Role of T lymphocytes in renal disease in HIV-transgenic mice

Collapsing glomerulopathy

Collapsing glomerulopathy (CG) has become an important cause of ESRD. First delineated from other proteinuric glomerular lesions in the 1980s, CG is now recognized as a common, distinct pattern of proliferative parenchymal injury that portends a rapid loss of renal function and poor responses to empiric therapy. Notwithstanding, the rise in disorders that are associated with CG, the identification of the first susceptibility genes for CG, the remarkable increase in murine modeling of CG, and promising preclinical testing of new therapeutic strategies suggest that the outlook for CG as a poorly understood and therapeutically resistant renal disease is set to change in the future. This focused review highlights recent advances in research into the pathogenesis and treatment of CG.

Modulation of podocyte phenotype in collapsing glomerulopathies

Podocytes are well-differentiated postmitotic cells whose function is largely based on their complex cytoskeletal architecture. In diseases with proteinuria, podocytes undergo morphologic changes. Podocytes react to an injurious stimulus by a reorganization of their foot process architecture that is independent of the primary injury and the cause of the proteinuria. Collapsing glomerulopathies, including the idiopathic and secondary forms due to HIV infection, have been previously considered a part of the focal sclerosing glomerulosclerosis (FSGS) spectrum. However, in contrast to FSGS, both forms of collapsing glomerulopathy are characterized by segmental and global collapse of the glomerular basement membrane (GBM) and by characteristic ultrastructural alterations in podocytes. These alterations include loss of the actin-based cytoskeleton, a dysregulated/dedifferentiated phenotype, cellular hypertrophy, and cell proliferation. These observations raise the following questions: 1) What mechanism causes glomerular collapse and do podocytes have a role? We recently proposed that in collapsing glomerulopathies the composition of the GBM is altered and contains more immature forms of collagen IV. These observations suggest that dedifferentiated/dysregulated podocytes may participate in remodeling the GBM composition, producing fetal collagen isoforms. 2) What is the pathomechanism underlying podocyte dysregulation? Although it is still unclear which etiologic factors are responsible for the idiopathic forms of collapsing glomerulopathy, in situ hybridization studies in a transgenic mouse model of HIV-associated collapsing glomerulopathy and on renal biopsies of patients with HIV-associated collapsing glomerulopathy demonstrated the presence of the HIV-1 RNA in podocytes and tubular epithelial cells. These findings suggest a direct link between viral gene expression and the dysregulation of the podocyte phenotype. 3) Another open question is how podocytes become infected in HIV-associated collapsing glomerulopathy. HIV-1 typically uses CD4 and a co-receptor such as CCR5 or CXCR4 to enter cells. So far, there is no demonstration of the expression of these receptors in podocytes. These negative findings, however, do not exclude the possibility that in the kidney another, CD4 independent, co-receptor may be used for viral cell entry. Finally, is it important to mention that collapsing glomerulopathies have a high prevalence in black patients, suggesting a link between racial background and the virus-related podocyte injury.

Suppression of HIV-1 expression by inhibitors of cyclin-dependent kinases promotes differentiation of infected podocytes

The glomerular lesions of HIV-associated nephropathy (HIVAN) are associated with the expression of HIV-1 in podocytes. Infected podocytes proliferate and lose several differentiation markers in vivo and in vitro, which suggests that HIV-1 gene expression induces these changes. Flavopiridol and roscovitine, newly identified inhibitors of cyclin-dependent kinase-9, markedly decrease HIV-1 promoter activity in cell lines of various lineages. In this study, the inhibitors were used to determine whether suppression of HIV-1 transcription in infected podocytes correlated with an inhibition of proliferation and a return to the differentiated phenotype. Dose-response analysis showed that both flavopiridol and roscovitine reversibly suppressed HIV-1 transcription in podocytes in vitro at an IC(50) of 25 nM and 3 microM, respectively. Despite equivalent suppression of HIV-1 transcription, roscovitine was a more effective inhibitor of podocyte proliferation than flavopiridol. Suppression of HIV-1 transcription by flavopiridol or roscovitine was marked by re-expression of the podocyte differentiation markers, synaptopodin and podocalyxin. These results suggest that inhibition of HIV-1 transcription decreases podocyte proliferation and permits the reexpression of differentiation markers. Thus, suppression of HIV-1 transcription by selective cyclin-dependent kinase-9 inhibitors may be a useful therapeutic strategy for the treatment of HIVAN.