Bone Disease in Nephropathic Cystinosis: Beyond Renal Osteodystrophy

Leptin signalling altered in infantile nephropathic cystinosis-related bone disorder

BACKGROUND

The CTNS gene mutation causes infantile nephropathic cystinosis (INC). Patients with INC develop Fanconi syndrome and chronic kidney disease (CKD) with significant bone deformations. C57BL/6 Ctns-/- mice are an animal model for studying INC. Hyperleptinaemia results from the kidney's inability to eliminate the hormone leptin in CKD. Ctns-/- mice have elevated serum leptin concentrations. Leptin regulates bone metabolism through its receptor that signals further via the hypothalamic melanocortin 4 receptor (MC4R). Leptin signalling may affect bone health in Ctns-/- mice.

METHODS

We first defined the time course of bone abnormalities in Ctns-/- mice between 1 and 12 months of age. We used both genetic and pharmacological approaches to investigate leptin signalling in Ctns-/- mice. We generated Ctns-/-Mc4r-/- double knockout mice. Bone phenotype of Ctns-/-Mc4r-/- mice, Ctns-/- mice and wild type (WT) mice at 1, 4, and 9 months of age were compared. We then treated 12-month-old Ctns-/- mice and WT mice with a pegylated leptin receptor antagonist (PLA) (7 mg/kg/day, IP), a MC4R antagonist agouti-related peptide (AgRP) (2 nmol, intracranial infusion on days 0, 3, 6, 9, 12, 15, 18, 21, 24, and 27), or vehicle (normal saline), respectively, for 28 days. Whole-body (BMC/BMD, bone area) and femoral bone phenotype (BMC/BMD, bone area, length and failure load) of mice were measured by DXA and femoral shaft biochemical test. We also measured lean mass content by EchoMRI and muscle function (grip strength and rotarod activity) in mice. Femur protein content of JAK2 and STAT3 was measured by ELISA kits, respectively.

RESULTS

Bone defects are present in Ctns-/- mice throughout its first year of life. The deletion of the Mc4r gene attenuated bone disorder in Ctns-/- mice. Femoral BMD, bone area, length, and strength (failure load) were significantly increased in 9-month-old Ctns-/-Mc4r-/- mice than in age-matched Ctns-/- mice. PLA and AgRP treatment significantly increased femoral bone density (BMC/BMD) and mechanical strength in 12-month-old Ctns-/- mice. We adopted the pair-feeding approach for this study to show that the protective effects of PLA or AgRP on bone phenotype are independent of their potent orexigenic effect. Furthermore, an increase in lean mass and in vivo muscle function (grip strength and rotarod activity) are associated with improvements in bone phenotype (femoral BMC/BMD and mechanical strength) in Ctns-/- mice, suggesting a muscle-bone interplay. Decreased femur protein content of JAK2 and STAT3 was evident in Ctns-/- mice. PLA or AgRP treatment attenuated femur STAT3 content in Ctns-/- mice.

CONCLUSIONS

Our findings suggest a significant role for dysregulated leptin signalling in INC-related bone disorder, either directly or potentially involving a muscle-bone interplay. Leptin signalling blockade may represent a novel approach to treating bone disease as well as muscle wasting in INC.

Cortical impairment and reduced muscle mass in children and young adults with nephropathic cystinosis

Nephropathic cystinosis is an orphan autosomal recessive lysosomal storage disease characterized by a deficiency of cystinosin, a cystine transporter protein, leading to tissue damage, primarily in the kidney and cornea. With the introduction of cystine-depleting therapy with cysteamine and the possibility to survive to adulthood, new challenges of skeletal complications are a concern, with sparse data available regarding bone development. The aim of the current study was to gain more information on bone density and geometry in these patients. Fifty-one patients (29 males, 22 females) with genetically proven nephropathic cystinosis were clinically evaluated with a medical history, physical examination, grip strength measurements, and biochemical and imaging studies. Bone mineral density, bone geometry, and muscle cross sectional area were measured, and muscle was evaluated. Results were compared with age- and gender-specific reference data. Z-scores for height (mean [M] = -1.75, standard deviation [SD] = 1.43), weight (M = -1.67, SD = 1.29), and BMI (M = -0.98, SD = 1.29) were lower than reference data. Medullary cross-sectional area (CSA) and cortical density z-scores were not compromised (M = 0.12, SD = 1.56 and M = -0.25, SD = 1.63, respectively), but cortical CSA z-scores and Strength-Strain Index (SSI) were reduced (M = -2.16, SD = 1.08, M = -2.07, SD = 1.08). Muscular deficits were reflected by reduced z-scores for muscle CSA (M = -2.43, SD = 1.27) and grip strength (M = -3.01, SD = 1.10), along with jump force (34% lower than reference value). Multiple regression analyses indicated an association of muscle mass with medullary CSA and SSI, but not with cortical CSA. While bone density parameters were normal, bone geometry was altered, resulting in a thinner cortex with possible impact on bone strength. Muscle weakness be partially responsible for altered bone geometry and could provide a potential treatment target.

