Selective expression of Knot/Collier, a transcriptional regulator of the EBF/Olf-1 family, endows the Drosophila sensory system with neuronal class-specific elaborated dendritic patterns

Impact of Drosophila LIM homeodomain protein Apterous on the morphology of the adult mushroom body

A LIM homeodomain transcription factor Apterous (Ap) regulates embryonic and larval neurodevelopment in Drosophila. Although Ap is still expressed in the adult brain, it remains elusive whether Ap is involved in neurodevelopmental events in the adult brain because flies homozygous for ap mutations are usually lethal before they reach the adult stage. In this study, using adult escapers of ap knockout (KO) homozygotes, we examined whether the complete lack of ap expression affects the morphology of the mushroom body (MB) neurons and Pigment-dispersing factor (Pdf)-positive clock neurons in the adult brain. Although ap KO escapers showed severe structural defects of MB neurons, no clear morphological defects were found in Pdf-positive clock neurons. These results suggest that Ap in the adult brain is essential for the neurodevelopment of specific ap-positive neurons, but it is not necessarily involved in the development of all ap-positive neurons.

MBL-1 and EEL-1 affect the splicing and protein levels of MEC-3 to control dendrite complexity

Transcription factors (TFs) play critical roles in specifying many aspects of neuronal cell fate including dendritic morphology. How TFs are accurately regulated during neuronal morphogenesis is not fully understood. Here, we show that LIM homeodomain protein MEC-3, the key TF for C. elegans PVD dendrite morphogenesis, is regulated by both alternative splicing and an E3 ubiquitin ligase. The mec-3 gene generates several transcripts by alternative splicing. We find that mbl-1, the orthologue of the muscular dystrophy disease gene muscleblind-like (MBNL), is required for PVD dendrite arbor formation. Our data suggest mbl-1 regulates the alternative splicing of mec-3 to produce its long isoform. Deleting the long isoform of mec-3(deExon2) causes reduction of dendrite complexity. Through a genetic modifier screen, we find that mutation in the E3 ubiquitin ligase EEL-1 suppresses mbl-1 phenotype. eel-1 mutants also suppress mec-3(deExon2) mutant but not the mec-3 null phenotype. Loss of EEL-1 alone leads to excessive dendrite branches. Together, these results indicate that MEC-3 is fine-tuned by alternative splicing and the ubiquitin system to produce the optimal level of dendrite branches.

Regulated alternative splicing of Dscam2 is required for somatosensory circuit wiring

Axon and dendrite placement and connectivity is guided by a wide range of secreted and surface molecules in the developing nervous system. Nevertheless, the extraordinary complexity of connections in the brain requires that this repertoire be further diversified to precisely and uniquely regulate cell-cell interactions. One important mechanism for molecular diversification is alternative splicing. Drosophila Down syndrome cell adhesion molecule (Dscam2) undergoes cell type-specific alternative splicing to produce two isoform-specific homophilic binding proteins. Regulated alternative splicing of Dscam2 is important for dendrite and axon patterning, but how this translates to circuit wiring and animal behavior is not well understood. Here, we examined the role of cell-type specific expression of Dscam2 isoforms in regulating synaptic partner selection in the larval somatosensory system. We found that synaptic partners in the nociceptive circuit express different Dscam2 isoforms. Forcing synaptic partners to express a common isoform resulted in nociceptive axon patterning defects and attenuated nocifensive behaviors, indicating that a role for Dscam2 alternative splicing is to ensure that synaptic partners do not express matching isoforms. These results point to a model in which regulated alternative splicing of Dscam2 across populations of neurons restricts connectivity to specific partners and prevents inappropriate synaptic connections.

Polycomb group genes are required for neuronal pruning in Drosophila

BACKGROUND

Pruning that selectively eliminates unnecessary or incorrect neurites is required for proper wiring of the mature nervous system. During Drosophila metamorphosis, dendritic arbourization sensory neurons (ddaCs) and mushroom body (MB) γ neurons can selectively prune their larval dendrites and/or axons in response to the steroid hormone ecdysone. An ecdysone-induced transcriptional cascade plays a key role in initiating neuronal pruning. However, how downstream components of ecdysone signalling are induced remains not entirely understood.

RESULTS

Here, we identify that Scm, a component of Polycomb group (PcG) complexes, is required for dendrite pruning of ddaC neurons. We show that two PcG complexes, PRC1 and PRC2, are important for dendrite pruning. Interestingly, depletion of PRC1 strongly enhances ectopic expression of Abdominal B (Abd-B) and Sex combs reduced, whereas loss of PRC2 causes mild upregulation of Ultrabithorax and Abdominal A in ddaC neurons. Among these Hox genes, overexpression of Abd-B causes the most severe pruning defects, suggesting its dominant effect. Knockdown of the core PRC1 component Polyhomeotic (Ph) or Abd-B overexpression selectively downregulates Mical expression, thereby inhibiting ecdysone signalling. Finally, Ph is also required for axon pruning and Abd-B silencing in MB γ neurons, indicating a conserved function of PRC1 in two types of pruning.

CONCLUSIONS

This study demonstrates important roles of PcG and Hox genes in regulating ecdysone signalling and neuronal pruning in Drosophila. Moreover, our findings suggest a non-canonical and PRC2-independent role of PRC1 in Hox gene silencing during neuronal pruning.

PP2A phosphatase regulates cell-type specific cytoskeletal organization to drive dendrite diversity

Uncovering molecular mechanisms regulating dendritic diversification is essential to understanding the formation and modulation of functional neural circuitry. Transcription factors play critical roles in promoting dendritic diversity and here, we identify PP2A phosphatase function as a downstream effector of Cut-mediated transcriptional regulation of dendrite development. Mutant analyses of the PP2A catalytic subunit (mts) or the scaffolding subunit (PP2A-29B) reveal cell-type specific regulatory effects with the PP2A complex required to promote dendritic growth and branching in Drosophila Class IV (CIV) multidendritic (md) neurons, whereas in Class I (CI) md neurons, PP2A functions in restricting dendritic arborization. Cytoskeletal analyses reveal requirements for Mts in regulating microtubule stability/polarity and F-actin organization/dynamics. In CIV neurons, mts knockdown leads to reductions in dendritic localization of organelles including mitochondria and satellite Golgi outposts, while CI neurons show increased Golgi outpost trafficking along the dendritic arbor. Further, mts mutant neurons exhibit defects in neuronal polarity/compartmentalization. Finally, genetic interaction analyses suggest β-tubulin subunit 85D is a common PP2A target in CI and CIV neurons, while FoxO is a putative target in CI neurons.

The Role of Even-Skipped in Drosophila Larval Somatosensory Circuit Assembly

Proper somatosensory circuit assembly is critical for processing somatosensory stimuli and for responding accordingly. In comparison to other sensory circuits (e.g., olfactory and visual), somatosensory circuits have unique anatomy and function. However, understanding of somatosensory circuit development lags far behind that of other sensory systems. For example, there are few identified transcription factors required for integration of interneurons into functional somatosensory circuits. Here, as a model, we examine one type of somatosensory interneuron, Even-skipped (Eve) expressing laterally placed interneurons (ELs) of the Drosophila larval nerve cord. Eve is a highly conserved, homeodomain transcription factor known to play a role in cell fate specification and neuronal axon guidance. Because marker genes are often functionally important in the cell types they define, we deleted eve expression specifically from EL interneurons. On the cell biological level, using single neuron labeling, we find eve plays several previously undescribed roles in refinement of neuron morphogenesis. Eve suppresses aberrant neurite branching, promotes axon elongation, and regulates dorsal-ventral dendrite position. On the circuit level, using optogenetics, calcium imaging, and behavioral analysis, we find eve expression is required in EL interneurons for the normal encoding of somatosensory stimuli and for normal mapping of outputs to behavior. We conclude that the eve gene product coordinately regulates multiple aspects of EL interneuron morphogenesis and is critically required to properly integrate EL interneurons into somatosensory circuits. Our data shed light on the genetic regulation of somatosensory circuit assembly.

Deterministic and Stochastic Rules of Branching Govern Dendrite Morphogenesis of Sensory Neurons

Dendrite morphology is necessary for the correct integration of inputs that neurons receive. The branching mechanisms allowing neurons to acquire their type-specific morphology remain unclear. Classically, axon and dendrite patterns were shown to be guided by molecules, providing deterministic cues. However, the extent to which deterministic and stochastic mechanisms, based upon purely statistical bias, contribute to the emergence of dendrite shape is largely unknown. We address this issue using the Drosophila class I vpda multi-dendritic neurons. Detailed quantitative analysis of vpda dendrite morphogenesis indicates that the primary branch grows very robustly in a fixed direction, though secondary branch numbers and lengths showed fluctuations characteristic of stochastic systems. Live-tracking dendrites and computational modeling revealed how neuron shape emerges from few local statistical parameters of branch dynamics. We report key opposing aspects of how tree architecture feedbacks on the local probability of branch shrinkage. Child branches promote stabilization of parent branches, although self-repulsion promotes shrinkage. Finally, we show that self-repulsion, mediated by the adhesion molecule Dscam1, indirectly patterns the growth of secondary branches by spatially restricting their direction of stable growth perpendicular to the primary branch. Thus, the stochastic nature of secondary branch dynamics and the existence of geometric feedback emphasize the importance of self-organization in neuronal dendrite morphogenesis.

Frameshift mutations of YPEL3 alter the sensory circuit function in Drosophila

A frameshift mutation in Yippee-like (YPEL) 3 was recently found from a rare human disorder with peripheral neurological conditions including hypotonia and areflexia. The YPEL gene family is highly conserved from yeast to human, but its members' functions are poorly defined. Moreover, the pathogenicity of the human YPEL3 variant is completely unknown. We generated a Drosophila model of human YPEL3 variant and a genetic null allele of Drosophila homolog of YPEL3 (referred to as dYPEL3). Gene-trap analysis suggests that dYPEL3 is predominantly expressed in subsets of neurons, including larval nociceptors. Analysis of chemical nociception induced by allyl-isothiocyanate (AITC), a natural chemical stimulant, revealed reduced nociceptive responses in both dYPEL3 frameshift and null mutants. Subsequent circuit analysis showed reduced activation of second-order neurons (SONs) in the pathway without affecting nociceptor activation upon AITC treatment. Although the gross axonal and dendritic development of nociceptors was unaffected, the synaptic contact between nociceptors and SONs was decreased by the dYPEL3 mutations. Furthermore, expressing dYPEL3 in larval nociceptors rescued the behavioral deficit in dYPEL3 frameshift mutants, suggesting a presynaptic origin of the deficit. Together, these findings suggest that the frameshift mutation results in YPEL3 loss of function and may cause neurological conditions by weakening synaptic connections through presynaptic mechanisms.

DeTerm: Software for automatic detection of neuronal dendritic branch terminals via an artificial neural network

Dendrites of neurons receive and process synaptic or sensory inputs. The Drosophila class IV dendritic arborization (da) neuron is an established model system to explore molecular mechanisms of dendrite morphogenesis. The total number of dendritic branch terminals is one of the frequently employed parameters to characterize dendritic arborization complexity of class IV neurons. This parameter gives a useful phenotypic readout of arborization during neurogenesis, and it is typically determined by laborious manual analyses of numerous images. Ideally, an automated analysis would greatly reduce the workload; however, it is challenging to automatically discriminate dendritic branch terminals from signals of surrounding tissues in whole-mount live larvae. Here, we describe our newly developed software, called DeTerm, which automatically recognizes and quantifies dendrite branch terminals via an artificial neural network. Once we input an image file of a neuronal dendritic arbor and its region of interest information, DeTerm is capable of labeling terminals of larval class IV neurons with high precision, and it also provides positional data of individual terminals. We further show that DeTerm is applicable to other types of neurons, including mouse cerebellar Purkinje cells. DeTerm is freely available on the web and was successfully tested on Mac, Windows and Linux.

An ancient role for collier/Olf/Ebf (COE)-type transcription factors in axial motor neuron development

BACKGROUND

Mammalian motor circuits display remarkable cellular diversity with hundreds of motor neuron (MN) subtypes innervating hundreds of different muscles. Extensive research on limb muscle-innervating MNs has begun to elucidate the genetic programs that control animal locomotion. In striking contrast, the molecular mechanisms underlying the development of axial muscle-innervating MNs, which control breathing and spinal alignment, are poorly studied.

METHODS

Our previous studies indicated that the function of the Collier/Olf/Ebf (COE) family of transcription factors (TFs) in axial MN development may be conserved from nematodes to simple chordates. Here, we examine the expression pattern of all four mouse COE family members (mEbf1-mEbf4) in spinal MNs and employ genetic approaches in both nematodes and mice to investigate their function in axial MN development.

RESULTS

We report that mEbf1 and mEbf2 are expressed in distinct MN clusters (termed "columns") that innervate different axial muscles. Mouse Ebf1 is expressed in MNs of the hypaxial motor column (HMC), which is necessary for breathing, while mEbf2 is expressed in MNs of the medial motor column (MMC) that control spinal alignment. Our characterization of Ebf2 knock-out mice uncovered a requirement for Ebf2 in the differentiation program of a subset of MMC MNs and revealed for the first time molecular diversity within MMC neurons. Intriguingly, transgenic expression of mEbf1 or mEbf2 can rescue axial MN differentiation and locomotory defects in nematodes (Caenorhabditis elegans) lacking unc-3, the sole C. elegans ortholog of the COE family, suggesting functional conservation among mEbf1, mEbf2 and nematode UNC-3.

CONCLUSIONS

These findings support the hypothesis that genetic programs controlling axial MN development are deeply conserved across species, and further advance our understanding of such programs by revealing an essential role for Ebf2 in mouse axial MNs. Because human mutations in COE orthologs lead to neurodevelopmental disorders characterized by motor developmental delay, our findings may advance our understanding of these human conditions.

