Strain-specific alterations in the skeletal response to adenine-induced chronic kidney disease are associated with differences in parathyroid hormone levels

Adenine-induced animal model of chronic kidney disease: current applications and future perspectives

Chronic kidney disease (CKD) with high morbidity and mortality all over the world is characterized by decreased kidney function, a condition which can result from numerous risk factors, including diabetes, hypertension and obesity. Despite significant advances in our understanding of the pathogenesis of CKD, there are still no treatments that can effectively combat CKD, which underscores the urgent need for further study into the pathological mechanisms underlying this condition. In this regard, animal models of CKD are indispensable. This article reviews a widely used animal model of CKD, which is induced by adenine. While a physiologic dose of adenine is beneficial in terms of biological activity, a high dose of adenine is known to induce renal disease in the organism. Following a brief description of the procedure for disease induction by adenine, major mechanisms of adenine-induced CKD are then reviewed, including inflammation, oxidative stress, programmed cell death, metabolic disorders, and fibrillation. Finally, the application and future perspective of this adenine-induced CKD model as a platform for testing the efficacy of a variety of therapeutic approaches is also discussed. Given the simplicity and reproducibility of this animal model, it remains a valuable tool for studying the pathological mechanisms of CKD and identifying therapeutic targets to fight CKD.

Two Weeks of Continuous Opioid Treatment in an Adenine-Induced Mouse Model of Chronic Kidney Disease Exacerbates the Bone Inflammatory State and Increases Osteoclasts

Patients with chronic kidney disease (CKD) report high pain levels, but reduced renal clearance eliminates many analgesic options; therefore, 30-50% of CKD patients have chronic opioid prescriptions. Opioid use in CKD is associated with higher fracture rates. Opioids may directly alter bone turnover directly through effects on bone cells and indirectly via increasing inflammation. We hypothesized that continuous opioid exposure would exacerbate the high bone turnover state of CKD and be associated with elevated measures of inflammation. Male C57Bl/6J mice after 8 weeks of adenine-induced CKD (AD) and non-AD controls (CON) had 14-day osmotic pumps (0.25-µL/hr release) containing either saline or 50-mg/mL oxycodone (OXY) surgically implanted in the subscapular region. After 2 weeks, all AD mice had elevated blood urea nitrogen, parathyroid hormone, and serum markers of bone turnover compared to controls with no effect of OXY. Immunohistochemical staining of the distal femur showed increased numbers of osteocytes positive for the mu opioid and for toll-like receptor 4 (TLR4) due to OXY. Osteocyte protein expression of tumor necrosis factor-α (TNF-α) and RANKL were higher due to both AD and OXY so that AD + OXY mice had the highest values. Trabecular osteoclast-covered surfaces were also significantly higher due to both AD and OXY, resulting in AD + OXY mice having 4.5-fold higher osteoclast-covered surfaces than untreated CON. These data demonstrate that opioids are associated with a pro-inflammatory state in osteocytes which increases the pro-resorptive state of CKD.

Inhibition of RANKL improves the skeletal phenotype of adenine-induced chronic kidney disease in mice

Skeletal fragility and high fracture rates are common in CKD. A key component of bone loss in CKD with secondary hyperparathyroidism is high bone turnover and cortical bone deterioration through both cortical porosity and cortical thinning. We hypothesized that RANKL drives high bone resorption within cortical bone leading to the development of cortical porosity in CKD (study 1) and that systemic inhibition of RANKL would mitigate the skeletal phenotype of CKD (study 2). In study 1, we assessed the skeletal properties of male and female Dmp1-cre RANKLfl/fl (cKO) and control genotype (Ranklfl/fl; Con) mice after 10 wk of adenine-induced CKD (AD; 0.2% dietary adenine). All AD mice regardless of sex or genotype had elevated blood urea nitrogen and high PTH. Con AD mice in both sexes had cortical porosity and lower cortical thickness as well as high osteoclast-covered trabecular surfaces and higher bone formation rate. cKO mice had preserved cortical bone microarchitecture despite high circulating PTH as well as no CKD-induced increases in osteoclasts. In study 2, male mice with established AD CKD were either given a single injection of an anti-RANKL antibody (5 mg/kg) 8 wk post-induction of CKD or subjected to 3×/wk dosing with risedronate (1.2 μg/kg) for 4 wk. Anti-RANKL treatment significantly reduced bone formation rate as well as osteoclast surfaces at both trabecular and cortical pore surfaces; risedronate treatment had little effect on these bone parameters. In conclusion, these studies demonstrate that bone-specific RANKL is critical for the development of high bone formation/high osteoclasts and cortical bone loss in CKD with high PTH. Additionally, systemic anti-RANKL ligand therapy in established CKD may help prevent the propagation of cortical bone loss via suppression of bone turnover.

Teriparatide and etelcalcetide improve bone, fibrosis, and fat parameters in chronic kidney disease model rats

Objectives

Chronic kidney disease (CKD) complicated by secondary hyperparathyroidism (SHPT) is associated with an increased risk of fragility fractures. Etelcalcetide (EC) is a treatment for SHPT that reduces serum parathyroid hormone (PTH) levels. However, the effects of combined treatment with osteoporosis drugs such as teriparatide (TPTD) remain unclear. This study investigates the combined effects of EC and TPTD on bone in CKD model rats.

Methods

The CKD model was established in 8-week-old male Wistar rats by feeding them a 0.75% adenine diet for 4 weeks. At 20 weeks of age, the rats were divided into 4 groups (N = 9-10 in each group): CKD group (vehicle administration), TPTD group (30 μg/kg, 3 times/week), EC group (0.6 mg/kg, daily), and Comb group (TPTD and EC combined). EC was injected for 12 weeks starting at 20 weeks of age, and TPTD was injected for 8 weeks starting at 24 weeks of age. After treatment, the followings were evaluated: bone mineral density, bone strength, biochemical tests, bone and fat histomorphometry, and micro-computed tomography.

Results

In CKD model rats, the combination of EC and TPTD was more effective in increasing cortical bone thickness and bone strength and inhibiting porosity. In addition, the combined treatment decreased bone marrow adiposity and fibrosis, and it increased bone mass and improved bone microstructure in trabecular bone.

Conclusions

With the observed benefits such as improved bone mass, bone strength, structural properties, and bone marrow adiposity, combination therapy may be a potential way to improve bone fragility in CKD.

Ex vivo exposure to calcitonin or raloxifene improves mechanical properties of diseased bone through non-cell mediated mechanisms

Raloxifene (RAL) reduces clinical fracture risk despite modest effects on bone mass and density. This reduction in fracture risk may be due to improved material level-mechanical properties through a non-cell mediated increase in bone hydration. Synthetic salmon calcitonin (CAL) has also demonstrated efficacy in reducing fracture risk with only modest bone mass and density improvements. This study aimed to determine if CAL could modify healthy and diseased bone through cell-independent mechanisms that alter hydration similar to RAL. 26-week-old male C57BL/6 mice induced with chronic kidney disease (CKD) beginning at 16 weeks of age via 0.2 % adenine-laced casein-based (0.9 % P, 0.6 % C) chow, and their non-CKD control littermates (Con), were utilized. Upon sacrifice, right femora were randomly assigned to the following ex vivo experimental groups: RAL (2 μM, n = 10 CKD, n = 10 Con), CAL (100 nM, n = 10 CKD, n = 10 Con), or Vehicle (VEH; n = 9 CKD, n = 9 Con). Bones were incubated in PBS + drug solution at 37 °C for 14 days using an established ex vivo soaking methodology. Cortical geometry (μCT) was used to confirm a CKD bone phenotype, including porosity and cortical thinning, at sacrifice. Femora were assessed for mechanical properties (3-point bending) and bone hydration (via solid state nuclear magnetic resonance spectroscopy with magic angle spinning (ssNMR)). Data were analyzed by two-tailed t-tests (μCT) or 2-way ANOVA for main effects of disease, treatment, and their interaction. Tukey's post hoc analyses followed a significant main effect of treatment to determine the source of the effect. Imaging confirmed a cortical phenotype reflective of CKD, including lower cortical thickness (p < 0.0001) and increased cortical porosity (p = 0.02) compared to Con. In addition, CKD resulted in weaker, less deformable bones. In CKD bones, ex vivo exposure to RAL or CAL improved total work (+120 % and +107 %, respectively; p < 0.05), post-yield work (+143 % and +133 %), total displacement (+197 % and +229 %), total strain (+225 % and +243 %), and toughness (+158 % and +119 %) vs. CKD VEH soaked bones. Ex vivo exposure to RAL or CAL did not impact any mechanical properties in Con bone. Matrix-bound water by ssNMR showed CAL treated bones had significantly higher bound water compared to VEH treated bones in both CKD and Con cohorts (p = 0.001 and p = 0.01, respectively). RAL positively modulated bound water in CKD bone compared to VEH (p = 0.002) but not in Con bone. There were no significant differences between bones soaked with CAL vs. RAL for any outcomes measured. RAL and CAL improve important post-yield properties and toughness in a non-cell mediated manner in CKD bone but not in Con bones. While RAL treated CKD bones had higher matrix-bound water content in line with previous reports, both Con and CKD bones exposed to CAL had higher matrix-bound water. Therapeutic modulation of water, specifically the bound water fraction, represents a novel approach to improving mechanical properties and potentially reducing fracture risk.

