trans

316 HELMINTH SURFACES
cuticle in these mutants, using deep-etched
replica analyses, show that the fibrous layer of
collagen is composed of parallel fibers instead
of the layers meeting at 60° angles seen in wildtype
strains. At the biochemical level, mutations
that effect this change result in removal
of a conserved cysteine, which inhibits formation
of tyrosine-derived non-reducible bonds
between collagen proteins. Other mutations in
sqt-1 result in markedly different phenotypes.
For example, sqt-1 mutations that affect conserved
arginines in a predicted cleavage site for
a subtilisin-like protease cause right-handed
helical twisting.
In addition to collagen and collagen-like proteins,
nematode cuticles contain at least one
glycoprotein, and small amounts of hyaluronic
acid and lipid. Lipid components that are localized
exclusively in the cortex have not been
chemically characterized. Also present in the
cuticle are chondroitin sulfate and several sulfated
mucopolysaccharides, which convey a
negative charge to the cuticle.
Developmental biology
The cuticle is a dynamic structure that is
replaced during each of four molts. The biochemical
events that control molting are not
well understood, but the hypodermis appears
to be the site of most cuticle protein formation.
Selective laser ablation of C. elegans hypodermal
cells before formation of the adult cuticle
leads to abnormalities in cuticle regions
directly above the lesions. Proteins recovered
from the hypodermis of A. suum differ from
those in the cuticle, indicating that considerable
processing of proteins probably occurs in
the cuticle layers prior to their incorporation.
Protocollagen proline hydroxylase, an enzyme
essential for cuticle protein synthesis, is present
in the hypodermis of adult A. suum.
During synthesis of the collagen matrix, individual
collagen genes are expressed in distinct
temporal progression. Chemical signals that
control the timing of cuticle molting are not well
understood. In insects, molting is hormonally
controlled, with the cholesterol derivative
20-hydroxyecdysone playing a critical role
through its interaction with nuclear hormone
receptor-class transcription factors, which are
activated in a cascading manner. C. elegans contains
about 270 genes that belong to the family
of nuclear hormone receptor-like transcription
factors. Most are uncharacterized, but at
least one, nhr-23, is expressed in the hypodermis
and has been implicated in molting. The
megalin-related protein, LRP-1, is a putative
sterol receptor that may play a critical role in
this process. LRP-1 is expressed on the apical
surface of the hypodermis. Mutations in lrp-1
cause arrested growth, usually between the
third and fourth molts, and an inability to shed
and degrade the old cuticle (i.e. the sheath, in
third stage larvae). The stage-specific switch
from larval to adult cuticle appears to correlate
with transcriptional activation of adultspecific
collagen genes and simultaneous
repression of larval-specific genes in hypodermal
cells. Several other genes that participate
in this process in C. elegans have been identified,
including the heterochronic genes, lin-29,
lin-4, lin-14 and lin-28.
Most species of nematode grow extensively
between molts, and some continue to grow
even after the final molt. To accommodate
that growth, the cuticle must expand concurrently.
Generally, cuticle thickness increases in
direct proportion to length of the parasite.
Most of the increase in thickness occurs in the
medial layer, which may become six to eight
times thicker as the adult parasite grows from
a length of 5 cm to 30 cm. The process by
which cuticular proteins permeate the scaffolding
of the growing cuticle to reach their
destinations and become integrated into the
structure is not known.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS