Reversing cortical porosity: Cortical pore infilling in preclinical models of chronic kidney disease

Combining raloxifene and mechanical loading improves bone composition and mechanical properties in a murine model of chronic kidney disease (CKD)

INTRODUCTION

Patients with chronic kidney disease (CKD) are at an alarming risk of fracture compared to age and sex-matched non-CKD individuals. Clinical and preclinical data highlight two key factors in CKD-induced skeletal fragility: cortical porosity and reduced matrix-level properties including bone hydration. Thus, strategies are needed to address these concerns to improve mechanical properties and ultimately lower fracture risk in CKD. We sought to evaluate the singular and combined effects of mechanical and pharmacological interventions on modulating porosity, bone hydration, and mechanical properties in CKD.

METHODS

Sixteen-week-old male C57BL/6J mice underwent a 10-week CKD induction period via a 0.2 % adenine-laced casein-based diet (n = 48) or remained as non-CKD littermate controls (Con, n = 48). Following disease induction (26 weeks of age), n = 7 CKD and n = 7 Con were sacrificed (baseline cohort) to confirm a steady-state CKD state was achieved prior to the initiation of treatment. At 27 weeks of age, all remaining mice underwent right tibial loading to a maximum tensile strain of 2050 μƐ 3× a week for five weeks with the contralateral limb as a non-loaded control. Half of the mice (equal number CKD and Con) received subcutaneous injections of 0.5 mg/kg raloxifene (RAL) 5× a week, and the other half remained untreated (UN). Mice were sacrificed at 31 weeks of age. Serum biochemistries were performed, and bi-lateral tibiae were assessed for microarchitecture, whole bone and tissue level mechanical properties, and composition including bone hydration.

RESULTS

Regardless of intervention, BUN and PTH were higher in CKD animals throughout the study. In CKD, the combined effects of loading and RAL were quantified as lower cortical porosity and improved mechanical, material, and compositional properties, including higher matrix-bound water. Loading was generally responsible for positive impacts in cortical geometry and structural mechanical properties, while RAL treatment improved some trabecular outcomes and material-level mechanical properties and was responsible for improvements in several compositional parameters. While control animals responded positively to loading, their bones were less impacted by the RAL treatment, showing no deformation, toughness, or bound water improvements which were all evident in CKD. Serum PTH levels were negatively correlated with matrix-bound water.

DISCUSSION

An effective treatment program to improve fracture risk in CKD ideally focuses on the cortical bone and considers both cortical porosity and matrix properties. Loading-induced bone formation and mechanical improvements were observed across groups, and in the CKD cohort, this included lower cortical porosity. This study highlights that RAL treatment superimposed on active bone formation may be ideal for reducing skeletal complications in CKD by forming new bone with enhanced matrix properties.

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.

Tracking changes of individual cortical pores over 1 year via HR-pQCT in a small cohort of 60-year-old females

Introduction

High-resolution peripheral quantitative computed tomography (HR-pQCT) is a powerful tool that has revolutionized 3D longitudinal assessment of bone microarchitecture. However, cortical porosity, a common characteristic of cortical bone loss, is still often determined by static evaluation of overall porosity at one timepoint. Therefore, we sought to 1) describe a technique to evaluate individual cortical pore dynamics in aging females over one year using HR-pQCT imaging and 2) determine whether formation and expansion of pores would exceed contraction and infilling of pores.

Methods

HR-pQCT (60.7 μm resolution) images were acquired one year apart at the distal tibia and distal radius in seven female volunteers (60-72 years of age). Baseline and one-year images were registered at each bone site and a custom software was used to quantify dynamic activity of individual cortical pores using the following categories: developed, infilled, expanded, contracted, and static.

Results

Over the one-year period, cortical pores actively developed, contracted, expanded, and infilled. More pores expanded and developed vs. infilled or contracted leading to increased pore area in both tibial and radial sites (p = 0.0034 and p = 0.0474, respectively). Closed pores in the tibia, those that were not connected to the endosteal or periosteal surfaces, were the most dynamic of any pores type (open/closed) at either bone site.

Conclusion

This study demonstrates an approach to longitudinally track individual cortical pore activity in tibial and radial sites. These data expand conventional parameters for assessing cortical porosity and show increased porosity in one year of aging is caused by newly developed pores and expansion of existing pores.

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.

