Podocytes have long been known to contain large intracellular vesicles, but the functional relevance of these vesicles has remained unclear. Findings now suggest that these vesicles are involved in macropinocytosis, enabling podocytes to internalize large amounts of extracellular fluid in response to albumin-bound free fatty acids (FFAs), and that the rate of internalization increases upon disruption of the filtration barrier. “FFAs bound to albumin can stimulate macropinocytosis by the podocyte in what could be both an adaptive and maladaptive response to proteinuria,” explains researcher Andrey Shaw. “These findings allowed us to build a model suggesting that proteinuria is sensed by the podocyte via signalling through FFA receptors.”

Macropinocytosis is a specialized endocytic process used by specific cell types to efficiently internalize extracellular fluids and large molecules. To investigate the mechanism by which podocytes handle proteins that cross the glomerular basement membrane, Shaw and colleagues studied fluid-phase uptake by injecting mice with fluorescently labelled tracers. Confocal imaging showed fluorescent labelling of podocytes, which was markedly increased in Lamb2−/− mice with nephrotic syndrome.

Credit: Courtesy of Björn Hartleben, Martin Helmstädter and Tobias B. Huber, University Hospital Freiburg, Germany

Podocytes cultured in the presence of a fluorescent tracer showed moderate amounts of uptake. Tracer uptake—particularly of high molecular weight tracers—increased considerably in the presence of bovine serum albumin (BSA). In contrast to conventional BSA, FFA-free BSA did not induce macropinocytosis. Using gene expression analyses combined with molecular and pharmacologic approaches, the researchers demonstrated that FFAs stimulate macropinocytosis through a pathway that involves GPCRs, the Gβ/Gγ complex and RAC1.

To test the role of FFAs on podocyte function in vivo, Shaw and colleagues placed mice on a high fat diet. Mice on this diet had elevated levels of plasma FFAs and were more susceptible to adriamycin-induced proteinuria, than were mice fed standard chow, suggesting that high FFA levels can aggravate podocyte injury. “Because albumin binds to fatty acids and keeps FFA levels low, one consequence of proteinuria is loss of albumin and loss of the buffering capacity of serum for fatty acids resulting in increased FFA levels,” says Shaw. “This would in turn lead to more macropinocytosis by the podocyte, initiating a vicious cycle and worsening proteinuria.”

The researchers plan to continue their investigations to determine the purpose of macropinocytosis in the podocyte. “We are interested in understanding whether this process functions to provide nutrients for the podocyte or to clear proteins that leak across the glomerular basement membrane,” explains Shaw. “We suspect that changes to the actin cytoskeleton induced by FFAs may help to initiate foot process effacement and increase podocyte loss.”