Directional migration of leading-edge mesoderm generates physical forces: Implication in Xenopus notochord formation during gastrulation

Gastrulation is a dynamic tissue-remodeling process occurring during early development and fundamental to the later organogenesis. It involves both chemical signals and physical factors. Although much is known about the molecular pathways involved, the roles of physical forces in regulating cellular behavior and tissue remodeling during gastrulation have just begun to be explored. Here, we characterized the force generated by the leading edge mesoderm (LEM) that migrates preceding axial mesoderm (AM), and investigated the contribution of LEM during Xenopus gastrulation. First, we constructed an assay system using micro-needle which could measure physical forces generated by the anterior migration of LEM, and estimated the absolute magnitude of the force to be 20-80nN. Second, laser ablation experiments showed that LEM could affect the force distribution in the AM (i.e. LEM adds stretch force on axial mesoderm along anterior-posterior axis). Third, migrating LEM was found to be necessary for the proper gastrulation cell movements and the establishment of organized notochord structure; a reduction of LEM migratory activity resulted in the disruption of mediolateral cell orientation and convergence in AM. Finally, we found that LEM migration cooperates with Wnt/PCP to form proper notochord. These results suggest that the force generated by the directional migration of LEM is transmitted to AM and assists the tissue organization of notochord in vivo independently of the regulation by Wnt/PCP. We propose that the LEM may have a mechanical role in aiding the AM elongation through the rearrangement of force distribution in the dorsal marginal zone.

The Caenorhabditis elegans homolog of the Opitz syndrome gene, madd-2/Mid1, regulates anchor cell invasion during vulval development

Mutations in the human Mid1 gene cause Opitz G/BBB syndrome, which is characterized by various midline closure defects. The Caenorhabditis elegans homolog of Mid1, madd-2, positively regulates signaling by the unc-40 Netrin receptor during the extension of muscle arms to the midline and in axon guidance and branching. During uterine development, a specialized cell called anchor cell (AC) breaches the basal laminae separating the uterus from the epidermis and invades the underlying vulval tissue. AC invasion is guided by an UNC-6 Netrin signal from the ventral nerve cord and an unknown guidance signal from the vulval cells. Using genetic epistasis analysis, we show that madd-2 regulates AC invasion downstream of or in parallel with the Netrin signaling pathway. Measurements of AC shape, polarity and dynamics indicate that MADD-2 prevents the formation of ectopic AC protrusions in the absence of guidance signals. We propose that MADD-2 represses the intrinsic invasive capacity of the AC, while the Netrin and vulval guidance cues locally overcome this inhibitory activity of MADD-2 to guide the AC ventrally into the vulval tissue. Therefore, developmental cell invasion depends on a precise balance between pro- and anti-invasive factors.

How to form and close the brain: insight into the mechanism of cranial neural tube closure in mammals

The development of the embryonic brain critically depends on successfully completing cranial neural tube closure (NTC). Failure to properly close the neural tube results in significant and potentially lethal neural tube defects (NTDs). We believe these malformations are caused by disruptions in normal developmental programs such as those involved in neural plate morphogenesis and patterning, tissue fusion, and coordinated cell behaviors. Cranial NTDs include anencephaly and craniorachischisis, both lethal human birth defects. Newly emerging methods for molecular and cellular analysis offer a deeper understanding of not only the developmental NTC program itself but also mechanical and kinetic aspects of closure that may contribute to cranial NTDs. Clarifying the underlying mechanisms involved in NTC and how they relate to the onset of specific NTDs in various experimental models may help us develop novel intervention strategies to prevent NTDs.

Mechanism of pectoral fin outgrowth in zebrafish development

Fins and limbs, which are considered to be homologous paired vertebrate appendages, have obvious morphological differences that arise during development. One major difference in their development is that the AER (apical ectodermal ridge), which organizes fin/limb development, transitions into a different, elongated organizing structure in the fin bud, the AF (apical fold). Although the role of AER in limb development has been clarified in many studies, little is known about the role of AF in fin development. Here, we investigated AF-driven morphogenesis in the pectoral fin of zebrafish. After the AER-AF transition at ∼36 hours post-fertilization, the AF was identifiable distal to the circumferential blood vessel of the fin bud. Moreover, the AF was divisible into two regions: the proximal AF (pAF) and the distal AF (dAF). Removing the AF caused the AER and a new AF to re-form. Interestingly, repeatedly removing the AF led to excessive elongation of the fin mesenchyme, suggesting that prolonged exposure to AER signals results in elongation of mesenchyme region for endoskeleton. Removal of the dAF affected outgrowth of the pAF region, suggesting that dAF signals act on the pAF. We also found that the elongation of the AF was caused by morphological changes in ectodermal cells. Our results suggest that the timing of the AER-AF transition mediates the differences between fins and limbs, and that the acquisition of a mechanism to maintain the AER was a crucial evolutionary step in the development of tetrapod limbs.

