PIWI proteins and PIWI-interacting RNAs function in Hydra somatic stem cells

Area of Science:

• Molecular Biology
• Developmental Biology
• Genetics

Background:

• PIWI proteins and PIWI-interacting RNAs (piRNAs) are crucial for animal fertility but their functions in somatic cells are largely unknown.
• The simple metazoan Hydra, with its well-characterized stem/progenitor cells, offers a unique model to study PIWI function outside the germline.

Purpose of the Study:

• To investigate the role and mechanism of PIWI proteins and piRNAs in the somatic stem/progenitor cells of Hydra.
• To explore the evolutionary origins of the PIWI-piRNA pathway.

Main Methods:

• Identified and characterized two PIWI proteins (Hywi and Hyli) and associated piRNAs in Hydra.
• Generated a Hydra transcriptome for piRNA mapping and analyzed small RNA populations across different cell lineages.
• Assessed the essentiality of Hywi function in somatic epithelial lineages.

Main Results:

• Hywi and Hyli are expressed in all Hydra stem/progenitor cells, with associated piRNAs exhibiting a ping-pong signature, indicating posttranscriptional regulation.
• piRNAs are significantly enriched in transposons, with transposon targeting primarily observed in the interstitial lineage.
• Putative non-transposon targets were identified for each lineage, and Hywi function was confirmed as essential in somatic epithelial cells.

Conclusions:

• The PIWI-piRNA pathway in Hydra's somatic stem/progenitor cells suggests an ancestral function in broader stem cell regulation.
• This study expands the understanding of PIWI-piRNA pathway roles beyond the germline in a nonbilaterian model organism.