Dendritic Cytoskeletal Architecture Is Modulated by Combinatorial Transcriptional Regulation in Drosophila melanogaster

Transcription factors (TFs) have emerged as essential cell autonomous mediators of subtype specific dendritogenesis; however, the downstream effectors of these TFs remain largely unknown, as are the cellular events that TFs control to direct morphological change. As dendritic morphology is largely dictated by the organization of the actin and microtubule (MT) cytoskeletons, elucidating TF-mediated cytoskeletal regulatory programs is key to understanding molecular control of diverse dendritic morphologies. Previous studies in Drosophila melanogaster have demonstrated that the conserved TFs Cut and Knot exert combinatorial control over aspects of dendritic cytoskeleton development, promoting actin and MT-based arbor morphology, respectively. To investigate transcriptional targets of Cut and/or Knot regulation, we conducted systematic neurogenomic studies, coupled with in vivo genetic screens utilizing multi-fluor cytoskeletal and membrane marker reporters. These analyses identified a host of putative Cut and/or Knot effector molecules, and a subset of these putative TF targets converge on modulating dendritic cytoskeletal architecture, which are grouped into three major phenotypic categories, based upon neuromorphometric analyses: complexity enhancer, complexity shifter, and complexity suppressor. Complexity enhancer genes normally function to promote higher order dendritic growth and branching with variable effects on MT stabilization and F-actin organization, whereas complexity shifter and complexity suppressor genes normally function in regulating proximal-distal branching distribution or in restricting higher order branching complexity, respectively, with spatially restricted impacts on the dendritic cytoskeleton. Collectively, we implicate novel genes and cellular programs by which TFs distinctly and combinatorially govern dendritogenesis via cytoskeletal modulation.

In vivo dendrite regeneration after injury is different from dendrite development

Thompson-Peer et al. show that, in intact Drosophila larvae, a discrete injury that removes all dendrites induces robust dendritic growth that recreates many features of uninjured dendrites. However, the growth and patterning of injury-induced dendrites is significantly different from uninjured dendrites.

Neurons receive information along dendrites and send signals along axons to synaptic contacts. The factors that control axon regeneration have been examined in many systems, but dendrite regeneration has been largely unexplored. Here we report that, in intact Drosophila larvae, a discrete injury that removes all dendrites induces robust dendritic growth that recreates many features of uninjured dendrites, including the number of dendrite branches that regenerate and responsiveness to sensory stimuli. However, the growth and patterning of injury-induced dendrites is significantly different from uninjured dendrites. We found that regenerated arbors cover much less territory than uninjured neurons, fail to avoid crossing over other branches from the same neuron, respond less strongly to mechanical stimuli, and are pruned precociously. Finally, silencing the electrical activity of the neurons specifically blocks injury-induced, but not developmental, dendrite growth. By elucidating the essential features of dendrites grown in response to acute injury, our work builds a framework for exploring dendrite regeneration in physiological and pathological conditions.

Dendritic diversification through transcription factor-mediated suppression of alternative morphologies

Neurons display a striking degree of functional and morphological diversity, and the developmental mechanisms that underlie diversification are of significant interest for understanding neural circuit assembly and function. We find that the morphology of Drosophila sensory neurons is diversified through a series of suppressive transcriptional interactions involving the POU domain transcription factors Pdm1 (Nubbin) and Pdm2, the homeodomain transcription factor Cut, and the transcriptional regulators Scalloped and Vestigial. Pdm1 and Pdm2 are expressed in a subset of proprioceptive sensory neurons and function to inhibit dendrite growth and branching. A subset of touch receptors show a capacity to express Pdm1/2, but Cut represses this expression and promotes more complex dendritic arbors. Levels of Cut expression are diversified in distinct sensory neurons by selective expression of Scalloped and Vestigial. Different levels of Cut impact dendritic complexity and, consistent with this, we show that Scalloped and Vestigial suppress terminal dendritic branching. This transcriptional hierarchy therefore acts to suppress alternative morphologies to diversify three distinct types of somatosensory neurons.

Microtubule nucleation and organization in dendrites

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.

The Krüppel-Like Factor Dar1 Determines Multipolar Neuron Morphology

Neurons typically assume multipolar, bipolar, or unipolar morphologies. Little is known about the mechanisms underlying the development of these basic morphological types. Here, we show that the Krüppel-like transcription factor Dar1 determines the multipolar morphology of postmitotic neurons in Drosophila. Dar1 is specifically expressed in multipolar neurons and loss of dar1 gradually converts multipolar neurons into the bipolar or unipolar morphology without changing neuronal identity. Conversely, misexpression of Dar1 or its mammalian homolog in unipolar and bipolar neurons causes them to assume multipolar morphologies. Dar1 regulates the expression of several dynein genes and nuclear distribution protein C (nudC), which is an essential component of a specialized dynein complex that positions the nucleus in a cell. We further show that these genes are required for Dar1-induced multipolar neuron morphology. Dar1 likely functions as a terminal selector gene for the basic layout of neuron morphology by regulating both dendrite extension and the dendrite-nucleus coupling.

SIGNIFICANCE STATEMENT

The three basic morphological types of neurons--unipolar, bipolar, and multipolar--are important for information processing and wiring of neural circuits. Little progress has been made toward understanding the molecular and cellular programs that generate these types since their discovery over a century ago. It is generally assumed that basic morphological types of neurons are determined by the number of dendrites growing out from the cell body. Here, we show that this model alone is insufficient. We introduce the positioning of nucleus as a critical factor in this process and report that the transcription factor Dar1 determines multipolar neuron morphology in postmitotic neurons by regulating genes involved in nuclear positioning.

Centrosomin represses dendrite branching by orienting microtubule nucleation

Neuronal dendrite branching is fundamental for building nervous systems. Branch formation is genetically encoded by transcriptional programs to create dendrite arbor morphological diversity for complex neuronal functions. In Drosophila sensory neurons, the transcription factor Abrupt represses branching via an unknown effector pathway. Targeted screening for branching-control effectors identified Centrosomin, the primary centrosome-associated protein for mitotic spindle maturation. Centrosomin repressed dendrite branch formation and was used by Abrupt to simplify arbor branching. Live imaging revealed that Centrosomin localized to the Golgi cis face and that it recruited microtubule nucleation to Golgi outposts for net retrograde microtubule polymerization away from nascent dendrite branches. Removal of Centrosomin enabled the engagement of wee Augmin activity to promote anterograde microtubule growth into the nascent branches, leading to increased branching. The findings reveal that polarized targeting of Centrosomin to Golgi outposts during elaboration of the dendrite arbor creates a local system for guiding microtubule polymerization.

