Ca2+-Mg2+-dependent ATP-ase activity and calcium homeostasis in children with chronic kidney disease

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Intracellular calcium increases in vascular smooth muscle cells with progression of chronic kidney disease in a rat model

Background

Vascular smooth muscle cells (VSMCs) exhibit phenotypic plasticity, promoting vascular calcification and increasing cardiovascular risk. Changes in VSMC intracellular calcium ([Ca 2+ ] i ) are a major determinant of plasticity, but little is known about changes in [Ca 2+ ] i in chronic kidney disease (CKD). We have previously demonstrated such plasticity in aortas from our rat model of CKD and therefore sought to examine changes in [Ca 2+ ] i during CKD progression.

Materials and Methods

We examined freshly isolated VSMCs from aortas of normal rats, Cy/+ rats (CKD) with early and advanced CKD, and advanced CKD rats treated without and with 3% calcium gluconate (CKD + Ca 2+ ) to lower parathyroid hormone (PTH) levels. [Ca 2+ ] i was measured with fura-2.

Results

Cy/+ rats developed progressive CKD, as assessed by plasma levels of blood urea nitrogen, calcium, phosphorus, parathyroid hormone and fibroblast growth factor 23. VSMCs isolated from rats with CKD demonstrated biphasic alterations in resting [Ca 2+ ] i : VSMCs from rats with early CKD exhibited reduced resting [Ca 2+ ] i , while VSMCs from rats with advanced CKD exhibited elevated resting [Ca 2+ ] i . Caffeine-induced sarcoplasmic reticulum (SR) Ca 2+ store release was modestly increased in early CKD and was more drastically increased in advanced CKD. The advanced CKD elevation in SR Ca 2+ store release was associated with a significant increase in the activity of the sarco-endoplasmic reticulum Ca 2+ ATPase (SERCA); however, SERCA2a protein expression was decreased in advanced CKD. Following SR Ca 2+ store release, recovery of [Ca 2+ ] i in the presence of caffeine and extracellular Ca 2+ was attenuated in VSMCs from rats with advanced CKD. This impairment, together with reductions in expression of the Na + /Ca 2+ exchanger, suggest a reduction in Ca 2+ extrusion capability. Finally, store-operated Ca 2+ entry (SOCE) was assessed following SR Ca 2+ store depletion. Ca 2+ entry during recovery from caffeine-induced SR Ca 2+ store release was elevated in advanced CKD, suggesting a role for exacerbated SOCE with progressing CKD.

Conclusions

With progressive CKD in the Cy/+ rat there is increased resting [Ca 2+ ] i in VSMCs due, in part, to increased SOCE and impaired calcium extrusion from the cell. Such changes may predispose VSMCs to phenotypic changes that are a prerequisite to calcification.

The Effect of Vitamin D3 Supplementation on Intracellular Calcium and Plasma Membrane Calcium ATPase Activity in Early Stages of Chronic Kidney Disease

Chronic kidney disease (CKD) is associated with increased concentration of intracellular calcium, which is pathological and may lead to irreversible damage of cell functions and structures. The aim of our study was to investigate the impact of 6 months vitamin D3 supplementation (14 000 IU/week) on free cytosolic calcium concentration ([Ca2+]i) and on the plasma membrane calcium ATPase (PMCA) activity of patients with CKD stage 2-3. PMCA activity of patients was also compared to that of healthy volunteers. Vitamin D3 supplementation of CKD patients resulted in the decrease of [Ca2+]i (119.79±5.87 nmol/l vs. 105.36±3.59 nmol/l, n=14, P<0.001), whereas PMCA activity of CKD patients (38.75±22.89 nmol Pi/mg/h) remained unchanged after vitamin D3 supplementation (40.96±17.74 nmol Pi/mg/h, n=14). PMCA activity of early stage CKD patients before supplementation of vitamin D3, was reduced by 34 % (42.01±20.64 nmol Pi/mg/h) in comparison to healthy volunteers (63.68±20.32 nmol Pi/mg/h, n=28, P<0.001). These results indicate that vitamin D3 supplementation had a lowering effect on [Ca2+]i and negligible effect on PMCA activity in CKD patients.

Erythrocyte caspase-3 levels in children with chronic kidney disease

OBJECTIVES

In chronic kidney disease (CKD), a number of intra- and extracellular factors, e.g., uremic toxins, mechanic, oxidative or osmotic stress - induce changes (rearrangements) in the structure of cytoplasmatic membrane, while also simultaneously deregulating blood cell metabolism and, in consequence, contributing to preliminary ageing and suicidal death of red blood cells (RBCs).The aim of the reported study was an evaluation of caspase-3 and lactate dehydrogenase activities and of ATP concentrations in erythrocytes as cellular responses to CKD progress.

