Pax6 localizes to chromatin-rich territories and displays a slow nuclear mobility altered by disease mutations

Ocular disease-associated mutations diminish the mitotic chromosome retention ability of PAX6

Transcription factors play a key role in maintaining cell identity. One mechanism of such cell memory after multiple rounds of cell division cycles is through persistent mitotic chromosome binding, although how individual transcription factors achieve mitotic chromosome retention is not completely understood. Here we show that PAX6, a lineage-determining transcription factor, coats mitotic chromosomes. Using deletion and point mutants associated with human ocular diseases in live-cell imaging analysis, we identified two regions, MCR-D1 and MCR-D2, that were responsible for mitotic chromosome retention of PAX6. We also identified three nuclear localization signals (NLSs) that contributed to mitotic chromosome retention independent of their nuclear import functions. Full mitotic chromosome retention required the presence of DNA-binding domains as well as NLSs within MCR-Ds. Furthermore, disease-associated mutations and NLS mutations changed the distribution of intrinsically disordered regions (IDRs) in PAX6. Our findings not only identify PAX6 as a novel mitotic chromosome retention factor but also demonstrate that the mechanism of mitotic chromosome retention involves sequence-specific DNA binding, NLSs, and molecular conformation determined by IDRs. These findings link mitotic chromosome retention with PAX6-related pathogenesis and imply similar mechanisms for other lineage-determining factors in the PAX family.

Enhancer function regulated by combinations of transcription factors and cofactors

Regulation of the expression of diverse genes is essential for making possible the complexity of higher organisms, and the temporal and spatial regulation of gene expression allows for the alteration of cell types and growth patterns. A critical component of this regulation is the DNA sequence-specific binding of transcription factors (TFs). However, most TFs do not independently participate in gene transcriptional regulation, because they lack an effector function. Instead, TFs are thought to work by recruiting cofactors, including Mediator complex (Mediator), chromatin-remodeling complexes (CRCs), and histone-modifying complexes (HMCs). Mediator associates with the majority of transcribed genes and acts as an integrator of multiple signals. On the other hand, CRCs and HMCs are selectively recruited by TFs. Although all the pairings between TFs and CRCs or HMCs are not fully known, there are a growing number of established TF-CRC and TF-HMC combinations. In this review, we focused on the most important of these pairings and discuss how they control gene expression.

The BAF complex interacts with Pax6 in adult neural progenitors to establish a neurogenic cross-regulatory transcriptional network

Numerous transcriptional regulators of neurogenesis have been identified in the developing and adult brain, but how neurogenic fate is programmed at the epigenetic level remains poorly defined. Here, we report that the transcription factor Pax6 directly interacts with the Brg1-containing BAF complex in adult neural progenitors. Deletion of either Brg1 or Pax6 in the subependymal zone (SEZ) causes the progeny of adult neural stem cells to convert to the ependymal lineage within the SEZ while migrating neuroblasts convert to different glial lineages en route to or in the olfactory bulb (OB). Genome-wide analyses reveal that the majority of genes downregulated in the Brg1 null SEZ and OB contain Pax6 binding sites and are also downregulated in Pax6 null SEZ and OB. Downstream of the Pax6-BAF complex, we find that Sox11, Nfib, and Pou3f4 form a transcriptional cross-regulatory network that drives neurogenesis and can convert postnatal glia into neurons. Taken together, elements of our work identify a tripartite effector network activated by Pax6-BAF that programs neuronal fate.

Identification of two independent nucleosome-binding domains in the transcriptional co-activator SPBP

Transcriptional regulation requires co-ordinated action of transcription factors, co-activator complexes and general transcription factors to access specific loci in the dense chromatin structure. In the present study we demonstrate that the transcriptional co-regulator SPBP [stromelysin-1 PDGF (platelet-derived growth factor)-responsive element binding protein] contains two independent chromatin-binding domains, the SPBP-(1551-1666) region and the C-terminal extended PHD [ePHD/ADD (extended plant homeodomain/ATRX-DNMT3-DNMT3L)] domain. The region 1551-1666 is a novel core nucleosome-interaction domain located adjacent to the AT-hook motif in the DNA-binding domain. This novel nucleosome-binding region is critically important for proper localization of SPBP in the cell nucleus. The ePHD/ADD domain associates with nucleosomes in a histone tail-dependent manner, and has significant impact on the dynamic interaction between SPBP and chromatin. Furthermore, SPBP and its homologue RAI1 (retinoic-acid-inducible protein 1), are strongly enriched on chromatin in interphase HeLa cells, and both proteins display low nuclear mobility. RAI1 contains a region with homology to the novel nucleosome-binding region SPBP-(1551-1666) and an ePHD/ADD domain with ability to bind nucleosomes. These results indicate that the transcriptional co-regulator SPBP and its homologue RAI1 implicated in Smith-Magenis syndrome and Potocki-Lupski syndrome both belong to the expanding family of chromatin-binding proteins containing several domains involved in specific chromatin interactions.

