Genetic basis of the formation and identity of type I and type II neurons in Drosophila embryos

A framework for analyzing the relationship between gene expression and morphological, topological, and dynamical patterns in neuronal networks

BACKGROUND

A key point in developmental biology is to understand how gene expression influences the morphological and dynamical patterns that are observed in living beings.

NEW METHOD

In this work we propose a methodology capable of addressing this problem that is based on estimating the mutual information and Pearson correlation between the intensity of gene expression and measurements of several morphological properties of the cells. A similar approach is applied in order to identify effects of gene expression over the system dynamics. Neuronal networks were artificially grown over a lattice by considering a reference model used to generate artificial neurons. The input parameters of the artificial neurons were determined according to two distinct patterns of gene expression and the dynamical response was assessed by considering the integrate-and-fire model.

RESULTS

As far as single gene dependence is concerned, we found that the interaction between the gene expression and the network topology, as well as between the former and the dynamics response, is strongly affected by the gene expression pattern. In addition, we observed a high correlation between the gene expression and some topological measurements of the neuronal network for particular patterns of gene expression.

COMPARISON WITH EXISTING METHODS

To our best understanding, there are no similar analyses to compare with.

CONCLUSIONS

A proper understanding of gene expression influence requires jointly studying the morphology, topology, and dynamics of neurons. The proposed framework represents a first step towards predicting gene expression patterns from morphology and connectivity.

Molecular cloning and characterization of homologs of achaete-scute and hairy-enhancer of split in the olfactory organ of the spiny lobster Panulirus argus

The olfactory organ of the Caribbean spiny lobster Panulirus argus maintains lifelong proliferation and turnover of olfactory receptor neurons (ORNs). Towards examining the molecular basis of this adult neurogenesis, we search for expression of homologs of proneural, neurogenic, and pre-pattern genes in this olfactory organ. We report here a homolog of the proneural Achaete-Scute family, called splash (spiny lobster achaete-scute homolog), and a homolog of the pre-pattern and neurogenic hairy-enhancer of split family, called splhairy (spiny lobster hairy). Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) indicates a molt stage dependence of the levels of expression of splash and splhairy mRNA in the olfactory organ, with higher expression in premolt than in postmolt or intermolt animals, which is positively correlated with rates of neurogenesis. splash and splhairy mRNA are expressed not only in the olfactory organ but also in other tissues, albeit at lower levels, irrespective of molt stage. We conclude that the expression of achaete-scute and hairy-enhancer of split in the proliferation zone of the olfactory organ of spiny lobsters and their enhanced expression in premolt animals suggest that they play a role in the proliferation of ORNs and that their expression in regions of the olfactory organ populated by mature ORNs and in other tissues suggests that they have additional functions.

MicroRNA-9a ensures the precise specification of sensory organ precursors in Drosophila

MicroRNAs (miRNAs) have been implicated in regulating various aspects of animal development, but their functions in neurogenesis are largely unknown. Here we report that loss of miR-9a function in the Drosophila peripheral nervous system leads to ectopic production of sensory organ precursors (SOPs), whereas overexpression of miR-9a results in a severe loss of SOPs. We further demonstrate a strong genetic interaction between miR-9a and senseless (sens) in controlling the formation of SOPs in the adult wing imaginal disc. Moreover, miR-9a suppresses Sens expression through its 3' untranslated region. miR-9a is expressed in epithelial cells, including those adjacent to SOPs within proneural clusters, suggesting that miR-9a normally inhibits neuronal fate in non-SOP cells by down-regulating Sens expression. These results indicate that miR-9a ensures the generation of the precise number of neuronal precursor cells during development.

The regulation of Notch signaling in muscle stem cell activation and postnatal myogenesis

The Notch signaling pathway is an evolutionarily conserved pathway that is critical for tissue morphogenesis during development, but is also involved in tissue maintenance and repair in the adult. In skeletal muscle, regulation of Notch signaling is involved in somitogenesis, muscle development, and the proliferation and cell fate determination of muscle stems cells during regeneration. During each of these processes, the spatial and temporal control of Notch signaling is essential for proper tissue formation. That control is mediated by a series of regulatory proteins and protein complexes that enhance or inhibit Notch signaling by regulating protein processing, localization, activity, and stability. In this review, we focus on the regulation of Notch signaling during postnatal muscle regeneration when muscle stem cells ("satellite cells") must activate, proliferate, progress along a myogenic lineage pathway, and ultimately differentiate to form new muscle. We review the regulators of Notch signaling, such as Numb and Deltex, that have documented roles in myogenesis as well as other regulators that may play a role in modulating Notch signaling during satellite cell activation and postnatal myogenesis.

