trans

322 HELMINTH SURFACES
The cuticle–hypodermis complex, intestine
and tubular system all appear to participate in
osmotic and volume regulation in nematodes.
However, studies that define the relative
contributions of each of these structures, or
the underlying mechanisms, are lacking.
Since osmotic regulation appears to occur over
hours, while turgor pressure changes over seconds,
it is unlikely that the two processes are
directly linked. The ability of nematodes to
withstand major osmotic changes in their natural
environment, as opposed to those observed
in the laboratory, probably depends more on
the permeability of the cuticle to water than
anything else. Marine nematodes are usually
isosmotic with seawater, and their cuticles are
highly permeable to water. Water permeability
in the marine nematode Europlus, for example,
is 4.4 m 3 H 2 O/m 2 body surface/hour, a value
over 100-fold greater than that recorded for
Aphelenchus avenue, a soil-dwelling species,
and over 200-fold greater than that for C. elegans,
another soil-dweller, or larvae of the
animal-parasitic species, Nippostrongylus muris
(brasiliensis) and Ancylostoma caninum. The
cuticle–hypodermis complex of adult A. suum
is also highly permeable to water, as demonstrated
by [ 3 H] 2 O flux studies. Water-filled pores
that traverse the cuticle of A. suum and other
species provide a highly accessible pathway for
water in the environment to reach the outwardfacing
membrane of the hypodermis.
The rates of transcuticular water flux reported
for most nematodes are hundreds-fold greater
than achievable by simple diffusion across lipid
membranes. In other organisms, water transport
across membranes occurs via intrinsic
membrane proteins called aquaporins. These
proteins have been most thoroughly studied
in human red blood cells and kidney proximal
tubules, but also exist in other vertebrate tissues.
Aquaporins are thought to form from two hemipores,
each containing three transmembrane
domains. Homologous proteins have been
identified in other organisms, including bacteria,
plants and insects. Each contains the
highly conserved motif Asn-Pro-Ala in the
putative pore-forming region of the protein.
Recently, a cDNA encoding a C. elegans aquaporin
was cloned and expressed in Xenopus
oocytes. Expression of the channel endowed
oocytes with greater permeability to water
and, to a lesser extent, urea, but not glycerol.
This gene was expressed only in early larvae of
C. elegans and was completely suppressed
before hatching.
Other genes predicted to encode aquaporins
have been found in the C. elegans genome
database and in cDNA libraries from various
parasitic nematodes, including Toxocara canis,
Brugia pahangi and O. volvulus. Definitive
studies of the functional properties of the proteins
encoded by these genes, as well as their
pattern of expression in nematode tissues, are
currently underway. Aquaporins probably play
a critical role in the transport of water across
the cuticle–hypodermis complex. Identification
of the rate-limiting barrier to water diffusion
across this tissue at either the hypodermis
or the epicuticle (or both) awaits experiments
on the localization and biophysical characterization
of water channels in these organisms.
Though aquaporins may provide an important
pathway for transport of water across the
cuticle–hypodermis complex to compensate
for changes in external osmotic conditions, it is
important to note that when active regulation
of water flux occurs, it is coupled in most metazoa
to movement of inorganic or organic solutes
across the same membranes, or to some metabolic
activity that raises or lowers the concentration
of organic solutes within the cells or
tissues. This first process is usually controlled
by ion channels that are specific for individual
ionic species. Several ion channels have been
suggested to contribute to osmotic and volume
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS