Knockdown of the complete Hox paralogous group 1 leads to dramatic hindbrain and neural crest defects

Abstract

The Hox paralogous group 1 (PG1) genes are the first and initially most anterior Hox genes expressed in the embryo. In Xenopus, the three PG1 genes, Hoxa1, Hoxb1 and Hoxd1, are expressed in a widely overlapping domain, which includes the region of the future hindbrain and its associated neural crest. We used morpholinos to achieve a complete knockdown of PG1 function. When Hoxa1, Hoxb1 and Hoxd1 are knocked down in combination, the hindbrain patterning phenotype is more severe than in the single or double knockdowns, indicating a degree of redundancy for these genes. In the triple PG1 knockdown embryos the hindbrain is reduced and lacks segmentation. The patterning of rhombomeres 2 to 7 is lost, with a concurrent posterior expansion of the rhombomere 1 marker, Gbx2. This effect could be via the downregulation of other Hox genes, as we show that PG1 function is necessary for the hindbrain expression of Hox genes from paralogous groups 2 to 4. Furthermore, in the absence of PG1 function, the cranial neural crest is correctly specified but does not migrate into the pharyngeal arches. Embryos with no active PG1 genes have defects in derivatives of the pharyngeal arches and, most strikingly, the gill cartilages are completely missing. These results show that the complete abrogation of PG1 function in Xenopus has a much wider scope of effect than would be predicted from the single and double PG1 knockouts in other organisms.

Global posterior prevalence is unique to vertebrates: A dance to the music of time?

“…It is also known that the Hox genes undergo cell-autonomous and non-cell-autonomous activating and repressive interactions in the gastrula and in early development. These include posterior prevalence (Hooiveld et al, 1999;Woltering and Durston, 2008;Durston et al, 2011;Iimura and Pourqui e, 2006) and anterior to posterior activation (Hooiveld et al, 1999;McNulty et al, 2005;In der Rieden et al, 2010;Durston et al, 2011). We suspect that these interactions are central in generating vertebrate temporal collinearity.…”

“…It has also been shown that loss of function for anterior Hox genes can downregulate more posterior ones. (McNulty et al, 2005). This may be the primary reason that vertebrates have correlated posterior prevalence and temporal collinearity, while Drosophila does not .…”

Gene Silencing and Polycomb Group Proteins: An Overview of their Structure, Mechanisms and Phylogenetics

“…Hox genes are critical developmental regulators and are expressed in distinct regions of the anterior-posterior axis (Alexander et al, 2009). Mutations in the Hox genes have been associated with various developmental disorders, such as limb malformations (Kmita et al, 2005;Zakany and Duboule, 2007) and neural crest defects (McNulty et al, 2005). The Hox genes are also specifically expressed in the allantois of placental mammals (Scotti and Kmita, 2012;Shaut et al, 2008).…”

Reverse genetics tools in zebrafish: A forward dive into endocrinology

“…Together with the high fecundity of zebrafish, morpholinos can be injected into several hundreds of embryos in one experiment and resulting phenotypes are readily observed in those embryos a few days later ( Figure 1). Furthermore, co-injection of a combination of up to three morpholinos can be performed to achieve double and triple knockdowns of genes of interest [31]. In addition, concentrations of morpholinos injected can be titrated in order to investigate dose-dependent resulting phenotypes.…”