Nucleotide depletion increases trafficking of gentamicin to the Golgi complex in LLC-PK1 cells

Genetics of cystogenesis in base-edited human organoids reveal therapeutic strategies for polycystic kidney disease

In polycystic kidney disease (PKD), microscopic tubules expand into macroscopic cysts. Among the world's most common genetic disorders, PKD is inherited via heterozygous loss-of-function mutations but is theorized to require additional loss of function. To test this, we establish human pluripotent stem cells in allelic series representing four common nonsense mutations, using CRISPR base editing. When differentiated into kidney organoids, homozygous mutants spontaneously form cysts, whereas heterozygous mutants (original or base corrected) express no phenotype. Using these, we identify eukaryotic ribosomal selective glycosides (ERSGs) as PKD therapeutics enabling ribosomal readthrough of these same nonsense mutations. Two different ERSGs not only prevent cyst initiation but also limit growth of pre-formed cysts by partially restoring polycystin expression. Furthermore, glycosides accumulate in cyst epithelia in organoids and mice. Our findings define the human polycystin threshold as a surmountable drug target for pharmacological or gene therapy interventions, with relevance for understanding disease mechanisms and future clinical trials.

Cell adhesion molecules in chemically-induced renal injury

Cell adhesion molecules are integral cell-membrane proteins that maintain cell-cell and cell-substrate adhesion and in some cases act as regulators of intracellular signaling cascades. In the kidney, cell adhesion molecules, such as the cadherins, the catenins, the zonula occludens protein-1 (ZO-1), occludin and the claudins are essential for maintaining the epithelial polarity and barrier integrity that are necessary for the normal absorption/excretion of fluid and solutes. A growing volume of evidence indicates that these cell adhesion molecules are important early targets for a variety of nephrotoxic substances including metals, drugs, and venom components. In addition, it is now widely appreciated that molecules, such as intracellular adhesion molecule-1 (ICAM-1), integrins, and selectins play important roles in the recruitment of leukocytes and inflammatory responses that are associated with nephrotoxic injury. This review summarizes the results of recent in vitro and in vivo studies indicating that these cell adhesion molecules may be primary molecular targets in many types of chemically-induced renal injury. Some of the specific agents that are discussed include cadmium (Cd), mercury (Hg), bismuth (Bi), cisplatin, aminoglycoside antibiotics, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), and various venom toxins. This review also includes a discussion of the various mechanisms, by which these substances can affect cell adhesion molecules in the kidney.

Loss of the homotypic fusion and vacuole protein sorting or golgi-associated retrograde protein vesicle tethering complexes results in gentamicin sensitivity in the yeast Saccharomyces cerevisiae

Gentamicin continues to be a primary antibiotic against gram-negative infections. Unfortunately, associated nephro- and ototoxicity limit its use. Our previous mammalian studies showed that gentamicin is trafficked to the endoplasmic reticulum in a retrograde manner and subsequently released into the cytosol. To better dissect the mechanism through which gentamicin induces toxicity, we have chosen to study its toxicity using the simple eukaryote Saccharomyces cerevisiae. A recent screen of the yeast deletion library identified multiple gentamicin-sensitive strains, many of which participate in intracellular trafficking. Our approach was to evaluate gentamicin sensitivity under logarithmic growth conditions. By quantifying growth inhibition in the presence of gentamicin, we determined that several of the sensitive strains were part of the Golgi-associated retrograde protein (GARP) and homotypic fusion and vacuole protein sorting (HOPS) complexes. Further evaluation of their other components showed that the deletion of any GARP member resulted in gentamicin-hypersensitive strains, while the deletion of other HOPS members resulted in less gentamicin sensitivity. Other genes whose deletion resulted in gentamicin hypersensitivity included ZUO1, SAC1, and NHX1. Finally, we utilized a Texas Red gentamicin conjugate to characterize gentamicin uptake and localization in both gentamicin-sensitive and -insensitive strains. These studies were consistent with our mammalian studies, suggesting that gentamicin toxicity in yeast results from alterations to intracellular trafficking pathways. The identification of genes whose absence results in gentamicin toxicity will help target specific pathways and mechanisms that contribute to gentamicin toxicity.

Cytoplasmic and intra-nuclear binding of gentamicin does not require endocytosis

Understanding the cellular mechanism(s) by which the oto- and nephrotoxic aminoglycoside antibiotics penetrate cells, and the precise intracellular distribution of these molecules, will enable identification of aminoglycoside-sensitive targets, and potential uptake blockers. Clones of two kidney cell lines, OK and MDCK, were treated with the aminoglycoside gentamicin linked to the fluorophore Texas Red (GTTR). As in earlier reports, endosomal accumulation was observed in live cells, or cells fixed with formaldehyde only. However, delipidation of fixed cells revealed GTTR fluorescence in cytoplasmic and nuclear compartments. Immunolabeling of both GTTR and unconjugated gentamicin corresponded to the cytoplasmic distribution of GTTR fluorescence. Intra-nuclear GTTR binding co-localized with labeled RNA in the nucleoli and trans-nuclear tubules. Cytoplasmic and nuclear distribution of GTTR was quenched by phosphatidylinositol-bisphosphate (PIP2), a known ligand for gentamicin. Cytoplasmic and nuclear GTTR binding increased over time (at 37 degrees C, or on ice to inhibit endocytosis), and was serially competed off by increasing concentrations of unconjugated gentamicin, i.e., GTTR binding is saturable. In contrast, little or no reduction of endocytotic GTTR uptake was observed when cells were co-incubated with up to 4 mg/mL unconjugated gentamicin. Thus, cytoplasmic and nuclear GTTR uptake is time-dependent, weakly temperature-dependent and saturable, suggesting that it occurs via an endosome-independent mechanism, implicating ion channels, transporters or pores in the plasma membrane as bioregulatory routes for gentamicin entry into cells.

Gentamicin traffics retrograde through the secretory pathway and is released in the cytosol via the endoplasmic reticulum

Previous mechanisms describing how aminoglycosides exert their cellular toxicity, including lysosomal accumulation, rupture, and release, cannot account for the rapidity and extent of the observed subcellular and organ effects. Using immunoamplification techniques and colocalization with epitopes of the endoplasmic reticulum (ER), we report rapid retrograde transport of gentamicin to the ER. Additionally, exposure times of 2 and 4 h in LLC-PK1cells produced cytosolic release and nuclear association. Cellular internalization and trafficking of aminoglycoside structural analogs, amine-containing cationic fluorescent dextrans of 3,000 molecular weight, corroborated these findings. However, identical anionic fluorescent dextrans, or larger cationic dextrans, of 10,000 molecular weight, failed to traverse from the ER into the cytosol or localize within the nucleus. These studies suggest that a pathway exists that transports internalized aminoglycosides, and other small-molecular-weight cationic compounds, in a retrograde manner through the Golgi complex and to the ER. From there, these compounds move into the cytosol for delivery throughout the cell. To quantify the potential toxic effects of cytosolic aminoglycoside release, experiments examining mitochondrial membrane potential in the continued presence of extracellular gentamicin were undertaken and demonstrated a significant reduction after 4 and 8 h. These observations provide a mechanism for the rapidly induced known cellular alterations, including aberrant vesicle fusion, mitochondrial toxicity/free radical generation, and decreased protein synthesis either by reduced transcription or translation after aminoglycoside exposure.