Multiple roles for notch in Drosophila myogenesis

ASC proneural factors are necessary for chromatin remodeling during neuroectodermal to neuroblast fate transition to ensure the timely initiation of the neural stem cell program

Background

In both Drosophila and mammals, the achaete-scute (ASC/ASCL) proneural bHLH transcription factors are expressed in the developing central and peripheral nervous systems, where they function during specification and maintenance of the neural stem cells in opposition to Notch signaling. In addition to their role in nervous system development, ASC transcription factors are oncogenic and exhibit chromatin reprogramming activity; however, the impact of ASC on chromatin dynamics during neural stem cell generation remains elusive. Here, we investigate the chromatin changes accompanying neural commitment using an integrative genetics and genomics methodology.

Results

We found that ASC factors bind equally strongly to two distinct classes of cis-regulatory elements: open regions remodeled earlier during maternal to zygotic transition by Zelda and less accessible, Zelda-independent regions. Both classes of cis-elements exhibit enhanced chromatin accessibility during neural specification and correlate with transcriptional regulation of genes involved in a variety of biological processes necessary for neuroblast function/homeostasis. We identified an ASC-Notch regulated TF network that includes likely prime regulators of neuroblast function. Using a cohort of ASC target genes, we report that ASC null neuroblasts are defectively specified, remaining initially stalled, unable to divide, and lacking expression of many proneural targets. When mutant neuroblasts eventually start proliferating, they produce compromised progeny. Reporter lines driven by proneural-bound enhancers display ASC dependency, suggesting that the partial neuroblast identity seen in the absence of ASC genes is likely driven by other, proneural-independent, cis-elements. Neuroblast impairment and the late differentiation defects of ASC mutants are corrected by ectodermal induction of individual ASC genes but not by individual members of the TF network downstream of ASC. However, in wild-type embryos, the induction of individual members of this network induces CNS hyperplasia, suggesting that they synergize with the activating function of ASC to consolidate the chromatin dynamics that promote neural specification.

Conclusions

We demonstrate that ASC proneural transcription factors are indispensable for the timely initiation of the neural stem cell program at the chromatin level by regulating a large number of enhancers in the vicinity of neural genes. This early chromatin remodeling is crucial for both neuroblast homeostasis as well as future progeny fidelity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01300-8.

The Notch signaling network in muscle stem cells during development, homeostasis, and disease

Skeletal muscle stem cells have a central role in muscle growth and regeneration. They reside as quiescent cells in resting muscle and in response to damage they transiently amplify and fuse to produce new myofibers or self-renew to replenish the stem cell pool. A signaling pathway that is critical in the regulation of all these processes is Notch. Despite the major differences in the anatomical and cellular niches between the embryonic myotome, the adult sarcolemma/basement-membrane interphase, and the regenerating muscle, Notch signaling has evolved to support the context-specific requirements of the muscle cells. In this review, we discuss the diverse ways by which Notch signaling factors and other modifying partners are operating during the lifetime of muscle stem cells to establish an adaptive dynamic network.

The Membrane-Bound Notch Regulator Mnr Supports Notch Cleavage and Signaling Activity in Drosophila melanogaster

The Notch signaling pathway is pivotal to cellular differentiation. Activation of this pathway involves proteolysis of the Notch receptor and the release of the biologically active Notch intracellular domain, acting as a transcriptional co-activator of Notch target genes. While the regulation of Notch signaling dynamics at the level of ligand-receptor interaction, endocytosis, and transcriptional regulation has been well studied, little is known about factors influencing Notch cleavage. We identified EP555 as a suppressor of the Notch antagonist Hairless (H). EP555 drives expression of CG32521 encoding membrane-bound proteins, which we accordingly rename membrane-bound Notch regulator (mnr). Within the signal-receiving cell, upregulation of Mnr stimulates Notch receptor activation, whereas a knockdown reduces it, without apparent influence on ligand-receptor interaction. We provide evidence that Mnr plays a role in γ-secretase-mediated intramembrane cleavage of the Notch receptor. As revealed by a fly-eye-based reporter system, γ-secretase activity is stimulated by the overexpression of Mnr, and is inhibited by its knockdown. We conclude that Mnr proteins support Notch signaling activity by fostering the cleavage of the Notch receptor. With Mnr, we identified a membrane-bound factor directly augmenting Notch intra-membrane processing, thereby acting as a positive regulator of Notch signaling activity.

