Paucimannosylation

The term "paucimannose" (occasionally spelled as "pauci-mannose") was coined in the early 1990s glycobiology literature. == Common paucimannosidic structural features across species, tissue and protein origin ==

Paucimannosidic glycans span the base composition Man1-3GlcNAc2. Additional modifications with Fuc, Xyl and/or Galactose (Gal) are common in mammals ref, plants and invertebrates, respectively. Paucimannosidic glycans expressed by insects and nematodes are particularly rich in structural diversity. == Tissue expression and (sub-) cellular localisation ==

Paucimannosylation has been extensively studied and documented in insects, nematodes and plants over the past decades. The paucimannosidic proteins are constitutively and broadly expressed across tissues in these organisms under normal physiology. It is widely recognised that paucimannosylation is a central component of the glycoproteome in these "lower" organisms. pathogen infection, inflammation and stemness. Insects Paucimannosidic glycans form the main component of the N-glycome of insects such as Drosophila melanogaster. Glycoprofiling of the venom component of the western honeybee, Apis mellifera, identified that paucimannosylation is a common modification of key proteins including hyaluronidase and phospholipase. Insect cells lines are frequently utilised for recombinant expression of mammalian glycoproteins, which therefore are decorated with paucimannosidic glycans e.g. mouse interferon-β, human IgG1 and calf alkaline phosphatase. Nematodes The model organism Caenorhabditis elegans classified under the phylum Nematoda is amongst the most studied invertebrate species in glycobiology. The literature clearly documents a repertoire of nematodal paucimannosidic glycans. Another model nematode, Pristionchus pacificus, was also documented to express common nematodal paucimannosidic glycans. In addition, there have been reports documenting the expression of paucimannosidic glycans by others parasitic nematodes such as Ascaris suum, Heligmosomoides polygyrus and Trichuris suis. Plants Most plant species studied to date are recognised to constitutively express paucimannosidic N-glycoproteins. The paucimannosidic N-glycoproteins are abundantly expressed in the vacuoles of plants such as the legume seeds of Lotus japonicus, the rice seeds and leaves of Oryza sativa. Literature has provided evidence for plant-specific paucimannosidic glycan structures modified with Xyl and Fuc. Such structures are found across the broad Streptophyta (land plants) and Chlorophyta (green algae) clade and in diatoms such as Phaeodactylum tricornutum. Vertebrates Paucimannosidic proteins have been reported in vertebrates such as quail, chicken and in mammals, and human tissues, but have subsequently been found also to decorate non-lysosomal glycoproteins. Particularly, the granules of human neutrophils are a principal source of paucimannosidic proteins. Paucimannosidic proteins were also observed in human monocytes and macrophages and paucimannosidic immunoglycopeptides were found to be presented by SARS-CoV-2 challenged dendritic cells. Species within other classes under Animalia related to vertebrates were also documented to express paucimannosidic proteins. with some observations of unusual plant- and invertebrate-like paucimannosidic glycan structures == Biosynthesis of paucimannosidic N-glycoproteins ==

