The endoderm gene regulatory network in sea urchin embryos up to mid-blastula stage

Alternative splicing in development and function of chordate endocrine systems: a focus on Pax genes

Genome sequencing has facilitated an understanding of gene networks but has also shown that they are only a small part of the answer to the question of how genes translate into a functional organism. Much of the answer lies in epigenetics-heritable traits not directly encoded by the genome. One such phenomenon is alternative splicing, which affects over 75% of protein coding genes and greatly amplifies the number of proteins. Although it was postulated that alternative splicing and gene duplication are inversely proportional and, therefore, have similar effects on the size of the proteome, for ancient duplications such as occurred in the Pax family of transcription factors, that is not necessarily so. The importance of alternative splicing in development and physiology is only just coming to light. However, several techniques for studying isoform functions both in vitro and in vivo have been recently developed. As examples of what is known and what is yet to be discovered, this review focuses on the evolution and roles of the Pax family of transcription factors in development and on alternative splicing of endocrine genes and the factors that regulate them.

Information processing at the foxa node of the sea urchin endomesoderm specification network

The foxa regulatory gene is of central importance for endoderm specification across Bilateria, and this gene lies at an essential node of the well-characterized sea urchin endomesoderm gene regulatory network (GRN). Here we experimentally dissect the cis-regulatory system that controls the complex pattern of foxa expression in these embryos. Four separate cis-regulatory modules (CRMs) cooperate to control foxa expression in different spatial domains of the endomesoderm, and at different times. A detailed mutational analysis revealed the inputs to each of these cis-regulatory modules. The complex and dynamic expression of foxa is regulated by a combination of repressors, a permissive switch, and multiple activators. A mathematical kinetic model was applied to study the dynamic response of foxa cis-regulatory modules to transient inputs. This study shed light on the mesoderm-endoderm fate decision and provides a functional explanation, in terms of the genomic regulatory code, for the spatial and temporal expression of a key developmental control gene.

High accuracy, high-resolution prevalence measurement for the majority of locally expressed regulatory genes in early sea urchin development

Accurate measurements of transcript abundance are a prerequisite to understand gene activity in development. Using the NanoString nCounter, an RNA counting device, we measured the prevalence of 172 transcription factors and signaling molecules in early sea urchin development. These measurements show high fidelity over more than five orders of magnitude down to a few transcripts per embryo. Most of the genes included are locally restricted in their spatial expression, and contribute to the divergent regulatory states of cells in the developing embryo. In order to obtain high-resolution expression profiles from fertilization to late gastrulation samples were collected at hourly intervals. The measured time courses agree well with, and substantially extend, prior relative abundance measurements obtained by quantitative PCR. High temporal resolution permits sequences of successively activated genes to be precisely delineated providing an ancillary tool for assembling maps of gene regulatory networks. The data are available via an interactive website for quick plotting of selected time courses.

Modularity and design principles in the sea urchin embryo gene regulatory network

The gene regulatory network (GRN) established experimentally for the pre-gastrular sea urchin embryo provides causal explanations of the biological functions required for spatial specification of embryonic regulatory states. Here we focus on the structure of the GRN which controls the progressive increase in complexity of territorial regulatory states during embryogenesis; and on the types of modular subcircuits of which the GRN is composed. Each of these subcircuit topologies executes a particular operation of spatial information processing. The GRN architecture reflects the particular mode of embryogenesis represented by sea urchin development. Network structure not only specifies the linkages constituting the genomic regulatory code for development, but also indicates the various regulatory requirements of regional developmental processes.

Network design principles from the sea urchin embryo

As gene regulatory network models encompass more and more of the specification processes underlying sea urchin embryonic development, topological themes emerge that imply the existence of structural network 'building blocks'. These are subcircuits which perform given logic operations in the spatial control of gene expression. The various parts of the sea urchin gene regulatory networks offer instances of the same subcircuit topologies accomplishing the same developmental logic functions but using different genes. These subcircuits are dedicated to specific developmental functions, unlike simpler 'motifs', and may indicate a repertoire of specific devices of which developmental gene regulatory networks are composed.