Worldwide disparities in access to treatment and investigations for nephropathic cystinosis: a 2023 perspective

BACKGROUND

Nephropathic cystinosis (NC) is a rare lysosomal disease, leading to early kidney failure and extra-renal comorbidities. Its prognosis strongly relies on early diagnosis and treatment by cysteamine. Developing economies (DEing) face many challenges when treating patients for rare and chronic diseases. The aim here is to evaluate the access to investigations and treatment in DEing, and to assess for potential inequalities with Developed Economies (DEed).

METHODS

In this international cross-sectional study, a questionnaire on access, price and reimbursement of genetic, biological analyses, and treatment was sent to nephrology centers worldwide during 2022.

RESULTS

A total of 109 centers responded, coming from 49 countries and managing 741 patients: 43 centers from 30 DEing and Economies in transition (TrE), and 66 from 19 DEed. In 2022, genetics availability was 63% in DEing and 100% in DEed, whereas intra leukocytes cystine levels (IL-CL) were available for 30% of DEing patients, and 94% of DEed patients, both increasing over the last decade, as has access to immediate release cysteamine and to cysteamine eye drops in DEing. However, delayed released cysteamine can be delivered to only 7% vs. 74% of patients from DEing and DEed, respectively, and is still poorly reimbursed in DEing.

CONCLUSIONS

Over the last decade, access to investigations (namely genetics and IL-CL) and to cysteamine have improved in DEing and TrE. However, discrepancies remain with DEed: access to delayed released cysteamine is limited, and reimbursement is still profoundly insufficient, therefore limiting their current use.

Chest configuration in children and adolescents with infantile nephropathic cystinosis compared with other chronic kidney disease entities and its clinical determinants

BACKGROUND

Infantile nephropathic cystinosis (INC) is a systemic lysosomal storage disease causing intracellular cystine accumulation, resulting in renal Fanconi syndrome, progressive kidney disease (CKD), rickets, malnutrition, and myopathy. An INC-specific disproportionately diminished trunk length compared to leg length poses questions regarding the functionality of the trunk.

METHODS

Thus, we prospectively investigated thoracic dimensions and proportions, as well as their clinical determinants in 44 pediatric patients with INC with CKD stages 1-5 and 97 age-matched patients with CKD of other etiology between the ages of 2-17 years. A total of 92 and 221 annual measurements of patients with INC and CKD, respectively, were performed, and associations between anthropometric and clinical parameters were assessed using linear mixed-effects models.

RESULTS

Patients with INC exhibited altered chest dimensions that were distinct from CKD controls, characterized by markedly increased chest depth to height and chest depth to chest width ratio z-scores (> 1.0), while those of patients with CKD were only mildly affected (z-score within ± 1.0). Ratio z-scores differed significantly between both patient groups from 2-6 years of age onward. The degree of chest disproportion in INC patients was significantly associated with both the degree of CKD and tubular dysfunction (e.g., low serum phosphate and bicarbonate) across three different age groups (2-6, 7-12, and 13-17 years).

CONCLUSION

Our data show an INC-specific alteration in thoracic shape from early childhood onward, which is distinct from CKD of other etiologies, suggesting early childhood subclinical changes of the musculoskeletal unit of the thoracic cage, which are associated with kidney function. A higher resolution version of the Graphical abstract is available as Supplementary information.

Amino acid metabolism in tumor: New shine in the fog?

Alterations in amino acid metabolism is closely related to the occurrence of clinical diseases. The mechanism of tumorigenesis is complex, involving the complicated relationship between tumor cells and immune cells in local tumor microenvironment. A series of recent studies have shown that metabolic remodeling is intimately related to tumorigenesis. And amino acid metabolic reprogramming is one of the important characteristics of tumor metabolic remodeling, which participates in tumor cells growth, survival as well as the immune cell activation and function in the local tumor microenvironment, thereby affecting tumor immune escape. Recent studies have further shown that controlling the intake of specific amino acids can significantly improve the effect of clinical intervention in tumors, suggesting that amino acid metabolism is gradually becoming one of the new promising targets of clinical intervention in tumors. Therefore, developing new intervention strategies based on amino acid metabolism has broad prospects. In this article, we review the abnormal changes in the metabolism of some typical amino acids, including glutamine, serine, glycine, asparagine and so on in tumor cells and summarize the relationship among amino acid metabolism, tumor microenvironment and the function of T cells. In particular, we discuss the current issues that need to be addressed in the related fields of tumor amino acid metabolism, aiming to provide a theoretical basis for the development of new strategies for clinical interventions in tumors based on amino acid metabolism reprogramming.

