Worm Breeder's Gazette 15(1): 51 (October 1, 1997)

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 analysis of dopaminergic neuron specification in the male tail

Robyn Lints, Scott W. Emmons

Albert Einstein College of Medicine, 1300 Morris Park Ave, Bronx, NY 10461

      Hermaphrodite and male C. elegans possess four pairs of sensory
neurons that contain the neurotransmitter dopamine, the CEPs, ADEs and
PDEs.  In addition, the male has three pairs of sex-specific
dopaminergic neurons, namely the A neurons of rays 5, 7 and 9 (1).
Behavioral analyses of males lacking these ray neurons and of cat-2
mutants, in which dopamine synthesis is significantly reduced, suggest
that dopamine facilitates the execution of one or more steps in male
mating behavior (2, 3).
      The existence of many male behavioral and developmental mutants
provides an opportunity to explore the genetic basis for the expression
of dopaminergic cell fate and function in the rays.  To this end we are
examining existing mutants for defects in the specification of the
dopaminergic neurons.  In order to follow the fate of dopaminergic
neurons in mutant backgrounds, we have developed a gfp expression
marker that specifically defines terminally differentiated dopaminergic
neurons.  This marker consists of a translational fusion between gfp
and a putative C. elegans tyrosine hydroxylase (TH) gene.  Tyrosine
hydroxylase is the first enzyme in the dopamine biosynthetic pathway
and is frequently used as a marker for dopaminergic cell fate in other
systems.  We identified this putative C. elegans TH gene in the genome
sequencing project database on the basis of its sequence similarity to
the rat and Drosophila tyrosine hydroxylase genes.  C. elegans TH
shares approximately 50% amino acid identity with the rat and
Drosophila genes and is located on cosmid C09D7 on linkage Group II.
The TH/gfp fusion is specifically expressed in all neurons previously
defined as dopaminergic by Sulston et al. (1975), including the neurons
of rays 5, 7 and 9.  Interestingly, the physical map position of this
gene corresponds closely to the genetic map position of cat-2, a
mutation which specifically disrupts dopamine synthesis (1).  We are
currently investigating whether this putative tyrosine hydroxylase gene
can rescue the cat-2 defect.
      In our first use of this TH/gfp reporter to screen for mutants
defective in dopaminergic cell specification we examined reporter gene
expression in TGF-beta pathway mutant backgrounds.  Mutations in
TGF-beta pathway components daf-4 and sma-2, -3, and -4 have been shown
to alter the identity of rays 5, 7 and 9 - the rays that contain the
dopaminergic neurons.  In these mutant backgrounds, the structural
cells of rays 5, 7, and 9 take on the identity of a neighbouring ray
(4).  Thus, TGF-beta pathway activity in rays 5, 7 and 9 is necessary
for the expression of their structural cell identity.  We were
interested in determining whether this pathway is also necessary for
expression of the neuronal cell identity of these rays - specifically
we asked whether this pathway is required for expression of the
dopaminergic fate of the A neuron.  To address this question, we
introduced our TH/gfp reporter into each of the following backgrounds:
daf-4, sma-2, -3, and -4 and examined the affects of these mutations on
expression of TH/gfp.  We find that in all these mutant backgrounds
expression of TH/gfp in dopaminergic neurons common to both sexes,
namely the CEPs, ADEs and PDEs, is unaffected.  In contrast, TGF-beta
pathway defects reduce or eliminate expression of TH/gfp in rays 5, 7
and 9.

1. Sulston, J. E., Dew, M., & Brenner, S. (1975)  J. Comp. Neurol. 163:
2. Loer, C. M. & Kenyon, C. J. (1993)  J. Neurosci. 13: 5407-5417.
3. Liu, K. S. & Sternberg, P. W. (1995)  Neuron 14: 79-89.
4. Savage, C., Das, P., Finelli, A. L., Townsend, S. R., Sun, C-Y.,
   Baird, S. E. & Padgett, R. W. (1996)  Proc. Natl. Acad. Sci. USA 93: