We investigated whether the intermediate filament protein and neural stem cell marker nestin characterizes the glomerular progenitor/reserve cell population immigrating the glomerulus after mesangial cell (MC) injury in the rat (antiThy1 nephritis). Nestin expression was investigated by immunohistochemistry and real-time PCR during anti-Thy1 nephritis. Migration and proliferation assays were used to characterize the function of nestin in isolated MCs after nestin knockdown by siRNA. After MC injury during anti-Thy1 nephritis, glomerular nestin was transiently increased during the repopulation phase. At the peak of mesangial proliferation and expansion (day 5) most OX-7-positive MCs expressed nestin largely colocalizing with the activation marker a-smooth muscle actin and the proliferation marker PCNA. In contrast to a healthy, non-injured mesangium in vivo, MCs in culture are considered to be in an 'activated, injured state' and express nestin in a generalized distribution with condensed localization around the nucleus as well as intensive staining of cell protrusions such as filopodia. During cell cycle, the percentage of MCs with high nestin levels was increased during S-and G2-phase. Blocking of nestin using specific siRNA resulted in inhibition of cell proliferation but not cell migration. In conclusion, nestin is constitutively expressed in podocytes, but is a marker for repopulating MCs after experimental MC injury in vivo. Nestin promotes MC proliferation in vitro, suggesting a supporting role for nestin during repair reaction.

Mesangial cell (MC) migration is essential during glomerular repair and kidney development. The aim of the study was to identify marker/player for glomerular progenitor/reserve cells migrating into the glomerulus after MC injury and during glomerulogenesis in the rat. Experimental mesangial proliferative nephritis was induced in Sprague Dawley rats by intravenous injection of OX-7 antibody. We investigated mRNA expression profiles in isolated glomeruli from on days 0, 1, 2, 3, and 5 after induction of anti-Thy1 nephritis using Affymetrix microarray technology. Using self-organizing maps, transgelin was identified as a new marker for repopulating glomerular cells. Expression of transgelin during anti-Thy1 nephritis was investigated by northern blot, real-time PCR, western blot, and immunohistochemistry. Migration and proliferation assays using isolated MCs after transgelin knockdown by siRNA were performed to investigate the potential role of transgelin during glomerular repopulation. Transgelin mRNA was not detected in healthy glomeruli. It was strongly upregulated during the repopulation process starting on day 1, continued to be increased until day 5 and disappeared on day 7. Transgelin was specifically expressed at the edge of the migratory front during glomerular repopulation as indicated by transgelin/OX-7 double staining. Transgelin expression was similar in migrating vs non-migrating MCs in vitro. Blocking of transgelin expression by siRNA treatment resulted in inhibition of MC migration and proliferation. Transgelin was also expressed in MCs during glomerulogenesis and in biopsies from patients with IgA nephritis. In conclusion, transgelin in the kidney is upregulated in repopulating MCs in vivo and supports their migratory and proliferative repair response after injury.

The growth-inhibitory effect of polymerized type I collagen is characterized by rapid changes of expression and/or activation of MAPK and G1-phase regulators and could result from the lack of alpha1beta1 integrin signaling in MCs on polymerized type I collagen. Conceivably, deposition of polymerized type I collagen might reflect a reparative response to control MC replication in glomerular inflammation.

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