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

ROR and RYK extracellular region structures suggest that receptor tyrosine kinases have distinct WNT-recognition modes

WNTs play key roles in development and disease, signaling through Frizzled (FZD) seven-pass transmembrane receptors and numerous co-receptors including ROR and RYK family receptor tyrosine kinases (RTKs). We describe crystal structures and WNT-binding characteristics of extracellular regions from the Drosophila ROR and RYK orthologs Nrk (neurospecific receptor tyrosine kinase) and Derailed-2 (Drl-2), which bind WNTs though a FZD-related cysteine-rich domain (CRD) and WNT-inhibitory factor (WIF) domain respectively. Our crystal structures suggest that neither Nrk nor Drl-2 can accommodate the acyl chain typically attached to WNTs. The Nrk CRD contains a deeply buried bound fatty acid, unlikely to be exchangeable. The Drl-2 WIF domain lacks the lipid-binding site seen in WIF-1. We also find that recombinant DWnt-5 can bind Drosophila ROR and RYK orthologs despite lacking an acyl chain. Alongside analyses of WNT/receptor interaction sites, our structures provide further insight into how WNTs may recruit RTK co-receptors into signaling complexes.

Extrinsic Factors Regulating Dendritic Patterning

Stereotypic dendrite arborizations are key morphological features of neuronal identity, as the size, shape and location of dendritic trees determine the synaptic input fields and how information is integrated within developed neural circuits. In this review, we focus on the actions of extrinsic intercellular communication factors and their effects on intrinsic developmental processes that lead to dendrite patterning. Surrounding neurons or supporting cells express adhesion receptors and secreted proteins that respectively, act via direct contact or over short distances to shape, size, and localize dendrites during specific developmental stages. The different ligand-receptor interactions and downstream signaling events appear to direct dendrite morphogenesis by converging on two categorical mechanisms: local cytoskeletal and adhesion modulation and global transcriptional regulation of key dendritic growth components, such as lipid synthesis enzymes. Recent work has begun to uncover how the coordinated signaling of multiple extrinsic factors promotes complexity in dendritic trees and ensures robust dendritic patterning.

Interplay between axonal Wnt5-Vang and dendritic Wnt5-Drl/Ryk signaling controls glomerular patterning in the Drosophila antennal lobe

Despite the importance of dendritic targeting in neural circuit assembly, the mechanisms by which it is controlled still remain incompletely understood. We previously showed that in the developing Drosophila antennal lobe, the Wnt5 protein forms a gradient that directs the ~45˚ rotation of a cluster of projection neuron (PN) dendrites, including the adjacent DA1 and VA1d dendrites. We report here that the Van Gogh (Vang) transmembrane planar cell polarity (PCP) protein is required for the rotation of the DA1/VA1d dendritic pair. Cell type-specific rescue and mosaic analyses showed that Vang functions in the olfactory receptor neurons (ORNs), suggesting a codependence of ORN axonal and PN dendritic targeting. Loss of Vang suppressed the repulsion of the VA1d dendrites by Wnt5, indicating that Wnt5 signals through Vang to direct the rotation of the DA1 and VA1d glomeruli. We observed that the Derailed (Drl)/Ryk atypical receptor tyrosine kinase is also required for the rotation of the DA1/VA1d dendritic pair. Antibody staining showed that Drl/Ryk is much more highly expressed by the DA1 dendrites than the adjacent VA1d dendrites. Mosaic and epistatic analyses showed that Drl/Ryk specifically functions in the DA1 dendrites in which it antagonizes the Wnt5-Vang repulsion and mediates the migration of the DA1 glomerulus towards Wnt5. Thus, the nascent DA1 and VA1d glomeruli appear to exhibit Drl/Ryk-dependent biphasic responses to Wnt5. Our work shows that the final patterning of the fly olfactory map is the result of an interplay between ORN axons and PN dendrites, wherein converging pre- and postsynaptic processes contribute key Wnt5 signaling components, allowing Wnt5 to orient the rotation of nascent synapses through a PCP mechanism.

