15594664
MASH1 maintains competence for BMP2-induced neuronal differentiation in post-migratory neural crest cells
BACKGROUND
The interplay between growth factors and transcription factors in vertebrate neurogenesis is poorly understood. MASH1 is a basic helix-loop-helix (bHLH) transcription factor that is essential for autonomic neurogenesis. Bone morphogenetic protein (BMP) 2, and its relative BMP4, have been shown to induce expression of MASH1 and to promote autonomic neuronal differentiation in neural crest stem cells. The relationship between expression of MASH1 and the neurogenic competence of neural crest cells has not been investigated, however.


RESULTS
We have examined the function of MASH1 in neurogenic competence using a population of immuno-isolated neural-crest-derived progenitor cells. Post-migratory neural crest cells isolated from fetal rat gut expressed Mash1, yet comprised a mixture of committed neuronal precursors and non-neuronal cells. The non-neuronal cells remained competent to differentiate to neurons, however, if challenged with BMP2. Such competence declines with time and is paralleled by a decline in Mash1 expression in the cells. Expression of endogenous Mash1 can be maintained by BMP2; in turn, constitutive expression of Mash1 from a retroviral vector maintains competence for neuronal differentiation in response to late addition of BMP2.


CONCLUSIONS
These data suggest that MASH1 promotes competence for neurogenesis, in a manner similar to its homologs, the proneural genes achaete-scute in Drosophila. They also reveal an unexpected feedback interaction between BMP2 and MASH1 during neuronal differentiation. MASH1 may play multiple roles at successive stages of development within a neurogenic lineage, only one of which is revealed by a loss-of-function mutation.
9317227
HLH proteins, fly neurogenesis, and vertebrate myogenesis
Yuh Nung Jan and Lily Yeh Jan Howard Hughes Medical Institute and the Departments of Physiology and Biochemistry University of California, San Francisco San Francisco, California 94143-0724 Remarkable similarities between the molecular mecha- nisms underlying Drosophila neurogenesis and vertebrate myogenesis have surfaced in recent studies. Transcription regulators that contain the helix-loop-helix (HLH) motif have been implicated in determining cell fate in both verte- brate and invertebrate development. For example, MyoD and related HLH proteins specify muscles in vertebrates, whereas achaete, scute, and other HLH gene products regulate the first steps of neural development in Dro- sophila. Specification of Neural Precursors in the Drosophila Wing imaginal Disc The early events of Drosophila neurogenesis are illus- trated in Figure 1. First, proneural genes are expressed in undifferentiated ectodermal cells (proneural clusters) (Cubas et al., 1991; Skeath and Carroll, 1991). These proneural genes are so named because they endow the cells that express them with the potential of developing into neural precursors (Ghysen and Dambly-Chaudiere, 1989); the location where proneural genes are expressed determines where the nervous system forms. Then, a sin- gle cell (or a subset of cells) from the proneural cluster expresses the proneural gene at a much higher level than the surrounding cells and becomes a neural precursor (Cubas et al., 1991; Skeath and Carroll, 1991). Other cells from the cluster are prevented from taking on the neural precursor fate, via a process of lateral inhibition that re- quires the function of neurogenic genes (reviewed by Ghysen et al., 1993). The proneural gene expression is transient; all neural precursors then show exclusive ex- pression of a panel of genes called neural precursor genes, which probably control differentiation of neural pre- cursors, their progeny, or both. Neural precursor specification has been characterized extensively in the wing imaginal disc, which gives rise to the wing and the thorax of the fruit fly. achaete and scute are proneural genes expressed in proneural clusters of the wing disc and are required for the formation of macro- chaetae (large sensory bristles) on the thorax. achaete and scute can each regulate the expression of the other. initially, some proneural clusters of the wing disc express achaete and others express scure. However, eventually both achaete and scufe are expressed in all proneural clusters of the wing disc (Martinez and Modolell, 1991). A subset of macrochaetae arises from proneural clusters that express achaete first; loss of achaete function re- moves both achaete and scute expression in these proneural clusters, leading to the elimination of precursors for those macrochaetae. A different subset of macrochae- tae arises from proneural clusters that express scute first
40826632
The MyoD family and myogenesis: Redundancy, networks, and thresholds
