Bmpr1bb is a novel gene involved in retinoic acid induced patterning of the zebrafish hindbrain

TLDR: It is proposed that Bmpr1bb is a downstream target of RA signalling, strongly downregulated during embryogenesis and specified to a specific region of the hindbrain, as being significantly downregulated in response to retinoic acid treatment.

Abstract: Retinoic acid signalling plays a critical role during zebrafish development. The teratogenic effects of retinoic acid have been demonstrated by embryonic deformation resulting from insufficient or excessive levels of this vitamin A derivative. During embryogenesis, bone morphogenetic proteins are closely linked to the physiological interpretation of RA gradients, particularly in the hindbrain. We describe an uncharacterized gene, Bmpr1bb, as being significantly downregulated in response to retinoic acid treatment. In situ expression demonstrates that Bmpr1bb is expressed ubiquitously at 10hpf, and is slowly downregulated until 48hpf where the expression is concentrated in the hindbrain. We propose that Bmpr1bb is a downstream target of RA signalling, strongly downregulated during embryogenesis and specified to a specific region of the hindbrain. Introduction Retinoic acid (RA) plays an important part in the development of pattern in the zebrafish hindbrain, a fact that has been appreciated since early studies of its teratogenic effects in vivo. RA also plays an important role in developing the anterior-posterior axis, inducing a combinatorial expression of Hox genes in the hindbrain (Kessel, Gruss 1991). Evidence for a RA gradient comes from studies demonstrating that excess of retinoic acid during embryogenesis disrupts development of the anterior hindbrain as a potent dorsalizing signal (Wilson et al. 2007), (Durston et al. 1989). RA deficiency also has teratogenic effects, most clearly demonstrated by vitamin A deficient animal models (White et al. 2000). Despite the importance of RA signalling in vertebrates, only a handful of genes are known to be involved in RA signalling (Duester 2008). To this end, we aimed to identify and characterize novel downstream targets of RA signalling in the vertebrate hindbrain. Based on RA/RA-antagonist microarray data, we hypothesize that we can identify and characterize a candidate gene that is part of a network of downstream targets responsive to RA levels. Within the cell, RA levels are tightly controlled, being either degraded by CYP26 in nontarget tissues, or bound to retinoic acid receptor (RAR) – retinoid X receptor (RXR) heterodimers and functioning as nuclear receptors (Fig 1) (Mark, Ghyselinck & Chambon 2009, Abu-Abed et al. 2001, Sakai et al. 2001). RA signalling in the developing hindbrain is intimately related to its effects on Hox gene function. Several Hox genes are direct targets of RA, and contain retinoic acid response elements (RAREs) in their promoter regions (Simeone et al. 1990) (also see (Langston, Gudas 1994) and references therein). Only a few direct targets of RA signalling have been described: Hox1, a small number of transcription factors (HNF-3α, Cdx1), and a number of genes directly involved in retinoid metabolism (CRABP1, CRABP2) (Balmer, Blomhoff 2002). Patterning of the embryonic dorsoventral axis of vertebrates requires signalling through bone morphogenetic proteins (BMPs). BMP ligands BMP2 and BMP7 function as heterodimers, catalyzing the assembly of a quadripartite transmembrane serine-threonine kinase receptor complex consisting of two type I and two type II receptors. Once the receptors are complexed, a phosphorylation cascade activates BMP-responsive Smads1/5 which act as transcription factors eliciting the downstream response (Feng, Derynck 2005). BMP signalling in vertebrates is tightly linked to RA levels. Indeed, exogenous RA has been shown to directly downregulate BMPs (Thompson et al. 2003). Bone morphogenetic protein receptors (BMPRs) are also ideal gene candidates for RA signalling, based on evidence for RA-induced Volume 1, Number 1 (2010) Eureka