Maintenance Of Calcium Homeostasis Research Articles

Mpp4 is required for proper localization of plasma membrane calcium ATPases and maintenance of calcium homeostasis at the rod photoreceptor synaptic terminals

Membrane palmitoylated protein 4 (Mpp4) is a member of the membrane-associated guanylate kinase family. We show that Mpp4 localizes specifically to the plasma membrane of photoreceptor synaptic terminals. Plasma membrane Ca(2+) ATPases (PMCAs), the Ca(2+) extrusion pumps, interact with an Mpp4-dependent presynaptic membrane protein complex that includes Veli3 and PSD95. In mice lacking Mpp4, PMCAs were lost from rod photoreceptor presynaptic membranes. Synaptic ribbons were enlarged, a phenomenon known to correlate with higher Ca(2+). SERCA2 (sarcoplasmic-endoplasmic reticulum Ca(2+) ATPase, type 2), which pumps cytosolic Ca(2+) into intracellular Ca(2+) stores and localizes next to the ribbons, was increased. The distribution of IP(3)RII (InsP(3) receptor, type 2), which releases Ca(2+) from the stores, was shifted away from the synaptic terminals. Synaptic transmission to second-order neurons was maintained but was reduced in amplitude. These data suggest that loss of Mpp4 disrupts a Ca(2+) extrusion mechanism at the presynaptic membranes, with ensuing adaptive responses by the photoreceptor to restore Ca(2+) homeostasis. We propose that Mpp4 organizes a presynaptic protein complex that includes PMCAs and has a role in modulating Ca(2+) homeostasis and synaptic transmission in rod photoreceptors.

Selective vitamin D receptor modulators and their effects on colorectal tumor growth

The active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], is an endocrine hormone whose classic role is the maintenance of calcium homeostasis. It is well documented that 1,25(OH)2D3 also has anti-tumor effects on a number of cancers and cancer cell lines including breast, colorectal, gastric, liver, ovarian, prostate, and non-melanoma skin cancers. Included in the anti-tumor activities of 1,25(OH)2D3 are its ability to cause antiproliferation, prodifferentation and decrease angiogenesis. Furthermore, through regulation of the plaminogen activator (PA) system and a class of proteolytic enzymes called matrix metalloproteinases (MMPs), 1,25(OH)2D3 reduces the invasive spread of tumor cells. Because of the calcemic limitations of using 1,25(OH)2D3 as a therapy, we have tested the effects of a novel Gemini vitamin D analogue, Deuterated Gemini (DG), on mouse colorectal cancer. We demonstrated that DG is more potent in reducing tumor volume and mass, compared to control and 1,25(OH)2D3. DG significantly prevented (100% reduction, p<0.05) the invasive spread of colorectal tumor cells into the surrounding muscle, and had no effect on serum calcium levels. Thus, DG acts as a selective vitamin D receptor modulator (SVDRM) by enhancing select anti-tumor characteristic 1,25(OH)2D3 activities, without inducing hypercalcemia. Thus, DG shows promise in the development of colorectal cancer therapies.

Effect of Leptin on Regulation of Renal 25-hydroxyvitamin D3 Metabolism and Maintenance of Calcium Homeostasis

Effect of Leptin on Regulation of Renal 25-hydroxyvitamin D3 Metabolism and Maintenance of Calcium Homeostasis

Prevention of osteoporosis by foods and dietary supplements. Vitamin D and bone metabolism

Vitamin D is an essential nutrient for maintenance of calcium homeostasis. In case of vitamin D deficiency, low vitamin D production rate in skin, insufficient metabolic activation of vitamin D, reduction of bone mass, and osteoporosis would be induced. In the last year, "Dietary Reference Intakes for Japanese, 2005" was published by Ministry of Health, Labour, and Welfare, Japan. This reference was based on the concept of the dietary reference intake (DRIs). The parameter for the assessment of vitamin D nutrition status was defined on the basis of the plasma levels of 25-hydroxyvitamin D, and established as "adequate intake" in the reference. The dietary vitamin D intake for prevention of osteoporosis will be more important according to the progress of examination of dietary vitamin D intake in Japanese.