The members of the MyoD family of myogenic transcriptional regulators (MyoD, Myf-5, MRF4, myogenin) have the remarkable property that, when expressed from a constitutive promoter, they can convert a large number of different cell types into muscle (reviewed by Buckingham, 1992; Emerson, 1990; Olson, 1990; Weintraub et al., 1991). In contrast, mice with homozygous gene-targeted mutations in either MyoD or Myf-5 produce fairly normal amounts of muscle (Rudnicki et al., 1992; Braun et al., 1992). This apparent paradox has now been resolved by two recent studies. In one, a simple genetic cross has shown that in the double homozygous mutants, in which both MyoD and Myfd are absent, no muscle is formed, no muscle markers are present, and no myogenin is transcribed (Rudnicki et al., 1993), indicating that MyoD and Myfd share an overlapping (redundant) function required for generating (or maintaining) muscle cell identity and activating myogenin. In the other study (Hasty et al., 1993; Nabeshimaet al., 1993), mice with a homozygous targeted mutation of myogenin contain myoblasts since these embryos produce normal amounts of MyoD RNA, but these cells fail to differentiate fully, indicating that myogenin has a unique function in the transition from a determined myoblast to a fully differentiated myotube. These results suggest a simple epistatic relation, shown in Figure 1. While MyoD and Myf-5 are postulated to be responsible for myoblast determination, markers for the determined myoblast state are limited; in fact, besides the expression of MyoD or Mfl-5, the only other reliable marker known is the capacity of cells to differentiate into myotubes when challenged. Thus, one wonders how MyoD and My65 actually establish a determined state, especially since known inhibitors of their transcription function (e.g., Id, Jun, Myc, etc.) are present at high levels in myoblasts. Perhaps other functions of MyoD and Myf-5 define the myoblast state or perhaps the myoblast form (which is unknown) of MyoD and Myf-5 (e.g., a homodimer or a heterodimer with Jun or Id) is capable of activating certain noncanonical promoters not yet identified. That there are indeed myoblast functions has now become very clear from recent knockout experiments. In myogenin knockouts, myoblasts are present in the appropriate positions and numbers in the embryo; they just fail to differentiate into muscle. However, in the MyoLIMyf-5 double knockout mice, cells fail to occupy the muscle compartments. Thus, myogenin-independent myoblast identity seems to be definable by the generation of approximately the right number of cells in the right positions of the body under the direction of MyoD-Myf-5 function. Regulation of Myogenic Genes In Vitro and In Vivo Tissue culture experiments also support the notion that Minireview
28506326
Cell interactions and gene interactions in peripheral neurogenesis.
The formidable complexity inherent in the development of an organism is the outcome of the interlacing or concatenation of a large number of discrete steps. Genetic analyses have revealed that each of these steps is a relatively simple developmental event resulting from the activity of a small battery of genes, often interacting with one another. This, at least, is the picture that emerges from work on developmental events as diverse as segmentation, dorsoventral organization, and segment determination in the fly embryo (Lewis 1978; NOsslein Volhard and Wieschaus 1980; for review, see St. Johnston and N/isslein-Volhard 19921. The expression of the appropriate battery of genes at the appropriate time and place is obviously of crucial importance. Thus, the question is How is this multiplicity of parallel programs kept in register? In the case of segmentation (spatial subdivision of the embryo), analysis has revealed that a succession of steps leads from simple anteroposterior heterogeneities in the distribution of a few key factors present in the oocyte to the reproducible establishment of 14 consecutive metameres shortly after cellularization of the embryo. At a late stage of this process, boundaries are set up between parasegmental metameres (for review, see Lawrence
22185747
Proneural clusters of achaete-scute expression and the generation of sensory organs in the Drosophila imaginal wing disc.
The proneural genes achaete (ac) and scute (sc) confer to Drosophila epidermal cells the ability to become sensory mother cells (SMCs). In imaginal discs, ac-sc are expressed in groups of cells, the proneural clusters, which are thought to delimit the areas where SMCs arise. We have visualized with the resolution of single cells the initial stages of sensory organ development by following the evolving pattern of proneural clusters and the emergence of SMCs. At reproducible positions within clusters, a small number of cells accumulate increased amounts of ac-sc protein. Subsequently, one of these cells, the SMC, accumulates the highest amount. Later, at least some SMCs become surrounded by cells with reduced ac-sc expression, a phenomenon probably related to lateral inhibition. Genetic mosaic analyses of cells with different doses of ac-sc genes, the sc expression in sc mutants, and the above findings show that the levels of ac-sc products are most important for SMC singling-out and SMC state maintenance. These products do not intervene in the differentiation of SMC descendants. The extramacrochaetae gene, an antagonist of proneural genes, negatively regulates sc expression, probably by interfering with activators of this gene.
23522925
The myoD gene family: nodal point during specification of the muscle cell lineage.
The myoD gene converts many differentiated cell types into muscle. MyoD is a member of the basic-helix-loop-helix family of proteins; this 68-amino acid domain in MyoD is necessary and sufficient for myogenesis. MyoD binds cooperatively to muscle-specific enhancers and activates transcription. The helix-loop-helix motif is responsible for dimerization, and, depending on its dimerization partner, MyoD activity can be controlled. MyoD senses and integrates many facets of cell state. MyoD is expressed only in skeletal muscle and its precursors; in nonmuscle cells myoD is repressed by specific genes. MyoD activates its own transcription; this may stabilize commitment to myogenesis.
