Focal segmental glomerulosclerosis (FSGS) is a clinical-pathologic syndromes characterized by the accumulation of fibrotic proteins in glomeruli, initially involving only some glomeruli (focal) and involving portions (segments) of the affected glomeruli. FSGS can be classified as follows: idiopathic FSGS, genetic FSGS and post-adaptive FSGS (associated with glomerular hypertrophy and hyperfiltration, and due to reduced renal mass, renal toxins, obesity, and sickle cell disease). A related syndrome is collapsing glomerulopathy, associated with podocyte hyperplasia whereas FSGS is associated with podocyte depletion. Collapsing glomerulopathy can be classified as HIV-associated or idiopathic. In order to define the molecular mechanisms responsible for HIV-associated collapsing glomerulopathy, we have established mice in which transgene expression can be regulated in the glomerular podocyte using an tetracycline-regulated system. We have used this system to express the HIV-1 accessory protein Vpr in the podocyte. These mice develop proteinuria beginning 4 weeks after treatment with tetracycline. Collapsing glomerulopathy appears at 8 weeks, progressing to global glomerulosclerosis and end-stage kidney disease. Podocyte phenotype is abnormal, with reduced expression of the differentiation marker synaptopodin and de novo expression of the injury marker desmin. Increased cell proliferation is present in the glomerular tuft, parietal epithelium, and tubular epithelium. These results demonstrate that Vpr is sufficient to induce HIV-associated collapsing glomerulopathy in transgenic mice. 1) Using these transgenic mice, we have established double transgenic podocyte cell lines, which bear the podocin/rtTA, and temperature-sensitive SV40 Tag transgenes to confer conditional immortalization. These cells were found to express characteristic podocyte markers, including podocin, nephrin, and WT1. We have introduced the Vpr gene into these cells to understand Vpr-induced cell injury. Our preliminary results indicate increased apoptosis with 12 hours of Vpr expression induced by doxycycline. The mechanisms appear to involve both caspase dependent and caspase independent pathways. Recently, we have completed a project in which we show that Vpr increases expression, in vivo (mice) and in vitro of the tubular sodium channel, promoting sodium excretion. This finding explains the clinical observation that patients with HIV-associated nephropathy often lack edema, in contrast to other patients with nephrotic proteinuria. This work will be subjected for publication in the next few months.
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