Asymmetric enrichment of GPR-1/GPR-2 at the posterior side of a C. elegans embryo during the first cleavage.

Proper positioning of the spindle during asymmetric cell division ensures correct partitioning of cellular determinants. In Caenorhabditis elegans embryos, displacement of the spindle towards the posterior side of a single-celled embryo results in asymmetric cleavage. Now, a study by Gotta et al. (Curr. Biol. 13, 1029–1037 (2003)) suggests that GPR-1 and GPR-2, two highly related G-protein regulators from C. elegans, may regulate spindle position through spatial asymmetries in G-protein signalling.

Earlier studies have shown the PAR proteins (required for polarization of the embryo) and heterotrimeric G proteins (consisting of α, β and γ subunits) regulate spindle asymmetry in the early embryo. Mutation of two Gα subunits – GOA1 and GPA16 – resulted in symmetric cell division, even though polarity markers were properly localized, suggesting that G-protein signalling may be involved in coupling polarity signals to spindle position. However, relatively little is known about how GOA-1 or GPA-16 are regulated or how they might govern spindle asymmetry.

In this study, Gotta et al. start by demonstrating that inactivation of GPR-1 and GPR-2 results in a symmetric first division. However, they also found that polarity markers such as PAR-2 and PAR-3, which are important for spindle positioning, are correctly localized. This suggests the GPR-1/GPR-2 might function downstream of PAR-2 and PAR-3. Because PAR-2 and PAR-3 are known to regulate pulling forces at the spindle, the authors then assayed for spindle forces. Using inhibitory RNA strategies (RNAi) to block gpr-1/gpr-2 expression, they determined that the forces at both spindle poles in RNAi-treated embryos treated were weak when compared with wild-type embryos. They also found that in single-celled wild-type embryos, GPR-1/GPR-2 was enriched at the pole, where spindle forces are stronger. In addition, the asymmetric distribution of GPR-1/GPR-2 was found to be dependent on PAR-2 and PAR-3. These observations begin to provide a framework for understanding how PAR-2 and PAR-3 direct generation of asymmetric forces at the spindle poles.

But how does GPR-1/GPR-2 affect G-protein signalling downstream of PAR-2 and PAR-3? The answer may lie in the fact that both GPR-1 and GPR-2 contain a GPR domain. Studies of mammalian and Drosophila melanogaster homologues have shown that the presence of this motif inhibits dissociation of GDP from the Gα subunit. Gotta et al. found that GOA-1 binds the GPR motif of both GPR-1 and GPR-2, and that binding specifically inhibited dissociation of GDP from GOA-1.

These findings lead the authors to propose that asymmetric spindle position may be generated by increased signalling through a GOA-1–GRP-1/GRP-2 complex at one pole which, in turn, results in stronger pulling forces at this spindle pole. Gotta et al. suggest that formation of this complex might result in activation of targets that directly regulate spindle force. The identification of downstream effectors of the GOA-1–GPR-1/GPR-2 complex is an important goal for the future and will undoubtedly provide greater insights into the underlying mechanisms that govern asymmetric spindle position.