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Gene editing in C . elegans using plasmid-based CRISPR reagents requires microinjection of many animals to produce a single edit. Germline silencing of plasmid-borne Cas9 is a major cause of inefficient editing. Here, we present a set of C . elegans strains that constitutively express Cas9 in the germline from an integrated transgene. These strains markedly improve the success rate for plasmid-based CRISPR edits. For simple, short homology arm GFP insertions, 50–100% of injected animals typically produce edited progeny, depending on the target locus. Template-guided editing from an extrachromosomal array is maintained over multiple generations. We have built strains with the Cas9 transgene on multiple chromosomes. Additionally, each Cas9 locus also contains a heatshock-driven Cre recombinase for selectable marker removal and a bright fluorescence marker for easy outcrossing. These integrated Cas9 strains greatly reduce the workload for producing individual genome edits.

Regeneration of injured neurons can restore function, but most neurons regenerate poorly or not at all. The failure to regenerate in some cases is due to a lack of activation of cell-intrinsic regeneration pathways. These pathways might be targeted for the development of therapies that can restore neuron function after injury or disease. Here, we show that the DLK-1 mitogen-activated protein (MAP) kinase pathway is essential for regeneration in Caenorhabditis elegans motor neurons. Loss of this pathway eliminates regeneration, whereas activating it improves regeneration. Further, these proteins also regulate the later step of growth cone migration. We conclude that after axon injury, activation of this MAP kinase cascade is required to switch the mature neuron from an aplastic state to a state capable of growth.

Synaptic vesicles can be released at extremely high rates, which places an extraordinary demand on the recycling machinery. Previous ultrastructural studies of vesicle recycling were conducted in dissected preparations using an intense stimulation to maximize the probability of release. Here, a single light stimulus was applied to motor neurons in intact Caenorhabditis elegans nematodes expressing channelrhodopsin, and the animals rapidly frozen. We found that docked vesicles fuse along a broad active zone in response to a single stimulus, and are replenished with a time constant of about 2 s. Endocytosis occurs within 50 ms adjacent to the dense projection and after 1 s adjacent to adherens junctions. These studies suggest that synaptic vesicle endocytosis may occur on a millisecond time scale following a single physiological stimulus in the intact nervous system and is unlikely to conform to current models of endocytosis. Neurons communicate with one another at junctions called synapses. When an electrical signal travels along a neuron and arrives at a synapse, vesicles filled with small neurotransmitter molecules fuse with the cell membrane and release the neurotransmitter. These chemicals rapidly bind to receptors on the downstream neuron that induce an electrical response in that cell. Vesicles can be consumed at prodigious rates, up to 500 a second, so the cell must recover the membrane rapidly and regenerate more vesicles filled with neurotransmitter. Experiments in the 1970s and 1980s suggested that when vesicles empty their contents into the synapse, they fuse completely with the membrane and are lost. To recover the membrane, the cell forms ‘pits’, by means of a coat protein called clathrin, which then bud off into the cell as new vesicles. It takes roughly 15–20 s for vesicles to be recycled in this way. By contrast, synapses with very high firing rates are thought to recycle vesicles through a faster process known as ‘kiss and run’, in which vesicles are not fully integrated into the membrane, but instead fuse transiently with it to form a reversible pore within about a second. However, these studies triggered vesicle release using conditions that are unlikely to occur naturally inside cells. Now, Watanabe et al. have used optogenetics to study vesicle recycling in response to single stimuli at the synapse between neurons and muscles in an intact living animal, the nematode C. elegans. The worms had been genetically modified to express a light-sensitive ion channel called channelrhodopsin in their motor neurons. Watanabe et al. used a single pulse of light to stimulate vesicle release, and then rapidly froze the worms before studying their synapses with electron microscopy. They found that vesicle recycling occurred at the edges of the synapse or at a specialized structure in the middle of the synapse. Vesicle recycling took less than 50 ms—much faster than anything previously observed. This ultrafast recycling is unlikely to occur via ‘kiss and run’ since recycling occurred at sites lateral to the sites of fusion and because the recycled vesicles were larger than the originals, implying that they had not simply re-formed after a brief fusion event. By using physiologically relevant stimuli in an intact animal, Watanabe et al. reveal that vesicles can be recycled at synapses much more rapidly than previously thought, suggesting that our current models of this process may need to be reassessed.

