Glial-derived TNF/Eiger signaling promotes somatosensory neurite sculpting

The selective elimination of inappropriate projections is essential for sculpting neural circuits during development. The class IV dendritic arborization (C4da) sensory neurons of Drosophila remodel the dendritic branches during metamorphosis. Glial cells in the central nervous system (CNS), are required for programmed axonal pruning of mushroom body (MB) γ neurons during metamorphosis in Drosophila. However, it is entirely unknown whether the glial cells are involved in controlling the neurite pruning of C4da sensory neurons. Here, we show that glial deletion of Eiger (Egr), orthologous to mammalian tumor necrosis factor TNF superfamily ligand, results in dendrite remodeling deficiency of Drosophila C4da sensory neurons. Moreover, the attenuation of neuronal Wengen (Wgn) and Grindelwald (Grnd), the receptors for TNF ligands, is also examined for defects in dendrite remodeling. We further discover that Wgn and Grnd facilitate dendrite elimination through the JNK Signaling. Overall, our findings demonstrate that glial-derived Egr signal links to the neuronal receptor Wgn/Grnd, activating the JNK signaling pathway and promoting developmental neuronal remodeling. Remarkably, our findings reveal a crucial role of peripheral glia in dendritic pruning of C4da neurons.

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.

The organization and function of the Golgi apparatus in dendrite development and neurological disorders

Dendrites are specialized neuronal compartments that sense, integrate and transfer information in the neural network. Their development is tightly controlled and abnormal dendrite morphogenesis is strongly linked to neurological disorders. While dendritic morphology ranges from relatively simple to extremely complex for a specified neuron, either requires a functional secretory pathway to continually replenish proteins and lipids to meet dendritic growth demands. The Golgi apparatus occupies the center of the secretory pathway and is regulating posttranslational modifications, sorting, transport, and signal transduction, as well as acting as a non-centrosomal microtubule organization center. The neuronal Golgi apparatus shares common features with Golgi in other eukaryotic cell types but also forms distinct structures known as Golgi outposts that specifically localize in dendrites. However, the organization and function of Golgi in dendrite development and its impact on neurological disorders is just emerging and so far lacks a systematic summary. We describe the organization of the Golgi apparatus in neurons, review the current understanding of Golgi function in dendritic morphogenesis, and discuss the current challenges and future directions.

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.

The GARP complex prevents sterol accumulation at the trans-Golgi network during dendrite remodeling

Membrane trafficking is essential for sculpting neuronal morphology. The GARP and EARP complexes are conserved tethers that regulate vesicle trafficking in the secretory and endolysosomal pathways, respectively. Both complexes contain the Vps51, Vps52, and Vps53 proteins, and a complex-specific protein: Vps54 in GARP and Vps50 in EARP. In Drosophila, we find that both complexes are required for dendrite morphogenesis during developmental remodeling of multidendritic class IV da (c4da) neurons. Having found that sterol accumulates at the trans-Golgi network (TGN) in Vps54KO/KO neurons, we investigated genes that regulate sterols and related lipids at the TGN. Overexpression of oxysterol binding protein (Osbp) or knockdown of the PI4K four wheel drive (fwd) exacerbates the Vps54KO/KO phenotype, whereas eliminating one allele of Osbp rescues it, suggesting that excess sterol accumulation at the TGN is, in part, responsible for inhibiting dendrite regrowth. These findings distinguish the GARP and EARP complexes in neurodevelopment and implicate vesicle trafficking and lipid transfer pathways in dendrite morphogenesis.

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.

A docked mutation phenocopies dumpy oblique alleles via altered vesicle trafficking

The Drosophila extracellular matrix protein Dumpy (Dpy) is one of the largest proteins encoded by any animal. One class of dpy mutations produces a characteristic shortening of the wing blade known as oblique (dpy^o ), due to altered tension in the developing wing. We describe here the characterization of docked (doc), a gene originally named because of an allele producing a truncated wing. We show that doc corresponds to the gene model CG5484, which encodes a homolog of the yeast protein Yif1 and plays a key role in ER to Golgi vesicle transport. Genetic analysis is consistent with a similar role for Doc in vesicle trafficking: docked alleles interact not only with genes encoding the COPII core proteins sec23 and sec13, but also with the SNARE proteins synaptobrevin and syntaxin. Further, we demonstrate that the strong similarity between the doc¹ and dpy^o wing phenotypes reflects a functional connection between the two genes; we found that various dpy alleles are sensitive to changes in dosage of genes encoding other vesicle transport components such as sec13 and sar1. Doc's effects on trafficking are not limited to Dpy; for example, reduced doc dosage disturbed Notch pathway signaling during wing blade and vein development. These results suggest a model in which the oblique wing phenotype in doc¹ results from reduced transport of wild-type Dumpy protein; by extension, an additional implication is that the dpy^o alleles can themselves be explained as hypomorphs.

