Elevations in Cortical Porosity Occur Prior to Significant Rise in Serum Parathyroid Hormone in Young Female Mice with Adenine-Induced CKD

Combined Romosozumab and Raloxifene treatment targets impaired bone quality in a male murine model of diabetic kidney disease

Comorbid diabetes and chronic kidney disease create a complex disease state with multi-faceted impacts on bone health, primarily reduced bone mass and tissue quality. To reduce fracture risk in this growing population, interventions are needed that target both bone mass and quality. Romosozumab (Romo) is an FDA-approved sclerostin inhibitor that has been shown to increase bone mass and strength in a murine model of combined diabetes and CKD (DKD), while Raloxifene (RAL) is a mild anti-resorptive used to treat osteoporosis that has also been shown to increase bone mechanical properties by increasing bone bound water content. We aimed to test whether combined RAL and Romo treatment could improve bone quality in our murine model of DKD more than either treatment alone. Using a previously established streptozotocin- and adenine-diet-induced model, male, C57BL/6J mice were randomly divided into four treatment groups and given daily subcutaneous injections of 100 μL vehicle (phosphorus buffered saline, PBS) or 0.5 mg/kg RAL. In addition, two groups were also given a weekly dose of Romo (10 mg/kg). Overall, Romo increased whole-bone strength and RAL improved tissue-level mechanical properties. Combined RAL-Romo treatment led to significantly higher cortical and trabecular bone mass compared to untreated controls. These morphological improvements created corresponding improvements in cortical bending strength and vertebral trabecular compression strength. These results suggest that combined RAL-Romo treatment provides both mass and quality improvements to DKD bone.

Role of sphingolipid metabolism signaling in a novel mouse model of renal osteodystrophy based on transcriptomic approach

BACKGROUND

Renal osteodystrophy (ROD) is a skeletal pathology associated with chronic kidney disease-mineral and bone disorder (CKD-MBD) that is characterized by aberrant bone mineralization and remodeling. ROD increases the risk of fracture and mortality in CKD patients. The underlying mechanisms of ROD remain elusive, partially due to the absence of an appropriate animal model. To address this gap, we established a stable mouse model of ROD using an optimized adenine-enriched diet and conducted exploratory analyses through ribonucleic acid sequencing (RNA-seq).

METHODS

Eight-week-old male C57BL/6J mice were randomly allocated into three groups: control group ( n = 5), adenine and high-phosphate (HP) diet group ( n = 20), and the optimized adenine-containing diet group ( n = 20) for 12 weeks. We assessed the skeletal characteristics of model mice through blood biochemistry, microcomputed tomography (micro-CT), and bone histomorphometry. RNA-seq was utilized to profile gene expression changes of ROD. We elucidated the functions of differentially expressed genes (DEGs) using gene ontology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, and gene set enrichment analysis (GSEA). DEGs were validated via quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

By the fifth week, adenine followed by an HP diet induced rapid weight loss and high mortality rates in the mouse group, precluding further model development. Mice with optimized adenine diet-induced ROD displayed significant abnormalities in serum creatinine and blood urea nitrogen levels, accompanied by pronounced hyperparathyroidism and hyperphosphatemia. The femur bone mineral density (BMD) of the model mice was lower than that of control mice, with substantial bone loss and cortical porosity. ROD mice exhibited substantial bone turnover with an increase in osteoblast and osteoclast markers. Transcriptomic profiling revealed 1907 genes with upregulated expression and 723 genes with downregulated expression in the femurs of ROD mice relative to those of control mice. Pathway analyses indicated significant enrichment of upregulated genes in the sphingolipid metabolism pathway. The significant upregulation of alkaline ceramidase 1 ( Acer1 ), alkaline ceramidase 2 ( Acer2 ), prosaposin-like 1 ( Psapl1 ), adenosine A1 receptor ( Adora1 ), and sphingosine-1-phosphate receptor 5 ( S1pr5 ) were successfully validated in mouse femurs by qRT-PCR.

CONCLUSIONS

Optimized adenine diet mouse model may be a valuable proxy for studying ROD. RNA-seq analysis revealed that the sphingolipid metabolism pathway is likely a key player in ROD pathogenesis, thereby providing new avenues for therapeutic intervention.

