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The canonical role of the transcription factor E2F is to control the expression of cell cycle genes by binding to the E2F sites in their promoters. However, the list of putative E2F target genes is extensive and includes many metabolic genes, yet the significance of E2F in controlling the expression of these genes remains largely unknown. Here, we used the CRISPR/Cas9 technology to introduce point mutations in the E2F sites upstream of five endogenous metabolic genes in Drosophila melanogaster . We found that the impact of these mutations on both the recruitment of E2F and the expression of the target genes varied, with the glycolytic gene, Phosphoglycerate kinase ( Pgk) , being mostly affected. The loss of E2F regulation on the Pgk gene led to a decrease in glycolytic flux, tricarboxylic acid cycle intermediates levels, adenosine triphosphate (ATP) content, and an abnormal mitochondrial morphology. Remarkably, chromatin accessibility was significantly reduced at multiple genomic regions in Pgk ΔE2F mutants. These regions contained hundreds of genes, including metabolic genes that were downregulated in Pgk ΔE2F mutants. Moreover, Pgk ΔE2F animals had shortened life span and exhibited defects in high-energy consuming organs, such as ovaries and muscles. Collectively, our results illustrate how the pleiotropic effects on metabolism, gene expression, and development in the Pgk ΔE2F animals underscore the importance of E2F regulation on a single E2F target, Pgk .

The canonical role of the transcription factor E2F is to control the expression of cell cycle genes by binding to the E2F sites in their promoters. However, the list of putative E2F target genes is extensive and includes many metabolic genes, yet the significance of E2F in controlling expression of these genes remains largely unknown. Here, we used the CRISPR/Cas9 technology to introduce point mutations in the E2F sites upstream of five endogenous metabolic genes inDrosophila. We found that the impact of these mutations on both the recruitment of E2F and the expression of the target genes varied, with the glycolytic gene,Phosphoglycerate kinase(Pgk), being mostly affected. The loss of E2F regulation onPgkgene led to a decrease in glycolytic flux, TCA cycle intermediates levels, ATP content and an abnormal mitochondrial morphology. Remarkably, chromatin accessibility was significantly reduced at multiple genomic regions inPgkΔE2Fmutants. These regions contained hundreds of genes, including metabolic genes that were downregulated inPgkΔE2Fmutants. Moreover,PgkΔE2Fanimals had shortened life span and exhibited defects in high-energy consuming organs, such as ovaries and muscles. Collectively, our results illustrate how the pleiotropic effects on metabolism, gene expression and development in thePgkΔE2Fanimals underscore the importance of E2F regulation on a single E2F target,Pgk.

In Drosophila, the wing disc‐associated muscle precursor cells give rise to the fibrillar indirect flight muscles (IFM) and the tubular direct flight muscles (DFM). To understand early transcriptional events underlying this muscle diversification, we performed single‐cell RNA‐sequencing experiments and built a cell atlas of myoblasts associated with third instar larval wing disc. Our analysis identified distinct transcriptional signatures for IFM and DFM myoblasts that underlie the molecular basis of their divergence. The atlas further revealed various states of differentiation of myoblasts, thus illustrating previously unappreciated spatial and temporal heterogeneity among them. We identified and validated novel markers for both IFM and DFM myoblasts at various states of differentiation by immunofluorescence and genetic cell‐tracing experiments. Finally, we performed a systematic genetic screen using a panel of markers from the reference cell atlas as an entry point and found a novel gene, Amalgam which is functionally important in muscle development. Our work provides a framework for leveraging scRNA‐seq for gene discovery and details a strategy that can be applied to other scRNA‐seq datasets., A single‐cell atlas of the Drosophila wing disc identifies the transcriptional profiles and markers for diverse cell types. The myoblasts exhibit distinct Notch‐dependent states of differentiation, and diversification into fiber fate specification.