Importance of gene dosage in controlling dendritic arbor formation during development

Proper dendrite morphology is crucial for normal nervous system functioning. While a number of genes have been implicated in dendrite morphogenesis in both invertebrates and mammals, it remains unclear how developing dendrites respond to changes in gene dosage and what type of patterns their responses may follow. To understand this, I review here evidence from the recent literature, focusing on the genetic studies performed in the Drosophila larval dendritic arborization class IV neuron, an excellent cell type to understand dendrite morphogenesis. I summarize how class IV arbors change morphology in response to developmental fluctuations in the expression levels of 47 genes, studied by means of genetic manipulations such as loss-of-function and gain-of-function, and for which sufficient information is available. I find that arbors can respond to changing gene dosage in several distinct ways, each characterized by a singular dose-response curve. Interestingly, in 72% of cases arbors are sensitive, and thus adjust their morphology, in response to both decreases and increases in the expression of a given gene, indicating that dendrite morphogenesis is a process particularly sensitive to gene dosage. By summarizing the parallels between Drosophila and mammals, I show that many Drosophila dendrite morphogenesis genes have orthologs in mammals, and that some of these are associated with mammalian dendrite outgrowth and human neurodevelopmental disorders. One notable disease-related molecule is kinase Dyrk1A, thought to be a causative factor in Down syndrome. Both increases and decreases in Dyrk1A gene dosage lead to impaired dendrite morphogenesis, which may contribute to Down syndrome pathoetiology.

Genome-Wide Mapping of Collier In Vivo Binding Sites Highlights Its Hierarchical Position in Different Transcription Regulatory Networks

Collier, the single Drosophila COE (Collier/EBF/Olf-1) transcription factor, is required in several developmental processes, including head patterning and specification of muscle and neuron identity during embryogenesis. To identify direct Collier (Col) targets in different cell types, we used ChIP-seq to map Col binding sites throughout the genome, at mid-embryogenesis. In vivo Col binding peaks were associated to 415 potential direct target genes. Gene Ontology analysis revealed a strong enrichment in proteins with DNA binding and/or transcription-regulatory properties. Characterization of a selection of candidates, using transgenic CRM-reporter assays, identified direct Col targets in dorso-lateral somatic muscles and specific neuron types in the central nervous system. These data brought new evidence that Col direct control of the expression of the transcription regulators apterous and eyes-absent (eya) is critical to specifying neuronal identities. They also showed that cross-regulation between col and eya in muscle progenitor cells is required for specification of muscle identity, revealing a new parallel between the myogenic regulatory networks operating in Drosophila and vertebrates. Col regulation of eya, both in specific muscle and neuronal lineages, may illustrate one mechanism behind the evolutionary diversification of Col biological roles.

The C. elegans COE transcription factor UNC-3 activates lineage-specific apoptosis and affects neurite growth in the RID lineage

Mechanisms that regulate apoptosis in a temporal and lineage-specific manner remain poorly understood. The COE (Collier/Olf/EBF) transcription factors have been implicated in the development of many cell types, including neurons. Here, we show that the sole Caenorhabditis elegans COE protein, UNC-3, together with a histone acetyltransferase, CBP-1/P300, specifies lineage-specific apoptosis and certain aspects of neurite trajectory. During embryogenesis, the RID progenitor cell gives rise to the RID neuron and RID sister cell; the latter undergoes apoptosis shortly after cell division upon expression of the pro-apoptotic gene egl-1. We observe UNC-3 expression in the RID progenitor, and the absence of UNC-3 results in the failure of the RID lineage to express a Pegl-1::GFP reporter and in the survival of the RID sister cell. Lastly, UNC-3 interacts with CBP-1, and cbp-1 mutants exhibit a similar RID phenotype to unc-3. Thus, in addition to playing a role in neuronal terminal differentiation, UNC-3 is a cell lineage-specific regulator of apoptosis.

Transcription factors and effectors that regulate neuronal morphology

Transcription factors establish the tremendous diversity of cell types in the nervous system by regulating the expression of genes that give a cell its morphological and functional properties. Although many studies have identified requirements for specific transcription factors during the different steps of neural circuit assembly, few have identified the downstream effectors by which they control neuronal morphology. In this Review, we highlight recent work that has elucidated the functional relationships between transcription factors and the downstream effectors through which they regulate neural connectivity in multiple model systems, with a focus on axon guidance and dendrite morphogenesis.

Intrinsic and extrinsic mechanisms of dendritic morphogenesis

The complex, branched morphology of dendrites is a cardinal feature of neurons and has been used as a criterion for cell type identification since the beginning of neurobiology. Regulated dendritic outgrowth and branching during development form the basis of receptive fields for neurons and are essential for the wiring of the nervous system. The cellular and molecular mechanisms of dendritic morphogenesis have been an intensely studied area. In this review, we summarize the major experimental systems that have contributed to our understandings of dendritic development as well as the intrinsic and extrinsic mechanisms that instruct the neurons to form cell type-specific dendritic arbors.

Double-bromo and extraterminal (BET) domain proteins regulate dendrite morphology and mechanosensory function

A complex array of genetic factors regulates neuronal dendrite morphology. Epigenetic regulation of gene expression represents a plausible mechanism to control pathways responsible for specific dendritic arbor shapes. By studying the Drosophila dendritic arborization (da) neurons, we discovered a role of the double-bromodomain and extraterminal (BET) family proteins in regulating dendrite arbor complexity. A loss-of-function mutation in the single Drosophila BET protein encoded by female sterile 1 homeotic [fs(1)h] causes loss of fine, terminal dendritic branches. Moreover, fs(1)h is necessary for the induction of branching caused by a previously identified transcription factor, Cut (Ct), which regulates subtype-specific dendrite morphology. Finally, disrupting fs(1)h function impairs the mechanosensory response of class III da sensory neurons without compromising the expression of the ion channel NompC, which mediates the mechanosensitive response. Thus, our results identify a novel role for BET family proteins in regulating dendrite morphology and a possible separation of developmental pathways specifying neural cell morphology and ion channel expression. Since the BET proteins are known to bind acetylated histone tails, these results also suggest a role of epigenetic histone modifications and the "histone code," in regulating dendrite morphology.

Developmental shaping of dendritic arbors in Drosophila relies on tightly regulated intra-neuronal activity of protein kinase A (PKA)

Dendrites develop morphologies characterized by multiple levels of complexity that involve neuron type specific dendritic length and particular spatial distribution. How this is developmentally regulated and in particular which signaling molecules are crucial in the process is still not understood. Using Drosophila class IV dendritic arborization (da) neurons we test in vivo the effects of cell-autonomous dose-dependent changes in the activity levels of the cAMP-dependent Protein Kinase A (PKA) on the formation of complex dendritic arbors. We find that genetic manipulations of the PKA activity levels affect profoundly the arbor complexity with strongest impact on distal branches. Both decreasing and increasing PKA activity result in a reduced complexity of the arbors, as reflected in decreased dendritic length and number of branching points, suggesting an inverted U-shape response to PKA. The phenotypes are accompanied by changes in organelle distribution: Golgi outposts and early endosomes in distal dendritic branches are reduced in PKA mutants. By using Rab5 dominant negative we find that PKA interacts genetically with the early endosomal pathway. We test if the possible relationship between PKA and organelles may be the result of phosphorylation of the microtubule motor dynein components or Rab5. We find that Drosophila cytoplasmic dynein components are direct PKA phosphorylation targets in vitro, but not in vivo, thus pointing to a different putative in vivo target. Our data argue that tightly controlled dose-dependent intra-neuronal PKA activity levels are critical in determining the dendritic arbor complexity, one of the possible ways being through the regulation of organelle distribution.

GM130 is required for compartmental organization of dendritic golgi outposts

Golgi complexes (Golgi) play important roles in the development and function of neurons [1-3]. Not only are Golgi present in the neuronal soma (somal Golgi), they also exist in the dendrites as Golgi outposts [4-7]. Previous studies have shown that Golgi outposts serve as local microtubule-organizing centers [8] and secretory stations in dendrites [6, 9]. It is unknown whether the structure and function of Golgi outposts differ from those of somal Golgi. Here we show in Drosophila that, unlike somal Golgi, the biochemically distinct cis, medial, and trans compartments of Golgi are often disconnected in dendrites in vivo. The Golgi structural protein GM130 is responsible for connecting distinct Golgi compartments in soma and dendritic branch points, and the specific distribution of GM130 determines the compartmental organization of dendritic Golgi in dendritic shafts. We further show that compartmental organization regulates the role of Golgi in acentrosomal microtubule growth in dendrites and in dendritic branching. Our study provides insights into the structure and function of dendritic Golgi outposts as well as the regulation of compartmental organization of Golgi in general.

Development of the embryonic and larval peripheral nervous system of Drosophila

The peripheral nervous system (PNS) of embryonic and larval stage Drosophila consists of diverse types of sensory neurons positioned along the body wall. Sensory neurons, and associated end organs, show highly stereotyped locations and morphologies. Many powerful genetic tools for gene manipulation available in Drosophila make the PNS an advantageous system for elucidating basic principles of neural development. Studies of the Drosophila PNS have provided key insights into molecular mechanisms of cell fate specification, asymmetric cell division, and dendritic morphogenesis. A canonical lineage gives rise to sensory neurons and associated organs, and cells within this lineage are diversified through asymmetric cell divisions. Newly specified sensory neurons develop specific dendritic patterns, which are controlled by numerous factors including transcriptional regulators, interactions with neighboring neurons, and intracellular trafficking systems. In addition, sensory axons show modality specific terminations in the central nervous system, which are patterned by secreted ligands and their receptors expressed by sensory axons. Modality-specific axon projections are critical for coordinated larval behaviors. We review the molecular basis for PNS development and address some of the instances in which the mechanisms and molecules identified are conserved in vertebrate development.

Dendrite architecture organized by transcriptional control of the F-actin nucleator Spire

The architectures of dendritic trees are crucial for the wiring and function of neuronal circuits because they determine coverage of receptive territories, as well as the nature and strength of sensory or synaptic inputs. Here, we describe a cell-intrinsic pathway sculpting dendritic arborization (da) neurons in Drosophila that requires Longitudinals Lacking (Lola), a BTB/POZ transcription factor, and its control of the F-actin cytoskeleton through Spire (Spir), an actin nucleation protein. Loss of Lola from da neurons reduced the overall length of dendritic arbors, increased the expression of Spir, and produced inappropriate F-actin-rich dendrites at positions too near the cell soma. Selective removal of Lola from only class IV da neurons decreased the evasive responses of larvae to nociception. The increased Spir expression contributed to the abnormal F-actin-rich dendrites and the decreased nocifensive responses because both were suppressed by reduced dose of Spir. Thus, an important role of Lola is to limit expression of Spir to appropriate levels within da neurons. We found Spir to be expressed in dendritic arbors and to be important for their development. Removal of Spir from class IV da neurons reduced F-actin levels and total branch number, shifted the position of greatest branch density away from the cell soma, and compromised nocifensive behavior. We conclude that the Lola-Spir pathway is crucial for the spatial arrangement of branches within dendritic trees and for neural circuit function because it provides balanced control of the F-actin cytoskeleton.

Cell-intrinsic drivers of dendrite morphogenesis

The proper formation and morphogenesis of dendrites is fundamental to the establishment of neural circuits in the brain. Following cell cycle exit and migration, neurons undergo organized stages of dendrite morphogenesis, which include dendritic arbor growth and elaboration followed by retraction and pruning. Although these developmental stages were characterized over a century ago, molecular regulators of dendrite morphogenesis have only recently been defined. In particular, studies in Drosophila and mammalian neurons have identified numerous cell-intrinsic drivers of dendrite morphogenesis that include transcriptional regulators, cytoskeletal and motor proteins, secretory and endocytic pathways, cell cycle-regulated ubiquitin ligases, and components of other signaling cascades. Here, we review cell-intrinsic drivers of dendrite patterning and discuss how the characterization of such crucial regulators advances our understanding of normal brain development and pathogenesis of diverse cognitive disorders.

Sensory-neuron subtype-specific transcriptional programs controlling dendrite morphogenesis: genome-wide analysis of Abrupt and Knot/Collier

The transcription factors Abrupt (Ab) and Knot (Kn) act as selectors of distinct dendritic arbor morphologies in two classes of Drosophila sensory neurons, termed class I and class IV, respectively. We performed binding-site mapping and transcriptional profiling of these isolated neurons. Their profiles were similarly enriched in cell-type-specific enhancers of genes implicated in neural development. We identified a total of 429 target genes, of which 56 were common to Ab and Kn; these targets included genes necessary to shape dendritic arbors in either or both of the two sensory subtypes. Furthermore, a common target gene, encoding the cell adhesion molecule Ten-m, was expressed more strongly in class I than class IV, and this differential was critical to the class-selective directional control of dendritic branch sprouting or extension. Our analyses illustrate how differentiating neurons employ distinct and shared repertoires of gene expression to produce class-selective morphological traits.

Genome-wide analysis of the bHLH gene family in planarians identifies factors required for adult neurogenesis and neuronal regeneration

In contrast to most well-studied model organisms, planarians have a remarkable ability to completely regenerate a functional nervous system from a pluripotent stem cell population. Thus, planarians provide a powerful model to identify genes required for adult neurogenesis in vivo. We analyzed the basic helix-loop-helix (bHLH) family of transcription factors, many of which are crucial for nervous system development and have been implicated in human diseases. However, their potential roles in adult neurogenesis or central nervous system (CNS) function are not well understood. We identified 44 planarian bHLH homologs, determined their patterns of expression in the animal and assessed their functions using RNAi. We found nine bHLHs expressed in stem cells and neurons that are required for CNS regeneration. Our analyses revealed that homologs of coe, hes (hesl-3) and sim label progenitors in intact planarians, and following amputation we observed an enrichment of coe(+) and sim(+) progenitors near the wound site. RNAi knockdown of coe, hesl-3 or sim led to defects in CNS regeneration, including failure of the cephalic ganglia to properly pattern and a loss of expression of distinct neuronal subtype markers. Together, these data indicate that coe, hesl-3 and sim label neural progenitor cells, which serve to generate new neurons in uninjured or regenerating animals. Our study demonstrates that this model will be useful to investigate how stem cells interpret and respond to genetic and environmental cues in the CNS and to examine the role of bHLH transcription factors in adult tissue regeneration.

Functional genomic analyses of two morphologically distinct classes of Drosophila sensory neurons: post-mitotic roles of transcription factors in dendritic patterning

Background

Neurons are one of the most structurally and functionally diverse cell types found in nature, owing in large part to their unique class specific dendritic architectures. Dendrites, being highly specialized in receiving and processing neuronal signals, play a key role in the formation of functional neural circuits. Hence, in order to understand the emergence and assembly of a complex nervous system, it is critical to understand the molecular mechanisms that direct class specific dendritogenesis.

