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Calcium signals drive cell shape changes during zebrafish midbrain-hindbrain boundary formation.

ABSTRACT: One of the first morphogenetic events in the vertebrate brain is the formation of the highly conserved midbrain-hindbrain boundary (MHB). Specific cell shape changes occur at the point of deepest constriction of the MHB, the midbrain-hindbrain boundary constriction (MHBC), and are critical for proper MHB formation. These cell shape changes are controlled by nonmuscle myosin II (NMII) motor proteins, which are tightly regulated via the phosphorylation of their associated myosin regulatory light chains (MRLCs). However, the upstream signaling pathways that initiate the regulation of NMII to mediate cell shape changes during MHB morphogenesis are not known. We show that intracellular calcium signals are critical for the regulation of cell shortening during initial MHB formation. We demonstrate that the MHB region is poised to respond to calcium transients that occur in the MHB at the onset of MHB morphogenesis and that calcium mediates phosphorylation of MRLC specifically in MHB tissue. Our results indicate that calmodulin 1a (calm1a), expressed specifically in the MHB, and myosin light chain kinase together mediate MHBC cell length. Our data suggest that modulation of NMII activity by calcium is critical for proper regulation of cell length to determine embryonic brain shape during development.

SUBMITTER: Sahu SU

PROVIDER: S-EPMC5385936 | biostudies-literature | 2017 Apr

REPOSITORIES: biostudies-literature

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Calcium signals drive cell shape changes during zebrafish midbrain-hindbrain boundary formation.

Sahu Srishti U SU Visetsouk Mike R MR Garde Ryan J RJ Hennes Leah L Kwas Constance C Gutzman Jennifer H JH

Molecular biology of the cell 20170201 7

One of the first morphogenetic events in the vertebrate brain is the formation of the highly conserved midbrain-hindbrain boundary (MHB). Specific cell shape changes occur at the point of deepest constriction of the MHB, the midbrain-hindbrain boundary constriction (MHBC), and are critical for proper MHB formation. These cell shape changes are controlled by nonmuscle myosin II (NMII) motor proteins, which are tightly regulated via the phosphorylation of their associated myosin regulatory light c ...[more]

PMID: 28148652

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Project description:BackgroundStudies in mouse, Xenopus and chicken have shown that Otx2 and Gbx2 expression domains are fundamental for positioning the midbrain-hindbrain boundary (MHB) organizer. Of the two zebrafish gbx genes, gbx1 is a likely candidate to participate in this event because its early expression is similar to that reported for Gbx2 in other species. Zebrafish gbx2, on the other hand, acts relatively late at the MHB. To investigate the function of zebrafish gbx1 within the early neural plate, we used a combination of gain- and loss-of-function experiments.ResultsWe found that ectopic gbx1 expression in the anterior neural plate reduces forebrain and midbrain, represses otx2 expression and repositions the MHB to a more anterior position at the new gbx1/otx2 border. In the case of gbx1 loss-of-function, the initially robust otx2 domain shifts slightly posterior at a given stage (70% epiboly), as does MHB marker expression. We further found that ectopic juxtaposition of otx2 and gbx1 leads to ectopic activation of MHB markers fgf8, pax2.1 and eng2. This indicates that, in zebrafish, an interaction between otx2 and gbx1 determines the site of MHB development. Our work also highlights a novel requirement for gbx1 in hindbrain development. Using cell-tracing experiments, gbx1 was found to cell-autonomously transform anterior neural tissue into posterior. Previous studies have shown that gbx1 is a target of Wnt8 graded activity in the early neural plate. Consistent with this, we show that gbx1 can partially restore hindbrain patterning in cases of Wnt8 loss-of-function. We propose that in addition to its role at the MHB, gbx1 acts at the transcriptional level to mediate Wnt8 posteriorizing signals that pattern the developing hindbrain.ConclusionOur results provide evidence that zebrafish gbx1 is involved in positioning the MHB in the early neural plate by refining the otx2 expression domain. In addition to its role in MHB formation, we have shown that gbx1 is a novel mediator of Wnt8 signaling during hindbrain patterning.

Project description:BackgroundIn vertebrates, the primordium of the brain is subdivided by the expression of Otx genes (forebrain/anterior midbrain), Hox genes (posterior hindbrain), and the genes Pax2, Pax5 and Pax8 (intervening region). The latter includes the midbrain/hindbrain boundary (MHB), which acts as a key organizer during brain patterning. Recent studies in Drosophila revealed that orthologous sets of genes are expressed in a similar tripartite pattern in the late embryonic brain, which suggested correspondence between the Drosophila deutocerebral/tritocerebral boundary region and the vertebrate MHB. To gain more insight into the evolution of brain regions, and particularly the MHB, I examined the expression of a comprehensive array of MHB-specific gene orthologs in the procephalic neuroectoderm and in individually identified neuroblasts during early embryonic stages 8-11, at which the segmental organization of the brain is most clearly displayed.Results and conclusionI show that the early embryonic brain exhibits an anterior Otx/otd domain and a posterior Hox1/lab domain, but that Pax2/5/8 orthologs are not expressed in the neuroectoderm and neuroblasts of the intervening territory. Furthermore, the expression domains of Otx/otd and Gbx/unpg exhibit a small common interface within the anterior deutocerebrum. In contrast to vertebrates, Fgf8-related genes are not expressed posterior to the otd/unpg interface. However, at the otd/unpg interface the early expression of other MHB-specific genes (including btd, wg, en), and of dorsoventral patterning genes, closely resembles the situation at the vertebrate MHB. Altogether, these results suggest the existence of an ancestral territory within the primordium of the deutocerebrum and adjacent protocerebrum, which might be the evolutionary equivalent of the region of the vertebrate MHB. However, lack of expression of Pax2/5/8 and Fgf8-related genes, and significant differences in the expression onset of other key regulators at the otd/unpg interface, imply that genetic interactions crucial for the vertebrate organizer activity are absent in the early embryonic brain of Drosophila.

Dataset Information

Calcium signals drive cell shape changes during zebrafish midbrain-hindbrain boundary formation.

Publications

Calcium signals drive cell shape changes during zebrafish midbrain-hindbrain boundary formation.

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