Trim33 conditions the lifespan of primitive macrophages and onset of definitive macrophage production.

Trim33 (Tif1γ) is a transcriptional regulator that is notably involved in several aspects of hematopoiesis. It is essential for the production of erythrocytes in zebrafish, and for the proper functioning and aging of hematopoietic stem and progenitor cells (HSPCs) in mice. Here, we have found that, in zebrafish development, Trim33 is essential cell-autonomously for the lifespan of the yolk sac-derived primitive macrophages, as well as for the initial production of definitive (HSPC-derived) macrophages in the first niche of definitive hematopoiesis, the caudal hematopoietic tissue. Moreover, Trim33 deficiency leads to an excess production of definitive neutrophils and thrombocytes. Our data indicate that Trim33 radically conditions the differentiation output of aorta-derived HSPCs in all four erythro-myeloid cell types, in a niche-specific manner.

Transient opening of tricellular vertices controls paracellular transport through the follicle epithelium during Drosophila oogenesis.

Paracellular permeability is regulated to allow solute transport or cell migration across epithelial or endothelial barriers. However, how cell-cell junction dynamics controls paracellular permeability is poorly understood. Here, we describe patency, a developmentally regulated process in Drosophila oogenesis, during which cell vertices in the follicular epithelium open transiently to allow paracellular transport of yolk proteins for uptake by the oocyte. We show that the sequential removal of E-cadherin, N-cadherin, NCAM/Fasciclin 2, and Sidekick from vertices precedes their basal-to-apical opening, while the subsequent assembly of tricellular occluding junctions marks the termination of patency and seals the paracellular barrier. E-cadherin-based adhesion is required to limit paracellular channel size, whereas stabilized adherens junctions, prolonged NCAM/Fasciclin 2 expression, blocked endocytosis, or increased actomyosin contractility prevent patency. Our findings reveal a key role of cell vertices as gateways controlling paracellular transport and demonstrate that dynamic regulation of adhesion and actomyosin contractility at vertices governs epithelial barrier properties.

Anisotropic organization of circumferential actomyosin characterizes hematopoietic stem cells emergence in the zebrafish.

Hematopoiesis leads to the formation of blood and immune cells. Hematopoietic stem cells emerge during development, from vascular components, via a process called the endothelial-to-hematopoietic transition (EHT). Here, we reveal essential biomechanical features of the EHT, using the zebrafish embryo imaged at unprecedented spatio-temporal resolution and an algorithm to unwrap the aorta into 2D-cartography. We show that the transition involves anisotropic contraction along the antero-posterior axis, with heterogenous organization of contractile circumferential actomyosin. The biomechanics of the contraction is oscillatory, with unusually long periods in comparison to other apical constriction mechanisms described so far in morphogenesis, and is supported by the anisotropic reinforcement of junctional contacts. Finally, we show that abrogation of blood flow impairs the actin cytoskeleton, the morphodynamics of EHT cells, and the orientation of the emergence. Overall, our results underline the peculiarities of the EHT biomechanics and the influence of the mechanical forces exerted by blood flow.

Trim33 is essential for macrophage and neutrophil mobilization to developmental or inflammatory cues.

Macrophages infiltrate and establish in developing organs from an early stage, often before these have become vascularized. Similarly, leukocytes, in general, can quickly migrate through tissues to any site of wounding. This unique capacity is rooted in their characteristic amoeboid motility, the genetic basis of which is poorly understood. Trim33 (also known as Tif1-γ), a nuclear protein that associates with specific DNA-binding transcription factors to modulate gene expression, has been found to be mainly involved in hematopoiesis and gene regulation mediated by TGF-ß. Here, we have discovered that in Trim33-deficient zebrafish embryos, primitive macrophages are unable to colonize the central nervous system to become microglia. Moreover, both macrophages and neutrophils of Trim33-deficient embryos display a reduced basal mobility within interstitial tissues, and a profound lack of a response to inflammatory recruitment signals, including local bacterial infections. Correlatively, Trim33-deficient mouse bone marrow-derived macrophages display a strongly reduced three-dimensional amoeboid mobility in fibrous collagen gels. The transcriptional regulator Trim33 is thus revealed as being essential for the navigation of macrophages and neutrophils towards developmental or inflammatory cues within vertebrate tissues.