KIBRA upregulation increases susceptibility to podocyte injury and glomerular disease progression

The intelligent podocyte: sensing and responding to a complex microenvironment

Podocytes are key components of the glomerular filtration barrier - a specialized structure that is responsible for the filtration of blood by the kidneys. They therefore exist in a unique microenvironment exposed to mechanical force and the myriad molecules that cross the filtration barrier. To survive and thrive, podocytes must continually sense and respond to their ever-changing microenvironment. Sensing is achieved by interactions with the surrounding extracellular matrix and neighbouring cells, through a variety of pathways, to sense changes in environmental factors such as nutrient levels including glucose and lipids, oxygen levels, pH and pressure. The response mechanisms similarly involve a range of processes, including signalling pathways and the actions of specific organelles that initiate and regulate appropriate responses, including alterations in cell metabolism, immune regulation and changes in podocyte structure and cognate functions. These functions ultimately affect glomerular and kidney health. Imbalances in these processes can lead to inflammation, podocyte loss and glomerular disease.

Transcriptional Regulators YAP and TAZ Have Distinct Abilities to Compensate for One Another in Podocytes

Key Points

Background

Kidney function depends on the filtration of enormous volumes of plasma, exposing the filtration barrier to mechanical forces. Podocytes must adapt to these forces for the lifetime of an organism as they cannot self-renew. The molecular mechanisms of podocyte adaptation to mechanical stress remain unclear. YAP and TAZ are key mechanotransducers that relay mechanical stimuli to control transcription.

Methods

We made use of podocyte-specific knockout mouse models for Yap (YAPpKO), Taz (TAZpKO), or both (YAPpKO/TAZpKO) and analyzed single-nucleus RNA sequencing data of isolated glomeruli to delineate the distinct and shared roles of YAP and TAZ in podocyte homeostasis.

Results

Here, we found that YAP and TAZ have only partially overlapping functions and compensatory potential in podocytes in vivo. YAPpKO mice displayed podocyte damage and progressive kidney failure. By contrast, TAZpKO animals did not develop any overt disease, while the combined deletion of Yap and Taz caused a neonatal lethal phenotype. Single-nucleus RNA sequencing analysis revealed that in both YAPpKO and TAZpKO mice, a subpopulation of podocytes showed a similar stress response driven by activator protein 1, revealing a protective compensatory mechanism. However, TAZ failed to compensate sufficiently for the loss of YAP, resulting in dysregulation of Rho-GTPases and subsequently the actin cytoskeleton in diseased YAPpKO. Furthermore, we observed loss of ERBB4 expression exclusively in YAPpKO, underscoring the role of ERBB4 signaling as additional layer of YAP-specific regulation in maintaining podocyte survival.

Conclusions

In summary, we identified common and distinct roles for the two transcriptional regulators in podocyte homeostasis. YAP and TAZ can compensate for the loss of the other in podocytes to preserve viability. Still, although YAP can entirely compensate for the loss of TAZ securing podocyte health, TAZ fails to maintain all the YAP-specific functions leading to podocyte injury.

Kibra knockdown inhibits the aberrant Hippo pathway, suppresses renal cyst formation and ameliorates renal fibrosis in nphp1KO mice

INTRODUCTION

Nephronophthisis (NPH) is an autosomal recessive interstitial cystic kidney disease, which is the most common genetic cause of end-stage renal disease (ESRD) in childhood. The Hippo pathway is regulated by the cilium and has been suggested to be linked to NPH. The aim of the study was to investigate the involvement of Hippo pathway in the pathogenesis of nphp1 defect-associated NPH (NPH1).

METHOD

Nphp1 knockout (nphp1KO) Madin-Darby Canine Kidney (MDCK) cells and nphp1KO C57BL/6J mice were generated via CRISPR gene editing strategy. The siRNAs targeting Kibra, MST1 and LATS1 were designed. An AAV9 vector was designed for Kibra knockdown. The expression and phosphorylation of core Hippo pathway molecules were evaluated. Pathological renal changes were evaluated via light microscopy respectively with haematoxylin-eosin and Masson staining.

RESULTS

In nphp1KO MDCK cells, nphp1KO mice and NPH1 patients' kidneys, Kibra, p-MST1/2, p-LATS and p-YAP exhibited a notable increase in levels, with an even greater elevation observed in renal cyst cells, indicating the Hippo pathway activated in these nphp1-deficient contexts. Nphp1 re-expression reversed the Hippo pathway activation in cells, indicating that the Hippo pathway activation is related to nphp1 deficiency in vitro. Meanwhile, in vitro, MST1 knockdown downregulated LATS1 and YAP phosphorylation, LATS1 knockdown downregulated YAP phosphorylation, suggesting the activation of the canonical Hippo pathway in nphp1-deficient contexts. Knockdown of the upstream regulator Kibra inhibited the Hippo pathway activation in both nphp1KO MDCK cells and mice. Following Kibra knockdown, the organisation of nphp1KO MDCK cells became more compact, the intensity of the actin fibres increased. Besides, decreased renal fibrosis and cyst formation were observed in nphp1KO mice.

