Hypomorphic expression of parathyroid Bmal1 disrupts the internal parathyroid circadian clock and increases parathyroid cell proliferation in response to uremia.

The molecular circadian clock is an evolutionary adaptation to anticipate recurring changes in the environment and to coordinate variations in activity, metabolism and hormone secretion. Parathyroid hyperplasia in uremia is a significant clinical challenge. Here, we examined changes in the transcriptome of the murine parathyroid gland over 24 hours and found a rhythmic expression of parathyroid signature genes, such as Casr, Vdr, Fgfr1 and Gcm2. Overall, 1455 genes corresponding to 6.9% of all expressed genes had significant circadian rhythmicity. Biological pathway analysis indicated that the circadian clock system is essential for the regulation of parathyroid cell function. To study this, a novel mouse strain with parathyroid gland-specific knockdown of the core clock gene Bmal1 (PTHcre;Bmal1flox/flox) was created. Dampening of the parathyroid circadian clock rhythmicity was found in these knockdown mice, resulting in abrogated rhythmicity of regulators of parathyroid cell proliferation such as Sp1, Mafb, Gcm2 and Gata3, indicating circadian clock regulation of these genes. Furthermore, the knockdown resulted in downregulation of genes involved in mitochondrial function and synthesis of ATP. When superimposed by uremia, these PTHcre;Bmal1flox/flox mice had an increased parathyroid cell proliferative response, compared to wild type mice. Thus, our findings indicate a role of the internal parathyroid circadian clock in the development of parathyroid gland hyperplasia in uremia.

A molecular circadian clock operates in the parathyroid gland and is disturbed in chronic kidney disease associated bone and mineral disorder.

Circadian rhythms in metabolism, hormone secretion, cell cycle and locomotor activity are regulated by a molecular circadian clock with the master clock in the suprachiasmatic nucleus of the central nervous system. However, an internal clock is also expressed in several peripheral tissues. Although about 10% of all genes are regulated by clock machinery an internal molecular circadian clock in the parathyroid glands has not previously been investigated. Parathyroid hormone secretion exhibits a diurnal variation and parathyroid hormone gene promoter contains an E-box like element, a known target of circadian clock proteins. Therefore, we examined whether an internal molecular circadian clock is operating in parathyroid glands, whether it is entrained by feeding and how it responds to chronic kidney disease. As uremia is associated with extreme parathyroid growth and since disturbed circadian rhythm is related to abnormal growth, we examined the expression of parathyroid clock and clock-regulated cell cycle genes in parathyroid glands of normal and uremic rats. Circadian clock genes were found to be rhythmically expressed in normal parathyroid glands and this clock was minimally entrained by feeding. Diurnal regulation of parathyroid glands was next examined. Significant rhythmicity of fibroblast-growth-factor-receptor-1, MafB and Gata3 was found. In uremic rats, deregulation of circadian clock genes and the cell cycle regulators, Cyclin D1, c-Myc, Wee1 and p27, which are influenced by the circadian clock, was found in parathyroid glands as well as the aorta. Thus, a circadian clock operates in parathyroid glands and this clock and downstream cell cycle regulators are disturbed in uremia and may contribute to dysregulated parathyroid proliferation in secondary hyperparathyroidism.

Circadian rhythms of mineral metabolism in chronic kidney disease-mineral bone disorder.

PURPOSE OF REVIEW: The circadian rhythms have a systemic impact on all aspects of physiology. Kidney diseases are associated with extremely high-cardiovascular mortality, related to chronic kidney disease-mineral bone disorder (CKD-MBD), involving bone, parathyroids and vascular calcification. Disruption of circadian rhythms may cause serious health problems, contributing to development of cardiovascular diseases, metabolic syndrome, cancer, organ fibrosis, osteopenia and aging. Evidence of disturbed circadian rhythms in CKD-MBD parameters and organs involved is emerging and will be discussed in this review. RECENT FINDINGS: Kidney injury induces unstable behavioral circadian rhythm. Potentially, uremic toxins may affect the master-pacemaker of circadian rhythm in hypothalamus. In CKD disturbances in the circadian rhythms of CKD-MBD plasma-parameters, activin A, fibroblast growth factor 23, parathyroid hormone, phosphate have been demonstrated. A molecular circadian clock is also expressed in peripheral tissues, involved in CKD-MBD; vasculature, parathyroids and bone. Expression of the core circadian clock genes in the different tissues is disrupted in CKD-MBD. SUMMARY: Disturbed circadian rhythms is a novel feature of CKD-MBD. There is a need to establish which specific input determines the phase of the local molecular clock and to characterize its regulation and deregulation in tissues involved in CKD-MBD. Finally, it is important to establish what are the implications for treatment including the potential applications for chronotherapy.

New Aspects of the Kidney in the Regulation of Fibroblast Growth Factor 23 (FGF23) and Mineral Homeostasis.

The bone-derived hormone fibroblast growth factor 23 (FGF23) acts in concert with parathyroid hormone (PTH) and the active vitamin D metabolite calcitriol in the regulation of calcium (Ca) and phosphate (P) homeostasis. More factors are being identified to regulate FGF23 levels and the endocrine loops between the three hormones. The present review summarizes the complex regulation of FGF23 and the disturbed FGF23/Klotho system in chronic kidney disease (CKD). In addition to the reduced ability of the injured kidney to regulate plasma levels of FGF23, several CKD-related factors have been shown to stimulate FGF23 production. The high circulating FGF23 levels have detrimental effects on erythropoiesis, the cardio-vascular system and the immune system, all contributing to the disturbed system biology in CKD. Moreover, new factors secreted by the injured kidney and the uremic calcified vasculature play a role in the mineral and bone disorder in CKD and create a vicious pathological crosstalk.

Circadian rhythm of activin A and related parameters of mineral metabolism in normal and uremic rats.

Activin A is a new fascinating player in chronic kidney disease-mineral and bone disorder (CKD-MBD), which is implicated in progressive renal disease, vascular calcification, and osteodystrophy. Plasma activin A rises early in the progression of renal disease. Disruption of circadian rhythms is related to increased risk of several diseases and circadian rhythms are observed in mineral homeostasis, bone parameters, and plasma levels of phosphate and PTH. Therefore, we examined the circadian rhythm of activin A and CKD-MBD-related parameters (phosphate, PTH, FGF23, and klotho) in healthy controls and CKD rats (5/6 nephrectomy) on high-, standard- and low-dietary phosphate contents as well as during fasting conditions. Plasma activin A exhibited circadian rhythmicity in healthy control rats with fourfold higher values at acrophase compared with nadir. The rhythm was obliterated in CKD. Activin A was higher in CKD rats compared with controls when measured at daytime but not significantly when measured at evening/nighttime, stressing the importance of time-specific reference intervals when interpreting plasma values. Plasma phosphate, PTH, and FGF23 all showed circadian rhythms in control rats, which were abolished or disrupted in CKD. Plasma klotho did not show circadian rhythm. Thus, the present investigation shows, for the first time, circadian rhythm of plasma activin A. The rhythmicity is severely disturbed by CKD and is associated with disturbed rhythms of phosphate and phosphate-regulating hormones PTH and FGF23, indicating that disturbed circadian rhythmicity is an important feature of CKD-MBD.

Klotho and activin A in kidney injury: plasma Klotho is maintained in unilateral obstruction despite no upregulation of Klotho biosynthesis in the contralateral kidney.