Cell cycle and cell-fate determination in Drosophila neural cell lineages

"Normal" development requires a finely tuned equilibrium between cell differentiation and cell proliferation. Important issues in development include whether the cell cycle controls the cell-fate determination and whether cell identity in turn regulates cell-cycle progression. Although, these issues are of general biological relevance, stereotyped Drosophila neural lineages are particularly suited to address these questions and have provided insights into the links between cell-cycle progression and cell-fate specification.

Control of central synaptic specificity in insect sensory neurons

Synaptic specificity is the culmination of several processes, beginning with the establishment of neuronal subtype identity, followed by navigation of the axon to the correct subdivision of neuropil, and finally, the cell-cell recognition of appropriate synaptic partners. In this review we summarize the work on sensory neurons in crickets, cockroaches, moths, and fruit flies that establishes some of the principles and molecular mechanisms involved in the control of synaptic specificity. The identity of a sensory neuron is controlled by combinatorial expression of transcription factors, the products of patterning and proneural genes. In the nervous system, sensory axon projections are anatomically segregated according to modality, stimulus quality, and cell-body position. A variety of cell-surface and intracellular signaling molecules are used to achieve this. Synaptic target recognition is also controlled by transcription factors such as Engrailed and may be, in part, mediated by cadherin-like molecules.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

The fragile X-related Gene Affects the Crawling Behavior of Drosophila Larvae by Regulating the mRNA Level of the DEG/ENaC Protein Pickpocket1

BACKGROUND

Fragile X syndrome is caused by loss-of-function mutations in the fragile X mental retardation 1 (FMR1) gene. How FMR1 affects the function of the central and peripheral nervous systems is still unclear. FMR1 is an RNA binding protein that associates with a small percentage of total mRNAs in vivo. It remains largely unknown what proteins encoded by mRNAs in the FMR1-messenger ribonuclear protein (mRNP) complex are most relevant to the affected physiological processes.

RESULTS

Loss-of-function mutations in the Drosophila fragile X-related (dfmr1) gene, which is highly homologous to the human fmr1 gene, decrease the duration and percentage of time that crawling larvae spend on linear locomotion. Overexpression of DFMR1 in multiple dendritic (MD) sensory neurons increases the time percentage and duration of linear locomotion; this phenotype is similar to that caused by reduced expression of the MD neuron subtype-specific degenerin/epithelial sodium channel (DEG/ENaC) family protein Pickpocket1 (PPK1). Genetic analyses indicate that PPK1 is a key component downstream of DFMR1 in controlling the crawling behavior of Drosophila larvae. DFMR1 and ppk1 mRNA are present in the same mRNP complex in vivo and can directly bind to each other in vitro. DFMR1 downregulates the level of ppk1 mRNA in vivo, and this regulatory process also involves Argonaute2 (Ago2), a key component in the RNA interference pathway.

CONCLUSIONS

These studies identify ppk1 mRNA as a physiologically relevant in vivo target of DFMR1. Our finding that the level of ppk1 mRNA is regulated by DFMR1 and Ago2 reveals a genetic pathway that controls sensory input-modulated locomotion behavior.

Enhanced locomotion caused by loss of the Drosophila DEG/ENaC protein Pickpocket1

Coordination of rhythmic locomotion depends upon a precisely balanced interplay between central and peripheral control mechanisms. Although poorly understood, peripheral proprioceptive mechanosensory input is thought to provide information about body position for moment-to-moment modifications of central mechanisms mediating rhythmic motor output. Pickpocket1 (PPK1) is a Drosophila subunit of the epithelial sodium channel (ENaC) family displaying limited expression in multiple dendritic (md) sensory neurons tiling the larval body wall and a small number of bipolar neurons in the upper brain. ppk1 null mutant larvae had normal external touch sensation and md neuron morphology but displayed striking alterations in crawling behavior. Loss of PPK1 function caused an increase in crawling speed and an unusual straight path with decreased stops and turns relative to wild-type. This enhanced locomotion resulted from sustained peristaltic contraction wave cycling at higher frequency with a significant decrease in pause period between contraction cycles. The mutant phenotype was rescued by a wild-type PPK1 transgene and duplicated by expressing a ppk1RNAi transgene or a dominant-negative PPK1 isoform. These results demonstrate that the PPK1 channel plays an essential role in controlling rhythmic locomotion and provide a powerful genetic model system for further analysis of central and peripheral control mechanisms and their role in movement disorders.