CONCLUSIONS

The canonical Hippo pathway is aberrantly activated in nphp1-deficient conditions. Kibra may serve as a crucial upstream regulator of nphp1 deficiency-related Hippo pathway activation. Kibra upregulation and activation of the Hippo pathway are involved in the pathogenesis of NPH1.

KEY POINTS

Canonical Hippo pathway activated in nphp1-deficient disease models and patients. Kibra was a key upstream molecule in regulating the activation of canonical Hippo pathway in nphp1-deficient disease models and patients and closely related to renal cyst formation and fibrosis in nphp1KO mice.

WWC1 upregulation accelerates hyperuricemia by reduction in renal uric acid excretion through Hippo signaling pathway

Hyperuricemia (HUA) is a metabolic disorder characterized by elevated serum uric acid (UA), primarily attributed to the hepatic overproduction and renal underexcretion of UA. Despite the elucidation of molecular pathways associated with this underexcretion, the etiology of HUA remains largely unknown. In our study, using by Uox knockout rats, HUA mouse, and cell line models, we discovered that the increased WWC1 levels were associated with decreased renal UA excretion. Additionally, using knockdown and overexpression approaches, we found that WWC1 inhibited UA excretion in renal tubular epithelial cells. Mechanistically, WWC1 activated the Hippo pathway, leading to phosphorylation and subsequent degradation of the downstream transcription factor YAP1, thereby impairing the ABCG2 and OAT3 expression through transcriptional regulation. Consequently, this reduction led to a decrease in UA excretion in renal tubular epithelial cells. In conclusion, our study has elucidated the role of upregulated WWC1 in renal tubular epithelial cells inhibiting the excretion of UA in the kidneys and causing HUA.

Role of biophysics and mechanobiology in podocyte physiology

Podocytes form the backbone of the glomerular filtration barrier and are exposed to various mechanical forces throughout the lifetime of an individual. The highly dynamic biomechanical environment of the glomerular capillaries greatly influences the cell biology of podocytes and their pathophysiology. Throughout the past two decades, a holistic picture of podocyte cell biology has emerged, highlighting mechanobiological signalling pathways, cytoskeletal dynamics and cellular adhesion as key determinants of biomechanical resilience in podocytes. This biomechanical resilience is essential for the physiological function of podocytes, including the formation and maintenance of the glomerular filtration barrier. Podocytes integrate diverse biomechanical stimuli from their environment and adapt their biophysical properties accordingly. However, perturbations in biomechanical cues or the underlying podocyte mechanobiology can lead to glomerular dysfunction with severe clinical consequences, including proteinuria and glomerulosclerosis. As our mechanistic understanding of podocyte mechanobiology and its role in the pathogenesis of glomerular disease increases, new targets for podocyte-specific therapeutics will emerge. Treating glomerular diseases by targeting podocyte mechanobiology might improve therapeutic precision and efficacy, with potential to reduce the burden of chronic kidney disease on individuals and health-care systems alike.

Inhibition of transcriptional coactivator YAP Impairs the expression and function of transcription factor WT1 in diabetic podocyte injury

Podocyte injury and loss are hallmarks of diabetic nephropathy (DN). However, the molecular mechanisms underlying these phenomena remain poorly understood. YAP (Yes-associated protein) is an important transcriptional coactivator that binds with various other transcription factors, including the TEAD family members (nuclear effectors of the Hippo pathway), that regulate cell proliferation, differentiation, and apoptosis. The present study found an increase in YAP phosphorylation at S127 of YAP and a reduction of nuclear YAP localization in podocytes of diabetic mouse and human kidneys, suggesting dysregulation of YAP may play a role in diabetic podocyte injury. Tamoxifen-inducible podocyte-specific Yap gene knockout mice (YappodKO) exhibited accelerated and worsened diabetic kidney injury. YAP inactivation decreased transcription factor WT1 expression with subsequent reduction of Tead1 and other well-known targets of WT1 in diabetic podocytes. Thus, our study not only sheds light on the pathophysiological roles of the Hippo pathway in diabetic podocyte injury but may also lead to the development of new therapeutic strategies to prevent and/or treat DN by targeting the Hippo signaling pathway.