In vertebrates, the nervous system is divided into distinct regions patterned in a head-to-tail direction. Embryologists have long been interested in working out the means by which this regionalization is achieved, and for years the dominant theory has involved a two-step mechanism in which neural tissues are first induced lengthwise down the entire body axis, followed by a transformation step where a second signal specifies the posterior identities of the neural tissue, which can be induced by a number of factors including members of the Wnt and FGF gene families, and retinoic acid.

While there is much evidence to support this model, a number of recent studies have indicated that the specification of the forebrain (an anterior structure, presumed to be induced by the initial signal in the activation-transformation model) requires additional regulatory inputs as well. New work by scientists in the Laboratory for Organogenesis and Neurogenesis (Group Director, Yoshiki Sasai) at the RIKEN Center for Developmental Biology (CDB; Kobe, Japan) showing anterior neural specifying activity in the African clawed frog, Xenopus laevis, lends weight to the revisionist argument.

In an article published in the July 13 issue of Developmental Cell, Takayuki Onai et al. report that XsalF, the Xenopus homolog of spalt, a homeotic gene known to function in anterior-posterior segment identity in Drosophila, regulates the expression of forebrain and midbrain-specific genes. A series of experiments in which XsalF was misexpressed, deleted and its function blocked, showed direct linkage between XsalF expression and forebrain/midbrain identity.

XsalF was originally identified in a screen of the frog anterior neural plate, a structure that appears early in neural development. Sequencing of the gene, and analysis of the timing and spatial pattern of its expression showed that it codes a transcription factor related to Spalt and is expressed in the incipient for
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Contact: Doug Sipp
sipp@cdb.riken.jp
81-78-306-3043
RIKEN Center for Developmental Biology
12-Jul-2004

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