18674284
Mammalian achaete-scute homolog 1 is transiently expressed by spatially restricted subsets of early neuroepithelial and neural crest cells.
Using monoclonal antibodies, we have examined the expression pattern of MASH1, a basic helix-loop-helix protein that is a mammalian homolog of the Drosophila achaete-scute proteins. In Drosophila, achaete-scute genes are required for the determination of a subset of neurons. In the rat embryo, MASH1 expression is confined to subpopulations of neural precursor cells. The induction of MASH1 precedes, but is extinguished upon, overt neuronal differentiation. MASH1 is expressed in the forebrain by spatially restricted domains of neuroepithelium and in the peripheral nervous system exclusively by precursors of sympathetic and enteric neurons. The features of early and transient expression, in spatially restricted subpopulations of neural precursors, are similar to those observed for achaete-scute. Thus, the amino acid sequence conservation between MASH1 and achaete-scute is reflected in a parallel conservation of cell type specificity of expression, similar to the case of mammalian MyoD and Drosophila nautilus. These data support the idea that helix-loop-helix proteins may represent an evolutionarily conserved family of cell-type determination genes, of which MASH1 is the first neural-specific member identified in vertebrates.
42685550
Dynamic expression of the murine Achaete-Scute homologue Mash-1 in the developing nervous system
The Drosophila Achaete-Scute Complex genes encode transcriptional regulators belonging to the basic-helix-loop-helix family which control early steps of development of the central and peripheral nervous systems. We have isolated two mouse homologues of Achaete-Scute Complex genes, Mash-1 and Mash-2, by using the conservation of the basic-helix-loop-helix domain in this family. In this article, we analyse the expression of Mash-1 from its onset during neurulation to adult stages by RNA in situ hybridization on whole mounts and sections. As was observed for the rat Mash-1 protein, mouse Mash-1 RNA expression is restricted to cells of the developing central and peripheral nervous systems. We have observed three successive phases in the distribution of Mash-1 transcripts in the developing central nervous system. Initially, between embryonic day 8.5 and 10.5, Mash-1 transcripts are found in restricted domains in the neuroepithelium of the midbrain and ventral forebrain, as well as in the spinal cord. Between embryonic day 10.5 and 12.5, Mash-1 expression pattern changes from a restricted to a widespread one. Mash-1 transcripts are then found at variable levels in the ventricular zone in all regions of the brain. From embryonic day 12.5 to post-natal stages, Mash-1 is also expressed in cells outside of the ventricular zone throughout the brain. In addition, Mash-1 is expressed during development of the olfactory epithelium and neural retina. Overall, its expression pattern suggest that Mash-1 plays a role at early stages of development of specific neural lineages in most regions of the central nervous system and of several lineages in the peripheral nervous system. We have also compared the expression of Mash-1 and mouse Notch because their Drosophila homologues have been shown to interact genetically. The two genes show very similar expression patterns, both spatially and temporally, in the early developing brain and in the retina, suggesting that both genes may participate in the development of the same neural lineages.
17847644
The cellular function of MASH1 in autonomic neurogenesis
Using primary cultures and immortalized multipotential stem cell lines derived from wild-type and Mash1 mutant neural crest cells, we have analyzed the cellular function of MASH1 in autonomic neurogenesis. We present evidence for the existence of a precursor expressing MASH1 and neuronal markers such as neurofilament, neuron-specific tubulin, and tetanus toxin receptor. This cell has a nonneuronal morphology. Differentiation of this precursor to neurons that express markers such as SCG10, peripherin, and neuron-specific enolase is dependent upon MASH1 function. These data imply that the differentiation of autonomic neurons from uncommitted neural crest cells occurs in several sequential steps. Moreover, they suggest that MASH1 does not commit multipotent cells to a neural fate, like its Drosophila achaete-scute counterparts, but rather promotes the differentiation of a committed neuronal precursor.
38282383
Distinct subpopulations of enteric neuronal progenitors defined by time of development, sympathoadrenal lineage markers and Mash-1-dependence.
Enteric and sympathetic neurons have previously been proposed to be lineally related. We present independent lines of evidence that suggest that enteric neurons arise from at least two lineages, only one of which expresses markers in common with sympathoadrenal cells. In the rat, sympathoadrenal markers are expressed, in the same order as in sympathetic neurons, by a subset of enteric neuronal precursors, which also transiently express tyrosine hydroxylase. If this precursor pool is eliminated in vitro by complement-mediated lysis, enteric neurons continue to develop; however, none of these are serotonergic. In the mouse, the Mash-1-/- mutation, which eliminates sympathetic neurons, also prevents the development of enteric serotonergic neurons. Other enteric neuronal populations, however, including those that contain calcitonin gene related peptide are present. Enteric tyrosine hydroxylase-containing cells co-express Mash-1 and are eliminated by the Mash-1-/- mutation, consistent with the idea that in the mouse, as in the rat, these precursors generate serotonergic neurons. Serotonergic neurons are generated early in development, while calcitonin gene related peptide-containing enteric neurons are generated much later. These data suggest that enteric neurons are derived from at least two progenitor lineages. One transiently expresses sympathoadrenal markers, is Mash-1-dependent, and generates early-born enteric neurons, some of which are serotonergic. The other is Mash-1-independent, does not express sympathoadrenal markers, and generates late-born enteric neurons, some of which contain calcitonin gene related peptide.
