mRNP granule proteins Fmrp and Dcp1a differentially regulate mRNP complexes to contribute to control of muscle stem cell quiescence and activation

Navigating translational control of gene expression in satellite cells

Satellite cells, named for their satellite position around the sarcolemma of the myofibre, are responsible for skeletal muscle regeneration. Satellite cells normally reside in a quiescent state, but rapidly activate the myogenic program and the cell cycle in response to injury. Translational control of gene expression has emerged as an important regulator of satellite cell activity. Quiescent satellite cells maintain low levels of protein synthesis and selectively translate specific mRNAs to conserve limited energy. Activated satellite cells rapidly restore global protein synthesis to meet the demands of proliferating myogenic progenitors that participate in muscle repair. We propose a model by which translational control enables rapid protein level changes in response to injury-induced environmental shifts, serving as both a brake mechanism during quiescence and an accelerator for injury response. In this Chapter, we navigate the processing, translation and metabolism of newly transcribed mRNAs. We review the modifications of mRNA that occur during mRNA processing in the nucleus of satellite cells, and illustrate how these modifications impact the translation and stability of mRNAs. In the cytoplasm, we review how pathways work in concert to regulate protein synthesis globally, while trans acting microRNAs and RNA binding proteins modify specific mRNA translation within a context of tightly regulated protein synthesis. While navigating translational control of gene expression in satellite cells, this chapter reveals that despite significant progress, the field remains nascent in the broader scope of translational control in cell biology. We propose that future investigations will benefit from incorporating emerging global analyses to study translational control of gene expression in rare satellite cells, and we pose unanswered questions that warrant future exploration.

Regulation of adult stem cell quiescence and its functions in the maintenance of tissue integrity

Adult stem cells are important for mammalian tissues, where they act as a cell reserve that supports normal tissue turnover and can mount a regenerative response following acute injuries. Quiescent stem cells are well established in certain tissues, such as skeletal muscle, brain, and bone marrow. The quiescent state is actively controlled and is essential for long-term maintenance of stem cell pools. In this Review, we discuss the importance of maintaining a functional pool of quiescent adult stem cells, including haematopoietic stem cells, skeletal muscle stem cells, neural stem cells, hair follicle stem cells, and mesenchymal stem cells such as fibro-adipogenic progenitors, to ensure tissue maintenance and repair. We discuss the molecular mechanisms that regulate the entry into, maintenance of, and exit from the quiescent state in mice. Recent studies revealed that quiescent stem cells have a discordance between RNA and protein levels, indicating the importance of post-transcriptional mechanisms, such as alternative polyadenylation, alternative splicing, and translation repression, in the control of stem cell quiescence. Understanding how these mechanisms guide stem cell function during homeostasis and regeneration has important implications for regenerative medicine.

A transcriptionally repressed quiescence program is associated with paused RNAPII and is poised for cell cycle reentry

Adult stem cells persist in mammalian tissues by entering a state of reversible quiescence/ G0, associated with low transcription. Using cultured myoblasts and muscle stem cells, we report that in G0, global RNA content and synthesis are substantially repressed, correlating with decreased RNA Polymerase II (RNAPII) expression and activation. Integrating RNAPII occupancy and transcriptome profiling, we identify repressed networks and a role for promoter-proximal RNAPII pausing in G0. Strikingly, RNAPII shows enhanced pausing in G0 on repressed genes encoding regulators of RNA biogenesis (Nucleolin, Rps24, Ctdp1); release of pausing is associated with their increased expression in G1. Knockdown of these transcripts in proliferating cells leads to induction of G0 markers, confirming the importance of their repression in establishment of G0. A targeted screen of RNAPII regulators revealed that knockdown of Aff4 (positive regulator of elongation) unexpectedly enhances expression of G0-stalled genes and hastens S phase; NELF, a regulator of pausing appears to be dispensable. We propose that RNAPII pausing contributes to transcriptional control of a subset of G0-repressed genes to maintain quiescence and impacts the timing of the G0-G1 transition.

Automated Quantification of Subcellular Particles in Myogenic Progenitors

Fluorescence microscopy is a powerful tool enabling the visualization of protein localization within cells. In this article, we outline an automated and non-biased way to detect and quantify subcellular particles using immunocytochemistry, fluorescence microscopy, and the program CellProfiler. We discuss the examination of two types of subcellular particles: messenger ribonucleoprotein (mRNP) granules, namely processing bodies and stress granules, and autophagosomes. Fluorescent microscopy Z-stacks are acquired and deconvolved, and maximum intensity images are generated. The number of subcellular particles per cell is then quantified using the described CellProfiler pipeline. We also explain how to isolate primary myoblast progenitor cells from mice, which were used to obtain the presented results. Last, we discuss the critical parameters to be considered for each of these techniques. Both mRNP granules and autophagosomes play important roles in sequestering intracellular cargo, such as messenger RNAs and RNA-binding proteins for mRNP granules and cytoplasmic waste for autophagosomes. The methods outlined in this article are widely applicable for studies relating to subcellular particle formation, localization, and flux during homeostasis, following stimuli, and during disease. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Immunofluorescence microscopy of messenger ribonucleoprotein granules in primary myoblasts Alternate Protocol: Immunofluorescence microscopy of autophagosomes in primary myoblasts Support Protocol: Isolation of primary myoblasts from mice Basic Protocol 2: Automated quantification of subcellular particles.