APbx1-dependent genetic and transcriptional network regulates spleen ontogeny

Abstract

The genetic control of cell fate specification, morphogenesis and expansion of the spleen, a crucial lymphoid organ, is poorly understood. Recent studies of mutant mice implicate various transcription factors in spleen development,but the hierarchical relationships between these factors have not been explored. In this report, we establish a genetic network that regulates spleen ontogeny, by analyzing asplenic mice mutant for the transcription factors Pbx1, Hox11 (Tlx1), Nkx3.2 (Bapx1) and Pod1 (capsulin, Tcf21). We show that Hox11 and Nkx2.5, among the earliest known markers for splenic progenitor cells, are absent in the splenic anlage of Pbx1homozygous mutant (-/-) embryos, implicating the TALE homeoprotein Pbx1 in splenic cell specification. Pbx1 and Hox11genetically interact in spleen formation and loss of either is associated with a similar reduction of progenitor cell proliferation and failed expansion of the splenic anlage. Chromatin immunoprecipitation assays show that Pbx1 binds to the Hox11 promoter in spleen mesenchymal cells, which co-express Pbx1 and Hox11. Furthermore, Hox11 binds its own promoter in vivo and acts synergistically with TALE proteins to activate transcription,supporting its role in an auto-regulatory circuit. These studies establish a Pbx1-Hox11-dependent genetic and transcriptional pathway in spleen ontogeny. Additionally, we demonstrate that while Nkx3.2 and Pod1 control spleen development via separate pathways, Pbx1genetically regulates key players in both pathways, and thus emerges as a central hierarchical co-regulator in spleen genesis.

PBX proteins: much more than Hox cofactors

“…Indeed, although Pbx is not necessary for the initiation step of organogenesis, Pbx is required in several other steps of organ development , Brendolan et al, 2005, DiMartino et al, 2001, Schnabel et al, 2003, Manley et al, 2004, Dutta et al, 2001. In mice where the Pdx1 gene lacks the PID-encoding portion, Dutta and colleagues have shown the promotion of a complete pancreatic genetic program.…”

Identification of novel genes and transcription factors involved in spleen, thymus and immunological development and function

“…Twenty-five of the tags expressed at higher levels in the developing spleen unambiguously mapped to transcription regulators using GO analysis. This list included Pbx1, 53 Tcf21 (capsulin) 1 and Fus (Pigpen), 53 which have been shown to Table 4). As observed in the spleen, many of these had extremely high significance factors including 2010001M09Rik protein (129.7), hypothetical protein LOC66167 (98.9) and RIKEN cDNA D930015E06 (94.6).…”

Functional characterization of a minimal sequence essential for the expression of human TLX2 gene

“…Despite a tissue-specific regulation mediated by different PBX species has been suggested, probably due to an in vivo differential recruitment of transcriptional co-factors, the DNAbinding properties of Pbx proteins appear similar in vitro (22). In fact, after demonstrating the binding of PBX2, but not PBX1 and PBX3, to TLX1 promoter in leukaemia cell lines (14), the analysis of the spleen organogenesis in mouse embryos has displayed the capability of TLX1 to act synergistically with PBX1 to bind its own promoter, thus auto-inducing its expression (23). This confirms that different PBX family members can act in the transactivation of the same target gene depending on the specific tissue or developmental stage and strengthens the hypothesis of overlapping functions of Pbx family members, at least in specific organ systems and in tissues showing similar expression patterns (19).…”