Carbohydrate Scaffolds for the Study of the Autism-associated Bacterium, Clostridium bolteae

Abstract

A large number of children in the autism spectrum disorder suffer from gastrointestinal (GI) conditions, such as constipation and diarrhea. Clostridium bolteae is a part of a set of pathogens being regularly detected in the stool samples of hosts affected by GI and autism symptoms. Accompanying studies have pointed out the possibility that such microbes affect behaviour through the production of neurotoxic metabolites in a so-called, gut-brain connection. As an extension of our Clostridium difficile polysaccharide (PS)-based vaccine research, we engaged in the discovery of C. bolteae surface carbohydrates. So far, studies revealed that C. bolteae produces a specific immunogenic PS capsule comprised of disaccharide repeating blocks of mannose (Manp) and rhamnose (Rhap) units: α-D-Manp-(1→[-4)-β-D-Rhap- (1→3)-α-D-Manp-(1→]n. For vaccinology and further immunogenic experiments, a method to produce C. bolteae PS conjugates has been developed, along with the chemical syntheses of the PS non-reducing end linkage, with D-Rha or L-Rha, α-D-Manp-(1→4)-α-D-Rhap- (1→O(CH2)5NH2 and α-D-Manp-(1→4)-α-L-Rhap-(1→O(CH2)5NH2, equipped with an aminopentyl linker at the reducing end for conjugation purposes. The discovery of C. bolteae PS immunogen opens the door to the creation of non-evasive diagnostic tools to evaluate the frequency and role of this microbe in autistic subjects and to a vaccine to reduce colonization levels in the GI tract, thus impeding the concentration of neurotoxins.

Keywords: Clostridium bolteae; TEMPO; autism; conjugate; diagnostic; gastrointestinal disorders; gut-brain axis; polysaccharide; synthesis; vaccine..

Fig. (1)

The structure of a proposed C. bolteae disaccharide repeating block: [→3)-α-D-Manp-(1→4)-β-D-Rhap-(1→].

Fig. (2)

Conjugation of the C. bolteae PS to CRM₁₉₇ protein. First, TEMPO-mediated oxidation activates the primary hydroxyl on the Man residue and then carbodiimide-like chemistry is used to conjugate the activated PS to the carrier protein CRM₁₉₇.

Scheme 1

Generation of C. bolteae disaccharide 1 equipped with an aminopentyl linker at the reducing end [30].

Scheme 2

Reagents and conditions [30]: (a) TMSOTf, CH₂Cl₂, -10°C, 69% (b) CAN, CH₃CN:H₂O, 0°C, 50%, (c) K₂CO₃, CH₂Cl₂, CCl₃CN, RT, 29%, (d) TMSOTf, 5-aminio-N-benzyloxycarbonyl pentanol, CH₂Cl₂, RT, 46%, (e) 
NH_{3 (l)}, Na _(s), THF, -78°C, 21%, (f) AcOH:H₂O, 80°C, 81%.