Methodology/Principal Findings

We have used the Drosophila dendritic arborization (da) neurons to gain systems-level insight into dendritogenesis by a comparative study of the morphologically distinct Class-I (C-I) and Class-IV (C-IV) da neurons. We have used a combination of cell-type specific transcriptional expression profiling coupled to a targeted and systematic in vivo RNAi functional validation screen. Our comparative transcriptomic analyses have revealed a large number of differentially enriched/depleted gene-sets between C-I and C-IV neurons, including a broad range of molecular factors and biological processes such as proteolytic and metabolic pathways. Further, using this data, we have identified and validated the role of 37 transcription factors in regulating class specific dendrite development using in vivo class-specific RNAi knockdowns followed by rigorous and quantitative neurometric analysis.

Conclusions/Significance

This study reports the first global gene-expression profiles from purified Drosophila C-I and C-IV da neurons. We also report the first large-scale semi-automated reconstruction of over 4,900 da neurons, which were used to quantitatively validate the RNAi screen phenotypes. Overall, these analyses shed global and unbiased novel insights into the molecular differences that underlie the morphological diversity of distinct neuronal cell-types. Furthermore, our class-specific gene expression datasets should prove a valuable community resource in guiding further investigations designed to explore the molecular mechanisms underlying class specific neuronal patterning.

Golden goal controls dendrite elongation and branching of multidendritic arborization neurons in Drosophila

Precise refinement of axonal and dendritic patterns is essential for the maturation of functional neuronal circuits. Although several transmembrane molecules have been shown to control the development of both axons and dendrites, the molecular mechanisms that regulate these different processes are poorly understood. Golden Goal (Gogo) is one of the molecules that are known to control the development of axons in the Drosophila visual system. In this study, we analyzed Gogo function in dendritic field formation of dorsal multidendritic arborization (md-da) neurons of the Drosophila Peripheral Nervous System. We showed that Gogo is required to restrain the growth of ddaC dendrites toward the midline in the embryo. During larval stages, Gogo promotes dendritic branching of the complex classIV ddaC neurons. However, over-expression of Gogo restrained dendritic branch formation in ddaC neurons, and this phenotype was enhanced by co-over-expression with Flamingo (Fmi), a partner of Gogo in axon guidance. These results suggest Gogo plays important roles in maintaining homeostasis of dendritic branching. Like axons, the cytoplasmic part of Gogo is required for its function in dendritic tree formation, suggesting that Gogo conveys information from extracellular cues to intracellular molecules that control dendrite development.

Bimodal control of dendritic and axonal growth by the dual leucine zipper kinase pathway

Knowledge of the molecular and genetic mechanisms underlying the separation of dendritic and axonal compartments is not only crucial for understanding the assembly of neural circuits, but also for developing strategies to correct defective dendrites or axons in diseases with subcellular precision. Previous studies have uncovered regulators dedicated to either dendritic or axonal growth. Here we investigate a novel regulatory mechanism that differentially directs dendritic and axonal growth within the same neuron in vivo. We find that the dual leucine zipper kinase (DLK) signaling pathway in Drosophila, which consists of Highwire and Wallenda and controls axonal growth, regeneration, and degeneration, is also involved in dendritic growth in vivo. Highwire, an evolutionarily conserved E3 ubiquitin ligase, restrains axonal growth but acts as a positive regulator for dendritic growth in class IV dendritic arborization neurons in the larva. While both the axonal and dendritic functions of highwire require the DLK kinase Wallenda, these two functions diverge through two downstream transcription factors, Fos and Knot, which mediate the axonal and dendritic regulation, respectively. This study not only reveals a previously unknown function of the conserved DLK pathway in controlling dendrite development, but also provides a novel paradigm for understanding how neuronal compartmentalization and the diversity of neuronal morphology are achieved.

High-resolution in vivo imaging of regenerating dendrites of Drosophila sensory neurons during metamorphosis: local filopodial degeneration and heterotypic dendrite-dendrite contacts

Neuronal circuits that are formed in early development are reorganized at later developmental stages to support a wide range of adult behaviors. At Drosophila pupal stages, one example of this reorganization is dendritic remodeling of multidendritic neurons, which is accomplished by pruning and subsequent regeneration of branches in environments quite distinct from those in larval life. Here, we used long-term in vivo time-lapse recordings at high spatiotemporal resolution and analyzed the dynamics of two adjacent cell types that remodel dendritic arbors, which eventually innervate the lateral plate of the adult abdomen. These neurons initially exhibited dynamic extension, withdrawal and local degeneration of filopodia that sprouted from all along the length of regenerating branches. At a midpupal stage, branches extending from the two cell types started fasciculating with each other, which prompted us to test the hypothesis that this heterotypic contact may serve as a guiding scaffold for shaping dendritic arbors. Unexpectedly, our cell ablation study gave only marginal effects on the branch length and the arbor shape. This result suggests that the arbor morphology of the adult neurons in this study can be specified mostly in the absence of the dendrite–dendrite contact.

Principles of branch dynamics governing shape characteristics of cerebellar Purkinje cell dendrites

Neurons develop dendritic arbors in cell type-specific patterns. Using growing Purkinje cells in culture as a model, we performed a long-term time-lapse observation of dendrite branch dynamics to understand the rules that govern the characteristic space-filling dendrites. We found that dendrite architecture was sculpted by a combination of reproducible dynamic processes, including constant tip elongation, stochastic terminal branching, and retraction triggered by contacts between growing dendrites. Inhibition of protein kinase C/protein kinase D signaling prevented branch retraction and significantly altered the characteristic morphology of long proximal segments. A computer simulation of dendrite branch dynamics using simple parameters from experimental measurements reproduced the time-dependent changes in the dendrite configuration in live Purkinje cells. Furthermore, perturbation analysis to parameters in silico validated the important contribution of dendritic retraction in the formation of the characteristic morphology. We present an approach using live imaging and computer simulations to clarify the fundamental mechanisms of dendrite patterning in the developing brain.

Fascin controls neuronal class-specific dendrite arbor morphology

The branched morphology of dendrites represents a functional hallmark of distinct neuronal types. Nonetheless, how diverse neuronal class-specific dendrite branches are generated is not understood. We investigated specific classes of sensory neurons of Drosophila larvae to address the fundamental mechanisms underlying the formation of distinct branch types. We addressed the function of fascin, a conserved actin-bundling protein involved in filopodium formation, in class III and class IV sensory neurons. We found that the terminal branchlets of different classes of neurons have distinctive dynamics and are formed on the basis of molecularly separable mechanisms; in particular, class III neurons require fascin for terminal branching whereas class IV neurons do not. In class III neurons, fascin controls the formation and dynamics of terminal branchlets. Previous studies have shown that transcription factor combinations define dendrite patterns; we find that fascin represents a downstream component of such programs, as it is a major effector of the transcription factor Cut in defining class III-specific dendrite morphology. Furthermore, fascin defines the morphological distinction between class III and class IV neurons. In fact, loss of fascin function leads to a partial conversion of class III neurons to class IV characteristics, while the reverse effect is obtained by fascin overexpression in class IV neurons. We propose that dedicated molecular mechanisms underlie the formation and dynamics of distinct dendrite branch types to elicit the accurate establishment of neuronal circuits.

