A knock-in Six2Cre line reveals transient interstitial potential in nephron progenitors

The developmental relationship between nephron progenitors and the renal interstitium remains unresolved, in part due to limitations of existing lineage tracing tools. The widely used transgenic Six2TGC line, which is routinely employed to target the nephron lineage, exhibits mosaic recombination and altered progenitor dynamics. To overcome these shortcomings, we generate a knock-in Six2Cre mouse allele that faithfully recapitulates endogenous Six2 expression, preserves nephron endowment, and achieves near-complete, non-mosaic recombination. Side-by-side lineage tracing with Six2Cre and Six2TGC, combined with RNA velocity analysis of single-cell RNA-sequencing datasets, reveals a brief interval around embryonic day 11 during which Six2-expressing mesenchymal nephron progenitors contribute to the renal interstitium. This contribution is transient and stage-restricted. These findings reveal an early dual potential within nephron progenitors and define a precise developmental window for dissecting mechanisms that coordinate nephron-interstitium integration.