Here we report the generation of a novel Cre transgenic mouse line, Tcf21-Cre, that allows gene excision in the metanephric mesenchyme and its derivatives, including interstitial cells and all epithelial components of the nephron from podocytes to distal tubules. It also results in gene deletion throughout the mesenchyme of other organs including lung, heart, gastrointestinal tract, pancreas, gonad and adrenal gland. Gene deletion begins at E10.5 in the condensing metanephric mesenchyme, and is consistently observed in its derivatives throughout renal development.
are most widely used to investigate the role of genes in the developing kidney and metanephric mesenchyme 
. Is there any advantage or need for Tcf21-Cre
? In contrast to Tcf21
expression is restricted to the cap mesenchyme. While this is useful to determine gene function in this specific compartment, Tcf21-Cre
provides a broader mesenchymal excision from both stromal and cap compartments as well as mesenchymal cells that give rise to periureteric bud smooth muscle cells. Similar to Six2-Cre
expression remains active in epithelial nephric derivatives but unlike Six2
is also expressed in interstitial lineages. As a first step in validation, we compared and contrasted phenotypes in mice lacking β-catenin following Six2
deletion. As expected, we observed many similarities but also important differences. Given the broader expression domain of Tcf21
, we observed hydroureters and a smooth muscle cell phenotype with down-regulation of Bmp4
. Similarly, gain of function studies of β-catenin
provided similarities but also some differences, with Tcf21-Cre
causing midline fusion of the kidneys in β-catenin GOF mutants.
How does Tcf21-Cre
compare to the Pax3-Cre
? Similar to Tcf21-Cre
reportedly excises in both epithelial and interstitial lineages that derive from cap and stromal metanephric mesenchyme. However, the extrarenal sites of Pax3-Cre activation are largely different, and include derivatives of neural crest and somites, such as dorsal root ganglia, skeletal muscle, adrenal medulla, and some subsets of colon epithelium, which do not overlap with the mesenchymal distribution of Tcf21-Cre
An additional benefit of Tcf21-Cre
includes the method of generation of the Cre line through homologous recombination in ES cells. This allows faithful recapitulation of expression pattern ensuring stable expression over subsequent breedings. Randomly inserted or BAC transgenic lines may or may not provide this asset. It is worthwhile to note that Tcf21
is not expressed in intermediate mesoderm 
, which suggests that the GFP expression of Tcf21-Cre; Z/EG
mice doesn't include an excision prior to the development of metanephros. In addition, we noted mosaicism of the Tcf21-Cre
expression and excision. Although this might be viewed as a negative (incomplete excision), we found it to be an asset in our experiments. As demonstrated with our proof of principle studies in this report, a wide spectrum of phenotypes can be observed in each litter ranging from severe to mild, allowing a more complete picture of gene function– i.e. permitting a dose response experiment in a single litter. Finally, the extrarenal expression of Tcf21
in other mesenchymal tissues allows insight from other organs. Although extrarenal expression of a Cre driver strain is often viewed as a weakness, it may provide additional information for the investigator. In this study, although we observed robust extrarenal phenotypes in all of the mutants, they did not preclude analysis of phenotype in kidney. One of the reasons for this is the relatively late onset of Tcf21 expression at mid-gestation. In turn, the development of aggressive embryonic sarcomas in both β-catenin GOF and Ptch1 LOF mutants points to a common genetic pathway – providing direction for future studies.
Utility of Tcf21-Cre
strain is further underscored by the phenotypes observed in the Ptch1
mutants. Although a role for Shh
pathway in renal cystic diseases is supported by dysregulation of Gli1
in nephronophthisis , there has not been any direct genetic evidence 
. Here, we show the development of a dramatic cystic renal phenotype in mice lacking the Ptch1
receptor with data to support a role for proliferation. Interestingly, we did not detect Shh pathway activation in the developing nephrons of Ptch1fl/fl
mice. It remains to be determined whether the nephrons found in the mutant kidney are derived from wild type cells that escape Cre excision due to the inability of Ptch1
null cells to form nephrons.
In summary, we report a new Cre driver strain that provides robust excision in the metanephric mesenchyme. It provides researchers with a useful tool to delete genes broadly from multiple metanephric mesenchyme subpopulations, resulting in phenotypes in the stromal cells, the cap mesenchyme and the periureteric bud cells.