Common aspects of the biosynthesis of paucimannosidic glycoproteins across species Similar to other N-linked glycan types, the biosynthesis of paucimannosidic proteins across most species has been documented to be facilitated by the actions of a limited set of glyco-enzymes including beta-N-acetylhexosaminidases (Hex) and alpha-mannosidases, through GnT-I-dependent and -independent truncation pathways. However, except for the well-studied D. melanogaster and other common insect model organisms, solid evidence for active involvement of Hex and/or the possible concerted usage of the GnT-I-independent pathway or alternative truncation pathways for paucimannosidic protein production remains unavailable across the diverse class of insects. Nematodes The model organism C. elegans is well studied; solid glycobiological literature have provided insights on the nematodal N-glycosylation machinery which shares many traits with other eukaryotic species. C. elegans is known to produce paucimannosidic proteins via a GnT-I-dependent route in which GnT-I firstly produces GlcNAc-capped glycoprotein intermediates. Further processing by two Hex isoenzymes (HEX-2 and HEX-3) encoded by two C. elegans genes (hex-2, hex-3) generate the unsubstituted C. elegans paucimannosidic glycans. Other glycoenzymes catalise further processing and structural diversity including α-Man II and α1,6- and α1,3-fucosyltransferases. Albeit less active, a GnT-I-independent α1,6-fucosyltransferase has also been observed for C. elegans, indicating that both the GnT-I-dependent and -independent pathways may contribute to the formation of paucimannosidic N-glycoproteins in worms. However, the biosynthetic processes underpinning the unusual non-sugar and core-modified paucimannosidic N-glycans in C. elegans remain to be elucidated. Plants Hexosaminidases (Hex) are important glycoside hydrolases for the generation of plant-specific paucimannosidic proteins across Plantae. HEXO1-HEXO3 have been reported to be key mediators of paucimannose expression in various plant species including Nicotiana benthamiana, A.thaliana and L. japonicus. Moreover, α1,3-fucosyltransferase (FUT11/12) and β1,2-xylosyltransferase as well as α-mannosidase II were also reported to play critical roles in the generation of the paucimannosidic proteins expressed by plants. From these two subunits, isoenzymes such as Hex A (one alpha and one beta subunit), Hex B (two beta subunits) and Hex S (two alpha subunits) are generated. Both Hex A and Hex B are reported to play important functional roles in human, Recently, granule-specific glycosylation was shown in neutrophils featuring prominent paucimannosylation in the azurophilic granules an observation that was suggested to arise from a "glycosylation-by-timing" mechanism yet to be documented. More widely across vertebrate species, the biosynthesis of paucimannosidic proteins remains largely unstudied. == Functions of protein paucimannosylation ==

Human The function of protein paucimannosylation remains largely unexplored in vertebrates. Recent literature however has emerged demonstrating that paucimannosylation play roles in mediating pathophysiological processes such as in inflammation, pathogen infection, cancer and in the development of stem cells and in normal homeostasis. For example, elevated expression of paucimannosidic proteins was shown in Mycobacterium tuberculosis infected macrophages, during preclampsia and on Tamm-Horsfall proteins secreted by human urothelial cells during urinary tract infections suggesting the involvement of paucimannosylation in those conditions. Additionally, sputum from individuals suffering from cystic fibrosis and airway infections were also observed to be rich in paucimannosidic proteins. Furthermore, paucimannosylation was reported to be prominent features of human neutrophils breast, blood, non-melanoma, liver, ovarian and prostate cancers. Enriched paucimannosidic glycoepitopes were found in the tumours when compared to the adjacent non-tumour tissues. Literature have also reported the presence of paucimannosylation in embryonic stem cells and neuronal stem cells, suggesting potential functional role(s) in these cells. Notably, deficiency of hexosaminidases results in clinically significant Tay-Sachs and Sandhoff diseases, which also implicates Hex and paucimannosidic proteins in those conditions. Endogenous and exogenous binding partners of mammalian paucimannosidic glycans have been suggested, and P. aeruginosa PA-IIL were also reported to play important roles in the adhesion and pathophysiology of these opportunistic pathogens. Insects In D. melanogaster, FDL-deficient mutants showed paucimannose-deficiency and, notably, caused locomotion defects in fruit flies, indicating that Hex and/or paucimannosidic proteins are involved, via elusive pathways, in essential fruit fly processes. As expected, the less-consequential monoallelic fdl mutation was shown to result in reduced paucimannosidic protein formation and caused a non-lethal, but still severe phenotype, by halting the generation of peripheral long-term memory neurons. Impaired generation of peripheral long-term memory neurons Taken together, these phenotypic observations suggest that the fruit fly paucimannosidic glycans, some of which overlap with the human repertoire, are pivotal in the development, immune function and survival processes of D. melanogaster. It was reported that T. castaneum abundantly expresses paucimannosidic proteins during its post-larval stages, recapitulating findings from other studies proposing that paucimannosidic proteins are strongly regulated during early development. In support, C. elegans hex-2 gene knock-out mutants displayed reduced paucimannosidic protein levels and altered sensitivity towards nematotoxic lectins relative to wild-type worms, a correlation suggesting involvement of paucimannosidic proteins in key C. elegans survival processes. Functionally, phosphocholine-containing paucimannosidic glycans were demonstrated to display immune-modulating roles in parasitic nematodes. Paucimannosidic glycans were suggested to play roles in the nematodal innate immune system by impacting the nematode's ability to fight and survive pathogenic bacteria == References ==