The epithelial cell tight junction separates apical and basolateral domains and is essential for barrier function. Disruption of the tight junction is a hallmark of epithelial cell damage and can lead to end organ damage including renal failure. Herein, we identify Gα12 activation by H 2 O 2 leading to tight junction disruption and demonstrate a critical role for Gα12 activation during bilateral renal ischemia/reperfusion injury. Madin–Darby canine kidney (MDCK) cells with inducible Gα12 (Gα12-MDCK) and silenced Gα12 (shGα12-MDCK) were subjected to ATP depletion/repletion and H 2 O 2 /catalase as models of tight junction disruption and recovery by monitoring transepithelial resistance. In ATP depleted cells, barrier disruption and recovery was not affected by Gα12, but reassembly was accelerated by Gα12 depletion. In contrast, silencing of Gα12 completely protected cells from H 2 O 2 -stimulated barrier disruption, a response that rapidly occurred in control cells. H 2 O 2 activated Src and Rho, and Src inhibition (by PP2), but not Rho (by Y27632), protected cells from H 2 O 2 -mediated barrier disruption. Immunofluorescent and biochemical analysis showed that H 2 O 2 led to increased tyrosine phosphorylation of numerous proteins and altered membrane localization of tight junction proteins through Gα12/Src signaling pathway. Gα12 and Src were activated in vivo during ischemia/reperfusion injury, and transgenic mice with renal tubular QLα12 (activated mutant) expression were delayed in recovery and showed more extensive injury. Conversely, Gα12 knockout mice were nearly completely protected from ischemia/reperfusion injury. Taken together, these studies reveal that ROS stimulates Gα12 to activate injury pathways and identifies a therapeutic target for ameliorating ROS mediated injury.

The polarized functions of epithelia require an intact tight junction (TJ) to restrict paracellular movement and to separate membrane proteins into specific domains. TJs contain scaffolding, integral membrane and signaling proteins, but the mechanisms that regulate TJs and their assembly are not well defined. Gα12 (GNA12) binds the TJ protein ZO-1 (TJP1), and Gα12 activates Src to increase paracellular permeability via unknown mechanisms. Herein, we identify Src as a component of the TJ and find that recruitment of Hsp90 to activated Gα12 is necessary for signaling. TJ integrity is disrupted by Gα12-stimulated Src phosphorylation of ZO-1 and ZO-2 (TJP2); this phosphorylation leads to dissociation of occludin and claudin 1 from the ZO-1 protein complex. Inhibiting Hsp90 with geldanamycin blocks Gα12-stimulated Src activation and phosphorylation, but does not affect protein levels or the Gα12–ZO-1 interaction. Using the calcium-switch model of TJ assembly and GST-TPR (GST-fused TPR domain of PP5) pull-downs of activated Gα12, we demonstrate that switching to normal calcium medium activates endogenous Gα12 during TJ assembly. Thrombin increases permeability and delays TJ assembly by activating Gα12, but not Gα13, signaling pathways. These findings reveal an important role for Gα12, Src and Hsp90 in regulating the TJ in established epithelia and during TJ assembly.

Mutations in PKD1 are the most common cause of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1 (polycystin-1 (PC1)) is a large transmembrane protein with a short intracellular C terminus that interacts with numerous signaling molecules, including G␣ 12 . Cyst formation in ADPKD results from numerous cellular defects, including abnormal cilia, changes in polarity, and dysregulated apoptosis and proliferation. Recently, we reported increased apoptosis in Madin-Darby canine kidney (MDCK) cells through G␣ 12 stimulation of JNK and degradation of the anti-apoptotic protein Bcl-2

Apoptosis is an essential mechanism for the maintenance of somatic tissues, and when dysregulated can lead to numerous pathological conditions. G proteins regulate apoptosis in addition to other cellular functions, but the roles of specific G proteins in apoptosis signaling are not well characterized. G␣ 12 stimulates protein phosphatase 2A (PP2A), a serine/threonine phosphatase that modulates essential signaling pathways, including apoptosis. Herein, we examined whether G␣ 12 regulates apoptosis in epithelial cells. Inducible expression of G␣ 12 or constitutively active (QL)␣ 12 in Madin-Darby canine kidney cells led to increased apoptosis with expression of QL␣ 12 , but not G␣ 12 . Inducing QL␣ 12 led to degradation of the anti-apoptotic protein Bcl-2 (via the proteasome pathway), increased JNK activity, and up-regulated I B␣ protein levels, a potent stimulator of apoptosis. Furthermore, the QL␣ 12 -stimulated activation of JNK was blocked by inhibiting PP2A. To characterize endogenous G␣ 12 signaling pathways, non-transfected MDCK-II and HEK293 cells were stimulated with thrombin. Thrombin activated endogenous G␣ 12 (confirmed by GST-tetratricopeptide repeat (TPR) pull-downs) and stimulated apoptosis in both cell types. The mechanisms of thrombin-stimulated apoptosis through endogenous G␣ 12 were nearly identical to the mechanisms identified in QL␣ 12 -MDCK cells and included loss of Bcl-2, JNK activation, and up-regulation of I B␣. Knockdown of the PP2A catalytic subunit in HEK293 cells inhibited thrombin-stimulated apoptosis, prevented JNK activation, and blocked Bcl-2 degradation. In summary, G␣ 12 has a major role in regulating epithelial cell apoptosis through PP2A and JNK activation leading to loss of Bcl-2 protein expression. Targeting these pathways in vivo may lead to new therapeutic strategies for a variety of disease processes.