The membrane protein Raw regulates dendrite pruning via the secretory pathway

Recent progress in dendritic pruning of Drosophila C4da sensory neurons

The brain can adapt to changes in the environment through alterations in the number and structure of synapses. During embryonic and early postnatal stages, the synapses in the brain undergo rapid expansion and interconnections to form circuits. However, many of these synaptic connections are redundant or incorrect. Neurite pruning is a conserved process that occurs during both vertebrate and invertebrate development. It requires precise spatiotemporal control of local degradation of cellular components, comprising cytoskeletons and membranes, refines neuronal circuits, and ensures the precise connectivity required for proper function. The Drosophila's class IV dendritic arborization (C4da) sensory neuron has a well-characterized architecture and undergoes dendrite-specific sculpting, making it a valuable model for unravelling the intricate regulatory mechanisms underlie dendritic pruning. In this review, I attempt to provide an overview of the present state of research on dendritic pruning in C4da sensory neurons, as well as potential functional mechanisms in neurodevelopmental disorders.

Opposing roles of Fos, Raw, and SARM1 in the regulation of axonal degeneration and synaptic structure

Introduction

The degeneration of injured axons is driven by conserved molecules, including the sterile armadillo TIR domain-containing protein SARM1, the cJun N-terminal kinase JNK, and regulators of these proteins. These molecules are also implicated in the regulation of synapse development though the mechanistic relationship of their functions in degeneration vs. development is poorly understood.

Results and discussion

Here, we uncover disparate functional relationships between SARM1 and the transmembrane protein Raw in the regulation of Wallerian degeneration and synaptic growth in motoneurons of Drosophila melanogaster. Our genetic data suggest that Raw antagonizes the downstream output MAP kinase signaling mediated by Drosophila SARM1 (dSarm). This relationship is revealed by dramatic synaptic overgrowth phenotypes at the larval neuromuscular junction when motoneurons are depleted for Raw or overexpress dSarm. While Raw antagonizes the downstream output of dSarm to regulate synaptic growth, it shows an opposite functional relationship with dSarm for axonal degeneration. Loss of Raw leads to decreased levels of dSarm in axons and delayed axonal degeneration that is rescued by overexpression of dSarm, supporting a model that Raw promotes the activation of dSarm in axons. However, inhibiting Fos also decreases dSarm levels in axons but has the opposite outcome of enabling Wallerian degeneration. The combined genetic data suggest that Raw, dSarm, and Fos influence each other's functions through multiple points of regulation to control the structure of synaptic terminals and the resilience of axons to degeneration.

Coordination of Pickpocket ion channel delivery and dendrite growth in Drosophila sensory neurons

Sensory neurons enable an organism to perceive external stimuli, which is essential for survival. The sensory capacity of a neuron depends on the elaboration of its dendritic arbor and the localization of sensory ion channels to the dendritic membrane. However, it is not well understood when and how ion channels localize to growing sensory dendrites and whether their delivery is coordinated with growth of the dendritic arbor. We investigated the localization of the DEG/ENaC/ASIC ion channel Pickpocket (Ppk) in the peripheral sensory neurons of developing fruit flies. We used CRISPR-Cas9 genome engineering approaches to tag endogenous Ppk1 and visualize it live, including monitoring Ppk1 membrane localization via a novel secreted split-GFP approach. Fluorescently tagged endogenous Ppk1 localizes to dendrites, as previously reported, and, unexpectedly, to axons and axon terminals. In dendrites, Ppk1 is present throughout actively growing dendrite branches and is stably integrated into the neuronal cell membrane during the expansive growth of the arbor. Although Ppk channels are dispensable for dendrite growth, we found that an over-active channel mutant severely reduces dendrite growth, likely by acting at an internal membrane and not the dendritic membrane. Our data reveal that the molecular motor dynein and recycling endosome GTPase Rab11 are needed for the proper trafficking of Ppk1 to dendrites. Based on our data, we propose that Ppk channel transport is coordinated with dendrite morphogenesis, which ensures proper ion channel density and distribution in sensory dendrites.

