Increased fibroblast growth factor-21 in chronic kidney disease is a trade-off between survival benefit and blood pressure dysregulation

Association between amorphous calcium-phosphate ratios in circulating calciprotein particles and prognostic biomarkers in hemodialysis patients

Calciprotein particles (CPPs) are circulating colloidal mineral-protein complexes containing crystalline and/or non-crystalline (amorphous) calcium-phosphate (CaPi). Serum CPP levels correlate with vascular stiffness and calcification in patients with chronic kidney disease (CKD). In vitro studies showed that CPPs containing crystalline CaPi were more arteriosclerogenic and inflammogenic than CPPs without containing crystalline CaPi. Thus, we hypothesized that not only the quantity but also the quality of CPPs (the phase of CaPi) might affect clinical outcomes. To test this hypothesis, we quantified amorphous CaPi ratio defined as the ratio of the amorphous CaPi amount to the total CaPi amount in serum CPPs from 183 hemodialysis patients and explored its possible correlation with serum parameters associated with prognosis of hemodialysis patients. Multivariate analysis revealed that the amorphous CaPi ratio correlated positively with hemoglobin and negatively with fibroblast growth factor-21 (FGF21), which remained significant after adjusting for the total CaPi amount. Because low hemoglobin and high FGF21 are associated with increased mortality, the present study warrants further studies to determine whether low amorphous CaPi ratio in circulating CPPs may be associated with poor prognosis in hemodialysis patients.

Blood DNA Methylation Predicts Diabetic Kidney Disease Progression in High Fat Diet-Fed Mice

Diabetic kidney disease (DKD) progresses at different rates among patients with type 2 diabetes mellitus (T2D). Early identification of patients with a higher risk of DKD progression is essential to improve prognosis. Epigenetic modifications, particularly DNA methylation, have been independently implicated in T2D and chronic kidney disease. The current study aimed to determine changes in blood DNA methylation that reflects and predicts DKD progression. C57BL/6 mice were fed a high-fat diet (HFD) from weaning and subclassified into two groups, HFD-1 and HFD-2, according to urinary kidney injury marker KIM-1/creatinine ratios (low vs. high) and histological abnormalities (mild–moderate vs. advanced). DNA methylation profiles were determined by reduced representative bisulfide sequencing (RRBS). Our results confirmed early and established DKD at week 9 and week 32, respectively. At week 32, advanced kidney injury was associated with dysregulation of methylation and demethylation enzymes in the kidney. Blood RRBS revealed 579 and 203 differentially methylated sites (DMS) between HFD-1 and HFD-2 animals at week 32 and week 9, respectively, among which 11 were common. The DMS in blood and kidney at week 32 were both related to organ development, neurogenesis, cell junction, and Wnt signalling, while the DMS in blood at week 9 suggested a specific enrichment of kidney development processes. In conclusion, our data strongly support the implication of early blood DNA methylation modifications and DKD progression in T2D that could be used to improve the disease’s prognostication.

The Multiple Roles of Fibroblast Growth Factor in Diabetic Nephropathy

Diabetic nephropathy (DN) is a common microvascular complication in the late stages of diabetes. Currently, the etiology and pathogenesis of DN are not well understood. Even so, available evidence shows its development is associated with metabolism, oxidative stress, cytokine interaction, genetic factors, and renal microvascular disease. Diabetic nephropathy can lead to proteinuria, edema and hypertension, among other complications. In severe cases, it can cause life-threatening complications such as renal failure. Patients with type 1 diabetes, hypertension, high protein intake, and smokers have a higher risk of developing DN. Fibroblast growth factor (FGF) regulates several human processes essential for normal development. Even though FGF has been implicated in the pathological development of DN, the underlying mechanisms are not well understood. This review summarizes the role of FGF in the development of DN. Moreover, the association of FGF with metabolism, inflammation, oxidative stress and fibrosis in the context of DN is discussed. Findings of this review are expected to deepen our understanding of DN and generate ideas for developing effective prevention and treatments for the disease.

FGF21 prevents low-protein diet-induced renal inflammation in aged mice

Low-protein (LP) diets extend lifespan through a comprehensive improvement in metabolic health across multiple tissues and organs. Many of these metabolic responses to protein restriction are secondary to transcriptional activation and release of FGF21 from the liver. However, the effects of an LP diet on the kidney in the context of aging has not been examined. Therefore, the goal of the current study was to investigate the impact of chronic consumption of an LP diet on the kidney in aging mice lacking FGF21. Wild-type (WT; C57BL/6J) and FGF21 knockout (KO) mice were fed a normal protein diet (20% casein) or an LP (5% casein) diet ad libitum from 3 to 22 mo of age. The LP diet led to a decrease in kidney weight and urinary albumin-to-creatinine ratio in both WT and FGF21 KO mice. Although the LP diet produced only mild fibrosis and infiltration of leukocytes in WT kidneys, the effects were significantly exacerbated by the absence of FGF21. Accordingly, transcriptomic analysis showed that inflammation-related pathways were significantly enriched and upregulated in response to LP diet in FGF21 KO mice but not WT mice. Collectively, these data demonstrate that the LP diet negatively affected the kidney during aging, but in the absence of FGF21, the LP diet-induced renal damage and inflammation were significantly worse, indicating a protective role of FGF21 in the kidney.NEW & NOTEWORTHY Long-term protein restriction is not advantageous for an otherwise healthy, aging kidney, as it facilitates the development of renal tubular injury and inflammatory cell infiltration. We provide evidence using FGF21 knockout animals that FGF21 is essential to counteract the renal injury and inflammation during aging on a low-protein diet.

Roles of fibroblast growth factor 21 in the control of depression-like behaviours after social defeat stress in male rodents

Fibroblast growth factor 21 (FGF21) modulates energy metabolism and neuroendocrine stress responses. FGF21 synthesis is increased after environmental or metabolic challenges. Detailed roles of FGF21 in the control of behavioural disturbances under stressful conditions remain to be clarified. Here, we examined the roles of FGF21 in the control of behavioural changes after social defeat stress in male rodents. Central administration of FGF21 increased the number of tyrosine hydroxylase-positive catecholaminergic cells expressing c-Fos protein, an activity marker of neurones, in the nucleus tractus solitarius and area postrema. Double in situ hybridisation showed that some catecholaminergic neurones in the dorsal medulla oblongata expressed β-Klotho, an essential co-receptor for FGF21, in male mice. Social defeat stress increased FGF21 concentrations in the plasma of male mice. FGF21-deficient male mice showed social avoidance in a social avoidance test with C57BL/6J mice (background strain of FGF21-deficient mice) and augmented immobility behaviour in a forced swimming test after social defeat stress. On the other hand, overexpression of FGF21 by adeno-associated virus vectors did not significantly change behaviours either in wild-type male mice or FGF21-deficient male mice. The present data are consistent with the view that endogenous FGF21, possibly during the developmental period, has an inhibitory action on stress-induced depression-like behaviour in male rodents.

FGF21 and Chronic Kidney Disease

The global nephrology community recognizes the increasing burden of kidney disease and its poor health outcomes in the general population. Given this, strategies to establish early diagnosis, improve understanding of the natural course and develop novel therapeutic interventions to slow progression and reduce complications are encouraged. Fibroblast growth factor 21 (FGF21), a member of the endocrine FGF subfamily, has emerged as a master homeostasis regulator of local and systemic lipid, glucose and energy metabolism. In addition, FGF21 should be considered an autonomic and endocrine regulator of stress responses in general. Promising results has been shown in both dysmetabolic animal models and metabolic disease patients after pharmacological administration of FGF21 analogs. The association of FGF21 with renal function has been studied for more than ten years. However, the functional role of FGF21 in the kidney is still poorly understood. This review summarizes the biological effects of FGF21 and discusses what is currently known about this hormone and chronic kidney disease, highlighting important gaps that warrant further research.

Scalable fabrication of renal spheroids and nephron-like tubules by bioprinting and controlled self-assembly of epithelial cells

Scalable fabrication concepts of 3D kidney tissue models are required to enable their application in pharmaceutical high-throughput screenings. Yet the reconstruction of complex tissue structures remains technologically challenging. We present a novel concept reducing the fabrication demands, by using controlled cellular self-assembly to achieve higher tissue complexities from significantly simplified construct designs. We used drop-on-demand bioprinting to fabricate locally confined patterns of renal epithelial cells embedded in a hydrogel matrix. These patterns provide defined local cell densities (cell count variance < 11 %) with high viability (92 ± 2 %). Based on these patterns, controlled self-assembly leads to the formation of renal spheroids and nephron-like tubules with a predefined size and spatial localization. With this, we fabricated scalable arrays of hollow epithelial spheroids. The spheroid sizes correlated with the initial cell count per unit and could be stepwise adjusted, ranging from Ø = 84, 104, 120 to 131 µm in diameter (size variance < 9 %). Furthermore, we fabricated scalable line-shaped patterns, which self-assembled to hollow cellular tubules (Ø = 105 ± 22 µm). These showed a continuous lumen with prescribed orientation, lined by an epithelial monolayer with tight junctions. Additionally, upregulated expression of kidney-specific functional genes compared to 2D cell monolayers indicated increased tissue functionality, as revealed by mRNA sequencing. Furthermore, our concept enabled the fabrication of hybrid tubules, which consisted of arranged subsections of different cell types, combining murine and human epithelial cells. Finally, we integrated the self-assembled fabrication into a microfluidic chip and achieved fluidic access to the lumen at the terminal sites of the tubules. With this, we realized flow conditions with a wall shear stress of 0.05 ± 0.02 dyne/cm² driven by hydrostatic pressure for scalable dynamic culture towards a nephron-on-chip model.

Aging and FGF23-klotho system

During the evolution of skeletons, vertebrates acquired the bone made of calcium phosphate. By keeping the extracellular fluid in a supersaturated condition regarding calcium and phosphate, vertebrates create the bone when and where they want simply by providing a cue for precipitation. To secure this strategy, a new endocrine system has evolved that strictly controls the extracellular phosphate concentration. In response to phosphate intake, fibroblast growth factor-23 (FGF23) is secreted from the bone and acts on the kidney through binding to its receptor Klotho to increase urinary phosphate excretion and maintain phosphate homeostasis. The FGF23-Klotho endocrine system, when disrupted, results in hyperphosphatemia and ectopic precipitation of calcium phosphate in mice and humans. In addition to disturbed phosphate homeostasis, mice lacking Klotho suffer from premature aging. They exhibit multiple organ atrophy, arteriosclerosis characterized by vascular calcification, cardiac hypertrophy, sarcopenia, cognition impairment, frailty, and a shortened life span associated with chronic non-infectious inflammation. Restoration of the phosphate balance by placing Klotho- or FGF23-deficient mice on low phosphate diet rescued them from the aging-like phenotypes, indicating that phosphate was responsible for the accelerated aging. The similar pathophysiology is universally observed in patients with chronic kidney disease (CKD), rendering advanced CKD a clinical model of accelerated aging. CKD patients bear colloidal nanoparticles containing calcium phosphate in the blood, which are termed calciprotein particles (CPPs). CPPs have the ability to induce cell damage and inflammation, potentially contributing to accelerated aging. Terrestrial vertebrates with the bone made of calcium phosphate may be destined to age due to ectopic calcium phosphate.

Endocrine fibroblast growth factors as potential biomarkers for chronic kidney disease

INTRODUCTION

Among the family of fibroblast growth factors (FGFs), FGF19, FGF21, and FGF23 act as circulating hormones and are called endocrine FGFs. FGF19 and FGF21 regulate bile acid and energy homeostasis, respectively, whereas FGF23 regulates vitamin D and phosphate homeostasis. Accumulating evidence suggests that FGF23 plays a critical role in disturbed mineral metabolisms, left ventricular hypertrophy, immunosuppression, inflammation, among others in patients with chronic kidney disease (CKD), highlighting the potential both as a biomarker and a therapeutic target. Several studies have also examined the potential role of FGF19 and FGF21 in CKD patients.

AREAS COVERED

In this review, we present a brief overview of the biology of FGF19, FGF21, and FGF23, and summarize recent clinical and experimental studies on the pathophysiological roles of endocrine FGFs, mainly FGF23, in CKD patients.

EXPERT OPINION

Among the endocrine FGFs, FGF23 represents the most promising biomarker in CKD patients. If future studies confirm that FGF23 is directly toxic in CKD patients, FGF23 could be regarded as a therapeutic target and its measurement would be valuable if applied in clinical practice. Despite their potentially important roles, the clinical relevance of FGF19 and FGF21 in CKD patients is unclear, and much more studies are required.