Dominant-negative mechanism of leukemogenic PAX5 fusions

PAX5 encodes a master regulator of B-cell development. It fuses to other genes associated with acute lymphoblastoid leukemia (ALL). These fusion products are potent dominant-negative (DN) inhibitors of wild-type PAX5, resulting in a blockade of B-cell differentiation. Here, we show that multimerization of PAX5 DNA-binding domain (DBD) is necessary and sufficient to cause extremely stable chromatin binding and DN activity. ALL-associated PAX5-C20S results from fusion of the N-terminal region of PAX5, including its paired DBD, to the C-terminus of C20orf112, a protein of unknown function. We report that PAX5-C20S is a tetramer, which interacts extraordinarily stably with chromatin as determined by Fluorescence Recovery After Photobleaching in living cells. Tetramerization, stable chromatin binding and DN activity all require a putative five-turn amphipathic α-helix at the C-terminus of C20orf112, and does not require potential corepressor binding peptides elsewhere in the sequence. In vitro, the monomeric PAX5 DBD and PAX5-C20S binds a PAX5-binding site with equal affinity when it is at the center of an oligonucleotide too short to bind to more than one PAX5 DBD. But, PAX5-C20S binds the same sequence with 10-fold higher affinity than the monomeric PAX5 DBD when it is in a long DNA molecule. We suggest that the increased affinity results from interactions of one or more of the additional DBDs with neighboring non-specific sites in a long DNA molecule, and that this can account for the increased stability of PAX5-C20S chromatin binding compared with wild-type PAX5, resulting in DN activity by competition for binding to PAX5-target sites. Consistent with this model, the ALL-associated PAX5 fused to ETV6 or the multimerization domain of ETV6 SAM results in stable chromatin binding and DN activity. In addition, PAX5 DBD fused to artificial dimerization, trimerization and tetramerization domains results in parallel increases in the stability of chromatin binding and DN activity. Our studies suggest that oncogenic fusion proteins that retain the DBD of the transcription factor (TF) and the multimerization sequence of the partner protein can act in a DN manner by multimerizing and binding avidly to gene targets, preventing the normal TF from binding and inducing expression of its target genes. Inhibition of this multimeriztion may provide a novel therapeutic approach for cancers with this or similar fusion proteins.

Pax6 represses androgen receptor-mediated transactivation by inhibiting recruitment of the coactivator SPBP

The androgen receptor (AR) has a central role in development and maintenance of the male reproductive system and in the etiology of prostate cancer. The transcription factor Pax6 has recently been reported to act as a repressor of AR and to be hypermethylated in prostate cancer cells. SPBP is a transcriptional regulator that previously has been shown to enhance the activity of Pax6. In this study we have identified SPBP to act as a transcriptional coactivator of AR. We also show that Pax6 inhibits SPBP-mediated enhancement of AR activity on the AR target gene probasin promoter, a repression that was partly reversed by increased expression of SPBP. Enhanced expression of Pax6 reduced the amount of SPBP associated with the probasin promoter when assayed by ChIP in HeLa cells. We mapped the interaction between both AR and SPBP, and AR and Pax6 to the DNA-binding domains of the involved proteins. Further binding studies revealed that Pax6 and SPBP compete for binding to AR. These results suggest that Pax6 represses AR activity by displacing and/or inhibiting recruitment of coactivators to AR target promoters. Understanding the mechanism for inhibition of AR coactivators can give rise to molecular targeted drugs for treatment of prostate cancer.

Functional analysis of missense mutations G36A and G51A in PAX6, and PAX6(5a) causing ocular anomalies

The PAX6 has been described a "master regulator of eye development". A specific ratio of PAX6, and its alternatively spliced isoform, PAX6(5a), has also been observed essential for optimal function. Mutations into PAX6 lead to a number of ocular, and neuronal defects of variable penetrance and expressivity but the mechanism is either poorly understood or underrepresented. This report describes analysis of functions of two missense mutations, G36A, and G51A, causing optic-nerve hypoplasia and optic-disc coloboma in humans, respectively. Mutations were created by site-directed mutagenesis. Products were detected by in-vitro translation and transient transfection to the cultured NIH-3T3 cells. Their DNA-binding, and transcriptional activation properties were analysed through electrophoretic mobility shift assay and luciferase reporter assay, respectively. Mutations induced changes in conformation and secondary structure of PAX6, and PAX6(5a) not only restrict to specific site of mutation in the paired-domain but extend to homeodomain, and transactivation domain. The PAX6-G36A showed reduced binding to PAX6-consensus binding sequence and PAX6(5a)-consensus binding sequence but its binding affinity to homeodomain binding sequence was unaffected. It showed significantly higher transactivation potential through PAX6-consensus binding sequence but reduced activity with PAX6(5a)-consensus binding sequence and homeodomain binding sequence containing luciferase reporters. The PAX6(5a)-G36A showed enhanced transactivation potential with PAX6-consensus binding sequence, PAX6(5a)-consensus binding sequence, and homeodomain binding sequence containing luciferase reporters. The binding affinity of PAX6(5a)-G36A was significantly higher to PAX6-consensus binding sequence, and PAX6(5a)-consensus binding sequence as compared to PAX6(5a) but remains unaffected to homeodomain binding sequence. The enhanced binding affinity was observed by PAX6-G51A to PAX6-consensus binding sequence, PAX6(5a)-consensus binding sequence, and homeodomain binding sequence. The transactivation potential was observed higher with PAX6-consensus binding sequence but significant reduction was evident with PAX6(5a)-consensus binding sequence, and homeodomain binding sequence containing luciferase reporters. The lower binding affinity to PAX6-consensus binding sequence and PAX6(5a)-consensus binding sequence was observed by PAX6(5a)-G51A but loss of binding affinity was detected to homeodomain binding sequence. However, PAX6(5a)-G51A showed significantly higher transactivation with PAX6-consensus binding sequence, PAX6(5a)-consensus binding sequence, and homeodomain binding sequence containing luciferase reporters. With the eye-specific α-A-crystallin promoter, PAX6-G36A and PAX6-G51A mutants were found to have higher ability to transactivate whereas PAX6(5a)-G36A and PAX6(5a)-G51A have lower transactivation potential compared to their respective wild type forms. Thus, variable DNA-binding and transactivation properties of the mutants with different PAX6-binding sequences provide an insight towards their variable penetrance and expressivity.