Cancer-associated Notch receptor variants lead to O-fucosylation defects that deregulate Notch signaling

Analysis of the Healthy Platelet Proteome Identifies a New Form of Domain-Specific O-Fucosylation

Platelet activation induces the secretion of proteins that promote platelet aggregation and inflammation. However, detailed analysis of the released platelet proteome is hampered by platelets' tendency to preactivate during their isolation and a lack of sensitive protocols for low abundance releasate analysis. Here, we detail the most sensitive analysis to date of the platelet releasate proteome with the detection of >1300 proteins. Unbiased scanning for posttranslational modifications within releasate proteins highlighted O-glycosylation as being a major component. For the first time, we detected O-fucosylation on previously uncharacterized sites including multimerin-1 (MMRN1), a major alpha granule protein that supports platelet adhesion to collagen and is a carrier for platelet factor V. The N-terminal elastin microfibril interface (EMI) domain of MMRN1, a key site for protein-protein interaction, was O-fucosylated at a conserved threonine within a new domain context. Our data suggest that either protein O-fucosyltransferase 1, or a novel protein O-fucosyltransferase, may be responsible for this modification. Mutating this O-fucose site on the EMI domain led to a >50% reduction of MMRN1 secretion, supporting a key role of EMI O-fucosylation in MMRN1 secretion. By comparing releasates from resting and thrombin-treated platelets, 202 proteins were found to be significantly released after high-dose thrombin stimulation. Complementary quantification of the platelet lysates identified >3800 proteins, which confirmed the platelet origin of releasate proteins by anticorrelation analysis. Low-dose thrombin treatment yielded a smaller subset of significantly regulated proteins with fewer secretory pathway enzymes. The extensive platelet proteome resource provided here (larancelab.com/platelet-proteome) allows identification of novel regulatory mechanisms for drug targeting to address platelet dysfunction and thrombosis.

In vivo evidence for GDP-fucose transport in the absence of transporter SLC35C1 and putative transporter SLC35C2

Slc35c1 encodes an antiporter that transports GDP-fucose into the Golgi and returns GMP to the cytoplasm. The closely related gene Slc35c2 encodes a putative GDP-fucose transporter and promotes Notch fucosylation and Notch signaling in cultured cells. Here, we show that HEK293T cells lacking SLC35C1 transferred reduced amounts of O-fucose to secreted epidermal growth factor-like repeats from NOTCH1 or secreted thrombospondin type I repeats from thrombospondin 1. However, cells lacking SLC35C2 did not exhibit reduced fucosylation of these epidermal growth factor-like repeats or thrombospondin type I repeats. To investigate SLC35C2 functions in vivo, WW6 embryonic stem cells were targeted for Slc35c2. Slc35c2[-/-] mice were viable and fertile and exhibited no evidence of defective Notch signaling during skeletal or T cell development. By contrast, mice with inactivated Slc35c1 exhibited perinatal lethality and marked skeletal defects in late embryogenesis, typical of defective Notch signaling. Compound Slc35c1[-/-]Slc35c2[-/-] mutants were indistinguishable in skeletal phenotype from Slc35c1[-/-] embryos and neonates. Double mutants did not exhibit the exacerbated skeletal defects predicted if SLC35C2 was functionally important for Notch signaling in vivo. In addition, NOTCH1 immunoprecipitated from Slc35c1[-/-]Slc35c2[-/-] neonatal lung carried fucose detected by binding of Aleuria aurantia lectin. Given that the absence of both SLC35C1, a known GDP-fucose transporter, and SLC35C2, a putative GDP-fucose transporter, did not lead to afucosylated NOTCH1 nor to the severe Notch signaling defects and embryonic lethality expected if all GDP-fucose transport were abrogated, at least one more mechanism of GDP-fucose transport into the secretory pathway must exist in mammals.

Protein O-glycosylation regulates diverse developmental and defense processes in plants

Post-translational modifications affect protein functions and play key roles in controlling biological processes. Plants have unique types of O-glycosylation that are different from those of animals and prokaryotes, and they play roles in modulating the functions of secretory proteins and nucleocytoplasmic proteins by regulating transcription and mediating localization and degradation. O-glycosylation is complex because of the dozens of different O-glycan types, the widespread existence of hydroxyproline (Hyp), serine (Ser), and threonine (Thr) residues in proteins attached by O-glycans, and the variable modes of linkages connecting the sugars. O-glycosylation specifically affects development and environmental acclimatization by affecting diverse physiological processes. This review describes recent studies on the detection and functioning of protein O-glycosylation in plants, and provides a framework for the O-glycosylation network that underlies plant development and resistance.

Two NOTCH1 O-fucose sites have opposing functions in mouse retinal angiogenesis

Previous in vitro studies demonstrated that Fringe glycosylation of the NOTCH1 extracellular domain at O-fucose residues in Epidermal Growth Factor-like Repeats (EGFs) 6 and 8 is a significant contributor to suppression of NOTCH1 activation by JAG1 or enhancement of NOTCH1 activation by DLL1, respectively. In this study, we sought to evaluate the significance of these glycosylation sites in a mammalian model by generating 2 C57BL/6J mouse lines carrying NOTCH1 point mutations, which eliminate O-fucosylation and Fringe activity at EGFs 6 (T232V) or 8 (T311V). We assessed changes to morphology during retinal angiogenesis, a process in which expression of Notch1, Jag1, Dll4, Lfng, Mfng, and Rfng genes coordinate cell-fate decisions to grow vessel networks. In the EGF6 O-fucose mutant (6f/6f) retinas, we observed reduced vessel density and branching, suggesting that this mutant is a Notch1 hypermorph. This finding agrees with prior cell-based studies showing that the 6f mutation increased JAG1 activation of NOTCH1 during co-expression with inhibitory Fringes. Although we predicted that the EGF8 O-fucose mutant (8f/8f) would not complete embryonic development due to the direct involvement of the O-fucose in engaging ligand, the 8f/8f mice were viable and fertile. In the 8f/8f retina, we measured increased vessel density consistent with established Notch1 hypomorphs. Overall, our data support the importance of NOTCH1 O-fucose residues for pathway function and confirms that single O-glycan sites are rich in signaling instructions for mammalian development.