Neural tube defects by NUAK1 and NUAK2 double mutation

BACKGROUND

NUAK1 and NUAK2, members of the AMP-activated protein kinase family of serine/threonine kinases, are prominently expressed in neuroectoderm, but their functions in neurulation have not been elucidated.

RESULTS

NUAK1 and NUAK2 double mutants exhibited exencephaly, facial clefting, and spina bifida. Median hinge point was formed, but dorsolateral hinge point formation was not apparent in cranial neural plate; neither apical constriction nor apico-basal elongation took place efficiently in the double mutants during the 5-10-somite stages. Concomitantly, the apical concentration of phosphorylated myosin light chain 2, F-actin, and cortactin was insignificant, and development of acetylated α-tubulin-positive microtubules was poor. However, the distribution of F-actin, cortactin, Shroom3, Rho, myosin heavy chain IIB, phosphorylated myosin light chain 2, α-tubulin, γ-tubulin, or acetylated α-tubulin was apparently normal in the double mutant neuroepithelia at the 5-somite stage.

CONCLUSIONS

NUAK1 and NUAK2 complementarily function in the apical constriction and apico-basal elongation that associate with the dorsolateral hinge point formation in cephalic neural plate during the 5- to 10-somite stages. In the double mutant neural plate, phosphorylated myosin light chain 2, F-actin, and cortactin did not concentrate efficiently in apical surfaces, and acetylated α-tubulin-positive microtubules did not develop significantly.

Multi-layered hypertrophied MEE formation by microtubule disruption via GEF-H1/RhoA/ROCK signaling pathway

BACKGROUND

Formation of the secondary palate is complex and disturbance during palatal fusion may result in cleft palate. The processes of adhesion, intercalation, and disappearance of medial edge epithelia (MEE) are characterized by morphological changes requiring dynamic cytoskeletal rearrangement. Microtubules are one of the cytoskeletal elements involved in maintenance of cell morphology. Microtubule-disrupting drugs have been reported to cause craniofacial malformations including cleft palate. The mechanisms underlying the failure of palatal fusion remain poorly understood. We evaluated the effect of nocodazole (NDZ), a drug that disrupts microtubules, on palatal fusion in organ culture.

RESULTS

NDZ caused failure of palatal fusion due to the induction of a multi-layered hypertrophied MEE in the mid-region of the secondary palatal shelves. Microtubule disruption increased RhoA activity and stress fiber formation. Pharmacological inhibition of the RhoA/ROCK pathway blocked multi-layered MEE formation. Partial prevention of hypertrophied MEE was observed with Y27632 and cytochalasin, but not with blebbistatin. NDZ induced re-localization of GEF-H1 into cytoplasm from cell-cell junctions.

CONCLUSIONS

The present study provided evidence that the GEF-H1/RhoA/ROCK pathway plays a pivotal role in linking microtubule disassembly to the remodeling of the actin cytoskeleton, which resulted in a multi-layered hypertrophied MEE and failure of palatal fusion.

The making of differences between fins and limbs

'Evo-devo', an interdisciplinary field based on developmental biology, includes studies on the evolutionary processes leading to organ morphologies and functions. One fascinating theme in evo-devo is how fish fins evolved into tetrapod limbs. Studies by many scientists, including geneticists, mathematical biologists, and paleontologists, have led to the idea that fins and limbs are homologous organs; now it is the job of developmental biologists to integrate these data into a reliable scenario for the mechanism of fin-to-limb evolution. Here, we describe the fin-to-limb transition based on key recent developmental studies from various research fields that describe mechanisms that may underlie the development of fins, limb-like fins, and limbs.

