Cell polarity and anaphase spindle positioning in the wildtype onecell stage C elegans embryo

In C. elegans, polarity along the AP embryonic axis is established shortly after fertilization (Goldstein & Hird 1996). A sperm component, which remains to be identified, provides an initial polarity cue that determines the future posterior of the embryo. This initial cue is then translated by the concerted action of six maternally required par genes (for ^artitioning-defective) to establish polarity along the AP axis (for review see Kemphues & Strome 1997). All six par genes have been cloned, and antibodies have been raised to most of the corresponding proteins (Boyd et al 1996, Etemad-Moghadam et al 1995, Guo & Kemphues 1995, Hung & Kemphues 1999, Levitan et al 1994, Watts et al 2000, K. Kemphues, personal communication). Strikingly, several of the PAR proteins have a polarized distribution along the AP axis in the one-cell stage embryo. Thus the PDZ-containing proteins PAR-3 and PAR-6 both localize to the anterior cortex (Etemad-Moghadam et al 1995, Hung & Kemphues 1999). Conversely, PAR-2, a ring-finger containing protein, and PAR-1, a Ser/Thr protein kinase, both localize to the posterior cortex (Boyd et al 1996, Guo & Kemphues 1995). These observations indicate that the polarized distribution of PAR proteins at the cell cortex is essential for establishing proper polarity along the AP embryonic axis.

Interestingly, homologues of some of the PAR proteins have been found in other metazoans where they also have a polarized distribution. For instance, in Drosophila, bazooka encodes a PAR-3 homologue that localizes to the apical surface of epithelia cells and neuroblasts; moreover, bazooka is required for proper polarity in these cells (Kuchinke et al 1998). In mammalian epithelial cells, the PAR-3 homologue ASIP is present in apically located tight junctions, while a PAR-1 homologue localizes to the basolateral domain (Bohm et al 1997, Izumi et al 1998). These observations suggest that the function of PAR proteins in establishing cell polarity may have been significantly conserved across metazoan evolution.

In the one-cell stage C. elegans embryo, establishment of polarity by the PAR proteins leads in turn, among other things, to an asymmetric position of the anaphase spindle along the AP axis. Spindle positioning can be followed with great spatial and temporal resolution in living embryos using time-lapse differential interference contrast (DIC) microscopy (Fig. 1). The spindle is initially set up roughly in the cell centre (Fig. 1, top panel). Then, as the spindle elongates during anaphase B, the anterior spindle pole stays relatively put with respect to overall AP polarity, while the posterior spindle pole is displaced slightly towards the posterior. This results in an asymmetric spindle position along the AP axis by the end of anaphase (Fig. 1, middle panel). As a result, the one-cell stage embryo divides asymmetrically, into a larger anterior blastomere and a smaller posterior one (Fig. 1, bottom panel). In embryos derived from par mutant hermaphrodites (hereafter referred to as par mutant embryos), posterior displacement does not take place and the first division is symmetric, most likely as a consequence of the earlier defects in establishing polarity along the AP axis (Kemphues et al1988).

0 0

Post a comment