[
Drugs,
1991]
Ivermectin, a derivative of avermectin B, is an orally effective microfilaricidal agent. It is the current drug of choice for treating patients infected with the nematode Onchocerca volvulus, which is a major cause of blindness in inhabitants of some tropical areas. Ivermectin is administered orally as a single dose of 150 micrograms/kg given annually. Skin and ocular microfilarial counts are dramatically reduced after the first dose, with some evidence for a resulting decrease in transmission of infection by the blackfly vector. With the exception of rare serious reactions such as severe systemic postural hypotension, ivermectin is generally well tolerated. The drug has the clear advantages of ease of administration and better tolerability compared with diethylcarbamazine and suramin, agents previously used to treat onchocerciasis. Thus, ivermectin is suitable for inclusion in mass treatment programmes and is the best therapeutic option presently available to combat onchocerciasis. As such it provides hope for many thousands of people at risk of becoming blind, and represents a major contribution to tropical medicine.
[
WormBook,
2006]
Heterotrimeric G proteins, composed of alpha , beta , and gamma subunits, are able to transduce signals from membrane receptors to a wide variety of intracellular effectors. In this role, G proteins effectively function as dimers since the signal is communicated either by the G alpha subunit or the stable G betagamma complex. When inactive, G alpha -GDP associates with G betagamma and the cytoplasmic portion of the receptor. Ligand activation of the receptor stimulates an exchange of GTP for GDP resulting in the active signaling molecules G alpha -GTP and free G betagamma , either of which can interact with effectors. Hydrolysis of GTP restores G alpha -GDP, which then reassociates with G betagamma and receptor to terminate signaling. The rate of G protein activation can be enhanced by the guanine-nucleotide exchange factor, RIC-8 , while the rate of GTP hydrolysis can be enhanced by RGS proteins such as EGL-10 and EAT-16 . Evidence for a receptor-independent G-protein-signaling pathway has been demonstrated in C. elegans early embryogenesis. In this pathway, the G alpha subunits GOA-1 and GPA-16 are apparently activated by the non-transmembrane proteins GPR-1 , GPR-2 , and RIC-8 , and negatively regulated by RGS-7 . The C. elegans genome encodes 21 G alpha , 2 G beta and 2 G gamma subunits. The alpha subunits include one ortholog of each mammalian G alpha family: GSA-1 (Gs), GOA-1 (Gi/o), EGL-30 (Gq) and GPA-12 (G12). The remaining C. elegans alpha subunits ( GPA-1 , GPA-2 , GPA-3 , GPA-4 , GPA-5 , GPA-6 , GPA-7 , GPA-8 , GPA-9 , GPA-10 , GPA-11 , GPA-13 , GPA-14 , GPA-15 , GPA-16 , GPA-17 and ODR-3 ) are most similar to the Gi/o family, but do not share sufficient homology to allow classification. The conserved G alpha subunits, with the exception of GPA-12 , are expressed broadly while 14 of the new G alpha genes are expressed in subsets of chemosensory neurons. Consistent with their expression patterns, the conserved C. elegans alpha subunits, GSA-1 , GOA-1 and EGL-30 are involved in diverse and fundamental aspects of development and behavior. GOA-1 acts redundantly with GPA-16 in positioning of the mitotic spindle in early embryos. EGL-30 and GSA-1 are required for viability starting from the first larval stage. In addition to their roles in development and behaviors such as egg laying and locomotion, the EGL-30 , GSA-1 and GOA-1 pathways interact in a network to regulate acetylcholine release by the ventral cord motor neurons. EGL-30 provides the core signals for vesicle release, GOA-1 negatively regulates the EGL-30 pathway, and GSA-1 modulates this pathway, perhaps by providing positional cues. Constitutively activated GPA-12 affects pharyngeal pumping. The G alpha subunits unique to C. elegans are primarily involved in chemosensation. The G beta subunit, GPB-1 , as well as the G gamma subunit, GPC-2 , appear to function along with the alpha subunits in the classic G protein heterotrimer. The remaining G beta subunit, GPB-2 , is thought to regulate the function of certain RGS proteins, while the remaining G gamma subunit, GPC-1 , has a restricted role in chemosensation. The functional difference for most G protein pathways in C. elegans, therefore, resides in the alpha subunit. Many cells in C. elegans express multiple G alpha subunits, and multiple G protein pathways are known to function in specific cell types. For example, Go, Gq and Gs-mediated signaling occurs in the ventral cord motor neurons. Similarly, certain amphid neurons use multiple G protein pathways to both positively and negatively regulate chemosensation. C. elegans thus provides a powerful model for the study of interactions between and regulation of G protein signaling.
