Mechanical forces in lymphatic vessel development: Focus

IntroductionA well-organized lymphatic system including proper lymph fluid absorption and drainage is imperative for maintaining interstitial fluid and protein homeostasis. The lymphatic system is composed of a blind-ended, unidirectional network that contains absorptive vessels, primary lymphoid organs such as thymus and bone marrow, secondary lymphoid organs such as lymph nodes, spleen, and Peyer’s patches, and lymphoid tissues as adenoids and tonsils (Ruddle and Akirav, 2009; Choi et al., 2012). Lymphatic fluid is collected from the interstitial space into lymphatic capillaries, and these lymphatic vessels merge and gradually thicken as lymphatic collecting vessels. Lymph is eventually drained at the angulus venosus, which is the junction of the subclavian vein and internal jugular vein (Ruddle and Akirav, 2009; Choi et al., 2012; Randolph et al., 2017; Oliver et al., 2020). The lymphatic system also plays crucial roles in lipid absorption and transportation from the digestive tract to the blood circulation, as well as immune cell transport from the interstitium into the venous circulation (Alitalo et al., 2005; Tammela and Alitalo, 2010; Petrova and Koh, 2020; Landau et al., 2021). Lymphatic dysfunction causes interstitial fluid imbalance and edema, nutrient malabsorption, and inflammatory pathologies (Tammela and Alitalo, 2010; Saito et al., 2013; Abouelkheir et al., 2017). Lymphangiogenesis, the formation of new lymphatic vessels from the preexisting lymphatic vessels, relates to various diseases and pathologies such as lymphedema, tumor metastasis, and chronic inflammatory diseases including rheumatoid arthritis (Detmar and Hirakawa, 2002; Alitalo, 2011; Masood et al., 2022); however, the molecular mechanisms that regulate lymphatic endothelial cell proliferation and migration via transcriptional regulation remain largely unknown. In this review, we provide an update on the current knowledge regarding the development of the lymphatic vasculature and its mechanical force signals, especially focusing on transcriptional regulatory mechanisms.The development of the lymphatic vascular systemThe development of the lymphatic vascular system initiates shortly after blood circulation is established in mouse embryos (Yang and Oliver, 2014). At embryonic day (E) 9.5, a subpopulation of lymphatic endothelial progenitor cells in the anterior cardinal vein start to express the Prospero-related homeobox 1 (PROX1) transcription factor, which is a master lymphatic vascular regulator (Wigle and Oliver, 1999; Francois et al., 2008; Ducoli and Detmar, 2021), and then differentiate into lymphatic endothelial cells (LECs) (Lee et al., 2009; Yamazaki et al., 2009; Srinivasan et al., 2010; Srinivasan and Oliver, 2011; Escobedo and Oliver, 2016; Petrova and Koh, 2018). By around E10.0, PROX1 positive lymphatic endothelial progenitor cells expressing vascular endothelial growth factor receptor (VEGFR) 3 sprout via stimulation with mesenchyme-derived VEGF-C ligand. These cells further migrate dorsolaterally from cardinal and intersomitic veins and establish primary lymph sacs and superficial lymphatic vessels identified as the jugular lymph sac by E11.5 (Wigle and Oliver, 1999; Karkkainen et al., 2004; Francois et al., 2012; Yang et al., 2012; Hagerling et al., 2013; Stritt et al., 2021). Another study also suggests LEC fate is decided during transition through the paraxial mesoderm (PXM) lineage. PXM-derived ECs selectively transdifferentiate from the cardinal vein to form LEC progenitors and form the lymphatic endothelium of multiple organs and tissues (Stone and Stainier, 2019). There is accumulating evidence that mesenchyme- or non-venous derived lymphatic progenitor cells contribute to the early lymphatic vasculature and the development of the lymphatic vascular network in various organs including the skin, heart, and mesentery (Bernier-Latmani et al., 2015; Klotz et al., 2015; Martinez-Corral et al., 2015; Stanczuk et al., 2015; Kazenwadel and Harvey, 2016; Ducoli and Detmar, 2021). This network spreads throughout the mouse embryo by E14.5 and subsequently goes through remodeling and maturation from E15.5-E16.0, forming the hierarchical structure of the lymphatic vascular network in which lymphatic capillaries merge to