A mechanistic link between histone mRNA homeostasis and piRNA biogenesis

Abstract

Germ cells play a fundamental role in evolution by containing all the necessary information for the development of a new organism. Numerous mechanisms have evolved to protect germ cells from unwanted changes, ensuring the correct transmission of information to the progeny. One such mechanism is the piRNA pathway, a highly conserved small RNA pathway best known for silencing transposable elements (TEs) in germ cells. In C. elegans, piRNAs predominantly target germline mRNAs and have only a modest effect on TEs regulation. Unlike other species, the loss of the piRNA pathway in C. elegans does not lead to acute sterility. piRNAs precursors are stabilised by binding to a complex known as PETISCO, which stimulates piRNA production. Surprisingly, although piRNAs are not essential for C. elegans development, PETISCO is crucial for their embryonic development. This apparent dual functionality is reflected by PETISCO’s two interacting proteins: PID-1 and TOST-1. While pid-1 mutants lack piRNAs and are viable, tost-1 mutants have piRNAs but are embryonic lethal. In this work, I found that PETISCO binds replication-dependent (RD) histone mRNAs and is essential for their homeostasis in the adult germline. This function depends on TOST-1, not PID-1, reflecting the dual functionality of PETISCO through its interacting proteins. Using various transgenic constructs, I found that histone expression is tightly regulated by the unique features of their transcripts, which complicates the investigation of their expression. Nonetheless, I demonstrated that altering histone concentrations, either through genetic depletion or by impairing degradation pathways, regulates embryogenesis and influences the fate of development or arrest. I propose that the maternal effect lethal phenotype of PETISCO mutants is a result of histone mRNA loss. Additionally, I found that PETISCO mutants exhibit widespread gene expression misregulation in early embryos and premature gene activation. I propose that PETISCO stabilizes maternal histone mRNA in the germline, ensuring their proper deposition into embryos, where they are essential for the earliest cell divisions. Overall, this study, combined with our previous work, proposes that PETISCO functions to stabilise inherently unstable transcripts: in the germ cells, piRNA precursors are stabilized to enable processing into piRNAs, while maternal RD histone mRNAs are stabilized to provide the early embryo with sufficient histone proteins. Evolutionary analysis of PID-1 and TOST-1 indicates that the RD histone mRNA stabilization function likely pre-dates PETISCO’s role in piRNA production. Hence, piRNA biogenesis may have evolved from a maternal mRNA stabilisation mechanism.