The receptor tyrosine kinase gene linotte is required for neuronal pathway selection in the Drosophila mushroom bodies

The making of the Drosophila mushroom body

The mushroom body (MB) is a computational center in the Drosophila brain. The intricate neural circuits of the mushroom body enable it to store associative memories and process sensory and internal state information. The mushroom body is composed of diverse types of neurons that are precisely assembled during development. Tremendous efforts have been made to unravel the molecular and cellular mechanisms that build the mushroom body. However, we are still at the beginning of this challenging quest, with many key aspects of mushroom body assembly remaining unexplored. In this review, I provide an in-depth overview of our current understanding of mushroom body development and pertinent knowledge gaps.

Importin-α2 mediates brain development, learning and memory consolidation in Drosophila

Neuronal development and memory consolidation are conserved processes that rely on nuclear-cytoplasmic transport of signaling molecules to regulate gene activity and initiate cascades of downstream cellular events. Surprisingly, few reports address and validate this widely accepted perspective. Here we show that Importin-α2 (Imp-α2), a soluble nuclear transporter that shuttles cargoes between the cytoplasm and nucleus, is vital for brain development, learning and persistent memory in Drosophila melanogaster. Mutations in importin-α2 (imp-α2, known as Pendulin or Pen and homologous with human KPNA2) are alleles of mushroom body miniature B (mbmB), a gene known to regulate aspects of brain development and influence adult behavior in flies. Mushroom bodies (MBs), paired associative centers in the brain, are smaller than normal due to defective proliferation of specific intrinsic Kenyon cell (KC) neurons in mbmB mutants. Extant KCs projecting to the MB β-lobe terminate abnormally on the contralateral side of the brain. mbmB adults have impaired olfactory learning but normal memory decay in most respects, except that protein synthesis-dependent long-term memory (LTM) is abolished. This observation supports an alternative mechanism of persistent memory in which mutually exclusive protein-synthesis-dependent and -independent forms rely on opposing cellular mechanisms or circuits. We propose a testable model of Imp-α2 and nuclear transport roles in brain development and conditioned behavior. Based on our molecular characterization, we suggest that mbmB is hereafter referred to as imp-α2^mbmB.

Glial Derived TGF-β Instructs Axon Midline Stopping

A fundamental question that underlies the proper wiring and function of the nervous system is how axon extension stops during development. However, our mechanistic understanding of axon stopping is currently poor. The stereotypic development of the Drosophila mushroom body (MB) provides a unique system in which three types of anatomically distinct neurons (γ, α’/β’, and α/β) develop and interact to form a complex neuronal structure. All three neuronal types innervate the ipsi-lateral side and do not cross the midline. Here we find that Plum, an immunoglobulin (Ig) superfamily protein that we have previously shown to function as a TGF-β accessory receptor, is required within MB α/β neurons for their midline stopping. Overexpression of Plum within MB neurons is sufficient to induce retraction of α/β axons. As expected, rescue experiments revealed that Plum likely functions in α/β neurons and mediates midline stopping via the downstream effector RhoGEF2. Finally, we have identified glial-derived Myoglianin (Myo) as the major TGF-β ligand that instructs midline stopping of MB neurons. Taken together, our study strongly suggests that TGF-β signals originating from the midline facilitate midline stopping of α/β neuron in a Plum dependent manner.

JNK signaling coordinates with ecdysone signaling to promote pruning of Drosophila sensory neuron dendrites

Developmental pruning of axons and dendrites is crucial for the formation of precise neuronal connections, but the mechanisms underlying developmental pruning are not fully understood. Here, we have investigated the function of JNK signaling in dendrite pruning using Drosophila class IV dendritic arborization (c4da) neurons as a model. We find that loss of JNK or its canonical downstream effectors Jun or Fos led to dendrite-pruning defects in c4da neurons. Interestingly, our data show that JNK activity in c4da neurons remains constant from larval to pupal stages but the expression of Fos is specifically activated by ecdysone receptor B1 (EcRB1) at early pupal stages, suggesting that ecdysone signaling provides temporal control of the regulation of dendrite pruning by JNK signaling. Thus, our work not only identifies a novel pathway involved in dendrite pruning and a new downstream target of EcRB1 in c4da neurons, but also reveals that JNK and Ecdysone signaling coordinate to promote dendrite pruning.

The Drosophila neurogenin Tap functionally interacts with the Wnt-PCP pathway to regulate neuronal extension and guidance

The neurogenin (Ngn) transcription factors control early neurogenesis and neurite outgrowth in mammalian cortex. In contrast to their proneural activity, their function in neurite growth is poorly understood. Drosophila has a single predicted Ngn homolog, Tap, of unknown function. Here we show that Tap is not a proneural protein in Drosophila but is required for proper axonal growth and guidance of neurons of the mushroom body, a neuropile required for associative learning and memory. Genetic and expression analyses suggest that Tap inhibits excessive axonal growth by fine regulation of the levels of the Wnt signaling adaptor protein Dishevelled.

Summary: Mammalian neurogenins are proneural factors, but the Drosophila homolog Tap is not, instead acting to prevent axonal outgrowth, likely by regulating the planar cell polarity pathway via Dishevelled.

Adult Drosophila sensory neurons specify dendritic territories independently of dendritic contacts through the Wnt5-Drl signaling pathway

Here, Yasunaga et al. use Drosophila class IV dendrite arborization (C4da) sensory neurons as a system to investigate how neurons specify dendritic territories during neuronal development. They show that, unlike the larval dendrites, adult C4da dendrites define the dendritic boundary independently of dendritic contacts and that Wnt5 derived from sternites is required for specification of the ventral boundaries of C4da dendrites. These findings provide novel insights into how dendritic territories of neurons develop and the role of the Wnt5–Drl signaling pathway in the contact-independent dendritic boundary specification.

Sensory neurons with common functions are often nonrandomly arranged and form dendritic territories in stereotypic spatial patterns throughout the nervous system, yet molecular mechanisms of how neurons specify dendritic territories remain largely unknown. In Drosophila larvae, dendrites of class IV sensory (C4da) neurons completely but nonredundantly cover the whole epidermis, and the boundaries of these tiled dendritic fields are specified through repulsive interactions between homotypic dendrites. Here we report that, unlike the larval C4da neurons, adult C4da neurons rely on both dendritic repulsive interactions and external positional cues to delimit the boundaries of their dendritic fields. We identify Wnt5 derived from sternites, the ventral-most part of the adult abdominal epidermis, as the critical determinant for the ventral boundaries. Further genetic data indicate that Wnt5 promotes dendrite termination on the periphery of sternites through the Ryk receptor family kinase Derailed (Drl) and the Rho GTPase guanine nucleotide exchange factor Trio in C4da neurons. Our findings thus uncover the dendritic contact-independent mechanism that is required for dendritic boundary specification and suggest that combinatory actions of the dendritic contact-dependent and -independent mechanisms may ensure appropriate dendritic territories of a given neuron.

The effect of neurospecific knockdown of candidate genes for locomotor behavior and sound production in Drosophila melanogaster

Molecular mechanisms underlying the functioning of central pattern generators (CPGs) are poorly understood. Investigations using genetic approaches in the model organism Drosophila may help to identify unknown molecular players participating in the formation or control of motor patterns. Here we report Drosophila genes as candidates for involvement in the neural mechanisms responsible for motor functions, such as locomotion and courtship song. Twenty-two Drosophila lines, used for gene identification, were isolated from a previously created collection of 1064 lines, each carrying a P element insertion in one of the autosomes. The lines displayed extreme deviations in locomotor and/or courtship song parameters compared with the whole collection. The behavioral consequences of CNS-specific RNAi-mediated knockdowns for 10 identified genes were estimated. The most prominent changes in the courtship song interpulse interval (IPI) were seen in flies with Sps2 or CG15630 knockdown. Glia-specific knockdown of these genes produced no effect on the IPI. Estrogen-induced knockdown of CG15630 in adults reduced the IPI. The product of the CNS-specific gene, CG15630 (a predicted cell surface receptor), is likely to be directly involved in the functioning of the CPG generating the pulse song pattern. Future studies should ascertain its functional role in the neurons that constitute the song CPG. Other genes (Sps2, CG34460), whose CNS-specific knockdown resulted in IPI reduction, are also worthy of detailed examination.

Wnt5 and drl/ryk gradients pattern the Drosophila olfactory dendritic map

During development, dendrites migrate to their correct locations in response to environmental cues. The mechanisms of dendritic guidance are poorly understood. Recent work has shown that the Drosophila olfactory map is initially formed by the spatial segregation of the projection neuron (PN) dendrites in the developing antennal lobe (AL). We report here that between 16 and 30 h after puparium formation, the PN dendrites undergo dramatic rotational reordering to achieve their final glomerular positions. During this period, a novel set of AL-extrinsic neurons express high levels of the Wnt5 protein and are tightly associated with the dorsolateral edge of the AL. Wnt5 forms a dorsolateral-high to ventromedial-low pattern in the antennal lobe neuropil. Loss of Wnt5 prevents the ventral targeting of the dendrites, whereas Wnt5 overexpression disrupts dendritic patterning. We find that Drl/Ryk, a known Wnt5 receptor, is expressed in a dorsolateral-to-ventromedial (DL > VM) gradient by the PN dendrites. Loss of Drl in the PNs results in the aberrant ventromedial targeting of the dendrites, a defect that is suppressed by reduction in Wnt5 gene dosage. Conversely, overexpression of Drl in the PNs results in the dorsolateral targeting of their dendrites, an effect that requires Drl's cytoplasmic domain. We propose that Wnt5 acts as a repulsive guidance cue for the PN dendrites, whereas Drl signaling in the dendrites inhibits Wnt5 signaling. In this way, the precise expression patterns of Wnt5 and Drl orient the PN dendrites allowing them to target their final glomerular positions.

Receptor tyrosine kinases in Drosophila development

Tyrosine phosphorylation plays a significant role in a wide range of cellular processes. The Drosophila genome encodes more than 20 receptor tyrosine kinases and extensive studies in the past 20 years have illustrated their diverse roles and complex signaling mechanisms. Although some receptor tyrosine kinases have highly specific functions, others strikingly are used in rather ubiquitous manners. Receptor tyrosine kinases regulate a broad expanse of processes, ranging from cell survival and proliferation to differentiation and patterning. Remarkably, different receptor tyrosine kinases share many of the same effectors and their hierarchical organization is retained in disparate biological contexts. In this comprehensive review, we summarize what is known regarding each receptor tyrosine kinase during Drosophila development. Astonishingly, very little is known for approximately half of all Drosophila receptor tyrosine kinases.

Transposition-driven genomic heterogeneity in the Drosophila brain

Recent studies in mammals have documented the neural expression and mobility of retrotransposons and have suggested that neural genomes are diverse mosaics. We found that transposition occurs among memory-relevant neurons in the Drosophila brain. Cell type-specific gene expression profiling revealed that transposon expression is more abundant in mushroom body (MB) αβ neurons than in neighboring MB neurons. The Piwi-interacting RNA (piRNA) proteins Aubergine and Argonaute 3, known to suppress transposons in the fly germline, are expressed in the brain and appear less abundant in αβ MB neurons. Loss of piRNA proteins correlates with elevated transposon expression in the brain. Paired-end deep sequencing identified more than 200 de novo transposon insertions in αβ neurons, including insertions into memory-relevant loci. Our observations indicate that genomic heterogeneity is a conserved feature of the brain.

Developmental changes in expression, subcellular distribution, and function of Drosophila N-cadherin, guided by a cell-intrinsic program during neuronal differentiation

Cell adhesion molecules (CAMs) perform numerous functions during neural development. An individual CAM can play different roles during each stage of neuronal differentiation; however, little is known about how such functional switching is accomplished. Here we show that Drosophila N-cadherin (CadN) is required at multiple developmental stages within the same neuronal population and that its sub-cellular expression pattern changes between the different stages. During development of mushroom body neurons and motoneurons, CadN is expressed at high levels on growing axons, whereas expression becomes downregulated and restricted to synaptic sites in mature neurons. Phenotypic analysis of CadN mutants reveals that developing axons require CadN for axon guidance and fasciculation, whereas mature neurons for terminal growth and receptor clustering. Furthermore, we demonstrate that CadN downregulation can be achieved in cultured neurons without synaptic contact with other cells. Neuronal silencing experiments using Kir(2.1) indicate that neuronal excitability is also dispensable for CadN downregulation in vivo. Interestingly, downregulation of CadN can be prematurely induced by ectopic expression of a nonselective cation channel, dTRPA1, in developing neurons. Together, we suggest that switching of CadN expression during neuronal differentiation involves regulated cation influx within neurons.

Drosophila melanogaster as a model system for studies of islet amyloid polypeptide aggregation

Background

Recent research supports that aggregation of islet amyloid polypeptide (IAPP) leads to cell death and this makes islet amyloid a plausible cause for the reduction of beta cell mass, demonstrated in patients with type 2 diabetes. IAPP is produced by the beta cells as a prohormone, and proIAPP is processed into IAPP by the prohormone convertases PC1/3 and PC2 in the secretory granules. Little is known about the pathogenesis for islet amyloid and which intracellular mechanisms are involved in amyloidogenesis and induction of cell death.

Methodology/Principal Findings

We have established expression of human proIAPP (hproIAPP), human IAPP (hIAPP) and the non-amyloidogenic mouse IAPP (mIAPP) in Drosophila melanogaster, and compared survival of flies with the expression driven to different cell populations. Only flies expressing hproIAPP in neurons driven by the Gal4 driver elav^C155,Gal4 showed a reduction in lifespan whereas neither expression of hIAPP or mIAPP influenced survival. Both hIAPP and hproIAPP expression caused formation of aggregates in CNS and fat body region, and these aggregates were both stained by the dyes Congo red and pFTAA, both known to detect amyloid. Also, the morphology of the highly organized protein granules that developed in the fat body of the head in hIAPP and hproIAPP expressing flies was characterized, and determined to consist of 15.8 nm thick pentagonal rod-like structures.

Conclusions/Significance

These findings point to a potential for Drosophila melanogaster to serve as a model system for studies of hproIAPP and hIAPP expression with subsequent aggregation and developed pathology.

Go signaling in mushroom bodies regulates sleep in Drosophila

STUDY OBJECTIVES

Sleep is a fundamental physiological process and its biological mechanisms are poorly understood. In Drosophila melanogaster, heterotrimeric Go protein is abundantly expressed in the brain. However, its post-developmental function has not been extensively explored.

DESIGN

Locomotor activity was measured using the Drosophila Activity Monitoring System under a 12:12 LD cycle. Sleep was defined as periods of 5 min with no recorded activity.

RESULTS

Pan-neuronal elevation of Go signaling induced quiescence accompanied by an increased arousal threshold in flies. By screening region-specific GAL4 lines, we mapped the sleep-regulatory function of Go signaling to mushroom bodies (MBs), a central brain region which modulates memory, decision making, and sleep in Drosophila. Up-regulation of Go activity in these neurons consolidated sleep while inhibition of endogenous Go via expression of Go RNAi or pertussis toxin reduced and fragmented sleep, indicating that the Drosophila sleep requirement is affected by levels of Go activity in the MBs. Genetic interaction results showed that Go signaling serves as a neuronal transmission inhibitor in a cAMP-independent pathway.

CONCLUSION

Go signaling is a novel signaling pathway in MBs that regulates sleep in Drosophila.

Pathogenic VCP/TER94 alleles are dominant actives and contribute to neurodegeneration by altering cellular ATP level in a Drosophila IBMPFD model

Inclusion body myopathy with Paget's disease of bone and frontotemporal dementia (IBMPFD) is caused by mutations in Valosin-containing protein (VCP), a hexameric AAA ATPase that participates in a variety of cellular processes such as protein degradation, organelle biogenesis, and cell-cycle regulation. To understand how VCP mutations cause IBMPFD, we have established a Drosophila model by overexpressing TER94 (the sole Drosophila VCP ortholog) carrying mutations analogous to those implicated in IBMPFD. Expression of these TER94 mutants in muscle and nervous systems causes tissue degeneration, recapitulating the pathogenic phenotypes in IBMPFD patients. TER94-induced neurodegenerative defects are enhanced by elevated expression of wild-type TER94, suggesting that the pathogenic alleles are dominant active mutations. This conclusion is further supported by the observation that TER94-induced neurodegenerative defects require the formation of hexamer complex, a prerequisite for a functional AAA ATPase. Surprisingly, while disruptions of the ubiquitin-proteasome system (UPS) and the ER-associated degradation (ERAD) have been implicated as causes for VCP-induced tissue degeneration, these processes are not significantly affected in our fly model. Instead, the neurodegenerative defect of TER94 mutants seems sensitive to the level of cellular ATP. We show that increasing cellular ATP by independent mechanisms could suppress the phenotypes of TER94 mutants. Conversely, decreasing cellular ATP would enhance the TER94 mutant phenotypes. Taken together, our analyses have defined the nature of IBMPFD-causing VCP mutations and made an unexpected link between cellular ATP level and IBMPFD pathogenesis.

Drosophila larvae establish appetitive olfactory memories via mushroom body neurons of embryonic origin

Insect mushroom bodies are required for diverse behavioral functions, including odor learning and memory. Using the numerically simple olfactory pathway of the Drosophila melanogaster larva, we provide evidence that the formation of appetitive olfactory associations relies on embryonic-born intrinsic mushroom body neurons (Kenyon cells). The participation of larval-born Kenyon cells, i.e., neurons that become gradually integrated in the developing mushroom body during larval life, in this task is unlikely. These data provide important insights into how a small set of identified Kenyon cells can store and integrate olfactory information in a developing brain. To investigate possible functional subdivisions of the larval mushroom body, we anatomically disentangle its input and output neurons at the single-cell level. Based on this approach, we define 10 subdomains of the larval mushroom body that may be implicated in mediating specific interactions between the olfactory pathway, modulatory neurons, and neuronal output.

Ryks: new partners for Wnts in the developing and regenerating nervous system

Conserved Ryk transmembrane proteins, tyrosine kinase-related Wnt receptors, are important during neurogenesis, axon guidance and synaptogenesis. Here, we review the increasingly complex biology of the Wnt/Ryk pathway, emphasizing the mechanisms by which Ryks transduce or sometimes block the Wnt signal. Recent studies reveal that Wnts signal through Ryk via multiple mechanisms, including nuclear translocation of their intracellular domains and pathways employing Src Family Kinases and members of the canonical Wnt pathway. We also discuss reports indicating that Wnt/Ryk axon guidance roles are evolutionarily conserved and Wnt/Ryk interactions are required for motoneuron target selection and synaptogenesis at the neuromuscular junction. Recent findings that injury-induced Wnt/Ryk pathway activation inhibits axon regeneration underscore the importance of further understanding this novel pathway.