Myogenic cells fates are antagonized by Notch only in asymmetric lineages of the Drosophila heart, with or without cell division

During the formation of the Drosophila heart, a combinatorial network that integrates signaling pathways and tissue-specific transcription factors specifies cardiac progenitors, which then undergo symmetric or asymmetric cell divisions to generate the final population of diversified cardiac cell types. Much has been learned concerning the combinatorial genetic network that initiates cardiogenesis, whereas little is known about how exactly these cardiac progenitors divide and generate the diverse population of cardiac cells. In this study, we examined the cell lineages and cell fate determination in the heart by using various cell cycle modifications. By arresting the cardiac progenitor cell divisions at different developing stages, we determined the exact cell lineages for most cardiac cell types. We found that once cardiac progenitors are specified, they can differentiate without further divisions. Interestingly, the progenitors of asymmetric cell lineages adopt a myocardial cell fate as opposed to a pericardial fate when they are unable to divide. These progenitors adopt a pericardial cell fate, however, when cell division is blocked in numb mutants or in embryos with constitutive Notch activity. These results suggest that a numb/Notch-dependent cell fate decision can take place even in undivided progenitors of asymmetric cell divisions. By contrast, in symmetric lineages, which give rise to a single type of myocardial-only or pericardial-only progeny, repression or constitutive activation of the Notch pathway has no apparent effect on progenitor or progeny fate. Thus, inhibition of Notch activity is crucial for specifying a myogenic cell fate only in asymmetric lineages. In addition, we provide evidence that whether or not Suppressor-of-Hairless can become a transcriptional activator is the key switch for the Numb/Notch activity in determining a myocardial versus pericardial cell fate.

Genetic control of dendritic morphogenesis in Drosophila

How the dendritic branching patterns of different neurons are specified is a fascinating question in developmental neurobiology. This question can now be addressed in detail in Drosophila, owing to technological advances that allow in vivo labeling of the dendrites of identifiable neurons. Recent genetic analyses in flies have uncovered several molecules, including transcription factors, cytoskeleton-associated proteins and membrane receptor-like molecules, that provide a glimpse into the complex regulatory network that controls dendritic morphogenesis.

Genetic manipulation of single neurons in vivo reveals specific roles of flamingo in neuronal morphogenesis

To study the roles of intracellular factors in neuronal morphogenesis, we used the mosaic analysis with a repressible cell marker (MARCM) technique to visualize identifiable single multiple dendritic (MD) neurons in living Drosophila larvae. We found that individual neurons in the peripheral nervous system (PNS) developed clear morphological polarity and diverse dendritic branching patterns in larval stages. Each MD neuron in the same dorsal cluster developed a unique dendritic field, suggesting that they have specific physiological functions. Single-neuron analysis revealed that Flamingo did not affect the general dendritic branching patterns in postmitotic neurons. Instead, Flamingo limited the extension of one or more dorsal dendrites without grossly affecting lateral branches. The dendritic overextension phenotype was partially conferred by the precocious initiation of dorsal dendrites in flamingo mutant embryos. In addition, Flamingo is required cell autonomously to promote axonal growth and to prevent premature axonal branching of PNS neurons. Our molecular analysis also indicated that the amino acid sequence near the first EGF motif is important for the proper localization and function of Flamingo. These results demonstrate that Flamingo plays a role in early neuronal differentiation and exerts specific effects on dendrites and axons.

Transgenic expression of numb inhibits notch signaling in immature thymocytes but does not alter T cell fate specification

The conserved adaptor protein Numb is an intrinsic cell fate determinant that functions by antagonizing Notch-mediated signal transduction. The Notch family of membrane receptors controls cell survival and cell fate determination in a variety of organ systems and species. Recent studies have identified a role for mammalian Notch-1 signals at multiple stages of T lymphocyte development. We have examined the role of mammalian Numb (mNumb) as a Notch regulator and cell fate determinant during T cell development. Transgenic overexpression of mNumb under the control of the Lck proximal promoter reduced expression of several Notch-1 target genes, indicating that mNumb antagonizes Notch-1 signaling in vivo. However, thymocyte development, cell cycle, and survival were unperturbed by mNumb overexpression, even though transgenic Numb was expressed at an early stage in thymocyte development (CD4(-)CD8(-)CD3(-) cells that were CD44(+)CD25(+) or CD44(-)CD25(+); double-negative 2/3). Moreover, bone marrow from mNumb transgenic mice showed no defects in thymopoiesis in competitive repopulation experiments. Our results suggest that mNumb functions as a Notch-1 antagonist in immature thymocytes, but that suppression of Notch-1 signaling at this stage does not alter gammadelta/alphabeta or CD4/CD8 T cell fate specification.

