Worm Breeder's Gazette 14(3): 34 (June 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.

Small Differences Between sma-2, sma-3 and sma-4

Cathy Savage, Richard W. Padgett

Waksman Institute, Piscataway, NJ 08855

        SMA-2, SMA-3, and SMA-4 are members of a family of cytoplasmic
signaling molecules, that we call the dwarfins, which act downstream of
TGF-b superfamily receptors. Other members of this family include the
Drosophila Mad gene, three of the C.elegans daf genes (D. Riddle, J.
Thomas, and G. Ruvkun, personal communications), and the human tumor
suppressor gene DPC4. We have previously described that sma-2, sma-3,
and sma-4 have very similar mutant phenotypes yet encode highly
homologous gene products. These results have led to a model in which at
least three dwarfin types are required together for signal transduction.
If this model is correct, we are left with the question: why are
multiple dwarfins required? More specifically, are there functional
differences between these genes? Our ongoing characterization of these
genes has begun to highlight some small differences between them.

        We have previously reported preliminary results on the
expression pattern of sma-2. From several different sma-2-lacZ or -gfp
constructs, we find that sma-2 is expressed nearly ubiquitously in
larval stages, and in adults most highly in the pharynx, seam cells, and
ventral hypodermis. We have now begun to look at sma-3-lacZ constructs.
Surprisingly, the expression patterns are slightly different. Again,
sma-3-lacZ is expressed very widely in larval stages, and shows
significant staining in the adult pharynx. In contrast to sma-2,
however, we see expression of sma-3 in intestinal cells in adults and
larvae. In addition, we have seen expression of sma-3 in lateral and
ventral hypodermal cells, but in many fewer animals and at much lower
intensity than with sma-2. These similarities and differences in
expression may allow us to begin to address whether and to what extent
these two gene products act together in signal transduction. Since the
staining of hypodermal cells with sma-3-lacZ is present in only a few
animals, we will ask the question of whether sma-3 expression is
temporally regulated, by looking more at staged animals and at sma-3-gfp
expression in living animals.

        To address the question of functional redundancy, we had
constructed the double and triple mutants sma-3 sma-2 and sma-4 sma-3
sma-2. We expected that if these genes were at least partially
redundant, then the double and/or triple mutant would show a more severe
phenotype. Instead, these mutant combinations showed no additional
phenotypes: they are small and have male tail abnormalities, crumpled
spicules and ray fusions. We concluded from this and other experiments
that these genes are not functionally redundant. In characterizing these
mutants more closely, however, we have noticed an unexpected result. The
double mutant sma-3 sma-2 shows a smaller body size and increased
penetrance of the male tail ray fusions than the single mutants. We
think that this increase in severity is likely because the existing
mutations are not nulls. The unexpected result is that, compared to the
double mutant, the triple mutant sma-4 sma-3 sma-2 is not as small and
has fewer ray fusions. So, even though sma-4 mutants have similar, not
opposite, phenotypes to those of sma-2 and sma-3, when we construct the
triple mutant, sma-4 acts like a suppressor. At present, we have no
clear explanation for this phenomenon, but it suggests that we must
consider models in which SMA-4 may have both positive and negative
effects on signal transduction.