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 transgenic animals expressing cytoplasmic ser-2p::GFP (green fluorescent protein) or plasma membrane DES-2::GFP in the PVDs (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.

Fig. 1.

PVD menorah sculpting by EFF-1. “c” denotes cell body, small droplets are autofluorescent gut granules, anterior is left, and dorsal is up. (A) Full stereotypic arborization pattern in young adult. (B) PVD in eff-1(hy21) showing disorganized menorahs. (Insets) Wild-type menorah (left) and eff-1(hy21) mutant (right) with excess branching from secondary stem (arrow) and abnormal retrograde migration of quaternary branch (asterisk). Scale bars represent 50 μm [(A) and (B)] and 10 μm [(B) insets]. (C) Menorah pattern map [primary to senary (1° to 6°) are blue, purple, red, green, orange, and yellow, respectively]. (D) Excess bifurcations from secondary stems in eff-1(hy21ts). (E) Increased retrograde branches in eff-1(hy21ts). In (D) and (E), *P < 0.0001, two-tailed t test. Data are mean ± SE. All experiments were compared with wild type at 20°C, n = 20, 260 menorahs. hy21 at 25°C, n = 6, 355 menorahs. hy21 at 15°C, n = 10, 322 menorahs. n, number of animals analyzed. We also performed a one-way analysis of variance (ANOVA) followed by Dunnett’s multiple comparison test (11).

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) mutant had a strong branching phenotype, whereas the oj55 mutant displayed a weaker effect, correlating with their respective epithelial fusion-defective phenotype (figs. S2 and S5). In wild-type menorahs, most sprouting of branches and bending of tertiary processes occurred at right angles to the branches of origin (Fig. 1A). In contrast, eff-1(hy21) mutant menorahs showed varying branching angles (Fig. 1B). We observed in the mutant a 10-fold increase in the number of branches sprouting from the secondary branch (Fig. 1D) and a 20-fold increase in the number of branches that erred and turned back 180° (Fig. 1E). These phenotypes suggest that EFF-1 sculpts and maintains right-angle nonoverlapping branches.

Cell-specific expression of eff-1 in the PVDs (des-2p::eff-1) partially rescued neuronal eff-1 phenotypes (Fig. 2C). In contrast, PVD patterning defects were not rescued by expression of eff-1 in the neighboring epidermal tissue (fig. S3; dpy-7p::eff-1). In eff-1(hy21) mutants expressing eff-1 in both neural and epidermal tissues (des-2p::eff-1+dpy-7p::eff-1), the rescue was not significantly stronger than the rescue observed with only the PVD-specific des-2p::eff-1 (fig. S3). These rescue experiments, together with expression of EFF-1::GFP in the PVDs (fig. S4), provide evidence that eff-1 controls branching cell autonomously. Moreover, eff-1 overexpression in the PVDs of wild-type animals reduced branching (Fig. 2D). The remaining branches were organized in a gradient starting from the cell body toward the head and tail, where no branches could be observed. Thus, eff-1 may play a role in mechanosensory neurons restricting branching in a dosage-dependent manner to produce dendrite simplification (Fig. 2G).

Fig. 2.

EFF-1 autonomously rescues PVD arborization and restricts branching. (A) eff-1(hy21) at 25°C had twice as many secondary and tertiary branches as the wild type. Scale bars represent 10 μm. (B) Menorahs in wild-type animals. (C) Expression of EFF-1 (des-2p::EFF-1) in the PVD in eff-1(hy21) mutants at 25°C partially rescued the menorah pattern and reduced the number of branches. (D) Overexpression of des-2p::EFF-1 in wild-type animals caused a menorah gradient starting from the PVD cell body until dendrites disappeared toward the posterior (arrows). (E) Apparent EFF-1 dosage–sensitive reduction in branching. *P < 0.0001, **P < 0.001, ***P < 0.05, two-tailed t test. We also performed ANOVA (11). Data are mean ± SE. eff-1(hy21), n = 6; number of wild-type animals, n = 6; eff-1(hy21);des-2p::EFF-1, 25°C, n = 5; des-2p::EFF-1, n = 6. (F) Quantification of the number of secondary to senary branches showed that secondary and tertiary branches were doubled in the mutant compared with wild type and that secondary to quinary branches were reduced when EFF-1 was expressed ectopically in the PVD. (G) A model for the maintenance of PVD branching in an eff-1 dosage–dependent manner. Low levels of eff-1 (A) increased the number of PVD branches, and elevated levels of eff-1 reduced branching (D).