DESIGN AND METHODS

Conservatively treated sixty (60) CKD children were enrolled into the study and divided, according to CKD progression (stage I-IV). The control group consisted of twenty-five (25) healthy children. The activity of caspase-3 (Casp-3) and lactate dehydrogenase (LDH) were spectrophotometrically assayed in haemolysed erythrocytes. Adenosine triphosphate (ATP(e)) concentrations were measured by means of a luciferin-luciferase kit.

RESULTS

A gradual increase of LDH and ATP levels was observed in transition from CKD stage I to stage III. In Group IV, the levels of those parameters were statistically significantly lower than in the control group. The activity of Casp-3 in Group I was comparable to that in healthy children. The highest activity of Casp-3 was observed in Group III.

CONCLUSIONS

1. The activity of caspase-3 in RBCs of CKD children grows with progression of the disease. 2. The lower LDH activities and the ATP concentration drop below the values characteristic for the control group, as observed in stage IV of CKD, indicate a compromised energy balance.

Purinergic P2X7 receptors participate in disturbed intracellular calcium homeostasis in peripheral blood mononuclear cells of patients with chronic kidney disease

BACKGROUND

P2X(7) receptors intervene with lymphocyte activation and are responsible for multiple processes, including calcium influx. Here, we studied the participation of P2X(7) receptors in disturbed intracellular calcium homeostasis regulation in early-stage chronic kidney disease (CKD).

METHODS

The study involved 20 healthy volunteers and 20 CKD stage 2-3 patients. The free cytosolic calcium concentration ([Ca(2+)](i)) was measured using fluorimetry. The P2X(7) pore function was evaluated by the fluorescent dye ethidium bromide.

RESULTS

In peripheral blood mononuclear cells (PBMCs) of patients, [Ca(2+)](i), intracellular calcium stores and the capacitative calcium entry were increased when compared with healthy subjects. The agonist of P2X(7) receptor BzATP caused a sustained increase in [Ca(2+)](i) in both groups, but the effect was smaller in patients. The antagonist at the P2X(7) receptor KN-62 reduced [Ca(2+)](i) in patients, but had no effect in healthy subjects. In patients, the permeability of ethidium bromide through P2X(7) pores, as well as through BzATP-activated and KN-62-inhibited pores, was distinct from permeability in healthy volunteers.

CONCLUSIONS

These results demonstrate that the calcium signaling pathway in PBMCs of CKD patients is defective already in CKD stage 2-3, and the pore-forming P2X(7) receptors are involved in these pathophysiological processes.

Ca(2+)-Mg (2+)-dependent ATP-ase activity in hemodialyzed children. Effect of a hemodialysis session

In the course of chronic kidney disease (CKD) the intracellular erythrocyte calcium (Ca (i) (2+) ) level increases along with the progression of the disease. The decreased activity of Ca(2+)-Mg(2+)-dependent ATP-ase (PMCA) and its endogenous modulators calmodulin (CALM), calpain (CANP), and calpastatin (CAST) are all responsible for disturbed calcium metabolism. The aim of the study was to analyze the activity of PMCA, CALM, and the CANP-CAST system in the red blood cells (RBCs) of hemodialyzed (HD) children and to estimate the impact of a single HD session on the aforementioned disturbances. Eighteen patients on maintenance HD and 30 healthy subjects were included in the study. CALM, Ca (i) (2+) levels and basal PMCA (bPMCA), PMCA, CANP, and CAST activities were determined in RBCs before HD, after HD, and before the next HD session. Prior to the HD session, the level of Ca (i) (2+) and the CAST activity were significantly higher, whereas bPMCA, PMCA, and CANP activities and the CALM level were significantly lower than in controls. After the HD session, the Ca (i) (2+) concentration and the CAST activity significantly decreased compared with the basal values, whereas the other parameters significantly increased, although they did not reach the levels of healthy children. The values observed prior to both HD sessions were similar. Ca (i) (2+) homeostasis is severely disturbed in HD children, which may be caused by the reduction in the PMCA activity, CALM deficiency, and CANP-CAST system disturbances. A single HD session improved these disturbances but the effect is transient.

Antenna-based optical imaging of single Ca2+ transmembrane proteins in liquids

Understanding the diversity of biological processes requires methods that can address single proteins in their natural environment and provide insights into structural and functional properties, as well as the local distribution of each individual protein. We use an optical antenna in the form of a single gold nanoparticle to localize incident laser radiation to 50 nm, significantly smaller than the diffraction limit of light. Our approach enables us to optically resolve individual plasma-membrane-bound Ca2+ pumps (PMCA4) immersed in aqueous environments and to determine the distribution of interprotein distances. We are able to correlate the protein maps with local topology. Improved antenna geometries will make it possible to resolve, identify, and probe single membrane proteins in live cells with true protein resolution of 5-10 nm.