. Interventions targeting CPPs are promising avenues for alleviating vascular remodelling and VC in CKD.

Future of Uremic Toxin Management

During the progression of chronic kidney disease (CKD), the retention of uremic toxins plays a key role in the development of uremic syndrome. Knowledge about the nature and biological impact of uremic toxins has grown exponentially over the past decades. However, the science on reducing the concentration and effects of uremic toxins has not advanced in parallel. Additionally, the focus has remained for too long on dialysis strategies, which only benefit the small fraction of people with CKD who suffer from advanced kidney disease, whereas uremic toxicity effects are only partially prevented. This article reviews recent research on alternative methods to counteract uremic toxicity, emphasizing options that are also beneficial in the earlier stages of CKD, with a focus on both established methods and approaches which are still under investigation or at the experimental stage. We will consequently discuss the preservation of kidney function, the prevention of cardiovascular damage, gastro-intestinal interventions, including diet and biotics, and pharmacologic interventions. In the final part, we also review alternative options for extracorporeal uremic toxin removal. The future will reveal which of these options are valid for further development and evidence-based assessment, hopefully leading to a more sustainable treatment model for CKD than the current one.

Calciprotein particles induce arterial stiffening ex vivo and impair vascular cell function

Calciprotein particles (CPPs) are an endogenous buffering system, clearing excessive amounts of Ca2+ and PO43- from the circulation and thereby preventing ectopic mineralization. CPPs circulate as primary CPPs (CPP1), which are small spherical colloidal particles, and can aggregate to form large, crystalline, secondary CPPs (CPP2). Even though it has been reported that CPPs are toxic to vascular smooth muscle cells (VSMC) in vitro, their effect(s) on the vasculature remain unclear. Here we have shown that CPP1, but not CPP2, increased arterial stiffness ex vivo. Interestingly, the effects were more pronounced in the abdominal infrarenal aorta compared to the thoracic descending aorta. Further, we demonstrated that CPP1 affected both endothelial and VSMC function, impairing vasorelaxation and contraction respectively. Concomitantly, arterial glycosaminoglycan accumulation was observed as well, which is indicative of an increased extracellular matrix stiffness. However, these effects were not observed in vivo. Hence, we concluded that CPP1 can induce vascular dysfunction.

Mineralo-organic particles inhibit influenza A virus infection by targeting viral hemagglutinin activity

Aim: Mineralo-organic particles, naturally present in human body fluids, participate in ectopic calcification and inflammatory diseases. These particles coexist with influenza A virus (IAV) in the same microenvironment during viral infection. Our objective was to investigate the functional consequences of the potential interactions between these particles and the virions.Materials & methods: We used in vitro models, including electron microscopy, fluorescence microscopy, hemagglutination assay and viral infection assays to examine the interactions.Results: Mineralo-organic particles bind to IAV virions through interactions involving particle-bound fetuin-A and mineral content, effectively engaging viral hemagglutinin. These interactions result in hindered viral infection.Conclusion: These findings uncover the novel interactions between mineralo-organic particles and IAV, highlighting the impact of virus microenvironment complexity.

Vascular Calcification Is Accelerated by Hyponatremia and Low Osmolality

BACKGROUND

Hyponatremia, frequently observed in patients with chronic kidney disease, is associated with increased cardiovascular morbidity and mortality. Hyponatremia or low osmolality induces oxidative stress and cell death, both of which accelerate vascular calcification (VC), a critical phenotype in patients with chronic kidney disease. Whether hyponatremia or low osmolality plays a role in the pathogenesis of VC is unknown.

METHODS

Human vascular smooth muscle cells (VSMCs) and mouse aortic rings were cultured in various osmotic conditions and calcifying medium supplemented with high calcium and phosphate. The effects of low osmolality on phenotypic change and oxidative stress in the cultured VSMCs were examined. Microarray analysis was conducted to determine the main signaling pathway of osmolality-related VC. The transcellular sodium and calcium ions flux across the VSMCs were visualized by live imaging. Furthermore, the effect of osmolality on calciprotein particles (CPPs) was investigated. Associations between arterial intimal calcification and hyponatremia or low osmolality were examined by a cross-sectional study using human autopsy specimens obtained in the Hisayama Study.

