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The Hippo pathway controls myofibril assembly and muscle fiber growth by regulating sarcomeric gene expression

ABSTRACT: Skeletal muscles are comprised of gigantic multinucleated cells called muscle fibers, which often span several centimetres in length. Each muscle fiber is densely packed with contractile force-producing myofibrils and ATP-producing mitochondria. During animal development the size of the individual muscle fibers must dramatically increase to match the growth of the animal and to connect growing skeletal elements. How such dramatic tissue growth is coordinated with growth of the contractile apparatus in the muscle fibers is not well understood. Here, we use the large Drosophila flight muscles to mechanistically decipher how muscle fiber growth is controlled during development. We isolated flight muscles from Drosophila melanogaster pupae at 24 h and 32 h APF of the genotypes wt, Dlg5-IR, Slmap-IR, Yorkie-CA, Hippo-IR and isolated total RNA for subsequent BRB-seq sequencing. Our study reveals that regulated activity of core members of the Hippo pathway, Hippo, Warts and Yorkie is required to support post-mitotic flight muscle growth. Interestingly, we identify Dlg5 and Slmap as important members of the STRIPAK phosphatase complex, which negatively regulates Hippo activity and therefore enables post-mitotic muscle growth. Mechanistically, we find that the Hippo pathway controls the timing and the levels of sarcomeric gene expression during muscle development and thus regulates the key components that mediate muscle fiber growth. Since Dlg5, STRIPAK and the Hippo pathway are conserved in mammals a similar mechanism may contribute to skeletal muscle or cardiac growth in humans.

ORGANISM(S): Drosophila melanogaster

PROVIDER: GSE158957 | GEO | 2020/10/03

REPOSITORIES: GEO

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Project description:Background: skeletal muscle is a complex, versatile tissue composed of a variety of functionally diverse fiber types. Although the biochemical, structural and functional properties of myofibers have been the subject of intense investigation for the last decades, understanding molecular processes regulating fiber type diversity is still complicated by the heterogeneity of cell types present in the whole muscle organ. Methodology/Principal Findings: we have produced a first catalogue of genes expressed in mouse slow-oxidative (type 1) and fast-glycolytic (type 2B) fibers through transcriptome analysis at the single fiber level (microgenomics). Individual fibers were obtained from murine soleus and EDL muscles and initially classified by myosin heavy chain isoform content. Gene expression profiling on high density DNA oligonucleotide microarrays showed that both qualitative and quantitative improvements were achieved, compared to results with standard muscle homogenate. First, myofiber profiles were virtually free from non-muscle transcriptional activity. Second, thousands of muscle-specific genes were identified, leading to a better definition of gene signatures in the two fiber types as well as the detection of metabolic and signaling pathways that are differentially activated in specific fiber types. Several regulatory proteins showed preferential expression in slow myofibers. Discriminant analysis revealed novel genes that could be useful for fiber type functional classification. Conclusions/Significance: as gene expression analyses at the single fiber level significantly increased the resolution power, this innovative approach would allow a better understanding of the adaptive transcriptomic transitions occurring in myofibers under physiological and pathological conditions. EDL and soleus muscles were incubated with type I collagenase to dissociate intact myofibres that were separated under stereo microscope from the bulk of hyper contracted fibres. Isolated myofibres were divided in two parts: one was immersed in Laemmli buffer for fibre typing; the other was placed in RNA extraction buffer for RNA amplification. We analyzed the transcription profiles of 10 biological replicas of type 2B single muscle fibres from EDL and 10 biological replicas of type 1 single muscle fibres from soleus. Microarray competitive hybridizations were carried out against an artificial control with a balanced composition of type 1 and type 2B fibres (20 hybridizations). Each oligonucleotide is spotted in two replicates on the glass slide, so for every data two intra-slide replicas are present. The control was created as follows: three couples of soleus and EDL muscles were removed from 3 different mice and treated with collagenase. Total RNA was extracted separately from EDL and soleus muscles. By mixing about 1/3 RNA from EDL and 2/3 RNA from soleus muscles the control had approximately the same contributions of type 1 and type 2B fibres. Purified RNA samples from single fibres and from control were amplified twice using the Amino Allyl MessageAmpM-bM-^DM-" II aRNA Amplification Kit (Ambion).

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The Hippo pathway controls myofibril assembly and muscle fiber growth by regulating sarcomeric gene expression

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