[
Sci STKE,
2003]
Examples of the activation of heterotrimeric G proteins in vivo by any means other than through activated cell surface receptors have been limited to pathophysiological phenomena. With the discovery of proteins apart from receptors that facilitate guanine nucleotide exchange and affect G protein subunit dissociation directly, however, the notion of receptor-independent modes of activation in normal circumstances has become a subject of great interest. Three recent publications, each focusing on G protein regulators (GPRs) in asymmetric positioning of the mitotic spindle in the early Caenorhabditis elegans embryo, provide substantial support for the likelihood of such a form of activation. The C. elegans proteins GPR-1 and GPR-2 each contain a G protein regulatory motif, which supports interaction with Galpha(i)-like subunits. Inactivation of the genes encoding GPR-1 and GPR-2 prevents the correct positioning of the mitotic spindle in the one- and two-cell embryo. This phenotype is identical to that achieved by inactivation of genes encoding the Galpha subunits GOA-1 and GPA-16. Because signaling in the one- and two-cell embryos is "intrinsic," the data suggest a GPR-dependent, receptor-independent mode of G protein activation. The GPRs interact preferentially with the guanosine diphosphate (GDP)-bound form of alpha subunits, and the GPR motif per se exhibits GDP dissociation inhibitor activity. The actions of the GPRs imply that GDP.Galpha.GPR is a key intermediate or effector in force generation relevant to
[
WormBook,
2014]
Polarity establishment, asymmetric division, and acquisition of cell fates are critical steps during early development. In this review, we discuss processes that set up the embryonic axes, with an emphasis on polarity establishment and asymmetric division. We begin with the first asymmetric division in the C. elegans embryo, where symmetry is broken by the local inactivation of actomyosin cortical contractility. This contributes to establishing a polarized distribution of PAR proteins and associated components on the cell cortex along the longitudinal embryonic axis, which becomes the anterior-posterior (AP) axis. Thereafter, AP polarity is maintained through reciprocal negative interactions between the anterior and posterior cortical domains. We then review the mechanisms that ensure proper positioning of the centrosomes and the mitotic spindle in the one-cell embryo by exerting pulling forces on astral microtubules. We explain how a ternary complex comprised of G (GOA-1/GPA-16), GPR-1/GPR-2, and LIN-5 is essential for anchoring the motor protein dynein to the cell cortex, where it is thought to exert pulling forces on depolymerizing astral microtubules. We proceed by providing an overview of cell cycle asynchrony in two-cell embryos, as well as the cell signaling and spindle positioning events that underly the subsequent asymmetric divisions, which establish the dorsal-ventral and left-right axes. We then discuss how AP polarity ensures the unequal segregation of cell fate regulators via the cytoplasmic proteins MEX-5/MEX-6 and other polarity mediators, before ending with an overview of how the fates of the early blastomeres are specified by these processes.
[
Med Sci (Paris),
2003]
The mechanisms orchestrating spatial cell division control remain poorly understood. In animal cells, the position of the mitotic spindle dictates cleavage furrow placement, and thus plays a key role in governing spatial relationships between resulting daughter cells. The one-cell stage Caenorhabditis elegans embryo is an attractive model system to investigate the mechanisms underlying spindle positioning in metozoans. In this review, the experimental advantages of this model system for an in vivo dissection of cell division processes are first discussed. Next, three lines of experiments that were conducted to dissect the mechanisms governing spindle positioning in one-cell stage C. elegans embryos are summarized. First, localized laser micro-irradiations were utilized to identify the forces acting on spindle poles during anaphase. This work revealed that there is a precise imbalance of pulling forces acting on the two spindle poles, with the forces acting on the posterior spindle pole being in slight excess, thus explaining the asymmetric spindle position achieved by the end of anaphase. Second, an RNAi-based fonctional genomic screen was carried out to identify novel components required for generating these pulling forces. This uncovered that
gpr-1/gpr-2, which encode GoLoco-containing proteins, as well as the previously identified Go subunits
goa-1/gpa-16, are required for generation of pulling forces on the spindle poles. Third, the
zyg-8 locus was identified by mutational analysis to play a distinct role during anaphase spindle positioning.
zyg-8 was found to encode a protein related to human Doublecortin, which is affected in patients with neuronal migration disorders. Moreover, ZYG-8 is a microtubule-associated protein that stabilizes microtubules against depolymerization. Together, these experimental approaches contribute to a better understanding of the mechanisms orchestrating spatial cell division control in metozoan organisms.