9368896
Induction and repression of mammalian achaete-scute homologue (MASH) gene expression during neuronal differentiation of P19 embryonal carcinoma cells.
MASH1 and MASH2, mammalian homologues of the Drosophila neural determination genes achaete-scute, are members of the basic helix-loop-helix (bHLH) family of transcription factors. We show here that murine P19 embryonal carcinoma cells can be used as a model system to study the regulation and function of these genes. MASH1 and MASH2 display complementary patterns of expression during the retinoic-acid-induced neuronal differentiation of P19 cells. MASH1 mRNA is undetectable in undifferentiated P19 cells but is induced to high levels by retinoic acid coincident with neuronal differentiation. In contrast, MASH2 mRNA is expressed in undifferentiated P19 cells and is repressed by retinoic acid treatment. These complementary expression patterns suggest distinct functions for MASH1 and MASH2 in development, despite their sequence homology. In retinoic-acid-treated P19 cells, MASH1 protein expression precedes and then overlaps expression of neuronal markers. However, MASH1 is expressed by a smaller proportion of cells than expresses such markers. MASH1 immunoreactivity is not detected in differentiated cells displaying a neuronal morphology, suggesting that its expression is transient. These features of MASH1 expression are similar to those observed in vivo, and suggest that P19 cells represent a good model system in which to study the regulation of this gene. Forced expression of MASH1 was achieved in undifferentiated P19 cells by transfection of a cDNA expression construct. The transfected cells expressing exogenous MASH1 protein contained E-box-binding activity that could be super-shifted by an anti-MASH1 antibody, but exhibited no detectable phenotypic changes. Thus, unlike myogenic bHLH genes, such as MyoD, which are sufficient to induce muscle differentiation, expression of MASH1 appears insufficient to promote neurogenesis.
11698606
Postmigratory neural crest cells expressing c-RET display restricted developmental and proliferative capacities
c-RET is an orphan receptor tyrosine kinase essential for enteric neurogenesis in mice and is involved in several human genetic disorders. RET is also one of the earliest surface markers expressed by postmigratory neural crest cells in the gut. We generated anti-RET monoclonal antibodies to isolate such cells. We find that RET+ cells are antigenically and functionally distinct from neural crest stem cells (NCSCs) characterized previously. Unlike NCSCs, which are RET- and MASH1-, most RET+ cells express MASH1. Moreover, unlike NCSCs, which are multipotent and have high proliferative capacity, many RET+ cells generate only neurons following a limited number of divisions. This behavior is observed even in the presence of glial growth factor, a polypeptide that suppresses neuronal and promotes glial differentiation by NCSCs. These data provide direct evidence for the existence of committed neuronal progenitor cells and support a model of neural crest lineage diversification by progressive restriction of developmental potential.
11497323
Alternative Neural Crest Cell Fates Are Instructively Promoted by TGFβ Superfamily Members
How growth factors influence the fate of multipotent progenitor cells is not well understood. Most hematopoietic growth factors act selectively as survival factors, rather than instructively as lineage determination signals. In the neural crest, neuregulin instructively promotes gliogenesis, but how alternative fates are determined is unclear. We demonstrate that bone morphogenic protein 2 (BMP2) induces the basic-helix-loop-helix protein MASH1 and neurogenesis in neural crest stem cells. In vivo, MASH1+ cells are located near sites of BMP2 mRNA expression. Some smooth muscle differentiation is also observed in BMP2. A related factor, transforming growth factor beta1 (TGFbeta1), exclusively promotes smooth muscle differentiation. Like neuregulin, BMP2 and TGFbeta1 act instructively rather than selectively. The neural crest and hematopoietic systems may therefore utilize growth factors in different ways to generate cellular diversity.
23193917
Isolation of a stem cell for neurons and glia from the mammalian neural crest
We have isolated mammalian neural crest cells using a monoclonal antibody to the low affinity NGF receptor, and established conditions for the serial propagation of these cells in clonal culture to assess their developmental potential. This analysis indicates that, first, single mammalian neural crest cells are multipotent, able to generate at least neurons and Schwann cells like their avian counterparts. Second, multipotent neural crest cells generate multipotent progeny, indicating that they are capable of self-renewal and therefore are stem cells. Third, multipotent neural crest cells also generate some clonal progeny that form only neurons or glia, suggesting the production of committed neuroblasts and glioblasts. Manipulation of the substrate alters the fate of the multipotent cells. These findings have implications for models of neural crest development in vivo, and establish a system for studying the generation of cellular diversity by a multipotent stem cell in vitro.