Mild Chronic Kidney Disease Associated with Low Bone Formation and Decrease in Phosphate Transporters and Signaling Pathways Gene Expression

The initial phases of molecular and cellular maladaptive bone responses in early chronic kidney disease (CKD) remain mostly unknown. We induced mild CKD in spontaneously hypertensive rats (SHR) by either causing arterial hypertension lasting six months (sham-operated rats, SO6) or in its' combination with 3/4 nephrectomy lasting two and six months (Nx2 and Nx6, respectively). Sham-operated SHRs (SO2) and Wistar Kyoto rats (WKY2) with a two-month follow-up served as controls. Animals were fed standard chow containing 0.6% phosphate. Upon follow-up completion in each animal, we measured creatinine clearance, urine albumin-to-creatinine ratio, renal interstitial fibrosis, inorganic phosphate (Pi) exchange, intact parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), Klotho, Dickkopf-1, sclerostin, and assessed bone response by static histomorphometry and gene expression profiles. The mild CKD groups had no increase in renal Pi excretion, FGF23, or PTH levels. Serum Pi, Dickkopf-1, and sclerostin were higher in Nx6. A decrease in trabecular bone area and osteocyte number was obvious in SO6. Nx2 and Nx6 had additionally lower osteoblast numbers. The decline in eroded perimeter, a resorption index, was only apparent in Nx6. Significant downregulation of genes related to Pi transport, MAPK, WNT, and BMP signaling accompanied histological alterations in Nx2 and Nx6. We found an association between mild CKD and histological and molecular features suggesting lower bone turnover, which occurred at normal levels of systemic Pi-regulating factors.

Upacicalcet, a positive allosteric modulator of the calcium-sensing receptor, prevents vascular calcification and bone disorder in a rat adenine-induced secondary hyperparathyroidism model

Secondary hyperparathyroidism (SHPT) is a major comorbidity of chronic kidney disease (CKD). Chronic elevation of PTH levels is associated with cortical bone deterioration and increase in the risk of fractures in CKD patients. Here, we evaluated the effect of repeated administration of upacicalcet, a novel positive allosteric modulator of the calcium-sensing receptor, in a rat model of adenine-induced renal failure, by determining serum levels of intact PTH (iPTH), calcium, phosphorus, creatinine, and urea nitrogen. Furthermore, parathyroid hyperplasia (parathyroid gland weight and Ki-67-positive cell density), ectopic calcification (calcium content in the thoracic aorta, kidney and heart and positive von Kossa staining in the thoracic aorta), and bone morphometry parameters (cortical porosity and fibrosis volume) were evaluated. Rats treated with either 0.2 mg/kg or 1 mg/kg upacicalcet exhibited significantly lower serum iPTH levels than CKD-control rats, as early as 7 days after the first dose. Repeated administration of upacicalcet reduced serum iPTH levels and inhibited parathyroid hyperplasia in rats with adenine-induced severe renal failure. Moreover, it suppressed ectopic calcification and cortical pore formation. In contrast, serum calcium and phosphorus levels were not significantly affected, suggesting a low risk of hypocalcemia, which often occurs with SHPT treatment. In conclusion, repeated administration of upacicalcet decreased serum iPTH levels and suppressed parathyroid hyperplasia in the adenine-induced CKD rat model of SHPT. Furthermore, ectopic calcification and cortical pore formation were suppressed without significant changes in blood mineral parameters. Upacicalcet safely inhibited the progression of SHPT in an adenine-induced CKD rat model.

