The Potential Influence of Uremic Toxins on the Homeostasis of Bones and Muscles in Chronic Kidney Disease

Molecular and Cellular Mechanisms Linking Chronic Kidney Disease and Sarcopenia in Aging: An Integrated Perspective

Chronic kidney disease (CKD) and sarcopenia are prevalent conditions among the aging population, contributing significantly to morbidity and mortality. CKD exacerbates sarcopenia through complex molecular and cellular mechanisms, including chronic inflammation, oxidative stress, uremic toxin accumulation, protein-energy wasting, and hormonal dysregulation. This review explores the interplay between CKD and sarcopenia, focusing on key pathways such as mTOR signaling, the AMPK-FOXO axis, and myostatin/activin pathways that regulate muscle protein metabolism. Additionally, mitochondrial dysfunction and impaired autophagy emerge as critical contributors to muscle wasting. Clinical implications include identifying biomarkers such as interleukin-6, tumor necrosis factor-alpha, myostatin, and Klotho for diagnosis and monitoring, while potential therapeutic strategies involve targeting the AMPK/mTOR pathway, enhancing mitochondrial function, and inhibiting myostatin activity. Emerging approaches, including multi-omics technologies and AI-driven personalized treatment models, offer innovative solutions for understanding and managing the CKD-sarcopenia axis. This review underscores the need for integrated therapeutic strategies and multidisciplinary collaboration to mitigate muscle wasting and improve outcomes in CKD patients. By bridging molecular insights with clinical applications, this work aims to inform future research and translational efforts in addressing this critical healthcare challenge.

Nutritional Intervention and Musculoskeletal Health in Chronic Kidney Disease

Chronic kidney disease (CKD) is a leading condition in terms of prevalence and overall health impact. With the increased life expectancy of the CKD population and the improvement in medical care, controlling musculoskeletal complications remains a tough challenge. Patients with CKD are prone to falls, fractures and sarcopenia, enhancing the risk of death. A multitude of mechanisms contribute to fractures, and treatment is suboptimal; therefore, prevention must stand out as a key step. This review aims to provide an overview of the most relevant data regarding the impact of nutrition on bone disorders and sarcopenia in CKD. The newest relevant studies emphasize that plant protein intake is associated with a lower production of uremic toxins, lower serum phosphorus levels, and stronger bones. We conclude that patients with CKD should adopt specific diets tailored to the presence of osteoporosis, renal osteodystrophy, and muscle wasting. Low-protein diets or plant-dominant diets containing an adequate amount of protein could be better choices for predialysis patients with CKD in order to protect their bones and muscles, whereas in the dialysis population, a higher protein intake could be essential to prevent osteoporosis and sarcopenia. In all patients with CKD, focusing on antioxidant food intake could provide a strong antiaging benefit through ensuring good musculoskeletal health.

Chronic Kidney Disease and Osteoarthritis: Current Understanding and Future Research Directions

Chronic kidney disease (CKD) is a significant public health concern. Osteoarthritis (OA), a common form of arthritis, has been shown to have a dramatically increased prevalence, particularly among individuals aged 40-50 and older, in the presence of CKD. Furthermore, CKD may exacerbate the progression and impact of OA. A survey study revealed that 53.9% of CKD patients undergoing long-term hemodialysis were diagnosed with OA. These findings underscore the potential association between CKD and OA. Uremic toxins, such as indoxyl sulfate, p-cresyl sulfate, transforming growth factor-β, and advanced glycation end-products, are regarded as potential risk factors in various CKD-related conditions, affecting bone and joint metabolism. However, whether these factors serve as a bridging mechanism between CKD and OA comorbidities, as well as their detailed roles in this context, remains unclear. Addressing the progression of OA in CKD patients and identifying effective treatment and prevention strategies is an urgent challenge that warrants immediate attention. This review focuses on describing and discussing the molecular pathological mechanisms underlying CKD-associated OA and the possible therapeutic strategies.

Hyperphosphatemia Contributes to Skeletal Muscle Atrophy in Mice

Chronic kidney disease (CKD) is associated with various pathologic changes, including elevations in serum phosphate levels (hyperphosphatemia), vascular calcification, and skeletal muscle atrophy. Elevated phosphate can damage vascular smooth muscle cells and cause vascular calcification. Here, we determined whether high phosphate can also affect skeletal muscle cells and whether hyperphosphatemia, in the context of CKD or by itself, is associated with skeletal muscle atrophy. As models of hyperphosphatemia with CKD, we studied mice receiving an adenine-rich diet for 14 weeks and mice with deletion of Collagen 4a3 (Col4a3-/-). As models of hyperphosphatemia without CKD, we analyzed mice receiving a high-phosphate diet for three and six months as well as a genetic model for klotho deficiency (kl/kl). We found that adenine, Col4a3-/-, and kl/kl mice have reduced skeletal muscle mass and function and develop atrophy. Mice on a high-phosphate diet for six months also had lower skeletal muscle mass and function but no significant signs of atrophy, indicating less severe damage compared with the other three models. To determine the potential direct actions of phosphate on skeletal muscle, we cultured primary mouse myotubes in high phosphate concentrations, and we detected the induction of atrophy. We conclude that in experimental mouse models, hyperphosphatemia is sufficient to induce skeletal muscle atrophy and that, among various other factors, elevated phosphate levels might contribute to skeletal muscle injury in CKD.