The Importance of Calcium-Binding Proteins in Childhood Diseases

Calcium is an essential component of bone. It is also required for many intracellular functions. Maintenance of calcium homeostasis is a dynamic process involving calcium absorption and excretion in the intestine, filtration and reabsorption in the kidney, and storage and mobilization in the skeleton. Nearly all of the body calcium (98%) is stored in slowly exchangeable skeletal hydroxyapatite crystals. It is the rapidly exchangeable calcium in recently deposited bone and in the extracellular, intracellular, and vascular spaces (2% of body calcium) that plays a critical role in intracellular communication, synthesis and release of neurotransmitters and hormones, muscle contraction, development, and differentiation, as well as aging and apoptosis. Therefore it is not surprising that a number of diseases in children and adults are associated with altered Ca 21 -homeostasis. 1-4 Pediatricians understand the role of calcium in bone development in children. They are also familiar with the Ca 21 -levels in body fluids and with the regulation of the extracellular calcium in the serum by integrated systems involving parathyroid hormone (PTH), vitamin D, calcitonin and the Ca 21 -signaling receptor (CaSR). Ionized calcium is metabolically active, and elevations may be seen in hyperparathyroidism, metastatic bone tumor, milkalkali syndrome, multiple myeloma, Paget disease, sarcoidosis, PTH-secreting tumors (paraneoplastic syndrome), and vitamin D intoxication. Lower than normal values may be seen in hypoparathyroidism, malabsorption, osteomalacia, pancreatitis, kidney failure, rickets, and vitamin D deficiency. 5,6 Pediatricians are also aware of 2 metabolic diseases: familial hypocalciuric hypercalcemia (FHH), and neonatal severe hypercalcemia, which are inherited disorders of the extracellular calcium sensing pathway caused by various mutations in the Ca 21 -signaling receptor. 7 Calcium is also essential in the coagulation system and in fibrin formation. 6 The vitamin K‐dependent proteins which participate in blood coagulation (prothrombin, various factors) contain stretches of g-glutamic acids residues (necessary for Ca 21 -binding). Lack of these residues in the vitamin K‐dependent proteins results in a loss of vitamin K response and severe bleeding episodes in early childhood. A future approach to treatment of this inherited metabolic Ca 21 -disorder (some forms of hemophilia) may be gene therapy. There are a number of other diseases in children associated with impaired extracellular Ca 21 -homeostasis, and this includes the calcifications observed in various tissues including the brain and type II citrullinemia (citrin deficiency), caused by a defect of the calcium-binding mitochondrial carrier protein. 1 Calcification caused by mineral imbalance involves a number of hormones and factors, and current research is exploring theories, including the possibility of a common cause between bone mineralization and vascular calcification, 4 resulting, it is hoped, in a more effective treatment. The purpose of this review is, however, to summarize recent advances in our understanding of abnormalities in the intracellular calcium sensing pathways, focussing on Ca 21 -binding proteins as diagnostic and predictive tools of childhood diseases.

Potential Role of Pancreatic and Enteric Hormones in Regulating Bone Turnover

Potential Role of Pancreatic and Enteric Hormones in Regulating Bone Turnover

Bone and the Kidney: A Systems Biology Approach to the Molecular Mechanisms of Renal Osteodystrophy

Despite its apparent static condition, the skeleton undergoes a permanent process of remodeling mediated by osteoblasts and osteoclasts. The activity of these cells is regulated by a plethora of factors, ranging from mechanical stress to the effects of hormones to the immune system. One well-studied regulatory system involves the maintenance of calcium homeostasis through a network whose main regulatory components include ionized calcium, phosphate, parathyroid hormone and active vitamin D. This system establishes the link between bone and kidney, as one of the kidney's endocrine functions is the activation of vitamin D, while electrolyte homeostasis is one of its excretory functions. Impaired renal function leads to disturbances in this regulatory system, resulting in the complex syndrome of renal osteodystrophy that affects the majority of patients with chronic renal failure. This review summarizes the current understanding of bone physiology on a molecular level, examines some of the pathological pathways related to renal disease, and concludes with an outlook on how the emerging field of systems biology may contribute to a more dynamic and quantitative understanding of the physiology and pathophysiology of renal bone disease.

Disruption of the endoplasmic reticulum by cytotoxins in LLC-PK1 cells

Prior induction of an endoplasmic reticulum stress response results in protection against reactive cytotoxins in the LLC-PK1 cell line. The purpose of this investigation was to determine therefore if the endoplasmic reticulum was disrupted by iodoacetamide, tert-butylhydroperoxide or sulfamethoxazole hydroxylamine. Toxic concentrations of the three toxins caused a dramatic loss of GRP94 protein within 3–8 h of exposure, while induction of GRP78 and calreticulin occurred at 8 and 24 h following exposure. There was no evidence of cytosolic elevation of calcium and neither dantrolene nor xestospongin were able to block the cytotoxicity of IDAM and TBHP. Exposure to the toxins led to DNA degradation and cleavage of procaspase-12. There was only evidence of procaspase-3 cleavage after TBHP exposure. These results demonstrate that the ER is disrupted by the reactive cytotoxins examined in LLC-PK1cells and suggest that the cytoprotection against low to moderate concentrations of cytotoxins observed following endoplasmic reticulum stress protein induction is likely due to a mechanism other than maintenance of calcium homeostasis.

Coactivation of the Human Vitamin D Receptor by the Peroxisome Proliferator-Activated Receptor γ Coactivator-1 α

The vitamin D receptor (VDR) belongs to the superfamily of steroid/thyroid hormone receptors that is activated by 1alpha,25-dihydroxyvitamin D(3). Traditional targets for 1alpha,25-dihydroxyvitamin D(3) action include tissues involved in the maintenance of calcium homeostasis and bone development and remodeling. Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha), a transcriptional coactivator that plays a role in mitochondrial biogenesis and energy metabolism, is predominantly expressed in kidney, heart, liver, and skeletal muscle. Because VDR and PGC-1alpha display an overlapping pattern of expression, we investigated the possibility that PGC-1alpha could serve as a coactivator for VDR. Transient cotransfection assays demonstrate that PGC-1alpha augments ligand-dependent VDR transcription when either full-length VDR or Gal4 DNA binding domain-VDR-ligand binding domain chimeras were analyzed. Furthermore, mammalian two-hybrid assays, coimmunoprecipitation analyses, and biochemical coactivator recruitment assays demonstrate a ligand-dependent interaction between the two proteins both in cells and in vitro. The coactivation potential of PGC-1alpha requires an intact AF-2 domain of VDR and the LXXLL motif in PGC-1alpha. Taken together, these results indicate that PGC-1alpha serves as a coactivator for VDR.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

PTH/PTHrP activity and the programming of skeletal development in utero.

There is increasing evidence that nutritional deficiency in utero adversely affects bone development and the risk of developing osteoporosis in later life. Although the mechanisms involved are unknown, circumstantial evidence points to an important role of PTH/PTHrP activity. It is recognized that PTH and PTHrP are critically involved in regulating fetal calcium homeostasis, actions that are mediated at least in part by PPR. As well as playing a central role in the maintenance of calcium homeostasis in the fetus, studies in transgenic mice show that PTH, PTHrP, and PPR exert similar effects on skeletal development in utero, acting to increase the size of the trabecular envelope and decrease that of the cortical envelopes. Taken together, these observations raise the possibility that stimulation of PTH/PTHrP activity in the fetus in response to calcium deficiency acts to increase the size of the trabecular envelope but to reduce that of the cortical envelope. Although any increase in trabecular bone at birth is likely to be relatively transient, a decrease in size of the cortical envelope may have a persistent effect on the trajectory of bone growth in subsequent childhood. Consistent with this proposal, preliminary findings from birth cohort studies suggest that maternal calcium intake and cord blood calcium levels are positively related to bone mass of the offspring as assessed later in childhood. Further studies are justified to determine whether alterations in fetal PTH/ PTHrP activity caused by calcium stress lead to a reduction in size of the cortical envelope at birth that persists into childhood and later adult life and to identify modifiable maternal factors that are responsible for these changes.

