Abstract:
Epithelial-mesenchymal transitions (EMTs) are fundamental and indispensable
to embryonic morphogenesis throughout the animal kingdom. At the onset of
gastrulation in the sea urchin embryo, micromere-derived primary mesenchyme cells
(PMCs) undergo an EMT process to ingress into the blastocoel, and these cells later
become the larval skeleton. Much has been learned about PMC specification in sea
urchin embryos. However, much less is known about how states of the sequentially
progressing PMC gene regulatory network (GRN) controls the EMT process during PMC
ingression. Transcriptional regulators such as Snail and Twist have emerged as
important molecules for controlling EMTs in many model systems. Sea urchin snail and
twist genes were cloned from Lytechinus variegates, and each has been experimentally
connected to the PMC regulatory network; these experiments demonstrate several
requirements for PMC ingression, and in doing so, begin to illustrate how a gene
regulatory network state controls morphogenesis.
Functional knockdown analyses of Snail with morpholino-substituted antisense
oligonucleotides (MASO) in whole embryos and chimeras demonstrated that Snail is
required in micromeres for PMC ingression. Investigations also show that Snail downregulates
cadherin expression as an evolutionarily conserved mechanism, and Snail
positively regulates a required endocytic clearance of epithelial membrane molecules
during EMT. Perturbation experiments indicate that Twist has accessory roles in
regulating PMC ingression, and possibly plays a maintenance role in PMC specification
network state. In addition, Twist also functions in the post-EMT network state,
particularly in directing PMC differentiation and skeletogenesis.
The recently annotated sea urchin genome accelerates the discovery of new genes
and holds strong promise of mapping out a complete canvas of the micromere-PMC gene
regulatory network. Using the genome resources we successfully cloned several newly
identified PMC genes, and found most of them to be expressed in micromeres just prior
to ingression of the nascent PMCs. Current experiments focus on the roles of these genes
in preparing for, executing, and/or controlling the mesenchymal behavior following PMC
ingression. The functions and inter-relationships of these genes will greatly augment our
understanding of how a gene regulatory network state controls a crucial morphogenetic
event.