20297598
Glial growth factor restricts mammalian neural crest stem cells to a glial fate
Growth factors and cytokines are thought to influence the development of uncommitted progenitor cell populations, but the issue of how these factors act on individual cells remains controversial. Such factors may act simply as selective mitogens or survival factors for cells that undergo lineage restrictions stochastically. Alternatively, they may instruct or bias multipotent cells to choose one lineage at the expense of others. Here we show that glial growth factor (GGF), previously defined as a Schwann cell mitogen, strongly suppresses neuronal differentiation of rat neural crest stem cells while promoting or allowing glial differentiation. Quantitative clonal analysis suggests that the action of GGF is likely to be instructive rather than selective. Taken together with the expression pattern of GGF, these data suggest a lateral signaling model for the diversification of cell types within developing peripheral ganglia.
8631773
A bipotential neuroendocrine precursor whose choice of cell fate is determined by NGF and glucocorticoids
Adrenal medullary endocrine (chromaffin) cells and sympathetic neurons both derive from the neural crest. We have found that the embryonic adrenal medulla and sympathetic ganglia are both initially populated by precursors expressing neural-specific genes. By birth, however, the medulla consists largely of chromaffin cells. In primary culture, the medullary precursors have three developmental fates: in NGF they continue to mature into neurons and survive, whereas in glucocorticoid they either extinguish their neuronal properties and exhibit an endocrine phenotype, or else continue to develop into neurons but then die. These data suggest that, in vivo, the adrenal medulla develops through both the glucocorticoid-induced differentiation of bipotential progenitors and the degeneration of committed neuronal precursors, which have migrated into the gland.
9885002
Antibody markers identify a common progenitor to sympathetic neurons and chromaffin cells in vivo and reveal the timing of commitment to neuronal differentiation in the sympathoadrenal lineage
Using specific antibody markers and double-label immunofluorescence microscopy, we have followed the fate of progenitor cells in the sympathoadrenal (SA) sublineage of the neural crest in developing rat embryos. Such progenitors are first recognizable in the primordial sympathetic ganglia at embryonic day 11.5 (E11.5), when they express tyrosine hydroxylase. At this stage, the progenitors also coexpress neuronal markers such as SCG 10 and neurofilament, together with a series of chromaffin cell markers called SA 1–5 (Carnhan and Patterson, 1991 a). The observation of such doubly labeled cells is consistent with the hypothesis that these cells represent a common progenitor to sympathetic neurons and adrenal chromaffin cells. Subsequent to E 11.5, expression of the chromaffin markers is extinguished in the sympathetic ganglia but retained by cells within the adrenal gland. Concomitant with the loss of the SA 1-5 immunoreactivity in sympathetic ganglia, a later sympathetic neuron-specific marker, B2, appears. In dissociated cell suspensions, some B2+ cells that coexpress SA 1 are seen. This implies a switch in the antigenic phenotype of developing sympathetic neurons, rather than a replacement of one cell population by another. The SA 1----B2 transition does not occur for the majority of cells within the adrenal primordium. In vitro, most B2+ cells fail to differentiate into chromaffin cells in response to glucocorticoid. Instead, they continue to extend neurites and then die. Taken together, these data imply that the SA 1----B2 transition correlates with a loss of competence to respond to an inducer of chromaffin differentiation. Thus, the development of SA derivatives is controlled both by environmental signals and by changes in the ability of differentiating cells to respond to such signals.
1623242
Isolation of monoclonal antibodies specific for human c-myc proto-oncogene product.
Six monoclonal antibodies have been isolated from mice immunized with synthetic peptide immunogens whose sequences are derived from that of the human c-myc gene product. Five of these antibodies precipitate p62c-myc from human cells, and three of these five also recognize the mouse c-myc gene product. None of the antibodies sees the chicken p110gag-myc protein. All six antibodies recognize immunoblotted p62c-myc. These reagents also provide the basis for an immunoblotting assay by which to quantitate p62c-myc in cells.
39185919
A new DNA binding and dimerization motif in immunoglobulin enhancer binding, daughterless, MyoD, and myc proteins
Two cDNAs were isolated whose dimerized products bind specifically to a DNA sequence, kappa E2, located in the immunoglobulin kappa chain enhancer. Both cDNAs share a region of extensive identity to the Drosophila daughterless gene and obvious similarity to a segment in three myc proteins, MyoD, and members of the Drosophila achaete-scute and twist gene family. The homologous regions have the potential to form two amphipathic helices separated by an intervening loop. Remarkable is the stringent conservation of hydrophobic residues present in both helices. We demonstrate that this new motif plays a crucial role in both dimerization and DNA binding.