Skeletal Muscle Injury in Chronic Kidney Disease-From Histologic Changes to Molecular Mechanisms and to Novel Therapies

Chronic kidney disease (CKD) is associated with significant reductions in lean body mass and in the mass of various tissues, including skeletal muscle, which causes fatigue and contributes to high mortality rates. In CKD, the cellular protein turnover is imbalanced, with protein degradation outweighing protein synthesis, leading to a loss of protein and cell mass, which impairs tissue function. As CKD itself, skeletal muscle wasting, or sarcopenia, can have various origins and causes, and both CKD and sarcopenia share common risk factors, such as diabetes, obesity, and age. While these pathologies together with reduced physical performance and malnutrition contribute to muscle loss, they cannot explain all features of CKD-associated sarcopenia. Metabolic acidosis, systemic inflammation, insulin resistance and the accumulation of uremic toxins have been identified as additional factors that occur in CKD and that can contribute to sarcopenia. Here, we discuss the elevation of systemic phosphate levels, also called hyperphosphatemia, and the imbalance in the endocrine regulators of phosphate metabolism as another CKD-associated pathology that can directly and indirectly harm skeletal muscle tissue. To identify causes, affected cell types, and the mechanisms of sarcopenia and thereby novel targets for therapeutic interventions, it is important to first characterize the precise pathologic changes on molecular, cellular, and histologic levels, and to do so in CKD patients as well as in animal models of CKD, which we describe here in detail. We also discuss the currently known pathomechanisms and therapeutic approaches of CKD-associated sarcopenia, as well as the effects of hyperphosphatemia and the novel drug targets it could provide to protect skeletal muscle in CKD.

The influence of uremic toxins on low bone turnover disease in chronic kidney disease

Uremic toxins play a crucial role in the development of low bone turnover disease in chronic kidney disease (CKD) through the induction of oxidative stress. This oxidative stress disrupts the delicate balance between bone formation and resorption, resulting in a decline in both bone quantity and quality. Reactive oxygen species (ROS) activate nuclear factor kappa-B and mitogen-activated protein kinase signaling pathways, promoting osteoclastogenesis. Conversely, ROS hinder osteoblast differentiation by facilitating the binding of Forkhead box O proteins (FoxOs) to β-catenin, triggering apoptosis through FoxOs-activating kinase phosphorylation. This results in increased osteoblastic receptor activator of nuclear factor kappa-B ligand (RANKL) expression and decreased nuclear factor erythroid 2-related factor 2 levels, compromising antioxidant defenses against oxidative damage. As CKD progresses, the accumulation of protein-bound uremic toxins such as indoxyl sulfate (IS) and p-cresyl sulfate (PCS) intensifies oxidative stress, primarily affecting osteoblasts. IS and PCS directly inhibit osteoblast viability, induce apoptosis, decrease alkaline phosphatase activity, and impair collagen 1 and osteonectin, impeding bone formation. They also reduce cyclic adenosine 3',5'-monophosphate (cAMP) production and lower parathyroid hormone (PTH) receptor expression in osteoblasts, resulting in PTH hyporesponsiveness. In summary, excessive production of ROS by uremic toxins not only reduces the number and function of osteoblasts but also induces PTH hyporesponsiveness, contributing to the initiation and progression of low bone turnover disease in CKD.

Co-Housing and Fecal Microbiota Transplantation: Technical Support for TCM Herbal Treatment of Extra-Intestinal Diseases Based on Gut Microbial Ecosystem Remodeling

Dysregulation of the gut microbial ecosystem (GME) (eg, alterations in the gut microbiota, gut-derived metabolites, and gut barrier) may contribute to the onset and progression of extra-intestinal diseases. Previous studies have found that Traditional Chinese Medicine herbs (TCMs) play an important role in manipulating the GME, but a prominent obstacle in current TCM research is the causal relationship between GME and disease amelioration. Encouragingly, co-housing and fecal microbiota transplantation (FMT) provide evidence-based support for TCMs to treat extra-intestinal diseases by targeting GME. In this review, we documented the principles, operational procedures, applications and limitations of the key technologies (ie, co-housing and FMT); furthermore, we provided evidence that TCM works through the GME, especially the gut microbiota (eg, SCFA- and BSH-producing bacteria), the gut-derived metabolites (eg, IS, pCS, and SCFAs), and intestinal barrier to alleviate extra-intestinal diseases. This will be beneficial in constructing microecological pathways for TCM treatment of extra-intestinal diseases in the future.