Human Podocytes Develop “Names” to Practice Self-Avoidance

Podocyte effacement and loss characterizes glomerulopathies such as diabetic nephropathy, lupus, and glomerular toxicity. Human primary podocyte-like epithelial cells cultured from urine (ECU) were characterized as a window to understand podocyte regeneration in these glomerulopathies. Cytokines TNF, and VEGF stimulated regrowth, whereas FGF-1 and IL1 inhibited growth, consistent with other podocyte or parietal epithelial cell models. Review of published micrographs indicated that podocyte foot processes in rodent kidneys interdigitated exclusively with foot processes from other cells, suggesting podocytes practice self-avoidance. In culture, ECU avoided close interactions with other cells arising from the same clone, until enough cells grew for the culture to become confluent. In contrast, ECU derived from multiple clones associated actively to form contiguous monolayers. Gene expression profiling of eight clonal ECU colonies using RNA sequencing revealed that each colony expressed a highly variable profile of the 53 Protocadherin genes. Variable combinations of the 53 Protocadherin PCDHA-, PCDHB-, and PCDHG- gene products are known to act homophilically to direct self-avoidance interactions in human neurons, so PCDH- genes are postulated to govern self-avoidance similarly in podocytes. Such protocadherin profiles could be considered cell names. Transcriptional profiles of ECUs suggest they may represent intermediate stages of Parietal Epithelial Cells (PECs) differentiating into podocytes. They further suggest that each podocyte practices self-avoidance, interdigitating selectively with other cells in the glomerulus. To enable further investigation of multi-clonal podocyte interactions, five podocyte clones were immortalized using temperature-sensitive SV40 Large T antigen. These results suggest that non-self interdigitation of new podocytes with remnant podocytes may facilitate replacement of dying podocytes and re-establishment of a functional filtration barrier. Eight primary colonies were isolated, each representing a clonal epithelial cell line from human urine. Primary ECU colonies containing approximately 50,000 – 150,000 cells were harvested 2-3 weeks after initial plating. Total RNA was isolated using Qiagen RNEasy Plus Mini kits (Qiagen, Hilden, Germany; Cat#:74106) including DNAse treatment to remove genomic DNA. Total RNA samples were selected for polyA+ transcripts using the Qiagen Oligotex midi kit. Resulting samples were depleted of ribosomal sequences using two rounds of RiboZero Gold™, Illumina Inc. RNAseq sequencing was performed using the TruSeq® kit and Illumina HiSeq 2500 instrument by the Frederick National Laboratories Sequencing Core, (Frederick MD, USA).