Calcimimetics Alter Periosteal and Perilacunar Bone Matrix Composition and Material Properties in Early Chronic Kidney Disease

Chronic kidney disease (CKD) affects 15% of Americans and greatly increases fracture risk due to elevated parathyroid hormone, cortical porosity, and reduced bone material quality. Calcimimetic drugs are used to lower parathyroid hormone (PTH) in CKD patients, but their impact on bone matrix properties remains unknown. We hypothesized that tissue-level bone quality is altered in early CKD and that calcimimetic treatment will prevent these alterations. To test this hypothesis, we treated Cy/+ rats, a model of spontaneous and progressive CKD-mineral and bone disorder (CKD-MBD), with KP-2326, a preclinical analogue of etelcalcetide, early in the CKD disease course. To measure tissue-level bone matrix composition and material properties, we performed colocalized Raman spectroscopy and nanoindentation on new periosteal bone and perilacunar bone using hydrated femur sections. We found that CKD and KP treatment lowered mineral type B carbonate substitution whereas KP treatment increased mineral crystallinity in new periosteal bone. Reduced elastic modulus was lower in CKD but was not different in KP-treated rats versus CTRL. In perilacunar bone, KP treatment lowered type B carbonate substitution, increased crystallinity, and increased mineral-to-matrix ratio in a spatially dependent manner. KP treatment also increased reduced elastic modulus and hardness in a spatially dependent manner. Taken together, these data suggest that KP treatment improves material properties on the tissue level through a combination of lowering carbonate substitution, increasing mineral crystallinity, and increasing relative mineralization of the bone early in CKD. As a result, the mechanical properties were improved, and in some regions, were the same as control animals. Therefore, calcimimetics may help prevent CKD-induced bone deterioration by improving bone quality in new periosteal bone and in bone tissue near osteocyte lacunae. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Cortical porosity development and progression is mitigated after etelcalcetide treatment in an animal model of chronic kidney disease

PURPOSE

Chronic kidney disease (CKD) leads to increased bone fragility and risk of fracture. Cortical deteriorations, including cortical porosity, are key factors in fracture susceptibility in CKD. Since secondary hyperparathyroidism is common in CKD individuals and contributes to cortical deterioration, we hypothesized that reducing parathyroid hormone (PTH) may modulate CKD-induced cortical porosity. The goal of this pilot study was to assess the effects of lowering PTH, via the preclinical analogue of the FDA-approved calcimimetic etelcalcetide (KP-2326), on the development and progression of cortical pores in the setting of CKD.

METHODS

Male Cy/+ Sprague Dawley rats with clinical biochemistries consistent with CKD (N = 8) were assigned to the study. At 30-32 weeks of age, cortical bone was assessed via In vivo μCT and blood collected for biochemistries to create baseline measures. Calcimimetic treatment with KP-2326 (KP) was then administered 3× weekly for 2-4 weeks. Cortical bone and biochemical parameters were repeated at study endpoint (33-37 wks of age). A group of age- and cohort-matched CKD rats (N = 4) were utilized as untreated controls.

RESULTS

Untreated CKD rats had significantly increased cortical porosity over time, while porosity in KP-treated CKD rats was not significantly changed over time. Individual pore analysis revealed that pore area was significantly higher for expanding pores in untreated CKD rats compared to KP-treated CKD rats. Mechanical properties of KP-treated animal femora were similar to historical values of age-matched CKD animals and lower than those of age-matched non-diseased animals.

CONCLUSION

Our pilot preclinical study demonstrates that etelcalcetide treatment can mitigate the progression of cortical bone changes in an animal model of CKD through suppression of pre-existing cortical pore expansion and limiting the size of new pore development. While stabilization of porosity is beneficial it remains likely that infilling of porosity will be needed to positively affect mechanical properties of bones in the setting of CKD.

Blueberry Polyphenols do not Improve Bone Mineral Density or Mechanical Properties in Ovariectomized Rats

Osteoporosis-related bone fragility fractures are a major public health concern. Given the potential for adverse side effects of pharmacological treatment, many have sought alternative treatments, including dietary changes. Based on recent evidence that polyphenol-rich foods, like blueberries, increase calcium absorption and bone mineral density (BMD), we hypothesized that blueberry polyphenols would improve bone biomechanical properties. To test this, 5-month-old ovariectomized Sprague-Dawley rats (n = 10/gp) were orally gavaged for 90 days with either a purified extract of blueberry polyphenols (0-1000 mg total polyphenols/kg bw/day) or lyophilized blueberries (50 mg total polyphenols/kg bw/day). Upon completion of the dosing regimen, right femur, right tibia, and L1-L4 vertebrae were harvested and assessed for bone mineral density (BMD), with femurs being further analyzed for biomechanical properties via three-point bending. There were no differences in BMD at any of the sites analyzed. For bone mechanical properties, the only statistically significant difference was the high dose group having greater ultimate stress than the medium dose, although in the absence of differences in other measures of bone mechanical properties, we concluded that this result, while statistically significant, had little biological significance. Our results indicate that blueberry polyphenols had little impact on BMD or bone mechanical properties in an animal model of estrogen deficiency-induced bone loss.

