mRNA localization mediates maturation of cytoplasmic cilia in Drosophila spermatogenesis

Abstract

Cytoplasmic cilia, a specialized type of cilia in which the axoneme resides within the cytoplasm rather than within the ciliary compartment, are proposed to allow for the efficient assembly of very long cilia. Despite being found diversely in male gametes (e.g., Plasmodium falciparum microgametocytes and human and Drosophila melanogaster sperm), very little is known about cytoplasmic cilia assembly. Here, we show that a novel RNP granule containing the mRNAs for axonemal dynein motor proteins becomes highly polarized to the distal end of the cilia during cytoplasmic ciliogenesis in Drosophila sperm. This allows for the incorporation of these axonemal dyneins into the axoneme directly from the cytoplasm, possibly by localizing translation. We found that this RNP granule contains the proteins Reptin and Pontin, loss of which perturbs granule formation and prevents incorporation of the axonemal dyneins, leading to sterility. We propose that cytoplasmic cilia assembly requires the precise localization of mRNAs encoding key axonemal constituents, allowing these proteins to incorporate efficiently into the axoneme.

RNA-binding protein Maca is crucial for gigantic male fertility factor gene expression, spermatogenesis, and male fertility, in Drosophila

“…CG31213 and JYalpha also showed both protein and mRNA level downregulation and CG5399 showed upregulation in maca null/null testes compared with the all other genotypes (Figs 5 and S4). kl-2, kl-3, and kl-5 are known to be essential for male fertility [27,29,39] and thus their downregulation in maca null/null testes can nicely explain the male sterility in maca null/null . Therefore, we decided to focus on kl-2, kl-3, and kl-5.…”

“…kl-2, kl-3, and kl-5 encode dynein proteins that are crucial components of the axoneme, the microtubule-based cytoskeletal structure that forms the core of a sperm flagellum [23][24][25][26][27][28]. The kl-2, kl-3, and kl-5 genes are transcribed during the spermatocyte growth and their mRNAs form cytoplasmic ribonucleoprotein (RNP) granules called kl-granules in late-stage spermatocytes [29]. kl-2, kl-3, and kl-5 mRNAs are suggested to start being efficiently translated only when axoneme elongates in the sperm elongation process in spermiogenesis.…”

The Microtubule Cytoskeleton during the Early Drosophila Spermiogenesis

“…Rather, the sperm axoneme of the Drosophila spermatids requires a cytoplasmic mode of assembly [ 3 , 4 ]. Accordingly, it has been demonstrated that the maturation of the axoneme relies on the local translation of specific key components from the cytoplasm that incorporate into the forming axoneme as it emerges from the ciliary cap [ 5 ]. The assembly and further elongation of the axonemal MTs would require tubulin translocation by diffusion through the ciliary cap.…”

Nucleo-cytoplasmic shuttling of splicing factor SRSF1 is required for development and cilia function

“…Recent evidence suggests the interesting possibility that spatially regulated translation may be occurring during motile ciliogenesis. During spermatogenesis, Drosophila mRNAs encoding motile ciliary proteins are stored in large cytoplasmic granules until their translation is required in the growing sperm axoneme, possibly regulating their translation (Fingerhut and Yamashita, 2020). In vertebrates, there is evidence of phase-separated cytoplasmic foci termed dynein axonemal particles (DynAPs) that contain assembly factors, chaperones and axonemal dynein proteins, that may provide a favorable environment for their orderly assembly and final transport onto cilia (Huizar et al, 2018).…”