Integrated network pharmacology, transcriptomics and metabolomics analysis to reveal the mechanism of salt Eucommia cortex in the treatment of chronic kidney disease mineral bone disorders via the PPARG/AMPK signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

The skeletal complications associated with chronic kidney diseases from stages 3-5 in individuals are called Chronic Kidney Disease-Mineral Bone Disorder (CKD-MBD), which increases the incidence of cardiovascular diseases drastically and affects the quality of life of patients seriously. Eucommia cortex has the effect of tonifying kidneys and strengthening bones, and salt Eucommia cortex is one of the most commonly used traditional Chinese medicines in the clinical treatment of CKD-MBD instead of Eucommia cortex. However, its mechanism still remains unexplored.

AIM OF THE STUDY

The aim of this study was to investigate the effects and mechanisms of salt Eucommia cortex on CKD-MBD by integrating network pharmacology, transcriptomics, and metabolomics.

MATERIALS AND METHODS

The CKD-MBD mice induced by 5/6 nephrectomy and low calcium/high phosphorus diet were treated with salt Eucommia cortex. The renal functions and bone injuries were evaluated by serum biochemical detection, histopathological analyses, and femur Micro-CT examinations. Differentially expressed genes (DEGs) between the control group and model group, model group and high-dose Eucommia cortex group, model group and high-dose salt Eucommia cortex group were analyzed by transcriptomic analysis. The differentially expressed metabolites (DEMs) between the control group and model group, model group and high-dose Eucommia cortex group, model group and high-dose salt Eucommia cortex group were analyzed by metabolomics analysis.The common targets and pathways were obtained by integrating transcriptomics, metabolomics and network pharmacology, which were identified and verified by in vivo experiments.

RESULTS

The negative impacts on the renal functions and bone injuries were alleviated with salt Eucommia cortex treatment effectively. Compared with CKD-MBD model mice, the levels of serum BUN, Ca and urine Upr were significantly decreased in the salt Eucommia cortex group. And the Integrated network pharmacology, transcriptomics and metabolomics analysis revealed that Peroxisome Proliferative Activated Receptor, Gamma (PPARG) was the only common target, mainly involved by AMPK signaling pathways. The activation of PPARG in the kidney tissue was significantly decreased in CKD-MBD mice but increased in the salt Eucommia cortex treatment. The AMPK signaling pathway were verified that AMPK expression levels were decreased in CKD-MBD mice but increased in the salt Eucommia cortex treatment.

CONCLUSIONS

Our study presented that salt Eucommia cortex alleviated the negative impact of CKD-MBD on the renal injury and bone injury of mice induced by 5/6 nephrectomy with the low calcium/high phosphorus diet effectively, which is highly likely achieved through the PPARG/AMPK signaling pathway.

Hyperglycinuria: diagnosis in middle age

Isolated hyperglycinuria is a rare disorder that is associated with osteoporosis and renal calculi. We report findings in a middle-aged, black woman who presented for renal function evaluation with a history of transient hypobicarbonataemia associated with topiramate therapy. She displayed the full triad of high urinary glycine, early-onset osteopenia despite normal reproductive hormones, and renal calculus with high urinary oxalate, phosphate and uric acid. Parathyroid hormone and fibroblast growth factor 23 were both normal. Formal genetic testing did not reveal mutations in SLC6A20, SLC6A18, SLC6A19, SLC36A2, the known genes associated with glycinuria; however, black individuals are poorly represented in the genetic databases. It may well be that otherwise unidentified mutations may be present or that topiramate may result in a lingering proximal tubule defect even after cessation of the drug.

Muscle and Bone Impairment in Infantile Nephropathic Cystinosis: New Concepts

Cystinosis Metabolic Bone Disease (CMBD) has emerged during the last decade as a well-recognized, long-term complication in patients suffering from infantile nephropathic cystinosis (INC), resulting in significant morbidity and impaired quality of life in teenagers and adults with INC. Its underlying pathophysiology is complex and multifactorial, associating complementary, albeit distinct entities, in addition to ordinary mineral and bone disorders observed in other types of chronic kidney disease. Amongst these long-term consequences are renal Fanconi syndrome, hypophosphatemic rickets, malnutrition, hormonal abnormalities, muscular impairment, and intrinsic cellular bone defects in bone cells, due to CTNS mutations. Recent research data in the field have demonstrated abnormal mineral regulation, intrinsic bone defects, cysteamine toxicity, muscle wasting and, likely interleukin-1-driven inflammation in the setting of CMBD. Here we summarize these new pathophysiological deregulations and discuss the crucial interplay between bone and muscle in INC. In future, vitamin D and/or biotherapies targeting the IL1β pathway may improve muscle wasting and subsequently CMBD, but this remains to be proven.