Effects of plasminogen activator inhibitor-1 deficiency on bone disorders and sarcopenia caused by adenine-induced renal dysfunction in mice

Chronic kidney disease (CKD) is a significant global health issue and often involves CKD-mineral and bone disorder (MBD) and sarcopenia. Plasminogen activator inhibitor-1 (PAI-1) is an inhibitor of fibrinolysis. PAI-1 has been implicated in the pathogenesis of osteoporosis and muscle wasting induced by inflammatory conditions. However, the roles of PAI-1 in CKD-MBD and sarcopenia remain unknown. Therefore, the present study investigated the roles of PAI-1 in bone loss and muscle wasting induced by adenine in PAI-1-deficient mice. CKD was induced in PAI-1+/+ and PAI-1-/- mice by administration of adenine for ten weeks. Muscle wasting was assessed by grip strength test, quantitative computed tomography (CT) analysis and muscle weight measurement. Osteoporosis was assessed by micro-CT analysis of femoral microstructural parameters. PAI-1 deficiency did not affect adenine-induced decreases in body weight and food intake or renal dysfunction in male or female mice. PAI-1 deficiency also did not affect adenine-induced decreases in grip strength, muscle mass in the lower limbs, or the tissue weights of the gastrocnemius, soleus, and tibialis anterior muscles in male or female mice. PAI-1 deficiency aggravated trabecular bone loss in CKD-induced male mice, but significantly increased trabecular bone in CKD-induced female mice. On the other hand, PAI-1 deficiency did not affect cortical bone loss in CKD-induced mice. In conclusion, PAI-1 is not critical for the pathophysiology of CKD-MBD or CKD-induced sarcopenia in mice. However, PAI-1 may be partly related to bone metabolism in trabecular bone in the CKD state with sex differences.

Combining anabolic loading and raloxifene improves bone quantity and some quality measures in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) increases fracture risk due to changes in bone quantity and quality caused by mutations in collagen and its processing proteins. Current therapeutics improve bone quantity, but do not treat the underlying quality deficiencies. Male and female G610C+/- mice, a murine model of OI, were treated with a combination of raloxifene and in vivo axial tibial compressive loading starting at 10 weeks of age and continuing for 6 weeks to improve bone quantity and quality. Bone geometry and mechanical properties were measured to determine whole bone and tissue-level material properties. A colocalized Raman/nanoindentation system was used to measure chemical composition and nanomechanical properties in newly formed bone compared to old bone to determine if bone formed during the treatment regimen differed in quality compared to bone formed prior to treatment. Lastly, lacunar geometry and osteocyte apoptosis were assessed. OI mice were able to build bone in response to the loading, but this response was less robust than in control mice. Raloxifene improved some bone material properties in female but not male OI mice. Raloxifene did not alter nanomechanical properties, but loading did. Lacunar geometry was largely unchanged with raloxifene and loading. However, osteocyte apoptosis was increased with loading in raloxifene treated female mice. Overall, combination treatment with raloxifene and loading resulted in positive but subtle changes to bone quality.

Romosozumab rescues impaired bone mass and strength in a murine model of diabetic kidney disease

As international incidence of diabetes and diabetes-driven comorbidities such as chronic kidney disease (CKD) continue to climb, interventions are needed that address the high-risk skeletal fragility of what is a complex disease state. Romosozumab (Romo) is an FDA-approved sclerostin inhibitor that has been shown to increase bone mineral density and decrease fracture rates in osteoporotic patients with mild to severe CKD, but its effect on diabetes-weakened bone is unknown. We aimed to test Romo's performance in a model of combined diabetes and CKD. 6-week old male C57BL/6 mice were randomly divided into control (CON) and disease model (STZ-Ad) groups, using a previously established streptozotocin- and adenine-diet-induced model. After 16 weeks of disease induction, both CON and STZ-Ad groups were subdivided into two treatment groups and given weekly subcutaneous injections of 100 μL vehicle (phosphorus buffered saline, PBS) or 10 mg/kg Romo. Mice were euthanized after 4 weeks of treatment via cardiac exsanguination and cervical dislocation. Hindlimb bones and L4 vertebrae were cleaned of soft tissue, wrapped in PBS-soaked gauze and stored at -20C. Right tibiae, femora, and L4s were scanned via microcomputed tomography; tibiae were then tested to failure in 4-pt bending while L4s were compression tested. Romo treatment significantly increased cortical and trabecular bone mass in both STZ-Ad and CON animals. These morphological improvements created corresponding increases in cortical bending strength and trabecular compression strength, with STZ-Ad treated mice surpassing vehicle CON mice in all trabecular mechanics measures. These results suggest that Romo retains its efficacy at increasing bone mass and strength in diabetic 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.

