Developmental cell fate choice in neural tube progenitors employs two distinct cis-regulatory strategies.

In many developing tissues, the patterns of gene expression that assign cell fate are organized by graded secreted signals. Cis-regulatory elements (CREs) interpret these signals to control gene expression, but how this is accomplished remains poorly understood. In the neural tube, a gradient of the morphogen sonic hedgehog (Shh) patterns neural progenitors. We identify two distinct ways in which CREs translate graded Shh into differential gene expression in mouse neural progenitors. In most progenitors, a common set of CREs control gene activity by integrating cell-type-specific inputs. By contrast, the most ventral progenitors use a unique set of CREs, established by the pioneer factor FOXA2. This parallels the role of FOXA2 in endoderm, where FOXA2 binds some of the same sites. Together, the data identify distinct cis-regulatory strategies for the interpretation of morphogen signaling and raise the possibility of an evolutionarily conserved role for FOXA2 across tissues.

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.

Ularcirc: visualization and enhanced analysis of circular RNAs via back and canonical forward splicing.

Circular RNAs (circRNA) are a unique class of transcripts that can only be identified from sequence alignments spanning discordant junctions, commonly referred to as backsplice junctions (BSJ). Canonical splicing is also linked with circRNA biogenesis either from the parental transcript or internal to the circRNA, and is not fully utilized in circRNA software. Here we present Ularcirc, a software tool that integrates the visualization of both BSJ and forward splicing junctions and provides downstream analysis of selected circRNA candidates. Ularcirc utilizes the output of CIRI, circExplorer, or raw chimeric output of the STAR aligner and assembles BSJ count table to allow multi-sample analysis. We used Ularcirc to identify and characterize circRNA from public and in-house generated data sets and demonstrate how it can be used to (i) discover novel splicing patterns of parental transcripts, (ii) detect internal splicing patterns of circRNA, and (iii) reveal the complexity of BSJ formation. Furthermore, we identify circRNA that have potential open reading frames longer than their linear sequence. Finally, we detected and validated the presence of a novel class of circRNA generated from ApoA4 transcripts whose BSJ derive from multiple non-canonical splicing sites within coding exons. Ularcirc is accessed via https://github.com/VCCRI/Ularcirc.