p73 as a Tissue Architect

Abstract

The TP73 gene belongs to the p53 family comprised by p53, p63, and p73. In response to physiological and pathological signals these transcription factors regulate multiple molecular pathways which merge in an ensemble of interconnected networks, in which the control of cell proliferation and cell death occupies a prominent position. However, the complex phenotype of the Trp73 deficient mice has revealed that the biological relevance of this gene does not exclusively rely on its growth suppression effects, but it is also intertwined with other fundamental roles governing different aspects of tissue physiology. p73 function is essential for the organization and homeostasis of different complex microenvironments, like the neurogenic niche, which supports the neural progenitor cells and the ependyma, the male and female reproductive organs, the respiratory epithelium or the vascular network. We propose that all these, apparently unrelated, developmental roles, have a common denominator: p73 function as a tissue architect. Tissue architecture is defined by the nature and the integrity of its cellular and extracellular compartments, and it is based on proper adhesive cell-cell and cell-extracellular matrix interactions as well as the establishment of cellular polarity. In this work, we will review the current understanding of p73 role as a neurogenic niche architect through the regulation of cell adhesion, cytoskeleton dynamics and Planar Cell Polarity, and give a general overview of TAp73 as a hub modulator of these functions, whose alteration could impinge in many of the Trp73 ^-/- phenotypes.

Keywords: actin cytoskeleton; cell adhesion; cell polarity; central nervous system development; neurogenic niche; p53-family; p73; tissue architecture.

FIGURE 1

The emergence of epithelia and the proposed relationship with p53 family members phylogeny. p53/p63/p73-ancestor proteins appear for the first time in Placozoan and Cnidaria. Coincidentally, these organisms are the first ones to fulfill the three criteria that distinguish the “true” epithelial phenotype: i) cells displaying aligned polarity; ii) cells connected by belt-forming junctions; and iii) cells associated with extracellular matrix, with a basal lamina. As vertebrates develop, the p53/p63/p73-ancestor gave rise to the three members of the p53 family. The phylogenetic tree is based on Timetree public knowledge-base. The pictures were created with BioRender.com. Photos were a courtesy of Robert Aguilar, Smithsonian Environmental Research Center, United States (https://commons.wikimedia.org/wiki/File:Nematostella_vectensis_(I1419)_999_(30695685804).jpg) and Bernd Schierwater, Institute of Animal Ecology and Cell Biology, Hannover (Germany). https://commons.wikimedia.org/wiki/File:Trichoplax_adhaerens_photograph.png.

FIGURE 2

p73 role as an architect of the SVZ neurogenic niche. (A) p73 is essential for the formation of the neurogenic pinwheels. In the absence of p73, EpCs show an aberrant membrane morphology and fail to organize into pinwheels, disrupting SVZ niche cytoarchitecture. Analysis of lateral ventricle wall whole-mounts of the indicated genotypes at postnatal day P160 immunostained for b-catenin (green), g-tubulin (red), and GFAP (blue). Pinwheel structures are marked by dotted yellow lines. Scale bars: 10 mm. (B) p73-deficient EpCs cell-junction defects compromise the integrity of the ependymal layer and halt the formation of the mono-stratified epithelium. In addition, cells with abnormal marker expression profiles are observed in p73KO brains. Coronal sections of the lateral wall of the lateral ventricle from P15 WT and p73KO mice were stained with the indicated antibodies and analyzed by confocal microscopy. Scale bars: 10 mm. (C) p73 is required for ciliogenesis and planar cell polarity establishment. p73KO cells displayed an abnormal cilia organization and basal body-cluster displacement. SEM (P7 mice) and confocal microscopy analysis (P15 mice, γ-tubulin, yellow; β-catenin, purple) of WT and p73KO lateral ventricle wall wholemounts. (D) p73 is also necessary for the formation of the polarized apical and sub-apical actin lattices in EpCs. Confocal images of WT and p73KO P15 wholemounts displaying as indicated: actin cytoskeleton (phalloidin, green), basal bodies (γ-tubulin, red) and the cell membrane (β-catenin, blue). Images from Dr. Marin’s research group (Gonzalez-Cano et al., 2016, Fuertes-Alvarez et al., 2018).

FIGURE 3

p73 is a central hub of cellular adhesion and cellular cytoskeleton dynamics. For the indicated cell models, putative TAp73 target genes (DEGs*) were obtained by comparison of differentially expressed genes (DEGs) derived from RNA-seq studies with candidate genes containing p73 binding peaks (*) according to ChIP-seq studies by Koeppel et al. (2011), Marshall et al. (2016), and Santos Guasch et al. (2018). A total number of 736 genes for Saos-2-Tet-On cells, 679 genes for MGCs, and 709 genes for E14-TAp73KO were analyzed with DAVID Bioinformatics Resources 6.8. Functional annotation clustering was performed and enriched biological GO terms for Saos2-Tet-On cells (A), MGCs (B) and E14-TAp73KO (C) are represented. Overlapping genes within GO terms related to “Cell-cell adhesion” and “Actin cytoskeleton” were furthered identified and represented for the three cell models (A’–C’). Comparison of the whole list of DEGs assigned to these clusters between the three cell models is shown (D,E). Publicly available datasets were analyzed in this study and can be found here: GSE15780, PRJNA310161; PRJNA437755. The pictures were created with BioRender.com.

FIGURE 4

Overview of the proposed p73 novel role as a tissue architect. We present a model in which p73 would act as a tissue architect by regulating transcriptional hubs involved in cellular adhesion and cytoskeleton dynamics. In determined spatio-temporal contexts, TAp73 will interplay with other transcriptional programs to orchestrate morphogenic processes like ciliogenesis and/or PCP, ensuring the correct overall tissular architecture in complex microenvironments such as neurogenic niche, the respiratory and reproductive epithelium and, maybe, the vascular network. The pictures were created with BioRender.com.