A gene regulatory network anchored by LIM homeobox 1 for embryonic head development.

Development of the embryonic head is driven by the activity of gene regulatory networks of transcription factors. LHX1 is a homeobox transcription factor that plays an essential role in the formation of the embryonic head. The loss of LHX1 function results in anterior truncation of the embryo caused by the disruption of morphogenetic movement of tissue precursors and the dysregulation of WNT signaling activity. Profiling the gene expression pattern in the Lhx1 mutant embryo revealed that tissues in anterior germ layers acquire posterior tissue characteristics, suggesting LHX1 activity is required for the allocation and patterning of head precursor tissues. Here, we used LHX1 as an entry point to delineate its transcriptional targets and interactors and construct a LHX1-anchored gene regulatory network. Using a gain-of-function approach, we identified genes that immediately respond to Lhx1 activation. Meta-analysis of the datasets of LHX1-responsive genes and genes expressed in the anterior tissues of mouse embryos at head-fold stage, in conjunction with published Xenopus embryonic LHX1 (Xlim1) ChIP-seq data, has pinpointed the putative transcriptional targets of LHX1 and an array of genetic determinants functioning together in the formation of the mouse embryonic head.

Programming of Multicellular Patterning with Mechano-Chemically Microstructured Cell Niches.

Multicellular patterning of stem-cell-derived tissue models is commonly achieved via self-organizing activities triggered by exogenous morphogenetic stimuli. However, such tissue models are prone to stochastic behavior, limiting the reproducibility of cellular composition and forming non-physiological architectures. To enhance multicellular patterning in stem cell-derived tissues, a method for creating complex tissue microenvironments endowed with programmable multimodal mechano-chemical cues, including conjugated peptides, proteins, morphogens, and Young's moduli defined over a range of stiffnesses is developed. The ability of these cues to spatially guide tissue patterning processes, including mechanosensing and the biochemically driven differentiation of selected cell types, is demonstrated. By rationally designing niches, the authors engineered a bone-fat assembly from stromal mesenchyme cells and regionalized germ layer tissues from pluripotent stem cells. Through defined niche-material interactions, mechano-chemically microstructured niches enable the spatial programming of tissue patterning processes. Mechano-chemically microstructured cell niches thereby offer an entry point for enhancing the organization and composition of engineered tissues, potentiating structures that better recapitulate their native counterparts.

The RNA-helicase DDX21 upregulates CEP55 expression and promotes neuroblastoma.

Approximately 25% of human neuroblastoma is caused by amplification of the MYCN oncogene, which leads to overexpression of N-Myc oncoprotein. The survival rate for this patient subtype is <50%. Here, we show that N-Myc protein bound to the DEAD-box RNA helicase DDX21 gene promoter and upregulated DDX21 mRNA and protein expression. Genome-wide differential gene expression studies identified centrosomal protein CEP55 as one of the genes most dramatically downregulated after DDX21 knockdown in MYCN-amplified neuroblastoma cells. Knocking down DDX21 or CEP55 reduced neuroblastoma cell cytoskeleton stability and cell proliferation and all but abolished clonogenic capacity. Importantly, DDX21 knockdown initially induced tumor regression in neuroblastoma-bearing mice and suppressed tumor progression. In human neuroblastoma tissues, a high level of DDX21 expression correlated with a high level of N-Myc expression and with CEP55 expression, and independently predicted poor patient prognosis. Taken together, our data show that DDX21 induces CEP55 expression, MYCN-amplified neuroblastoma cell proliferation, and tumorigenesis, and that DDX21 and CEP55 are valid therapeutic targets for the treatment of MYCN-amplified neuroblastoma.

TWIST1 Homodimers and Heterodimers Orchestrate Lineage-Specific Differentiation.

The extensive array of basic helix-loop-helix (bHLH) transcription factors and their combinations as dimers underpin the diversity of molecular function required for cell type specification during embryogenesis. The bHLH factor TWIST1 plays pleiotropic roles during development. However, which combinations of TWIST1 dimers are involved and what impact each dimer imposes on the gene regulation network controlled by TWIST1 remain elusive. In this work, proteomic profiling of human TWIST1-expressing cell lines and transcriptome analysis of mouse cranial mesenchyme have revealed that TWIST1 homodimers and heterodimers with TCF3, TCF4, and TCF12 E-proteins are the predominant dimer combinations. Disease-causing mutations in TWIST1 can impact dimer formation or shift the balance of different types of TWIST1 dimers in the cell, which may underpin the defective differentiation of the craniofacial mesenchyme. Functional analyses of the loss and gain of TWIST1-E-protein dimer activity have revealed previously unappreciated roles in guiding lineage differentiation of embryonic stem cells: TWIST1-E-protein heterodimers activate the differentiation of mesoderm and neural crest cells, which is accompanied by the epithelial-to-mesenchymal transition. At the same time, TWIST1 homodimers maintain the stem cells in a progenitor state and block entry to the endoderm lineage.