Tyrosine phosphorylation and proteolytic cleavage of Notch are required for non-canonical Notch/Abl signaling in Drosophila axon guidance

Notch signaling is required for the development and physiology of nearly every tissue in metazoans. Much of Notch signaling is mediated by transcriptional regulation of downstream target genes, but Notch controls axon patterning in Drosophila by local modulation of Abl tyrosine kinase signaling, via direct interactions with the Abl co-factors Disabled and Trio. Here, we show that Notch-Abl axonal signaling requires both of the proteolytic cleavage events that initiate canonical Notch signaling. We further show that some Notch protein is tyrosine phosphorylated in Drosophila, that this form of the protein is selectively associated with Disabled and Trio, and that relevant tyrosines are essential for Notch-dependent axon patterning but not for canonical Notch-dependent regulation of cell fate. Based on these data, we propose a model for the molecular mechanism by which Notch controls Abl signaling in Drosophila axons.

Acting on identity: Myoblast fusion and the formation of the syncytial muscle fiber

The study of Drosophila muscle development dates back to the middle of the last century. Since that time, Drosophila has proved to be an ideal system for studying muscle development, differentiation, function, and disease. As in humans, Drosophila muscle forms via a series of conserved steps, starting with muscle specification, myoblast fusion, attachment to tendon cells, interactions with motorneurons, and sarcomere and myofibril formation. The genes and mechanisms required for these processes share striking similarities to those found in humans. The highly tractable genetic system and imaging approaches available in Drosophila allow for an efficient interrogation of muscle biology and for application of what we learn to other systems. In this article, we review our current understanding of muscle development in Drosophila, with a focus on myoblast fusion, the process responsible for the generation of syncytial muscle cells. We also compare and contrast those genes required for fusion in Drosophila and vertebrates.

Specification of the somatic musculature in Drosophila

The somatic muscle system formed during Drosophila embryogenesis is required for larvae to hatch, feed, and crawl. This system is replaced in the pupa by a new adult muscle set, responsible for activities such as feeding, walking, and flight. Both the larval and adult muscle systems are comprised of distinct muscle fibers to serve these specific motor functions. In this way, the Drosophila musculature is a valuable model for patterning within a single tissue: while all muscle cells share properties such as the contractile apparatus, properties such as size, position, and number of nuclei are unique for a particular muscle. In the embryo, diversification of muscle fibers relies first on signaling cascades that pattern the mesoderm. Subsequently, the combinatorial expression of specific transcription factors leads muscle fibers to adopt particular sizes, shapes, and orientations. Adult muscle precursors (AMPs), set aside during embryonic development, proliferate during the larval phases and seed the formation of the abdominal, leg, and flight muscles in the adult fly. Adult muscle fibers may either be formed de novo from the fusion of the AMPs, or are created by the binding of AMPs to an existing larval muscle. While less is known about adult muscle specification compared to the larva, expression of specific transcription factors is also important for its diversification. Increasingly, the mechanisms required for the diversification of fly muscle have found parallels in vertebrate systems and mark Drosophila as a robust model system to examine questions about how diverse cell types are generated within an organism.