23486487
DNA binding and transcriptional regulatory activity of mammalian achaete-scute homologous (MASH) proteins revealed by interaction with a muscle-specific enhancer.
The MASH genes are vertebrate homologues of achaete-scute, genes required for neuronal determination in Drosophila. The sequence of MASH1 and MASH2 contains a basic helix-loop-helix (bHLH) motif that is present in other transcriptional regulators such as MyoD and E12. In the absence of an authentic target for the MASH proteins, we examined their DNA binding and transcriptional regulatory activity by using a binding site (the E box) from the muscle creatine kinase (MCK) gene, a target of MyoD. Like myogenic bHLH proteins, the MASH proteins form heterooligomers with E12 that bind the MCK E box with high affinity in vitro. Unexpectedly, however, MASH1 and MASH2 also activate transcription of both exogenous and endogenous MCK in transfected C3H/10T1/2 fibroblasts. However, they do not induce myogenesis. Myogenic activity is not exclusively a property of the MyoD basic region, as substitution of this domain fails to confer myogenic activity on MASH1. These data suggest that different bHLH proteins may activate overlapping but distinct sets of target genes in the same cell type.
29339773
Interactions between heterologous helix-loop-helix proteins generate complexes that bind specifically to a common DNA sequence
A DNA binding and dimerization motif, with apparent amphipathic helices (the HLH motif), has recently been identified in various proteins, including two that bind to immunoglobulin enhancers (E12 and E47). We show here that various HLH proteins can bind as apparent heterodimers to a single DNA motif and also, albeit usually more weakly, as apparent homodimers. The HLH domain can mediate heterodimer formation between either daughterless, E12, or E47 (Class A) and achaete-scute T3 or MyoD (Class B) to form proteins with high affinity for the kappa E2 site in the immunoglobulin kappa chain enhancer. The achaete-scute T3 and MyoD proteins do not form kappa E2-binding heterodimers together, and no active complex with N-myc was evident. The formation of a heterodimer between the daughterless and achaete-scute T3 products may explain the similar phenotypes of mutants at these two loci and the genetic interactions between them. A role of E12 and E47 in mammalian development, analogous to that of daughterless in Drosophila, is likely.
29514374
The protein Id: A negative regulator of helix-loop-helix DNA binding proteins
We have isolated a cDNA clone encoding a novel helix-loop-helix (HLH) protein, Id. Id is missing the basic region adjacent to the HLH domain that is essential for specific DNA binding in another HLH protein, MyoD. An in vitro translation product of Id can associate specifically with at least three HLH proteins (MyoD, E12, and E47) and attenuate their ability to bind DNA as homodimeric or heterodimeric complexes. Id is expressed at varying levels in all cell lines tested. In three cell lines that can be induced to undergo terminal differentiation, Id RNA levels decrease upon induction. Transfection experiments indicate that over-expression of Id inhibits the trans-activation of the muscle creatine kinase enhancer by MyoD. Based on these findings, we propose that HLH proteins lacking a basic region may negatively regulate other HLH proteins through the formation of nonfunctional heterodimeric complexes.
19459236
Mammalian achaete-scute homolog 1 is required for the early development of olfactory and autonomic neurons
The mouse Mash-1 gene, like its Drosophila homologs of the achaete-scute complex (AS-C), encodes a transcription factor expressed in neural precursors. We created a null allele of this gene by homologous recombination in embryonic stem cells. Mice homozygous for the mutation die at birth with apparent breathing and feeding defects. The brain and spinal cord of the mutants appear normal, but their olfactory epithelium and sympathetic, parasympathetic, and enteric ganglia are severely affected. In the olfactory epithelium, neuronal progenitors die at an early stage, whereas the nonneuronal supporting cells are present. In sympathetic ganglia, the mutation arrests the development of neuronal precursors, preventing the generation of sympathetic neurons, but does not affect glial precursor cells. These observations suggest that Mash-1, like its Drosophila homologs of the AS-C, controls a basic operation in development of neuronal progenitors in distinct neural lineages.