In a new paradigm of etiology related to chronic kidney disease-mineral and bone disorder (CKD-MBD), kidney injury may cause induction of factors in the injured kidney that are released into the circulation and thereby initiate and maintain renal fibrosis and CKD-MBD. Klotho is believed to ameliorate renal fibrosis and CKD-MBD, while activin A might have detrimental effects. The unilateral ureter obstruction (UUO) model is used here to examine this concept by investigating early changes related to renal fibrosis in the obstructed kidney, untouched contralateral kidney, and vasculature which might be affected by secreted factors from the obstructed kidney, and comparing with unilateral nephrectomized controls (UNX). Obstructed kidneys showed early Klotho gene and protein depletion, whereas plasma Klotho increased in both UUO and UNX rats, indicating an altered metabolism of Klotho. Contralateral kidneys had no compensatory upregulation of Klotho and maintained normal expression of the examined fibrosis-related genes, as did remnant UNX kidneys. UUO caused upregulation of transforming growth factor-ß and induction of periostin and activin A in obstructed kidneys without changes in the contralateral kidneys. Plasma activin A doubled in UUO rats after 10 days while no changes were seen in UNX rats, suggesting secretion of activin A from the obstructed kidney with potentially systemic effects on CKD-MBD. As such, increased aortic sclerostin was observed in UUO rats compared with UNX and normal controls. The present results are in line with the new paradigm and show very early vascular effects of unilateral kidney fibrosis, supporting the existence of a new kidney-vasculature axis.

Fibroblast Growth Factor (FGF) 23 Regulates the Plasma Levels of Parathyroid Hormone In Vivo Through the FGF Receptor in Normocalcemia, But Not in Hypocalcemia.

The calcium and phosphate homeostasis is regulated by a complex interplay between parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and calcitriol. Experimental studies have demonstrated an inhibitory effect of FG23 on PTH production and secretion; the physiological role of this regulation is however not well understood. Surprisingly, in uremia, concomitantly elevated FGF23 and PTH levels are observed. The parathyroid gland rapidly loses its responsiveness to extracellular calcium in vitro and a functional parathyroid cell line has currently not been established. Therefore, the aim of the present investigation was to study the impact of FGF23 on the Ca2+/PTH relationship in vivo under conditions of normocalcemia and hypocalcemia. Wistar rats were allocated to treatment with intravenous recombinant FGF23 and inhibition of the FGF receptor in the setting of normocalcemia and acute hypocalcemia. We demonstrated that FGF23 rapidly inhibited PTH secretion and that this effect was completely blocked by inhibition of the FGF receptor. Furthermore, inhibition of the FGF receptor by itself significantly increased PTH levels, indicating that FGF23 has a suppressive tonus on the parathyroid gland's PTH secretion. In acute hypocalcemia, there was no effect of either recombinant FGF23 or FGF receptor inhibition on the physiological response to the low ionized calcium levels. In conclusion, FGF23 has an inhibitory tonus on PTH secretion in normocalcemia and signals through the FGF receptor. In acute hypocalcemia, when increased PTH secretion is needed to restore the calcium homeostasis, this inhibitory effect of FGF23 is abolished.

Kidney fibroblast growth factor 23 does not contribute to elevation of its circulating levels in uremia.

Fibroblast growth factor 23 (FGF23) secreted by osteocytes is a circulating factor essential for phosphate homeostasis. High plasma FGF23 levels are associated with cardiovascular complications and mortality. Increases of plasma FGF23 in uremia antedate high levels of phosphate, suggesting a disrupted feedback regulatory loop or an extra-skeletal source of this phosphatonin. Since induction of FGF23 expression in injured organs has been reported we decided to examine the regulation of FGF23 gene and protein expressions in the kidney and whether kidney-derived FGF23 contributes to the high plasma levels of FGF23 in uremia. FGF23 mRNA was not detected in normal kidneys, but was clearly demonstrated in injured kidneys, already after four hours in obstructive nephropathy and at 8 weeks in the remnant kidney of 5/6 nephrectomized rats. No renal extraction was found in uremic rats in contrast to normal rats. Removal of the remnant kidney had no effect on plasma FGF23 levels. Well-known regulators of FGF23 expression in bone, such as parathyroid hormone, calcitriol, and inhibition of the FGF receptor by PD173074, had no impact on kidney expression of FGF23. Thus, the only direct contribution of the injured kidney to circulating FGF23 levels in uremia appears to be reduced renal extraction of bone-derived FGF23. Kidney-derived FGF23 does not generate high plasma FGF23 levels in uremia and is regulated differently than the corresponding regulation of FGF23 gene expression in bone.

Effect of chronic uremia on the transcriptional profile of the calcified aorta analyzed by RNA sequencing.

The development of vascular calcification (VC) in chronic uremia (CU) is a tightly regulated process controlled by factors promoting and inhibiting mineralization. Next-generation high-throughput RNA sequencing (RNA-seq) is a powerful and sensitive tool for quantitative gene expression profiling and the detection of differentially expressed genes. In the present study, we, for the first time, used RNA-seq to examine rat aorta transcriptomes from CU rats compared with control rats. Severe VC was induced in CU rats, which lead to extensive changes in the transcriptional profile. Among the 10,153 genes with an expression level of >1 reads/kilobase transcript/million mapped reads, 2,663 genes were differentially expressed with 47% upregulated genes and 53% downregulated genes in uremic rats. Significantly deregulated genes were enriched for ontologies related to the extracellular matrix, response to wounding, organic substance, and ossification. The individually affected genes were of relevance to osteogenic transformation, tissue calcification, and Wnt modulation. Downregulation of the Klotho gene in uremia is believed to be involved in the development of VC, but it is debated whether the effect is caused by circulating Klotho only or if Klotho is produced locally in the vasculature. We found that Klotho was neither expressed in the normal aorta nor calcified aorta by RNA-seq. In conclusion, we demonstrated extensive changes in the transcriptional profile of the uremic calcified aorta, which were consistent with a shift in phenotype from vascular tissue toward an osteochondrocytic transcriptome profile. Moreover, neither the normal vasculature nor calcified vasculature in CU expresses Klotho.

The vascular secret of Klotho.

Klotho is an evolutionarily highly conserved protein related to longevity. Increasing evidence of a vascular protecting effect of the Klotho protein has emerged and might be important for future treatments of uremic vascular calcification. It is still disputed whether Klotho is locally expressed in the vasculature or whether its vascular effects arise uniquely from its presence in the circulation.

Key role of the kidney in the regulation of fibroblast growth factor 23.

High circulating levels of fibroblast growth factor 23 (FGF23) have been demonstrated in kidney failure, but mechanisms of this are not well understood. Here we examined the impact of the kidney on the early regulation of intact FGF23 in acute uremia as induced by bilateral or unilateral nephrectomy (BNX and UNX, respectively) in the rat. BNX induced a significant increase in plasma intact FGF23 levels from 112 to 267 pg/ml within 15 min, which remained stable thereafter. UNX generated intact FGF23 levels between that seen in BNX and sham-operated rats. The intact to C-terminal FGF23 ratio was significantly increased in BNX rats. The rapid rise in FGF23 after BNX was independent of parathyroid hormone or FGF receptor signaling. No evidence of early stimulation of FGF23 gene expression in the bone was found. Furthermore, acute severe hyperphosphatemia or hypercalcemia had no impact on intact FGF23 levels in normal and BNX rats. The half-life of exogenous recombinant human FGF23 was significantly prolonged from 4.4 to 11.8 min in BNX rats. Measurements of plasma FGF23 in the renal artery and renal vein demonstrated a significant renal extraction. Thus the kidney is important in FGF23 homeostasis by regulation of its plasma level and metabolism.

A potential kidney-bone axis involved in the rapid minute-to-minute regulation of plasma Ca2+.

BACKGROUND: Understanding the regulation of mineral homeostasis and function of the skeleton as buffer for Calcium and Phosphate has regained new interest with introduction of the syndrome "Chronic Kidney Disease-Mineral and Bone Disorder"(CKD-MBD). The very rapid minute-to-minute regulation of plasma-Ca(2+) (p-Ca(2+)) takes place via an exchange mechanism of Ca(2+) between plasma and bone. A labile Ca storage pool exists on bone surfaces storing excess or supplying Ca when blood Ca is lowered. Aim was to examine minute-to-minute regulation of p-Ca(2+) in the very early phase of acute uremia, as induced by total bilateral nephrectomy and to study the effect of absence of kidneys on the rapid recovery of p-Ca(2+) from a brief induction of acute hypocalcemia. METHODS: The rapid regulation of p-Ca(2+) was examined in sham-operated rats, acute nephrectomized rats (NX), acute thyroparathyrectomized(TPTX) rats and NX-TPTX rats. RESULTS: The results clearly showed that p-Ca(2+) falls rapidly and significantly very early after acute NX, from 1.23 ± 0.02 to 1.06 ± 0.04 mM (p < 0.001). Further hypocalcemia was induced by a 30 min iv infusion of EGTA. Control groups had saline. After discontinuing EGTA a rapid increase in p-Ca(2+) took place, but with a lower level in NX rats (p < 0.05). NX-TPTX model excluded potential effect of accumulation of Calcitonin and C-terminal PTH, both having potential hypocalcemic actions. Acute TPTX resulted in hypercalcemia, 1.44 ± 0.02 mM and less in NX-TPTX rats,1.41 ± 0.02 mM (p < 0.05). Recovery of p-Ca(2+) from hypocalcemia resulted in lower levels in NX-TPTX than in TPTX rats, 1.20 ± 0.02 vs.1.30 ± 0.02 (p < 0.05) demonstrating that absence of kidneys significantly affected the rapid regulation of p-Ca(2+) independent of PTH, C-PTH and CT. CONCLUSIONS: P-Ca(2+) on a minute-to-minute basis is influenced by presence of kidneys. Hypocalcemia developed rapidly in acute uremia. Levels of p-Ca(2+), obtained during recovery from hypocalcemia resulted in lower levels in acutely nephrectomized rats. This indicates that kidneys are of significant importance for the 'set-point' of p-Ca(2+) on bone surface, independently of PTH and calcitonin. Our results point toward existence of an as yet unknown factor/mechanism, which mediates the axis between kidney and bone, and which is involved in the very rapid regulation of p-Ca(2+).

Circulating FGF23 levels in response to acute changes in plasma Ca(2+).

The regulation of fibroblast growth factor 23 (FGF23) synthesis and secretion is still incompletely understood. FGF23 is an important regulator of renal phosphate excretion and has regulatory effects on the calciotropic hormones calcitriol and parathyroid hormone (PTH). Calcium (Ca) and phosphate homeostasis are closely interrelated, and it is therefore likely that Ca is involved in FGF23 regulation. It has recently been reported that dietary Ca influenced FGF23 levels, with high Ca increasing FGF23. The mechanism remains to be clarified. It remains unknown whether acute changes in plasma Ca influence FGF23 levels and whether a close relationship, similar that known for Ca and PTH, exists between Ca and FGF23. Thus, the aim of the present study was to examine whether acute hypercalcemia and hypocalcemia regulate FGF23 levels in the rat. Acute hypercalcemia was induced by an intravenous Ca infusion and hypocalcemia by infusion of ethylene glycol tetraacetic acid (EGTA) in normal and acutely parathyroidectomized rats. Intact plasma FGF23 and intact plasma PTH and plasma Ca(2+) and phosphate were measured. Acute hypercalcemia and hypocalcemia resulted as expected in adequate PTH secretory responses. Plasma FGF23 levels remained stable at all plasma Ca(2+) levels; acute parathyroidectomy did not affect FGF23 secretion. In conclusion, Ca is not a regulator of acute changes in FGF23 secretion.

Parathyroid hormone-related peptide plasma concentrations in patients on hemodialysis.

BACKGROUND: Uremic patients develop hyperplasia of the parathyroid glands due to disturbances in the mineral metabolism. The hyperplastic parathyroids are associated with significant expression of parathyroid hormone (PTH)-related peptide (PTHrP). PTHrP has been shown to have an autocrine/paracrine function in the parathyroids, but it is still uncertain if PTHrP is a secretory product of the gland and thereby possess endocrine actions. In cells of severe adenomatous secondary hyperparathyroidism PTHrP and PTH have been found to be co-localized in the same secretory granules. PTH and PTHrP act through the same receptor, the PTH1R, and it has been shown experimentally that PTHrP enhances the PTH secretory response to hypocalcemia, indicating a link between the two hormones. METHODS: Together with a number of parameters involved in mineral homeostasis plasma PTHrP was measured before hemodialysis in 90 patients and in 15 healthy subjects. Plasma PTH was determined in order to examine the possible relationship between the two peptides. RESULTS: In hemodialysis patients mean plasma PTHrP, 4.2 ± 2.1, was significantly lower than that of healthy subjects, 8.3 ± 1.1 pmol/L, p < 0.0001. No relationship was found between plasma PTHrP and PTH in hemodialysis patients. Gender, PTX, specific treatments and diagnoses had no impact on PTHrP concentrations. CONCLUSION: Thus PTHrP is measurable in hemodialysis patients, but its secretion might not be part of a regulated mineral homeostatic process and may not derive from the uremic hyperplastic parathyroid glands.

