trans

FURTHER READING 337
Also, a mutation in let-653, which encodes a
mucin-like protein, results in lethal arrest
concurrent with vacuole formation anterior to
the lower pharyngeal bulb, in a position consistent
with dysfunction of the tubular apparatus.
The extent to which these physiological
and genetic observations on C. elegans are
relevant to parasitic species is largely undetermined.
The existence of tubular structures
in parasitic nematodes is well documented.
Recent studies suggest that these structures
may be more elaborate than originally thought.
For example, anterior regions of the tubular system
in O. volvulus include paired glomeruluslike
structures in the lateral cords that appear
to connect to the cuticle through canals formed
by projections from the basal zone of the cuticle.
Anatomically, these structures resemble
organs with an osmoregulatory or excretory
function. However, assigning them a function
in homeostatic regulation will require additional
evidence, such as localization within their
membranes of ion channels or aquaporins.
There is no direct evidence that regulation of
water or solute transport across the tubule system
in nematodes is controlled by neuroeffectors
or hormones as analogous processes are
in higher eukaryotes. However, several observations
suggest this possibility. As noted above
for C. elegans, the excretory ampulla in larvae
of some parasitic species, including H. contortus,
pumps more rapidly under hyposmotic
conditions. The rate of filling of the ampulla is
controlled by the excretory valve, which is innervated
and presumably under some form of
neural control.
CONCLUSION
Helminth surfaces serve numerous critical
roles. Indeed, it is possible to assign a role to
surface structures in almost every aspect
of helminth physiology. Knowledge of the
biology of helminth surfaces has advanced
more slowly than that of their nervous systems,
probably because most modern anthelmintics
target the latter specifically, and these compounds
have provided an economic stimulus
for many ‘pharmacology-driven’ investigations
on helminth neurobiology. It is likely, however,
that future exploitation of helminth surfaces
in anthelmintic and vaccine discovery and
development will be aided greatly by knowledge
derived from C. elegans genomics. These
approaches will facilitate the characterization
of surface proteins in a variety of ways. Among
these will be approaches referred to generally
as ‘reverse pharmacology’, whereby genes for
candidate target proteins, selected from electronic
databases, will be cloned and then overor
underexpressed in C. elegans, which will
allow their functions to be studied against the
background of the full complement of ‘normal’
genes. This approach will also be extended
to parasites as orthologous genes are identified.
This genetic approach will be augmented
by others, including selective laser ablation of
cells that express proteins of interest. As illustrated
by several examples in this chapter,
these approaches are already being used with
great success in C. elegans, and their extension
to parasitic species requires only additional
data on the genome sequences of those species.
FURTHER READING
Arme, C. (1988). Ontogenetic changes in helminth
membrane function. Parasitology 96, S83–S104.
Beames, C.G. and King, G.A. (1972). Factors influencing
the movement of materials across
the intestine of Ascaris. In: Van den Bossche, H.
(ed.) Comparative Biochemistry of Parasites,
New York: Academic Press, pp. 275–282.
Bird, A.F. (1991). The Structure of Nematodes, 2nd
edn, New York: Academic Press.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS