Worm Breeder's Gazette 14(4): 53 (October 1, 1996)

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.

Genetic Screens to Elucidate the TGF-b Signaling Pathway

Cathy Savage, Srikant Krishna, Stephen Cohen, Dan Collins, Sapana Patel, Guangcheng Cao, Richard W. Padgett

Waksman Institute, Rutgers University, Piscataway, NJ 08855

        We have previously described our analysis of a TGF-b pathway
regulating body size in C. elegans. This pathway includes the TGF-b type
II receptor DAF-4, the type I receptor SMA-6, and the homologous
downstream components SMA-2, SMA-3, and SMA-4. We continue to be
interested in identifying novel components of this signaling pathway. To
this end, we have been conducting a series of genetic screens. Here we
present an update on two of these screens.

I. The Small Screen

At least four TGF-b signaling components have been identified by their
Small phenotypes: the downstream components encoded by sma-2, sma-3, and
sma-4, and the type I receptor gene sma-6. To identify other components
of the pathway, it seemed reasonable to begin by isolating Small
mutants, since this type of screen had not been done systematically
before. We therefore screened F2 progeny of EMS-mutagenized N2 worms,
and isolated 85 Small mutants from ~17,000 mutagenized genomes. We are
characterizing these mutations by first doing a complementation test
with the triple mutant sma-4 sma-3 sma-2. Alleles that complement this
chromosome are being mapped to chromosomes by STS mapping, using
described Tc1 polymorphisms. Then, these alleles are being placed into
complementation groups. Fine structure mapping, analysis of male tail
phenotypes, and eventually molecular characterization will follow.
Alleles that fail to complement known genes include: 2 alleles of sma-1,
38 alleles of sma-2, sma-3, or sma-4, and 1 allele of sma-6.  An
additional 20 alleles have been mapped to chromosomes and are being
placed into complementation groups.

II. Suppressors of sma-6

Mutations in sma-6 presumably result in a loss of signaling activity of
the type I receptor in the pathway and hence yield the small phenotype.
The CB1482 sma-6 strain was screened for  suppressors in the pathway,
which were identified by wild-type or long phenotypes.  The reasoning
behind the screen was derived from two possibilities.  First, components
downstream of the receptor in the pathway, when constitutively active,
may be able to circumvent the requirement for full sma-6 receptor
signaling potential and thereby restore transduction.  In addition,
negative regulators of the pathway, when mutant, may revert small
animals via higher signal throughput.  It is reasonable to argue that
mutations which augment the level of signaling in the pathway would be
easier to generate against a weak mutant background.  Therefore, the
CB1482 (obtained from the CGC) strain was used because it is a
relatively mild sma-6 mutant biochemically and phenotypically. However,
a practical consideration to this approach is the increased difficulty
in the identification of suppressed animals.  We screened through
approximately 19k genomes and obtained nine suppressors, which we are
currently mapping and characterizing.  Additionally, we are performing
the screen with a stronger allele of sma-6.