Myocardial protection in adult cardiac surgery: current options and future challenges.

Current techniques of myocardial protection are evolving with the use of less conventional modalities of cardioplegia and have reduced the morbidity and mortality of cardiac operations. Blood cardioplegic solutions appear superior to cold cardioplegia in terms of myocardial protection and adjuncts as glutamate/aspartate enhancement, antioxidant supplementation, nitric oxide donors and maintenance of calcium homeostasis seem effective. In the near future, further experimental and clinical investigations about pharmacological preconditioning, sodium-hydrogen exchangers inhibition and gene therapy need to be addressed to well define their potential role in the improvement of current techniques of myocardial protection that are suboptimal in high-risk clinical settings.

Glutamate Decreases Mitochondrial Size and Movement in Primary Forebrain Neurons

Mitochondria are essential to maintain neuronal viability. In addition to the generation of ATP and maintenance of calcium homeostasis, the effective delivery of mitochondria to the appropriate location within neurons is also likely to influence their function. In this study we examined mitochondrial movement and morphology in primary cultures of rat forebrain using a mitochondrially targeted enhanced yellow fluorescent protein (mt-eYFP). Mt-eYFP-labeled mitochondria display a characteristic elongated phenotype and also move extensively. Application of glutamate to cultures results in a rapid diminution of movement and also an alteration from elongated to rounded morphology. This effect required the entry of calcium and was mediated by activation of the NMDA subtype of glutamate receptor. Treatment of cultures with an uncoupler or blocking ATP synthesis with oligomycin also stopped movement but did not alter morphology. Interestingly, application of glutamate together with the uncoupler did not prevent the changes in movement or shape but facilitated recovery after washout of the stimuli. This suggests that the critical target for calcium in this paradigm is cytosolic. These studies demonstrate that in addition to altering the bioenergetic properties of mitochondria, neurotoxins can also alter mitochondrial movement and morphology. We speculate that neurotoxin-mediated impairment of mitochondrial delivery may contribute to the injurious effects of neurotoxins.

Potassium-dependent sodium-calcium exchange through the eye of the fly.

In this review, we describe the characterization of a Drosophila sodium/calcium-potassium exchanger, Nckx30C. Sodium/calcium (-potassium) exchangers (NCX and NCKX) are required for the rapid removal of calcium in excitable cells. The deduced protein topology for NCKX30C is similar to that of mammalian NCKX, with 5 hydrophobic domains in the amino terminus separated from 6 at the carboxy-terminal end by a large intracellular loop. NCKX30C functions as a potassium-dependent sodium-calcium exchanger and is expressed in adult neurons and during ventral nerve cord development in the embryo. Nckx30C is expressed in a dorsal/ventral pattern in the eye-antennal disc, suggesting that large fluxes of calcium may be occurring during imaginal disc development in the larvae. NCKX30C may play a critical role in modulating calcium during development as well as in the removal of calcium and maintenance of calcium homeostasis in adults.

Cerebellar neurons lacking complex gangliosides degenerate in the presence of depolarizing levels of potassium

Mice engineered to lack GM2/GD2 synthase (GalNAc-T), with resultant deficit of GM2, GD2, and all gangliotetraose gangliosides, were originally described as showing a relatively normal phenotype with only a slight reduction in nerve conduction. However, a subsequent study showed that similar animals suffer axonal degeneration, myelination defects, and impaired motor coordination. We have examined the behavior of cerebellar granule neurons from these neonatal knockouts in culture and have found evidence of impaired capacity for Ca2+ regulation. These cells showed relatively normal behavior when grown in the presence of physiological or moderately elevated K+ but gradually degenerated in the presence of high K+. This degeneration in depolarizing medium was accompanied by progressive elevation of intracellular calcium and onset of apoptosis, phenomena not observed with normal cells. No differences were detected in cells from normal vs. heterozygous mice. These findings suggest that neurons from GalNAc-T knockout mice are lacking a calcium regulatory mechanism that is modulated by one or more of the deleted gangliosides, and they support the hypothesis that maintenance of calcium homeostasis is one function of complex gangliosides during, and perhaps subsequent to, neuronal development.

