The canonical WNT signaling pathway plays a crucial role in patterning of the embryo during development, but little is known about the specific developmental events which are under WNT control. To understand more about how the WNT pathway orchestrates mammalian organogenesis, we studied the canonical ␤-catenin-mediated WNT signaling pathway in kidneys of mice bearing a ␤-catenin-responsive TCF/␤Gal reporter transgene. In metanephric kidney, intense canonical WNT signaling was evident in epithelia of the branching ureteric bud and in nephrogenic mesenchyme during its transition into renal tubules. WNT signaling activity is rapidly downregulated in maturing nephrons and becomes undetectable in postnatal kidney. Sites of TCF/␤Gal activity are in proximity to the known sites of renal WNT2b and WNT4 expression, and these WNTs stimulate TCF reporter activity in kidney cell lines derived from ureteric bud and metanephric mesenchyme lineages. When fetal kidney explants from HoxB7/GFP mice were exposed to the canonical WNT signaling pathway inhibitor, Dickkopf-1, arborization of the ureteric bud was significantly reduced. We conclude that restricted zones of intense canonical WNT signaling drive branching nephrogenesis in fetal kidney. nephrogenesis; ␤-catenin; branching morphogenesis THE WNT FAMILY is comprised of 19 secreted glycoproteins which act as short-range intercellular signaling molecules, recognizing one of the 10 frizzled receptors expressed at the surface of nearby target cells. The canonical signaling pathway is activated by WNTs which bind to cognate frizzled receptors heterodimerized with LRP5 or LRP6 coreceptors (2). Activated receptors recruit dishevelled protein (Dvl) and inhibit degradation of cytoplasmic ␤-catenin via the GSK3␤-axin-APC complex (11). When its degradation is blocked, cytoplasmic ␤-catenin is available to translocate to the nucleus, dimerize with partners belonging to the T-cell factor (TCF) family, and activate target genes. TCF recognition motifs have been well-studied, allowing design of vectors (e.g., TOPFlash) which drive transcription of reporter genes in response to canonical WNT signaling activity (32). In general, canonical ␤-catenin/TCF signaling is thought to activate gene targets (e.g., c-myc) involved in cell proliferation (3, 27).More than 35 years ago, Unsworth and Grobstein (33) reported that tissue from spinal cord could induce formation of renal tubules when cocultured with isolated metanephric mesenchyme. In 1994, Herzlinger et al. (12) found that WNT1-expressing NIH3T3 cells were also able to induce tubule formation in the coculture assay, suggesting that the canonical (␤-catenin-mediated) WNT signaling pathway is essential for mammalian nephrogenesis. However, the precise function of canonical WNT signaling in renal development is unknown. Surprisingly, WNT1 is not present in the developing kidney, but numerous other WNTs are transiently expressed in specific cell lineages (34). Several of these are able to activate the canonical signaling pathway in other context...

DICER1 is an endoribonuclease central to the generation of microRNAs (miRNAs) and short interfering RNAs (siRNAs). Germline mutations in DICER1 have been associated with a pleiotropic tumour predisposition syndrome and Wilms tumour (WT) is a rare manifestation of this syndrome. Three WTs, each in a child with a deleterious germline DICER1 mutation, were screened for somatic DICER1 mutations and were found to bear specific mutations in either the RNase IIIa (n = 1) or the RNase IIIb domain (n = 2). In the two latter cases, we demonstrate that the germline and somatic DICER1 mutations were in trans, suggesting that the two-hit hypothesis of tumour formation applies for these examples of WT. Among 191 apparently sporadic WTs, we identified five different missense or deletion somatic DICER1 mutations (2.6%) in four individual WTs; one tumour had two very likely deleterious somatic mutations in trans in the RNase IIIb domain (c.5438A>G and c.5452G>A). In vitro studies of two somatic single-base substitutions (c.5429A>G and c.5438A>G) demonstrated exon 25 skipping from the transcript, a phenomenon not previously reported in DICER1. Further we show that DICER1 transcripts lacking exon 25 can be translated in vitro. This study has demonstrated that a subset of WTs exhibits two 'hits' in DICER1, suggesting that these mutations could be key events in the pathogenesis of these tumours.No conflicts of interest were declared. pleuropulmonary blastoma (PPB) syndrome (OMIM 601200) [7,8]. Several genes have been identified as being somatically mutated in WT, including WT1 , CTNNB1 , IGF2 , TP53 , WTX , DIS3L2 , FBXW7 and MYCN [5,9]. There are overlapping distributions of molecular abnormalities at 11p15, WTX , WT1 and CTNNB1 in WT [9,10], but other genes could be contributing to the aetiology of WT [3,[11][12][13]. WT histopathology is heterogeneous and tumours associated with different types of nephrogenic rests and histologies tend to show different underlying genetic changes [14,15]. A revised model for WT ontogeny takes into account genetic data (WT1 status, 11p15 imprint control region 1 methylation status, gene expression patterns of other genes) and histological data [14]. Nevertheless, each WT is thought to be of monoclonal origin [16].

Cystinosis is a rare disease caused by homozygous mutations of the CTNS gene, encoding a cystine efflux channel in the lysosomal membrane. In Ctns knockout mice, the pathologic intralysosomal accumulation of cystine that drives progressive organ damage can be reversed by infusion of wildtype bone marrow-derived stem cells, but the mechanism involved is unclear since the exogeneous stem cells are rarely integrated into renal tubules. Here we show that human mesenchymal stem cells, from amniotic fluid or bone marrow, reduce pathologic cystine accumulation in co-cultured CTNS mutant fibroblasts or proximal tubular cells from cystinosis patients. This paracrine effect is associated with release into the culture medium of stem cell microvesicles (100–400 nm diameter) containing wildtype cystinosin protein and CTNS mRNA. Isolated stem cell microvesicles reduce target cell cystine accumulation in a dose-dependent, Annexin V-sensitive manner. Microvesicles from stem cells expressing CTNSRed transfer tagged CTNS protein to the lysosome/endosome compartment of cystinotic fibroblasts. Our observations suggest that exogenous stem cells may reprogram the biology of mutant tissues by direct microvesicle transfer of membrane-associated wildtype molecules.