O-fucose monosaccharide of Drosophila Notch has a temperature-sensitive function and cooperates with O-glucose glycan in Notch transport and Notch signaling activation

Notch (N) is a transmembrane receptor that mediates the cell-cell interactions necessary for many cell fate decisions. N has many epidermal growth factor-like repeats that are O-fucosylated by the protein O-fucosyltransferase 1 (O-Fut1), and the O-fut1 gene is essential for N signaling. However, the role of the monosaccharide O-fucose on N is unclear, because O-Fut1 also appears to have O-fucosyltransferase activity-independent functions, including as an N-specific chaperon. Such an enzymatic activity-independent function could account for the essential role of O-fut1 in N signaling. To evaluate the role of the monosaccharide O-fucose modification in N signaling, here we generated a knock-in mutant of O-fut1 (O-fut1(R245A knock-in)), which expresses a mutant protein that lacks O-fucosyltransferase activity but maintains the N-specific chaperon activity. Using O-fut1(R245A knock-in) and other gene mutations that abolish the O-fucosylation of N, we found that the monosaccharide O-fucose modification of N has a temperature-sensitive function that is essential for N signaling. The O-fucose monosaccharide and O-glucose glycan modification, catalyzed by Rumi, function redundantly in the activation of N signaling. We also showed that the redundant function of these two modifications is responsible for the presence of N at the cell surface. Our findings elucidate how different forms of glycosylation on a protein can influence the protein's functions.

bHLH-O proteins are crucial for Drosophila neuroblast self-renewal and mediate Notch-induced overproliferation

Drosophila larval neurogenesis is an excellent system for studying the balance between self-renewal and differentiation of a somatic stem cell (neuroblast). Neuroblasts (NBs) give rise to differentiated neurons and glia via intermediate precursors called GMCs or INPs. We show that E(spl)mγ, E(spl)mβ, E(spl)m8 and Deadpan (Dpn), members of the basic helix-loop-helix-Orange protein family, are expressed in NBs but not in differentiated cells. Double mutation for the E(spl) complex and dpn severely affects the ability of NBs to self-renew, causing premature termination of proliferation. Single mutations produce only minor defects, which points to functional redundancy between E(spl) proteins and Dpn. Expression of E(spl)mγ and m8, but not of dpn, depends on Notch signalling from the GMC/INP daughter to the NB. When Notch is abnormally activated in NB progeny cells, overproliferation defects are seen. We show that this depends on the abnormal induction of E(spl) genes. In fact E(spl) overexpression can partly mimic Notch-induced overproliferation. Therefore, E(spl) and Dpn act together to maintain the NB in a self-renewing state, a process in which they are assisted by Notch, which sustains expression of the E(spl) subset.

Control in time and space: Tramtrack69 cooperates with Notch and Ecdysone to repress ectopic fate and shape changes during Drosophila egg chamber maturation

Organ morphogenesis requires cooperation between cells, which determine their course of action based upon location within a tissue. Just as important, cells must synchronize their activities, which requires awareness of developmental time. To understand how cells coordinate behaviors in time and space, we analyzed Drosophila egg chamber development. We found that the transcription factor Tramtrack69 (TTK69) controls the fates and shapes of all columnar follicle cells by integrating temporal and spatial information, restricting characteristic changes in morphology and expression that occur at stage 10B to appropriate domains. TTK69 is required again later in oogenesis: it controls the volume of the dorsal-appendage (DA) tubes by promoting apical re-expansion and lateral shortening of DA-forming follicle cells. We show that TTK69 and Notch compete to repress each other's expression and that a local Ecdysone signal is required to shift the balance in favor of TTK69. We hypothesize that TTK69 then cooperates with spatially restricted co-factors to define appropriate responses to a globally available (but as yet unidentified) temporal signal that initiates the S10B transformations.

