Mechanisms of the epithelial-to-mesenchymal transition in sea urchin embryos

Abstract

Sea urchin mesenchyme is composed of the large micromere-derived spiculogenetic primary mesenchyme cells (PMC), veg2-tier macromere-derived non-spiculogenetic mesenchyme cells, the small micromere-derived germ cells, and the macro- and mesomere-derived neuronal mesenchyme cells. They are formed through the epithelial-to-mesenchymal transition (EMT) and possess multipotency, except PMCs that solely differentiate larval spicules. The process of EMT is associated with modification of epithelial cell surface property that includes loss of affinity to the apical and basal extracellular matrices, inter-epithelial cell adherens junctions and epithelial cell surface-specific proteins. These cell surface structures and molecules are endocytosed during EMT and utilized as initiators of cytoplasmic signaling pathways that often initiate protein phosphorylation to activate the gene regulatory networks. Acquisition of cell motility after EMT in these mesenchyme cells is associated with the expression of proteins such as Lefty, Snail and Seawi. Structural simplicity and genomic database of this model will further promote detailed EMT research.

Keywords: cell surface-to-cytoplasm signal transduction; extracellular matrix; gene regulatory network; neuronal mesenchyme cell; non-spiculogenetic mesenchyme cell; primary mesenchyme cell; sea urchin.

Figure 1.

Cleavage pattern and fate of blastomeres. (A) The 16-cell stage embryo is comprised of the animal most 8 mesomeres (deep green), equatorially positioned 4 macromeres (light green) and the vegetal most 4 micromeres (orange). (B) The 32-cell stage embryo. The vegetal most small-micromeres cleaved unequally to produce 4 vegetal most small-micromeres (red; S-micro) and 4 large-micromeres on the animal side (orange; L-micro). The vegetal side of macromeres is presumptive NSMC (yellow area). (C) Swimming blastula. The mesomere-derived ectodermal cells comprise the animal hemisphere (deep green). The animal side of the macromeres comprises veg1-tier ectoderm (light green) and the ancestor of archenteron in gastrulae (yellow and red). The vegetal side of the macromeres comprises veg2-tier ectoderm that is ancestor of NSMC (yellow). The large micromeres adjacent to veg2-tier cells are ancestors of PMC (orange), and the small micromere descendants that comprise central area of the vegetal plate (red) become primordial germ cells (PGC).

Figure 2.

Schematic process of EMT in primary mesenchyme cell (PMC) formation at the vegetal plate of swimming blastula based on transmission electron microscope images. (1) Presumptive PMC (blue; P) in the vegetal ectoderm posses an apical cilium (C) that extend through the hyaline layer (H). The basal surface is lined with basal lamina (BL). B, basal body. (2) The presumptive PMC shows apicobasal pulsatile movement and loses apical cilium. The apical surface near the adherens junction (AJ; red circles with arrows) protrudes short cell processes. (3) The basal surface bulges into the blastocoel (BC) through the basal lamina, and the apical cytoplasm constricts associating with apicobasal elongation of the cell body. The hyaline layer is raised above the apical surface of presumptive PMC (arrow). (4) The apical cytoplasmic constriction proceeds further associating with dissolution of AJs. (5) After sealing apical gap that is formed between adjacent ectodermal cells, the constricted apical cytoplasm is retracted to cell body and completes shifting of the cell body into the blastocoel. After Katow and Solursh.

Figure 3.

A Hyalin repeat (HYR) domain structure. HYR domain is comprised of 3 aromatic amino acids (red), 3 negatively charged acidic residues (purple) and cysteine residues (green) occupy at N- and C-termini.

Figure 4.

PMC specification GRN. Reproduced with permission by Development (dev.biologists.org). Detail is in the text. Source: Davidson Lab website which is updated periodically as new information becomes available at http://sugp.caltech.edu/endomes/.

Figure 5.

Combined sub-circuits diagram. Five sub-circuits of the basal lamina remodeling (green lines), the motility (red lines), the apical constriction (yellow lines), the apicobasal polarity (purple lines), and the de-adhesion (blue lines) are combined to one diagram. Reproduced with permission by Development (dev.biologists.org).

Figure 6.

Signal transduction in PMC migration. Extracellular growth factor (GF)-like ligands bind to GFR or Non-GF ligands to receptor type PTKs transmits signal to Ras via phosphorylation. Ras signal is transmitted to Raf-1 of MAPK pathway and then to MAPK/extracellular signal-related kinase (ERK) kinase (MEK1/2) and ERK1/2 in the pathway. The signal transferred to MAPK-activated protein kinase (MAPKAPK) that phosphorylates myosin light chain (MLC). MLC phosphorylation activates cell proliferation, cell migration and spiculogenesis except pigmentation, which is activated by ERK1/2. After Katow and Aizu.

Figure 7.

Spatial pattern of PMC ring around archenteron for spiculogenesis after migration in 2D embryo. (A) Two-D embryo after 4 h 30 min in incubation. Archenteron invaginated at the center of vegetal plate. (B) PMC migration pathway to their destinations traced by a computer-assisted tracking device. Blue lines, toward primary loci; Red lines, secondary loci; D-cable, dorsal PMC cable; L-PL, left side PMC locus; R-PL, right side PMC locus; V-cable, vegetal PMC cable. Dotted circle, approximate periphery of vegetal plate (VP). After Katow et al.

Figure 8.

GRN of pigment cell/immune cell differentiation at the vegetal plate. The vegetal plate of mesenchyme blastula is partitioned to the aboral field (green) and the oral field (blue) around small micromeres (green yellow) in the center. Aboral ectoderm is not shown here. A rectangle below projected from the oral filed by double arrow is blastocoelar cells specified at the downstream of gataC and scl. Inset, schematic left side view of mesenchyme blastula. An, animal pole; PMC, primary mesenchyme cell. Detail is in the text.

Figure 9.

NSMC specification at the vegetal plate of gastrula. The center of the vegetal plate is occupied by small micromere-derived ancestor of PGCs (deep green) that is surrounded by lower veg2-tier-derived ancestor of pigment cells on the aboral field (light green). They transcribe Gcm/Pks. Upper veg2-tier-derived ancestor of blastocoelar cells locate on the oral field (deep blue) and transcribe Gata/Scl. Ancestor of muscle cells (red) locates between ancestors of spicule tip cells (yellow) and coelomic pouch cells (yellow green) on the aboral side in upper veg2-tier-derived area. Detail is in the text.

Figure 10.

NSMC specification GRN. Reproduced with permission by Dr. Eric Davidson. Detail is in the text. Source: Davidson Lab website is updated periodically as new information becomes available at http://www.spbase.org/endomes/#EndoNetworkDiagrams.

Figure 11.

Schematic summary of NMC (green) ingression.⁴ (1) NMCs appear in the entire ectoderm and ingress into the blastocoel in the mesenchyme blastula stage along with PMCs (blue) by losing Eith-2 (red lines in ectoderm). Some of NMCs also detected in the wall of archenteron from the early gastrula stage (2) to the mid-gastrula stage (3). (4) NMCs in the blastocoel constitute network by the prism larva stage. NMC descendants alone indicated. NSMC descendants are not indicated. A, animal pole; AO, aboral side; O, oral side; V, vegetal pole.