Dendritic structural plasticity

Dendrites represent the compartment of neurons primarily devoted to collecting and computating input. Far from being static structures, dendrites are highly dynamic during development and appear to be capable of plastic changes during the adult life of animals. During development, it is a combination of intrinsic programs and external signals that shapes dendrite morphology; input activity is a conserved extrinsic factor involved in this process. In adult life, dendrites respond with more modest modifications of their structure to various types of extrinsic information, including alterations of input activity. Here, the author reviews classical and recent evidence of dendrite plasticity in invertebrates and vertebrates and current progress in the understanding of the molecular mechanisms that underlie this plasticity. Importantly, some fundamental questions such as the functional role of dendrite remodeling and the causal link between structural modifications of neurons and plastic processes, including learning, are still open.

Control of dendritic morphogenesis by Trio in Drosophila melanogaster

Abl tyrosine kinase and its effectors among the Rho family of GTPases each act to control dendritic morphogenesis in Drosophila. It has not been established, however, which of the many GTPase regulators in the cell link these signaling molecules in the dendrite. In axons, the bifunctional guanine exchange factor, Trio, is an essential link between the Abl tyrosine kinase signaling pathway and Rho GTPases, particularly Rac, allowing these systems to act coordinately to control actin organization. In dendritic morphogenesis, however, Abl and Rac have contrary rather than reinforcing effects, raising the question of whether Trio is involved, and if so, whether it acts through Rac, Rho or both. We now find that Trio is expressed in sensory neurons of the Drosophila embryo and regulates their dendritic arborization. trio mutants display a reduction in dendritic branching and increase in average branch length, whereas over-expression of trio has the opposite effect. We further show that it is the Rac GEF domain of Trio, and not its Rho GEF domain that is primarily responsible for the dendritic function of Trio. Thus, Trio shapes the complexity of dendritic arbors and does so in a way that mimics the effects of its target, Rac.

Candidate molecular mechanisms for establishing cell identity in the developing retina

In the developing nervous system, individual neurons must occupy appropriate positions within circuits. This requires that these neurons recognize and form connections with specific pre- and postsynaptic partners. Cellular recognition is also required for the spacing of cell bodies and the arborization of dendrites, factors that determine the inputs onto a given neuron. These issues are particularly evident in the retina, where different types of neurons are evenly spaced relative to other cells of the same type. This establishes a reiterated columnar circuitry resembling the insect retina. Establishing these mosaic patterns requires that cells of a given type (homotypic cells) be able to sense their neighbors. Therefore, both synaptic specificity and mosaic spacing require cellular identifiers. In synaptic specificity, recognition often occurs between different types of cells in a pre- and postsynaptic pairing. In mosaic spacing, recognition is often occurring between different cells of the same type, orhomotypic self-recognition. Dendritic arborization can require recognition of different neurites of the same cell, or isoneuronal self-recognition. The retina is an extremely amenable system for studying the molecular identifiers that drive these various forms of recognition. The different neuronal types in the retina are well defined, and the genetic tools for marking cell types are increasingly available. In this review we will summarize retinal anatomy and describe cell types in the retina and how they are defined. We will then describe the requirements of a recognition code and discuss newly emerging candidate molecular mechanisms for recognition that may meet these requirements.

Convergent local identity and topographic projection of sensory neurons

Development of sensory neural circuits requires concurrent specification of neuron modality, position, and topographic projections. However, little is understood about how controls over these distinct parameters can unify in a single developmental sequence. To address this question, we have used the nociceptive class IV dendritic arborization neurons in the Drosophila larval body wall as an excellent model that allows precise spatiotemporal dissection of developmental-genetic control over sensory neuron positioning and wiring, and subsequent analysis of its functional significance for sensorimotor behavior. The class IV neurogenetic program is intrinsic to the anterior domain of the embryonic parasegment epithelium. Along the ventrolateral axis of this domain, nociceptive neuron induction requirements depend upon location. Near the ventral midline, both Hedgehog and Epithelial growth factor receptor signaling are required for class IV neurogenesis. In addition, close to the ventral midline, class IV neurogenesis is preceded by expression of the Iroquois factor Mirror that promotes local nociceptive neuron differentiation. Remarkably, Mirror is also required for the proper routing of class IV topographic axonal projections across the midline of the CNS. Manipulation of Mirror activity in class IV neurons retargeted axonal projections and caused concordant changes in larval nociceptive escape behavior. These findings indicate that convergent sensory neuron specification, local differentiation, and topographic wiring are mediated by Mirror, and they suggest an integrated paradigm for position-sensitive neural development.

Different levels of the Tripartite motif protein, Anomalies in sensory axon patterning (Asap), regulate distinct axonal projections of Drosophila sensory neurons

The axonal projection pattern of sensory neurons typically is regulated by environmental signals, but how different sensory afferents can establish distinct projections in the same environment remains largely unknown. Drosophila class IV dendrite arborization (C4da) sensory neurons project subtype-specific axonal branches in the ventral nerve cord, and we show that the Tripartite motif protein, Anomalies in sensory axon patterning (Asap) is a critical determinant of the axonal projection patterns of different C4da neurons. Asap is highly expressed in C4da neurons with both ipsilateral and contralateral axonal projections, but the Asap level is low in neurons that have only ipsilateral projections. Mutations in asap cause a specific loss of contralateral projections, whereas overexpression of Asap induces ectopic contralateral projections in C4da neurons. We also show by biochemical and genetic analysis that Asap regulates Netrin signaling, at least in part by linking the Netrin receptor Frazzled to the downstream effector Pico. In the absence of Asap, the sensory afferent connectivity within the ventral nerve cord is disrupted, resulting in specific larval behavioral deficits. These results indicate that different levels of Asap determine distinct patterns of axonal projections of C4da neurons by modulating Netrin signaling and that the Asap-mediated axonal projection is critical for assembly of a functional sensory circuit.

