Detailed analysis of chick optic fissure closure reveals Netrin-1 as an essential mediator of epithelial fusion.
eLife
Hardy, H., Prendergast, J. G., Patel, A., Dutta, S., Trejo-Reveles, V., Kroeger, H., Yung, A. R., Goodrich, L. V., Brooks, B., Sowden, J. C., Rainger, J.
2019; 8
Abstract
Epithelial fusion underlies many vital organogenic processes during embryogenesis. Disruptions to these cause a significant number of human birth defects, including ocular coloboma. We provide robust spatial-temporal staging and unique anatomical detail of optic fissure closure (OFC) in the embryonic chick, including evidence for roles of apoptosis and epithelial remodelling. We performed complementary transcriptomic profiling and show that Netrin-1 (NTN1) is precisely expressed in the chick fissure margin at the fusion plate but is immediately downregulated after fusion. We further provide a combination of protein localisation and phenotypic evidence in chick, humans, mice and zebrafish that Netrin-1 has an evolutionarily conserved and essential requirement for OFC, and is likely to have an important role in palate fusion. Our data suggest that NTN1 is a strong candidate locus for human coloboma and other multi-system developmental fusion defects, and show that chick OFC is a powerful model for epithelial fusion research.
View details for DOI 10.7554/eLife.43877
View details for PubMedID 31162046
Abstract
During brainstem development, newborn neurons originating from the rhombic lip embark on exceptionally long migrations to generate nuclei important for audition, movement, and respiration. Along the way, this highly motile population passes several cranial nerves yet remains confined to the CNS. We found that Ntn1 accumulates beneath the pial surface separating the CNS from the PNS, with gaps at nerve entry sites. In mice null for Ntn1 or its receptor DCC, hindbrain neurons enter cranial nerves and migrate into the periphery. CNS neurons also escape when Ntn1 is selectively lost from the sub-pial region (SPR), and conversely, expression of Ntn1 throughout the mutant hindbrain can prevent their departure. These findings identify a permissive role for Ntn1 in maintaining the CNS-PNS boundary. We propose that Ntn1 confines rhombic lip-derived neurons by providing a preferred substrate for tangentially migrating neurons in the SPR, preventing their entry into nerve roots.
View details for DOI 10.1016/j.celrep.2018.01.068
View details for Web of Science ID 000424981600005
View details for PubMedID 29444422
View details for PubMedCentralID PMC5877811
Abstract
The vestibular system of the inner ear detects head position using three orthogonally oriented semicircular canals; even slight changes in their shape and orientation can cause debilitating behavioral defects. During development, the canals are sculpted from pouches that protrude from the otic vesicle, the embryonic anlage of the inner ear. In the center of each pouch, a fusion plate forms where cells lose their epithelial morphology and the basement membrane breaks down. Cells in the fusing epithelia intercalate and are removed, creating a canal. In mice, fusion depends on the secreted protein netrin 1 (Ntn1), which is necessary for basement membrane breakdown, although the underlying molecular mechanism is unknown. Using gain-of-function approaches, we found that overexpression of Ntn1 in the chick otic vesicle prevented canal fusion by inhibiting apoptosis. In contrast, ectopic expression of the same chicken Ntn1 in the mouse otic vesicle, where apoptosis is less prominent, resulted in canal truncation. These findings highlight the importance of apoptosis for tissue morphogenesis and suggest that Ntn1 may play divergent cellular roles despite its conserved expression during canal morphogenesis in chicken and mouse.