Cerebellar neurons lacking complex gangliosides degenerate in the presence of depolarizing levels of potassium.

Mice engineered to lack GM2/GD2 synthase (GalNAc-T), with resultant deficit of GM2, GD2, and all gangliotetraose gangliosides, were originally described as showing a relatively normal phenotype with only a slight reduction in nerve conduction. However, a subsequent study showed that similar animals suffer axonal degeneration, myelination defects, and impaired motor coordination. We have examined the behavior of cerebellar granule neurons from these neonatal knockouts in culture and have found evidence of impaired capacity for Ca2+ regulation. These cells showed relatively normal behavior when grown in the presence of physiological or moderately elevated K+ but gradually degenerated in the presence of high K+. This degeneration in depolarizing medium was accompanied by progressive elevation of intracellular calcium and onset of apoptosis, phenomena not observed with normal cells. No differences were detected in cells from normal vs. heterozygous mice. These findings suggest that neurons from GalNAc-T knockout mice are lacking a calcium regulatory mechanism that is modulated by one or more of the deleted gangliosides, and they support the hypothesis that maintenance of calcium homeostasis is one function of complex gangliosides during, and perhaps subsequent to, neuronal development.

Plasma membrane Ca2+-ATPase in excitable and nonexcitable cells.

There is a significant number of data confirming that the maintenance of calcium homeostasis in a living cell is a complex, multiregulated process. Calcium efflux from excitable cells (i.e., neurons) occurs through two main systems--an electrochemically driven Na+/Ca2+ exchanger with a low Ca2+ affinity (K0.5 = 10-15 microM), and a plasmalemmal, specific Ca2+-ATPase, with a high Ca2+ affinity (K0.5 < 0.5-1 microM), whereas in nonexcitable cells (i.e., erythrocytes) the calcium pump is the sole system responsible for the extrusion of calcium ions. The plasma membrane Ca2+-ATPase (PMCA) is a ubiquitously expressed protein, and more than 26 transcripts of four PMCA genes are distributed in a tissue specific manner. Differences in the structure and localization of PMCA variants are thought to correlate with specific regulatory properties and may have consequences for proper cellular Ca2+ signaling. The regulatory mechanisms of calcium pump activity have been studied extensively, resulting in a new view of the functioning of this important molecule in the membranes.

Correlations between bone mineral density and circulating bone metabolic markers in diabetic patients

Diabetic osteopenia, a known chronic complication of diabetes, has been demonstrated with alterations in the calcium–vitamin D endocrine system. In order to investigate the relationship between the decrease of bone density and the altered mineral metabolism in non-insulin-dependent diabetes mellitus (NIDDM), 104 male clinical-proven NIDDM patients were studied. All patients were examined on metacarpal bone mineral density (m-BMD) by means of computed X-ray densitometry (CXD) methods. The values of the Z-score of m-BMD were significantly lower than those of age-matched controls ( P<0.01). There was a positive correlation between m-BMD and serum calcium levels ( P<0.01), but a negative correlation was conversely observed between m-BMD and serum intact parathyroid hormone (PTH) ( P<0.01), indicating that the negative calcium balance under diabetic conditions could cause the decrease of m-BMD in NIDDM. Interestingly, since a significantly positive correlation was found between circulating levels of calcium and parathyroid hormone-related peptide (PTHrP) ( P<0.05) but not PTH, it seems likely that PTHrP plays a more compensatory role on the maintenance of calcium homeostasis than PTH under diabetic conditions. Based on these observations, the CXD method would be useful in measuring the mineral density of cortical bone, and would also be beneficial to investigate underlying pathogenetic mechanism of diabetic osteopenia.

