Worm Breeder's Gazette 13(2): 100 (February 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Recently, we fortuitously revealed phalloidin-stained, ring-shaped structures shared by sister cells in the early embryo. The detection of these structures requires formaldehyde fixation followed by 100% acetone extraction, a treatment that extracts other phalloidin-binding components including most of the cortical actin cytoskeleton in early nematode embryos. The time of appearance, position, morphology and number of phalloidin-positive rings within embryos suggest these organelles are persistent remnants from prior cell divisions. We refer to these rings as cell division remnants (CDRs). Preliminary characterization of these structures reveals the following:
(i) A given embryo always has one fewer CDR than the total number of cells or a number equal to the number of cell divisions. (ii) The CDRs persist through many cell cycles and appear to be shared by all sister cells. (iii) The dimensions and position of the CDRs are similar to the initial size and location of the transient, CP-actin complexes that coincide with the onset of centrosome rotation in the P(1) lineage(1). For example, the CDR from first cleavage is located at a central site in the midfocal plane between AB and P(1) early in the 2-cell stage. Later, nuclear movements in the dividing AB cell push the CDR off-center in a manner which resembles the movement of the transient CP-actin complex during the 2-3 cell transition. (iv) The diameter of CDRs is variable (0.25-2.0 µm) and unpredictable. In general, older CDRs are smaller but there is no obvious correlation between ring diameter and the cell cycle. (v) The CDR derived from first cleavage is invariably shared by EMS and ABa at the 4-cell stage, EMS and ABal at the 6-cell stage and MS and ABal at the 8-cell stage. At least some of the above observations suggest that once a CDR is formed, it is not divisible by subsequent cell divisions.
Are CDRs involved in cell fate decisions? At the 4-cell stage, a signal from the EMS blastomere is required to instruct the anterior AB daughter to express a fate different from its sister, this interaction requires maternal expression of the glp-1 gene product (2,3). The glp-1 protein is found in the membrane of both AB-derived cells at the 4-cell stage(4) in agreement with previous experiments which show these two cells have equivalent potential. Paradoxically, the EMS cell contacts both AB-derived cells at the 4-cell stage, yet only the anterior AB daughter expresses the EMS-induced fate change. How an EMS-derived signal is directed to the appropriate Glp-1 expressing AB descendant remains unclear. The invariant "path" of the persistent CDR from first cleavage to the 6-cell stage provides an internal, asymmetrically-positioned structure which could be exploited to distinguish between equivalent sites of contact between AB-derived cells and EMS at the 4 and 6-cell stages. The AB-EMS lineage interactions specify the dorsal-ventral(2) and left-right(5,6) embryonic axes at the 4 and 6-cell stages, respectively. We propose a model whereby the asymmetrically positioned CDR could be exploited to deliver a signal from EMS to specific AB descendants despite apparently equivalent contacts.
A CDR model for the specification of the d-v embryonic axis in C. elegans. Fig 1. shows a schematic view of an experiment done by Priess and Thompson(2). At the 2-cell stage, segregation of determinants (shaded region) along the a-p axis in P(1) define the EMS and P(2) "ends" of the cell. Left unperturbed, embryos develop as shown on the left. On the right, the spindle in the AB cell is reoriented by physical manipulation with a blunt micropipette. One consequence of this manipulation is reorientation of the P(1) spindle. We propose that because the centrosome in P(1) is tethered to a CDR from first cleavage (bold circle between cells) the CDR, its associated cortex and determinants, and the anterior centrosome are repositioned as a unit. As a result, the absolute position of the EMS cell changes. At the stage, a signal from EMS is received by the AB daughter that shares the CDR from first cleavage; this interaction defines the d-v axis.
A CDR model for the specification of the l-r embryonic axis in C. elegans. Fig. 2 shows a schematic view of an experiment done by Woods(5). EMS and its descendant MS are shaded to emphasize orientation. In all cases, anterior is left and ventral comes out of the page. In unoperated embryos, nuclear movements in the AB cells are skewed such that AB daughters on the left are slightly more anterior than their sisters on the right. If the skew of the AB spindles is reoriented prior to cytokinesis (right column), the l-r axis is switched giving rise to an enantiomeric embryo and adults. Reorientation of the AB cell positions after cytokinesis does not reverse the l-r axis(5). We propose the physical manipulation changes the absolute position of the CDR derived from first cleavage. This change might reroute an EMS-derived signal that makes ABal different from ABar and reverse the l-r axis. A correlation between the position of the CDR and cell fate specification can now be tested. The prediction is that the embryo manipulations described in Figs. 1 and 2 should invariably reposition the CDR upon successful reversal of the embryonic axes. We plan to test the models diagrammed in Figs. 1 and 2 and to investigate the composition of the CDRs using existing antibodies to various cytoskeletal components.
REFERENCES: (l) Waddle et al, 1993 Worm Mtg, Abs#2.
(2) Priess and Thompson, 1987
(3) Priess et al, 1987
(4) Kimble et al, 1992, CSH Symp Quant Biol 57:401
(5) Wood, 1991 Nature 349:536.
(6) Schnabel, 1991, Mech Dev 34:85.