Mummy, A UDP-N-acetylglucosamine pyrophosphorylase, modulates DPP signaling in the embryonic epidermis of Drosophila

Abstract

The evolutionarily conserved JNK/AP-1 (Jun N-terminal kinase/activator protein 1) and BMP (Bone Morphogenetic Protein) signaling cascades are deployed hierarchically to regulate dorsal closure in the fruit fly Drosophila melanogaster. In this developmental context, the JNK/AP-1 signaling cascade transcriptionally activates BMP signaling in leading edge epidermal cells. Here we show that the mummy (mmy) gene product, which is required for dorsal closure, functions as a BMP signaling antagonist. Genetic and biochemical tests of Mmy’s role as a BMP-antagonist indicate that its function is independent of AP-1, the transcriptional trigger of BMP signal transduction in leading edge cells. pMAD (phosphorylated Mothers Against Dpp) activity data show the mmy gene product to be a new type of epidermal BMP regulator – one which transforms a BMP ligand from a long- to a short-range signal. mmy codes for the single UDP-N-acetylglucosamine pyrophosphorylase in Drosophila, and its requirement for attenuating epidermal BMP signaling during dorsal closure points to a new role for glycosylation in defining a highly restricted BMP activity field in the fly. These findings add a new dimension to our understanding of mechanisms modulating the BMP signaling gradient.

Regulation of BMP Signaling by O-GlcNAcylation

“…Dpp signaling expansion in sxc mutants is identical to that which we documented previously in mmy 1 mutants (Humphreys et al, 2013). In this regard, we observed that while pMad immunoreactivity extends to a maximum average depth of five cells in the epidermis of wild-type embryos, immunoreactivity extends to a maximum average depth of ten cells in similarly staged sxc mutants ( Figure 3G).…”

“…The sxc-encoded OGT is required for embryonic development mmy codes for the single UDP-N-acetylglucosamine pyrophosphorylase in Drosophila (Schimmelpfeng et al, 2006), and its requirement for attenuating epidermal BMP signaling during dorsal closure points to a previously unrecognized role for glycosylation in defining a restricted BMP activity field in the fly (Humphreys et al, 2013). UDP-GlcNAc serves as the precursor for a diverse set of glycosyl modifications that are catalyzed by a similarly diverse set of transferase enzymes (Lairson et al, 2008).…”

“…Third, the JNK reporter line, dpp 151H is properly expressed in the LE of sxc embryos ( Figure 3L). It is also notable that the expression domain of the dpp transcriptional inhibitor brk is unaffected in sxc mutants ( Figure 3M-N), as it is similarly unaffected in all Dpp/BMP signaling antagonists characterized to date (Humphreys et al, 2013).…”

“…First, Jun accumulates normally in LE cells in sxc mutants ( Figure 3H-J, H'-J'). Second, Jun protein levels are not elevated in sxc mutants, particularly in comparison to a known JNK antagonist, raw ( Figure 3K) (Humphreys et al, 2013). Third, the JNK reporter line, dpp 151H is properly expressed in the LE of sxc embryos ( Figure 3L).…”

“…Three of the four antagonists we and others have studied thus far (raw, puckered [puc], and ribbon [rib]) function at the level of JNK (Byars et al, 1999). mmy, in contrast, targets the Dpp/BMP signaling pathway directly (Humphreys et al, 2013). None are part of the brinker (brk) repression system (Deignan et al, 2016;Minami et al, 1999).…”