Reduced fragmentation of apoptotic chromatin is associated with nephritis in lupus-prone (NZB x NZW)F1 mice

Objective. Antinucleosome autoantibodies are pathogenic factors in lupus nephritis. Defects in apoptotic pathways may result in increased levels of apoptotic nucleosomes. The objectives of this study were 1) to determine whether low molecular weight oligonucleosomes are present in the kidneys of autoimmune (NZB x NZW)F-1 mice, 2) to analyze whether the presence of glomerular membrane-associated TUNEL-positive electron-dense structures reflect the existence of low molecular weight oligonucleosomes, and 3) to determine an eventual temporal relationship between glomerular electron-dense structures, oligonucleosomes, and proteinuria in these mice. Methods. DNA was isolated from mouse HUM hybridoma cells and from the kidneys of normal BALB/c mice in which apoptosis was induced by camptothecin and from the kidneys of (NZB x NZW)F-1 mice at ages 4 weeks, 8 weeks, 20 weeks, and 2:26 weeks (nephritic mice). The DNA fragmentation pattern was determined with an Agilent bioanalyzer. An electron microscopy-based TUNEL assay was performed to detect apoptotic chromatin in glomerular membranes, and immunoelectron microscopy was used to determine antibody binding. Transcription levels for nucleases associated with apoptosis and necrosis were determined by real-time polymerase chain reaction. Results. DNA from camptothecin-treated cell lines and BALB/c mouse kidneys, but not that from untreated (NZB x NZW)F-1 mouse kidneys, demonstrated DNA cleavage consistent with apoptotic fragmentation. DNA from (NZB x NZW)F-1 mice was devoid of apoptotic fragmentation, irrespective of the age of the mice, whereas TUNEL-positive chromatin particles were detected in glomerular membranes in nephritic mice. Renal DNase I transcription was reduced in nephritic mice. Nucleosomal DNA fragmentation in response to camptothecin exposure was highly reduced in (NZB x NZW)F-1 mouse kidneys compared with that in their normal counterparts. Conclusion. The results of this study demonstrate that TUNEL-positive chromatin particles are deposited in the glomeruli of nephritic (NZB x NZW)F-1 mice, due to reduced fragmentation and clearance of chromatin.