SALL1 Modulates CBX4 Stability, Nuclear Bodies, and Regulation of Target Genes.

Development is orchestrated through a complex interplay of multiple transcription factors. The comprehension of this interplay will help us to understand developmental processes. Here we analyze the relationship between two key transcription factors: CBX4, a member of the Polycomb Repressive Complex 1 (PRC1), and SALL1, a member of the Spalt-like family with important roles in embryogenesis and limb development. Both proteins localize to nuclear bodies and are modified by the small ubiquitin-like modifier (SUMO). Our results show that CBX4 and SALL1 interact in the nucleoplasm and that increased SALL1 expression reduces ubiquitination of CBX4, enhancing its stability. This is accompanied by an increase in the number and size of CBX4-containing Polycomb bodies, and by a greater repression of CBX4 target genes. Thus, our findings uncover a new way of SALL1-mediated regulation of Polycomb bodies through modulation of CBX4 stability, with consequences in the regulation of its target genes, which could have an impact in cell differentiation and development.

The role of SUMOylation during development.

During the development of multicellular organisms, transcriptional regulation plays an important role in the control of cell growth, differentiation and morphogenesis. SUMOylation is a reversible post-translational process involved in transcriptional regulation through the modification of transcription factors and through chromatin remodelling (either modifying chromatin remodelers or acting as a 'molecular glue' by promoting recruitment of chromatin regulators). SUMO modification results in changes in the activity, stability, interactions or localization of its substrates, which affects cellular processes such as cell cycle progression, DNA maintenance and repair or nucleocytoplasmic transport. This review focuses on the role of SUMO machinery and the modification of target proteins during embryonic development and organogenesis of animals, from invertebrates to mammals.

Truncated SALL1 Impedes Primary Cilia Function in Townes-Brocks Syndrome.

Townes-Brocks syndrome (TBS) is characterized by a spectrum of malformations in the digits, ears, and kidneys. These anomalies overlap those seen in a growing number of ciliopathies, which are genetic syndromes linked to defects in the formation or function of the primary cilia. TBS is caused by mutations in the gene encoding the transcriptional repressor SALL1 and is associated with the presence of a truncated protein that localizes to the cytoplasm. Here, we provide evidence that SALL1 mutations might cause TBS by means beyond its transcriptional capacity. By using proximity proteomics, we show that truncated SALL1 interacts with factors related to cilia function, including the negative regulators of ciliogenesis CCP110 and CEP97. This most likely contributes to more frequent cilia formation in TBS-derived fibroblasts, as well as in a CRISPR/Cas9-generated model cell line and in TBS-modeled mouse embryonic fibroblasts, than in wild-type controls. Furthermore, TBS-like cells show changes in cilia length and disassembly rates in combination with aberrant SHH signaling transduction. These findings support the hypothesis that aberrations in primary cilia and SHH signaling are contributing factors in TBS phenotypes, representing a paradigm shift in understanding TBS etiology. These results open possibilities for the treatment of TBS.

MiR-24 induces chemotherapy resistance and hypoxic advantage in breast cancer.

Breast cancer remains one of the leading causes of cancer mortality among women. It has been proved that the onset of cancer depends on a very small pool of tumor cells with a phenotype similar to that of normal adult stem cells. Cancer stem cells (CSC) possess self-renewal and multilineage differentiation potential as well as a robust ability to sustain tumorigenesis. Evidence suggests that CSCs contribute to chemotherapy resistance and to survival under hypoxic conditions. Interestingly, hypoxia in turn regulates self-renewal in CSCs and these effects may be primarily mediated by hypoxic inducible factors (HIFs). Recently, microRNAs (miRNAs) have emerged as critical players in the maintenance of pluripotency and self-renewal in normal and cancer stem cells. Here, we demonstrate that miR-24 is upregulated in breast CSCs and that its overexpression increases the number of mammospheres and the expression of stem cell markers. MiR-24 also induces apoptosis resistance through the regulation of BimL expression. Moreover, we identify a new miR-24 target, FIH1, which promotes HIFα degradation: miR-24 increases under hypoxic conditions, causing downregulation of FIH1 and upregulation of HIF1α. In conclusion, miR-24 hampers chemotherapy-induced apoptosis in breast CSCs and increases cell resistance to hypoxic conditions through an FIH1-HIFα pathway.

Role of histidine 148 in stability and dynamics of a highly fluorescent GFP variant.

The armory of GFP mutants available to biochemists and molecular biologists is huge. Design and selection of mutants are usually driven by tailored spectroscopic properties, but some key aspects of stability, folding and dynamics of selected GFP variants still need to be elucidated. We have prepared, expressed and characterized three H148 mutants of the highly fluorescent variant GFPmut2. H148 is known to be involved in the H-bonding network surrounding the chromophore, and all the three mutants, H148G, H148R and H148K, show increased pKa values of the chromophore. Only H148G GFPmut2 (Mut2G) gave good expression and purification yields, indicating that position 148 is critical for efficient folding in vivo. The chemical denaturation of Mut2G was monitored by fluorescence emission, absorbance and far-UV circular dichroism spectroscopy. The mutation has little effect on the spectroscopic properties of the protein and on its stability in solution. However, the unfolding kinetics of the protein encapsulated in wet nanoporous silica gels, a system that allows to stabilize conformations that are poorly or only transiently populated in solution, indicate that the unfolding pathway of Mut2G is markedly different from the parent molecule. In particular, encapsulation allowed to identify an unfolding intermediate that retains a native-like secondary structure despite a destructured chromophore environment. Thus, H148 is a critical residue not only for the chromophoric and photodynamic properties, but also for the correct folding of GFP, and its substitution has great impact on expression yields and stability of the mature protein.