Bone and heart health in chronic kidney disease: role of dentin matrix protein 1

Sclerostin, Osteocytes, and Wnt Signaling in Pediatric Renal Osteodystrophy

The pathophysiology of chronic kidney disease-mineral and bone disorder (CKD-MBD) is not well understood. Specific factors secreted by osteocytes are elevated in the serum of adults and pediatric patients with CKD-MBD, including FGF-23 and sclerostin, a known inhibitor of the Wnt signaling pathway. The molecular mechanisms that promote bone disease during the progression of CKD are incompletely understood. In this study, we performed a cross-sectional analysis of 87 pediatric patients with pre-dialysis CKD and post-dialysis (CKD 5D). We assessed the associations between serum and bone sclerostin levels and biomarkers of bone turnover and bone histomorphometry. We report that serum sclerostin levels were elevated in both early and late CKD. Higher circulating and bone sclerostin levels were associated with histomorphometric parameters of bone turnover and mineralization. Immunofluorescence analyses of bone biopsies evaluated osteocyte staining of antibodies towards the canonical Wnt target, β-catenin, in the phosphorylated (inhibited) or unphosphorylated (active) forms. Bone sclerostin was found to be colocalized with phosphorylated β-catenin, which suggests that Wnt signaling was inhibited. In patients with low serum sclerostin levels, increased unphosphorylated "active" β-catenin staining was observed in osteocytes. These data provide new mechanistic insight into the pathogenesis of CKD-MBD and suggest that sclerostin may offer a potential biomarker or therapeutic target in pediatric renal osteodystrophy.

Matricellular proteins in cutaneous wound healing

Cutaneous wound healing is a complex process that encompasses alterations in all aspects of the skin including the extracellular matrix (ECM). ECM consist of large structural proteins such as collagens and elastin as well as smaller proteins with mainly regulative properties called matricellular proteins. Matricellular proteins bind to structural proteins and their functions include but are not limited to interaction with cell surface receptors, cytokines, or protease and evoking a cellular response. The signaling initiated by matricellular proteins modulates differentiation and proliferation of cells having an impact on the tissue regeneration. In this review we give an overview of the matricellular proteins that have been found to be involved in cutaneous wound healing and summarize the information known to date about their functions in this process.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Dynamic Molecular Profiles of Bone Marrow-Derived Osteoblasts at the Single-Cell Level

Osteogenesis is an important process of bone metabolism, and abnormal osteogenesis leads to various skeletal system diseases. Osteoblasts, the main cells involved in bone formation, are central elements in the study of bone metabolic diseases. Single-cell RNA sequencing is an important tool for studying the transcriptome of cells and can help to elucidate various cellular and molecular functions at the single-cell level, providing new avenues for life science research. Here we explore the heterogeneity of osteoblasts and try to reveal the developmental trajectory of osteoblasts, thereby contributing to efforts to describe the mechanism of osteogenesis. In this study, single-cell sequencing data of murine bone marrow cells were used to identify osteoblasts. Finally, osteoblasts were divided into four groups, each differing in characteristic genes and signal pathways. We also identify clues of the changes of some genes in the process of osteoclast formation, providing directions for further study. Collectively, our findings suggest that bone marrow osteoblasts can be divided into several subgroups, which represent different stages of cells, and that the specific genes of each subgroup respond to the molecular mechanisms of cell development. This data will likely be of great help in resolving diseases of the skeletal system.

The Role of DMP1 in CKD-MBD

PURPOSE OF REVIEW

Chronic kidney disease-mineral and bone disorder (CKD-MBD) has become a global health crisis with very limited therapeutic options. Dentin matrix protein 1 (DMP1) is a matrix extracellular protein secreted by osteocytes that has generated recent interest for its possible involvement in CKD-MBD pathogenesis. This is a review of DMP1 established regulation and function, and early studies implicating DMP1 in CKD-MBD.