The absence of excess branching in wild-type animals may reflect a situation where only the appropriate branches initiate outgrowth. Alternatively, an excess of branches may be generated and extended but at some later point undergo retraction, pruning, or fusion to repair branching errors. To determine how EFF-1 restricts branching, we followed the behavior of branches by using three-dimensional (3D) live-imaging confocal microscopy in wild-type animals overexpressing des-2p::eff-1. The tips of tertiary to senary menorah branches first contacted the muscle cells, and then the dendrites detached from the menorahs by fission events (Fig. 3A; arrowheads) causing spontaneous dendrite break-off (Fig. 3B; arrowheads). Thus, following dynamic outgrowth, fission events eliminate extra branches.

Fig. 3.

Dendrite autofusion and fission trim menorahs. Scale bars represent 5 μm, (A) and (B); 20 μm and insets 5 μm, (C); and 0.5 μm, (D). (A) Trimming of dendrites that break off (arrows) and fission of branches in des-2p::EFF-1 animal (arrowheads). m, body wall muscle. (B) Fission of a branch in des-2p::EFF-1 animal (arrowhead). (C) eff-1(hy21ts) animal grown at 25° downshifted to 15° for 4 hours and then upshifted to 25° for 2 hours. Loops can be seen (arrows and insets). (D) Four TEM prints from wild-type animal “N2U” (6) showing the route of a PVD fusion process at the edge of the ventral nerve cord. Prints 1572 and 1597 show quaternary dendrites (arrowheads, red pseudocolor) passing between a thin layer of hypodermis and body wall muscle (M). PVD branch emerged from one quaternary dendrite at 1572, joined the minor fascicle of the ventral cord between 1578 and 1592, and ran ventralward to autofuse with another quaternary dendrite at 1596 and 1597 (arrows). Whereas this fusion process of PVD spans about 3 μm along the anterior-posterior axis, the marked neuron processes (42, 51, 58, and 96) were traced for thousands of serial sections back to their cell bodies. Dorsal is down (fig. S9; www.wormimage.org). (E) Cartoon representing one menorah (red) with eight terminal dendrites autofused along the dorsal midline (arrows).

To further analyze eff-1–dependent remodeling of menorahs, we grew the eff-1(hy21ts) worms at the restrictive temperature and shifted them to the permissive temperature in the early L4 stage. The defective arborization pattern was static at the nonpermissive temperature. Four hours after down-shifting to the permissive temperature, we observed branches that met, touched, and retracted. Some branches appeared to be stably connected, forming loops of different sizes and shapes (43 stable loops, n = 18; fig. S6). Because EFF-1 is a fusogen, we hypothesized that transient meeting and attachment of branches expressing EFF-1 may have resulted in loop formation because of interbranch fusion. To distinguish between interbranch fasciculation and fusion, we analyzed the 3D structure of the loops by generating confocal z stacks, projections, and rotations. The loops were stably connected over time (55 loops, n = 18 animals; movie S1). Next, we imaged DES-2::GFP in loops and found that fluorescent particles moved freely through them. To test whether the loops resulted from eff-1 activity, we incubated temperature-sensitive mutants at the restrictive temperature to generate hyperbranching, then we shifted them to the permissive temperature to induce expression of active EFF-1. After generating loops for 3.5 hours, we upshifted the worms back to the restrictive temperature to stabilize the loops (fig. S8). We obtained a two- to threefold increase in the number of loops (119 stable loops in n = 16 animals; Fig. 3C). We also observed symmetric fluorescence recovery of photobleached areas within the loops (fig. S7 and movie S2), indicating that the loops are continuous. Thus, eff-1 induces loop formation by neurite autofusion and/or fasciculation.

