LET-99 opposes Gα/GPR signaling to generate asymmetry for spindle positioning in response to PAR and MES-1/SRC-1 signaling

Abstract

cell division. In C. elegans embryos, homologs of receptorindependent G protein activators, GPR-1 and GPR-2 (GPR-1/2), function together with Gα (GOA-1 and GPA-16) to generate asymmetric spindle pole elongation during divisions in the P lineage. Although Gα is uniformly localized at the cell cortex, the cortical localization of GPR-1/2 is asymmetric in dividing P cells. In this report, we show that the asymmetry of GPR-1/2 localization depends on PAR-3 and its downstream intermediate LET-99. Furthermore, in addition to its involvement in spindle elongation, Gα is required for the intrinsically programmed nuclear rotation event that orients the spindle in the one-cell. LET-99 functions antagonistically to the Gα/GPR-1/2 signaling pathway, providing an explanation for how Gα-dependent force is regulated asymmetrically by PAR polarity cues during both nuclear rotation and anaphase spindle elongation. In addition, Gα and LET-99 are required for spindle orientation during the extrinsically polarized division of EMS cells. In this cell, both GPR-1/2 and LET-99 are asymmetrically localized in response to the MES-1/SRC-1 signaling pathway. Their localization patterns at the EMS/P 2 cell boundary are complementary, suggesting that LET-99 and Gα/GPR-1/2 signaling function in opposite ways during this cell division as well. These results provide insight into how polarity cues are transmitted into specific spindle positions in both extrinsic and intrinsic pathways of asymmetric cell division.

The adhesion GPCR latrophilin – a novel signaling cascade in oriented cell division and anterior-posterior polarity

“…So far, several studies have very elegantly demonstrated the involvement of G proteins, but in a GPCR-independent context, for instance via GEF proteins 23 and G-protein regulators. 24 Although a GPCRdependent and G protein-mediated signaling pathway in these processes has not been unambiguously defined there are some clear indications for their existence. 25,26 LAT-1 might fill this gap and contribute an additional signal via a G s protein/adenylyl cyclase introducing a new level of regulation.…”

Polarity Control of Spindle Positioning in the C. elegans Embryo

“…As mentioned above, RNAi of gpb-1 and gpc-2 causes abnormal centrosome movements and spindle orientation. Based on combined RNAi with Gα, this phenotype results from hyperactive Gα that is no longer sequestered by Gβγ, and not from loss of Gβγ-specific effector signaling (Tsou et al 2003). Spindle-severing experiments showed that gpb-1 RNAi increases spindle-pulling forces in the anterior (Afshar et al 2004(Afshar et al , 2005.…”

“…The localization of GPR-1/GPR-2 is quite dynamic and likely subjected to regulatory mechanisms. GPR-1/GPR-2 and LIN-5 become anteriorly enriched during polarity establishment and prophase of the first mitotic division, which contributes to pronuclear centration (Tsou et al 2003;Park and Rose 2008). Subsequently, GPR-1/GPR-2 and LIN-5 redistribute to become higher at the posterior cortex in mitosis.…”

“…LET-99 is a DEP (Dishevelled/EGL-10/Pleckstrin) domain protein that antagonizes cortical localization of Gα-GPR-1/GPR-2 (Tsou et al 2003;Bringmann et al 2007;Park and Rose 2008). The let-99 mutant phenotype resembles aspects of the gpb-1 RNAi phenotype and indicates negative regulation of cortical Gα-GPR-1/GPR-2.…”

“…The identification of the LET-99 band has resulted in a cortical force model with three instead of two (anterior and posterior) domains. Negative regulation of force generation in the posterior LET-99 band (possibly in combination with asymmetric distribution of cortical GPR-1/GPR-2) results in net anterior pulling forces during pronuclear centration and net posterior pulling forces from metaphase onwards (Tsou et al 2003;Couwenbergs et al 2004;Krueger et al 2010). This indicates the possibility that not anterior-posterior GPR-1/GPR-2 asymmetry per se, but rather reduced pulling on astral microtubules that reach the LET-99 lateral band, is responsible for a net increase in posterior pulling forces and spindle displacement in mitosis.…”