The mushroom body of adult Drosophila characterized by GAL4 drivers

The mushroom body is required for a variety of behaviors of Drosophila melanogaster. Different types of intrinsic and extrinsic mushroom body neurons might underlie its functional diversity. There have been many GAL4 driver lines identified that prominently label the mushroom body intrinsic neurons, which are known as "Kenyon cells." Under one constant experimental condition, we analyzed and compared the the expression patterns of 25 GAL4 drivers labeling the mushroom body. As an internet resource, we established a digital catalog indexing representative confocal data of them. Further more, we counted the number of GAL4-positive Kenyon cells in each line. We found that approximately 2,000 Kenyon cells can be genetically labeled in total. Three major Kenyon cell subtypes, the gamma, alpha'/beta', and alpha/beta neurons, respectively, contribute to 33, 18, and 49% of 2,000 Kenyon cells. Taken together, this study lays groundwork for functional dissection of the mushroom body.

Src family kinases are required for WNT5 signaling through the Derailed/RYK receptor in the Drosophila embryonic central nervous system

Members of the RYK/Derailed family have recently been shown to regulate axon guidance in both Drosophila and mammals by acting as Wnt receptors. Little is known about how the kinase activity-deficient RYKs transduce Wnt signals. Here, we show that the non-receptor Src family tyrosine kinases, SRC64B and SRC42A, are involved in WNT5-mediated signaling through Derailed in the Drosophila embryonic central nervous system. Analysis of animals lacking SRC64B and SRC42A reveals defects in commissure formation similar to those observed in Wnt5 and derailed mutants. Reductions in SRC64B expression levels suppress a Wnt5/derailed-dependent dominant gain-of-function phenotype, and increased levels of either SRC64B or SRC42A enhance Wnt5/derailed-mediated axon commissure switching. Derailed and SRC64B form a complex, which contains catalytically active SRC64B, the formation or stability of which requires SRC64B kinase activity. Furthermore, Derailed is phosphorylated in a SRC64B-dependent manner and coexpression of Derailed and SRC64B results in the activation of SRC64B. The mammalian orthologs of Derailed and SRC64B also form complexes, suggesting that Src roles in RYK signaling are conserved. Finally, we show that coexpression of WNT5 and Derailed has no apparent effect upon TCF/LEF-dependent transcription, suggesting that the WNT5/Derailed signaling pathway is unlikely to directly regulate canonical Wnt pathway targets. Together, these findings indicate that the Src family kinases play novel roles in WNT5/Derailed-mediated signaling.

Receptor tyrosine phosphatases regulate birth order-dependent axonal fasciculation and midline repulsion during development of the Drosophila mushroom body

Receptor tyrosine phosphatases (RPTPs) are required for axon guidance during embryonic development in Drosophila. Here we examine the roles of four RPTPs during development of the larval mushroom body (MB). MB neurons extend axons into parallel tracts known as the peduncle and lobes. The temporal order of neuronal birth is reflected in the organization of axons within these tracts. Axons of the youngest neurons, known as core fibers, extend within a single bundle at the center, while those of older neurons fill the outer layers. RPTPs are selectively expressed on the core fibers of the MB. Ptp10D and Ptp69D regulate segregation of the young axons into a single core bundle. Ptp69D signaling is required for axonal extension beyond the peduncle. Lar and Ptp69D are necessary for the axonal branching decisions that create the lobes. Avoidance of the brain midline by extending medial lobe axons involves signaling through Lar.

Respective roles of the DRL receptor and its ligand WNT5 in Drosophila mushroom body development

In recent decades, Drosophila mushroom bodies (MBs) have become a powerful model for elucidating the molecular mechanisms underlying brain development and function. We have previously characterized the derailed (drl; also known as linotte) receptor tyrosine kinase as an essential component of adult MB development. Here we show, using MARCM clones, a non-cell-autonomous requirement for the DRL receptor in MB development. This result is in accordance with the pattern of DRL expression, which occurs throughout development close to, but not inside,MB cells. While DRL expression can be detected within both interhemispheric glial and commissural neuronal cells, rescue of the drl MB defects appears to involve the latter cellular type. The WNT5 protein has been shown to act as a repulsive ligand for the DRL receptor in the embryonic central nervous system. We show here that WNT5 is required intrinsically within MB neurons for proper MB axonal growth and probably interacts with the extrinsic DRL receptor in order to stop axonal growth. We therefore propose that the neuronal requirement for both proteins defines an interacting network acting during MB development.

Drosophila dMRP4 regulates responsiveness to O2 deprivation and development under hypoxia

For most vertebrates, oxygen is a prerequisite for survival. Although we have previously shown that Drosophila melanogaster is hypoxia tolerant, how this species senses O(2) deprivation and how it survives oxygen-limiting conditions are as yet poorly understood. We began to address this question by testing for anoxic responsiveness in Drosophila adult flies following overexpression of existing EP lines. In this screen, we identified Drosophila CG14709 gene as a homolog of the human multidrug resistance protein 4 (MRP4/ABCC4) that is tightly regulated to oxygen. Ubiquitous expression of dMRP4 in adult flies resulted in increased sensitivity to anoxia as they had longer recovery time from anoxic stupor. When exposed to 4% oxygen chronically (throughout its lifespan), constitutive expression of dMRP4 in larvae caused larval lethality due to growth arrest. Mutations of dMRP4 led to a hypersensitive response to acute anoxia in adult flies but had less impact on larval survival under chronic hypoxia compared with dMRP4 overexpression. Selective expression of this gene in neurons, but not in glia or muscles, mirrored the same phenotype as the ubiquitous one. Thus, our data suggest novel roles for MRP in vivo: 1) dMRP4 regulates the sensitivity to acute or chronic O(2) deprivation, and 2) dMRP expression in neurons is sufficient to induce the sensitivity to O(2) in the whole organism.

