Isthmus-to-midbrain transformation in the absence of midbrain-hindbrain organizer activity

Abstract

In zebrafish acerebellar (ace) embryos, because of a point mutation in fgf8, the isthmic constriction containing the midbrain-hindbrain boundary (MHB) organizer fails to form. The mutants lack cerebellar development by morphological criteria, and they appear to have an enlarged tectum, showing no obvious reduction in the tissue mass at the dorsal mesencephalic/metencephalic alar plate. To reveal the molecular identity of the tissues located at equivalent rostrocaudal positions along the neuraxis as the isthmic and cerebellar primordia in wild-types, we undertook a detailed analysis of ace embryos. In ace mutants, the appearance of forebrain and midbrain specific marker genes (otx2, dmbx1, wnt4) in the caudal tectal enlargement reveals a marked rostralized gene expression profile during early somitogenesis, followed by the lack of early and late cerebellar-specific gene expression (zath1/atoh1, gap43,tag1/cntn2, neurod, zebrin II). The Locus coeruleus(LC) derived from rostral rhombomere 1 is also absent in the mutants. A new interface between otx2 and epha4a suggests that the rostralization stops at the caudal part of rhombomere 1. The mesencephalic basal plate is also affected in the mutant embryos, as indicated by the caudal expansion of the diencephalic expression domains of epha4a,zash1b/ashb, gap43 and tag1/cntn2, and by the dramatic reduction of twhh expression. No marked differences are seen in cell proliferation and apoptotic patterns around the time the rostralization of gene expression becomes evident in the mutants. Therefore,locally distinct cell proliferation and cell death is unlikely to be the cause of the fate alteration of the isthmic and cerebellar primordia in the mutants. Dil cell-lineage labeling of isthmic primordial cells reveals that cells, at the location equivalent of the wild-type MHB, give rise to caudal tectum in ace embryos. This suggests that a caudalto-rostral transformation leads to the tectal expansion in the mutants. Fgf8-coated beads are able to rescue morphological MHB formation, and elicit the normal molecular identity of the isthmic and cerebellar primordium in ace embryos. Taken together, our analysis reveals that cells of the isthmic and cerebellar primordia acquire a more rostral, tectal identity in the absence of the functional MHB organizer signal Fgf8.

Fibroblast Growth Factor Receptors Cooperate to Regulate Neural Progenitor Properties in the Developing Midbrain and Hindbrain

“…Remarkably, even in the Fgfr1 cko ;Fgfr2 cko ;Fgfr3 null mutants the entire r1 was not deleted or transformed into Otx2-positive tissue. This is similar to the phenotype of the zebrafish Fgf8 mutants and suggests that the posterior r1 is unique in being less dependent on IsO-derived signals (Jaszai et al, 2003).…”

“…In both cases, the same brain structures fail to develop, and cell death increases, especially on the dorsal side (alar plate) of the midbrain-r1 region. Instead of increased cell death, in the zebrafish Fgf8 mutants the isthmic region in the anterior r1 transforms into midbrain identity (Jaszai et al, 2003). In the conditional Fgf8 mouse mutants, the posterior border of the midbrain, as determined by Otx2 expression, shifts slightly (Chi et al, 2003).…”

Zebrafish limb development is triggered by a retinoic acid signal during gastrulation

“…A similar example of such plasticity is provided by the late rescue of the isthmic fold demarcating the junction between optic tectum and cerebellum in the brains of acerebellar (fgf8) mutant embryos. Acrylic beads soaked in recombinant Fgf protein are able to rescue the wild-type structure of this brain region, although the defect in the mutant originates much earlier (Reifers et al, 1998;Jaszai et al, 2003).…”

Toward a developmental neurobiology of autism

“…5-HT neurons form caudal to the MHB in response to the same two signals, but preceded by an earlier signal, Fgf4, from the primitive streak [Ye et al, 1998;Hynes and Rosenthal, 1999]. NA neurons of the LC are also generated caudal to the MHB, in response to signaling by Shh, Fgf8, and BMPs [Crossley et al, 1996;Morin et al, 1997;Guo et al, 1999;Vogel-Hopker and Rohrer, 2002;Jaszai et al, 2003;Lam et al, 2003;Eddison et al, 2004]. As discussed below, specification of neurotransmitter phenotypes of MA neurons requires a complex signaling through networks of transcription factors, which is only beginning to be elucidated [Briscoe et al, 1999;Pattyn et al, 1999Pattyn et al, , 2000Pattyn et al, , 2004.…”