Morphological analysis of Drosophila larval peripheral sensory neuron dendrites and axons using genetic mosaics

Nervous system development requires the correct specification of neuron position and identity, followed by accurate neuron class-specific dendritic development and axonal wiring. Recently the dendritic arborization (DA) sensory neurons of the Drosophila larval peripheral nervous system (PNS) have become powerful genetic models in which to elucidate both general and class-specific mechanisms of neuron differentiation. There are four main DA neuron classes (I-IV)(1). They are named in order of increasing dendrite arbor complexity, and have class-specific differences in the genetic control of their differentiation(2-10). The DA sensory system is a practical model to investigate the molecular mechanisms behind the control of dendritic morphology(11-13) because: 1) it can take advantage of the powerful genetic tools available in the fruit fly, 2) the DA neuron dendrite arbor spreads out in only 2 dimensions beneath an optically clear larval cuticle making it easy to visualize with high resolution in vivo, 3) the class-specific diversity in dendritic morphology facilitates a comparative analysis to find key elements controlling the formation of simple vs. highly branched dendritic trees, and 4) dendritic arbor stereotypical shapes of different DA neurons facilitate morphometric statistical analyses. DA neuron activity modifies the output of a larval locomotion central pattern generator(14-16). The different DA neuron classes have distinct sensory modalities, and their activation elicits different behavioral responses(14,16-20). Furthermore different classes send axonal projections stereotypically into the Drosophila larval central nervous system in the ventral nerve cord (VNC)(21). These projections terminate with topographic representations of both DA neuron sensory modality and the position in the body wall of the dendritic field(7,22,23). Hence examination of DA axonal projections can be used to elucidate mechanisms underlying topographic mapping(7,22,23), as well as the wiring of a simple circuit modulating larval locomotion(14-17). We present here a practical guide to generate and analyze genetic mosaics(24) marking DA neurons via MARCM (Mosaic Analysis with a Repressible Cell Marker)(1,10,25) and Flp-out(22,26,27) techniques (summarized in Fig. 1).

The seven-pass transmembrane cadherin Flamingo controls dendritic self-avoidance via its binding to a LIM domain protein, Espinas, in Drosophila sensory neurons

Members of the Flamingo cadherin family are required in a number of different in vivo contexts of neural development. Even so, molecular identities downstream from the family have been poorly understood. Here we show that a LIM domain protein, Espinas (Esn), binds to an intracellular juxtamembrane domain of Flamingo (Fmi), and that this Fmi-Esn interplay elicits repulsion between dendritic branches of Drosophila sensory neurons. In wild-type larvae, branches of the same class IV dendritic arborization neuron achieve efficient coverage of its two-dimensional receptive field with minimum overlap with each other. However, this self-avoidance was disrupted in a fmi hypomorphic mutant, in an esn knockout homozygote, and in the fmi/esn trans-heterozygote. A functional fusion protein, Fmi:3eGFP, was localized at most of the branch tips, and in a heterologous system, assembly of Esn at cell contact sites required its LIM domain and Fmi. We further show that genes controlling epithelial planar cell polarity (PCP), such as Van Gogh (Vang) and RhoA, are also necessary for the self-avoidance, and that fmi genetically interacts with these loci. On the basis of these and other results, we propose that the Fmi-Esn complex, together with the PCP regulators and the Tricornered (Trc) signaling pathway, executes the repulsive interaction between isoneuronal dendritic branches.

How does morphology relate to function in sensory arbors?

Sensory dendrites fall into many different morphological and functional classes. Polymodal nociceptors are one subclass of sensory neurons, which are of particular note owing to their elaborate dendritic arbors. Complex developmental programs are required to form these arbors and there is striking conservation of morphology, function and molecular determinants between vertebrate and invertebrate polymodal nociceptors. Based on these studies, we argue that arbor morphology plays an important role in the function of polymodal nociceptors. Similar associations between form and function might explain the plethora of dendrite morphologies seen among all sensory neurons.

Turtle functions downstream of Cut in differentially regulating class specific dendrite morphogenesis in Drosophila

Background

Dendritic morphology largely determines patterns of synaptic connectivity and electrochemical properties of a neuron. Neurons display a myriad diversity of dendritic geometries which serve as a basis for functional classification. Several types of molecules have recently been identified which regulate dendrite morphology by acting at the levels of transcriptional regulation, direct interactions with the cytoskeleton and organelles, and cell surface interactions. Although there has been substantial progress in understanding the molecular mechanisms of dendrite morphogenesis, the specification of class-specific dendritic arbors remains largely unexplained. Furthermore, the presence of numerous regulators suggests that they must work in concert. However, presently, few genetic pathways regulating dendrite development have been defined.

Methodology/Principal Findings

The Drosophila gene turtle belongs to an evolutionarily conserved class of immunoglobulin superfamily members found in the nervous systems of diverse organisms. We demonstrate that Turtle is differentially expressed in Drosophila da neurons. Moreover, MARCM analyses reveal Turtle acts cell autonomously to exert class specific effects on dendritic growth and/or branching in da neuron subclasses. Using transgenic overexpression of different Turtle isoforms, we find context-dependent, isoform-specific effects on mediating dendritic branching in class II, III and IV da neurons. Finally, we demonstrate via chromatin immunoprecipitation, qPCR, and immunohistochemistry analyses that Turtle expression is positively regulated by the Cut homeodomain transcription factor and via genetic interaction studies that Turtle is downstream effector of Cut-mediated regulation of da neuron dendrite morphology.

Conclusions/Significance

Our findings reveal that Turtle proteins differentially regulate the acquisition of class-specific dendrite morphologies. In addition, we have established a transcriptional regulatory interaction between Cut and Turtle, representing a novel pathway for mediating class specific dendrite development.

Coe genes are expressed in differentiating neurons in the central nervous system of protostomes

Genes of the coe (collier/olfactory/early B-cell factor) family encode Helix-Loop-Helix transcription factors that are widely conserved in metazoans and involved in many developmental processes, neurogenesis in particular. Whereas their functions during vertebrate neural tube formation have been well documented, very little is known about their expression and role during central nervous system (CNS) development in protostomes. Here we characterized the CNS expression of coe genes in the insect Drosophila melanogaster and the polychaete annelid Platynereis dumerilii, which belong to different subgroups of protostomes and show strikingly different modes of development. In the Drosophila ventral nerve cord, we found that the Collier-expressing cells form a subpopulation of interneurons with diverse molecular identities and neurotransmitter phenotypes. We also demonstrate that collier is required for the proper differentiation of some interneurons belonging to the Eve-Lateral cluster. In Platynereis dumerilii, we cloned a single coe gene, Pdu-coe, and found that it is exclusively expressed in post mitotic neural cells. Using an original technique of in silico 3D registration, we show that Pdu-coe is co-expressed with many different neuronal markers and therefore that, like in Drosophila, its expression defines a heterogeneous population of neurons with diverse molecular identities. Our detailed characterization and comparison of coe gene expression in the CNS of two distantly-related protostomes suggest conserved roles of coe genes in neuronal differentiation in this clade. As similar roles have also been observed in vertebrates, this function was probably already established in the last common ancestor of all bilaterians.

Differential regulation of dendritic and axonal development by the novel Krüppel-like factor Dar1

Dendrites and axons are two major neuronal compartments with differences that are critical for neuronal functions. To learn about the differential regulation of dendritic and axonal development, we conducted a genetic screen in Drosophila and isolated the dendritic arbor reduction 1 (dar1) mutants, which display defects in dendritic but not axonal growth. The dar1 gene encodes a novel transcription regulator in the Krüppel-like factor family. Neurons lacking dar1 function have severely reduced growth of microtubule- but not F-actin-based dendritic branches. In contrast, overexpression of Dar1 dramatically increased the growth of microtubule-based dendritic branches. Our results suggest that Dar1 promotes dendrite growth in part by suppressing the expression of the microtubule-severing protein Spastin. Our study thus uncovers a novel transcriptional program for microtubule regulation that preferentially controls dendrite growth.