RECENT FINDINGS

Patients and mice with CKD show perturbations of DMP1 expression in bone, associated with impaired osteocyte maturation, mineralization, and increased fibroblast growth factor 23 (FGF23) production. In humans with CKD, low circulating DMP1 levels are independently associated with increased cardiovascular events. We recently showed that DMP1 supplementation lowers circulating FGF23 levels and improves bone mineralization and cardiac outcomes in mice with CKD. Mortality rates are extremely high among patients with CKD and have only marginally improved over decades. Bone disease and FGF23 excess contribute to mortality in CKD by increasing the risk of bone fractures and cardiovascular disease, respectively. Previous studies focused on DMP1 loss-of-function mutations have established its role in the regulation of FGF23 and bone mineralization. Recent studies show that DMP1 supplementation may fill a crucial therapeutic gap by improving bone and cardiac health in CKD.

Efficacy and Safety of Evocalcet Evaluated by Dialysate Calcium Concentration in Patients with Secondary Hyperparathyroidism Undergoing Hemodialysis

Purpose

Evocalcet is a novel oral calcimimetic drug that has demonstrated similar efficacy to cinacalcet in regulating serum parathyroid hormone (PTH), calcium, and phosphate levels, with fewer upper gastrointestinal tract-related adverse drug reactions (ADRs) in patients with secondary hyperparathyroidism undergoing hemodialysis in Japan. We investigated the efficacy and safety of once-daily oral evocalcet under different dialysate calcium concentrations.

Patients and Methods

A post hoc analysis by dialysate calcium concentration (2.5, 2.75, and 3.0 mEq/L) was performed using data from a previous Phase 3 study that included cinacalcet as an active control. Efficacy endpoints were the proportion of patients who achieved the target intact PTH levels of ≥60 and ≤240 pg/mL between Week 28 and Week 30; time-course changes in serum intact PTH; calcium and phosphorus levels, bone turnover markers, and fibroblast growth factor 23 (FGF23) over the 30-week study period. Safety endpoints were overall ADRs and hypocalcemia- and upper gastrointestinal tract-related ADRs.

Results

A total of 634 patients were included in the analysis. Levels of intact PTH, calcium, phosphate, bone turnover markers, and FGF23 showed improvement in all sub-groups, irrespective of dialysate calcium concentration. The incidence of upper gastrointestinal tract-related ADRs was significantly lower in the evocalcet group than the cinacalcet group with dialysate calcium concentrations of 2.75 and 3.0 mEq/L (p<0.05 for both concentrations).

Conclusion

Evocalcet was effective and safe in regulating the levels of serum intact PTH, calcium, and phosphate in patients with secondary hyperparathyroidism undergoing hemodialysis, irrespective of dialysate calcium concentration.

Osteocyte-Mediated Translation of Mechanical Stimuli to Cellular Signaling and Its Role in Bone and Non-bone-Related Clinical Complications

PURPOSE OF REVIEW

Osteocytes comprise > 95% of the cellular component in bone tissue and produce a wide range of cytokines and cellular signaling molecules in response to mechanical stimuli. In this review, we aimed to summarize the molecular mechanisms involved in the osteocyte-mediated translation of mechanical stimuli to cellular signaling, and discuss their role in skeletal (bone) diseases and extra-skeletal (non-bone) clinical complications.

RECENT FINDINGS

Two decades before, osteocytes were assumed as a dormant cells buried in bone matrix. In recent years, emerging evidences have shown that osteocytes are pivotal not only for bone homeostasis but also for vital organ functions such as muscle, kidney, and heart. Osteocyte mechanotransduction regulates osteoblast and osteoclast function and maintains bone homeostasis. Mechanical stimuli modulate the release of osteocyte-derived cytokines, signaling molecules, and extracellular cellular vesicles that regulate not only the surrounding bone cell function and bone homeostasis but also the distant organ function in a paracrine and endocrine fashion. Mechanical loading and unloading modulate the osteocytic release of NO, PGE₂, and ATPs that regulates multiple cellular signaling such as Wnt/β-catenin, RANKL/OPG, BMPs, PTH, IGF1, VEGF, sclerostin, and others. Therefore, the in-depth study of the molecular mechanism of osteocyte mechanotransduction could unravel therapeutic targets for various bone and non-bone-related clinical complications such as osteoporosis, sarcopenia, and cancer metastasis to bone.