Droj2 Facilitates Somatosensory Neurite Sculpting via GTP-Binding Protein Arf102F in Drosophila

Developmental remodeling of neurite is crucial for the accurate wiring of neural circuits in the developing nervous system in both vertebrates and invertebrates, and may also contribute to the pathogenesis of neuropsychiatric disorders, for instance, autism, Alzheimer's disease (AD), and schizophrenia. However, the molecular underpinnings underlying developmental remodeling are still not fully understood. Here, we have identified DnaJ-like-2 (Droj2), orthologous to human DNAJA1 and DNAJA4 that is predicted to be involved in protein refolding, as a developmental signal promoting dendrite sculpting of the class IV dendritic arborization (C4da) sensory neuron in Drosophila. We further show that Arf102F, a GTP-binding protein previously implicated in protein trafficking, serves downstream of Droj2 to govern neurite pruning of C4da sensory neurons. Intriguingly, our data consistently demonstrate that both Droj2 and Arf102F promote the downregulation of the conserved L1-type cell-adhesion molecule Neuroglian anterior to dendrite pruning. Mechanistically, Droj2 genetically interacts with Arf102F and promotes Neuroglian downregulation to initiate dendrite severing. Taken together, this systematic study sheds light on an unprecedented function of Droj2 and Arf102F in neuronal development.

Establishment of a novel axon pruning model of Drosophila motor neuron

Developmental neuronal pruning is a process by which neurons selectively remove excessive or unnecessary neurite without causing neuronal death. Importantly, this process is widely used for the refinement of neural circuits in both vertebrates and invertebrates, and may also contribute to the pathogenesis of neuropsychiatric disorders, such as autism and schizophrenia. In the peripheral nervous system (PNS), class IV dendritic arborization (da) sensory neurons of Drosophila, selectively remove the dendrites without losing their somas and axons, while the dendrites and axons of mushroom body (MB) γ neuron in the central nervous system (CNS) are eliminated by localized fragmentation during metamorphosis. Alternatively, dendrite pruning of ddaC neurons is usually investigated via live-cell imaging, while dissection and fixation are currently used for evaluating MB γ neuron axon pruning. Thus, an excellent model system to assess axon specific pruning directly via live-cell imaging remains elusive. Here, we report that the Drosophila motor neuron offers a unique advantage for studying axon pruning. Interestingly, we uncover that long-range projecting axon bundle from soma at ventral nerve cord (VNC), undergoes degeneration rather than retraction during metamorphosis. Strikingly, the pruning process of the motor axon bundle is straightforward to investigate via live imaging and it occurs approximately at 22 h after pupal formation (APF), when axon bundles are completely cleared. Consistently, the classical axon pruning regulators in the Drosophila MB γ neuron, including TGF-β signaling, ecdysone signaling, JNK signaling, and the ubiquitin-proteasome system are also involved in governing motor axon pruning. Finally, our findings establish an unprecedented axon pruning mode that will serve to systematically screen and identify undiscovered axon pruning regulators. This article has an associated First Person interview with the first author of the paper.

Spatiotemporal Control of Neuronal Remodeling by Cell Adhesion Molecules: Insights From Drosophila

Developmental neuronal remodeling is required for shaping the precise connectivity of the mature nervous system. Remodeling involves pruning of exuberant neural connections, often followed by regrowth of adult-specific ones, as a strategy to refine neural circuits. Errors in remodeling are associated with neurodevelopmental disorders such as schizophrenia and autism. Despite its fundamental nature, our understanding of the mechanisms governing neuronal remodeling is far from complete. Specifically, how precise spatiotemporal control of remodeling and rewiring is achieved is largely unknown. In recent years, cell adhesion molecules (CAMs), and other cell surface and secreted proteins of various families, have been implicated in processes of neurite pruning and wiring specificity during circuit reassembly. Here, we review some of the known as well as speculated roles of CAMs in these processes, highlighting recent advances in uncovering spatiotemporal aspects of regulation. Our focus is on the fruit fly Drosophila, which is emerging as a powerful model in the field, due to the extensive, well-characterized and stereotypic remodeling events occurring throughout its nervous system during metamorphosis, combined with the wide and constantly growing toolkit to identify CAM binding and resulting cellular interactions in vivo. We believe that its many advantages pose Drosophila as a leading candidate for future breakthroughs in the field of neuronal remodeling in general, and spatiotemporal control by CAMs specifically.