Functional Analysis of saxophone, the Drosophila Gene Encoding the BMP Type I Receptor Ortholog of Human ALK1/ACVRL1 and ACVR1/ALK2

Abstract

In metazoans, bone morphogenetic proteins (BMPs) direct a myriad of developmental and adult homeostatic events through their heterotetrameric type I and type II receptor complexes. We examined 3 existing and 12 newly generated mutations in the Drosophila type I receptor gene, saxophone (sax), the ortholog of the human Activin Receptor-Like Kinase1 and -2 (ALK1/ACVRL1 and ALK2/ACVR1) genes. Our genetic analyses identified two distinct classes of sax alleles. The first class consists of homozygous viable gain-of-function (GOF) alleles that exhibit (1) synthetic lethality in combination with mutations in BMP pathway components, and (2) significant maternal effect lethality that can be rescued by an increased dosage of the BMP encoding gene, dpp 1. In contrast, the second class consists of alleles that are recessive lethal and do not exhibit lethality in combination with mutations in other BMP pathway components. The alleles in this second class are clearly loss-of-function (LOF) with both complete and partial loss-offunction mutations represented. We find that one allele in the second class of recessive lethals exhibits dominant-negative behavior, albeit distinct from the GOF activity of the first class of viable alleles. On the basis of the fact that the first class of viable alleles can be reverted to lethality and on our ability to independently generate recessive lethal sax mutations, our analysis demonstrates that sax is an essential gene. Consistent with this conclusion, we find that a normal sax transcript is produced by sax P , a viable allele previously reported to be null, and that this allele can be reverted to lethality. Interestingly, we determine that two mutations in the first class of sax alleles show the same amino acid substitutions as mutations in the human receptors ALK1/ACVRl-1 and ACVR1/ALK2, responsible for cases of hereditary hemorrhagic telangiectasia type 2 (HHT2) and fibrodysplasia ossificans progressiva (FOP), respectively. Finally, the data presented here identify different functional requirements for the Sax receptor, support the proposal that Sax participates in a heteromeric receptor complex, and provide a mechanistic framework for future investigations into disease states that arise from defects in BMP/TGF-b signaling.T HE type I serine-threonine receptors of the transforming growth factor-b (TGF-b)/bone morphogenetic protein (BMP) signaling pathway are critical for the transduction and specificity of signals initiated by the secreted ligands of this superfamily. A dimeric ligand elicits a vast range of biological responses by binding to the extracellular domain of a heterotetrameric receptor complex comprising two type I and two type II serine/threonine kinase receptors that then transduce a signal by phosphorylation of intracellular transcriptional regulators (reviewed by Yamashita et al. 1994;Weis-Garcia and Massague 1996; Kirsch et al. 2000a,b;Shi and Massague 2003; ten Dijke and Hill 2004). Both receptor types have a cysteine-rich, ligandbinding extracellular domain, ...

Hyperactive BMP signaling induced by ALK2R206Hrequires type II receptor function in aDrosophilamodel for classic fibrodysplasia ossificans progressiva

“…1F). This cell‐based assay makes use of a lacZ reporter construct under the control of the brinker silencer element ( brkS ), which is known to quantitatively repress transcription in response to Mad‐mediated signaling (Müller et al,2003; Bangi and Wharton,2006b; Twombly et al,2009). S2 cells transfected with a plasmid construct encoding the Drosophila ligand Gbb exhibit a reduction in β‐galactatosidase (β‐gal) activity, reflecting the repression of lacZ transcription as a result of an increase in BMP signaling (Fig.…”

Investigations of Activated ACVR1/ALK2, a Bone Morphogenetic Protein Type I Receptor, That Causes Fibrodysplasia Ossificans Progressiva

“…Various animal models in different species (including Drosophila , zebrafish, chicken, and mice) have been useful in understanding in vivo effects of BMPs in ectopic bone formation and have shed light on the pathophysiology of FOP as well as nonsyndromic forms of posttraumatic heterotopic ossification (Fukuda et al ., 2006; Glaser et al ., 2003; Kan et al ., 2004, 2009; Lounev et al ., 2009; Shen et al ., 2009; Twombly et al ., 2009; Urist, 1965; Wozney et al ., 1998; Yu et al ., 2008b). Here we will focus on zebrafish and mouse models.…”

Granting immunity to FOP and catching heterotopic ossification in the Act

“…As in vertebrates, elevated basal BMP pathway signaling associated with mACVR1 in Drosophila is BMP ligand-independent [51]. Wild type ACVR1 can antagonize, as well as promote BMP signaling while mACVR1 can only promote signaling with or without ligand [44,45].…”