Worm Breeder's Gazette 11(4): 95

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.

Defective Trans-Hypodermal Cell Anchorage in him-4 Mutants

Ed Hedgecock

Figure 1

Neurons and muscles adhere to the hypodermis via contacts with the 
basolateral cell membranes or basal lamina, respectively.  For 
stronger anchorage, many cells induce assembly of transhypodermal 
cytoskeletal elements, generally intermediate filaments (IFs), that 
couple them mechanically to the cuticle(1).  
In him-4 mutants, trans-hypodermal anchorage of diverse cell types 
is defective.  e1266 and e1267, isolated by D.  Riddle following ICR 
mutagenesis, and rh166, obtained by EMS mutagenesis, all share three, 
seemingly unrelated, phenotypes: (1) elevated meiotic nondisjunction 
of the X chromosome (Him phenotype), (2) abnormal linker cell 
migration (Mig phenotype), and (3) defective static attachment of 
cells to the body wall.  The Him defect is comparable in all three 
strains: hermaphrodites have about 5% males among their self-progeny (
In mutant males, the linker cell often fails to reflex and instead 
continues anterior along the ventral muscles or hypodermis onto the 
head (2,3).  This phene occurs in about 60% of e1267 males but only 
30% of rh166 males (Figure 1).  In the remaining males, the migration 
is normal until approaching the cloaca; there the linker cell may slow 
and stop before connecting.  Premature stopping is more frequent in 
e1267 (perhaps 100% penetrant) than rh166 males (about 50%).  The Mig 
phenotype explains why e1267 males are sterile, i.e., the vas deferens 
is never joined to the cloaca; if this is the sole cause, then some 
30% of rh166 males should prove fertile.  In wild type males, the 
distal gonad does not elongate during larval growth; in mutants, the 
distal arm often meanders posteriorly during L4 stage.  This is not an 
active migration led by the distal tip cell but passive extension 
resulting from germ cell proliferation.  Distal extension is not 
simply the result of removing the steric hindrance provided by the 
reflexed proximal gonad; this phene is not observed in mig-7 males 
with failed linker cell reflexion.
Wildtype ALM/PLM axons are anchored via the chain: neuronal membrane 
- extracellular mantle - hypodermal basolateral membrane - IF 
structure - hypodermal apical membrane - cuticle (Figure 2).  The 
mantle may be a hypodermal secretion induced by neuronal contact; the 
IF structure is a double track of IF proteins induced by the overlying 
axon.  In mec-1 mutants, both mantle and IFs are absent; axons float 
unanchored on the hypodermis and are functionless (4).  In him-4 
mutants, mantle is present as judged by peanut lectin staining but the 
IF structure is probably absent; axons float on the hypodermis.  
Unlike mec-1, these mutants are not overtly touch insensitive.  In 
late L4 hermaphrodites, certain uterine (use), vulval (type E), and 
muscle cells (vm1, um1) attach to the seam syncytium as an orderly 
complex (5,6).  These cells induce tonofilaments in the hypodermis 
that anchor them to the cuticle (see Wood (1988) Fig.  20, p.116).  An 
outline of the vulval-uterine-seam attachment complex, probably 
corresponding to the use-seam limits, can be discerned by Nomarski 
microscopy as a thin-line with down turned ends running through seam 
cells V3.ppppp and V4.pappp.  In him-4 adults, the uterus often everts 
through the vulva.  The use-seam attachment is incomplete or absent in 
these mutants; vulval E cells still form some attachment to the seam.
The rectum sometimes everts during molts, killing him-4 larvae; the 
cellular mechanism is not known.  Gaps between body muscle, possibly a 
weak mua phenotype, are seen occasionally.  Several of the him-4 
phenotypes would be lethal if more expressive or penetrant: (1) 
hermaphrodite broods are small because uterine eversion kills the 
young adults, (2) the rectum sometimes everts during molts, (3) gaps 
between body muscles, possibly a weak mua phenotype, have been seen 
very occasionally.  Thus him-4 affects essential events and may itself 
be an essential gene.  A primary defect in the assembly of trans-
hypodermal IF anchorages could explain most cellular phenotypes.  
Dynamic IF structures are also present in the nucleus and might 
conceivably affect chromosome segregation.  Understanding the relation 
of the Him, Mig, and Mua-like phenotypes may require a biochemical 
understanding of the him-4 product.
[See Figure 1]

Figure 1