In vivo longitudinal 920 nm two-photon intravital kidney imaging of a dynamic 2,8-DHA crystal formation and tubular deterioration in the adenine-induced chronic kidney disease mouse model

Chronic kidney disease (CKD) is one of the most common renal diseases manifested by gradual loss of kidney function with no symptoms in the early stage. The underlying mechanism in the pathogenesis of CKD with various causes such as high blood pressure, diabetes, high cholesterol, and kidney infection is not well understood. In vivo longitudinal repetitive cellular-level observation of the kidney of the CKD animal model can provide novel insights to diagnose and treat the CKD by visualizing the dynamically changing pathophysiology of CKD with its progression over time. In this study, using two-photon intravital microscopy with a single 920 nm fixed-wavelength fs-pulsed laser, we longitudinally and repetitively observed the kidney of an adenine diet-induced CKD mouse model for 30 days. Interestingly, we could successfully visualize the 2,8-dihydroxyadenine (2,8-DHA) crystal formation with a second-harmonics generation (SHG) signal and the morphological deterioration of renal tubules with autofluorescence using a single 920 nm two-photon excitation. The longitudinal in vivo two-photon imaging results of increasing 2,8-DHA crystals and decreasing tubular area ratio visualized by SHG and autofluorescence signal, respectively, were highly correlated with the CKD progression monitored by a blood test showing increased cystatin C and blood urea nitrogen (BUN) levels over time. This result suggests the potential of label-free second-harmonics generation crystal imaging as a novel optical technique for in vivo CKD progression monitoring.

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.

Increased PHOSPHO1 expression mediates cortical bone mineral density in renal osteodystrophy

Patients with advanced chronic kidney disease (CKD) often present with skeletal abnormalities, a condition known as renal osteodystrophy (ROD). While tissue non-specific alkaline phosphatase (TNAP) and PHOSPHO1 are critical for bone mineralization, their role in the etiology of ROD is unclear. To address this, ROD was induced in both WT and Phospho1 knockout (P1KO) mice through dietary adenine supplementation. The mice presented with hyperphosphatemia, hyperparathyroidism, and elevated levels of FGF23 and bone turnover markers. In particular, we noted that in CKD mice, bone mineral density (BMD) was increased in cortical bone (P < 0.05) but decreased in trabecular bone (P < 0.05). These changes were accompanied by decreased TNAP (P < 0.01) and increased PHOSPHO1 (P < 0.001) expression in WT CKD bones. In P1KO CKD mice, the cortical BMD phenotype was rescued, suggesting that the increased cortical BMD of CKD mice was driven by increased PHOSPHO1 expression. Other structural parameters were also improved in P1KO CKD mice. We further investigated the driver of the mineralization defects, by studying the effects of FGF23, PTH, and phosphate administration on PHOSPHO1 and TNAP expression by primary murine osteoblasts. We found both PHOSPHO1 and TNAP expressions to be downregulated in response to phosphate and PTH. The in vitro data suggest that the TNAP reduction in CKD-MBD is driven by the hyperphosphatemia and/or hyperparathyroidism noted in these mice, while the higher PHOSPHO1 expression may be a compensatory mechanism. Increased PHOSPHO1 expression in ROD may contribute to the disordered skeletal mineralization characteristic of this progressive disorder.

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.

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.

Serum PTH, PTH1R/ATF4 pathway, and the sRANKL/OPG system in bone as a new link between bone growth, cross-sectional geometry, and strength in young rats with experimental chronic kidney disease

