Biology subjects

Ramat, A.

Publications and source records attributed to Ramat, A..

5 recordsLinked to original sources

bioRxiv2023

Germ granule higher-order organization coordinates their different functions

Most RNP condensates are composed of heterogeneous immiscible phases. However, how this multiphase organization contributes to their biological functions remains largely unexplored. Drosophila germ granules, a class of RNP condensates, are the site of mRNA storage and translational activation. Here, using super-resolution microscopy and single-molecule imaging approaches, we show that germ granules have a biphasic organization and that translation occurs in the outer phase and at the surface of the granules. The localization, directionality and compaction of mRNAs within the granule depend on their translation status, translated mRNAs being enriched in the outer phase with their 5end oriented towards the surface. Altering germ granule biphasic organization represses translation. These findings demonstrate the importance of RNA granule architecture in organizing different functions, highlighting the functional compartmentalization of RNA granules and the key role of higher-order organization.

cell biology↗

bioRxiv2023

piRNAs are regulators of metabolic reprogramming in stem cells

Stem cells preferentially use glycolysis instead of oxidative phosphorylation and this metabolic rewiring plays an instructive role in their fate; however, the underlying molecular mechanisms remain largely unexplored. PIWI-interacting RNAs (piRNAs) and PIWI proteins have essential functions in a range of adult stem cells across species. Here, we show that piRNAs and the PIWI protein Aubergine (Aub) are instrumental in activating glycolysis in Drosophila germline stem cells (GSCs). High glycolysis is required for GSC self-renewal and aub loss-of-function induces a metabolic switch in GSCs leading to their differentiation. Aub directly binds glycolytic mRNAs and Enolase mRNA regulation by Aub depends on its 5UTR. Furthermore, deletion of a piRNA target site in Enolase 5UTR leads to GSC loss. These data reveal an Aub/piRNA function in translational activation of glycolytic mRNAs in GSCs, and pinpoint a new mode of regulation of metabolic reprogramming in stem cells based on small RNAs.

developmental biology↗

bioRxiv2019

The PIWI protein Aubergine recruits eIF3 to activate translation in the germ plasm

Piwi-interacting RNAs (piRNAs) and PIWI proteins are essential in germ cells to repress transposons and regulate mRNAs. In Drosophila, piRNAs bound to the PIWI protein Aubergine (Aub) are transferred maternally to the embryo and regulate maternal mRNA stability through two opposite roles. They target mRNAs by incomplete base-pairing, leading to both their destabilization in the soma, and stabilization in the germ plasm. Here, we report a function of Aub in translation. Aub is required for translational activation of nanos mRNA, a key determinant of the germ plasm. Aub physically interacts with the poly(A) binding protein PABP and the translation initiation factor eIF3. Polysome gradient profiling identifies Aub role at the initiation step of translation. In the germ plasm, PABP and eIF3d assemble in foci that surround Aub-containing germ granules, and Aub acts with eIF3d to promote nanos translation. These results reveal a new mode of mRNA regulation by Aub, highlighting PIWI protein versatility in mRNA regulation.

developmental biology↗

bioRxiv2018

The BTB-ZF transcription factor Tramtrack 69 shapes neural cell lineages by coordinating cell proliferation and cell fate

Cell diversity in multicellular organisms relies on coordination between cell proliferation and the acquisition of cell identity. The equilibrium between these two processes is essential to assure the correct number of determined cells at a given time at a given place. Here, we show that Tramtrack-69 (Ttk69, a BTB-ZF transcription factor ortholog of the human PLZF factor) plays an essential role in controlling this balance. In the Drosophila bristle cell lineage, producing the external sensory organs composed by a neuron and accessory cells, we show that ttk69 loss of function leads to supplementary neural-type cells at the expense of accessory cells. Our data indicate that Ttk69 (1) promotes cell-cycle exit of newborn terminal cells by downregulating cycE, the principal cyclin involved in S-phase entry and (2) regulates cell fate acquisition and terminal differentiation by downregulating the expression of hamlet and upregulating that of Suppressor of Hairless, two transcription factors involved in neural-fate acquisition and accessory-cell differentiation, respectively. Thus, Ttk69 plays a central role in shaping neural cell lineages by integrating molecular mechanisms that regulate progenitor cell-cycle exit and cell-fate commitment.\n\nSummary statementTramtrack-69, a transcription factor orthologous to the human tumor-suppressor PLZF, plays a central role in precursor cell lineages by integrating molecular mechanisms that regulate progenitor cell-cycle exit and cell-fate determination.

developmental biology↗

bioRxiv2017

aPKC-mediated displacement and actomyosin-mediated retention polarize Miranda in Drosophila neuroblasts

Cell fate generation can rely on the unequal distribution of molecules during progenitor cell division in the nervous system of vertebrates and invertebrates. Here we address asymmetric fate determinant localization in the developing Drosophila nervous system, focussing on the control of asymmetric Miranda distribution in larval neuroblasts. We used live imaging of neuroblast polarity reporters at endogenous levels of expression to address Miranda localization during the cell cycle. We reveal that the regulation and dynamics of cortical association of Miranda in interphase and mitosis are different. In interphase Miranda binds directly to the plasma membrane. At the onset of mitosis, Miranda is phosphorylated by aPKC and displaced from the PM. After nuclear envelope breakdown asymmetric localization of Miranda requires actomyosin activity. Therefore, Miranda phosphorylation by aPKC and differential binding to the actomyosin network are required at distinct phases of the cell cycle to polarize fate determinant localization.

cell biology↗