A novel murine model of combined insulin-dependent diabetes and chronic kidney disease has greater skeletal detriments than either disease individually

Diabetes and chronic kidney disease (CKD) consistently rank among the top ten conditions in prevalence and mortality in the United States. Insulin-dependent diabetes (IDD) and CKD each increase the risk of skeletal fractures and fracture-related mortality. However, it remains unknown whether these conditions have interactive end-organ effects on the skeleton. We hypothesized that combining IDD and CKD in mice would cause structural and mechanical bone alterations that are more deleterious compared to the single disease states. Female C57BL6/J mice were divided into four groups: 1) N = 12 Control (CTRL), 2) N = 10 Streptozotocin-induced IDD (STZ), 3) N = 10 Adenine diet-induced CKD (AD), and 4) N = 9 Combination (STZ+AD). STZ administration resulted in significantly higher blood glucose, HbA1c (p < 0.0001), and glucose intolerance (p < 0.0001). AD resulted in higher blood urea nitrogen (p = 0.0002) while AD, but not STZ+AD mice, had high serum parathyroid hormone (p < 0.0001) and phosphorus (p = 0.0005). STZ lowered bone turnover (p = 0.001). Trabecular bone volume was lowered by STZ (p < 0.0001) and increased by AD (p = 0.003). Tissue mineral density was lowered by STZ (p < 0.0001) and AD (p = 0.02) in trabecular bone but only lowered by STZ in cortical bone (p = 0.002). Cortical porosity of the proximal tibia was increased by AD, moment of inertia was lower in both disease groups, and most cortical properties were lower in all groups vs CTRL. Ultimate force, stiffness, toughness, and total displacement/strain were lowered by STZ and AD. Fracture toughness was lower by AD (p = 0.003). Importantly, Cohen's D indicated that STZ+AD most strongly lowered bone turnover and mechanical properties. Taken together, structural and material-level bone properties are altered by STZ and AD while their combination resulted in greater detriments, indicating that improving bone health in the combined disease state may require novel interventions.

Cortical porosity occurs at varying degrees throughout the skeleton in rats with chronic kidney disease

Cortical porosity develops in chronic kidney disease (CKD) and increases with progressing disease. Cortical porosity is likely a prominent contributor to skeletal fragility/fracture. The degree to which cortical porosity occurs throughout the skeleton is not fully known. In this study, we assessed cortical bone porosity via micro-computed tomography at multiple skeletal sites in rats with progressive chronic kidney disease. We hypothesized that cortical porosity would occur in long bones throughout the body, but to a lesser degree in flat bones and irregular bones. Porosity was measured, using micro-CT, at 17 different skeletal sites in 6 male rats with CKD. Varying degrees of porosity were seen throughout the skeleton with higher porosity in flat and irregular bone (i.e. parietal bone, mandible) vs. long bones (p = 0.01) and in non-weightbearing bones vs. weightbearing bones (p = 0.01). Porosity was also higher in proximal sites vs. distal sites in long bones (p < 0.01 in all comparisons). There was large heterogeneity in porosity within skeletal sites across rats and within the same rat across skeletal sites. Correlations showed cortical porosity of the proximal tibia was positively associated with porosity at the other sites with the strongest correlation to the parietal bone and the weakest to the ulna. Overall, our data demonstrates varying and significant cortical bone porosity across the skeleton of animals with chronic kidney disease. These data point to careful selection of skeletal sites to assess porosity in pre-clinical studies and the potential for fractures at multiple skeletal sites in patients with CKD.

Cortical porosity is elevated after a single dose of zoledronate in two rodent models of chronic kidney disease

PURPOSE

Patients with chronic kidney disease (CKD) have high risk of fracture in part due to cortical bone deterioration. The goal of this study was to assess the impact of two different bisphosphonates and dosing regimens on cortical microstructure (porosity, thickness, area) and bone mechanical properties in animal models of CKD.

METHODS

In experiment 1, Male Cy/+ (CKD) rats were treated with either a single dose or ten fractionated doses of zoledronate at 18 weeks of age. Fractionated animals received 1/10th of single dose given weekly for 10 weeks, with study endpoint at 28 weeks of age. In experiment 2, male C57Bl/6 J mice were given dietary adenine (0.2%) to induce CKD. Bisphosphonate treated groups were given either a single dose of zoledronate or weekly risedronate injections for 4 weeks. Cortical microstructure was assessed via μCT and mechanical parameters evaluated by monotonic bending tests.