Ergocalciferol treatment and aspects of mineral homeostasis in patients with chronic kidney disease stage 4-5.

BACKGROUND: Focus on non-classical effects and possible less side effects of treatment with nutritional vitamin D, raises the expectation of possible benefits from treating chronic kidney disease (CKD) patients with ergocalciferol (vitamin D2). Treatment with 1,25(OH)2 vitamin D (calcitriol) induces elevated fibroblast growth factor 23 (FGF23), while epidemiological studies have found positive effects of nutritional and 25(OH)vitamin D on mortality in CKD. Disturbed mineral homeostasis in CKD is correlated to adverse outcome and cardiovascular mortality. The objective was to examine the possible effects of treatment with high doses of ergocalciferol on parameters of mineral homeostasis in predialysis CKD patients. METHODS: A total of 43 adult patients with CKD stage 4-5, not receiving vitamin D supplementation, were studied, and allocated by simple randomization to either an intervention (n = 26) or a control group (n = 17). The intervention group received ergocalciferol, 50.000 IU/week for 6 weeks. Plasma FGF23, creatinine, parathyroid hormone (PTH), phosphate and ionized calcium were obtained at baseline and after the 6 weeks. RESULTS: The intervention group had a significant increase in 25(OH)D2 concentration from < 10 to 90 ± 4 nmol/L, while 1,25(OH)2D (62 ± 6 at baseline and 67 ± 6 pmol/L at 6 weeks) remained stable. No changes were seen in the circulating vitamin D concentrations in the control group. After the 6 weeks of treatment no significant changes were seen in concentration of creatinine, phosphate, ionized calcium, PTH and FGF23 remained stable. CONCLUSION: No harmful effects of short-term treatment with high-dose ergocalciferol were observed on markers of mineral homeostasis and FGF23 in CKD patients stage 4-5.

High dose intravenous iron, mineral homeostasis and intact FGF23 in normal and uremic rats.

BACKGROUND: High iron load might have a number of toxic effects in the organism. Recently intravenous (iv) iron has been proposed to induce elevation of fibroblast growth factor 23 (FGF23), hypophosphatemia and osteomalacia in iron deficient subjects. High levels of FGF23 are associated with increased mortality in the chronic kidney disease (CKD) population. CKD patients are often treated with iv iron therapy in order to maintain iron stores and erythropoietin responsiveness, also in the case of not being iron depleted. Therefore, the effect of a single high iv dose of two different iron preparations, iron isomaltoside 1000 (IIM) and ferric carboxymaltose (FCM), on plasma levels of FGF23 and phosphate was examined in normal and uremic iron repleted rats. METHODS: Iron was administered iv as a single high dose of 80 mg/kg bodyweight and the effects on plasma levels of iFGF23, phosphate, Ca2+, PTH, transferrin, ferritin and iron were examined in short and long term experiments (n = 99). Blood samples were obtained at time 0, 30, 60, 180 minutes, 24 and 48 hours and in a separate study after 1 week. Uremia was induced by 5/6-nephrectomy. RESULTS: Nephrectomized rats had significant uremia, hyperparathyroidism and elevated FGF23. Iron administration resulted in significant increases in plasma ferritin levels. No significant differences were seen in plasma levels of iFGF23, phosphate and PTH between the experimental groups at any time point within 48 hours or at 1 week after infusion of the iron compounds compared to vehicle. CONCLUSIONS: In non-iron depleted normal and uremic rats a single high dose of either of two intravenous iron preparations, iron isomaltoside 1000, and ferric carboxymaltose, had no effect on plasma levels of iFGF23 and phosphate for up to seven days.

Frontiers in Bone Metabolism and Disorder in Chronic Kidney Disease.