During brain development, the processes of nerve cells, axons and dendrites, grow over long distances to find and connect with each other to form synapses in precise locations. Understanding the mechanisms that control the growth of these neurites is important for understanding normal brain functions like neuronal plasticity and neural diseases like autism. Although much progress has been made by studying the development of axons and dendrites separately, the mechanisms that guide neuronal processes to their final locations are still incompletely understood. In particular, careful observation of converging pre- and postsynaptic processes suggests that their targeting may be coordinated. Whether the final targeting of axons and dendrites are functionally linked and what molecular mechanisms may be involved are unknown. In this paper we show that, in the developing Drosophila olfactory circuit, coalescing axons and dendrites respond to the extracellular Wnt5 signal in a codependent manner. We demonstrate that the converging axons and dendrites contribute different signaling components to the Wnt5 pathway, the Vang Gogh and Derailed transmembrane receptors respectively, which allow Wnt5 to coordinately guide the targeting of the neurites. Our work thus reveals a novel mechanism of neural circuit patterning and the molecular mechanism that controls it.

Differential Changes in the Number and Morphology of the New Neurons after Chronic Infusion of Wnt7a, Wnt5a, and Dkk-1 in the Adult Hippocampus In Vivo

In the adult hippocampus of many mammals, a particular microenvironment in the neurogenic niche regulates the proliferation, self-renewal, and differentiation of neuronal stem cells. In this proliferative niche, a variety of molecules provide a finely regulated molecular signaling that controls stem cell properties. During development, Wnt signaling has been implicated in cell fate determination and proliferation, in the establishment of cell polarity, as well as a cue for axonal growth and dendrite orientation. In the adult brain, this pathway also participates in the stem cell self-renewal and neuronal differentiation. However, the effects of the chronic Wnt signaling modulation in the adult hippocampus, through the infusion of Wnt7a, Wnt5a, and Dkk-1, on the rate of neurogenesis and on the induction of neurite arborization have not been studied. In this study, we show that Wnt7a and Wn5a further increased the rate of newly generated neurons. However, Wnt5a exerted additional effects by promoting neurite growth and neurite misorientation in the dentate gyrus of adult rats. The chronic exposure to Dkk-1 also generated aberrant location of growing neurites. These results suggest that the interplay of canonical and non-canonical Wnt ligands participates in neuronal stem cell proliferation and in the establishment of proper neurite maturation. Anat Rec, 302:1647-1657, 2019. © 2019 American Association for Anatomy.

The Wnt receptor Ryk is a negative regulator of mammalian dendrite morphogenesis

The unique dendritic architecture of a given neuronal subtype determines its synaptic connectivity and ability to integrate into functional neuronal networks. It is now clear that abnormal dendritic structure is associated with neuropsychiatric and neurodegenerative disorders. Currently, however, the nature of the extrinsic factors that limit dendritic growth and branching within predetermined boundaries in the mammalian brain is poorly understood. Here we identify the Wnt receptor Ryk as a novel negative regulator of dendritic arborisation. We demonstrate that loss of Ryk in mouse hippocampal and cortical neurons promotes excessive dendrite growth and branching in vitro. Conversely, overexpression of wildtype Ryk restricts these processes, confirming that Ryk acts to restrain dendrite arborisation. Furthermore, we identify a hitherto uncharacterized membrane proximal subdomain crucial for Ryk-mediated suppression of dendrite morphogenesis, suggesting that it may act through a novel signalling pathway to constrain dendrite complexity. We also demonstrate that Ryk performs a similar function in vivo as Ryk haploinsufficient postnatal animals exhibit excessive dendrite growth and branching in layer 2/3 pyramidal neurons of the somatosensory cortex. These findings reveal an essential role for Ryk in regulating dendrite complexity and raise the intriguing possibility that it may influence neural plasticity by modifying dendritic structure.

Semaphorin-1a prevents Drosophila olfactory projection neuron dendrites from mis-targeting into select antennal lobe regions