A systematic analysis of microtubule-destabilizing factors during dendrite pruning in Drosophila

It has long been thought that microtubule disassembly, one of the earliest cellular events, contributes to neuronal pruning and neurodegeneration in development and disease. However, how microtubule disassembly drives neuronal pruning remains poorly understood. Here, we conduct a systematic investigation of various microtubule-destabilizing factors and identify exchange factor for Arf6 (Efa6) and Stathmin (Stai) as new regulators of dendrite pruning in ddaC sensory neurons during Drosophila metamorphosis. We show that Efa6 is both necessary and sufficient to regulate dendrite pruning. Interestingly, Efa6 and Stai facilitate microtubule turnover and disassembly prior to dendrite pruning without compromising the minus-end-out microtubule orientation in dendrites. Moreover, our pharmacological and genetic manipulations strongly support a key role of microtubule disassembly in promoting dendrite pruning. Thus, this systematic study highlights the importance of two selective microtubule destabilizers in dendrite pruning and substantiates a causal link between microtubule disassembly and neuronal pruning.

The Nrf2-Keap1 pathway is activated by steroid hormone signaling to govern neuronal remodeling

The evolutionarily conserved Nrf2-Keap1 pathway is a key antioxidant response pathway that protects cells/organisms against detrimental effects of oxidative stress. Impaired Nrf2 function is associated with cancer and neurodegenerative diseases in humans. However, the function of the Nrf2-Keap1 pathway in the developing nervous systems has not been established. Here we demonstrate a cell-autonomous role of the Nrf2-Keap1 pathway, composed of CncC/Nrf2, Keap1, and MafS, in governing neuronal remodeling during Drosophila metamorphosis. Nrf2-Keap1 signaling is activated downstream of the steroid hormone ecdysone. Mechanistically, the Nrf2-Keap1 pathway is activated via cytoplasmic-to-nuclear translocation of CncC in an importin- and ecdysone-signaling-dependent manner. Moreover, Nrf2-Keap1 signaling regulates dendrite pruning independent of its canonical antioxidant response pathway, acting instead through proteasomal degradation. This study reveals an epistatic link between the Nrf2-Keap1 pathway and steroid hormone signaling and demonstrates an antioxidant-independent but proteasome-dependent role of the Nrf2-Keap1 pathway in neuronal remodeling.

The membrane protein Raw regulates dendrite pruning via the secretory pathway

Neuronal pruning is essential for proper wiring of the nervous systems in invertebrates and vertebrates. Drosophila ddaC sensory neurons selectively prune their larval dendrites to sculpt the nervous system during early metamorphosis. However, the molecular mechanisms underlying ddaC dendrite pruning remain elusive. Here, we identify an important and cell-autonomous role of the membrane protein Raw in dendrite pruning of ddaC neurons. Raw appears to regulate dendrite pruning via a novel mechanism, which is independent of JNK signaling. Importantly, we show that Raw promotes endocytosis and downregulation of the conserved L1-type cell-adhesion molecule Neuroglian (Nrg) prior to dendrite pruning. Moreover, Raw is required to modulate the secretory pathway by regulating the integrity of secretory organelles and efficient protein secretion. Mechanistically, Raw facilitates Nrg downregulation and dendrite pruning in part through regulation of the secretory pathway. Thus, this study reveals a JNK-independent role of Raw in regulating the secretory pathway and thereby promoting dendrite pruning.