Methods are reported for the combination of fluorescence nanoscopy using either stimulated emission depletion microscopy (STED) or photoactivated localization microscopy (PALM) with electron microscopy, to achieve correlative imaging in which the super-resolved fluorescence signal is placed in the context of cellular ultrastructure. A complete portrait of a cell requires a detailed description of its molecular topography: proteins must be linked to particular organelles. Immunocytochemical electron microscopy can reveal locations of proteins with nanometer resolution but is limited by the quality of fixation, the paucity of antibodies and the inaccessibility of antigens. Here we describe correlative fluorescence electron microscopy for the nanoscopic localization of proteins in electron micrographs. We tagged proteins with the fluorescent proteins Citrine or tdEos and expressed them in Caenorhabditis elegans, fixed the worms and embedded them in plastic. We imaged the tagged proteins from ultrathin sections using stimulated emission depletion (STED) microscopy or photoactivated localization microscopy (PALM). Fluorescence correlated with organelles imaged in electron micrographs from the same sections. We used these methods to localize histones, a mitochondrial protein and a presynaptic dense projection protein in electron micrographs.

Erik Jorgensen and colleagues report a highly efficient method for generating single-copy transgene insertions in C. elegans . Notably, these single-copy transgenes are expressed at endogenous levels and can be expressed in the female and male germlines. At present, transgenes in Caenorhabditis elegans are generated by injecting DNA into the germline. The DNA assembles into a semistable extrachromosomal array composed of many copies of injected DNA. These transgenes are typically overexpressed in somatic cells and silenced in the germline. We have developed a method that inserts a single copy of a transgene into a defined site. Mobilization of a Mos1 transposon generates a double-strand break in noncoding DNA. The break is repaired by copying DNA from an extrachromosomal template into the chromosomal site. Homozygous single-copy insertions can be obtained in less than 2 weeks by injecting approximately 20 worms. We have successfully inserted transgenes as long as 9 kb and verified that single copies are inserted at the targeted site. Single-copy transgenes are expressed at endogenous levels and can be expressed in the female and male germlines.

Most neurotransmission is mediated by action potentials, whereas sensory neurons propagate electrical signals passively and release neurotransmitter in a graded manner. Here, we demonstrate that Caenorhabditis elegans neuromuscular junctions release neurotransmitter in a graded fashion. When motor neurons were depolarized by light-activation of channelrhodopsin-2, the evoked postsynaptic current scaled with the strength of the stimulation. When motor neurons were hyperpolarized by light-activation of halorhodopsin, tonic release of synaptic vesicles was decreased. These data suggest that both evoked and tonic neurotransmitter release is graded in response to membrane potential. Acetylcholine synapses are depressed by high-frequency stimulation, in part due to desensitization of the nicotine-sensitve ACR-16 receptor. By contrast, GABA synapses facilitate before becoming depressed. Graded transmission and plasticity confer a broad dynamic range to these synapses. Graded release precisely transmits stimulation intensity, even hyperpolarizing inputs. Synaptic plasticity alters the balance of excitatory and inhibitory inputs into the muscle in a use-dependent manner.

Activation of voltage-gated calcium channels at presynaptic terminals leads to local increases in calcium and the fusion of synaptic vesicles containing neurotransmitter. Presynaptic output is a function of the density of calcium channels, the dynamic properties of the channel, the distance to docked vesicles, and the release probability at the docking site. We demonstrate that at Caenorhabditis elegans neuromuscular junctions two different classes of voltage-gated calcium channels, CaV2 and CaV1, mediate the release of distinct pools of synaptic vesicles. CaV2 channels are concentrated in densely packed clusters ~250 nm in diameter with the active zone proteins Neurexin, α-Liprin, SYDE, ELKS/CAST, RIM-BP, α-Catulin, and MAGI1. CaV2 channels are colocalized with the priming protein UNC-13L and mediate the fusion of vesicles docked within 33 nm of the dense projection. CaV2 activity is amplified by ryanodine receptor release of calcium from internal stores, triggering fusion up to 165 nm from the dense projection. By contrast, CaV1 channels are dispersed in the synaptic varicosity, and are colocalized with UNC-13S. CaV1 and ryanodine receptors are separated by just 40 nm, and vesicle fusion mediated by CaV1 is completely dependent on the ryanodine receptor. Distinct synaptic vesicle pools, released by different calcium channels, could be used to tune the speed, voltage-dependence, and quantal content of neurotransmitter release.