RESULTS

Low osmolality exacerbated calcification of the ECM (extracellular matrix) of cultured VSMCs and mouse aortic rings. Oxidative stress and osteogenic differentiation of VSMCs were identified as the underlying mechanisms responsible for low osmolality-induced VC. Microarray analysis showed that low osmolality activated the Rac1 (Ras-related C3 botulinum toxin substrate 1)-Akt (protein kinase B) pathway and reduced NCX1 (Na-Ca exchanger 1) expression. Live imaging showed synchronic calcium ion efflux and sodium ion influx via NCX1 when extracellular sodium ion concentrations were increased. An NCX1 inhibitor promoted calcifying media-induced VC by reducing calcium ion efflux. Furthermore, low osmolality accelerated the generation and maturation steps of CPPs. The cross-sectional study of human autopsy specimens showed that hyponatremia and low osmolality were associated with a greater area of arterial intimal calcification.

CONCLUSIONS

Hyponatremia and low osmolality promote VC through multiple cellular processes, including the Rac1-Akt pathway activation.

Vitamin K for Vascular Calcification in Kidney Patients: Still Alive and Kicking, but Still a Lot to Learn

Patients with chronic kidney disease (CKD) suffer disproportionately from a high burden of cardiovascular disease, which, despite recent scientific advances, remains partly understood. Vascular calcification (VC) is the result of an ongoing process of misplaced calcium in the inner and medial layers of the arteries, which has emerged as a critical contributor to cardiovascular events in CKD. Beyond its established role in blood clotting and bone health, vitamin K appears crucial in regulating VC via vitamin K-dependent proteins (VKDPs). Among these, the matrix Gla protein (MGP) serves as both a potent inhibitor of VC and a valuable biomarker (in its inactive form) for reflecting circulating vitamin K levels. CKD patients, especially in advanced stages, often present with vitamin K deficiency due to dietary restrictions, medications, and impaired intestinal absorption in the uremic environment. Epidemiological studies confirm a strong association between vitamin K levels, inactive MGP, and increased CVD risk across CKD stages. Based on the promising results of pre-clinical data, an increasing number of clinical trials have investigated the potential benefits of vitamin K supplementation to prevent, delay, or even reverse VC, but the results have remained inconsistent.

Liver and spleen predominantly mediate calciprotein particle clearance in a rat model of chronic kidney disease

Calciprotein particles (CPPs) provide an efficient mineral buffering system to prevent the complexation of phosphate and calcium in the circulation. However, in chronic kidney disease (CKD), the phosphate load exceeds the mineral buffering capacity, resulting in the formation of crystalline CPP2 particles. CPP2 have been associated with cardiovascular events and mortality. Moreover, CPP2 have been demonstrated to induce calcification in vitro. In this study, we examined the fate of CPP2 in a rat model of CKD. Calcification was induced in Sprague-Dawley rats by 5/6 nephrectomy (5/6-Nx) combined with a high-phosphate diet. Control rats received sham surgery and high-phosphate diet. Twelve weeks after surgery, kidney failure was significantly induced in 5/6-Nx rats as determined by enhanced creatinine and urea plasma levels and abnormal kidney histological architecture. Subsequently, radioactive and fluorescent (FITC)-labeled CPP2 ([89Zr]Zr-CPP2-FITC) were injected intravenously to determine clearance in vivo. Using positron emission tomography scans and radioactive biodistribution measurements, it was demonstrated that [89Zr]Zr-CPP2-FITC are mainly present in the liver and spleen in both 5/6-Nx and sham rats. Immunohistochemistry showed that [89Zr]Zr-CPP2-FITC are predominantly taken up by Kupffer cells and macrophages. However, [89Zr]Zr-CPP2-FITC could also be detected in hepatocytes. In the different parts of the aorta and in the blood, low values of [89Zr]Zr-CPP2-FITC were detectable, independent of the presence of calcification. CPP2 are cleared rapidly from the circulation by the liver and spleen in a rat model of CKD. In the liver, Kupffer cells, macrophages, and hepatocytes contribute to CPP2 clearance.NEW & NOTEWORTHY Calciprotein particles (CPPs) buffer calcium and phosphate in the blood to prevent formation of crystals. In CKD, increased phosphate levels may exceed the buffering capacity of CPPs, resulting in crystalline CPPs that induce calcification. This study demonstrates that labeled CPPs are predominantly cleared from the circulation in the liver by Kupffer cells, macrophages, and hepatocytes. Our results suggest that targeting liver CPP clearance may reduce the burden of crystalline CPP in the development of vascular calcification.

