Synthesis of N-Glycans and Azasugars to Target Disease

<p>In this thesis two aspects of carbohydrate research will be discussed. First, the total synthesis of N-glycans found on allergens that are known to stimulate an allergic immune response and second, the synthesis of iminosugars in an attempt to extend the scope of the PGF-methodology. Asthma affects 235 million people worldwide, with New Zealand ranking among the highest in the world. Although there is a good understanding of how allergens trigger the immune system on a “macroscopic” level, how an allergen’s molecular structure causes such an allergic response remains unknown. Upon close review of carbohydrates present on the allergens that are known to give an allergic T helper (Th 2) immune response, a common structure has been identified. The structure consists of a complex type N-glycan made up of a pentasaccharide core (Man3(GlcNHAc)2), with additional 1,3-linked α-L-fucose and 1,2-linked D-xylose cappings. As part of a structure relationship study this heptasaccharide and structural derivatives thereof have been synthesised. The synthesis of these N-glycans will allow a more detailed study of the role of these defined structures in triggering an allergic immune response. The second part of this thesis focuses on the protecting group free (PGF) synthesis of iminosugars, which are potent glycosidase inhibitors and are currently used in the treatment of a variety of diseases. Synthetic strategies for the synthesis of iminosugars involve the use of protecting groups, which are necessary to block potential competing reactive centres within a molecule during the multistep synthesis. The disadvantage, however, is that the installation of protecting groups introduces additional steps to the total synthesis, which inevitably leads to lower yields and the generation of waste. Within our group, PGF methodologies have been developed which allow for the synthesis of a variety of iminosugars. The work presented in this thesis extends the scope of this methodology for the synthesis of an important class of iminosugars, the 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidines. For the purpose of introducing an additional hydroxymethyl group, a ketose starting material was required, and therefore an efficient Vasella/reductive amination reaction using ketoses was developed. Additionally, iodocyclisation and carbamate annulation of the intermediate alkenylamines provided successful entry to the 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidines, exemplified by the efficient 6-step synthesis of 2,5-dideoxy-2,5-imino-L-iditol and 2,5-dideoxy-2,5-imino-D-mannitol (DMDP).</p>

Synthesis of N-Glycans and Azasugars to Target Disease

<p>In this thesis two aspects of carbohydrate research will be discussed. First, the total synthesis of N-glycans found on allergens that are known to stimulate an allergic immune response and second, the synthesis of iminosugars in an attempt to extend the scope of the PGF-methodology. Asthma affects 235 million people worldwide, with New Zealand ranking among the highest in the world. Although there is a good understanding of how allergens trigger the immune system on a “macroscopic” level, how an allergen’s molecular structure causes such an allergic response remains unknown. Upon close review of carbohydrates present on the allergens that are known to give an allergic T helper (Th 2) immune response, a common structure has been identified. The structure consists of a complex type N-glycan made up of a pentasaccharide core (Man3(GlcNHAc)2), with additional 1,3-linked α-L-fucose and 1,2-linked D-xylose cappings. As part of a structure relationship study this heptasaccharide and structural derivatives thereof have been synthesised. The synthesis of these N-glycans will allow a more detailed study of the role of these defined structures in triggering an allergic immune response. The second part of this thesis focuses on the protecting group free (PGF) synthesis of iminosugars, which are potent glycosidase inhibitors and are currently used in the treatment of a variety of diseases. Synthetic strategies for the synthesis of iminosugars involve the use of protecting groups, which are necessary to block potential competing reactive centres within a molecule during the multistep synthesis. The disadvantage, however, is that the installation of protecting groups introduces additional steps to the total synthesis, which inevitably leads to lower yields and the generation of waste. Within our group, PGF methodologies have been developed which allow for the synthesis of a variety of iminosugars. The work presented in this thesis extends the scope of this methodology for the synthesis of an important class of iminosugars, the 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidines. For the purpose of introducing an additional hydroxymethyl group, a ketose starting material was required, and therefore an efficient Vasella/reductive amination reaction using ketoses was developed. Additionally, iodocyclisation and carbamate annulation of the intermediate alkenylamines provided successful entry to the 2,5-dihydroxymethyl-3,4-dihydroxypyrrolidines, exemplified by the efficient 6-step synthesis of 2,5-dideoxy-2,5-imino-L-iditol and 2,5-dideoxy-2,5-imino-D-mannitol (DMDP).</p>

Allergen-induced dendritic cell migration is controlled through Substance P release by sensory neurons

SUMMARYDendritic cells (DCs) of the cDC2 lineage are necessary for the initiation of the allergic immune response and in the dermis are marked by their expression of CD301b. CD301b+ dermal DCs respond to allergens encountered in vivo, but not in vitro. This suggests that another cell in the dermis may sense allergens and relay that information to activate and induce the migration of CD301b+ DCs to the draining lymph node. Using a model of cutaneous allergen exposure, we show that allergens directly activate TRPV1+ sensory neurons leading to itch and pain behaviors. Allergen-activated sensory neurons release the neuropeptide Substance P, which stimulates proximally located CD301b+ DCs through MRGPRA1. Substance P induces CD301b+ DC migration to the draining lymph node where they initiate Th2 differentiation. Thus, sensory neurons act as primary sensors of allergens, linking exposure to activation of allergic-skewing DCs and the initiation of the allergic immune response.