Essential roles for four cytoplasmic intermediate filament proteins in Caenorhabditis elegans development

Abstract

The structural proteins of the cytoplasmic intermediate filaments (IFs) arise in the nematode Caenorhabditis elegans from eight reported genes and an additional three genes now identified in the complete genome. With the use of double-stranded RNA interference (RNAi) for all 11 C. elegans genes encoding cytoplasmic IF proteins, we observe phenotypes for the five genes A1, A2, A3, B1, and C2. These range from embryonic lethality (B1) and embryonic͞ larval lethality (A3) to larval lethality (A1 and A2) and a mild dumpy phenotype of adults (C2). Phenotypes A2 and A3 involve displaced body muscles and paralysis. They probably arise by reduction of hypodermal IFs that participate in the transmission of force from the muscle cells to the cuticle. The B1 phenotype has multiple morphogenetic defects, and the A1 phenotype is arrested at the L1 stage. Thus, at least four IF genes are essential for C. elegans development. Their RNAi phenotypes are lethal defects due to silencing of single IF genes. In contrast to C. elegans, no IF genes have been identified in the complete Drosophila genome, posing the question of how Drosophila can compensate for the lack of these proteins, which are essential in mammals and C. elegans. We speculate that the lack of IF proteins in Drosophila can be viewed as cytoskeletal alteration in which, for instance, stable microtubules, often arranged as bundles, substitute for cytoplasmic IFs.

Intermediate filaments in Caenorhabditis elegans

“…The cytoplasmic C. elegans IF polypeptides are distinguished by an elongated coil 1b with 42 extra residues which is typical for protostomia and is also a feature shared by all lamins [Weber et al, 1989;Dodemont et al, 1990Dodemont et al, , 1994. Furthermore, the A-and B-type IFs also contain an additional, 110 amino acid-long globular Ig-like fold lamin-homology segment in the tail domain [Weber et al, 1989;Karabinos et al, 2001]. These features argue for a common ancestry of lamins and cytoplasmic C. elegans IFs.…”

“…Based on the pioneering work of the Weber lab [e.g., Karabinos et al, 2001Karabinos et al, , 2002Karabinos et al, , 2003Karabinos et al, , 2004 two major expression groups can be defined for cytoplasmic IFs ( Table I). The first group is characterized by the obligatory presence of IFB-1 which forms heteropolymers with any IFA and is therefore co-expressed with different IFAs in various cell types [Karabinos et al, 2003; Table I].…”

“…We think that this and the fact that all IFCs, IFDs as well as IFP-1 and IFB-2 are present in the intestinal apical terminal web justify a common grouping. The IF-rich terminal web is a characteristic feature of the subapical region of adluminal metazoan intestinal cells [Hirokawa et al, 1982] consisting of a complex mesh of various cytoskeletal and [Karabinos et al, 2003] Early larval Essential for viability after L1 [Karabinos et al, 2001] IFA-1b: amphids, c some neurons, c pharyngeal-intestinal valve, c vulva, c rectum c [Karabinos et al, 2001[Karabinos et al, , 2003] IFA-2 (5 MUA-6) Hypodermis, b,c amphids, c uterus, b,c vulva, c rectum c [Hapiak et al, 2003;Karabinos et al, 2002Karabinos et al, , 2003 Early larval Hypodermal integrity, attachment of muscles to the body wall [Karabinos et al, 2001;Hapiak et al, 2003] IFA-3…”

“…Hypodermins (only embryonic and larval), c ventral nerve cord (only larval), c rectum c [Karabinos et al, 2001[Karabinos et al, , 2003 cytoskeleton-associated polypeptides [e.g., Ku et al, 1999]. Whether the intestinal C. elegans IF polypeptides also form heteropolymers has not been investigated.…”

“…The contribution of IFA-2, IFA-3, and IFB-1 to epidermal morphogenesis, elongation, and muscle attachment has been emphasized in this context lending strong support for a crucial mechanical function of IFs [Plenefisch et al, 2000;Karabinos et al, 2001Karabinos et al, , 2004Hapiak et al, 2003;Woo et al, 2004].…”