Common Dietary Modifications in Preclinical Models to Study Skeletal Health

Bone is a highly dynamic tissue that undergoes continuous remodeling by bone resorbing osteoclasts and bone forming osteoblasts, a process regulated in large part by osteocytes. Dysregulation of these coupled catabolic and anabolic processes as in the case of menopause, type 2 diabetes mellitus, anorexia nervosa, and chronic kidney disease is known to increase fracture risk. Recent advances in the field of bone cell metabolism and bioenergetics have revealed that maintenance of the skeleton places a high energy demand on these cells involved in bone remodeling. These new insights highlight the reason that bone tissue is the beneficiary of a substantial proportion of cardiac output and post-prandial chylomicron remnants and requires a rich supply of nutrients. Studies designed for the specific purpose of investigating the impact of dietary modifications on bone homeostasis or that alter diet composition and food intake to produce the model can be found throughout the literature; however, confounding dietary factors are often overlooked in some of the preclinical models. This review will examine some of the common pre-clinical models used to study skeletal biology and its pathologies and the subsequent impact of various dietary factors on these model systems. Furthermore, the review will include how inadvertent effects of some of these dietary components can influence bone cell function and study outcomes.

The combination of aging and chronic kidney disease leads to an exacerbated cortical porosity phenotype

PURPOSE

Chronic kidney disease (CKD) and aging are each independently associated with higher fracture risk. Although CKD is highly prevalent in the aging population, the interaction between these two conditions with respect to bone structure and mechanics is not well understood. The purpose of this study was to examine cortical porosity and mechanical properties in skeletally mature young and aging mice with CKD.

METHODS

CKD was induced by feeding 16-week and 78-week male mice 0.2% adenine (AD) for six weeks followed by two weeks of maintenance on a control diet for a total study duration of eight weeks of CKD; control (CON) animals of each age were fed a standard diet. Serum biochemistries, μCT imaging, and mechanical properties via four-point bending were assessed at the endpoint.

RESULTS

Phosphorus, parathyroid hormone, and blood urea nitrogen were elevated in both ages of AD mice compared to age-matched CON; aging AD mice had PTH and BUN values higher than all other groups. Femoral cortical porosity was more than four-fold higher in aging AD mice compared to young AD mice and more than two-fold higher compared to age-matched controls. Structural and estimated material mechanical properties were both lower in aging mice, but there were no significant interactions between AD treatment and age.

CONCLUSION

These data show an interaction between CKD and aging that produces a more severe biochemical and cortical bone phenotype. This highlights the importance of studying mechanisms and potential interventions in both young and aged animals to translate to a broader spectrum of CKD patients.

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

Chronic kidney disease (CKD) leads to loss of cortical bone through cortical thinning and the development of cortical porosity. The goal of this current study was to assess cortical bone alterations to adenine-induced chronic kidney disease (CKD) in two strains of mice with known genetic differences in cortical thickness. We hypothesized that C3H mice with thicker cortices and baseline levels of intracortical remodeling would have greater cortical porosity in response to adenine-induced CKD compared to B6 animals.

METHODS

Female C57BL/6 J (B6) and C3H/Hej (C3H) at 16-weeks of age were given a diet with 0.2% adenine to induce CKD for 6 weeks followed by a control diet for 4 weeks. Age- and strain-matched controls were fed the control diet without adenine for the 10-week period (n = 8 per group per strain).

RESULTS

Both strains of adenine-fed mice had elevated blood urea nitrogen, demonstrating compromised kidney function, compared to strain-matched controls, but only B6 adenine mice had statistically higher parathyroid hormone (PTH), greater cortical porosity, high bone turnover rate, a greater percentage of osteocytes positive for RANKL and IL-17, and lower osteocyte apoptosis compared to B6 controls. C3H mice had intracortical remodeling present in both control and adenine mice, while B6 mice had intracortical remodeling present only in adenine mice. Adenine mice of both strains had lower cortical thickness and a higher percentage of osteocytes positive for TNF-α compared to controls.

CONCLUSION

While both strains of mice had biochemical markers of kidney disease, only B6 mice developed a phenotype with significantly elevated PTH, high bone turnover, and cortical porosity development. This work, in a model of progressive CKD, further confirms the role of chronically elevated PTH in the development of cortical porosity and demonstrates adenine-induced increases in PTH contribute to intracortical remodeling in B6 mice. Adenine-induced changes that occurred in both strains of mice, notably lower cortical thickness and a higher percentage of osteocytes expressing TNF-α, indicate potential PTH-independent responses to CKD.

Adenine-induced chronic kidney disease induces a similar skeletal phenotype in male and female C57BL/6 mice with more severe deficits in cortical bone properties of male mice

Chronic kidney disease (CKD) causes bone loss, particularly in cortical bone, through formation of cortical pores which lead to skeletal fragility. Animal models of CKD have shown variability in the skeletal response to CKD between males and females suggesting sex may play a role in this variation. Our aim was to compare the impact of adenine-induced CKD on cortical parameters in skeletally mature male and female C57Bl/6 mice. After 10-weeks of adenine-induced CKD, both male and female adenine mice had high serum parathyroid hormone (PTH), high bone turnover, and cortical porosity compared to non-CKD controls. Both sexes had lower cortical thickness, but only male mice had lower cortical bone area. CKD imparted greater deficits in mechanical properties of male mice compared to female mice. These data demonstrate that both male and female mice develop high PTH/high bone turnover in response to adenine-induced CKD and that cortical bone phenotypes are slightly more severe in males, particularly in mechanical properties deficits.