AP-2 is the core-organizing element in clathrin-mediated endocytosis. During the formation of clathrin-coated vesicles, clathrin and endocytic accessory proteins interact with AP-2 in a temporally and spatially controlled manner, yet it remains elusive as to how these interactions are regulated. Here, we demonstrate that the endocytic protein NECAP 1, which binds to the α-ear of AP-2 through a C-terminal WxxF motif, uses an N-terminal PH-like domain to compete with clathrin for access to the AP-2 β2-linker, revealing a means to allow AP-2-mediated coordination of accessory protein recruitment and clathrin polymerization at sites of vesicle formation. Knockdown and functional rescue studies demonstrate that through these interactions, NECAP 1 and AP-2 cooperate to increase the probability of clathrin-coated vesicle formation and to control the number, size, and cargo content of the vesicles. Together, our data demonstrate that NECAP 1 modulates the AP-2 interactome and reveal a new layer of organizational control within the endocytic machinery.

Many cell movements proceed via a crawling mechanism, where polymerization of the cytoskeletal protein actin pushes out the leading edge membrane. In this model, membrane tension has been seen as an impediment to filament growth and cell motility. Here we use a simple model of cell motility, the Caenorhabditis elegans sperm cell, to test how membrane tension affects movement and cytoskeleton dynamics. To enable these analyses, we create transgenic worm strains carrying sperm with a fluorescently labeled cytoskeleton. Via osmotic shock and deoxycholate treatments, we relax or tense the cell membrane and quantify apparent membrane tension changes by the membrane tether technique. Surprisingly, we find that membrane tension reduction is correlated with a decrease in cell displacement speed, whereas an increase in membrane tension enhances motility. We further demonstrate that apparent polymerization rates follow the same trends. We observe that membrane tension reduction leads to an unorganized, rough lamellipodium, composed of short filaments angled away from the direction of movement. On the other hand, an increase in tension reduces lateral membrane protrusions in the lamellipodium, and filaments are longer and more oriented toward the direction of movement. Overall we propose that membrane tension optimizes motility by streamlining polymerization in the direction of movement, thus adding a layer of complexity to our current understanding of how membrane tension enters into the motility equation.

Zinc-finger nucleases are chimeric proteins consisting of engineered zinc-finger DNA-binding motifs attached to an endonuclease domain. These proteins can induce site-specific DNA double-strand breaks in genomic DNA, which are then substrates for cellular repair mechanisms. Here, we demonstrate that engineered zinc-finger nucleases function effectively in somatic cells of the nematode Caenorhabditis elegans. Although gene-conversion events were indistinguishable from uncut DNA in our assay, nonhomologous end joining resulted in mutations at the target site. A synthetic target on an extrachromosomal array was targeted with a previously characterized nuclease, and an endogenous genomic sequence was targeted with a pair of specifically designed nucleases. In both cases, ≈20% of the target sites were mutated after induction of the corresponding nucleases. Alterations in the extrachromosomal targets were largely products of end-filling and blunt ligation. By contrast, alterations in the chromosomal target were mostly deletions. We interpret these differences to reflect the abundance of homologous templates present in the extrachromosomal arrays versus the paucity of such templates for repair of chromosomal breaks. In addition, we find evidence for the involvement of error-prone DNA synthesis in both homologous and nonhomologous pathways of repair. DNA ligase IV is required for efficient end joining, particularly of blunt ends. In its absence, a secondary end-joining pathway relies more heavily on microhomologies in producing deletions.