The G1 and G2 variants of APOL1 are associated with an increased risk of kidney disease; however, the mechanism by which these risk variants cause kidney injury is unclear. New findings show that, unlike the G0 reference allele, the G1 and G2 variants form cytotoxic cation channels at the surface of cells, which triggers an influx of Na+ and Ca+ across the plasma membrane and leads to cell death. “The discovery that cation influx has an important role in APOL1-associated cytotoxicity can help us elucidate the downstream pathways by which G1 and G2 induce cell death,” explains Joseph Giovinazzo. “For example, Ca+ has been linked to activation of a multitude of signalling and cell death pathways, cytoskeletal rearrangements and podocyte foot process effacement, and it is possible that any of these processes are induced downstream of Ca+ influx.”

Using confocal immunofluorescence microscopy, the researchers show that following release from the endoplasmic reticulum, all three variants traffic to the plasma membrane. At the plasma membrane, use of live-cell microscopy with Ca+ sensors and APOL1 impermeable cations showed that cytoplasmic influx of Ca+ and Na+ precedes the swelling and lysis of cells that express the G1 and G2, but not the G0 variant. The researchers propose that a regulatory mechanism exists that prevents G0 cytotoxicity. Artificial acidification of the non-toxic G0 variant after plasma membrane localization activated the channel and induced cell death. “Our work suggests that the APOL1 risk variants may be more pH sensitive and therefore form channels more readily than non-risk variants,” says Giovinazzo. “Our live-cell microscopy and time course experiments have also allowed us to build a timeline of events through which the risk variants lead to cell death. Perhaps most importantly, the channel activity of the risk variants at the plasma membrane could make these variants amenable to therapeutic targeting. We anticipate that future efforts will be directed towards understanding the channel structure and the development of compounds to block APOL1 channel activity.”