Role of biophysics and mechanobiology in podocyte physiology

Abstract

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.

Figure 1.. Role of podocyte morphology in glomerular biomechanics.

(A) Depiction of podocytes wrapping around a segment of the glomerular capillary (glomerular basement membrane, GBM, not shown). The cell body floats in the urinary space whereas the primary processes wrap around the capillary in a perpendicular orientation. The terminal foot processes branch off from primary processes parallel to the capillary and represent the only points of anchorage to the GBM. Red arrows denote the direction of tension generated by actomyosin-containing primary processes. The enlarged section highlights transmission of intracellular forces within the primary processes and foot processes. Grey arrows denote tension in the noncontractile foot processes generated through their connection to the primary processes. (B) Scanning electron microscope (SEM) image of podocytes wrapping around a section of glomerular capillary. (C) A membrane-extracted SEM image of podocytes wrapping around a section of glomerular capillary showing the actin cytoskeleton. (D) Transmission electron microscope (TEM) image showing the glomerular filtration barrier.

Figure 2.. Effects of external mechanical stresses on podocytes.

Podocytes are exposed to several types of mechanical stresses, including flow-induced shear stress, bending caused by apicobasal stress differences and tensile stress induced by capillary dilation. These different stress elements act as stimuli that produce distinct cellular effects through various target mechanosensors and transducers. Modulation of intracellular forces that can balance these external forces underlies the biophysical resilience of podocytes.

Figure 3.. Podocyte intracellular mechanical forces.

Within podocytes, several intracellular forces balance external mechanical stresses, which are detected through force-sensitive proteins, to maintain the glomerular filtration barrier. A combination of contractile and noncontractile structures ensure cytoskeletal integrity and adhesion to the glomerular basement membrane (GBM). For example, contractility of the primary processes that run orthogonal to the glomerular capillaries leads to torsional forces that balance against capillary dilation while keeping the GBM compressed.

Figure 4.. Pathways influencing podocyte biomechanics.

(A) Under basal conditions of the health podocyte foot processes, α₃β₁ integrins are more active than α_Vβ₃ integrins. In the context of glomerular disease, ligands such as suPAR can bind to and activate α_Vβ₃ integrin, leading to altered mechanobiological signaling and proteinuria. APOL1 variants G1 and G2 have a high affinity to suPAR activated α_Vβ₃ integrin and further contribute to podocyte dysfunction. (B) EPB41L5 mediated force transmission from focal adhesions, through sources such as tensile stress, recruits adaptor proteins, such as PDLIM5 and ACTN4, to focal adhesions. At the focal adhesion, YAP bound to PDLIM5 undergoes tyrosine phosphorylation by c-Src, promoting YAP nuclear translocation and activation of the TEAD family of transcription factors. One of the YAP-TEAD transcriptional targets, ARHGAP29, inhibits RhoA in a negative feedback loop to counterbalance RhoA activation downstream of EPB41L5. Moreover, EPB41L5 signaling modulates extracellular matrix assembly and secretion, which in turn influences force transmission through activation of integrins. (C) In the canonical Hippo pathway, YAP nuclear translocation is controlled by a kinase cascade, whereby MST1/2 phosphorylates LATS1/2, which in turn phosphorylates a YAP serine, promoting YAP cytoplasmic sequestration by 14-3-3 proteins and eventual degradation. When the Hippo pathway is inactive, unphosphorylated YAP enters the nucleus and serves as a transcriptional coactivator of TEAD family of transcription factors. (D) Small Rho GTPases are critical for the generation and maintenance of podocyte morphology through downstream spatial control of cytoskeletal-associated processes. RhoA, Rac1 and Cdc42 are directly regulated by several guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). They are also downstream of mechanosensitive cation channels, such as TRPC5 and TRPC6. Additionally, ligands can modulate Rho GTPases spatially by recruiting GEFs and GAPs to a receptor, such as SLIT2 that can recruit SRGAP1 to ROBO2, or modulate their activity through downstream signaling, such as SEMA3 leading to downstream RhoA inactivation upon binding to Plexin A.