Phosphate in Cardiovascular Disease: From New Insights Into Molecular Mechanisms to Clinical Implications

Hyperphosphatemia is a common feature in patients with impaired kidney function and is associated with increased risk of cardiovascular disease. This phenomenon extends to the general population, whereby elevations of serum phosphate within the normal range increase risk; however, the mechanism by which this occurs is multifaceted, and many aspects are poorly understood. Less than 1% of total body phosphate is found in the circulation and extracellular space, and its regulation involves multiple organ cross talk and hormones to coordinate absorption from the small intestine and excretion by the kidneys. For phosphate to be regulated, it must be sensed. While mostly enigmatic, various phosphate sensors have been elucidated in recent years. Phosphate in the circulation can be buffered, either through regulated exchange between extracellular and cellular spaces or through chelation by circulating proteins (ie, fetuin-A) to form calciprotein particles, which in themselves serve a function for bulk mineral transport and signaling. Either through direct signaling or through mediators like hormones, calciprotein particles, or calcifying extracellular vesicles, phosphate can induce various cardiovascular disease pathologies: most notably, ectopic cardiovascular calcification but also left ventricular hypertrophy, as well as bone and kidney diseases, which then propagate phosphate dysregulation further. Therapies targeting phosphate have mostly focused on intestinal binding, of which appreciation and understanding of paracellular transport has greatly advanced the field. However, pharmacotherapies that target cardiovascular consequences of phosphate directly, such as vascular calcification, are still an area of great unmet medical need.

Calciprotein Particles Induce Cellular Compartment-Specific Proteome Alterations in Human Arterial Endothelial Cells

Calciprotein particles (CPPs) are indispensable scavengers of excessive Ca2+ and PO43- ions in blood, being internalised and recycled by liver and spleen macrophages, monocytes, and endothelial cells (ECs). Here, we performed a pathway enrichment analysis of cellular compartment-specific proteomes in primary human coronary artery ECs (HCAEC) and human internal thoracic artery ECs (HITAEC) treated with primary (amorphous) or secondary (crystalline) CPPs (CPP-P and CPPs, respectively). Exposure to CPP-P and CPP-S induced notable upregulation of: (1) cytokine- and chemokine-mediated signaling, Ca2+-dependent events, and apoptosis in cytosolic and nuclear proteomes; (2) H+ and Ca2+ transmembrane transport, generation of reactive oxygen species, mitochondrial outer membrane permeabilisation, and intrinsic apoptosis in the mitochondrial proteome; (3) oxidative, calcium, and endoplasmic reticulum (ER) stress, unfolded protein binding, and apoptosis in the ER proteome. In contrast, transcription, post-transcriptional regulation, translation, cell cycle, and cell-cell adhesion pathways were underrepresented in cytosol and nuclear compartments, whilst biosynthesis of amino acids, mitochondrial translation, fatty acid oxidation, pyruvate dehydrogenase activity, and energy generation were downregulated in the mitochondrial proteome of CPP-treated ECs. Differentially expressed organelle-specific pathways were coherent in HCAEC and HITAEC and between ECs treated with CPP-P or CPP-S. Proteomic analysis of mitochondrial and nuclear lysates from CPP-treated ECs confirmed bioinformatic filtration findings.