TLR4 signaling cascades leading to increases in albumin permeability, nephrin ectodomain shedding, and apoptosis in MPC-5 podocytes.

Proteinuria is a cardinal symptom of kidney disease, but common transient proteinurias may reflect a normal physiological response to distant tissue damage or infection. Here we observed that activation of Toll-like receptor-4 (TLR4) using the selective agonist CRX572 induced three distinct functional effects in cultured podocytes. TLR4 activation evoked a reversible increase in albumin flux (PAlb) across a confluent layer of podocytes in 60 m or less. At 24 h matrix metalloproteinase-dependent shedding of nephrin ectodomains was detected. Finally, apoptosis occurred after 72 h. These effects were blocked by TAK242, a selective TLR4 antagonist. TLR4 activation engaged multiple intracellular transduction cascades. Activation of Erk was observed at 30 m, increased reactive oxygen species (ROS) was seen at 60 m and 24 h, and activation of NFκB was seen with 24 h of TLR activation. The increase in PAlb required increased ROS and activation of Erk. The subsequent nephrin ectodomain shedding also required Erk signaling but did not require increased cytosolic ROS or NFκB activation. Finally, TLR4-evoked apoptosis required both increased ROS and activation of NFκB but did not require Erk activation. Innate immune signaling through TLR4 may alter glomerular function via parallel pleiotropic signaling cascades in podocytes.