Specific Isoforms of the Guanine-Nucleotide Exchange Factor dPix Couple Neuromuscular Synapse Growth to Muscle Growth

A large reverse-genetic screen identifies numerous regulators of testis nascent myotube collective cell migration and collective organ sculpting

Collective cell migration is critical for morphogenesis, homeostasis, and wound healing. Migrating mesenchymal cells form tissues that shape the body's organs. We developed a powerful model, exploring how Drosophila nascent myotubes migrate onto the testis during pupal development, forming the muscles ensheathing it and creating its characteristic spiral shape. To define genes regulating this, we used RNA sequencing (RNA-seq) to identify genes expressed in myotubes during migration. Using this dataset, we curated a list of 131 ligands, receptors, and cytoskeletal regulators, including all Rho/Ras/Rap1 regulators, as candidates. We then expressed 279 short hairpin RNAs (shRNAs) targeting these genes and examined adult testes. We identified 29 genes with diverse roles in morphogenesis. Some have phenotypes consistent with defective migration, while others alter testis shape in different ways, revealing the underlying logic of testis morphogenesis. We followed up on the Rho-family GEF dPix in detail. dPix knockdown drastically reduced migration and thus muscle coverage. Our data suggest different isoforms of dPix play distinct roles in this process and reveal a role for its partner Git. We also explored whether dPix regulates Cdc42 activity or cell adhesion. Our RNA-seq dataset and genetic analysis provide an important resource for the community to explore cell migration and organ morphogenesis.

Postsynaptic BMP signaling regulates myonuclear properties in Drosophila larval muscles

The syncytial mammalian muscle fiber contains a heterogeneous population of (myo)nuclei. At the neuromuscular junction (NMJ), myonuclei have specialized positioning and gene expression. However, it remains unclear how myonuclei are recruited and what regulates myonuclear output at the NMJ. Here, we identify specific properties of myonuclei located near the Drosophila larval NMJ. These synaptic myonuclei have increased size in relation to their surrounding cytoplasmic domain (size scaling), increased DNA content (ploidy), and increased levels of transcription factor pMad, a readout for BMP signaling activity. Our genetic manipulations show that local BMP signaling affects muscle size, nuclear size, ploidy, and NMJ size and function. In support, RNA sequencing analysis reveals that pMad regulates genes involved in muscle growth, ploidy (i.e., E2f1), and neurotransmission. Our data suggest that muscle BMP signaling instructs synaptic myonuclear output that positively shapes the NMJ synapse. This study deepens our understanding of how myonuclear heterogeneity supports local signaling demands to fine tune cellular function and NMJ activity.

A large reverse-genetic screen identifies numerous regulators of testis nascent myotube collective cell migration and collective organ sculpting

Collective cell migration is critical for morphogenesis, homeostasis, and wound healing. During development migrating mesenchymal cells form tissues that shape some of the body's organs. We have developed a powerful model for examining this, exploring how Drosophila testis nascent myotubes migrate onto the testis during pupal development, forming the muscles that ensheath it and also creating its characteristic spiral shape. To define genes that regulate this process, we have carried out RNAseq to define the genes expressed in myotubes during migration. Using this dataset, we curated a list of 131 ligands, receptors and cytoskeletal regulators, including all Rho-family GTPase GAPs and GEFs, as candidates. We then used the GAL4/UAS system to express 279 shRNAs targeting these genes, using the muscle specific driver dMef2>GAL4, and examined the adult testis. We identified 29 genes with diverse roles in testis morphogenesis. Some have phenotypes consistent with defects in collective cell migration, while others alter testis shape in different ways, revealing some of the underlying logic of testis morphogenesis. We followed up one of these genes in more detail-that encoding the Rho-family GEF dPix. dPix knockdown leads to a drastic reduction in migration and a substantial loss of muscle coverage. Our data suggest different isoforms of dPix play distinct roles in this process, reveal a role for its protein partner Git. We also explore whether cdc42 activity regulation or cell adhesion are among the dPix mechanisms of action. Together, our RNAseq dataset and genetic analysis will provide an important resource for the community to explore cell migration and organ morphogenesis.

Postsynaptic BMP signaling regulates myonuclear properties in Drosophila larval muscles

The syncytial mammalian muscle fiber contains a heterogeneous population of (myo)nuclei. At the neuromuscular junction (NMJ), myonuclei have specialized positioning and gene expression. However, it remains unclear how myonuclei are recruited and what regulates myonuclear output at the NMJ. Here, we identify specific properties of myonuclei located near the Drosophila larval NMJ. These synaptic myonuclei have increased size in relation to their surrounding cytoplasmic domain (scaling), increased DNA content (ploidy), and increased levels of transcription factor pMad, a readout for BMP signaling activity. Our genetic manipulations show local BMP signaling affects muscle size, nuclear size, ploidy, and NMJ size and function. In support, RNA sequencing analysis reveals that pMad regulates genes involved in muscle growth, ploidy (i.e., E2f1), and neurotransmission. Our data suggest that muscle BMP signaling instructs synaptic myonuclear output that then positively shapes the NMJ synapse. This study deepens our understanding of how myonuclear heterogeneity supports local signaling demands to fine tune cellular function and NMJ activity.

Dysregulation of BMP, Wnt, and Insulin Signaling in Fragile X Syndrome

Drosophila models of neurological disease contribute tremendously to research progress due to the high conservation of human disease genes, the powerful and sophisticated genetic toolkit, and the rapid generation time. Fragile X syndrome (FXS) is the most prevalent heritable cause of intellectual disability and autism spectrum disorders, and the Drosophila FXS disease model has been critical for the genetic screening discovery of new intercellular secretion mechanisms. Here, we focus on the roles of three major signaling pathways: BMP, Wnt, and insulin-like peptides. We present Drosophila FXS model defects compared to mouse models in stem cells/embryos, the glutamatergic neuromuscular junction (NMJ) synapse model, and the developing adult brain. All three of these secreted signaling pathways are strikingly altered in FXS disease models, giving new mechanistic insights into impaired cellular outcomes and neurological phenotypes. Drosophila provides a powerful genetic screening platform to expand understanding of these secretory mechanisms and to test cellular roles in both peripheral and central nervous systems. The studies demonstrate the importance of exploring broad genetic interactions and unexpected regulatory mechanisms. We discuss a number of research avenues to pursue BMP, Wnt, and insulin signaling in future FXS investigations and the development of potential therapeutics.

An exon junction complex-independent function of Barentsz in neuromuscular synapse growth

The exon junction complex controls the translation, degradation, and localization of spliced mRNAs, and three of its core subunits also play a role in splicing. Here, we show that a fourth subunit, Barentsz, has distinct functions within and separate from the exon junction complex in Drosophila neuromuscular development. The distribution of mitochondria in larval muscles requires Barentsz as well as other exon junction complex subunits and is not rescued by a Barentsz transgene in which residues required for binding to the core subunit eIF4AIII are mutated. In contrast, interactions with the exon junction complex are not required for Barentsz to promote the growth of neuromuscular synapses. We find that the Activin ligand Dawdle shows reduced expression in barentsz mutants and acts downstream of Barentsz to control synapse growth. Both barentsz and dawdle are required in motor neurons, muscles, and glia for normal synapse growth, and exogenous Dawdle can rescue synapse growth in the absence of barentsz. These results identify a biological function for Barentsz that is independent of the exon junction complex.

Synaptic homeostats: latent plasticity revealed at the Drosophila neuromuscular junction

Homeostatic signaling systems are fundamental forms of biological regulation that maintain stable functionality in a changing environment. In the nervous system, synapses are crucial substrates for homeostatic modulation, serving to establish, maintain, and modify the balance of excitation and inhibition. Synapses must be sufficiently flexible to enable the plasticity required for learning and memory but also endowed with the stability to last a lifetime. In response to the processes of development, growth, remodeling, aging, and disease that challenge synapses, latent forms of adaptive plasticity become activated to maintain synaptic stability. In recent years, new insights into the homeostatic control of synaptic function have been achieved using the powerful Drosophila neuromuscular junction (NMJ). This review will focus on work over the past 10 years that has illuminated the cellular and molecular mechanisms of five homeostats that operate at the fly NMJ. These homeostats adapt to loss of postsynaptic neurotransmitter receptor functionality, glutamate imbalance, axonal injury, as well as aberrant synaptic growth and target innervation. These diverse homeostats work independently yet can be simultaneously expressed to balance neurotransmission. Growing evidence from this model glutamatergic synapse suggests these ancient homeostatic signaling systems emerged early in evolution and are fundamental forms of plasticity that also function to stabilize mammalian cholinergic NMJs and glutamatergic central synapses.

Think Globally, Act Locally: Scaling the Growth of Motor Neurons

During organismal growth, body parts expand proportionally with one another and with the body as a whole, but the signals mediating this scalar expansion have been elusive. In this issue of Developmental Cell, Ho and Treisman uncover a signal transduction pathway that coordinates muscle growth and neuromuscular junction expansion.