Optimizing renal transporter immunodetection: consequences of freeze-thaw during sample preparation

Abstract

Renal transporters (co-transporters, channels, claudins) mediate homeostasis of fluids and electrolytes and are targets of hormonal and therapeutic regulators. Assessing renal transporter abundance with antibody probes by immunoblotting is an essential tool for mechanistic studies. While journals require authors to demonstrate antibody specificity, there are no consensus guidelines for kidney sample preparation leading to lab-to-lab variability in immunoblot results. In this study, we determined the impact of sample preparation, specifically freeze-thawed (Froz) versus freshly-processed (Fresh) kidneys (female and male rats and mice) on immunoblot signal detection of fifteen renal transporters, and the impact of protease inhibitors during homogenization. In female Sprague Dawley rat kidneys homogenized with: aprotinin, Na2EDTA, PMSF, and phosphatase inhibitors, immunodetection signals were ~50% lower in Frozen versus Fresh samples for most transporters. Inclusion of additional inhibitors (Roche cOmpleteTM Protease Inhibitor, "+") only partially increased transporter immunoblot signals to near Fresh levels. In male Sprague Dawley rats, immunoblot signal density was lower in Froz+ versus Fresh+ despite additional inhibitors. In C57BL/6 male mice, immunoblot signals from proximal tubule transporters were lower in Froz vs Fresh by ~25-50% and was greater in Froz+. In contrast, female mice exhibited selective transporter signal degradation in Froz not improved with additional protease inhibitors. Thus, kidney sample preparation variables, including freeze-thaw and protease inhibition, have substantial transporter-specific effects on quantification of renal transporter abundance by immunoblot. These findings underscore the critical importance of assessing and reporting the impact of sample preparation protocols on transporter recovery to ensure robust rigor and reproducibility