Jagged1 is a competitive inhibitor of Notch signaling in the embryonic pancreas

Pancreatic endocrine cells originate from precursors that express the transcription factor Neurogenin3 (Ngn3). Ngn3 expression is repressed by active Notch signaling. Accordingly, mice with Notch signaling pathway mutations display increased Ngn3 expression and endocrine cell lineage allocation. To determine how the Notch ligand Jagged1 (Jag1) functions during pancreas development, we deleted Jag1 in foregut endoderm and examined postnatal and embryonic endocrine cells and precursors. Postnatal Jag1 mutants display increased Ngn3 expression, alpha-cell mass, and endocrine cell percentage, similar to the early embryonic phenotype of Dll1 and Rbpj mutants. However, in sharp contrast to postnatal animals, Jag1-deficient embryos display increased expression of Notch transcriptional targets and decreased Ngn3 expression, resulting in reduced endocrine lineage allocation. Jag1 acts as an inhibitor of Notch signaling during embryonic pancreas development but an activator of Notch signaling postnatally. Expression of the Notch modifier Manic Fringe (Mfng) is limited to endocrine precursors, providing a possible explanation for the inhibition of Notch signaling by Jag1 during mid-gestation embryonic pancreas development.

The HLH protein Extramacrochaetae is required for R7 cell and cone cell fates in the Drosophila eye

Notch signaling is one of the most important pathways in development and homeostasis, and is altered in multiple pathologies. Study of Drosophila eye development shows that Notch signaling depends on the HLH protein Extramacrochaetae. Null mutant clones show that extramacrochaetae is required for multiple aspects of eye development that depend on Notch signaling, including morphogenetic furrow progression, differentiation of R4, R7 and cone cell types, and rotation of ommatidial clusters. Detailed analysis of R7 and cone cell specification reveals that extramacrochaetae acts cell autonomously and epistatically to Notch, and is required for normal expression of bHLH genes encoded by the E(spl)-C which are effectors of most Notch signaling. A model is proposed in which Extramacrochaetae acts in parallel to or as a feed-forward regulator of the E(spl)-Complex to promote Notch signaling in particular cellular contexts.

Antagonistic function of Lmd and Zfh1 fine tunes cell fate decisions in the Twi and Tin positive mesoderm of Drosophila melanogaster

In this study we show that cell fate decisions in the dorsal and lateral mesoderm of Drosophila melanogaster depend on the antagonistic action of the Gli-like transcription factor Lame duck (Lmd) and the zinc finger homeodomain factor Zfh1. Lmd expression leads to the reduction of Zfh1 positive cell types, thereby restricting the number of Odd-skipped (Odd) positive and Tinman (Tin) positive pericardial cells in the dorsal mesoderm. In more lateral regions, ectopic activation of Zfh1 or loss of Lmd leads to an excess of adult muscle precursor (AMP) like cells. We also observed that Lmd is co-expressed with Tin in the early dorsal mesoderm and leads to a reduction of Tin expression in cells destined to become dorsal fusion competent myoblasts (FCMs). In the absence of Lmd function, these cells remain Tin positive and develop as Tin positive pericardial cells although they do not express Zfh1. We show further that Tin repression and pericardial restriction in the dorsal mesoderm facilitated by Lmd is instructed by a late Decapentaplegic (Dpp) signal that is abolished in embryos carrying the disk region mutation dpp(d6).

Drosophila mind bomb2 is required for maintaining muscle integrity and survival

We report that the Drosophila mind bomb2 (mib2) gene is a novel regulator of muscle development. Unlike its paralogue, mib1, zygotic expression of mib2 is restricted to somatic and visceral muscle progenitors, and their respective differentiated musculatures. We demonstrate that in embryos that lack functional Mib2, muscle detachment is observed beginning in mid stage 15 and progresses rapidly, culminating in catastrophic degeneration and loss of most somatic muscles by stage 17. Notably, the degenerating muscles are positive for apoptosis markers, and inhibition of apoptosis in muscles prevents to a significant degree the muscle defects. Rescue experiments with Mib1 and Neuralized show further that these E3 ubiquitin ligases are not capable of ameliorating the muscle mutant phenotype of mib2. Our data suggest strongly that mib2 is involved in a novel Notch- and integrin-independent pathway that maintains the integrity of fully differentiated muscles and prevents their apoptotic degeneration.

