Key Points• BMP9 is required for lymphatic valve formation.• Mice deficient in Bmp9 exhibit reduction in lymphatic draining efficiency.Lymphatic vessels are critical for the maintenance of tissue fluid homeostasis and their dysfunction contributes to several human diseases. The activin receptor-like kinase 1 (ALK1) is a transforming growth factor-b family type 1 receptor that is expressed on both blood and lymphatic endothelial cells (LECs). Its high-affinity ligand, bone morphogenetic protein 9 (BMP9), has been shown to be critical for retinal angiogenesis. The aim of this work was to investigate whether BMP9 could play a role in lymphatic development. We found that Bmp9 deficiency in mice causes abnormal lymphatic development. Bmp9-knockout (KO) pups presented hyperplastic mesenteric collecting vessels that maintained LYVE-1 expression. In accordance with this result, we found that BMP9 inhibited LYVE-1 expression in LECs in an ALK1-dependent manner. Bmp9-KO pups also presented a significant reduction in the number and in the maturation of mesenteric lymphatic valves at embryonic day 18.5 and at postnatal days 0 and 4. Interestingly, the expression of several genes known to be involved in valve formation (Foxc2, Connexin37, EphrinB2, and Neuropilin1) was upregulated by BMP9 in LECS. Finally, we demonstrated that Bmp9-KO neonates and adult mice had decreased lymphatic draining efficiency. These data identify BMP9 as an important extracellular regulator in the maturation of the lymphatic vascular network affecting valve development and lymphatic vessel function. (Blood. 2013;122(4):598-607) IntroductionThe lymphatic vasculature is essential for the maintenance of normal fluid balance and for the immune response. It consists of a network of vessels that drain protein-rich lymph from the extracellular space back to the blood circulation, which absorbs dietary fatty acids and is involved in the traffic of immune cells.1 Lymphatic vessels may also serve as a conduit to lymph nodes and thereby participate in systemic metastasis of cancer cells. Hypoplasia, disruption or dysfunction of the lymphatic vessels, impair the ability of the lymphatic vasculature to collect and transport fluids and lead to lymphedema. 1,2The mammalian lymphatic system has been shown to originate from embryonic veins. Lymphatic vessel development starts at embryonic days (E) 9.5 to 10.5 in mice. After formation of the primary lymph sacs, further expansion leads to the formation of a primary lymphatic vascular plexus that, through subsequent remodeling and maturation, will provide a hierarchical network of lymphatic capillaries and lymphatic collecting vessels.1,3 During this vessel specification, maturation of collecting vessels is accompanied by the downregulation of lymphatic marker molecules such as LYVE-1, the acquisition of partial smooth muscle cell coverage, and the formation of intraluminal valves. [4][5][6] Lymphatic valves are essential components that ensure unidirectional lymph flow. They develop from E15.5 to early postnatal d...

The transcription factor E2F1 has a key function during S phase progression and apoptosis. It has been well-demonstrated that the apoptotic function of E2F1 involves its ability to transactivate pro-apoptotic target genes. Alternative splicing of pre-mRNAs also has an important function in the regulation of apoptosis. In this study, we identify the splicing factor SC35, a member of the SerRich Arg (SR) proteins family, as a new transcriptional target of E2F1. We demonstrate that E2F1 requires SC35 to switch the alternative splicing profile of various apoptotic genes such as c-flip, caspases-8 and -9 and Bcl-x, towards the expression of pro-apoptotic splice variants. Finally, we provide evidence that E2F1 upregulates SC35 in response to DNA-damaging agents and show that SC35 is required for apoptosis in response to these drugs. Taken together, these results demonstrate that E2F1 controls pre-mRNA processing events to induce apoptosis and identify the SC35 SR protein as a key direct E2F1-target in this setting. Cell Death and Differentiation (2008) 15, 1815-1823 doi:10.1038/cdd.2008 published online 19 September 2008 Pre-mRNA splicing is an essential step for the expression of most genes in higher eukaryotic cells. This process has emerged as an important mechanism of genetic diversity as about 74% of human genes undergo alternative splicing, leading to the production of various protein isoforms. 1 SC35 belongs to the serine/arginine-rich (SR) protein family, one of the most important class of splicing regulators. Members of the SR family have a modular structure consisting of one or two copies of an N-terminal RRM (RNA-recognition motif) followed by a C terminus rich in serine and arginine residues known as the RS domain. They act at multiple steps of spliceosome assembly and participate in both constitutive and alternative splicing. 2 Together with most of the other splicing factors, SR proteins localize to nuclear subregions termed nuclear speckles. 3 Extensive serine phosphorylation of the RS domain has an important function in the regulation of both the localization and the activities of SR proteins. 4 Although the splicing functions of SR proteins have been well documented in vitro, their roles and physiological targets in vivo are less well known. However, based on gene targeting experiments demonstrating that they are required for cell viability and/or animal development, SR proteins undoubtedly control essential biological functions.Apoptosis is one of the cellular processes in which alternative splicing has an important regulatory function. Indeed, a remarkable number of transcripts that encode proteins involved in the apoptotic pathway are subjected to alternative splicing. This usually drives the expression of proteins with opposite functions, either pro-or anti-apoptotic. 5 Interestingly, changes in SR protein phosphorylation have been observed upon apoptotic stimulation following activation of the Fas receptor. 6 In addition, in vitro overexpression experiments have suggested a potential role for ...