Building discontinuous liver sinusoidal vessels (original) (raw)
Sinusoids are fenestrated discontinuous capillaries found in the liver, spleen, and bone marrow. These three organs all have hematopoietic roles at various stages of development; therefore, sinusoids are well adapted to facilitate the movement of hematopoietic progenitor or stem cells and mature blood cells between the circulatory system and these tissues. During embryonic development, erythro-myeloid progenitors derived from the yolk sac or definitive hematopoietic stem cells generated in the yolk sac, placenta, umbilical cord, and/or aorta-gonad-mesonephros (AGM) region migrate through the circulatory system and populate the developing fetal liver (5, 6). Within the liver, hematopoietic stem cells proliferate and egress to colonize the spleen, thymus, and bone marrow, which becomes the primary site of hematopoiesis after birth (7). The fetal liver is also a key site of definitive erythropoiesis, and mature red blood cells begin exiting the liver and entering the circulatory system by E12.5 to oxygenate the rapidly growing embryo (8). Monocytes in the fetal liver also give rise to a subset of tissue macrophages that require diaphragmed fenestrations in sinusoidal endothelium in order to exit the fetal liver and populate tissues throughout the body (9). Interestingly, sinusoidal morphology changes during rodent development: by late gestation, junctional proteins are downregulated and fenestrations lose diaphragms and increase in size and number (10). The implications of these alterations in sinusoidal morphology on fetal liver function are poorly understood.
In this issue, Géraud et al. provide important insight into the transcriptional regulation of discontinuous liver sinusoid morphology (11). This group had previously determined that the transcription factor GATA4 is enriched in rat liver sinusoidal endothelial cells (LSECs) compared with rat lung microvascular endothelial cells (LMECs) (12). Géraud and colleagues have now employed transgenic mouse lines to define the function of GATA4 in murine LSECs (11). Specifically, they developed a line of mice harboring Cre driven by the stabilin 2 (Stab2) promoter (Stab2-Cre), which is expressed in mature LSECs (11), and exploited an existing Lyve1-Cre line (13), which is active in embryonic LSECs (14). Deletion of a floxed Gata4 allele in either line resulted in mutant embryos with hypoplastic livers, anemia, and prenatal lethality (11). Unlike the liver, other organs were not obviously affected by Gata4 deletion in either the Stab2-Cre or Lyve1-Cre line (11). GATA4-deficient livers exhibited microvessels with features of continuous capillaries rather than those of discontinuous sinusoids seen in WT livers (11). In particular, upregulation of endothelial cell junction proteins (CD31 and VE-cadherin) on LSECs was observed by immunostaining, and a more robust basement membrane was detected by electron microscopy and by immunostaining for extracellular matrix components (11). No effect of Gata4 deletion on LSEC fenestrae was reported, but it would be interesting to know whether this transcription factor affects the number or size of fenestrae or the presence of a diaphragm on the pores. As LSECs are highly endocytic (15), it would also be informative to know whether GATA4 impacts endocytic vesicle density on embryonic LSECs.
Géraud et al. also exploited primary rat LSECs and LMECs to establish a transcriptional profile for discontinuous and continuous capillaries, respectively (11). As rodent LSECs rapidly de-differentiate in culture, precluding knockdown or overexpression studies, Géraud and colleagues manipulated GATA4 expression in human umbilical vein endothelial cells (HUVECs), which are transcriptionally similar to continuous LMECs (11). Upon GATA4 overexpression, HUVECs acquired a transcriptional profile more similar to discontinuous LSECs, indicating that GATA4 can promote a discontinuous LSEC gene program across species and in different endothelial cell types (11).
Finally, Géraud et al. reported that the emergence of continuous capillary features in Gata4 mutant livers preceded liver hypoplasia and anemia (11). Erythro-myeloid progenitor cells that typically colonize the fetal liver from the circulation around E10.5 were markedly reduced in livers but elevated in the blood of Gata4 mutants at E11.25 (11). Hematopoietic stem cell populations followed a similar aberrant localization pattern in the liver and blood of E13.25 Gata4 mutants (11). These progenitor and hematopoietic stem cells from mutant embryos appeared to have no intrinsic defects, as these populations expanded and differentiated in transplantation experiments and in vitro (11). Instead, Géraud and colleagues propose that the continuous capillaries that aberrantly displace discontinuous sinusoids in Gata4 mutant livers prevent progenitor and hematopoietic stem cells from colonizing the mutant liver, where they would subsequently undergo expansion and differentiation (Figure 1).
Gata4 deletion alters liver sinusoid morphology. Genetic deletion of the transcription factor Gata4 from liver sinusoidal endothelial cells (LSECs) causes upregulation of endothelial cell junction proteins and robust deposition of basement membrane proteins that prevent circulating hematopoietic progenitor or stem cells from colonizing the fetal liver (11). The consequences of this transition from discontinuous sinusoidal to continuous capillary morphology are liver hypoplasia, anemia, and lethality of Gata4 mutant embryos (11).