Alternative cleavage of the bone morphogenetic protein (BMP), Gbb, produces ligands with distinct developmental functions and receptor preferences

Abstract

The family of TGF-β and bone morphogenetic protein (BMP) signaling proteins has numerous developmental and physiological roles. They are made as proprotein dimers and then cleaved by proprotein convertases to release the C-terminal domain as an active ligand dimer. Multiple proteolytic processing sites in Glass bottom boat (Gbb), the BMP7 ortholog, can produce distinct ligand forms. Cleavage at the S1 or atypical S0 site in Gbb produces Gbb15, the conventional small BMP ligand, whereas NS site cleavage produces a larger Gbb38 ligand. We hypothesized that the Gbb prodomain is involved not only in regulating the production of specific ligands but also their signaling output. We found that blocking NS cleavage increased association of the full-length prodomain with Gbb15, resulting in a concomitant decrease in signaling activity. Moreover, NS cleavage was required for Gbb-Decapentaplegic (Dpp) heterodimer-mediated wing vein patterning but not for Gbb15-Dpp heterodimer activity in cell culture. Gbb NS cleavage was also required for viability through its regulation of pupal ecdysis in a type II receptor Wishful thinking (Wit)-dependent manner. In fact, Gbb38-mediated signaling exhibits a preference for Wit over the other type II receptor Punt. Finally, we discovered that Gbb38 is produced when processing at the S1/S0 site is blocked by linked glycosylation in third instar larvae. Our findings demonstrate that BMP prodomain cleavage ensures that the mature ligand is not inhibited by the prodomain. Furthermore, alternative processing of BMP proproteins produces ligands that signal through different receptors and exhibit specific developmental functions.

Regulation of neuroblast proliferation by surface glia in the Drosophila larval brain

“…S3A and a previous work 59 , Gbb binds to heparin-sepharose beads. Thus, similar to many other BMP members, Gbb is a heparin-binding protein 59 , characteristic of HS-dependent factors.…”

“…There are multiple proteolytic processing sites in Gbb, which produce distinct mature ligand forms, including gbb 1 5 and Gbb38, and the amount of each product expressed in S2 cells is generally low. When a few cleavage sites are blocked, a larger amount of Gbb38 is produced 59 . S2 cells were transfected with pAW-Gbb[mS1mS0]-HA and incubated for 72 hours.…”

Drosophila ML-DmD17-c3 cells respond robustly to Dpp and exhibit complex transcriptional feedback on BMP signaling components

“…Overall, these results suggest that the combined action of Tkv- and Sax-containing BMP receptors is necessary to fully decode the Dpp signal in vivo. In addition, Sax- and Wit- containing receptor complexes play principal roles in responding to the Scw and Gbb ligands, respectively [62, 65, 67, 68].…”

“…Several outstanding questions regarding the integrated response of cells to stimulation by BMPs remain unanswered. For instance, BMP receptors are thought to exist as heterotetramers [1], and little is known of how the composition of such complexes affects signal transduction, specifically as it pertains to that of distinct ligand heterodimers [11, 61, 68]. In addition, we have demonstrated that depleting specific BMP receptors results in differential effects on known direct transcriptional targets ( Dad , brk ).…”

Specificity, versatility, and control of TGF-β family signaling

“…Cleavage by furins occurs generally but not always in the endoplasmic reticulum, and may be constitutive for the TGF-βs, yet regulated for some TGF-β family proteins (19). Such regulation is apparent with Gbb, the Drosophila homolog of BMP-5, −6 and −7, and illustrates an underappreciated scenario of differential proteolytic processing of the precursor as basis for tissue-specific activities (20–22). The pro-segments act as chaperones required for folding and secretion of TGF-β family dimers (19, 23).…”