Ptpmeg is required for the proper establishment and maintenance of axon projections in the central brain of Drosophila

Ptpmeg is a cytoplasmic tyrosine phosphatase containing FERM and PDZ domains. Drosophila Ptpmeg and its vertebrate homologs PTPN3 and PTPN4 are expressed in the nervous system, but their developmental functions have been unknown. We found that ptpmeg is involved in neuronal circuit formation in the Drosophila central brain, regulating both the establishment and the stabilization of axonal projection patterns. In ptpmeg mutants, mushroom body (MB) axon branches are elaborated normally, but the projection patterns in many hemispheres become progressively abnormal as the animals reach adulthood. The two branches of MB α/βneurons are affected by ptpmeg in different ways; ptpmegactivity inhibits α lobe branch retraction while preventing β lobe branch overextension. The phosphatase activity of Ptpmeg is essential for bothα and β lobe formation, but the FERM domain is required only for preventing α lobe retraction, suggesting that Ptpmeg has distinct roles in regulating the formation of α and β lobes. ptpmeg is also important for the formation of the ellipsoid body (EB), where it influences the pathfinding of EB axons. ptpmeg function in neurons is sufficient to support normal wiring of both the EB and MB. However, ptpmeg does not act in either MB or EB neurons, implicating ptpmeg in the regulation of cell-cell signaling events that control the behavior of these axons.

Differential microarray analysis of Drosophila mushroom body transcripts using chemical ablation

Mushroom bodies (MBs) are the centers for olfactory associative learning and elementary cognitive functions in the Drosophila brain. As a way to systematically elucidate genes preferentially expressed in MBs, we have analyzed genome-wide alterations in transcript profiles associated with MB ablation by hydroxyurea. We selected 100 genes based on microarray data and examined their expression patterns in the brain by in situ hybridization. Seventy genes were found to be expressed in the posterodorsal cortex, which harbors the MB cell bodies. These genes encode proteins of diverse functions, including transcription, signaling, cell adhesion, channels, and transporters. Moreover, we have examined developmental functions of 40 of the microarray-identified genes by transgenic RNA interference; 8 genes were found to cause mild-to-strong MB defects when suppressed with a MB-Gal4 driver. These results provide important information not only on the repertoire of genes that control MB development but also on the repertoire of neural factors that may have important physiological functions in MB plasticity.

rap gene encodes Fizzy-related protein (Fzr) and regulates cell proliferation and pattern formation in the developing Drosophila eye-antennal disc

The rap (retina aberrant in pattern) gene encodes the Fizzy-related protein (Fzr), which as an activator of the ubiquitin ligase complex; APC/C (anaphase promoting complex/cyclosome) facilitates the cell cycle stage-specific degradation of cyclins. Loss-of-function mutations in rap cause unscheduled accumulation of cyclin B in the developing eye imaginal disc, resulting in additional mitotic cycles and defective patterning of the developing Drosophila eye. Targeted mis-expression of rap/fzr in the eye primordial cells causes precocious cell cycle exit, and smaller primordial eye fields, which either eliminate or drastically reduce the size of the adult eye. Although mitosis is inhibited in the mis-expression animals, cells with abnormally large nuclei form tumor-like structures from continued endoreplication, cell growth and retinal differentiation. Interestingly, overexpression of Rap/Fzr in the eye primordia also increases the size of the antennal primordium resulting in the induction of ectopic antennae. These results suggest that Rap/Fzr plays an essential role in the timely exit of precursor cells from mitotic cycles and indicate that mechanisms that regulate cell cycle exit are critical during pattern formation and morphogenesis.

Mental retardation genes in drosophila: New approaches to understanding and treating developmental brain disorders

Drosophila melanogaster is emerging as a valuable genetic model system for the study of mental retardation (MR). MR genes are remarkably similar between humans and fruit flies. Cognitive behavioral assays can detect reductions in learning and memory in flies with mutations in MR genes. Neuroanatomical methods, including some at single-neuron resolution, are helping to reveal the cellular bases of faulty brain development caused by MR gene mutations. Drosophila fragile X mental retardation 1 (dfmr1) is the fly counterpart of the human gene whose malfunction causes fragile X syndrome. Research on the fly gene is leading the field in molecular mechanisms of the gene product's biological function and in pharmacological rescue of brain and behavioral phenotypes. Future work holds the promise of using genetic pathway analysis and primary neuronal culture methods in Drosophila as tools for drug discovery for a wide range of MR and related disorders.

Thirty years of olfactory learning and memory research in Drosophila melanogaster

The last 30 years have witnessed tremendous progress in elucidating the basic mechanisms underlying a simple form of olfactory learning and memory in Drosophila. The application of the mutagenic approach to the study of olfactory learning and memory in Drosophila has yielded insights into the participation of a large number of genes in both the development of critical brain regions as well as in the physiology underlying the acquisition, storage, and retrieval of memory. Newer sophisticated molecular-genetic tools have further allowed for the specification and functional dissection of the neuronal circuitry involved in these processes at a systems level. With these advances in our understanding of the genes, neurons, and circuits involved in learning and memory, the field of Drosophila memory research is nearing a state of integration of the bottom up and top down approaches to understanding this form of behavioral plasticity.

Receptor related to tyrosine kinase RYK regulates cell migration during cortical development

Mammalian RYK is a receptor related to tyrosine kinase without detectable catalytic activity. We have previously reported that rat RYK is dominantly expressed in neural progenitor cells and mature neurons in the developing central nervous system. Mouse RYK has been found to bind to EphB2/B3 receptors, which have diverse functions during development. In this study, we demonstrated that RYK, EphB2, EphB3, ephrinB1, and ephrinB2 are expressed in embryonic brain. In vitro analysis using COS-7 cells revealed binding between rat RYK and EphB3, and that the RYK deletion mutant without extracellular leucine-rich motifs lacked this binding ability. To investigate the function of RYK in vivo, embryonic cortical slice cultures were analyzed after electroporation of expression plasmids for RYK or its deletion mutants. The results showed that overexpression of RYK suppressed cell migration from the ventricular zone to the pial surface, however, overexpression of the RYK deletion mutant without leucine-rich motifs had no effect on cell migration. These results suggest that RYK regulates cell migration during mammalian cortical development through the binding to Eph receptors.