Impaired glucose tolerance in vitamin d deficiency can be corrected by calcium

Vitamin D 3, via its active metabolite 1α,25-dihydroxyvitamin D 3, helps maintain normal calcium levels in the body. Apart from the maintenance of calcium homeostasis, the active form of vitamin D 3 is now known to be involved in a number of other functions including that of pancreatic β cells. Low serum insulin levels and impaired glucose tolerance in a vitamin D-deficient state have been reported in experimental animals. Hypocalcemia is a major consequence of vitamin D deficiency. Whether the impairment observed is due to vitamin D deficiency per se or is secondary to low calcium is still a matter of controversy. The present study was conducted to delineate the roles of vitamin D and calcium in glucose intolerance associated with vitamin D deficiency in vivo. It was found that supplementation with either vitamin D 3 or high calcium alone to vitamin D-deficient rats could correct the defects. In addition, insulin sensitivity was found to be enhanced in the vitamin D-deficient group compared with vitamin D control or calcium-supplemented groups. Hence the present study demonstrates that calcium per se in the absence of vitamin D increases insulin secretion and normalizes intolerance to glucose seen in vitamin D deficiency.

Review of risk factors for osteoporosis with particular reference to a possible aetiological role of dietary salt

Laboratory animal, clinical and epidemiological studies in the published literature have been reviewed in order to establish whether excessive salt intake is an important risk factor for the development of osteoporosis and whether an intervention strategy based on salt restriction would be beneficial in the prevention of osteoporosis. Genetic factors appear to be far more important than the combination of nutritional, hormonal, environmental and lifestyle factors in the pathogenesis of osteoporosis. The most important single non-genetic factor is oestrogen deficiency in postmenopausal women. Preventive measures should be aimed at maximizing peak bone mass at skeletal maturity and retarding bone loss thereafter. Apart from postmenopausal oestrogen deficiency, various factors have been incriminated as risk factors for osteoporosis, and these include age at menarche, age at and years since menopause, insufficient physical exercise, alcohol, smoking, low calcium intake, low or high protein intake and high intake of phosphorus, sodium or caffeine. Many of the risk factors are considered to be weak, although when combined they could impact significantly on bone health. Increased intakes of various nutritional factors (potassium, magnesium, zinc, vitamin C), fibre and alkaline-producing fruit and vegetables favour adult bone health. Calcium homeostasis is normally well regulated such that increased calcium loss via the urine leads to increased calcium absorption from the gut. However, the duration of this adaptive process may be greater than that of many of the studies demonstrating that increased salt intake leads to both increased sodium and calcium in the urine. In any case, higher urinary calcium output appears to be seen only in a minority of humans in response to increased salt intake. As numerous factors—genetic, nutritional, hormonal and lifestyle—are involved in the maintenance of calcium homeostasis, it is difficult to devise human studies which adequately take into account all the important factors. Another difficulty is that many past studies have relied on imprecise methods for the measurement of bone resorption. Nor have studies based on the use of the laboratory rat produced clear answers to the problem because the rat, as a species, is uniquely deficient in its ability to handle the relevant minerals. Limited studies to date indicate that increased sodium intake neither exerts a consistent effect on various biomarkers of bone health nor leads to irreversible changes in the bone modelling process in men or in pre- or postmenopausal women. We conclude from the available evidence that increased sodium (or salt) intake is not an important risk factor for osteoporosis and that a reduction of salt intake from 9 to 6 g/day in the diet would not be beneficial as an intervention measure in the prevention of osteoporosis. More research is needed to (i) assess the effects (especially long-term) of various nutrients including sodium on bone health, (ii) assess the long-term value of any intervention strategy involving reduced intake of a particular nutrient such as sodium; and (iii) determine whether subpopulations exist particularly in the elderly (e.g. sodium-responsive subjects) in which adaptation to sodium-induced hypercalciuria may be compromised. General prudence dictates that excessively high levels of dietary salt should be eschewed by those persons with raised blood pressure or a limited range of genetic disorders. However, for the generally healthy person there is no sound evidence that the consumption of salt at the present average level of 9 g/day constitutes a risk factor for osteoporosis.