Cell movements of the deep layer of non-neural ectoderm underlie complete neural tube closure in Xenopus

In developing vertebrates, the neural tube forms from a sheet of neural ectoderm by complex cell movements and morphogenesis. Convergent extension movements and the apical constriction along with apical-basal elongation of cells in the neural ectoderm are thought to be essential for the neural tube closure (NTC) process. In addition, it is known that non-neural ectoderm also plays a crucial role in this process, as the neural tube fails to close in the absence of this tissue in chick and axolotl. However, the cellular and molecular mechanisms by which it functions in NTC are as yet unclear. We demonstrate here that the non-neural superficial epithelium moves in the direction of tensile forces applied along the dorsal-ventral axis during NTC. We found that this force is partly attributable to the deep layer of non-neural ectoderm cells, which moved collectively towards the dorsal midline along with the superficial layer. Moreover, inhibition of this movement by deleting integrin β1 function resulted in incomplete NTC. Furthermore, we demonstrated that other proposed mechanisms, such as oriented cell division, cell rearrangement and cell-shape changes have no or only minor roles in the non-neural movement. This study is the first to demonstrate dorsally oriented deep-cell migration in non-neural ectoderm, and suggests that a global reorganization of embryo tissues is involved in NTC.

TRIM proteins in development

TRIM proteins play important roles in several patho-physiological processes. Their common activity within the ubiquitylation pathway makes them amenable to a number of diverse biological roles. Many of the TRIM genes are highly and sometimes specifically expressed during embryogenesis, it is therefore not surprising that several of them might be involved in developmental processes. Here, we primarily discuss the developmental implications of two subgroups of TRIM proteins that conserved domain composition and functions from their invertebrate ancestors. The two groups are: the TRIM-NHL proteins implicated in miRNA processing regulation and the TRIM-FN3 proteins involved in ventral midline development.

The microtubule-associated C-I subfamily of TRIM proteins and the regulation of polarized cell responses

TRIM proteins are multidomain proteins that typically assemble into large molecular complexes, the composition of which likely explains the diverse functions that have been attributed to this group of proteins. Accumulating data on the roles of many TRIM proteins supports the notion that those that share identical C-terminal domain architectures participate in the regulation of similar cellular processes. At least nine different C-terminal domain compositions have been identified. This chapter will focus on one subgroup that possess a COS motif, FNIII and SPRY/B30.2 domain as their C-terminal domain arrangement. This C-terminal domain architecture plays a key role in the interaction of all six members of this subgroup with the microtubule cytoskeleton. Accumulating evidence on the functions of some of these proteins will be discussed to highlight the emerging similarities in the cellular events in which they participate.

TRIM family: Pleiotropy and diversification through homomultimer and heteromultimer formation

The TRIM family is composed of multidomain ubiquitin E3 ligases characterized by the presence of the N-terminal tripartite motif (RING, B-boxes, and coiled coil). TRIM proteins transfer the ubiquitin moiety to specific substrates but are also involved in ubiquitin-like modifications, in particular SUMOylation and ISGylation. The TRIM family members are involved in a plethora of biological and physiological processes and, when altered, are implicated in many pathological conditions. Growing evidence indicates the pleiotropic effect of several TRIM genes, each of which might be connected to very diverse cellular processes. As a way to reconcile a single family member with several functions, we propose that structural features, that is, their ability to homo- and hetero-di(multi)merize, can increase and diversify TRIM ubiquitin E3 ligase capability.

TRIM-9 functions in the UNC-6/UNC-40 pathway to regulate ventral guidance

TRIpartite Motif (TRIM) family proteins are ring finger domain-containing, multi-domain proteins implicated in many biological processes. Members of the TRIM-9/C-I subfamily of TRIM proteins, including TRIM-9, MID1 and MID2, have neuronal functions and are associated with neurological diseases. To explore whether the functions of C-I TRIM proteins are conserved in invertebrates, we analyzed Caenorhabditis elegans and Drosophila trim-9 mutants. C. elegans trim-9 mutants exhibit defects in the ventral guidance of hermaphrodite specific neuron (HSN) and the touch neuron AVM. Further genetic analyses indicate that TRIM-9 participates in the UNC-6-UNC-40 attraction pathway. Asymmetric distribution of UNC-40 during HSN development is normal in trim-9 mutants. However, the asymmetric localization of MIG-10, a downstream effector of UNC-40, is abolished in trim-9 mutants. These results suggest that TRIM-9 functions upstream of MIG-10 in the UNC-40 pathway. Moreover, we showed that TRIM-9 exhibits E3 ubiquitin ligase activity in vitro and this activity is important for TRIM-9 function in vivo. Additionally, we found that Drosophila trim-9 is required for the midline attraction of a group of sensory neuron axons. Over-expression of the Netrin/UNC-6 receptor Frazzled suppresses the guidance defects in trim-9 mutants. Our study reveals an evolutionarily conserved function of TRIM-9 in the UNC-40/Frazzled-mediated UNC-6/Netrin attraction pathway.