Chronic Kidney Disease (CKD) is a progressive condition that affects 10-15% of the adult population, a prevalence expected to increase worldwide [...].

The Calcified Vasculature in Chronic Kidney Disease Secretes Factors that Inhibit Bone Mineralization.

Vascular calcification and bone disorder progress simultaneously in chronic kidney disease (CKD). Still, how the complex pathological mechanisms are linked is only sparsely understood. Up to now, the focus has been on the disturbed bone metabolism in developing vascular calcification. However, our group has recently demonstrated that vascular calcification has negative effects on bone formation and mineralization as shown in the bone of normal recipient rats transplanted with the calcified aorta from CKD rats. In the present in vitro study, the hypothesis of a direct crosstalk between the vasculature and bone was examined. Calcified aortas from 5/6 nephrectomized rats and normal aortas from control rats were excised and incubated ex vivo. The calcified aorta secreted large amounts of sclerostin, dickkopf-1 (Dkk1), and activin A. Both normal and calcified aortas secreted frizzle-related protein 4 (SFRP4). Aorta rings were co-incubated with the osteoblast-like cell line UMR-106. The calcified aorta strongly inhibited calcium crystal formation in UMR-106 cells, together with a significant upregulation of the mineralization inhibitors osteopontin and progressive ankylosis protein homolog (ANKH). The strong stimulation of osteopontin was blocked by lithium chloride, indicating involvement of Wnt/ß-catenin signaling. The present in vitro study shows detrimental effects of the calcified aorta on bone cell mineralization. These findings support the hypothesis of an active role of the calcified vasculature in the systemic CKD-mineral and bone disorder (CKD-MBD), resulting in a pathological vascular-bone tissue crosstalk. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The secretory response of parathyroid hormone to acute hypocalcemia in vivo is independent of parathyroid glandular sodium/potassium-ATPase activity.

The involvement of sodium/potassium-ATPase in regulating parathyroid hormone (PTH) secretion is inferred from in vitro studies. Recently, the α-klotho-dependent rapid recruitment of this ATPase to the parathyroid cell plasma membrane in response to low extracellular calcium ion was suggested to be linked to increased hormone secretion. In this study, we used an in vivo rat model to determine the importance of sodium/potassium-ATPase in PTH secretion. Glands were exposed and treated in situ with vehicle or ouabain, a specific inhibitor of sodium/potassium-ATPase. PTH secretion was significantly increased in response to ethylene glycol tetraacetic acid-induced acute hypocalcemia and to the same extent in both vehicle and ouabain groups. The glands were removed, and inhibition of the ATPase was measured by (86)rubidium uptake, which was found to be significantly decreased in ouabain-treated parathyroid glands, indicating inhibition of the ATPase. As ouabain induced systemic hyperkalemia, the effect of high potassium on hormone secretion was also examined but was found to have no effect. Thus, inhibition of the parathyroid gland sodium/potassium-ATPase activity in vivo had no effect on the secretory response to acute hypocalcemia. Hence, the suggested importance of this ATPase in the regulation of PTH secretion could not be confirmed in this in vivo model.

New Insights to the Crosstalk between Vascular and Bone Tissue in Chronic Kidney Disease-Mineral and Bone Disorder.

Vasculature plays a key role in bone development and the maintenance of bone tissue throughout life. The two organ systems are not only linked in normal physiology, but also in pathophysiological conditions. The chronic kidney disease-mineral and bone disorder (CKD-MBD) is still the most serious complication to CKD, resulting in increased morbidity and mortality. Current treatment therapies aimed at the phosphate retention and parathyroid hormone disturbances fail to reduce the high cardiovascular mortality in CKD patients, underlining the importance of other factors in the complex syndrome. This review will focus on vascular disease and its interplay with bone disorders in CKD. It will present the very late data showing a direct effect of vascular calcification on bone metabolism, indicating a vascular-bone tissue crosstalk in CKD. The calcified vasculature not only suffers from the systemic effects of CKD but seems to be an active player in the CKD-MBD syndrome impairing bone metabolism and might be a novel target for treatment and prevention.