In vivo evidence for a regulatory role of the kinase activity of the linotte/derailed receptor tyrosine kinase, a Drosophila Ryk ortholog

The RYK subfamily of receptor tyrosine kinases is characterised by unusual, but highly conserved, amino acid substitutions in the kinase domain. The linotte/derailed gene encodes a Drosophila RYK subfamily member involved in embryonic and adult central nervous system development. Previous studies have shown that the kinase activity of this receptor is not required in vivo for its embryonic function. In this study, we have investigated the role of the cytoplasmic domain and the kinase activity of the linotte/derailed receptor tyrosine kinase in adult brain development. Our results indicate that these domains are not essential for adult brain development but they are required for the proper regulation of the activity of this receptor. This sheds light on a regulatory role for the kinase activity of a RYK subfamily member.

Drosophila retained/dead ringer is necessary for neuronal pathfinding, female receptivity and repression of fruitless independent male courtship behaviors

Mutations in the Drosophila retained/dead ringer (retn) gene lead to female behavioral defects and alter a limited set of neurons in the CNS. retn is implicated as a major repressor of male courtship behavior in the absence of the fruitless (fru) male protein. retn females show fru-independent male-like courtship of males and females, and are highly resistant to courtship by males. Males mutant for retn court with normal parameters, although feminization of retn cells in males induces bisexuality. Alternatively spliced RNAs appear in the larval and pupal CNS, but none shows sex specificity. Post-embryonically, retn RNAs are expressed in a limited set of neurons in the CNS and eyes. Neural defects of retn mutant cells include mushroom body beta-lobe fusion and pathfinding errors by photoreceptor and subesophageal neurons. We posit that some of these retn-expressing cells function to repress a male behavioral pathway activated by fruM.

Defective neuronal development in the mushroom bodies of Drosophila fragile X mental retardation 1 mutants

Fragile X mental retardation 1 (Fmr1) is a highly conserved gene with major roles in CNS structure and function. Its product, the RNA-binding protein FMRP, is believed to regulate translation of specific transcripts at postsynaptic sites in an activity-dependent manner. Hence, Fmr1 is central to the molecular mechanisms of synaptic plasticity required for normal neuronal maturation and cognitive ability. Mutations in its Drosophila ortholog, dfmr1, produce phenotypes of brain interneurons and axon terminals at the neuromuscular junction, as well as behavioral defects of circadian rhythms and courtship. We hypothesized that dfmr1 mutations would disrupt morphology of the mushroom bodies (MBs), highly plastic brain regions essential for many forms of learning and memory. We found developmental defects of MB lobe morphogenesis, of which the most common is a failure of beta lobes to stop at the brain midline. A similar recessive beta-lobe midline-crossing phenotype has been previously reported in the memory mutant linotte. The dfmr1 MB defects are highly sensitive to genetic background, which is reminiscent of mammalian fragile-X phenotypes. Mutations of dfmr1 also interact with one or more third-chromosome loci to promote alpha/beta-lobe maturation. These data further support the use of the Drosophila model system for study of hereditary cognitive disorders of humans.

Mammalian Ryk Is a Wnt Coreceptor Required for Stimulation of Neurite Outgrowth

The Ryk receptor belongs to the atypical receptor tyrosine kinase family. It is a new member of the family of Wnt receptor proteins. However, the molecular mechanisms by which the Ryk receptor functions remain unknown. Here, we report that mammalian Ryk, unlike the Drosophila Ryk homolog Derailed, functions as a coreceptor along with Frizzled for Wnt ligands. Ryk also binds to Dishevelled, through which it activates the canonical Wnt pathway, providing a link between Wnt and Dishevelled. Transgenic mice expressing Ryk siRNA exhibit defects in axon guidance, and Ryk is required for neurite outgrowth induced by Wnt-3a and in the activation of T cell factor (TCF) induced by Wnt-1. Thus, Ryk appears to play a crucial role in Wnt-mediated signaling.

Silencing the Drosophila ribosomal protein L14 gene using targeted RNA interference causes distinct somatic anomalies

The Drosophila Minutes are haploinsufficient mutations that are defective in ribosomal protein (rp) production, resulting in short, thin bristles, delayed development and recessive lethality. In a Minute fly, the amount of rp gene messenger RNA (mRNA) is reduced to >or=50% of the normal amount of gene product, and becomes rate limiting for ribosome biogenesis, cell proliferation and growth. Haploinsufficiency increases the vulnerability to complete loss of gene function (homozygous null state) if hit by a second mutation. Because of the homozygous lethality, it has only been possible to study the effects of Minute mutations in heterozygous animals. To be able to study the consequences of a loss-of-function of an rp gene (0%>mRNA<50%) in developing and differentiated cells we used heritable RNA interference (RNAi) in combination with the yeast GAL4/UAS binary system to spatiotemporally knock down the ribosomal protein L14 (RpL14) gene. We show, at the RNA and phenotypic levels, that RNAi efficiently reduces RpL14 gene expression throughout development, causing lethality and distinct and dramatic somatic anomalies in both developing and differentiated cells.

Gain-of-function screen identifies a role of theSrc64oncogene inDrosophilamushroom body development

Mushroom bodies (MB) are substructures in the Drosophila brain that are essential for memory. At present, MB anatomy is rather well described when compared to other brain areas, and elucidation of the genetic control of the development and projection patterns of MB neurons will be important to the understanding of their functions. We have performed a gain-of-function screen in order to identify genes that are involved in MB development. We drove expression of genes in MB neurons by crossing 2407 GAL4-driven UY element lines to lines containing an MB GAL4 source and UAS-GFP elements, and looked for defects in the MB structure. We have molecularly identified the genomic regions adjacent to the 26 positive UY insertions and found 18 potential genes that exhibit adult MB gain-of-function phenotypes. The proteins encoded by these candidate genes include, as well as genes with yet unknown function, transcription factors (e.g., tramtrack), nanos RNA-binding protein, microtubule-severing protein, vesicle trafficking proteins, axon guidance receptor, and the Src64 cytoplasmic protein tyrosine kinase. These genes are involved in key features of neuron cell biology. In three cases, tramtrack, nanos, and Src64, we show that the open reading frame located directly downstream of the UY P element is indeed the expressed target gene. Loss-of-function mutations of both ttk and Src64 lead to MB phenotypes proving that these genes are involved in the genetic control of MB development. Moreover, Src64 is shown here to act in a cell-autonomous fashion and is likely to interact with the previously-identified linotte/derailed receptor tyrosine kinase in MB development.