Sequoia, a tramtrack-related zinc finger protein, functions as a pan-neural regulator for dendrite and axon morphogenesis in Drosophila

Morphological complexity of neurons contributes to their functional complexity. How neurons generate different dendritic patterns is not known. We identified the sequoia mutant from a previous screen for dendrite mutants. Here we report that Sequoia is a pan-neural nuclear protein containing two putative zinc fingers homologous to the DNA binding domain of Tramtrack. sequoia mutants affect the cell fate decision of a small subset of neurons but have global effects on axon and dendrite morphologies of most and possibly all neurons. In support of sequoia as a specific regulator of neuronal morphogenesis, microarray experiments indicate that sequoia may regulate downstream genes that are important for executing neurite development rather than altering a variety of molecules that specify cell fates.

Lineage diversity in the Drosophila nervous system

The detailed descriptions of cellular lineages in the Drosophila nervous system have provided the foundations for an in-depth genetic analysis of the mechanisms that regulate fate decisions at every cell cycle.

The selector gene cut represses a neural cell fate that is specified independently of the Achaete-Scute-Complex and atonal

The peripheral nervous system (PNS) of Drosophila offers a powerful system to precisely identify individual cells and dissect their genetic pathways of development. The mode of specification of a subset of larval PNS cells, the multiple dendritic (md) neurons (or type II neurons), is complex and still poorly understood. Within the dorsal thoracic and abdominal segments, two md neurons, dbd and dda1, apparently require the proneural gene amos but not atonal (ato) or Achaete-Scute-Complex (ASC) genes. ASC normally acts via the neural selector gene cut to specify appropriate sensory organ identities. Here, we show that dbd- and dda1-type differentiation is suppressed by cut in dorsal ASC-dependent md neurons. Thus, cut is not only required to promote an ASC-dependent mode of differentiation, but also represses an ASC- and ato-independent fate that leads to dbd and dda1 differentiation.

Lineage, cell polarity and inscuteable function in the peripheral nervous system of the Drosophila embryo

The stereotyped pattern of the Drosophila embryonic peripheral nervous system (PNS) makes it an ideal system to use to identify mutations affecting cell polarity during asymmetric cell division. However, the characterisation of such mutations requires a detailed description of the polarity of the asymmetric divisions in the sensory organ lineages. We describe the pattern of cell divisions generating the vp1-vp4a mono-innervated external sense (es) organs. Each sensory organ precursor (SOP) cell follows a series of four asymmetric cell divisions that generate the four es organs cells (the socket, shaft, sheath cells and the es neurone) together with one multidendritic (md) neurone. This lineage is distinct from any of the previously proposed es lineages. Strikingly, the stereotyped pattern of cell divisions in this lineage is identical to those described for the embryonic chordotonal organ lineage and for the adult thoracic bristle lineage. Our analysis reveals that the vp2-vp4a SOP cells divide with a planar polarity to generate a dorsal pIIa cell and a ventral pIIb cell. The pIIb cell next divides with an apical-basal polarity to generate a basal daughter cell that differentiates as an md neurone. We found that Inscuteable specifically accumulated at the apical pole of the dividing pIIb cell and regulated the polarity of the pIIb division. This study establishes for the first time the function of Inscuteable in the PNS, and provides the basis for studying the mechanisms controlling planar and apical-basal cell polarities in the embryonic sensory organ lineages.

A genetic programme for neuronal connectivity

What is the nature of the genetic programme that allows neurons to extend their axons and connect to other neurons with a high degree of specificity? Work on the sensory neurons of the fly has shown how the control of neuronal identity is embedded in the general developmental programme of the organism. The ongoing analysis of pathfinding mutants suggests plausible mechanisms for the translation of neuronal identity into axonal behaviour.