Peripheral Blood Mononuclear Cells (PBMCs) to Dissect the Underlying Mechanisms of Bone Disease in Chronic Kidney Disease and Rare Renal Diseases

PURPOSE OF REVIEW

To describe the methods that can be used to obtain functional and mature osteoclasts from peripheral blood mononuclear cells (PBMCs) and report the data obtained with this model in two peculiar diseases, namely pediatric chronic kidney disease-associated mineral and bone disorders (CKD-MBD) and nephropathic cystinosis. To discuss future research possibilities in the field.

RECENT FINDINGS

Bone tissue undergoes continuous remodeling throughout life to maintain bone architecture; it involves two processes: bone formation and bone resorption with the coordinated activity of osteoblasts, osteoclasts, and osteocytes. Animal models fail to fully explain human bone pathophysiology during chronic kidney disease, mainly due to interspecies differences. The development of in vitro models has permitted to mimic human bone-related diseases as an alternative to in vivo models. Since 1997, osteoclasts have been generated in cell cultures, notably when culturing PBMCs with specific growth factors and cytokines (i.e., M-CSF and RANK-L), without the need for osteoblasts or stromal cells. These models may improve the global understanding of bone pathophysiology. They can be been used not only to evaluate the direct effects of cytokines, hormones, cells, or drugs on bone remodeling during CKD-MBD, but also in peculiar genetic renal diseases inducing specific bone impairment.

Grafting of polymers via ring-opening polymerization for electrochemical assay of alkaline phosphatase activity

As an important hydrolytic enzyme, abnormal activity of alkaline phosphatase (ALP) is closely associated with a variety of diseases. It has been identified as an important diagnostic indicator for clinical hepatobiliary and bone diseases. Herein, a novel electrochemical sensor based on signal amplification strategy through ring-opening polymerization (ROP) has been developed to assay of ALP activity. First of all, 3-mercaptopropanoic acid (MPA) was employed as a cross-linking agent to attach O-phosphoethanolamine to the electrode surface via amide bond. Then, ALP catalyzed the hydrolysis of phosphate monoester structures to hydroxyl groups, which could initiate ROP reaction. The polymer grafted on the electrode surface contains a large number of ferrocene electroactive molecules, which effectively increased the signal output of the electrochemical sensor and improved the sensitivity of ALP activity detection. Under optimum conditions, this electrochemical sensor rendered a satisfactory linear dependence over the range from 20 to 120 mU mL^-1, with a low detection limit of 0.66 mU mL^-1. Furthermore, this strategy presented satisfactory selectivity and interference resistance in human serum sample, and compared with clinical data, the relative error of the results obtained by this method was less than 5%. Thus, this method showed considerable potential for the detection of ALP activity in clinical application.

Response to Cysteamine in Osteoclasts Obtained from Patients with Nephropathic Cystinosis: A Genotype/Phenotype Correlation

Bone complications of cystinosis have been recently described. The main objectives of this paper were to determine in vitro the impact of CTNS mutations and cysteamine therapy on human osteoclasts and to carry out a genotype-phenotype analysis related to osteoclastic differentiation. Human osteoclasts were differentiated from peripheral blood mononuclear cells (PBMCs) and were treated with increasing doses of cysteamine (0, 50, 200 µM) and then assessed for osteoclastic differentiation. Results are presented as median (min-max). A total of 17 patients (mainly pediatric) were included, at a median age of 14 (2–61) years, and a eGFR of 64 (23–149) mL/min/1.73 m². Most patients (71%) were under conservative kidney management (CKM). The others were kidney transplant recipients. Three functional groups were distinguished for CTNS mutations: cystinosin variant with residual cystin efflux activity (RA, residual activity), inactive cystinosin variant (IP, inactive protein), and absent protein (AP). PBMCs from patients with residual cystinosin activity generate significantly less osteoclasts than those obtained from patients of the other groups. In all groups, cysteamine exerts an inhibitory effect on osteoclastic differentiation at high doses. This study highlights a link between genotype and osteoclastic differentiation, as well as a significant impact of cysteamine therapy on this process in humans.