The progression of chronic kidney disease (CKD) in children is associated with deregulated parathyroid hormone (PTH), growth retardation, and low bone accrual. PTH can cause both catabolic and anabolic impact on bone, and the activating transcription factor 4 (ATF4), a downstream target gene of PTH, is related to its anabolic effect. Osteoprotegerin (OPG) and receptor activator of NF-κB ligand (RANKL) are PTH-dependent cytokines, which may play an important role in the regulation of bone remodeling. This study aimed to evaluate the impact of endogenous PTH and the bone RANKL/OPG system on bone growth, cross-sectional geometry and strength utilizing young, nephrectomized rats. The parameters of cross-sectional geometry were significantly elevated in rats with CKD during the three-month experimental period compared with the controls, and they were strongly associated with serum PTH levels and the expression of parathyroid hormone 1 receptor (PTH1R)/ATF4 genes in bone. Low bone soluble RANKL (sRANKL) levels and sRANKL/OPG ratios were also positively correlated with cross-sectional bone geometry and femoral length. Moreover, the analyzed geometric parameters were strongly related to the biomechanical properties of femoral diaphysis. In summary, the mild increase in endogenous PTH, its anabolic PTH1R/ATF4 axis and PTH-dependent alterations in the bone RANKL/OPG system may be one of the possible mechanisms responsible for the favorable impact on bone growth, cross-sectional geometry and strength in young rats with experimental CKD.

Cortical bone development, maintenance and porosity: genetic alterations in humans and mice influencing chondrocytes, osteoclasts, osteoblasts and osteocytes

Cortical bone structure is a crucial determinant of bone strength, yet for many years studies of novel genes and cell signalling pathways regulating bone strength have focused on the control of trabecular bone mass. Here we focus on mechanisms responsible for cortical bone development, growth, and degeneration, and describe some recently described genetic-driven modifications in humans and mice that reveal how these processes may be controlled. We start with embryonic osteogenesis of preliminary bone structures preceding the cortex and describe how this structure consolidates then matures to a dense, vascularised cortex containing an increasing proportion of lamellar bone. These processes include modelling-induced, and load-dependent, asymmetric cortical expansion, which enables the cortex's transition from a highly porous woven structure to a consolidated and thickened highly mineralised lamellar bone structure, infiltrated by vascular channels. Sex-specific differences emerge during this process. With aging, the process of consolidation reverses: cortical pores enlarge, leading to greater cortical porosity, trabecularisation and loss of bone strength. Each process requires co-ordination between bone formation, bone mineralisation, vascularisation, and bone resorption, with a need for locational-, spatial- and cell-specific signalling pathways to mediate this co-ordination. We will discuss these processes, and a number of cell-signalling pathways identified in both murine and human genetic studies to regulate cortical bone mass, including signalling through gp130, STAT3, PTHR1, WNT16, NOTCH, NOTUM and sFRP4.

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.

Osteoporosis and Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD): Back to Basics

Osteoporosis is defined as a skeletal disorder of compromised bone strength predisposing those affected to an elevated risk of fracture. However, based on bone histology, osteoporosis is only part of a spectrum of skeletal complications that includes osteomalacia and the various forms of renal osteodystrophy of chronic kidney disease-mineral and bone disorder (CKD-MBD). In addition, the label "kidney-induced osteoporosis" has been proposed, even though the changes caused by CKD do not qualify as osteoporosis by the histological diagnosis. It is clear, therefore, that such terminology may not be helpful diagnostically or in making treatment decisions. A new label, "CKD-MBD/osteoporosis" could be a more appropriate term because it brings osteoporosis under the official label of CKD-MBD. Neither laboratory nor noninvasive diagnostic investigations can discriminate osteoporosis from the several forms of renal osteodystrophy. Transiliac crest bone biopsy can make the diagnosis of osteoporosis by exclusion of other kidney-associated bone diseases, but its availability is limited. Recently, a classification of metabolic bone diseases based on bone turnover, from low to high, together with mineralization and bone volume, has been proposed. Therapeutically, no antifracture treatments have been approved by the US Food and Drug Administration for patients with kidney-associated bone disease. Agents that suppress parathyroid hormone (vitamin D analogues and calcimimetics) are used to treat hyperparathyroid bone disease. Antiresorptive and osteoanabolic agents approved for osteoporosis are being used off-label to treat CKD stages 3b-5 in high-risk patients. It has now been suggested that intermittent administration of parathyroid hormone as early as CKD stage 2 could be an effective management strategy. If confirmed in clinical trials, it could mitigate the retention of phosphorus and subsequently the rise in fibroblast growth factor 23 and may be beneficial for coexisting osteoporosis.