The proneural gene amos promotes multiple dendritic neuron formation in the Drosophila peripheral nervous system

In the Drosophila peripheral nervous system, proneural genes direct the formation of different types of sensory organs. Here, we show that amos is a novel proneural gene that promotes multiple dendritic (MD) neuron formation. amos encodes a basic-helix-loop-helix (bHLH) protein of the Atonal family. During embryonic development, amos is expressed in patches of ectodermal cells, and the expression is quickly restricted to sensory organ precursors. Loss of amos function eliminates MD neurons that remain in ASC;atonal mutants. Misexpression of amos generates ectopic MD and other types of neurons. Amos interacts with the ubiquitously expressed Daughter-less protein in vivo and in vitro. Our final misexpression experiments suggest that a domain located outside the DNA-binding domain of Amos determines the MD neuronal specificity.

The COE transcription factor Collier is a mediator of short-range Hedgehog-induced patterning of the Drosophila wing

BACKGROUND

The secreted Hedgehog (Hh) proteins have been implicated as mediators of positional information in vertebrates and invertebrates. A gradient of Hh activity contributes to antero-posterior (A/P) patterning of the fly wing. In addition to inducing localised expression of Decapentaplegic (Dpp), which in turn relays patterning cues at long range, Hh directly patterns the central region of the wing.

RESULTS

We show that short-range, dose-dependent Hh activity is mediated by activation of the transcription factor Collier (Col). In the absence of col activity, longitudinal veins 3 and 4 (L3 and L4) are apposed and the central intervein is missing. Hh expression induces col expression in a narrow stripe of cells along the A/P boundary through a dual-input mechanism: inhibition of proteolysis of Cubitus-interruptus (Ci) and activation of the Fused (Fu) kinase. Col, in cooperation with Ci, controls the formation of the central intervein by activating the expression of blistered (bs), which encodes the Drosophila serum response factor (D-SRF), the activity of which is required for the adoption and maintenance of the intervein cell fate. Furthermore, col is allelic to knot, a gene involved in the formation of the central part of the wing. This finding completes our understanding of the sectorial organisation of the Drosophila wing.

CONCLUSIONS

Col, the Drosophila member of the COE family (Col/Olf-1/EBF) of non-basic, helix-loop-helix (HLH)-containing transcription factors, is a mediator of the short-range organising activity of Hh in the Drosophila wing. Our results support the idea that Hh controls target gene expression in a concentration-dependent manner and highlight the importance of the Fu kinase in this differential regulation. The high degree of evolutionary conservation of the COE proteins and the diversity of developmental processes controlled by Hh signalling raises the possibility that the specific genetic interactions depicted here may also operate in vertebrates.

The Drosophila sanpodo gene controls sibling cell fate and encodes a tropomodulin homolog, an actin/tropomyosin-associated protein

Notch signaling is required in many invertebrate and vertebrate cells to promote proper cell fate determination. Mutations in sanpodo cause many different neuronal peripheral nervous system precursor cells to generate two identical daughter neurons, instead of a neuron and sibling cell. This phenotype is similar to that observed when Notch function is lost late in embryonic development and opposite to the numb loss-of-function phenotype. Genetic interaction studies show that sanpodo is epistatic to numb. sanpodo encodes a homolog of tropomodulin, an actin/tropomyosin-associated protein. Loss of sanpodo leads to an aberrant F-actin distribution and causes differentiation defects of actin-containing sensory structures. Our data suggest that an actin-based process is involved in Notch signaling.

The mammalian numb phosphotyrosine-binding domain. Characterization of binding specificity and identification of a novel PDZ domain-containing numb binding protein, LNX

Numb is a phosphotyrosine-binding (PTB) domain-containing protein implicated in the control of cell fate decisions during development. A modified two-hybrid screen in yeast was used to identify Numb PTB domain-interacting proteins important for Numb function. Here we report the identification of a novel protein, LNX, which interacts specifically with the Numb PTB domain. Two differentially expressed LNX messages encode overlapping proteins with predicted molecular masses of 80 kDa (LNX) and 70 kDa (LNX-b). LNX and LNX-b contain unique amino-terminal sequences and share four PDZ domains. The unique amino-terminal region of LNX includes a RING finger domain. The Numb PTB domain binding region of LNX was mapped to the sequence motif LDNPAY, found in both protein isoforms. Mutational analysis of LNX and peptide competition experiments showed that phosphorylation of the tyrosine residue within this motif was not required for binding to the Numb PTB domain. Finally, we also provide evidence that tyrosine phosphorylation of the LDNPAY sequence motif in LNX could generate a binding site for the phosphorylation-dependent binding of other PTB domain-containing proteins such as SHC. We speculate that LNX may be important for clustering PTB-containing proteins with functionally related transmembrane proteins in specific membrane compartments.