To further demonstrate that neurites can fuse with each other, we turned to higher resolution images of menorah branches and membranes. By using archival serial section transmission electron micrographs [TEMs (6)], we identified arborizations derived from the PVDs. In transverse sections of an adult hermaphrodite (N2U), 30- to 80-nm-diameter branches sandwiched between the body wall muscles and the hypodermis were observed (Fig. 3D, arrowheads). We reconstructed parallel neurites corresponding to the quaternary branches of a menorah from serial sections (Fig. 3E and figs. S9 to S11). In 10 examples of PVDs and FLPL/R (highly arborized neurons anterior to the PVDs), we found 2 to 12 branches fusing at the midline and forming fused longitudinal processes (Fig. 3, D and E, arrows). In no cases did we observe distal dendrites fasciculate; instead these fused if they reached one another. Thus, neurite-neurite auto-fusion plays a role in PVD and FLP arborization.

In addition to loop formation, dendrites were highly dynamic, erring, growing, and retracting in wild-type worms (Fig. 4A and movie S3). PVDs overexpressing eff-1 showed similar retractions but, unlike in the wild type, showed limited branch growth (Fig. 4, B and C). Similarly, when eff-1(hy21ts) animals were downshifted from the restrictive to the permissive temperature, we observed a 20-fold increase in the number of retracting branches and a 5-fold reduction in branch growth compared with eff-1 mutants at 25°C (Fig. 4, D to G, and movie S1). In contrast, upshifting eff-1(hy21ts) worms resulted in excess neurite growth (Fig. 4F). Thus, we propose a model in which EFF-1 autonomously induces retraction of branches to simplify menorahs.

Fig. 4.

EFF-1–dependent retraction of branches. (A, B, D, and E) Time-lapse confocal projections of L4 and young-adult animals. (A) Growth of a wild-type menorah (movie S3). Growing tertiary branches (arrows). (B) Retraction of branches in des-2p::EFF-1 animal (arrowheads). (C) Number of branches growing and retracting in des-2p::EFF-1 and wild type. Branches first showed dynamic movements but eventually either shortened or lengthened (dynamic branching ending with growth or retraction). **P < 0.01, ***P < 0.05, two-tailed t test. Data shown as mean ± SE. Wild type, n = 3; des-2p::EFF-1, n = 2. (D) L4 eff-1(hy21ts) at 25°C, branches were static. (E) Retracting branch in L4 eff-1(hy21ts) grown at 25°C and shifted to 15°C for 4 hours (arrowhead; movie S1). (F) Number of branches showing growth and retraction in eff-1(hy21) grown at 25°C and downshifted as in (E). *P < 0.001, **P < 0.01, two-tailed t test. Data are mean ± SE. eff-1(hy21) 25°C, n = 3; eff-1(hy21) 25° to 15°C, n = 4 animals. (G) Wild-type branches grew (0.8 nm/s; N = 5 branches) and retracted (0.9 nm/s; N = 3 branches). In eff-1(hy21) temperature-shifted animals, branches retracted (0.9 nm/s; N = 10 branches). Data are mean ± SE. Scale bars represent 5 μm.

EFF-1 is both a sculptor of epithelial organs by cell fusion (10) and a menorah sculptor by controlling dendrite bending, retraction, and fusion. The activities of this fusogen may be due to its ability to induce membrane curvature, a process that is thought to constitute a major driving force in membrane fusion and fission (12–14). Proteins capable of bending membranes, such as atlastins (15, 16) and dynamins (17), can induce tubulation, fusion, and fission (12, 13). Three mechanistic principles may form and maintain branched tubes in the cytoplasm and in extracellular branched filopodia or neuronal arbors such as menorahs: first, assembly of specialized proteins on membranes; second, membranous tube formation involving growth and bifurcation of tubes; and third, membrane bending followed by membrane fusion and fission restricts excessive branching. How can EFF-1 control mechanistically different processes such as dendrite fusion and retraction? Different isoforms and interactions may account for diverse activities. For example, trans interactions between EFF-1 on dendrites will cause autofusion, whereas assembly of large EFF-1 complexes on dendrites may induce actin-mediated retraction.