Caenorhabditis elegans PIG-1/MELK Acts in a Conserved PAR-4/LKB1 Polarity Pathway to Promote Asymmetric Neuroblast Divisions

Abstract

Asymmetric cell divisions produce daughter cells with distinct sizes and fates, a process important for generating cell diversity during development. Many Caenorhabditis elegans neuroblasts, including the posterior daughter of the Q cell (Q.p), divide to produce a larger neuron or neuronal precursor and a smaller cell that dies. These size and fate asymmetries require the gene pig-1, which encodes a protein orthologous to vertebrate MELK and belongs to the AMPK-related family of kinases. Members of this family can be phosphorylated and activated by the tumor suppressor kinase LKB1, a conserved polarity regulator of epithelial cells and neurons. In this study, we present evidence that the C. elegans orthologs of LKB1 (PAR-4) and its partners STRAD (STRD-1) and MO25 (MOP-25.2) regulate the asymmetry of the Q.p neuroblast division. We show that PAR-4 and STRD-1 act in the Q lineage and function genetically in the same pathway as PIG-1. A conserved threonine residue (T169) in the PIG-1 activation loop is essential for PIG-1 activity, consistent with the model that PAR-4 (or another PAR-4-regulated kinase) phosphorylates and activates PIG-1. We also demonstrate that PIG-1 localizes to centrosomes during cell divisions of the Q lineage, but this localization does not depend on T169 or PAR-4. We propose that a PAR-4-STRD-1 complex stimulates PIG-1 kinase activity to promote asymmetric neuroblast divisions and the generation of daughter cells with distinct fates. Changes in cell fate may underlie many of the abnormal behaviors exhibited by cells after loss of PAR-4 or LKB1.A SYMMETRIC cell divisions produce daughter cells with distinct fates, a process important for generating cell diversity in organisms as different as bacteria, plants, and animals. In Caenorhabditis elegans, asymmetric divisions of neuroblasts often produce cells fated to die. The posterior daughter of the Q cell (Q.p) neuroblasts, for example, divide to produce a neuronal precursor and a cell that dies, but how this fate asymmetry is generated in these divisions is poorly understood. PIG-1 regulates asymmetric neuroblast divisions that generate apoptotic cells by controlling spindle positioning, myosin distribution, and daughter cell fate (Cordes et al. 2006;Ou et al. 2010). PIG-1 is the sole C. elegans ortholog of MELK (maternal embryonic leucine zipper kinase), a serine/ threonine kinase that has been implicated in many developmental processes including stem cell renewal, apoptosis, cell cycle progression, and spliceosome assembly (Davezac et al. 2002;Vulsteke et al. 2004;Nakano et al. 2005;Lin et al. 2007;Jung et al. 2008). MELK and PIG-1 represent a subgroup of a large family of serine/threonine kinases that include molecules like PAR-1 and SAD-1, which regulate cell polarity, and AMPKs, which regulate metabolic processes (Bright et al. 2009). These family members are often regulated directly by the LKB1 kinase (Lizcano et al. 2004).Loss of the tumor suppressor LKB1 causes Peutz-Jeghers syndrome, a disease in humans that is characterized...

Programmed Cell Death DuringCaenorhabditis elegansDevelopment

“…It has been proposed that this is achieved through the asymmetric presence or activity in the two daughter cells of activators and/or repressors of the cell death fate and egl-1 transcription (Guenther and Garriga 1996;Frank et al 2005;Hatzold and Conradt 2008;Chien et al 2013). Indeed, mutations that affect the abilities of mothers of cells programmed to die to divide asymmetrically can affect the cell death fate of their daughters (Guenther and Garriga 1996;Cordes et al 2006;Hatzold and Conradt 2008;Ou et al 2010;Singhvi et al 2011;Chien et al 2013;Gurling et al 2014;Teuliere et al 2014). Therefore, events that lead to the polarization of mothers of cells programmed to die and that are required for their abilities to divide asymmetrically are fundamentally important for cell death specification.…”

“…The Q lineage: A number of genes have been identified that are required for the asymmetric divisions of the left and right posterior daughter of the Q cell (Q.p) during the first larval stage (L1 stage) and the apoptotic deaths of their smaller daughters (Cordes et al 2006;Ou et al 2010;Singhvi et al 2011;Chien et al 2013;Gurling et al 2014;Teuliere et al 2014). [Q.pL and Q.pR divide asymmetrically and each gives rise to a larger daughter, Q.pa, which further divides to generate two neurons (AVM/PVM and SDQL/SDQR, respectively), and a smaller daughter, Q.pp, which dies (Sulston and Horvitz 1977).]…”

“…[Q.pL and Q.pR divide asymmetrically and each gives rise to a larger daughter, Q.pa, which further divides to generate two neurons (AVM/PVM and SDQL/SDQR, respectively), and a smaller daughter, Q.pp, which dies (Sulston and Horvitz 1977).] These genes define three pathways that contribute to the ability of Q.p to divide asymmetrically by size and/or fate: the Par-1-like gene 1 (pig-1) pathway (Cordes et al 2006;Chien et al 2013), the ADP-ribosylation factor (arf) pathway (Singhvi et al 2011;Teuliere et al 2014), and a target of ERK kinase MPK-1 (toe-2)-dependent pathway (Gurling et al 2014). The pig-1 pathway is comprised of the genes abnormal embryonic partitioning of cytoplasma 4 (par-4) LKB1, pig-1 MELK, yeast STE20 related adaptor protein homolog 1 (strd-1) STRAD, and the mouse embryo scaffolding protein homolog mop-25.2 MOP25.…”

“…The identification of these pathways suggests that events at the plasma membrane as well as membrane trafficking events (potentially in an endocytic compartment) play an important role in the polarization of Q.p and its ability to divide asymmetrically. What remains to be determined is how these three pathways interact, what signal or signals they help to transduce, and how their functions relate to the programming of the cell death fate and the activation of egl-1 transcription in the smaller daughter cell, in particular (Guenther and Garriga 1996;Frank et al 2005;Cordes et al 2006;Hatzold and Conradt 2008;Ou et al 2010;Singhvi et al 2011;Chien et al 2013;Gurling et al 2014;Teuliere et al 2014).…”

‘Biogeneric’ developmental processes: drivers of major transitions in animal evolution

“…In brewers yeast, a member of Opisthokonta, a phylogenetic group that contains the holozoans as well as the fungi, the cytoplasmic protein Mo25 is essential for elongating growth and regulation of cell division [36]. This protein also exists in animals, where it mediates A/B-related cell shape changes during embryogenesis [37]. In regulating cell polarity, Mo25 operates in conjunction with the enzyme Lkb1 [38], which not only has a homologue in brewers yeast, but also in the more distantly related social amoebae Dictyostelium discoideum, where it is also involved in morphogenesis [39].…”