The Effect of Single Versus Group μCT on the Detection of Trabecular and Cortical Disease Phenotypes in Mouse Bones

Micro‐computed tomography is a critical assessment tool for bone‐related preclinical research, especially in murine models. To expedite the scanning process, researchers often image multiple bones simultaneously; however, it is unknown if this impacts scan quality and alters the ability to detect differences between experimental groups. The purpose of this study was to assess the effect of multibone scanning on detecting disease‐induced changes in bone microarchitecture and mineral density by group scanning two murine models with known skeletal defects: the Col1a2^G610C/+ model of osteogenesis imperfecta and an adenine‐induced model of chronic kidney disease. Adult male femurs were scanned individually and in random groups of three and eight in a Bruker Skyscan 1172 and 1176, respectively, then assessed for standard trabecular and cortical bone measures. Although scanning methodology altered raw values, with trabecular microarchitecture values more affected than cortical properties, a disease phenotype was still detectable in both group and solo scans. However, tissue mineral density in both trabecular and cortical bone was significantly impacted by group versus solo scanning. Researchers may be able to use small groupings in a single μCT scan to expedite preclinical analyses when the overall bone phenotype is large to decrease costs and increase speed of discoveries; however the details of scanning (single or group) should always be reported. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

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

PURPOSE

Chronic kidney disease (CKD) patients have a high incidence of fracture due in part to cortical porosity. The goal of this study was to study cortical pore infilling utilizing two rodent models of progressive CKD.

METHODS

Exp 1: Female C57Bl/6J mice (16-week-old) were given dietary adenine (0.2%) to induce CKD for 10 weeks after which calcium water supplementation (Ca-H2O; 1.5% and 3%) was given to suppress PTH for another 4 weeks. Exp 2: Male Cy/+ rats were aged to ~30 weeks with baseline porosity assessed using in vivo μCT. A second in vivo scan followed 5-weeks of Ca-H2O (3%) supplementation.

RESULTS

Exp 1: Untreated adenine mice had elevated blood urea nitrogen (BUN), parathyroid hormone (PTH), and cortical porosity (~2.6% porosity) while Ca-H2O lowered PTH and cortical porosity (0.5-0.8% porosity). Exp 2: Male Cy/+ rats at baseline had variable porosity (0.5%-10%), but after PTH suppression via Ca-H2O, cortical porosity in all rats was lower than 0.5%. Individual pore dynamics measured via a custom MATLAB code demonstrated that 85% of pores infilled while 12% contracted in size.

CONCLUSION

Ca-H2O supplementation causes net cortical pore infilling over time and imparted mechanical benefits. While calcium supplementation is not a viable clinical treatment for CKD, these data demonstrate pore infilling is possible and further research is required to examine clinically relevant therapeutics that may cause net pore infilling in CKD.

Measuring Bone Volume at Multiple Densities by Micro-computed Tomography

Bone strength is controlled by both bone mass, and the organization and quality of the bone material. The current standard method for measuring bone mass in mouse and rat studies is micro-computed tomography. This method typically uses a single threshold to identify bone material in the cortical and trabecular regions. However, this single threshold method obscures information about the mineral content of the bone material and depends on normal morphology to separately analyze cortical and trabecular structures. To extend this method to identify bone mass at multiple density levels, we have established a protocol for unbiased selection and application of multiple thresholds using a standard laboratory-based micro-computed tomography instrument. This non-invasive method can be applied to longitudinal studies and archived samples and provides additional information about bone structure and strength.

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.

The CKD-MBD Syndrome: Hysteresis in PTH Involvement and PTH Administration for Its Management

Chronic kidney disease (CKD) disturbs mineral homeostasis, leading to mineral and bone disorders (MBD). CKD-MBD is a significant problem and currently available treatment options have important limitations. Phosphate retention is thought to be the initial cause of CKD-MBD but serum phosphate remains normal until the late stages of CKD, due to elevated levels of the phosphaturic hormone fibroblast growth factor-23 (FGF-23), and parathyroid hormone (PTH). Reduction of 1,25-dihydroxy-vitamin D (1,25[OH]₂ D) concentration is the next event in the adaptive response of the homeostatic system. We argue, and provide the rationale, that calcium retention which takes place concurrently with phosphate retention, could be the reason behind the hysteresis in the response of PTH. If indeed this is the case, intermittent administration of PTH in early CKD could prevent the hysteresis, which arguably leads to the development of secondary hyperparathyroidism, and provide the platform for an effective management of CKD-MBD. © 2020 American Society for Bone and Mineral Research (ASBMR).