Nephrotic syndrome (NS) is one of the most common kidney diseases seen in children. It is characterized by massive protein leakage across the kidney's filtration barrier, and dramatic structural changes in podocytes, which in part comprise this barrier. These changes include retraction of the actin-rich podocyte foot processes with disruption of their actin filaments, suggesting that the podocyte actin cytoskeleton is critical to the maintenance of the filtration barrier. Glucocorticoids are the primary therapy used to treat NS, although neither the target cell nor their mechanism of action in NS is known. Due to the absence of inflammatory cells in the glomerulus during idiopathic NS and the known immunosuppressive properties of glucocorticoids, soluble mediators presumably released from circulating lymphocytes have historically been presumed to be responsible for disease. Similarly, the clinical efficacy of glucocorticoids has been presumed to result from their effects on lymphocytes. In contrast to this theory, however, we have recently identified a novel direct protective effect of glucocorticoids on podocytes in culture. We have found that glucocorticoids can both protect and enhance the recovery of cultured podocytes from two mechanistically distinct types of injury, including: 1) Puromycin aminonucleoside, a podocyte toxin commonly used to induce experimental NS in animals, and 2) Direct disruption of the actin cytoskeleton by the actin monomer-sequestering agent, latrunculin A. We have also found that RU486, a compound that blocks glucocorticoid receptor-mediated transcriptional activation, can abolish the glucocorticoid-induced podocyte protection, suggesting that these protective effects are mediated via the giucocorticoid receptor. Based on these novel observations, we hypothesize that the beneficial clinical effects of glucocorticoids in NS are mediated, at least in part, by a direct action on podocytes to induce alterations in the expression of specific podocyte proteins. To test this hypothesis, we will: 1) Use a proteomic analysis of cultured murine podocytes to identify novel or known podocyte proteins whose expression is significantly altered by direct treatment with glucocorticoids both before and following induction of podocyte injury, and 2) Confirm, by quantitative Western blotting, the changes in expression of selected podocyte proteins following the above treatments, and determine if these treatments induce changes in their intracellular distribution within podocytes. Identification of a potential mechanism for glucocorticoid action in the treatment of NS would permit the development of more highly targeted and less toxic therapies for this very common form of kidney disease.
| Eichler, Tad; Ma, Qing; Kelly, Caitlin et al. (2006) Single and combination toxic metal exposures induce apoptosis in cultured murine podocytes exclusively via the extrinsic caspase 8 pathway. Toxicol Sci 90:392-9 |
