RIC-8 (Synembryn): a novel conserved protein that is required for G(q)alpha signaling in the C. elegans nervous system

Abstract

Recent studies describe a network of signaling proteins centered around G(o)alpha and G(q)alpha that regulates neurotransmitter secretion in C. elegans by controlling the production and consumption of diacylglycerol (DAG). We sought other components of the Goalpha-G(q)alpha signaling network by screening for aldicarb-resistant mutants with phenotypes similar to egl-30 (G(q)alpha) mutants. In so doing, we identified ric-8, which encodes a novel protein named RIC-8 (synembryn). Through cDNA analysis, we show that RIC-8 is conserved in vertebrates. Through immunostaining, we show that RIC-8 is concentrated in the cytoplasm of neurons. Exogenous application of phorbol esters or loss of DGK-1 (diacylglycerol kinase) rescues ric-8 mutant phenotypes. A genetic analysis suggests that RIC-8 functions upstream of, or in conjunction with, EGL-30 (G(q)alpha).

Figure 1. One Possible Arrangement of the Components of the G_oα–G_qα Signaling Network

Model based on the findings of Koelle and Horvitz (1996), Hajdu-Cronin et al. (1999), Miller et al., (1999), Nurrish et al. (1999), and Lackner et al. (1999). An alternative arrangement, in which GOA-1 acts through EAT-16, is equally consistent with the available data (Hajdu-Cronin et al., 1999). Arrows denote activation. Bars at the end of a line denote inhibition. Proteins that promote or activate synaptic transmission are shown in green blocks. Reducing the function of these proteins results in aldicarb resistance and reduced locomotion rates. Proteins that inhibit synaptic transmission are shown in red blocks. Reducing the function of these proteins results in aldicarb hypersensitivity and hyperactive locomotion. Neurotransmitter inputs into this pathway are not specified.

Figure 2. ric-8 and egl-30 (G_qα) Reduction-of-Function Mutants Share Similar Phenotypes

Photographs comparing wild-type C. elegans (N2) with strains con taining reduction-of-function mutations in snt-1 (a synaptotagmin homolog) (Nonet et al., 1993), egl-30, or ric-8. Note that snt-1 mutants have a partially coiled posture, are smaller in size, and have fewer eggs in their uteri than do wild type. In contrast, egl-30 and ric-8 mutants, in addition to being aldicarb resistant, are both bloated with eggs and exhibit decreased body flexion phenotypes that are characteristic of the subclass of aldicarb resistance mutants with defects in the G_oα–G_qα signaling network.

Figure 3. ric-8 Encodes a Novel Protein that Is Conserved in Vertebrates

(A) Amino acid alignment of mouse (top), Drosophila (middle), and C. elegans (bottom) synembryn. Gray and black backgrounds represent similar and identical residues, respectively. (B) The ric-8 gene, transcript, and mutations. Schematic of ric-8 gene structure and mutations. Boxed regions are exons, black boxed regions indicate coding sequence, and lines indicate introns. The break in the intron near the middle of the gene indicates that this intron has not been completely sequenced. The positions and descriptions of the three ric-8 mutations are indicated. The md1909 mutant has a 1.6 kb Tc1 transposon insertion following amino acid 221. The md303 mutant has a C-to-A change at nucleotide 832 of the cDNA that converts alanine 275 to glutamic acid. md1712, isolated as an intragenic suppressor of md303, converts leucine 267 to phenylalanine.

Figure 4. RIC-8 Appears to Function Upstream of EGL-30 (G_qα) or in a Parallel Intersecting Pathway

(A) egl-30(ad805) and ric-8(md303) exhibit a similar degree of aldicarb resistance. Shown are the aldicarb dose-response curves for wild type, egl-30(ad805), and ric-8(md303). One hundred percent represents the number of progeny produced from a starting population of L1 larvae over a 96 hr period in the absence of aldicarb. Curves are representative of duplicate experiments. (B) The aldicarb resistance of ric-8(md303) is suppressed to near wild-type levels by overexpression of egl-10+. Shown are aldicarb dose-response curves of ric-8(md303); egl-10+[nls51] animals and control strains. Note that the aldicarb sensitivity of ric-8(md303); egl-10+[nls51] animals is close to that of wild type. Curves are representative of duplicate experiments. (C) The locomotion rate of md303 is rescued to near wild-type levels by eliminating or blocking GOA-1 function. Shown are the mean locomotion rates of wild type, ric-8(md303), egl-30(ad805), goa-1(n363), egl-10+ [nIs51], and two double mutant combinations. Note that the locomotion rate of goa-1(n363); ric-8(md303) is close to that of wild type. Error bars represent the standard error of the mean in a population of five to ten young adults.

Figure 5. Loss of DGK-1 (Diacylglycerol Kinase) or Exogenous Application of Phorbol Ester Results in a Striking Rescue of ric-8 Mutant Phenotypes

(A) ric-8; dgk-1 double mutants are hypersensitive to aldicarb. Aldicarb sensitivity is quantified by measuring population growth rates of wild-type and mutant strains on various concentrations of aldicarb. Shown are aldicarb dose-response curves for wild type, ric-8(md303), dgk-1(sy428), and ric-8(md303); dgk-1(sy428). Note that ric-8(md303) is strongly resistant to aldicarb, while dgk-1(sy428) is strongly hypersensitive to aldicarb. The aldicarb sensitivity of the ric-8; dgk-1 double mutant is similar to that of the dgk-1 single mutant. Curves are representative of duplicate experiments. (B) The mean locomotion rate of ric-8; dgk-1 double mutants is close to wild-type. The mean locomotion rate of ric-8(md303) is 2.6% that of the wild-type rate. dgk-1(sy428) mutants, on the other hand, exhibit hyperactive locomotion. ric-8; dgk-1 double mutants have intermediate locomotion rates that are close to that of wild type. Error bars represent the standard error of the mean in a population of ten young adult animals. A supplementary video (http://www.neuron.org/cgi/content/full/27/2/289/DC1) shows that ric-8; dgk-1 double mutants also exhibit normal body flexion and coordinated locomotion. (C) Effect of phorbol esters on wild type and ric-8. Shown are photographs of N2 (wild type) and ric-8(md303) on agar plates containing carrier (no drug) or 10 mM phorbol myristate acetate. Note that the phorbol ester induces a hyperflexive posture in both wild type and the ric-8 mutant. In addition, note that ric-8 mutants on phorbol esters are no longer bloated with eggs. (D) Phorbol esters induce hyperactive locomotion in wild type and rescue ric-8 locomotion rates to near wild-type levels. Shown are the mean locomotion rates of N2 (wild type) and ric-8(md303) on agar plates containing carrier (no drug) or 10 μM phorbol myristate acetate. Error bars represent the standard error of the mean in a population of eight young adult animals.

Figure 6. Localization of RIC-8 in Juvenile C. elegans

(A) An affinity-purified RIC-8 antibody recognizes proteins of 62 kDa (major band) and 64 kDa (minor band) in a Western blot of total C. elegans protein. A parallel control blot of total C. elegans protein (probed with a RIC-8 antibody that had been preadsorbed to purified, recombinant RIC-8) was devoid of immunoreactivity (data not shown). (B) Wild-type C. elegans juvenile stained an antibody to RIC-8. Staining (green) is seen in the head and tail ganglia, as well as near the nuclear membrane of nonneuronal cells, including germ cell nuclei. (C) Control immunostaining experiment: a wild-type C. elegans juvenile stained with a RIC-8 antibody that was preadsorbed to purified, recombinant RIC-8. To confirm nervous system permeabilization, this animal was double stained with antibodies to CHA-1 (choline acetyltransferase; data not shown).

Figure 7. RIC-8 Is Concentrated in the Nervous System of Adult Animals

(A) Head region of a wild-type C. elegans adult stained with an antibody to RIC-8. Note the prominent staining of the head ganglia, amphid process, and ventral nerve cord. (B) Close-up view of a section of the head ganglia. White arrowheads indicate examples of several strongly stained neurons in which RIC-8 immunoreactivity is present throughout the cytoplasm of the cell. Note the variability in RIC-8 staining intensity between different neurons. (C) RIC-8 is also present at some cholinergic synapses in the ventral cord. Close-up view of a region near the vulva that was double stained with RIC-8 (green staining) and CHA-1 (red staining), a marker that is localized to cholinergic synapses (represented by bright puncta) (Janet Duerr, personal communication). Regions of overlap show up as yellow. Arrowheads indicate a subset of synapses along the ventral nerve cord that are positive for both RIC-8 and CHA-1. (D) Expanded view of a region of head ganglia showing RIC-8 staining in the nerve ring, which contains axonal processes. (E) Head region of a ric-8(md1909) adult stained with an antibody to RIC-8. Note that the nervous system staining is decreased, though not absent, in this transposon insertion mutant.