The Osteocyte as the New Discovery of Therapeutic Options in Rare Bone Diseases

Osteocytes are the most abundant (~95%) cells in bone with the longest half-life (~25 years) in humans. In the past osteocytes have been regarded as vestigial cells in bone, since they are buried inside the tough bone matrix. However, during the last 30 years it has become clear that osteocytes are as important as bone forming osteoblasts and bone resorbing osteoclasts in maintaining bone homeostasis. The osteocyte cell body and dendritic processes reside in bone in a complex lacuno-canalicular system, which allows the direct networking of osteocytes to their neighboring osteocytes, osteoblasts, osteoclasts, bone marrow, blood vessels, and nerves. Mechanosensing of osteocytes translates the applied mechanical force on bone to cellular signaling and regulation of bone adaptation. The osteocyte lacuno-canalicular system is highly efficient in transferring external mechanical force on bone to the osteocyte cell body and dendritic processes via displacement of fluid in the lacuno-canalicular space. Osteocyte mechanotransduction regulates the formation and function of the osteoblasts and osteoclasts to maintain bone homeostasis. Osteocytes produce a variety of proteins and signaling molecules such as sclerostin, cathepsin K, Wnts, DKK1, DMP1, IGF1, and RANKL/OPG to regulate osteoblast and osteoclast activity. Various genetic abnormality-associated rare bone diseases are related to disrupted osteocyte functions, including sclerosteosis, van Buchem disease, hypophosphatemic rickets, and WNT1 and plastin3 mutation-related disorders. Meticulous studies during the last 15 years on disrupted osteocyte function in rare bone diseases guided for the development of various novel therapeutic agents to treat bone diseases. Studies on genetic, molecular, and cellular mechanisms of sclerosteosis and van Buchem disease revealed a role for sclerostin in bone homeostasis, which led to the development of the sclerostin antibody to treat osteoporosis and other bone degenerative diseases. The mechanism of many other rare bone diseases and the role of the osteocyte in the development of such conditions still needs to be investigated. In this review, we mainly discuss the knowledge obtained during the last 30 years on the role of the osteocyte in rare bone diseases. We speculate about future research directions to develop novel therapeutic drugs targeting osteocyte functions to treat both common and rare bone diseases.

Chronic prolonged hyponatremia and risk of hip fracture in elderly patients with chronic kidney disease

BACKGROUND

Chronic prolonged hyponatremia (CPH) is a risk factor for hip fracture in the general population. Whether CPH increases hip fracture risk in chronic kidney disease (CKD) patients is unknown.

METHODS

Case-control study in patients over 60 years of age with stage 3 or greater CKD. Patients who had a hip fracture were referred to as cases (n = 1236) and controls had no hip fracture (n = 4515). Patients were classified as having CPH if serum sodium was <135 mEq/L on at least two occasions separated by a minimum of 90 days prior to the diagnosis of hip fracture (cases) or at any time during the study period (controls). Conditional logistic regression models were used to test the association between CPH and hip fracture. Analyses were conducted for patients with and without osteoporosis and falls and for patients with age >70 years versus ≤70 years.

RESULTS

CPH was present in 21% of cases and 10% of controls (p < 0.001; sodium level: 131-134 mEq/L). In univariate logistic regression analysis, CPH was associated with higher odds of hip fracture (odds ratio [OR] 2.44, (95% [CI] 2.07-2.89). In a multivariate model adjusted for comorbidities, medications and laboratory parameters CPH association with higher odds of Hip fracture was attenuated but remained significant (OR 1.36, 95% CI 1.04-1.78). The association between CPH and risk of hip fracture was consistent in patients with or without osteoporosis and falls and across the age strata.

CONCLUSION

Chronic prolonged hyponatremia is a risk factor for hip fracture in CKD patients older than 60 years of age.