Hemidesmosome-intermediate filament complexes play an essential role in maintaining adhesion and transmitting force between tissues at sites of mechanical stress. They are prominently observed in the epidermis where muscle force is transmitted across this tissue to the cuticle. Mutations affecting proteins in these complexes have been previously shown to result in failure of the mechanical linkage between muscle and cuticle at varying stages of development. Mutations in
mua-1 and
mua-2 result in postembryonic epidermal-cuticle separation.
mua-1 encodes one of the three members of the Kruppel-like family (KLF) of transcription factors in C. elegans. In vertebrates, these proteins are involved in regulating tissue specific gene expression in a wide variety of developmental events, and we predict that MUA-1 is likely to be regulating attachments between epidermis and cuticle in response to developmental cues.
mua-1 mutations also result in prolapse of the uterus through the vulval opening suggesting MUA-1 may also regulate attachment of the uterus to the epidermal seam. A
mua-1p::gfp reporter construct is expressed throughout development in the epidermis, excepting the seam, and at the L4-adult transition in the utse,
uv1,
uv2, and
uv3. Preliminary results show nuclear localization of a MUA-1::GFP translation fusion in epidermal cells, consistent with its predicted regulatory role. We are currently testing whether other proteins expressed at the uterine seam are mis-expressed or their pattern disturbed in
mua-1 mutants. Positional cloning mapped
mua-2 to a region of DNA covered by the YAC Y66D12. Initial observations suggest that
mua-2 encodes a novel single pass transmembrane receptor, based on weak transgene rescue and the finding of a frame-shift associated with
mua-2(
rh174) DNA. Furthermore, RNAi results in defects similar to those seen in mutant
mua-2 homozygotes, however a clear Mua (muscle detachment) phenotype in RNAi exposed worms was not observed. We are currently sequencing a second
mua-2 allele to confirm the predicted identity, and constructing a rescuing MUA-2::GFP translational fusion reporter to test the predicted epidermal expression. In addition, we identified a second gene in the C. elegans genome with extensive sequence similarity to the presumptive
mua-2 gene. We are testing through RNAi analysis in wild-type and in
mua-2 mutant animals whether the protein encoded by this second gene plays a role in cell attachment or interacts with MUA-2.