Lrrn1 is required for formation of the midbrain–hindbrain boundary and organiser through regulation of affinity differences between midbrain and hindbrain cells in chick

Kyoko Tossell,Laura C Andreae,Chloe Cudmore,Emily Lang,Uma Muthukrishnan,Andrew Lumsden,Jonathan D Gilthorpe,Carol Irving

doi:10.1016/j.ydbio.2011.02.002

Kyoko Tossell, Laura C Andreae + Show 6 more

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https://doi.org/10.1016/j.ydbio.2011.02.002

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Abstract

The midbrain–hindbrain boundary (MHB) acts as an organiser/signalling centre to pattern tectal and cerebellar compartments. Cells in adjacent compartments must be distinct from each other for boundary formation to occur at the interface. Here we have identified the leucine-rich repeat (LRR) neuronal 1 (Lrrn1) protein as a key regulator of this process in chick. The Lrrn family is orthologous to the Drosophila tartan/capricious (trn/caps) family. Differential expression of trn/caps promotes an affinity difference and boundary formation between adjacent compartments in a number of contexts; for example, in the wing, leg and eye imaginal discs. Here we show that Lrrn1 is expressed in midbrain cells but not in anterior hindbrain cells. Lrrn1 is down-regulated in the anterior hindbrain by the organiser signalling molecule FGF8, thereby creating a differential affinity between these two compartments. Lrrn1 is required for the formation of MHB — loss of function leads to a loss of the morphological constriction and loss of Fgf8. Cells overexpressing Lrrn1 violate the boundary and result in a loss of cell restriction between midbrain and hindbrain compartments. Lrrn1 also regulates the glycosyltransferase Lunatic Fringe, a modulator of Notch signalling, maintaining its expression in midbrain cells which is instrumental in MHB boundary formation. Thus, Lrrn1 provides a link between cell affinity/compartment segregation, and cell signalling to specify boundary cell fate.

Highlights

The midbrain–hindbrain boundary (MHB) is an organising centre that is crucial for the formation of tectum and cerebellum from midbrain and hindbrain, respectively (Chi et al, 2003; Marin and Puelles, 1994; Martinez et al, 1995; Reifers et al, 1998)
In order to investigate the expression of Lrrn1 in the chick MHB in more detail, we performed a series of whole mount in-situ hybridisations on embryos between the 4 and 25 somite stages (HH8−15), over the time period that the midbrain and hindbrain compartments form
From stage HH9 Lrrn1 was down-regulated in the MHB region, with high levels remaining both anteriorly and posteriorly in the neural tube (Fig. 1A)

Summary

Introduction

The MHB is an organising centre that is crucial for the formation of tectum and cerebellum from midbrain and hindbrain, respectively (Chi et al, 2003; Marin and Puelles, 1994; Martinez et al, 1995; Reifers et al, 1998). It arises at Hamburger Hamilton stage (HH) in the chick when morphological constrictions begin to appear along the length of the neural tube, sub-dividing it into smaller units upon which patterning signals can bestow specific regional identities (Lumsden and Krumlauf, 1996). The Notch modifier, Lunatic Fringe (LFng), is a glycosyltransferase that

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