The Fusogen EFF-1 Controls Sculpting of Mechanosensory Dendrites

Abstract

The mechanisms controlling the formation and maintenance of neuronal trees are poorly understood. We examined the dynamic development of two arborized mechanoreceptor neurons (PVDs) required for reception of strong mechanical stimuli in Caenorhabditis elegans. The PVDs elaborated dendritic trees comprising structural units we call "menorahs." We studied how the number, structure, and function of menorahs were maintained. EFF-1, an essential protein mediating cell fusion, acted autonomously in the PVDs to trim developing menorahs. eff-1 mutants displayed hyperbranched, disorganized menorahs. Overexpression of EFF-1 in the PVD reduced branching. Neuronal pruning appeared to involve EFF-1-dependent branch retraction and neurite-neurite autofusion. Thus, EFF-1 activities may act as a quality control mechanism during the sculpting of dendritic trees.Morphologies of dendritic trees vary from one neuronal type to another, and the pattern of these arbors determines the manner in which a neuron processes its synaptic or sensory input. However, little is known regarding the mechanisms controlling the outgrowth and maintenance of dendritic trees (1-4). Two mechano-receptors in Caenorhabditis elegans (PVDR and PVDL; right and left, respectively) are responsible for an avoidance response triggered by strong mechanical stimuli to the body (5). The complete neural system of C. elegans has been considered to comprise only simple-patterned neurons (6). However, recent studies show that the PVDs have a more complex morphology (7,8).Here, we established a genetic system to dissect the mechanisms of branch generation and plasticity of arborized neurons in C. elegans. To determine branching patterns, we imaged † To whom correspondence should be addressed. podbilew@tx.technion.ac.il (B.P.), ishemer@email.unc.edu (G.S.). * Present address: Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. Note added in proof:After we submitted this report, showed that axotomized PLM sensory neurons fail to reconnect in eff-1 mutants. (table S1). The PVDs contained repetitive structural units reminiscent of multibranched candelabras or menorahs (Fig. 1A). Although the number of menorah branches varied, the menorahs appeared to develop in a stepwise manner from the L2 larva to the adult ( fig. S1). The stereotypical menorah structure is likely to form a functional unit necessary for the PVDs mechanosensory activities.Mutations in the cell fusion gene eff-1 (9,10) affected the pattern of PVDs arborization, resulting in disorganized and hyperbranched phenotypes (Fig. 1B). Moreover, eff-1(ok1021) mutant animals showed reduced sensitivity to strong mechanical stimuli (53%, n = 106) (11). To characterize menorah disorganization in eff-1 mutants, we quantified the number of processes at different degrees of the branching order (primary to senary branches; Fig. 1C and fig. S2). The frequency of secondary and tertiary branching was doubled in the eff-1(hy21) mutant compared with wild type. The eff-1(hy21)...

AFF-1 fusogen can rejuvenate the regenerative potential of adult dendritic trees via self-fusion

“…The fusion protein EFF-1 mediates dendrite retraction and auto-fusion of excess branches by a novel cell-autonomous pruning mechanism (Oren-Suissa et al, 2010). Thus, this fusogen maintains the structural units by trimming excess branching during normal developmental arborization (Oren-Suissa et al, 2010). AFF-1, a paralog of EFF-1, is a second C. elegans fusogen displaying a more restricted tissue distribution pattern (Avinoam and Podbilewicz, 2011;Sapir et al, 2007); AFF-1 was not found to be involved in PVD remodeling during development (Oren-Suissa et al, 2017).…”

“…The fusion protein EFF-1 mediates dendrite retraction and auto-fusion of excess branches by a novel cell-autonomous pruning mechanism (Oren-Suissa et al, 2010). Thus, this fusogen maintains the structural units by trimming excess branching during normal developmental arborization (Oren-Suissa et al, 2010).…”

“…The PVD develops in a stereotypic fashion from the L2 larval stage to the young adult (Oren-Suissa et al, 2010). The fusion protein EFF-1 mediates dendrite retraction and auto-fusion of excess branches by a novel cell-autonomous pruning mechanism (Oren-Suissa et al, 2010).…”

“…1A) (Oren-Suissa et al, 2010). These dendritic trees arise and are dynamically maintained through several intrinsic and extrinsic genetic pathways (Chatzigeorgiou et al, 2010;Cohen et al, 2014;Dong et al, 2013Dong et al, , 2015Husson et al, 2012;Liu and Shen, 2011;Maniar et al, 2012;Oren-Suissa et al, 2010;Salzberg et al, 2013;Smith et al, 2010;Wei et al, 2015).…”

C. elegans bicd-1, homolog of theDrosophiladynein accessory factorBicaudal D, regulates the branching of PVD sensory neuron dendrites

“…To date, mec-3, which encodes a LIM homeodomain transcription factor expressed in PVD neurons (Way and Chalfie, 1988;Way and Chalfie, 1989), as well as eff-1, which encodes a protein essential for cell fusion, are the only genes known to be required for the elaboration of PVD dendritic arbors (Oren-Suissa et al, 2010;Tsalik et al, 2003). By carrying out two RNAi screens, a small-scale candidate-based screen and a large-scale screen of chromosome IV, we identified 11 genes that either promote or suppress PVD dendrite branching.…”