Worm Breeder's Gazette 13(4): 38 (October 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.

Reciprocal Changes in Expression of lin-12 and lag-2 Prior to Commitment of Z1.ppp and Z4.aaa.

Hilary A. Wilkinson1, Kevin Fitzgerald2, Iva Greenwald2

1 Current address: Merck Research Laboratories, Rahway, NJ 07065
2 HHMI and Dept. of Biochemistry and Molecular Biophysics, Columbia University College of Physicians and Surgeons, New York, NY 10032

  As the result of interactions between two cells, Z1 .pppand Z4 .aaa,one becomes
the anchor cell (AC) and the other becomes a ventral uterine precursor cell (W). This
interaction, the AC/VU decision, is a simple example of lateral specification. Genetic
studies have established that the activities of lin-12 and lag-2 control the AC/VU
decision (Greenwald et al., 1983; Lambie and Kimble, 1991; Tax et al., 1994). It is
thought that lin-12 encodes the receptor and lag-2 encodes the ligand that mediates
communication between Z1 .pppand Z4 .aaa(Yochem et al., 1988; Seydoux et al., 1989;
Tax et al., 1994), and that activation of lin-12 leads to adoption of the W fate (Greenwald
et al., 1983; Greenwald and Seydoux, 1990). Genetic mosaic analysis has indicated that
the decision between the AC and W fates involves a stochastic variation in ligand
and/or receptor activity in Z1 .pppand Z4 .aaathat is subsequently amplified by a
feedback mechanism (Seydoux and Greenwald, 1989).
  We have generated lin-12 ::lacZand lag-2 ::lacZreporter genes and have used
them to study the expression of lin-12 and lag-2 during the time that interactions
between Z1 .pppand Z4 .aaaspecify their fates. Our results suggest that transcriptional
regulation of both receptor and ligand is involved in the AC/VU decision.
  The analysis of the expression pattern of the reporter genes led us to make three
conclusions. First, we found that the expression patterns of lin-12 and lag-2 change
in Z1 .pppand Z4 .aaain a reciprocal manner. Initially, lin-12 and lag-2 are expressed
in both Z1 .pppand Z4 .aaa.However, expression of lin-12 is maintained in the
presumptive W but not in the presumptive AC, and expression of lag-2 is maintained
in the presumptive AC but not in the presumptive W. Second, we found that the
expression patterns of lin-12 and lag-2 change in uncommitted cells: when the gonad
primordium is forming, Z1 .pppand Z4 .aaaare generally not committed (Kimble, 1981),
yet at this time, generally only one of the two cells express lin-12 or lag-2 .Third, we
found that activation of lin-12 has opposite effects on lin-12 and lag-2 expression:
mutations that activate lin-12 maintain expression of lin-12 and repress expression of
lag-2 during primordium formation, and lin-12 null mutations eliminate expression
of lin-12 and maintain expression of lag-2 during primordium formation.
  We have also shown that this positive autoregulation is necessary to specify the W
fate. We had previously identified a potential cis-acting regulatory sequence, LCS1 ,by
comparing the sequence of the C. elegans and C. briggsae 5' flanking regions
(H.A.W., J. Yochem, J. Leegwater-Kim and I.G., unpublished observations). We
found that LCS1 is necessary for the maintenance of lin-12 expression in the
presumptive W: deletion of LCS1 from the lin-12 ::lacZreporter gene specifically
abolishes lin-12 expression in the presumptive W during primordium formation, but
does not affect the initial expression in both Z1 .pppand Z4 .aaa.Furthermore, LCS1 is
necessary for the AC/VU decision: deletion of LCS1 specifically abolishes the ability of
an otherwise lin-12 (+)transgene to restore the AC/VU decision to mutants lacking
endogenous lin-12 activity.
  Although we have demonstrated that positive autoregulation of lin-12 activity is
necessary to specify the W fate, it is possible that transcriptional regulation is not the
only component of the feedback mechanism for this cell fate decision. In addition,
most other lin-12 -mediateddecisions (which do not involve cells with naturally
variable fates), do not appear to use transcriptional regulation to generate or amplify
differences between equivalent cells. For example, lin-12 appears to be continuously
expressed in all six VPCs from the L2 stage through their division in the L3 stage
(H.A.W. and I.G., unpublished observations).