Stem cell regulation by the Hippo pathway

The Hippo pathway controls a switch between retinal progenitor cell proliferation and photoreceptor cell differentiation in zebrafish

The precise regulation of numbers and types of neurons through control of cell cycle exit and terminal differentiation is an essential aspect of neurogenesis. The Hippo signaling pathway has recently been identified as playing a crucial role in promoting cell cycle exit and terminal differentiation in multiple types of stem cells, including in retinal progenitor cells. When Hippo signaling is activated, the core Mst1/2 kinases activate the Lats1/2 kinases, which in turn phosphorylate and inhibit the transcriptional cofactor Yap. During mouse retinogenesis, overexpression of Yap prolongs progenitor cell proliferation, whereas inhibition of Yap decreases this proliferation and promotes retinal cell differentiation. However, to date, it remains unknown how the Hippo pathway affects the differentiation of distinct neuronal cell types such as photoreceptor cells. In this study, we investigated whether Hippo signaling regulates retinogenesis during early zebrafish development. Knockdown of zebrafish mst2 induced early embryonic defects, including altered retinal pigmentation and morphogenesis. Similar abnormal retinal phenotypes were observed in zebrafish embryos injected with a constitutively active form of yap [(yap (5SA)]. Loss of Yap's TEAD-binding domain, two WW domains, or transcription activation domain attenuated the retinal abnormalities induced by yap (5SA), indicating that all of these domains contribute to normal retinal development. Remarkably, yap (5SA)-expressing zebrafish embryos displayed decreased expression of transcription factors such as otx5 and crx, which orchestrate photoreceptor cell differentiation by activating the expression of rhodopsin and other photoreceptor cell genes. Co-immunoprecipitation experiments revealed that Rx1 is a novel interacting partner of Yap that regulates photoreceptor cell differentiation. Our results suggest that Yap suppresses the differentiation of photoreceptor cells from retinal progenitor cells by repressing Rx1-mediated transactivation of photoreceptor cell genes during zebrafish retinogenesis.

Combining small molecules for cell reprogramming through an interatomic analysis

The knowledge available about the application and generation of induced pluripotent stem cells (iPSC) has grown since their discovery, and new techniques to enhance the reprogramming process have been described. Among the new approaches to induce iPSC that have gained great attention is the use of small molecules for reprogramming. The application of small molecules, unlike genetic manipulation, provides for control of the reprogramming process through the shifting of concentrations and the combination of different molecules. However, different researchers have reported the use of "reprogramming cocktails" with variable results and drug combinations. Thus, the proper combination of small molecules for successful and enhanced reprogramming is a matter for discussion. However, testing all potential drug combinations in different cell lineages is very costly and time-consuming. Therefore, in this article, we discuss the use of already employed molecules for iPSC generation, followed by the application of systems chemo-biology tools to create different data sets of protein-protein (PPI) and chemical-protein (CPI) interaction networks based on the knowledge of already used and new reprogramming cocktail combinations. We further analyzed the biological processes associated with PPI-CPI networks and provided new potential protein targets to be inhibited or expressed for stem cell reprogramming. In addition, we applied a new interference analysis to prospective targets that could negatively affect the classical pluripotency-associated factors (SOX2, NANOG, KLF4 and OCT4) and thus potentially improve reprogramming protocols.

Mechanotransduction and fibrosis

Scarring and tissue fibrosis represent a significant source of morbidity in the United States. Despite considerable research focused on elucidating the mechanisms underlying cutaneous scar formation, effective clinical therapies are still in the early stages of development. A thorough understanding of the various signaling pathways involved is essential to formulate strategies to combat fibrosis and scarring. While initial efforts focused primarily on the biochemical mechanisms involved in scar formation, more recent research has revealed a central role for mechanical forces in modulating these pathways. Mechanotransduction, which refers to the mechanisms by which mechanical forces are converted to biochemical stimuli, has been closely linked to inflammation and fibrosis and is believed to play a critical role in scarring. This review provides an overview of our current understanding of the mechanisms underlying scar formation, with an emphasis on the relationship between mechanotransduction pathways and their therapeutic implications.

Paracrine cellular and extracellular matrix interactions with mesenchymal progenitors during pulmonary alveolar septation

Alveolar development in humans primarily occurs postnatally and requires a carefully orchestrated expansion of distal epithelial and mesenchymal progenitor populations and coordinated differentiation, to create a highly segmented gas-exchange surface. The regulation of alveolarization normally assimilates cues from paracrine cell-cell, cell-extracellular matrix, and mechanical interactions which are superimposed on cells and the extracellular matrix through phasic respiratory movement. In bronchopulmonary dysplasia, the entire process is precociously initiated when cellular and extracellular components are adapted to the saccular stage where movement and circulation are much more limited. This review focuses on mesenchymal cells (fibroblasts, endothelial cells, and pericytes), and epithelial cells are primarily discussed as sources of growth factor ligands or recipients of ligands produced by mesenchymal cells. Some interstitial fibroblasts differentiate to contractile myofibroblasts, containing a smooth muscle-actin rich cytoskeleton, which connects with tensile and elastic elements in the extracellular matrix, and together comprise a load-bearing network that diffuses mechanical forces during respiration. Other interstitial fibroblasts assimilate neutral lipid droplets, which regulate the differentiation of distal epithelial progenitors and surfactant production by alveolar type 2 cells. Pericytes organize and reinforce the capillary network as it expands to match the coverage of type 1 epithelial cells. Hyperoxia and the mechanical load imposed by positive pressure mechanical ventilation disrupt these paracrine interactions, leaving thickened alveolar walls, airways and arterioles, thereby diminishing gas-exchange surface area. Better understanding of these mechanisms of alveolar septation will lead to more effective treatments to preserve and perhaps augment the surface usual sequence of events that drive alveolarization.

