Caenorhabditis elegans Embryo: Establishment of Asymmetry

Abstract

In Caenorhabditis elegans, asymmetry is established in the one‐cell embryo in response to the position of the sperm provided centrosome. Asymmetric contraction of actin and myosin at the cortex leads to the localisation of the conserved PAR (partitioning defective) polarity proteins into anterior and posterior cortical domains. The PAR proteins and associated protein kinases generate cytoplasmic gradients of polarity mediators, which in turn regulate the anterior/posterior cytoplasmic localisation of downstream cell fate regulators. The PAR proteins also regulate a conserved G protein pathway that coordinates the division plane with the anterior/posterior axis. Thus, the first division is asymmetric and the daughter cells have different developmental fates. During the next few divisions, anterior/posterior asymmetries and cell–cell signalling events establish the dorsal/ventral and left–right axes of the embryo and further refine cell fates.

Key Concepts:

  • The C. elegans embryo exhibits an invariant pattern of asymmetric cell divisions that generates diverse cell types.

  • Asymmetric divisions are regulated by the conserved PAR proteins, which become localised in anterior and posterior cortical domains during polarity establishment after fertilisation.

  • Polarity establishment occurs in response to an unknown cue associated with the sperm centrosome, which causes an actomyosin cortical flow that localises the anterior PAR proteins.

  • PAR polarity cues create a cytoplasmic gradient of the polarity mediators MEX‐5 and MEX‐6, which act together with other polarity mediators to generate the asymmetric localisation of cell fate determinants.

  • Asymmetric localisation of cell fate determinants is accomplished mainly through protein degradation and translational regulation.

  • The localisation of cell fate determinants combined with cell–cell signalling events leads to specification of different developmental programmes in cells of the early embryo.

  • A conserved G protein pathway functions with the microtubule motor dynein to position the mitotic spindle on the polarity axis during asymmetric divisions.

Keywords: asymmetric division; polarity; PAR; actomyosin; spindle position; protein degradation; translational control

Figure 1.

Asymmetric cell divisions produce blastomeres with distinct fates. Posterior is to the right. (a–e) The invariant division pattern leads to a precise arrangement of cells and the birth of six founder cells. The names of the founder cells and the germ‐line precursor cells (P cells) are labelled, and cells derived from each founder are indicated by colour. (a) One‐cell. (b) Two‐cell. (c) Four‐cell. (d) Eight‐cell. (e) For simplicity, the 16‐cell embryo at bottom does not show the daughter cells of the fourth AB cleavage, which occurs at about the same time as the P3 division. (f) Diagrammatic representation of the early lineage, showing the major cell types produced by each founder cell. Horizontal lines indicate cell divisions, and the lengths of the vertical lines indicate relative differences in cell cycle rates of each blastomere.

Figure 2.

Establishment of polarity in the early embryo. Posterior is to the right in all embryos. (a) Just after fertilisation, PAR‐3/PAR‐6/PKC‐3 are initially localised uniformly at the cortex, and contractions (indentations) occur throughout the cortex. (b) The position of the male pronucleus and associated centrosomes results in a local cessation of cortical contractile activity at the closest cortical region, but cortical contractions continue in the anterior. (c) Cortical cytoplasm (curved arrows), NMY‐2 and anterior PARs begin to flow towards the anterior, and PAR‐2 and PAR‐1 localise to the posterior cortex. Internal cytoplasm flows towards the posterior (straight arrows). The posterior PAR domain expands as actomyosin contractions and PAR‐3/PAR‐6/PKC‐3 become localised to the anterior. (d) The female pronucleus migrates towards the sperm pronucleus as contraction culminates in a medial pseudocleavage furrow. (e) By the time of pronuclear meeting, P granules are enriched in the posterior half of the embryo. (f) First division is intrinsically asymmetric resulting in an anterior cell AB and a posterior cell P1 that differ in cytoplasmic inheritance and PAR protein domains. PAR‐2 and PAR‐1 are initially present around the entire P1 cortex, but as the cell cycle progresses, PAR protein asymmetry and cellular polarity are reestablished.

Figure 3.

Localisation of polarity mediators, cell fate determinants and signalling pathways in the early embryo. Colours are used to indicate the localisation of different proteins as indicated; the hatched nucleus indicates high levels of POP‐1 in addition to the other proteins; arrows show the direction of cell–cell signalling events. For simplicity, SKN‐1 and PAL‐1 are shown together in posterior nuclei. However, SKN‐1 levels decrease after the four‐cell stage. In contrast PAL‐1 is not present until the four‐cell stage and persists in 8–12 cell stage embryos after SKN‐1 disappears. Similarly, MEX‐5 and MEX‐3 are both enriched in the AB lineage. However, MEX‐5 is also present in an anterior/posterior gradient of each P cell by division and persists in the somatic daughter for several cycles, and MEX‐5 signal in the AB cells diminishes beginning at the four‐cell stage. MEX‐1, MEX‐3 and PIE‐1 are also present on P granules in the P cells.

Figure 4.

Division patterns are determined by centrosome migrations and nuclear rotation. All embryos are left views, except (g) which is a dorsal view. Posterior is to the right. The small green circles represent centrosomes, green lines represent astral microtubules and arrows indicate directions of centrosome movements. The number of astral microtubules drawn is reduced for clarity. Blue circles represent interphase or prophase nuclei, blue bars are chromosomes in metaphase or anaphase. (a) The female pronucleus meets the sperm pronucleus in the posterior. (b) The pronuclei centre and rotate so that the first spindle (c) is aligned on the long axis of the embryo. In two‐cell embryos (d), centrosomes migrate until they are on opposite sides of the nuclei. In P1, there is a subsequent 90° nuclear rotation (e) to align the spindle (f) on the anterior/posterior axis. At the four‐cell stage, centrosomes migrate onto the left/right axis (g) and the spindle forms on this axis in ABa and ABp (h, only the spindle pole is visible). In EMS, the nucleus then rotates 90° onto the anterior/posterior axis (h and i). A 45° nuclear rotation orients the P2 spindle towards the site of contact with EMS, but division is dorsal/ventral due to constraints of cellular arrangements and the eggshell (not shown).

Figure 5.

Model for PAR‐dependent spindle positioning. Gα/GPR signalling is required for forces that pull on microtubules from the cortex (+), whereas LET‐99 inhibits force (−). Note that LET‐99 and GPR/LIN‐5 localise to the entire cortex, but only the regions with highest levels are indicated for simplicity. (a) During nuclear rotation, GPR and LIN‐5 show a slight enrichment at the anterior cortex and LET‐99 localises to the cortex in a posterior band. Inhibition of GPR‐1/2 localisation in the band would result in reduced force on lateral‐posterior microtubules compared to the force acting on microtubules contacting the other regions of the cortex. As a result, the pronuclear centrosome complex would move anteriorly and rotate. (b) At metaphase/anaphase, GPR‐1/2 and LIN‐5 are highest at both poles and lowest in the LET‐99 region. The resulting asymmetry of forces acting on the posterior pole displaces the spindle. (c) Gα associates with LIN‐5 through the adaptor GPR. LIN‐5 is thought to recruit dynein. Anchored to the cortex through the Gα/GPR/LIN‐5 complex, dynein may supply force via cortical‐contacting astral microtubules. Additionally, microtubules almost immediately depolymerise on contacting the cortex, which may also assist in cortical force generation.

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Web Link

WormBook: The online review of C elegans biology. http//www.wormbook.org

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Espiritu, Eugenel B, and Rose, Lesilee S(Apr 2013) Caenorhabditis elegans Embryo: Establishment of Asymmetry. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001506.pub3]