Retinoic acid signaling is directly activated in cardiomyocytes and protects mouse hearts from apoptosis after myocardial infarction.

Retinoic acid (RA) is an essential signaling molecule for cardiac development and plays a protective role in the heart after myocardial infarction (MI). In both cases, the effect of RA signaling on cardiomyocytes, the principle cell type of the heart, has been reported to be indirect. Here we have developed an inducible murine transgenic RA-reporter line using CreERT2 technology that permits lineage tracing of RA-responsive cells and faithfully recapitulates endogenous RA activity in multiple organs during embryonic development. Strikingly, we have observed a direct RA response in cardiomyocytes during mid-late gestation and after MI. Ablation of RA signaling through deletion of the Aldh1a1/a2/a3 genes encoding RA-synthesizing enzymes leads to increased cardiomyocyte apoptosis in adults subjected to MI. RNA sequencing analysis reveals Tgm2 and Ace1, two genes with well-established links to cardiac repair, as potential targets of RA signaling in primary cardiomyocytes, thereby providing novel links between the RA pathway and heart disease.

R-spondin signalling is essential for the maintenance and differentiation of mouse nephron progenitors.

During kidney development, WNT/ß-catenin signalling has to be tightly controlled to ensure proliferation and differentiation of nephron progenitor cells. Here, we show in mice that the signalling molecules RSPO1 and RSPO3 act in a functionally redundant manner to permit WNT/ß-catenin signalling and their genetic deletion leads to a rapid decline of nephron progenitors. By contrast, tissue specific deletion in cap mesenchymal cells abolishes mesenchyme to epithelial transition (MET) that is linked to a loss of Bmp7 expression, absence of SMAD1/5 phosphorylation and a concomitant failure to activate Lef1, Fgf8 and Wnt4, thus explaining the observed phenotype on a molecular level. Surprisingly, the full knockout of LGR4/5/6, the cognate receptors of R-spondins, only mildly affects progenitor numbers, but does not interfere with MET. Taken together our data demonstrate key roles for R-spondins in permitting stem cell maintenance and differentiation and reveal Lgr-dependent and independent functions for these ligands during kidney formation.

Kidneys filter waste out of the bloodstream to produce urine. Each kidney contains many structures called nephrons which separate the waste from the blood. The number of nephrons in a kidney varies between people, and those with low numbers have a higher risk of chronic kidney disease. Nephrons are formed before birth from a specific group of so-called progenitor cells. Each of these cells can either divide to make others like itself, or it can specialize to make nephron cells. At the end of embryonic kidney development, all the progenitor cells become nephron cells. Cells that specialize to become part of a nephron first go through a change called a mesenchyme-to-epithelial transition. Epithelial cells move less than mesenchymal cells, and also develop a clear structure where the two ends of the cell adapt to different roles. Evidence suggests that a cell communication process called WNT/ß-catenin signaling controls this transition. Yet the details of how this transition is controlled are not fully understood. One way to activate WNT/ß-catenin signaling is with R-spondin proteins, which have been found in developing kidneys. Vidal et al. studied R-spondins during the embryonic development of kidneys in mice. Removing R-spondins stopped the progenitor cells from producing more of themselves and increased the number that died. The R-spondins were also needed for the progenitor cells to specialize as nephron cells through the mesenchyme-to-epithelial transition. Further results revealed that R-spondins activate WNT/ß-catenin signaling in these cells, even though the proteins that usually act as R-spondin receptors (called LGR4/5/6) could be removed without affecting the results. This suggests that R-spondins interact with different receptor proteins during kidney development. These findings highlight the role of R-spondins and WNT/ß-catenin signaling in kidney development. Future studies will seek the receptor proteins that R-spondins interact with in kidneys. They may also look to understand how R-spondins balance their different roles in progenitor cells and during cell specialization. These results in mice could also be extended to determine their relevance in human health and disease, including chronic kidney disease, which is responsible for more deaths than breast or prostate cancer.

The Adult Adrenal Cortex Undergoes Rapid Tissue Renewal in a Sex-Specific Manner.

Evolution has resulted in profound differences between males and females that extend to non-reproductive organs and are reflected in the susceptibility and progression of diseases. However, the cellular and molecular basis for these differences remains largely unknown. Here we report that adrenal gland tissue renewal is highly active and sexually dimorphic, with female mice showing a 3-fold higher turnover than males. Moreover, in males, homeostasis relies on proliferation of cells within the steroidogenic zone, but females employ an additional stem and/or progenitor compartment situated in the adrenal capsule. Using lineage tracing, sex reversal models, gonadectomy, and dihydrotestosterone treatments, we further show that sex-specific stem cell activity is driven by male hormones that repress recruitment of Gli1+ stem cells from the capsule and cell proliferation. Taken together, our findings provide a molecular and cellular basis for adrenal sex dimorphism that may contribute to the increased incidence of adrenal diseases in females.

Genetic and Molecular Insights Into Genotype-Phenotype Relationships in Osteopathia Striata With Cranial Sclerosis (OSCS) Through the Analysis of Novel Mouse Wtx Mutant Alleles.

The X-linked WTX/AMER1 protein constitutes an important component of the ß-catenin destruction complex that can both enhance and suppress canonical ß-catenin signaling. Somatic mutations in WTX/AMER1 have been found in a proportion of the pediatric kidney cancer Wilms' tumor. By contrast, germline mutations cause the severe sclerosing bone dysplasia osteopathia striata congenita with cranial sclerosis (OSCS), a condition usually associated with fetal or perinatal lethality in male patients. Here we address the developmental and molecular function of WTX by generating two novel mouse alleles. We show that in addition to the previously reported skeletal abnormalities, loss of Wtx causes severe midline fusion defects including cleft palate and ectopic synostosis at the base of the skull. By contrast, deletion of the C-terminal part of the protein results in only mild developmental abnormalities permitting survival beyond birth. Adult analysis, however, revealed skeletal defects including changed skull morphology and an increased whole-body bone density, resembling a subgroup of male patients carrying a milder, survivable phenotype. Molecular analysis in vitro showed that while ß-catenin fails to co-immunoprecipitate with the truncated protein, partial recruitment appears to be achieved in an indirect manner using AXIN/AXIN2 as a molecular bridge. Taken together our analysis provides a novel model for WTX-caused bone diseases and explains on the molecular level how truncation mutations in this gene may retain some of WTX-protein functions. © 2018 American Society for Bone and Mineral Research.

A knock-in mouse line conditionally expressing the tumor suppressor WTX/AMER1.

WTX/AMER1 is an important developmental regulator, mutations in which have been identified in a proportion of patients suffering from the renal neoplasm Wilms' tumor and in the bone malformation syndrome Osteopathia Striata with Cranial Sclerosis (OSCS). Its cellular functions appear complex and the protein can be found at the membrane, within the cytoplasm and the nucleus. To understand its developmental and cellular function an allelic series for Wtx in the mouse is crucial. Whereas mice carrying a conditional knock out allele for Wtx have been previously reported, a gain-of-function mouse model that would allow studying the molecular, cellular and developmental role of Wtx is still missing. Here we describe the generation of a novel mouse strain that permits the conditional activation of WTX expression. Wtx fused to GFP was introduced downstream a stop cassette flanked by loxP sites into the Rosa26 locus by gene targeting. Ectopic WTX expression is reported after crosses with several Cre transgenic mice in different embryonic tissues. Further, functionality of the fusion protein was demonstrated in the context of a Wtx null allele.