RESULTS

Exp 1: CKD rats had higher blood urea nitrogen (BUN) and parathyroid hormone (PTH) compared to NL littermate controls. Single dose zoledronate had significantly higher cortical porosity in CKD S.Zol (2.29%) compared to NL control (0.04%) and untreated CKD (0.14%) (p = 0.004). Exp 2: All adenine groups had significantly higher BUN and PTH compared to control mice. Mice treated with single dose zoledronate (Ad + Zol) had the highest porosity (~6%), which was significantly higher compared to either Ad or Ad + Ris (~3%; p < 0.0001) and control mice had the lowest cortical porosity (0.35%). In both experiments, mechanics were minimally affected by any bisphosphonate dosing regimen.

CONCLUSION

A single dose of zoledronate leads to higher cortical porosity compared to more frequent dosing of bisphosphonates (fractionated zoledronate or risedronate). Bisphosphonate treatment demonstrated limited effectiveness in preventing cortical bone microstructure deterioration with mechanical parameters remaining compromised due to CKD and/or secondary hyperparathyroidism irrespective of bisphosphonate treatment.

Analysis of bone in adenine-induced chronic kidney disease model rats

Objectives

The purpose of this study is to investigate the stage of chronic kidney disease (CKD) in adenine-induced CKD model rats by serum analyses, and to examine bone mineral density (BMD), bone strength, and microstructure of trabecular and cortical bone in these rats.

Methods

Eight-week-old, male Wistar rats (n = 42) were divided into 2 groups: those fed a 0.75% adenine diet for 4 weeks until 12 weeks of age to generate CKD model rats (CKD group); and sham rats. The CKD and sham groups were sacrificed at 12, 16, and 20 weeks of age (n = 7 in each group and at 12, 16, and 20 weeks), and various parameters were evaluated, including body weight, renal wet weight, muscle wet weight, renal histology, biochemical tests, BMD, biomechanical testing, and micro-computed tomography (CT). The parameters were compared between the 2 groups at the various time points.

Results

In the CKD model rats, at 20 weeks of age, serum creatinine, phosphorus, and intact-PTH levels were elevated, and serum calcium levels were normal, indicating that the CKD was stage IV and associated with secondary hyperparathyroidism. Decreased BMDs of the whole body and the femur were observed as bone changes, and micro-CT analysis showed deterioration of bone microstructure of the cortical bone that resulted in decreased bone strength in the cortical and trabecular bone.

Conclusions

These CKD model rats showed stage IV CKD and appear appropriate for evaluating the effects of several treatments for CKD-related osteoporosis and mineral bone disorder.

CKD Stages, Bone Metabolism Markers, and Cortical Porosity Index: Associations and Mediation Effects Analysis

Chronic kidney disease (CKD) has a significant negative impact on bone health. However, the mechanisms of cortical bone deterioration and cortical porosity enlargement caused by CKD have not been fully described. We therefore examined the association of CKD stages with cortical porosity index (PI), and explored potential mediators of this association. Double-echo ultrashort echo-time magnetic resonance imaging (UTE MRI) provides the possibility of quantifying cortical porosity in vivo. A total of 95 patients with CKD stages 2-5 underwent 3D double-echo UTE-Cones MRI (3.0T) of the midshaft tibia to obtain the PI. PI was defined as the ratio of the image signal intensity of a sufficiently long echo time (TE) to the shortest achievable TE. Parathyroid hormone (PTH), β-CrossLaps (β-CTX), total procollagen type I amino-terminal propeptide (T-P1NP), osteocalcin (OC), 25-hydroxyvitamin D (25OHD), and lumbar bone mineral density (BMD) were measured within one week of the MRI. Partial correlation analysis was performed to address associations between PI, eGFR and potential mediators (PTH, β-CTX, T-P1NP, OC, 25OHD, BMD, and T-score). Multiple linear regression models were used to assess the association between CKD stages and PI value. Then, a separate exploratory mediation analysis was carried out to explore the impact of CKD stages and mediators on the PI value. The increasing CKD stages were associated with a higher PI value (P_trend < 0.001). The association of CKD stages and PI mediated 34.4% and 30.8% of the total effect by increased PTH and β-CTX, respectively. Our study provides a new idea to monitor bone health in patients with CKD, and reveals the internal mechanism of bone deterioration caused by CKD to some extent.