Focal adhesion kinase (FAK) is a nonreceptor tyrosine kinase that plays a critical role in cell motility. Movement and retraction of podocyte foot processes, which accompany podocyte injury, suggest focal adhesion disassembly. To understand better the mechanisms by which podocyte foot process effacement leads to proteinuria and kidney failure, we studied the function of FAK in podocytes. In murine models, glomerular injury led to activation of podocyte FAK, followed by proteinuria and foot process effacement. Both podocyte-specific deletion of FAK and pharmacologic inactivation of FAK abrogated the proteinuria and foot process effacement induced by glomerular injury. In vitro, podocytes isolated from conditional FAK knockout mice demonstrated reduced spreading and migration; pharmacologic inactivation of FAK had similar effects on wild-type podocytes. In conclusion, FAK activation regulates podocyte foot process effacement, suggesting that pharmacologic inhibition of this signaling cascade may have therapeutic potential in the setting of glomerular injury.
By using ligands with various affinities for the T-cell receptor (TCR) and by altering the contribution of the CD45 tyrosine phosphatase, the effects of the potency of TCR-induced signals on the function of small GTPases Ras and Rap1 were studied. T cells expressing low-molecular-weight CD45 isoforms (e.g., CD45RO) exhibited the strongest activation of the Ras-dependent Elk-1 transcription factor and the highest sensitivity to the inhibitory action of dominant negative mutant Ras compared to T cells expressing high-molecular-weight CD45 isoforms (ABC). Moreover, stimulation of CD45RO؉ , but not CD45ABC ؉ , T cells with a high-affinity TCR ligand induced suboptimal Elk-1 activation compared with the stimulation induced by an intermediateaffinity TCR-ligand interaction. This observation suggested that the Ras-dependent signaling pathway is safeguarded in CD45RO؉ expressors by a negative regulatory mechanism(s) which prohibits maximal activation of the Ras-dependent signaling events following high-avidity TCR-ligand engagement. Interestingly, the biochemical activity of another small GTPase, the Ras-like protein Rap1, which has been implicated in the functional suppression of Ras signaling, was inversely correlated with the extent of Elk-1 activation induced by different-affinity TCR ligands. Consistently, overexpression of putative Rap dominant negative mutant RapN17 or the physiologic inhibitor of Rap1, the Rap GTPase-activating protein RapGAP, augmented the Elk-1 response in CD45RO ؉ T cells. This is in contrast to the suppressive effect of RapN17 and RapGAP on CD45ABC ؉ T cells, underscoring the possibility that Rap1 can act as either a repressor or a potentiator of Ras effector signals, depending on CD45 isoform expression. These observations suggest that cells expressing distinct isoforms of CD45 employ different signal transduction schemes to optimize Ras-mediated signal transduction in activated T lymphocytes.
Background: Humoral immunity against the protease inhibitor serpin B13 is associated with partial protection from type 1 diabetes. Results: Anti-serpin B13 antibodies up-regulate the cleavage of CD4 and CD19 molecules in lymphocytes residing in pancreatic islets and lymph nodes. Conclusion: Antibodies prevent serpin B13 from neutralizing proteases, thereby impairing leukocyte function. Significance: Enhancement of humoral immunity against serpin B13 should impede the progression of pathologic changes in type 1 diabetes.
Intracellular (clade B) ovalbumin (ov)-serpin protease inhibitors play an important role in tissue homeostasis by protecting cells from death in response to hypoosmotic stress, heat shock and other stimuli. Whether these serpins influence immunological tolerance and the risk for autoimmune diseases is not known. We found that a fraction of young autoimmune diabetes-prone non-obese diabetic (NOD) mice had elevated levels of autoantibodies against a member of clade B family known as serpinB13. High levels of anti-serpinB13 antibodies were accompanied by low levels of anti-insulin autoantibodies, reduced numbers of islet-associated T-cells and delayed onset of diabetes. Exposure to anti-serpinB13 monoclonal antibody (mAb) alone also decreased islet inflammation and co-administration of this reagent and a suboptimal dose of anti-CD3 mAb accelerated recovery from diabetes. In a fashion similar to that discovered in the NOD model, a deficiency in humoral activity against serpinB13 was associated with early onset of human type 1 diabetes. These findings suggest that in addition to limiting exposure to proteases within the cell, clade B serpins help to maintain homeostasis by inducing protective humoral immunity.
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