SummaryInDrosophila, the wing disc-associated adult muscle precursors (AMPs) give rise to the fibrillar indirect flight muscles (IFM) and the tubular direct flight muscles (DFM). To understand early transcriptional events underlying this muscle diversification, we performed single cell RNA-sequencing experiments and built a cell atlas of AMPs associated with third instar larval wing disc. Our analysis identified distinct transcriptional signatures for IFM and DFM precursors that underlie the molecular basis of their divergence. The atlas further revealed various states of differentiation of AMPs, thus illustrating previously unappreciated spatial and temporal heterogeneity among them. We identified and validated novel markers for both IFM and DFM precursors at various states of differentiation by immunofluorescence and genetic tracing experiments. Finally, we performed a systematic genetic screen using a panel of markers from the reference cell atlas as an entry point and found a novel gene,Ama, which is functionally important in muscle development. Thus, our work provides a framework of leveraging scRNA-seq for gene discovery and therefore, this strategy can be applied to other scRNA-seq datasets.

SUMMARY The importance of the retinoblastoma tumor suppressor protein pRB in cell cycle control is well established. However, less is known about its role in differentiation during animal development. Here, we investigated the role of Rbf, the Drosophila pRB homolog, in adult skeletal muscles. We found that the depletion of Rbf severely reduced muscle growth and altered myofibrillogenesis but only minimally affected myoblast proliferation. We identified an Rbf-dependent transcriptional program in late muscle development that is distinct from the canonical role of Rbf in cell cycle control. Unexpectedly, Rbf acts as a transcriptional activator of the myogenic and metabolic genes in the growing muscles. The genomic regions bound by Rbf contained the binding sites of several factors that genetically interacted with Rbf by modulating Rbf-dependent phenotype. Thus, our results reveal a distinctive role for Rbf as a direct activator of the myogenic transcriptional program that drives late muscle differentiation., Graphical Abstract, In Brief Inactivation of the tumor suppressor RB, an obligatory step in most cancers, results in unrestrained cell cycle progression. Zappia et al. show that Rbf, the RB Drosophila ortholog, directly activates the metabolic program that accompanies muscle development. This work expands the understanding of the plethora of Rbf functions.

The E2F transcription factor is a key cell cycle regulator. However, the inactivation of the entire E2F family in Drosophila is permissive throughout most of animal development until pupation when lethality occurs. Here we show that E2F function in the adult skeletal muscle is essential for animal viability since providing E2F function in muscles rescues the lethality of the whole-body E2F-deficient animals. Muscle-specific loss of E2F results in a significant reduction in muscle mass and thinner myofibrils. We demonstrate that E2F is dispensable for proliferation of muscle progenitor cells, but is required during late myogenesis to directly control the expression of a set of muscle-specific genes. Interestingly, E2f1 provides a major contribution to the regulation of myogenic function, while E2f2 appears to be less important. These findings identify a key function of E2F in skeletal muscle required for animal viability, and illustrate how the cell cycle regulator is repurposed in post-mitotic cells., The transcriptional regulators E2F/Dp play a critical role in cell-cycle regulation, but it is unclear why E2F-deficient flies die. Here, the authors show this is linked to the function of E2F in adult Drosophila skeletal muscle, with the contribution of E2f1 being most important in post-fusion muscle.

To understand the consequences of the complete elimination of E2F regulation, we profiled the proteome of Drosophila dDP mutants that lack functional E2F/DP complexes. The results uncovered changes in the larval fat body, a differentiated tissue that grows via endocycles. We report an unexpected mechanism of E2F/DP action that promotes quiescence in this tissue. In the fat body, dE2F/dDP limits cell-cycle progression by suppressing DNA damage responses. Loss of dDP upregulates dATM, allowing cells to sense and repair DNA damage and increasing replication of loci that are normally under-replicated in wild-type tissues. Genetic experiments show that ectopic dATM is sufficient to promote DNA synthesis in wild-type fat body cells. Strikingly, reducing dATM levels in dDP-deficient fat bodies restores cell-cycle control, improves tissue morphology, and extends animal development. These results show that, in some cellular contexts, dE2F/dDP-dependent suppression of DNA damage signaling is key for cell-cycle control and needed for normal development.