SCL: From the origin of hematopoiesis to stem cells and leukemia Eric Lécuyer, Trang Hoang  Experimental Hematology  Volume 32, Issue 1, Pages 11-24 (January 2004) DOI: 10.1016/j.exphem.2003.10.010

Figure 1 Functions played by SCL during ontogeny of the hematopoietic and vascular systems and during adult hematopoiesis. (A): SCL functions during the establishment of the hematopoietic system. Mesodermally derived hematopoietic cells appear in both extra-embryonic (YS blood islands) and intra-embryonic (AGM region) hemogenic sites. In both locations, hematopoietic cells are thought to arise from a precursor (hemangioblast or hemogenic endothelium) that can also generate endothelial cells. Primitive hematopoietic cells, as well as an initial wave of definitive hematopoietic progenitors [33], are seemingly first produced in the YS blood islands, whereas the first detectable adult repopulating HSC are generated in the AGM region (reviewed in [42]). Cells from both of these regions are thought to contribute to the seeding of the fetal liver, the main site of definitive hematopoiesis in the fetus, although this remains a matter of considerable debate. SCL−/− embryos exhibit a complete absence of both primitive and definitive hematopoietic cells (bold lines) [12–15]. Furthermore, hematopoietic-specific rescue of SCL−/− embryos has revealed that SCL is also required for angiogenic remodeling of the primary capillary plexus in the YS [49]. In tissue culture, SCL−/− ES cells are unable to generate BL-CFCs [52,53], the in vitro equivalent to the hemangioblast, although this remains to be clarified in vivo. (B): SCL expression and function during definitive hematopoiesis. Within the definitive hematopoietic hierarchy, SCL is expressed in HSC, multipotent progenitors, as well as cells of the erythroid, megakaryocytic, and mastocytic lineages (gray shading). Conditional knockout studies (bold lines) have shown that SCL is essential for the differentiation down the erythroid and megakaryocytic pathways, whereas its requirement in the mast cell lineage remains to be characterized (question marks). Transgenic mice ectopically expressing SCL and LMO2 in the lymphoid lineage exhibit a block in T-cell differentiation at the transition of DN to DP cells and develop T cell tumors [68,121,125,127], similar to the phenotype observed in E2A−/− mice [159]. DN, double negative; DP, double positive; SP, single positive cells for CD4/CD8 expression. Experimental Hematology 2004 32, 11-24DOI: (10.1016/j.exphem.2003.10.010)

Figure 2 Schematic representation of the SCL protein and SCL-containing complexes. (A): In addition to the distinctive bHLH region, the SCL protein harbors a putative proline-rich transactivation domain (AD) between positions 117 and 175 [106]. The positively charged basic (b) region is required for DNA binding, while the HLH motif mediates important protein-protein interactions. The production of different N-terminally truncated isoforms of SCL (arrows) is regulated by an upstream open reading frame (uORF) [110], and SCL can also be phosphorylated (P) on serine residues in response to different signaling pathways [99–105]. (B): A multifactorial complex containing SCL, E2A, LMO2, Ldb-1, and GATA-1 was first identified using a binding site amplification procedure with MEL cell nuclear extracts [146]. Within this complex, LMO2 is thought to act as a bridge between DNA-bound SCL/E2A heterodimers and GATA-1, as these factors were found to associate with a composite Ebox-GATA motif. (C): Using a similar approach with extracts of leukemic T cells from LMO2-transgenic mice, a distinct complex was identified that could bind a bipartite Ebox-Ebox motif [148]. In this context, Ldb-1 homodimerization is thought to exert a bridging function between distinct SCL/E2A heterodimers. (D): In T-ALL cell lines, SCL and LMO1/2 have been shown to activate transcription of the retinaldehyde dehydrogenase 2 (RALDH2) gene through their recruitment by promoter-bound GATA-3 [149]. (E): SCL can also inhibit the transcription of target genes regulated by E2A/HEB homo/heterodimers during lymphoid cell differentiation, such as the pre-Tα gene [168]. (F): A complex containing SCL, E2A, LMO2, Ldb-1, and GATA-1/-2 has been shown to activate the c-kit promoter through functional interaction with promoter-bound Sp1 [97]. It remains to be determined whether additional cis elements help to recruit SCL and its partners to the c-kit promoter, as indicated by dotted arrows. The Rb protein is thought to contribute to c-kit promoter repression during erythroid cell differentiation by associating with SCL and its partners [183], although the mechanism of repression remains unclear. Experimental Hematology 2004 32, 11-24DOI: (10.1016/j.exphem.2003.10.010)