Aberrant activation, nuclear localization, and phosphorylation of Yes-associated protein-1 in the embryonic kidney and Wilms tumor

BACKGROUND

The Yes-associated-protein-1 (YAP1) is a novel, direct regulator of stem cell genes both in development and cancer. FAT4 is an upstream regulator that induces YAP1 cytosolic sequestering by phosphorylation (p-Ser 127) and therefore inhibits YAP1-dependent cellular proliferation. We hypothesized that loss of FAT4 signaling would result in expansion of the nephron progenitor population in kidney development and that YAP1 subcellular localization would be dysregulated in Wilms tumor (WT), an embryonal malignancy that retains gene expression profiles and histologic features reminiscent of the embryonic kidney.

METHODS

Fetal kidneys from Fat4(-/-) mice were harvested at e18.5 and markers of nephron progenitors were investigated using immunohistochemical analysis. To examine YAP1 subcellular localization in WT, a primary WT cell line (VUWT30) was analyzed by immunofluorescence. Forty WT specimens evenly distributed between favorable and unfavorable histology (n = 20 each), and treatment failure or success (n = 20 each) was analyzed for total and phosphorylated YAP1 using immunohistochemistry and Western blot.

RESULTS

Fat4(-/-) mouse fetal kidneys exhibit nuclear YAP1 with increased proliferation and expansion of nephron progenitor cells. In contrast to kidney development, subcellular localization of YAP1 is dysregulated in WT, with a preponderance of nuclear p-YAP1. By Western blot, median p-YAP1 quantity was 5.2-fold greater in unfavorable histology WT (P = 0.05).

CONCLUSIONS

Fetal kidneys in Fat4(-/-) mice exhibit a phenotype reminiscent of nephrogenic rests, a WT precursor lesion. In WT, YAP1 subcellular localization is dysregulated and p-YAP1 accumulation is a novel biomarker of unfavorable histology.

The Hippo pathway effectors TAZ/YAP regulate dicer expression and microRNA biogenesis through Let-7

MicroRNAs (miRNAs) are genome-encoded small double-stranded RNAs that have emerged as key regulators of gene expression and are implicated in most aspects of human development and disease. Canonical miRNA biogenesis involves processing of ∼70-nucleotide pre-miRNA hairpins by Dicer to generate mature ∼22-nucleotide miRNAs, which target complementary RNA sequences. Despite the importance of miRNA biogenesis, signaling mechanisms controlling this process are poorly defined. Here we demonstrate that the post-transcriptional regulation of Dicer is controlled by the cell density-mediated localization of the Hippo pathway effectors TAZ (transcriptional co-activator with PDZ-binding motif) and YAP (Yes-associated protein) (TAZ/YAP). We show that nuclear TAZ/YAP, which are abundant at low cell density, are required for efficient pre-miRNA processing. Knockdown of TAZ/YAP in low density cells, or density-mediated sequestration of TAZ/YAP into the cytoplasm, results in the defective processing of pre-miRNAs. Strikingly, one exception is Let-7, which accumulates upon loss of nuclear TAZ/YAP, leading to Let-7-dependent reduction in Dicer levels. Accordingly, inhibition of Let-7 rescues the miRNA biogenesis defects observed following TAZ/YAP knockdown. Thus, density-regulated TAZ/YAP localization defines a critical and previously unrecognized mechanism by which cells relay cell contact-induced cues to control miRNA biogenesis.

The hippo-yes association protein pathway in liver cancer

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide and the third leading cause of cancer mortality. Despite continuing development of new therapies, prognosis for patients with HCC remains extremely poor. In recent years, control of organ size becomes a hot topic in HCC development. The Hippo signaling pathway has been delineated and shown to be critical in controlling organ size in both Drosophila and mammals. The Hippo kinase cascade, a singling pathway that antagonizes the transcriptional coactivator Yes-associated protein (YAP), plays an important role in animal organ size control by regulating cell proliferation and apoptosis rates. During HCC development, this pathway is likely inactivated in tumor initiated cells that escape suppressive constrain exerted by the surrounding normal tissue, thus allowing clonal expansion and tumor development. We have reviewed evolutionary changes in YAP as well as other components of the Hippo pathway and described the relationships between YAP genes and HCC. We also discuss regulation of transcription factors that are up- and downstream of YAP in liver cancer development.

Mesenchymal stem cell and chondrocyte fates in a multishear microdevice are regulated by Yes-associated protein

Mechanical cues exert considerable influence on the fates of stem cells and terminally differentiated chondrocytes. The elucidation of the interactions between cell fate and mechanical cues in nuclear mechanotransduction will provide new clues to modulate tissue homeostasis and regeneration. In this study, we used an integrated microfluidic perfusion device to simultaneously generate multiple-parameter fluid shear stresses to investigate the role of fluid flow stimuli in the regulation of Yes-associated protein (YAP) expression and the fates of mesenchymal stem cells (MSCs) and primary chondrocytes. YAP expression was regulated by the level of fluid flow stimulus in both MSCs and chondrocytes. An increase in the magnitude of stimulation enhanced the expression of YAP, ultimately resulting in an increase in osteogenesis and a decrease in adipogenesis for MSCs, and initiating dedifferentiation for chondrocytes. Cytochalasin D not only repressed nuclear YAP accumulation in the flow state, but also abrogated flow-induced effects on MSC differentiation and the chondrocyte phenotype, resulting in MSC adipogenesis and the maintenance of the chondrocyte phenotype. Our findings reveal the connection between YAP and MSC/chondrocyte fates in a fluid flow-induced mechanical microenvironment and provide new insights into the mechanisms by which mechanical cues regulate the fates of MSCs and chondrocytes.