PBX proteins: much more than Hox cofactors.

Pre-B cell leukaemia transcription factors (PBXs) were originally identified as Hox cofactors, acting within transcriptional regulation complexes to regulate genetic programs during development. Increasing amount of evidence revealed that PBX function is not restricted to a partnership with Hox or homeodomain proteins. Indeed, PBXs are expressed throughout murine embryonic development and are involved in several developmental pathways including Hox-independent mechanisms. This review summarizes what is known about PBX partnerships and proposes to position PBXs as central developmental factors whose role consists of integrating transduction signals, in order to regulate gene expression programs during development.

Identification of a new type of PBX1 partner that contains zinc finger motifs and inhibits the binding of HOXA9-PBX1 to DNA.

PBX1 belongs to the TALE-class of homeodomain protein and has a wide functional diversity during development. Indeed, PBX1 is required for haematopoiesis as well as for multiple developmental processes such as skeletal patterning and organogenesis. It has furthermore been shown that PBX1 functions as a HOX cofactor during development. More recent data suggest that PBX1 may act even more broadly by modulating the activity of non-homeodomain transcription factors. To better understand molecular mechanisms triggered by PBX1 during female genital tract development, we searched for additional PBX1 partners that might be involved in this process. Using a two hybrid screen, we identified a new PBX1 interacting protein containing several zinc finger motifs that we called ZFPIP for Zinc Finger PBX1 Interacting Protein. We demonstrated that ZFPIP is expressed in embryonic female genital tract but also in other PBX1 expression domains such as the developing head and the limb buds. We further showed that ZFPIP is able to bind physically and in vivo to PBX1 and moreover, that it prevents the binding of HOXA9/PBX complexes to their consensus DNA site. We suggest that ZFPIP is a new type of PBX1 partner that could participate in PBX1 function during several developmental pathways.

PBX1 intracellular localization is independent of MEIS1 in epithelial cells of the developing female genital tract.

While studies have highlighted the role of HOXA9-13 and PBX1 homeobox genes during the development of the female genital tract, the molecular mechanisms triggered by these genes are incompletely elucidated. In several developmental pathways, PBX1 binds to MEINOX family members in the cytoplasm to be imported into the nucleus where they associate with HOX proteins to form a higher complex that modulates gene expression. This concept has been challenged by a recent report showing that in some cell cultures, PBX1 nuclear localization might be regulated independently of MEINOX proteins (Kilstrup-Nielsen et al., 2003). Our work gives the first illustration of this alternative mechanism in an organogenesis process. Indeed, we show that PBX1 is mostly cytoplasmic in epithelial endometrial cells of the developing female genital tract despite the nuclear localization of MEIS1. We thus provide evidence for a control of PBX1 intracellular distribution which is independent of MEINOX proteins, but is cell cycle correlated.

A conserved non-homeodomain Hoxa9 isoform interacting with CBP is co-expressed with the 'typical' Hoxa9 protein during embryogenesis.

Various Hox genes are known to produce alternative transcripts encoding different isoforms whose physiological relevance during development is not yet understood. In this work, we analysed two different Hoxa9 mRNAs encoding a full-length protein (Hoxa9) or a protein lacking the homeodomain (Hoxa9T). First, we demonstrated that these transcripts are conserved from birds to mammals. We then showed that both transcripts are present throughout embryogenesis and that Hoxa9T transcript is particularly abundant in embryonic genital tract, kidney, forelimb and tail. We further found that both isoforms are able to interact with CBP, suggesting a competition between Hoxa9 and Hoxa9T with this protein.