ReviewThe midbrain–hindbrain boundary organizer
Introduction
The initial subdivision of the neural plate, or regionalization, is the first step towards generating cellular diversity in the vertebrate brain. The subdivision is reflected by gene expression in restricted domains along the length of the neural primordium. As development proceeds, this rough subdivision is further refined within each region, ultimately generating the multitude of cell types in the central nervous system (CNS). Both vertical signals from the mesoderm to the overlying ectoderm [1] and planar signals travelling in the plane of the ectodermal epithelium are thought to be involved in generating cell diversity [2], [3], [4].
Patterning of the neural primordium also involves neuroepithelial organizers—special groups of cells that produce secreted molecules and thus control the cell fate of the surrounding cells. The two best-studied organizers are the anterior neural ridge (ANR, or row 1 [the first row of cells in the zebrafish neural plate]) acting on the forebrain neural plate [5], [6], [7), and the midbrain–hindbrain boundary organizer (MHB organizer, or isthmic organizer) acting on the midbrain and hindbrain primordium [8], [9], [10].
The MHB organizer was initially identified through transplantation experiments in chick embryos. When MHB tissue is transplanted into the caudal forebrain of chick embryos, the surrounding host tissue switches fate and adopts an isthmic or midbrain character [11], [12]; in the rhombencephalon, MHB tissue induces cerebellar fate [13]. These experiments suggested that this tissue also acts as an organizing center in its normal location at the MHB. This review focuses on recent progress in understanding how the midbrain–hindbrain boundary organizer develops and functions.
Several genes, encoding either transcription factors (Engrailed [En], Pax, Otx and Gbx families) or secreted proteins (Wnt and Fgf [fibroblast growth factor] families), are expressed within the midbrain–hindbrain territory at early embryonic stages (Fig. 1). Several groups have generated mutations in these genes in mice through gene targeting [9], [10]. Mutagenesis screens in zebrafish have yielded acerebellar (ace), a probable null-allele of fgf8, an allelic series of no isthmus (noi) alleles in the pax2.1 gene [14], [15], [16], and several mutants in which molecular identification is ongoing. The different mutants lack MHB structures and/or neighboring brain territories to varying degrees, as listed in Table 1. From the mutant analysis, several regulatory steps are distinguished in MHB development. During the establishment phase, a crucial first step is the subdivision into an Otx2- and a Gbx2-expressing domain (see below). At this interface between Otx2 and Gbx2, at least three signaling pathways become activated independently of each other, as monitored by the expression of the wnt1, pax2.1 and fgf8 genes (Fig. 2a) [15], [16]. Establishment is followed by the maintenance phase, during which expression of the above genes comes to depend on each other. Perturbation of any one gene disrupts the continued development of the MHB. During this period, Fgf8 expression is activated at the MHB, thus probably endowing these cells with organizing capacity (Fig. 2b).
Section snippets
The Otx–Gbx interface and positioning of the isthmic organizer — or how much of a fly wing is the MHB?
The establishment of organizing centers is thought to require the prior specification of two distinct, adjacent cell populations. Local cellular interactions then result in the production of molecules with longer-range signaling properties [17]. This phenomenon has been studied extensively, for example, at the anterior–posterior compartment boundary of the fly wing. How are the two cell populations that generate the MHB organizer defined? During normal CNS development, one of the earliest
Fgfs and their role at the MHB
Once the organizer is positioned properly, secreted Fgf8 and Wnt1 proteins from the organizer are thought to mediate its organizing influence on the surrounding neural tissue. Wnt1 functions as a mitogen and to maintain expression of En genes, but is unable to mimic the activity of the organizer when misexpressed [51], [52]. Fgf8 is expressed at the right time and place to mediate the organizing activity [16], [20], [53]. In contrast to Wnt1, the ectopic application of Fgf8 protein mimics the
Vertebrate brains are different
Studies in amphioxus indicate that the MHB organizer is probably a vertebrate-specific invention [72], although part of this genetic machinery (Pax2 expression) may be conserved in ascidians [73]; hence, it is of particular interest to understand the actions and genetic regulation of this organizer and how this could generate the various brain morphologies in different species. From the available evidence so far, the genetic network controlling MHB development appears to be very similar in
Conclusions
Results discussed in this review suggest that two distinct phases in MHB development can be recognized. The first phase is a phase of establishment that involves the consecutive or parallel activation of different factors (Otx2, Gbx2, Fgf, Wnt1, Pax, En) at the Otx–Gbx interface. It remains to be determined which signal(s) creates the Otx–Gbx interface during gastrulation, and how this interface causes the ordered activation of MHB organizer genes around it. The second phase is a maintenance
Acknowledgements
The authors thank Klaus Lun, Steffen Scholpp, Antonio Simeone and Horst Simon for many stimulating discussions and critical reading of the manuscript. We apologize for not being able to cite all relevant primary papers due to space constraints. This work was supported by research grants to Michael Brand (Max-Planck-Society, EU Biotech, Deutsche Forschungsgemeinschaft) and an Institut National de la Santé et de la Recherche Médicale fellowship to Muriel Rhinn.
References and recommended reading
Papers of particular interest, published within the annual period of review,have been highlighted as:
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