Year of Publication

2014

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Molecular and Cellular Biochemistry

First Advisor

Dr. Salvatore J. Turco

Abstract

The abundant cell-surface lipophosphoglycan (LPG) of Leishmania parasites plays a central role throughout the eukaryote’s life cycle. A number of LPG-defective mutants and their complementing genes have been isolated and have proven invaluable in assessing the importance of LPG and related glycoconjugates in parasite virulence. While ricin agglutination selection protocols frequently result in lpg- mutants, one L. donovani variant we isolated, named JABBA, was found to be lpg+. Procyclic (logarithmic) JABBA expresses significant amounts of a large-sized LPG, larger than observed from procyclic wild-type but similar in size to LPG from wild-type from metacyclic (stationary) phase.

Structural analysis of the LPG from logarithmically-grown JABBA by capillary electrophoresis protocols revealed that it averaged 30 repeat units composed of the unsubstituted Gal(β1,4)Man(α1)-PO4 typical of wild-type L. donovani. Analysis of JABBA LPG caps indicated that 20% are the disaccharide Glc(β1,2)Man, trisaccharide Gal(β1,4)[Glc(β1,2)]Man, and tetrasaccharide Gal(β1,4)[Glc(β1,2)Man(α1,2)]Man in addition to wild-type Gal(β1,4)Man and Man(α1,2)Man and Gal(β1,4)[Man(α1,2)]Man terminating caps, These glucose containing isoforms were absent in stationary parasites.

Consistent with these structural observations, analyses of the relevant glycosyltransferases in JABBA microsomes involved in LPG biosynthesis showed a two-fold increase in elongating mannosylphosphoryltransferase activity and up-regulation of a β-glucosyltransferase activity. The β-glucosyltransferase in both JABBA and wild-type in vitro produced a β-glucosidase sensitive and β-galactosidase sensitive trisaccharide, indicative of the mannose of repeating units and caps being used in substrate, novel in comparison to other Leishmania species. Furthermore, the caps of JABBA LPG are cryptic in presentation as shown by the loss of binding by the lectins ricin, peanut agglutinin and concanavalin A and reduced accessibility of the terminal galactose residues to oxidation by galactose oxidase.

These results indicate that LPG from JABBA is intriguingly similar to the larger LPG in wild-type parasites that arises following the differentiation of the non-infectious procyclic promastigotes to infectious, metacyclic forms, and has a unique β-glucosyltransferase not active in vivo in wild-type parasites.

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