Hypertranscription: the invisible hand in stem cell biology.

Stem cells are the fundamental drivers of growth during development and adult organ homeostasis. The properties that define stem cells - self-renewal and differentiation - are highly biosynthetically demanding. In order to fuel this demand, stem and progenitor cells engage in hypertranscription, a global amplification of the transcriptome. While standard normalization methods in transcriptomics typically mask hypertranscription, new approaches are beginning to reveal a remarkable range in global transcriptional output in stem and progenitor cells. We discuss technological advancements to probe global transcriptional shifts, review recent findings that contribute to defining hallmarks of stem cell hypertranscription, and propose future directions in this field.

A fibronectin gradient remodels mixed-phase mesoderm.

Physical processes ultimately shape tissue during development. Two emerging proposals are that cells migrate toward stiffer tissue (durotaxis) and that the extent of cell rearrangements reflects tissue phase, but it is unclear whether and how these concepts are related. Here, we identify fibronectin-dependent tissue stiffness as a control variable that underlies and unifies these phenomena in vivo. In murine limb bud mesoderm, cells are either caged, move directionally, or intercalate as a function of their location along a stiffness gradient. A modified Landau phase equation that incorporates tissue stiffness accurately predicts cell diffusivity upon loss or gain of fibronectin. Fibronectin is regulated by WNT5A-YAP feedback that controls cell movements, tissue shape, and skeletal pattern. The results identify a key determinant of phase transition and show how fibronectin-dependent directional cell movement emerges in a mixed-phase environment in vivo.

Hedgehog signalling in Foxd1+ embryonic kidney stromal progenitors controls nephron formation via Cxcl12 and Wnt5a.

Congenital anomalies of the kidney and urinary tract (CAKUT) are characterised by a spectrum of structural and histologic abnormalities and are the major cause of childhood kidney failure. During kidney morphogenesis, the formation of a critical number of nephrons is an embryonic process supported, in part, by signalling between nephrogenic precursors and Foxd1-positive stromal progenitor cells. Low nephron number and abnormal patterning of the stroma are signature pathological features among CAKUT phenotypes with decreased kidney function. Despite their critical contribution to CAKUT pathogenesis, the mechanisms that underlie a low nephron number and the functional contribution of a disorganised renal stroma to nephron number are both poorly defined. Here, we identify a primary pathogenic role for increased Hedgehog signalling in embryonic renal stroma in the genesis of congenital low nephron number. Pharmacologic activation of Hedgehog (Hh) signalling in human kidney organoid tissue decreased the number of nephrons and generated excess stroma. The mechanisms underlying these pathogenic effects were delineated in genetic mouse models in which Hh signalling was constitutively activated in a cell lineage-specific manner. Cre-mediated excision of Ptch1 in Foxd1+ stromal progenitor cells, but not in Six2+ nephrogenic precursor cells, generated kidney malformation, identifying the stroma as a driver of low nephron number. Single-cell RNA sequencing analysis identified Cxcl12 and Wnt5a as downstream targets of increased stromal Hh signalling, findings supported by analysis in human kidney organoids. In vivo deficiency of Cxcl12 or Wnt5a in mice with increased stromal Hh signalling improved nephron endowment. These results demonstrate that dysregulated Hh signalling in embryonic renal stromal cells inhibits nephron formation in a manner dependent on Cxcl12 and Wnt5a. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Absolute scaling of single-cell transcriptomes identifies pervasive hypertranscription in adult stem and progenitor cells.

