Background Schistosomiasis remains widespread in many regions despite efforts at its elimination. By examining changes in the transcriptome at the host-pathogen interface in the snail Biomphalaria glabrata and the blood fluke Schistosoma mansoni, we previously demonstrated that an early stress response in juvenile snails, manifested by induction of heat shock protein 70 (Hsp 70) and Hsp 90 and of the reverse transcriptase (RT) domain of the B. glabrata non-LTR- retrotransposon, nimbus, were critical for B. glabrata susceptibility to S. mansoni. Subsequently, juvenile B. glabrata BS-90 snails, resistant to S. mansoni at 25°C become susceptible by the F2 generation when maintained at 32°C, indicating an epigenetic response. Methodology/Principal findings To better understand this plasticity in susceptibility of the BS-90 snail, mRNA sequences were examined from S. mansoni exposed juvenile BS-90 snails cultured either at 25°C (non-permissive temperature) or 32°C (permissive). Comparative analysis of transcriptomes from snails cultured at the non-permissive and permissive temperatures revealed that whereas stress related transcripts dominated the transcriptome of susceptible BS-90 juvenile snails at 32°C, transcripts encoding proteins with a role in epigenetics, such as PIWI (BgPiwi), chromobox protein homolog 1 (BgCBx1), histone acetyltransferase (BgHAT), histone deacetylase (BgHDAC) and metallotransferase (BgMT) were highly expressed in those cultured at 25°C. To identify robust candidate transcripts that will underscore the anti-schistosome phenotype in B. glabrata, further validation of the differential expression of the above transcripts was performed by using the resistant BS-90 (25°C) and the BBO2 susceptible snail stock whose genome has now been sequenced and represents an invaluable resource for molecular studies in B. glabrata. A role for BgPiwi in B. glabrata susceptibility to S. mansoni, was further examined by using siRNA corresponding to the BgPiwi encoding transcript to suppress expression of BgPiwi, rendering the resistant BS-90 juvenile snail susceptible to infection at 25°C. Given transposon silencing activity of PIWI as a facet of its role as guardian of the integrity of the genome, we examined the expression of the nimbus RT encoding transcript at 120 min after infection of resistant BS90 piwi-siRNA treated snails. We observed that nimbus RT was upregulated, indicating that modulation of the transcription of the nimbus RT was associated with susceptibility to S. mansoni in BgPiwi-siRNA treated BS-90 snails. Furthermore, treatment of susceptible BBO2 snails with the RT inhibitor lamivudine, before exposure to S. mansoni, blocked S. mansoni infection concurrent with downregulation of the nimbus RT transcript and upregulation of the BgPiwi encoding transcript in the lamivudine-treated, schistosome-exposed susceptible snails. Conclusions and significance These findings support a role for the interplay of BgPiwi and nimbus in the epigenetic modulation of plasticity of resistance/susceptibility in the snail-schistosome relationship.

The human telomerase reverse transcriptase (hTERT) is the catalytic sub-unit of the ribonuclear protein, telomerase. Together with telomerase RNA, the enzyme complex participates in the maintenance of telomeres at the proximal ends of chromosomes, adding species-specific repeats to the 3’end of the telomere. The regulation of hTERT is tightly linked to the cell cycle and cell differentiation states governing either malignancy or senescence, making it a prospective therapeutic target of cell proliferation in cancer. Malignancy behaves like a parasitic disease in that it only progresses by depending on biochemical and molecular pathways of the host. The snail host/schistosome relationship provides a facile model to examine the regulation of the cancer transcriptome, such as the gastropod homolog of hTERT. To test this hypothesis in relation to the development of larval Schistosoma mansoni in the Biomphalaria glabrata, we utilized an in-silico approach to identify the hTERT homolog of B. glabrata. The human hTERT amino acid sequence (ID 014746) shows a strong homology (E-value of 2e86) to the B. glabrata ortholog (733 amino acids, accession XP_013074763.1). BLASTp analyses using S. mansoni as the query suggested that the parasite lacks a cognate TERT. To study the regulation of the snail-like hTERT in relation to schistosome development, transcriptome analysis was performed which revealed a temporal regulation of the telomerase before and during S. mansoni infection, with an upregulation of B. glabrata hTERT transcription evident by 30 minutes after exposure to the parasite. The anti-telomerase drugs, BPPA and BIBR at 100 ng/mL before infection blocked shedding of parasite cercariae. These findings indicate that the schistosome may rely on the telomerase of its host for asexual reproduction, development and proliferation.

BackgroundSchistosomiasis remains widespread in many regions despite efforts at its elimination. By examining changes in the transcriptome at the host-pathogen interface in the snail Biomphalaria glabrata and the blood fluke Schistosoma mansoni, we previously demonstrated that an early stress response in juvenile snails, manifested by induction of heat shock protein 70 (Hsp 70) and Hsp 90 and of the reverse transcriptase (RT) domain of the B. glabrata non-LTR-retrotransposon, nimbus, were critical for B. glabrata susceptibility to S. mansoni. Subsequently, juvenile B. glabrata BS90 snails, resistant to S. mansoni at 25°C become susceptible by the F2 generation when maintained at 32°C, indicating an epigenetic response.Methodology/Principal FindingsTo better understand this plasticity in susceptibility of the BS90 snail, mRNA sequences were examined from S. mansoni exposed juvenile BS90 snails cultured either at 25°C (permissive temperature) or 32°C (non-permissive). Comparative analysis of transcriptomes from snails cultured at the non-permissive and permissive temperatures revealed that whereas stress related transcripts dominated the transcriptome of susceptible BS90 juvenile snails at 32°C, transcripts encoding proteins with a role in epigenetics, such as PIWI (BgPiwi), chromobox protein homolog 1 (BgCBx1), histone acetyl transferase histone deacetylase (HDAC) and metallotransferase (MT) were highly expressed in those cultured at 25°C. To further determine a role for BgPiwi in B. glabrata susceptibility to S. mansoni, siRNA corresponding to the BgPiwi encoding transcript was utilized to suppress expression of BgPiwi, rendering the resistant BS90 juvenile snail susceptible to infection at 25°C. Given transposon silencing activity of PIWI as a facet of its role as guardian of the integrity of the genome, we examined the expression of the nimbus RT encoding transcript at 120 min after infection of resistant BS90 piwi-siRNA treated snails. We observed that nimbus RT was upregulated, indicating that modulation of the transcription of the nimbus RT was associated with susceptibility to S. mansoni in BgPiwi- siRNA treated BS90 snails. Furthermore, treatment of susceptible snails with the RT inhibitor lamivudine, before exposure to S. mansoni, blocked S. mansoni infection concurrent with downregulation of the nimbus RT transcript and upregulation of the BgPiwi encoding transcript in the lamivudine-treated, schistosome-exposed susceptible snails.Conclusions and SignificanceThese findings support a role for the interplay of BgPiwi and nimbus in the epigenetic modulation of plasticity of resistance/susceptibility in the snail-schistosome relationship.