Involvement of Mesangial Cells Expressing α-Smooth Muscle Actin During Restorative Glomerular Remodeling in Thy-1.1 Nephritis

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Abstract
The function of actin cytoskeleton in mesangial cells (MCs) during the recovering process of injured glomeruli is not fully understood. MCs in injured glomeruli express α-smooth muscle actin (α-SMA), which is not detected in normal glomeruli. We focused on the localization of α-SMA in MCs of Thy-1. 1 nephritic rat. Expression of α-SMA in the injured glomeruli peaked at day 5 after antibody injection and then declined gradually. At day 5, MCs, where α-SMA was localized at their cytoplasmic processes situated in various positions, occupied the expanded mesangium. MCs expressing α-SMA tended to be located at the peripheral region close to the glomerular basement membrane (GBM) or endothelial cells at day 8. Localization of α-SMA within the peripheral MCs was restricted to the cytoplasmic processes radiating toward the GBM and touching it with their tips at day 8. These α-SMA-containing processes are suitable to transmit the contractile force to GBM and may contribute to normalize the expanded glomerular volume. In addition, an actin-binding protein, drebrin, was localized in all MC processes extending toward various directions throughout the course of nephritis, suggesting that drebrin is involved in the formation of MC processes.