Sugar-free frosting, a homolog of SAD kinase, drives neural-specific glycan expression in the Drosophila embryo

Abstract

SUMMARYPrecise glycan structures on specific glycoproteins impart functionalities essential for neural development. However, mechanisms controlling embryonic neural-specific glycosylation are unknown. A genetic screen for relevant mutations in Drosophila generated the sugar-free frosting (sff) mutant that reveals a new function for protein kinases in regulating substrate flux through specific Golgi processing pathways. Sff is the Drosophila homolog of SAD kinase, which regulates synaptic vesicle tethering and neuronal polarity in nematodes and vertebrates. Our Drosophila sff mutant phenotype has features in common with SAD kinase mutant phenotypes in these other organisms, but we detect altered neural glycosylation well before the initiation of embryonic synaptogenesis. Characterization of Golgi compartmentation markers indicates altered colocalization that is consistent with the detected shift in glycan complexity in sff mutant embryos. Therefore, in analogy to synaptic vesicle tethering, we propose that Sff regulates vesicle tethering at Golgi membranes in the developing Drosophila embryo. Furthermore, neuronal sff expression is dependent on transcellular signaling through a non-neural toll-like receptor, linking neural-specific glycan expression to a kinase activity that is induced in response to environmental cues.

SAD kinases control the maturation of nerve terminals in the mammalian peripheral and central nervous systems

“…Although our survey is incomplete, we conclude that SADs promote maturation of many, and perhaps all, types of synapses in mice. Favoring this idea, mutations of SAD orthologs in C. elegans and Drosophila also lead to presynaptic defects (16,37). Although presynaptic maturation was impaired in all four types of synapses examined, the defects varied among synaptic types.…”

Vertebrate protein glycosylation: diversity, synthesis and function

“…For example, administration of epithelial growth factor (EGF) and platelet-derived growth factor (PDGF) to HeLa cells activates a Golgi-localized SRC kinase that induces a redistribution of the enzymes responsible for initiating O -linked glycosylation from the cis -Golgi to the ER, where they drive increased mucin glycosylation 86 . Other Tyr and Ser/Thr kinases have recently been implicated as regulators of Golgi morphology and, in some cases, protein glycosylation 87–90 . These emerging paradigms, in which growth factors, cytokines, chemokines and other signalling molecules stimulate changes in Golgi dynamics, promise to reveal stronger mechanistic links between cell signalling and glycomic diversity.…”

N-Glycosylation in Regulation of the Nervous System

“…Downregulation of sugar-free frosting , a gene encoding a Drosophila homolog of SAD kinase that regulates secretory flux through the Golgi, inhibits synthesis of the HRP glycoepitope (α3-linked core fucose) and increases the amount of hybrid and complex N-glycan structures. Sugar-free frosting mutations lead to neuromuscular junction defects in larvae and locomotor abnormalities in adult flies (Baas et al 2011). Meigo, a putative nucleotide sugar transporter, appears to specifically regulate the targeting of neurite projections in the olfactory system by affecting N-glycosylation of ephrin (Sekine et al 2013).…”