Dynamic Hippo pathway activity underlies mesenchymal differentiation during lung alveolar morphogenesis

Alveologenesis, the final stage in lung development, substantially remodels the distal lung, expanding the alveolar surface area for efficient gas exchange. Secondary crest myofibroblasts (SCMF) exist transiently in the neonatal distal lung and are crucial for alveologenesis. However, the pathways that regulate SCMF function, proliferation and temporal identity remain poorly understood. To address this, we purified SCMFs from reporter mice, performed bulk RNA-seq and found dynamic changes in Hippo-signaling components during alveologenesis. We deleted the Hippo effectors Yap/Taz from Acta2-expressing cells at the onset of alveologenesis, causing a significant arrest in alveolar development. Using single cell RNA-seq, we identified a distinct cluster of cells in mutant lungs with altered expression of marker genes associated with proximal mesenchymal cell types, airway smooth muscle and alveolar duct myofibroblasts. In vitro studies confirmed that Yap/Taz regulates myofibroblast-associated gene signature and contractility. Together, our findings show that Yap/Taz is essential for maintaining functional myofibroblast identity during postnatal alveologenesis.

Cadherins and catenins in cancer: connecting cancer pathways and tumor microenvironment

Cadherin-catenin complexes are integral components of the adherens junctions crucial for cell-cell adhesion and tissue homeostasis. Dysregulation of these complexes is linked to cancer development via alteration of cell-autonomous oncogenic signaling pathways and extrinsic tumor microenvironment. Advances in multiomics have uncovered key signaling events in multiple cancer types, creating a need for a better understanding of the crosstalk between cadherin-catenin complexes and oncogenic pathways. In this review, we focus on the biological functions of classical cadherins and associated catenins, describe how their dysregulation influences major cancer pathways, and discuss feedback regulation mechanisms between cadherin complexes and cellular signaling. We discuss evidence of cross regulation in the following contexts: Hippo-Yap/Taz and receptor tyrosine kinase signaling, key pathways involved in cell proliferation and growth; Wnt, Notch, and hedgehog signaling, key developmental pathways involved in human cancer; as well as TGFβ and the epithelial-to-mesenchymal transition program, an important process for cancer cell plasticity. Moreover, we briefly explore the role of cadherins and catenins in mechanotransduction and the immune tumor microenvironment.

Stretch increases alveolar type 1 cell number in fetal lungs through ROCK-Yap/Taz pathway

Accurate fluid pressure in the fetal lung is critical for its development, especially at the beginning of the saccular stage when alveolar epithelial type 1 (AT1) and type 2 (AT2) cells differentiate from the epithelial progenitors. Despite our growing understanding of the role of physical forces in lung development, the molecular mechanisms that regulate the transduction of mechanical stretch to alveolar differentiation remain elusive. To simulate lung distension, we optimized both an ex vivo model with precision cut lung slices and an in vivo model of fetal tracheal occlusion. Increased mechanical tension showed to improve alveolar maturation and differentiation toward AT1. By manipulating ROCK pathway, we demonstrate that stretch-induced Yap/Taz activation promotes alveolar differentiation toward AT1 phenotype via ROCK activity. Our findings show that balanced ROCK-Yap/Taz signaling is essential to regulate AT1 differentiation in response to mechanical stretching of the fetal lung, which might be helpful in improving lung development and regeneration.

Reducing YAP expression in Pkd1 mutant mice does not improve the cystic phenotype

The Hippo pathway is a highly conserved signalling route involved in organ size regulation. The final effectors of this pathway are two transcriptional coactivators, yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (WWTR1 or TAZ). Previously, we showed aberrant activation of the Hippo pathway in autosomal-dominant polycystic kidney disease (ADPKD), suggesting that YAP/TAZ might play a role in disease progression. Using antisense oligonucleotides (ASOs) in a mouse model for ADPKD, we efficiently down-regulated Yap levels in the kidneys. However, we did not see any effect on cyst formation or growth. Moreover, the expression of YAP/TAZ downstream targets was not changed, while WNT and TGF-β pathways' downstream targets Myc, Acta2 and Vim were more expressed after Yap knockdown. Overall, our data indicate that reducing YAP levels is not a viable strategy to modulate PKD progression.