Development and evolution of the insect mushroom bodies: towards the understanding of conserved developmental mechanisms in a higher brain center

The insect mushroom bodies are prominent higher order neuropils consisting of thousands of approximately parallel projecting intrinsic neurons arising from the minute basophilic perikarya of globuli cells. Early studies described these structures as centers for intelligence and other higher functions; at present, the mushroom bodies are regarded as important models for the neural basis of learning and memory. The insect mushroom bodies share a similar general morphology, and the same basic sequence of developmental events is observed across a wide range of insect taxa. Globuli cell progenitors arise in the embryo and proliferate throughout the greater part of juvenile development. Discrete morphological and functional subpopulations of globuli cells (or Kenyon cells, as they are called in insects) are sequentially produced at distinct periods of development. Kenyon cell somata are arranged by age around the center of proliferation, as are their processes in the mushroom body neuropil. Other aspects of mushroom body development are more variable from species to species, such as the origin of specific Kenyon cell populations and neuropil substructures, as well as the timing and pace of the general developmental sequence.

Drosophila clock can generate ectopic circadian clocks

Circadian rhythms of behavior, physiology, and gene expression are present in diverse tissues and organisms. The function of the transcriptional activator, Clock, is necessary in both Drosophila and mammals for the expression of many core clock components. We demonstrate in Drosophila that Clock misexpression in nai;ve brain regions induces circadian gene expression. This includes major components of the pacemaker program, as Clock also activates the rhythmic expression of cryptochrome, a gene that CLOCK normally represses. Moreover, this ectopic clock expression has potent effects on behavior, radically altering locomotor activity patterns. We propose that Clock is uniquely able to induce and organize the core elements of interdependent feedback loops necessary for circadian rhythms.

Mutation of linotte causes behavioral defects independently of pigeon in Drosophila

The Drosophila linotte1 mutation was isolated from a genetic screen designed to identify learning and memory genes. For some authors, this mutation affects a novel gene specifically involved in adult learning and memory, whereas for others, it is an allele of the derailed receptor tyrosine kinase gene (the linotte/derailed gene) involved in nervous system development. Here, we show that the original derailed mutation induces a memory phenotype. We also report that a new null mutation, lioexc21, affecting specifically the linotte/derailed gene causes behavioral defects, which can be partially rescued by expression of a lio+/drl+ transgene. The data presented here suggest that the memory phenotype of linotte and derailed mutants is a consequence of abnormal brain development due to loss of function of the linotte/derailed encoded receptor tyrosine kinase.

Cell-type-specific expression of protein tyrosine kinase-related receptor RYK in the central nervous system of the rat

The mammalian RYK is an orphan receptor that contains a catalytically inactive tyrosine-kinase-related domain. Its Drosophila homolog, Lio/Drl, is required for axon pathfinding in developing brain. Our previous study suggested that RYK mRNA is expressed in nestin-positive progenitor cells and neurons. In the present study, immunohistochemistry has been used to further localize RYK in the central nervous system of rats to identify the lineage of the RYK-expressing cells. In the embryonic forebrain, RYK colocalized with nestin in the ventricular zone and with MAP2 in the cortical plate, suggesting that RYK is expressed in neural progenitor cells and neurons. Localization of RYK in embryonic spinal cord also suggested its expression in both cell types. In primary cultures of rat cerebrum, RYK expression was observed in all neurons, as well as in a significant population of oligodendrocytes, O-2A progenitor cells, and type-2 astrocytes. However, no RYK expression was detected in type-1 astrocytes or microglia. Multipotent neural stem cell line MNS-70 was also analyzed for expression of RYK, and most of the cells were positive for both RYK and nestin in the undifferentiated stage. In the differentiated stage, expression of RYK was detected in the neurons, but not in type-1 astrocytes. In conclusion, RYK is expressed in nestin-positive progenitor cells and neurons, and in a certain population of oligodendrocytes, O-2A progenitor cells, and type-2 astrocytes in developing CNS. These findings show that expression of RYK in rat CNS is tightly regulated in a cell-type-specific manner.

What can we teach Drosophila? What can they teach us?

A number of single gene mutations dramatically reduce the ability of fruit flies to learn or to remember. Cloning of the affected genes implicated the adenylyl cyclase second-messenger system as key in learning and memory. The expression patterns of these genes, in combination with other data, indicates that brain structures called mushroom bodies are crucial for olfactory learning. However, the mushroom bodies are not dedicated solely to olfactory processing; they also mediate higher cognitive functions in the fly, such as visual context generalization. Molecular genetic manipulations, coupled with behavioral studies of the fly, will identify rudimentary neural circuits that underly multisensory learning and perhaps also the circuits that mediate more-complex brain functions, such as attention.

Flies, genes, and learning

Flies can learn. For the past 25 years, researchers have isolated mutants, engineered mutants with transgenes, and tested likely suspect mutants from other screens for learning ability. There have been notable surprises-conventional second messenger systems co-opted for intricate associative learning tasks, two entirely separate forms of long-term memory, a cell-adhesion molecule that is necessary for short-term memory. The most recent surprise is the mechanistic kinship revealed between learning and addictive drug response behaviors in flies. The flow of new insight is likely to quicken with the completion of the fly genome and the arrival of more selective methods of gene expression.

The Drosophila castor gene is involved in postembryonic brain development

castor (cas) encodes a zink finger protein expressed in a subset of Drosophila embryonic neuroglioblasts where it controls neuronal differentiation. We show here that cas is expressed at larval and pupal stages in brain cell clusters where it participates in the elaboration of the adult structures. In particular using the MARCM system (mosaic analysis with a repressible cell marker), we show that cas is required postembryonically for correct axon pathfinding of the central complex (CX) and mushroom body (MB) neurons. The linotte (lio) gene encodes a transmembrane protein expressed at larval/pupal stage in a glial structure, the TIFR, and interacts with the no-bridge (nob) gene. We show that cas interacts genetically with lio and nob. These interactions do not involve direct transcription regulation but probably cellular communication processes.