The nanos translational control element represses translation in somatic cells by a Bearded box-like motif

Developmental control of translation is frequently mediated by regulatory elements that reside within 3' untranslated regions (3' UTRs). Two stem-loops within the nanos 3' UTR translational control element (TCE) act independently to direct translational repression of maternal nanos mRNA in the ovary or embryo. We have previously shown that the nanos TCE can also function in select somatic sites. Using an ectopic expression screen, we now identify a new site of TCE function, the dorsal pouch epithelium. Analysis of TCE mutants reveals that TCE activity in the dorsal pouch does not depend on either of the stem-loops required for maternal TCE function, but instead requires a third feature-a sequence that closely matches the Bearded box, a regulatory motif found in the 3' UTRs of several Notch pathway genes. In addition, we identify pleiohomeotic mRNA as an endogenous candidate for regulation by Bearded box-like motifs in the dorsal pouch. Together, these results suggest that the TCE has appropriated a conserved regulatory motif to expand its function to somatic tissues.

Notch signaling patterns Drosophila mesodermal segments by regulating the bHLH transcription factor twist

One of the first steps in embryonic mesodermal differentiation is allocation of cells to particular tissue fates. In Drosophila, this process of mesodermal subdivision requires regulation of the bHLH transcription factor Twist. During subdivision, Twist expression is modulated into stripes of low and high levels within each mesodermal segment. High Twist levels direct cells to the body wall muscle fate, whereas low levels are permissive for gut muscle and fat body fate. We show that Su(H)-mediated Notch signaling represses Twist expression during subdivision and thus plays a critical role in patterning mesodermal segments. Our work demonstrates that Notch acts as a transcriptional switch on mesodermal target genes, and it suggests that Notch/Su(H) directly regulates twist, as well as indirectly regulating twist by activating proteins that repress Twist. We propose that Notch signaling targets two distinct 'Repressors of twist' - the proteins encoded by the Enhancer of split complex [E(spl)C] and the HLH gene extra machrochaetae (emc). Hence, the patterning of Drosophila mesodermal segments relies on Notch signaling changing the activities of a network of bHLH transcriptional regulators, which, in turn, control mesodermal cell fate. Since this same cassette of Notch, Su(H) and bHLH regulators is active during vertebrate mesodermal segmentation and/or subdivision, our work suggests a conserved mechanism for Notch in early mesodermal patterning.

Control of Myoblast Fusion by a Guanine Nucleotide Exchange Factor, Loner, and Its Effector ARF6

Myoblast fusion is essential for the formation and regeneration of skeletal muscle. In a genetic screen for regulators of muscle development in Drosophila, we discovered a gene encoding a guanine nucleotide exchange factor, called loner, which is required for myoblast fusion. Loner localizes to subcellular sites of fusion and acts downstream of cell surface fusion receptors by recruiting the small GTPase ARF6 and stimulating guanine nucleotide exchange. Accordingly, a dominant-negative ARF6 disrupts myoblast fusion in Drosophila embryos and in mammalian myoblasts in culture, mimicking the fusion defects caused by loss of Loner. Loner and ARF6, which also control the proper membrane localization of another small GTPase, Rac, are key components of a cellular apparatus required for myoblast fusion and muscle development. In muscle cells, this fusigenic mechanism is coupled to fusion receptors; in other fusion-competent cell types it may be triggered by different upstream signals.

Dynamic gene expression during the onset of myoblast differentiation in vitro

Skeletal myogenesis is a well-studied differentiation process. However, despite the identification and functional characterization of the myogenic basic HLH transcription factors, molecular details are still lacking. With the advent of microarray technology, it has become possible to look at changes in gene expression profiles in a biological process on an unprecedented scale. In this study, we applied this technology to profile gene expression during the in vitro differentiation of an established myoblast cell line, C2C12. We report over 1500 genes whose expression is altered when these cells differentiate, including 624, or about 40% of the total number of genes, with unknown functions. This analysis reveals the existence of 12 groups of coordinately regulated genes that are expressed in temporal waves of gene expression prior to the transcriptional induction of myogenin. Among these are multiple families of transcription factors that are important for the process of myogenesis. In addition, the induction of the Notch signaling pathway suggests that previously unappreciated intercellular signaling occurs during myogenic differentiation. These results provide a molecular description of the skeletal myogenesis up to the activation of myogenin.

