Podocytes in health and glomerular disease

Abstract

Podocytes are highly specialized, terminally differentiated cells in the glomerulus of the kidney and these cells play a central role in blood filtration. In this review, we comprehensively describe the cell biology of podocytes under healthy conditions and in glomerular disorders wherein podocyte injury is a major pathological mechanism. First, the molecular mechanisms that maintain podocyte actin cytoskeleton structure, permanent cell cycle exit, and metabolism under healthy conditions are described. Secondly, the mechanisms of podocyte injury, including genetic alterations and external insults that ultimately disrupt podocyte actin cytoskeleton dynamics or interrupt podocyte quiescence and mitochondrial metabolism are discussed. This understanding forms the basis of described potential therapeutic agents that act by modulating dysregulated podocyte cytoskeleton organization, prevent or reverse cell cycle re-entry, and re-establish normal mitochondrial energy production. Lastly, the application of modern techniques such as single cell RNA sequencing, super resolution microscopy, atomic force microscopy, and glomerular organoids is improving the resolution of mechanistic podocytopathy knowledge. Taken together, our review provides critical insights into the cellular and molecular mechanisms leading to podocyte loss, necessary for the advancement of therapeutic development in glomerular diseases.

Keywords: actin cytoskeleton; differentiated; glomerular disease; metabolism; podocytes; slit diaphragm; therapeutic targets.

FIGURE 1

Podocyte morphology. (A) Diagram representation of a glomerulus. Podocytes are specialized epithelial cells comprised of cellular projections called foot processes which wrap around the surface of glomerular capillaries. (B) Transmission electron microscopy image of a podocyte. The cell body of podocytes branch into major processes (MP), which in turn form the foot processes (FP’s) that are in direct contact with the underlying glomerular basement membrane (GBM). On the capillary lumen side are fenestrated endothelium (FE). Adjacent foot processes form a space in-between called the slit diaphragm (SD).

FIGURE 2

Podocytes are postmitotic cells comprised of a highly organized actin cytoskeleton, and these two cellular properties are supported by mitochondrial processes. (A) Podocyte foot processes are comprised of parallel actin filaments which are anchored to the cell membrane and to the underlying glomerular basement membrane by various actin-binding and actin-associated proteins (A) adapted from Lasagni et al., 2013; Curr Mol Med). (B) Podocytes undergo cell cycle arrest at the G0 resting phase. Within the nucleus, this terminally differentiated state is maintained through increased expression of cell cycle checkpoint proteins such as cyclin dependent kinase inhibitors and simultaneous decreased expression of proteins that promote cell cycle entry and proliferation. (C) The podocyte metabolic demands are largely due to the maintenance of their highly organized cellular structure and terminally differentiated state. Podocyte mitochondria have several adaptations to meet dynamic changes in metabolic demands, including biogenesis, fusion and fission to increase mitochondrial numbers and self-degradation through autophagy to remove damaged mitochondria and prevent excessive oxidative stress.

FIGURE 3

Podocyte foot process effacement and podocyte loss resulting from adriamycin injury. (A,B) Transmission electron microscopy images of podocytes from a healthy mouse glomerulus shows normal podocyte foot process morphology, characterized as interdigitating finger-like projections (red arrows) from the cytoplasm of podocytes which are in direct contact with the underlying glomerular basement membrane (A). In mice with podocyte injury due to adriamycin, the characteristic interdigitating pattern of podocyte foot processes is lost (red arrows), and instead are observed as a continuous, flattened structure lining the glomerular basement membrane. (C,D) Immunofluorescence staining for podocyte nuclear marker Wilms Tumour-1 (WT-1) demonstrates decreased number of podocytes (white arrows) in adriamycin-injured mice (D) compared to healthy controls (C).

FIGURE 4

Mechanisms of podocyte loss and potential treatments. Disorganization of the podocyte actin cytoskeleton, cell cycle re-entry and mitotic catastrophe, and mitochondrial dysfunction are three cellular mechanisms that underlie podocyte loss. These mechanisms are also modifiable by potential treatments that can modulate the severity of podocyte injury.