Nogo-B fosters HCC progression by enhancing Yap/Taz-mediated tumor-associated macrophages M2 polarization

Tumor-associated macrophages (TAMs) and their M2-type extremely promote tumor angiogenesis, invasion and metastasis, including hepatocellular carcinoma (HCC). Nogo-B is expressed in most tissues and participates in macrophage polarization. However, whether Nogo-B is involved in the polarization and the effects of TAMs has been unclear. The expression of Nogo-B in TAMs of HCC patients is significantly increased, which correlated with the poor prognosis of the patients with HCC. Coincidentally, HCC conditioned medium (HCM) facilitated Nogo-B expression and the M2 phenotype of macrophages. Nogo-B knockdown Nogo-B significantly suppressed the M2-type polarization of macrophages and inhibited HCC cells proliferation both in vivo and in vitro. Furthermore, interference of Nogo-B facilitates macrophage-mediated apoptosis of tumor cells. Nogo-B meaningfully enhanced IL4-stimulated the alternative activation of macrophages as well as expression of the transcriptional regulators Yes-associated protein (Yap)/transcriptional coactivator with PDZ-binding motif (Taz). An inhibitor of Yap, Verteporfin, could block Nogo-B-Yap/Taz-mediated macrophages M2 polarization. Nogo-B expression in macrophages facilitates tumor-associated macrophages M2 polarization and protumoral effects of TAMs in HCC. Targeting Nogo-B/Yap/Taz in macrophages could provide a new therapeutic strategy in HCC therapy.

Dchs1-Fat4 regulation of osteogenic differentiation in mouse

In human, mutations of the protocadherins FAT4 and DCHS1 result in Van Maldergem syndrome, which is characterised, in part, by craniofacial abnormalities. Here, we analyse the role of Dchs1-Fat4 signalling during osteoblast differentiation in mouse. We show that Fat4 and Dchs1 mutants mimic the craniofacial phenotype of the human syndrome and that Dchs1-Fat4 signalling is essential for osteoblast differentiation. In Dchs1/Fat4 mutants, proliferation of osteoprogenitors is increased and osteoblast differentiation is delayed. We show that loss of Dchs1-Fat4 signalling is linked to increased Yap-Tead activity and that Yap is expressed and required for proliferation in osteoprogenitors. In contrast, Taz is expressed in more-committed Runx2-expressing osteoblasts, Taz does not regulate osteoblast proliferation and Taz-Tead activity is unaffected in Dchs1/Fat4 mutants. Finally, we show that Yap and Taz differentially regulate the transcriptional activity of Runx2, and that the activity of Yap-Runx2 and Taz-Runx2 complexes is altered in Dchs1/Fat4 mutant osteoblasts. In conclusion, these data identify Dchs1-Fat4 as a signalling pathway in osteoblast differentiation, reveal its crucial role within the early Runx2 progenitors, and identify distinct requirements for Yap and Taz during osteoblast differentiation.

yap1b, a divergent Yap/Taz family member, cooperates with yap1 in survival and morphogenesis via common transcriptional targets

Yap1/Taz are well-known Hippo effectors triggering complex transcriptional programs controlling growth, survival and cancer progression. Here, we describe yap1b, a new Yap1/Taz family member with a unique transcriptional activation domain that cannot be phosphorylated by Src/Yes kinases. We show that yap1b evolved specifically in euteleosts (i.e. including medaka but not zebrafish) by duplication and adaptation of yap1. Using DamID-seq, we generated maps of chromatin occupancy for Yap1, Taz (Wwtr1) and Yap1b in gastrulating zebrafish and medaka embryos. Our comparative analyses uncover the genetic programs controlled by Yap family proteins during early embryogenesis, and show largely overlapping targets for Yap1 and Yap1b. CRISPR/Cas9-induced mutation of yap1b in medaka does not result in an overt phenotype during embryogenesis or adulthood. However, yap1b mutation strongly enhances the embryonic malformations observed in yap1 mutants. Thus yap1 ^-/-; yap1b ^-/- double mutants display more severe body flattening, eye misshaping and increased apoptosis than yap1 ^-/- single mutants, thus revealing overlapping gene functions. Our results indicate that, despite its divergent transactivation domain, Yap1b cooperates with Yap1 to regulate cell survival and tissue morphogenesis during early development.

The signaling protein Wnt5a promotes TGFβ1-mediated macrophage polarization and kidney fibrosis by inducing the transcriptional regulators Yap/Taz

M2 macrophage polarization is known to underlie kidney fibrosis. We previously reported that most of the members of the Wnt family of signaling proteins are induced in fibrotic kidneys. Dysregulation of the signaling protein Wnt5a is associated with fibrosis, but little is known about the role of Wnt5a in regulating M2 macrophage activation that results in kidney fibrosis. Here, using murine Raw 264.7 cells and bone marrow-derived macrophages, we found that Wnt5a enhanced transforming growth factor β1 (TGFβ1)-induced macrophage M2 polarization as well as expression of the transcriptional regulators Yes-associated protein (Yap)/transcriptional coactivator with PDZ-binding motif (Taz). Verteporfin blockade of Yap/Taz inhibited both Wnt5a- and TGFβ1-induced macrophage M2 polarization. In mouse models of kidney fibrosis, shRNA-mediated knockdown of Wnt5a expression diminished kidney fibrosis, macrophage Yap/Taz expression, and M2 polarization. Moreover, genetic ablation of Taz in macrophages attenuated kidney fibrosis and macrophage M2 polarization in mice. Collectively, these results indicate that Wnt5a promotes kidney fibrosis by stimulating Yap/Taz-mediated macrophage M2 polarization.

