Retinoic Acid Signaling Coordinates Macrophage-Dependent Injury and Repair after AKI.

Retinoic acid (RA) has been used therapeutically to reduce injury and fibrosis in models of AKI, but little is known about the regulation of this pathway and what role it has in regulating injury and repair after AKI. In these studies, we show that RA signaling is activated in mouse and zebrafish models of AKI, and that these responses limit the extent of injury and promote normal repair. These effects were mediated through a novel mechanism by which RA signaling coordinated the dynamic equilibrium of inflammatory M1 spectrum versus alternatively activated M2 spectrum macrophages. Our data suggest that locally synthesized RA represses proinflammatory macrophages, thereby reducing macrophage-dependent injury post-AKI, and activates RA signaling in injured tubular epithelium, which in turn promotes alternatively activated M2 spectrum macrophages. Because RA signaling has an essential role in kidney development but is repressed in the adult, these findings provide evidence of an embryonic signaling pathway that is reactivated after AKI and involved in reducing injury and enhancing repair.

Delayed treatment with PTBA analogs reduces postinjury renal fibrosis after kidney injury.

No therapies have been shown to accelerate recovery or prevent fibrosis after acute kidney injury (AKI). In part, this is because most therapeutic candidates have to be given at the time of injury and the diagnosis of AKI is usually made too late for drugs to be efficacious. Strategies to enhance post-AKI repair represent an attractive approach to address this. Using a phenotypic screen in zebrafish, we identified 4-(phenylthio)butanoic acid (PTBA), which promotes proliferation of embryonic kidney progenitor cells (EKPCs), and the PTBA methyl ester UPHD25, which also increases postinjury repair in ischemia-reperfusion and aristolochic acid-induced AKI in mice. In these studies, a new panel of PTBA analogs was evaluated. Initial screening was performed in zebrafish EKPC assays followed by survival assays in a gentamicin-induced AKI larvae zebrafish model. Using this approach, we identified UPHD186, which in contrast to UPHD25, accelerates recovery and reduces fibrosis when administered several days after ischemia-reperfusion AKI and reduces fibrosis after unilateral ureteric obstruction in mice. UPHD25 and 186 are efficiently metabolized to the active analog PTBA in liver and kidney microsome assays, indicating both compounds may act as PTBA prodrugs in vivo. UPHD186 persists longer in the circulation than UPHD25, suggesting that sustained levels of UPHD186 may increase efficacy by acting as a reservoir for renal metabolism to PTBA. These findings validate use of zebrafish EKPC and AKI assays as a drug discovery strategy for molecules that reduce fibrosis in multiple AKI models and can be administered days after initiation of injury.

BMP and retinoic acid regulate anterior-posterior patterning of the non-axial mesoderm across the dorsal-ventral axis.

Despite the fundamental importance of patterning along the dorsal-ventral (DV) and anterior-posterior (AP) axes during embryogenesis, uncertainty exists in the orientation of these axes for the mesoderm. Here we examine the origin and formation of the zebrafish kidney, a ventrolateral mesoderm derivative, and show that AP patterning of the non-axial mesoderm occurs across the classic gastrula stage DV axis while DV patterning aligns along the animal-vegetal pole. We find that BMP signalling acts early to establish broad anterior and posterior territories in the non-axial mesoderm while retinoic acid (RA) functions later, but also across the classic DV axis. Our data support a model in which RA on the dorsal side of the embryo induces anterior kidney fates while posterior kidney progenitors are protected ventrally by the RA-catabolizing enzyme Cyp26a1. This work clarifies our understanding of vertebrate axis orientation and establishes a new paradigm for how the kidney and other mesodermal derivatives arise during embryogenesis.