Hypertranscription supports biosynthetically demanding cellular states through global transcriptome upregulation. Despite its potential widespread relevance, documented examples of hypertranscription remain few and limited to early development. Here, we demonstrate that absolute scaling of single-cell RNA-sequencing data enables the estimation of total transcript abundances per cell. We validate absolute scaling in known cases of developmental hypertranscription and apply it to adult cell types, revealing a remarkable dynamic range in transcriptional output. In adult organs, hypertranscription marks activated stem/progenitor cells with multilineage potential and is redeployed in conditions of tissue injury, where it precedes bursts of proliferation during regeneration. Our analyses identify a common set of molecular pathways associated with both adult and embryonic hypertranscription, including chromatin remodeling, DNA repair, ribosome biogenesis, and translation. These shared features across diverse cell contexts support hypertranscription as a general and dynamic cellular program that is pervasively employed during development, organ maintenance, and regeneration.

Chd1 protects genome integrity at promoters to sustain hypertranscription in embryonic stem cells.

Stem and progenitor cells undergo a global elevation of nascent transcription, or hypertranscription, during key developmental transitions involving rapid cell proliferation. The chromatin remodeler Chd1 mediates hypertranscription in pluripotent cells but its mechanism of action remains poorly understood. Here we report a novel role for Chd1 in protecting genome integrity at promoter regions by preventing DNA double-stranded break (DSB) accumulation in ES cells. Chd1 interacts with several DNA repair factors including Atm, Parp1, Kap1 and Topoisomerase 2ß and its absence leads to an accumulation of DSBs at Chd1-bound Pol II-transcribed genes and rDNA. Genes prone to DNA breaks in Chd1 KO ES cells are longer genes with GC-rich promoters, a more labile nucleosomal structure and roles in chromatin regulation, transcription and signaling. These results reveal a vulnerability of hypertranscribing stem cells to accumulation of endogenous DNA breaks, with important implications for developmental and cancer biology.

A case for revisiting Nodal signaling in human pluripotent stem cells.

Nodal is a transforming growth factor-ß (TGF-ß) superfamily member that plays a number of critical roles in mammalian embryonic development. Nodal is essential for the support of the peri-implantation epiblast in the mouse embryo and subsequently acts to specify mesendodermal fate at the time of gastrulation and, later, left-right asymmetry. Maintenance of human pluripotent stem cells (hPSCs) in vitro is dependent on Nodal signaling. Because it has proven difficult to prepare a biologically active form of recombinant Nodal protein, Activin or TGFB1 are widely used as surrogates for NODAL in hPSC culture. Nonetheless, the expression of the components of an endogenous Nodal signaling pathway in hPSC provides a potential autocrine pathway for the regulation of self-renewal in this system. Here we review recent studies that have clarified the role of Nodal signaling in pluripotent stem cell populations, highlighted spatial restrictions on Nodal signaling, and shown that Nodal functions in vivo as a heterodimer with GDF3, another TGF-ß superfamily member expressed by hPSC. We discuss the role of this pathway in the maintenance of the epiblast and hPSC in light of these new advances.

Hedgehog-GLI signaling in Foxd1-positive stromal cells promotes murine nephrogenesis via TGFß signaling.

Normal kidney function depends on the proper development of the nephron: the functional unit of the kidney. Reciprocal signaling interactions between the stroma and nephron progenitor compartment have been proposed to control nephron development. Here, we show that removal of hedgehog intracellular effector smoothened (Smo-deficient mutants) in the cortical stroma results in an abnormal renal capsule, and an expanded nephron progenitor domain with an accompanying decrease in nephron number via a block in epithelialization. We show that stromal-hedgehog-Smo signaling acts through a GLI3 repressor. Whole-kidney RNA sequencing and analysis of FACS-isolated stromal cells identified impaired TGFß2 signaling in Smo-deficient mutants. We show that neutralization and knockdown of TGFß2 in explants inhibited nephrogenesis. In addition, we demonstrate that concurrent deletion of Tgfbr2 in stromal and nephrogenic cells in vivo results in decreased nephron formation and an expanded nephrogenic precursor domain similar to that observed in Smo-deficient mutant mice. Together, our data suggest a mechanism whereby a stromal hedgehog-TGFß2 signaling axis acts to control nephrogenesis.