Characterization of Schistosome Tegumental Alkaline Phosphatase (SmAP)

Abstract

Schistosomes are parasitic platyhelminths that currently infect over 200 million people globally. The parasites can live for years in a putatively hostile environment - the blood of vertebrates. We have hypothesized that the unusual schistosome tegument (outer-covering) plays a role in protecting parasites in the blood; by impeding host immunological signaling pathways we suggest that tegumental molecules help create an immunologically privileged environment for schistosomes. In this work, we clone and characterize a schistosome alkaline phosphatase (SmAP), a predicted ∼60 kDa glycoprotein that has high sequence conservation with members of the alkaline phosphatase protein family. The SmAP gene is most highly expressed in intravascular parasite life stages. Using immunofluorescence and immuno-electron microscopy, we confirm that SmAP is expressed at the host/parasite interface and in internal tissues. The ability of living parasites to cleave exogenous adenosine monophosphate (AMP) and generate adenosine is very largely abolished when SmAP gene expression is suppressed following RNAi treatment targeting the gene. These results lend support to the hypothesis that schistosome surface enzymes such as SmAP could dampen host immune responses against the parasites by generating immunosuppressants such as adenosine to promote their survival. This notion does not rule out other potential functions for the adenosine generated e.g. in parasite nutrition.

Tegumental Phosphodiesterase SmNPP-5 Is a Virulence Factor for Schistosomes

“…We noted two patterns of immunogold particle localization-clustered and scattered-in the tegument. While scattered immunogold particles have been seen in other immunolocalization experiments involving tegumental molecules (4,16,17), the report of the clustered pattern presented here is, to our knowledge, unique. We speculate that the clustered particles overlay membranous bodies within the tegument called multilaminate vesicles (MLV).…”

Virulence factors of schistosomes

“…The GPIanchored ADP-ribosyl cyclase enzyme also on the external surface, whilst apparently unable to generate cyclic adenosine diphosphate ribose, may catabolize extracellular NAD(þ) to prevent its use by host enzymes that facilitate immune responses [50]. It has also been suggested that the adenosine generated from adenosine monophosphate by tegumental alkaline phosphatase acts as a localised immunosuppressant [51]. Lastly, the tegumental phosphodiesterase, SmNPP-5, has actually been directly implicated as a virulence factor by the use of RNAi treatment to suppress gene expression in parasites, followed by demonstration of their greatly reduced ability to establish after administration to mice [1].…”

Schistosoma comparative genomics: integrating genome structure, parasite biology and anthelmintic discovery

“…Accordingly, schistosomes may prevent their hosts from focusing immunological mediators in their vicinity [82]. In support of this notion, RNAi of adult tegumental alkaline phosphatase (SmAP) prevented production of the anti-inflammatory adenosine from an exogenously added precursor (AMP) [83]. …”