# Deciphering pachytene piRNA function

> **NIH NIH R01** · UNIVERSITY OF PENNSYLVANIA · 2021 · $322,000

## Abstract

Deciphering pachytene piRNA function
PROJECT SUMMARY / ABSTRACT
 An enigmatic class of small RNAs appears at the pachynema of Meiosis I of mammalian
spermatocytes. They are processed from long, non-coding RNAs, bind to Piwil1 (in mouse commonly
known as Miwi) protein, and are termed pachytene piwi-interacting (pi) RNAs. Miwi/piRNAs are
essential for spermiogenesis and male fertility. Our laboratory discovered that in diverse species, Piwi
proteins loaded with piRNAs are symmetrically dimethylated on specific arginines by the
methylosome, and mediate interaction with Tudor domain containing (Tdrd) proteins. Miwi binds
directly to Tdrd6, a protein that contains six canonical Tudor domains, and together form the core of
the chromatoid body, a large, cytoplasmic, non-membrane bound structure that contains numerous
mRNAs along with pachytene piRNAs.
 Pachytene piRNAs are very abundant; they are not conserved even among closely related species;
their sequence diversity is enormous; and their function still remains a mystery. Hypotheses about
their roles need to reconcile two seemingly contradictory properties: like microRNAs, pachytene
piRNAs are loaded to an Argonaute protein, Miwi, and can serve as guides to bind RNA targets.
Unlike microRNAs, their sequence diversity is so enormous that they can bind any mRNA at multiple
sites, thus losing sequence-driven specificity. In this application we propose a radically new conceptual
framework to crack the enigma of pachytene piRNA function. We will test the hypothesis that
pachytene piRNAs play a critical role in sorting transcripts, by dynamically trapping non-spermiogenic
mRNAs in Miwi-piRNA-Tdrd6 assemblies, which form the core of the chromatoid body. In our model,
multivalent interactions between Miwi/piRNAs, which bind with partial complementarity to mRNAs,
and between Miwi and the multiple Tudor domains of Tdrd6, nucleate the chromatoid body that
sequesters trapped mRNAs for eventual elimination during spermiation. The model predicts that
longer mRNAs are preferentially trapped as they contain more binding sites for piRNAs, while
spermiogenic mRNAs that need to be translated to drive spermatid differentiation should be shorter to
avoid trapping. We will also test whether the multivalent interactions in Tdrd6-Miwi/piRNA-mRNA
assemblies lead to liquid-liquid phase separations that underlie the formation of the chromatoid body.
 We are confident that the multiple, orthogonal, in vitro and in vivo approaches that we propose
will illuminate expected and unexpected outcomes and truly uncover the elusive function of
mammalian pachytene piRNAs.

## Key facts

- **NIH application ID:** 10135998
- **Project number:** 5R01GM123512-04
- **Recipient organization:** UNIVERSITY OF PENNSYLVANIA
- **Principal Investigator:** ZISSIMOS MOURELATOS
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $322,000
- **Award type:** 5
- **Project period:** 2018-08-01 → 2023-04-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10135998

## Citation

> US National Institutes of Health, RePORTER application 10135998, Deciphering pachytene piRNA function (5R01GM123512-04). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10135998. Licensed CC0.

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