Revelations of the RYK receptor

Significant progress has been made over the last decade in elucidating the mechanisms employed by receptor protein tyrosine kinases (RTKs) in transducing extracellular signals critical for the regulation of diverse cellular activities. Nevertheless, revealing the biological significance of a subset of the RTKs that contain catalytically inactive protein tyrosine kinase domains has proven more elusive. ErbB3 has served as the prototype for models of catalytically inactive RTK function, performing the role of signal diversification in heterodimeric receptor complexes with other ErbB subfamily members. The receptor related to tyrosine kinases (RYK) is unique amongst the catalytically inactive RTKs. Based on structural or functional properties of the extracellular domain, RYK cannot be classified into an existing RTK subfamily. Recent genetic analyses of mouse Ryk and its Drosophila orthologue derailed have defined a role for this novel subfamily of receptors in the control of craniofacial development and neuronal pathway selection, respectively. Recent biochemical data lead us to propose a model that involves RYK in signal crosstalk and scaffold assembly with Eph receptors. This model is consistent with the established roles of Eph receptors and ephrins in craniofacial and nervous system morphogenesis. BioEssays 23:34-45, 2001.

Deceiving appearances: signaling by "dead" and "fractured" receptor protein-tyrosine kinases

The mechanisms by which most receptor protein-tyrosine kinases (RTKs) transmit signals are now well established. Binding of ligand results in the dimerization of receptor monomers followed by transphosphorylation of tyrosine residues within the cytoplasmic domains of the receptors. This tidy picture has, however, some strange characters lurking around the edges. Cases have now been identified in which RTKs lack kinase activity, but, despite being "dead" appear to have roles in signal transduction. Even stranger are the cases in which genes encoding RTKs produce protein products consisting of only a portion of the kinase domain. At least one such "fractured" RTK appears to be involved in signal transduction. Here we describe how these strange molecules might function and discuss the questions associated with their evolution. BioEssays 23:69-76, 2001.

no-bridge and linotte act jointly at the interhemispheric junction to build up the adult central brain of Drosophila melanogaster

The Drosophila transmembrane protein Linotte (Lio) is expressed in a glial transcient interhemispheric fibrous ring (TIFR), which was hypothesised to serve as scaffold for adult brain formation during metamorphosis. We isolated TIFR specific enhancers from the lio locus and showed that only four interhemispheric cells give rise to this complex fibrous structure. We showed that lio controls the TIFR differentiation, and confirmed the major role played by this structure in central brain metamorphosis since its destruction by apoptosis led to a pronounced adult phenotype, which included defects of lio and no-bridge (nob) mutants. lio interhemispheric expression was specifically affected in a nob(1) context, confirming that nob plays a key role in adult brain development via the TIFR.

Early development of the Drosophila mushroom body: the roles of eyeless and dachshund

The mushroom body (MB) is a uniquely identifiable brain structure present in most arthropods. Functional studies have established its role in learning and memory. Here we describe the early embryonic origin of the four neuroblasts that give rise to the mushroom body and follow its morphogenesis through later embryonic stages. In the late embryo, axons of MB neurons lay down a characteristic pattern of pathways. eyeless (ey) and dachshund (dac) are expressed in the progenitor cells and neurons of the MB in the embryo and larva. In the larval brains of the hypomorphic ey(R) strain, we find that beside an overall reduction of MB neurons, one MB pathway, the medial lobe, is malformed or missing. Overexpression of eyeless in MBs under the control of an MB-specific promoter results in a converse type of axon pathway abnormality, i.e. malformation or loss of the dorsal lobe. In contrast, loss of dachshund results in deformation of the dorsal lobe, whereas no lobe abnormalities can be detected following dachshund overexpression. These results indicate that ey and dachshund may have a role in axon pathway selection during embryogenesis.

The retinal determination gene, dachshund, is required for mushroom body cell differentiation

The dachshund gene of Drosophila encodes a putative transcriptional regulator required for eye and leg development. We show here that dachshund is also required for normal brain development. The mushroom bodies of dachshund mutants exhibit a marked reduction in the number of (α) lobe axons, a disorganization of axons extending into horizontal lobes, and aberrant projections into brain areas normally unoccupied by mushroom body processes. The phenotypes become pronounced during pupariation, suggesting that dachshund function is required during this period. GAL4-mediated expression of dachshund in the mushroom bodies rescues the mushroom body phenotypes. Moreover, dachshund mutant mushroom body clones in an otherwise wild-type brain exhibit the phenotypes, indicating an autonomous role for dachshund. Although eyeless, like dachshund, is preferentially expressed in the mushroom body and is genetically upstream of dachshund for eye development, no interaction of these genes was detected for mushroom body development. Thus, dachshund functions in the developing mushroom body neurons to ensure their proper differentiation.

Axon routing across the midline controlled by the Drosophila Derailed receptor

In nervous systems with symmetry about the midline, many neurons project axons from one side to the other. Although several of the components controlling midline crossing have been identified, little is known about how axons choose the appropriate pathway when crossing. For example, in the Drosophila embryo axons cross the midline in one of two distinct tracts, the anterior or posterior commissure (AC or PC, respec tively). Here we show that the Derailed (Drl) receptor tyrosine kinase is expressed by neurons that project in the AC, and that in the absence of Drl such neurons often project abnormally into the PC. Conversely, misexpression of Drl in PC neurons forces them to cross in the AC. The behaviour of Drl-misexpressing neurons and the in vivo binding pattern of a soluble Drl receptor probe indicate that Drl acts as a guidance receptor for a repellent ligand present in the PC. Our results show that Drl is a novel component in the control of midline crossing.

A tyrosine kinase screen of mouse vestibular maculae

Receptor tyrosine kinases allow extracellular signals to influence intracellular events, while other tyrosine kinases are involved in intracellular signalling. They may therefore be involved in the development, maintenance and repair of the sensory epithelia of the inner ear, since these are believed to be affected by inter- and intracellular signalling. In order to analyse possible tyrosine kinases expressed in sensory areas of the inner ear, a reverse transcription polymerase chain reaction screen of microdissected sensory epithelia was undertaken, using primers targeted at conserved sequences in tyrosine kinase domains. Tissue was taken from the maculae of the mouse vestibular organs, and consisted mainly of hair cells and their supporting cells. Of 80 clones sequenced, 49 coded for tyrosine kinases, and 11 for other known molecules. Further analysis of one of the sequences, for FGF receptor 4, showed a novel variant, expressed in the inner ear and elsewhere, with a variation in the intracellular domain which suggests differential activation of known signalling pathways. Other clones coded for tyrosine kinases expected to be involved in cell surface and intracellular signalling. The technique forms a powerful tool for analysing a range of the tyrosine kinases expressed, and provides a starting point for the analysis of cell-cell signalling in the inner ear.