View details for DOI 10.1242/dev.144519
View details for Web of Science ID 000411081100016
View details for PubMedID 28851705
View details for PubMedCentralID PMC5612249
Abstract
Newborn neurons often migrate before undergoing final differentiation, extending neurites, and forming synaptic connections. Therefore, neuronal migration is crucial for establishing neural circuitry during development. In the developing spinal cord, neuroprogenitors first undergo radial migration within the ventricular zone. Differentiated neurons continue to migrate tangentially before reaching the final positions. The molecular pathways that regulate these migration processes remain largely unknown. Our previous study suggests that the DCC receptor is important for the migration of the dorsal spinal cord progenitors and interneurons. In this study, we determined the involvement of the Netrin1 ligand and the ROBO3 coreceptor in the migration.By pulse labeling neuroprogenitors with electroporation, we examined their radial migration in Netrin1 (Ntn1), Dcc, and Robo3 knockout mice. We found that all three mutants exhibit delayed migration. Furthermore, using immunohistochemistry of the BARHL2 interneuron marker, we found that the mediolateral and dorsoventral migration of differentiated dorsal interneurons is also delayed. Together, our results suggest that Netrin1/DCC signaling induce neuronal migration in the dorsal spinal cord.Netrin1, DCC, and ROBO3 have been extensively studied for their functions in regulating axon guidance in the spinal commissural interneurons. We reveal that during earlier development of dorsal interneurons including commissural neurons, these molecules play an important role in promoting cell migration.
View details for DOI 10.1186/s13064-016-0074-x
View details for PubMedID 27784329
View details for PubMedCentralID PMC5081974
Abstract
Netrin 1 (Ntn1) is a multifunctional guidance cue expressed in the ventricular zone and floor plate of the embryonic neural tube. Although Ntn1 is best known for acting as an axon guidance cue through Dcc and neogenin receptors, it is also thought to regulate neuronal survival and blood vessel development through Unc5 family receptors. However, the Ntn1 gene trap mutant mouse does not display all the phenotypes predicted from in vitro assays or analyses of mice lacking predicted receptors. Since the gene trap strain still produces wild-type Ntn1 protein, it is unclear whether the absence of phenotypes reflects the activity of alternative cues or of residual Ntn1. To resolve the full contribution of Ntn1 to development, we generated a null allele of Ntn1 and re-examined tissues exhibiting phenotypic discrepancies between receptor mutants and Ntn1 hypomorphs. We found that in Ntn1 null animals commissural axons rarely cross the midline, resulting in a strongly enhanced phenotype relative to Ntn1 hypomorphs, which retain many axons with normal trajectories. Thus, low levels of Ntn1 can account for persistent attraction to the midline in hypomorphs. By contrast, Ntn1 null mice do not show all of the phenotypes reported for Unc5 receptor mutants, indicating that Ntn1 is not necessarily the dominant ligand for Unc5 family members in vivo and ruling out primary roles in survival or angiogenesis.
View details for DOI 10.1242/dev.128942
View details for Web of Science ID 000366359900008
View details for PubMedID 26395479
View details for PubMedCentralID PMC4647218
Abstract
The transforming growth factor-? (TGF-?) signaling pathway serves critical functions in CNS development, but, apart from its proposed neuroprotective actions, its physiological role in the adult brain is unclear. We observed a prominent activation of TGF-? signaling in the adult dentate gyrus and expression of downstream Smad proteins in this neurogenic zone. Consistent with a function of TGF-? signaling in adult neurogenesis, genetic deletion of the TGF-? receptor ALK5 reduced the number, migration and dendritic arborization of newborn neurons. Conversely, constitutive activation of neuronal ALK5 in forebrain caused a marked increase in these aspects of neurogenesis and was associated with higher expression of c-Fos in newborn neurons and with stronger memory function. Our findings describe an unexpected role for ALK5-dependent TGF-? signaling as a regulator of the late stages of adult hippocampal neurogenesis, which may have implications for changes in neurogenesis during aging and disease.
View details for DOI 10.1038/nn.3732
View details for PubMedID 24859199
Abstract
When the axons that carry signals to muscles are growing, they rely on help from Frizzled3-a protein that is known to perform a number of other important functions in cells-to reach their final destination.
View details for DOI 10.7554/eLife.01845
View details for PubMedID 24347551
View details for PubMedCentralID PMC3865645
