The structural and functional properties of neurons have intrigued scientists since the pioneering work of Santiago Ramón y Cajal. Since then, emerging cutting-edge technologies, including light and electron microscopy, electrophysiology, biochemistry, optogenetics, and molecular biology, have dramatically increased our understanding of dendritic properties. This advancement was also facilitated by the establishment of different animal model organisms, from flies to mammals. Here we describe the emerging model system of a Caenorhabditis elegans polymodal neuron named PVD, whose dendritic tree follows a stereotypical structure characterized by repeating candelabra-like structural units. In the past decade, progress has been made in understanding PVD's functions, morphogenesis, regeneration, and aging, yet many questions still remain.

Complex dendritic trees are a distinctive feature of neurons. Alterations to dendritic morphology are associated with developmental, behavioral and neurodegenerative changes. The highly-arborized PVD neuron of C. elegans serves as a model to study dendritic patterning; however, quantitative, objective and automated analyses of PVD morphology are missing. Here, we present a method for neuronal feature extraction, based on deep-learning and fitting algorithms. The extracted neuronal architecture is represented by a database of structural elements for abstracted analysis. We obtain excellent automatic tracing of PVD trees and uncover that dendritic junctions are unevenly distributed. Surprisingly, these junctions are three-way-symmetrical on average, while dendritic processes are arranged orthogonally. We quantify the effect of mutation in git-1, a regulator of dendritic spine formation, on PVD morphology and discover a localized reduction in junctions. Our findings shed new light on PVD architecture, demonstrating the effectiveness of our objective analyses of dendritic morphology and suggest molecular control mechanisms.

AFF-1-coated virus infects skin cells and rarely muscle cells in C. elegans• Epithelial cell-specific infection requires eff-1 activity • Pseudoviral infection depends on the amount of eff-1 in target cells • Ectopic EFF-1 in muscle cells induces their fusion and viral infectionAbstract Viral-based vectors are widely used for fundamental and translational research; however, production of a tissue-specific targeting vector is often a challenging step. Here, we develop a cell type-specific delivery system, based on vesicular stomatitis virus lacking the viral fusion protein (fusogen) and pseudotyped with AFF-1, a C. elegans cell-to-cell bilateral fusogen from the fusexin family. Following injection of AFF-1-coated virus into the C. elegans pseudocoelom that functions as a circulatory system, we observe specific infection of different cells that express EFF-1 or AFF-1 fusogens. EFF-1-expressing epidermal cells are preferentially infected, whereas mononucleated body wall muscles (BWMs) that do not express any fusogen are rarely infected. The epidermal cells infection is abolished when the host cells do not express EFF-1, while ectopic expression of EFF-1 on BWMs induces their abnormal cell-cell fusion, produces paralyzed-dumpy animals and enhances muscle infection by AFF-1-coated viruses. Thus, our data uncover that AFF-1-coated pseudotyped viruses can be retargeted to desired cells in a controlled manner and serve as a cell-specific delivery system in nematodes with potential applications in regenerative processes, therapies such as dendritic reconnection following injuries, and studies of diverse cellular and viral fusogens.VSV-G is a class III viral fusogen whereas the Caenorhabditis elegans EFF-1 and AFF-1, which fuse somatic cells during development, are structural homologs of class II viral proteins and gamete HAP2(GCS1) fusogens from the fusexin family [14][15][16][17][18][19][20][21][22]. EFF-1 and AFF-1 also participate in maintaining neuronal architecture and neuronal reconnection following injury [23][24][25][26][27]. Studied viral glycoproteins, including VSV-G, use a unilateral fusion mechanism that depends on the expression of receptors only on the target cells [28,29]. In contrast, EFF-1 and AFF-1 are bilateral fusogens-their presence is required on the membranes of both apposing cells to mediate fusion [6,16,18,30,31]. These two fusogens can act in either a homotypic or a heterotypic manner [6,16,18,21] and mediate heterotypic cell fusion of Sf9 insect and Baby Hamster Kidney (BHK) cells [6,16]. Finally, VSV viruses containing a GFP substituting the VSV-G coding sequence (VSV∆G) [32,33] that are coated with AFF-1 (VSV∆G-AFF-1) specifically infect AFF-1 or EFF-1 expressing BHK cells [6]. Thus, in contrast to the pseudotyped virus coated with the native, unilateral G glycoprotein (VSVΔG-G), infection by VSVΔG-AFF-1 requires fusogen expression on the host cell membrane. To date, however, VSVΔG coated with AFF-1, EFF-1 or any other non-viral fusogen have not been tested for infection in a living organism ...