Lats1/2 Regulate Yap/Taz to Control Nephron Progenitor Epithelialization and Inhibit Myofibroblast Formation

In the kidney, formation of the functional filtration units, the nephrons, is essential for postnatal life. During development, mesenchymal progenitors tightly regulate the balance between self-renewal and differentiation to give rise to all nephron epithelia. Here, we investigated the functions of the Hippo pathway serine/threonine-protein kinases Lats1 and Lats2, which phosphorylate and inhibit the transcriptional coactivators Yap and Taz, in nephron progenitor cells. Genetic deletion of Lats1 and Lats2 in nephron progenitors of mice led to disruption of nephrogenesis, with an accumulation of spindle-shaped cells in both cortical and medullary regions of the kidney. Lineage-tracing experiments revealed that the cells that accumulated in the interstitium derived from nephron progenitor cells and expressed E-cadherin as well as vimentin, a myofibroblastic marker not usually detected after mesenchymal-to-epithelial transition. The accumulation of these interstitial cells associated with collagen deposition and ectopic expression of the myofibroblastic markers vimentin and α-smooth-muscle actin in developing kidneys. Although these myofibroblastic cells had high Yap and Taz accumulation in the nucleus concomitant with a loss of phosphorylated Yap, reduction of Yap and/or Taz expression levels completely rescued the Lats1/2 phenotype. Taken together, our results demonstrate that Lats1/2 kinases restrict Yap/Taz activities to promote nephron progenitor cell differentiation in the mammalian kidney. Notably, our data also show that myofibroblastic cells can differentiate from nephron progenitors.

Arhgef7 promotes activation of the Hippo pathway core kinase Lats

The Hippo pathway regulates tissue growth and organ size, and inactivation contributes to cancer. Signals flow through Mst/Lats kinases, which phosphorylate and promote cytoplasmic localization of the transcriptional regulators Yap and Taz to inhibit transcription. Here, we identify the multidomain-containing guanine nucleotide exchange factor (GEF) Arhgef7, or βPix, as a positive Hippo pathway regulator. We show that βPix, which localizes to the cytoplasm, binds both Lats and Yap/Taz and thereby promotes Lats-mediated phosphorylation of Yap/Taz in a GEF-independent manner. βPix is required downstream of both cell density sensing and actin cytoskeletal rearrangements, and we demonstrate that loss of βPix expression in normal mammary epithelial cells strongly reduces Yap/Taz phosphorylation, promotes nuclear localization and increases target gene expression. Conversely, increased expression of βPIX in breast cancer cell lines re-couples the Hippo kinase cassette to Yap/Taz, promoting localization of Yap/Taz to the cytoplasm and inhibiting cell migration and proliferation. These studies thus define βPix as a key component that links the Hippo kinase cassette to Yap/Taz in response to multiple upstream Hippo pathway activators.

Stretch-activated ion channel Piezo1 directs lineage choice in human neural stem cells

Neural stem cells are multipotent cells with the ability to differentiate into neurons, astrocytes, and oligodendrocytes. Lineage specification is strongly sensitive to the mechanical properties of the cellular environment. However, molecular pathways transducing matrix mechanical cues to intracellular signaling pathways linked to lineage specification remain unclear. We found that the mechanically gated ion channel Piezo1 is expressed by brain-derived human neural stem/progenitor cells and is responsible for a mechanically induced ionic current. Piezo1 activity triggered by traction forces elicited influx of Ca(2+), a known modulator of differentiation, in a substrate-stiffness-dependent manner. Inhibition of channel activity by the pharmacological inhibitor GsMTx-4 or by siRNA-mediated Piezo1 knockdown suppressed neurogenesis and enhanced astrogenesis. Piezo1 knockdown also reduced the nuclear localization of the mechanoreactive transcriptional coactivator Yes-associated protein. We propose that the mechanically gated ion channel Piezo1 is an important determinant of mechanosensitive lineage choice in neural stem cells and may play similar roles in other multipotent stem cells.