Specification of cell fates within the salivary gland primordium

The Drosophila salivary gland is a simple tubular organ derived from a contiguous epithelial primordium, which is established by the activities of the homeodomain-containing proteins Sex combs reduced (SCR), Extradenticle (EXD), and Homothorax (HTH). EGF signaling along the ventral midline specifies the salivary duct fate for cells in the center of the primordium, while cells farther away from the source of EGF signal adopt a secretory cell fate. EGF signaling works, at least in part, by repressing expression of secretory cell genes in the duct primordium, including fork head (fkh), which encodes a winged-helix transcription factor. FKH, in turn, represses trachealess (trh), a duct-specific gene initially expressed throughout the salivary gland primordium. trh encodes a basic helix-loop-helix PAS-domain containing transcription factor that has been proposed to specify the salivary duct fate. In conflict with this model, we find that three genes, dead ringer (dri), Serrate (Ser), and trh itself, are expressed in the duct independently of trh. Expression of all three duct genes is repressed in the secretory cells by FKH. We also show that SER in the duct cells signals to the adjacent secretory cells to specify a third cell type, the imaginal ring cells. Thus, localized EGF- and Notch-signaling transform a uniform epithelial sheet into three distinct cell types. In addition, Ser directs formation of actin rings in the salivary duct.

Reciprocal regulatory interactions between the Notch and Ras signaling pathways in the Drosophila embryonic mesoderm

Convergent intercellular signals must be precisely integrated in order to elicit specific biological responses. During specification of muscle and cardiac progenitors from clusters of equivalent cells in the Drosophila embryonic mesoderm, the Ras/MAPK pathway--activated by both epidermal and fibroblast growth factor receptors--functions as an inductive cellular determination signal, while lateral inhibition mediated by Notch antagonizes this activity. A critical balance between these signals must be achieved to enable one cell of an equivalence group to segregate as a progenitor while its neighbors assume a nonprogenitor identity. We have investigated whether these opposing signals directly interact with each other, and we have examined how they are integrated by the responding cells to specify their unique fates. Our findings reveal that Ras and Notch do not function independently; rather, we have uncovered several modes of cross-talk between these pathways. Ras induces Notch, its ligand Delta, and the epidermal growth factor receptor antagonist, Argos. We show that Delta and Argos then synergize to nonautonomously block a positive autoregulatory feedback loop that amplifies a fate-inducing Ras signal. This feedback loop is characterized by Ras-mediated upregulation of proximal components of both the epidermal and fibroblast growth factor receptor pathways. In turn, Notch activation in nonprogenitors induces its own expression and simultaneously suppresses both Delta and Argos levels, thereby reinforcing a unidirectional inhibitory response. These reciprocal interactions combine to generate the signal thresholds that are essential for proper specification of progenitors and nonprogenitors from groups of initially equivalent cells.

Lola regulates midline crossing of CNS axons in Drosophila

The pattern and level of expression of axon guidance proteins must be choreographed with exquisite precision for the nervous system to develop its proper connectivity. Previous work has shown that the transcription factor Lola is required for central nervous system (CNS) axons of Drosophila to extend longitudinally. We show here that Lola is simultaneously required to repel these same longitudinal axons away from the midline, and that it acts, in part, by augmenting the expression both of the midline repellant, Slit, and of its axonal receptor, Robo. Lola is thus the examplar of a class of axon guidance molecules that control axon patterning by coordinating the regulation of multiple, independent guidance genes, ensuring that they are co-expressed at the correct time, place and relative level.

