Abstract Chronic liver disease frequently leads to debilitating bone fractures, yet the mechanistic link between liver injury and skeletal fragility remains elusive. This study aimed to investigate a liver–bone axis where dysregulated iron flux, mediated by hepatocyte ferroportin (FPN), drives osteoporosis during the progression of cirrhosis. We utilized a mouse model of progressive liver disease and generated hepatocyte-specific FPN deletion mice to examine systemic iron homeostasis. Bone quality and microarchitecture were evaluated via micro-CT and histological analysis. To assess fracture risk and mechanical integrity, we employed engineering-based finite element simulations and physical mechanical testing. Our data reveal a critical window during liver fibrosis where compensatory mechanisms counteract iron toxicity. However, upon progression to cirrhosis, this homeostatic balance collapses, leading to massive toxic iron accumulation in the skeleton. This creates a pro-inflammatory environment that suppresses bone formation and accelerates resorption. Biomechanical simulations and testing demonstrate that combined deficits in bone material properties and architecture create distinct stress concentration zones, markedly reducing load-bearing capacity. Strikingly, hepatocyte-specific deletion of FPN attenuated liver fibrosis and restored systemic iron balance, thereby preserving bone microarchitecture and mechanical performance. Hepatocyte FPN-regulated iron homeostasis is a central mediator of the liver–bone axis. Cirrhosis-induced iron overload is a major contributor to skeletal mechanical failure, identifying FPN as a key regulator of bone health in chronic liver disease.
Abstract Distal-less homeobox genes Dlx5 and Dlx6 are key regulators of osteoblast differentiation during skeletal development, but their role in postnatal bone remodeling remains poorly understood. To investigate this, we conditionally deleted Dlx5/6 in Osterix-lineage cells after 4 wk of birth using an inducible Osx-Cre system. Dlx5/6Δ/Δ Osx-Cre and control mice were analyzed in terms of bone architecture (micro-CT), remodeling (histomorphometry), gene expression profiling, and single-cell RNA sequencing. Postnatal deletion of Dlx5/6 induced a complex skeletal phenotype characterized by radial bone hypertrophy and altered cortical remodeling. Enhanced periosteal activity was associated with increased Osx expression in periosteal cells, confirmed by BaseScope and immunofluorescence analyses. In parallel, cortical bone became thinner and more porous, while trabecular bone displayed reduced bone formation and thinner trabeculae, despite paradoxical accumulation within the diaphysis. These alterations were associated with increased Osx expression in periosteal and diaphyseal compartments, indicating abnormal spatial regulation of osteolineage cell expansion and bone remodeling. Single-cell transcriptomic analysis in BM cells identified altered mesenchymal stromal cell populations with dysregulated extracellular matrix and Wnt signaling pathways, including downregulation of the Wnt antagonist Sfrp1, validated in situ by RNAscope. Consistently, osteoblasts derived from Dlx5/6Δ/Δ Osx-Cre displayed impaired terminal differentiation in vitro, with reduced expression of late osteogenic markers and decreased matrix mineralization. Collectively, these findings identify Dlx5/6 as critical regulators of postnatal bone homeostasis that coordinate osteolineage expansion and terminal osteoblast maturation during postnatal bone remodeling.
Bone remodeling is regulated by circadian rhythm, yet the molecular mechanisms linking circadian clock components to osteoclast function remain largely undefined. We previously showed that osteoclast-specific deletion of Per1, a core circadian regulator, increased osteoclastogenesis and reduced bone mass in male mice, whereas deletion of Per2 had little effect. Here, we investigated Per1;Per2 conditional double-knockout (dKO) mice and unexpectedly found the opposite phenotype. In contrast to Per1 deficiency, dKO suppressed osteoclastogenesis and increased bone mass specifically in male mice, revealing previously unrecognized functional interactions between PER1 and PER2 in osteoclasts. Transcriptomic analyses showed that dKO osteoclasts preferentially downregulated innate immunity genes, including both positive and negative regulators of osteoclastogenesis. Chromatin immunoprecipitation and reporter assays further identified innate immunity genes as downstream targets of PER-dependent circadian regulation. These findings indicate that PER1 and PER2 regulate osteoclastogenesis through coordinated control of a network of immune genes with opposing effects on osteoclast differentiation, rather than acting as simple promoters or inhibitors of bone resorption. More broadly, this study establishes innate immunity as a mechanistic interface between the circadian clock and osteoclast function, providing a framework for understanding how circadian disruption influences bone remodeling and inflammatory bone diseases.
The synovial joints are essential for skeletal function and dexterity and do so by virtue of their components and structure. The capsule protects and insulates the joint from the surrounding environment, articular cartilage provides mechanical resilience and endurance, the ligaments stabilize motion, and the synovial cavity and fluid sustain frictionless sliding. These and other fundamental aspects of synovial joint biology are well known and recognized. However, how the joints form during embryogenesis and acquire their components and functional attributes remains unclear. Happily, the last few years of research have provided new insights into limb joint development. In particular, new light has been shed on: morphogenetic mechanisms establishing the mesenchymal interzone which is the cradle of joint tissue progenitors; spatio-temporal processes endowing the joint progenitors with distinct developmental and tissue forming potentials; osmoregulatory and hydraulic forces set by Na/K-ATPase ion pumps that bring about joint cavitation and fluid accrual; cellular and signaling mechanisms establishing articular cartilage's zonal organization; and potentials of embryonically-derived progenitors residing in adult joints to engage in repair. This review describes and comments on those new studies, frames them within the broader field of joint research, and identifies lingering questions. Clinically, the treatment of joint disease remains ineffective and new directions and strategies are needed to move forward. Studies on synovial joint development are poised to provide such new directions and bring about future regenerative tools based on principles and mechanisms of developmental biology. This review is meant to highlight the potentials of such basic-to- translational-to-clinical strategies and provides an overall framework for research goals ahead.
Patients receiving dialysis face a 5-fold higher risk of skeletal fractures compared to the general population, along with increased mortality and a longer hospital stay after a fracture. However, fracture risk varies among patients, and reliable methods to assess risk are limited. Using linked healthcare databases, we conducted a population-based cohort study of adults aged 40-90 years receiving maintenance dialysis with an available parathyroid hormone (PTH) value in Ontario, Canada (2010-2017). Patients were followed for 3 years or until death, first major fracture, or provincial emigration. Using Fine and Gray subdistribution hazards models accounting for the competing risk of death, we developed a fracture prediction tool (Dial-Frac) that included demographic, comorbidity, and laboratory information. Models were compared using measures of discrimination, calibration, and model fit and were internally validated using 10-fold cross-validation. The cohort included 11,599 patients receiving dialysis. Mean age was 66 years; 39% were female, and 12% had a previous fracture. Over 3 years, 839 patients (7.2%) experienced a fracture (44% of which occurred in the first year); hip fracture was most common (299/839). The final prediction model included age, sex, previous fracture, previous kidney transplant, proton pump inhibitor use, and PTH and serum albumin concentrations. The time dependent area under the receiver operating curve for fracture at 1 and 3 years were 0.79 and 0.71, respectively, indicating good discrimination. We developed Dial-Frac, an easy-to-implement equation that predicts fracture risk in patients receiving maintenance dialysis. It predicts risk at 1 and 3 years without requiring bone density measurements or additional tests, helping to identify high-risk patients for further screening, preventive interventions and clinical trial participation. Next steps include external validation.
