Circulating autotaxin levels in healthy teenagers: Data from the Vitados cohort.

Autotaxin (ATX) is a secreted enzyme with a lysophospholipase D activity, mainly secreted by adipocytes and widely expressed. Its major function is to convert lysophosphatidylcholine (LPC) into lysophosphatidic acid (LPA), an essential bioactive lipid involved in multiple cell processes. The ATX-LPA axis is increasingly studied because of its involvement in numerous pathological conditions, more specifically in inflammatory or neoplastic diseases, and in obesity. Circulating ATX levels gradually increase with the stage of some pathologies, such as liver fibrosis, thus making them a potentially interesting non-invasive marker for fibrosis estimation. Normal circulating levels of ATX have been established in healthy adults, but no data exist at the pediatric age. The aim of our study is to describe the physiological concentrations of circulating ATX levels in healthy teenagers through a secondary analysis of the VITADOS cohort. Our study included 38 teenagers of Caucasian origin (12 males, 26 females). Their median age was 13 years for males and 14 years for females, ranging from Tanner 1 to 5. BMI was at the 25th percentile for males and 54th percentile for females, and median blood pressure was normal. ATX median levels were 1,049 (450-2201) ng/ml. There was no difference in ATX levels between sexes in teenagers, which was in contrast to the male and female differences described in the adult population. ATX levels significantly decreased with age and pubertal status, reaching adult levels at the end of puberty. Our study also suggested positive correlations between ATX levels and blood pressure (BP), lipid metabolism, and bone biomarkers. However, except for LDL cholesterol, these factors were also significantly correlated with age, which might be a confounding factor. Still, a correlation between ATX and diastolic BP was described in obese adult patients. No correlation was found between ATX levels and inflammatory marker C-reactive protein (CRP), Body Mass Index (BMI), and biomarkers of phosphate/calcium metabolism. In conclusion, our study is the first to describe the decline in ATX levels with puberty and the physiological concentrations of ATX levels in healthy teenagers. It will be of utmost importance when performing clinical studies in children with chronic diseases to keep these kinetics in mind, as circulating ATX might become a non-invasive prognostic biomarker in pediatric chronic diseases.

X-Linked Hypophosphatemia, Not Only a Skeletal Disease But Also a Chronic Inflammatory State.

CONTEXT: X-linked hypophosphatemia (XLH) is a rare genetic disease caused by a primary excess of fibroblast growth factor 23 (FGF23). FGF23 has been associated with inflammation and impaired osteoclastogenesis, but these pathways have not been investigated in XLH. OBJECTIVE: This work aimed to evaluate whether XLH patients display peculiar inflammatory profile and increased osteoclastic activity. METHODS: We performed a prospective, multicenter, cross-sectional study analyzing transcript expression of 8 inflammatory markers (Il6, Il8, Il1ß, CXCL1, CCL2, CXCR3, Il1R, Il6R) by real-time quantitative polymerase chain reaction on peripheral blood mononuclear cells (PBMCs) purified from total blood samples extracted from patients and healthy control individuals. The effect of native/active vitamin D on osteoclast formation was also assessed in vitro from XLH patients' PBMCs. RESULTS: In total, 28 XLH patients (17 children, among them 6 undergoing standard of care [SOC] and 11 burosumab therapy) and 19 controls were enrolled. Expression of most inflammatory markers was significantly increased in PBMCs from XLH patients compared to controls. No differences were observed between the burosumab and SOC subgroups. Osteoclast formation was significantly impaired in XLH patients. XLH mature osteoclasts displayed higher levels of inflammatory markers, being however lower in cells derived from the burosumab subgroup (as opposed to SOC). CONCLUSION: We describe for the first time a peculiar inflammatory profile in XLH. Since XLH patients have a propensity to develop arterial hypertension, obesity, and enthesopathies, and because inflammation can worsen these clinical outcomes, we hypothesize that inflammation may play a critical role in these extraskeletal complications of XLH.

Autotaxin/Lysophosphatidic Acid Axis: From Bone Biology to Bone Disorders.

Lysophosphatidic acid (LPA) is a natural bioactive phospholipid with pleiotropic activities affecting multiple tissues, including bone. LPA exerts its biological functions by binding to G-protein coupled LPA receptors (LPA1-6) to stimulate cell migration, proliferation, and survival. It is largely produced by autotaxin (ATX), a secreted enzyme with lysophospholipase D activity that converts lysophosphatidylcholine (LPC) into active LPA. Beyond its enzymatic activity, ATX serves as a docking molecule facilitating the efficient delivery of LPA to its specific cell surface receptors. Thus, LPA effects are the result of local production by ATX in a given tissue or cell type. As a consequence, the ATX/LPA axis should be considered as an entity to better understand their roles in physiology and pathophysiology and to propose novel therapeutic strategies. Herein, we provide not only an extensive overview of the relevance of the ATX/LPA axis in bone cell commitment and differentiation, skeletal development, and bone disorders, but also discuss new working hypotheses emerging from the interplay of ATX/LPA with well-established signaling pathways regulating bone mass.

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 IL1ß 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.

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.

Expression of the type 1 lysophosphatidic acid receptor in osteoblastic cell lineage controls both bone mineralization and osteocyte specification.

Lysphosphatidic acid (LPA) is a major natural bioactive lipid mediator whose biological functions affect multiple organs. These include bone as demonstrated by global Lpar1-knockout mice (Lpar1-/-) which present a bone growth defect. LPA acts on all bone cells including osteoblasts, that are responsible for bone formation, and osteoclasts, which are specialized cells that resorb bone. LPA appears as a potential new coupling molecule during bone remodeling. LPA1 is the most ubiquitous LPA receptor among the six LPA receptor family members (LPA1-6). To better understand the specific role of LPA via its receptor LPA1 in osteoblastic cell lineage we generated osteoblast-specific Lpar1 knockout mice (Lpar1-∆Ob) by crossing Lpar1flox/flox and Osx:Cre+ mouse lines. Lpar1-∆Ob mice do not recapitulate the bone defects of Lpar1-/- mice but revealed reduced bone mineralization and decreased cortical thickness, as well as increased bone porosity associated with an augmentation in the lacunae areas of osteocyte and their apoptotic yield. In vitro, primary Lpar1-∆Ob and immortalized cl1-Ob-Lpar1-/- osteoblasts revealed a remarkable premature expression of alkaline phosphatase, reduced cell proliferation associated with decreased YAP-P nuclear accumulation, and reduced mineralization activity. Osteocyte specification is markedly impaired as demonstrated by reduced expression of early (E11) and late (DMP1, DKK1, SOST) osteocyte markers ex vivo in enriched osteocytic fractions of Lpar1-∆Ob mouse bone explants. In addition, E11 expression and dendrite formation induced by FGF2 are markedly impaired in both primary Lpar1-∆Ob and immortalized cl1-Ob-Lpar1-/- osteoblasts. Taken together these results suggest a new role for LPA in bone mass control via bone mineralization and osteocyte function.