The immunoglobulin-like protein Hibris functions as a dose-dependent regulator of myoblast fusion and is differentially controlled by Ras and Notch signaling

Hibris (Hbs) is a transmembrane immunoglobulin-like protein that shows extensive homology to Drosophila Sticks and stones (Sns) and human kidney protein Nephrin. Hbs is expressed in embryonic visceral, somatic and pharyngeal mesoderm among other tissues. In the somatic mesoderm, Hbs is restricted to fusion competent myoblasts and is regulated by Notch and Ras signaling pathways. Embryos that lack or overexpress hbs show a partial block of myoblast fusion, followed by abnormal muscle morphogenesis. Abnormalities in visceral mesoderm are also observed. In vivo mapping of functional domains suggests that the intracellular domain mediates Hbs activity. Hbs and its paralog, Sns, co-localize at the cell membrane of fusion-competent myoblasts. The two proteins act antagonistically: loss of sns dominantly suppresses the hbs myoblast fusion and visceral mesoderm phenotypes, and enhances Hbs overexpression phenotypes. Data from a P-homed enhancer reporter into hbs and co-localization studies with Sns suggest that hbs is not continuously expressed in all fusion-competent myoblasts during the fusion process. We propose that the temporal pattern of hbs expression within fusion-competent myoblasts may reflect previously undescribed functional differences within this myoblast population.

Serrate and Notch specify cell fates in the heart field by suppressing cardiomyogenesis

Notch signaling mediates numerous developmental cell fate decisions in organisms ranging from flies to humans, resulting in the generation of multiple cell types from equipotential precursors. In this paper, we present evidence that activation of Notch by its ligand Serrate apportions myogenic and non-myogenic cell fates within the early Xenopus heart field. The crescent-shaped field of heart mesoderm is specified initially as cardiomyogenic. While the ventral region of the field forms the myocardial tube, the dorsolateral portions lose myogenic potency and form the dorsal mesocardium and pericardial roof (Raffin, M., Leong, L. M., Rones, M. S., Sparrow, D., Mohun, T. and Mercola, M. (2000) Dev. Biol., 218, 326–340). The local interactions that establish or maintain the distinct myocardial and non-myocardial domains have never been described. Here we show that Xenopus Notch1 (Xotch) and Serrate1 are expressed in overlapping patterns in the early heart field. Conditional activation or inhibition of the Notch pathway with inducible dominant negative or active forms of the RBP-J/Suppressor of Hairless [Su(H)] transcription factor indicated that activation of Notch feeds back on Serrate1 gene expression to localize transcripts more dorsolaterally than those of Notch1, with overlap in the region of the developing mesocardium. Moreover, Notch pathway activation decreased myocardial gene expression and increased expression of a marker of the mesocardium and pericardial roof, whereas inhibition of Notch signaling had the opposite effect. Activation or inhibition of Notch also regulated contribution of individual cells to the myocardium. Importantly, expression of Nkx2. 5 and Gata4 remained largely unaffected, indicating that Notch signaling functions downstream of heart field specification. We conclude that Notch signaling through Su(H) suppresses cardiomyogenesis and that this activity is essential for the correct specification of myocardial and non-myocardial cell fates.

Drosophila SNS, a member of the immunoglobulin superfamily that is essential for myoblast fusion

The Drosophila sticks-and-stones (sns) locus was identified on the basis of its mutant phenotype, the complete absence of body wall muscles and corresponding presence of unfused myoblasts. The genetic location of the mutation responsible for this apparent defect in myoblast fusion was determined by recombination and deficiency mapping, and the corresponding wild-type gene was isolated in a molecular walk. Identification of the SNS coding sequence revealed a putative member of the immunoglobulin superfamily (IgSF) of cell adhesion molecules. As anticipated from this homology, SNS is enriched at the membrane and clusters at discrete sites, coincident with the occurrence of myoblast fusion. Both the sns transcript and the encoded protein are expressed in precursors of the somatic and visceral musculature of the embryo. Within the presumptive somatic musculature, SNS expression is restricted to the putative fusion-competent cells and is not detected in unfused founder cells. Thus, SNS represents the first known marker for this subgroup of myoblasts, and provides an opportunity to identify pathways specifying this cell type as well as transcriptional regulators of fusion-specific genes. To these ends, we demonstrate that the presence of SNS-expressing cells is absolutely dependent on Notch, and that expression of SNS does not require the myogenic regulatory protein MEF2.