Elucidating how appropriate neurite patterns are generated in neurons of the olfactory system is crucial for comprehending the construction of the olfactory map. In the Drosophila olfactory system, projection neurons (PNs), primarily derived from four neural stem cells (called neuroblasts), populate their cell bodies surrounding to and distribute their dendrites in distinct but overlapping patterns within the primary olfactory center of the brain, the antennal lobe (AL). However, it remains unclear whether the same molecular mechanisms are employed to generate the appropriate dendritic patterns in discrete AL glomeruli among PNs produced from different neuroblasts. Here, by examining a previously explored transmembrane protein Semaphorin-1a (Sema-1a) which was proposed to globally control initial PN dendritic targeting along the dorsolateral-to-ventromedial axis of the AL, we discover a new role for Sema-1a in preventing dendrites of both uni-glomerular and poly-glomerular PNs from aberrant invasion into select AL regions and, intriguingly, this Sema-1a-deficient dendritic mis-targeting phenotype seems to associate with the origins of PNs from which they are derived. Further, ectopic expression of Sema-1a resulted in PN dendritic mis-projection from a select AL region into adjacent glomeruli, strengthening the idea that Sema-1a plays an essential role in preventing abnormal dendritic accumulation in select AL regions. Taken together, these results demonstrate that Sema-1a repulsion keeps dendrites of different types of PNs away from each other, enabling the same types of PN dendrites to be sorted into destined AL glomeruli and permitting for functional assembly of olfactory circuitry.

In the Drosophila olfactory system, olfactory projection neurons (PNs) are derived from four neural stem cells (called neuroblasts) during the development. Intriguingly, these PNs generate complex dendritic patterns within the primary olfactory center of the brain, the antennal lobe (AL), to relay odorant information from olfactory sensory neurons in the periphery to neurons in higher olfactory centers. In this study, we investigate how various types of PNs use a repulsive transmembrane protein Semaphorin-1a (Sema-1a) to establish appropriate dendritic patterns within the AL. Previously, Sema-1a was proposed to globally control initial PN dendritic targeting along the dorsolateral-to-ventromedial axis of the AL. In contrast, we disclose an unknown role of Sema-1a, in which this neuronal protein acts to keep dendrites of various types of PNs produced from different neuroblasts away from select AL regions, thereby enabling the dendrites of the same types of PNs to sort correctly into destined glomeruli within the developing AL for assembly of the functional olfactory neural circuitry.

Epithelia-derived wingless regulates dendrite directional growth of drosophila ddaE neuron through the Fz-Fmi-Dsh-Rac1 pathway

Background

Proper dendrite patterning is critical for the receiving and processing of information in the nervous system. Cell-autonomous molecules have been extensively studied in dendrite morphogenesis; however, the regulatory mechanisms of environmental factors in dendrite growth remain to be elucidated.

Results

By evaluating the angle between two primary dendrites (PD-Angle), we found that the directional growth of the primary dendrites of a Drosophila periphery sensory neuron ddaE is regulated by the morphogen molecule Wingless (Wg). During the early stage of dendrite growth, Wg is expressed in a group of epithelial cells posteriorly adjacent to ddaE. When Wg expression is reduced or shifted anteriorly, the PD-Angle is markedly decreased. Furthermore, Wg receptor Frizzled functions together with Flamingo and Dishevelled in transducing the Wg signal into ddaE neuron, and the downstream signal is mediated by non-canonical Wnt pathway through Rac1.

Conclusions

In conclusion, we reveal that epithelia-derived Wg plays a repulsive role in regulating the directional growth of dendrites through the non-canonical Wnt pathway. Thus, our findings provide strong in vivo evidence on how environmental signals serve as spatial cues for dendrite patterning.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0228-0) contains supplementary material, which is available to authorized users.

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.

Development of dendritic form and function

The nervous system is populated by numerous types of neurons, each bearing a dendritic arbor with a characteristic morphology. These type-specific features influence many aspects of a neuron's function, including the number and identity of presynaptic inputs and how inputs are integrated to determine firing properties. Here, we review the mechanisms that regulate the construction of cell type-specific dendrite patterns during development. We focus on four aspects of dendrite patterning that are particularly important in determining the function of the mature neuron: (a) dendrite shape, including branching pattern and geometry of the arbor; (b) dendritic arbor size;

Regulation of dendrite morphogenesis by extrinsic cues

Dendrites play a central role in the integration and flow of information in the nervous system. The morphogenesis and maturation of dendrites is hence an essential step in the establishment of neuronal connectivity. Recent studies have uncovered crucial functions for extrinsic cues in the development of dendrites. We review the contribution of secreted polypeptide growth factors, contact-mediated proteins, and neuronal activity in distinct phases of dendrite development. We also highlight how extrinsic cues influence local and global intracellular mechanisms of dendrite morphogenesis. Finally, we discuss how these studies have advanced our understanding of neuronal connectivity and have shed light on the pathogenesis of neurodevelopmental disorders.