Golgi Outposts Nucleate Microtubules in Cells with Specialized Shapes

Classically, animal cells nucleate or form new microtubules off the perinuclear centrosome. In recent years, the Golgi outpost has emerged as a satellite organelle that can function as an acentrosomal microtubule-organizing center (MTOC), nucleating new microtubules at distances far from the nucleus or cell body. Golgi outposts can nucleate new microtubules in specialized cells with unique cytoarchitectures, including Drosophila neurons, mouse muscle cells, and rodent oligodendrocytes. This review compares and contrasts topics of functional relevance, including Golgi outpost heterogeneity, formation and transport, as well as regulation of microtubule polarity and branching. Golgi outposts have also been implicated in the pathology of diseases including muscular dystrophy, and neurodegenerative diseases, such as Parkinson's disease (PD). Since Golgi outposts are relatively understudied, many outstanding questions regarding their function and roles in disease remain.

Homeostatic Roles of the Proteostasis Network in Dendrites

Cellular protein homeostasis, or proteostasis, is indispensable to the survival and function of all cells. Distinct from other cell types, neurons are long-lived, exhibiting architecturally complex and diverse multipolar projection morphologies that can span great distances. These properties present unique demands on proteostatic machinery to dynamically regulate the neuronal proteome in both space and time. Proteostasis is regulated by a distributed network of cellular processes, the proteostasis network (PN), which ensures precise control of protein synthesis, native conformational folding and maintenance, and protein turnover and degradation, collectively safeguarding proteome integrity both under homeostatic conditions and in the contexts of cellular stress, aging, and disease. Dendrites are equipped with distributed cellular machinery for protein synthesis and turnover, including dendritically trafficked ribosomes, chaperones, and autophagosomes. The PN can be subdivided into an adaptive network of three major functional pathways that synergistically govern protein quality control through the action of (1) protein synthesis machinery; (2) maintenance mechanisms including molecular chaperones involved in protein folding; and (3) degradative pathways (e.g., Ubiquitin-Proteasome System (UPS), endolysosomal pathway, and autophagy. Perturbations in any of the three arms of proteostasis can have dramatic effects on neurons, especially on their dendrites, which require tightly controlled homeostasis for proper development and maintenance. Moreover, the critical importance of the PN as a cell surveillance system against protein dyshomeostasis has been highlighted by extensive work demonstrating that the aggregation and/or failure to clear aggregated proteins figures centrally in many neurological disorders. While these studies demonstrate the relevance of derangements in proteostasis to human neurological disease, here we mainly review recent literature on homeostatic developmental roles the PN machinery plays in the establishment, maintenance, and plasticity of stable and dynamic dendritic arbors. Beyond basic housekeeping functions, we consider roles of PN machinery in protein quality control mechanisms linked to dendritic plasticity (e.g., dendritic spine remodeling during LTP); cell-type specificity; dendritic morphogenesis; and dendritic pruning.

Characteristics and Functions of the Yip1 Domain Family (YIPF), Multi-Span Transmembrane Proteins Mainly Localized to the Golgi Apparatus

Yip1 domain family (YIPF) proteins are multi-span, transmembrane proteins mainly localized in the Golgi apparatus. YIPF proteins have been found in virtually all eukaryotes, suggesting that they have essential function(s). Saccharomyces cerevisiae contains four YIPFs: Yip1p, Yif1p, Yip4p, and Yip5p. Early analyses in S. cerevisiae indicated that Yip1p and Yif1p bind to each other and play a role in budding of transport vesicles and/or fusion of vesicles to target membranes. However, the molecular basis of their functions remains unclear. Analysis of YIPF proteins in mammalian cells has yielded significant clues about the function of these proteins. Human cells have nine family members that appear to have overlapping functions. These YIPF proteins are divided into two sub-families: YIPFα/Yip1p and YIPFβ/Yif1p. A YIPFα molecule forms a complex with a specific partner YIPFβ molecule. In the most broadly hypothesized scenario, a basic tetramer complex is formed from two molecules of each partner YIPF protein, and this tetramer forms a higher order oligomer. Three distinct YIPF protein complexes are formed from pairs of YIPFα and YIPFβ proteins. These are differently localized in either the early, middle, or late compartments of the Golgi apparatus and are recycled between adjacent compartments. Because a YIPF protein is predicted to have five transmembrane segments, a YIPF tetramer complex is predicted to have 20 transmembrane segments. This high number of transmembrane segments suggests that YIPF complexes function as channels, transporters, or transmembrane receptors. Here, the evidence from functional studies of YIPF proteins obtained during the last two decades is summarized and discussed.