ARTICLE TOPIC COLLECTION: Func%onal Implica%ons of Piwi Proteins and piRNAs in Stem Cell Maintenance and Development

Fernando Claro Linares Patricia Rojas Ríos ^*

• Department of Genetics, Faculty of Biology, University of Seville, Seville, Spain

In this mini review, we discussed the func%onal roles of PIWI proteins and their associated small RNAs, piRNAs, in regula%ng gene expression within stem cell biology. Guided by piRNAs, these proteins transcrip%onally and post-transcrip%onally repress transposons using mechanisms such as the ping-pong amplifica%on cycle and phasing to protect germline genomes. Ini%ally iden%fied in Drosophila melanogaster, the piRNA pathway regulate germline stem cell self-renewal and differen%a%on via cellautonomous and non-cell-autonomous mechanisms. Precisely, in GSCs, PIWI proteins and piRNAs regulate gene expression by modula%ng chroma%n states and directly influencing mRNA transla%on. For instance, the PIWI protein Aubergine loaded with piRNAs promotes and represses transla%on of certain mRNAs to balance self-renewal and differen%a%on. Thus, the piRNA pathway exhibits dual regulatory roles in mRNA stability and transla%on, highligh%ng its context-dependent func%ons. Moreover, PIWI proteins are essen%al in soma%c stem cells to support the regenera%ve capacity of highly regenera%ve species, such as planarians. Similarly, in Drosophila intes%nal stem cells, the PIWI protein Piwi regulates metabolic pathways and genome integrity, impac%ng longevity and gut homeostasis. In this case, piRNAs appear absent in the gut, sugges%ng piRNA-independent regulatory mechanisms. Together, PIWI proteins and piRNAs demonstrate evolu%onary conserva%on in stem cell regula%on, integra%ng TE silencing and gene expression regula%on at chroma%n and mRNA levels in soma%c and germline lineages. Beyond their canonical roles, emerging evidence reveal their broader significance in maintaining stem cell proper%es and organismal health under physiological and pathological condi%ons.