form pre-collecting and collecting lymphatic vessels (Coso et al., 2014; Norden and Kume, 2020) (Figure 1).FIGURE 1. The developmental process of lymphatic vessel. Lymphatic endothelial progenitor cells in the cardinal vein begin to express PROX1 (known as a master lymphatic vascular regulator) at E9.5 and then differentiate into LECs expressing VEGFR3. VEGF-C/VEGFR3 signaling promotes the sprouting and migration of PROX1-positive LECs, which leads to the formation of lymph sacs and initial lymphatic plexus by E11.5. Extended sprouting and migration of LECs from the initial lymphatic structures give rise to the hierarchical lymphatic vessel network. Non-venous origin lymphatic endothelial progenitor cells also contribute to the early lymphatic vasculature and the development of its network. This network undergoes remodeling and maturation during E15.5-E16.0 and forms the organized lymphatic vascular network including lymphatic capillaries and pre-collecting and collecting lymphatic vessels. Arrows show the direction of lymph flow. LEC, lymphatic endothelial cell; VEC, venous endothelial cell; VEGF, vascular endothelial growth factor.Lymphatic vessels are composed of lymphatic capillaries, which are also called initial lymphatics, and collecting lymphatic vessels. The basement membrane of lymphatic capillaries is discontinuous without lining of any pericytes or lymphatic smooth muscle cells (SMCs); therefore, they work for collecting excess plasma and interstitial fluid leaked from blood vessels (Norden and Kume, 2020). In contrast, collecting lymphatic vessels have lymphatic valves to prevent the backflow of lymph, and smooth muscle to transport lymph fluid by contraction (Kume, 2015). The development of the lymphatic vascular network is conducted by several critical signaling pathways including lymphangiogenic signaling such as the VEGF-C/D-VEGFR3 and Angiopoietin (Angpt)-tunica interna endothelial cell kinase (TEK, also known as Tie2) pathways (Potente and Makinen, 2017). Two transcription factors, the SRY-Box transcription factor 18 (SOX18) and the chick ovalbumin upstream promoter transcription factor 2 (COUP-TFII), also play an important role in lymphatic specification via the induction of PROX1 expression, whereas different pathways such as Notch, retinoic acid, and Wnt/beta-catenin signaling are involved in this process (Nicenboim et al., 2015; Ducoli and Detmar, 2021). VEGFR3 also regulates PROX1 by establishing a feedback loop necessary to maintain the identity of LEC progenitor cells, and VEGF-C-mediated activation of Vegfr3 signaling is required to maintain PROX1 expression in LEC progenitor cells (Srinivasan et al., 2014). In collecting lymphatic vessels, platelet-derived growth factor B (PDGFB) regulates lymphatic SMC recruitment, but PDGFB overexpression is insufficient to mediate recruitment to lymphatic capillaries (Wang et al., 2017).A recent study has demonstrated the deficiency of Folliculin, a tumor suppressor, causes ectopic expression of PROX1 in venous endothelial cells (VECs), leading to the misconnection of blood and lymphatic vessels (Tai-Nagara et al., 2020). In LEC-biased VECs deficient for Folliculin, the basic helix-loop-helix transcription factor E3 (TFE3) translocate into the nucleus, binds to a regulatory element of the PROX1 gene, and induces its ectopic venous expression (Tai-Nagara et al., 2020). Thus, in mice, it has been shown that the transition of lymphatic specification and differentiation from venous cell fate is tightly controlled during development. Importantly, the development of the zebrafish anal fin begins along with the formation of lymphatic vessels, but not blood vessels. Following the progressive loss of lymphatic markers during the anal fin growth, these vessels subsequently acquire a blood vessel fate leading to the connection to blood circulation. Thus, this specialized blood vessel formation occurs through LEC transdifferentiation (Das et al., 2022). Single-cell RNA-sequencing analysis in this study further reveals that the loss of lymphatic fate results in the upregulation of several blood endothelial markers, such as VEGFR1, Delta-like (DLL) 4, and SRY-box (SOX) 17. Of note, mosaic overexpression of SOX17 in zebrafish ECs results in reduced lymphatic gene
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