Controls in patterning and diversification of somatic muscles during Drosophila embryogenesis

Recent genetic studies in Drosophila have provided important insights into the pathways determining the formation and diversification of body wall muscles. These pathways control a progressive subdivision of the mesoderm, ultimately leading to the specification of individual cells, the muscle founders, which are endowed with genetic programs capable of generating distinct muscle fibers. A network of activities of transcriptional regulators, signaling pathways, and lineage genes is beginning to emerge which controls successive steps of this muscle patterning and differentiation process.

LOLA has the properties of a master regulator of axon-target interaction for SNb motor axons of Drosophila

The proper pathfinding and target recognition of an axon requires the precisely choreographed expression of a multitude of guidance factors: instructive and permissive, positive and negative, and secreted and membrane bound. We show here that the transcription factor LOLA is required for pathfinding and targeting of the SNb motor nerve in Drosophila. We also show that lola is a dose-dependent regulator of SNb development: by varying the expression of one lola isoform we can progressively titrate the extent of interaction of SNb motor axons with their target muscles, from no interaction at all, through wild-type patterning, to apparent hyperinnervation. The phenotypes we observe from altered expression of LOLA suggest that this protein may help orchestrate the coordinated expression of the genes required for faithful SNb development.

R-esp1, a rat homologue of drosophila groucho, is differentially expressed after optic nerve crush and mediates NGF-induced survival of PC12 cells

The differential display reverse transcription polymerase chain reaction method was used to detect alterations in gene expression in the superior colliculus after optic nerve crush in adult rats. One of the most prominent changes observed was the selective induction of R-esp1, a homologue of the Drosophila enhancer of split locus (Groucho). Therefore, we studied the influence of R-esp1 on nerve growth factor (NGF)-induced cell survival of PC12 cells. Overexpression of R-esp1 promotes cell survival even in the absence of NGF and, conversely, it is reduced by antisense-mediated inhibition of R-esp1 expression. In conclusion, we propose a novel model in which R-esp1 protein mediates the NGF-signaling pathway.

Requirement for the Drosophila COE transcription factor Collier in formation of an embryonic muscle: transcriptional response to notch signalling

During Drosophila embryogenesis, mesodermal cells are recruited to form a stereotyped pattern of about 30 different larval muscles per hemisegment. The formation of this pattern is initiated by the specification of a special class of myoblasts, called founder cells, that are uniquely able to fuse with neighbouring myoblasts. We report here the role of the COE transcription factor Collier in the formation of a single muscle, muscle DA3([A])(DA4([T])). Col expression is first observed in two promuscular clusters (in segments A1-A7), the two corresponding progenitors and their progeny founder cells, but its transcription is maintained in only one of these four founder cells, the founder of muscle DA3([A]). This lineage-specific restriction depends on the asymmetric segregation of Numb during the progenitor cell division and involves the repression of col transcription by Notch signalling. In col mutant embryos, the DA3([A]) founder cells form but do not maintain col transcription and are unable to fuse with neighbouring myoblasts, leading to a loss-of-muscle DA3([A]) phenotype. In wild-type embryos, each of the DA3([A])-recruited myoblasts turns on col transcription, indicating that the conversion, by the DA3([A]) founder cell, of ‘naive’ myoblasts to express its distinctive pattern of gene expression involves activation of col itself. We find that muscles DA3([A]) and DO5([A]) (DA4([T]) and DO5([T])) derive from a common progenitor cell. Ectopic expression of Col is not sufficient, however, to switch the DO5([A]) to a DA3([A]) fate. Together these results lead us to propose that specification of the DA3([A]) muscle lineage requires both Col and at least one other transcription factor, supporting the hypothesis of a combinatorial code of muscle-specific gene regulation controlling the formation and diversification of individual somatic muscles.