5574968
Time of origin of neurons in the murine enteric nervous system: Sequence in relation to phenotype
The hypothesis was tested that developing enteric neurons withdraw from the cell cycle in a sequence related to their phenotype. The birthdays of immunocytochemically identified myenteric and submucosal neurons were determined in the murine duodenum and jejunum. [3H]thymidine ([3H]TdR) was injected into timed pregnant mice or pups at 4–8 hour intervals over a 24 hour period. Pups were killed on postnatal day 30 (P30). [3H]TdR incorporation was detected by radioautography in enteric neurons, which were phenotypically identified by the simultaneous detection of the immunoreactivities of 5‐hydroxytryptamine (5‐HT), choline acetyl transferase (ChAT), neuropeptide Y (NPY), enkephalin (ENK), calcitonin gene‐related peptide (CGRP), and vasoactive intestinal peptide (VIP). The dates of the earliest withdrawal from the cell cycle of neurons containing these markers were determined, as well as the length of time during which the identified neurons continued to be born, and the date on which their rate of birth was maximal. The birthdates of myenteric neurons that contained 5‐HT (E8–E14, peak at E10) or ChAT (E8–E15, peak at E12) tended to be earlier than those that contained ENK (E10–E18, peak at E14), NPY (E10–E18, peak at E15), VIP (E10–P5, peak at E15), or CGRP (E10–P3, peak at E17). For any given immunocytochemically defined neuronal phenotype, submucosal neurons tended to be born later than their myenteric counterparts and submucosal neurons that contained neuropeptides were born later than those that contained only ChAT immunoreactivity. The day (E8) on which the first 5‐HT‐ and ChAT‐immunoreactive neurons became postmitotic is earlier than the day (E9) on which the colonization of the bowel by crest‐derived cells has been detected. The population of neural precursors that colonizes the gut, therefore, is heterogeneous; many cells are proliferating, but a specific subset, which will ultimately give rise to serotoninergic or cholinergic neurons, is already postmitotic. Neurons continued to be born throughout fetal life and even after birth. Consequently, terminally differentiated neurons coexist in the developing enteric nervous system with dividing neural precursor cells. This observation is consistent with the idea that early developing neurons could affect the development of enteric neural precursors; moreover, they also demonstrate that it is possible to add neurons to the enteric plexuses even after the neural circuits on which the bowel depends have become functional.
35331181
Neuronal cell fate specification in Drosophila

Recent work indicates that the Drosophila nervous system develops in a progressive process of cell fate specification. Expression of specific proneural genes in clusters of cells (the proneural clusters) in the cellular blastoderm endows these cells with the potential to form certain types of neural precursors. Intercellular interactions that involve both proneural genes and neurogenic genes then allow the neural precursors to be singled out from the proneural clusters. Expression of neural precursor genes in all neural precursors is likely to account for the universal aspects of neuronal differentiation, such as axonal outgrowth. Selective expression of certain neuronal-type selector genes further specifies the type of neuron(s) that a neural precursor will produce.
7861221
Neural enhancer-like elements as specific cell markers in Drosophila.
We have analysed four strains of Drosophila melanogaster which each carry the transposon P[lac,ry+] at a unique genomic location. In one of the strains, P[lac,ry+]A37, all the peripheral neurones that we can identify express the P-lac fusion protein; in at least some cases, and the support cells associated to particular neurones are also labelled. Expression of the fusion protein can be detected in subepidermal cells of the body segments as early as 4-5 h of development, according to a precise and reproducible pattern. On the basis of genetic evidence, we propose that these cells are precursors of sense organs, implying that the development of the peripheral nervous system overlaps in time with the development of the central nervous system. In the other three strains, the fusion product is expressed in unique subsets of cells of the peripheral nervous system, as well as in some other tissues.
22899086
Competence to develop sensory organs is temporally and spatially regulated in Drosophila epidermal primordia.
The Drosophila adult cuticle displays a stereotyped pattern of sensory organs (SOs). Its deployment requires the expression of the achaete (ac) and scute (sc) genes. Their products confer to cells of epidermal primordia (imaginal discs and histoblasts) the ability to become SO precursors (SOPs). In imaginal discs, ac and sc expression is spatially restricted to cell clusters within which one or a few cells become SOP(s). With the help of ubiquitous sc expression provided at different developmental times by a heat shock‐sc (HSSC) chimeric gene, we have analyzed the response of epidermal primordia to the proneural action of the sc product, and have tested whether the patterned distribution of ac/sc products is necessary to position SOs correctly within the epidermis. Each primordium responds to HSSC expression by developing SOs only during a characteristic developmental period. In the absence of the endogenous ac and sc genes, most SOs induced by HSSC are of the correct type and are located in wild type positions. These results indicate that the capacity of primordia to respond to sc is temporally and spatially regulated, that specification of the type of SO does not depend on ac/sc, and that SO positioning utilizes topological information independent of the spatially restricted distribution of ac/sc products.
4536636
Genesis of the Drosophila peripheral nervous system.
The formation of a sense organ in Drosophila is a progressive process which begins with the local acquisition of a 'proneural' state. Mutations altering different steps of this process reveal the existence of several distinct operations, some of which are mediated by transcriptional regulation while others involve cell interactions.
30089446
Positive autoregulation of the myogenic determination gene MyoD1
Transfection of cDNA expression vectors encoding either MyoD1 or myogenin into 10T1/2 cells converts them to myogenic cells. We show that transfection of 10T1/2 cells with the MyoD1 cDNA activates expression of endogenous MyoD1 mRNA, indicating that MyoD1 is subject to positive autoregulation. This activation of endogenous MyoD1 mRNA was also observed in Swiss 3T6 cells, but not in several other fibroblast or adipoblast cell lines transfected with the MyoD1 cDNA. In addition, transfection of the MyoD1 cDNA leads to activation of myogenin expression, and transfection of the myogenin cDNA leads to activation of MyoD1 expression. Thus, MyoD1 and myogenin appear to function in a positive autoregulatory loop that could either: account for or contribute to the stability of myogenic commitment; or amplify the level of expression of both MyoD1 and myogenin above a critical threshold that is required for activation of the myogenic program.
