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

Abstract

Skeletal muscles are composed of gigantic cells called muscle fibers, packed with force-producing myofibrils. During development the size of individual muscle fibers must dramatically enlarge to match with skeletal growth. How muscle growth is coordinated with growth of the contractile apparatus is not understood. Here, we use the large Drosophila flight muscles to mechanistically decipher how muscle fiber growth is controlled. We find that regulated activity of core members of the Hippo pathway is required to support flight muscle growth. Interestingly, we identify Dlg5 and Slmap as regulators of the STRIPAK phosphatase, which negatively regulates Hippo to enable post-mitotic muscle growth. Mechanistically, we show that the Hippo pathway controls timing and levels of sarcomeric gene expression during development and thus regulates the key components that physically mediate muscle growth. Since Dlg5, STRIPAK and the Hippo pathway are conserved a similar mechanism may contribute to muscle or cardiomyocyte growth in humans.

An Analysis of Differentially Expressed Coding and Long Non-Coding RNAs in Multiple Models of Skeletal Muscle Atrophy

“…Interestingly, upregulated mRNAs under the four muscle atrophy conditions were related to the negative regulation of leukocyte activation and regulation of myotube differentiation pathways ( Figure 4 C). These pathways include genes identified as negative ( Cdkn1a and Ctsl ) [ 25 , 26 ] and positive ( Abcc8 , Csrp3 , Dlg5 , Gdf5 , Gpnmb , Il4ra , and Runx1 ) [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ] regulators of muscle atrophy, supporting the validity of our analysis. Consistent with previous studies of each muscle atrophy condition [ 35 , 36 , 37 , 38 , 39 ], mRNAs with decreased expression in all muscle atrophy conditions were related to the terms of metabolism of carbohydrates and spermine metabolic processes ( Figure 4 D).…”

AnnoMiner: a new web-tool to integrate epigenetics, transcription factor occupancy and transcriptomics data to predict transcriptional regulators

“…Genes with log2FC in absolute value greater than 2 and FDR smaller than 0.01 were considered as differentially expressed. For integration with phenotypic data, we made use of data published on muscle morphogenesis and function in Drosophila (58) and flight muscle growth in Drosophila (47). Supplementary Table S6 for all genotypes used.…”

“…We selected these peaks to retrieve associated genes and then integrated the BRB-seq data for 24 h APF yki knockdown (yki-IR 24 h), as well as 24 h and 32 h APF constitutively active yki, respectively (yki-CA 24 h, yki-CA 32 h; Supplementary Table S7). Upon knock-down of yki, already at 32 h APF, a severe myofibril assembly defect had been observed (47). Interestingly, AnnoMiner identified Yki and Sd binding sites in the TSS of two genes essential for muscle function and development, which were downregulated in yki knock-down muscles at 24 h APF.…”

“…This genes code for the sarcomeric proteins Tropomyosin 1 (Tm1) and the Nesprin-family protein Muscle-specific protein 300 kDa (Msp300), which is important to link the myofibrils to the nuclei (65) (Figure 8 b). The gain-of-function yki phenotype (yki-CA) is characterized by premature expression of sarcomeric proteins resulting in muscle fiber hyper-compaction (47). At 24 h APF scalloped (sd) itself is the only direct target gene of the Yki/Sd complex which is differentially expressed in yki-CA (Figure 8 c).…”

“…Thus, AnnoMiner correctly predicted a new role for Trl, as well as CG14655 as transcriptional regulators in a specific muscle type in Drosophila. Finally, we used AnnoMiner to predict direct transcriptional targets for the likewise enriched TFs Yorkie (Yki) and Scalloped (Sd) required for flight muscle growth (47).…”