Probes for INS

Hepatocytes are replenished gradually during homeostasis and robustly after liver injury1, 2. In adults, new hepatocytes originate from the existing hepatocyte pool3-8, but the cellular source of renewing hepatocytes remains unclear. Telomerase is expressed in many stem cell populations, and mutations in telomerase pathway genes have been linked to liver diseases9-11. Here we identify a subset of hepatocytes that expresses high levels of telomerase and show that this hepatocyte subset repopulates the liver during homeostasis and injury. Using lineage tracing from the telomerase reverse transcriptase (Tert) locus in mice, we demonstrate that rare hepatocytes with high telomerase expression (TERTHigh hepatocytes) are distributed throughout the liver lobule. During homeostasis, these cells regenerate hepatocytes in all lobular zones, and both self-renew and differentiate to yield expanding hepatocyte clones that eventually dominate the liver. In response to injury, the repopulating activity of TERTHigh hepatocytes is accelerated and their progeny cross zonal boundaries. RNA sequencing shows that metabolic genes are downregulated in TERTHigh hepatocytes, indicating that metabolic activity and repopulating activity may be segregated within the hepatocyte lineage. Genetic ablation of TERTHigh hepatocytes combined with chemical injury causes a marked increase in stellate cell activation and fibrosis. These results provide support for a 'distributed model' of hepatocyte renewal in which a subset of hepatocytes dispersed throughout the lobule clonally expands to maintain liver mass.

Abstract

Although hundreds of cytosolic or transmembrane molecules form the primary cilium, few secreted molecules are known to contribute to ciliogenesis. Here, homologous secreted metalloproteases ADAMTS9 and ADAMTS20 are identified as ciliogenesis regulators that act intracellularly. Secreted and furin-processed ADAMTS9 bound heparan sulfate and was internalized by LRP1, LRP2 and clathrin-mediated endocytosis to be gathered in Rab11 vesicles with a unique periciliary localization defined by super-resolution microscopy. CRISPR-Cas9 inactivation of ADAMTS9 impaired ciliogenesis in RPE-1 cells, which was restored by catalytically active ADAMTS9 or ADAMTS20 acting in trans, but not by their proteolytically inactive mutants. Their mutagenesis in mice impaired neural and yolk sac ciliogenesis, leading to morphogenetic anomalies resulting from impaired hedgehog signaling, which is transduced by primary cilia. In addition to their cognate extracellular proteolytic activity, ADAMTS9 and ADAMTS20 thus have an additional proteolytic role intracellularly, revealing an unexpected regulatory dimension in ciliogenesis.

Craniofacial development requires a set of patterning codes that define the identities of postmigratory mesenchymal cells in a region-specific manner, in which locally expressed morphogens, including fibroblast growth factors (FGFs) and bone morphogenetic proteins (BMPs), provide instructive cues. Msx2, a bona fide target of BMP signaling, is a transcription factor regulating Runx2 and osterix (Osx), whose mutations are associated with cranial deformities in humans. Here we show that Msx2 defines osteo-chondro precursor cells in specific regions of the craniofacial mesenchyme at the postmigratory stage, particularly in the mandibular process and the posterior cranial vault. Analysis of Msx2-creER mice revealed that early mesenchymal cells in proximity to the BMP4-expressing mesenchyme were marked upon tamoxifen injection, and their descendants contributed to diverse types of mesenchymal cells in the later stage, such as chondrocytes and perichondrial cells of the transient cartilage, as well as osteoblasts and suture mesenchymal cells. By contrast, Osx-creER marked osteoblast precursors at the later stage, and their descendants continued to become osteoblasts well into the postnatal stage. Therefore, Msx2 marks spatially restricted populations of mesenchymal precursor cells with diverse differentiation potential, suggesting that extrinsic molecular cues can dictate the nature of postmigratory mesenchymal cells in craniofacial development.