37862808
Cross-regulatory interactions between the proneural achaete and scute genes of Drosophila.
The achaete (ac) and scute (sc) genes of Drosophila allow cells to become sensory organ mother cells. Although ac and sc have similar patterns of expression, deletion of either gene removes specific subsets of sensory organs. This specificity was shown to reside in the peculiar regulation of ac and sc expression. These genes are first activated in complementary spatial domains in response to different cis-regulatory sequences. Each gene product then stimulates expression of the other gene, thus generating similar patterns of expression. Therefore, removal of one gene leads to the absence of both proneural gene products and sensory organs in the sites specified by its cis-regulatory sequences.
21333147
Spatial regulation of proneural gene activity: auto- and cross-activation of achaete is antagonized by extramacrochaetae.
The spatially restricted activities of achaete (ac) and scute (sc) are thought to define proneural clusters of potential sensory organ precursor cells in the imaginal discs of Drosophila. These genes encode transcriptional regulators of the basic helix-loop-helix (bHLH) class. We have found that direct, positive transcriptional autoregulation by the ac protein and cross-regulation by sc are essential for high-level expression of the ac promoter in the proneural cluster pattern and that autoactivation is important for the bristle-promoting function of the ac gene. These auto- and cross-regulatory activities are antagonized in a dose-dependent manner by the inhibitory HLH protein encoded by the extramacrochaetae (emc) gene. We have found that emc is expressed in the wing imaginal disc in a pattern strongly complementary to that of the proneural clusters. Our results indicate that emc plays an essential early role in defining territories of bristle-forming potential.
15150284
Advanced mammalian gene transfer: high titre retroviral vectors with multiple drug selection markers and a complementary helper-free packaging cell line.
We report the development of an advanced system for transfer and expression of exogenous genes in mammalian cells based on Moloney murine leukemia virus (Mo MuLV). Extensive deletion/mutagenesis analysis to identify cis-acting signals involved in virus transmission has led to the design of a family of novel, highly efficient retroviral vectors and a partner helper-free packaging cell line. The pBabe retroviral vector constructs transmit inserted genes at high titres and express them from the Mo MuLV Long Terminal Repeat (LTR). Each of these vectors has been constructed with one of four different dominantly acting selectable markers, allowing the growth of infected mammalian cells in the presence of G418, hygromycin B, bleomycin/phleomycin or puromycin, respectively. The high titre ecotropic helper free packaging cell line, omega E, was designed in conjunction with the pBabe vectors to reduce the risk of generation of wild type Mo MuLV via homologous recombination events. The omega E cell line was generated with separate gagpol and ecotropic env expression constructs with minimal sequence overlap and decreased sequence homology achieved by 'codon wobbling'. Homologous env coding sequences were deleted from the pBabe vectors without diminishing recombinant vector titre. Together, the pBabe vectors and omega E cell line should prove useful in experiments where highest frequencies of gene transfer, or concomitant expression of several different genes within a single cell are required with minimal risk of helper virus contamination.
18598374
A conserved family of nuclear phosphoproteins localized to sites of polymerase II transcription
An antibody was identified previously that recognizes sites of polymerase II transcription on lampbrush chromosomes, puffs on polytene chromosomes, and many small granules in the nucleoplasm of all cells tested. This antibody binds a conserved family of phosphorylated polypeptides in vertebrate and invertebrate cells. We developed a method for purifying these proteins that involves differential solubility in MgCl2. We isolated a Drosophila cDNA encoding one of the proteins using information obtained from microsequencing. In vivo expression studies show that this protein is concentrated on sites of polymerase II transcription and that it is highly phosphorylated. The protein shares a high degree of homology with proteins involved in alternative splicing of pre-mRNA suggesting the possibility that this protein plays a role in pre-mRNA splicing.
10287752
Production of high-titer helper-free retroviruses by transient transfection.
The generation of high-titer, helper-free retroviruses by transient transfection has been achieved by using the highly transfectable 293T cell line into which are stably introduced constructs that express retroviral packaging functions. The resulting ecotropic virus packaging cell line BOSC 23 produces infectious retrovirus at > 10(6) infectious units/ml of supernatant within 72 hr after CaPO4-mediated transfection. A stringent assay for replication-competent virus showed that no helper virus was present. The system can produce high titers of retroviral vectors expressing genes that are extremely difficult to propagate at high titer in stable producer lines. This method should facilitate and extend the use of helper-free retroviral gene transfer, as well as be useful for gene therapy.