SUMMARY Stem cell niches provide a microenvironment to support the self-renewal and multi-lineage differentiation of stem cells. Cell-cell interactions within the niche are essential for maintaining tissue homeostasis. However, the niche cells supporting mesenchymal stem cells (MSCs) are largely unknown. Using single-cell RNA sequencing, we show heterogeneity among Gli1+ MSCs and identify a subpopulation of Runx2+/Gli1+ cells in the adult mouse incisor. These Runx2+/Gli1+ cells are strategically located between MSCs and transit-amplifying cells (TACs). They are not stem cells but help to maintain the MSC niche via IGF signaling to regulate TAC proliferation, differentiation, and incisor growth rate. ATAC-seq and chromatin immunoprecipitation reveal that Runx2 directly binds to Igfbp3 in niche cells. This Runx2-mediated IGF signaling is crucial for regulating the MSC niche and maintaining tissue homeostasis to support continuous growth of the adult mouse incisor, providing a model for analysis of the molecular regulation of the MSC niche.

Progenitor cells are crucial in controlling organ morphogenesis. Tooth development is a well-established model for investigating the molecular and cellular mechanisms that regulate organogenesis. Despite advances in our understanding of how tooth crown formation is regulated, we have limited understanding of tooth root development. Runt-related transcription factor 2 (RUNX2) is a well-known transcription factor in osteogenic differentiation and early tooth development. However, the function of RUNX2 during tooth root formation remains unknown. We revealed in this study that RUNX2 is expressed in a subpopulation of GLI1+ root progenitor cells, and that loss of Runx2 in these GLI1+ progenitor cells and their progeny results in root developmental defects. Our results provide in vivo evidence that Runx2 plays a crucial role in tooth root development and in regulating the differentiation of root progenitor cells. Furthermore, we identified that Gli1, Pcp4, NOTUM, and Sfrp2 are downstream targets of Runx2 by integrating bulk and single-cell RNA sequencing analyses. Specifically, ablation of Runx2 results in downregulation of WNT inhibitor NOTUM and upregulation of canonical WNT signaling in the odontoblastic site, which disturbs normal odontoblastic differentiation. Significantly, exogenous NOTUM partially rescues the impaired root development in Runx2 mutant molars. Collectively, our studies elucidate how Runx2 achieves functional specificity in regulating the development of diverse organs and yields new insights into the network that regulates tooth root development.

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Glechomae Herba is a traditional Chinese medicine used for the treatment of urolithiasis, cholelithiasis, and urinary tract infections in China. Identification of chemical constituents is helpful to discover the potential active ingredients. However, this significant work is stymied by complex chemical constituents. Therefore, an ultra high performance liquid chromatography coupled to quadrupole‐time‐of‐flight tandem mass spectrometry analysis with diagnostic product ions and neutral loss filtering strategy was established for chemical profiling of Glechomae Herba. The diagnostic product ions and neutral loss filtering strategy simplified spectral elucidation. A total of 120 compounds, including 10 chlorogenic acids, 10 gallic acids, 21 phenylpropionic acids, and 77 flavonoids, were reasonably identified in Glechomae Herba. Sixty‐five constituents were first discovered in Glechomae Herba. Four types of chlorogenic acids (caffeoylquinic acid, feruloylquinic acid, p‐coumaroylquinic acid, and di‐caffeoylquinic acid), three types of galloylglucoses (di‐O‐galloyl‐glucose, tri‐O‐galloyl‐glucose, and tetra‐O‐galloyl‐glucose), three types of phenylpropionic acid skeletons (p‐coumaric acid, caffeic acid, and rosmarinic acid) and five types of flavonoid aglycone skeletons (apigenin, kaempferol, luteolin, quercetin, and chrysin) were identified in Glechomae Herba. The results indicated that the developed strategy was feasible and rational technique for identifying the complex chemical constituents in Glechomae Herba.