Quantification of renal low-molecular-weight protein degradation in the intact rat

Spatiotemporal organisation of protein processing in the kidney

The kidney regulates plasma protein levels by eliminating them from the circulation. Proteins filtered by glomeruli are endocytosed and degraded in the proximal tubule and defects in this process result in tubular proteinuria, an important clinical biomarker. However, the spatiotemporal organization of renal protein metabolism in vivo was previously unclear. Here, using functional probes and intravital microscopy, we track the fate of filtered proteins in real time in living mice, and map specialized processing to tubular structures with singular value decomposition analysis and three-dimensional electron microscopy. We reveal that degradation of proteins requires sequential, coordinated activity of distinct tubular sub-segments, each adapted to specific tasks. Moreover, we leverage this approach to pinpoint the nature of endo-lysosomal disorders in disease models, and show that compensatory uptake in later regions of the proximal tubule limits urinary protein loss. This means that measurement of proteinuria likely underestimates severity of endocytotic defects in patients.

Polesel et al. visualize plasma protein filtration, uptake and metabolism in the kidneys of living mice in real-time. They reveal coordinated activity of different specialized tubular segments, with major compensatory adaptations occurring in disease states.

Drug-targeting to the kidney: renal delivery and degradation of a naproxen-lysozyme conjugate in vivo

A renal-specific controlled release of an active drug may enable a reduction of the required dose and may provide a reduction of extra-renal toxicity. To achieve renal specific targeting of the NSAID naproxen, the low-molecular-weight protein (LMWP) lysozyme was employed as carrier since it is mainly taken up and catabolized in the proximal tubules of the kidney. A conjugate was synthesized with an average coupling degree of 2 mol naproxen per 1 mol lysozyme in which the drug was directly coupled to the protein via a peptide bond. First, we investigated whether naproxen conjugation affects the renal disposition of lysozyme. As native lysozyme, the conjugate was predominantly and rapidly (within 20 min) taken up by the kidney. The subsequent decrease in renal content reflecting the renal degradation of the conjugated lysozyme molecules appeared also to be similar to that of native lysozyme with a half life of four hours. Second, the effect of lysozyme conjugation on the body distribution of naproxen was studied. An important observation with regard to the aimed reduction in extra-renal side effects was that no detectable amounts of free naproxen were present in the plasma after administration of conjugate. Conjugation of naproxen to lysozyme resulted in a pronounced (70-fold) increase of naproxen accumulation in the kidney. In agreement with the protein disposition study, the conjugate was rapidly taken up by the kidney and subsequently degraded. In conclusion, renal selective targeting of the NSAID naproxen can be obtained by conjugation with the LMWP lysozyme. This concept of drug delivery to the kidney has the potential to improve drug efficacy and safety.